The beneficiation and chlorination of bauxite by gas-solid reaction

Recent developments in the production of aluminium metal by the electrolysis of chloride sale melts have indicated that this technique may now result in significant cost savings if applied on an industrial scale. A prerequisite of the electrolysis stage is the development of an economical method to produce high purity aluminium chloride from aluminous ores. The purpose of this project was to investigate the feasibility of producing low-iron aluminium chloride from Weipa bauxite via gas-solid reaction. A critical aspect of selecting an improved route over those previously proposed was the attempt to attain attractive operating conditions at reaction temperatures below 8-900°C. The thermodynamics of bauxite beneficiation were investigated using computer techniques. Complex gas-solid equilibria were predicted which enabled several promising beneficiation schemes to be studied experimentally. One of the most promising techniques for iron separation from alumina was found to require the sulphidisation of iron oxide using SO₂/CO gas mixtures followed by the chlorination volatilisation of the iron sulphide intermediate. This technique was investigated in detail and was found to result in very rapid reaction rates at the relatively low temperatures of 6-700°C. The chlorination of bauxite, beneficiated bauxite and alumina by CO/Cl₂ mixtures was studied in detail for temperatures below 1000°C. It was found that an optimum reaction rate was attainable in the region 6-800°C such that no improvement was obtained until the temperature was raised above 1100°C. The optimum reaction rate was obtained by controlled calcination of the alumina to a high surface area. The reaction kinetics were a complex function of gaseous diffusion and the physical properties of the solid. The reaction kinetics have been described in terms of a Zone Model and the reaction is considered to take place via an activated complex adsorbed on the solid surface. The most significant result was that a laboratory scale scheme was developed for the beneficiation and chlorination of bauxite such that a temperature of 750°C was not exceeded. The reaction times were less than 1 hour for each stage, the final iron level was 0.3% or below, and the alumina losses were about 2%.