Where does metal come from?

Australia is enjoying a mining boom. The economies of China and India have a voracious appetite for many of Australia’s metals. Australia has developed a reputation for innovation in both mining and metallurgy.

 

Metallurgy is the art and science of making metals and alloys into forms suitable for practical use. Some metals are found in their native or free elemental form (eg, copper, silver, gold), and a limited number can be produced from seawater, notably magnesium.

 

However, most metals are extracted from naturally occurring mineral compounds found at or near the earth’s surface through a process called mining. In Australia most nonferrous metals (eg, copper, zinc, nickel, lead) are produced by processing sulphide ores.

 

The steps in extracting metals from their ores are:

 

1. mineral processing, which processes ore into concentrates and tailings (wastes);

 

2. chemical extraction, which separates concentrates into metals and slag and leach residues;

 

3. and refining, which produces refined metal and by-products.

 

Mineral Processing

 

Mineral processing involves the chemical and physical treatment of the ore to upgrade the valuable mineral component before the metal-extraction stage. This processing eliminates as much of the waste portion as possible. Two principal steps are involved: comminution and separation.

 

Mineral Comminution

 

Comminution is liberation of the desired mineral (eg, nickel sulphide, copper sulphide) from the waste by crushing and grinding. The ore is crushed, by special machines called jaw crushers and cone crushers. The crushed ore is then ground in fine grinding mills to particle sizes less than 75 micro metres.

 

Grinding is usually carried out in water to minimize dust and improve recovery. The degree of fine grinding depends on the size and distribution of the valuable mineral in the ore. There are many different types of fine grinding equipment on the market.

 

Separation

 

Separation of the desired mineral from the gangue produces a concentrate and a discard portion (often called tailings). Separation is performed by taking advantage of differences in the physical and chemical properties of the minerals. These physical properties include,: weight, density, radioactivity, magnetism, electrical conductivity and surface reactivity.

 

The most commonly used property, surface reactivity, has proven to be very selective, particularly for sulphide ores. The separation process based on this property is called flotation.

 

In flotation, air is bubbled through the pulp (a mixture of finely ground ore and water) in a tank which is agitated vigorously to break the air into fine bubbles. The valuable mineral is picked up by the bubble and floated to the surface to form a stable froth, which is skimmed off.

 

The concentrates produced through mineral processing are drastically reduced in bulk and weight compared to the original ore. For example, 100 kg of copper ore containing as little as 1% metal in the form of a sulphide can be upgraded to 4 kg of concentrate containing 25% copper, a reduction of 96% in the weight of the initial ore.