For many years, raw high alumina refractory materials have been widely used in the chemical, building materials, and refractory industries. Through the process of electric arc furnace smelting, brown fused alumina can be produced by separating some of the oxide components in the alumina at a temperature of about 2000℃ in the smelting furnace. High alumina refractory materials contain some naturally occurring titanium oxide, and as the grade of high alumina refractory materials increases, the content of titanium oxide in the material also increases significantly. Due to the strong thermodynamic stability of titanium oxide, it is impossible to reduce it completely during the brown fused alumina production process, which results in a titanium content of up to 3% and an alumina content of only about 95% in the brown fused alumina raw materials. The titanium oxide phase is the main impurity phase in brown fused alumina raw materials, and is also the main reason for giving the raw materials excellent toughness.
Due to the production process, the highly stable titanium, magnesium, and calcium elements in brown fused alumina for abrasive materials are difficult to completely reduce, resulting in small amounts of Ti2O3, Ti solid solution, titanium iron silicon alloy low-melting glass phase, and calcium aluminate solid solution in the raw materials in addition to the corundum component. The SEM image of the polished cross-section of the brown fused alumina particles shows the distribution of each phase in the raw material: Ti2O3, Ti solid solution, and titanium iron silicon alloy are distributed in the form of granular phases within the corundum particles; the low-melting phase is mainly distributed in the gaps between two corundum particles; while calcium aluminate solid solution is mainly distributed at the corners of three corundum particles.
As a raw material for refractories, brown fused alumina for refractory still suffers from the problem of pulverization and cracking compared to other corundum quality raw materials. This is mainly due to the inadequate removal of reducing agent coke in the brown fused alumina production process, which leads to the formation of Aluminum Carbide in the raw material, causing cracking and peeling of the finished product due to the pulverization of Aluminum Carbide upon contact with water.
The production of the brown fused alumina for refractory is essentially a high-temperature reduction process of the Fe2O3, SiO2, TiO2, K2O, Na2O components in the bauxite ore. At 2000℃, the bauxite ore and iron are first transformed into a mixed melt. Under the action of the reducing agent carbon, each component in the melt is reduced, with Fe2O3 being the first to be reduced. SiO2 in the melt is reduced after Fe2O3, and the reduction process Fe in the melt will also participate in the reaction to form ferrosilicon, which will precipitate to the bottom of the furnace and separate from the corundum molten mass due to its high density after formation.
After the reduction of Fe2O3 and SiO2 in the melt, TiO2 begins to be reduced. Titanium has multiple valences, which leads to the possible existence of titanium oxide in bauxite ore in the forms of TiO2, TiO3, Ti2O3, and Ti3O5, with TiO2 and Ti2O3 being the most stable. At the same time, when carbon acts as a reducing agent, the titanium oxide cannot be directly reduced to the element Ti, but is reduced to form Ti(C,N,O) solid solution under the action of a small amount of N in the air. The actual reduction products are Ti2O3 and Ti(C,N,O) solid solution.
In the previous steps, Fe2O3 is reduced to produce metallic iron. With the participation of this part of the metallic iron, some TiO2 and Ti2O3 in the melt are reduced to elemental titanium and form titanium iron alloy together with the metallic iron. Due to the large density of titanium iron alloy, it can precipitate to the bottom of the furnace and achieve separation from the corundum molten mass. After the reduction process is completed, the solid-phase related to the melt is precipitated during the cooling of the melt, while the remaining impurity oxides in the liquid phase form calcium aluminate solid solution and low-melting glass phase, which remains in the gaps between brown fused alumina particles.