**1. Introduction**

Crystallization from melt is one of the separation and concentration techniques used for organic compound solutions [1]. Nevertheless, under certain conditions, several organic compounds and green solvents, such as ionic liquids and deep eutectic solvents, can undergo thermal degradation or hydrolysis [2–6]. Even though it has been reported that low-pressure evaporation and distillation [7–9] can be used as a concentration method in the recycling of ionic liquids from aqueous ionic liquid solutions, the main drawback of such processes is that they are high-energy separation methods due to the high latent heat of evaporation. Consequently, there is a need to find more feasible concentration methods, which also allow application at low-temperature ranges.

Melt crystallization is a low-energy separation method that typically uses a low processing temperature close to room temperature. This is an advantage when working with thermally unstable substances or organic compounds that tend to react and decompose at higher temperatures [10]. Freeze concentration of an aqueous solution is one type of melt-based crystallization method and this is defined as the separation of formed ice from the aqueous solution.

Melt crystallization methods can be generally classified as either layer crystallization or suspension crystallization [11]. In static layer crystallization, the formation and growth of the ice layer occurs at the sub-cooled surface of a crystallizer from stagnant aqueous solution. In contrast, with the suspension crystallization technique, ice crystals form and grow within a sub-cooled solution present inside a crystallizer equipped with a scraper.

Moreover, a recent study [12] has shown that the combination of evaporation and freeze crystallization processes as a new method of recycling an ionic liquid from an aqueous solution is more energy efficient than evaporation-based concentration, primarily as a result of the lower latent heat of freezing when compared with the latent heat of evaporation.

In the present work, an ionic liquid (IL), 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]), was used as a model compound for the investigation of the freeze concentration method. [DBNH][OAc] has been shown to be an efficient solvent for dissolving birch-based cellulose [13] and is considered to be a promising industrial solvent due to its safety, low environmental impact, economic viability, and production of high-quality fiber even from low-refined unbleached pulps [14–17] with good spinnability [18]. Nevertheless, the main challenge to the wider application of IL is that it can be a relatively expensive organic solvent and, therefore, it must be efficiently recycled from aqueous solution as a way to reduce costs [19].

In this work, freeze crystallization techniques as a concentration method for IL recycling were investigated. The solid–liquid equilibria were determined between [DBNH][OAc] and water by the differential scanning calorimetry (DSC) technique.
