Journal of Colloid and Interface Science 256, 153–158 (2002)

doi:10.1006/jcis.2002.8271

Kaolin Flotation
Sharad Mathur1
Engelhard Corporation, 108 Briarcliff Road, Gordon, Georgia 31031

Received July 18, 2001; accepted January 31, 2002; published online August 13, 2002

KAOLINITE STRUCTURE, SOURCES, AND NATURE

The effectiveness and efficacy of flotation diminishes with the size OF COLORED IMPURITIES
of the particles. However, the kaolin industry is unique in success-
fully using flotation with fine particle size feed. Kaolin is valued as a The mineral kaolinite is an aluminum silicate and can be rep-
white pigment and therefore the goal in kaolin flotation circuits is to resented by the formula (OH)8 Si4 Al4 O10 . Pure kaolinite has the
remove the colored impurities to result in an acceptable brightness
composition, expressed as oxides: 46.54% SiO2 , 39.50% Al2 O3 ,
product. Kaolin flotation originated with a novel carrier flotation
process and later on carrierless flotation techniques were developed.
and 13.96% H2 O. In actual fact commercial kaolins differ some-
The surfactants that are utilized to impart selectivity are based on what from the above analysis because of the presence of acces-
fatty acids. The purpose of this paper is to provide a comprehensive sory minerals and possible substitutions of other elements within
review of the reagent schemes used in the kaolin industry, the un- the crystal lattice.
derstanding of which has been significantly aided by research work The two main producing areas of pigment kaolinite are
conducted in Prof. Somasundaran’s lab. C 2002 Elsevier Science (USA) the Cornwall area of England and the fall line of Georgia
Key Words: flotation; fine particle; kaolin; titanium; oleic acid; and South Carolina in the United States. Recently, Brazil is
hydroxamate oxide. on the way to becoming a major resource and supplier of
kaolin.
The kaolin of Cornwall originates from in situ weathering of
INTRODUCTION granitic rock while the Georgia–South Carolina and Brazilian
kaolins are sedimentary deposits. As a result of the different
Kaolin clay, consisting largely of the mineral kaolinite, is modes of origin, the two types of kaolin present quite different
widely used as a white pigment. In the United States, for in- problems in beneficiation for color improvement. This paper will
stance, pigment kaolin production was nearly 8.5 million tons focus only on flotation methods currently in practice in Georgia
in 1999 (3). The estimated revenue for this production volume for removal of colored impurities.
was close to $1.8 billion. Since pure kaolinite should be white, brightness value of 95
Although much of the kaolin is used as an inert filler where or better should be possible if discoloring impurities could be
product specifications are not very rigid, a large quantity of removed. It may be mentioned here that the brightness scale is
kaolin is converted to products where very restrictive speci- calibrated with respect to pure MgCO3 which has been arbitra-
fications apply. These specifications set severe limits on such rily assigned a value of 100.
properties as viscosity in a water suspension, particle size dis- Anatase titanium dioxide is one of the major colored impuri-
tribution, color, and brightness. The term “brightness” refers to ties in the kaolins of the southeastern United States. Although
the reflectance of the pigment to blue light (11). Since even the commercial, synthetic, pigment anatase is pure white with a
brightest commercial kaolins have a somewhat yellowish color, brightness close to 100, the anatase occurring in kaolin is beige
the blue reflectance (or brightness) is a reasonably good measure to dark reddish brown. The color of the anatase is due to sub-
of their nearness to a perfectly white material. stitution of other cations such as Fe within its lattice which are
There are several beneficiation techniques such as size classi- generally present at levels of less than 5%.
fication methods, magnetic separation, flotation, selective floc- As anatase titanium dioxide is removed from kaolin, the
culation, and bleaching which are used in the kaolin industry for brightness of the remaining kaolin is improved as shown in
removal of colored impurities that result in improving the bright- Fig. 1. All titanium removal processes give the brightness vs
ness of kaolin. This paper will focus solely on one beneficiation TiO2 content relationship shown in Fig. 1; however, the curve
method—flotation—used to improve the color and brightness may be displaced slightly when kaolins from different sources
of Georgia kaolin and will review the advantages and problems are treated. In addition to the titanium dioxide in the anatase
associated with it. form, there is also some present in the rutile form. The rutile
titanium dioxide occurs mostly in coarse particles. Since, in
1 Fax: 478-628-5827. E-mail: Sharad Mathur@Engelhard.com. the normal wet processing of kaolin, the extreme coarse particle
153 0021-9797/02 $35.00

C 2002 Elsevier Science (USA)
All rights reserved.
154 SHARAD MATHUR

92 REAGENT SCHEMES FOR REMOVAL OF COLORED

90 IMPURITIES BY FLOTATION
GEB, %

88
86 Much effort has been put into improving the flotation response
84 of fine feeds over the last several years. There are no general
82 flotation methods that have been developed for beneficiation of
0 0.5 1 1.5 2 fine particles. Each industry has developed its own way of deal-
ing with the problem of flotation of fine particles. In the kaolin
TiO2, %
industry, selectivity in flotation is based traditionally on activator
FIG. 1. Relationship between %TiO2 in the kaolin and its GEB. ion–fatty acid chemistry, a common reagent scheme in the flota-
tion of oxide particles. Recently, specific interaction between
the collector molecule and the surface ion has become the basis
fraction is discarded, the rutile is not a significant discoloring for achieving selectivity through the usage of hydroxamic acid
impurity in the final product. based reagents and surface iron on anatase particles.
In addition to rutile and anatase, many other minerals are All kaolin deposits do not respond to the same extent to the
found associated with Georgia kaolins. These include goethite, flotation process. The presence of trace impurities, particle size
graphite, kyanite, marcasite, muscovite, pyrite, quartz, siderite, distribution of kaolin and anatase, and extent of liberation of
smectite (montmorillonite), tourmaline, staurolite, and zircon. anatase are the key factors that govern the selectivity and yield of
Many of these minerals are potential discoloring agents for clay, the flotation process. Coarse white clays respond better to flota-
but they normally occur in very small quantity or in the coarse tion than East Georgia kaolin deposite which have a finer particle
particles. size, higher surface area, and higher TiO2 content. Again, pre-
The removal of colored impurities by flotation entails the fractionating the flotation feed to improve the brightness on a
problems associated with fine particle flotation. desired particle population may be used to control the economics
of the flotation process.
The underlying surface chemical interactions between the col-
WHY FINE PARTICLE SIZE IS A PROBLEM IN FLOTATION lector molecule and the minerals providing selectivity in the
flotation processes developed in the kaolin industry is provided
Fine particles or slimes (less than 10 µm) may be naturally below.
occurring constituents of a mineral or ore or may be artificially
produced during the grinding of the mineral or ore to a suitable
Ultraflotation
size for mineral liberation. In many instances slimes cannot be
avoided and removed, when present, for economic or practical Carrier mineral flotation or “ultraflotation” was the first suc-
reasons. For example, kaolin clay is a naturally slimed mineral, cessful anatase removal process used with kaolin on a large scale
consisting predominantly of particles 2 µm or finer, the clay at Engelhard Corporation and was put into commercial operation
being mechanically associated with very finely divided color in 1961 (8). In this process the conditioning of kaolin is accom-
body impurities which detract from the value and utility of the plished with two essential reagents, tall oil and calcium carbon-
clay in its pigmentary applications. Therefore, the beneficiation ate. The dissolved calcium ions act as activator for the mineral
of kaolin entails using methods that are effective in the fine surfaces to adsorb the tall oil, which cooperatively permit im-
particle range. proved beneficiation of the very finely divided feed. The tall
It has been observed that certain materials will not float in oil reagentized carrier calcium carbonate particles capture the
a froth flotation process when ground to an exceedingly fine tall oil coated ultrafine anatase particles. Since the carrier min-
size, although they will float under the same conditions when eral is relatively coarse in particle size as compared to anatase,
provided in coarser grain size. The loss in flotability of fine parti- to be removed, flotation is accomplished through standard sub-
cles has been attributed to the difference in the physico-chemcial aeration cells,. The schematic of the carrier flotation process is
properties of the fines compared to the coarse (6). First, the small shown in Fig. 2. Calcite form of calcium carbonate is a satis-
mass and momentum of the fine particles cause these to report factory carrier mineral because of its low cost, excellent flota-
to the froth either due to entrainment in the liquid or mechanical tion response, ready availability, and ease of removal from the
entrapment within the particles being floated. Second, the large froth.
specific surface area of fine particles results in excessive collec- Wang and Somasundaran (14) clarified the cooperation be-
tor consumption. Third, surface and electrochemical properties tween tall oil and calcium carbonate in ultraflotation. According
of fine particles tend to be different from the properties of coarse to these authors the “piggyback phenomenon” of attachment of
particles of the same material. The higher solubility or leaching anatase onto calcite is due to hydrophobic bonding between the
of cations from fine particles owing to the high specific surface oleate layers adsorbed on the mineral surfaces. This conclusion
energy may lead to unwanted activation of nonfloatable minerals is based on electrokinetic measurements that show both anatase
and thus loss of selectivity. and calcite to be negatively charged in the presence of oleate
 KAOLIN FLOTATION 155

ence of activator ions such as calcium. The scrubbing action,

according to Cundy, consists of cleaning the minerals of their
contaminants and is derived from relatively high solids under
high-speed agitation. It is, however, more likely that this pro-
cess facilitates liberation of the colored impurities from kaolin
and thus prepares the slurry for conditioning with the oleic acid
in the presence of calcium ions. Again, as with the ultraflotation
process, the high-speed agitation also probably leads to surface
coating of only the anatase particles and thus selectivity in flota-
tion. The collector-coated anatase particles may be selectively
coagulated under the high-speed agitation, which effectively in-
creases the particle size and allows flotation.
In the ECCI process, the kaolin crude is conditioned with oleic
acid and calcium salts under alkaline conditions. The flotation
solids are reduced to 15–20% and standard subaeration flotation
FIG. 2. Schematic of ultraflotation (after Fuerstenau (5)). cells are used to remove the colored impurities.

Titanium Removal and Extraction Process (TREP)

collectors. This thus rules out the possibility of electrostatic in-
The TREP was developed at Freeport Kaolin (assets of
teraction between the two minerals.
Freeport Kaolin were acquired by Engelhard Corporation) and
Wang and Somasundaran (14) also provided an explanation
is also a carrierless flotation which uses oleic acid as a condition-
for the beneficial effect of high-speed agitation during the condi-
ing reagent in the presence of a calcium activator under acidic
tioning process in ultraflotation. Dissolved calcium ions adsorb
conditions (16).
on both kaolinite and anatase surfaces, facilitating the adsorp-
A major improvement of the TREP over the previous kaolin
tion of tall oil reagent. The collector adsorption on kaolinite
flotation processes was flotation at the same high solids as during
is relatively weak as compared to its adsorption on calcite and
conditioning, i.e., in excess of 25%. This lends to the benefit of
anatase. Therefore, upon high shear agitation, the collector coat-
higher throughput and lower dewatering costs. The other unique
ing on kaolinite is not formed while it remains strong on anatase,
features of the TREP compared to previous flotation methods
allowing for selectivity to be achieved in the flotation process.
are an indigenously developed high-intensity conditioner (2)
Experimental evidence for this explanation is that in the pres-
and a column-like flotation cell (1). In the TREP conditioner
ence of sodium oleate the ζ -potential of kaolinite decreases with
the temperature of the conditioned slurry reached is in excess of
increasing agitation while that of anatase is not affected.
200◦ F. The conditioned product is then treated with a defloccu-
The recovery of the ultraflotation may be improved by cen-
lant prior to being floated in the column flotation cell described
trifuging the froth to separate the coarse calcium carbonate–
in the patent by Bacon (1).
titaia aggregates from the entrained kaolin (7). A mechanical
The high-conditioning temperature facilitates higher solubil-
separation process such as the centrifuge or a hydrocyclone will
ity of oleic acid and also decreases the pK a of the oleic acid.
not accomplish the separation if all of the particles were dis-
Consequently, even though the pH of the conditioner feed is
persed. The presence of a flotation collector to agglomerate the
in the range of 6.1 to 6.3, attachment of oleic acid occurs to
impurities is essential to mechancial separation and enhances
the anatase particles through bridging with Ca ions. The condi-
the overall recovery of kaolin.
tioning under acidic pH distinguishes the TREP from the ECCI
process.
ECCI Process
A typical result of the TREP process is given in Table 1 where
English China Clay International (ECCI) developed the first GEB is the GE brightness.2
carrierless flotation process for removal of titaniferous impuri- The TREP is sensitive to the dispersant added prior to con-
ties (4). The two essential features of this process are high-energy ditioning and flotation. Young et al. (16) showed that organic
scrubbing of the kaolin slurry at 40–60% solids and the pres- dispersants such as polyacrylates or inorganic dispersants such
as sodium polyphosphates added prior to conditioning inhibit
flotation (see Table 2). The results dramatically show that the ad-
TABLE 1 dition of a polyacrylate dispersant prior to conditioning is very
Effect of TREP on Beneficiation of Kaolin (16) detrimental to TiO2 removal. The polyacrylate salt added before
Sample % TiO2 GEB
2 GEB is GE brightness and is a measure of the blue reflectance of pigments.
Feed 1.76 84.7
It is measured at an effective wavelength of 457 nm and is distributed throughout
TREP product 0.53 89.1
the spectral range of 400–500 nm.
156 SHARAD MATHUR

TABLE 2 gested to be a successful approach for improving the flotation

Effect of Polyacrylate Dispersant Added Prior to Conditioning of fine particles due to the following reasons (5).
on Flotation (16)
1. Specific chemical interactions between the collector ion or
Product % TiO2 molecule and metallic cation sites on the surface;
Sample Dispersant type Dosage (%) (% TiO2 ) GEB removal 2. The residual concentration of collector in chemisorbing
Feed — 1.83 85.6 —
systems is low, which obviates the excessive collector consum-
ption.
TREP product “N” brand sodium 0.11 0.53 90.9 71.0
silicate Fuerstenau and Pradip (6) present an excellent review of
TREP product “N” brand sodium 0.11 1.50 87.0 18.0 the hydroxamate collectors. Yoon and Hilderbrand (19) first
silicate
patented a successful kaolin flotation process based on hydro-
Na polyacrylate 0.09
xamate collectors. The hydroxamate collectors can be used ef-
Note. The N-brand sodium silicate has a modulus (weight ratio Na2 O/SiO2 ) fectively at pH values above 6, at which the dispersion of clay
of 3.22. is readily achieved. The amounts of these reagents required for
flotation are considerably less than those typically used in the
conditioning provides only one-fourth of the TiO2 removal ob- conventional tall oil flotation process. Also, the hydroxamate
tained otherwise. However, the conditioner product could be best collectors possess frothing properties so that no frothers may be
dispersed for flotation with polyacrylate rather than an inorganic necessary for flotation.
dispersant as illustrated in Table 3. Cytec patented a feasible manufacturing process (12) for hy-
These results show that the polyacrylate dispersant added to droxamates and the product reagent S6493 is currently used by
the conditioned product results in almost twofold TiO2 removal Thiele Kaolin. The hydroxamate collector is supplied as a water-
during flotation compared to no dispersant. Further, the inor- in-oil microemulsion. Conditioning solids can be as high as 70%
ganic dispersants offer only a marginal improvement over no wt% and the flotation solids between 15 and 45% solids with
dispersant addition prior to flotation. the usuage of hydroxamate collector.
The flotation with hydroxamate collectors consists of similar
Hydroxamate Flotation
prior basic steps as described with the other processes such as
The above-mentioned flotation processes are based on the use dispersing of the clay slurry and conditioning with the collector.
of the fatty acid or tall oil type of collectors, which require the The distinguishing features with the hydroxamate collectors are
use of divalent or trivalent activator cations. The presence of relative insensitivity to the dispersant type for dispersing clay
activator ions makes the process sometimes difficult to control slurry and high solids during conditioning and flotation (15). The
because of the necessity to maintain a proper balance between conditioning pH is generally maintained between 8 and 10 since
the amounts of collector and activator added. For instance, an at lower pH values the process is not that efficient and pH higher
excessive use of activators can induce coagulation of the clay than 10 results in excessive frothing which may inhibit effective
particles and makes the separation difficult. Further, activators separation. In many flotation systems which use hydroxamate
may also cause the flotation of the clay particles themselves as collectors the optimum pH has been shown to be between
rather than the colored impurities, resulting in a poor separation pH 9 and pH 9.5, which is similar to the pK a of hydroxamic
efficiency and a loss of clay recovery. It is, therefore, desirable acid (6). The following results (Table 4) indicate that kaolin is
to have a collector for colored impurities that does not require no exception.
activators. Two variations on the usage of hydroxamates in flotation have
Hydroxamates, (R–C(O)–NH–OM), where R is an alkyl, been developed and consist of
aryl, or alkylaryl group and M represents an alkali or alkaline
(i) extending hydroxamate usuage with oleic acid (10) and
metal or hydrogen, are a class of chelating chemicals and thus
they adsorb by chemisorption. Chemisorption has been sug-
TABLE 4
TABLE 3 Effect of Conditioning pH on Flotation of Anatase from a Middle
The Efficiency of Various Dispersants Added after Conditioning Georgia Crude (15)
on Flotation in the TREP (16)
Sample pH % TiO2 Clay yield (wt%)
Dispersant % TiO2
Sample Dispersant dosage (%) % TiO2 GEB removed Feed — 1.45 100
Float product 6.2 1.08 94.6
Feed — — 1.76 84.7 — Float product 6.8 0.95 93.8
TREP product None — 1.12 86.0 36.4 Float product 7.4 0.84 94.7
TREP product TSPP/Na2 CO3 0.1 1.06 86.8 39.8 Float product 8.2 0.82 91.2
TREP product Na silicate 0.1 0.95 87.3 46.0 Float product 8.9 0.71 89.4
TREP product Na polyacrylate 0.1 0.53 89.1 69.9 Float product 9.6 0.66 93.2
 KAOLIN FLOTATION 157

TABLE 5
Effect of Blended Hydroxamate and Tall Oil versus the Individual Collectors on TiO2 Removal and Clay Yield (10)

Amount of TiO2 removed

Collector Dosage (%) % TiO2 product Yield of clay (%) by flotation (%) Coefficient of separation

Tall oil (Westvaco L-5) 0.15 0.30 64.9 81.0 0.46

Cytec S6493 hydroxamate 0.1 0.28 86.0 81.9 0.68
Tall oil (Westvaco L-5) 0.05
Cytec S6493 hydroxamate 0.05 0.31 86.6 80.0 0.67

(ii) pretreating the crude with hydroxamate in a clay–water slurry (9). This provides flexibility in the choice of the disper-
system prior to addition of the deflocculant (9). sant for the flotation feed. All the flotation schemes described so
far utilize the coarse-grained Middle Georgia crude. The patent
The blended co-collector system serves to reduce the dosage
of Norris and Yordan (9) also shows the prior addition of hy-
of hydroxamate significantly while maintaining a similar level
droxamate provides improved effectiveness when conditioning
of TiO2 removal and clay yield. The industrial implication is
fine-grained East Georgia crudes as illustrated in Table 6.
a lower collector cost since the hydroxamates are more expen-
sive than the tall oil. Table 5 illustrates that the co-collector of
hydroxamate and tall oil removes the same amount of anatase SUMMARY
as the individual collectors. However, the efficiency of the co-
collector system is the same (measured by the coefficient of sepa- A review of the current reagent schemes for removal of col-
ration) of the hydroxamate chemistry while using only half of the ored impurities in kaolin by flotation shows that the kaolin indus-
dosage of alkyl hydroxamates and only one-third of the dosage of try has been at the forefront of adapting the flotation technology
tall oil. to fine particle processing. Still there are many technological
The coefficient of separation (C.S.) is an index used to mea- challenges that need to be addressed and include:
sure the flotation process performance (13). and is defined as
(i) variable response of kaolins from different sources;
C.S. = (% Yield of Clay + % of TiO2 removed by flotation (ii) high-conditioning energy requirements;
(iii) low flotation solids which entails a large amount of water
− 100)/100. [1] removal;
(iv) higher iron removal.
Thus, the C.S. varies from zero for no separation to 1 for perfect
separation. According to Norris and Yordan (9), the C.S. varies A fundamental understanding of the industrial practice is cer-
from 0.3 to 0.75 in the case of beneficiation of kaolin by froth tainly needed to refine the existing processes as well as develop
flotation. new reagent schemes. The move to specific collector chemistry
The co-collector usage circumvents the use of activators such such as hydroxamates has shown a lot of promise in designing
as the divalent salts normally required with oleic acid (10). This robust flotation processes. The hydroxamate collector is rela-
is an interesting phenomenon and merits further investigation. tively insensitive to the dispersant type and pH which offer a
The advantage of prior addition of hydroxamate to condition lot of forgiveness during processing. The downside is the higher
the kaolin is that any type of deflocculant can be used on this chemical cost of the reagent.

TABLE 6
REFERENCES
Effect of Order of Addition of Deflocculant and Hydroxamate
Collector on Beneficiation of East Georgia Crude (after Norris and 1. Bacon, F. C., “Froth Flotation Apparatus and Process,” U.S. Patent
Yordan (9)) 4,472,271, 1984.
2. Bacon, F. C., and Brooks, R. L., “High Intensity Conditioning Mill and
% TiO2 removed % yield Method,” U.S. Patent 4,483,624, 1984.
Reagents % pH by flotation of clay C.S. 3. China Clay Producers Association (CCPA), 1999.
4. Cundy, E. G., “Processing of Clay,” U.S. Patent 3,450,257, 1969.
Sodium silicate 0.45 5. Fuerstenau, D. W., in “Fine Particle Processing” (P. Somasundaran, Ed.),
Soda ash 0.1 8.6 60.7 67.3 0.28 Vol. 1, Chap. 35, pp. 669–705. AIME, New York, 1980.
S6493 0.2 6. Fuerstenau, D. W., and Pradip, in “Reagents in the Mineral Industry”
S6493 0.2 (M. J. Jones and R. Oblatt, Eds.), pp. 161–168. The Institution of Min-
SHMP 0.2 8.0 80.0 82.2 0.62 ing and Metallurgy, London, 1984.
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Separation of Finely-Divided Minerals by Addition of Selective Collector
Note. SHMP = sodium hexametaphosphate. Reagent and Centrifugation,” U.S. Patent 5,358,120, March 1994.
158 SHARAD MATHUR

8. Greene, E. W., Duke, J. B., and Hunter, J. L., “Selective Froth Flotation of 13. Wang, Y. H. C., and Somasundaran, P., in “Fine Particles Processing”
Ultrafine Minerals or Slimes,” U.S. Patent 2,990,958, July 4, 1961. (P. Somasundaran, Ed.), Vol. 2, Chap. 57, pp. 1112–1128. AIME,
9. Norris, J. A., and Yordan, J. L., “Process for Conditioning Kaolin New York, 1980.
Clays Prior to Removing Impurities,” U.S. Patent 5,685,899, Nov. 11, 14. Wang, Y. H. C., and Somasundaran, P., in “Transactions of AIME,”
1997. Vol. 272, pp. 1970–1974. AIME, New York, 1983.
10. Shi, J. C. S., and Yordan, J. L., “Process for Removing Impurities from 15. Yoon, R. H., and Hilderbrand, T. M., “Purification of Kaolin Clay by Froth
Kaolin Clays,” U.S. Patent 5,522,986, June 4, 1996. Flotation Using Hydroxamate Collectors,” U.S. Patent 4,629,556, Dec. 16,
11. TAPP1 Standard 646 OS75. 1986.
12. Wang, S. S., and Nagraj, D. R., “Novel Collectors and Processes for Making 16. Young, R. H., Morris, H. H., and Brooks, R. L., “Method of Treating Clay
and Using Same,” U.S. Patent 4,871,466, Oct. 3, 1989. to Improve its Whiteness,” U.S. Patent 4,492,628, Jan. 8, 1985.