Investigation into the modes of action of extractants for base metal cations and metalate anions

Abstract

This thesis involves the design and development of reagents for the recovery of base metals (specifically zinc, nickel and cobalt) in hydrometallurgical solvent extraction processes. The work aims to demonstrate how ligand design can affectively tune the strength and selectivity of extractants to achieve efficient recovery of the desired base metals. Chapter 1 reviews current solvent extraction processes used in extractive metallurgy, encompassing both the well established technologies developed for sulfate streams as well as those more recently explored for treating chloride streams. Also reviewed, is the nature of the chemical binding involved in the three key modes of extraction; namely cation transport, anion transport and metal salt transport. Chapter 2 summarises the methodologies that have been established during this research for the appropriate testing of these reagents. Chapter 3 deals exclusively with the processing of zinc sulfide ores with an aim to design reagents to achieve concentration and separation of zinc in chloride hydrometallurgical circuits. The amido functionalised reagents that are reported have a common structural feature with ligands that have been previously studied by the Tasker group (Ross J. Ellis Thesis, University of Edinburgh, 2009). A six membered chelate ring is formed by a protonated amino nitrogen atom and an amido oxygen atom in a sequence of the type R2HN+-CH2-NR-CO-R. This differs from those previously studied which have a sequence of the type R2HN+-CH2-CHR-CO-NR2. The pro-ligands (L) operate via an anion exchange mechanism (Equation 1) whereby two protonated ligands (LH+) coordinate to the outersphere of anionic zinc(II) or iron(III) chloridometalates from acid chloride solutions using both N-H and C-H hydrogen-bond donors. pH dependent solvent extraction experiments have concluded that this reagent series achieves zinc(II) loading with pH0.5 values that are competitive with the previous ligand series (Ross J. Ellis Thesis, University of Edinburgh, 2009). Chloride concentration dependent solvent extraction experiments have demonstrated that the reagents show an unusually good selectivity for ZnCl4 2- over chloride or FeCl4 - in equilibrium of the type; yLorg + yH+ +MClx y- ⇌ [(LH)yMClx](org) (1) The development of bidentate and tridentate pyrazolone-based pro-ligands for the extraction of nickel and cobalt from mixed metal sulfate streams is considered in Chapters 4 and 5. These reagents (LH) operate via metal cation transport, where an inner-sphere complex of nickel(II) or cobalt(II) is formed with the ligand (L-) see Equation 2. A combination of N, O and S donors has been incorporated into 1-phenyl-3-methyl-4- acylpyrazol-5-ones and their respective 4-acylpyrazolone oximes in order to tune the bidentate ligands (L-) for optimal coordination with nickel(II) or cobalt(II). Substituent effects have also been investigated, by synthesising a series of 1-(2-X-phenyl)-3-methyl-4- acylpyrazol-5-one oximes [X = Cl, H]. Substitution in the 3-position of the phenol group in phenolic oximes has been reported to increase extractant strength for copper by two orders of magnitude (Ross S. Forgan Thesis, University of Edinburgh, 2008). Similar improvements were not observed in this study. The nature of this effect has been attributed to buttressing of hydrogen-bonds, where the substituent forms a stabilising, bifurcated hydrogen-bond between the oximic hydrogen and the pyrazolonic oxygen. yLHorg + My+ ⇌ [(L)yM]org + yH+ (2) Tridentate analogues of the oxime reagents above have been prepared as imines derived from anilines contained o-O, S or N donor atoms. It was hoped that these would give high spin octahedral nickel(II) complexes in extraction processes. They proved to be weak extractants. Chapter 6 focuses of the development of bidentate pyrazolethiones for the selective extraction of cobalt from manganese in acidic sulfate streams. These reagents have been designed to favour coordination to metals in a tetrahedral geometry as shown by L. Emeleus and A. Smith for copper and zinc (Lucy Emeleus Thesis, University of Edinburgh 1999 and Andrew Smith Thesis, University of Edinburgh 2000).