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Chemistry of the Ironmaking by Blast Furnace Process

• satyendra
• November 22, 2013
• 1 Comment
• blast furnace, Boudouard reaction, CO, CO/CO2 ratio, CO2, Ellingham diagram, Reaction, reduction, slag, Solution loss reaction,

Chemistry of the Ironmaking by Blast Furnace Process

The modern blast furnace (BF) operating with a low coke rate is an efficient processing unit primarily because of the intrinsic characteristics of a counter-current gas-solids reactor. A successful use of this concept needs that each of the materials charged to the furnace be of uniform physical character, and have a uniform composition. In addition, each material is to retain this good physical character as it moves down through the furnace to where melting occurs.

As iron oxide, coke, and slag forming materials move down through the stack of the furnace, several important exchange processes take place. Heat is removed from the ascending furnace gases which mainly consist of carbon monoxide (CO), carbon dioxide (CO2), and nitrogen (N2) and transferred to the descending burden materials. Oxygen (O2) is removed from the descending iron oxides and transferred to the ascending reducing gases. Hence within this very efficient counter-current reactor, chemical reactions take place and as the charge descends the temperature of the burden materials increases, fusion of the reduced iron, iron oxide and slag-forming materials begins, and finally liquid metal and slag collect in the hearth of the furnace. Majority of the coke charged to the furnace is burned with oxygen in the hot air blast at the tuyeres to provide both heat and the reducing agent CO.

When the burden materials and coke which are charged into the top of the BF descend through the stack, they are preheated by the hot gases ascending from the tuyeres. As a result of this preheat, the coke burns with great intensity when it reaches the lower portion of the furnace adjacent to the tuyeres and comes in contact with the hot-blast air. However, because of the very high temperature (around 1,650 deg C) and the large quantity of carbon (C) present as coke, the CO2 formed is not stable and immediately reacts with additional carbon to form CO. As a result, the combustion of carbon (coke) in the BF can be expressed by the chemical equation 2C + O2 = 2CO; delta H = +110,458 kJ/kmol. In the operation of modern BF, between 250 kilograms (kgs) and 400 kgs of carbon react in this manner for every ton of hot metal produced. This reaction is the main source of heat for the smelting operation and also produces a reducing gas (CO) which ascends into the furnace stack where it preheats and reduces majority of the iron oxide in the burden as it descends to the hearth.

Any moisture (H2O) in the blast air also reacts with some of the carbon in the coke in the combustion zone. This reaction does not produce heat as combustion does but, rather, consumes heat. However, for every unit of carbon, this reaction produces more reducing gas than that produced when carbon is burned in air. When carbon burns in air, it produces only one unit of CO, but when it reacts with H2O, it produces one unit of CO and one unit of hydrogen (H2). Hence, in certain instances, where the inherent reduction rate of the burden materials is lower than normal and where a relatively high hot blast temperature is available (between 1,000 deg C and 1,200 deg C), it has been thought to be advantageous to keep the moisture content of the blast at a uniformly high level by moisture (steam) additions to increase the reducing power of the BF gas. Auxiliary fuel injection provides a similar advantage. This chemical reaction is expressed by the equation C + H2O = CO + H2; delta H = +131,378 kJ/kmol. An additional benefit is derived from the introduction (or increase) of hydrogen in the furnace reducing gases. As the percent hydrogen increases, the density of the gas decreases. This results in an equivalent volume of reducing gas providing less resistance to burden decent.

The ascending gases start to reduce the iron oxide of the burden in the upper portion of the BF where the temperature is below 925 deg C. At this temperature, chemical equilibrium prevents all of the CO and H2 from being used for reduction (the equilibrium CO / CO2 ratio is around 2.3 for the reduction of wustite, if the ratio falls below this value iron is reoxidized. Hence, the molecular ratio of CO or H2 to iron oxide is to be around three times the amount shown by the stoichiometric reactions (i) 1/2 Fe2O3 + 3/2 CO = Fe + 3/2 CO2; delta H = +12,866 kJ/kmol, (ii) 1/3 Fe3O4 + 4/3 CO = Fe + 4/3 CO2; delta H = +3940 kJ/kmol, (iii) FeO + CO = Fe + CO2; delta H = –16,108 kJ/kmol, (iv) 1/2 Fe2O3 + 3/2 H2 = Fe + 3/2 H2O; delta H = +48,953 kJ/kmol, (v) 1/3 Fe3O4 + 4/3 H2 = Fe + 4/3 H2O; delta H = +51,042 kJ/kmol, and (vi) FeO + H2 = Fe + H2O; delta H = +25,104 kJ/kmol.

In the past, this type of reduction was called indirect reduction in contrast to the type occurring at higher temperatures which was called direct reduction. However, this nomenclature has become confusing because these same chemical reactions are called direct reduction in describing the DRI processes like Wiberg, the HIB, the FIOR and similar processes. For this reason, these terms are not normally used as they were in the past.

The portion of iron oxide which is not reduced in the upper part of the furnace where the temperature is relatively low is to be reduced in the lower part where the temperature is very high. Since CO2 and H2O are not stable at these temperatures in the presence of large quantities of coke, they react with carbon almost as rapidly as they form. As a result, the overall reduction reaction in this part of the furnace can be represented by reaction FeO + C = Fe + CO; delta H = +156,482 kJ/kmol no matter whether H2 or CO is the reactant. This reaction is obtained by algebraically adding either reactions FeO + CO = Fe + CO2; delta H = –16,108 kJ/kmol, and CO2 + C = 2CO; delta H = +172,590 kJ/kmol or reactions FeO + H2 = Fe + H2O; delta H = +25,104 kJ/kmol, and H2O + C = CO + H2; delta H = +131,378 kJ/kmol.

The reduction reaction FeO + C = Fe + CO absorbs a large quantity of heat, and hence, the larger the amount of reduction occurring in this way, the larger the quantity of heat is to be supplied to the furnace. This reaction also produces CO, which is the gas used in the reduction reactions taking place in the BF stack. In the majority of the cases, the most efficient operation is achieved when roughly one-third of the reduction is done according to reaction FeO + C = Fe + CO and the balance according to reactions Fe2O3 + 3 CO = 2 Fe + 3 CO2 through FeO + H2 = Fe + H2O.

The heat for the process is not produced entirely by the combustion of coke, since at the majority of the BFs roughly 40 % is supplied from the sensible heat of the hot blast air. A considerable portion of the fuel can be economically injected through the tuyeres as natural gas, tar, fuel oil, or coal in either pulverized or granular form. In such cases, the carbon in the fuel burns to CO, but because of the large amount of coke present, the hydrogen remains as H2 and is not oxidized until it reduces iron oxide somewhere above the tuyeres.

The iron-bearing components in the charge to the furnace are the simple oxides of iron, Fe2O3, and Fe3O4. The natural ore normally are hematite (Fe2O3) or magnetite (Fe3O4). Pellets are principally Fe2O3. Iron ore sinter can range in composition from Fe2O3 and Fe3O4 to fused mixtures containing magnetite, fayalite, 2FeO.SiO2, and di-calcium ferrite. The reduction of iron oxides normally takes place in steps. The reactions with CO are given by equations (i) 3Fe2O3 (s) + CO (g) = 2Fe3O4 (s) + CO2 (g); delta H -48 kJ, (ii) Fe3O4 (s) + CO (g) = 3FeO (s) + CO2 (g); delta H -21.7 kJ, and (iii) FeO (s) + CO (g) = Fe (s) + CO2 (g); delta H -11 kJ. These reactions are accomplished at successively higher temperatures, and farther down the furnace.

Successively higher percentages of CO are needed to complete these reactions by the rising gases. It is to be recognized that it is not possible for the entire CO in the gases to be converted to CO2 for each reaction. For example, there is an equilibrium ratio as given by the constant K3 for the three equations and K3 = P CO2 / P CO which is temperature dependent. At 800 deg C, the equilibrium gas mixture contains around 65 % CO and 35 % CO2. If the CO2 content exceeds this value in the gases in contact with FeO and solid iron at this temperature, iron present tends to be oxidized back to FeO. Accordingly, to force these reactions to occur, there is required to be a considerable concentration of CO in the gases at each step as indicated in Fig 1, and it is not possible to convert CO completely to CO2 by the reactions. Fig 1 shows the stability diagram for Fe-C-O and Fe-H-O systems. The S curve in Fe-C-O system represents ‘solution loss’ or Boudouard or reaction.

Fig 1 Fe-C-O and Fe-H-O systems

Because of hydrogen in the auxiliary fuels and moisture from the fuels and the air blast, the gases leaving the tuyeres can also contain upto 2 % or 3 % hydrogen. Steam can be added to the hot air blast as an aid in controlling the furnace. The reduction of steam by carbon in the coke and fuels proceeds by the overall reaction H2O (g) + C (s) = CO (g) + H2 (g); delta H = 131.3 kJ. This reaction is endothermic whereas the oxidation of carbon by oxygen in the blast to form CO by the equation C (s) + 1/2 O (g) = CO (g); delta H = -110.5 kJ is exothermic. The reduction of iron oxides by hydrogen also proceeds by steps (i) 3Fe2O3 (s) + H2 (g) = 2Fe3O4 (s) + H2O (g); delta H = -7.1 kJ, (ii) Fe3O4 (s) + H2 (g) = 3FeO (s) + H2O (g); delta H 62.9 kJ, and (iii) FeO(s) + H2 (g) = Fe (s) + H2O (g); delta H =30.2kJ.The temperature affects the equilibria of these reactions.

The water gas shift reaction CO2 (g) + H2 (g) = H2O (g) + CO (g); delta H = 41.2 kJ can take place among the various species in the gas phase to redistribute the oxygen and bring the hydrogen bearing and carbon bearing gas species into equilibrium. This reaction needs very little heat and the equilibrium constant (P H2O.P CO) / (P H2.P CO2) is unity at 825 deg C. The gases in the stack react with the carbon of the coke as well as with the oxides of iron in the charge. The overall reaction of CO and CO2 with carbon as graphite is the ‘solution loss’ or Boudouard reaction CO2 (g) + C (s) = 2CO (g); delta H 172.4 kJ. The equilibrium of the reaction is shifted strongly to the right at temperatures above 750 deg C. Below 600 deg C the equilibrium is strongly to the left, resulting in the deposition of carbon as soot in the furnace burden 2CO (g) = C (s) + CO2 (g); delta H = -172.4. The ‘S’ shaped curve leading from the lower left to the top centre of Fig 1 represents the equilibrium. A gas whose temperature and composition place it above the line tend to deposit carbon by the second reaction, and one whose composition and temperature place it below the line oxidizes carbon in accordance with the first reaction.

The principal effects of the carbon solution reaction at high temperatures are a relative reduction of heat generated at the tuyeres where it is needed and an increase in the concentration of CO in the gases at regions of the furnace above 700 deg C. This latter condition is particularly desirable as it increases the volume of the gases and aids in heat transfer. It is to be noted that the combination of the solution loss reaction and the reaction FeO (s) + CO (g) = Fe (s) + CO2 (g); delta H -11 kJ corresponds to the ‘direct’ reduction of FeO by carbon given by equation FeO (s) + C (s) = Fe (s) + CO (g); delta H = 131.3 kJ. It is be evident from Fig 1 that the gases passing up the stack cannot normally be in equilibrium with carbon in the coke and the iron oxides in the descending burden. The actual relationship between gas composition and temperature in the BF stack depends to a great extent on the actual practice employed.

Relative stability of oxides

The relative stability of various oxides is plotted against temperature in Ellingham diagram (Fig 2). Ellingham diagram is very useful for understanding the behaviour of oxides in the BF. The relative stability is measured in terms of the free energy of formation of the oxides. The higher the negative free energy of formation of the oxide, the higher is the oxide stability. This means that oxides which are located in the upper part of the diagram have a relatively low stability, while oxides located in the lower portion of the diagram have a high stability. Oxides located in the centre of the diagram have a moderate stability. Oxides with a relatively low stability include potassium oxide, sodium oxide, phosphorus oxide and iron oxide. Oxides with a moderate stability include manganese oxide, chromium oxide, silica, and titanium oxide. . Oxides with a high stability include alumina, magnesia, and lime.

Fig 2 Ellingham diagram

It is also useful to consider this diagram in terms of the affinity of an element for oxygen. For example, elements which are located at the top of the diagram have a low affinity for oxygen, while elements located towards the bottom of the diagram have a high affinity for oxygen. This means that oxides at the top are relatively easy to reduce, while those at the bottom are difficult to reduce. This is shown by the line for the formation of phosphorus oxide which lies above the line for formation of iron oxide at temperatures corresponding to those found in the BF hearth. This implies that phosphorus oxides has a lower stability than iron oxide and hence, since reducing conditions in the furnace are sufficient to reduce iron oxide, essentially all of the phosphorus entering the furnace end up in the hot metal. On the other hand, stable oxides such as alumina, magnesia, and lime are not reduced under BF conditions, and end up in the slag phase. Oxides with a moderate stability such as manganese oxide, chromium oxide, silica, and titanium oxide are partially reduced to give some manganese, chromium, silicon and titanium dissolved in the hot metal, while the remaining unreduced oxide constitutes part of the slag.

The Ellingham diagram is constructed on the basis that a pure element at unit activity reacts with one of mole of oxygen gas to form pure oxide at unit activity. The thermodynamic term ‘activity’ is a particularly useful concept for discussing the behaviour of elements dissolved in liquid iron, or oxides dissolved in liquid slag. For example, when small concentrations of elements such as oxygen or sulphur are dissolved in liquid steel, their activity can frequently be taken as equal to their concentration in percent. However, in the presence of high concentrations of other elements, for example, carbon in hot metal, the activity of sulphur is higher than the concentration, while the activity of oxygen is less than the concentration. In such cases, it is important to distinguish between activity and concentration. · The concentration of a component in solution is a measure of how much of the component is present. · The activity of a component in solution is a measure of how the component actually behaves.

All the lines on the Ellingham diagram except those involving carbon, have a positive slope, indicating that the oxide stability decreases with increasing temperature. The lines for the oxides of potassium oxide, sodium oxide, magnesia, and lime, each show a show increase in slope at the temperatures corresponding to the boiling points of the respective metals. The line for the formation of CO2 from carbon and oxygen has almost zero slope indicating little change in stability with increasing temperature, while that for CO has a strong negative slope which means that the stability of CO actually increases as the temperature increases. The lines for the two oxides of carbon cross at around 700 deg C. Above this temperature, CO is more stable than CO2 while at lower temperatures, CO2 is more stable than CO.

Carbon-oxygen reactions

The pre-heated air blast injected through the tuyeres at a temperature of around 1,000 deg C to 1,200 deg C and 0.2 MPa to 0.3 MPa pressure, produces a pear shaped reaction zone in front of each tuyere. The temperature in this region is around 2,000 deg C and rapid reaction first occurs between excess oxygen and coke to give CO2. This is an exothermic reaction (C + O2 = CO2). Immediately outside this zone, there is no longer free oxygen available and the CO2 reacts with excess coke to give CO (CO2 + C = 2CO). This is known as the Boudouard reaction and is endothermic. Combining these two reactions gives the reaction for partial combustion of carbon with oxygen to provide CO. (2C + O2 = 2CO). The heat evolved in the formation of one mole of CO2 is around three and one half times that for the formation of one mole of CO and one measure of the efficiency of the BF is the degree of conversion of carbon in the coke to CO2.

Below 700 deg C, CO2 is more stable than CO and the second reaction proceeds to the left (2CO = C + CO2). This reaction is frequently referred to as the carbon deposition. Above 700 deg C, CO is more stable than CO2 and the second reaction proceeds to the right. This reaction is sometimes called the carbon solution loss reaction and in this sense implies a negative behaviour. On the other hand the reaction represents a regeneration of reducing gas within regions of the furnace above 700 deg C. This is one of the important functions of coke within the BF and is particularly desirable as it increases the volume of the gases and helps in heat transfer. However, this reaction is endothermic and when it occurs within the tuyere zone, it creates a cooling effect within a location where high temperatures are important.

The effect of temperature on the equilibrium reaction between coke and a gas mixture containing CO and CO2 at 0.1 MPa and also 0.3 MPa pressure, which is more typical of modem BF practice, is shown in Fig 3. To the right of the graph, CO is more stable than CO2, while at lower temperatures, to the left of the graph, CO2 is more stable than CO. From this figure it is clear that above 1,000 deg C, the percentage of CO2 in equilibrium with coke is essentially zero. On the other hand, at temperatures below 400 deg C, the concentration of CO is small. Hence, as the temperature decreases between 1,000 deg C to 400 deg C, the stability of CO decreases while the stability of CO2 increases and the partial pressure of both gases in equilibrium with coke is considerable.

Fig 3 Effect of temperature on CO content of CO / CO2 gas mixture in equilibrium with carbon

The gases leaving the top of the furnace are normally at around 200 deg C and if equilibrium is achieved with coke, the ratio of CO to CO2 is to be around 10 to the power -5. In fact, the ratio is normally between 1 and 3, i.e. the gas is very much more reducing than that predicted from equilibrium considerations and full use is not being made of the reducing potential of the gas. This implies that the coke rate is in excess of the theoretical requirements. This lack of equilibrium between the gases and coke can be attributed mainly to the high gas velocity in the stack. The gas retention time in the furnace is only around 10 seconds, and extremely high velocities can occur, particularly in loosely packed, coke rich regions. Another factor is that the gas temperature drops by around 1,800 deg C as it rises through the furnace and so there is little opportunity for equilibrium to be maintained.

The carbon deposition reaction

Since the CO content of the gas within the stack of the BF at temperatures below 1,000 deg C is considerably higher than it is required to be, there exists a driving force for the carbon deposition, or sooting reaction to proceed. This driving force is particularly strong between 500 deg C and 700 deg C. A gas with a temperature and composition above the line in Fig 3 tends to deposit carbon by the reaction 2CO = C + CO2, and one with a composition and temperature below the line oxidizes carbon in accordance with the reaction CO2 + C = 2CO. Fortunately carbon deposition reaction is sluggish and equilibrium is never achieved, otherwise serious clogging of the spaces within the burden at the top of stack can occur.

This in turn can lead to irregular flow of the reducing gases and uneven descent of the burden. Even for partial reaction, a suitable catalytic surface is needed, upon which the carbon can nucleate and grow. Iron particles, partial reduced iron ore, and iron carbide have all been suggested as possible catalysts. The reaction appears to be enhanced by hydrogen and water vapour while nitrogen and sulphur compounds, for example, ammonia, hydrogen sulphide and carbon di-sulphide act as inhibitors. Zinc oxide and alkaline compounds oppose the inhibiting effect of sulphur, and although the concentration of these compounds in the furnace is normally small, they volatilize at high temperatures in the hearth and condense again in the cooler regions of the stack. The cumulative effect is that such compounds can offset the influence of sulphur. The carbon deposited by the reaction is in a very finely divided form and some can be accommodated within the pores of the iron ore particles and cared back down the stack again. This can affect the reduction process in several ways.

Because of the active nature of the carbon and its close association with the ore, reduction by solid carbon can take place at lower temperature than that needed for the reduction by coke, particularly since coke cannot penetrate the pores and reduction can only take place at points of contact between the solid particles. The rate of such reduction depends upon the rate of diffusion of oxygen from the interior of the particle to the point of contact. In the upper part of the furnace, the reduction by coke is negligible, compared with gaseous reduction. It becomes considerable only above around 1,000 deg C, when the gaseous reactions are impeded by slag formation. In contrast, reduction by precipitated carbon can occur at temperatures as low as 800 deg C.

The formation of CO during reaction within the pores tends to open up deep fissures within the particle, hence increasing the gas-solid contact area, and increasing the efficiency of gaseous reduction. When CO2 is produced within the pores of a particle by the gaseous reduction reaction, it can be rapidly regenerated to CO by reaction with the carbon in the pores, hence allowing the reaction to continue.

Unfortunately, the carbon deposition reaction can also have certain adverse effects. The reaction can cause splitting of refractories by deposition on active iron spots, in regions where the temperature is around 500 deg C to 550 deg C, for example in the outer shells at lower levels in the stack, or within the inner shells at the upper levels. If excessive, carbon deposition can cause ore pellets or sinter to crumble into powder and this can cause irregular gas flow and uneven descent of the burden.

Since the carbon deposition reaction is exothermic, the temperature of the exit gases is increased. Although the overall effect of the carbon deposition reaction can be debatable, certain facts remain. The reaction does decrease the CO / CO2 ratio of the exit gases. The reaction recirculates a certain amount of carbon, which otherwise is to be carried out of the furnace, hence increasing the time available for reaction with carbon and increasing the chemical efficiency of the reduction process.

Reduction of iron oxides

The reduction of iron oxides by CO can be represented by the reactions (i) 3Fe2O3 + CO = 2Fe3O4 + CO2, (ii) Fe3O4 + CO = 3FeO + CO2, and (iii) FeO + CO = Fe + CO2. These reactions are accomplished at increasingly higher temperatures and as shown in Fig 1, with increasingly higher percentages of CO. This means that reactions (i) and (ii), which are relatively easy to achieve, can take place within the upper regions of the furnace. Reaction (iii) which entails the removal of the last amount of oxygen from the iron, is in fact the most difficult to achieve and hence takes place further down the furnace where the temperatures are higher and the CO content of the reducing gases is higher. Below 570 deg C, the non-stoichiometric wustite phase (FexO) is unstable and it is possible to reduce magnetite directly to iron.

At any particular temperature, there is a minimum CO content in the gas mixture needed for reduction of a specific oxide. This means that it is not possible for the entire CO in the gases to be converted to CO2 if the reduction reactions are to continue. For example, at 800 deg C the equilibrium gas mixture in contact with FeO and solid iron contains about 65 % CO and 35 % CO2. If the CO2 content of the gases exceeds this value at this temperature, iron tends to be oxidized back to FeO. Accordingly, for these reactions to occur, there is to be a minimum concentration of CO in the gases at each step as indicated in Fig 1, and it is not possible to convert CO completely to CO2 by these reactions. Fortunately at these temperatures the CO2 produced by the reduction reactions is unstable in the presence of coke and CO is regenerated based on reaction CO2 + C = 2CO so that the reduction reactions can continue. It is worth noting that the combination of this reaction with reaction (iii) corresponds to the ‘direct’ reduction of FeO by carbon (FeO + C = Fe + CO), and this is a strongly endothermic reaction.

The reduction of iron oxides can also take place by hydrogen which is generated by the partial combustion of auxiliary fuels injected through the tuyeres to produce two reducing gases, CO and hydrogen. Hydrogen is also produced when steam is added to the blast as an aid in controlling the furnace. While the oxidation of carbon by oxygen in the hot air blast to form CO is exothermic, the reduction of moisture by coke to form CO and hydrogen (H2O + C = CO + H2) is strongly endothermic.

The reduction of iron oxides by hydrogen again proceeds in a sequential manner. The reactions are (i) 3Fe2O3 + H2 = 2Fe3O4 + H2O, (ii) FeO + H2 = Fe + H2O, and (iii) Fe3O4 + H2 = 3FeO + H2O. The effect of temperature on these reaction equilibria is shown in Fig 1. While reaction (i) is slightly exothermic, reactions (ii) and (iii) are endothermic. The presence of hydrogen, which because of its small size has a high diffusivity, markedly reduces the density and viscosity of the BF gases and, particularly at high temperatures, enhances the reduction of low reducibility raw materials. The water gas shift reaction (CO2 + H2 = H2O = CO) can take place between the different components in the gas phase to bring the hydrogen bearing and carbon bearing gases into equilibrium.

It is evident from Fig 1 that the gases passing up the furnace cannot be in equilibrium with carbon in the coke and at the same time in equilibrium with iron oxides in the descending burden. Above about 800 C the reaction of the gases with carbon is more rapid than with oxides and the equilibrium between coke and the gas phase is probably approached fairly closely. Measurements of the temperatures and compositions of gases in operating furnaces indicate that they tend to fall between the CO/CO2-C line and the FeO/Fe line above 800 deg C, cut the FeO/Fe line between 600 deg C and 800 deg C and then remain at or just above the Fe3O4/Fe line. At temperatures below 600 deg C, the very rapid gas flow allows little time for reaction with solids and the CO content of the gas is far in excess of that which is in equilibrium with the coke.

If the iron oxide is chemically associated with other oxides, its activity in the BF is decreased. This means the iron oxide is more difficult to reduce and the CO/CO2 ratios needed is higher than those normally considered here. For example with ferrous silicate, the minimum CO /CO2 ratio needed for reduction at 700 deg C is to be increased from around 1.5 to around 22, i.e. from around 60 % CO to almost 96 % CO on a carbonaceous gas basis. Since combined oxides are more difficult to reduce, higher temperatures are needed for reduction and hence the amount of reduction achieved with CO before slag formation occurs is decreased. This implies an increase in coke rate since the quantity of reduction needed in the lower part of the furnace is increased.

Reactions in the bosh and hearth

Reduction of other oxides – The reduction of oxides more stable than iron oxide such as manganese oxide and silica do not take place in the BF if the products are pure metals since the reaction MnO + CO = Mn + CO2 has, at equilibrium, a percentage of CO very close to 100 %. That is, the efficiency of reduction is extremely low and enormous quantities of gas is needed for very small amounts of manganese reduced. The situation with silica is even more extreme since it is a very stable oxide. However, by dissolving the manganese and silicon in iron, the reactions MnO + CO = Mn (dissolved in iron) + CO2, and SiO2 + 2CO = Si (dissolved in iron) + 2CO2 are moved somewhat to the right so that there is a distribution of manganese and silicon between metal and slag which is a function of the slag composition and of the temperature. Since the reduction of both of these elements is endothermic, the quantity of each in the hot metal increases with temperature and the extent of the reactions is to some degree is controlled by controlling the temperature in the hearth of the furnace. Of higher importance is the fact that the CO2 produced by these reactions is to react by the Boudouard reaction and causes an increase in the coke consumption.

The quantity of manganese reduced clearly also depends on the quantity in the charged ore. Ores with upto 2 % manganese give much higher than normal manganese contents in hot metal with consequent higher coke rates per ton of hot metal produced. Silicon ‘swings’ caused by erratic burdening of the furnace or by temperature variations can also have another serious effect, since the silicon reduced into the hot metal it is to be depleted from the slag, hence increasing the basicity ratio and changing the melting point and fluidity of the slag sometimes dramatically.

Effects of Silicon monoxide (SiO) formation – For several years it was considered that silica and manganese oxide are reduced directly from the slag by reaction with carbon in iron according to the reactions (i) SiO2 (slag) + 2C = Si + 2CO (g), and MnO (slag) + C = Mn + CO (g). It was thought that liquid iron droplets picked up silicon as they passed through the slag phase and on into the hearth. Various studies however, have shed new light on these reactions and also those involving sulphur. Several laboratory studies together with plant data have shown that at the temperature of the combustion zone, around 2,000 deg C, SiO gas is produced during the combustion of coke by the reaction SiO2 (coke ash) + CO = SiO (gas) + CO2. Combining this equation with the reaction for coke oxidation [CO2 + C (coke) = 2CO] yields the overall reaction SiO2 (coke ash) + C (coke) = SiO (gas) + CO. While the presence of FeO in slag is likely to make SiO formation from slag very difficult, an additional source of silica is to be reduced silica-rich slag adhering to the coke particles. Following these reactions, silicon is transferred to iron droplets by reaction with SiO in the gas phase [SiO (gas) + C = Si + CO]. As iron droplets containing silicon pass through the slag layer, some of the silicon is oxidized by iron oxide and manganese oxide, and taken up by the slag [2FeO (slag) + Si = SiO2 (slag) + 2Fe, 2 MnO (slag) + Si = SiO2 (slag) + 2Mn.

Reduction of phosphorus – It is expressed by the reaction P2O5 + 5C = 2P + 5CO; delta H = +995,792 kJ/kmol. The final reduction of phosphorus also takes place only at very high temperatures. However, unlike manganese and silicon the phosphorus is essentially completely reduced. For this reason, virtually all of the phosphorus in the charge is dissolved in the hot metal. The only means of controlling the phosphorus content of the hot metal is by limiting the quantity charged to the furnace.

Behaviour of sulphur – Sulphur is a troublesome element in BF operations since hot metal for steelmaking is to be low in sulphur. Levels of 0.035 % to 0.02 % are normal. The reaction by which sulphur is removed from liquid iron (S) into the slag (S) is frequently represented by the reaction S + (CaO) + C = (CaS) + CO (g) Where sulphur (S) and carbon (C) in the metal react with lime (CaO) dissolved in the slag to form calcium sulphide in the slag and CO gas. The distribution of sulphur between slag and metal, (S) /S, is strongly influenced by a number of factors as described here. Increasing the basicity of the slag (CaO / SiO2 ratio) tends to raise the thermodynamic activity of CaO in the slag which pushes reaction to the right. An increased oxygen potential in the system pushes the reaction to the left. This is shown by rewriting the reaction S + (CaO) = (CaS) + O. This effect is very strong, and the presence of even small concentrations of FeO in the slag seriously limits the sulphur ratio (S) / S. Fortunately both silicon and carbon raise the thermodynamic activity of sulphur in hot metal by 5 times to 7 times. Accordingly, sulphur in hot metal is 5 times to 7 times easier to remove than it is from liquid steel which contains relatively little carbon and silicon.

Assuming sulphur in coke ash is present as CaS, the reaction which can occur with SiO in the combustion zone to form volatile SiS  is CaS (coke ash) + SiO (gas) = CaO + SiS (gas). To a lesser extent, some CS gas can form by the reaction CaS (coke ash) + CO = CaO + CS (gas).

Sulphur transfer from these volatile species to liquid iron droplets then takes place within the bosh zone. A study has shown that when iron droplets containing silicon and sulphur are allowed to fall through the liquid slag, in the absence of MnO, the silicon content of the hot metal actually increases, and there is no transfer of sulphur. In the presence of MnO, silicon is removed from the metal by reaction and manganese transfers from slag to metal together with sulphur transfer from metal to slag take place. Based on the various results available, the sequence of reactions in the bosh and hearth are (i) the formation of SiO and SiS in the combustion zone, (ii) the transfer of silicon and sulphur to metal and slag droplets in the bosh, (iii) the oxidation of silicon by FeO and MnO in the slag as the iron droplets pass though the slag layer, and (iv) the desulphurization of metal droplets as they pass through the slag layer.

The sulphur distribution ratios found in the BF normally varies between 20 and 120. On the other hand experiments have shown that when metal and slag samples from BF are remelted in graphite crucibles at 0.1 MPa CO, the distribution ratio increases to between 120 and 220, depending on the slag basicity. This suggests that the oxygen potential of the system is higher than is to be expected for C-CO equilibrium in the furnace hearth. Hence, while thermodynamic conditions favour sulphur removal from the hot metal within the BF, kinetic considerations imply that the reaction can be more readily accomplished outside the furnace by external desulphurization.

Reaction of less abundant elements

In addition to the elements (that is Fe, P, Mn, Si, Al, Ca, Mg and S) which are normally considered in reporting the chemical composition of an iron-bearing material, there are a number of less abundant elements which undergo chemical reactions in the BF. Some of these can cause considerable operating difficulty and some can contaminate the product and make it unsuitable for certain steelmaking applications. The source of these elements is not only from natural iron ores, but also from waste materials such as scrap, steelmaking dust, and grindings etc., which are recycled through the BF. Some of the more important of these elements are arsenic, barium, chlorine, chromium, cobalt, copper, fluorine, lead, molybdenum, nickel, potassium, sodium, tin, titanium, vanadium and zinc.

Alkalis and zinc – Sodium, potassium and zinc, frequently called the ‘rogue elements’, can cause serious operating problems in the BF and are to be monitored and carefully controlled if stable conditions are to be maintained. The alkali metals enter the BF as the constituents of the gangue in the ore and also as a part of the coke ash, normally as silicates. In the stack of the furnace, the silicates react as per the equations (i) K2SiO3 + CO = 2K + SiO2 + CO2 and (ii) Na2SiO3 + CO = 2Na + SiO2 + CO2.

In the BF, the potassium reaction can take place above 500 deg C, while the sodium reaction occurs at around 600 deg C. At temperatures of around 900 deg C, the alkali metals are above their boiling point so they join the gas phase. However, as these gases start to rise up the furnace, the metal becomes unstable with respect to other compounds which can form and cyanides, oxides, and carbonates all start to precipitate from the gas phase as very fine fumes or mists, since the cyanides are liquid over a wide temperature range. These fine particles of solid and liquid can deposit on the iron ore particles, the coke, and the furnace wall, with some, of course, being swept out with the BF gas and being captured in the dust catching system. Particularly the liquid alkali compounds can penetrate the brick lining of the furnace and cause serious deterioration and spalling. As well, these compounds can build upon the wall and cause scaffolding, hanging, and slipping.

The alkalis which land on the iron and coke are carried to the lower part of the furnace. There, they are again reduced to the metal which rises up the stack as a gas, forms the same alkali compounds, and repeats the cycle, joining new material in the process. The reduction needs carbon, increasing the coke rate, and cooling the furnace, and the recycling material can build up to the point where it degrades the coke in the furnace, causing it to break into small pieces and increasing the reactivity of the coke to CO2.

This increased reactivity can again reduce the temperature of the furnace and decrease the heat efficiency of the whole system. The high concentration of alkalis in the furnace also affects the strength and reduction characteristics of the iron bearing materials, causing dramatic swelling and catalyzing carbon deposition on the pellets. These deleterious reactions with both the coke and the ore can have serious impacts on the gas permeability in the furnace and on the stability of the BF operation.

Fortunately, the alkali oxides are very basic oxides and can be fluxed with SiO2 in acid slags and removed from the furnace. Normally, decreasing the slag basicity can carry increasing quantities of alkali away in the slag. This is in direct contrast to sulphur removal, where increasing the slag basicity increases the sulphur removal. When majority of desulphurizing takes place in the BF, there is a conflict between the attainment of low sulphur and removal of alkalis and the basicity of the furnace is carefully controlled to balance both the problems. With external desulphurization, this is no longer a problem and the furnace can normally be burdened to minimize alkali attack.

Zinc normally originates in steelmaking off-gas dust from furnaces using galvanized scrap which in some fashion has been recycled to the BF. Occasionally, the zinc content of iron ores or coal ash can be also a considerable source. Behaving not unlike sodium, zinc is reduced from the oxide or ferrite at around 600 deg C, forms a vapour which subsequently forms oxides or carbonates that can react with the sidewalls or be carried down the furnace on coke or ore to be reduced and further cycled, consuming coke at each turn. Zinc which escapes as a fume in the gas stream, enters the BF filter-cake, making it unsuitable to recycle if present in a high enough percentage. Unlike the alkalis, zinc is not captured to any extent in the slag and can only effectively be removed by decreasing the input and allowing the recycling vapour to slowly leave through the gas phase.

Clearly, the best protection against alkali metals and zinc is to ensure that the absolute minimum is part of the BF feed. Because of the tendencies of these elements to circulate in the furnace, they are unseen and unknown consumers of coke and cause refractory, ore and coke problems. Unfortunately, the symptoms of the problem are not always evident until the problem is of fairly major proportions and then needs fairly drastic measures, such as eliminating certain feed materials, to affect a solution.

Lead and titanium – Lead is seldom a problem in the BF but occasionally enough can enter a BF through the ore or sinter to cause a problem. Lead is very easily reduced in the iron BF and falls to the bottom of the hearth which normally has a chilled hot metal layer which protects the hearth refractories. Lead has virtually no solubility in the hot metal so it forms a low melting point liquid pool on which the hot metal floats, and hence promotes more rapid attack on the hearth. In certain furnaces where this problem is known to occur, a second tap-hole, deeper than the iron notch, can be used to periodically tap the lead.

Titanium is an even more stable oxide than silica but in the BF it can form extremely stable carbides and nitrides. The titanium compounds, if present in small quantities can be effective in forming a light protective layer on the hearth surfaces and prolong the life of the hearth. For this reason, titani-ferrous ores are added judiciously to sinter mixes. However, at high concentrations, these same compounds can stiffen the slag while building up a heavy hearth layer, reducing the hearth capacity of the furnace. As with zinc, the best solution is to reduce the input and slowly eliminate the titanium from the furnace.

Arsenic – Arsenic is found in a number of iron ores. The behaviour of arsenic is very much like that of phosphorus, in that it is almost completely reduced and dissolves in the hot metal. It increases the fluidity of the hot metal and hence, it appears to increase the wear of refractories. It is not completely removed during the steel refining process and imparts brittleness to the finished steel.

Barium – Barium is chemically similar to calcium and occurs as a very basic oxide in some iron and manganese ores. It is not reduced in the BF but becomes part of the slag, increasing the slag basicity. It can cause difficulty in controlling the metal composition if the operator is not aware of its presence.

Chlorine – Chlorine occurs as alkali chlorides in several iron ores and as a contaminating compound in ores processed with sea water. Chlorine is also present in some coals used for injection. In the high temperature zone of the BF, these compounds are volatized and as they rise toward the top of the furnace they condense around cooling plates and cause corrosion. They can also condense in uptakes and down-comers where they form accretions which can eventually restrict the passage of the top gas, or react to form HCl (hydrochloric) acid and attack the gas cleaning system.

Chromium – Chromium is found in some ores and is reduced to a certain extent depending on the basicity of the slag and the operating temperature. Normally, around 50 % to 60 % of the chromium is reduced into the hot metal.

Cobalt, copper, and nickel – Cobalt, copper, and nickel occurs in several different ores. They are also present in iron-bearing tailings from the copper industry which are sometimes sintered and used in the BF to recover the iron. All three of these elements are reduced almost completely into the hot metal and are not oxidized in the steel refining process. As a result, in operations which produce steel which is to meet stringent ductility specifications, such ores cannot be used.

Fluorine – Fluorine compounds are found in several ores and behave somewhat like chlorine compounds. The ability of HF (hydrofluoric) acid to attack the gas cleaning system is well known.

Molybdenum and tungsten – Molybdenum and tungsten occur very rarely and only in such minute quantities that they can be ignored. If any compounds of these elements are present in the BF, they are at least 90 % reduced into the hot metal.

Tin – Tin is an element which enters the BF mostly by way of recycled materials such as scrap or sintered dusts. It is almost entirely reduced and dissolves in the hot metal.

Vanadium – Vanadium occurs and behaves in a manner somewhat similar to chromium. Around 50 % of the vanadium in the burden is reduced and enters the hot metal.

Selenium and tellurium – Selenium and tellurium, though somewhat rare, can be present in some raw materials. In their reactions they are similar to sulphur but possess an even greater tendency to remain with the metal.

Fluxes

Limestone charged to the furnace is calcined by the reaction CaCO3(s) = CaO(s) + CO2 (g); delta H = 177.8 kJ at around 800 deg C. Magnesium carbonate in the dolomite of the charge is calcined by a similar reaction MgCO3(s) = MgO(s) + CO2(g);  delta H = 167.4 kJ at 50 deg C to 100 deg C lower temperatures. These reactions result in several undesirable conditions in the furnace. The first is that they need considerable heat and the second is that CO2 is released in the furnace. The additional CO2 raises the oxygen potential of the gases which inhibits the final step in the reduction of the iron ore, i.e., FeO to Fe. It also favours ‘solution’ of carbon from the coke by the equation CO2 (g) + C(s) = 2CO (g).

A considerable improvement in the furnace operations is achieved when ‘self-fluxing’ agglomerates of iron-ore concentrates are the principal iron-bearing charge to the furnace. Limestone and dolomite can be added to the feed of sintering machines and pelletizing furnaces. When the sinter is fired and the pellets are indurated, the fluxes are calcined and reacted with iron oxides to form calcium-ferrites and other more complex compounds. The CaO and MgO carried into the BF by these agglomerates are then free of CO2.

Slags

The fundamentals of the BF slag are complex. At around 40 %, oxygen is the largest single element in slag. Slag is, hence, an oxide system and ionic in nature. The oxide system which forms the basis for BF slags is the lime-silica-alumina (CaO-SiO2-Al2O3) system modified due to the presence of certain percent of MgO in the slag. Fig 4 show phase diagram of CaO-Al2O3-SiO2-10 % MgO system. Due to the nature of the BF process, slag formation is a multi-step process involving considerable changes in composition and temperature. The four primary components of slags form numerous compounds which result in a wide range of chemical and physical properties. The lesser components of slag are of particular interest with respect for hot metal chemistry and furnace control, and add to the complexity of the physico-chemical properties of the slag.

Fig 4 Phase diagram of CaO-Al2O3-SiO2-10 % MgO system

Slags with compositions in the region of 40 % SiO2, 48 % CaO, and 12 % A12O3 have low melting points, i.e., 1,300 deg C, and are appropriate for the control of sulphur and silicon in the hot metal. Frequently 6 % to 10 % MgO is used in place of an equivalent quantity of CaO to lower the viscosity of the slag. Small quantities of MnO, FeO, Na2O, and K2O etc. help to lower the melting point of the slag.

Essentially there are two slags in the furnace. The first is the ‘primary, or bosh, or early’ slag which is formed principally from the gangue constituents in the ores and agglomerates and CaO and MgO from the calcined fluxes, or the self-fluxing portions of the agglomerates. This slag is relatively basic compared to the final slag and contains some iron oxide. The ‘final or hearth’ slag is formed by the union of the early slag with constituents of the coke ash which are freed from the coke when it is burned before the tuyeres. This final slag continues to have its composition modified as it passes down into the hearth and mixes with liquid iron which also is flowing down into the crucible. There is an adjustment in the silica content of the slag, iron oxide can be reduced from it and it can absorb sulphur from the coke and liquid iron.

The formation of slags in the slag-formation zone is very furnace specific due to the impact of burden properties and furnace operation. The slag formation zone begins at the cohesive zone, where softening of burden begins, and continues down to below the tuyere elevation. The slag formation zone hence includes the cohesive zone, active coke zone, deadman, and raceway. The slag formed in the upper part of the slag formation zone is called the ‘bosh’ or ‘primary’ slag, and the slag leaving the zone at the bottom is the ‘hearth’ slag. The Primary slag is normally assumed to be made up of all burden slag components including the iron oxides not reduced in the granular zone, but does not include the ash from the coke or injected coal. The slag composition changes as it descends in the furnace due to the absorption of the coke ash and coal ash, sulphur and silicon from the gas, and the reduction of the iron oxide. The temperature of the slag increases of the order of 500 deg C as it descends to the tuyere elevation. These changes in composition and temperature can considerably impact the physical properties of the slag, specifically the liquidus temperature and the viscosity.

The slag produced in slag formation zone collects in the slag layer in the hearth zone, filling the voids in the hearth coke and ‘floating’ on the hot metal layer. The hot metal passes through the slag layer to reach the hot metal layer. The high surface area between the hot metal and slag as the hot metal passes through the slag layer enhances the kinetics of the chemical reactions. These reactions result in considerable changes in the hot metal chemistry. In particular the (Si) and (S) contents prior to entering the slag layer are much higher than those in the hot metal layer.