Silicon and Polyethylenglycol recycling process   

 Most of PV wafers are produced from silicon (Si) (multi- or mono-crystalline)the production of these wafers happens by cutting silicon ingots by wire-saws.These wire-saws are using slurry that consist PEG (polyethylenglycol) as liquid and silicon-carbide (SiC) as sawing material. The thicknesses of the wafers vary from 100 μm - 180μm. So 50% or more of the silicon after the sawing process has been finished is part of the slurry. The production of silicon is very expensive so it could be profitable to reclaim silicon and silicon-carbide out of the slurry to reuse the silicon-dust for the production of wafers. It is recommended that as the first step the largest amount of the PEG has to be removed out of the slurry by centrifugation.


M.E.L. started together with the in the gases market well known German company MESSER Group GmbH, Sulzbach, Germany  a cooperation to develop a method for reclaiming the silicon-carbide, silicon and PEG from abrasive slurries wherein the abrasive slurries are first subjected  to a method for high pressure extraction by means of liquid or supercritical carbon-dioxide wherein components comprising oil and polyethylene are dissolved in carbon-dioxide. The resulting intermediate product containing silicon-carbide, silicon and in powder (dust)form is subjected under vacuum or inert gas atmosphere to a separating processf or separating into a fraction comprising silicon-carbide and a fraction of comprising silicon. Said separating process preferably consists of separatingin a fine screen, a sieve, or a cyclone. In particular high purity silicon can be reclaimed by means of this method.


A main advantage of the extraction by supercritical carbon-dioxide is the property of carbon-dioxide to be an inert gas that protects the silicon-powder  during the process against oxidization so the amount of silicon-oxide (SiO)/ silicon-dioxide (SiO2) in the extracted material is dramatically reduced.


MESSER Group GmbH and M.E.L: asked for international patents..




Silicon ingot before sawing