Optimization Of Process Parameters For The Purification Of Zinc From Pickling Waste Liquid By Tributyl Phosphate Extraction

Hot dipped galvanizing is one of the most effective ways to slow down the environmental corrosion of steel,but the process of hot dipped galvanizing results in a significant amount of pickling waste liquid,which contains high concentrations of acid,large amounts of iron,zinc and trace amounts of other metals.The primary element influencing how pickling waste liquid is disposed of and used is heavy metal zinc.The resource usage of pickling waste liquid containing zinc has been hampered by the difficulty of recovering zinc while keeping iron.It is a feasible process route to separate iron and zinc by tributyl phosphate（TBP）extraction to achieve resourceful recovery of heavy metal zinc and product utilization of iron ions in the pickling waste liquid.In this paper,the optimization of the process parameters of tributyl phosphate extraction for the purification of heavy metal zinc in pickling waste liquid was investigated,the maximum extraction capacity of the extractant TBP at different zinc concentrations,the relationship between the extraction ratio O/A（O/A is the volume ratio of organic phase to aqueous phase）and the utilization rate of TBP were investigated,based on which the counter-current extraction process parameters（the number of stages and ratio O/A）were optimized.Finally,the process parameters were applied to the actual pickling waste liquid for verification,and the extraction mechanism of TBP on zinc was also analyzed by infrared spectroscopy.The main results obtained from this study are as follows:1.The maximum extractive capacity of TBP was correlated with the concentration of zinc in the pickling waste liquid,and the maximum extractive capacity increased with the increase of zinc ion concentration.The maximum extractive capacity of TBP was 26.3 g/L,35.3 g/L,41.3 g/L,46.9 g/L,50.5 g/L,60.3 g/L and 65.1 g/L for simulated pickling wastes liquid with zinc concentrations of 5 g/L,10 g/L,15 g/L,20 g/L,25 g/L,35 g/L and 50 g/L,respectively,under the conditions of 0.5 ratio O/A,5%free acid content（w/w）,2 min extraction time and room temperature.2.Ratio O/A is an important parameter in the extraction process,which is closely related to TBP utilization and zinc removal rate.When determining the ratio O/A parameters in the extraction process,the relationship between TBP utilization and Zn removal rate should be considered comprehensively.In the case of single extraction,the zinc removal rate increased accordingly with increasing ratio O/A,but the TBP utilization gradually decreased.For the simulated pickling waste liquid with Zn concentrations of 5 g/L,10 g/L,15 g/L,20 g/L,25 g/L,35 g/L and 50 g/L,the TBP utilization rate was higher at the ratio O/A of 0.1,which exceeded 80%,but the Zn removal rate was relatively low,with the highest not exceeding 50%and the lowest only about 10%.3.In order to ensure the zinc removal rate while maximizing the utilization of TBP,multi-stage counter-current extraction process is usually used in practical engineering.In this paper,multi-stage counter-current extraction parameters were optimized for different Zn concentrations of simulated pickling waste liquid,and the optimized O/A and number of stages were able to achieve Zn concentration below 1.5 g/L and TBP utilization of about 90%in the extracted residue.For the simulated pickling waste liquid with a zinc concentration of 50 g/L,the zinc removal rate was stabilized at 99%after four-stage counter-current extraction at a free acid content of 5%（w/w）,an extraction time of 2 min,and the temperature of room temperature,the ratio O/A of 0.8,and the zinc concentration in the extractive residue was stabilized below 0.5 g/L,with a corresponding TBP utilization rate of 94%.The zinc removal rate was stabilized at 96%,94%,90%,97%,98%and 98%after the three-stage counter-current extraction of the simulated pickling waste liquid with zinc concentrations of 5 g/L,10 g/L,15 g/L,20g/L,25 g/L and 35 g/L,respectively,ratio O/A of 0.2,0.25,0.35,0.45,0.55 and 0.65,respectively.The zinc concentrations in the extracted residue were lower than 0.2 g/L,0.6 g/L,1.5 g/L,0.6 g/L,0.5 g/L,0.7 g/L,and the corresponding TBP utilization rates were 89%,100%,92%,100%,87%,and 89%,respectively.4.The optimized countercurrent extraction process parameters were verified using the actual pickling waste liquid.Under the optimal conditions,the zinc removal rate of the actual pickling waste liquid with a zinc concentration of 50 g/L exceeded 98%and the zinc concentration in the extracted residue was stabilized below 0.9 g/L after four-stage counter-current extraction at the O/A of 0.8.The actual pickling waste liquid with a zinc concentration of 20 g/L was extracted at three stages of counter-current extraction under the condition of 0.45 of the O/A,and the zinc removal rate exceeded 94%,and the zinc concentration in the extracted residue was stabilized below 1.2 g/L.5.Using infrared spectroscopy and slope method,the extraction mechanism of TBP on zinc ions in hydrochloric acid pickling waste liquid,the form of P=O bonding and zinc ion complexation is used,there is H⁺involved in the reaction of TBP zinc extraction reaction,H⁺can complex with P=O in TBP,the form of extractant in organic phase after TBP zinc extraction is mainly ZnCl₂·2TBP,and there exists HZnCl₃·3TBP form.