Ball Mills
Introduction
Ball Mills, and other forms of tumbling mills such as tube mills and rod mills, form a significant class of equipment used for the size reduction of a wide variety of materials such as minerals, ores, coal, carbon products and general chemicals. These operate by introducing the material to be milled into a rotating vessel which contains a grinding medium (eg steel balls or rods) and the material is ground (wet or dry) by attrition through the tumbling action of the mill. Slow rotational speeds combined with simple and robust construction make them ideal for grinding hard and abrasive materials where continuity of service, low maintenance and minimum downtime are a necessity. There are many different mill designs, each configured for a specific purpose, and a selection are described in more detail below. Various lining materials can be used to cope with the abrasive nature of the raw material. Ball mills can be designed to operate continuously (fed at one end of the system and discharged at the other), or on a batch basis for smaller, or intermittent, volume requirements.
System Configuration
Ball mills can be operated either on a stand-alone basis (“open-circuit grinding”) or in conjunction with a classifier which returns oversize product for further grinding (“closed circuit grinding” – see figure 1). With open-circuit systems, finished product is obtained from a single pass through the mill and typically produces a relatively wide particle size distribution. Special care has to be taken selecting the geometry of the mill (length, diameter, rotational speed etc) to ensure that the desired product size is obtained. Combined with the right classifier, closed-circuit grinding is capable of achieving a much narrower particle size distribution, with an accurate control of top-size. A closed system minimizes over-grinding, resulting in improved efficiency, and allows a smaller mill to be used for any given capacity.
Air Swept Milling
Applicable to dry closed-circuit milling only, a further variation can be the introduction of an air flow through the mill (“air-swept milling”). This provides a form of elementary pre-classification and typically produces narrower size distributions but less fine particles than its non air-swept equivalent. Typically suitable for end product falling in the range of 50 to 200 microns, this system can also be used to achieve a limited drying capability, handling product of up to 8% moisture content.
Mill Types
Grate Discharge Ball Mill
Used in wet and dry grinding in both open and closed circuits, a grate discharge ball mill (sometimes known as a “diaphragm discharge mill”) incorporates a slotted discharge diaphragm with lifters at the discharge end of the mill. The diaphragm, or “grate”, serves to retain the balls and any coarse material, with finished product passing through.
Air Swept Ball Mill
Used in dry closed-circuit grinding, air-swept ball mills allow fines to be extracted from the mill as soon as they are produced. The air flow temperature can be varied and limited drying can be achieved.
Conical Ball Mills
Used wet or dry, closed or open circuit, conical ball mills provide a classifying action within the body of the mill due to particles of greater size tending to reside at a point of greater diameter. This characteristic delivers increased efficiency and lower power consumption. Conical mills may also be air-swept when run dry.
Rod Mill
Normally used for grinding a relatively coarse product and charged with rods instead of balls, the introduction of coarse feed effectively spreads the rods at the input end of the mill, with the greatest crushing force, therefore, being applied to the coarsest particles as the rods tumble.
Tube Mill
A parallel mill of large length to diameter ratio, a tube mill is used for wet or dry grinding in open circuit. This is a grate discharge mill which may be fitted with additional internal diaphragms of different specifications to control the flow of material through the mill.
Screen Mill Discharge
The British Rema Screen Discharge Mill is designed for continuous operation and produces a relatively coarse product with minimum generation of superfines. The grinding chamber is formed from a set of replaceable cast steel perforated grinding plates bolted to shaft-mounted end-plates fitted with renewable segmented steel liners. The chamber is totally surrounded by two concentric sets of perforated screens. The chamber contains a steel ball charge. Material enters through an axially placed feed nave. Ground material passes through the perforated grinding plates onto the inner screen surface. Material then passes through this screen continuing to the outer screen which controls the final product size. Oversize (or semi-ground) material retained on each screen is returned to the grinding chamber through a series of return plates.
Product is collected within a surrounding dust-tight casing with integrated discharge hopper.
Batch Mill
Batch mills are appropriate for relatively small scale or intermittent operation where continuous milling would be inappropriate and enable extended and variable residence/grinding times to be accommodated.
