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Long-term acid generation containing heavy metals from the tailings of a closed mine and its countermeasures
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| Title: | Long-term acid generation containing heavy metals from the tailings of a closed mine and its countermeasures |
| Other Titles: | 休廃止鉱山における鉱滓からの重金属を含む酸性水の長期溶出とその対策 |
| Authors: | KHOEURN, Kimleang Browse this author |
| Issue Date: | 25-Mar-2019 |
| Publisher: | Hokkaido University |
| Abstract: | Acid mine drainage (AMD) from abandoned mines is a well known problem all over the world. The AMD is mainly produced by the oxidation of pyrite and metal sulfides. A significant amounts of heavy metals (e.g., Zinc (Zn), copper (Cu), iron (Fe), and others) are released and could affect the surrounding environment as well as human health when they are ingested through food, crop, and contaminated water. Numerous researches have been conducted to solve these problem by neutralizing the AMD (e.g., calcium hydroxide (Ca(OH)2), sodium hydroxide (NaOH), etc.). However, it remains unclear when the treatment would stop. To answer this question, leaching of heavy metals and acid generation from tailings dams were studied. In this dissertation, an in depth study of the mechanisms controlling the long-term leaching of heavy metals, such as Zn, Cu, and Fe from the tailings was conducted. First, the tailings dam was characterized and speciation of heavy metals was studied. Second, the column experiments were carried out at a different irrigation rate. Finally, chicken eggshell as a low-cost neutralizer for the AMD was applied. The dissertation contains 5 chapters.
Chapter 1 introduces the background of AMD from the tailings and gives the motivation, importance, and objectives of the study.
Chapter 2 addresses the mechanism of AMD generation in tailings from an abandoned mine site and predicts the evolution of Zn, Cu, and Fe concentrations. Batch leaching experiments and sequential extractions were conducted to investigate the leaching behavior of these contaminants from the tailings and to understand their solid-phase partitioning. Acid-base accounting and principal component analysis (PCA) were used to confirm factors affecting Zn, Cu, and Fe leaching and acid formation based on the leaching experiments. There were strong positive correlations between Zn, Fe, and electrical conductivity (EC) or sulfate ion (SO42-), indicating that pyrite and sphalerite are the major minerals releasing Zn and Fe. This agreed with the PCA results. In the upper part of the tailings, the water-soluble and sulfide fractions of Zn, Cu, and Fe were almost flushed out, whereas they remained high deeper in the tailings. This implies that the tailings will likely continue to release these contaminants (Zn > Cu > Fe) for a long time unless remedial measures are taken.
After identifying the high content of heavy metals and ions in the deeper tailings samples, Chapter 3 addresses the understanding of long-term acid generation and leaching of Zn, Cu, and Fe. The unweathered tailings samples at depths of 1 to 3 m were collected from a tailings dam of a closed mine. The mechanisms of long-term weathering of the tailings were assessed through leaching of heavy metals by three column experiments. Mineralogical and chemical constituents, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), sequential extraction of the tailings, and chemical analysis of the leachates were carried out to determine the processes responsible for the leaching of Zn, Cu, Fe, and SO42-. The contents of Zn, Cu, and Fe in the tailings were mainly associated with ion exchangeable and sulfide fractions. The pH values of the leachates from the columns were 3.0－3.7 throughout the column experiments over 84 weeks, and approximately 35-45% of Zn, 15-23% of Cu, 2.5-4% of Fe and 20-25% of S were leached out. Higher concentrations of Zn, Cu, Fe, and SO42- at the beginning of the experiments were observed, which could be attributed to the dissolution of soluble sulfate minerals present in the tailings. This indicates that the formation and dissolution of secondary soluble sulfate minerals contributed to Zn and Cu leaching. After stabilizing the leaching concentrations of Zn, Cu, Fe, and SO42-, they were still released from the tailings until the end of the experiments. This continuous leaching suggests the oxidation of pyrite and sulfide minerals. During these processes, ferrihydrite, goethite, lepidocrocite, and maghemite were formed and these minerals also acted as a sink for Zn and Cu by adsorption and/or co-precipitation. These results mean the significance of the long-term behavior of heavy metals released from mine tailings dams.
Chapter 4 focuses on the evaluation the effectiveness of the chicken eggshell for the neutralization of AMD and removal of the heavy metals. The eggshell were collected from kitchen wastes, and then washed, dried, grinded, and stored in air-dried containers before use. The batch adsorptions were conducted at different contact times, particle sizes, and dose of eggshell. The morphology of the eggshell before and after batch adsorption was analyzed by SEM-EDX. The results showed that pH reached 6.7 and the highest removal efficiency of Zn (60%) , Cu (98%), Fe (99%), and SO42- (10%) were obtained at contact time of 24 h, particle size of 75-150μm, and dose of 1 g/25 mL. The removal efficiency of heavy metals were in order Fe > Cu > Zn > Mn. The removal process of Cu and Fe was rapid and reached equilibrium at 24 h. This means that the removal of Cu and Fe is due to precipitation whereas that of Zn and Mn is due to adsorption. Based on the results, the chicken eggshells can be used for neutralization and removal of the heavy metals from the actual AMD.
Chapter 5 gives the overall conclusions of the research and suggestions for future study on the AMD as well as heavy metals from the tailings. The results could provide helpful information on the management of tailings dams after mine closure. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第13655号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 教授 五十嵐 敏文, 教授 佐藤 努, 教授 廣吉 直樹, 准教授 原田 周作 |
| Degree Affiliation: | 工学院（環境循環システム専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/74046 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 工学院(Graduate School of Engineering)
学位論文 (Theses) > 博士 （工学）
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