A low cost, low energy route to solar grade silicon from rice hull ash (RHA), a sustainable source
Polycrystalline silicon, with impurity levels lower than those of the SEMI III standard for solar grade silicon feedstock (≈99.9999% pure), was produced using rice hull ash (RHA) as a biogenic silica source. The RHA is first purified using very simple, low cost, low energy, acid milling/boiling water wash purification steps and pelletization followed by carbothermal reduction using an experimental 50 kW electric arc furnace (EAF) operated at 1700–2100 °C in batch mode. Typical processing involves adding 3.6 kg of pellets to the EAF followed by introduction of an additional 3.6 kg charge every 6 h after the start of carbothermal reduction. This approach produces up to 1.6 kg of silicon per batch. Purities, determined by inductively coupled plasma optical emission spectrometry (ICP-OES), were reproducibly found to be 99.9999 wt% (6 Ns) with B contents of ≈0.1 part per million by weight. This process escapes multiple process steps including the intermediacy of metallurgical grade silicon and the production and reduction of chlorosilanes as currently used in the Siemen's process. Furthermore, burning rice hulls to produce electricity and RHA, generates more energy than required for the overall process. Finally, the carbon footprint for the process discussed here is very low. The rice plant “fixes” CO2 as it grows. The recovered hull contains sufficient amounts of this carbon that it can be burned to generate electricity returning part of this carbon to the atmosphere as CO2. The carbon retained in the RHA is still from fixed CO2 and provides the carbon source (especially in the Path 2 process) for carbothermal reduction returning the remaining carbon to the atmosphere as CO2. A further point is that the alternative of landfilling with RHA or especially rice hulls would lead to generation of methane, a known green house gas. Thus, one might even argue that the carbon footprint for the process described here is actually negative.