| 其他摘要 | The exploitation of abundant lead-zinc deposits have significantly contributed to the rapid development of economy and society in our country, however inevitably led to serious environmental pollution. Acid mine drainage (AMD) and heavy metal elements pollution have become two main environmental problems. What the people worried about is that, the leach solution of the mine tailings still affect the ecological environment around the mining district, even the mines were shut down in a few decades or centuries or even longer. Sphalerite is not only one of the main ores in Pb-Zn deposits, but also the main Cadmium-host ore. Cadmium is the toxic heavy metal element after Mercury. During mining, separating and smelting of Pb-Zn ores, plenty of waste rocks and tailings containing sulfide minerals were exposed to surface.The acid mine drainage and some heavy metal elements such as Zn, Cd, were released to water-body, and soils by reaction between water, rock and gas (oxidation and leaching), thus damage the environment in the mining district, and harm the human health. Based on studies of weathering and leaching on Pb-Zn ores and oxidation kinectics of sphalerite under supergene conditions, we can understand the weathering and leaching path of ores and minerals, oxidative rates and effect factors , and reveal the rule of releasing, movement and enrichment of heavy metal elements, and establish the rate law of sphalerite oxidation. It will offer scientific theories for forecasting, evaluating and controlling the mining environment pollution.
In this dissertation, the weathering-leaching experiment of ores and oxidative kinetic experiment of sphalerite were carried out on a typical example of the Niujiaotang Cd-rich zinc deposit, and the environment of the mining district was investigated to study supergene geochemistry and its environmental effects. From the above studies, the following conclusions were drawn:
1. The weathering-leaching experiment shows leaching solution was neutral-alkaline. Numerous cations (Zn2+、Cd2+、Ca2+、Mg2+ etc)and anions(SO42-、CO32-) were released out, meanwhile deposits containing mainly gypsum were produced. Zn and Cd dominantly exist in these deposits, with little elements in the leaching solution.
2. The leaching rate of Zn and Cd from pyrite-bearing zinc ore was lower than the half oxidative ore, indicating that the half oxidative or oxidative zinc ore weathers and leaches more easily.
3. Taking Fe2(SO4)3 as oxidant, oxidative rates increase with increasing ferric concentration and temperature, deceasing pH value, and the releasing rates of Zn and Cd were generally similar. The apparent activation energy is 41.75kJ.mol-1 and 42.51 kJ.mol-1 for Zn and Cd, respectively, indicating that the rate is chemically controlled. In limited time（less than 60 h），the oxidative rate of sphalerite decreases with increasing content of pyrite because of competition relationship between sphalerite and pyrite with Fe3+. Taking oxygen (O2 ) as oxidant, oxidative mechanism change with variation in pH value. In the pH range of 2.0-6.0, the rates decrease with increasing pH value, while the rates increase with increasing pH value in a range of 6.0-7.8. Both Fe3+ and oxygen play an important role during oxidation of sphalerite, but the latter is more important .
4. Most of the dissolution curves of Zn and Cd are very similar, and their apparent activation energy are close, suggesting that Zn and Cd are similar in geochemistry. Nevertheless, the released rates of Zn and Cd exhibit difference during dissolution of sphalerite: the rate of Cd is faster than Zn under the acidic condition, but the rate of Cd is slower than Zn under the neutral-alkalic condition. It was mainly because that Zn and Cd belong to ⅡB system, showing geochemical similarity and difference.
5. On the basis of data regression, a rate expression of sphalerite oxidation was produced as follows:
or
The rate law of sphaerite would be applied for calculating Zn and Cd during sphalerite oxidation. This may provide theories for forecasting, evaluating and controlling mining environment pollution.
6. The investigating results of the mining district environment indicate that water body was neutral-alkaline, and water pollution was not serious. Neither Zn or Cd concentration exceeds the Chinese National standard. However, the soils, river deposits and plants were polluted in a certain extent.
7. This study indicates that oxidation of sphalerite and weathering of Pb-Zn ores result in discharge with low pH and high-metal concentration, and pollute environment of the mining district , finally harm the human heath. In order to prevent and control environment pollution, some measures can be made as below. Firstly, increasing pH value, can decrease the oxidative rate of sulfides such as sphalerite. Secondly, avoiding expose of waste rocks and tailing ores to oxidative conditions containing air and ferric, can decrease the oxidative rate of sulfides (e.g., sphalerite) . |
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