Occurrence of Metals

Metals almost never sit around waiting to be picked up in their pure, shiny form. Only a handful of unreactive elements — carbon, sulphur, gold, and the noble gases — are found "free" in nature. Everything else is locked up in compounds inside the earth's crust, and the first job of metallurgy is figuring out which compound to dig for.

Abundance sets the stage

Metals differ enormously in how common they are:

• Aluminium is the most abundant metal in the crust, and in fact the third most abundant element overall — roughly 8.3% by weight. It shows up as a major component of igneous minerals such as mica and clays. Even gemstones borrow it: ruby and sapphire are really impure $Al_2O_3$, coloured by traces of Cr and Co respectively.
• Iron is the second most abundant metal. Its chemistry is unusually versatile, which is why it ends up in so many compounds and so many everyday uses — and biology depends on it too (haemoglobin being the obvious example).

From mineral to ore

A mineral is simply a naturally occurring chemical substance found in the crust and won by mining. Any given metal is usually locked up in several different minerals, but only a few of them are actually worth mining — rich enough, or accessible enough, to be commercially sensible. A mineral that clears that bar is called an ore. So every ore is a mineral, but not every mineral is an ore.

Table 6.1 lists the principal ores of the four metals this chapter keeps coming back to:

Why one ore is chosen over another

Having several candidate ores for the same metal means a choice has to be made, and that choice is driven by what's chemically feasible and commercially viable:

• Aluminium is extracted almost exclusively from bauxite — it's abundant and the extraction route built around it (Section 6.2.4) works cleanly.
• Iron is usually pulled from its oxide ores. They're plentiful, and — just as importantly — they don't release polluting gases the way iron pyrites does when it's roasted (roasting $FeS_2$ generates $SO_2$).
• Copper and zinc can be sourced from any of the ores in the table above, and the choice comes down to local availability and cost.

Metallurgy as an overall process

The complete scientific and industrial sequence used to pull a metal out of its ore is called metallurgy. It draws on several branches of chemistry at once, but almost every metal's extraction follows the same broad script:

1. Concentration of the ore — stripping away the unwanted rocky material.
2. Isolation of the metal from the concentrated ore (usually a reduction step).
3. Purification (refining) of the isolated metal.

An ore is essentially never pure — it comes mixed with earthy, commercially worthless material called gangue, and separating gangue from the valuable mineral is where the story begins.

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The sections that follow walk through this script in order: first the physical tricks used to concentrate an ore, then the thermodynamic and electrochemical reasoning behind converting the concentrate into metal.