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Discussion on grinding process of thermal paper coating (Ⅲ)

Discussion on grinding process of thermal paper coating (Ⅲ)

  • May 18,2020.

Discussion on grinding process of thermal paper coating (Ⅲ)

Author: LiMeng, Lei

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


3 - Grinding process of thermal paper coating

3.1 Fineness of thermal paper coating

The increase in fineness helps to improve the quality of thermal paper and reduce the amount of coating on the paper per unit area. However, if the final particle fineness is reduced from D50≤0.65µm to D50≤0.35µm, the quantity of Bead Mill required may be doubled, and this increases the cost of investment. How to achieve a better input-output ratio, will need to comprehensive consideration of various factors.

3.2 Grinding medium (Zirconia Bead) and grinding method

The diameter of the grinding medium is determined according to the fineness of the feed material and the fineness of the discharge material. Usually the diameter of the grinding media should be the feeding particles Dmax * 20-25, and the discharge particles D50 * 1000. In the two-stage series process of thermal paper coating, the diameter of the beads in the former bead mill is generally 1.5-1.7mm, which is more suitable for grinding the coating feed particles to D50=1-2µm. If the variety of thermal coatings is single and the production volume is large, it is recommended to use one bead mill to grind the thermal paper coating to D50≈20-30µm (this is easy to do), and then use several bead mills (required very large energy input) to cycle grind the material to D50≤0.5-0.7µm or finer.

3.3 Selection of Bead Mill type

If thermal paper coating grinding use tandem grinding process. the first bead mill is mainly responsible for grinding the material from hundreds of dozens of micron to a few microns. The energy needed in this process, is much less than the energy for grinding the material from a few microns to hundreds of nanometers. Disk Bead Mill can completely this task well.

FIG (1): Structure of PU Disk Bead Mill

The next use peg type nano bead mill to grind the material to sub-nanometer range, and further narrow the particle size distribution. Large flow circulation grinding process is necessary. Both theory and practice can prove that, in unit time, the more cycles of grinding, the finer the particles, the better the particle size distribution, then the larger flow rate the bead mill will need. For large flow rate circulation grinding of material, usually use peg type bead mill.

Curve 1: the relationship between the increase of cycle times and the distribution of particle size

FIG (2): Structure of SuperMaxZeta PU Peg Bead Mill

This Nano Peg Bead Mill has the following characteristics:

* Adopted the highly efficient separation system OCS (Open Canister Separation System), and the flow area of tube type centrifugal separator in the system is super large. Such as PUHLER PHN SuperMaxZeta type nano mill, with hollow rotor design, thereby giving space to place super slotted pipe separator. Since the rotor inside the OCS of the dynamic separation system can effectively block the grinding beads away from the stirring rotor, there is almost no grinding medium around separator, that makes the material can be smoothly in and out of grinding chamber.

* Peg Bead Mill has the characteristics of high power and large specific energy input, so there has high energy density per unit volume in the grinidng chamber. The production efficiency of high energy density (2 kW/dm3) peg type nano bead mill production is 3 to 5 times than ordinary bead mill, even if the material stays in grinding chamber for a short time, it still can get sufficient grinding. Such as PHN SuperMaxZeta 60 (about 60 litres), motor power is 75 kW, has equal power to disk bead mill PHE 200 (about 200 litres), and it is suitable for grinding of sub-micron and nano materials.

(To be continued)

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