The production of low-cost solar grade silicon from rice husk.

Rice husk, an agricultural waste product obtained in large quantities in many countries including Nigeria, is very rich in siliceous materials. It has been known for several decades that, with careful processing, rice husk can be a source of metallurgical grade silicon. In Nigeria this would have the benefit of transforming large volumes (> 600,000 tonnes per annum) of agricultural waste into a partial solution to that country's issue with energy distribution.In this work, silica of between 95.24% and 98.03% purity has been prepared from RHA (ashed at 700°C, 800°C, 900°C and 1000°C for either 5hrs or 12 hours). Additionally, the silica value was boosted by use of hydrometallurgical purification process. The improved purification processes yielded 99.18% and 99.51% of silica. Removal of many metallic trace impurities was significant: MgO (98.33%), AI2O3 (96.77%), Mn3O4 (80%), SO3 (55%), CaO (97.92%), B (73.91%) and P2O5 (88.34%) are removed by leaching. Impurities such as Na2O, Fe2O3 and K2O are almost completely leached out beyond detection of the XRF after the final processing step.Metallothermic reduction of the purified RHA with magnesium was investigated and post hydrometallurgical purification to further eliminate all soluble impurity. XRF and EDS showed P was reduced below their detection limit. The XRD showed that RHA transformation from amorphous to crystalline material depends on temperature and time. TEM investigation shows that derived silicon consist of agglomerate polycrystalline materials. TG analysed the the devolatilization, combustion and mass gain in RHA. The effectiveness of each stage of hydrometallurgical process in removing impurity elements was presented. While the hydrometallurgical purification of RHA is effective in removing impurities such as Ti and Fe to levels below the limits of detection of X-ray fluorescence (XRF), B levels was reduced to 22 ppm. Solvent refining process was done using Sn as a selected gettering metal for B in silicon.