Simultaneous recycling of both desalination reject brine water and phosphogypsum waste in manufacturing K2Mg(SO4)2·6H2O fertilizer

Abstract

The purpose of this paper is twofold. The first is to convert phosphogypsum (PG) waste, mainly composed by gypsum (CaSO4·2H2O), into potassium sulfate (K2SO4) compound, which help to reduce the environmental impact and open new ways and valuable chain for the industrial manufacturing of phosphoric acid. The second is to recover MgSO4·7H2O from desalination reject brine water (RBW), thereby minimizing its associated environmental impact. The novelty of the current study is to simultaneously use the recovered salts K2SO4 and MgSO4·7H2O to produce K2Mg(SO4)2.6H2O (K2MgS6), considered as a double fertilizer. Additionally, Solid–Liquid Equilibria (SLE) are frequently applied to several industry domains. SLE are an interesting outline to visualize the precipitation, separation, and purification of a solid phase and the pathways by which crystallization can occur. The SLE of the ternary phase diagrams K2SO4-MgSO4-H2O at 25 °C and 0 °C were especially used to successfully determine the operating conditions and the design of a crystallization process during the PG/RBW conversion into K2MgS6. Several characterization techniques (i.e., XRD, DTA/DTG, SEM/EDS, FTIR) were employed to identify the solids formed during this process. An analysis on the distribution of natural radionuclides and heavy metals was carried out to confirm the effectiveness of the developed process. The main conclusion of this study was that K2MgS6 fertilizer can be manufactured by combining PG and RBW from desalination plants. Furthermore, the formed fertilizer, K2MgS6, is highly recommended for many applications in the agriculture sector.