Polysilicon High-Temperature Off-Gas Treatment

Process Background

Polysilicon is the core raw material for photovoltaic and semiconductor industries and has experienced rapid capacity expansion in recent years.

Both the modified Siemens process and the silane-based process involve extensive filtration and separation units throughout production, including silicon powder collection, chlorosilane circulation and scrubbing, and circulating liquid purification and reuse.

Effective filtration is essential to maintain stable operation, heat transfer efficiency, and overall process safety.

Typical Contaminants and Solid Impurities

During polysilicon production, filtration systems are required to handle a wide range of solid contaminants, including:

• Unreacted silicon powder particles
• Solid impurities carried by circulating process liquids
• Dust and particulates introduced with feed materials
• Solid by-products generated during synthesis reactions

The presence of these particles directly affects process stability, heat exchanger performance, and equipment reliability.

High-efficiency filtration and separation systems protect critical equipment while contributing to improved production yield.

High-Temperature Off-Gas Purification Applications

Off-gas filtration is applied across multiple stages of polysilicon production and plays a critical role, particularly under current market conditions where cost reduction and operational efficiency are essential.

Typical Application Points

• Flue gas filtration from waste gas and waste liquid treatment units
• Reduction furnace off-gas separation
• Hydrogenation furnace off-gas separation
• Silicon powder silo vent and dust collection

Reduction Furnace Off-Gas Separation

After dry separation of SiCl₄ hydrogenation gas and SiHCl₃ reduction gas, hydrogen (H₂) and hydrogen chloride (HCl) are recovered for reuse.

Before hydrogen is recycled, it must be filtered to remove unreacted silicon powder and graphite powder generated during the process. This ensures hydrogen purity and enables stable and efficient hydrogenation and reduction reactions.

Key Challenges

High-Risk Hydrogen Service

• Hydrogen is a hazardous gas requiring leak-tight system design
• Minimized maintenance intervention to reduce operational risk

Fine and Hard Particulates

• Graphite and silicon powder particles are extremely fine and abrasive
• Require stable pore structures and consistent filtration performance

Elevated Off-Gas Temperature

• Off-gas temperatures are high and require thermal-resistant filtration systems
• Avoiding gas cooling prevents corrosion and reduces energy consumption

Filtration Solutions

• Automatic back-pulse cleaning systems to reduce maintenance frequency
• Sintered metal fiber filter elements with stable pore size distribution
• Typical filtration ratings of 0.1–0.5 μm
• Direct high-temperature filtration without pre-cooling

Waste Gas and Waste Liquid Thermal Treatment

Waste gas and waste liquid treatment remains one of the most challenging aspects of polysilicon production.

Traditional wet scrubbing systems are associated with high operating costs, significant resource consumption, and increasing environmental compliance pressure.

Dry Separation and Thermal Treatment Solution

A dry separation approach is applied in which waste gas and waste liquid are combusted inside the furnace, generating solid SiO₂ powder.

High-temperature filtration systems are used to separate and collect SiO₂ particles.

Key Benefits

• Recovery of valuable SiO₂ by-products, creating additional economic value
• Lower operating costs compared to traditional wet scrubbing
• Reliable compliance with environmental emission standards

Typical Filtration Technologies Applied

• High-temperature sintered metal fiber filters
• Automatic back-pulse filter systems
• Dry dust separation units
• High-efficiency solid–gas separation systems