Upgradation of locally available silica for the production of solar grade silicon

Abstract

The purpose of the work was to initiate a systematic investigation within the country on the possibility of producing solar grade (SoG) silicon from local silica deposits and to achieve that the objective was to identify a suitable deposit of silica within the country and to develop techniques for the purification of silica of this deposit so as to make it suitable for the production of SoG-Si. The characterization and upgradation processes were employed on five sand samples, collected from Dalia (district Nilphamari), Patgram (district Lalmonirhat), Bipinganj (district Netrokona), Kuakata (district Patuakhali) and Sharighat (district Sylhet). The minimum required percentage of silica for the production of SoG-Si is >98%. The received sand samples contained SiO2 ranging from around 65% to 97%. The characterization was performed on the basis of material composition, particle size, density, sphericity, moisture content, material composition and crystallinity on the samples. All five sand samples contain minimum amount of moisture and negligible organic matter. In case of bulk density deviation was observed from standard value of 2.65g/cc. Variation was observed in particle size distribution for Patgram, Dalia and Bipinganj sand. Most of Kuakata sand is sized ~ 75μm and for Sylhet it is 0.2mm. The sample containing minimum impurity and perfect quartz crystal form, collected from Bipinganj was found to have high potential. The sample from Sylhet containing ~ 86% SiO2 was also considered as a strong contender for further processing. Traditional beneficiation methods like attrition, acid washing and alkali washing were performed on the samples. Based on stereomicroscopic observation and XRF result the optimum level was identified for each beneficiation step e.g. for attrition optimum stirring speed and time was found to be 800 rpm and 30 minutes respectively. Finally a methodology was developed for the purification of the samples which lead to the upgraded point of 99.45% and 95.31% SiO2 for Bipinganj and Sylhet sand sample respectively. The result concludes Bipinganj sand can be used as raw material for the production solar grade silicon.