Ind. Eng. Chem. Process Des. Dev. 1980, 19, 490-494
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COMMUNICATIONS
Size Reduction of Crystals in Slurries by the Use of Crystal Habit Modifiers Crystal slurries undergo a significant size reduction when gently agitated in their saturated solutions containing traces of a habit-modifying addfie. This phenomenon, for which only speculative mechanisms can at present be proposed, could be utilized in low-energy comminution, e.g., for hazardous substances.
Introduction Many industrial crystalline products consist of nonuniform, polycrystalline agglomerates which cannot easily be dispersed into their primary crystalline particles without the use of drastic techniques, such as comminution or the application of ultrasonic irradiation to suspensions in liquids (Mullin and Ang, 1974). Some degree of dispersion can often be effected if polycrystalline aggregates are slurried in their own saturated solution. For example, some of the crystallites adhering to the faces of the larger crystals may be washed off or weak agglomerates may be disengaged. The overall result is a slight but measurable shift toward a smaller product mean size. However, in the present study, the surprising observation has been made that when a small quantity of an impurity, which is a specific habit modifier for the salt in question, is added to a gently agitated slurry of the salt in its own saturated solution, a considerable size reduction can take place. The use of additives, e.g., hydrocarbons, organo-silicon liquids, fatty acids, etc., as grinding aids for the dry milling of abrasive substances is quite common (Snow and Luckie, 1976). However, the present method, involving the use of habit-modifying additives as size-reduction agents, does not appear to involve any mechanical grinding; consequently, its potential applications range from the processing of materials where dry grinding is hazardous to the preparation of fine-grained slurries for the facilitation of pumping. Crystal habit modifiers may be of very diverse character: multivalent cations, complexes, surface active agents, soluble polymers, biologically active macromolecules, fine particles of sparingly soluble salts, and so on (Buckley, 1951; Mullin 1972). In many instances the modifying activity of an additive is due to its adsorption on specific crystal faces, thus retarding their growth (Davey and Mullin, 1974). Sometimes their action is more complex, depending upon structural compatibility between the additive and the major constituent (Sarig et al., 1975) or on the intervention of the impurity at the nucleation stage (Sarig and Ginio, 1976). The mechanisms involved in the size reduction of crystals in slurries, induced by habit modifiers, are undoubtedly even more complex. Experimental Section The substances used in the present study were all simple soluble salts (Analar grade, BDH, Ltd.) with admixtures of generally well-known specific crystal habit modifiers belonging to the following classes: (1)inorganic complex: potassium ferrocyanide (K,Fe(CN),) with sodium chloride (NaC1); (2) divalent cation: lead chloride (PbC12) with potassium chloride (KC1); (3) organic molecule: urea 0196-4305/80/1119-0490$01.00/0
Table I. Correlation between t h e Median Size of Sodium Chloride Crystals and t h e Experimental Conditions
population
A0 B C D E Fb G H
[Fe( CN),J4-, time, PPm h
0 12 5 30
48 48 72 72
0 10
72 72
The original feedstock. feedstock.
median size, Pm
3 50 24 0 120 118 103 275 185 140
reduction of the median, %
0 50 50 57 0 24
Close-sized (250-300 pm)
(CO(NH,),) with ammonium bromide (NH,Br); (4) sparingly soluble salt: calcium hydrogen phosphate (CaHPOJ with ammonium perchlorate (NH4C104). The experimental technique used was very simple. Basically, crystals of the salt under investigation were gently agitated in about 300 mL of its saturated solution ( 5 7 % by weight slurry) in a 500-mL flask fitted with a magnetic stirrer and a reflux condenser. All runs were carried out at 30 "C. After a specified period of time, the slurry was filtered, the crystals washed and dried, and their size distribution determined by sieving. Several runs could be made simultaneously in the same thermostat bath so that the behavior of pure solutions and solutions containing active additives could be studied side by side. An Inorganic Complex Modifier The cumulative size distributions of three populations of sodium chloride crystals are shown in Figure 1. Population A represents the original crystals (feedstock). Population B was obtained by adding 130 g of sodium chloride crystals (population A) to 300 mL of water and agitating for 48 h. The excess solids (-20 g) gave a 5% by weight slurry of sodium chloride in its saturated solution. Population C resulted from a run similar to B, i.e. -5% w/w slurry, except that the solution also contained 12 ppm of [Fe(CN),I4-, i.e., 1 2 pg of ferrocyanidelg of water. Runs B and C were carried out simultaneously. The median sizes, estimated from Figure 1,are A = 350 pm, B = 240 pm, and C = 120 pm. The coefficient of variation, CV, which gives a measure of the crystal size distribution, is reduced slightly from about 38 to 32% for populations B and C, respectively. To test the effects of both additive concentration and time, two additional experiments were carried out: (D)5 ppm and (E) 30 ppm of ferrocyanide both agitated for 72 h. The results are listed in Table I. Comparison between 0 1980 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 3, 1980 491
I 100
200 300 Clpt.al IIZ.. pm
500
Figure 1. The effect of additives on the size distribution of sodium chloride crystals: A, original feedstock; B, population A stirred in pure satwated solution for 48 b; C, population A stirred for 48 h in saturated solution containing 12 ppm of ferrocyanide (a hahit modifier for NaC1). The solid circles (0)represent the product resulting from population A being stirred for 48 h in saturated solution containing 30 ppm of ferric chloride (a non-habibmodifierfor NaC1).
runs B and C shows the significant effect of the crystal habit modifier on size reduction. Comparison between C, D, and E suggests that this effect is dependent on both time and additive concentration, but confirmation and quantification of this conclusion would obviously require further experimentation. The feedstock used in this first series of experiments contained a significant proportion of agglomerated crystals. It was necessary, therefore, to establish if the additives were merely facilitating the breakdown of agglomerates or if other mechanisms were involved. Accordingly, a slurry was prepared from a close sieve-cut (250-300 pm) of sodium chloride crystals. The particles in this fraction (population F) contained very few agglomerates. Two 300-mL portions of saturated sodium chloride solution were prepared: (a) with water and (b) with water containing 10 ppm of ferrocyanide; 20 g of the close-sieved crystals were added to each flask and stirred for 72 h. The sieve analyses of the resulting crystals indicated median sizes of (a) 185 Gm (population G ) and (b) 140 pm (population H),i.e., a further 24% reduction in size due to the presence of the additive. Although this effect is less pronounced than that experienced with the broad size-spread feedstock (Table I), it does show that agglomerate dispersion is not the exclusive mechanism of the size reduction. Dissolution is clearly ruled out as a cause of the size reduction because the presence of trace quantities of [Fe(CN),]‘ does not affect the equilibrium solubility of sodium chloride in water to any significant extent. Furthermore, the effect of breakdown manifests itself soon after the achievement of equilibrium saturation by the appearance of minute crystals which develop a “haze” in the solution. In an attempt to determine if the size reduction effect is specific to the crystal habit-modifying additive, a multivalent cation impurity, FeCl,, which has no habitmodifying effect on sodium chloride, was used in place of ferrocyanide. The striking result is shown in Figure 1;no size reduction occurred. Scanning electron micrographs of the crystalline products allow a glimpse into the complexity of the processes involved. Figure 2a shows the original feedstock crystals of sodium chloride (mainly rounded agglomerates). Figure 2b shows the product obtained by stirring the original feedstock for 48 h in pure saturated solution (clean-cut crystals with overgrowths oriented along cubic lines). The crystals resulting from being stirred in the presence of 12 ppm of ferrocyanide (Figure 2c) are clearly single particles, but generally rounded.
b
5-
1.0
n
Figure 2. Scanning electron microgapha (X40) of the 18IF210-&m sieve fraction from various populations of sodium chloride crystals (see Figure 1): a, original feedstock (A); b, product crystals (B) obtained by stirring A in saturated solution without additive; e, product crystals (C) (X40) obtained by stirring A in saturated solution in the presence of 12 pprn of ferrocyanide.
At higher magnification, some features of similarity are noted between the crystals in Figures 2b and Zc. The step pattern in Figure 3a, a crystal produced in the absence of additive, is matched by that in Figure 3b on a rounded corner of a crystal produced in the presence of 12 ppm of ferrocyanide, although the steps on the former are about 15 times deeper than those on the latter. It is reasonably certain that the features seen on these crystals are “growth” rather than “dissolution” steps. The dissolution of the almost shapeless agglomerates of Figure 2a would be very unlikely to result in the sharply outlined cubic shapes of Figures 2b, 3a, and 3 b the corners and edges of crystals are usually the first to dissolve. It would appear, therefore, that some form of ripening process takes place in contact with the saturated solution, both with and without additives present. The different step heights in Figures 3a and 3b suggest that the additive has a significant effect on the growth process. The crystal surfaces in Figure 2c exhibit several quite different growth patterns at higher magnification, e.&, rounded comers (Figure 3b), numerous cubic outgrowths
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Ind. Eng. Chem. Process Des. Dev., Val. 19, No. 3, 1980
Figuie 3. Surface details of sodium chloride crystals: a, crystal (-300 pm) from pure solution (sample B) (XlZO); b, rounded corner on a crystal (-200 pm) from 12 ppm of ferrocyanide in solution (sample C) (X1600); c, flat surface on a crystal (-200 rrm) from 12 ppm of ferrocyanide in solution (sample C) (x800); d, flat surface on a crystal (-200 pm) from 12 ppm of ferrocyanide in solution (sample C ) (X4oW).
(Figure 3 4 , and surfaces containing holes (Figure 3d). It is unlikely that any of these patterns result from the drying-up of adhering mother liquor as the crystals were washed with acetone on the filter. The slurry made up from the close-sieved (250-300 pm) feedstock (population F) was reduced by agitation to a population of 185 median size (G) in pure solution and in the presence of 10 of ppm ferroto one of 140 pm (H) cyanide. An interesting visual difference between these two products is seen in Figure 4. The product without additive (10% larger than 250 pm) shows (Figure 4a) a few well-formed cubes with slightly rounded corners and a number of cubic agglomerates, whereas the product with ferrocyanide (4% larger than 250 pm) shows (Figure 4h) a few near-perfect cubes with many irregular agglomerates of much smaller component particles.
A Divalent Cationic Modifier Lead salts in minute concentration are known to he effective habit modifiers for potassium chloride (Buckley, 1951; Mullin, 1972). There is spectrophotometric evidence (Glasner and Skurnik, 1965) that an appreciable proportion of any lead ions present as impurity (at the ppm level) in concentrated chloride solutions form chloride complexes. Consequently, the mechanism of habit modification of KC1 by PhC12, whether by poisoning (adsorption) or the formation of heteronucleic, may not differ substantially from the mechanism of sodium chloride modification by ferrocyanide (Glasner and Zidon, 1974). The interaction of PbClz with KC1 is ohviously quite different from that of FeC1, with NaCl (no habit modification). It is significant that Fe3+does not form stable chloride complexes and is most probably hydrolyzed in saturated sodium chloride solution. An excess of 20 g of KC1 was stirred for 48 h in 300 mL of saturated KC1 solution. One flask contained pure solution and another contained solution with 10 ppm of Ph2+. The median sizes of the products were 180 and 105 pm,
Figure 4. Sodium CI iloride (X16) produced from a close-riieved (250-300 pm)feedstock (population F): a, from pure solutioin (GI; mF.irnia. _. in .nilstinn IU) I"._1.".. \__,. b, from 10 ppm of fen-..
...
respectively, Le., a 31% reduction of the median size due to the presence of PIP. A n Organic Molecular Modifier Ammonium bromide generally crystallizes from aqueous solution in dendritic form. Dendrites are not particularly noticeable in the typical industrial product, hut the granular salt is exceedingly prone to caking and the use
99
40
M 20
/I
.
Ind. Eng. Chem. Process Des. Dev.. VoI. 19, No. 3, 1980 493
-
,
30 20
I
10
i 30 20
:10 3 30 ..6 20
: 10
L
C M 20 1 0
10
0 60
100
crp1.a Kx)
200 300 400 5 0 0 6 0 0
Figure 5. Size reduction of ammonium bromide crystals in the presence of 40 ppm of urea.
of urea as an anticaking agent has been suggested. Two flasks each containing 20 g excess ammonium bromide crystals in 300 mL of saturated solution were agitated for 48 h. To one of the solutions 12 mg of solid urea (40 ppm) was added. The median size of the product crystals stirred in the pure solution was 360 pm and that in the presence of urea was 180 pm (50% reduction). In addition to reducing the median size, stirring in the presence of the crystal habit modifier narrowed the size distribution: the coefficient of variation was reduced from about 32 to 22% (Figure 5). A Sparingly Soluble Modifier The use of sparingly soluhle salts as crystal habit modifiers is rather rare. However, one example is the use of calcium hydrogen phosphate in the production of smooth spherulites of ammonium perchlorate (Sarig and Shakked). In this particular case, most of the CaHP04 remains in suspension and a satisfactory mechanism for this effect has not yet been elucidated. Another example is the effect of calcium fluoride, precipitated in situ, which reduces the crystal size of calcium hydrogen phosphate crystals produced from apatite. In this case, heterogeneous nucleation on the solid impurity particles, acting as substrates, has been suggested as the operating mechanism (Sarig et al., 1976). In the present study, ammonium perchlorate was used with calcium hydrogen phosphate as the additive, hut the difference between this system and those described above was the modifier:salt ratio. The 5-40 ppm concentrations of the soluhle additives represented 5oo-pm fractions) produced from: a, pure solution (X40); b and e, solution with 1 % CaHPO, in suspension (X16) and (XSO), respectively.
a more dramatic effect was produced when 1% of CaHPO, was used the multi-nodal coarse-grain feedstock was transformed into an almost mononodal population, where the size of the main fraction was reduced by a factor of 5 (Figure 6d). But even in this case enough of the largest fraction “remained to constitute 1.8% of the new popu-
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Ind. Eng. Chem. Process Des. Dev. 1980, 19, 494-497
lation. However, the agglomerated clusters shown in Figure 7b provide convincing evidence that the expression “remained” is erroneous. The +500-pm product crystals obtained after stirring the feedstock in the presence of 1% CaHP04 are really bunches of about 50 pm sized globules (Figure 7c). The large-size fraction in the product was therefore not the equivalent fraction preserved from the original feedstock; it was newly formed by the agglomeration of smaller particles. Discussion The discovery of the phenomenon of “size reduction induced by specific additives” opens a new field of study. The distinguishing feature of the technique is that a significant size reduction can be achieved by gently agitating crystals in a slurry of their own solution containing traces of certain substances, generally active habit modifiers. The required energy input is relatively low. The present investigation itself is far from being complete. In order to establish general principles, many other systems should be studied. The important variables (additive concentration, slurry density, time, mode of agitation, etc.) and their separate effects should be investigated systematically. Such information, while having considerable practical value, would also help to elucidate the mechanisms involved. For three of the salts studied in the present work, the median sizes of the resultant crystals produced in the presence of a habit modifier were some 50% smaller than those produced in the equivalent slurry without additive. In the case of ammonium perchlorate the size reduction was even more pronounced, but less easily quantified (Figure 6). The presence of growth steps on large crystals of sodium chloride which had been agitated in pure solution (Figure 3a) indicates salt deposition, although the system was originally saturated. It is reasonable to suppose that deposition was preceded by dissolution and, if so, this points to a kind of ripening process. Similar evidence for ripening may be seen in Figures 3b and 3c for crystals produced in the presence of ferrocyanide, but the nature of the ripening is undoubtedly modified in the latter case since the growth steps are very much deeper. Ripening, of course, implies a shift of the particle population to a larger mean size, so
if ripening does occur its effects must be massively obscured by other processes which lead to a size reduction. Agglomerate breakdown cannot be the only cause of size reduction because a shift toward smaller sizes was also achieved with nonagglomerated, close-sized feedstocks (Table I). Additional processes, influenced by habitmodifying impurities must therefore be operating. For example, it is possible that, in addition to mass deposition (growth) following dissolution, some secondary nucleation occurs. The resulting small crystals would lower the mean size of the population, although some could form aggregated clusters (Figures 4b, 7b, and 7c) or be deposited on crystal faces (Figure 34. Any secondary nucleation process would very likely be strongly modified by the active impurities. Another possibility is that the added impurities could interact with surface defects, e.g., Griffith cracks (Griffith, 1920; Strickland-Constable, 1979), into the tips of which stress is concentrated, and render the crystals prone to disintegration. However, such proposed mechanisms can only be regarded as speculative at this stage. Acknowledgment The authors are indebted to the Science Research Council for the financial support which enabled this work to be carried out. Literature Cited Buckley. H. E. ”Crystal Growth”, Longmans: London, 1951. Davey, R. J.; Mullln, J. W. J . Cryst. Growth 1074, 23, 183; 26, 45. Glasner. A,; Skurnik (Sarlg), S. Isr. J . Chem. 1965, 3 , 143. Glasner, A.; Zldon, M. J . Ctyst. Growth 1074, 295. Griffith, A. A. Phil. Trans. R . SOC. London, Ser. A 1920, 227, 163. Mullin, J. W. “Crystallization”, 2nd ed;Butterworths: London, 1972. Mullin, J. W.; Ang, H-M. Chem. Ind. (London) 1074, 622. Sarig, S.; Ginio, 0. J . Phys. Chem. 1076, 80, 256. Sarlg, S.; Glasner, A.; Epsteln, J. A. J . Cryst. Growth 1075, 28, 295. Sarig, S.; Leshem, R.; Ben-Yosef, N. Chem. Eng. Sci. 1976, 1061. Sarig, S.; Shakked, Z.,unpublished work. Snow, R. H.;Luckie, P. T. Powder Technoi. 1076, 13, 33. Strickland-Constable, R. F. J . Chem. SOC. Faraday Trans. 1070, 75, 921.
Department o f Chemical and Biochemical Engineering University College London London W C l E 7JE, England
Sara Sarig John W.Mullin*
Received for review April 24, 1979 Accepted January 28, 1980
Deacidification of Glyoxal by Liquid Ion Exchange Continuous countercurrent extraction of acidic glyoxal with trioctylamine dissolved in isopropyl acetate was studied as an alternative to conventional deacidification using anion-exchange resins. The effects of selected variables on the extent of deacidification, the quality of the product, and the composition of the waste stream were studied. A bench-scale pilot unit was operated continuously to produce glyoxal of acceptable quality. Introduction Glyoxal produced by the nitric acid oxidation of aqueous acetaldehyde contains in the crude state significant amounts of acids such as acetic, formic, glyoxylic, and unreacted nitric. After processing to remove the bulk of acetic and formic acids, crude glyoxal still contains 1-3% acidity, calculated as acetic acid. These acids must be removed in order to produce a salable product. Various methods of purifying glyoxal have been examined. One method is to convert glyoxal to a volatile or extractable derivative followed by purification and regeneration (Wessendorf et al., 1976; Sommer and Wes0196-4305/80/1119-0494$01.00/0
sendorf, 1975; Lange, 1942). Another is purification by electrodialysis (Asaki et al., 1970). In commercial practice, however, purification seems to be conducted exclusively by fixed-bed ion exchange (Tsunemitsu and Tsujino, 1966; Merz et al., 1966; Gabrielson and Samuelson, 1952). Deacidification of glyoxal by ion exchange has two major drawbacks: (1)the resin is rapidly exhausted because the acidity of the glyoxal is high; (2) waste disposal costs are high because the salts of the acids produced during regeneration of the resin are highly dilute. These drawbacks potentially could be minimized by continuous anion exchange where a moving bed of resin 0 1980 American Chemical Society
