The kinetics of the dissolution of zinc in aqueous solutions of ammonia and ethylenediamine under oxygen pressure were investigated at a temperature of 25°C. The variables investigated included free amine concentration, oxygen pressure and stirring rate. Two regions of kinetic control were observed having different dependences on oxygen pressure. At low oxygen pressures, the rate of dissolution of zinc is first order in oxygen pressure, dependent on stirring rate and independent of the free amine concentration. At higher oxygen pressures, an adherent oxide layer is formed on the zinc surface and the rate of dissolution is first order with respect to free amine concentration and independent of oxygen pressure. These results indicate that at low oxygen pressures the rate is controlled by the diffusion of oxygen to the zinc surface; and at high oxygen pressures, the rate of reaction is chemically controlled at the film-solution interface. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Thesis (MTech. degree in Metallurgic Engineering)--Tshwane University of Technology, 2008. / The purpose of this study is to develop a feasible process for extracting gold and silver from an electrolytic zinc refinery residue. Composite samples of electrolytic zinc refining lead-silver residues containing traces of gold (1.35 g/t) and significant amounts of silver (155 g/t) were subjected to batchwise cyanide leaching to assess the feasibility of extracting gold and silver. This project was concluded with a recommendation that a pilot Residue Leach Plant should be commissioned to beneficiate gold and silver from the residue dam for a better estimation of lime, cyanide consumptions and gold and silver recoveries. The return on capital investment will be high at current gold price and operational cost is low. It is also recommended to investigate the economics of cyanide leaching of a pre-concentrate obtained from froth flotation.

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The kinetics of zinc extraction from perchlorate solutions with di(2-ethylhexyl) phosphoric acid in n-heptane have been measured using the rotating diffusion cell technique. The extraction of zinc is controlled by the mass transfer of reactants (Zn²⁺ and D2EHPA) to the interface. At low zinc concentrations, the system is controlled by the aqueous transport of Zn²⁺ to the interface; at higher zinc concentrations transport of D2EHPA becomes rate controlling. For the range of D2EHPA concentrations examined, the transport of D2EHPA is rate controlling. Bulk pH has a negligible effect, except perhaps at the lowest pH values examined, where there may be a slight decrease in extraction rate. This decrease was attributed to less favourable thermodynamics at low interfacial pH values. It appears that the chemical reaction rate is fast enough that it has a negligible effect on the overall extraction rate. A basic mathematical model was developed which is adequate for predicting the extraction rate under variable conditions of zinc concentration, D2EHPA concentration, and pH. The effect of using a partially loaded organic extractant was also investigated, and the system was found to be mass transfer controlled. An extended mathematical model was developed which predicts that the speciation of organic complexed zinc changes with increasing preload, and at high loadings the direction of ZnL₂HL and ZnL₂(HL)₂ flux reverses, with these species providing extractant to the interface. At very high loadings, ZnL₂HL provides almost all the extractant to the interface. Experimental studies of the effect of temperature on the rate of zinc extraction resulted in a calculation of the activation energy which was consistent with a diffusion controlled mechanism. Finally, the effect of different filter pore sizes on extraction was examined. The extraction rate decreases significantly with a very small filter pore size, while there appeared to be little or no effect for larger filter pore sizes. For the filter pore size used in this study, it was therefore concluded that the filter pores do not pose an additional resistance to mass transfer. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This work was undertaken to study the nature of the deformation mechanisms in polycrystalline zinc and cadmium over a temperature range from 77°K to 300°K. It has been observed that the only non basal slip system which is observed under normal light microscopy is that of second order pyramidal [ll22] <ll23>. At temperature above [formula omitted], the amount of non basal slip is greater in zinc than in cadmium. The amount of twinning, substructure formation and grain boundary migration is comparable in both systems. Negative work hardening beyond the U.T.S. at temperatures above Tн = .4 is associated with recrystallization. In both systems at temperatures below Tн = .26 a region of temperature and strain rate independent linear work hardening occurs. The extent of linear hardening increases with decreasing temperature below Tн = .26. Above Tн = .26, polycrystalline hardening in both systems is parabolic from yield on and the rate of hardening at a given value of strain decreases with increasing temperature. Cadmium single crystals showed a similar trend in that below .26 both [formula omitted] remained constant. However above .26 there was a steady decrease in the shear hardening rates. It was observed that the Cottrell-Stokes law is obeyed only in the linear hardening regions of polycrystals and in Stage II hardening of single crystals below .26. When dynamic recovery occurs [formula omitted] increases with increasing strain. It has been observed that below .26 the linear hardening rate in cadmium decreased with increasing grain size ( constant specimen dimensions) so that [formula omitted] The value of [formula omitted] was shown to correspond to the tensile hardening rate during Stage II single crystal deformation. The tensile hardening rate was used because of the extensive twinning found to be associated with Stage II hardening. The grain size dependence of 0 has been interpreted in terms of a grain size dependence of the extent of [ll22] <1123> slip. It was found that during linear hardening in both zinc and cadmium the difference in flow stress at two different temperatures is a reversible difference implying that the dislocation configurations produced with increasing strain do not vary in nature or extent with temperature. Under such conditions it is possible to formulate a mechanical equation of state. Extensive rate theory measurements have been made in both systems in order to attempt an evaluation of the rate controlling mechanisms both during linear hardening and.during dynamic recovery. The former has tentatively been associated with intersection. Dynamic recovery on the other hand has been linked to the loop annealing observations of Price. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the Degree of Master of Science in Engineering. / Laboratory experimentation were conducted to investigate the regeneration and recycling of nitric acid during leaching in froth (LIF) processing of Gamsberg zinc sulphide concentrate. Two experiments were performed in a mechanically stirred batch reactor at 80- 90°C to determine the leaching kinetics of the zinc sulphide concentrate. 93 to 97% of zinc was extracted into spent zinc sulphate electrolyte containing 30 to 40g/1 nitric acid during 40 to 60 minutes of leaching without regeneration of nitric acid. Between 60.72. and 97.79% of the consumed nitric acid was regenerated after 45 minutes of batch L/F experiments carried out at 80·90oC and 50kPa. The results show that the degree of nitric acid regeneration is inversely proportional to the concentration of nitric acid in the leaching solution. Zinc extraction ranged between 56.31 and 78.37% whilst 40.29 to 50.99% of the initial sulphide sulphur was oxidised to elemental sulphur. In the continuous L/F experiments conducted at 80- 90°C and 100·200kPa, the degree of nitric acid regeneration varied from 33.63 to 97.22%. Overall zinc extraction was about 62% whilst 47% of the sulphide sulphur reported as elemental sulphur after 60 minutes of processing. A five-stage L/F processing of the concentrate was carried out in which the flotation phenomenon was used for selective separation of the floatable fraction from the nonfloatable fraction. About 40 to 80% of the consumed nitric acid was regenerated, 88.94% of zinc was extracted whilst 55.65% of the sulphide sulphur reported is elemental sulphur after 77 minutes of processing. The recoveries of zinc and elemental sulphur increased to over 92% and 58% respectively when the leach residue was subsequently leached in the batch reactor. / Andrew Chakane 2019

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A variety of iron precipitates formed in the electrolytic zinc industry were studied to estimate the possibility of magnetic filtration to augment conventional thickening/filtering systems. The precipitates included K,NH(,4),Na jarosites, (alpha) and (beta) goethite, (alpha) hematite and an industrially produced NH(,4) jarosite. The wet Frantz Isodynamic Separator was used to generate magnetic susceptibility data. Susceptibilities ranged from (kappa) = 6.9 x 10('-5) (--->) 15.5 x 10('-5) emu/cm('3)0e. Particle size of the precipitates ranged from 25 (mu)m to < 1 (mu)m. Magnetic filtration breakthrough curves were produced using a high gradient magnetic filtration technique. The role of fluid viscosity and velocity as well as magnetic field was studied. / All the precipitates proved filterable. A comparison was made of a physical and an empirical model of the breakthrough curves in the sizing of the magnetic filters required to treat thickener overflow at the CEZ Valleyfield plant. / For a volumetric flow of 100m('3)/hr electrolyte, containing 15g/L NH(,4) jarosite, 2 magnets of 2.6m diameter and 0.35m bed depth are required to lower solids contents to the present operating level of 3g/L.