Research Of Regenerating Water-based Polysilicon Cutting Fluid

With the increasing global energy and environmental pollution, more and more country is paying much more attention on cleaning energy. The importance of solar energy utilization and construction has being stetted off a wave in worldwide. Therefore, the solar industry chain has caused a range of products grows rapidly. In recent years, the development of solar cells in China has made great development; it also contributed its main raw materials of polysilicon solar industry. The polysilicon chiping technology needed in the photovoltaic industry, which polysilicon cutting fluid is required, as a result, the amount of polysilicon cutting fluid make a surge. The cutting fluid can only use repeatly in several times in the polysilicon cutting machine, after which will gradually wear out. Presently, polysilicon cutting fluid is mainly water-based both at home and abroad, of which the polyethylene glycol and silicon carbide are the mainly components. Therefore, the rapid development of the whole solar industrial chain will inevitably lead to the production of large quantity of waste cutting fluid.Water-based polysilicon cutting fluid waste is a black liquid and very viscous mixture which greatly exceeds the value of COD wastewater discharge standards, which emissions is prohibited and there is no efficient waste treating and recovery method. So large of waste piled up in domestic manufacturers. With the continuous development of producting and piling up over many years, it has become the stumbling block to development. To find a simple and effective method to recovery polyethylene glycol and silicon carbide from waste cutting fluid is urgently. Firstly, the paper analyzed and determined the main impurities content in silicon water-based cutting fluid. 1. Its impurities include polysilicon and Fe2O₃ and a small amount of metal oxide. 2. The mixed acid (HF+HNO₃+HCl) for determining total impurities in silicon carbide. The acid determination result of the SiC waste sand is the total content of impurities 3. The result of alkalis determination is the real polysilicon impurity content, 40% NaOH is used for determination of silicon impurities. 4. Fe₂O₃ content was determined by spectrophotometer. 5. New sand content of SiC is 99.79%, Fe₂O₃ content of 0.09%; other metal oxides are 0.12%. The content of waste sand are respectively SiC 88.85%, Fe₂O₃ 1.53%, Si 9.42%, 0.20% other metal impurities. The size distribution of silicon carbide powder is an important indicator configured by factory to determine whether the cutting fluid can be used. Finally, the carbide powder distribution was evaluated by laser particle size analyzer and the particle size of waste cutting fluid dose not fit the standard. The particles less than 1μm silicon are silicon impurities and particle size distribution from 4 to 14μm new sand particles, and the closer distribution the more silicon carbide powder coating of impurities.This study on regeneration of PEG mainly uses the solid-liquid separation method. The vacuum distillation and filtration and concentration and gravity sedimentation and centrifugal sedimentation methods were compared. Apart from the vacuum distillation, only diluted by water can get cleared liquate. The fflocculation settled at the bottom after boiled the liquate for 10mins and placed it for 3 to 5 days, then the PEG solution can get by pouring the upper supernatant solution without Fe +3, then distilled and concentrated by T=100℃. At last the recovery rate is above 50% and 60% respectively by filtration and centrifugal sedimentation methods, both of which are most relate to the new PEG.. To regenerate silicon carbide from the waste sand which remained from recycled PEG was studied. The mainly method is to remove impurities of the polysilicon and Fe₂O₃ impurities. The acid and alkali dissolved impurity effect on the recovery method was evaluated separately. For the are that the single HF method obtained silicon carbide powder can make Fe2O3 down to 0.15% or less, but the product still contains more than 3% silicon impurities. For the mixed acid method, the single factor and orthogonal experiment of removing silicon and Fe2O3 were studied with the main indicator of total silicon and Fe2O3 content. The order of impacting factors on purifying is HNO3 concentration>HF concentration>reaction time>reaction temperatures. Optimum level of impurity is HF concentration is 3mol/L, HNO3 concentration is 2.0mol/L, reaction temperature is 35℃, silicon removal time is70mins; For the alkaline purifying method, the order of removing impurity was studied respectively, And both the technology orders for removing the silicon is basically the same conditions. In addition, the silicon removing conditions were optimized by orthogonal experiments from which the effecting order of re moving silicon factors is reaction temperature>NaOH concentration of>reaction time, and the optimum level is that NaOH concentration is 3%, temperature is 70℃, time is 2h, However, if the silicon removed first the Fe2O3 removing conditions will be much sample. Both the acid and alkali methods are two good ways to purify. And the impurity of Si will down to 0.3% by acid method while the alkaline can make it to below 0.5%. Both of which can make the Fe2O3 content and silicon carbide particle size distribution achieve the requirements, which can be used to make polysilicon cutting fluids...