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One Arrastra Gold Operation in Costa Rica
By Charles Kubach, Mine-Engineer.Com
December 28, 2012
A small gold mine operating in Coast Rica is using ancient technology to recover gold. The ore is from a alluvial deposit as well as a small vein. The vein is mined with stoppers, while the alluvial deposit is dug out with hand tools, picks and shovels. then they place the ore in an Arrastra and add mercury to grind the ore, which will bond with any free gold it comes into contact (amalgamate) and form a gold-mercury amalgam. The ore is ground for one hour, then the fines are washed out, more ore is added and ground for another hour. The ground particle size is 20-30 mesh. This small mine processes 15 ton per day, and they work 6 days per week. The ore would be considered high grade today as it assays between 10 to 15 grams per ton (approximately 1/2 ounce/ton) of gold. However, sulfide gold will not bond with mercury and is lost to the tailings. The mine currently producte about $1,500,000 from gold sales per year (average $1700/ounce). Their tailings average 6 grams per ton of sulfide gold ore, and 180 grams of mercury per ton of ore. This will leave 810 Kg of mercury per year and 868 pounces of gold in the tailings per year, on average. That is about $1.5 million at $1,700/ounce for gold. If the tailings could be roasted and the fumes quenched, the mercury could be recovered, the gold could be recovered and the mercury laden tailings could be restored to a non toxic fill. If 80% of the gold could be recovered, it would pay for the cleanup, and all would be well with the regulators and tree huggers. However tailings piles would have to be placed on impermeable liners, to prevent mercury from migrating to the soil beneath the tailings pile. Below is a brief history of the Arrastra.
The arrastra is at least 500 years old, and was used by gold mills to grind high grade concentrates to recover the gold, usually by the addition of mercury. Currently, the arrastra is used by small mines predominantly in Central and South America. Often it is powered by the use of donkeys, but many are powered by gasoline or electric motors.
A manufactured arrastra consists of a of a round slightly conical shaped tank, several feet in height, with large steel grinding shoes being rotated across the bottom, attached to a arm, which is powered by a motor. The ore slides beneath the shoes and is ground between the bottom of the show and the tank floor. As one could imagine, there is a bit of wear associated with a arrastra, since most gold ores are very abrasive. It can take 5-7 hours to get the ore to 100 -200 mesh. About 1 hour for a 20-30 mesh product. Once the ore is ground to a pre-determined liberation particle size, mercury is added and grinding is continued for an hour or two. This has resulted in a lot of mercury contamination in many gold producing countries, however, in Costa Rica, they have come up with a method to prevent this mass mercury contamination. The local miners mine the ore, concentrate if and grind the concentrates to -30 mesh in a arrastra, then take it to a mill for final recovery using mercury or other methods. When mercury is used at the mill, proper procedures are taken to minimize or eliminate mercury contamination, and wide spread tailings contaminated with mercury are eliminated.
The old Denver arrastra was 30 inches in diameter and would process 300 pounds of ore per batch (8 hours). Locally produced arrastra's are commonly made from cement, and grinding shoes are large stones with flat bottoms, attached to a wood beam and rotated by human or donkey power. Occasionally they will be attached to a gasoline engine to turn the arrastra. A typical small mine will produce 15 tons per day of concentrates, averaging 10-15 grams per ton gold. Typical tailings from a arrastra will depend upon the sulfide content containing gold, since this is not recovered by use of mercury. In some areas it will range 6 grams of gold per ton of spent tailings. It can range form 0 to 8 grams per ton, depending upon the sulfide gold ore present in the deposit. Typical tailings mercury content is around 180 grams of mercury per ton of tailings.
In a year a typical small mine can generate 4,500 tons of ore, which becomes tailings after processing. Mercury can be recovered, virtually 100%, by heating it to 900 degrees F, and using a condenser to cool the vapors and collect the mercury. This device is called a mercury retort. If a mercury retort is properly used in conjunction with the use of mercury, there is virtually no mercury contamination from the gold mercury amalgam. For the past 20 or so years, many countries have discouraged the use of arrastra's and mercury to prevent large contamination of the soil and water. Centrifugal concentrator's, such as the Falcon concentrator have been favored for small miners, and these work quite well to concentrate the ore, producing a high value concentrate, containing free gold, some heavy minerals and the gold-sulfide complex. The centrifugal concentrators are quite popular for concentrating gold ore in areas with good water supplies, and give over 90% recoveries. The problem small miners have, is the lack of funds to purchase one and be trained on its operation. If this problem were solved, almost no small miner would find the need to use mercury, however it always comes down to money, or the lack of it.
A Link To Falcons Small Mine Centrifugal Concentrators (Icon)
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