Study On The Extraction Behavior And Microscopic Mechanism Of Copper(â…¡), Nickel(â…¡) And Zinc(â…¡) Ions In Ammoniacal Solution

The development of new technologies used for high efficient extraction of non-traditional ores, which include low grade ore, tailings, and drosses etc., is an urgent target in order to solve the shortage problem of strategic non-ferrous metal resources in china. Among many metallurgical methods, the "ammoniacal leaching—solvent extraction—acidic electrowinning"(AL-SX-AE) is one of the most promising technologies used to treat the low-grade complex oxide minerals. And solvent extraction is the most key process in this technology. Therefore, the detailed insights into the extraction mechanism are essential to optimize extractant formula and design the extraction process.In this paper, the home-made β-diketone extractant,1-phenyl-4-ethyl-1,3-octanedione (denote as HA), was used to extract copper(II), nickel(II) and zinc(II) from ammonia-ammoniacal sulfate solution. The distribution behavior of water and ammonia in the organic phase has been studied in the absence and presence of metal ions. The coordination structure of the species in both aqueous and organic phases was characterized by using of UV-Vis spectroscopy, FT-IR spectroscopy, X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The extraction mechanism has been elucidated in view of the extraction equilibrium and structural characteristics of species in both phases. The obtained data are helpful for the development and application of "AL-SX-AE" technology.1. The extraction behavior and microscopic mechanism of Cu(II) in ammonia-ammoniacal sulfate solution have been investigated.It was found that the1-phenyl-4-ethyl-1,3-octanedione is a superior extractant for Cu(II) in ammoniacal solution. The formation of copper ammonia complexes inhibits the extraction reaction of Cu(II), resulting in that the extraction efficiency decreases sharply at pH>8.5. Water molecules can not be coordinated with copper extracts in the organic phase, but a small amount of ammonia molecules can be co-extracted into organic phase with copper extracts. The extracted copper complexes (CuA2) are four coordinate structure with a square planar configuration, which is independent of the aqueous pH. However, as the pH increases, the coordination structure of copper ions in ammoniacal solution can be gradually transformed from a six-coordinated octahedral geometry into a distorted square planar configuration, which is the essential reason why the extraction efficiency of copper(Ⅱ) decreases.2. The extraction behavior and microscopic mechanism of nickel(Ⅱ) in ammonia-ammoniacal sulfate solution have been investigated.It was found that the extraction behavior of nickel(Ⅱ) is strongly dependent on the aqueous pH. When the pH is lower than8.5, the extraction efficiency of nickel(Ⅱ) increases with the increase of pH, but decreases at8.5<pH<9.5, then dramatically increases again when pH is larger than9.5. Water and ammonia molecules can be co-extracted with nickel extracts into organic phase to form the hydrated and ammonia-coordinated nickel extracts, thus depressing the distribution of nickel complexes in a non-polar hydrocarbon solvent because of their higher hydrophilicity. Moreover, the green nickel extracts precipitate at pH>9.5, which should be the essential reason why the extraction efficiency of nickel dramatically increases when pH is larger than9.5. This extraction behavior of Ni(Ⅱ) is unfavorable for the industrial practical application. The composition of soild nickel extracts is NiA2·H2O·NH3and the coordination structure is identified with an octahedral configuration. The structure of nickel extracts in organic phase is identical to that of the solid extracts. In aqueous phase, ammonia molecule will substitute successive-ly water molecule of Ni(Ⅱ) to form more stable nickel ammonia species with an octahedral configuration, thus inhabiting the extraction reaction of Ni(Ⅱ). Especially, the formation of Ni(NH3)52+and Ni(NH3)62+decreases the extraction efficiency of nickel significantly.3. The extraction behavior and microscopic mechanism of zinc(Ⅱ) in ammonia-ammoniacal sulfate solution have been investigated.It was found that the extraction equilibrium of zinc is very sensitive to the aqueous pH. Especially, the extraction efficiency of zinc decreases sharply when pH is larger than7.35. Water and ammonia molecules can be co-extracted into organic phase by coordinated with zinc extracts. Thus, several equilibrium species could co-exist in the organic phase, i.e, ZnA2, hydrated ZnA2and ammonia-solvated ZnA2. The resulting hydrated and ammonia-coordinated zinc extracts have higher hydrophilicity, thereby depressing the distribution of the extracted zinc complexes in a non-polar hydrocarbon solvent. The coordination structure of ZnA2is identified with a tetrahedral geometry. The hydrated ZnA2and ammonia-coordinated ZnA2are penta-coordinate structure. In the ammoniacal solution, as the pH increases, ammonia molecule will substitute successively water molecule of zinc ion to form more stable zinc ammonia species. The coordination structure of zinc ions can be gradually transformed from a six-coordinated octahedral geometry into a four-coordinated tetrahedral configuration, thus the extraction efficiency of zinc will be sharply decreased at pH>7.35.4. The solvent effect on the extraction of zinc(Ⅱ) in ammoniacal solution has been investigated with n-octanol, toluene, and nonane as solvents.It was found that the increase of polarity of the solvent can promote the extraction reaction of zinc(Ⅱ) in ammoniacal solution. However, the extraction efficiency of zinc in octanol system still decreases sharply at pH>7.3. And the concentration of water and ammonia in octanol system dramatically increases with the increase of zinc concentration. The strong solvent effect in octanol system could be attributed to the role of hydrogen bonding between zinc extracts and octanol molecules, and the distribution of hydrated ZnA2and ammonia-solvated ZnA2into polar solvent can be improved. The zinc extract is a penta-coordinate structure.5. The synergistic effect on the extraction of zinc(Ⅱ) in ammoniacal solution has been investigated with tributyl phosphate (TBP), trioctyl-phosphine oxide (TOPO) and tributylphosphane (TBuP) as synergists.It was found that the addition of neutral phosphorus-containing ligands (denote as B) can significantly promote the extraction reaction of zinc(Ⅱ) in ammoniacal solution. And the extraction performance is stable at7.26<pH<8.38for both TOPO and TBuP systems. Water and ammonia can not be extracted into organic phase with zinc extracts. The strong synergistic effect could be attributed to the increase of stability and hydrophobicity of zinc extracts, because the neutral ligands can replace water and ammonia molecules in hydrated ZnA2and ammonia-solvated ZnA2to form stable penta-coordinated synergistic adducts.6. The extraction behavior of zinc(Ⅱ) from ammoniacal solution into four hydrophobic ionic liquids ([OMIM]PF6,[BMIM]PF6,[BMIM]NTf2and [OMIM]NTf2) was investigated with HA as the extractant.It was found that hydrophobic ILs combined with HA can be used to extract zinc(Ⅱ) from ammoniacal solutions. The extraction behavior of zinc(Ⅱ) is dependent on the zinc species in ammoniacal solutions and the hydrophobicity of ILs. The extraction efficiency of zinc decreases in the order of [BMIM]PF6>[BMIM]NTf2>[OMIM]PF6>[OMIM]NTf2. The zinc extract in four ionic liquids is mainly the penta-coordinated structure, and the average coordination number of the zinc extracts decreases with the increase of the hydrophobicity of the ILs. The good extraction performance of [BMIM]PF6system can be attributed to the interaction between zinc extracts with ionic components of the ionic liquid.