3.1.3. Effect of Concentration Factor α on Crystal Shape

The main source of impurities within the product are derived from the adherent mother liquor, which could be reduced to a certain extent by the washing of the crystal cake in a centrifuge. The amount of adherent mother liquor is defined by the physical parameters of the solution, like density and (especially) viscosity on the one hand, and on the crystal size, shape and surface conditions of the solid crystals on the other.

Further to its effect on the final moisture of the cake, the shape of the crystalline product is very important for parameters such as the tendency to build up dust, free-flowing ability and storage stability.

It was observed that an increasing impurity level led to a change in the crystal shape from a compact shape to an increasingly needle-like shape (Figure 7a–d). As the tests were executed with the original feed solution, containing a defined matrix of impurities, no comparison to the product crystallized from a pure solution is available. Identifying the responsible impurity from the matrix of different impurities was not within the scope of the test, as this is fixed by upstream processes and cannot easily be adjusted.

The needle-like shape of the product crystals creates various difficulties, as follows:


It was observed that a concentration factor of α = 3 already led to an elongated crystal shape at a still sufficient purity. The elongated shape is the defining parameter, with regards to limiting the concentration factor of the industrial unit.

**Figure 7.** (**a**) Crystals after washing for a concentration factor of two. (**b**) Crystals after washing for a concentration factor of three. (**c**) Crystals after washing for a concentration factor of four. (**d**) Crystals after washing for a concentration factor of five.
