Extraction of alumina from coal fly ash generated from a selected low rank bituminous South African coal

Abstract

Vast quantities of coal ash, originating from the steam generation coal-based process have accumulated over the years. Continuous research is conducted to identify opportunities for the utilization of fly ash. This coal ash typically contains 30% alumina, 1.5% titania, 2.5% hematite, 9.5% lime and 60% silica. Due to the availability of alumina in fly ash and large quantities of alumina imported by South Africa annually, a project was initiated to evaluate the possibility of extracting the alumina from fly ash. The mullite-containing fly ash was mixed with calcium oxide and subsequently calcined at a temperature ranging from 1000 to 1200 °C to yield acid or base soluble calcium aluminate. The calcined ash was leached with a sulphuric acid solution to produce a solution containing iron, aluminium and titanium species. An aluminium extraction efficiency of 85% was achieved when the sintered pellets were leached with sulphuric acid using an acid concentration of 6.12 mol dm −3 at 80 °C for 4 h. The leached residue, from the sulphuric acid leaching, could be considered as a co-product in this process and could possibly be suitable for use as a lightweight aggregate in masonry concrete applications or cement production. Purification methods such as precipitation, solvent extraction and crystallization were evaluated to selectively separate both iron and titanium ions from the aluminium containing aqueous solution. The solvent extraction method was found to be superior for the removal of iron and titanium from the aluminium-containing aqueous solution in comparison with all of the purification methods evaluated in this study. The major final product obtained with the solvent extraction method contained approximately 99.4% alumina, whilst the minor product contained 97% titanium oxide. The alumina product can be used for the production of aluminium chemicals and in refractories, while the titanium oxide product can possibly be used as a white pigment in the production of paints.