The performance of a jet mill can be evaluated based on several key parameters such as particle size reduction efficiency, energy consumption, and product quality.
Airflow crushing is the use of high-speed airflow or superheated steam energy for ultrafine crushing of solid materials; it is one of the most commonly used ultrafine crushing methods, widely used in non-metallic minerals, pharmaceuticals, chemicals, metallurgy, new materials, ceramic materials, lithium materials, rare earth materials, rare metals, quartz, flame retardant materials, and other industries of ultrafine processing of materials.
Different from ball mills, vibration mills, mechanical impact mills, and other ultrafine crushing equipment, airflow crushing equipment has its own unique structure and process parameters, such as the type and shape of the air mill size, air flow rate, working pressure (nozzle inlet pressure), the type and size of the nozzle, the arrangement of the nozzle (spatial location), the type of grading wheel and rotational speed, etc., will affect the effect of the crushing effect to varying degrees.
Types of Jet Mill
For different kinds of jet mills, there are bound to be differences in structure and parameters, which leads to differences in the crushing process and crushing mechanism.
Target jet mills are unsuitable for crushing materials with high hardness (Mohs hardness greater than 5); the product pollution is more serious, but its crushing efficiency is high, and the output is large.
Flat air mill, circulating tube jet mill impact strength is low, unsuitable for crushing materials with high hardness, simplified grading area to improve yield. Still, the product size distribution is often wider.
Pair-jet type air jet mill, fluidized bed type air jet mill has the characteristics of fine particle size and narrow distribution, and the product is less polluted.
Therefore, according to different requirements, choosing different kinds of air flow crushing equipment, whether for optimizing the crushing process or saving energy, is very necessary.
Jet mill size
Compared with other comminution equipment, air flow comminution equipment has a low yield and high energy consumption, suitable for processing demanding and high value-added products. However, the impact of mill size on the crushing efficiency makes developing large-scale airflow crushing equipment very promising.
Studies have shown that by increasing the power of the air mill, crushing efficiency will be greater. Improvement of air mill power depends on the enlargement of the mill size; it can be predicted that the jet mill will be the future development trend.
Working pressure (nozzle inlet pressure)
The working pressure of the air mill is an important factor affecting the jet stream’s speed; often, the greater the working pressure, the higher the jet stream’s speed. In the case of smooth airflow, particles with a flow better the higher the speed of the jet stream, often accelerated particles of its collision velocity is higher, and thus the greater the degree of crushing, the smaller the product size.
However, under high inlet pressure, the particles impact each other more violently, the sub-particles produced by particle fragmentation have sharp edges, and the degree of roundness is lower than that under low pressure.
In addition, when the working pressure is too high, the particle size of the product particles drop is not obvious. At the same time, the energy consumption increases sharply, so the pressure generally taken should be reasonable. When increasing the inlet pressure does not result in a correspondingly high throughput, and sufficient fineness, energy consumption, cost, and mill efficiency must be considered.
Feed rate and holding capacity
The feeding rate and grinding chamber gas-particle two-phase flow of particle concentration and particle holding capacity are extremely relevant; feeding rate is low, the concentration of particles in the grinding chamber is low, the particles carried by the average kinetic energy is high, the product size may be finer; when the feeding rate is high, the concentration of particles in the grinding chamber, the particles of the collision rate is high, the collision strength is low, the crushing rate is reduced or increased are possible. Therefore, it is necessary to consider the balance between the collision probability of particles and the average kinetic energy carried by the two and choose the best feeding rate.
The relationship between the holding capacity and the median diameter of the product is a “fishhook” curve, i.e., there is a holding capacity (range) that minimizes the median diameter of the product particles.
Air Jet Nozzles
The nozzle is the formation of high-speed jet airflow components; nozzle type and size largely determine the speed, shape, and stability of the jet airflow, while the spatial distribution of the nozzle affects the particle acceleration and collision area of the flow field.
In a general fluidized bed air mill, usually using Laval nozzles, the product fineness will be correspondingly smaller than that obtained using other nozzles. Smooth, meeting the airflow parameters of the nozzle shape, is conducive to obtaining the minimum energy loss of high-speed, smooth, concentrated jet airflow, thus improving crushing efficiency.
Crushing agent
Airflow crushing using the workpiece is generally dry and oil-free air or superheated steam; in some special circumstances, it needs to crush flammable, explosive materials and use nitrogen and inert gases as a workpiece.
The workpiece is different; its jet velocity and crushing flow field will also be different; using different workpieces to improve the airflow crushing process is an important research direction in the future.
Classification device type and its parameters
The classification device is used to control the particle size distribution of airflow-crushing product components; through the classification device, the requirements of the fine powder become the final product, and the future to meet the fineness requirements of the particles are isolated by the classification device, and back to the grinding chamber for crushing. The classification device makes the finished powder finer and more compact in size distribution and reduces the over-grinding of particles, which is important for improving the crushing efficiency.
Classification wheel type (horizontal and vertical), blade structure, and flow field will affect the classification effect; generally, only through the classification wheel speed can control the classification of particle size. Classification wheel speed is an optimal value so that the product size is the finest.
Types of materials
Various material particles due to their different physical and chemical properties, the crushing process has some differences. Usually, for the lower hardness of the material, it is easier to crush; in the case of lower mill requirements and energy consumption, you can get the required fineness of the product; while the higher hardness of the material, its crushing to be much more difficult, and thus the mill’s working conditions require more (working pressure and classification device, etc.), the product size will be relatively coarse.
The addition of grinding aid and dispersant
Adding grinding aids is conducive to improving product fineness, increasing crushing efficiency, reducing energy consumption, improving classification efficiency and precision, and improving product performance.
The appropriate type and amount of dispersant can help to improve the dispersion of particles in the grinding chamber, thus improving the crushing efficiency and product fineness; in addition, the high-performance dispersant may be an important prerequisite for the preparation of nanoscale particles using airflow pulverization in the future.
