Artisanal Miners of the Amazon: Part 2, The Effects of Mercury

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.

Artisanal Miners of the Amazon: Part 1

To start, I would like to share some photos taken across the Amazon Basin, primarily within the Guiana Shield. Artisanal miners exploit alluvial gold shedding off of gold-rich regions (likely eroded orogenic gold deposits) of the amazon and in some cases mine in-situ gold deposits in chemically weathered environments. Their methods are simple yet effective. Often pumping a few days worth of slurry over a sluice box, panning the trapped concentrate and repeating until the area is depleted of accessible gold. 

Fine gold in a pan concentrate. Additional work is required to separate the gold from iron-rich minerals or "black sands". 

Hard work and skill can pay off big. These workers are part of a modern day gold rush, prospecting in remote areas, pushing the boundaries of known gold occurrences.  

An artisanal miner shows off his spoils. He explains,  four miners working a single stream for a month produced the amount shown. Others are not as fortunate, junior explorers included. 

Quicksilver. Despite the health hazard, mercury is still used in the recovery of fine gold. Here excess mercury in a pan is poured into a submerged cup for later use.

The current dependence on mercury to recover gold has caused environmental concerns. If used irresponsibly mercury can contaminate soil and drainages with devastating long-term effects. Development and promotion of efficient gravity recovery methods, such as centrifugal concentrators, are important to help eliminate the risks of mercury contamination. 

A gold amalgam is produced by mixing a gold-rich concentrate with mercury. The mercury is later removed by heating in a retort. 

Governments are pushing for the replacement of mercury recovery techniques with the use of gravity recovery methods. I plan to expand on these methods in future posts.

For more information on mercury and its effects check out the Mineralogical Association of Canada Short course series Volume 34, Mercury: Sources, Measurements, Cycles and Effects.

Brodie