In this work a process has been developed for the transformation of gypsum from coal-based thermal plants into sodium sulfate. The current lack of market of gypsum and the traditional use of mineral gypsum makes this process an interesting alternative within the circular economy strategy to reuse a waste from the power industry. The process consists of the synthesis and crystallization of sodium sulfate and the possibility of producing the hydrated and/or the anhydrous crystal. This section is integrated within the desulfurization unit by regenerating a fraction of the raw material required, CaCO3. The optimal operating conditions for the evaporation and crystals recovery have been computed using a mathematical programming approach. The economics of the process is attractive for the production of the anhydrous crystal, 0.21 (sic)/kg with an investment of 36 M(sic), as well as for the production of the decahydrated one, 0.06 (sic)/kg and 26 M(sic) of investment, processing the gypsum of a 350 MW power plant.