silicon for the chemical and solar industry XIII

Conclusion

Increased conversion of carbon in the raw materials to SiC before they enter the hot

zone around the electrode tip increases the silicon yield. High SiO reactivity

combined with small enough particles and a fairly even gas distribution are important

with respect to this. High temperature in the hot zone is important for the silicon yield

because the SiO/CO ratio of the gas here decreases with temperature. It is believed

that this is achieved by selecting electrical set points that give an energy distribution

between the charge and the metal pool that results in a chemical conversion in the hot

zone that equals the material transport from the cavity walls around the electrode tip.

The optimal electrical set points change with the raw material mix and state of the

furnace. The problems in measuring basic process parameters and changes in the raw

materials are among the most challenging aspects of the process control. Long time

constants for some important sub processes are also very demanding to handle.

There are large amounts of measurements and observations that can be used to help

the operators and metallurgists in understanding what is happening inside the furnace.

Some of these are described here. The more complex ones, often found by combining

several trend curves and observations, are those that really make the process

interesting and provide improved operations. The different producers wisely keep

these tools and knowledge within their own company.

References

1. Schei A., J. Kr. Tuset & H. Tveit, Production of High Silicon Alloys, Tapir Forlag,

Trondheim, Norway 1998, ISBN 82-519-1317-9.

2. Bullón J., M. Lage, A. Bermudez & F. Pena, The New Compound Electrode:

Current Situation and Thermoelectric Studies, Infacon 8, Bejing, China, 1998.

3. Tveit H., T. Halland, K-I Landrø, S. T. Johansen & B. Ravary, The Tapping

Process in Silicon Production, Silicon for the Chemical Industry VI, Loen,

Norway, 2002.

4. Nygaard L. & B. Andresen, Refining of Silicon Metal, Silicon for the Chemical

Industry VI, Loen, Norway, 2002.