John E. Dutrizac

Characterizing gold in refractory sulfide gold ores and residues

Gold is present in refractory sulfide gold ores mainly in arsenian pyrite and arsenopyrite, where it occurs in both the chemically bonded state and as nano-size grains of metallic gold. During roasting or pressure oxidation, the sulfide matrix is destroyed and essentially all the gold is converted to the metallic form. The liberated gold is readily dissolved in conventional cyanide media, although a few residual gold particles, commonly encapsulated by films of silica-rich gel or by calcium sulfate, are detected. Chloridecontaining gold ores can generate soluble gold chlorocomplexes during hydrometallurgical processing and the dissolved gold can be sequestered on the surfaces of associated carbon particles, resulting in reduced gold recoveries.

COMPARISON OF MEASURED AND MINERALOGICALLY PREDICTED VALUES OF THE SOBEK NEUTRALIZATION POTENTIAL FOR INTRUSIVE ROCKS 1

Twelve specimens of intrusive rocks, ranging from granitic to ultramafic, were ground and subjected to the static-test neutralization-potential (NP) protocol so that the results could be compared with those computed by using the NP values previously obtained by Sobek tests of the constituent minerals. The quantitative mineralogy of the rocks was determined by Rietveld refinements of X-ray powder diffraction data, and was supplemented by optical microscopy, fizz tests, and analyses of total carbon to determine the presence of carbonate minerals. Despite the igneous nature of the suite, most samples were found to be carbonate-bearing; optical microscopy and fizz tests of the coarse (minus 6 mm) fractions were observed to be more sensitive to the presence of carbonates than was the minus 60-mesh fraction that is used in the Sobek protocol. For some minerals, notably olivine and serpentine, the acid-digestion period in the Sobek test has a pronounced effect on the resulting NP, and this part of the test protocol is in need of new standardization. Mineralogical prediction of the NP values is sensitive to the composition of the plagioclase because these feldspars are typically a major component of igneous rocks and the NP of the calcic end- member, anorthite, is about 12 times that of the sodic end-member, albite.

Characterization of ferric arsenate-sulfate compounds: Implications for arsenic control in refractory gold processing residues

Abstract 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 (FeAsO4·0.75H2O). 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.