Solid state transformations of (NH4,Pb)-clinoptilolite through heating

Three clinoptilolite-based materials, having Pb2+/NH4+ ratios of 1.60, 0.56 and 0.16, were prepared starting from a powder containing ≈90% of NH4-clinoptilolite. The samples were submitted to a set of thermal treatments of 2 hours up to 1000 °C. Further treatments were performed at 900 and 1000°C for 4, 8, 16 and 32 h. (NH4, Pb)-clinoptilolite underwent dehydration, de-ammoniation, and dehydroxylation, the latter accompanied by amorphization that occurred between 600-700°C, whereas crystallization of new phases started at 800-900°C. The products, obtained through solid state reactions, were: i) Pb-feldspar + cristobalite/tridymite + glass, when Pb2+/NH4+ ≥ 0.16; ii) mullite + cristobalite/tridymite + glass, when 0 ≤ Pb2+/NH4+ ≤ 1.60, hence both Pb-feldspar and mullite were obtained for 0.16 ≤ Pb2+/NH4+ ≤ 1.60. XRD data showed that, for a given composition, temperature and duration of the heating influenced the quantity of the phases formed, but also the occupancy of Pb- and T-sites of Pb-feldspar. Crystalline products (amorphous 10-12%) were obtained from all samples. When Pb2+/NH4+ ≥ 0.56, the highest amounts of Pb-feldspar and the lowest quantities of residual glass were recorded after 32 h at 900 °C, but a 2-hour treatment at 1000 °C allowed to speed up the transformation, confining over 70% of lead within feldspar. The sample with Pb2+/NH4+ = 0.16 required 32 h at 1000 °C to reduce glass at 10%, but most of the lead (≈ 70%) remained in the amorphous fraction.