Roel Cruz

Electrochemical oxidation of arsenopyrite in acidic media

Abstract Arsenopyrite (FeAsS) is a common sulfide mineral in base metals and precious metals ores and concentrates. The treatment of these kinds of ores involves frequently an oxidation step to improve metal recoveries. The study of the mechanisms occurring during the arsenopyrite oxidation is then necessary to optimize industrial processes, like bioleaching of arsenopyrite-bearing concentrates using autotrophic acidophilic bacteria. In this work, an electrochemical approach was used to study the arsenopyrite oxidation in acidic media. Cyclic voltammetry and chronoamperometry techniques were performed using a carbon paste electrode (CPE) and an acidic growth medium as electrolyte solution. This electrochemical study allows to determine that the oxidation of arsenopyrite in an acidic growth medium is performed in two steps. The first step corresponds to the initial surface oxidation of arsenopyrite to produce realgar (As 2 S 2 ) and ferrous ions in solution ( E V SCE ). During the second step, a catalytic oxidation of these interfacial products appears to solubilize finally H 3 AsO 4 and ferric ions ( E > 0.55 V SCE ). The presence of elemental sulfur on the arsenopyrite surface was not detected in this study. This fact is associated to the potential zone where this electrochemical study was performed. However, the study here reported allows to have some advances in the understanding of the arsenopyrite oxidation in acidic media.

Galena weathering under simulated calcareous soil conditions.

Exploitation of polymetallic deposits from calcareous mining sites exposes galena and others sulfides to weathering factors. Galena weathering leads to the formation of lead phases (e.g., PbSO(4), PbCO(3)) with a higher bioaccessibility than galena, thus increasing the mobility and toxicity of lead. Despite the environmental impacts of these lead phases, the mechanisms of galena oxidation and the transformation of lead secondary phases, under neutral-alkaline carbonated conditions, have rarely been studied. In this work, an experimental approach, combining electrochemical and spectroscopic techniques, was developed to examine the interfacial processes involved in the galena weathering under simulated calcareous conditions. The results showed an initial oxidation stage with the formation of an anglesite-like phase leading to the partial mineral passivation. Under neutral-alkaline carbonated conditions, the stability of this phase was limited as it transformed into a cerussite-like one. Based on the surface characterization and the formation of secondary species, the weathering mechanisms of galena in calcareous soil and its environmental implications were suggested.

Surface characterization of arsenopyrite in acidic medium by triangular scan voltammetry on carbon paste electrodes

Abstract To optimize the industrial processes involving chemical or biological oxidation of arsenopyrite and to control acid rock drainage it is necessary to perform a precise characterization of the mineral surfaces. In this work we have demonstrated that a voltammetric study with carbon paste electrodes (CPE) provide a possible alternative method to allow a rough characterization of the arsenopyrite surface. By these means the presence of sulfur or FeAsO 4 passive layers on the surface is shown, when arsenopyrite is previously oxidized by either chemical, biological and biological assisted methods. The presence of sulfur or FeAsO 4 was confirmed by XRD and IRD surface determinations. On the other hand the heterogeneity of the precipitates formed during the previous oxidation process was detected by CPE-mineral voltammetry studies. These observations were confirmed by SEM photomicrographs of the surface treated arsenopyrite.

Electrochemical Study of Orpiment ( As2 S 3 ) and Realgar ( As2 S 2 ) in Acidic Medium

Electrochemical characterizations of high purity natural crystals of orpiment (As 2 S 3 ) and realgar (AS 2 S 2 ) were performed in the presence of a bacteria-free, acidic bacterial growth medium for Thiobacillus ferrooxidans, name M2. A carbon-paste electrode (CPE), composed of a mixture of pure mineral grains with graphite powder and silicon oil as a binder, was used for the experiments. Reproducibility of the mineral surfaces was assured by monitoring the open-circuit potential (OCP) of the initial CPE-mineral-electrolyte interface. The voltammetric study revealed that the oxidation processes for both of these arsenic-bearing minerals involves two steps: an initial oxidation of the sulfide mineral to H 3 AsO 8 and HSO 4 - , followed by the oxidation of H 3 AsO 3 to H 3 AsO 4 . The reduction processes for the two minerals, however, differ. While the reduction of orpiment occurs in a dual step process, for realgar, the process is achieved in a single step.

An experimental study of iron sulfides weathering under simulated calcareous soil conditions

In calcareous sites, hard rock mining activities release pyrite (FeS2), pyrrhotite (Fe1−xS) and other sulfides to soils. The sulfides then undergo weathering processes, generating acid rock drainage and secondary compounds. Despite the potentially important environmental impacts, very few studies have considered the mechanisms of pyrite and pyrrhotite weathering and the transformation of secondary compounds under neutral-alkaline carbonated conditions. In this study, we used an experimental approach combining electrochemical, microscopic and spectroscopic techniques to examine the interfacial processes involved in pyrite and pyrrhotite weathering under simulated calcareous soil conditions. The results showed an initial oxidation step with the formation of variable amounts of surface sulfur compounds (e.g., polysulfides, Sn2−, and elementary sulfur, S0) and acid generation, leading to significant modification of the oxidative behavior of the minerals. The surface changes that occurred as a result of mineral weathering provoked transient enhancement of pyrite reactivity and progressive passivation in the pyrrhotite system. Iron sulfides weathering was found to involve the formation of an intermediate siderite (FeCO3)-like compound, preceding the predominant formation of K-jarosite (K·Fe3(SO4)2(OH)6) and/or ferric oxyhydroxide (α, γ-FeOOH) compounds, depending on the surface acid condition reached in the systems. Mechanisms of pyrite and pyrrhotite weathering in calcareous soils are suggested on the basis of surface characterization and chemical analysis of the leachates generated, and the environmental implications are discussed.

Electrochemical and spectroscopic study of interfacial interactions between chalcopyrite and typical flotation process reagents

Comparative voltammetry and differential double-layer capacitance studies were performed to evaluate interfacial interactions between chalcopyrite (CuFeS2) and n-isopropyl xanthate (X) in the presence of ammonium bisulfite/39wt% SO2 and caustic starch at different pH values. Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, contact angle measurements, and microflotation tests were used to establish the type and extent of xanthate adsorption as well as the species involved under different mineral surface conditions in this study. The results demonstrate that the species that favor a greater hydrophobicity of chalcopyrite are primarily CuX and S0, whereas oxides and hydroxides of Cu and Fe as well as an excess of starch decrease the hydrophobicity. A conditioning of the mineral surface with ammonium bisulfite/39wt% SO2 at pH 6 promotes the activation of surface and enhances the xanthate adsorption. However, this effect is diminished at pH ≥ 8, when an excess of starch is added during the preconditioning step.

An Overview of Reclaimed Wastewater Reuse in Gold Heap Leaching

ABSTRACT The intensive use of groundwater by the gold mining industry is an important sustainability concern, especially in arid and semiarid regions where groundwater is a scarce resource. Alternatives, such as water reuse, treatment and recycling, have been implemented to overcome this issue. The potential use of reclaimed wastewater in gold heap leaching, without decreasing the process efficiency, has not yet been considered. Hence, this review focuses on the physicochemical, biological, and chemical features (organic matter, microbial loads, metal ions, and anions) of reclaimed wastewater that may limit its use in gold heap leaching.

Arsenopyrite weathering under conditions of simulated calcareous soil

Mining activities release arsenopyrite into calcareous soils where it undergoes weathering generating toxic compounds. The research evaluates the environmental impacts of these processes under semi-alkaline carbonated conditions. Electrochemical (cyclic voltammetry, chronoamperometry, EIS), spectroscopic (Raman, XPS), and microscopic (SEM, AFM, TEM) techniques are combined along with chemical analyses of leachates collected from simulated arsenopyrite weathering to comprehensively examine the interfacial mechanisms. Early oxidation stages enhance mineral reactivity through the formation of surface sulfur phases (e.g., Sn2−/S0) with semiconductor properties, leading to oscillatory mineral reactivity. Subsequent steps entail the generation of intermediate siderite (FeCO3)-like, followed by the formation of low-compact mass sub-micro ferric oxyhydroxides (α, γ-FeOOH) with adsorbed arsenic (mainly As(III), and lower amounts of As(V)). In addition, weathering reactions can be influenced by accessible arsenic resulting in the formation of a symplesite (Fe3(AsO4)3)-like compound which is dependent on the amount of accessible arsenic in the system. It is proposed that arsenic release occurs via diffusion across secondary α, γ-FeOOH structures during arsenopyrite weathering. We suggest weathering mechanisms of arsenopyrite in calcareous soil and environmental implications based on experimental data.

Kinetic Analysis of the Decomposition of the KFe3(SO4)2-x(CrO4)x(OH)6 Jarosite Solid Solution in Ca(OH)2 Medium

The decomposition of the solid solution of potassium jarosite with chromium(VI) in Ca(OH)2 media was studied in the present work. According to experimental results, the incorporation of CrO42- into the crystal structure of jarosite resulted in a solid solution with the following approximate formula: [K0.86(H3O)0.14]Fe2.67[(SO4)1.23(CrO4)0.77][(OH)5.01(H2O)0.99]. The experimental data describe a reaction based on the shrinking core model with chemical control for spherical particles. A reaction order of n = 0.67 and an activation energy (Ea) of 63.75 kJ mol-1 were obtained in the induction period (tind). The progressive conversion period is characterized by the diffusion of K+, SO42- and CrO42- ions into the solution. In this stage, n = 1.99 with respect to OH-, and Ea = 51.56 kJ mol-1. The CrO42- diffusion is slower compared to that of sulfate, a slight amount of chromate is adsorbed in the layer of the solid residue consisting on Fe(OH)3. Finally, the equations that satisfactorily describe the reaction process were established from the data obtained.

Chemical treatment of the intra-canal dentin surface: a new approach to modify dentin hydrophobicity

Objective: This study evaluated the hydrophobicity of dentin surfaces that were modified through chemical silanization with octadecyltrichlorosilane (OTS). Material and Methods: An in vitro experimental study was performed using 40 human permanent incisors that were divided into the following two groups: non-silanized and silanized. The specimens were pretreated and chemically modified with OTS. After the chemical modification, the dentin hydrophobicity was examined using a water contact angle measurement (WCA). The effectiveness of the modification of hydrophobicity was verified by the fluid permeability test (FPT). Results and Conclusions: Statistically significant differences were found in the values of WCA and FPT between the two groups. After silanization, the hydrophobic intraradicular dentin surface exhibited in vitro properties that limit fluid penetration into the sealed root canal. This chemical treatment is a new approach for improving the sealing of the root canal system.