When stamp mills are used for dry crushing, double discharge mortars are provided (see Fig. 50), and the screens are put low down, giving a small depth of discharge. These arrangements are necessitated by the difficulty of discharging the crushed ore from the mortar, the only means of doing this being the dashing of the dry ore against the screens, due to the fall of the stamps. Expediting the delivery of the ore by a blast of air forced into the mortar, or by suction from outside, is not often attempted. The result of the slowness of discharge is that with a given fineness of the mesh of the screens, dry crushing gives a finer product than wet crushing. This is an advantage with many ores, in respect that the gold is more readily accessible to the solutions, but, on the other hand, the product is more difficult to leach. The output in dry crushing is much lower than in wet crushing by stamps, and the cost per ton of ore is stated to be from 50 to 100 per cent. more.
One of the difficulties in dry crushing is the production and loss of dust, which is generally richer than the bulk of the ore, for the reason that the sulphides are more brittle than the gangue. This difficulty is partly met by surrounding the crushing machine with an outer casing, and drawing off the escaping dust with air suction. Nevertheless, some dust escapes, involving a loss of values and also some injury to the machinery and to the health of the workmen.
Dry crushing by stamps is largely used in the treatment of silver ores by pan-amalgamation or by hyposulphite (thiosulphate) leaching. It was introduced on a large scale in 1894 for crushing the oxidised quartzose ores from the upper levels of the Waihi and other Ohinemuri (Upper Thames) Mines, New Zealand, and was finally abandoned in favour of wet crushing in 1904, partly owing to the fact that the character of the ore had changed with the working-out of the upper levels.
At the Waihi Mill the ore was dried and partly roasted in kilns, with wood as fuel, and then broken down in rock-breakers before being fed to the stamps. The stamps weighed 1,000 lbs. each, and gave 94 blows a minute with a 6½-inch drop. The screens were 30-mesh steel wire (900 holes to the square inch), and the output was about 1¼ tons per stamp per day. Over 77 per cent, of the product was fine enough to pass a 90-mesh sieve. The ore was then charged into shallow leaching vats and cyanided, the solutions being run in from below. The depth of the charge was from 20 to 30 inches, and vacuum leaching was used. A depth of three feet of ungraded pulp was found to be unmanageable, and no attempts were made to separate the slimes. According to James, the cost at the Waihi Mill was stated to be 9.58d. per ton less for dry stamping than for wet stamping on the same ores.