Effect Of Pre-oxidation Research Articles

Effects of pre-oxidation on the hydrogen-rich reduction of Panzhihua ilmenite concentrate powder: Reduction kinetics and mechanism

The objective of this study was to investigate the effect of pre-oxidation treatment on the hydrogen-rich reduction of Panzhihua ilmenite concentrate powder on the reduction kinetics and mechanisms. In this study, reduction processes of raw and pre-oxidized ilmenite concentrates reduced by H2/CO mixtures (H2/CO = 0.4 and H2/CO = 2.5) at 950, 1000, and 1050 °C were systematically investigated using a thermal analyzer. The hydrogen-rich reduction rate was accelerated by the pre-oxidation treatment of raw ilmenite concentrate and an increase in H2 in H2/CO mixtures. The hydrogen-rich reduction of raw and pre-oxidized ilmenite concentrates can be divided into two stages according to the first derivatives of the reduction process: Fe3+ to Fe2+ and Fe2+ to Fe. The results of the research on the model function demonstrated that the reaction mechanism of raw and pre-oxidized ilmenite concentrates were three-dimensional diffusion models in the first stage. In the second stage, the reaction mechanism of the raw ilmenite concentrate was a phase boundary reaction model, and for the pre-oxidized ilmenite concentrate, it was a chemical reaction model. Furthermore, the apparent activation energies of both raw and pre-oxidized ilmenite concentrates under different reducing atmospheres were determined and compared. It was demonstrated that pre-oxidation treatment could effectively reduce the apparent activation energies of the reduction reaction. It is feasible to combine the advantages of the pre-oxidation treatment and hydrogen-rich reduction with a higher hydrogen ratio for the efficient reduction of Panzhihua ilmenite concentrate.

Effect of Pre-Oxidation on a Ti PVD Coated Ferritic Steel Substrate during High-Temperature Aging

A PVD coating is often applied on the surface of metallic alloys to improve their high-temperature resistance. In the present work, a thin titanium layer (1.2 µm) was deposited by PVD on the surface of a stainless steel substrate before high-temperature exposure (800 °C in ambient air). The underlying idea is that metallic Ti converts into Ti oxide (TiO2) during high-temperature aging at 800 °C, thereby slowing down the substrate oxidation. The stability of the coating with and without substrate pre-oxidation was investigated. Morphological, structural, and chemical characterizations were performed and completed by simulation of the film growth and measurement of the mechanical state of the film and the substrate. In the case of the sample that was not pre-oxidized, the oxidation of the steel was slowed down by the TiO2 scale but spallation was observed. On the other hand, when the steel was pre-oxidized, TiO2 provided more significant protection against high-temperature oxidation, and spalling or cracking did not occur. A combination of different kinds of stress could explain the two different behaviors, namely, the mechanical state of the film and the substrate before oxidation, the growing stress, and the thermal stress occurring during cooling down.

Effects of air volume and pre-oxidation on re-ignition characteristics of bituminous coal

Coal spontaneous combustion (CSC) seriously threatens the safety of coal mines. To reduce the spontaneous combustion risk of residual oxidized coal in goaf near coal seam and thick coal seam, it is necessary to study the re-ignition process of coal in depth. The re-ignition process of bituminous coal under different pre-oxidation temperatures and air volume rates was tested by temperature-programmed experiments. The gas index and re-ignition characteristic parameters of low-temperature oxidized coal were analyzed. The results show that CO production, C2H4 concentration, and Graham's fire coefficient (R2, R3) can be used as indicator gases to predict the CSC process. Meanwhile, the oxygen consumption rate and heat release intensity of raw coal and oxidized coal have a good exponential relationship with the coal temperature. In addition, during the slow oxidation stage of bituminous coal, the pre-oxidation process and the increased air volume can increase the oxygen consumption and heat release and reduce the activation energy, which aggravates the risk of spontaneous combustion. The results can enrich the research on the re-ignition characteristics of oxidized coal and can be applied to the management and control of CSC.

Effect of Pre-Oxidation on High-Temperature Oxidation Behavior of Al-Si Coating on Nickel-Based Superalloy.

Under the condition of long-time high-temperature oxidation, oxidation in Al-Si coating will lead to degradation of the coating. To solve this problem, the Al-Si coating was treated before the oxidation experiment at different pre-oxidation temperatures and times. The structure, morphology and element distribution of the oxide film were characterized by XRD, SEM and EPMA, and the oxidation kinetics curves were drawn. The results show that the oxidation resistance of the pre-oxidized samples is improved in the process of constant temperature oxidation at 1000 °C. When the pre-oxidation temperature is set at 950 °C, the mass gain of the samples is the lowest, and the accelerated decomposition of Cr2O3 caused by high pre-oxidation temperature is avoided. After oxidation for 300 h, the oxide film on the surface is still continuous and dense without peeling. The samples pre-oxidized at 950 °C for 7 h show the best oxidation resistance in a 1000 °C constant temperature oxidation process, more α-Al2O3 oxides are generated on the coating surface and the element diffusion between the matrix and the coating caused by overly long oxidation time is avoided.

Effect of pre-oxidation on copper-lead bulk concentrate flotation separation with sodium polyacrylate as galena depressant

In this work, adsorption and flotation tests were performed to investigate the suitability of sodium polyacrylate (PAAS) as a galena depressant in a Cu-Pb separation system and the effect of pre-oxidation on it. Adsorption studies show that PAAS effectively adsorbed on galena in the pH range 8–9, during the interaction between (R-COO-)n and Pb(OH)+. However, the adsorption of PAAS on chalcopyrite was approximately 6.8-fold less than on galena. Insoluble lead oxides formed on the galena surface after pre-treated with hydrogen peroxide (H2O2), which promoted the adsorption of Pb(OH)+ on galena, thereby providing additional adsorption sites and enhancing the adsorption of PAAS on galena. For Cu-Pb bulk concentrate containing 3.03 % Cu and 62.32 % Pb, the use of PAAS combined with H2O2 as a galena depressant for flotation separation yielded a Cu concentrate with 28.02 % Cu and a recovery of 82.56 %, containing 5.54 % Pb. The separation efficiency reached 65.44 %, comparable to that of potassium dichromate (K₂Cr₂O₇, 68.84 %). Compared with traditional chromium-containing depressants, this work avoids the pollution of metal chromium to humans and the environment, and provides a better reference for similar research.

The effect of pre-oxidation on the hot corrosion behavior of CoNiCrAlHf alloy in Na2SO4 environment at elevated temperature

The effect of pre-oxidation on the hot corrosion behavior of CoNiCrAlHf alloy in Na2SO4 environment at elevated temperature

Fast biodegradation of long-chain alkanes in heavily polluted soil by improving C/H conversion after pre-oxidation

This study aimed to explore the effects of Fenton pre-oxidation under different conditions on the indigenous bacterial communities and the mechanism of soil C/H conversion and rapid degradation of long-chain alkanes. The soil total petroleum hydrocarbons (TPH) content for this study was 27998 ± 883 mg/kg of soil. The results showed that the half-life period of the rapid long-chain degradation group was 266 days faster than that of the slow long-chain degradation group. After pre-oxidation, the number of bacteria in the fast group (5.07 log CFU/g) was higher than slow group (2.57 log CFU/g). Further studies showed that the consumption of ammonia nitrogen in the long-chain rapid degradation group was 1.37 times higher than that in the long-chain slow degradation group. The mapping characterization by DEEM revealed a high level of humic acid-like and organic matter produced by microbial metabolism in the fast group, and the long-chain degradation group had higher C/H conversion (35%), thus improving the degradation rate of long-chain alkanes (47%). High biomass and ammonia nitrogen consumption promoted higher C/H conversion. After proper pre-oxidation, the bacterial community changed, with an average relative abundance of 34% for Actinobacteria. This study has good engineering application value.

The Effect of Pre-Oxidation on the Reducibility of Chromite Using Hydrogen: A Preliminary Study

The majority of ferrochrome (FeCr) is produced through the carbothermic reduction of chromite ore. In recent years, FeCr producers have been pressured to curve carbon emissions, necessitating the exploration of alternative smelting methods. The use of hydrogen as a chromite reductant only yields water as a by-product, preventing the formation of carbon monoxide (CO)-rich off-gas. It is however understood that only the Fe-oxide constituency of chromite can be metalized by hydrogen, whereas the chromium (Cr)-oxide constituency requires significantly higher temperatures to be metalized. Considering the alternation of chromite’s spinel structure when oxidized before traditional smelting procedures, the effects on its reducibility using hydrogen were investigated. Firstly, the effect of hydrogen availability was considered and shown to have a significant effect on Fe metallization. Subsequently, spinel alternation induced by pre-oxidation promoted the hydrogen-based reducibly of the Fe-oxide constituency, and up to 88.4% of the Fe-oxide constituency was metallized. The Cr-oxide constituency showed little to no reduction. The increase in Fe-oxide reducibility was ascribed to the formation of an exsolved Fe2O3-enriched sesquioxide phase, which was more susceptible to reduction when compared to Fe-oxides present in the chromite spinel. The extent of Fe metallization of the pre-oxidized chromite was comparable to that of unoxidized chromite under significantly milder reduction conditions.

Effect of pre-oxidation of dispersed catalysts on heavy oil cracking

Heavy and extra-heavy oils are characterized by a high content of resins, asphaltenes, and sulfur, which hinders oil refining. The purpose of this paper is to study the effect of NiCrWC catalyst surface pre-oxidation on the composition of products of heavy oil catalytic cracking. The optimal conditions for the pre-oxidation of the catalyst are a temperature of 450 °C and a duration of 2 hours in air. The products of catalytic cracking are analyzed by content of saturated and aromatic hydrocarbons, resins and asphaltenes, gas chromatography, X-ray diffraction, and scanning electron microscope spectrometry methods. Pre-oxidation of a NiCrWC catalyst at 450 °C leads to a decrease in the content of macromolecular components, sulfur, by-products, and an increase in the yield of a fraction with a boiling point up to 360 °C. The decrease in sulfur content in the course of heavy oil upgrading is caused both by the interaction of metal oxides on the surface of NiCrWC with S-containing compounds of the feedstock, and by the removal of sulfur in the form of gas. A hypothetical mechanism for the cracking of heavy oil components over pre-oxidized catalysts is proposed.

Investigation of Electrical Transitions in the First Steps of Spark Plasma Sintering: Effects of Pre-Oxidation and Mechanical Loading within Copper Granular Media.

Spark Plasma Sintering (SPS) has become a conventional and promising sintering method for powder consolidation. This study aims to well understand the mechanisms of densification encountered during SPS treatments, especially in the early stages of sintering. The direct current (DC) electrical behavior of copper granular medium is characterized. Their properties are correlated with their microstructural evolutions through post-mortem scanning electron microscope (SEM) observations to allow a thorough understanding of the involved Branly effect that is suspected to occur in SPS. The electrical response is studied by modifying the initial thickness of the oxide layer on particles surfaces and applying various mechanical loads on the granular medium. Without load and at low current, the measured quasi-reversible behavior is connected to the formation of spots at the microcontacts between the particles. By increasing the current, the Branly transition from an insulating to a conductive state suddenly occurs. The insulating oxide layer is destroyed, and micro-bridges are created. The application of a mechanical pressure strongly modifies the DC Branly effect. Increasing low stress leads to a strong decrease in the breakdown field. For high-applied pressure, successive drops in the electric field are detected during the electrical transition. These successive drops are induced by microcracking of the insulating oxide layer.

Enhanced reduction of ilmenite ore by pre-oxidation in the view of pore formation

The promotion effect of pre-oxidation on reduction of ilmenite was revealed from the view of pore formation. The result shows that the enhancement of pre-oxidation on reduction of ilmenite ore is attributed to the formation of porous structure during the reduction process. Reduction of ilmenite is inhibited by iron-rutile topology structure formed on the particle surface. By contrast, porous structure ilmenite is regenerated during the ferric-to-ferrous reduction stage of the pre-oxidized ilmenite with dominant reduction rate over diffusion rate of produced ferrous ω ≥ 33.2 mol/m4. The newly formed porous structure ilmenite enlarges particle surface area with facilitated gas-solid contact, thus it is responsible for the enhancement of the reduction of pre-oxidized ilmenite. A novel formation mechanism of porous structure is also proposed based on the competition and coordination between the reduction rate and diffusion rate determined by reduction potential and temperature.

A Study on Pre-Oxidation of Petroleum Pitch-Based Activated Carbons for Electric Double-Layer Capacitors.

Electric double-layer capacitors (EDLCs) are an excellent electrochemical energy storage system (ESS) because of their superior power density, faster charge–discharge ability, and longer cycle life compared to those of other EES systems. Activated carbons (ACs) have been mainly used as the electrode materials for EDLCs because of their high specific surface area, superior chemical stability, and low cost. Petroleum pitch (PP) is a graphitizable carbon that is a promising precursor for ACs because of its high carbon content, which is obtained as an abundant by-product during the distillation of petroleum. However, the processibility of PP is poor because of its stable structure. In this study, pre-oxidized PP-derived AC (OPP-AC) was prepared to investigate the effects of pre-oxidation on the electrochemical behaviors of PP. The specific surface area and pore size distribution of OPP-AC were lower and narrower, respectively, compared to the textural properties of untreated PP-derived AC (PP-AC). On the other hand, the specific capacitance of OPP-AC was 25% higher than that of PP-AC. These results revealed that pre-oxidation of PP induces a highly developed micropore structure of ACs, resulting in improved electrochemical performance.

Effect of pre-oxidation on galena in the flotation separation of chalcopyrite from galena with calcium lignosulfonate as depressant

In this work, to achieve efficient separation of chalcopyrite from galena in Cu–Pb sulfide minerals, pre-oxidation by sodium persulfate (Na2S2O8) was used to magnify the differences in the floatability of two minerals in the presence of calcium lignosulfonate (CLS). The micro-flotation results of chalcopyrite and galena showed that pre-oxidation by Na2S2O8 could strengthen the depression performance of CLS on galena. The strengthening mechanism of Na2S2O8 to CLS adsorption onto the galena surface was revealed by local electrochemical impedance spectroscopy (LEIS), zeta-potential analysis, and X-ray photoelectron spectroscopy (XPS). LEIS showed that oxidation by Na2S2O8 contributed to the adsorption of CLS on the galena surface. XPS analysis demonstrated that the main oxidation species on the galena surface were PbSO4 and Pb(OH)+ in the presence of Na2S2O8 and that CLS interacted with oxidized galena by chemisorption. Zeta potential analysis indicated that a positive oxidation species was adsorbed on the galena surface; moreover, the oxidation species Pb(OH)+ endowed CLS on the galena surface with excellent adsorption performance. These findings are also supported by the results of the solution chemistry calculation. Galena also exhibits terrible surface reactivity and responds poorly to xanthate collectors after interaction with Na2S2O8 and CLS.

Beneficial effects of flame pre-oxidation on the oxidation behavior of NiCoCrAlHf alloy at 1050℃

Beneficial effects of flame pre-oxidation on the oxidation behavior of NiCoCrAlHf alloy at 1050℃

Effect of Long-Term Pre-oxidation on the Corrosion Rate of 316L Stainless Steel in a High-Temperature Water Environment

Effect of Long-Term Pre-oxidation on the Corrosion Rate of 316L Stainless Steel in a High-Temperature Water Environment

Effect of pre-oxidation treatment on the hot corrosion behavior of pack-cemented aluminide coatings on the K438 alloy in salt mixture

The hot corrosion test was carried out for the pack-cemented aluminide coatings on the K438 Ni-based alloy containing 50 wt.% NaCl + 50 wt.% Na2SO4 salt mixture at 1050 ℃ in air. This study aimed to add the salt mixture only once to the surface of the as-received and pre-oxidation aluminide coatings to investigate the influence on the subsequent and long-term hot corrosion behavior of coatings. The specimens were studied using a combination of gravimetry and several surface-analytical techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), focused ion beam (FIB), and transmission electron microscopy (TEM). The oxide scale on the surface of the pre-oxidation aluminide coating was denser and maintained better integrity after hot corrosion. The rod-like precipitate phase of CrS was mainly formed in the substrate of the two coatings. The sulfur was concentrated around Cr2O3 and corrosion cavities were observed in the oxide scale on the surface of the as-received aluminide coatings. Furthermore, a schematic mechanism of the hot corrosion process of the aluminide coatings was proposed, emphasizing the effect of pre-oxidation.

Elucidating radical-mediated pyrolysis behaviors of preoxidized lignins

Effect of lignin preoxidation on subsequent radical-mediated pyrolysis was discussed in this study. Technical hot-water-extracted lignin was preoxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in diverse degrees and pyrolyzed under different temperatures. Characterizations indicated that preoxidation increased lignin oxygen contents and converted α-hydroxyls to α-carbonyls. These structural modifications caused by preoxidation reduced the thermal stability and pyrolysis reactivity of lignin, shifting lignin thermal decomposition to the low temperature region and inhibiting lignin pyrolysis into bio-oil fractions. However, recognition of species and yields of specific compounds via analytical pyrolysis declared that although preoxidation reduced product yields, it did not alter the reaction pathways. The fixed bed experiments proved the above findings and gave the gas compositions, mainly CO2 derived through decarbonylation. Both radicals in chars and bio-oils were monitored, and char radical concentrations were proportional to the preoxidation degrees. This work sorted out the performances of lignin pyrolysis after preoxidation and determined their negative effects.

Hot Corrosion Behavior of Ti45Al8.5Nb Alloy: Effect of Anodization and Pre-oxidation

Hot Corrosion Behavior of Ti45Al8.5Nb Alloy: Effect of Anodization and Pre-oxidation

Effects of Air Volume and Pre-Oxidation on Re-Ignition Characteristics of Bituminous Coal

Effects of Air Volume and Pre-Oxidation on Re-Ignition Characteristics of Bituminous Coal

Effects of hydrogen peroxide preoxidation on clarification and reduction of the microbial load of groundwater and surface water sources for household treatment

Abstract Household water treatment (HWT) technologies are a promising strategy for addressing the waterborne diseases burden. However, in order to be efficient, these are often limited to water quality and require it to not exceed a certain threshold of physicochemical and microbiological contamination. Additionally, some popular HWTs, such as chlorination, are related to by-product formation. Preoxidation may improve source water quality, and hydrogen peroxide (H2O2) is an oxidant that has not been deeply explored in this specific application, so it could be an innovative approach to HWTs. We investigated effects of H2O2 preoxidation in two natural source waters (surface and groundwater), spiked with a high level of microorganisms. Clarification results suggested this pretreatment may improve life of HWTs. Reduction in microbial load of groundwater was considered ineffective, but 5-min H2O2 preoxidation at 15 mg L−1 led to >4.0 log10 inactivation of Phi X174 coliphage and >3.0 of Escherichia coli in surface water. We believe this performance was increased due to the presence of catalysts in the river water. This raised the point that water quality may be not only impairing, but potentially beneficial to the main HWT and characterization is crucial prior to the implementation of any technologies.