Pre-oxidation Time Research Articles

Achieving high-thermal-conductivity brazed joint between carbon-based composites and Mo-Cu alloys by increasing the heat transfer area

Carbon-fiber-reinforced carbon matrix (Cf/C)-Mo30Cu brazed joints play an important role in the cooling systems of thermonuclear reactors. However, the limited contact area of heterogeneous interfaces severely limits the heat transfer efficiency. To overcome this drawback, we prepare three-dimensional porous interfaces by pre-oxidizing the Cf/C composite. Results show that circular gaps are formed between the carbon fibers and the pyrolyzed carbon after pre-oxidization at 600 °C in air. Fster braze penetration in the Cf/C composite is achieved, and the heat-transfer area across the interface is dramatically increased. The room-temperature thermal conductivity of the joints reaches a maximum value of 146 W·m−1·K−1 at a pre-oxidation time of 2 min; this value is 30 % higher than that obtained without treatment. The enhancement in thermal conductivity is mainly attributed to the increased contact area at the interface between the brazing seam and the Cf/C matrix, which provides more channels for heat transfer. This method of significantly improving the thermal conductivity is an important guide for the thermal management of thermonuclear reactors.

Non-monotonic dependence of high-temperature stability of stone wool fibres on pre-oxidation time and temperature

Stone wool is a sustainable, fire-safe insulation material, and its fire-safety depends on its high-temperature stability (HTS). It is known that the HTS of stone wool fibres can be enhanced by pre-oxidation (i.e., heat treatment in air) at temperatures below or at its glass transition temperature (Tg). However, so far, a quantitative trend of HTS with varying pre-oxidation temperature (Tpre-ox) and time (tpre-ox) remains unknown. In this study, we quantified the change of HTS of stone wool fibres with a Tg of 677 °C as functions of both Tpre-ox and tpre-ox through a hot-stage microscope. The derived results revealed striking non-monotonic trends in the HTS of stone wool fibres. The HTS first increased with Tpre-ox from 500 to 630 °C for tpre-ox of 15 minutes, but declined beyond 630 °C. The HTS was significantly improved upon pre-oxidation for merely 2 minutes, at 677 °C, then dropped with extending tpre-ox to 16 hours, and finally increased again with tpre-ox up to 16 days. The origin of the non-monotonic trend was explored by characterizing the crystallization behaviour, thermal expansion, surface composition, and surface morphology in stone wool fibres after pre-oxidation.

High-temperature oxidation and gas thermal shock studies of IC10 simulated specimens with thermal barrier coatings

PurposeThe purpose of this study is to investigate the effects of high-temperature oxidation tests and gas thermal shock tests on IC10 simulated components with thermal barrier coatings under different temperatures and oxidation times.Design/methodology/approachIn the high-temperature oxidation test, specimens were oxidized at three different temperatures of 850, 980, and 1,100 °C for durations of 10, 20, 50, 100, 200, and 300 h, respectively. In the gas thermal shock test, specimens were pre-oxidized for 10, 20, 50, and 100 h, followed by a high-temperature gas thermal shock test at 1,100 °C.FindingsIn the high-temperature oxidation tests, with increasing oxidation time, the oxidation layer thickened, and the air-film holes diameter decreased. The microstructure of the bond coat transitioned from strip-like to block-like, and internal cracks transformed from numerous and short to larger and deeper. Below the bond coat, a noticeable disappearance layer of strengthening phase appeared, with increasing thickness. The strengthening phase in the substrate transitioned from regular square shapes to circles as temperature increased. In gas thermal shock tests at 1,100 °C, the oxidation weight gain ratio increased with longer pre-oxidation times, whereas the erosion weight loss ratio gradually decreased.Originality/valueThe originality and significance of this study lie in its departure from the typical subjects of high-temperature oxidation and thermal shock tests. Unlike common research targets, this study focuses on IC10 simulative specimens with thermal barrier coatings and air-film holes. Furthermore, it investigates the effects of varying temperatures and oxidation durations.

Effect of Direct and Indirect Pre-Oxidation of Gold Ore from Doup North Sulawesi using Ozone Gas on the Gold Extraction in the Cyanidation Process

Refractory gold ore is a type of gold ore that is difficult to process when treated through a conventional cyanidation process. Pre-oxidation using ozone gas is one of the pre-treatment methods that can be used to improve gold recovery from sulfide-type refractory ore. This study used gold ore samples from Doup, North Sulawesi, Indonesia, to study the effect of direct and indirect pre-oxidation using ozone gas on gold recovery and subsequently treated by a cyanidation process. Direct and indirect pre-oxidation experiments were carried out at different pHs and using two raw materials for ozone generation (air and O2). The slurry obtained from the pre-oxidation experiments was filtered, and the filtrate was analyzed for the concentration of dissolved Cu, Fe, and Zn using atomic absorption spectroscopy (AAS); meanwhile, the residue from each pre-oxidation experiment was leached by cyanidation to obtain a pregnant leaching solution (PLS), which was analyzed for its gold concentration using AAS. The percentages of dissolved Cu, Fe, and Zn during pre-oxidation increased with the pre-oxidation time when the acidity was maintained at pH 2, using pure O2 as the ozone raw material. The direct pre-oxidation process at pH 2 using air as the ozone raw material increased gold extraction from 79.08% to 82.78% in the cyanidation process. The pre-oxidation experiments performed without controlling the pH resulted in lower gold dissolutions than the pre-oxidation conditions at pH 2. Possible causes of the lower gold dissolution were discussed.

Experimental study on the effect of room temperature pre-oxidized time on spontaneous combustion characteristics of coal

The presence of different types of coal at room temperature can lead to self-heating of coal, potentially resulting in spontaneous combustion. To investigate the effect of ambient temperature pre-oxidation (BL) time on the self-combustion characteristics of different coal types, synchronous thermal analysis (STA) and Fourier-transform infrared spectroscopy (FTIR) experiments were conducted. The results of the synchronous thermal analysis experiments indicate that ambient temperature pre-oxidation for 3 months (BL3), BL6, and BL9 coals exhibit faster oxidation reactions compared to the original coal, while BL12 coal shows slower oxidation than the original coal. Among these, BL9 coal demonstrates the most significant changes in oxidation reaction characteristics, with the fastest oxidation reaction time being 35.36 min, which is 1.38 min faster than the original coal. To support this observation, a comparison was made between the relative content of active functional groups in the original coal and BL coal. The study revealed that the BL process affects the relative content of hydroxyl groups, aromatic hydrocarbons, aliphatic hydrocarbons, and oxygen-containing functional groups, thereby influencing the coal-oxygen reaction process. This suggests that pre-oxidized coal, compared to the original coal, has a larger pore structure, which plays a dominant role in promoting coal self-combustion in the first 9 months of the BL process. As BL time continues to increase, the continuous reaction of active functional groups at room temperature leads to excessive consumption, resulting in a more significant role in inhibiting coal self-combustion. The research results provide valuable insights for predicting the spontaneous combustion risk of oxidized coal.

Effects of operating conditions on iron (hydr)oxides evolution and ciprofloxacin degradation in potassium ferrate-ozone stepwise oxidation system

In this study, a stepwise oxidation system of potassium ferrate (K2FeO4) combined with ozone (O3) was used to degrade ciprofloxacin (CIP). The effects of pH and pre-oxidation time of K2FeO4 on the evolution of K2FeO4 reduction products (iron (hydr)oxides) and CIP degradation were investigated. It was found that in addition to its own oxidation capacity, K2FeO4 can also influence the treatment effect of CIP by changing the catalyst content. The presence of iron (hydr)oxides effectively enhanced the mineralization rate of CIP by catalyzing ozonation. The pH value can influence the content and types of the components with catalytic ozonation effect in iron (hydr)oxides. The K2FeO4 pre-oxidation stage can produce more iron (hydr)oxides with catalytic components for subsequent ozonation, but the evolution of iron (hydr)oxides components was influenced by O3 treatment. It can also avoid the waste of oxidation capacity owing to the oxidation of iron (hydr)oxides by O3 and free radicals. The intermediate degradation products were identified by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Besides, the degradation pathways were proposed. Among the degradation products of CIP, the product with broken quinolone ring structure only appeared in the stepwise oxidation system.

Bisulfite-activated permanganate oxidation plus coagulation as a pretreatment of SWRO desalination lines to enhance boron rejection

To address the concern of high level of residual boron in the filtrate of seawater reverse osmosis (SWRO), bisulfite-activated permanganate technology (PM/BS) combined with post-coagulation/sedimentation (C/S) was evaluated as a pre-treatment strategy. The results revealed that the combination (PM/BS + C/S) can remove 37.1 % of boron under optimal conditions. For PM/BS pretreatment, there was an optimal dosage of BS (450 μM) and PM (90 μM) and pre-oxidation time (5 min) to assist C/S in achieving boron removal. For post-C/S, the optimal flocculant dose (10 mg/L), GT value (GrapidT = 45948 and GslowT = 12480), and precipitation time (20 min) were also determined. BS/PM pretreatment weakened the dependency on strict GT control and shortened the precipitation time required. Moreover, compared to C/S alone, BS/PM + C/S showed a remarkable improvement in the elimination of dissolved organic matter (19.8 %) and nutrients (19.8 % for assimilable organic carbon, 30.8 % for total nitrogen, and 16.0 % for total phosphorus), which reduced membrane fouling. The synergistic mechanism for enhanced boron removal includes destroying the organic coating of colloid or suspended particles, changing the structure of dissolved organic matter, generating MnO2 particles in situ as a boron absorbent and coagulation aid, and regulating the growth and development of flocs. A pilot-scale test integrating PM/BS into a typical SWRO process yielded an average boron rejection rate of 95.7 %. This study indicates that PM/BS + C/S may be a promising pretreatment technology for SWRO to control the amount of residual boron in the filtrate.

Multiscale thermal behavioral characterization of spontaneous combustion of pre-oxidized coal with different air exposure time

To comprehensively investigate the multi-scale characteristics of coal spontaneous combustion, in situ infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were utilized. The samples were pre-oxidized at 70 °C for 2, 4, 6, and 8 h (the raw coal and above samples were named R0, R2, R4, R6, and R8), then the secondary oxidation experiments were carried out. The results showed that the contents of –OH, –C=O, and –C=C- increased with the pre-oxidation time. The aromatic structure of pre-oxidized coal is more susceptible to oxygen erosion during secondary oxidation. The migration and transformation of –OH, –C=O are the underlying causes of water evaporation and gas desorption during low-temperature oxidation. The spontaneous combustion behaviors in stage I and stage II have different reaction mechanisms and kinetic characteristics. In light of Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS), the apparent activation energy of stage I is R0>R2>R4>R6>R8, and stage II is R6>R8>R2>R4>R0. Long pre-oxidation time in stage I will inhibit coal spontaneous combustion, while the opposite is true in stage II. By the Pearson correlation analysis, –OH is the key structure affecting the low-temperature endothermic stage of coal, revealing the correlation between the multi-scale thermal behavior of pre-oxidized coal spontaneous combustion.

Microstructure and self-healing mechanism of B4C– TiB2– SiC composite ceramic after pre-oxidation behaviour

As a kind of structural ceramic, ultrahigh hardness B 4 C ceramics are sensitive to flaws, especially surface flaws, which cause mechanical property degradation. The present study intended to achieve flaw self-healing with the pre-oxidation treatment method at 1073 K for 15–60 min, which effectively improved the flexural strength of the B 4 C– TiB 2 – SiC composite ceramic. The changes in surface morphology and flexural strength over pre-oxidation time were studied in detail, and schematic diagrams of pre-oxidation strengthening process were systematically proposed. The research results indicated that the pre-oxidation treatment could heal pores and cracks because glassy oxide B 2 O 3 , which has a low melting point, could flow and fill flaws on composite ceramic surfaces efficiently and, in turn, significantly improve mechanical performance. The flexural strength of the composite ceramic reached a maximum of 684 MPa at the optimum pre-oxidation time of 45 min, which was an increase of 28.77% compared with that of the untreated composite ceramic. Further increasing the pre-oxidation time resulted in a decrease in flexural strength due to pores forming on surfaces again, which was induced by the production of more CO gas and the intensive evaporation of the B 2 O 3 glass phase.

Oxidation behaviour of interlocking SiC–Si coating for graphite prepared by preoxidation and gaseous silicon infiltration process

An interlocking SiC–Si coating on graphite was fabricated by a method combining preoxidation and gaseous silicon infiltration processes. The effect of preoxidation time on the microstructures and oxidation behaviour of the coating was investigated. After preoxidation of graphite at 900 °C for 10 min, the thickness of the as-obtained SiC–Si coating increased from 35 μm to 80 μm, and the penetration depth of the SiC–Si phase was up to several hundred microns. The isothermal oxidation test at 1500 °C suggested that the mass loss of coated graphite decreased from 5.42% after 6 h to 1.28% after 110 h, and the 15 times thermal shock test between 1500 °C and room temperature suggested that the mass loss decreased from 13.20% to −0.34% because of preoxidation. The improved oxidation performance is ascribed to the fact that the interlocking structure of the SiC–Si coating alleviates crack propagation during oxidation, and a large amount of glassy SiO2 produced on the thick SiC–Si layer migrates and seals the defects in the coating. It is believed that the construction of an interlocking structure offers a technological basis to produce oxidation protective coatings for carbon-based materials in high-temperature applications.

Permanganate/Bisulfite Pre-Oxidation of Natural Organic Matter Enhances Nitrogenous Disinfection By-Products Formation during Subsequent Chlorination

The permanganate/bisulfite (PM/BS) process is a novel oxidation process, which can degrade micropollutants within several seconds. As natural organic matter (NOM) ubiquitously exists in an aquatic environment, the PM/BS process will inevitably react with NOM, which may impact the disinfection-by-products (DBPs) formation during subsequent chlorination. This study investigated the effect of PM/BS pre-oxidation of NOM on DBP formation. It was found that TOC removal reached a plateau when the molar ratio of PM to BS was 1:5. Increasing ratios of PM to BS decreased the intensity and area of fluorescence spectroscopy. PM and BS doses, pre-oxidation time, pH of solutions and concentration of Br− impacted the formation potential of various DBPs. PM/BS pre-oxidation decreased the formation of TCM while increasing the yields of N-DBPs, thus increasing the risk of water quality. Calculated toxicity analysis showed that a general increase in CTI was observed with PM/BS pre-oxidation, indicating that PM/BS pre-oxidation had a negative effect on risk control of overall cytotoxicity. Although the PM/BS process could accelerate the degradation of micropollutants, the elevated DBPs formation, especially highly toxic N-DBPs, needs enough attention to control water-quality risk.

Influences of the pre-oxidation time on coal secondary spontaneous combustion behaviors by temperature-programmed technique

ABSTRACT To reveal the influence of pre-oxidation time on coal spontaneous combustion behaviors, temperature-programmed experiments were carried out to analyze the kinetics characteristics of coal primary and secondary spontaneous combustion from the macro viewpoint. By the growth-rate analysis of gaseous compounds, the critical temperature of raw coal (Y0) spontaneous combustion was 70°C. At the critical temperature, isothermal oxidation was performed for 2 h, 4 h, 6 h, and 8 h, and the processed samples were named Y2, Y4, Y6, Y8. It was found that the changing trend of CO concentration increased at first and then decreased with the increase in isothermal pre-oxidation time. Two maximum peak values were found near the 1st and 4th hour in the production of CO2 gas. Spontaneous combustion characteristic parameters and apparent activation energy of oxidized coal showed staged characteristics compared to raw coal. Below 70°C, the order of spontaneous combustion characteristic parameters and the apparent activation energy was Y0< Y2< Y4< Y6< Y8. Above 70°C, the order was Y6< Y8< Y2< Y4< Y0. Isothermal pre-oxidation at the critical temperature could slow down coal secondary low-temperature oxidation and speed up high-temperature oxidation. When isothermal pre-oxidation is at the critical temperature, the pre-oxidation time has double effects on the high-temperature oxidation of oxidized coal.

Insight on Exogenous Calcium/Magnesium in Weakening Pyrite Floatability with Prolonged Pre-Oxidation: Localized and Concomitant Secondary Minerals and Their Depression Characteristics

In this study, we investigated the localized and concomitant precipitation of calcium (Ca)/magnesium (Mg)-bearing species and iron oxides/oxyhydroxides, and their depression characteristics to the pyrite floatability in flotation process at pH 9 and pH 10.5 with prolonged pre-oxidation. Contrary to the depression characteristics at pH 9, the incipient (within aeration times of 30 min) depression of pyrite floatability in Ca/Mg-bearing solutions was more obvious at pH 10.5, while the subsequent decline was only slightly when the pre-oxidation time was expanded to 120 min and 360 min. The competitive adsorption among Ca/Mg-bearing species and potassium amyl xanthate (PAX, C6H11OS2K, collector) at specific sites onto the pyrite surface was demonstrated by the regularly decreased zeta potential of the pyrite surface pretreated in Ca/Mg-bearing solutions. Further scanning electron microscopy-energy dispersive spectrometry demonstrated the concomitant secondary Ca/Mg/Fe-bearing precipitates on the pyrite surface. X-Ray photoelectron spectroscopy suggested strong reprecipitation of iron oxides/oxyhydroxides on the pyrite surface via acid–base complexation among Ca/Mg hydroxy species and iron hydroxy species. Incipient occupation efficiency of specific reaction sites by Ca/Mg-bearing species, which were mainly controlled by the metastable distribution of Ca/Mg hydroxy species and their electrostatic affinity with pyrite surface, was the crucial factor that influenced the competitive adsorption of xanthate and pyrite floatability. More obvious incipient depression at pH 10.5 rather than at pH 9 contributed to more effective Ca/Mg-bearing species and their higher affinity to pyrite surface at pH 10.5. The localized and concomitant precipitation of secondary Ca/Mg/Fe-bearing species leads to a slightly increased hydrophilic coverage upon the pyrite surface, thus a slowly decreased pyrite floatability with increasing pre-oxidation time.

Synthesis of self-regenerating NiAl-Al2O3 composite coatings

The aluminothermic reactions involved either in the mixtures or the stack of NiO and Al powders were elucidated in a previous work (Troncy et al., 2020) [1]. The present work now investigates the formation of potential self-regenerating coatings using the aluminothermic reaction between nickel oxide and aluminum. The use of this exothermic reaction allows the simultaneous formation of both alumina and nickel aluminides. The coatings were obtained by first pre-oxidizing the nickel substrate, then spraying an aluminum slurry before applying an adequate heat treatment. The most promising self-regenerating coatings display a first composite layer, made of an intermetallic compound (Al3Ni or NiAl) and alumina, and a second layer with a higher concentration of oxide. The first layer is expected to play the role of an Al reservoir coating and the second one that of a diffusion barrier to slow down the Al flux to maintain the Al reservoir in the coating. The effects of preoxidation time and of temperature on the resulting coating microstructures are discussed as a function of time.

The Influence of Humidity Content on Ferritic Stainless Steels Used in Solid Oxide Fuel Cell Under Dual Atmosphere Conditions at 600°C

In the last few years, the interest in the development and application of green energy harvesting, storage and production technologies, boosted the research activities to fulfill the demand of better performing, more reliable and cost-effective materials. In the field of Solid Oxides Fuel Cells (SOFC), many efforts have been spent on the interconnects, that are used to electrically connect single cells to obtain stacks for higher output voltages. Interconnects are exposed simultaneously to fuel gas (i.e., H2) and oxidizing (humid air) atmospheres (dual atmosphere) at a working temperatures in the range between 600°C and 800°C. Nowadays, chromia forming ferritic stainless steel (FSS) has been selected as material for interconnects due to its ease of fabrication, affordability and electrical conductivity. In air the material forms a protective chromia layer that reduces further corrosion. However, in dual atmosphere conditions, FSS commonly show fast and unexpected degradation, on the air side, especially at 600°C.The mechanism behind the dual atmosphere effect is still not fully clear, but previous works highlighted the importance of pre-oxidation time, exposure temperature, humidity and surface finishing. Until now, major efforts have been spent in studying the long-term behaviour, with the aim to identify a long-lasting interconnect material. To successfully overcome the effects of dual atmosphere-induced corrosion, it is of fundamental importance to investigate how the corrosion phenomenon starts and evolves. In this work, the initial stages of dual atmosphere breakaway corrosion at 600°C were investigated for selected FSS alloys. The samples were exposed under controlled conditions which resemble those of SOFC (Ar-5%H2 + 3%H2O//FSS//air + 3%H2O). The oxide scales were further investigated by means of Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) analysis. Experiments with different exposure times were performed (i.e., up to 12 hours) and the influence of temperature ramping was studied.Several parameters were evaluated, like the effects of chromium content, the role of sample thickness and the influence of trace elements in different alloys and different batches of the same alloy. Implications of grain orientation at the surface, as well as of pre-oxidation treatment were investigated too.

Tailoring a Phenolic Resin Precursor by Facile Pre-oxidation Tactics to Realize a High-Initial-Coulombic-Efficiency Hard Carbon Anode for Sodium-Ion Batteries.

As the leading anode material for sodium-ion batteries (SIBs), hard carbon (HC) still faces the puzzle of low initial Coulombic efficiency (ICE) in achieving commercialization. From the perspective of precursors, the low ICE has been attributed to the large specific surface area and porosity produced by the rapid decomposition of polymers during the carbonization. Therefore, increasing the cross-linking degree of precursors will be an effective shortcut to improve the ICE. Herein, a facile pre-oxidation tactic was successfully employed to tailor the cross-linking degree of phenolic resin precursors to precisely control the specific surface area of the obtained HC. As the pre-oxidation time is increased, the optimal HC with the lowest specific surface area shows an ICE elevated by 22.2% (from 62.5 to 84.7%) compared to the original pre-oxidation HC and delivers a high reversible capacity of 334.3 mAh g-1 at 20 mA g-1. Besides, the pre-oxidation also introduces abundant carbonyl groups, which increase the disorder degree of HC and supply abundant adsorption sites of Na+, thus enhancing the rate performance. When matched with a layered O3-NaNi1/3Fe1/3Mn1/3O2 cathode, the full cell achieves an energy density of ca. 256.2 Wh kg-1 with superior rate performance. This work sheds light on the positive effect of pre-oxidation in elevating the ICE of HC and provides effective guidance to achieve a high ICE for other HC materials.

Optimization of pre-oxidation time for corrosion resistance of cobalt based coating alloy in mixed sulphates

PurposeThis study aims to optimise the effect of pre-oxidation on hot corrosion behaviour of Tribaloy T-900 at 900 °C in mixed Na2SO4 and K2SO4.Design/methodology/approachPrior to hot corrosion experiment, pre-oxidation treatments were carried in ambient air at 900 °C for 1, 5 and 10 h, respectively. The hot corrosion experiments were performed in a box type furnace at 900 °C. Both surfaces of specimens were brushed with saturated salt solution of 75 wt.% Na2SO4 + 25 wt.% K2SO4. After brushing, the salt-coated specimens were placed in electric stove to ensure drying of salt. After drying, presence of 3 mg/cm2 salt on specimen’s surface was ensured through weighting.FindingsThe 1-h pre-oxidation treatment prior to hot corrosion showed superior hot corrosion resistance against molten salt attack. An optimum pre-oxidation time of 1 h helped timely formation of protective Cr2O3 layer and inhibited the formation of less stable and porous surface oxides of Ni and Mo during hot corrosion.Originality/valuePre-oxidation effect on hot corrosion behaviour of refractory metal (such as Mo in investigated alloy) containing alloy has never been reported previously. Refractory metals oxide (e.g. MoO3) could transform the corrosion phenomena to catastrophic failure through acidic fluxing.

Comparative study of conventional and microwave heating of polyacrylonitrile-based fibres

Abstract The effects of the conventional heating method and the microwave heating method on polyacrylonitrile-based fibres in the temperature range of 180–280 °C were investigated. Fourier transform infrared spectroscopy, X-ray wide-angle scattering, Raman spectroscopy, energy-dispersive spectrometer, scanning electron microscopy and bulk density were used to characterise the properties of the samples. Results show that the microwave heating method can shorten the pre-oxidation time, reduce pre-oxidation temperature and reduce the number of surface defects. The pre-oxidised fibres obtained by the microwave heating method exhibit not only good crystallite size but also a smooth surface. Atomic morphology and molecular arrangement are orderly inside the fibre. The FT-IR spectrum shows that the oxidation reaction occurs at 220 °C, and the CI value of PAN fibers stabilised by microwave heating is the larger than the fibers stabilised by conventional heating. XRD analysis shows that fibers stabilised by microwave heating have low stack domains. The SEM and Raman spectra indicate that hydrogen peroxide can improve the surface finish of the fibers and reduce defects. Microwave heating can reduce the pre-oxidation temperature by about 20 °C and shorten the heating time. The economic benefits of using this method are significantly improved.

Influences of the pre-oxidation time on the microstructure and flexural strength of monolithic B4C ceramic and TiB2-SiC/B4C composite ceramic

B 4 C is a structural ceramic, which is very sensitive to cracks and flaws. The present study aims to improve its mechanical performance using pre-oxidation process. The monolithic B 4 C ceramic and TiB 2 -SiC/B 4 C (B 4 C-TSC) composite ceramic were fabricated using spark plasma sintering technique, and they were pre-oxidized at 800 °C in the air. The effect of pre-oxidation time on microstructure and flexural strength was systematically investigated. The results indicated that 30 min was the optimum pre-oxidation time for both B 4 C ceramic and B 4 C-TSC ceramic. At this pre-oxidation time, the maximum flexural strength of 601.5 and 785.2 MPa were obtained for B 4 C ceramic and B 4 C-TSC ceramic, respectively. The improved flexural strength was primarily ascribed to the formation of glassy oxide layer during the pre-oxidation process and, in turn, the crack healing effect on the surface of the ceramics. Further increasing pre-oxidation time reduced flexural strength due to the pore formation induced by a large amount of CO gas and an intensive evaporation of B 2 O 3 glass phase. • The monolithic B 4 C and TiB 2 -SiC/B 4 C ceramic were fabricated by spark plasma sintering followed by pre-oxidation at 800 °C. • The in-situ formed oxide glass (B 2 O 3 ) can effectively heal the cracks and flaws on the surfaces of the ceramics. • The flexural strength is 43% and 72% increase compared to the corresponding untreated ceramics at 800 °C for 30 min. • The optimum pre-oxidized time is about 30 min as the two kinds of ceramics are processed at 800 °C.

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.