Much of the environmental damage from small-scale mining comes as a result of improper handling of mercury. Because of its low cost and availability, mercury is preferred by small-scale miners for amalgamating gold. Alternative technologies, such as retorts, can provide a closed system in which excess mercury is recovered, rather than washed away in streams or rivers. A new, inexpensive device developed in the Brazilian Amazon by a German company also shows promise. Consisting of little more than a modified sluice box, this new technology has a gravity trap that fits at the end of the sluice box and a sealed crucible that prevents mercury from being released into the atmosphere. The trap works by slowing down water flow so that excess mercury settles out before reaching the river and the sealed crucible aids in recovering liquid mercury from a condenser.
Ued in conjunction with one another, both devices, which have been tested in Ecuador and Colombia, have proved to recover 95 percent of mercury used and 5-10 percent of gold that would have otherwise been washed away. Each item costs approximately $30 and can be made from scrap metals. [94] Unfortunately, such techniques are not in use in Venezuela, possibly because few incentives encourage small-scale miners to adopt more environmentally friendly techniques, and information on these practices is not well disseminated.
Corporate mining operations
The few gold-mining companies operating in Venezuela extract the resource using open-pit and underground mining techniques. To create an open-pit mine, heavy equipment is used to excavate a pit as large as 2,000 feet deep into the rock, removing large quantities of waste rock – materials that lack significant quantities of gold and other metals – and ore in the process. The ground water around the future pit must be pumped out using an elaborate network of wells, so that the excavation can proceed downward below the water table. [95] Industrial underground mines, of which there are two in Venezuela, require the company to dig shafts and to transport gold-bearing rock to the surface.
Once the ore has been removed from the ground, by either open-pit or underground mining techniques, it must be further processed to extract the gold. At a global level, gold processing has generally involved heap and other leaching or mill flotation processes. All of these processes involve the use of cyanide.96 In heap leaching, ore is removed from the ground and put in piles that are sprayed with a dilute solution of cyanide, which combines with the gold. The gold is later stripped from the cyanide and recovered. In mill flotation processes, gold-bearing ore is taken to a mill where it is crushed and treated with numerous chemicals including sodium cyanide, amines, lime, sodium sulfide, acids and other organic compounds, to extract the gold and possibly other metals. Many of the chemicals, if present in sufficient concentrations, are toxic to terrestrial and aquatic life. [97]
At a mill site, two types of wastes are generated: waste rock, and tailings, which is the material left over after processing. Tailings of pulverized rock and processing chemicals are usually sent to an impoundment as slurry. In industrial countries, modern mining practices require that tailings be disposed of in lined facilities, which can be constructed of synthetic or natural materials. Sometimes tailings are discarded directly into the environment without being held in an impoundment site (see discussion on BHP and Placer Dome’s operations in Papua New Guinea).
References and notes
94. A. Coghlan, “Cleaner Gold Improves Miners’ Prospects,” New Scientist (April 6, 1996), p. 11; A. Coghlan, “Midas Touch Could End Amazon’s Pollution,” New Scientist (June 27, 1997), p. 10.
95. W.H. Dennen, Mineral Resources: Geology, Exploration, and Development, (Taylor and Francis: New York, 1989), p. 121.
96. A. Smith and T. Mudder, “The Environmental Geochemistry of Cyanide,” in G.S. Plumlee and M.H. Logsdon (eds), vol. 6, Society of Economic Geologists. 1993.
97. J.S. Scott and J.C. Ingles, “Removal of Cyanide from Gold Mill Effluents,” in Proceedings of Canadian Mineral Processors Thirteenth Annual Meeting, Ottawa, Canada, Jan. 20-22, 1981.
