Characterization of calcium and magnesium salt removal and inorganic salt crystallization deposition in supercritical water oxidation system

Supercritical water oxidation (SCWO) is a state-of-the-art treatment technology for highly concentrated, difficult-to-degrade organic wastes, but salt deposition leading to equipment clogging and corrosion is a key issue limiting the further development of this technology. In this work, pre-removal of calcium and magnesium salts was studied. The findings indicate that the solubility of CaCO₃ increases with temperature between 25 and 120 °C, which hinders calcium ion removal. However, above 120 °C, the temperature effect on reducing solubility becomes more significant. Direct addition of solid NaOH generates Mg(OH)₂ colloids, leading to incomplete reactions, so NaOH solution is recommended for practical use. Then the phase behavior of inorganic salts under sub/supercritical conditions was studied for the wastewater after the removal of calcium and magnesium salts, and the solubility and salt precipitation rate models were established for each temperature condition with the continuous experiments, and the solubility and crystallization behaviors of the inorganic salts in the wastewater after the calcium and magnesium salts removal were obtained. The study showed that salt crystal precipitation was observed at 550 °C and 25 MPa, while no precipitation occurred at 380 °C during preheating. It is suggested that 380 °C or lower be used as a reference for preheating temperature. Subsequent analysis revealed that the main salts deposited were NaCl and Na₂SO₄, with high Cl⁻ content causing corrosion of the metal substrate. The work provides important experimental and theoretical support for the treatment of high hardness oily wastewater, especially in the application of SCWO technology with certain guiding significance.