The size reduction operation in the plant takes place as a sequence of crushing and grinding processes. In crushing, particles are reduced in size to such a level that grinding can be conducted, while grinding allows to achieve the desired liberation size.
Besides, the process of comminution can be simple or complicated depending on the nature of the ore and the suitable degree of liberation. For example, hard rocks that come from blasting can be big as 1m in size, and require multiple stages of crushing and grinding. In addition, if the valuable mineral is finely disseminated in the ore the grinding size can be down to 40 microns, which can be achieved by two grinding stages at least.
Initial size reduction is accomplished by Jaw, cone, and gyratory crushers. It can be applied in the mine site as well as at the start of the milling plant. Medium size grains require jaw, cone, and gyratory crushers as well as roll crushers to be reduced in size. The comminution of smaller coarse particles is done by Cone, roll. Hammer crushers and rotary breakers.
Grinding is conducted in mills. There are three grinding stages, coarse grinding (hundreds of microns), fine (tens of mictons) and ultrafine grinding (<10 microns). Tumbling mills, rotary breakers, and pendulum mills are mostely used.
Both crushing and grinding require energy to reduce the ore size. However, in grinding operations we consume much more eneregy than crushing, up to 100 times ( thousands of kw compared to tens in crushing).
Additionally, some chemical reagents can be used to minimize the energy consumption of grinding, because grinding energy is minimum at pH of the point of zero electrical charges of the material surface which is known as Rebinder effect.
Wears are most important in grinding compared to crushing. Because as far we go to finer sizes wears increase due to more collosion requirment for further size reduction. Besides, the grinding media generates extra wears production
The demand for high pressure grinding rolls (HPGRs) increases recently, as they respond to features requirements. Skewing designs for HPGRs are investigating for reducing wear and promoting better grinding.