The cost of grinding ores by the tube mill may be gaged by the following list of mills grinding gold ores to practically 200-mesh screen, except in the case of the Homestake where 100 is sought. to pay more attention to this point.

As this last item shows an unusually low cost for tube mill grinding even with 30 per cent. 200-mesh, a further analysis may be of interest.

Nine months operation until lining was changed, 5 by 14-ft. tube mill

Pebbles per ton of ore crushed               $0.0120
Lining per ton of ore crushed (El Oro)      0.0197
Repair labor per ton of ore crushed         0.0003
Power per ton of ore crushed                    0.0410
Total                                                              $0.0730

Man running tube mill also looks after 12 vanners, two Wilfleys, one pump, two sets of rolls and two elevators. The mill grinds approximately to 30-mesh with feed 17.5 per cent, on 6-mesh for an average of 30 hp.

The cost sheet of the regrinding plant at the Homestake mine, South Dakota, is here given as being of particular interest. The feed contains 25 per cent, on 50-mesh screen and is ground without classification.

Regrinding the %-mesh steam stamp product at the Winona mill, Michigan, the Hardinge conical mills in 1914 showed the following results:

Costs Cents per ton ground
Power at 1.054 cts. per kilowatt-hour      13.64
Labor                                                               0.65
Pebbles at 0.8 ct. per pound                       2.08
Silex lining at 1.436 cts. per pound           0.09
Steel lining at 5.242 cts. per pound           0.33
Sundry charges                                             0.43
Total                                                               17.22

It is possible to reduce the cost of tube mill grinding at the expense of the primary crusher, but the careful millman will so proportion the work of his various machines that the whole cost of reduction will be at the lowest compatible with economical extraction of metal.

The output of a tube mill will average about 10 tons of ground sand per ton of pebble charge per day, so that knowing the interior dimensions of the mill and the line at which the load of pebbles is kept we may find the weight of pebbles in the mill by the table already given and by multiplying by 10 we arrive at an approximation of the amount of sand ground to say 85 per cent. 200-mesh, which is an average screen analysis.

The output per horsepower will vary with the size of feed and discharge. Grinding to 90-mesh as practiced in South Africa the capacity is commonly reckoned at 11/2 tons per horsepower, while the El Oro output at 150 to 200-mesh will average 1.2 tons per horsepower. The following list, taken from data at hand, is approximate.

For an average quartz ore grinding to the usual mesh for cyaniding we would consider 1.4 tons per horsepower to be a fair average.

The following table shows the cost of tube mjll grinding compared with the cost of crushing with primary and intermediate machinery. This table may be useful when it is desired to have a means of comparing the cost of grinding with a tube mill in place of stamps.