Bentonite
is naturally occurring plastic clay of great commercial importance
and it falls mainly under montmorillonite group (OH4
Si8 Al4 O20 nH2O).
It is characterized by possession of exchangeable ions. Two types
of bentonite are generally identified. One is called swelling
or sodium bentonite and the other non-swelling or calcium bentonite
depending upon whether the exchangeable iron is constituted by
sodium or calcium.
Bentonite
is mainly user as a binder in medium and high-grade casting,
in foundry industry, in palletizing iron ore etc., as a component
of cement and concrete mix in construction of dams, canals,
reservoirs, as a component of drilling mud in water well and
oil well drilling. It is also used in paper, ceramic, lubricant,
soap and detergent, pharmaceutical, cosmetic, insecticide, etc.
Besides, it is used for purification, clarification and decoloration
of oil, sewage, industrial waste liquors, food and beverage
etc. Its suitability as an additive to soil for preservation
of fertility has also been established.
The
BIS (IS: 6186 – 1986) has prescribed specifications for bentonite
as given below:
|
S. No. |
Characteristics |
Requirement for
Type - 1 Type
- 2
High
offshore |
|
i |
Moisture,
percent by mass |
5.0
- 12.0 |
12.00
max |
12.00
max |
|
ii |
pH |
9.0
- 10.5 |
-- |
-- |
|
iii |
Gel
formation index |
to
pass test |
to
pass test |
to
pass test |
|
iv |
Swelling
power |
to
pass test |
to
pass test |
to
pass test |
|
v |
Fineness:
DRY
a.
To pass through 150 micron IS sieve percent by mass. min.
To
pass through 75 micron IS sieve percent by mass. min.
b.
Wet:
Retained
on 150 micron IS sieve, percent by mass. max.
To
pass through 45 micron IS sieve, percent by mass. min.
|
--
95
0.04
90 |
98
90
--
98 |
98
--
--
-- |
|
vi |
Viscosity
at 30o C, Centipoise, min.
a.
Apparent
b.
Plastic |
--
-- |
15
6 |
--
-- |
|
vii |
Filtration
loss, ml, max. |
-- |
15 |
15 |
|
viii |
Sand
content, percent by mass. max. |
-- |
2 |
2 |
Type – I Suitable for chemical industries including decolorizing
of petroleum and vegetable oil and rubber industries with some
additional requirements.
Type
– II Suitable for oil well drilling. Type II shall have additional
requirement for Bentonite for rubber industry.
| S. No. |
Characteristic |
Requirement |
| i. |
Loss
on ignition (other than loss on drying) percent by mass.
max. |
6.0 |
| ii. |
Matter
soluble in water, percent by mass max. |
4.0 |
| iii. |
Copper
(as CuO), percent by mass max.
|
0.01 |
| iv. |
Manganese
(as MnO) percent by mass max. |
0.01 |
However,
in special practice the concerned industries are not strictly
adhering to the specifications standardized by the BIS. The
specifications that are in vogue in the industry are discussed
below:
(A)Binder:
For
the bentonite, which is used as a binder, the distinguishing parameters
are swelling capacity, gelling index, gel time, Base Exchange
capacity, liquid limit and green compressive strength. It has
been observed that bentonite used for medium-grade casting and
for palletizing of iron ore is required to have similar properties,
similarly the bentonite used in foundry industry and in civil
construction works (water well, drilling and dam, canals and reservoirs,
etc.) should have similar properties. The bentonite required for
high-grade casting must be better in quality than the bentonite
used in other industries as a binder. The distinguishing particular
of specifications of bentonite for use as a binder for different
purpose are given below:
| S. No. |
Particulars |
Medium-grade
Casting & palletizing |
Foundry industry
water well drilling & dam canals & reservoirs |
High-grade casting
|
| 1.
|
Swelling
capacity
(2gm.
In 100ml D.W) |
12
- 14 |
16
- 20 |
25
- 45 |
| 2. |
Gelling
index (MgO test) |
20
- 25 |
35
- 45 |
65
- 72 |
| 3. |
Gel
time (minutes) |
7
– 10 |
5
– 7 |
Instant |
| 4. |
Base
exchange capacity Mgg./100 g |
60
– 65 |
60
– 70 |
75
– 100 |
| 5. |
Liquid
limit (%) |
250
– 275 |
285
– 315 |
335
– 600 |
| 6. |
Green
compressive strength PSI (Bent/H2O: 7/3) |
7
– 8 |
8
– 9.5 |
9.5
- 12 |
| 7. |
Type
of bentonite |
Sodium
bentonite |
Sodium
bentonite |
Sodium
bentonite |
Oil
Well Drilling: -
The
principal consumer of bentonite of the grade in India is the
ONGC, while some quantities are exported also. For indigenous
use, ONGC has its own specification whereas the others use the
ones prescribed by API. The two standards (i. e. ONGC
and API) are more of less, the same expect slight the same except
two parameters, i.e. yield point and moisture. The two
specifications are given below:
| S.
No. |
Particulars |
Indigenous
use (ONGC) |
Export
(API) |
|
1. |
Apparent
viscosity |
15.0
Cps min. |
15.0
Cps min. |
|
2. |
Plastic
viscosity |
8.00
Cps min. |
8.00
Cps min. |
|
3. |
Yield
point |
4
x Pv maximum |
4
x Pv maximum |
|
4. |
Filtrate
|
15.0
mls. max. |
15.0
mls. max. |
|
5. |
Wet
Screen Analysis on U. S. Sieve No. 200 |
2.5%
maximum |
2.5%
maximum |
|
6. |
Moisture
as shipped from Point of manufacture |
12.0%
maximum |
10%
maximum |
(C) Rubber:
The distinguishing particulars of the specification
followed by the user industries are as given below:
|
1. |
Grit |
Should
be free |
|
2. |
Size |
Should
pass through 200 mesh sieve |
|
3. |
Moisture |
45%
maximum |
|
4. |
L.
O. I. |
8.5%
maximum |
So
far as the other uses of bentonite, i.e., in paper ceramic,
lubricant, pharmaceuticals, etc. are concerned; there is no cut and dry specification agreed upon between
suppliers and users.
Data
on detailed physical tests are lacking in most cases and even
where available, they have not been linked up with blocks of
reserves. Hence in the inventory as on 1-1-85 the dispatch data
from the mines to different industries were chiefly relied upon
for classification of reserves. In other words if the ROM from
a mine is dispatched to foundry industry, then reserves of that
mine and of adjacent freehold deposits have been considered
to be foundry grade and so on. On this basis, the reserves in
the inventory as on 1.1.85 have been classified intro the following
grades.
| (i) |
Foundry |
| (ii) |
Oil well drilling |
| (iii) |
Poor bendable |
|
(iv) |
Unclassified
(decolorizing agent, Cosmetics, medicine, refining agent
for vegetable oils and fats, paper, rubber, etc.) |
After
examining the specifications of standardized by BIS and the current
trends in the user industries and also keeping in view the distinguishing
characteristics of the bentonite being used in different industries,
the group recommends the following end use classification of reserves.
A. Binder:
| S. No. |
Particulars |
Medium-grade
Casting & palletizing
|
Foundry industry
water well drilling & dam canals & reservoirs |
High-grade casting
|
| 1.
|
Swelling
capacity
(2gm.
In 100ml D.W) |
12
- 14 |
16
- 20 |
25
- 45 |
| 2. |
Gelling
index (MgO test) |
20
- 25 |
35
- 45 |
65
- 72 |
| 3. |
Gel
time (minutes) |
7
– 10 |
5
– 7 |
Instant |
| 4. |
Base
exchange capacity Mgg./100 g |
60
– 65 |
60
– 70 |
75
– 100 |
| 5. |
Liquid
limit (%) |
250
– 275 |
285
– 315 |
335
– 600 |
| 6. |
Green
compressive strength PSI (Bent/H2O: 7/3) |
7
– 8 |
8
– 9.5 |
9.5
- 12 |
B.Oil Well Drilling:
| S.
No. |
Particulars |
Indigenous
use (ONGC) |
| 1. |
Apparent
viscosity |
15.0
Cps min. |
| 2. |
Plastic
viscosity |
8.00
Cps min. |
| 3. |
Yield
point |
4
x Pv maximum |
| 4. |
Filtration
|
15.0
mls. max. |
| 5. |
Wet
screen analysis on U. S. sieve No. 200 |
2.5%
maximum |
| 6. |
Moisture
as shipped from point of manufacture |
12.0%
maximum |
