Effect of Impurities and additives on Crystal Growth

Henry Hatakka^*, Hannu Alatalo and Seppo Palosaari

Lappeenranta University of Technology, Department of Chemical Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland.
^* Corresponding author
Introduction

This contribution provides a review of effects of impurities on crystal growth.

Impurity can be any substance other than the material being crystallized. Therefore even the solvent from which the crystals are grown can be considered to be an impurity. When impurities are added specifically to produce a desired morphological effect they are referred to as additives.

The presence of impurities or additives in a crystallization system can have a radical effect on crystal growth, nucleation, macrostep formation, agglomeration and on the uptake of foreign ions in the crystal structure. Different impurity - crystallization chemical -pair have a different effects on the crystal growth; some impurities can suppress growth entirely, some may enhance growth, and some may exert a highly selective effect, acting only on certain crystallographic faces. Nevertheless, the specific impurity may effect differently on different crystallization chemical. The influence of impurity or additive can be caused by various mechanisms, such as of the presence of metal ions, and of tailor-made and multifunctional additive. The concentration with which the impurities have an effect may range, depending on the system, from very low ppm-level concentration to cases where the impurity concentration may be even 50 percent by weight.


Fundamentals

Crystallization as a unit operation can be separated to a crystallizer proper and ancillary equipment related to feed, energy and separation. The different factors describing a crystallization process are shown in Figure 1. Crystal growth is limited by heat transfer, by mass transfer, or by surface reaction which can be effected by impurities. Basic methods to find out the effects of impurities on crystal growth are morphology analysis, determination of purity and crystal size.


Figure 1. Overview of crystallization as a unit operation.


Ionic Interaction

Ionic interactions in aqueous systems are of considerable importance in crystallization. It is well known that commonly occurring ionic impurities such as Cr³⁺, Al³⁺ and Fe³⁺ have a pronounced effect on the growth of simple inorganic salts from aqueous solution (Mullin and Garside [1]). Handbook of Industrial Crystallization [2] has listed most common additives affecting the growth of crystals, and they are shown in Table I.

TABLE I.

Additives affecting the growth of crystals according to Handbook of Industrial Crystallization [2].

These additives in Table I are mostly metal ions which can be purposely added, but are more often unavoidably present in the crystallizing solution. According to van Rosmalen et al [3], even those metal ions which only have a slight influence on the crystal growth tend to be incorporated into the host crystal. This is very important in industrial practice to uptake of divalent heavy metal ions in mineral precipitates, e.g. for the purification of waste-water streams. This uptake can proceed by coprecipitation of various compounds, by interstitial incorporation or by isomorphous substitution. The uptake of foreign ions is determined by the thermodynamics of the system, i.e. by equilibria, and by the kinetics of the growth process.


Tailor-Made Additives

An important class of additives are so called, "Tailor-made" additives, which are designed to interact in very specific ways with selected faces of crystalline materials. These additives are designed to contain some chemical groups or moieties that mimic the solute molecule and are thus readily adsorbed at growth sites on the crystal surface.

Organic compounds lend themselves more easily to the purpose of designing structurally specific additives than the simpler ionic compounds because of organic compounds more complex nature. Only a few examples of tailor-made additives are known for inorganic and mineral salts.


Multifunctional Additives

For ionic compounds so called multifunctional additives are often applied to achieve the desired effects. This kind of additives are for example phosphoric acids, polycarboxylic acids, polysulfonic acids, and all kind of low as well as high molecular weight copolymers with various acidic groups. Multifunctional additives are capable of forming bonds with cationic species at crystal-liquid interfaces. An advantage of these types of additives is their performance at concentrations as low as 10 to 50 ppm, the exact dosage depending on the process conditions like the pH.


Habit Modification

Crystal habit or morphology is often used quite loosely to describe the shape and aspect ratio of crystals. From a process development and troubleshooting standpoint, the principal impact of crystal habit is on the bulk physical properties of the product. For example, crystal habit plays a part (often along with size distribution) in defining the ease and effectiveness of solid/liquid separation, bulk density, powder flow characteristics, breakage, and dustiness.

The overall shape of a growing crystal is determined by the relative rates of growth of its various faces. The slower the growth rate the larger the face. In general the growth rate of a surface will be controlled by a combination of structurally related factors, such as intermolecular bonds and dislocations, and external factors such as supersaturation, temperature, solvent and impurity concentration. The individual crystal faces each have their own growth-rate dependence on temperature and supersaturation. Therefore, altering the temperature and supersaturation history can affect crystal habit. The extent of changes that can be induced in this way can be investigated by changing experimental conditions. As a general rule, crystals become more extreme in habit as supersaturation levels increase. Impurities operate by binding to growth sites, and thereby reducing the crystal growth rate. Since different crystal faces can have different characteristics due to the orientational order imposed by the crystal lattice, specific impurities can bind effectively to some faces, but not others. These face-specific interactions result in modification of the crystal habit, and some impurities can have significant effects even at trace levels. Comparison of crystallization from pure and impure solutions may indicate whether or not significant habit modification is caused by the impurities in the test solutions.

Certain crystal habits are disliked in commercial crystals because they give the crystalline mass a poor appearance; others make the product prone to caking, induce poor flow characteristics or give rise to difficulties in the handling or packaging of the material. A granular or prismatic crystal habit is usually desired, but there are also specific occasions when other morphologies, such as plates or needles, may be wanted. Mullin [4] has listed some examples of habit modification and they are shown in Table II.

TABLE II.

Some crystal habit modifications according to Mullin [4, p. 255].

The trace presence of foreign cations can exert an influence on the crystal habit of inorganic salts. Some act by simple substitution in the lattice, e.g. Cd²⁺ for Ca²⁺ in calcium salts or Ca²⁺ for Mg²⁺ in magnesium salts, as a result of similar ionic radii and charge. Trivalent cations, particularly Cr³⁺ and Fe³⁺, have a powerful effect on the morphology of salts such as ammonium and potassium dihydrogenphosphates. Complex cations, like Fe(CN)₆^4-, have a remarkable influence on NaCl, producing large, hard dendrites instead of small cubic crystals at concentrations of less than 1 ppm.

There are literally thousands of reports in the scientific literature concerning the effects of impurities on the growth of specific crystals. Comprehensive reviews on the influence of additives in the control of crystal morphology have been made by Kern [5], Boistelle [6], Davey [7], Botsaris [8], Nancollas and Zawacki [9], van Rosmalen et al. [3] and Davey et al. [10].


Predicting the Influence of Additives and Impurities

The first generally applicable quantitative measure of relative face growth rates via calculation of E_att, the attachment energy, was provided by the Hartman-Perdok technique. Calculations were simplified by reducing crystal structures to chains of strong bonds and identifying the slowest growing faces as those lying parallel to at least two bond chains. The attachment energy is then the energy released when a new layer of thickness d_hkl is added to the (hkl) face. The lower the attachment energy the slower the face is assumed to grow. The morphology of crystals can be calculated by attachment energy using various approximations:

• Donnay Harker. This approach makes a very simple assumption, namely that the binding energy between crystal planes is inversely proportional to the interplanar spacing. Thus the relative growth rate of a series of faces can be assessed purely on the basis of their structures.
• Hartman and Perdok. By examining crystal structures and identifying chains of strong bonds within them it was possible to make calculations more specific by use of available summation techniques and assuming that ions were point charges.
• Specific force fields. Initially Bennema but more recently Docherty and Roberts have attempted to make the calculations more precise by using available potential functions to describe the interactions between molecules and ions. The van der Waals and electrostatic contributions to the overall interaction energies of adjacent molecules are calculated separately by summation of the interaction E_ij between all the individual non-bonded atoms which constitute molecules.

To find out the effect of additives or impurities above approximations should be extended to include the influence of additives on attachment energies and hence morphology. To do this additive molecules are substituted for substrate molecules in the growth slice taking each lattice site in turn. This gives new values for the slice E_s1 and attachment energies E_att. The binding energy at a surface site can be calculated by equation

Eb = ES1 + Eatt

(1)

It is now possible to calculate the change in binding energy due to the inclusion of an additive (or an impurity) in the slice. The lower this change E_b the higher the probability is it to be influenced by an additive. These calculations can only be carried out with any certainty, without availability of crystallographic data, for a tailor-made additives which are slightly modified substrate molecules. This is because tailor-made additive molecule is assumed to have identical conformation to the substrate. The modified part of the molecule of the additive may also be fixed if the molecule is rigid but may adopt a variety of conformations if, for example free rotation about one or more bonds is possible. Roberts and Docherty [11] have extended this approach to include calculations of attachment energy when a tailor-made impurity is already in a surface site.


Acknowledgements

Thanks are due to the Technology Development Centre, Kemira Oy, Cultor Ltd., and Orion Corporation Fermion for financial support.

References

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10. Davey, R.J., Polywka, L.A., Maginn, S.J., The Control of Morhology by Additives: Molecular Recognition, Kinetics and Technology, Advances in Industrial Crystallization, (Garside, J., Davey, R.J., Jones, A.G. eds.), Butterworth-Heinemann, Oxford, 1991, 150-165
11. Docherty, R., Roberts, K.J., Modelling the Morphology of Molecular Crystals, Thesis, University of Strathclyde, 1989