M. del C. Ruiz

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.

Determination of Nb, Ta, Fe and Mn by X-ray fluorescence

An analytical methodology was developed for the quantification of niobium, tantalum, iron and manganese in samples of ores and solid residues coming from ore pressure leaching, using different acids. The adopted instrumental technique was X-ray fluorescence on pelleted solid samples. Comparison of the analytical data obtained by application of this method to ore and leaching residue samples with those obtained by a reference method showed that the experimental results were statistically acceptable.

Formation of niobium-tantalum pentoxide orthorhombic solid solutions under chlorine-bearing atmospheres

This paper studies, by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS), the structural changes suffered by amorphous hydrated Nb oxide and mixtures of amorphous and crystalline Nb and Ta oxides subjected to thermal treatments in air and chlorine atmospheres. The air heating of amorphous Nb2O5·nH2O leads to different crystalline phases depending on the working temperature. The hexagonal phase of Nb2O5 is obtained at 773 K and the monoclinic phase β-Nb2O5 is obtained at 1173 K. The thermal treatment of amorphous Nb oxide in chlorine atmosphere decreases the temperature at which phase γ-Nb2O5 appears and at 1173 K the stable monoclinic phase α-Nb2O5 is obtained. Air calcination for 5 hours at temperatures between 973 and 1273 K of different amorphous Nb2O5·nH2O - Ta2O5·nH2O mixtures does not lead to solid solution of these oxides. Thermal treatment in chlorine atmosphere of amorphous Nb and Ta oxides leads to the formation of Nb2O5 and Ta2O5 orthorhombic solution in one hour at 973 K and in 24 hours at 1223 K, when starting from crystalline oxides. The effect of chlorine is due to the dissolution-recrystallization of the metallic chlorides and the oxygen formed, when the system evolves to chemical equilibrium between solid phases and gaseous chlorine.

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.

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.

Carbochlorination of Argentinian Tantalo-Columbites

Abstract The extraction of Nb and Ta from tantalo-columbites of San Luis, Argentina has been studied at the laboratory scale. The ore mixed with carbon was chlorinated in a fixed bed reactor operated with downcoming flow and pressures close to atmospheric. The variables investigated were: ore/carbon ratio, mixing time and procedure; temperature and reaction time; particle size and previous ore treatment with HCl, EDTA and NaOH solutions. The results obtained showed that: the optimum percentage of carbon varies with temperature; Nb and Ta recovery is hardly affected by the mixing time; the influence of the mixing procedure is important; recovery increases both with higher temperature and with longer reaction times, and decreases with larger particle size; in general, ore leaching led to a decrease in the solid reactivity to the chlorination process. The solids were characterised by BET, XRD, XRF, SEM and optical microscopy. Quantitative chemical analyses were carried out by XRF, ICPAES, gravimetry and UV-Visible spectrometry.