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A novel system for leaching of reconstituted cores of soils taken from horizons of <I>Calluna</I> moorland podzol profiles derived from granites has been used to see if absolute, or at least, relative, weathering rates assessed by simple leaching techniques and by using the PROFILE model agreed. Initial base cations are first removed by leaching with ammonium acetate. Agreement was reasonable, but the results highlighted problems posed by mineralization effects in column-based procedures. A successful attempt was made to develop a procedure to overcome this problem, based upon pre-oxidation with H₂O₂. This procedure was tested on a selection of B and C horizon soils from podzol profiles, and shown to give weathering rates which correlated well with those derived using the PROFILE model. The equilibrated soil cores remaining at the end of the study were then used for testing the effect of acidification of infiltrating water upon weathering rates of base cations. Weathering rate did not increase regularly to combat the acid input. Intact soil core microcosms, fitted with rhizon samplers at three depths and subjected to appropriate simulated precipitation, have been used to elucidate how land use influences the mobility of copper and zinc. Three land uses were considered, improved pasture, semi-improved pasture, and <I>Calluna</I> moorland. Copper and zinc concentrations were also measured in river water samples collected at 59 points from throughout the River Dee network under diverse flow conditions, with a view to elucidating catchment factors influencing the mobilisation of these two elements into river water. The results demonstrated large inputs of copper arising as a consequence of arable land use, and suggested that TOC in drainage water from moorland soils mobilises copper from mineral sediments in the river network. Both zinc and copper concentrations were increased under high discharge conditions.

Field scale variability of soil texture can influence crop yield and movement of soil water in the field. The objective of this study was to investigate the spatial structure of soil texture in relation to the variability of nitrate-N leaching using geostatistics. Soil textural fractions showed strong spatial autocorrelations from surface to 60 cm depth. Random variability of soil texture increased with depth. Soil water content, as well as total carbon, total nitrogen and soil organic carbon of top 15 cm, also showed spatial autocorrelations similar to soil texture. Elevation, relative slope position and vertical distance to channel network showed significant influence on the distribution of soil texture. Soil texture at 90 cm depth correlated best with cumulative percolated water and cumulative nitrate leached in field lysimeters. Our results showed that soil layers with low hydraulic conductivity control the water and nitrate movement through the soil profile.

Efflorescence is the presence of insoluble salts upon the surface of concrete or masonry products. The aims of this project were to understand efflorescence and to evaluate metakaolin as an additive for efflorescence prevention. Computer modelling was used in order to simulate the effect of alkali upon relevant phases. Alkali was shown to depress the solubility of calcium hydroxide via the common ion effect, increase, in general, the solubility of calcium carbonate via the foreign ion effect and dramatically increase the solubility and rate of uptake of carbon dioxide into solution. The raindrop model is an attempt to simulate the processes leading to efflorescence. A raindrop falling onto a roof tile is assumed firstly to leach ions from the tile, secondly, to absorb atmospheric carbon dioxide and thirdly to precipitate calcium carbonate. The raindrop model calculates that the formation of efflorescence is primarily dependant upon leaching rather than drying effects. Sensitivity studies show that even when significant sulphate is leached, calcium carbonate remains the prevalent efflorescent phase. Metakaolin, a type of burnt clay pozzolan, had previously been cited as preventing efflorescence. However, this study showed that the performance of metakaolin is affected by the presence of micaceous and other impurity phases which are activated upon heating to metakaolinization temperatures, and readily release alkalis. This study found that the main products of metakaolin hydration in alkaline solutions are gehlenite hydrate and amorphous C-A-S-H gels. The latter was shown to absorb alkalis, particularly potassium, from solution. Results are presented of wet-dry and renewal of leachant leaching tests on two metakaolin blends and a neat OPC paste control. Wet-dry leaching, judged to be the most realistic leach test, produces leaching trends not noted in other leaching tests.

MgCl2 brines present a number of potential advantages for the processing of saprolite ores for nickel production. Concentrated MgCl2 solutions enhance the activity of acid used, allow atmospheric leaching at elevated temperature and inhibit magnesium dissolution, which reduces acid consumption and increases metal selectivity. However, with a chloride system it is economically requisite to recover hydrochloric acid, conventionally accomplished by pyrohydrolysis. This work was performed in conjunction with a novel flowsheet for the processing on saprolite ores, which recovers HCl by the precipitation and subsequent decomposition of magnesium hydroxychlorides, alleviating some of the issues with pyrohydrolysis. Leaching and iron control experiments have been conducted in concentrated MgCl2 brines, up to 4.5 m, to determine the most amenable process conditions. It was determined that > 95% extraction of metals was possible using both aqueous and gaseous HCl. In addition, the feasibility of iron control by precipitation with MgO addition was proven.

An electrochemical cell consisting of a flow-through yttria-stabilized zirconia (YSZ) sensor and a flow-through Ag/AgCl reference electrode has been employed to measure pH of high-temperature acidic sulphate solutions relevant to the pressure acid leaching (PAL) of nickel laterites. In a previous study, this cell was used to measure pH of H2SO4, Al2(SO4)3-H2SO4 and MgSO4-Al2(SO4)3-H2SO4 solutions at 250oC. In this work, the solutions range in complexity from the binary MgSO4-H2SO4, NiSO4-H2SO4, and Al2(SO4)3-H2SO4, through the ternary MgSO4-Al2(SO4)3-H2SO4 and NiSO4-Al2(SO4)3-H2SO4, to the PAL process solutions, whereas the temperature ranges from 200oC to 250oC. The measured and theoretical pH values typically agree within less than 0.1 pH unit and 0.2 pH units in synthetic solutions and PAL solutions, respectively. This is an improvement over the results of the previous study in synthetic solutions, which show differences between theory and experiment as high as 0.4 pH units. The conversion of measured potentials into pH values is based on the new mixed-solvent electrolyte (MSE) speciation model of the OLI Systems software calibrated independently based on solubility measurements. Both Henderson’s equation and the exact definition of the diffusion potential were employed in treating the obtained experimental data. Experimental pH values calculated using the diffusion potentials evaluated by either approach are essentially the same. This finding suggests that Henderson’s equation, which is based on readily available limiting ionic mobilities, can be effectively used. Lithium chloride is found to be a suitable alternative to sodium chloride as the reference electrode solution for the measurement of pH of aluminium-containing solutions, because it did not induce precipitation of aluminium as an alunite-type compound. The experimental results indicate that the high-temperature behaviour of Ni, Co and Mn sulphates can be satisfactorily approximated with that of MgSO4. The experimental findings also support the postulation that acid should be added to a PAL process so that the solution pH is around 1 at the leach temperature, regardless of the feed composition. The cell can be used for hydrometallurgical process research and development on a laboratory scale with very satisfactory performance, provided that a well-behaved YSZ sensor is available.

MgCl2 brines present a number of potential advantages for the processing of saprolite ores for nickel production. Concentrated MgCl2 solutions enhance the activity of acid used, allow atmospheric leaching at elevated temperature and inhibit magnesium dissolution, which reduces acid consumption and increases metal selectivity. However, with a chloride system it is economically requisite to recover hydrochloric acid, conventionally accomplished by pyrohydrolysis. This work was performed in conjunction with a novel flowsheet for the processing on saprolite ores, which recovers HCl by the precipitation and subsequent decomposition of magnesium hydroxychlorides, alleviating some of the issues with pyrohydrolysis. Leaching and iron control experiments have been conducted in concentrated MgCl2 brines, up to 4.5 m, to determine the most amenable process conditions. It was determined that > 95% extraction of metals was possible using both aqueous and gaseous HCl. In addition, the feasibility of iron control by precipitation with MgO addition was proven.

Landfill emissions are the major environmental impact associated with the landfilling of solid wastes. These emissions which are mainly gases and leachate are a result of the anaerobic biochemical breakdown of the waste in landfills. The long term emissions from these solid wastes in landfills can be significantly reduced by Mechanical Biological Pretreatment of the waste prior to landfilling. Mechanical Biological Pretreatment of solid wastes is aimed at reducing the long term polluting potential from landfills through accelerated stabilization of the organic constituent material present in the waste body by biological degradation before the waste is placed in landfills. This research is part of a broader investigation on the applicability of Mechanical Biological Pretreatment of solid wastes prior to disposal in the South Africa Waste Management context. General waste disposed at Bisasar Road Landfill Site in Durban which has been pretreated for 16 weeks in passively aerated windrows was collected and screened using a rotatory drum screen to generate an over-sieved waste fraction with particles of diameter greater than 50mm and an under-sieved waste fraction characterized by particles diameter less than 50mm. The long term behavior of these heavily pretreated wastes was then simulated using large anaerobic reactors (Lysimeter) in relation to grain size distribution. A third lysimeter containing general waste without pretreatment collected from Bisasar Road Landfill Site was also set up in order to study the effect of pretreatment on solid waste samples. The results of this research were then used to make recommendation on the appropriateness of Mechanical Biological in the South Africa Waste Management context. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2007

This study investigates the occurrence and chemical composition of leachate at Clayton South and Brooklyn Municipal waste landfills in Melbourne, Victoria. Both are ‘modern’ municipal waste landfills, being engineered and managed consistent with current regulatory requirements. These landfills accept only putrescible and solid inert waste, but not soluble chemical, hazardous, liquid or prescribed industrial waste. (For complete abstract open document)