BENTONITE 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 exchangeable ions.
Properties
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.
Industrial Applications
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 :
- It gives the fluid a viscosity several times that of water and thixotrophy.
- It 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:
- By the percolation method.
- By the contact method.
Use as foundry sands :
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 been used.
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:
- Percolation
- Contact method
In 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 bleaching property.
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 the standard.
Procedure
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 properties:
- Bleaching efficiency
- Settling efficiency
- Filtrability
The 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
| Mechanical Analysis | At least 99% should pass through 200 mesh sieve. |
| Chemical composition | SiO2/Sesquioxide ratio in clay fraction should not be less than 3.5 and more than 4.5. |
| Mineral content | 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. of bentonite. |
| Swelling index | The ratio of the weight of water to that of minimum weight of bentonite to produce a gel must be more than 8. |
| Swelling test | The swelling should not be less than 6cc in water per gram of bentonite. |
| Thixotrophy | 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. |