A novel approach to eliminate the negative effects of SO 42- and NH4+ on vanadium extraction from the high-calcium shale during the internal recycling of vanadium industrial wastewater

In this study, the inhibition of SO42- and NH4+ on vanadium extraction from the high-calcium shale during the internal recycling of vanadium industrial wastewater (the raffinate and the precipitated liquor) was investigated and a novel approach was proposed to eliminate the negative effects. The main inhibitions of SO42- and NH4+ occur in the leaching process, there was no negative effect on the solvent extraction of vanadium solutions and the precipitation of vanadium products. The enrichment of NH4+ during the internal recycling of vanadium industrial wastewater leads to a decrease in vanadium leaching efficiency of less than 10 % due to the deduction of effective free F- caused by the complexation of NH4+ and F-. There was a nearly 39 % reduction of vanadium leaching efficiency when the concentration of cyclically enriched SO42- reaches to 250 g/L, and the suppressed secondary ionization of H2SO4 and the production of a large amount of gypsum were the main reasons for this phenomenon. A novel procedure of using HCl solution to preferentially remove calcium from high-calcium shale, and then using the obtained CaCl2 solution to treat the industrial wastewater can effectively eliminated the inhibitory effects of SO42- and NH4+ on vanadium leaching efficiency of Ca-removal shale during the wastewater recycling. The use of HCl solution for pre-calcium removal increased the vanadium leaching efficiency to 86.66 %. Meanwhile, a by-product of 2-hydrated calcium sulfate with a 99.9 % purity can be obtained. Therefore, a process was proposed which can recycle vanadium wastewater, this process is highly efficient, beneficial and environmentally friendly.