Impurity optimized silicon is needed for the advancement of terrestrial photovoltaic power generation. In this study the possibility of producing solar grade silicon from rice husks has been pursued. An integrated process flowsheet was developed and practiced that included initial leaching, reduction of Rice husk ash (RHA) and post-reduction purification of silicon. Metallothermic reduction of purified RHA with magnesium was investigated within the temperature range of 500-950 oC. The reduction product was purified by two stage acid leaching sequence. Analysis of the final silicon powder product by XRD and ICP-OES showed crystalline silicon with boron content to be less than 3ppm- corresponding to reduction by a factor greater than 10, whilst the phosphorus content was reduced by a factor of over 20 and reaching less than 73ppm. The effects of temperature, magnesium amount and leaching agents were optimized in this study. A one step test melting was also carried out to convert the silicon powder into silicon chunks.

The improvement in mechanical strength offered by ultra fine- (UF) and nanocrystalline (NC) sized grains is very attractive for potential applications of structural metals. Accumulative Roll-Bonding (ARB) is one of the promising new techniques for producing bulk UF grained metals. There are numerous reports on the monotonic mechanical behavior of various ARBed metals, however there are few, if any, on the cyclic deformation behavior of such metals. The primary objective of this study is to investigate the cyclic deformation behaviour and the related micro-mechanisms of ARBed metals from a fundamental perspective. To achieve this, the microstructure and the deformation behavior of commercial purity aluminum, OFHC copper, and DLP copper after ARB processing have been systematically characterized. The as-ARBed microstructure is found to be composite natured, with constituents of different grain sizes. The three constituents are: (i)UF grained matrix, (ii)NC primary discontinuities, and (iii)conventional sized pre-existing coarse grains. Due to this composite nature, three different cyclic strain accommodation mechanisms were found in the ARBed OFHC copper: (i)conventional dislocation patterns in the large grains, (ii)reactivation of pre-existing shear bands, and (iii)stress/strain driven grain coarsening at sites of strain localization. The order of activation of the mechanisms can be described with a composite approach based on activation energy. The occurrence of grain coarsening is the major contributor to the cyclic softening response observed in OFHC copper. Conversely, the lesser extent of cyclic softening in the other two metals is likely due to the higher microstructure stability of the initial as-ARBed materials. The microstructure stability is believed to be the primary influencing factor for the extent of grain coarsening and cyclic softening. The applied cyclic plastic strain is a secondary influencing factor, although this is generally overshadowed by the limitation of grain coarsening due to the short cyclic lifespan of these metals. The occurrences of shear banding and grain coarsening reported in the present ARBed metals are similarly reported for UF grained metals from other processes, e.g. ECAPed metals. Thus, its relationship to the cyclic deformation response and governing factors are believed to be applicable for UF grained metals in general.

Selective oxidation and sulfation roasting of nickel concentrate followed by leaching was investigated as a novel route for nickel production. In the oxidation roasting stage, the iron species in the nickel concentrate was preferentially oxidized to form iron oxides, leaving non-ferrous metals (Ni, Cu, Co) as sulfides. The roasted product was then sulfation roasted to convert the sulfides of the latter metals into water-soluble sulfates. The sulfates were then leached into solution for further recovery and separation from iron oxides. The oxidation of nickel concentrate was firstly studied by means of thermogravimetric and differential thermal analysis over a wide temperature range. A reaction scheme was deduced, in which preferential oxidation of iron sulfide species occurred over a wide temperature range up to about 700 ºC, forming a Ni1-xS core with iron oxide shell. A batch fluidized bed roaster was then constructed to study the oxidation and sulfation roasting of nickel sulfide concentrate. Oxidation roasting tests were carried out at temperatures between 650 °C and 775 °C. It was found that low temperatures (e.g. 650 °C) are favorable for the preferential oxidation of iron sulfide species while minimizing the formation of nickeliferous oxides, i.e. trevorite and NiO. Several parameters were varied in the sulfation roasting experiments, including the sulfation gas flowrate, sulfation roasting temperature, the addition of Na2SO4, sulfation roasting time, and the oxidation roasting temperature. Under optimized conditions of sulfation gas composition (95% air, 5% SO2), temperature (700 °C), Na2SO4 addition (10 wt%) and time (150 min), the conversions to sulfates were 79% Ni, 91% Cu, and 91% Co. Only 5% Fe forms water-soluble sulfate. The residue from the leaching of calcine in water contained 49% Fe and 10% Ni, which is a suitable feedstock for the production of ferronickel alloys. Therefore, further studies were also conducted to evaluate the reduction behavior of the residue with CO, H2 and graphite.

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.