The leaching of covellite (CuS), chalcocite (Cu₂S), chalcopyrite (CuFeS₂) and bornite (Cu₅FeS₄) was carried out in a shaking autoclave in perchloric acid solutions and using moderate pressures of oxygen. The temperature range of investigation was 105-140°C. It was found that covellite, chalcocite and bornite leach at approximately similar rates, with chalcopyrite being an order of magnitude slower. It was found that chalcocite leaching can be divided into two stages, first, the rapid transformation to covellite with an activation energy 1.8 Kcal/mole, followed by a slower oxidation stage identified as covellite-dissolution with an activation energy of 11.4 Kcal/ mole. These two stages of leaching were also observed in bornite with chalcocite (or digenite) appearing as an intermediate step. No such, transformations were observed in covellite or chalcopyrite. Two separate reactions were recognized as occuring simultaneously for all four minerals during the oxidation process: an electrochemical reaction yielding elemental sulphur and creating pits on the mineral surface, and a chemical reaction producing sulphate. The first reaction dominates in strongly acidic conditions, being responsible for about. 85% of the sulphur released from the mineral, but the sulphate to elemental sulphur ratio in solution increases with decreasing acidity. Above 120°C the general oxidation process is inhibited by molten sulphur coating the mineral particles; the sulphate producing reaction, however, is not reduced above this temperature. For chalcopyrite the activation energies for the sulphate producing reaction and mineral solubilized were found to be 16.0 Kcal/mole and 11.0 Kcal/mole, respectively. It is suggested that chalcopyrite may leach partially via the .formation of transient covellite on the surface but since its leaching rate is faster than that for chalcopyrite, no covellite is detected after leaching. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The direct dissolution of various samples of hematite and goethite in hydrochloric, sulphuric and perchloric acids using a particulate (-65, + 150 mesh) feed has been investigated. The reductive dissolution of hematite samples using acidified sulphur dioxide solutions was also investigated. Goethite and hematite leached by a common mechanism with comparable rates. The various shapes of leaching curves obtained for different hematite samples can be correlated with their grain size to particle size ratio. An increasing rate of dissolution was found for a large number of grains per particle, and an approximately constant rate was observed for particles containing relatively few grains per particle. The effect of acid concentration on the rate of dissolution for different acids has led to the modification of a previously proposed chemical mechanism dependent on the complexing power of the anion of the acid with iron. The increase in dissolution rate for a weak complexer was proportional to the acid concentration e.g. HCIO₄, and for a strong complexer, if was proportional to the acid concentration squared e.g. HCl. The rate of dissolution in the presence of sulphur dioxide was extremely rapid compared with the acid dissolution. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Leaching of chalcopyrite by Thiobacillus ferrooxidans under standard microbiological leaching conditions resulted in simultaneous solubilization of copper, iron and sulfur. The sulfide portion was oxidized preferentially over iron in solution suggesting a direct attack mechanism by the bacteria on the mineral particles. Copper extractions were low, 29-44%, with maximum specific copper extraction rates of the order of 0.001-0.006 h⁻¹ and cell yields per unit of copper released 4-43mg TOC/mg Cu. Leaching under redox-controlled conditions required a minimum pulp density, ca. 200 g/1, to result in elemental sulfur production. Some unknown factor, resulting from biological leaching activity under standard conditions and transferred with the liquid phase of the inoculum, was needed for the leach to occur under redox-controlled conditions. Copper extractions of 44-100% were achieved. Maximum specific copper extraction rates were of the order of 0.002-0.007 h⁻¹ with cell yield per unit of copper released of the order of 0.12-3.32mg TOC/g Cu for batch cultures. Ferrous iron in solution appeared to be the energy source for cell growth under redox-controlled conditions. The cells' sulfide oxidizing capacity seemed to be inhibited at the metabolic regulation level and not at the enzyme synthesis level. Cells growing under the standard conditions underwent a lag phase, upon transfer to the redox-controlled medium. During this lag phase low metal solubilization rates and low S⁰ production occurred, but when cell growth started, the leaching rates increased and the mineral dissolved rapidly. Electron diffraction X-ray analyses were carried out to investigate the role of silver in the controlled-redox leaching. No silver was observed to be on the surface of the chalcopyrite particles before or after the initial activation stage of the controlled-redox process. Silver deposits were observed after many hours of leaching. A mathematical model to describe the kinetics of microbial leaching, using a shrinking particle concept as its basis, was developed. When tested against data from the literature for leaching of Zinc from ZnS concentrate it was able to predict particle size as a function of leach time. It also gave reasonable predictions of particle size as a function of leach time for standard leaching of chalcopyrite but failed to predict accurate values for particle size as a function of leach time for the controlled-redox process. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Water, flowing through beds of refuse in a sanitary landfill, will leach organic and inorganic substances from the fill. These leached substances may be a source of pollution for receiving surface or ground waters. The leachate, before it is diluted by the receiving water, can usually be classed as a very strong waste water; that is, the levels of the waste water parameters COD, Suspended Solids, low dissolved oxygen and turbidity are many times those found in normal, municipal waste water. Added to these foregoing parameters are possible high levels of toxic chemicals and metals. It is now generally recognized that the leachate from refuse landfills should be controlled, and in some recently designed landfills, leachate collection is incorporated into the overall design. Toxic chemicals and metals are not adequately removed from waste waters by the standard biological sewage treatment processes; thus, the collected landfill leachate often requires pretreatment before it can be discharged to a municipal sewer system. If it is to be discharged to a natural receiving water, it requires more complete treatment. It was the purpose of this research to attempt to develop a physical-chemical treatment system for landfill leachate, such that the effluent might be safely discharged to a biological treatment plant or a natural receiving water. To deal with the extremely large number of possible chemical reagents, and to a lesser extent, physical methods available-, it was first necessary to select a number of primary candidates from prior information and theory available in the literature; secondly, it was advantageous to use a statistically designed experimental programme for screening those candidates chosen. In the screening process, no changes in the physical parameters screened, such as duration and speed of mixing or duration of settling, were found to be significant, if normal minimum times and usual speeds were used. Four chemical reagents, lime, ozone, ferric sulfate, and alum were indicated as having a potentially significant effect on the leachate-contained Total Solids (TS), Total Carbon (TC), Turbidity (Turb), Cadmium (Cd), Copper (Cu), Iron (Fe), Zinc (Zn), Potassium (K), Calcium (Ca), Sodium (Na), Phosphorus and the acid-base relationship as expressed by the term pH. The follow-up experiments determined that only two of the above four reagents were significantly effective in removal of the afore-named pollutants, as well as Manganese (Mn), Lead (Pb), Colour, Chemical Oxygen Demand (COD), the components of Total Carbon (TC) Total Inorganic Carbon (TIC) and Total Organic Carbon (TOC), and the components of Total Solids (TS)—Suspended Solids (SS) and Dissolved Solids (DS). All of the multivalent metals, except Calcium, were significantly removed from this wastewater by pH adjustment with lime, with additional minor removals by oxidation with ozone. Dissolved organic materials were not removed by pH adjustment and only removed in approximate stoichiometric amounts by reaction with ozone. In these experiments, the polymers tested were not effective in the removal of the named pollutants. Ozone is indicated to be an effective disinfectant, but highly sensitive to the COD of the leachate. An ozone-COD ratio, which determines the quantity of applied ozone necessary for the oxidation of some of the dissolved metals and for disinfection, as a function of the contained COD, is proposed for this leachate. The possibility of the application of this ozone-COD ratio is put forth, subject to further investigation. / Applied Science, Faculty of / Civil Engineering, Department of / Unknown

In the biostabilization phase, a BOD₅:N:P loading of 100:3.2:1.1 was found to be "adequate" for treatment, while the standard nutrient loading of 100:5:1 was found to be "excessive". This was evident by the much higher nitrite-nitrate concentration in the effluent of the BOD₅:N:P = 100:5:1 reactor. Organic removal by the first stage units was excellent. BOD₅ and COD removals of at least 99.4 and 96.4 percent, respectively, were achieved under all conditions investigated, except for the two units close to washout conditions (the 5-day sludge age units at 10° and 5°C). Temperature and sludge age also had minimal effects on the removal of metals, except under the two conditions mentioned above; removals were greater than 90 percent, for most of the metals monitored. The reactors only reduced magnesium concentrations by 32.5 to 52.7 percent, mainly because the mixed liquor pH's (about 8.5) were not high enough for magnesium precipitation as magnesium hydroxide. For the lime-magnesium polishing step, samples were dosed with lime to pH levels of 10.0, 10.7, and 11.4. Magnesium doses of 0, 10, 20, 35 and 50 mg/L were then added to the samples at each pH level. In general, removals of impurities were not enhanced significantly by these magnesium additions. This was due, in part, to the initial low concentrations of contaminants; in addition, there already existed greater than 20 mg/L of magnesium in the samples. Aerobic biostabilization, at a sludge age greater than 15 days, at BOD₅:N:P = 100:3.2:1.1, and liquid temperatures of at least 3°C, followed by lime precipitation (to pH greater than or equal to 10.0), is capable of reducing most contaminants of a medium strength leachate (BOD₅ = 8090 mg/L) to levels below local (Province of British Columbia) pollution control objectives. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

The extraction of nickel and cobalt, from nickeliferous laterite, together with the hydrothermal precipitation of hematite has been investigated. In order to emphasize the relevance and significance of this process, an appraisal is made of the state of the nickel, cobalt and iron industries. A compilation of the annual production of the respective ores on the world market is included with an examination of the future uses and demand of nickel and cobalt. Solubility relationships for iron (III) compounds in aqueous solution are reviewed in terms of pH, solution composition and temperature. The thermodynamic data used at elevated temperatures between 60°C (333°K) and 200°C (473°K) have been estimated by using the "Entropy correspondence principle" method of Criss and Coble. A sample calculation is shown in Appendix A. The effects of (a) temperature; (b) ferric chloride concentration, (c) hydrochloric acid concentration and (d) pulp density were studied in order to evaluate extraction conditions. Generally, metal extraction increased with temperature and ferric chloride concentration. At 423°K, over 90 percent of the nickel was extracted with a ferric chloride concentration greater than 1M. Since appreciable amounts of gangue dissolved under most conditions, thereby consuming acid, a discussion on the recovery of hydrochloric acid is presented. Filtration of the precipitated hematite has proved difficult, because of the very fine nature of the particles. An overview of the nucleation and growth of particles in supersaturated solutions has therefore been included. This phenomenon is used to describe the phase changes which occurred during leaching experiments, and to propose an approach by which coarser particles might be achieved. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The oxygen pressure leaching of pyrite has been studied in sulfuric and perchloric acid using an oxygen-18 tracer technique. The results are consistent with an entirely electrochemical mechanism. The leaching potential of a pyrite pulp has been measured as 0.699V SHET*(110°C, 1M H2S04, 176 psi 02) and potentiostatic experiments at that potential have yielded the same reaction product ratios as observed in oxygen pressure leaching. The ratio of sulfate to elemental sulfur produced during pyrite leaching has been found to be a function of the leaching mixed potential. The yield of sulfate is increased with increasing potential until all the mineral sulfur is converted to the sulfate form at potentials above 1.0V. A mechanistic model of pyrite leaching has been advanced, which includes the electrochemical formation of a protective sulfur film as its basis. The theory has been supported by polarization studies in combination with Eh-pH diagrams. Chalcopyrite leaching has been studied using the same 0¹⁸ technique. Again the results are consistent with an electrochemical mechanism. The ratio of the sulfate to elemental sulfur in the reaction products has been observed to increase with higher oxygen pressures. The mixed potential of leaching is also increased at higher pressure indicating that the same type of mechanism observed in the pyrite work is operational for chalcopyrite. Molybdenite leaching has also been discussed in terms of the electrochemical model derived for pyrite with good results, and the mechanism therefore appears to have applications in many, if not most, sulfide systems. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Bibliography: pages 125-136. / The recent focus on bacterial leaching as a preoxidation step in the treatment of refractory gold bearing sulphide ores and concentrates, has created the need for kinetic models to adequately describe bacterial leaching reactor performance. This work is a kinetic study of the bacterial leaching of a refractory gold bearing, pyrite concentrate. The study includes the presentation of two mechanistically based, the shrinking particle and propagating pore (Hansford and Drossou, 1986), batch reactor kinetic models. These models are derived for single stage continuous reactor description. In addition, the empirical logistic growth model (Pinches et al., 1987) is presented for both batch and continuous reactor description. The models are correlated with the experimental data. Three narrow size fractions of the pyrite concentrate were subjected to batch and continuous bacterial oxidation, using a Thiobacillus ferrooxidans culture. Time profile data of the pyrite oxidation were obtained for the batch reactor study. Similarly, retention time profile data of pyrite oxidation was obtained for the single stage continuous reactor. The .gold extraction as a function of sulphide oxidation as well as fraction arsenic leached, was established.

Indiana University-Purdue University Indianapolis (IUPUI) / Phosphorus is an essential plant nutrient and an important contributor to the eutrophication of aquatic ecosystems. Studies have shown that gypsum (CaSO4∙H2O) applications can potentially reduce phosphorus export from agricultural fields. Most studies have examined the effect of gypsum application rates on treatment effectiveness, but limited research has been conducted to determine how the timing of gypsum application can affect soil phosphorus mobility and phosphorus leaching. A greenhouse experiment was conducted to address this question and further our understanding of the effect of gypsum addition on soil phosphorus chemistry. For the experiment, two soil types with different background phosphorus levels (low P, high P), and three different time intervals between gypsum and phosphorus fertilizer application (2, 28 and 56 days) were applied. A total of 18 soil columns (L: 15 cm; diam: 10 cm) packed with sieved soil were treated with gypsum (3.9 g) and separated into three sets corresponding to each of the phosphorus application times. An equal number of columns not treated with gypsum were also included to serve as controls. Phosphorus fertilizer (0.34 mg P cm-1) was added as KH2PO4 solution. Rainwater (58 mL) was applied every 2-4 days to generate leachate that was collected and analyzed for ortho-P, total P, and SO4-2. At the end of each time series, the set of soil columns were sliced into 2-4 cm increments, and water extractable and bicarbonate extractable phosphorus (Olsen-P) was determined to examine downward phosphorus movement. Results of the study showed that Olsen-P levels were not affected by the gypsum treatment, indicating no interference of gypsum treatment with the P-supplying capacity of soils. The gypsum treatment reduced water-extractable P levels in the high-P soil, but treatment effect was not significant in the low-P soil. Likewise, in the high-P soil, gypsum treatment resulted in leachate ortho-P reduction during the second and third period of collection. For the low-P soil, there was no significant reduction in ortho-P. Overall, these results indicated that the beneficial effect of gypsum on phosphorus export from agricultural fields is dependent on soil-P status and time interval between gypsum amendment and P fertilizer application.

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