Ammonium sulphate and/or ammonium bisulphate as extracting agents for the recovery of aluminium from ultrafine coal fly ash

We recently showed that the selective extraction of aluminium from the amorphous phase of a South African ultrafine coal fly ash can be achieved via thermochemical treatment with ammonium sulphate for 1 h followed by aqueous dissolution, as an alternative to conventional hydrometallurgical processes. In this study, insight gained from the previous work was applied to investigate and compare total vs selective aluminium extraction efficiencies using ammonium sulphate or ammonium bisulphate either on its own, or as a mixture of the two salts as extracting agents during a 2 h thermal treatment process. The effects of (i) ash-to-extractant mass ratio and (ii) temperature during thermal treatment on extraction efficiency was investigated. While a maximum, but non-selective, recovery of 46.6% total aluminium was achieved using ammonium bisulphate at 400 °C, the most technically appropriate results for selective recovery yielded 37.3% aluminium, with only 0.3% silicon, 0.1% titanium and 3.9% iron having been co-extracted when using ammonium sulphate at a processing temperature of 600°C. Extraction of most of the calcium and magnesium could not be prevented. Using mixtures of ammonium salts as extracting agents during thermochemical treatment may however introduce technical difficulties on large scale. Our results indicate that any of the two ammonium salts could be used on their own during thermochemical treatment. Thermochemical treatment of coal fly ash using ammonium salts may therefore represent a promising technology for extracting aluminium from South African coal fly ash.