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.

Grain size distribution of salts during crystallisation process

Monitoring of crystal size changes during the entire production process

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.

  • Control and regulation of crystallization process until ideal grain size has been achieved
  • Recognition and specific influence of crystallization phases (e.g. detection of seeding point)
  • Production of crystal sizes specifically for different salt applications
  • Prevention of fine crystals and undesirable dust formation

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Application strengths

  • Acoustic measuring method for opaque or highly concentrated media
  • In-situ analysis of particle size distribution and crystal concentration in undiluted original state
  • Repeatable and high resolution measurement using freely selected measurement parameters
  • High analysis frequency with up to 1,000 analyses/day without interruption over several months, no cleaning or maintenance necessary
  • Simple installation by flanging into process pipe or reactor
  • High durability and wear resistance even under extreme process conditions, e.g. high temperature and pressure, chemically aggressive media or abrasive solids
  • Low-maintenance and user-friendly
  • Direct integration into process control systems

Customer benefits

  • High resolution real-time control of dynamic crystallization processes
  • Monitoring of real in situ grain size distribution without sampling
  • Product quality optimization through process control
  • Better understanding of process
  • Increasing process efficiency by precise control of crystallization dynamic
  • Control of precipitated product yield through simultaneous determination of solids

Typical configuration



Ultrasonic Extinction | Wet
Size range: < 0.1 µm - 3,000 µm
Flow rate: > 10,000 l/h
Analysis volume: 10 l/h - 1,000 l/h

Suited for

Typical applications

Applied for control of