Abstract
One of the effective reuse application for waste salt, sodium chloride (NaCl), from a near-zero discharge process is the production of sodium hydrate (NaOH) by ion-exchange membrane electrolysis. However, the organic contaminants must be effectively removed from the waste salt before reuse to prevent the rise of cell voltage. In this study, a low-cost near-zero discharge integrated process was proposed for purifying and reusing waste NaCl from the hydrazine hydrate (N2H4) manufacturing process. In the integrated process, water washing followed by activated carbon adsorption was applied to purify the waste salt to meet the requirements for NaOH production using ion-exchange membrane electrolysis. Washing the salt at a water/salt ratio of 5:8, 88.5% of dissolved organic carbon (DOC) was removed, with 25% loss of salt in water washing. Saturated salt solution prepared from the salt after water washing was treated with activated carbon at a dose of 500 mg/L to further remove 10% DOC. Then the salt in the washing wastewater was obtained through evaporation and crystallization and further purified using pyrolysis, with a DOC removal over 99.17% at a temperature >500 °C. Evaluation using an ion-exchange membrane electrolysis system running for 42 h showed, that the voltage in treating purified salt by water washing and activated carbon adsorption was almost the same as that for pure salt, suggesting that the substances causing the rise of cell voltage were mostly removed. Meanwhile, though direct pyrolysis exhibited a DOC removal rate as high as 99.29% at a temperature >500 °C, the treatment cost is estimated to be 500 RMB/t, while the integrated process treatment cost is estimated to be 224.38 RMB/t. This study proposes a feasible way to reuse the waste salt from N2H4 manufacturing process.
Published Version
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