Molecular orbital calculations on 18 hydrosulfide molecules containing selected main group X-cations yield minimum energy bond lengths, R_t(XS), that reproduce those observed in chemically similar sulfide crystals. A least-squares analysis shows that R_t(XS) can be estimated by the equation R = l.83(s/r)^-0.21, where s is the Pauling bond strength and r is the row number of the X-cation in the periodic table, with 98% of the variation of R_t(XS) being explained in terms of a linear dependence on R. In addition, R serves to rank observed XS bond lengths in sulfide crystals for main group X-cations for rows 1 through 5 of the periodic table to within 0.12Å on average, with R accounting for 96% of the variation in the observed bond lengths. Bonded radii obtained from electron density maps calculated for the molecules show that the radii of both the X-cations and S atom increase with R_t(XS). A similar trend has been found to hold for the bonded radii and the R_t(XO) bond lengths calculated for hydroxyacid molecules (Finger and Gibbs 1985). The radius of S is smaller (1.16Å) when bonded to highly electronegative atoms like 4-coordinate As and larger (1.67Å) when bonded to a considerably less electronegative atoms like 4-coordinate Li but is smaller than Shannon's (1.70Å) crystal radius and Pauling's ionic crystal radius (1.84Å). / M.S.

The interactions of O-isopropyl-N-ethylthionocarbamate (IPETC) and O-isobutyl-N-ethoxycarbonylthionocarbamate (IBECTC) with Cu₂S, CuFeS₂ and FeS₂ have been characterized using thermodynamic calculations, electrochemistry, microflotation tests, contact angle measurements, FTIR, and UV spectroscopy. Pearson’s theory of hard and soft acids and bases (HSAB) has also been applied to these flotation systems, through the use of Drago’s acid-base concept and flow microcalorimetry. The results of the thermodynamic calculations and electrochemical measurements suggest that IPETC and IBECTC adsorption on copper and chalcocite are dependent on potential. This has been verified by contact angle and in-situ spectroelectrochemical measurements. Microflotation tests with these thionocarbamates show that the floatability of Cu₂S and CuFeS₂ is dependent on pH. The floatability of FeS₂ only becomes significant at acidic conditions and high collector additions. Spectroscopic measurements also show that thionocarbamate adsorption is dependent on pH and is most favored on Cu₂S followed by CuFeS₂ and FeS₂. FTIR results indicate that IPETC is adsorbed on Cu°, Cu₂S, and CuFeS₂ through a coordination of the sulfur atom with the surface Cu. IBECTC adsorption on these substrates involves the coordination of Cu with both sulfur and oxygen atoms to form a six-membered chelate ring. Adsorption of these collectors cannot remove or prevent the formation of sulfoxy oxidation products on the FeS₂ surface, unlike the case with the xanthate-pyrite system. This may explain the improved selectivity of IPETC and IBECTC over xanthates. Infrared reflection-absorption spectroscopic studies show that KEX is preferentially adsorbed on Cu° over IPETC and IBECTC. Between IPETC and IBECTC, the latter is more favorably adsorbed than the former. Kinetic studies using UV spectroscopy show that the rate of thionocarbamate adsorption is highest on Cu₂S followed by CuFeS₂ and FeS₂. IBECTC adsorption on each sulfide mineral is relatively faster than IPETC, indicating the higher collecting power of IBECTC. The HSAB concept suggests that the interaction of thionocarbamates (soft bases) with sulfide minerals that are classified as soft acids should be favored. The C/E ratios of Cu₂S and FeS₂ were determined to be 0.86 and 0.52, respectively. This indicates that FeS₂ is a harder acid than Cu₂S, thus providing an explanation for the observed selectivity of both IPETC and IBECTC against FeS₂. / Ph. D.

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Thesis (PhD (Process Engineeering))--University of Stellenbosch, 1995. / The hydrometallurgical leaching of sulphide concentrates was introduced in the 1950's. Generally the leaching mechanisms of these processes are not understood fundamentally. The reasons for this are the inherently complex nature of sulphide chemistry and that the sulphide concentrates usually consist of highly intergrown sulphide minerals. The leaching kinetics of sulphide concentrates where only one metal-sulphide mineral occurs have been investigated intensively, but not for sulphide concentrates with more than one metal-sulphide mineral. The behaviour of these mixed metal-sulphide minerals has mostly been investigated on plant scale to qualitatively determine the leaching trends of the process. The consequence of the relatively unKnown leaching mechanism and kinetics is that these processes are not controlled efficiently. This study was conducted on the acid-oxygen pressure leaching of Ni-Cu matte (the first stage leach process at the Ni-Cu refinery of Impala Platinum Ltd.). As a first step to improve the control efficiency of the process, the process must be stabilised. Therefore, an off line computer simulation program is proposed to control the repulping section of the plant that has previously been controlled solely by an operator. Controlling the repulping section is very important, because conditions exist in the repulping tanks for leaching to occur. This causes perturbations in the pulp entering the pressure leach autoclave. Due to the fast reaction kinetics of the matte in the pressure leach autoclave the perturbations entering the autoclave will influence the performance of the acid-oxygen pressure leach process. The simulation program was tested on the plant and indicated that considerable improvement in the stability of the operation could be achieved. In obtaining a better understanding of the behaviour of this process, it is essential that key variables and trends are identified. A methodology is proposed to analyse and model this ill-defined and poorly understood process from historical data by v artificial neural networks (ANN), inductive learning by decision trees and statistical techniques. The back propagation neural network, learning vector quantization neural network and the decision trees yielded comparable classification rates between 73% and 84%, and could serve as a basis for the adjustment of operating conditions to improve the efficiency of the process. The relative importance of the process variables is determined by a method of sensitivity analysis and together with the statistical mean, the effect of an increase or decrease in the variable on the process is quantified. These results are substantiated by experimental findings. A leaching mechanism for the acid-oxygen pressure leach of Ni-Cu matte is postulated. The leaching sequence of the nickel and copper sulphides is Ni3Sr Ni7S" NiS-Ni3S4, and CU2S-CU31SWCU1.BS-CUS, respectively. Ni7Sa and CU31 S1a are intermediate nickel and copper sulphide phases that form during the leaching process. Ni alloy has a galvanic effect on the sulphide minerals which inhibits the overall leaching rate and results in the formation of H2S and the intermediate nickel and copper sulphides (Ni7Sa and CU31 S1a). A semi-empirical kinetic model was developed based on the chemical reaction rate expressions of the leaching mechanism. This model can accurately simulate the batch leaching process for variations in the oxygen partial pressure, oxygen flowrate, temperature, particle size, initial acid concentration and pulp density. A sensitivity analysis on the model indicated that for a matte with a lower initial Ni alloy content the leaching rate of nickel is much faster.

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The discovery of Ni-, Cu-, and Zn-bearing Fe-sulfides from comet 81P/Wild 2 represents the strongest evidence, in the Stardust collection, of grains that formed in an aqueous environment. Crystalline sulfide assemblages from Wild 2 and the hydrothermally altered CI chondrite Orgueil were investigated. Structural and compositional characterizations of the sulfide grains from both collections reveal similarities. The Stardust samples include a cubanite (CuFe₂S₃) grain, a pyrrhotite [(Fe,Ni)₁₋ₓS]/pentlandite [(Fe,Ni)₉S₈] assemblage, and a pyrrhotite/sphalerite [(Fe,Zn)S] assemblage. Similarly, the CI-chondrite sulfides include individual cubanite and pyrrhotite grains, cubanite/pyrrhotite assemblages, pyrrhotite/pentlandite assemblages, as well as possible sphalerite inclusions within pyrrhotite grains. The cubanite is the low temperature orthorhombic form, which constrains temperature to a maximum of 210°C. The Stardust and Orgueil pyrrhotites are the 4C monoclinic polytype, which is not stable above ~250°C. The combinations of cubanite and pyrrhotite, as well as pyrrhotite and pentlandite, signify even lower temperatures. The crystal structures, compositions, and petrographic relationships of these sulfides constrain formation and alteration conditions. Taken together, these constraints attest to low-temperature hydrothermal processing. The hydrothermal conditions under which cubanite forms were investigated through thermodynamic modeling and experimental synthesis. A thermodynamic model for cubanite was developed to constrain its formation environment. Cubanite was synthesized under hydrothermal conditions consistent with those predicted for the CI-chondrite parent body. The similarity between Stardust and CI-chondrite sulfides suggest similar fluid conditions may have existed on the comet at some point in its history. The presence of cubanite in the Stardust collection has implications for large scale issues such as: heat sources in the comet-forming region; aqueous activity on cometary bodies; and the extent and mechanisms of radial mixing of material in the early nebula. The Wild 2 sulfides are most likely the products of low-temperature aqueous alteration and provide evidence of radial mixing of material from the inner solar system to the comet-forming region and possible secondary aqueous processing on the cometary body.

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