Recovery of sodium sulphate from a reverse osmosis concentrate of silica-industry wastewater by eutectic freeze crystallisation technology

Resources recovery from wastewater is pursued as it facilitates a circular economy, which a society runs with a minimal environmental impact. Eutectic freeze crystallisation (EFC) has been identified as a potential technology that can assist to achieve such goal. High quality of salt and ice can be retrieved with this treatment with a note of high energy efficiency. In this paper, based on an actual case of a Spanish silica production company, freeze and eutectic freeze crystallisation are found to be applicable to a complex RO concentrate, which involved Na, Mg, Ca, Sr, Ba, SiO2. Quality ice and mirabilite products were also produced, which the impurities attached can be further washed away easily. The influence of the impurities on the eutectic point of sodium sulphate solution is investigated. A conservative recovery rate of 42wt% and 55wt% are obtained respectively for mirabilite and ice. Heat transfer characteristics of the equipment are also conducted, the boundary layers in the working solution and in the coolant have a clear influence on the overall heat transfer. A heat transfer of 404W/K-m2 is obtained for the EFC cooling, which is found to be comparable to past study. Energy consumption for a complete EFC process on the RO concentrate is estimated to be 0.1437 kWh/kg-solution, which is only 22% of the energy consumed in heat crystallisation. Column crystalliser is suggested to be a possible method to resolve impurities accumulation issue. Torque was also identified to indicate nucleation and crystallisation during the process. A crystal size distribution model is developed and can be used to evaluate or predict the performance of an EFC system. During the thesis study, a successful demonstration and training was provided to the staff from Fundació CTM Centre Tecnològic, which is the respondent for the Spanish case and will later perform a EFC experiment in the treatment plant.Civil Engineering | Environmental Engineerin