Application of ball mill grading production line on silicon micropowder

Abstract: This paper mainly introduces the influence of various factors on the yield and quality of ultrafine processing of silicon micropowder in the silica micropowder production system of ball mill and classifier; and how to apply the cooperation between the two, media ball grading, classifier protection and Adjustment of parameters, etc. If applied properly, it has low energy consumption, high efficiency and good purity.

Key words: ball milling, grading, silicon micropowder, parameter matching, wear protection

Entering the 21st century, with the ultra-fine processing of non-metallic minerals in China, ultra-fine pulverization and grading technology has become one of Zui's important deep processing technologies, which is of great significance to the development of modern high-tech industries.

The complete set production line of ball mill plus classifier refers to the continuous working system that uses dry crushing and re-classification of materials; its output is large, the equipment is easy to operate, the maintenance cost is low, the grinding medium and liner are flexible, and the material is processed with high purity. The characteristics of small pollution, reliable operation of the whole equipment and stable product quality are obvious. It is especially suitable for ultra-fine processing of silicon micropowder, with high whiteness, good gloss and stable quality indicators. However, since the function and focus of the ball mill and the classifier are different, the coordination problem in the production system is particularly important. When properly matched, the functions can be achieved, the advantages are complementary, the efficiency is high, the cooperation is not good, the function is restricted, and the energy consumption is high. ,low efficiency.

The process is generally:

Since ball mills and classifiers are usually produced by two companies, there are often differences in concepts such as production. The output of the ball mill is affected by various conditions such as the feeding fineness, the effective diameter after the lining of the grinding body, the rotation speed of the ball mill, the selection and grading of the grinding medium, the loading amount, the effective length of the grinding body, the feeding amount, and the like. The output should be affected by various conditions such as powder concentration, turbine classifier speed, air volume wind pressure, classification efficiency, particle size distribution, product fineness, etc. The two devices should have the following relationship:
The output of the classifier = the capacity of the classifier - the output of the graded ball mill after the classification = the feed amount of the raw material + the return amount of the coarse material after the classification, the processing amount of the classifier = the output of the ball mill, so it is necessary to select and design the equipment. Consider the match between the two.

1. The influence of various factors of ball mill on the output and quality of silicon micropowder

1.1 Effective diameter and ball mill speed after ball mill lining

The lining plate used in the ball mill is generally made of alumina lining and ball lining. The ball milling media are also mostly alumina ball and ball stone. Regardless of which liner and liner are used, the effective diameter of the ball mill will be reduced. In the case where the mill speed has been determined, the smaller the effective diameter, the worse the grinding effect and the lower the yield. Therefore, when selecting the diameter of the barrel of the mill, the problem of the effective diameter reduced by the lining plate should be fully considered to prevent the parabola of the material from being reduced in the grinding, which is not conducive to material grinding.

1.2 Selection of grinding media and liners

The selection of grinding media and liners is also a very important factor that restricts the quality and yield of silicon micropowder. The use of alumina lining and media ball has the advantages that the alumina lining plate is thin, the effective diameter inside the grinding cavity is large, and the media ratio is large, which is beneficial to the pulverization and grinding of the material. The output is correspondingly higher than that of the ball lining and the ball. The disadvantage is to increase the percentage of Al2O3 in the quality index of the silicon micropowder. Due to the different degrees of wear of the alumina lining and the ball, the service life is limited, so the cost is increased. Moreover, the degree of heat generation during grinding is higher than the temperature of the ball lining and the ball, and the whiteness of the silicon powder is lowered due to the increase of the grinding temperature, thereby affecting the product quality of the silicon powder.

The use of ball lining and media, due to the thickness of the lining, the ball mill's effective diameter is reduced by more than 20 cm, and the contact between various spheroidal media is very irregular, and even some face-to-face contact, It is not conducive to the grinding and grinding of materials, affecting the production of silicon micropowder. Since the ball and the lining plate are made of ball stone, the service life is generally 30kg per ton of ball stone. Therefore, it is necessary to regularly add the medium ball stone to the ball mill, but the wear amount can be converted into the silicon powder product, so there is basically no cost. cost. Since the main component of the spheroid itself is SiO2, it does not cause changes in the material quality index, and the heat generation rate is lower than that of the alumina lining and the medium ball, and does not affect the quality of the silicon powder. In addition, the price of the ball is low and the investment is small.

1.3 Grading and filling rate of grinding media

The gradation and filling rate of the grinding media of the ball mill will also affect the yield of the silicon micropowder, and the gradation and filling rate should be generally in the following range. (taking ball stone as an example)

Ball mill grinding body four-level map

Ball mill grinding body three-level map

Ball mill filling rate calculation:
H/D=0.64 0.63 0.62 0.61
¢=35.8 36.2 37.8 38.8
D: effective diameter ¢: filling rate (%)
Abrasive load calculation:
G: grinding body loading t
L: effective length of the ball mill
P: the bulk density of the abrasive body depends on the size of the media ball
P average = 2.7K (K is the space factor K ≈ 0.7)
P average = 2.7 × 0.7 ≈ 1.89
Average ball diameter:
D ball = D1G1+D2G2+D3G3+.../G1+G2+G3+...
D1-3 medium diameter media balls
G1-3 Quality of each media ball

2. Influence of various factors of grader on the yield and quality of silica micropowder

2.1 Main adjustment parameters of the classifier

The turbine classifier of the classifier rotates to form a stable centrifugal force field, and adjusts the particle size of the product by adjusting the number of impellers of the classifier, the impeller speed, the wind pressure of the induced draft fan, the air volume and the size of the secondary air purifier. And the output, if the air volume is too small, the product size is easy to be thinner, the output is reduced, the number of impeller blades is too small, it is easy to form a high rotation speed, which is not conducive to grading, the product quality is reduced; the secondary hood pottery opening is too small, The fine powder contained in the coarse material is not cleanly extracted, which affects the classification efficiency.

2.2 Wear resistance of classifier

Due to the high hardness of quartz, the equipment is seriously worn and the products are contaminated. The equipment must be wear-resistant and protective, especially the electronic grade high-purity silicon micro-powder. The impurity content is less than several ppm, and the protection method of the equipment is very important.
It is very effective and reliable to use ceramic or organic wear-resistant materials for the static parts of all cavities and pipes. However, due to its high-speed rotation, the graded rotor has a complicated structure, and the wear and tear is serious, and the protection is difficult. At present, the rotor of the vertical classifier is difficult to be ceramized due to its large volume; and the balance after the tile is lowered, the tile is easily detached at high speed, the service life is shortened, and the reliability is lowered. Horizontal classifier rotors are small in size, usually using integral ceramic wheels or alloy wheels, but because of their relatively high rotational speed, the wear is still heavy, there is still some metal pollution to the product, and the impeller is expensive, especially the multi-rotor structure. The replacement repair cost is high and the running cost is high.

Qingdao Mai Cologne Powder Technology Equipment Co., Ltd. uses a special polymer wear-resistant material to make the graded impeller. The limit wear amount allowed under the good balance operation is greatly improved, and only the local wear part is replaced, so it is made compared with the alloy impeller and the ceramic impeller. The cost is greatly reduced, and the service life is greatly improved compared with the surface spray impeller and the plastic impeller, thereby greatly reducing the running cost, and the zero metal pollution of the material has a very high cost performance. It has been widely used in Anhui, Jiangsu, Shandong, Hebei, Hunan, Liaoning, Shaanxi and other places, and has been updated and modified for users of several original classifiers. Through long-term operation comparison, taking crystalline silicon micropowder as an example, the maintenance cost of sorting 10,000 tons of 320 mesh products is about 0.3 million yuan, which is only a dozen of other impellers.

3, the cooperation between the ball mill and the classifier

In the dry superfine pulverizing and grading production system composed of the ball mill and the classifier, because the ball mill has a large output and a high powder concentration, the powder completely relies on the negative pressure suction, and enters the centrifugal force field grading area formed by the rotation of the classifying impeller to classify; And the products that have been ground in the ball mill cavity should be discharged out of the mill in time, so the air consumption is large and the pressure loss is increased. The air volume indicator should be fully considered when selecting the draft fan to form the required stable wind pressure. In the dry superfine pulverizing and grading silicon micropowder production line of ball mill and air classifier designed and manufactured for Anhui Fengcheng Powder Material Factory and Fengyang Lida Powder Material Factory, the pulse bag collection with large filtration area is adopted. The impeller rotor with special material is wear-resistant and pollution-free, which improves the production quality of silicon micropowder. Correctly adjust the air volume and wind pressure of the whole system, so that the qualified products in the mill can be taken away in time, the grinding efficiency of the ball mill is improved, the efficiency of product classification is high, and the output is large. Continuous production and processing of 10,000 tons of system stability, the effect is obvious.

4, the correct connection method can improve the efficiency and stability of the system

Since the ball mill is always in a high concentration discharge state during the continuous operation of the ball mill, and the ball mill is in operation, the grinding chamber is always in a semi-negative pressure state, so the classifier is mostly used in the connection with the discharge pipe. Directly connected, leaving a secondary air inlet, and adding an air volume adjustment valve to adjust the air volume and wind pressure in the mill; and the material dispersion in the classifier feed pipe. (as the picture shows)

If the opening degree of the regulating valve is too small, it will easily cause the wind pressure in the ball mill to be too large, forming a "wind sweep", which affects the grinding effect in the ball mill; it is also not conducive to the material conveying by the classifier, and it is easy to form a blocking material. If the opening is too large, the material in the mill will be over-grinded and the output will be reduced.

In short, in the dry superfine pulverizing and grading production system consisting of ball milling and grading, the correct coordination and coordination of the two is particularly important. If the application is unreasonable, it will form functional constraints, high energy consumption and low efficiency. When properly coordinated, we can give full play to our respective advantages, so that the advantages of the entire system are complementary, with more effort and greater efficiency.

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