Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.11, pp.1007-1025, 2011 Printed in the USA. All rights reserved
Significance of Impurity Mineral Identification in the Value Addition of
Kaolin – A Case Study with Reference to an Acidic Kaolin from India
S. Ramaswamy* and P. Raghavan
National Institute for Interdisciplinary Science and Technology (NIIST),
Council of Scientific and Industrial Research (CSIR), Industrial Estate P.O
Thiruvananthapuram-695 019, Kerala, India
*Corresponding Author :
Kaolin or china clay is a versatile industrial mineral with wide technological applications and is
abundantly available in India. The major mineral in kaolin is kaolinite (Al
O). The
common ancillary / impurity minerals occurring with kaolin include parent rocks like feldspar
and mica, quartz, ferruginous, titanoferrous and carbonaceous materials. The most deleterious
impurities in kaolin are iron minerals which imparts colour to the white kaolin. Iron exists as
oxides, hydroxides, oxy hydroxides, sulphides and carbonates along with iron stained
quartz/anatase and mica in kaolin. Kaolin finds extensive applications in paper, paint, rubber,
ceramics, plastics etc. One of the highest value additions for kaolin is as pigment in paper and
paint industries. The optical properties are important for pigment applications and removal of
the iron impurity is very important to improve this property. Extensive research has been carried
out on the nature of iron impurities present in kaolin, which leads to the conclusion that iron is
present as a part of the kaolinite or ancillary mineral (mica or titania) structure, which can be
termed as “structural iron” or as independent iron minerals such as oxides, hydroxides, oxy-
hydroxides, sulphides and carbonates, which can be termed as “free iron” [1]. The present
paper discusses the iron speciation studies carried out on a typical china clay sample collected
from Koraput district of Orissa State in the Union of India. Studies have shown that the major
impurity mineral species is in “pyritic” (Iron sulphide) form along with other hydroxides, oxy-
hydoxides and oxides of Iron. Presence of limonite is also observed in the sample. The
identification/quantification of the impurity minerals have played a crucial role in the selection /
modification and sequentialisation of beneficiation processes and subsequent processing studies
have shown that the sample can be value added to ceramic grade.
Key words: Kaolin, Impurity minerals, Beneficiation, Pyrite, Value addition
1008 S. Ramaswamy and P. Raghavan Vol.10, No.11
Impurity mineral identification and their removal are the two important aspects in the value
addition of kaolin. Ferruginous and titanoferrous minerals are the common coloring impurities
present in kaolin and iron exists as oxides, hydroxides, oxy hydroxides, sulphides and carbonates
along with iron stained quartz / anatase and mica. Goethite (α-FeOOH) is yellow to brown in
colour and hematite (α-Fe
) is brownish red. Pyrite and ilmenite are black and give gray color
to kaolin. Iron stained titania (titanoferrous) gives dirty yellow color to kaolin. The ancillary
impurities especially those of iron strongly influence the physico-chemical properties of kaolin
and adversely affect the qualities of the finished products. Extensive studies have been carried
out on the relationship between the total iron content in the kaolin, the structural order of the iron
species and the surface reactivity of kaolin [2,3]. The advent of sophisticated spectroscopic
analytical techniques has made it easy to understand the state of iron and its effect on the
properties of kaolin [4]. The crystallo-chemical characteristics and the assembly of kaolinites
with the associated iron oxyhydroxides are closely linked to the geological conditions in which
the kaolins are formed.
Iron minerals in kaolin are often found to be of low concentrations, having different particle sizes
and sometimes they are found to be more or less amorphous in nature. The difficulties associated
with the low concentration of iron in kaolin, the complexity of the natural material matrix from
which kaolins are obtained and the effect of pre-concentration or extraction methods often
require the use of multiple advanced analytical techniques to characterize the materials. Hence, it
becomes essential to separate/concentrate the iron impurities for characterizing the same
especially for the mineral content and valence states. Free and crystalline iron oxides are
frequently separated from kaolin by various physical methods (such as particle size separation,
density gradient separation, magnetic separation) and chemical methods such as selective
dissolution technique for identification and quantification. The Citrate Dithionite Bicarbonate
(DCB) treatment of the kaolin sample can be used to get information on the quantity of “free
iron” which can be removed by chemical leaching.
In addition to X-ray Diffraction Analysis (XRD), the traditional tool for mineral identification,
advanced analytical techniques like Electron Paramagnetic Resonance Spectroscopy (EPR),
Moss Bauer spectroscopy, FT-IR can be used to identify (i) the nature of iron present (free or
structural), crystal defects, the oxidation state of the iron present etc., where as Electron Probe
Micro Analysis (EPMA) of the sample gives the distribution of elements on the particles. High
Resolution Transmission Electron Microscopy fitted with Energy Dispersive Spectroscopy (HR-
TEM EDS) can be used to carry out the atomic level microanalysis of the samples.
Electron Paramagnetic Resonance (EPR) spectroscopy is used to characterize Fe
ions that
contain unpaired electrons [5,6]. EPR spectra distinguishes two forms of Fe
viz., (i) isolated
Vol.10, No.11 Significance of Impurity Mineral Identification 1009
ions isomorphously substituted for Al
within the kaolinite structure (“dilute” structural
) and (ii) poorly understood domains in which Fe
ions reside in close proximity to one
another. Fe
occurring in these domains are referred to as “concentrated / clustered” Fe
. Dilute
structural iron exhibits a paramagnetic signal at low magnetic field. The characteristic EPR
resonance lines of kaolinite are found in low magnetic (F1) and high magnetic (F2) fields with
“g” values near 4.0 and 2.0 respectively. The resonances at F1 are attributable to structural Fe
and the wide resonance at g =2.0 is due to the contaminants of free Fe
and iron
oxyhydroxides. Iron removal by various methods completely eliminates or weakens this
resonance. Thus EPR spectral method can be used to assess the effectiveness of iron removal
techniques. The intensities of the signals are related to the concentration of the corresponding
paramagnetic species and this will vary from one sample to the other in significant manner. Moss
Bauer spectroscopy is a very important technique which can be used for the identification and
characterization of less crystalline iron oxide and oxy-hydroxide minerals present in kaolin
samples. The information deduced from the Mossbauer spectral analysis are useful to understand
the oxidation state and the coordination environments of the iron in mineral phases. The isomer
shift () and quadrupole splitting (E) values give the information about the iron species. The
infrared spectrum of a clay mineral is sensitive to the chemical composition, isomorphous
substitution and the size and shape of the mineral particles and it provides the fundamental
information for the mineral identification [7,8].
While Scanning Electron Microscopy (SEM) pictures give the morphology and size of the
particles, EPMA gives information about the various mineral phases present in the kaolin
sample. The HR-TEM analysis gives information on nano particles and with EDS facility, it
provides the chemical assay (in atomic percentage) of individual particles and this information
can be used for the identification of various mineral phases. Weaver [9] has used this technique
for studying the titanoferrous impurity (Ti,Fe)O
present in Georgia kaolin. TEM-EDS have also
been used to understand the incorporation of iron in anatase and kaolinite structure [10].
In the present study, the iron species/minerals in the impurity concentrates separated from the
kaolin by different methods are identified by chemical, mineralogical (XRD and Rational
analysis), spectroscopic (EPR, Mossbauer and FT-IR ) and microscopic (SEM, HRTEM-EDS
and EPMA techniques). The Raw kaolin and products of beneficiation have been subjected to
EPR spectral studies. The impurity mineral phases concentrated and separated from kaolin by
different physico-chemical techniques and these impurity concentrates were analyzed by
Mossbauer spectroscopy, FT-IR spectroscopy, EPMA and HRTEM-EDS. The Scanning Electron
Microscopic analysis of the Raw and final product sample was also carried out. The chemical
and physical and mineralogical characterization of the Raw and selected beneficiated samples
were also carried out to understand the maximum value addition possible for the kaolin sample.
1010 S. Ramaswamy and P. Raghavan Vol.10, No.11
In the present study, bulk sample of the kaolin was collected from the Koraput district of Orissa
State in India. Representative samples were prepared by coning quartering method after through
2.1 Concentration of Impurity Minerals
The impurity minerals were concentrated from the kaolin by the following methods ie. (a)
Sieving - The mineral impurities in the size range <300 µm, > 45 µm were concentrated by
sieving the sample through 300 µm and 45 µm BSS Test sieves and the sample is designated as
IM1 (b) Panning – Here a dilute suspension (10% w/w) of the clay was prepared in distilled
water and stirred well with a low speed mechanical stirrer. The clayey portion was removed by
decantation after washing it with distilled water several times. The heavier fractions in the
remaining mixture are separated and concentrated by panning using a metallic pan. The
“panned” impurity concentrate is labeled as IM2 (c) Alkali Treatment - The iron oxides (IM3)
were concentrated by digesting the clay (< 45 µm fraction) with 5M NaOH solution [11] and (d)
the ferro/ferri magnetic materials in the kaolin have been separated using a hand magnet (IM4).
2.2 Value Addition Studies
The Raw kaolin sample has been found to be highly acidic and addition of water to the clay
makes pH of the slurry in the range 1-2. Hence, the conventional method cannot be made use of
for the size classification studies. Hence, a modified method was adopted for the processing of
this sample. In the modified method, the raw clay is mixed with water and the clay slurry is
subjected to soft mixing using a low speed mechanical stirrer. The slurry was then screened
through 300 µm BSS Sieve and the <300 µm fraction slurry was size classified using a set of
Mozley hydrocyclones (H/C) viz., 2” stub and 1” cyclones. The 2” stub cyclone overflow solids
(SCP1) correspond to the fraction below 45 µm and the overflow solids of 1”cyclone is the Final
Size Classified Product (FSCP ie., fraction below 2 µm). FSCP sample was subjected to DCB
Treatment [12] and the product sample is designated as FSCP-DCBTP. All the above operations
were carried out at conditions optimized in the laboratory.
2.3 Characterization Methods
The physical, chemical and mineralogical properties were determined by standard methods
[13,14]. The XRD patterns were taken using X’pert Pro PANalytical X-ray diffractometer with
Cu Kα radiation using Ni as filter at a setting of 40 kV and 30 mA. Rational analysis was done
by calculating the mineral content from the chemical assay (Bennett and Reed, 1971). Sulphur
was determined as per the standard procedure [15]. JEOL JSM 5600V SEM was used for
Vol.10, No.11 Significance of Impurity Mineral Identification 1011
studying the morphology of the samples. High resolution TEM J 1210 JEOL was used to carry
out micro analysis of various elements. EPMA studies were carried out using EPMA Scanning
Electron Microscope, JEOL Model (JXA-8100), Japan. FTIR Perkin – Elmer spectrophotometer
was used for the spectral studies of the impurity concentrates in the IR region. EPR studies were
carried out using EPR Varian model E-112 spectrophotometer, while the Mossbauer studies were
done using Mossbauer spectrometer (
Co source in Rh matrix as the Mössbauer source). The
optical properties ie., brightness and “L a b” color values (in ISO units) were measured using
Color Touch spectrophotometer (Technidyne Corporation, USA). Brightness represents the % of
reflectance of light at a wavelength of 457 nm. Particle size distribution was found out by
Sedigraph 5100 model, Micromeritics, USA.
3.1 Characterization of the Raw Kaolin
The Raw clay is soft and gray in color with blackish impurities. It is easily slaking and the pH of
the clay is found to be quite low (1.74). The general properties of the Raw clay is given in Table
1. XRD analysis data shows the presence of pyrite in the sample. The highly acidic nature of the
clay may be due to the presence of pyrite particles. The oxidation of pyrite by ferric iron is much
faster than by oxygen [16].
+ H
O -- Fe
+ 2SO
+ 2H
+ ¼ O
+ H
-- Fe
+ ½ H
+ 14 Fe
+ 8 H
O 15 Fe
+ 2 SO
+ 16 H
The matter soluble in water is on the higher side ie., 4.14% and the reason for the high value can
be attributed to mobilization of metals such as Fe
from the clay by H
. The matter soluble
in acid is 9.06% and this high acid soluble value may be due to the presence of some soluble
salts. Specific gravity of the Raw clay is slightly higher than that of pure kaolinite which is
possibly due to the presence of heavy mineral impurities like pyrite. The CEC of the clay is
found to be low, 2.1meq/100g clay. This is attributed to its low pH which causes protonation and
formation of a positive charge on the surface of the clay minerals [17]. The clay is kaolinitic as
indicated by the silica and alumina content which are close to the theoretical values of kaolinite
mineral. However, the LOI value is found to be on a higher side (18.17%) which can be
attributed to the presence of carbonaceous matter or decomposable minerals resulting in weight
loss on heating. The iron and TiO
content in the sample are found to be relatively high (3.76 and
1.60% respectively). XRD pattern of the clay showed that pyrite is one of the major impurities.
Rational analysis data also confirms the presence of pyrite in the sample. The pyrite content in
the sample is estimated to be 3.87% and correspondingly the sulphur content is 2.07 %. Particle
size distribution analyses given in Table 1 show that the percentage of fines are moderately high
1012 S. Ramaswamy and P. Raghavan Vol.10, No.11
(49.90% < 2µm fraction). The brightness (45.87), “L” (67.43) and “HW” (27.17) values are
found to be very poor. The low “L” and HW values show the presence of black/dark colored
particles in the clay. The greenish tinge of the sample (represented by “-a” value) is indicative of
the absence of reddish iron compounds in the sample. The “b” (-0.44) and “HY” (0.94) values
show that the overall yellowness of the sample is low. This indicates that the low brightness of
the sample is due to the presence of the black/dark colored minerals. Due to the high acidity of
the sample and the precipitation of the water soluble colouring matter on further water addition/
pH modification, the conventional wet processing of this clay is found to be difficult. Hence, a
thorough water wash was given to the clay before blunging and size classification.
3.2. Impurity Mineral Identification Studies
3.2.1. Chemical and mineralogical studies
The impurity minerals IM1, IM2 and IM3 have been characterized for their chemical assay and
mineralogical properties and the salient results are given in Table 2. The IM1 sample is found to
contain appreciable amount of iron and sulphur. The LOI is slightly high compared to that of
kaolinite mineral, indicating the presence of volatiles other than the water of crystallinity. From
the XRD analysis (Figure 1), quartz, pyrite and kaolinite are found to be the major minerals with
rutile and anatase in minor quantities. The iron and sulphur contents confirm the presence of
pyrite and the high loss on ignition value can again be attributed to the presence of pyrite
particles. The high percentage of iron
(8.28%) and the pyrite content (~ 65% of the total pyrite in
Raw kaolin) in the sample indicates the extent of concentration of iron impurity minerals along
with quartz during size separation at 45 microns (Table 1). The Rational analysis also supports
these findings. The chemical assay of the IM2 sample shows the presence of appreciable
quantities of quartz and heavy minerals (of “Fe” and “Ti”). XRD analysis shows that pyrite and
goethite are the major minerals present in the sample along with other minerals such as quartz,
rutile, anatase, ilmenite, kaolinite and graphite in minor quantities. Considerable quantity of
pyrite is found to be present and is evident from the chemical assay, rational analysis and XRD
findings. The high percentages of iron
(33.4%) and TiO
(11.77%) indicates that iron and
titanium impurity minerals are getting concentrated to a great extent during panning. The IM3
sample is found to be enriched with the iron mineral phases, particularly pyrite and this is
evident from the chemical assay of the sample. The titania content is low since part of it may be
getting leached into the alkali on heating. XRD analysis data shows the presence of pyrite, quartz
and goethite as major minerals along with anatase and rutile as minor phases. The XRD analysis
of the magnetic fraction, IM4 (separated by hand magnet) confirms the presence of magnetite,
rutile, goethite, hematite, anatase and ilmenite as the major phases along with minor quantities
pseudo rutile.
Vol.10, No.11 Significance of Impurity Mineral Identification 1013
Table 1 Properties of Raw and beneficiated samples
Properties Raw kaolin SCP1 FSCP FSCP-DCBTP
Chem.Assay (% wt)
Total iron
Physical Properties
Particle Size Distribution, wt.%,
< 2µm
Optical Properties (% ISO)
Mineralogy (XRD)
Major phases
Minor phases
Rational Analysis (wt. %)
Muscovite mica
Paragonite mica
Carbonaceous matter
Free alumina
K, Q
Q-Quartz; K-Kaolinite; P-Pyrite; A-Anatase; R-Rutile; H-Hematite
1014 S. Ramaswamy and P. Raghavan Vol.10, No.11
Table 2 Chemical assay and mineralogy of impurity mineral concentrates
Detectabe Limit (BDL) < 0.01 %
K – Kaolinite; Q – Quartz; R – Rutile; P – Pyrite; G-Goethite;
A –Anatase; Gr-Graphite; I-Ilmenite; H-Hematite
Properties IM1 IM2 IM3
Chem.Assay (% wt.)
Total Iron
Major phases
Minor phases
Rational Analysis
(Mineral % weight)
Muscovite mica
Paragonite mica
Q, P, K
R, A,
P, G
Q, R, A, I,
K, Gr
Q, P
A, R, H
Vol.10, No.11 Significance of Impurity Mineral Identification 1015
K – Kaolinite; Q – Quartz; R – Rutile; P – Pyrite; G-Goethite; A –Anatase; Gr-Graphite
I-Ilmenite; H-Hematite
Figure 1. XRD patterns of (a) IM1 (b) IM2 (c) IM3 and (d) IM4 samples
3.2.2. Spectroscopic and microscopic studies Spectroscopic studies
Moss Bauer spectral analysis
The Mossbauer spectral study of the impurity mineral concentrated by panning (IM2) of the
kaolin has been carried out at room temperature and the spectrum is given in Figure 2. The
sample is found to show isomer shifts of 0.35 & 0.88 mm/sec and quadrupole splitting of 0.64
and 1.32 mm/sec. respectively. The one having the lower isomer shift and quadrupole splitting is
due to the Fe(III) in the kaolin lattice [18] and the doublet with larger isomer shift and
quadrupole splitting values is due to Fe(II) in pyrite [19]. The sample is found to contain
appreciable quantities of pyrite.
Figure 2 Mossbauer spectra of IM2 sample
Intensity (arbitrary
2θ (degrees)
1016 S. Ramaswamy and P. Raghavan Vol.10, No.11
FT-IR spectral analysis
The impurity concentrate samples ie.,IM1 and IM2 were studied by FT-IR spectroscopy and the
IR spectra’s in the 1200 - 350 cm
region are given in Figure 3. The IR spectrum of the samples
shows the characteristic bands of kaolinite, quartz along with iron minerals. As expected, in the
IR spectra of both the impurity concentrates, the bands due to kaolinite mineral are very weak
and the features associated with the iron minerals are found to be very prominent. IR spectra
show the presence of goethite, hematite, maghemite, along with quartz and kaolinite in samples
IM1 & IM2. Lepidocrocite was also present in IM2 sample. Hematite, an anhydrous oxide,
occurs in two morphological forms, namely, a platy (kidney ore - Hk) and a more equant form
(specularite - Hs). Their spectra are generally similar, but show considerable differences in detail
due to the differences in crystal size and particularly in shape. The features of the hematite bands
show that the mineral is in the specularite form (Hs).
K–Kaolinite; Q–Quartz; G-Goethite; H-Hematite: M-Maghemite; L-Lepidocrocite
Figure 3 IR spectra of (a) IM1 and (b) IM2 samples (400 - 1200 cm
) Microscopic studies
HRTEM - EDS analysis
High Resolution Transmission Electron Microscopic analysis of the IM1 sample was done to get
an atomic level chemical composition of the impurity minerals (Figure 4). The picture shows
near usual kaolinite platelets (particles B, C, E, F) along with very fine pyrite particle (D) of
dimension <50nm sticking to the kaolinite particle. The high iron and sulphur contents in
particles ‘A ‘ & ‘G’ indicate that they are rich in pyrite content. Particle ‘A’ is rich in iron and is
found to contain more iron than that required for the pyrite formation. This indicates the presence
Vol.10, No.11 Significance of Impurity Mineral Identification 1017
of iron minerals other than pyrite in the sample. The chemical assay of the sample also supports
the presence of non-pyritic iron in the sample.
X 5000 120 kV J1210
Figure 4. HR TEM-EDS picture of IM1 sample
SEM analysis
Scanning Electron Microscopic analysis (SEM) pictures of the Raw clay is presented in Figure 5
and it shows the presence of aggregates of pseudo hexagonal kaolinite particles along with well
crystallized pyrite particles of typical octahedral shape.
O 64.96 65.78 63.70 54.73 63.85 64.70 27.02
Si 1.78 18.16 19.05 16.78 18.53 18.11 3.59
Al 2.53 16.46 16.64 15.19 17.21 17.45 4.80
Fe 15.90 0.31 0.54 4.67 0.33 0.29 21.34
S 9.53 0.00 0.00 7.37 0.05 0.03 40.57
Ti 0.01 0.03 0.00 0.00 0.02 0.00 0.43
Na 4.46 0.00 0.06 0.26 0.00 0.00 0.82
K 0.64 0.05 0.13 0.26 0.24 0.15 0.91
Mg 0.10 0.00 0.00 0.10 0.00 0.00 0.00
Ca 0.07 0.00 0.00 0.05 0.07 0.00 0.00
Cl 0.11 0.03 0.14 0.32 0.00 0.21 1.39
P 0.22 0.00 0.00 0.27 0.16 0.00 0.00
1018 S. Ramaswamy and P. Raghavan Vol.10, No.11
Pyrite particles
Figure 5. SEM picture of Raw sample
EPMA analysis
EPMA pictures of IM1 along with the weight percentages of the constituents are given in Figure
6. Analysis shows that kaolinite is the major mineral present in the sample along with the minor
quantities of pyrite and hematite. The sample is also found to contain traces of illite, ilmenite,
rutile and gypsum.
Figure 6. Electron micrograph of IM1 sample
3.3. Quantification of Iron Minerals in the Sample
3.3.1. Inferences from preliminary laboratory studies
Mineralogical (XRD) analysis has shown that the colouring and water soluble impurity species
getting precipitated at near neutral pH during pH modification is limonite. Though the quantity
of this species was only 0.4% (with respect to Raw sample), it is found to adversely affect the
overall shade of the material due to the formation of very fine reddish coloured coating on the
( weight %)
12.70 31.60
1.63 14.65
2.84 0.86 1.27 35.03
Vol.10, No.11 Significance of Impurity Mineral Identification 1019
clay surface during drying. Hence, the removal these iron species was an important step before
the size classification of the sample. Also, laboratory level studies on the separation and
quantification of the pyrite mineral has shown that ~ 65% (with respect to raw sample) of the
pyrite particles are above 45 µm in size and their removal at this particular size range is highly
3.3.2. Quantification of “Free Iron” in the FSCP sample by DCB treatment
The iron content and optical properties of the beneficiated samples are given in Table 1. DCB
treatment gives information on the quantity of “free iron” in the sample, which can be removed
by chemical leaching using suitable reducing agents. In the present study, DCB treatment of the
FSCP sample was carried out and it is found to be highly effective in removing the “free iron
species” from the sample. After DCB treatment, the iron content in the FSCP DCBTP sample
has come down from 0.81% to 0.35% and this has led to an appreciable improvement in its
optical properties. The iron removal shows that ~57% of the total iron in the clay is”free” in
nature and the rest is present in the structure of either kaolinite or ancillary mineral (mica or
titania). The brightness/whiteness of kaolin is dependent on the overall effect of the “L a b
color values. The “Lvalue and HW of the sample increase after DCB treatment by ~3.5 and
~34 units and it gives an idea about the extent of removal of the dark colored impurity minerals.
Also, the decrease in the “b” and “HY” values (~ 5.5 and 8.8 units respectively) confirms the
removal of the coloring iron impurities such as hematite and goethite. The sample is still found
to contain 0.35% of iron. Since the sample has got good optical properties, it is possible that the
iron remaining after DCB treatment may be present as part of the kaolinite structure and their by
not appreciably affecting the overall brightness of the sample. It is also worth mentioning that
chemical leaching has not effected any changes in the TiO
EPR spectroscopy can be used to assess the effectiveness of iron removal techniques and the
EPR spectral data of the samples also support the above findings. In the present work, FSCP and
FSCP-DCBTP samples have been studied by EPR spectroscopy to understand the removal of
“free iron” by DCB treatment. The EPR spectra of the corresponding samples are given in Figure
7. Both the samples exhibited EPR lines at F1 and F2 regions, but these lines are found to be
weak in nature. Sharper lines are observed in the F1 region ( g ~4.83 & 4.25) for FSCP sample
with a more intense line at g~ 2.54 along with g~2.02 line. The intense line atg~2.54 indicate
the presence of “free iron” impurities. In the case of FSCP DCBTP sample, the intensity of this
line (at g ~2.56) has decreased considerably, thus confirming the removal of free iron
contaminants by DCBT. The reduction in iron content and the sharp improvement in brightness
of the sample support the findings. Chemical assay and XRD analysis shows that the EPR silent
pyrite is the major iron impurity in the clay. The other lines in the F1 region of the spectra are
found to be sharper. Both the lines at Fe
& Fe
sites in F1 region are due to Fe
substituting for Al
in the kaolinite lattice but they have different symmetry.
1020 S. Ramaswamy and P. Raghavan Vol.10, No.11
Figure 7. EPR spectra of (a) FSCP and (b) FSCP DCBTP samples
3.4. Significance of Identification and Quantification of the Impurity Minerals
All the above observations and inferences have really played a crucial role in incorporating the
required modifications in the conventional beneficiation flow sheet (Figure 8). In the modified
process flow sheet (Figure 9), the conventional high speed stirring was replaced with low speed
mixing in order to avoid the breaking down of the coarse pyrite particles and their spreading to
the finer size ranges. Also an additional operation of water washing technique (three consecutive
washes) involving a set of operations ie., water addition, mild mixing, settling of the clay mass
and decanting the coloured supernatant liquid was introduced before size separation to remove
the water soluble colouring species. After removing the water soluble colouring impurities, the
kaolin can be value added by employing techniques like hydrocycloning followed by reductive
bleaching using suitable reducing agents like sodium dithionite. In nutshell, the identification and
Vol.10, No.11 Significance of Impurity Mineral Identification 1021
quantification of the impurity mineral species has played an important role in the
selection/modification and sequentialisation of beneficiation process.
Raw clay
Mixing with water
Stirring at high rpm for max. de-aggregation
Screening/ size separation using a Set of Hydrocyclones
Figure 8. Conventional Flow sheet for size separation of Kaolin
Final Size classified Product (FSCP)
1022 S. Ramaswamy and P. Raghavan Vol.10, No.11
Raw clay
Water addition
Mild Mixing (at low rpm for de-aggregation)
(to avoid size reduction of coarse pyrite particles)
W 1
W 2
W 3
pH modification using NaOH solution (to neutral pH)
Screening/ size separation using a set of hydrocyclones
Dried Final Product
Figure 9. Modified flow sheet for size separation of Koraput Kaolin
W1, W2 & W3
– First, Second & Third “water wash” respectively.
“Water wash” consists of settling of the clay mass, decantation of the coloured supernatant liquid
and water addition with mild mixing.
Final Size classified Product
Vol.10, No.11 Significance of Impurity Mineral Identification 1023
3.5. Evaluation of Properties of the Beneficiated Samples
Beneficiation of the samples was carried out after incorporating the necessary modifications in
the conventional process flow sheet based on the nature and quantity of the impurity species.
Water washing, screening and size classification were effective in removing water soluble
colouring impurities and most of the pyrite and coarse particles in the sample. This increased the
kaolinite content in the sample and led to the enrichment of fines to 73.9% from 49.9% (Table
1). The rational analysis data also supports these findings. Brightness and “L a b” color
components are influenced by iron minerals and iron bearing anatase. Iron decreases brightness
and “L” values whereas anatase increases “b” to cause yellowness. The brightness, lightness (L)
and Hunter whiteness (HW) values of raw sample are found to be very poor due to the presence
of black/dark coloured particles in the clay. Size classification increases the brightness of
samples appreciably ie., by 22.3 (SCP1) and 24.6 (SCP2) units. Similarly, there is a sharp
increase in the lightness “L” value (~21 units) and moderate increase in the “HW” (~11 and 16
units for SCP1 and FSCP respectively) of the beneficiated samples which are due to the removal
of the black colored pyrite impurities. The appreciable reduction in the iron content of SCP1 and
FSCP also supports the same (Table 1). DCB treatment improves the brightness substantially
(~13.0 units) and during this process the iron content reduces from 0.81% to 0.35%, indicating
that part of the iron in the sample is “free” and leachable.
a) The sample taken for the present study is kaolinitic in nature and found to contain different
types of impurity mineral species.
b) Characterization studies showed that the major impurity mineral species in the sample is
pyrite (Iron sulphide) along with minor ancillary minerals such as goethite, lepidocrocite,
maghemite, ilmenite, hematite, rutile, anatase and pseudo rutile. Presence of water soluble
colouring iron species like limonite is also detected.
c) Most of the “pyritic” particles in the sample” is found to be in coarse size ranges.
d) The basic information about the type and nature of the impurity minerals present in the kaolin
have really played a crucial role in incorporating the required modifications in the conventional
beneficiation flow sheet. This shows that the identification and quantification of the impurity
mineral species has got an important role to play in the selection/modification and
sequentialisation of beneficiation processes.
e) Evaluation of the properties (chemical assay, particle size distribution and brightness) of the
product sample has shown that the sample can be value added to ceramic grade.
1024 S. Ramaswamy and P. Raghavan Vol.10, No.11
The authors are thankful to the Director, NIIST, Trivandrum for his permission to communicate
this work. They are also grateful to Dr.Peter Koshy for providing SEM pictures and
Dr.Syamaprasad and Mr.Gurusamy for providing the X-ray diffraction analysis data of the
samples. Thanks are also due to Prof. Storr of the Univ.of Greifswald, Germany for the HRTEM-
EDS results. The authors are also thankful to Dr.Mohapatra, IIMT, Bhubaneswar for the EPMA
pictures; Dr.Sambasiva Rao, University of Pondicherry for the EPR spectra and Dr.Tripathi, JNV
University, Jodhpur for the Mossbauer spectral data.
[1] Jepson, W.B., 1988. Structural iron in kaolinites and in associated ancillary minerals, In :
Iron in Soils and Clay Minerals (J.W.Stucki, B.A.Goodman and U.Schwertmann,
editors). NATO Advanced Science Institutes Series, D.Riedel Publishing Company,
Dordrecht, Holland, pp. 467-53
[2] Cases J.M., Lietard O., Yvon J. and Delon J.F., 1982. Etude des properties
crystallographiques, morphologiques et superficielles de kaolinites desordonnees, Bull. Soc.
Fr. Min. Crist., 105, pp. 439-457
[3] Cases J.M.l, Cunin P.,Grillet Y., Poinsingnon C. and Yvon J., 1986). Methods of
analyzing morphology of kaolinite : relations between crystallographic and morphological
properties. Clay Minerals, 21, pp. 55-68
[4] Brindley, G.W., Kao,C.C., Harrison, J.L., Lipsicas, M., and Raythata, R.,1986. Relation
between structural and other characteristics of kaolinite and dickite. Clay and Clay
Minerals, 34, pp. 239-249
[5] Komusinski J., Stoch L. and Dubiel S.M., 1981. Application of election paramagnetic and
Mossbauer spectroscopy in the investigation of kaolin group minerals, Clays and Clay
Minerals, 29, pp. 23-30
[6] Balan A., Allard T., Biozot, B., Morin G. and Muller J.P., 2000. Quantitative measurement
of paramagnetic Fe
in kaolinite. Clays and Clay Minerals, 48, pp. 439
[7] Lazarev A.N., 1974). The dynamics of crystal lattices, in The Infrared Spectra of Minerals
(ed. V.C.Farmer), Mineralogical Society, London, pp. 69-86
[8] Rendon J.L and Serna C.J., 1981. IR spectra of powder Hematite; effects of particle size and
shape, Clay Minerals, 16, pp. 375-382
[9] Weaver C.E., 1976. The nature of TiO2 in in kaolinite, Clays and Clay Minerals, 24, pp.
215- 218
[10] Jepson W.B and and Rowse J.B., 1975. The composition of kaolinite – an electron
microprobe study, Clays and Clay Minerals, 23, pp. 310-317
[11] Singh, B. and Gilkes, R.J., 1992. Properties and distribution of iron oxides and their
association with minor elements in the soils of south-western Australia. Journal of Soil
Vol.10, No.11 Significance of Impurity Mineral Identification 1025
Science, 43, pp. 77-98
[12] Mehra, O.P., Jackson M.L., 1960. Fe Oxide removal from soil and clays by a dithionite-
citrate system buffered with sodium carbonate. Clays and Clay Minerals, 7, pp. 317-327
[13] Bennett H and Reed R.A., 1971. Chemical methods of silicate analysis. A handbook
Academic Press Ind., London, pp. 272
[14] Searle, A.B., Grimshaw, R.W., 1960. Chemistry and Physics of Clays. Western Printing
Services Ltd., Bristol, UK. pp. 944
[15] Jeffery et al G.H., Bassett J., Mendham J. and Denney R.C.(1989),Vogel’s Text book of
Quantitative Chemical Analysis, 741, John Wiley and Sons, USA
[16] Dold B., Fontbote, L., 2000. A mineralogical and geochemical study of element mobility
in sulfide mine tailings of ‘Fe’ oxide cu-au deposits from the Punta del Cobre belt,
northern Chile. Chemical Geology 189 (3), pp. 135-163
[17] Foth H. D., Ellis B.G., 1996. Soil Fertility, Lewis Publishers, CRC Press, Inc. New York,
[18] Fysh S.A., Cashion J.D. and Clark P.E., 1983. Mossbauer Effect studies on iron in kaolin.I,
Structural iron, Clays and Clay Minerals, 31, pp.285-292
[19] Ruby C., Refait PH., Genin J.M.R., Delineau T. and Yvon J., 1999. Evidence of structural
Fe(II) ions in Font-Bouillant kaolinites : a Mossbauer study, Clay Minerals, 34, pp.515-518