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Polysilicon plant waste recycling

By understanding how waste streams are created and recycling processes available, polysilicon plant managers can bring their facilities into closed-loop operation, say Carl Merkh and Xiaojing Sun of Dynamic Engineering. Here’s how, and why, to recycle at polySi fabs.

October 17, 2011 -- Polysilicon production generates waste, and recycling that waste can be advantageous. An understanding of how waste streams are created and how to remove them through recycling, will enable polysilicon manufacturing with a closed-loop design. The economics of recycling are also discussed along with the use of an ion-exchange catalyst for recycling.

Polysilicon production overview

Polysilicon is required by the photovoltaic (PV) industry as a raw material for the manufacture of solar cells. Semiconductor makers also use polysilicon.

Prior to 2008, a polysilicon shortage triggered new capacity startups for making both solar- and electronic-grade polysilicon. Efforts are now underway to minimize waste materials generated while reducing the production cost of polysilicon. Trichlorosilane (TCS or SiHCl3) is utilized for most of the polysilicon produced in the world using the Siemens reactor process.

Here comes the waste

There are two distinct processes for producing trichlorosilane: the chlorination process and the hydrochlorination process. Both processes produce waste streams that can be recycled. One significant waste stream produced by these processes contains dichlorosilane (DCS or SiH2Cl2), which is generated in multiple units of a polysilicon plant that uses the Siemens reactor process. In addition to DCS, silicon tetrachloride (STC or SiCl4) is also produced in even greater quantities by multiple units and is recycled back to one or more units. Because DCS is very dangerous to store in large quantities and has a very low boiling point, a method must be found to recover the silicon loss due to DCS generation. Storage of large quantities of STC is not as dangerous as storing DCS, and it is more easily transportable for potential sale as a by-product.

The method developed by DEI for handling DCS incorporates a redistribution reactor (see the figure) in the trichlorosilane manufacturing process to provide a path to convert waste DCS back to TCS. To be effective, excess STC is required for the reaction with DCS to produce additional TCS. When optimized, 95 % of original DCS in the feed is typically converted to TCS with the current DEI DCS redistribution reactor technology.

Figure. Dynamic Engineering Inc. Redistribution Reactor.

Economics

DCS is formed in two ways in the polysilicon production process. First, DCS is a common by-product for both the TCS chlorination reactor and hydrochlorination reactor. It is removed together with other light impurities through the distillation columns. Normally, this DCS stream is handled as a waste stream to the plant scrubber or waste hydrolysis system. Second, in the deposition of TCS in the CVD reactors, DCS is also a by-product in the off-gas, of which a portion is often removed to improve operation of the deposition process. When DCS is removed, a large waste stream can be created. This waste stream can be minimized if the DCS is converted back to TCS. This method allows for a closed-loop plant design.

Benefits

Many polysilicon producers are seeking technologies to improve their process economy by reducing waste and energy consumption. So, what are the benefits of utilizing the DCS redistribution process? First, DCS is recycled back into the process to generate TCS rather than being discarded. Recycling increases the TCS plants output and yield, which has a net savings for the plant. Second, basic/caustic reactants such as NaOH or Ca(OH)2 are saved in the typical waste neutralization. It also reduces the emission of NaCl in the waste water, SiO2 solid waste, and water consumption for hydrolyzation. This waste reduction results in a more environmentally sustainable and greener operating process. Third, STC is consumed in this process instead of being sent back to the STC thermal converter or hydrochlorination reactor, which saves electrical power consumption and improves throughput for non-recycled streams. See the table for DCS redistribution process savings based on a 1,000 MTA plant.

Name

1000 MTA Plant

Materials Savings

Financial Savings

DCS Recovered

1,000 MTA

Saves 800 MTA NaOH for neutralization

@ $300/ton = $240,000/year

STC Consumed

1,700 MTA

Saves 3.6 KWH/kg to convert to TCS with a Thermal Converter

@ $0.06/KWH = $367,200/year

Or saves 1,600 MTA NaOH for neutralization

@ $300/ton = $480,000/year

TCS Produced

2,295 MTA (85% TCS Recovery) Waste = 405 MTA

Savings vs. 2295 MTA sourced from supplier

@ $1.40/kg TCS = $3,213,200/year.

405 MTA TCS to waste requires 360 MTA NaOH for neutralization

@ $300/ton = $108,000/year

 

Net savings = $3,105,000

Total Value

 

Up to $3,825,000/year in savings

Additionally, although a distillation column is needed for this redistribution reactor process, the capital and utility cost is offset by savings in the neutralization and waste water treatment.

DCS safety issues

A review of hazards listed in the MSDS for DCS associated with storage, handling, and transporting DCS reveals the following information:

  • Poisonous, flammable, corrosive liquid and gas under pressure;
  • Under ambient conditions, is a colorless gas with an irritating, choking odor;
  • Contact with water or moist air liberates HCl gas;
  • Can form explosive mixtures at low or high concentrations with air (4.6 - 4.8) Vol % lower limit, or (94 -98) Vol % upper limit; and
  • May ignite on contact with air or water.

DCS has a normal boiling point of 8.2°C and must be under pressure to be stored as a liquid. Because of these considerations, extraordinary precautions must be taken to ensure any process using DCS for feedstock must be thoroughly tested for leaks before DCS is introduced. The process must be documented leak proof. The system must also be completely free of oxygen and moisture and any other known residual oxidants prior to introduction of chlorosilanes.

Storage of large quantities of DCS should always be minimized and transportation discouraged for operator safety and liability risk due to potential DCS leaks.

For safety and economic reasons, the DCS redistribution reactor provides an easy and quick means for disposing of DCS and recycling the contained silicon back to the polysilicon manufacturing process.

Ease of operation

The chemical reaction occurring in the reactor is presented as follows:

SiCl4 + H2SiCl2 ? 2 HSiCl3

or

STC + DCS ? 2 TCS

This reaction is typically performed in a continuous process flow at near ambient temperatures. The reactants STC and DCS flow though a reactor bed filled with a catalyst for the reaction to produce TCS. The reactants are fed into one end of the reactor, and the TCS product comes out the other end along with the excess STC present. This continuous flow process is very easy to control and maintain.

Testing

To ensure that high rates of DCS conversion to TCS are maintained, periodic analytical testing of the chlorosilane composition of reactants fed into the reactor bed and of the product exiting the reactor bed must be performed. Reduced DCS conversion efficiency may be caused by the catalyst becoming deactivated because of adsorption of impurities in the chlorosilane feed streams over a period of time. Collecting liquid or gas samples of chlorosilane mixtures is time consuming and labor intensive because of the required safety precautions. Handling, transporting, and storage of the chlorosilane samples to be analyzed are also major cost considerations before a lab analysis is performed for chlorosilane composition. Typical lab analysis for chlorosilane composition is performed by a gas chromatograph with an atomic emission detector. The sample injection must be performed in a closed-loop system with a nitrogen environment by using an autosampler in a laboratory on site. Transporting samples to an off-site laboratory can cause sample contamination, and has the safety hazards associated with handling, storing, and transporting chlorosilane samples.

However, an in-line NIR process analyzer is a much simpler way of monitoring the chlorosilane composition fed in and out of the reactor using In-line flow cells connected to a near-infrared spectrophotometer. To utilize this automated measurement system, one transmission flow cell is installed in the feed stream and another is installed on the exit stream of the reactor to determine feed and product chlorosilane compositions. Fiber optic cables transmit the NIR spectrum through the process fluids to a near-infrared spectrophotometer. After this system is set up and calibrated using a reference method, precise determination of the chlorosilane composition percentages (DCS, TCS, and STC) may be determined.

The in-line NIR process analyzer does not require a sample collection, which eliminates exposure to operations and lab personnel for handling, transporting, and storage of hazardous chlorosilane samples for lab analysis. The DCS conversion efficiency can be effectively determined by the in-line NIR process analyzer signals with spectra data dumps and daily reports being generated continuously.

Conclusion

A process-centric approach was taken to address dichlorosilane waste streams in a polysilicon plant. The focus on minimizing waste and energy expenses while optimizing the production of TCS has enabled polysilicon plants to effectively use DCS waste streams instead of disposal. This process eliminates the need to store DCS, which is costly and hazardous, by utilizing a DCS redistribution reactor to recycle the DCS with excess STC. To ensure high rates of DCS conversion to TCS, periodic analytical testing must be performed. An automated and simpler means of testing is achieved with a NIR analyzer, which requires no sample collection because transmission flow cells are installed in the flow stream.

Carl Merkh received his BS in chemical engineering from Lowell Technological Institute and MS in chemical engineering from Lowell U. He is the author/coauthor of five patents and has two patents pending. Carl is the quality assurance manager at Dynamic Engineering, Inc., 654 N. Sam Houston Parkway E, Suite 200, Houston, TX 77060 USA; ph.: 281-617-0099; cmerkh@dynamicengineer.com.

Xiaojing Sun received her BS in chemical engineering from Dalian U. in China and an MS from Mississippi State U. She is a project manager/engineer at Dynamic Engineering Inc.

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