Recovery and Utilisation of Kerf Waste from Silicon Wafering Process
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While the green credentials of solar photovoltaic technology are often touted, there is still substantial room for improvement in the silicon solar photovoltaic value chain. In particular, silicon kerf which is a byproduct of the wafering process represent a loss of nearly half of the high purity silicon feed in value chain. Thus recycling and recovery of the kerf presents opportunities for waste reduction as well as product development. The current study characterizes silicon kerf, as well as investigates two possible applications for the kerf; conversion to the engineering material silicon carbide via high temperature reaction with carbon monoxide and remelting of the kerf to stock silicon which can be returned to the value chain. The kerf particles were found to be small platelets, with mean diameter approximately 0.9 μm, having an overall metallic impurity level below 1000 ppm and oxygen content under 2% by weight, corresponding to an approximate oxide layer thickness of 10 nm on the kerf particles. Reaction with carbon monoxide yielded powders with various hues which depended upon maximum temperature exposure, and larger particles being present due the agglomeration of the kerf at high temperature. Partial conversion of the kerf silicon to silicon carbide and silicon oxides was detected. Nanofibres were also found on the exposed powder sample surfaces, becoming more prominent with higher temperature exposure. The melting of small batches of kerf was carried out successfully at 1500°C, with segregation of some carbide and oxide impurities towards the ingot surfaces, and the bulk material of the ingot appearing relatively pure. The kerf required relatively little treatment prior to melting, although the low density of the powder is likely to present practical handling difficulties at larger scales.