Process mineralogy of refractory gold ore in thiosulfate solutions

A mineralogical characterization was carried out in samples of refractory gold ore with 59 g/ton, according to fire assays, which were taken from a previous leaching process with ammonium thiosulfate. The potential window used to evaluate the cathodic and anodic processes was chosen knowing that gold and copper reduction, and thiosulfate and Cu(I) species oxidation occurs in it. The leaching process consisted of a pretreatment of the mineral (in ammonium hydroxide and copper sulfate during 12 h with and without air bubbling) followed by leaching (in ammonium thiosulfate solution and EDTA at a pH ∼ 10 during 24 h with mechanical stirring at a rate of 250 rpm) which rendered a gold-release of 81.57%. When oxidants and stabilizers like Cu(II), NH3 and EDTA are present in thiosulfate based leaching solutions, the chemistry turns complex, and the partial reduction of copper, and redissolution of gold may occur. This fact and the low gold concentration make the electrowinning process especially difficult. Characterization techniques such as polarized light microscopy, X-Ray Diffraction, Rietveld quantification, and Automated Digital Imaging System, were chosen to track the process mineralogy of the sample, and the results prior to the leaching revealed association, size, and shape of each mineral occurrence; it was found that the mineral composition was pyrite crystals, exhibiting subhedral and inequigranular morphology (75%); galena crystals (10%); sphalerite (1.8%); chalcopyrite and arsenopyrite crystals (<3%); and free gold (<1%). The rest of the sample was comprised of aluminosilicate phases such as quartz and muscovite. After the leaching process, the main feature found was some accumulation of individual pyrite grains with corrosion grooves, and decrease in the particle size; on the other hand, the dissolution of the accompanying phases, with respect to the galvanic interactions, evidenced the next sequence of dissolution: I/E = pyrite > galena > M3 > sphalerite. Rietveld quantification suggested changes in the ratio of the main phases: pyrite decreased from 75.6% up to 65.7%; quartz increased from 21.1% up to 31.3%; galena decreased from 10.4% up to 7.6%; sphalerite decreased from 1.8% up to 1.1%. It is worth saying that the presence of precipitated amorphous phases was not observed.