Flue gas gypsum

R. Zisselmar, Pressure agglomeration of flue gas gypsum with roller presses , Zement-Kalk-Gips, 38 (5), 243-9 (1985). 

H. Stahl and H. Jurkowitsch, Briquetting of flue-gas gypsum , m Agglomeration 85 (ed. C. E. Capes), Proceedings of Fourth International Symposium on Agglomeration, Toronto, Canada, The Iron and Steel Society, Inc., Warrendale, Pa., USA, 1985, pp. 565-70. 

REFERENCE Wirsching, F., Drying and Agglomeration of Flue Gas Gypsum," Tht Chemistry and Technology of Gypsum, ASTM STP 861. R. A. Kuntze. Ed.. American Society for Testing and Materials, 1984, pp. 160-173. 

Flue gas gypsum is obtained as finely divided crystals in an aqueous reaction medium. Depending on die process, the size and shape of the crystals vary from 1 to 200 pm and from a cubic to a rod shaped structure, as shown in Figs. 2a... 

FIG. 1—Reactions of sulfur dioxide with limestone to produce flue gas gypsum. 

Moist finely divided flue gas gypsum has a free water content of approximately 10% in the form of adsorbed surface water. Furthermore, as a calcium sulfate dihydrate it contains about 20% chemically bonded water—water of crystallization or combined water. Flue gas gypsum has no hygroscopic water. 

The energy levels of the two stages are quite distinct from each other, so that proper reaction steps will produce a completely dry flue gas gypsum without removal of any of the combined water. 

Laboratory tests on the drying and calcination of moist finely divided flue gas gypsum in relation to temperature and time have shown that the drying behavior... 

FIG. 2—(a) Particle structure of flue gas gypsum cubic shaped, bulk density 1.2 tonlm and (b) particle structure of flue gas gypsum rod shaped, bulk density 0.5 ton/m. ... 

FIG. 3—Drying behavior curves of flue gas gypsum in relation to temperature and time. 

FIG. 4—RdationsUp btiween exhaust gas temperature and drying and calcination of the flue gas gypsum. 

In choosing the type of drier for drying the flue gas gypsum it was found that of all the cocurrent driers available, such as rapid driers, flash driers, or contact driers, the directly heated cocurrent rapid drier was the most suitable. Its design and mode of operation are described in Fig. 5. 

The dried flue gas gypsum can subsequently be calcined in kettles to hemihydrate for the manufacture of gypsum plasterboard as it is done especially in North America and in Japan. For its use in the cement industry or in the manufacture of machine-applied plaster or hand-applied plaster the dry flue gas gypsum first has to be agglomerated. This will be explained in the next section. 

Agglomeration of finely divided gypsum, such as phosphogypsum or flue gas gypsum, into a lumpy product can be done by any one of the following processes ... 

These processes are briefly discussed below. Technically and economically the compacting press was found to be the best for the agglomeration of flue gas gypsum. 

HG. 7—Agglomeration of flue gas gypsum by means ef an extrusion press producing granules. 

FIG. 12—Comparison of finely divided flue gas gypsum and compacted flue gas gypsum. 

Figure 12 shows a comparison of finely divided flue gas gypsum with compacted flue gas gypsum. Special difficulties are caused by the very different bulk densities of the moist finely divided flue gas gypsum from approximately O.S to 1.2 ton/m . The process of compaction will overcome this deficiency and will produce a uniform product with a bulk density of about 1.0 to l.l ton/m ... 

From what has been said, and considering all the flue gas gypsum agglomeration processes, it can be concluded that if a compacting press is used for compaction a product is obtained that is of a lumpy structure very similar to that of lumpy natural gypsum. 

It is mainly the monoclinic structure of the calcium sulfate dihydrate lattice that makes it possible for flue gas gypsum to be compacted without having to use bonding agents or additives, while the original particle size and structure are completely changed. 

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