In nature, minerals exist in physical and chemical combinations with each other. To separate minerals of commercial interest from the host rock both physical and chemical methods are employed. Most minerals are mined in the form of large rocks. Others like the ilmenite, rutile, zircon, leucoxene, heavy minerals or alluvial placer deposits of gold are found decimated amongst sand in beaches or in riverbeds. To access the minerals in the host rocks, they have to be crushed and even ground. When a maximum amount of the mineral of interest is separated by comminution from the parent rock, that size is usually known as the liberation size. The aim of comminution is to maximise the liberation of the mineral from the host rock. Usually the concentration of useful minerals in host rocks are low, therefore large tonnages of host rocks have to be mined to recover sufficient quantities of the useful mineral to make the operation commercially viable. The first step in the recovery process of minerals from the host rocks therefore is to reduce the size of rock by crushing and grinding.
2. Design of Size Reduction Processes
The process of size reduction is normally designed to take place in single stage open circuit, single stage closed circuit or multiple stage open or closed circuit In some cases a combination of these methods are adopted. In a single stage, single pass, open circuit size reduction operation, the product consists of a range of particle sizes which seldom achieves the desired degree of liberation. Hence second or even third stages of size reduction are often necessary to progressively reduce the remaining particle size to liberate mineral particles to an acceptable degree (Figure 1).
In closed circuit, the product from the stage of size reduction is separated into relatively fine and coarse fractions. The coarser fraction is then collected and recrushed in the same unit as seen in Figure 2. In so doing the load on the equipment for size reduction is increased and a circulating load is established, but the total number of units required for obtaining the same degree of size reduction is less.
Several options in design exist in closed circuit grinding set-ups. The two most commonly used devices for size reduction are the crushers and grinding mills. The crushers are normally fed with rocks, up to about 1 meter in size, while the grinders are usually fed with rocks crushed down to a maximum size of about 50 mm. Larger rocks produced at the mines are initially separated by grizzlies, broken by hammers and then fed to the crushers. The mechanisms of size reduction during crushing and grinding are different. The chief difference being that in crushing operations the size reduction is more by compression and impact and less by attrition while in grinding, the forces of attrition are much greater. The grinding operation is rather complex and its complexity can roughly be illustrated by Figure 3. Spherical balls or cylindrical rods are mostly used as the grinding media. These media cascade within a mill and impinge on the ore thus providing a crushing action. As the balls and rods tumble within tubular mills, they provide a grinding action and forces of attrition, all of which result in further reduction of the size of the rock particles. Impact breakage occurs as balls or rods drop into the toe of the charge and abrasion or attrition occurs as the layer of balls or rods slides over each other or against the mill liner.
In designing a plant for size reduction the two main features of interest are:
1. The power required for size reduction
2. The choice of crushers and grinders
The power or energy required is the sum of the work required to crush or grind the rock as well as rotate the mill. The power required depends on the hardness of the rock, the initial size and the final product size required to achieve reasonable liberation of the mineral of interest, from the host rock.