Sunday, June 3, 2012

Concentration of Gold Ores - a Key to Reducing Mercury Use in Artisanal and Small Scale Gold Mining

In artisanal gold mining, mercury is essentially used to concentrate the gold. It does this by bonding with the gold to form a heavy amalgam that is then easily separated from the other minerals. It follows therefore, that if mercury is used as a mechanism of concentration, producing concentrates of gold ores through other means can enable reduction or even elimination of mercury use. In other words, a good process of concentration can replace mercury and lead to big environmental and health improvements.

Concentrating gold ores before applying mercury greatly reduces the mass of material that comes into contact with mercury. This reduces the amount of material that becomes contaminated and also the amount of mercury that is required to be applied. And of course, if concentration is done with sufficient sophistication and knowledge, mercury is not needed at all.

As an important first step along the way to eliminating mercury, concentrating avoids a worst practice called whole-ore-amalgamation, where mercury is brought into contact with 100% or the ore and losses of mercury to the environment can be very high - as high as 50 parts mercury for every part gold produced (more often ranging from 20:1 to 4:1). Simple concentration methods reduce the amount of mercury required for amalgamation to roughly 1 part mercury per 1 part of gold recovered. A vast improvement. Better yet, almost all of the mercury that is applied to a good concentrate ends up in the gold-mercury amalgam and if this mercury is recycled, for example by using a retort, losses can be further decreased by an additional 95%. Operating in this way, only 1 part mercury is used for each 20 parts of gold produced. 

That is hundreds of times less mercury used and lost to the environment

Common methods of concentration that are, to a variable degree, accessible to artisanal and small scale gold miners are: panning, magnets, sluicing, spirals, vortexes, centrifuges, shaking tables, and flotation - in increasing order of complexity and cost.

Choosing a technology to produce a concentrate depends on the type of ore, access to capital, and having the knowledge and organisational skills with which to acquire and operate the equipment. Access to capital and expertise is often affected by legal status – with illegal or informal status being a significant barrier for miners to acquire and operate better equipment, and by extension a barrier to eliminate the use of mercury.

There is no doubt that helping artisanal miners gain access to the equipment and knowledge required to adequately concentrate their gold ores will help them reduce and ultimately eliminate the use of mercury. Technically speaking, replacing mercury with other processes of concentration can certainly be done. It is in fact, exactly what the large scale mining sector has done over the last decades. 

They used to use mercury and now they don't - and they get more gold without it 

With some innovation, finance, and most importantly, some political will, the same can be true for artisanal gold miners.