Seawater bittern, a huge reserve containing numerous mineral materials, is an essential source for producing various industrial salts. In this work, the formation of a double salt between magnesia abundant in the seawater bittern and ammonium sulfate was studied via SEM and EDX analyses. In the next step, to optimize the efficiency of the double salt separation, the effects of technological parameters such as concentration of reactants and their ratio, as well as reaction time and temperature, were investigated. Afterward, the design of the experimental method was utilized to quantify the influence of these parameters. Based on the validation of the obtained model according to statistical criteria, only the ratio of reactants and reaction time have been shown to have decisive roles in the yield of the double salt recovery. The optimal yield reached a value of about 95%, demonstrating the feasibility of the applied technique for further processes, i.e., industrial production.