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A comparison of secondary nuclei produced by contact of different growth faces of potash alum crystals under supersaturated solutions
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ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 166 (1996) 1068-1073
A comparison of secondary nuclei produced by contact of different growth faces of potash alum crystals under supersaturated solutions Manijeh M. Reyhani *, Gordon M. Parkinson A.J. Parker Cooperative Research Centre for Hydrometallurgy, School of Applied Chemist~', Curtin UniversiO' of Technology, GPO Box U 1987, Perth 6001, Western Australia, Australia
Abstract Secondary nuclei of potash alum crystals may easily be produced by gentle crystal contact. In this investigation, crystal faces of the {100}, {110} and {111} families were identified in a parent crystal, and gentle contact between these and a solid surface in a slightly supersaturated solution of potash alum produced many secondary nuclei of the same orientation. Breeding of the large number of particles produced by contact between a parent crystal and a glass surface under supersaturated aqueous solution was directly observed by optical microscopy with an in situ, thermostatted cell. A strong correlation was found between the symmetry of the nuclei produced and that of the parent crystal face. Ex situ scanning (SEM) and transmission electron microscopic (TEM) measurements were also carried out to study this type of secondary nuclei, produced from a known surface geometry. In these cases, many small nuclei in the size range of 50 nm to 1 /xm were produced and studied. The larger crystals displayed morphologies commensurate with that of the parent face; the very small nuclei, whilst frequently showing very poorly ordered boundaries, nonetheless were highly ordered internally, as shown by electron diffraction, the symmetry observed reflecting that of the parent face.
1. Introduction Secondary nucleation is the main source of new crystal nuclei in an industrial crystallizer [1-3]. It involves the presence of crystals and their interaction with the environment such as crystallizer walls, impellers, etc. Although several theories [3-10] have been proposed to explain secondary nucleation, the exact source of contact nuclei has not yet been determined. The proposed theories fall into two cate-
* Corresponding author. Fax: +61 9 351 2300.
gories: one traces the origin of the secondary nuclei to the parent crystal, involving the generation of new particles by microattrition of a growing crystal surface [5-7]. In the second, the source of nuclei is thought to be an ordered, intermediate layer of solute adjacent to a growing crystal surface [8-11]. An additional third possible source for secondary nuclei is also proposed by Denk and Botsaris [1] arising from ordering of the solute molecules near the surface of the crystal leading to a high local supersaturation. The presence of an intermediate layer as a source of secondary nuclei was initially proposed by Powers [8], who suggested fluid shear as a mechanism for
0022-0248/96/$15.(X) Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)001 20-0
M.M. Reyhani, G.M. Parkinson /Journal of C~stal Growth 166 (1996) 1068-1073
the generation of secondary nuclei. Several investigators verified this notion by sliding a crystal along a solid surface, and observing the nucleation and crystallization behaviour. It was found that a growing crystal was necessary for the generation of new nuclei and that the nucleation rate was a function of the contact energy and the degree of supersaturation. Berglund and Larson [9] studied contact nucleation by sliding the (100) face of a growing parent crystal along a glass plate in a citric acid monohydrate system. No nuclei were formed when a crystal was contacted in a saturated solution. Therefore, the mechanism of contact nucleation did not appear to be due to physical breakage of the parent crystal in the sliding contact. However, direct observation of secondary nuclei production in the potash alumwater system by Garside and Larson [5] has shown that the secondary nuclei are produced as a result of attrition of a crystal surface by a solid object, even at very low contact energies. The aim of this work is to characterize the extent to which structural information about the parent crystal is transferred during contact nucleation using the potash alum-water system and hence contribute to the understanding of secondary nucleation.
2. Experimental procedure Direct optical microscopic observations of the secondary nucleation occurring by contact between a parent crystal and a glass surface under supersaturated aqueous solution were carried out with an in situ, thermostatted cell [5]. A schematic diagram of the cell is shown in Fig. 1. It is constructed of stainless steel in a circular cross section with an internal diameter of 30 mm. Solution is held in the upper section with a capacity of about 5 ml while thermostatically controlled water is circulated through the lower section to provide temperature control. A pure (99.5%) aluminium potassium sulphate, KAI(SO4) 2 • 12H20, potash alum-water system was used. This particular system was used because the properties of this system are well defined and it is easy to grow well-formed crystals. A slightly supersaturated solution was generated in the cell by cooling a saturated solution, and gentle contact was made in the solution between an identified surface of the
1069
7
/
" . - ~ \ \
\
5
J
Fig. 1. Schematic diagram of the static cell used to observe contact nucleation. (1) Parent crystal; (2) Thermocouple; (3) Solution; (4) Water; (5) Supporting rod; (6) Glass.
parent crystal and the glass surface. The contact was created by turning and moving the supporting rod, thereby creating a gentle touch between the parent crystal and the glass. A parent crystal was glued to the supporting rod in such a way that by turning it the required face of the crystal would be in contact with the glass. The production of secondary nuclei of the same orientation was observed. Ex situ scanning (SEM) and transmission electron microscopic (TEM) measurements were carried out on the Philips 505, Jeol 6400 and Philips 430 instruments. In these cases, a crystal of potash alum, held under a slightly supersaturated solution, was brought into contact in a known orientation with either an SEM stub or a TEM grid coated with a thin carbon film. Immediately after contact, the support was removed from the solution, and the excess liquid was rapidly drained
1070
M.M. Reyhani, G.M. Parkinson / Journal of Crystal Growth 166 (1996) 1068-1073
off. Many small nuclei in the size range of 50 nm to 1 /xm were produced and studied.
(a)
3. Results
Fig. 2a shows a scanning electron micrograph of a typical crystal obtained by cooling a supersaturated solution of potash alum, showing the morphology of these crystals. Fig. 2b illustrates the morphology and well-defined (100), (110) and (111) facets of the crystal [12]. Formation of the new nuclei after contact with the (100) and (111) faces in the thermostatted cell is shown in Fig. 3a and 3b, respectively. As shown in the micrographs, the newly formed nuclei predominantly exhibit the same symmetry as the
200~
Fig. 3. (a) Secondary nuclei produced in the thermostatted cell after contact with the (I 11) face. (b) Secondary nuclei produced in the thermostatted cell after contact with the (100) face.
(b)
Fig. 2. (a) Scanning electron mlcrograph showing a typical potash alum crystal formed by cooling of a saturated solution. (b) Schematic diagram of the potash alum crystal showing the different crystal faces.
contacted face of their parent crystal. In the case of (111) contact breeding, nearly all the new crystals have the same morphology. However, using the (100) contact to produce secondary nuclei resulted in some other new crystals with (111) morphology, which can be observed in Fig. 3b. This could be due to additional primary crystallization induced by cooling; the (111) faces are the most predominant in freely grown crystals, indicating that they have the lower free energy, and hence would tend to be the favoured surface in primary nuclei. In order to study the secondary nucleation process in more detail, further experimental observations were carried out by ex situ scanning and transmission electron microscopy at increasingly higher magnifications. Contacts made from the known surface geometry of a parent potash alum crystal in the slightly supersaturated solution with an aluminium SEM stub produced nuclei of the same symmetry as
M.M. Reyhani, G.M. Parkinson / Journal of Co'stal Growth 166 (1996) 1068-1073
the contact face, as can be observed in Fig. 4. Figs. 4 a - 4 c show the different morphologies obtained as the nuclei formed. The strong correlation between the symmetry of the parent crystal and the nuclei produced is indicative of the transfer of information from the parent crystal to the nuclei. TEM confirmed the results obtained by optical and scanning microscopy and has shown that the very fine nuclei produced by contact nucleation still
1071
carry the same information as the parent contact crystal. Fig. 5a-5c show bright field images of the nuclei obtained after the (111), (110) and (100) contacts were made with a TEM grid in the slightly supersaturated aqueous solution. A new particle of 50 nm size produced by contact from the (111) face of the crystal is shown in Fig. 6. The selected area diffraction pattern for this particle confirms that even though the particle has poorly defined boundaries, it
Fig. 4. Scanning electron micrographsshowing secondarynuclei produced by contacting the different faces of the parent crystal: (a) (111); (b) (1 lO); (c) (100).
1072
M.M. Reyhani, G.M. Parkinson / Journal of Crystal Growth 166 (1996) 1068-1073
Fig. 6. (a) Transmission electron micrograph showing a bright field image of the small particle formed after (111) contact nucleation. (b) Selected area diffraction pattern from the particle in (a), showing the (111) orientation.
is nonetheless highly crystalline and has the (111) orientation.
4. Discussion Fig. 5. Transmission electron micrographs showing bright field images of the secondary nuclei produced by contact with the different faces of the parent crystal: (a) (111); (b) (110): (c) (100).
Optical, s c a n n i n g and transmission electron microscopy studies have shown that the m o r p h o l o g y of crystals of potash alum produced by secondary nu-
M.M. Reyhani, G.M. Parkinson/ Journal of Crystal Growth 166 (1996) 1068-1073
cleation is determined by the nature of the surface of the parent crystal that is contacted. The ease of production of secondary nuclei from different crystal faces correlates with the natural occurrence of these faces in freely grown crystals, i.e. ( 1 1 1 ) > ( 1 1 0 ) > (100), and is thus related to the surface energy or rate of growth of these faces [12]. When supersaturated solutions are allowed to nucleate freely on the substrate used, predominantly the (111) orientation is observed. It is likely that the occurrence of primary nucleation is responsible for the observation of some (111) nuclei after nuclei are produced on contacting (110) and (100) faces; however, contacting these faces on a fiat surface will also necessarily result in the edges of (111) faces being touched (see Fig. 2b), which may transfer some (111)-oriented information to the substrate. Because of the finite time required to remove the substrate from the supersaturated solution during sample preparation for SEM and TEM studies, some growth has necessarily occurred between contact and observation. Nonetheless, particles as small as 50 nm have been observed, and electron diffraction indicates that these particles are highly crystalline. This work shows that there is a strong correlation between the symmetry of the contacting face of the parent crystal and the nuclei produced, indicating that there is a transfer of structural information. Whether this information comes directly from the mechanical attrition of the solid, parent crystal, or via some ordered, intermediate layer needs further investigation.
1073
Acknowledgements This work has been supported under the Australian Government's Cooperative Research Centres Programme, and this support is gratefully acknowledged.
References [1] E.G. Denk, Jr. and G.D. Botsaris, J. Crystal Growth 13/14 (1972) 493. [2] J. Mathis-Lilley and K.A. Berglund, AICHE J. 31 (1985) 865. [3] M. Liang, R.W. Hartel and K.A. Berglund, j. Eng. Sci. 42 (1987) 273. [4] M.K. Cerreta and K.A. Berglund, J. Crystal Growth 102 (1990) 869. [5] J. Garside and M.A. Larson, J. Crystal Growth 43 (1978) 694. [6] R. Wissing, M. Elwenspoekand B. Degens, J. CrystalGrowth 79 (1986) 614. [7] K. Shimizu, K. Tsukamoto, J. Horita and T. Tadaki, J. Crystal Growth 69 (1984) 623. [8] E.C. Powers, Nature 178 (1956) 139. [9] K.A. Berglund and M.A. Larson, AICHE Symp. Series (1982) 9. [10] P. Elankovanand K.A. Berglund, Appl. Spectrosc. 40 (1986) 712. [11] P. Elankovan and K.A. Berglund, AICHE J. 33 (1987) 1844. [12] J.N. Sherwood and T. Shripathi, Faraday Disc. 95 (1993) 173.
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 166 (1996) 1068-1073
A comparison of secondary nuclei produced by contact of different growth faces of potash alum crystals under supersaturated solutions Manijeh M. Reyhani *, Gordon M. Parkinson A.J. Parker Cooperative Research Centre for Hydrometallurgy, School of Applied Chemist~', Curtin UniversiO' of Technology, GPO Box U 1987, Perth 6001, Western Australia, Australia
Abstract Secondary nuclei of potash alum crystals may easily be produced by gentle crystal contact. In this investigation, crystal faces of the {100}, {110} and {111} families were identified in a parent crystal, and gentle contact between these and a solid surface in a slightly supersaturated solution of potash alum produced many secondary nuclei of the same orientation. Breeding of the large number of particles produced by contact between a parent crystal and a glass surface under supersaturated aqueous solution was directly observed by optical microscopy with an in situ, thermostatted cell. A strong correlation was found between the symmetry of the nuclei produced and that of the parent crystal face. Ex situ scanning (SEM) and transmission electron microscopic (TEM) measurements were also carried out to study this type of secondary nuclei, produced from a known surface geometry. In these cases, many small nuclei in the size range of 50 nm to 1 /xm were produced and studied. The larger crystals displayed morphologies commensurate with that of the parent face; the very small nuclei, whilst frequently showing very poorly ordered boundaries, nonetheless were highly ordered internally, as shown by electron diffraction, the symmetry observed reflecting that of the parent face.
1. Introduction Secondary nucleation is the main source of new crystal nuclei in an industrial crystallizer [1-3]. It involves the presence of crystals and their interaction with the environment such as crystallizer walls, impellers, etc. Although several theories [3-10] have been proposed to explain secondary nucleation, the exact source of contact nuclei has not yet been determined. The proposed theories fall into two cate-
* Corresponding author. Fax: +61 9 351 2300.
gories: one traces the origin of the secondary nuclei to the parent crystal, involving the generation of new particles by microattrition of a growing crystal surface [5-7]. In the second, the source of nuclei is thought to be an ordered, intermediate layer of solute adjacent to a growing crystal surface [8-11]. An additional third possible source for secondary nuclei is also proposed by Denk and Botsaris [1] arising from ordering of the solute molecules near the surface of the crystal leading to a high local supersaturation. The presence of an intermediate layer as a source of secondary nuclei was initially proposed by Powers [8], who suggested fluid shear as a mechanism for
0022-0248/96/$15.(X) Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)001 20-0
M.M. Reyhani, G.M. Parkinson /Journal of C~stal Growth 166 (1996) 1068-1073
the generation of secondary nuclei. Several investigators verified this notion by sliding a crystal along a solid surface, and observing the nucleation and crystallization behaviour. It was found that a growing crystal was necessary for the generation of new nuclei and that the nucleation rate was a function of the contact energy and the degree of supersaturation. Berglund and Larson [9] studied contact nucleation by sliding the (100) face of a growing parent crystal along a glass plate in a citric acid monohydrate system. No nuclei were formed when a crystal was contacted in a saturated solution. Therefore, the mechanism of contact nucleation did not appear to be due to physical breakage of the parent crystal in the sliding contact. However, direct observation of secondary nuclei production in the potash alumwater system by Garside and Larson [5] has shown that the secondary nuclei are produced as a result of attrition of a crystal surface by a solid object, even at very low contact energies. The aim of this work is to characterize the extent to which structural information about the parent crystal is transferred during contact nucleation using the potash alum-water system and hence contribute to the understanding of secondary nucleation.
2. Experimental procedure Direct optical microscopic observations of the secondary nucleation occurring by contact between a parent crystal and a glass surface under supersaturated aqueous solution were carried out with an in situ, thermostatted cell [5]. A schematic diagram of the cell is shown in Fig. 1. It is constructed of stainless steel in a circular cross section with an internal diameter of 30 mm. Solution is held in the upper section with a capacity of about 5 ml while thermostatically controlled water is circulated through the lower section to provide temperature control. A pure (99.5%) aluminium potassium sulphate, KAI(SO4) 2 • 12H20, potash alum-water system was used. This particular system was used because the properties of this system are well defined and it is easy to grow well-formed crystals. A slightly supersaturated solution was generated in the cell by cooling a saturated solution, and gentle contact was made in the solution between an identified surface of the
1069
7
/
" . - ~ \ \
\
5
J
Fig. 1. Schematic diagram of the static cell used to observe contact nucleation. (1) Parent crystal; (2) Thermocouple; (3) Solution; (4) Water; (5) Supporting rod; (6) Glass.
parent crystal and the glass surface. The contact was created by turning and moving the supporting rod, thereby creating a gentle touch between the parent crystal and the glass. A parent crystal was glued to the supporting rod in such a way that by turning it the required face of the crystal would be in contact with the glass. The production of secondary nuclei of the same orientation was observed. Ex situ scanning (SEM) and transmission electron microscopic (TEM) measurements were carried out on the Philips 505, Jeol 6400 and Philips 430 instruments. In these cases, a crystal of potash alum, held under a slightly supersaturated solution, was brought into contact in a known orientation with either an SEM stub or a TEM grid coated with a thin carbon film. Immediately after contact, the support was removed from the solution, and the excess liquid was rapidly drained
1070
M.M. Reyhani, G.M. Parkinson / Journal of Crystal Growth 166 (1996) 1068-1073
off. Many small nuclei in the size range of 50 nm to 1 /xm were produced and studied.
(a)
3. Results
Fig. 2a shows a scanning electron micrograph of a typical crystal obtained by cooling a supersaturated solution of potash alum, showing the morphology of these crystals. Fig. 2b illustrates the morphology and well-defined (100), (110) and (111) facets of the crystal [12]. Formation of the new nuclei after contact with the (100) and (111) faces in the thermostatted cell is shown in Fig. 3a and 3b, respectively. As shown in the micrographs, the newly formed nuclei predominantly exhibit the same symmetry as the
200~
Fig. 3. (a) Secondary nuclei produced in the thermostatted cell after contact with the (I 11) face. (b) Secondary nuclei produced in the thermostatted cell after contact with the (100) face.
(b)
Fig. 2. (a) Scanning electron mlcrograph showing a typical potash alum crystal formed by cooling of a saturated solution. (b) Schematic diagram of the potash alum crystal showing the different crystal faces.
contacted face of their parent crystal. In the case of (111) contact breeding, nearly all the new crystals have the same morphology. However, using the (100) contact to produce secondary nuclei resulted in some other new crystals with (111) morphology, which can be observed in Fig. 3b. This could be due to additional primary crystallization induced by cooling; the (111) faces are the most predominant in freely grown crystals, indicating that they have the lower free energy, and hence would tend to be the favoured surface in primary nuclei. In order to study the secondary nucleation process in more detail, further experimental observations were carried out by ex situ scanning and transmission electron microscopy at increasingly higher magnifications. Contacts made from the known surface geometry of a parent potash alum crystal in the slightly supersaturated solution with an aluminium SEM stub produced nuclei of the same symmetry as
M.M. Reyhani, G.M. Parkinson / Journal of Co'stal Growth 166 (1996) 1068-1073
the contact face, as can be observed in Fig. 4. Figs. 4 a - 4 c show the different morphologies obtained as the nuclei formed. The strong correlation between the symmetry of the parent crystal and the nuclei produced is indicative of the transfer of information from the parent crystal to the nuclei. TEM confirmed the results obtained by optical and scanning microscopy and has shown that the very fine nuclei produced by contact nucleation still
1071
carry the same information as the parent contact crystal. Fig. 5a-5c show bright field images of the nuclei obtained after the (111), (110) and (100) contacts were made with a TEM grid in the slightly supersaturated aqueous solution. A new particle of 50 nm size produced by contact from the (111) face of the crystal is shown in Fig. 6. The selected area diffraction pattern for this particle confirms that even though the particle has poorly defined boundaries, it
Fig. 4. Scanning electron micrographsshowing secondarynuclei produced by contacting the different faces of the parent crystal: (a) (111); (b) (1 lO); (c) (100).
1072
M.M. Reyhani, G.M. Parkinson / Journal of Crystal Growth 166 (1996) 1068-1073
Fig. 6. (a) Transmission electron micrograph showing a bright field image of the small particle formed after (111) contact nucleation. (b) Selected area diffraction pattern from the particle in (a), showing the (111) orientation.
is nonetheless highly crystalline and has the (111) orientation.
4. Discussion Fig. 5. Transmission electron micrographs showing bright field images of the secondary nuclei produced by contact with the different faces of the parent crystal: (a) (111); (b) (110): (c) (100).
Optical, s c a n n i n g and transmission electron microscopy studies have shown that the m o r p h o l o g y of crystals of potash alum produced by secondary nu-
M.M. Reyhani, G.M. Parkinson/ Journal of Crystal Growth 166 (1996) 1068-1073
cleation is determined by the nature of the surface of the parent crystal that is contacted. The ease of production of secondary nuclei from different crystal faces correlates with the natural occurrence of these faces in freely grown crystals, i.e. ( 1 1 1 ) > ( 1 1 0 ) > (100), and is thus related to the surface energy or rate of growth of these faces [12]. When supersaturated solutions are allowed to nucleate freely on the substrate used, predominantly the (111) orientation is observed. It is likely that the occurrence of primary nucleation is responsible for the observation of some (111) nuclei after nuclei are produced on contacting (110) and (100) faces; however, contacting these faces on a fiat surface will also necessarily result in the edges of (111) faces being touched (see Fig. 2b), which may transfer some (111)-oriented information to the substrate. Because of the finite time required to remove the substrate from the supersaturated solution during sample preparation for SEM and TEM studies, some growth has necessarily occurred between contact and observation. Nonetheless, particles as small as 50 nm have been observed, and electron diffraction indicates that these particles are highly crystalline. This work shows that there is a strong correlation between the symmetry of the contacting face of the parent crystal and the nuclei produced, indicating that there is a transfer of structural information. Whether this information comes directly from the mechanical attrition of the solid, parent crystal, or via some ordered, intermediate layer needs further investigation.
1073
Acknowledgements This work has been supported under the Australian Government's Cooperative Research Centres Programme, and this support is gratefully acknowledged.
References [1] E.G. Denk, Jr. and G.D. Botsaris, J. Crystal Growth 13/14 (1972) 493. [2] J. Mathis-Lilley and K.A. Berglund, AICHE J. 31 (1985) 865. [3] M. Liang, R.W. Hartel and K.A. Berglund, j. Eng. Sci. 42 (1987) 273. [4] M.K. Cerreta and K.A. Berglund, J. Crystal Growth 102 (1990) 869. [5] J. Garside and M.A. Larson, J. Crystal Growth 43 (1978) 694. [6] R. Wissing, M. Elwenspoekand B. Degens, J. CrystalGrowth 79 (1986) 614. [7] K. Shimizu, K. Tsukamoto, J. Horita and T. Tadaki, J. Crystal Growth 69 (1984) 623. [8] E.C. Powers, Nature 178 (1956) 139. [9] K.A. Berglund and M.A. Larson, AICHE Symp. Series (1982) 9. [10] P. Elankovanand K.A. Berglund, Appl. Spectrosc. 40 (1986) 712. [11] P. Elankovan and K.A. Berglund, AICHE J. 33 (1987) 1844. [12] J.N. Sherwood and T. Shripathi, Faraday Disc. 95 (1993) 173.