Potential Synthesis of Solar-Grade Silicon from Rice Husk Ash

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 [1]. The question remains as to whether rice husk ash (RHA) can be purified by a cost-effective, low technology route to produce solar-grade silicon suitable for use in photovoltaic devices. 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, high purity silica has been prepared from RHA (ashed at 700°C for 5 and 12 hours) using a hydrometallurgical process. We report on the effect of natural variations in the rice husk composition on the effectiveness of the hydrometallurgical purification; the effectiveness of each stage of the hydrometallurgical process in removing impurity elements. 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 and P levels need to be reduced to < 1017cm-3 well below the detection limits of XRF. The resultant silica has been subsequently reduced to metallurgical-grade silicon (MG-Si) by direct reduction using Mg powder.