Silicon Tetrachloride Reclamation in Polysilicon Industry
Year:2010 ISSUE:10
COLUMN:HEALTH, SAFETY & ENVIRONMENT
Click:351    DateTime:Nov.02,2010
Silicon Tetrachloride Reclamation in Polysilicon Industry    
By Amy Lee   

China's current polysilicon market   

China's polysilicon production capacity reached 80 000 t/a in 2009, while the output was only 17 000 tons, which means the overall utilization rate of polysilicon capacity was as low as about 20%. But such a low operating rate essentially arose from neither the slack demand nor the oversupply. In fact, the demand for polysilicon is booming in China. To satisfy its domestic need, China imported about 20 000 tons of polysilicon in 2009, more than those it produced, and the figure is expected to be 25 000 - 30 000 tons in 2010.   
   This phenomenon has little connection to product quality. The manufacture of polysilicon that used in photovoltaic industry has no high requirement in technology. China-produced photovoltaic polysilicon products can compare favorably with those made abroad. The high production cost is the real crucial one. For some well respected multinational enterprises, the production cost is about US$15-25/kg, while in China, the figure is US$35/kg for large-scale manufacturers and even reaches US$70-100/kg for the numerous small plants. But the selling price of polysilicon in the international market has maintained at about US$50/kg for a long time, so there is no margin for most of the Chinese small-scale producers. They have no choice but suspend their operation.   
   The high production cost in these plants can be blamed on the low reclamation rate of silicon tetrachloride - a byproduct in polysilicon production. Among the various polysilicon manufacturing techniques, Siemens process is the most popular one in China. On the average level for this process, producing one ton of polysilicon generates about 15 tons of silicon tetrachloride.   

Difficulties for reclamation   

Unfortunately, unlike the foreign advanced companies which have realized closed-recycle for polysilicon production and zero discharge of silicon tetrachloride, most of the silicon tetrachloride generated in China cannot be reclaimed. This causes a waste of silicon resources and also has negative effect on the environment.
   Facing technical bottleneck in the reclamation of silicon tetrachloride, most of the Chinese enterprises have to extend the product chain to produce downstream chemicals as to cut cost. The major varieties of the downstream chemicals include gas-phase silica white (SiO2), silicon optical fibers, silicone, etc. But the demand for average-grade gas-phase silica white in both domestic and international market is saturated. There is still market gap for the high grade silica white, but technology is still on the way of its scalization. The consumption of silicon tetrachloride in the production of photovoltaic (PV) grade silicon tetrachloride and silicone is very limited. In the past few years, some enter prises supply silicon tetrachloride to the downstream manufacturers freely and even pay the transport themselves. In recent years, the situation became much better. It still has a long way to go for polysilicon involvers to treat the increasing amount of silicon tetrachloride by developing downstream products.
    "The output of polysilicon is projected to reach 40 000 tons in China in 2010, which will come along with 400 000 - 600 000 tons of silicon tetrachloride. So, for further development, enterprises have to attach great importance to the silicon tetrachloride reclamation. At present, reclamation is the best way for the treatment of silicon tetrachloride. Silicon tetrachloride can be converted to trichlorosilane for the production of polysilicon. Other segments that use silicon tetrachloride as material can search further development in cooperation with polysilicon manufacturers," said Yi Zhengyi, general manager of Sichuan Yongxiang Polisilicon Co., Ltd.
   There are about four processes for the reclamation of silicon tetrachloride in polysilicon production: cold-hydrogenation, thermal hydrogenation, hydrochlorination and plasma hydrogenation. Most of the Chinese polysilicon units that started operation before 1990 use the thermal hydrogenation process. It was severely checked since it has huge energy consumption and causes great waste of resources. Cold-hydrogenation is respected to be a mature process for the conversion of silicon tetrachloride to trichlorosilane, and most newly-built units in recent years adopt this process. However, there have not enough people with qualified technical level for this process' successive running of operation with low production cost. Cheng Huiting, engineer of Hualu Engineering & Technology Co., Ltd. said," there's only one polysilicon producer in Jiangsu province using the cold-hydrogenation process with proprietary intellectual property rights. It has an annual polysilicon output of about 12 000 tons per year. Most of the others use the modified Siemens process which can also realized closed-circuit of silicon tetrachloride. The overall polysilicon output with the closed-circuit process has occupied about 90% of the total polysilicon output national wide. For the Chinese enterprises, it is difficult to realize continuous operation using this imported process."
    Shen Zuxiang, an engineer from Chengdu Shuling Technology Development Co., Ltd. did not agree with this. "Silicon tetrachloride reclamation is an exacting work that requires lots through the whole system. We can not fondly believe the imported technology; we should have our own judgment."