Solubility and Metastable Zone Width Measurement of NaCO Hydrate Phases in the NaCO-NaOH-HO System as a Basis for a Novel Carbon-Negative Soda Ash Production Strategy.

This study supports a carbon-negative soda ash production process by integrating CO direct air capture in NaOH solutions with crystallization. It presents solid-liquid equilibrium (SLE) and metastable zone width (MSZW) data for NaCO hydrates in the NaCO-NaOH-HO system, relevant to the so-called CODA process. Due to limited previous SLE data, new solubility models were developed. SLE data were determined for decahydrate (3.2-17.1 °C, 0-10 wt % NaOH) and monohydrate (35-85 °C, 0-15.7 wt % NaOH), and concentrations were measured by pH-titration with automated dosing. To prevent undesired primary nucleation in future industrial processes, MSZW was determined for both hydrates for the first time: by cooling crystallization for decahydrate and vacuum evaporative crystallization for monohydrate. Results show that decahydrate exhibits a wide, stochastic MSZW (Δ : 20.9-3 K), narrowed by lower temperatures but widened by NaOH. In contrast, monohydrate has a comparatively small MSZW (average supersaturation ratio: 1.08), independent of both evaporation rate and NaOH presence.