PREPARATION OF PHOSPHORIC-ACID WASTE GYPSUM FOR FURTHER PROCESSING TO MAKE BUILDING-MATERIALS

In some countries around the world there are very large quantities of synthetic gypsum stored in stockpiles, especially phosphoric acid gypsum. As a rule this gypsum has a free moisture content of between 25 and 30%, so when it is being processed it is absolutely essential to arrange the use of energy, i.e. the passage of heat through the predrying and subsequent calcining systems, to achieve the smallest possible consumption of energy. A method of treating chemical gypsum is described which has been tested over the years and which shows how moist, and in some cases still impure, gypsum can be treated effectively and simply to obtain a building plaster which is highly suitable for all manufactured gypsum products, such as hemihydrate plaster, plasterboard and high temperature gypsum plaster for flooring plaster, etc. This article describes a well tried purification process which works on the principle of dividing the particle size fractions. A purification effect of more than 75% relative to the starting material is achieved merely by separation of the coarse particles, which are rich in P2O5 and HF compounds, from the very fine particles to which organic materials and P2O5 bound in the crystal lattice tend to adhere. It also shows how the waste water from a gypsum purification plant can be treated. This article makes particular reference to the drying and calcining process with an indirectly heated rotary calciner with which it is possible not only to calcine all fine grained synthetic gypsums but also to produce high temperature gypsum plaster. The material treated in this rotary calciner at temperatures up to 550-degrees-C and residence times of approximately 45 min is very suitable as the basic gypsum plaster for manufacturing flow screed. This process is also finding increasing use for onward processing of gypsum from flue gas desulphurization systems, so-called FGD gypsum.