Crystallisation of Salts

Real-time particle size control of crystallization processes

The production of salt and other crystalline substances happens via crystallization processes whereby the thermodynamic equilibrium of solubility in saturated salt liquor is disturbed leading to production of particles. There are basically two types, cooling crystallization and evaporation crystallization. In both cases the crystal growth requires crystallization seeds. Dissolved substances eventually precipitate off the liquor as solid crystals with a high degree of purity.

Through meaningful control of process parameters like pressure, temperature or residence times, specific crystal sizes can be produced which define the product’s properties. Salts like fertilizer or road salt need to have good ballistic properties and are therefore produced as narrowly distributed grains of up to 5 mm with small particles, responsible for dust occurrence, avoided on principle. Other applications like industrial chemicals or active pharmaceutical ingredients with the focus on reactivity and bioavailability often require fine crystals that offer a large surface area.

Crystallization processes, where sample extraction and sample dilution would change the particle size, ultrasonic extinction with OPUS can prove its worth. Particle size measurement in mother liquor under process conditions offers observation and control of the crystallization process in real time.

OPUS succeeds in analysing the crystals in the original state without sampling and dilution. However, besides pure sample handling, the frequency of crystallisation analysis often plays an important role. Particularly, continuous crystallisation processes are subject to a strong process dynamic due to the change of supersaturation and sub-saturation.

The trend diagram clearly shows this process dynamic by the sawtooth fluctuations in the particle size of sodium chloride. In subcritical conditions, the crystals mature to large particles until a constant saturation degree of the crystal solution is reached and the free surface becomes too small for further growth. A spontaneous crystallisation of the finest primary crystals occurs and the particle size values suddenly drop. The high analysis frequency of the OPUS probe of approx. 1,000 measurements per day guarantees a reliable monitoring of the current process situation at any time and allows targeted pinpoint interventions, e.g. with seed crystals.