Crystallization of calcium sulfate dihydrate in the presence of some metal ions
Introduction
Crystallization of calcium sulfate dihydrate (gypsum) is of importance in view of their applications in a number of industrial and environmental precipitation processes. There are three main forms of calcium sulfate: calcium sulfate dihydrate or gypsum (CaSO4·2H2O), calcium sulfate hemihydrate (CaSO4·0.5H2O) and anhydride (CaSO4). With increasing temperature, the solubility of all calcium sulfate forms decreases. This is the cause of calcium sulfate scale formation on heat transfer surface [1]. Crystallization can take place on foreign substance or dust particles in the solution and it is very difficult to reproduce the results of such studies [2]. Earlier, many authors studied the growth of seed crystals of gypsum in super-saturation solutions [3], [4], [5], [6], [7] crystallization of gypsum on other crystal surfaces [8] and the precipitation on heated metal surface [9]. The factors that govern this mechanism of precipitation and dissolution of the sparingly soluble salts are therefore, of considerable interest, especially the influence of anions and cations which may exert a marked effect on the rate of precipitation, either through adsorption or by lattice substitution [10].
In the present study, experiments were made to investigate the influence of the presence of cadmium, copper, ferric, and chromium ions, degree of saturation on the reaction rate. Effect of mixed additives were also studied. In addition, direct adsorption experiments were done to determine the affinity constant of the metal ions on the crystal surface of gypsum.
Section snippets
Materials and methods
Solutions of calcium chloride, sodium sulfate, sodium chloride, cadmium nitrate, copper nitrate, ferric chloride and chromium nitrate were prepared using either reagent grade (Fisher Scientific Co., and J.T. Baker Co.) or ultra pure (Alfa Chemical Co.) chemicals with triply distilled deionized water. The solutions were filtered through 0.22 μm millipore filters, which had been prewashed in order to remove any residual wetting agents or surfactants. Then they were analyzed (±0.1%) by atomic
Results and discussion
Concentrations of the ionic species in the mixed solutions of sodium sulfate and calcium chloride at any instant during the scaling experiments were calculated from the measured calcium ion concentrations using the successive approximation methods described previously [12]. Allowance was made for the presence of the ion-pair CaSO4, by using the thermodynamic association constant K (CaSO4)=169 l mol−1 at 25 °C, using the temperature coefficient results of Martynova et al. [13]. Activity
Conclusion
The analysis of the results show that the presence of some metal ions in the reaction medium was inhibiting the rate of precipitation of calcium sulfate dihydrate (gypsum). The degree of inhibition may depend on the concentration of metal ions and degree of saturation. Precipitation is inhibited and a precipitation rate is decreased in the following order: Cd>Cu>Fe>Cr. Complete inhibition of precipitation of gypsum was not found in any concentration of these metal ions and degree of saturations.
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