Solar energy has received considerable attention over the past few decades, due to its importance as a green and renewable energy. Low-cost solar-grade silicon production is critical for the widespread use of solar cells. Conventional routes (e.g., modified Siemens process: chlorosilane and hot filament) have still dominated the production of solar-grade silicon. The metallurgical route offers benefits in the productivity and cost, but efficient removal of boron is one of the most daunting challenges in front of us. This paper reviews thermodynamic and kinetic properties (solubility, diffusivity, diffusion coefficients, mass transfer rate, and activity coefficient) of boron and recent research topics (slag treatment, solvent refining, gas injection, plasma treatment, and acid leaching) for boron removal.@en

A sustainable simultaneous synthesis method of TiB2-Ti5Si3 mixtures is developed as an alternative recovery route of silicon waste from the photovoltaics industry. TiO2-CaO-B2O3 ternary was reduced by molten silicon at 1500 °C, producing TiB2 within 3 h and Ti5Si3 within 10 h. The lab-scale experiments were designed to study the reaction kinetics of the reaction. SiO2 diffusion rate controlling kinetic model was derived to describe the reaction process. The reaction rate constant was recommended to be ∼5 × 10-4 s-1. B2O3 content in slag was suggested to have an insignificant effect on the reaction rate constant, whereas more titanium and boron could be reduced by silicon with higher B2O3 content in slag. Stability of the reaction interface was simulated using phase field model. A decreased interfacial tension in B2O3-containing system was indicated to be the reason for emulsification of phases.@en

The increasing amount of silicon waste generated from the rapid developing photovoltaic industry calls for an economical silicon recycling process. The present work proposes a facile process with which silicon waste and ironmaking slag containing TiO2 were used as raw materials to produce titanium silicides, a promising high added value material. The process was experimentally investigated in lab scale. The result shows that a high CaO/SiO2 ratio in slag promotes the reaction. TiSi2 and Ti5Si3 could be synthesized as principal products within 0.5 and 3 h with CaO/SiO2 = 1.31 in mass, respectively. CaO-SiO2 slag was produced as byproducts. Kinetic analysis indicates that silicon diffusion in slag is the rate-determining step of the reaction process. The reaction rate constant is around 1.0 × 10-4 s, and the effective diffusion film thickness in slag side is around 10-3 cm at the silicon-slag interface. Slag basicity is suggested to increase to 1.31 for a faster silicon diffusion and further promotion of the reaction rate.@en

Ammonia gas blowing has been successfully applied to the silicon refining process for boron removal. Molten iron catalyst could effectively promote the boron removal ratio in the progress. It inspired the attempt of combining ammonia blowing with other refining treatments. Oxygen resources were introduced into the process in order to further decrease the boron content in silicon. However, neither slagging + ammonia blowing nor humidification + ammonia blowing shows a significant positive improvement in the silicon refining process, respectively. The experimental results implied that the introduction of oxygen sources into the ammonia system would decrease boron nitride activity or produce Si2N2O which retard the efficient generation of boron-bearing volatiles.@en

Nanometer-sized hematite was prepared via a two-step process. In the first step, FeSO4·7H2O was oxidized to Fe2(SO4)3 by oxygen in an acidic solution. In the second step, the Fe2(SO4)3 was reduced to nanosize hematite with sulfur vapor at 550 °C. The hematite has good thermal stability up to 500 °C and good colloid stability in water-based paint. Its properties satisfy the requirements of the international standard ISO 1248-A-I-1-a for an iron oxide red pigment. Graphical abstract: [Figure not available: see fulltext.]@en

To improve the boron-removal efficiency of metallurgical-grade silicon by increasing the reaction rate, a combined method with the 30 mol pct CaO-23.3 mol pct SiO2-46.7 mol pct CaCl2 slag treatment and ammonia injection at 1723 K to 1823 K was proposed. For 1 hour and at 1823 K, the maximum removal efficiency of boron was 98 pct, and the final boron concentration in silicon decreased to 1.5 ppmw by the present method without the introduction of the iron catalyst. A kinetic model was also established to clarify the reaction mechanism and rate-limiting steps of this complicated boron-removal process. In this model, the rate-limiting step is the mass transfer of boron oxide at the interface between the slag and silicon phase.@en