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The lignin hydrothermal processing is an important option but a full understanding of the role played by the water molecules in the depolymerization of lignin is still lacking. In order to clarify the role of the water molecules in the depolymerization of lignin, the evolution of chemical bonds, microstructural changes, and possible mechanisms of product generation were compared during the pyrolysis process under vacuum and water conditions using Reactive Molecular Dynamics Simulation. Compared with vacuum conditions, the role of water changes with temperature, identifying three stages: promotion (1200-1800 K)-inhibition (2100-2400 K)-promotion (2700-3000 K). Also compared with vacuum conditions, hydrothermal processing can promote the cleavage of the ether bonds while inhibiting the destruction of carbocycles. Water molecules promote the depolymerization of lignin into more C

Silicon kerf generated during the diamond-wire slicing process contains various impurities and are considered industrial waste. In order to utilize the waste, a feasible process to purify silicon kerf by a combination of slag treatment and acid leaching was developed. The results demonstrated that the Na2O SiO2 slag refining can effectively enhance the yield of the silicon kerf, and the Si yield was found to be 63.2%. The combined processes were able to reduce the total concentration of major impurities (Fe, Al, Ca, Ni, B, and P) from 6998 to 58 ppmw with a removal efficiency of 99.2%. The purity of the recovered Si was upgraded to from 93.3% to 99.98%. The mechanism suggests that impurity removal was facilitated by both the oxidation and the segregation of impurities to intermetallic precipitate phases. These precipitates can then be eliminated by acid leaching, and the extraction process of metallic impurities was mainly dominated by the surface chemical reaction.

Bauxite residue, also known as red mud, is generated during alumina production and is an abundant industrial waste material. Continuously increasing environmental concerns, together with scarcity of traditional mineral resources, have created a thrust to re-use the material. Red mud contains significant amounts of iron oxide and sodium hydroxide, hence a highly basic (pH > 10) slurry. In this research, the use of red mud as starting material for preparation of iron refining fluxes was evaluated. Red mud based fluxes and hot metal were equilibrated in graphite crucibles at the temperature range of 1300 ºC to 1400 °C and oxygen partial pressures range of 10-2 atm to 10-6 atm. It was found that the sulphide capacity increases with lime addition to a maximum 32 wt% CaO and decreases with increasing A12O3, TiO2 and SiO2 content in the fluxes saturated with lime. An iron foil equilibrium technique was employed to obtain precise measurements of phosphorus distribution between carbon saturated iron and red mud based fluxes. The measurements indicate that the equilibrium phosphorus distribution ratio initially increases with rise in FeO or CaO concentration of the fluxes and then drops. The melting behavior of the fluxes was also studied by visualizing the deformation of flux pellets as they were heated using a high temperature microscopy technique. Measurements of characteristic temperature for different fluxes indicated the melting property is a function of slag basicity. Therefore, optical basicity was used to establish a correlation between basicity of the red mud based fluxes and their melting properties.

Titanium (Ti) and calcium (Ca), as trappers, were introduced into copper-alloyed metallurgical grade silicon (MG-Si). The effects of trappers on the segregation behavior and removal efficiency of phosphorus (P) were investigated. It was found that P tended to enrich in the TiSi2 and CaCu2Si2 phases, and quenching above the eutectic temperature of Si-Cu alloy is conducive to the P removal. The corrosion process of the P-enriched phases indicated that Ca addition was in favour of P removal, while Ti addition was opposite. The results suggested that though the addition of Ti and Ca could effectively enrich P in the Si-Cu alloy, the acid sensitivity of P-enriched phases would affect the P removal.

Enhanced removal of P from Si-Cu alloy was obtained by Ca addition. The effectiveness of Ca addition on the phase reconstruction and P gettering in Si-50 wt%Cu alloy was investigated. It was shown that the introduction of Ca can achieve the phase reconstruction of Si-50 wt%Cu alloy by the formation of CaCu2Si2 and CaCu11Si5 phases, which were precipitated in the Cu3Si phase. Dense P was found to homogeneously precipitated in the CaCu2Si2 phase, indicating that the increase in the amount of the CaCu2Si2 phase would collect more P. It was found that the extraction fraction of P was proportional to Ca content in Si-50 wt% Cu alloy. The removal fraction of P was increased from 27% to 82% when the Si-Cu system was doped with 5 wt% Ca. The thermodynamic analysis of P removal was investigated, showing that the Ca addition could effectively decrease the segregation coefficient of P between solid Si and the Si-Cu melt. It was concluded that Ca could be used as an impurity trapper to facilitate the P removal in the Si-Cu alloy.

Silicon kerf waste can be used as a secondary resource for the recovery of high purity silicon for solar cells production. Boron is amongst the most deleterious impurities in silicon and its concentration should be strictly controlled. Slag treatment is considered an effective method to extract impurities, especially boron, from silicon kerf. The study investigated the feasibility and optimization of a process for producing high-purity silicon from sawing waste by slag treatment. The slag system studied was Na2O-SiO2 and the effects of holding time, slag composition, and slag:kerf ratio on boron distribution were investigated. The optimum conditions for refining of silicon kerf at 1923 K were found to be a slag of 65 wt.% Na2O−35 wt.% SiO2 and treatment for 30 min. This resulted in reduction of boron concentration in silicon from 8.6 to 1.0 ppmw, corresponding to a removal efficiency of 88%. Investigation of the boron transfer mechanism indicated that mass transfer of boron in slag is likely the rate controlling step of the overall process. The mass transfer coefficients for boron in silicon and slag were 3.6 × 10−6 cm⋅s−1 and 5.8 × 10−6 cm s−1, respectively.

In this study, titanium was introduced to a Cu-Si alloy as an impurity getter to redistribute boron. A small amount of Ti was melted with Cu-Si alloy, followed by cooling of the alloy to precipitate purified Si crystals which were later separated through multi-step leaching. The effects of Ti content on the distribution of various elements in different phases and microstructure of the solidified alloy were investigated. It was found that Ti in Cu-Si alloy facilitates boron removal by several mechanisms including formation of TiB2 and decreasing the segregation coefficient of B between Si and Cu-Si. Addition of Ti to a Cu-50 wt%Si alloy, compared to silicon metal, showed a significant increase in B removal. An increase in Ti in the range of 1–5% increased B removal, reaching ∼85% at 5% Ti addition. Ti in excess of that required to form TiB2 reacts with Si to generate TiSi2, which collects impurities such as P and Al.

Electrowinning of Si from cryolite-based melts is a possible solution for mass production of high purity silicon. The required cell potential to deposit Si on a cathode of interest, copper, is fundamental information that needs to be measured for controlling the co-deposition of impurities. In this study, the potential was measured using cyclic voltammetry in a cryolite-6 pct SiO2 melt. The deposited Si on copper forms a Cu-Si alloy in which the activity of Si affects the decomposition potential. The measurements were carried out in a range of Si concentrations, allowing the prediction of potential change during the actual deposition process, where the Si content changes constantly. The results were compared with the values obtained from cyclic voltammetry on an inert electrode, graphite, to investigate if the formation of Cu-Si alloy is responsible for decreasing the potential.