Gypsum Precipitation from an Aqueous Solution in the Presence of

Dipartimento di Chimica, Ingegneria Chimica e Materiali, UniVersita` dell'Aquila, Monteluco di Roio, 67040. L'Aquila (AQ), Italy, Dipartimento di Inge...
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Gypsum Precipitation from an Aqueous Solution in the Presence of Nitrilotrimethylenephosphonic Acid Marina Prisciandaro,† Emilia Olivieri,‡ Amedeo Lancia,*,‡ and Dino Musmarra§ Dipartimento di Chimica, Ingegneria Chimica e Materiali, UniVersita` dell’Aquila, Monteluco di Roio, 67040 L’Aquila (AQ), Italy, Dipartimento di Ingegneria Chimica, UniVersita` degli Studi di Napoli “Federico II”, Piazzale Tecchio 80, 80125 Napoli (NA), Italy, and Dipartimento di Ingegneria CiVile, Seconda UniVersita` degli Studi di Napoli, Real Casa dell’Annunziata, Via Roma 29, 81031 AVersa (CE), Italy

The effect of nitrilotrimethylenephosphonic acid (NTMP) on calcium sulfate dihydrate nucleation has been studied at 25 °C in a batch crystallizer with a related optical device. This paper reports the measurement of the induction period done under different supersaturation ratios ranging from 2.60 to 4.99, while varying the NTMP concentration in the interval 0.005-0.1 g/L. The comparison of the interfacial tension values estimated in the presence of NTMP with respect to those previously obtained without any additive in the mother liquor and in the presence of citric acid, by using the nucleation-controlled mechanism, suggests that NTMP is more effective in retarding gypsum nucleation than citric acid. Besides, the examination by optical microscopy of the crystals formed indicates that NTMP also modifies the crystal habit of gypsum, resulting in less elongated platelike particles. Introduction The addition of additives in a supersaturated solution is an usual technique employed to modify nucleation and crystal growth mechanisms. A number of papers reported that the calcium sulfate dihydrate formation is reduced in the presence of additives such as polyelectrolytes,1 citric acid,2 and organophosphorus compounds.3 The presence of additives may also induce desirable changes in crystal structure, size, and morphology and thus improve the filtration rate and productivity when calcium sulfate dihydrate is obtained as a byproduct of various chemical processes such as wet production of phosphoric acid and flue gas desulfurization (FGD).4 The study of the effect of different additives on the gypsum nucleation can be carried out by evaluating the induction period, tind, defined as the time that elapses between the onset of supersaturation and the first changes in the system physical properties due to the formation of a solid phase.6 This time depends on solution supersaturation, temperature, and experimental techniques employed to detect the formation of the solid phase, and it is the sum of two components: the nucleation time (tn), related to the appearance of the critical nuclei, and the growth time (tg), connected to the growth process that leads from critical nuclei to measurable crystals. Depending on the relative values of these two time periods, the induction time can be influenced by nucleation alone (tn . tg; nucleationcontrolled induction period), by both mechanisms (tn = tg; nucleation- and growth-controlled induction period), or by growth alone (tn , tg; growth-governed induction period).5,6 While tg can be estimated from a kinetic growth expression, tn is more difficult to quantify. Nevertheless, it is possible to discriminate whether the appearance of the new solid phase is controlled by nucleation or by growth or both, on the basis of the dependence of tind on supersaturation. In particular, if the process that takes place is truly homogeneous nucleation (i.e., * To whom correspondence should be addressed. Tel.: [+39](81)7682243. Fax: [+39](81)5936936. E-mail: [email protected]. † Universita` dell’Aquila. ‡ Universita` degli Studi di Napoli “Federico II”. § Seconda Universita` degli Studi di Napoli.

it occurs in a clear solution under the effect of supersaturation alone), tind is inversely proportional to the nucleation rate, defined as the number of nuclei formed in solution per unit time and volume. In this case, as shown by Mullin7 and So¨hnel and Garside,6 tind dependence on supersaturation allows one to determine the interfacial tension among crystals and the surrounding solution. Actually, the dependence of the induction time on the supersaturation ratio (σ) assumes different shapes, depending on the prevailing mechanism, nucleation or growth; namely, if tn . tg or tn = tg, a linear relationship between log(tind) and (log σ)-2 does exist, with a different value of the regression slope; on the contrary, when tn , tg, the correlation is between log(tind) and (log σ)-1.6,8 Among phosphonic acid derivatives, nitrilotrimethylenephosphonic acid (NTMP) is the first one used as a cost-effective gypsum-scale inhibitor in circulating cooling water treatment, industrial boilers, and heat exchangers.9 NTMP markedly retards the rate of crystallization and produces the conditions for the appearance of a retard in nucleation, whose duration increases with an increase in the additive concentration. This has been explained in terms of two effects: direct chelation with the crystal lattice ions in the solution and adsorption on the crystal surfaces either generally or on particular faces or crystal sites. An adsorption process appears to offer a more feasible explanation of the effect because the foreign ions are usually present at a very much smaller concentration than that of the crystal ion.10 Moreover, the addition of NTMP modifies the crystal habit from the common needlelike form to a less elongated one.11 In particular, Amathieu and Boistelle12 reported that NTMP strongly adsorbs onto gypsum, forming bonds between the calcium ions and the phosphonate groups, accompanied by hydrogen bonding between another oxygen atom of the same group and the water molecules closer to the calcium ions. The strong adsorption of NTMP onto gypsum could possibly account for the lack of long needlelike morphology of the crystals formed.3 The surface coverage needed for growth inhibition of gypsum crystals in the presence of phosphonates is of about 4-5%, and

10.1021/ie050615a CCC: $33.50 © 2006 American Chemical Society Published on Web 02/11/2006

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Figure 1. Schematic diagram of the experimental apparatus.

the adsorption process is irreversible, as reported by Weijen and Van Rosmalen.13,14 Klepetsanis and Koutsoukos3 determined the effect of NTMP, N,N,N′,N′-ethylenediaminetetramethylenephosphonic acid (ENTMP), and 1-hydroxyethylidene-1,1-diphosphonic acid (EHDP) on the kinetics of spontaneous precipitation of calcium sulfate dihydrate at 30 °C, resulting in NTMP and ENTMP increasing drastically the induction times preceding precipitation, while the rates of precipitation were reduced by 90% at a concentration below 1 µM. The different effectiveness of the tested organophosphorus compounds may reflect differences in the extent of adsorption on the growing supercritical nuclei. More recently, El-Shall et al.15 studied the effect of NTMP on gypsum nucleation under conditions similar to those found in the wet process of phosphoric acid production. The results indicate that NTMP increases the induction time at all studied supersaturation ratios because of the decrease of the regular crystal growth. Moreover, the surface energy is decreased while the nucleation rate is increased if compared with the baseline in the presence of NTMP. Although the effects of NTMP on gypsum crystallization are well described, few data have been reported for homogeneous nucleation and significant parameters such as interfacial tension between gypsum crystals and the mother solution in the presence of NTMP. Therefore, the main objective of the present work is the measurement of the induction period (tind) for gypsum nucleation when NTMP is added in solution as an additive, by using a well-assessed laser light scattering technique previously devised.16 The measured induction period values are used to estimate the values for the interfacial tension between gypsum crystals and the mother solution, for each selected NTMP concentration level. The obtained values are then compared to those estimated in the absence of additives in the mother liquor16 and in the presence of citric acid2,17 at the same NTMP concentration level. 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 1.1 × 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-bladed polypropylene stirrer, with the rotation rate ranging between 1

and 10 s-1. 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.18 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; because we are interested in induction time measurements, it is reasonable to think that crystal nuclei are small enough to be undamaged while passing through the pumping device, though located after that of the measure cell. The cell, made of quartz, is 0.07 m long, with a square section of 0.01 m2 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, whose diameter is 2 mm, is vertically polarized. On the path of the laser beam, placed at 45° with respect to its direction, a beam splitter is provided in order to divide the laser beam into two parts; one is used to illuminate the measure cell, while the other, collected by a photodiode, allows one to check the stability and intensity of the laser beam (I0). The signal of the scattered light (Isca) is collected by two lenses of focal lengths 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 mixing clear aqueous solutions of reagent-grade CaCl2‚2H2O, Na2SO4 (Applichem, Darmstadt, Germany), and N(CH2PO3H2)3 (Acros Organics, Geef, Belgium) in bidistilled water. The dissolved Ca2+ ion concentration was measured by ethylenediaminetetraacetic acid titration using Murexide (Applichem, Darmstadt, Germany) as an indicator, while the SO42- ion concentration was measured by means of turbidity measurements carried out in a spectrophotometer (Hach model 2010). The NTMP concentration was determined by converting it into the orthophosphate form with potassium persulfate and a UV lamp and analyzing the free orthophosphate by means of a spectrophotometer (Hach model 2010). After their preparation, the three solutions were filtered, by using a 0.45-µm filter (Millipore, model HVLP 4700) and a vacuum pump (Vacuubrand, model MZ4C), to eliminate all

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foreign material inevitably present in the solution. A NTMP aqueous solution was added to the Na2SO4 solution and then fed to the reactor. The equimolar concentration of CaCl2‚2H2O and Na2SO4 in the reactor varied between 65 and 145.2 mol/ m3, while the NTMP concentration (cNTMP) varied in the interval 0.005-0.1 g/L (1.7 × 10-2-3.4 × 10-1 mol/m3). As for the tind calculations, three NTMP levels were explored, namely, cNTMP ) 0.005 g/L, cNTMP ) 0.01 g/L, and cNTMP ) 0.05 g/L, and the relative pH values were 4.1, 3.8, and 3.1. The lower level (cNTMP ) 0.005 g/L) was selected by considering the dosage range of NTMP typically used as a scale inhibitor in process equipment.9 The supersaturation ratio was calculated considering the liquid-solid equilibrium between Ca2+ and SO42- ions and solid CaSO4‚2H2O, as described by the equation

Ca2+ + SO42- + 2H2O ) CaSO4‚2H2O

(1)

so that it is

σ)

[Ca2+][SO42-][H2O]2 Ksp

(2)

where [J] represents the activity of the J species (J ) Ca2+, SO42-, and H2O) expressed as the product of the molality (mJ) and the activity coefficient (γJ) and Ksp is the solubility product of gypsum. The value of Ksp was calculated as a function of temperature by means of the following relationship:19

ln(Ksp) ) 390.96 - 152.62 log T - 12545.62/T + 0.08T (3) The activity coefficients in the supersaturated solution were calculated from a modified Deybe-Hu¨ckel equation reported in the Appendix. All experiments have been carried out for various values of NTMP concentrations (0.005-0.1 g/L) while changing the supersaturation ratio in the range 2.60-4.99. The temperature was fixed in all experimental runs at the value of 25 °C. The values for pH were in the range 3.1-4.1. The induction period was evaluated by measuring the intensity of scattered and transmitted light signals as a function of time. Such signals have been processed to evaluate tind by adopting two parallel procedures, one graphical and the other numerical. These procedures, described in detail elsewhere,16 gave quite similar ((10%) results. Gypsum crystals grown in the presence of 0.1 g/L NTMP were analyzed by an optical microscope (Zeiss, Axioscope FS), equipped for transmitted light work, and then compared to those grown in solution without any additive. Results and Discussion Parts a and b of Figure 2 show the curves of Itrans/I0 (upper curves) and Isca/I0 (lower curves) as a function of time for three different values of the supersaturation ratio each (σ ) 4.83, 4.66, and 4.16) in the presence of NTMP added in solution (Figure 2a, cNTMP ) 0.01 g/L; Figure 2b, 0.05 g/L). For the same NTMP concentration, Figure 2 shows the effect of supersaturation on the induction period, which, as expected, decreases with increasing σ;7 but more interesting is the effect of NTMP, which strongly retards gypsum nucleation when it is added in solution. As an example, for σ ) 4.16, tind passes from 2.65 min for cNTMP ) 0.01 g/L to 8.50 min when cNTMP grows to the value of 0.05 g/L, with a percentage increase of about 70%.

Figure 2. Intensities of the scattered (Isca/I0) and transmitted (Itrans/I0) light, as a function of time for different values of the supersaturation ratio in the presence of NTMP (T ) 25 °C): (a) cNTMP ) 0.01 g/L; (b) cNTMP ) 0.05 g/L.

Parts a-c of Figure 3 show the dependence of the induction period on the supersaturation ratio for three levels of NTMP concentration, namely, cNTMP ) 0.005, 0.01, and 0.05 g/L, respectively. It can be observed that the induction period for gypsum nucleation continuously decreases with an increase in supersaturation. The fitting of experimental data has been carried out by using the following semiempirical correlation,20 reported as a continuous line in Figure 3a-c:

tind ) K/σr

(4)

where K and r are empirical constants. By a regression analysis, the following values for the exponent r were determined: r ) 8.7 (cNTMP ) 0.005 g/L); r ) 8.5 (cNTMP ) 0.01 g/L); r ) 8.2 (cNTMP ) 0.05 g/L). Experimental data of the induction period plotted as a function of supersaturation levels can be adequately used, as suggested by So¨hnel and Garside,6 to discriminate whether the appearance of the new solid phase is controlled by nucleation and/or by growth. Moreover, always on the basis of the dependence of tind on supersaturation, it is possible to distinguish between homogeneous and heterogeneous nucleation phenomena. This is worth noting in the estimation of some characteristic parameters of primary nucleation, such as the interfacial tension. To this purpose, at first the following equation derived from homogeneous nucleation equations, applicable when the appearance of the new solid phase is controlled by nucleation, was considered:5,8

log(tind) ) C +

D T 3(log σ)2

(5)

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Figure 3. Induction period as a function of the supersaturation ratio (T ) 25 °C): (a) cNTMP ) 0.005 g/L; (b) cNTMP ) 0.01 g/L; (c) cNTMP ) 0.05 g/L.

Figure 4. Induction period as a function of the supersaturation ratio (T ) 25 °C): (a) cNTMP ) 0.005 g/L; (b) cNTMP ) 0.01 g/L; (c) cNTMP ) 0.05 g/L. Symbols: (4) homogeneous nucleation; (O) heterogeneous nucleation; (0) transition.

where C is an empirical constant and D is given by

D)

βγs3Vm2NAf(φ) (2.3R)3ν2

(6)

in which β is a shape factor, γs is the interfacial tension, NA is Avogadro’s number, R is the gas constant, Vm is the molar volume, ν is the number of ions in which the molecule is dissociated, and f(φ) is a correction factor, which takes into account the heterogeneous nucleation; in particular, according to So¨hnel and Mullin,5 when purely homogeneous nucleation takes place, it is f(φ) ) 1, while when heterogeneous nucleation occurs, it is f(φ) < 1. Consequently, a change in the slope of the experimental results may indicate a transition from homogeneous to heterogeneous nucleation mechanisms. In Figure 4 a-c, the distinction between the two nucleation mechanisms is reported for three levels of NTMP concentration at the temperature of T ) 25 °C and the passage from homogeneous to heterogeneous nucleation is well visible. To gain values relative to the homogeneous nucleation region solely, this distinction has been carried out for all NTMP concentration levels studied, and the data belonging to the homogeneous nucleation region for all levels studied are reported in Figure 5. The estimation of the interfacial tension values, carried out by linearizing the experimental data according to eqs 5 and 6, gives the three values of 41.2 mJ/m2 (cNTMP ) 0.005 g/L), 46.5 mJ/m2 (cNTMP ) 0.01 g/L), and 54.0 mJ/m2 (cNTMP ) 0.05 g/L), whose average value is 47.2 mJ/m2 (in eq 6, it was considered β ) 16π/3, assuming a spherical particle, and Vm ) 74.69 cm3/mol). Because it is not possible to distinguish a priori which mechanism controls the induction period, experimental data have

Figure 5. Induction period as a function of the supersaturation ratio (T ) 25 °C): (O) cNTMP ) 0.05 g/L; (4) cNTMP ) 0.01 g/L; (×) cNTMP ) 0.005 g/L.

been also interpolated according to the other relationships, valid when tn = tg and tn , tg. For the three NTMP concentration levels selected (0.005, 0.01, and 0.05 g/L), rather different values of the interfacial tension are found, less comparable with interfacial tension values found in the literature for calcium sulfate dihydrate. The values of γs in the presence of NTMP obtained by using the nucleation-controlled mechanism, if compared with those obtained in the absence of any additive16 and in the presence of citric acid17 with the same kinetic model, confirm that NTMP is a much stronger retardant than the others studied. The comparison is reported in Table 1: it can be noted that, for the same additive concentration, γs in the presence of NTMP is increased about 20-25% with respect to the value estimated in the presence of citric acid.

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Table 1. Estimated Interfacial Tension Values for Calcium Sulfate Dihydrate in Aqueous Solutions at T ) 25 °C additive NTMPa NTMPa NTMPa a

c (g/L)

γs (mJ/m2)

0.005 0.01 0.05

additive acidb

41.2 46.5 54.0

citric citric acidb citric acidb baselinec

c (g/L)

γs (mJ/m2)

0.01 0.05 0.10 0.00

37.0 41.0 43.8 31.9

This work. b Prisciandaro et al. (2003). c Lancia et al. (1999).

Table 2. Interfacial Tension Values for Calcium Sulfate Dihydrate in Aqueous Solutions Available in the Literature T (°C)

γs (mJ/m2)

ref

T (°C)

γs (mJ/m2)

ref

41.1 39.3 76.0 23.2 95.0 18.0 5.79 5.65

8a

80 80 80 20 30 25 25

8.4 8.7 6.7 14.3 13.9 11.0 8.9

32 32d 32e 33 33 34 34

25 25 30 30 25 25 80 80

8a 28 29 30 31 15b 15c

a With NaCl in solution. b Values calculated by using the experimental data reported by El-Shall et al., 2002, for the baseline. c Values calculated by using the experimental data reported by El-Shall et al., 2002, with NTMP in solution. d With CTAB in solution. e With SDS in solution.

Table 2 reports the values for the interfacial tension collected from the literature. It is well recognizable from the data dispersion that a good agreement among researchers has not been found, nor for the values in the absence of any additive. In addition, the only value in the presence of 100 ppm NTMP is that found by El-Shall and co-workers,15 but it is worth noting that it was calculated by the slopes of the experimental data presented in their work by using eq 5 reported in their paper because the authors did not directly perform the calculation. Furthermore, a qualitative effect of NTMP toward the shape of gypsum crystals may also be interesting: the habit modification induced by NTMP on gypsum crystals is shown in Figure 6. For the same supersaturation ratio (σ ) 3.88), the upper image shows long needlelike particles grown in the absence of any additive in the mother liquor, while the lower image refers to the less elongated platelike particles, similar to those obtained in the presence of citric acid,21 formed when NTMP is added in solution (cNTMP ) 0.1 g/L). This change in the crystal’s shape may be useful in the case in which gypsum precipitation is followed by a solid-liquid separation, such as in the limegypsum FGD process: a crystal block or plate shape seems to have strong positive outcomes on the sludge dewatering characteristics.22,23 Conclusions In this paper, the effect of NTMP acid on gypsum nucleation kinetics is studied; NTMP appears to be an effective retardant, even if compared with other tested efficacious additives, such as citric acid, proving itself to be a significant additive in studying gypsum fouling problem avoidance. The interfacial tension values have been estimated by using the nucleationcontrolled mechanism for the induction period, and the results are that, for the three NTMP concentration levels selected (0.005, 0.01, and 0.05 g/L), the average value of 47.2 mJ/m2 was obtained, higher than that estimated both in the absence of any additive (31.9 mJ/m2) and in the presence of citric acid (40.6 mJ/m2) for the same temperature T ) 25 °C. Moreover, optical microscope images have shown that less elongated platelike particles are obtained with respect to the case of any additive absence, having this circumstantial important outcome

Figure 6. Optical microscope images of gypsum crystals: upper image, without NTMP; lower image, cNTMP ) 0.1 g/L.

on the sludge dewatering characteristics as in the lime-gypsum FGD processes. Appendix The values of activity coefficients (γJ) for calcium and sulfate ions as a function of ionic strength were calculated for calcium sulfate by means of the model of Debye-Hu¨ckel, extended to ionic strength up to 0.6 mol/dm3.

log γJ ) -AzJ2

(

xFI - bFI 1 + axFI

)

(A.1)

where zJ is the charge of J species, FI is the solution ionic strength (mol/dm3), a (dm3/2/mol1/2) and b (dm3/mol) are two constants, and A (dm3/2/mol1/2) is a temperature-dependent constant. The ionic strength is defined by the following relationship:

FI )

1

n

cJzJ2 ∑ 2J)1

(A.2)

where cJ is the concentration of J species (mol/dm3), while the dependency of A on the temperature is expressed by

A ) 1.82 × 106(T)-3/2

(A.3)

where T is the absolute temperature (K) and  is the dielectric constant. For the constant a in eq A.1, the value a ) 1 has been used, while more arguments have been reserved to the choice of the b constant value. In the activity coefficient calculations, for the system under experimentation, a value of b derived from the comparison with a more accurate method, Bromley’s method,24 valid in solution up to 6 M ionic strength has been carried out. As a matter of fact, the constant b has no prefixed value, but its value is

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typically determined by the ionic solution under observation, by matching ionic calculations and experimental results.25 In particular, in this case, the value of b has been found to match the Debye-Hu¨ckel extended equation results and Bromley’s method results for a solution of CaCl2‚2H2O and Na2SO4 in the absence of any foreign substance. This practice has been made necessary for two reasons: first, in the supersaturation ratio range in which experiments have been carried out, the ionic strength sometimes is above the limit of applicability of eq A.1 as is; last, the application of Bromley’s method16,26,27 requires the knowledge of certain specific constants available in the literature only for sparingly soluble salts, and therefore not for NTMP, very soluble in water. The procedure has been carried out for different temperatures, finding each time the value of b that gives the better match between the two approaches (Debye-Hu¨ckel extended method and Bromley’s method) by using the least-squares method. The values of b as a function of temperature for the system under examination have been interpolated by using the following equation:

b ) -0.30922 + 0.0010331T

(A.4)

For the calculation of the water activity, the correlation by Bromley,24 based on the osmotic coefficient φ, was used. The relationship between the water activity coefficient (aw) as a function of the osmotic coefficient is the following:

φ)

-1000 ln aw Mw ImI

∑

(A.5)

where Mw is the water molecular weight and mI is the molality of the I species present in solution. Nomenclature Symbols a ) constant in eq A.1, dm3/2/mol1/2 aw ) water activity coefficient A ) temperature-dependent constant in eq A.1, dm3/2/mol1/2 b ) constant in eq A.1, dm3/mol c ) concentration, mol/m3 C ) constant in eq 5 D ) constant in eq 5, K3 f(φ) ) correction factor in eq 6 FI ) ionic strength, mol/m3 I ) intensity of light, W/m2 K ) constant in eq 4, s Ksp ) solubility product, mol4/kg4 m ) molality, mol/kg Mw ) water molecular weight, g/mol NA ) Avogadro’s number, 1/mol r ) constant in eq 4 R ) gas constant, J/mol‚K t ) time, s tind ) induction period, s T ) absolute temperature, K Vm ) molar volume, m3/mol z ) electric charge Greek Letters β ) shape factor γ ) activity coefficient γs ) interfacial tension, J/m2  ) dielectric constant

λ0 ) wavelength, m ν ) number of ions σ ) supersaturation ratio φ ) osmotic coefficient Subscripts g ) growth J ) chemical species n ) nucleation NTMP ) nitrilotrimethylenephosphonic acid sca ) scattering trans ) transmitted w ) water 0 ) relative to incident light Literature Cited (1) O ¨ ner, M.; Dogan, O ¨ .; O ¨ ner, G. The influence of polyelectrolytes architecture on calcium sulphate dihydrate growth retardation. J. Cryst. Growth 1998, 186, 427. (2) Prisciandaro, M.; Lancia, A.; Musmarra, D. Citric acid retarding effect on gypsum nucleation. Chem. Eng. Trans. 2002, 1, 677. (3) Klepetsanis, P. G.; Koutsoukos, P. G. Kinetics of calcium sulphate formation in aqueous media: effect of organophosphorus compounds. J. Cryst. Growth 1998, 193, 156. (4) Lancia, A.; Prisciandaro, M.; Musmarra, D. Calcium Sulfate. Contribution in Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; John Wiley: New York, 2002. (5) So¨hnel, O.; Mullin, J. W. Interpretation of crystallization induction periods. J. Colloid Interface Sci. 1988, 123, 43. (6) So¨hnel, O.; Garside, J. Precipitation; Butterworth-Heinemann Ltd.: Oxford, U.K., 1992. (7) Mullin, J. W. Crystallization, 3th ed.; Butterworth-Heinemann Ltd.: Oxford, U.K., 1993. (8) 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. (9) Screening Information Data Set (SIDS) for High Volume Chemicals, 1993. http://www.chem.unep.ch/irptc/sids/OECDSIDS/6419198.pdf. (10) Liu, S. T.; Nancollas, G. H. The crystal growth of calcium sulphate dihydrate in the presence of additives. J. Colloid Interface Sci. 1973, 44, 422. (11) Tadros, M. E.; Mayes, I. Linear growth rates of calcium sulphate dihydrate crystals in the presence of additives. J. Colloid Interface Sci. 1979, 72, 245. (12) Amathieu, L.; Boistelle, R. Improvement of the mechanical properties of set plasters by mean of four organic additives inducing {101} faces. J. Cryst. Growth 1986, 79, 169 (as cited in ref 3). (13) Weijen, M. P. C.; Van Rosmalen, G. M. The role of additives and impurities in the crystallization of gypsum. In Industrial Crystallization 84; Jancic, S. J., de Jong, E. J., Eds.; Elsevier: Amsterdam, The Netherlands, 1984; pp 61-66 (as cited in ref 15). (14) Weijen, M. P. C.; Van Rosmalen, G. M. Adsorption of phosphonates on gypsum crystals. J. Cryst. Growth 1986, 79, 157 (as cited in ref 15). (15) El-Shall, H.; Rashad, M. M.; Abdel-Aal, E. A. Effect of phosphonate additive on crystallization of gypsum in phosphoric and sulfuric acid medium. Cryst. Res. Technol. 2002, 37, 1264. (16) Lancia, A.; Prisciandaro, M.; Musmarra, D. Measuring of the induction period for calcium sulphate dihydrate precipitation. AIChE J. 1999, 45, 390. (17) Prisciandaro, M.; Lancia, A.; Musmarra, D. The retarding effect of citric acid on calcium sulfate nucleation kinetics. Ind. Eng. Chem. Res. 2003, 42, 6647. (18) Zacek, S.; Nyvlt, J.; Garside, J.; Nienow, A. W. A stirred tank for continuous crystallization studies. Chem. Eng. J. 1982, 23, 111. (19) Marshall, L. W.; Slusher, R. Thermodynamics of calcium sulphate dihydrate in aqueous sodium chloride solutions, 0-110°. Phys. Chem. 1966, 70, 4015. (20) Packter, A. The precipitation of calcium sulphate dihydrate from aqueous solutionsInduction period, crystal number and final size. J. Cryst. Growth 1974, 21, 191. (21) Olivieri, E. Cristallizzazione del solfato di calcio biidrato in presenza di acido nitrilotrimetilenfosfonico. M.S. Thesis, University of Naples, Naples, Italy, 2003.

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ReceiVed for reView May 24, 2005 ReVised manuscript receiVed October 7, 2005 Accepted October 18, 2005 IE050615A