The grinding of base metal ores is done in water to which certain synthetic chemicals and oils are added. Then the pulp which results from it is swirled around in rectangular tanks arranged in series. These tanks are known as flotation cells. Controlled air and further chemicals, which are called flotation reagents, are added. The bubbles in the pulp are formed by air, the flotation reagents coat the metal sulphides (but not the waste particles) and cause them to stick to the bubbles which, in turn, carry the sulphides to the top of the tanks.
The bubbles carrying sulphide are scraped from the top of the flotation machines, while the material that is worthless is left behind and sinks to the bottom and is discharged.
A recent innovation, designed to improve flotation recoveries, has been to use column flotation cells – 33 feet tall (10-metre) tanks in which a tall column of froth is able to develop. Unwanted minerals from the froth are washed with water sprayers at the top, so that only minerals that are chemically attracted to the bubbles are retained.
After, the bubbles that bear metal, which is presently known as a concentrate, have the water removed from them before they are shipped. This is done typically by pressure filters. This dewatering step is carried out to reduce the weight of the concentrate if the shipping distance is large and to prevent freezing (in rail cars, for example) during the shipment of winter. By employing a variety of flotation reagents, different kinds of metal sulphides can be floated or separated one at a time. This way, if an ore contains copper, zinc and iron sulphides, it is possible for each metal to have its own separate concentrate.
Flotation is occasionally applied to gold ores, to make a gold flotation concentrate that is afterwards treated with cyanide, with roasting or without it. This method is especially applicable when the metal is very fine and intimately associated with minerals containing arsenic or sulphur. It is possible to malee, in this form, say, 11 tons (10 tonnes) of gold concentrate from 330 tons (300 tonnes) of mill feed. This concentrate can be economically treated much more intensely (longer agitation and finer grinding) than the original ore.