Removal of iron from kaolin and quartz: dissolution with organic acids

Abstract

Several minerals are of commercial interest due to their high level of purity. These include quartz sand which is used for glass manufacture and kaolin which is used as a coating material. However, these minerals contain traces of iron, usually as oxides, which cause discolouration and affect their commercial value. Mineral acids can be used to remove the iron but these are expensive, not always effective and produce effluents which require treatment for the removal of the iron. The objective of the work was to investigate the use of organic acids as a cheaper and more environmentally acceptable alternative.

The initial work involved the characterisation of the samples followed by leaching with both chemically and microbially derived organic acids. For the preparation of microbially produced organic acids, the micro-organism Aspergillus niger was used, with molasses as the carbon source. The chemically derived acids investigated were citric, oxalic and ascorbic.

The best results for both quartz and kaolin have been achieved with a mixture of citric and oxalic acids and at a ratio of 2: 1 and temperature of 90°C, the iron removal was 37% and 40% respectively. This has reduced the residual iron contents to an acceptable level.

After leaching with organic acids the effluents contain ferrous iron, sulphate and residual oxalates/citrates. Testwork on a synthetic solution of these constituents has been carried out using an upflow anaerobic sludge blanket (UASB) reactor. Methanogenic bacteria in the sludge decompose the oxalates to C0₂ and CH₄, heterotrophic sulphate reducing bacteria cause the reduction of sulphate to sulphide and the sulphides of iron and other metals are precipitated in the presence of the HS^- anion.