Fundamental Study On The Recycling Of Silicon From The Wiresawing Slurry By The Wet Process

Compared with fossil energy, solar energy has obviously advantages, such as pollution-free, reserves unlimited, and can be generated nearly anywhere at any scale. Therefore, the efficiency utilization of solar energy is essential for the global continuous development. Especially in recent years, more and more countries have paid attention to the solar energy photovoltaic industry and the governments have issued a series of policies which are favorable to the development of the polysilicon production. As the core component of the solar modules, the pollysilicon production has been consistent growth of global. The main process of producing silicon wafer is by multi-wire slicing, which uses polyethylene glycol(PEG) as the suspension agent and SiC particles as the abrasive material. After slicing of the polysilicon ingot into wafers, about 40 wt% silicon scrap powder will be produced into the wiresawing slurry and mixed with SiC and a small amount of scrap metal from the steel wires. The efficient and economical recovery of high purity silicon powder is significantbecause it could easy up environmental pollution and silicon resources wasting. Besides, the recycled silicon powder as raw material to remelt into the solar grade silicon which realize the high value circulation use and decrease the cost of production. To achieve this goal, two challenges must be solved, i.e., the first one is to separate the Si/SiC mixture powder effectively to obtain the Si-rich powder, and the second one is to remove the tiresome harmful impurities such as B, P to meet SoG-Si requirements. Based on these two problems, theoretical analysis and experimental investigation of recycling high purity silicon powder from the wiresawing slurry are discussed in detail in this paper.The chemical compositions, occurrence forms and component contents of the silicon cutting waste powder were investigated at first. The content of Si, SiC and other impurities of the mixed powder, which was pretreated by removal of PEG, are 24.9wt%,72.2wt% and 3.1wt%, respectively. The aggregation of Si and SiC particles in the silicon wiresawing waste powders may be caused by the frictional effects in the cutting process, or the drying and dehydration process.For the separation of Si and SiC particles, based on the different particle size, density, volume, and surface charge with different pH value in the aqueous solution, the way of centrifugation treatment was investigated. Tuning the particle surface potential combined with centrifugation can effectively separate the two kinds of particles. The terminal velocity of SiC and Si was about 1000 times different, especially in the centrifugal field, the particles were in the state of accelerated motion, enhanced the efficiency of the centrifugation. The obtained Si-rich powder contains more than 91.8 wt% Si and SiC-rich phase contains more than 95.2 wt% SiC.In addition, a novel process is proposed to separate Si and SiC particles effectively based on the different particle sizes. The average particle diameters of Si and SiC were 1μm and 9μm. so with the proper pore size of the millipore membrane, the two kinds of particles can be separated effectively by the microfiltration separation process. After tuning the surface charge of the particles in the solution, the Si recovery ratio of the collected Si-rich powder can be reach 76.2%.The B and P were often found in the waste wiresawing slurry, and their complete removal is very significant to the effective recycling of silicon scrap for solar energy cell. The source of them was confirmed from the steel wire used as the cutting after systematical study. The cutting wires must be brass plated before drawing to proper diameter during the production. Surface treatment of steel wires contains phosphating and covering borax to improve the corrosion resistance and the drawing property. The wires were irrevocably scuffed after using, and the steel scraps will be mixed in the cutting slurries. This finding suggested that using an appropriated method instead of phosphating can avoid the introduction of B and P, and the subsequent procedure was simple and low in cost.The Si-rich powders obtained by the centrifugation process, in which the major metal impurities was Fe, and B,P came from sawing wires distributed with Fe. And these impurities were mixed among the particles, instead of existing in the inner silicon crystal. So acid washing way was being proposed to the remove Fe, B and P. The reaction temperature of the acid leaching process has the significant on the removal efficiency of scraped Fe, B and P. The leaching behavior of these impurities in the hydrochloric acid solution is the diffusion-controlled process. Microwave heating for acid leaching may have many advantages which can promote and strengthen this process. But there are still two problems to solve, one was acid consumption is lager, the other was the residual impurities of B and P cannot meet the requirement of SoG-silicon. Therefore, the process of pre-dispersion and magnetic separation was attempted. The aggregation of the fine Si-rich powder which prevented the impurities exposing to the acid solution, was brought big trouble to the further purification. The aggregates in the Si-rich powder were dispersed by surface potential tuning combined with ultrasonic wave and microwave treatment.According to the DLVO theory and experimental results, the electrostatic repulsion of the particles waslargest when the pH value of solution reached over 13.0. and the fine particles can be kept sufficiently dispersed, which was good for the further separation by magnetic field with the 90%removal of iron impurity.The residual iron impurity can be further completely removed by the microwave acid leaching with the mixing acid solutions of hydrofluoric and hydrochloric acid. By combiningthese enhanced purification techniques, other bothersome impurities such as B and P can also be reduced to the requirements of SoG-silicon.This subject was targeted at the separation and purification of high purity silicon powder from the silicon wiresawing waste. The process of tuning the particle surface potential combined with centrifugation, magnetic separation and acid leaching was adopted to solve the two problems successfully. The separation of Si/SiC and thoroughly removal of B/P, laid a foundation for silicon wiresawing waste high-value utilized.