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Silicon purification by plasma torch

Objective

Attractive results have been obtained in laboratory using thermal plasma to purify silicon. The present studies consist in developing an industrial pilot unit to prepare solar grade silicon feedstock. That will lead to present an economical evaluation of the process.
Attractive results have been obtained in the laboratory using thermal plasma to purify silicon. The present studies involve developing an industrial pilot unit to prepare solar grade silicon feedstock. An economical evaluation of the process will follow.

Studies have led to the identification of the main mechanisms involved in silicon purification. The plasma process is based upon the associated chemical and thermal properties. The first action is the melting of the silicon, the second one the extraction of the impurities in the slag melted material and at the slag plasma interfaces. The plasma properties depend on the inductive plasma composition (argon plus hydrogen plus oxygen): hydrogen increases heat conductivity of the argon plasma and the heat transfer between the plasma and the material. The redox properties depend on oxygen flow: in particular, boron is eliminated as boron oxide. Extraction mechanisms of impurities at the plasma material interface depend on the molten bath temperature, viscosity, and the slag chemical composition.

Most of these results have been obtained with a 7 kW power plant. A 25 kW power unit was also built. The sample section has been increased from 2.2 cm{2} to 15 cm{2}. The size extrapolation has induced heavy technical modification especially on the torch and the reactor materials. The main components of the industrial installation (induction power, torch, reactor, gas environment and process control) can be determined from the adaptation of the present equipment.

Once the pilot equipment is installed, purified samples (50 to 100 cm{2}) will be used as feedstock in the POLIX ingot process leading to an economic evaluation of the plasma purification process.
Silicon purification by thermal plasma has been developed by the laboratory of plasma Technical Reactors (ENSCP) of Prof. Amouroux for 8 Years.

On the laboratory scale, the plasma purification efficiency has been proved:

- purification grade 10E5 10E6
-linear silicon treatment rate : 1m/h
- type of silicon
electronic silicon rejects (out of photovoltaic specifications )
selected metallurgical silicon
rejects of photovoltaic casting. rk=(01,02)

Studies have led to identify the main mechanisms involved in silicon purification. The plasma process is based upon the associated chemical and thermal properties. The first action is the melting of the silicon, the second one the extraction of the impurities at the slag-melted material, and slag-plasma interfaces. The plasma properties depend on the inductive plasma composition (Ar + H2 + O2): hydrogen increases heat conductivity of the argon plasma, and also the heat transfer the plasma and the material. The redox properties depend on oxygen flow; in particular, boron is eliminated as boron oxide. Extraction mechanisms of impurities at the plasma-material interface depend on The molten bath temperature and viscosity, and on the slag chemical composition.

Most of these results have been obtained with a 7 kW power plasma. A 25 kW power unit was also built. The sample section has been increased from 2.2 cm2 to 15 cm2. The size extrapolation has induced heavy technical modifications specially on the torch and on the reactor materials. The adaptation of this equipment allows to determine the main components of the industrial installation : induction power, torch, reactor, gas environment and process control.

Then the pilot equipment will be built by CFEI and installed in PHOTOWATT. The purified samples (50 to 100 cm2) will be used as feedstock in the POLIX ingot process. The project will lead to evaluate economically the plasma purification process.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Photowatt International SA
Address
33 Rue Saint-honoré Zone Industrielle Champ Fleuri
38300 Bourgoin-jallieu
France