From the preceding discussion it should be obvious that grindability tests such as those performed by Bond are approximately valid for a relatively narrow size range only. Extrapolation of the resulting data for size range covered by coarse and fine crushing as well as by very fine and superfine grinding offers little-if any-dependable information at all.
It may be argued that crushing, even with all the conventional crushing steps combined, requires so little energy (usually of the order of 1 k ~ h / t t)h at even precise information is of questionable practical value in the design of a crushing plant . In a search for truth, such arguments should be disregarded.
To obtain truly dependable experimental information on the various steps of size reduction, the tests should be performed in accord with the basic principles of the respective industrial methods of size reduction. Crushing should be investigated with a machine such as a jaw crusher , coarse grinding (rod milling) in a set of rolls, and fine grinding in a ball mill . The fineness of the feed and product in each step should be defined in clear and simple terms. Each step should represent (for example) a 10 to 1 reduction in product size x. In all cases, the net power should be evaluated and the net energy consumption calculated.
By a procedure outlined previously, three indices would be obtained: one for crushing (from 10 to 1 cm), one for coarse grinding (from 1 cm to 1 mm), and one for fine grinding (from 1 to 0.1 mm). Each of the indices would be characteristic for its respective size range. On the basis of this information, a cumulative net energy-size curve could be drawn.
Each mineral, rock, and ore will have its own characteristic size reduction curve. From this curve the net energy required for any conventional crushing and grinding operation can be very closely obtained. By knowing the mechanical efficiency of the machinery and the tonnage treated, sizes of the machinery and motors can now be selected at a good accuracy.