| Summary: | Leaching of a limonitic laterite (1.49% Ni, 0.33 % Co, 2.29% Mn, 51.8% Fe) using ferrous sulphate as reductant was studied in ammoniacal solutions. Studied parameters included temperature, agitation, ferrous sulphate concentration, solids content, ammonia concentration and a comparison between ammonium sulphate and chloride. Tests were performed in a batch cell with temperatures between 20 and 80ºC at atmospheric pressure. Leaching kinetics for nickel were different to cobalt and manganese. Nickel was favoured by temperature in ammonium sulphate reaching 66% extraction in 3 hours, whereas in ammonium chloride it showed slower but steady kinetics with an extraction of 75% in 24 hours. Cobalt extraction in ammonium sulphate was low at high temperature and solids content; however, at low solids content (4% w/w) and high temperature (80°C) cobalt extraction reached 80% in 4 hours. With 11% solids at the same temperature, extraction decreased to 20%. In presence of ammonium chloride, temperature had a positive impact on cobalt reaching 80% in 8 hours at 23% w/w solids. Cobalt appeared to re-precipitate in presence of ammonium sulphate. Manganese behaved similarly to cobalt, however, in presence of ammonium sulphate, it only reached 60% extraction after 8 hours at 80°C at low solids. In presence of ammonium chloride, manganese reached 65% in 3 hours while increasing solids was not detrimental. Manganese suffered adsorption and/or co-precipitation in ammonium sulphate and chloride. This decrease in extraction exhibited by cobalt and manganese was attributed to co-precipitation and/or adsorption in iron oxides/hydroxides. Nickel extraction appeared aided by ferrous sulphate. Cobalt and manganese improved in presence of higher ammonia/ammonium, agreeing with the observation that cobalt and manganese are more prone to co-precipitation/adsorption; for which they prefer lower ferrous sulphate and higher ammonia/ammonium in order to stabilize their ammines. Feed and solid residue analyses using chemical assays, x-ray diffraction, scanning electron microscopy and mineralogy, provided better understanding of limonite reduction. The main phase in the residue was magnetite. Differences in behaviour between nickel and cobalt/manganese suggest a two-stage process, and chloride solutions seem promising due to better stability of cobalt and manganese. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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