Desilication of Sodium Aluminate Solutions from the Alkaline Leaching of Calcium-Aluminate Slags

The desilication of sodium aluminate solutions prior to precipitation of aluminum trihydroxides is an essential step in the production of high purity alumina for aluminum production. This study evaluates the desilication of sodium aluminate solutions derived from the leaching of calcium-aluminate slags with sodium carbonate, using CaO, Ca(OH)2, and MgO fine particles. The influence of the amount of CaO used, temperature, and comparisons with Ca(OH)2 and MgO were explored. Laboratory scale test work showed that the optimal conditions for this process were using 6 g/L of CaO at 90 C for 90 min. This resulted in 92% of the Si being removed with as little as 7% coprecipitation of Al. The other desilicating agents, namely Ca(OH)2 and MgO, also proved effective in removing Si but at slower rates and higher amounts of Al co-precipitated. The characteristics of solid residue obtained after the process indicated that the desilication is via the formation of hydrogarnet, Grossular, and hydrotalcite dominant phases for CaO, Ca(OH)2 and MgO agents, respectivel