ACID AND HEAT TREATMENT OF ARMENIAN CLINOPTILOLITE

Abstract

Acid treatment of natural zeolites is often used to "improve" their porous structure and properties, which are also affected by thermal treatment. The influence of hydrochloric acid solutions with concentrations up to 2 N and calcination at temperatures up to 800℃ on the structure and properties of clinoptilolite-containing tuff from the Armenian Nor-Kohb deposit, selected for the creation of new bactericidal zeolite filter materials for purification and disinfection of water from various sources, was studied by the chemical analysis, powder X-ray diffraction patterns, adsorption of water, benzene and nitrogen. It was found that an acidic environment leads to significant dealumination (Si/Al atomic ratio increases from ≈5 to ≈9.4) and changes in cationic composition without decationization leading to a decrease in the adsorption capacity for water vapor; acidic solutions do not lead to amorphization of the zeolite microporous crystal structure, but gradually dissolve it. Acid treatment also "opens" micropores for large non-polar nitrogen molecules, increases the adsorbing surface area and causes changes in the mesopore system, leading to an increase in their size. Heating leads to the transformation of partially dehydrated zeolite into the metastable phase of heulandite B at 457℃ followed by possible formation of zeolite-like mineral wairakite (500℃) and formation of anorthite-like feldspar (600℃); the content of the amorphous phase increases with increasing temperature, but the zeolite crystalline structure is maintained even after annealing at 800℃. The specific porosity of heulandite (0.17 cm³/g) is maintained up to a temperature of 700℃, the adsorption capacity of micropores for water vapor (>4 mmol/g) does not change up to a temperature of 500℃, at higher calcination temperatures, the volume of micropores available for nitrogen molecules and the surface area also decrease, and the average diameter of nano-sized pores increases.