Mercury is used in gold mining to separate fine gold particles from sediment and crushed rock. The process exploits a simple chemical property: liquid mercury bonds with gold on contact, forming a heavy amalgam that can be easily collected while the remaining dirt and sand are washed away. An estimated 2,000 tonnes of mercury are consumed this way each year, almost entirely in artisanal and small-scale gold mining operations across dozens of countries.
How the Amalgamation Process Works
The basic technique has barely changed in centuries. A miner crushes gold-bearing ore or collects river sediment, then adds liquid mercury directly to the material. Mercury has an unusual affinity for gold: when the two metals touch, gold dissolves into the mercury and forms a silvery, paste-like mixture called an amalgam. The miner agitates the mixture by hand, in a bucket, or in a simple rotating drum, giving the mercury time to contact as many gold particles as possible.
Once the amalgam has formed, the miner separates it from the surrounding waste material, often by panning or sluicing. Because the amalgam is far denser than sand or clay, it settles to the bottom while lighter material washes away. The result is a small ball or lump of mercury-gold amalgam, typically containing roughly equal parts of each metal by weight.
Burning Off the Mercury
The next step is the most dangerous. To isolate the gold, the miner heats the amalgam above 400°C, usually with a blowtorch or over an open flame. At that temperature, mercury vaporizes and leaves behind a porous lump called sponge gold. This sponge gold typically runs 16 to 22 karats in purity, depending on the ore and how well the process was carried out.
In most operations, the mercury vapor simply escapes into the air. When amalgam is heated without any containment, virtually all of the mercury is lost to the atmosphere. A device called a retort, essentially a sealed container with a condensation tube, can capture more than 90% of the vapor and return it to liquid form for reuse. But retorts remain uncommon. Many miners lack access to them, don’t trust them, or find them too slow for the volume of amalgam they process daily.
Health Effects on Miners and Communities
Mercury vapor is the primary health threat. Inhaled mercury passes from the lungs into the bloodstream and crosses into the brain, where it is converted into a form that cannot diffuse back out. It accumulates in neurological tissue over time, and the damage it causes can persist for years after exposure stops.
Chronic exposure produces a condition called mercurialism. Symptoms include tremors, memory loss, irritability, extreme shyness, and persistent insomnia. In artisanal mining communities, airborne mercury levels routinely exceed safe limits, and the people doing the burning are not the only ones affected. Family members, neighbors, and anyone nearby during the heating process inhales the vapor as well. Studies of former miners have found neurological symptoms lingering years after they stopped working with mercury, because the trapped form of the metal stays in the brain indefinitely.
How Mercury Contaminates Water and Fish
The mercury released into the air doesn’t simply disappear. Elemental mercury vapor can travel vast distances around the globe before settling back to earth. Once it lands in or near water, a chain of chemical transformations begins. Sunlight and atmospheric chemistry convert some of the mercury into an ionic form that dissolves easily in water. This dissolved mercury sinks to the sediment layer at the bottom of lakes, rivers, and wetlands.
There, bacteria that thrive in the transition zone between oxygen-rich and oxygen-poor sediment convert the mercury into methylmercury, an organic compound that is far more dangerous than the original metal. Methylmercury is the only form of mercury that bioaccumulates through the food chain, and it does so at staggering rates. Concentrations in fish tissue can be a million to a hundred million times higher than concentrations in the surrounding water. Every bit of mercury found in consumable fish tissue is methylmercury. It is also the only form that crosses into the human brain after ingestion, making contaminated fish a direct route of neurological exposure for communities that may be hundreds of miles from any mining site.
Why Miners Still Use Mercury
Mercury is cheap, widely available, and requires no electricity or specialized equipment. A miner working a remote riverbed with a shovel and a pan can carry a small bottle of mercury and extract gold that would otherwise be too fine to collect by hand. Gravity-based methods like panning and sluicing work well for larger flakes and nuggets, but very fine gold particles, sometimes called “flour gold,” slip through. Mercury captures those particles efficiently, and for a miner earning a few dollars a day, that difference matters.
The technique also produces visible, immediate results. A miner adds mercury, collects an amalgam, burns it, and holds gold in hand within hours. Alternative methods often require more steps, more equipment, or more time, all of which carry costs that small-scale operations struggle to absorb.
Mercury-Free Alternatives
Several methods can reduce or eliminate the need for mercury entirely, and they center on one principle: concentrating gold by exploiting its density. Gold is roughly 19 times denser than water and far heavier than the sand and rock surrounding it. Effective concentration can make mercury unnecessary.
- Sluicing: Water washes ore down angled platforms lined with carpet or textured mats. Gold sinks and is trapped by the lining while lighter material flows away.
- Shaking tables: Crushed ore feeds onto a vibrating, grooved table. Water moves lighter minerals off the edge while grooves direct gold toward collection points on the side.
- Spiral concentrators: A spinning pan uses water spray to wash light particles outward and downward while dense gold travels along spiral grooves toward a central collection point.
- Vortex concentrators: Water pumped into a bowl creates a rotating current that lifts lighter material up and out through a central drain, leaving gold on the bottom.
- Centrifuges: Mechanized versions of gravity separation that spin material at high speed to amplify the density difference between gold and waste.
These methods work best in combination. A miner might run material through a sluice first, then process the heavy concentrate on a shaking table. The goal at each stage is to reduce the volume of material while keeping the gold, so the final separation step handles only a small amount of highly concentrated feed.
International Efforts to Phase Out Mercury
The Minamata Convention on Mercury, a global treaty named after the Japanese city devastated by mercury poisoning in the 1950s, requires countries where artisanal miners use mercury amalgamation to take steps toward reducing and, where feasible, eliminating that use. Countries with significant artisanal mining are expected to develop national action plans outlining how they will transition miners to safer methods.
Progress has been slow. Eighteen countries that submitted national action plans reported a combined baseline of 352 tonnes of mercury use per year, a fraction of the estimated global total of roughly 2,000 tonnes. The gap reflects the difficulty of reaching millions of miners spread across remote regions in dozens of countries, many operating informally or illegally. Changing the practice requires not just providing alternative equipment but also training, financing, and building trust with communities that have relied on mercury for generations.