Comparison of wet grinding and dry grinding

In wet grinding, the nano powder is mixed with a suitable solvent to prepare a suitable material. In order to avoid the phenomenon of powder agglomeration during the grinding process, it is necessary to add appropriate dispersants or auxiliary agents as grinding aids. If you want the final nano-scale product to be a powder instead of a slurry, you need to consider how to filter the large particles in the slurry first and how to dry the filtered slurry to obtain a nano-scale powder. Therefore, when the nano-scale powder is obtained by wet grinding, how to choose the appropriate solvent, auxiliary, filtration method and drying method will also affect whether the nano-scale powder can be successfully obtained.

Whether the powder wet grinding can successfully achieve the purpose of grinding or dispersing mainly depends on the selection of the size and material of the grinding medium (zirconia beads). The selected grinding media must be 0.1-0.4 mm or less. At the same time, in order to prevent such a small grinding medium from being affected by the thrust of the slurry moving in the direction of the spindle during the grinding process, it will be blocked near the filter screen and cause the grinding chamber to stop due to too high pressure, the linear speed of the stirring rotor needs to be More than 10 m/sec or more. At the same time, the slurry viscosity is adjusted to below 100 cps so that the movement of the grinding media is not affected by the slurry viscosity. At the same time, the solid content of the slurry should also be controlled below 35% to prevent the increase in viscosity due to the increase in the specific surface area of ​​the powder during the grinding process and the use of small zirconium beads cannot be continued. Of course, in order to prevent the 0.3-0.4mm zirconium beads from flowing out of the grinding chamber from the dynamic separator or plugging on the filter screen, the filter screen gap needs to be adjusted to 0.1 mm.

Dry and wet grinding

In order to quickly meet the powder wet grinding particle size requirements and make the grinder run normally, the required control rules and parameters are as follows:

1. Choose the appropriate grinding media according to the required particle size requirements. If you need to meet the nanometer requirements and avoid the loss of grinding media, choose the greater the Mohs hardness, the better, and the surface of the grinding media should be a true circle, no pores, and a size of 0.05-0.4 mm. The smaller the grinding medium, the better the grinding effect and the smaller the required specific energy value. The choice of the grinding medium will determine whether it can be successfully ground to the desired particle size.

2. Adjust the appropriate stirring rotor speed according to the size of the grinding medium and the viscosity of the slurry. Generally, for nano-level grinding, the speed needs to be above 12.5 m/sec.

3. Control the temperature of the grinding slurry. Generally, the grinding temperature of nano-scale slurry needs to be controlled below 45°C. The main parameters that affect the slurry temperature are the control of the rotor speed, the filling rate of the grinding medium, the size of the heat exchange area of ​​the grinding barrel, the cooling water condition and the flow rate

4. Select the appropriate dynamic separation system gap according to the size of the grinding media. Generally, the gap is 1/2 to 1/3 of the diameter of the zirconium bead.

5. Adjust the pump speed. Within the acceptable pressure range of the grinding barrel, the higher the pump speed, the better. In this way, the number of grinding times of the slurry through the grinder can be increased in the same grinding time to obtain a narrower particle size distribution.

Generally, to obtain nano-sized powder, it is necessary to use a grinder to grind dozens of times, or even hundreds of times, to reach nano-sized powder.

Dry grinding means that the water content of the material does not exceed 4% during the grinding operation, while wet grinding is to suspend the raw material in the carrier liquid stream for grinding, and appropriate additives such as dispersing agents are added to help the grinding. When the moisture content of the material exceeds 50% in the wet grinding machine, the problem of dust flying can be overcome. In food processing, the ground material is often used as a preparatory operation for leaching to make the components easy to dissolve, so it is quite suitable for wet grinding. However, wet operation generally consumes more energy than dry operation, and the equipment wears more seriously.

When the dry grinder grinds the powder, the temperature of the powder will rise sharply due to the introduction of a large amount of energy. When the powder particles are refined, how to avoid explosion problems is difficult for the grinder to control. Therefore, the wet grinding method came into being, and the powder obtained by wet grinding is the most effective and economical method. It avoids the high cost of chemical nano-powder manufacturing, and also avoids the shortage of mechanical dry grinding fineness that is difficult to reach nano-level powder. Generally, the particle size of dry grinding can only be grind to about 8um, if you want to reach finer or nanometer level, you have to use wet grinding.

Dry grinding means that the water content of the material does not exceed 4% during the grinding operation, while wet grinding is to suspend the raw material in the carrier liquid stream for grinding, and appropriate additives such as dispersing agents are added to help the grinding. When the moisture content of the material exceeds 50% in the wet grinding machine, the problem of dust flying can be overcome. In food processing, the ground material is often used as a preparatory operation for leaching to make the components easy to dissolve, so it is quite suitable for wet grinding.