General Principles of Isolation of Elements (Class 12)

The four stages

1. Concentration — removing gangue from the ore.
2. Conversion to oxide — because oxides are easiest to reduce.
3. Reduction — obtaining the crude metal.
4. Refining — purifying it.

Concentration methods, and what each exploits

Froth flotation deserves attention because it carries the most detail. Sulphide ore particles are preferentially wetted by oil and rise with the froth, while gangue is wetted by water and sinks. Collectors make the ore surface more oil-attracting, froth stabilisers keep the froth intact, and depressants selectively prevent one sulphide from floating so two sulphides can be separated. Depressant questions are common.

Calcination and roasting

Reduction, and how thermodynamics decides the method

The Ellingham diagram plots the standard free energy of formation of oxides against temperature. Its use rests on one principle: a metal whose oxide line lies lower can reduce the oxide of any metal whose line lies higher, because the overall free energy change is then negative.

Three features of the diagram are examined:

• Most lines slope upward, because forming a solid oxide from a solid metal and gaseous oxygen reduces entropy, and ΔG = ΔH − TΔS becomes less negative as temperature rises.
• The carbon monoxide line slopes downward, because that reaction increases the number of gas molecules, so ΔS is positive. This is why carbon becomes a better reducing agent at high temperature and eventually crosses below other lines.
• A sharp change in slope marks a phase change of the metal or its oxide.

The downward carbon monoxide line is the single most useful fact on the diagram, since it explains why coke reduction works industrially for so many metals despite failing at lower temperatures.

Which method for which metal

Highly reactive metals require electrolysis precisely because their oxide lines lie so low that nothing else can get beneath them — a good way to phrase the answer when asked why.

Refining

• Distillation for low-boiling metals.
• Liquation for a metal that melts below the impurities.
• Electrolytic refining — impure metal as anode, pure as cathode. Impurities either dissolve into solution or collect below the anode as anode mud, which is where valuable byproducts are recovered.
• Zone refining — based on impurities being more soluble in the molten than the solid phase, so a moving molten zone sweeps them to one end. Used where extreme purity is needed.
• Vapour phase refining — the metal is converted to a volatile compound which is then decomposed, giving very pure metal.

Zone refining and vapour phase refining are the two most frequently asked, and both are answered by naming the principle rather than describing apparatus.

Frequently asked questions

Why are sulphide ores roasted rather than reduced directly?

Because oxides are much easier to reduce than sulphides. Roasting converts the sulphide to the oxide first, making the subsequent reduction feasible.

Why does the carbon monoxide line slope downward?

Because the reaction produces more moles of gas than it consumes, so ΔS is positive and the −TΔS term makes ΔG more negative as temperature rises.

What is a depressant for?

To prevent one sulphide from floating during froth flotation so that two sulphide ores present together can be separated.

Why is electrolysis needed for reactive metals?

Their oxides are so stable that no available chemical reducing agent can supply the necessary free energy change. Electrolysis supplies the energy electrically instead.

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