Bentonite


Pure bentonite is a creamy yellowish to pale buff or grayish coloured mineral with
specific gravity 2.0-2.2. Its refractive index is 1.447-1.550. The important criteria determining
its industrial uses are as follows:
1. Chemical composition: The approximate chemical composition of bentonite is: 45-
65% SiO2, 14-25% Al2O3, 3-9% FeO+Fe2O3, 2.0-3.5% MgO, 1-5% CaO, 0.40-
2.51% Na2O, 0.5-1.5% K2O, 0.8-2.0% TiO2, and Na/Ca 4.55-2.50 (Na-bentonite) or
0.16-0.0001 (Calcium bentonite).
2. pH value: Presence of sodium/calcium ensures that bentonite is alkaline, i.e., its pH
value is above 7.
3. Plasticity: Bentonite is highly plastic, and consequently it has high viscosity.
4. Swelling/adsorptive power: Bentonite has marked swelling and adsorptive properties.
The high adsorption is due to the 3-layered structure of montmorillonite. The sodium
bentonite is more swelling than the calcium bentonite. The former may absorb up to
5 times its weight of water, and increase in volume by up to 15 times its dry bulk (or
may even be higher after processing). The swelling/adsorptive power of calcium
bentonite can also be raised to this level by activating it with acid. When mixed with
water, it forms into a viscous and highly plastic gel. This property is measured and
expressed in terms of four parameters, namely: (i) swelling capacity, (ii) swelling
index of gel value, (iii) gelling time, and (iv) gel formation index. Specific testing
procedures for bentonite have been standardized. For testing swelling capacity, 2 gm
of bentonite is slowly poured into 100 ml of distilled water and allowed to remain for
24 hours after which the volume of the gel formed in millilitres is measured. To test
swelling index, the minimum weight of bentonite that will form gel in 10 ml of
distilled water in 24 hours is determined by trial, and the swelling index is the
number obtained after dividing 10 by the weight in gramme. Gelling time is tested by
the time in minutes taken by 2.5 gm of dried bentonite to form gel in 25 ml of
distilled water. For testing gel formation index, 1.4 gm of dried bentonite, 0.2 gm of
MgO and 2.6 gm of alumina are first thoroughly mixed, then 100 ml of distilled
water is added and again thoroughly shaken for 1 hour to ensure complete suspension of the particles. After allowing the suspended particles to settle for 24 hours, the
volume of gel formed is measured which is the gel formation index.

5. Permeability: Because of its surface properties of water-binding, it has very low
permeability. This is so because it adsorbs water and does not allow the water to
penetrate into it.
6. Dispersion: When dispersed in water, bentonite rapidly breaks down into miniscule
particles, even up to 0.1 micron size. The particles of sodium bentonite are smaller
and they remain in suspension practically indefinitely, while those of calcium
bentonite are a little coarser and they settle down after some time. The mechanism of
this breaking down is not clearly known, but it is believed to have some connection
with the silicon, aluminium and magnesium contents of montmorillonite. The
tetravalent Si++++ ion is replaced by the trivalent Al+++ ion which, in its turn, is
replaced by divalent Mg++ ion resulting in weakening of the charge and consequently
the bond.
7. Base/cation exchange capacity: It means the quantity of positively charged ions
(cations) that a clay mineral can accommodate on its negatively charged surface, and
it is expressed as milli-equivalents per 100 gm (equivalent weight is the molecular
weight of an element divided by its valency). Bentonite in general, and sodium
bentonite in particular, possesses excellent base exchanging property. Sodium or
potassium is exchanged readily for calcium or magnesium. The base exchanging
power is further increased because bentonite breaks down readily into small particles
in a liquid, thus making available a very large surface area for adsorption by virtue of
which the exchange of ions takes place. The methylene blue (MB) test measures the
active clay present by determining the cation exchange capacity of a sample of
bentonite. The number of exchangeable ions present are determined by replacing
these ions with methylene blue dye.
8. Viscosity: Viscosity is that property of a liquid which is a measure of its internal
resistance to deform under shear stress, and it is measured by the stress in dynes/cm2
or Pascal (Pa) required to be applied to overcome that resistance and maintain a
velocity of flow of one centimetre per second. This unit of measurement of viscosity
is Poise which is 1 gm.cm.sec or 1 Pascal second (Pa.sec). It is often expressed in
centipoise (cP). It is sometimes specified for bentonite meant for use as a suspension
in water.
9. Toxicity: Bentonite is non-poisonous and harmless.
10. Fusibility: The fusion temperature of bentonite ranges between 13300C and 14300C.