Surface mining is the primary source of gold and silver. It requires extensive blasting as well as rock, soil, and vegetation removal to reach load deposits. Ore is removed from the mine and transported to milling and beneficiating plants for concentrating the ore, smelting, and/or refining.
When the gold or silver ores lie close to the surface, they often can be uncovered by stripping away a layer of dirt, sometimes only a few feet thick. The ore is mined from large sites by progressive extraction along steps or benches. The benches provide access to progressively deeper ore, as upper-level ore is depleted. After the soil and overlying rock are cleared, the ore is drilled and blasted. The portion of the ore body to be removed is first drilled in a specific pattern, the holes loaded with explosive mixtures and fragmented. Following blasting, the fractured ore is loaded by huge mechanical shovels, hydraulic excavators, or front-end loaders onto large dump trucks or railroad hopper cars.
Surface Placer Mining
Placer mining is a method of obtaining gold from sand and gravel using nearby water supplies and consists of two types: hydraulicking or dredging. The type of placer mining depends on the size of the mineral.
In hydraulic mining, or “hydraulicking,” a stream of water under great pressure is directed against the base of the placer gravel bank using pipes and large nozzles called giants. The water caves the bank, disintegrates the gravel, and washes the broken material to and through sluice boxes situated in convenient positions downslope. Hydraulic mining totally disturbs large surface areas, puts much loose debris into the drainage system, and involves large surface water runoff that may cause substantial damage downstream. Many of the western states passed laws years ago to closely control “hydraulicking,” and few substantial deposits of placer gravel remain that could be mined economically within the restraints of this legislation.
Another placer mining method is dredging. Large alluvial deposits are mined by floating washing plants capable of excavating the gravel, processing it, and stacking the tailings away from the dredge pond. Bucket line and dragline dredges have been used. The bucket line dredges are larger and more efficient, consisting of a continuous line of buckets that scoop the material from the gravel bank at the edge of the dredge pond, raising it to the top of the washing plant mounted in the hull. Dragline dredges are smaller and less efficient, employing a single bucket. This bucket digs the gravel and is swung over the feed hopper of a floating washing plant similar to the layout in a bucket line dredge, although usually smaller.
Dredging temporarily involves total disturbance of the ground surface, although with careful planning and engineering of the operation it is possible to plan for restoration of the surface, and perhaps even to improve some aspects of the flood plain or nearby river channel. It is not possible to restore the land to the precise original contour, for the swell factor of the gravel increases volume 20 percent or more. In many areas in the West, particularly near major construction projects or cities, clean gravel placer tailings are valuable for manufacture of aggregate or crusher run in fills of various kinds, and can be considered a resource in their own right. In a few areas, people traveling through areas of old placer tailings, expecting the area to be a wasteland, are pleased to find a great variety of fishing and water sport recreation available, and thriving wildlife in the habitat that has been created. Placer deposits can be thoroughly explored before floating the dredge. Such operations lend themselves to thorough planning, and it is possible to do a considerable amount of reclamation at only a slight increase in overall operating costs.
Underground Room-and-Pillar Mining
Underground mining occurs if the ore grade or quality is sufficient to justify more targeted mining. The room-and-pillar method involves extracting the ore by carving a series of rooms while leaving pillars of ore to support the mine roof.
Precious metals may be recovered from the gold ore or from refining processes of base metals such as copper and lead. Because these are distinct recovery methods, they are discussed separately.
Precious Metal Recovery from Ores
In 2000, gold was produced at 641 load mines; about a dozen large placer mines; and numerous small placer mines. In addition a small amount of domestic gold was produced as a byproduct of processing base metals.
Silver was produced in the United States from precious-metal ores at about 30 lode mines and from base-metal ores at about 24 lode mines. Fewer placer operations recovered silver in 2000, and the quantity recovered was less than 1 percent of the total domestic production. 3 184.108.40.206 Gravity Separation
Gravity separation relies on density differences to separate desired materials from host rock. Devices used include gold pans, sluices, shaking tables, and jigs. Gravity separation alone is used at most placer mines, and in combination with other methods and some lode miners.
Much of the extracted ore must be milled to prepare it for further recovery activities. Uniformly sized particles may be obtained by crushing, grinding, and wet or dry classification. The degree of milling performed on the ore depends on the gold and silver
concentration of the ore, mineralogy and hardness of the ore, the mill’s capacity, and the next planned step for recovery. Milled ore is pumped to the next operation unit in the form of a slurry. Fugitive dust generated during crushing and grinding activities is usually collected by air pollution control devices and recirculated into the beneficiation circuit. Most mills use water sprays to control dust from milling activities. After milling, sulfide ores may be subjected to chlorination, bio-oxidation, roasting, or autoclaving. Chlorination is not commonly used to oxidize sulfide ores because of high equipment maintenance costs caused by the corrosive nature of the oxidizing agent. Biooxidation of sulfide ores employs bacteria to oxidize the sulfur-bearing minerals.
Roasting of sulfide ores involves heating the ores in air to convert them to oxide ores and break up their physical structure, allowing leaching solutions to penetrate and dissolve the gold. In effect, roasting oxidizes the sulfur in the ore, generating sulfur dioxide that can be captured and converted to sulfuric acid. Roasting temperatures are dependent on the mineralogy of the ore, but range as high as several hundred degrees Celsius. Roasting of carbonaceous ores oxidizes the carbon to prevent interference with leaching and reduced gold recovery efficiency.
Autoclaving (pressure oxidation) is a relatively new technique that operates at lower temperatures than roasting. Autoclaving uses pressurized steam to start the reaction and oxygen to oxidize sulfur-bearing minerals. Heat released from the oxidation of sulfur sustains the reaction.
Because ores with a high proportion of small particles may retard the percolation of the lixiviant, agglomeration is used to increase particle size. This operation includes mixing the crushed ore with portland cement and/or lime, wetting the ore evenly with weak cyanide solution to start leaching before the heap is built, and mechanically tumbling the ore mixture so fine particles adhere to larger particles.
Cyanidation – Leaching
Currently, cyanide leaching is the chief method use. In this technique, sodium or potassium cyanide solution is either applied directly to ore on open heaps or is mixed with a fine ore slurry in tanks. Heap leaching is generally used to recover gold from lowgrade ore, while tank leaching is used for higher grade ore. Cyanidation is the primary means of recovery of fine gold and silver. In this process, solutions of sodium or potassium cyanide are brought into contact with an ore, which may or may not have required extensive preparation prior to leaching. Gold and silver are dissolved by cyanide in solutions of high pH in the presence of oxygen. There are three general methods of contacting ores with leach solutions: (1) heap leaching, (2) vat leaching, and (3) agitation leaching. Heap leaching and vat leaching account for most gold and silver recovery. These leaching methods are discussed in detail below.
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