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Paktunc D.,CanmetMINING | Paktunc D.,University of Ottawa | Majzlan J.,Friedrich - Schiller University of Jena | Huang A.,University of Ottawa | And 3 more authors.
American Mineralogist | Year: 2015

Arseniosiderite and yukonite are among the important arsenate minerals occurring as secondary alteration products in relation to the oxidation of arsenopyrite and arsenian pyrite and as discrete grains in some gold ores, mine tailings, and contaminated soils. Characteristics of these Ca-Fe arsenate species are not well known and our understanding of the conditions promoting their formation and dissolution is limited. Long- and short-range structural characteristics and thermodynamic properties of the Ca-Fe arsenates forming in the Ca-Fe(III)-As(V)-NO3 system were determined to better predict the mineralogical transformations taking place in neutralized sludge and tailings environments, and their influence on arsenic mobilization. Yukonite and arseniosiderite readily form from solutions with highly variable compositions at a wide pH range from slightly acidic to alkaline conditions. Calcium concentrations corresponding to molar Ca/(Ca+Fe+As) ratios as low as 0.1 appear to be adequate for their formation. Our experimental results confirm observations in natural settings and mine tailings where scorodite is progressively replaced by yukonite and arseniosiderite. The initial amorphous precipitates made of small oligomeric units of edge-sharing FeO6 octahedra with bridging arsenate evolve to yukonite through the establishment of corner linkages between the FeO6 chains. Yukonite represents a nanocrystalline precursor and Ca-deficient variety of arseniosiderite. Formation of arseniosiderite is kinetically controlled with faster development of crystallinity at neutral to slightly acidic pH and slower kinetics under alkaline conditions. Calorimetric measurements provided an enthalpy of formation value of -1950.3 ± 3.1 kJ/mol and standard entropy of 237.4 ± 4.4 J/(mol·K) for arseniosiderite [with composition Ca0.663Fe1.093(AsO4)(OH)1.605·0.827H2O], the corresponding Gibbs free energy of formation is -1733 ± 3.4 kJ/mol. A rough estimate of the thermodynamic properties of yukonite is also provided. Arseniosiderite is a stable arsenate between pH 3.5 and 7.5 in solutions saturated with respect to soluble Ca minerals such as calcite, gypsum, anorthite, or Ca-montmorillonite. Arsenic release from mine wastes and contaminated soils can be effectively controlled by arseniosiderite and the conditions promoting its formation such as lime-treatment leading to gypsum saturation in ferric arsenate solutions would prove to be desirable for stabilizing arsenic in the form of arseniosiderite in mine wastes.

Paktunc D.,CanmetMINING | Paktunc D.,University of Ottawa | Manceau A.,French National Center for Scientific Research | Dutrizac J.,CanmetMINING
American Mineralogist | Year: 2013

Ferrihydrite is the main form of ferric iron in surficial environments and a key reactive nanoparticle that regulates nutrient availability and the mobility of metal(loid) contaminants, yet its structure is not completely elucidated. Two models exist to date: the "f-phase" in which Fe is fully octahedral and the "akdalaite-model" possessing 20% of the Fe atoms in tetrahedral coordination. In this study, germanium was used as a structural probe to re-examine the validity of the latter model. Germanium-bearing ferrihydrites containing 0.2, 0.6, 1.4, 2.2, 2.9, 3.8, 12, and 15 wt% Ge were synthesized in the laboratory at 25 and 65 °C. X-ray diffraction analyses showed all the precipitates to be six-line ferrihydrite. Semi-quantitative energy-dispersive X-ray microanalyses (TEM) indicate that the precipitates made from solutions having Fe/Ge molar ratios of two and four have Fe/Ge atomic ratios of 3.8-3.9 and 4.4-5.1, respectively, which suggest a limit of Ge uptake in ferrihydrite of about 20 at% relative to total cations. Based on TEM examinations, these high Ge-bearing ferrihydrites are homogenous and consist of equant and plate-like crystallites about 5-6 nm in size. Furthermore, it appears that higher Ge concentrations in solution have no significant effect on the crystallite size, supporting the incorporation of Ge in the ferrihydrite structure. Extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that the Fe atoms in both the low and high Ge-bearing ferrihydrites are in octahedral coordination and that Ge occurs in the ferrihydrite structure by filling the empty tetrahedral sites and coordinating to 4 edge-sharing FeO6 trimers through sharing a common oxygen (Ge-O-Fe linkage). Incorporation of the Ge tetrahedra in the ferrihydrite structure requires redistribution of Fe occupancy along the alternating O/OH layers while forming an ordered distribution of octahedral Fe and tetrahedral Ge. The local structure around Ge mimics a Keggin-like motif in two different, yet equivalent, orientations. It appears that the split diffraction peak at 1.46 and 1.51 Å is a characteristic feature of Ge-rich ferrihydrite and suggests that it is a fingerprint of increased order due to significant Ge incorporation in the tetrahedral sites. The findings can be rationalized in terms of the incorporation of Ge in the so-called "f-phase" of the classical ferrihydrite model, and demonstrate the flexibility of the model in terms of accommodating a Keggin-like cluster without the need of imposing unrealistic constraints as in the akdalaite model. Direct comparison of the imaginary parts of the Fourier transforms for ferrihydrite and maghemite further confirms the absence of tetrahedral Fe in ferrihydrite. The absence of tetrahedral Fe substantiates the use of goethite-like or akaganeite-like models to describe the polyhedral structure of ferrihydrite used in modeling sorption reactions at the ferrihydrite-water interface.

Paktunc D.,CanmetMINING | Majzlan J.,Friedrich - Schiller University of Jena | Palatinus L.,ASCR Institute of Physics Prague | Dutrizac J.,CanmetMINING | And 3 more authors.
American Mineralogist | Year: 2013

A combination of techniques, including powder X-ray diffraction (XRD), electron microprobe analysis (EPMA), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAFS), is used to characterize the common ferric-arsenate-sulfate compounds, which could result from the pressure oxidation of refractory gold ores at elevated temperatures. Three general types of precipitate are identified; namely, arsenate-bearing basic ferric sulfate [FeSO4(OH) and designated as BFS], ferric arsenate-sulfate [an extensive solid solution Fe(AsO4)0.2-0.7(SO4)0.7-0.2(OH)0.7-0.2 and designated as FAS], and hydrated ferric orthoarsenate (FeAsO40.75H2O). The crystal structure of FAS is solved by precession electron-diffraction experiments. The structures of BFS and FAS are constructed from octahedral Fe3+ chains, which are cross-linked by sulfate and arsenate tetrahedra. Extensive substitution of arsenate for sulfate occurs in both types of compounds with charge neutrality being maintained by variations in the (OH) content. The XAFS spectra indicate that the local structures of both BFS and FAS are made of corner-linked single chains of FeO6 octahedra where the chains are linked by AsO4 or SO4 tetrahedra forming alternating layers of FeO6 octahedra and AsO4 or SO4 tetrahedra. Preliminary toxicity characteristics leaching procedure (TCLP) testing of the precipitates indicates that FAS with a molar ratio As/(As+S) ratio of =0.5 could be an acceptable material for disposal in a tailings impoundment, whereas more As-rich FAS and BFS may require further treatment. The results for the laboratory-prepared precipitates are compared with those obtained on three residues from the processing of refractory gold ores. The major As-carrier in one of the residues is FAS, whereas As-bearing goethite and hematite are the dominant As-carriers in the other two residues. Thus, the mineralogical characteristics of the residues dictate the appropriate arsenic management and disposal options in the processing of refractory gold ores.

Deschenes G.,CanmetMINING | Guo H.,Eldorado Gold Corporation | Xia C.,CanmetMINING | Pratt A.,CanmetMINING | And 3 more authors.
Minerals | Year: 2012

The high solubility of copper sulphide minerals is an issue in the cyanidation of gold ores. The objective of this study was to quantify the effect of individual copper sulphide minerals on the Hunt process, which showed advantages over cyanidation. High purity djurleite, bornite and chalcopyrite, with a P70 of 70–74 microns, were mixed with fine quartz and gold powder (3–8 micron) to obtain a copper concentration of 0.3%. The ammonia-cyanide leaching of slurry with djurleite proved to be more effective than cyanidation; producing comparable extraction of gold (99%), while reducing the cyanide consumption from 5.8 to 1.2 kg/t NaCN. Lead nitrate improved the Hunt leaching. The lower cyanide consumption is associated to a significant reduction of copper dissolved. XPS surface analysis of djurleite showed that lead nitrate favored the formation of Cu(OH)2 species. Lead was also detected on the surface (oxide or hydroxide). Sulphide and copper compounds (cyanide and sulphide) were reaction products responsible for inhibiting the dissolution of gold. Lead nitrate added in the Hunt leaching of bornite produced 99% gold extraction. Surface reaction products were similar to djurleite. The cyanide consumption (~4.4 kg/t NaCN) was not reduced by the addition of ammonia. Cyanidation of chalcopyrite showed a lower consumption of cyanide 0.33 kg/t NaCN compared to 0.21 kg/t NaCN for Hunt. No significant interferences were observed in gold leaching with a slurry containing chalcopyrite. © 2012 by the authors; licensee MDPI, Basel, Switzerland.

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