Refractory materials for aluminum melting furnace linings are usually kept at temperatures below 1000°C. Fluorspar or barite and other sintering aids and anti-wetting agents are added to these refractory materials to promote low-temperature sintering of the materials and improve resistance to aluminum liquid penetration. However, when the furnace is overheated for some reason, these additives can cause premature corrosion of the refractory materials. In order to overcome the problem of premature corrosion of refractory materials for aluminum melting furnace linings, a (CaO-Al2O3) calcium aluminate refractory castables with calcium aluminate as aggregate and calcium aluminate cement as binder was developed. Usually CAA is made by melting raw materials with a material ratio of Al2O3 to CaO CaO/Al2O3=1 or 0.7 (molar ratio) and crushing into different particle sizes. Since CAA is made by melting process, its apparent porosity is almost zero. In CAA materials, the mineral phase is mainly CaO-Al2O3, with a melting point of 1600℃.
Calcium aluminate cement (abbreviated as CAC) can be prepared by sintering and electric melting processes, and its chemical composition is about CaO27%, Al2O371%. The results of the comparative crucible self-etching test of CAA castables and ordinary castables at 800℃, 72h with AZ8GU aluminum alloy show that CAC-bonded castables have excellent anti-penetration ability. On the contrary, CAC-bonded plate-like alumina castables can be deeply penetrated into its structure by aluminum alloy melt, and there is obvious corundum (α-Al2O3) generation. The test results also show that the aluminum liquid penetrates deeply into the clay castable with anti-wetting agent, even the clay castable with CAA added.
It has been confirmed that the interaction between the alloy melt and the refractory material in the aluminum smelting furnace is mainly the penetration of the melt into the pores in the refractory structure of the furnace lining, resulting in the reduction of SiO2 and/or aluminum oxides to form corundum (α-Al2O3). In the case of sodium-rich alloys or sodium-rich alkali salts, β-Al2O3 is formed, which causes the aluminum-rich refractory lining to peel off and prematurely damage. Since CAA materials are almost not penetrated by aluminum liquid, there is almost no problem of structural peeling and accelerated damage to the refractory lining.
The results of the comparative test also show that the permeability of CAA low-cement refractory castables without anti-wetting agents is the same as that of traditional alumina-based low-cement castables with BaSO4 (anti-wetting agent), and the former can be used at a temperature of up to 1350℃, which ensures that even when the furnace is overheated, it will not cause premature corrosion of the refractory lining. This shows that CAA low-cement castables with CAA as aggregate and Calcium aluminate cement as binder are fully compatible with the use conditions of aluminum smelting furnaces. The results of other studies (500℃, 100h CO erosion resistance test) show that iron-free CAA has excellent CO erosion resistance after heating to 800℃, indicating that CAA castables with CAA as aggregate and CAC as binder can also be used in the petrochemical industry.
