Metallurgy: One Mark Questions, Book Back Answers and Notes

About Metallurgy

Medium ~120 min study

Very few metals are found free in nature. Most occur as compounds mixed with sand, clay and rock, so getting the metal out takes a series of physical and chemical steps. This unit takes you through those steps in the same order a plant would use them, starting with how to decide which mineral is worth working as an ore.

The chemistry behind the steps comes from thermodynamics and electrochemistry. The Ellingham diagram is the main tool here. It shows how the free energy of formation of a metal oxide changes with temperature, and from it you can read off whether carbon, carbon monoxide or another metal will reduce a given oxide, and at what temperature. Where no chemical reducing agent is good enough, electrolysis takes over.

Exam questions from this unit usually ask you to justify a choice. Why is froth flotation used for a sulphide ore? Why is a sulphide roasted before reduction? Why does zone refining work? If you can give the reason behind each process, not just its name, the MCQs and short answers become much easier.

What you'll learn

• Name the common ores of the metals discussed in the textbook.
• Choose a concentration method from the physical properties of the ore and gangue.
• Use the Ellingham diagram to pick a reducing agent for a metal oxide.
• Write the reactions that take place during roasting and calcination.
• Describe the electrolytic refining of copper, naming the anode, cathode and electrolyte.
• Explain why zone refining is used for semiconductor-grade germanium and silicon.

Before you start

• The periodic table and the reactivity series of metals.
• Writing chemical formulas and balancing simple redox equations.
• Basic physical separation methods such as filtration and distillation.

Metallurgy explained

How a Metal Is Extracted, Step by Step

Minerals and Ores

Most metals are too reactive to stay uncombined, so they occur in the earth's crust as minerals. A mineral is any naturally occurring substance that contains the metal. An ore is a mineral from which the metal can be extracted easily and profitably. Bauxite is an ore of aluminium; clay also contains aluminium but is not used as an ore. So every ore is a mineral, but not every mineral is an ore.

Concentration of the Ore

The unwanted earthy material in an ore is called gangue, and it has to be removed first. The method depends on how the ore and gangue differ. Gravity separation uses a difference in density, froth flotation works for sulphide ores, magnetic separation works when one component is magnetic, and leaching dissolves the ore in a suitable reagent while the gangue stays behind.

Conversion to the Oxide

Oxides are easier to reduce than sulphides or carbonates, so the concentrated ore is usually turned into an oxide. Roasting means heating the ore in excess air; it converts sulphides to oxides and drives off volatile impurities as oxides such as SO₂. Calcination means heating in the absence of air or in a limited supply of air; it decomposes carbonates and hydrated oxides, for example ZnCO₃ to ZnO and CO₂.

Reduction to Crude Metal

The oxide is then reduced. Carbon and carbon monoxide are the usual reducing agents. A more reactive metal can also be used, as in the aluminothermic process, where aluminium reduces Cr₂O₃ to chromium. Which agent works depends on how stable the metal oxide is.

The Ellingham Diagram

The Ellingham diagram plots the standard free energy change of formation of metal oxides against temperature. A reduction is feasible when the overall free energy change is negative. On the diagram, a metal whose oxide line lies lower can reduce the oxide of a metal whose line lies above it at that temperature. This is how the choice of reducing agent and furnace temperature is justified.

Electrochemical Extraction

Metals such as sodium and aluminium form very stable compounds that carbon cannot reduce at a practical temperature. These are extracted by electrolysis of a molten compound, and the metal is deposited at the cathode. Aluminium, for instance, is obtained by electrolysing alumina dissolved in molten cryolite. The electrode potentials of the species present decide what is discharged.

Refining the Crude Metal

Crude metal still carries impurities. Distillation suits volatile metals like zinc and mercury. Liquation suits low-melting metals like tin. In electrolytic refining the impure metal is the anode and pure metal collects on the cathode, as with copper. Zone refining gives very pure germanium and silicon, and vapour phase refining, such as the Mond process for nickel and the van Arkel method for zirconium and titanium, also gives high purity.

Uses of the Metals

The unit ends with the uses of common metals. Iron and aluminium are used in construction and transport, copper in electrical wiring because it conducts well, zinc for galvanising iron, and gold in jewellery and electronics. Each use follows from a property of the metal.

Common mistakes to avoid

• Mixing up roasting and calcination. Roasting uses excess air and is for sulphide ores; calcination uses little or no air and is for carbonates and hydrated oxides.
• Calling every mineral an ore. A mineral becomes an ore only when extraction from it is economical.
• Reading the Ellingham diagram the wrong way round. The metal whose oxide line is lower is the one that can act as the reducing agent.
• Forgetting the flux in smelting. Without it the gangue does not form slag and cannot be separated from the molten metal.
• Suggesting carbon reduction for sodium or aluminium. Their oxides are too stable, so they are extracted by electrolysis.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

Why is froth flotation used for sulphide ores?

Sulphide ore particles are wetted by oils such as pine oil, while the gangue is wetted by water. When air is blown through a mixture of powdered ore, water and oil, the sulphide particles stick to the air bubbles and rise with the froth. The gangue sinks, so the two are separated.

What does a flux do in metallurgy?

A flux is added during smelting to combine with the gangue left in the ore. The product is slag, a fusible material that floats on the molten metal and can be removed. An acidic flux such as silica removes basic impurities like FeO, and a basic flux such as lime removes acidic impurities.

How does the Ellingham diagram help in choosing a reducing agent?

It shows how the free energy of formation of each oxide changes with temperature. At a given temperature, a metal whose oxide line lies below another line can reduce that higher oxide, because the overall free energy change is negative. You can also read off the temperature above which carbon becomes a good reducing agent for a particular oxide.

When is leaching used instead of a physical method?

Leaching is used when the ore dissolves in a particular reagent and the impurities do not. It suits low-grade ores and metals present in very small amounts. Gold and silver, for example, are leached with dilute sodium cyanide solution in the presence of air.

What is the principle of zone refining?

Impurities are more soluble in the molten metal than in the solid. A circular heater is moved slowly along a rod of impure metal. As the molten zone moves, pure metal crystallises behind it and the impurities travel with the melt to one end, which is then cut off.

Why is aluminium extracted by electrolysis?

Aluminium oxide is very stable, so carbon cannot reduce it at a temperature that is practical in industry. Electrolysis supplies the energy needed to reduce Al3+ ions, and aluminium is deposited at the cathode. Sodium is obtained by electrolysis for the same reason.

What is the difference between an ore and a mineral?

A mineral is any naturally occurring substance that contains a metal, free or combined. An ore is a mineral from which the metal can be extracted conveniently and at a profit. All ores are minerals, but not all minerals are ores.