Gypsum, disodium pentacalcium sulfate, and anhydrite solubilities in concentrated sodium chloride solutions

Abstract

The solubility of gypsum and anhydrite have been determined at 65 "C in aqueous 3.0 to 6.1 m NaCl solutions. Gypsum solubility at 30 "C has been determined in 5.9 m NaCl and in saturated sodium chloride. At 65 "C gypsum reacts with sodium chloride solutions more concentrated than 3.7 tn NaCl to form disodium pentacalcium sulfate, Na2Ca,(S04)a.3H20, which is isomorphous with calcium sulfate hemihydrate. The solubility of this double salt has been determined at 65 "C in aqueous 4.0 to 6.3 m NaCl solutions.Canadian Journal of Chemistry, 48, 3733 (1970) Introduction5CaS04.3H20, which was shown to be petro~l t h~~~h many phase equilibrium studies have graphically identical with that found by Hill and been made of the geologically important CaS04-Wills (9) in the CaS04-Na2S04-H20 system. NaC1-H2O system, the relative solubilities of Without X-ray analysis Marshall and Slusher (1) gypsum, hemihydrate, and anhydrite in concen-Were unaware that the solid disodium pentatrated sodium chloride brines are uncertain at calcium sulfate was isomor~hous with the 65 OC. ~1 1 recent solubility measurements in sodium-containing calcium sulfate hemihydrate concentrated sodium chloride solutions have reported to be non-stoichiometric by Powell (8). been made (1-5) by determining the saturation TO better define the CaSO4-NaCl-HzO system calcium ion concentration and assuming an equal at 65 OC, we have determined the solubilities of concentration for the dissolved sulfate ion. gypsum and anhydrite and for the first time have Arguments for this equality have been given by measured the solubility of the double salt PreZen (2). While this assumption may be true for pared in situ with a range of concentrated sodium gypsum solubility in concentrated brines at chloride solutions at 65 "C. The uncertainty of ambient temperatures, the equivalence of calcium the formula and structure of the double salt and sulfate is contrary to observation above f~r m e d from the reaction of gypsum with con--65 OC, where the sulfate ion levels are shown to centrated sodium chloride brines at higher tembe significantly lower than those for calcium (6) ; peratures has been unequivocall~ resolved by our this early work provides the first evidence for the identification of disodium pentacalcium sulfate reaction of gypsum with concentrated sodium by chemical, X-ray, and differential thermal chloride brines to form a double salt.analyses. Eipeltauer (7) and Powell (8) described the Experimental reaction of gypsum with concentrated brine at

Phosphogypsum Transition to α‐Calcium Sulfate Hemihydrate in the Presence of Omongwaite in NaCl Solutions Under Atmospheric Pressure

“…4), it can be inferred that omongwaite (Na 2 Ca 5 (SO 4 ) 6 Á3H 2 O) loses its crystal water at 100°C-280°C as demonstrated by an obvious endothermic region, while the mixture of crystalline HH and omongwaite (Na 2 Ca 5 (SO 4 ) 6 Á3H 2 O) displayed complicate thermal properties. This is in agreement with previously published differential thermal analysis of dehydration endotherm for Na 2 Ca 5 (SO 4 ) 6 Á3H 2 O at 235 ± 2°C, 19 indicating the presence of omongwaite in the reaction product.…”

“…Because of the concentrated NaCl solution under which the PG dehydration reaction was conducted, formation of omongwaite (Na 2 Ca 5 (SO 4 ) 6 ·3H 2 O) was postulated. In 1970, the reaction of DH with concentrated sodium chloride brines had been unequivocally resolved as being disodium pentacalcium sulfate using chemical, X‐ray, and differential thermal analyses . Na 2 Ca 5 (SO 4 ) 6 ·3H 2 O was named omongwaite that can be described as a HH structure in which one out of six Ca 2+ ions are replaced by Na + and a second Na + ion occupies a position near those sites .…”

“…In 1970, the reaction of DH with concentrated sodium chloride brines had been unequivocally resolved as being disodium pentacalcium sulfate using chemical, X-ray, and differential thermal analyses. 19 Na 2 Ca 5 (SO 4 ) 6 Á3H 2 O was named omongwaite that can be described as a HH structure in which one out of six Ca 2+ ions are replaced by Na + and a second Na + ion occupies a position near those sites. 20 The characteristic XRD peaks of omongwaite (Na 2 Ca 5 (SO 4 ) 6 Á3H 2 O) overlapped with HH and cannot be clearly identified (Fig.…”

“…Sodium chloride is a commonly used salt, whose aqueous solution may be a potential medium for gypsum dehydration reaction. Glew and Hames indicated that gypsum can react with concentrated sodium chloride brine to form a double salt Na 2 SO 4 ·5CaSO 4 ·3H 2 O. In 2008, Na 2 Ca 5 (SO 4 ) 6 ·3H 2 O was named omongwaite and was described as a HH structure, as a result of topotactic replacement during interaction of DH with concentrated solutions .…”

Phase Stability of Calcium Sulfate in the System NaCl/monoethylene glycol/water

“…By using the XRD analysis software -Jade and the XRD spectra to perform the match, the most possible phase is glauberite (CaNa 2 (SO 4 ) 2 ), which has been reported as the intermediate during the transformation from gypsum to anhydrite in concentrated salt aqueous solutions [28][29][30] . Comparing the XRD spectra of solid phase obtained from the solution with 6 m NaCl/95 wt% MEG/water with that from other solutions, CaNa 2 (SO 4 ) 2 was observed only in solutions with 6 m NaCl/95 wt% MEG/water due to the highest concentration of NaCl and MEG in our experiments.…”