Briefly describe the problem of agglomeration and dispersion of ultrafine powder
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The dispersion of ultrafine powders in ultrafine powder technology is undoubtedly the most critical technology. The grading, particle size measurement, mixing, storage and transportation operations are all largely dependent on the degree of dispersion of the particles.
1 Reasons for reunion
1.1 Intermolecular forces cause particle agglomeration
As we all know, there is always Van der Waals gravitation between molecules, which is short-range force. However, for a system composed of a very large number of molecular aggregates, there are interactions between a plurality of molecules, and the effective spacing of the inter-particle molecular forces can be more than 50 nm, which is a long-range force. The molecular force between the ultrafine powder particles is the root cause of particle agglomeration.
1.2 Electrostatic force between particles causes agglomeration
Most of the particles in the dry air are naturally charged. The maximum amount of charge obtained by the particles is limited by the breakdown strength of the surrounding medium, which is about 1.7 x 10 Å electrons/crn2 in dry air. When the charged particles are in contact with other objects, the surface charge of the particles attracts the opposite charge of the other party, causing residual charge on the surface of the object, thereby generating a contact potential difference.
1.3 Bonding of particles in wet air
When the relative humidity of the air exceeds 65%, the water vapor begins to condense on the surface of the particles and between the particles, and the adhesion between the particles is greatly enhanced by the formation of a liquid bridge. The liquid bridge adhesion is mainly composed of the capillary pressure generated by the curved surface of the liquid bridge and the adhesion caused by the surface tension.
2 particle dispersion pathway
2.1 Surface modification method
In recent years, many researchers at home and abroad have used the surface modification method to study the dispersion of ultrafine powder particles. Although the surface can improve the anti-agglomeration properties of ultrafine powder particles, the surface of the modified particles promotes the original properties. , it has a great impact on its application, and sometimes even has a huge negative effect. A method of collecting and preserving nanoparticles using an organic solvent has been developed, which can greatly reduce the agglomeration of nanoparticles in a solvent, but does not solve the problem of agglomeration in air.
2.2 Mechanical dispersion method
Mechanical dispersion refers to the mechanical agitation of particles agglomerated, which is currently the most widely used dispersion method. The necessary condition for mechanical dispersion is that the mechanical force (referring to the shear and compressive stress of the fluid) should be greater than the adhesion between the particles. Usually, the mechanical force is formed by a high-speed rotating impeller or a nozzle of a high-speed airflow and a strong turbulent motion of the airflow caused by the impact. This method is mainly to improve the dispersion efficiency by improving the dispersion equipment. Mechanical dispersion is easier to achieve, but since it is a mandatory dispersion method, the mutually bonded particles, although they can be dispersed in the dispersion, have no change in the force between the particles, and are quickly reagglomerated after being discharged from the disperser. Another problem with mechanical dispersion is that brittle materials may be pulverized, and the dispersion effect of mechanical equipment is reduced.
2.3 Dry dispersion method
In humid air, the liquid bridge formed between the particles is the main cause of particle agglomeration, thus preventing the liquid bridge from being produced or eliminated.
The resulting liquid bridge action is the primary means of ensuring particle dispersion. In almost all relevant production processes, warm drying pretreatment is used to remove the moisture of the material and ensure the looseness of the particles. Drying is a simple and easy method. At present, the drying equipment of domestic mineral products mainly uses rotary kiln, drying pit, cylinder dryer, electric drying oven, far infrared dryer and so on. These equipments can dry materials, but the equipment covers a large area, the capital construction investment is large, the degree of automation is low; the operating environment is harsh; the equipment operation energy consumption is large, the heat utilization rate is low; the product loss is large.
2.4 Electrostatic dispersion method
Electrostatic dispersion refers to the charge of the same polarity of the powder particles according to the needs of the production technology, and the Coulomb repulsion between the charged particles is used to achieve complete and uniform dispersion between the particles. The key to electrostatic dispersion is how to fully charge the particles. At present, the methods for charging the particles mainly include contact charging, inductive charging, corona charging, etc., and the particles can be charged by electron beam irradiation.
