.

Solvent recovery
is easy

Disadvantages of SCF
High fixed cost

.Highpressure required
is

Leaching
Many biological and organic and organic sub.
in a mixture of different
components in a
separate the desired solute constituentsolic
or
undesirable
solute component from the solid pha
is contacted with liquid phase. The two
contact and the solute can diffusefrom the solidar
phases
to
resulting in the separation of the components I
solid. This separation process is called orig
or simply leaching.
solid-liquida
Various operations n leaching process are
given

Washing
In leaching when an undesirable
component is reme
a solid with water, the is
pocess called washing.
Deoction
It refers specially to the use of solvent at its
temperature.

Elution/Elutriation
When the soluble material is largely on the surface of ir
solid and is washed off by the solvent, the operation is
Elution or Elutriation.

Applications of Leaching
Leaching may be used either for the production
concentration solution of a valuable solid material orin
to free an insoluble solid from a soluble material wiul

it is contaminated.
The main applications of leaching are

or Biological Substances produe

In the industries many.

biological and food process by Lg
separated from their original natural eructure
structure
are
solid Leaching.
 Mass Transfer

mples

ofsugar from sugar beets with hot water Nvis Y (under flow)
vegetable oil, organic solvents
of
such as
anddeth
ether are used to extract the
a c e t o n e

exane, oil from

cOvabean, suntlower seeds, cotton seeds etc.
peanuts,s o y a b
T i el i n e
pharm rmaceutical ndustry many different N
the
In lucts are obtained Nvls X
,p h a r m a c e u t i c a l
by leaching plant ( o v e rp l o w )

and stems.
Production of soluble"Instant" coffee.
olable tea is produce by caching tea leaves with water. (0,0) XY (1.0)
Thnnin
isremoved from trec barks by leaching with
waler
YvisX

ching for Organic and Inorganic

Materials
Leachingprocess is usedlextensively in the metal processing
Y =X

industry.
e useful metals usually occur in mixture with very large
nts of undesirable constituents and leaching is used
amo

the metal as soluble salts.

o remove X
Gold is leached from its ore using aq. NaCN solution.
Fig. 5.58 Curves in leaching process
,Cobalt and Nickel salts are leached from their ores by
H.SO-NH 0 , mixture. where, N kginsoluble/kgsolution
NOH is leached from a slurry of CaCO, and NaOH X = weight fraction C in overflow (B-free basis)
prepared by reacting Na,CO, with Ca(OH),. = weight fraction in underflow (B-free basis)
. Copper salts are leached from ground ores containing other This curve represents a case where solute C is infinitely soluble
minerals by H,SO, or Ammoniacal liquor. in a given solvent A, thus X and Y will have values over the
entire range 0.0 and 1.0. The curve JKL represents separated
Two Steps of Leaching solid (underflow/wet solid).
Te separation of a soluble constituent from a solid by means Curve MNP represents the clear solution withdrawn (overflow]
dasolvent, i.e., leaching consists of two steps.
which is above the N=0 axis, which indicates thatthe overflow
Contacting the solid with the selective solvent in order to contains some amount of B. This indicates that B is slightly
dissolve the soluble solute in the solvent. soluble in solvent or a partially settled solution is withdrawn
Separationof insolublephases. so that some amount of insoluble solid is also carmied away.
Here the tie lines are not vertical. This is because
Braphical Representation of Equilibrium . ifinadequate contact time with leaching solvent to dissolve
anditions in Leaching all solute is provided.
case of Leaching operation, equation data is plottedN . i f adsorption ofthe solute occurs.
Ic. of insoluble solid B in any mixture) and xy (conc. of . i f the solute Cis soluble in insoluble solid B.
ne C in overflow and underflow), we get these types of
e.g., soyabean oil (C) hexane (A) soyabean meal (B).
rves.
 Nv'a Y
Any mixture M to the right of line P'S will setula
ule
saturatedsolution anda slury U,
whose comp to give
ad
upon position of M. ONon depen
Point Trepresents the composition of pure solute ad
or settling of saturated ution. Since the tie ier ranas
of PS are shown vertical no
adsorption occursa lines to
the
M X, Yp
liquids are clear.
overi
1.0p Examplo 1. Seeds containing 20%
by weight oil.
extracted in a counter current
plant and 90% ofofarethetn
is recovered in a solution
oil. If the seeds are contacted with 50"% by containing
Yv/o
X
fresh
kg of solution is removed in the underflow in solvent and
with every 2 kg of insolub
matter, ass0oriel
theoretical stages required. determin
X Sol. Basis 100 kg seeds as feed
Fig. 5.59 Curve without adsorption of solute Feed C Oil 20% 20 kg
=

B insoluble 80% 80 kg
=

Curve represents a case where =

adsorption of solute does not A Solvent in seeds = 0

occur, so that the clear solution is withdrawn F
(overflow) and (A+ C) in seeds 0 =

+20=20 kg
the solution associated with the solid in the underflow have
YF
C 20
the same composition as the overflow and tie lines are vertical A+C 20 1
and thus ay* curve coincides with the 45°
diagonal and the B

distribution coeflicient m 1
NE A-C 204
Solvent Oil in solvent 0
The overfow contains no insoluble solid B either dissolved A = Pure solvent =
Pkg
or suspended. So, that the overflow line N v/s Oil +Solvent 0+ Pkg
Xis represented
by X-axis. Xn1 weight fraction of oil in
solvent=0
Figure represents a case where solute Chas alimited solubility
N =
kg insoluble
k gsolvent 0
, in solvent A. No clear solution stronger than x, can be
obtained so that the tie lines joining Overflow
slurry and saturated The strong leach solution
solutions must converge as shown. leaving the unit contain, 50% oil as ovetow
and contains no insoluble
A = 100 50 50%

N v/s Y 50
Co fraction of oil = X =10o 0.50

B
N = 0= A+C
N
Underflow
1 kg of solution is retained for
every 2 kg of insoluble matter trom eac
stage le., constant underflow and therefore the ratio of BA+Cis constart
N vWs is horizontal y to X are
Y
1.0

Based on 100 kg seeds

Insoluble matter in leached seeds Insoluble in feed
1 kg solution is rotained for 2
=

60 k9
kg insoluble
i.e., for Bo kg insolublo solution retained 40 kg
Oll recovored 90%
Oll In sollds 10% 10202
100 kg
Solvont 40-Oll 40 2 38 kg
1.0 wolght fractlon of oll In leached
2
Fig.5.60 Curve with limited solubility of solute Soeds 4n 2+3R
+3A 0.05
 Irawing the cunve
lear spsfora X and N v/s Y
In these cases crushing and grinding of the ore is used to
vs increase the rate of leaching since the soluble portions are
ends Draw
N = 2 . 0 o nY a x i s
made more accessible to solvent.
4)
tape
.Lcate F@p N)F(1,
0) Animal and Vegetable Materials
leh Lpcate R, 1 Biological materials are cellular in structure and the soluble
low Late R, X, 0)=R, (0.5, 0)
(05, 2) constituents are generally found inside the cells. The rate of
1cateE, (YN)= extended Join E, - R+1 and leaching may be comparitively slow because the cell walls
Join FR,
and
extended to
provide another resistance to diffusion. However to gring the
Dbe ,

the tended line meet

to at AR.
neet
R, draw a vertical tie line to cut N biological material sufficiently small, to expose the contents
10ht ,Through v/sY of individual cells is impractical.
nd atE R, E and R, line on the same line. Join Sugar beets are cut into thin wedge shaped slices for leaching
on The points Plot E,AR so that the distance required for the water solvent to diffuse in
at R,.
the pcut Nv/s X vertical to cut N v/s at E, order to reach individual cells in reduced.
hrough R, draw For the leaching of pharmaceutical products from leaves,
the procedure point E, is reached. Every tie line
till
Repeat stems, and roots, drying of the material before extraction helps
represent a stage.
in rupturing the cell walls. Thus, the solvent can directly
F(1,4) dissolve the solute.

(0.05,2) Factors Affecting Rate of Leaching

E, EE E N v/s Y The selection of equipment for extraction is influenced by the
factors which are responsible for limiting the extraction rate.
There are four important factors to be considered.

.Particle Size The size, the greater is the interfacial

smaller the
Ap.2A0.40.5 0.6 0.8 1.0 area between the solid and liquid and therefore the higher is
the rate of transfer of material and smaller is the distance the
X solute must diffuse with in the solid.
Solvent The liquid chosen should be a good selective solver
and its viscosity should be sufficiently low for it to circulat
used initially,
freely. Generally a relatively pure solvent will be
proceeds the concentration of solute
although as the extraction
of extraction will decrease, first because
ertiow will increase and rate
the
the concentration gradient will be reduced and secondly
solution will generally become more viscous.
fraction of oil in solution on B free basis material is
Fig. 5.61 Weight Temperature In most cases, the solubility of the
being extracted will increase with temperature give higher
to a

Preparation of Solids for Leaching rate of extraction.

.Agitation of the Fluid Agitation of the solvent is important

to be
Belore extraction to be started, the solids has prepared therefore the
because this increases the eddy diffusion and
Some common
to the bulk of
Paon
pending on their properties and types. transfer of material from the surface of particles
fine particles
prepared for leaching of
art are as
dustries where solids are the solution. Further agitation of suspensions
sedimentation and more effective use
in mode of
illows prevents
interfacial surface.

Inorganic and Organic Materials

Tte method of preparation of the solid depends
to a large
Single Stage Leaching
soluble constituent present,
t upon the proportion of the solid, nature of solid
Notation Scheme
nbution throughout the original A Pure solvent
ad the original particle size B Insolublesolid
matrix of insoluble C
5Oluble material is surrounded by contact and dissolve Weight fraction of C in solid or slurry
,the solvent must diffuse inside to
Y =

A+C
Soluble material and then diffuse out.
 must lie on
same line
and M
effluent solution from E, R
X =
Weight fraction of Cin For
stu

C Ne
stage
A+C

N =
Mass B N Fir
sta
Mass (A+ C) Nv/s Y Solute

For pure solvent A, N=0, X=0

Solvent to be leached
Leached solid Na M

B= mass insoluble Nv/s X

B= mass insoluble
F= mass (A+ E mass (A +)
N= mass B/mass(A+0)
(Ro
Ne= mass B(A+C) X AA 1.0
X= mass C/mass (A+) YF mass CI(A+C) gbalas
XY=Weight fraction C, B-free basis
Leaching solid
leaching solvent
Fig. 5.63 Single stage leaching's graph
5.62 Schematic view of single stage leaching
Fig.
Ro = A+C
R = (A+C)
Multistage Cross Current
C C
XA+C Leaching
XoA+C B
This is simply an extension of single stage leaching in uk:
the
the leached ssolids are essively contacted wit
F NF leached This
leaching solvent. This may may bebe performed inin a bate fresh
BFN =EN ...0) performedaa batch
batch or
continuous fashion. Here the leached solids lids from an from any
Solute 'C" balance act as the feed solids to the next, so more and more amou. stage
FY+ RoXo = EY1 + R,X1 ..11) solute is remained from the solids as we move alono
the
Solvent A balance given cascade.
F1-Y)+Ro (1 - Xo)= E (1- Y) + R, (1-X) ii) Figure shows three stage cross current leaching operation n

Solution balance gives which unequal amounts of leaching solvent of some

composition are used in all the three stages.
F+Ro E +R =M4 .iv)
Mixing the solids to be leached and leaching solvent produce Composited
a mixture of B-free mass M such that R,X RX R,X, Soludon leach
Solids to
B B be leacheds 1 2) 3)5Y Mu
NM, F +RoM ..(V) solids Lea
RoX RX
Now, FY+Roko =
YM, M Leaching Consi
solvent COnta
P+ R, X
YM F+Ro ..(vi) Fig. 5.64 Multistage cross current leaching one o

.PointF (Yp, Np) represents the solids to be leached Assume that there are no insolubles in the leach soluicn Stage
i.e., feed (overflow). So that B in solids to be leached is equal to B's to be
Point leached solid from all the stages. (Over
R, (Xo,. 0) represents the leaching solvent
Point M, (Yu, Material balance of insolubles (B) for is
NM, ) represents the mixture (solids ++ any stage n
solvent that can split into overflow and underflow) B =
E-1 N,-1= E,N,
F, Rg and M, must lie on same For stage 1 B
straight line EN =N F
FM Solution A+C balance for any stage is n
F M, R En-1 + Eon E , +R, =Ma

In the-
not l
 Firstage2 En-1
Eo Fednbes B
F Solvent balance for entire plant is
F+Ro, =
E +Ri= M F+Ryp1 = Ri + Eyp=M ..i)
..(iii) Solute C balance
E +Ro,
E+R2=M
=
firstage3
balance for any stage n is vN) FY + Ryp1 Xwp= RX +ENp YNp =MYMi)
Solte
where, M represents the B-free mixture obtained by mixing
E,- Y-1+ Ron Xo solids to be leached and leaching solvent.
YMn
M .(V) The coordinates of point M are

YMn
E-Y-1+Ron X N B
F+ Rp+ B
..(ii)
En-1+Ron F+RP+1
is
thalance for any stage
n
FYp+ Ryp+i XNps
B En-1 Nn-1 E,N, =NMn M, Yy ..iv)
B
F+Ryp+1
B Now
NMn equation (i) can be rearranged as
M E,-1 t on
F-RI ENp- RyP+ 1=AR
=

AR =
difference point

N E
ch
esh Na
I a Nv/s Y N v/s Y

age
of M
he

Nv/s X
n (R
X, R R1
ne 1.0 Nv/sX
XY=Weight fraction C, B-free basis HN1 Rs/R2 /R
Fig. 5.65 Multi stage cross current leaching xY

Multistage Counter Curent

Leaching
Consider a multistage counter current leaching system
Containing n stages as shown in the figure. Calculations in
0Re of the stage is given below
SMage 1 represents one end of the system at which the solids
o be leached are fed and from which strong leach solution AR
oicrilow) of highest solute concentration leavs the system. Fig. 5.67 Multistage counter current leaching

E ENP-1 ENP
N N2 N. YNP-1 YNP, B
NNp Leached
In-Situ Leaching
|NNP-1 solid It is a process of recovering minerals such as copper and
Leached uranium through bore holes, drilled into the deposit. It
RNp Np,1 solvent involves pumping of a leachate solution into the body via a
X XNp XNp borehole, which circulates through the porus rock dissolving
Fig. 5.66 Multistage counter current leaching9 the ore and is extracted via a second borehole. It is used
flow sheet it is assumed that solid B is insoluble and is regularly in the removal of salts from deposits below the earth's
the
M lost Surface.
in clear solution.
 res Equipments for Leaching
It has been applied to the leaching of low grade cOpper orc F r e s hs a

little 0.21 copper and to ores as 335 mt below Some of the equipments used for leachi re given bel, thirough
Containing as as
ftowsda
the earth surface. Bollman Extractor
a n df u l l

tight vertical
chamber isn whichae
It consists of vapour
a which i
Heap Leaching metais
of perforated baskets are attached to a chain speedin
It is an industrial mining process to extract precious baskets are provided with perforations at the conveyor r a t i oi s

and copper compounds from ore.

is crushed into small

chunks and heaped
on an are conveyed
in the perforated basket down oottheom.fvgn I tconsis

The mixed ore the ore the left in the chamber.

is pumped over up on
t w Os e c

mpervious ground. The leach liquor

This process takes
The baskets
are loaded with flaky solids when i sfillec

collected as it drains from the heap. eachhba

and come at the top. They are sprayed near the tono sprayec

large time.
pyrite ores in this manner
with the dilute
solution of oil in solvent (halt the chwante solvent

e8., Copper has been

in heaps containing
leached
as much as 2.2 x
from
101 of ore, using over
they move downward the solids are leached
d. miscela
The
s oi te x

require 7 years
It maj
to sprayed percolates throughthe solids and collects at section

20000 ms of leach liquor per day. the final

reduce copper content of such heaps from 2 to
0.3%.
of the chamber as strong solution of the 6il a Thesol

removed out of the chamber from the o vapour

ded as and the

Shank System bottom.
which

(Counter Current Multiple Contact The partially extracted solids travel upward and ar t
are lee remov

withdrawn solution is taken further counter currently with a fresh solvent to obta
In counter current scheme final solution of oil at the bottom of chamber
concer

and the fresh solvent is

from contact with the freshest solid
added to solid from which most of
the solute has already been
Half Mircella
leached. Fresh solvent tank
Dry flokes
Infigure a Shank system is shown, it is operated in following tank feed hopper

manner
Assume at the time of inspecting the system
in (a) that fig.
tank 1
it has been operated for some time. Tank 6 is empty,
and tank 1
to 5 are filled with solid, tank 5 most recently
to 5 are also filled with leach
D r y flakes
for longest time. Tanks 1
liquid and the most concentration as in tank 5. Since it is
feed hopper
Hopper
in contact with freshest solid. Fresh solvent just added to
Do

ten 1.
Withdraw the concentration solution from tank 5, transfer
the liquid from tank 4 to 5, 3 to 4, 2 to 3 and from 1 to 2.
Add fresh solid to tank 6. Screw
Refer to Fig. (b) Discard spent solid from tank. Transfer conveyor
the liquid from 5 to 6, 4 to 5, 3 to 4 and from 2 to 3. Add
Rat
fresh solvent to tank 2.
It co
Continue the operation in same manner.
shap
hing
Solu

Fresh
LLig-flow Spent soled Com

solid -6) Basket

vap
Ast
Sump hop
Sump
Cone. und
Sol fror
oil

(a)
A)
(b) Halt muscella
misce
cOD
pu
Fig. 5.68 Shank system cu
Fig. 5.69 Bollman extractor
 uent is sprayed near the top which then
meal. On this side meal moves upward andpercolates
S
At the end of one rotation, the exhausted sceds from each
drougth
t)
solution compartment are dumpedin one of the compartments of lower
rd. When the baskets are at top
lo
fousownward.
they are inverted tank from where they are continuously taken away.
ullyexhaustedsolid is discharged into the

is removed by screw conveyor. hopper from

Usually conveyor
tion per hour and solvent to fresh
one revolutic
meal
Senes
T. The
peedi

patio
i so n e

vertical vessel which is divided into

Consists o f c y l i n d r i c a l

Soid
nt and oons by a start portion. The upper portion of the vessel
Spray Fresh
solvent
Pump
S uith a charge of flaked seeds. A fresh solvent is
over the see with the help of a distributor. The
baske ayed
solvent
percolates through the bed of solids and while doing Last compartment
amber i textracts oil1 seeds and finally it drains in the lower Exhausted seads
la). As section through perforated plate. Full misculla
Seeds Cormpartment
blution in lower ction is continuously boiled to produce
to tank

ottom The
solutio

thesteam heat.The vapours are fed to a condenser Fig. 5.71 Ratocel extractor
and is
9 ondensed liquid is fed to a phase separator from Miscella is not withdrawn from compartment-1 as it may
atthe the
from the bottom and solvent is
is removed
ch water contain suspended solids. These are filtered by passing through
nved from the top and is fed back to the vessel. The is
ached remov the bed of seeds in compartment-2 from where miscella
centrated solutio of the oil is removed from the bottom. removed. Last compartment is of large size to provide
dilute Phase Condenser
Solvent distributor sufficient time for drainage.
separator
Pure
solvent
Dorr Thickener
but
It is simply a device for separating the solids from liquids,
it is in extremely useful in washing of fine solids e.g., leaching
Leached Water t
Vapours of gold from ores.
solids drain

|Particuld Solvent+water vapours Overflow

Perforated L
plate Steam
Downcomes
Shaft
forsol Condensate
Heating Arm
gel
Product Blades
Fig. 5.70 Batch plant for extraction of oil
Discharge cone
Ratocel Extractor with scrapper
divided into sector 5.72 Dorr thickener
Iconsists of shoot, vertical cylindrical tank Fig.
is provided with diameter shallow tank. It is
shaped compartments. Each compartment seeds and drains It consists of a flat bottomed large
the rakes driven from a central
hinged perforated bottoms that supportsabove a stationary provided with slow moving
radial
5olution. This tank slowly rotates thin slurry of liquid and
are enclosed in
shaft for removing the sludge. The
tank and both the tanks centre by the tank through feed
Compartmented
suspended solids in fed at the
below the surface of liquid.
1apour tight housing. well at the depth of 0.3 to 1 m
passes under feeding
a
fill the tank and slowly
AS the tank rotates each compartment seeds and then The solids settle from the liquid which
with the prepared settled sludge towards the discharge
opper where it is filled solvent for leaching. The solvent revolving rakes direct the
of the sludge is taken out. The clarified liquid
deT the series of sprays downward through seeds, extracts conc. from where
the overflow which runs around
Om each spray percolates is continuously removed from
Full then drained into appropriate the solids are continuously
i and
during percolation the top edge of the tank. Thus,
Soelle which it is continuously then inwards to the sludge outlet; the
Ompartment of the lower tank from moving downward and
flow counter
that seeds and solvent and then rapidly drawn.
uped to the next spray so liquid is moving upwards
CuITently