Comminution process can be regarded as an assembly of single fractures, and as a natural result the behaviour of the materials in single fracture an essential role in the performance of the comminution system. In this paper glass spheres and irregular shaped limestone particles with different grain sizes have been comminuted under various impact velocities and the behaviour of these materials in high velocity comminution has been investigated and the results have been analysed for the size distribution of the products. Using these results the authors have tried to estimate a performance of a high velocity impact comminution system by simulation study. A summary of the results is as follows:
(1) Size distribution of the product is with a unique function of the impact velocity, and can be normalized in terms of the feed size.
(2) The results of simultion indicates that the multistage comminution system of relatively low impact velocity can make the best use of comminution energy.
(3) Impact comminution experiments on limestone particles showed that the peak of the size distribution of the products is consistent with the crystalline grain size of the materials.

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Grinding kinetics which was a field of active development in the 1960's has become settled for practical applications towards the 1970's. A serious problem was a possible reduction in efficiency for large-scale operations. Research was also extended to the comminution of non-brittle materials including wastes.
Another tide was an intensive research activities in the production of very fine materials. Endless effort is made for the improvement in the efficiency of size reduction. In this context special attention should be paid to the revival of roller mills for the grinding of raw materials in cement production.
Reference is also made to the mill lining techniques, development of the centrifugal mill and the computer control of milling operations.

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