812 J SCI IND RES VOL 67 OCTOBER 2008Journal of Scientific & Industrial Research

Vol. 67, October 2008, pp.812-818

*Author for correspondence

E-mail: senguptap@rrljorhat.res.in

SEM-EDX characterization of an iron-rich kaolinite clay

Pinaki Sengupta*, Pradip C Saikia and Prakash C Borthakur

Materials Science Division, North East Institute of Science and Technology (NEIST), Jorhat 785 006

Received 27 March 2008; revised 14 July 2008; accepted 04 August 2008

Kaolin clay from Deopani deposit of Assam contains high amount of iron. Kaolinite particles, characterized by SEM-

EDX, are pseudohexagonal and arranged in face-to-face pattern. Clay particles are coated with iron- and titanium-bearing

minerals, which can be separated by wet high intensity magnetic separator Titaniferrous impurities present as coatings on

kaolinite particles are difficult to remove by oxalic acid treatment.

Keywords: Classification, Industrial minerals, Leaching, Magnetic separation, Particle morphology

Introduction

Kaolinite (white if pure) clay, one of the most

versatile industrial minerals, is mostly used as ceramic

raw material, coating and filler pigment for paper, filler

for paint, rubber, insecticide etc., and also used in

catalyst manufacture, in formulation of medicine,

cosmetics, etc. Mined kaolin usually contains silica (as

quartz) as major contaminant and Fe- and Ti-bearing

minerals that impart colour1 as other contaminants.

Kaolin occurs in Deopani of Karbi Anglong district,

Assam, India. The deposit is estimated to contain 1.0

million tonnes of workable kaolinite2. Physico-chemical

characteristics of clay and its beneficiated products,

using chemical, X-ray diffraction (XRD) and Fourier

transform infra red (FTIR) analysis, have been reported3.

Fe content, chief colour imparting component of clay,

could be reduced from 9.48% to about 1.00% by size

separation (SS) and oxalic acid leaching (OAL) due to

favourable Fe-mineralogical form (sideritic).

Beneficiated fractions are suitable for using as ceramic

raw material, filler material for paper, rubber, plastic,

paint etc3.

This paper reports characteristics and particle

morphology of clay and its beneficiated products using

scanning electron microscopy (SEM) and energy

dispersive X-ray spectroscopy (EDX).

Materials and Methods

Kaolin clay was collected from exposed faces of

Deopani deposit, located at 26014/27// to 26014/39// N

latitudes, 93045/05// to 93046/05// E longitude and extend

over an area of 0.0348 km2 and 0.40 km2 in block I and

block II respectively. Thickness of deposits varies from

2.40-23.20 m (av 7.77 m) in block I and 0.95-7.60 m (av

2.99 m) in block II. Overburden (0-16.0 m) consists of

soil, shale, limestone, sandstone and coal. Kaolinization

occurs in a lensoid pattern in granite body along NE-SW

direction, which approx. coincides with foliation plan of

granitic gneisses2. Precambrian granites and granitic

gneisses with different degrees of weathering dominate

the area and are overlain by Tertiary sediments2.

Representative clay samples were prepared from bulk

clay following Indian standard method4. Clay was

suspended in water, stirred, and a fraction (–53 µm) was

separated by sieving and further utilized for beneficiation

by SS and OAL. A wet high intensity magnetic separator

(WHIMS) was used to separate nonmagnetic and

magnetic fractions of clay. Fe and Ti contents of samples

were estimated by wet chemical analyses and X-ray

fluorescence (XRF) methods. Mn content of clay was

determined by following standard techniques5.

SEM and EDX analysis were recorded by using LEO

S430 scanning electron microscope coupled with energy

dispersive X-ray analyzer model Oxford LINK ISIS.

Samples were prepared by dispersing dry powder on

double sided conductive adhesive tape. Samples were

coated with carbon by arc discharge method for SEM-

EDX. Samples were scanned in secondary electrons (SE)

for morphology and back scattered electrons (BSE) mode

for compositional image6. Particles with white patches

SENGUPTA et al: SEM-EDX CHARACTERIZATION OF AN IRON-RICH KAOLINITE CLAY 813

(spot marked as Fe) were analyzed by EDX to ascertain

presence of Fe and Ti.

Results and Discussion

Chemical analysis3 of crude clay sample, its –53 µm

fraction and some other fractions show that Fe, Ti and

Mn can be separated by SS, WHIMS treatment and

OAL (Table 1). Table 2 shows Fe2O

3 content of coarse

(–53 µm +10 µm), medium (–10 µm +4 µm) and fine

(–4 µm) fractions of the as such clay and those obtained

after leaching with 1M oxalic acid solution3. Removal

of iron from different size fractions of clay by oxalic

acid treatment, as reported earlier3, is presented in

Fig. 1. XRD had shown that crude clay contains

kaolinite and quartz as major mineral constituents,

siderite as major iron bearing impurities along with

small amounts of mica and goethite and/or hematite3.

SE image of -53 µm fraction (as such) of clay

(Fig. 2a) shows kaolinite particles of varying sizes that

are arranged in face-to-face patterns. Some individual

well crystalline pseudohexagonal edges of kaolinite as

well as some rolled and rough edged kaolinite particles

are also observed. Some kaolinite particles are below 1

µm size. Spongy quartz (Fig. 2a) might have resulted due

to soil environment of Assam and particularly of Karbi

Anglong district, which is acidic, hot moist subhumid

and excessively drai-ned7. Hot and humid weathering

environment also favours weathering of labile minerals

to kaolin group minerals8. Formation of sericitic mica

type mineral, an intermediate between micas and clay

minerals, are also possible9. Some patches associated with

wedge shaped minerals, which coat kaolinite particles as

impurities, are also observed. Ti-bearing rock forming

minerals associated with varying amounts of Fe and Mn

form wedge shaped minerals10. EDX spectrum of marked

patches (Fig. 2b) shows presence of Fe and Ti along with

minor amounts of Mn.

Among SE images of magnetic (Fig. 3) and

nonmagnetic (Fig. 4) fractions of -53 ¼m clay, obtained

by WHIMS treatment, magnetic portion showed presence

of very high amount of patches (Fig. 3a). The fraction is

rich in quartz and clay platelets, which are highly coated

and stacked together. EDX analysis showed presence of

Table 1—Oxide composition (wt %) of Deopani kaolin and its beneficiated fractions3

Sample SiO2

Al2O

3Fe

2O

3TiO

2MnO

2CaO MgO LOI

Crude clay 58.44 19.11 9.48 0.59a 0.267 0.54 0.38 10.98

-53 µm fraction 47.20 35.75 2.86 0.90 0.022 0.17 - 12.23

-53 µm, nonmagnetic 45.04 38.67 1.10 0.70 - - - 12.65

-53 µm, magnetic 23.92 33.50 20.94 8.13 0.528 - - 11.05

Fine fraction obtained* 45.35 38.31 1.01 0.57 - - - 13.75

*Fe2O

3: oxalic acid = 1.0: 0.4 mol ratio; Fine fraction: - 4 µm

aTiO2 content of the crude clay was determined separately and included in the table

Table 2—Fe2O

3 content of beneficiated fractions3

Fe2O

3: oxalic acid Fe

2O

3 content, wt %

(M) Coarse Medium Fine

1.0:0.0 3.38 2.52 2.21

1.0:0.4 2.60 1.53 1.01

Coarse: - 53 µm + 10 µm; Medium: - 10 µm + 4 µm;

Fine: - 4 µm

0

Fine

Middle

Coarse

15

30

45

60

75

0 0.5 1 1.5 2

Acid conc (Fe/oxalic acid), mol

ratio

Fe

rem

ov

ed, %

Acid conc (Fe/oxalic acid), mol ratio

Fig. 1—Removal of Fe from Deopani kaolin by oxalic acid

814 J SCI IND RES VOL 67 OCTOBER 2008

very high amount of Fe, Ti and Mn in this fraction

(Fig. 3b, Table 1). Kaolinite platelets are clearly visible

in nonmagnetic fraction (Fig. 4a), which is

comparatively free from patches. EDX spectrum

(Fig. 4b) also shows very low Fe content. Although

chemical analysis showed presence of Ti (Table 1),

EDX pattern did not indicate presence of Ti-bearing

minerals in nonmagnetic portion.

SE image of coarse fraction, separated from clay

without acid leaching, shows presence of some large and

spherical particles of quartz (Fig. 5 a and b). Rounded

nature of quartz particles and spherical grains indicate

recycled nature and maturity of sediment accumulation11.

Quartz particle at higher magnification shows spongy

surface and microcracks. One of the blocks shows

exfoliation at one end, which may be attributed to presence

Fig.. 2— a) SE image of -53 µ fraction of Deopani kaolin: pseudohexagonal kaolinite in face-to-face arrangement (→); rolled and

rough edged kaolinite ( ); spongy or porous quartz surface ( ); wedge shaped titaniferrous mineral

( ); b) EDX analysis (qualitative) of marked patch

Fig. 3—(a) SE image of magnetic portion of WHIMS treated Deopani kaolin. (b) EDX analysis

(qualitative) of marked patch

SENGUPTA et al: SEM-EDX CHARACTERIZATION OF AN IRON-RICH KAOLINITE CLAY 815

of sericitic mica12. Stacks of kaolinite are also observed.

Coarse fraction also shows patches, rich in Ti and Fe

bearing minerals and containing traces of Mn as

indicated by EDX pattern (Fig. 6), which coats quartz

particles and stacks of kaolinite.

SE images of medium (Fig. 7) and fine fractions (Fig.

8) separated from clay without acid leaching show that

kaolinite platelets have broken edges. Large kaolinite

particles in medium fraction are coated with very fine

particles, which are well-formed booklets of kaolinite

layers. In fine fraction, some relatively big sized kaolinite

particles are coated with patches containing wedge

shaped titaniferrous minerals10. EDX analysis shows that

Fig. 4— a) SE image of nonmagnetic portion of WHIMS treated Deopani kaolin; (b) EDX analysis (qualitative) of

marked patch

Fig. 6—EDX analysis (qualitative) of coarse

fraction of unleached Deopani kaolin

Fig. 5—SE image of coarse fraction of Deopani kaolin: (a) lower and (b) higher magnification: spherical quartz particle (→) ;

rounded corners (→ ); exfoliation ( ); rouleau or stacks ( )

816 J SCI IND RES VOL 67 OCTOBER 2008

Fig. 7—(a) SE image of medium fraction of unleached Deopani kaolin: kaolinite particles with broken edges (→) ;

booklets of kaolinite ( ); wedge shaped titaniferrous mineral ( ). (b) EDX analysis (qualitative) of marked patch

Fig. 8—(a) SE image of fine fraction of unleached Deopani kaolin. (b) EDX analysis (qualitative) of marked patch

Fig. 9—(a) SE image of coarse fraction of acid leached Deopani kaolin. (b) EDX analysis (qualitative) of marked patch

(a) (b)

(a) (b)

(a) (b)

SENGUPTA et al: SEM-EDX CHARACTERIZATION OF AN IRON-RICH KAOLINITE CLAY 817

Fe, Ti and Mn contents of coatings decreases in order:

coarse > medium > fine fraction (Figs 6-8).

SE image of coarse fraction contains large quartz

particles with surfaces etched by acid solution, due to

dissolution of Fe bearing minerals, exposing pores and

channels. Amounts of patches in different fractions

decrease in order of coarse (Fig. 9)> medium (Fig. 10) >

fine (Fig. 11) fraction. It is less in acid treated fractions

than corresponding untreated fractions. Patches are

maximum in untreated coarse fraction (Fig. 5) and

minimum in acid treated fine fraction (Fig. 11). Fe and

Ti content of patches, revealed by EDX, follow same

trend as with patches in these fractions (Figs 9-11).

Untreated coarse fraction contains maximum Fe and Ti

(Fig. 5) and acid treated fine fraction contain minimum

amount of these impurities (Fig. 11). Removal of Fe by

acid treatment is highest in fine fraction followed by

medium and coarse fraction (Fig. 1). SEM-EDX

investigation reveals that it is difficult to remove

completely coatings of titaniferrous minerals from

kaolinite particles.

Fig. 10—(a) SE image of medium fraction of acid leached Deopani kaolin. (b) EDX analysis (qualitative) of marked patch

Fig. 11—(a) SE image of fine fraction of acid leached Deopani kaolin. (b) EDX analysis (qualitative) of marked patch

(a) (b)

(a) (b)

818 J SCI IND RES VOL 67 OCTOBER 2008

Conclusions

SEM-EDX investigation shows that iron-rich kaolin

from Deopani deposits of Assam, India contains

pseudohexagonal kaolinite particles in face-to-face

arrangement, quartz and titaniferrous minerals. A large

amount of clay particles are coated with patches

containing Fe, Ti and Mn. Highly coated particles can

be separated as magnetic fraction by WHIMS treatment.

Fe content of coatings decreases in the order: coarse >

medium > fine fraction. SEM investigation indicates that

amounts of patches in acid treated fractions are less than

corresponding untreated fractions and it is minimum in

acid treated fine fraction. EDX analysis also shows that

Fe and Ti content of patches are less in acid leached

fractions and are minimum in fine fraction of clay.

Although, acid treatment considerably removes Fe and

Ti bearing minerals, kaolinite particles still remains

coated with it.

Acknowledgements

Authors thank Ministry of Coal and Mines, Govt of

India, New Delhi for financial grant and Directorate of

Geology and Mining, Govt of Assam, Guwahati, India

for providing kaolin sample. Authors also thank Mr D

Bordoloi, Technical Officer, Dr P Kotoky, Scientist and

Dr P G Rao, Director, NEIST, Jorhat, for helpful

discussion and encouragement in the work. Thanks are

also due to Dr N J Saikia and Mr D J Bharali, NEIST,

Jorhat.

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