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HEAP LEACHING TECHNOLOGY – GREENER AND CHEAPER
Editorial Focus by Harold Hough Dec/Jan 2010
As gold prices go through the roof, some experts are saying that the world’s supply of gold has run out. That’s not true, but the new demand for gold and other metals is sparking a new resurgence in the one technology that is known for milking the smallest amount of gold out of a low grade ore – heap leaching. At the same time, heap leaching promises to be the “green” alternative in an era where carbon emissions must be controlled.
Although heap leaching really came into its own in Nevada in the 1980s, when the amount of gold processed by heap leaching grew by 500%, its history is much older. Using cyanide to extract gold was a process discovered by three Scottish gentlemen: Dr. Robert Forrest, Dr. William Forrest, and John MacArthur in the 1880s. But leaching metals out of ore goes back nearly 500 years. Mines in Hungary recycled copper-bearing solutions through waste heaps in the 1550s, and Spanish miners percolated acid solutions through large heaps of oxide copper ore on the banks of the Rio Tinto in 1752.
Gold heap leaching was first suggested as a process by the US Bureau of Mines in 1967. A few years later the Carlin Gold Mining Company in Nevada opened up the first commercial gold heap leaching operation in the United States. Today, ore that contains as little as 0.02 ounces of gold per ton is commercially extractable, thanks in part to small, portable, inexpensive heap leaching equipment.
Leaching is opening up new fields on a regular basis. Recently, Orezone Gold Corp. announced that it received excellent leaching results at the Bomboré Project, one of the largest gold resources in Burkina Faso. Since heap leaching requires less capital investment and is faster to bring on line, Orezone is focusing on developing the project as a heap leach operation.
Although gold has made the most news with heap leaching, other ores are being treated with this technology.
SILVER LEACHING. Silver ore like gold ore works best with cyanide solutions. The solution percolates through the heap and leaches out the precious metal in a process that can take several weeks. The solution containing the precious metals (pregnant solution) continues percolating through the crushed ore until it reaches the liner at the bottom of the heap where it is piped into a recovery system that often contains activated charcoal. After separating the precious metals from the pregnant solution, the dilute cyanide solution is re-used.
COPPER HEAP LEACHING. This method is essentially similar to the method used in gold and silver heap leaching, except it uses sulfuric acid to extract copper from its ores instead of cyanide. There are also copper leaching processes that use ammonia. Since the quantity of copper in ore is greater than gold, the amount of solution used in the process is usually larger and the recovery system must handle larger amounts of metal and liquid.
NICKEL HEAP LEACHING. Nickel heap leaching has become more attractive lately for the same reasons it did in gold, silver, and copper industries - low capital costs. Other methods like high pressure acid leaching are faster, but required a massive capital investment. On the other hand, heap leaching takes up to 200 days, and can produce nickel for much less. As with copper leaching, nickel can be extracted with dilute sulfuric acid, mixed hydroxide products, or ammonia. However, the chemical compositions of nickel ores have their own problems. Iron, which inhibits the heap leaching process by using too much acid, is often found in nickel ores. And, as with gold and copper ores, too much clay will cause the solution to “channel” through the ore and not thoroughly soak the heap.
LEACHING OTHER METALS. Ammonia promises to be an excellent leaching solution for other metals. Ammonia, unlike acid doesn’t react with ferrosilicates. As a result, zinc can be leached from mixed oxide ores. And, uranium, which has been leached since the 1950s, appears to be another candidate for ammonia leaching.
BIOLEACHING. Not all ores are amenable to traditional heap leaching. For instance, many ores are found near volcanic regions and hot springs. These often contain sulfur compounds, which inhibits the chemical processes used in leaching.
Mining engineers discovered that some bacteria can modify these sulfide compounds to allow leaching. Now mines can “infect” their heaps with one of these strains of bacteria. This in turn, allows conventional heap leaching technologies to break the ore down and allow the valuable metals to be extracted.
If there has been one problem with heap leaching, it is the negative image it has with environmentalists, primarily because gold heap leaching uses cyanide. However, heap leaching does have a positive environmental aspect that may encourage its use in some parts of the world. Heap leaching usually has a smaller carbon footprint than traditional energy intensive extraction methods. In addition, heap leach pads are next to the mine, which cuts transportation related pollution. As a result, mines can actually become “greener” by using heap leaching, managing solution leakage, planting trees, and sequestering carbon.
Heap leaching has always had the advantage in low capital costs and the ability to bring a mine on-line quickly. However, with its potential as a green mining method, it looks like heap leaching’s future is brighter than its past.
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