Ind. Eng. Chem. Res. 2001, 40, 2335-2339
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GENERAL RESEARCH Calcium Sulfate Dihydrate Nucleation in the Presence of Calcium and Sodium Chloride Salts† Marina Prisciandaro,‡ Amedeo Lancia,*,§ and Dino Musmarra⊥ Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` dell’Aquila, Monteluco di Roio, 67040 L’Aquila, Italy, Dipartimento di Ingegneria Chimica, Universita` di Napoli “Federico II”, P.le Tecchio 80, 80125 Napoli, Italy, and Istituto di Ricerche sulla Combustione, CNR, P.le Tecchio 80, 80125 Napoli, Italy
Calcium sulfate dihydrate primary nucleation was experimentally studied in a batch crystallizer with a related optical device at a supersaturation ranging from 2 to 4 and a fixed temperature T ) 25 °C. The mother liquor was supersaturated with calcium sulfate in which calcium or sodium chloride salts were added to approach flue gas desulfurization stack-gas liquor. The experimental results were compared with those obtained in the absence of foreign substances in solutions, showing that chloride salts influence gypsum nucleation by increasing the induction period, thus retarding nucleation kinetics. Introduction The crystallization of CaSO4‚2H2O (gypsum) has been widely studied in order to highlight the effects of a large number of additives on its spontaneous precipitation, which is an unwanted occurrence in many scale control applications, such as desalination and industrial water treatment. Primary nucleation can be studied by evaluating the induction period, defined as the time which elapses between the onset of supersaturation and the formation of critical nuclei, or embryos (clusters of loosely aggregated molecules which have the same probability of growing to become crystals or dissolving to disappear into the mother solution). An extensive literature analysis has shown that a wide number of papers are focused on the effects that specific additives have on growth kinetics of gypsum crystals, especially from seeded solutions (e.g., with a secondary nucleation mechanism), while fewer papers have been published on the effect of additives on primary nucleation kinetics of gypsum, and therefore on the induction period, even though trace additives may affect the nucleation kinetic, significantly. In fact, many papers, mainly focused on the inhibition action exerted by organic and/or inorganic species, are available in the literature,1-7 but none of them gives quantitative measurements of the induction period for gypsum nucleation. As a matter of fact, few data are available for tind for gypsum nucleation, and most of them have been obtained by means of chemical or photographical techniques, which are not fast and † A reduced version of this work was presented at the 14th International Symposium on Industrial Crystallization, Cambridge, U.K., Sept 12-16, 1999. * Corresponding author. Telephone: [39](081)768-2243. Fax: [39](081)239-1800. E-mail:
[email protected]. ‡ Universita ` dell’Aquila. § Universita ` di Napoli “Federico II”. ⊥ CNR.
sensitive enough to give reliable measurements of relatively short induction periods.8,9 Eventually, in this literature analysis no paper concerning the effect of dissolved impurities on the induction period for gypsum nucleation in conditions typical of a flue gas desulfurization (FGD) process was found. Among the organic species whose effect on gypsum precipitation was tested, polymeric additives were the most used, and mainly polyelectrolytes,4 polyphosphates, and phosphonates.5,7 All cited authors agree on the circumstance that these polymeric species are effective in retarding the kinetics of gypsum crystal nucleation and growth, and their action depends on the pH and on the polymer concentration. In particular, O ¨ ner et al.6 compared the retarding influence of acid acrylate and methacrylate polymers on the growth of gypsum crystals, and they found that the nature of the polymer used as an additive has a strong result toward its action on the growth rate; e.g., the homopolymer has the highest inhibiting efficiency followed by the block copolymer and the random copolymer. Moreover, in the case of copolymers an increase in the effectiveness was found with an increase in molecular weight in the range 2000-18 000, allowing authors to presume the existence of an optimum molecular weight for each kind of polymer at which a maximum inhibiting effect would be obtained. As regards inorganic additives, aluminum ions were the most active among those studied,2,3 which enhance the degree of agglomeration of gypsum crystals and reduce the overall growth and nucleation rate but above a certain concentration cause the induction period to decrease. In a previous paper, Lancia et al.10 devised a new laser light scattering technique for the measurement of tind, which allows one to have a continuous on-line measurement of the light scattered and absorbed by the crystal suspension, giving a reliable and accurate evaluation of this parameter. The technique was used to evaluate the activation energy and the interfacial tension between gypsum crystals and the surrounding
10.1021/ie000391q CCC: $20.00 © 2001 American Chemical Society Published on Web 04/24/2001
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Figure 1. Schematic diagram of the experimental apparatus.
aqueous solution in the absence of any kind of foreign substance. The main object of the present paper is the measurement of the induction period for gypsum nucleation when an excess of chlorine exists in solutions; this is a condition similar to those encountered in a typical FGD process, where a lot of foreign ions may exert their influence on the kinetics of precipitation. The induction period values, measured by using the experimental technique previously set up (Lancia et al.10), are compared to those previously obtained in the absence of extraneous ions in the mother liquor. Experimental Apparatus and Procedure The experimental apparatus consists of a stirred reactor with a related optical device and is schematically shown in Figure 1. The reactor is a batch cylindrical crystallizer, made of glass, with a working volume of 1.0 × 10-3 m3 and a diameter of 0.09 m. The crystallizer is surrounded by a water jacket for temperature control; stirring is provided by a two-blade polypropylene stirrer, with rotation rate ranging between 1 and 10 s-1. Stimulus-response tests with an indicator ensured the perfect mixing of the liquid phase into the reactor. An off-take tube, placed at half of the working height of the vessel, allows one to remove samples of the suspension; the position of the tube has been chosen to ensure that the content of the exit stream is the same as the content of the reactor (Zacek et al.11). The stream removed by the off-take tube is sent, by a peristaltic pump, to an analysis flow-through cell and then is conveyed again to the crystallizer. The cell, made of quartz, is 0.07 m long, with a square section of 0.01 m length per side and 0.0025 m thickness. A 10 mW He-Ne laser beam (λ0 ) 632.8 nm) is focused on the cell, orthogonal to its walls; the beam is vertically polarized with a diameter of 2 mm. On the path of the laser, placed at 45° with respect to the beam direction, a beam splitter is provided in order to divide the laser beam into two parts: one is focused on the measure cell, while the other, collected by a photodiode, allows one to check the stability and the intensity of laser beam (I0). The signal of the scattered light (Isca) is collected by two lenses of focal length 120 and 50 mm, at 90° with respect to the laser beam; this signal is sent, through a quartz optical fiber which ends on an interferential filter, to a photomultiplier tube, connected to a power supply with voltage variable in the range of 0-1000 V. The signal of the transmitted light (Itrans) is collected by a photodiode located beyond the cell, at 0° with
respect to the laser beam. The two analogue signals of scattered and transmitted light, together with I0, are collected by a recorder device. Supersaturated solutions of calcium sulfate were prepared by using two clear equimolar aqueous solutions of reagent-grade CaCl2‚2H2O and Na2SO4 in bidistilled water. After their preparation, the two solutions were filtered, by using a 0.22 µm filter (Millipore) and a vacuum pump, to eliminate all foreign material inevitably present in the solution and then mixed directly into the reactor. The concentrations of CaCl2‚ 2H2O and Na2SO4 in the two feed solutions varied between 44 and 105 mol/m3. The dissolved Ca2+ ion concentration in the solution was measured by EDTA titration using Idranal Pellets (Riedel-de Haen) as an indicator, while the SO42- ion concentration was measured by means of turbidity measurements carried out in a spectrophotometer (Hach, DREL/5). With the aim to highlight the influence of Cl- ions on the induction period, the experiments were performed, keeping the supersaturation constant and varying the concentration of Cl- ions, by increasing the concentration of chloride salts such as CaCl2 and NaCl. The supersaturation (S) was calculated considering the following liquid-solid equilibrium between Ca2+ and SO42- ions and solid CaSO4‚2H2O:
Ca2+ + SO42- + 2H2O ) CaSO4‚2H2O
(1)
So that it is
S)
aCa2+aSO42-aw2 Kps
(2)
where aJ is the activity expressed as the product of the molality (mJ) and the activity coefficient (γJ) of the J species (J ) Ca2+, SO42-, and water) and Kps is the solubility product of gypsum. The value of Kps was calculated as a function of temperature by means of the relationship
ln(Kps) ) 390.9619 - 152.624 log T - 12545.62/T + 0.0818493T (3) obtained by Marshall and Slusher12 for calcium sulfate dihydrate in aqueous sodium chloride solutions from 0 to 110 °C (see also works by Barba et al.13 and Budz et al.3). The procedure used for activity coefficient calculations in the supersaturated solution is reported in a previous work (Lancia et al.10). The excess of Cl- ions with respect to equimolar conditions has been defined as follows:
E)
[Cl-] - [Cl-eqm] [Cl-eqm]
(4)
where Cl- is the molar concentration of chlorine in solution and Cl-eqm is the chlorine introduced in solution through the equimolar feed as CaCl2‚2H2O. Four different series of experiments were performed. Namely, the first has been realized in CaCl2‚2H2O and Na2SO4 equimolar conditions by varying the supersaturation; the second and the third have been carried out by varying the excess of Cl- added as CaCl2 and NaCl, respectively; the fourth has been found by realizing a fixed excess of chlorine (E ) 0.5) through the addition
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Figure 2. Intensities of the scattered (Isca) and transmitted (Itrans) light, as a function of time for different values of supersaturation and equimolar feed. a curves: S ) 3.58. b curves: S ) 2.94. c curves: S ) 2.60.
Figure 4. Effect of Na+ and Ca2+ on intensity curves. (a) S ) 2.43. A curves: no additive. B curves: E ) 0.3, cCaCl2 ) 15.54 mol/ m3. C curves: E ) 0.3, cNaCl ) 28.08 mol/m3. (b) S ) 2.60. D curves: no additive. E curves: E ) 0.3, cCaCl2 ) 17.21 mol/m3. F curves: E ) 0.3, cNaCl ) 30.85 mol/m3.
generally described in the literature by the following semiempirical correlation (Packter15), on which the curves correlating experimental data in Figure 3 are based:
tind ) K/Sr
Figure 3. Induction period as a function of supersaturation. 4: E ) 0. O: E ) 0.5. - -: eq 5.
of NaCl and varying the supersaturation in solution. The concentration of Cl- in feed solutions ranged from 1 to 133 mol/m3, considering all of the experimental series. The temperature was kept constant in all experiments at the value T ) 25 °C. The induction period was evaluated by measuring the intensity of scattered and transmitted light as a function of time and by adopting two parallel procedures, one graphical and the other one numerical, as described in detail elsewhere (Lancia et al.10), which gave quite similar ((10%) results. Results and Discussion The experimental results belonging to the first series (obtained with CaCl2 and Na2SO4 equimolar feed, e.g., without addiction of chlorine excess) are reported in Figures 2 and 3. In particular, in Figure 2 the curves of Isca/I0 and Itrans/I0 are reported as a function of time for different supersaturation values (S ) 2.60, 2.94, and 3.58). In this figure the induction period is easily recognized as the time delay before a linear decrease of the signal Itrans/I0 with time, caused by the going on of the precipitation phenomenon, and correspondingly for the signal Isca/I0, the induction period is individuated as the time delay before a signal increase with time. Figure 2 shows that, as expected, for a fixed temperature the induction period is strongly influenced by supersaturation (e.g., see Mullin14). The dependence of the induction period on supersaturation is reported in the log-log plot of Figure 3 for E ) 0 and 0.5. The figure shows that in both cases tind continuously decreases with increasing S. The dependence of the induction period on supersaturation is
(5)
where K and r are empirical constants that have been estimated by a nonlinear regression analysis to be K ) 1.3 × 105 s and r ) 5.6 when E ) 0. Experiments belonging to the second and third series are reported in Figures 4a,b, 5, and 6; all of them have been carried out by keeping the initial supersaturation value constant and by exploring the same range of values of chlorine excess in the reaction vessel (0.01 < E < 1). Figure 4a,b shows the effect of calcium chloride and sodium chloride on the Isca/I0 and Itrans/I0 curves, and therefore on the induction period, for E ) 0.3 and S ) 2.43 (Figure 4a) and for E ) 0.3 and S ) 2.60 (Figure 4b). It can be seen that, for a fixed value of supersaturation, the presence of chloride salts in solution has a strong effect on tind, which always increases, thus retarding spontaneous nucleation. In particular, for the same value of chlorine excess, the addition of NaCl makes nucleation slower than the addition of CaCl2: as shown in Figure 4a, a concentration of NaCl ) 28.08 mol/m3 (C curves) gives rise to spontaneous nucleation for an induction time tind ) 3100 s, which is not detectable on the X-axis scale chosen to show all curves in the same plot. This time is remarkably delayed with respect to the cases of the absence of added salts (A curves) and of the presence of CaCl2 (B curves). In Figure 5 all results belonging to the second experimental series are reported; the figure shows the influence of Ca2+ on the induction period for two different values of supersaturation (S ) 2.43 and 2.60). It can be seen that the presence of Ca2+ ions in solution affects the behavior of the induction period, which, with increasing CaCl2 concentration, initially grows, reaches a maximum, and then decreases, remaining always above the value relative to equimolar conditions, represented by the first experimental point obtained with an excess E ) 0. This behavior is particularly evident for S ) 2.60. In Figure 6 all results belonging to the third experimental series are reported; the figure shows the influence of Na+ on the induction period for S ) 2.43 and 2.60. From the plot it can be observed that the induction
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the gypsum solubility curve, calculated in a previous work (Prisciandaro et al.17) by using the equilibrium equations and the mass balances among different species in solution, was used. That curve reports that the calcium sulfate dihydrate solubility continuously decreases while the calcium concentration increases in the range 25-200 mol/m3; this result could justify the increase in the induction period experimentally obtained (Figure 5). Conclusions Figure 5. Effect of Ca2+ on the induction period. Void symbols: equimolar conditions.
The induction period for primary nucleation can be reliably used as a parameter able to evaluate the effectiveness of various kinds of additives on nucleation kinetics. In this work the effect of chloride salts (NaCl and CaCl2) on calcium sulfate dihydrate nucleation was studied, and experimental results have shown that the influence is a consequence of the different ions (Na+ or Ca2+) added in solution to increase the Cl- concentration with respect to the equimolar feed. In particular, the addition of CaCl2 or NaCl always increases tind, thus retarding spontaneous nucleation, but NaCl has a stronger effect on primary nucleation. The experimental results were interpreted by means of the behavior of gypsum solubility in NaCl and CaCl2 water solutions. List of Symbols
Na+
Figure 6. Effect of equimolar conditions.
on the induction period. Void symbols:
period is indeed influenced by the presence of Na+, and even in this case the trend of tind is always above its value relative to equimolar conditions (E ) 0). The comparison between Figures 5 and 6 shows in a clear way that, for the same range of chlorine excess explored, the influence of Na+ on tind is stronger than that of Ca2+. With the aim of making an easier comparison, in Figure 3 experimental results belonging to the fourth series are reported. This figure illustrates the dependence of the induction period on supersaturation for T ) 25 °C and a fixed value of chlorine excess (see eq 4), namely, E ) 0.5; moreover, it is possible to observe the comparison between the two situations E ) 0 (Lancia et al.10) and 0.5. In the case of E ) 0.5, the values of the constants in eq 5 are K ) 1.1 × 105 min and r ) 5, which are quite similar to values obtained in the case of E ) 0, indicating that the nature of the dependence of the induction time on supersaturation for a fixed value of temperature is almost independent of the composition of mother liquor, but the curve is translated at higher values of induction times, strengthening the fact that the presence of a chlorine excess has a vigorous effect on the induction period value. To interpret the results experimentally obtained, the effect of chloride salts on the calcium sulfate solubility has been investigated. Concerning the behavior of the induction period as a function of the NaCl concentration, the continuous decrease of tind can be due to an increase in the calcium sulfate solubility in NaCl solutions (Barba et al.13). Similar results were obtained by He et al.16 that found a decrease in the induction period for calcium sulfate dihydrate as the NaCl concentration increases up to 3.0 × 103 mol/m3. For what concerns the behavior of the induction period as a function of the CaCl2 excess concentration,
a ) activity, mol/m3 E ) excess of chlorine ion defined by eq 4 I ) intensity of light, W/m2 K ) constant in eq 5, s Kps ) solubility product, mol4/kg4 m ) molality, mol/kg r ) constant in eq 5 S ) supersaturation t ) time, s tind ) induction period, s T ) absolute temperature, K Greek Symbols γ ) activity coefficient λ0) wavelength, m Subscripts eqm ) equimolar J ) generic species in solution sca ) scattering trans ) transmitted w ) water 0 ) relative to incident light
Literature Cited (1) Etherthon, D. L.; Randolph, A. D. Nucleation/growth rate kinetics of gypsum in simulated FGD liquors: some process configurations for increasing particle size. AIChE Symp. Ser. 1981, 77, 87. (2) Sarig, S.; Mullin, J. W. Effect of impurities on calcium sulphate precipitation. J. Chem. Technol. Biotechnol. 1982, 32, 525. (3) Budz, J.; Jones, A. G.; Mullin, J. W. Effect of selected impurities on the continuous precipitation of calcium sulphate (gypsum). J. Chem. Technol. Biotechnol. 1986, 36, 153. (4) Amjad, A. B.; Hooley, J. Influence of polyelectrolytes on the crystal growth of calcium sulphate dihydrate. J. Colloid Interface Sci. 1986, 111, 496. (5) Klepetsanis, P. G.; Koutsoukos, P. G. Kinetics of calcium sulfate formation in aqueous media: effect of organophosphorus compounds. J. Cryst. Growth 1998, 193, 156.
Ind. Eng. Chem. Res., Vol. 40, No. 10, 2001 2339 (6) O ¨ ner, M.; Dogan, O ¨ .; O ¨ ner, G. The influence of polyelectrolytes architecture on calcium sulfate dihydrate growth retardation. J. Cryst. Growth 1998, 186, 427. (7) Wilson, M. P.; Rohl, A. L.; McKinnon, A. J.; Gale, J. D. An experimental and molecular modelling investigation into the inhibition of gypsum crystallisation by phosphonate additives. Proceedings of Industrial Crystallization, Cambridge, U.K., Sept 12-16, 1999. (8) So¨hnel, O.; Mullin, J. W. A method for the determination of precipitation induction periods. J. Cryst. Growth 1978, 44, 377. (9) So¨hnel, O.; Mullin, J. W. Interpretation of crystallization induction periods. J. Colloid Interface Sci. 1988, 123, 43. (10) Lancia, A.; Musmarra, D.; Prisciandaro, M. Measurement of the induction period for calcium sulfate dihydrate precipitation. AIChE J 1999, 45, 390. (11) Zacek, S.; Nyvlt, J.; Garside, J.; Nienow, A. W. A stirred tank for continuous crystallization studies. Chem. Eng. J. 1982, 23, 111. (12) Marshall, W. L.; Slusher, R. Thermodynamics of calcium sulfate dihydrate in aqueous sodium chloride solutions, 0-110°.
J. Phys. Chem. 1966, 70, 4015. (13) Barba, D.; Brandani, V.; Di Giacomo, G. A thermodynamic model of CaSO4 solubility in multicomponent aqueous solutions. Chem. Eng. J. 1982, 24, 191. (14) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann Ltd.: Oxford, U.K., 1993. (15) Packter, A. J. Chem. Soc. 1968, 859 (cited by So¨hnel and Mullin8). (16) He, S.; Oddo, J. E.; Tomson, M. B. The nucleation kinetics of calcium sulfate dihydrate in NaCl solutions up to 6 m and 90 °C. J. Colloid Interface Sci. 1994, 162, 297. (17) Prisciandaro, M.; Lancia, A.; Musmarra, D. The influence of typical gypsum impurities on gypsum nucleation. Proceedings of Industrial Crystallization, Cambridge, U.K., Sept 12-16, 1999.
Received for review April 6, 2000 Revised manuscript received February 16, 2001 Accepted February 23, 2001 IE000391Q
