Exchange of ultra-fine nano grinding technology fo

Based on the author's experience in grinding machine sales for several years, the main application fields of nano grinding can be distinguished from 1998. Before 1998, the problem faced by the business community was how to improve the efficiency of dispersion grinding to reduce labor costs, such as dye, coating, ink and other industries. After 1998, the bottleneck of industrial technology is how to obtain micro (nano) materials and how to disperse nano materials into final products, such as TFT-LCD, inkjet ink, electronics, magnetic materials, medicine, biopharmaceutical and cell crushing, oxide, food and other industries

pollution control is equally important, whether it is the need of traditional industries to improve grinding efficiency and speed or the need of high-tech industries to refine nano materials. Therefore, fine, fast and less pollution has become the most important topic of the new generation of decentralized grinding technology

this paper will discuss the current situation and development of nano grinding, the principle of nano dispersed grinding technology, the structure of nano grinding machine, the source of existing equipment, application examples and precautions, conclusions and suggestions

current situation and development of nano scale dispersion grinding technology

◆ chemical method (bottom up)! Physical mechanical grinding (top down)

with the lightness, thinness and shortness of 3C products and the white hot application of nano materials, how to apply ultra-fine grinding technology to the manufacture and dispersion grinding of nano materials has become an important topic at present. There are two ways to get nano powder by using the electronic universal experimental machine in our company. One is the bottom-up manufacturing method of chemical methods, such as chemical precipitation method, sol gel method, etc. The other method is physical method, which changes the powder particles from large to small (top down), such as mechanical ball milling method

so far, most of the chemical methods or bottom up nano powder manufacturing methods have been studied in the academic community and have achieved fruitful results, and several nano powders can be obtained. However, its manufacturing cost is sometimes quite high, and it is not easy to scale up, and the obtained particle size distribution is also large. So far, enterprises still use physical and mechanical grinding (top down) method to get nano-sized powder. The top down method is easy to obtain nano powder with small particle size distribution. At the same time, the production cost is relatively low, and the parameters are easy to control. The R & D experiment will benefit from the parameter scaling up obtained by this machine to the mass production machine. Only the top down method can grind to 30 nm at present, but it can meet the needs of the industry

◆ dry grinding! Wet grinding

for nano powder manufacturers, it is certainly hoped that the final nano powder can be obtained by dry grinding. However, if the powder is ground by mechanical grinding, the temperature of the powder will rise rapidly due to the introduction of a large amount of energy during the grinding process, and when the particles are refined, how to avoid explosion-proof problems is difficult for the grinder to control. Therefore, generally speaking, the particle size of dry grinding can only be ground to 8um. If you want to get the particle size below 8um, you must use wet grinding

the so-called wet grinding is to mix the nano powder with appropriate solvent to prepare appropriate materials. In order to avoid the phenomenon of powder agglomeration in the grinding process, it is necessary to add appropriate dispersants or additives as grinding aids. If you want the final nanoscale finished product to be powder instead of slurry, you need to consider how to filter the large particles in the slurry first and how to dry the filtered slurry to get nanoscale powder. Therefore, how to select appropriate solvents, additives, filtration methods and drying methods will affect the key technology of successfully obtaining nano-sized powder when wet grinding is used to obtain nano-sized powder

◆ grinding, dispersing

as the name suggests, grinding is defined as the use of shear force, friction or impactforce to grind powder from large particles into small particles. Dispersion is defined as coating the nano powder with the added solvents, additives, dispersants, resins, etc., so as to achieve the purpose of complete separation, wetting, distribution, uniformity and stabilization of particles. It is important for users to disperse or grind nano powders according to the relevant binding parameters set by the experimental items and requirements, because the phenomenon of van der Waals force and Brownian motion is gradually obvious and important in the process of powder size changing from large to small. Therefore, how to choose the appropriate additives to avoid the re agglomeration of the powder and how to choose the appropriate grinder to control the grinding slurry temperature to reduce or avoid the influence of Brownian motion will become the key technology for the success of the wet grinding dispersion method to obtain nano powder grinding and dispersion

dispersion grinding principle of nano powder

wet grinding by mechanical method is the most effective and economic method to obtain nano powder. In this paper, the principle and process of wet grinding and dispersion methods will be discussed in depth. For the convenience of explanation, this paper will take the nanoscale high-speed stirred bead mill in Figure 1 as an example

Figure 1 example of nano scale high-speed stirred bead mill

the grinder in Figure 1 is a closed system, in which grinding balls (grinding media) of appropriate material size are placed in the grinding room of the grinder. Its principle is shown in Figure 2

Figure 2 Schematic diagram of the principle of the nano high-speed stirring ball mill

the motor uses the belt drive stirring rotor to generate shear force from the movement of the grinding ball. The slurry has a dispersive grinding effect due to the shear force generated by the relative movement with the grinding ball during the movement of the pump thrust to the grinding chamber. When the particle size is smaller than the filter gap of the dynamic large flow separator that separates the grinding ball and the slurry in the grinding room, the slurry will be extruded to the discharge barrel groove by centrifugal force to obtain the dispersed grinding effect. If the above process has not met the particle size requirements, the above actions can be repeated, which is usually called cyclic grinding until the particle size meets the requirements

Figure 3 operation process of nano high-speed stirred bead mill

the above process is represented and discussed in the flow chart of Figure 3, and the description is as follows:

◆ slurry pretreatment and pre mixing

whether the system can successfully achieve the purpose of grinding or dispersion mainly depends on the size of grinding medium (i.e. grinding ball) and the selection of material. Based on the author's experience in planning and actual commissioning of hundreds of factories, the selected grinding ball needs to be 0 4 mm or less. At the same time, in order for such a small grinding ball to block forward near the filter without being affected by the thrust of the slurry moving in the x-axis direction during the grinding process, resulting in the shutdown of the grinding chamber due to high pressure, the linear speed of its mixing rotor needs to be more than 10 m/sec. At the same time, the slurry viscosity control is adjusted to less than 100 CPS, so that the motion of the grinding ball is not affected by the slurry viscosity. At the same time, the solid content by weight of the slurry should also be controlled below 35% to prevent the viscosity from rising due to the increase of the specific surface area of the powder during the grinding process, and the small grinding ball cannot be used anymore. Of course, to avoid 0 4mm grinding balls flow out of the grinding chamber from the dynamic separator or plug on the filter, so the filter clearance needs to be adjusted to 0.1 mm

the above relationship can be summarized as shown in Table 1

in order to meet the requirements of Table 1 above, during pretreatment or pre mixing, it is necessary to prepare the slurry before grinding according to the following rules, and sort it out as follows:

● first determine the final particle size requirement to be ground (target fineness)

● prepare the slurry viscosity, solid content, start fineness (final required fineness) and particle size of target before grinding, and meet the requirements of Table 1

● the mixing rotor speed of the pre mixing or pretreatment system should be designed for high speed. It is recommended that the tangent speed be m/sec to avoid slurry sedimentation or uneven problems

◆ grinder part

in order to quickly meet the requirements of grinding particle size and make the grinder operate normally, the required control rules and parameters are as follows:

● select the appropriate grinding ball according to the requirements of the required particle size. For example, if you need to meet the requirements of nano scale and avoid grinding ball loss, you need to choose yttrium stabilized zirconia grinding ball. The greater the Mohs hardness, the better. The surface of the grinding ball needs to be true round, without pores, and the size of the grinding ball is 0 4 mm。 Whether the grinding ball is properly selected or not will determine whether it can be successfully ground to the desired particle size

● adjust the appropriate mixing rotor speed according to the size of the grinding ball and the viscosity of the slurry. Generally, the speed of nano grinding needs to be more than 12.5 m/sec

● control the grinding slurry temperature. Generally, the grinding temperature of nano slurry should be controlled below 45 ℃. The main parameters that affect the slurry temperature are the control of rotor speed, ball filling rate, heat exchange area of grinding barrel, cooling water conditions and flow

● select the appropriate clearance of dynamic separation system according to the size of grinding ball. The general clearance is 1/2 ~ 1/3 of the grinding ball diameter

● adjust the pump speed. Within the acceptable pressure range of the grinding barrel, the higher the speed of the pump, the better. In this way, it is possible to increase the number of times the slurry passes through the grinder at the same grinding time to obtain a narrower particle size distribution

● record the kW value of electric energy consumed by the grinder

● when sampling, record the specific energy value of each sample, and after analyzing the particle size, make the relationship between the thermal conductivity energy and the average particle size of such products, so as to facilitate the future scale up

● stop the machine when the required specific energy value is reached. At this time, in principle, the required average particle size of grinding dispersion has been achieved

◆ generally, nano powder can be obtained by grinding with a grinder for dozens of times, or even hundreds of times. In order to save manpower and facilitate automatic and unmanned operation, the author recommends using the recirculation operation mode for nano powder grinding

● the main considerations are as follows:

(1) the size of the circulating barrel should not be too large. Generally, if the large flow of the grinder is 3000 l/hr, the maximum capacity of the moving cylinder is 500 L

generally, the size of the circulating bucket should be 1/5 ~ 1/10 of the maximum allowable flow of the grinder, and the smaller the better. In this way, the slurry in the circulating tank can be increased to be grinded by the grinder at the same time to obtain a better particle size distribution

(2) the circulating drum needs to be designed with mixing blades, and the mixing speed should not be too fast, with m/sec as the appropriate to avoid bubbles

the circulating barrel groove needs to be designed with a hot jacket layer to increase the grinding efficiency. If you want to get nano powder dispersion grinding efficiently, the above pretreatment, grinding machine and circulating barrel all need to be equipped, and none of them is indispensable

4) method to determine the average particle size (D50). If the slurry formula is fixed and the grinder operating conditions are also fixed, the average particle size will be determined by the specific energy value. The specific energy E value is defined as follows:

e: specific energy, in kwh/t

p: power consumption, in kw

p0: invalid power consumption. When the grinding ball has not been added, start the grinder to consume power, in kw

m': flow, The unit is ton/HR

cm: solid composition, the unit is%

from the above, the physical meaning of specific energy is the power consumed per ton of powder per hour

Figure 4 specific energy consumption and