The Danish pebble
The Danish pebble is dark gray in color, hard and free from fracture planes which makes it an ideal grinder ; the French pebble is of lighter color and wears faster, while the Newfoundland pebble is more liable to break than either of the two mentioned due to minute cracks or planes of fracture. The dark gray pebbles appear to wear better than those of lighter hue, whether they be Danish or French.
Pebbles, to give the maximum efficiency, should be round, and where irregular pieces of local quartz or other rock is used in lieu of flint pebbles considerable work must be done on them in the mill before they become good grinders. The small pieces broken off in this process are too small for grinders and too large for being ground effectively, causing increased wear on the liners and increased power consumption. When local quartz is used, the sharp edges should be broken off or the quartz should be revolved in a roughing machine which then leaves the rock in better condition for grinding. While flint pebbles are used in most mills for the reasons stated, in other mills the use of mine rock has been found economical. It is more a question of freight rates than grinding efficiency. At the El Tigre mill, Mexico, the consumption of Danish pebbles was about one-tenth of the amount of mine rock, while the wear on the liners was less and a better product was obtained by the use of the former. At the Purisima Grande mill, Mexico, hard pieces from the mine about 4 in. in diameter were used consuming about 1J^ tons a day.
At the Santa Gertrudis " flint pebbles were used for a considerable time, but their increasing cost led to the adoption of mine rock entirely to replace the pebbles. The mine-rock supply is obtained mechanically in the crushing plant and is sent separately over the regular conveyors to a compartment in the battery bin, from which it is transferred by chute to the primary tube mill floor, where it is distributed by car. Part of the rock is introduced into the tube mill through the feeder. As the trunnion opening of the mills is not as large as it should be, rocks over 5 in. size as well as occasionally large boulders, 12 to 15 in., required in the primary mills for efficient grinding, are loaded into the mills through the manholes once a day; 130 Ib. of mine rock is required for each ton of ore milled and is credited to the total tonnage treated."
The economical grinding point is taken at 75 per cent, through 200-mesh. The following table is interesting as the grinding is done with mine rock and at a plant with a capacity of 1000 tons daily.
Moisture 35 to 40 per cent. Operating without return, 175 tons of ore pass through tube mill per 24 hr. Operating with return, in closed circuit, 200 tons of ore per 24 hr.
