Black Alumina oxide powder

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.

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.

1. 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.
2. 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.