Process water re-use is a common practice in Australian coal preparation plants. It is an effective solution to on site water scarcity and minimizes environmental impact. This paper investigates the effect of inorganic mineral salts in the process water on the thermoplastic properties of metallurgical coals after a short period of mild oxidation. Two different metallurgical coals were treated with the process water received from a coal preparation plant and exposed to air at ambient temperature. The thermoplastic properties of the samples treated with salt solutions were measured using Gieseler plastometry. The results suggested that the fluidity of coal decreased as the concentration of inorganic salts increased. When different salt solutions were used, the degree of fluidity reduction was found to be similar at the same concentration. Examination of the coal surface with SEM-EDS analysis showed the existence of both sodium and chlorine on the surface of process water treated coal while the surface analysis by X-ray photoelectron spectroscopy (XPS) indicated a relatively insignificant chemical reaction between the inorganic ions present in the water and the coal surface after rinsing the samples. Salts appeared to precipitate on non-maceral matter and act as an inert (non-plastic) additive. (C) 2019 Elsevier B.V. All rights reserved.