Reduction of Porous Silica Pellets by Electrodeoxidation in Molten Salts
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1210-Q08-17
Reduction of Porous Silica Pellets by Electrodeoxidation in Molten Salts Emre Ergul, Ishak Karakaya, Metehan Erdogan Middle East Technical University Metallurgy and Materials Engineering Dep. 06531 Ankara / TURKEY
ABSTRACT Direct electrochemical reduction of porous SiO2 pellets in molten CaCl2 salt and CaCl2NaCl salt mixture were investigated by applying 2.8 V potential. The study focused on the effects of temperature, powder size and cathode contacting materials. Starting materials and electrolysis products were characterized by X-ray diffraction analysis and scanning electron microscopy. Due to reactive nature of silicon, different cathode contacting materials were used to test the extent of reactions between silicon produced at the cathode and the contacting materials. X-ray diffraction patterns showed that silicon produced at the cathode reacted with nickel, and iron in stainless steel to form Ni-Si and Fe-Si compounds respectively. Besides, studies revealed that higher temperature and smaller particle size had positive effects in increasing reduction rate. The results were interpreted from variation of current versus time graphs under different conditions, microstructures and compositions of the reduced pellets. INTRODUCTION Silicon plays an important role in production of photovoltaic cells. Industrial solar grade silicon (SOG-Si) production involves several steps, starting from metallurgical grade silicon (MG-Si). These steps are applied to increase the purity of MG-Si, first by converting into a compound such as SiHCl3, SiCl4, SiH2Cl2, or SiH4 and then transforming silicon compound back to high purity silicon [1]. Due to high production cost of solar grade silicon, recent researches related with production of SOG-Si have focused on new production methods. These studies mostly based on direct electrochemical reduction of silicon compounds either by molten salt electrolysis [2] or by electrodeoxidation of metal oxides, called FFC Cambridge process [3]. The electrodeoxidation involving the reduction of insulating SiO2 plate to Si metal in molten calcium chloride at 850 °C was first reported by Nohira and his coworkers [4]. Then, porous SiO2 pellets were reduced by Jin et. al. [5]. In the light of these studies, several works related with reduction of solid SiO2 to Si were reported [6-11]. The reduction process of SiO2 at the cathode is expressed as: SiO2 +4e−→ Si + 2O2−
(1)
Here O2- ions that form in the vicinity of the three phase interphase (metal-oxideelectrolyte) at the cathode diffuse through the electrolyte to the anode. Previous studies reported
O2- transportation as the rate determining step [7] of electrodeoxidation process. Furthermore, precipitation of CaO in the catholyte at more negative potentials [9], was attributed to the saturation of O2- ions due to limited transportation in the electrolyte. All the studies use graphite as anode which reacts with O-2 carried to anode/electrolyte interface to produce CO2 at the anode. Calcium carbonate (CaCO3) formation reported [10] in CO2 co
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