Marcos Monroy

Effect of sulfide impurities on the reactivity of pyrite and pyritic concentrates: a multi-tool approach

Sulfide mineral oxidation, primarily pyrite and pyrrhotite, generates acid mine drainage during weathering. Successful management of acid generating wastes entails the suppression of the initiation of oxidation reactions. The reactivity of pyrite depends on ore mineralogy, including the effects of associated sulfide impurities. The electrochemical surface characterization study using cyclic voltammetry with carbon paste electrodes containing minerals particles (CPE-Mineral) is an effective tool for demonstrating how the various mineral characteristics work together to influence the overall reactivity of the mineral. This study was supported by chemical, mineralogical and leachate chemistry data. The results show that the presence of other sulfides in contact with pyrite at the beginning of the weathering process is the most important parameter affecting pyrite reactivity, which is likely to be oxidized and passivated. In more advanced stages of leaching, mineral coatings which passivate the pyrite surfaces tend to play the most important role in defining the reactivity of pyrite. The electrochemical response of pyritic samples in conjunction with the evolution of the chemical quality of the leach solution in the simple experimental device here used, could then provide valuable information on acid mine drainage generation.

Cyclic voltammetry applied to evaluate reactivity in sulfide mining residues

Oxidation of sulfide present in mining residues can generate contaminating acid effluent known as acid rock drainage. Prediction and control of acid rock drainage are critically important to the mining industry because of the environmental impact resulting from the sulfide oxidation. Due to its particular reaction kinetics, once acid drainage has begun, it is very difficult to control without a substantial economic investment. For this reason, efficient prediction and prevention programs, which monitor mining waste reactivity, are required to limit the oxidation of sulfide-bearing residues before damage to the environment occurs. In this work, the authors evaluated mining waste reactivity under oxidizing conditions as a function of its voltammetric behavior before and during alteration under simulated natural conditions produced in the laboratory. This method is supported by conventional mineralogical characterization of the mineral samples and chemical quality of the effluents produced during the simulated alteration process kinetics.

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 characterization of galena under simulated carbonate rich weathering conditions

An electrochemical and mineralogical study of galena oxidation was carried out under similar conditions to those found on semiarid carbonate rich soils, which are characteristic of important Mexican mining sites. Investigation of galena behavior in calcareous and alkaline conditions is important to establish strategies for remediation, prevention or control of pollution associated with weathering of this mineral. Voltammetric analyses of galena oxidation lead to the proposition that Pb(OH) 2 and Pb 1-x S 1-y layers are formed initially. Subsequently, oxidation of these species produces a PbO 2 layer and SO 4 2- . Electrochemical and mineralogical results indicate a low mobility of Pb, due to Pb(OH) 2 and PbCO 3 formation during a slow oxidation process.

Electrochemical and Spectroscopic Analysis of the Arsenopyrite (FeAsS) Oxidation under Calcareous Soil Conditions

In Mexico, the U.S.A. and Australia there are important calcareous mining soil. The mineralogy of this soil is dominated by carbonates rich deposits containing sulfide minerals type skarn or vein associated with economic values (e.g., Au, Zn, Cu) (1, 2). Arsenopyrite is perhaps the most important source of arsenic in calcareous soil (1, 3). In some of these sites important concentrations of arsenic have been identified in water and soil systems, presumably associated with the arsenopyrite weathering process, spite of the semi-alkaline carbonated conditions in this kind of soil (3, 4). Additionally, it has been found the formation of ferric arsenate phases like scorodite, as well as ferric compounds with sorbed arsenic on arsenopyrite crystals (4). This fact shows the feasibility of the arsenopyrite weathering in this kind of soil; however, the mechanisms and interfacial processes implied during the arsenopyrite weathering are not clear. Therefore, the aim of this study was to evaluate the oxidation process of arsenopyrite, and to elucidate the oxidation products under similar weathering conditions than those found in calcareous soil. This goal was reached by using electrochemical, geochemical and spectroscopic studies of arsenopyrite before and after leaching with a carbonated solution. The devices for arsenopyrite weathering process of consisted of Buchner funnels (mini-cells) designed to promote the natural oxidation of sulfides by applying two humid cycles per week, with similar results as those obtained with humid cells proposed by ASTM (1999). A similar methodology was successfully employed for evaluate the reactivity in pyritic waste from Mexico and Canada (5). The geochemical study suggests important dissolution cycles of arsenic and iron from arsenopyrite (Fig. 1). In addition, these processes were accompanied by sulfates generation and consumption of total alkalinity, however, not any significant changes of pH was observed during these processes in system. These results could be indicative of the precipitation of iron (II) compounds, such as siderite (FeCO3). The voltammetric study of arsenopyrite pristine sample indicated the initial oxidation of arsenopyrite under calcareous conditions. It was found the formation of surficial metal-deficient layers (e.g., Fe1xAs1-yS) and, in agreement with geochemical study, probably the formation of ferrous carbonates. However, higher potentials brought the formation of ferric hydroxide, as indicated in positive-going sweeps in Fig. 2 (peak R1, without the addition of H2AsO4 in system). Additionally, if the electrochemical system is previously enrich with H2AsO4species, the formation of ferric hydroxide is suppressed and the interaction between As(V) and iron species seems to occur (Fig. 2). Figure 1. Chemical evolution of leachate for As (●) and iron (▲) as a function of the alteration time of arsenopyrite.