Application Status and Research Progress of Ultrafine Mixing Mill

1 Overview
Stirring mill is a high-efficiency ultra-fine grinding equipment that has been used in industries such as paints, inks, pigments, paper coatings, ceramic glazes, magnetic materials, plastic fillers, functional materials and difficult-to-treat gold mines. With the rapid development of new materials, there is an increasing need for ultra-fine non-metallic industrial mineral powders. For example, the paper industry needs ultra-fine kaolin , ultra-fine calcium carbonate (-2μm ≥ 70~90% ); the plastic industry needs ultra-fine weight. calcium carbonate and talc, wollastonite powder, mica powder (-10μm ~ 20μm); zirconium silicate ceramic glaze industry needs ultra-fine (a five micron); the SIC fine ceramics industry needs ultra-fine, ultra-fine Al ₂ O ₃ (-1μm) and so on. These have greatly stimulated the development of ultra-fine grinding equipment, especially the development of agitating mills.

In recent years, some researchers have fully affirmed that the mixing mill is a high-efficiency and ultra-fine grinding equipment, and carried out some basic research work to manufacture some industrial equipment. However, there are few studies on how to improve the energy utilization of the agitator mill itself and the optimal fine grinding process for different specific uses of industrial minerals. Since CRIMM successfully developed in the 1980s vertical spiral mixing mill, after 10 years of improvement and perfection, has been used in gold and then grinding, fine grinding red iron oxide, graphite purification stripping, electrolytic manganese and wet grinding Kaolin fine grinding and other industries. Then after several years of experimental research and comprehensive absorption tower tower grinding (vertical spiral stirring mill), sand mill, batch mixing mill and ring gap grinding equipment, and the development of a new ultra-fine agitating mill, the device has been gradually applied in TSP, kaolin, silicon carbide, garnet, electrolytic manganese dioxide, ferrite slurry, and the aluminum industry.

2 Development status of mixing mill
2.1 Types of mixing mill  
The types of mixing mills are shown in Table 1 . Table 1 Classification of mixing mill

2.2 Analysis of existing mixing mill
Tower mill 1, the invention is by Dr. River Japan terminus of the heavy wins in 1952, has been applied in gold, lead-zinc ore regrinding operations gradually in recent years, non-metallic mineral processing industry applications. The Changsha Research Institute of Mining and Metallurgy has carried out a large number of experimental research work to manufacture a vertical spiral mixing and grinding machine. The machine features: medium and low speed, high medium filling height, spiral agitator, energy during fine grinding or superfine grinding. High utilization rate. However, the barrel is too high, and maintenance and overhaul are not convenient;

When performing ultrafine grinding, it is difficult to increase the speed due to the use of a spiral agitator.

 

Figure 1 Tower mill schematic
The sander (peeling machine) is shown in Figure 2 . Also known as high-speed agitator mills, this type of equipment is widely used in the paint industry. However, the speed is high, and the energy density distribution in the cylinder is not uniform; due to the disc stirrer, the glass beads or ceramic beads are subjected to frictional force on the disc, and the energy loss is large, and the energy utilization rate of the rod stirrer is lower than that of the rod stirrer. High-speed rotation, disc, tube wall wear is serious, heat is large, and the casing must be provided with a cooling jacket. The advantage is that the height of the cylinder is suitable, the weight of the switchboard is light, and the maintenance and repair are convenient.

Figure 2 sand mill schematic     

Rod type agitating mill, or periodic agitating mill, such as the Attritor mixing mill of UP Company of the United States, as shown in Figure 3 , in fine ceramics, electronic materials, powder metallurgy and other industries. Because of the rod stirring structure, the grinding efficiency is higher; as the diameter of the barrel increases, the speed difference between the grinding center and the top of the stirring rod is too large, so the energy transmitted to the medium is not equal, the energy in the barrel The distribution is not uniform; in addition, the number of stir bars is too small, so that the frequency of energy delivered to the medium is low. The machine has the advantages of simple structure and convenient maintenance. With the use of an external circulation pump, the material can be ground to a few microns.

Figure 3 Schematic diagram of the rod stirring mill
The annulus-type agitating mill solves the problem of uneven energy density, high energy utilization rate, and uniform particle size. However, the effective volume is small and the equipment is difficult to enlarge. Limited to applications in the finishing industry such as fine ceramics, abrasives and specialty inks and coatings. For the preparation of industrial mineral powders, processing equipment with a large amount of processing is required. Therefore, it is imperative to develop a large-scale ultra-fine agitating mill with fine particle size and large processing capacity for deep processing of industrial mineral powder.

3 Research and development of new high-efficiency ultra-fine agitating mill
Based on the above analysis and according to our long-term experimental research and industrial application practice, we designed a new ultra-fine agitating mill-vertical rod-type mixing mill with high energy utilization rate, convenient operation and maintenance, and capable of grinding industrial minerals with large processing capacity. , as shown in Figure 4.

The equipment consists mainly of a screw stirrer, a cylinder, a drive and a frame. The agitator adopts a spiral rod structure, the diameter of the main shaft is large, and the number of rods on the agitator is large, which increases the chance of stirring the small medium ball. The agitator can be made of alloy steel, polyurethane, ceramic or hard alloy. The cylinder body adopts a special-shaped structure and can be made of alloy steel, polyurethane and stainless steel. The cylinder shell is provided with a cooling water jacket. The medium ball can be made of glass beads, ceramic beads, steel balls or carbide balls. The diameter of the ball is φ1~φ10mm . If a thinner ball is needed.

During operation, the upper transmission drives the agitator to agitate the media ball at a suitable speed ( 3~6m/s ), mainly using friction, shearing and impact between the ball media to grind the material.

Figure 4 New ultra-fine mixing mill
4 process design
4.1 dry grinding process
Dry process is the production of 600 to 2500 mesh size industrial mineral powder, are: heavy calcium carbonate, talc, light burned magnesia powder. The dry micro-grain classifier must be used in production, and the fineness of the product depends on the classifier. The typical process is shown in Figure 5.
The process has low operating cost and low power consumption, and the drying equipment in the wet method can be omitted, and the surface modification can be simultaneously performed during grinding. However, it still needs to be solved: the problem of agglomeration after industrial mineral grinding; wear resistance of the stirrer and anti-iron pollution; heat loss during grinding.

Figure 5 dry grinding process
1--Height, 2--grinder, 3--classifier, 4--collector, 5-fan.

4.2 wet grinding process
The wet grinding process mainly produces ultra-fine industrial minerals, such as high-grade paper-coated heavy calcium products (-2μm ≥ 90%), mainly one-stage (intermittent), multi-stage, cyclic and closed-circuit grinding (grinding + classification) .

The most ideal wet grinding industry is closed-circuit grinding. This process ensures that the product has no over-grinding, improves grinding efficiency and ensures the stability of the final product. However, there is currently no high-efficiency ultra-fine wet grading equipment , and the fine-grained concentration is too late after grading , and a large-scale concentrating tank needs to be provided. The wet super-segmentation equipment mainly has small diameter ( φ10 ) cyclones, horizontal spiral classifiers and butterfly classifiers.

Circulating grinding is a very good fine grinding process, as shown in Figure 6 , especially for large-volume circulating grinding. The fine grinding product has narrow particle size distribution and good heat dissipation effect. The concentration of wet grinding products can reach 60~70%. It can directly enter the flash drying, multi-purpose drying and other equipment to dry, reducing the compression equipment, the process investment is more economical.

Figure 6 Cycle grinding process    

Multi-stage grinding is a series of several mixing mills. As shown in Figure 7, according to the grinding fineness, medium balls of different diameters can be added in each mixing mill, which improves the grinding efficiency and heat dissipation conditions. However, the process is large and suitable for large-scale industrial production. In addition, if any equipment in the process fails, the production line will have to stop, which will affect normal production.

Figure 7 Multi-stage grinding process

1-- Paste bucket , 2.4.7 .10-- pump , 3.6.9-- intermediate barrel ,5.8.11-- mixing mill , 12-- sieve , 13-- storage bucket

A section of grinding or intermittent grinding is only suitable for applications where the product size is small or the amount of processing is small, and it is less used in non-metallic industrial minerals. At present, the closed-circuit grinding process and the large-volume cycle grinding process are relatively advanced wet grinding processes.

5 Special requirements for fine grinding of industrial minerals
Industrial minerals, such as heavy calcium, talc, wollastonite, mica, kaolin, etc., have many different uses due to their different purity, fineness, particle size distribution, particle size and color, and are widely used in plastics. , paper, paint, chemical, metallurgy, ceramics industry. Therefore, special requirements are imposed on the ultrafine grinding equipment and its process, not a purely simple particle size reduction process. For example, the preparation of pearlescent pigment mica powder requires a large diameter to thickness ratio, a smooth surface, no scratches, and no secondary pollution. This requires that the stirring mill speed should not be too high, the surface of the medium ball is smooth, and the material is peeled off by applying shearing and softening. Wollastonite is used as a rubber and plastic filler. It requires a large aspect ratio and a fine particle size. The material thus produced has strong rigidity and toughness. The ceramic powder needs to have ultrafine (-1 μm ), high purity ( 99.9% ), high dispersion, narrow particle size distribution, and other shaped particle shapes. As the coating grade heavy calcium carbonate for papermaking, ultrafine (-2μm ≥ 90 % ), whiteness ≥ 90% , purity CaCO ₃ ≥ 98% , narrow particle size distribution, certain viscosity concentration, high dispersibility and the like are required.
Due to the specific requirements of the industrial sector for ultra-fine industrial mineral powders, this determines that ultra-fine grinding equipment and processes must meet:
1) product fineness, particle size distribution, particle size shape;
2) No secondary pollution can occur during grinding;
3) Great processing capacity;
4) High energy efficiency.