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

A completely new reaction for conversion of FGD gypsum into FGD anhydrite was investigated in the research project which forms the basis for this article. The reaction takes place with moist, finely divided, FGD gypsum with the catalytic action of small quantities of sulphuric acid at temperatures around 100 to 200-degrees-C Moisture-free FGD anhydrite with an orthorhombic crystalline structure is obtained. The conversion of the crystalline lattice of calcium sulphate dihydrate into calcium anhydrite II takes place directly through the formation of new minerals. 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 sulphate dihydrate is suitable as the starting material. The FGD gypsum with 10% residual moisture, which is already in a finely divided crystalline state when it is generated in the power station, is particularly 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-industrial scale and the design principles were worked out for large-scale 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 particle sizes around 2 microns. and has novel technological properties. Investigations were carried out into its potential applications for the cement industry and as a raw material for producing fillers. In principle it is suitable for the cement industry. Application as a filler allows the FGD gypsum to extend its uses outside the traditional areas of the gypsum industry. Initial trials indicate that after a processing procedure, which was also newly developed in the laboratory, FGD anhydrite possesses the characteristic features necessary for a high grade filler.