Ana E. Bohé

Oxidation of carbons in the presence of chlorine

The oxidation reaction of carbon black, sucrose carbon and graphite in the presence of chlorine was studied by thermal analysis (TGA and DTA). The oxidation rate was shown to depend upon the characteristics of each carbon. Heating in chlorine caused different degrees of mass increase in each of the three carbons, with two reaction zones due to physisorption and chemisorption of chlorine on the carbon surface. Burning of the carbons in oxygen gave the following reactivity order: carbon black>sucrose carbon>graphite. Burning of the carbons in the presence of chlorine showed its inhibiting effect, being strongest in carbon black. Oxidation in absence of chlorine started at 525, 560 and 660°C for carbon black, sucrose carbon and graphite, respectively. When chlorine was present in the gaseous phase, oxidation started at 650, 590 and 770°C. Therefore, the reactivity order in the presence of oxygen was sucrose carbon>carbon black>graphite.

Zirconium recovery from zircaloy shavings

A chlorination process for recovering Zr from zircaloy scrap has been studied. Zircaloy chlorination was possible at temperatures as low as 220 °C. The scale microstructure and its effect on the zircaloy reactivity was analysed using Thermogravimetric analysis (TGA), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS) and scanning electron microscopy (SEM) techniques. A solid-solid phase transformation took place into the oxide scale during the zircaloy chlorination. Zirconium, as ZrCl4(g), was separated from the oxide scale and chlorides of Cr and Fe. The effect of the reaction temperature was also analysed.

Chlorination of lanthanum oxide.

The reactive system La(2)O(3)(s)-Cl(2)(g) was studied in the temperature range 260-950 °C. The reaction course was followed by thermogravimetry, and the solids involved were characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The results showed that the reaction leads to the formation of solid LaOCl, and for temperatures above 850 °C, the lanthanum oxychloride is chlorinated, producing LaCl(3)(l). The formation of the oxychloride progresses through a nucleation and growth mechanism, and the kinetic analysis showed that at temperatures below 325 °C the system is under chemical control. The influence of diffusive processes on the kinetics of production of LaOCl was evaluated by studying the effect of the reactive gas flow rate, the mass of the sample, and the chlorine diffusion through the boundary layer surrounding the solid sample. The conversion curves were analyzed and fitted according to the Johnson-Mehl-Avrami description, and the reaction order with respect to the chlorine partial pressure was obtained by varying this partial pressure between 10 and 70 kPa. The rate equation was obtained, which includes the influence of the temperature, chlorine partial pressure, and reaction degree.

Effect of reaction temperature on the chlorination of a Sm2O3–CeO2–C mixture

Abstract The intrinsic kinetics of the chlorination of an hematite-titania-carbon mixture was studied by means of scanning electron microscopy and isothermal and non-isothermal thermogravimetry between 580 and 1123 K. It was observed that various reactions take place simultaneously and that a carbon surface deactivation occurs between 733 and 820 K. The latter was attributed to chemisorption of iron chlorides on the carbon surface.

Chlorination of niobium and tantalum ore

The reaction of chlorine with columbite concentrate, a niobium and tantalum ore, was studied by thermogravimetry between 300° and 950°C. Nonisothermal and isothermal measurements were performed. Morphological evolution of solid reactants and elemental composition of particles were analyzed by SEM and EDXS, respectively. The growth of crystals, high in Ta and Nb content, was observed. The Ta content in remaining samples was greater as the chlorination temperature increased. A scheme of the reaction mechanism is proposed.

Enhancement of the martensitic transformation of tetragonal zirconia powder in the presence of iron oxide

Abstract The martensitic t → m phase transformation in air is enhanced by the presence of iron oxide during heating and follows the same mechanism as in the absence of iron. X-ray diffraction and scanning electron microscopy show the phase transformation and the morphology changes occurring during heating.

β-Ta2O5 Carbochlorination with Different Types of Carbon

Abstract The effect of the type of carbon on β- Ta2O5 chlorination with Cl2-Ar and Cl2-Ar-O2 gas mixtures was studied by isothermal and non-isothermal thermogravimetric measurements. The carbons used were carbon black, sucrose carbon and graphite and the characterization techniques were BET area, scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was observed that the carbochlorination reaction reactivity increased with higher carbon percentages. On the other hand, the carbochlorination reaction was favoured both chemically and kinetically by the carbon with the highest reducing activity according to the following reactivity order: carbon black > sucrose carbon > graphite. On a fait des recherches à propos de l’effet du type de charbon sur la chloration de β- Ta2O5 avec des mélanges gazeux de Cl2-Ar et Cl2-Ar-O2, en faisant des mesures thermogravimétriques isothermiques et non isothermiques. Les charbons utilisés ont été carbon black, surcrose carbon et graphite. Les techniques de caractérisation utilisées ont été BET, microscopie électronique de balayage (SEM) et diffraction de rayons X (XRD). On a observé qu’un accroissement dans le pourcentage de charbon augmente la réactivité de la réaction de carbochloration. D’autre part, la réaction de carbochloration est favorisée chimiquement et cinétiquement par le charbon plus réducteur, selon l’ordre suivant de réactivité: carbon black > surcrose carbon > graphite.

Cu-Zn-Al alloys characterization: phases thermal stability and dezincation

As a first stage in the study of chlorination of ternary alloys, characterization studies involving thermodynamics and temperature effects are necessary to know the changes in the structure, which might arise from the heating that precedes the chlorination reactions. The initial beta-alloy was quenched from 900oC. After different non-isothermal treatments mass loss was observed due to a dezincation process at temperatures above 70oC0. It was found that the heating rate affects the Zn loss starting temperature. Occurrence of low temperature phases due to thermal treatments was observed. The peaks observed in the sample XRD profiles allow to identify the presence of alpha and gamma phases for temperatures below 525oC, and the beta phase for higher temperatures. At 240oC the beta phase disappears after 9 hours, while at 315oC the beta peak was absent after 2 hours. XRD and SEM techniques were used to analyse non-isothermal measurements and thermal treatments.

THERMAL BEHAVIOR OF GALENA ORE IN CHLORINE ATMOSPHERES

The thermal behavior of a purified galena in a chlorine surrounding was determined. The kinetic of the chlorination of galena and the stoichiometry of this reaction between 723 to 973 K (450 to 700°C) were measured by thermogravimetry. The microstructural characterization of the solid residues were determined by SEM, EDXS and XRD. The ΔHvap of the produced lead chloride volatilization was obtained by the application of Clausius-Clapeyron equation considering the process in an equilibrium state.

Gasification Studies on Argentine Solid Fuels

As the global population growth and energy demand are steadily raising and the industry is forced to reduce the greenhouse gas emissions due to the global warming, there is an increasing pressure to improve the overall efficiency of the energy production systems. In this challenging framework, a renewed interest on coal gasification technologies has recently emerged worldwide, since they offer the potential of clean and efficient energy. One attractive characteristic of coal gasification technology is the possibility of coproduction of electricity, hydrogen, liquid fuels and high-value chemicals that contributes to the improvement of power generation efficiency compared with conventional pulverised coal fired plants as well as the reduction of emissions of greenhouse gases and particulates to the atmosphere (Minchener, 2005). Gasification has also the additional advantage of accommodating a wide range of feed stocks, including low-cost fuels like petroleum coke, biomass, and municipal wastes (Higman & Van der Burgt, 2003).