High purity silicon materials prepared through wet-chemical and electrochemical approaches

Abstract

A new approach to produce high purity solar grade silicon (SOG-Si) using wet-chemical and electrochemical processes was proposed. First, diatomaceous earth was used as a source of silica and wet-chemical purification by acid leaching and solvent extraction processes were applied for eliminating heavy metal and light element such as boron, respectively. In particular, the solvent extraction using channel flow reactor demonstrated extremely high efficiency for eliminating boron to 7N (99.99999%) level purity. Secondly, the high-purity silica was reduced to silicon by the direct electrolysis method using molten CaCl 2 as an electrolyte and Si plate as cathode. We proposed the continuous electrolysis system by feeding silica granules to the Si cathode set at the bottom of the cell. It was confirmed that the reduction proceeded steadily to form crystalline Si with a sufficient reduction rate. We also attempted electrodeposition of Si films and have developed a process using KF-KCl + K 2 SiF 6 molten salt, from which uniform layer of Si with a thickness larger than 100 mm could be formed.

Mechanism of Electrolytic Reduction of SiO2at Liquid Zn Cathode in Molten CaCl2

“…For the past two decades, we have been studying the direct electrolytic reduction of solid SiO 2 to Si in molten CaCl 2 as a new production process of SOG-Si [17][18][19][20] . Here, since purification of SiO 2 up to 6-7N is possible at low cost 21,22 , such purified SiO 2 is assumed to be used as the raw material 22 . In this process, electrochemical reduction of insulating SiO 2 is realized by using a SiO 2 contacting electrode, which provides the three-phase zone of conductor/SiO 2 /CaCl 2 17 .…”

Optimization of the Granulation Binders of High-Purity Carbothermic Reduction for Solar-Grade Silicon

“…Figure 5 shows the images and grain properties of granules with four different binders: PVA, CMC, starch, and sucrose. The size of the granules with PVA were larger than those with other binders, while the average bulk densities of PVA, CMC, starch, and sucrose granules were (0.88 g/cm 3 ), (0.99 g/ cm 3 ), (1.18 g/cm 3 ), and (1.70 g/cm 3 ), respectively. In this granulation method, the formation of granules starts gradually by agglomeration during the rolling motion.…”

“…16,17) By means of adding SiC as the initial raw material, a partial amount of SiC is consumed in eqs. (5) and (3). To promote SiO gas and SiC ef ciently under the reaction in eq.…”

“…1) Various routes of Si production have been investigated, including the Siemens, Union Carbide, and uidized bed processes 2) and electrolytic production. 3) For the production process of solar-grade silicon (SoG-Si) with a purity of 6N or higher, some reduction processes have multiple reduction processes with high temperature treatment. The energy cost of these reduction processes depends on the count of high temperature processes in the total process.…”

Electrolytic Production of Silicon Using Liquid Zinc Alloy in Molten CaCl2

“…[13][14][15] To improve the productivity of the process, we recently studied the electrochemical reduction of SiO 2 granules set on a bottom cathode in molten CaCl 2 , analogous to the Hall-Héroult Al production process. 16 Many research groups, including ours, have investigated direct electrochemical reduction of SiO 2 for SOG-Si production.…”