Thermal behaviour of natural and cation-exchanged clinoptilolite from Sardinia (Italy)

The thermal behavior of two clinoptilotites from an epiclastic and a pyroclastic deposit of central-northern Sardinia and of their exchanged forms (Li, Na, K, Cs, Mg, Ca, Sr and ammonium) were investigated by differential thermal analysis and thermogravimetry up to 1000degreesC. Their thermal stability was studied by evaluating the residual crystallinity (expressed as rehydration capacity) after 2 It thermal treatments at 450, 600 and 900degreesC. The water loss at 1000degreesC was linearly related to the radius (r) and the charge (z) of the exchangeable cations by the equations r(2)/z or r(3)/z, which are proportional to the inverse of the charge density over the surface or to the charge density over the volume of the cations. The cation composition plays a crucial role in determining the thermal behavior of clinoptilolite. The presence of cations such as Cs or K, which have low surface or volume charge densities, was found to increase the thermal resistance. In particular, the crystallinity of Cs- and K-exchanged forms of both clinoptilolites was not affected by thermal treatment at 450degreesC and was only slightly reduced by thermal treatment at 600degreesC.