Silicon Refining by Solidification from Liquid Si–Zn Alloy and Floating Zone Method

This study evaluated the refining ability of a proposed production process for solar-grade silicon utilizing the electrolytic reduction of SiO₂ on a liquid zinc electrode in molten salt. The distribution behaviors of impurity elements during the precipitation of solid silicon from a liquid Si–Zn alloy were studied by thermodynamic calculations at 923 K. In the precipitation experiment, silicon granules were recovered from a liquid Si–Zn alloy, which was prepared from metallurgical-grade silicon. The impurity removal ratios exceeded 99% for C, Al, and Ca, and 90% for Fe. High removal ratios were attained for B and O as well. As the post-processing, a silicon ingot was produced from the precipitated silicon granules by the floating zone method. The Zn residue in the precipitated silicon was completely evaporated during the floating zone refining. The total content of metallic elements (Al, Ca, Fe, Ti, and Zn) was lower than 0.2 ppmw, even though metallurgical-grade silicon was used as the starting material

Electrolytic Production of Silicon Using Liquid Zn Alloy Cathode in Molten CaCl₂

2019 Liquid Metal Processing & Casting Conference (LMPC2019), Birmingham, UK, 8-11 September, 2019.A new electrolytic production process for solar-grade Si has been proposed utilizing a liquid Si–Zn alloy cathode in molten CaCl₂. The process consists of three major processes: electrolysis, precipitation, and refining. One of the advantages of the process is the attainability of high purity by application of a solidification refining from the liquid Si–Zn alloy. The residual zinc is easily removed afterwards due to its high volatility. To establish this process, the behavior of liquid Zn metal in molten CaCl₂ at 1123 K was investigated. Evaporation of Zn metal was largely suppressed by immersion into the molten salt, which enabled the use of a Zn electrode despite its high vapor pressure. Based on the results of cyclic voltammetry, the reduction of SiO₂ on a liquid Zn cathode was conducted by potentiostatic electrolysis at 0.9 V vs. Ca²⁺/Ca. Precipitated Si particles were recovered in the solidified Zn matrix

Mechanism of Electrolytic Reduction of SiO₂ at Liquid Zn Cathode in Molten CaCl₂

The reaction mechanism of electrolytic reduction of SiO₂ at a liquid Zn cathode in molten CaCl₂ was investigated with the aim of establishing a new production process of solar-grade Si. Three types of Zn/SiO₂ contacting electrodes were prepared depending on the objectives. Cyclic voltammetry suggested two reduction mechanisms of SiO₂ at a Zn electrode. One is a direct electrolytic reduction that proceeds at potentials more negative than 1.55 V vs. Ca²⁺/Ca. The other is an indirect reduction by liquid Ca–Zn alloy at potentials more negative than 0.85 V. The both reduction mechanisms were confirmed to proceed at 0.60 V by electrolysis and immersion experiments. Impurity analysis by ICP-AES was conducted for the Si prepared by potentiostatic electrolysis at 0.60 V, and confirmed that the concentrations of the metal elements and P were lower than the target levels for primary Si before directional solidification process

A New Electrolytic Production Process of Silicon Using Liquid Zn Alloy Cathode in Molten Salt

PRiME 2016/230th ECS Meeting, October 2, 2016 - October 7, 2016, Honolulu, HI.Electrolytic production process for solar-grade Si utilizing liquid Si–Zn alloy cathode in molten CaCl₂ has been proposed. Toward the establishment of the process, the behavior of liquid Zn metal was investigated in molten CaCl₂ at 1123 K. The evaporation of Zn metal was largely suppressed by an immersion into molten salt, which enables the use of Zn electrode in spite of the high vapor pressure of Zn. The cyclic voltammetry suggested the reduction of SiO₂ at 1.45 V vs. Ca²⁺/Ca on a Zn cathode. After the potentiostatic electrolysis at 0.9 V, Si particles with diameters of 2–30 µm were precipitated in the solidified Zn matrix by the slow cooling process of the produced liquid Si–Zn alloy. The alloying rate between solid Si and liquid Zn was measured as 4.56 μm s⁻¹, and it linearly decreased with the Si content in liquid Zn phase

Communities of Arbuscular Mycorrhizal Fungi in the Roots of Pyrus pyrifolia var. culta (Japanese Pear) in Orchards with Variable Amounts of Soil-Available Phosphorus

We examined the colonization rate and communities of arbuscular mycorrhizal fungi (AMF) in the roots of Pyrus pyrifolia var. culta (Japanese pear) in orchards to investigate the effect of phosphorus (P) fertilization on AMF. Soil cores containing the roots of Japanese pear were collected from 13 orchards in Tottori Prefecture, Japan. Soil-available P in the examined orchards was 75.7 to 1,200 mg kg−1, showing the extreme accumulation of soil P in many orchards. The AMF colonization rate was negatively correlated with soil-available P (P <0.01). AMF communities were examined on the basis of the partial fungal DNA sequences of the nuclear small-subunit ribosomal RNA gene (SSU rDNA) amplified by AMF-specific primers AML1 and AML2. The obtained AMF sequences were divided into 14 phylotypes, and the number of phylotypes (species richness) was also negatively correlated with soil-available P (P <0.05). It was also suggested that some AM fungi may be adapted to high soil-available P conditions. Redundancy analysis showed the significant effects of soil pH, available P in soil, and P content in leaves of P. pyrifolia var. culta trees on AMF distribution. These results suggested that the accumulation of soil-available P affected AMF communities in the roots of Japanese pear in the orchard environment