THE SOLAR THERMAL-DECOMPOSITION OF TIO2 AT TEMPERATURES ABOVE 2200 K AND ITS USE IN THE PRODUCTION OF ZN FROM ZNO

A 2 kW concentrating solar furnace was used to study the thermal decomposition of TiO2 at temperatures of 2300-2800 K in an Ar atmosphere at 1 bar. The condensed products may be described as TiOx; the minimum values of x that we obtained experimentally were 1.91, 1.86, and 1.83 for temperatures of 2300, 2500, and 2700 K, respectively. The final products, after the temperature was reduced to room temperature, were mixtures of TinO2n-1. The liquid mixture is well characterized as an ideal mixture of oxides of the form TinO2n-1 and TiO2 with n ranging from 4 to 1. The rate of decomposition is limited by the rate at which oxygen diffuses from the liquid gas interface. We have demonstrated that this rate is well described by a numerical model that couples the equations of chemical equilibrium to those of steady-state mass transfer, for which the rate of mass transfer is given by Fick's law for ordinary diffusion. The, decomposition reaction can be used in a two-step solar process for producing Zn from ZnO. We have calculated maximum efficiencies for such a process.