Copper is found in natural ore deposits around the world. This page explains the production route taken from ore-containing rock to a final product that is the highest-purity commercial metal in existence and used in a wide variety of applications essential to modern living.
Copper minerals are found throughout the earth's crust. They occur in both sedimentary and igneous rocks. The outer 10 km of the crust contains 33 g of copper for every tonne of rock and in some places volcanic activity, millions of years ago, deposited molten copper in one location. It is these areas which are mined today because they contain enough copper to make mining profitable. As well as the valuable copper there is much waste rock (called gangue) which has to be removed.
An ore is a rock containing enough valuable mineral to make it worth extracting. In the case of copper, it is worth extracting when there is about 2 kg of copper per 1,000 kg of ore (0.2%). Copper minerals are found in over one hundred varieties, although only a few have been worked for copper on a large scale. The most abundant ores are chalcopyrite and bornite, which contain both copper and iron sulphides. These account for about 80% of the world's known ores.
The following gives an overview of how copper is extracted from its ore and converted into pure metal.
The ore is removed from the ground in either open pit or underground mines. Underground - sinking a vertical shaft into the earth to reach the copper ore and driving horizontal tunnels into the ore. Open-pit - 90% of ore is mined using the open pit method. Ores near the surface can be quarried after removal of the surface layers.
An ore is a rock that contains enough metal to make it worthwhile extracting.
The ore is crushed, then ground into powder.
The ore is enriched using a process called froth flotation. Unwanted material (called gangue) sinks to the bottom and is removed.
This is where the chemical reactions start. The powdered, enriched ore is heated in air between 500°C and 700°C to remove some sulphur and dry the ore, which is still a solid called calcine.
Smelting with fluxes
A flux is a substance which is added to the ore to make it melt more easily. The solid calcine is heated to 1200°C and melts. Some impurities are removed forming a matte (a mixture of liquid copper and iron sulphide).
Conversion of matte
Air is blown into the liquid matte forming blister copper, so called because the gas bubbles trapped in the solid form blisters on the surface.
The blister is cast into anodes for electrolysis.
The copper is purified to 99.99% by electrolysis.
The production route described above shows the progression from a rock containing about 0.2% copper to a copper cathode of 99.99% purity.
Leaching offers an alternative to copper mining. First, the ore is treated with dilute sulphuric acid. This trickles slowly down through the ore, over a period of months, dissolving copper to form a weak solution of copper sulphate. The copper is then recovered by electrolysis. This process is known as SX-EW (solvent extraction/electrowinning).
Advantages of these processes are:
- Much less energy is used than in traditional mining
- No waste gases are given off
- Low capital investment
- Ability to be operated economically on a small scale.
It can be used on ore with as little as 0.1% copper - for this reason leaching extraction is growing in importance. It is estimated that in 2011 SX-EW represented 17% of total copper refined production.
Another important source of copper is recycled scrap, described as secondary copper production, which in 2011 accounted for 41% of total refined copper production. The use of scrap is one example illustrating the sustainable nature of copper.