Oxidative Dissolution Of Arsenopyrite Research Articles

Arsenic release from arsenopyrite oxidative dissolution in the presence of citrate under UV irradiation

Arsenopyrite oxidative dissolution is one of the most important sources of arsenic (As) pollution in the soils and waters around sulfide mining areas. Sunlight and low-molecular-weight organic acids in the environment affect the redox behavior of sulfide minerals. In this work, the As release from arsenopyrite was studied in the presence of citrate under UV irradiation, and the effects of dissolved oxygen and citrate concentrations and pH on As release rate were also investigated. The results indicated that As release from the oxidative dissolution of arsenopyrite is affected by the complexation between citrate and dissolved iron ions. Under dark conditions in air atmosphere, dissolved oxygen, Fe(III)-citrate and the active intermediate product O2− facilitated the release of As at pH 7.0, and the As release rate increased first and then decreased with increasing pH from 5.0 to 9.0. Under UV irradiation in air atmosphere at pH 7.0, the reactive oxygen species (ROS) including O2− and OH generated by Fe(III)-citrate through the photo-Fenton reaction accelerated the As release and oxidation. However, Fe(III)-citrate photolysis led to the rapid flocculation and precipitation of dissolved iron ions, inhibiting the further oxidation of arsenopyrite. With increasing pH from 5.0 to 9.0, the As release rate gradually decreased under UV irradiation. Increases in the concentrations of citrate and dissolved oxygen promoted the formation of Fe(III)-citrate and ROS in the reaction system under both UV irradiation and dark conditions. The present work expands our understanding of the geochemical behavior of As in near-neutral pH environment.

Study on Pre-oxidation of a High-Arsenic and High-Sulfur Refractory Gold Concentrate with Potassium Permanganate and Hydrogen Peroxide

For arsenic-containing refractory gold concentrate, gold recovery by leaching is extremely difficult due to the encapsulation of gold in arsenopyrite and pyrite. In this work, the pre-oxidation of a high-arsenic and high-sulfur refractory gold concentrate in the acidic potassium permanganate or hydrogen peroxide solutions has been investigated. The studied parameters include pre-oxidation time, acidity, pre-oxidation temperature, ethylene glycol addition and oxidant concentration. When potassium permanganate of 0.25 mol/L is used as oxidant, 61.4% of arsenic and 51.8% of iron can be removed, respectively. The results of kinetics analysis indicate that the oxidative dissolution of arsenopyrite during the pre-oxidation process is controlled by the product layer diffusion, with an activation energy of 7.92 kJ/mol. The stability of hydrogen peroxide can be significantly improved by increasing both sulfuric acid concentration and ethylene glycol addition when hydrogen peroxide is used as oxidant. Under optimum conditions, arsenic removal ratio of 57.2% and iron removal ratio of 51.9% have been achieved with hydrogen peroxide of 2.5 mol/L, sulfuric acid of 0.75 mol/L and ethylene glycol of 30 mL/L. The gold extraction of refractory concentrate is only 11.5% in the copper–ammonia–thiosulfate leaching solutions without pre-oxidation. A gold extraction of 71.8% and significant decrease in thiosulfate consumption from 28.2 to 5.7% have been observed for the concentrate after pre-oxidation.

Electrochemical behaviour of the dissolution and passivation of arsenopyrite in 9K culture medium

The behaviour of arsenopyrite (FeAsS) in the acidic iron-free 9K culture medium was investigated using a combination of electrochemical and analytical techniques. The results demonstrated that the oxidative dissolution of arsenopyrite was accompanied by its surface passivation from the beginning of the arsenopyrite dissolution at potentials from OCP to 1000 mV, which limited further dissolution of arsenopyrite. The dissolution was complex and different under different applied potentials. The leaching kinetics of Fe, As and S, as well as the physicochemical properties of the formed passive film, varied with the potentials. A dissolution rate was found to be likely Fe > As > S from FeAsS. The passive film consisted mainly of As2S2 and As2S3 (≤800 mV), and S0 (>800 mV). The passive film was also shown to have a two-layer structure, i.e. a porous outer layer and a compact inner barrier layer that appeared to impart its strongest resistance to the dissolution of arsenopyrite at 750 mV. An applied potential higher than 800 mV could observably improve the dissolution of the film and thus reduce its passivating action.

Arsenic release and speciation during the oxidative dissolution of arsenopyrite by O2 in the absence and presence of EDTA

The oxidative decomposition of arsenopyrite is an important source of As in surface environment. This study investigated the oxidative dissolution of arsenopyrite by O2 and aqueous arsenic transformation at different pHs, dissolved oxygen (DO) contents, and temperatures in the absence and presence of EDTA. The oxidative dissolution was greatly inhibited at neutral and alkaline pH in the absence of EDTA. However, in the presence of EDTA, the oxidative dissolution rate increased linearly from pH 4 to 7. The highest dissolution rate was 3–4 times higher than that at pH 4 and 1–2 orders of magnitude higher than that at pH 7 in the absence of EDTA. This is possibly due to the lack of Fe oxyhydroxides on the surface of arsenopyrite. In the pH range of 7–10, the oxidative dissolution rate decreased linearly, possibly due to the formation of goethite and/or hematite coating. The oxidation of released arsenite (AsIII) to arsenate (AsV) took place simultaneously during the oxidative dissolution of arsenopyrite in the presence of dissolved Fe without EDTA, while no obvious aqueous AsIII oxidation was observed in the presence of EDTA, indicating that aqueous Fe species play an important role in AsIII oxidation.

Geochemistry of mine waters draining a low-sulfide, gold-quartz vein deposit, Bralorne, British Columbia

Abstract The Bralorne and Pioneer mines, now inactive, produced over 4 million ounces of Au from an orogenic lode Au deposit located on the eastern edge of the Coastal Mountains of SW British Columbia. Between 2007 and 2009, drainage from a recently developed exploration adit was investigated in order to better understand and anticipate potential environmental management issues associated with the development of this type of deposit in the future. Portal discharge rate and specific conductance were monitored continuously over a 14-month period during which 36 water samples were collected. Additional samples were collected from flooded workings within the adit. Concentrations of As and Sb at the portal range as high as 1738 and 316 μg/L, respectively, while those in the mine pool reach 3304 and 349 μg/L, respectively. Effluent chemistry is mildly alkaline (pH = 8.7) and is dominated by Na, Ca, Mg, HCO3 and SO4. Geochemical inverse modeling of effluent composition indicates weathering of albite (2515 kg/a), ferroan dolomite (718 kg/a), pyrite (456 kg/a), arsenopyrite (23 kg/a) and stibnite (2 kg/a). Modeled sulfide reaction coefficients, normalized by their corresponding host rock concentrations, suggest that oxidation of arsenopyrite is 25 times slower than that of pyrite whereas oxidation of stibnite is 1.5 times faster. Oxidative dissolution of arsenopyrite and stibnite releases 10.6 kg/a of As and 1.1 kg/a of Sb of which 57% and 46%, respectively, are sorbed to ferrihydrite and gibbsite on the bed of the shallow channel through which the mine pool drains to the portal. Although mass balance calculations predict the formation of sufficient ferrihydrite to sorb 100% of the As dissolved in the mine pool, this attenuation process was ineffective possibly because the precipitated sorbents settled to the bottom of the water column or because of competition for sorption sites from Ca and HCO3. The dissolved Sb/As molar ratio in portal effluent (0.082) is much greater than the Sb/As ratio of the mineralization (0.002) because of slower arsenopyrite oxidation and somewhat lesser sorption of Sb.

Sulfide oxidation observed using micro-Raman spectroscopy and micro-X-ray diffraction: The importance of water/rock ratios and pH conditions

Oxidative dissolution of arsenopyrite and pyrite from two former mining sites located in the French Massif Central has been studied to determine some of the critical parameters controlling the formation of secondary phases. Micro-Raman spectroscopy (μRS) and micro-scanning X-ray diffraction (μSXRD) were used for mineralogical identification and for mapping the distribution of these alteration products. The two mining sites are characterized by different pH conditions and sedimentary environments. Enguialès exhibits acidic conditions (pH ~ 3) and consists of coarse-grained tailings on a steep slope, which have been leached by meteoric waters and represent an oxidizing environment with a high solid/solution ratio. In contrast, the Cheni site shows near neutral conditions (pH ~ 6.3) and consists of muddy tailings deposited in a settling basin, which exemplify an oxidizing environment with a low solid/solution ratio. Amorphous or poorly crystalline iron arsenate (e.g., amorphous scorodite or parasymplesite-like phase) are among the first products precipitated from oxidized arsenopyrites. These iron arsenates are highly mobile and are frequently observed in association with non-arsenian pyrites, where in some instances they have matured into more crystalline forms. Arsenic may also be trapped by amorphous or poorly crystalline iron (oxy)hydroxides, as has been observed for goethite or jarosite in the latter stages of sulfide oxidation. This study also shows that rings of elemental sulfur are formed around altered sulfides, but only when conditions are near neutral.

Arsenic speciation in the tailings impoundment of a gold recovery plant in Siberia

The recovery of gold from ores containing arsenopyrite releases significant amounts of arsenic into the environment. Arsenic releases primarily result from the processing and oxidation of stored mine tailings. This investigation examines: (1) the distribution of arsenic, iron, manganese and their chemical species in sediment cores extracted from a tailings impoundment in the Kemerovo region of southwestern Siberia; and (2) the weathering of arsenic minerals collected from its tailings dam. The data demonstrate that during storage of the sediments within the impoundment, arsenic is predominantly present as residual arsenopyrite and, to a lesser degree, as substances co-precipitated with iron hydroxide. The arsenic released from the sediment results from an oxidative dissolution of arsenopyrite and reductive dissolution of Fe oxides.

Oxidative Dissolution of Arsenopyrite by Mesophilic and Moderately Thermophilic Acidophiles

The purpose of this work was to determine solution- and solid-phase changes associated with the oxidative leaching of arsenopyrite (FeAsS) by Thiobacillus ferrooxidans and a moderately thermoacidophilic mixed culture. Jarosite [KFe(3)(SO(4))(2)(OH)(6)], elemental sulfur (S), and amorphous ferric arsenate were detected by X-ray diffraction as solid-phase products. The oxidation was not a strongly acid-producing reaction and was accompanied by a relatively low redox level. The X-ray diffraction lines of jarosite increased considerably when ferrous sulfate was used as an additional substrate for T. ferroxidans. A moderately thermoacidophilic mixed culture oxidized arsenopyrite faster at 45 degrees C than did T. ferroxidans at 22 degrees C, and the oxidation was accompanied by a nearly stoichiometric release of Fe and As. The redox potential was initially low but subsequently increased during arsenopyrite oxidation by the thermoacidophiles. Jarosite, S, and amorphous ferric arsenate were also formed under these conditions.