Energy utilisation

One of the first important investigations into the distribution of the energy fed into a crusher was carried out by OWENS(13) who concluded that energy was utilised as follows:

a) In producing elastic deformation of the particles before fracture occurs.

b) In producing inelastic deformation which results in size reduction.

c) In causing elastic distortion of the equipment.

d) In friction between particles, and between particles and the machine.

e) In noise, heat and vibration in the plant, and

f) In friction losses in the plant itself.

Owens estimated that only about 10 per cent of the total power is usefully employed. In an investigation by the U.S. BUREAU OF MINES(14), in which a drop weight type of

crusher was used, it was found that the increase in surface was directly proportional to the input of energy and that the rate of application of the load was an important

factor. This conclusion was substantiated in a more recent investigation of the power consumption in a size reduction process which is reported in three papers by KWONG

et al.(15), ADAMS et al.(16) and JOHNSON et al.(17). A sample of material was crushed by placing it in a cavity in a steel mortar, placing a steel plunger over the sample and

dropping a steel ball of known weight on the plunger over the sample from a measured height. Any bouncing of the ball was prevented by three soft aluminium cushion wires under the mortar, and these wires were calibrated so that the energy absorbed by the system could be determined from their deformation. Losses in the plunger and ball were assumed to be proportional to the energy absorbed by the wires, and the energy actually used for size reduction was then obtained as the difference between the energy of the ball on striking the plunger and the energy absorbed. Surfaces were measured by a water or air permeability method or by gas adsorption. The latter method gave a value approximately double that obtained from the former indicating that, in these experiments, the internal surface was approximately the same as the external surface. The experimental results showed that, provided the new surface did not exceed about 40 m2/kg, the new surface produced was directly proportional to the energy input. For a given energy input the new surface produced was independent of:

a) The velocity of impact,

b) The mass and arrangement of the sample,

c) The initial particle size, and

d) The moisture content of the sample.

Between 30 and 50 per cent of the energy of the ball on impact was absorbed by the material, although no indication was obtained of how this was utilised. An extension of

the range of the experiments, in which up to 120 m2 of new surface was produced per kilogram of material, showed that the linear relationship between energy and new surface no longer held rigidly. In further tests in which the crushing was effected slowly, using a hydraulic press, it was found, however, that the linear relationship still held for the larger increases in surface.

In order to determine the efficiency of the surface production process, tests were carried  out with sodium chloride and it was found that 90 J was required to produce 1 m2 of new surface. As the theoretical value of the surface energy of sodium chloride is only 0.08 J/m2, the efficiency of the process is about 0.1 per cent. ZELENY and PIRET(18) have reported calorimetric studies on the crushing of glass and quartz. It was found that a fairly constant energy was required of 77 J/m2 of new surface created, compared with a surface-energy value of less than 5 J/m2. In some cases over 50 per cent of the energy supplied was used to produce plastic deformation of the steel crusher surfaces.

The apparent efficiency of the size reduction operation depends on the type of equipment used. Thus, for instance, a ball mill is rather less efficient than a drop weight type of crusher because of the ineffective collisions that take place in the ball mill. Further work(5) on the crushing of quartz showed that more surface was created per

unit of energy with single particles than with a collection of particles. This appears to be attributable to the fact that the crushing strength of apparently identical particles may vary by a factor as large as 20, and it is necessary to provide a sufficient energy concentration to crush the strongest particle. Some recent developments, including research and mathematical modelling, are described by PRASHER(6).