The leaching behaviour of enargite was studied under atmospheric pressure. The kinetics of enargite dissolution appear to be extremely slow under the conditions studied using both small scale shake flasks and bench scale stirred tank reactors. An enargite concentrate and a purer mineral specimen were utilized and the effect of numerous variables including temperature, particle size, acidity, oxygen flow rate, iron addition, redox potential and pyrite addition were investigated. The initial rate of copper dissolution from enargite was relatively fast up to about 40% copper extraction but this was followed by a much slower dissolution rate. The first stage is thought to be reaction rate controlled while the latter stage appears to be diffusion controlled. The activation energy was determined to be 32 kJ/mol for the concentrate and 33 kJ/mol for the pure specimen in the temperature range of 55-85°C. In the presence of high concentrations of copper sulphate, enhanced copper recovery was observed for the pure enargite specimen. This could be the result of a change in the reaction product layer on the enargite surface and perhaps the formation of intermediate Cu2S. Previous work has shown that adding excess pyrite enhances copper extraction from enargite but these results could not be reproduced. Particle size reduction appears to be the most effective method to increase extraction rates, however only 70% copper recovery and 35% arsenic recovery was achieved after 24 hours of leaching at d80=2μm. At these reaction rates, it appears that enargite dissolution at atmospheric pressure will not be an economical process compared to other technologies such as high temperature/pressure oxidation or roasting. / Thesis (Master, Mining Engineering) -- Queen's University, 2010-05-02 23:12:46.528

Potential new ore deposits containing significant levels of enargite, a copper arsenic sulphide mineral, are being considered for development. The processing of high arsenic copper concentrates directly in copper smelters is difficult due to environmental concerns. This thesis investigates a process using sulphation roasting as an alternative method for processing enargite concentrates; copper is recovered from the calcine by acid leaching, gold is extracted from the leach residue by conventional cyanidation and arsenic is either fixed in the calcine or precipitated from process emissions. In this research, sulphation roasting between the temperatures of 300-800oC, with varying oxygen and sulphur dioxide partial pressures, was investigated. Experiments indicated that high levels of copper extraction, as well as arsenic fixation, could be achieved from the produced calcines through hydrometallurgical processes. At operating temperatures between 400-550oC copper sulphate, copper arsenate, iron sulphate, hematite and iron arsenate form in the calcine, as well as some arsenic being volatilized as arsenic trioxide. At processing temperatures between 475-575oC, greater than 80% of the arsenic was retained in the calcine as copper and iron arsenates. Copper arsenate would be weak-acid soluble and fixed in an effluent treatment plant along with arsenic captured in the wet-gas scrubber bleed solution. As operating temperatures increase above 650oC copper sulphates were converted into oxysulphates, oxides and ferrites, hematite production was favoured, and arsenic was primarily volatilized. Increasing the sulphur dioxide addition in the reaction atmosphere resulted in additional sulphate formation and increased sulphate stability at higher temperatures. Sulphation roaster heat balances were developed for calcines produced at two temperatures, 500 and 750oC. They indicated that while high copper extraction and arsenic fixation rates could be achieved, the sulphation roasting reactions are highly exothermic and significant cooling water would need to be added. Due to these issues, it is likely that partial roasting operations would be economically favourable in greenfield operations. However, niche applications of this process in operations with existing copper SX/EW facilities in good acid markets, have the potential to be economically favourable. / Thesis (Master, Mining Engineering) -- Queen's University, 2012-08-17 20:14:36.292

<div>Global warming poses an existential threat to humanity and is inevitable unless significant efforts are made to eliminate its root causes. The need to replace fossil fuels with renewable sources has been obvious for many years, yet the world still receives the vast majority of its energy from non-renewable reservoirs. Harnessing solar radiation is the most promising route to ensure a carbon-free energy future, as the sun is the sole source of energy that can meet humankind’s energy demands for generations to come.<br></div><div><br></div><div>The most widely recognized technology associated with the sun is a photovoltaic (PV) cell, which converts electromagnetic radiation directly into electricity that can either be used immediately or stored for later use. Silicon-based solar cells currently dominate (>90% market share) the global PV market, driven in part due to parallel research in the microelectronics industry. However, silicon is an indirect bandgap material, resulting in inflexible solar modules, and it requires high capital expenditures and high energy inputs for terawatt scale manufacturing.</div><div><br></div><div>The remainder of the commercial PV market consists of thin-film technologies based on Cu(In,Ga)Se₂ (CIGSe) and CdTe. These materials have a direct bandgap, so they can be used in flexible applications, and they are readily scalable due to their amenability to low-cost, roll-to-roll manufacturing. The power conversion efficiencies of CIGSe and CdTe cells have exceeded 20% and are nearing those of silicon cells, but concerns over the long-term supply of indium and tellurium cast doubt on whether these materials can be deployed at large scales. Alternative materials, such as Cu₂ZnSnS_4-xSe_x (CZTSSe), have been researched for many years; the allure of a material with earth abundant elements and properties similar to CIGSe and CdTe was quite enticing. However, recent work suggests that CZTSSe is fundamentally limited by the formation of defects and band tails in the bulk material, and the efficiencies of CZTSSe-based devices have been saturated since 2013.</div><div><br></div><div>New materials for the PV market must meet several criteria, including constituent earth abundant elements, outstanding optoelectronic properties, and low propensity for defect formation. In this regard, the copper-arsenic-sulfur family of materials is an attractive candidate for PV applications. Cu, As, and S are all earth abundant elements with sufficiently different ionic radii, suggesting high defect formation energies. In addition, previous computational work has suggested that several ternary phases, most notably enargite Cu₃AsS₄, have appropriate bandgaps, high absorption coefficients, and high predicted efficiencies in a thin-film PV device. The system must be investigated experimentally, with attention not only paid to synthesis and device performance, but also to characteristics that give clues as to whether high efficiencies are achievable.</div><div><br></div><div>This dissertation studies the Cu-As-S system in the context of thin-film photovoltaics, with an emphasis on Cu₃AsS₄ and detours to related materials discussed when appropriate. The first synthesis of Cu₃AsS₄ thin-films is reported using solution-processed nanoparticles as precursors. Initial device efficiencies reach 0.18%, which are further boosted to 0.35% through optimization of the annealing procedure. Several limitations to the initial approach are identified (most notably the presence of a carbonaceous secondary phase) and addressed through post-processing treatments and ligand exchange. Cu₃AsS₄ is also rigorously characterized using a suite of optoelectronic techniques which demonstrate favorable defect characteristics that motivate continued research. The current limitations to Cu₃AsS₄ performance stem from improper device architecture rather than material properties. Further development of Cu-As-S thin films must focus on identifying and fabricating ideal device architectures in parallel with continued improvements to film fabrication.</div><div><br></div><div>This dissertation ultimately demonstrates high promise for Cu₃AsS₄ as a thin-film PV material. It also may serve as an example for other researchers studying new materials, as the examination of fundamental optoelectronic properties early in the material’s development phase is key to ensure that limited scientific resources are invested into the compounds with the highest potential impact on society.<br></div>

<p dir="ltr">Transitioning to a carbon-neutral future will require a broad portfolio of green energy generation and storage solutions. With the abundant availability of solar radiation across the Earth’s surface, energy generation from photovoltaics (PVs) will be an important part of this green energy portfolio. While silicon-based solar cells currently dominate the PV market, temperatures exceeding 1000 °C are needed for purification of silicon, and batch processing of silicon wafers limits how rapidly Si-based PV can be deployed. Furthermore, silicon’s indirect band gap necessitates absorber layers to exceed 100 µm thick, limiting its applications to rigid substrates.</p><p dir="ltr">Solution processed thin-film solar cells may allow for the realization of continuous, high-throughput manufacturing of PV modules. Thin-film absorber materials have direct band gaps, allowing them to absorb light more efficiently, and thus, they can be as thin as a few hundred nanometers and can be deposited on flexible substrates. Solution deposition of these absorber materials utilizing molecular precursor-based inks could be done in a roll-to-roll format, drastically increasing the throughput of PV manufacturing, and reducing installation costs. In this dissertation, solution processed synthesis and the characterization of two emerging direct band gap absorber materials consisting of earth abundant elements is discussed: the enargite phase of Cu₃AsS₄ and the distorted perovskite phase of BaZrS₃.</p><p dir="ltr">The enargite phase of Cu₃AsS₄ (ENG) is an emerging PV material with a 1.42 eV band gap, making it an ideal single-junction absorber material for photovoltaic applications. Unfortunately, ENG-based PV devices have historically been shown to have low power conversion efficiencies, potentially due to defects in the material. A combined computational and experimental study was completed where DFT-based calculations from collaborators were used inform synthesis strategies to improve the defect properties of ENG utilizing new synthesis techniques, including silver alloying, to reduce the density of harmful defects.</p><p dir="ltr">Chalcogenide perovskites are viewed as a stable alternative to halide perovskites, with BaZrS₃ being the most widely studied. With a band gap of 1.8 eV, BaZrS₃ could be an excellent wide-bandgap partner for a silicon-based tandem solar cell.Historically, sputtering, and solid-state approaches have been used to synthesize chalcogenide perovskites, but these methods require synthesis temperatures exceeding 800 °C, making them incompatible with the glass substrates and rear-contact layers required to create a PV device. In this dissertation, these high synthesis temperatures are bypassed through the development of a solution-processed deposition technique.A unique chemistry was developed to create fully soluble molecular precursor inks consisting of alkaline earth metal dithiocarboxylates and transition metal dithiocarbamates for direct-to-substrate synthesis of BaZrS₃ and BaHfS₃ at temperatures below 600 °C.</p><p dir="ltr">However, many challenges must be overcome before chalcogenide perovskites can be used for the creation of photovoltaic devices including oxide and Ruddlesden-Popper secondary phases, isolated grain growth, and deep level defects. Nevertheless, the development of a moderate temperature solution-based synthesis route makes chalcogenide perovskite research accessible to labs which do not have high temperature furnaces or sputtering equipment, further increasing research interest in this quickly developing absorber material.</p>