1, ultrafine powder overview
There is no unified definition of ultrafine powder, foreign general particle size is less than 3μm for ultrafine powder, domestic general particle size is less than 30μm for ultrafine powder, and further subdivided into micron (1 ~ 30μm), sub-micron (1 ~ 0.1μm) and nano (0.001 ~ 0.1μm).
Ultrafine powder in optics, magnetism, acoustics, electricity and mechanics and other properties are very different from the normal, so it is widely used in medicine, chemical industry, electronics, energy and building decoration and other fields. These differences in nature can be attributed to two aspects: on the one hand, the volume effect caused by the smaller particle size; On the other hand, the surface effect is caused by the increase in the proportion of the number of atoms on the surface of the particle.
In principle, the preparation of ultrafine powder is mainly divided into two kinds: chemical synthesis method and physical crushing method, of which physical crushing method is the main preparation method at present, which has the advantages of simple process, large yield and low cost. The related equipment mainly includes stirring ball mill, vibrating ball mill, planetary ball mill, Raymond mill, colloid mill, air mill and so on.
2. Introduction of ultrafine powder processing technology and equipment
(1) Mixing mill
The stirring device in the stirring mill mainly has three kinds: spiral type, rod type and disk type. Screw type, rod type mixing mill is mainly vertical, disk type mixing mill has vertical and horizontal 2 kinds.
Ordinary horizontal ball mill cylinder rotation consumes large energy, the problem of low grinding efficiency is more prominent, and the stirring mill cylinder is stationary, high energy utilization rate, while the grinding medium diameter is smaller, the product particle size is finer, the stirring line speed is high, the unit volume energy density is high, when the particle size of the material to be ground is less than 100μm, the fine grinding efficiency is much higher than that of the ordinary ball mill. It is smaller and more energy efficient for the same processing capacity.
(2) high-speed mechanical impact grinding
The high speed mechanical impact mill uses the rotor rotating at high speed to transfer momentum to the material, so that the material and the liner collide violently to get the ultrafine powder. The grinding, shearing and eddy current effects generated in this process can promote the formation of a new surface of the powder. The pulverized material is driven by the main air flow into the classification area, and the fine powder particles that meet the requirement of fineness are discharged through the classification rotor with the air flow and collected by the collection device. The unqualified coarse powder particles are driven back to the cylinder wall by the rotor blade under the action of centrifugal force, and return to the crushing zone for further crushing.
This equipment is widely used in medium and low hardness calcite, marble, chalk, talc and other materials with convenient feeding, small footprint, high crushing efficiency and adjustable crushing size. However, it has a thermal effect in the crushing process and is not suitable for heat-sensitive and low melting point materials.
This equipment mainly relies on the high-speed operation of parts for crushing, which will cause serious wear of parts, and the pollution of materials is inevitable, so the development of high wear-resistant materials is an important way to promote the development of such equipment. The high-speed mechanical impact mill is usually operated by a matching unit, which integrates the three processes of crude crushing, fine crushing and screening of traditional Chinese medicine into one set of equipment, which can improve production efficiency and reduce labor intensity. The whole system is also equipped with dust collection device, which can improve the working environment and avoid material waste.
(3) Air flow shredder
The principle of the air mill is to use the high-speed air flow to make the material and impact parts collide, impact, shear and other effects to crush. According to the different structure or working mode, the air flow mill can be divided into: flat type, circulation tube type, collision type, target type, fluidized bed type.
The product has fine particle size, narrow distribution, clean and smooth particle surface, regular particle shape, good dispersion and high activity. Air mill is widely used in processing non-metallic minerals with low hardness, such as chemical raw materials, food, medicinal materials and so on.
(4) Vibration mill
Vibration mill is a super fine grinding equipment with ball or rod as the medium, including grinding medium, grinding bowl and eccentric vibration exciting device. The fineness of the products processed by vibration grinding can reach several microns, which is widely used in the ultra-fine powder processing of building materials, metallurgy, chemical industry, ceramics, glass, refractory materials and non-metallic minerals.
Vibration mill can be divided into intermittent type and continuous type according to different operation modes, divided into single cylinder type and multi-cylinder type according to different number of grinding cylinders, and divided into inertia type and rotary type according to different vibration characteristics.
The working principle of the vibration mill is that the grinding medium is used to impact, grind, shear and other effects on the material in the cylinder of high frequency vibration, so that the material is crushed in a short time.
Vibration mill has the advantages of compact structure, small volume, small mass, simple operation, easy maintenance, low energy consumption, high output, uniform product particle size, etc., the disadvantage is loud noise, large size vibration mill on the spring, bearing and other machine parts of high technical requirements.
(5) Raymond mill
Raymond mill has the advantages of small footprint, low energy consumption and less capital investment, and occupies an important position in grinding equipment. The working principle of Raymond mill is as follows: under the influence of centrifugal force, the mill roller is firmly pressed on the mill ring, the blade picks up the material and sends it to the gap between the mill ring and the mill roll, the powder is formed by rolling the material, and then the powder is blown up by the fan into the analytical machine, the material that meets the fineness requirements enters the cyclone separator for separation and collection, and the unqualified material falls back to the grinding chamber and then grinds.
With the widespread application of non-metallic minerals in ultrafine powders, the fineness control of non-metallic minerals is becoming more and more strict. In this context, the lack of traditional Raymond mill has caused trouble to related equipment manufacturers and mining products enterprises. For example, the product fineness is low, the mechanical failure rate is high, the amount of pollution is large, and the collection and separation of the finished powder cannot meet the expectations.
(6) Colloid mill
Colloid mill is a kind of fluid ultrafine grinding machine, which also has the functions of crushing, dispersing, emulsifying, homogenizing and stirring. Colloidal mill is generally divided into vertical, horizontal, pipe and so on.
The working principle of the colloid mill is: the rotating tooth (rotor) and the fixed tooth (stator) are relatively high-speed rotation, and the material is subjected to strong shear, friction, vibration and other effects when passing through the two teeth, so as to achieve the ultrafine grinding effect. Colloid mill is mainly used in food, paint, cosmetics, medicine, daily chemical industry and other industries.
The main advantages of colloid mill are simple structure, easy maintenance and maintenance, suitable for high viscosity, large particles of material grinding, but it is not allowed to mix quartz sand, broken glass, metal chips and other hard substances, the size of the processed products can reach a few microns or even 1μm below. The main disadvantage of colloid mill is that the flow rate of centrifugal movement is not constant, and changes significantly with the viscosity of the material; The high heat generated by the high-speed friction between the rotor, the stator and the material denatures the material, and the surface is easy to wear, resulting in a significant decrease in the thinning effect.
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