Trace Elements in the Si Furnace-Part II: Analysis of Condensate in Carbothermal Reduction of Quartz

Abstract

Silicon feedstock for production of solar-grade silicon should be as pure as possible to decrease the cost of manufacturing of solar cells. Impurities in quartz, carbonaceous materials, electrodes, and refractories are mostly present in the form of oxides. These oxides can be reduced to volatile gaseous compounds in presence of SiO(g) and CO(g) atmosphere and potentially leave the furnace or stay in the condensed reaction products, metal, and slag. This work investigates the conditions under which volatile impurities report to the gas phase in laboratory experiments with lumpy and pelletized mixtures of SiO2, SiC, and Si at 1923 K and 2123 K (1650 °C and 1850, respectively, were carried out. The volatile compounds were generated by the reduction of quartz and collected in the form of condensate. The effects of the reaction temperature, quartz type, charge composition, pellets, and lumps on the composition of the condensate were studied. The trace elements in the charge input, reacting charge, and condensate were analyzed using inductively coupled plasma (ICP)-mass spectroscopy (MS) and X-ray diffraction (XRD). CO(g) and SiO(g), which are the major components in reduction reactions, formed four types of condensate: white, brown, green, and orange. The condensate constituents were amorphous SiO2, 3C:SiC, Si, and α-quartz. Each impurity present in the quartz charge entered the gas phase during quartz reduction and was detected in the condensate. Al and Fe show limited volatility. The volatility of Mn, P, and B depends on the charge mix: a higher PCO enhances the concentration of these elements in the gas phase. Fluid inclusions, common in hydrothermal quartz, enhance the distribution of the contaminants to the gas phase. Industrial campaigns on Si and Fe-Si production confirm the experimental results. Silicon feedstock for production of solar-grade silicon should be as pure as possible to decrease the cost of manufacturing of solar cells. Impurities in quartz, carbonaceous materials, electrodes, and refractories are mostly present in the form of oxides. These oxides can be reduced to volatile gaseous compounds in presence of SiO(g) and CO(g) atmosphere and potentially leave the furnace or stay in the condensed reaction products, metal, and slag. This work investigates the conditions under which volatile impurities report to the gas phase in laboratory experiments with lumpy and pelletized mixtures of SiO 2 , SiC, and Si at 1923 K and 2123 K (1650°C and 1850°C), respectively, were carried out. The volatile compounds were generated by the reduction of quartz and collected in the form of condensate. The effects of the reaction temperature, quartz type, charge composition, pellets, and lumps on the composition of the condensate were studied. The trace elements in the charge input, reacting charge, and condensate were analyzed using inductively coupled plasma (ICP)-mass spectroscopy (MS) and X-ray diffraction (XRD). CO(g) and SiO(g), which are the major components in reduction reactions, formed four types of co...

Trace Elements in the Si Furnace. Part I: Behavior of Impurities in Quartz During Reduction

“…The hydrothermal quartz is a new high-purity source from a Norwegian deposit; it had been examined in previous studies. [27,28] The electronic grade silicon was supplied by Wacker, and the ultra-pure SiC was produced by reduction of hydrothermal quartz with high-purity carbon black. Ultra-pure saccharine by Danisco AS C 12 H 22 O 12 was used as a binder in the pellet production.…”

“…The process of making pellets was described elsewhere. [27] The reactions were studied in ultra pure graphite crucibles of the isotropic type IG-110 supplied by Toyo Tanso and in ultra-pure argon which contained H 2 O <3.0 ppmw; O 2 <2.0 ppmw. Table I summarizes the chemical compositions of SiC, Si, and graphite provided by suppliers and determined at NTNU.…”

“…The melting point of the hydrothermal quartz was estimated in a reducing atmosphere for a 3-mm particle size to be 2092 K (1819°C). [27] As quartzite contains more contaminants than hydrothermal quartz (Table II) and the impurities in quartz lower the melting point of quartz, [29] quartzite is expected to have a lower melting point than hydrothermal quartz. Therefore, a temperature of 1923 K (1650°C) was chosen to study the behavior of both quartz-types in their solid states.…”

“…[27] V. CONCLUSIONS This article, which is the first of a series of two, investigated the behavior of the impurities in quartz when quartz reacted with SiC and Si in reducing atmosphere.…”