Gypsum dehydration process

Isa and Oruno describe a method that enables identification of intermediates (in the gypsum dehydration process) more easily. The method involves the use of simultaneous TG-DTG-DTA under various sealed atmospheres corresponding to three systems—open completely, sealed, and quasi-sealed. Endothermic DTA peaks appear earlier (129 and 133°C) than the point of decreasing TG. This technique, resembling the quasi-isothermal and isobaric thermogravimetry (Q-TG), is superior to the latter in that it needs less of the sample. 

Section 8.0-A Three Step Gypsum Dehydration Process... 

The dehydration process through which gypsum proceeds can be described (44) as follows ... 

Effect of Impurities on CaS04 Transformation. The transition from gypsum to orthorhombic anhydrite is slow but occurs even at ambient temperatures (44). The relatively large concentration of finely divided sodium chloride present in the plaster in the tomb of Nefertari may have facilitated the dehydration process. The presence of a hygroscopic material, such as sodium chloride, can help promote dehydration reactions. Also, impurities within the lattice of a crystalline structure can weaken the lattice (46, 47) and thereby accelerate thermodynamically favored reactions. These points suggest a strong correlation between the extent to which the dehydration reaction proceeds and sodium chloride concentration, but they do not exclude the possibility that dehydration can take place in the absence of salt. 

The main or sole constituent of plaster of Paris is calcium sulfate hemihydrate. If natural gypsum is used as the starting material the plaster may also contain some anhydrite, calcite, or clay minerals. The binder consists of individual particles whose size and shape corresponds to those of the starting dihydrate. They exhibit a distinct internal porosity and a relatively large specific surface area as a result of the dry dehydration process. 

The sulphates - mainly the products of the hypergenic processes - very easily undergo the diagenetic processes, in which the dominant role is played by hydration (gypsification) of anhydrite and dehydration (anhydritization) of gypsum both processes are reversible and the reaction takes place as follows ... 

Reactions (1) and (2) are dehydration processes. The exothermic reaction (3) represents a lattice modification from the hexagonal to orthorhombic form. Decreases in peak temperatures (150 to 123°C and 197°C to an undetectable position) occurred when the gas pressure in the cell was reduced to 1 torr (Fig. 2). The peak for the exothermic effect was not sensitive to changes in gas pressure. The DTA curve for j8-CaS04 H2O has a sharp endothermic peak at 195°C and a small exothermic peak at 375°C (Fig. 3). The position and shape of the small exothermic peak were not significantly affected by changes in atmospheric pressures within the DTA cell. The endothermic peak has the same pressure-temperature behavior previously noted for the second endothermic curve of gypsum (a shift occurs from 195°C to 132°C for 760 torr and 1 torr respectively). 

Natural gypsum is either extracted in open cast or underground mines. It is ground, homogenized and then thermally dehydrated, the process depending upon the required properties of the final product. 

The clinker formation process is initiated by the dehydration of gypsum to anhydrite at around 100-120°C, followed by a decomposition of clay minerals at around 300-600°C. The decaibonization of the calcium carbonate that is present starts at about 700°C, and is completed before the temperature reaches 900°C. 

Tlie process is carried out in a rotary kiln that is similar to cement kilns. When gypsum is used, it is first dehydrated in a separate kiln. When phosphogypsum is used, part of the fluorine is discharged and may be... 

The only known commerdal process for produdng alpha hemihydrate from phosphogypsum is the Giulini process that wm developed in West Germany [281 and was used in two plants in that country and one in fre-land. As mentioned previously, alpha hemihydrate is formed by dehydration of gypsum in an aqueous suspension in an autodave. 

Alternative processes at the time were the Bradford-Carmichael process, in which lead ore was mixed with dehydrated gypsum (plaster) as a binder, was formed into lumps and then processed in the converter, the Savelsberg Process, which fired ore and limestone over a fuel bed in a converter to produce a sintered material as blast furnace feed. 

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