The refractory nature of gold ores is often associated with gold finely disseminated in sulphide minerals, such as pyrite, at conventional grind sizes. Conventional milling can liberate the pyrite from the gangue allowing a low mass pyritic concentrate to be produced by a process such as flotation. However, direct leaching of the concentrate results in poor gold extractions as the cyanide lixivant is unable to contact the gold locked or included within the pyrite

Ultra Fine Grinding

The traditional approach for such refractory material has been to liberate the locked gold by chemically destroying the pyrite through oxidation. Roasting, pressure oxidation, and bacterial oxidation all employ various degrees of temperature, pressure and catalysis to react the pyrite with oxygen to produce an iron oxide and sulphur by-products. This method efficiently liberates finely disseminated gold or gold in solid solution.

Whilst such oxidative reactions are metallurgically sound and are capable of achieving high metal recoveries, the environmental aspects of treating the reaction products can alter the economics of the process.

Fine Grinding

An alternative, applicable to the liberation of disseminated gold from the host mineral, is to continue the grinding process to further reduce the particle size of the host mineral thereby exposing a part of the gold surface for contact with cyanide solution. A benefit of this technique is that the host mineral is not destroyed in an oxidative chemical reaction with the resultant problems of treatment of the reaction products. Such fine grinding, however, has proven to be increasingly energy intensive with each size reduction step. In pit blasting, primary crushing, secondary crushing, SAG and ball milling are all able to exploit natural fracture planes in the ore allowing breakage along these features. As progressive size reduction occurs, the naturally occurring minerals are liberated and a point reached where the crystal structure of the mineral has to be broken for further size reduction to be achieved. This may present a significant barrier to further breakage with higher power intensities required to achieve a breakage event.

In the past, inefficiencies associated with conventional milling have made fine grinding unattractive to the mineral processing industry. The desired grinds for the harder minerals could only be achieved by prolonged milling with resultant low throughput and high power consumption. The use of smaller media in closed circuit can assist tumbling mills to achieve fine grinds but remain fundamentally limited in the manner in which they impart kinetic energy to the media as well as having large “dead zones” where little media movement occurs (Kalra,1999). As finer media is used, the kinetic energy imparted to the media lessens thus significantly reducing the available energy transfer in a media/particle contact event.

Ultra Fine Grinding

UFG mills overcome these limitations by the use of rotating stirrers inside a stationary mill shell. Ultra fine grinding mills have been in use for many years in a large number of every day applications such as pharmaceuticals, dyes, clays, paint and pigments before being used in the mineral processing industry. They usually fine grind in a range of l μm to 10μm, and impart a significantly increased surface area as well as other potentially desirable properties such as colour, ease of absorption into the blood stream and increased chemical reactivity.

The use of UFG grinding in the minerals processing industry is a relatively new development being based on the smaller low mass, batch UFG mills being used by other industries for high value products.

The chief requirement of the minerals processing industry was a mill that could process quantities in the order of several tonnes per hour in continuous operation whilst maintaining cost efficiency in power and media usage.

Two basic types of UFG mills are available, the vertical stirred mill and the horizontal stirred mill. Both use rotating stirrers within a stationary mill shell to impart kinetic energy to a fine media charge (usually sand). The breakage mechanism is the same for the two mills, the differences being related to stirrer speed, method of media retention, and size of currently available mills.