Operators in the metallics industry, sometimes forget that pebble mills are an important factor in fine grinding. However, pebble mills are not extinct, as is shown by reference to a recent paper on ore milling, wherein it was shown that five of the 44 so-called “ball” mills mentioned actually were pebble mills. If the nonmetallics industry included, pebble mills would be found to have a really important place.

In both wet and dry grinding, pebble mills have been used on plant problems in the Rolla laboratory and experience with them has overcome some of the early prejudices. One of these arose from the belief that pebbles had to be much larger than steel balls. The results of an experiment in which the balls and pebbles were of the same size, given in table 21, show that dolomite yielded about the same type of grinding with each medium and the efficiency of the pebbles was only 18 percent below that of the balls. The pebbles were irregular in shape. Their failure to present spherical surfaces might have reduced their effectiveness. (As is well known, large quantities of quartzite “pebbles” are used in tube mills on the Rand. They are waste sorted out and are effective and economical.)

With this adverse disparity must be considered the low density of the pebbles, so that they contributed only 69 percent of the total weight of pebbles, ore, and water, all of which required power to impart action to them. In the rod mill with a large discharge (tables 3 and 4) and “rods” with the same density as that of pebbles, the ratio of weight of rods to weight of ore and water was more favorable because of the smaller amount of voids. In table 21, as in table 3, the powers are proportional to the weight of media plus ore plus water. The results with the chert, shown in the left side of table 21, require more critical consideration. Undoubtedly the pebbles failed to grind the coarse material because they were too small or too light. The pebbles failed to grind the coarse chert, whereas the grinding by the balls was about the best on record for the particular speed and charge employed. The balls were known to be the best size for the chert, and it is apparent that the pebbles should have been larger. If the balls had smaller and the pebbles had been correspondingly smaller, then the pebbles might have displayed the same deficiency they showed with chert in all, the tests indicate that pebbles should be larger than balls for a comparable type of grind.

The pebbles gave about 37 percent of the capacity and about 81 percent of the efficiency secured by the balls. The efficiency of pebbles in wet grinding (table 21) was about the same as that of ball in dry grinding (table 14).