[en] The EPR technique was used to study SeO^-₄ in K₂SeO₄ in the temperature range of 130-93K where the crystal presents an incommensurable phase, characterized by the wave vector q-vector = (1 - δ) a^*/3. By fitting of line form with EPR the β parameter (critical exponent) could be determined. The found values were: β = 0.33 +- 0.03 and 2 β = 0.58 +- 0.06. A summary of the experimental values of β found in literature is presented. The mean value of these measurements β = 0.347 +- 0.03 fits to the calculated value β = 0.3455 +- 0.0020 for the 3d XY model. It was possible to detect a change in the modulation of the plane wave regime to multisoliton regime through the δ parameter obtained also by fitting of line form. This parameter is related to soliton density (n_s), ''lock-in'' incommensurable transition order parameter. Close to T_c, n_s does not follow theoretical predictions and saturates in a value different than zero. This result was interpreted as due to mesh defects which fix phase modulation and create metastable states that may lead to a chaotic state between the multisolitons phase and ''lock-in'' phase. (A.C.A.S.)

[en] Published in summary form only

No abstract available

[en] Gold mining residues associated with sulphides may contain high levels of antimony, and the release of this metalloid may lead to environmental impacts due to its potentially toxic character. Within this context, the recovery of antimony from residues becomes an interesting option under the social, economic and environmental points of view. The present work has the aim to identify the occurrence of antimony and its association in the tailings from the treatment of a refractory gold ore. The characterization of the tailings obtained from the cyanidation of a refractory gold ore previously submitted to pressure oxidation was conducted using X-ray diffraction, Energy Dispersive X-ray Fluorescence, sizing particle analysis, RAMAN spectroscopy, coupled scanning electron microscopy and transmission electron microscopy, in addition to wet chemical analysis. The particle size analyses showed solids below 30μm, with a mean particle diameter of 8.6μm. The results of X-ray diffraction and chemical composition indicate that the tailings consist predominantly of silicates, such as feldspars and micas; no single phase or compound of antimony has been identified. Through the electron microscopy (scanning and transmission), it was observed that the antimony is associated with iron-containing microparticles aggregated with larger silicate particles. Raman spectroscopy indicated that hematite and feroxyhyte are the predominant iron phases. Transmission electron microscopy analyzes showed that antimony is widespread in iron microparticles. (author)

No abstract available

[en] Highlights: • The As release from FeAsS oxidation is slightly affected by O_2(aq) concentration. • The galvanic coupling with pyrite significantly increases the As release. • The coupling increased the steady-state, As release rates in O₂-saturated solutions. • Detection of fine amorphous layer (As-bearing Fe oxy-hydroxide) on oxidized FeAsS. • Similar products were detected in the absence and presence of FeS₂ in the reactor. Arsenopyrite (FeAsS), the most common arsenic-bearing mineral, is usually found associated with pyrite (FeS₂) in gold mining tailings. This work examined the galvanic effect of FeS₂ on As release from FeAsS oxidation in circumneutral media under oxygen-depleted and oxygen-saturated conditions. The oxidation experiments were conducted with a flow-through reactor in the absence of FeS₂ particles and in the presence of different contents of this sulfide. The results indicated that the permanent, physical contact between FeAsS and FeS₂ particles causes an increase in the accumulated As release, mainly under O₂-saturated conditions. At 30% wt. FeS₂, the increases relatively to FeS₂-free conditions were 82% and 117% in O₂-depleted and O₂-saturated solutions, respectively. At steady-state, the As release rates increased from (4.9 ± 0.5)× 10⁻⁴ µmol m⁻² s⁻¹ (0% wt. FeS₂) to (1.1–1.9)× 10⁻³ µmol m⁻² s⁻¹ (5–30% wt. FeS₂) under O₂-saturated conditions. Analysis of FeAsS samples after oxidation revealed oxidized particles partially or entirely covered by precipitates with different sizes, shapes and compositions (e.g., As−S−bearing ferrihydrite, elemental sulfur, and As−O phases). A fine (3–4 nm thick) amorphous layer of S−As−bearing ferric oxy-hydroxide was also identified on oxidized FeAsS, with Fe(III) and As(III) species.