Kinetic study of Hubnerite (MnWO4) chlorination

The kinetics of Argentinean Hubnerite (MnWO4) chlorination using gaseous chlorine as chlorination agent was studied between 750 and 950 °C. The relative mass change during the chlorination reaction was continuously monitored using a high resolution thermogravimetric system. The starting temperature for the reaction of the manganese tungstate with chlorine was determined at about 650 °C. The influence of gaseous flow rate, sample mass, chlorine partial pressure, and temperature on the reaction rate was analyzed. The dependence of the reaction rate with sample mass clearly indicates that the reaction is not occurring under chemical control, so the reaction proceeds under mixed control for sample masses greater than 0.5 mg. In those conditions, an apparent activation energy of 198 ± 9 kJ mol−1 was obtained with an isoconversional method. Concerning the influence of chlorine partial pressure, it was determined that pressures greater than 35 kPa do not modify the kinetic regime. For the experiment at 850 °C, it was found that the chlorination rate was proportional to a potential function of the partial pressure of chlorine whose exponent is around 0.85. Finally, a global rate equation that includes these parameters was developed.

► The Hubnerite (MnWO4) chlorination reaction begins at 650 °C.
► The reaction is affected by mass transfer through the sample pores at t < 850 °C.
► Apparent activation energy obtained was 198 ± 9 kJ mol−1.
► Below 850 °C the Hubnerite chlorination reaction is under mixed control.
► A global kinetic expression for Hubnerite chlorination reaction was obtained.