Neutralisation of arsenic bearing BIOX ® liquors

Abstract

Neutralisation studies have been carried out on arsenic bearing liquors arising from the bacterial oxidation of two Australian pyrite/arsenopyrite concentrates, Harbour Lights and Wiluna. The continuous two-state neutralisation process, employing pure limestone to a final pH of 5 in Stage 1, and pure lime to a final pH of 8 – 11 in Stage 2, produced neutralisation liquors containing ≤0.04 ppm arsenic. Characterisation of the neutralisation residues, by means of chemical analyses and infra-red spectroscopy, indicated that the arsenic was generally precipitated in the form of basic ferric arsenate compounds, FeAsO 4.xFe(OH) 3. Co-precipitation of amorphous calcium arsenate, Ca 3(AsO 4) 2, only occured at a relatively high neutralisation pH (∼10), and then only in conjunction with low Fe/As mole ratios ( ≤ 3 1 ). Stability tests were carried out on the neutralisation residues using both the standard toxicity characteristic leaching procedure (TCLP) of the US Environmental Protection Agency and a modified stability procedure, which allowed for stability testing to be carried out over a wider pH range (3 – 10). The arsenic values in the TCLP extracts were all well within the EPA legistative limit of 5 ppm, indicating that the neutralisation residues are sufficiently stable for disposal on a slimes dam.