ABSTRACT
Acid mine drainage (AMD) formation is mainly caused by the oxidation of pyrite. Carrier-microencapsulation (CME) using metal-catecholate complexes has been proposed to passivate sulfide minerals by forming surface-protective coatings on their surfaces. Among the various metal-catecholate complexes, Ti-catecholate formed stable coatings having superior acid-resistance, but a thick enough passivating film required considerable time (ca. 14 days) to grow. Meanwhile, Fe-catecholates can form Fe-oxyhydroxide coatings within 2 days, however, they are less stable than Ti-based coating. To address these drawbacks of using a single metal-complex, this study investigated the concurrent use of Fe-catechol and Ti-catechol complexes for accelerating the formation of stable passivating coating on pyrite. Compared with a single metal-complex system, the coating formation was significantly accelerated in mixed system. Linear sweep voltammetry showed the simultaneous decomposition of [Fe(cat)]+ and [Ti(cat)3]2- as the main reason for improved coating formation. Electrochemical properties of coatings formed by single and mixed complex systems, confirmed by electrochemical impedance spectroscopy and cyclic voltammetry, indicated the coating formed in the mixed system had higher resistance and more electrochemically inert than the other cases. The simultaneous use of Fe-catechol and Ti-catechol complexes enhanced pyrite passivation by accelerating metal-complex decomposition and forming more stable coating composed of Fe2TiO5.
Subject(s)
Iron , Titanium , Catechols , SulfidesABSTRACT
Pyrite, a common gangue mineral in complex sulfide ores and coals, is rapidly oxidized in water by ferric ions and dissolved oxygen to form a very acidic and heavy metal-laden leachate called acid mine drainage (AMD). Carrier-microencapsulation (CME) using Ti4+, Si4+, and Al3+ was reported as a promising new approach to prevent pyrite oxidation by forming a passivating barrier on the pyrite surface. In CME, the presence of Fe3+-catecholate complexes is unavoidable but their effects on pyrite oxidation remain unclear. In this study, the effects of Fe3+-catecholate complexes on pyrite oxidation were investigated. Formations of mono-, bis-, and tris-catecholate complexes of Fe3+ were verified by UV-Vis spectrophotometry and their speciation with pH was consistent with thermodynamic considerations. Linear sweep voltammetry was conducted to evaluate the redox properties of Fe3+-catecholate complexes, and the results indicate that ligands in the three complexes were sequentially oxidized until Fe3+ is released. Coating formation on pyrite was confirmed after treatment with mono- and bis-catecholate complexes. Results of SEM-EDX and ATR-FTIR indicate that the coating is composed primarily of iron oxyhydroxide phases. The results of leaching experiments showed that pyrite oxidation was suppressed by Fe3+-catecholate complexes via two mechanisms: (1) electron donating effects of the complexes, and (2) formation of a protective coating on pyrite. The results provide not only a better understanding of the effects of Fe3+-catecholate complexes on pyrite oxidation but also some possible applications of Fe3+-based CME such as the suppression of pyrite oxidation to prevent AMD formation and depression of pyrite floatability in mineral processing.
