Showing papers on "Chromium(III) oxide published in 1998"

Solubility and phase behavior of Cr(III) oxides in alkaline media at elevated temperatures

Abstract: A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of Cr2O3 and FeCr2O4 in alkaline sodium phosphate, sodium hydroxide, and ammonium hydroxide solutions between 21 and 288°C. Baseline Cr(III) ion solubilities were found to be on the order of 0.1 nmolal, which were enhanced by the formation of anionic hydroxo and phosphato complexes. At temperatures below 51°C, the activity of Cr(III) ions in aqueous solution is controlled by a Cr(OH)3·3H2O solid phase rather than Cr2O3; above 51°C the saturating solid phase is γ-CrOOH. Measured chromium solubilities were interpreted via a Cr(III) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from least-squares analyses of the data. The existence of four new Cr(III) ion complexes is reported: Cr(OH)3(H2PO4)−, Cr(OH)3(HPO4)2−, Cr(OH)3(PO4)3−, and Cr(OH)4(HPO4)-(H2PO4)4−. The last species is the dominant Cr(III) ion complex in concentrated, alkaline phosphate solutions at elevated temperatures.

Kinetics of chlorination and oxychlorination of chromium (III) oxide

Abstract: Thermogravimetric analysis (TGA) is used to study the kinetics of chlorination of Cr2O3 with Cl2+N2 and Cl2+O2 gas mixtures in the temperature range of 550 °C to 1000 °C. The reactivity of Cr2O3 toward the chlorine-oxygen gas mixture is higher than that toward the chlorine-nitrogen one. Chlorination of Cr2O3 proceeds with an apparent activation energy of about 86 kJ/mol between 550 °C and 1000 °C. The apparent reaction order with respect to chlorine is about 1.23 at 800 °C. At temperatures lower than 650 °C, the shrinking sphere model is the most appropriate for describing the reaction kinetics. Oxychlorination of Cr2O3 is characterized by an apparent activation energy of about 87 and 46 kJ/mol for temperatures lower than 650 °C and higher than 700 °C, respectively. At 800 °C and using a Cl2+O2 gas mixture, the maximum reaction rate is obtained when the Cl2/O2 molar ratio is equal to 4, confirming the formation of chromium oxychloride. At this temperature, the reaction orders with respect to chlorine, oxygen, and Cl2+O2 are about 1.08, 0.23, and 1.29, respectively. Mathematical fitting of the experimental data is discussed.

Hydrolysis of Chromium(III) Ion and Solubility of Chromium(III) Oxide in High Temperature Water

Abstract: Recently, the chemical behaviour of chromium species in the primary circuit of water-cooled nuclear reactors has gained a great deal of attention because of its important role in accumulation of radioactivity onto the surface of piping systems. To elucidate the behaviour of chromium compounds, the thermodynamic approach plays an important role. However, there is no thermodynamic data set of chromium ion at high temperatures. Thus, the acquisition of experimental data was considered to be essential. In this work, values of hydrolysis constants and standard free energy of formation of chromium(III) ion at high temperatures were evaluated experimentally.

Kinetics of carbochlorination of chromium (III) oxide

Abstract: Kinetics of the carbochlorination of Cr2O3 has been studied with Cl2+CO gas mixtures between 500 °C to 900 °C using thermogravimetric analysis. The apparent activation energy is about 100 kJ/mol. Mathematical fitting of the experimental data suggests that the shrinking sphere model is the most adequate to describe the carbochlorination mechanism of chromium oxide and that is controlled by the chemical reaction. In the temperature range of 550 °C to 800 °C, the reaction order is about 1.34 and is independent of temperature. Changing the Cl2+CO content from 15 to 100 pct increases the reaction rate and does not affect the reaction mechanism. Similarly, changing the ratio of Cl2/(Cl2+CO) from 0.125 to 0.857 does not modify the carbochlorination mechanism of Cr2O3. In these conditions, the reaction rate passes through a maximum when using a chlorinating gas mixture having a Cl2/(Cl2+CO) ratio of about 0.5.

Study of porous structure of pillared clays : 1. Montmorillonite fixed by chromium(III) oxide

Abstract: The porous structure of montmorillonite fixed by chromium oxide was studied by nitrogen, water vapor, and benzene adsorption. The adsorption data obtained were analyzed in detail by the α s -method and the theory of volume filling of micropores. The fixation of acid-treated montmorillonite K10 by hydroxochromium cations resulted in the formation of the pillared clay with considerable mesoporosity. Additional micropores appeared, and new polar adsorption sites were formed. The role of new absorption sites belongs to hydroxochromium cations or chromium oxide clusters localized in the interlayer space of the pillared clay. A microporous material was found to be formed mostly by the fixation of Oglanli bentonite. The calcination of the synthesized pillared clays at 673 K resulted in an increase in the average width of their micropores and mesoporosity. The porous montmorillonite structure fixed by chromium oxide can be purposefully formed using clays of different chemical composition and physical and chemical properties.

Refractory oxides containing aluminium and barium

Abstract: Oxides containing aluminium and barium, optionally with chromium, are refractory with several possible industrial uses. A gel precursor of an oxide having the formula BaO.n(Al2xCr2yO3), where 1