Ind. Eng. Chem. Res. 1994,33, 2187-2197
2187
Steady-State Transitions and Polymorph Transformations in Continuous Precipitation of Calcium Carbonate D. Chakraborty, V. K. Agarwal, S. K. Bhatia; and J. Bellare Department of Chemical Engineering, Indian Institute of Technology, Powai, Bombay 400 076, India
The continuous precipitation of calcium carbonate is studied here with on-line particle size distribution evaluation and in-situ video visualization. Morphological characterizations of collected samples using differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and cryo-SEM were also performed. It is found that in a scaled reactor the nucleation is homogeneous and predominantly of vaterite which transforms to calcite within the reactor itself, and subsequently outside the reactor during sample collection and drying. In an unscaled reactor the precipitation is predominantly heterogeneous with both vaterite and calcite being nucleated. The heterogeneously nucleated material is morphologically more perfect and is therefore slower t o transform and has a lower growth rate. As a result of the transformation of vaterite, conventional characterizations such as DSC, XRD, and SEM, involving sample collection, filtration, and drying, at room temperature are unreliable. More meaningful determinations are available from cryo-SEM studies of rapidly frozen samples in which the transformation and subsequent nucleation is arrested.
Introduction The precipitation of calcium carbonate has received much attention in the literature because of its numerous applications. It is a basic raw material in the manufacture of paints, textiles, plastics, adhesives, and rubber, as well as several other products. It also has important applications in water treatment and energy production technology. Further, calcium carbonate precipitation in boiler, desalination, and transportation facilities is of concern as its concentration in many natural waters exceeds the saturation level. In all these instances the morphology of the calcium carbonate, as well as its particle size distribution, plays a vital role (Stumm and Morgan, 1970;Aubrey, 1954). Consequently, control of these characteristics of the precipitate can usually lead to improved operation or product properties. Perhaps the most impressive demonstration of these features of calcium carbonate precipitation is offered by nature itself where there are many instances in which fine control of morphology and particle size is advantageously exploited. This is employed, for example, by shell fish such as mollusc to form their shell by nucleating and depositing calcium carbonate of controlled morphology in an organized manner on specific sites (Mann, 1988). Another example occurs in avian eggshells in which oriented calcite is nucleated in fibrous disulfide-linked proteins (Mann, 1988). Calcium carbonate, like many other crystalline compounds, exhibits polymorphism with a thermodynamically stable form, calcite, and two metastable crystalline varieties, vaterite and aragonite, at any given temperature and pressure (Brookset al., 1950;Wray and Daniels, 1957). Among these, calcite belongs to the hexagonal-rhombohedral crystal system with a structure which may be viewed as a distortion of the cubical sodium chloride structure (Hurlbut, 1971). More than 300 different forms have been described varying from nearly cubical (or distorted cubes) to rhombohedral. Vaterite belongs to the hexagonal system and is most commonly spherulitic or disclike, while aragonite belongs to the orthorhombic system and is often needlelike. Depending on the conditions prevailing during precipitation, the morphology and therefore the physical properties of the product can vary. As a result of its
* Author to whom correspondence may be addressed.
importance, mentioned above, determination of the relationship between precipitation conditions and the product morphology for calcium carbonate is a problem that has lured the attention of scientists for decades (Brooks et al., 1960; Wray and Daniels, 1957; Groot and Duyvis, 1966)and still continues to do so (Crenshaw, 1982; Kitano and Hood, 1985; Mann et al., 1988; Dalas and Koutsoukos, 1989). Among these, Brooks et al. (1950) reported the precipitation of calcite, aragonite and vaterite in a batch precipitator. They also observed that vaterite is less stable than aragonite and gradually transforms to calcite, particularly when moist. The proportion of each of the forms in the precipitate was noted to be dependent on the ratio of reagent concentrations in their experiments which utilized the double decomposition reaction between sodium carbonate and calcium chloride. Similar observations were subsequently made by Wray and Daniels (1957), in the temperature range 30-70 "C, who also noted that vaterite is favored at the lower temperatures and calcite in the range 40-50 OC. The gradual transformation of vaterite into calcite in a supersaturated solution, during the precipitation itself, has also been reported in the literature (Sohnel and Mullin, 1982). Many of these studies have also indicated that impurities such as Pb2+, Mg2+,Mn2+,and Cr3+can affect the growth kinetics and morphological mix of the precipitate (Wray and Daniels, 1957; Sohnel and Mullin, 1982; Peters and Chang, 1983). Most recently Hostomsky and Jones (1991) have reported studies in a continuous precipitator at room temperature which indicated the formation of vaterite at high pH (19.5) and agglomerated calcite at low pH (8.5). At high supersaturations even an amorphous gelatinous precipitate was reported. Such a gel was previously also observed by Sohnel and Mullin (1982) and was found to transform to a conventional crystalline precipitate containing vaterite and calcite. In other work Yagi et al. (1984) have reported the variation of CaC03 product morphology with the concentration of Ca(OH)2 when reacted with COz. It is evident from the above discussion that the morphologicalproperties of precipitated calcium carbonate have been given some attention in the literature. A common feature of all of these studies, however, is that they have relied on scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies of the precipitate powder obtained from the supersaturated solution after
0888-5885/94/2633-2187$04.50/00 1994 American Chemical Society
2188 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994
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Figure 1. Schematic of continuous precipitation apparatus.
filtration and subsequent drying of the residue. In view of the suspicion of gradual transformation of the metastable vaterite to calcite it would seem that such room temperature procedures cannot be relied on, particularly in those cases where only calcite is observed in the dried powder. In the latter instance it may indeed be that the precipitate was originally nucleated in vateritic form but had transformed in the solution, or during subsequent separation and drying, before examination. Thus, while not quantitative, the procedures may not always be reliable even for qualitative conclusions. More definitive conclusions may be obtained by on-line examination of the suspension or by examination of frozen samples, in which transformation is arrested, by cryo-scanning electron microscopy (cryo-SEM). Both of these techniques are employed in the study reported here. An additional feature not given due attention is the homogeneous or heterogeneous character of the precipitation process. It is well-known that calcium carbonate precipitation is accompanied by extensive scaling of internal surfaces. Thus, the precipitation is homogeneous as well as heterogeneous. Schierholz and Stevens (1975) prevented wall deposition by periodically scraping the walls in their continuous system, but this may affect the morphology of the product. The influence on kinetics has been subsequently acknowledged by Swinney et al. (1982), although Dabir et al. (1982) have suggested that scaling may not directly influence the particle size distribution. Nevertheless, no direct link between precipitator behavior, particularly product morphology, and the scaling has been established. There are, however, reports of heterogeneous nucleation of calcium carbonate (Dalas and Koutsoukos, 1984: Mann et al., 1991) which suggested that considerations of lattice compatibility with the substrate and stereochemical correspondence are important in determining the morphology. Thus, under conditions of scale development, where heterogeneous nucleation can dominate, the morphology can be different from that under homogeneous bulk nucleation conditions.
In the present article we report our experiments on the continuous precipitation of calcium carbonate in a mixed suspension mixed product removal (MSMPR) system, using the double decomposition reaction between calcium nitrate and sodium carbonate. Steady-state transitions between heterogeneous and homogeneous nucleation regimes, accompanying scaling, are demonstrated, and the product morphology in each regime is investigated. The morphology is studied using on-line video examinations and cryo-scanning electron microscopy in addition to conventional room temperature XRD and SEM of the filtered and dried powders. Consistent with the suggestion above it is seen that the latter results can sometimes be misleading due to morphological transformation of the precipitate. Experimental Apparatus and Procedure ContinuousPrecipitation. Continuous precipitation experiments were conducted with the double decomposition reaction between calcium nitrate and sodium carbonate. The precipitation was carried out in a well-stirred glass-jacketed reactor with a working volume of 800 mL. It was equipped with a four-bladed stainless steel impeller and four equally spaced baffles to ensure good mixing. The design was similar to that employed by Fitchett and Tarbel(1990) and permitted high impeller speeds without significant air entrainment or vortexing. The precipitator temperature was controlled at 25 OC by circulating water from a constant temperature bath through itsjacket. Figure 1provides a schematic diagram of the experimental setUP * Feed stocks were prepared from high purity anhydrous sodium carbonate and calcium nitrate, using single distilled water whose trace impurity analysis is given in Table 1. Tables 2 and 3 give the specifications of the reagents used. Reactant solutions, stored at controlled temperature (25 "C)in 40-Ltanks, were continuously fed at constant rates by means of peristaltic pumps, and the product slurry was
Ind. Eng. Chem. Res., Vol. 33, No. 9,1994 2189 Table 1. Concentration of Trace Impurities in Distilled Water
ion Na2+ Ca2+ Fez+
amount, ppm 0.42 0.0 0.044
ion Mgz+ Ni2+ Cr2+
amount, ppm 0.310 0.15 0.096
Table 2. Specifications of Calcium Nitrate Powder
manufacturer quality type analysis, % Ca(N03)r4H20 chloride (Cl) sulfate (sod) heavy metals (as Pb) iron (Fe)
Merck purified
parameter inlet supersaturation, Xi inlet Ca2+/C03Gmole ratio, Ri residence time, 7 (min) stirring speed (rpm) temperature ("C)
xi =
range 250-3200 0.01-10 1.0-12 100-1250 25
,
[Ca2+1,[CO~~-I
(1)
KSP
98 0.005 0.02 0.002 0.001
Table 3. Specifications of Anhydrous Sodium Carbonate Powder
manufacturer quality type analysis, % Na2C03 insoluble matter chloride (Cl) nitrate (Nos) phosphate (PO,) silicate (Si02) sulfate (SO,) aluminum (Al) calcium (Ca) magnesium (Mg) potassium (K) ammonium ("4) iron (Fe) lead (Pb) copper (Cu)
Table 4. Range of Experimental Parameters
Qualigens (India) analytical reagent 99.9 0.01
0.004 0.002 0.001 0.0025 0.0025 0.001 0.01 0.002 0.01 0.0001 0.0005 0.0005 0.0005
continuously removed by a similar but variable speed pump. The speed of this pump was continuously controlled to maintain a constant head in the reactor which was monitored by a differential pressure transmitter. An additional outlet provided at the bottom permitted a small stream of about 20 mL/min to be continuously withdrawn for on-line particle size distribution analysis. The analyzer comprised a Galai CIS-1 laser sizer equipped with a flowthrough cell as well as a microscope camera and a video monitor for visualizing particles passing through the measuring zone of the cell. The particle size distribution, total number density, and volumetric concentration were determined on-line. In addition, off-line particle size analysis was conducted using the magnetically stirred cell module of the instrument, for samples pippetted out of the reactor. The instrument permitted analysis of particles in the 0.5-150 pm range for slurry concentrations in the range 103-108 particles/mL. In order to study the particle morphology, room temperature XRD and SEM, differential scanning calorimetry (DSC), and low temperature SEM were used. For room temperature XRD and SEM, as well as for DSC, outlet samples accumulated in a flask were collected over a 0.2 pm cellulose nitrate filter paper through which the slurry was passed by applying a vacuum on one side. For the experiments the reactor was thoroughly cleaned and washed before each run to remove any adhering scale from a prior experiment. The variables whose effect was examined in the study were the feed supersaturation level, the ratio of molar feed rates of calcium and carbonate ions, the residence time, and the stirring speed. Table 4 gives the range of values of the different parameters chosen for the study. The inlet supersaturation is defined here as the ratio of product of total calcium and carbonate ion concentrations, based on a well-mixed feed, to their solubility product. Thus,
where [Ca2+Itand [C032-It are the total inlet calcium and carbonate ion concentrations based on a well-mixed and completely dissociated feed. At the reaction temperature of 25 OC the solubility product Ksphas a value of 1.416 X lo4 ((g mol)/L)2for vaterite, corresponding to a solubility of the polymorph of 0,0119 (g mol)/L). All the supersaturations reported here are based on this solubility product. Further details of the experimental apparatus and procedure are available elsewhere (Chakraborty, 1994; Chakraborty and Bhatia, 1994b). Cryo-Scanning Electron Microscopy. Low temperature scanning electron microscopy of the precipitate was performed using a JEOL cryo-SEM Model JSM-6400. The sample preparation involved removing a small amount of CaC03 slurry from the reactor using a syringe and immediately sandwiching a drop between two copper plates. The sample was then quickly frozen by rapidly plunging it into a bath of liquid freon-22 (melting point of 113 K) using a specially designed shooting device equipped with a trigger-operated mechanism. The sample was removed from the plunging device and clamped on the cryo-SEM holder in a liquid nitrogen bath. Subsequently, the sandwich was transferred to the microscope and fractured on the coating stage with the help of a cryofracture knife attached to the microscope. The sample adhering to the lower plate was then observed. More details of the plunging device and procedure for the cryoSEM are available elsewhere (Agarwal, 1993).
Results and Discussion Steady-StateTransitions. A. Nearly Stoichiometric Operation. Using the apparatus and procedure discussed above, several continuous precipitation experiments were conducted in which on-line particle size analysis was performed at regular intervals of about 30 min, and the dynamics as well as persistence of the steady state was tracked. In addition, for several experiments, off-line analysis was also carried out. A steady-state was considered to be achieved if the particle size distribution (PSD) was unchanged and persisted over an interval of at least five residence times. Figure 2a and b shows the variation of the PSD with stirrer speed for several runs at an inlet supersaturation of 2500, a ratio of total inlet calcium to carbonate ionic concentrations of 0.8, and a mean residence time of 6 min. An interesting aspect of these figures is the steady-state multiplicity at stirring speeds of 250 and 450 rpm. In these runs two steady states are evident, with those in Figure 2a having a finer PSD and appearing first. These small particle steady states appeared shortly after the start of the run (within 2-5 residence times) and persisted for another 5-10 residence times before a transition to the larger particle size steady state in Figure 2b occurred. No further transitions occurred subsequently, and this steady state was stable and persisted for over 40 residence times, after which the run was stopped. At the lower rpm of 100, however, only the small-particle steady state (LSS) in Figure 2a was observed and was stable without transition, while at 625
2190 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994
6
5 E T 4 W
E 3 m SIZE, prn
2 1
DIMENSIONLESS TIME, t/'T
Figure 3. Temporal behavior of observed mean particle size under nearly stoichiometric conditions.
SIZE, prn
Figure 2. Steady state particle size distributions at various stirrer speeds. (a) LSS (b) USS.
rpm only the large-particle steady state (USS) in Figure 2b was observed and was stable. It may be noted that a gradual shift in the PSD and a change in crystal habit during the course of a run has been noted earlier by Swinneyet al. (1982);however, our findings showed the transitions to be abrupt and occurring within an interval of 1-3 residence times. In the runs reported in Figure 2a and b the reactor was initially filled with supersaturated solution corresponding to a well-mixed feed. To study the effect of the initial condition, experiments were also done in which the reactor was initially filled with water. However, the results were similar and no firm correlation with initial condition could be established. Nevertheless, in some runs the LSS did not appear for given operating conditions but did so in other runs for the same set of conditions. Further, when the LSS existed, and was manifested, it always appeared first, followedlater by the transition to the USS. Another distinctive difference between the two steady states is evident from the degree of nonlinearity of the PSD curves in Figure 2a and b on the semilogarithmic coordinates used. For a nonaggregatingprecipitate of uniform morphology and growth rate it is well established that under steady MSMPR conditions the PSD follows (Randolph and Larson, 1988).
n(L) 0: exp(-L/GT)
(2)
where n(L)is the population density at size L, G the growth rate, and T the mean residence time. Clearly, deviations from this behavior will be manifested as a nonlinearity of the fractional oversize plot (which is also readily seen to follow the above exponential form) in semilogarithmic coordinates. In this context then the LSS curves such as in Figure 2a were found to show greater nonlinearity, indicative of a larger significance of aggregation or more than one polymorph or a combination of both. This was verified by on-line video and SEM examinations to be discussed later. Results of model fits to be presented elsewhere (Chakraborty and Bhatia, 1994a,b) were also consistent with this observation. However, it may be noted that the increasing nonlinearity with an increase in rpm in Figure 2a is consistent with an increase in aggregation
Table 5. Experimental Parameters for Various Runs inlet stirring mean curve superspeed residence Caz+/C03S initial no. saturation (rprn) time T (min) ratio Ri conditiona 625 0.81 water 2136 625 0.81 2136 ss 400 0.79 2702 ss 400 1.04 2251 ss 1920 100 0.80 ss 400 1339 11.03 water 625 10.31 ss 1466 0.09 625 2101 water 250 0.10 1477 ss 4
SS represents supersaturatedsolution.
rate. A similar effect was noticed with an increase in residence time (Chakraborty and Bhatia, 1994b). The above apparent multiplicity of steady states and the transition between them is most clearly evident from an examination of the dynamic behavior, as depicted in Figure 3. In this figure the temporal variation of the mean particle size in the acquisition range, given as (3)
has been illustrated for several runs covering the range of conditions listed in Table 4. Here the lower limit of 0.5 pm is set by instrument limitations whereby particles smaller than 0.5 pm are not counted. Table 5 lists the conditions for the various runs for which the dynamics is discussed here. Of the experiments at Ri around 0.8-1 (correspondingto curves 1-5, given in Figure 3) two initially reach a steady state (LSS)after three-four residence times (curves 1 and 3) which holds for a further five-eight residence times. In this state the observed mean particle size is in the region of about 1pm. Subsequently, however, a sudden transition is noticed to a new steady state (USS) in which the mean particle size is about 5-6 pm. This steady state continued in further operation for more than 30 residence times and was found to be stable. On the other hand, curves 2,4, and 5 show no transition with the initially realized steady state being stable over a long period. For curves 2 and 4 the steady state corresponds well with the large particle USS of curve 1. Indeed, the runs for curves 1 and 2 differ only in terms of initial condition, and the small difference in mean size at the USS may be related to variation in scaling rate to be discussed below. The steady-state number densities for these runs match well as shown in Figure 4. Similarly the
Ind. Eng. Chem. Res., Vol. 33, No. 9,1994 2191 ;r 102 -
( a1
1
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10
3
10
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lo4A
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Figure 4. Temporal behavior of observed particle density under nearly stoichiometric conditions.
runs correspondingto curves 3 and 4, which differ only in terms of a small variation in the inlet total calcium ion to carbonate ion concentration ratio (c.f. Table 5), yield a very similar USS as seen in Figures 3 and 4. However, curve 4 does not possess the initial small-particle steady state (LSS) in curve 3. Interestingly, the run with the relatively low stirring speed of 100 rpm yielded an LSS which was stable and showed no change or transition over a period of operation of 35 residence times, as depicted by curve 5 of Figures 3 and 4. Initially the above multiplicity and associatedtransitions between the steady states appeared somewhat puzzling, particularly in view of the unpredictability and short duration of the LSS. However, it soon became apparent that the transition was related to scale formation on the internal surfaces of the reactor. This inference was suggested by a visual observation that at the USS the internal reactor surfaces were scaled and essentially fully coated with a layer of precipitate. On the other hand at the LSS the scale was initially not present and built up only during its later stages. For the low rpm run corresponding to curve 5, however, there was very little scaling throughout the entire duration of the experiment of about 35 residence times. Thus, it became clear that when all or most of the internal surface is exposed to the bulk solution the LSS is obtained and a transition to the USS occurs as the surface layering becomes significant and rapidly approaches completion. These observations suggested that the LSS corresponds to domination by heterogeneous nucleation, while in the USS the nucleation is largely homogeneous. In support it may be seen in Figure 4 that the number density in the LSS is in the region of about 107 mL-1, while in the USS it is around lo5 mL-* (i.e., lower by a factor of almost 100). For the low supersaturations as in Table 5 the much higher rate for heterogeneous nucleation, facilitated by a lower free energy barrier on impurities and external surface sites, is wellknown (Garside, 1985;Randolph and Larson, 1988;Dirksen and Ring, 1991), and in our case this is manifested prior to scale formation. The duration of the LSS thus corresponds to an induction period following which the surface scale has nucleated and begins to grow rapidly. The scale growth is accompanied by a rapid decline in available surface sites, and the rate of heterogeneous nucleation correspondingly decreases. During this period of rapid scale growth the transition to the USS occurs and homogeneous nucleation then predominates due to lack of available sites which are effectively blocked off by the layer of scale. In confirmation of this an experiment was performed in which a small clean rubber tube was dropped into the reactor operating at a stable USS a t 625 rpm. Immediately the particle size distribution become finer
1:-41 ?lo 0
I
7
, 1L 21
I
I
I
I
28 35 42 49 56 63 70
SIZE, p m Figure 5. Comparison of on-line and off-line particle size distributions. (a) LSS and (b)USS.
(as in the LSS) and the number density increased from 2.9 X lo4to 5.3 X 105mL-l. Due to the small surface area of the tube the increase is only 20 fold, as opposed to the 100-fold increase when the reactor is unscaled. Subsequently, however, after a period of about 8 residence times the original USS was regained, at which time the rubber tube was found to be coated with scale. Another interesting demonstration of the role of exposed surfaces is offered by a comparison of on-line and off-line determinations of the PSD, as in Figure 5a and b, for a run for which the LSS and USS were manifested. For the LSS the on-line determined PSD (which is the one used in the above discussion) matches well with the off-line PSD (c.f. Figure 5a) obtained by transferring a small amount of slurry into aglass cuvette which is inserted into the off-line cell of the instrument. The added heterogeneous nucleation induced by the unscaled cuvette surface is small compared to the existing population in the slurry, and the PSD is only marginally lower (in the region below 0.01 % cumulative oversize). There was only a small increase in population density (by about 20%) measured in the off-line cell, and this was attributed to the added heterogeneousnucleation on ita surfaces. On the other hand an attempt at off-line measurement of the PSD for the USS yielded a significantly finer distribution, as seen in Figure 5b, and a 10fold increase in number density. This anomaly was attributable to subsequent nucleation and growth in the off-line cuvette, outside the reactor. Thus, it is clear that for the low number density suspensions obtained from homogeneous nucleation of the precipitate, as in the USS, off-line PSD analysis is strongly affected by subsequent heterogeheousnucleation and precipitation and therefore unreliable. The persistence of the LSS at the low stirring speed of 100 rpm (curve 5 ) is an interesting feature of the results. For the duration of the run (upto 35 residence times) negligible scaling was observed and no transition to the USS occurred. This behavior was reproduced in several repeat runs. Had the run been continued for much longer, scale buildup followed by the transition to the homogeneously nucleating USS may perhaps have occurred; however, this aspect was not explored here. Thus, the question of whether the LSS at low rpm persisted because
2192 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 5 ,
1-1
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Figure 6. Temporal behavior of (a) observed mean particle size and (b) observed particle density for a run at high total calcium to carbonate ion concentration ratio.
of a large induction time for scale formation or was due to a very low scale growth rate is an interesting and open problem for subsequent study. In contrast,at highstirring speeds the scaling was much more rapid, and at 1250 rpm the LSS was never observed in several repeat experiments with only the USS being initiallymanifestedand remainii stable. However, the existence of the USS a t low stirring speeds was confirmed by an experiment at 100 rpm, with the reactor prescaled from a previous run at a higher rpm, with Ri = 1and Xi = 3000. This directly yielded the large particle size and low number density steady state (USS). Thus, the LSSwas effectivelybypassed, although this does not indicate whether or not the scale would have formed at all at 100 rpm, starting with a clean reactor and given sufficient time. A point to note, however, is that there was no noticeable increase in the amount of scale during the 20 residence time duration of the run, indicating that at low stirring speeds such as 100 rpm the scale growth rate is small. It should be noted that the shift from heterogeneousto homogeneous nucleation, due to consumption of heteronuclei, is a known phenomenon (Dirksen and Ring, 1991) but has been considered in the past only in the context of batch systems. The current work suggests the possibility of sustaining steady-state operation with any desired nucleation regime in a continuous operation, by suitably adjusting the process conditions. B. Non-StoichiometricOperation. Having identified the nucleation regimes and the associated steadystate transitions under nearly stoichiometric conditions (Ri = 0.8 or l), experiments were performed at high and low values of Ri to study the effect of this parameter. The runs at an inlet total calcium to carbonateion concentration ratio of 10 (Le., Ri = 10) in Table 5 (curves 6 and 7) yielded only an intermediate particle size (2-2.5 pm mean size) steady state (VSS)with negligible scaling,and these steady states were stable up to at least 40 residence times; although for the 625 rpm case the steady state was attained very late (after about 25 residence times). Figure 6a and b illustrate the dynamics for these cases. The steady state
m
fz
1 04
-
0
5
10
15
20
>5
DlMENSlONLESS TIME, t/"C
Figure 7. Temporal behavior of (a) observed mean particle size and (b) observed particle densityfor a run at low totalcalcium to carbonate ion concentration ratio.
(VSS)at the high value of Ri = 10 is similar to the LSS although the mean particle size is now higher at about 2-fold, while the number density is lower by a factor of 2-3 than those typical of the LSS in Figure 3 and 4. Nevertheless the particle concentration is still 50-100 times that typical of the USS and appears to suggest the possibility of heterogeneous nucleation. However, very similar results were also obtained from operation with a prescaled reactor, so that homogeneous or secondary nucleation seemed the most likely possibility. On the other hand, experiments at low Ri (0.09 or 0.11, corresponding to curves 8 and 9 in Table 5, yielded only the USS accompanied by rapid scaling. The dynamic behavior is depicted in Figure 7a and b, and this figure shows the rapid establishment of the homogeneous nucleation regime and a USS which was stable for the run duration of 20-25 residence times. It is clear from the above results that while reactor scaling is inhibited by increasing the calcium to carbonate ion concentration ratio in the feed, under these conditions heterogeneous nucleation as in the LSS does not occur and a different mechanism such as homogeneous or secondary nucleation is likely. On the other hand reduction in Ri results in faster scaling and rapid establishment of the homogeneous nucleation regime. This behavior is best explained by the surface adsorption of calcium ions which is known (Huang et al., 1991) to result in slowing of flocculation of CaC03 suspensions and therefore in improved dispersions. This occurs because of an increase in the t potential and the related increase in interparticle repulsion. In the present case therefore it is likely that increased adsorption of calcium ions on the internal surfaces of the reactor such as the glass walls and the impeller surface, as well as on the small number of CaC03 particles adhering to them, at higher values of Rj, inhibits scale growth as well as heterogeneous nucleation. Consequently homogeneous or secondary nucleation dominates. On the other hand at low values of Ri much more of the particles are initially heterogeneously nucleated on
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2193 Table 6. Outlet Conditions for Various Steady States
outlet curve steady Ca*+X 109 no. state ((gmol/L) R, 0.691 1 LSS 1.406 0.889 0.563 1 uss 0.592 2 USS 9.828 0.786 0.45 3 LSS 0.185 0.15 3 USS 0.944 2.11 4 uss 0.643 0.48 5 LSS 13.200 741.50 6 VSS 603.71 7 VSS 13.300 8 USS 0.0011 0.0073 0.179 0.0148 9 USS
X
202 99 115 96 16 29 61 17 21 120 154
c,X 104 ssx 104 0.32 0.67 0.33 1.10 1.03 0.24 0.65 1.35 0.28 1.77 0.43 1.56 0.13 1.19 0.53 0.39 0.61 0.41 0.31 0.046 0.02 0.15
the surface, leading to more rapid scaling and early establishment of the USS. Clearly, since alarge population of fine precipitate particles is often required in practice, operation a t high Ri (or VSS) would seem to be more desirable, particularly as this is not sensitive to scaling. C. Correlation with Outlet Conditions. While we have correlated the nature of the steady state obtained with the inlet supersaturation and ratio of total calcium to carbonate ion concentrations, it may be recognized that in an MSMPR system it is more pertinent to use instead the actual reactor or outlet values. To this end liquid phase calcium analysis of fitrate samples from eachsteady state was obtained for the different runs reported in Table 5. The determination of total calcium levels was conducted using inductively coupled plasma atomic absorption spectroscopy (ICP-AES),and the concentration of calcium and carbonate ions in dissociated form was estimated using a modification of the liquid phase ionic equilibrium model of Nancollas and Reddy (1971). More details of the calculations are provided elsewhere (Chakraborty and Bhatia, 1994b). From the estimated free calcium and carbonate ion concentrations the outlet supersaturation was calculated from A=
[Ca2+1[C03z~
KBP
(4)
Table 6 lists the results for the various steady states obtained in the runs indicated in Table 5. Also reported in Table 6 is the steady state solid fraction (by volume) in the outlet slurry as estimated from the liquid phase calcium balance and that reported by the Galai laser sizer. In the latter case the estimate is based on the assumption of spherical particles, and is therefore subject to errors when the product contains significant amounts of calcite (which is nearly cubic or rhombic) in addition to vaterite. The solid fraction E, follows 0
E,
= aJL3n(L) dL
(5)
0
with the shape factor u having the value of unity for cubes and 0.524 (i.e., ~ / 6 )for spheres, which clearly needs modification to accommodate mixtures of these. On the other hand the liquid phase equilibrium model yielded significant sensitivity of estimated outlet supersaturations to the small measurement errors (-2%) in outlet calcium levels at highvalues ofRi (Chakraborty andBhatia, 1994b). Nevertheless, for the runs corresponding to curves 6 and 7 the outlet solid fractions are in reasonable agreement, indicating that for high Ri runs the product is most likely spherical vaterite (as confirmed below), and negligible scaling occurs as already observed. On the other for the USS the estimated solid fraction from calcium analysis
appears to be typically an order of magnitude more than that estimated from the PSD. Such a large difference cannot be attributed to the incorrect assumption of the shape factor in the PSD-based result (which can at most be lower by a factor of d 6 ) and suggests that scale growth accounts for the major part of the precipitation in the USS. The large increase in scale at the USS was indeed observed in our runs. For the LSS,however, the solid fraction based on the calcium balance is roughly 2 times that based on the PSD, except for the curve 5, suggesting shape-related errors in the latter case in these steady states. This was indeed verified for a large number of runs exhibiting the LSS (Chakraborty, 1994). It may be noticed in Table 6 that there is no significant demarcation of supersaturation for the different steady states. However, it does appear that a t any outlet supersaturation, but low free calcium levels, a steady state with a relatively high scaling rate is obtained (c.f. values for curves 6 and 7 and the USS of curve 3). This is clearly due to less adsorption of calcium ions under these conditions (Huang et al., 1991). Similarly, at any outlet free calcium level a steady state with a relatively low rate of scale growth is obtained a t low outlet supersaturations.
Morphological Studies The existence of the two different steady states with distinctly different characteristic particle sizes and nucleation regimes also suggests the likelihood of different morphologies between them. The differences in morphologies could also explain the widely different growth rates which are evident between the LSS and USS,as the mean particle size in the latter case is 3-5 times that in the LSS. While the growth rate is expected to be dependent on the outlet supersaturation, our measurements of outlet calcium concentrations using the ICPAES technique, to be discussed in detail elsewhere (Chakraborty and Bhatia, 199413) in conjunction with kinetic modeling, yielded only small differences in outlet supersaturation, as given in Table 6, which could not reconcile the large difference in growth rates. Consequently the possibility of morphological differences appeared important, although this could be compounded by varying defect structures between the heterogeneous and homogeneously nucleated particles. Imperfections and defects on crystal surfaces are known to enhance growth rates (Garside 1985;Dirksen and Ring, 1991)and are also held responsible for the phenomenon of growth rate dispersion due to the their random occurrence (Garside, 1985). In light of these considerations morphological characterizations using SEM, cryo-SEM, XRD, and DSC for the homogeneously and heterogeneously nucleated particles were undertaken. Since the Galai CIS-1 laser sizer, used on-line for our experiments, is equipped with a microscope camera and video monitor, the particles could also be visualized on the screen during the progress of the run. In general the video imMes showed only spherical vaterite or rhombic calcite, and their mixtures, when the light beam was focused in the bulk slurry passing through the on-line cell. These observations are consistent with the results of Wray and Daniels (1957) who reported predominantly vaterite and some calcite in their CaC03 precipitated at 30 "C. No aragonite was reported by them at this temperature, although it was observed a t higher temperatures of 40 OC and above. However, in our on-line observations needlelike aragonite was noticed only when the beam was focused on the glass wall of the flow-through cell. Thus, it appeared that some aragonite was nucleating on and adhering to surfaces, while the bulk precipitate comprised vaterite
2194 Ind. Eng. Chem. Res., Vol. 33. No. 9,1994
4
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Figwe 8. SEM micrographs of precipitate particlea obtained fmm operationat LSS. (a, top) R w m temperature SEM and (b, bottom) cryo-SEM.
and calcite. This was ala0 supported by other evidence in which numerous cryo-SEMexaminationsof rapidly fmzen specimens of the bulk suspension never showed any needlelike aragonite. However, room temperature SEM, XRD, and DSC of samples, filtered and dried after collection of the reactor effluents in a flask, did show the presence of aragonite. This could only be explained by the subsequent nucleation of aragonite on the flask and other surfaces duringand after collection. Similarly, DSC studies were also inconclusive and generally showed the peak for transformation of aragonite to calcite a t 480 'C, in addition to a vaterite to calcite peak (Rao et al., 1968) a t 365 OC. Often, for the homogeneoua nucleation case (the USS)the vaterite to calcite peak a t 365 "C was very weak, indicating the presence of very little vaterite. However, our r w m temperature SEM as well as cryoSEM characterizations presented below suggested that a significant amount of spherical vaterite was also formed but was rapidly transforming to rhombic and nearly cubic calcite within the reactor itself. Thus, on subsequent filtering and drying the transformation was usually complete. Heterogeneous Nucleation Regime. For the heterogeneously nucleated precipitate of the U S ,on-line video images showed mostly vaterite and some calcite, while the DSC yielded no peak at 365 OC indicating that transformation to calcite was already complete before the calorimetry. As indicated above, the presence of rhombic and nearly cubical calcite in the LSS was also suggested by the factor of 2 difference in outlet solid fractions estimated by calcium balance and from the PSD. Figure Sa shows a micrograph obtained from room temperature SEM with the reactor operated at 100 rpm with hi = 2700, Ri = 0.8, and r = 6 min, for which the LSS was attained. Considerable aggregation is seen, consistent with the nonlinearity in Figure 2a, and the primary well-formed rhombic
Figure 9. SEM micrograph of precipitate particles obtained from operationat hightotalCaz'lC0~2ratio. (a.tnp) Room temperature SEM and (b. bottom) Cryo-SEM.
and nearly cubical calcite particles are evident in the micrograph. Many of these rhombs and distorted cubes have sharp edges and are unlikely to have resulted from transformation but are probably nucleatedascalcite. Some small vaterite spheres also appear in the lower left portion and other parts, and this suggests that both vaterite and calcite are nucleated in the reactor. Some rhombic and odd-shaped particles suggestiveof transformation arealso seen. Even more clear evidence of the presence of vaterite was obtained from cryo-SEM of a sample obtained from a run operated a t the LSS, as depicted in Figure ab. However, at room temperature, on examination of stored samplesobtainedafterfiltrationanddrying, thesevaterite particles appear to be transforming. Perhaps some transformation alsooccurs in the suspension in the reactor itself. Nevertheless, the well-formed vaterite and calcite suggest surfaces with relatively few imperfections and defects, which is consistent with the lower growth rate in the LSS (Figure 2). Another feature consistent with the PSDs in Figure 2a is the increase in aggregation with rpm whichwasevidentinvideovisualitionsandismanifested as increased nonlinearity in the figure. More details and model fits under aggregating conditions are available elsewhere (Chakraborty and Bhatia, 1994a,b). Homogeneous Nucleation Regime. Figuregadepicts a micrograph obtained from r w m temperature SEM of the precipitate from a run with a high value of Ri. For this run the conditions were Ri = 7, hi = 2300, r = 6 min, and stirring speed = 625 rpm. Predominantlyvaterite spheres and some disks, in the 1-3 pm range, are clearly evident without the presence of any calcite. The spherical shape is consistent with the observation above that the outlet solid fraction estimated from the liquid phase calcium balance is consistent with that calculated from the PSD using a shape factor of 4 6 . Some needlelike aragonite is also seen, but this was evidently nucleated subsequently duringcollectionastheon-lineimagesonthevideomonitor did not show any aragonite. In confirmation cryo-SEM
Ind. Eng. Chem. Res., Vol. 33, No.9,1994 2195
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ofthesuspensionalsoyieldedonlytheaphericalandsome disklike vaterite as depicted in Figure 9b. Thus, in this ease stable vaterite is produced which does not spontane ously change its morphology even when stored for long periods; as our room temperature SEM was done several weeks after filtering and drying the sample. In addition, room temperature SEM of a freeze-dried sample also gave an identical result but did not show the aragonite needles, again indicating these to be nucleated outside the reactor. Further, in all cases the vaterite particles were seen to be well formed and relatively free of surface imperfections. Consequently the growth rate is lower than that in the USS (c.f. Figures 6 and 7 ) where surface defects are much more prominent. The high degreeof surface imperfections for the homogeneous nucleated material of the USS will be subsequently evident. A distinctive feature of the micrographs in Figure 9 is the absence of any aggregation. This is most likely due to the adsorption of Ca2+on the surface (Huang et al., 1991)which, at the high value of the Ca2+/C032 ratio (Ri) used, is able to stabilize the suspension by enhancing interparticle repulsion. The absence of aggregation is also confirmed by the linearity of the PSD on semilogarithmic coordinates, as shown in Figure 10, for a run at 625 rpm with Xi = 2300, Ri = 7, and T = 1.5 min. The solid line in the figure is a fit of eq 2, yielding G = 0.68 pm/min and B = 1.21 X 10' mL-' min-1. These results were also obtained by fitsof the population balance equation (PBE) with aggregation using the MWR solution (Bhatia and Chakraborty, 1992). which yielded a negligibly small constant aggregation kernel. More details of the fits of the PBE with aggregation are provided elsewhere (Chakraborty and Bhatia, 1994b). For the homogeneously nucleated particles of the USS the results were most intriguing. Asmentionedpreviously DSC results often indicated very little vaterite, as evidenced by a weak peak at 365 "C and at other times a stronger one for runs under similar conditions. The XRD analyses were also inconclusive and showed the presence of all three polymorphs. All of these were again suggestive of transformations outside the reactor. However, the online video images clearly showed primary vaterite spheres aswellasintermediateshapes betweenrhombsandspherea which appeared to be the result of morphological transformation and some aggregates. Figure l l a shows a roam temperature SEM micrograph of the precipitate obtained from a run at 625 rpm with Xi = 2600, Ri = 0.5, and r = 6 min, depicting spheres with numerous surface abnormalities, as well as a few rhombs or distorted cubes and
Figure 11. SEMmicrographsofpreeipitateparticlesobtainedfrom operation at USS. (a, top) R w m temperature SEM, Ri = 0.5, and (b. bottom) r w m temperature SEM. Ri = 0.8.
intermediate shapes. These imperfections are consistent with the high growth rate in the USS and for the spherical vaterite may also be expected to lead to more rapid morphological transformation in the reactor suspension itself. With an increase in rpm, although on-line video images did still reveal some vaterite spheres, the SEM micrographs, such as that shown in Figure l l b , yielded mostly calcitic rhombs and distorted cubes, with many imperfect edges and surfaces, which appeared to be the result of transformation from vaterite. For this run a t 1250rpm the inlet conditions were Xi = 2100, Ri = 0.8, and T = 7 min. Thus, the transformation appeared to be more rapid in the suspension a t the higher stirring speed, most likely due to the higher surface shear. Even more convincing evidence of this was obtained by cryo-SEM which could capture intermediate shapes of vaterite transforming in the suspension. Figure 12a and b shows several such particles in various stages of transformation, with edges gradually appearing and the shape changing to rhombic. Figure 12c shows a close-up of a rhomb, evidently obtained from transformation (note the curved upper and lower faces), with several surface defects. For the run to which the micrographs in Figure 12 correspond the stirring speed was 625 rpm with Xi = 2500, Ri = 1,and T = 3 min. For the homogeneous nucleation at the USS,discussed above, the inlet total calcium to carbonate ion concentration ratio for the runs was of the order of unity (in the range 0.5-1). Under these conditions the aggregation tendency in the USS also appeared to be less than that in the LSS as seen from Figures 8a and lla. This is also evident from a greater linearity of the oversize plots for the USS in Figure 2. However, on lowering the value of Ri to 0.1 the aggregation of the homogeneously nucleated particles in the USS was increased considerably, as evidenced in the video monitor. In this case the on-line visualizations showed spherical vaterite as well as well-
2196 Ind. Eng. Chem. Res., Vol. 33, No.9,1994
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