A low cost, low energy route to solar grade silicon from rice hull ash (RHA), a sustainable source

Abstract

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" CO 2 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 CO 2 . The carbon retained in the RHA is still from fixed CO 2 and provides the carbon source (especially in the Path 2 process) for carbothermal reduction returning the remaining carbon to the atmosphere as CO 2 . 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.

Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic, Green, and Sustainable Sources

“…If we only consider precipitated silica, then our process does not produce CO 2 or Na 2 SO 4 by-products,t hus making it ag reen, low carbon footprint, [32] low-temperature,a nd lowcost route to high-purity ppt SiO 2 .F inally,t he resulting carbon-enriched RHA, when treated with dilute HCl, provides ah igh-purity starting material for direct carbothermal reduction to produce silicon metal with 99.9999 %p urity without further purification. [32][33][34]…”

Comparative assessment of chemical treatments on extraction potential of commercial grade silica from rice husk

“…This silica is reported to be used for production of silicon‐based material such as concrete, laboratory glass wares, absorbents silicon carbide, silicon nitride, and zeolites . Silica extracted from RHA can also be used for production of metallurgical grade silicon (MG‐Si) and solar grade silicon (purity level equivalent to 99.99% and 99.9999%, respectively) reported by several authors …”

“…[7][8][9] Silica extracted from RHA can also be used for production of metallurgical grade silicon (MG-Si) and solar grade silicon (purity level equivalent to 99.99% and 99.9999%, respectively) reported by several authors. 10,11 Silica is generally found in the earth crust that exhibits either crystalline or amorphous structure. Silica extracted from RHA is usually in amorphous state with high surface area.…”

Vielfältige Nanostrukturen aus Reishülsen‐Biomasse für Energieanwendungen

“…Um die Leistung der Solarzellen weiter zu steigern, versuchte man, die Reinheit des RH‐Siliciums zu verbessern. Kürzlich haben Marchal und Mitarbeiter einen nachhaltigen Weg entwickelt, um Solarsilicium mit hoher Reinheit (99.9999 %) aus RH (1.6 kg Silicium pro Charge) herzustellen …”