Ball mills are employed in the comminution stage as grinding machines (size reduction). The purpose of grinders in the mining industry is to reduce the feed material size in order to liberate the minerals from the barren rock. Ball mills are the most common grinding machine employed in the mining industry.
Grinding occurs in a single stage, or multiple stages. Multiple stages may include a rod mill followed by a ball mill (two stage circuit), or a semi-autogenous grinding (SAG) mill followed by a ball mill (two stage circuit). Smaller plants tend to add extra crushing stages in order to operate a single grinding stage only.
The following process description is based upon a ball mill used in the hard rock mining industry for liberating minerals from ore, but the operating principle for ball mills used in other industries is the same.
For both wet and dry ball mills, the ball mill is charged to approximately 33% with balls (range 30-45%). Pulp (crushed ore and water) fills another 15% of the drums volume so that the total volume of the drum is 50% charged. Pulp is usually 75% solid (crushed ore) and 25% water; pulp is also known as slurry.
An electric motor is used to rotate the ball mill. As the ball mill rotates, the balls stick to the inner surface of the drum due to the centrifugal force created within the drum. At a certain angle, the weight of the balls overcomes the centrifugal force holding them against the drum and they begin to tumble back to the centre line of the ball mill (this area is known as the toe). In this manner, the ore is reduced in size by both attrition (ore rubbing against other bits of ore) and impact (balls impacting with the ore).
The ore moves gradually through the mill then exits through the discharge port. The discharge port may be covered by a grate to prevent oversized ore exiting the mill, or it may have no grate (overflow type ball mill).
Ball mills may operate in a closed-circuit, or open-circuit. Closed circuits return a certain amount of the ball mills output back to the ball mill for further size reduction. A typical closed system grinds the ore between two to three times.
Hydro-cyclones installed directly after the ball mill ensure only over-sized material is returned to the ball mill. Other types of classifiers can be used (rake and spiral classifiers), but the hydro-cyclone is now one of the most common.
Critical speed is defined as the point at which the centrifugal force applied to the grinding mill charge is equal to the force of gravity. At critical speed, the grinding mill charge clings to the mill inner surface and does not tumble.
Most ball mills operate at approximately 75% critical speed, as this is determined to be the optimum speed. The true optimum speed depends upon the drum diameter. Larger drum diameters operate at lower than 75% critical speed whilst smaller drum diameters operate at higher than 75% critical speed.
Irrespective of the type of grinding machine employed, grinding is a low efficiency and power intensive process. For this reason, the grinding stage of a mineral processing plant may account for up to 40% of total operating costs.
As a general rule of thumb, the larger the diameter of the ball mill drum, the more efficient the grinding process will be. This rule of thumb stops though once the diameter of the drum reaches approximately 4m (13.1 feet).
The operating principle of the ball mill consists of following steps. In a continuously operating ball mill, feed material fed through the central hole one of the caps into the drum and moves therealong, being exposed by grinding media. The material grinding occurs during impact falling grinding balls and abrasion the particles between the balls. Then, discharge of ground material performed through the central hole in the discharge cap or through the grid (mills with center unloading the milled product and mills with unloading the milled product through the grid).
In filling mill by grinding balls on 40 50% and non-smooth liner, the outer layers slip is virtually absent, but the sliding of the inner layers one on another observed in various modes of operation mill. In a monolayer filling mill by grinding media, they rotate around their axis parallel to the drum axis of rotation. Grinding media are not subjected to a circular motion by a smooth lining, even at high speeds. In a multilayer filling mill by grinding media, depending on the rotational speed, there is possible one of the following modes the grinding media motion:
Cascade mode motion of grinding balls carried out at low drum speed. At start-up of a mill, the grinding material rotated by a certain angle and grinding balls start to move by closed path. The curved surface of natural slope is close to the plane inclined at some angle to the horizontal. This angle is equal to a limit angle of rotation. In this mode, the ground material remains in this position, but the grinding balls continuously circulate, rise on circular trajectory and cascade roll to the reference point. There is a zone or core in the central trajectory of the grinding material. This zone is inactive. In cascade mode grinding occurs as a result of crushing and abrasive actions by grinding balls. This mode used in the ball mill with a central discharge.
Waterfall mode motion of grinding media in the mill carried out by the drum rotation speed, ensures the transfer all of the grinding balls layers from a circular to a parabolic trajectory. In this mode, grinding balls rise on circular trajectory and at certain points deviate from it and make a free flight by a parabolic curve.
Weight of grinding balls should be sufficient to grind the largest pieces of crushed material. For efficient operation of ball mills necessary to observe the right balance between balls size and feed material size. If the feed material contains many large lumps and grinding balls cant crush them, it leads to a gradual accumulation them between the balls. As a result, mill suspends own operation. In these cases, need to reduce the size of crushed material or increase the size of the balls. By increasing the grinding balls size, decreases the mill working surface and reduced mill productivity. It is important to follow the degree of drum filling by grinding balls, because with a large filling rising grinding balls collide with falling balls.
Established impact of design mills and lining forms on their productivity. Mills operating with low pulp level, have better productivity than mills with high pulp level. Particularly, productivity of mills with unloading the milled product through the grid approximately 15% higher productivity mills with center unloading the milled product. Productivity mills with smooth lining less than productivity mills with ribbed liner. Mill productivity also depends on other factors: number of the drum rotations, the grinding fineness, humidity and size of the crushed material, timely removal the finished product.
Ball mills characterized by high energy consumption. When the mill idles, the energy consumption is approximately equal to the energy consumption with full mill capacity. Therefore, the work of the mill with partial load conditions is unprofitable. Energy consumption for ball mills is a function of many factors: the physical properties of the ground material its specific gravity and hardness; the degree of drum filling by grinding balls; the number of drum rotations, etc. Ball mills have low efficiency no more than 15%. Energy is mainly consumed on the wear of grinding balls and mill housing, friction; heating the material etc.
The advantages of ball mill there are large unit capacity, achievement degree of fineness corresponding to a specific surface of 5000 cm2 / g, simple construction, high reliability and well designed scientific justification.
The disadvantages of ball mills include their considerable metal consumption and deterioration grinding media, as well as a lot of noise. Most of the energy useless lost during ball mill operation, leading to low it efficiency. But even a significant specific energy consumption for grinding material compensates beneficial effect by using mill. This does not exclude a search energy saving solutions for milling, and this handled by experts from around the world.
The ball mill is a hollow drum closed with loading and unloading end caps, filled with grinding media and rotated around its axis. The drum of the ball mill (Pic. 1) is a hollow cylinder of steel, lined inside with armor lining plates which protect it from impact and friction effects of the balls and the grinding material. The shape of drum liners has a significant impact on mills work. Drum Liners of ball mills operated on a large source material have ribs. For mills operated on the fine materials use lining with small ribs or quite smooth. Height, mutual arrangement and shape of the ribs define force of adhesion the grinding media with the drum and the results of the mills work. It is important when the character of the lining surface did not change harshly during its deterioration.
End caps molded integrally with the hollow pins. On pins planted supporting bandages. The drum is supported by two self-aligning roller through these bandages. Material loaded into the mill through the hopper. The mill is driven by a motor through a clutch, gearbox and flexible coupling. Grinding media followed to the direction of the drum rotation during its rotation, lifted to a certain height and freely fell down or rolled down.
At mills with center unloading the milled product removed by a free sink through a hollow unloading trunnion. It is necessary for the pulp level in the drum to be above the level of the lower generating unloading trunnion. Therefore mills with center unloading sometimes called drain type mills or mills with high pulp level. The unloading funnel has a slightly larger diameter than the loading funnel to create slope and for maintain a high pulp level in the mill.
Mills with unloading through the grid have a lifting device and it forced to unload the milled product. Therefore, in this type of mills the slurry level may be lower than the unloading trunnion level. Mills with unloading through the grid sometimes called mills with forced unloading or mills with a low level of pulp. This type of ball mill has a grid with openings for unloading crushed material and located in the unloading end of the drum. The grid has radial rib-lifters on side facing to the unloading end cover. Rib-lifters divide the space between the grid and the end cover on a sectorial chambers opened in the trunnion. By rotating the drum ribs act as elevators wheel and raise the pulp to the level of unloading trunnion. This device allows to maintain a low pulp level in the mill and reduces the spent time of the material in it due to the decrease of volume of the pulp.
Depending on the shape of the drum there are cylindrical mills and cylinder-conical mills. Cylindrical mills classified into three types: short mills, long mills and pipe mills. Short ball mills have a drum length less than drum diameter or equal to drum diameter. Long ball mills have a drum length more than one drum diameter, but less than three drum diameters. Pipe ball mills have a drum length longer than three drum diameters.
A cylindrical ball mill used for grinding the coarse material. This mill should have a short length because the balls distributed uniformly over the entire length of the mill and during rotation obtained the same pulse. The drum diameter of the cylindrical ball mill should be the greater, the larger the pieces of crushed material.
The balls impact on the milled material longer at the pipe ball mills. The drum of these mills lined with flint blocks inside or flint pebbles on the cement. Material continuously fed by the drum axis through a hopper at one end and leaves at the opposite end of the drum through end wall or holes on drum walls. Pipe ball mill (Pic. 3) provided with a drive with construction similar to the drive of rolling mills. The central drive shaft has milled protrusions and depressions at the ends and enters them into the corresponding coupling. In this arrangement the axial displacement of the mill is not transmitted to the reducer or motor.
The housing of the cylinder-conical ball mill consist of two cones and a short cylindrical part between them (Pic. 4). This change in the shape of a cylindrical mill is highly advisable, because achieved the proportionality between the force and useful resistance. The peripheral speed of the conical mill drum gradually decreases in the direction from the cylindrical part to the discharge outlet in the same direction reduced lifting angle of balls inside the mill, and consequently decreases their kinetic energy. The amount of milled pieces also gradually decreases as approaching to the place of unloading and this reduces energy consumption for grinding.
Should be noted, the productivity of ball mills depend on the drum diameter and ratio between the diameter and length of the drum. At the short ball mill grinding is a more rough and for grinding fineness a lot of material has to be returned from classifier to the mill, it leads to mill overload. In long ball mills the grinding occurs only at the front and the rest of the balls in the drum only increases power consumption.
Drum mills have one grinding chamber (short and long ball mills) and two or more grinding chambers (long and pipe ball mills). Single-chamber continuous mills are the main equipment at mining and processing plants.