THE SEMIDRY ACID-ANHYDRITE PROCESS - THE USE OF FLUE-GAS DESULFURIZATION (FGD) GYPSUM BY DEVELOPMENT OF A NEW PROCESS FOR THE PRODUCTION OFFGD ANHYDRITE

A completely new reaction for conversion of FGD gypsum into FGD anhydr ite was investigated in the research project which forms the basis for this article. The reaction takes place with moist, finely divided, FG D gypsum with the catalytic action of small quantities of sulphuric ac id at temperatures around 100 to 200-degrees-C Moisture-free FGD anhyd rite with an orthorhombic crystalline structure is obtained. The conve rsion of the crystalline lattice of calcium sulphate dihydrate into ca lcium anhydrite II takes place directly through the formation of new m inerals. This conversion is developed into a new process called the '' Semidry Acid-Anhydrite Process''. The reaction and its mechanism were first investigated in laboratory trials. Any finely divided calcium su lphate dihydrate is suitable as the starting material. The FGD gypsum with 10% residual moisture, which is already in a finely divided cryst alline state when it is generated in the power station, is particularl y advantageous as for this application it does not have to be dried or ground first. The process development was carried out up to a semi-in dustrial scale and the design principles were worked out for large-sca le plants at power station sites. The directly heated rotary tube kiln proved to be a suitable reaction unit. The FGD anhydrite is obtained in this process as a dry, finely divided, product with reproducible pa rticle sizes around 2 microns. and has novel technological properties. Investigations were carried out into its potential applications for t he cement industry and as a raw material for producing fillers. In pri nciple it is suitable for the cement industry. Application as a filler allows the FGD gypsum to extend its uses outside the traditional area s of the gypsum industry. Initial trials indicate that after a process ing procedure, which was also newly developed in the laboratory, FGD a nhydrite possesses the characteristic features necessary for a high gr ade filler.