Rate Of Pyrite Oxidation Research Articles

Electrochemical Oxidation Mechanism of Pyrite in a Copper(II)-Citrate-Sodium Thiosulfate Composite System

Pyrite is an important gold carrier during gold leaching, but it is readily oxidized, which causes environmental problems such as acidic mine drainage. Thus, it is necessary to study the oxidation mechanism of pyrite in a gold-leaching electrolyte. The surface oxidation reaction of pyrite is a multi-electron transfer electrochemical oxidation process. Based on this, electrochemical technology was used to explore the electrochemical oxidation mechanism of pyrite in a Cu2+-Cit3−-S2O3 2− system. The surface oxidation products of the pyrite were characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The experiments showed that the thiosulfate concentration did not change the oxidation mechanism of pyrite at 0.30 V under the conditions of this experiment. Increasing the concentration and potential of thiosulfate accelerated the oxidation rate of pyrite. The electrochemical oxidation of pyrite in the Cu2+-Cit3−-S2O3 2− system occurred in two stages. At low potentials in a passivated state, the process was diffusion-controlled, and the surface oxidation rate was slow. When the potential exceeded 0.50 V, the passivation film on the surface was penetrated, allowing the oxidation reaction on the surface of pyrite to continue. The results of this experiment are useful for deepening the understanding of the oxidation mechanism of pyrite in electrolytes.

Role of Oxidants in Metal Extraction from Sulfide Minerals in a Deep Eutectic Solvent.

Metallurgical applications of deep eutectic solvents (DESs), known as ionometallurgy, have received significant research attention in recent years. While many studies claim that DESs are generally green and enhance process efficiency, others believe that industrial applications of ionometallurgy are generally not viable. Here, we report on leaching experiments of a sulfide flotation concentrate using ethaline, a chloride-based DES, in the presence of common oxidants. Following a mineral-based approach, we compare results with those obtained from aqueous chloride solutions to assess the influence of the leaching medium. We aim to contribute to a basic understanding of key differences between DESs and aqueous solutions and hope that this will help to make informed decisions about the suitability of DESs for leaching applications. Experiments were performed on a feed concentrate comprising a mixture of sulfide minerals along with substantial concentrations of Au, Ag, and Te. We found similar leaching behaviors for ethaline and aqueous solutions in nonoxidative leaching. However, when oxidizing agents were introduced, ethaline exhibited higher leaching efficiencies. Notably, the oxidation rate of pyrite in ethaline was very low, while chalcopyrite exhibited high oxidation rates. Furthermore, the results highlight significant variations in leaching rates depending on the type of oxidant, with the highest rate observed for I2, followed by CuCl2, and FeCl3. H2O2 and O2 were less effective. The leaching of gold-silver tellurides was possible in ethaline. This could be of particular significance, given that Au-Ag-Te compounds pose challenges in conventional cyanide treatment.

Experimental timing of pyrite oxidation under various leaching conditions: consequences for rates of weathering in geological profiles

Pyrite (FeS2) is one of the most abundant sulfides on Earth and has already been studied in numerous ways for decades because of its rapid oxidation and the associated environmental impacts. This study proposes a new experimental physico-chemical approach (air, tridistilled water and water drip exposure) to determine the oxidation rate of pyrite using surface and depth data via XPS (X-ray Photoelectron Spectroscopy) analyses. Our experimental study of almost pure pyrite reveals a maximum oxidation rate of 11.7 ± 1.8 nm day−1 for drip exposure with precipitation of sulfates or Fe-oxides depending on the experimental condition. The oxidation rates obtained under various experimental conditions may be extrapolated to weathering rates of different zones of supergene profiles/ores (leached zone, saprolite and cementation zone). The extrapolation suggests a maximum rate of 4.3 ± 0.6 m Ma−1, which is consistent with data obtained by isotope dating of weathering profiles. Under geological conditions however, the oxidation rate of pyrite may be influenced by additional parameters, such as the nature of the host rock, its porosity/permeability, the climate, the influence of an oxidizing environment, and the mineralization of secondary minerals.

Experimental Study on the Oxidation Reaction of Coal-Pyrite and Mineral-Pyrite with the Participation of Fe(III) and Bacteria under Acidic Conditions

As one of the crucial factors contributing to coal spontaneous combustion, the oxidation of pyrite is a complex process involving multiple reactions, particularly in the presence of oxidants (Fe3+ and O2) and bacteria. However, experimental results based on mineral-pyrite are not entirely applicable to coal-pyrite due to their differences in formation environments and compositions. This study selected two types of coal-pyrite and one type of mineral-pyrite as research to conduct oxidation experiments with the participation of oxidant (Fe3+) and bacteria (Acidithiobacillus ferrooxidans), respectively, to obtain the following conclusions. Under natural conditions, the chemical oxidation rate of pyrite is slow, but the addition of oxidant Fe3+ and bacteria can significantly accelerate the oxidation rate. The promotion effect of oxidant Fe3+ on the oxidation reaction is stronger than that of bacteria. Under the same conditions, the oxidation rate of coal-pyrite samples is slightly higher than that of mineral-pyrite, due to the relatively higher impurities content, poorer crystal structure, and humic acid in the coal seams. Additionally, different compositions of coal-pyrite samples can lead to various oxidation degrees under different conditions. Therefore, the oxidation process and mechanism of pyrite in coal seams are complex and affected by many factors, which need further study to prevent coal spontaneous combustion accurately and effectively.

Stable Isotopes and Bayesian Tracer Mixing Model Reveal Chemical Weathering and CO2 Release in the Jiulongjiang River Basin, Southeast China

AbstractLinks between erosion, chemical weathering, and atmospheric CO2 can modulate the global climate over geologic times, but the role of chemical weathering on Earth's long‐term carbon cycle is still unclear. Although silicate weathering by carbonic acid could sequester CO2, the pyrite oxidation coupled to carbonate dissolution could provide a transient source of CO2 at the same time. Landscapes with low physical erosion rate cover the majority of exposed land area, however, the concomitant weathering of silicate, carbonate, and sulfide occurring in the low erosion rate terrains is not well understood. Here, we investigate these questions by examining the river water chemistry from a catchment characterized by complex lithology and low erosion rate. Using a Bayesian tracer mixing model, elemental and isotopic ratios, we quantitatively identify the proportion of cations from silicate and carbonate weathering as well as the acid sources during weathering. We propose that the sulfuric acid from pyrite oxidation boosts both silicate and carbonate weathering, and the coupling between pyrite oxidation and silicate weathering is more pronounced as the pyrite oxidation rates increases. In doing so we determine the effect of weathering on the CO2 balance in our study area, and find that the CO2 release from pyrite oxidation coupled to carbonate dissolution overwhelms the CO2 drawdown from silicate weathering on the time scale between marine carbonate precipitation and sulfate reduction (approximately 106–107 years). Our analysis indicates that the lithology and distribution of minerals play an important role on the chemical weathering in low erosion rate terrains.

Strategies for anti-oxidative stress and anti-acid stress in bioleaching of LiCoO2 using an acidophilic microbial consortium.

High metal ion concentrations and low pH cause severely inhibit the activity of an acidophilic microbial consortium (AMC) in bioleaching. This work investigated the effects of exogenous spermine on biofilm formation and the bioleaching efficiency of LiCoO2 by AMC in 9K medium. After the addition of 1mM spermine, the activities of glutathione peroxidase and catalase increased, while the amount of H2O2, intracellular reactive oxygen species (ROS) and malondialdehyde in AMC decreased. These results indicated that the ability of AMC biofilm to resist oxidative stress introduced by 3.5g/L Li+ and 30.1g/L Co2+ was improved by spermine. The activity of glutamate decarboxylase was promoted to restore the intracellular pH buffering ability of AMC. Electrochemical measurements showed that the oxidation rate of pyrite was increased by exogenous spermine. As a result, high bioleaching efficiencies of 97.1% for Li+ and 96.1% for Co2+ from a 5.0% (wv-1) lithium cobalt oxide powder slurry were achieved. This work demonstrated that Tafel polarization can be used to monitor the AMC biofilm's ability of uptaking electrons from pyrite during bioleaching. The corrosion current density increased with 1mM spermine, indicating enhanced electron uptake by the biofilm from pyrite.

Conventional versus microwave-assisted roasting of sulfidic tailings: Mineralogical transformation and metal leaching behavior

Roasting is often required to convert sulfidic minerals into their sulfate and/or oxide forms with the aim to increase the extractability of targeted metals. In this study, sulfidic tailings of Neves Corvo Cu-Zn-Pb-mine (Portugal) underwent conventional roasting (CR) and microwave-assisted roasting (MR) to compare the effect of both heating technologies on the roasting behavior. Upon roasting in air, transformations of mineral phases in the tailings were studied by different techniques using quantitative X-ray diffraction, mineral liberation analysis, Raman spectroscopy and thermogravimetry-differential scanning calorimetry coupled to mass spectrometry. Also, the leaching behavior of elements from the roasted tailings was assessed in water. It was shown that CR and MR induce different reaction pathways for mineral transformations during roasting. While CR led to indirect pyrite oxidation through intermediate sulfate formation, direct pyrite oxidation was the main transformation pathway during MR. This change in reaction pathway can be attributed to the mineral-selective heating induced by microwaves, which in particular accelerates the oxidation rate of pyrite. In agreement with the proposed roasting mechanism, the leaching behavior in water showed that CR resulted in a higher extraction of Cu, Zn, Pb, As and Fe from the tailings – since their sulfate phases are more abundant – compared to MR at the same roasting temperature. Overall, tailings treated by CR and MR within the temperature ranges of 500 °C to 550 °C for 1 h gave optimal leaching efficiencies of the studied metals (water leaching at room temperature for 30 min; stirring speed of 600 rpm; solid-to-liquid ratio of 10 mL · g−1). Furthermore, thermodynamic equilibrium calculations showed that hydrolysis of soluble Fe(III) species strongly controls the pH of the leachates. In comparison to MR tailings, more acidic leachates were generated for CR tailings because of a more pronounced formation of ferric sulfate.

Differential flotation of pyrite and Arsenopyrite: Effect of pulp aeration and the critical importance of collector concentration

• Differential oxidation of pyrite and arsenopyrite achieved with pulp aeration. • Recovery of pyrite and arsenopyrite was high when singularly treated. • Xanthate adsorption removed high FeOOH removal from arsenopyrite compared to pyrite. • Adsorption of xanthate on heavily oxidised arsenopyrite surface was slow. • Selectivity was lost at high xanthate concentration. Differences in the oxidation rate and extent of model pyrite and arsenopyrite minerals, augmented by pulp aeration, was investigated using dissolved oxygen demand tests and X-ray photoelectron spectroscopy (XPS) analysis. Flotation studies of single mineral admixture coupled with zeta potential measurements, UV–Vis spectroscopy analysis of collector concentration and ex-situ metal ions in solution after collector addition were ascertained to shed light on reasons for observed window for selective flotation. The highest selectivity was achieved with low collector concentration of 1 × 10 -5 M PAX and selectivity index of 5.2 after 6 min flotation with collector concentration playing a critical role in attaining a good selectivity. Fast adsorption of xanthate and simultaneous oxidation to dixanthogen on slightly oxidised pyrite surface starved heavily oxidised arsenopyrite surface of the collector species, at low collector concentration, leading to its depression. The study was extended to the flotation of an auriferous refractory ore where similar observations were made. 62.7% rejection of pyrite was achieved in the staged flotation process.

Evolution of pyrite oxidation from a 10-year kinetic leach study: Implications for secondary mineralisation in acid mine drainage control

Acid rock drainage (ARD) is generated when mineral sulfides, primarily pyrite (FeS2), are exposed to weathering. Pyrite oxidation and dissolution may also result in the precipitation of secondary sulfate minerals, commonly found in weathered mine wastes, that release ‘stored acidity’ on subsequent dissolution. To better understand the release of this stored acidity, a kinetic leach column experiment using a synthetic pyritic (5 wt%) mine waste was conducted for a period of 524 weeks. The first 260 weeks watering/flushing was carried out using Milli-Q water during which the leachate pH decreased from 6.0 to 2.0, pyrite passivation layers present initially were removed and secondary sulfate minerals formed. The rate at which acidity was generated from secondary sulfate mineral dissolution was significantly greater than from pyrite oxidation after the calcite-saturated silicate water treatment during the period from 260 to 440 weeks. This suggests that in weathered pyritic wastes, both the amount of acidity and the rate at which it is released may be dominated by acid sulfate minerals rather than residual pyrite. Importantly, it was found that passivation strategies appropriate for reducing pyrite oxidation rates were not effective for reducing the rate of acid release from secondary mineralisation. Significant amounts of lime-saturated water (with high alkalinity) were required to dissolve the secondary sulfate minerals containing the stored acidity before circum-neutral conditions necessary for pyrite passivation could be achieved. It follows therefore that it is necessary to quantify the amount of stored acidity in secondary minerals so that appropriate remediation and acid management strategies can be designed and implemented to achieve long-term sustainable ARD control and mine closure.

Geochemistry of natural acid rock drainage in the Judith Mountains, Montana, part 2: Seasonal and spatial trends in Chicago Gulch

Three streams radially draining Red Mountain, a prominent peak in the Judith Mountains of central Montana, are naturally acidic in their headwaters and experience a rise in pH as they transition from hydrothermally altered quartz monzonite to sedimentary bedrock in their lower reaches. A previous study (Williams et al., 2015) examined the geochemistry of two of these streams, Armells Creek and Collar Gulch, using detailed synoptic sampling with continuous tracer-injection. Here we present results of two synoptic sampling events in the third stream, Chicago Gulch. Data are compared between the three watersheds, and estimates of solute flux (load divided by watershed area) are presented.Filtered and unfiltered metal concentrations and loads are quantified with distance downstream in Chicago Gulch under low-flow (August) and high-flow (May) conditions. Headwater streams in the watershed have pH < 4 and extensive deposits of ferricrete and Fe-bog deposits. Weakly acidic springs (pH > 4) contain >10 mg/L of Fe2+ which quickly oxidizes and precipitates as hydrous ferric oxide (HFO), releasing protons that lower the pH of the stream. Geochemical modeling suggests the actively forming HFO is a mix of schwertmannite and ferrihydrite, both of which are slowly converting to goethite which cements the ferricrete deposits. White, hydrous aluminum oxide (HAO) flocs form where stream pH transitions from <5 to >5, and a comparison of filtered and unfiltered samples indicates strong partitioning of Cu and Pb into the suspended particles. Dissolved loads of Zn, Cd, Mn, and Tl (thallium) diminish slightly in the lower reaches of Chicago Gulch where pH rises above 7.All water samples collected in the upper reaches of Chicago Gulch exceed the US-EPA water quality standards for protection of aquatic life for Pb, Cd, Zn, and Cu, and human health standards for Tl. Lead concentrations are especially high in the headwaters of Chicago Gulch, indicating the possible presence of an unmined Pb-sulfide deposit. Estimates of the load of dissolved sulfate at baseflow conditions in each stream on Red Mountain, normalized to their respective surface areas of hydrothermally altered bedrock, are remarkably similar, and indicate a steady state pyrite oxidation rate of 11.9 mol FeS2/h/km2. Prediction of pre-modern pH using the methods of Nimick et al. (2009) indicates that the current pH regimes of all three streams draining Red Mountain are broadly similar to conditions when ancient ferricretes were deposited. Because the Judith Mountains were never glaciated in the Pleistocene, it is possible that similar hydro-geochemical conditions have existed in the area for 10's to 100's of thousands of years.

Geochemical behaviour of benign desulphurised waste rocks for mine drainage control and sustainable management

Large volumes of waste rock (WR) are generated in open-pit mines. WR is characterised by high anisotropy in its physical, chemical, mineralogical, and hydrogeological properties. WR is often deposited in unsaturated piles on the mine surface. Mineralogical investigations defined the diameter of physical locking of sulphides (DPLS) allowing the WR to be separated into two fractions: i) a reactive fine fraction (<2.4 mm), and ii) an inert coarse fraction (>2.4 mm). Environmental desulphurisation carried out on the reactive fine fraction decreased the risk of contamination caused by WR oxidation. The geochemical behaviour of three benign desulphurised lithologies (altered greywacke (AGR), carbonated greywacke (CGR) and carbonated porphyry (CPO)) and their corresponding fine fractions (<2.4 mm) were assessed using kinetic weathering cells (WCs). These tests confirmed the effectiveness of environmental desulphurisation to prevent acid mine drainage formation. In fact, geochemical analyses of the leachates from WCs showed that pH values ranged between 7.4 and 8.9. Moreover, instantaneous concentrations of iron and zinc were below 0.5 mg/L and 3 mg/L, respectively. Pyrite oxidation rates, which represent sample reactivity, were higher for head samples (3.01, 4.86, and 2.54 μmol/kg/day for AGR, CGR, and CPO lithologies, respectively) in comparison to the benign desulphurised materials (2.32, 2.4, and 2.25 μmol/kg/day for AGR, CGR, and CPO lithologies, respectively). Pyrite in the dismantled material was only partially liberated or almost encapsulated, making it mineralogically unavailable for oxidation. The benign desulphurised material with low initial sulphide content was potentially non-acid generating at the long-term scale. Upstream environmental desulphurisation has been confirmed as an effective strategy for WR management and valorisation.

Oxidative Dissolution of Sulfide Minerals in Single and Mixed Sulfide Systems under Simulated Acid and Metalliferous Drainage Conditions.

Chalcopyrite, galena, and sphalerite commonly coexist with pyrite in sulfidic waste rocks. The aim of this work was to investigate their impact, potentially by galvanic interaction, on pyrite oxidation and acid generation rates under simulated acid and metalliferous drainage conditions. Kinetic leach column experiments using single-minerals and pyrite with one or two of the other sulfide minerals were carried out at realistic sulfide contents (total sulfide <5.2 wt % for mixed sulfide experiments), mimicking sulfidic waste rock conditions. Chalcopyrite was found to be most effective in limiting pyrite oxidation and acid generation with 77-95% reduction in pyrite oxidation over 72 weeks, delaying decrease in leachate pH. Sphalerite had the least impact with reduction of pyrite dissolution by 26% over 72 weeks, likely because of the large band gap and poor conductivity of sphalerite. Galena had a smaller impact than chalcopyrite on pyrite oxidation, despite their similar band gaps, possibly because of the greater extent of oxidation and the significantly reduced surface areas of galena (area reductions of >47% for galena vs <1.5% for chalcopyrite) over 72 weeks. The results are directly relevant to mine waste storage and confirm that the galvanic interaction plays a role in controlling acid generation in multisulfide waste even at low sulfide contents (several wt %) with small probabilities (≤0.23%) of direct contact between sulfide minerals in mixed sulfide experiments.

Effect of salinity on the kinetics of pyrite dissolution in oxygenated fluids at 60 °C and implications for hydraulic fracturing

Shale hydraulic fracturing involves the injection of oxygenated acidic fluids which trigger redox driven and acid driven reactions, in particular with pyrite and carbonates. These reactions alter the shale pore and fracture network and mobilize potentially harmful elements. Hence, understanding the kinetics of pyrite oxidation at shale reservoir conditions is critical. Here, we investigate the effect of salinity on the rate of pyrite oxidative dissolution by fluids of pH 1 at 60 °C, and 0–3.4 mol L−1 NaCl. We determine a pyrite dissolution rate of about 1.2 × 10−9 mol m−2 s−1 in non-saline oxygenated acidic fluids at 60 °C, consistent with previously established rate equations, considering an activation energy of 57 kJ mol−1. Our results suggest the following correlation between the pyrite dissolution rate r (in mol m−2 s−1) at 60 °C and Eh (0.580–0.700 V), log(r) = 4.47 Eh – 12.03. This reaction order is lower than that established in previous studies involving pyrite reaction with Fe3+ enriched fluids in the presence of dissolved O2. Furthermore, we calculate that the pyrite oxidation rate varies from 0.8 × 10−10 to 1.3 × 10−10 mol m−2 s−1 at 25 °C, based on salinity-determined dissolved oxygen content. The mixed flow reactor experimental results suggest a dependence of pyrite oxidation rate on Cl− activity aCl (in mol kg−1) at 60 °C for fluids with 0.6–3.4 mol L−1 NaCl and Eh of 612 ± 8 mV as follows, log(r) = −0.58 log(aCl) – 9.42. The pyrite oxidation rate varies within less than half an order of magnitude for the salinity range tested, and hence, the impact of salinity is less important than the Eh and temperature. The improved understanding of factors controlling pyrite oxidation kinetics may help in predicting environmental and engineering impacts of injection of hydraulic fracturing fluids in shale.

Reprocessing feasibility of polymetallic waste rock for cleaner and sustainable mining

Mining operation produces significant volumes of waste rocks (WR) characterised by high heterogeneity in their physical, mineralogical and hydrogeological properties. The purposes of this study were to assess the acid-generating potential, evaluate reprocessing feasibility of polymetallic WR using desulphurisation process (i.e., gravity separation and flotation) and study the geochemical behaviour of desulphurised materials. The fresh WR was collected in the waste dump from Langlois mine at Lebel-sur-Quévillon (Quebec, Canada). The WR sample was divided into six fractions using wet screening: D < 0.053 mm, 0.053 < D < 0.3 mm, 0.3 < D < 0.85 mm, 0.85 < D < 2.4 mm and 2.4 < D < 5 mm. Chemical and mineralogical characterisations showed that fine fractions (D < 2.4 mm) were enriched in sulphide minerals (i.e., 4.6 wt% pyrite, 3.5 wt% sphalerite, etc.) and carbonates (i.e., 3.7 wt% calcite and 5.1 wt% ankerite) compared to coarse fractions (1.2 wt% pyrite, 1.1 wt% sphalerite, 3.2 wt% calcite, 5.2 wt% ankerite). The acid-generating potentials of different fractions were evaluated using acid-base accounting (ABA) and net acid generation (NAG) tests. Only one size fraction (F2) was acid generating and others were classified as uncertain. The degree of liberation of sulphides in the size fractions greater than 2.4 mm was negligible (<16%). Consequently, for this WR sample, 2.4 mm was defined as the diameter of physical locking of sulphide (DPLS). Based on DPLS, the WR were separated into two fractions; reactive (<2.4 mm), non-reactive (<2.4 mm) and then submitted to kinetic tests. Geochemical analyses of leachates of the coarse fraction (2.4 < D < 10 mm), the total sample (D < 5 mm) and –DPLS fractions (D < 2.4 mm) and the benign desulphurised materials (D < 2.4 mm) showed that pH values remained between 6.62 and 8.11 and the instantaneous concentrations of metals such as iron (Fe), copper (Cu) and lead (Pb) were below environmental limits (D019, Quebec, Canada), except for zinc (Zn) leached from the total sample which indicates a risk of contaminated neutral drainage. The pyrite oxidation rate within the coarse fraction was low (16.6 μmol/kg/day) compared to fine fraction (35.8 μmol/kg/day). A combined gravity–flotation process gave better results to decontaminate the waste rock (D < 2.4 mm), with a benign desulphurised material reaching a low sulphur content (0.23 wt%) and high sulphur recovery (89 wt%). The final concentrates from the combined gravity–flotation process showed high base metals (Zn, Cu and Pb) recovery (more than 87 wt%).

Effect of sodium carbonate on alkaline self-leaching of gold from flotation gold ore

Oxygen pressure alkaline self-leaching of gold is an environmentally friendly and smokeless leaching process. But the insoluble iron oxide generated will precipitate on the surface of parent minerals, thus reducing the oxidation reaction and gold leaching efficiency. Simultaneously, due to the different pH requirements of pyrite oxidation and gold leaching, the pH must be strictly controlled to achieve the maximum efficiency. In this work, dissolution effect and buffering effect of sodium carbonate in the gold self-leaching process were carried out. As the addition of sodium carbonate, soluble iron-carbonate complexes would be formed to remove passivation layer and expose fresh surface of pyrite, resulting in improvement pyrite oxidation rate. Meanwhile, the buffering capacity of sodium carbonate can avoid the thiosulfate decomposition and improve the concentration of thiosulfate. The maximum gold extraction efficiency of gold-bearing flotation concentrate could achieve 91.56% under the following condition: a concentration of sodium hydroxide, sodium carbonate and sodium carboxymethyl cellulose at 0.6 M, 0.3 M and 100 mg/dm3, respectively; stirring speed at 800 rpm; temperature at 308.15 K; oxygen pressure at 0.5 Mpa; leaching time of 18 h and a liquid-solid ratio of 5:1.

A comparative study on the micro-surface characteristics at black shale initial oxidation stage

The pyrite oxidation is crucial to the overall black shale oxidation process. A. ferrooxidans was documented an effective oxidation ability on pure pyrite, but its role in black shale oxidation is unclear. In this study, a comparative study of acid solution and A. ferrooxidans on the micro-surface characteristics at the initial stage (7 days) was conducted on black shale slices, a comprehensive approach combining the micro-morphologies, micro-structures, micro-environmental pH and micro-surface elemental content were investigated by using polarizing microscopies, SEM, fluorescent staining and EDX line scan analysis. The pyrite oxidation rate was employed to the index for black shale oxidation degree, and analyzed by XRD, aqueous pH, oxidation-reduction potential (ORP), ferrous and ferric ions concentrations measurement. The results show that the micro-surface characteristics are different in acid solution and A. ferrooxidans groups, which significantly impact the pyrite oxidation rate. A. ferrooxidans promote the jarosite formation and elemental C accumulation on the rocks micro-surface, which is assumed to inhibit further reactions. Two reaction phases named “pyrite oxidized phase” and “jarosite formation phase” are proposed to occur in the initial stage of A. ferrooxidans oxidizing black shale. These findings provide experimental data to evaluate the micro-surface reactions during black shale oxidation process.

Kinetics of Pyrite Oxidation in Simulated Mine Tailings Pond Conditions

Pyrite oxidation is the predominant source of acid mine drainage (AMD), which is one of the severe environmental impacts of mining. AMD destroys flora and fauna in the nearby ecosystem, and it causes grave health problems to the people. To further the understanding and help in the mitigation of AMD, this electrochemical study investigates the effect of the type and concentration of oxidant, and the presence of impurity (chalcopyrite) to the rate of pyrite oxidation, while simulating the conditions in tailings ponds. Open Cell Potential (OCP) and Electrochemical Impedance Spectroscopy (EIS) analysis were done using a three-electrode cell set-up, which is composed of a pyrite rotating disc electrode (RDE), mercury/mercurous sulfate reference electrode in saturated potassium sulfate, and a platinum counter electrode, with a sulfuric acid working electrolyte. Polarization resistance obtained after fitting an electrical circuit to the EIS data was related to the rate of pyrite oxidation, while OCP was used to describe its oxidation mechanism. It was found out that the oxidation of pyrite by ferric ions preferentially occurred rather than by dissolved oxygen. The parameter setting with the lowest relative oxidation rate were: electrolyte with no ferric addition, and with nitrogen purging; and RDE with 10 % chalcopyrite addition. Reaction orders calculated for the effect of ferric ions and dissolved oxygen were equal to 1; but ferric has a higher slope value in the plot. Presence of chalcopyrite in the RDE caused the preferential oxidation of chalcopyrite instead of the pyrite due to their galvanic interaction. To minimize pyrite oxidation, ferric and dissolved oxygen concentration should be minimized, and chalcopyrite impurities should be increased. Keywords: Electrochemical Impedance Spectroscopy, Acid Mine Drainage, Kinetics, Rotating-Disc Electrode, Pyrite

Temporal Changes of Pyrite Oxidation Rate in Bolivian Sulphidic Mining Wastes

Since the 70's, when huge sulphidic open pits were developed, the acidic rock drainage (ARD) become the leader problem of the sulphidic mining industry. Although the recycling is an essential technology, it cannot cover the demand alone, thus mining activity needs to continue. Acidity in mine drainage commonly requires most of the attention, but the main problem is the caused elevated level of metal mobility and leaching, which are generated by the increased rates of sulphide weathering under acidic conditions. The Itos mine is a polymetallic vein deposit in Bolivia, had been mined for silver and tin until 1990, leaving behind much and huge tailings and mine waste heaps, where quite often the pyrite content exceeds 10 %. Serious ARD effects take place in the mine waste heaps. These processes can be well characterized with the pH 1 or 2 of the seepage water, which forms serious alteration in the waste itself and the neighbouring rocks. In three consecutive years, the pyrite oxidation rate was investigated on the same 7 samples by humidity cell test. 5-6 months pauses were left between the humidity cell test periods, which mimics the alternation of wet and dry periods, typical for the place. The results give much more information, than the oxidation rate in the individual test periods, showing the changes by time. This applied method gave good result to characterize the behaviour of the waste in long-term. The column test was complemented with mineralogical analyses, such as electron probe micro analysis. The mineralogical and column test analyses show, that the changes of the pyrite oxidation rate split the samples into three different groups, one where the oxidation rate decreases, second where it increases with time and the third where oxidation rate is maximal and stays stable for several years.

Temporal Changes of Pyrite Oxidation Rate in Bolivian Sulphidic Mining Wastes

Temporal Changes of Pyrite Oxidation Rate in Bolivian Sulphidic Mining Wastes

The Effect of Particle Size on Mine Waste Sulfide Oxidation Rates and Conceptual Treatment Costs

Acid and metalliferous drainage (AMD) oxidation reaction rates were determined using oxygen consumption rates in a high sulfur overburden rock from the Australian Latrobe Valley coal provinces, and a mid to low range sulfur waste rock from a porphyry copper deposit in Papua, New Guinea. Nine grain sizes were tested, ranging from material retained by a 0.038 mm sieve through to a 40 mm sieve. Oxygen consumption rates in sealed cells were measured to establish pyrite oxidation rates (POR) in each sample. The oxygen consumption rates displayed a strong exponential reaction correlation with particle size for the material. The POR was found to range from 0.28 to 10.90 wt%/year FeS2 for the materials tested. A relationship between particle size and POR was established for comparative purposes. The smaller grind sizes included in this study extend the AMD/POR particle size data set available in the literature and will assist with geochemical engineering for designing tailings storage facilities. The potential economic and mine design ramifications of additional reactivity of fine mine materials is assessed and discussed using a unit cost framework for applying neutralizing materials.