The usual method of grinding that has been adopted by ASM in SBM is stamp milling. SBM is perhaps the only country where outdated but appropriate stamp mills are in common use. Despite of being less efficient than ball mills, stamp milling is an accepted technology in SBM as the entire process is VISIBLE and TRANSPARENT. Stamp mills have the advantages of not requiring prior crushing beyond what can be achieved manually with sledgehammers, of being relatively easily cleaned out between ore batches and of being robust and simple to operate. On the other hand "traditional" stamp mills are large devices, slow to erect (requiring cranes, etc), relatively high capital and relatively inefficient.
These mills are made by a number of SBM companies, but cost and size dictates that they are normally owned and operated as "custom" mills, where several artisanal miners will use a single mill, owned normally by an entrepreneur, the miller. The mill mortar box – generally of deep, rectangular cast iron construction - must be cleaned out between ore batches. Cleaning is easy but not quick, and usually involves manually digging out all the accumulation in the mortar box between the stamp dies. The Stone Crusher owners normally also extract heavy fees from the ASMs.
The Small Mining Supplies developed a much smaller, single-stamp mill - the "Katanka" (so named because of the noise it makes) which is intended to provide one mill per miner, or per co-operative (Fig. 1). The mill frame is of flanged pipe construction rather than heavy, single wooden beams, and is designed to be transportable by small truck when disassembled. Assembly is possible using shears legs and a shuttering kit is provided to enable casting of foundations, or even supply of pre-cast block foundations which simply require setting and leveling.
The Katanka has a cylindrical mortar box that provides the advantage of more useful (multi-directional) splash-back of ore onto the stamp die. The mortar box is of steel-lined with a bolted front face that is easily removed for cleaning. Production rate, while lower than the big 3 or 5 stamp mills, will be more suitable, for demonstration and even for the scale of ASM operations
The discharge screen size is altered easily by changing the mesh in the discharge splash box. Typical size is about 0.6-0.8mm aperture, at which P80 is probably about 0.3-0.4mm. The specific benefit of the Katanka is the fact that it is small enough as a single stamp to warrant one-man-one-mill. "Normal" stamp mills have 3 or 5 stamp, and a Katanka 3 or 5 stamp mill would not offer any benefit over the mills currently available in SBM. These are very large and well beyond the means of most ASMs, and also need very much bigger foundations and frames as a result of the odd numbers of stamps, compression strokes, etc. The Katanka has pre-cast foundation blocks. At each site the ground needs minor excavation, leveling and compaction, placement of the pre-cast foundation blocks and erection of mill. Casting of cement foundations is NOT necessary.
How Stamp Mills Work:
A stamp mill is a large mechanical device used to crush ore and extract the desired metals from rock – the host material. It uses heavy steel stamps to pound and break apart rock, releasing the valuable metals from worthless rock, allowing for the extraction of gold and silver for further refining.
The basic design of a stamp mill has been used for thousands of years for a variety of crushing applications. Its traditional use has been for the processing of mineral extraction, whether it is copper, silver, gold, or any other metal contained within host rock. The first stamp mill in the United States was built in 1829 near Charlotte, North Carolina.
Stamp mills during the early gold rush days were generally powered by water, although sometimes steam engines were used as a power source. Their construction typically involves a series of heavy metal stamps arranged in a wooden frame called a stamp battery. A system of rotating shaft and cam is used to raise and lower the stamps on top of the ore. Stamps were usually built in banks of five and the number of banks built and used varied with the amount of ore needing to be processed. Smaller mills were often composed of only one bank of five, some were composed of two banks of five and this one on the museum property was one of the largest – a 20-stamp mill, four banks of five. The stamps themselves are extremely big and heavy, made from steel or cast iron heavy enough to pulverize the ore beneath. The stamps are repeatedly raised and dropped onto ore that is fed into the mill, until the coarse chunks of ore are reduced to finer material capable of further processing.
The stamp mill provided an invaluable need for the early gold miners. Unlike placer deposits that produced gold nuggets already separated from rock, lode gold deposits were of little value if the gold could not be efficiently removed from the host rock. With the ore crushed into fine powder and the gold released from its host, the amalgamation process with mercury or cyanide could be done, allowing for the final extraction of the gold.