Decrease In Oxidation Rate Research Articles

Study of the Role of Nitrogen in the Oxidation of Titanium-Based Alloys by Changing the Reaction Gas

AbstractThe role of nitrogen in the oxidation of Ti-2W, Ti-10Al-2W (at.%) and Ti6242S was investigated using experiments in air and in Ar-20%O2, and two-stage experiments where the reaction gas was switched from one mixture to the other. When switching from Ar-20%O2 to air, the oxidation rates first increased during a short period, then decreased. This surge of mass gain following the introduction of air was attributed to N pickup, forming a nitride layer and a N-enriched zone in the alloy, below the oxide layer. The subsequent decrease of oxidation rate was attributed to the formation of nitride and/or N-rich zone, which both act as diffusion barriers for oxygen. Switching from air to Ar-20%O2 caused an increase in the oxidation rate of the W-containing alloys, which was attributed to the consumption of this barrier. The gas change had no significant effect on the oxidation rate of Ti6242S, which formed a much thinner nitride layer in air. The faster the nitride layer grows, the faster it is consumed when removing N from the reaction gas, probably because of a higher diffusion rate of N in W-doped TiO2 compared to TiO2 formed on Ti6242S.

Numerical investigation of soot formation in methane/n-heptane laminar diffusion flame doped with hydrogen at elevated pressure

The addition of hydrogen in a dual-fuel mode based on natural gas and diesel offers great potential for improving engine efficiency and reducing emissions. Therefore, detailed numerical simulations have been used to explore the effects of soot formation characteristics of natural gas-diesel dual-fuel engines doped by hydrogen at elevated pressure. Numerical simulations are compared with available experimental data, the effects of H2 addition on soot formation in laminar CH4 + n-heptane, H2 + CH4 + n-heptane, FH2 + CH4 + n-heptane diffusion flames at 2, 4, 6 and 8 atm are simulated, FH2 is added to separate the H2 chemical effects. The results show that the dilution and thermal effects of H2 inhibit soot formation, while the chemical effects of H2 promote soot formation under different pressures. The addition of H2 increases the concentration of H and OH radicals, which promotes the formation of C2H2 and benzene (A1). By simplifying the reaction path and analyzing the production rate of A1, the mechanism of H2 addition to promote A1 formation is illustrated. The concentrations of polycyclic aromatic hydrocarbon (PAH) pyrene (A4), benzo(ghi)fluoranthene (BGHIF) and benzo(a)pyrene (BAPYR) are inhibited at higher pressures that result in lower inception and condensation rates. Therefore, the increase of hydrogen abstraction acetylene addition (HACA) rate is the main reason for the increase of soot formation under chemical effects. While the O2 oxidation rate decreases and the OH oxidation rate increases, that is due to the decrease of O2 concentration and the increase of OH concentration in the soot formation area under chemical effects.

Effect of Temperature and Water Content on the Oxidation Behaviour and Cr Evaporation of High-Cr Alloys for SOFC Cathode Air Preheaters

High temperature alloys are being investigated for use in cathode air pre-heater for solid oxide fuel cell systems because of their high thermal conductivity, formability, manufacturability, and superior mechanical properties. However, it is well-known that high temperature alloys often contain a high concentration of Cr which presents a risk of evaporation and contaminating the cathode of the solid oxide fuel cell (SOFC). The oxidation and Cr2O3 evaporation mechanisms of the alloys including alloy 625, SS309, and alloy 318 have been investigated by varying temperatures and water content of the exposure atmosphere. For the influence of water content, the alloys were isothermally exposed at 850 °C in dry air and air containing 1%, 3% and 9% of H2O at a high flow rate for 168 h. For the influence of temperature, the alloys were isothermally exposed at 650 °C, 750 °C and 850 °C in a 6.0 L/min air stream containing 3% H2O for 168 h. The results of this study show that Cr2O3 evaporation and oxidation rates were dramatically reduced with decreasing temperatures for alloy 625 and SS309. Alloy 318 exhibited a decreased oxidation rate with decreasing temperature, but it demonstrated a reverse trend to the temperature-dependent Cr2O3 evaporation compared to alloy 625 and SS309. The major effect of water vapour on the three tested materials appeared to be the further enhancement of Cr2O3 evaporation.

Influence of Temperature and Water Vapour on the High-Temperature Oxidation and Cr Evaporation Behaviour of High-Cr Alloys for SOFC Cathode Air Pre-Heaters

The oxidation and Cr2O3 evaporation mechanisms of the stainless steels Inconel 625, SS309 and AluChrom 318 have been investigated by varying temperatures and water content of the exposure atmosphere. The finding of this research demonstrates that Cr2O3 evaporation and oxidation rates were significantly lowered with decreasing temperature for Inconel 625 and SS309. AluChrom 318 showed a decreased oxidation rate with decreasing temperature, but it demonstrated a reverse trend to the temperature-dependent Cr2O3 evaporation compared to Inconel 625 and SS309. The major influence of water vapour on the three tested materials appeared to be the further increase of Cr2O3 evaporation.

An organic silicone composite coating for protection of Ti-6Al-4V alloy: Oxidation behavior at 600 ºC in dry air

A new organic silicone composite coating modified by Al, ZrO2, quartz and PbO-SiO2-Al2O3 glass powders was developed to protect Ti-6Al-4V alloy. During oxidation at 600 °C in dry air, the coating was oxidized and formed an inorganic composite coating composed of amorphous silica framework and fillers which greatly decreased oxidation rate of substrate. Al2SiO5 formed in coating due to reaction of Al with glass. Glass powders with softening point of 385 °C may play important roles in improving coating protectiveness because they can flow to fill the cavities formed by oxidation of organic components in coating at 600 °C.

Dynamic modeling of Si(100) thermal oxidation: Oxidation mechanisms and realistic amorphous interface generation

Silicon and its native oxide SiO2 have been utilized in semiconductor technology since the 1950s and are still crucial for the development of novel device technologies today. Recent theoretical and experimental studies show that the fabrication of high-quality interfacial layers becomes critical for reliable operation of modern nanoscale devices. This paper presents a first-principles based approach to theoretically assess the thermal oxidation process of the technologically relevant Si(100) surface in the ultra-thin layer regime below 2 nm. The oxidation process is dynamically modeled by means of ab-initio molecular dynamics and density functional based tight binding simulations. We qualitatively explain the experimentally well-known but poorly understood decrease of oxidation rate in the initial oxidation stage as a complex interplay between various oxidation mechanisms such as fast O2 dissociation at the surface, slower oxygen integration mediated by molecular precursor states and O2 diffusion through the oxide. Our model combines previously reported experimental insights into a comprehensive picture of Si oxide growth. Strong evidence for an immediate amorphization of the oxide surface layer was found and identified as a direct consequence of lattice vibrations. Furthermore, our modeling method is a novel approach for the generation of realistic, amorphous interface structures that is based on the stepwise oxidation of the crystalline Si surface and could be extended to other materials systems as well.

Phosphorous and Aluminum Implantation for MOSFET Manufacturing: Revisiting Implantation Dose Rate and Subsequent Surface Morphology

In this work, we study the impact of the dose rate on the electrical properties of aluminum (p-body, p+-body-contact) and phosphorous (n-source/drain) implanted 4H-SiC. We find no significant differences for dose rates ranging from 1×1011 cm-2s-1 to 2−7×1012 cm-2s-1. AFM scans across implanted and non-implanted regions after thermal oxidation and subsequent oxide etching reveal a clear dependence of the oxidation rate on the conduction type and doping concentration. In addition, we observe an increasing (decreasing) oxidation rate for increasing doping concentrations of the n-type (p-type) ion implanted areas.

Microstructure refinement and oxidation behavior improvement in Mo modified TiAl4822 alloy prepared by vacuum sintering

In this paper, mechanical milling and vacuum hot-pressing sintering were used to fabricate TiAl4822 and TiAl4822-4 wt%Mo alloy. Hot compression and high temperature oxidation behavior at 800 °C and 900 °C were investigated respectively. Microstructure of the as sintered sample, morphology and composition of the oxidized surface was analyzed by SEM, TEM and EDX. The result shows that a refined microstructure with multi-scaled α2 lamella is exhibited in TiAl4822-4 wt%Mo alloy. The size of both equiaxed γ phase and α2/γ colony is finer in Mo modified alloy. Mo addition results in ∼6% lattice misfit by SAED analysis. Obvious improvement of hot compression properties and retardation of high temperature oxidation is displayed in Mo modified alloy. The compressive yield strength at 800 °C of TiAl4822-4 wt%Mo reaches 554.6 MPa which is higher than the Mo free counterpart. The weight increment and oxidation rate of TiAl4822-4 wt%Mo alloy after 900 °C/250 h is 3.74 mg/cm2 and 0.060 mg2cm−4h−1 which is 32.97% lower than TiAl4822 alloy. The increase of hot compressive strength is originated from grain refinement effect and solid solution strengthening effect introduced by lattice misfit. The decrease of oxidation rate in Mo modified alloy is probably attributed to the lower vacancy density of oxygen.

Y-Hf co-doped AlCoCrFeNi2.1 eutectic high-entropy alloy with excellent oxidation and spallation resistance under thermal cycling conditions at 1100 °C and 1200 °C

The oxidation behavior of AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) with Y-Hf co-doping under thermal cycling conditions at 1100 °C and 1200 °C is studied in comparison to the conventional NiCoCrAlYHf alloy (CNA). The oxidation rates of EHEA at two temperatures are much lower than those of CNA. Additionally, the resistance to Al2O3 scale spallation for the EHEA is much superior to that for the CNA. It is well demonstrated that increasing the interfacial toughness is more beneficial to improve the scale spallation resistance in comparison to the decrease in oxidation rate.

Does Hyperglycemia Cause Oxidative Stress in the Diabetic Rat Retina?

Diabetes, being a metabolic disease dysregulates a large number of metabolites and factors. However, among those altered metabolites, hyperglycemia is considered as the major factor to cause an increase in oxidative stress that initiates the pathophysiology of retinal damage leading to diabetic retinopathy. Diabetes-induced oxidative stress in the diabetic retina and its damaging effects are well known, but still, the exact source and the mechanism of hyperglycemia-induced reactive oxygen species (ROS) generation especially through mitochondria remains uncertain. In this study, we analyzed precisely the generation of ROS and the antioxidant capacity of enzymes in a real-time situation under ex vivo and in vivo conditions in the control and streptozotocin-induced diabetic rat retinas. We also measured the rate of flux through the citric acid cycle by determining the oxidation of glucose to CO2 and glutamate, under ex vivo conditions in the control and diabetic retinas. Measurements of H2O2 clearance from the ex vivo control and diabetic retinas indicated that activities of mitochondrial antioxidant enzymes are intact in the diabetic retina. Short-term hyperglycemia seems to influence a decrease in ROS generation in the diabetic retina compared to controls, which is also correlated with a decreased oxidation rate of glucose in the diabetic retina. However, an increase in the formation of ROS was observed in the diabetic retinas compared to controls under in vivo conditions. Thus, our results suggest of diabetes/hyperglycemia-induced non-mitochondrial sources may serve as major sources of ROS generation in the diabetic retina as opposed to widely believed hyperglycemia-induced mitochondrial sources of excess ROS. Therefore, hyperglycemia per se may not cause an increase in oxidative stress, especially through mitochondria to damage the retina as in the case of diabetic retinopathy.

Effect of pre-oxidation and Al amount on the oxidation behavior of healing agents in YSZ-Mo(Si1-x,Alx)2 composite

Efficient and easy to produce self-healing agents have significant role on the development of self-healing composites. Herein, we report preoxidized Self-propagating high-temperature synthesis (SHS) synthesized Al-alloyed MoSi2 for self-healing agent in thermal barrier coating system. The raw materials of Mo, Si and Al with molar aspect ratio of (1:1.6:0.4) and (1:1.2:0.8) were used to produce Mo(Si1-x,Alx) via SHS method. The healing agents were used to produce YSZ- Mo(Si1-x,Alx) composite by spark plasma sintering (SPS) method. Effect of peroxidation and Aluminum amount were studied after iso-thermal oxidation at 1050 °C with FE-SEM, EDX and XPS analysis. Oxide shell were grown on the surface of healing agent powders after preoxidizing. Oxide form of aluminum, silicon and molybdenum are available in oxide shell after preoxidation. Selective oxidation of Aluminum led to produce firm and continues Alumina oxide film on the surface of healing agents. Peroxidation is effective way to reserve the healing agent with molar aspect ratio of (1:1.2:0.8) by decreasing oxidation rates of them at working temperature and has not significant effect on healing agent with (1:1.6:0.4) molar aspect ratio. There is much prospect that this research will led to producing novel self-healing thermal barrier coatings by modification of healing agents composition in order to have the most appropriate properties at working condition.

Co-oxidation of Antarctic krill oil with whey protein and myofibrillar protein in oil-in-water emulsions.

Antarctic krill oil (AKO) is usually encapsulated by the protein materials, enhancing its oxidative stability. Proteins exhibit immense effect on lipid oxidation and induce protein-lipid co-oxidation. This study aimed at elucidating the co-oxidation mechanism of AKO and whey protein (WP) or myofibrillar protein (MP) in oil-in-water emulsions. The estimations of malondialdehyde (MDA) content, phospholipid molecular species, and pyrrole content resulted in increased and decreased oxidation rate of AKO (especially phosphatidylethanolamine) by WP and MP, respectively. Meanwhile, protein concentration, sulfhydryl content, the loss of tryptophan fluorescence intensity, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis results demonstrated that AKO promoted WP oxidation but inhibited MP oxidation. Further, the antioxidative abilities of seven common antioxidants were evaluated. Ascorbyl palmitate showed the most substantial antioxidative effect for both AKO and proteins (about 70% decrease of MDA content and 30% decrease of the decrease ratio of tryptophan fluorescence intensity). This finding supported that different proteins could exhibit different pro/anti-oxidative effects on lipid oxidation, especially for marine lipids abundant in phospholipids and polyunsaturated fatty acids. Besides, MP could also act as antioxidant in MP AKO emulsions, further extending its application from traditional surfactants. PRACTICAL APPLICATION: AKO is usually encapsulated by the protein materials, enhancing its oxidative stability. The results demonstrated MP could inhibit AKO oxidation, and vice versa, especially when ascorbyl palmitate was added at the same time. As a result, this finding explored a new potential wall material with antioxidative ability for the encapsulated products of AKO.

(Invited) Investigation of High Temperature Oxide Scale Using Chemical Reactor

IntroductionNi aluminide forms highly oxidation-resistant Al2O3 on its surface when it contains a certain concentration of aluminum. However, when the Al content is too high, the melting point of the alloy decreases. As a result, the high temperature strength decreases. Furthermore, the material becomes brittle and the workability deteriorates. Studies have reported that it is effective to add Cr to produce alumina even with a low Al concentration.In this study, the oxidation behavior of Ni-Al and Ni-Al-Cr alloys in Ar-H2O was investigated using a gas sensor. Then, the effect of Cr concentration is clarified from the oxidation rate and cross-sectional structure.ExperimentalFig.1 shows the oxidation test equipment used in this study. A hydrogen sensor and an oxygen pump sensor were installed downstream the electric furnace. The partial pressure of hydrogen and the oxidation current (hydrogen concentration) of hydrogen in the discharged gas were measured. A CaZr0.9In0.1O3 tube was used for the hydrogen sensor, and a yttria-stabilized zirconia tube (8 mol% Y2O3-ZrO2) was used for the oxygen ion pump and sensor.The hydrogen sensor measures the electromotive force and substitutes it into the Nernst equation, shown in equation (1), is used to obtain the hydrogen partial pressure. E=(RT/2F) ln (P H2(ref)/P H2(mea)) (1)Where R: gas constant, T: temperature, F: Faraday constant, E: measured electromotive force, : hydrogen partial pressure in reference gas (Ar-1.01 vol.%H2), : measured hydrogen partial pressure.Fig. 2 shows details of the oxygen pump-sensor. In an oxygen pump-sensor, composed of a pump section and a sensor section, the electromotive force is measured by the sensor part and the electromotive force is set so that the initial oxygen partial pressure is obtained. When a current is applied to the pump section, the YSZ tube acts an oxide ion conductor, and oxygen is supplied to the atmosphere inside the pump section tube by the reaction shown in the figure. Then, the oxygen partial pressure of the pump section is maintained at the initial oxygen partial pressure by potentiostat. The calculation of the oxidation rate using the oxygen pump-sensor is performed according to Faraday's law, as seen in equation (2). J= I / 4F (2)Where, F: Faraday constant, I: applied current, and J: amount of oxygen supplied to the measurement system by the oxygen pump. The amount of hydrogen was calculated from the supplied amount of oxygen J, and the oxidation rate was calculated.ResultsNi-5 wt.% Al showed a sharp increase in oxidation rate at 900℃. Thereafter, a linear increase in the oxidation rate was observed during the heating process. On the other hand, the Ni-5 wt.% Al-5 wt.%Cr alloy showed the same sharp increase in oxidation rate at 900℃, but the value reached was much lower than that of Ni-5 wt.% Al. Additionally, during the heating process from 900℃ to 1250℃, the oxidation rate peaked briefly, then declined, before resuming a linear increase. In its cross-sectional structure, thick and thin oxides were formed. For the thin layers , NiAl2O4 was formed on the surface and Al2O3 below. The oxidation rate of Ni-5 wt.% Al-10 to 20 wt.% Cr, the oxidation rate when the temperature reached 900℃ was very low. Looking at its cross-sectional structure, Cr oxide was formed on the surface, and Al2O3 below. When Cr oxide formed on the surface, a sharp decrease in the oxidation rate was observed during the heating process. Al2O3 transforms from θ to α around 1100℃. Thus, the oxidation rate decreases with the transformation of Al2O3 from θ to α during the heating process. Furthermore, the rate of oxidation during the heating process decreased more when chromium oxide was formed on the surface than when Ni oxide was formed on the surface. This reveals the relationship between the oxidation rate and the effect of Cr on Al2O3.Conclusions(1) Ni-5Al showed a rapid increase in the oxidation rate at 900℃, and after that a linear increase in the oxidation rate was observed during the heating process.(2) For the Ni-5Al-5Cr alloy, the oxidation rate during the heating process from 900℃ to 1250℃ slowed at first, then dipped, before resuming a linear increase. Its scale structure consisted of thick and thin oxides. In the thin layers, NiAl2O4 was formed on the surface and Al2O3 was formed below. Figure 1

Effect of atmosphere of SO2 coexisted with oxidizing gas on mercury removal under oxy-fuel condition

The effects of oxidizing gas and SO2 in oxy-fuel combustion on mercury adsorption by modified corn stalk char were studied. Results showed that after 1% NH4Cl modification, the number of surface pores and oxygen-containing functional groups increased, which contributed to the improvement of adsorption capacity. O2 could promote the adsorption of mercury by heterogeneous oxidation and oxygen-containing functional groups. The addition of HCl could obviously improve the adsorption effect of adsorbent as the result of the chlorine-containing function groups on the surface of biomass char and the effect of Cl2 produced by Deakin reaction. SO2 could be oxidized to SO3 by oxygen atoms on the surface of adsorbent, which competed with the active sites and inhibited adsorption. SO2 could also directly react with Hg0 to produce HgSO4. The increase of O2 would alleviate the inhibition effect of SO2. SO2 could react with O2 by some ways and thus resulted in the decrease of oxidation rate owing to the consumption of O2. The existence of SO2 would weaken the oxidation effect under HCl through the reaction with Cl2 produced by HCl and O2, and SO2 would compete with HCl for the active sites on the surface of adsorbent.

Degradation of crystal violet (CV) from aqueous solutions using ozone, peroxone, electroperoxone, and electrolysis processes: a comparison study

In this study, the degradation of crystal violet (CV) was investigated by different oxidation processes; ozone (OZ), peroxone (PO), electrolysis (E), electrolysis/H2O2 (ECP), electroperoxone (EPO), and electrolysis/peroxene/H2O2 (EPOP). Main parameters including contact time, pH, CV concentration, and effect of scavengers were studied. The results showed that all processes were capable of CV decolorization. Among these processes, peroxone and electrolysis/peroxene/H2O2 with efficiency about > 90% were more effective than other ones. A significant decrease in oxidation rate of CV was observed by adding scavengers. This fact was due to higher affinity to react with free radicals. To confirm degradation of CV, FTIR spectra and nitrate ion level were taken. Also, to recognize toxicity the treated wastewater was tested against coliform bacteria. Growths of Escherichia coli in EMB medium were observed. As a result, it confirms that the treated effluent can be discharged to environment.

Investigation of Alumina Formation and Oxidation Rate of Ni–5 wt%Al–X wt%Cr Alloy Using Hydrogen Sensor and Oxygen Pump Sensor

The oxidation rates of Ni–Al and Ni–Al–Cr alloys in an Ar–12 vol%H2O atmosphere during heating were determined by a hydrogen sensor and an oxygen pump sensor installed at the subsequent stage. The effect of Cr on the Ni–Al alloy was studied. As a result, the Ni–5 wt%Al showed a sharp increase in oxidation rate at 900 °C. Thereafter, a linear increase in the oxidation rate was observed during its heating. On the other hand, the Ni–5 wt%Al–5 wt%Cr alloy showed a sharp increase in the oxidation rate at 900 °C, but the value was lower than that of Ni–5 wt%Al. During the heating from 900 to 1250 °C, it showed a discontinuous behavior. In the cross-sectional structure, a thick oxide and thin oxide were formed. For the thin part, NiAl2O4 was formed on the surface and Al2O3 was formed below it. The oxidation rate of Ni–5 wt%Al–10 to 20 wt%Cr alloys when the temperature reached 900 °C was very low. For the cross-sectional structure, Cr oxide was formed on the surface, and Al2O3 was formed below it. When Cr oxide was formed on the surface, a sharp decrease in the oxidation rate was observed during the heating. It is considered that the oxidation rate decreases by the transformation of Al2O3 from θ to α during the heating. Furthermore, the rate of oxidation during the heating decreased more when chromium oxide was formed than when Ni oxide was formed on the surface. This revealed the relationship between the oxidation rate and the effect of Cr on the Al2O3.

Oxidation Behavior and Electrical Properties of De-siliconized AISI 430 Alloy with Mn1.5Co1.5O4 Coating for Solid Oxide Fuel Cell Interconnect

The effect of de-siliconization on the oxidation behavior of AISI 430 stainless steel used for solid oxide fuel cell interconnect application was investigated. De-siliconization treatment was conducted via heating steel parts in an H2 environment. The de-siliconized substrates were then coated with a Mn1.5Co1.5O4 spinel coating, using wet spray method. For comparison, a similar coating process was applied on the as-received AISI 430 stainless steel specimens. Oxidation kinetics of coated interconnects were evaluated at 700, 800 and 900 °C in air. Results showed that the de-siliconization surface treatment decreased oxidation rates, with kinetic rates (g2 cm−4 s−1) of 4.39 × 10−14 at 700 °C, 1.71 × 10−13 at 800 °C and 1.62 × 10−12 at 900 °C, i.e., 55, 48 and 22% lower compared to the as-received coated substrates, respectively. Scanning electron microscopy images and EDS analysis confirmed the formation of SiO2-rich layer beneath the surface oxide scale for the oxidized specimens. The thickness of the formed scale for the de-siliconized specimens was noticeably lower than that of as-received ones. Step-wise electrical conductivity measurements showed that the de-siliconized specimens, compared to the as-received specimens, had higher electrical conductivity (~ 30–40%), implying lower oxidation degrees of the substrate and thus, better electrical performance under cathodic service condition of SOFC.

Особенности роста поверхностной окалины в процессе окисления на воздухе сплава V-Cr-Ta-Zr

Investigation results of features of surface scale growth on samples of V-Cr-Ta-Zr vanadium alloy at a short duration of oxidation in air are presented. It is shown that at the initial stages of oxidation, regardless of the samples microstructure, the growth rate of surface scale can be an order of magnitude higher than with long-term processing. It has been established that scale is characterized by open porosity formed as a result of sticking of small plate-like V2O5 flakes together. It has been suggested that the decrease in oxidation rate is a consequence of an increase in the scale thickness, which helps to slow down the supply of oxygen to the sample surface on which the processes of nucleation and growth of scales occur, as well as diffusion saturation of the surface layer of the vanadium alloy with oxygen.

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.

Stable sulphur and oxygen isotopes as indicators of sulphide oxidation reaction pathways and historical environmental conditions in a Cu–W–F skarn tailings piles, south-central Sweden

Improved remediation strategies or predictive modelling of acid mine drainage (AMD) sites, require detailed understanding of the sulphide oxidation reaction pathways, as well as pollutant-source characterisation. In this study, ore minerals, solids and water-soluble fractions of an oxidising Cu–W–F skarn tailings in Yxsjöberg, Sweden, were chemically and isotopically (δ34S and δ18O) characterised to reveal sulphate (SO42−) sources, sulphide oxidation reaction pathways and historical environmental conditions in the tailings. δ34S was additionally used to trace the weathering of danalite [(Fe,Mn,Zn)4Be3(SiO4)3S], a rare and unstable sulphur-bearing silicate mineral containing high concentrations of beryllium (Be) and zinc (Zn). Eighteen subsamples from a drill core of the tailings were subjected to batch leaching tests to obtain water-soluble fractions, which reflected both existing pore-waters and easily-soluble secondary minerals. The tailings were categorised into three geochemical zones: (i) oxidised zone (OZ), (ii) transition zone (TZ) and (iii) unoxidised zone (UZ), based on prevailing pH, elemental concentrations and colour. The upper OZ (UOZ) showed a sharp depletion of sulphur (S) and relatively higher δ18OSO4 values (−3.0 to +0.1‰) whereas the underlying lower OZ (LOZ) showed S accumulation and lower δ18OSO4 values (−4.6 to −4.2‰). The higher δ18OSO4 suggested the role of atmospheric oxygen, O2 (as oxidant), contribution of evaporated rainwaters and/or evaporation in the upper zones of the tailings. The lower δ18OSO4 values were indicative of ferric iron (Fe3+) as oxidant and the possible incorporation of 16O into SO42− during its formation, most probably from snow melt or depleted rainwater. The δ34SSO4 values in the OZ (+2.3 to +2.4‰) suggested SO42− from pyrrhotite oxidation in the UOZ which has been subsequently mobilised to the LOZ. Low δ34S fractionation (+0.2 to +1.9‰) between SO42− in the OZ and pyrrhotite, as well as the low δ18OSO4 values in the LOZ suggested the complete oxidation of pyrrhotite by Fe3+, signalling that previously, a low pH (<3) prevailed in the tailings. Mineralogical observations confirmed that pyrrhotite was completely oxidised in the UOZ, with the formation of hydrous ferric oxides (HFOs) coatings. The observed current high δ18OSO4 and pH (3.9–4.5) values in the UOZ were attributed to decreased oxidation rate and silicate buffering, limiting the availability of aqueous Fe3+ and subsequent formation of HFOs. The δ34SSO4 signatures of the water-soluble SO42− in the TZ and UUZ suggested the dissolution of gypsum which precipitated from a leachate from the weathering of danalite in the UOZ. In the middle UZ, the δ34SSO4 (−0.8 to +0.6‰) and δ18OSO4 (−1.8 to −1.0‰) signatures corresponded to SO42− from a mixture of pyrite, pyrrhotite and chalcopyrite oxidation by O2 at the LOZ (i.e. oxidation front). Negative δ34S fractionation values (−3.0 to −1.6‰) between these minerals and the water-soluble SO42− were attributed to the potential formation of intermediate S species, due to the partial oxidation of the sulphides. Consequently, the S accumulation in the LOZ could be due to the likely formation of the intermediate S species and secondary pyrite identified in this zone. The lower UZ coincided with the groundwater table and registered consistent negative δ34SSO4 (−2.6 to −1.8‰) and δ18OSO4 (−7.6 to −4.4‰) values. These signatures were hypothesised to be controlled by SO42− from the mineralisation of organic S in peat underneath the tailings and/or H2S oxidation, with possible contribution from sulphide oxidation in the tailings. This study highlights the usefulness of δ34S and δ18O as tracers of geochemical processes and environmental conditions that have existed in the tailings.