New Dissolved Nitrogen Predispersed Solvent Extraction Method, 2: Selectivity M. R. Tavakoli Mohammadi, S. M. Javad Koleini,* and M. Abdollahy Mineral Processing Department, Tarbiat Modares University, Tehran, Iran * S Supporting Information ABSTRACT: Dissolved Nitrogen Predispersed Solvent Extraction (DNPDSE) is a new method in which the two-phase mixture operation is based on bubble dispersion of the organic phase, instead of its drop dispersion into the aqueous phase. This substitution is intended to increase the two-phase contact area (to improve recovery of metal ions from the dilute solutions) and also to enhance the buoyancy force of organic phase (to improve separation of two phases). However, the performance mode of this method can increase the recovery of interfering elements, because of increased surface contact, entrainment, and precipitate flotation, and thus have an unfavorable effect on the selectivity of this method. Therefore, selectivity of this innovative method was compared with the conventional solvent extraction (SX) method. The results of experiments conducted on the dilute and dense copper, zinc, and iron synthetic solutions within the copper extraction pH values (1.2-2.1) showed a lack of zinc recovery (i.e., no effect on total selectivity) in both methods and increased iron recovery (i.e., effect on partial selectivity) only in the DNPDSE method. The results also showed that increased iron recovery was not just dependent on increased contact area of the two phases, and gel-like precipitates observed on the surface of organic phase indicated precipitation of Fe ions and their transfer onto the surface by bubble entrainment and precipitate flotation. 1. INTRODUCTION Solvent extraction (SX) is an extremely useful chemical process in hydrometallurgy with extensive expansion, good acceptance, and high efficiency for the recovery of most elements in the periodic table. Selectivity is one of the most important characteristics of this method. This feature effectively enables both the concentration processes (selective removal of metal) and the purification processes (selective removal of impurity). 1 Selectivity in this method indicates the capability of organic phase for selective transfer of metal ions from impure aqueous leach solution to pure electrolyte used in electrotwinning process. This feature can be affected by extraction chemistry, entrainment of aqueous leach solution in charged organic phase, and impurities transfer of leach solution as cured in charged organic phase and stripping circuit. 2 Dissolved Nitrogen Predispersed Solvent Extraction (DNPDSE) is a new method in SX field devised with the aim of improving the performance of equipments used in this field and increasing their operational power, especially for dilute solutions. Increased contact area of the two phases and enhanced buoyancy force of the organic phase were two important features attainable to achieve the mentioned objective. Since colloidal liquid aphrons (CLAs), colloidal gas aphrons (CGAs), and ordinary air bubbles were used to execute this method, it was expected that an increased extraction percentage of interfering elements adversely affects its selectivity, because of increased contact area, entrainment, and precipitate flotation. Therefore, this research is intended to compare the selectivity of SX and DNPDSE methods in dilute and dense solutions. To achieve this objective, synthetic aqueous dilute and dense solutions made from reagent-grade CuSO 4 · 5H 2 O, ZnSO 4 ·7H 2 O, and Fe 2 (SO 4 ) 3 were used. The selection of zinc and iron as the elements associated with copper was done according to to the extraction isotherms obtained for some elements, presented in the Results and Discussion section. In the DNPDSE method, a decreased concentration of metal ions in the aqueous phase was not just due to the extraction method, and the CLA dilution water was also effective in reducing its concentration; hence, it was impossible to determine distribution coefficient and separation factor to evaluate selectivity for this method. In addition, reduced concentration of iron in the aqueous phase was not merely due to the extraction and CLA dilution processes; precipitate flotation also occurred at pH values in which iron precipitated. Therefore, it was impossible to determine the iron concen- tration in the organic phase for these pH values. Therefore, the best way to compare the selectivity of SX and DNPDSE methods in dilute and dense solutions was via a comparison of the actual recovery diagrams for Cu, Fe, and Zn ions in various pHs; the results are presented in this paper. 2. EXPERIMENTAL WORK In the first part of this study, in which the performance of this new method has been compared with the conventional SX method, general principles and execution of SX and DNPDSE methods have been presented. Therefore, in this part, only reagents and solutions used are introduced. 2.1. Reagents and Solutions. Synthetic aqueous solution containing reagent-grade CuSO 4 ·5H 2 O (Merck, Germany), ZnSO 4 ·7H 2 O (Fluka, Switzerland), Fe 2 (SO 4 ) 3 (Merck, Ger- Received: January 12, 2012 Revised: February 15, 2013 Accepted: February 15, 2013 Published: February 15, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 3852 dx.doi.org/10.1021/ie300114u | Ind. Eng. Chem. Res. 2013, 52, 3852-3857