M.Tech. / The efficiency of hydrochloric acid leaching of gold from low grade gold-bearing material has been investigated in the presence of sodium hypochlorite (NaOCl) to produce in situ chlorine gas which is an oxidizing agent able to oxidize gold metal to gold soluble forms: Au+ and Au3+. The effect of concentrations of HCl, NaOCl, and mixtures HCl + NaOCl was sought. An investigation on chlorine species was conducted to predict their stability areas. The reactions involved and their mechanisms were established. Prior to leaching, the feed was subjected to fire assay, Atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), X-ray fluorescence (XRF), and Fourier transform infrared spectroscopy (FTIR) analyses, to quantify the various elements, and to find out the chemical composition and the different mineralogical phases which are present. The main minerals found in the feed were: quartz, pyrite, muscovite-2M1, (M); and clinochlore. The grade was found to be 0.62 g/t. It was found from the hydrochloric acid leaching process that most of metals constituents (iron and potassium) of the feed were leached and consumed all the hydrochloric acid. Therefore, leaching of gold from low grade gold-bearing materials in aqueous chlorine solution is not an effective process for now. Hydrochloric acid leaching of cobalt and copper from four oxidized cobalt-bearing ores was studied in the presence of FeCl2 to produce in situ Fe2+ that is a reducing agent able to reduce Co3+ to Co2+ which is the soluble form of cobalt. The effects of the concentrations of HCl, FeCl2, mixture HCl + FeCl2, leaching time, particle size, and the dissolution kinetics were investigated. Prior to leaching the feeds were characterized with XRD, XRF, FTIR, AAS, Scanning electron microscopy (SEM), and gravimetric analysis. Cobalt contents were found to be between 2.59% and 39.76% in the four ores namely, high Cu ore, high Co ore, low Co ore, and high Co ore with mica. Effect of FeCl2 concentrations in HCl solutions involving 35.06 g; 70.13 g; and 105.18 g corresponding to 0.25 M; 0.5 M; and 0.75 M respectively, were studied and the reactions mechanism established. It was found that an increase in FeCl2 concentration increases the Co dissolution due to the reduction of Co3+ into Co2+ which is soluble. High Co extractions of 99%, 99%, 99%, and 95% were obtained when mixtures 1 M HCl + 105.18 g FeCl2 were used as lixiviants for high Cu ore, high Co ore, low Co ore, and high cobalt ore with mica, respectively. Copper recoveries of 88%, 77%, 75%, and 86% were obtained for high Cu ore, high Co ore, low Co ore, and high Co ore with mica, respectively. The increase in FeCl2 concentration did not have any important effect on the Cu extraction. Mechanisms of involved reactions were established. The order of cobalt leaching reaction was found to be 2 with respect to cobalt concentration. Optimum leaching conditions were found to be: leaching time: 30 minutes, reagent concentrations: mixture 1 M HCl + 105.18 g FeCl2, S/L ratio = 1:20, stirring speed avoiding the vortex, room temperature, pH values from 2.28 to 0.03, and ORP values from 0.402 to 0.322 V.

No description available.

Heap leaching is a hydrometallurgical process used for the extraction of minerals within complex and typically low-grade ores. An important factor in the mineral dissolution process is the contact efficiency between the irrigation fluid (lixiviant) and the targeted mineral, which is influenced by both the solid and fluid properties of the system. One of the principal challenges related to the contact efficiency is preferential flow, cited to result in low extraction rates and in extreme cases, heap failure. Preferential flow reveals itself on two scales in drip irrigated heaps, referred to as the bed and solution scale. The bed scale takes a macro view of the heap and deals with uneven wetting profiles characterized by the presence of wet and dry sections. Linked to this is capillary suction effects which play an important role in the establishment of fluid flow profiles within the heap. The solution scale focuses on preferential flow behaviour in the wetted sections of the heap characterized by variations in the residence times of fluid elements. Such variations produce fast flowing, slow flowing and stagnant solution pools. Therefore, ideal solution flow behaviour in a heap result in uniform wetting at the bed scale and plug flow behaviour with similar fluid residence times at the solution scale. Though bed scale preferential flow can be visually observed, diagnosing symptoms at the solution scale typically requires the generation, analysis, and modelling of residence time distribution (RTD) curves. The main objectives of this study were to firstly explore the effects that important material and fluid properties have on the steady state fluid flow profiles in drip irrigated beds characteristic of those used in laboratory scale column leaching studies and quantified using step tracer tests. This is based on the underlying principle that the movement of inert tracer molecules within an irrigated bed at steady state is identical to the solution flow path within the bed. The second objective was to test the ability of nine empirical and semi-empirical solute transport models to adequately fit the generated flow profiles or RTD curves. The third was to compare the magnitudes of the quantified model parameters to ascertain the level of solution scale preferential flow in the different beds and determine the adequate level of model complexity needed to describe their flow profiles which facilitates identifying the controlling variables within the system. Properties of the loading material that were identified as potentially most impactful with respect to heap operations were: porosity, wettability, particle shape and size distribution. Therefore, four different materials with unique inherent characteristics were selected for this study: glass beads (GB - spherical and non-porous), glass shards (GS - irregularly shaped and non-porous), greywacke (GW - irregularly shaped, porous, and highly wettable) and malachite ore (MO - irregularly shaped, highly porous, low wettability and non-uniform composition). In terms of fluid properties, current models have already established a correlation between the concentration of dissolved chemical species within a fluid and its viscosity. This was relevant due to the variety of lixiviant compositions used in previous heap hydrology studies and the fact that the composition also varies with time within a reactive heap. To study the effects of this parameter on the establishment of flow profiles, glycerol was used as a viscosity modifier to formulate solutions with viscosities ranging from 0.8 to 2.2 cP, representative of the range experienced in heap leaching systems due to varying SO4 2- concentrations. The packed beds were characterized using their bulk densities, void age, total liquid holdups, total bed saturations, 24-hour drain-down moisture percentages, solution and tracer breakthrough times. Beds containing both narrow and mixed particle size fractions were tested. The nine solute transport models used for RTD modelling included three compartmental model configurations (CM-1, CM-2, CM-3) and the tanks-in-series (TIS) model, all empirical in nature. The five semi-empirical models selected were the advection-dispersion (AD), piston exchange (PE), piston exchange-diffusion variant (PE-D) and piston dispersion and exchange (PDE) models. A novel model formulation called the piston dispersion and exchange-diffusion variant (PDE-D) model was also coded and tested, which incorporated both the longitudinal dispersion coefficient as well as a diffusional flux mass transfer mechanism. The CM-2, AD and TIS were mono porosity models assuming all solution volumes within the beds were actively flowing which limited their ability to account for solution scale preferential flow. The CM-1, CM-3, PE, PE-D, PDE, PDE-D models were dual porous, accounting for the presence of either dead or stagnant solution volumes. The model parameters used to account for preferential flow in the RTD profiles included: the fraction of dead to total solution volume, dynamic to total saturation fraction, number of TIS, ratio of parallel continuously stirred tank volumes, longitudinal dispersion coefficient, overall mass transfer coefficient and maximum diffusional pore length. The cumulative RTD responses for the bed systems composed of narrow size fractions were noticeably impacted by particle size. These systems displayed symptoms of increased solution channelling behaviour at steady state, based on their relatively short tracer breakthrough times, as the average particle size was increased from ∼1 to 15 mm. The incorporation of semi-empirical models which could account for stagnant volumes. The main comparative modelling results across all systems studied showed that the PDE and PDE-D models were the top performers, based on a model fit analysis. This was due to their dual porous nature and relatively higher levels of complexity. The mono porosity models (CM-2, TIS and AD) performed the worst due to their inability to account for isolated and immobile liquid volumes. However, when 10 mm during agglomeration will aid in increasing the fraction of mobile (actively flowing) liquid within the heap due to the increased presence of macro voids. High levels of particle porosities (>2.5 m2/g) will also aid in this aspect. This is proposed to be due to greater void network connectivity with an increase in porosity facilitating better mass transfer. These insights were obtained through the analysis of experimentally generated data and model simulations. They have provided a better understanding of the movement of fluid molecules within drip irrigated beds, which is essential for improved leaching performance. Building on this, the next step is to consider the effects of scale up and reactive systems on both empirical and simulated data.

Determining optimum N fertilization rate and timing is critical to improve yields and economic sustainability for no-till winter wheat (Triticum aestivum L.) in the Virginia Coastal Plain. Little data are available evaluating N management strategies, optimum N rate prediction methods, or potential NO3 leaching under no-till wheat in soils and climate similar to the mid-Atlantic region. The objectives of our research were: (1) to determine economic optimum N fertilization rates and timings; (2) evaluate selected methods for predicting optimum N rates at GS 25 and GS 30; and (3) measure NO3 leaching loss under selected N management strategies. Eleven experiments over three years evaluated N rates at GS 25 and GS 30. Six experiments over two years evaluated pre-plant and December or GS 25 N rates under no-till winter wheat in farm fields of the Coastal Plain region of Virginia. Nitrate leaching was measured under selected pre-plant and December or GS 25 N application rates. All sites represented common Coastal Plain soil types and cultural practices for no-till wheat production. Yield data were used to calculate economic optimum N rates for a range of combinations of N management strategies. Optimum N rates were regressed against tiller density at GS 25, and wheat tissue N content and SPAD chlorophyll meter readings at GS 30, to determine their effectiveness as predictors of the optimum N rate at GS 25 or GS 30. Tiller density was an effective predictor of optimum GS 25 N rate in these split application management strategies. However, wheat tissue N contents and SPAD chlorophyll meter readings were not effective predictors of optimum N application at GS 30. Yields across all experimental designs were affected by planting date. Timely planted no-till wheat consistently produced higher yields compared to late planted. Sites under continuous no-till production for 8 years or more also produced higher yields than sites under continuous no-till production for less than 8 years. Including an additional December N application with the more traditional N management strategy of pre-plant, GS 25 and GS 30 N applications improved yields. Nitrate leaching loss at selected pre-plant and December or GS 25 N rates was not higher than background check plot levels under timely planted no-till wheat. Additionally, economic optimum N application rates and timings at these sites did not produce NO3 leaching losses above background levels under timely or late planted wheat, except at one late planted site. These data indicate N application rates and timings in no-till wheat can be managed for improved economic sustainability and reduced environmental impact. / Ph. D.

The overall goal of this research was to improve simulation of soil phosphorus (P) transport and transformations in GLEAMS 3.0, a non-point source model that simulates edge-of-field and bottom-of-root-zone loadings of nutrients from climate-soil-management interactions to assess management alternatives. The objectives of this research were to identify the state of the science for P transport and transformations, determine appropriate relationships for inclusion in GLEAMS, and determine if modifications to GLEAMS improved predictions of P loss in runoff, sediment, and leachate. The state of the science review revealed numerous equations available to predict dissolved P loss in runoff and leachate from a soil's nutrient status. These equations use a single variable to predict P loss and were developed for site-specific conditions based on empirical data. Use of these equations in GLEAMS is not reasonable as transport factors must also be considered when predicting P loss. Results from the sensitivity analysis showed that GLEAMS prediction of leached P were extremely sensitive to changes in the P partitioning coefficient (CPKD). Runoff PO₄-P output was slightly to moderately sensitive, sediment PO₄-P was moderately sensitive to sensitive, and sediment organic P was moderately sensitive to changes in CPKD whereas plant uptake of P was insensitive to slightly sensitive. The weakness of GLEAMS to estimate CPKD has been documented. Upon further investigation, it was determined that CPKD was highly over-estimated in GLEAMS as compared to measured values found during the literature review. Furthermore, this over-estimation caused under-estimation of the P extraction coefficient (BETA P); the value of BETA P remained constant at 0.10 and did not vary over the simulation period. Expressions for CPKD and BETA P were modified in GLEAMS. Data from three published studies (Belle Mina, Gilbert Farm, and Watkinsville) were used in the analyses of three modifications to GLEAMS: GLEAMS BETA P, GLEAMS CPKD, and GLEAMS BETA P+CPKD. GLEAMS BETA P investigated the change in BETA P as a function of soil clay content, GLEAMS CPKD attempted to improve GLEAMS' estimation of CPKD, and GLEAMS BETA P+CPKD assessed the combined effects of changes to BETA P and CPKD. Over the respective study periods, GLEAMS over predicted runoff PO₄-P for Belle Mina by 193 to 238% while under-predicting runoff PO₄-P at Gilbert Farm by 41% and Watkinsville by 81%. Sediment P was over-predicted by GLEAMS for Belle Mina by 225 to 233% and Gilbert Farm by 560%, while sediment P was under-predicted by 62% at Watkinsville. Leached PO₄-P was both over- and under-predicted by GLEAMS; Belle Mina was the only data set with observed leached P values. Simulation results from the model changes were inconclusive. There was no clear evidence supporting use of one model over another. Modifications increased predicted dissolved P in runoff and leachate, while decreasing predicted sediment-bound P in runoff. The original GLEAMS model best predicted runoff and leached PO₄-P at the Belle Mina sites. GLEAMS CPKD was the best predictor of runoff PO₄-P and sediment P at Gilbert Farm. GLEAMS BETA P+CPKD best predicted runoff PO₄-P at Watkinsville. Overall, the proposed improvements to GLEAMS did not improve GLEAMS predictions. In conclusion, GLEAMS should not be used for quantitative estimates of hydrology, sediment, and nutrient loss for specific management practices. As recommended by the GLEAMS model developers, GLEAMS should only be used to predict relative differences in alternative management systems. It is recommended that future research focus on developing a better correlation between CPKD, clay mineralogy and content, and organic matter content, as CPKD has been identified as a vital component of the GLEAMS P sub-model that requires further examination. / Master of Science

Concrete corrosion has serious societal and economic impacts and is an important concern in a utility's overall corrosion control strategy. Though concrete based pipes and linings are only restricted to the distribution mains, they still make up a large percentage of the drinking water infrastructure at about 17% of its total length. An improved understanding of the corrosion mechanisms involved steps that can be taken to mitigate concrete corrosion are very important. This study examined the role of phosphate chemicals, water chemistry, temperature and gaseous cavitation on the degradation of cement-based pipes and linings. It also provides information for utilities to make informed decisions regarding the use, effectiveness, and application of phosphate corrosion inhibitors relative to concrete corrosion control. Under low alkalinity and low pH conditions, considered to be highly aggressive in the literature, we noticed very substantial corrosion of concrete in laboratory experiments. At high pH and high alkalinity conditions, the buildup of scale (e.g., calcium carbonate) on the inside of the pipe is the major concern. The addition of phosphate inhibitors strongly influenced both concrete corrosion and scaling. At low alkalinity the addition of zinc orthophosphate or polyphosphate reduced corrosion of concrete. The addition of orthophosphate under low alkalinity conditions increased aluminum leaching and could push aluminum concentrations above the EPA SMCL threshold. At high alkalinity conditions the addition of orthophosphate is highly effective at reducing scaling, and aluminum leaching was not a concern. The presence of high concentrations of magnesium and silicon could form magnesium aluminum oxyhydroxides and magnesium silicates which could act as a protective scale on the concrete surface. However, this precipitate forms only at pH values above 9.5. The effectiveness of this protective scale in reducing corrosion of concrete was not established unambiguously in this research. Temperature plays a key role in corrosion of concrete. Calcite solubility increases at lower temperatures however at higher temperatures corrosion of concrete increases, which implies that corrosion of concrete is not driven by calcite solubility. At higher alkalinities scaling of concrete is higher at lower temperatures. This indicates that calcite solubility controls scaling of concrete at higher alkalinities. Tests with gaseous cavitation indicate that corrosion of concrete does not increase in the presence of gaseous cavitation. Vaporous cavitation is more detrimental to concrete than gaseous cavitation. / Master of Science

Glass corrodes in aqueous environments. The corrosion process is well-understood for many circumstances involving long periods of time at room temperature as well as processes that involve conventional heating, but the effect of microwave energy on glass corrosion has never been fully investigated. It was suspected that microwaves may alter or accelerate the aqueous corrosion processes that occur in glass which contribute to migration into foods or other materials. Lithium disilicate (Li2O-2SiO2) and commercial soda-lime glass were corroded using both conventional and microwave heating in this study. The results did not clearly show substantial differences in corrosion under the test conditions, but leave open the possibility of an altered mechanism in some circumstances. These findings suggest the need for testing at a lower microwave frequency, specifically 2.45 GHz. / Master of Science

The aim of this study was to extract nickel and cobalt from the lateritic nickel ores of G&ouml / rdes region by hydrometallurgical methods under the optimum conditions. Limonitic and nontronitic types of G&ouml / rdes lateritic nickel ores were used during experiments. Agitative and column leaching experiments at atmospheric pressure were conducted with various parameters / these were duration, temperature and initial sulfuric acid concentration of leach solution. It was shown that in agitative leaching, under the optimum conditions that were determined as 24 hours of leaching at 95&deg / C with initial sulfuric acid concentration of 192.1 g/L for nontronite and 240.1 g/L for limonite, nickel and cobalt extractions were 96.0% and 63.4% for nontronite / 93.1% and 75.0% for limonite, respectively. Overall acid consumptions of ores were calculated as 669 kg H2SO4/ton dry ore for nontronitic type nickel ore and 714 kg H2SO4/ton dry ore for limonitic type nickel ore. Column leaching experiments also showed that nickel and cobalt could be extracted from both ore types by heap leaching. Nontronite type of laterite was found to be more suitable for column leaching by sulfuric acid. In column leaching, the calculated nickel and cobalt extractions were 83.9% and 55.2% for nontronite after 122 days of leaching with 100 g/L sulfuric acid concentration. Acid consumption of nontronite was found to be 462 kg H2SO4/ton dry ore.

In the production of base metals by smelting of sulphide ore concentrates, large amounts of iron are rejected with iron silicate slags. These slags contain Ni, Cu and Co in concentrations up to several percent units. Extraction of the entrapped base metals using high pressure oxidative acid leaching (HPOXAL) was investigated in this work. Crystalline slags containing fayalite (Fe2SiO4), magnetite (Fe3O4), silica (SiO2) and matte (MeSn<1) were found to be highly amenable to leaching at 250°C, 90 psi (6.2 bar) O2 partial pressure and 70 g/L initial H2SO4. Extractions of Ni, Co and Cu exceeded 90% within 15-20 min and arrived at 95-97% after 45 min. The residues of leaching were identified as aggregates of crystalline hematite (Fe2O3) and amorphous silica. Dissolution of fayalite and magnetite was shown to be acid driven. Since HPOXAL operates with substoichiometric additions of sulphuric acid (10-20% of the stoichiometric requirement), acid regeneration facilitated by iron oxidation and hydrolysis is crucial to high rates of leaching. Low acidities (<10 g/L) were shown to cause precipitation of ferrous sulphate, slowing acid regeneration and slag dissolution. Elevated acidities (>70 g/L) result in excessive concentrations of Fe(III) in the leach solution, complicating downstream processing. The use of pyrrhotite tailings, an environmentally hazardous waste, as a substitute of sulphuric acid in slag leaching was investigated. Oxidative co-leaching of pyrrhotite tailings with naturally cooled converter slag at 250°C, 90 psi (6.2 bar) O2, 68 g/L equivalent H2SO4 was shown to have kinetics comparable to adding sulphuric acid with final extractions reaching 95-97% in 45 min. Granulation of slag melt can produce an amorphous solid solution of SiO2 and metal oxides. Amorphous slag is not amenable to HPOXAL due to the formation of a passive layer of silica. Leaching of amorphous slag at low temperatures was shown to proceed nearly to completion. The difference in the leachability of amorphous slag at high and low temperatures is explained in terms of the rate of silicic acid re-polymerization leading to closure of pores in the leached layer.

In the production of base metals by smelting of sulphide ore concentrates, large amounts of iron are rejected with iron silicate slags. These slags contain Ni, Cu and Co in concentrations up to several percent units. Extraction of the entrapped base metals using high pressure oxidative acid leaching (HPOXAL) was investigated in this work. Crystalline slags containing fayalite (Fe2SiO4), magnetite (Fe3O4), silica (SiO2) and matte (MeSn<1) were found to be highly amenable to leaching at 250°C, 90 psi (6.2 bar) O2 partial pressure and 70 g/L initial H2SO4. Extractions of Ni, Co and Cu exceeded 90% within 15-20 min and arrived at 95-97% after 45 min. The residues of leaching were identified as aggregates of crystalline hematite (Fe2O3) and amorphous silica. Dissolution of fayalite and magnetite was shown to be acid driven. Since HPOXAL operates with substoichiometric additions of sulphuric acid (10-20% of the stoichiometric requirement), acid regeneration facilitated by iron oxidation and hydrolysis is crucial to high rates of leaching. Low acidities (<10 g/L) were shown to cause precipitation of ferrous sulphate, slowing acid regeneration and slag dissolution. Elevated acidities (>70 g/L) result in excessive concentrations of Fe(III) in the leach solution, complicating downstream processing. The use of pyrrhotite tailings, an environmentally hazardous waste, as a substitute of sulphuric acid in slag leaching was investigated. Oxidative co-leaching of pyrrhotite tailings with naturally cooled converter slag at 250°C, 90 psi (6.2 bar) O2, 68 g/L equivalent H2SO4 was shown to have kinetics comparable to adding sulphuric acid with final extractions reaching 95-97% in 45 min. Granulation of slag melt can produce an amorphous solid solution of SiO2 and metal oxides. Amorphous slag is not amenable to HPOXAL due to the formation of a passive layer of silica. Leaching of amorphous slag at low temperatures was shown to proceed nearly to completion. The difference in the leachability of amorphous slag at high and low temperatures is explained in terms of the rate of silicic acid re-polymerization leading to closure of pores in the leached layer.