Discussion flotation and mineral processing with industry metallurgist from around the world.

Wetting and not wetting or water-absorbing is the principle of flotation. Some minerals are wetted, whereas others are not wetted. Wetting is a variable property of minerals and depends upon the surface properties of the solid, liquid, and gaseous conditions. Minerals that are wetted do not float, whereas those which are not wetted float well. Therefore, all metal sulphides and most non-metallic minerals float, while the gangue or enclosing mineral sinks—galena and quartzite, for example. Gold of minus 40 mesh also floats, and such flaky materials as graphite and molybdenite are good floaters. With the recent developments in flotation reagents it is now possible to separate non-metallics.

In general practice, ores are crushed and ground fine in water (at least through 100 mesh and 25 to 50 per cent solids in the pulp), an oil or chemical is added to the pulp and thoroughly mixed, and the sulphides become attached to the reagent and float, but the gangue, peculiarly, is wetted by the water and sinks. During this operation, air is beaten into the pulp by rapidly driven propellers or agitators, and the oil-coated sulphides attach themselves to the bubbles of air and are carried upward. The consumption of oil and chemicals is generally low.

To quote an Australian engineer, in whose country the practicability of flotation was first done on a large scale:

The art of flotation is to cause one mineral in a pulp to float while the other minerals do not float. In a mixture of galena, sphalerite, pyrite, and quartz, it is generally practicable to float the three useful minerals in succession.

In floating copper ores and lead-zinc ores, depression of the iron pyrite is important; but in floating gold-silver ores, the free gold and the iron pyrite are floated. The surface of iron pyrite may be altered while broken underground or on its way to the mill or during cyanidation and therefore needs special attention.

Reagents

Flotation reagents may be classified in three principal groups: frothers, collectors, and conditioning agents. The third group includes acids and alkalies for acidity or alkalinity control, sulphidizing agents, activating agents, and depressing agents. Excepting a few, all reagents are added as 5 to 10 per cent solutions; oils and acid are undiluted. They are fed regularly to the grinding circuit, to the flotation circuit, or to the conditioning tank.

Collectors are the reagents that enable the desired mineral to become attached to the air bubbles in the pulp and thus float. The quantity of collectors used varies from 1/100 to ½ pound per ton of ore.

For sulphide minerals the most commonly used collectors are the xanthates, usually potassium or sodium ethyl xanthate, and the corresponding amyl salts. For certain ores, thiocarbanilid, T-T mixture, and Aerofloat (phosphocresylic acid) are highly effective. Various coal-tar creosotes, coal tars, and pine tars are also used.

Frothers are the reagents necessary to produce the mineral- carrying froth. The quantity consumed is 1/20 to 1/5 pound per ton of ore.

For sulphide flotation various wood oils have been used, the most common being pine oil, particularly steam-distilled. Cresylic acid is a common frother of another type. Frothers should be used in as small quantity as is necessary to produce the best concentrates. Excess frothers give misleading results, in that too much froth is produced, gangue minerals may be floated, selectivity is poor, and recovery is misleading.

In non-sulphide flotation, frothers used may be the same as for sulphides, or sodium oleate, Emulsol X-1, Ninol, or an alcohol derivative may be used. When oleic acid is used, it alone may produce enough froth and also act as a collector.

It will be found, in general, that most frothers act as collectors to some extent; but if proper quantities are used, this tendency is not important.

Conditioning or modifying or regulating agents include soda ash, time, sodium silicate, caustic soda, sodium sulphide, sulphuric acid, and others—½ to 10 pounds per ton for the acidity or alkalinity of pulp; sodium sulphide for sulphidizing—½ to 5 pounds per ton; copper sulphate and sulphuric acid for activating —½ to 2 pounds per ton; sodium cyanide, starch, lime, and others for depressing—1/100 to 10 pounds per ton.

Figure 126 A and B shows a flotation machine at work, and Fig. 127 shows what is going on in the cells.

 

All the firms mentioned make tests on ores and give advice on the flotation of ores. Some of them make testing machines.