Mill/Milling

(i) In mining, the term mill has come to cover the broad range of machinery inside the mineral treatment plant where the gold is separated from the ore. The type of mill will vary from mine to mine depending on the type of ore and processing method. Each mill, in short, is tailor-made. It often represents the largest single capital cost in the development of a new mine and the milling may account for up to half the working costs if a complex ore is involved.


Ore being crushed and milled in tube mills until it resembles talcum powder
(Credit: courtesy The Chamber of Mines of South Africa)

The scale of the milling operation can be immense: in South Africa nearly 80 million tonnes of ore is milled each year to produce about 400 tonnes of gold. However, to meet the need of small-scale open-pit mines in Australia, mobile milling plants have been developed that can be readily moved from one site to another.

Before it reaches the mill itself, the ore will often have been through primary and secondary crushers to break it down into more manageable pieces. The initial stage of milling is to grind this broken ore into a fine powder by passing it through rotating cylinders filled with steel balls, known as ball mills, or with short steel rods, known as rod mills. In the emerging rock dust most of the tiny particles of gold contained in the ore will have been exposed. The gold dust then goes into the cyanidation mill, where it is dissolved out into a solution; a technique originally pioneered with the MacArthur Forrest process.

The gold is recovered from this solution either by the more traditional method of adding zinc dust, which has the effect of taking gold’s place, allowing it to be precipitated out, or, increasingly, by the modern technique of carbon-in-pulp.

The next stage is smelting. The gold is heated in a furnace with silica, borax and soda ash which soaks up most of the impurities, forming black molten slag which rises to the top of the furnace while the heavier gold settles to the bottom. This gold is poured into ore bars, usually with a minimum fineness of 850 fine that are shipped from the mine to be refined.

(ii) In the semi-fabrication of gold products, the term mill is applied to a variety of processes in rolling. A breakdown mill is used for the fast reduction of cross-sectional dimensions of sheet or strip by hot or cold work. In a finishing mill, rolls of sheet or strip gold alloys are carefully prepared to impart good surface qualities. A tape mill is used for continuous rolling of narrow tapes of thin material that may be used for chain making.