Grinding Circuit

Ore from the storage bins is fed, together with water, to the first circuit in the mill building. This is known as the grinding circuit and consists of one or more ball mills or rod mills. As each mill revolves, the ore within rolls over itself, causing it to be crushed and ground. The steel balls (or steel rods) in the mill assist this process.

A particularly economic form of grinding, used today wherever possible, is autogenous grinding. This process involves the use of rock-against-rock contact to crush and grind ore to the required size, thus, the cost 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 discharged from a grinding mill. Some will be too coarse or too fine for the chemical separation of their constituents to work effectively.

If the crushing and grinding process is not carefully controlled, some ore particles get reduced to sub-micron sizes. These are called slimes and may interfere with subsequent treatment processes. That is 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 then returned to the grinding mills. While traditional classifiers consist of a box set on a slope and a mechanism for moving material up the incline, newer plants 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, the process is known as a closed circuit. The ground ore produced will have a certain maximum size and the amount of “fines” is limited. Usually a ball mill grinds in closed circuit for maximum control and efficiency.

A rod mill may be used to prepare feed for a ball mill, in which case it may operate without a classifier, or, as it is termed, in open circuit.

The goal of the mill circuit is to crush and grind the ore down to what is termed its size of liberation — the maximum size to which the rock must be ground to ensure that individual mineral grains are separated from one another.