A worker identifies a gold-rich layer and breaks it up for later washing.
Why Use Mercury?
- Bonds with Au forming an amalgam
- Process is quick relative to other recovery methods
- Cost is lower than other recovery methods
The simple low cost design of sluice boxes make them easy to maintain. Slurry is introduced through the barrel at the top controlling the flow and distribution of material over the sluice box surface.
Mercury Loss
- Between 10 to 30% of mercury used in artisanal mining is lost to the environment.
- It is estimated that 196,000 tonnes of Hg was lost in Central and South America from 1570 - 1900 (Nriagu, 1993)
The Effects Include:
Exposure by ingestion, inhalation or contact with skin can lead to:
- damage of the digestive, nervous and immune systems
- kidney and / or lung failure
- complications with fetus development
Artisanal miners are known for exploiting near surface gold deposits. In some cases, to increase the recovery of fine gold these miners will use mercury in their process. Mercury loss is common and can lead to hazardous effects to the local environment.
The sluice box, a trademark tool for extracting gold from alluvial deposits.
Mercury in the Environment
- Bacteria can form organic methylmercury
- Both mercury and methylmercury enter the food chain
- methylmercury is able to migrate through cell walls
The sluice box surface. Expanded steel creates turbulence cells when water flows over, trapping heavy minerals. A layer of prospector's matting lines the bottom of the box, this layer is removed for cleaning after a 2-3 day cycle.
The chart shows the biomagnification of 1 ppm of mercury in water. Mercury is not easily removed from an organism causing it to bioaccumulate, concentrations then increase as the heavy metal is passed through the food chain. (Environment Canada website)
There are gravity recovery methods that allow for the extraction of fine gold without the use of mercury. However these methods have higher start-up and operation costs versus traditional methods. Government regulation and incentives towards the use of gravity recover methods is required to reduce the unnecessary use of mercury in the future.
A worker washes a gold-rich clay layer into a sump that is later pumped over a sluice box. Mercury is often introduced into the environment at this stage making management and recovery difficult.
Reference
Nriagu, J.O., 1993, Legacy of Mercury Pollution: Nature, v. 363, p. 589.