Grinding Circuit
Ore from the storage bins, together with water is fed, to the first circuit in the mill building. This is known as the grinding circuit and consists of one or more rod mills or ball mills. As every mill revolves, the ore that is within rolls over itself, causing it to be ground and crushed. The steel rods (or steel balls) in the mill assist this process efficiently.
Autogenous grinding is a particularly economic form of grinding, which is used today wherever the possibility permits. This process involves the use of rock-against-rock contact to grind and crush ore to the required size, that way, the expense of periodically replacing steel balls is eliminated.
Semi-autogenous grinding refers to the addition of some steel balls to the grinding stage to supplement the rock-on-rock breakage.
It is natural to expect that there will be considerable variation in the size of particles which are discharged from a grinding mill. Some of them will be too fine or too coarse for the chemical separation of their constituents to work in an effective manner.
If the process of grinding and crushing is not carefully controlled, some ore particles get reduced to sub-micron sizes. These are called slimes and may interfere with subsequent processes of treatment. That is the reason why many crushing and grinding circuits are now controlled by computers.
Particles that are too coarse are separated from the balance of the material in a classifier and are then returned to the grinding mills. While traditional classifiers consist of a box set on a slope and a mechanism for moving the material up the incline, the plants which are newer may use a hydrocyclone, which separates various sizes of particles by spinning, as in a centrifuge.
When a ball mill and classifier work together as a unit, this process is known as a closed circuit. The ground ore which is produced will have a certain máximum size and the amount of "fines" is limited. In most cases a ball mill grinds in closed circuit for máximum control and efficiency.
A rod mill has the possibility of being used to prepare feed for a ball mill, in which case it may operate without a classifier or, as it is also termed, in open circuit.
The goal of the mill circuit is to grind and crush the ore down to what its size of liberation is termed - the máximum size to which the rock must be ground to ensure that individual mineral grains are separated from one another.
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