Iron making

Smarajit SarkarDepartment of Metallurgical and Materials EngineeringNIT Rourkela

Ahindra Ghosh and Amit Chatterjee: Ironmaking and Steelmaking Theory and Practice, Prentice-Hall of India Private Limited, 2008Anil K. Biswas: Principles of Blast Furnace Ironmaking, SBA Publication,1999R.H.Tupkary and V.R.Tupkary: An Introduction to Modern Iron Making, Khanna Publishers.R.H.Tupkary and V.R.Tupkary: An Introduction to Modern Steel Making, Khanna Publishers.David H. Wakelin (ed.): The Making, Shaping and Treating of Steel (Ironmaking Volume), The AISE Steel Foundation, 2004.Richard J.Fruehan (ed.): The Making, Shaping and Treating of Steel (Steeelmaking Volume), The AISE Steel Foundation, 2004.A.Ghosh, Secondary Steel Making Principle & Applications, CRC Press 2001.R.G.Ward: Physical Chemistry of iron & steel making, ELBS and Edward Arnold, 1962.F.P.Edneral: Electrometallurgy of Steel and Ferro-Alloys, Vol.1 Mir Publishers,1979 B. Ozturk and R. J. Fruehan,: "Kinetics of the Reaction of SiO(g) with Carbon Saturated Iron": Metall. Trans. B, Vol. 16B, 1985, p. 121. B. Ozturk and R. J. Fruehan: "The Reaction of SiO(g) with Liquid Slags, Metall. Trans.B, Volume 17B, 1986, p. 397. B. Ozturk and R. J. Fruehan:.Transfer of Silicon in Blast Furnace": , Proceedings of the fifth International Iron and Steel Congress, Washington D.C., 1986, p. 959. P. F. Nogueira and R. J. Fruehan: Blast Furnace Softening and Melting Phenomena - Melting Onset in Acid and Basic Pellets", , ISS-AIME lronmaking Conference, 2002, pp. 585.

There are as many as two thousand odd varieties of steels in use. These specifically differ in their chemical composition. However, a couple of hundred varieties are predominantly in use. The chemical composition of steels broadly divide them into two major groups, viz. (i) plain carbon steels and (ii) alloy steels.

The plain carbon steels are essentially alloys of iron and carbon only whereas, if one or more of elements other than carbon are added to steel in significant amounts to ensure specific better properties such as better mechanical strength, ductility, electrical and magnetic properties, corrosion resistance and so on it is known as an alloy steel. These specifically added elements are known as alloying additions in steels.

Steels may contain many other elements such as AI, Si, Mn, S, P, etc. which are not added specifically for any specific purpose but are inevitably present because of their association in the process of iron and steelmaking and can not be totally eliminated during the known process of iron and steelmaking. These are known as impurities in steel.

Every attempt is made to minimise them during the process of steelmaking but such efforts are costly and special techniques are required for decreasing their contents below a certain level in the case of each element.

For cheaper variety of steels therefore their contents at high levels are tolerated. These high. levels are however such that the properties of steels are not significantly adversely affected. These tolerable limits of impurities are considered as 'safe limits' and the impurity levels are maintained below these safe limits.

For example, for ordinary steels sulphur contents up to 0.05% are tolerable ,whereas for several special steels the limit goes on decreasing to as low as 0.005% or even lower. For most high quality steels now the total impurity level acceptable is below 100 ppm and the aim is 45 ppm.

Plain carbon steels are broadly sub-divided into four major types based on their carbon contents. These are not strict divisions based on carbon contents but are generally broad divisions as a basis of classification. This division is definitely useful. These are:

(i) Soft or low carbon steels up to 015% C (ii) Mild steels in the range 015-035% C (iii) Medium carbon steels in the range 035-065% C (iv) High carbon steels in the range 065-175% C

The alloy steels are broadly sub-divided into three groups on the basis of the total alloying elements present. This division is also only a broad division and not a rigid one. This is :

(i) Low alloy steels up to 5% total alloying contents (ii) Medium alloy steels 5-10% total alloying (iii) High alloy steels above 10% total alloying

B.F. process is the first step in Producing Steel From Iron Oxide.

This Would remain so probably at least for the first quarter of the century despite Speedy depletion of Coking coal reservesEnhanced adoption of alternate routes for iron making for ultimate conversion to steel.

The B.F. works on a counter current principleAscending hot gases meet Descending solid chargeThe charge includes Iron bearing materials (ore, sinter, pellets), coke & flux (Lime stone, Dolomite)The ascending gases cause reduction of Iron oxide in the Iron bearing materials while progressively heating it. The result is Production of Liquid slagLiquid MetalB.F. Gas of considerable calorific value

All the reduced elements join the metal. A typical composition of the Metal (Iron) produced in Blast Furnace is presented below.

The Slag is a low melting chemical compound formed by the chemical reaction of the gangue and the flux in the charge.

All unreduced ones join the slag

The major constituents of the slag include the followingAl2O3 20.45%CaO 32.23%SiO2 33.02%MgO 9.95%S 0.89%MnO 0.54%TiO2 1.01%FeO 0.41%K2O+Na20 1%Trace Oxides 0.5% (Curtsey TATA STEEL)

Blast furnace productivity depends upon an optimum gas through flow as well as smooth and rapid burden descent. The character of the gas and stock movements is intimately associated with the furnace lines. The solid materials expand due to heating as they descend and their volume contracts when they begin to soften and ultimately melt at high temperatures in the lower furnace. cont

A further volume contraction occurs when the solid coke burns before the tuyeres. An enormous volume of the combustion gas has to bubble through the coke grid irrigated with a mass of liquid metal and slag. An optimum furnace profile should cater to the physical and chemical requirements of counter flow of the descending solid, viscous pasty or liquid stock and the ascending gases at all places from the hearth to the top cont

Only then, an optimum utilization of the chemical and thermal energies of the gases as well as a smooth, uniform and maximum iron production with minimum coke rate will be realized.

In an integrated steel works the capacity of the Blast Furnace depends uponThe capacity of the works.The process of steelmaking adopted.The ratio of hot metal and steel scrap in the charge.Consumption of foundry iron in the works.Losses of iron in the ladle and the casting machine.The number of furnaces to be installed

Stock line: The distribution pattern at the top.Charge or stock level in the furnace throatThe materials or the stock or the burden should be properly distributed for uniform distribution of the ascending gas.Zero stock line: Horizontal plane formed by bottom of big bell when closed. 6ft stock level for instance located 6ft below zero stock line.

This is a unique design in which large bell is replaced by a distributor chute with 2 hoppers A rotating chute is provided inside the furnace top cone Advantages: Greater charge distribution flexibility more operational safety and easy control over varying charging particles Less wearing parts: easy maintenance

The advantages accruing from improved distribution control can be summarised as follows: Increased productivity, decreased coke rate, improved furnace life .Reduced refractory erosion Improved wind acceptance and reduced hanging as well as slips Improved efficiency of gas utilisation and its indirect reduction Lower silicon content in hot metal and consistency in the hot metal quality Reduced tuyere losses and minimisation of scaffold formation Lower dust emission owing to uniform distribution of fines.

As has been made clear that even the most efficient of the modern blast furnace would produce an effluent gas containing a significant proportion of CO which could not be used for iron oxide reduction. The actual CO content may vary around 20-30% by volume. This has a calorific value of nearly 900 kcal/m3. The quantity of gas produced depends upon the amount of fuel burnt. For one tonne of coke burnt nearly 4000 m3 of effluent gas may be produced. Hence a blast furnace requiring 1000 t of coke per day would generate nearly 4 x 106 m3 of gas with a total energy content of 3600 x 106 kcal which is nearly equivalent to 500 t of coke.

The effluent gas from the furnace cannot directly be used as a fuel since a substantial quantity of dust from the burden is also discharged along with. It may lead to accumulation of dust and wear in the equipment using the gas. The gas is, therefore, cleaned before its use and in so doing the sensible heat of the gas is invariably lost. The chemical heat of the cleaned gas is what is utilised.

The average dust content may vary in the range of 7-30 g/m3. In general cleaning is carried out in three stages viz. coarse, semi-fine and fine cleaning. The coarse cleaning is done in dust catchers and cyclones in dry condition. The dust content of the coarse cleaned gas is nearly 5-10 g/m3. The semi-fine cleaning is carried out in scrubbers, ventury washers, cyclone separators, centrifugal disintegrators, feld washers or even in electrostatic precipitators. The dust content is thereby reduced to 05-15 g/m3. Fine cleaning is carried out mainly by electrostatic precipitators or at times by high speed rotary disintegrators, The dust content is thereby reduced down to 0.01 g/m3 The semi-fine and fine cleaning is carried out either in wet or dry condition. Wet methods are generally preferred to dry methods for their better efficiency and smooth working.

Two adjacent uptakes are joined together to form one single duct and the two such ducts, thus formed, are connected to form only one duct which carries the gas downwards into the dust catcher. The downcoming pipe or duct is called downcomer.A bleeder valve is a safety device, which opens automatically or is opened, to release extra pressure developed inside the furnace and thereby eliminate the danger of explosion. The uptakes and the downcomers are steel pipes and are lined from inside with firebricks. The sizes of the uptakes and downcomers and the angle of their joints are such that gas flows out of the furnace smoothly without any hindrance.

The uptakes should be located on the furnace-top periphery at those points which are not directly vertically above the iron-notch, slag notch, blast main entrance to the bustle pipe, etc. These are active points of the furnace and if the uptakes are located right above these points it may cause uneven distribution of the gas through the burden. The entire design should also ensure that minimum of dust is carried form the furnace with the gases.

It essentially consists of a tall cylindrical structure comprising of a combustion chamber and heat regenerator unit of checker bricks. The clean blast furnace gas is burnt in the combustion chamber and the hot products of combustion later heat up the checker bricks. In this case the stove is said to be on 'on-gas' and is maintained on gas until the checker bricks are heated to a certain temperature.

Firing is stopped and cold blast is passed through checkers which impart the heat stored in them and there by produce preheated blast. The stove is said to be 'on blast'. It can continue heating the blast till a certain minimum temperature of the blast is obtainable. The stove is again put on gas and the cycle is repeated.

The stove design and the number of stoves, employed should ensure a steady supply of preheated blast to the furnace. This duty demands that the amount of heat generated by way of combustion of gas per unit time should be adequate to heat up the required amount of blast to the required temperature per unit time, taking into account the usual efficiency of heat transfer via checker system and the usual heat losses from the system.

The thermal efficiency of the stove varies between 75-90%. The checker work cools more rapidly whereas it takes longer time to heat it up. In practice a stove may be on gas for 2-4 hours and on blast for 1-2 hours. For an uninterrupted steady supply of blast at specified temperature therefore a battery of at least three stoves is necessary. A two stove system is quite unsatisfactory and hence three or four stove system is preferred.

The checkerwork has to absorb maximum heat at faster rate while heating and should desorb heat equally rapidly to the incoming cold blast. The larger the weight of bricks the more will be its heat storing capacity. The larger is the surface area exposed as flues the faster is the heat exchange with gas. The bricks should have maximum weight with maximum surface area of flues i.e. maximum openings to allow free passage of gases. It has been found that a ratio of weight of bricks in kilogram to heating surface in square metres of about 5-6 in minimum. Below this structural difficulties may arise.

The checker bricks are supported on steel grids which in turn are supported by cast iron or steel columns. Since the maximum temperature during combustion is generated near the dome and since the top portion of checker bricks have to stand higher temperatures, with progressively decreasing value downwards, the quality of checker bricks used also very accordingly. Heavy duty firebricks are essential for dome construction. The top 3-6 m height of the checkers is made up of higher alumina bricks or semi-silica bricks while the remainder as of good quality firebricks.

It is the volume of Blast Furnace occupied by the charge materials and the products , i.e. the volume of furnace from the stock line to the tap hole.Useful volume = the furnace capacity C.U.U.V.C.U.U.V = coefficient of utilization of useful volume.The value of C.U.U.V. varies in a wide range from 0.48-1.50 m3/ton of pig iron

V =k D2HV=Useful volumeH=Total heightD=Diameter at the bottom of the shaftK=A coefficient usually lies with in the range of 0.47 to 0.53. High value is for slim profile.

Total height = useful height +distance between stock line and the charging platform (it is governed by the construction of gas off-take and charging platform, this dimensions varies from 3 to 4m.) Useful height= height from the tapping hole to the stock line.The height of the blast furnace is mainly governed by the strength of the raw materials, particularly that of coke. cont

The strength of the coke charged to the furnace should be sufficient to withstand the load of raw materials without getting crushed. Coke provides permeability(in the dry as well as wet zones )and also mechanical support to the large charge column, permitting the gases to ascend through the voids. Total height (H)= 5.55V0.24Useful height (H0) =0.88H

Diameter: The belly /bosh parallel is the cylinder that connects the tapers of the shaft and the bosh. Its diameter, dbll, and the ratio of this diameter to the useful or inner height of the furnace as well as to the diameter of the hearth play an important role in the operation of the furnace. The correct descent of the stock, ascent of the gas and efficient utilization of the chemical and thermal energies of the gas depend greatly upon these ratios.

The importance of an adequate belly diameter lies in the fact that softening and melting of the gangue and formation of the slag occurs in this region. An increase in the diameter facilitates gas passage through the sticky mass and also slows down stock movement, thus increasing the residence time for indirect reduction.However, the belly diameter cannot be increased arbitrarily as it is directly related to bosh angle, bosh height, hearth and throat diameters and useful height.

The belly height depends upon the softenability of the ferrous burden and also on the shaft angle desired. If the slag fusion occurs at higher temperatures and in a narrow temperature range as in the case of pre-fluxed burden, the hydraulic resistance decreases in the vertical cross-section and the belly height can be correspondingly reduced. dbelly =0.59 (V)0.38

HbelIy = 0.07H

The hearth is designed such that its volume between the iron notch and tuyeres is sufficient to hold the molten metal and the slag. The dia of hearth depends upon:The intensity of coke consumption.The quality of burden.The type of iron being produced.

D hearth =0.32 V0.45

A very approximate relationship between the coke burning rate and hearth diameter is given by the following equation: D = c Q 0.5 D = hearth diameter, m Q = coke throughput, tonnes/24h c = throughput coefficient which varies between 0.2-0.3 depending upon burden preparation.

For highly prepared burden, the value of c = 0.2 has been achieved in modern large furnaces . Therefore, for a furnace planned to produce 10,000 THM per day with a coke rate of 500 kg/THM, i.e., a coke throughput of 5,000 tonnes per day, the hearth diameter should be about 14.1 m. The value will be 21.2 m if the value of c=0.3.

With increasing diameter of the hearth, the gas penetration must be ensured by providing adequate bed permeability with the use of mechanically strong, rich, pre-fluxed burden of uniform size and low slag bulk as well as strong lumpy coke. The Hearth height should be 10% of the total height of the furnace

The shaft height must be sufficient to allow the heating, preparation and reduction of ore before the burden reaches the bosh. In the upper regions of the shaft , volume changes due to increase in temperature and carbon deposition. These demand an outward batter for smooth flow of materials. In the lower region of the shaft , the material starts fusing and tends to stick to the furnace wall. So to counteract the wall drag an outward butter is necessary.

Stack height Hstack = 0.63 H- 3.2 m

Stack angle

The stack angle usually ranges from 850 to 870(i) 850 for weak and powdery ores; (ii) 860 for mixture of strong and weak, lumpy or fine ores; (iii) 870 for strong, lumpy ore and coke.

The variations in the angles are necessary for obtaining an adequate peripheral flow which is an essential pre-requisite for forcing of the blast furnace. Since the ore hump is located in the intermediate zone and it moves almost vertically downwards pushing the lighter coke towards the wall and the axis. A smaller shaft angle in the case of weak and powdery ore helps to loosen the periphery.

Stack angle can be calculated from the formulaStack angle ()= Cot-1(D-d1/2xStack Height) Where, D= Bosh parallel Diameterd1= Throat Diameter

Bosh angle can be calculated from the formulaBosh angle ()= Cot-1(D-d/2xBosh Height) Where, D= Bosh parallel Diameterd= Hearth Diameter

When the raw materials are charged into the blast furnace, little volume change takes place for a few meters of their descent and hence the walls of the throat are generally parallelThroat diameter can not be too small as it has to allow the enormous volume of the gas to pass through at a reasonably low velocity to maintain adequate solid gas contact and to decrease the dust emission, throat hanging and channeling. Cont..

Throat diameter can not be too wide as it may compact the charge. A certain velocity and lifting power of gas is necessary for losening the charge at top.

Throat Diameter d throat =0.59 V0.35Where, V= useful volume

A considerable amount of slag and iron descends to the hearth through the inter-tuyere zones. If they do so without having been adequately heated, the thermal state of the hearth may be disturbed with attendant high sulphur in iron, sluggish slag movement, erratic metal analysis, frequent tuyere burning, etc.

The distance between the adjacent tuyeres around the hearth circumference should be such as to obtain, as far as possible, a merging of the individual combustion zones of each tuyere into a continuous ring.

The number of tuyeres mainly depend upon the diameter of the hearth. The diameter of the tuyeres depend upon the blast volume.The following formulae can be used to determine the number of tuyeresPavlov: n = 2d +1Rice:n = 2.6d-0.3Tikhomirov et al : n = 3d-8Where n= Number of tuyeres, d=hearth diameter

Capacity (THM/Day)Parameter200030005000Useful Volume (m3)170025504250Total Height (m)33.0836.4641.22Useful Height (m)29.1132.0836.27Bosh Parallel Dia (m)9.9611.6214.11Bosh Parallel Height (m)2.322.552.89Bosh Height (m)4.374.815.44Hearth Dia (m)9.110.9213.74Hearth Area (m2)65.0493.66148.27Hearth Height (m)3.3083.6464.122Stack/Shaft Height (m)17.6419.7722.77Throat Dia (m)6.877.859.29Bosh Angle (0)84.3285.8488.05Stack Angle (0)8584.5583.96Nos. of Tuyeres202534

Richness: Richness means the percentage of metallic iron in the ore. e.g. In order to produce a tonne of pig iron about1.5tonnes of ore is required in Australia (68% Fe), about 2 tonnes are required in India (55-60%) and nearly 3 tonnes are required in U.K. (30-35%)Composition of the gangue : The composition of gangue associated with an ore may reduce the value of an otherwise rich ore or in some case may even enhance that of a lean ore.

e.g. Value of an ore is drastically reduced by the presence of alkali oxides , reduced to some extent by the presence of alumina and is in fact enhanced by the presence of lime and/or magnesia.Location: The location of an ore, both geographical and geological, is very importantTreatment and preparation needed before smelting

Cold strength Porosity Decrepitation Low-temperature breakdown under reducing conditions (LTB) Hot compression strength Softening temperature and range Swelling and volume change High-temperature bed permeability under compressive load and reducing conditions.

Cold strength measurement comprises of tumbler or drum test for abradibility, shatter test for impact and compression test for load during storage. Tumbler or drum test: It measures the susceptibility of ferrous materials (coke as well) to breakage due to abrasion during handling, transportation, charging on to the blast furnace bells as well as inside the furnace itself. In this test, a certain weight of the material within a selected size range is rotated in a drum of given size for a given time with certain number of revolutions.

The abrasion strength is given by the percentage weight of + 6.3 mm surviving the test and dust index by the percentage of - 0.6 mm. For good pellets the respective percentages are 85-95 and 3-7, for sinters 60-80 and 5-10 and for ores they vary greatly, 60-95 and 2-25.

In order to minimize the amount of fines delivered to the furnace, a practice attracting an interest is to deliberately subject the materials, especially coke and sinter, to mechanical breakdown and stabilize the charge, e.g., by means of vibrating screens. They break where the bonds are weak and the undersize screened out. However, it cannot be helped if any fines are generated between charging into the skip car and then into the furnace.

Shatter test: It measures the susceptibility to breakdown due to impact during loading, unloading and charging into the furnace. In this test a certain weight of material is allowed to fall on a steel plate from a certain height for a pre-determined number of times and the amount of undersize measured. For strong sinters the percentage +10mm surviving is above 80. Compression test: It is used mainly for pellets. Pellets, unreduced or reduced to various degrees, are subjected to compressive load at ambient or high temperatures and the percentage of + 5 mm yield measured and correlated with blast furnace performance.

Porosity: While ores and pellets possess mostly open pores, in sinters there are macro- and micro-pores as well as open and closed pores (cut off from outside and cannot be reached by gas). True porosity and hence closed porosity can be determined from open porosity which can be measured from the true and bulk densities. Although reducibility increases with increasing open porosity, the latter changes continuously during reduction on load. Generally, a high initial porosity results in earlier softening of the material.

Decrepitation : When iron bearing materials are suddenly exposed to the exhaust gas temperature at the stock level on charging, breakdown may occur due to thermal shock. This is known as decrepitation. Experimentally it is measured by dropping a known weight of material in a furnace previously heated to a temperature level of 400600C, under normal atmosphere, inert atmosphere or under mildly reducing conditions. After the charge attains the temperature it is removed, cooled and sieved to measure the breakdown.

In a typical test 500 g of 20-40 mm size undried ore is dropped in a furnace previously heated to a temperature level of 400C and retained there for 30 min under a flow rate of 5000 litres of nitrogen per hour. The sample is then removed, cooled and the percentage of 05 mm and -56 + 05 mm material in the product is determined by sieving.It is believed that ores with more than 10% porosity will not decrepitate.

Low-Temperature Breakdown Test (L.T.B.T.)It has been observed in the experimental blast furnace that the iron bearing materials do disintegrate at low temperatures under mildly reducing conditions, that is in the upper part of the stack, affecting the furnace permeability and consequently the output adversely. It is believed that deposition of carbon in this region of the stack is also a contributory factor although with sinters the breakdown has been associated with the presence of micro-cracks. In essence the test consists of subjecting the charge to static bed reduction at low temperatures in a rotating furnace for a fixed duration. The percentage of fines generated is quoted as the L. T.B. T. index.

Reducibility is the ease with which the oxygen combined with iron can be removed indirectly. A higher reducibility means a greater extent of indirect reduction that may be obtained in the blast furnace resulting in a lowered coke rate and higher productivity.

Reducibility of ferrous materials is characterized by theirfractional oxygen removal rates in gaseous reducing atmosphere. The percent degree of reduction orpercent fractional oxygen removal is given by

Wheren0= number of moles of oxygen originally combined with iron only; n = number of moles of oxygen left combined with iron after experimental time, t.

A schematic representation of relationship between reduction at 40% degree of reduction and 60% degree of oxidation levels,

particle size porosity crystal structurepore size volume change impurities

Reduction of natural hematite ores by CO or H2 starts between 200-5000C, depending upon the physical characteristics and mineralogical composition. However, the rate below 5000C is sluggish. Hematite is more reducible than magnetite although the amount of oxygen to be removed per unit weight of iron is about 12 percent higher in the former. The better reducibility of hematite may be due to: formation of porous wustite from hematite, easily accessible to reducer gas whereas magnetite forms dense wustite during reduction;

tendency of hematite to break down and expose larger surface due to expansion in volume during reduction to magnetite ; pores in hematite are more elongated and the microporosity larger; magnetite has larger grain size and is more closely packed; a higher value of overall rate constant for wustite reduction since the wustite lattice formed during reduction of hematite exhibits a higher degree of disorder than that formed from magnetite.

Chemical Influence

It is well known that the reduction rate of wustite is critical in the overall kinetics of iron oxide reduction.The equilibrium partial pressure or concentration of CO2 would decrease if aFeO is lowered by solution and/or compound formation. Hence, the reduction rate would also decrease.

Natural ores can contain iron oxides as compounds with gang materials, such as, 2FeO.Si02, FeO.AI203, FeO.Cr203, FeO.TiO2 etc where wustite exists in a state of low activity. The activity of wustite can also decrease when it undergoes sintering with the impurities present, such as SiO2, Al2O3 etc.

The reduction rate of ore increases with increase in linear velocity of the reducing gas due to the reduction of the boundary layer thickness at the bulk-gas/particle interface. After a critical gas velocity is reached, there is no further increase in the rate with increasing gas velocity since the overall rate becomes controlled or limited by other processes. The figure shows that the limit is only 0.4 m/s. The figure also shows that the critical velocity is independent of the degree of oxidation. In blast furnace, the linear gas velocity does not affect the reduction rate since it ranges between 1-20 m/s and is often exceeded.

For the reduction of iron ores the reducing gas has to diffuse into the interior of the body where transformations can occur. In general, the reduction rate increases with temperature but the degree depends upon the mechanism of the reaction . The overall reduction rate depends upon the relative contributions of chemical control and gaseous mass transport and hence depends upon the particular reactions occurring and the reaction temperature. Since chemical reaction has higher activation energy than gaseous diffusion, the former will increase at a much greater rate with increase in temperature than the latter.

Hence, a stage will arrive where diffusion will become rate-controlling. Depending upon the degree of reduction, at lower temperatures of about 500-600C, the chemical reaction rate controls the reduction rate forming what is known as the kinetic region in the blast furnace. At temperatures above 600C, gaseous diffusion becomes the dominant rate controlling mechanism. The temperature regime in the blast furnace shaft is such that it can be assumed a zone of mixed-control exists.

In the blast furnace , the reducing gas is predominantly CO with varying amounts of hydrogen depending upon the moisture content of the blast and other blast additives like fuel oil or natural gas. Study shows that a mixture of CO and hydrogen appears to be a more efficient reductant than either of them.

The function of coke in the blast furnace is five-fold, namely, (i) it acts as a fuel by providing for the thermal requirements in the furnace, the reaction being, 2C + O2 = 2CO: H0 = - 2300 kcal/kg.C On complete combustion to CQ2 the heat evolved is 8150 kcallkg.C. Thus only about 28 percent of the obtainable heat is supplied by coke; (ii) it provides CO for the reduction of iron oxides; (iii) it reduces the oxides of metalloids, such as, Mn, Si, P and others if present; (iv) it carburizes the iron and lowers its melting point; (v) it provides permeability (in the dry as well as the wet zones) and also mechanical support to the large charge column, permitting the gases to ascend through the voids.

Coke is the universal fuel used in the blast furnace. It acts both as a reductant as well as a supplier of heat. It also comprises the major portion of iron production cost. Now-a-days other fuels are also being used as part replacement of coke. These fuels cannot be charged from the top and as such they are injected into the furnace through the tuyeres along with the blast. In some countries, especially in Brazil, charcoal is used as a blast furnace fuel.

Coke size: Coke comprises about 50-60 percent of the volume of the charge material. The coke size is important as it provides permeability in the dry as well as in the wet bosh zone The coke size is always 3-4 times larger than the ore size, since coke is partially burnt as it descends. It also has a lower density, and hence a greater tendency for fluidisation. Of course, in the lower bosh region of a blast furnace, coke is the only solid that remains, and which helps to support the burden. The optimum size range for lump ore is 10-30 mm and for coke is 40-80 mm. Since the coke size becomes smaller as it descends through the blast furnace due to mechanical breakdown, gasification, attrition, etc., the factor of prime importance is the strength of coke.

Coke strength: Mechanically considered, it is the quality cohesion that prevents the coke from collapsing and tends to avoid the formation of small particles. High cohesion or strength is related to several coke making properties. On the basis of breakage by impact, compression or abrasion, the coke strength should be assessed both at ambient as well as high temperatures. Studies of the structure of different coke samples show that the best varieties have a regular distribution of pores: with adequate thickness and hardness of the walls between the pores and are free from cracks generated internally. Such a structure ensures withstanding of high compressive forces and high temperatures in the all-important lower furnace.

The strength of coke produced in the coke-ovens is influenced by:blending ratio of coals of varying caking components and proportion of the fibrous portion; particle size and distribution of charging coal; coke-oven temperature and combustion conditions; moisture and addition of oil; soaking time; width, height and method of heating.

It is defined as the ability of coke to react with O2, CO2 or steam (H2O). More reactive cokes have higher thermal values of their volatile matter. Coke of high reactivity ignites easily and gives rapid pick up of fuel bed temperature. However, low reactivity coke gives a higher fuel bed temperature than a highly reactive coke Reactivity is inversely proportional to the absolute density. It is affected by the presence of easily reducible iron compounds in ash. Coke of high reactivity is obtained from weakly caking coals or blends. Strongly coking, high rank coals produce coke with low reactivity.

For blast furnace coke, size and hardness are more important than reactivity. Satisfactory hearth temperature is obtained with unreactive coke containing little breeze. Reactivity of coke is measured by Critical Air Blast method and is reported as Critical air blast (CAB) value of coke. The CAB value of coke is the minimum rate of flow of air in ft3/minute necessary to maintain combustion in a column of closely graded material (14 to 25 B.S.) which is 25 mm deep and 40 mm in diameter. The typical CAB value for oven coke is 0.065 ft3/minute. More reactive coke has got lower CAB value.

Another modern and current method of expressing the reactivity and strength of coke is Coke Reactivity Index (CRI) and Coke Strength After Reaction (CSR) which is being followed in Indian steel plants. Coke Reactivity Index (CRI).

To determine CRI, 200 gm of coke sample (size + 20 - 25 mm) is taken in a stainless steel tube and heated in electric furnace to 1100C. CO2 gas at 5 kg/cm2 pressure is passed through the coke bed for two hours. CO formed (by reaction C + CO2 = 2CO) is burnt in a burner and is exhausted out. Carbon of coke reacts with CO2 (depending upon the reactivity level of the coke) and there is a loss of weight of coke depending upon its reactivity. More is the loss in weight of the coke, reactivity is more. % loss in weight of coke is reported as coke reactivity index (CRl). Ideal CRI value of a good blast furnace coke should be about 20%. Typically CRI of Indian blast furnace coke is about 25%.

Coke Strength after Reaction (CSR). The left out coke from the CRI determination test is rotated for 60 rotation in a micum drum. And the % of coke retained on a 10 mm size screen is reported as coke strength after reaction (CSR). Stronger the coke, more is its CSR value. Ideal value of CSR for blast furnace coke is a minimum of about 55%. Typically CSR of Indian blast furnace coke is about 60-65.

Agglomeration of Iron Ore FinesAbout 65 75 % of iron ore gets converted into fines ( - 5 mm ) during various operations from mining to conversion into CLO. Majority of these fines are exported to other countries at throwaway price resulting in greater financial loss to the nation. Most widely used methods for the agglomeration of these fines to render them useful for BF are Sintering and Pelletization.Sintering Sintering is essentially a process of heating of mass of fine particles to the stage of incipient fusion for the purpose of agglomerating them into lumps.

To increase the size of ore fines to a level acceptable to the BFTo form a strong and porous agglomerate To remove volatiles like CO2 from carbonates, S from sulphide ores etcTo incorporate flux in the sinterTo increase the BF output and decrease the coke rate

Iron ore sintering is carried out by putting a mixture Iron bearing fines mixed with solid fuels on a permeable bed. The top layer of sinter bed is heated up to the temperature of 1200 - 13000C by a gas or oil burner. The combustion zone initially develops at the top layer and travels through the bed raising its temperature layer by layer to the sintering label. The cold blast drawn through the bed cools the already sintered layer and gets itself heated.

In the combustion zone, bonding takes place between the grains and a strong and porous aggregate is formed. The process is over when the combustion zone reaches the lowest layer of the bed. The screened under size sinter is recycled and over size is sent to B.F.

Two types of bonds may be formed during sintering. Diffusion or Recrystallization or Solid State Bond : It is formed as a result of recrystallization of the parent phase at the point of contact of two particles in solid state and hence the name. Slag or Glass Bond: It is formed as a result of formation of low melting slag or glass at the point of contact of two particles, depending upon the mineral constitution, flux addition, etc. As a result the sinter can have three different types of constituents: Original mineral which has not undergone any chemical or physical change during sintering. Original mineral constituents which have undergone changes in their physical structure without any change in their chemistry. Recrystallization is the only change at some of the particle surfaces. Secondary constituents formed due to dissolution or reactions between two or more of the original constituents

The proportion of each of the physical and chemical change during sintering depends upon the time-temperature cycle of the process. The higher is the temperature more will be the proportion of new constituents by way of solutions and interactions whereas lower is the temperature and longer is the duration more is the process of recrystallization in solid state. The more is the slag bonding, stronger is the sinter but with less reducibility and, more is the diffusion bonding, more is the reducibility but less is the strength. Since ores are fairly impure slag bond predominates. On the other hand in rich sinters slag bond is of minor importance.

The area under the time-temperature curves essentially determines the nature and strength of the bonds developed during sintering of a given mix. For a given mix it is most unlikely the bonds of sufficient strength will be formed below a certain temperature level within a reasonably short time. Hence the area under the curve above a certain temperature, which may be around 1000C for iron ores, is the effective factor in deciding the extent of sintering

rather than the whole area under the curve from room temperature to the combustion temperature level. The nature of the time-temperature graph will depend upon the rate of heating and cooling of a given mix. The nature of this graph is of paramount importance in assessing the sintering response. The factors that affect this curve are then the variables of the process and which should be adjusted properly for obtaining effective sintering.

Bed permeability Total volume of air blast drawn through the bed Particle size of iron oreThickness of the bedRate of blast drawn through the bed Amount and quality of solid fuel incorporated in the sinter mixtureChemical composition of ore finesMoisture content in the charge

During sintering, heat exchange takes place between the solid charge and air drawn. At any time, the air takes the heat from combustion zone and then transfers to the lower layer of the bed. For faster rate of heat exchange, the volume of air drawn should be more. If suction rate of air is too high, transfer of heat may become less efficient. On the other hand, the flame front will not move down the bed properly if suction is less. Higher the bed permeability, more will be the air drawn. But, higher permeability leads to loss of strength in the resulting sinter due to reduction in bond strength. Hence a compromise is made between these two factors. It is usual practice to draw about 700 1100 m3 of air/ton of charge.

An increase in particle size increases bed permeability and the volume of air drawn. Strength of sinter gets reduced with an increase in particle size of the ore due to reduction in contact area.For effective sintering, the use of larger ore lumps is undesirable. Iron ore size > 10mm is rarely preferred.Higher proportion of 100 mesh size fines adversely affects the bed permeability. Better is that 100 mesh size fraction should be screened off and used for pelletization. Ideal size of iron ore for sintering is 0.07 10 mm.

During mining and ore dressing operations, especially where very fine grinding is necessary for wet concentration, a large amount of - 0.05 mm fines is generated which are not amenable to sintering because of very low permeability of the bed. They can, however, be agglomerated by balling them up in the presence of moisture and suitable additives like bentonite, lime, etc. into 8-20 mm or larger size. These green pellets are subsequently hardened for handling and transport by firing or indurating at temperatures of 1200-1350C.

Pelletisation essentially consists of formation of green balls by rolling a fine iron bearing material with a critical amount of water and to which an external binder or any other additive may be added if required. These green balls of nearly 8-20 mm size are then dried, preheated and fired, all under oxidising conditions, to a temperature of around 1250-1350C. Bonds of good strength are developed between the particles at such high temperatures.

The pelletisation process consists of the following steps: Feed preparation. Green ball production and sizing. Green ball induration: (a) Drying (b) Pre-heating (c) Firing Cooling of hardened pellets.

The observations on ball formation that eventually led to the development of the theory of balling are as follows: Dry material does not pelletise and presence of moisture is essential to roll the powder into balls. Excessive water is also detrimental. Surface tension of water in contact with the particles plays a dominant role in binding the particles together. Rolling of moist material leads to the formation of balls of very high densities which otherwise is attainable by compacting powder only under the application of a very high pressure: The ease with which material can be rolled into balls is almost directly proportional to the surface area of particles, i.e. its fineness.

The capillary action of water in the interstices of the grains causes a contracting effect on them. The pressure of water in the pores of the ball is sufficiently high so as to compact the constituent grains into a dense mass. The compressive force is directly proportional to fineness of the grains since the capillary action rises with the decrease in pore radius and the latter decreases with increasing fineness. An optimum moisture is important since too little of water introduces air inclusions in the pores and too much of water would cause flooding and destruction of capillary action. The optimum moisture content usually lies between 5-10 percent or more, the finer the grains the larger the requirement.

Besides the bonds formed due to surface tension mechanical interlocking of particles also pays a significant role in developing the ball strength. Maximum strength of a green ball produced from a given material will be obtained by compacting the material to the minimum porosity and with just sufficient water to saturate the voids. The rolling action during pelletisation is beneficial in reducing the internal pore space by effecting compaction and mechanical interlocking of the particles.

From fundamental studies it has been concluded that there are three different water-particle systems: The pendular state, when water is present just at the point of contact of the particles and surface tension holds the particles together. The funnicular state, when some pores are fully occupied by water in an aggregate system. The capillary state, when all the pores are filled with water but there is no coherent film covering the entire surface of the particles.

The ball formation is a two stage process, i.e. nucleation or seed formation and their growth. The formation of balls on a pelletiser depends primarily on the moisture content. Seeds are formed only if critical moisture level is maintained and without which the process cannot proceed properly. Growth takes place by either layering or assimilation. It has been observed that the size of the balls produced in a pelletiser from a charge containing right amount of moisture depends on the time and speed of the pelletiser, i.e. number of revolution.. Three regions can be clearly observed, during ball formation. : Nucleii formation regionTransition region Ball growth region.

When a wet particle comes in contact with another wet or dry particle a bond is immediately formed between the two. Similarly several such particles initially join during rolling to form a highly porous loosely held aggregate and crumbs which undergo re-arrangement and partial packing in short duration to form small spherical, stable nucleii. This is the nucleation period, a pre-requisite for ball formation since these very nucleii later grow into balls.

After nucleii are formed they pass through a transition period in which the plastic nucleii further re-arrange and get compacted to eliminate the air voids present in them. The system moves from a pendular state through funicular state to the capillary state of bonding. Rolling action causes the granules to densify further. The granules are still plastic with a water film on the surface and capable of coalescing with other granules. The size range of granules in this region is fairly wide.

The plastic and relatively wet granules grow if they are favorably oriented. In this process some granules may even break because of impacts, abrasion, etc. Growth takes place by two alternative modes. growth by assimilation is possible when balling proceeds without the addition of fresh feed material. growth by layering is possible when balling proceeds with the addition of fresh feed material.

Growth by AssimilationIf no fresh feed material is added for balling the rolling action may break some of the granules, particularly the small ones, and the material coalesces with those which grow. The bigger the ball the larger it will grow under these conditions. Since smaller granules are weaker they are the first victim and growth of the bigger balls takes place at their expense.Growth by Layering Growth of the seeds is said to be taking place by layering when the balls pick up material while rolling on a layer of fresh feed, The amount of material picked up by the balls is directly proportional to its exposed surface, i.e. the increase in the size of the balls is independent of their actual size. Growth by layering is more predominant in the disc pelletisers and growth by assimilation is more predominant in drum pelletisers, at least beyond the feed zone.

In general natural lumpy ore or sinter or pellets or a suitable combination of two or more of these form the burden.. The modern large capacity furnaces necessarily need fully prepared burden to maintain their productivity since the required blast furnace properties cannot just be met by natural lumpy ore. The selection of the process of agglomeration, whether sintering or pelletising, will depend upon the type of ore fines available, the location of the plant and other related economic factors involved. Sintering is preferred if the ore size is -10 mm to + 100 mesh and if it is -100 mesh pelletising is generally adopted. Pelletising in fact requires ultrafines of over 75% of -325 mesh. These processes are therefore not competitive.

Minimum closure of pores by fusion or slagging; open pore system; very good reducibility due to high microporosity . Porosity of sinter is 10-18% and that of pellets is 20-30%. The shape of pellets is near spherical and hence bulk permeability of the burden is much better than that obtained from sinter which is non-uniform in shape. The shape, size and low angle of repose give minimal segregation and an even charge distribution in the furnace.

More accessible surface per unit weight and more iron per unit of furnace volume because of high bulk density, 3-3.5 tonnes/m3 .Larger surface and increased time of residence per unit weight of iron give better and longer gas/solid contact and improved heat exchange; Degradation of sinter during its transit is much more than that of pellets. The sinter therefore has to be produced nearby the blast furnace plant while pellets can be carried over a long distance without appreciable degradation. Ease in handlingIt should also be noted that If high rates of productivity demand elimination of fines and since sinter happens to contribute more to the generation of fines than that of pelllets, the later will have to be chosen as the burden in preference to sinter.

The installation cost of a pelletising plant will be 30-40% more than that of sintering plant of an equal size. The operating cost of sintering is slightly less than that of pelletising. Difficulty of producing fluxed pellets. Swelling and loss of strength inside the furnaceFluxed pellets break down under reducing conditions much more than acid and basic sinters and acid pellets.Strong highly fluxed sinters, especially containing MgO, are being increasingly preferred to pellets.

Burden distribution is one of the key operating parameters influencing blast furnace performance, particularly the productivity and the coke rate. The proper distribution of burden materials improves bed permeability, wind acceptance, and efficiency of gas utilisation.

In a typical Indian blast furnace equipped with a bell-less (Paul Wurth) distribution system, the decrease in coke rate that is due exclusively to burden distribution was found to be 1012 kg/thm.

Design of the blast furnace and its charging device (effect of these factors is constant).

Angle and size of the big bell. Additional mechanical device(s) used for obtaining better distribution. Speed of lowering of large bell.

Inconsistency in physical properties of charge materials (deficiencies caused by this should be eliminated by improving quality of the burden.

Size range of the various charge materials Angle of repose of raw materials and other physical characteristics of the charge. Density of charge materials.

Level, system and sequence of charging, programme of revolving the distributor (conditions determining major means of blast furnace process control from top).

Distribution of charge on the big bell Height of the big bell from the stock-line i.e. charge level in the furnace throat. Order and proportion of charging of various raw materials.

The density of three important raw materials viz. the ore, the coke and the limestone are quite different. The heaviest is iron ore with around 5-6 glcc, the lightest is coke with density of around 15 glcc and the limestone is intermediate with-a value of density around 30-35 glcc. It means that the rolling tendency of coke particles is maximum and that of the ore is minimum. Since the density values cannot be altered, the sizes may be so chosen that their differential rolling tendencies are offset to some extent.

When a multi-particle material is allowed to gently fall on a horizontal plane it tends to form a conical heap. The base angle of this cone is known as angle of repose of that material. This angle depends upon the particle size, its surface characteristics, moisture content, shape, size distribution, etc.

The problem of very dense ores is serious from the point of view of their sluggish reduction rates rather than their tendency towards segregation. Such ores are therefore invariably crushed and sintered to obtain more porous agglomerates before charging these in the furnaces.

For an iron ore of 10-30 mm size, with an average mean size of 18 mm, the angle of repose is around 33-35. For coke of 27-75 mm size, with an average size of 45 mm, the same is around 35-38. Similarly the angle of repose for sinter is in the range of 31-34 and for pellets it is around 26-28.

The higher is the angle of repose the more it has the tendency to form ridges on charging in a blast furnace. The more dried is the ore and the more it is free from fines the less pronounced is the angle of repose and thus less is the tendency towards segregation. The clayey ores tend to form ridges because of their high angle of repose. The effective way to reduce the angle of repose of any iron ore is to eliminate the fines, dry the ore if wet and to wash off clay, if any, adhering the ore.

On dumping, as the materials fall on the stock surface, they take a parabolic path and mainly two different profiles of the accumulated mass emerge depending upon whether the particles hit the in-wall directly(V- shape) or the stock surface (M-shape)

The M-profile itself is generally obtained if the material strikes the stock surface. This happens when the bell/throat diameter ratio is small (larger bell-inwall distance) or the charging distance is small . It is clear that the peak of the M-contour approaches the inwall (hence the peripheral permeability decreases) as the charging distance increases and ultimately the M changes to V profile.

Right at the top of the furnace is the granular zone that contains the coke and the iron bearing materials charged, sometimes along with small quantities of limestone and other fluxes. The iron-bearing oxides charged get reduced to wustite and metallic iron towards the lower end of the granular zone. As the burden descends further, and its temperature rises on account of contact with the ascending hot gases, softening and melting of the iron-bearing solids takes place in the so-called cohesive zone (mushy zone).

Further down the furnace, impure liquid iron and liquid slag are formed. The absorption of carbon lowers the melting point of iron drastically. For example, an iron alloy containing 4 wt. % carbon melts at only 1185C.. In the cohesive zone and below it, coke is the source of carbon for carburisation of liquid iron. However, carbon directly does not dissolve in liquid iron at this stage. The possible mechanism of carburisation of iron entails the formation of CO by gasification of carbon, followed by the absorption of carbon by the reaction: 2CO(g) = [C]in Fe+ CO2(g)

Coke is the only material of the blast furnace charge which descends to the tuyere level in the solid state. It burns with air in front of the tuyeres in a 1-2 m deep raceway around the hearth periphery. Beyond the raceway there is a closely packed bed of coke, the central coke column or dead man's zone. The continuous consumption of coke and the consequent creation of an empty space permit the downward flow of the charge materials. The combustion zone is in the form of a pear shape, called 'raceway' in which the hot gases rotate at high speeds carrying a small amount of burning coke in suspension.

The raceway is a vital part of the blast furnace since it is the heat source in a gigantic reactor and at the same time a source of reducing gas.The salient features of Combustion zone are summarized below: The force of the blast forms a cavity the roof of which is formed of loosely packed or suspended coke lumps and the wall more closely packed. The CO2 concentration tends to increase gradually from the centre and reaches a maximum value just before the raceway boundary where most of the combustion of coke occurs according to: C+O2 (air) =CO2+94450 cal

The temperature of the gas rises as the coke consumption proceeds and reaches a maximum just before the raceway boundary. Thereafter, it falls sharply as the endothermal reduction of CO2 by C proceeds; CO2 +C =2CO-41000 calThe concentration of CO2 fall; rapidly from the raceway boundary and the gasification is completed within 200-400 mm from the starting point of the reaction.

The primary slag of relatively low melting point which forms in the lower part of the stack or in the belly consists of FeO-containing silicate and aluminates with varying amounts of lime which has become incorporated depending upon the degree of calcination undergone . As the slag descends, ferrous oxide is rapidly reduced by carbon as well as by CO. As the lime is continually absorbed, the original FeO-Si02-AI203 system rapidly changes to the CaO-Si02-AI203system with some minor impurities accompanying the burden. The dissolution of lime and the approach to the CaO-Si02-Al203 system is more pronounced,.

As the liquid primary slag runs down the bosh and loses its fluxing constituent FeO, the liquidus temperature also increases. If, therefore, the slag has to remain liquid it must move down to hotter parts of the furnace as rapidly as its melting point is raised. As the reduction of FeO is almost complete above the tuyeres the resulting bosh slag, composed mainly of CaO-Si02-AI203The hearth slag is formed on dissolution of the lime which was not incorporated in the bosh and on absorption of the coke ash released during combustion. The formation is more or less complete in the combustion zone.

This slag runs along with the molten iron into the hearth and accumulates there and forms a pool with the molten metal underneath. During the passage of iron droplets through the slag layer, the slag reacts with the metal and a transference of mainly Si, Mn and S occurs from or to the metal, tending to attain equilibrium between themselves as far as possible.

0.81 kg. C is required for indirect reduction of 1 kg. Fe from Fe203 and about 1790 kcal of heat is evolved in the process. for direct reduction of 1 kg. Fe, only 0.23 kg. C is consumed but results in an absorption of 656 kcal of heat.

Below 600C : Pre-heating and pre-reduction600 -950C: Indirect reduction of iron oxides by CO and H2 9500C to softening temperature: Direct reduction; gasification of carbon (solution loss reactions) by CO2 and H2 becomes prominent.

The formation of cohesive layers or partially reduced and partially molten iron oxide takes place. The coke slits provide passage for gaseous flow. Dripping or Dropping Zone Semi fluidized region in which liquids drip and fragments of cohesive layers drop. Zone through which liquids trickle down to the hearth. It is the final stage of iron oxide reduction

Blast, injectants and coke are converted to hot reducing gas. This gas reduces the ore as it moves counter currently towards the top of the furnace. Hearth

It is a container for liquids and coke where slag/metal! coke/gas reactions take place. Metal droplets pass through the slag/coke layer. Liquid metal/coke layer in which chemical reactions take place only to a small extent.

fluidization of small particles when the local gas velocity is excessive; diminution of void age due to swelling and softening-melting; flooding of slag in the bosh zone when the slag volume and gas velocity are excessive.

The charge in the blast furnace descends under gravity against the frictional forces of solids and buoyancy of gas. With increasing gas velocity, the pressure drop increases approximately quadratically until the upward thrust of the gas and downward thrust of the solids are held in balance. When this critical velocity is exceeded (the point of incipient fluidization), the packing in the bed becomes loose, the finer particles begin to teeter and the pressure drop ceases to increase, i.e., the resistance to gas flow drops (due to increase in void age at places where the fines become suspended).

The mechanism of the softening-melting phenomena is schematically illustrated in previous Figure. It is evident that with the onset of softening, the voidage in the bed decreases and the bed becomes more compact (origin of the terminology cohesive). As a consequence, further indirect reduction of iron oxide by gases becomes increasingly difficult. Upon melting, dripping of molten FeO-containing slag through the coke layers increases the flow resistance through the coke slits and the active (i.e. dripping) coke zone because of loss of permeability.

The cohesive zone has the lowest permeability. Hence, for proper gas flow: Ts should be as high as possible The thickness of the cohesive zone should be as small as possible. This thickness depends on the difference between Ts and T m (Tm - Ts), and therefore, the difference should be as low as possible.

Gas flow through Granular zone:For resistance to gas flow, more important than the particle diameter is the relative size of the materials in the bed. In a mixed bed of widely varying particle size, the small particles land in the interstices of the large ones and decrease the void age . Starting with large uniform spheres, the void age decreases as the small ones are introduced and the bed becomes more and more compact as the proportion of the latter increases. The bed is most dense, i.e., the voidage is minimum when 60-70 percent of the total volume of the particles consists of the large ones for about all the cases.

The m increases on either side of the minimum, i.e., with increasing or decreasing volume fraction of the small particles (approaching more uniformity of the size distribution). The voidage decreases greatly as the ratio ds/ d1 decreases. This shows that for a good and uniform permeability and low resistance to gas flow in a mixed bed, the size fractions should be as narrow as possible. One can easily visualize the adverse effects of multi-granular bed of particles of varying diameter on the voidage.

A narrow size distribution has the following advantages: charge permeability increases and the gas distribution is more uniform with better utilization of the chemical and thermal energies of the gases; more even material distribution at the stock level and less material segregation in the shaft during descent; gas flow is not impeded if the size ratio is within limits but at the same time gives rise to a tortuous flow of gases with continuous changing of flow directions, providing a larger gas/solid contact time.

The fraction of iron bearing material below the limiting size is therefore termed as 'fines' by the blast furnace technologists and is invariably eliminated by screening at every possible stage. From the point of view of reduction the maximum top size of an iron bearing material should be as low as possible, since the rate of reduction decreases, perhaps exponentially, with increasing size. The size range of materials charged in the blast furnace represents a compromise to give both good stack permeability and adequate bulk reducibility.

Gas flow in wet zone:

Wet zones consist of the coke beds in the bosh and belly regions, i.e. inactive coke zone, active coke zone, and the coke slits in the cohesive zone. Here molten iron and molten slag flow downwards through the bed of coke. This reduces the free cross section available for gas flow, thus offering greater resistance, thereby increasing the pressure drop. An extreme situation arises when, at high gas velocity, the gas prevents the downward flow of liquid. This is known as loading. With further increase in gas velocity, the liquid gets carried upwards mechanically, causing flooding.

Lump ores, sinter and pellets disintegrate into smaller pieces during their downward travel through the blast furnace owing to the weight of the overlying burden, as well as abrasion and impact between the burden materials. It has been found that this tendency gets aggravated when the oxides are in a reduced state. Reduction of hematite into magnetite occurs in the upper stack at 500-600C, and this is accompanied by volume expansion even to the extent of 25%. This results in compressive stresses being developed and contributes significantly to breakdown of the iron oxides. Blast furnace operators prefer a low RDI (below 28 or so) since the adverse effect of high RDI has been clearly demonstrated in practice.

Scientists have tried to estimate pressure drop in blast furnace. However, they are approximate. Moreover, they are only for the granular zone and coke zones. The situation in the cohesive zone is very complex, and reliable theoretical estimates are extremely difficult to come by.

Therefore, for practical applications in blast furnaces, an empirical parameter, called Flow Resistance Coefficient (FRC) has become popular. The FRC for a bed is given as where the gas flow rate is for unit cross section of the bed, i.e. either mass flow velocity or volumetric flow velocity .

FRC=1/ bed permeability The FRC for a furnace can be empirically determined from measurements of pressure drop and gas flow rate. Since it is possible to measure pressures at various heights within a furnace, the values of FRC for individual zones can also be determined.

These measurements have indicated that FRCs for the granular, cohesive, coke + tuyere zones are approximately 20%, 50% and 30% of the overall furnace FRC. This means that the cohesive zone is responsible for the maximum flow resistance and pressure drop, to a very large extent.

Decreasing the extent of SiO formation by: Lowering ash in coke, and the coke rate Lowering RAFT Lowering the activity of Si02 in coke ash by lime injection through the tuyeres.

Decreasing Si absorption by liquid iron in the bosh by enhancing the absorption of Si02 by the bosh slag. This can be achieved by: Increasing the bosh slag basicity. Lowering the bosh slag viscosity..

Removal of Si from metal by slag-metal reaction at the hearth by: Lowering the hearth temperature Producing a slag of optimum basicity and fluidity.

Desulphurisation of metal droplets through slag-metal reaction in the furnace hearth : (CaO) + [S] + [C ]= (CaS) + CO (g)Desulphurisation through the coupled reaction: (CaO) +[S] +[ Mn] = (CaS) + (MnO) (CaO) + [S] + [ Si] = (CaS) + 1/2 (SiO2)

Sulphur pick-up through the vapour-phase reaction: CaS( in coke ash) + SiO (g) = SiS(g) + CaO FeS( in coke ash) + SiO (g) = SiS(g) + CO(g) +[Fe] In the bosh and belly regions, SiS decomposes asSiS(g) = [Si] + [S]

Reducing slag i.e. FeO content should be lowHigh basicityHigh temperature, since desulphurisation is an endothermic reactionKinetic factor Contact surface of metal and slag ( by agitation)Fluidity of slag( by adding MgO , MnO)Time of desulphurisation

0.8-0.9t0.5-0.6t1.7-1.8t2500 m3 0.6t 1tFuelReducing agent supplyPermeable bed (spacer)3200m3 + 80kg dust

The efficiency of operation of a blast furnace may be measured in terms of coke rate which should of course be as low as possible. The achievement of a satisfactory coke rate depends on optimising the extent to which the carbon deposition reaction proceeds. If the top gas is high in C02 sensible heat is carried from the furnace as a result of the exothermic reaction.2CO=CO2+CIf on the other hand the top gas is high in CO, chemical heat leaves the furnace.

CO2 emission

IndustryContribution %Power51Transport16Steel10other23

The purpose of HTP is to introduce more oxygen to burn more carbon by blowing more air and at the same time maintaining the linear gas velocity (and pressure drop) identical to that in the conventional practice without any formation of channels, maldistribution of gas, increase in coke rate or flue dust emissionAdvantages:For the same volume flow rate, a greater mass of air (hence, oxygen) can be blown with HTP; higher output;

A major benefit that is so obvious is increased production rate because of increased time of contact of gas and solid as a result of reduced velocity of gases through the furnace. Increased pressure also increases the reduction rate of oxide;Suppression of Boudouard reaction (C02 + C= 2CO) and hence savings in fuel; More uniform distribution of gas velocity and reduction across furnace cross-section; smoother furnace operation due to increased permeability;

less flue dust losses, less variation of coke input, better maintenance of the thermal state of the hearth, more uniform iron analysis; More uniform operation with lower and more consistent hot metal silicon content have been claimed to be the benefit of high top pressure;

Bhilai Steel Plant (operative), RSP yet to implement

SiO2 +C ={SiO} +{CO}From above equation it can be seen that partial pressure of SiO can be brought down by increasing the partial pressure of CO; in other words the SiO2 reduction reaction can be discouraged by application of top pressure which enables a higher blast pressure and hence an increase in partial pressure of CO.

The blast volume and therefore the coke throughput can be increased by 30 percent with the maintenance of identical pressure drop and gas velocity conditions in the blast furnace by increasing the top pressure to 2.1 from 1.1 ata and bottom pressure to 3.5 from 2.5 ata under the given blowing conditions.

'raceway adiabatic flame temperature This is the highest temperature available inside the furnace. There is temperature gradient in vertical direction on either side of this zone. This temperature is critically related to the hearth temperature known as operating temperature of the furnace. It is equally related to the top gas temperature such that the hot raceway gasses have to impart their heat to the descending burden to the extent expected and leave the furnace as off-gases at the desired temperature.

The primary purpose of using injectants with the blast is profitability which depends upon the relative price of coke and injectants and the amount of coke that can be saved per unit of the latter, i.e., upon the replacement ratio:

*******Recrystallization is a process by which deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility. Thus, the process may be introduced as a deliberate step in metals processing or may be an undesirable byproduct of another processing step. The most important industrial uses are the softening of metals previously hardened by cold work, which have lost their ductility, and the control of the grain structure in the final product.

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Iron making

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