Effect of dry recovery gas content on reduction

The specific impact is unknown, but in manufacturers that use the CDI process, exhaust gases containing DCS (dichlorodihydrosilane) are likely to be present; generally, when the exhaust gases are impure, there is a higher possibility of trace amounts of HCL being present.

Dichlorodihydrosilicon is certainly present in the recycled exhaust gas; after dry recovery, it is sent to a purification tower, where it escapes along with the exhaust gas at the top of the tower. A trace amount of it may end up in the product that goes into the reduction process. Long-term monitoring and observation of the growth conditions inside the reduction furnace show that this has little impact on those conditions.

The question posed by the original poster is a bit vague. It’s normal for the gas entering the dry recovery system to contain dichlorodihydrosilane; it is only possible to determine the performance of the reduction furnace based on the amount of this substance present. If the gas at the output end of the dry recovery system contains dichlorodihydrosilane, that’s a serious problem – not only must the impact on the reduction furnace be taken into account, but also the activated carbon adsorption tower.

Let me share what I know: DCS is generated during the reduction process. DCS is also a raw material used in the production of polysilicon, and its pyrolysis temperature is lower than that of TCS (around 800 degrees). Foreign polysilicon manufacturers do not separate DCS during the distillation process; instead, it remains together with TCS as part of the mixture undergoing reduction to achieve equilibrium. Currently, some domestic companies still lack effective control over this reduction process – specifically, the pressure, temperature, current, and feed rate of the reduction furnace, as well as the ratio of H to TCS in the feed, have not been adjusted to their optimal levels. If DCS is not separated, then as the feed rate increases during the later stages of reduction, some of the DCS will pyrolyze before it can settle on the silicon rods to form silicon powder particles!

The exhaust gas from the reduction furnace is subjected to dry recovery to produce a mixture of trichlorosilane and silicon tetrachloride. After dry distillation of this mixture, it is returned to the reduction furnace. The DCS level in this mixture is between 5% and 8%; both the pyrolysis temperature and the reduction temperature of DCS are lower than those of TCS. Therefore, I believe that the DCS content has an impact on the reduction reaction. With a high DCS content, does deposition on the reduction furnace increase? Do the silicon powder particles increase as well? Does the conversion rate increase or decrease? Does the safety factor increase or decrease?

If the DCS content is high, the one-way conversion rate of TCS decreases, increasing operating costs.

The gas recovered by dry method contains dichlorodisilane and hydrogen chloride gases; after filtration through activated carbon and purification in a distillation tower, the amount of low-boiling substances is very small, or even none, so it has little impact on the reduction process.