| Vanadium-containing shale is an important vanadium-bearing resource in China.Acid leaching of vanadium-containing shale with H2SO4 is the most common method in vanadium metallurgical plants because of its high vanadium recovery and low pollution.However,the consumption of sulfuric acid in the acid leaching process is high.A quantity of residual free sulfuric acid is present in acid leaching solution after leaching with H2SO4,and the acidity of the acid leaching solution is relatively high.The content of sulfuric acid is about 30~150 g/L and p H value of acid leaching solution is usually less than 0.5.Whereas,The optimum range of p H value for purification and enrichment of vanadium is about 1.8~2.5.Therefore,before purification and enrichment of vanadium,p H value of acid leaching solution should be adjusted to 1.8~2.5.At present,a conventional and popular method to adjust p H value of acid leaching solution is neutralization with ammonia water or lime.However,such a neutralization by alkali process will generate a large quantity of highly concentration ammonia-nitrogen wastewater or vanadium-containing gypsum which cause ensuing disposal problem and serious environmental pollution.Moreover,valuable resources such as sulfuric acid,base and part of vanadium are lost.In this study,coupling diffusion dialysis with bipolar membrane electrodialysis was employed as a substitute for neutralization by alkali to separate and recover sulfuric acid from acidic vanadium-containing leaching solution,aiming to achieve the recovery and utilization of the residual sulfuric acid to reduce acid consumption and the increase of p H value of the acid leaching solution.The diffusion dialysis results showed that DF120-III anion exchange membrane showed good performance in separating sulfuric acid from acidic vanadium-containing leaching solution.The optimum operating conditions for this process included a flow rate of 12 m L/min and flow rate ratio of water to feed of 1-1.1.Under the optimum operating conditions,the sulfuric acid recovery reached 71.12%,water osmosis rate could be controlled at 14.95%,and vanadium rejection was 95.50%.The values of the rejection of impurity ions,such as Al,Fe,Mg,K,F,P,and S were 99.04%,97.37%,98.01%,85.12%,98.33%,91.16%,and 69.96%,respectively.The contents of the impurity ions in the recovered acid were relatively low,it could be reused in the acid leaching process.The p H value of the acid leaching solution rose to 1.0 from 0.04,not meeting the p H value for purification and enrichment of vanadium,a further treatment of the acid leaching solution is needed.The sulfuric acid recovery and the p H value of acid leaching solution can not be further enhanced by the single diffusion dialysis process.One of the reasons is that it is important to control the water osmosis in the diffusion dialysis process.Water osmosis diluted the acid leaching solution and reduced the vanadium concentration,which was disadvantageous to separation and enrichment of vanadium.The control of water osmosis was disadvantageous to the recovery of sulfuric acid.The other is that the high rejections of F in the form of complexes(Al F2+,Al F2+)and P in the form of incompletely dissociated acid(H3PO4,H2PO4-)hindered the separation of H~+.Therefore,bipolar membrane electrodialysis was adopted for secondary treatment of the acid leaching solution.The results showed that the p H value of the acid leaching solution could rose to 1.8~2.5 from about 1.0,meeting the p H value for purification and enrichment of vanadium.In the bipolar membrane electrodialysis process,the amount of the precipitation generated was less than 3 grams per liter acid leaching solution;the loss rate of vanadium was about 5%;the electric energy consumption was less than 0.1 k Wh/mol H~+. |
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