What is the gas refining process for quartz glass?
Gas refining involves using hydrogen or other hydrogen-containing combustible gases as the energy source, combined with oxygen and special burners (referred to as “lamps” in the quartz glass industry). This process, carried out on specialized equipment, is used to melt quartz glass into various products.
Powder gas refining directly into pipe or transparent crucible
Initially, large-diameter transparent quartz glass tubes and crucibles were produced by directly melting crystal powders using simple equipment. This process had the following features:
Advantages: Simple equipment, no need for secondary heating, lower comprehensive energy consumption.
Disadvantages: The resulting tubes had uneven dimensions and wavy surfaces.
Although this technique is largely obsolete, some manufacturers retain it for small batches or special products. The product quality heavily depends on the operator’s skill.
Vertical Quartz Glass Ingot-Making Process
The vertical ingot-making process excels in producing large-diameter ingots. It allows for the installation of multiple burners and feed pipes, and enables the ingot to move in translation, rotation, and descent, ensuring uniform internal quality. However, this method is not suitable for creating very long ingots.
Principles of Vertical Ingot Making
In this process, a furnace with an opening at the bottom is used. A quartz glass target holder is fed upwards, while a hydrogen-oxygen burner at the top of the furnace heats powdered raw materials fed through a central pipe. The target holder rotates and descends simultaneously, maintaining a constant high-temperature zone for melting.
As raw materials are heated, they form a molten layer on the target holder. By balancing feeding speed and descent speed, the molten layer is consistently positioned in the high-temperature zone. Quartz glass’s short viscosity allows it to naturally form a cylindrical ingot under the temperature constraints of the furnace.
Key process parameters include:
- Ingot Target Diameter (D) – Defines the size of the ingot.
- Temperature (T) – Comprising flame temperature (T₁), furnace temperature (T₂), and ingot surface temperature (T₃).
- Feeding Speed (M) – Determines material input.
- Descent Speed (V) – Ensures uniformity in the ingot shape.
- Gas Flow Rates (QH, Qo) – The ratio and flow of hydrogen and oxygen influence flame temperature and melting efficiency.
Process Control and Mathematical Model
The mutual dependence of variables like temperature, feeding rate, and descent rate makes the process complex. While most manufacturers rely on experience, Liu Xuefeng’s mathematical model provides a theoretical approach. The model uses a second-order partial differential equation (similar to a vibration equation) to describe the relationship between parameters such as yield rate, heat levels, and ingot cross-section. This allows for optimization of process conditions to achieve uniform diameter and high-quality ingots.
Typical Parameters for a 200mm Diameter Ingot
- Regas Pressure: 0.6 MPa
- Drop Speed: 0.3 mm/min
- Furnace Temperature: 1600°C
- Oxygen Pressure: 0.6 MPa
- Feeding Amount: 20 g/min
- Hydrogen-Oxygen Ratio: ~2:1
- Furnace Mouth Diameter: 280 mm
Common Defects and Mitigation
The production process of gas-refined quartz glass ingots is relatively long, usually about 72 hours. If the process is to be stable, defects must be avoided. There are usually three types of defects, namely: bubbles, spots and diameter expansion and contraction.
Bubbles
Common bubbles include laminar bubbles, spot bubbles and dispersed bubbles. Laminar bubbles are numerous and small, densely clustered on a certain cross section of the melting block. This is mainly caused by excessive feeding at a certain time. The solution is to use a feeding device with good control performance. Spot bubbles often appear around the spots with spots. This is caused by poor raw material quality, and good quality raw materials should be replaced. Dispersed bubbles are mostly large and appear randomly in the melting block. The reasons for this type of bubble are more complicated, which may be a raw material problem or a process problem, and requires specific analysis.
| Chinese Bubble Standards | |||
|---|---|---|---|
| Bubble Diameter (mm) | Level 1 (Count) | Level 2 (Count) | Level 3 (Count) |
| 0.1–0.2 | 20 | 40 | 60 |
| 0.2–0.5 | 4 | 9 | 28 |
| >0.5 | 0 | 5 | 7 |
| German Bubble Standards | |
|---|---|
| Level | Bubble Diameter Range (mm²) |
| 0 | 0–0.03 |
| 1 | 0.03–0.10 |
| 2 | 0.10–0.25 |
| 3 | 0.25–0.50 |
| 4 | 0.50–1.00 |
| 5 | 1.00–2.00 |
| 6 | 2.00–4.00 |
| 7 | 4.00–8.00 |
| 8 | 8.00–16.00 |
Spot
The common colors of the specks in the melting ingot are: grayish white, grayish brown and black. From the analysis results in the table, it can be seen that the specks are grayish white when there is no iron, black when there is iron, and grayish brown when there is iron, calcium, aluminum, etc. The sources of the specks are: brought in by the raw materials, brought in by the hydrogen and oxygen pipelines, and brought in by the falling of the furnace materials. Therefore, in order to eliminate or reduce the specks, in addition to selecting good materials, it is also necessary to regularly clean the pipelines and pipe fittings, treat the furnace materials, and prevent them from falling off in the structural design.
| No. | Spot Color | Chemical Composition (Mass Fraction, %) | |||||
|---|---|---|---|---|---|---|---|
| Si | S | Cl | Ca | Fe | Al | ||
| 1 | Grayish White | 30.16 | 9.64 | 1.07 | 5.34 | 51.15 | |
| 2 | Grayish Brown | 39.76 | 2.56 | 3.31 | 52.02 | 1.14 | |
| 3 | Grayish Brown | 43.96 | 1.37 | 0.41 | 52.89 | 1.37 | |
| 4 | Grayish Brown | 45.49 | 52.91 | 0.75 | 0.85 | ||
| 5 | Black | 46.51 | 53.14 | 1.16 | |||
| 6 | Black | 45.96 | 52.86 | 0.35 | |||
Diameter expansion and contraction
Controlling the uniform diameter of quartz glass ingots during gas refining is one of the most challenging aspects of the process, yet it is critical. Uniformly sized ingots save time and materials in subsequent cold processing. With the rapid development of the electronics industry, the demand for large, high-quality quartz glass materials has increased, making uniformly sized ingots indispensable.
Balancing Diameter and Quality
Maintaining consistent diameter and internal quality often presents conflicting requirements. During forming, the furnace must provide an appropriate longitudinal temperature field. The highest temperature zone should not be excessively high to prevent the glass from becoming too fluid and flowing uncontrollably. Simultaneously, a sharp downward temperature gradient is needed for solidification. However, to avoid bubbles, the melting zone must maintain sufficiently high temperatures to quickly melt the raw materials and allow gas to escape. Balancing these factors remains a key challenge for process engineers
Due to differences in furnace structure and temperature measurement points, providing an accurate temperature control value is difficult. Optical temperature measurement, which monitors the surface temperature of the ingot, has been suggested as a reliable method. However, sudden changes in process parameters, such as the ingot’s position, can significantly alter these readings.
| Item | Furnace Temperature | Feed Rate | Descent Speed |
|---|---|---|---|
| Parameter Change | High | High | Fast |
| Parameter Change | Low | Low | Slow |
| Ingot Diameter | Large | Large | Small |
| Ingot Diameter | Small | Small | Large |
Factors Influencing Ingot Diameter
Several factors impact the diameter of the ingots, including furnace temperature, descent speed, and feed rate. Among these, furnace temperature is the most influential. Higher temperatures lower the viscosity of quartz glass, causing the melt to flow outward and downward under centrifugal and gravitational forces, enlarging the ingot diameter. Conversely, an increase in feed rate can also enlarge the diameter, but insufficient temperature may cause unmelted raw materials and bubble formation. Similarly, descent speed affects diameter—faster speeds can shrink the diameter, while slower speeds can lead to expansion. To achieve uniform ingot diameter, maintaining stable process parameters throughout the operation is essential.
Equipment for Vertical Ingot Making
The vertical ingot-making process relies on specialized equipment to ensure precision and quality. The ingot-making machine provides both rotation and precise vertical movement, with a harmonic reducer minimizing creeping and ensuring smooth low-speed motion for accurate positioning. The furnace is designed for high thermal efficiency and uniform temperature distribution, equipped with thermocouples and observation ports for real-time monitoring. Hydrogen-oxygen burners with porous plates enhance combustion efficiency and minimize gas waste, while precision feeders ensure consistent material input, improving yield and ingot quality.
Advanced control systems play a critical role in the process. Pressure and flow meters monitor gas inputs, while cutting-edge temperature sensors, such as optical or infrared types, regulate both the furnace and ingot surface temperatures. By integrating this advanced equipment and precise process control, vertical ingot-making enables the production of high-quality quartz glass products, suitable for demanding applications.
Horizontal weight making
The horizontal ingot-making method offers the advantage of producing very long ingots, though the diameter tends to be smaller. Another benefit is that impurities from the insulation furnace liner are less likely to mix into the melt. German companies such as HERAEUS Quartz Glass produce ingots up to 8 meters long with a diameter of 80 mm using this method. SCHOTT also employs the horizontal method to produce synthetic quartz glass ingots with diameters reaching 200 mm. In China, the horizontal method for synthetic quartz glass ingot production has been in use since the early 1980s and remains widely applied today.
Both domestic and international melting equipment for the horizontal method share similar designs, as illustrated in Figure 3-22. However, differences in burner and furnace design significantly affect melting efficiency. For example, HERAEUS Quartz Glass claims that producing 1 kg of ingot requires only 15 m³ of hydrogen, whereas domestic operations consume up to 22 m³.
Continuous milling
The continuous ingot production technology, invented by Saint-Gobain Quartz Glass Company in France, has been patented in the United States. This process divides the furnace into upper and lower parts, with the lower furnace mouth sealed to form a molten pool supported by insulating bricks on a baseplate. The pool’s conical base facilitates smooth ingot extraction, with controlled wall temperatures to minimize crystallization that could affect the ingot’s dimensions. The upper furnace houses multiple burners, and the molten pool and the ingot mold and clamps rotate synchronously to ensure uniform hydroxyl content, which is critical for maintaining a consistent glass refractive index. Clamps below the pool extract the ingot at speed matching the melting rate, cycling between gripping and releasing while maintaining straight-line extraction. If higher uniformity is required, the entire column or furnace may move in the X and Y directions, aligning the process with large vertical lathe mechanics.
Refractory Material Selection
The choice of refractory materials is critical. While high-quality zirconia typically meets the requirements, yttria-stabilized zirconia provides superior corrosion resistance, justifying its higher cost through extended furnace life. At the start of the process, a pre-fabricated target holder is inserted into the furnace mouth, with the base of the molten pool layered with glass fragments to act as seed material. Once the temperature reaches the melting point, feeding begins, and ingot pulling starts when the pool surface reaches the desired level.
Hot-Top Process for Thick-Walled Tubes
Thick-walled quartz glass tubes, used as precursors for large-diameter optical fiber and semiconductor pipes, are essential in modern applications. Common dimensions include an outer diameter of 150 mm and a wall thickness of 35 mm. Methods for producing these tubes vary: Germany’s HERAEUS melts elongated ingots horizontally, reheats them in resistance furnaces, and extrudes thick-walled tubes using a heat-resistant head and outer mold. France’s Saint-Gobain employs high-frequency plasma flames to deposit powder onto a central rod. In the 1990s, a Chinese-developed hot-top process for thick-walled tubes achieved direct production without reheating, earning a domestic patent. A comparative analysis of these methods is shown in Table .
| Manufacturing Method | Precision | Internal Quality | Energy Consumption | Equipment Complexity | Maximum Tube Dimensions (mm) |
|---|---|---|---|---|---|
| German Method | High | High | High | Complex | φ150×φ80×40 |
| French Method | Medium | Medium | High | Complex | φ165×φ75×15 |
| Chinese Method | Low | Medium | Low | Simple | φ165×φ70×20 |
Hot-Top Process Overview
The hot-top process enhances vertical ingot-making by introducing a central rod along the target holder’s axis. This rod is fixed axially but rotates synchronously with the holder, creating hollow ingots. The process relies on the stable rheological properties of quartz glass between 1650°C (softening point) and 1800°C. In this range, the glass exhibits consistent flow characteristics under low shear stress. By applying a controlled deformation force, the hot-top process achieves precise shape transformation. Unlike ingot production, this method requires specific furnace temperature gradients to stabilize the molten glass during flow and rapidly cool it for optimal shaping. Ideal and actual furnace temperature profiles are depicted in Figure.
Furnace Temperature Profiles
The furnace temperature profile is crucial for the hot-top process. As shown in Figure , the ideal profile maintains a stable and appropriate temperature gradient around the molten material, ensuring smooth flow. After the rheological process, a sharp temperature drop is necessary for effective shaping. The axial distance (H) from the observation port centerline is used as a reference, with the temperature steadily decreasing along the axis. While ideal profiles are challenging to achieve in practice, optimization of furnace design and precise control significantly enhance tube quality.
Ideal and actual furnace temperature field
The horizontal plane at the center of the observation hole is taken as the zero point, and H represents the axial distance from the zero point downward.
Powder gas refining ingot reheating top forming thick wall pipe process and equipment
The method involves first using gas refining to melt elongated ingots, which are then heated in a resistance furnace to form thick-walled tubes. The small ingot diameter allows easy heating to deformation temperature. An ingot is pulled out of the furnace using a cart equipped with clamps. The furnace mouth is fitted with an outer shaping tool, and a water-cooled steel rod with a graphite head is positioned at the center of the clamps. The cart moves along horizontal rails. With shaping tools applied to both the interior and exterior, the resulting thick-walled quartz glass tubes have highly uniform inner and outer diameters, with lengths reaching up to 4 meters. Minimal grinding is needed before the tubes can be drawn, ensuring high efficiency. An illustration of the equipment is shown in Figure
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