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Enhanced metal recovery from a modified caron leach of mixed nickel-cobalt hydroxide

Jones, Andrew (2013) Enhanced metal recovery from a modified caron leach of mixed nickel-cobalt hydroxide. PhD thesis, Murdoch University.

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In the last 20 years nickel laterites have become a popular resource due to the economic expansions of China and India, an improvement in processing technologies and the large unexploited orebodies around the world. The development of a split process, producing a metal hydroxide intermediate, is becoming popular as it lowers technical risk and capital costs. Following on from Cawse, BHP Billiton have been instrumental in developing this process, and produced a mixed hydroxide precipitate for approximately a year (2008) at Ravensthorpe in Western Australia, which was processed in an ammonia solution at the existing Yabulu refinery in Townsville Queensland. This PhD project focused on the ageing of the precipitate which would occur during transportation, and the subsequent leaching in an ammonia-ammonium carbonate solution with a sulphide (CoNiS) reductant.

Metal ion hydroxides were discovered to precipitate within the pores of magnesium hydroxide (precipitant). This meant that the precipitate particle size was relatively large, oxidation of cobalt and manganese occurred throughout the particles and the dissolution rate followed a shrinking core model. Although cobalt and manganese oxidation was envisaged to be a problem, only ~8% of cobalt and ~52% of manganese oxidised in a Ravensthorpe sample after 12 weeks and was leached in 45 minutes in the presence of a reductant. All oxidation occurred during precipitation, filtration and preparation of the precipitate.

The formation of stable slow leaching nickel-magnesium hydroxide and hydrotalcite-like structures did affect nickel and cobalt recoveries. Reducing the incorporation of magnesium, increasing the manganese concentration and drying the precipitate all reduced the effect of the nickel-magnesium hydroxide. Drying the precipitate could result in a saving in transportation costs, while increasing the manganese concentration would lower reagent costs and energy consumption. Aluminium, chromium(III) and sulphate concentrations needed to be minimised to reduce the effect of hydrotalcitelike structures. Sulphate may need to be precipitated from solution prior to metal hydroxide precipitation.

The reaction mechanism of the reduction of high valent metal ions by mixed cobalt nickel sulphide reductant (CoNiS) produced on-site at Yabulu was investigated. The extent of reduction was directly related to the Co:S ratio, however the presence of NiS was crucial as it had a faster rate of dissolution and introduced sulphur species into solution. The ideal ratio of cobalt to nickel was between 2:1 and 3:1. The site survey of Yabulu revealed the potential of the leach liquors needed to be monitored to ensure cobalt existed in the trivalent state, which is more soluble. HPLC (High Performance Liquid Chromatography) results showed that numerous cobalt ammine species were present in solution. As unwanted cobalt precipitation is a major cause of lower metal recoveries and the final product is influenced by solution chemistry, the results will help improve cobalt recovery and product grade.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Engineering and Information Technology
Supervisor(s): Senanayake, Gamini and Welham, Nicholas
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