Thermodynamic And Kinetic Study Of Hafnium Oxide Chlorination In The Presence Of Carbon

Hafnium metal is widely used in nuclear industry for fabrication of neutron absorbing elements (control rods) and in metallurgy as a component of superalloys. The magnesium metallothermic reduction of hafnium tetrachloride is the best method for the metal production in terms of process conditions, purity of the product, and economic characteristics. However, synthesis of HfCl4 is a separate task. Direct reaction of hafnium oxide with chlorine gas is thermodynamically impossible. For this reason, chlorination is performed in the presence of a reducing agent such as carbon. In the present work thermodynamic analysis of hafnium oxide chlorination by gaseous chlorine in the presence of carbon was performed at temperatures between 450 and 1000°C. It was found that the main reaction products are gaseous hafnium tetrachloride, carbon monoxide and carbon dioxide. Rising temperature leads to increasing carbon monoxide partial pressure, while that of CO2 decreases. In the above temperature range, the variations of enthalpy, entropy, and Gibbs energy change of the chlorination reaction were calculated and the composition of the gas phase under the conditions of thermodynamic equilibrium was determined. However, the content of the components in real gas–vapor mixtures may significantly differ from the calculated equilibrium values due to kinetic limitation. A series of thermogravimetic experiments was performed under controlled atmosphere using a custom-built set-up based on a Mettler AT20 balances connected to a computer, and intended for the continuous measurement of the sample weight in gases (including aggressive) or in liquids. The kinetics of chlorination of hafnium oxide in the presence of carbon was investigated at temperatures ranging from 600 to 950°C. It was found that below 700°C the process is limited by the chemical reaction on the surface of nonporous solid spherical particles resulting in the formation of a volatile product. Above 700°C, the rate of the process is determined by the mass transfer of the gaseous substances. From the dependences of the logarithm of the reaction rate on the reciprocal temperature the values of the apparent activation energy (Ea) of the chlorination process were calculated. The value of Ea at 600–700°C equals to 65±5 kJ/mol, while at higher temperatures, 700–950°C, it is several times lower and equals to 14±7 kJ/mol. This confirms that at 600-700°C the chlorination proceeds in the kinetic mode, and the process speed is limited by the chemical reaction rate. At 700–950°C the rate of the chemical reaction is significantly higher and the slowest stage is the diffusion of Cl2 and CO to the reaction interface or removal of gaseous HfCl4.