Slag and its Role in Blast Furnace Ironmaking
Slag and its Role in Blast Furnace Ironmaking
Blast furnace (BF) is the oldest (more than 700 years old) of the various reactors which are being used in the steel plants. It is used for the production of liquid iron (hot metal). The blast furnace is a complex high temperature counter current reactor and is in the shape of a shaft in which iron bearing materials (ore, sinter/pellet) and coke are alternately charged at the top along with flux materials (limestone, dolomite etc.) to create a layered burden in the furnace. Preheated air is blown in from the lower part of the furnace through tuyeres. This hot air reacts with the coke to produce reducing gases. Descending ore burden (iron oxides) is reduced by the ascending reducing gases and is melted to produce hot metal. The gangue materials and coke ash melt to form slag with the fluxing materials. The liquid products (hot metal and slag) are drained out (tapped) from the furnace at certain intervals through the tap hole. The quality of hot metal obtained is dependent on the formation of the slag and its mineralogical transformations. A good quality slag is necessary for a quality hot metal. The slag is a mixture of low melting chemical compounds formed by the chemical reaction of the gangue of the iron bearing burden and coke ash with the flux materials in the charge. All unreduced compounds such as silicates, aluminosilicates, and calcium alumino silicate etc. also join the slag.
It is well known that the components of slag namely silica (SiO2) and alumina (Al2O3) increase the viscosity whereas the presence of calcium oxide reduces the viscosity. The melting zone of slag determines the cohesive zone of blast furnace and hence the fluidity and melting characteristics of slag play a major role in determining the blast furnace productivity. Initially iron rich slag is formed and thereafter due to the assimilation of calcium oxide (CaO) and magnesium oxide (MgO) from flux materials, the composition of slag varies. As the slag trickles down, it assimilates SiO2 and Al2O3 of ash, generated from combustion of the coke. The process of trickling down depends on fluidity (low viscosity) of the slag, which further is governed by its composition and temperature.
The slag must have the affinity for absorbing impurities i.e. gangue from charge along with other deleterious impurities which affect the quality of hot metal. It is essential to know the behaviour of slag in terms of the chemical composition, the mineralogical constitution and its ability to react and trap the minor impurities. Also the slag should be free flowing at the operating temperature with high slag-metal separation without entrapping metal. Hence the various properties of the final product are directly influenced by the composition of the slag. Thus the physicochemical properties of slag play an important role in the operation of the blast furnace.
The blast furnace final slag is produced during the production of hot metal is mainly considered to be a mixture of the four oxides namely (i) SiO2, (ii) Al2O3, (iii) CaO, and (iv) MgO. The minor components of slag includes (i) ferrous oxide (FeO), (ii) manganese oxide (MnO), titanium oxide (TiO2), alkalis (K2O and Na2O), and sulphur bearing compounds. There are four kinds of slags with distinct compositions are produced at different regions inside the blast furnace. (Fig 1). These are (i) primary slag, (ii) bosh slag, (iii) tuyere slag, and (iv) final slag. These four types of slags are respectively generated in the (i) cohesive zone, (ii) dripping zone, (iii) raceway and (iv) hearth. It is the final slag which is tapped and hence for good tapping it is necessary that it should have proper fluidity (low liquidus temperature and low viscosity).
Fig 1 Kinds of BF slags and regions of their generation
For the smooth operation of the blast furnace, the slag is required to satisfy the following conditions.
- The volume of the slag is to be as low as feasible.
- The slag is to have capacity for the removal of the alkalis,
- It is to fulfil the requirements for the desulphurisation.
- The composition of the primary slag must be uniform.
- Slag formation is to be confined to a limited height of the blast furnace and the slag must be stable.
- The slag is to provide good permeability in the zone of slag formation.
- The melting point of the slag is to be neither too high nor too low.
- The final slag is to be fluid enough so that it is possible to drain it through tap hole,
The iron bearing material layers start softening and melting in the cohesive zone under the influence of the fluxing agents at the prevailing temperature which greatly reduces the layer permeability that regulates the flow of materials (gas/solid) in the furnace. It is the zone in the furnace bound by softening of the iron bearing materials at the top and melting and flowing of the same at the bottom. A high softening temperature coupled with a relatively low flow temperature would form a narrow cohesive zone lower down the blast furnace. This would decrease the distance travelled by the liquid in the furnace there by decreasing the silicon pick-up. On the other hand the final slag that trickles down the bosh region to the hearth of the furnace, is to be a short slag that starts flowing as soon as it softens. Thus fusion behaviour is an important parameter to evaluate the effectiveness of the BF slag.
The slag fluidity in a blast furnace affects softening-melting behaviour in the cohesive zone, permeability in the lower part of a furnace due to liquid hold-up in the dripping zone, liquid flow in the furnace hearth, and the ability of the drainage of the slag through the tap hole. It also affects its desulphurization ability. The slag fluidity is affected by temperature and composition, with the latter influenced by the ore gangue minerals and ash materials of the coke and the pulverised coal.
Blast furnace slags mainly belongs to three slag systems namely (i) tertiary system of CaO–Al2O3–SiO2, (ii) quaternary system of CaO– Al2O3–SiO2–MgO, and (iii) quinary system of CaO–Al2O3–SiO2– MgO–TiO2. Generally, the major operating region of blast furnace slag for good fluidity in the quinary system (SiO2-Al2O3-CaO-MgO-TiO2) liquidus diagram is the melilites phase (solid solutions of akermanite, Ca2MgSi2O7, and gehlenite, Ca2Al2SiO7).
Blast furnace slag composition has very important bearing on its physicochemical characteristics which affects the degree of desulphurisation, smoothness of operation, slag handling, coke consumption, gas permeability, heat transfer, hot metal productivity and its quality etc. The slag properties which affect most are viscosity, sulphide capacity, alkali capacity and liquidus temperature. These properties have great influence on the overall blast furnace process. Viscosity of the slag is strongly influenced by the chemical composition, structure and the temperature.
Slag viscosity is a transport property that relates to the reaction kinetics and the degree of reduction of the final slag. Slag viscosity also determines the slag– metal separation efficiency, and subsequently the metal yield and impurity removal capacity. In operation, the slag viscosity is indicative of the ease with which slag could be tapped from the furnace, and therefore relates to the energy requirement and profitability of the process.
If the furnace controls has the ability to predict the slag viscosity and liquidus temperature then it has the potential to optimize the analysis and decision-making control during the operation of the blast furnace. In such case it replaces the use of rules of thumb pertaining to slag compositions. For this several efforts have been made in the past to measure and model viscosities for different slag systems.
Liquid slag can be classified as a Newtonian fluid with the shear viscosity being independent of the shear rate, and therefore named dynamic viscosity. Viscosity is largely influenced by bonding and the degree of polymerisation, with SiO2 and Al2O3 contributing to higher viscosities with their highly covalent bonds. In contrast, monoxides such as CaO and MgO exhibit ionic behaviour, leading to the destruction of silicate chains and lowering of the viscosity. These is true only for the liquid slag-phase system, and in the multiphase system, an increase in monoxides leads to higher activities of solid phases and possible solids precipitation, which increases the effective (observed) viscosity.
In a typical operation where it is possible to alter the slag composition, changes in the composition usually have opposing effects. For example, the achievement of lower viscosity at higher basicities will likely be associated with the adverse effect of increased liquidus temperature. In addition to the effects on physicochemical properties, the slag basicity also influences the sulphur (and to some extent the phosphorus) removal capacity of the slag, and the silicon content of the hot metal. Higher basicities lead to higher sulphur values in the slag and lower silicon values in the metal.
Low Al2O3 slags generally have low viscosity, high sulphide capacity, and low liquidus temperature as well as lower slag volume than high Al2O3 slag with Al2O3 normally more than 15 %. High Al2O3 slag is encountered mainly with Indian blast furnaces because of high Al2O3/SiO2 ratio in the iron ore as well as in the sinter and high ash content in the coke. These slags are having high viscosities.
The viscosity of liquid slag is determined primarily by its temperature and the chemical composition. The temperature dependence of viscosity over a given temperature range is usually described by Arrhenius equation as given below.
N = A exp (E/RT)
N = Slag viscosity
A = Pre- exponential term
E = Activation energy of viscous flow
R = Gas constant
T = Absolute temperature
Silicate slags are built up of Si4+ cations which are surrounded by 4 oxygen anions arranged in the form of a regular tetrahedron. These SiO4 4- tetrahedra are joined together in chains or rings by bridging oxygen. Viscous flow in slag depends on the mobility of ionic species in the system, which, in turn depends upon the nature of the chemical bond and the configuration of ionic species. The interionic forces in the case of slags depend on the sizes and charges of ions involved. Thus, it is natural to expect that stronger interionic forces lead to an increase in viscosities. In the case of silicate melts with high silica contents, the polymeric anions cause a high viscosity. With increase of the metal oxide concentrations, the Si-O bonds progressively breakdown and size of network decreases accompanied by lowering of the viscosity of slags. It has been shown that the addition of alkali oxides up to 10-20 mole % leads to a drastic fall in the viscosities due to de-polymerization.
In the case of blast furnace slags, alumina is always present and the AlO4 5- groups form polymer units with SiO4 4-. In the slags containing CaO-MgO-SiO2-Al2O3, alumina increases the viscosity as is done by the silica. On the other hand, lime and magnesia, the suppliers of oxygen, have the opposite effect on viscosity.
Viscosity of slags depends upon composition and temperature. Low viscosity not only helps to govern reaction rates by its effect on the transport of ions in the liquid slag to and from the slag/metal reacting interface. It also ensures a smooth running of the blast furnace. Both an increase of basic oxides and that of temperature above the liquidus temperature of the slag decrease viscosity. In the case of the system CaO-MgO-SiO2-Al2O3, alumina and silica are not equivalent on molar basis in their effect although both increase the viscosity of these melts. The effect of the alumina on viscosity depends on the lime content of slag. This is because Al3+ can replace Si4+ in the silicate network only if associated with 1/2 Ca2+ to preserve electrical neutrality.
Fusion behaviour of the slag is described in terms of four characteristic temperatures namely (i) the initial deformation temperature (IDT) symbolising surface stickiness, important for movement of the material in the solid state, (ii) the solid state (ST) symbolising plastic distortion, indicating start of plastic distortion, (iii) the hemispherical temperature (HT), which is also the fusion or liquidus temperature, symbolising sluggish flow, playing a significant role in the aerodynamics of the furnace and heat and mass transfer, and (iv) the flow temperature (FT) symbolising liquid mobility.
The slag formed in the cohesive zone is the primary slag which is formed with FeO as the primary fluxing constituent. The solidus temperature, fusion temperature, solidus-fusion interval are being significantly affected by FeO. This slag is completely different from the final slag where the fluxing is primarily caused due to the presence of basic constituents like CaO or MgO. While it is not possible to obtain primary slag from the blast furnace, it is always possible to prepare a synthetic slag in the laboratory resembling the primary slag and study its flow characteristics. The final slag is a slag with a small difference between the ST and FT. Such a slag acquires liquid mobility and trickles down the furnace away from the site where it starts distorting plastically, as soon as possible. This action exposes fresh sites for further reaction and is supposedly responsible for enhanced slag-metal reaction rates, influencing the blast furnace operations and the quality of the metal.
The flow-characteristic of blast furnace slags is strongly influenced by the extent of reduction of iron oxide at low temperature (in the granular zone) besides being influenced by the composition, and the quality and the quantity of the gangue in the iron bearing materials. The CaO/SiO2 ratio and the MgO content of the blast furnace slag greatly influence its softening-melting properties. The availability of MgO later in the process often results in small temperature range of cohesive zone resulting in better permeability of the bed which in turn influences the coke consumption and quality of hot metal produced.
The increase of Al2O3 in the iron ore not only affects the strength of the sinter, but also its characteristics at high-temperatures in the cohesive zone. The Al2O3 concentration in the slag is considered to be a factor that degrades the slag fluidity and increases the liquidus temperature. The effects of high alumina in the slag are as follows.
- High alumina slag has got high viscosity for constant basicity (CaO/SiO2). However with an increase of basic oxides and that of temperature above the liquidus temperature of slag, the viscosity of high alumina slag decreases to some extent.
- Higher alumina slag has greater tendency towards silicon reduction and there is tendency towards increase of hot metal silicon level.
- The sulphur content of the hot metal tends to increase with the increase in the alumina content of the slag. Hence the high alumina slag contributes to less efficient desulphurization.
- The pressure drop in the dripping zone increases as the Al2O3 concentration in the slag increases. Even if the ratio CaO/SiO2 increases the pressure drop in the dripping zone increases.
The deteriorating effect of high alumina in the slag is offset by increasing its MgO content. The alumina concentration in the slag is semi-empirically set in many countries at the upper limit of around 16 % in order to avoid the accumulation of the iron slag and the deterioration of permeability in the lower part of the blast furnace.