Jet mill nozzle and crushing efficiency

Air jet pulverization is a device that uses the energy of high-speed airflow (300-500m/s) or superheated steam (300-400°C) to cause particles to collide, collide and rub each other, thereby causing particle smashing. The high-speed airflow is generated by the nozzles installed around the pulverizer, which rapidly expands after ejecting high-pressure air or high-pressure hot airflow. Due to the large velocity gradient near the nozzles, most of the pulverization occurs near the nozzles. The collision frequency between particles in the pulverizing chamber is much higher than the collision between particles and the wall, which means that the main pulverization effect of jet pulverization is the impact or collision between particles. The control of the particle size of the product by the jet mill mainly depends on the particle size of the raw material, the crushing pressure, the feeding pressure, and the feeding speed. The logical relationship between the jet mill and these parameters is specifically: the smaller the particle size of the raw material, the higher the crushing efficiency, on the contrary, the larger the particle size of the raw material, the relatively low crushing effect. When the crushing pressure and the feed pressure are constant, the product will be finer when the feed rate is reduced, and the product will become coarser when the feed rate is increased. In the case of a certain feed rate, the particle size of the product will be finer when the crushing pressure is increased, and the product will become coarser when the crushing pressure is reduced. Therefore, the particle size control is achieved by adjusting the parameters of the jet mill during the crushing process to achieve different crushing fineness. Before crushing, the relationship between the feed rate and the pressure needs to be measured and then the appropriate crushing parameters are determined to meet the customer's particle size requirements. .

 

The advantage of the jet mill is that the ground material does not produce pollution. After crushing, the compressed supersonic air velocity decreases and the volume increases. It is an endothermic process and has a cooling effect on the material. It is especially suitable for ultrafine grinding. The jet mill uses supersonic airflow to accelerate the particle generation speed, collide with each other or collide with the target to grind the material to achieve the grinding effect. Several evenly distributed auxiliary nozzles are arranged around the main nozzle to accelerate the material particles around the main nozzle to enter the central area of ​​the main stream to obtain a larger collision velocity. A feed nozzle is arranged in the center of the main nozzle, and the fluidized particles in the fluidized bed are directly sucked into the center of the main nozzle to obtain a high collision velocity.

 

There are five types of jet mills used in industry: flat type, circulating tube type, target type, opposing type, and fluidized bed type.

 

Factors affecting the crushing effect of jet mill:

 

It is understood that the pulverization effect of the jet mill is affected by the gas-solid ratio, feed particle size, working fluid temperature, working fluid pressure and crushing aids.

 

(1) Gas-solid ratio

 

If the gas-solid ratio is too small, the kinetic energy of the air flow will be insufficient, which will affect the fineness of the product. On the contrary, if the gas-solid ratio is too high, it will not only waste energy, but even deteriorate the dispersion performance of certain materials.

 

(2) Feed size

 

When crushing hard materials, there are also strict requirements for the particle size of the feed. For titanium powder, it is necessary to control 100-200 mesh when crushing the calcined material; the crushed material after surface treatment is generally 40-70 mesh, not exceeding 2-5 mesh.

 

(3) Working fluid temperature

 

When the temperature of the working fluid is too high, the flow rate of the gas will increase. Taking air as an example, the critical speed at room temperature is 320m/s. When the temperature rises to 480℃, the critical speed can be increased to 500m/s, that is, the kinetic energy increases by 150%. Therefore, increasing the temperature of the working fluid is beneficial to improve the crushing performance. effect.

 

(4) Working fluid pressure

 

The pressure of the working fluid is the main parameter that produces the jet flow velocity, and it is also the main parameter that affects the pulverization fineness.

 

Generally speaking, the higher the working fluid pressure, the faster the speed and the greater the kinetic energy. The technical staff said that this mainly depends on the crushability and fineness requirements of the material.

 

(5) Crushing aid

 

In the pulverizing process of the jet mill, if the appropriate pulverizing aid is added, not only the efficiency of pulverization can be improved, but also the dispersion performance of the product in the medium can be improved.