Alumina and its Role in Iron and Steelmaking
Alumina and its Role in Iron and Steelmaking
Alumina is a chemical compound of aluminum (Al) and oxygen (O2) with the chemical formula aluminum oxide (Al2O3). It is the most commonly occurring of several aluminum oxides. It is significant in its use to produce aluminum metal. It is being used as an abrasive material because of its hardness. It is also being used as a refractory material owing to its high melting point.
Aluminum oxide is an amphoteric substance. It can react with both acids and bases, acting as an acid with a base and a base with an acid, neutralizing the other and producing a salt. It is insoluble in water.
Aluminum oxide has a white solid appearance and is odorless. The molar mass of aluminum oxide is 101.96 grams per mole. Specific gravity of alumina is 3.986. It is insoluble in water. Melting point of aluminum oxide is 2072 deg C while the boiling point is 2977 deg C.
Alumina affects the processes of producing iron and steel during the production of iron and steel. Besides alumina is a very important refractory material for the lining of furnaces and vessels in iron and steel plants.
Role of alumina in ironmaking
Alumina during ironmaking enters the process through impurities in the input materials mainly iron ore.
Alumina affects the sintering of iron ore. The most harmful effect of alumina is to worsen the RDI (reduction degradation index) value of sinter. RDI value increases as the alumina content rises. It is seen that within a 10 % to 10.5 % CaO content range, an increase of 0.1 % in the alumina content raises the RDI by 2 points. The strength and quality of sinter deteriorate as the alumina content rises. Alumina promotes the formation of SFCA (silico ferrite of calcium and aluminum), which is beneficial for sinter strength, but the strength of the ore components is lower, since a high alumina content in their lattice has been reported to be the main cause of the observed lower strength . Alumina increases the viscosity of the primary melt which forms during the sintering process, leading to a weaker sinter structure with more interconnected irregular pores.
High alumina in the iron ore raises the levels of alumina in blast furnace slag. To operate a blast furnace with high alumina slag is quite difficult and need a different type of skill from the blast furnace operators since with the increase in the Al2O3 content of the slag, the blast furnace operation has problems such as excess accumulation of molten slag in the blast furnace hearth and increase in the pressure drop at the lower part of the furnace. Hence it is important to keep good slag fluidity in the blast furnace operation for keeping good permeability and good drainage of slag during tapping.
There are four kind of slags with distinct compositions are produced at different regions inside the blast furnace due to a series of reduction reactions. These four types of slags, namely primary slag, bosh slag, tuyere slag and final slag, are generated respectively in the cohesive zone, dripping zone, raceway and hearth. The slag fluidity in a blast furnace affects softening melting behaviour in the cohesive zone, permeability in the lower part of the furnace due to the 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. Good tapping is dependent mainly on the final slag which should have low liquidus temperature, low viscosity and wide fluidity. The slag fluidity is affected by temperature and composition of the slag.
High alumina in BF slag has many adverse effects. The characteristics of slag having high alumina are as follows.
- The viscosity of molten slag is dependent basically on its chemical composition and on its temperature. Slag viscosity is an important process variable of the blast furnace process. It is the transport property of the slag that relates to the reaction kinetics and the degree of reduction of the final slag. Low viscosity helps to govern the reaction rates by its effect on the transport of ions in the liquid slag to and from the slag/metal interface. It also determines the slag metal separation efficiency, the metal yield and impurity removal capacity. It also ensures a smooth running of the furnace. 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.
- In BF operation, the slag drainage phenomenon in the furnace hearth is a fluidization phenomenon dominated by viscosity. The slag drainage rate decreases as the slag viscosity increases.
- Higher alumina slag has greater tendency towards silicon reduction and there is tendency towards increase of hot metal silicon level. This can be either due to rise in the equilibrium concentration of silicon or not attaining the equilibrium levels at all.
- 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. It is observed that not only is the equilibrium distribution of sulphur between metal and slag is affected adversely but the rate of attaining such a distribution is also markedly slower. Hence there is slower pick up of sulphur by the high alumina slag since the sulphur equilibrium is not being attained within the blast furnace,.
- 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 pressure drop is mainly caused by the effect of wettability as a result of the slag static hold up, little due to the effects of dripping slag viscosity and crystalline temperature. The permeable resistance in the cohesive zone increases as the Al2O3 content of the slag increases.
The increase of permeable resistance in the cohesive zone can be suppressed by increase of MgO in the burden. Physical effects of increased MgO content in the slag are just the opposite to those of alumina. MgO helps in the maintenance of good slag drainage from the hearth during tapping. As the level of MgO increases in the high alumina slag, the sulphur content of hot metal improves for a given range of silicon. This is probably due to the higher fluidity of the high MgO slag. High MgO slag is advantageous for the control of both sulphur and silicon. Decrease of slag basicity is also helpful. To offset the deteriorating effect of high alumina slag operation of the blast furnace, the following measures are important.
- Since increase in slag MgO improves the hearth drainage rate at high alumina slag operation, MgO in the slag is to be kept at a level which is more than the minimum level. Higher MgO level in the slag also improves the permeability in the cohesive zone of the blast furnace.
- To suppress the increase in the pressure drop in the dripping zone, it is important to decrease the slag hold up by the decrease of CaO2/SiO2. Permeability of the dripping zone is improved by decreasing the slag CaO/SiO2 ratio in the slag to around 1 %.
One other method for reducing the effect of high alumina in slag is to dilute the level of alumina in the slag to lower concentrations by addition of extra slag forming materials in the blast furnace burden. However this results into higher slag volume and involves higher flux and coke rates and lower productivity of the blast furnace. This method can be used for control only as an occasional remedy.
Role of alumina in steelmaking
Alumina in steel during steelmaking comes through deoxidation of steel with aluminum as well as due to wear of alumina refractory lining. Aluminum is also added to the steel for the control of grain size. When alumina is carried into steels from refractories, then the inclusions tend to be large and isolated. Optically alumina inclusions appear as stringers, often with ‘comet tails’ due to polishing.
Alumina inclusions deteriorate the properties of steel. Alumina is solid at steelmaking temperature and is brittle in nature. On rolling, alumina inclusions break up which is a serious surface defect. The alumina may accumulate in the continuous casting nozzle causing clogging of the nozzle. This affects the steel flow rate to the mould.
Alumina inclusions are dendritic when formed in a high oxygen environment and often coalesce to create irregular shaped ‘alumina clusters’ as a result of the collision of smaller particles. These clusters significantly affect the mechanical properties of steel, especially fracture sensitive properties such as toughness and fatigue life, and may also result in the generation of surface defect.
Alumina inclusions occur as deoxidation products. Pure alumina has a melting point of 2072 deg C, i.e., these alumina inclusions are present in a solid state in liquid steel. The addition of calcium to steel which contains such inclusions changes the composition of these inclusions from pure alumina to calcium aluminates. The melting point of the calcium aluminate decreases as the CaO content increases, until liquid oxide phases occur at about 22 % of CaO, i.e., when the CaO.2Al2O3 compound is first exceeded at 1600 deg C. The liquid phase content continues to increase as CaO content rises further and is 100 % at 35 % of CaO. The minimum melting temperature for the liquid calcium aluminates is around 1400 deg C, i.e., such liquid calcium aluminates may be present in liquid form until, or even after, the steel solidifies.
Alumina as refractory material
Alumina refractories are the part of alumina- silica (SiO2) group of refractories and belong to the Al2O3-SiO2 phase equilibrium system. They differ from fire clay refractories in term of Al2O3 content and normally have Al2O3 content greater than 45 %.
Al2O3-SiO2 refractories are manufactured from a blend of sized raw material aggregates and clays by mixing, forming, drying, and firing. These are traditional processes in manufacturing any ceramic product. Since refractories are used at elevated temperatures, they are particularly sensitive to contamination and particle size segregation. By minimizing contamination and particle size segregation refractories with a smaller variance in physical properties can be produced.
Alumina refractories can be categorized in four categories (Fig 1). These are (i) refractories containing with 50 % and 60 % of Al2O3, (ii) refractories containing with 70 % of Al2O3, (iii) refractories containing with 80 % and 85 % of Al2O3, and (iv) refractories containing with 90 % and 99 % of Al2O3.
Fig 1 Categories of alumina refractories
Refractories containing 50 % and 60 % Al2O3 show improved refractoriness over fireclay products. There are two fundamental mineral mixtures used in the production of these refractories, and the physical properties of the refractories depend, in part, on which mineral mixture has been used in the manufacture.
The most straightforward way to produce these refractories is to use with 50 % or 60% Al2O3 aggregates (i.e., bauxitic kaolin or andalusite). Another way to produce these refractories is to use a mixture of bauxite and fireclay. This latter method is called the bauxite dilution method since the bauxite (at 88 % Al2O3) is diluted with calcined fireclay and raw clay (contains around 40 % Al2O3) to produce the required Al2O3 content. Properties of the refractories produced by bauxite dilution method are generally inferior.
It is important to note that refractories containing bauxite or andalusite typically show high reheat expansion while clay refractories do not. In refractories containing andalusite and also containing clay, this tendency for high reheat expansion may not be observed. Thus, there is a fundamental difference in the refractories within the same class with respect to permanent expansion characteristics. In linings requiring the extreme tightness (as in rotary kiln applications) the reheat expansion is extremely important in good lining life.
By contrast, high reheat expansion may be associated with high spalling tendency, i.e., low spalling resistance. In this regard, refractories produced from bauxitic kaolin, i.e., ‘clay base’, may have superior spalling resistance. This is because of their finer texture, namely smaller average pore size, and due to the absence of permanent expansion reactions on heating.
The TiO2 content of refractories may indicate the fact that they contain calcined bauxite aggregate (if around 2.5 %). Bauxite can also be recognized on a broken or saw-cut surface as a gray-appearing aggregate to the naked eye.
Refractories containing 70 % of Al2O3 have become a workhorse in industrial furnaces since second half of 1940s because of their high use, high duty ratings, and durability in many processes where slag corrosion or other reactions take place. These refractories can be produced either from bauxitic clays having 70 % Al2O3 or by using appropriate mixtures of calcined bauxite (88 % Al2O3) and fireclay (around 40 % Al2O3). As in the case of 60 % Al2O3 refractories, the mineral base of the firebrick makes a significant difference in physical properties and thermal response of 70 % Al2O3 refractories.
The refractories based on calcined bauxitic clays show much higher reheat expansion (PLC) and higher spalling loss than the refractories based on bauxitic clay. On the other hand, the refractories based on calcined bauxite have superior performance where erosion resistance is required.
The microstructure examination s of the two types of 70 % Al2O3 refractories show that in the refractories made from calcined bauxitic clays, the coarse aggregate particles are surrounded by a finely textured matrix. The distribution of medium size particles around coarse aggregates is excellent. This microstructure suggests excellent spalling resistance. By contrast, the refractories made from calcined bauxite have a completely different appearance. The microstructure is dominated by the dark bauxite particles. A glassy matrix containing mullite surrounds all particles. A comparison of the microstructures provides graphic evidence that the refractories of same Al2O3 percentage can have different reheat (PLC) properties.
Refractories with 80 % and 85 % Al2O3 were originally developed for use in aluminum smelting and holding furnaces. It is rare that they find application in other types of furnaces. These refractories are based on calcined bauxite, as it is the closest mineral in Al2O3 content to their overall composition. The resistance to aluminum attack is, in part, due to the resistance of the bauxite to solution in molten aluminum and to salt fluxes that may cover the metal bath.
These refractories have not been successful in ferrous industry applications. The reason may be the relatively poor refractoriness of the bond phase (glass and mullite) holding together highly refractory calcined bauxite aggregate particles. In an aggressive slagging situation, the bauxite aggregate is eroded out of the refractory brick, and wear rates are usually unacceptably high.
Refractories with 90 % and 99 % Al2O3 are amongst the highest strength and erosion resistant refractories. They are made from synthetic Al2O3 aggregates, and some types may contain fused Al2O3 for special erosion resistance. There are several distinct types of 90 % Al2O3 refractories. The fused Al2O3 provides very high erosion resistance to flowing liquid steel. The microstructure of this type of refractory shows fused alumina aggregate particles (white with rounded black pores) are surrounded by a gray matrix containing a lighter mullite phase. The type called as ‘tabular alumina—corundum matrix’ is made from coarse super calcined Al2O3 aggregates and reactive calcined Al2O3 fines to produce a ‘direct bonded’ microstructure where Al2O3 to Al2O3 bonding (corundum to corundum) predominates. This provides an obvious increase in hot modulus of rupture (HMOR). The microstructure of this type of refractories shows tabular Al2O3 aggregate particles which reside, and connected to the matrix through bonds with smaller corundum crystals. There is a practical limit on Al2O3 content of around 96 % Al2O3 (contains about 3.7 % SiO2) in refractory brick for the highest-temperature applications. At compositions of higher Al2O3 content, the products cannot be sintered in conventional gas fired kilns at sufficient temperatures to have good density and PLC (reheat) properties. While 99 % Al2O3 refractory bricks exist, they are primarily used for low temperature applications such as in chemical processes.