Chemical Activation of a Kaolinite under Acid and Alkaline Conditions

Four metakaolins were prepared by calcination of a natural Spanish kaolin (Navalacruz deposit, Zamora province) at 600, 700, 800, and 900 °C. These four metakaolins, more reactive than the parent kaolin, were submitted to acid and alkaline activations. Acid activation was carried out with 6 M HCl at room temperature and at 90 °C under reflux conditions, with times of treatment of 6 and 24 h. Treatment at room temperature did not produce any alteration in the structure or in the properties of the metakaolins. Treatment under reflux conditions for 6 h led to the removal of most of the octahedral Al 3+ cations, and the formation of an amorphous silica phase, with surface area up to 219 m 2 /g, because of a high development of both the internal and the external surface of the solids. Acid treatment under stronger conditions (reflux, 24 h) also removed the octahedral cations, but provoked an amorphization of the silica obtained ( S ≈ 23 m 2 /g). The metakaolin prepared by calcination at 900 °C showed a lower reactivity than those synthesized at the other temperatures considered. The alkaline treatments were carried out with 1 M and 5 M KOH solutions, the treatments with the concentrated solution, both at room temperature and, especially, under reflux, led to the dissolution of the metakaolins and to the formation of well-crystallized K−F zeolite. The solids obtained by acid treatments are promising as adsorbents and catalyst supports, while treatment in alkaline conditions may be a good method for preparing K−F zeolite.