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Galvanic microencapsulation: a new technique to suppress pyrite oxidation

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Title: Galvanic microencapsulation: a new technique to suppress pyrite oxidation
Other Titles: ガルバニックマイクロエンキャプシュレーション:黄鉄鉱酸化抑制のための新しい方法
Authors: Seng, Sophea Browse this author
Issue Date: 25-Mar-2019
Publisher: Hokkaido University
Abstract: Acid mine drainage (AMD) is one of the biggest environmental concerns for centuries. This problem is generally caused by the exposure of sulfide minerals to oxygen and water. This process can occur naturally, however, mining industries are the major producer of such effluents. AMD is very acidic and contains high concentrations of hazardous heavy metals such as copper (Cu), lead (Pb), and zinc (Zn) as well as toxic metalloids like arsenic (As) and selenium (Se). The most widely used technique to mitigate the negative environmental impacts of AMD is via neutralization. In this technique, basic materials such as limestone are added to AMD to increase its pH and precipitate most of the heavy metal as metallic oxyhydroxides. Even though this technique is effective, AMD generation could continue for several decades or even centuries, so this approach is unsustainable. Microencapsulation is a promising and potentially more sustainable approach because it limits AMD production by suppressing pyrite oxidation directly through the formation of a passivating coating on the mineral. This study developed a new technique to prevent pyrite oxidation called “Galvanic Microencapsulation”. Chapter 1 gives a general introduction of acid mine drainage (AMD) formation and its mechanism as well as reviews AMD remediation, and prevention techniques. The statement of the problem and the objective of the study are highlighted to understand the key points and the structure of the entire research. The prevention techniques to prevent AMD are formation by controlling the migration of water, flooding and sealing of underground mine, alkaline amendment, alkaline recharge trenches, anaerobic and anaerobic wetland, limestone ponds, in-pit disposal, and encapsulation. Among these techniques microencapsulation to prevent pyrite from being oxidized by forming a protective coating on pyrite surface is recently gaining attention. The original microencapsulation techniques introduced by Evangelou (1995) used hydrogen peroxide (H2O2) to oxidize Fe2+ to Fe3+ for the formation of insoluble ferric phosphate on pyrite. Although this technique effectively suppressed pyrite oxidation, H2O2 was unable to selectively target pyrite in real, complex wastes leading to unnecessarily large consumption of expensive reagents. Moreover, handling and storage of H2O2 are both difficult, especially in large-scale applications. Carrier microencapsulation (CME) is, a microencapsulation technique that uses redox-sensitive metal(loid)-organic complexes to carry the coating material to the surface of pyrite where the complexes are adsorbed and decomposed, releasing the insoluble metal(loid) ion of the complex that is rapidly precipitated to form a protective coating on pyrite. Because pyrite dissolves via an electrochemical mechanism, the redox-sensitive metal (loid)-organic complexes have been shown to selectively target pyrite even in a complex system containing arsenopyrite and quartz. Chapter 2 introduces a new encapsulation technique called galvanic microencapsulation (GME). The effects of GME on pyrite oxidation using zero-valent aluminum (ZVAl) or zero-valent iron (ZVI) at various dosages and under different conditions (i.e., leaching time and pH) were investigated. Pyrite oxidation was suppressed in the presence of ZVAl or ZVI. Galvanic interaction between pyrite and ZVAl in the first 3 days was negligible, which could be attributed to the Al-oxyhydroxide coating on ZVAl. After 7 days, however, ZVAl exhibited substantial suppressive effects on pyrite oxidation. In comparison, the suppressive effects of ZVI on pyrite oxidation were observed after just 1 day. Pyrite oxidation suppression mechanisms by GME during leaching were elucidated by electrochemical measurements. Cyclic voltammetry and chronoamperometry measurements showed that the suppressive effects of ZVAl and ZVI were predominantly due to galvanic interactions. In chapter 3, coating formation technique for prolonged suppressive effects of GME on pyrite oxidation using phosphate and ZVI was developed. In Chapter 2 galvanic interaction was observed, however, the coating was not observed on the pyrite surface. In the presence of phosphate, suppression of pyrite oxidation by ZVI was dramatically improved because of the combined effects of galvanic interactions and coating formation. When phosphate was added, ferric phosphate was formed as a protective coating on pyrite. In chapter 4, GME was applied not only to improve the separation efficiency during coal cleaning but also to suppress pyrite oxidation. GME treatment was carried out before flotation to change the surface of pyrite from hydrophobic to hydrophilic by phosphate coating. Iron phosphate coating was induced in a ball mill grinding system using steel balls as the electron donor and then followed by flotation. The results showed improvement of flotation results and suppression of pyrite oxidation. Finally, chapter 5 gives the general conclusions of this study.
Conffering University: 北海道大学
Degree Report Number: 甲第13659号
Degree Level: 博士
Degree Discipline: 工学
Examination Committee Members: (主査) 准教授 伊藤 真由美, 教授 廣吉 直樹, 教授 五十嵐 敏文, 准教授 坂入 正敏
Degree Affiliation: 工学院(環境循環システム専攻)
Type: theses (doctoral)
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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