Crystallization Routes and Metastability of α-Calcium Sulfate Hemihydrate in Potassium Chloride Solutions under Atmospheric Pressure

2009 ◽  
Vol 54 (3) ◽  
pp. 719-725 ◽  
Author(s):  
Baohong Guan ◽  
Xianfa Ma ◽  
Zhongbiao Wu ◽  
Liuchun Yang ◽  
Zhuoxian Shen
Keyword(s):  
Potassium Chloride ◽  
Atmospheric Pressure ◽  
Calcium Sulfate ◽  
Chloride Solutions ◽  
Calcium Sulfate Hemihydrate

EFFECT OF WATER VAPOR ON THE FORMATION OF MODIFICATIONS

1965 ◽  
Vol 43 (9) ◽  
pp. 2522-2529 ◽  
Author(s):  
R. A. Kuntze
Keyword(s):  
Water Vapor ◽  
Vapor Pressure ◽  
Atmospheric Pressure ◽  
Calcium Sulfate ◽  
Water Vapor Pressure ◽  
Exothermic Peak ◽  
Vapor Pressures ◽  
Ambient Water ◽  
Rate Of Heating ◽  
Calcium Sulfate Hemihydrate

The two recognized forms of calcium sulfate hemihydrate can be identified by the position of a relatively small exothermic peak in their differential thermograms. Hemihydrates prepared at various water vapor pressures up to 760 mm Hg were found to produce this exothermic peak in a position which is characteristic for the β-form. These results indicate that α-hemihydrate cannot be made at atmospheric pressure, as was previously suggested on the basis of heat solution measurements. The typical differential thermogram of α-hemihydrate is only obtained with material made by dehydration in solution or by autoclaving. The effect of ambient water vapor pressure on the position of the three peaks that occur in the differential thermogram of CaSO4•2H2O has also been studied. It was found that the incipient temperature of the second endothermic peak, corresponding to the transition of hemihydrate to soluble anhydrite, is displaced independent of the rate of heating from 145 °C to 187 °C with increasing water vapor pressures up to 760 mm Hg. This indicates that, for each temperature, a threshold water vapor pressure exists, above which soluble anhydrite cannot be formed.

The Effect of Process Conditions on the Preparation of α-Calcium Sulfate Hemihydrate from FGD Gypsum Using the Hydrothermal Method under Atmospheric Pressure

2011 ◽  
Vol 250-253 ◽  
pp. 881-889 ◽  
Author(s):  
Xian Feng Liu ◽  
Jia Hui Peng ◽  
Chen Yang Zou ◽  
Leng Bai ◽  
Mei Li
Keyword(s):  
Hydrothermal Method ◽  
Reaction Temperature ◽  
Salt Concentration ◽  
Atmospheric Pressure ◽  
Calcium Sulfate ◽  
Ph Value ◽  
Fgd Gypsum ◽  
Rate Of Reaction ◽  
Percent Conversion ◽  
Calcium Sulfate Hemihydrate

This paper studies the laws of crystal growth, percent conversion and the rate of reaction of α-calcium sulfate hemihydrate from FGD gypsum under different conditions using the hydrothermal method under atmospheric pressure. The crystal morphology was observed by using SEM, polarizing microscope profile, and percent conversion and the rate of reaction were obtained by assaying crystal water content and calculating. The results showed, (1) with the increase of reaction temperature, the dehydration rate increased and the formed α-calcium sulfate hemihydrate crystal had a larger particle size; (2) with the increase of salt concentration or slurry concentration, the formed α-calcium sulfate hemihydrate crystal was smaller, percent conversion and the rate of reaction was nearly unchanged; (3) with the increase of pH value of solution, the rate of reaction increased and percent conversion was nearly unchanged, and with pH value ranging from 5 to 7 the formed α-calcium sulfate hemihydrate crystal was crassitude. In conclusion, the perfect technological parameters were as follows: reaction temperature ranging from 95°C to 100°C, salt concentration ranging from 15% to 20%, slurry concentration ranging from 15% to 20%, pH value ranging from 5 to 7, and reaction time not exceeding 90min.

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

1970 ◽  
Vol 48 (23) ◽  
pp. 3733-3738 ◽  
Author(s):  
D. N. Glew ◽  
D. A. Hames
Keyword(s):  
Sodium Chloride ◽  
Calcium Sulfate ◽  
Double Salt ◽  
Chloride Solutions ◽  
Sodium Chloride Solutions ◽  
Calcium Sulfate Hemihydrate

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 m NaCl to form disodium pentacalcium sulfate, Na2Ca5(SCO4)6•3H2O, 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.

Effect of Organic Diacid Carbon Chain Length on Crystal Morphology of α-Calcium Sulfate Hemihydrate in Preparation from Flue Gas Desulphurization Gypsum

2012 ◽  
Vol 253-255 ◽  
pp. 542-545 ◽  
Author(s):  
Xian Feng Liu ◽  
Jia Hui Peng ◽  
Jian Xin Zhang ◽  
Jin Dong Qu ◽  
Mei Li
Keyword(s):  
Hydrothermal Method ◽  
Flue Gas ◽  
Atmospheric Pressure ◽  
Calcium Sulfate ◽  
Crystal Morphology ◽  
Carbon Chain ◽  
Carbon Chain Length ◽  
Key Technology ◽  
Calcium Sulfate Hemihydrate ◽  
Flue Gas Desulphurization

Modifying crystal morphology was the key technology of the preparation of α-calcium sulfate hemihydrate from flue gas desulphurization gypsum using the hydrothermal method under atmospheric pressure. For exploring crystal modifier, the crystal morphology of α-calcium sulfate hemihydrate in preparation from flue gas desulphurization gypsum with various organic diacid was studied, by polarizing microscope profile observation. The results showed, When the space between two carboxyls was triplicate length of C-C, the organic diacid had the best effect of modifying crystal morphology.

Preparation of α-Calcium Sulfate Hemihydrate from Phosphogypsum in CaCl2 Solution under Atmospheric Pressure

2012 ◽  
Vol 554-556 ◽  
pp. 570-574 ◽  
Author(s):  
Bao Guo Ma ◽  
Xiao Hong Ru ◽  
Si Wen Lu ◽  
Kai Bo Zou
Keyword(s):  
Phase Transition ◽  
Reaction Time ◽  
Atmospheric Pressure ◽  
Calcium Sulfate ◽  
Product Phase ◽  
Suitable Temperature ◽  
Common Ion ◽  
The Common ◽  
Common Ion Effect ◽  
Calcium Sulfate Hemihydrate

With the growing demand of phosphoric fertilizer a great quantity of phosphogypsum (PG) was produced in China.To utilization of PG in an energy-saving way,the phase transition of PG from DH to α-HH in CaCl2 solution under atmospheric pressure was conducted, the reaction time of phase transition was significantly shortened by higher reaction temperature and CaCl2 content of the mixtures. The suitable temperature and CaCl2 addition of the mixture were 95 °C and 24 wt.% respectively, morphology of the corresponding dehydration product phase is rod-like shape. Preparation of α-HH from PG in hot CaCl2 solution at atmospheric pressure was based on dissolution- crystallization mechanism. The increasing addition of CaCl2 in solution makes the solubility of CaSO4 decrease due to the common ion effect.

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