Eradicating mercury from artisanal and small-scale mining

Materials World magazine
,
25 Mar 2019

The UN launched a new initiative to eradicate mercury-based mining methods, replacing with alternatives for safer mining. Shardell Joseph finds out more.

A US$180m project was launched by the United Nations (UN) with the goal of improving the conditions of artisanal miners across eight countries, while slashing harmful mercury emissions. Recognising the health risks of exposure to mercury, the Global Environment Facility-backed Global Opportunities for the Long-term Development (GEF GOLD) programme will aid the transition to safer alternative methods of mineral processing in mining.

Artisanal and small-scale gold mining (ASGM), accounting for 20% of the world’s annual gold production, is the largest source of man-made mercury emissions, releasing up to 1,000t of mercury into the atmosphere annually. This exposes 10-19 million people from more than 70 countries to risks of brain damage, vision and hearing loss, and delayed childhood development.

Reducing these emissions, however, has proven extremely difficult. In many countries, ASGM relies on methods that use elemental mercury. Although it does not produce 100% pure gold, ASGM widely uses this approach as it eliminates the bulk of impurities.

Mercury amalgamation

‘Amalgamation is a simple process, which works reasonably well with almost any gold ore and needs very little to almost no investment. Even the poorest miners can afford it,’ says ASGM expert and Alliance for Responsible Mining Board Member, Felix Hruschka. Warning of the consequences, he adds, ‘as mercury is lost during the amalgamation process into air, water and soil, most mercury ends up in waterbodies. Fishing communities living downstream of mining sites are also often severely affected’.

Mercury in mining has been a method used for centuries, tracing back to Ancient Rome. Because of its atomic structure and low boiling point, it has always been effective for extracting gold in a process of amalgamation and distillation.

For whole ore amalgamation, rocks and ore are crushed into small pieces by hand until they have reached a coarse consistency. With water, this is then added into motorised mills or trommels. Because of the need for water, the equipment is often built near waterways, subjecting the natural resources to severe mercury pollution.

To extract the gold from the crushed ore, liquid mercury is added directly to the trommel and mixed for several hours, creating a dense mercury-gold amalgam, which separates from the finer crushed rocks and sand. The amalgam is then isolated by hand and excess liquid mercury is recovered for reuse. A mercury-gold amalgam ball is produced – typically between 40-80% mercury by mass.

‘One of the advantages of mercury is that it is relatively easy to procure, it is cheap and fast,’ says Jerome Stucki, Project Manager responsible for the GEF GOLD projects implemented by UNIDO. ‘This means that a miner can go and extract rock during the day, crush it for some hours, and it is already extracted with gold so they can sell it.’

Using gravity

ASGM provides millions of jobs and huge economic benefits to communities – therefore mercury in mining cannot be reduced or eradicated without finding practical and cost effective alternatives.

There are various different techniques that have been considered as an alternative use by ASGM operations. Favoured by the GEF GOLD project, gravitational concentration is the main one. This type of processing separates minerals according to their different densities. Elemental gold has a specific gravity of 19.3m and a typical ore has a specific gravity of about 2.6m.

All gravity concentration devices create movement between the gold and host rock particles which separates the heavy materials from the lighter ones.

The gravity concentration process is done effectively when applied to a narrow particle size range. For success, the most important factor is the liberation of gold from gangue minerals. The most popular gravity concentration devices are sluice boxes, rocker boxes, jigs, spirals, shaking tables, centrifugal concentrators and dry washers.

This technique is low in capital and operation costs, can be applied to relatively coarse particles as well as finer materials, a reagent is not required, and has little to no environmental concerns.

‘For these people, it’s a livelihood, and sometimes they know mercury is dangerous or toxic, but they don’t have any alternative,’ Stucki says.

‘For every kg of rock they are extracting, they are managing to extract 40-50% of the gold and the rest goes to waste. What we are trying to do is introduce these techniques that will allow them to recover more gold. We are helping the miners to recover more gold and to help them sell it at a price that is closer to the international gold price.’