Copper mining. From ore to copper.

page 3

From ore to copper


		On the previous page, we introduced the main stages in producing copper. Here we will look at the steps before any chemical reactions take place.

Picture 3. An open pit mine. The size of the huge lorries give you an idea of the scale.

Mining

Copper can be extracted from its ore by:

1. Traditional mining

	underground	sinking a vertical shaft into the Earth to an appropriate depth and driving horizontal tunnels into the ore.
	open pit	90% of ore is mined by this method. Ores near the surface can be quarried after removal of the surface layers.

2. Leaching

The ore is treated with dilute sulphuric acid. This trickles slowly through the ore dissolving copper to form copper sulphate. The copper is recovered by electrolytic refining. Advantages of this process are:

much less energy is use than in traditional mining
no waste gases are given off
it can be used on ores with as little as 0.1% copper - for this reason, leaching extraction is growing in importance.

Picture 4. Map of copper production. Compare this with the pie chart of production.

Picture 5 . Cuprite mineral and a close up of malachite.

What's in an ore?

There are many types of copper ore found throughout the world. Picture 4 shows the distribution of these deposits. About 80% of all copper extracted comes from sulphide ores.

A typical ore contains only 0.5% to 2.0% copper. It is a measure of the value of copper that it is worth extracting it from such small concentrations. The ore is a mixture of minerals and rock (called gangue). The most common copper minerals are:

Mineral	Formula	Appearance	% copper in mineral
Cuprite	Cu₂O	Red, earthy	see Qn 2
Chalcocite	Cu₂S	Dark grey, metallic	see Qn 2
Bornite	Cu₅FeS₄	Golden brown, metallic	63
Malachite	CuCO₃Cu(OH)₄	Bright green, earthy	58
Azurite	2CuCO₃Cu(OH)₄	Blue, glassy	55
Chalcopyrite	CuFeS₂	Golden yellow, metallic	35

The first step is to physically remove the gangue.

Picture 6 . Froth flotation.

	Concentrating the ore

		The ore is crushed, ground and then enriched (concentrated) by Froth Flotation. The powdered ore is mixed with a special paraffin oil which makes the copper mineral particles water repellent. It is then fed into a bath of water containing a foaming agent which produces a kind of bubble bath. When jets of air are forced up through the bath, the water repellent copper mineral particles are picked up by the bubbles of foam. They float to the surface making a froth. The unwanted waste rock (gangue) falls to the bottom and is removed. The froth is skimmed off the surface and the enriched ore (mainly the copper mineral) is taken away for roasting. The mixture of water, foaming agent and paraffin is recycled. After this stage the enriched ore now contains about 25% copper by mass.