Extraction of silica from rice husk for the production and optimization of metallurgical-grade silicon for potential engineering applications

Conventional methods of silicon production; and carbothermic reduction, primarily rely on quartz or silica sand, which involves high-energy consumption and poses significant environmental challenges. Amidst this backdrop, the exploration of alternative sources for silicon extraction has become imperative. The utilization of rice husks which are considered renewable resources are produced every year in Nigeria. Despite its non-commercial value and causing environmental pollution, its ash content is rich in silica (SiO₂) and has become an economic potential source for the production of silicon. The high-purity metallurgical-grade silicon (MG-Si) was prepared by employing a successive acid leaching process followed by the reduction of rice husk ash (RHA) using magnesium as the reducing agent. The experiment was designed using a two-level-three-factor Box-Behnken Design (BBD), and 17 experimental runs were generated with operating process conditions of annealed temperature of 700-800^oC, ratio of HF:H₂SO₄ (1-9) and RHA: Mg ratio 1-1.5. The optimized MG-Si yield of 98.015 wt% was obtained at the process conditions of 799.365°C calcination temperature, 1:8.987 HF:H₂SO₄ ratio, and RHA: Mg ratio of 1:1.381. The produced MG-Si were characterized using XRF analysis and the results obtained indicated a MG-Si purity of 97.06wt% Si and 2.89wt% impurity respectively. Also, the XRF result showed that the impurities detected were Fe, K, Ca, Al, Ti, Cu, and Mg. The R² value of 0.9810 was achieved while the adjusted R² values of 0.9565 for the metallurgical grade silicon yield were in reasonable agreement with the predicted R² values of 0.9429, since their difference is less than 0.2. All of these validations showed that the experimental data for metallurgical grade silicon yield from rice husk ash matched the model's projected value accurately. Thus, optimizing the silicon production process has proven to be instrumental in achieving higher yields and better product quality. This not only enhances the overall efficiency of the production process but also contributes to the economic viability of silicon extraction from unconventional sources.