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 thenbroken 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 6-inch drop. The screens were 30-mesh steel wire (900 holes to the square inch), and the output was about 1 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 toseparate 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.
A stamp is a metal that is moved up and down by a battering arm, pulverizing ores that are fed into it. They were mechanically operated machines that were used to crush ores by pounding till they extract the desired ore.
Gold pans and sluice boxes were necessary tools for the placer miner, but they were of little use in the large mines. Gold veins were locked up in rock, and ores needed to be grinded up to separate out the gold. While there have been other crude tools used to grind ore over the centuries, it was the use of the stamp mill that changed everything.
One of the earliest users were miners from Persia. The use of the stamp mills was also evident in most Islamic states in Spain and Africa. Although they were different than the more modern stamp design, they achieved the same effect.
The early miners that arrived during the California Gold Rush mostly focused on the rich and easy-to-mine placer deposits in creeks and rivers, but as the years went on it was evident that there was a lot more gold hidden in those mountains.
There was a huge demand for metals such as gold and silver, this powered the need for the improved stamp mills. The miners specifically liked them because they could pound the minerals into very fine powder thus enabling gravity concentration.
Stamp milling process especially that which involved gold mining had to have zero contact with grease or oil as the stamp fell into the stamp die, such a situation could be hazardous as the fine gold would be lost in the separation process.
They generally operated by the stamp being lifted by the camshaft, it would then fall down by means of gravity onto the stamp die. The ore would be brought to the mill by a ore car using a winch, it would then be gratted by a metal grate known as a grizzy into fine and coarse material.The finer material would move to the ore chute while the larger pebbles would be directed to the ore crusher till they are crushed into fine gravel.
The stamp battery worked by compressing the ore and by the abrasion of the rocks as they reacted to the impact of the stamp. Once the ore was turned into smaller particles it was then mixed with water, salt and mercury and crushed further by rotating iron plates. This would continue until it turned into a slurry or slime substance.
A mercury coated copper table would then be used to evenly spread the slurry. At this stage, gold would adhere to the mercury through the process of amalgamation while the other impurities would move to a shaking and vibrating table called the concentration table.
This table was designed to ensure that any gold remaining in the sand would be recovered. Later on, the gold and the mercury would be scrapped off the table and separated through chamois and retort process. These two process would be repeated till all the gold has been removed.
Several stamps were used on a stamp mill. Single stamps, 3-stamp, 5-stamp, and even 20-stamp mills were in operation at some of the larger mines. As mines grew larger and extracted more ores, the need for bigger stamping operations was evident. It was common for successful mines to upgrade their equipment throughout the years.
Bringing these stamps into remote gold mines was no small task. These were big, massively heavy pieces of iron. Even when they were deconstructed they were still heavy and needed to be brought in piece-by-piece to remote gold mines. The cost was immense, which is one reason that so many mines failed even though they had rich ores. Stamping was an added cost just like any other.
Stamp mills were large and cumbersome tools, but they were an absolute necessity for the hard rock miners. While it was possible to crush ores using more rudimentary methods, the mechanized process used with stamp mills meant that large quantities of rock could be pulverized.