Ind. Eng. Chem. Res. 2004, 43, 5411-5417
5411
Inhibition of Calcium Carbonate Nucleation with Aminophosphonates at High Temperature, pH and Ionic Strength Jihui Guo and Steven J. Severtson* University of Minnesota, Department of Wood and Paper Science, 2004 Folwell Avenue, St. Paul, Minnesota 55108
Results of a study on the ability of aminophosphonates to inhibit CaCO3 nucleation are reviewed. Species discussed include ethylene-, butylene-, pentylene-, and hexamethylene-diaminetetramethylenephosphonate. These species form a homologous series differing only in the number of methylene linkages separating amine functional groups in the backbone structures of the molecules. Performance was gauged by the temperature increases required to induce rapid nucleation in solutions of pH and ion composition similar to those used in the isolation of cellulose fiber for papermaking. Aminophosphonates were found to increase nucleation temperatures sharply at concentrations lower than those typically required with other types of antiscalants. The inhibition ability of the aminophosphonate homologous series decreases with increasing number of methylene linkages. Comparison of performances for the analogues ethylenediaminetetramethylenephosphonate (EDTMP) and ethylenediaminetetraacetate (EDTA) indicates that distinctly different interactions are responsible for the observed inhibition. A semiempirical equation developed from classical nucleation theory demonstrates that interfacial interactions are primarily responsible for the observed inhibition for EDTMP, whereas solution-phase complexation of calcium ions was previously shown to account for the influence of EDTA. Also studied were binary mixtures involving EDTMP, EDTA, and poly(maleic acid) (PMA). It is shown that the ability of EDTA to raise nucleation temperatures is significantly enhanced by the presence of either EDTMP or PMA. Mixtures of EDTMP and PMA, both of which are believed to function through interfacial interactions, provide inhibition performances between those observed for the individual species when applied at the same concentration. Introduction Calcium carbonate scale formation is a costly problem for paper producers. Scale deposits form in heat exchangers, screens, and pipes and on surfaces of other process equipment, disrupting normal operations and reducing productivity.1-6 Especially prone to CaCO3 deposits are pulping operations involved in the isolation of cellulose fiber from wood. Process waters or liquors used in alkaline pulping stages are at high temperatures and pH and are normally supersaturated in calcium carbonate. Chemical treatments known as antiscalants are an effective compliment to approaches such as process and equipment modifications for controlling inorganic deposit problems. These treatments impede mechanisms that result in the formation of crystalline deposits.7-13 In this paper, results are reviewed from a study on the ability of aminophosphonates to inhibit CaCO3 nucleation under high-temperature and highpH conditions. Although aminophosphonates have been used extensively by various industries to control scale deposit problems, mechanisms by which they slow processes such as nucleation are not well understood. Previous studies on their influence on nucleation and crystal growth kinetics indicate that adsorption plays a key role in the observed inhibition. These species are known to adsorb strongly to a wide variety of mineral surfaces including CaCO3.14-17 Regardless, it is unclear the extent to which nonsorbed species contribute to the observed performance. * To whom correspondence should be addressed. Tel.: (612) 625-5265. Fax: (612) 625-6286. E-mail:
[email protected].
In a previous publication, it was demonstrated that solution-phase interactions that reduce supersaturation by sequestration can be distinguished from those that likely involve interfacial interactions, which can impede kinetics at a constant supersaturation. This was achieved by fitting the nucleation data using the following form of classical nucleation theory18
T-3/2 )
[
] ( ) [ ]
ln(Jmax /J ) B
1/2
ln
γCa2+γCO32Ksp
ln(Jmax /J ) B
+
1/2
ln(CCa2+CCO32-) (1)
Here, J is the number of nuclei formed per unit volume and unit time; Jmax is the frequency factor; B is a collection of variables characterizing the volume, geometry, and interfacial energy of the developing nuclei [) 4βa3γ3ν2/(27βυ2kB3)]; γCa2+ and γCO32- are the aqueous activity coefficients for calcium and carbonate ions, respectively; Ksp is the solubility product for a particular CaCO3 phase; and CCa2+ and CCO32- are the dissolved calcium and carbonate ion concentrations, respectively. The temperature at which rapid nucleation is induced, T, can be determined by heating a supersaturated solution, mimicking a common pathway for deposit formation, and monitoring the concentration of dissolved calcium.19,20 The temperature corresponding to a significant drop in calcium concentration is identified as the “nucleation temperature”. According to eq 1, plotting the inverse of measured nucleation temperatures raised to the 3/2 power versus the natural logarithm of the
10.1021/ie049787i CCC: $27.50 © 2004 American Chemical Society Published on Web 07/15/2004
5412 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
additives for controlling scale issues in pulping operations. It also provides general insights into the interactions involved in inhibiting the formation of CaCO3 crystalline deposits using aminophosphonates, aiding in the identification of new additives. Materials and Experimental Methods
Figure 1. Name abbreviations and structures of acid forms for aminophosphonates and aminocarboxylates.
calcium and carbonate ion products should produce a linear relationship. Assuming that eq 1 remains valid during the heating process and that the parameters in it are not strongly dependent on temperature over the range to which it is applied, information on the interaction by which an additive inhibits the nucleation process can be obtained through its use. It was demonstrated that the relatively small increase in nucleation temperatures caused by sequestrants such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) can be modeled using eq 1 by accounting for the reduction of dissolved calcium due to complexation interactions. The addition of poly(acrylic acid) (PAA) or poly(maleic acid) (PMA), on the other hand, produces substantially greater increases in nucleation temperatures, which appears to be the result of direct interactions between these species and the developing nuclei. The influence of these types of inhibitors can be taken into account through changes in the [ln(Jmax /J )/B]1/2 value in eq 1. These changes are likely to be a result of modifications at the nuclei-solution interface. The inhibition of CaCO3 nucleation with various aminophosphonates was studied under conditions representative of those used in the isolation of cellulose fiber from wood. Tested aminophosphonates (Figure 1) include ethylenediaminetetramethylenephosphonate, butylenediaminetetramethylenephosphonate, pentylenediaminetetramethylenephosphonate, and hexamethylenediaminetetramethylenephosphonate. These chemicals, denoted here as EDTMP, BDTMP, PDTMP and HDTMP, respectively, comprise a homologous series in which species differ from each other in the number of methylene linkages between amine functional groups in their backbone structures. These differences are shown to have a significant impact on the efficacies of the member molecules. Although the structures in this series are more similar to those of the aminocarboxylates, the interactions by which they inhibit CaCO3 nucleation are more similar to those reported for PAA and PMA. Even in the case of the analogues EDTMP and EDTA, the abilities of the molecules to raise nucleation temperatures and the interactions by which this is achieved are shown to be distinctly different and independent of each other. Mixtures of aminophosphonates and aminocarboxylates take advantage of these differences with the interfacial interactions enhancing the ability of complexation interactions to inhibit nucleation through a reduction in supersaturationsa phenomenon that apparently has not previously been reported. The information presented in this paper is useful in identifying
Chemicals. Reagent-grade Na2CO3 (anhydrous), NaOH (50% aqueous solution), CaCl2 (anhydrous), HCl, and H2SO4 were purchased from Fisher Scientific (Pittsburgh, PA). Ethylenediaminetetraacetic acid (EDTA) [60-00-4] was purchased from Avocado Inc. (Ward Hill, MA). The aminophosphonates were obtained from Solutia Inc. (St. Louis, MO) as aqueous solutions. The aminophosphonates tested included ethylenediaminetetramethylenephosphonate (EDTMP) [7651-992], butylenediaminetetramethylenephosphonate (BDTMP) [56399-18-9], pentylenediaminetetramethylenephosphonate (PDTMP) [31513-20-9], and hexamethylenediaminetetramethylenephosphonate (HDTMP) [38820-596]. Poly(maleic acid) (PMA) [26099-09-2], a 50% w/w aqueous solution, was purchased from Polysciences Inc. (Warrington, PA). Its molecular weight distribution was determined using matrix-assisted laser desorption/ ionization (MALDI) spectroscopy. The measured weightaverage and number-average molecular weights and polydispersity were 771 g/mol, 745 g/mol, and 1.04, respectively. Whenever possible, concentrations of tested chemicals are reported in terms of their active acid (mass concentration of the acid form of the active species) to allow direct comparisons of performance. Measurement of Nucleation Temperatures. The ability of organic additives to inhibit the onset of CaCO3 nucleation was determined by monitoring the temperature required to induce rapid nucleation carried out in a stirred 2-L model 4522 Parr reactor (Moline, IL). The reactor was equipped with a water-cooled extraction port, allowing samples to be withdrawn while the system was under pressure. In all experiments, solutions consisted of 5 g/L NaOH, 50 mg/L Ca2+ (introduced as CaCl2), and variable levels of Na2CO3 (0.5-3 g/L) in deionized water producing pH’s greater than 13 and ionic strengths ranging from 0.14 to 0.22 M. This composition is representative of the process liquors found in pulping and bleaching operations used to produce fiber for papermaking. In nucleation experiments, a known concentration of organic additive was introduced and the solution was stirred at a rate of 100 rpm with two six-bladed axial impellers (2.28-in. tipto-tip diameter, 45° pitched blades) and heated at 1 °C/ min. This rate was selected according to a study in which the heating rate was varied over the range of values available to our equipment (∼0.25-3 °C/min.). It was found that, between 0.5 to 2 °C/min, there was little change in measured nucleation temperatures, which were highly repeatable ((2 °C). The technique used to detect the onset of nucleation involved the monitoring of filtered calcium. At fixed time periods, approximately 15-mL samples were drawn. Approximately 3 mL of the sample was passed through a 0.45µm-pore-size polytetrafluoroethylene (PTFE) filter membrane. One-milliliter aliquots of the filtered and unfiltered samples were pipetted into separate 15-mL disposable centrifuge tubes containing 5 mL of a 4% (mass/mass) HCl solution, and the calcium concentrations were measured using a Perkin-Elmer (Shelton, CT) 100 AAnalyst atomic absorption spectrometer. The concen-
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5413
Figure 2. Temperature necessary to induce rapid nucleation as a function of active acid concentration for PMA, the homologous series of aminophosphosphonates, and EDTA up to (a) 10 and (b) 100 mg/L.
tration of solution calcium that passes through the filter membrane is referred to here as the dissolved calcium. Using X-ray diffraction, crystals isolated from the reactor following the heating process were found to be composed of CaCO3 in its calcite phase. Results and Discussion Influence of Aminophosphonates on Calcium Carbonate Nucleation. Figure 2a is a plot of nucleation temperature as a function of the active acid concentration for PMA, the homologous series of aminophosphonates, and EDTA (single point at 10 mg/L). As discussed previously, differences exist in the mechanisms by which carboxylate containing polymeric treatments such as PMA affect CaCO3 nucleation versus those involved for aminocarboxylates such as EDTA. The aminocarboxylates function mainly by complexing calcium in the solution phase to reduce supersaturation, while the effects of PMA and PAA are attributed to interactions that slow the kinetics of growth processes at interfaces of developing nuclei. The data indicate that, for the aminophosphonates, more than calcium complexation is involved in elevating nucleation temperatures. EDTMP and EDTA are chemical analogues that differ in their acid functional groups, phosphonic and carboxylic, respectively. It is well-established that EDTA complexes dissolved calcium species with a 1:1 molar stoichiometry.21 If it is assumed that EDTMP also complexes with dissolved calcium with a 1:1 stoichiometry, supersaturation should be reduced by a greater extent for the same mass by EDTA with a molecular weight of 292 g/mol than by EDTMP, which has a
molecular weight in its acid form of 436 g/mol. However, nucleation temperatures are considerably higher for EDTMP. For example, at 10 mg/L, the measured nucleation temperatures for EDTA and EDTMP were 34 and 106 °C, respectively. It is clear that, in terms of effectiveness, the addition of aminophosphonates raises nucleation temperatures in a manner more similar to that found for polymers such as PMA and PAA. As for these additives, results here indicate that EDTMP would be considered a threshold treatment under these test conditions because it provides inhibition far in excess of what would be expected for the solution-phase binding of calcium. This is also true for other members of the aminophosphonate homologous series. The curves shown in Figure 2a are for antiscalant concentrations up to 10 mg/L, where aminophosphonates are commonly applied. As can be seen, all of the aminophosphonates demonstrate a high efficiency, but performance levels off at relatively low concentrations. This is a general finding for the aminophosphonates tested under our test conditions. Also, aminophosphonates can precipitate as calcium salts, diminishing their ability to function as antiscalants. These behaviors are more clearly demonstrated in Figure 2b, which shows nucleation temperature data up to 100 mg/L. The figure shows that the PMA sample has a lower efficiency than the aminophosphonates but, in this case, is more effective. As was discussed in a recent publication,20 the performance of a polymeric additive strongly depends on its molecular weight distribution. This dependency often manifests itself as a local minimum in the nucleation temperature versus additive concentration curve. Here a monodisperse PMA sample was selected to avoid this phenomenon. Also, this PMA has a high performance relative to those of other polymers tested using this technique. The shape of the curve for PMA after the initial sharp rise has been attributed to complexation interactions.20 It was suggested that polyelectrolyes, such as ionized PMA, complex metals according to charge density, i.e., one complexed calcium ion per pair of ionized carboxylic acid functional groups. The stability of these types of interactions is unclear, but they appear to raise measured nucleation temperatures and, thus, effectively reduce supersaturation. For the homologous series of aminophosphonates, the molar complexation stoichiometry with dissolved calcium can be assumed to be 1:1. Thus, their efficiency at complexing calcium is significantly lower than that of the polyelectrolytes, and this interaction likely makes only minor contributions to the observed increases in nucleation temperatures. Given the complexity of the nucleation process and the interactions that can occur between organic additives and forming CaCO3 nuclei, it is difficult to draw conclusions about the specific inhibition mechanisms involved. However, the empirical results shown in Figure 2a,b are consistent with what has previously been proposed for the interactions between nucleating crystals and dissolved organic molecules and polymers. Threshold treatments, which are virtual at concentrations well below a stoichiometic match with the metal of the scaling salt, are believed to function primarily through their adsorption at the interfaces of developing nuclei to slow the incorporation of solutes.22-27 In the case of molecules such as aminophosphonates, these interactions can be quite selective, involving the blockage of specific growth sites. For polymeric species such
5414 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
Figure 4. Ratio of ln(Jmax /J ) and B from eq 1 as a function of additive concentration for EDTA, EDTMP, and BDTMP. Table 1. Parameters for the Fit Using Eq 1 of Nucleation Data Collected in the Presence of EDTMP and BDTMP
[
ln(Jmax /J )
conc (mg/L) 0
Figure 3. Plots of eq 1 for various calcium and carbonate concentrations in the absence of an inhibitor and with a range of (a) EDTMP and (b) BDTMP concentrations.
as PMA, selective interactions are likely not a necessary requirement because of their ability to act as surface obstacles. The fact that the aminophosphonates are significantly more efficient than PMA is consistent with these observations. It can also be speculated that the lower effectiveness of the aminophosphonates relative to PMA is due to their higher adsorption specificity and the limited availability of appropriate sites. It is also observed that the inhibition performance of the aminophosphonates in the homologous series decreases with increasing numbers of methylene linkages in the backbone (i.e., EDTMP, BDTMP, PDTMP, and HDTMP). This indicates that the spacing between phosphonic acid groups plays an important role in determining the ability of the aminophosphonates to inhibit CaCO3 nucleation. Fit of Nucleation Data for Aminophosphonates Using Classical Nucleation Theory. In Figure 3a, the inverse of the temperature required to induce detectable nucleation raised to the 3/2 power is plotted as a function of the natural logarithm of the product of the calcium and carbonate ion concentrations for liquors containing no inhibitor and various concentrations of EDTMP. A separate line with a different slope and intercept can be distinguished for each concentration. Parameters from the fits of the lines using eq 1 are listed in Table 1. Figure 3b shows similar data for BDTMP. Parameters from the fits of these lines are also listed in Table 1. According to eq 1, the ratio between the slope and intercept of each line is equal to the natural logarithm of the product of the aqueous activity coefficients for calcium and carbonate ions divided by the solubility product of the phase of CaCO3 forming. These
B
] [ 1/2
ln(Jmax /J ) B
] ( 1/2
ln
γCa2+γCO32-
(×106 K-3/2)
(×106 K-3/2)
13.3 ( 0.2
336 ( 4
Ksp
) ( ln
γCa2+γCO32Ksp
)
25.2 ( 0.7
0.5 1 5 10 50 100
11.4 10.7 10.1 9.8 9.1 8.9
EDTMP 289 271 254 247 234 230
25.4 25.3 25.1 25.2 25.7 25.8
0.5 1 5 10 50 100
11.6 10.8 10.2 10.0 9.4 9.3
BDTMP 291 275 258 252 241 239
25.1 25.5 25.3 25.2 25.6 25.7
values are relatively constant over the temperature range tested and are similar to the values reported for PAA and PMA, which ranged from 25.0 to 26.1. As was discussed previously, in addition to the explicit temperature dependency indicated by eq 1, there is an implicit dependency on temperature for the various parameters, and the use of this form of classical nucleation theory with heating curve data is not strictly correct.18 However, for the broad calcium and carbonate ion product range used here to test the influence of organic additives, the nucleation temperature range is relatively small, less than 30 °C in most cases. It is expected that, over this temperature range, changes for equation parameters will be small. According to eq 1, the slope of the T -3/2 versus ln(CCa2+CCO32-) line raised to the second power is the ratio of ln(Jmax /J ) and B. The decrease in this parameter with increasing concentration of an organic additive is identified as an indication of interactions that inhibit the onset of detectable nucleation other than simply solution-phase complexation with dissolved calcium, in particular interactions between additives and forming CaCO3 embryos that modify the interface. From Table 1, it can be seen that this value decreases with the addition of the aminophosphonates. This decrease is sharp for the lowest concentrations and much lower for concentrations of 10 mg/L and above. Figure 4 shows a plot of this parameter for both EDTMP and BDTMP as
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5415
Figure 5. Plot of eq 1 for a fixed product of calcium and carbonate ion concentrations in the presence of 5 mg/L EDTMP and various concentrations of EDTA. The solid lines are the best fits of nucleation data for untreated liquor and liquor treated with 5 mg/L EDTMP. Open circles are data uncorrected for calcium complexation with EDTA. Closed circles are data corrected for complexation.
a function of their active acid concentrations. Also shown are the results for EDTA to demonstrate the influence of a species that is known to inhibit the onset of nucleation by solution-phase complexation of the calcium, and thus, it has no effect on the ln(Jmax /J )/B term. The data demonstrate why organophosphonates are commonly applied at low concentrations. The ability of the chemicals to further raise nucleation temperatures, and thus further reduce the ln(Jmax /J )/B parameter, is minimal beyond about 10 mg/L. In fact, most of the effect of aminophosphonates is observed for concentrations less than about 5 mg/L even under the high supersaturation conditions utilized here. Using eq 1, Figure 5 further demonstrates the separation of solution-phase and interfacial interactions for EDTA and EDTMP, respectively. Data for various EDTA concentrations uncorrected for the complexation of dissolved calcium are shown (open circles) for experiments carried out with no inhibitor and in a mixture with 5 mg/L of EDTMP. The solid lines are the best fits of nucleation data from Figure 3 for untreated liquor and for liquor containing 5 mg/L of EDTMP. The closed circles are the data corrected for the reduction in dissolved calcium due to complexation with EDTA. These data fall on the solid lines. A linear correlation with a slope of 1 and an intercept of 0 was confirmed (r 2 ) 0.99) between the molar residual calcium concentration associated with the heating curves used to produce Figure 5 and the added molar concentration of EDTA. These results demonstrate the independence of the interfacial interactions proposed for EDTMP and the complexation interactions associated with EDTA. This distinction is important. Terminology such as chelation or sequestration is often used when describing at least a portion of the influence of aminophosphonates in industrial process streams supersaturated with CaCO3. Results reviewed here indicate that these types of interactions play little if any role in the observed ability of these chemicals to inhibit the onset of rapid nucleation processes and likely the formation of scale deposits. Influence of Mixtures Containing Aminophosphonates on Calcium Carbonate Nucleation. Mix-
Figure 6. Nucleation temperature as a function of the EDTAto-calcium molar ratio for solutions containing either no other inhibitors, 5 mg/L PMA, or 5 mg/L EDTMP. The dotted lines show nucleation temperatures when no EDTA is present.
tures containing aminophosphonates and other additives are sometimes formulated to reduce the cost of an antiscalant treatment or to obtain synergistic performance. For the treatment of pulping and bleaching process streams, the performances of additives in the presence of other dissolved organics are important because these liquors contain extracted wood polymers that can also interact with calcium and carbonate to affect nucleation and crystallization processes.19 A previous publication discussed the role of adsorption fractionation in determining the performance of polydisperse PAA samples in slowing CaCO3 nucleation kinetics.20 There, it was speculated that, under competitive conditions, higher molecular weight PAAs, which are less effective at inhibiting nucleation, dominate the surfaces of growing nuclei. Thus, raising the concentration of a polydisperse PAA can actually decrease measured nucleation temperatures because species with a higher affinity for developing CaCO3 surfaces tend to provide lower inhibition performance. Although competitive effects involving wood polymers have not been specifically studied, it is likely that these waste macromolecules play a significant role in determining the locations for deposit formation in pulping, bleaching, and black liquor evaporation operations. Wood polymers, in particular kraft lignin, would be present at substantially higher concentrations than additives introduced to kraft process liquors to control scale and produce much less of an inhibitory influence on CaCO3 nucleation. It is possible that their presence interferes with the ability of additives to function as antiscalants. This is consistent with the lower than expected inhibition of CaCO3 precipitation in laboratory kraft pulping reactions observed for PAA, PMA, and commercial polymeric treatments containing acrylic and maleic acid monomers.15 The influence of a mixture of antiscalants is dependent on the mechanisms involved in inhibiting nucleation rates for the component additives. For species that function through the solution-phase complexation of metals (i.e., sequesterants), the effect of a mixture is simply the sum of the performances provided by the individual components. The combination of a species that functions through interfacial interactions with one or more sequesterants enhances the influence of the solution-phase complexation mechanism on nucleation rates. This is demonstrated in Figure 6, which shows
5416 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
Figure 8. Nucleation temperature as a function of PMA weight fraction in mixtures of EDTMP and PMA at total concentrations of 0.5, 1, and 5 mg/L.
Figure 7. Temperature necessary to induce rapid nucleation as a function of active acid concentration up to (a) 10 and (b) 100 mg/L for EDTMP, PMA, and their equal mass fraction combination. The dotted curve indicates the calculated additive performance for a 50:50 (mass) mixture of EDTMP and PMA.
nucleation temperature as a function of EDTA concentration (EDTA and Ca2+ molar ratio) for solutions containing no other inhibitors, 5 mg/L PMA, or 5 mg/L EDTMP. (Given the high concentration of sequesterant required to achieve significant inhibition, it was not possible to provide a broad range of component concentrations.) The increase in nucleation temperature with increasing concentration is significantly better when EDTA is combined with EDTMP or PMA. For example, for the same range of EDTA concentration, 0-600 mg/ L, the increases in nucleation temperatures were 19, 30, and 34 °C with no other inhibitor, with PMA, and with EDTMP, respectively. These data are simply another way of demonstrating the results shown in Table 1 and Figure 5, i.e., the modification of the [ln(Jmax /J )]/B term in the presence of additives such as EDTMP and PMA and the independence of the interfacial and solution-phase interactions. Here, these effects combine to provide inhibition ability beyond the sum of their individual performances. The inhibition performance of a binary mixture of species that function through interfacial interactions appears to lie somewhere between the performances of the two components. Mixtures involving polymers are complex because of the possible contribution from complexation interactions at higher concentrations. This is less of an issue for aminophosphonates and for polymers at low concentrations. Figure 7 shows a plot of nucle-
ation temperature versus concentration for a mixture of EDTMP and PMA at equal mass fractions for concentrations up to 10 mg/L (Figure 7a) and 100 mg/L (Figure 7b). Also shown are the curves for EDTMP and PMA. The dotted curve in Figure 7a shows the calculated additive performance for a 50:50 (by mass) mixture of EDTMP and PMA. Judging from these results and those presented previously for PAA mixtures, it is possible that the additive line might serve as a gauge of which species in a mixture plays a dominant role in determining the observed performance. This might result, for example, from differences in adsorption behavior as was found for various mixtures of PAA molecular weight distributions. Figure 8 is a plot of nucleation temperature as a function of the PMA mass fraction for various total mixture concentrations. Also shown are the additive lines (dotted), i.e., sums of component mass fractions multiplied by nucleation temperatures, for the pure component at a given concentration. The data produce curves that are above the additive lines and concave down. This is another possible indication of which chemical is dominating the influence of the nucleation process. In this case, the results indicate that EDTMP is somewhat less affected by competitive interactions with PMA. Thus, it might provide for their greater inhibition influence on CaCO3 nucleation in laboratory kraft reactions relative to carboxylic acid-containing polymeric treatments.15 Acknowledgment The authors thank Sheldon Verrett of Solutia Inc. for providing aminophosphonate samples and his help in characterizing the samples. This research was supported by Solutia Inc. and the Minnesota Agricultural Experimental Station Project Number MIN-43-050. Nomenclature J ) nucleation rate according to classical nucleation theory Jmax ) frequency factor kB ) Boltzmann constant, 1.38 × 10-23 J/K Ksp ) solubility product of calcium carbonate S ) supersaturation of calcium carbonate T ) absolute temperature CCa2+ ) calcium concentration in the solution phase CCO32- ) carbonate concentration in the solution phase
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5417 βa ) area conversion factor ) 4π for a sphere βυ ) volume conversion factor ) 4π/3 for a sphere γ ) surface free energy per unit area γCa2+ ) activity coefficient of Ca2+ ions γCO32- ) activity coefficient of CO32- ions ν ) molecular volume of the precipitated nuclei
Literature Cited (1) Hartler, N.; Libert, J. Structure and Composition of Scale in Continuous Kraft Digesters. Sven. Papperstid. 1972, 75, 65. (2) Berry, L. Black Liquor Scaling in Multiple Effect Evaporators. Tappi J. 1966, 49, 68A. (3) Cowan, J. C.; Weintritt, D. J. Water-Formed Scale Deposits; Gulf Publishing Company: Houston, TX, 1976. (4) Markham, L. D.; Bryce, J. R. Formation of Calcium Carbonate Scale in a Continuous Digester. Tappi J. 1980, 63, 125. (5) Schmidl, W.; Frederick, W. J. Current Trends in Evaporator Fouling. In 1998 TAPPI International Chemical Recovery Conference Proceedings; TAPPI Press: Atlanta, GA, 1998; Vol. I, p 367. (6) Markham, L. D.; Bryce, J. R. G. Formation of Calcium Carbonate Scale in a Kamyr Digester, In TAPPI 1979 Pulping Conference Proceedings; TAPPI Press: Atlanta, GA, 1979; p 17. (7) Severtson, S. J.; Duggirala, P. Y.; Carter, P. W.; Reed, P. E. Mechanism and Chemical Control of CaCO3 Scaling in the Kraft Process. Tappi J. 1999, 82, 167. (8) Smith, B. R.; Sweett, F. Scale Control by Polyacrylates: Relation to Crystallization Inhibition. Desalination 1971, 9, 277. (9) Giannimaras, E. K.; Koutsoukos, P. G. The Crystallization of Calcite in the Presence of Orthophosphate. J. Colloid Interface Sci. 1987, 116, 423. (10) Reddy, M. M.; Hoch, A. R. Calcite Crystal Growth Rate Inhibition by Polycarboxylic Acids. J. Colloid Interface Sci. 2001, 235, 365. (11) Naka, K.; Chujo, Y. Control of Crystal Nucleation and Growth of Calcium Carbonate by Synthetic Substrates. Chem. Mater. 2001, 13, 3245. (12) Wada, N.; Kanamura, K.; Umegaki, T. Effect of Carboxylic Acids on the Crystallization of Calcium Carbonatre. J. Colloid Interface Sci. 2001, 233, 65. (13) Pach, L.; Duncan, S.; Roy, R.; Komarneni, S. Morphological Control of Precipitated Calcium Carbonates and Phosphates by Colloidal Additives. J. Mater. Sci. 1996, 31, 6565. (14) He, S.; Oddo, J. E.; Tomson, M. B. The inhibition of Gypsum and Barite Nucleation in NaCl Brines at Temperatures from 25 to 90 °C. Appl. Geochem. 1994, 9, 561.
(15) Guo, J.; Severtson, S. J. Influence of Organic Additives on Calcium Carbonate Precipitation during Kraft Pulping. Tappi J. 2002, 1, 21. (16) Nowack, B. Environmental Chemistry of Phosphonates. Water Res. 2003, 37, 2533. (17) Kavanagh, A. M.; Rayment, T.; Price, T. J. Inhibitor Effects on Calcite Growth at Low Supersaturations. J. Chem. Soc., Faraday Trans. 1990, 86, 965. (18) Guo, J.; Severtson, S. J. Application of Classical Nucleation Theory to Characterize the Influences of Carboxylate-Containing Additives on CaCO3 Nucleation at High Temperature, pH, and Ionic Strength. Ind. Eng. Chem. Res. 2003, 42, 3480. (19) Severtson, S. J.; Guo, J. Influence of Ozonized Kraft Lignin on the Cryatallization of CaCO3. J. Colloid Interface Sci. 2002, 249, 423. (20) Loy, J. E.; Guo, J.; Severtson, S. J. Role of Adsorption Fractionation in Determining the CaCO3 Scale Inhibition Performance of Polydisperse Sodium Polyacrylate. Ind. Eng. Chem. Res. 2004, 43, 1882. (21) Skoog, D. A.; West, D. M.; Holler, F. J. Fundamentals of Analytical Chemistry, 7th ed.; Saunders College Publishing: Orlando, FL, 1996. (22) Reddy, M. M.; Nancollas, G. H. Calcite Crystal Growth Inhibitory by Phosphonates. Desalination 1973, 12, 61. (23) O ¨ ner, M.; Dogˇan, O ¨ .; O ¨ ner, G. The Influence of Polyelectrolytes Architecture on Calcium Sulfate Dihydrate Growth Retardation. J. Crystal Growth 1998, 186, 427. (24) Zhang, J. W.; Nancollas, G. H. Mechanisms of Growth and Dissolution of Sparingly Soluble Salts. In Review in Minerology 23; Hochella, M. F.; White, H. F., Eds.; Minerological Society of America: Washington, DC, 1990; pp 365-396. (25) Boistelle, R.; Astier, J. P. Crystallization Mechanism in Solution. J. Crystal Growth 1988, 90, 14. (26) Backfolk, K.; Lagerge, S.; Rosenholm, J.; Eklund, D. Aspects on the Interaction between Sodium Carboxymethylcellulose and Calcium Carbonate and the Relationship to Specific Site Adsorption. J. Crystal Growth 2002, 248, 5. (27) Giannimaras, E. K.; Koutsoukos, P. G. Precipitation of Calcium Carbonate in Aqueous Solutions in the Presence of Oxalate Anions. Langmuir 1988, 4, 855.
Received for review March 18, 2004 Revised manuscript received May 17, 2004 Accepted May 25, 2004 IE049787I
