Metals and Non-metals

The matter around us is either pure or impure. Elements and compounds are pure substances; mixtures are impure. Of the 118 known elements, most (about 94) are metals, about 18 are non-metals, and only a handful are metalloids. You studied their properties in earlier classes, and in Unit 14 you saw where each class sits in the periodic table.

Only a few metals, the least reactive ones, are found in nature as the free metal: gold, platinum, and sometimes silver and copper. We say they occur in the native (free) state. Reactive metals such as sodium, potassium, calcium and iron have joined with other elements long ago, and are found only as compounds: oxides, sulphides, carbonates and others.

The elements and compounds found naturally in the Earth’s crust are called minerals. Minerals are the sources of metals. A mineral is:

• natural (formed by nature, not made by people),
• inorganic (not made by living things),
• solid and usually crystalline (its particles are arranged in a regular pattern),
• pure, with a fixed chemical formula: hematite is always Fe₂O₃.

A rock is different: it is a mixture of one or more minerals. Granite, for example, is a rock made of the minerals quartz, feldspar and mica. Coal is not a mineral either: it was made from ancient plants, so it is organic. (In everyday and mining language, rocks and fuels like these are counted among the “mineral resources”.)

About 90% of the Earth’s crust is made of silicate minerals (compounds of silicon, oxygen and metals). Besides these, there are sulphide, oxide, carbonate, sulphate and phosphate minerals, and these are where most metals come from.

Rock straight from a mine is mostly worthless material, with the useful mineral scattered through it. Some minerals hold a lot of metal that can be got out easily and cheaply; others hold only a little, or hold it so tightly that getting it out would cost more than the metal is worth.

A mineral from which a metal can be extracted in large amounts, easily and at low cost is called an ore of that metal. Minerals from which the metal can’t be extracted profitably are not ores. So:

All ores are minerals, but not all minerals are ores.

Ores of iron

Ore	Formula	Note
hematite	Fe₂O₃	the chief ore of iron; up to 70% iron
magnetite	Fe₃O₄	magnetic; 72% iron, the richest of all
siderite	FeCO₃	a carbonate
limonite	2Fe₂O₃·3H₂O	hydrated (water-holding) iron oxide; yellow-brown
iron pyrite	FeS₂	“fool’s gold”; not used for iron, because of its sulphur

Hematite is the main ore of iron: it is plentiful and rich in iron. Pure hematite is 70% iron by mass (2 × 56 = 112 parts iron in 160), and magnetite is a little richer, 72%; hematite is the chief ore because there is so much of it.

Ores of aluminium

Ore	Formula
bauxite	Al₂O₃·2H₂O
cryolite	Na₃AlF₆
corundum	Al₂O₃

Bauxite is the chief ore of aluminium; it contains about 40–60% alumina (aluminium oxide, Al₂O₃), so the aluminium metal in it is about half as much. Cryolite is used too, but not as a source of the metal: it is melted with the alumina to lower its melting point, from over 2,000 °C to about 950 °C, in the factory where aluminium is made. Corundum is very hard; red corundum is ruby, and blue is sapphire.

Ores of copper

Ore	Formula
chalcocite (copper glance)	Cu₂S
chalcopyrite (copper pyrite)	CuFeS₂
cuprite	Cu₂O
malachite	Cu₂(OH)₂CO₃

Chalcopyrite (copper pyrite) is the main ore of copper; it contains about 34.5% copper.

Ores of silver

Ore	Formula
argentite (silver glance)	Ag₂S
horn silver	AgCl
ruby silver	Ag₃SbS₃
silver copper glance	(Ag,Cu)₂S

Argentite is the chief ore of silver; it contains about 87% silver.

Gold

Gold is so unreactive that it is found in the native (pure) state: as grains and nuggets in veins of quartz rock, and as specks in the alluvial sand and gravel that rivers wash down from those rocks. Getting gold is mostly a matter of separating it from rock and sand, not of breaking a compound.

To get a metal, its ore is first dug out of the ground. Taking useful substances and minerals out of the Earth’s crust is called mining: coal, petroleum, gold and the ores of metals are all mined. Then the metal is extracted from its ore in a series of steps.

Metallurgy is the science of the properties, production and purification of metals, including the step-by-step extraction of metals from their ores. Generally, these are the main steps:

Grinding

This is the first step. The big lumps of ore are crushed into small pieces, then ground into a powder between the rollers or balls of crushing machines. This is grinding (also called crushing or pulverization). A powder has a huge surface area, which makes every later step faster (you saw why in Unit 15).

1. Concentration

The ground ore is still mixed with mud, sand, rock and other unwanted material, called gangue (say “gang”). Removing the gangue, so that the percentage of metal in the ore goes up, is concentration, also called ore dressing. The method depends on how the ore and the gangue differ:

• Gravity (hydraulic) separation, when the ore is heavier than the gangue. The powder is washed with a stream of water: the light sand and clay are carried away, and the heavy ore particles settle. This is how hematite and tin ore are concentrated, and how a gold panner works.
• Magnetic separation, when one of them is magnetic. The powder falls past a magnetic roller: magnetic particles (like magnetite) stick to it and drop off on one side, non-magnetic ones fall straight down.
• Froth flotation, when one of them is wetted by water (hydrophilic, “water-loving”) and the other is not (hydrophobic, “water-fearing”). The powder is stirred into water with a little pine oil, and air is blown through. Sulphide ore particles, which are not wetted by water, stick to the oily bubbles and float up in a froth, which is skimmed off; the sand and rock, wetted by water, sink.

2. Oxidation: roasting and calcination

It is easiest to get a metal out of its oxide, so a concentrated ore that is not already an oxide is first changed into one. There are two ways.

a. Roasting

The ore is heated strongly in a supply of air (oxygen), below its melting point. Roasting is used for sulphide ores: the sulphur burns off as sulphur dioxide. Zinc blende (zinc sulphide, ZnS) becomes zinc oxide:
$$\text{2ZnS} + \text{3O}_2 \xrightarrow{\Delta} \text{2ZnO} + \text{2SO}_2$$

b. Calcination

The ore is heated strongly without air (or with very little). Calcination is used for carbonate ores (and to drive the water out of hydrated ores like bauxite): carbon dioxide is driven off. Calamine (zinc carbonate) and limestone:
$$\text{ZnCO}_3 \xrightarrow{\Delta} \text{ZnO} + \text{CO}_2 \qquad \text{CaCO}_3 \xrightarrow{\Delta} \text{CaO} + \text{CO}_2$$

3. Reduction

Reduction is the removal of oxygen from the metal oxide. The oxide is heated with a reducing agent, a substance that takes the oxygen for itself, such as carbon (coke), carbon monoxide or hydrogen:
$$\text{CuO} + \text{H}_2 \xrightarrow{\Delta} \text{Cu} + \text{H}_2\text{O} \qquad \text{PbO} + \text{C} \xrightarrow{\Delta} \text{Pb} + \text{CO}$$
$$\text{Fe}_2\text{O}_3 + \text{3CO} \xrightarrow{\Delta} \text{2Fe} + \text{3CO}_2$$

How a metal is reduced depends on how tightly it holds on to oxygen, which follows the reactivity series you met in Unit 15:

Metal	How its oxide is reduced
K, Na, Ca, Mg, Al (very reactive)	their oxides are very stable: only electrolysis (passing electricity through the molten compound) works
Zn, Fe, Pb, Cu (middle)	heating with carbon, carbon monoxide or hydrogen; zinc oxide only with carbon: ZnO + C → Zn + CO
Hg, Ag (unreactive)	their oxides are unstable: heat alone breaks them, 2HgO → 2Hg + O₂
Au, Pt	found native; no reduction needed

Smelting

Usually the reduction is done hot enough to melt the metal. Heating an ore above the metal’s melting point with a reducing agent, so that the molten metal separates from the remaining impurities, which leave as gases or float off as a glassy waste called slag, is smelting. Smelting is a kind of reduction.

4. Refining

The metal from reduction is still impure, so it is refined to make it pure. There are several methods; two are:

a. Distillation

The impure metal is heated until it boils; its vapor is cooled and condensed back into pure metal, leaving the impurities behind. This works for metals with low boiling points, such as mercury (357 °C) and zinc (907 °C).

b. Electro-refining

Electro-refining uses electrolysis to purify metals such as copper, silver and gold. A block of the impure metal is made the anode (connected to the positive terminal of the battery), and a thin sheet of pure metal is made the cathode (the negative terminal). Both hang in a solution of a salt of the same metal, the electrolyte, in a vessel called a voltameter.

When the current flows, metal atoms of the anode lose electrons and go into the solution as ions; at the cathode, ions from the solution gain electrons and are deposited as pure metal. For copper:
$$\text{at the anode: } \text{Cu} \rightarrow \text{Cu}^{2+} + \text{2e}^- \qquad \text{at the cathode: } \text{Cu}^{2+} + \text{2e}^- \rightarrow \text{Cu}$$
The anode slowly wears away and the cathode grows. The impurities either stay dissolved or fall below the anode as anode mud.

1. Choose the correct option

2. Give reasons

3. Write the differences between

4. Answer these questions