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FROTHS AND FROTHING AGENTS
by R. B. Booth and W. L. Freyberger

Froth flotation i s a chemically induced method fo r beneficiating or up-
grading an ore, which utilizes a layer o r column of froth a s a separating
medium to segregate and remove the valuable minerals f rom the worthless
gangue components of a finely ground o re suspended in water. The object of
this chapter i s to discuss the theoretical aspects of froth formation, the vari-
ous 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 bub-
bles, and the rejection of nonfloatable minerals; and 3 ) separation of the
floatable minerals f rom the nonfloatable minerals. As 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 particle-
bubble 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 for the purpose of creating a froth in a flotation separation. How-
ever, some col lectors produce froths that a r e adequate for some flotation
operations. Although this chapter is chiefly concerned with the specific
f ro thers used in flotation, some discussion of frothing collectors is included.
Otherwise, col lectors (or promoters) and modif iersare not discussed a t length;
f o r information on these types of flotation chemicals, the reader i s referred
to other sect ions 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 Ear ly in the f i rs t period, large quanti-
t ies of fatty and oily mater ials, up to 10 to 20% of the weight of the ore, were
used a s a means of separating the sulfide and oxide components f rom the
gangue minera ls in o re pulps. Later, various gases replaced these large
quantities of oil a s a buoyant and separat ing medium, decreasing oil require-
ments 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.

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consumption, inherent differences in the frothing and collecting power of var i-
ous 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, esters, and fatty acids and their soaps
found application a s frothing agents. Monohydroxylated, water-insoluble com-
pounds 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 rs t historical
period, a trend was established toward promoters of definite chemical com-
position by the use of certain types of oils containing sulfur (either naturally
or 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 to 6C alcohols; sodium and ammonium dialkyl
(2C to 6C) and cresyl dithiophosphates; and mercaptobenzothiazole deriva-
tives.

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 sa l ts 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 (alkoxy-
substituted paraffins such a s t r i e th~x~bu tane ) .

Most of the sulfide promoters, both oily and water-soluble, which were
developed in the second historical period of flotation, a re 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 ree
cresylic acid) produce froth, sufficient for conducting flotation operations in
some cases. The collectors utilized in the co~~centrat ion of nonmetallic ores
a re froth formers and may be used with o r without auxiliary frothing agents.

PHYSICAL ASPECTS OF FROTHING

SURFACE ACTIVITY:
When aqueous solutions a re 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 na-
ture 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 as 'molecules' for the sake
of simplicity. Of course, it will be remembered that many frothing materials such as
the soaps are ionic in aqueous solution.

Page 9

Higher frothing power i s exhibited by a normal alcohol rather than by its
isomers. Also, [email protected] frothing power i s shown by aliphatic alcohols rather
than by corresponding aromatic alcohols. The frothing power of such aro-
matic compounds i s enhanced by saturation of thedouble bonds in the aromatic
nucleus; saturation tends to minimize the differences in frothing capacity be-
tween an aromatic and a corresponding aliphatic compound. In the case of
terpene compounds, frothing is improved by increasing the number of double
bonds in the molecule; saturation of these bonds causes a decrease in froth-
ing.

The side chain on the benzene r ing of an aromatic frother exerts an influ-
ence on i ts frothing power. An increase in frothing i s noted if a methyl (CH3)
group i s added, but further increase in the length of the side chain produces
only limited improvement in frothing capacity.

Since froths should exhibit l imited stability and break down on removal
f rom the flotation cell, the hydrocarbon portions of the frother should contain
no more than 8 C atoms in a single chain o r branch. Water solubility is in-
fluenced a lso by these nonpolar portions of thefrother molecule. Thus, in the
case of the aliphatic alcohols used a s frothers, the optimum chain length i s
5-8 C atoms. The frothing character ist ics of cresylic acids vary with the
length of the alkyl groups on the benzene ring in the nonpolar segment of the
frother, a factor which a lso influences boiling range and water solubility.

As indicated above, the polar groups included in frother molecules a r e
hydroxyl (-OH) and ether linkages (-0-). These groups do not form stable
bonds a t mineral surfaces, a property which is in sharp contrast to the
surface-bonding character ist ics of functional groups in promoters, i.e., the
sulfhydryl group common to strong sulfide promoters and the carboxylate,
sulfate, sulfonate, and amine groups in nonsulfide promoters. Thus, the com-
pounds commonly used a s frothers do not possess strong collecting teriden-
cies.

In addition to the above compounds which a r e utilized in flotation for the
specific purpose of creat ing a froth, other compounds, employed primari ly as
collectors for nonsulfide minerals, 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 usedas anionic collectors for non-
sulf ides a r e fatty acids, rosin acids, combination fatty-rosin acids a s found
in crude and refined tal l oils, naphthenic acids, and related compounds derived

TABLE 3. General Types of Compounds Used as Frothing Collectors

Chemical Type Type Formula Representative Compounds
-

Long chain RCOOH Fatty or rosin
carboxylates RCOONa acids such as oleic acid, tall oil,

etc.

Long chain
sulfonates or
sulfates

RSO, Na
ROS03 Na

Oil and water-soluble petroleum
sulfonates

Long chain RNHz Octadecylamine (free base or as
amines acetate)
-

Page 10

from petroleum sources. These compounds are used a s collectors chiefly as
the free carboxylic acid and occasionally a s soaps. Oleic acid was wed ex-
tensively as 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 as oleic, linoleic, and linolenic acids.

The chief sulfonate collectors used in the flotation of nonmetallics are the
petroleum sulfonates (sodium salts) of both the water-soluble and the oil-
soluble types, which are 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 nonsul-
fides, but are employedin some instances as emulsifiersfor hydrocarbon oils
and insoluble anionic collectors such as the fatty acids and tall oils.

Long chain amines, about 12 to 20 C atoms in chain length, are the princi-
pal cationic promoters employed in the flotation of silica, silicates, and salts
such as potassium chloride. Modified amines, e.g., the condensates of poly-
Blkylene polyarnines with fatty acids, a re also in commercial use.

PRACTICAL ASPECTS OF FROTHING

GENERAL SPECIFICATIONS OF FROTHERS:
Selection of a frother for any particular flotation operation will be contin-

gent 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 miner-
als 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, especial-
ly in the case of ore slimes.

4) Cost and availability should be satisfactory for large-scale use.
5) Low chemical activity and limited collecting tendency should be ex-

hibited; 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% as 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 as frothers.

THE FUNCTIONS OF A FROTH IN FLOTATION:
The froth generated by the action of a flotation machine acts as a separat-

ing medium to segregate and remove the valuable mineral particles from the
gangue particles of the ore. These functions of the froth at the surface of the
pulp in a flotation cell are illustrated in Fig. 3, which is a modification of a
similar illustration originally presented by ~ a g g a r t l from observations ob-
tained in a glass-walled flotation cell. Fig. 3 represents in an idealized man-
ner the flotation of copper minerals (black) from gangue (light colored).

Page 18

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 f rother per ton of o r e treated) a r e l isted
in three separate columns, thus providing the reader withthe range of quanti-
ties of frother required (columns 1, 3) a s well a s the average quantity con-
sumed (column 2) in any particular type of flotation.

The data in Table 5 suggest that frother usage in general var ies between
about 0.01 and 0.3 lb per ton of ore. Usage ~f pine oil, cresyl ic acid, and
methylisobutylcarbinol in sulfide flotation (column 1) tends to run somewhat
higher than for the higher alcohols and the synthetic f rothers, both on the
average and with regard to the maximum amounts used. Additions of cresyl ic
acid would appear to be the greatest of any of the f ro thers listed.

With the exception of coal flotation where frother consumption is higher
than elsewhere, usage does not vary widely f rom one type of flotation circui t
to another.

In summary, the past 50 years have seen 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 in the sharpness and selectivity
achieved in such separations. Frothers have played an important par t in th is
trend, and operators now have considerable latitude in the choice of a f rother
to meet the requirements of any particular flotation application. The c lass i -
cal types of frothers, monohydroxylated compounds such a s pine oi l and
cresylic acids which were developed in the ear ly applications of flotation,
sti l l find wide use in modern practice. Synthetic f rothers suchasthe aliphatic
alcohols, water-soluble polypropylene glycol derivatives, and nonhydroxylated
frothers such a s triethoxybutane have replaced these original f ro thers in
many operations. Of interest i s the widespread use of combinations of f roth-
e rs , 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

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Page 19

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