CHARACTERISATION OF RANGER MINE WATER TREATMENT SLUDGE

摘要

Process water from ERA's Ranger Uranium Mine (Northern Territory, Australia) requires treatment to meet stringent environmental water quality criteria prior to discharge into the environment. Prior to treatment, the process water is acidic and contains high concentrations of sulfates, aluminium, magnesium and manganese in addition to residual uranium. One concept that has been considered for a process water treatment plant would use milk of lime and reverse osmosis to treat 1.5GL per annum to discharge 1GL of high purity water, with a brine balance and 85 000 tonnes of sludge by-product, the latter requiring stable long-term storage. The low pH of the process water currently stored in a tailings dam precludes this as a storage option unless re-leaching can be prevented. The water treatment sludge obtained during trials was characterized via XRD, XRF, TGA and laser sizing to identify both the mineralogy and morphology of the material with the aim of understanding its properties and possible interactions with tailings and hence possible options for its secure disposal. A variety of approaches to either modify the treatment process or the sludge to obtain a more stable material for long-term storage were explored. One promising treatment used a novel precipitation via sodium aluminate (e.g. spent aluminate liquor from Bayer process alumina refining) to form hydrotalcites, layered double hydroxide minerals of the general form Mg_6Al_2(CO_3)(OH)_(16)-n(H_2O). This method utilises both the aluminium and magnesium present in the original process water and robustly allows for variations in the divalent to trivalent cation ratio from 2:1 to 3:1. Moreover, the formation of hydrotalcites in mine process water has a number of advantages over conventional treatment techniques such as rapid formation, easy dewatering, increasing stability above pH 4-5 and co-precipitation/incorporation of oxyanions other than carbonate.