Characteristics, mechanisms and measurement methods of dissolution and deposition of inorganic salts in sub-/supercritical water

The efficient and harmless treatment of hypersaline organic wastes has become an urgent environmental problem. Compared to traditional thermochemical methods, supercritical water oxidation has been proven to be an efficient organic waste treatment technology due to the advantages of low cost, high degradation rate, no secondary pollutants, etc. However, the solubilities of inorganic salts drop rapidly near the critical point of water, and some sticky salts form easily agglomerates and then adhere to internal surfaces of reactor and pipeline, causing plugging and inhibition of heat transfer. Hence, the characteristics, mechanisms and measurement methods of the dissolution and deposition of inorganic salts in sub-/supercritical water are summarized and analyzed systematically and comprehensively in this work, intending to provide a valuable guide for salt deposition prevention and subsequent research directions. Firstly, a new classification form of inorganic salt is put forward based on melting point. The phase equilibriums of brine systems are then analyzed in detail. Six theories concerning dissolution mechanisms are discussed deeply and various measurement methods of salt solubility are also supplemented. Furthermore, salt deposition characteristics and related measurement technologies are summarized. Notably, a new idea “hydrothermal molten salt” system is reviewed which may provide a solution for salt deposition in sub/supercritical water. Finally, an outlook for the follow-up researches is prospected and some suggestions are proposed.