NEUTRALIZATION OF ARSENIC BEARING BIOX(R) LIQUORS

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-stage 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 less-than-or-equal-to 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, FeAsO4.xFe(OH)3. Co-precipitation of amorphous calcium arsenate, Ca3(AsO4)2, only occured at a relatively high neutralisation pH (approximately 10), and then only in conjunction with low Fe/As mole ratios (less-than-or-equal-to 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 legislative limit of 5 ppm, indicating that the neutralisation residues are sufficiently stable for disposal on a slimes dam.