Recapture system limits mercury exposure from gold purification

According to the World Gold Council, about 195,300 tons (177,200 tonnes) of gold have been dug out of the ground in all of human history. Countless lives have been lost obtaining the rare metal, and in the developing world, which currently accounts for 20 percent of the world's gold production, small-scale mining and smelting under primitive conditions poses a major health hazard. To help alleviate this, a team from Argonne National Laboratory and the US Environmental Protection Agency (EPA) are developing a prototype mercury capture system (MCS) to reduce heavy metal pollution.

A common method used by small miners in South America and other places is mercury extraction, which exploits the ability of mercury to "dissolve" gold and form an amalgam. This way, particles of gold can be separated from crushed ore and floated away. When the mercury is boiled away, the gold remains behind. This process is now outmoded in the mainstream mining industry, where it's been superseded by cyanide and other methods that are cheaper, more effective, less expensive, and much safer, but is still in common use in the developing world.

The use of mercury is more than a theoretical hazard. The removal of the mercury in smelting the gold is often incomplete, so when goldsmiths melt gold in their city shops, more mercury is released. Worse, the protections against airborne mercury are notoriously poor with only five percent of gold shops in Brazil having any mercury control systems. Usually, the shops have little more than extraction hoods that empty directly into the open air, and Argonne says that Peruvian shops have no protection at all. Not surprisingly, the mercury levels in and around gold shops in the developing world generally exceed safe levels.

The upshot of this is that these small operations account for 700 tons (635 tonnes) of airborne mercury being released each year. The heavy metal falls back to earth, where it is inhaled, is picked up on exposed food on market stalls, and washes away into streams, where it enters the food chain. Its this latter problem that drew the attention of the EPA, because it can get into food exports, which eventually reach US tables, as well as airborne aerosols that can drift into US water systems.

Argonne and the EPA's answer is the prototype MCS, which consists of a converted steel drum that condenses and recaptures the mercury vapor with 80 percent effectiveness. An Argonne/EPA team worked with locals in the Amazon region to refine the design of the MCS and to build prototypes, which cost about US$500 each. These were then installed in several gold shops along with fume hoods and exhaust chimneys.

The prototype is made of a 55-gallon drum fitted with a circulating fan, baffles for collecting the mercury, and a piping system to collect and remove the liquid metal. The over-large hood, the drum, and the baffles provide the mercury vapor plenty of time to condense, which is the key to the device's efficiency. The team says that the MCS is affordable and can be retrofitted to existing shop hood ventilation systems.

"We wanted to design a system to address the significant mercury exposure affecting both the shops and the homes that purify gold," says Argonne environmental systems engineer Margaret MacDonell. "Many people have been burning mercury for decades because that is the way their grandparents or neighbors did it. We helped develop a better way of thinking about the process."

The video below explains the MCS project.

Source: Argonne National Laboratory