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No description available.

No description available.

A method was developed for measuring the rate of leaching of lignin macromolecules from the fibre walls of unbleached kraft pulp fibres suspended in water. From a comparison of a theoretical analysis with experimental results, it was concluded that the leaching process was governed by a diffusion mechanism. Very low and widely polydisperse diffusion coefficients were observed for this system. The molecular weights of the leached lignins were found to increase with wash time. A comparison of the molecular dimensions with pore size indicated that a flat disk-like conformation, rather than a spherical conformation was appropriate for the lignin macromolecule. From 20(DEGREES) C to 70(DEGREES)C the change in the intrafibre diffusion coefficient with temperature varied directly as the ratio of the absolute temperature to the viscosity of water. Above 70(DEGREES)C there was a much larger dependence of the diffusion coefficient on temperature. The rate of leaching was also affected by the presence of electrolytes in the wash liquid; leaching was found to decrease with an increase in the ionic strength of the electrolyte. A conceptual model for this leaching effect is presented. The model offers a qualitative explanation for the experimental results noted above.

Enhancement of the magnetic susceptibility of pyrite particles is observed following a mild oxidative pressure leach. This increase is studied for a range of experimental conditions. The increase in susceptibility is attributed to the formation of a thin layer of ferromagnetic oxides. This layer of leach reaction products has been extensively studied using various analytical techniques. The species identified are (gamma)-Fe(,2)O(,3) and Fe(,3)O(,4), which supports a previously postulated reaction model. / This leach treatment significantly increases the magnetic susceptibility of coal-pyrite permitting improved high-gradient magnetic desulfurization of a high pyritic sulfur coal. / The leach treatment also enhances the magnetic susceptibility of other iron-containing mineral sulfides. However, application of this leach pretreatment in sulfide/sulfide separations is limited due to galvanic coupling effects which passivate the susceptibility increase reaction. In the presence of oxides, coal and probably other non-sulfides passivation does not occur and the susceptibility of the iron-bearing sulfide increases. The potential exists for the application of the leach as a pretreatment in enhancing fine iron-containing sulfide/non-sulfide magnetic separations.

This study was carried out to investigate the flotation and leaching behaviour of bornite and chalcopyrite when they are together as principal sulphide minerals. Bench scale flotation performance was examined both in the absence and in the presence of a collector, PEX. In the absence of a collector, chalcopyrite floated significantly better than bornite. In the presence of a collector there was a reversal in selectivity, with bornite recovery presiding over that of chalcopyrite. Chalcopyrite recovery was marginally improved from the collectorless flotation case while bornite experienced vastly improved recoveries in the presence of a collector to surpass the recovery of chalcopyrite. The gap between floatability of bornite and chalcopyrite became more prominent under oxygen-deficient conditions induced by nitrogen flotation, which significantly decreased flotation rate of both minerals, but the floatability of bornite was still much higher than that of chalcopyrite. It is suggested that galvanic interactions promote the interaction of bornite with a collector while retarding interactions of chalcopyrite with the collector. Collectorless flotation of chalcopyrite observed under oxidizing conditions using air is thought to be prevalent even in the presence of a collector obscuring the selectivity of the process. Leaching of the mixtures of bornite and chalcopyrite at mass ratios of 1:1, 1:3 and 3:1 are performed under various conditions to investigate the possible changes in leaching behaviour in relation to galvanic interactions. Leaching at 90°C in the presence of 0.1 M ferric ions in 0.1 M sulphuric acid provided the best results with 90% copper extraction from the mixtures in 6 hours. Observed copper extractions suggested increased copper extraction from chalcopyrite with this approach. The introduction of cupric and ferrous ions into solution, together with variation of solution potential made it difficult to assess in details on the effects of galvanic interactions due to these parameters being known to improve copper extraction from chalcopyrite. Galvanic interactions together with the presence of cupric ions were held mainly responsible for the increased copper extraction. / Thesis (Master, Mining Engineering) -- Queen's University, 2014-01-11 00:25:35.365

Thesis (PhD)--University of South Australia, 2008.

Thesis (PhD)--University of South Australia, 2008.

Thesis (PhDApSc(MineralsandMaterials)--University of South Australia, 2002.

A fundamental study was undertaken in order to establish the mechanisms of the leaching of pre-reduced nickel laterites in ammonia-ammonium carbonate solution. Although the process has undergone various improvements since it was first introduced, the total recoveries are still relatively low, averaging at 80% for nickel and 45-50% for cobalt. The lack of sound fundamental information regarding the mechanisms and the kinetics of the dissolution of nickel and cobalt from iron-alloy grains produced by the reduction roasting of lateritic ores is seen as the main obstacle to establishing the reasons for the lack of adequate recovery and to defining alternative processing strategies which would lead to improved extraction and better process optimisation. Based on fimdamental electrochemical studies and bench scale leaching tests, as well as on-site measurements at an industrial plant where this process is applied, the work presented in the thesis introduces significant new evidence and sheds more light on the understanding of the mechanism of the dissolution process and on the reasons for the low extractions of nickel and cobalt. It has been established that the oxidative dissolution of the iron-alloy grains formed during the reduction roasting takes place primarily via a reaction involving reduction of dissolved cobalt (111) to cobalt (11), and also that the reduction of water is not a significant component of the reactions involved in the dissolution process. Open-circuit potential measurements with various metals and iron-alloys selected to model the reduced material in various solutions which simulated the leaching conditions were also conducted and revealed that passivation takes place during the leaching process. This brings to light a possible new reason for the low recoveries of nickel and cobalt which has not been previously considered. In addition, a kinetic study was conducted, in which the dissolution rates for the various metals, iron-alloys and solutions were measured as a function of time, and the results of which shed more light on the development of the process of passivation. The passivation itself is attributed to formation of iron-oxide on the surface of the dissolving iron-alloy grains, which restricts the transfer of electrons to the reduced metal and brings the oxidative leaching reaction to an end. It has been shown that the passivation can occur in two ways. On the one hand, particularly high concentration of dispersed and dissolved oxygen in the leaching reactors causes passivation by overcoming the limiting current density for the oxidation of iron to divalent iron ions and taking the potential to the region where the direct oxidation to trivalent ions becomes favourable resulting in the formation of an iron-oxide film on the surface. On the other hand, in normally aerated solutions, the passivation of the dissolving ironalloy grains occurs due to the formation of a cobalt and nickel sulphide layer, as a byproduct of the reduction of thiosulphate on the metal surface, which in turn gradually shrinks the area available for the oxidation of iron, again leading to a situation where the oxidising agents present in the solution under standard aerated conditions become sufficient to shift the mixed potential to the region where the iron-oxide forming reaction becomes favourable. The presence of thiosulphate has a decisive role in the second type of passivation. What is more, besides preventing further dissolution of the nickel and cobalt locked in the iron-alloy matrix, the loss of nickel and cobalt by precipitation in the presence of thiosulphate can further reduce the recovery of these metals. For this reason, a method was developed for the oxidation and removal of thiosulphate fkom the actual plant liquor.