Crystallisation

Question 1

What are the two main types of binary mixtures?

Answer 1

-Eutectics (easy to separate)
-Solid solutions

Question 2

What is phase equilibria affected by?

Answer 2

-Pressure
-Temp

-in binary mixtures affected by conc and temp, pressure is negligible usually

Question 3

What is the eutectic point?

Answer 3

It is the point on the graph which corresponds to the composition of the mixture with the lowest crystallisation temp.

Question 4

What is an unsaturated/dilute solution?

Answer 4

-It is in a stable state
-No crystals are presents
-No potential for crystals to form

Question 5

What is a saturated solution?

Answer 5

-In a stable state
-Point where solid matter will start forming
-Solid and liquid phases are in equilibrium with conc. of dissolved material (equilibrium concs.)

Question 6

What is a supersaturated solution?

Answer 6

-In an unstable state
-System is beyond equilibrium
-Conc. of dissolved species is greater than the equilibrium conc.
-Necessary for crystallisation

Question 7

What are the 3 general steps for a crystallisation process?

Answer 7

1) Preparation of a solution containing the species to be crystallised
2) Using this solution to prepare a supersaturated solution
3) Precipitation of the crystals from the supersaturated solution

Question 8

What are the 3 ways supersaturation can be achieved?

Answer 8

-Temp change (cooling)
-Solvent removal
-Addition of a 3rd component

Question 9

When is a temp change used for supersaturation

Answer 9

-Used for systems where solubility decreases with temp.
-Involves cooling through the solubility curve

Question 10

When is a solvent removal used for supersaturation?

Answer 10

-Used for systems where solubility is not affected by temp changes
-Involves evaporating part of the solvent

Question 11

When is the addition of a 3rd component used for supersaturation

Answer 11

-Used if neither cooling or evaporation is desirable
-Added component may form a mixed solvent of reduced solubility, with the original solvent (salting)
-Added component may react chemically with the original solute to form an insoluble substance (precipitation)
-Can lead to very large supersaturations

Question 12

What does metastable mean in terms of supersaturation

Answer 12

-Spontaneous nucleation (crystal formation) is improbable

Question 13

What does labile mean in terms of supersaturation

Answer 13

-Spontaneous nucleation (crystal formation) is probable, but not inevitable

Question 14

What is the equilibrium solubility curve?

Answer 14

Where the transition from the stable to the metastable region occurs

-Can be determined experimentally

Question 15

What is the super-solubility/ maximum supersaturation curve?

Answer 15

Where the transition from the metastable to labile region occurs

-Can’t be accurately determined as it is affected by factors such as rate of supersaturation, degree of agitation and presence of impurities

Question 16

How can supersaturation in the labile zone be achieved?

Answer 16

-Cooling without loss of solvent, conc. is constant
Spontaneous nucleation will only occur once in the
labile region
-Removal of solvent (evaporation), temp is constant
Spontaneous nucleation may occur before the labile
region is reached

Question 17

What is primary nucleation?

Answer 17

-Occurs in the absence of crystals
-Likely, but no inevitably, to occur in labile region
-Unlikely to occur in the metastable region
-Two types, homogeneous and heterogeneous

Question 18

What is secondary nucleation?

Answer 18

-Only occurs in the presence of crystals of the same species
-Can occur in the metastable region
-Secondary nuclei generated due to damage to growing crystals from shear fields or contact between other crystals or equipment
-Most dominant type of crystallisation in industrial crystallisers

Question 19

When does homogeneous primary nucleation occur?

Answer 19

-Result of bimolecular collisions and interactions in a supersaturated fluid
-Can cause the spontaneous formation and build-up of lattice-structured bodies (nuclei)
-Formed nuclei may or may not achieve thermodynamic stability
-Nucleation rate increases dramatically when a critical supersaturation ratio is surpassed

Question 20

When does heterogeneous primary nucleation occur?

Answer 20

-Initiated by the presence of suspended particles of ‘foreign’ substances
-Mechanism not fully understood, likely begins with adsorption of crystalline species
-Crystalline bodies are created which grow into macro-crystals
-In industrial crystallisers, most primary nucleation is heterogeneous
-Nucleation occurs at lower supersaturation ratio than in homogeneous due to foreign particles

Question 21

What are the two specific process stages within crystallisation

Answer 21

-Diffusional step
Solute transported from bulk fluid through solution
boundary layer to crystal surface
-Deposition step
Adsorbed solute ions/molecules at crystal surface are deposited and integrated into the crystal lattice

Question 22

At high levels of molecule integration, how is this relationship between crystal growth and mass transfer coefficients effected?

Answer 22

-KG = kd, kr = 0
-Crystallisation is diffusion controlled

Question 23

At low levels of diffusional resistance, how is this relationship between crystal growth and mass transfer coefficients effected?

Answer 23

KG = kr, kd = 0
-Crystal growth is deposition controlled

Question 24

What is the mother liquor?

Answer 24

The saturated solution to be separated from the crystallised product

Question 25

What is the magma?

Answer 25

The slurry of crystals and mother liquor

Question 26

What assumptions are made in estimating the theoretical yield of crystals?

Answer 26

-Crystallisation only occurs once supersaturation is reached
-Crystallisation proceeds until the saturation limit
-There is no retention of mother liquor by crystals
-No loss of crystals during washing