Rod mills are used for grinding hard minerals. This type of mill is normally used as the first stage of a milling process to provide a reduced size feedstock for a further milling process. Ball mill (Fig 1) is same kind of mill as rod mill , except that it is filled with balls instead of rods.
In various fields of the process industry, reduction of size of different materials is a basic unit operation. The basic idea is to reduce particle sizes of material under handling by cutting or breaking those to smaller pieces. The usual reason for size reduction is normally the need for smaller size, but it can also be desired shape, size or number of particles. The functions of mechanics in machines made for size reduction are usually based on the principles namely (i) compression, (ii) impact, (iii) attrition , and (iv) shear.
Grinding mills are size reductions machines that often follow crushers in the processes where finer products are desired after crushing. Different grinding machines are usually named as mills, for example rod mills, ball mills, and attrition mills. Because of the name, verb milling is also been used as a synonym for grinding.
Milling, sometimes also known as fine grinding, pulverizing or comminution, is the process of reducing materials to a powder of fine or very fine size. It is distinct from crushing or granulation, which involves size reduction of a material to a smaller size. Milling is used to produce a variety of materials which either have end uses themselves or are raw materials or additives used in the manufacture of other products.
A wide range of mills has been developed each for particular applications. Some types of mills can be used to grind a large variety of materials whereas others are used for certain specific grinding requirements.
Material grinding is quite often an integral part of an industrial process, whether carried out on a large or small scale and in some cases the grinding mill may be the single most costly item for in the process of operation. Installing a grinding mill which is suitable for the purpose, is one of the main requirements for cost effective and trouble free processing of materials, if a grinding stage is involved. Most types of readily available hammer mills for agricultural grinding are not suitable for grinding of minerals.
The characteristics of the material, which are to be taken into account during the selection of the grinding mill, are (i) hardness, (ii) brittleness, (iii) toughness, (iv) abrasiveness, (v) stickiness, (vi) softening and melting temperature, (vii) structure (e.g. close grained or cellular), (viii) specific gravity, (ix) free moisture content, (x) chemical stability, (xi) homogeneity, and (xii) purity.
The hardness of a material is probably the most important characteristic to be considered when deciding on what type of mill to choose. Trying to grind a material which is too hard results either in costly damage to the mill or an expensive maintenance requirement. Generally for harder materials, specialized and expensive type of grinding mills are used. In addition if a particular mill can be used over a range of hardness scales, the harder the material the lower the throughput for a given size requirement.
Another characteristic of a material that is to be known is the brittleness, which is the degree to which a material breaks easily. Most minerals are brittle, as opposed to metals which are ductile, although some to a greater degree than others. Brittleness does not equate with hardness as brittle materials can be hard or may not be hard. It is not easy to grind materials which are not brittle to some degree.
In case of dry milling, free moisture content of the material is to be as low as possible. In practice, this can be a problem, especially in humid regions where the moisture can cause the material to stick to the grinding media. Different mills behave in different ways with moist materials and in some cases drying of the raw materials may be required.
The final size of the material after grinding is important. Usually required degree of fineness is specified which is normally stipulated that a proportion of the material is finer than a particular size. Usually this is 90 % or 95 % but may be 99 % for particularly demanding application.
The grinding mills are categorized in three groups, namely (i) tumbling mills, (ii) roller mills, and (iii) very fine grinding mills which include (a) high speed pulverizing or hammer mill, (b) vibrating mill, (c) ) pin mill, (d) turbo mill, (e) fluid energy mill, (f) stirred media mill.
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