Recovery Of Hydrochloric Acid From Spent Pickling Solution Using Solvent Extraction

The steel industry produces a lot of spent pickling solutions every year. The solutions would damage the environment due to the existence of ferrous salt and free residual acid. Therefore, resource and comprehensive utilization of the spent pickling solutions are the inevitable choices. To solve this problem, a study on the solvent extraction, which was used to separate the hydrochloric acid from ferrous salt, was carried out in this work. The solvent extraction technology provides a new way to recycle the hydrochloric acid in spent pickling solutions.At first, the extraction rate of hydrochloric acid by three typical organic solvents, including Trioctylamine(TOA), tri-butyl phosphate(TBP) and iso-amyl alcohol, were tested and compared. The iso-amyl alcohol was considered as an inappropriate extractant due to its lowest extraction rate. In this work, the initial content of hydrochloric acid and ferrous ion in feed solution were 69.4 g/L and 95.3 g/L, respectively. The results of experiment showed, stirring time of 20 min, 50% TOA and 20% 2-octanol in volume, temperature in 20 ? and 2/1 of O/A ratio was the optimum technological condition for extraction of hydrochloric acid. The extraction rate can be 100%. The organic phase after extraction can be stripped in to chloride by ammonia and used as nitrogen fertilizer.In this paper, as a potentially energy saving alternative, solvent extraction was used for the recycling of aqueous hydrochloric acid(HCl) solutions aiming at spent pickling solution mainly containing ferrous iron besides HCl. Three extractant systems were taken into consideration and Tributylphosphate(TBP) was selected as the extractant due to its appropriate acid extraction rate and ease in stripping. In current study, the extraction of HCl using TBP as solvent has been studied. More specifically, various parameters were optimized for the maximum extraction of HCl such as time for equilibration, concentration of organic extractant, temperature, O/A ratio and ferrous iron concentration etc. The mixed time was chosen as 10 minutes, the concentration of TBP was fixed at 100%, and the extraction temperature was 20 ?. Additionally, the enthalpy change of HCl extraction by TBP was-9.133 kJ·mol-1. Ferrous iron was found having salting-out effect on HCl extraction and the salting-out empirical equation was obtained. TBP was found having good selectivity when extracting HCl from aqueous solution containing ferrous iron. Multi-stage countercurrent extraction was carried out using string class experiment method to increase the acid extraction rate. After 6-stage countercurrent extraction, the acid extraction rate grew up to 89.6%. 3-stage countercurrent extraction was used to striping the HCl loaded organic phases and the striping rate was above 98%. Furthermore the HCl concentration in finally recycled acid aqueous solution was 71.1 g/L, higher than the feed solution. The HCl recycling rate after 6-stage extraction and 3-stage striping was almost about 88%.