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Abstract:

Rapid depletion of high-grade ores, stringent environmental regulations and global movement towards a circular economy has highlighted the importance of metal recovery from waste materials, including the residues generated by the metallurgical industry. Recovery of toxic and valuable metals from the industrial process residues is complex because the metals are often present in very low concentrations and often locked in complex matrices. Hence it is important to develop a process that can selectively recover the metal(s) of interest, while the undesired metals remain in the solid residue. In this PhD thesis, organic lixiviants were used to selectively recover valuable metals from industrial process residues. By selecting a suitable organic lixiviant, it is possible to attain high reactivity and selectivity towards the metal(s) of interest because non-hydrated anions have a great affinity to bind to certain metal ions. If required, water can be added to the organic lixiviant to avoid precipitation of the dissolved metal complexes because the high solvating power of water can keep them solubilized. If the lixiviants are composed mainly of organic solvents with no or very limited water, the process can be referred to as a solvometallurgical process. Solvometallurgy is a new branch of extractive metallurgy that uses non-aqueous solvents instead of aqueous solutions. The first part of this PhD thesis shows the development of novel solvometallurgical processes to selectively recover lead and zinc from jarosite of the zinc industry. Jarosite is a by-product of zinc hydrometallurgy plants. It contains iron, lead, zinc, and low concentrations of valuable metals such as indium, germanium and silver. Two processes were developed to recover valuable metals from the jarosite. In the first process, ionic liquids Aliquat 336 ([A336][Cl]) and Cyphos IL 101 ([C101][Cl]), equilibrated with 0.5 mol L–1 hydrochloric acid, were used to selectively leach lead and zinc from the iron-rich residue. The dissolved metals were recovered from the pregnant leach solution (PLS) by selective precipitation-stripping with an aqueous ammonia solution, and the ionic liquid was reused for leaching a new batch of jarosite. In the second process, concentrated methanesulfonic acid (MSA) was used to leach lead and zinc from jarosite. The solubilized lead and zinc remained dissolved in the PLS whereas the solubilized iron precipitated due to the low solubility of iron methanesulfonate salts in pure MSA. The dissolved metals were recovered by vacuum distillation and the MSA was successfully reused for three leaching cycles. In the second part of the PhD thesis, organic lixiviants were used to recover valuable metals from secondary lead smelter residues. Two processes were developed: one on iron-rich matte and slag, and the other on lead-rich dross. In the first process, EDTA was used as a lixiviant to recover lead from the matte and slag. These residues are composed mostly of iron and lead, with some amounts of tin, antimony, nickel and zinc. By using 0.05 mol L-1 EDTA solution in water, highly selective leaching of lead over iron was achieved: the lead was fully leached from the residues when contacted three times by a fresh EDTA solution, with minimal co-dissolution of iron. The dissolved lead was recovered by precipitation using ammonium sulfide, and the EDTA was successfully reused for leaching of fresh residues. In the second process, antimony was selectively leached from a lead-rich dross, using 2 mol L–1 hydrochloric acid in ethanol as a lixiviant. The antimony in the PLS was recovered by hydrolytic precipitation using water, producing a pure antimony(III) oxide chloride (Sb4O5Cl2). The ethanol in the remaining PLS was distilled to be reused for leaching of more drosses. In conclusion, leaching by organic lixiviants exhibited high selectivity towards the metal(s) of interest with minimal co-dissolution of matrix metals. The high selectivity of the organic lixiviant was due to the selective reactivity of the organic lixiviant towards the target mineral, or the selective solubility of the dissolved metal in the organic lixiviants. The high cost of using organic lixiviants was offset by recycling and reusing them several times.