Once mined, the ore is crushed and ground to liberate the copper-bearing mineral from the gangue material. Crushing typically involves a primary gyratory crusher in the pit. The reduced product is then conveyed to a crushing plant where the material passes through a secondary and/or a tertiary crushing cycle. Following crushing, the size of the ore is further reduced by milling. Traditionally, the first stage of milling is accomplished by a large number of rod mills or ball mills. A recent trend over the past twenty years is the development of huge semi-autogenous (SAG) mills (9-10 m in diameter) that allow the ore to grind itself with the help of a small amount of large (12.5 cm) steel balls. One SAG mill has replaced the work once performed by dozens of rod or ball mills. Final grinding involves smaller ball mills, which produce a final product size of typically less than 100 microns.
During the final milling of the ore, reagents called collectors are added to the slurry which preferentially absorb onto copper sulfide minerals. These chemicals cause the sulfide particles to become hydrophobic (literally means "afraid of water"). The copper minerals are then concentrated by flotation when air bubbles are introduced into the slurry. The hydrophobic particles attach themselves to the air bubbles and float to the surface of the slurry. Chemicals called frothers are typically added to stabilize the particle laden bubbles at the top of the slurry thus creating a froth. The copper rich froth is then removed by an overflow mechanism. After de-watering, the resulting concentrate contains approximately 25-35% copper and is sent to a smelter for pyrometallurgical processing.
For more information check out the DocCopper's Copper Book List.
Go to smelting or return to the conventional processing page.