Second Instalment
This series of articles is being published by the Stabroek Business in support of the initiative being undertaken by the Guyana Gold and Diamond Miners Association (GGDMA) to enhance public awareness of the mining sector’s thrust to reduce and eventually remove mercury from local gold-mining operations
Many technologies exist that do not involve the use of mercury in the gold recovery process. These include – Gravity Concentration: Sluice Boxes, Shaking Table, Centrifuges; Complexing Techni-ques: Chlorine Processing, Cyanide Processing (Dangerous), Other Halides, Flotation Methods; Agglomeration Processes and Electrolytic Processes. All of these processes have advantages and disadvantages, which would favour the selection of one or the other. Most are friendlier to the environment and human health in comparison to the mercury amalgamation process; this makes them the preferred methods by environmental regulators. Practitioners of alternative techniques have identified two of the obstacles preventing transition away from mercury use to alternative methods as cost and know-how.The key activity needed to promote the use of mercury free technologies is education and awareness.
The main principle of recovering gold (and diamonds)in alluvial mining in Guyana is gravity concentration.
Gravity concentration
Gravity concentration methods separate minerals based on differences in densities. Gravity separation is more effective for narrow particle size ranges.
Main advantages of gravity concentrators are:
1. Relatively simple pieces of equipment with low capital and operating costs
2. Little or no reagent is required
3. Can be applied to both coarse particles and fine particles
Gravity concentration techniques include panning, sluice boxes, shaking tables and centrifuges.
Panning
Panning is the most ancient form of gravity concentration; pictures of gold pans appear on Egyptian monuments as old as 2900 BC. The circular or back-and-forth shaking of ore and water in a pan causes the ore to stratify, so the heavy minerals such as gold settle to the bottom of the pan while the lighter gangue can be washed off the top. Coarse gold grains can be sorted, hand-picked and recovered without the use of mercury. There are different types of pans – conical, flat bottomed, with riffles, etc. Pans can be made of plastic, wood, steel, aluminium, brass, etc. The type of pan mainly used in Guyana is the batel.
Sluice boxes
Although they may be considered inefficient relative to modern gravity concentrators, sluice boxes are popular because of low capital and operating costs. They do require an ample supply of water, however. They can also process significantly more ore than simple panning.
· The operational parameters involved in using a sluice box are simple:
· The slope of the boxes is established by trial and error;
· Slurry rates between 250kg/min and 1400kg/min can provide high recoveries for finer particles, provided the sluice has sufficient length;
· Multi Stage Sluice Boxes (about 3 boxes) can be used to capture various gold particle size ranges;
· Regular clean up of the mesh on the box is essential to avoid packing, which may result in mineral losses;
· The cleanup process can be performed WITHOUT using mercury;
· Sorting can be done by hand to remove coarse particles;
· Time consuming but cleaner;
· Sluices are batch-type equipment and the operation must stop periodically to remove the concentrates;
· Final concentrate can be smelted directly to produce gold ingots.
Clean gold sluice
Magnetic sheets can be used in aluminium sluice boxes.
The floors of the boxes are usually lined with Magnetite (mineral found in the ore); good material to recover fine particles of gold.
Gold is trapped below the magnetite layer; the sluice can be scraped and washed into a pan.
The magnet is used to remove the magnetite, leaving behind a high grade of gold.
This all boils down to low cost and high concentration ratios.
Borax
An alternative to mercury for amalgamation of gold is the use of borax. This method is simple, inexpensive, does not require advanced technical equipment and is non-toxic. The basic principle behind the method is that borax reduces the melting point of gold. The melting point of gold is 1064oC, which is much higher temperature than can be obtained by most inexpensive burners. By adding borax, the melting temperature decreases. The gold concentrate from a batel, or sluice box, etc is dried and then mixed with three times as much (by volume) of borax powder.
The borax-concentrate mixture is then placed in a plastic bag which is then put in a ceramic bowl together with a few pieces of charcoal – all this is heated with a blow-torch and within a few minutes all the other heavy minerals is separated from the molten gold.
