Carbothermal Reduction of Quartz and Carbon Pellets at Elevated Temperatures
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INTRODUCTION
SILICON production is carried out in submerged arc furnaces (SAFs) by reduction of quartz with carbon materials. At present, quartz and carbon materials are charged into the furnace in the form of lumps. Carbonaceous materials, like coal, low-temperature coke/ char, and charcoal, are commonly used as reduction materials in the process.[1,2] The overall process of silicon production can be described by Reaction [1]. SiO2 þ 2C ¼ Si þ 2CO:
½1
The silicon process can be divided into three parts: the combustion zone, the low-temperature zone, and the high-temperature zone. The combustion zone is over the charge layer where the combustion reactions happen. Generally, the temperature in the low-temperature zone is 800 °C to 1750 °C, and above 1700 °C to 1900 °C in the high-temperature zone. When the charge materials enter the low-temperature zone, the quartz and carbon materials meet the SiO and CO gases produced in the high-temperature zone. Reactions [2] through [4] occur in the low-temperature zone. Carbon materials react with some of the SiO gas to produce SiC according to Reaction [2]. Reaction [2] is the most important reaction in the low-temperature
FEI LI and MERETE TANGSTAD are with the Department of Material Science and Engineering, Norwegian University of Science and Technology, Trondheim, N-7491, Norway. Contact e-mail: [email protected] ELI RINGDALEN is with the SINTEF Materials and Chemistry, Trondheim,7465, Norway. Manuscript submitted March 14, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
zone because it captures SiO gas from the high-temperature zone. This reaction also produces the intermediate product SiC, which is the key reactant for silicon forming reactions in the high-temperature zone. SiOðgÞ þ 2 CðsÞ ¼ SiCðsÞ þ COðgÞ DH1773 K ð1500 CÞ ¼ 20:8 Wh/mol
2SiO ¼ SiðlÞ þ SiO2 ðlÞ DH1773 K ð1500 CÞ ¼ 169 Wh/mol
3 SiOðgÞ þ COðgÞ ¼ 2SiO2 ðs; lÞ þ SiCðsÞ DH1773 Kð1500 CÞ ¼ 390 Wh/mol:
½2
½3
½4
Based on the importance of Reaction [2], the reactivity of carbonaceous materials toward SiO gas has been studied by several researchers.[3–7] In general, the trend of SiO reactivity for different carbon materials is that charcoal and lower-rank coal are more reactive than metallurgical coke and higher-rank coal. When agglomerated quartz and carbon are charged into the SAF furnace, quartz and carbon particles have large contact area and Reaction [5] also occurs in the low-temperature zone, in addition to Reactions [2] through [4]. This is an endothermic reaction and will thus change the temperature in the low-temperature zone. According to the theoretical heat balance calculation, when the amount of agglomerates in the charge increases, the temperature in the low-temperature zone decreases, and SiO loss consequently decreases.[8] The energy consumption per ton Si reduced decreases, while
the Si yield increases. The energy content in the off-gas lost to the environment, or recovered in an energy recovery system, also decreases.[1,8] Reactions in quartz and carbon agglomera
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