Effect Of Pre-oxidation Research Articles

Leveraging strategies to increase gold cyanidation in the presence of sulfide minerals — Packed-bed electrochemical reactor approach

Several leveraging strategies were explored to improve gold leaching during cyanidation in the presence of pyrite, chalcopyrite, sphalerite and chalcocite. Galena, used a source of lead, was found to largely neutralize the negative effect of sulfide minerals dissolution on gold leaching, especially in the cases of pyrite, chalcopyrite and sphalerite. Pyrite–galena galvanic interactions improved gold leaching while for chalcopyrite, sphalerite and chalcocite active galvanic interactions with galena were found to be detrimental to gold leaching. Pre-oxidation of metal sulfides prior to cyanidation significantly increased gold leaching rate when gold–pyrite galvanic interactions were disabled, while their enablement identified pyrite pre-oxidation as counter-productive. Pre-oxidation of mixed mineral systems of pyrite–chalcopyrite and pyrite–sphalerite showed that enabling galvanic interactions between the associated minerals during pre-oxidation controlled gold leaching in the subsequent cyanidation process. For pyrite, lead nitrate addition during pre-oxidation resulted in a significant increase of gold leaching, whereas its addition to subsequent cyanidation canceled the positive effects of pre-oxidation on gold dissolution.

Effect of moderate pre-oxidation on the removal of Microcystis aeruginosa by KMnO 4–Fe(II) process: Significance of the in-situ formed Fe(III)

This study developed a novel KMnO 4–Fe(II) process to remove the cells of Microcystis aeruginosa, and the mechanisms involved in have been investigated. At KMnO 4 doses of 0–10.0 μM, the KMnO 4–Fe(II) process showed 23.4–53.3% higher efficiency than the KMnO 4–Fe(III) process did. This was first attributed to the moderate pre-oxidation of M. aeruginosa by KMnO 4, achieved by dosing Fe(II) after a period of pre-oxidation, to cease the further release of intracellular organic matter (IOM) and the degradation of dissolved organic matter (DOM). The extensive exposure of M. aeruginosa to KMnO 4 in KMnO 4–Fe(III) process led to high levels and insufficient molecular weight of DOM, inhibiting the subsequent Fe(III) coagulation. Additionally, Fe(II) contributed to lower levels of the in-situ formed MnO 2, the reduction product of KMnO 4 which adversely affected algae removal by Fe(III) coagulation. However, the in-situ formed Fe(III), which was derived from the oxidation of Fe(II) by KMnO 4, in-situ MnO 2, and dissolved oxygen, dominated the remarkably high efficiency of KMnO 4–Fe(II) process with respect to the removal of M. aeruginosa. On one hand, in-situ formed Fe(III) had more reactive surface area than pre-formed Fe(III). On the other hand, the continuous introduction of fresh Fe(III) coagulant showed higher efficiency than one-off dosage of coagulant to destabilize M. aeruginosa cells and to increase the flocs size. Moreover, the MnO 2 precipitated on algae cell surfaces and contributed to the formation of in-situ formed Fe(III), which may act as bridges to enhance the removal of M. aeruginosa.

Effect of pre-oxidation of Fe3Al on its corrosion resistance in sulfur and chlorine contaminated syngas

Pre-oxidation treatment of Fe3Al alloy, a material used in high temperature gas filtration, is common practice to improve corrosion resistance. This work evaluates the various pre-oxidation procedures in terms of the quality of the protective layers formed, using microstructural analysis and corrosion tests. The effect of atmosphere on the oxidation kinetics at 950 °C and the oxide layer morphology was determined. The corrosion resistance of Fe3Al filters was investigated in simulated coal-derived synthesis gas in the temperature range 600–800 °C. It was found that the corrosion resistance of Fe3Al was significantly improved by the development of the surface oxide layer, and that the quality of oxide layer is dependent on the oxidation treatment conditions, such partial oxygen pressure, pO2. The best corrosion resistance was achieved with a two-stage oxidation treatment, involving initial pre-oxidation at low pO2 and final pre-oxidation at high pO2. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.

The effects of pre-oxidation on the sintering and mechanical property of powder injection moulded SiC material

Usually, injection moulded SiC green parts are debound in inert atmosphere or vacuum, which induces the residual carbon and increases forming cycle and production cost. In this paper, injection moulded SiC with Al 2O 3 and Y 2O 3 as sintering assistant was thermal debound in air and Ar, respectively. The paper investigates the effects of pre-oxidation during debinding stage on the sintering and mechanical property of SiC material. During sintering, the oxide SiO 2 is in favour of the shrinkage of debound samples at lower temperature. After sintering, the linear shrinkage of sintered samples with pre-oxidation is bigger than the sample without pre-oxidation. Test results by TEM and XRD indicate that SiO 2 disappear from the inside of the sintered samples. The loss of SiO 2 decreases the content of Al 2O 3, which affects the formation of YAG (Y 3Al 5O 12). Sintered Sic samples contain α-SiC phase and intergranular phase. There is no hetero-phase between the boundaries of α-SiC phase and intergranular phase. The bending and compression strength values of sintered samples with pre-oxidation reach to 537 MPa and 2.89 GPa, respectively. These values approach the strength of sintered samples without pre-oxidation (594 MPa and 3.0 GPa).

Effects of Potassium Permanganate Preoxidation Followed with Coagulation on the Fluorescence Spectrum of Algae

This study was performed to illustrate the effects of preoxidation by potassium permanganate on the fluorescence spectrum of Microcysis aeruginosa, a kind of blue-green algae. The results showed there were four dominate excitation/emission (Ex/Em) wavelength pairs, 230/334, 280/312, 280/334 and 620/642 nm/nm in the fluorescence spectra of algae and their derived organic matters (AOM). Coagulation resulted in desorption of the organic matter adsorbed in algae cells and produced two new fluorophores centered at 275/460 and 400/460 nm/nm. Potassium permanganate preoxidation could reduce the fluorescence intensity of algae and dissolved algal organic matters effectively and the descent rate increased with increasing potassium permanganate dosage.

Effects of Platinum on the Hot Corrosion Behavior of Hf-Modified γ′-Ni3Al + γ-Ni-Based Alloys

The establishment of a protective α-Al2O3 scale is critical for providing high temperature protection from oxidation and hot corrosion, thereby improving lifetimes of advanced gas turbine engine components. Recent work by our group has shown that a wide range of Pt + Hf-modified γ′-Ni3Al + γ-Ni alloy compositions form a very adherent and slow-growing Al2O3 scale and exhibit excellent oxidation resistance. The main thrust of the present study was to understand the effects of Pt addition on the Type I (900 °C) and Type II (705 °C) hot corrosion (HC) behavior of model Hf-modified γ′ + γ alloy compositions. The salt used to bring about hot corrosion was Na2SO4. It was found that the Type I HC resistance of γ′ + γ alloys improved with up to about 10 at.% Pt addition, but then decreased significantly with increasing Pt content up to 30 at.% (the maximum level studied); however, under Type II HC conditions the resistance of γ′ + γ alloys progressively improved with increasing Pt content up to 30 at.%. The effect of pre-oxidation on hot corrosion resistance was also examined, and the results indicated that pre-oxidation generally improved Type II HC resistance for the test duration studied.

Mercury removal from aqueous solution by adsorption on activated carbons prepared from olive stones

The textural characterization of a series of activated carbons prepared from olive stones, by carbonization at different temperatures (400, 550, 700 and 850 °C) and thermal activation with CO2, has been investigated using N2 adsorption at −196 °C and CO2 adsorption at 0 °C. The effect of pre-oxidation of the carbonized precursor has also been studied, using temperature-programmed decomposition (TPD), to evaluate the effect of oxygen content of the chars in the performance of the obtained activated carbons for mercury removal. The adsorption of Hg(II) cations from aqueous solutions at room temperature by the prepared activated carbons was studied. Experimental results show that all samples exhibit a large microporosity (pore diameter below 0.56 nm). The amount of surface oxygen groups increased after pre-oxidation treatment, this enhancing the Hg(II) uptake (up to 72%). It can be concluded that these groups make the support more hydrophilic, thus providing a more efficient adsorption of Hg(II). The formation of a great amount of surface oxide groups such as carboxyl, phenol and lactone alters the surface charge properties of the carbon, this enhancing the surface-Hg(II) interaction.

Effect of chlorination and ozone pre-oxidation on the photobacteria acute toxicity for dissolved organic matter from sewage treatment plants

Effect of chlorination and ozone pre-oxidation on the photobacteria acute toxicity for dissolved organic matter (DOM) from sewage treatment plants was investigated in the study. Results showed that ozone pre-oxidation enhanced the photobacteria acute toxicity of the water samples. DOM before and after ozone pre-oxidation were fractionated by resins into six kinds of hydrophobic and hydrophilic organics. The six fractions were chlorinated individually and the photobacteria acute toxicity before and after chlorination was tested. It was found that the percentage of hydrophilic organics in DOM significantly increased after ozone pre-oxidation and hydrophilic organics exhibited remarkably higher acute toxicity that hydrophobic organics. In view of potentiometric titration and fourier transform infra-red analysis, the hydrophilic organics showed a rather higher content of ph?OH structure than hydrophobic organics.

HCl-Induced High Temperature Corrosion of Stainless Steels in Thermal Cycling Conditions and the Effect of Preoxidation

Gaseous HCl released during combustion is one reason for the severe materials degradation often encountered in power generation from waste and biomass. In this study, three stainless steels (the low alloyed EN 1.4982, the standard EN 1.4301 and the higher alloyed EN 1.4845) were tested by repeated thermal cycling in an environment comprising N2–10%O2–5%H2O–0.05%HCl at both 400 and 700 °C. The materials were exposed with ground surfaces and preoxidised at 400 or 700 °C. A positive effect of preoxidation is evident when alloys are exposed at 400 °C. Oxide layers formed during preoxidation effectively suppress chlorine ingress for all three materials, while chlorine accumulation at the metal/oxide interface is detected for surface ground specimens. The positive effect of preoxidation is lost at 700 °C and corrosion resistance is dependent on alloying level. At 700 °C metal chloride evaporation contributes significantly to the material degradation. Based on the results, high temperature corrosion in chlorinating environments is discussed in general terms.

Ammonia modification of activated carbon to enhance carbon dioxide adsorption: Effect of pre-oxidation

A commercial granular activated carbon (GAC) was subjected to thermal treatment with ammonia for obtaining an efficient carbon dioxide (CO2) adsorbent. In general, CO2 adsorption capacity of activated carbon can be increased by introduction of basic nitrogen functionalities onto the carbon surface. In this work, the effect of oxygen surface groups before introduction of basic nitrogen functionalities to the carbon surface on CO2 adsorption capacity was investigated. For this purpose two different approaches of ammonia treatment without preliminary oxidation and amination of oxidized samples were studied. Modified carbons were characterized by elemental analysis and Fourier Transform Infrared spectroscopy (FT-IR) to study the impact of changes in surface chemistry and formation of specific surface groups on adsorption properties. The texture of the samples was characterized by conducting N2 adsorption/desorption at −196°C. CO2 capture performance of the samples was investigated using a thermogravimetric analysis (TGA). It was found that in both modification techniques, the presence of nitrogen functionalities on carbon surface generally increased the CO2 adsorption capacity. The results indicated that oxidation followed by high temperature ammonia treatment (800°C) considerably enhanced the CO2 uptake at higher temperatures.

Effect of pre-oxidation on microcracks in graphite foams

The formation mechanisms of microcracks in graphite foams were investigated by varying the preoxidation temperature of the precursor mesophase pitch. Results indicated that after the mesophase pitch was preoxidized, its quinolineinsoluble component became the dominant fraction, which leads to a decrease in heat stress gradient and as a result, a decrease in the amount, length and width of microcracks.

Effect of Pre-Oxidation at 800 °C on the Pitting Corrosion Resistance of the AISI 316L Stainless Steel

In situ X-ray diffraction was used to identify the oxides formed on the AISI 316L (containing 2% Mo) stainless steel during isothermal oxidation at 800 °C, in air. Good oxidation behavior was observed on this steel when considering kinetics, structural characteristics and scale adherence. It was demonstrated that molybdenum plays a protective role in that it hinders the outward iron diffusion and leads to the lower growth rate and the better scale adherence. The oxide scale was then composed of Cr2O3 with a small amount of Mn1,5Cr1,5O4 at the external interface. Pre-oxidation of the AISI 316L also improved its aqueous corrosion resistance. No pitting corrosion occured during the corrosion test. Aqueous corrosion testing also showed that the oxide scale formed at 800 °C is crack-free and still adherent after cooling to room temperature.

Effect of pre-oxidation on the hot corrosion of CoNiCrAlYRe alloy

The effect of pre-oxidation on the resistance to hot corrosion was examined by corroding the CoNiCrAlYRe alloy at 900 °C in molten Na 2SO 4. Preoxidized specimens featured strong adhesion of oxide scale with uniform multi-layered structure. The time of pre-oxidation was crucial for controlling Al content sufficient for subsequent hot corrosion. However, direct corrosion yielded a defective and non-protective oxide scale, which allowed detrimental penetration of sulfur into substrate. Sulfur migrating along phase boundary was trapped by yttrium to diminish slightly sulphidation. Thus, two advantages of proper pre-oxidation treatment were presented, as keeping repairing for Al 2O 3 scale and inhibiting sulfur penetration.

ChemInform Abstract: The Effect of Preoxidation of Chromia and Alumina Forming Alloys on Erosion in Laboratory Simulated Fluidized-Bed Conditions.

ChemInform Abstract: The Effect of Preoxidation of Chromia and Alumina Forming Alloys on Erosion in Laboratory Simulated Fluidized-Bed Conditions.

Effect of chlorination and ozone pre-oxidation on the photobacteria acute toxicity for dissolved organic matter from sewage treatment plants

The effect of chlorination and ozone pre-oxidation on the photobacteria acute toxicity for dissolved organic matter (DOM) from sewage treatment plants was investigated in this study. The results show that ozone pre-oxidation enhanced the photobacteria acute toxicity of the water samples. DOM before and after ozone pre-oxidation was fractionated by resins into six kinds of hydrophobic and hydrophilic organics. The six fractions were chlorinated individually and the photobacteria acute toxicity before and after chlorination was tested. It was found that the percentage of hydrophilic organics in DOM significantly increased after ozone pre-oxidation and hydrophilic organics exhibited remarkably higher acute toxicity than hydrophobic organics. In view of potentiometric titration and fourier transform infrared (FTIR) analysis, the hydrophilic organics showed a rather higher content of ph-OH structures than hydrophobic organics.

Synergic Removal on <i>Chironomus kiiensis larvae</i> in Urban Water Source with Pre-Oxidation

Chironomus kiiensis larvae which cannot be exterminated by conventional disinfection process propagates prolifically in eutrophic water body, and it therefore turns to be a potential problem encountered in drinking water quality. In this work, the quantitative experimental studies were carried out on removal of Chironomus kiiensis larvae in raw water by coagulation-sedimentation process. The coagulation jar test showed that the Chironomus kiiensis larvae could be partially removed from water by coagulation-sedimentation process. Based on it, removal effect of pre-oxidation combined with coagulation-sedimentation process on Chironomus kiiensis larvae was evaluated. The results showed that chlorine dioxide pre-oxidation possessed better removal performance than prechlorinion, and Chironomus kiiensis larvae in the raw water could be completely removed by chlorine dioxide pre-oxidation in combination with the coagulation-sedimentation process at chlorine dioxide dosage of 0.8mg/L. The pH in the range of 6-8 did not affect the inactivation efficiency of chlorine dioxide, whereas pH 10 resulted in around 10% decrease in removal rate. Meanwhile, the removal rate of Chironomus kiiensis larvae improved with the temperature increasing within the range investigated of 15-30°C. The removal rate was reduced by 6.7% when temperature reduced from 30°C to 15°C.

Effect of precursor preoxidation on the structure of phenolic resin-based activated carbon spheres

Activated carbon spheres with 3D hierarchical porous structure were prepared from phenol–formaldehyde resins with oxidation treatment in air and physical activation in an inert atmosphere. Based on the results of thermogravimetric analysis, infrared spectrometer (IR), scanning electron microscopy (SEM) and nitrogen adsorption/desorption, the effect of preoxidation on the morphology and structure of activated carbon spheres was investigated. The results show that decomposition and crosslinking reactions occur during the preoxidation and the structural changes of precursor generated by the preoxidation lead to differences in the pore structure of activated carbon spheres. The carbon spheres exhibit the unique 3D hierarchical porous structure, high specific surface area of 1897 m 2/g and high pore volume of 2.22 cm 3/g.

Electrochemical behavior of gold cyanidation in the presence of a sulfide-rich industrial ore versus its major constitutive sulfide minerals

Abstract A detailed study on the relative importance of passivation phenomena and galvanic interactions during gold cyanidation was carried out. Mineral disc electrodes consisting of a sulfide-rich industrial ore and major sulfide components were prepared along with an Au electrode (gold/silver alloy) in use for gold leaching rate tests. These leaching tests that were conducted by hyphenating gold and mineral disc electrodes conjointly in one electrochemical cell or in two separate electrochemical cells objectified both passivation-induced setbacks as well as boosts by Au/mineral galvanic interactions on gold dissolution. To decipher the role of sulfide ores on gold cyanidation, a systematic study was performed by monitoring the leaching behavior of an Au disc electrode successively immersed in slurries of industrial ore and its major sulfide constituents, i.e., pyrite, sphalerite and chalcopyrite. The tested mineral constituents and ore exhibited an inhibiting effect on gold leaching, decreasing in the following order: chalcopyrite > sphalerite > industrial ore > pyrite. Pre-oxidation of the industrial sulfide ore prior to cyanidation improved the gold leaching rate. However, in spite of noticeable reductions in cyanide consumption, no beneficial effect of pre-oxidation on gold leaching was observed for the major sulfide (ore) constituents when tested separately. Although cooperating permanent galvanic interactions between gold and main constitutive minerals in the industrial ore prompted higher gold leaching rates, predictability of the latter from lab-controlled leach tests of the nearly pure constitutive sulfide minerals still remain premature.

The Effects of Pre-Oxidation and Alloy Chemistry of Austenitic Stainless Steels on Glass/Metal Sealing

An oxidation treatment is often performed on austenitic stainless steel prior to joining to alkali barium silicate glass to produce hermetic seals. The thin oxide formed during this pre-oxidation step acts as a transitional layer and a source of Cr and other elements that diffuse into the glass during the subsequent bonding process. Pre-oxidation is performed in a low pO2 atmosphere to avoid iron oxide formation; the final oxide is composed of Cr2O3, MnCr2O4 spinel, and SiO2. Significant heat-to-heat variations in the oxidation behavior of austenitic stainless steel were observed in this work, resulting in inconsistent glass/metal seal behavior. Twenty-five (25) stainless steel heats were examined including 304L, 316L, and experimental high sulfur alloys similar to 303SS. The objectives were to characterize the oxidation kinetics, the oxide morphologies, and compositions that affect glass/metal adhesion. It was found that poor glass sealing is associated with a more continuous layer of SiO2 at the metal/oxide interface. The effects of alloy chemistry, in particular Mn and Si concentrations, on glass/metal sealing behavior were empirically determined. A criterion based on the Mn/Si ratio was developed for use in selecting heats with good glass/metal bonding characteristics. To test this criterion, four other austenitic stainless steels were evaluated: 21-6-9 (also known by original Armco Steel Co. trade name Nitronic® 40), 22-13-5 (Nitronic® 50), Nitronic® 60, and Gall-Tough® (Carpenter Technology Corp.). These alloys have compositions significantly different from 300-series alloys, but they were still found to comply with the compositional guidelines developed for predicting glass/metal adhesion.

Effect of chemical preoxidation coupled with in-line coagulation as a pretreatment to ultrafiltration for algae fouling control

Algae fouling can cause a transmembrane pressure (TMP) increase or fl ux decrease during ultrafi ltration of surface waters. In this study, chemical preoxidation coupled with in-line coagulation was investigated as a pretreatment step for algae fouling control. The coupled strategy was able to control the fl ux decline. Also, the treated water quality could be improved. Chemical preoxidation by potassium permanganate composites (PPC) and chlorine (Cl>2) removed algae cells by both cell death and adsorption, which could also alleviate the load on the ultrafi ltration unit. During the coupled treatment, the electrostatic forces between algae cells and the fl ocs weaken. The cells could be packed by in situ formed hydrous manganese dioxide and fl ocs. The fl ocs would be trapped on the cake layer and the algae fouling for ultrafi tlration could be controlled.