CO-CO2 Atmosphere Research Articles

Carbon deposition on iron surfaces in CO–CO2 atmosphere

Thermodynamic calculations and thermogravimetric (TG) analysis were performed in order to understand the mechanism of carbon deposition on the surfaces of iron particles during the reduction of iron ore in a CO–CO2 atmosphere. The results of the thermodynamic equilibrium phase analysis indicate that the phases of the carbon deposition process can be predicted on the basis of the carbon potential, reaction temperature and gas pressure. The optimal thermodynamic conditions for carburisation are a low temperature (T < Tm) and a high carbon potential (αc>1). TG analysis is performed in a gas mixture of 65 vol.-% CO and 35 vol.-% CO2 at 650, 706 and 750°C. Cementite (Fe3C) is generated as an intermediate product, which acts as a catalyst for carbon deposition. Carbon deposition is inhibited at high temperatures (T>791°C) owing to the high stability of Fe3C. When the reaction temperature is higher than the thermodynamic limit for the formation of Fe3C, carbon deposition cannot occur. A mechanism for carbon deposition is proposed based on the experimental results.

Formation of epigenetic graphite inclusions in diamond crystals: experimental data

To elucidate the conditions of formation of epigenetic graphite inclusions in natural diamond, we carried out experiments on high-temperature treatment of natural and synthetic diamond crystals containing microinclusions. The crystal annealing was performed in the CO–CO2 atmosphere at 700–1100 °C and ambient pressure for 15 min to 4 h. The starting and annealed diamond crystals were examined by optical microscopy and Raman spectroscopy. It has been established that the microinclusions begin to change at 900 °C. A temperature increase to 1000 °C induces microcracks around the microinclusions and strong stress in the diamond matrix. The microinclusions turn black and opaque as a result of the formation of amorphous carbon at the diamond–inclusion interface. At 1100 °C, ordered graphite in the form of hexagonal and rounded plates is produced in the microcracks. A hypothesis is put forward that graphitization in natural diamond proceeds by the catalytic mechanism, whereas in synthetic diamond it is the result of pyrolysis of microinclusion hydrocarbons. The obtained data on the genesis of graphite microinclusions in diamond are used to evaluate the temperature of kimberlitic melt at the final stage of formation of diamond deposits.

Effect of coexisting inorganic chlorides on lead volatilization from CaO–SiO2–Al2O3 molten slag under municipal solid waste gasification and melting conditions

The effect of inorganic chlorides on volatilization of lead compounds from CaO–SiO2–Al2O3 molten slag under simulating conditions of gasification and melting of municipal solid waste was studied.It was found from the experiments that the volatilization of lead compounds from CaO (30wt.%)–SiO2 (50wt.%)–Al2O3 (20wt.%) molten slag in a lab-scale electric furnace at 1773K in N2, N2–O2 or N2–CO–CO2 atmosphere was promoted by the addition of NaCl, KCl and CaCl2 to the slag containing PbO, in the order of CaCl2>NaCl>KCl. The rate of volatilization and the amount of volatilized lead compounds were the highest by employing CaCl2 in N2–CO–CO2 atmosphere, due to combined reduction of PbO to metallic lead (Pb) and chlorination of PbO to PbCl2. By heating the molten slag at 1773K for about 10min, almost 100% of PbO was removed by adding CaCl2 to the slag at a molar ratio of (Cl)/(Pb)=27. However, the heating of molten slag at 1773K for 10min was not sufficient for complete removal of PbO, even by adding an excessive amount of NaCl and KCl to the slag at a higher molar ratio of (Cl)/(Pb)=110.From the equilibrium calculations on the main elements involved in municipal solid waste in shaft-type gasification/melting furnace, almost 100% of lead compounds were predicted to volatilize as Pb, PbS and PbCl2. The ratio of PbCl2 increased with the increase in chlorine (Cl) content in the waste. It was considered by equilibrium calculations that increased volatilization of lead compounds was attributed to the pronounced formation of PbCl2 by the reaction of PbO with HCl which was released from NaCl, KCl and CaCl2.

Thermodynamics of Selenium and Tellurium in Calcium Ferrite Slags

New measurements have been made on the equilibrium distribution of selenium and tellurium between molten slag and metal phases under a controlled CO-CO2 atmosphere. Distribution coefficients of Se and Te between magnesia-saturated calcium ferrite slag and copper or silver alloy were measured at 1473 to 1673 K and oxygen patrial pressures of 10−11 to 10−0.68 atm. For the calcium ferrite slag, the highest distribution coefficients were recorded at the highest lime content, under strongly reducing conditions and at higher temperature. The selenide and telluride capacities were also calculated for each slag and were found to increase with increasing temperature and increasing lime activity. Activity coefficients of Se in the slag phase were found to be much lower than those of Te, and values for both minor elements decreased considerably with increasing activities of lime and ferrous oxide in the slag. Selenium showed strong negative deviation from the ideal behavior, while Te showed positive deviation up to 1573 K.

High-Temperature Oxidation Behavior of Chemical Vapor Deposited Silicon Carbide.

The transition behavior from active to passive oxidation and from passive oxidation to bubble formation for CVD SiC was studied by thermogravimetry at wide-ranged oxygen partial pressures in O2-Ar, CO2-Ar and CO-CO2 atmospheres at temperatures from 1600 to 2000 K. In O2-Ar, the mass loss rate (i.e., active oxidation rate) increased with increasing PO2, and at a critical PO2 the active-to-passive transition (i.e., mass loss to mass gain) was clearly observed. In CO-CO2, the mass loss was significant at a low PCO2/PCO ratio and then the mass loss decreased with increasing PCO2/PCO ratio. The transition from mass loss to mass gain was gradual, being contrast to that in O2-Ar. The transition mechanism in O2-Ar and CO-CO2 was explained by Wagner model, volatility diagram and numerical thermodynamic calculation.The transition from passive oxidation to bubble formation was observed at more than 1985K in O2-Ar at PO2>5kPa. At PO2<5kPa the transition temperature decreased with decreasing PO2 in O2-Ar and CO2-Ar. The bubble formation might be initiated by the stoichiometric reaction between SiO2 and SiC forming SiO and CO vapors at the SiO2/SiC interface. The transition temperature could be calculated from the condition that the equilibrium SiO and CO vapor pressures exceed the ambient pressure.

High photorefractive sensitivity at 860 nm in reduced rhodium-doped KNbO_3

We show that high-temperature reduction in a CO-CO(2) atmosphere increases the photorefractive sensitivity of KNbO(3):Rh at 860nm by 4 orders of magnitude compared with that of the as-grown crystal. The effective trap density is increased by a factor of 3, and the photoconductivity by a factor of 30, and the photorefractive response at a grating spacing of 0.15 mu;m is accelerated by a factor of 400. The grating buildup time at a grating spacing of 0.7 microm and an intensity of 1Wcm(-2) is 0.5 s, a value comparable with that of as-grown KNbO(3):Fe at visible wavelengths. The optical and photorefractive parameters of Rh-doped KNbO(3) subjected to reduction treatment are characterized for wavelengths of 0.48-1.064 microm .

CO-CO&lt;SUB&gt;2&lt;/SUB&gt;雰囲気における固体鉄への浸炭速度

In order to construct a mathematical model of a moving bed type ironmaking process using iron scrap as the burden materials, the carburization rate from CO-CO2 gas mixture into solid iron was studied experimentally and theoretically.The mass change of an iron sample during carburization was measured by a thermo-balance over a temperature range of 973 to 1423K and CO concentration of 70 to 100%. In CO atmosphere, carburization rate increased significantly with temperature over 973K, however it decreased over 1273K. In CO-CO2 atmosphere, carburization scarcely proceeded over 10% CO2 concentration at 1273K.The carburization rate equation was derived by considering CO dissociation and CO2 formation as two elemental processes. Rate constants were obtained from experimental data. The rate equation obtained agreed well with experimental results.

Active‐to‐Passive Transition and Bubble Formation for High‐Temperature Oxidation of Chemically Vapor‐Deposited Silicon Carbide in CO–CO2 Atmosphere

Oxidation behavior of chemically vapor‐deposited SiC in CO─CO2 atmospheres (0.1 MPa) was investigated using a thermogravimetric technique at temperatures from 1823 to 1923 K. Active or passive oxidation was observed depending on temperature and CO2/CO partial pressure ratio (Pco2/Pco). The critical Pco2/Pco value for the transition was 1O2 times as large as a theoretical value calculated from the Wagner model. In the passive oxidation above 1873 K, SiO2 bubbles were grown. The expansion and rupture of bubbles caused cyclic rapid mass gain and mass loss.

The solubility of copper in lime-saturated and calcium ferrite-saturated liquid iron oxide

The solubility of copper in lime-saturated and calcium ferrite-saturated liquid iron oxide has been measured at 1300 °C by equilibrating copper-gold alloys with the melts in CO-CO2 atmospheres of oxygen potentials 10−7, 10−8, 10−9, and 10−10 atm. It was found that copper exhibits Henrian behavior in the oxide melts and that the solubility is given by $$wt pct Cu = 17.6a_{CuO_{0.5} } $$

Oxidation of SiC-ZrO2 composites

Planar silicon carbide-zirconium oxide composites were oxidized at temperatures between 910 and 1050 °C in oxygen, argon and CO-CO2 atmospheres. In addition, bare SiC substrates were oxidized in oxygen. The bare substrates showed parabolic oxidation kinetics. The oxidation kinetics of the composites were more difficult to interpret because of the solid state reaction between the ZrO2 and the SiO2, but were approximately parabolic.

Carbon transport through oxide scales on Fe-Cr alloys

The penetration of carbon through Cr2O3 layers was studied for a series of different Fe-Cr alloys using a radioactive tracer method. Preoxidized samples were exposed at 900°C for 700 hr in a H2 -H2O -CO-CO2 atmosphere tagged with14C; carbon penetration profiles were then determined, and the lateral distribution of carbon was observed by autoradiography. Even minute amounts of carbon (⩾0.05 ppm) within the scale and in the alloy could be detected. The carbon uptake into different Fe-Cr alloys decreased with increasing Cr content to a minimum for the alloys with 12.5–20% Cr, indicating low porosity and good adherence of the Cr2O3 layers. Poor scale adherence was observed for Fe-10% Cr but could be improved by Ce additions. Porosity increased with contents >20% Cr of the alloys. Pore formation could be induced by impurities, e.g., SiC particles distributed on the surface.

Interfacial rates of reaction of CO2 with liquid iron silicates, silica-saturated manganese silicates, and some calcium iron silicates

Measurements of the rates of dissociation of CO2 have been made by the14CO2-CO isotope exchange technique on liquid iron silicates, calcium iron silicates, and silica-saturated manganese silicates as functions of temperature and imposed equilibrium CO2/CO ratio. It is shown that the rates of reduction of the liquid iron silicates and an iron oxide-rich slag in CO-CO2 atmospheres are consistent with the rates of isotope exchange, indicating a common rate determining step. The dependences of the apparent first order rate constant on the oxygen activity for the dissociation of CO2 on silica-saturated iron silicates and an equimolar “FeO”-CaO-SiO2 melt are found to be closely consistent with the ability to transfer two charges to the adsorbing or dissociating CO2 molecule, as was previously found for liquid iron oxide and lime-saturated calcium ferrites. Apparent rate constants at fixed oxygen activity are found to increase generally with the basicities of the melts.

The effect of the nature of the gas phase on the surface properties of liquid tin in contact with the (100) face of solid electrolyte ZrO2Y2O3

Abstract Using the method of measuring the electrolytic conductivity G between the electrode being investigated and the auxiliary electrode as a function of the potential φ, a study has been made of the surface properties of liquid tin in contact with solid electrolyte 0.90 ZrO20.10 Y2O3 in an atmosphere of COCO2 as well as in helium. The G-φ curves obtained exibit two free surface energy (FSE) maxima in the gas mixture COCO2 at 900°C and only one FSE maximum in the helium atmosphere. In the COCO2 atmosphere the transition from the (100) face of the solid electrolyte 0.90 ZrO20.10 Y2O3 single crystal to the sintered material displaces the potential of the first maximum by 0.07 V towards the negative region, that of the second maximum by 0.06 V. It has been found that the change of the gas phase COCO2 for the gas phase H2H2O (hydrogen with an insignificant admixture of water) affects very largely the magnitude of the potentials that correspond to the FSE maximum. The latter circumstance is a demonstration that the adsorption phenomena play a significant part.

Distribution Equilibria of Minor Elements between Liquid Copper and Calcium Ferrite Slag

The distribution of Ag, Co, Ni, Zn, Sn, Pb, As, Sb and Bi between calcium ferrite slag and liquid copper was measured at 1523 K under controlled CO–CO2 atmospheres. The distribution ratios, defined by %X in slag/%X in metal, were plotted against the oxygen potential and compared with those obtained for silicate slag. From the slopes of such plots, reasonable oxide forms dissolved in slag, XOv, were estimated. The increasing content of CaO in ferrite slag results in increase in the distribution ratio for As or Sb, but in decrease in the ratio for Pb. The effects of the composition change in the alloy phase and the temperature dependence on the distribution ratios were evaluated thermodynamically. It was proved that a mono-nuclear atom base expression is reasonable to express the oxide form dissolved in slag, such as CuO0.5, AsO1.5 etc. Moreover, from the iron contents in a Cu-X alloy, the interaction parameters between Fe and X in liquid copper were derived.

Analysis of the Activity of as in FeO–Fe2O3–SiO2Slag and the Distribution of as Between Slag and Molten Copper

AbstractThe distribution coefficient of arsenic between slag and copper and the activity of arsenic in silica saturated FeO-Fe2O3-SiO2 slag were measured at temperatures between 1473 and 1536 K. The distribution experiments were conducted under a controlled CO–CO2 atmosphere with an oxygen potential of 10−10 atm. The observed distribution coefficient for arsenic was found to be 250; the coefficient being defined as the ratio of the mole fraction of As in the metal to that in the slag, assuming the slag to be the FeO-FeO1.5–SiO2 system. Temperature had no discernible effect on the distribution coefficient, although the measured values of the coefficient were subject to considerable scatter due to analytical error.The activity of arsenic in slag was measured by equilibrating arsenic vapor with slag sealed in an evacuated tube. Analysis of the equilibrium constant for the chemical reaction X ·As2(g) = As2X (slag)yields:logN As = log [2XK eq/γ°As2X (P°As2 )X ] + X log P As2 .where N As is the mole fraction of...

Distribution equilibria of Sn, Se and Te between FeO-Fe2O3-SiO2-AI2O3-CuO0.5 slag and metallic copper

The distribution of tin, selenium and tellurium between alumina-containing fayalitic slags and metallic copper was measured at 1200 and 1300°C under controlled CO-CO2 atmosphere with oxygen partial pressure (pO2) in the rangePO2 = 10-6 to 10-11 atm (1 atm = 1.013 x 102 kPa). The solubility of Sn in slag was observed to increase linearly with increasing P1/2O2. It was deduced that Sn is present in the slag in the form of SnO or Sn2 and the activity coefficient of SnO in the slag was calculated to be 1.9 at 1200°C and 0.8 at 1300°C. The solubility of Se in the slag decreases with increasing oxygen partial pressure up topO2l = 4 x 10−8 atm, but above this oxygen partial pressure it becomes practically constant and the ratio (pet Se in slag/pet Se in copper) = 0.018 (at 1200°C) and 0.036 (at 1300°C). The solubility of Te shows a similar variation with oxygen partial pressure and the ratio (pet Te in slag/pet Te in copper) = 0.026 (at 1200°C) and 0.032 (at 1300°C) abovepol2 = 106 atm. A concept of molecular dissolution of chalcogen elements in slag was developed on the basis of thermodynamic properties of slag, and the observed solubility of Se and Te was explained in terms of the chemical stability of the molecular cluster FeSe and FeTe iη the slag.

Structural changes and kinetics in the gaseous reduction of hematite

The mechanism of the gaseous reduction of hematite grains to magnetite was studied. Gravimetric measurements were carried out for the reduction of Carol Lake hematite pellets and grains in CO-CO2 atmospheres over the temperature range 500 to 1100°C. The pore size distribution in the reduced magnetite was measured by mercury porosimetry. Partially reduced grains were examined by optical microscopy. At temperatures below 800°C, the reduction of a hematite grain to magnetite occurred at a well-defined shrinking-core interface. The average pore size in magnetite formed at 600°C was found to be 0.03 μm. An estimate of the rate of CO diffusion through pores of this size indicated that the reaction rate at 600°C was controlled by a step near the hematite-magnetite interface. At temperatures above 800°C, the reaction mechanism became altered due to the preferential growth of magnetite along a single direction in each hematite grain. The reduction rate decreased with an increase in temperature, and no microporosity was present in magnetite formed at 1000°C and above. It was postulated that the reaction rate was controlled by the rate of formation of fresh nuclei and by their rate of subsequent growth.

High resistivity BaTiO3 ceramics sintered in CO-CO2 atmospheres

A barium titanate ceramic containing 13.5 mol % calcium zirconate was doped with up to 3 mol % of oxides of various metals that were considered likely on the basis of ionic size and valence to enter the small cation lattice with charge less than 4+. It was hoped in this way to compensate electrically for loss of oxygen during sintering in CO-CO2 mixtures, so as to obtain high resistivity dielectrics and allow the use of base metal electrodes in a monolithic capacitor. Doping with approximately 0.5 mol % Mn, Co, or Mg produced the highest resistivities, and dielectrics with nickel electrodes and relative permittivity up to 10 000 were obtained with resistivity in excess of 1012 Ω cm at room temperature. When the doped ceramic sintered in contact with nickel, the grain structure and permittivity-temperature characteristics depended on the oxygen partial pressure of the sintering atmosphere, apparently influenced by dissolution of Ni into the ceramic.

ChemInform Abstract: EFFECT OF OXYGEN ON THE REACTION BETWEEN COPPER AND SAPPHIRE

AbstractDer Einfluß des Sauerstoffpotentials auf die Benetzung und die Grenzflächenspannung zwischen geschmolzenem Cu und Sapphir wird in einer CO‐CO2‐Atmosphäre untersucht.

Effect of Oxygen on the Reaction Between Copper and Sapphire

The effect of oxygen potential on the wetting behavior and interfacial energy between Cu and sapphire was studied using the sessile drop technique in a CO‐CO2 atmosphere. A linear relation was found between γSL and logpO2 (atm) from 10−16 to 10−5. Beyond 10−5 atm γSL approached a constant value asymptotically. A barrier surface layer was proposed to explain this change. The Gibbs adsorption equation was used to evaluate the characteristics of the interfaces. Formation of a Cu2O film at the liquid‐vapor interface and a CuAlO2 film at the solid‐liquid interface is suggested. The work of adhesion reached a maximum at ∼ 0.01 at.% oxygen, corresponding to pO2∼ 10−9atm. Measurements of the basal radius as a function of oxygen content were used to evaluate the role of oxygen in promoting spreading. Spreading on sapphire is directly proportional to the logarithm of oxygen present in the molten Cu drops.