| 其他摘要 | Mercury is a global pollutant due to its long residence time in the atmosphere. Zinc smelting is regarded as an important anthropogenic mercury source to the atmosphere. In developed countries, strict pollution-targeted controlling measures are administered during the smelting of zinc so that Hg pollution problem can be effectively controlled. Due to economy and the technology most zinc-smelting corporations in our country do not implement the reclaim of flue gas Hg, we could not use the abroad research results. China is the biggest zinc ingots country around the world. For instance, the zinc production of China account for twenty-five percent of the worlds production in 2005. Although our nation is the biggest zinc ingot producing country, research related to this field lags behind very much. Recently, the fact that our country zinc smelting releases Hg into the atmosphere gives rise to international controversy. Moreover, Hg release during the smelting process poses important threat to local ecosystem. Now people pay more and more attention to the mercury pollution caused by zinc smelting.
The first objective of this study is to conduct the case studies on mercury in the zinc ore and zinc concentrate of the different types lead-zinc mine and zinc concentrate of different zinc smelting factories, so as to understand the mercury in zinc ore and zinc concentrate. The second objective of this study is to conduct the case studies on mercury in different products and byproducts, so as to demonstrate the mercury in zinc concentrate distributes in the different steps, e.g. calcine and dust, washing water, wet dust, during the smelting process. Then we can calculate the mercury emission factors to atmosphere using the mass balance. The third objective of this study is to assess the environmental impact and environmental geochemistry of those smelting areas, total Hg and MeHg in soils, plants, and different Hg species in stream-waters in artisanal and industrial zinc smelting area were determined.
1. The study on Hg in zinc ore and zinc concentrate
Hg concentrations in zinc ore and zinc concentrate at fankou lead-zinc mine and lanping lead-zinc mine were 64.5-107 mg∙kg-1, 588-602 mg∙kg-1 and 0.23-0.40 mg∙kg-1, 1.91-3.33 mg∙kg-1 respectively. Hg concentration in zinc ore and zinc concentration of fankou lean-zinc mine is 100 times than that of lanping lead-zinc mine. Zinc concentration, came from different mine, in different zinc smelting companies varied 0.10 mg∙kg-1 to 1100 mg∙kg-1. The mixed zinc concentration in different zinc smelting companies ranged from 37 mg∙kg-1 to 105 mg∙kg-1. Zinc-lead ore in different zinc-lead mines have different mercury concentration because the mines belong to different tye of zinc-lead mines and the mines have different geological element concentration.
2. The distribution of the mercury in zinc concentrate in different steps and mercury emission factors during the zinc smelting process
A and B smelting factory which adopt different zinc smelting technics and eliminating mercury technics indicat that the mercury in zinc concentrate distributed in different steps during the course of zinc smelting, however main restrictive factor is whether reclaim the mercury in the flue gas or not. Before the flue gas reach the mercury removal equipment, mercury control technology for zinc mine, removal efficiency in these steps, e.g. calcine and dust, water washing, wet dust, between A and B zinc smelting factories is similar. After the flue gas pass the equipment more than half of the mercury is eliminated in A zinc smelting factory. It is result to distinct difference of A and B zinc smelting factory between the mercury which is absorbed by acid and the mercury which is emitted to atmosphere.
The Hg emission factors are 113 g Hg t-1 zinc, 49 g Hg t-1 zinc, 243 g Hg t-1 and 5.7 g Hg t-1 zinc along artisanal zinc smelting process without any environmental protection measures, B and C zinc smelting factories without mercury removal equipment and A zinc smelting factory with mercury removal equipment during the smelting process. Take into account all kinks of Hg emission factors and the zinc production came from different methods, we estimated that 61.3-177.5 t mercury emitted to atmosphere during the zinc smelting process each yeah.
3. Mercury pollution to the ecosystem caused by zinc smelting
Two factors affect the Hg concentration in agricultural soil. One is mercury in waste calcines, the other is the deposition of the reactive Hg and particulate Hg at ambient air. Hg concentrations in agricultural soil are 72-355 μg·kg-1 and 259-2990 μg·kg-1 in atisanal zinc smelting area and industrial zinc smelting area, respectively. The results indicate that mercury pollution to environment caused by industrial zinc smelting is more severe than that caused by artisanal zinc smelting. The degree of methylation became worse due to the higher MeHg and higher rate between MeHg and THgin in soil neaby the smelting sits in Weining County. Sludge with high Hg concentrations, produced by sewage treatment in industrial zinc smelting factories, is the potential mercury sources to the local environment.
Similar to the results for Hg in soils, surface waters are heavily impacted by zinc smelting activities in Weining County. Discharging the treated waste water, which has high Hg concentrations, is the mercury sources to local rivers. The rate between dissolved MeHg and dissolved THg in paddy water is higher than that in normal water. This implies that the paddy conditions may be favorable for methylation in paddy water.
Atmospheric Hg concentrations in artisanal zinc smelting areas are strongly correlated with zinc smelting activities. Total gaseous Hg concentration near the zinc smelting workshop is 5220 ng∙m-3, significantly elevated. There is a positive relationship between Hg concentrations in corn leaves tissue and the total gaseous Hg concentrations in the ambient air, indicating that the mercury in foliage tissue originated from the atmosphere. Uptake of atmospheric Hg by leaves is the predominant pathway for corn plant to accumulate Hg.
Agricultural rice and vegetables collected from industrial zinc smelting area present highly elevated total Hg concentrations when compared to corn collected from artisanal zinc smelting area. In artisanal zinc smelting area, Hg concentration in all corn grain tissue is lower than the national guidance limit for foodstuff of China that is 20 μg kg-1, however in industrial zinc smelting area Hg concentration in some rice exceed the maximum Hg concentration recommended by the Chinese National Standard Agency for food sources, and total Hg concentrations in all capsicum exceed the National Standard of 10 µg·kg-1 for edible vegetations. This will give rise to potential danger to local people when people eat the contaminated food and vegetation. |
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