Kinetics of yttrium oxide carbochlorination

The chlorination kinetics of the Y2O3–sucrose carbon system was studied by thermogravimetry. This work is a continuation of a previous one in which the reaction stages and the stoichiometry of each reaction have been determined. The influence of carbon content, total flow rate, sample initial mass and chlorine partial pressure was evaluated. The effect of carbon content on the reactive mixture was studied between 6.7 and 70% (carbon mass/total mass). The results showed that the reaction rate of each stage is strongly increased as the carbon content increases and the range of occurrence of the stages depends on the amount of carbon in the solid reactive mixture. The formation reaction of YOCl (STAGE I) is chemically controlled for temperatures lower than 700 °C with average effective activation energies of 165 ± 6 and 152 ± 7 kJ/mol for 8.7 and 16.7%C, respectively. The formation of the YOCl follows a nucleation and growth mechanism, with a combination of continuous nucleation and site saturation, and anisotropic growth controlled by diffusion. The kinetics of STAGE I can be expressed by the following global rate equation that includes the variables analyzed:dαdt=k0Bexp−EaRgTpCl2{n(1−α)[−ln(1−α)]}(n−1)/nwhere k0B = 1.9 × 104, n = 1.20 for 8.7%C, and k0B = 8.4 × 103, n = 1.14 for 16.7%C.STAGES II and III correspond to the YOCl carbochlorination to form YCl3, being these stages kinetically different. It was not possible to obtain kinetic parameters for these stages. The reaction rate of STAGE II is affected by diffusion of Cl2 through the gas film surrounding the sample and mass changes in STAGE III have two opposite components: formation and evaporation of liquid YCl3.

Research highlights▶ Chlorination kinetics of Y2O3–C system was studied by thermogravimetry. ▶ The influence of carbon content, flow rate, sample mass and pCl2 were evaluated. ▶ Reaction proceeds through three successive stages until the formation of YCl3(l). ▶ STAGE I is the formation of YOCl(s) and is under chemical control for T < 700 °C. ▶ STAGE I follows a nucleation and growth model. Kinetics parameters were obtained.