79    A UNIQUE METHOD FOR NEUTRALIZING METAL
CHLORIDE WASTE PRODUCED IN THE
PURIFICATION OF POLYSILICON
William C. Breneman, Chief Design and Process Engineer
Electronics Division
Union Carbide Corporation
Washougal, Washington 98671
David M. Reeser, Environmental Process Manager
RUST International Corporation
Portland, Oregon 97225
INTRODUCTION
Union Carbide Corporation has constructed a polycrystalline silicon purification facility on a site
near Moses Lake, Washington. The plant, covering 40 acres, contains two facilities. The first, utilizing proprietary technology, converts metallurgical-grade silicon to ultra-high-purity silane. Using
another proprietary process, the other facility converts the silane to semiconductor-grade polycrystalline silicon.
Process design for the facilities was the work of a combined team of engineers from Union Carbide
Corporation and RUST International Corporation. The silane area process was developed from
original Union Carbide work done for the Department of Energy. The polysilicon facility process was
based on a proprietary license of the Komatsu Corporation of Japan. Process design of the waste
treatment systems was the work of the process team based on original concepts of the authors.
Design of the waste treatment facilities was accomplished by first developing a preliminary flow
diagram. Because of uncertainties in part of the process, a research laboratory in Salt Lake City, Utah
was rented and product reaction tests were made. The process flow diagrams were revised in accordance with the findings of these tests. After finalization of the process flow diagram, a conceptual
styrofoam model of the treatment facility was constructed, followed by a W to 1' scale construction
model. Construction began in July 1982, with startup of the plant occurring in the fall of 1984.
The facility was awarded both the Washington State and Pacific Northwest regional industrial
pollution control awards for 1985 by the Pacific Northwest Pollution Control Association.
PROCESS DESCRIPTION
Polycrystalline silicon is a basic element of the electronics industry. Prior to this facility, it had most
commonly been made by chlorosilane vapor deposition using a power-intensive technology developed
when electricity and fuel were inexpensive. Union Carbide Corporation's process, however, is based
on recently developed, low-energy technology and builds on the firm's manufacturing experience with
chlorosilane intermediates.
Metallurgical silicon is first fed to a pressurized, heated vessel where it is reacted with a mixture of
hydrogen and chlorosilanes to yield a higher silicon content chlorosilane liquid. This liquid is alternately distilled and catalytically rearranged to yield silane and unconverted chlorosilanes. Reclaimed
chlorosilanes are recycled back to the initial reactor to react with more metallurgical silicon. The
silane is decomposed in a heated vessel to yield both a high- purity silicon product and hydrogen,
which is also totally recycled back to the initial reactor. After cooling, the product silicon is removed
in rod form, crushed, cleaned, and packaged for shipment.
The impurities extracted from the silicon raw material are concentrated internally in this closed loop
process and are periodically purged either to the flame hydrolyzer or a batch hydrolyzer for neutralization in a lime slurry. The research and development of a controllable hydrolyzation process are
subjects of major import in this paper and are also the subjects of a process patent application now
pending in the U.S. Patent Office. The products of hydrolysis, Si02, and HC1, are scrubbed before
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