Hydrothermal molten salt: A hydrothermal fluid in SCWO treatment of hypersaline wastewater

A hydrothermal fluid system proposed in this paper is composed of immiscible molten salts and sub-/supercritical water (also named as hydrothermal molten salt, HyMoS). It can avoid salt deposition and equipment blocking in the process of SCWO treatment of hypersaline glyphosate wastewater, which provides an exalting technical idea for preventing and controlling salt deposition occurred in various fields of thermophysics and thermochemistry. Aiming to understand the behavior characteristics and rules of multiple salts in SCWO process and explain the formation mechanism of HyMoS, the main salt constituents (NaCl and Na₂HPO₄) in glyphosate wastewater were taken as research objects. XJTU – SCWO continuous pilot plant was used to systematically investigate their dissolution, transportation and deposition characteristics during SCWO. The phase equilibrium rules and melt properties of them in sub-/supercritical water was acquired via on-line record and observation on the Visual phase equilibrium apparatus. The result verified the inhibition effect of low-melting salts on salt crystallization and deposition, obtained the behavior characteristics of NaCl, Na₂HPO₄ and their mixtures, and established the mechanism model of salt melting and redissolving in sub-/supercritical water during temperature rising and dropping, respectively. Furthermore, the dissolution-melt-crystallization phase diagram of Na₂HPO₄ in sub-/supercritical water was established for the first time.