EXPT. NO.
:1 DATE:
CRYSTALLIZATION
(Batch Crystallization)
Aim: To find the yield of crystals in batch crystallizer.
Apparatus: 500ml beaker, measuring cylinder, weighing machine, hot plate.
Chemicals: Copper sulphate powder.
Theory: Crystallization is the unit operation that involves the separation of solute from
its solution in the form of crystals. It is normally carried out in process industries, from
solution in the form of almost pure crystals. Crystallization usually involves concentration of
solution and cooling of solution until the solute concentration becomes greater than its
solubility at that temperature then the solute comes out of the solution in form of pure
crystals. The performance of the process is evaluated in terms of size, shape, yield and purity
of crystals. A saturated solution is defined as the one, which is in equilibrium with excess of
solid solute of a given temperature. Supersaturation is the quantity of the solute present in the
solution in which crystals are growing as compared with the quantity of the solute that is in
equilibrium with the solution. The amount of crystals formed depends upon the difference in
saturation concentration, since once the crystallization begins, the extra solute held in the
solution due to supersaturation also comes out of the solution in the form of crystals. There
are two steps involved in crystallization.
1) Nucleation
2) Crystal growth
The generation of solid phase is called nucleation while growth is the increase in the size of
this nucleus with the layer-by-layer addition of solute. Supersaturation is the common driving
force for nucleation and crystal growth. The number of nuclei and the rate of growth of
crystals does not depend upon the temperature of operation and of less number of nuclei and
hence large size crystals are formed whereas rapid cooling results into the formation of large
number of nuclei, giving yield containing large number of tiny crystals. In crystallization
supersaturation is obtained by
1) Adding the solute
2) Evaporating part of solvent
3) Cooling the saturated solution
Mier's supersaturation theory
There is definite relationship between the solubility curve and super solubility curve. Both
the curves are approximately parallel to each other as shown;
If the solutions having composition and temperature indicated by point C is cooled in the
direction shown by arrow, it first crosses the solubility curve AB and crystal formation will
not begin until the solution is super cooled considerably past the curve AB. According to
Mier's theory, crystallization will start in the neighborhood of point D and concentration of
the solution then follows roughly along curve DE for an initially unseeded solution. Curve
PQ represents the limit where this spontaneous crystal formation starts. According to Mier's
theory under normal conditions, nuclei can not form and crystallization can not occur in area
between solubility curve and super solubility curve i.e., at any position short of point D along
line CD.
Procedure :
1) Take 60 gm of CuSO4 powder in 500 ml beaker.
2) Add 200 ml water and stir well.
3) Heat the solution gently until all the powder dissolves in it. This will be the saturated
solution.
4) Continue heating and evaporate nearly half of the solution so that the solution
becomes super saturated.
5) Cool the beaker slowly.
6) After two hours the crystals of CuSO4 forms. Remove the mother liquor.
7) Dry the crystals and weigh it.
Observations:
Weight of CuSO4 powder taken = gm
Weight of water =
Weight of beaker + crystals =
Weight of crystals obtained =
Calculations :
The yield of the crystals can be calculated as
% yield = Weight of crystals obtained
________________________________
100
Weight of crystals in feed
Result : The %yield of the crystals _____________
Precautions : Cooling should be slow.
R1 R2 R3 R4 Total Signature
(5 Marks) (5 Marks) (5 Marks) (5 Marks) (20Marks)
