Recovery of Na2SO4 from remediation of wastewater and reuse for preparation of sodium 4-nitrotoluene-2-sulfonate (NTSNa)

A new strategy is proposed based on the recovery of Na2SO4 after the four-effect evaporation of wastewater originating from the preparation of 4,4′-dinitrostilbene-2,2′-disulfonic acid by the oxidation of sodium 4-nitrotoluene-2-sulfonate (NTSNa), and the reuse of the reclaimed Na2SO4 for producing NTSNa. The recovery of high-purity Na2SO4 from wastewater was achieved by calcining the waste sodium sulfate after the four-effect evaporation. The reclaimed Na2SO4, instead of NaOH, was used in the preparation of NTSNa as the starting material. The effects of temperature and calcination time on the decomposition of organic compounds were investigated, and the optimum conditions of reclaiming Na2SO4 were found to be calcining for 2 h at 700°C. Furthermore, in the continuous preparation of NTSNa, the favorable mol ratio (NTS to Na2SO4) should be ≥3.3 and the optimum operation conditions of the filtrate-recycle process were determined, which economized in raw materials and was environmentally friendly.