Research status and prospects of airflow pulverizing equipment

1 Introduction
The jet mill, also known as jet mill, refers to a device that uses high-speed airflow (300-500 m/s) or superheated steam (300-400 °C) to make particles collide, collide, and rub against each other to achieve ultra-fine crushing. Compared with other types of pulverizers, it has the following advantages: 1 The product has good particle size. The average particle size ( D 50 ) of the material is generally below 5 μm and the particle size distribution is narrow. 2 The product has high purity and is especially suitable for the pulverization of materials such as medicines that are not allowed to be contaminated. 3 can pulverize low melting point and heat sensitive materials. 4 product particles have high activity and good dispersion. 5 The production process is continuous, the production capacity is large, and the degree of automation is high.

2 Classification and evaluation of airflow pulverizer
There are many types of jet mills, and there is currently no uniform classification standard. According to the manner in which the particles are pulverized, they can be divided into a single air flow type and a multi-flow type, and can be classified into a high speed air flow type and a hot steam flow type according to the type of the fluid medium. According to the ZBG90005-87 professional standard-superfine pulverizing machine terminology formulated by the former Ministry of Chemical Industry, the current jet mills are divided into five types, namely: flat jet mill, O jet mill (circulating), target airflow A pulverizer, a countercurrent jet mill, and a fluidized bed jet mill.

2.1 flat jet mill
The flat-disc type jet mill (Micro-Jet-Mill) is an air jet mill developed in the early stage. It is a kind of crushing machine that uses the collision, shearing and friction between the particles and the inner wall of the crushing chamber. equipment. Its main component is a disc crushing chamber, which is arranged on the jet ring at a certain angle (6-24) high-pressure working nozzles, jet feeders, finished traps, etc. at an angle to the plane of the crushing chamber. The machine has a simple structure, convenient operation and self-grading function, and is particularly suitable for crushing brittle soft materials.

The outstanding shortcoming is that the crushing chamber is seriously worn, which constitutes certain pollution to the product, and the limit particle size is relatively high. Qian Haiyan of Nanjing University of Chemical Technology by Al ₂ O3 and crushing experiments showed that SiO _2, Al ₂ O ₃ particle size limit is 4.8μm, SiO ₂ particle size limit is 3.5μm. Li Guanchang of Changsha Mining Research Institute has achieved a certain success by reducing the pulverization particle size by setting a staged impeller in the central area. It is reported that the average particle size of the ultrafine zircon powder produced can reach 0.4 to 3 μm.

At present, the manufacturers of this model mainly include Sturterant Mill Company of the United States, Jet Pulverizer Company, Apex Construction Company of the United Kingdom, Fresh Enterprise of Japan (STJ type), Shanghai No. 3 Factory of Shanghai Chemical Machinery (QS type), Nanjing University of Science and Technology. Jiangsu Province Superfine Powder Engineering Technology Research Center Test Base (STJ type and GQF type).

2.2 Circulating jet mill
The JOM circulating airflow pulverizer (Jet-O-Mizer) is mainly composed of an O-shaped circulation pipe, a high-pressure working nozzle, a venturi pipe, and a feeding injector. After the material enters the circulation pipe, the material is pulverized by friction collision between the particles and the pipe wall.

This model, although small in size, large capacity, but the wear of the pipe wall and is inappropriate for pulverizing a high purity and high hardness requirements of the material, usually cemented, high abrasion resistant material (alumina, superhard alloy Etc.). At present, the main manufacturers include Reduction, Aljet Engineering (PJ), Japan Fresh Enterprise, and Shanghai Chemical Machinery (QSB, QON Series).

2.3 target airflow pulverizer
The target jet mill is a jet mill that pulverizes the material by impacting the impact particles (impact target, impact ring, etc.) at high speed.

The fixed target jet mill is one of the earliest airflow pulverizers. The high-speed airflow entrains the material particles to hit the fixed target plate at high speed to crush the material. There is also a movable target jet mill with a cylindrical shape and a slow rotation to make the target wear more uniform. Because the impact of the entrained material on the target is very strong, the erosion of the target is generally very serious, causing certain pollution to the product. The engineers and technicians of Nanjing University of Science and Technology used SiC, tungsten carbide, nitriding materials and corundum to make the smashing test. After continuously spraying the quartz for 10 hours, the target eroded a jet hole of 3 to 10 mm deep. . Therefore, the industrial application of the impact target jet mill is limited. Generally used only to deal with coarser particles, and the application is less, has been eliminated.

Domestic baking Lu Chan et al developed a new type of jet mill plate (Practical Chinese patent CN2300452), a reflecting plate disposed lining machine, and designed circulating cooling water jacket, no accumulation of powder, magnesium aluminum is widely used in Smashing of alloys and rubber.

2.4 collision type jet mill
The collision type jet mill is a device that uses two high-speed jets to collide with each other to pulverize the solid particles therein, and successfully solves the problem of severe wear of the impact parts by the high-speed airflow. The Mayak type developed by Majac Corporation of the United States. A jet mill (Majac JetPulverizer) is representative.

This type of machine has a large production capacity and a large impact strength, and can crush various materials of hard, brittle, and toughness with a Mohs hardness of 9.5 or less. Higher purity products can be produced by avoiding the wear of high velocity jets on fixed impact components. The models produced with mica powder fineness of up to 325 mesh or less. The current main models include the Japanese TJ series and the domestic QLM type.

2.5 fluidized bed airflow mill
The Fluidized Bed Opposed JetMill was first developed by the Apline Company of Germany in 1981 (AFG type). It is the leading model of the current jet mill and is widely used and has many models.

The fluidized bed type jet mill can be divided into gravity feed type and screw feed type according to the feeding mode. Its working principle is that the material uses two or three-dimensional nozzles (3~7) to spray the airflow impact energy, and the airflow expansion is crushed by the collision and friction generated by the fluidized bed floating and tumbling, and Driven by the negative pressure airflow through the turbine-type classification device provided at the top, the fine powder discharge machine is trapped by the cyclone separator and the bag collector, and the coarse powder is returned to the crushing zone by gravity to continue to be crushed.
Its outstanding advantages are low noise, small footprint, high product fineness ( d 97 up to 3~10μm), narrow particle size distribution and low energy consumption . Compared with flat airflow pulverizer, it can save energy by 30%~40. %, high pulverization efficiency, using Al2O3, SiC and other materials as wearable parts, thus producing products with a Mohs hardness greater than 10 and high purity products.

However, this model covers many nozzles, flow field diversification, material layer fluidization, horizontal grading and other technologies, and it is not without its disadvantages. Its shortcomings are mainly manifested in: 1 The bottom of the crushing chamber is convenient for unloading. More designed into a conical shape, but for a two-dimensional flow field model, once the material conveyed by the feeding device enters the blind zone, it cannot be pulverized; 2 the flow field in the pulverizing chamber can only partially realize fluidization. The upper part of the nozzle and the edge of the pulverizing chamber cannot form a fluidization; 3Because the material is fluidized in the pulverizing chamber, it can be bound and crushed by the airflow, so there is a certain requirement for the upper limit of the feed size. For dense, fibrous, sheet-like materials, it is more difficult to pulverize. There are also many manufacturers, mainly Germany's Apline (AFG), Natzsch-Condux (CGS), Shanghai Chemical Machinery No. 3, Kunshan Superfine Grinder Factory (QYF).

The above describes several commonly used jet mills, each with advantages and disadvantages. In comparison, the fluidized bed jet mill has obvious advantages, representing the mainstream direction of the development of jet mill equipment.

3 Outlook
At present, the development and production of ultra-fine airflow pulverizing equipment in China has been greatly developed, and has made important contributions to China's powder industry. However, compared with developed countries, there is still a big gap, mainly in the machine of independent innovation. Most of the manufacturers are still at the stage of copying similar foreign products; large-scale, specialized, and capable of producing high-purity, high-fineness products. Therefore, the development of ultra-fine airflow crushing equipment in China should pay attention to the following aspects:
(1) Strengthen basic theoretical research to guide the independent innovation of equipment development.
(2) On the basis of existing equipment, vigorously develop research on process optimization and optimize process flow. For large-scale production of powder, it can be developed specifically for specific models.
(3) Strengthen the research and development of automatic control and on-line testing equipment and related software supporting powder machinery, in order to improve the automation level of equipment and reduce energy consumption.