Chemical and Biomolecular Engineering - Theses
Permanent URI for this collection
Search Results
1 - 4 of 4
-
ItemGeopolymerisation of aluminosilicate minerals( 2002-04)Geopolymerisation can transfer A1 and Si containing wastes into geopolymers with a high mechanical strength and high fire, acid and bacteria resistances. This thesis investigates the geopolymerisation of natural aluminosilicate minerals which cover a wide range of structures, minerals groups, hardness, densities and A12O3 and SiO2 contents. The study of fifteen selected A1-Si minerals shows that the framework structured A1-Si minerals with a high CaO content and a lower K2O content, have a higher potential to undergo geopolymerisation in KOH solution with resulting geopolymers possessing a higher compressive strength. The NaOH solution is observed to leach more A1 and Si complexes from the solid reactants than KOH solution, while KOH solution produces geopolymers with a higher mechanical strength than NaOH solution does. (For complete abstract open document)
-
ItemThe development of an aqueous leaching process for the treatment of spent pot lining( 2001)Spent pot lining (SPL) is the failed cathode lining of the electrolytic cells used in the production of aluminium metal via the Hall-Heroult process. It consists of carbon, refractory brick material and various residual components of the electrolytic bath. SPL is classified as a hazardous waste because it contains significant quantities of leachable fluoride, predominantly present as the compounds sodium fluoride, cryolite and calcium fluoride, and cyanide, usually present as sodium cyanide and sodium ferrocyanide. As such, it cannot be disposed of in a conventional manner. Moreover, major restrictions are also imposed on the handling, transportation and storage of SPL. Although numerous processes for treating this solid have been investigated, few have proceeded beyond pilot plant stage. Of those that have, virtually all employ high temperatures to destroy cyanide by oxidation. However, only a small number of these attempt to recover the fluoride values for recycling. Currently, the safe treatment/disposal of SPL constitutes a major concern for the aluminium smelting industry because there is, as yet, no universally accepted technology for the treatment of this waste. The research presented in this thesis attempts to address the SPL problem. It is specifically aimed at the development of an efficient aqueous leaching process for the detoxification of SPL in a manner which ultimately enables the recovery and recycling of fluoride. “Wet de-lined” SPL, from a smelter operated by the Tomago Aluminium Company in New South Wales, Australia, was used throughout this study. Importantly, it was established early in the investigation that subjecting the SPL to a water leach, in the first instance, results in the dissolution of most of the cyanide and the complete extraction of sodium fluoride and sodium carbonate from the solid. The removal of sodium fluoride proved to be a significant result because that compound was found to constitute approximately 50% by weight of the fluoride initially present in the SPL. A thorough investigation on the feasibility of using aqueous fluoro-acids as extractive reagents for the remaining fluoride and recoverable aluminium values, present in water washed SPL, was subsequently undertaken. It was discovered that hydrofluoric acid, fluorosilicic acid, fluoroboric acid and monohydroxyfluoroboric acid each react with the oxidic compounds present in water washed SPL, such as Al(OH)3, NaAISiO4, SiO2 and CaCO3, to give fluoridated products, such as Na5Al3FI4, Na2SiF6 and CaF2, the precipitation of which are highly dependent upon the initial concentration of acid used. An in depth discussion on the possible mechanism of formation of these products is given in this thesis and it is concluded that the primary factors contributing to their precipitation are the overall fluoride ion concentration in solution and the presence of a large quantity of Na+ in the SPL. Significantly though, it was established that regardless of the initial concentration of fluoro-acid used, or the level of extraction achieved, the overall fluoride content of the water washed SPL increased with respect to that originally present prior to treatment with the acids. This result alone excludes the possible use of fluoro-acids as effective leachants for SPL. Consequently, it was decided to implement a totally different approach towards the treatment of water washed SPL. It was concluded that a more effective leaching strategy should focus solely on the total extraction of fluoride without attempting to recover the abundant aluminium values also present in the material. As such, an investigation on the leaching of fluoride from water washed SPL by aqueous solutions containing Al3+ was undertaken. Aqueous Al3+ proved to be a highly effective leachant in that it was demonstrated that approximately 95% of the fluoride in water washed SPL could be extracted by suitable control of reagent concentration and reaction temperature. Furthermore, it is hypothesized that optimization of other reaction parameters, such as SPL particle size, may lead to even higher levels of fluoride extraction. Once the efficacy of aqueous Al3+ to leach fluoride from water washed SPL was established, it was decided to investigate a possible means of generating Al3+ leachates from readily available materials. It was shown that a cheap and abundant mineral called nepheline syenite, which is used extensively in the glass manufacturing industry could be easily decomposed by dilute aqueous mineral acids to generate solutions of Al3+ which are capable of dissolving cryolite and calcium fluoride. These two fluorides are the main fluorides present in water washed SPL. Moreover, it was demonstrated from "model" compound studies that by suitable adjustment of the molar ratio of total fluoride to total aluminium, in leaching mixtures containing cryolite, calcium fluoride and aqueous Al3+, selective dissolution of the cryolite can be achieved. The option of selectively extracting cryolite from water washed SPL may simplify the procedure by which fluoride is ultimately recovered from the leachates. Finally, as a conclusion to the work presented in this thesis, a description of the unit operations of a conceptual process for the treatment of SPL is given. Included in the description are various chemical options which could be used to recover fluoride from the leachates in a form amenable towards recycling back into the aluminium smelting process.
-
ItemThe leaching and adsorption behaviour of gold ores( 2000)There are many non-idealities in gold processing which are becoming increasingly important as simple free-milling oxide ores are depleted. During cyanidation and adsorption onto activated carbon, these effects include competing metal-cyanide complexes, preg-robbing and the mineralogy of the ore. This thesis considers these areas from an experimental and modelling viewpoint. In this research it is shown that a variety of metal-cyanide species can be used to leach gold from certain refractory ores. A mechanism is proposed which involves the dissociation and precipitation of these metal-cyanide complexes, where the cyanide from the dissociation of the complex is used to leach gold from the ore. The stepwise dissociation of cyanide from copper-cyanide complexes can also be used to leach gold from a variety of oxide and sulphide ores. The copper cyanide's fourth ligand readily dissociates at low cyanide concentrations for use in gold leaching. The extent of preg-robbing onto mineral surfaces was also shown to be related to the metal-cyanide complexes in solution, with these complexes serving to inhibit preg-robbing. However, free cyanide was found to be much more important in preventing preg-robbing. This phenomenon occurs in cyanide-deficient solutions and the extent of preg-robbing is related to the cyanide consuming ability of the ore, rather than simply the ore's mineralogy. Activated carbon was found to compete strongly with the ore for aurocyanide in solution, although when no cyanide was present the kinetics of the two processes governed the distribution of gold between the ore and the carbon. Gold was found to reduce at the chalcopyrite surface and a mechanism for this reduction was proposed. For pyrite, a combination of reduction and physical adsorption was observed. The effect of preg-robbing on the enhanced extraction of gold in the presence of activated carbon was closely examined for a number of different ore types. For the Telfer oxide ore, which was not found to be preg-robbing, enhanced gold extraction was observed at low cyanide concentrations, but not at high concentrations, as the gold was completely extracted by cyanide alone. The flotation feed and copper concentrate showed an enhancement in gold extraction due to the activated carbon adsorbing gold that would otherwise have been preg-robbed. However, this mechanism did not explain the behaviour of the pyrite concentrate. For this ore the enhanced extraction was due to a combination of equilibrium and preg-robbing effects. Diagnostic leaching was used to show that mineralogy was not an important factor in the enhanced gold extraction in the presence of activated carbon for the ores. Kinetic models were developed for the leaching of gold from the ore, adsorption onto activated carbon and preg-robbing at the ore surface. A variable order empirical model of the Mintek form was used to describe leaching, while adsorption was modelled using the film-transfer model. A model of this form was also used for preg-robbing using the Freundlich isotherm to describe the equilibrium between the solution and the surface of the ore. These models successfully accounted for the individual sub-processes and when combined into an overall batch model were used to show the effect of preg-robbing on gold extraction from ores of different mineralogy. The effect of preg-robbing was found to be closely related to the kinetics of the preg-robbing and adsorption processes. This batch model was extended to a continuous model that simulated three leaching and seven carbon-in-Ieach tanks. The effect of processing the oxide and sulphide ores studied was examined and the model shown to satisfactorily describe the extraction of gold from the Telfer oxide ore. A sensitivity analysis of the effect of process changes on this ore was then undertaken, where the correct operating response to preg-robbing ores and process changes were determined.
-
ItemImproving the sustainability of mining waste management( 2007)The thesis examines the issue of minimizing large, long-term tailings management liabilities and preventing those liabilities from being bequeathed to taxpayers. It commences by questioning whether demand for metals is likely to wane and thus render the issue moot, finding that demand is likely to increase; combined with declining ore grades the issue of tailings management will become more prominent. The public’s perception of the industry has resulted in an imperative to attend to sustainability issues that has begun to be addressed with managerial tools. Technical approaches for tailings management were examined, concluding that improved disposal methods are indeed available but are not utilized, largely due to the initial capital expenditure required despite potential long-term cost savings and risk mitigation. The possibility of mandating technological solutions through regulations is considered, as well and non-regulatory approaches for achieving environmental outcomes. The example of the United States illustrated that regulation is reactive, expensive, often ineffective and focuses on minimum requirements rather than best practice. The politicized nature of mining can render it difficult to for governments to proactively manage environmental impacts despite the fact that the environment is essential for society’s longevity. The global nature of the industry is highlighted, further emphasizing the weaknesses of regulatory controls, particularly through under-resourced developing country governments; international environmental regulations are unlikely to eventuate. International environmental law is nascent; the most promising advances in environmental management have been made under voluntary initiatives. There are few mechanisms to ensure transnational accountability. The crux of the thesis lies within the examination of current financial reporting requirements; such requirement were found to perpetuate short-term managerial attitudes by requiring arguably arbitrary discounting of long-term liabilities. The emphasis of accounting standards on legal liabilities is a disincentive for best practice implementation. Accounting standards sellers promote market-based measures for the convenience of investors despite the possible adverse consequences of industry failing to accurately quantify and be held accountable to long-term environmental costs. Market-based measurement objectives promote under-evaluation of liabilities by requiring the inclusion of third-party-related factors. It was shown from case studies that standardized methods of estimating actual future tailings remediation costs are possible, supporting the development of an industry-specific accounting standard to address long-term cost management issues. Finally, an improved financial assurance mechanism is proposed to ensure that the mining company sets aside actual funds in a trust for closure; financial reporting auditing requirements could be combined with financial assurance verification to ensure that adequate provision is made for closure. By requiring the accurate estimation of and provision for future remediation costs, financial reporting and assurance requirements will promote the uptake of more sustainable tailings management procedures.