IN SITU TIME-RESOLVED SYNCHROTRON DIFFRACTION STUDIES OF HIGH TEMPERATURE BAYER DIGESTION

The more extreme conditions and short residence time of the high temperature Bayer digestion stage makes a clear description of digestion chemistry and kinetics difficult. In situ time-resolved diffraction using a synchrotron source is well documented for studying hydrothermal dissolution and crystallization mechanisms. This provides a unique method for elucidating digestion chemistry and kinetics, which is possible as the significantly higher photon energy and flux permits X-rays to penetrate appropriately designed autoclaves. However, currently available apparatus at synchrotron facilities (globally) were unable to cope with the temperature/pressure, corrosive liquor or requirements of dual reagent feed, as found in Bayer digestion. We have developed and used a pressure vessel that models high temperature Bayer digestion; this investigation has been driven by a desire to improve process efficiency and intensity. The ‘inconel’ pressure vessel has a very thin-walled base permitting the transmission of photons through the vessel and solution contents. Scattered X-rays collected as a function of time from the growing or dissolving crystals provided kinetic data followed from the inception of reagent mixing. Without a high energy X-ray synchrotron source this would otherwise be impossible.