Calcium sulfate Phases

Continuous Crystallization of Calcium Sulfate Phases from Phosphoric Acid Solutions... 

This work represents an attempt to elucidate the complex nature of proper drying and its effect on the composition of gypsum containing various calcium sulfate phases. 

Figure 2.17 Anhydrous calcium sulfate crystals grown in pure phosphoric acid (top) and acid containing 0.8% aluminum fluoride (bottom). [Reprinted with permission from G.J. Witkamp and G.M. Van Rosmallen (1990). Continuous Crystallization of Calcium Sulfate Phases from Phosphoric Acid Solutions, Crystallization as a Separation Process. A.S. Myerson and K. Toyokura (eds.), ACS Symposium Series, no. 438, pp, 392. 1990 American Chemical Society.]...

The viability of the process is based on the low solubility of calcium sulfate phases in aqueous solutions. Upon controlled mixing of the solutions calcium sulfate crystals are produced and hydrochloric acid is regenerated [8]. Research on the determination of process operating windows such as temperature and concentration levels for the production of high quality calcium sulfate dihydrate (DH) or the valuable calcium sulfate a-hemihydrate (HH) phase instead of formation of low value anhydrite (AH) [9,10] has been undertaken. 

Quantification of Calcium Sulfate Phase Transformations In order to quantitatively describe the progress of phase conversion ratios of absolute intensities of characteristic peaks of each phase s XRD pattern were compared. This method is generally considered suitable for quantitative phase analysis of mixtures [35]. To aid phase quantification, calibration curves based on prepared mixtures with known composition were used and the respective intensity ratios calculated. The intensity ratios were calculated from the intensity of peaks at 26 29.10725.43°, 29.71°/25.43° and... 

T. Feldmann and G.P. Demopoulos, Phase Transformation Kinetics of Calcium Sulfate Phases in Strong CaC -HCl solutions, Hydrometallurgy, 2012 (Article In Press), DOT 10.1016/j.hydromet.2012.08.015... 

Z. Li, and G. Demopoulos, Model-based Construction of Calcium Sulfate Phase Transition Diagrams in the HCl-CaCl2-H20 System between 0 and 100°C, Industrial Engineering and Chemistry Research, 45 (2006), 4517-4524. 

Two main categories of the wet process exist, depending on whether the calcium sulfate is precipitated as the dihydrate or the hemihydrate. Operation at 70—80°C and 30% P20 in the Hquid phase results in the precipitation of CaSO 2 filterable form 80—90°C and 40% P20 provide a filterable CaSO O.5H2O. Operation outside these conditions generally results in poor filtration rates. A typical analysis of wet-process acid is given in Table 4. For more detailed discussion of the wet-process acid, see Fertilizers. 

The composition of the Hquid phase during the early hydration of Portiand cements is controlled mainly by the solution of calcium, sulfate, sodium, and potassium ions. Very Httie alumina, siHca, or iron are present in solution. Calcium hydroxide, as calcium oxide, and gypsum, as calcium sulfate, alone have solubihties of about 1.1 and 2.1 g/L at 25°C, respectively. In the presence of alkaHes released in the first 7 min, the composition tends to be governed by the equiHbrium ... 

ASTM C845 Type E-I (K) expansive cement manufactured ia the United States usually depends on aluminate and sulfate phases that result ia more ettriagite formation duriag hydration than ia normal Portland cements. Type K contains an anhydrous calcium sulfoaluminate, C A SI. This cement can be made either by iategraHy burning to produce the desired phase composition, or by intergrinding a special component with ordinary Portland cement clinkers and calcium sulfate. 

About 250 ml of a reaction mixture obtained by the electrolytic reduction of nitrobenzene in sulfuric acid solution and containing about 23 grams of p-aminophenol by assay is neutralized while at a temperature of 60° to 65°C, to a pH of 4.5 with calcium carbonate. The calcium sulfate precipitate which forms is filtered off, the precipitate washed with hot water at about 65°C and the filtrate and wash water then combined. The solution is then extracted twice with 25 ml portions of benzene and the aqueous phase is treated with 0.5 part by weight, for each part of p-aminophenol present, of activated carbon and the latter filtered off. The activated carbon is regenerated by treatment with hot dilute caustic followed by a hot dilute acid wash, and reused a minimum of three times. 

The supersaturation is too low in all experiments to be measured accurately, but it seems reasonable to assume that the effect of residence time is imposed through the kinetics. Another observation is that the D-value for cadmium uptake in anhydrite is about ten times higher than in HH or DH. An explanation for this higher D seems to be related to the crystal structures of the calcium sulfates. Only the AH structure matches with an anhydrous CdSO phase, while no hemi- or dihydrate phase of CdS04 exists. 

For uptake proceeding by isomorphous substitution, the partition coefficient D depends on thermodynamic parameters such as ionic radius of the impurity ions and the phases of the calcium sulfate as well as on kinetics. 

Type and amount of aluminates, calcium sulfate and free lime present in the mixture governs the rate at which ettringite forms. The presence of lime is reported to be essential for both the initial and subsequent stages of ettringite formation because it maintains a solution phase saturated with calcium ions [81]. Crystal habit, particle size and range of particle size in CSAs determine its rate of hydration and duration of expansion (Fig. 6.11). 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

Strelyuk described a thin-layer chromatographic method for the qualitative detection of dipyridamole [62], Data were presented on the use of various reagents for the detection by solution color change and precipitate formation in the thin-layer chromatography of dipyridamole. Thin-layer chromatography was on silica gel-calcium sulfate dihydrate, with methanol aqueous ammonia or benzene-dioxane being used as the mobile phase with detection at 270-330 nm. The Rvalues were 0.7-0.75 and 0.5-0.55 for the two solvent systems, and the detection limit was... 

Bis[3-phcnyl-l-rf-propyl] Ditellurium1 A suspension of 266 mg (2.0 mmol) 3-phenylpropanal and 1.75 g (4.0 mmol) aluminum telluridein 10 ml tetrahydrofuran is cooled to - 78°. A solution of 0.21 m/(4.0 mmol) of concentrated (98%) sulfuric acid in 1.75 ml (96 mmol) deuterium oxide is added to the chilled suspension. The mixture is wanned to 20° and then heated to the reflux temperature with continuous stirring over a 30 min period. The mixture is then refluxed for 1.5 h. After cooling to 20°, the black precipitate is removed by filtration. The filtrate is washed with aqueous sodium hydrogen carbonate solution and dried with calcium sulfate. The mixture is filtered, the solvent removed from the filtrate, and the residue chromatographed with hexane/benzcnc (1 1) as the mobile phase. The product was obtained as a dark-red oil in 70% yield. 

---