A green process for producing Na2S from waste Na2SO4 through hydrogen agglomerate fluidized bed reduction of BaSO4

A green process is proposed to produce Na2S from waste Na2SO4 through hydrogen agglomerate fluidized bed reduction of BaSO4. In the new process, BaSO4 acts as the circulating medium, and the synthesis of Na2S from waste Na2SO4 is performed via a two-step approach. First, BaSO4 is reduced to BaS by hydrogen at high temperatures. Then, the reduction product BaS is put into the Na2SO4 solution to prepare Na2S through the metathetical reaction. Simultaneously, insoluble BaSO4 is regenerated by filtration and drying, and it can be reused in the reduction reaction. Experimental results showed that the looping material BaSO4 could be reduced to a conversion rate above 80% at 900–950 °C in a conical fluidized bed by agglomerate fluidization. The introduction of Na2SO4 could greatly accelerate the reduction rate of BaSO4, and detailed analyses of the reduction kinetics indicated that the addition of Na2SO4 could strongly catalyze the reduction of BaSO4 by reducing the apparent reaction activation energy, which was realized through the synergistic effect of Na2SO4 and Na2S. The consumption of BaSO4 in the reduction reaction was nearly equal to that regenerated in the metathetical reaction, and the reactivity of BaSO4 remained almost the same as the original reactivity during multiple cycles, which indicated the strong recyclability of BaSO4 in the new process. After dehydration treatment of the Na2S solution by evaporation concentration, cooling crystallization, and vacuum drying, anhydrous Na2S of high purity was obtained eventually. Compared to traditional coal-based reduction approaches, the new process is more environmentally friendly, highly efficient, and low-cost.