MINERAL FLOTATION BY ARTHUR F. TAGGART
Introduction Appearing as it did in an art (ore dressing) that utilized mechanical principles almost exclusively in its operation, and developing empirically, flotation was first investigated from the purely physical standpoint. And because the controlling phenomena are almost wholly chemical, progress in understanding was correspondingly slow. Since the chemical aspect of the process has been recognized, however, investigation has made rapid strides, and a sufficient background of knowledge has now been built up so that methods of research and control founded on established chemical principles may be confidently applied. There are a number of socalled flotation processes, but the essential differences between these methods narrow down on analysis, leaving two fundamental groups, which may be designated as bubble-column processes and pulp-body proFIQ.I cesses respectively. The former Diagrammatic sketch of bubble-column flotation are the simpler both in respect to operation and investigation, and may be taken as the type for study. Fig. I presents diagrammatically a common bubble-column operation of the present day. Finely ground (minus - 0.2-mm.) sulphide ore mixed with three to four times its weight of water (the mixture is called pulp), is flowed continuously into one end of a porous-bottomed trough-like tank. Lime ( I t o 4 lb. per ton of ore) has usually been added to the pulp during grinding. Xanthate (0.05 to 0.2 lb. per ton) goes in from a few seconds to a few minutes before the pulp reaches the flotation cell, Pine oil (0.05-0.1lb. per ton) is introduced a t the head of the cell with the stream of pulp. Air is blown in at the bottom, as pictured. A watery froth carrying the bulk of the suIphide mineral (concentrate) overflows continuously; the residual impoverished solid with water (tailing) also discharges continuously. Buoyancy of the conceatrate particles is brought about by forming aggregates thereof with the air bubbles. The lime and xanthate are added to induce this aggregation. Pine oil imparts stability to the bubbles.
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Levitation In froth flotation of sulphide ores the material that floats is the specifically heavier part. This fact excludes the hypothesis that, the overflowing solid in the bubble-column cell is lifted by the rising fluid current induced by the air bubbles. For if such were the mechanism, it would be the specifically lighter material that overflowed. The particles that overflow are lifted by the bubbles. The reason for the bouyancy of the bubble-solid aggregates is, of course, simple displacement. But the means of attachment of bubble to particle is more difficult to determine. There will occur to the mind of anyone whose recollection runs back to his days in physical laboratory the floating-needle experiment, and-not too
FIG. 2 A floating needle
FIG.3 Probable m a n ement of phases in a bub%le column
literally-the same force that held the needle a t the surface of the water there holds the concentrate particles in the bubble films. But it is distinctly questionable whether tension of the same interface is effective in both cases. With the needle it was the air-water surface tension, the needle being in equilibrium under the forces pictured in Fig. 2 . It is doubtful, however, whether the concentrate particles in a bubble-column froth are in the air-liquid interface; the weight of the evidence is that they are in the interface between a layer of oily liquid forming an inner sheath of the bubble, and the surrounding water. (See Fig. 3 .)
Experimental I . A portion of bubble film may be picked up by dipping a wire ring 1/4 to 3/8-in. diameter into the bubble. If fresh film carrying solid is thus obtained and examined immediately a t 30- to 50-X magnification, the air-liquid interface appears unbroken. After 30 seconds to a minute liquid appears to draw away from points of the solid particles that project toward the lens, and these points then protrude into the air. The conclusion would seem to be that the method of examination is capable of distinguishing between an unbroken liquid-gas contact and a solid-gas contact, and that it is the former that exists at the surface, and probably, therefore, in the body of an operating bubble column. 2. A bubble column will not operate if an oil or other substance capable of forming an oily film on the bubbles is missing. 3. The force of the adheaion between solid particles and bubbles in a bubblecolumn is apparently less than that between the solid particles and the bubbles in a pulpbody operation. (See p. 143.)I n this latter type of process the solid particles are in the airliquid interface. (See Experiment 30, p. 146.) Since the air-liquid surface tension of the
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overflow liquid in a properly conducted frothing operation is normally upwards of 60 dynes per em., while the interfacial tension of the oil used against water is probably not over half that figure, the observed difference in adhesion is significant. 4. If, say, I cc. of a non-frothing oil such as Nujol is placed with 5 gm. of a deslimed sulphide ore and 25 cc. of water in a 50-cc. test-tube and the latter closed with a clean cork (not with the thumb) and shaken vigorously three or four times, the oil is broken up into relatively coarse droplets, and microscopic examination (IO- to 20-x) shows that sulphide particles are held suspended by these droplets, as is the particle in Fig. 3. If now a minute amount of a spreading oil (see below) is introduced and the tube is again shaken three or four times, the oil droplets differ from those before observed in that, in addition to their solid load, many or all contain air bubbles. These now are obviously in the exact condition pictured in Fig. 3, except that the oil sheaths are relatively thicker. Successive shakings produce progressive thinning of the oil sheaths and more bubbles. Final prolonged shaking will, if conditions are right, produce a mineral-carrying froth of such great bubble surface that the oil layers are so thin as to be invisible under the magnification prescribed. But since the solid particles were seen to be carried, so long as the oil sheath was visible, a t the oil-water interface, and since we know, from surface-tension considerations, that the oil film persists, the conclusion seems reasonable that the adhering solid particles remain in the oil-water interface.
Frothing If pine oil is eliminated from the operation pictured in Fig. I, all other conditions remaining the same, the volume of the column of bubbles overlying the body of pulp will be materially smaller, so much so that there will be no overflow, and visual evidence of concentration (seep. 143) will be lacking. The pine oil has two functions; it stabilizes the bubble films so that they persist and build up to the overflow level, and it furnishes the oil sheath around the bubbles for holding the solid particles. The frothing action of a reagent involves spreading of the reagent at the air-water interface, with consequent increase in concentration of the reagent in overflow as compared with the residual liquid. Only those substances which spread are useful frothers. They spread because they lower the surface tension of the air-liquid interface. Their effectiveness as frothers is in proportion to the intensity of their effect on this surface tension, when present in low concentrations. Soluble substances that raise the surface tension of water also have been reported to exert some frothing effect, but are not practical frothers in the flotation art. Experimental Fig. 4 presents results of surface-tension measurements on overflow and residue liquids from a series of flotation operations in a pneumatic cell, using pine oil as a frother and no other reagents added. Fig. 5 shows the relation between weight of overflow (solid plus water) and difference in surface tension between overflow and residue. The range in quantities of pine oil present, between 20 and 40 mg. per 1.. is the usual operating range for this reagent, hence it is to be concluded that a reagent that will lower the surface tension between 3 and 5 dynes per cm. at such low concentrations has requisite frothing power. 6. Solutions of sulphuric acid, acid sodium sulphate, and of sodium chloride and sulphuric acid, of I to z per cent strength, have been used as sole reagents in froth flotation of certain high-grade sulphide ores. The effect of these inorganic substances on the surface tension of the solutions at the concentrations used is an elevation of the order of a dyne per cm. It is distinctly doubtful whether the froths obtained were due to the inorganic 5.
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reagents directly. Sensible amounts (from the flotation standpoint) of lubricating oils and greases are present in practically all ores that have been machine mined and milled to flotation size. Such relatively strong inorganic solutions as those under discussion, hot, as they were used, would tend to dislodge such oils from the gangue of constituents of the ores, break up the soaps in the greases, and thus free organic reagents for frothing and collecting, and it is, in all probability, these that were effective.
FIG.4 Surface tensions of overflow and residual liquors from a bubblecolumn operation 10
A
'
2(BC)T, and the film is a pure liquid, EF will move downward until the film breaks; if W
