built up in the vessel which is fluidised to some extent by the gases from the burner tube and additionally self fluidised by the water vapour evolved by the calcining gypsum at this point the burner rating is increased and the temperature allowed to rise to a calcination set point of about 150 u00b0 c or other appropriate temperature corresponding to the particle product to be made the calcined stucco discharged at the weir assembly is found to be of good quality and adjustments of the grinding and calcining parameters enable stucco of particular characteristics to be produced for example for such different uses as bag plaster or the manufacture of plasterboard rotary horizontal grinding mills of the type employed here are usually operated with a moderated feed of raw material such mills do not ordinarily work well when flooded with material moreover a fluidised bed of the kind employed in this invention for example of gypsum exerts a considerable pressure at its base in a practical example the back pressure at the outlet of the burner tube branches 22 with a fluidised bed of gypsum may be 35 inches 890 mm wg or more it is most surprising that the grinding or milling equipment described will operate satisfactorily when immersed in a fluidised bed of material and when subjected to the pressure involved whereas earlier mills of this type have introduced large quantities of air at the base in order to carry ground particles to the classifier equipment and to prevent the escape of material such amounts of air.