Pull one compound out of a liquid mixture using a second liquid. Learn it by watching the ternary diagram get drawn, dot by dot, line by line.
The idea
Why extraction exists
Distillation splits things by boiling point. When boiling point stops working, you switch to a solvent that grabs the compound you want. That switch is extraction.
Three times boiling point fails
Azeotrope
The vapour and liquid reach the same composition. Distillation cannot cross that point, no matter how tall the column.
Close boilers
When two compounds boil within a few degrees, the column needs huge height and heat. Often not worth it.
Heat-sensitive
Antibiotics, vitamins, proteins break down when boiled. Extraction works cold, so the product survives.
The one-line definitionAdd a solvent that likes your target compound. It pulls the compound out of the original liquid into a new liquid layer. Then you separate the two layers by density. No boiling required.
The three players and the two products
A = solute (what you want, e.g. acetone)C = carrier (holds the solute, e.g. water)S = solvent (the grabber, e.g. TCE)
Feed (A in C) meets solvent S, they mix, then settle into two layers. The solvent-rich layer that leaves with the solute is the extract. The leftover carrier layer is the raffinate.
The idea
Reading the ternary triangle
Three compounds, one chart. Every point inside the triangle is a mixture of A, C and S. This is the map every extraction calculation is drawn on.
binodal (solubility) curvetie lineplait point
two ways to draw the same thing
Equilateral vs right triangle
The equilateral triangle is the true picture: all three components share the chart equally. The right triangle holds the exact same curve but is easier to plot on graph paper, so the worked examples use it. Flip the switch above to compare.
how to read a corner
Each corner is 100% of one thing
One corner is pure carrier C, one is pure solute A, one is pure solvent S. Slide toward a corner, you get more of it. On the edge opposite a corner, that component is zero.
the dome + where it closes
Inside the curve it splits
Outside the binodal curve is one clear liquid. Inside, it separates into two layers at the ends of a tie line. Where those ends meet is the plait point.
Two shapes you will meet
Type I: only one pair of compounds will not mix (C and S). One dome. This is your acetone case.
Type II: two pairs will not mix. The two-phase band stretches across. Fewer worries about the plait point.
The only two rules you need
Rule 1 · mixing rule
Mix two streams, land on the line
Blend feed F and solvent S and the mixture M sits on the straight line joining them. More solvent slides M toward S.
Rule 2 · lever-arm rule
The line is a see-saw
M balances on the line like a see-saw. The side that is shorter carries more mass. That gives the amounts of extract and raffinate.
Picking the solvent
A good solvent scores well on all of these. The first two matter most.
1 · Selectivity
Grabs the solute A, leaves the carrier C behind. This is the whole point.
2 · Low mutual solubility
Barely mixes with the carrier, so the two layers stay clean and separate.
3 · Recoverable
Easy to boil off the solute afterwards, and it forms no azeotrope with A.
4 · Density gap
Different enough in density from the carrier that the layers settle quickly.
5 · Interfacial tension
Enough that drops coalesce and separate, not so much that mixing is hard.
6 · Safe and cheap
Low toxicity, not too flammable, low cost, and gentle on the equipment.
Where azeotrope comes backPoint 3 is the azeotrope link. You recover the solvent by distilling the extract. If the solvent formed an azeotrope with the solute, that final distillation would fail too, and you would be stuck. So recoverability rules out any solvent that azeotropes with A.
Play with it
The Ternary Lab
Drag the orange dot anywhere in the triangle. Read its make-up live. Drop it inside the dome and the tie line snaps in, showing the two layers it splits into.
your point Mextract Eraffinate Rbinodaltie line
Drag the orange dot
A · solute0.17
C · carrier0.17
S · solvent0.67
Two phases: it splits
Extract E (A, S)0.18, 0.80
Raffinate R (A, S)0.09, 0.01
Extract share83%
Jump to:
C is whatever is left over, because A + C + S = 1. Read A along the bottom, S up the left side.
What the tie line is telling youWhen your point lands inside the dome, the mixture cannot stay as one liquid. It separates into the two layers sitting at the ends of the tie line: the extract E (solvent-rich, up high) and the raffinate R (carrier-rich, down low). The see-saw split tells you how much of each you get.
Design · Example 1
Single stage, drawn step by step
One mixer, one settler. Watch each point land on the triangle and read what it means before the next one appears.
ProblemFeed is 100 kg of 50% acetone in water. Add 200 kg of trichloroethane (TCE) solvent, mix once, let it settle. Find the two layers and the percent of acetone pulled out.
F feedS solventM mixE extractR raffinate
Answer
Design · Example 2
Cross-current: fresh solvent twice
Take the leftover raffinate and wash it again with new solvent. The same total solvent, split into two doses, pulls out far more.
ProblemSame feed: 100 kg, 50% acetone. Stage 1 gets 100 kg fresh TCE. Its raffinate goes to stage 2, which gets another 100 kg fresh TCE. Find the overall percent extracted, and compare it to using all 200 kg at once.
feed to a stagemix Mextractraffinate
Answer
Design · Example 3 · Hunter-Nash
Counter-current staircase
Feed and solvent enter from opposite ends and pass each other. This is the method students find hardest, so it is built one line at a time here.
ProblemFeed 100 kg, 30% acetone. Solvent 40 kg TCE enters from the far end. Leave only 4% acetone in the final raffinate. How many stages are needed?
A naming noteYour slides call this the Ponchon-Savarit method. In separations textbooks the counter-current triangle construction is the Hunter-Nash method. Same picture, correct name, so it matches the textbook and any search you run.
extract Enraffinate Rntie line (a stage)operating line to P
Answer
Play with it
The live solver
Move the sliders and the whole diagram redraws. Good for checking homework and for building a feel for what more solvent, or a leaner target, actually does.
0.50
200
0.04
Mixing point sits inside the dome
Try thisIn single-stage mode, pull the solvent slider down. Watch the mixing point M slide toward the feed and drop out of the dome: below a certain amount of solvent there is too little solvent to make two layers. That threshold is the minimum solvent. In counter-current mode, lower the target and watch the staircase add steps.
In the real world
Where extraction actually runs
This is not only a textbook exercise. The same triangle and tie lines run inside refineries, mines, pharma plants and water-treatment units every day.
Aromatics from fuel
Refinery scale · sulfolane
A: benzene, toluene, xyleneC: other hydrocarbonsS: sulfolane
Reformate is a soup of aromatics and non-aromatics of nearly the same boiling point. Sulfolane selectively dissolves the aromatics, so they can be lifted out as a clean stream for plastics and fuel blending.
Why not distillation: the aromatics and non-aromatics boil within a degree or two and form azeotropes, so a column cannot split them.
Copper from ore
About a quarter of world copper
A: copper ionsC: acid leach waterS: oxime in kerosene
Low-grade ore is leached with acid into a weak, dirty copper solution. An oxime reagent (the LIX family) in kerosene grabs only the copper, concentrates it, and hands it to electrowinning to make pure copper sheet. This is the SX-EW route.
Why extraction: the leach liquor is far too dilute and impure to treat directly. Extraction both purifies and concentrates the copper.
Nuclear fuel recycle
PUREX · tributyl phosphate
A: uranium, plutoniumC: nitric acid solutionS: TBP in kerosene
Spent fuel is dissolved in nitric acid. Tributyl phosphate in kerosene selectively pulls out the uranium and plutonium, leaving the radioactive fission-product waste behind in the water. This is the PUREX process.
Why extraction: boiling intensely radioactive material is out of the question. Extraction does the separation gently and remotely.
Penicillin recovery
Pharmaceutical · butyl acetate
A: penicillinC: fermentation brothS: butyl or amyl acetate
Penicillin is made by mould in a water broth. Drop the pH and the penicillin moves into butyl acetate. Raise the pH later and it moves back into clean water. A pH switch drives it in and out of the solvent.
Why not distillation: penicillin falls apart when heated, so it can never be boiled. Extraction is done cold and fast.
Acetic acid recovery
Chemical plants · ester solvent
A: acetic acidC: waterS: ethyl acetate
Many processes leave acetic acid dissolved in a lot of water. An ester solvent pulls the acid out of the water, and the acid is then recovered from the much smaller solvent stream.
Why not distillation: boiling water away from dilute acetic acid wastes enormous energy because the two are close-boiling. Extraction first is far cheaper.
Cleaning phenol from water
Effluent treatment
A: phenolC: wastewaterS: MIBK or ether
Coke-oven and refinery water carries dissolved phenol, which is both a pollutant and worth money. A solvent lifts the phenol out, cleaning the water and recovering the phenol at the same time.
Why extraction: the phenol is too dilute to distil economically, and the water must be cleaned before discharge.
The pattern behind all sixEvery case is the same shape: a compound you want is trapped in a liquid you cannot easily boil, so a chosen solvent lifts it out. Selectivity and recoverability decide the solvent, exactly as in the theory tab.
Check yourself
Practice, quiz & formula sheet
Try the traps that catch people in exams. Reveal each answer only after you have committed to one.
Warm-up problems
Practice 1 · read the split
A mixture sits inside the dome. Its tie line ends at E (the extract) and R (the raffinate), and the mixing point M is much closer to E than to R. Which layer do you get more of?
More extractThe amount of a layer is set by the arm on the opposite side of M. The amount of extract goes with length RM, and the amount of raffinate goes with length ME. M sitting close to E makes ME short and RM long, so you get more extract. Simple rule to remember: M sits nearer the layer you get more of.
Practice 2 · stages
In a counter-current design you draw the staircase and count 4 tie lines before reaching the target raffinate. How many equilibrium stages is that?
4 stagesEach tie line is one equilibrium stage. The number of stages is simply the number of tie lines you step off. Operating lines just carry you from one stage to the next; they are not counted.
Quick quiz
One-page formula sheet
Lever-arm rule (amounts)
amount of Eamount of R = length RMlength ME
M is the mix of the two layers. Shorter arm carries more mass.
Mixing rule (overall balance)
F + S = M = E + R
Feed plus solvent equals the mix, which settles into extract plus raffinate.
Component balance (on solute A)
F·xF + S·yS = E·yE + R·xR
Distribution coefficient & selectivity
KA = yA in extractxA in raffinate·β = KAKC
K says how strongly the solvent pulls A. Selectivity β compares its pull on A versus the carrier C. Bigger β is better.
Counter-current difference point
F − E1 = RN − S = P
Every operating line passes through the one difference point P. Stages = number of tie lines.
