Bentonite
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

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