Different types of crystallisation units can be proposed by Kestner, in
different types of crystallisers :
Crystallisation by concentration
This
process concerns firstly products having a solubility which does not vary
appreciably with the temperature: sodium chloride, ammonium sulphate, sodium
carbonate, citric acid, …
Another application concerns the recovery –
or elimination – of products from liquid effluents, often very dilute :
eluates, residual substances from reaction tanks, solution stripped in
absorption towers, ….
In most cases, the actual crystallisation unit is
combined with a pre-concentration stage.
Crystallisation
by cooling
When the solubility curve varies with the
temperature (direct solubility), the technique of crystallisation by cooling
may be applied. The operating temperature is usually obtained by applying a
vacuum to the hot concentrated feed solution, or by indirect heat exchange with
a cold source (water or thermal fluid). Due to the risk of solids build-up on
the heat exchange surfaces, the vacuum cooling solution is retained in most
cases. This technique is applied for example in the production of boric acid,
melanin, ammonium molybdate, glauber salt, …
Crystallisation by reaction or by equilibrium
displacement
Crystallisation and chemical reaction are two
single operations which may occasionally be combined.
Kestner (backed by
its experience gained in the manufacture of synthetic products such as ammonium
sulphate, ammonium perchlorate or ammonium phosphate) can offer equipment which
combine these two features. In most of the cases mentioned here, the question
of energy efficiency often becomes a major factor and a specific study is
devoted to the integration of the crystalliser in the process as a whole, for
each project.
Kestner has also an in-depth experience in the
design of elutriation legs which covers:
- Classification of crystals in the leg, to produce a narrow crystal
size.
- Cooling of the crystals by contact with cold mother liquor, to get less
suspension flashing in the dewatering equipment.
- Increase in crystals purity by replacement of mother liquor by a feed
stream to displace the impurities sticking to the crystals.