Technical knowledge

Technical knowledge

Nano Color Paste (Ⅱ)

Nano Color Paste (Ⅱ)

  • April 24,2020.

Nano Color Paste ()

-- PUHLER nano grinding technology exchange

Author: LiMeng Lei.

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


2 - Dispersing and grinding principle of nanometer powder

The wet grinding by mechanical method is the most efficient and economical method to obtain nano powder.

Here we use Figure (1) - PUHLER® grinding machine as an example to make and explanation.

grinding machine

FIG (1) The grinding machine is a closed system. In the grinding chamber of the grinding machine, appropriate material and size's grinding balls are placed (Grinding media).

Nano Mill

FIG (2): PHN 10 Peg grinding system of Nano Mill is used in automotive repair paint, integrated grinding system workstation, and DCS signal data collection.

The motor uses a belt driven stirring rotor to generate shear force from the movement of the grinding ball. During the movement of the slurry from the pump to the grinding chamber, the slurry produces a dispersing and grinding effect due to the shear force generated by the relative movement of the grinding ball. When the particle size is smaller than the gap of the separator and the grinding media, the slurry will be extruded by the centrifugal force to the discharge tank groove to obtain the dispersing and grinding effect.The above process is a grinding pass. If it still not reached the particle size requirements, the above action can be repeated, generally it is called circulating grinding...until the particle size meets the requirements.

The above flow can be represented by the flowchart of the following figure (3) as follows:

FIG (3) Nano Grinding process

2.1 Slurry pretreatment and Pre-mixing

Whether the system can successfully achieve the purpose of grinding or dispersing mainly depends on the size of grinding medium and the proper selection of material. According to the author's experience, the selected grinding ball should be 0.1-0.4mm or less, and the linear speed of the stirring rotor should exceed 10m/s, the slurry viscosity is under 100 CPS, the solid content by weight should also be kept below 35 percent. In order to avoid the 0.3-0.4mm grinding ball flow out from the dynamic separator or stuck on the sieve, so the sieve slot width should be 0.1mm.

The above relationships can be summarized in the following Table (1):

Table (1) Grinding ball selection reference of high speed Nano Bead Mill

Slurry before grinding should be prepared according to the following rules in the pretreatment or pre-mixing.

2.1.1 Determine the target fineness.

2.1.2 Prepare the slurry viscosity, solid content, start fineness, target particle size and meet the requirement of Table (1).

2.1.3 The totating speed of the stirring rotor of the pre-mixing or pretreatment system shall be designed for high speed. The tangential velocity is recommended to be 2-13m/SEC to avoid slurry precipitation or uneven problems.

2.2 Grinding machine part

In order to quickly meet the grinding particle size requirements and make the grinding machine operate normally, the control rules and parameters are as follows:

2.2.1 Select the appropriate grinding bead according to the required particle size. For example, yttrium-stabilized zirconia beads should be selected for nano scale grinding. The higher the Mohs hardness, the better. The surface of the bead grinding should be a true circle with no pores and the size of the ball is 0.05-0.4mm. Properly selection of grinding bead will determine whether it can be successfully ground to the desired particle size.

2.2.2 Adjust the appropriate stirring rotor speed according to the size of the grinding bead and the viscosity of the slurry. Generally, the rotation speed should be above 12.5m/s for nano grinding.

2.2.3 Control the temperature of grinding slurry. Usually the grinding temperature of nano slurry should be controlled below 45℃. The main effects s affecting slurry temperature are rotor speed, grinding bead filling rate, heat exchange area of grinding chamber, cooling water condition and flow rate.

2.2.4 Select appropriate slotted pipe sieve according to the size of the grinding bead. Generally, the slotted pipe is 1/2 ~ 1/3 of the diameter of the grinding bead.

2.2.5 Adjust the Pump speed. Within the range of acceptable pressure, the faster the Pump speed, the better.

2.2.6 Record the power kW value required by the Grinding machine.

2.2.7 Record the specific energy value of each sample when sampling, and after analyzing the particle size. The relation between specific energy and average particle size was worked out to facilitate Scale up in the future.

2.2.8 Stop when the required specific energy value is reached. At this point, the required average particle size of grinding dispersion has been met.

2.3 Circulating Tank part

Generally, it will need to grind dozens of times, or even hundreds of times to get the nano powder. In order to save manpower and facilitate automation and unmanned operation, the author strongly recommends recirculation operation mode to do nano-scale powder grinding

The main considerated points are as follows:

2.3.1 Size of the circulating tank should not be too large. When the largest flow rate of a grinding machine is 3000 l/hr, the maximum capacity of the moving tank is 500 l, and the size of the circulating tank is 1/5 ~ 1/10 of the maximum allowable flow rate of the grinding machine, and the smaller the better.

2.3.2 Circulation tank should be designed with a stirring blade, and the stirring speed should not be too fast, 0-3m/s should be used to avoid bubbles.

2.3.3 Circulation tank shall be designed with a hot jacket layer to increase the grinding efficiency.

(To be continued)

© Copyright: 2020 PUHLER (Guangdong)Smart Nano Technology Co.,Ltd. All Rights Reserved.

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