Recently, the demand for solar-grade silicon (SOG-Si) with 6-7N (99.9999~9%) level of purity is dramatically increasing due to its popularity of photovoltaic power generation. The conventional process, Siemens method, is majorly used for SOG-Si production, which, however, consumes huge amount of energy and time. So, there is a need to develop an alternate production process for SOG-Si. As the cost-effective process, we are now focusing on a wet chemical process for purification of silica, followed by electrochemical reduction of silica to SOG-Si ^[1]. This purification process consists of two main steps: (i) acid leaching including dissolution & precipitation treatment of diatomaceous earth as an abundant Si source and (ii) liquid-liquid extraction for precise elimination of lighter element impurities that are possible to influence the semiconductor property of Si. Aim of the present work is to increase the extraction efficiency of each process to be optimized and combining them to complete the whole process.

First, we optimized the acid leaching process. This process performs pH adjustments of strong alkaline solution with diatomaceous earth dissolved to separate impurity-rich phase and silica. It has been shown that more than 90% of the heavy element (Fe, Al, etc.) impurities precipitate by decreasing pH ^[2]. However, since NaOH was used in the preparation of initial alkaline solution, much amount of Na impurities turned out to be included in the final silica product. For this, we employed the mixture of NaOH and Tetramethylammonium hydroxide (TMAH) to lower the concentration of Na in the alkaline solution preparation. Experimental results suggested that some amount of Na ion was necessary to be included in the solution in order to provide sufficient dissolution rate of silica. This led us to the optimal concentration of NaOH and TMAH in the mixture: 0.05 M NaOH and 1.40 M TMAH. Further purification, experiment results concluded that this mixture effectively prevented the inclusion of Na impurity, resulting in 5N level of purity when the process was repeated 3 times.

In spite of the achievement shown above, the lighter element, in particular, B is hard to be eliminated from the leaching process, because the pH dependence of solubility of B is similar to that of Si. To further eliminate this impurity, the liquid-liquid extraction process is employed. Since our previous results indicated that the effective mixing of aqueous and organic phase leads to the best efficiency of B elimination, we first designed flow type reactor with 3D printing technology. The reactor was designed to have 2 characteristic structures: a throat like structure that shows gradually diverging part after converging (converging/diverging structure) near Y shaped inlets and obstacles structure on downstream ^[3]. The high-speed camera demonstrated the reactor offered a fully dispersed flow regime, which leaded to the efficient solution mixing.

Based upon these results, we carried out a model experiment, using silica powder of 99.9 % purity with trace amount of B was dissolved in NaOH solution (aqueous phase) and injected into one of the inlet of the liquid-liquid extraction reactor. The extractant, 2,2,4-trimethyl-1,3-pentanediol (TMPD) solution (organic phase) was injected into the other inlet. The extraction was repeated 5 times at 380 ml/min and the efficiency was found to be 99.7% leaving less than 0.1 ppm of B in silica (7N purity).

From the above results, it is concluded that we have achieved efficient process to prepare 7N level silicon, by combining optimized leaching and liquid-liquid extraction with designed reactor. This could be a promising method to produce SOG-Si with lower cost than the present process at industries.

References:

[1] T. Homma, N. Matsuo, X. Yang, K. Yasuda, Y. Fukunaka and T. Nohira: Electrochimica Acta 179 (2015) 512–518.

[2] M. Bessho, Y. Fukunaka, H. Kusuda and T. Nishiyama: Energy & Fuels, 23, 4160–4165 (2009).

[3] Y. V. Akash, M. Mimura, M. Kunimoto, Y. Fukunaka and T. Homma: ECS Transactions, 80 (10) 1441-1446 (2017).