PFI:AIR - TT: Cost-Effective Membrane-Based Green Electrolytic Process for Solar and Semiconductor Grade Silicon Production

这个PFI：空气技术转换项目的重点是将最先进的硅生产工艺从能源密集型和对环境有害的工艺转变为具有成本效益的绿色工艺。新工艺在能源成本方面比目前的做法高出一个数量级，而且排放的二氧化碳(CO2)也远低于现有最有效的冶金工艺的一半。该项目的目标是生成必要的工艺数据，以评估这种基于固体氧化物薄膜的绿色电解工艺用于半导体和太阳能级硅生产的可扩展性和成本效益。在成功实施后，这一进程将比目前的进程显示出几个优势。它们包括：与还原二氧化硅(SiO_2)所需的自由能相比，能耗非常低；使用几乎不需要或不需要前处理的廉价原材料；无碳热还原，每公斤硅产品排放10公斤二氧化碳，其污染物通常会使硅的纯度从天然石英岩中的99.6%降至冶金级(MG)硅中的97-98%；通过三氟化硼(BF3)挥发去除固有的硼；工艺中不存在任何碳或氯；并且不存在导致全氟化硅和/或高氯碳排放的阳极效应。目前生产硅的方法有：流态化还原、高纯二氧化硅碳热还原、渣/晶精炼、液态硅电解、氯化物和氟化物电解法。这些工艺的一些主要限制包括广泛的原材料前处理、低产量、有害的环境影响和大量的能源需求。在所提出的工艺中，将使用一端封闭的氧离子导电稳定氧化锆(SOM)管从放置在SOM管内的惰性阳极中分离出溶解在熔剂中的纯二氧化硅(SiO_2)。为了确保产品的纯度，在较低的施加电压下使用二次阴极的预还原步骤将被用来去除比硅更具电负性的杂质。去除含有杂质的二次阴极，然后采用液态锡阴极，提高外加电位以还原二氧化硅。还原后的硅将进入液态锡阴极中溶解。与硅相比，电负性较小的杂质离子将留在熔剂中。因此，电负性杂质较多和较少的杂质氧化物不会与二氧化硅一起还原。在高温下，硅在液态锡中的原子百分含量超过95%，但在较低温度下，纯硅和锡是不相容的。这将允许在电解后采用定向凝固来生产高纯度硅锭，并证明这是一种从商业上可获得的二氧化硅来源大规模生产硅的成本效益高的无碳方法。该项目将为研究生和本科生提供与我们的工业合作伙伴合作的研究机会，并使技术更接近商业化。它还将提供一套丰富的案例研究材料，用于引入本科生和研究生课堂教学。清洁金属公司英飞凌和半导体级硅的消费者SunEdison半导体将参与这项研究计划，以评估从商业上可获得的二氧化硅来源开始大规模生产硅的绿色技术的质量、可扩展性和成本效益。

This PFI: AIR Technology Translation project focuses on translating technology that will change the state-of-the-art silicon production process from an energy-intensive and environmentally detrimental one into a cost-effective green process. The new process is an order of magnitude better in energy cost than current practices, and also emits well below half the carbon dioxide (CO2) of the most efficient existing metallurgical processes. The goal of the project is to generate the necessary process data to evaluate scalability and cost-effectiveness of this solid-oxide-membrane-based green electrolytic process for semiconductor and solar grade silicon production. Upon successful implementation, this process will demonstrate several advantages over current processes. They include: very low energy usage relative to free energy required for silicon dioxide (SiO2) reduction; use of inexpensive raw materials that require little to no pre-treatment; no carbothermic reduction, which emits 10 kg of CO2 per kg silicon (Si) product and whose contaminants typically reduce purity of silicon from 99.6% in natural quartzite to 97-98% in metallurgical grade (MG) Si; possibility of inherent boron removal by borium triflouride (BF3) volatilization; absence of any carbon or chlorine in the process; and there are no anode effects resulting in perfluro and/or perchloro carbon emissions. Current methods for Si production include, fluidized bed reduction processes, carbothermic reduction of high-purity silica, slag/crystal refining, liquid Si electrorefining, and electrolysis of chlorides and fluorides. Some of the major limitations of these processes include extensive raw materials pre-processing, low yields, detrimental environmental impact and substantial energy requirements. In the proposed process, a one-end-closed oxygen-ion-conducting stabilized zirconia (SOM) tube will be used to separate pure silica (SiO2) dissolved in molten flux from an inert anode placed inside the SOM tube. To ensure product purity, a pre-reduction step using a secondary cathode at lower applied potentials will be employed to remove impurities that are more electronegative than Si. The impurity-laden secondary cathode will be removed, and then employing a liquid tin cathode the applied potential will be increased to reduce silica. The Si reduced will go into solution in the liquid tin cathode. Less electronegative impurity ions compared to Si will remain in the flux. Thus impurity oxides of both more and less electronegative impurities are not reduced along with silica. Si is over 95 atom% soluble in liquid tin at high temperatures but at lower temperatures pure Si and Sn are immiscible. This will allow directional solidification to be employed after electrolysis to produce high-purity Si ingots and demonstrate this as a cost-effective carbon-free method for mass production of Si from commercially available sources of silica. This project will provide research opportunities for graduate and undergraduate students to work with our industrial partners and move the technology closer towards commercialization. It will also provide a rich set of case-study materials for introduction into both undergraduate and graduate classroom teaching.Infinium, a clean metals company, and SunEdison Semiconductors, consumer of semiconductor grade silicon will be engaged in the research program to assess quality, scalability and cost-effectiveness of the green technology for mass production of silicon starting from commercially available sources of silica.