Remediation process of region, formally exploited for chemical leaching of uranium, is based on pumping water contaminated mostly by residuals of sulfuric acid from the relevant geological horizons. The contaminated water is then treated in a chemical plant. In the treatment process evaporators play the key role. Presently operating evaporators – three units – are of falling film type. Heating service is provided by compressed vapor that is generated by evaporators. For driving the compressors and associated circulating pumps of each unit electric power of 3,4 MW is required. In the evaporators treated solution is distributed into individual tubes by nozzles of cyclone type that are mounted at the top end of tubes. In the evaporators feed solution contains calcium sulfide of low concentration. Once the calcium sulfide concentration in the evaporating process reaches the state of saturation, even locally, fouling is initiated. Thus the solids start to grow on the inner surface of the tube wall and the nozzle’s openings become restricted. The evaporator performance – evaporating rate – gradually declines and when it reaches the bottom limit the evaporator has to be shut down. This is followed by carrying out the cleaning procedure. At the time when the evaporators started operating (1996) the period between shut downs lasted around a half a year. However, gradually, this period has become substantially shorter – presently it may even last one week only. The thesis proposes two ways of approaching the present hardly acceptable state. First, replacement of distributing nozzles by a set of properly perforated sieve trays secures the uniform distribution of solution into each individual tube. Second, evaporators’ duty will be taken over by newly build multi-flash concentrator that will substitute the presently operating evaporators. Configuration of this new equipment is derived from widely used multi-flash distillation of sea water. Heat required by the concentrator’s end heaters is drawn from a gas turbine exhaust gas. Steam provided by Heat Recovery Steam Generator is supplied to the condensing heat exchangers installed on the evaporator’s feed stream. Generator coupled to the gas turbine, fueled by natural gas, provides the electric power of 30 MW for export.

The purpose of this work was to recover uranium from residue originating from the production of molybdenum -99. Carbonate solutions containing hydrogen peroxide or oxygen gas as oxidants were used as leachants. Experimental parameters included: different peroxide and carbonate concentrations, dissolution time and temperature. Results indicate complete dissolution of the residue at 60 oC, after thirty minutes, in ammonium carbonate solution enriched with hydrogen peroxide. Almost two hours were needed to achieve the same extent in the presence of oxygen. The yield and rate of uranium extraction were found to increase as a function of both temperature, in the range of 25-60 oC, and hydrogen peroxide concentration. The leaching kinetics were determined for various leaching conditions and the activation energy was found to be 45.5kJ/mol. The order of reaction with respect to uranium concentration was found to be unity. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / unrestricted

Includes abstract. / Includes bibliographical references (leaves 74-79). / Historically the platinum group metals (PGMs) have been, and are still being dissolved by means of rather aggressive methods, e.g. aqua regia. Limited research has been conducted into the dissolution of the PGMs using different oxidizing agents. The dissolution of gold on the other hand has been afforded extensive research, and numerous papers and review articles have been published on the subject. The last number of years has seen the biggest application by volume of the PGMs as part of autocatalysts towards the degradation of harmful motor vehicle exhaust gases. This has subsequently sparked research into the recovery of specifically platinum, palladium, and rhodium from spent autocatalysts. Currently pyrometallurgical recovery of PGMs is being employed predominantly. A hydrometallurgical process on the other hand is, based on current technology, still a rather aggressive process that makes for high maintenance costs and an unpleasant environment. Gold has traditionally been dissolved by making use of cyanide, which is still the major route for gold dissolution. Due to environmental concerns lixiviants such as thiosulphate (S2O3 2-), thiourea (H2NCSNH2), and thiocyanate (SCN-) are gaining acceptance due to them being more environmentally friendly and giving good recoveries. These ‘softer’ alternatives have however not been tested on the PGMs. It is therefore the aim of this study to obtain an improved understanding of the leaching of the PGMs using lixiviants less aggressive than aqua-regia. These lixiviants include (i) SCN-, (ii) S2O3 2-, (iii) H2NCSNH2, and (iv) AlCl3/HCl. A thermodynamic study highlighted the fact that thermodynamic data for platinum-, palladium- and rhodium complexes are basically non-existent. To therefore obtain a clearer thermodynamic understanding of the leaching of the platinum group metals by means of these alternative lixiviants, future detailed speciation and thermodynamic investigations need to be conducted. An exploratory electrochemical investigation focusing on open circuit potentials and potentiodynamic scans, showed AlCl3 / HCl / NaOCl to be a good candidate for the leaching of the platinum group metals followed by SCN- / Fe3+ and CS(NH2)2 / Fe3+. Actual leach results, employing virgin autocatalysts as sample material, again highlighted the potential of AlCl3 / HCl / NaOCl as being a good lixiviant system. The surprise package, however, has been SCN- / Fe3+ that rendered very good results for Pd and Pt.

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Philosophiae Doctor - PhD / The catalytic support materials used in the present study are zeolite HBEA and MCM-41. These high silica zeolites were synthesised from coal fly ash (CFA) waste via a novel approach that involved a fusion step, acid assisted silica extraction and removal of Al, Ca and Na from the silica by treatment with oxalic acid. The generated silica was converted to HBEA and MCM-41 via conventional hydrothermal treatment. The metal incorporation onto HBEA was done via two approaches namely; liquid phase ion exchange (LIE) and wet impregnation (WI) while the loading on MCM-41 was only done via WI since the material does not possess exchange sites. The metal solution precursors were AMD and Fe extracted from CFA (FeAsh) via acid leaching followed by pH regulation by concentrated NaOH. This is the first time these solutions were tested as possible metal precursors in catalyst synthesis. / 2021-08-30

A process oriented modeling of an irrigated rice field in a semi arid area of Mali has been done with the help of computational tool CoupModel. The model has been used to simulate two levels of irrigation rates, in an attempt to test and see adequacy of a recommended irrigation rate and its environmental impact over the current management. A simpler simulation to represent less water demanding crops like sorghum or millet has also been done to indicate extent of the excess water and as alternative crop cultivation. Important processes and parameters to represent a rice cropping system have been identified and simulation was run for a 12 year period. Results show an irrigation amount of 916 mm delivers an overall 6 % increased yield. Results from the reduced irrigation also show a better output in surface runoff, nitrogen leaching and uptake, photosynthetic water use efficiency and fertilizer efficiency. Soil nitrogen and carbon storage shows nearly the same trend. Only nitrous oxide (N2O) emission rate increased by 13 % in the case of reduced irrigation. Simulation done for the other crops also shows a reasonable yield of sorghum or millet can be obtained with 46 % of water used for current rice irrigation.

Manganese (Mn) is an important element in soil, it occur natural in minerals and precipitated as Mn-oxides. Several factors could decide the solubility and mobility of Mn in soil water. In this study, the impact of road de-icing salts (NaCl) on manganese mobilization and transport to groundwater in roadside soils has been investigated by leaching tests. Generally, in the salt solution leachates, the water-soluble concentrations of Mn tended to increase with elevated salt concentrations, suggesting that ion exchange mainly affected the mobilization. The process was also attributed to the complexion with Cl. Associated with exchangeable concentration of Mn and soil properties such as pH and acidity, the mobilizations of Mn varied. Mn-oxides can dissolve when reduced condition exists, therefore the oxalate extractable Mn was extracted to estimate the change of redox potential condition in roadside soils. The redox potential of soil samples is higher in general. Redox condition has little effect on the Mn solubility and mobility in this research. Although groundwater samples indicated that only a few periods and sites were under threaten of elevated concentrations of manganese, there is still great risk of transport of high water-soluble concentrations of Mn in roadside soils to groundwater, especially the areas exposed to de-icing salts. In addition, lower value of Mn concentrations in groundwater for considering good drinking water quality for the well-being of children should be paid more attention to.

The impact of long term management strategies of irrigation with saline water in semi-arid region of Gordonia, South Africa is the highest interest to optimize water consumption, soil conservation, and crop yield for sustainable water allocations to human food production and ecosystem without irreversible damages to soil and water body. An integrated ecosystem assimilation, in shape of soil-water storage model based on physical approach for 30-year simulation run defined in form of digital ecosystem modelling with help of CoupModel tool to assemble together the most important underlying processes of soil hydraulics, irrigation demands, leaching fraction, evapotranspiration, salt transport. Two scenarios of water management strategy; surface as traditional and drip as subsurface irrigation considered to apply water and salt into the ecosystem model. Gaining high food production for human with respect to ecosystem sustainability, in each water management scenario studied by evaluating general and detailed result from water and salt balance for the entire simulation period plus long term nitrogen and carbon turnover as crop yield indicator. Non-productive water losses, salt accumulation in root zone, carbon and nitrogen turnover, salt transport to aquifer via deep percolation observed thoroughly. Decline in crop yield due to water and salt stress, conducted by monitoring biomass production with respect to water consumption and soil osmotic pressure in root zone. Drip scenario had better functionality to perform less water wastage by decreasing soil evaporation as non-productive water loss almost 40 %, however productive water consumption decreased 20 % due to insufficient leaching fraction and also salt accumulation increased in root zone. Precipitation had a significant role to accomplish leaching deficiency and removing salt from root zone. Salt accumulation flushed out from root zone by more leaching, though resulting more water wastage and more possibility of salinization threatening beneath aquifer. Ecosystem in terms of soil-water and plant responding differently facing salinity in different water management practices and salt as source of pollution could either stabilized in soil by accumulating in root zone causing anthropogenic soil desertification or percolate to beneath aquifer resulting aquifer salinization.

On a catchment scale, chloride (Cl-) leached from forest soil due to dry deposition and interaction between vegetation and soil could attribute to the Cl- mass balance. The availability of Cl- data from monitoring works enables the assessment of net ecosystem budgets of Cl-, which compares Cl- amount from wet deposition to that in stream export. Some studies have shown that Cl- imbalances could occur in some catchments and one of such catchments with negative budget is Kringlan. The Kringlan catchment is a sub-basin located on the western boundary of Norrström basin and for this project, a southern sub-catchment with a size of 54.5 km2 within Kringlan catchment was chosen to be studied. To fulfill the Cl- budget, one possible internal source of Cl- leached from forest soil was added and tested. Moreover, a field trip to the study site was performed and soil samples were taken back for leaching test analysis. The evaluation of Cl- transport through the catchment requires the water flow and MIKE SHE was selected as the tool for flow simulation in an integrated manner. Lastly, the Particle Tracking (PT) module helped to analyze the Cl- transport through saturated zone in a time span of 100 years for a simplified scenario. Results show that the calibrated chloride concentration to groundwater leached from forest soil (-1 m to -5 m below the ground) was 200 mg/l. A total chloride leaching amount from forest subsurface was calculated as 4287 tons during calibration stage of 5 years. However, only 265 tons of chloride for 5 years could be transported out from the catchment for the calibration period, which corresponds to a leaching flux rate of 1.3g/m2/yr. It was found that the flow discharge rather than the Cl- concentration dominated the Cl- transport loads at the outlet, both in variation and magnitude. For future studies, some other possible internal sources, e.g., mineral weathering, road salting and etc. need to be researched.