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汞元素赋存之地理环境模型(03)  

2017-11-09 07:20:05|  分类: 不归类文章 |  标签: |举报 |字号 订阅

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The replacement deposits hosted in quartzite consist of stratiform zones of cross cutting ore consisting of cinnabar, native mercury, pyrite, calcite, and quartz. The grade and thickness of ore bodies is highest near the margin of the volcanic craters and decreases systematically away from the craters. Mercury is the only ore metal in these deposits and the only other trace metal present is zinc. No other mineral deposit type is associated with these mercury deposits.

MINERALOGIC CHARACTERISTICS

The primary ore mineral in all mercury deposit types is cinnabar, (HgS). Elemental mercury is also common in all deposit types but generally in small amounts. Only in one deposit of the Almaden type, Las Cuevas, Spain, is elemental mercury the primary ore mineral. The high temperature polymorph of mercury sulfide, metacinnabar, commonly is present in silica-carbonate-type mercury deposits and in a few of these deposits, it is the primary ore mineral. More rarely, under extreme oxidizing and low pH conditions, mercury sulfates and chlorides may form as coatings on surface exposures. These yellow and green minerals are photosensitive and typically turn black on exposure to the sun and can be easily confused with manganese oxides. Mercury silicates and chromates also are present in silica-carbonate-type deposits. All of these minerals can be present in very small amounts except in a few silica-carbonate-type deposits where they comprise the main ore minerals. In hot-spring type mercury deposits, mercury sulfate, chloride, and oxide minerals are present in the upper parts of the ore bodies and these phases may form during supergene alteration of the ore body. Because these phases are very soluble, they commonly are preserved in sealed vugs and fracture coatings within the upper parts of ore bodies. The mineral corderoite, a mercury chloro-sulfide, is the dominant ore mineral at only one hot-spring type mercury deposit, the McDermitt deposit, Nevada.

Other sulfide minerals present in silica-carbonate type deposits include pyrite, and marcasite and in hot-springtype deposits, pyrite and native sulfur are present. In silica-carbonate deposits iron sulfides comprise from 5-10 percent by volume of the ores and altered country rock except where these deposits are localized adjacent to mafic volcanic centers and sulfidation of the country rock results in up to 50 percent iron sulfides being present in the ores. Pyrite and marcasite are the primary acid-generating sulfides in silica-carbonate type deposits. In hot-spring-type and Almaden-type deposits, pyrite is the primary acid generating sulfide. Cinnabar is the most insoluble of the metal sulfides under ambient oxidizing conditions, and as a result, its acid-generating capacity is very limited. Other sulfide minerals present in silica-carbonate and hot-spring-type deposits are stibnite, realgar, and orpiment, but the small amount of these phases contributes minimally to acid generation.

MINING AND PROCESSING METHODS

Mercury mining operations typically are small and their surface impact covers an area of a square kilometer or less. In the larger mercury districts, deposits commonly are localized along the same regional fault zone and mines may extend for several kilometers along the mineralized structure. Most mines were underground operations with open stope mining of high-grade ore shoots. Waste rock often was used to backfill underground workings resulting in only a small fraction of waste rock being brought to the surface. The high-grade ore zones usually have sharp contacts with low-grade ore or unmineralized country rock. Because low-grade ore is only rarely mined, stopes tend to follow the outline of high-grade ore bodies and are irregular in shape. Ore bodies that were present in the near surface were mined by open pit methods but only a few mines have been exclusively open pit operations.

The primary processing method for mercury ores utilizes the comparatively simple and inexpensive process of heating ores above the upper stability temperature of cinnabar to volatilize mercury and sulfur followed by a condensing process to recover elemental mercury. This procedure permitted the recovery of elemental mercury at most mine sites without the need to ship ores to a central processing facility. At some of the mines, a beneficiation process was used to minimize the amount ore being processed. Beneficiation processes included screening, jigging, tabling, or flotation, the latter process especially being used at the more recently developed mines. In the mid-1960's, flotation plants located at Gabbs and Mina, Nevada, processed ores from several deposits within the Great Basin mercury belt. The largest of the flotation plants was established in 1975 at the McDermitt mine, Nevada, where ores from the unusual ore body consisting of both cinnabar and corderoite, Hg3S2Cl2, were concentrated prior to roasting in a Herreshoff furnace.

Mercury ores have been heated in a variety of retorts and several types of furnaces. Furnaces differ from retorts in that furnaces internally heat the ores, mix fuel and mercury vapor, and process a continuous feed of ore. Retorts consist of one or more pipes that contain a single charge of ore. In retorts, the ore is externally heated and vapors from the fuel do not mix with the volatilized mercury. Retorts generally have a small capacity, about 500 pounds of ore per pipe. Because of their low cost, retorts have been utilized at small mines having small but high-grade ore bodies. Two basic types of retorts have been utilized. The Rossi type consists of several iron tubes that serve both as ore roasting and condensing systems. The D type retorts, are "D" shaped in cross section and are either used for roasting ore and condensing, and in the Pan type retorts, the roasting is done in a separate chamber. In some of the larger mines with ore processing furnaces, retorts were utilized to purify sooty mercury recovered from the furnace condensing system. Furnaces that have been utilized include the Scott, Herreshoff, and several types of rotary furnaces. The capacity of furnaces ranged from less than 10 tons to 350 tons per day. The rotary furnace was the most commonly utilized. It consists of a rotating, inclined iron cylinder into which ore is continuously fed and from which the roasted ore, calcine, is removed. Mercury vapors and other gases are drawn from the upper end of the cylinder into a dust collector (cyclone) and then processed in a condenser before being released through a stack. The Herreshoff furnace was used at larger mines and consists of a series of circular hearths into which ore is continuously fed and mechanically advanced to successive hearths of higher temperature. In the Herreshoff furnace, calcine is removed from the base and gases are fed through a dust collector and then a condensing system. The Scott furnace generally was built from brick and consisted of a series of baffles upon which the ore migrated downward as it was heated. The calcines were drawn from the base. The Scott furnaces initially were inefficient until considerable ore had been processed through them and the bricks had become saturated with elemental mercury. At the end of mining, the bricks from the furnace were processed in a retort to recover the mercury.

 

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