FROTH FLOTATION CONCENTRATION C. C. D E W I n Michigan College of Mining and Metallurgy, Houghton, Rlich.
references to the story recorded by Herodotus (6Q),who tells of Byzantian maidens recovering gold dust by dipping birds’ feathers, smeared with pitch, in the mud. Evidently even Herodotus told the story with his tongue in his cheek, for he seeks to justify himself with: “If this be true, I know not; I but write what is said”. Sagui (84) claims that the ancient Greeks, about 2000 years ago, used a skin flotation process for the recovery of minerals from crushed ores. However, it was not until 1860 that Haynes (51) recognized the differences between the wettability of minerals and that of gangue. His work probably presents the first successful method of agglomeration concentration. In 1866 Everson (31) found that the Haynes method could be substantially improved by the addition of acidic substances and neutral salts to the water in the flotation cell. I n 1901-03, Froment (35) Delpratt (22) and Potter (81) modified Everson’s procedure by introducing gas bubbles into the water suspension of agglomerated mineral particles and gangue. 1905 found Schwarz (85) advocating the use of sodium sulfide as a conditioning reagent in the recovery of oxidized minerals. In 1906 Sulman, Picard, and Ballot (88) succeeded in reducing the quantity of oil needed for mineral agglomeration by the introduction of violent agitation as a means of more thoroughly contacting the gas bubbles with the oiled mineral particles. I n 1909 Greenway, Sulman, and Higgins (47‘) introduced the use of such soluble frothing agents as ketones, fatty acids, esters, and pine oil to replace the oil formerly considered necessary for mineral flotation. I n 1912 Lowry and Greenway (70) discovered and patented the use of bichromates as a depressant in the flotation of galena. This was one of the first patents relative t o selective flotation. In 1913 Bradford (12) working with the accumulated tailings and slimes a t Broken Hill, Australia, discovered that the addition of copper sulfate increases the flotability of sphalerite. Bradford (13) is also credited with the use of sulfur dioxide as a depressant for sphalerite. Perkin (79) in 1921 introduced the use of very slightly soluble organic flotation reagents containing trivalent nitrogen, divalent sulfur, and other nonoleaginous compounds. I n 1924 Keller ( 5 8 ) ,working for the Minerals Separation Company,.patented the use of less than one per cent of a soluble organic compound, particularly the xanthates] as a mineral flotation reagent. I n 1925 soaps were first used as flotation reagents for nonsulfide minerals. The work of Perkin and Keller is particularly significant in that it marks the point a t which mineral flotation became a chemical science. After Perkin’s work alkaline flotation circuits were standard practice in the flotation of sulfide ores. The discovery of many ways of modifying the flotative properties of minerals by the addition of specific chemicals to the flotation cell followed in rapid succession. Some of these chemicals are: lime to inhibit pyrite flotation, sulfites discovered by Pallanch (767, cyanides with or without zinc sulfides by Hellstrand sulfate by Sheridan and Griswold (86),
The discoveries leading to the successful operation of the froth flotation process for the recovery of minerals are reviewed briefly. Several theories of mineral flotation as related to surface chemistry are discussed, with the opinions of various investigators as well as those of the author. The important physicochemical characteristics of flotation reagents are reported, including those used for the recovery of metallic sulfides, oxides, and carbonates. The current practice in metallic mineral recovery for some of the more important minerals is reviewed, and the concentration of nonmetallic minerals by froth flotation as well as the agglomeration concentration of these minerals is discussed. Some of the important research problems related to mineral flotation which remain unsolved are summarized.
F
LOTATION is a process of ore concentration-that is, a method of segregating minerals in an ore into several products (36). The process of the concentration of mineral values of an ore by froth flotation involves several steps: (a) the ore is crushed to such a state of subdivision that the mineral values shall be substantially separated from the accompanying gangue material; (b) the crushed ore is suspended in m-ater to which has been added suitable addition agents, the presence of which, in conjunction with violent agitation and the addition of air, causes the mineral particles to adhere to the rising stabilized air bubbles; (c) these bubbles, lined with their accompanying mineral particles] are skimmed off the top of the agitated suspension in the flotation cell. The concentrate of mineral particles may then be subjected to further purification by the same process or, after dewatering, may be suitably treated t o recover the valuahle constituent of the mineral concentrate. Petersen (80) has presented in tabular form much of the pertinent control data for many of the successful flotation processes now in operation.
History of Flotation Two other types of flotation processes have found some commercial usage in the past-skin flotation (112) and bulk oil flotation (29). These developments, coming as they did in the earlier stages of the more modern method of froth flotation, were of value in that they turned the attention of workers toward the greater possibilities of froth flotation. Perhaps the most general statement that might be made regarding any of these three types of flotation is that mineral flotation is the science of making minerals selectively dirty so that they may readily be picked up. Certainly no consideration of the general aspects of flotation is complete without
(53).
In 1935, 49,087,920 tons (75) of nonferrous ore were mined in the United States. About 34,400,000 tons of this amount were concentrated wholly or in part by froth flotation proc652
MAY, 1940
SEPARATION OPERATIONS
esses. Of this ore, 26,051,709 tons, concentrated in 184 plants, consumed 85,494,458 pounds of chemical reagents, o r 3.282 pounds per ton.
Flotation Cells and Reagent Feeders Bosqui ( 1 1 ) classifies and compares the types of commercial flotation cells according to the method of introduction of the air : Mechanical a. Forced aeration b. Xatural sub-aeration
Pneumatic a. Low-pressure open pipe b. High-pressure porous-diaphragm type
I n the minerals separation cell compressed air is supplied below the agitator. The Denver Sub-A and Fagergren machines draw in the air by the vortex action of the agitator. The low-pressure open-pipe pneumatic cell is exemplified by the air-lift principle used in the Foresster and Southwestern cells. The first of the high-pressure diaphragm cells was the Callow cell, which in its improved form is known as the MacIntosh cell. The Geco cell combines the forced aeration and mechanical stirring principles. The critical concentrations of reagents necessary for good results in mineral flotation practice have resulted in the development of slow-moving belt systems for the introduction of dry reagents, and of the disk-cup and roll-strip feeders for solutions of reagents. The disk cup, the most widely used of the solution feeders, has a series of detachable cups arranged around the periphery of the disk, which, driven a t a constant speed, allows the cups to pick u p and discharge solutions at a fixed rate. The second type of solution feeder consists of a constant-speed cylindrical roll, against which may bear the ends of thin metal strips of varying width. The roll revolves toward the strip end and allow the strip to skim off a fixed quantity of the liquid film.
633
tributes the remarkable stability of some two-phase froths to the existence of solid films possessing a definite orientation. Foulk (53) studied the relation of frothing with regard to the difference between the static and the dynamic surface tensions of both sulfuric acid and of sugar solutions. De Witt and Makens (25) showed that of the purified components of pine oil, a-terpineol, the most efficient frothing agent, has a surface tension-molar concentration curve with the most negative slope. There are not yet sufficient data to show that any organic compound is a useful flotation frother if, when dissolved in water, it decreases the surface tension of the water. However, the Gibbs adsorption theory clearly indicates that one may always expect the adsorption behavior at a liquid-gas interface to be dependent on the bulk molal concentration of the solution, or a function of that quantity, and on the negativity of the slope of the surface tension-bulk molal concentration curve. I n this connection it should be noted that the concentration of the frothing agent a t a liquid-air interface is many times greater than in the main body of the solution, and that the actual surface-tension lowering of such a solution skimmed from such a n interface may be expected to be of a magnitude much greater than that caused by the bulk concentration of the solution. Bancroft (1) indicates that a sudden change in surface tension, either negative or positive in sign, causes frothing. Any sudden change in the size of an air bubble would necessarily involve an instantaneous change in the concentration of frothing agent a t the liquidgas interface. This phenomenon is doubtless one of the reasons why an almost proportional relationship has thus far been found to exist between frothing power and the difference between static and dynamic surface tensions. Such a n hypothesis does not in any way deny the initial applicability of the Gibbs adsorption theory to the selection of the most
Functional Reagents As in the development of all applied arts and sciences, that of mineral flotation has brought with it a nomenclature peculiarly its own. Four functional groupings of the uses of reagents have become well established. These are frothers, collectors, activators, and depressants or inhibitors. Wark (102) defines these functional reagents thus:
-4frother is a substance (generally organic) which, when dissolved in water, enables it to form a more or less stable froth with air.
A collector for any mineral is a substance (generally organic) which induces it t o float at the air-water interface and, in the presence of a frother, to form a more or less stable mineralized froth. An activator for any mineral is a substance (generally inor.ganic) the addition of which induces flotation in the presence of some collector that otherwise is without effect on the mineral. A depressant for any mineral is a substance (generally inorganic) the addition of which prevents a collector from functioning as such for that mineral.
Frothers The theory of the action of frothers has been developed by Bartsch ( 3 ) ,Foulk (%), and others (25). Edser (B), Edser (28), using the Gibbs adsorption theory, concluded that frothing is dependent, on the surface tension-concentration curves having a definite slope. Bartsch (5’) proved that the solid-liquid-gas froth systems have a much greater stability than two-phase systems. Bartsch’s work indicates significantly that only those minerals conditioned for flotation stabilize the froth. Minerals that can be completely wetted do not perform this function. Talmud (93, 94, 96) at-
Cuuilesy Denier E q u i p m e n t C o m p n n y
DESVER SUB-A (FAHRESN 4I.D)
F I . O T i T I O N ‘?EI.I.R
654
INDUSTRIAL AND ENGINEERING CHEMISTRY
efficient frothing agent; it is obvious that the frothing agent with the most negative slope of surface tension-molal concentration curve will likewise give the greatest change of surface tension with concentration and therefore should show the greatest difference between static and dynamic surface tensions. The principal frothing agents used in 1935 were pine oil, cresylic acid, and orthotoluidine (75). I n 184 plants the total consumption of these reagents in concentrating 26,051,709 tons of ore was 4,011,479 pounds, an average consumption of 0.154 pound per ton of ore.
Collectors The theory of collectors and the mechanism of film formation have been the subjects of much work and of an almost equal amount of healthy difference of opinion which has been supported by seemingly admirable experimental data. Taggart and his collaborators (90,91,92) contend that only those molecules capable of forming insoluble precipitates are deposited on the surface of the mineral. If this generalization be accepted, then adsorption a t mineral interfaces occurs only from saturated solutions of difficultly soluble substances, or from double decomposition reactions a t and with mineral surface atoms. There appear to be ample data to disprove this generalization, but Taggart’s statement that the compounds adhering in such surface coatings have a much lower solubility than the normal solubilities of these compounds is well substantiated. Although Taggart’s theory does not acceptably explain all the known cases of flotation, it does account for not a few cases where oxidized minerals are concerned (91,109). Gaudin and Schuhmann (43) showed that in the flotation of chalcocite the effective film is not copper xanthate. Other data disproving the generality of Taggart’s are found in the work of Gaudin and Hansen (39) on the adsorption of the heptoate radical by calcite. Held and Samochwalov (6.2), studying the adsorption of laurate ion from barium laurate on barium sulfate, found that adsorption takes place from unsaturated barium sulfate solutions. Kolthoff and Rosenblum (6.2) likewise report data on the adsorption of wood violet on lead sulfate from dilute solutions. Wark and Wark (99) found that amines are adsorbed by sulfide minerals without compound formation. Clearly, the objections to Taggart’s theory have their genesis in the work of Horovitz and Paneth (66) as interpreted by Freundlich (34). According to Freundlich (34) the work of Horovitz and Paneth indicates the adsorption of salts from solutions not saturated with those salts. The original observations of Horovitz and Paneth were that adsorption of radioactive elements is dependent on the degree of the insolubility of the salts formed by the anion of the crystal lattice of the adsorbing solid with the radioactive element. Fajans and von Beckrath (52) confirmed these observations. I n this connection the work of Bartell and Miller ( 2 ) on hydrolytic adsorption, later amplified by Miller ( 7 4 , seems to be significant. Since then the several types of adsorption have been summarized by Kolthoff (61). The discovery of the use of the cationic reagents as flotation collectors shows the lack of universal applicability of Taggart’s theory to film formation on minerals. Some of these reagents are trimethylcetyl ammonium bromide, discovered by Wark ( I O l ) , lauryl pyridinium sulfate and other specific quaternary ammonium salts by Bertsch and Stober (IO), quaternary ammonium and phosphonium, and ternary phosphonium compounds by Lenher (68),and primary amines by Wark and Wark (110) and Kirby (59). Recently De Witt and von Batchelder (24) showed that salicylaldoxime, heptaldoxime, and octaldoxime are effective flotation reagents in
VOL. 32, NO. 5
acid circuits for oxides, sulfides, and carbonates of copper. Although it is known that certain chelate compounds, such as salicylaldoxime, form exceedingly insoluble copper salts ( S O ) , it has not yet been exactly determined whether heptaldoxime and octaldoxime behave similarly. The properties of an effective collector may be summarized as follows (24) : (1) The reagent must be selectively adsorbed on or react with the surface layer of the mineral particle; (2) any reaction product must adhere tenaciously to the mineral surface; (3) the reagent used must not precipitate from the solution any material which, adsorbed on the surface of the gangue, will cause the latter t o float as if it were the sought mineral; (4) the slopes of the surface tension-concentration curves of water solutions of collectors measured to date are negative. The objective of items 1 and 2 is apparent in that the purpose of a flotation reagent, or collector, is to cover the mineral at least partially with a film of an organic compound. Item 3 requires that the heavy metal ions be substantially absent from the solution or that the solubility product of the metallic compound which might be precipitated by the addition of the flotation reagent be so large that it will have no sensible effect on the subsequent flotation. Or, if the precipitation does occur, the particles in a colloidal state of subdivision must have an electric charge of the same magnitude and sign as the gangue material.
Item 4 is cited to call attention once again t o the possible qualitative application of the Gibbs adsorption theory t o selection of collectors (26, 98, 107). According t o Taggart (89) some two thousand flotation reagents had been patented by 1926. Petersen (80) reviewed the more important reagents patented since 1926. Each year many more possible collectors are suggested. Less than fifty are actually used in commercial practice. Of these fifty reagents, four general types find preponderant usagenamely, the sodium salts of the alkyl xanthates, alkyl thiophosphates and alkyl carboxylic acids, and distillation products, such as coal-tar creosotes, wood-tar creosotes, petroleum products, and blast-furnace oils (75). In 1935 the total amount of ore concentrated in 183 plants by the first three of these collectors was 21,508,698 tons (75). This ore consumed 2,578,895 pounds of synthetic reagents, or 0,099 pound per ton of ore treated; 1,111,545 pounds of distillation products were used in 35 plants as collectors for 4,543,011 tons of ore, or 0.245 pound per ton of ore treated.
Activators The activation of a mineral is usually accomplished by the additon of an inorganic compound which, adsorbing as anion or cation or reacting a t the mineral surface, renders it readily amenable to the use of a collector. Ever since Bradford (1.2) discovered the use of copper sulfate as an activator in the flotation of zinc blend, this reagent has remained one of the most popular activators, even though other metallic compounds have been proposed for the same purpose. This, reagent also activates pyrite. The reaction a t the zinc blend surface is best explained by the reaction: ZnS
+ CuSOl = CuS + ZnSOl
Sodium sulfide was introduced by Schwarz (85) in 1905 as an activator for oxidized heavy-metal minerals. With malachite the surface reaction is: CUCO~.CU(OH)~ + 2NazS = 2CuS
+ 2NaOH + Na&OJ
Contributions to this fruitful field of flotation research have been made by many investigators, notable among whom are Gaudin (57, 39, 40, 4.2, IC), Halbich (481, Wark and Wark (IOO),Berl and Schmitt (9), Kraeber and Boppel (63). I n 1935, 551,106 pounds of sodium sulfide (76) were used to sulfidize 973,117 tons of ore at 16 plants, an average consump-
MAY, 1940
SEPARATION OPERATIONS
65.5
Taylor and Bull (96) compared the toxic action of cations with the solubility of sulfides.
Colloidal Effects in Flotation
Courtesy, American Cyanamzd
FAGERGREN FLOTATION CELLS tion of 0.57 pound per ton; 5,692,270 pounds of copper sulfate were used to activate 6,670,477 tons of ore a t 108 plants, an average consumption of 0.853 pounds per ton.
Depressants The function of a depressant is to change the nature of the mineral surface so that a collector cannot react or adsorb thereon. It is significant that a depressant which prevents the action of one collector may not prevent the efTectiye action on another collector of different chemical structure (101). Among the most important depressants used are sodium sulfide, calcium oxide, sodium cyanide, sodium silicate, sodium dichromate, sodium permanganate, zinc sulfate, and other soluble sulfates. The theories of the action of depressants and the data concerning their use have been recently reviewed by Wark (104). To be effective as depressants both sodium cyanide and sodium sulfide must be used in alkaline solutions. Thus, in their study of the effect of sodium cyanide and calcium oxide on the flotation of sulfide minerals, Tucker, Gates, and Head (97) found that in neutral solutions containing 0.5 pound of sodium cyanide per ton, 21 per cent of pyrite is floated when ethyl xanthate is used as collector and pine oil as frother; on the other hand, in a 0.02 per cent calcium oxide solution containing a like amount of sodium cyanide, only 2 per cent of the pyrite is floated with the same collector and frother. Wark and Cox (107) found that there is a critical pH value for the use of alkalies as depressants. This pH is dependent on the nature of the mineral, the collector and its concentration, and the temperature. These investigators likewise found that there is a critical concentration for cyanide (108) and hydrosulfide ions (109) with each mineral. Gaudin (37, 39, 40, 42, 44) contributed many fundamental data on effective depressants for sulfide and oxide minerals.
The reason for grinding the ore is to free the mineral from the gangue. In the grinding operation many ultrasmall particles are formed. Gaudin (38, 41) studied the three following factors which affect the flotation of too finely ground ore: (a) The increased surface area requires considerably more collector, (b) The chance of a small ore particle contacting the large air bubble is reduced. (c) The smaller the particle, the older is the surface; therefore the Rotation collector should be added in the grinding circuit (this is now common practice). Wark (105) adds three more significant factors: (a) The interfacial area necessary to float the increased number of fine particles must be increased proportionately; a decrease in size by one tenth requires ten times as much froth area. ( b ) The small radius of curvature of the particle almost prevents the particle from contacting any bubble but a small one; large bubhles grow a t the expense of small bubbles, (c) Adsorption of ions on small Corporation particles is apt to give them a high surface charge. The problem of handling slimes was studied by Taggart, Taylor, and Ince (91) and by Del Giudice (11)who presents a chemical theory to explain the action of one mineral on another. Wark (106) obtained results inconsistent with Del Giudice’s premises. Hydrophilic colloids depress gangue minerals by dispersing them. Sodium silicate appears to be one of the most satisfactory reagents for this purpose; reports on its use have been made by Patek (77, 78), Lottermoser and Rumpelt (69), and Halbich (49). Bartsch (3, 4) studied the negative effect of saponin on the flotation of sulfides. Weinig and Cuthbertson (111) patented the use of glue as a depressing reagent.
Flotation Theory
It is generally accepted that the mineral must be a t least partially and selectively coated with an adherent film of organic material. To some investigators it seems to be fairly well established that no minerals possess an “inherent or natural floatability”; others insist that this property is possessed by certain minerals. However, even the most easily floated sulfide ores exhibit no contact angle with water when cleaved under water out of contact with air. Experimental work (23) has shown that the contact angles developed in the presence of air on freshly cleaved copper sulfide surfaces and copper-xanthate-coated copper sulfide surfaces may be brought to a zero contact angle by the removal of the air and the substitution of a water vapor atmosphere for the air. Other minerals which exhibit finite contact angles with water in the presence of noncondensable gases have been shown, in accordance with Wark’s experimental work, to exhibit no contact angle when cleaved 01 polished under water (98). The object, then, of coating a mineral with an insoluble or tenacious organic film is to give it the property of exhibiting a finite contact angle in the presence of air-i. e., to make it possible for the air to displace the water partially from
INDUSTRIAL AND ENGINEERING CHEMISTRY
656
the coated mineral surface in order to make the mineral particle surface less easily wetted by water. The equation relating to interfacial energies per unit area is asg - r.92 = d g 008 6
where osg, usZ, and ulg refer to the solid-gas, solid-liquid, and liquid-gas interfaces, respectively, and B is the contact angle. When t? = 0, cosine 8 = 1, t.hen usg - usZ = d q at equilibrium. T o produce this condition it is necessary and sufficient that ops
> os2 + .?47
The lower the value of olg, the greater is the spreading tendency: the greater ugs, the greater is tlie tendency to spread. In the first case liquids of low surface eiiergy can replace liquids of high interfacial energy at the solid-liquid interface. I n the second case the greater tendency to spread is brought about by the liquid having to cover a wider area before equilibrium is reached hetveen the solid-liquid and liquid-gas interfaces. Therefore, it is apparent that, in the case of solids with a high soli&gas interfacial energy, liquids possessing a low liquid-gas interfacial energy preferentially wet such solids. This explains credibly why sulfide surfaces having high solid-gas interfacial energies are, as compared to silica of low solid-gas interfacial energy, preferentially wetted by liquids having a low liquid-gas interfacial energy. The Gihbs equation (h-),
was specifically derived to account for the excess concentration, U , of soluble materials a t liquid-gas interfaces. It may he wishful thinking to attempt to apply this equation, even in a qualitative sense, to any explanation .n.ith regard to a filmed mineral s u r f a c w i . e., a liquid-solid interface. Howeyer, from the thermodynamic point of vievI., those oompuim~is
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which effect the greater lowering in the free surface energy of their solutions will be selectively adsorbed at an interface. The reported behavior of organic collectors and frothers used in flotation studies supports this conclusion. If we examine carefully tlie contact sone between a filmed mineral surface and a stabilized froth bubble, we find that the gas bas a t least partially displaced the water from the mineral surface. The p l a r ends of the adsorbed molecules adhere to the mineral surface and the hydrocarbon ends of these adsorbed molecules project into the surrounding gas phase. This orientation of polar hydrocarbons has been accounted for by Langmuir (64). If, then, with the foregoing qualifications in mind, we apply tlie information to be gained from a coiisiiieration of the Gibbs equation, we arrive at the conclusion that in a homologous series of film-forming organic reagents the compound passessing the most negative slope of svrface tensionconcentration curves will be the best film-forming agent. This is in accordance with Traubc’s rule and is amply snhstantiated by experimental data (26, 98, 103). Translated into coinmercial terms, the steeper the negative slope of the surface tension-concentration curve of the filmforming reagent, the more effective that flotation reagent will he; that is, fewer moles of reagent per ton of ore will bc needed to accomplish the same recovery. It can readily be understood that such a compound will not always be the cheapest. from the point of view of pounds per ton used to obtain the given percentage recovery of miqeral value. I h a l l y , as the length oi the hydrocarbon chain increases, the frotliing tendency of the resgent becomes more apparent and the froth formed is so tough that it carries up with it rnoclianically not only the mineral sought hut also much of the gangue material.
Selective Differential Flotation
In general, oxidation of tlie surfaces of sulfide minerals will render them incapable of being floated with a given r;iilfirle collector and frother. The acci1rat.e control of this oxidat,ion effect permits selcct,ive differential flotation in many cases where sulfides are concerned. However, other reagents, such as sodium cyanide, effect their depressing action by aetualty dissolving the activating film-e. g., copper sulfateactivated sphalerite. Siiice it has been established that critical concentrations of depressants, activators, and collectors at certain pH values exist for each mineral, it is not surprising to find combinations of these reagents used in the process of selectively separating mixtures of mineral snlfides. One of the accepted practices is first to float sll of the mineral values at once with as littie inclusion of gangue as possible-i. e., molybdenite and pyrite (Climax Molybdenum Company). With this concentratein hand, thematerial is refloated to separate the several components of the mixture, the proper activator or depressant and collectors being used with due regard to concentration, pH value, and temperature. I n other cases the mineral sulfides are floated one at a time-e. g., chalcopyrite from pyrite (Utah Copper Company). Flotation of Oxidized Minerals The flotation of oxidized minerals has elaiined the attention of many investigators, notably Gaudin (57, 39), Taggart (SS), Wark (IO,?), and I’atek (76, 77). Some of these oxidized minerals, such as anglesite (PbSO.,), cerussite (PhCO,),
MAY, 1940
SEPARATION OPERATIONS
malachite [CuC03.Cu(OH),], and azurite [BCuCOa.Cu(OH)r],respond well to higher concentrations of the xanthate type of collector than are necessary for the flotation of sulfides. Presumably the additional collector is used in precipitating dissolved heavy metals from the solution, for these minerals are more soluble than the sulfides. Traube’s rule applies in the selection of the collector, the longer alkyl-group compounds being the more efficient collectors; however, the difficultly soluble collectors of this type are notably inefficient ( 6 7 ) . The fatty acids and soaps, as well as the sodium salts of the sulfated alcohols, find application as collectors of oxidized ores in flotation. Gaudin (36) showed that greater amounts of shorter alkyl-group fatty acids are required for the flotation of malachite than of the longer alkyl-group fatty acids. Manganese oxide ores are now being successfully concentrated by a method developed by the Cuban-American Manganese Corporation (19). Keck and Jasberg (57) found that of the unsaturated fatty acids, oleic acid gives superior results in the flotation of magnetite and hematite. Rroadbridge and Edser (15),Lawrence and DeVaney (67), Belash (5-8), Luyken and Bierbrauer (71) have also contributed methods for the flotation of phosphates. The use of chelate compounds and of normal oximes as collectors for cuprite, malachite, and azurite was reported by De Witt and von Batchelder (24). Sodium sulfide, first discovered by Schmarz (85), is much used as an activator for those oxide ores which allow themselves to be transformed a t the surface into insoluble, adherent sulfide coatings. Mineral oxides thus filmed respond fairly well to the usual sulfide ore collectors and frothers. However, the precipitated sulfide coatings are much more easily oxidized than the natural sulfide surfaces; hence relatively large concentrations of sodium sulfide are sometimes used (as much as 10 pounds of sodium sulfide per ton). Ralston (83) reviewed the present state of knowledge of the flotation and agglomeration of the nonmetallic minerals. This report refers to the flotation of zircon, barite, chromite, cassiterite, beryl bauxite, scheelite, ilmenite, rutile, manganese oxides, iron ore, as well as some thirty-five other important nonmetallic substances-notably phosphates, graphite, limestone, coal, sulfur, salt-crystal mixtures, fluorspar, talc, and feldspar. One of the most recent notable advances in cement technology involving the flotation of calcium carbonate from siliceous limestone was made by Breerwood (14). This process, used by the Valley Forge Cement Company, has made available for cement manufacture large quantities of otherwise low-grade cement rock.
Agglomeration Concentration Agglomeration concentration is accomplished by causing flocculation of the mineral values into massive granules. These granules are held together by means of an addition agent such as a fatty acid or oil. Haynes (51), Froment (35), and Cattermole (16) had noted and made use of this phenomenon, but no real industrial application was carried out until it was found that, on a Wilfley table, the oil-flocculated granules were washed over the table by the cross water and thus left the undesired gangue material in the riffles; this amounts simply to a filming-levitation operation. The importance of the application of these principles to phosphate recovery has caused much litigation, notably in the case of the Christiansen (18) and the Chapman and Littleford (17) patents, owned respectively by the Southern Phosphate Corporation and the Phosphate Recovery Corporation. Other contributors to this field of investigation are Klosky (60), Singleton ( 8 7 ) , Hagood (bo), Green and Wilbur (46),
657
McCoy, Wright, and Hall (73). Limestone was concentrated by agglomeration a t the plant of the Universal Atlas Cement Company by Diener, Clemmer, and Cooke (27). At Carlsbad, N. Mex., silvite and halite were thus separated by Coghill, DeVaney, Clemmer, and Cooke (20). Coal was recovered by Trent (82) by an agglomeration process using hydrocarbon oils as the agglomeration agent.
Future Flotation Research The present magnitude and the evident commercial success of the froth flotation process and of its allied process, agglomeration concentration, affords an assurance that further progress will be made in promoting a more thorough understanding of the principles involved. It is impossible for any one person t o lay out all of the problems confronting this field of applied science. However, it seems that the problem of how to calculate or measure accurately the absolute values of the interfacial tension liquid-solid and gas-solid remains the most important unsolved fundamental problem concerned with flotation theory. Except in isolated instances little trouble is now encountered in the selective flotation of sulfide ores. But the problems of selective flotation of oxidized ores and the flotation of nonmetallic minerals mill require much research. Selective agglomeration separations will likewise command the attention of investigators. The study of the preferential adsorption of various substances on mineral surfaces, both metallic sulfides and metallic oxides, as well as on other nonmetallic minerals, and the method of removal of these adsorbed substances from such surfaces will give much fruitful information relevant to flotation. Such work will undoubtedly result in the development of many new and useful flotation reagents. A study of the function of the Jones-Ray effect (66, 66) in froth flotation may present some interesting results. The continued and extensive work of McBain (72) on the Gibbs adsorption theory and of Langmuir (65) on molecular films will undoubtedly provide important information relevant to froth flotation and agglomeration concentration. Electron-beam diffraction studies may help to decide more effectively the nature of surfaces prepared for flotation. Sometime the perennial controversy should be settled between the insoluble chemical film theory of Taggart and his collaborators and the application of the Horovitz-Paneth adsorption dictum as interpreted by Freundlich, Wark, and others, for both sides have succeeded in satisfactorily accounting for themselves in many instances. From the side lines one ventures the opinion that both are partly right, but that it may take considerable research to settle the question finally. Since this controversy has so far been one of the principal spurs to the progress of the theory and practice of mineral flotation, one feels justified, perhaps, in hoping that it will continue to a vigorous old age.
Acknowledgment
No one can write on the subject of mineral flotation without frequently consulting the excellent writings of Wark (IO$), Taggart (89), Gaudin (36), Petersen (80), and the numerous Reports of Investigations of the United States Bureau of Mines, Transactions of the American, Institute of M i n i n g and Metallurgical Engineers, the “Trend of Flotation” published by the Colorado School of Mines, the Reports of Investigations of the Canadian Bureau of Mines, and the Oficial Gazette of the United States Patent Ofice, as well as articles that have appeared in scientific and technical magazines devoted to surface chemistry and flotation technology.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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