Pressure oxidation of refractory gold ores containing carbonate minerals is conducted under neutral/alkaline conditions in order to promote fast kinetics, reduced reagent consumption and suppressing the formation of elemental sulphur and CO2 (which reduces the effectiveness of the process). In this work, both the addition of sodium carbonate and the presence of silica were investigated during the pressure oxidation of pyrite in the presence of calcium carbonate. It was found that the shift to an alkaline leaching environment favours the formation of soluble sulphate products over anhydrite (an industrial scale), but that the increase in kinetics is likely due to an increase in pH and carbonate/bicarbonate concentrations. The presence of silica in the autoclave induces the formation of an in situ iron oxyhydroxide silicate coating and a significant reduction in pyrite oxidation, which was minimized by addition of sodium carbonate.

Pressure oxidation of refractory gold ores containing carbonate minerals is conducted under neutral/alkaline conditions in order to promote fast kinetics, reduced reagent consumption and suppressing the formation of elemental sulphur and CO2 (which reduces the effectiveness of the process). In this work, both the addition of sodium carbonate and the presence of silica were investigated during the pressure oxidation of pyrite in the presence of calcium carbonate. It was found that the shift to an alkaline leaching environment favours the formation of soluble sulphate products over anhydrite (an industrial scale), but that the increase in kinetics is likely due to an increase in pH and carbonate/bicarbonate concentrations. The presence of silica in the autoclave induces the formation of an in situ iron oxyhydroxide silicate coating and a significant reduction in pyrite oxidation, which was minimized by addition of sodium carbonate.

In the past few decades, several aqueous oxidants have been employed (e.g., permanganate, persulfate) to remediate petroleum hydrocarbons. However, the majority of the research in this field has been focused primarily on the use of oxidants in treating fresh water at low groundwater temperature. In this study, bench experiments were carried out to investigate the effectiveness of persulfate (PS) as an oxidant to remediate petroleum hydrocarbons in alternative settings (saline environments at high groundwater temperature). Benzene, Toluene, Ethylbenzene, Xylenes (BTEX), Trimethylbenzenes (TMBs), and Naphthalene were the target organic compounds investigated. Three important aspects were examined during this laboratory study: 1) the evaluation of (alkaline activated and non-activated) persulfate as a chemical oxidation agent; 2) the investigation of the effect of different temperatures (10°C versus 30°C); and 3) the examination of the effect of different persulfate concentration (20 versus 100 g/L) on the reactivity of persulfate. The results showed the high potential of persulfate to remediate the target contaminants under certain conditions. In general, alkaline-activated persulfate showed a higher potential than the non-activated persulfate. However, precipitations of calcium hydroxide were observed due to the reaction between sodium hydroxide and the high concentration of calcium which will limit the use of alkaline-activated persulfate in this particular groundwater setting The results also showed that the initial concentration of persulfate and the system temperature can play important roles in enhancing the effectiveness of PS to oxidize the target contaminants. For instance, the oxidation rate of the target contaminants was seen to be dramatically increased by increasing the persulfate addition from 20 to 100 g/L as well as with increasing the system temperature from 10°C to 30°C. However, increasing both factors (temperature and concentration) accelerated the decomposition rate of PS. Lowering the system pH was tremendously successful in order to enhance the oxidation rate of all compounds. Moreover, the expected effect of the radicals scavenging at acidic pH by Cl- and Br – ,which was reported in the literatures (e.g., Pignatello et al., 2006; Grebel et al., 2010; Suri et al., 2010), was not observed in this study which might be attributed to the contribution of the produced halogen radicals to the contaminant oxidation.

No description available.

Thesis (PhD) -- University of South Australia, 1998

The internal oxidation behaviour of Ni-base and Fe-base alloys containing approximately 5 at% Al and both with and without low concentration Cr additions in flowing low-oxygen atmospheres at 1273 K was studied. There were two groups of Febase alloys; ferritic alloys that were Fe-Al-Cr and others that also contained approximately 9.3 at% Ni in order to make them austenitic. Ni?base alloys were oxidised in oxygen partial pressures of either 4.6 ?? 10-11 atm or 9.8 ?? 10-13 atm and Febase alloys were oxidised in an oxygen partial pressure of 1.2 ?? 10-16 atm The aim of this investigation was to examine the effect of internal oxidation on Fe- and Ni-base alloys containing Al or Al with Cr. The morphology of the precipitates formed and rates of reaction were of interest. Oxidation of the ferritic Fe-base alloys produced internal oxidation only at lower solute concentrations. In these alloys steady state diffusion-controlled precipitation was prevented from occurring due to the formation of an oxide barrier at the reaction front, and cracking off of the internal oxidation zone. In all of the austenitic alloys (Ni-base and y-Fe-base) internal oxidation was observed after all exposures. In y-Fe-base alloys and in Ni-base alloys oxidised at the higher oxygen partial pressure (4.6 ?? 10-11 atm) precipitation zones were found to widen according to parabolic kinetics, indicating diffusion control. In Ni-base alloys oxidised at 9.8 ?? 10-13 atm, precipitation zones were observed to widen according to parabolic kinetics up to 40.9 hours. However, the rate slowed for longer reaction times due to coalescence of precipitates at the reaction front. The rate of internal oxidation decreased with increasing Cr, and thus total solute, concentration. The parabolic rate constants measured for internal oxidation were higher than predicted by Wagner's theory of internal oxidation, which is consistent with observations in previous studies. Kinetics were accelerated by the presence of elongated precipitates, aligned approximately normal to the alloy surface. Chromium alloy additions led to precipitate coarsening, and at lower oxygen partial pressures, to loss of elongated morphology. The precipitates formed were found to be a mixture of M2O3 and AM2O4, where M represents either Al or Cr, and A represents either Fe or Ni. Both oxide forms were detected at all depths within the internal oxidation zone. However, Cr-containing oxides were limited to the part of the internal oxidation zone closer to the alloy surface, while Al-containing oxides were present at all depths. This is consistent with thermodynamic predictions.

The oxidation of refractory gold concentrates containing arsenopyrite and pyrite and the simultaneous dissolution of gold in aerated alkaline solutions at ambient temperatures and pressures without the addition of cyanide has been studied. It involves the following aspects: the chemistry of the oxidation of pure arsenopyrite and pyrite minerals in aerated alkaline solutions; the kinetics of oxidation of arsenopyrite and the simultaneous dissolution of gold in such solutions; the kinetics of simultaneous dissolution of gold during the alkaline oxidation of refractory gold concentrates; the electrochemistry of gold in alkaline solutions containing thiosulfate or monothioarsenate; the effect of copper on the leaching of gold in alkaline thiosulfate solutions; and the leaching of gold in alkaline solutions with thioarsenites. The nature and proportions of the products of the oxidation of arsenopyrite in aerated alkaline solutions have been studied using high pressure ion chromatography techniques that have shown that thiosulfate and a new species, monothioarsenate, are the main oxidation products of arsenopyrite apart from arsenate and sulfite. The alkaline oxidation of pyrite primarily yields thiosulfate and sulfite. A kinetic investigation of the oxidation of arsenopyrite with air or oxygen has shown that the initial rate of arsenopyrite oxidation is proportional to the concentration of dissolved oxygen. A reaction mechanism for the oxidation of arsenopyrite has been proposed, which involves an anodic oxidation of the mineral involving hydroxyl ions coupled to a cathodic process for oxygen reduction which is partially controlled by mass transfer of dissolved oxygen to the mineral surface. Detailed studies of the dissolution behaviour of gold in aerated alkaline solutions in the presence of thiosulfate or monothioarsenate by electrochemical and leaching methods have demonstrated that the dissolution rate is very low as compared to that of gold in alkaline cyanide or ammoniacal thiosulfate solutions. It has been found that copper ions catalyze the dissolution of gold in the thiosulfate solutions in the absence of ammonia. The leaching experiments also have shown that gold may dissolve in alkaline thioarsenite solutions, which provides a possible new process option for the leaching of gold. The oxidation of refractory arsenical gold concentrates in aerated alkaline solutions results in the formation of thiosulfate, arsenate and sulfate as well as the dissolution of gold, copper and iron. It appears that the dissolution of gold is due to the complex reactions of gold with thiosulfate ions promoted by the catalytic effect of copper ions. Up to 80% of the gold may be extracted during the oxidation of selected refractory arsenical

Zugl.: Furtwangen, Fachhochsch., Diplomarbeit, 1995

Thesis (Ph. D.)--University of New South Wales, 2004. / Also available online.

Thesis (M.S.)--University of Nevada, Reno, 2005. / "August 2005." Includes bibliographical references (leaves 81-82). Online version available on the World Wide Web.