Refractory selection criteria for aluminum smelting & holding furnaces
Low silica, iron oxide and calcium oxide content
Sufficient mechanical strength
Effect of different alloy to the refractory materials
Hard alloy contains high Mg, Zn, Cu, and low Si. Generally speaking, refractory materials are susceptible to corrosion
Soft alloy contains high Si, which is beneficial to refractory materials
Therefore, in order to reduce the problem of corundum tumor and to achieve a long life of the lining, the appropriate refractory material can be selected from the following aspects:
1. Select refractory materials containing non-wetting agent which mainly have two effects:
A. The non-wetting agent has a large wetting angle and does not react with the molten aluminum;
B. The non-wetting agent can make the material have a very low porosity after sintered, low porosity will prevent the penetration of molten aluminum.
The figure below showed the results of the anti-corrosion test of the two materials with the same alumina content, but with or without the non-wetting agent. The effect of the agent can be clearly seen;
2. Reduce the content of silica in refractory materials, especially dissociative silica. The higher proportion of silica in the material, the easier it is to cause corundum tumor infiltration; the ALU-FLO 94AR and ALU-FLO 87AR are developed and produced by SFR have very low silica content, which is recommended to use in harsh environments, and they can extend the life of the lining efficiently;
3. Apply low cement and low water addition castables. Traditional castables contain more CaO and water, so the material has poor density, many pores, and molten aluminum is easy to penetrate. The salts used in the smelting process, such as CL salt and F salt, will react with CaO in cement and damage the material structure. SFR’s all materials that are in contact with aluminum are low-cement castables, which maximizes the density of the material, reduces the porosity, and improves the resistance of the material to molten aluminum penetration.
In the process of using the lining, due to a large number of cracks in the furnace wall, spalling, and the expansion of the side wall would cause deformation of the furnace steel structure, or the temperature of the furnace wall is too high in the later period of use, and the heat dissipation is serious. All of these are related to the occurrence of corundum tumor. The main chemical reactions as follow:
3 (Al2O3-SiO2) + 8Al → 6Si + 13Al2O3
Aluminum reacts with the silicon oxide in the lining, reducing Si, causing the aluminum molten to increase silicon, and at the same time increasing the proportion of aluminum oxide in the refractory material. This reaction is an expansion reaction and will destroy the structure of the lining (as the picture shows).
Alcoa alkali corrosion resistance test for roof and upper furnace wall material K2CO3 + NaCO3 at 900 ℃ for 5 hours
The carbonate alkali mainly forms a low-melting-point phase with the alumina oxide and silica oxide of the lining, thereby reducing the service life, and the carbonate further reacts with the cement hydration phase:
Na2CO3 + CAH10 → CaCO3 + Na2O.Al2O3 + 10H2O
SFR selects low impurity content (low Fe2O3 & Na2O), low cement and ultra-low cement castables in the upper wall and bottom area, and meanwhile adopts micro-powder technology and new water-reducing agent technology to make the material very dense. The ULTRA-FLO 71 and ALU-FLO 80 which SFR choose for the upper furnace wall have good resistance to alkali corrosion.