Solving Polysilicon Purification Challenges: Key Processes and Equipment Selection Guide for Rotary

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Solving Polysilicon Purification Challenges: Key Processes and Equipment Selection Guide for Rotary

The photovoltaic and semiconductor industries have an increasingly urgent demand for high-purity polysilicon. However, traditional purification processes have long been plagued by three major challenges: high energy consumption, difficulty in removing impurities, and poor stability of finished products. Alumina microbeads, as ideal high-temperature support media, can prevent secondary pollution but often suffer from wear and failure due to improper control of process parameters. The rotary tube furnace, with its core advantages of uniform heating and sufficient material mixing, is becoming a key equipment to address this industry pain point. Combining actual test data, this article breaks down the optimal solution for the collaborative processing of polysilicon and alumina microbeads, while explaining the key points of equipment selection.

1. Industry Pain Points: Three Core Obstacles in Polysilicon Purification

The initial purity of industrial-grade polysilicon is mostly 99.99% (4N). Among them, metal impurities such as Fe and Cu, as well as non-metallic impurities such as B and P, directly affect the conversion efficiency of photovoltaic modules and the performance of semiconductor chips. The traditional improved Siemens method has three major limitations:
High energy consumption: Producing 1 ton of polysilicon requires more than 120 kWh of electricity, which is not in line with the trend of green manufacturing;
Incomplete impurity removal: Low-volatility impurities such as B and P are difficult to separate through simple distillation;
Serious material loss: Polysilicon particles are prone to adhesion and cracking during high-temperature treatment, with a yield rate of only 60%-70%.
The introduction of alumina microbeads was originally an optimization direction. Their α-Al₂O₃ composition has a melting point of 2050℃ and a compressive strength of over 200MPa, which can provide uniform support for polysilicon. However, in practical applications, 80% of enterprises will encounter problems such as microbead wear and fluctuations in polysilicon purity. The root cause lies in the mismatch between process parameters and equipment performance.

2. Verified by Experiments: Optimal Solutions for Four Key Process Parameters

Based on hundreds of experiments with the RTF-1200 type rotary tube furnace, we have identified the key parameters for the treatment of polysilicon and alumina microbeads (mass ratio 1:3). These parameters can increase the purity of polysilicon to over 99.9995% (5N5) while ensuring that the number of repeated uses of microbeads exceeds 50 times.

2.1 Heating Rate: 5℃/min Balances Efficiency and Purity

The heating rate directly affects impurity diffusion and material integrity:
Too slow (≤2℃/min): The diffusion power of impurity atoms is insufficient, and the residual amounts of Fe and Cu are 3 times higher than the optimal value;
Too fast (≥8℃/min): The temperature difference between the surface and interior of polysilicon particles reaches more than 80℃, the cracking rate rises to 15%, and the compressive strength of microbeads drops to 185MPa (initial 200MPa);
Optimal 5℃/min: Impurities are fully released, the particle integrity rate reaches 98%, and the microbead structure is undamaged.
This benefit comes from the 30-segment program temperature control system of the rotary tube furnace, which can achieve precise temperature control of ±1℃ and avoid local overheating.

2.2 Holding Temperature: 1200℃ Achieves a Win-Win for Purification and Stability

The effect of temperature on polysilicon purity shows an inverted U-curve:
1000-1200℃: As the temperature increases, the contents of B and P decrease from 0.0008% and 0.0006% to 0.00005% and 0.00004%;
＞1200℃: Polysilicon undergoes slight melting and adhesion, which hinders the further release of impurities. At the same time, high temperature may cause a slight reaction between polysilicon and the corundum furnace tube, introducing a small amount of Al impurities;
Key control point: A furnace body using silicon-molybdenum rod heating elements can accurately maintain a constant temperature of 1200℃, which is an advantage that a quartz tube furnace body cannot achieve.
For alumina microbeads, at 1200℃, their XRD pattern completely matches the standard α-Al₂O₃ pattern, with no scratches on the surface, and the compressive strength remains above 195MPa.

2.3 Holding Time: 4h Balances Energy Consumption and Effect

The core of holding time is "sufficient without redundancy":
2h: Impurities are not fully diffused, and the removal rate of B and P is only 60%;
4h: The impurity content tends to be stable. Extending the time to 8h further does not improve the purity but increases energy consumption by 40%;
Adaptation suggestion: Equip with a paperless recorder for real-time monitoring and automatically trigger the cooling program to avoid manual misoperation.

2.4 Rotation Speed: 10r/min Optimizes Mixing and Wear

The rotation speed determines the material uniformity and microbead service life:
5r/min: Mixing is insufficient, the surface roughness of polysilicon reaches 2.5μm, and there are obvious depressions;
10r/min: The surface flatness is improved to 0.8μm, particles are evenly dispersed, and there is no obvious wear on microbeads;
＞15r/min: Material collision intensifies, the agglomeration rate of polysilicon reaches 12%, and the compressive strength of microbeads drops to 180MPa.
The tiltable furnace body design can further optimize material flow. By adjusting the tilt angle to assist in controlling the discharge speed, it forms a dual regulation with the rotation speed.

3. Equipment Selection: Three Core Indicators for Rotary Tube Furnaces
The key to the implementation of the process lies in equipment performance. When selecting equipment, focus on the following three points:

3.1 Heating and Temperature Control System
Heating element: For temperatures above 1200℃, silicon-molybdenum rods are preferred, which have better oxidation resistance than resistance wires and a service life twice as long;
Temperature control accuracy: Must reach ±1℃. A 32-segment PID temperature controller with an RS-485 interface is recommended to support remote monitoring;
Furnace lining material: Using alumina polycrystalline fibers, the heating rate is 30% faster than that of traditional refractory bricks, and energy is saved by 25%.

3.2 Furnace Tube and Transmission System
Furnace tube material: High-purity corundum tubes are suitable for polysilicon purification, avoiding the problem of high-temperature softening of quartz tubes;
Rotation mechanism: Gear + reducer transmission is more stable than belt transmission, and the speed adjustment range is preferably 0-30r/min;
Sealing performance: Double water-cooled "O"-ring sealing ensures that the purity of the argon atmosphere is ≥99.999% and prevents oxidation.

3.3 Safety and Expansion Configuration
Necessary functions: Over-temperature power-off alarm and automatic heat cut-off when the furnace door is opened to avoid safety accidents;
Optional configurations: Automatic feeder and angle measuring instrument, suitable for industrial continuous production;
Data traceability: Equipped with a USB interface recorder to meet the quality traceability requirements of the photovoltaic industry.

4. Conclusion
The core of polysilicon purification is "precise process parameters + matched equipment performance". The rotary tube furnace, with its advantages of uniform heating and flexible regulation, is perfectly suitable for the collaborative processing scenario of alumina microbeads. By selecting equipment with high-precision temperature control and a stable transmission system, and combining the optimal parameters of 5℃/min heating rate, 1200℃ holding temperature, 4h holding time, and 10r/min rotation speed, dual optimization of purity and cost can be achieved.

Zhengzhou Protech Technology Co.,LTD is a professional manufacturer specializing in tube furnaces, muffle furnaces, atmosphere furnaces, and vacuum furnaces. We are committed to providing targeted solutions to meet your diverse heating equipment needs.
For customized heating solutions tailored to your specific requirements, feel free to get in touch with us:
WhatsApp: +86 17719806024
Email: info@lab-furnace.com

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Solving Polysilicon Purification Challenges: Key Processes and Equipment Selection Guide for Rotary

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