Ball mill liner and ball selection

First, about the liner

Traditional high-manganese steel ball mill liner unfit, which disadvantages are: creep and readily extensible, deformable surface against the bow of a phase change caused, in a ball mill not be obtained over the operating condition of work hardening. The former is the root cause of powder leakage and broken bolts, while the latter indicates that the expected anti-wear performance cannot be achieved. It must be stated here that even if partial work hardening is obtained, it cannot be used as a sign of the degree of wear resistance, because the work hardening here is a kind of hardening after "destruction", which is different from the initial hardness of the lining board. .

Hard means wear, which is indisputable. The 1970s and 1980s, a large number of domestic and foreign Release liner high hardness, ball materials, such as high chromium cast iron, Ni-hard cast iron, high carbon alloy steel, ductile iron martensite, and some medium carbon alloy steel and the like, but Since the toughness values ​​of these materials are relatively low, some 琢k values ​​are only about 5J, and they are not widely used due to the constraints of the traditional lining. The rupture of the lining has become the biggest obstacle to the application of high hardness and high anti-wear materials.

The fundamental error of traditional lining design has been unable to adapt to the development of anti-wear materials. Its installation structure mainly has two forms, one is the "dove tail groove occlusion, tightening wedge fixing" widely used in thermal power plants; the other is used in every lining widely used in mining, cement, chemical and large thermal power plants. The way the bolts are fixed to the cylinder respectively.

Whether the cooperation of the dovetail slot is ideal, we will not talk about it. Tightening the wedge above the liner will first be worn and the tightening wedge will function entirely from a "hanging bolt". Hanging bolts are the most unstable type of fastening. The most unstable lifting bolts and the first worn-out tightening wedges are used to stabilize all the liners of the mill. This is undoubtedly a mistake.

Then the lining plate is directly bolted? We don’t talk about whether it is reasonable and troublesome to put a thousand holes in the grinding machine manufacturing process, or talk about a thousand bolts to make a thousand leak points, and the worker twists a thousand big bolts. Is it hard? Just discuss whether a hole should be designed on the wear-resistant part. This hole will produce thousands of crack sources during casting and heat treatment. The hole is the most dangerous and weakest position of the workpiece. The weak position fixes the entire lining plate to withstand the enormous force of dozens of tons and more than one hundred tons of grinding balls in long-term operation, which is extremely unreasonable design. A large number of examples prove that the rupture of the lining is the main reason for the failure of the lining, and most of the rupture occurs through the screw holes.

In view of the characteristics of high-hardness materials, we analyzed and compared the structural forms of dozens of linings and the reasons for their wear and failure, and designed a "combined self-solid bolt-free ball mill liner" - Chen's liner!

The meaning of the combination has two aspects: one refers to the form of the lining structure; the second refers to the overall form of the lining installation. The meaning of self-solidation also has two aspects: one is that the lining itself is self-reinforcing; the other is self-tightening after installation.

In the design of the composite liner, the liner is composed of two or three separate parts of the plate and the rivet. The function of the plate body is anti-wear, and the material is a high-hardness alloy; the riveting plate is a tough alloy, which functions to stabilize the plate body and apply pre-stress to the plate body.

The combined self-solid lining plate utilizes the geometry of the mill itself, adopts the mechanical structure to carry out the bimetal combination, uses the ball mill to produce the grinding ball in the operation, carries out the pre-stress self-solidation, realizes the overall strengthening of the lining plate, thereby ensuring high hardness and low toughness. Material liner stability and reliability. The material of the combined lining is suitable for high-chromium cast iron, nickel hard cast iron, high and medium carbon alloy steel, medium and low chromium cast iron, martensite sphere and other high hardness materials.

At present, the composite lining has been widely used in more than 20 provinces and cities in China, and most of the power plants in Hebei, Hubei, Jiangsu, Guangdong, Shanghai and other provinces have adopted composite lining. The biggest advantage of the combination liner is that it can give full play to the maximum quenching hardness of the material used - the maximum anti-wear performance, and the maintenance is almost zero. The most efficient use of the limited resources on the earth should be called the “green lining”.

In addition, the liner has an important choice, which is the surface shape of the work surface. The interaction between the surface shape of the working surface and the grinding ball determines the activity of the grinding medium in the mill and determines the maximum grinding capacity of the ball mill.

Such as ball mill grinding coal mine and power plant corrugated liner in favor of active grinding media, but did not consider the maximum grinding capacity between the liner and the ball; cement the traditional ladder liner did not consider the liner and the ball The maximum grinding capacity between the two does not adjust the activity of the grinding medium in the mill to a more ideal state.

The author believes that the surface shape of the lining of power plants and cement plants should be combined. Because the change in the fineness of the material from the feed end to the discharge end is very large. Therefore, the movement of the grinding medium from the feed end to the discharge end should also be changed. The change of the movement mode is determined by the surface shape of the mill liner. Therefore, the choice of the surface shape of the power plant coal mill and the cement cylinder lining should be in combination. The feed end should be a combination of comminution and grinding, and the discharge end is relatively sharply ground.

The material of the ball mill liner of power plant and cement plant should be selected from materials such as chrome molybdenum alloy steel or high chromium cast iron with high carbon content. One problem to be noted here is the concept of "working conditions", not the higher the alloying, the better!

Since 1987, the author has achieved an increase of more than 5% in output through the transformation of hundreds of cement mills, up to about 20%, and noise reduction of 3 to 5 decibels. The benefit of increasing production by 5% is staggering because its cost is almost just raw materials. The shape of the surface of the lining is only one choice: to select the ideal grinding ball working state, and to pursue the maximum grinding capacity of the ball mill.

The material of the cement mill liner should be recommended for high alloys. For example: Cr12, Cr15, Cr18, Cr20, Cr24, etc., according to the working conditions of the mill and the economic situation of the enterprise. The use of these high-hardness, high-abrasion-resistant linings not only reduces maintenance, reduces labor intensity, but more importantly, these highly wear-resistant material linings can maintain the design state for a long time and maintain optimal working conditions. The largest range of increased production, creating maximum economic benefits. Especially in recent years, with the application of the pre-grinding roller press, the diameter of the largest grinding ball in the cement mill has dropped to about 30 mm, which provides good working conditions for the application of high hardness and high alloy liner.

The double-medium quenching medium and high carbon chromium-molybdenum alloy steel liner has a good cost-effective ratio in the cement mill, and the service life is generally about two years. In the 2.2m cement mill, the service life of the partition plate has also reached 18 months, which is ideal.

Even in power plant coal mills, I do not advocate the use of high manganese steel liners. Because the coal quality is relatively loose, the largest grinding ball in most power plant mills is 60mm. Therefore, in the coal mill, the high manganese steel liner is difficult to get better work hardening, and can not exert the expected anti-wear performance. The ideal lining of the coal mill of the power plant is medium and high carbon chrome molybdenum alloy steel or high chrome cast iron above Cr18.

Mills used in mines are generally short and thick. Such as 32/36, 43/63 and so on. The thickness of the liner is also relatively thick. Due to the large size of the mine mill, the grinding balls are also large, and most of them adopt the wet process. Therefore, the operating conditions of the liner are very harsh. The author suggests using some high-hardness materials as much as possible under the premise of reliability. For example, chromium-molybdenum alloy steel containing 0.4% to 0.9% carbon or cast iron material containing 10% to 18% chromium has a hardness of more than HRC50.

Large-scale grinding mills, self-grinding machines or semi-autogenous grinding machines in mines use stabilized austenitic high-manganese steel liners from a stable and reliable point of view.

Second, on the grinding ball

At present, the grinding balls widely used at home and abroad are mostly high-carbon high-alloy casting grinding balls and high-carbon low-alloy forging grinding balls (or casting grinding balls), and most of the dry production processes in thermal power plants and cement industries. Use high and low chrome cast iron balls. In the wet process of mining, high-carbon low-alloy grinding balls are often used.

There are many types of domestic grinding balls. Due to the defects of the market, the use of grinding balls is not determined according to the process benefits, and the randomness is very large. The main varieties are chromium-based high-alloy cast irons. Such as: Cr8, Cr12, Cr15, Cr18, Cr24, Cr26, etc., low chromium cast iron such as: Cr2 and martensitic ductile iron. Martensitic ductile iron and low-chromium cast iron are of the same grade in terms of wear resistance, and their differences are in the production process. The high-alloy cast iron has excellent anti-wear properties. The higher the alloy, the better the anti-wear performance and the good economic benefits. Some people had misunderstood, high chrome ball cement mill application is good, but not suited to power plants, thermal power plants in fact the country has a good record of the extensive use of high chromium ball, for example: Wuxi, Yueyang, Xingtai, Hengyang, Zou County, MA After the first-class power plants use high-chromium balls and no longer wish to use other balls, high-chromium cast iron grinding balls are rapidly spreading in power plants. In the dry grinding process of power plants, cement plants and other dry grinding processes, the ton wear of high chrome balls is only 15 to 50 grams. The use of high chrome balls not only saves costs, reduces worker labor, reduces mechanical and electrical consumption, but also reduces waste of resources. It should be unknowingly said: high chrome balls can be called environmentally friendly grinding balls and green grinding balls.

The varieties of forged balls and rolled balls are mainly high carbon chromium molybdenum alloys and medium and low carbon manganese alloys. Forging and rolling ball ball simple process, low manufacturing cost, economic considerations mine general class of low wet process ball chromium alloy, the wear is generally 500 to 700 g in iron ore, copper ore, molybdenite. Increasing the hardness of low chromium alloy balls is the most effective way to reduce wear. We have increased the hardness of low-chromium balls to about HRC55, and its wear in copper mines is around 550 grams. Due to the production process, the quality of the forged ball should be slightly better than the rolling ball.

If the diameter is greater than 110mm, I recommend the use of high carbon chromium molybdenum low alloy forged balls, mainly considering that the crushing rate of large diameter cast balls may be higher. The carbon content of the forged ball is generally about 1%, and the chromium content is less than 2%. The hardness of low-chromium cast balls should generally be around HRC48. The hardness of low-chromium cast iron balls should be increased as much as possible to improve economic efficiency. The hardness of the low chromium cast iron ball produced by the author's company and the Indian manufacturer is as high as HRC55, and the difference between the core and the surface hardness is very small. The diameter of the grinding ball in a thermal power plant is generally not greater than 60 mm. Cement and mines should be sized according to the diameter of the mill, the surface shape of the liner and the particle size of the feed. There is only one grading principle: to reduce the diameter of the grinding ball as much as possible, because the ball mill is a grinding machine, and if it relies on it to bear the breaking effect, even if it is only a small part, it is very inappropriate.

Under the premise of material selection, the grading of the grinding ball is very important. The ideal grinding ball grading plus the ideal lining surface shape allows the mill to achieve maximum efficiency. The correct choice of material is the fundamental guarantee for the ideal grinding ball grading, the ideal lining surface shape to maintain the best state for a long time.

Third, the liner and grinding ball

For a long time, a less correct statement has been widely spread, that is, the hardness of the grinding ball is higher than the liner by HRC3~5. Asked the reason, but nowhere to test.

The lining and grinding balls are undoubtedly a mutual relationship, and they form a three-body abrasive wear together with the materials. But what is better? It should be said that it is best to borrow the concept of "base hole system" or "base shaft system" in mechanical design. As far as the ball mill is concerned, the impact of the liner on the production is much larger than that of the grinding ball, so it should be "substrate system". Therefore, among the selection of the lining and the grinding ball, the lining should be determined first. The service life of the lining must be guaranteed over two overhaul periods, and maintenance should be avoided as much as possible. Priority is given to those liners that are highly reliable and resistant to abrasion. For example, the combination of self-solid high-chromium cast iron lining, the service life of the power plant coal mill can reach 20 years, the cement mill life is 10 years, and almost no maintenance.

In the double medium quenching, the alloy steel lining has a service life of more than 8 years in the coal mill of the power plant, 3 years of cement grinding, and about 10 months of iron ore. Even the double-medium quenched low-chromium alloy steel liner can last for more than 6 years in the power plant coal mill, the cement mill is about 2 years, and the service life of the iron ore mill is more than 8 months.

The hardness of the liner and the grinding ball should be above HRC50, mainly because the hardness of the hard phase in the abrasive is Mohs hardness of 6~7, which is estimated to be close to HRC60. Theoretically, the hardness of the grinding medium is greater than or equal to 8% of the hardness of the abrasive. Ten has the ideal anti-wear effect.

After the lining plate determines the grinding ball, it is easy to choose. As long as the service life of the lining plate guarantees two overhaul periods, it should be preferred that the anti-wear performance of the grinding ball is the best. If the liner and the grinding ball can be replaced for 10 years, then of course it is ideal. One principle: reduce costs, reduce the labor intensity of workers, and improve the operating rate of ball mills.

Under the premise of "substrate system", the grinding ball is allowed to have a hardness HRC3~5 higher than that of the lining plate. This is for the high-manganese steel lining plate that should be eliminated, and the concept of circulation is fundamentally different.

Fourth, choose the grinding ball countermeasures

1. High manganese steel material and traditional ball mill lining form are not suitable. The epoch-making design of combined self-solid lining plate has great superiority, completely solves the problem of broken bolts and powder leakage, and reduces maintenance by more than 98%. The combination lining can fully exert the quenching hardness of various high hardness materials, so that it can exert the best anti-wear performance.

2. In the dry process, power plants and cement plants should give priority to double-medium quenching medium and low chromium alloy steel liners or high chromium cast iron and high chromium cast steel liners, and select high hardness high chromium cast iron grinding balls. Double-medium quenching medium and low chromium alloy steel has good economic benefits; in the mine wet process, from the economic point of view, it is preferred to use high and medium carbon alloy steel liner or Cr10~Cr20 series cast iron with lower carbon content. Liner and high hardness low chromium alloy grinding balls. However, grinding balls with a diameter greater than 110 mm should be considered for high carbon alloy steel forging balls.

3. The matching of the lining plate and the grinding ball should be based on the viewpoint of “substrate system”. Under the premise that the service life of the lining plate is guaranteed to be two overhaul periods, the anti-wear performance of the grinding ball should be selected the best. The hardness of the lining plate should be greater than HRC50, the hardness of the chrome-molybdenum alloy casting grinding ball should be greater than HRC52, and the hardness of the wrought alloy steel grinding ball should be greater than HRC55. Generally choose the grinding ball with the best anti-wear performance.

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