1. R Sadanandam
Solvent Extraction is a branch of hydrometallurgy for separation and purification of various metals.
It involves material transfer between two immiscible phases ( Organic & Aqueous phases).
Distribution ratio D (partition coefficient)
Separation factor β = D1/D2 where D1 > D2

2. Typical examples of SX systems
Agent to transfer metal from aqueous to organic phases called as extractant.
Solvating Extractants – TBP, DBBP, MIBK, Diethyl ether.
Cation exchange extractants – D2EHPA, PC-88A, carboxylic acids, LIX 84I.
Anion exchange extractants – Alamine 336, Aliquot 336.

3. SX Block Diagram Solvent Regeneration Extraction (O/A)1 n1
Scrubbing (O/A)2 n2
Stripping (O/A)3 n3
Scrub solution
pH/ Acidity
Strip solution
Metal concentration
pH/ Acidity
Product solution
Raffinate
Loss of Metals of interest
Loss of solvent
Environmental considerations.
Feed
Metal concentration
pH/ Acidity

4. Extraction – Transfer of metal ion of interest along with other metal impurities from aqueous feed to organic phase
Scrubbing – Stripping of impurity metal ions (organic to aqueous phase)
Stripping – Stripping of metal ion of interest (organic to aqueous phase)

5. Purity and recovery in SX is dependent on -
Composition of aqueous face (feed, scrub and strip solutions) namely concentration of metal ions, salts, complexing agents and redox environment.
Composition of organic phase (solvent, diluent and phase modifier)
pH/ acidity of feed, scrub and strip solutions.
Number of stages in extraction, scrubbing and stripping units.
O/A ratios in the three units.

6. McCabe Thiele's diagram
It gives interrelationship between phase ratio and no. of counter current stages under optimized conditions.
It consist of an equilibrium isotherm and an operating line/ curve and the procedure of arriving at no. of stages.

7. Each of the three units have typical McCabe Thiele's diagrams which are generated from lab data.
Procedure for collecting isotherm data
Isotherm Organic aqueous
Extraction Barren feed
Scrubbing impure loaded scrub solution
Stripping pure loaded strip solution

8. Types of McCabe Thiele's diagrams
Linear
Log – log
Types of equilibrium isotherms
Single metal
Mixed metals
Derivation of equations for operating lines
Extraction
yo.O + xn.A = xr.A + yn.O (Material balance)
yn = (A/O)xn + [y0 - xr.(A/O)]
y = mx+c
Limiting cases
I y0=0 (completely barren organic)
yn = (A/O)xn - xr.(A/O)
When xn = 0, yn= - xr.(A/O)
When yn= 0, xn = xr
Operating line with an intercept of xr.

9. II y0 >0
yn = (A/O)xn + [y0 - xr.(A/O)], when the2nd term becomes zero, we get
yn= (A/O) xn
log yn = log(A/O) + log xn
y0 – xr. (A/O) = 0
xr = y0.(O/A)
Operating line
y0 – xr. (A/O) > 0
Then all the terms in the equation have a role in shaping the operating curve on a log –log plot.
Taking logarithm on boh sides we get,
log yn = log {(A/O) xn +( y0 – xr. (A/O) )} = log{{(A/O) xn + b}
Log – log Mc Cabe Thiele’s plot indicates pinching effect at raffinate end in addition to that at feed side.
Derivation of equations for operating lines for scrubbing/ stripping
yn.O + xs.A = xn.A + ys.O (Material balance)
yn = (A/O)xn + [ys - xs.(A/O)]

10. I xs = 0 (metal free scrub/strip solutions) yn = (A/O)xn + ys
Limiting cases
I xs = 0 (metal free scrub/strip solutions)
yn = (A/O)xn + ys
Operating line with an intercept of ys
II ys = 0 (fully barren organic)
yn = (A/O)xn - xs.(A/O)
This becomes operating line when xs=0 and operating curve (on a log-log plot) when xs>0 as in spent electrolyte solution in SX-EW circuit.
log yn = log {(A/O) xn - xs. (A/O) )}
log yn = log {(A/O) xn-b}

11. Effect of operating parameters in solvent extraction
O/A,y0,xr(expected), xs
Depending on the values of y0 and xr(expected) we get an operating curve and not operating line as usually observed in drawing McCabe Thiele’s diagram.
When SX – EW is adopted in the process flowsheet as in Ni stripping in DOD process, we get operating curve from McCabe Thiele’s approach compared to stripping with an acid solution where the operating line is obtained.
Irrespective of the number of stages in extraction, the raffinate concentration is decided exclusively by y0 and distribution coefficient at the raffinate end, which happens to be maximum and constant as we approached very low organic concentration in the extraction isotherm.
From a study of Kremser’s equation for scrubbing / stripping, it is evident that the concentration of scrubbed / stripped organic depends only on xs
and the distribution coefficient at the scrub / strip inlet end.
Since xr depends on y0 and mr (distribution coefficient at raffinate end), stripping should be done efficiently to achieve acceptable xr.

12. D1 = D2 > D3>D4>D5>D6

13. When McCabe Thiele’s diagrams are used for extraction, scrubbing and stripping for a given feed and levels of purity and recovery, no. of stages in each module get fixed.
An alternate procedure for arriving at the no. of stages in solvent extraction system is use of Kremser’s equations.
Kremser’s equation:
Extraction:
(Xr – Yo /mr) / (Xf-Yo/mr) = (ε –1) / (εn+1 –1)         (When ε ≠1)
     
(Xr – Yo /mr) / (Xf-Yo/mr) = 1 / (n+1)              (When ε =1)
Scrubbing / stripping:
(Ys-Xs.ms) / (Yf-Xs.ms) = [ (1/ ε ) –1 ] / [ (1/ ε )n+1 – 1] ( When ε ≠ 1)   
(Ys-Xs.ms) / (Yf-Xs.ms) = 1 / (n +1)              (When ε =1)
ε = Dgm*(O/A)

14. Xf metal ion concentration in the feed
Xr metal ion concentration in the raffinate that can be tolerated.
Xs metal ion concentration in the scrub/ strip solution
yf metal ion concentration in the
unscrubbed/unstripped organic phase .
ys metal ion concentration in scrubbed/stripped organic phase that can be tolerated.
mr distribution ratio at raffinate end.
ms distribution ratio at scrub/strip inlet end.
ε (Geometric mean of distribution ratios) x O/A

15. Procedure for calculating the Dgm
Dgm = (D1. D2. D3. D4)1/4

16. Iron extraction by Alamine 336 in Cl- medium
Feed: 20.0 g/L Fe3+, 23.2 g/L Co2+, 5 g/L Al3+ in ~1.5 M Cl-
Organic: 15 vol% alamine 336, 5% isodecanol in kerosene.
Isotherm data: with O/A = 2.5/1, a requirement of three stages is shown in McCabe Thiele’s approach.
[Fe3+]aq
[Fe3+]org Kd
9.30
8.50
0.91
2.22
5.78
2.60
0.24
1.58
6.58
0.01
o.17
17.00

17. Calculation of stages by Kremser’s equation
xr / xf = (ε –1) / (εn+1 –1) , Dgm = (k1. k2. k3. k4)1/4
xf = 20.0 g/L, xr = g/L,
Dgm = 4.0, O/A = 2.5/1, ε = 10.0
0.010/20.0 = (10-1)/(10n+1 – 1)
1/2000 = 9 / (10n+1 – 1)
(10n+1 – 1) = 18000
n + 1 = 4.25
n = 3.25

18. Calculation of stages by Kremser’s equation for SACC
Extraction
xf = xf (Fe+Al) = g/L,
xr = xr (Fe+Al) = g/L,
Dgm = 3.03, O/A = 3/1, ε = 9.09,
y0 = 0.05 g/L, m = 15.5
(xr – y0/m)/ (xf – y0/m) = (ε –1) / (εn+1 –1)
Substituting the values of various terms we get n = 4.5
By changing O/A to 4/1 we get n = 4.0 which is adopted in the plant.

19. Stripping: stripping of Fe & Al from D2EHPA / TBP mixture
Dgm = 0.73, O/A = 0.4, 1/ε = 3.42,
yf = 5.8 g/L, ys = 0.05 g/L
(ys-xs.ms) / (yf -xs.ms) = [ (1/ ε ) –1 ] / [ (1/ ε )n+1 – 1]
Xs = 0 because we strip with acid solution.
Calculation shows n = 3.6, a 4 stage mixer – settler unit is used for stripping.

20. Mixed - metal isotherm are used when two or more metals are simultaneously extracted, scrubbed or stripped.
e.g. Fe(III) + Cu(II) + Zn(II) DOD work
Fe(III) + Al(III) cobalt work
Fe(III) + Cu(II) + Al(III) Alnico work
Log-log McCabe Thiele’s diagram is used to give equal weightage to each range of concentration from ppm to g/L. This approach clearly bring out the concept of operating curves.
Achieving purity and recovery in a SX process is dependent on various parameters, use of McCabe Thiele’s approach or Kremser’s equations help us in properly selecting the no. of stages in extraction, scrubbing and stripping section of a SX system.

21. When extraction data is collected for Fe + Al at a given equilibrium pH, the isotherm has a classical shape. But when operation is done in mixer – settler unit there is a pH variation across the stages, hence there is a change in the shape of the extraction isotherm.
When electrowinning unit is coupled with stripping unit of SX system. xs is finite and hence we get operating curve both when plotted on linear graph as well as log – log graph.

22. McCabe Thiele diagram for Extraction of Fe + Al

23. McCabe Thiele’s diagram for Fe + Al extraction in mixer – settler unit for SACC

24. McCabe Thiele's Diagram for Fe+Al Stripping using 2 mol/L HCl

25. FLOWSHEET FOR RECOVERY OF COBALT OXALATE FROM SACC PELLETS
pH adjustment (pH~0.8)
Solvent extraction with D2EHPA & TBP
Loaded D2EHPA
(Fe3+, Al3+)
Raffinate (Ni2+, Co2+)
Ion Exchange Process
Ni2+ free raffinate (Co2+)
Oxidation of Fe2+
Leach liquor
leaching
Oxalic acid
Precipitation, Filtration
Co oxalate powder
Regeneration of Solvent & reuse in SX
SACC pellets H2SO HNO3
Stripping with 2M HCl