超冶金级硅的制备研究

【作者】 于站良；

【导师】 戴永年； 谢克强；

【作者基本信息】 昆明理工大学 ， 冶金物理化学， 2010， 博士

【摘要】 太阳能是一种取之不尽用之不竭的能源,目前太阳能电池每年以30%的速度增长。太阳能电池目前主要是采用电子级硅为原料生产,然而电子级硅材料在生产过程中存在高能耗、高污染、高成本等问题,为了快速健康地发展太阳能电池产业,必须寻找一条不依赖现有电子级硅工艺的低成本新工艺。作者所在的课题组根据冶金级硅中杂质的分布特点提出了采用真空冶金综合法——由冶金级硅经超冶金级硅直接制备太阳能级多晶硅的新工艺。此工艺不依赖于现有传统的Siemens方法,而且具有低成本、低能耗、低环境成本等特点。在此新工艺中,湿法预处理是一个很重要的过程：一方面,它能够彻底去除冶金级硅中大部分的金属杂质,降低后续真空熔炼过程的难度；另一方面,它是一个杂质混匀的过程,此过程也有利于杂质在后续真空熔炼过程中有效的去除。本论文以云南省某工业硅厂生产出来的产品为代表研究冶金级硅中杂质分布的特点,并提出了冶金级硅湿法预处理去除金属杂质的新工艺,并进行了较系统研究。(1)利用电子扫描电镜分析了冶金级硅锭的不同位置的杂质分布。分析结果表明冶金级硅中的杂质在冷凝过程中出现了严重的偏析现象,它们主要以小颗粒状的杂质团随机分布在硅锭的边缘位置,而处在硅锭中间位置的杂质主要沉积在硅锭的晶界部位而形成一条条狭长的杂质带,或以大颗粒状的杂质团分布在硅锭中部。通过增加硅锭厚度,缩小铸锭面积,加快浇注速度等手段,可以使更多的杂质有足够的时间扩散到中心位置形成大颗粒杂质团,以有利于杂质与酸溶液发生反应而被去除。(2)杂质Al在冶金级硅锭冷却时没有形成新相,而是以金属相沉积在冶金级硅的晶界部位或以杂质团随机分布于冶金级硅锭中。随着pH值的不断减小,即盐酸溶液浓度的不断增加,溶液中[Fe2+]的溶解度在不断增加,溶液中[Al3+]也不断增加。(3)研究了常压浸出过程浸出剂种类及其浓度、硅粉粒度、反应温度、反应时间、液固比等多种因素对冶金级硅中杂质Fe、Al去除效率的影响。实验确定最佳的浸出工艺条件为：盐酸浓度6 mol/L,硅粉粒度≤50μm,反应温度60℃,反应时间3天,液固比2：1。在此反应条件下浸出,杂质Fe的去除效率超过了70%,杂质Al的去除效率为60%左右,规模为5 kg的实验研究具有较好的重现性。(4)研究了高压浸出过程盐酸浓度、硅粉粒度、反应温度、反应压强、液固比、反应时间等多种因素对冶金级硅中杂质Fe、Al、Ca、Ti等元素去除效率的影响。实验确定的最佳工艺条件为：硅粉粒度≤50μm,反应温度150-160℃,反应压强1.5-1.6 MPa,盐酸浓度4 mol/L,液固比4：1,保温时间2小时。在此最佳反应条件下,杂质Fe的去除效率超过80%,杂质Al的去除效率达到75%,杂质Ca的去除效率达到90%。而高压浸出对冶金级硅中杂质Ti的去除效果不佳。多次重现性实验研究发现,高压浸出处理后的冶金级硅中杂质Fe、Al、Ca的残余量分别小于290 ppmw、295 ppmw、30 ppmw。超冶金级硅的纯度可达99.95%。(5)研究了冶金级硅高压浸出动力学,建立了硅粉粒度、反应压强、反应温度和盐酸浓度对杂质Fe、Al去除效率影响的动力学方程。计算了Fe、Al的表观活化能分别为Ea Fe=46.908 kJ/mol,Ea Al=34.067 kJ/mol,表明Fe、Al在高压浸出过程中均受混合控制。Fe、Al的表观反应级数分别为0.899、0.346。最终建立了冶金级硅粉在高压浸出时去除Fe、Al的动力学模型：以80个实验结果分别验证此动力学模型,吻合甚好。(6)超冶金级硅生产各环节的生产成本估算结果表明超冶金级硅的生产成本低于1.73万元／吨,因此采用湿法冶金技术生产超冶金级硅能够获得比较可观的经济效益。更多还原

【Abstract】 Solar energy already has shortly been in great demand since it is inexhaustible and cleaner than any conventional energy resources. At present, an expensive grade silicon for semiconductor is used for a solar cell to convert solar energy into electri-city. However, the amount of supply is limited and we have to develop an innovative process, which using metallurgical grade silicon (MG-Si) as a starting material for production of solar grade silicon (SOG-Si) is believed to be one of the ways to make solar cells less expensive.According to the distribution of the impurities in metallurgical grade silicon,a new method for production of solar grade silicon at low cost from MG-Si via vacuum metallurgical technique was proposed by the research team consisted of the author, which is a promising process to feedstock solar grade silicon material independence of traditional Siemens method. In this procedure, pre-treatment of MG-Si using hydro-metallurgical route to remove metallic impurities such as iron and aluminum to reduce the production cost is an important process. One hand, it could remove mostly of metallic impurities for reduce the difficulty of follow-up vacuum melting process of this new method. On other hand, it is a process of blending impurities for being con-ductive to remove impurities in the vacuum melting process.In this paper, a novel process for pre-treatment of MG-Si on removal metallic impu-rities under hydrometallurgical route was proposed according to the distribution of the impurities in metallurgical grade silicon from Yunnan province, and the related theory was investigated.(1) The distribution of impurities in different positions in MG-Si was analyzed by SEM. The results of SEM showed that most of metallic impurities at the periphery of MG-Si ingot are mainly randomly distributed in the edge of silicon ingot as the small granular groups, and the mostly metallic impurities at the middle of MG-Si ingot mainly deposit in some narrow band of impurities or form some big granular groups. It can make more impurities have enough time to spread to the center of silicon ingot or form large particle size of impurities by the means such as increasing the thickness of silicon ingots, reducing ingot size, speeding up the casting speed when the silicon fluid is cooling. After silicon ingot crushing, most of impurities can expose and have the relation with acid. The chemical content of MG-Si from a MG-Si factory in Yun-nan province showed that the content of Fe, Al, Ca, Ti in MG-Si was 1500 ppmw, 1082 ppmw,320 ppmw,120 ppmw, respectively.(2) Aluminum in silicon cannot form new phase with the cooling of silicon fluid, but deposit in the grain boundary with metal phase or distribute in silicon ingot ran-dom as graininess.The thermodynamics of removal of iron and aluminum from MG-Si with hydrometallurgical route was analyzed. The solubility of Fe2+ in solution of acid would increase with the increasing of concentration of hydrochloric aicd. The solubi-lity of Al3+ in solution of acid would increase with the increasing of concentration of hydrochloric aicd.(3) With the scale of experimental extended constantly, the influences of many factors such as hydrochloric acid, reaction temperature and reaction time in the atmos-pheric pressure on the removal efficiencies of iron and aluminum impurities was sys-tematic studied. The optimum operating conditions for leaching in hydrochloride was acid concentration 6 mol/L at 60℃for 3days with 50μm in diameter for raw material ground and with liquid to solid ratio of 2:1. The optimum operation conditions of in-fluences on the removal efficiency of iron and aluminum impurities were up to 70% and 60%, respectively.(4) The influences of many factors such as hydrochloric acid, reaction tempe-rature, reaction pressure, liquid to solid ratio and reaction time in the pressure leach-ing on the removal efficiencies of some impurities, such as Fe, Al, Ca and Ti, was systematic studied. The optimum operating conditions for leaching in hydrochloric was acid concentration 4mol/L at 423～433K for 2 hours with 50μm in diameter for raw material ground and with liquid to solid ratio of 4:1 in the pressure of 1.5～1.6MPa. The optimum operation conditions of influences on the removal efficiency of Fe, Al, Ca were up to 80%,75% and 90%, respectively. But Ti was not well removed in the pressure leaching. The residual content of Fe, Al and Ca in silicon powder after pres- sure leaching of reproducible experiment was less than 290 ppmw,295 ppmw and 30 ppmw, respectively.(5) The kinetics of pressure leaching of MG-Si powders was studied. The kinetics equations of particle size, temperature、pressure and concentration of hydrochloric acid are found respectively. The apparent reaction activation energy of removal of iron & aluminum was Ea Fe=46.908kJ/mol, Ea Al=34.067kJ/mol, respectively. The apparent reaction order of removal of iron & aluminum with pressure leaching in hydrochloric was 0.899、0.346, respectively. The kinetics model of Fe&Al extraction from MG-Si by pressure leaching was found respectively:The models were proved by 80experiments, respectively. They indicate that the calculated consequences and experimental consequences were inosculation very well.(6) The economy analysis on the pre-treatment of MG-Si(production of upgrade metallurgical grade silicon) showed that the production cost was controlled less than 1.73×104 RMB/ton.It can get great economic benefit for production of upgrade metallurgical grade silicon using hydrometallurgical routes in industry.更多还原