Influence of multiphasic systems on salt(s) solubility in supercritical water: the case of NaCl and NaCl-Na2SO4

Supercritical water oxidation processes (SCWO) have been developed as an alternative technology to treat toxic and/or complex chemical wastes with a very good efficiency. However, one main limitation of the SCWO process comes from the precipitation of inorganic compounds. When dealing with supercritical water conditions (T ≥ 374 °C, p ≥ 22.1 MPa), the polarity of water drops and inorganics, such as salts, are no longer soluble. This precipitation phenomenon results in a solid salt deposition in the reactor, which can lead to clogging and interruption of the continuous process. Considering a specific salt, beside its intrinsic solubility in supercritical water, the presence of other salts or other compounds in the media can modify its precipitation behaviour. With the use of a specific experimental set-up, we propose in the present work to study the solubility of NaCl (type I salt) in supercritical water, and the influence of its phase diagram on its precipitation behaviour as well as on the solubility of a second salt, Na2SO4 (type II salt). The complex and dynamic aspect of NaCl precipitation has been studied through the use of a continuous solubility measurement set-up, revealing unexpected results.