Purification of coal fly ash leach liquor for alumina recovery

Rampou, Mohau
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Over the years, much work has been done on the extraction of alumina from coal fly ash (CFA). Previous works were motivated by the results of the quantitative analysis of CFA that showed a high content of alumina; comparatively ranking second to the primary bauxite ore. Thereafter, research studies have been dedicated to finding efficient routes of recovering alumina from CFA. Amongst recent studies, a 2-step leach process developed by researchers from the University of Witwatersrand gave promising recovery results. The current work is an extension of the 2-step leaching process, where an attempt is made to purify the leach liquor recovered from this leaching process in order to generate a purer aluminum solution by removing iron and titanium, which are considered as the two major impurities. The aim is to obtain an impurity free solution that can be used to generate smelter grade alumina. In this study, solvent extraction using Primene JMT in kerosene was applied to remove the impurities from the leach solution. Optimum parameters tested during the solvent extraction process included contact time, extractant concentration and the organic to aqueous (O/A) ratios. As expected, most of the titanium was extracted in the organic phase and much of the aluminum remained in the aqueous phase. However, the results showed that most of the iron, which occurred in solution as iron (III) and iron (II), also remained in the aqueous phase. This was an unwanted occurrence, as this residual iron would then co-precipitate with the alumina leading to a contaminated product. Two approaches were then applied for a more effective separation of iron from aluminum. The first approach (Route 1) considered the reduction of the iron (III) species into iron (II) prior to solvent extraction for titanium removal. Alumina crystallization then followed and since iron (II) precipitates at higher pH than aluminum, there was reduced contamination of the alumina product. The second approach (Route 2) firstly considered solvent extraction to remove titanium ions and any iron ions. This was followed by the reduction of residual iron (III) to iron (II). In both approaches, aluminum was crystallized out and iron (II) precipitated using (NH4)2SO4. Titanium (IV) was stripped from the organic phase using NH4OH. The results obtained showed a successful purification of the coal fly ash leach liquor for both processes. Route 1’s disadvantage is that it used more metal iron filings for iron (III) reduction to iron (II) than route 2. Route 2’s disadvantage is that due to the 2-stage stripping applied, it used more excess reagents, therefore implying higher operational costs than route 1. As a result, Route 1 was considered for further tests. The final alumina product generated with the route 1 purification process had a 99.4% Al purity. This purity level meets the smelter grade alumina product specifications. The grade of the final product therefore, indicates that the applied process route was successful.
Thesis is submitted in partial fulfilment for the degree of Master of Science in Engineering to the Faculty of Engineering and the Built Environment, School of Chemical and Metallurgical Engineering at the University of the Witwatersrand, Johannesburg, 2018
Rampou, Mohau (2018) Purification of coal fly ash leach liquor for alumina recovery, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/25724>