Manual for Training Artisanal and Small-Scale Gold Miners

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24 GMP - Manual for Training Artisanal and Small-Scale Gold Miners machine and a supplier of hammers (made of cast iron). Some miners use leaf springs from truck suspensions or pieces of railway rails as hammers. In South America and China, hammer mills are very popular among artisanal gold miners. One inconvenience is that particles of gold can become retained inside of the mill and at the end of the cycle the mill has to be opened and vigorously cleaned with water. • Stamp mill This is another crusher that has the name of “mill”. Stamp mills operate simply by the crushing action of the cast-metal stamp heads as they fall onto the ore at the bottom of the mortar box. “Normal" stamp mills have 3 or 5 stamps. The stamps are lifted via the cast iron collars by cams driven by a pulley and flywheel. Feed is typically -50mm, loaded into the top of the mortar box by shovel, and discharge is typically -0.5 to 1mm via a fixed screen on the back of the box. Stamp mills can be operated either dry or wet, and can be either manually, electrically, or diesel driven. During operation, the base (die) of the stamps becomes deformed and quite often miners have to stop the crushing process to grind off the lips formed on the steel base. The crushing capacity of a 3-stamp mill is around 0.3 to 0.5 tonnes/h depending on the hardness of the ore. The cleaning of a stamp mill is easy but not quick, and usually involves manually digging out all the material accumulated in the mortar box between the stamp dies. Gold, as a heavy mineral, can be accumulated in the mortar box. Fig. 3.11 – Three-stamp mill in Zimbabwe discharging crushed material into a centrifuge Fig. 3.12 – Miner grinding the base of stamps inside a mill
GMP - Manual for Training Artisanal and Small-Scale Gold Miners In Zimbabwe, stamp mills are an accepted technology because the entire process is VISIBLE. Miners discharge the crushed product through a 0.5 to 1 mm screen directly into a centrifuge or onto an amalgamating copper plate. Gold liberation at 1 mm particle size is often poor, and is the main reason why Zimbabwean miners often recover less than 30% of the total gold. Grinding Grinding is undertaken on already crushed material to achieve adequately fine particle size necessary to liberate the most gold possible. Grinding typically uses some sort of tumbling mill: a round metal barrel driven either manually or mechanically and filled with a grinding media such as steel balls, rods or hard pebbles. Grinding can be undertaken dry or wet. Dry grinding reduces the wear on mill shells, liners and balls, but requires up to 30% more power than wet grinding. Common types of grinding mills are: • Chilean-type ‘Muller’ Mills Fig. 3.13 – Discharge screen This type of mill combines crushing and grinding. It utilizes two or three wheels running in a circular trough, driven by one of the wheels or by a central boss gear. The mill can either grind batches (where ground ore is taken out periodically and replaced with fresh feed) or operate continuously (for example where a continuous stream of water washes the ground ore out of the mill onto a sluice). The feed size is usually smaller than 10 cm. Some miners in Ecuador crush the ore in a small jaw crusher to 2 to 5 cm and feed this into the Chilean mill. They use a nylon screen (mesh size as small as 0.1 mm) at the discharge of the mill. The mill operates with water (usually a pulp of 20% solids) and the material discharged through the nylon screen goes to sluice boxes lined with mats. The process is continuous and the output depends on the size of the mill and the grain size of the product. Typically the production is between 0.2 to 0.5 tonne/h for product ground below 0.2 mm. Chilean mills are very efficient, but unfortunately many miners in South America and China add mercury to the mill - the mercury becomes finely pulverized (floured) and is lost with the tailings. 25
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Manual for Training Artisanal and Small-Scale Gold Miners

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