Phosphorus separation from metallurgical-grade silicon by magnesium alloying and acid leaching

Research on Carbon Emission of Solar Grade Polysilicon Produced by Metallurgical Route Using Digital Simulation Technology

Under the enormous pressure of carbon reduction, we need to have a clear understanding of the environmental impact of the energy-intensive and high-emission polysilicon industry. With the rapid development of technology, we now have the ability to monitor the inflow and outflow of materials in enterprises, so as to obtain the life cycle inventory required for environmental impact assessment. And solve the problems of large data collection workload and long working cycle encountered in conventional life cycle assessment. By combining digital simulation technology and life cycle assessment, we analyze carbon dioxide (CO2) emission in each production process of 1 kg solar grade polysilicon (SoG-Si) by metallurgical route (MR) in detail. We not only analyze four typical production processes of MR, namely slag refining, hydrometallurgy, directional solidification and electron beam refining. The production process of metallurgical grade silicon is also analyzed. It is obtained that the production of 1 kg SoG-Si by MR will produce 69.77 kg CO2. The contribution analysis shows that the CO2 produced by electron beam refining, metallurgical silicon smelting, secondary directional solidification and primary directional solidification is more significant, reaching 38.47%, 20.88%, 15.84% and 14.50%, respectively. The sensitivity analysis shows that the sensitivity of electric power in the process of electron beam refining, secondary directional solidification, primary directional solidification and metallurgical silicon smelting is significant, reaching 38.47%, 15.77%, 14.45% and 13.81%, respectively. In addition, according to the analysis results, the improvement suggestions to reduce CO2 emission are given.

A new strategy for de-oxidation of diamond-wire sawing silicon waste via the synergistic effect of magnesium thermal reduction and hydrochloric acid leaching

With the rapid development of photovoltaic industry, the amount of diamond-wire sawing silicon waste (DWSSW) increased dramatically. Recently, how to achieve cost-effective deoxygenation of DWSSW should be the most critical issue in silicon waste recycling. Herein, a new strategy for de-oxidation from DWSSW, employing Mg vapor as de-oxidation agent and hydrochloric acid (HCl) as leaching agent, under low temperature was suggested. The de-oxidation mechanism and the mineralogical evolution of different phases during the magnesium thermal reduction and acid leaching were clarified. Results showed that the impurity oxygen in DWSSW exists as amorphous SiO₂ on the surface of Si particles, which can be eliminated by transforming it into crystalline MgO using magnesium thermal reduction and followed by acid leaching. As the reduction condition was controlled at 1333 K for 2 h, the residual oxygen content in DWSSW was 0.9643% and the de-oxidation fraction attained 93%. Additionally, the removal rate of impurities Al and Fe were 99.06% and 92.20%, respectively, and the Si recovery rate achieved 88%. Taking into consideration the recoverability of Mg, the cost of the process will be reduced significantly, making this process more viable in industry.

Thermodynamics Analysis and Removal of P in a P-(M)-H2O System

In order to efficiently remove phosphorus, thermodynamic equilibrium diagrams of the P-H₂O system and P-M-H₂O system (M stands for Fe, Al, Ca, Mg) were analyzed by software from Visual MINTEQ to identify the existence of phosphorus ions and metal ions as pH ranged from 1 to 14. The results showed that the phosphorus ions existed in the form of H₃PO₄, H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻. Among them, H₂PO₄⁻ and HPO₄²⁻ were the main species in the acidic medium (99% at pH = 5) and alkaline medium (97.9% at pH = 10). In the P-Fe-H₂O system ((P) = 0.01 mol/L, (Fe³⁺) = 0.01 mol/L), H₂PO₄⁻ was transformed to FeHPO₄⁺ at pH = 0–7 due to the existence of Fe³⁺ and then transformed to HPO₄²⁻ at pH > 6 as the Fe³⁺ was mostly precipitated. In the P-Ca-H₂O system ((P) = 0.01 mol/L, (Ca²⁺) = 0.015 mol/L), the main species in the acidic medium was CaH₂PO₄⁺ and HPO₄²⁻, and then transformed to CaPO₄⁻ at pH > 7. In the P-Mg-H₂O system ((P) = 0.01 mol/L, (Mg²⁺) = 0.015 mol/L), the main species in the acidic medium was H₂PO₄⁻ and then transformed to MgHPO₄ at pH = 5–10, and finally transformed to MgPO₄⁻ as pH increased. The verification experiments (precipitation experiments) with single metal ions confirmed that the theoretical analysis could be used to guide the actual experiments.