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Discussion on grinding process of thermal paper coating (Ⅳ)

Discussion on grinding process of thermal paper coating (Ⅳ)

  • May 22,2020.

Discussion on grinding process of thermal paper coating ()

Author: LiMeng, Lei

(Germany PUHLER Group, Puhler International Holding Group-PUHLER (Guangdong) Smart Nano Technology Co.,Ltd - General Manager)


3.4 Material of bead mill chamber and grinding tool

Thermal paper coatings use water as solvent, so bead mill chamber and grinding tools need to be rust-proof. Since polyurethane material (PU) has excellent wear resistance,  and no pollution, so the agitator shaft, grinding disc, peg and bottom plate of the bead mill used for grinding thermal paper coating are all coated with polyurethane material on the metal skeleton. Besides the rust prevention, the inner chamber needs to consider cooling. Usualy it is made of stainless steel with hard chrome plating . Another is cold-casting steel, which contains super hard metals such as molybdenum, tungsten, and formed with special process, has excellent wear resistance, it has been used to make cannon tube abroad. With a certain anti-rust function, and good thermal conductivity, cold-casting steel is commonly used as a grinding inner chamber in large-scale bead mills for grinding titanium dioxide (water-based materials).

3.5 Configuration of Thermal Paper Coating Bead Mill

The type, size, quantity and layout of the required bead mill, in addition to meet the coating production requirements of output, type and variety, reduce the investiment cost, give full play to the function of bead mill, are all important factors need to pay attention to.

A full understanding of the performance and function of bead mill, product properties and energy changes during the grinding process will help to arrange the thermal paper coating process properly. The production capacity of bead mill can be obtained initially through the sample grinding experiment, then the formulated grinding process can be further rationalized in actual production.

In the grinding of thermal paper coating, the bead mill can be arranged as:

* Single machine cycle grinding

* Two machines in series, the front is a Disk Bead Mill, the back is a Peg Bead Mill.

* Multiple machines are connected in series. Generally, one high-speed dispersing machine is used to stir and mix or one emulsifying machine, then, a large-capacity Disk Bead Mill is used for rough grinding process, by several Peg Bead Mills in parallel for the fine nano grinding process. In the tandem series mode, the front Disk Bead Mill use bigger grinding beads, at the same time, it is necessary to find the proper balance point of particle grinding fineness so that all the bead mills in series can fully perform their functions.

3.6 Optimization of grinding process

The optimization of the grinding process can be achieved from the linear speed of the bead mill, the particle size of the beads, the bead filling rate, solid content and flow rate.

Linear speed of the bead mill

The disk or peg of bead mill transfers the energy to the grinding beads in the form of linear speed, so that it gets 1/2 MV2 energy (where V is the linear speed), the grinding beads then impacted and grind the material finer with this energy.

If the linear speed is increased from V1 = 12m/s to V2 = 15m/s, then V22/V12 = 15 x 15/12 x 12 = 1.56, which means the energy input of the bead mill is increased by 56%, greatly improveing the grinding efficiency.

The particle size of the beads

If the bead materials are the same, the ratio of the number of beads with two different particle sizes in thesame volume is N2/N1=R23/R13(Derivation omitted). R is the diameter of the beads, N is the number of the beads in the chamber.

If the particle size of the bead is reduced from 0.5mm to 0.35mm, then 0.5x0.5x0.5 / 0.35x0.35x0.35 = 2.915. That is, the reduction of the particle size will increase the number of the beads in the grinding chamber to 2.915 timesthe original. This will greatly improve the effective impact and number of grinding times. In addition, the finer the beads, the higher the probability of fine particles in the material being grinded. Both factors are conducive to the improvement of the bead mill grinding efficiency and the finer grinding of the material.

Bead Filling Rate

If the bead filling rate is increased from 75% to 85%, then 85/75 = 1.133. That is, the number of beads in the grinding barrel is increased to 113.3% original, which improves the grinding efficiency.

Solid Content

In case of material fluidity permit, the higher the solid content, the higher the probability of beads meet the material, the higher the grinding efficiency, and the better the protection of the grinding chamber and the reduction of bead wear.

Flow Rate

The larger the flow rate of the bead mill, the more cycles per unit time will increase, and the degree of movement of the material inside the chamber will be correspondingly increased, the mutual collision between material and grinding medium and the intensity of grinding will also be strengthened. All these factors make the distribution curve of the material particle size tend to be thinner, and the target D50 is easy to reach.

Due to the optimization of the process parameters of bead mill, the production capacity of the peg type and the disk type bead mill can be further improved..

4 - Postscript

The grinding process of the bead mill and the configuration of the bead mill seem to be simple, but after carefully studying and observing and recording the phenomenon in the actual operation process, and then applying some laws of physical chemistry to establish some physical models and mathematical derivation, still can find many useful rules. Following these rules, the functions of bead mill can be better played, to reduce investment costs and operating costs, and increasing production capacity and product quality will be the future direction.



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