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About the grinding medium...

About the grinding medium...

  • January 21,2020.

About the grinding medium...


1. Selection of grinding beads:

With the material fineness requirements are higher, the use of bead mill is more common, and the grinding mediums in the market are more, how to choose the most suitable grinding medium for your own production technology and conditions, is very critical. Here are some simple analyses.


1.1 Chemical composition

According to the material, grinding medium can be divided into glass beads, ceramic beads (including zirconium silicate beads, zirconia beads, alumina beads, rare earth metal stable zirconia beads, etc.), steel beads, and so on.

Due to the differences in chemical composition and manufacturing processes, it determine the crystal structure of the grinding beads, the dense crystal structure ensures the high strength of the beads, high wear resistance and low ink absorption, etc. The percentage content of each component determines the specific gravity of the grinding beads, and the high specific gravity guarantees the high efficiency of grinding. The natural wear of the grinding beads' chemical composition during the grinding process will have a certain impact on the performance of the slurry, so in addition to considering the low wear rate, chemical elements are also to be considered. Such as when grinding tape powder or other electronic component paste, metal Fe, Cu and other elements should be avoided, then the grinding beads containing Fe2O3 or CuSO4 and other components should not be selected, so the zirconium bead is the common choice of this industry. For example, in pesticides, pharmaceutical and biochemistry, heavy metals are to be avoided, while PbO is the most common component.

In a word, the chemical composition of the grinding beads determines some physical properties (hardness, density, wear resistance) and the contamination of the slurry from the self-wear is one of the consider factors for the choice of grinding medium.


1.2 Physical Properties

1.2.1 The density of grinding bead

Density is expressed as specific gravity (true specific gravity) and bulk weight (false specific gravity) in the usual documents. The molecular weight and percentage composition of various oxides determine the grinding bead’s density. The common used grinding bead’s density is shown as below.

Type

Glass bead

Zirconium   silicate bead

PureZirconium bead

Alumina bead

Steel bead

Rare earth zirconium bead

Specific gravity

2.5

4.3

5.5

5.7

7.8

6.1

Bulk weight

1.5

2.5

3.2

3.4

4.

3.7

Under normal circumstances, the larger the specific gravity of the grinding bead, the larger the impulse, the higher the grinding efficiency. And the the wear of the contact parts of the bead mill (cylinder, dispersing plate, etc.) are relatively large, so the slurry viscosity and the flow rate to become the key. Low density grinding bead for low viscosity slurry, high density grinding bead for high viscosity slurry.

1.2.2 Hardness of grinding bead

Mohs hardness (Mohs) is a commonly used index. The harder the grinding bead, the lower the wear rate of the bead.

Although the grinding bead with high hardness has more wear on the contact parts, it can reach the optimal optimization point by adjusting the filling amount of the grinding bead, the viscosity of the slurry, the flow rate and other parameters.

1.2.3 Size of grinding bead

The size of the grinding bead determines the number of contact points between the bead and the material. The more contact points of the bead with small particle size, the higher the theoretical grinding efficiency. On the other hand, when grinding materials with relatively large particles in the initial test, such as 100 micron slurry, D=1mm beads may not be suitable, because the impact energy of small beads cannot reach the energy of sufficient grinding and dispersing, at this point, larger beads should be used.


1.3 Notes for first use of grinding medium

1.3.1 Choose zirconium beads or glass beads according to the viscosity of the material.

1.3.2 Choose suitable size beads according to particle size of raw materials and fineness required by products.

1.3.3 Check whether the separator or mesh aperture of the bead mill is properly set and selected. The clearance shall be one third of the diameter of the smallest bead. For example, use 1.2-1.4mm beads with a gap of 0.4mm.

1.3.4 Try to avoid opening the bead mill in dry condition, which may cause unnecessary wear and tear of beads and accessories.

1.3.5 Beads of different brands should not be mixed.

1.3.6 Try to avoid mixing different size beads.


1.4 How to determine the size of zirconium beads

1.4.1 Requirements for grinding equipment:

1.4.1.1 Sieve screen separation: minimum diameter of bead = gap of screen *1.5;

1.4.1.2 Ring type separation: minimum diameter of bead = gap of ring *3;

1.4.2 Technology process requirements:

1.4.2.1 Initial diameter of material: minimum diameter of bead = initial diameter of material *30~50;

1.4.2.2 Final diameter of material = bead diameter

1.4.3 The bead diameter of 1.5mm is a better choice.


2. Crushing of grinding beads

After we choose the grinding bead, how to use it correctly, avoid the crushing of the grinding bead, try to extend the life of the grinding medium and the contact parts of the bead mill.


2.1 How to solve the broken problem of grinding beads

First identify the appearance of "broken" beads. If the particles are smooth rather than sharp like pieces of broken glass, they are naturally worn beads. These beads may be ground into various shapes such as disks, ellipses, or small beads. This condition may be called abrasion, only the degree of wear is different.

From the point of view of force, when the dispersing disk transfer energy to the beads, if the liquid viscosity is very low, when using the solvent or water clean the bead mill, the material thrust is not strong enough, then the beads may hit the dispersing disk and broken. This is one of the important reason that we recommend to shorten the cleaning time as much as possible. If the machine is washed with a low viscosity liquid, the beads may also wear out other beads or contact parts of the bead mill. In order to obtain the longest service life of the grinding beads and bead mill, it is better to use resin for cleaning and maintain sufficient material viscosity in the production process.

Many people used to come across the case of bead breakage after new grinding beads loading during the trial. The following analysis of this problem and explore the solution.

1) Improper installation of internal contact parts of the bead mill. Loose dispersing disk or sharp angle on the edge of the dispersing disk, there is foreign body inside the bead mill, loose valve, etc., all of which may cause grinding bead breakage.

2) Wear or improper installation of bead mill contact parts. The dynamic separator or sieve may be worn, the sieve may be broken or installed in reverse. If the grinding medium passes through the separator and enters the feed pump, the pump will crush the beads before being blocked or shut down.

3) Backpressure: when the feed pump is turned off, the residual pressure in the bead mill presses the beads into the pump, when the pump is restarted, it will crush the beads. One-way valves are not always reliable and sometimes allow the beads to pass before the valve is closed.

4) Grinding bead backlog: if the grinding beads are accumulated in the bottom of the bead mill or as a result of the work pump too fast speed lead to the grinding beads are concentrated in the outlet of the horizontal bead mill, will appear such situation. The solution is to start the bead mill with on - off - on - off inching to loosen the backlog of grinding beads.

5) Dispersing disk wear: this situation may be more difficult to determine. The worn edge of the disk is uneven and the edge of the disk is where the linear velocity of the disk rotates the most and may form a strong eddy current.

In addition, for a vertical bead mill, the grinding beads are concentrated near the edge of the lower dispersing disk due to their own weight. This is possible to cause the grinding beads broken. The wear of the dispersing disk is too large mainly due to the feed pump speed is too slow, grinding bead density is too large. This problem can be solved by replacing badly worn parts and loading the same batch of grinding beads.

6) Mix up with the grinding beads produced by different manufacturers. The density of products may vary from different manufacturers. This will increase the wear between the beads. The solution is never to mix different grinding beads. If they are suspected to have been mixed, they can only be thrown out.

7) Used beads were accidentally added to the bead mill.

8) Garbage or some small worn beads: there is often garbage or some small worn beads attached to the sieve, shaft, bracket, or other machine corners. When new grinding beads are added, these items may slip from the attached positions into the newly added beads, this creates the illusion that the new beads broke immediately, but actually they are the natural wear and tear of the previous beads. If it is not serious, let these little things pass naturally after a certain amount of time. Otherwise, you must thoroughly discharge the old beads and replace them with new ones.

9) The grinding beads can be cleaned with water, solution or resin according to the formula. Keep low speed when cleaning, press the "start - turn off" key for intermittent cleaning.


2.2 The principle of filling grinding medium

If the grinding efficiency of the bead mill is found to be reduced, it is a possible signal to add beads.

According to their own process, the user can master the actual loss rate of beads, and make regular filter the bead and add.

Sift the small beads out in a sieve about two-thirds of the bead's diameter. For example, use a sieve of 0.8mm when bead is 1.2-1.4mm, and then add the balance quantity beads.


2.3 How to fill the grinding medium

Due to the natural wear and tear, the size of the grinding beads will become smaller and smaller, in order to maintain a uniform filling amount and avoid the small beads blocking or entering the separation device, according to the service life of the grinding medium and the conditions of the user's own process to filter the beads and add a certain amount of grinding medium. It is recommended that after 100-200 working hours, screening and add appropriate amount of new beads.


3. Developing trend of grinding medium:

There are various kinds of grinding balls (beads) in the grinding medium market. How to recommend suitable grinding medium to users and how users choose the best grinding medium have become a problem that cannot be ignored in the grinding industry. This article tries to make the following points from the market orientation:


3.1 The partical size used is getting smaller

From the birth of the world's first agitator ball mill (Attritor), which used larger grinding beads, to the development of vertical bead mill, horizontal bead mill and all kinds of ultra-fine bead mill with improved functions, which used smaller and smaller grinding medium. It is mainly driven by two reasons:

u Material final fineness is micron: since the material is dispersed and grinded by contact with the moving grinding medium, the smaller the particle size of the grinding medium is, the more the contact point is, and the higher the grinding effect and the smaller the grinding fineness is finally achieved. For example, a brand of zirconium silicate beads with a diameter of 2mm is about 20,000pcs per liter, while the size of 1mm beads is about 80,000pcs, which is four times the former. When large size zirconium beads are used to grind a product, but the particle size of the material fails to reach the fineness requirement no matter how many passes of grinding, when change to smaller beads, the effect was significantly improved.

u The improvement of the separation device of the bead mill makes it possible to use ultrafine grinding beads: the minimum useable grinding bead size has become an important index for evaluating the quality grade of bead mill. The separators range from the static, conventional flat Nickel sieve to the Johnson sieve with triangular beams and to the dynamic, high-flow ring separator and sleeve type Cartridge sieve at PUHLER. Except extending the service life, the partical size of useable grindng medium are becoming smaller and smaller without significantly affecting the flow of materials. Representatives of Cartridge sieve (e.g. High Speed Grinder of Premier, American) and representatives of the ring separator (such as the dyno-mill laboratory in Switzerland) both can use the smallest beads as 0.2mm in size. The dynamic large flow centrifugal separation system developed by PUHLER (with international and domestic invention patent certificate) changes the separation principle from separating bead size to separating bead density, and pushes the minimum particle size of grinding beads to a new limit.


3.2 The specific gravity is heavier

Comparison of common grinding beads


Glass bead

Stone bead

Zirconium   silicate bead

PureZirconium bead

Chromme steel bead

Specific Gravity

2.5kg/dm³

2.9kg/dm³

4.5kg/dm³

6.1kg/dm³

7.8kg/dm³

Mohs Hardness

6

6.5

7.7

9

7

Compressive strength

0.45KN

0.50KN

0.75KN

2.0KN

0.6KN

As can be seen from the above table, the proportion of common grinding beads such as glass beads, stone beads, zirconium silicate beads, pure zirconium beads and chromium steel beads increases successively, and their hardness, compressive strength and wear resistance increase successively in addition to chromium steel beads. According to the dynamic formula P=mv, the impulse P of the bead is proportional to the mass m of the bead. The higher the density, the greater the movement energy of the bead, and the higher the grinding efficiency.

Because inorganic pigments with high toughness (such as carbon black, phthalein blue, iron oxide, etc.) need ultra-fine grinding, in addition to the pursuit of higher grinding efficiency and ideal gloss, using high-density grinding beads has become a solution. Bead mill manufacturers have complied with this requirement with the following two reasons:


  • The generation of high input energy density bead mill: one of the development trends of modern bead mill is that the volume of grinding chamber becomes smaller, and the configured motor power becomes larger, the input energy density increases sharply. Relevant compared parameters are shown as the follows.



Representative of traditional bead mill

Representative of modern bead mill

Brand

Premier, America

PUHLER

Model

HM-15

Puhler PHN 06

Grinding chamber volume

15L

6.0 L

Power of motor

15KW

15 KW

Energy density

1KW/L

3.85 KW/L

According to the above table, the energy density of modern bead mill is 4 times or more than that of traditional bead mill. This makes it possible to efficiently drive high-density grinding beads such as pure zirconium beads. At the same time due to the high energy distribution of the grinding area, only high compressive strength and wear resistant grinding medium (such as pure zirconium beads) can be used.


  • Quality Improvement of the bead mill contact parts: contact parts of bead mill (such as inner grinding chamber,  dispersing disk, pegs and separation device, etc.) adopted hard wear-resisting carbide (like tungsten carbide) and ceramic (such as silicon carbide, zirconium oxide, etc.) of high performance material, it can withstand the impact and friction caused by the high energy and hardness of the pure zirconium beads.



3.3 The size deviation of grinding beads is getting smaller

For the bead mill, the grinding beads with uniform particle size show its superiority, it has certain contribution to the narrow distribution of material fineness, on the other hand, it greatly reduces the production of broken beads, reduces the wear of bead mill contact parts and avoids the pollution to the products. The particle size deviation of Nanor grinding medium commonly used in the market remains at 0.2mm. Uniform beads due to the more complex production process, so the market price is relatively high.

In conclusion, the diversity of grinding products and the complexity of production technology determine the diversity of grinding medium. A variety of grinding beads, such as glass beads, stone beads, zirconium silicate beads, pure zirconium beads and chromium steel beads can co-exist harmoniously, the difference is the market share.


4. Others

4.1 Standard for Zirconium beads


Standard

Ingredient

ZrO2 58%,SiO2 37%,Others 5%

Specific Gravity

4.0 g/cc

Bulk weight

2.4 g/cc

Mohs Hardness

>7

Degree of sphericity

>90%

Air content

0

Compression strength

>9710 bar


4.2 Partical size of Zirconium beads:

0.6~0.8 mm

1.2~1.4 mm

1.8~2.0 mm

0.8~1.0 mm

1.4~1.6 mm

2.0~2.2 mm

1.0~1.2 mm

1.6~1.8 mm

2.2~2.4 mm


4.3 Safety device

A contact pressure gauge is installed at the inlet and a contact thermometer is installed at the outlet, prevents the machine from starting without an grinding chamber or a front cover. Capacitive switches are used to monitor the cleaning fluid level in the mechanical seal, and contact pressure gauge are used to monitor the shaft seal pressure.

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

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