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Origin of secondary nucleation as revealed by isotopic labelling
Journal of Crystal Growth 69 (1984) 623—626 North-Holland, Amsterdam
623
LETtER TO THE EDITORS ORIGIN OF SECONDARY NUCLEATION AS REVEALED BY ISOTOPIC LABELLING K. SHIMIZU Department of Applied Chemistry, Iwate University, Morioka 020, Japan
K. TSUKAMOTO Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Aoba, Sendai 980, Japan
J. HORITA Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro - ku, Tokyo 152, Japan
and T. TADAKI Department of Chemical Engineering, Tohoku University, Aoba, Sendai 980, Japan Received 7 May 1984; manuscript received in final form 15 September 1984
Isotopic measurement by mass spectroscopy was for the first time applied to the secondary nucleation products formed in a supersaturated H 20—alum solution in the presence of a seed crystal containing D20. It was shown that fine particles were chipped off from the seed into the solution and act as centres for secondary nucleation.
Crystal nucleation has been classified as primary nucleation when it takes place without the help of seed crystals and as secondary nucleation when seed crystals are present, in a supersaturated solution [1.-7]. Although in industrial crystallization secondary nucleation is considered to be the more important for controlling the size distribution of the product crystals [8,9], it has not been well understood. It was recently proposed by the present authors [10], based on quantitative measurements of the number of secondary nuclei coupled with surface observation of seed crystals, that minute crystalline particles adhering to the surface of a seed crystal act as centres for secondary nuclei, an idea which had been proposed originally by Lal et al. [5] and Strickland-Constable [6]. Since this conclusion was derived from indirect experimental evidence, a direct experiment was planned to test the hypothesis by means of isotopic labelling with
K-alum seeds containing D20 instead of H20 in the form of hydrate water. Seed crystals for secondary nucleation were prepared in both H20 solution and D20 solution by overgrowing K-alum onto smaller seeds (1680— 2000 ~smin diameter and 0.003 g in weight) which had been grown in a H20 solution. When the seeds were grown from the D20 solution, using 99.7% pure D2O, the seeds (D-seeds) contained D2O in the form of hydrate water instead of H20. K-alum solutions for secondary nucleation experiments were prepared using distilled H 20 at 20°C by stirring the solution continuously for 2 h. After ageing the solution for 1 h without agitation, the upper part of the solution was poured into a nucleation cell, fig. 1. Secondary nucleation experiments were then performed at a superaturation corresponding to an undercooling ~T = 2.0°Cby the following methods employing the nucleation cell: (1) Nucleation by initial breeding; a seed
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
624
K. Shimizu et al.
GLASS ROD
—-*
~
/
Origin of secondary nucleation as revealed by isotopic labelling
In all three cases, the secondary nuclei were kept to be grown larger in the same supersaturated solution for another 30 mm, followed by measur-
[j
I —
ing the D/H ratio of water contained both in the nuclei and in the solution. The measurement was not performed for each small nucleus, 10 ~sm in diameter, but for numbers of the collected small crystal, 20 mg. Before measurement the hydrate water in the secondary nuclei was dehydrated in vacuum by heating the collected nuclei up to approximately 300°C for 3 h. The H20 and D20
— ——
SEED HOLDER
L
measurement metallic uranium. of the These D/Hgases ratiowere on aused McKinneyfor the
___________ K-ALUM SOLUTION
~- FILTER
CUIJM
c”_~ VESSEL
were reduced by H2 and D2 gases at 750°C with
—~
_______________
Fig. 1. Nucleation cell made of a 50 ml glass bottle, equipped with a glass rod (2 mm in diameter. 110 mm long and 0.57 g in weight) and a seed holder,
error of measurement for the D/H ratio is ±0.3 x 1O~. type In mass tablespectrometer. Ia the D/H The ratios overall of the experimental secondary nuclei formed by contact nucleation using pure seed and the H20—alum solution are shown. These values are considered to be an average D/H ratio for ordinary distilled water and chemical reagents. In table lb the D/H ratios of the secondary nuclei formed by the present experiment using D seeds are shown, in which one will notice the marked increase in D/H ratios as compared to the values in table la. This suggests that either D of the D seed is exchanged by H in the supersaturated H2O—alum solution from which the nuclei were formed or the surface of the D seed has been chipped off in minute particles which acted as the centre for secondary nucleation.
holder, at the tip of which a seed was fixed, was immersed in the solution for 5 s. (2) Nucleation by fluid shear; the seed holder with a seed was rotated at 60 rpm for 30 mm. (3) Nucleation by contact; the contact of the glass rod with the crystal surface was achieved by pounding the seed surface with the glass rod, one per minute for 5 mm. In each case holder with seed and glass rod takenthe outseed from the solution immediately afterwere the above treatment. After the nucleation experiment, only the solution was pumped out with the help of vacuum pressure through a filter with a pore size the 1 filter. filtrate in the glassnuclei vesselbehind was then of ~sm, The leaving the secondary on used for isotopic measurement.
Table 1 D/H ratios of water, both in the secondary nuclei and in the H2o solution (a) With H20 seeds
_____________________________________________ Species Nuclei from pure seeds H 20 solution (b) With D 20 seeds
D/H ratio 6 145.9x10 6 146.1x10
Species
D/H ratio
By By fluid initialshear breeding By contact nucleation
154.9x106 631.0x10 3520.0x106
6 _______________________________________________
_________________________________________________
K. Shimizu et al.
/
Origin of secondary nucleation as revealed by isotopic labelling
To investigate which mechanism is appropriate, the exchange rate of D/H ratio between a D seed and the pure H20 solution was measured. A D seed was at first partially dissolved in distilled water so as to entirely remove the possible fine Crystallites adhering to the surface [10], and then was hung by a nylon string in another seed was reduced by 0.004 g on average. The D content of the solution was measured after 30 mm. As shown in table 2, only a slight increase of the D/H ratio in the solution could be discerned. Since the longest experimental duration in the present experiment was 30 mm, the data in table lb are not due to the effect of isotopic exchange between the D seed and the H20 solution but due to the chipped off particles from the D seed during secondary nucleation. The above conclusion is further supported by the much higher D/H ratios in table lb. The fact that the D/H ratio for contact nucleation is much higher than for the other treatments indicates that mechanically chipped off particles of the seed with a glass rod act as the centres of secondary nuclei and are integrated into the nucleation products. Since the number of nuclei by contact was also found to be much larger than by the other two methods [10], the present higher D/H ratio is consistent with this interpretation, As will be published, scanning electron microscopy and differential interference contrast microscopy after Nomarski reveal such a typical feature around the place where the surface was pounded with a glass rod as the surface is locally broken into pieces of small crystallites. This is further support for the importance of the mechanically chipped off particles. According to our investigation, fluid shear has the least power to chipp off the surface of the seed, The D/H ratio for the fluid shear method is much lower than the other two values, though it is higher Table 2 Results of a blank test, in order to measure the exchange rate of D between the D seeds and the H20 solution Duration (nun)
D/H ratio
0 30
146.1 x 146.7 x
io~ ~
6
625
than those in table la. This can be explained in the following manner. Since fluid shear provides strong movement of the supersaturated solution, it is probable that rapid overgrowth of pure K-alum onto the seed crystal may take place before the majority of fine crystallites leave the D seed surface, because the growth rate of a crystal is expected to become larger due to faster volume diffusion of molecules to the seed when the seed is rotated in a solution. As shown in ref. [10], the number of secondary nuclei by the fluid shear method was larger than by initial breeding, though the absolute number was largely influenced by the state of the seed surface. Considering this fact, the particles chipped off from a seed by initial breeding might be larger in size than by fluid shear. It is interesting to investigate whether or not the number of nuclei is equal to the number of fine particles which are chipped off from the surface of a seed crystal. Although at this moment it is impossible to count the latter number, the former number is considered to be less than the latter. This is because only the particles with larger radius than that of critical radius of a nucleus [11] would act as nucleation centres and grow larger. In conclusion, by applying the isotopic labelling method, direct evidence was obtained that the chipped off particles of a seed crystal have acted as nucleation centres and were then incorporated into the secondary nuclei. The authors would like to express their thanks to Professor S. Matsuo for discussions on the isotopic measurements and to Professor I. Sunagawa for discussions and for his critical reading and revision of the manuscript. This work is partly supported by a Grant-in-Aid for Fundamental Researches from the Japan Ministry of Education.
References [11 R.F. Strickland-Constable and R.E.A. Mason, Nature 197 (1963) 897. [21 W.L. McCabe, in: Chemical Engineers Handbook, Ed. J.H. Perry (McGraw-Hill, New York, 1953) p. 1056. NS’vlt, in: Industrial Crystallization (Verlag Chemie, Weinheim, 1982) p. 79.
[31J.
626
[41M.D.
K. Shimizu et al.
/
Origin of secondary nucleation as revealed by isotopic labelling
Cisc and AD. Randolph, AIChE Symp. Ser. 121 (1972) 42. ~5] D.P. Lal, R.E.A. Mason and R.F. Strickland-Constable, J. Crystal Growth 5 (1969) 1. [6] R.F. Strickland-Constable, J. Chem Soc. Faraday Trans. I. 75 (1979) 921. [7] C.Y. Sung, J. Estrin and G.R. Yougquist, AIChE J. 19 (1973) 857.
[81 J. Garside, Chem. Eng. Commun. 4 (1980) 393. [9] MA. Larson, Chem. Eng. Commun. 12 (1981) 161. [10] K. Shimizu, N. Kubota, K. Tsukamoto, I. Sunagawa, T. Yonemoto and T. Tadaki, J. Crystal Growth 69 (1984) 115. [11] M. Ohara and R.C. Reid, in: Modeling Crystal Growth Rates from Solution (Prentice-Hall, London. 1973) p. 19.
623
LETtER TO THE EDITORS ORIGIN OF SECONDARY NUCLEATION AS REVEALED BY ISOTOPIC LABELLING K. SHIMIZU Department of Applied Chemistry, Iwate University, Morioka 020, Japan
K. TSUKAMOTO Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Aoba, Sendai 980, Japan
J. HORITA Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro - ku, Tokyo 152, Japan
and T. TADAKI Department of Chemical Engineering, Tohoku University, Aoba, Sendai 980, Japan Received 7 May 1984; manuscript received in final form 15 September 1984
Isotopic measurement by mass spectroscopy was for the first time applied to the secondary nucleation products formed in a supersaturated H 20—alum solution in the presence of a seed crystal containing D20. It was shown that fine particles were chipped off from the seed into the solution and act as centres for secondary nucleation.
Crystal nucleation has been classified as primary nucleation when it takes place without the help of seed crystals and as secondary nucleation when seed crystals are present, in a supersaturated solution [1.-7]. Although in industrial crystallization secondary nucleation is considered to be the more important for controlling the size distribution of the product crystals [8,9], it has not been well understood. It was recently proposed by the present authors [10], based on quantitative measurements of the number of secondary nuclei coupled with surface observation of seed crystals, that minute crystalline particles adhering to the surface of a seed crystal act as centres for secondary nuclei, an idea which had been proposed originally by Lal et al. [5] and Strickland-Constable [6]. Since this conclusion was derived from indirect experimental evidence, a direct experiment was planned to test the hypothesis by means of isotopic labelling with
K-alum seeds containing D20 instead of H20 in the form of hydrate water. Seed crystals for secondary nucleation were prepared in both H20 solution and D20 solution by overgrowing K-alum onto smaller seeds (1680— 2000 ~smin diameter and 0.003 g in weight) which had been grown in a H20 solution. When the seeds were grown from the D20 solution, using 99.7% pure D2O, the seeds (D-seeds) contained D2O in the form of hydrate water instead of H20. K-alum solutions for secondary nucleation experiments were prepared using distilled H 20 at 20°C by stirring the solution continuously for 2 h. After ageing the solution for 1 h without agitation, the upper part of the solution was poured into a nucleation cell, fig. 1. Secondary nucleation experiments were then performed at a superaturation corresponding to an undercooling ~T = 2.0°Cby the following methods employing the nucleation cell: (1) Nucleation by initial breeding; a seed
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
624
K. Shimizu et al.
GLASS ROD
—-*
~
/
Origin of secondary nucleation as revealed by isotopic labelling
In all three cases, the secondary nuclei were kept to be grown larger in the same supersaturated solution for another 30 mm, followed by measur-
[j
I —
ing the D/H ratio of water contained both in the nuclei and in the solution. The measurement was not performed for each small nucleus, 10 ~sm in diameter, but for numbers of the collected small crystal, 20 mg. Before measurement the hydrate water in the secondary nuclei was dehydrated in vacuum by heating the collected nuclei up to approximately 300°C for 3 h. The H20 and D20
— ——
SEED HOLDER
L
measurement metallic uranium. of the These D/Hgases ratiowere on aused McKinneyfor the
___________ K-ALUM SOLUTION
~- FILTER
CUIJM
c”_~ VESSEL
were reduced by H2 and D2 gases at 750°C with
—~
_______________
Fig. 1. Nucleation cell made of a 50 ml glass bottle, equipped with a glass rod (2 mm in diameter. 110 mm long and 0.57 g in weight) and a seed holder,
error of measurement for the D/H ratio is ±0.3 x 1O~. type In mass tablespectrometer. Ia the D/H The ratios overall of the experimental secondary nuclei formed by contact nucleation using pure seed and the H20—alum solution are shown. These values are considered to be an average D/H ratio for ordinary distilled water and chemical reagents. In table lb the D/H ratios of the secondary nuclei formed by the present experiment using D seeds are shown, in which one will notice the marked increase in D/H ratios as compared to the values in table la. This suggests that either D of the D seed is exchanged by H in the supersaturated H2O—alum solution from which the nuclei were formed or the surface of the D seed has been chipped off in minute particles which acted as the centre for secondary nucleation.
holder, at the tip of which a seed was fixed, was immersed in the solution for 5 s. (2) Nucleation by fluid shear; the seed holder with a seed was rotated at 60 rpm for 30 mm. (3) Nucleation by contact; the contact of the glass rod with the crystal surface was achieved by pounding the seed surface with the glass rod, one per minute for 5 mm. In each case holder with seed and glass rod takenthe outseed from the solution immediately afterwere the above treatment. After the nucleation experiment, only the solution was pumped out with the help of vacuum pressure through a filter with a pore size the 1 filter. filtrate in the glassnuclei vesselbehind was then of ~sm, The leaving the secondary on used for isotopic measurement.
Table 1 D/H ratios of water, both in the secondary nuclei and in the H2o solution (a) With H20 seeds
_____________________________________________ Species Nuclei from pure seeds H 20 solution (b) With D 20 seeds
D/H ratio 6 145.9x10 6 146.1x10
Species
D/H ratio
By By fluid initialshear breeding By contact nucleation
154.9x106 631.0x10 3520.0x106
6 _______________________________________________
_________________________________________________
K. Shimizu et al.
/
Origin of secondary nucleation as revealed by isotopic labelling
To investigate which mechanism is appropriate, the exchange rate of D/H ratio between a D seed and the pure H20 solution was measured. A D seed was at first partially dissolved in distilled water so as to entirely remove the possible fine Crystallites adhering to the surface [10], and then was hung by a nylon string in another seed was reduced by 0.004 g on average. The D content of the solution was measured after 30 mm. As shown in table 2, only a slight increase of the D/H ratio in the solution could be discerned. Since the longest experimental duration in the present experiment was 30 mm, the data in table lb are not due to the effect of isotopic exchange between the D seed and the H20 solution but due to the chipped off particles from the D seed during secondary nucleation. The above conclusion is further supported by the much higher D/H ratios in table lb. The fact that the D/H ratio for contact nucleation is much higher than for the other treatments indicates that mechanically chipped off particles of the seed with a glass rod act as the centres of secondary nuclei and are integrated into the nucleation products. Since the number of nuclei by contact was also found to be much larger than by the other two methods [10], the present higher D/H ratio is consistent with this interpretation, As will be published, scanning electron microscopy and differential interference contrast microscopy after Nomarski reveal such a typical feature around the place where the surface was pounded with a glass rod as the surface is locally broken into pieces of small crystallites. This is further support for the importance of the mechanically chipped off particles. According to our investigation, fluid shear has the least power to chipp off the surface of the seed, The D/H ratio for the fluid shear method is much lower than the other two values, though it is higher Table 2 Results of a blank test, in order to measure the exchange rate of D between the D seeds and the H20 solution Duration (nun)
D/H ratio
0 30
146.1 x 146.7 x
io~ ~
6
625
than those in table la. This can be explained in the following manner. Since fluid shear provides strong movement of the supersaturated solution, it is probable that rapid overgrowth of pure K-alum onto the seed crystal may take place before the majority of fine crystallites leave the D seed surface, because the growth rate of a crystal is expected to become larger due to faster volume diffusion of molecules to the seed when the seed is rotated in a solution. As shown in ref. [10], the number of secondary nuclei by the fluid shear method was larger than by initial breeding, though the absolute number was largely influenced by the state of the seed surface. Considering this fact, the particles chipped off from a seed by initial breeding might be larger in size than by fluid shear. It is interesting to investigate whether or not the number of nuclei is equal to the number of fine particles which are chipped off from the surface of a seed crystal. Although at this moment it is impossible to count the latter number, the former number is considered to be less than the latter. This is because only the particles with larger radius than that of critical radius of a nucleus [11] would act as nucleation centres and grow larger. In conclusion, by applying the isotopic labelling method, direct evidence was obtained that the chipped off particles of a seed crystal have acted as nucleation centres and were then incorporated into the secondary nuclei. The authors would like to express their thanks to Professor S. Matsuo for discussions on the isotopic measurements and to Professor I. Sunagawa for discussions and for his critical reading and revision of the manuscript. This work is partly supported by a Grant-in-Aid for Fundamental Researches from the Japan Ministry of Education.
References [11 R.F. Strickland-Constable and R.E.A. Mason, Nature 197 (1963) 897. [21 W.L. McCabe, in: Chemical Engineers Handbook, Ed. J.H. Perry (McGraw-Hill, New York, 1953) p. 1056. NS’vlt, in: Industrial Crystallization (Verlag Chemie, Weinheim, 1982) p. 79.
[31J.
626
[41M.D.
K. Shimizu et al.
/
Origin of secondary nucleation as revealed by isotopic labelling
Cisc and AD. Randolph, AIChE Symp. Ser. 121 (1972) 42. ~5] D.P. Lal, R.E.A. Mason and R.F. Strickland-Constable, J. Crystal Growth 5 (1969) 1. [6] R.F. Strickland-Constable, J. Chem Soc. Faraday Trans. I. 75 (1979) 921. [7] C.Y. Sung, J. Estrin and G.R. Yougquist, AIChE J. 19 (1973) 857.
[81 J. Garside, Chem. Eng. Commun. 4 (1980) 393. [9] MA. Larson, Chem. Eng. Commun. 12 (1981) 161. [10] K. Shimizu, N. Kubota, K. Tsukamoto, I. Sunagawa, T. Yonemoto and T. Tadaki, J. Crystal Growth 69 (1984) 115. [11] M. Ohara and R.C. Reid, in: Modeling Crystal Growth Rates from Solution (Prentice-Hall, London. 1973) p. 19.