THE ELMORE VACUUM FLOTATION PROCESS
If water taken from an open vessel where it has been exposed to the air at ordinary temperature and pressure be confined in a glass vessel and subjected to a nearly complete vacuum, it will at once be evident that the water contained a gas in solution, because, as the vacuum increases, the gas which in this case is air will be seen forming in minute bubbles on the walls of the vessel, or at any other available points. At sea level and at ordinary temperature the quantity of air which open water holds in solution amounts to about 2.2% of its volume. This solution of air is explained by the law of Henry and Dalton. The amount of a gas which will go into solution in a liquid varies in general directly as the pressure and inversely as the temperature.
If we subject a pulp of acidified water and crushed ore to an operation whereby a small amount of oil has been applied to the sulphide particles in a suitable mixing vessel, and then subject this oiled, acidified pulp to a vacuum, the air in solution in the water is at once released, and attaches itself to the convenient points and angles of the oiled sulphide particles in the form of small bubbles, which gradually increase in size until they are sufficiently large to overcome by their buoyancy the force of gravity acting on the sulphide particles. When of sufficient size for this purpose the bubbles rise to the surface of the pulp, carrying along with them the sulphide particles ; at the surface a froth, composed of air bubbles and mineral, forms, and is removed over a lip.
Francis E. Elmore invented an apparatus in which this operation is performed. This apparatus is unique in its features, and it is allowable to marvel at the ingenuity displayed in its invention.
The apparatus is shown in the accompanying sketch, Fig. 31. The pulp from the crushing mill flows continuously into the mixer A, into which also is introduced small quantities of oil, and, if required, of acid also, at the point B. The agitation is done by the rotation of the beaters C. The agitated pulp flows continuously from the mixer into the funnel D. The concentrate discharge-pipe E and the tailing discharge-pipe F are both sealed with water in the tanks G and H respectively. The upper end of the feed-pipe D enters the centre of the conical separating vessel I. Upon the application of a vacuum through pipe J, the pulp from the mixer is caused to ascend the feed-pipe and fill the conical chamber I. The rate of flow of the pulp down the pipe F being slightly less than that up the feed-pipe D, a small amount of the liquid overflows the lip of the annular space K, this quantity of liquid being sufficient to carry floating sulphides down the pipe E into the tank G. The rakes L are caused to rotate slowly by means of the worm and whe'el M, the angle of the rake-blades being such as to cause the water-wetted gangue in the pulp to travel from the centre to the periphery of the conical chamber, whence the gangue continuously discharges down the pipe F. The feed-pipe D is usually about 25 to 30 ft. long, the tailing and the concentrate pipes E and F being a few feet longer, so that in effect the feed-pipe and tailing-pipe form the long and the short leg of a syphon ; thus the power is supplied by the falling column of pulp in the tailing-pipe. So long as a continuous flow of pulp is supplied to the mixer, a continuous and entirely automatic discharge of tailing and concentrate is secured. The annular space K is surrounded by a thick glass cylinder, or by a metal cylinder with one or more thick glass windows, through which the discharge of the sulphides over the lip of the annular ring may be observed.
Fig. 32 is a photograph of the separating vessel I. The capacity of each unit is from 30 to 50 tons per day, depending on the nature of the ore. The apparatus is constructed of metal, with the exception of the glass windows, and the mixer is a simple wooden trough with wooden beaters. The power required for a 5-ft. unit is said to be 2 to 2j h.p., including that required for driving the vacuum pump, mixer, and separator. The apparatus is usually constructed in standard units, the conical separating chamber of which is 5 ft. diameter. Including the mixer, conical separating chamber, vacuum pump, all piping, etc., complete, the unit costs, London, approximately 350. The capacity of the apparatus naturally varies with the kind of ore to be treated, the fineness to which the ore has to be ground, and other conditions ; but a 5-ft. machine will ordinarily treat from 25 to 45 tons of crude ore per 24 hours.
The cost of the plant is low for so large a capacity. No expensive foundations are required, because the beaters in the mixing cylinder only make 30 to 40 r.p.m., and the rakes in the conical separating chamber only one to two revolutions per minute. Vibration is, therefore, entirely absent. No sizing of the pulp is required, and the operation of the plant is entirely automatic.
The quantity of oil and acid required is not the same for all ores, but usually amounts to not more than a few pounds, say from 3 to lolb. per ton of ore treated. A large variety of oils and similar substances have been tried and found suitable : Californian crude oil, Texas crude oil, Texas residuum, fuel oil, Russian crude, Borneo, Sumatra and similar cheap oils, tars, blast-furnace oils, olive-oil residues, oleic acid, kerosene, a variety of light oils, and fish oils.
