Ultrafine grinding equipment employs mechanical force and grinding media (such as steel balls, ceramic balls, or zirconia balls) to grind materials into micrometer levels and classify them. Due to its excellent processing performance, it is widely used in high-end coatings, food, pharmaceuticals, chemicals, building materials, medicinal materials, mining, and other industries. Powder equipment, especially ultrafine grinding equipment, plays a crucial role in these applications.
But with numerous excellent mills available, how should one make a choice? This article compiles introductions, classifications, and application scenarios of equipment such as agitator mills, airflow mills, and sand mills for mutual learning and exchange.
1. Agitator Mills
(1). Introduction to Agitator Mills
Agitator mills refer to a type of ultrafine grinding equipment consisting of a stationary cylindrical body filled with grinding media and a rotating agitator. The cylinder of the agitator mill is generally equipped with a cooling jacket. During material grinding, cooling water or other cooling media can be circulated within the cooling jacket to control the temperature rise during grinding. The inner wall of the grinding cylinder can be lined with different materials or equipped with fixed short shafts (rods) of various shapes according to different grinding requirements to enhance the grinding effect. The agitator is the most crucial component of the agitator mill and comes in types such as shaft-rod type, disc type, perforated disc type, cylindrical type, annular type, and spiral type. Among these, spiral and rod-type agitator mills are mainly vertical, while disc-type agitator mills come in both vertical and horizontal forms.
(2). Classification and Application of Agitator Mills
Vertical Agitator Mills
The representative of vertical agitator mills is the vertical spiral agitator mill. The agitator is the core component, and the lining plate is the main wear-resistant part. Grinding mainly relies on the gravity of grinding media (such as high-chrome balls or ceramic balls with a diameter of 12 to 30mm) and the friction brought by the spiral agitator to achieve fine grinding. During operation, the linear speed of the agitating device is relatively low, ranging from 2.5 to 4.0 m/s, and the product granularity can reach around 20μm.
Horizontal Agitator Mills
The typical representative of horizontal agitator mills is the IsaMill. It primarily consists of eight grinding discs mounted on a shaft, forming eight separate grinding mineral chambers. During operation, the disc's linear speed is between 21 to 23 m/s, and the energy input density can reach 300 kW/m3. It mainly utilizes extremely high grinding and energy density to achieve ultrafine grinding, significantly enhancing ore grinding efficiency.
(3). Different Agitator Grinding Principles and Applications
The agitation shaft drives the agitating arms to move at high speed, forcing the media balls in the grinding barrel to move irregularly with the material being ground. The material and media balls are crushed due to mutual collisions, shearing, and friction. The grinding action mainly occurs between the grinding media and the material. In comparison, a typical horizontal ball mill with a rotating drum consumes more energy. When the particle size of the material to be ground is less than 100μm, the efficiency of fine grinding in agitator mills is much higher than that in ordinary ball mills. Agitator mills of the same processing capacity have smaller specifications and are more energy-efficient.
In terms of applications, agitator mills are generally used to process most non-metallic minerals: talc, heavy calcium carbonate, kaolin, graphite, bentonite, etc. They can also prepare various color pigments: litharge, malachite, vermilion, red clay, silica, barytes, and mica powder, among others. A disadvantage is that there is a certain drying cost in production.
Based on different operation modes, agitator mills can also be classified into intermittent, circulation, and continuous types. Despite classification differences, each type of equipment has its advantages in production.
Circulation-type Agitator Mill: Due to the continuous rapid passage of slurry through the rotating grinding media layer and sieve, qualified fine-grade products are promptly discharged, avoiding the aggregation of fine particles caused by over-grinding. This process achieves high grinding efficiency and yields finely graded grinding products. The circulating cylinder also has mixing and dispersing effects and allows the addition of dispersants or grinding aids within the circulation tank. Moreover, since the slurry spends a short time in the grinding barrel each time and the newly pumped slurry into the circulation barrel is sufficient to balance the temperature rise inside the grinding barrel, this type of agitator mill does not require cooling for the grinding barrel.
Dry Vertical Continuous Agitator Mill: It can use spiral feeding and discharging on the feeding and discharging equipment. Compared with high aspect ratio agitator mills, the cylinder of this type is relatively 'short' but has a larger diameter. The interior of the cylinder is made into a polygonal shape (hexagonal or octagonal), increasing the probability of particle grinding and the intensity of grinding. Due to the short retention time of the slurry, on one hand, the material is not easily over-ground; on the other hand, the temperature rise during the grinding process is minimal, minimizing the occurrence of stickiness and thus ensuring higher grinding efficiency. In recent years, this type of agitator mill has been increasingly applied in the deep processing production of heavy calcium carbonate slurries and ultrafine calcined kaolin and other non-metallic mineral materials.
Horizontal Agitator Mill: With its unique disc-style agitator, it eliminates the shaking of the mill during operation and ensures the uniform distribution of grinding media throughout the entire grinding chamber, thereby increasing energy utilization and grinding efficiency. Additionally, the horizontal agitator mill can use a power medium separation screen to eliminate blockages and screen wear caused by the media. Consequently, this type of agitator mill has a higher energy density and achieves higher grinding efficiency.
2. Airflow Mill
(1) Introduction to Airflow Mill
An airflow pulverizer, also known as an airflow mill or jet mill, is one of the primary ultrafine grinding equipment. By utilizing internal classification functions and external classification devices, an airflow mill can process powder products with d97=3~5μm, with production ranging from several tens of kilograms to several tens of tons per hour. The products of airflow pulverization have characteristics such as narrow particle size distribution, regular particle shapes, and high purity. Currently, there are several machine models and more than dozens of specifications for airflow mills, including flat (disc) type, circulating tube type, opposed jet type, fluidized bed reverse jet type, cyclone type, and target type. Commonly used types include flat, fluidized bed, and opposed jet types.
(2) Classification and Application of Airflow Mill
The airflow mill processes materials into fine particles between 1-30μm, maintaining feed particles under 1mm. It's great for ultrafine processing of non-metallic minerals like rare earth, marbles, kaolin, and talc. The particle size limit depends on airflow solid content and inversely affects energy use. Low solid content allows for 5-10μm d95; pre-crushing achieves an average 1μm product. These products have concentrated, smooth particles with high purity, activity, and dispersibility. The cooling effect from compressed gas suits grinding low-melting-point materials.
Flat-type Airflow Mill: Utilizing high-pressure airflow, the flat-type airflow mill operates by circulating materials at high speeds through impacts, collisions, and friction. It's simple to operate but may damage the chamber and contaminate products when grinding harder materials, leading to significantly increased energy consumption for smaller product sizes.
Fluidized Bed-type Airflow Mill: Commonly used in industries for ultrafine grinding and shaping, it accelerates materials within the chamber using high-pressure airflow, achieving grinding and subsequent grading, while coarser materials are returned for further grinding.
Opposed Jet-type Airflow Mill: This equipment achieves grinding through high-speed collisions between materials and airflow, producing finer product sizes and reduced wear and contamination. However, it demands larger installation spaces, has high energy consumption, and a wider particle size distribution, suitable for grinding hard, brittle, or viscous materials.
3. Sand Mill
(1) Introduction to Sand Mill
The sand mill, another form of agitating or bead mill, got its name from initially using natural sand as the grinding medium. It primarily relies on the high-speed rotation between the grinding medium and the material for its grinding operation and can be categorized into open-type and closed-type, each available in both vertical and horizontal configurations.
In general, the distinction between horizontal and vertical sand mills lies in the larger sand capacity and higher grinding efficiency of the horizontal type, along with relatively easier disassembly and cleaning. In terms of application, sand mills find wide usage in fields such as coatings, dyes, paints, inks, pharmaceuticals, nano fillers, magnetic powders, ferrites, photosensitive films, pesticides, papermaking, cosmetics, and other areas requiring efficient nano-particle grinding.
Apart from the roles of rolling, stamping, grinding, and mutual collisions seen in rolling ball mills, sand mills possess additional characteristics:
1. Mixing and rotating the powder material with medium-sized balls to avoid dead zones during grinding, ensuring a more uniform mixture of crushed powders and slurries.
2. Enhanced grinding effects with increased rotational speeds.
3. The use of smaller-sized "sand" with a larger sand-to-material ratio increases the probability of contact, thereby boosting efficiency. These features result in better particle size distribution and average particle size compared to conventional rolling ball mills.
Vertical Sand Mill: Comprising a feeding system, grinding cylinder, disc, transmission, and control system, these mills are easier to manufacture due to avoiding sealing issues, lowering production costs. They suit products with lower quality demands but higher production needs.
In terms of fineness, gravity affects the grinding medium in vertical sand mills, resulting in uneven filling and less effective grinding. However, their design prevents spindle deformities, improving grinding efficiency somewhat.
Horizontal Closed-type Sand Mill: With a different rotor design from vertical mills, these have higher manufacturing costs but ensure sealed material handling, maintaining product purity. They excel in precision grinding compared to vertical sand mills, overcoming gravity's impact on the medium for superior grinding results meeting fineness requirements.
Conclusion:
Regardless of how the powder industry evolves, mechanical grinding remains one of the primary methods to achieve ultrafine powders. Moreover, with increasing demands for material fineness, the choice of grinding media becomes crucial. For instance, in mineral grinding, vertical and horizontal stirred mills initially utilized steel balls, but they faced the drawback of producing coarser ground materials. Nowadays, the use of zirconia composite balls enhances efficiency, reduces wear, and produces finer D50 discharge particle sizes. For mineral grinding, zirconia composite balls with densities of 3.7-3.8g/cm³, 4.0-4.1g/cm³, 4.5-4.7g/cm³ are commonly employed. Horizontal or vertical sand mills typically use zirconia balls, where industry demands for material fineness have led to the use of zirconia beads as small as 0.05mm, 0.1mm in dual-power, screenless sand mills.
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