The Ultimate Guide to Ball Mills

2019-08-14 18:20:34XinHai

Ball mill is a common grinding equipment in concentrator, the main role is to provide qualified grinding products for the subsequent dressing process.

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The ball grinding process is a process in which the steel ball rotates in the cylinder at a certain speed and rises with the wall of the cylinder. After rising to a certain height, it will hit the raw material under the action of gravity, and then rises with the wall of the cylinder again and again to grind the raw material. 

Working principle of ball mill

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The ball mill is usually composed of a horizontal cylinder, a hollow shaft and a grinding head. The barrel is a long cylinder, which is equipped with ball grinding medium. The barrel is made of steel plate and fixed by lining plate and barrel. Grinding medium is generally steel ball, and according to different diameters and a certain proportion into the cylinder.

According to the size of the grinding material, the material is loaded into the barrel by the hollow shaft at the feeding end of the ball mill. When the ball mill barrel rotation, ball grinding medium due to inertia, centrifugal force, friction, so that it attached to the barrel liner is taken away by the barrel. When it is brought to a certain height, it is dropped due to its own gravity, and the falling ball grinding medium breaks the materials in the cylinder like a projectile.

Materials from the feeding device through the feeding hollow shaft screw evenly into the first bin of the mill.There is a stepped lining board or corrugated lining board in the bin, and steel balls of various specifications are installed inside. Centrifugal force is generated by rotation of the cylinder body to bring the steel balls to a certain height and then they fall, causing heavy impact and grinding effect on the materials. After the material is roughed in the first bin, it enters the second bin through the single compartment board, which is inlaid with flat lining board and contains steel balls to further grind the material. The powder is discharged through the unloading grates to complete the grinding operation.

The ball grinding medium also slips during the rotation of the cylinder. The media slip will grind the material. When the grinding of large particles, in order to effectively use the grinding effect, the grinding body cylinder with a compartment plate separated into two sections, that is, double bin. When the material enters the first warehouse, it is broken by steel ball. When the material enters the second warehouse, the steel section grinds the material. The material with fine grinding quality is discharged from the hollow shaft at the discharge end. When grinding materials with small feed particles, such as sand no. 2 slag and coarse fly ash, the cylinder of the mill can be separated into a single silo grinding,and the grinding body can also be steel section.

Raw materials are ground through a hollow shaft neck into a hollow cylinder containing grinding media of various diameters(steel balls or gravel, etc.). When the cylinder rotates at a certain speed around the horizontal axis, the medium and raw material installed in the cylinder will reach a certain height along with the cylinder under the action of centrifugal force and friction force. When its own gravity is greater than the centrifugal force, it falls or rolls down from the inner wall of the cylinder, crushing the ore. At the same time, in the process of mill rotation,the sliding motion of grinding medium also produces grinding effect on raw materials. The ground material is discharged through the hollow shaft neck.


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Common Types of Ball mill

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The common industrial ball mills mainly include cement ball mill, tubular ball mill, ultra-fine laminating mill, cone ball mill, ceramic ball mill, intermittent ball mill, overflow ball mill, grid ball mill, wind discharge ball mill, double bin ball mill, energy saving ball mill.

1. According to the shape of the ball mill cylinder (the ratio of length to diameter) :

Short cylinder ball mill: the cylinder length(L) is less than two times of the cylinder diameter(D),that is, the ball mill with L≤2D is the short cylinder ball mill. It is usually a single-bin structure, mainly used for coarse grinding or first-stage grinding. Because of its high operation efficiency, two or three ball mills can be used in series at the same time, which has a wide application.

Medium cylinder ball mill: The cylinder length of the ball mill is equal to three times of cylinder diameter.

Long cylinder ball mill: The cylinder length of the ball mill is equal to or larger than four times of cylinder diameter. Generally, the long cylinder ball mill is equipped with two or four bins.

2. According to the grinding medium loaded in the ball mill:

The grinding medium loaded into the steel ball mill is mainly steel section or steel ball. This ball mill has large grinding intensity, light structure and stable rotation speed.

The grinding medium in the gravel ball mill mainly includes pebbles, gravel, sand, porcelain balls,etc. The gravel ball mill adopts porcelain material or granite as the lining board, which is widely used in the field of colored cement, white cement and ceramics.

3. According to the discharging method of ball mill:

End discharge ball mill: This ball mill takes its head and end as the inlet and outlet respectively.

Central discharge ball mill: the inlet of medium unloading ball mill is at both ends, and the outlet is in the middle of the ball mill.

4. According to the ore discharge of ball mill:

Overflow ball mill: Discharge the ore through hollow shaft.

 Grid ball mill: Discharge the ore through the plaid plate.

Peripheral ball mill: Discharge the ore through the periphery of the cylinder.

Center driving ball mill: The driving power end of this ball mill is in the center of the ball mill. And the motor realizes the operation of the ball mill through reducer. During the operation,the hollow shaft in the center of the ball mill drives the mill body to rotate under the driving of the power system.

Edge driving ball mill: the motor drives the gear on the edge of the cylinder through the reducer to drive the cylinder movement.

6. According to the operation characteristics of ball mill:

Wet ball mill: Adding water when feeding, the discharging material is discharged when it is in a certain concentration of slurry. The wet ball mill forms the closed circuit operation with the hydraulic classification equipment in the closed circuit system.

Dry ball mill: Some discharging material is extracted with the air,and the ball mill forms the closed circuit with the wind classification equipment.


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The Grinding Media Of Ball Mill – Steel Balls

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The steel balls in ball mill are consumable, which need to be supplied from now and on.

1. The filling rate of steel balls

Steel balls undertake the main job in grinding, therefore, the number of steel balls in ball mill is highly relative with the grinding strength. 

In terms of physical phenomena, the number of steel balls are large, the grinding area is large as well. On the contrary, the number of steel balls are small, the grinding strength is also small. In terms of mechanics phenomena, the more steel balls in ball mill,the bigger consumed power of ball mill, the bigger ball mill productivity.

Besides, there are mine and water in ball mill, so the practical power is different with idea power in a gap. When the ball mill is in high-speed rotating, the steel balls are in throwing-down state, which is in a complicate situation.

Under a certain filling-in rate, the steel balls will go into throwing-down state with the larger and larger rotating speed. However, under different filling-in rate, the rotating speed for the change is different. The bigger filling-in rate, the higher rotating speed for the changing. 

Above all, we can know that, under different rotating speed,there are different suitable filling-in speed for steel balls, rather than the higher the better. The suitable filling-in rate is determined by mineral processing experiment. 

2. The size of steel balls

Mine was grinded in ball mills depending on the impact breaking force from steel ball throwing down and the friction between mines. The impact breaking force, the grinding capacity and the ball mill capacity are relative with the size of steel balls. 

The experiment results show that, at the same amount, the size of steel balls is in direct proportion to the impact breaking force, the grinding capacity and the ball mill capacity. It is inversely proportional to the single steel ball energy and grinding fineness in a certain scope. If using too large steel balls, the grinding fineness can’t be reached. 

Therefore, in the case of larger hardness, larger particle size and larger grinding particle size, appropriately increasing the size of the steel ball can significantly increase production, energy saving and consumption reduction.

3. the material of steel balls

We all know that, the steel balls are easily damaged. The different material of steel ball has different quality and hardness, and also different grinding damaging.

Now, the main materials of steel balls are as follows:

 

Common steel ball materials

Specific

1

Low chromium cast ball

Hardness, impact force, and breaking rate are relative with the metallographic structure of steel.

The higher chromium content, the better wear-resistance.

2

Middle chromium cast ball

3

High chromium cast ball

4

Forging steel ball

Large impact toughness;

Not easy to cause deformation;

Low broken rate.

5

Hot rolled steel ball

New technology, geometric circle;

Size precision;

High hardness, and good mechanical property;

Wear-resistance, not easy to cause deformation and cracking.


In summary, cast ball is suitable for dry grinding. Cast ball is common grinding medium for ball mill. Hot rolled steel balls are replacing cast balls little by little as high quality steel ball. When choosing the material of steel balls, it should be decided by minerals and economic situation of processing plant.

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How to Add Steel Balls into Ball Mill Correctly?

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As one of the commonly used equipment in mineral processing plants, the number and proportion of steel balls in the ball mill affect not only the grinding efficiency and production capacity of the ball mill, but also the service life of the ball mill.

During grinding operation, the grinding efficiency of the ball mill generally depends on the sum of the work done by each steel ball (ie, the number of impacts of the steel ball and the impact force generated by the steel ball).

Therefore, in order to give full play to the grinding efficiency of each steel ball, it is necessary to ensure that the steel balls moving along different trajectories do not collide as much as possible.

When the number of steel balls in the ball mill is too large,the steel balls will be pressed against each other, so that some steel balls would fall before reaching the required height, and the grinding ability cannot be fully exerted. And at the same time, too many steel balls would increase the kinetic energy consumption and shorten the service life of the ball mill.

When the number of steel balls in the ball mill is too small, although the grinding capacity of each steel ball is maximized, the overall grinding capacity is limited, thereby affecting the grinding efficiency of the ball mill.

Therefore, maintaining a reasonable balance of the amount of steel balls is a very important part when using ball mill. So, what is the most suitable number of steel balls in the ball mill during the operation of the ball mill?

Assuming the rotation speed of the ball mill is fixed, when the volume of the steel ball accounts for only 10% of the volume of the ball mill cylinder, the steel balls in the ball mill mainly moves in the form of sliding, and there is no any tumbling phenomenon.

When the amount of steel balls gradually increases up to 30%,the steel balls in the ball mill are still mainly sliding, but the phenomenon of tumbling has begun to occur.

When the steel ball volume exceeds 40%, the steel ball in the ball mill will be mainly in the form of tumbling.

When the amount of steel balls accounts for 50% of the total volume of the ball mill cylinder, the surface of the steel ball is on the diameter line of the cylinder of the ball mill, so it is easy to produce tumbling phenomenon, which is the condition required for the production of the ball mill.

Therefore, it is more appropriate to maintain the steel ball volume between one-third and one-half, and nearly one-half is the most ideal.

According to the experimental results, when the filling amount of the steel balls in the cylinder of the ball mill is 40%, the capacity of the ball mill is the highest. But in actual production, the filling rate of the ball in the cylinder of the ball mill is mostly about 35%, which can keep the volume of the steel balls in the cylinder slightly lower than the horizontal centerline of the ball mill.

Now, we have known about the optimal number of steel balls which can ensure the ball mill's grinding efficiency and output.

In the actual production of mineral processing plant, after determining the appropriate number of steel balls, we also need to choose the appropriate proportion of ball loading, including the size of the steel ball,the different grades of ball diameters, and the proportion of steel balls of various specifications.

Reasonable steel ball grading is the key to maximize the grinding capacity of the ball mill, which will have a direct impact on the grinding efficiency, and then affect the ball mill output.

In the production of the processing plant, the more common steel ball ratio of ball mill is two-stage ratio and multi-stage ratio. Generally speaking, the multi-stage ratio follows the principle of less large and small steel balls and more medium-sized steel balls, namely, less steel balls at both ends, and more in the middle. And the so-called two-stage ratio is to use two kinds of steel balls with different sizes and large diameter difference for grading.

Usually, the gap between the large steel balls is filled by small steel balls to sufficiently increase the bulk density of the steel balls. The large steel balls can impact crush the material, which greatly improves the impact capacity and impact times of the ball mill. And the small steel balls fill the gap between the large steel balls, increase the bulk density of the steel balls, and control the material flow rate well. At the same time, the coarse particulate material in the void is extruded in time to be placed in the impact zone of the large steel balls.

However,if there are too many large steel balls in the cylinder of ball mill, the grinding particle size will be too coarse, that is under-grinding. On the other hand, if there are too many small steel balls in the cylinder of ball mill, the surface area is increased, resulting in over-grinding phenomenon. Moreover, due to too few large steel balls, the impact force is insufficient, which may cause over-grinding and under-grinding. Therefore,reasonable steel balls loading ratio can improve the grinding efficiency of the ball mill.

In short, the number and ratio of ball mill steel balls is a relatively complicated technical problem. Each processing plant must carefully analyze its actual situation. By consulting professional ball mill manufacturers and long-term research, accumulation to find out the values suitable for their own plants, which must be just right and without any deviation.

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Ball Mill Liner

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Theshape of the ball mill liner has a great influence on the work of the ballmill. At present commonly used ball mill liners are wedge shape (tower shape),wave shape, convex shape, smooth shape, ladder shape, strip shape and so on.

Whenthe coarse material is processed with ball mill, the ball mill liner should bewedge shape, convex shape, ladder shape or strip shape.Because they can raise the grinding media to a higher height,thereby enhancing the impact of the media and improving the production capacityof the ball mill, which are often used in the primary grinding.

Whenthe ball mill for fine grinding, it is preferable to use a smooth or wave ballmill liner. Because they make the medium slide androll, the grinding and stripping action is strengthened, which is suitable forthe second grinding.

Usuallythe thickness of the ball mill liner is determined by the diameter of the ballmill, typically 50-150 mm.

The smooth ball mill liner is used for the fine grinding bin, and the rising height of the grinding media depends on thestatic friction coefficient with the ball mill liner.

The strip ball mill liner is used in the first bin, which can make the grinding mediarise higher and have greater impact energy.

The convex ball mill liner is similar to the strip ball mill liner.

The wave ball mill liner is suitable for rod and ball grinding.

The ladder ball mill liner is suitable for crushing bin. Itis widely used, which can raise the same balls to the same height and wears thesurface of the liner evenly. Moreover, the ball mill liner pinning ability actson the grinding media of the other layers, reducing the sliding wear between the liner and the outer most grinding media, and preventing sliding and wear between the different levels of the grinding media.

The hemispherical ball mill liner is suitable for crushing bin.

The wave ball mill liner is a boltless liner for fine grinding, with small peaksand tightness.

The classifying ball mill liner allows the steel balls to be automatically classified.

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How to Change Ball Mill Liner?

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The ball mill is an indispensable equipment in the beneficiation operation, because it is necessary to grind the ore to a suitable selected particle size before extract high-grade concentrate from the ore. Needless to say, it takes a lot of time and effort to choose a suitable ball mill, from the pre-purchase consultation, to the smooth installation, and to the test run of the ball mill. Moreover, the price of ball mills is relatively expensive among many beneficiation equipment. Therefore,after successfully purchasing a ball mill, it is necessary to carry out daily maintenance and maintenance of the ball mill. The replacement of the ball mill liner is an indispensable maintenance work.

Under normal conditions, if it is not improperly operated and maintained, the ball mill will not be easily damaged. Of course, when the ball mill works for a long time, the barrel and the lining of the ball mill will also have some wear. When the lining wear depth is serious, it will seriously affect the working efficiency and safe operation of the ball mill. At this time, it is necessary to replace the new ball mill liner. The ball mill liner is intended to protect the barrel of the ball mill in order to reduce and avoid direct impact and friction of the body and abrasive minerals on the barrel of the ball mill. The form of the ball mill liner is not uniform. Different types of liner can be used to adjust the movement state of the grinding body, improve the grinding efficiency of the ball mill, and reduce the consumption of metal. Therefore, the protection effect of the ball mill liner on the ball mill cannot be ignored. Xinhai mining and dressing experts with more than 20 years of mineral processing experience believe that when the wear of the ball mill liner is very serious, replacing the ball mill liner is an effective method to solve the problem of liner wear. So how do you replace the ball mill liner? Next, we will briefly explain the correct ball mill liner replacement method from the entire process of ball mill liner replacement.

1.  Before installing the ball mill liner, you need to understand the performance of the ball mill liner. Consider the material and shape of the ball mill liner, wear resistance, impact resistance, etc., and choose the appropriate liner of the original ball mill to replace it. Xinhai mining experts suggest that new ball mill liners can be ordered from the original manufacturer.

2. The replacement of the ball mill liner requires a commanding expert to arrange and coordinate the entire replacement process. Moreover, all ball mill operating workers need to understand the job drawings and disassembly and installation requirements, and the operation of the entire process can be fully understood.

3. It is necessary to circulate the remaining slurry in the ball mill as much as possible, remove the rubberpad of the waste liner, clean the protruding point of the wall of the ball mill, and remove dust and descale the cylinder. When cleaning, it is necessary to fix the barrel of the ball mill. The work shop needs to have good ventilation and ventilation conditions, and the temperature is suitable. Then the installer can enter the barrel of the ball mill to start the operation.

4. When removing the old ball mill liner, first remove the used liner screws, then remove the old ball mill liner in one line and one line, and then lift the used ball mill liner. When removing the ball mill liner, the personnel should pay attention to safety, pay attention to the position, and avoid being injured by the ball mill liner.

5. The size of the ball mill is large, and it is necessary to carry out lifting work during the process of replacing the ball mill liner. The hoisting ball mill liner is the responsibility of the hoist. The lifting process requires attention to the operation of the wire rope and the hook.

6. The new ball mill liner should be installed to ensure that the gap is within the required range. Fix the spiral of the liner. Carefully seal the packing and gasket to prevent leakage of mineral powder. If there is no gasket, it should be wrapped around the corresponding position. Two rounds of cotton cord or hemp plus lead oil. At the same time, it is necessary to apply a layer of cement mortar on the inner wall of the cylinder, and screw it tightly before solidification. It should be noted that the thin end of the step liner should be consistent with the direction of the arrow in the direction of rotation of the ball mill.

7.  After the replacement, the ball mill operator needs to confirm the unmanned, working tools and excess debris in the cylinder,and must carefully check the inside and outside of the cylinder before closing the empty cover.

The replacement of the ball mill liner is a very cumber some and technically high-quality work. In addition to the above mentioned replacement techniques, the personal safety of the staff is particularly needed. Correct ball mill liner replacement operation can improve the use time of the ball mill and ensure the safe operation of the ball mill. In order to improve the safety factor of the use of the ball mill, Xinhai Mining and dressing experts suggest that the replacement process of the entire ball mill liner should be properly guided and participated by the technicians of the ball mill manufacturer.

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What is the Critical Speed of Revolution of Ball Mill?

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The outermost ball in the ball mill just rotates with the cylinder without falling, and the number of revolutions of the ball mill at this time is called the critical speed of revolutions. The critical speed of revolutions of various ball mills is proportional to its diameter D. It is not easy to determine the absolute value of the critical speed.At present, only the formula for calculating the theoretical critical speed of revolutions is widely used.

The formula for calculating the critical speed of revolutions:

N0=42.2/√ ̄D

In the formula:

D—the inner diameter of the ball mill, M;

N0 - critical speed of revolutions, r /min.

When the non-smooth ball mill liner is used, the grinding ball filling ratio is 40-50%, and the grinding concentration is relatively large, the calculation result of the formula is relatively close to the actual one.


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