Mineral beneficiation-ore comminution

The operation of applying a force on the particle to break it is called size reduction.

Sinonine deeply know that Comminution is a general term for size reduction that may be applied without regard to the actual breakage mechanism involved.

In any industrial comminution operation, the breakage of any individual particle is occurring simultaneously with that of many other particles. The breakage product of any particle is intimately mixed with those of other particles. Thus an industrial comminution operation can be analyzed only in terms of a distribution of feed particles and product particles. However, each individual particle breaks as a result of the stresses applied to it and it alone.

1 FRACTURE

Fracture in the particle occurs as a result of application of a force. When a force is applied on a particle, stress will develop within the particle. When this stress exceeds ultimate stress, the particle will break. Let us consider a particle subjected to two opposing forces by a concentrated load as shown in Figure 1.

Figure 1 Compressive forces.

The principal stress in the z-direction is a compressive stress throughout the particle. The principal stress in the x- and y-directions is a compressive stress adjacent to the load points but a tensile stress within the particle. This tensile stress is lower than compressive stress. As the tensile strength is as little as 1/10 of compressive strength,

the fracture occurs primarily because of the tensile stress which results in breakage into a small number of large pieces. Due to the compression adjacent to the loading points, it results a large number of small pieces.

Compressive force can be applied at either a fast or a slow rate. Under the conditions of slow compression, energy applied is just sufficient to load comparatively few regions of the particle to the fracture point and only a few particles result. Their size is comparatively close to the original particle size. Under the conditions of rapid loading such as in high velocity impact, applied energy is well in excess of that required for fracture. Many areas in the particle are overloaded and the result is a comparatively large number of particles with a wide size distribution. Impact causes immediate fracture with no residual stresses.

Attrition or abrasion fracture occurs when a force (shear force) acts parallel to the surface of the particle. Due to insufficient energy applied on the particle, localized stress occurs and a small area is fractured to give very fine particles.

Another type of fracture is chipping. In this chipping, the edges or corners of a particle will break due to the application of oblique forces, generally impact forces, on the particle.

In practice, these events do not occur in isolation. For example, when the particles are crushed by compression as in the case of a crusher, coarse particles will be produced resulting from the induced tensile stress, fine particles will be produced resulting from compressive stress near the points of loading and by attrition due to particle interaction. All these types of forces and fractures, and sizes of the particles after fracture, are shown in Figure 2.

Figure 2. Mechanism of fracture.

It can be summarized that all types of forces exist in any size reduction operation even though individual size reduction units are predominantly designed for application of one type of force.

2. OBJECTIVES OF COMMINUTION

The following are some of the objectives of comminution:

1 Reduction of large lumps into small pieces.

2 Production of solids of desired size range.

3 Liberation of valuable minerals from gangue minerals.

4 Preparation of feed material for different beneficiation operations.

5 Increasing the surface area for chemical reaction.

6 Convenience in handling and transportation.

The energy consumed for the comminution operation is high when compared to other operations such as screening, beneficiation, dewatering, conveying etc. In Mineral and Mining Industries. Hence attention needs to be paid to minimize the production of fines (finer than required) which will consume additional power for reducing to fines.

3.TYPES OF COMMINUTION OPERATIONS

The run-of-mine ore is quite coarse and cannot be reduced to fine size in one stage. It may require three or more stages. Each stage requires separate equipment. The comminution operations are divided into two broad groups as follows:

1.Crushing

 Crushing is a size reduction operation wherein large lumps are reduced to fragments or smaller particles.

2.Grinding

Grinding is considered as size reduction of relatively coarse particles to the ultimate fineness.

The machines used for crushing and grinding are entirely different. It is to be noted that the energy required for comminution of unit mass of smaller particles is more than the energy required for unit mass of coarser particles. However, the energy required to reduce coarser particle is more than that of smaller particle. Hence the machines used for crushing (crushers) must be massive and rugged and the machines used for grinding (mills) must be capable of dispersing energy over a large area. In crushers, the breakage forces are applied either by compression or impact whereas in grinding mills shear forces are predominantly applied.

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Sinonine Technology Team