Journal of Scientific & Industrial Research
VoL58, June 1 999, pp 436-442
Surface Properties of Indian Hematite and Bauxite and Their Coating
Mechanism with Colloidal Magnetite
S Prakash' and B Das
Regional Research Laboratory, Bhubaneswar 751 0 1 3
and
R Venugopal
Indian School of Mines, Dhanbad 826 004
Received : 05 August 1 998; accepted : 1 7 November 1 998
The selective coating of colloidal magnetite on hematite and bauxite for enhancing the magnetic separation techniques which depends
on surface and chemical phenomena is studied. The mechanism of colloidal magnetite interaction with hematite and bauxite is investi­
gated through electrokinetic and IR studies. The point of zero charge and electrophoretic mobilities of pure hematite, bauxite and
colloidal magnetite are determined. The effect of various ions such as PO·\ ' SiO/ and the surfactant such as sodium oleate on surface
properties of hematite and bauxite are examined. IR spectroscopy studies indicate that oleate ions are chemically bonded with hematite
while it is only physically adsorbed on the surface of bauxite at certain pH. The zeta-potential also correlates the chemical interaction of
oleate on hematite. The separation of quartz from hematite and hematite from bauxite is carried out by applying selective colloidal
magnetite coating followed by magnetic separation techniques. Hematite is separated in magnetic fraction for both the cases.
Introduction
Hematite is an important raw material widely used for
the production of iron and steel. This can be separated
from its ore by various methods. In Indian iron ore wash­
ing plants, generally washing is being practiced to sepa­
rate the associated gangues. However, during the proc­
ess of washing around 8 million tonnes of superfines from
different washing plants of India with 48-60 per cent Fe
are being generated and thrown into the iron ore tailing
ponds without any further utility ) . Attempts have been
made by different workers to recover the iron values lost
by various techniques with l imited success2-4• Similarly,
bauxite is also an economic non-metallic mineral used
as a raw material for producing alumina and in refrac­
tory making. The low grade bauxite contains variety of
gangue minerals, viz. Fe-oxides, silicates, and carbon­
ates. Amongst them, the separation of iron is problem­
atic one, causing difficulty in down stream processes5-7•
Roasting followed by leaching is the most general method
used for dissolution of bauxite in commercial operations.
Coating and carrier methods for enhancing the mag­
netic and flotation separation methods have however re*
Author for correspondence
ceived little attention8 •9. We have been working in our
laboratory to enhance the magnetic response of weakly
magnetic minerals through selective magnetic coating
techniques. In this paper an attempt has been made to
understand the surface chemistry and coating mechanism
of oleate colloidal magnetite on hematite and bauxite
through electrokinetic, IR study and the effect of other
ions such as silicate and phosphates for separation in a
large scale.
Experimental
The experimental study broadly consist of careful
preparation of pure minerals, electrokinetic potential
measurements, IR spectrophotometry and wet high in­
tensity magnetic separation studies.
Sample
The hematite and bauxite sample, as c lean as possible
were used for the experimental studies. The chemical
analysis of the samples is given in Table t . The samples
used were obtained from Orissa Mining Corporation. Se­
lected pieces were crushed and hand ground in a mortar.
A portion of the sample was reduced to - 500 11m. After
PRAKASH
et al.
:
SURFACE PROPERTIES OF INDIAN HEMATITE & BAUXITE
desliming in laboratory tap water a closed sized fraction
of -7S+30�m was separated out by hand sieving. The
bauxite sample was washed with O.SN hydrochloric acid
to eliminate any trace of iron present in the surface. This
was fol lowed by washing and rewashing with distilled
water till no trace of chloride ions was detected in rins­
ing water. On the other hand the hematite sample was
passed in a low intensity magnetic separator to remove
any contaminated magnetite in it. Quartz sample was also
of mineral origin and supplied by Saroj Mining Corpora­
tion, Karnataka. The chemical analysis shows that the
sample contains 99.6 per cent Si02 • The size and prepa­
ration of the material used in the magnetic separation
experiment was similar to bauxite and hematite.
Reagents
Reagents used such as perchloric acid, sodium hydrox­
ide, sodium silicate, sodium hexa metaphosphate, and
potassium nitrate were all AR grade reagents. Fresh so­
lutions of sodium oleate (analar grade) having known con­
centrations were prepared in situ.
Magnetic separation studies were carried out on the
closed sized fractions. A portion of the sample (-7S�m)
was further ground and microsieved to obtain -5 �m
materi a l . Electromagnetic measurement and IR
spectroscopy were conducted with the help of -S�m ma­
terial .
Methods
Preparation of Oleate Magnetite
The colloidal magnetite was prepared by the combi­
nation of I : I ferrous and ferric salts. The salts were dis­
solved in distilled water and heated slowly to 70° C. S g
of sodium hydroxide dissolved in SO cc of water was
added slowly with constant stirring into the warm solu­
tion. The precipitated magnetite thus obtained was washed
several times, centrifuged to remove salts and sodium
hydroxide. The total volume after washing was made up
to 1 00 cc. The oleate colloidal magnetite was prepared
by the addition of 2 x 10-2M sodium oleate at pH I I and
boiling until the magnetite is fully dispersed.
Electrokinetic Measurements
The electrophoretic mobility of the minerals and oleate
magnetite were measured by using the zetameter, Rank
Brother model, mark-II, UK and the mobility'S were con­
verted into zeta-potentials using the H elmholtz-
437
Smoluchowsky equation. Samples for the zetapotential
tests were prepared by making a solution of the desired
pH by adding the pH regulators, i.e., HCI0 , or NaOH.
4
0.01 g of the sample was added into the 1 00 ml required
solution and it was conditioned for 1 h at the room tem­
perature. In all the experiment KN03 (2 x 1 0-5 M) was
used as the supporting electrolyte. The mobility of the
suspension was determined by adding portion of the so­
lution into the given cell connected with two Pt electrodes.
IR Spectroscopy
The IR spectra was recorded by a JASCO Fourier trans­
form IR model S3oo spectrophotometer in 4oo-4000cm­
I . The sample used in the spectrophotometer studies was
prepared by grinding in an agate motor to a very fine
power. One to 2 g of the mineral sample was conditioned
at a given pH for I h with the surfactant and the oleate
magnetite. After conditioning, the solids were separately
dried at SOoC and the FTIR measurements were taken.
Magnetic Separation Studies
The magnetic separation studies were carried out by a
wet magnetic separator supplied by Rapid Box Mag sepa­
rator, England. The magnetic intensity could be varied
with current and grid gaps. The slurry was prepared by
adding different proportions of minerals and required
quantities of distilled water. The pH of the slurry was
adjusted by adding NaOH or HCI0 which was moni­
4
tored throughout the experiment. Oleate magnetite or
magnetite fines were added in desired quantity into the
suspension. A small quantity of sodium hexa meta phos­
phate (0.0 I per cent) was added as dispersant whenever
necessary. The mineral mixture was then conditioned for
1 0 min. The slurry was than passed through the magnetic
separator slowly. The magnetic fraction was retained on
the grid whereas the non-magnetic fraction was collected
at the bottom. The magnetic fraction was taken out of the
grid by flushing and washing with water in the absence
of the magnetic field. The magnetic and non-magnetic
fraction thus collected were dried separately and analysed
for Fe, Si0 , Alp etc. to determine the quality and
2
3'
recovery.
Results and Discussion
Electrokinetic Studies
The results of the electrokinetic potential variation of
hematite, bauxite, magnetite and oleate magnetite as a
function of pH using KN03 as the supporting electrolyte
J SCI IND RES VOL.58 JUNE 1 999
438
ro �------�--�
oo,-·----�
c nil
1O:�
10
>
E
• 1.7 x
M
A 3.4 X 10_ M
;
o 6.8 x
M
�
�
c
11
�
o
Q.
12
•
Hemat ite
[] Bauxite
A Magneiite
o Magnetite
.e
..
N
12
I
+ Na oleate
L-----�----p�H--�
-601
Figure I - Zeta potential of hematite, bauxite, magnetite and
magnetite coated with sodium oleate at different pH,
(2 x 1 0-3 KNO J )
-
Figure 3
Zeta potential of bauxite as a function of pH at different
concentration of sodium hexametaphosphate
30
60 r---�----.
'E
40
20
c
A
•
o
nil
1.7
'E
x 10-5M
. 3.4 X \05 M
5.i x
1
---,-----o---:
ni-:1 -------,
t:.
•
10 '
o
L�
lIi5 M
'-' II
\O�
til
3.4 x 10_ M
5
5.1 X 10 M
w
"
I
- 501 L
-------------'.
----
pH
Figure 4 - Zeta potential of hematite as a function of pH and
different sodium silicate concentration
Figure 2 - Zeta potential of bauxite as a function of pH at different
concentration of sodium silicate
are given in Figure I . The point of zero charge of hematite
is found to be around 7.2 and that of bauxite is around
8.6. The zero point of charge of hematite reported in the
literature is at 7.6 to 8.0 (ref. I 0) and that of bauxite is at
pH 8.5 to 9.4 (ref. I I ). For oxide minerals, H+ and OH­
ions have since been considered to be potential deter­
mining ions. The various concentrations of these ions
change the m agn i tu de and s i gn of electrokinetic
potentials. So these ions are also the potential determin­
ing ions for hematite and bauxite, as reported earlier'2.
The pzc of magnetite obtained in the present investiga­
tion is around pH 6.8. Above this pH, magnetite has got
negative charge. The pzc of synthetic oleate magnetite is
about 5.5. The synthetic compound resulted a shift in
iso-electric point.
Effect of Phosphate and Silicate Ions
Figure 2 and 3 show the electrokinetic potential of
bauxite in the presence of sodium hexameta phosphate
(SHMP) and silicate ions. Both these ions play a vital
role in the dispersion and thereby the separation of min­
erals. It is seen that the adsorption of silicate ions or
hexameta phosphate ions makes the surface more nega-
30
c nil
20
10
>
E
0
�
�
c
.!
0
0.
"
...
..
N
-10
5
10
"
12
-20
-
3
-40
- so
-
Figure
4
:;
• 1 . 7 X 10 101
o 3.4 . ,0 M
-5
Co 5 . 1 X I) M
60L----_
----�
�-pH
- Zeta potential of hematite as a function of pH and
sodium hexametaphosphate concent.ration
tively charged bauxite and thereby the particles remain
in dispersed phase due to mutual repulsion of the strongly
negative charged particles and even at lower pH, the par­
ticles remain in suspension. Therefore, either of these
ions can be used as a dispersant in the separation of
hematite from bauxite by selective magnetic coating.
Similar observations are made with hematite-sodium
silicate (Figure 4) or hematite hexameta phosphate (Fig­
ure 5). The electropotential suddenly decreases to nega­
tive sides after the addition of the 1 0-5M concentration
of sodium silicate. Further addition of the reagent dose
PRAKASH
e/ al.
:
SURFACE PROPERTIES OF INDIAN HEMATITE & BAUXITE
not change the potential in a significant way. However
the mineral possess a constant negative potential of 3 1 35 mv in the pH range 5 to 9 . Only at pH 3.5 the poten­
tial drops from + 1 8 mv to around - 1 0 to 1 5 mv. Sodium
silicate is therefore one of the most widely used modify­
ing reagent in flotation.
...
The zeta-potential-pH curve for hematite in the pres­
ence of d i fferen t c oncentrat i o n of sodium h e x a
metaphosphate is shown in Figure 5. It suggests that the
point of zero zeta potential has shifted from pH 6.8 to
more on acidic side. From the examination of these
curves, it is noted that the addition of hex a metaphosphate
to the hematite suspension has produced a change in zeta­
potential at a l l the pH values studied . The hex a
metaphosphate has acted as a good dispersing agent of
hematite. The interaction between hematite and reagent
may also be due to complex formation as phosphates and
polyphosphates are good complexing agents for a wide
variety of metal cations 1 3 .
Effect ofSodium Oleate on the Zeta-potential ofHematite
Figure 6 shows the values of zeta-potential of hematite
at different initial concentration of sodium oleate with
respect to pH at constant ionic strength. In a selective
magnetic coating, surfactants are used along with mag­
netite or separately as oleate magnetite in order to con­
trol surface charge or better magnetite coatin'g of the sur­
face. The adsorption of oleate and magnetite takes place
either through the electrostatic interaction or by the chemi­
cal interaction between the surfactant and mineral. In the
present case, an increase in negative potential starts
around 1 O·5M concentration of oleate and continues to
increase with further addition of oleate. The increase in
negative potential clearly demonstrates the successive
oleate adsorption on the surface of the mineral. In the
present case the negative oleate ion can only be adsorbed
on the mineral surface chemically resulting in the net
increase of negative surface charge of the mineral indi­
cating specific adsorption .
The zeta potential obtained in the presence of sodium
oleate (5x 1 0-5 M) as a function of silicate and meta phos­
phate concentration have also been studied. It shows the
surface potential is the same -3 1 mv until the 1 0-4 M meta
phosphate concentration and further level of these two
ions increases in negative zeta potential. As the increase
of this potential is marginal, it can be concluded that low
effect of these ions might have masked in the presence of
oleate similar to apatite and ca1citeI4• It has been stated
439
that adsorption of sodium oleate on hematite greatly re­
duces after treatment with modulus 3.3 sodium silicate.
On the other hand, it has been stated that the presence of
oleate could possibly cause desorption of silicates due to
high chemical interaction between the cations and the
oleate ions.
Results of Infra-red Spectroscopy
The spectra of hematite, hematite conditioned with
oleate magnetite at pH 7.2 and the spectra of oleate mag­
netite are shown in the Figure 7. Hematite is character­
ised by the presence of peaks around 4 1 0-4 1 5 cm-I, 450470 cm-I and 670-680 cm- I . The absorption bands were
detected at 1 559 cm-I , and 1 525 cm-I due to the -C-C and
-CH -CH-C- indicates the formation of ferric oleate (Fig­
2
ure 7 III). Similar peaks detected at 1 590 cm l and addi­
tional peak at 1 034 cm-I recorded in the case of hematite
interacted with oleate magnetite prove the chemical in­
teraction of oleate on mineral surface. The peak observed
at 1 034 cm-I has been attributed due to -C-O-C cross­
linking between the hydrocarbon chains. IR study car­
ried out earlier proved that ferrous oleate and aluminum
oleate had the characteristic peak at 1 595 cml and 1 6 1 5
cm l, respectively l5. Figure 8 shows the IR spectra of
bauxite and bauxite coated with oleate magnetite. Gibbsite
the main mineral for bauxite is charatersied by the stretch­
ing vibration due to OH group at 36 1 5 cm-I, 3460 c m - I
and O H bending vibration at 1 005- 1 035 cm-I and 970990 cm -I (ref. 1 6). The IR spectra of bauxite did not alter
when bauxite sample was conditioned with oleate mag­
netite. There is no chemical interaction between bauxite
and oleate species. So the adsorption, if any, may be due
to physical adsorption . The mineral conditioned with
oleate magnetite was washed several times with distilled
water and its IR spectra was recorded again. The IR spec­
tra was unchanged and no new spectra appeared.
Result of Magnetic Separation Studies
The separation of hematite from quartz was performed
by wet magnetic separation methods by applying the coat­
ing techniques. Both oleate magnetite prepared syntheti­
cally and finely divided natural magnetite (-5/lm) were
used as the coating agents. The best results obtained un­
der the optimum conditions are summarized in Table 1
and 2. A synthetic mixture of hematite to quartz ( I : I )
was used. The experiments were conducted at natural pH
at which zeta-potential of hematite is also found to be
zero and the maximum adsorption of oleate coating takes
J SCI IND RES VOL.58 JUNE 1 999
440
Table I - Chemical analysis of hematite and bauxite
per cent
Si02
Al p]
CaO
Fep3
MgO
Material
0.92
1 .85
97.02
1.14
Hematite
Bauxite
0,06
0.05
1 .20
64.85
LOI
�
0.35
32.0
0.03
0.03
Table 2 - Results of magnetic separation of hematite and quartz mixture
using colloidal magnetite ( H : Q I : I , pH:7.2, S HMP 0.0 1 per cent)
Weight.
per cent
Mag intensity,
k gauss
Details
Magnetic I
Grade, per cent
Hematite
Quartz
3.0
35. 1
95.3
Recovery, per cent
Hematite
Quartz
4.7
66.8
3.3
5.0
47.5
94.2
6.0
89.3
5.5
Magnetic JII
6.5
7.8
93.5
92.6
6.9
Magnetic IV
5 1 .6
53.3
7.7
96.3
98.6
46.7
1 .5
98.5
1 .4
6.9
8.0
92.0
Magnetic
II
Non-magnetic
..
20
..
12
c:
:! o
a.
2
to
N
_
-801 �------________________�
pH
Figure 6 - Zeta potential of hematite as a fuhction of pH and sodium
oleate concentration
place at this pH than at the other pH values17• The results
indicate that a Fe2 03 grade of 9 1 per cent with more than
97 per cent recovery was obtained by using a magnetic
intensity of around 7.8 k gauss. Generally hematite is
feebly magnetic in nature and a magnetic intensity of 1 31 4 k gauss is required for its effective separation. Since
the intensity required is much less than the hematite sepa­
ration by a magnetic separation it can be concluded that
magnetite response of hematite has been increased by
the addition of a small proportion of colloidal magnetite.
The addition of sodium oleate as a surfactant with col­
loidal magnetite further helped for selective attachment
with hematite surface.
The magnetic separation with natural magnetite also
suggested that a Fe 03 recovery of 90.3 per cent was
2
to
u
c
o
�
E
..
c
�
t-
4IDl
lSOO
)000
2500
2000
Wave-
1500
'
1000
sao
'number ( em-I I
Figure 7 - FTIR spectra of: (I) Hematite, (II) Hematite coated with
oleate magnetite, and (JII) Oleate magnetite
PRAKASH
et al.
: SURFACE PROPERTIES OF INDIAN HEMATITE & BAUXITE
44 1
Table 3 - Results of magnetic separation of hematite-bauxite mixture
(H:B 1 : 1 , pH:7.2, SHMP O.OI per cent)
Magnetic Intensity,
k gauss
Weight,
per cent
Magnetic I
3.0
1 2.2
87.2
Magnetic II
5.0
44.4
92.8
Magnetic III
Magnetic IV
6.5
7.8
54. 1
92.8
55.4
90.7
9.3
99. 1
1 0.5
44.6
1 .0
99.0
0.9
89.5
Details
Non-magnetic
Product per cent
Alp]
Fep3
1 2.8
Recovery, per cent
Fep]
AIP3
2 1 .0
3.2
7.2
8 1 .3
6.5
7.8
" 98.5
8.5
in Table 3. A 99 per cent AIP with 90 per cent recovery
3
has bean reported in the non-magnetic fraction. In the
absence of SHMP, it has been found that some bauxite
was reported in magnetic fraction .
•
"
c:
"
Conclusions
II!
E
..
c:
l!
t-
-J!.
3000
2000 "
Waye number
I c m- ' )
1000
Figure 8 - FTIR spectra of: (I) Bauxite and (II) Bauxite coated with
oleate magnetite
Separation of quartz from hematite or hematite from
bauxite can be obtained by using a small fraction of col­
loidal magnetite in a low to medium intensity of mag­
netic separator. The selective coating and the correspond­
ing separation mostly depends on the surface properties
of the minerals. Both phosphate and silicate ions play a
role in the zetapotential of the minerals. It has also been
concluded that colloidal magnetite is chemically bonded
with hematite, whereas it is physically adsorbed with the
bauxite.
Acknowledgement
,
achieved assaying around 98 per cent Fe20 by using
3
around 6 kg/tonne of magnetite'8. The quantity required
is very high compared to the oleate magnetite. However,
it has been established that ground magnetite can be used
as the coating agents in the system like other minerals8 •9•
The authors are thankful to Prof. H S Ray, Director,
Regional Research Laboratory, Bhubaneswar for his keen
interest in the study and granting permission to publish
this paper.
For the separation of bauxite from hematite which
is a very potential impurity in the ore body, a mixture of
hematite and bauxite was fed into the magnetic separator
using the colloidal oleate magnetite. In this case, sodium
hexa metaphosphate and sodium silicate ware used in
order to change the surface properties of bauxite. The
sign of zeta-potential becomes negative. Therefore the
probability of interaction between oleate magnetite with
bauxite decreases. As a result, hematite is separated eas­
ily from bauxite which has got a chemical interaction
with oleate. However, SHMP gave far better results than
sodium silicate. The results of the study using 0.0 1 2 g/kg
of oleate magnetite and 0.01 per cent SHMP are presented
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