Indian Journal of Chemical Technology
Vol. II , July 2004, pp 512-517
Destabilization characteristics of talc in distilled and fresh waters
A Ozkan
Selcuk University, Department of Mining Engineering, 42031 Konya, Turkey
Received 19 August 2003; revised received 19 February 2004; accepted 8 April 2004
The destabilization characteristics of talc were investigated in distilled and fresh water containing Ca 2+, Mg2+, Na' and K'
ions. The stability of the talc suspension remained almost constant in distilled water at all pH values, while it sharply decreased
with these cations (i.e. in fresh water) at pH values above 10.5. In addition, the effects of A-95 (anionic), C-521 (cationic) and N100 (non-ionic) tlocculants on the destabilization of the suspension were also investigated in distilled and fresh waters. In fresh
water, anionic and non-ionic tlocculants were very effective on the talc suspensions at pH 7.2 and at pH values greater than 10.5.
The stability of talc suspension was decreased by tlocculation and also coagulation effect at pH levels greater than 10.5. The
cationic tlocculant caused weak destabilization of the suspension with distilled water in acidic conditions. However, this polymer
was ineffective on the talc suspensions in fresh water at almost all pH levels.
IPC Code: C08J 3/00, CO I F 5/00
Keywords: Talc, destabilization, stability, cations, polymeric tlocculants, magnesium silicate
Talc is a hydrous magnesium silicate with the theoretical
chemical formula Mg,Si 40 lo (OH) 2. The chemical
composition of talc ores found in nature, however, is
quite variable l . Commercial talc may contain related
sheet silicates such as chlorite and serpentine, and
carbonates such as magnesite, dolomite and calcite2.
When talc mineral is broken, two types of surfaces are
formed, one surface results from the easily broken layers
and the other from the rupture of ionic bonds within
these layers. These surfaces are called as 'faces' and
'edges', respectively. The faces are hydrophobic, while
the edges are hydrophilic . The ratio of hydrophilic and
hydrophobic surface sites affects the electrokinetic
properties and thus flotation, flocculation and other
wetting related processes of the mineral' .
Dispersion control is an important step in the
processing of solid particle suspension. Some unit
operations of particle processing require the fine particles
suspended in liquid being stably dispersed, e.g .
classification and fine particle separation. However,
stable particle aggregation is needed for others, such as
solid-liquid separation. Destabilization of fine particles
can be achieved by neutralizing the electrical charge of
E-mail: [email protected]; Fax: +90-332-2410635
A part of this study was published as a technical note in Minerals
Engineering Journal.
the interacting particles, which is coagulation or by
bridging the particles with polymolecules , called
flocculation 4 . Bridging is considered to be a consequence
of the adsorption of the segments of flocculant
macromolecules onto the surfaces of more than one
particle. Such bridging links the particles into loose
flocs, and incomplete surface coverage ensures that there
is sufficient unoccupied surface available on each
particle for the adsorption during the collisions of chain
segments attached to the particles 5 .
Polymeric flocculants are used very effectively to
destabilize fine particle suspension. The advantage of
polymeric flocculants is their ability to produce large,
stronger flocs comparing to those obtained by
coagulation. Theoretically, the flocculants may be
applied either after destabilizing the suspension by
coagUlation, or without prior destabilization . The
flocculants are known to be not very effective in treating
stable suspensions and so the first option which involves
prior destabilization by coagulation is always betterh. x.
Most flocculants are synthetic polymers based on
repeating units of acrylamide and its derivatives, which
may contain either cationic or anionic charge. Both
anionic and non-ionic polymers can be manufactured
with very high molecular weights, and thus are capable
of forming large, rapid-settling, good-compacting flocs
OZKAN : DESTABILIZATION CHARACfERISTICS OF TALC
SI3
Table 1- The chemical composition of the talc sample used
MgO
63 .82
29.28
0.59
0.56
0.55
and they are widely used in mining and processing
applications. Cationic polymers have lower molecular
weight and rarely used in the mining areaX,9 , Since most
suspended particles are negatively charged, cationic
polymers seems a common choice, But, the charge on a
bridging polymer appears to be of secondary importance,
High molecular weight polymers are extremely effective
in promoting floc growth in suspensions. Elimination
of charge barriers by pH control or the addition of a low
molecular weight polyelectrolyte permits small flocs to
form and the bridging action of the polymer serves to
link these into substantially larger units 7 . 1O .
In industrial water, calcium and magnesium ions are
inevitably present. Ca2+ and Mg2+ ions are released by
dissolution of carbonate and silicate minerals and they
are present in their highest amount in the suspension at
neutral pH range ll . While Ca2+ and Mg2+ ions are not
very effective in the neutral pH range, the less hydrated
Ca(OHt and Mg(OH)2 precipitate formed at high pH
values are responsible for the coagulation of silicate
suspensions 11.12. In addition, Ca 2+ and Mg2+ ions are
reported to enhance frother adsorption on talc surfaces,
increasing floatability, so that it is noticed that the
highest natural floatability is obtained in the neutral pH
range. The talc flotation recovery, on the other hand,
sharply decreases in the alkaline region. This behaviour
is attributed to the presence of carbonate ions released
from carbonate minerals in high amounts at such pH
values 13 • Since talc mineral has many uses as powdered
form in the manufacture of paint, paper, roofing material,
rubber as well as in cosmetics in the form of face and
talcum powder and electrical appliances, etc ., it is
important to beneficiate the talc from most ores
involving all steps of mineral processing.
The objective of this work was to determine the
destabilization properties of talc mineral in distilled
water, and fresh water containing Ca2+, Mg2+, Na+ and
K+ ions. Therefore, this paper presents the findings of
destabilization characteristics of talc and is expected to
be consistent with the industrial applications .
0. 14
CaO
Loss of ignition
0.06
5.00
Experimental Procedure
Materials
Pure talc mineral obtained from Sivas, Turkey, was
used in the experimental study . The chemical
composition of talc sample is given in Table I . The
sample was dry ground in a laboratory size ceramic ball
mill of 128 mm internal diameter and 2S00 mL vo.1ume.
The sample ground in the mill was taken out at certain
grinding times and sieved to obtain -38 Ilm size fraction.
Thus, the sample was not exposed to over-grinding and
contamination of product due to abrasion of the balls
was kept at a minimum level. Anionic (A-9S) , cationic
(C-S21) and non-ionic (N- 100) Superfloc fiocculants,
obtained from the American Cyanamid Company, were
used for the flocculation of talc. These synthetic
polymers are based on acrylamide and its derivatives.
A-9S is basically a polymer of acrylamide-acrylic acid;
C-S21 is a polymer of acrylamide-quaternised
aminoalkylacrylate and N-I 00 is polyacrylamide. The
molecular weights of A-9S and N-I 00 polymers are in
the range of S-ISXI06 and that of C-S21 is 2-6XI06.
The polymers prepared as 0.01 % solutions were used
in this study. Sodium hydroxide and sulphuric acid
(Merck Company), were used for pH adjustment. In the
experiments, mono distilled and fresh waters were
employed. Natural pH values of distilled and fresh
waters were 6 .0 and 7 .2 , respectively. Specific
conductance values of distilled and fresh waters were
determined as 7 and 203 IlS/cm, respectively. The
concentrations of dissolved cationic and anionic species
in fresh and distilled waters are given in Table 2.
Method for destabilization studies
The destabilization experiments were carried out
in a 600 mL cylindrical cell with four baffles using O.S
g solid and SOO mL water. The dispersed suspension,
adjusted to the desired pH, was first conditioned at SOO
rpm for S min . The flocculant was added to the .
suspension at an impeller speed of SOO rpm. After 3
min, the stirring speed was reduced to 100 rpm for 2
min, to allow floc growth. After a settling time of I
514
INDIAN 1. CHEM. TECHNOL., JULY 2004
Table 2--Concentrations of dissolved cationic and anionic species in fresh and mono distilled waters
Concentrations, mg/L
Na+
Sr2+
Fresh
0.38
Distilled
K+
Ca 2+ Mg2+ CO ) 2-
HCO-)
Cl-
SO/
NH)
N0 2
NO-)
PO/
Fe 2+
F-
Zn 2+
0.002
2.10
0.041
0.006
0.55
0. 13
4.03 0.41 46.10 5.05
water
4.50
105.90
4 .90
9.05
0.009
0. 15 0.02 0.96 0.21
water
0.005
0.22
0.85
0.20
0.01
min, 20 mL of supernatant was taken out, at a fixed
distance of 5 cm below the air-liquid interface, by a
special system for turbidity measurements. The turbidity
of the supernatant was measured by an Orbeco-Hellige
Model 966 turbidimeter. The destabilization
experiments were carried out at 20± I DC . The
experimental procedure of the destabilization tests is
shown in Fig. I.
The performance of the destabilization process was
assessed using a formula [Eq. (1)], defined previously 14,
and modified to allow for the measurement of
supernatant turbidity;
suspension stability
= Td / ~) X
100
0.01
Sample
I
+
Flocculant
pH conditioning: 5 min
~ Impeller speed : 500 rpm
c:;:.c:,
~
I
I
Flocculant conditioning: 3 min
+/" Impeller speed: 500 rpm
Floc growth: 2 min
~Impellerspeed: 100rpm
~ / " Settling time: I min
n
l=J-.
Taken out 20 mL of sup em at ant
for turbidity measurement
... (1 )
where ~) is the turbidity (nephelometric turbidity unit)
of well dispersed suspension of talc at I gIL of solid
concentration and Td is the turbidity of supernatant when
sedimentation is assisted by a f10cculant or pH variation.
~) value was determined as 480 NTU.
Fig. I-Diagram of destabilization of talc suspension.
400 , - - - - -_ _ _ _ _ _ _ _ _ _---,
::l
f-
Z
300
i-
Results and Discussion
Figure 2 represents the variation of supernatant
turbidity with settling time to evaluate the natural
stability of talc suspension in fresh water whose pH is
7.2. It is seen that supernatant turbidity value decreased
rapidly for the first 10 min of settling time, thereafter it
continued to decrease steadily reaching 32 NTU in 60
min. This turbidity value corresponded to approximately
7% of suspension stability.
The stability of talc suspension as a function of
pH in distilled and fresh waters is shown in Fig. 3. As
seen from Fig. 3, the stability of talc suspension
increased slowly with increasing pH values in distilled
water, however, it decreased sharply between a pH of
1.6 and 1.9. The stability of talc suspension in fresh
water (i.e. with cations) increased with increasing pH
between 7.2 and 10.5, and it reached a maximum value
at pH 10.5. After this point, the stability of the
'i3
:e3
200
§
til
E
";:s0..
100
(/)
o
10
20
30
40
50
60
Settling time (min)
Fig. 2-Variation of supernatant turbidity with settling time in
fresh water.
suspension decreased sharply to 32% at a pH of 12.2.
Particularly, the coagulation in the presence ofCa2+and
Mg2+ ions becomes significantI1.l2.15.16 at pH values
beyond 10. The species distribution diagrams 17 for
calcium and magnesium ions are presented in Fig. 4.
These diagrams seem to explain the results obtained for
these ions in the destabilization experiments of talc. In
515
OZKAN: DESTABILIZATION CHARACTERISTICS OF TALC
100..-------------:-----,
"$-
10·) ' . - - - - - - . : : - - - - - - - - - - - ,
ee"
80
~
Ci(OH>:l<s)
:c
s 60
10·6.'---,----,,---.':-I..---.J
U'l
10
'o"
'in
40
o
2
6
4
8
10
12
14
pll
Fig. 3-Effect of pH on the suspension stability of talc in distilled
and fresh waters.
70..-------------------------------,
60
~
50
~
40
c
.9 30
~
~
11
14
pH
00
~
Fig. 4-Species distribution diagrams for (a) 10-3 M Cal< and (b)
10 4 M Mg2+.
- :.' - Distilled water
- - Fresh water
~
12
pH
c
8.
~
II
- - A-95
20
CIl
~
N-IOO
10
0.0
0.2
0.4
0.6
0.8
1.0
Flocculant concentration, mg/L
Fig. 5--Effect of the concentration of A-95 and N-I 00 polymers
on the suspension stability of talc in fresh water.
the high pH range, Ca2+ ion is specifically adsorbed on
the negatively charged sites on the mineral. This reduces
the negative surface charge and leads to coagulation of
suspension. Mostly, suspended particles, including
talc)3·,x.'9, are negatively charged 7 and in general the
particle surface charge becomes more negative with
increasing pH value 9. Both calcium and magnesium ions
exist as free ions at neutral pH range, but as the pH
increases, the concentrations of these species decrease
and hydrolysis products of these cations become
significant. This firstly results in the formation of the
monohydroxyl complex Ca(OH)+ in the suspension. As
the pH increases, the concentration of this less hydrated
Ca(OH)+ increases rapidly (Fig. 4) . The less hydrated
Ca(OH)+ specifically adsorbs on silicate surfaces and
repulsive hydration forces are eliminated 20. In addition,
Mg2+ ion, attached by heterocoagulation as Mg(OH)2
precipitate (say above pH 10), were responsible for the
coagulation of silicate suspension 11 .20. In fact, a little
adsorption of Ca2+ occurs throughout the pH range20 of
4 to 10. In acidic suspensions, stability of talc suspension
in the presence of cations increases slowly with
decreasing pH, but below pH of 2.1, the stability
decreased slightly.
The effect of A-95 and N-IOO polymers as a
function of concentration on the suspension stability of
talc in fresh water at pH 7.2 is shown in Fig. 5. It can be
seen that both A-95 and N-IOO had similar effect on
flocculation of talc suspension at pH 7.2. The suspension
stability decreased sharply with increasing flocculant
concentration and approximately a constant value was
reached beyond 0.1 mg/mL for both the polymers . In
the experiments where the cationic polymer (C-521) was
used, the flocculation of talc did not occur at pH 7.2. In
addition, at higher concentration of C-521 (10-50 mgt
L), talc suspensions were not flocculated and also these
high concentrations caused the stabilization of talc
suspension. These polymers also represented similar
effects on the talc suspensions in distilled water at neutral
pH.
The effect of pH on the suspension stability of talc
by A-95, C-521 and N-100 polymers in distilled water
is shown in Fig. 6. As seen from Fig. 6, N-I 00 polymer
was not very sensitive to the pH change of the
suspension. The suspension stability values increased
with increasing pH in the alkaline region for A-95
polymer. C-521 polymer destabilized weakly the
suspension in acidic region, but it caused the stabilization
in alkaline region. It has been noticed 9 that the pH range
for cationic polymers can be quite variable, but often is
on the acid pH side, non-ionic polymers are not much
affected by pH changes and flocculation at high pH is
difficult with anionic polymers, which need a certain
amount of divalent or trivalent ions available in the
particle system to act as a bridge between the negatively
charged particle surface and the negative charge of the
anionic polymer.
516
INDIAN J. CHEM . TECHNOL., JULY 2004
100
100
80
~
~
&
:.0
60
~
c:
.~
c:
- '.- No polymer
~'
---- A-95, 0 .1 mg/mL
~
:;;
- - - N-100, 0.1 mg/mL
c:
.~
- ' - C-521, I mg/mL
40
..,
c
Co
\\
- C-.
No polymer
____ 1\ -95, 0.1 rngiL
60
- - - N-IOO, 0.1 mg/ L
40
~
- - C-;2J , I mgi L
'"~
::J
v>
::J
en
80
(/)
20
20
\~
0
0
0
2
4
6
8
10
12
0
14
Fig. 6-Effect of pH on the suspension stability of talc with
A-95, C-521 and N-I 00 polymers in distilled water.
Figure 7 shows the effect of pH on the suspension
stability of talc by A-95, C-521 and N-I 00 polymers in
fresh water containing cations. As shown in Fig. 7, the
lowest suspension stabilities of talc were obtained at
pH 7.2 and above pH 10.5 for A-95 and N-IOOpolymers.
When the pH was higher than 10.5, the destabilization
of talc was assisted by the coagulation phenomenon,
that is enhanced destabilization was caused by both the
flocculant and the cationic promotion due to the presence
of Ca2+ and Mg2+ ions. Hence, it can be said that the
charge on the talc surfaces decreased and thus, the
destabilization was supported by the coagulation effect
due to decreasing repulsive forces between the particles.
Except for pH values greater than 9, talc suspensions
were not destabilized by C-521 polymer at almost all
pH levels. The suspension stability values with C-521
above pH 10.5 were higher than the suspension stability
values obtained with cations only. This was attributed
to a decrease in the negative charge on the talc surfaces
when cations are present in the Ca(OHt and Mg(OH)2
form, and eventually charge may be reversed from
negative to positive. This has been found to be valid for
many mineral systems, including talcl()-1 2. IX. Thus, it may
be said that the stabilization occurred partially due to
repulsive forces between the positively charged talc
particles and the cationic polymer_
Conclusions
Industrial water always contains cations and their
effects should be taken into account in mineral
suspension systems. While the talc suspension was not
destabilized in distilled water by the pH changes, it was
strongly destabilized by coagulation mechanism due to
hydrolyzed forms of cations in fresh water at pH values
2
4
6
10
12
14
pH
pH
Fig. 7-Effect of pH on the suspension stability of talc with
A-95, C-521 and N-IOO polymers in fresh water.
above 10.5. Calcium ion, adsorbed as Ca(OH)+' and
magnesium ion, attached by heterocoagulation as
Mg(OH)2 precipitate, were responsible for the
destabilization of talc suspension in fresh water.
Therefore, talc suspensions can be destabilized by use
of simple additive such as lime in industrial applications.
A-95 (anionic) and N-100 (non-ionic) polymers were
more effective on the talc suspensions than C-52l
(cationic) polymer in both distilled and fresh waters. In
addition, the effects of A-95 and N-l 00 polymers varied
depending on pH of the suspension in distilled and fresh
waters. Particularly, the destabilization of talc
suspension with these polymers was improved by
coagulation effect in fresh water at pH values greater
than 10.5. That is, these polymers were greatly effective
in promoting floc growth in previously destabilized talc
suspension by cations in fresh water.
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II
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