Predicting the Effects of Impurities

The deliberate addition of an impurity is intended to cause a particular effect, such as a reduction in mineral scale formation or a change in morphology. This effect comes from changes in the kinetic process involved, e.g. for a change in morphology the relative growth rates of the different crystal faces have to change. The function of the additive is thus very specific: the morphology changes if the growth of a specific face is blocked. This is illustrated in Table 1 for a number of desired additive effects. The first column gives the desired effect, the second the kinetic process involved and the third the action acquired by the additive. Table 2 shows whether an interaction with a specific face or all faces is required, at what particular moment the additive should be added and whether it blocks a face or acts as a template. This table shows the close relationship between additives used for different purposes. An anticaking agent is simply an antiscaling agent added after crystallization, and an additive used to block a specific crystal face can in some cases also act as a template for that same face.

Identification of an unintentional impurity and understanding of its effect on the crystallization process requires a kind of reverse engineering. Many impurities are formed as by-products during the synthesis of the compound to be crystallized. By observing how the crystallization process changes in the presence of impurities, the kinetic process involved can be identified. The effect on the process then can be matched with the expected (predicted) effect of one of the impurities present. This is a rather complicated procedure because not all the impurities present that can act on the kinetic processes involved will be known.

The desired effects of additives are changes in the crystal quality or the process parameters. The crystal quality is an overall term for a variety of crystal parameters (morphology, purity, etc.). Changes in the process conditions due to additives, such as better filterability, make the process more efficient. Figure 2 shows the effects of the impurities on the crystal quality and the process conditions. The working directions for both the rational design and the reverse engineering are shown. Key to this figure is knowledge of the interaction of a given compound with the crystalline interface. This interaction determines whether an impurity can, e.g. block growth or induce nucleation of a given face. After evaluating the influence of a given impurity on the morphologically most important faces, the overall effect on the crystal growth and the outgrowth of

Table 1 Additive effects on the crystal growth process

Desired effect

Anticaking

Antiscaling

Changing morphology (filtration, flowability) Control of polymorphism Changing crystal size distribution

Kinetic process involved

Dissolution and regrowth of crystalline material

Complete blocking of nucleation and/or growth

Ratio of growth rates of different faces Nucleation of a given polymorph Changing nucleation/growth ratio

Action required Complete blocking of growth Complete blocking of growth Block specific faces

Mimicing a face of the desired polymorph Block specific faces, or block all faces nuclei can be deduced. From these effects the influence on the crystal quality and the product handling characteristics and ultimately the product properties can be determined. The starting point for designing and studying additives is therefore a study of the molecular structure of the crystalline interface.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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