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M. Tech. Chemical Engineering / The depreciation of high grade sulphide ores has resulted to the use of low grade laterites to meet the global nickel demand. Available low grade laterite ore reserves in South Africa are underutilised. Suitable metallurgical process for recovery of nickel from these reserves is not yet implemented because the mineralogy is not well understood. Hence, in this research the mineralogy and recovery of nickel from low grade laterites was studied through electrochemical process. The research shows that it is feasible to selectively recover nickel from the low grade laterite ore using electrochemical leaching method. It is recommended that optimization of the electrochemical cell design be looked into to evaluate the possibility of scaling up for industry application.

Falconbridge Dominicana C. Por A. operating in the Dominican Republic produces nickel as an iron-nickel alloy from laterite ore. Material that is rejected from their mining operations due to its incompatible chemistry is currently being stockpiled against mine depletion. Recent interest in recuperating nickel from this reject material led to the examination of the suitability of carbothermic reduction followed by magnetic separation for nickel recovery from this reject ore. Five parameters were considered in a Design of Experiment framework, namely: heating time; temperature; material type, coke addition and oxygen partial pressure. Magnetic separation was conducted with a Davis Tube and a low intensity hand magnet. / It was found that this reject material can successfully be treated using a carbothermic reduction process to produce 50 to 75 mum diameter metallic components within a gangue matrix. The recovery of nickel ranged from 61.7% to 21.9% to 16.4% in the high, low and non magnetic fractions, respectively, separated from the reduced material. The grade of nickel ranged from 1.97 wt% Ni to 1.58% to 0.75 wt% Ni in the high, low and non magnetic fractions, respectively. / Heating temperature had the greatest positive influence on the performance of the process followed by material type and heating time. Oxygen activity and coke addition were seen not to influence the performance. The Boudouard reaction was considered to be the rate controlling step and a maximum possible operating temperature was considered to depend on the physical properties of the material, specifically the softening temperature.

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D.Tech. (Extraction Metallurgy) / Please refer to full text to view abstract

M.Tech (Metallurgy) / Nickel in South Africa is scarce. Other than as a by-product from platinum mining, its only source is the Uitkomst Complex, a satelite of the Bushveld Igneous Complex, located between Machadodorp and Barberton and mined by Nkomati Nickel. Pentlandite occurs as the main nickel-bearing mineral, disseminated within a sulfide matrix of pyrrhotite, pyrite and chalcopyrite. Accessories are of chromite and platinum in solid solution with the sulfides. The sulfides are hosted in mainly ultramafic rocks. The presence of talc is of particular nuisance. Head grade is in the decline, from earlier 0.7 to more recently 0.3% or even lower. Standard froth flotation yields a concentrate of 7 – 9% nickel at a recovery of up to 70%. A higher nickel recovery from such low – grade feed stock would be desirable. To achieve these two methods were investigated and reported in the following:  Non-oxidative leaching as an alternative to flotation, using strong hydrochloric and sulfuric acid and also aqueous ammonia solutions at various liquid-to-solid ratios. Only the degree of nickel dissolution over time at room temperature was studied and no attempt was made to recover the dissolved metal from solution.  Froth flotation after pre-treatment with microwaves at various power levels and over various periods of time of irradiation. It was surmised that a certain amount of inter-granular cracking could thus be achieved that would improve the flotation behavior in respect of grade and recovery. An extensive literature study, in particular, of the nature of microwaves, their interaction with matter and possible metallurgical benefits derived as a consequence of such interaction, forms part of the investigation and is reported in detail. A lot of theoretical and experimental work has been done in this respect, although the obtained results were not conclusive.

M.Tech. (Chemical Engineering) / Please refer to full text to view abstract

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The aim of this investigation was to extend the knowledge of the bacterial leaching of copper and zinc sulphides into the area of nickel sulphide leaching. By far the major portion of both theoretical and practical expertise which is available in the field of bacterial leaching is based on the treatment of copper and zinc sulphides. As yet there is little information available on the bacterial leaching of nickel sulphides to meet the growing interest in this field both in South Africa and elsewhere. To a large degree, it was necessary to start from basic principles in this novel extension of bacterial leaching technology so that the work covers a fairly wide field in general rather than one particular aspect in detail. A strain of 'nickel adapted' Thiobacillus ferrooxidans was isolated from the tailings dam of a disused nickel mine. The growth characteristics of this strain were studied in some detail on sulphur using both batch and continuous techniques. This was done as it was considered that growth on sulphur would provide useful information which could be correlated with the mineral leaching results. The mineral pentlandite (NiFeS2) was chosen as the one with which to work because of its economic importance. This was prepared in a highly purified form from a concentrate of the Rhodesian Shangani deposit. Bacterial leaching tests in both batch and continuous operation were then carried out in order to define the effects of various physico-chemical parameters on the leaching of nickel from this mineral. As a preliminary to these tests, a detailed chemical kinetic study in the absence of bacteria of the leaching of nickel was carried out using similar physico-chemical conditions. The results of the bacterial and chemical leaching tests were then compared and used to postulate a mechanism and model for the process. It was found that the rate of leaching of nickel from pentlandite in acid ferric sulphate solutions was directly proportional to the concentration of ferric ions and speed of agitation of the stirrer and to the square root of the oxygen concentration. The form of the rate expression was interpreted in terms of a mixed diffusive and chemical rate controlling mechanism. Bacterial growth rates on flowers of sulphur were found to be controlled by the rate of dissolution of oxygen from the gas bubbles into the bulk solution. When this latter condition was made non-rate limiting, it was found that growth rates were still dependent on the rate of agitation, implying mass transport control by another mechanism. The batch bacterial leaching results showed a linear pattern of nickel leaching and bacterial growth, with a marked dependence on oxygen concentration and rate of agitation. A mechanism in accordance with the batch data was postulated, which proposed that the rate of bacterial leaching was proportional to the concentration of bacteria attached at the mineral surface and to the square of the oxygen concentration. The rates of bacterial leaching were computed by taking the difference between the overall measured leach rate and the chemical leach rate based on the chemical kinetic data. The leach rates in continuous bacterial leaching were higher than those predicted from the batch data. This effect was interpreted in terms of higher specific growth rates being achieved in continuous operation. An economic assessment was made of the process based on the optimum leach rates obtained in continuous leaching and found to show some promise. / Thesis (Ph.D.)-University of Natal, Durban, 1974.