THE SEMIDRY ACID-ANHYDRITE PROCESS - THE USE OF FLUE-GAS DESULFURIZATION (FGD) GYPSUM BY DEVELOPMENT OF A NEW PROCESS FOR THE PRODUCTION OFFGD ANHYDRITE
F. Wirsching et al., THE SEMIDRY ACID-ANHYDRITE PROCESS - THE USE OF FLUE-GAS DESULFURIZATION (FGD) GYPSUM BY DEVELOPMENT OF A NEW PROCESS FOR THE PRODUCTION OFFGD ANHYDRITE, ZKG international. Edition B, 47(5), 1994, pp. 278-286
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.