| Solvent extraction is an important process in hydrometallurgy.The extraction of cations has been studied thoroughly,however the extraction of anions has not been studied systematically and deeply.In fact,48 metallic elements and 15 nonmetal can form anions through acid radical or coordination ions.Therefore,it is essential to study systematically the extraction of anions for expanding the application of anion extraction and perfecting the theoretical system of solvent extraction.Halide ions are typical anions.They are congener elements,and the extraction of halide ions is helpful to study the law of gradual change in anion extraction.Moreover,many central ions can form complex anions with halide ions,so it is significant to study the extraction of anions with halide ions as the representative.Halide ions are also common anions in nature and industrial production.Fluoride ion is harmful impurity in a variety of solutions.For acidic high-fluoride solutions,conventional defluoridation technologies are either inefficient or costly.Chloride system is an important hydrometallurgical medium,and high-chloride solutions are usually generated by utilizing the minerals or secondary resources containing chloride.The typical processes are mainly aimed at the removal of low-chloride,so economical and efficient methods are urgent for high-chloride solution.Bromine and iodine are valuable elements,but the existing technologies are backward,hence it is critical to develop advanced process.In view of the above-mentioned problems,efficient processes for fluoride removal,chloride separation and iodine extraction have been developed based on the complexation extraction of halide ions.The law of complexation and extraction have also been investigated.The main results are as follows:Boric acid was selected as the complexation reagent since boron coordinated with fluoride strongly,and the generated borofluoride ion is an easily extracted species.The loading capacity increased by four times when boric acid was added.FT-IR spectroscopic analysis was utilized for describing the extraction mechanism,and the peak appeared at 1040 cm"1 was the characteristic peak of BF4-.The extraction ratio was found to increase with the increase in the boron fluoride molar ratio,Alamine336 concentration,and phase volume ratio.Fluoride decreased from 5 g/L to 0.05 g/L after two-stage cross-flow extraction,with an extraction efficiency of 99%.For the simulated electrolyte,the majority of metallic ions were insensitive to the extraction,with the exception of Al3+and Fe3+.Zinc sulfate was selected as the complexation reagent since zinc coordinated with chloride strongly.The extraction ratio increased with increases in Alamine336 concentration,phase volume ratio and Zn/Cl molar ratio.However,temperature and initial pH had negligible effects on the extraction ratio.After a two-stage counter-current extraction,the concentration of Cl in the raffinate could be reduced from 100 g/L to 10 g/L.Chloride was mainly extracted as ZnC13-when Zn/Cl molar ratio was 0.23,as predicted by theoretical calculations based on the cumulative stability constants.The result was confirmed experimentally by saturated method and slope method.An efficient process was proposed for the recovery of iodine via solvent extraction of triiodide and iodide.The extraction ratio increased with the increasing of H2O2 dosage,phase volume ratio and Alamine336 concentration,whereas it decreased with the increase in equilibrium pH and temperature.The extraction ratio reached up to 91%after only one stage extraction.High selectivity was found since the molar ratio of I/Cl in the loaded organic phase increased by 600 times compared with the initial aqueous.When the oxidation ratio of I-was less than 75%,the distribution ratio of I3-was always more than that of I2,indicating that the recovery of I-and I3-was easier than I2.UV-vis spectra analysis was also utilized for describing the extraction mechanism.The UV spectrum of the loaded organic phase was different from that of I2,showing that the main extracted species were I-and I3-.The central ion with large ionic potential and the ligand with large electronegativity can form stable complexes through ionic bond,while the central ion with small ionic potential and the ligand with small electronegativity can form stable complexes through covalent bond.The hydration energy of ions can be used to characterize the contact probability of ions with extractants.The lower the charge and the larger the radius of ions,the smaller the hydration energy,the easier it is to contact with extractants.From the aspects of electrostatic attraction,steric hindrance,collision theory and extraction energy,it is explained that the lower valence of complex anion is,the more stable the neutral extraction complexation is.It forms a bond with an atom of the extractant when cation is extracted,however,it forms a bond with the entire cation of the extractant when anion is extracted.The former has a stronger bonding strength than hydration,and the latter bonding strength is less or close to hydration.In order to improve the extraction efficiency and selectivity,suitable complex reagent is selected to form low valence complex anion for target element,or to form high valence complex anion for impurity element. |
PDF Full Text Request