Prevention of anhydrite formation during gypsum processing

When fine grinding raw fines in a digester or in a rotating bowl, it is easier to get burnout in the form of soluble anhydrite, which reduces the quality of the finished product. Burnout, for example, in a digester can be local, depending on the temperature and condition of the bottom of the boiler, usually cast iron.

In shape, the bottom is usually made spherical, assembled from individual cast-iron segments abutting against a corner steel support ring. The joints of the segments are sealed with asbestos soaked in fire-resistant clay. But often joints, and often bottoms, burn out if they were not replaced in time.

Fire seeps through the joints and burns through the plaster. The same thing happens when segments burn through. The agitator in the boiler, from which chains move along the bottom of the boiler, must thoroughly mix the gypsum powder, preventing it from burning to the bottom.

The temperature of the powder in the boiler should be controlled, preferably automatically, by means of a thermocouple embedded in the boiler and having leads to a self-recording device, the pen of which writes a temperature curve on a paper circle. In rotary kilns, gypsum stone, ground to a fineness of 0-5, 0-8 mm, is usually subjected to heat treatment. Due to the short stay of the fired material in the furnace (no more than 25-30 minutes), large particles of gypsum may contain particles of raw dihydrous gypsum, which is a harmful impurity in gypsum, when it exits the furnace.

Therefore, in rotary kilns, raw fines should be fired as much as possible of the same grain composition. This is what they do in large industries: they disperse the grinding product into fractions and each of them is fired according to its own regime.

All this is done because gypsum is very sensitive to temperature: even at a relatively low temperature, significant changes occur in it.