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Cryst. Res. Technol. 40, No. 9, 860 – 866 (2005) / DOI 10.1002/crat.200410446

Effect of cetyl pyridinium chloride additive on crystallization of gypsum in phosphoric and sulfuric acids medium H. El-Shall1, M. M. Rashad2, and E. A. Abdel-Aal*2 1 2

Engineering Research Center for Particle Science & Technology, Department of Materials Science and Engineering, University of Florida, Gainesville, FL, USA Central Metallurgical Research & Development Institute (CMRDI), P.O. Box: 87 Helwan, Cairo, Egypt

Received 14 September 2004, revised 10 October 2004, accepted 16 December 2004 Published online 15 August 2005 Key words crystallization growth, calcium sulfate dehydrate, crystal size distribution, induction time, cetyl pyridinium chloride CPC. PACS 61.66.Hq A basic study was carried out to understand effect of Cetyl Pyridinium Cloride (CPC) on calcium sulfate dihydrate (gypsum) crystallization. Induction time was measured under different supersaturation ratios ranging from 1.222 to 1.979. This is the time elapsed between the achievement of supersaturation and the appearance of a solid phase. The results show that, the induction time decreases exponentially with increasing the supersaturation ratio. In addition, the surface energy increases with CPC compared to the baseline (without CPC). Number of molecules required for formation of stable nucleus are calculated to be from 3 to 41 molecules depending on supersaturation ratios and presence of CPC. Interestingly, with addition of the CPC, the mean diameter of the crystals and the median increase. The growth rate was found dependent on the supersaturation with and without CPC. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1

Introduction

Crystallization of calcium sulfate dihydrate (gypsum) plays a very important role in phosphoric acid production by the wet-process. The increase of gypsum crystal growth means increase in the filtration rate of gypsum, i.e. increase in the productivity. Phosphoric acid is mainly produced by dihydrate process in which phosphate concentrate [Ca10F2(PO4)6] is leached with sulfuric acid (H2SO4) and weak phosphoric acid to produce phosphoric acid and gypsum (CaSO4·2H2O) as a by-product. Crystallization of calcium sulfate dihydrate (gypsum) occurs as the leaching is taking place. After leaching the slurry is filtered and counter – current washed to separate the phosphoric acid from gypsum cake. The simple reaction for this process is as follows (1): Ca10F2(PO4)6 + 14H3PO4 → 10Ca(H2PO4)2 + 2HF 10Ca(H2PO4)2 + 10H2SO4 + 20H2O → 20H3PO4 +10CaSO4·2H2O Ca10F2(PO4)6 + 10H2SO4 + 20H2O → 6H3PO4 + 10CaSO4·2H2O + 2HF The reaction is fast. It takes from 2 to 10 minutes depending on phosphate reactivity and process conditions. However, the crystallization of gypsum extends for long time 2-8 hours [1]. It is known that the filtration rate depends on the characteristics of filter cake such as crystal size, size distribution and morphology of the crystals. Therefore, enhancing the formation of large and uniform gypsum crystals is desired in achieving better filtration rate in phosphoric acid manufacture. Tests were conducted to examine the effect of two nonionic surfactants (Crysmod and Hiflo) on the filterability of gypsum crystals [2-4]. Results indicated that addition of surfactant material during nucleation stage was effective in increasing the rate of gypsum filtration. The rate of gypsum filtration increased 25% and 43% with Crysmod and Hiflo surfactants, respectively. The overall P2O5 recovery also increased by more than 1%. The presence of surfactants will reduce the hydration status of calcium ions. Since the dehydration kinetics determines the gypsum crystal growth rate, larger crystals with low surface area can be produced with the addition of surfactants. ____________________

* Corresponding author: e-mail: [email protected] © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Rocha et al. [5] showed that the alkyl benzene sulfonate increases the filtration rate of gypsum because of effecting on the crystal nucleation and growth rate. Liu and Nancollas (6) showed that the influence of sodium dodecyl sulfate upon gypsum crystallization can be explained in terms of two effects: 1. direct chelating with the crystal lattice ions in the solution, 2. adsorption on the crystal surface either generally or on particular crystal faces or crystal sites. An adsorption process appears to offer a more feasible explanation of the effect since the surfactant ions are present usually at a very smaller concentration than that of the crystal ions. Schroeder et al. [7] explained the effect of sodium salts of alkylbenzene sulfonic acid C10-C14 (sulphapol) on gypsum crystallization may be a consequence of a. the change in the interfacial free energy or b. mass transport due to surfactant adsorption on the crystal surface. Semina and Kopyleva [8] studied the electrical surface properties of phosphogypsum in the presence of surfactants. The data presented showed the potential of phosphogypsum precipitates has the zero values and the potential is not dependent on the concentration of the surfactant in the suspension, the pH of the liquid phase or the ionic strength of the solution. The values of potential of gypsum precipitate are due to the modification surface by adsorbed surfactant molecules. Free et al. [9] showed the presence of poly(ethylene oxide) and branched sodium dodecyl benzene sulfonate (BSDBS) delay the measurable onset of phosphogypsum nucleation by associating with calcium ions and acting as an effective buffer. The reduced rate of nucleation is believed to be responsible for observed increase in particle size and the corresponding increase in the rate of phosphogypsum filtration. Torocheshnikov et al. [10] showed that the presence of surfactant has resulted in reduction of P2O5 in the precipitated calcium sulfate and this is attributed to the surfactant effect in reducing the capture of phosphate ions from solutions by crystallizing gypsum crystals. In another study, Kopyleva [11] found that the presence of surfactant increases the solubility of calcium phosphate in wet process acid of 10-40% P2O5 concentration and at temperature of 40-75°C. Organic and inorganic additives play an important role in crystallization. They alter the surface properties of the crystals. In addition, the additives change nucleation, growth, shape of the crystals and their agglomeration or dispersion behavior [12-16]. The main objective of this work is to study effect of cetyl pyridinium chloride CPC 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 of phosphoric acid production. Calculation of surface energy, nucleation rate, free energy and critical nucleus size with and without surfactant is another objective of this study.

2

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, CPC (cetyl pyridinium chloride) surfactant [C21H38Cl.H2O from Fisher Scientific Co. is used. The primary nucleation of calcium sulfate dihydrate with and without CPC 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/CPC solution 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 growth efficiency (E) Growth efficiency (E) is calculated by the following relation [17]: E=

100(G1 − G0 ) G0

Where Go: The crystal growth rate in absence of CPC, µm/min, G1: The crystal growth rate in presence of CPC, µm/min. G is calculated as follows: © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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H. El-Shall et al.: Effect of cetyl pyridinium chloride additive on crystallization of gypsum

G=

d90 at t2 − d90 at t0 t 2 − t0

Where: d90: diameter of crystals passing 90 vol. %, to: is the time corresponding to start-up of the test at which d90 equals to zero, t2 : is the corresponding time intervals at which samples for crystal size, distribution were taken. Calculation of supersaturation ratio (S) The supersaturation ratio (S) was calculated (18) as follows: S=

c c*

Where: S: Supersaturation ratio, c: Calcium sulfate dihydrate concentration, %, c*: Calcium sulfate dihydrate (solute) solubility under the applied conditions = 0.83 (1). 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 [19-20]: log (tind ) = A +

B T (log 2 S ) 3

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

B=

βγ 3Vm2 N A f (θ ) (2.3RT )3

Where β 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/ [log2 S] over a range of high supersaturation ratios (1.40-1.979) 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 [18-19]. Calculation of Nucleation Rate (Js), Free Energy Change (∆Gcr) and Critical Nucleus Radius (r) Based on classic homogenous nucleation, it can easily calculate the nucleation rate, i.e., the number of nuclei formed per unit time per volume by applying the following relation: ⎡ − βγ 3Vm2 N A f (θ ) ⎤ Js = F exp ⎢ ⎥ 3 2 ⎣ ( RT ) ln S ⎦

Where Js is the nucleation rate and F is a frequency constant and is known as the pre-exponential factor and has a theoretical value of 1030 nuclei/cm3.sec [20]. By known the surface energy of gypsum crystals (γ), it can easily determine the nucleation rate with and without CPC. The difficulty with applying the above equation is that it predicts the nucleation rate only at high supersaturation ratio [21]. So, 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 [12,19]: Js = F exp [-∆Gcr/KT] Where K is Boltzman constant and T is the absolute temperature. By known the free energy change (∆Gcr), the radius of the critical nucleus (r) can be calculated from the following equation: ∆Gcr = 4/3 π r2 γ The number of molecules in the critical nucleus as [22] i = 4 π r3/3 Vm © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


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863

Results

Calcium sulfate dihydrate was prepared according to the following reaction equation: CaH4(PO4)2.H2O + H2SO4 + H2O → 2H3PO4 + CaSO4.2H2O ↓ The gypsum crystals grow at 80 oC in phosphoric acid solution containing 25% P2O5 and 2% free sulfuric acid with and without CPC. The experiments were done at different supersaturation ratios and results of turbidity, induction time, crystal size distribution, crystal growth rate measurements are presented. Effect of supersaturation ratio on induction time with and without CPC Induction times were determined at different supersaturation ratios with and without CPC and given in Table 1. These results confirm that 100 ppm CPC consistently decreases 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 is decreased. Correlation between supersaturation ratio and induction time Relation between log induction time and 1/log2 supersaturation ratio with and without 100 ppm CPC is given in Fig. 1. The calculated surface energies are 5.98 and 7.07 mJ/m2 without and with CPC, respectively. It is clear that, the surface energy is increased with addition of CPC. Decreasing the surface energy leads to increasing the nucleation rate of gypsum crystals [23]. Generally, the surface energy for more soluble salts is less than that for less or sparingly soluble salts [21].

Fig. 1 Relation between log induction time and 1/log supersaturation, with and without 100 ppm CPC.

Fig. 2 Effect of supersaturation ratio on the nucleation rate with and without 100 ppm CPC.

2

Fig. 3 Effect of supersaturation on the radius of nucleus with and without 100 ppm CPC.

Table 1 Effect of CPC on the induction time (T) of gypsum crystals (at different supersaturation ratios). Item

Without CPC With CPC * in minutes

1.222 T* 80 70

1.40 T 55.2 42.8

1.502 T 16.3 11.0

Supersaturation 1.60 1.70 T T 9.2 6.3 5.9 3.0

1.80 T 4.1 2.2

1.979 T 2.9 1.3

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Table 2 Effect of CPC on nucleation rate, free energy change 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 CPC 1.11 4.58 9.94 16.3 22.9 33.5

With CPC 0.10 0.85 2.85 6.13 10.3 18.5

Free Energy change for Formation of Critical Nucleus Size ∆Gcr x 10-21, Joule Without CPC With CPC 21.5 33.8 16.0 23.0 10.8 17.4 8.43 13.7 6.77 11.2 4.92 8.31

Radius of Critical Nucleus cm x 10-8 Without CPC 9.27 7.99 6.57 5.80 5.20 4.43

With CPC 10.7 8.81 7.67 6.80 6.15 5.30

Table 2 and Figs. 2&3 show nucleation rate, free energy change for formation of critical nucleus size and radius of critical nucleus of gypsum crystals with and without CPC 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 CPC additive (Fig. 2). Moreover, addition of CPC decreases the nucleation rate at all studied supersaturation ratios compared with the baseline (without CPC). 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 0.85 x 1028 nuclei/cm3.sec and 4.58 x 1028 nuclei/cm3.sec with and without CPC addition, respectively. The free energy change for formation of critical nucleus size is decreased with increasing the supersaturation ratio. It is also increased with addition of CPC. In parallel, the radius of critical nucleus is decreased with increasing the supersaturation and is increased with addition of CPC (Fig. 3). The radius of critical nucleus is increased with CPC by a percentage ranged from 10.3 % to 19.6 % compared with the baseline at the studied supersaturation ratios. Effect of CPC on Number of Molecules in the Critical Nucleus Number of molecules in the critical nucleus are calculated at different supersaturation ratios with and without 100 ppm of CPC surfactant. The results are given in Table 3. These data indicate that the number of molecules required for formation of stable nucleus are decreased with increasing supersaturation ratios. On the other hand, this number is increased in the presence of CPC surfactant.

Fig. 4 Crystal size distribution of gypsum with 25 ppm CPC at 1.502 supersaturation ratio.

Effect of CPC on crystal size distribution Crystal size distribution of the formed crystals at 1.502 supersaturation ratio with and without different concentrations of CPC are measured. Fig. 4 shows an example of crystal size distribution using 25 ppm CPC concentration at 1.502 supersaturation ratio. Mean diameter and d90 of the gypsum crystals at 1.503 supersaturation ratio and different time intervals with and without CPC are © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


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given in Table 4. These data show that mean diameter and d90 are increased with addition of CPC and with time. This means that, CPC increases the crystal growth. Lower CPC concentrations give higher mean diameter and d90. Mean diameter is increased with 100 ppm CPC by about 42% compared with the baseline at 1.502 supersaturation ratio and after 15 minutes retention time. Table 3 Effect of CPC on number of molecules in the critical nucleus (at different supersaturation ratios). Supersaturation 1.40 1.502 1.60 1.70 1.80 1.979

Number of Molecules in the Critical Nucleus Without CPC With CPC 27 41 17 23 10 15 7 11 5 8 3 5

Table 4 Effect of CPC concentration on the crystal size distribution at supersaturation of 1.502. CPC Concentration, ppm 0 12 25 50 100

1 min 0.771 4.119 2.343 1.959 1.429

Mean Diameter, µm 5 min 1.144 4.225 2.826 2.130 1.982

15 min 2.211 4.328 2.993 2.438 3.137

1 min 1.789 8.971 5.053 4.617 3.582

d90, µm 5 min 2.872 9.215 6.222 4.940 4.103

15 min 4.992 9.218 6.407 5.351 5.468

Table 5 Effect of CPC concentration on the gypsum crystal growth rate and growth efficeincy at supersaturation 1.502. CPC Concentration, ppm 0 12 25 50 100

Crystal Growth Rate, µm/min. 1 min 5 min 15 min 1.789 0.574 0.333 8.971 1.843 0.615 5.053 1.244 0.427 4.617 0.988 0.357 3.582 0.821 0.365

1 min 401.4 182.4 158.1 100.2

Growth Efficiency, % 5 min 15 min 221.1 84.7 116.7 22.0 72.1 7.2 43.0 9.6

Fig. 5 Photomicrographs of gypsum with and without different cpc surfactant concentrations. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Effect of CPC on crystal growth rate The calculated crystal growth rate and growth efficiency with and without CPC are given in Table 5. These data indicate that the crystal growth rate is enhanced with CPC. Again, lower CPC concentrations give higher crystal growth rate. In addition, the crystal growth rate is decreased with time with and without CPC. The crystal growth rate can be decreased to zero after attainment of crystallization equilibrium. The obtained growth efficiencies are higher at lower CPC concentrations. Growth efficiency of the formed gypsum crystals with 12 ppm CPC are 4 to 8 times higher than that with 100 ppm CPC. The growth efficiencies are ranged from about 7 % to 85% at supersaturation of 1.502 and after 15 minutes. Effect of CPC on gypsum morphology Fig. 5 shows photomicrographs of gypsum crystals with and without different concentrations of CPC. It is clear that the formed crystals are larger with CPC compared to the baseline (without additive). With decreasing CPC concentration from 100 ppm to 12 ppm, larger crystals are obtained. The sizes of gypsum crystals with 1.2 ppm CPC are more or less the same as with 25 ppm CPC.

4

Conclusions

Effect of cetyl pyridinium chloride CPC on calcium sulfate dihydrate (gypsum) crystallization is studied. The results indicate that CPC decreases the induction time at all the supersaturation ratios studied due to increase the regular crystal growth. Surface energy is increased in the presence of CPC compared with the baseline. Nucleation rate is decreased in the presence of CPC compared with the baseline. The Critical nucleus diameter and hence size is larger with addition of 100 ppm CPC. The crystal growth rate is higher with CPC compared with the baseline. The crystal growth rate is decreased with time at all the studied CPC concentrations and at 1.502 supersaturation ratio. The number of molecules required for formation of stable nucleus is calculated to be from 3 to 27 molecules without CPC surfactant and 5 to 41 molecules with surfactant. Interestingly, addition of 12 ppm CPC gives the highest mean diameter, d90, crystal growth rate, growth efficiency and largest size of formed crystals.

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