Artisanal and Small-Scale Gold Mining Without Mercury
In many countries, elemental mercury is used in artisanal and small-scale gold mining. Mercury is mixed with gold-containing materials, forming a mercury-gold amalgam which is then heated, vaporizing the mercury to obtain the gold.
This process can be very dangerous and lead to significant mercury exposure and health risks. In some jurisdictions, mercury use may be illegal or restricted in certain ways. The Minamata Convention on Mercury, a global agreement for reducing mercury pollution, recognizes the risks of using mercury in artisanal and small-scale gold mining, and calls upon nations to reduce, and where feasible eliminate mercury use in this sector.
Although many miners use mercury in artisanal and small-scale gold mining, it is possible to safely and economically recover gold without it. Mercury-free techniques are safer for miners, their families and local communities. They may also help miners market their gold at higher prices.
Using Concentration Methods
Concentration means increasing the amount of gold in ore or sediment, by selectively removing lighter particles. If employed effectively, concentration methods can eliminate or greatly reduce the need for mercury.
Before concentration can begin, ore must be crushed or milled to liberate gold particles from rock and to decrease grain size. Concentration works best when grain size of the milled material or sediment is relatively consistent, so that most particles are of similar size. An appropriate grain size can be achieved using screens or sieves. Once the gold-containing material has the appropriate grain size, one (or several) of the methods described below can be employed to concentrate gold bearing material.
Most concentration methods rely on the high density of gold relative to other minerals in ore or alluvium mixture. These are referred to as gravity methods. Magnetic or chemical properties can also be exploited to enhance concentration.
Each mining operation is unique. Concentration methods must be selected after considering factors such as the type of ore or sediment, other minerals present, gold particle size, and the availability of water and electricity.
Panning uses water to separate heavy gold particles from other lighter particles within a medium sized pan. In this process sediment or ore thought to contain gold is placed in a wide, curved pan along with water. The miner moves the pan in a series of motions designed to eject lighter sediments. The density of gold keeps it on the bottom of the pan as lighter material is ejected along with water. After a series of successful iterations have been completed, gold will be exposed on the bottom of the pan for the miner to recover.
Panning works best when gold is coarse and well liberated. Under right conditions, panning can produce high grade concentrates or even liberated gold. Then miners can employ gold recover methods such as direct smelting (described below), although many panning operations lead to directly recoverable gold.
Panning offers miners a low cost method of gravity concentration but it requires time and skill to be effective. One of the major drawbacks to panning is that miners must pan small amounts of concentrate. Therefore, panning is often done after other methods of gravity concentration such as sluicing have completed.
Sluices use water to wash ore or alluvium down a series of angled platforms. As water washes sediment down a sluice, gold particles sink and are captured by material covering the bottom of the sluice, often carpets. Sluices are usually inclined at 5 to 15 degree angle. As moving water travels down a sluice, it generates greater force and keeps gold particles from sinking easily. For this reason most gold is captured at the beginning of the sluice. Carpets or other capturing devices on the bottom of sluices can be removed and washed in a bucket to remove the captured dense material.
Sluice design can lead to higher gold recovery if the force of the water traveling through the sluice is decreased. A series of rifles can help break the flow to improve recovery. A zig zag sluice also achieves this by creating a drop between the first and second platform that disrupts the velocity of the water as it travels down the sluice.
A simpler alternative to the zig zag sluice is a combination of two sluice surfaces. The first is tilted at a steeper angle then the second, decreasing the velocity of the water as it hits the second sluice, increasing gold recovery.
Sluices can be relatively expensive or affordable depending on the complexity of their design. Simple sluices can be a single angled platform a few feet in length and others can be very elaborate.
Having an available and consistent water supply is necessary to have a functioning sluice operation. This can be done with piping, drums, buckets, or natural flowing water bodies. A constant flow will be better than a bucket-driven flow.
Sluices are good at concentrating large amounts of ore and sediment in a relatively short time but often do not yield concentrates with high amounts of gold. The resulting concentrate must usually undergo further methods of concentration, such as panning.
Shaking tables are elevated tables tilted to one side with raised ridges running horizontally down their length. Mineral feed (crushed ore or sediment) and water are released at one end of the table. The water washes the feed down the table. As the material is washed down the table, specialized grooves trap gold and direct it to collection points on the side of the table as lighter minerals are washed away. During this process, the table is continually shaken by a motor to agitate the material and aid in the separation of gold particles.
Shaking tables are very effective and can concentrate sizeable amounts of ore at a time, providing high grade concentrates and liberated gold, but they are also relatively expensive and require some experience to operate.
Spiral concentrators are specialized pans tilted on an angle with spiraled grooves. The spiral grooves in the pan lead toward the center where a hole is connected to a container to catch material.
A motor is used to rotate the pan continually as concentrate is fed onto the pan by an operator. A pipe extending horizontally across the pan sprays water along the surface of the pan as the concentrator spins. The water washes lighter particles down the spiral concentrator into a bucket while denser particles, including gold, are carried by the spiral grooves toward the hole in the center of the concentrator.
After this process is repeated multiple times, the operator is left with a high grade concentrate, and often liberated gold. Spiral concentrators are relatively easy to operate but do represent a larger capital investment than panning or sluices.
Vortex concentrators use a rotating flow of water to separate lighter materials from a concentrate and remove them via a raised drain hole.
A vortex concentrator is a circular tub with water input on the side of the tub and a raised drain in the center. The tub is filled with water until it reaches the level of the drain hole. Then concentrate is added in a thin layer around the bottom of the bowl. Water is then pumped into the side input, creating a rotating vortex of water that drains in the center. The vortex pulls lighter material up from the bottom of the bowl and out the drain hole. Dense materials such as gold remain in the bottom of the tub. After the miner sees only gold left on the bottom of the bowl the water source is turned off and the gold is ready to be removed.
Miners must pay attention to the amount of water flow going into the tub. If it is too great the velocity of the water will carry gold particles out of bowl and this will lead to losses of gold. When vortexes are operated correctly, the result is a fine gold concentrate that is usually very high grade. Vortexes are easy to operate and are good at capturing fine gold that is hard to extract through other methods.
A centrifuge is a vessel that rotates about a central point. It is used to separate materials in a mixture by density. To separate gold particles from a concentrate, concentrate is fed into the centrifuge through a pipe at the top of the machine in a slurry of around 60-75% water and 40-35% solids. The material collects in a vessel in the center of the machine where high speed rotation creates force that moves the material up the sides of the vessel’s walls. As the material is pushed up the sides of the bowl’s wall, denser material like gold is caught in ridges while lighter material is ejected from the vessel.
Centrifuges operate in cycles that can be preprogrammed or determined manually depending on the equipment and the material processed. After a cycle is completed, the miner can then extract gold from the ridges of the centrifuge vessel. For small scale centrifuges, cycles usually last around 0.5-2 hours.
Operating a centrifuge takes skill as it must be tuned to the material it is processing. This is accomplished by adjusting feed grain size, rate of feed, rotation velocity, and cycle duration. Centrifuges can be more effective at concentrating gold than other methods of gravity concentration but are generally more expensive.
Other Concentration Methods
Magnets can be used to remove magnetic minerals such as magnetite from concentrate. They can be used after or in conjunction with other method of concentration. One technique for extracting magnetic minerals is to place hand held magnets on the bottom of a pan containing dried concentrate to separate metallic from non metallic material. Care must be taken to avoid losing gold particles during the separation. It can be helpful to cover the magnet with a piece of paper. After magnetic minerals are attracted to the surface of the paper, it is removed to easily discard the metallic material.
Flotation is usually used by large scale miners but can also be applied in small scale operations. It is a process that works best for processing complex ore types, especially ores that are difficult to process using gravity methods. In flotation, a mixture of slurry (crushed ore and water) and frothing agents are added into a flotation machine. A tube releases air into the tank of the machine and an agitator creates air bubbles at the bottom of the tank.
Minerals that are hydrophilic, such as gold, attach to the bubbles’ surface and are brought up to the top of the tank. Other minerals fall to the bottom of the tank and are discarded as tailings. Bubbles containing gold and other hydrophilic minerals accumulate at the top of the water level as froth. This froth is then scraped off to create a concentrate of gold and other hydrophilic minerals. Flotation creates high quality concentrates and is good at capturing fine gold. Flotation usually requires a substantial amount of capital investment.
There are a variety of possible frothing agents. Depending on the chemical, specific precautions must be taken when employing the method to protect human health, and waste materials must be disposed of appropriately.
Separation methods, like the ones described above, if employed properly, should yield a high-grade concentrate with a large proportion of gold relative to other materials. However, this gold still needs to be separated from the other remaining minerals before it can be sold. At this point in the process, direct smelting can often be employed as the final stage of gold recovery.
In direct smelting high-grade concentrate is heated until the gold melts. The liquid is then cooled to form a solid mass of gold dore, a semi pure gold alloy, that can reach upwards of 95% purity.
To perform direct smelting, a miner, community mining consortium, or processing shop employs a crucible, a high temperature bowl designed for smelting. Gold concentrate is combined with a flux, such as borax or other materials of mixtures, in the crucible. The flux acts to decrease the melting temperature and viscosity of non-gold minerals in the concentrate so they are more easily separated from the gold during the cooling process. Small amounts of concentrates, usually around 50 to 100 grams, are used in direct smelting. Concentrate and flux are heated in the crucible to the temperature at which gold melts, 1065 degrees Celsius. Miners usually use a blow torch to generate heat. After this temperature is maintained for some time the material in the crucible will melt and can then be poured out of the crucible to cool and harden into gold dore.
If concentrate is of poor quality (low gold percentage and high amounts of other minerals) the melt may need to be poured into another structure, such as a cupple, to separate the impurities from the gold. When melt is poured into the cupple, gold and other metals will sink as silicates and other minerals rise to the top, forming slag. When these materials harden the gold can be easily separated from the slag. The slag frequently contains addition gold that can be recovered through reprocessing.
Chemical leaching makes use of the chemical properties of gold to leach it from ore, concentrate, or tailings. This process is mostly used in large scale mining operations but has been increasingly adopted in small scale mining because of its high gold recovery rate and low cost. The best practices for chemical leaching are a combination of pre-concentration and mill leaching, as they lead to the least amount of waste, a short processing time for miners, and high gold recoveries. First, concentration is done through gravity techniques. Then the concentrate is simultaneously milled and leached. When chemical leaching is employed, it is important for miners to handle the chemicals properly and ensure that they are properly used and stored to avoid health and environmental concerns.
Cyanide is often the preferred chemical used in leaching. Cyanide is highly toxic and great care must be taken when using it. However, in contrast to mercury, cyanide is does not persist in the environment. Cyanide leaching must not be used on tailings where mercury is present, as soluble mercury-cynanide complexes will form, mobilizing mercury to great distances.
- Reducing Mercury Pollution from Artisanal and Small-Scale Gold Mining
- UNEP: A Practical Guide to Reducing Mercury Use in Artisanal and Small-Scale Gold Mining (PDF) (76 pp, 7.1 M, About PDF)
- UNEP Global Mercury Partnership
- Artisanal Gold Council video on mercury-free mine in Burkina Faso