Grinding tests of limestone were performed using the Hardgrove's testing machine. Energy consumed (E kg. m) by grinding was obtained by the measurement of torque of the axis of the machine. Specific surface area increased (ΔS_vcm²/cm³) by grinding was calculated by the size analysis of the products. Grindability was shown as the ratio of ΔS_v to E.
As a result, the grindability indices of 17 samples of limestone from 14 quarries were ranging 8-18(cm²/cm³)/(kg.m). These values of limestone were larger than those of quartz, feldspar, fluorite, kaolin and bentonite, and smaller than that of talc, and two to five times as large as that of quartz.

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In this report, the mass balance equation of comminution process is investigated from the view point of that the selection function in this equation is influenced by the size distribution in a mill.
Though in several reports some questions have been pointed out about the assumption that the selection function is independent of the size distribution of powder in a mill, few reports try to investigate this problem experimentally.
In this report, the calculation of the selection function are made in every change of the size distribution by grinding using the method proposed by Austin et al.
The results and discussions are summarized as follows;
(1) The value of selection functions obtained is influenced not only by the particle size but also by the amount of weight retained on the size.
2) These influences are characterized by each grinding condition such as rod loading weight or pulp density in a mill.
3) A mathematical model, in which the selection function is modified by the size distribution in a mill, assimilates closely to the experimental results of a closed circuit grinding as well as batch one.
4) Some examples of this mathematical model applications are also shown, where the plug flow in a mill is assumed.

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In this report, the measurement of the energy transfer in comminution that is one of the effective methods to analyse the grinding process is discussed by comparing the observed energy efficiency of cominution in various kinds of mills.
The mills used were a vibration mill, a stamp mill, a grinding device with two rods as a grinding medium, and a Hardgrove machine.
The energy efficiencies of comminution in the mills, except the vibration mill, were calculated from the data of the breakage function.
The following results are derived.
1) The mechanical energy loss in grinding process is about 70-80%
2) When the materials are ground in the state of particulate bed, the energy loss is about 70%.
3) When a particulate bed of the ground material flows through among the grinding mediums, the energy loss is about 50-70%.
From the calculation of the energy efficiency of a practical milling process, the energy efficiency is estimated as about 1.8-4.5%.

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