Abstract:
South African coal fly ash represents an untapped secondary resource of aluminium. Continuous research is conducted
to develop suitable chemical and/or geometallurgical processes for aluminium extraction, preferably accompanied
by minimal silicon extraction. The thermochemical treatment of a South African ultrafine coal fly ash
was investigated to test the feasibility of recovering aluminium using ammonium sulphate, a widely available,
low-cost, recyclable chemical agent. The optimum processing conditions were determined to be a temperature
of 500 °C and a fly ash to ammonium sulphate weight ratio of 2:6 when a reaction time of 1 h was used. Water
leaching of the reaction product obtained under these conditions resulted in the selective recovery of 95.0% aluminium
from the amorphous phase, with b0.6% Si extracted. Mullite was unlikely to have reacted with the
extracting agent. Except for Si, the process was not element-selective, but the extraction of iron could be minimized
by increasing the treatment temperature to 600 °C without compromising Al extraction. Thermochemical
treatment using ammonium sulphate may therefore represent a promising technology for extracting aluminium
from coal fly ash, which could be subsequently converted to value-added products such as alumina.
