Nadia Petrova

Thermal decomposition products of hydrotalcite-like compounds: low-temperature metaphases

Synthetic hydrotalcite-like samples with composition [M1 − x2+Alx3+(OH)2](CO3)x/2 ·nH2O, where M = Mg, Ni, x = 0.25 and 0.33, n = 2x and 1.5x, are studied by XRD, DTA, TG and IR spectroscopy after heating in the temperature interval 120–260 °C and rehydration in air and water. Structural models of the two metaphases obtained are proposed. Metahydrotalcite-D (HT-D) is formed at 140–180 °C by the reversible dehydration of the interlayer. Metahydrotacite-B (HT-B) is formed at temperatures 240–260 °C as a result of the dehydroxilation of a part of OH groups of the brucite-like layer and inclusion of two oxygenes from the CO3-group in the same layer. The HT-B has a specific crystal structure. Rehydration does not restore the initial structural state but leads to the formation of a phase (HT-B-r) characterized with increased thickness of the interlayer, high content of water and a low temperature of dehydration. The properties of HT-B depend on the ratio M2+: Al of the initial sample. The substitution Mg ↔ Ni does not influence the properties of the metaphases in the low temperature region.

Hydration heats of zeolites for evaluation of heat exchangers

The enthalpy of hydration of purified clinoptilolite from Beli Plast, Bulgaria, and various cation-exchanged types such as Na-, K-, Ca- and Mg-clinoptilolite was determined by the adiabatic water-vapor absorption calorimeter. The hydration enthalpy becomes more exothermic in the sequence K, Na, Ca, Mg depending on hydration energy values of exchanged cations. Na-clinoptilolite would be an efficient heat exchanger in a wide temperature range of dehydration, 180–300°C, while Mg-clinoptilolite in higher temperatures, 300–350°C.

Entropy of Zeolitic Water

Zeolite-water heat-pump system is suitable for effective use of low temperature heat sources such as solar energy and waste heats from factories, that is,for energy saving. The heat exchange function of zeolite owes obviously to the nature of the zeolitic water, the state of which can be described in terms of the entropy value as an independent component of H2O. Most entropy values of zeolitic water have been given so far to be intermediate between those of liquid water (69.9 J mol-1 K-1 at 298 K) and ice (41.5 J mol-1 K-1 at 273 K).The present calorimetric measurements proved, however, that the entropy value for Mg-exchanged A-type zeolite is so small, even at the ambient temperature, as to be compared with the residual entropy of ice at 0 K.

Zeolitization of glasses: a calorimetric study☆

Abstract The construction of calorimetric ampoules for work in hydrothermal conditions makes it possible to investigate the enthalpy of mineral crystallization. The potential of the equipment is demonstrated for the case of zeolite synthesis from glasses. Two synthetic Na- and NaK-glasses, and dacite pumice were used as starting materials. Aqueous solutions of 2 N and 6 N NaOH and KOH were prepared. The crystallization was performed in a Kalve calorimeter DAC-1A at 95°C. Three peaks are observed in the e.m.f. curves due to: wetting of the glass, glass hydration and dissolution, and crystallization of zeolites. Their position versus time and their intensity depend on the experimental conditions. The described calorimetric equipment is very useful for the low-temperature hydrothermal synthesis of zeolite minerals under isothermal conditions and provides a means of controlling the crystallization process.

Influence of crystalline seeds on the zeolitization of volcanic ashes: a calorimetric study

A direct calorimetric study of zeolitization of volcanic ashes is discussed. It is shown that the addition of seeds influences the mineralogical composition of the syntesized products and the zeolitization kinetics and enthalpy.

Matching of the water states of products and zeolite during contact adsorption drying

ABSTRACT The mixing of dehydrated zeolites with product particles leads to drying of the product, owing to the decrease in relative humidity of the environment and increase the temperature of the mixture, due to the release of adsorption heat. The analysis of the conventional one-stage contact drying process showed a significant mismatching in the water states of the product and the zeolite during drying. A two-stage drying process is proposed for harmonization of the water state of both product and sorbent. The significant advantage of the two-stage drying over one-stage regarding the degree of drying of the product, the utilization of the zeolite adsorption capacity and the duration of the process were experimentally confirmed.