With the expanding application of non-metallic mineral resources across various sectors of the economy and society, the exploitation of these resources has significantly intensified. Since many of these non-metallic minerals are utilized in powder form, the industry demands higher processing technologies, particularly for achieving ultrafine grades. In this article, we will explore several types of mills to prepare ultrafine powders.
1. Ultrafine Non-Metallic Mineral Powders
Ultrafine powders refer to a series of superfine materials with particle sizes ranging from the micron to the nanometer scale. The widespread application of non-metallic mineral powders in modern high-tech new materials is predicated on their unique functionalities. The performance of most non-metallic mineral functions relies on particle size, size distribution, and particle shape.
Ultrafine powders possess excellent physical and chemical properties. They include large specific surface area, high surface activity, fast chemical reaction speed, low sintering temperature with high sintered body strength, good filling and reinforcing performance, and high hiding power. Consequently, many application fields require non-metallic mineral raw materials to have very fine particle sizes (micron or sub-micron).
Currently, according to the consensus within China’s mineral processing industry, ultrafine powder is defined as powder with 100% of particles smaller than 30μm. Based on particle size, ultrafine powders can be categorized into: micron-grade (1–30μm), sub-micron-grade (0.1–1μm), and nano-grade (0.001–0.1μm). Ultrafine non-metallic mineral powders exhibit significant advantages in optical, magnetic, acoustic, electrical, and mechanical properties and are widely used in medicine, chemicals, electronics, energy, and other fields.
2. Processing of Ultrafine Non-Metallic Mineral Powders
There are numerous methods for preparing ultrafine powders. Based on the formation medium, they can be classified into three categories: gas-phase, liquid-phase, and solid-phase methods. Gas-phase methods include high-frequency induction heating and plasma preparation techniques, suitable for producing ultrafine powders with high purity, narrow particle size distribution, small size, and uniform distribution. Liquid-phase methods mainly include chemical liquid reduction, sol-gel, ultrasonic atomization, and hydrothermal methods. Solid-phase methods primarily employ mechanical crushing.
From the principle of preparation, methods can be divided into chemical and physical methods.
Chemical methods
Chemical methods involve chemical reactions, forming the desired ultrafine powder from ions or atoms through nucleation and growth. The advantages are the ability to produce powders with high purity, small particle size, narrow size distribution, and good morphology. Disadvantages include low yield, high cost, and complex processes.
Physical methods
Physical methods involve crushing materials through mechanical force. Advantages are lower cost, relatively simple processes, high output, and suitability for large-scale industrial production. Furthermore, mechanochemical effects generated during crushing can increase powder activity.
Currently, in the processing of ultrafine non-metallic mineral powders, physical methods are the primary preparation technique. Generally, the process of converting raw materials into ultrafine powder mainly involves two steps: crushing and sınıflandırma. Material first enters an ultrafine grinding equipment for crushing. Due to variations in particle structure, the energy required for crushing differs, and the forces experienced within the equipment are unequal. Thus, the resulting fine particles vary in shape and size, with only a portion meeting the size requirements. In practical production, extending the crushing time is often used to ensure thorough particle size reduction to meet standards. However, this not only increases energy consumption but may also lead to over-grinding. Therefore, it is crucial to separate out particles that meet the size specification promptly, making ultrafine classification technology highly important in the ultrafine powder preparation process.
3. Research Status of Ultrafine Grinding Equipment
Commonly used ultrafine grinding equipment currently includes impact mills, stirred mills, jet mills, and vibration mills.
3.1 Impact Mill
Impact mills are ultrafine grinding devices that use a high-speed rotating rotor (with rods, hammers, blades, etc.) around a horizontal or vertical axis to generate intense impact and shear forces on the material. This causes strong impact collisions between the material and the mill wall/fixed parts. It can also cause collusion between particles, leading to particle breakage. They are suitable for medium-hardness materials like talc, marble, and calcite. Feed size is generally within 8mm, and product fineness can reach 3–74μm.
Main advantages of impact mills include adjustable fineness, simple structure, compact installation, easy operation, small footprint, and high efficiency. A disadvantage is the potential for overheating during high-speed operation. Therefore, equipment improvements could consider cooling methods. Additionally, using materials with good compressive and wear-resistant properties is advisable to avoid significant part wear.
3.2 Stirred Mill
Stirred mills are one of the promising ultrafine grinding equipment types, available in various forms. Based on installation, they can be vertical, horizontal, or disc stirred mills. Based on process, they include spiral, batch, circulation, and continuous stirred mills.
Overall, stirred mills primarily operate by rotating an agitator shaft to stir the grinding media (steel balls, zirconia balls, ceramic balls, corundum balls, gravel, etc.) and material filled inside the chamber, causing them to move. They are often used for deep processing of non-metallic minerals and pigment preparation. Feed size is typically within 3mm, and product fineness ranges from 0.1 to 45μm. Key process parameters affecting grinding efficiency include agitator speed, slurry concentration, material-to-ball ratio, media properties, and retention time.
Compared to traditional ball mills, stirred mills offer significant advantages: ① The mill chamber is stationary while the agitator rotates, solving the issue of high energy loss associated with the rotating drum of a bilyalı değirmen. ② Unlike ball mills which combine impact and grinding, stirred mills rely mainly on grinding supplemented by minimal impact, reducing energy consumption from noise, heat, and vibration, thereby improving fine grinding efficiency. ③ Stirred mill equipment generally has a relatively small footprint, simple structure, and convenient installation.
3.3 Vibration Mill
Vibration mills use balls or rods as media and can process products down to a few microns. They work by utilizing the grinding media inside a high-frequency vibrating chamber to impart impact, grinding, and shear forces on the material, achieving rapid particle size reduction. They are widely used in chemicals, metallurgy, building materials, ceramics, refractories, and non-metallic minerals for ultrafine powder processing. Feed size is usually within 6mm, and product fineness ranges from 1 to 74μm. Vibration mills offer advantages like small size, low energy consumption, high output, compact structure, simple operation, easy Bakım, and uniform product size. Disadvantages include high noise levels and demanding part requirements.
3.4 Jet Mill
Jet mills operate by accelerating compressed air through nozzles; the resulting jets propel particles at high speed, causing them to collide, frictionally interact, and shear, leading to comminution. They are mostly used for ultrafine grinding of non-metallic minerals with medium hardness or below, such as marble, kaolin, and talc, and also in health foods, rare earths, and chemical raw materials. Feed size is generally controlled below 1mm, and product fineness ranges from 1 to 30μm, but production capacity is relatively small.
Jet mills offer advantages like high automation and large capacity. Disadvantages include high equipment cost, large footprint, high energy consumption, limited availability of very high fineness products, significant part wear, and a lack of independent innovative models. As jet mills are the most researched type in China with the most complete model range and relatively mature technology, they are very popular in the market. Currently, they can be classified based on structure or operation into: horizontal disc (flat) jet mills, circulating tube jet mills, opposed jet mills, target jet mills, and fluidized bed jet mills.
3.5 Other Equipment
Planetary Ball Mill: Structures are mainly vertical and horizontal. Individual grinding jars are geometrically similar to those of ordinary ball mills, but the key difference is that each jar in a planetary mill rotates on its own axis while simultaneously revolving around a central axis. The high grinding capability and energy utilization efficiency of planetary ball mills make them widely used in ultrafine powder preparation, mechanochemistry, and mechanical alloying. Suitable for high-hardness materials, fineness can reach 0.1μm. Characteristics include prone wear of grinding media and suitability for small-batch production.
- Ring-Roller Mill: Essentially a small-to-medium-sized ultrafine grinding equipment. Over years of development, its application scope has gradually expanded, showing increasingly evident advantages. In terms of specific application effects, the process operation is relatively simple, the crushing ratio is relatively large, and the unit energy consumption during processing is low, meeting energy-saving and environmental protection requirements. Based on current applications in non-metallic mineral processing, feed size should not exceed 20mm. Internal classification devices allow for effective adjustment of product fineness according to standards during processing.
Regardless of powder industry developments, the primary means of obtaining ultrafine non-metallic mineral powders remains mechanical crushing. “A craftsman must sharpen his tools to do his job well.” In the future, strengthening basic theoretical research, increasing technological investment, optimizing process flows based on existing equipment, continuously enhancing innovation capability, and developing high-tech, green, economical, low-energy-consumption, low-emission ultrafine grinding equipment with high added value are essential.
4. Research Status of Ultrafine Classification Equipment
Ultrafine powder classification separates particles based on the different motion trajectories induced by forces like centrifugal force, gravity, and inertial force acting on different sized particles in a medium, directing them into respective collection devices.
Based on the medium used, ultrafine classification is generally divided into dry and wet methods. Wet classification uses liquid as the dispersion medium, offering high classification precision and good uniformity. However, subsequent steps like drying and wastewater treatment limit its development.
Three types of Air Classifers
Based on different classification principles, dry classification equipment can be divided into three types: inertial, jet, and centrifugal.
- Inertial classifiers separate particles by utilizing their different inertia under an applied force, causing them to move in different directions.
Air classifiers comprehensively apply the Coanda effect, inertial classification, and rapid classification principles.
Centrifugal force is stronger than gravity, making centrifugal classifiers a widely developed type. They can be further divided into forced vortex and free vortex types based on the flow field within the centrifugal force field.
Despite some drawbacks, dry classification’s cost-effectiveness and simplicity lead to its widespread use in powder preparation.
Currently, the most widely used classification equipment in industrial production is the turbo hava sınıflandırıcı. Based on the installation form of the classification wheel, they can be vertical wheel type and horizontal wheel type.
In vertical wheel classifiers, the classification wheel is typically cantilever-mounted. Operation can encounter issues like imbalance with larger wheel sizes, susceptibility of the main shaft and bearings to fracture under excessive load, and poor sealing. In horizontal wheel classifiers, the classification wheel is installed horizontally, allowing stable operation even at high speeds, with significantly improved sealing. For the same processing capacity, horizontal classifiers are more compact, offering advantages like low vibration and long service life.
With the development of vortex air classifiers, scholars have made improvements through extensive theoretical and practical research, effectively enhancing classification precision and efficiency. For example, adding tertiary air streams to elutriate coarse particles descending along the cone improves separation efficiency.
Besides optimizing structural dimensions and operational parameters based on existing rotor classifiers, some scholars have also developed new dynamic classifiers by adding internal components or changing the direction of air flow inlets and outlets.
Based on current research, future directions will focus on three areas: classifying flow field regulation, separation process optimization, and hybrid flow type coupling. With increasingly mature technologies and equipment available, companies must now carefully select and optimize these solutions. The priority is controlling process parameters and adjusting workflows to boost capacity and profitability.
Destansı Toz
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