FLOTATION PROCESS IN METAL MINING, The. Flotation is a metallurgic process in which valuable metallic minerals are extracted from ores by methods that cause the mineral particles of the pule to float. A pulp is a mixture of finely crushed ore and water. It has been assumed that this new process is a physical contradiction of the old methods of concentration, in which the force of gravity was the principal factor. Such an interpreta tion is not permissible because gravity has not been outlawed by flotation; on the contrary the new process is dependent upon the action of the same force. The mineral particles that ascend in the new apparatus are as obedient to gravity as the particles that descend in the old machines. The difference lies in the inter play of molecular forces, notably surface-ten sion, which is a phase of cohesion, that is, the attraction that binds molecules of like kind to each other. In consequence, the surface of a liquid acts as if it were a stretched membrane or an elastic film.
The earliest practicable method of flotation was invented by F. E. Elmore in 1898. It was based directly upon the buoyancy of oil. This is a simple manifestation of gravity, whereby an oil, lighter than water, will rise to the surface of a pulp and carry with it any mineral parti cles that have become entrained in it. The oil plays the part of a raft or boat. Owing to the small flotative margin offered by common oils, which have a specific gravity of about 0.9 as against the 1 of water, this method involved the use of so much oil that it proved imprac ticable in metallurgic practice. As first used it required a ton of oil to treat a ton of ore.
Next, the direct effect of surface-tension in water was utilized in methods invented in skustralia, where they were patented by G. D. )elprat and C. V. Potter in 1902. Such proc :sses come under the heading of filrn-sus )ension; they are typified by the floating of i needle on water. In the familiar experiment small needle is greased, although it will float without the aid of grease. Little or no 311 is employed in these processes, which show that bubbles of air entangled in the pulp, or of gas generated therein by the addition of acid, are effective for flotation.
Then came the third and much the most im portant development of this branch of metal lurgy. Several investigators, notably, 1-1.. L.
Sulman and H. F. K. Pickard, made use of the experience obtained in the Australian mills and, in 1904, developed a process in which bubbles of air became the principal agent. This phase of flotation may be called bubble-levita tion ; it depends upon the attachment of bubbles of air to the metallic minerals in preference to the gangue, such preference representing an affinity or selectiveness that is enhanced by the presence of a small proportion of oil, even less than 1 per cent on the weight of the ore. Whether the result be due to the oiling of the bubble-film or to the oiling of the metallic sur face of the mineral particles is not yet clear, but it is recognized that the oil affects the sur face-tension of the water of the pulp in such a way as to strengthen the bubble-films and cause the bubbles to last longer. The bursting of a bubble is due to the contractile force of surface-tension, and it must be moderated if the bubble is to last long enough to perform its metallurgic function.
The process now in use on a big scale in the United States is one in which a multitude of bubbles, developed in a pulp of ore, is utilized as a means for separating the valuable minerals from the valueless gangue. This separation is based on the fact that sundry minerals (especially sulphides) exhibit a preference for air-bubbles when these are made in a liquid modified for the purpose by the addition of a substance that produces a variable surface tension. The bubbles result from the dis persion of air in the pulp, whether entrained by agitation or blown through the porous bot tom of the vessel in which the operation is conducted. These bubbles rise, and the vari able surface-tension produced by the modifying agent added to the water hinders the coales cence of the bubbles while they are rising. The modifying agent may be a constituent of various oils or it may be any one of many other substances, including chlorides of the alkalis; it enters into the bubble-films between water and air, where the mineral particles are caught and held at the interface. The mineral:laden bubbles, arriving at the surface, aggregate into a froth, which is withdrawn, to be collected as a concentrate, which is then drained, filtered, and dried previous to reduction by smelting in the ordinary way.