Literature Review on IronMaking

1

CHAPTER TWO

LITERATURE REVIEW ON IRONMAKING

The properties of different iron ores are discussed. The features, raw

materials, modifications, and disadvantages of blast furnace are covered.

Commercially applied Direct reduction processes for iron production are well-

described. The industrially proven smelting reduction processes

arepresented/explained.

2.1 INTRODUCTION

In order to model and optimize certain new process for ironmaking, this

should be preceded by presenting the different global techniques used in the

field. Each of the three principal categories of ironmaking ; blast furnace,

direct reduction, and direct smelting have advantages and disadvantages.

Moreover, the direct reduction and smelting reduction comprise a lot of

different processes which are competing to reach the degree of commercial

application. Big steel complexes can successfully use the blast furnace

together with one of the new techniques to utilize the advantages of both.

2.2 PRINCIPAL IRON BEARING MATERIALS

As will be shown later, the nature of iron bearing materials is very

important parameter which can cause the success or failure of certain

technology. Nearly all technologies put constraints for the nature of iron

bearing materials to be used in the production of iron

2

2.2.1 Different Iron Ores

The ores of iron occur exclusively as its oxides. [6]

Hematite is the most

plentiful iron mineral mined, followed by magnetite, Goethite “limonite”,

siderite, ilmenite, and pyrite. [1]

The following table compares the different iron ores in terms of chemical

composition, percentage iron, color, crystal structure, and specific gravity.

Table 2.1 Principal Iron Bearing Materials[7]

Parameter Hematite Magnetite Goethite

(Limonite)

Siderite Ilmenite Pyrite

Chemical

Name

Ferric

Oxide

Ferrous-

Ferric

Oxide

Hydrous Iron

Oxide

Iron

Carbonate

Iron-

Titanium

Oxide

Iron

Sulfide

Chemical

Formula

Fe2O3 Fe3O4 HFeO2 FeCO3 FeTiO3 FeS2

Iron, wt% 69.94 72.36 62.85 48.2 36.8 46.55

Color Steel gray

to red

Dark gray

to black

Yellow or

brown to

nearly black

White to

greenish

gray to

black

Iron-

black

Pale

brass-

yellow

Crystal

Structure

Hexagonal Cubic Orthorhombic Hexagonal Hexagonal Cubic

Specific

Gravity

5.24 5.18 3.3-4.3 3.83-3.88 4.72 4.95-

5.1

Limonite is famous with its extreme reducibility; thus, it is often employed

as a mixture with less reducible ores. On the other hand, Magnetite is

characterized by special magnetic behavior which causes easy separation of

the ore impurities. Moreover, Magnetite is denser than other ores, thus it is

3

considered a resistance to the reducing gases. Thus, most of the reduction

takes place by the solid carbon. The latter causes higher time needed for

complete reduction. To treat this, high fuel consumption is needed.[6]

2.2.2 Impurities in Iron Ores

Beside the iron oxide contained in any ore, there are also different

percentages of impurities (called gangue) which will affect the purity of the

produced iron especially if one of the direct reduction technologies is to be

used. Thus, direct reduction technologies have big constraints on the purity of

iron ore.

The following table shows the composition of 2 different iron ores; one

from a Swedish mine and the other from an Egyptian mine. It is apparent that

the iron ore from both minesgreatly differ from each other. It is also apparent

that the iron percentage in the Egyptian mine is low, and thus it isn’t a suitable

raw material for the production of DRI.

Table 2.2 Iron Ore composition from 2 different mines

Composition Swedish Ore[8]

Egyptian Ore[9]

Fe 66.74 58

Al2O3 0.25 2

CaO 0.47 0.7

K2O 0.035 0.3

MgO 1.45 0.02

MnO 0.085 3.5

Na2O 0.041 0.3

P2O5 0.021 0.2

S 0.0005 0.1

SiO2 2 8

TiO2 0.195 0.02

4

V2O5 0.21

BaO 1.5

2.2.3 Classification of Iron Ores

The most famous classification of iron ores is Bessemer versus non-

Bessemer. A Bessemer ore is one in which the phosphorous is low enough to

make the pig iron only contains 0.1 % phosphorous or less. Iron ores can be

also classified as manganiferous, siliceous, high-Phosphorous, low-

phosphorous, etc.[6]

Iron ores can also be classified according to the particle form. So, it is

frequent to hear that the iron ore is classified into lump, fine, pellets, and

sinter.

2.2.4 Pelletisation

Pelletisation is a process of agglomeration of iron ore fines. In this process,

the particles smaller than 200 μm of which about 50% with 50 μm size are

converted into 12-15 mm pellets with nodular shape.

These iron ore fines are mainly generated at mines. However, further

grinding is also needed to reach the size stated above. For efficient pelletizing

process, the iron ore should be of very high quality (low gangue). Thus, low

grade ore fines should be grinded and cleaned.[10]

The ground ore is mixed with the proper amount of water and binder,

normally bentonite, hydrated lime, or organic material, and then is rolled into

small balls 9–15 mm in diameter in a balling drum or disk. These green (wet)

pellets are dried, then are heated to 1200–1375oC to bond the small particles,

and finally are cooled. As shown in figure 2.2, the heating can be done on a

traveling grate, in a shaft furnace, or by a combination of a traveling grate and

5

a rotary kiln (grate–kiln). The traveling grate and grate–kiln are the most

commonly used pelletizing processes.[1]

Figure 2.1 Iron Ore Pellets

Figure 2.2 Pellet-Firing System: a) Shaft furnace, b) Grate furnace,c)

Travelling grate furnace [10]

2.2.5 Sintering

Sintering is a principal section in integrated steel plants. Sintering consists

of igniting a mixture of wet iron-bearing limestone and cokefines on a

traveling grate to produce a clinker-like aggregate (sinter) suitable for use in

6

Water

Mashing Stage

Fine Scrubbers

ESP

Thickener

Sludge

WaterTreatment Immobilisation

Depot

Slag

Recycling

Fe-Components

Sinter Machine

Process Air

Cleaned Water

DischargeWater

Sludge Tank

Floating Sludgeto BF

Water

Nat.GasReheating

EmissionMonitoring

Fan

Main Fan

Quench

MashingWater

SludgeWater

the blast furnace. The iron-bearing fines can include iron ore fines, flue dust,

or other steel mill wastes. The traveling grate is shaped like an endless loop of

conveyor belt. The bed of material on the grate is first ignited by passing under

an ignition burner that is fired with natural gas and air; then, as the grate

moves slowly toward the discharge end, air is pulled down through the bed. As

the coke fines burn in the bed, the heat generated sinters the particles. At the

discharge end of the machine, the sinter is crushed to remove extra large

lumps, then cooled, and is then finally screened. [1]

When the coke fines are combusted on the grate, partial fusion takes place

for the charge. On cooling, the different mineral phases crystallize and bond

the structure together to form a strong sinter.[11]

The sinter bed permeability mainly controls the performance of sintering. It

was found that water addition, particle size distribution, ore porosity are very

important factors in determining the bed’s permeability.[12], [13]

Figure 2.3 Flow Sheet of Iron Ore Sintering [14 ]

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2.3 Blast Furnace

The blast furnace is the dominant unit operation for iron production till now.

Continuous improvements to the blast furnace have enabled it to compete with

the fast growing new direct reduction and smelting reduction technologies.

2.3.1 Blast Furnace Process Description

As shown in Figure 2.4, blast furnace is a steel reactor standing

approximately the height of a 10-story building. It has a refractory brick lining

to enable it to withstand the intense heat generated within.

As shown in Figure 2.5, a hopper at the top discharges raw materials into

the furnace by using a pressurized gas seal system known as double bell. [1]

This system prevents gases and dust from escaping into the atmosphere. The

charge consists of alternating loads of coke and a mixtureof iron bearing

materials (iron ore, pellets, sinter) and flux (mainly limestone, and

dolomite).[3]

These solids form a column that descends through the furnace

with a total residence time of about eight hours.[15]

Around the circumference

of the furnace near the bottom, water cooled nozzles (called tuyeres) inject

preheated air, often enriched with oxygen, into the furnace. In most cases,

gaseous, liquid, or powdered fuel are introduced together with the preheated

air. The heated air burns the injected fuel, and coke to produce the heat

required by the process, and to provide reducing gases that removes oxygen

from the ore. [16]

The reducing gases rapidly ascend through the column and are

expelled through a pair of stacks at the top in less than 20 seconds. [3]

The

reduced iron melts, and settles in the bottom. The flux combines with the

impurities of the ore and coke to produce slag which also melts and

accumulates on the top of the molten iron. The gases exiting the furnace are

cleaned, and then are utilized in different ways.

8

The charging to blast furnaces is continuous, while the tapping of the hot

metal and slag is performed in batches. So, the process is considered semi-

continuous.

Figure 2.4 Blast Furnace

9

Figure 2.5 Double Bell System Used in Blast Furnaces

2.3.2 Blast Furnace Tapping

In the terminology of blast furnace, getting the hot metal and slag from the

furnace’s bottom is called tapping. As stated before, tapping takes place in

batches. The latter range from 3 to 5 hours.

Every product has a tapholewhere the product emerges from. A taphole is

built into the refractory lining of the blast furnace. To get each product, a

tapping machine is used as shown in figure 2.6. The latter utilizes a

tapholedrill to drill a hole through the taphole material as shown in figure 2.7.

After getting the product, and to close the holes again, the same tapping

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machine utilizes a mud gun is used to plug the holes with a quick-hardening

clay.[1]

Figure 2.6 Tapping MachineFigure 2.7 Blast Furnace Directly

After Tapping

The slag’s hole is about 1.4 meters above the hot metal’s hole. From the

operating experiences, the slag is tapped before the hot metal. [17]

As shown in figure 2.8, Hot metal is poured into refractory-lined railcars.

The latter are used for transportation to the steelmaking section.

Slag is either transferred as a liquid in inverted bell-shaped rail cars, or

poureddirectly into a slag pit adjacent to the blast furnace. After solidification,

slag have multiple uses. The slag can be crushed, sized and sold for road

ballast. In addition, the slag may be granulated using a water spray to make a

by-product suitable for cement industry. Finally, it can also be used for the

production of rock wool insulation.[1]

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Figure 2.8 ARailcar Used for Transporting Hot Metal to the Steelmaking

Facilities

2.3.3 Blast Furnace Gas Cycle

As shown in figure 2.9, Off-gases (top gas) leave the top of the furnace

through uptake pipes,reverse direction in the downcomer, and enter the dust

catcher. In the latter, dust is separated from the gases. The dust is emptiedinto

a rail car for transport to a sinter plant for recycle or to a landfill.

Figure 2.9 Downcomer and Dust Catcher Used in the BF Gas Cycle

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Afterleaving the dust catcher, the off-gas is washed in a venturi scrubber to

get rid of the remaining solid particles, and to condense the water vapor to

achieve higher gas calorific value.[18]

The cleanedgas can be utilized in

different ways:

a)Steam generation and Power generation (as shown in figure 2.10)

b) Firingsteelmaking furnaces

c) Firing of coke ovens in the coke-processing nearby plant.

d) Firing the blast furnacestoves.

The blast furnace stoves (also known as cawpers) are used for preheating

the air used in the blast furnace. There are usually three or four stoves lined

with refractory materials. Mainly air passes by one of the stoves, and the

others are being heated by the combustion of blast furnace gases. Thus, the

stoves alternate between absorbing heat generated by combustion ofthe blast

furnace off-gas and releasing heat to the cold blast air as it passesdown

through the stove.After leaving the stoves, the hot blast enters a large

refractory-lined bustlepipe which distributes the air on thetuyeres.[1]

Figures

2.11, 2.12, and 2.13 show the air system in the blast furnace.

Figure 2.10 Blast Furnace Gas Utilized in Power Generation

13

Figure 2.11 TuyeresAround the Blast Furnace

Figure 2.12 Large Air Pipe Connected to Tuyeres

14

Figure 2.13 Stoves Used for Preheating the Air

2.3.4 Production and Processing of Coke

As shown in the blast furnace process description, coke is a main raw

material. Coke is produced from coking coals in a plant nearby the blast

furnace inside the integrated steel complex.

The process of producing coke involves heating coal in absence of oxygen

to about 2000oF. The heating of the coal is done in narrow, rectangular, silica brick

ovens. The ovens stand in groups of 10 to 100 or more, called batteries.Many

of the organicsubstances inside the coal volatize at thattemperature, leaving

the coke behind. The volatilized gas is then subjected to sequentiallylower

temperature direct contact condensing chambers, which capture tar (amixture

of many relatively heavy organic com-pounds), oils, light oils, and finally low-

molecular-weight gases. Coke is pushed into a quenching car that transports it

15

to quenching towers. Here, the coke is sprayed with water to lower the

temperature. About 35% of this water evaporates. The remaining water drains to

a settling basin, where the coke fines are removed.[19]

Because of the direct contact of water with the coke oven gases, wastewater

streams from the coking plant contain high concentrations of ammonia,

phenol, sulfides, thiocyanates, and cyanides. Moreover, airborne emissions

also include SOx, NOx, ammonia, and particulate matters.[20]

The process of transforming coal into coke is a very complex process which

isn’t fully understood. The most known theory is:

“The more fusible components soften, the more volatile vaporize, and the less

thermally stable decompose. The decomposition of other components, with the

deposition of the decomposition products within the pores of the mass

contributes to the strength of the coke”. [21]

2.3.5 Iron Ore Properties (1st Raw Material)

As stated before, the ores of iron occur exclusively as its oxides where

hematite is the most famous form. Iron ore is charged into the blast furnace in

the form of pellets, sinter, orlump ore. Total iron content in the iron oxides

normally ranges from 60 to 66%,but may be as low as 50% in low quality ores

or in sinter using high quantitiesof recycled materials. The impurities that can

accompany iron oxide has been previously stated in Table 2.2.

Size control is important in order to guarantee appropriate bed permeability

inside the blast furnace,so fines (_6 mm) should be kept less than 2%. [1]

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2.3.6 Coke Properties (2nd

Raw material)

Coke has 3 primary functions in the blast furnace. First, the coke is a

reductant. It is gasified with the hot air to produce CO rich reducing gas that

converts the iron ore feed into iron. Second, the coke is a fuel. It provides

sufficient heat to melt the iron and the slag and promote the endothermic

reactions involved in the blast furnace. Third, the coke serves as a packed bed.

It provides a self-supporting porous bed that facilitates the contact between the

descending iron ore charge, and the ascending reducing gases. It also

facilitates the drainage of molten iron and slag phases.

If coal is used instead of coke, the volatile matter inside the coal will rapidly

plug the bed once being volatilized.[2]

Moreover, the coke has higher

mechanical strength than coal, and hence promotes stable operation.[1]

The composition of coke is known by conducting proximate analysis where

4 parameters are measured: Fixed Carbon, volatiles, ash content, and moisture.

Mainly Ash, phosphorous, and sulphur are the major impurities in

coke.Sulphur is the most undesirable impurity in this industry. Despite being

partially removed during coking, still coke can contain about 1% sulphur.[6]

Size control is important in order to guarantee appropriate bed permeability,

so the minimum used size is about 15 mm.[17]

Through continuous research, and as an attempt to reduce the coke

consumption in the blast furnace, it is customary now to see solid, liquid, or

gaseous fuels, eg, coal, fuel oil, ornatural gas, may be added to the hot air blast

at the tuyeres to replace some of thecoke. [1]

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2.3.7 Flux Properties (3rd

Raw Material)

If iron ores are reduced without flux, the impurities of the iron ore (mainly

silica and alumina) will react with iron oxides to form double silicates of iron

which is a heavy loss of iron. By the addition of a fluxing material (e.g.:

limestone), silica and alumina will have a great tendency to react with lime

than iron.

The coke forms ash when burnt in the furnace. The ash formed is composed

largely of silica and alumina like the impurities of the iron ore. Flux also reacts

with coke ash.

Flux also reacts with sulphur, and thereby reducesits concentration in the

hot metal. Most of the sulphur originates from coke, and it has the ability to

transfer from the coke to the iron product if no flux is available.

All the products of the flux reactions form the slag phase which is also

tapped from the blast furnace. However, high temperatures are preferred as the

slag will more desulphurize iron at high temperatures.[6]

Fluxes are usually added in the form of either limestone or dolomite.

Thefluxes provide the basic constituents (CaO and MgO) needed to balance

theacid constituents (SiO2 and Al2O3) from the coke and ore.[1]

The main flux impurities are silica, alumina, sulphur, and phosphorous. The

presence these impurities will reduce the % of lime and magnesia, and this will

require additional amount of flux to get rid of them. The use of impure flux

material will cause the formation of more slag, which need more heat to be

melted, and thus the fuel consumption increases.

Mainly sulphur and phosphorous aren’t found in high percentages in fluxes.

However, if they are high, they will be detrimental to the quality of the iron

produced. Thus, in this case the use of low grade fluxing material isn’t

possible.[6]

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2.3.8 Air Properties (4th

Raw Material)

Air may also be considered a raw material. Over 1.5 ton of air is required to

produce 1 ton of hot metal.[1]

2.3.8.1 Modifications in temperature and composition of the air

The air supply to the tuyeres has been greatly modified all over the years. It

was known that preheating the air will lead to less coke consumption, so it was

always a research objective. Long time ago, and when the researchers first

think about preheating the air, higher temperatures were really produced in the

combustion zones, but it was apparent that the heat transfer to the charge

wasn’t rapid enough to use this heat. Consequently, the idea of preheating the

air wasn’t advantageous in this period.

After more research, the researchers pointed out that this poor performance

might be caused by the undesirable melting of unreduced burden materials and

subsequent resolidification during reduction by solid carbon. Moreover, the

use of well-sized dense ores will cause more rapid heat transfer.

Between the years of 1950 to 1955, it becomes apparent to the operators that

if the moisture content of the air was increased, the higher air temperatures

could be used satisfactorily. The added moisture promotes the endothermic

water gas reaction (C + H2O = CO + H2), and thus the temperature in the

combustion zones isn’t extremely high, and consequently the furnace runs

more smoothly. The coke is consumed by air through combustion, and by

steam through water gas reaction, and thus the rate of heat transfer and rate of

reduction were improved.

After further research, and as stated above, it was possible to inject

auxiliary fuels as pulverized coal, fuel oil, and natural gas with the air through

the tuyeres in order to decrease the coke consumption.Moreover, and after

reaching good understanding of the process, and good control on the

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temperature in the combustion zone, it is now possible to use oxygen enriched

air as high as 30%. Oxygen enrichment causes high production rates. However,

a good economic analysis is needed to determine the optimum percentage of

oxygen enrichment.[22]

So, recently the air used in the tuyeres is preheated to between 900 and

1300o C, and is mainly associated with moisture, oxygen, and auxiliary fuels.

2.3.8.2 Modifications in the blast furnace stoves

As stated above, the blast furnace stoves (also known as cawpers) are used

for preheating the air used in the blast furnace. They are considered

regenerative heat exchangers that convert the chemical energy of the blast

furnace exiting gases into sensible heat of the entering air.

Before discovering how to reach high air temperatures without causing

problems in the combustion zone, most blast furnaces had more stove capacity

than required.

However, after getting more experience, and discovering that high air

moisture can lead to better utilization of high air temperatures, many operators

used the highest hot blast temperature that the stoves could produce. However,

this has caused a control problem. If the furnace suddenly becomes cold, they

couldn’t use the stoves as a control parameter. The solution to this problem

was the control of the air’s moisture content. So, if the furnace becomes

suddenly cold, the air’s moisture content was decreased and vice versa.

However, the stoves always suffer from the problem of dust and alkali

attack on the refractories. Some reports stated that 60% of the dust and 80% of

the alkalis from the gas remain in the stove.[22]

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2.3.9 Blast Furnace Reactions

From an overall heat and mass balance point of view, the blast furnace can

be divided into 4 zones:

a) Combustion zone

As the coke descends through the furnace, it is heated by the ascending

gases. When it reaches the raceway in front of the tuyeres, it reacts

immediately with the oxygen in the hot blast air according to equation 1.

Equation1, however, is actually the combination of coke combustion (equation

2) and cokegasification (equation3, also referred to as solution loss).

C + 0.5 O2 CO (1)

C + O2 CO2 (2)

C + CO2 2 CO (3)

C + H2O CO + H2 (4)

Coke gasification occurs just outside the raceway area where gaseous

oxygenis no longer available to completely combust the CO to CO2. This

reaction goesessentially to completion at temperatures between 1500 to

2100oC. The net heateffect is exothermic; however, and as stated before, the

endothermic water gas reaction (equation 4)allows control of the temperature

in front of the tuyeres by controlling the moisturein the hot blast.

b) Melting (fusion) zone and final reduction of wustite:

21

H2 and CO from the previous reactions rise through the burden, contact

wustite (FeO) formed from previous reduction reactions in the upper part of

the furnace, and reduces it to iron.

CO + FeO CO2 + Fe (5)

H2+FeO H2O + Fe (6)

The iron absorbs carbon through contact with the coke, and melts owing to

itsdecreased melting point. Equations 3 and4 combine with equations 5 and 6

in a cyclewhich effectively regenerates CO. Owing to the highly endothermic

nature ofequation 3, the gases cool as they rise in the furnace.

c) Thermal reserve zone:

Once the gases have cooled toabout 925oC, the thermodynamics for

equation 3 are no longer favorable. Becausethe predominant reaction is now

equation 5 which is slightly exothermic, andbecause the mildly endothermic

equation 6 occurs to a much lesser extent, thegases do not cool appreciably,

resulting in a thermal reserve zone. The net relativeamounts of CO2 and H2O

produced by reduction are determined by equilibrium of the water gas reaction

(Equation 4).

d) Reduction of hematite to wustite (upper shaft):

Only the slightest amounts of CO or H2 are required to reduce hematite to

Wustite.

CO + Fe2O3 CO2 + FeO (7)

22

H2 + Fe2O3 H2O + FeO (8)

Moreover, calcination and magnesium carbonate decomposition (from the

flux) takes place in this zone:

CaCO3 CaO + CO2 (9)

MgCO3 MgO + CO2 (9)

In this zonethe gas temperature falls off rapidly because of cooling by the

incoming materials,evaporation of moisture, and the net endothermic nature of

the abovereactions.

In addition to the principal reactions discussed, several others are also important,

including:

Fluxing of the sulfur into the slag,

S + CaO + C CaS + CO (10)

Reduction of other metallic oxides,

MnO + C Mn + CO (11)

SiO2 + C Si + 2 CO (12)

P2O5 + 5 C 2 P + 5 CO (13)

Equations 10–13 result from contact between hot metal and slag, where

theproduced manganese, silicon,and phosphorus are dissolved into the hot metal.[1]

Finally, it is to be noted that factors affecting the rate of reactions in the

blast furnace are the reactivity of the reductant (fuel), particle size of the ore

23

and the reductant, and the intimate contact between the ore and the reductant.

Reactivity of the fuel is the form of the carbon which reacts faster than another

at any given temperature. High reactivity of the fuel is not always necessary

because of the high temperatures already available in the furnace. So, the

reactions will mainly depend at the rate of transfer of oxygen. [21]

2.3.10 Role of Computerized Programs in the Blast Furnace

Inside the blast furnace, many different unit operationsoccur simultaneously

including heat and mass transfer, reduction by gases, reduction by solid

carbon, high temperature gas generation, and finally smelting and liquid

drainage.[23]

The countercurrent flow of gas and solids include heat transfer

from gases to solids, and oxygen transfer from solids to gases.[24]

Moreover, the packed bed created by coke in the blast furnace differ than

the common packing features in the chemical engineering applications as

follows:

- The superficial velocities of molten slag and metal in the furnace are very

low, about 0.08 mm/s, compared with those in chemical processes.

- The packing material in the furnace is crushed coke while artificial

packings with much higher porosity are commonly used in chemical processes.

- The liquids are more than 2.5 times as heavy as the packing, coke, in the

furnace while the packings are usually heavier than the liquids in chemical

processes.[25]

Thus, the blast furnace is a very complex reactor which needs rigorous

control in order to reach smooth operation. The accumulating experiences in

operating the blast furnace have resulted in the increased dependence on

computerized control.

Modern furnaces rely on computerized controls for weighing and

chargingthe raw materials. The ore and coke are charged in alternating

24

batches, so asto create distinct layers within the furnace which promote

permeability for therising gases.

Computer controls are also used for stove operation, to control deliveryof

the hot air. High air temperatures are generally desirable, as thesereduce the

coke rate. Control of the flame temperature in the raceway is effectedby

controlled additions to the hot blast, primarily of moisture. The effect of

adding auxiliary fuels should be well covered in the computerized control .

Moreover, Controlling slag chemistry is crucial in operating the furnace. In

general, increasing the basicity of the slag promotes sulfur removal but

decreases alkali removal. [1]

Increasing the basicity preserves the refractories,

but increases the slag's viscosity, and thus results in lower fluidity.[17]

So, all

these factors should be balanced by proper control on the amounts of limestone

and dolomite added.

2.3.11 The Challenge of Coal Injection

Among all the ironmaking processes, the blast furnace still holds the

dominant position. Despite that the new technologies have already started

competing with the blast furnace; experts believe that the blast furnace will

remain the principal method for iron making. This is essentially because of the

developments that have taken place over the years in the technology, and the

engineering aspects. [26]

However, experts also see that the blast furnace would only be competitive

on the long run in case of increasing the coal usage, and decreasing the

dependence on coke.[5]

This trend has really started all over the world, and

coke rates of 300 kg/THM (Ton Hot Metal) with 190-200 kg/THM of coal

injection have already been achieved. [27]

It is also foreseen that coal injection

of 250 – 280 kg / THM should be possible.

25

The coal injection is adopted till the maximum possible extent ensuring

stable operation of the furnace. The important aspects of coal injection include

coal quality, granulometry of coal, and influence of hot air temperature.

However, still more research is needed to decrease the consumption of coke

for the technology to be sustainable. The recent research trend is to increase

the oxygen enrichment in the entering hot air. [5]

2.3.12 Operational Features of Large Blast Furnaces in the World

The following table compares the size, productivity, iron ore form, and the

auxiliary fuel used in different blast furnaces. The hearth diameter is

equivalent to the bottom diameter.

Table 2.3 Comparison of different blast furnaces in terms of size,

productivity, iron ore form, and the auxiliary fuel used[5]

Country Furnace Hearth

Diameter, m

Productivity

t/m3/d

Burden Form

Auxiliary

Injected

Fuel

Finland Rautaruukki

BF1, BF2 7.2 3.4 Sinter Oil

Sweden SSAB

Trunplat BF2 8.5 3.5 Pellets Coal

Canada Dofasco BF4 8.5 2.9 Pellets Oil

USA Rouge C 8.6 3.1 Pellets Gas

Belgium Sidmar BF A 10 2.9 Sinter Coal

Japan Nisshin Kure

BF1 10.5 2.4 Sinter Coal

Australia BHP Port

Kembla BF5 12 2.5 Sinter Gas

China Wuhan BF5 12.2 2 Sinter Coal

26

Netherlands

Hoogovens

BF7 13.8 2.7 Sinter + Pellets Coal

Hoogovens

BF6 11 2.9 Sinter + Pellets Coal

The following table compares the top pressure, oxygen enrichment, coke

rate, auxiliary fuel rate, slag rate, and hot air temperature in different blast

furnaces. PCI refers to pulverized coal injection.

Table 2.4 Comparison of different blast furnaces in terms of top pressure,

oxygen enrichment, coke rate, auxiliary fuel rate, slag rate, and hot air

temperature[5]

Parameter Posco

(Korea) BF6

Hoogovens

(Netherlands)

BF7

Kimitusu 3

(Japan)

Nippon steel

(Japan)

Productivity,

t/m3/d

2.66 2.9 2.7 2.47

Top Pressure,

kg/cm2

2.5 1.67 2.25 2.2

% oxygen

enrichment 1.6 / 2 4.9 4 2.4

Burden 85% Sinter

15% Lump

50% Sinter

50% Pellets

50% Sinter

50% Pellets

93% Sinter

7% Pellets

Coke rate, kg

/ THM 390 339 365 392

PCI rate,

kg / THM 100 161 125 71 (Oil)

Slag rate,

kg / THM 320 236 236 286

Blast

Temperature o C

1200 1180 1180 1278

27

Moreover, it has become customary to use the slag resulting from the

steelmaking section in the blast furnace. This slag contains iron, manganese,

and an excess of calcined basic oxides with a ratio of bases to acids of 2 or 3

to 1. If this slag isn't used in the blast furnace, an additional cost will be

required to dispose of it. However, this slag also contains phosphorous, so the

added amount of this slag is often limited by the allowable phosphorous

content in the pig iron product.[22]

2.3.13 Environmental Analysis for the Blast Furnace Technology

The blast furnace route for steel production is always criticized by the

experts for its bad environmental impacts. As the environmental awareness in

the world is enhanced, the pressures on the blast furnace route are increased.

The pollutants are mainly generated from the sintering plant, and coking

plants.

2.3.13.1 Sintering Plant Pollutants

Emissions from the sintering process arise primarily from materials-

handling operations, which result in airborne dust, and from the combustion

reaction on the traveling grate.

Combustion gases from the latter source contain CO, CO2, SOx, and NOx,

together with particulate matter. The concentrations of these substances vary

with the quality of the fuel and raw materials used and combustion conditions.

Atmospheric emissions also include volatile organic compounds (VOCs)

formed from volatile material in the coke breeze, dioxins and furans, formed

from organic material under certain operating conditions, and oily mill

scale.Iron sintering has been identified as a source of polychlorinated

dibenzoparadioxins (PCDD) and polychlorinated dibenzofurans (PCDF).

28

Combustion gases are most often cleaned in electrostatic precipitators

(ESPs), which significantly reduce dust emissions but have minimal effect on

the gaseous emissions. Water scrubbers, which are sometimes used for sinter

plants, may have lower particulate collection efficiency than ESPs but higher

collection efficiency for gaseous emissions. Significant amounts of oil in the

raw material feed may create explosive conditions in the ESP. Sinter crushing

and screening emissions are usually controlled by ESPs or fabric filters.

Wastewater discharges, including runoff from the materials storage areas,

are treated in a wastewater treatment plant that may also be used to treat blast

furnace wastewater.[28]

2.3.13.2 Coking Plant Pollutants

The coke oven is a major source of fugitive air emissions. Table 2.5 shows

the approximate amounts of emissions resulting for every ton of coke

producedif there is no vapor recovery system.

Most of these emissions are released from doors during coking and when the

coke is pushed from the oven on the way to the quench tower. [1]

Table 2.5Approximate amounts of emissions resulting for every ton of coke

produced in the absence of vapor recovery system [20]

Pollutant Amount produced in kg per ton of coke

particulate matter (PM) 0.7 to 7.4

SOx 0.2 to 6.5

NOx 1.4

Ammonia 0.1

VOCs 3 (including 2 kg of benzene)

As shown above in the process description of the coke processing, the plant

also produces rich wastewater streams. Wastewater is generated at an average

rate ranging from 0.3 to 4 cubic meters per ton of coke processed.[1]

Table

29

2.6shows the approximate wastewater concentration resulting from the plant.

PAH stands for polycyclic aromatic hydrocarbons.

Table 2.6Approximate wastewater concentration resulting from the coking

plant[20]

Pollutant Concentration in mg/lit

BOD 1000

COD 1500 – 1600

TSS 200

Benzene 10

Phenol 150-2000

Ammonia 0.1-2

Cyanide 0.1-0.6

PAH 30

2.3.13.3 Blast Furnace Pollutants

As mentioned above, blast furnace gas is scrubbed before being used as a

fuel. Thewastewater stream from the scrubbing process contains iron oxide and

carbon particulates. Moreover, it also contains ammonia and cyanide which

were absorbed from the gases. [1]

Another source of pollution occurs during tapping the blast furnace where

appreciable amount of dust emerges. Finally, when the blast furnace gas is

used as a fuel, SOx and NOx result from the combustion.

2.3.13.4Greenhouse Gas Emissions for the Whole Industry

Generally, the largest future environmental problem facing the world’s steel

industryis that of greenhouse gas emission, specifically carbon dioxide.

30

The preparationof sinter fines or pellets uses a large amount of electricity.

The production of electricity is primarily based on coal and oil, whichends up

as CO2 and water. The reduction of iron ore is largely based on the use

ofcarbon, which ultimately ends up as CO2. Limestone used ultimately

dissociatesinto CaO and CO2. Thus, the industry produces a tremendous

amount of carbondioxide.

The blast furnace route followed by the basic oxygen furnace (BOF) used

for steelmaking results in emission of about one ton of CO2 per ton of

steel.Thus, the industry is one of the largest contributors to greenhousegas

emissions.

Unfortunately, currently there is no economic substitute for thereductant and

energy requirements of the industry. So the only choice to reduce these

emissions is to improvethe energy efficiency of the existing plants and

processes. [1]

2.3.14 Drawbacks of the Blast Furnace

Despite being the predominant technology for iron production, and despite

being modified all the over the years, the blast furnace route for iron

production suffers from a lot of disadvantages that enabled the new

technologies to compete with it. The drawbacks of the blast furnace can be

summarized in the following points:

There is continuing restructuring of the industry. In 1985, less than 50% of

the steel industry was in private hands. This percentage increases gradually,

resulting in increased competitive pressures. The capital cost of a

conventional integrated iron and steelmaking complex is very high, and

31

very large new plants are required to assure profit. This investment is

difficult to be afforded by the private investors.

On the other hand, mini-mills can produce quality products at competitive

cost at a much smaller scale. Thus, this option is more attractive to the

nowadays investors.[2]

The inherent dependence of the blast furnace process on the scarce coking

coal is a great disadvantage for the process.[5]

Blast Furnaces are relatively inflexible in terms of output and any reduction

in production results in a negative effect on the metal quality, both in terms

of chemistry and temperature.[4]

Since all the different unit operations takes place in a single reactor, there

are no means of ascertaining the efficiencies of individual process steps

taking place in the blast furnace.[5]

Coke ovens and sinter plants required for blast furnace operation are major

sources of environmental pollutants, and there are currently a lot of

pressures against the use of this route in iron production.[2]

2.4 General Overview on Direct Reduction

Direct reduction (DR) includes many processes in which iron ore in the

form of lump or pellets is reduced in the solid state by either solid or gasesous

reducing agents. Reformed natural gas or non-coking coal is generally used as

the reductant. [5]

In the DR processes, the final product is solid. So, the gangue won't be

separated from the iron product as was the case in blast furnace. Consequently,

and as DRI retains the chemical purity of the iron ore from which it is

produced, the iron ore should be very low in residual elements such as copper,

chrome, tin, nickel, and molybdenum.

32

Direct reduced iron (DRI) can be produced in pellet, lump, or briquette

form. When produced in pellets or lumps, DRI retains the shape and form of

the iron oxide material fed to the DR process.[1]

2.4.1 Metallization

Metallization is defined as the percent of total iron in the DRI which has

been converted to metallic iron. For example, DRI having a total iron content

of 92% and a metallic iron content of 85% has 92.4% metallization.

% 𝑀𝑒𝑡𝑎𝑙𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛 = 𝑀𝑒𝑡𝑎𝑙𝑙𝑖𝑐 𝐹𝑒

𝑇𝑜𝑡𝑎𝑙 𝐹𝑒∗ 100

Although it is theoretically possible to convert all of the iron oxide in iron

ore to metallic iron, it is not economically feasible. The reduction reaction

slows significantly in the last stages and low production rates would result in

case of reaching complete metallization. Moreover, in practice, it is

advantageous to retain a small amount of iron oxide in the DRI. During

melting in an electric arc furnace (used for steelmaking), the iron oxide in DRI

reacts with carbon in the DRI to form metallic iron and carbon monoxide. The

carbon monoxide foams the slag during steelmaking, and this improves the

operation of the electric furnace. [1]

DRI normally should at least have 85% metallization. Processes producing

solid <85% metallized, are classified as prereduction processes. The partially

reduced product, called prereduced iron, is not acceptable for steelmaking but

can be used as a feed for iron smelting.

2.4.2 Use of DRI in Electric Arc Furnaces (EAF)

As stated before, the world is currently witnessing a gradual shift from

integrated steel plants (using the Blast furnace for iron production followed by

33

Basic Oxygen Furnace for steel production) to mini-mills. The latter mainly

use electric arc furnace (EAF) for steelmaking. With advantages of lower

investment costs coupled with better flexibility towardsthe input raw materials,

energy inputs and steel grade mix, the world steelproduction through EAF

route is projected to touch the magic figure of 50% soon.[29]

EAF-based plants currently represent about one third of global steelmaking

capacity. Theirshare has been growing at 5% year on year, and will

systematically increase in the future, asnearly all new steel production outside

China is through the EAF process.[30]

Over 95% of the world’s DRI production is consumed in electric arc furnace

steelmaking. The primary use of DRI is as a clean supplement or replacement

for the ferrousscrap charge in EAF.The desired portion of DRI used in the

EAF charge depends on economics, thetype of steel being produced, and the

available scrap quality.[1]

2.4.3 Use of DRI in Blast Furnace

DRI can be added to the blast furnace burden to increase furnace

productivity and reduce coke requirements. It can be used for short-term

increases in blast furnace output when a facility is short of hot metal during

times of high steel demand, or when one of several blast furnaces is down for

maintenance.[1]

2.4.4 DRI Oxidation

The removal of oxygen from the iron oxide during direct reduction leaves

voids, giving the DRI a spongy appearance when viewed through a microscope

as shown in figure 2.14. This is why DRI is also called sponge iron. The latter

is characterized by having very small size of grains. [31]

Thus, DRI in these

34

forms tends to have lower apparent density, greater porosity, and more specific

surface area than iron ore.

Figure 2.14 Micrograph of sponge iron

Because of the high porosity, and specific surface area, DRI is subjected to

oxidation during transportation. In general, oxidation of DRI takes place in

two forms: Reoxidation andCorrosion. [32]

Reoxidation occurs when the metallic iron in hot DRI reacts with oxygen

inthe air to form either Fe3O4 or Fe2O3. The reaction continues as long as the

DRIremains hot and sufficient oxygen is available. Because reoxidation

reactions areexothermic and DRI is a good insulator, it is possible that once

reoxidation beginsinside a pile, the DRI temperature increases and accelerates

the reoxidation rate.Although the inner core of the pile may reach temperatures

up to the fusion pointof iron, the maximum temperature of the outer parts of

the pile will be muchlower because of heat dissipation.

Corrosion occurs when the metallic iron in DRI is wetted with fresh or

saltwater and reacts with oxygen from air to form rust, Fe(OH)3. The corrosion

reactionscontinue as long as water is present. Because water evaporates at

approximately100oC, corrosion reactions have a low temperature limit even

though thereactions are exothermic. Small amounts of hydrogen may be

35

generated whenDRI reacts with water. [1]

This can form an explosive mixture if

it is stored in closed environment.[30]

Allowing DRI to become wet does not necessarily cause it to

overheat.Whenlarge piles of DRI are wetted with rain, the corrosion reactions

are limited to theouter surface area of the pile and the resultant heat from the

corrosion reactionsis dissipated into the atmosphere. However, if water

penetrates into the pilefrom the bottom, or if wet DRI is covered with dry DRI,

the heat from corrosionreactions can build up inside the pile to the point where

rapid reoxidationbegins. DRI saturated with water can cause steam explosions

if it is batch chargedinto an electric arc furnace.

Thus, the key to avoid oxidation is simply keeping the material cool and

dry. [1]

2.4.5 DRI Briquetting

Several methods of passivating DRI to make it more resistant to reoxidation

and corrosion have been developed, but none has been as effective as hot

briquetting. When DRI is hot briquetted, it is called HBI.

HBI is produced by molding hot (700oC) DRI into pillow-shaped

briquettesusing roll press. HBI has the following advantages:

It is almost twice as dense as non-briquettedDRI and it has substantially

less surface area, which makes it 100 times moreresistant to reoxidation.

It is stronger and more massive, making it more resistantto fines generation.

It takes up less volume for storage and shippingowing to its high bulk

density.[1]

Figure 2.15 shows the difference between the HBI and Pellet DRI.

36

Figure 2.15 HBI and Pellet DRI

2.4.6 Broad Classification of DR Processes

DR Processes are mainly classified according to the type of reductant used:

Reformed Natural gas, or non-coking coal. Table 2.7 shows a comparison

between gas-based and coal-based DR processes.

Table 2.7 Comparison between Gas-based and Coal-based DR Processes

Parameter Gas-Based Coal-Based

Reaction Kinetics Faster Slower

Temperatures Needed Lower Higher

Product's Purity Higher Lower

Energy Consumption Lower Higher

Raw Materials[31]

Natural Gas and Pellets Non-coking coal and Lump ore

Capital Cost[33]

Higher Lower

37

2.4.7 Global DRI Production

As stated before, the blast furnace share in ironmaking decreases with time.

The following curve shows the growth of DRI production globally from 1975

to 2007.[31]

Figure 2.16 Growth of global DRI production from 1975 to 2007

The DRI production isn't localized in certain part in the world; however, it

is divided between the different regions as shown in figure 2.17. [34]

Figure 2.17 2008 World DRI production by region (Mt)

38

2.5 Gas-Based DR Processes

In 2008, 75% of the DRI production was from the gas-based processes. [34]

In the gas based processes, the reduction of iron oxide is carried out by a

mixture of CO and H2 at a temperature of about 750-950°C. The reducing gas

is produced by reformation of natural gas. [35]

2.5.1 HYL I Process

The process was first developed in Mexico, and the first industrial scale unit

was built in Monterrey (Mexico) in 1957 in a capacity of 200 metric tons per

day. After that, the process has undergone several modifications. By 1980,

fifteen HYL plants with a total capacity of about 10 million metric ton (11

million net ton) per year of DRI were scheduled to be in operation in world

wide.[35]

2.5.1.1 Main Reactions Taking Place

Steam Reforming:

CH4 + H2O CO+ 3 H2 (1)

Iron Ore Reduction:

H2 + Fe2O3 H2O + 2 FeO (2)

CO + Fe2O3 CO2 + 2 FeO (3)

H2 +FeO H2O + Fe (4)


2.5.1.2 Process Description

As shown in figure 2.18, the reducing section consists of a set of four

reactors, three of which are in operation while the fourth is in the turn-around

39

position. The HYL process is a cyclical batch operation, and three on line

reactors operates in series. The reduction of the charge is performed in two

stages, an initial reduction stage and main reduction stage. Cooling and

carbonization (Fe3C) and the final adjustment of metallization are performed in

the third stage. Each stage of operation takes about three hours. A rigorous

system of valves permits the reactors to be connected in any desired order so

that inside any reactor, initial reduction or main reduction or cooling and

carburization can take place.[35]

Natural gas is reformed using excess steam in the presence of catalyst. The

resulting reducing gases are quenched so as to condense water, and increase

the calorific value.[36]

The flow of reducing gas is counter-current of the iron oxide reduction

stage. The quenched fresh gas from the reformer is used first in the reactor

where cooling and carburization takes place (in which DRI will be produced).

The gas then flow through the reactor that is in the main reduction stage, and

finally through the reactor that has most recently been charged for the initial

reduction stage. Reduction is accomplished according to reduction (2) to (5).

Because water is produced during reduction, the reducing gas is quenched

when it leaves each reactor to condense the water and enhance the reduction

potential of the gas. Before entering the reactors in the reduction stage, the

quenched process gas is heated to about 815°C in an indirectly heated gas fired

furnace. The gas is further heated to about 1050oC by the combustion of

residuals unreformed hydrocarbons with the controlled injection of air at the

entrance to the reactor. The reducing temperature in the HYL process is above

980oC.

[35]

40

2.5.1.3 Current Status

At present, 3 HYL I plants (Located in Venezuela, Indonesia, and Iran)

having 9 modules with total plant capacities varying from 1 to 2 Mtpa, are in

operation. However, and because of being a batch process, the process is no

longer attractive, and is now contributing by less than 1% of the global DRI

production despite contributing by about 40% in the early eighties.[36]

Figure 2.18 Flow Sheet of HYL I process for DRI production

2.5.2 MIDREX Process

Surface combustion division of Midland developed the Midrex Process. In

the mid-1960s the Midrex division became a subsidiary of Korf industries. The

first commercial Midrex plant was installed near Portland Oregon and started

41

production in 1969. The plant included two shaft reduction furnaces of 3.4 m

(11.2 feet) inside diameter and had a total capacity of 300000 metric tons per

year. The average energy consumption of this plant was about 15 million

kilojoules per metric ton (12.9 million BTU per ton of DRI). Many difficult

engineering and operating problems were solved during the first several years

of operation of this plant. By 1983, more than twenty Midrex modules were

installed having a total capacity of about 9 million metric ton per year (9.9

million net ton per year).[35]


Reforming:

CH4 + H2O CO+ 3 H2 (1)

CH4 + CO2 2 CO+ 2 H2 (2)

Iron Ore Reduction:

H2 + Fe2O3 H2O + 2 FeO (3)

CO + Fe2O3 CO2 + 2 FeO (4)




As shown in figure 2.19, reducing process gas, about 95% combined

hydrogen and carbon monoxide, enters the reducing furnace through a bustle

pipe and ports located at the bottom of the reduction zone. The reducing gas

temperature ranges between 760 & 950oC. The reducing gas flows

countercurrent to the descending solids. [35]

The latter may be lump ore or

pellets; however, pellets are preferred owing to their superior physicochemical

42

characteristics. [36]

Iron oxide reduction takes place according to the reduction

reactions above.

The partially spent reducing top gas, containing about 70% carbon

monoxide plus hydrogen, flow from an outlet pipe located near the top of the

DRI furnace into the top gas scrubber where it is cooled and scrubbed to

remove the dust particles. The largest portion (about two third) of the top gas

is recompressed, enriched with natural gas, preheated to about 400oC and

piped into the reformer tubes. In the catalyst tubes, the gas mixture is purified

to form carbon monoxide and hydrogen according to the reforming reactions

above. The hot reformed gas (over 900oC) which has been restored to about

95% carbon monoxide plus hydrogen is then recycled to the DRI furnace.

The balance top gas (about one third) provides fuel for the burner in the

reformer. Hot flue gas from the reformer is used in the heat recuperates to

reheat combustion air for the reformer burners and also to preheat the process

gas before reforming. The addition of heat recuperates to these gas streams has

enhanced process efficiency, helping to decrease energy consumption to about

11.5 million kilojoules per metric ton of DRI.

Cooling gases flow countercurrent to the burden in the cooling zone of shaft

furnace.The gas then leaves at the top of the cooling zone and flow through the

cooling gas scrubber. The cleaned and cooled gas is compressed, passed

through a demister, and is recycled to the cooling zone.[35]

When incorporating hot briquetting in the MIDREX process, the cooling

gascircuit is eliminated, and the hot DRI is continuously discharged from the

shaftfurnace into a hopper and directly fed into a hot briquetting machine. The

resultingHBI is continuously discharged from the hot briquetting machine,

separatedinto individual briquettes, and cooled.[1]

43

Figure 2.19 Flow Sheet of MIDREX Process for DRI Production

2.5.3 HYL III Process

The HYL-III process of Hylsa of Monterrey, Mexico evolved from the

original HYL process by retaining the catalytic reformer, the gas reheater, and

the off gas handling system which condenses water and remove particulates.

However, in the HYL III process, a single shaft furnace with a moving bed is

used in place of the four original fixed bed reactors.[35]


Steam Reforming:

CH4 + H2O CO+ 3 H2 (1)

Iron Ore Reduction:

H2 + Fe2O3 H2O + 2 FeO (2)

CO + Fe2O3 CO2 + 2 FeO (3)



44


The HYL III process is similar to the MIDREX process,however, it uses a

conventional steam reformer and pressurized shaft furnace.

As shown in figure 2.20, sized iron ore (pellet or lump) is charged via lock

hoppersinto a pressurized shaft furnace wherein the ore is heated, reduced,

carburized,and cooled as it descends by gravity. The cooled

productisdischarged via a rotary valve and lock hoppers onto a conveyor belt.

In thecase of hot briquetting, the cooling gas circuit is eliminated and the hot

DRI isdischarged through lock hoppers into the hot briquetting units.

Fresh reducing gas is generated by reforming natural gas with steam.

Thenatural gas is preheated in the reformer's stack, desulfurized to less than 1

ppm sulfur. It is thenmixed with superheated steam, further preheated to 620oC

in the reformer's stack, andthen reformed in alloy tubes filled with nickel-

based catalyst at a temperatureof 830oC. The reformed gas is quenched to

remove water vapor, mixed withclean recycled top gas from the shaft furnace,

reheated to 925oC in an indirectfired heater, and injected into the shaft furnace.

For high (above 92%) metallizationa CO2 removal unit is added in the top

gas recycle line in order to upgradethe quality of the recycled top gas and

reducing gas.[1]

2.5.3.3 Some Process Features

The process can utilize high sulfur feed natural gas since it is equipped with

sulfur removal step.

Utilizing a CO2 removal circuit (typically PSA) in the circulating gas

system results in more positive control for the CO to H2 ratio in the

reducing gas. This allows controlling the degree of metallization and/or

carbon content of the final product.

45

The higher gas pressure system reduces the tendency for bed fluidization,

and thus permits higher capacity.

Selective elimination of H2O and CO2 from the reducing gas circuit allows

maximum recycle of reducing gases to the furnace

High pressure steam generated in the reformer is mainly used for generating

electric power for the plant.[16]

Figure 2.20 Flow Sheet of HYL III Process for DRI Production

2.5.4 ENERGIRON Process

46

For more than 50 years, HYL (now Tenova HYL) has developed

technologiesdesigned to improve steelmaking competitiveness and

productivity for steelfacilities.[37]

ENERGIRON brings the combined support of two premier companies –

Tenova and Danieli – to the world iron and steel industry. ENERGIRON is the

innovative HYL direct reduction technology jointly developed by Tenova and

Danieli.[38]


There are 3 sources for generating reducing gases in this scheme; self-

reforming in the furnace, feeding natural gas as make-up to the reducing gas

circuit, and injecting oxygen at the furnace's inlet.

Reforming and Oxidation:

CH4 + H2O CO+ 3 H2

CO2 + H2 CO+ H2O

CH4 + 0.5 O2 CO + 2 H2

2 H2 + O2 2 H2O

Iron Ore Reduction:

H2 + Fe2O3 H2O + 2 FeO (2)

CO + Fe2O3 CO2 + 2 FeO (3)




47

As shown in figure 2.21, the natural gas stream is mixed with the reducing

gas recycle stream from the CO2 removal system. The reducing gas stream is

passed through the gas heater where it is heated up to 930oC. The reducing gas

temperature is further increased up to about 1020oC after the partial

combustion with oxygen before the furnace. The rest of the process is the same

as HYL III.

Since all reducing gases are generated in the reduction section,

takingadvantage of the catalytic effect of the metallic iron inside the shaft

furnace,optimum reduction efficiency is attained, and thus an external

reducing gasreformer is not required. This is called zero-reformer process

(ZR).

The basic scheme can also use the conventional steam-natural gas

reformingequipment, which has long characterized the process. Other reducing

agentssuch as hydrogen, gases from gasification of coal, petroleum coke and

similar fossilfuels and coke-oven gas, among others, are also potential sources

ofreducing gas depending on the particular situation and availability.[37]

Figure 2.21 Flow Sheet of ENERGIRON process for DRI production

48

2.5.4.3 Some Process Features

HYTEMP system allows the immediate transportation of hot DRI emerging

from the furnace to the EAF by the means of pneumatic transport system.

The ENERGIRON process can process high sulfur iron ores asthe sulfur is

eliminated along with the CO2 in the CO2 absorption system,which is part

of the reduction circuit.

Mixtures of pellets and lump ores can be processed in ratios ranging

from100% pellets to 100% lump ores, depending on the ore characteristics.

Plantsin Brazil (Usiba) and In India (VikramIspat) are operating with 100%

lump ore.

Additionally, the low gas velocity inside the shaft, due to the high

operationpressure, diminishes fines carry over, and thus reducing ore losses

from the plant.

For the production of high quality DRI,i.e. 94% metallization, 3.5% carbon

anddischarged at 700°C, the thermal energyconsumption is only 9.15

million BTU per ton of DRI which is lower than other technologies.

A remarkable advantage of this processscheme is the wider flexibility for

DRIcarburization, which allows attainingcarbon levels up to 5.5%. The

effect of increasing carbon content on the energy consumption in the

subsequent EAF is shown in figure 2.22.

For the ENERGIRON plant, the NOx is below environmental limits as there

is no need for huge air preheating. This is because of the very high energy

integration available in the process. [37]

49

Figure 2.22Effect of increasing carbon content and temperature of DRI

on the energy consumption in EAF

2.5.5 Global Gas-Based DRI Production

As stated above, 75% of the DRI productionin 2008 was from the gas-based

processes. As shown in figure 2.23, MIDREX technology is the dominating,

followed by HYL/Energiron.[34]

Figure 2.23 2008 World Gas-based DRI Production by Process

50

2.6 Coal-Based DR Processes

In 2008, 25% of the DRI production was from the coal-based processes. [34]

Despite not being a big percentage, the share of coal-based processes in DRI

production is gradually increasing.

This may be attributed to the high global reserves of coal which exceeds the

natural gas as shown in figure 2.24.

Figure 2.24 Global energy reserves

Moreover, experts are sure that on the long run coal will continue to be less

expensive and its price will be more stable than other forms of energy.

Historically, this is true and shown in figure 2.25. [23]

51

Figure 2.25 Fluctuations of crude oil and coking coal historically

In coal-based processes, rotary kilns are used as the reducing reactor. The

basic principle of all rotary kiln processes is to reduce iron ore (lump or

pellets) in a rotary kiln where coal is fed from both ends to produce the

reducing conditions and generate the heat required for reduction along the

kiln's length. The main differences in the individual processes are related to

the control system especially for temperature.[31]

2.6.1 General Process Description of Rotary Kiln Technologies

In all coal-based DR processes, lump ore or pellets (or both) together with

coarse fraction of non-coking coal are fed to the inlet end of the rotary kiln.

The size ranges of lump ore, pellets, and non-coking coal are respectively 4-20

mm, 9-20 mm, and 6-20 mm. This coal is referred to as co-current coal, and it

acts as a reducing agent, and as a major heat supplier to the kiln owing to its

combustion. A finer fraction of coal (-6 mm) is also injected from the

discharge end of the kiln using primary air as the carrier gas. This coal is

52

called countercurrent coal, and it helps in completing the reduction, and

supplying heat.

A fluxing material like limestone or dolomite should also be added in order

to control the sulfur pick up by the reduced materials from the coal ash. The

flux is mainly in fine form (-4 mm), and is added with the countercurrent coal.

In these processes, optimizing the temperature of the bed charge is crucial.

At the inlet end, the temperature should be high enough so that the reduction

reactions proceed rapidly. On the other hand, the temperature should be low

enough to prevent the fusion of the coal ash. This is achieved by conserving a

balance between the solid-bed temperature, and the temperature in the

atmosphere above the bed (normally at least 100-150oC higher).

[31]This is

mainly achieved by burning combustibles released form the bed using

secondary air. The latter is blown by fansthrough burner tube space uniformly

along the length of the kiln.[35]

The product from the kiln is mainly a mixture of DRI and char. The

product's temperature is about 950-1000oC, and it is cooled in an indirectly

water-cooled rotary cooler to about 120oC.After that, the DRI is separated

from the coal char using magnetic separators, and finally screening is

performed. The separated char is mainly recycled as a feed material.

Waste gases leaving the kiln at the inlet end pass through a dust chamber

and a post-combustion chamber, before being cooled and cleaned in

electrostatic precipitators, scrubbers, or bag filters. Alternatively, the clean

kiln gases can be used in waste heat boilers to utilize the sensible heat in

producing steam. [5]

Figure 2.26 shows a schematic representation of DRI production in rotary

kilns. Figure 2.27 shows an approximate material balance for DRI production

in rotary kilns.[31]

53

Figure 2.26 A schematic representation of DRI production in rotary kilns

Figure 2.27 An approximate material balance for DRI production in rotary

kilns.

54

2.6.2 Encountered Reactions during coal-based DR Processes

The main reactions that take place within the rotary kiln are the frequent

reduction reactions.

CO + Fe2O3 CO2 + 2 FeO (1)


Reaction 2 takes place in the last 30% of the kiln's length.

The carbon monoxide results from combustion of coal in the presence of

controlled amounts of air

C + 0.5 O2 CO (3)

The produced carbon dioxide resulting from the reduction reacts quickly

with the carbon present in coal to produce carbon monoxide according to the

famous boudouard reaction

C + CO2 2 CO (4)

This cycle continues to maintain the reducing conditions prevailing in the

kiln.

Moreover, coal pyrolysis takes place inside the kiln, where volatiles tend to

evolve till about 600oC. However, it is to be noted that most of these volatiles

don't have real contribution to the actual process of reduction. Part of these

volatiles is being combusted by the secondary air injected to the kiln. This

combustion transfers heat to the charge directly by radiation, and also by

conduction from the kiln lining. [16]

55

Figure 2.28 shows a schematic cross section area of rotary kiln together

with the encountered reactions.[31]

Figure 2.28 A schematic cross section area of a rotary kiln t ogether with

the encountered reactions

2.6.3 Comparison between Different Rotary Kiln DR Processes in

Commercial Use

The coal-based DR processes are similar to great extent. The main

industrially applied processes are SL/RN, Codir, DRC, Jindal, and SIIL.

Before discussing every process separately, the main differences between them

are shown in table 2.8.[31]

56

Table 2.8 Main differences between commercial rotary kiln DR processes

Parameter SL/RN Codir DRC Jindal

% P in Ore As low as

possible

As low as

possible

As low as

possible Up to 0.04

Size range of iron-

bearing materials,

mm

10-30 (Lump)

10-16 (Pellets)

Wide size range

acceptable 5-25 6-18

Energy

Requirements,Gj/t

DRI

20 15 16.5-18.5 17.5-19

Flux, kg/t DRI 50-70 50-90 40 40

2.6.4 SL/RN Process

RN process (for Republic Steel Company and Nations Lead Corporation)

was developed originally in Norway, primarily to recover TiO2 from titanium

bearing ore for the production of paint pigments. However, further

development showed that other iron bearing ores could also be treated

successfully to produce iron. Subsequently, a pilot plant was built in the

United States, and in 1964 LurgiChemie acquired the RN patents and

developed the technology further with the Steel Company of Canada Ltd.

(Stelco) to the SL/RN process.Currently, the process is also known as Lurgi

(Outokumpo) process.[35]

The process description is the same as that shown in section 2.6.1. The main

features of this process are:

Flexibility to the type of iron bearing materials which can be used such as

lump ore, pellets, ilmenite, iron sands, and steel plant wastes

57

Use of wide variety of solid fuels ranging from anthraciteto lignite

including charcoal. [5]

Finally, it worth noting that this process is the mother of all the other coal-

based DR processes, and it is the most widely applied.[31]

2.6.5 DRC Process

The DRC process of the DAVY Reduction Corporation (DRC)was

developed on the basis of the Hockin process of synthetic titanium production.

The first commercial furnace was constructed for Scaw Metals in Germiston,

South Africa. Operations at Scaw Metals started in July of 1983. [35]



The direction of the opening in the secondary air tubes in the kiln's

preheating zone ensures co-current flow with respect to the kiln gases. On

the other hand, the openings in the reduction zone are positioned to give

countercurrent flow with respect to the kiln gases. It is claimed that this

method for air injection inside the kiln insures optimum heat transfer.

The trajectory of the fine coal injection can be altered by changing the

inclination of the delivery pipe.[31]

2.6.6 CODIR Process

The CODIR (Coal Ore Direct Reduction) process was developed in

Germany. Commercial operation of the process began in 1974 when a 120,000

tpa plant was commissioned at the Dunswart iron and steel works, South

Africa.

58



Countercurrent injection of coarse coal in the size range of 6-25 mm, going

up even to 35 mm using highly reactive coals (rather than below 6 or 10

mm in other rotary kiln processes).

In this process, most of the coal is injected from the discharge end of the

kiln (which is completely opposite to the other rotary kiln processes)

The coarse coal particles which settle in high temperature zone of the kiln

are mixed with the burden while releasing volatiles. As a result, the

volatiles in the countercurrent injected coal participate to reduction, and

thus amount of coal needed becomes lower.

CODIR coolers use a direct mist of water inside the rotary cooler rather

than indirect water sprays on the cooler shell. The water mist is controlled

in such a manner that the hot sponge iron is cooled without significant loss

in metallization. This has led to significant reduction in amount of cooling

water required.

The energy consumption in case of course coal injection is only15 Gj/t

compared to 20 Gj/t with finer coal injection.

Because of these modifications, the flow sheet has slightly been changed to

be as shown in figure 2.29.[31]

59

Figure 2.29 General Flow Sheet of the CODIR process for DRI production

2.6.7 Jindal Process

The Jindal process for DRI production was developed by Jindal strips Ltd.in

a 15,000 tpa pilot plant. Based on that, Jindal strips installed a commercial

plant which started production in March 1991. Today, Jindal Steel and Power

has six kilns of 300 tpd and four kilns of 500 tpd (total capacity 1.37 tpa).

Thus, it is the largest coal based DRI plant in the world.



55-60% of the total coal is injected from the discharge end. It is claimed

that as a result, the C/Fe ration in the feed end is as low as 0.42-0.44 which

is well below the normal figures in the other coal-based DR processes

(0.46-0.48).

60

Coal up to 30% ash can be used successfully.

The placement of secondary air tubes within the kiln is completely different

from the other technologies (no details are available).

For the first time in the world, the blast furnace gas has been used in this

process, and has resulted in better reduction, and reduction on specific coal

consumption.[31]

2.6.8 Advantages of Rotary Kiln Processes

Rotary Kiln can effectively mix the solid charge as it undergoes

simultaneous heating and reduction. Intimate mixing of the charge helps in

diluting CO2 formed around the iron ore particles, and this helps the

reduction reactions to proceed.

Since large freeboard space is available above the solid charge in any kiln,

the gas phase can tolerate the presence of heavily dust-laden gases. In gas-

based processes, generation of dust can lead to channeling.

Rotary kilns are commercially proven, and there is a lot of operating

experiences with it especially in cement industry.

The temperature of iron oxide reduction is much lower than that of blast

furnace (1000oC against 1300-1600

oC). As a result, less energy is required

for reduction [31]

2.6.9 Disadvantages of Rotary Kiln Processes

The productivity is very low compared to shaft furnaces in gas-based DR

processes. In the latter, yield is up to 5 times more than the rotary kilns for

the same inner volume. Thus, for large capacity plants, multiple rotary kilns

are needed.

61

The fact that the reactor rotates at 0.4-0.5 rpm makes it difficult to

incorporate process control and quality control systems. Moreover, the

engineering of such kilns is difficult.

The fact of cooling the product in order to perform magnetic separation is a

huge source of energy losses. Thus, these processes exhibit very low energy

efficiency.

Because of the repeated fall and rise of the charge during rotation, the

solids undergo size degradation. Thus, the coarser particles tend to float on

the top of the charge, and the fines tend to settle at the bottom, and thereby

increasing the tendency of adhering to refractory lining. The latter gives

rise to ring formation. Once rings are formed, uniform movement of the

charge becomes difficult, and shutdown of the kiln becomes a must.[31]

2.6.10 Share of Different Processes in Global DRI Production

The coal-based DR processes are growing gradually. In 2003, their

contribution in the global DRI production was only 10.1%. The corresponding

figures for 2004, 2005, 2006, 2007, and 2008 were 12.1%, 15%, 19.7%, 22.6%

and 25.3% respectively.

The breakup of production in different coal-based rotary kiln processes in

2006 is shown in the following table. [31]

Table 2.9 Share of different coal-based processes in global 2006 DRI

production

Process Production in Mt

SL/RN 1.83

DRC 0.63

CODIR 0.52

Jindal 1.32

62

2.7 Smelting Reduction

As stated before, smelting reduction (SR) is the 3rd

route of ironmaking after

the blast furnace, and direct reduction. Smelting reduction produces pig iron

like the blast furnace; however, it has different features.

The gradual world shift from the integrated steel plants using the Blast

Furnace – Basic Oxygen furnace route to smaller mini-mills essentially based

on EAF was the main driving force for research and development in the field

of SR. [36]

2.7.1 Advantages of SR with respect to Blast Furnace

SR processes use non-coking coal as fuel and reductant instead of the

scare coal used in blast furnace.

SR processes are viable at lower production capacities, and this copes

with the gradual world shift from the integrated steel plants to smaller

mini-mills essentially based on EAF.

SR processes are more environmentally-friendly compared to blast

furnace.

Some SR processes use un-agglomerated ore over 8mmin size as the

ferrousfeedstock. This wasn't possible in blast furnace. [39]

In SR, the same phenomena taking place in the blast furnace occurs;

however, they can take place separately in 2 or 3 units, and this

assures better process control. [40]

2.7.2 Advantages of SR with respect to DR

SR processes are characterized by operating at high temperatures so as to

produce molten iron. These high temperatures have the following advantages:

Faster rates of reaction

63

Prevention of sticking problems associated with solid state reactions

in DR Processes

Having liquid phase in the reactor has the following advantages:

Increased transport rates owing to convection

Remarkable increase in the conversion rate because of the higher

contact area resulting from the dispersed nature of phases.[5]

2.7.3 Use of Hot Metal in Electric Arc Furnaces (EAF)

In mini-mills, DRI or hot metal (HM) can be used as scrap substitutes in

EAF. As shown in figure 2.30, charging of HM as a scrap substitute leads to

great decrease in energy consumption, whereas, DRI causes a slight increase.

However, a maximum of 50% replacement is specified for the HM, as the

energy content will be too large for EAF steelmaking. As shown in figure 2.31,

at higher percentages, productivity decreases, as the decarburization of HM

will be the dominant operation in the EAF.[5]

Thus, this can be considered anther advantage of SR with respect to DR.

Figure 2.30 Effect of different scrap substitutes on EAF energy

consumption

64

Figure 2.31 Effect of different scrap substitutes on EAF productivity

2.7.4 Raw Materials for SR Processes

In both blast furnace and SR, the cost of raw materials constitutes about

60% of the total cost of producing hit metal. Thus, efforts have been made and

will continue to utilize less expensive raw materials.

However, already the quality specifications for raw materials used in SR

processes are less stringent than those used in blast furnace. Most SR

processes therefore use low grade fine ores, iron bearing wastes, and cheap

thermal coals.[36]

2.7.5General Features of SR

Figure 2.32 shows a schematic diagram of SR. The phenomena taking place

in the blast furnace are mainly divided on 2 stages. Moreover, the reactants

65

aren't introduced together. Thermal non-coking coal is used as a fuel and

reductant.

As stated before, liquid phase formation helps in having higher reaction

rates. The SR processes are characterized by reduction of molten FeO by CO.

It has been concluded that the controlling step for this reaction is mass

transfer. The overall reaction rate is proportional to the square root of the gas

flow rate. Therefore, for SR processes, one of the objectives is to increase the

amount of gas available for reduction.This will be mainly achieved by

increasing the amount of used coal.[36]

Post combustion mainly refers to the

secondary oxygen or air introduced in the process for combusting volatiles for

heat production and volatiles cracking.

Figure 2.31 Schematic representation of SR technology

2.7.6 Single Stage SR

In an ideal single SR reactor, all the reduction reaction should take place

together in the liquid state in a single step as shown in figure 2.32. Since the

reactor is fed with iron ore without any pre-reduction, the energy

consumptionin single stage SR is higher. The energy consumed is translated

66

into coal, and thus high amounts of oxygen will also be required to react with

coal.

Consequently, single stage SR processes are often in efficient and

economically unattractive despite having low capital cost. In actual practice,

most SR processes utilize at least 2 reactors for effective process control.[36]

Figure 2.32 Block Diagram for single stage SR

2.7.7 Two-Stage SR

2.7.7.1 General Idea

Here 2 separate reactors are involved, one for pre-reduction and the other

for smelting and the rest of the reduction as shown in figure 2.33. The off-gas

from the smelting stage is utilized for pre-reduction. Hence, there must be a

close match between these 2 stages to assure efficient operation.[36]

Figure 2.33 Block Diagram for two-stage SR

67

2.7.7.2 Reactions Encountered in Pre-reduction Stage

Iron Ore Reduction:

H2 + Fe2O3 H2O + 2 FeO (1)

CO + Fe2O3 CO2 + 2 FeO (2)



Carburization:

3 Fe + 2 CO Fe3C + CO2 (5)

3 Fe + CO + H2 Fe3C + H2O (6)

2.7.7.3 Reactions Encountered in Smelting Stage

C + 0.5 O2 CO (1)

C + O2 CO2 (2)

C + CO2 2 CO (3)

C + H2O CO + H2 (4)

2 H2 + O2 2 H2O (5)

CO + H2O CO2 + H2 (6)

H2 + Fe2O3 H2O + 2 FeO (7)

CO + Fe2O3 CO2 + 2 FeO (8)



C in HM + FeO in slag Fe in HM + CO (11)

CO+ FeO in slag Fe in HM + CO2 (12)

In addition, pyrolysis of coal, volatiles composition, and evaporation of

moisture takes place. [40]

68

2.7.8 COREX Process

Among the newly developed SR processes, COREX is the leader both in

terms of capacity and number of plants across the world which has adopted

this technology.[40]

COREX produces a high quality HM using non-coking coal

and pure oxygen in an environmentally-friendly process.

2.7.8.1 History of COREX

The COREX process is a technology developed by Voest Alpine, Austria

(Now Siemens VAI) and Korf Engineering, Germany. The success achieved in

the early experiments led to the commissioning of a pilot plant at Kehl in

Germany in 1981 with a capacity of 60,000 tpa. The pilot plant was operated

for six years, during which various grades of different iron ore forms as well

as different types of coal were tested.

Successful performance of the pilot plant encouraged the process developers

to set up a commercial unit. It was felt that a maximum scale up factor of 5

will be successful. Thus, a COREX unit was installed in 1988 in South Africa

in Iscor's Pretoria Works with a capacity of 300, 000 tpa.[40]

2.7.8.2 COREX Process Description

As shown in figure 2.34, the COREX process is based upon areduction shaft

for iron ore reduction, and a meltergasifier for coal gasification and iron

melting.

In the reduction shaft, the iron oxide feed is in the form of lump ore or

pellets. As in blast furnace, the reduction gas (originated from the

meltergasifier as will be shown below) moves counter-currently to the

descending burden.In the reduction shaft, about 75-95% metallization is

achieved. The off-gases are cleaned and then used as high caloric export gas.

The solid product from the reduction shaft is discharged via screw conveyors,

and transported via feed legs into the meltergasifier. [16]

69

In the meltergasifier,the pre-reduced iron is further heated and melted to

separate iron from slag. Hot metal is tapped at a temperature of approximately

1400-1500oC in a manner similar to the blast furnace.Inside the meltergasifier,

the non-coking coal is introduced at room temperature, and it is dried and

devolatilized along the reactor. In the bottom, it is combusted using pure

oxygen in order to generate carbon monoxide essential for reduction. The

evolved coal volatiles are cracked in the top of the reactor, and thus huge

environmental problems are prevented. The reducing gases exit the

meltergasifier at about 1000-1100oC. They are cooled to 800-900

oC, dedusted

in a hot dust cyclone, and conveyed back to the reduction shaft.[40]

Figure 2.34 Flow Sheet of COREX process for HM Production

2.7.8.3 Limitations for Economic Production

The export gas from the reduction shaft is produced in large amounts-

typically 1650 – 1700 Nm3/THM. Efficient utilization of COREX export gas is

the only way that hot metal production can be made techno-economically

70

feasible. The export gas can be used in different ways including heating

processes in steel plants, and power generation.[40]

2.7.8.4 Commercial Production

Currently, 4 plants are utilizing COREX process for pig iron production.

These plants are:

(i) Saldanha Steel Works, South Africa (0.8 mtpa);

(ii) Jindal South West Steel, Toranagallu Works, India (2 *0.8 mtpa);

(iii) Posco, Pohang Works, Korea (0.8 mtpa);

(iv) Baosteel, China (1.2 mtpa).

Figure 2.35 COREX plant in Saldanha Steel Works, South Africa

71

2.7.9 FINEX Process

FINEX is a SR process that uses ore fines in a series of fluidized bed

reactors for initial pre-reduction followed by a meltergasifier to produce pig

iron.[36]

2.7.9.1 History of FINEX

Since 1992, Siemens VAI and the Korean steel producer Posco have been

jointly developing the FINEX process.[41]

The objective of the process was

mainly utilizing the ore and coal fines generated during processing of the feed

required by the COREX unit in Posco.[36]

A 150 ton/day pilot plant was

installed after the success achieved with smaller 15ton/day unit. [16]

After that,

commercial unit of 1.5 mtpa at Pohang, Korea was commissioned in 2007 and

is in operation since then.[42]

This is the only operating FINEX process till now.

2.7.9.2 FINEX Process Description

As shown in figure 2.36, fine iron ore is preheated and reduced to fine direct

reduced iron (DRI) in a three or four stage fluidized bed reactor system. The

upper reactor stage serves primarilyas a preheating stage. In the succeeding

stages the iron ore is progressively reduced to fine DRI. The fine DRI will be

compacted and then charged in the form of hot compacted iron (HCI) intothe

meltergasifier. The charged HCI is subsequently reduced to metallic iron and

melted. The heat needed for reduction and melting is supplied by coal

gasification with oxygen. The reduction gas, also produced by the coal

gasification, ispassed through the fluidized bed reactors in a similar idea to

COREX process.

72

The generated FINEX export gas is ahighly valuable product and can

befurther used for DRI/HBI production,electric energy generation or

heatingpurposes. The hot metal and slagproduced in the meltergasifier

isfrequently tapped from the hearth as inblast furnace or COREX process. [41]

Figure 2.36 Flow Sheet of FINEX process for HM Production

Figure 2.37 FINEX plant, Posco, Pohang Works, Korea

73

2.8 Groupings of Ironmaking processes

To summarize the previous part, different groupings of the process will be

presented according to different points of comparison.[16]

2.8.1 According to Reduction Process Type

2.8.2 According to Reduction Agent Type

74

2.8.3 According to Production Capacity

2.8.4 According to Product Type

75

2.9 COREX Process for Pig Iron Production

As this thesis deals with optimization and modeling of COREX process, the

following section aims to zoom into the process from different point of views.

2.9.1 Detailed Process Description

As shown in figure 2.38, COREX consists of two reactors, the reduction

shaft and the melter-gasifier. The reduction shaft is placed above the melter-

gasifier.

2.9.1.1 Reduction Shaft Process Description

Iron ore, pellets and additives (limestone and dolomite) are continuously

charged into the reduction shaft via lock hopper system located on the top of

the shaft. Some amount of coke is also added to the shaft to avoid clustering of

the burden inside the shaft due to sticking of ore/pellets and to maintain

adequate bed permeability.

The reduction gas is injected through the bustle located about 5 meters

above the bottom of the shaft at about 850oC and over 3-bar pressure. The

specific reduction gas flow is about 1200Nm3/ton of iron bearing burden

charged to the shaft. The gas moves in the counter current direction to the top

of the shaft and exits from the shaft at around 250oC. Percentage metallization

ranges from 75% to 95%, and the solid product is termed as DRI.

Subsequently, six screws discharge the DRI from the reduction shaft into the

melter-gasifier.

2.9.1.2 Melter-Gasifier Process Description

As shown in figure 2.39, the melter-gasifier can be divided into three main

reaction zones:

Gaseous free board zone (upper part or dome)

76

Moving bed (middle part above oxygen tuyeres). It is also called char

bed.

Fluidized bed (in the transition area between the moving bed and the

free board zone)

The Hearth zone which is the lower part below oxygen tuyeres can also be

considered as the fourth zone.

The hot DRI at around 600-800oC along with partially calcined limestone

and dolomite are continuously fed into the melter-gasifier through DRI down

pipes. The DRI down pipes are uniformly distributed along the circumference

near the top of the melter-gasifier so as to ensure uniform distribution of

material over the char bed. Additionally non-coking coal, iron ore fines, flux

fines and some coke are continuously charged by means of lock hopper

system. The operating pressure, in the melter-gasifier is in excess of 3 bars.

Oxygen plays a vital role in COREX process for generation of heat and

reduction gases. It is injected through the tuyeres, which gasifies the coal char

generating CO. The hot gases ascend upward through the char bed.[43]

It is to be

noted that the gas velocity in the lower part of the gasifier is adjusted to

maintain a stable fluidized bed before the secondary oxygen injection. [40]

The

sensible heat of the gases is transferred to the char bed, which is utilized for

melting iron and slag and other metallurgical reactions. The hot metal and slag

are collected in the hearth and tapped in a manner similar to the blast furnace.

The dome temperature is maintained between 1000oC to 1100

oC, and this

assures cracking of all the volatile matter releases from the coal. The gas

generated inside the melter-gasifier contains fine dust particles, which are

separated in hot gas cyclones. The dust collected in the cyclones is recycled

back to the melter-gasifier through the dust burners, where the dust is

combusted with secondary oxygen. There are four of these dust burners

located around the circumference of the melter-gasifier above the char bed.

The gas from the melter-gasifier is cooled to the reduction gas temperature

77

(850oC) through the addition of cooling gas. A major part of this gas is

subsequently fed to the reduction shaft. The excess gas is used to control the

plant pressure. This excess gas and the reduction shaft top gas are mixed and

termed as COREX export gas.[43]

Figure 2.38 Flow Sheet of COREX Process for HM Production

Figure 2.39 Zones in the Melter-Gasifier

78

2.9.2 Gas Cycle and Specifications in COREX Process

Figure 2.40 shows the gas flow diagram in COREX process.[40]

Stream

number 3 refers to the cooling gas. It appears from the table attached that the

amount of dust is greatly reduced in the process due to the presence of the hot

gas cyclone, and the water scrubbers. By comparing the compositions of

stream 4 and stream 5, it is also apparent that the water scrubbers help in

increasing the calorific value of the gas by condensing the water vapor.

Figure 2.40 Gas Cycle and Specifications in COREX Process

2.9.3 COREX Export Gas

Unless export gas is well-utilized, the process won't be cheaper than the blast

furnace route. [36]

Large volumes of export gas are generated from the process-

79

typically 1650 – 1700 Nm3/THM. The export gas can be utilized in the following

ways:

Heating purposes is a steel plant (in rolling mills)

Power Generation (as shown in figure 2.41 a)

The production of oxygen needed in COREX plant (as shown in figure 2.41

a)

Synthesis gas in the chemical industry

Reductant to produce DRI in any gas-based DR processes (as shown in

figure 2.41 b)

Can be used internally in the process as reducing gas after carbon dioxide

removal. This results in appreciable reduction in the consumption of coal

and oxygen (as shown in figure 2.42)[40]

Figure 2.41 Export gas from 3000 tpd COREX plant used in: a) combined

cycle power plant (th: thermal, el: electrical) and b) DRI production

80

Figure 2.42 In-Process utilization of COREX export gas

2.9.4 Raw Material Requirements for COREX Process

To achieve reliable operation, there are specifications for the different raw

materials to be used in COREX process.

2.9.4.1 General Requirements

Table 2.10 summarizes the preferred and tolerable grain size, and some

chemical properties of the raw materials used in COREX process.

Table 2.10 Raw materials requirements for COREX process

Specification Preferred Tolerable

Coal

a) % Volatile Matter

b) % Ash

c) % Sulfur

d) Grain size, mm

20 - 30 (Water free) 15 - 36 (Water free)

5 - 12 (Water free) 10 - 25 (Water free)

0.4 - 0.6 0.5 - 1.5

5 - 40 (50% should be +10)

Lump Ore

a) Fe%

b) Grain size, mm

62 - 65 55 (min.)

8 - 20 6 - 30

Pellets

a) Fe%

62 - 65 58 (min.)

81

b) Grain size, mm 8 - 16 6 - 30

Sinter

a) Fe%

b) Grain size, mm

50 - 55 45 - 50 (min.)

10 - 30 6 - 45

Limestone, dolomite

Grain size, mm

8 – 16

Limestone, dolomite fines

Grain size, mm

4 - 10

The chemical requirements of coal can also be illustrated in figure 2.42. The

figure defines the regions where the coal can be used safely, and the regions

where blending should be performed to ensure safe operation. The numbers

shown are for various tested coals.[5]

Figure 2.43 classification of coal according to their ash and volatile matter

content

82

2.9.4.2 COREX Insensitivity to Alkali Content

One of the most attractive features of COREX is its insensitivity to the

alkali content of the raw materials, so there is no buildup of alkalis as is the

case in the blast furnace. Since pure oxygen is used, no cyanides are formed at

the tuyere level, rather K2CO3 and Na2CO3 vapors are formed from the

reaction with CO2. These vapors are non toxic and are discharged via the

cooling gas scrubbers at the same input quantity. Moreover, if some alkali dust

emerges from the melter-gasifier, the water used in the scrubbers dissolve

them, and thereby prevents accumulation.[5]

2.9.4.3 Use of Coke in COREX Process

Despite announcing in the beginning that COREX can operate totally

without coke, the actual practice has shown that coke quantities in the

operating plants are in the range of approximately 2-10 % of the coal charge.

Whereas POSCO plant in South Korea showed that it is possible to operate the

COREX plant at zero coke for long periods (coke consumption in total 1999

was in average 19 kg/t HM) by a careful operation and treatment of the raw

materials. However, SALDANHA plant of South Africa and JINDALplant of

India could not reach that.

Another important aspect has to be taken into account regarding to the

required coke quality: In case coke is charged to the COREX plant, the coke

quality is different to the coke qualities being typically used in the blast

furnace. In terms of blast furnace operation, only low quality coke is required

for COREX, which has also a big effect to the specific coke price. It is equal to

coke breeze.

Finally, coke can be seen as an "additive" for the processas to the most

extent thermal coal is directly used. Moreover, coke will be of minor economic

impact.[44]

83

2.9.5 Factors Affecting the Efficiency of COREX Process

In the reduction shaft, the metallization degree of the DRI and the calcination of

the additives are strongly dependent on the following parameters:[43]

Amount and quality of the reduction gas flow

Temperature of the reduction gas

Reducibility of the iron bearing burden

Average particle size and the distribution of the solids charged

The efficiency of the whole process depends on the following parameters:

Size and chemical analysis of the raw materials especially the coal

Low CO2 percentage in the reduction gas so as to ensure higher

metallization of the DRI

Optimum distribution of oxygen between the tuyeres and dust burners

Permeability of the char bed

High system pressure

2.9.6 Environmental Analysis for COREX Process

The elimination of coke-making operations and sintering has made COREX

Process a very environmentally-friendly process. The latter is one of the most

salient features for this process.

Table 2.11 shows the differences between the gaseous emissions and

aqueous effluents between a modern blast furnace, and an operating COREX

unit. [5]

It is apparent that COREX is a very clean technology with respect to

the blast furnace.

84

Table 2.11 Comparison between pollutants emerging from a blast furnace

and a COREX unit

Table 2.12 compares the sulfur input and output balance in blast furnace and

COREX unit. (45)

It is apparent that despite that COREX can sustain an input

amount of sulfur twice as high as a blast furnace, the sulfur content in the hot

metal is similar to that ofthe blast furnace. This is because during the

gasification of coal in the melter-gasifier, sulfur is converted predominantly to

H2S. Moreover, small amount of SOx is formed by the combustion of H2S with

the oxygen in the dust burner region.The above H2S and SOx along with

reducing gases enter the reduction shaft furnace where the following reactions

take place:

CaO + H2S CaS + H2O

(Ca,Mg)O + H2S (Ca, Mg)S + H2O

4 CaO + 4 SO2 3 CaSO4 + CaS

Through these reactions, sulfur is captured in the calcined additives and

then is fed into the melter-gasifier and finally dissolves into a molten-slag

phase.[46]

Blast Furnace COREX Process

1) Aqueous Effluents in mg/THM

a) Ammonia

b) Phenol

c) Sulphide

d) Cyanide

590 50

80 0-1

60 7

20 1

2) Gaseous Emissions in mg/THM

a) NOx

b) SOx

c) Dust

1900 21

1600 26

427 39

85

Table 2.12 Comparison between sulfur balance in a blast furnace and a

COREX unit

2.9.7 Advantages of COREX Process

COREX process holds a lot of advantages, and this is why more than one

steel plant in the world uses this process in ironmaking. This is also why this

process is nearly the only competent to the blast furnace in the field of pig iron

production. The advantages can be summarized in the following points:

The dependence on thermal coal instead of coke allows conserving of the

scarce non-coking coal.[40]

The process is viable at lower production capacities, and this copes with the

gradual world shift from the integrated steel plants to smaller mini-mills

essentially based on EAF. [39]

The process is very environmentally friendly (as proved above).

The raw material requirement is not as stringent as in blast furnace, and

despite that the quality of the produced hot metal is not affected.

Sulfur input / output Blast Furnace COREX Process

1) Inputsin kg sulfur/THM

a) Ore

b) Fuel

Total

0.15 0.035

2.6 4.682

2.75 4.717

2) Outputs in kg sulfur/THM

a) Slag

b) HM

c) Sludge/ Dust

Total

2,42 4.051

0.18 0.19

0.15 0.476

2.75 4.717

86

Correction of hot metal and slag composition is easier and faster in COREX

than in blast furnace as additions can be made through the meltergasifier.

The calorific value of COREX gas is 2.6 - .27 times higher than that of

Blast furnace.

The start of the COREX furnace is easier after a shut down, and can reach

the rated capacity in one hour.

Specific melting capacity of COREX is about twice that of the blast

furnace. [47]

The cost advantage of COREX process with respect to the blast furnace

(after utilizing the export gas) varies from 10-20%. It is 10% at POSCO in

South Korea, and 19% at Jindalin India.[40]

COREX is suitable for two different steelmaking routes, the EAF route at

Saldanha Steel, South Africa, and the basic oxygen furnace (BOF) route at

Jindal, India.[4]

COREX is suitable for mini-mills and integrated steel plants. [5]

2.9.8 Disadvantages of COREX Process

Beside the various advantages of COREX, the process has also some

drawbacks which can be summarized in the following points:

The process can only have a maximum of 10% ore fines in the charge.

High volatile coals can't be used directly, and they must be blended with

low volatile ones.

The process won't be economically viable if the export gas isn't well

utilized.[42]

87

2.9.9 Case Study – Jindal

Jindal Vijayanagar Steel Limited (JVSL) is a great example of COREX

process success. The company started its integrated steel operation in 1999,

based on COREX with a capacity of 0.8 mtpa. After success of the first Corex

unit, JVSL added the second module in 2001. After that, the company further

increased the production capacity into 3.8 mtpa by the commissioning of 2

blast furnaces. Thus, the company utilizes the advantages of both COREX and

the blast furnace in a great synergetic way. [47]

2.9.9.1 COREX Export Gas

The export gas from both COREX units is used in the generation of

electrical energy in two adjacent power plants each of 130 MW capacity, as

well as for the production of pellets in a pelletising plant of 3 MTpa. About

50% of these pellets will be processed in Corex plants, and the rest will be sold

to third parties. Using 70% pellets instead of 100 % lump ore increased the

metal output. [5]

During the decision making process of installing COREX modules in the

company, they have deduced that buying electricity from the state grid would

have meant paying Rs 4.32 per unit. On the other hand, generating power from

a COREX unit cost only Rs 2.60 per unit. This meant power costs reduced by

almost 40 per cent.

On the other hand, and during the decision making process of increasing the

production capacity, they have deduced that there will be surplus amount of

export gas if COREX is to be applied. In that case, they didn't find a feasible

application for this surplus gas, and thus, the decision was to use blast furnace

instead. The chief executive officer of JVSL says that setting up a plant using

Corex involves an assumption that there’s an assured buyer for the excess

power produced. This shows that the decision of returning back to blast

furnace doesn't necessarily mean that blast furnace is better than COREX.[48]

88

2.9.9.2 Using Iron Ore Fines

Undersized iron ore (size 6-12 mm) is being charged directly into the

COREX melter-gasifier. It was realized that the surplus heat available in the

free board could be utilized for reduction of iron ore fines. Addition of fines

via the coal line increases the hot metal productivity, generates extra reduction

gas for the shaft and helps in controlling the process parameters more

uniformly. On a monthly average basis, maximum 15.5% of the total iron

bearing material has been substituted by iron ore fines addition.[43]

2.9.9.3 Recycling of various by products and plant wastes

The drive towards reduction in hot metal price has prompted JVSL to adopt

innovative measures for recycling of various by products and plant wastes.

Some of these are:

Use of BOF Slag

Mill scale

Limestone and Dolomite fines [43]

2.9.9.4 Improvement in Plant Operation

Through continuous research, and by gaining more operating experiences,

the plant's performance was greatly enhanced as shown in Table 2.13.[43]

Table 2.13 Progress of COREX performance in JVSL

Year Production,

mt

Fuel Consumption,

kg / THM

Hot Metal

Temperature, oC

% S in

HM

1999-2000 0.4 1163 1491 0.06

2000-2001 0.77 1071 1503 0.037

2001-2002 1.52 1082 1497 0.037

2002 - 2003 1.46 1041 1497 0.029

2003-2004 1.36 1000 1487 0.027

89

2.9.9.5 Synergetic Combination of COREX and Blast Furnace

JVSL has initiated a great engineering trend of having both COREX and

blast furnace operating in one integrated steel plant. The synergy of COREX

and blast furnace has helped JVSL to maximize the utilization of solid waste

and thereby reduced production cost of pig iron.

As shown in figure 2,44, the non-coking coal used in COREX is screened so

that the lump coal is fed to COREX, and the coal fines ( - 6 mm) are fed to the

blast furnace as pulverized coal injection. Moreover, out of the total coke

produced, the lump coke us fed to the blast furnace, nut coke (6-25 mm) is fed

to COREX, and the coke breeze (-6 mm) is fed to the sinter plant.

More than 70% of the plant wastes such as flux fines, mill scale, and BOF

slag are recycled into COREX either directly or indirectly through the pellet

and sinter plants. Moreover, the COREX export gas is used as a backup in the

blast furnace stoves, boilers, and in sinter and pellet plant. [49]

Figure 2.44 Synergy of COREX and Blast Furnace

90

2.9.10 Case Study – SALDANHA

An Integrated Compact Mill (ICM), based on a COREX C-2000 (2000 tpd)

unit in combination with a COREX Gas based Direct Reduction (DR) Plant,

was started-up in 1999 at SALDANHA STEEL, on the west coast of South

Africa.

Export gas from the COREX plant is used for the production of direct

reduced iron (DRI) in an adjacent DR plant using a MIDREX shaft furnace

and LINDE Vacuum Pressure Swing Absorption plant (VPSA) for the removal

of CO2. The DR plant is operated with a mixture of about 65 per cent lump ore

and 35 per cent pellets.

The COREX plant at SALDANHA STEEL was started up on December

1998 and is operated with mainly local iron ores comprising lump ore (80 -

100%), pellets (0 - 20%) and local coal. All of the required additives are also

supplied locally.[44]

2.10 COREX Macroscopic Analysis

Because of having a multi-component multi-phase system, the macroscopic

analysis is very important to reach better understanding of the process, and

assess the effect of different parameters so as to be able to optimize it.

2.10.1 Reduction Shaft

The most important parameter is the permeability of the burden inside the

shaft. To ensure good permeability, the particle size of the iron bearing

materials, and the amount of fines generated before and after reduction should

be adjusted. Despite having better strength than lump, the particle size of

pellets decreases along the shaft because of attrition, and reaction. The cold

crushing strength (CCS) is the parameter which measures the pellets' strength.

91

As this parameter decreases, the pellets crumble and decreases the bed

voidage, and consequently pressure drop increases. The latter will cause

channeling, and low metallization. Statistical analysis has been used to study

this phenomenon, and the following curve results.[50]

Figure 2.45 Influence of CCS on the reduction shaft's pressure drop

2.10.2 MelterGasifier

Most of the research focus on the melter-gasifier as it is the place where

coal pyrolysis, and combustion takes place. Moreover, it is the place where the

products are tapped.

2.10.2.1 Effect of Coal Size

92

A decrease in the mean particle size of the coal decreases the permeability

of the bed causing gas channeling and high pressure drop as shown in figure

2.46. [50]

Moreover, the

undersize coal combusts

faster, and thus a proportion

of the combustion heat

is not fully used in melting;

however, it raises the dome's

temperature. This

results in increasing the

fuel rate and raising the

silicon percentage in hot metal.[51]

. It was deduced that the optimum mean

particle size is 20-22 mm.[50]

Figure 2.46 Influence of the mean particle size on the meltergasifier's

pressure drop

2.10.2.2 Fuel Rate

Regression analysis using multiple variables was used to get an equation for

the fuel rate (coal + coke) in COREX process

93

𝐹𝑅 = 1712 − 8.1 × 𝑀𝑅 + 12.1 × 𝑀𝑜𝑖𝑠𝑡 − 1.28 × 𝑀𝑒𝑡𝑙𝑛 + 0.21 × 𝑆𝑅

+ 2.45 × 𝑈𝑆 + 1.1 × 𝑉𝑀 − (4.1 × 𝐶𝐶𝑆𝑅)

Where FR is the fuel rate (kg/THM), MR is melting rate (t/h), Moist is %

moisture in coal, Metln is % metallization, SR is slag rate (kg/THM), US is %

undersize coal (-6 mm), VM is % volatiles in coal, and CCSR is coal char

strength after reaction.

CCSR is an important factor in characterizing coal, and it determines to

great extent the amount of coke needed.[50]

2.10.2.3 Factors Affecting Coke Addition

Heat stability is also an important index of coal forCOREX

processoptimisation. Usually the higher thevalue, the better the process as high

stability coal candecrease coke consumption in the meltergasifier.[52]

The heat stability is measured using the following experiment. One

kilogram of coal is put into drying oven. Thiscoal should have a grain size

between 5 and 15 mm. Thisoven is heated to 1100oC where the coal will

bemaintained in the furnace for 0.5 h without air. Afterthat, the coal is taken

out and screened using 6 mmscreen. RW+6 is the percentage of coal bigger than

6 mmobtained after this process.[53]

For high heat stability of smelting coal, its

RW+6 reaches 98%, and this value can theoretically cancel the addition of coke

to the process.[52]

In another study, statistical analysis of daily plant performance data using

more than 500 points has been carried out, and the following regression

equation was the result:

𝐶𝑜𝑘𝑒 𝑅𝑎𝑡𝑒 = 409.3 + 13.56 × 𝑀𝑐𝑜𝑘𝑒 − 0.11 × 𝑀𝑅 + 0.21 × 𝑆𝑅

− 3.08 × 𝐶𝑆𝑅 − 5.15 × 𝑃𝐴𝑙2𝑂3 − 4.18 × 𝑃𝑀𝑔𝑂 − (1.14 × 𝑃𝑚 )

Where Mcoke is the % moisture in coke, MR is the melting rate in t/h, SR is

the slag rate in kg/THM, CSR is the char strength after reaction of the coal

94

blend, PAl2O3 and PMgO are respectively the percentage of Al2O3 and MgO in

the slag, and Pm is the percentage metallization from the reduction shaft.[54]

Another study has discussed this issue from another point of view, where 2

factors for the used thermal coal will be able to assess the amount of coke to

be added. These 2 factors are charstrength after reaction (CSR) and char

reactivity index (CRI). Their significance can be explained as follows:

During coal descent in the fixedbed, char is gasified with CO2, where CO is

produced from the famous Boudouard reaction. Ifthe latter occurs quickly in

thechar bed, carbon consumption increases in intensity toform a small

combustion area, which leads to unevengas distribution and poor heat balance

of the char bed.[55]

The quick consumption can be attributed to particle size of char (CSR), and

char reactivity (CRI). It is apparent from figure 2.47 that with increasing CSR

thecoke rate decreases. Thus, CSR of char above 45 and CRI below 30% are

recommended. [54]

Figure 2.47Effect of coal's CSR on coke rate

2.10.2.4 Effect of amount of volatile matter in coal

If the coalvolatile matter is low as seen from figure 2.48, the gasesgenerated

from such coal would be insufficientfor carrying out the reduction ofiron

bearing materials in the shaft. On the other hand, if volatile matteris high, it

would increase the heat demand inside themelter–gasifier dome and cause

95

problems such as highfuel rate and more tarry material like CH4would

begreater than2% as shown in figure 2.49. High amount of tarry

materialcreates problem in the gas cleaning system.[54]

Figure 2.48 Effect of coal's VM on reduction gas and fuel rate

Figure 2.49 Effect of coal's VM on % CH 4 in the reduction gas

96

2.10.2.5 Effect of Moisture

As coal's moisture increases, fuel rate increases, oxygen used increases, and

high reduction gas flow rate results. 1% increase in moisture may increase

about 10 kg carbon per THM.[54]

Figure 2.50 shows the effect of moisture on

the dry fuel rate.

Figure 2.50 Effect of coal's moisture on the dry fuel rate

2.10.2.6 Minimization of Energy Consumption

To minimize energy consumption, amount of gases produced in the melter-

gasifiershould be the same amount needed in the reduction shaft. It was found

that % Metallization of 85-95% is the optimum for minimized energy

consumption. [52]

2.10.2.7 Effect of %MgO and %Al2O3 on the slag

Experiments have proved thatthe viscosity ofslag decreases with increasing

level of MgO and hencewhen Al2O3in slag is to be increased to reduce slag

97

ratewhich also tends to increase viscosity, MgO in slag needs to be increased.

to counter balance this effect.[56]

The following figures also show that increasing % MgO in the slag tends to

reduce the slag rate and fuel rate. [54]

Figure 2.51 Effect of %MgO and % Al2O3 on the slag rate

98

Figure 2.52 Effect of %MgO and % Al2O3 on the dry fuel rate

2.10.2.8 Modeling of COREX process for optimisation ofoperational parameters

One of the most important studies was conducted to build a macroscopic

model able to predict the changes taking place on altering any of the input

variables. A complex mass and energy balance model has been developed as

illustrated in figure 2.53. The model has been validated for slag rate and %CO

inthe generator gas.

Figure 2.53Block diagram showing a macroscopic model for COREX

2.11 COREX Microscopic Analysis

Due to the enclosed process and the limitations of measuringinstruments, it

is difficult to directly study the different velocity, temperature and

composition profiles inside each part of the process. [57]

Thus, to reach the

99

previous objective, microscopic modeling was performed for both the

reduction shaft and melter-gasifier.

2.11.1 Reduction Shaft

A two-dimensional mathematical model was developed to describe iron

oxides reduction in the reduction shaft. Combined with mass, momentum and

heat transfers between gas and solid phases in steady state, the model

calculates and demonstrates the basic characteristics of the shaft furnace, such

as velocity, pressure, temperature fields of relevant phases and species’ mass

fraction distributions.

It was proved that the reduction from magnetite (Fe3O4) to wustite (FeO)

occupies most part of the furnace, and the reduction degree of burden located

near wall is comparatively higher than that close to center.

The model has also been applied to determine the influences of down pipe

gas on reduction behaviors. When flowing toward the center from DRI down

pipe, the high temperaturereducing gas brings more energy along the pathway,

thus helping to increase the reductionreaction rate and enhance DRI average

metallization level.[57]

2.11.2 MelterGasifier

The firstdeveloped model for the melter-gasifier was one-dimensional (1-

D), and focused on coal pyrolysis. The reactor was classified into 3 zones

which are the freeboard, fluidized bed, and moving bed. [39]

After that, and in

another paper, the model was used to studythe effect of operational parameters

like the bed height,C/O ratio, top pressure, and steam injection on the

processperformance.[58]

This model hasassumed that combustion took place instantaneously at the

tuyere plane (zero-dimensionalmodel was assumed for the tuyere region). This

reduces thecombustion-zone height to zero. But, the combustion zonecould be

100

quite large, since pureoxygen is used for combustion. Thereby, the

simplifyingassumption ignores one of the important aspects of the process.

The modelfurther assumed that the ore and flux arenonreactive and gases in

the free board are in equilibrium.However, their resultsshow general

agreement with the plant data for the temperature of the hot metal and

composition and temperature of the top gas.[39]

The most recent model has modified the modeling of the tuyere level by

using twodimensional(2-D) modeling of the combustion zone. The 2-D zone

extends fromthe tuyere region to 0.5 m above that, and the remainingpart is 1-

D as shown in figure 2.54.The model is based on multiphase conservation of

mass, momentum, and heat. The fluidized bed has been treatedas 1-D. Partial

equilibrium is calculated for the free board. The calculated temperature of the

hotmetal, the top gas, and the

chemistry of the top gas agree

with the reported plant data.

The modelhas been used to study

the effects of bed height,

injection of impure O2, coal

chemistry, and reactivityon the

process performance.[59]

101

Figure 2.54 Schematic diagram of the computational domains in the melter -

gasifier

102

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Literature Review on IronMaking

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