Dissolution Characteristics of Inorganic Salts in Sub/supercritical Water: Type 1 and Type 2 Salts

Supercritical water’s unique properties enable organic decomposition and inorganic synthesis, but salt precipitation risks clogging. This study investigates the solubility, supersolubility, and metastable zone width (MSZW) of type 1 and type 2 salts using salt bed dissolution (cooling) and precipitation (warming) methods. At supercritical conditions (25 ± 0.1 MPa), type 1 salts show a moderate solubility decline (1–2 orders of magnitude, to ∼10^–1to 10° mmol·kg^–1), while Type 2 salts exhibit a drastic drop (3–4 orders, to ∼10^–3to 10^–2mmol·kg^–1). Type 1a salts display a wide metastable zone between the critical point and three-phase equilibrium, whereas type 1b salts show stable MSZW. Type 2a and 2b salts exhibit minimal dissolution differences. Most salts (except type 1a) have a wider MSZW under transcritical conditions but are narrower under supercritical conditions. These findings clarify salt-specific precipitation behaviors, aiding in clogging prediction and mitigation strategies for supercritical water processes.