Crystallization

What is Crystallization

Formation of solid particles within a homogenous phase

• e.g. formation of solid NaCl from salt solution

• Formation of ice, snow from water vapor

• Natural minerals, e.g. gemstones, diamonds

Significance of study of crystallization:

• Crystals are very pure form of materials

• Crystals formed from impure solution are still pure

• Very practical method of obtaining very high purity

material in satisfactory condition for storage

Snow flakes

Rock salt crystalsNatural diamond crystal

• Magma: two phase mixture of mother liquor and crystals of all sizes

• Mother liquor: the part of a solution left over after crystallization

• Supersaturated solution: solution with concentration greater than equilibrium concentration

• Invariant crystal: the crystal maintains geometric similarity

• Characteristic length: definition of size of crystal

• Overlapping principle: different faces of crystal grow at a different rate and later the shape and appearance of the crystal

– Faces with low velocity survive crystal growth

Terms

sp is surface area of the crystal vp is the volume of the crystal

p

p

s

vL

6

Equilibrium Curve

• Solubility is usually a strong function of temperature, e.g. KNO3, MnSO4

• Most inorganic substances crystallize with water of crystallization– In some systems several hydrates might be formed

• Fundamental driving force for crystallization is change in chemical potential but mostly expressed as concentration driving force

KNO3

Solu

bili

ty

Temperature

NaCl

MnSO4

Solubility-supersolubility diagram

Supersaturation

From solutions

• solution with concentration greater than equilibrium concentration

• Generated using – Temperature change (cooling or heating)

– Evaporating the solvent

– Addition of third component (salting out, precipitation)

• Degree of supersaturation is given by

• Saturation ratio is given by

• Relative supersaturation:

*ccycm

*c

cS

1

Sc

c*

Example

• At 293 K, a supersaturated solution of sucrose contains 2.45 kg sucrose/kg water. If the equilibrium saturation value is 2.04 kg/kg water, what is the supersaturation ratio in terms of kg/kg water and kg/kg solution?

Formation of Crystals

• Rupture of existing crystals

• Nucleation

– Primary nucleation (homogenous nucleation)• Solute molecules clustering to form aggregates or embryos that grow into

nuclei

– Secondary nucleation (heterogeneous nucleation)• Contact nucleation

– Breakage of dendritic growths on the crystals

– Catalytic effect of solid particles in the supersaturated solution

• Fluid shear nucleation

– Sonication, shear due to impellers, etc

Formation of Crystals

• Caking

– Frequently cake or cement together on storage

– Depend on crystal size, shape, moisture content, storage conditions

– Minimized by airtight packing, deposition of inert dust on crystals

• Washing

– To remove mother liquor content and for removal of impurities

– Crystals are separated using filtration, centrifugation

– Cake is washed with another insoluble liquid

– Multiple washing stages might be required

Crystal Growth

Design of crystallizers

• Set supersaturation limit in the crystallizer

• Calculate solution recirculation rate from material balance

• Estimate maximum growth rate assuming cubic crystals for given supersaturation

• Calculate crystal growth for different desupersaturations

• Mass of crystals in suspension and the suspension volume are calculated assuming a value for the voidage which is often about 0.85

• Solution up-flow velocity is calculated for very small crystals– Usually measured exerimentally

• Crystallizer area and diameter are calculated, and height is estimated using (volume of suspension/cross-sectional area)

Equipment

• Cooling Crystallizers

– Agitated vessels

– Scraped surface crystallizers

• Evaporating crystallizers

– Vacuum cooling crystallizers

Equipment

• Cooling Crystallizers

– Agitated vessels

– Scraped surface crystallizers

• Evaporating crystallizers

– Vacuum cooling

– Forced circulation

Forced circulation Swenson crystallizer

Equipment

• Cooling Crystallizers

– Agitated vessels

– Scraped surface crystallizers

• Evaporating crystallizers

– Vacuum cooling

– Forced circulation

– Fluidized bed

Oslo fluidized bed crystallizer