[Technology] What problems should be paid attention to when preparing high aspect ratio wollastonite by fluidized bed jet mill?

Wollastonite products can be divided into two categories: high aspect ratio wollastonite and fine wollastonite. High aspect ratio ultrafine wollastonite needle powder has high application value in industry, such as plastic filler. It can improve the strength and dimensional stability of the products; the wollastonite acicular powder for papermaking fillers is also required to achieve the fineness of lum; the finer the wollastonite acicular powder added in the paint and paint, the better the performance The more obvious. Therefore, the processing and preparation of ultra-fine wollastonite acicular powder with high aspect ratio has become a research hotspot of the world's wollastonite processing enterprises.

The crystalline structure of wollastonite determines the properties of even fine particles that are fibrous or needle-like. Wollastonite has a length-to-diameter ratio of 5:1, a β-form of 20:1, and a maximum of 30:1. The length-to-diameter ratio varies greatly depending on the pulverization method. To increase the aspect ratio of wollastonite products, the key is to maintain the original crystal structure of the mineral by suitable pulverization during the pulverization process. At present, the equipment for superfine pulverization of wollastonite acicular powder mainly includes mechanical impact pulverizer, airflow mill (flat, circulating, impact, fluidized bed, spray type), agitator mill, Raymond Grinding, vibration grinding, etc. Practice has shown that the shearing and rubbing action always has the tendency to peel the particles along the crystal cleavage plane parallel to the direction of force action, which determines that the fluidized bed type jet mill is the best equipment for preparing high aspect ratio wollastonite. .

1. Working principle of fluidized bed jet mill

The jet mill grinds the material by the high-speed movement of the air stream to cause strong impact collision and friction shear between the material particles and between the particles and the wall. The fluidized bed jet mill is a combination of the most advanced multi-nozzle technology, fluidized bed technology and turbine grading technology, which realizes diversification of flow field, fluidization of material layer and grading and horizontalization. The fluidized bed jet mill grinds the powder with a fluid-compressed air as the working medium, and the fluid velocity is as high as 300-500 m/s.

The working principle is as follows: the compressed air is accelerated into a supersonic flow through the Laval nozzle and then injected into the crushing zone to fluidize the material. In the comminution zone, the accelerated material meets at the intersection of the nozzles. Here, the particles collide with each other and pulverize. The pulverized material is transported to the classification zone by the negative pressure rising gas flow, and the fine powder which meets the particle size requirement is screened by the inner grading wheel, and the coarse powder which does not satisfy the particle size requirement is returned to the pulverization zone to continue pulverization. The qualified fine powder is collected into the cyclone with the airflow, and the dusty gas is filtered and purified by the bag filter and discharged into the atmosphere.

2. Process characteristics of fluidized bed jet mill

The process characteristics of the fluidized bed jet mill are: high-speed particles do not collide with the inner wall of the crushing process during the pulverization process, and the target material does not pass through the nozzle. Therefore, the wear is extremely light and the product is not contaminated. The upper part of the main unit is equipped with horizontal grading wheels to prevent coarse particles from entering the finished product. Due to the principle of fluidized bed (airflow expansion, fluidized bed suspension boiling, collision, friction for pulverization) and internal horizontal turbine classification device, compared with other types of airflow mill, the airflow condition is better, energy saving 30 -40%. The product has a narrow particle size distribution and no large particles, high degree of automation, low noise and compact structure.

3. Influence of process parameters of fluidized bed jet mill on the aspect ratio of wollastonite

(1) Effect of grading wheel speed on the aspect ratio of wollastonite

The rotational speed of the fluidized bed airflow grading wheel has a significant influence on the aspect ratio of wollastonite products. Liu Haixin et al. studied the influence of different grading wheel speeds on the aspect ratio of wollastonite products. The results show that: As the speed of the grading wheel increases, the aspect ratio of wollastonite increases gradually. After reaching a peak, the aspect ratio will decrease rapidly.

(2) Influence of jet pulverization pressure on the aspect ratio of wollastonite

When the speed of the grading wheel is constant, the average length of the product is significantly shorter as the pulverizing pressure increases, and the average diameter does not fluctuate significantly. Therefore, as the pulverizing pressure increases, the ash is gray. The stone length to diameter ratio is reduced.

(3) Effect of heating time and temperature on the aspect ratio of wollastonite

In order to improve the pulverization efficiency, heat-assisted pulverization of a pair of ore is carried out to induce grain boundary fracture, reduce material strength and improve grindability. The crystal structure of wollastonite causes it to crack along the Ca-O bond during calcination, and free Ca2+ ions and O2+ ions appear to fill the lattice gap, so that it needs to break the wollastonite during the airflow collision and pulverization process. The energy is small, so the particles are easily broken, and the time required for the crushing is short, so that the qualified grade-level wollastonite product leaves the crushing chamber in time, and the aspect ratio of the wollastonite product is protected in time. Therefore, the wollastonite is heated and then pulverized by a jet mill. It is found that the suitable heating temperature is 600-800 ° C, and it is prolonged with the heating time. The variation of the aspect ratio of the wollastonite product is first. Increase and decrease, that is, to obtain a high aspect ratio wollastonite product, there is an optimum heating temperature and heating time.

(4) Effect of classification of raw materials on the aspect ratio of wollastonite

The wollastonite raw materials are first classified, and then treated by a jet mill, which can well protect the aspect ratio of the wollastonite products, and the wollastonite products obtained at different fractions are relatively uniform. Feeding the material into the jet mill after classification is an effective step in producing high aspect ratio wollastonite.

Taking full advantage of the inherent characteristics of wollastonite to obtain ultra-fine powder of wollastonite with high aspect ratio and increasing its added value is the premise for deep processing and utilization of wollastonite. The wollastonite pulverization process is different in the way of the force, and the pulverized products have different morphologies. When the force applied to the wollastonite particles during the pulverization process is shearing force, the desired high aspect ratio wollastonite can be obtained. The jet mill pulverizes the pressure of compressed air to form a high-speed airflow trajectory in the pulverization chamber, which is mainly shearing. It is characterized by high-speed collision and friction between wollastonite materials to achieve pulverization and protect the wollastonite crystal morphology. .

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