The Flotation Process

Until quite recently copper ore had to be hand-picked if the extraction of the metal was to be economical. In fact, in the last century, ores of 4 percent grade were regarded as almost valueless. Today such ores (which would be regarded as relatively rich in copper) are scarce; the great majority of mines contain the metal in finely disseminated particles which aggregate anything from 2 percent down to 1 percent or even 0.8 percent of copper. Thus to obtain metallic copper, up to 99 percent of the material mined must be removed as waste. Only the flotation process mentioned above could make this practicable; and even so, it requires large companies with huge plants, continuous working and immense capital. This explains why, in order to smelt 4 million tons of new copper in 1963, nearly 400 million tons of ore had to be handled and treated in various ways.

Flotation depends on the fact that the grains of some minerals, and especially of metals, differ in the degree to which their surfaces can be wetted by a suitable solution. The powdered product of grinding the ore is fed into a series of tanks called flotation cells which are filled with a solution containing various oils capable of forming a froth. Air is pumped into each cell, the solution being agitated to bring froth of bubbles to the surface. In the case of copper minerals, the particles adhere to this froth, which is separated, whereas the great bulk of the useless material sinks to the bottom of the tanks. By varying the chemical conditions within the cells different results can be obtained, thereby making flotation highly selective in skilled hands.