Once the auriferous ore is extracted from the mine is sent to the processing plant where the objective is to obtain a gold concentrate or a bullion. There is a sterile part called tailings which are many times an issue very important at the moment of beginning any metallurgical project. The processing plant employs essentially two operations: comminution and concentration. Sometimes, cyanidation is included as part of the process. In all cases, comminution has as unique objective to liberate gold particles from the non- valuable minerals (gangue). This operation involves crushing and grinding steps. It is believed that grinding is often the greatest energy consumer in a mineral processing plant. As it is this process which achieves particles liberation, it is also important to obtain a good separation between particles. Thus, in order to obtain clean concentrates is necessary to grind the ore fine enough to liberate the associated minerals. So grinding is a compromise between high grade concentrates and operating costs.
The classical comminution equipment is related to crushing machines such as jaw crushers, conical and gyratory. The selection depends of the ore characteristics and the grinding equipment to be used in the next stage. Recently, has been studied with special interest the applicability of microwave devices because rapid heating of ore minerals in a microwave transparent matrix generates thermal stress of sufficient magnitude to create micro-cracks, some of which occur at mineral grain boundaries. Inducement of micro-cracking before beneficiation is likely to improve liberation.
An element very important is the grinding circuit which is constituted by SAG mills, ball mills, rod mills and cyclones. There is not a unique way to design the circuit, but he objective is to assure the liberation of valuable minerals. The rod mill has been universally available for many years, and yet in many cases has been bypassed in service by other equipment. However, the rod mill can provide better product than the newer, more sophisticated machine. The efficient grinding of different particle sizes in the feed to a ball mill requires the provision of a variety of ball sizes, each ball size maximizing the breakage of a certain particle size.