In this application, bentonite is used for decolourizing
purpose. Both swelling and non-swelling varieties are used for this. Non-swelling bentonite is
required to be activated. By virtue of its base-exchanging property and ability to swell, it
absorbs colouring matter. So, its base exchanging power as well as swelling index should be
high. It should be fine-sized, because greater the fineness, larger is the available surface area,
and consequently, more will be the efficiency of decolourization. But at the same time, it
should not be highly colloidal and should settle down after doing its job. For this matter,
content of alumina should be within some optimum limits and that of other water-soluble
salts which may be present in the combined water, should be as low as possible.
The industry generally specifies 200 mesh (approximately 50 micron) size, SiO2/Al2O3
ratio between 3.5 and 4.5, base-exchanging power 70 milli-equivalents per 100 gm, and
swelling index above 8.
