Bentonite and Fuller's Earth
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and FULLER'S EARTH are the two important naturally occurring clays of
great commercial importance possessing inherent bleaching properties.
They are, therefore commonly called bleaching clays. They fall mainly
under montmorillonite group containing a varying amount of
attapulgite. The montmorillonite is designated by the chemical formula
(OH4).Si8.O20.nH2O. The clay minerals of montmorillonite and
attapulgite groups are distinguished by their adsoption
characteristics. The chemical composition of these clays is similar to
that of other common clays and it is rarely that these clays can be
identified by chemical analysis. It is by X-rays, D.T.A., electron
microscopic as well as by staining techniquished. The natural activity
in the clay is determined by actual trials. The other important
property of the montmorillonite group of minerals is possession of
Two types of bentonite are generally identified. One is called the
swelling type or sodium bentonite, which has single water layer
particles containing Na+ as the exchangeable ion. The other has double
water layer particles with Ca++ as the exchangeable ion. It is called
calcium bentonite or non-swelling type. Na+ or Ca++ is exchanged by
Mg++ or Fe++. A third type of montmorillonite has been identified with
zero water layer particles and is probably electrostatically neutral.
Calcium bentonite is usually referred to as fuller's earth by a number
of authorities because chemically and also in physical properties it
is identical to calcium-montmorillonite.
In the early years, all naturally occurring activated clays having
good bleaching properties were called fuller's earths. The word
fuller's earth has been named after the practise of fulling or
cleaning the grease and stains from wool and cloth.
The essential difference between bentonite and fuller's earth is in
their modes of occurrence and other physical properties. Bentonite is
regarded to have been formed by the alteration of volcanic ash
deposits, mostly in upper Cretaceous formations. Fuller's earth
represent a shaly facie of Tertiary rock.
Bentonites having low iron content, have been found to be good
catalytic agents in petroleum refining. The bentonites having Ca and /
or Mg as exchangeable ions are good decolourizers. Bentonites can
absorb water to a greater extent than ordinary plastic clays. Fuller's
earth, on the other hand, is non-plastic or semi-plastic in character.
It has a foliated structure. Dry or dehydrated fuller's earth adheres
strongly to the tongue. The absorption of water in sodium bentonite
proceeds with a considerable increase in volume (as much as 14 times
its original volume) creating an excellent gel and viscous material
which is invaluable for the preparation of drilling muds and in
grouting of dams, wells etc. Sodium bentonite has an excellent
thixotropic property, i.e. the gel becoming stiff on standing and
reverting to fluidity when shaken. The swelling type bentonite when
dispersed in water, separates into suspendible flakes which are all
finer than 0.5 micron. Calcium bentonite yields about 35% finer than
0.5 micron. Calcium bentonite yields about 35% finer than 0.5 microns.
The difference in bentonite and other clays lies in lattice structure.
The sheet of atoms in bentonite are much thinner and more easily
separable in water. That is why bentonite occupies more surface area
than other clays. This property is known as dispersibility, which is
unique to swelling type of bentonite.
Use as grouting material :
Bentonite has great water binding ability and consequently very low
permeability to water. It has been found, that the prmeability of the
soil is reduced considerably when substituted by sodium bentonite.
Hence, this material is often employed in construction engineering ot
make a porous medium water-tight. It can be used alone or with some
other grouting material.
Use in drilling muds :
Drilling muds consist of water to which sodium bentonite and
pulverized barytes are added. Such muds are prepared mainly for deep
drilling, like oil-well drilling. Bentonite imparts two properties :
gives the fluid a viscosity several times that of water and
seals the wall of the holes, thus preventing water loss.
The quantity of bentonite used is variable depending upon the depth of
the hole to be drilled. Generally one tonne of bentonite is used to
prepare about 100 barrels of mud.
Use as decolourizer :
Decolourizing bentonites are those which carry Ca and / or Mg as an
exchangeable ion. They are used in the decolourization of animal and
vegetable facts (like ground-nut, castor-oil and Vanaspati) and
petroleum oil, lubricants, paraffins and other waxes. These are
decolourized in two ways:
as foundry sands :
the percolation method.
the contact method.
Bentonite is utilized in foundry to bind the sand grains into desired
shapes. Bentonite helps in retaining the mechanical shape of the mould
by making the particles of sands adhere and also making the surface
impermeable. Strength and fusion point are the two important
properties desired for selecting bentonite. Generally, the swelling
type of bentonite is used though other types of bentonites have also
Use in cosmetic and pharmaceutical preparations :
Bentonite gels are used as a carrier for a number of cosmetic
preparatios, tooth-pastes, creams for skin and other similar products.
For the preparation of cosmetic creams, bentonite is generally used as
a paste formed with water and glycerine. Bentonite when intimately
mixed with water in the proportion of one to four gives a pasty mass
with the consistency of a heavy grease and in this form it is used for
the preparation of medicinal ointments. Bentonite in the natural state
is non-poisonous and harmless; thus it finds use in tooth-paste and
even in the preparation of lipstick.
The swelling type of bentonite is finding increasing use in the
manufacture of insecticides and paints. The latest use has been its
development as a bonding agent in pelletizing iron ore fines in the
USA. The taconite agglomeration plant in north-eastern Minnesota
consumes a considerable quantity of bentonite.
Tests and specifications
Decolourizing power of a clay is determined by:
both the processes the bleaching property of a clay is tested against
the volume and colour of the filtrate obtained through a clay of known
In the percolation method a comparison is made between the
decolourizing property of the clay under test and a standard clay
whose decolourizing power is known. For this purpose, standard
ground-nut oil is taken and is allowed to pass through a column
containing granular clay under examination. The decolourized oil is
collected and its volume is measured. The procedure is repeated using
a column of standard clay and the decolourized oil is collected. By a
comparison of the yield in both cases, the efficiency of
decolourization of the clay under test can be determined with respect
to the standard clay.
In the contact process, the decolourizing power of the clay is
determined by comparing the colour produced by mixing the clay with a
standard oil stock, agitating the mixture at a given temperature and
filtering the clay from the oil. The amount of a standard clay
required to produce the same colour being known, the decolourizing
power of the clay being treated can be expressed as a percentage of
Weigh accurately about 200 gm. of the alkali refined ground oil in a
round-bottom flask. Immerse the flask in a water-bath maintained at 90º
to 95ºC, slowly bubble carbon dioxide gas through the oil. When
the temperature of the oil has reached 90ºC momentarily remove
the cork and introduce 2.0 gm. of the material.
Start the stirrer and adjust its speed to about 350 rpm. The bottom
end of the stirrer should be adjusted to a level at which intimate
mixing is ensured. Maintain the temperature at 90ºC for 20
minutes and then stop stirring. Remove the flask from the water-bath,
allow the oil to cool in the atmosphere of carbon dioxide and filter
the oil through filter paper Watman No. 1. Determine oil in the glass
cell using a Lovibond tintometer.
In order to obtain a better insight into the performance of the
materials, it is recommended that the procedure mentioned be repeated
with at least three additional quantities of the material, say, 1.0
gm., 3.0 gm., and 4.0 gm., and the values for decolourizing power
plotted against the percentage of material added.
By agreement between the purchaser and the seller, any vegetable oil,
other than alkali-refined groundnut oil of agreed colour intensity,
may be used for this test. In such a case, it may also be necessary to
use a glass cell of some other thickness and also to express the
colour reading in terms of factor other than (Y+5R).
Clay, generally of 200 mesh is used in the contact process and 60 to
80 mesh in the percolation process. A bleaching clay is evaluated for
three percolation process. A bleaching clay is evaluated for three
clays should not only possess adequate decolourizing power but also
settle very readily to the bottom of the treater leaving, within an
hour, a clear or nearly supernatant oil or wax amounting to not less
than 85% of the volume treated. The spent earth should not unduly
impede the filtration of the bottom portion of the treater which has a
high concentration of the spent earth and this should be filtered in
the final stage to avoid excessive treatment loss of valuable oil /
wax. Since there is no single oil standard by which efficiency of the
various earths can be gauged, the relative value of competitive clays
must be established on representative commercials oils.
The importance of oil-retention value of the earths to be tested is
second only to decolourization value, since the oil held up or absobed
in the clay after use is not customarily recovered. Therefore, the
loss must be written off to the operation when the clay is discarded.
This particular test is not so important with the percolation type
clay, because revivification is generally practised, but it is of
utmost importance for contacting clays, where oil frequently is not
recovered from spent clays.
Chemically activated clays have a low oil retention value compared to
naturally active clays and because of this a small quantity of
material is required to obtain decolourization. Hence for commercial
use, even the naturally active clays are chemically reactivated to
have suitable bleaching and oil retention properties.
Specifications desired for oil well drilling purpose
||At least 99% should pass through 200 mesh sieve.
||SiO2/Sesquioxide ratio in clay fraction should
not be less than 3.5 and more than 4.5.
||The mineral content in the clay should not be
less than 70% by weight on the oven dry basis. The water soluble
salts, should be less than 3%.
|Base exchange capacity
||Not less than 70 million equivalents per 100 gm.
||The ratio of the weight of water to that of
minimum weight of bentonite to produce a gel must be more than 8.
||The swelling should not be less than 6cc in water
per gram of bentonite.
||The time of settling down of 6 percent suspension
of sodium bentonite in a test-tube of ½" inner diameter
should not be more than one hour.
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