Occurrence and Extraction of Metals

In this lesson you will

• Define mineral, ore and gangue, and explain the need for concentration/enrichment.
• Group metals into low, medium and high reactivity and state how each group is typically extracted.
• Distinguish roasting from calcination and explain why sulphide/carbonate ores are converted to oxides before reduction.
• Write the equation for the thermit reaction and state a practical use.
• Explain electrolytic reduction for highly reactive metals and electrolytic refining for purifying an extracted metal.

Where metals come from

The earth’s crust is the main source of metals; seawater also holds dissolved salts such as sodium chloride and magnesium chloride. Elements and compounds that occur naturally in the crust are called minerals; wherever a mineral is concentrated enough to extract its metal profitably, it is called an ore.

Extraction follows the reactivity series

Knowing where a metal sits in the reactivity series tells you how it must be extracted:

• High reactivity (K, Na, Ca, Mg, Al) — never found free; extracted by electrolysis of the molten compound.
• Medium reactivity (Zn, Fe, Pb, Cu) — found as oxides, sulphides or carbonates; extracted by reduction with carbon after converting the ore to an oxide.
• Low reactivity (Ag, Au, and often Cu) — unreactive enough to occur as free elements.

Before any of this, mined ore is contaminated with soil, sand and other impurities called gangue, which must be removed (concentration/enrichment) using differences in the physical or chemical properties of the ore and the gangue.

Extracting metals low in the series

Very unreactive metal oxides can be reduced by heat alone. Cinnabar (HgS), the ore of mercury, is first heated in air to mercuric oxide, which further heating breaks down straight to mercury metal. Copper found as Cu2S can likewise be obtained just by heating in air.

Extracting metals in the middle of the series

Moderately reactive metals are usually found as sulphides or carbonates, which are harder to reduce directly than an oxide. So the ore is first converted to an oxide: a sulphide ore by roasting (heating strongly in excess air), a carbonate ore by calcination (heating strongly with limited air). The resulting oxide is then reduced to the metal using a suitable reducing agent such as carbon.

Sometimes a reactive metal is used as the reducing agent instead of carbon, in a highly exothermic displacement. The industrially important example is the thermit reaction: iron(III) oxide reacts with aluminium to give molten iron and aluminium oxide, releasing so much heat that the iron comes out liquid — used to weld railway tracks and cracked machine parts together on-site.

Extracting metals at the top of the series

The most reactive metals — sodium, magnesium, calcium, aluminium — have more affinity for oxygen than carbon does, so carbon simply cannot reduce their oxides. These metals are obtained instead by electrolytic reduction of their molten compounds: the metal is deposited at the cathode (e.g. Na⁺ + e⁻ → Na) while chlorine is liberated at the anode (2Cl⁻ → Cl2 + 2e⁻). Aluminium is obtained the same way, by electrolytic reduction of molten aluminium oxide.

Refining the extracted metal

Metal produced by any of these reduction routes is not yet pure. The most common purification method is electrolytic refining: the impure metal becomes the anode, a thin strip of pure metal becomes the cathode, and a solution of a salt of that metal is the electrolyte. As current passes, pure metal dissolves off the anode and an equivalent amount deposits on the cathode. Soluble impurities pass into the electrolyte; insoluble impurities fall to the bottom under the anode as anode mud.

Key terms

Mineral

An element or compound that occurs naturally in the earth’s crust.

Ore

A mineral that contains a high enough percentage of a particular metal that the metal can be profitably extracted from it.

Gangue

The unwanted impurities (soil, sand, and similar material) mixed with an ore as mined, which must be removed before extraction.

Roasting

Heating a sulphide ore strongly in excess air to convert it into the metal oxide.

Calcination

Heating a carbonate ore strongly in limited air to convert it into the metal oxide.

Key relationships

• Metals at the top of the reactivity series (K, Na, Ca, Mg, Al) are never found free in nature and are extracted by electrolysis of their molten compounds.
• Metals in the middle (Zn, Fe, Pb, Cu) occur as oxides, sulphides or carbonates and are extracted by reducing their oxide with carbon.
• Metals at the bottom (Ag, Au, and often Cu and Pt) are unreactive enough to occur as free elements in nature.
• Sulphide ores are converted to oxides by roasting; carbonate ores are converted to oxides by calcination.
• Very reactive metals (Al, Ca, Na) can themselves reduce the oxide of a less reactive metal in a highly exothermic displacement — the thermit reaction is the industrial example.
• In electrolytic refining, the impure metal is the anode, a thin strip of pure metal is the cathode, and insoluble impurities settle out as anode mud.

Formulae

What you should be able to do

Recognise

• Whether a described extraction step is roasting, calcination, carbon reduction, or electrolytic reduction.
• The thermit reaction from its reactants/products or its railway-track use.

Explain

• Why carbonate and sulphide ores are converted to oxides before reduction.
• Why carbon cannot be used to extract sodium, magnesium, calcium or aluminium.
• What anode mud is and why it collects only insoluble impurities.

State

• The definitions of mineral, ore and gangue.
• Two metals found in nature in the free (native) state.
• The three reactivity-based extraction groups and the extraction method for each.

NCERT alignment

This topic corresponds to NCERT Class 10 Science, Chapter “Metals and Non-metals”, sections 3.4, 3.4.1, 3.4.2, 3.4.3, 3.4.4, 3.4.5, 3.4.6. Explanations, visuals and worked material here are original.

Quick revision

• Mineral = naturally occurring element/compound; Ore = a mineral rich enough to extract the metal profitably; Gangue = the impurities that must be removed first.
• High reactivity (K,Na,Ca,Mg,Al) → electrolysis; medium (Zn,Fe,Pb,Cu) → reduction with carbon after roasting/calcination; low (Ag,Au) → found native.
• Roasting = sulphide ore + excess air + heat → oxide. Calcination = carbonate ore + limited air + heat → oxide.
• Thermit reaction: Fe2O3 + 2Al → 2Fe(molten) + Al2O3 + heat — used to join railway tracks.
• Highly reactive metals extracted by electrolytic reduction of the molten compound (metal at cathode, non-metal at anode).
• Electrolytic refining: impure metal = anode, pure metal strip = cathode, metal-salt solution = electrolyte, insoluble impurities = anode mud.