Synthesis of porous silicate materials using sodium fluride as fluxing agent

Abstract

Foam glass products are very actively used for thermal and chemical protection of oil and gas infrastructure instead of polymer materials. The main trend in foam glass technology is a search for new raw materials. A presence of refractory alumina in the raw materials necessitates the introduction of substances that reduce the melting temperature of silicate masses (fluxes). The aim of this work was to study the processes that occur during the heat treatment of foam glass batches when sodium fluoride NaF is added as a flux. The synthesis of porous materials was carried out by powder method according to the hydrate mechanism based on the interaction between silicate raw materials and an alkali solution. The initial batch compositions were developed, and their heat treatment was performed. TG/DSC analysis showed that NaF does not affect mass change processes, including the removal of physically bound water, crystalline hydrated water from hydrosilicates, chemically bound water, and decomposition of organic and carbonate impurities. There is also an endothermic effect of the “β-quartz→α  -quartz” transition for both compositions. However, there is a sharp endothermic jump in the composition with the NaF addition at temperatures above 600 оC due to exposure to sodium fluoride. It was shown that ions from fluoride dissociation are concentrated in a less polymerized microphase, sharply changing its viscosity properties and the viscosity of the melt as a whole. It was set that most of the silicate mass is in an X-ray amorphous glassy state. The main crystalline phase is α -quartz in an amount of not more than 25%, and albite crystals can also be distinguished. As a result of studying the internal structure and density of the synthesized samples, it was found that the introduction of sodium fluoride significantly accelerates the process of silicate mass melting, which makes it possible to obtain samples with a density of 313 kg/m3 at 750 оC and subsequently leads to the breakthrough of pore walls, pore coalescence, and severe sedimentation of the foam, which complicates manufacturing process of high-quality materials. Thus, the use of sodium fluoride in the synthesis of porous silicate materials can be recommended in a small amount as an active fluxing agent.This research was carried out by the Platov South-Russian State Polytechnic University (NPI) and funded by the Russian Science Foundation under the Agreement No. 19-79-00015 “Understanding some general regularities related to the synthesis of porous materials on the basis of man-made and natural silicates” (led by B. M. Goltsman).