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汞元素赋存之地理环境模型(04)  

2017-11-09 07:23:30|  分类: 不归类文章 |  标签: |举报 |字号 订阅

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Condensing systems, which cool the vapor from the furnaces in order to separate the liquid mercury from other stack gases, range from simple single pipe to complex multiple pipe condensers. The condensing system usually consists of a pipe 8-15 inches in diameter and several hundred feet in length that is folded into inverted U shapes. The bases of the condensing system pipes are immersed in water tank and equipped with a drain hole at the base so that condensed elemental mercury can be collected in the tank. Both air- and water-cooled condensing systems have been utilized. In the more simple retorts, the condensing system consists of an iron pipe 2-4 inches in diameter and 5-10 feet long sloping downward from the top of the retort to a collection tank. About two thirds of the product recovered from the condensers consists of elemental mercury and the remainder consists of fine soot composed of rock dust, mercury sulfides and sulfates, fuel soot, water, and finely dispersed elemental mercury. In the older mines this material was dried on steam tables. However, this process released mercury to the atmosphere and often was a serious source of mercury poisoning. Calcium carbonate was used to aid in the mechanical coagulation of finely condensed particles of mercury from soot and collected from the table through a small pipe leading to a collecting tank. In the larger and more modern mines, mechanical mixers or a centrifuge was employed to remove mercury from the soot.

Mercury has been bottled at the processing site in iron flasks that contain 76 pounds of elemental mercury. More recently the metric ton flask has been used. Inefficiencies and spillage during the bottling process has caused mercury contamination of mill sites and calcines. For a more complete discussion of the processing techniques, the reader is referred to Von Bernewitz (1937) and Schuette (1931, 1938).

ENVIRONMENTAL CONCERNS ASSOCIATED WITH MINERAL PROCESSING

Because of the inefficiencies in the roasting process, mercury vapor and mercury- enriched particles were released to the atmosphere and deposited down wind from furnace and retort sites causing local enrichment of mercury in soils. Mercury content is highest in organic rich, surface soils, ranging from 5 to 25 μg/g (ppm), and decreases to background levels at a maximum depth of 0.5 meters. A typical vertical profile of soils impacted by long term furnace release is shown in figure 2 from the New Idria district, the second largest producer of mercury in North America. The background concentration of mercury, 100 ng/g (ppb), is reached at a depth of 33 cm. The leaching and transport of mercury into the soil column indicates that the mercury phases deposited from furnace release are soluble and include elemental mercury and other chloride and sulfates of mercury (see discussion below on speciation). In the larger mercury districts where processing occurred over a long period, the loading of mercury to soils adjacent to the mine site constitutes a potential source of mercury- enriched sediment. Soils naturally accumulate mercury deposited from the global atmospheric pool of mercury, and as result, even soils present outside of mineralized areas have mercury concentrations that reflect increased mercury deposition since the inception of the industrial period. Although soils from mercury mine areas have significantly elevated levels of total mercury, the ratio of methylmercury to total mercury is lower in mine areas than in the baseline sites in unmineralized areas (Bailey and others, 1999). This is because an enzyme-catalyzed (organomercurial lyase) microbial demethylation reaction breaks the Hg-C bond to produce Hg (II) and CH4 (Summers, 1986), thus preventing high concentrations of methylmercury from accumulating in these soils (Hines and others, 1999).

汞元素赋存之地理环境模型(04) - 不在眉头愁 - 吉建斌的网易博客

Figure 2. Profile of mercury concentration in soil impacted by release of particulate mercury and vapor from mercury mines in the New Idria mercury district, California.

The potential environmental impact of mercury-mine wastes varies considerably depending on the total mercury concentration and speciation of phases present in the mine waste. Mercury mine wastes, listed in increasing content of mercury, include waste rock, low grade ore, mine tailings, condenser soot, and cyclone dust. Dust generated during roasting of the ores was collected in a cyclone prior to the mercury-rich vapor entering the condensing system at the larger mines that employed rotary furnaces. Cyclone dust has a distinctive maroon color and can contain up to several weight percent mercury. Soot that accumulates in the condensing columns represents the fine material that escaped the cyclone collectors or that was directly released from the furnace to the condenser at mines that did not have collectors. This fine-grained, well-sorted material can contain up to several tenths of a percent of mercury that is present as elemental mercury and as mercury sulfate and chloride phases. Condenser soot was generally collected and reprocessed in a retort. A major environmental concern is posed by soot that was not reprocessed and was discarded on waste dumps, because of its high concentration of soluble mercury phases. Condenser soot has the consistency of fine silt and can be readily transported by the wind throughout mine site. The mercury vapor flux from condenser soot to the atmosphere also can be very high especially under hot and sunny weather conditions where it can pose a risk to humans through the inhalation pathway.

Mercury mine ore tailings, termed calcines, have a characteristic red color that results from the oxidation of iron sulfides during the roasting of the sulfide ores. The term calcine comes from the common historic practice of adding lime (CaO) to the mercury ore as a desulfurizer. Calcines were typically deposited adjacent to the furnace site or discarded into nearby stream channels. Flood events periodically removed the tailings from the stream channel thus providing space for continuous tailings disposal during the life of the mine. As a result, mercury mine tailings commonly occur within stream channels and in overbank deposits for several to tens of kilometers downstream from mine sites. Transport of mercury from mercury mine sites where the source of mass loading is from mine tailings occurs primarily in the particulate phase and most of the mercury flux occurs during peak flow events (Whyte and Kirchner, 1999; Whyte, 1998). Mine tailings can impact waters at great distances the mine area. For example, downstream from the Idrija mine in Slovenia, the second largest mercury district in the world, sediments in the River Soca and in the seawater of the Gulf of Trieste are contaminated with inorganic mercury more than 50 km from the mine area (Horvat et al. 1999). Sediments contaminated with tailings are a concern because they are a source of elemental mercury and ionic mercury that can become methylated in downstream aquatic environments. For example, inorganic mercury phases in mine tailings from the Idrija mine introduced into Gulf of Trieste contribute to methylation of mercury in bottom seawater sediments (Horvat et al. 1999).

 

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