Iron making

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  • 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.
  • Iron making

    1. 1. Smarajit SarkarDepartment of Metallurgical and Materials Engineering NIT Rourkela
    2. 2.  Ahindra Ghosh and Amit Chatterjee: Ironmaking and Steelmaking Theory and Practice, Prentice- Hall of India Private Limited, 2008 Anil K. Biswas: Principles of Blast Furnace Ironmaking, SBA Publication,1999 R.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.
    3. 3.  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.
    4. 4.  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.
    5. 5.  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 tech-niques are required for decreasing their contents below a certain level in the case of each element.
    6. 6.  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 signifi-cantly 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.
    7. 7. Plain carbon steels are broadly sub-divided into fourmajor types based on their carbon contents. These arenot strict divisions based on carbon contents but aregenerally broad divisions as a basis of classification.This division is definitely useful. These are:(i) Soft or low carbon steels up to 0·15% C(ii) Mild steels in the range 0·15-0·35% C(iii) Medium carbon steels in the range 0·35-0·65% C(iv) High carbon steels in the range 0·65-1·75% C
    8. 8. The alloy steels are broadly sub-divided into three groupson the basis of the total alloying elements present. Thisdivision 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
    9. 9.  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 reserves ◦ Enhanced adoption of alternate routes for iron making for ultimate conversion to steel.
    10. 10.  The B.F. works on a counter current principle Ascending hot gases meet Descending solid charge The 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 slag ◦ Liquid Metal ◦ B.F. Gas of considerable calorific value
    11. 11. All the reduced elements join the metal. A typicalcomposition of the Metal (Iron) produced in BlastFurnace is presented below.
    12. 12.  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 following ◦ Al2O3 – 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)
    13. 13. Smarajit SarkarDepartment of Metallurgical and Materials Engineering NIT Rourkela
    14. 14.  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.
    15. 15.  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…
    16. 16.  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.
    17. 17. o In an integrated steel works the capacity of the Blast Furnace depends upon The 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
    18. 18. 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.
    19. 19.  This is a unique design in whichlarge bell is replaced by a distributorchute with 2 hoppers A rotating chute is provided insidethe furnace top cone Advantages:  Greater charge distribution flexibility  more operational safety and easy control over varying charging particles  Less wearing parts: easy maintenance
    20. 20. 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.
    21. 21. As has been made clear that even the most efficient of themodern blast furnace would produce an effluent gas containing asignificant proportion of CO which could not be used for ironoxide reduction. The actual CO content may vary around 20-30%by volume. This has a calorific value of nearly 900 kcal/m 3. Thequantity of gas produced depends upon the amount of fuel burnt.For one tonne of coke burnt nearly 4000 m 3 of effluent gas maybe produced. Hence a blast furnace requiring 1000 t of coke perday would generate nearly 4 x 106 m3 of gas with a total energycontent of 3600 x 106 kcal which is nearly equivalent to 500 t ofcoke.
    22. 22. The effluent gas from the furnace cannot directly beused as a fuel since a substantial quantity of dust fromthe burden is also discharged along with. It may leadto accumulation of dust and wear in the equipmentusing the gas. The gas is, therefore, cleaned before itsuse and in so doing the sensible heat of the gas isinvariably lost. The chemi-cal heat of the cleaned gasis what is utilised.
    23. 23. 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 0·5-1·5 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.
    24. 24.  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.
    25. 25.  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.
    26. 26. It essentially consists of a tall cylindrical structurecomprising of a combustion chamber and heatregenerator unit of checker bricks. The clean blastfurnace gas is burnt in the combustion chamberand the hot products of combustion later heat upthe checker bricks. In this case the stove is said tobe on on-gas and is maintained on gas until thechecker bricks are heated to a certaintemperature.
    27. 27. Firing is stopped and cold blast is passed throughcheckers which impart the heat stored in them andthere by produce preheated blast. The stove issaid to be on blast. It can continue heating theblast till a certain minimum temperature of theblast is obtainable. The stove is again put on gasand the cycle is repeated.
    28. 28. The stove design and the number of stoves, employedshould ensure a steady supply of preheated blast to thefurnace. This duty demands that the amount of heatgenerated by way of combustion of gas per unit timeshould be adequate to heat up the required amount ofblast to the required temperature per unit time, takinginto account the usual efficiency of heat transfer viachecker system and the usual heat losses from thesystem.
    29. 29. The thermal efficiency of the stove varies between75-90%. The checker work cools more rapidlywhereas it takes longer time to heat it up. In practicea stove may be on gas for 2-4 hours and on blast for1-2 hours. For an uninterrupted steady supply ofblast at specified temperature therefore a battery ofat least three stoves is necessary. A two stovesystem is quite unsatisfactory and hence three orfour stove system is preferred.
    30. 30. The checkerwork has to absorb maximum heat at faster rate whileheating and should desorb heat equally rapidly to the incoming coldblast. The larger the weight of bricks the more will be its heatstoring capacity. The larger is the surface area exposed as fluesthe faster is the heat exchange with gas. The bricks should havemaximum weight with maximum surface area of flues i.e. maximumopenings to allow free passage of gases. It has been found that aratio of weight of bricks in kilogram to heating surface insquare metres of about 5-6 in minimum. Below this struc-turaldifficulties may arise.
    31. 31. The checker bricks are supported on steel grids which in turnare supported by cast iron or steel columns. Since themaximum temperature during combustion is generated nearthe dome and since the top portion of checker bricks have tostand higher temperatures, with progressively decreasingvalue downwards, the quality of checker bricks used also veryaccordingly. Heavy duty fire-bricks are essential for domeconstruction. The top 3-6 m height of the checkers is made upof higher alumina bricks or semi-silica bricks while theremainder as of good quality firebricks.
    32. 32. 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
    33. 33. 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.
    34. 34.  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… …
    35. 35. The strength of the coke charged to thefurnace should be sufficient to withstand theload of raw materials without gettingcrushed. Coke provides permeability(in thedry as well as wet zones )and alsomechanical support to the large chargecolumn, permitting the gases to ascendthrough the voids.Total height (H)= 5.55V0.24Useful height (H0) =0.88×H
    36. 36. Diameter:The belly /bosh parallel is the cylinder thatconnects the tapers of the shaft and the bosh.Its diameter, dbll, and the ratio of this diameter tothe useful or inner height of the furnace as wellas to the diameter of the hearth play animportant role in the operation of the furnace.The correct descent of the stock, ascent of thegas and efficient utilization of the chemical andthermal energies of the gas depend greatly uponthese ratios.
    37. 37. The importance of an adequate belly diameter lies in thefact that softening and melting of the gangue andformation of the slag occurs in this region. An increase in the diameter facilitates gas passagethrough the sticky mass and also slows down stockmovement, thus increasing the residence time for indirectreduction.However, the belly diameter cannot be increasedarbitrarily as it is directly related to bosh angle, boshheight, hearth and throat diameters and useful height.
    38. 38. 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)0.38HbelIy = 0.07×H
    39. 39. The hearth is designed such that its volumebetween the iron notch and tuyeres is sufficientto 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
    40. 40. A very approximate relationship betweenthe coke burning rate and hearth diameteris given by the following equation: D = c Q 0.5 D = hearth diameter, m Q = coke throughput, tonnes/24h c = throughput coefficient which variesbetween 0.2-0.3 depending upon burdenpreparation.
    41. 41. For highly prepared burden, the value ofc = 0.2 has been achieved in modern largefurnaces .There-fore, for a furnace planned toproduce 10,000 THM per day with a cokerate of 500 kg/THM, i.e., a coke throughputof 5,000 tonnes per day, the hearthdiameter should be about 14.1 m.The value will be 21.2 m if the value ofc=0.3.
    42. 42. With increasing diameter of the hearth,the gas penetration must be ensuredby providing adequate bedpermeability with the use ofmechanically strong, rich, pre-fluxedburden of uniform size and low slagbulk as well as strong lumpy coke.The Hearth height should be 10% of thetotal height of the furnace
    43. 43. The shaft height must be sufficient to allow theheating, preparation and reduction of ore beforethe burden reaches the bosh. In the upperregions of the shaft , volume changes due toincrease in temperature and carbon deposition.These demand an outward batter for smoothflow of materials. In the lower region of theshaft , the material starts fusing and tends tostick to the furnace wall. So to counteract thewall drag an outward butter is necessary.
    44. 44. Stack height Hstack = 0.63 H- 3.2 m Stack angleThe 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.
    45. 45. 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.
    46. 46. Stack angle can be calculated from the formulaStack angle (α)= Cot-1(D-d1/2xStack Height)Where, D= Bosh parallel Diameterd1= Throat DiameterBosh angle can be calculated from the formulaBosh angle (β)= Cot-1(D-d/2xBosh Height)Where, D= Bosh parallel Diameterd= Hearth Diameter
    47. 47.  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 parallel Throat 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..
    48. 48.  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 V0.35 Where, V= useful volume
    49. 49. 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.
    50. 50.  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.
    51. 51. 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 tuyeres Pavlov: n = 2d +1 Rice: n = 2.6d-0.3 Tikhomirov et al : n = 3d-8Where n= Number of tuyeres, d=hearth diameter
    52. 52. Capacity → 2000 3000 5000 (THM/Day)Parameter↓Useful Volume (m3) 1700 2550 4250Total Height (m) 33.08 36.46 41.22Useful Height (m) 29.11 32.08 36.27Bosh Parallel Dia (m) 9.96 11.62 14.11Bosh Parallel Height (m) 2.32 2.55 2.89Bosh Height (m) 4.37 4.81 5.44Hearth Dia (m) 9.1 10.92 13.74Hearth Area (m2) 65.04 93.66 148.27Hearth Height (m) 3.308 3.646 4.122Stack/Shaft Height (m) 17.64 19.77 22.77Throat Dia (m) 6.87 7.85 9.29Bosh Angle (0) 84.32 85.84 88.05Stack Angle (0) 85 84.55 83.96Nos. of Tuyeres 20 25 34
    53. 53. Richness: Richness means the percentage ofmetallic iron in the ore. e.g. In order to produce atonne of pig iron about1.5tonnes of ore is requiredin Australia (68% Fe), about 2 tonnes are requiredin India (55-60%) and nearly 3 tonnes are requiredin U.K. (30-35%)Composition of the gangue : Thecomposition of gangue associated with an oremay reduce the value of an otherwise rich ore orin some case may even enhance that of a leanore.
    54. 54. 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 important Treatmentand preparation needed before smelting
    55. 55.  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.
    56. 56. 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, trans-portation, 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.
    57. 57.  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.
    58. 58.  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.
    59. 59.  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.
    60. 60.  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.
    61. 61. • 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.
    62. 62. Porosity: While ores and pellets possess mostly open pores, insinters there are macro- and micro-pores as well as open andclosed pores (cut off from outside and cannot be reached bygas).True porosity and hence closed porosity can be determined fromopen porosity which can be measured from the true and bulkdensities.Although reducibility increases with increasing open porosity, thelatter changes continuously during reduction on load. Generally,a high initial porosity results in earlier softening of the material.
    63. 63. Decrepitation : When iron bearing materials are suddenly exposed to the ex-haust 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 400-600°C, 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.
    64. 64. • 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 400°C 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 0·5 mm and -5·6 + 0·5 mm material in the product is determined by sieving.• It is believed that ores with more than 10% porosity will not decrepitate.
    65. 65. • 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 400°C 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 0·5 mm and -5·6 + 0·5 mm material in the product is determined by sieving.• It is believed that ores with more than 10% porosity will not decrepitate.
    66. 66. 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 dura- tion. The percentage of fines generated is quoted as the L. T.B. T. index.
    67. 67.  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.
    68. 68. Reducibility of ferrous materials is characterized by theirfractional oxygen removal rates in gaseous reducingatmosphere. The percent degree of reduction orpercent fractional oxygen removal is given byWheren0 = number of moles of oxygen originally combined with iron only;n = number of moles of oxygen left combined with iron after experi-mental time, t.
    69. 69. A schematic representation of relationship between reduction at40% degree of reduction and 60% degree of oxidation levels,
    70. 70.  particle size porosity crystal structure pore size volume change impurities
    71. 71. 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 500 0C is sluggish. Hematite is more reducible than magnetite although the amount of oxy-gen 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;
    72. 72.  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.
    73. 73. Chemical InfluenceIt is well known that the reduction rate of wustite is critical in theoverall kinetics of iron oxide reduction.The equilibrium partial pressure or concentration of CO2 woulddecrease if aFeO is lowered by solution and/or compoundformation. Hence, the reduction rate would also decrease.
    74. 74. 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 lowactivity. The activity of wustite can also decrease when it undergoes sintering withthe impurities present, such as SiO2, Al2O3 etc.
    75. 75. 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.
    76. 76. For the reduction of iron ores the reducing gas has to diffuseinto the interior of the body where transformations can occur.In general, the reduction rate increases with temperature butthe degree depends upon the mechanism of the reaction .The overall reduction rate depends upon the relativecontributions of chemical control and gaseous masstransport and hence depends upon the particular reactionsoccurring and the reaction temperature. Since chemicalreaction has higher activation energy than gaseous diffusion,the former will increase at a much· greater rate with increasein temperature than the latter.
    77. 77. Hence, a stage will arrive where diffusion will become rate-controlling. Depending upon the degree of reduction, at lower temperatures of about 500-600°C, the chemical reaction rate controls the reduction rate forming what is known as the kinetic region in the blast furnace. At temperatures above 600°C, 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.
    78. 78. In the blast furnace , the reducing gas ispredominantly CO with varying amounts ofhydrogen depending upon the moisture content ofthe blast and other blast additives like fuel oil ornatural gas. Study shows that a mixture of CO andhydrogen appears to be a more efficient reductantthan either of them.
    79. 79. 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.COn complete combustion to CQ2 the heat evolved is 8150 kcallkg.C. Thusonly 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 ifpresent;(iv) it carburizes the iron and lowers its melting point;(v) it provides permeability (in the dry as well as the wet zones) and alsomechanical support to the large charge column, permitting the gases toascend through the voids.
    80. 80.  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.
    81. 81. Coke size: Coke comprises about 50-60 percent of the volume of thecharge material. The coke size is important as it providespermeability in the dry as well as in the wet bosh zone The coke sizeis always 3-4 times larger than the ore size, since coke is partiallyburnt as it descends. It also has a lower density, and hence a greatertendency for fluidisation. Of course, in the lower bosh region of ablast furnace, coke is the only solid that remains, and which helps tosupport the burden. The optimum size range for lump ore is 10-30mm and for coke is 40-80 mm. Since the coke size becomessmaller as it descends through the blast furnace due to mechanicalbreakdown, gasification, attrition, etc., the factor of prime importanceis the strength of coke.
    82. 82. Coke strength: Mechanically considered, it is the quality cohesion thatprevents the coke from collapsing and tends to avoid the formation ofsmall particles. High cohesion or strength is related to several cokemaking properties. On the basis of breakage by impact, compressionor abrasion, the coke strength should be assessed both at ambient aswell as high temperatures. Studies of the structure of different cokesamples show that the best varieties have a regular distribution of pores:with adequate thickness and hardness of the walls between the poresand are free from cracks generated internally. Such a structure ensureswithstanding of high compressive forces and high temperatures in theall-important lower furnace.
    83. 83. 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.
    84. 84.  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.
    85. 85.  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 ft 3/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.
    86. 86. 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 1100°C. CO 2 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%.
    87. 87. Coke Strength after Reaction (CSR). The left out cokefrom the CRI determination test is rotated for 60 rotationin a micum drum. And the % of coke retained on a 10mm size screen is reported as coke strength afterreaction (CSR). Stronger the coke, more is its CSRvalue. Ideal value of CSR for blast furnace coke is aminimum of about 55%. Typically CSR of Indian blastfurnace coke is about 60-65.
    88. 88.  Agglomeration of Iron Ore Fines About 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.
    89. 89.  To increase the size of ore fines to a level acceptable to the BF To form a strong and porous agglomerate To remove volatiles like CO2 from carbonates, S from sulphide ores etc To incorporate flux in the sinter To increase the BF output and decrease the coke rate
    90. 90. 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.
    91. 91. In the combustion zone, bonding takes placebetween the grains and a strong and porousaggregate is formed. The process is over whenthe combustion zone reaches the lowest layer ofthe bed. The screened under size sinter isrecycled and over size is sent to B.F.
    92. 92. 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
    93. 93. The proportion of each of the physical and chemical change duringsintering depends upon the time-temperature cycle of the process.The higher is the temperature more will be the proportion of newconstituents by way of solutions and interactions whereas lower isthe temperature and longer is the duration more is the process ofrecrystallization in solid state.The more is the slag bonding, stronger is the sinter but with lessreducibility and, more is the diffusion bonding, more is thereducibility but less is the strength. Since ores are fairly impureslag bond predominates. On the other hand in rich sinters slagbond is of minor importance.
    94. 94. The area under the time-temperature curvesessentially determines the nature andstrength of the bonds developed duringsintering of a given mix. For a given mix it ismost unlikely the bonds of sufficientstrength will be formed below a certaintemperature level within a reasonably shorttime. Hence the area under the curve above acertain temperature, which may be around1000°C for iron ores, is the effective factor indeciding the extent of sintering
    95. 95. rather than the whole area under the curve fromroom temperature to the combustion temperaturelevel. The nature of the time-temperature graph willdepend upon the rate of heating and cooling of agiven mix. The nature of this graph is of paramountimportance in assessing the sintering response. Thefactors that affect this curve are then the variables ofthe process and which should be adjusted properlyfor obtaining effective sintering.
    96. 96.  Bed permeability Total volume of air blast drawn through the bed Particle size of iron ore Thickness of the bed Rate of blast drawn through the bed Amount and quality of solid fuel incorporated in the sinter mixture Chemical composition of ore fines Moisture content in the charge
    97. 97. 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.
    98. 98.  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.
    99. 99. During mining and ore dressing operations, especiallywhere very fine grinding is necessary for wetconcentration, a large amount of - 0.05 mm fines isgenerated which are not amenable to sinteringbecause of very low permeability of the bed. They can,however, be agglomerated by balling them up in thepresence of moisture and suitable additives like bentonite,lime, etc. into 8-20 mm or larger size. These green pelletsare subsequently hardened for handling and transport byfiring or indurating at temperatures of 1200-1350°C.
    100. 100. Pelletisation essentially consists of formation of greenballs by rolling a fine iron bearing material with a criticalamount of water and to which an external binder or anyother additive may be added if required. These greenballs of nearly 8-20 mm size are then dried, preheatedand fired, all under oxidising conditions, to a temperatureof around 1250-1350°C. Bonds of good strength aredeveloped between the particles at such hightemperatures.
    101. 101. 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.
    102. 102. 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.
    103. 103. The capillary action of water in the interstices of the grains causes acontracting effect on them. The pressure of water in the pores of theball is sufficiently high so as to compact the constituent grains into adense mass. The compressive force is directly proportional tofineness of the grains since the capillary action rises with thedecrease in pore radius and the latter decreases with increasingfineness. An optimum moisture is important since too little ofwater introduces air inclusions in the pores and too much of waterwould cause flooding and destruction of capillary action. Theoptimum moisture content usually lies between 5-10 percent ormore, the finer the grains the larger the requirement.
    104. 104.  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.
    105. 105. 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.
    106. 106. 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. :o Nucleii formation regiono Transition regiono Ball growth region.
    107. 107. When a wet particle comes in contact with another wetor dry particle a bond is immediately formed between thetwo. Similarly several such particles initially join duringrolling to form a highly porous loosely held aggregateand crumbs which undergo re-arrangement and partialpacking in short duration to form small spherical, stablenucleii. This is the nucleation period, a pre-requisite forball formation since these very nucleii later grow intoballs.
    108. 108. After nucleii are formed they pass through a transition periodin which the plastic nucleii further re-arrange and getcompacted to eliminate the air voids present in them. Thesystem moves from a pendular state through funicular stateto the capillary state of bonding. Rolling action causes thegranules to densify further. The granules are still plastic with awater film on the surface and capable of coalescing with othergranules. The size range of granules in this region is fairlywide.
    109. 109. 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.
    110. 110. Growth by AssimilationIf no fresh feed material is added for balling the rolling action may breaksome of the granules, particularly the small ones, and the materialcoalesces with those which grow. The bigger the ball the larger it will growunder these conditions. Since smaller granules are weaker they are the firstvictim and growth of the bigger balls takes place at their expense.Growth by LayeringGrowth of the seeds is said to be taking place by layering when the ballspick up material while rolling on a layer of fresh feed, The amount ofmaterial picked up by the balls is directly proportional to its exposedsurface, i.e. the increase in the size of the balls is independent of theiractual size.Growth by layering is more predominant in the disc pelletisers andgrowth by assimilation is more predominant in drum pelletisers, atleast beyond the feed zone.
    111. 111. In general natural lumpy ore or sinter or pellets or a suitable com-bination oftwo or more of these form the burden.. The modern large capacity furnacesnecessarily need fully prepared burden to maintain their productivity sincethe required blast furnace properties cannot just be met by natural lumpyore. The selection of the process of agglomeration, whether sintering orpelletising, will depend upon the type of ore fines available, the location ofthe 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 requiresultrafines of over 75% of -325 mesh. These processes are there-fore notcompetitive.
    112. 112.  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.
    113. 113.  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 handling It 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.
    114. 114. o The installation cost of a pelletising plant will be 30-40% more than that of sintering plant of an equal size.o The operating cost of sintering is slightly less than that of pelletising.o Difficulty of producing fluxed pellets.o Swelling and loss of strength inside the furnaceo Fluxed pellets break down under reducing conditions much more than acid and basic sinters and acid pellets.o Strong highly fluxed sinters, especially containing MgO, are being increasingly preferred to pellets.
    115. 115. Smarajit SarkarDepartment of Metallurgical and Materials Engineering NIT Rourkela
    116. 116.  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.
    117. 117.  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 10–12 kg/thm.
    118. 118.  Design of the blast furnace  Angle and size of the big bell. and its charging device  Additional mechanical (effect of these factors is device(s) used for obtaining constant). better distribution.  Speed of lowering of large bell.
    119. 119.  Inconsistency in physical properties of  Size range of the various charge materials charge materials (deficiencies caused by this should be  Angle of repose of raw eliminated by improving quality of the bur-den. materials and other physical characteristics of the charge.  Density of charge materials.
    120. 120.  Level, system and  Distributionof charge sequence of on the big bell charging, programme  Height of the big bell of revolving the from the stock-line i.e. distributor (conditions charge level in the determining major furnace throat. means of blast  Order and proportion furnace process of charging of various control from top). raw materials.
    121. 121.  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 1·5 glcc and the limestone is intermediate with-a value of density around 3·0-3·5 glcc. It means that the rolling tendency of coke particles is maxi- mum 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.
    122. 122.  When a multi-particle material is allowed to gently fall on a hori-zontal 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.
    123. 123.  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.
    124. 124.  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°.
    125. 125.  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.
    126. 126.  On dumping, as the materials fall on the stock surface, they take a para-bolic 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)
    127. 127.  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.
    128. 128.  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).
    129. 129.  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 1185°C.. 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)
    130. 130.  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 mans 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.
    131. 131. 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
    132. 132.  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 cal The 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.
    133. 133.  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-Si0 2-Al203 system is more pronounced, .
    134. 134.  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-AI203 The 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.
    135. 135.  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.
    136. 136.  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.
    137. 137.  Below 600°C : Pre-heating and pre-reduction 600 -950°C: 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.
    138. 138.  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
    139. 139.  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.
    140. 140.  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.
    141. 141.  The charge in the blast furnace descends under gravity against the fric-tional 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).
    142. 142.  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.
    143. 143. 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. 
    144. 144. Gas flow through Granular zone: For resistance to gas flow, more important thanthe particle diameter is the relative size of thematerials in the bed. In a mixed bed of widely varying particle size, thesmall particles land in the interstices of the largeones and decrease the void age . Starting with large uniform spheres, the void agedecreases as the small ones are introduced andthe bed becomes more and more compact as theproportion of the latter increases. The bed is most dense, i.e., the voidage isminimum when 60-70 percent of the total volume ofthe particles consists of the large ones for about allthe cases.
    145. 145. The € m increases on either side of theminimum, i.e., with increasing or decreasingvolume fraction of the small particles(approaching more uniformity of the sizedistribution). The voidage decreases greatly as theratio d s / d 1 decreases. This shows that for a good and uniformpermeability and low resistance to gas flowin a mixed bed, the size fractions should beas narrow as possible . One can easily visualize the adverseeffects of multi-granular bed of particles ofvarying diameter on the voidage.
    146. 146. A narrow size distribution has the following advantages: charge permeability increases and the gas distribution ismore uniform with better utilization of the chemical andthermal energies of the gases;more even material distribution at the stock level and lessmaterial segregation in the shaft during descent;gas flow is not impeded if the size ratio is within limits butat the same time gives rise to a tortuous flow of gases withcontinuous chang-ing of flow directions, providing a largergas/solid contact time.
    147. 147. The fraction of iron bearing material below the limiting sizeis therefore termed as fines by the blast furnace technologistsand is invariably eliminated by screening at every possiblestage. From the point of view of reduction the maximum top size ofan iron bearing material should be as low as possible, since therate of reduction de-creases, perhaps exponentially, withincreasing size.  The size range of materials charged in the blast furnacerepresents a compromise to give both good stack permeabilityand adequate bulk reducibility.
    148. 148. 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.
    149. 149. Lump ores, sinter and pellets disintegrate into smaller pieces during theirdownward travel through the blast furnace owing to the weight of theoverlying burden, as well as abrasion and impact between the burdenmaterials.It has been found that this tendency gets aggravated when the oxides are ina reduced state. Reduction of hematite into magnetite occurs in the upperstack at 500-600°C, and this is accompanied by volume expansion even tothe extent of 25%.This results in compressive stresses being developed and contributessignificantly to breakdown of the iron oxides.Blast furnace operators prefer a low RDI (below 28 or so) since the adverseeffect of high RDI has been clearly demonstrated in practice.
    150. 150.  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.
    151. 151.  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 .
    152. 152.  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.
    153. 153.  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.
    154. 154. Smarajit SarkarDepartment of Metallurgical and Materials Engineering NIT Rourkela
    155. 155.  Decreasing the extent of SiO formation by:o Lowering ash in coke, and the coke rateo Lowering RAFTo 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:o Increasing the bosh slag basicity.o Lowering the bosh slag viscosity..
    156. 156.  Removal of Si from metal by slag-metal reaction at the hearth by:o Lowering the hearth temperatureo Producing a slag of optimum basicity and fluidity.
    157. 157.  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] + ½[ Si] = (CaS) + 1/2 (SiO2)
    158. 158.  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 as SiS(g) = [Si] + [S]
    159. 159.  Reducing slag i.e. FeO content should be low High basicity High temperature, since desulphurisation is an endothermic reaction Kinetic factor • Contact surface of metal and slag (↑ by agitation) • Fluidity of slag(↑ by adding MgO , MnO) Time of desulphurisation
    160. 160. 3200m3 0.8-0.9t + 0.5-0.6t 80kg dust 1.7-1.8t •Fuel •Reducing agent supply •Permeable bed (spacer)2500 m3 0.6t 1t
    161. 161. The efficiency of operation of a blast furnace may bemeasured in terms of coke rate which should of coursebe as low as possible. The achievement of a satisfactorycoke rate depends on optimising the extent to which thecarbon deposition reaction proceeds. If the top gas ishigh in C02 sensible heat is carried from the furnace as aresult of the exothermic reaction.2CO=CO2+CIf on the other hand the top gas is high in CO, chemicalheat leaves the furnace.
    162. 162. Industry Contribution % Power 51 Transport 16 CO2 emission Steel 10 other 23
    163. 163. 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 emission Advantages: ◦ For the same volume flow rate, a greater mass of air (hence, oxygen) can be blown with HTP; higher output;
    164. 164.  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;
    165. 165.  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
    166. 166. SiO2 +C ={SiO} +{CO}From above equation it can be seen that partialpressure of SiO can be brought down by increasingthe partial pressure of CO; in other words the SiO2reduction reaction can be discouraged by applicationof top pressure which enables a higher blast pressureand hence an increase in partial pressure of CO.
    167. 167. The blast volume and therefore the cokethroughput can be increased by 30percent with the maintenance of identicalpressure drop and gas velocity conditionsin the blast furnace by increasing the toppressure to 2.1 from 1.1 ata and bottompressure to 3.5 from 2.5 ata under thegiven blowing conditions.
    168. 168. raceway adiabatic flame temperature‘This is the highest temperature available inside thefurnace. There is temperature gradient in verticaldirection on either side of this zone. This temperatureis critically related to the hearth temperature known asoperating temperature of the furnace. It is equallyrelated to the top gas temperature such that the hotraceway gasses have to impart their heat to thedescending burden to the extent expected and leavethe furnace as off-gases at the desired temperature.
    169. 169.  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:

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