Development and evaluation of alumina calcination.
This thesis focuses on a number of aspects governing the transformation of gibbsite,
via intermediate phases, to a-alumina. These aspects include the size and morphology
of the gibbsite grains, the influence of additions of foreign elements, the effect of a
mechanical treatment of the gibbsite prior to calcination, and combinations of these
factors. The materials were characterisedb y scanninge lectron microscopy, X-ray
diffraction and surfacea rea measurementsF. or someo f the calcined materials an
attempt was made to sinter the powders to a dense body to investigate if any of the
treatments during calcination had an effect on this process.
The literature review covers the current state of understanding of the production of
bulk alumina powder by the Bayer process and the phase changes seen on calcination
of precursors to the stable a-alumina phase. A detailed description of the phase
changes is given and the various routes and conditions necessary for the
transformations to occur are considered. The transformations are examined in relation
to the morphology of the crystals and the variables controlling the phase
transformation route are discussed.
Calcination in air showed that the size of the gibbsite grain governs the calcination
route taken to reach a-alumina. The standard gibbsites used in this work show a
mixed calcination sequence transforming both via the boehmite phase, followed by
the y, 8 and 0 phases, and via the x and K phases. The formation of boehmite is
attributed to retention of water vapour within the grainDifferences in morphology of the starting materials showed that for the range of
materials seen, the morphology of the grain is less important than its size. The super
fine material confirmed that a small grain size transforms via the non-boehmite route
only, with the other gibbsites taking intermediate routes as for the standard gibbsites.
Of the additions made prior to calcination, aluminium fluoride was found to reduce
the transformation temperature to a-alumina by approximately 300°C. Other additions
had little effect on the transformation temperature although a reduction in grain size
was seen with aluminium chloride. It was found that good mixing of the alumina
fluoride was essential to obtain reliable and reproducible results. This is due to the
small amounts of additive that are needed and the sensitivity of the process to
concentration variations. Mineralisation of a range of gibbsites showed that the
presence of sodium in the starting material was crucial in reducing the calcination
temperature. This led to the conclusion that the sodium and fluoride react to form a
liquid phase. The presence of a liquid phase increases the mobility of the aluminium
and oxygen atoms resulting in a reduction of the transformation temperature. Fluoride
additions to the gibbsites with different morphologies showed that the presence of
sodium was the governing factor in reduction of the transformation temperature.
Milling of the starting materials showed that there was a small reduction in the
transformation temperature between some of the phases. The energy involved in
milling leads to activation of the gibbsite. This activation takes the form of a
reduction in the grain size and in a reduction of the crystallinity seen in the XRD
patternFluoride additions during the calcination of sapphire with a standard gibbsite powder
showed preferential grain growth. It was possible to initiate growth of small plate-like
crystals on the polished surface of a piece of sapphire parallel to the basal plane.
Crystal growth was also seen in scratches on a polished surface perpendicular to the