What is the ball mill?
A ball mill is key equipment to crush the material after it is crushed. This type of mill is filled with a certain number of steel balls as a grinding medium in its cylinder.
It is widely used in cement, silicate products, new building materials, refractories, chemical fertilizer, black and non-ferrous metal mineral processing, glass ceramics and other production industries, dry or wet grinding of various ores, and other grindable materials. Ball mill is suitable for grinding all kinds of ores and other materials. It is widely used in mineral processing, building materials, and the chemical industry. It can be divided into dry grinding and wet grinding. According to the different ways of ore discharge, it can be divided into grid type and overflow type.
Ball mill working principle
The ball mill is composed of a horizontal cylinder, feeding and discharging hollow shaft, grinding head, and other parts. The cylinder is a long cylinder, which is equipped with a grinding body. The cylinder is made of steel plate, which is fixed with the cylinder by steel liners. The grinding body is generally a steel ball, which is loaded into the cylinder according to different diameters and a certain proportion. The grinding body can also be used as a steel section. The grinding material is selected according to the particle size. The material is loaded into the cylinder by the hollow shaft at the feed end of the ball mill. When the cylinder of the ball mill rotates, the grinding body is attached to the liner of the cylinder due to the action of inertia, centrifugal force, and friction, and is taken away by the cylinder. When it is brought to a certain height, it is thrown down due to its own gravity, and the falling grinding body is like a projectile The material in the cylinder will be broken.
The material enters the first bin of the mill evenly through the feeding device through the feeding hollow shaft screw. There are stepped lining plates or corrugated lining plates in the bin, which are filled with various specifications of steel balls. The rotation of the cylinder generates centrifugal force to bring the steel balls to a certain height and then fall down, which has a heavy impact and grinding effect on the material. After rough grinding in the first bin, the material enters the second bin through a single-layer partition board. The bin is inlaid with a flat lining board and steel balls to further grind the material. The powder is discharged through the discharging grate plate to complete the grinding operation.
During the rotation of the cylinder, the grinding body also has the phenomenon of sliding. In the process of sliding, the material is ground. In order to make effective use of the grinding effect, the grinding body cylinder is divided into two sections with a partition board when the material is finely ground with 20 mesh. That is to say, it becomes a double bin. When the material enters the first bin, it is broken by the steel ball. When the material enters the second bin, the steel section has no effect on the material For grinding, the qualified materials are discharged from the hollow spindle at the discharge end. When grinding the materials with small feed particles, such as sand No.2 slag and coarse fly ash, the mill cylinder can be a single bin cylinder mill without partition, and the grinding body can also be made of the steel section.
The raw material is fed into the hollow cylinder through the hollow Journal for grinding. The cylinder is filled with grinding media of various diameters (steel ball, steel bar or gravel, etc.). When the cylinder revolves around the horizontal axis at a certain speed, the medium and raw materials in the cylinder will fall or roll off the inner wall of the cylinder when the gravity is greater than the centrifugal force under the action of centrifugal force and friction, and the ore will be broken due to the impact force. At the same time, in the process of mill rotation, the sliding motion of grinding media also produces a grinding effect on raw materials. The ground material is discharged through the hollow journal.
Ball mill classification
Classification by length to diameter：
- Short cylinder ball mill: the length of the cylinder L is less than 2 times the diameter of the cylinder D, that is, the ball mill with L ≤ 2D is a short cylinder ball mill, which is usually of a single bin structure. It is mainly used for rough grinding operations or primary grinding operations, with high operation efficiency. It can realize the simultaneous use of 2-3 ball mills in series and has a wide range of applications.
- Medium cylinder ball mill: when the cylinder length L = 3D, it is a medium ball mill.
- Long cylinder ball mill: when the cylinder length L ≥ 4D, it is a long ball mill. It is generally divided into 2-4 warehouses.
Classification by discharge method:
- Tail discharge ball mill: the tail discharge ball mill takes its head and tail as the inlet and outlet of its abrasive respectively. When the ball mill is working, the staff will feed the abrasive from the inlet end, and then unload it from the other end.
- Medium discharge ball mill: the inlet of the medium discharge ball mill is at both ends, and the outlet is in the middle of the mill. The workers usually feed the abrasive from both ends and then discharge it from the middle of the cylinder.
Classification by discharge mode:
- Overflow type: discharge ore through the hollow shaft.
- Grid type: discharge ore through grid plate.
- Peripheral type: discharge ore around the cylinder.
Classification by transmission mode:
- Central drive ball mill: the power end of this kind of mill is in the center of the machine body, and the motor realizes the operation of the ball mill through the reducer. During operation, the hollow shaft in the center of the ball mill drives the grinding body to rotate under the drive of the power system.
- Edge drive ball mill: the motor drives the gear at the edge of the cylinder through the reducer to drive the cylinder.
Classification by operation characteristics:
- Wet ball mill: feed at the same time add water, discharge into a certain concentration of slurry and discharge, in the closed-circuit system and hydraulic classification equipment to form a closed-circuit operation.
- Dry ball mill: Some discharge materials are extracted by airflow, and the mill and the wind classification device form a closed circuit. For example, the cement mill adopts self-flow discharge.
Ball mill maintenance
The maintenance and overhaul of the ball mill is a regular work, the quality of the maintenance work directly affects the operation rate and service life of the ball mill, so how to correctly maintain and overhaul in the process of use is introduced as follows:
- All lubricating oil should be drained out when the ball mill is put into continuous operation for one month, cleaned thoroughly, and replaced with new oil. In the future, the oil will be changed every 6 months in combination with intermediate maintenance.
- Check the lubrication condition and oil level height of each lubricating point at least every 4 hours.
- When the mill is running, the temperature rise of lubricating oil of the main bearing shall not exceed 55 ℃.
- When the mill is in normal operation, the temperature rise of the drive bearing and reducer shall not exceed 55 ℃ and the maximum temperature shall not exceed 60 ℃.
- The large and small gears are driven smoothly without abnormal noise. Adjust the clearance timely if necessary.
- The ball mill is running smoothly without strong vibration.
- The motor current shall be free from abnormal fluctuation.
- All the connecting fasteners are free of looseness, and the joint surface is free of oil leakage, no water leakage, and no ore leakage.
- Steel balls shall be added in time according to the wear condition.
- If abnormal conditions are found, stop grinding and maintenance immediately.
- The mill liner plate should be replaced when it is worn by 70% or has 70mm long crack.
- Replace the liner plate if the bolt of the liner is damaged and the liner plate is loose.
- The main bearing should be replaced when it is seriously worn.
- The grate plate of the grid ball mill should be replaced when it is worn to be welded and repaired.
- The gear face of the big gear can be turned over for further use after it is worn to a certain extent.
- The pinion should be replaced for severe wear.
- When the screw of the feed and outlet is worn, it shall be welded timely and replaced if it is worn to be unable to repair.
- The anchor bolts shall be repaired in time if they are loose or damaged.
The maintenance of the ball mill is regular work, and the quality of maintenance directly affects the operating rate and service life of the ball mill. In order to find out the defects and eliminate the hidden dangers in time to ensure the normal operation of the mill, in addition to daily maintenance, it is necessary to stop grinding regularly (it is recommended to check the important parts such as hollow shaft, main bearing, cylinder, reducer, big and small gear, etc.) carefully and make detailed records. According to the defect situation, the appropriate treatment and arrangement of medium repair and overhaul plan shall be made according to the priority.
Common problems and solutions
Problem 1: When the ball mill is running, there is a regular knocking sound, and the sound is very loud.
Solution 1: It is that some of the liner bolts are not tightened. When the ball mill rotates, the mill liner hits the ball mill barrel. Judge the parts of the mill liner of the ball mill according to the sound, find out the loose bolts and fasten them separately.
Problem 2: The temperature of the ball mill and motor bearing is higher than the specified.
Solution 2: Try to touch the bearing by hand, which is partial or full temperature is too high, check and handle the ball mill from the following points:
- Whether the lubricating oil grade used is consistent with the factory instruction of the equipment.
- The lubricating oil used has deteriorated.
- If the lubricating oil does not enter the lubricating point directly, the oil quantity is insufficient and the heating will be caused.
The above problems should be dealt with according to their causes. Only if the side clearance of the bearing bush is too small or the bottom contact angle is too large, the grinding cylinder must be jacked up with an oil jack, and the bearing bush should be pulled out from one side of the shaft and scraped separately.
Problem 3: Bearing heating of ball mill reducer.
Solution 3: In addition to checking the bearing temperature rise of the ball mill, check whether the exhaust hole of the reducer is blocked and dredge the exhaust hole.
Problem 4: Vibration occurs when the motor speed reducer of the ball mill is started.
Solution 4: Adjust the clearance between the two wheels to make the two shafts concentric. Fasten the coupling bolts symmetrically with the same torque. When the rotor is unbalanced, the rotor of the ball mill shall be pulled out for static balance.
Problem 5: Huge vibration occurs when the ball mill reducer drives the mill.
Solution 5: When the mill is installed with ball mill liners, there is no secondary grouting or the anchor bolt after the secondary grouting is not fastened properly. The winch is used to rotate the mill barrel, resulting in one end of the mill barrel displacement, and the two axes are not in a straight line, which causes vibration after the reducer drives the mill.
Treatment method: to readjust, so that the ball mill axis and reducer axis in the same plane axis line. The large-scale ball mill has a large volume and heavy weight, which makes the foundation sink and displace. Set monitoring settlement points beside the foundation; conduct observation, and adjust when settlement is found.
Problem 6: Abnormal running sound of ball mill reducer.
Solution 6: The sound of the normal operation of the ball mill reducer should be uniform and stable. If there is a slight knocking sound or hoarse friction sound on the gear and there is no obvious change during the operation, you can continue to observe and find out the cause, and stop the ball mill for treatment. If the sound is getting louder and louder, stop the ball mill immediately for an inspection.
It is worth noting that the balance wheel and intermediate wheel of the ball mill reducer are not installed according to the specified meshing tooth elevation, which will cause the high-speed shaft pinion of the ball mill to drive the large gear of the intermediate shaft on one side, while the pinion of the intermediate shaft of the ball mill drives the balance wheel, and the balance wheel of the ball mill turns to drive the intermediate shaft on the other side so that the reducer of the ball mill does not form both sides of the load sharing rotation, which occurs It’s dangerous to make a noise.
10 tips for purchasing a ball mill
There are 10 tips for purchasing a ball mill:
- Ensure the production capacity of the mill. The selected grinding equipment can achieve the required output under the condition of ensuring the required grinding fineness.
- The production capacity should be higher than the design. The change of hardness and fineness of ore should be considered in the design. Generally, if the ore in the deep part of the deposit becomes hard or fine, the selected grinding machine should be able to adapt to it and ensure smooth operation in the initial stage.
- A grinding test must be done. When there is no actual data as the basis in the design, it is necessary to conduct a grinding test, especially for the large-scale concentrators. The selection calculation and scale-up of the grinding machine should be carried out from the basic data obtained.
- Large-scale equipment should be considered properly. Large-scale equipment is the trend of design since the 21st century. Due to the large-scale equipment, the total weight of the equipment is light, the floor space is less, the production system is less, the operator and auxiliary system are less, and the corresponding investment and production cost are low. However, the operation and management of large-scale equipment require a high level. If the operation rate is slightly reduced, the output of the concentrator will be greatly reduced.
- Choose equipment with a high operation rate. In the same type of equipment, there are good and bad, old and new. Sometimes the operation rate of the rod mill and the autogenous mill is lower, but they are suitable under specific conditions. In principle, the equipment with a high operation rate should be selected to reduce the maintenance and shutdown time.
- Understand the hardness, grade, and other properties of minerals, try to select an energy-saving ball mill.
- Know the required fineness of mineral grinding, according to the fineness of their own mineral requirements, select the aperture of the closed plate and the position of the bin.
- According to the requirement of output, the diameter and length of the ball mill should be selected properly.
- Choose a new generation of modified ball mill, it’s better to buy it from a professional ball mill manufacturer corresponding to your mineral.
- Select the modulus of big teeth and small teeth of ball mill, as well as the manufacturer of motor and subtractor. According to the price of the ball mill, the material of ball mill liners and the thickness of steel plate of the grinding body is required.
Popular ball mills in China markets
There are some popular ball mills in China:
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What are the ball mill liners?
In a ball mill, feed head liners, output head liners, shell liners, discharge liners, and lifting liners are called ball mill liners.
What is the function of ball mill liners?
There are 3 functions of ball mill liners:
- The mill liner is close to the cylinder, forming a “barrier” between the cylinder and ore and medium, which plays a good buffer role.
- Change the movement state of mineral material and play a good role in grinding aid. In actual production, there are many designs and shapes of liners. Take corrugated liners and flat liners as examples, corrugated liners can lift the mineral material and medium, and flat liners can play a good role in finishing the finished grain shape.
- Strengthen the rigidity of the cylinder and improve its compression resistance.
What is the material for ball mill liners?
The selection of the material of construction is a function of the application, abrasivity of ore, size of mill, corrosion environment, size of balls, mill speed, etc. liner design and material of construction are integral and cannot be chosen in isolation. A list of the primary material of construction is given, with the particular uses and strengths of each.
- Austenitic Manganese steel (AMS): This is used for grid liners and generally smaller mills. Its great advantage is that it works hardens under stress, yet the substrate remains tough and can withstand extreme impact without fracture. Its primary disadvantage is that it spreads with impact, so solid liners begin to squeeze together and become extremely difficult to remove, and can damage a mill shell if the stress is allowed to build up to an extreme level.
- High Carbon Chrome Moly Steel (325 to 380BHN): This steel is now considered the main material used for SAG mill liners. There are a number of variations with either different carbon or chrome contents. The variations tend to have a bearing on the size of the liner and its section thickness. There is ongoing development within this area as the size of the liners is outstripping the properties provided by the standard high chrome-moly steels.
- Low Carbon Chrome Moly Steel (300 to 370BHN): Was generally used for mill liners (AG, SAG and Ball) prior to the movement to higher carbon content steels. It has excellent wear characteristics with some impact resistance, generally now used for discharge grates where slightly better impact resistance is required compared to the higher carbon chrome-moly steels or for thinner section liners.
- High Chrome Irons (+600BHN) Cr Iron: It is considered to have superior wear abrasion characteristics, and is generally used in rod and ball mills. It is more cost-competitive but more brittle than chrome-moly white irons.
- Chrome Moly White Irons (600 to 700BHN) WI: This casting material is considered to be the ultimate developed and used to date for abrasion resistance in Milling. It is commonly used in cement mills and some of the largest Ball Mills in the world and where performance has not been bettered to date.
- Ni-hard Iron (550BHN): The use of this type of material generally began with Rod Mills and Ball Mills, where impacts were considered low enough for this brittle yet highly abrasive resistant wear material to perform well. However, it is now considered obsolete in light of the use of high chrome irons and chrome-moly white iron.
How many types of ball mill liners?
There are 5 types of liners in the ball mill, which include:
- Shell liners. Shell liner thickness can vary, which helps both in balancing mill capacity as well as extending ball mill liner life.
- Lifter bars. Available in a range of profiles and dimensions, lifter bars increase the efficiency of the milling operation by maximizing the agitation of the ore.
- Head liners. Head liners are protecting the output and feed doors.
- Grate plates. Grate plates are available in a range of aperture sizes and configurations.
- Central cones. These components assist in the controlled discharge of the material, maintaining mill capacity across the operation. Cones are provided in segments, which are then assembled inside the mill. This enables easier handling and manageability.
Reasons for wear of ball mill liners?
There are many reasons that affect the wear of ball mill liners, such as:
- The ball mill liner’s structure is not reasonable. It can be seen from the analysis of the wear of the lining plates of ball grinders that if the bolt holes of the lining plates are square, they are liable to cause stress concentration in this area and cause serious damage to the lining plates in production. In addition, improper matching between the liner and the cylinder wall during installation and failure to meet the qualified requirements of casting defects, mechanical properties, chemical composition, etc., will cause the liner to be damaged too quickly in production.
- Non-uniform feeding. Non-uniform feeding may cause the silo to idle, resulting in steel balls directly rubbing against the cylinder and the liner, which aggravates the wear of the liner and causes greater damage to the liner.
- The friction of the ball mill liners with steel balls and materials. When the ball grinder works, the lining plate is subject to the dropping the impact of steel balls and materials as well as the sliding wear of steel balls. Therefore, the friction between mill liners and material, as well as between mill liners and steel balls, is also an important reason for serious damage to the mill liners.
- Unqualified liner. Unqualified liner material and heat treatment process will greatly reduce the fatigue strength of the liner. For example, the hardness of high manganese steel after heat treatment is usually low, especially since the yield strength of this steel is low, which makes the square head bolt of ball grinder subject to great shear force and leads to frequent breakage of the bolt for fixing liner.
- Effects of operating conditions. On the one hand, if the working temperature of the ball grinder is high, including the ambient temperature and friction, the lining plate will heat up, which is easy to cause plastic deformation under impact and grinding of steel balls and materials. On the other hand, if the feed quantity of the ball grinder is not adjusted in time, the material in the ball grinder will be reduced. Some steel balls are directly attacked by friction with the lining plate, and the increase of impact force will aggravate the wear of the ball mill liners.
How to install ball mill liners?
In the working process of the mill, the liner will be constantly worn and shortened its service life. There are certain installation steps and precautions when installing the liner.
Ball mill liner installation steps:
- Preparation before installation. Check whether the on-site construction lighting conditions comply with the safety process and meet the safety regulations; prepare lifting tools; clean up the site and prepare the tools and appliances used in good condition; open the entrance and exit explosion-proof doors and manhole doors for ventilation.
- Installation sequence. Install the fan-shaped liner at the end cover first. The fan-shaped liner of the installation end cover is installed from the lower part and then filled up by half a circle from both sides. After each part is installed, the bolts should be connected, but the nuts should not be tightened. Then, rotate the cylinder 180°, install the remaining half-turn, and finally tighten all the bolts. Then install the cylinder liner, first install the wedge-shaped iron directly below the big tank, fasten it with bolts, and fix the two ends with the wedge-shaped iron, and use the lifting equipment to rotate the big tank 90°. Then install the 1/4 circle lining plate of the big tank, and then fasten it with wedge iron, and then use the lifting equipment to rotate the big tank 180°, and install the last 1/4 circle lining plate.
Matters needing attention for ball mill liner installation
1. Before installing the liner, the dust in the cylinder should be removed. When the ball mill installs the liner, a layer of 1:2 cement mortar should be applied between the inner wall of the cylinder and the liner, and the liner bolts should be tightened while wet. The gaps between the liners are also smoothed with sand and grout.
2. Before installing the liner of the ball mill, the liner should be inspected and trimmed. The back and the periphery of the liner should be smooth and flat. The bolt holes should be thoroughly sanded and cast flash removed so that the bolts can penetrate smoothly.
3. The bolts for fixing the liner should be carefully filled with gaskets to prevent the leakage of the slurry.
4. Note that the direction of the liner should not be reversed. The long grinding machine generally divides the grinding cylinder into front and rear bins by the partition board. The feed end is the first bin, and the first bin is the step liner. When installing, the thin end of the step liner should be in the same direction as the arrow of the turning direction when the mill is working. The grate hole of the compartment board is tapered, and the large end of the cone mouth should be consistent with the discharge direction of the ball mill during installation. The cylinder liner cannot form an annular gap.
5. When installing the partition board, wear all the bolts, and then tighten the bolts gradually in three times. At the same time, note that the connecting bolts on the central disc in the cylinder body should be tightened first, and then the bolts on the cylinder body should be tightened. After all the bolts are tightened, the nut of the bolt on the central disk of the cylinder is “spot welded” to prevent loosening.
6. The cement mortar can only be put into trial run after it has solidified to meet the strength requirements. If the bolts are found to be loose during operation, they must be tightened in time.
How to exchange ball mill liners?
- Requirements for ball mill liner installation. Before installing the ball mill liner, you need to understand the performance of the ball mill liner, consider the material and shape of the liner, wear resistance, impact, etc., select the appropriate liner of the original ball mill model to replace. Qiming Casting experts suggest that new liners can be ordered from the original manufacturer.
- It is required to replace the liner of the ball mill. The replacement of the liner of the ball mill requires a commanding expert to arrange and coordinate the entire replacement process. Moreover, all ball mill workers need to understand the operation drawings, disassembly and installation requirements, and understand the entire process operation.
- Cleaning requirements for the ball mill lining. It is necessary to circulate the remaining slurry in the ball mill as much as possible, remove the waste lining rubber pad, clean the protruding points of the ball mill barrel wall, and perform dust and scale removal on the barrel. During the cleaning operation, the cylinder of the ball mill needs to be fixed, and the workshop needs to have good ventilation conditions and the temperature is appropriate, and then the installer can enter the cylinder of the ball mill to start work.
- Disposal of the old liner of the ball mill. When removing the old liner in the ball mill, you should first remove the screws of the old liner, then remove the old liner in one line, and then lift the old liner out. When removing the liner, personnel need to pay attention to safety, pay attention to standing, and avoid being injured by the liner.
- Hoisting requirements for the liner of the ball mill. The volume of the ball mill is relatively large, so during the process of replacing the liner, lifting operations are required. Hoisting of the ball mill liner is the responsibility of the crane worker, and attention should be paid to the operation of the wire rope and hook during the lifting process.
- Ball mill liner installation time requirements. The installation of the new liner of the ball mill should ensure that the gap during the installation is within the required range. Fix the spiral of the liner, and carefully pad the sealing packing and gasket to prevent leakage of mineral powder, if there is no sealing gasket At the same time, two loops of cotton rope or hemp and lead oil should be wrapped around the corresponding position. At the same time, a layer of cement mortar needs to be applied to the inner wall of the cylinder and screwed 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 rotation direction when the mill is working.
- It is required after the ball mill liner is installed. After the replacement is completed, employees must confirm that there are no people, working tools, and extra debris in the ball mill cylinder, and carefully inspect the cylinder body and surroundings before closing the empty cover.