1.0 Introduction
Size reduction is simply the process of reducing/minimizing the particle size of a substance to a finer state of sub-division to smaller pieces, to coarse particles or to powder (fine particles).
The term size reduction is applied into all the ways in which particles of solid are cut down or broken into smaller pieces. This solid are reduced by different method for different purposes. Chunks of crude are crushed to workable size, synthetic chemicals are ground into powder. Today, size reduction plays an important role in the production of a wide range of product from foods and pharmaceutical to process. The engineer needs the reason that size reduction or combination is usually carried out; to increase the surface area of the material. This will help maximize the area of solid in contact with liquid or gas phase around it, which enhances reaction, dissolution, catalytic effect etc.
Hence, the rate of combustion of solid particles is proportional to the area presented to the gas, although a number of secondary factors can be involved. Example is leaching, the rate of extraction increases when the area of contract between the solvent the solid in increased.
The solids are reduced by different methods for different purposes. This makes it essentially a preliminary operation in most processing industries. Large solids mostly from the mines are crushed and further ground to workable sizes. The crude iron are from the are, the coal obtained from coal mines, the calcium carbonate used in cement production, the pulp for paper making and so on, are few examples of raw material that require size reduction to desired dimensions of size and shape for easier handling and free flow of processed materials. For liquids, the breaking down of liquid droplets is known as ATOMIZATION. This involves the reduction of the continuous sheet of liquid into tiny droplets which from sprays. This is effectively utilized in drying, where spray of slurry is formed in the spray dryers, an operation mostly used in soap-factory for soap production.
Size reduction is used to improve solvent efficiency, in this process it reduces the distance traveled to extract solute from the solid in which it is embedded. Size reduction is carried out in many processing industries to enhance or rather, ensure easy handling of materials. For instance, in transporting a substance through a conveyor belt, it is much easier to handle small particles due to convenience and economy of the process. It is obvious that small particles exert lesser impact on any system and energy free flow in process equipments. For large particles, the movement from one side to another in the factory is relatively attendant complexity in the design and operation, all is indispensable to making and the operation is practiced at al spheres of human endeavour.
There are four mechanisms by which size reduction may be achieved:
- Impact particle conclusion by a single rigid force.
- Compression particles disintegrating by two force
- Shear produced by a flied or by particle – particles interaction.
- Attrition arising from particles scraping against one another or against a rigid force.
This course provide the attended with an understanding of the characteristic opportunities and benefit of literature solid particles dispersed system using communuting size reduction dispersion technology.
Mechanism of Size Reduction
Size reduction in the separation of particles into two or more parts with simultaneous creation of addition surface. When a single lump of material is subjected to an impact, it breaks into a few relatively large particles and number of fine particles. The size of higher particle is more closely connected. Size reduction obviously requires energy.
It is consumed in size reduction apparatus or a much hither rate than would be predicated from the new surface are created, by a factor of about 1000.
Therefore, the energy required to effect size reduction is related to the internal structure of eh material. Reduction is a very inefficient process from the point of energy utilization because the supplies to communition of a material is maximized, larges particles material with increase in the number of fines but not the size.
The method by which size reduction can be achieved is a complex one and lacks any analytical interpretation. It has been difficult and seemly impossible to obtain theoretic events of action on the behaviour of the constitution particles when they are subject to an applied force of size reduction. However, the dear statement of facts reveals that whenever a lamb of materials is subjects to a force beyond the bond strength of constituent atoms, the strain that developed may eventually leads to fracture and breaking of the material to relatively smaller units of size. The exact method by which this fracture occurs as suggested by PIRET is that the exerted forces (more especially compression force) initially creates small flows in the material. If the energy concentration exceeds a certain critical value in relation to the bond strength of the material these flows will grow rapidly and producing many more branches. The process consists of two parts-the first is used for opening up small fissures which are already present and the second is for forming new surfaces. Remarkably there is a close similarly between chemical reaction and size reduction operation.
In both causes, a critical energy must be exceeded before the occurrence while each of them varies linearly with time. A small force will cause fracture provided it is maintained for a sufficient time.
The product of size reduction ranges from coarse, intermediate to fine particles. The ration of these sizes is closely related to the nature of constituent particles vis-a-vis the assisting bond strength of the parent material.
The size of the product decrease progressively with amount of energy impacted. For a given amount of stress impacted, the size of the emerging product are determined by internal structure of the material. Some of eh solid material have structural defects and impurities. It the one set, fracturing takes place along the weak joints or cracks in the material as well as areas where impurities are existing and later extends to other region. For instance, a lump of coal containing a number of cracks and other imperfection will therefore require minimum application of force resulting to various sizes of time product this case is quite different from strong and homogeneous body which is inferred to have balanced distribution of bonding strength of the constituent particles. Generally, the size of the larger particle is more closely connected with the process size reduction. While the size of the fine particles related to the internal structural of the material.
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