RECOVERY BY FLOTATION OF MINERAL PARTICLES OF

RECOVERY BY FLOTATION OF MINERAL PARTICLES OF COLLOIDAL SIZE A. M. GAUDIN

AND

PLAT0 MALOZEMOFF

Ore Dressing Laboratories of the Montana School of Mines, Butte, Montana Received September 19, 1932

I n the early stages of the development of the flotation process for concentrating ores, the belief arose that a t last a process of ore concentration was available for the successful treatment of the finest particles (I), which in water suspension are collectively known as “slime.” It was thought that all mineral particles that are too fine to be recovered by gravity concentration and yet comprise a wide range of sizes float equally well. And, indeed, it is true that flotation is applicable to a wider size range than any of the three principal methods of gravity concentrationjigging, tabling, and vanning. Nevertheless the introduction of ball-mill grinding made it increasingly obvious that particles of all sizes do not float equally well or rapidly. The presence of an upper size limit beyond which flotation is impossible was recognized even before the advent of seZective flotation (as contrasted with collective flotation). The existence of a lower size limit beyond which flotation is difficult was suspected, but the limitations of laboratory sizing technique made it difficult t o more than surmise that particles of colloidal size are refractory to modern selective flotation operation (2). Definite reduction in floatability with reduction in particle size beyond a critical size of maximum floatability was recently demonstrated (3). The size limits within which recovery by flotation is good were found to be more or less peculiar to each mineral. I n general, however, the optimum size of mineral particles for concentration by flotation is between 50 and 10 microns, and the recovery is markedly lower for particles finer than 5 microns. This dependence of flotation upon the size of particles was found to hold true in practical mill operations as well as in the flotation of synthetic mineral mixtures. About 10 per cent by weight of the ore treated in a modern mill is ground too fine to be recovered efficiently by flotation. The economic loss thus involved is appreciable. It is therefore of considerable economic importance as well as of scientific interest to look into the possible causes for this behavior of fine particles and to devise, if possible, a more effective means of recovering them by flotation. 597

598

A. M. GAUDIN AND PLAT0 MALOZEMOFF

This paper deals with the study of the flotation behavior of exceedingly fine particles-those which in modern practice can be recovered only with difficulty. The size range of these particles embraces the entire scale from 5 microns to the truly colloidal particles which have as their limiting size the unit crystal of the mineral (4). In the size range considered the particles of near-colloidal size (5 to 0.5 microns) obey in kind, if not in degree, the same general laws of flocculation and dispersion (5) as the truly colloidal particles. Thus it is proper to examine and interpret the behavior of these fine particles in the light of colloid chemistry. CAUSES FOR NON-FLOTATION OF COLLOIDAL PARTICLES

In the study of the effect of particle size on flotation a number of hypotheses were considered (3) to account for the non-flotation of particles of colloidal size. Of these hypotheses the most likely is that very fine particles do not float because it is difficult for them to come in contact with gas bubbles. In recent years the emphasis laid upon the physicochemical properties of the mineral surfaces as determinants of flotation behavior have subordinated the equally important mechanical problem of bringing gas and solid together. The importance of gas-solid attachment is self-evident from a careful definition of flotation.1 Attachment of mineral particles to bubbles usually results from contact becoming established by direct encounter of particles with bubbles. It can be shown (6) that the probability of encounter between a mineral particle and a bubble varies directly as the size of the particle, provided the particle is small compared to the bubble. I n other words, the finer the particle the poorer its chance of being recovered. Also, it appears likely that mineral recovery can be enhanced by flocculating the mineraL2 A further consideration, more directly deriving from customary colloid phenomena, also points to the desirability of flocculating the mineral which is to be floated. It is known that fine mineral particlessuspended in an electrolyte are electrically charged (7); it is logically certain, furthermore, that the charge on particles of the same kind is of the same sign. 1 Flotation is a process of separation of mixed dissimilar solid particles, applied to the concentration of finely ground ores in aqueous pulp. The separation is caused by the selective adhesion of some species of solids to gas bubbles and the simultaneous adhesion of other species of solids to the aqueous solution; segregation of the resulting froth from the remaining pulp yields the desired separation. a In this article the term “floccule” is used t o describe an aggregation of solid particles suspended in a liquid in the absence of gas bubbles. In flotation literature the term “flocculation” has been used by some to refer t o the formation of highly mineralized gas-solid aggregates. This confusing terminology is regrettable.

RECOVERY BY FLOTATION O F MINERAL PARTICLES

599

Accordingly, if it is proposed to cause electrically charged (dispersed) particles to adhere to a somewhat mineralized gas bubble, it should be expected that the mineral particles already attached to the bubble will prevent the adherence of more particles (8) and, hence, will prevent the formation of highly mineralized bubbles. Since flocculation in conducting media usually involves neutralization or great reduction of the charges responsible for dispersion, flocculation of the mineral may help flotation by eliminating or decreasing the electric charge a t the surface of the particles. From these considerations of the mechanics of gas-solid attachment, it seems likely that if fine particles can be made to lose their identity as such by coalescing into floccules, they may be expected to behave as coarse, uncharged particles, that is, to be more readily recovered by flotation. MINERAL FLOCCULATION

The obvious means of flocculating a dispersed solid suspended in an electrolyte is t o add to the electrolyte suitable soluble salts capable of discharging the dispersed phase. Another means of flocculating the mineral is to form an insoluble heteropolar coating on the surface of the mineral, oriented in such a way as to make the transition from one phase to another abrupt (9). This is suggested by the following considerations. Dispersion can result either if the two phases are of like polarity or if there is established between them an intermediate zone or atmosphere of molecules or ions which is of a polarity intermediate between that of the two phases, so that a gradual transition is set up. Conversely, if the phases are very dissimilar, or if a film can be caused to develop on the solid, of a vastly different polarity than the liquid, the transition between the film-coated solid and the liquid will be abrupt and flocculation will tend to occur. Some of the experimental vindication for this argument is as follows: H. E. Kamprath (10) studied the behavior of polar and non-polar minerals in polar and non-polar liquids. He found, for example, that silicate minerals, which are polar, flocculate in a non-polar liquid, but that in a heteropolar, nonconducting liquid they remain dispersed. One of the heteropolar liquids used was butyl alcohol, in which dispersion can hardly be attributed to electric charges. It would appear as though butyl alcohol molecules adhere to the solid in definite orientation so as to cause the surface of the coated silicate particles to have the same polarity as the liquid. That dispersion can be obtained under certain conditions without the presence of substantial electric charges was recently confirmed by Basil C . Soyenkoff (ll),who concluded that " . . . the majority of colloid dispersions in hydrocarbons either are uncharged or carry only a small fraction (less than of the charge possessed by the particles in water."

600


Kamprath also found that graphite, which is non-polar, flocculates in polar liquids, but remains dispersed in non-polar liquids. The same behavior was noticed in the case of galena, provided it is kept from oxidizing. Thus, when galena is ground in water in a nitrogen atmosphere, it flocculates invariably, which may be considered to derive from its low polarity as compared with water. EFFECT OF FLOCCULATION BY USE OF ELECTROLYTES ON FLOTATION OF GALENA

I n order to ascertain the effect of flocculation by electrolytes on flotation it was thought wise t o study extensively a simple system whose normal behavior is otherwise well known. Accordingly, mixtures of pure galena with granite (Butte quartz monzonite) were ground wet, long enough (twenty-four hours) to reduce all particles to 5 microns and finer. Various electrolytes were added to the pulp, after grinding, in the hope of improving flotation under standard conditions (12) (2.0 lb. of potassium amyl xanthate and 0.16 lb. of terpineol per ton). None of the substances investigated (lime, sodium hydroxide, aluminum sulfate, sodium carbonate) improved the recovery, although flocculation was obtained in many instances. To study the problem further, galena and granite were ground separately for the time required to insure sufficient reduction in size, flocculating agents were added, the pulps were mixed, and flotation operations were conducted. Potassium n-amyl xanthate (2 to 4 lb. per ton) was used as collector, and terpineol (0.04 to 0.16 lb. per ton) as frother. The experiments showed that suitable flocculation of the mineral actually permits flotation of extremely fine mineral particles. Thus, if fine galena be first completely flocculated and then mixed with gangue minerals, an effective separation by flotation may be obtained. On the contrary, if the mineral be first completely dispersed and then mixed with gangue, recovery of the mineral is decidedly poor. These experiments, however, do not simulate practice, as the sulfide and gangue minerals were ground separately, and flocculated (or dispersed) separately, so that the minerals exercised minimum influence on each other’s behavior and that the system perhaps had not time to come to equilibrium. However, the experiments yield the valuable information that fine mineral particles are capable of ready attachment to bubbles if flocculated. To apply this important experimental result to practice, care would have to be exercised to flocculate selectively the mineral to be floated, or else compound floccules of mineral and gangue would result. Such compound floccules would of course be a hindrance rather than a help to the separation of the minerals from each other. Thus, of the four possible

601


states of aggregation of the minerals, that of flocculation of the mineral, together with dispersion of the gangue, is the most desirable. The other three, of course, are dispersion of mineral with dispersion of gangue, dispersion of mineral with flocculation of gangue, and flocculation of mineral with flocculation of gangue. However, if the mineral and the gangue are ground together it is difficult to obtain simultaneously satisfactory flocculation of the sulfide and dispersion of the gangue. Dissimilar mineral particles tend to precipitate on the surface of one another (13), possibly because of the’occurrence of unlike charges at their surfaces. Thus, some fine galena adheres to the surface of relatively coarse quartz, and, conversely, quartz adheres to galena. This phenomenon appears to take place in the grinding mill as soon as particles are formed fine enough to be affected by the charges that TABLE 1 Comparison between amvl xanthate and amyl dixanthogen as collectors for colloidal galena

Time of grind (hours). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Place of addition of reagent.. . . . . . . . . . . . . . . . . . . . . . . . . . . Quantity of reagent (pounds per ton) . . . . . . . . . . . . . . . . . . Time required for rougher flotation (minutes) . . . . . . . . . . Rougher concentrate (lead, per cent). . . . . . . . . . . . . . . . . . . Tailing (lead, per cent). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lead recovery (per cent), . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rougher selectivity index* Lead: Granite . . . . . . . . . . . . . . . Cleaner concentrate (lead, per cent) . . . . . . . . . . . . . . . . . . . Cleaner tailing (lead, per cent). . . . . . . . . . . . . . . . . . . . . . . .

XANTHATE

DIXANTHOGEN

24 Flotation machine 4.0 60 43.1 4.4 80.5 4.0

30 Pebble mill 3.0 5 39.3 0.30 98.7 15.5 69.4 1.10

-

* For method of calculation see A. M . Gaudin: Flotation, p. 526 (see reference 5).

obtain at their surface. Hence, in order to prevent this action, it appears necessary to have present a t the time when the fine particles are produced the agency that will cause the desired selective flocculation; that is, the selectively flocculating agent should be present in the grinding mill during the grinding. The search for a means to cause simultaneously the selective flocculation of the sulfide mineral and the dispersion of the gangue by the use of electrolytes was abandoned after much experimentation because the action of electrolytes did not appear to be selective enough. FLOCCULATION AND FLOTATION O F COLLOIDAL SULFIDE MINERALS, USING CERTAIN SULFUR-BEARING HETEROPOLAR COMPOUNDS

Remarkably successful flotation of colloidal sulfide minerals may be obtained by the use of certain heteropolar organic compounds. These

602


compounds have a structure such as to make possible the formation of oriented coatings of molecular dimensions on the sulfide minerals, and thereby produce selective flocculation of the sulfide minerals. This is true provided the flocculating-floating compounds are added to the mineral mixture before grinding. I n this connection it should be stated that in the experiments discussed in the preceding section a sulfur-bearing heteropolar compound was used in each test ?s a flotation collector. The reagent used was potassium namyl xanthate, a reagent known to be exceptionally effective in the flotation of coarse sulfide minerals. Nevertheless, under normal conditions this compound is not capable of floating slimed galena. On the contrary, n-amyl dixanthogen, a non-ionized oxidation product “of n-amyl xanthate, under certain conditions is capable of recovering colloidal galena. This discovery led to a broadening of the investigation to include non-ionized as well as ionized organic compounds containing single- or double-bonded sulfur atoms and an amyl, a phenyl, or a cresyl hydrocarbon group. Some of the reagents investigated and their formulas are as follows: 0

Potassium n-amyl thiocarbonate. ............................

I/

KSCOCiH11 S

II

Potassium n-amyl dithiocarbonate (xanthate), . . . . . . . . . . . . . . . KSCOC6H11 S

Potassium isoamyl trithiocarbonate ..........................

I1

KSCSC~H~I 0

0

n-Amyl formate disulfide, .......................

s s

I/ I/

n-Amyl thjoformate disulfide (dixanthogen).

......... C~H~~OCSSCOC~H

s s I1 II

Isoamyl dithioformate disulfide. ...................... C5H11SCSSCSCsHll Isoamyl disulfide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CjHllSSCiHll Isoamyl sulfide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CJLISCSHII Isoamyl mercaptan. ............................................ C~HHSH p-Thiocresol.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HSCeHdCHs SH Phenyl thiourethane. ..................................

I/ I

C2HbOCNCbH5

603


S Thiocarbanilide.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

/I

CeHbNCNCeHs

I I

H H Tri-o-cresyl thiophosphate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(CeHL%O)sPS

Of the above, the first six belong to a group of which xanthate and dixanthogen are representative. The compounds in this group other than the xanthate and dixanthogen were especially prepared in order to clarify the mechanism of attachment of the compound to the mineral. For this reason the double-bonded, or thione, sulfur atoms in xanthates and dixanthogen, which are commonly regarded by flotation engineers as being at least partly responsible for the attachment of minerals to bubbles, were replaced by oxygen atoms t o form formate disulfide and potassium thiocarbonate; also, the oxygen atoms were entirely eliminated by replacing them with sulfur atoms in the case of dithioformate disulfide and potassium trithiocarbonate. With the exception of the potassium salts, the compounds enumerated are but sparingly soluble in water, but since the organic solvents necessary to dissolve the agents may interfere in an undetermined manner with flotation, the undiluted agents were added directly t o the grinding mill so as to be in intimate contact with the mineral throughout the grinding operation. Flotation was conducted in a 500-g. Fahrenwald flotation machine. The products were analyzed for the valuable metal content. A flotation operation was arbitrarily termed successful when the tailing assayed less than 0.5 per cent (in valuable metal), and if the recovery exceeded 96 per cent. (Ordinarily, flotation under standard conditions, with the reagents added after grinding, yields a tailing having a metal content exceeding 3 per cent). It was found unnecessary to use any frother in most of the flotation operations. The mineral mixtures were ground in porcelain jars (“assay” pebble mills) long enough to be reduced to the near-colloidal size. Investigation was confined to five of the commonest mineral sulfides (galena, chalcocite, chalcopyrite, pyrite, and sphalerite) and to the gangue minerals occurring in quartz monzonite (quartz, andesine, magnetite, biotite, hornblende). The grinding was conducted in contact with air in the case of every reagent and of every mineral mixture. Identical tests, except for grinding in contact with nitrogen, were also conducted. The tests in which the minerals were ground in nitrogen were undertaken to supply some information concerning the very peculiar behavior of some mineralreagent combinations. It is well known that sulfide minerals are readily oxidized. It has been currently assumed (14) that surface oxidation of

604


sulfide minerals is prerequisite for modern flotation. Obviously, even in the case of particles ground in the presence of a restricted amount of oxygen -as in the tests under consideration-much oxidation may take place during a period of grinding of many hours. Also, change in the reagent may take place when the reagent is added so as to be in contact with a mineral and air for many hours during grinding. Clearly, when the mineral is ground in the presence of nitrogen, such reactions are largely inhibited and the reagent then acts on relatively unchanged sulfide surfaces. Table 2 summarizes the floatability of the different minerals with some of the more effective agent's. Flotation of colloidal sphalerite can be accomplished by none of the reagents listed, unless, of course, the mineral is first activated.3 Of the compounds which are similar in structure to amyl xanthate and dixanthogen (thioformate disulfide), namely, the thiocarbonate, trithiocarbonate, formate disulfide, and dithioformate disulfide, none showed

TABLE 2 Flotation of slimed sulfide minerals ground in contact with air when using some heteropolar sulfur-bearing compounds GALENA

Potassium amyl xanthate . . . . . . . . . . . . . . . . . . Amyl dixanthogen . . . . . . . . . . . . . . . . . . . . . . . . . Isoamyl disulfide. . . . . . . . . . . . . . . . . . . . . . . . . . Isoamyl sulfide. . . . . . . . . . . . . . . . . . . . . . . . . . . . Isoamyl mercaptan. . . . . . . . . . . . . . . . . . . . . . . . Thiocresol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phenyl thiourethane . . . . . . . . . . . . . . . . . . . . . . .

CAALCO-

CEALCO-

CITE

PYRITE

Good Good Good None Good Good None

Good Good Fair Good Good

.~

Fair Good None None None None Good

-

l -

i

PYRITE

_____ Good Good None None

-

-

any great promise to collect colloidal galena. The formate disulfide showed some tendency to collect galena, but it was too weak to be considered satisfactory. The action of these substances on other minerals was not investigated. Isoamyl disulfide was effective for chalcocite only. Isoamyl sulfide promoted the flotation of chalcocite and pyrite only when these minerals were ground in nitrogen. Its effect was nil when the minerals were ground in air. It was effective for flotation of chalcopyrite irrespective of the atmosphere in which the mineral was ground. Isoamyl mercaptan was effective only for chalcocite and chalcopyrite. Its action on the flotation of galena and sphalerite was nil. Its action on pyrite was not investigated. 3 By activation is meant the formation on the surface of the mineral of a layer which will react with the collector t o which the unactivated mineral surface fails to respond.

RECOVERY BY FLOTATION OF MINERAL PARTICLES

605

Thiocresol exhibited good collecting properties in the case of chalcocite, but none a t all in the case of galena and sphalerite. Its action on other minerals was not studied. The experimental results related above give little grounds from which sweeping generalizations can be drawn for predicting the action of the same compound on different minerals, or for designating a definite compound as one likely to be effective in promoting selective slime flocculation and the collection of floccules in flotation. In the present state of our knowledge, all that can be said is that the action of a given compound seems, to a large extent, to be specific to each mineral. It is hoped, however, that significant broad conclusions will be possible following the experimentation now in course. DRY VERSUS WET FROTHS

During the investigation with the organic compounds as selective flocculators a curious phenomenon was observed: in the case of some of the reagents a peculiar “dry” froth was obtained in the flotation operation. This unusual type of froth occurred if an amount of agent greater than a certain critical amount was used; when the amount of the agent used was less than this critical amount, the froth had the normal or “wet” appearance. The “dry” froth contains very little interbubble water and is in the form of a thick, dry mass which is made up of extremely fine bubbles, difficultly distinguishable by eye. Whenever a dry froth was obtained, most of the mineral floated in the dry mass in one minute, whereas flotation of colloidal material with the usual wet froth requires twenty minutes or longer. What mineral did not float in the dry froth came up in the form of a dull dry film, which formed persistently at the surface of the pulp for some time and which had to be raked off painstakingly. This remainder could not be induced to float in the form of a froth even with the addition of large amounts of frother. By the use of reagents giving the usual type of froth, a good recovery of colloidal mineral in the cleaning operation is always difficult, but whenever a dry froth is obtained the cleaner tailing is but slightly higher in metal content than the rougher tailing. Moreover, owing to the dry character of the dry froth, the concentrate obtained is very clean, devoid of the usual adulterating interbubble gangue suspension. Most of the froth remained indefinitely in the dry condition, resembling stiff whipped cream in plasticity. The small part that did disintegrate settled to the bottom of the water layer in very large floccules. Instead of cleaning the rougher concentrate by flotation, therefore, simply draining or filtering off the water (containing dispersed gangue suspension) could be used as a substitute. Wet froths obtained by the use of heteropolar reagents were examined to determine whether the mineral floats in the form of floccules or whether

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it floats as dispersed mineral particles attached to gas bubbles. Samples of froth were dried, then pieces were broken off and the thickness of the bubble wall was measured. This thickness was found to vary between 4 and 7 microns. The dried bubble was then broken up, and the individual particles in it were likewise measured after dispersion in pure terpineol. Most of the particles were smaller than half a micron. From a microscope examination of slides of the pulp, the diameter of the average floccular unit of the mineral was determined to be in the neighborhood of 4 t o 7 microns. Hence, it appears that the bubbles in the froth are made up of the same floccules that occur in the pulp. Similar determinations could not be obtained with dry froths because of the minuteness of the bubbles. From the partial disintegration of dry froths to form very coarse floccules, and from their dull luster it appears likely that they consist of floccules even larger than those occurring in wet froths. It was observed that coarse floccules float before finer flccules, a sequence analogous to that observed for the flotationof dispersed particles (3). SUMMARY

1. Hypotheses were considered to account for the non-flotation of colloidal particles. It is believed that non-flotation of colloidal sulfide mineral particles is due to their being unable to come in contact with gas bubbles because of fine size and state of dispersion. 2. Selective flocculation of colloidal mineral makes its flotation more complete and easier. 3. Owing to the possibility of mutual flocculation of fine gangue and sulfide minerals in a flotation pulp, it appears necessary, in order to-effect selective flocculation, to act on the mineral particles at the time they are produced in the grinding operation. 4. Flotation of colloidal sulfide mineral particles may be successfully accomplished by using as reagents certain heteropolar sulfur-bearing organic substances. These agents effect selective flocculation of the sulfide mineral particles. REFERENCES (1) MEGRAW,H. A . : The Flotation Process, p. 157. McGraw-Hill Book Co., New York (1916). (2) MEQRAW, H. A. : The Flotation Process, pp. 28, 143. HOOVER, T. J.: Concentrating Ores by Flotation, p. 158. Mining Mag., London (1912). TAGGART, A. F.: Handbook of Ore-Dressing, p. 859. Wiley and Sons, New York (1927).

(3) GAUDIN,A. M., GROH,J. O., AND HENDERSON, H. B.: Am. Inst. Mining Met. Engrs., Tech. Pub. 414 (1931). Effect of Particle Size on Flotation. BRUNNER, J. J. : Effect of Particle Size on Flotation; Part I1 Master of Science Thesis, Montana School of Mines.


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(4) GROSS, J., AND ZIMMERLEY, S. R.: Am. Inst. Mining Met. Engrs., Milling Methods, p. 30 (1930). Crushing and Grinding, 11.-Relation of Measured Surface of Crushed Quartz to Sieve Sizes. (5) GAUDIN,A. M. : Flotation, pp. 44-9. McGraw-Hill Book Co., New York (1932). (6) GAUDIN,A. M.: Flotation, pp. 88-90. (7) INCE,C. R.: Am. Inst. Mining Met. Engrs., Milling Methods, p. 261 (1930). A Study of Differential Flotation. CALLOW, J. M.: Mining Sci. Press 111, 854 (1915). Notes on Flotation. Kolloidchem. Beihefte 2, 84 (1914); Z. physik. Chsm. 89, 91 (1914). (8) TAYLOR, N. W.: Trans. Electrochem. SOC.60, 305 (1931). Discussion of “An Outline of Some Physicochemical Problems of Flotation” by A. M. Gaudin. (9) MCBAIN,J. W. : Colloid Symposium Monograph IV, 7 (1926). A Survey of the Main Principles of Colloid Science. FAHRENWALD, A. W.: Trans. Electrochem. SOC.60,. 311 (1931). Solubility, Peptization, Wetting and Flotation. (10) KAMPRATH, H. E. : A Study of the Dispersion and Flocculation of Pure Mineral Suspensions in Liquids. Master of Science Thesis, Montana School of Mines, 1931. (11) SOYENKOFF, BASILC.: J. Phys. Chem. 34, 2519 (1930). Benzene Dispersions of Basic Soaps of Nickel and Iron. J. Phys. Chem. 36, 2993 (1931). Hydrocarbons as Dispersion Media: A Review. (12) GROH,J. O., AND HENDERSON, H. B.: Effect of Particle Size on Flotation. Master of Science Thesis, Montana School of Mines. (13) TAGGART, A. F., TAYLOR, T. C., A N D INCE,C. R.: Am. Inst. MiningMet. Engrs., Milling Methods, p. 285 (1930). Experiments with Flotation Reagents. (14) TAGIGART, A. F., TAYLOR, T. C., A N D KNOLL,A. F.: Am. Inst. Mining Met. Engrs., Milling Methods, p. 217 (1930). Chemical Reactions in Flotation.

THE J O U R N A L OF PHYSICAL CHEMISTRY, VOL. XXXVII, NO. 5

RECOVERY BY FLOTATION OF MINERAL PARTICLES OF

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