Trace Elements in the Si Furnace. Part I: Behavior of Impurities in Quartz During Reduction ELENA DAL MARTELLO, GABRIELLA TRANELL, OLEG OSTROVSKI, GUANGQING ZHANG, OLA RAANESS, RUNE BERG LARSEN, KAI TANG, and PRAMOD KOSHY Quartz and carbonaceous materials, which are used in the production of silicon as well as electrodes and refractories in the silicon furnace, contain trace elements mostly in the form of oxides. These oxides can be reduced to gaseous compounds and leave the furnace or stay in the reaction products—metal and slag. This article examines the behavior of trace elements in hydrothermal quartz and quartzite in the reaction of SiO 2 with Si or SiC. Mixtures of SiO 2 (quartz or quartzite), SiC, and Si in forms of lumps or pellets were heated to 1923 K and 2123 K (1650°C and 1850°C) in high purity graphite crucibles under Argon gas flow. The gaseous compounds condensed in the inner lining of the tube attached to the crucible. The phases present in the reacted charge and the collected condensates were studied quantitatively by X-ray diffraction (XRD) and qualitatively by Electron Probe Micro Analyzer (EPMA). Contaminants in the charge materials, reacted charge and condensate were analyzed by Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS). Muscovite in the mineral phase of quartz melted and formed two immiscible liquid phases: an Al-rich melt at the core of the mineral, and a SiO 2 -rich melt at the mineral boundaries. B, Mn, and Pb in quartz were removed during heating in reducing atmosphere at temperature above 1923 K (1650°C). Mn, Fe, Al and B diffused from quartz into silicon. P concentration was under the detection limit. Quartzite and hydrothermal quartz had different initial impurity levels: quartzite remained more impure after reduction experiment but approached purity of hydrothermal quartz upon silica reduction. DOI: 10.1007/s11663-012-9717-4 Ó The Minerals, Metals & Materials Society and ASM International 2013 I. INTRODUCTION IMPURITIES in silicon affect the final efficiency of silicon solar cells. The required purity of solar grade silicon was brought forward by Coletti and co-workers. [1] The carbothermic production of silicon is an important source of contamination: quartz, carbonaceous materi- als, as well as electrode and refractories carry detrimen- tal impurities which transfer to silicon. Impurities in carbon material are mainly the oxides and sulfides present in the ash. Because carbon is very porous, the impurities are easily exposed to the furnace gases up in the furnace, and the volatile compounds which form can be easily removed when the charge is still at the top. [2–4] The behavior of the impurities in quartz during the carbothermic reduction is still not well understood. The impurities present in the quartz do not have easy access to the reducing atmosphere compared with carbon because the quartz is not porous. The quartz melts at around 1973 K (1700°C) and may not be melted in the production furnace until just above the cavity around the cavity, created by the electric arc. Consequently, the oxides of the additional elements embedded in the quartz are believed to be accessible for reduction once the quartz reaches the cavity, [5] and any volatile com- pound which may form might not leave the furnace because of the interaction with the upper charge layer. If cracks formed by quartz thermal expansion are invaded by reducing gases, then the impurities can be exposed earlier, however, this is a topic yet to be investigated. SiO(g), CO(g), C(s), and SiC(s) are reducing agents for the impurity oxides present in the charge material. Oxides of Ni, Co, Fe, Pb, Cu, Cr, Mn, Zn, Na, and K are easily reduced to metals, while oxides of Ca, Al, Ti, and Mg are stable at temperatures and conditions of quartz reduction. Impurities can be removed from quartz with a gas phase if the reduced or oxidic species are volatile. The vapor pressure of the reduced species is also influenced by the activities of the species in the raw materials and reaction products and by the reaction temperature. Lumpy quartz contains impurities in form of (1) mineral inclusions, (2) liquid inclusions, and (3) trace elements in the quartz lattice. In a recent process developed for production of solar grade silicon from pure raw materials, [6] quartz is charged in the furnace in form of pellets of intermixed SiO 2 and SiC. Quartz in ELENA DAL MARTELLO, Ph.D., GABRIELLA TRANELL, and RUNE BERG LARSEN, Professors, are with NTNU, Trondheim, Norway. Contact e-mails: dalmarte@yahoo.it, dalmarte@material. ntnu.no OLEG OSTROVSKI, Professor, and PRAMOD KOSHY, Research Associate, are with UNSW, Sydney, NSW, Australia. GUANGQING ZHANG, Lecturer, is with UOW, Wollongong, NSW, Australia. OLA RAANESS, Senior Adviser, and KAI TANG, Research Scientist, are with SINTEF, Trondheim, Norway. Manuscript submitted: June 15, 2012. Article published online January 29, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 44B, APRIL 2013—233