Bentonite

Bentonite is one of the two naturally occurring minerals of montmorillonite group with decolorizing and absorbing properties, the other minerals of this group being fuller’s earth. On exposure, almost all Bentonite assume a slightly darker shade due to absorption of moisture. Two types of Bentonite viz., swelling and non-swelling types are commonly known in trade. Sodium Bentonite is of swelling type while calcium Bentonite is non-swelling. Bentonite is believed to have been formed by the alteration of volcanic ash deposits, mostly of Upper Cretaceous Age. The commercial importance of Bentonite depends more on its physico-chemical properties such as Base Exchange capacity, thixotropy, settling time, swelling power, pH value, etc. then on chemical composition. Treated Bentonite has been used in palletizing of iron ore fines, in ceramic nuclear fuels and ceramic moderators.

     Sodium and calcium Bentonite are distinctly characterized by high ration of their dominant exchangeable base. Intermediate type Bentonite may have either sodium or calcium base dominant but in low or nearly equal ratios-mixed Bentonite may exhibit low, high or equal ration of sodium and calcium iron but contain a higher percentage of non-clay material and/or other clay minerals. Plasticity and bonding strength of all bentonite and dilatency (swelling capacity) viscosity (resistance to flow), thixotropy (getting strength) of sodium bentonite are all due to colloidal nature of bentonite. Sodium bentonite (high swelling-high gel) are strongly hydroscopic exhibiting a great affinity for water.

     Non-swelling bentonite (sub-bentonite meta-bentonite) are calcium bentonite. Calcium bentonite is more common than sodium bentonite because calcium is higher in the replacement series.

The Kutch variety of bentonite is natural sodium based with very 

high base-exchange capacity, very high plasticity and swelling. This type of bentonite is confined only to Western part of Kutch district in India.

     OIL-WELL DRILLING :- Bentonite is used for oil well drilling operation.  The consumption of Bentonite varies from field to field.  Approximately around 15 MT of Bentonite is required for drilling 1000M or well considering this norm, the estimated requirement of Bentonite is as under for different years.

     DRILLING MUDS :- Bentonite is valuable as a constituent of drilling mud’s owing to its thickening and suspending properties and its ability to act as a seal.  Bentonite used for this purpose is generally of the sodium or swelling type and of lower grade than that used for oil refining.  A number of tests have been formulated by American Petroleum Institute to assist in the selection and evaluation of suitable material.

     IRON & PELLESTISATION :- Steel technology is undergoing rapid changes and more stress is being laid on the utilization of iron pellets and sinters in place of nature ore.  Pellets being harder, withstand transportation without breaking up and these is already good amount of international trade in this commodity.Bentonite is used as binding materials in the preparation peelers

     PELLETIZING ORES, FLUXES, FUELS, etc. :- In the beneficiation of some ores for metallurgical use, it is frequently necessary to pulverize the ore to carry out separations and concentrations.  In some cases such ores can not be used until the fine material has been palletized or agglomerated into larger units.  The fine product can only be used in the blast furnace after it is palletized into units of the order of 1 in or more in dia.  Pellets can be prepared by mixing coke, laconite ore and a small percentage of bentonite (sodium montmorillonite), then moistening, extruding or rolling to form pellets, followed by roasting at 200 to 400 F and firing at 1650 F to 1800 F.  Quicklime may also be used as an ingredient.  The amount of bentonite used is reported to be about 0.5% of the weight of the ore.  It is not known how thoroughly other types of clay have been tried in palletizing this iron ore.

     DETERGENTS :- Bentonite of the swelling type is a constituent of many proprietary detergent sold in Great Britain, the United States and Germany for scouring textiles.One user in Great Britain requires a while or nearly white products which will from a viscous solution with ten times its weight of water.  The product must be of such fineness that 98% will pass a 120 mesh sieve and 90% in 240 mesh sieve whilst the residue must not contain grit or hand particles.  The loss on the drying at 100 C is limited to 15 percent, the pH of 25 percent suspension in distilled water must note be less than 8 or more than 9 when determined by the glass electrode.  The bentonite must also have a packing density of not less than 0.85 and 2 mg. of the mineral must swell to 10ml. when treated with water in accordance with the specification requirement.  Bentonite is also used in certain soaps to the extent of about 25 percent.

     FOUNDRY INDUSTRY :- Bentonite is used in the foundry industry as a moulding send binder in the cast iron, grey iron malleable iron and steel casting. As per the rough consumption norms, around 10MT if Bentonite is required for manufacture of 100MT of gray iron/cast iron and malleable iron whereas in case of alloys and steel castings it is around 20MT per 100MT.

     FOUNDRY USES :- The third largest use for Bentonite is in foundry practice, where the material is employed as a bounding agent and conditioner in molding send used for casting ferrous and non-ferrous metals.  In fact, it has been claimed that its use for this purpose is standard practice in most Canadian and United States foundries.  Bentonite is states to be particularly useful as a bond with the high silica sands used in steel foundries and as an ingredient in core ashes in which it serves to keep the carbonaceous ingredients in suspension

     Bentonite used as a bonding agent may be either the sodium swelling type or a calcium Bentonite, according to requirements.  The selection of a suitable Bentonite is a matter which has received much attention, particularly in the United State.  The cementing or bonding power, the amount of moisture necessary to develop maximum green strength, the high dry-bond strength, the effect on the permeability of the mold, the refractoriness, the life during repeated use are all important considerations.  The American Foundry men’s Association has laid down methods by which these factors shall be determined for a given Bentonite.  Bentonite for incorporation in foundry sands are usually sold in powder from 90-95 percent of which will pass a 200 mesh sieve.

     REFINING OILS & FATS :- Considerable quantities of Bentonite are used in refining petroleum products and vegetable oils and in treatment of fats and greases.  In treating petroleum, both the swelling and non-swelling types are employed, the latter often after activation to remove tarry complexes and dissolved colorings matter, the last named by selective absorption.  The quantity required upon the nature of the products treated and varies from about one lb per barrel of petrol up to 100 lb per barrel of dark, heavy lubricating oil.

     HORTICULTURAL USES :- Bentonite is sometimes used in emulsion sprays for horticultural proposed and as a carrier or dilment for poisons such as nicotine sulphate, pyzetheum, and rotenone colloidal mixtures of bentonite and sulphur are employed under various trade names, as fungicides and animal drips.  Dust spray mixtures as similar composition are largely used in some areas of the United State for scattering by aero plane.

     CERAMICS :- Small quantities of Bentonite are sometimes incorporation in white ware ceramic bodies to increase plasticity and modules of rupture both before and after firing and also as a suspending agent in glaze mixtures.  Colour is of minor importance but a maximum swelling capacity is most desirable.  A qualitative test sometimes applied to Bentonite for these used is to mix the minerals with 5% of Mgo agitate the mixture with 25 times its weight of distilled water and allow it to send for 24 hours.  Under these conditions the best grade Bentonite will form a stiff jelly which will have a pH value of about 11.5.

     OTHER INDUSTRIES :- There are a number of other important areas like bleaching of oil and insecticides which consume Bentonite.  Small quantities are consumed in refractory and ceramics, asbestos products, rubber, paints and pharmaceuticals and civil engineering work.

     OTHER USES :- Miner used for Bentonite include the employment of an alkali Bentonite mixed with glycerin, in some facial beauty creams, in the de-inking of old newsprint as a thixotropic agent to prevent settling of mixed paints, in bleaching crude sulphur and as an addition to Portland cement and pozzolanas, to increase their workability and decreases them permeability by solutions.

     Some of the swelling types of Bentonite have been successfully used for making earthen dams and reservoirs more resistant to percolating water.  The method is to mix the soil with 5-10 percent of dry Bentonite and apply a layer of the mixture, several inches thick to the surface of the walls and bottom of the reservoir.