Effect of phosphonate additive on crystallization of

Cryst. Res. Technol.

37

2002

12

1264–1273

H. EL-SHALL*1, M. M. RASHAD2, E. A. ABDEL-AAL2 1

Engineering Research Center for Particle Science & Technology, Department of Materials Science and Engineering, University of Florida, Gainesville, FL, USA 2 Central Metallurgical Research & Development Institute, Helwan, Cairo, Egypt

Effect of phosphonate additive on crystallization of gypsum in phosphoric and sulfuric acid medium Understanding the mechanisms of growth and inhibition during crystallization of calcium sulfate is of primary importance for many industrial applications. For instance, inhibition of the crystallization process may be required to prevent scale formation in pipes, boilers, heat exchangers, reactors, reverse osmosis membrane surfaces, cooling water systems, secondary oil recovery utilizing water flooding techniques and desalination evaporators, etc. On the other hand, control growth and morphology of gypsum crystals is desired in achieving higher filtration rate and higher productivity of phosphoric acid from phosphate rocks. In this regard, this basic study is carried out to understand effect of Aminotris (methylenephosphonic acid (ATMP) on calcium sulfate dihydrate (gypsum) crystallization. The time elapsed between the achievement of supersaturation and the appearance of a solid phase (termed as induction time) is measured under different supersaturation ratios ranging from 1.018 to 1.979. The data are used to calculate the surface energy, critical nucleus size, and crystal growth rates of gypsum under different conditions. The results show that, the induction time decreases exponentially with increasing the supersaturation ratio. In addition, the surface energy decreases with ATMP compared to the baseline (without ATMP). Interestingly, with addition of the ATMP, the crystals mean and median diameters are found to decrease. The inhibition efficiency ranges from 16% to 59% depending on supersaturation ratio. Keywords: crystallization inhibition, calcium sulfate dihydrate, crystal size distribution, induction time, aminotris (methylenephosphonic acid). (Received July 22, 2002; Accepted September 24, 2002)

Introduction Understanding the mechanism of growth and inhibition during crystallization of calcium sulfate is of primary importance for industrial application. For instance, inhibition of the crystallization process may be required to prevent scale formation in pipes, boilers, heat exchangers, reactors, reverse osmosis membrane surfaces, cooling water systems, secondary oil recovery utilizing water flooding techniques and desalination evaporators as reported by van Rosmalen and his coworkers [1]. On the other hand, growth of gypsum crystals is desired in achieving higher filtration rate and higher productivity of phosphoric acid from phosphate rocks according to El-Shall et al [2]. Crystallization is a result of three consequent stages: supersaturation, nucleation and crystal growth. Supersaturation is characterized by existence of ordered aggregates (embryos or clusters) of molecules of various sizes in solution [3]. When an aggregate exceeds a particular size (critical nucleus size), it becomes stable nucleus that grows in size spontaneously. Nucleation can be classified as primary * corresponding author: [email protected] © 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0232-1300/02/1212-1264 $ 17.50+.50/0

Cryst. Res. Technol. 37 (2002) 12

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(when no seed crystals are present) or secondary (in presence of seed crystals or in presence of growing primary crystals) [4]. Organic and inorganic additives play an important role in crystallization. They alter the surface properties of the crystals. In addition, additives may affect nucleation rate, crystal growth, and / or shape of the crystals and their agglomeration or dispersion behavior [5]. For example, Klepetsanis and Koutsoukos [6] prepared calcium sulfate dihydrate crystals by mixing solutions of sodium sulfate and calcium nitrate at pH 7.0 and 30oC in presence and absence of organophosphorus compounds. They found that the induction time has highly increased and the rate of precipitation has decreased. In another study, Oner et al [7] reported that increase in temperature lead to substantial decrease in the induction time at a constant supersaturation state. Liu and Nancollas [8] studied the crystallization kinetics of calcium sulfate dihydrate seed crystals, which were prepared by slow drop-wise addition of calcium chloride solution to sodium sulfate solution at 25oC. The presence of ENTMP (N,N,N-,N-ethylene diaminotetra methylene phosphonic acid) and TENTMP (N,N,N-,N-triethylene diamine tetra (methylene phosphonic acid)) markedly retarded the rate of crystallization. This was attributed to the adsorption of these additives on available growth sites of the growing crystal surfaces. In a later study, Tadros and Mayes [9] showed that addition of NTMP {Nitrilotris (Methylene Phosphonic acid)) retarded the crystallization of calcium sulfate dihydrate and modifed the habit of the crystals from the common needle-like form due to inhibition of normal growth on the {111} faces of calcium sulfate dihydrate crystals. The growth occurred mainly on {110} faces instead of {111} faces. More recently, Bosbach et al [10] showed that the inhibition using phosphonic acid derivatives [Hydroxyethylene 1, 1-diphosphonic acid (HEDP) and ENTMP] was attributed to adsorption of these molecules on step-edges of the crystals which resulted in a change in the gypsum morphology. The change in gypsum morphology was dependent on the inhibitor concentration and the reactivity of step edges. These results agreed with the data obtained by He et al [11] indicating that the nucleation of calcium sulfate in NaCl solutions was inhibited by the addition of HDTMP (Hexamethylenediaminetetra (methylene phosphonic acid). In the same study, He and his coworkers obtained a prolonged induction time with the additive. The degree of inhibition was found to depend on the pH of the solution and the lattice ion molar ratio, in addition to the degree of supersaturation and temperature. The above studies may be explained by the results of Weijnen and van Rosmalen [12, 13] who studied the adsorption of phosphonates on gypsum crystals. They found that the surface coverage needed for growth inhibition was 4-5% and the adsorption process was irreversible. Moreover, the inhibitor concentrations needed to block the gypsum growth process were pH dependent. They suggested the following mechanisms for phosphonate adsorption: • Chemical bonding (complexation) of the Ca2+ ions at the crystal surface with the phosphonate group. • Formation of hydrogen bridges between the hydroxyl groups and either the sulfate ions or the crystal water molecules at the gypsum crystal surface. As far as the adsorption kinetics is concerned, van Rosmalen et al [14] reported that adsorption of phosphonate could be completed within about one minute. To our knowledge, however, no data have been reported on the effect of phosphonates on gypsum nucleation and/ or crystallization parameters under conditions similar to that found in wet-process of

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H. EL-SHALL et al.: Crystallization of gypsum

phosphoric acid production. Therefore, the main objective of this work is to study effect of Aminotris (methylenephosphonic acid) [ATMP] on the induction time, nucleation, crystal size distribution and growth rate of the formed gypsum crystals in phosphoric and sulfuric acids medium simulates to some extent the industrial conditions. Experimental Pure chemicals including phosphoric and sulfuric acids and calcium hydrogen phosphate monobasic (CaH4(PO4)2.H2O), from Fisher Scientific Co. are used for this study. In addition, Aminotris (methylenephosphonic acid) [N(CH2PO3H2)3] from Pfaltz & Bauer Incorporation is used. The primary nucleation of calcium sulfate dihydrate with and without ATMP was followed by turbidity measurement. Turbidity and induction time measurements For turbidity measurement, 500 ml of phosphoric and sulfuric acids solution (27.5% P2O5 & 2.5%H2SO4) was added in 800-ml beaker and heated to 80°C using a water bath. Then, the desired amounts of dissolved calcium hydrogen phosphate monobasic in 100 ml phosphoric acid (20% P2O5), sulfuric acid (32.5%) and 50 ml of deionized water or water containing ATMP were added simultaneously. The reaction was kept at 80°C with constant agitation. The turbidity of the resulting solution was measured at different time intervals during the course of the reaction using a HACH 2100A Turbidimeter. Each experiment was repeated 2 times and averages of the results are represented. A graph of time vs. turbidity was plotted. The time corresponding to the point of intersection of the two asymptotic lines represents the induction time. Crystal size distribution measurement During the experiment and after 5 and 15 min from the start-up, 3ml slurry was taken and dispersed in 100 ml methanol. Then, the size distribution of formed gypsum crystals was determined using Coulter Laser Diffraction Analyzer model LS230. Calculation of inhibition efficiency (E) Inhibition efficiency (E) is calculated by the following relation: E=

100 (G 0 -G1 ) G0

(1)

where: G0 : The crystal growth rate in the absence of ATMP, µm/min G1 : The crystal growth rate in the presence of ATMP, µm/min G is calculated as follows [15]: G=

d90 at t 2 -d90 at t1 t 2 -t1

(2)


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where: d90: diameter of crystals passing 90 vol.%. t1 & t2: the corresponding time intervals at which samples for crystal size distribution were taken. Calculation of supersaturation ratio (S) The supersaturation ratio (S) was calculated [16] as follows: Supersaturation ratio (S) =

c c*

(3)

where: c : Calcium sulfate dihydrate concentration, % c*: Calcium sulfate dihydrate (solute) solubility under the applied conditions = 0.83 [17] Calculation of Surface Energy (γ) The surface energy (interfacial tension) between the crystals and the aqueous solution is a fundamental parameter for understanding the rate of both nucleation and crystal growth. Based on the classic homogenous nucleation theory, the induction time can be related to the supersaturation using the following correlation [11, 18]: log(t ind )=A+

B T3 (log S)2

(4)

Where A is an empirical constant (dimensionless) and B depends on the number of variables, and is given by: B=

ßγ 3 Vm2 N A f (θ ) (2.3R)3

(5)

Where ß is a geometric (shape) factor of 16π/3 for the spherical nucleus, ƒ(θ) is a correction factor, when purely homogeneous nucleation takes place ƒ(θ) = 1 and when heterogenous nucleation occurs ƒ(θ) = 0.01. Vm is the molar volume (74.69 cm3 mol-1 for gypsum), T is the absolute temperature (K) and R is the gas constant (J/mol. K), γ is the surface energy (J/m2), NA is the Avogadro’s number (mol-1). Plotting of log tind against 1/[log S]2 over a range of high supersaturation ratios (1.401.979) for a fixed temperature gives a straight line with slope (B), relative to homogenous nucleation. As a matter of fact, the change of nucleation mechanism produces change in the slope of B [11, 18]. Calculation of Nucleation Rate (Js), Free Energy (∆Gcr) and Critical Nucleus Size (r) Based on classic homogenous nucleation, the nucleation rate, i.e., the number of nuclei formed per unit time per volume can be calculated by applying the following relation:

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 -ßγ 3 Vm2 N A f (θ )}  Js=Fexp   3 2  (RT) ln S 

(6)

Where Js is the nucleation rate and F is a frequency constant and is known as the preexponential factor and has a theoretical value of 1030 nuclei/cm3 sec [3]. The surface energy of gypsum crystals (γ) can be used to determine the nucleation rate with and without ATMP. The difficulty with applying the above equation is that it predicts the nucleation rate only at high supersaturation ratio [19]. Thus, it is applied at supersaturation ratios ranged from 1.40 to 1.979. The free energy change ∆Gcr for the formation of critical nucleus size can be calculated from the following Arrhenius type equation [3, 16]: Js = F exp [-∆Gcr/KT]

(7)

where K is Boltzman constant and T is the absolute temperature. Then, the radius of the critical nucleus (r) is calculated from the following equation: ∆Gcr = 4/3 π r2 γ

(8)

Results Calcium sulfate dihydrate was prepared according to the following reaction: CaH4(PO4)2.H2O + H2SO4 + H2O à 2H3PO4 + CaSO4.2H2O ↓ Gypsum crystals were allowed to grow at 80oC in phosphoric acid solution containing 25% P2O5 and 2% free sulfuric acid with and without ATMP at several supersaturation values. The obtained results of turbidity, induction time, crystal size distribution, crystal growth rate measurements are presented below. Effect of ATMP on gypsum turbidity at different supersaturation ratios Effect of ATMP (100 ppm) on gypsum turbidity at 1.018 –1.979 supersaturation ratios is studied. An example of the obtained results at a supersaturation ratio of 1.222 is given in Fig. 1. It is clear that, ATMP has inhibited the crystallization of gypsum as indicated by longer induction time or because the suspension reaches a turbidity of 1000 NTU slower than the baseline. Effect of supersaturation ratio on induction time with and without ATMP Induction times were determined at different supersaturation ratios with and without ATMP and given in Table 1. These results confirm that 100 ppm ATMP consistently increases the induction time to a greater degree than the baseline at all the studied supersaturation ratios. In all these cases, as the supersaturation ratio has increased, the induction time was decreased.


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Fig. 1: Effect of Time on Turbidity of Calcium Sulfate Dihydrate (Supersaturation 1.222, 100 ppm ATMP)

Fig. 2: Relation between Log Induction Time and 1/(log Supersaturation Ratio) with and without 100 ppm ATMP Table 1: Effect of ATMP on the Induction Time of Gypsum Crystals (At Different Supersaturation Ratios)

Item

Baseline With ATMP * in minutes

1.018 T* 140 151

1.222 T 80 94

1.40 T 55.2 59.1

Supersaturation 1.502 1.60 T T 16.3 9.2 20.0 11.8

1.70 T 6.3 8.4

1.80 T 4.1 5.9

1.979 T 2.9 3.6

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Correlation between supersaturation ratio and induction time Relation between log induction time and 1/log2 supersaturation ratio with and without 100 ppm ATMP is given in Fig. 2. From the data presented in Table 2, it is clear that, the surface energy is reduced by addition of ATMP. Decreasing the surface energy leads to increasing the nucleation rate of gypsum crystals [3]. Generally, the surface energy for more soluble salts is less than for that for less or sparingly soluble salts [15]. Table 2 and Figs. 3 & 4 show nucleation rate, free energy for formation of critical nucleus size and radius of critical nucleus of gypsum crystals with and without ATMP at high supersaturation ratios ranged from 1.40 to 1.979. It is clear that, the nucleation rate is increased with increasing supersaturation ratio with and without ATMP additive as seen in Fig. 3. Table 2: Effect of ATMP on Nucleation Rate, Free Energy for Formation of Critical Nucleus Size and Radius of Critical Nucleus of Gypsum Crystals (At Different Supersaturation Ratios)

Supersaturation

1.40 1.502 1.60 1.70 1.80 1.979

Nucleation Rate, nuclei/cm3.sec x 1028 Without With ATMP ATMP 1.11 2.68 4.58 8.40 9.94 15.6 16.3 23.3 22.9 30.5 33.5 41.5

Free Energy for Formation of Critical Nucleus Size ∆Gcr x 10-21, Joule

Radius of Critical Nucleus, cm x 10-8

Without ATMP

Without ATMP 9.27 7.99 6.57 5.80 5.20 4.43

21.5 16.0 10.8 8.43 6.77 4.92

With ATMP 17.2 11.7 8.64 6.69 5.38 3.88

With ATMP 8.60 7.09 6.09 5.36 4.81 4.08

Fig. 3: Effect of Supersaturation Ratio on the Nucleation Rate with and without 100 ppm ATMP.


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Moreover, addition of ATMP increases the nucleation rate at all studied supersaturation ratios compared with the baseline. High nucleation rate means that a high number of formed nuclei are obtained. These nuclei have relatively lower chance to grow to large crystals compared to lower number of formed nuclei grow under the same conditions. The nucleation rates at supersaturation ratio of 1.502 are 8.40 x 1028 nuclei/cm3 sec and 4.58 x 1028 nuclei/cm3 sec with and without ATMP additive, respectively. The free energy for formation of critical nucleus size is decreased with increasing the supersaturation ratio. It is also decreased with addition of ATMP. In parallel, the radius of critical nucleus is decreased with increasing the supersaturation and with addition of ATMP as shown in Fig. 4. The radius of critical nucleus has decreased by 7.2% – 11.3% in presence of ATMP as compared to the baseline (without ATMP addition) at the studied supersaturation ratios.

Fig. 4: Effect of Supersaturation on the Radius of Nucleus with and without 100 ppm ATMP.

Effect of ATMP on crystal size distribution Mean diameter and d90 of the gypsum crystals at different supersaturation ratios and different time intervals with and without ATMP are given in Table 3. These data show that with addition of 100 ppm ATMP the formed crystals are smaller in size than the baseline (without additive). This means that, ATMP decreases the crystal growth. Moreover, d90 is decreased with ATMP addition by a percentage ranging from 25% to 50% as compared to the baseline at the studied supersaturation ratios and after 15 minutes retention time. In addition, at higher supersaturation, the formed gypsum crystals are smaller due to increasing nucleation rate rather than increasing the regular crystal growth.

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H. EL-SHALL et al.: Crystallization of gypsum Table 3: Effect of ATMP on Gypsum Crystal Size Distribution (at Different Supersaturation Ratios and after 15 minutes Time)

Item Without ATMP With ATMP Without ATMP With ATMP Without ATMP With ATMP

Supersaturation Ratio 1.018 1.018 1.222 1.222 1.502 1.502

Mean Diameter, µm 4.926 2.717 2.464 2.120 2.211 1.470

d90 *, µm 10.090 5.020 5.306 3.989 4.992 3.471

d90: diameter of crystals passing 90% by volume.

Effect of ATMP on crystal growth rate The calculated crystal growth rate and inhibition efficiency with and without ATMP are given in Table 4. These data indicate that the crystal growth rate is lower with ATMP. Generally, the growth rate is lower as supersaturation ratio is decreased. The inhibition efficiency ranged between 16% to 59% at the studied supersaturation ratios. Table 4. Effect of ATMP on Gypsum Crystal Growth Rate and Inhibition Efficiency (At Different Supersaturation Ratios)

Item

Supersaturation Ratio

Without ATMP With ATMP Without ATMP With ATMP Without ATMP With ATMP Without ATMP With ATMP

1.018 1.018 1.222 1.222 1.502 1.502 1.979 1.979

Crystal Growth Rate, µm/min 0.181 0.153 0.234 0.096 0.212 0.158 0.483 0.310

Inhibition Efficiency, % 16 59 26 36

Conclusions Effect of Aminotris (methylenephosphonic acid) [ATMP] on calcium sulfate dihydrate (gypsum) crystallization is studied. The results indicate that: • ATMP increases the induction time at all the supersaturation ratios studied due to decrease the regular crystal growth. • Surface energy is decreased in the presence of ATMP compared with the baseline. • Nucleation rate is increased in the presence of ATMP compared with the baseline • The Critical nucleus diameter and hence size is smaller with addition of 100 ppm ATMP. • The crystal growth rate is lower with ATMP compared with the baseline. • The crystal growth rate is generally lower at lower supersaturation ratio with and without ATMP additive.


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• Interestingly, with addition of 100 ppm ATMP, the crystal inhibition efficiency increases up to 59%. Acknowledgments The Engineering Research Center (ERC) for Particle Science and Technology at University of Florida, the National Science Foundation grant # EEC-94-02989 and grant # INT-9810983, and the Industrial Partners of the ERC are acknowledged for the partial financial support.

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