Leaching and Crystallization

1
Leaching and Crystallization
Last lecture covered Ternary Liquid-Liquid
extractions. Ternary phase diagrams. A
procedure to determine the product compositions
and flow rates of a liquid-liquid extraction
separation.
This lecture will cover Ideal leaching
Non-ideal leaching Crystallization
2
Ideal Leaching
Leaching is a solid-liquid separation method
which involves preferential solvation ofone
component, the solute, of a solid mixture into a
liquid solvent. Diffusion of the solute
through the solid is slow and complicates the
separation process in practice because
equilibrium is difficult to attain . It is
also difficult to completely disengage the solid
and liquid phases.
The ideal case is one where all of B is leached
out of solid A.
Diffusion through solids is slow
slow
B
A
C
B
Solid exits wet
A,B
A
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Ideal Leaching
Solvent C
Solid-freeliquid
Overflow B,C
Underflow B,C (liquid) A (solid)
Solid Feed A (carrier) B (solute)
Solid-liquidslurry (B only in liquid)
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Ideal Leaching
Overflow L1, yL1, xL1
Solvent yS0, xS0
Liquid B and C No Solids
Liquid Solvent C
Solid Carrier A and Solute B
Liquid B and C Solid A (insoluble)
Solids yL0, xL0
Underflow S1, yS1, xS1
Solute completely dissolves
Ratio of solid to liquidin the underflow is a
constant
XS solute/(solvent solute) in overflow YS
solute/(solvent solute) in underflow YI
carrier/(solvent solute carrier) in overflow
or underflow y fraction of solvent in overflow
or underflow x fraction of solute in overflow
or underflow
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Ideal Leaching
Nomenclature XS solute/(solvent solute) in
overflow YS solute/(solvent solute) in
underflow YI carrier/(solvent solutecarrier)
in overflow or underflow y fraction
of solvent in overflow or underflow x fraction
of solute in overflow or underflow
Givens The fraction of solids in the underflow
YI. All of the solute is dissolved (ideal
leaching). The composition of the solid feed. The
fraction of solids in the overflow YI 0.

• YI carrier/(solvent solute carrier) is
  constant so if we increase the solvent flow rate,
  the
• flow rate of the overflow increases while the
  underflow remains constant.
• If solvent flow rate increases, the liquid
  becomes less concentrated in solute.
• The composition of the overflow and the liquid
  in the underflow are the same.
• Since all the solute is dissolved, adding
  solvent only dilutes the liquid in the solute.
  The dilution
• is the same for the overflow and underflow
  liquids since the liquids have the same
  concentration.
• Since the underflow also has insoluble solid A
  the concentration of B in the underflow is lower.

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Ideal Leaching
y fraction of solvent in overflow or
underflow x fraction of solute in overflow or
underflow
Overflow y solvent/(solventsolute) x
solute/(solventsolute) xy1 y 1 x
So overflow has slope 1
Underflow y solvent/(solventsolutecarrier)
x solute/(solventsolutecarrier) YI
carrier/ (solventsolutecarrier) xyYI 1
YI is a constant. y 1 x YI
So underflow has slope of 1
The overflow and underflow lines both have slope
of 1
Overflow y solvent/(solventsolute) x
solute/(solventsolute) y/x
solvent/solute Underflow y solvent/(solventsol
utecarrier) x solute/(solventsolutecarrier)
y/x solvent/solute
overflow
y
underflow
x
So tie lines joining the overflow and underflow
lines pass through the origin.
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Ideal Leaching
1) Locate the feed point L0. Locate the underflow
line using the fraction of solids given. Draw
overflow and underflow lines 2) Draw the mixing
line.3) Locate the mixing point M using the
inverse Lever Rule 4) Draw the tie line through
M. 5) Determine the compositions of the flow
streams 6) Locate the overflow and underflow
rates L1 and S1 using the inverse lever rule mass
balances
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Example
Caffeine is leached out of coffee grounds using
water. If 1000kg/hr of pure water ismixed with
1000 kg/hr of ground coffee beans which are 80
by weight insoluble and 20 caffeine,
calculate the amounts and compositions of
underflow and overflowfrom an ideal separator. A
test of the underflow shows that it is 70 solids.
Step 1. Draw the y-x diagram with the overflow
line. Draw the underflow line parallel to the
overflow line and intersecting 0.3. Locate feed
L0 at solidmixture composition and S0 at the
solvent concentration.
S0
1
Solvent point
overflow line
y
underflow line
0
YI
L0
1
x
Feed point
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Example
Step 2 Draw the mixing line through the S0, and
L0. The mixing point will lie onthis line and
depend on the ratio of solvent to solid feed.
Step 3 Apply the lever rule to a solute
(caffeine) balance or solvent (water) balance.
Solute balance
Solvent balance
overflow
yM
underflow
xM
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Example
Step 4 Draw the tie line by extending a line from
the origin through the mixingpoint to the
overflow line. Step 5 From point M we know y/x
5. Using the equations of the overflow
and underflow lines allows us to determine the
stream composition.
Overflow
S0
1
L1
overflow
y
Underflow
M
S1
underflow
0
1
L0
x
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Example
Step 6 Determine the overflow and underflow flow
rates
S0
1
L1
y
M
overflow
S1
underflow
0
1
L0
x
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Non-Ideal Leaching
Often, the conditions in a leaching system are
such that equilibrium is not obtained. The
overflow and underflow curves become non-ideal.
S0
1
L1
y
M
S1
overflow
underflow
0
1
L0
x
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Ideal Leaching
For systems with low solute to carrier ratios, a
finite amount of solvent may be needed to create
a two phase equilibrium condition.
S0
1
L1
y
M
S1
M
0
1
L0
x
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Crystallization
Separation using crystallization involves cooling
a liquid solution intoa two-phase solid-liquid
equilibrium condition. The solid precipitate
isvery concentrated in the higher melting point
material and can be separatedfrom the remaining
liquid solution.
T
L
L Solid Naph
M
L Solid Benz
Solid Naph Solid Benz
benzene
naphthalene
xnaphthalene
The mass of the solid naphthaleneproduced is
determined using the lever rule. MF is the mass
of the original feed. The solid concentrationis
usually assumed to be pure.
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Summary
This lecture covered Ternary Ideal
leaching. Graphical interpretation of mass
balances and equilibrium. A procedure to
determine the product compositions and flow
rates of a leaching separation. Simple
description of crystallization separations