There are three main stages to mineral processing in the mining industry. These are:
- Comminution (crushing and grinding).
- Beneficiation (separation and concentration).
- Smelting and refining.
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.
If a ball mill uses little or no water during grinding, it is a ‘dry’ mill. If a ball mill uses water during grinding, it is a ‘wet’ mill.
A typical ball mill will have a drum length that is 1 or 1.5 times the drum diameter. Ball mills with a drum length to diameter ratio greater than 1.5 are referred to as tube mills.
Ball mills may be primary or secondary grinders. Primary grinders are fed from crushers such as a jaw crusher. Secondary grinders are fed from other grinders e.g. a rod mill.
Ball mills are usually either of the grate (diaphragm) or overflow type.
Ball Mill Components
Ball mills consist of the following components:
- Inlet – crushed ore is fed to the ball mill through the inlet. A scoop ensures the feed to the mill is constant.
- Discharge – reduced feed exits the mill through the discharge.
Ball Mill Components
- Drum – the drum is the cylindrical casing of the ball mill. The inside of the drum is fitted with manganese steel alloy plates (‘armour’) that protect the steel shell from abrasion; it is also possible to use rubber as armour rather than manganese steel.
View Inside Ball Mill (armour plates are the square shapes)
- Ring Gear – a gear ring installed on the outer periphery of the drum.
- Electric Motor – a motor used to rotate the drum. The motor drive train leads to a gearbox than a ring gear. The motor is usually fitted with a variable speed drive (VSD) to control the rpm of the ball mill.
- Gearbox – used for speed reduction from the motor to the ball mill.
- Balls – usually manufactured from manganese alloy steel, but the material depends upon for what material the mill will grind (chrome steel alloy and rubber also available). The size of the balls depends upon the size of the drum, typical ball sizes are two to three inches in diameter (four-inch balls are also possible).
- Bearings - smaller ball mills use anti-friction cylindrical roller bearings. Larger ball mills use trunnion bearings.
How Ball Mills Work
The below video is an extract from our Mechanical and Electrical Engineering Explained Online Video Course.
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.
Crushed ore is fed to the ball mill through the inlet; a scoop (small screw conveyor) ensures the feed is constant.
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 drum’s 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).
As the balls wear due to abrasion, they are replaced with new balls (‘green’ balls).
If large particle sizes are retrieved from the ball mill, the process is known as a ‘coarse grind’. If smaller particle sizes are retrieved from the ball mill, the process is known as a ‘fine grind’.
If hard pebbles are used instead of steel balls, the mill is referred to as a ‘pebble mill’.
If rods are used instead of steel balls, the mill is referred to as a ‘rod mill’.
Closed-Circuit or Open-Circuit
Ball mills may operate in a closed-circuit, or open-circuit. Closed circuits return a certain amount of the ball mill’s 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).
Ball mills are usually designed with a maximum reduction factor/ratio of 60:1 although it is possible to achieve reduction ratios of up to 70:1.
The electric drive for a ball mill should be able to handle loads where the ball mill is charged by up to 45% with balls.