IRONMAKING IN BLAST FURNACES

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30 -

20 -

10 -

0 -

5 -

-

Fe2O3

Scrap

Hot metalRefining

Liquid steel

Direct molten steelmaking

Blast furnace and Smelting ReductionDirect

reductionRemelting DRI, HBI

Oxy

gen

%C

arb

on

%

Temperature OC

Iron ore

Scrap

IRON ORE TO LIQUID STEEL

BLAST FURNACE IRONMAKING

Amongst all the ironmaking processes, the blast furnace still holds the dominant position.

The blast furnace has remained up-to-date and competitive with the new technologies.

Hot metal production rates of 8000-10,000 tpd, fuel rates of around 450-470 kg/thm (270-275 kg coke plus 175-225 kg coal), furnace availability ranging between 95-98% and campaign life of 15-20 years are benchmarks today.

SIZE AND NUMBER OF BLAST FURNACES IN THE WORLD

130

59

8063

38 35 34

821

9 7 40

20

40

60

80

100

120

140

BF inner volume, m3

Nu

mb

er o

f B

Fs

In 2007, there were around 490 BFs varying in size from 500 to 6000 m3 inner volume

BLAST FURNACES IN INDIA

BF size, m3 (inner vol.)

No. of Furnaces

Combined inner vol., ‘000 m3

Production, Mpta

> 3000 4 6.4 4.0

2000-3000 8 17.2 8.5

1500-1999 8 14.1 6.5

1000-1499 15 16.2 7.5

500-999 8 3.9 3.0

< 500 ~ 30 ~ 5.5 ~ 2.5

Total 68 61 32

INCREASING BLAST FURNACE SIZE IN INDIASteelworks Inner vol.,

m3

Capacity, Mtpa

Big BFs JWS 4019 3.0

Tata Steel - ‘G’ (after upgrdation)

2308 1.8

Tata Steel - ‘H’ 3800 2.5

Tata Steel - ‘I’ 3800 2.5

Big BFsUnder project planning stage

Vizag # 3 ~ 4000 3.0

Bhilai # 0 ~ 3800 2.7

IISCO ~ 3200 2.2

Bokaro 2 – rebuild ~ 2600 1.8

Tata Steel – KPO ~ 4000 x 2 6.4

Medium and small BFs under construction

JSW, JSPL, Bhusan 1680 x 2 3.0

Various others < 500 2.0

MACRO-FEATURES OF A BLAST FURNACE

The furnace is a refractory lined steel shell filled with material viz. coke, iron ore, sinter, pellets, flux, etc. from the stockline down to the bottom. The process goes on continuously for several years till the furnace is shut down for repairs and modification. The inputs and outputs are represented per metric ton (i.e. tonne) of hot metal.

Preheated air at 1000-1250O C is blown through tuyeres into the furnace. It may be enriched with some pure oxygen, moisture. Most modern furnaces also inject pulverised coal. Exothermic combustion of coke and coal by oxygen of air gasifies carbon into CO and also provides heat. The highest temperature zone of the furnace (1900-2000OC) is at the level of tuyeres – the raceway.

8H Blast FurnaceH Blast FurnaceCinder Ladle

SLAG

SGDP

Granulated

RMH(Iron Ore & Fluxes)

Sinter Plant(Sinter)

HMCPCL(Coke)

Torpedo

O2

Steam

Cold Air Stoves

Hot Air

Charge

Stock House

Belt Charging

Coal Injection

LD#1 & LD#2

Casthouse

Hot Metal

Process MappingProcess Mapping

BLAST FURNACE PLANT

Top Gas Iron Ore

Hot Blast

Hot Metal

Heating

Reduction

Melting

Job Sub -steps

Bosh GasTool

Function that BF must satisfy

BLAST FURNACE PROBING AND CONTROL

For containing heat, lining is important. It is subjected to:

• Carbon monoxide attack.

• Action of alkali and other vapours high temperature.

• Abrasion by moving solid charges.

• Attack by molten slag and metal.

• Effect of furnace design and operation.

There have been attempts to use silicon carbide bricks in the bosh region as inner refractory lining.

Alumino-silicates and carbon are refractory materials most commonly employed for BF lining.

Ordinary fireclay bricks containing 40-45% Al2O3 are used in the upper stack. 60% Al2O3 (known as high duty fireclay) is employed for lower stack, belly and bosh.

BLAST FURNACE REFRACTORY LINING

Carbon is the popular refractory in the hearth. It has very high thermal conductivity. The hearth is cooled by water.

HOT BLAST STOVES

TYPICAL COST BREAK UP FOR HOT METAL

(25.4%) Ore & Scrap

(1.2%) Fluxes

(47.4%) Coke

(3.7%) Coal & Tar injection(4.7%) Blowing

cost

(5.1%) Others

(6.2%) Labour & Admn.

(2.5%) Maintenance

(2.5%) Relining(1.3%)

Refractories

DEVELOPMENTS IN BF IRONMAKING

TECHNOLOGICAL IMPROVEMENTS IN BFs IN GERMANY AND EFFECT ON COKE RATE

IMPROVEMENTS IN BLAST FURNACES

Maximum size of blast furnaces stabilised at about 15 m hearth diameter; inner volume of 5000-6000 m3.

Maximum productivity achieved 2.8-2.9 t/m3/day using conventional raw materials. Maximum output is 12000 tpd; equivalent to 4 Mtpa.

Coke consumption (without coal injection or other fuel) is at best about 450 kg/thm, i.e 3.15 Gcal or 12.5 GJ, with recoverable excess BF gas of energy value 3-4 GJ.

Iron ore beneficiation becoming mandatory for reduction of slag volume from 300-350 kg/thm to 200 kg or even 100-150 kg/thm using high grade pellets (66-68%).

Injection of hydrocarbons through the tuyeres generates H2 and CO in the combustion zone. H2 gives several additional benefits, such as:

• Faster gaseous reduction of iron oxides.

• Higher thermal conductivity of the gas and consequently, faster heat transfer to the solid burden.

• Better bed permeability in the furnace, since hydrogen has a much lower density than CO and N2.

AUXILIARY FUEL INJECTION INTO BLAST FURNACES

Country Coke, kg/thm

Coal, kg/thm

Oil, kg/thm

Others, kg/thm

Total, kg/thm

Japan 414 98.8 - 1.2 514

USA 413 36.5 10.0 40 499.5

France 351 125.6 3.5 - 480.1

Germany 359 51 63 - 473

Italy 353 129.5 15 - 497.5

Netherlands 357 141 - - 498

UK 394 43 55 - 492

India 480 120 - - 600

TOTAL REDUCING AGENTS IN 1995

Today, in many countries, coke consumption even as low as 270-290 kg/thm has been achieved at coal injection rates of 190-220

kg/thm, with a coke to coal replacement ratio in the range of0.9-1.08.

COAL INJECTION INTO BLAST FURNACE

• Pulverised coal injection (PCI) is a of considerable current interest.

• In most cases, 1 kg coal at best replaces 1 kg coke, referred to as Replacement ratio. Sometimes, RR can be more than 1.

• Typically, coal is ground to about 80% below 75 micron (0.075mm).

• Coal injection is normally accompanied by suitable oxygen enrichment of the air blast.

• Coal injection rates above 100 kg coal/thm are quite common now-a-days and some modern furnaces have reached a level as high as 250 kg/thm.

• Choice of appropriate coal in terms of its ability to combust easily in the raceway, depends on the nature of the coal (particularly its volatile matter content), particle size distribution and mode of injection. All these factors influence the Replacement ratio.

INCREASE IN GLOBAL AVERAGE PCI RATE

Higher PCI calls for better coke.Higher PCI calls for better coke.

Avg. PCI rate in 2008-09 :Japan, Korea, Taiwan – 120, China – 190, EU – 215,

Tata Steel India – 160, JSPL, India – 130-140, Baotou, China – 150.

PRESSURE DROP AT DIFFERENT COKE RATES

LIMITATION OF COAL COMBUSTION IN THE RACEWAY

CHARGING SYSTEM: BELL TYPE

BELL-LESS TOP WITH ROTATING

CHUTE

LATEST IS GIMBLE TOP CHARGING

SYSTEM

ADVANTAGES ACCRUED FROM IMPROVED BURDEN DISTRIBUTION

• Increased productivity, decreased coke rate, improved furnace life.

• Improved wind acceptance and reduced hanging as well as slips.

• Improved efficiency of gas utilisation and indirect reduction.

• Lower silicon content in hot metal and consistency in the hot metal quality.

• Reduces tuyere losses and minimisation of scaffold formation.

• Reduced dust emission owing to uniform distribution of fines.

CENTRAL WORKING AND WALL WORKING BLAST FURNACES

MONITORING BURDEN DISTRIBUTION

The monitoring system for assessing distribution includes:

• Heat flux monitoring equipment to measure the heat flow in different zones (both above and under the burden).

• Profile meters for the measurement of surface profiles. • Thermocouples in the throat, stack and bosh regions to

measure temperature. • Stack pressure monitoring and pressure drop measurement

along the furnace height. • Special instruments such as infrared probes to monitor the

burden surface temperature, devices in the stack region to measure individual layer thicknesses and local descent rate, and tuyere probes to sample materials at the tuyeres level.

• Mathematical models for charge distribution control, overall heat and mass balance and interpretation of probe data.

Thank you

PART - II

IRONMAKING IN BLAST FURNACES

Mechanism of Reduction, Blast Furnace Reactions,

Zones in a BF

The reduction of iron oxides by CO and H2 is traditionally known as Indirect Reduction in blast furnace ironmaking.

This is meant to distinguish it from the reduction by solid carbon, which is called Direct Reduction.

Gas-solid reactions are much faster than reactions between two solids. Therefore, maximum of indirect reduction is the goal. Utilisation of hydrogen as a reductant has definite advantage. Disadvantage is -----.

NOMENCLATURE OF REACTIONS IN A BF

BLAST FURNACE REACTIONS

As the solid charges descend downwards, major reactions may be classified into the following categories viz.:

• Removal of moisture from the raw materials.• Reduction of iron oxides by CO.

• Gasification of carbon by CO2.

• Dissociation of CaCO3 (where raw limestone added).

• Reduction of FeO by carbon.• Reduction of some other oxides of ore by carbon.• Combustion of coke and coal in front of tuyeres.

The outputs from the furnace are:• Molten iron (i.e. hot metal)• Molten slag• Gas at a temperature of around 200OC, containing CO, CO2, N2, moisture and dust particles.

Boudouard reaction: 2CO = 2CO2 + C

Gasification reaction: 2C + O2 = 2CO

Solution loss reaction: C + CO2 = 2CO

Endothermic reaction

Exothermic reaction

Water gas shift reaction: CO + H2O = H2 + CO2

Mild Exothermic reaction

Endothermic/ Exothermic (beyond 1000°C) reaction

IMPORTANT BLAST FURNACE REACTIONS

• Transfer of reactant gas to the solid surface (CO or H2) across the gas boundary layer around the piece of solid.

• Inward diffusion of reactant gas through the pores of the solid chemical reaction

• Outward diffusion of the product gas (CO2 or H2O) through the pores.

FeO (s) + H2 = Fe (s) + H2O (g)

FeO (s) + CO = Fe (s) + CO2 (g)

3 Fe2O3(s) + CO (g) = 2 Fe3O4 (s) + CO2 (g) Fe3O4(s) + CO (g) = 3 FeO (s) + CO2 (g) 3 Fe2O3(s) + H2 (g) = 2 Fe3O4 (s) + H2O (g) Fe3O4(s) + H2 (g) = 3 FeO (s) + H2O (g)

• Transfer of the product gas from the solid surface into the bulk gas across the boundary layer.

MECHANISM OF IRON OXIDE REDUCTION

Reaction kinetics of iron ore reduction determines the rate at which iron oxides are converted to metallic iron.

The rate of any chemical reaction increases as the temperature increases. As a result, reaction kinetics is not generally a matter of great concern in blast furnaces.

IRON OXIDE REDUCTION

This is contrary to the reaction rates in DR processes. Owing to lower temperatures, the reactions are slower. Hence, the production rate of DR processes is generally lower than that in blast furnaces.

Iron oxide reduction is complex because the oxide charged undergoes a series of changes, step-by-step before the conversion to the final product.

Chemical reactions are either homogeneous (if a single phase is involved) or heterogeneous (if two or more phases are involved).

The slowest step in the entire process chain determines the overall reaction rate and is referred to as the rate controlling step.

The solid-state reduction of iron oxides is heterogeneous, involving solid and gas phases separated by an interface.

IRON OXIDE REDUCTION

For the chemical reactions to occur, the reactants must reach the interface and the products must move away.

The movements of reactants and products are affected by several factors – any of which can be rate controlling.

RATE CONTROLLING STEPS IN IRON OXIDE REDUCTION

Rate of iron oxide reduction depends on the rates of heat and mass transfer across the gas-flow boundary layer at the outer surface of the solid phase. When the reaction rate is controlled by this factor, it is known as "Boundary Layer Control".

Rate of diffusion of reducing gas inwards and product gas outwards through the reduced iron layer can control the rate of reduction. This phenomenon is generally associated with large ore particles, and known as "Gaseous Diffusion Control" or "Iron Pore Control."

Chemical reaction at the wustite-iron interface can be rate controlling. In such a case, the rate of reduction per unit area of the remaining iron-oxide surface is found to be constant with time. Mechanism is known as "Interfacial Reaction Control" or "Phase Boundary Reaction Control".

When both gaseous diffusion control and interfacial reaction control combine to influence the rate of reduction, the mechanism is referred to as "Mixed Control."

SCHEMATIC OF IRON ORE REDUCTION

Topo-chemical type reduction Partially reduced porous iron oxide

REDUCTION OF IRON OXIDES

Importance of porosity assessed by microscopic examination of reduced iron oxide. Topo-chemical reduction has three concentric layers – magnetite, wustite, metallic iron – each layer same shape as outer surface in case of dense oxides. Porous oxides has similar structure in individual particles.

REDUCTION OF IRON OXIDE BY H2 VS. CO

Reduction by hydrogen Reduction by CO

• Initially, reduction by hydrogen is faster than carbon monoxide.• Magnetite reduced by hydrogen contains grains of wustite completely

enveloped in dense layers of metallic iron. In CO reduction, metallic iron layers consist of almost pure pearlite.

• Carbon can diffuse very rapidly in austenite so that at the interface between austenite and wustite, carbon is available to complete the reduction. In case of hydrogen, reduction is incomplete.

• Solid-state diffusion of ferrous iron through wustite much greater than gaseous diffusion of hydrogen or CO through ore particles. Therefore, solid-state diffusion in the stack region is not the rate controlling step in BF ironmaking.

STRUCTURAL CHANGES IN IRON OXIDE REDUCTION

Hematite → Magnetite → Wustite → Metallic iron • In hematite, oxygen atoms arranged in close-packed hexagon structure.• In magnetite and wustite, the structure is FCC.• In first stage of reduction, oxygen atoms undergo major readjustment.• Results in 25% increase in volume, opens-up structure, facilitates redn.• During magnetite to wustite transformation, oxygen lattice is unchanged.• Iron atoms diffuse to fill vacant sites in lattice; volume change is small.• Wustite has variable composition – composition changes from equilibrium

with magnetite to equilibrium with metallic iron. • Nucleation and growth of iron crystals results in shrinkage and large

increase in porosity of the metallic phase.• Transformation of hematite to magnetite – 25% increase in volume. Further,

7-13% increase during transformation to wustite. Followed by shrinkage to metal phase.

• Total increase in volume during complete reduction of hematite:25-27%.• For magnetite ores, no volume increase; 4-5% shrinkage in final product.• Explains why reducibility of magnetite is very poor.• Often, magnetite first oxidised to hematite and then reduced.

REDUCTION OF METAL OXIDES BY CO

REDUCTION OF IRON OXIDE BY CO

REDUCTION OF IRON OXIDE BY HYDROGEN

BF NOMENCLATURE

Hei

gh

t o

f B

F

ZONES IN A BLAST FURNACE

Extends from the stock line to the mantle level. In this zone the burden is completely solid. The charge gets heated from 200°C at the stock line to 1100-1200°C at the bottom of the stack. Most of reduction occurs by gas-solid contact.

DETAILS OF THE ZONES Stack: Wall slopes outwards in downwards direction

Belly: The cylindrical portion below the stack

Metallic burden begins to soften and fuse as it travels.

Bosh: Below the belly and sloping inwards in downwards direction

Burden begins to melt except coke. Gangue and flux combine to form the slag. The furnace walls are either parallel and then taper down, or are entirely tapering down resulting in reduction the sectional area by about 20-25% . This is because of decrease in the apparent volume of the charge.

Tuyere or Combustion Zone: End of slope; parallel walls

Except central column of coke, entire charge is molten. Oxygen of the blast burns coke to CO. Number of combustion zones, in front of each tuyere exists. Thus, there is a ‘runway’ or ‘race-way’ in front of each tuyeres which is first horizontal and then becomes vertical while expanding.

Hearth: Below the tuyere region and cylindrical

Some coke descends into hearth to form the ‘deadman’. Entire charge is molten and stratifies into slag and metal layers – tapped separately.

TUYERE AREA IN A BLAST FURNACE

MECHANISM OF SILICON REDUCTION

CONCEPT OF RAFT

Change in Operating Variable Change in RAFT , 0C

Blast temperature raised by 1000C +82

Blast oxygen raised by 1% +53

Blast moisture raised by 5g/Nm3 -28

1% methane added to blast -56

From sensible heat of the flame, its temperature is calculated. This is known as Raceway Adiabatic Flame Temperature (RAFT).

Heat content of flame = mass of gas in the flame average specific heat of gas (RAFT 298)

BF productivity is defined as tonnes of hot metal produced per day per cubic metre of inner/working volume.

Productivity can be increased by: • Screening of solid charges before charging into the furnace to

eliminate fines below a certain size• Agglomeration of fines by sintering, pelletising • Proper top charging device to make the distribution of burden

size as uniform as possible in any horizontal section. • Use of better quality coke. • Use of higher hot blast temperature. • Use of oxygen enriched blast. • Use of higher top pressure. • Use of superior quality iron oxide burden. • Improved facilities for metal and slag evacuation.

PRODUCTIVITY OF BLAST FURNACES

PRODUCTIVITY AS FUNCTION OF SLAG RATE

ACCEPTABLE COKE FOR BLAST FURNACES

• Room temperature strength : M10 7.0 (max.)• High temperature strength: CSR 64 (min.), CRI 25

(max.).• Chemistry: Ash 15-17% min., Alkali 0.35% max.• Size : Suit iron oxide feed. Size at tuyere level?

Suitability assessed in terms of: Room temperature strength High temperature strength Chemistry Size Reactivity

For blast furnaces in India, ‘acceptable’ values are:

Lower productivity of Indian furnaces essentially on account of poor coke. Emphasis on coke quality.

TYPICAL COST BREAK UP FOR HOT METAL

(25.4%) Ore & Scrap

(1.2%) Fluxes

(47.4%) Coke

(3.7%) Coal & Tar injection(4.7%) Blowing

cost

(5.1%) Others

(6.2%) Labour & Admn.

(2.5%) Maintenance

(2.5%) Relining(1.3%)

Refractories

PART - III

SMELTING REDUCTION

ALTERNATIVE METHOD OF IRONMAKING

YearCoke rate,

kg/thm

Injectant, kg/thm

Total reductant,

kg/thmComments

1950 1000 0 1000 Lean local ores

1965 600 0 600 Rich seaborne ores

1970 525 50 575 Oil injection/high blast temperature/ oxygen enrichment

1980 500 50 550 High top pressure/burden distribution and permeability control

1990 400 125 525 Coal injection/improved sinter coke quality

2000 325 175 500 Increased coal/gas/oil injection

2010 250 250 500 Continued use of metallics in the burden

PROGRESSIVE REDUCTION IN BF COKE CONSUMPTION OVER THE YEARS

DEPENDENCE OF BLAST FURNACES ON COKE

Parameter Case A Case B Case C Case D

Coal, kg/thm 0 106 149 173

Coke, kg/thm 482 376 334 305

Total fuel, kg/thm 482 482 483 478

Production, t/m2 hearth area/ d

59.4 59.2 61.1 58.8

Blast temperature, oC 1129 1141 1159 1177

Humidity, g/Nm3 30 16 10 6

Oxygen in blast, % 21 21 22.4 22.4

Gas utilisation, % 49.2 50.6 51.0 48.2

Hot metal temperature, oC 1487 1475 1478 1482

Si in hot metal, % 0.24 0.27 0.28 0.33

FORECAST OF TECHNOLOGIES TO BE ADOPTED FOR HOT METAL PRODUCTION

Conventional BF route

New SR technologiesNon-coking coal Fine

ore

Pellets Lump ore

Sinter Pellets Lump ore

Coking coal

2000 2025-2030YEAR

100

75

50

25

Future

Fine ores

Scrap substitutes

Non-cokingcoal, NG, synthetic

gases

Lump ore,sinter,pellets

Coke from metallurgical coal and PCI

Scrap

DRI using NG, non- coking

coal

SR hot metal using non-

coking coal

Lump ore Coke from metallurgical coal

Conventional

Scrap

Recent

EMERGING TRENDS IN INPUTS TO IRON/STEELMAKINGEMERGING TRENDS IN INPUTS TO IRON/STEELMAKING

FUNDAMENTALS OF SMELTING REDUCTION(Critical in economics of all SR processes)

Pre-ReductionPre-reduction

Unit

Off-Gas

Pre-reduced Ore

Smelting Reduction

Vessel

Post Combustion

Final Reduction

Melting

Coal Gasification

Coal

Hot Reducing Gas

Oxygen/Air

Hot Metal

Iron Ore/ Pellets

Pre-reduction Degree, Extent of Post Combustion and Heat Transfer Efficiency are critical for success of all SR processes.

Process

Oxide Feed Reductant Product

Blast furnace including mini blast furnace

Lump ore, sinter, pellets

Coke, coal, oil, tar, natural gas

Hot metal essentially for BOF steelmaking

Smelting reduction

Ore fines, lump ore, waste iron oxides

Coal, oxygen, electricity

Hot metal (synthetic hot metal) for EAF / EOF steelmaking

RAW MATERIALS USED IN BLAST FURNACESAND IN SMELTING REDUCTION

Ore

Slag

O2

Coal

Metal

Reductionand

melting

Gas

Single stage

DRI

Slag

O2

Coal

Metal

Gas

Reduction

Melting

Ore Gas

DRI

Slag

O2

Coal

Metal

Gas

Reduction

Melting

Ore Gas

Gasification

GasCoke

Coke

Three stage

Two stage

FLOWSHEET OF SINGLE STAGE, TWO STAGE AND THREE STAGE SR PROCESSES

• SR involves both reduction and smelting, i.e. melting accompanied by chemical reaction(s).

• In an ideal SR reactor, in the strictest sense, all the reduction reactions should take place in the liquid state in a single step.

• Any SR process involves the extraction of metal values from the ore following liquid phase reduction by non-coking coal.

• In actual practice, most SR processes utilise two steps – the removal of oxygen from the ore in the solid state to varying extents in step one, followed by removal of the remaining oxygen in liquid phase reduction reactions in step two.

• Compared with DR processes, the principal advantage of high temperature operation in SR processes is: faster rates of reaction and prevention of sticking problems associated with solid-state reactions (intrinsic in DR processes).

SALIENT FEATURES OF SR (1)

• Smelting reduction processes are thus either two-step processes with separate pre-reduction and smelting reduction steps (such as Corex and HIsmelt),

• Simpler one-step processes involving simultaneous reduction and smelting still not fully proven.

• All SR processes consume fairly large volumes of coal that generates large amount of export gas, the effective utilisation of the generated by-product gas is extremely important.

• Generally, the use of export gas makes or breaks the cost structure of SR processes .

SALIENT FEATURES OF SR (2)

• Unless the net export gas from any Corex plant can be utilised (extent of generation 1650 Nm3/thm; calorific value 7500KJ/Nm3) the process itself becomes totally uneconomical.

• If no credit is given to the off- gas, the cost of the hot metal made can be as much as 50% higher than blast furnace hot metal.

• Adequate credit can be obtained by selling co-generated electrical power from the Corex off gas.

• Another use of the export gas is in shaft furnace, DRI production adjacent to the Corex furnace.

SALIENT FEATURES OF SR (3)

REDUCTION STEPS IN SR

• Reduction by solid carbon

• Reduction by carbon dissolved in Iron

• Reduction of molten FeO by CO

A liquid boundary layer is expected to exist on the slag side of the slag / carbon interface, FeO must be transported to the nucleation site in the following manner:

FeO (l) + C (s) = Fe (l) + CO (g)FeO (bulk) transported to FeO (slag /gas interface)

A halo is formed, after which the following steps come into play:

FeO + CO = Fe + CO2 (at gas / slag interface) CO2 (gas /slag interface) transported to CO2 (carbon /gas interface).

CO2 + C = 2CO (at carbon / gas interface)CO (carbon / gas interface) transported to CO (gas / slag interface).

REDUCTION OF SLAGS BY SOLID CARBON

REDUCTION OF SLAGS BY CARBON DISSOLVED IN IRON

Taking desulphurisation as an example, a three step mechanism has been proposed:

FeS(iron) = FeS (slag)

FeS (slag) + CaO (slag) = FeO (slag) + CaS(slag)

FeO (slag) + C (iron) = Fe(l) + CO(g)

REDUCTION OF MOLTEN FeO BY CO

• Reduction rate of slags is independent of the FeO concentration in the range 67.7 to 48.0%.

• FeO less than 48%, the reduction is dependent on the fraction reacted and the partial pressure of CO.

• Reduction rate of iron oxide by CO follows a first order rate law for the reversible reduction of stoichiometric FeO.

The rate equation is:(R’ / Ao) = exp (-32300 / RT – 1.37). (1.0 - 0.7aSiO2). (aFeO. pCO - aFe. pCOk)

where, R’ = Rate constant, mol/cm2 .s Ao = Reaction surface area, cm2

T = Reaction temperature, KThe activation energy was determined to be 135 kJ/ mol.

0

5

10

15

20

25

30

35

Fastmelt Hismelt Corex Redsmelt BlastFurnace

En

erg

y, G

J/th

m

Gas Credit

Net Energy

NET ENERGY CONSUMPTION AND GAS CREDIT OF SR PROCESSES vis-a-vis BF

Vertical Shafts• MBF – Capacity : 30,000-1,125,000 tpa (covers very wide range)• Corex – 5 operating plants, Capacity : 300,000-900,000 tpa First and leading SR process. Very high volumes of off-gas; some coke often

used. Coal properties can be varied over a small range (Indian coal?)• Finex – Capacity : 1.2-1.5 mtpa. Process development complete. Very

promising. • Tecnored – Capacity : 150,000 tpa. Process still under development

MAJOR SMELTING PROCESSES

• ITmk3 – Capacity : 0.5 mtpa. Slag separation by partial melting is unique.

Bath Smelting Processes• HIsmelt – Capacity : 600,000-1,200,000 tpa. Process almost ready.• Ausmelt – Capacity : up to 2.5 mtpa. Process not proven. • Romelt – Capacity : 200,000-1,000,000 tpa. Russian process with tremendous promise, but no plant despite efforts, including in India and Japan

Rotary Hearth Furnace (RHF)

RHF Combined with Melting / Smelting• Inmetco – Capacity - 60,000 tpa. Suitable for zinc-bearing iron ores.• FastMelt – 2 operating plants mainly for smelting solid wastes from ISPs. Capacity – 150,000-1,000,000 tpa

COMMERCIALISED SR PROCESSES

• Corex – Many operating units; most popular SR process.

Typical iron oxide Lump ore Pellets Sinter

Fetotal 62 - 65 62 - 65 50 - 55

Grain size, mm 8 - 20 8 - 16 10 - 30

• HIsmelt – Nucor, Rio Tinto, Mitsubishi and Shougang (of PR of China) agreed to construct a 0.8 Mtpa plant at Kwinana in Western Australia after pilot plant tests were completed at the same location. The plant had produced more than 37,000 tonnes of hot metal in total till March 2006. Has been stopped thereafter.

FLOW SHEET OF THE COREX PROCESS

• It has outstanding environmental superiority in comparison with the blast furnace process in terms of generation of dust, SOx, NOx, phenol, cyanides, etc.

SOME KEY FEATURES OF COREX

• Is a shaft furnace-based process. It can accept high alkali containing ores without any build-up inside the reactor.

• Specific melting capacity is higher than in blast furnaces-productivity of the order of 3-3.5 t/m3/d can be achieved.

• The process is capable of operating at 50-115% of its nominal capacity.

• It takes only half an hour to stop the plant and four hours to restart it, whenever required.

• Hot metal quality is comparable with blast furnaces (C=4.0-4.5%, Si=0.30-0.80%, S=0.02-0.09%, P depends on inputs).

FLOW SHEET OF THE FINEX PROCESS

• Posco developed Finex process to utilise iron ore/non-coking coal fines not suitable for charging to their Corex plant.• Corex in Posco uneconomical owing to restrictions on coal and

iron oxide size (top size and under size). Also, often required 10% coke to ensure hearth permeability.

• Finex actually able to utilise iron ore in the form of fines.

• After fluidised bed reduction, fine DRI is hot briquetted before melting in a melter gasifier (similar to Corex).

• Non-coking coal is also briquetted before use in melter gasifier.

• Hot metal composition same as BF.

• Range of complete scale of production yet to be established.

SOME KEY FEATURES OF FINEX

Finex is certainly of interest to ISPs. Whether it can be used to supply limited tonnages of hot metal to EAFs?

FLOW SHEET OF HISMELT PROCESS

HISMELT FLOW OVERVIEW

• Incorporates many BF features – hot blast stoves, air blast, etc.

• Can be single stage process; better, with separate pre-reduction.

• Pre-reduction of iron oxide and oxygen enrichment of hot air (1200°C) blast provide substantial productivity enhancement.

• Hot metal contains 4.3 ± 0.15% carbon, phosphorus and silicon levels are extremely low viz. < 0.05% P and <0.05 Si%.

• Process can use high phosphorus iron ore – can be of significance in India since many Indian ores have high phos.

• Coals with 50-70% FC, 10-38% VM and 5-12% ash have been used in Pilot/Demo plant. Typical coal rate reported – 640-690 kg/thm.

• Utilisation of export gas not as critical as in the case of processes like Corex on account of in-bath smelting.

SOME KEY FEATURES OF HISMELT

Hot metal production economics under Indian conditions still not proven. Demo plant not proven on sustained basis at full capacity.

PROCESS FLOW SHEET OF ITmk3

SOME KEY FEATURES OF ITmk3• First step produces iron nuggets in a very short reduction time

(10-12 min) along with some slag removal in a RHF.• High-purity iron nuggets contain 96-97% iron and 2.5-3.0% C,

0.05 S, i.e. nearly same composition as blast furnace pig iron. Coal consumption reported to be 500 kg/t nuggets.

• The process is flexible as far as the type of iron ore that can be used. Magnetite, haematite as well as pellets made of taconite (low-grade iron ore found in USA) have been processed.

• Emits at least 20% less carbon dioxide than blast furnaces.• Overall less NOx, SOx and particulate matter emissions.• Easy start-up, shut down and change in production rate.

Over last 10-12 years has gone through laboratory, small pilot plant and demonstration plant testing. Nuggets melted in EAFs. A commercial plant (0.5 mtpa) Mesabi Nuggets at

North-Eastern Minnesota, USA has been started. Could be of immense interest to Indian Secondary Producers in future.

SUMMARY OF SMELTING REDUCTION• SR designates processes in which iron oxide is reduced at high

temperature to liquid iron essentially using non-coking coal and oxygen.

• SR processes utilise several distinct thermo-chemical configurations.

• In all SR processes, a substantial portion of the iron oxide charged enters into the slag phase and is reduced by solid carbon particles, gaseous CO, and carbon dissolved in iron.

• SR reactions are by nature quite complex – major reasons are elevated temperature, multiple phases and intense bath turbulence.

• It is essential to optimise post-combustion ratio (PCR), degree of pre-reduction, and heat transfer efficiency (HTE).

Ideal SR process should be of single-stage configuration to recover iron from iron ore directly through liquid phase reduction.

THANK YOU

BF and SRIronmaking

IIT, Kharagpur, 26th October 2010