News

Blast Furnace Slag and its Role in Furnace Operation

• satyendra
• March 12, 2022
• 2 Comments
• blast furnace, Blast Furnace Slag, Final slag, Granular zone, Hearth slag, Oxide system, Primary slag, Slag basicity, Slag structure,

Blast Furnace Slag and its Role in Furnace Operation

The importance of blast furnace (BF) slag in achieving smooth operation of the BF is illustrated by the old saying ‘if you take care of the slag, the furnace will take care of the rest’. There has been a tremendous quantity of work has been done on the BF slag studying the properties, formation mechanisms, and impacts on the furnace operations.  A good quality slag is necessary for producing good quality of hot metal in the BF.

BF slag practice is required to meet certain requirements for efficient BF operation. These requirements include (i) it is to absorb all unreduced non-volatile components of the burden and remove them from the BF, (ii) it is to be liquid of low viscosity, (iii) it is to be able to absorb the sulphur primarily contained in the fuels, (iv) it is to contain as small quantity of iron oxide as possible in order to increase the yield of the hot metal, (v) its volume is to be as low as possible without affecting the desulphurization, (vi) the temperature range where the burden components become cohesive is to be narrow for ensuring better  permeability of the burden column, and (vii) its quality is to be such so that it can be processed into saleable material. These requirements are in part complementary and in part mutually exclusive. It is hence necessary to state priorities.

Fortunately, there are general relationships which provide a more practical view of the nature of slags which can be used on a daily basis. It is important, however, to have a basic understanding of the fundamental nature of the BF slag to understand the general relationships.

Fundamental of BF slag

The fundamentals of the BF slag are complex. At around 40 %, oxygen is the largest single element in slag. Slag is, hence, a oxide system and ionic in nature. 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 the BF slag are SiO2 (silica), CaO (lime), MgO (magnesia), and Al2O3 (alumina). These four components of the BF slag 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 add to the complexity of the physico-chemical properties of slag.

The fundamental of BF slag include issues related to the BF process. These issues include the slag formation, flow in the hearth, the molecular structure of slag and how the structure relates the chemical indices known as basicity, slag solidification, and the impact of changes of the thermal state of the furnace on slag composition.

Slag formation – BF is a pressurized, counter-current heat exchanging, refluxing, gas-solid-liquid, packed bed reactor. It has three primary functions, namely (i) reduction of iron oxides to metallic iron (ii) fusion of the metallic iron and oxides, which provides for the (iii) separation of the impurities of the burden and fuel from the liquid iron. These characteristics of the process lead to the division of the furnace into three vertical zones with respect to slags (i) namely (i) granular zone, (ii) slag formation zone, and (iii) hearth zones. These three zones and some specific reactions for each zone are given in Fig 1.

Fig 1 Blast furnace zones and zone reactions

The granular zone is located in the upper part of the furnace where all charged components are in solid phases. The granular zone is bounded by the stock line on the top and by the start of the formation of liquid phases, the cohesive zone, on the bottom. As the burden descends through the granular zone, it is heated by gases from the lower part of the furnace and a part of the reduction of the iron oxides is performed. The quantity of reduction which occurs in the granular zone is a function of the nature of the iron bearing materials, burden distribution, and the gas composition and flow patterns.

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 third zone is the slag layer in the hearth of the furnace. The slag produced in slag formation zone collects in the slag layer, 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 improves 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. The formation of slags in the slag formation zone is very furnace specific due to the impact of burden properties and furnace operation.

Slag structure – The conceptualization of slag structure (Fig 2) is based upon the structure formed by silica. On the molecular level, the silicon atom is located in the centre of a tetrahedron surrounded by four oxygen atoms, one oxygen atom at each comer of the tetrahedron as shown in Fig 2a. Each oxygen atom is bonded to two silicon atoms and hence each oxygen atom is a corner of two tetrahedrons. The sharing of oxygen atoms results in a polymer or network in three dimensions in the crystalline state where all corners are shared (Fig 2b). As silica is heated, some of the corner bonds are broken but the polymer nature of the structure is maintained even when molten as shown in Fig 2c.

Fig 2 Conceptualization of slag structure

The addition of metallic oxides, such as CaO and MgO breaks down the polymer structure. These oxides act as oxygen donors, replacing an oxygen atom in one corner of a tetrahedron and breaking the tetrahedron-to-tetrahedron comer bond (Fig 2d). The breakdown of the polymer structure continues with the addition of more metal oxides until the molar ratio of metal oxides to silica equals two, at which point all tetrahedron-to-tetrahedron comer bonds are broken (Fig 2e). The molar ratio of two is the ortho-silicate composition, 2CaO-SiO2, 2MgO-SiO2, and CaO-MgO-SiO2. Al2O3 acts in a similar fashion as SiO2 in forming polymers and accepting oxygen atoms from basic oxides. Oxides which accept oxygen, SiO2 and Al2O3, are termed as acid oxides. Oxides which donate oxygen, CaO and MgO are termed s basic oxides.

Slag basicity – It is very useful when relating the properties of a multi-component system to its composition to develop an index based upon the composition. The problem in developing an index is how to reflect the significance of each component of the system in the index. The different nature of the acid oxides (A) and basic oxides (B) has been used in the development of slag composition indices, normally termed basicities. Examples of basicity indices which have been developed are (i) excess bases = {(CaO) + (MgO)} – {(SiO2) + (Al2O3)}, (ii) basicity (B/A) = {(CaO) + (MgO)} / {(SiO2) + (Al2O3)}, (iii) bell’s ratio = {(CaO) + 0.7(MgO)} / {0.94(SiO2)+ 0.18(Al2O3)}, and (iv) optical basicity = {(CaO) + 1.11(MgO) + 0.915(SiO2) + 1.03 (Al2O3)} / {(CaO) + 1.42(MgO) + 1.91(SiO2) + 1.69(Al2O3)}.

Basicity indices can be grouped into general categories namely (i) differences between the quantity of bases and acids, as shown in equation (i) above, (ii) bases to acids ratios based upon the weight percentages, as shown in equation (ii) above, (iii) bases to acids ratios based upon the molar concentrations, as shown in equation (iii) above, and (iv) sum of the basicity of each component and its molar concentration, as shown in equation (iv) above. As to be expected based on the previous description of slag structure, those indices which reflect the molecular nature of the slag composition, equations (iii) and equation (iv) tend to be better predictors of slag properties. However, as the index defined by equation (ii) is probably the most commonly used definition.

Temperature impact – [Si], basicity, and slag volume – There is increase the quantity of [Si] with the increase of the hot metal temperature in the BF as shown in Fig 3. The quantity of [Si] increase for a given temperature increase varies from furnace to furnace, but the trend is the same for all the furnaces. As the [Si] increases, the (SiO2) decreases and hence the basicity increases and the slag volume decreases. The quantity of increase in the basicity for a specific increase in [Si] is a function of the slag volume. Fig 3a shows the change in B/A for initial slag volumes of 200 kgs/tHM (kilograms per ton of hot metal) and 300 kgs/tHM and for the [Si] and hot metal temperature relationship given on the figure. The normal trend demonstrated here is that the larger is the slag volume the smaller is the change in B/A for the same change in [Si] or hot metal temperature.

Fig 3 Temperature impact and slag solidification

Slag solidification – The common definition of melting temperature only applies to a single component system such as water, where only liquid water exists above the melting temperature and only solid water exists below the melting temperature. Slag is a multi-component system and, hence, do not have the common definition of melting temperature except at specific compositions. Majority of the slag compositions have both solid and liquid phases present over a range of temperatures. The lowest temperature at which only the liquid phase exists for a specific composition is called the liquidus temperature.

Fig 3b shows the solidification path of a slag is illustrated on the simplified phase diagram. Start with slag of composition Cstart at temperatures where only liquid slag exists. As the slag cools, moving down vertically on the diagram, the composition of the liquid slag does not change until the intersection with the ‘liquidus line’. The intersection with the ‘liquidus line’ is the liquidus temperature for the composition Cstart. A very small amount of the solid compound forms at the liquidus temperature on the left. Three changes continue as the temperature is further reduced below the liquidus temperature namely (i) more of the solid compound is formed, (ii) the quantity of liquid slag decreases, and (iii) the composition of the liquid slag changes, moving towards the right along the ‘liquidus line’. In the example, where the compound formed is 2CaO.SiO2, the basicity of the liquid slag decreases as the slag is cooled since 2CaO.SiO2 contains around twice as much CaO as SiO2.

The solidification path shows how a compound can be formed even when the liquid slag composition is considerably different than the composition of the compound. The weight ratio of CaO to SiO2 = 1.86 for the compound di-calcium silicate, 2CaO.SiO2. While no BF has ever been successfully operated using slags with a CaO to SiO2 approaching 1.86, considerable quantities of di-calcium silicate can be formed in the slags of operating BFs. The formation of sufficient di-calcium silicate results in a solid slag which breaks down into dust upon cooling, known as a ‘falling’ or ‘dusting’ slag. The breakdown is caused by the 10 % volume expansion of di-calcium silicate as it goes through a phase change at 675 deg C. The guideline reported for avoiding a ‘falling’ slag is (CaO) less than 0.9(SiO2) + 0.6(A2O3) + 1.75(S).

It is important to remember that phase diagrams are based upon equilibrium conditions. Equilibrium conditions imply that the cooling rate is slow relative to the rate of the reactions, such as the formation of di-calcium silicate. The solidification path described above is ‘by-passed’ if the cooling rate is very high as in slag granulation and, to a lesser extent, slag pelletization. The rapid cooling locks the composition in a solid glass phase, where the kinetics of the reactions is too slow for the compounds to form.

Slag flow In the hearth – The control of the slag level in the hearth is important for maintaining stable furnace operation, especially as the hot metal production rates have been increased. High slag levels result in increasing blast pressure and bosh wall working, and disrupting the uniform descent of the burden. One of the issues in controlling the slag level is slag flow in the hearth during the tapping. In the hearth, slag flow to the tap hole is more difficult than the flow of hot metal to the tap hole. Hot metal flow has a larger driving force due the higher density of hot metal compared to the slag. The hot metal flow path is thought to be primarily through ‘coke free’ regions below and / or around the deadman coke. The slag flow path to the tap hole is through deadman coke.

Fig 4 shows the configuration of the hearth and a possible sequence of stages of the hearth during tapping which lead to a false dry hearth condition at the end of the tapping. The surface of the hot metal is thought to remain relatively flat across the entire hearth area throughout the tapping due to the high density of hot metal and the ‘coke free’ path to the tap hole. The slag surface can be considerably lower in the region about the tap hole than at other regions of the hearth. When the slag tapping rate is higher than the slag flow rate across the hearth to the tap hole region, a depletion of slag occurs in the tap hole region and the slag surface begins to curve down towards the tap hole as shown in the step 4 of Fig 4. The slag depletion continues until there is no slag at the tap hole and the furnace appears to be dry when there is still considerable slag remaining in the hearth as shown in the step 5 of Fig 4. Minimizing the resistance to slag flow in the hearth minimizes the slag remaining in the hearth at the end of a tapping. Resistance to slag flow in the hearth is reduced as the porosity of the hearth coke bed is increased and the slag viscosity is reduced.

Fig 4 Configuration of the hearth and a possible sequence of stages of the hearth during tapping

Oxide system

Around 95 % of the slag consists of SiO2, CaO, MgO, and Al2O3. The requirement of low viscosity can be met by a variety of components in this quaternary system. Ignoring the presence of MgO, the phase diagram of the ternary oxide system CaO-Al2O3- SiO2 (Fig 5) shows a low melting temperature region which is parallel to the CaO-SiO2 binary with low Al2O3 content. This region extends from high SiO2 content to the saturation isotherm for 2CaO.SiO2 and then for essentially constant CaO content toward high Al2O3 content. The MgO content of the slag does not substantially affect the relative position of the low melting temperature region and only affects the absolute values of the melting temperatures.

Fig 5 Phase diagram of the oxide system CaO-Al2O3- SiO2

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 6 shows the phase diagram of CaO-Al2O3-SiO2-10 % MgO system.

Fig 6 Phase diagram of CaO-Al2O3-SiO2-10 % MgO system

The compositions of BF slags as encountered under various operating conditions are shown in Fig 7. The desulphurization of hot metal increases with slag basicity, i.e. with increasing CaO and / or MgO content, region 1 in Fig 7 can, hence, be used only for processing low sulphur burden. Since the gangue constituents normally form a low basicity slag, region 1 largely represents the slag composition without addition of fluxes. The furnace can be operated at a relatively low temperature because of the low melting points. Region 2 is reached for low iron content burden with acid gangue constituents. This mode of operation prevails, and needs extensive desulphurization of the hot metal outside of the BF. The attainment of a basicity which results in adequate desulphurization within the furnace needs a large lime (CaO) addition which leads to a high slag volume and hence a higher coke rate. Region 3 represents the world wide preferred slag compositions for large blast furnaces. In this case, depending on the alumina content, dolomite is to be added to satisfy the needed MgO contents.

Fig 7 Compositions of BF slags as encountered under various operating conditions

Tab 1 shows the optimum components of the BF slag. Slags with higher basicities (B) as shown in Tab 1 do favour optimum softening conditions. The softening and the melting range of the gangue constituents is around 80 deg C to 130 deg C for B= 0.5, and around 20 deg C to 50 deg C for B = 2. Because of the higher melting temperature of the highly basic slag and of extra energy needed due to the larger quantity of the flux addition, the slag basicity is hence maintained at around 1.2.

Slag properties

The physical and chemical properties of slags are primarily a function of the slag composition and temperature. The following describes these relationships for the purpose of developing general trends.

Liquidus temperatures – The relationships of liquidus temperature and composition for the four primary components of slag are represented on a quaternary phase diagram. Fig 8 has been generated from ternary planes of the quaternary phase diagram. Figures 8a and 8b are not phase diagrams. There are two general trends derived from these figures. The first is that the liquidus temperatures increase with increases in (Al2O3) and B/A, and the second is that the (MgO) in the range of 8 % to 14 % tends to minimize the increase in liquidus temperature caused by the increase in either (Al2O3) or B/A.

Fig 8 Relationship liquidus temperature, basicity, and alumina

Viscosity – Viscosity is a measure of the quantity of force needed to change the form of a material and is reported in unit called ‘poise’. The higher the viscosity, the more force is needed to cause a liquid to flow. For comparison purposes considering that at 20 deg C the viscosity of water is 0.01002 poise, a typical acceptable slag viscosity is around 2 to 5 poise, and the viscosity of the liquid SiO2 is of the order of 100,000 poise. The high viscosity of liquid SiO2 is caused by the polymer structure. The breakdown of the polymer structure by the basic oxides lowers the viscosity. The decrease in the viscosity of all liquid slags with increasing the B/A is shown Fig 9a. In general, the viscosity of any liquid / solid mixture increases as the quantity of suspended solids increases. The impact of temperature on slag viscosity is considerably higher at temperatures below the liquidus temperature than above the liquidus temperature as shown in Fig 9b.

Fig 9 Relationship between viscosity, B/A and temperature

There are two general trends which are seen for the viscosity. Above the liquidus temperature, the viscosity of liquid slags decreases with increasing temperature and B/A. At temperatures below the liquidus temperature, the viscosity decreases with increasing temperature, and decreasing B/A.

Sulphur partition ratio – The BF ironmaking is a very good desulphurizing process compared to the steelmaking process because of the difference in the oxygen potential of the slags of the processes. The effect of the oxygen potential on desulphurization can be shown using equation (CaO) + [S] = (CaS) + (FeO), where the oxygen potential is indicated by the (FeO). The higher the (FeO) the more the reaction is driven to the left and the higher the [S]. Steelmaking slags with (FeO) of 15 % to 25 % are, hence, weaker desulphurizing slags than the BF hearth slags with (FeO) of less than 1 %.

Essentially all the sulphur into the BF leaves the furnace in the hot metal and slag. A relationship for the prediction of [S] can be developed based upon a mass balance of sulphur for one ton of hot metal, as per the equation (i) below, and the defined term sulphur partition as per the equation (ii) below. The prediction of [S], by equation (iii) below, is derived by the substitution of [S] from the equation (i) into equation (ii) and then solving for [S].

Equation(i) is St = [S] /100  x 1,010 + (S) /100 x Svol where 1,010 is the kgs of hot metal in a ton of hot metal including a 1 % yield loss, St is the sulphur loading which is the total weight of sulphur in kgs/tHM. Svol is the slag volume which is the weight of slag in kgs/tHM. Equation (ii) Sp = (S) /[S] where Sp is the sulphur partition ratio. Equation (iii) is [S] = St x 100 / (Sp x Svol + 1,010).

The slag Sp can be predicted based upon equation (iv) Sp = 147.7 x BB + 37.7 x [Si] – 190 and equation (v) BB = {(CaO) + 0.7(MgO)} / {0.94(SiO2) + 0.18(Al2O3)}. Here BB is the basicity as defined by the bell’s ratio. It is to be noted that the coefficients in equation (iv) have been developed from regression analysis of a specific furnace. Equation (iv) and equation (v) have been used to construct Fig 10(a), and equation (iii), equation (iv), and equation (v) have been used in the construction of Fig 10 (b).

Fig 10 Sulphur partition between slag and metal

The general trends which can be derived from the above equations and figures are (i) [S] decreases with decreasing St and increasing Sp and Svol, (ii) however, Sp normally increases with B/A, (iii) CaO is a better desulphurizer than MgO, and (iv) Al2O3 has a smaller effect on Sp than SiO2.

Alkali capacity – A ‘refluxing’ or ‘recycling’ phenomena occurs in the BF due to the counter-current flow of gases versus solids / liquids, particularly for sulphur, zinc, and alkalis. The recycling of the alkali potassium (K) is shown in Fig 11a. The recycling phenomena is when an element travels down the furnace in a solid or liquid phase, reacts to form gas species in the higher temperature regions of the furnace, then travels back up the furnace as gases, where it reacts and is absorbed by the solid / liquid phases in the lower temperature region of the furnace. The recycling results in much higher internal concentrations of the recycled element than the concentration going in or out of the furnace. For example the internal loading of K can be 10 kgs/tHM, when the materials being charged contain only 2 kgs/tHM.

Alkalis have no beneficial, but many deleterious effects on the BF. Alkalis are absorbed by refractories, coke, and ore causing degradation of the refractories and coke, and ore swelling. Alkalis can also form scabs which can peel off upsetting the thermal condition of the furnace, or build up and constrict burden and gas flow. Alkalis cannot be avoided as they are contained in all coals, cokes, and to a lesser extent ores. The alkali loading is to be minimized wherever possible.

A portion of the alkalis leave the furnace in the top gas, the quantity being a function of the top temperature profile. The remaining alkalis are to be removed in the slag. The ability of slag to remove alkalis from the furnace is referred to as the alkali capacity of the slag. The relationships of alkali capacity to slag composition and temperature are shown in Fig 11b. In general the alkali capacity increases with lower B/A, and with lower temperature.

Fig 11 Alkali recycling and alkali capacity of slag

Silica activity – The [Si] produced is dependent upon the burden materials, furnace operation, and slag chemistry. The impact of the slag chemistry is shown in the equation [Si] = (SiO2) x GSiO2 / GSi x Keq / (P to the power 2)co. Here CO is carbon mono-oxide. This equation is developed from the equilibrium constant, equation Keq = {ASi x (P to the power 2)co} / { ASiO2 x Ac}, for the reaction given in equation (SiO2) + 2C = [Si] + 2COgas, the definitions of the activities of (SiO2) and [Si], equation ASiO2 = (SiO2) x GSiO2 and equation ASi = [Si] x GSi, and assuming that the activity of the carbon in the hearth is equal to one. The trend implied by Equation [Si] = (SiO2) x GSiO2 / GSi x Keq / (P to the power 2)co is that the [Si] decreases as the (SiO2) decreases.

Slag design factors

In some of the BFs, a typical slag composition which is formed from the gangue in the ore and ash of the coke is 9 % CaO, 5 % MgO, 75 % SiO2, and 10 % Al2O3. A slag of this composition has a liquidus temperature of the order of 1,600 deg C and does not flow well even above its liquidus temperature. Hence, CaO and MgO are added to the burden to ‘flux’ the gangue and ash resulting in acceptable liquidus temperatures and flow characteristics.

Basic slag design is the selection of the types and quantities of fluxes to be used with a burden and coke to produce a slag of acceptable properties. Burden and coke selections are largely driven by economic issues such as local versus imported sources and degree of beneficiation. These economic driving forces have resulted in a wide range of slag compositions throughout the world.

The general factors to be considered in designing a slag for normal operation are (i) liquidus temperature, that is, the slag is to be completely liquid in the hearth and cast house, (ii) viscosity, that is, the slag is to have a low viscosity, high fluidity, so as to drain from the hearth and down the cast house runners, (iii) sulphur capacity that is the Sp is to be sufficient to produce hot metal with sulphur contents within specifications, (iv) alkali capacity, that is, the slag alkali capacity is to be sufficient to prevent alkali build up in the furnace, (v) hot metal silicon control, that is, the effect of the slag chemistry on the [Si] is to be considered, (vi) slag volume, that is, the slag volume is to be high enough to contribute to the stability of the slag properties and hot metal quality, but not so high as to require excessive fuel or contribute to furnace instability, (vii) robust properties, that is, the slag properties are to be as insensitive to variations in normal variations in furnace operation as possible, specifically hot metal temperature, and (viii) end use, that is, the requirements of the end use of the slag is to be considered.

Slag design is to recognize that the above factors are not independent and that the design always involves a balancing of the above factors to resolve the conflicting trends (Tab 2).

Two examples of the slag design are described below. In the first example (Tab 3) , the issue is to increase the alkali removal without increasing the [S]. The resolution of the issue is to increase the slag volume through the use of additional SiO2 in the burden, while decreasing the slag basicity.

The issue in the second example (Tab 4) is to lower the [Si] without negatively impacting the other properties of slag and furnace operation. The resolution of the issue is to decrease the (SiO2) by increasing the (Al2O3) using quartzite, a high (Al2O3) burden material, while holding the (CaO) and (MgO) constant. The change in slag chemistry results into a decrease of both [Si] and [S].

Slag after the BF

The use of BF slag is driven by the economics of processing and market demand. In the place, where the processing and marketing is performed by the organization producing the slag, the markets tend to be local in nature with minimal processing. As per the present trend, independent organizations take ownership of the liquid slag at the end of the slag runner which has led to wider markets with more extensive processing. The product slag can be classified by the rate of cooling.

Air-cooled slags are those produced with low cooling rates. These are slags which are solidified in pits and frequently cooled with water sprays. The largest uses for air cooled slag are in road construction, railroad ballast, and aggregate. Air-cooled slag has also been used in the production of cement, mineral wool insulation, roofing, and glass.

Pelletized and granulated slags are those produced with high cooling rates. Pelletized slag is produced by pouring liquid slag onto a rotating drum, sometimes with water. Granulated slags are produced by either pouring the liquid slag directly into a large slag pit of water or through the use of high pressure water sprays which breaks the slag up into droplets. Rapidly cooled slags have been used for the same applications as air-cooled slags. The high glass content of rapidly cooled slags makes it particularly suitable for Portland cement production.