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Minerals 2017, 7(1), 11;

Thermal Transformation of NH4-Clinoptilolite to Mullite and Silica Polymorphs

Department of Natural and Territorial Sciences, University of Sassari, Via Piandanna 4, 07100 Sassari, Italy
Author to whom correspondence should be addressed.
Academic Editor: Annalisa Martucci
Received: 14 December 2016 / Revised: 10 January 2017 / Accepted: 13 January 2017 / Published: 19 January 2017
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Clinoptilolite is a natural zeolite used for the abatement of ammonium in the treatment of urban wastewater. By considering that mullite was obtained through thermal treatment of NH4-exchanged synthetic zeolites, this work aimed to evaluate if this phase can be obtained from NH4-clinoptilolite. A material containing about 90 wt % of clinoptilolite, prepared using a Sardinian zeolite-rich rock, was NH4-exchanged and subjected to treatments up to 1200 °C. After dehydration, de-ammoniation, and dehydroxylation processes, the clinoptilolite structure collapsed at 600 °C. An association of mullite, silica polymorphs, and glass, whitish in color, was obtained for treatments between 1000 and 1200 °C. The higher degree of crystallinity was reached after a 32 h heating at 1100 °C: mullite 22 wt %, cristobalite 59 wt %, tridymite 10 wt %, glass 9 wt %. It is possible to speed up the kinetics of the transformation by increasing the temperature to 1200 °C, obtaining the same amount of mullite in 2 h, but increasing the residual amorphous fraction (16 wt %). These results indicate that NH4-clinoptilolite could represent a raw material of potential interest in the ceramic field, in particular in the production of acid refractory, opening scenarios for a possible reuse of clinoptilolite-based exchangers employed in ammonium decontamination. View Full-Text
Keywords: zeolite; clinoptilolite; mullite; ammonium; ammonia; cristobalite; ceramic; refractory; thermal treatment; waste zeolite; clinoptilolite; mullite; ammonium; ammonia; cristobalite; ceramic; refractory; thermal treatment; waste

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Brundu, A.; Cerri, G.; Sale, E. Thermal Transformation of NH4-Clinoptilolite to Mullite and Silica Polymorphs. Minerals 2017, 7, 11.

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