Objectives and problems to be solved:
The major objective of the RE-Si-CLE project is the demonstration of technologies that allow the recycling of 75% of the silicon rejects contained in exhausted wire cutting slurries. On an economical level the project goal is the demonstration of recycling costs below 20€/kg Si. In addition the project developments should allow the PV industry to reduce their industrial wastes from currently 30tons of exhausted slurries per MW produced PV power to 5 tons. During the slicing of silicon blocks into thin photovoltaic wafers by multiple wire saw technology, around 34% of the silicon (or 5 tons of silicon per MW produced PV power), that entered the PV production as silicon feedstock, is lost in form of fine silicon powders which accumulate in the wire sawing slurries. These slurries usually consist of a liquid, either polyethylene glycol (PEG) or oil, and fine silicon carbide grains, which due to their high mechanical hardness are responsible for the cutting action at the places where the wire interacts with the silicon block. As a result, after a certain number of wire cutting cycles and depending on the applied cutting processes, the exhausted slurries contain up to 10% of silicon.
The technical objectives of the project can be summarised as follows:
I. separation of silicon powders from exhausted wire sawing slurries by a combination of mechanical and chemical treatments
II. Conditioning and purification of the separated silicon by plasma evaporation of residual impurities to arrive at suitable silicon feedstock
III. Demonstration of highly performing solar cells (with of 16%) on multi-crystalline wafers produced from the recycled silicon Work performed and achievements to date.
The following points have been achieved during the first 12 months of the project work:
- Granulometric and chemical analysis of different types of fresh and exhausted slurries
- Application and adaptation of existing equipment for the recycling of SiC and the liquid in order to produce a Si rich paste-like residue mix called "sludge" with the highest SiC and liquid removal yield possible
- Chemical and mechanical approach to remove Fe and other metals from the sludge, general feasibility of the processes
- Pre-fusion treatment to remove the residual liquid from the sludge and to arrive at solid silicon pieces for the plasma purification
-Definition of most effective plasma purification conditions (highest performance and minimal Si losses), in terms of processing windows by chemical modelling of the plasma purification process
- Calculation of acceptable impurity concentrations in the silicon at different stages of the recycling and purification treatment in order to arrive at high efficient solar cells.
Funding SchemeCSC - Cost-sharing contracts
79110 Freiburg (In Breisgau)
38300 Bourgoin Jallieu