The selection of grinding equipment requires a consideration of the purpose in grinding, the characteristics of the material to be ground, equipment available for grinding, and the manner in which it can be applied best.
How is Equipment Selection Made
It is obvious that coal to be used as a pulverized fuel will be pulverized dry. It is just as obvious that cement clinker must be processed into cement by dry grinding,
Some materials must be ground without contact with metallic surfaces, which would contaminate the product. In these instances, grinding equipment is restricted to pebble mills and autogenous mills, and grinding may be wet (using water, oils, acid solutions) or dry. Also, where very fine products are wanted in a dry state, ceramic-lined, fluid-energy mills are used.
H – Hard, abrasive (silica, corundum, calcined coke)
MH – Medium hard (limestone, barite, apatite)
S – Soft, non-abrasive (talc, gypsum, bituminous coal)
T – Tough, fibrous (mica, asbestos, wood)
G – Granular (closely sized)
F – Fine (< 150 microns, 100 mesh)
SF – Superfine (<30 microns, 400 mesh)
Experience and History
Thousands of grinding mills of all types, grinding many different materials, provide much of the knowledge of grinding performance which is used as a guide in selecting the size of equipment, as well as the type. This accumulated performance information is available to those interested, from technical publications, handbooks, and grinding equipment manufacturers’ records.
To apply experience and history, information on the mineralogy, chemical analysis, and (possibly) geology of the geographical area for the mineral to be ground, is essential. This enables those with experience to make comparisons with available grinding performance data, and judge with.surprising accuracy the equipment needed to meet a specific grinding requirement.
Grindability is a measure of the relative resistance to grinding of materials. It provides the means for predicting power, which determines grinding mill size for a specific grinding requirement.
The first grindability determinations were made using a small anvil and hand hammer. The breaking characteristics of an unknown material were compared with a known by similar treatment under the hammer. It is obvious that the technique, skill, experience and judgment of the wielder of the hammer are paramount. Many grinding equipment recommendations made using this technique supplemented by reference to known grinding history are performing as predicted.
Laboratory tests are more involved and require a much larger material sample than any of the grindability tests. The test samples required vary from several hundred pounds to several tons, depending upon the uniformity of the ore, the grinding characteristics and the product fineness requirement. The results from laboratory tests may be extrapolated directly without reference to other grinding data.
The following are illustrative: Test Plant results for a limonite ore were 6.3 net horsepower hours per ton, grinding a 12-mesh scalped feed to 95% passing 100 mesh. Plant results for a 45-ton per hour, ball mill-air classifier circuit are approximately 7.8 gross horsepower hours per ton for the grinding mill. Computed losses reduce this to about the same net horsepower hours per ton indicated by the test plant work.