Source of Nuclei in Contact Nucleation as Revealed by Crystallization

Chapter 3

Source of Nuclei in Contact Nucleation as Revealed by Crystallization of Isomorphous Alums Downloaded by UCSF LIB CKM RSCS MGMT on November 23, 2014 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0667.ch003

Manijeh M . Reyhani and Gordon M . Parkinson

A. J. Parker Cooperative Research Centre for Hydrometallurgy, School of Applied Chemistry, Curtin University of Technology, GPO Box U 1987, Perth 6001, Western Australia

Potash alum, KAl(SO ) 12H O, and chrome alum, KCr(SO ) 12H O, crystallize from their respective aqueous solutions as regular octahedra in almost identical forms and are known to be isomorphous. Alum crystals are also easily formed by secondary crystallization. Gentle crystal contact between a parent alum crystal and a solid surface under supersaturated aqueous solution produces a large number of secondary nuclei. In this study, the formation of secondary nuclei of potash alum crystals in its supersaturated solution by contacting a known crystal surface of chrome alum with a TEM grid, and, conversely, the formation of chrome alum crystals in its supersaturated solution by contacting a known crystal surface of potash alum are reported. These experiments have been used to study closely the source of nuclei in contact nucleation. The results obtained using analytical transmission electron microscopy will be discussed. 4 2·

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Contact nucleation is probably the most important source of secondary nuclei in an industrial crystallizer (1-4). New crystals are formed due to the prior presence of other growing crystals. The source of nuclei in contact nucleation has been studied by several investigators and two major theories have been proposed (5-11). One assumes that the source of nuclei is an ordered, intermediate layer of solute adjacent to a growing crystal surface (5-8) and the other suggests the origin of the secondary nuclei is from the parent crystal, involving the generation of new particles by microattrition of a growing crystal surface (9-11). Potash alum, KA1(S0 ) .12H 0, and chrome alum KCr(S0 ) .12H 0 crystallize from their respective aqueous solutions as regular octahedra in almost identical forms and are known to be isomorphous. Alum crystals are also easily formed by 4

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© 1997 American Chemical Society

In Separation and Purification by Crystallization; Botsaris, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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Source of Nuclei in Contact Nucleation

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secondary nucleation. The objective of this work is to study more closely the source of nuclei in contact nucleation by making use of the isomorphous relationship between the crystal structures of potash and chrome alums, and the power of the analytical transmission electron microscope (TEM) to differentiate between very small volumes of the chemically different components.

Downloaded by UCSF LIB CKM RSCS MGMT on November 23, 2014 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0667.ch003

Experimental Two aqueous systems have been studied: pure (99.5%) aluminium potassium sulphate, KA1(S04)2-12 H2O, (potash alum-water) system and pure (99.6%) chromium aluminium sulphate, KCr(S04)2.12 H2O, (chrome alum-water). A 3 mm diameter, 200 mesh copper grid coated with carbon film was brought into gentle contact with specific growth faces of a potash alum crystal in a supersaturated solution of chrome alum and conversely with a chrome alum crystal in a supersaturated solution of potash alum. Immediately after contact, the grids were removed from the solution, and the excess liquid was rapidly drained off. Nuclei were produced in the range of 50ηπι-5μιη and were studied using a Philips 430 analytical transmission electron microscope, and analysed by electron diffraction and energy dispersive X-ray analysis (EDX).

Results Figure 1 shows the morphology of a typical alum crystal (12). Three distinctive crystal faces of (100), (110) and (111) are shown in this figure. Potash alum and chrome alum crystals are isomorphous and both exhibit this structure. Both materials form smooth overgrowths onto crystals of each other in supersaturated solutions. Production of secondary nuclei by contact nucleation in potash alum has been previously reported in a number of papers, as it is an ideal system for such studies (9, 13). Contacting an identified surface of a potash alum crystal, for example the (100), (110) and (111) growth faces with a solid surface in a supersaturated solution of the same alum resulted in the production of many secondary nuclei of an identical orientation (13). Similar experiments have been carried out in this work by choosing a chrome alum solution when the potash alum parent crystal is used and a potash alum solution when a chrome alum parent crystal is used. Figure 2a shows one of the crystals formed by contacting a (111) face of a chrome alum crystal with a T E M grid in a supersaturated potash alum solution. As shown in figure 2b, an E D X spectrum taken from this crystal shows no trace of chromium, indicating that there has been no transfer of solid in the formation of the new crystal. The equivalent experiment, ie, contacting potash alum crystal in a supersaturated solution of chrome alum, resulted in the production of chrome alum crystals without any trace of aluminium coming from the solid crystal.



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Figure 1 Schematic diagram of the observed morphology of alum crystals showing different crystal faces.



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Figure 2a Transmission electron micrograph, showing a bright field image of a secondary nucleus produced by contact with a (111) face of a chrome alum parent crystal.

1200-· looo 800 600 Al 400-

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KeV Figure 2b A n E D X spectrum taken from the secondary nucleus produced by contact with a (111) face of a chrome alum parent crystal.


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Examination of the very fine particles produced by this method (for example, those of 150 nm in size, shown in figures 3a & 3b) also showed no trace of solid transfer from the contacted crystal. This is important because at such small particle sizes the presence of any embedded material would not be obscured by the absorption of emitted X-rays. These nuclei were produced in a potash alum solution by contacting a chrome alum (111) crystal face (figure 3a) and in a chrome alum solution by contacting a potash alum (100) face (figure 3b). Figures 4a & 4b show the E D X spectra taken from these particles. Although the small nuclei do not show the external form of the crystal orientation as clearly as is observed at lower magnification with larger crystals (figure 2a), nonetheless they do exhibit the same orientation, as revealed by electron diffraction, which also indicates that they have a well ordered crystalline structure.

Discussion The above results favour the proposition that the intermediate ordered layer of solute adjacent to a growing crystal surface is the source of secondary nuclei in this case (5-8). However, there is the possibility that some transfer of solid material may have occurred and it has not been detected. This could be due either to the small size of the fragment transferred or to the quick overgrowth of the parent crystal in supersaturated solution; ie, if a sufficiently thick overgrowth of the counter material from solution occurred prior to contact, then any solid transferred would not contain the characteristic element from the source crystal, and hence would not be detected. The degree of supersaturation and control of the rate of growth over the parent crystal are thus very important factors. To clarify this, potash alum crystals, which have lower solubility than chrome alum, have been used as parent crystals in a range of chrome alum solutions that were undersaturated and supersaturated. Results show that in the case of an undersaturated solution of chrome alum, no nuclei of potash alum are formed from the solid crystal, whereas nuclei of chrome alum were detected from both saturated and supersaturated solutions of chrome alum by contacting the parent crystal of potash alum crystal. Comparing these results indicates that the secondary nuclei that are produced truly come from the surface between the original crystal and the solution and not from a thick overgrowth (14).

Acknowledgments

This work has been supported under the Australian Government's Cooperative Research Centres Programme, and this support is gratefully acknowledged.



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Figure 3a Transmission electron micrograph showing a bright field image of a small secondary nucleus produced by contact with a (111) face of a chrome alum parent crystal in a supersaturated potash alum solution.

Figure 3b Transmission electron micrograph showing a bright field image of a small secondary nucleus produced by contact with a (100) face of a potash alum parent crystal in a supersaturated chrome alum solution.


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KeV Figure 4a A n E D X spectrum taken from the small secondary nuclei produced by contact with a (111) face of a chrome alum parent crystal in a supersaturated potash alum solution (see figure 3a).

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KeV Figure 4b A n E D X spectrum taken from the small secondary nuclei produced by contact with a (100) face of a potash alum parent crystal in a supersaturated chrome alum solution (see figure 3b).


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Literature Cited 1. E . G . Denk, Jr. and G. D. Botsaris, J. of Crystal Growth 1972, 13/14, 493. 2. J. Mathis-Lilley and K. A. Berglund, AIChE J. 1985, Vol. 31, No.5, 865. 3. M . Liang, R. W . Hartel and K. A . Berglund, J. Engineering Science 1987 Vol. 42, No.11, 273. 4. M . K. Cerreta and K. A. Berglund, J. of Crystal Growth 1990, 102, 869. 5. E . C. Powers, Nature 1956, 178, 139. 6. K. A . Berglund, M . A. Larson, AIChE Symposium Series, 1982 9. 7. P. Elankovan and K. A . Berglund, Applied Spectroscopyl986, Vol. 40, No. 5, 712. 8. P. Elankovan and K. A. Berglund, AIChE J, 1987, Vol.33, No.11, 1844. 9. J. Garside, and M . A . Larson, J. of Crystal Growth 1978, 43, 694. 10. R. Wissing, M . Elwenspoek and B. Degens, J. of Crystal Growth 1986, 79, 614. 11. K. Shimizu, K. Tsukamoto, J. Horita and T. Tadaki, J. of Crystal Growth 1984, 69, 623. 12. N . Sherwood and T. Shripathi, Faraday Discussion 1993., 95, 173. 13. M . M . Reyhani and G. M . Parkinson, J. of Crystal Growth, in press, 1996. 14. M .M.Reyhani and G. M . Parkinson, to be published.


Source of Nuclei in Contact Nucleation as Revealed by Crystallization

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