A Heat and Mass Transfer Model of a Silicon Pilot Furnace
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NTRODUCTION
SILICON is found in high quantities in the earth’s crust, second only to oxygen as an element.[1] It naturally occurs in quartz and quartzite (both as silicon dioxide) and other rock, from which it can be extracted. To produce silicon, a submerged-arc furnace[1–3] is continually fed with carbon and quartz.[1,4–6] Liquid silicon is tapped from the base of the furnace[1,4,5,7] and undergoes further processes to produce silicon with the desired purity and size distribution for the customers’ needs. The hot off-gases that rise through the furnace are cooled, extracted, and filtered. Conventionally, raw materials are added in a homogenized batch at roughly 15 minutes intervals. Ideally, they should be added to balance the average rate at which material is consumed. However, in practice, a solid crust region builds up, forming a ceiling-like structure above a gas cavity. This crust is composed of a mixture of carbon, molten quartz, silicon carbide, and condensate. The presence of the crust slows
BENJAMIN M. SLOMAN, COLIN P. PLEASE and ROBERT A. VAN GORDER are with the Mathematical Institute, University of Oxford, Oxford, OX2 6GG, UK. Contact e-mail: [email protected] AASGEIR M. VALDERHAUG, ROLF G. BIRKELAND and HARALD WEGGE are with the Elkem Technology, Fiskaaveien 100, 4621 Kristiansand, Norway. Manuscript submitted December 11, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
the motion of solids and liquids to the lower part of the furnace, restricting their access to the heat required to facilitate the necessary reactions. The crust rises in the furnace, since material that melts and reacts away from the lower interface is replenished with additional material deposited on the top. Since the products from the lower part of the crust are tapped out of the furnace base or convected away as gas, this rising crust corresponds to a volumetric increase in the gas cavity. It is possible for the crust to be so thick and the cavity pressure so high that gas escapes rapidly out of the taphole, in a disastrous situation known as ‘blowout.’[1,8] In order to counter the crust build-up, operators ‘stoke’ the furnace, where the crust is manually broken up by a bar from a ‘stoking car’ on an hourly basis. Research into silicon furnaces is thus important to improve operational efficiency and see if crust formation can be controlled. Taking measurements in a silicon furnace is extremely difficult due to the high temperatures involved. The cost of lost production prohibits experiments on industrial furnaces. Thus, to gain insight into the furnace environment, Elkem have carried out experiments on ‘Pilot Furnace Process Simulators.’ These are cylinders of inner height 43 cm and inner diameter 13 cm which are filled with raw material and heated from room temperature in an induction furnace. Once the furnaces have cooled down, at the end of the experiment, epoxy is injected, so that material stays in the final state when they are cut open. Photographs of the results are shown in Figure 1. Similar experiments are descri
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