ACID ACT IVAT ION OF A SPANISH SEPIOL ITE" PHYSICOCHEMICAL CHARACTERIZAT ION, FREE SILICA CONTENT AND SURFACE AREA OF PRODUCTS OBTAINED

and thermal analyses, along with SEM, TEM and nitrogen adsorption isotherms at 77 K. The free silica was digested and determined in all samples. Several samples were seen to have specific surface areas up to 450 m2/g, with a maximum value of 549 m2/g in the sample treated with 1.25 wt % HCI for 48 h. A sudden decrease in specific surface areas was observed when free silica was digested. The physicochemical behaviour of clay minerals is studied because of its relation to their adsorbing and/or catalytic properties. This behaviour is governed by the extent and nature of their external surface which can be modified by suit-able acid and thermal treatments; these treat-ments are called activation treatments if they produce an increase in the specific surface area and/or in the number of acid centres in the clay mineral. Acid and thermal treatments increase the catalytic and adsorbing activity of some clay minerals, and further and stronger treatments decrease this activity (Grim, 1962; Fern~inde