Cristalização de flor de sal em soluções hipersalinas naturais induzida por evaporação em diferentes condições de umidade e plasma atmosférico

Abstract

Sea water presents itself as an inexhaustible resource from which it’s possible to extract drinking water and numerous minerals with high importance for technological and environmental development. Due to its complexity and unique chemical composition, the recovery of minerals present in seawater is a well-studied process. Even today, the main method of extracting these minerals, in the form of salts, is natural evaporation in artificial ecosystems, with NaCl as the most commercially explored product. A very specific type of NaCl, still not well understood, produced by few salt industries in Brazil is Sal Flower (Fleur de sel), crystals that are formed strictly on the surface of natural brines. Its crystallization depends essentially on the local climatic conditions that constantly change the thermodynamics of the process, making it impossible to control its production. In this context, another method still little studied, but which has already presented positive results regarding crystallization of particles on the surface of brines, consists of the application of discharges at atmospheric pressure, atmospheric plasma, on the surface of the solution. Based on this, this work aims to investigate the influence of thermal gradient generated in different humidity conditions on the crystallization mechanism on the surface of natural brines, as well as to study the performance of atmospheric plasma discharge in contact with the surface of these solutions. For this study, samples of natural hypersaline solutions, with a density of 26 °Bé, were obtained directly from the crystallization tanks of a saline industry and subjected to a thermal gradient generated between the ambient air and its interior. The average relative humidity of the air was controlled at two levels and the formation of crystals on the surface was monitored together with the density, total salt concentration, electrical conductivity and pH of the solution. The influence of plasma on the crystallization of the hypersaline solution was evaluated through the application of luminescent discharges in an atmosphere of air on the surface of drops of this same solution. The crystals obtained in these experiments were characterized in terms of their mass through weighing on an analytical balance, morphology using scanning electron microscopy (SEM), chemical composition by atomic absorption spectroscopy, atomic emission spectrometry and Morh titration and the microstructure was assessed by X-ray diffraction (XRD). The analyzes showed that the thermal gradient enabled the formation of crystals on the solution surface in both humidity conditions. The moisture effect was significant for the evaporation rate, mass and morphology of the crystals formed on the surface. The chemical composition varied according to the density over the evaporation time, in which the amount of Ca2+, K+ , Clions decreased and Mg2+ and SO4 2- increased. The plasma acted significantly in the crystallization, noticed by the considerable increase in the mass of the solids present in the solution after the treatment and by observation of its performance under a stereoscopic magnifying glass. These crystals showed differences in their morphology and chemical composition that varied both with density and with the application of plasma, which acted by reducing the concentrations of Na + and increasing those of K+, SO4 2-, Mg2+ and Ca2+. The microstructures of the materials showed differences in crystallinity and phases present in relation to humidity, density and plasma effect, with presence of NaCl phases peaks being more significant. In this way, it was possible to understand not only the formation of salt flower, but also its morphology, microstructure and chemical composition. In addition, plasma has proved to be an extremely efficient method for extracting minerals from brines and can be studied as an advanced desalination technique.