Froths and Frothing Agents

August 30, 2017 | Author: Renzo Chavez | Category: Ether, Alcohol, Physical Chemistry, Chemistry, Chemical Compounds
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FROTHS AND FROTHING AGENTS by R. B. Booth and W. L. Freyberger

Froth flotation i s a chemically induced method f o r beneficiating o r upgrading a n ore, which utilizes a l a y e r o r column of froth a s a separating medium to segregate and remove the valuable minerals from the worthless gangue components of a finely ground o r e suspended in water. The object of this chapter i s to discuss the theoretical aspects of froth formation, the various frothing agents employed in flotation, and the effect of chemical structure on froth formation, and to correlate these factors with flotation technology. The flotation method of separation is conducted in three main steps: 1 ) selective chemical modification of the surface of specific mineral particles to effect floatability o r nonfloatability; 2) contact between a i r bubbles and mineral particles, the selective adherence of floatable minerals to these bubbles, and the rejection of nonfloatable minerals; and 3 ) separation of the floatable minerals from the nonfloatable minerals. A s indicated above, these three operations a r e achieved by the use of chemicals. Frothers play their most important role in the second and third steps by influencing particlebubble contact and by affecting the degree of separation obtained in the froth column. Frothers, a s defined by flotation operators, a r e compounds that a r e used specifically f o r the purpose of creating a froth in a flotation separation. However, s o m e collectors produce froths that a r e adequate for some flotation operations. Although this chapter is chiefly concerned with the specific f r o t h e r s used in flotation, some discussion of frothing collectors is included. Otherwise, collectors (or promoters) and modifiersare not discussed a t length; f o r information on these types of flotation chemicals, the reader i s referred to other sections of this book o r to other s0urces.l HISTORICAL DEVELOPMENT O F FROTHING AGENTS Historically, there a r e two distinct phases in the development of frothing agents and other flotation chemicals: 1 ) 1860 to 1920-oil flotation, and 1921 to date-chemical flotati0n.2~3 E a r l y in the f i r s t period, large quantities of fatty and oily materials, up to 10 to 20% of the weight of the ore, were used a s a m e a n s of separating the sulfide and oxide components f r o m the gangue m i n e r a l s in o r e pulps. Later, various gases replaced these large quantities of oil a s a buoyant and separating medium, decreasing oil requirements to l e s s than 1%of the weight of the ore. With this reduction in oil R. B. Booth, Manager, Mining Chemicals Research Laboratory, American Cyanamid Co., Stamford, Conn. W. L. Freyberger, Research Metallurgist, Mining Chemicals Research Laboratory, American Cyanamid Co., Stamford, Conn.

consumption, inherent differences in the frothing and collecting power of various oils were noted. Frothing properties became associated with compounds known to contain specific chemically functioning groups such a s hydroxyl (-OH), carbonyl ( -CO), ester ( -COOR), and carboxyl ( -COOH), and specific compounds such a s alcohols, ketones, e s t e r s , and fatty acids and their soaps found application a s frothing agents. Monohydroxylated, water-insoluble compounds such a s pine oils, cresols, and the aliphatic alcohols came into general use a s frothers in flotation operations. Toward the end of the f i r s t historical period, a trend was established toward promoters of definite chemical composition by the use of certain types of oils containing sulfur (either naturally o r by direct sulfurization). This trend continued by the later application of aromatic m i n e s such a s naphthylamines, toluidines, and thioureas such a s thiocarbanilide. Modern chemical flotation, beginning in 1925, has been concerned with the following trends: 1) Development of more specific water-soluble collectors: sodium and potassium xanthates from 2C t o 6C alcohols; sodium and ammonium dialkyl (2C to 6C) and cresyl dithiophosphates; and mercaptobenzothiazole derivatives. 2) Extension of flotation techniques to nonmetallic ores: separation of nonsilicate, nonsulfide minerals from quartz and silicate gangues by means of such anionic collectors a s fatty acids and long chain sulfonates; and flotation of silica, silicates, and soluble s a l t s with cationic collectors. 3) Continued use of oily materials in flotation: a s collectors (xanthoyl formates, diaryldithiophosphoric acids, thionocarbamates) for sulfides; and conjoint use of hydrocarbon oils with the above nonmetallic collectors. 4 ) Trends in frothers: continued utilization of the monohydroxylated types; development of hydroxylated, water-soluble frothers (polypropylene glycols and derivatives); and use of nonhydroxylated frothers (alkoxysubstituted paraffins such a s t r i e t h ~ x ~ b u t a n e ) . Most of the sulfide promoters, both oily and water-soluble, which were developed in the second historical period of flotation, a r e nonfrothing and generally require the conjoint use of specific frothers. Certain of the longer chain, water-soluble dialkyl and dicresyl dithiophosphate salts and the oily dicresyldithiophosphoric acid (the last two of which may contain some f r e e cresylic acid) produce froth, sufficient for conducting flotation operations in of nonmetallic o r e s some cases. The collectors utilized in the co~~centration a r e froth formers and may be used with o r without auxiliary frothing agents. PHYSICAL ASPECTS O F FROTHING SURFACE ACTIVITY: When aqueous solutions a r e made of heteropolar organic compounds such a s those used for frothers, it i s found that, a s the concentration of the solute is raised from zero to the saturation limit, the surface tension of the solution decreases to a minimum value, usually much lower than the value for pure water. Thisdecrease in the surface tension i s a result of the heteropolar nature of the compounds which leads to preferential adsorption of the frother molecules* at the air-solution interface, the amount of adsorption increasing *In the rest of this section, the frothers will be referred to a s 'molecules' for the sake of simplicity. Of course, it will be remembered that many frothing materials such a s the soaps are ionic in aqueous solution.

with increasing concentration. At the liquid-gas interface, surface-active molecules a r e arranged with their hydrophilic, polar groups immersed in the aqueous solution, while the hydrophobic, nonpolar hydrocarbon chains extend into the gas phase.4 These molecules in the surface layer a r e in dynamic equilibrium with those in the solution, much like the equilibrium between the molecules of a vapor and its liquid. The degree of adsorption of such surface-active molecules is related t o the decrease in surface tension through the Gibbs adsorption equation,4,5

where rl is the surface excess concentration of the solute (in this case the frother) in terms of mol per cm2; y is the surface tension in terms of dynes per cm; C i i s the bulk concentration of the solute; R i s the gas constant, 8.31~10-~ ergs per deg-mol; and T is the absolute temperature. The term &/&is the slope of the curve relating surface tension and concentration of the frother. Like all rigorously derived thermodynamic relations, the proper form of Eq. 1 involves the activity of the solute rather than the concentration. However, since most frothers a r e nonionic and a r e generally employed at mol per 1, the substitution of concentration concentration levels below for activity is acceptable. ~ a t relating a ~ surface tension to concentration for aqueous solutions of some alcohols similar to those used a s frothers a r e presented in Fig. 1. The curves apply to aqueous solutions of n-pentanol (CSHIIOH),n-hexan01 (Cs HISOH), n-heptanol (C7HlsOH), and n-octanol (C8H170H). It i s evident that for this s e r i e s of homologs, the curves become much steeper as the number of carbon atoms in the alcohol is increased. Actually, according to









0 0.01 0.02 0.03 0.04 OD5 0 ALCOHOL C O N C E N T R A T I O N , MOLES/LITER

Fig. 1-Surface tension of aqueous solutions of some aliphatic alcohols. Data from Posner e t a1.6

0.2 0.4 0.6 0.8 CONCENTRATION, G/LITER


Fig. 2-Surface tension of aqueous solutions of pine oil and some of i t s major com onents. Data from DeWitt and Makens.


Traube's rule, ay/acl i s threefold greater for each added CH, group.4 Since the value of i3y/acl in Eq. 1 is a measure of the steepness of the curves, and since ay/acl is negative for the curves shown in Fig. 1, it follows that 1) the surface excess concentration of these alcohols i s always positive, that is, these alcohols a r e positively adsorbed a t the surface; and 2) the adsorption a t a given concentration increases with increasing chain length. These r e sults a r e typical of those obtained with surface-active compounds. There is, however, a great lack of such information concerning the frothers used in flotation, particularly for the polyglycols and alkoxyalkanes. Data available7 for aqueous solutions of pineoil and of some of i t s components a r e reproduced in Fig. 2. The two components, borneol and fenchyl alcohol, constitute about 15 to 20% of pine oil; a-terpineol constitutes about [email protected],9 The properties of these three components of pine oil appear to be very similar. SOLUBILITY OF FROTHERS IN WATER: The solubilities of various frothers and alcohols similar to those used a s frothers a r e listed in Table 1. These data demonstrate that, with the exception of polyglycol derivatives, the compounds commonly used a s frothers a r e only slightly soluble in water, the solubilities ranging from about 0.7 gpl (borneol) to about 17 gpl (methylamyl alcohol). It might be noted that addition of 0.10 lb of frother per ton of o r e to a flotation slurry containing 33% solids by weight corresponds to a concentration of frother in the solution of 25 mg per 1, considerably lower than the solubility limit of any of the compounds listed in Table 1. TABLE 1. Solubility of Some Flotation Frothers in Water

Frother 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Solubility. G per 1000 G of Solution

n -pentanol1° Isoamyl alcohollO 'n -hexan0110 Methylamyl alcoholl1 n -heptanol10 ~eptanol-312 n -0ctanoll0 Octanol-2 (capryl alcohol)l0 Pine oilll a-terpineol1° Borneo111 Cresylic acid11 1,1,3-tr~ethox~butanell Polypropylene glycol,12 mol wt 400-450

21.9 26.7 6.24 17.0* 1.81 4.5 0.586 1.28* 2.50* 1.98* 0.74 1.66* -. 8* Complete

Temperature, OC 25 25 25 20 25 20 25 25 25 15-20 25 20 20 20

*Solubility in terms of g per 1000 ml of water.

STABILITY OF FROTH: The stability of froths o r foams i s in large part a function of the rapidity with which the surface tension can vary when a bubble i s subjected t o mechanical stress. As a bubble i s stretched o r compressed, the surface

tension must be capable of a corresponding rapidincrease o r decrease to offs e t the shock. This quality is r e f e r r e d to a s the 'Gibbs elasticity'. Because of this requirement, it is often found that the maximum froth stability a r i s e s , not in the region of minimum s u r f a c e tension, but in a region where the s u r f a c e tension is capable of rapid change with concentration. For example, a f r o t h e r concentration of 25 m g p e r 1 corresponds to a molar concentration of the o r d e r of 1-3x10-' mol p e r 1 f o r f r o t h e r s commonly in use. The data in Figs. 1 and 2 show that the s u r f a c e tensions corresponding to such frother concentration a r e f a r above the minimum values, and may correspond t o s u r f a c e tension reductions of only a few dynes per centimeter from the value f o r pure water. These concentrations do correspond to regions where the surface tension is s t i l l capable of rapid change with concentration. Although it i s generally found that foam stability i s maximum where the s u r f a c e tension can vary rapidly with concentration, this is not always the case. F o r example, soap solutions often produce very stable foams f r o m solutions having a s u r f a c e tension n e a r the minimum value.4 Such behavior is probably related to the slowness of diffusion of soap molecules a t the s u r face. The contribution t o the Gibbs elasticity from lack of equilibrium in an aqueous solution of l a u r y l alcohol and sodium lauryl sulfate has been disc u s s e d by Sporck. l 3 Since a pure liquid a t constant temperature and p r e s s u r e possesses a cons t a n t value f o r the s u r f a c e tension, i t i s evident f r o m the above discussion that pure liquids cannot foam. n l studied ~ how foam stability Recently, Brown, Thurman, and ~ c ~ a i have is related to viscosity of the f i l m f o r m i n g t h e foam and to the a i r permeability of that film. They found that froth stability i n c r e a s e s with increasing viscosity and with d e c r e a s i n g permeability. They further postulated that syst e m s producing a s t a b l e foam would be those containing two surface active substances, one of which s e r v e s a s a r e s e r v o i r of material in the bulk of the solution, the other to condense the adsorbed layer a t the interface s o a s to r a i s e the viscosity and d e c r e a s e the permeability. Presumably, this condensation o c c u r s through hydrogen bonding. These authors also suggest that a m o r e useful m e a s u r e of foam stability than that based on volume change alone would a l s o include a factor f o r the change of specific surface of the foam with time."14 DeWitt and Makens attribute the foam stiffening prop e r t i e s of the terpinolene, pinene, and dipentene components of pine oil to their ability to r a i s e the viscosity of the foam.7 The effect of t r a n s p o r t of the s u b s t r a t e on foam stability has been cons i d e r e d by E w e r s and ~ u t h e r l a n d . 1 5 They postulate that such motion arising f r o m establishment of a s u r f a c e tension gradient due to shock t o the film i s of p r i m a r y importance to the stability of a foam. Under the influence of such a gradient, the s u b s t r a t e m a t e r i a l will move from a region of low s u r f a c e tension t o one of a higher value. When such motion is toward the center of a disturbed a r e a , i.e., when the s u r f a c e tension i s highest a t the center of the disturbance, the film will be stable, and vice versa. ~ r o b e l l lh a s studied the r a t e of adsorption a t the air-solution interface f o r a number of commonly used f r o t h e r s by measuring the variation of s u r f a c e tension with t i m e f o r aqueous solutions of the reagents. He found that solutions of pine oil and triethoxybutane show appreciable variation in s u r f a c e tension with t i m e , indicating a low r a t e of migration of molecules to the interface. On the other hand, solutions of isoamyl alcohol and methylamyl alcohol appear to r e a c h equilibrium in a few seconds. This might explain in

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part the difference in frothing behavior of these compounds as discussed below. Pine oil and triethoxybutane produce relatively stable froths while the alcohols produce a more ephemeral froth. THEORETICAL ASPECTS OF THE INFLUENCE OF FROTHERS ON FLOTATION There has not been a great deal of study of the influence exerted by frothe r s on the actual flotation process. Leja and ~ c h u l m a n l shave studied the interactions of frothers and collectors in a number of systems. They concluded that long chain xanthates (12C) could condense with long chain alcohols (16C) or alkyl sdlfates (12C) to give very stable films on a Langmuir trough. They also claim that the alcohols will prevent oxidation of the xanthates. Further studies demonstrated the degree of interaction of such reagents a s ethyl and amyl xanthates with alcohol frothers (5C to 10C, cresols, and terpineol). These interactions influenced both flotation recovery of galena and froth volume. Finally they postulate a mechanism of bubble-particle attachment which involves first the formation of diffuse monolayers of associated and unassociated collector and frother molecules. a t both the air-liquid and solid-liquid interfaces. On contact, these layers penetrate one another and the molecules at the air-liquid interface attach themselves to the solid surface, thus increasing the hydrophobic properties of the solid surface. The layers also condense further due to interaction of the penetrating molecules, helping to stabilize the froth. Should the layers a t the two interfaces be too highly condensed prior to bubble-particle contact, that contact is prevented. They also attribute the failure of 'over oiled' particles to float to the inability of the two layers to interact due to the presence of the interfering oil layer. In earlier work, Taggart and ~ a s s i a l i s l vreported that induction times for particle-bubble contact in the measurement of contact angle were greatly reduced by the presence of undissolved frother (cresol). However, they attributed this result to the formation of an oil film on the air bubble and attached to it through the frother molecules. (such an arrangement wouldrequire the hydrophilic, polar groups in the frother to be dissolved in the frother.) The particle-bubble attachment subsequently takes place through the oil layer. Leja and Schulman suggest that this is unlikely.16 As brought out in the above discussion, experimental evidence has been presented which indicates that frother molecules are adsorbed at a collectorcoated mineral surface.l6,18 It i s suggested that this adsorptian takes place through formation of weak, inter-molecular bonds between the collector and frother at the mineral surface. However, the conditions of the tests were in general not very close to those met in actual flotation circuits, particularly in sulfide flotation. Conditioning times were usually quite long and the r e agents studied were not typical of those used in practice. ~ e j a l 8studied the effect of addition of ore and of reagents on the surface tension of a flotation liquor. He postulated a relationship between the observed increases in surface tension which occurred on addition of the flotation collectors and activators and the abstraction of frother from solution by the floatable sulfides. No data are presented concerning the effects of the flotation reagents alone on surface tension. This whole problem of the interaction of frothers and collectors has not been adequately studied. Sutherland and ~ a r k l have g supported the position that there is no interaction between frothers and collectors. They base their

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view on the evidence that addition of frother to a flotation system will, in general, have no effect on the contact angle, since a contact can be established between an a i r bubble and a mineral surface in the absence of a frother. However, a s pointed out by ~ e j a , l 8the results of static measurements may not be completely translatable t o the actual flotation process which i s a dynamic system. The resolution of this particular question would be an important contribution t o the fields of flotation science and technology. One further influence of f r o t h e r s on particle-bubble attachmentwas studied by Fuerstenau and ~ a y m a n . 2 0 These investigators found that the presence of a frother tends to maintain sphericity and smoothness in the shape of a i r bubbles rising up a column of water. In the absence of the frother, bubbles of the s a m e s i z e assumed very much m o r e distorted shapes and underwent violent oscillations which would probably cause solid particles to be repelled f r o m the bubbles. The presence of a frother was also found to slow the r i s e of the bubbles in the tube. w r o b e l a l studied the effect of frother additions on the power required by a laboratory flotation machine. He found that for frother additions up to about 20 m g per 1 (approximately the concentration encountered in flotation) the power requirement decreased by about 1096, but further increases in concentration had no effect. Of the f r o t h e r s studied, pine oil, cresylic acid, and terpineol had the least effect; triethoxybutane had the greatest. Wrobel r e lates this effect on power requirement t o the extent to which a frother dec r e a s e s the s u r f a c e tension of the solution. In the same paper21 Wrobel presents data concerning the effect of frother concentration on the rougher concentrate grade and on the 'wetness' of the froth. The latter function i s defined a s follows: Wetness


Wt of water in froth Wt of galena in froth


cu f t of a i r per min


[ 21

Choosing data f o r frother concentrations of 24 m g per 1, he found that the o r der of increasing concentrate grade is pine oil, a-terpineol, cresylic acid, methylamyl alcohol, isoamyl alcohol, and triethoxybutane. Exactly the oppos i t e o r d e r was found f o r increasing wetness of froth. Urfortunately, no data on galena recovery, a r e presented, nor does the study include the frothers derived f r o m polypropylene glycol. CHEMICAL PROPERTIES O F FROTHERS The general types of compounds used a s specific frothers in flotation a r e listed in Table 2. Alcohols used a s f r o t h e r s a r e heteropolar and contain a single polar group which is hydroxyl (OH). These compounds generally show limited solubility in water. The polypropylene glycols contain two hydroxyl groups which tend t o induce water solubility; these compounds and their short chain ethers also reflect the solubilizing influence of the repetitive ether linkages in their structures. Triethoxybutane is a frother which contains no hydroxyl groups and has limited solubility in water. The aliphatic alcohols used a s frothers have chain lengths of 5 to 8 C atoms and include mixed amyl alcohols, methylisobutylcarbinol (methylamyl alcohol), and technical grade heptanols and octanols, and mixtures of certain of these alcohols. Capryl alcohol, octanol-2, is an excellent frother.

TABLE 2 . General Types of Compounds Used a s Specific Frothers Chemical Type

Type Formula

Representative Frothers Aliphatic Alcohols: R i s a straight or branched 5C - 8C chain



Cresylic Acids: R i s a benzene ring with short chain alkyl substituents Pine Oils: structure

Hydroxylated polyethers

Alkoxy substituted paraffins

R i s a terpene



Polypropylene glycols of low mol wt

R' O(R0)xH

Methoxytripropylene glycol.

(R' a X R


Provided there i s sufficient branching in the chain, alcohols up to the 10 to 12 C-atom range give froths which in volume and texture resemble those of the lower alcohols, but have not found wide application because of price. Pine oi18,9 produced from turpentine o r from pine-stump wood by distillation o r solvent extraction, i s a mixture of terpene alcohols, ketones, ethers, and hydrocarbons. The alcohol components, CY and P-terpineols, fenchyl alcohol, and borneol, and camphor ( a ketone) a r e the chief frothing components. The terpene hydrocarbons, though nonfrothers, a r e reported to improve the froths produced by the above compounds a s well a s enhancing collecting action on sulfide minerals. Cresylic acids22 obtained from the byproduct coking of coal and from the thermal cracking of petroleum, a r e used a s flotation frothers. Recovered from phenolic fractions by caustic extraction followed by acidification and distillation, the fractions used in flotation a r e mixtures of isomeric methyl phenols and xylenols with some higher boiling phenols and s m a l l quantities of phenol, water, pyridine bases, neiltral oils, and sulfur-bearing compounds. The petroleum cresylics a r e more widely used thanthose obtained from coal. Polypropylene glycol frothers a r e of two related types, the polyglycols themselves and their lower ethers.23,24 Both types a r e soluble in water and, accordingly, represent a departure from the criterion that flotation frothers should exhibit only limited solubility in water? The polypropylene glycols of low molecular weight, prepared by reacting propylene oxide with propylene glycol, a r e excellent frothers. The ethers of polypropylene glycol, a s typified by the compound methoxytripropylene glycol, a r e prepared by reacting propylene oxide and an alcohol under pressure; they also a r e potent frothers. Triethoxybutane and related compounds25,26 a r e synthetic, nonionic frothers, containing no hydroxyl groups. These compounds show limited solubility in water, a r e readily dispersible, and a r e persistent frothers over a wide pH range. Chemical structure exerts a marked influence on froth volume, texture, and stability. The following factors27 have been noted in the c a s e of the classical monohydroxylated frothers, which a r e currently the most widely used in flotation practice.

Higher frothing power i s exhibited by a normal alcohol r a t h e r than by its i s o m e r s . Also, h i @ e r frothing power i s shown by aliphatic alcohols rather than by corresponding a r o m a t i c alcohols. The frothing power of such a r o matic compounds i s enhanced by saturation of thedouble bonds in the aromatic nucleus; saturation tends to minimize the differences in frothing capacity between a n aromatic and a corresponding aliphatic compound. In the c a s e of terpene compounds, frothing is improved by increasing the number of double bonds in the molecule; saturation of these bonds causes a d e c r e a s e in frothing. The side chain on the benzene r i n g of a n aromatic frother e x e r t s a n influence on i t s frothing power. An i n c r e a s e in frothing i s noted if a methyl (CH3) group i s added, but further i n c r e a s e in the length of the side chain produces only limited improvement in frothing capacity. Since f r o t h s should exhibit limited stability and break down on removal f r o m the flotation cell, the hydrocarbon portions of the frother should contain no m o r e than 8 C a t o m s in a single chain o r branch. Water solubility is influenced a l s o by these nonpolar portions of thefrother molecule. Thus, in the c a s e of the aliphatic alcohols used a s frothers, the optimum chain length i s 5-8 C atoms. The frothing c h a r a c t e r i s t i c s of cresylic acids vary with the length of the alkyl groups on the benzene ring in the nonpolar segment of the f r o t h e r , a factor which a l s o influences boiling range and water solubility. As indicated above, the polar groups included in frother molecules a r e hydroxyl (-OH) and e t h e r linkages (-0-). These groups d o not f o r m stable bonds a t mineral s u r f a c e s , a property which is in s h a r p contrast to the surface-bonding c h a r a c t e r i s t i c s of functional groups in promoters, i.e., the sulfhydryl group common to s t r o n g sulfide promoters and the carboxylate, sulfate, sulfonate, and amine groups in nonsulfide promoters. Thus, the compounds commonly used a s f r o t h e r s d o not possess strong collecting teridencies. In addition to the above compounds which a r e utilized in flotation for the specific purpose of c r e a t i n g a froth, other compounds, employed primarily as collectors f o r nonsulfide m i n e r a l s , show a marked tendency to produce froth. The general types of compounds that may be classed a s frothing-collectors a r e found in Table 3. The main froth-producing carboxylates u s e d a s anionic collectors for nonsulfides a r e fatty acids, r o s i n acids, combination fatty-rosin acids a s found in c r u d e and refined tall oils, naphthenic acids, and related compounds derived TABLE 3. General Types of Compounds Used a s Frothing Collectors Chemical Type

Type Formula

Representative Compounds

Long chain carboxylates


Fatty or rosin acids such a s oleic acid, tall oil, etc.

Long chain sulfonates or sulfates


Oil and water-soluble petroleum sulfonates

Long chain amines


Octadecylamine (free base or a s acetate)



from petroleum sources. These compounds a r e used a s collectors chiefly a s the free carboxylic acid and occasionally a s soaps. Oleic acid was w e d extensively a s a collector when flotation was first introduced for the treatment of nonsulfide ores, but has been replaced by the lower priced fatty acids of low rosin content obtained by refining of tall oils. These collectors contain unsaturated fatty acids such a s oleic, linoleic, and linolenic acids. The chief sulfonate collectors used in the flotation of nonmetallics a r e the petroleum sulfonates (sodium salts) of both the water-soluble and the oilsoluble types, which a r e obtained mainly as byproducts from the refining of white oils and the sulfonation of other hydrocarbon fractions. Other sulfonates and sulfates, such as the alcohol suliates, sulfonated glyceride oils, and other surface-active agents have not found extensive use in the flotation of nonsulfides, but a r e employedin some instances a s emulsifiersfor hydrocarbon oils and insoluble anionic collectors such a s the fatty acids and tall oils. Long chain amines, about 12 to 20 C atoms in chain length, are the principal cationic promoters employed in the flotation of silica, silicates, and salts such a s potassium chloride. Modified amines, e.g., the condensates of polyBlkylene polyarnines with fatty acids, a r e also in commercial use. PRACTICAL ASPECTS OF FROTHING GENERAL SPECIFICATIONS OF FROTHERS: Selection of a frother for any particular flotation operation will be contingent upon how closely a compound fulfills the following ideal requisites in any particular separation.27 1) Low concentrations should produce continuously a froth of sufficient volume and toughness to act a s a medium of separation of the floated minerals from the ore pulp. 2) The froth should break readily after being removed from the flotation cell to allow concentrates tobe flushed or pumped to reflotation (cleaning) or to recovery by thickening-filtration. 3) Froth texture should allow for elimination of gangue particles, especially in the case of ore slimes. 4) Cost and availability should be satisfactory f o r large-scale use. 5) Low chemical activity and limited collecting tendency should be exhibited; collecting activity if shown should be selective for the minerals to be floated. 6) Sensitivity to pH change and dissolved salt content of flotation pulp should be low. Limited solubility in water, frequently specified in the range of 0.02 to 0.05% a s exhibited by the classical monohydroxylated frothers of the alcohol type,% is obviously not a rigid requirement in view of the application of the water-soluble polypropylene glycol derivatives a s frothers. THE FUNCTIONS OF A FROTH IN FLOTATION: The froth generated by the action of a flotation machine acts a s a separating medium to segregate and remove the valuable mineral particles from the gangue particles of the ore. These functions of the froth a t the surface of the pulp in a flotation cell a r e illustrated in Fig. 3, which i s a modification of a similar illustration originally presented by ~ a g g a r t from l observations obtained in a glass-walled flotation cell. Fig. 3 represents in an idealized manner the flotation of copper minerals (black) from gangue (light colored).

Fig. 3-Schematic representation of a vertical section through a flotation froth. Froth column a s a separating medium. Solid squares are copper minerals (floated) and open triangles are gangue minerals (rejected).

This d i a g r a m is a c r o s s section of a froth column, ranging vertically f r o m the froth-pulp interface to the upper portion of the froth, which is removed by skimming o r by overflowing f r o m the flotation cell. The agitating-aerating action of the flotation machine d i r e c t s o r e pulp toward the b a s e of this froth column. The separation of the valuable copper m i n e r a l s (black in the diagram) f r o m the gangue minerals (light colored) takes place a s shown. Concentration of the d a r k minerals occurs throughout the column and finally r e a c h e s a maximum for the operation in the mineralized l a y e r a t the top of the froth, while the r e v e r s e occurs in the gangue content of the f r o t h column. The above observations may be confirmed by withdrawing samples of the m i n e r a l s a t v a r i o u s levels in the underlying pulp and in the froth column and t l shown that, in the flotaassaying these samples. F o r example, ~ a ~ g a rhas tion of a copper o r e , the copper content was 0.67 t o 0.76% Cu for samples taken f r o m the pulp and 0.77% f o r s a m p l e s taken a t the pulp-froth contact. F o r s a m p l e s taken a t successively higher levels in the froth column, the copper contents w e r e 2.07, 4.70, and 12.43% Cu and finally 38.8% Cu in the mine r a l i z e d l a y e r at the top of the froth column (also s e e Fig. 6, Chapter 18 0. Fig. 3 a l s o s e r v e s t o i l l u s t r a t e the effect of froth texture o r structure on the r e c o v e r y of floated p a r t i c l e s and the grade of the flotation concentrate. It will be immediately apparent that small-bubbled, closely knit froths will be conducive to high r e c o v e r y ; s u c h f r o t h s support heavy mineral loads and p r e vent drop-out of values f r o m the froth. On the other hand, a loosely textured, larger-bubbled froth will tend to r e l e a s e gangue p a r t i c l e s f r o m the bubble aggregate and tend to produce concentrates of higher grade. The functioning of a froth column a s a separating medium usually i s well illustrated in conducting flotation testing on a sulfide o r e in a laboratory machine such a s the F a g e r g r e n type. On addition of a frother and collector, a column of bubbles 1 to 2 in. in depth commonly f o r m s and supports a well flocculated l a y e r of sulfides a t i t s surface. Removal of this surface layer by skimming gradually produces a b a r r e n froth which frequently p e r s i s t s after the m i n e r a l load h a s been taken off. The mineralized froth, after being skimmed f r o m the cell, b r e a k s down immediately. In some cases s i m i l a r frothing conditions occur in the t r e a t m e n t of nonsulfides. From this condition, ideal not only f o r the separation of the floated minerals from the body of

the pulp but also for upgrading of the concentrate, froths range to much lower volumes and even to heavily flocculated curds supported by a cushion of bubbles just sufficient to allow the scraping of the concentrated minerals from the underlying pulp. This latter condition sometimes i s noted when froths a r e required to carry heavy mineral loads. In extreme cases, the heavily mineralized froths become relatively dry and brittle and flow with difficulty. FACTORS INFLUENCING FROTHING: A multiplicity of variables affect flotation and several of these influence frothing. As indicated above, the volume, structure, and stability of a froth will depend on the type of frother used and also on the type of collector and modifier used. The efficiency of any frother will ultimately be judged on the recovery of values and the grade of the flotation concentrate obtained, factors dependent upon the cooperative roles played by the frother-collector or frother-collector-modifier combination and the type of flotation machine used. The type of ore solids, the degree of comminution, and the presence of slimes, either naturally occurring o r produced in the grinding o r flotation operation also affect frothing. Frequently, it i s difficult to create a froth in the treatment of coarsely ground ores in which essentially no slimes a r e present; excessive quantities of slimes on the other hand impart bulk and rigidity to froths, rendering them difficult to break down and handle inlarge-scale operations. Frothing i s influenced also by the presence of dissolved inorganic salts and organic matter present in the ore, the pH of the o r e pulp, and the presence of oils and greases used a s lubricants in mining and pretreatment operations. Oily materials including oily promoters generally show amarked tendency to flatten froth in a flotation cell. Impurities in frothers themselves, even in small quantities, sometimes have a marked effect on frothing power and structure. Highly insoluble components frequently a r e observed to float off in the froth, to concentrate in localized areas at the froth surface, and to depress frothing. THE FROTHING PROPERTIES OF COMMON =OTHERS: The frothing action of the main substances usedas specific frothers and a s frothing collectors in flotation a r e described briefly a s follows. Pine oil generates a small-bubble froth of closely knit texture, which breaks down readily on removal from the flotation cell. This froth structure does not allow fallout of mineral grains s o readily a s more loosely constructed froths and in some cases does not yield a s high-grade concentrates a s other frothers but has atendency to produce higher recoveries. Excessive quantities of pine oil tend to flatten flotation froth, decrease i t s volume, and cause effervescence at the surface. The froths resulting from the use of cresylic acids a r e generally similar in structure to those of pine oil, but a r e of somewhat larger bubble size. Froth volume i s decreased when an excess of frother i s applied and effervescence results. The froths created by the aliphatic alcohols a r e usually larger in bubble size and of less compact structure than those from pine oil and cresylic acids. The addition of large quantities of these frothers to the flotation cell tends to form a closer structure, but does not decrease froth volume a s markedly as noted in the case of pine oil o r cresylic acid. In many instances, the froth produced by the alcohols i s not a s lasting a s those produced by other frothers and requires staged additions to maintain it a t full volume. This tendency i s an advantage in some differential flotation operations where more than a sin-

gle flotation circuit i s involved and excessive c a r r y over of frother from the f i r s t to subsequent circuits i s not desired. Also, the close froth structure is conducive to gangue elimination and, particularly in the case of o r e s containing high percentages of slimes, improves concentrate grade. The polypropylene glycols and their short chain ethers, which a r e actually polymers of propylene oxide and a r e nonionic surface active agents, produce compact, lasting froth structures. These froths, however, break down readily after being removed from the flotation cell. Unlike the frothers of limited water-solubility, the polyglycol-derived frothers do not tend to flatten o r kill frothing when added in large amounts. In such cases, these frothers, being water soluble, produce m o r e tightly knit froths, but do not cause effervescence from the froth surface. These frothers a r e in some cases more selective than the classical insoluble frothers and duplicate their performance a t lower concentrations, a factor which in certain operations balances the higher cost of the polyglycol compounds. Triethoxybutane produces froths resembling those of pine oil. Such froths show l e s s tendency to flatten and effervesce when excessive quantities a r e used and i n t h i s r e s p e c t a r e similar to those produced by the polyglycol frothe r s . Triethoxybutane and related f r o t h e r s at the present time find fairly extensive use in African flotation plants on ores of various types. The froths resulting from the use of fatty acids, sulfonates, and amine collectors in the flotation of nonsulfide o r e s a r e closely textured, stable-bubble aggregates which frequently a r e difficult to disintegrate even by the application of water jets and sprays. Flotation operators seek to avoid voluminous overfrothing and permanent, lathery froth structures, since these a r e not conducive to the production of high grade concentrate and a r e difficult to handle. The addition of f r o t h e r s along with the nonsulfide collectors willat times improve these froth conditions, particularly if large quantities of slimes a r e present in the o r e pulp. OTHER FROTH-PRODUCING SUBSTANCES: Although a variety of compositions f r o m natural and synthetic sources exhibit frothing properties and a r e described inthe flotation patent literature a s f r ~ t h e r s ,m~o s t of these materials have found only limited use in large scale practice, either a s frothers o r a s froth modifiers. For example, creosotes a r e used conjointly with frothers to improve load-bearing capacity of froths f o r coarse-sized mineral granules o r middling particles. Also, hydrocarbons a r e used to extend frothers such a s the alcohols and cresylic acids and also to modify and stabilize froth structure. Kerosene and fuel oil often effectively control the voluminous overfrothing caused on occasion by saponified fatty acids and tall oils discussed above. SURVEY O F PRACTICAL USAGE O F FROTHERS In o r d e r to determine whether any pattern exists in the usage of frothers in the milling industry, a study was made of the reagent practice in almost 400 m i l l s located throughout the world (with the exception of the Soviet union). The mills were grouped accordingto the type of o r e being treated and the major product o r products of flotation. The number of mills using a particular f r o t h e r o r combination of f r o t h e r s w a s determined for each group. This basi s of comparison was chosen in preference to determining the number of tons of o r e treated with a given frother in o r d e r to avoid placing overwhelming

stress on one frother only because itwas used in one or two very large mills, while a great number of smaller mills had other choices. The information in this survey was obtained directly ftom the operators involved by the field representatives of the Explosives and Mining Chemicals Dept. of American Cyanamid Co. and those of its Canadian and international affiliates. Deliberately, no attempt i s made to connect such data with any particular flotation operation, since the study was madefor the purpose of observing trends in frother usage rather than the specific practice on any single ore. Some selection was made in listing the mills for purposes of this survey. Only those mills were listed which treated 100 tpd or more, and the millmust have been operated at some time during the past five years. When classifying the frother usage for a given mill, it was also necessary to make somewhat arbitrary decisionsinabout 1% of the cases. Most of these instances arose when one or more of the liquid dicresyldithiophosphates was used a s a collector along with a frother other than cresylic acid. In such instances, the frother usage was listed a s a combination of the frother plus cresylic acid, since these dithiophosphate collectors a r e based on cresylic acid or contain some free cresylic acid. Most of these cases involved the use of methylisobutylcarbinol (MIBC) or one of the polyglycol frothers along with the dicresyldithiophosphate collectors. If only a dicresyldithiophosphate was added to the circuit, the frother was listed a s cresylic acid. Finally, in some cases of selective flotation of complex ores, such a s lead-zinc ores, where no frother was added to the second flotation circuit, it was assumed that the frother added in the first circuit was effective in the second. If the frother added to the second circuit was different from that added to the first, it was assumed that the new frother added to the second circuit was the only effective one in that circuit. The results of the survey a r e summarized in Tables 4 and 5, which list the frother practice for each of nine different general types of flotation. In Table 4 the heading of each column gives, along with a description of the type of flotation, the total number of mills included in the group. The different types of ore included in each column heading a r e more explicitly identified in footnotes in the table. The data listed in each column in the table give the ercenta e s of the total number of mills using a particular frother. Thus, of the 35 mills classed a s treating complex copper ores (column 4) use methylisobutylcarbinol a s a frother, and 23% use pine oil. The data were normalized in this way to simplify the reading of the table. Frother identification is given in the first column of the table. The frothers a r e listed singly or in four major combinations (pine oil plus cresylic acid, polyglycols plus pine oil and/or cresylic acid, MIBC plus pine oil and/or cresylic acid, and a miscellaneous group including all other combinations). Before discussing the data presented in Table 4, it would be well to recall that many factors influence the choice of frother, not all of these arising from purely metallurgical considerations. For instance, in60W of the cases involving selective flotation of sulfides, the same frother, or no frother at all, i s added to the second flotation circuit a s was added to the first. It is reasonable to suspect that in some of these instances this situation arises from factors other than the behavior of the frother in flotation circuit. Again, it was observed while making the survey, that for some mining companies operating more than one mil1,not necessarily on the same type of ore, the same frother was used in all of the mills. Mention of these observations is innoway meant


TABLE 4. Frother Usage Simple Complex Gold Copper Copper Lead Zinc Ores,* 0res,t Ores,t ores! Ores,ll 52 Mills 66 Mills 35 Mills 95 Mills 104 Mills Frother Pine Oil

Percentage 15





Bulk Pyrite Sulfide ores,# Ores, Coal** 14 Mills 23 Mills 1 5 Mills of

Total 50

Amine Flotation, 21 Mills

Fatty Acid, Soap, Sulfonate Flotation, 61 Mills

Mills 17


30 (Mica; Feldspar)


8 11 8 27 17 22 13 --5 Cresylic acid Pine oil and cresylic acid 19 5 3 2 6 -9 ---Methylisobutylcarbinol (MIBC) -14 26 29 33 -25 53 25 (KCl) 11 Other alcohols 4 3 3 -4 7 4 7 5 (KC1) 5 Polyglycol types 13 21 8 9 16 7 9 13 5 (Feldspar) 7 Triethoxybutane (TEB) 15 8 3 --7 ---------30 (Quartz) 64 None 9 Combinations (other than pine oil and cresylic acid): Polyglycols and -4 5 8 2 2 ----a ) pine oil --2 ---bj cresylic acid 8 1 3 2 ---------c ) pine oil andcresylic acid 6 MIBC and --5 6 2 1 -9 -a) pine oil 2 -b) cresylic acid 4 3 6 12 3 -5 ------C ) pine oil andcresylic acid -----7 -Miscellaneous 2 7 3 2 3 -9 -5 *Ores from which gold i s the only important product recovered by flotation, probably in most cases in pyrite. t o r e s f r o m which copper i s the product of major importance. 49 o r e s classed a s containing copper only, 17 ores classed a s containing some gold and/or silver. $Ores from which two important products a r e obtained, generally by selective flotation. 14 Cu-MoS2ores; 13 Cu-Zn ores; 2 Cu-CO ores; 6 Cu-Ni ores. 8 4 lead o r e s ; 71 Pb-Zn ores; 15 Pb-Cu-Zn ores. In the last named subgroup i t was assumed that a lead, copper float was aimed primarily a t recovering the lead. 11'8 zinc ores; 70 Pb-Zn ores, 1 3 Pb-Cu-Zn ores; 13 Cu-Zn ores. (Zinc frother unidentified in one Pb-Zn o r e and two Pb-Cu-Zn ores.) # ~ u l ksulfide flotation from ore, not followed by differential flotation. Eight instances of flotation of sulfides from tungsten ores. **Survey limited to domestic mills in this group.

TABLE 5. Frother Requirements, Lb Per Ton of Ore Treated

Sulfide Ores All Types

Fatty Acid, Soap, Sulfonate Flotation

Amine Flotation














Pine oil













Creeylic acid







Methylisobutylcarbinol (MIBC)







Other alcohols








Polyglycol types












*Data from one mill only. t ~ r o t h e radded only when needed.

No mills



No mills


0.06* 0.07

0.16 No mills




No mills


No mills

to imply that such operation i s not optimum. However, factors thus a r e i l l u s trated which could not be taken into account in making this survey and which may s e r v e to place s o m e limitation on the utility of the data. In addition to the f a c t o r s just described, t h e r e a r e those related to o r e properties and mill operation discussed e a r l i e r which could not be considered in the survey. In spite of the complexities involved, Table 4 is a practical method of surveying the f r o t h e r s in c u r r e n t u s e on various types of o r e s a n d allows certain useful generalizations to be drawn. The data, a s s e t forth in Table 4, show s o m e trends in frother usage depending on the type of o r e being floated. Pine oil would s e e m to be the m o s t widely used single frother. It holds a position of importance in a l l types of flotation with the notable exceptions of lead flotation, where i t s use i s very s m a l l , and of zinc flotation, where it i s of secondary importance. In other types of flotation, pine oil finds use, either alone o r in combination, in one third to one half of the mills studied. The observations on the u s e of pine oil suggest that this reagent finds its biggestapplication in flotation p r o c e s s e s when recovery is of major importance. The g r e a t e s t single u s e of c r e s y l i c a c i d occurs in lead flotation, where it i s extensively used e i t h e r alone o r in conjunction with MIBC. Cresylic acid a l s o finds important usage in pyrite flotation and bulk sulfide flotation. Methylisobutylcarbinol, MIBC, is of major importance in the selective flotation of complex o r e s (columns 4, 5, 6). This frother is also of major i m portance in bulk sulfide flotation, but s e e m s to find little use in the flotation of gold and p y r i t e o r e s and simple copper ores. It is widely used in domestic coal flotation plants. The difference in usage of MIBC between simple copper o r e s and complex ones will be noted. I t s use a s an auxiliary frother in the amine flotation of potash s a l t s is of interest. This use could a r i s e because the flotation is being done in solutions of high s a l t content, o r from o r e s with relatively l a r g e amounts of clay o r fine gangue. The u s e s of MIBC would s e e m to r e f l e c t the selectivity of i t s f r o t h s whichstructurally lend themselves to good self -cleaning properties. None of the alcohol f r o t h e r s other than MIBC has a markedly important single u s e a t present. The polyglycols have theirbiggest single use in the flotation of simple copper o r e s . This application i s to be compared with their secondary importance in floating complex copper o r e s . These f r o t h e r s a r e a l s o relatively important in the flotation of gold and zinc ores. Triethoxybutane, TEB, finds use in gold flotation, but is of l e s s e r importance elsewhere. One factor contributing to this may be i t s limited availability t o d a t e outside the African continent. The u s e of auxiliary f r o t h e r s in amine flotation circuits i s interesting. As indicated, pine oil and MIBC a r e the major f r o t h e r s used with amine collectors. P i n e oil was found t o be used primarily in mica-feldspar flotation c i r c u i t s , while MIBC, a s mentioned above, i s used primarily in the flotation of potash salts. On the other hand, no f r o t h e r s a r e important in the flotation of quartz. (The q u a r t z flotation c i r c u i t s included in this survey were with one exception connected with the flotation of phosphate rock.) T h e r e would appear t o be no f r o t h e r s of special importance for use a s f r o t h modifiers in fatty acid, soap, o r sulfonate flotation circuits; 64% of the m i l l s used a frothing collector only. In Table 5 d a t a a r e given on the quantities of f r o t h e r s added in different types of flotation circuits, a s d e s c r i b e d by the column headings. Data a r e

tabulated only for mills that used only one frother in a given flotation circuit. For each frother and each type of flotation, the minimum, average, and the maximum quantities used (pounds of frother per ton of o r e treated) a r e listed in three separate columns, thus providing the r e a d e r withthe range of quantities of frother required (columns 1, 3) a s well a s the average quantity consumed (column 2) in any particular type of flotation. The data in Table 5 suggest that frother usage in general varies between about 0.01 and 0.3 lb per ton of ore. Usage ~f pine oil, cresylic acid, and methylisobutylcarbinol in sulfide flotation (column 1) tends to run somewhat higher than f o r the higher alcohols and the synthetic frothers, both on the average and with regard to the maximum amounts used. Additions of cresylic acid would appear to be the greatest of any of the f r o t h e r s listed. With the exception of coal flotation where frother consumption is higher than elsewhere, usage does not vary widely f r o m one type of flotation circuit to another. In summary, the past 50 y e a r s have s e e n a broad extension of the flotation method in the treatment of o r e s of various types. This expansion has been accompanied by marked improvements i n the sharpness and selectivity achieved in such separations. Frothers have played an important p a r t in t h i s trend, and operators now have considerable latitude in the choice of a frother to meet the requirements of any particular flotation application. The c l a s s i cal types of frothers, monohydroxylated compounds such a s pine oil and cresylic acids which were developed in the e a r l y applications of flotation, still find wide use in modern practice. Synthetic frothers s u c h a s t h e aliphatic alcohols, water-soluble polypropylene glycol derivatives, and nonhydroxylated frothers such a s triethoxybutane have replaced these original f r o t h e r s in many operations. Of interest i s the widespread use of combinations of frothe r s , a factor which reflects not only the complexity of the problem of finding a proper frother, but alsothe diligence of flotation operators in seeking i t out. REFERENCES 1. A. F. Taggart:

Handbook of Mineral Dressing, John Wiley and Sons, Inc., New

York, 1948. 2. A. M. Gaudin: Flotation, 2nd Ed., McGraw-Hill Book Co., Inc., New York, 1957.

3. J. J. Bikerman, J. M. P e r r i , R. B Booth, and C. C. Currie: Foams: Theory and Industrial Applications. Reinhold Publishing Corp., b'?w York, 1953. 4. N. K. Adam: The Physics and Chemistry of Surfaces. 3 r d Ed., Oxford University P r e s s , London, 1941. 5. J. W . Gibbs: Collected Works, Yale University P r e s s , New Haven, 1948, vol. 1.

6. A. M. Posner, J. R. Anderson, and A. E. Alexander: J. Colloid Sci., 1952, vol. 7, p. 623. 7. C. C. DeWitt and R. F. Makens: J. Am. Chem. Soc., 1932, vol. 54, p. 455. 8. W. T. Bishop: AIME Trans., 1946, vol. 169, p. 379. 9. S. C Sun: AIME Trans., 1952, vol. 193, p. 65; Mining Engineering, January 1952. 10. A. Seidell: Solubilities of Organic Compounds, 3 r d Ed., D. Van Nostrand Co., Inc., New York, 1941, vol. 2. 11. S. A. Wrobel: Bull. Inst. of Mining & Metallurgy, 1951-1952, vol. 61, p. 505.

12. Carbide and Carbon Chemicals Co., Synthetic Organic Chemicals, 13th Ed., 1952. 13. C. R. Sporck: J, Am. Oil Chemists' Soc., 1953, vol. 30, p. 190.

14. A. G. Brown, W. C. T h u r m a n , and J. W. McBain: J. Colloid Sci., 1953, vol. 8, pp. 491, 508. 15. W. E. E w e r s a n d K. L. Sutherland: A u s t r a l i a n J. Sci. R e s e a r c h , 1952, vol. A5,p. 697. 16. J. L e j a and J. H. Schulman: AIME T r a n s . , 1954, vol. 199, p. 221; Mining Engineer&, F e b r u a r y , 1954. 17. A. F. T a g g a r t a n d M. D. H a s s i a l i s : AIME Tech. Pub. 2078. 18. J. Leja:

B u l l . 1957, , vol.

607, p. 425.

19. K. L. Sutherland a n d I. W. Wark: P r i n c i p l e s of Flotation, Australasian Inst. of Mining & Metallurgy, Melbourne, 1955. 20. D. W. F u e r s t e n a u a n d C. H. W a y m a n Engineering, J u n e 1958.

AIME T r a n s . , 1958, vol. 211, p. 694;Mininff

21. S. A. Wrobel: Mine a n d Q u a r r y Eng., 1953, vol. 19, p. 314. 22. W. A. B a t e s a n d R. J. M i l l e r : AIME T r a n s . , 1946, vol. 169, p. 385. 23. E. C. T v e t e r (to Dow C h e m i c a l Co.):

U.S. Patent No. 2611485, 1952.

24. R. B. Booth a n d J. M. Dobson (to A m e r i c a n Cyanamid Co.): 2695101, 1954.

U.S. P a t e n t NO.

25. J. H. P o t g l e t e r van A a r d t (to R. F. Powell and National C h e m i c a l P r o d u c t s Co.): U.S. P a t e n t No. 2561251, 1951. 26. R. F . Powell a n d H M. Stanley: U.S. P a t e n t No. 2591289, 1952. 27. S. A. Wrobel: C h e m . T r a d e J . , 1952, vol. 131, p. 824.

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