REPLACING MERCURY IN GOLD MINING

Sep 17, 2017 AFC Column, Features / Columnists

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(Continued from last Sunday’s article titled: Preserving miners’ health & the environment)
In keeping with Guyana’s commitments to create a truly Green state, this country will be represented at the first Conference of the Parties to the Minamata Convention scheduled for Geneva, Switzerland from September 24 to 29.
The Minamata Convention is an international treaty designed to protect human health and the environment from anthropogenic emissions and releases of mercury and mercury compounds. This chemical is used in gold mining and it poses serious threats to human health and the mining environment.
The name Minamata was given to a neurological disorder which was caused by mercury poisoning in the Japanese city with the same name. The disease was discovered in 1956 and a nearby chemical plant was blamed for causing it by emitting untreated wastewater into the Minamata Bay.
Guyana was one of the first countries to sign on to the legally binding agreement on October 10, 2013. Even though the then government had claimed support from the broad mining sector including the Guyana Women Miners’ Organisation (GWMO), the latter later issued a disclaimer stating that the women miners had not been consulted before the signing. However, the ladies were all too familiar with the dangers of Mercury and the benefits of mining without it, so they joined the eradication campaign.
Last week we began to examine alternatives to extracting and amalgamating gold in the field without using Mercury. We told you about 2 gravity concentration methods – panning and sluicing. Today we continue with suggestions assisted by www.epa.gov:
Shaking Tables
These are elevated tables tilted to one side with raised ridges running horizontally down their length. Crushed ore or sediment mixed and water is applied at one end of the table. The water washes the ‘feed’ down the table and specialized grooves trap the gold and direct it to collection points on the side of the table. 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.
Spiral Concentrators
Spiral concentrators are specialized pans tilted at an angle with spiraled grooves that lead toward the center where a hole is connected to a container to catch material.
A motor rotates the pan as the concentrate (ore) is fed onto it. A pipe extending horizontally across the pan sprays water along its surface 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 repeating this process several times, the operator is left with a high grade concentrate, and liberated gold. Spiral concentrators are relatively easy to operate but they do require a larger capital investment than pans or sluices.
Vortex Concentrators
Vortex concentrators use a rotating flow of water to separate lighter materials and remove them via a raised drain hole.
It is basically a circular tub with a water inlet on the side of the tub and a raised drain in the center. The tub is allowed to fill with water until it reaches the level of the drain hole and then concentrate is added in a thin layer around the bottom of the bowl. Water is then pumped into the side inlet 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 at the bottom.
It is important to pay attention to the amount of water flowing into the tub. If the water pressure is too high, the velocity of the water will carry the gold particles out through the drain along with the rest of the ore. When a vortex is operated correctly, the result is a fine gold concentrate that is usually very high grade. Vortices are easy to operate and are good at capturing fine gold that is hard to extract through other methods.
Magnets
Magnets can be used to remove minerals such as magnetite from concentrate/ore. 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. This is to separate metallic materials. Care must be taken to identify gold particles during the separation process. You can cover the magnet with a piece of paper that will easily throw off the metallic minerals when it is removed from the magnet.
Flotation
Flotation is usually used by large scale miners but it could be applied in small scale operations. It is a process that works best for processing complex ore types, especially ore that is difficult to process using gravity methods. In flotation, slurry (mixture of crushed ore and water) and a frothing agent is put 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.
There are many suitable frothing agents. Depending on the chemical, specific precautions must be taken when the frother to protect miners’ and residents’health, and waste must be disposed of in a manner that does no harm to the environment.
Minerals that are hydrophilic, such as gold, attach themselves to the bubbles created by the injected air and float to the top of the tank. Other minerals fall to the bottom to be discarded as tailings. Bubbles containing gold and other hydrophilic minerals accumulate on the surface of the water as froth or flow over the sides. The bubbles dissipate and a concentrate of gold and other hydrophilic minerals is formed.
Flotation creates high quality concentrate and is good at capturing fine gold. This method usually requires considerable capital investment, however.
(To be continued)

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