Solvometallurgy for recovering metals from secondary resources

Metals are mainly recovered from primary resources such as ores, concentrates or from secondary resources such as waste residues. Solvometallurgy uses non-aqueous media for the recovery of valuable metals from these sources. Unit operations of a simplified solvometallurgical flowsheet are: 1) solvoleaching, in which metals from the solid material are transferred into a non-aqueous solution (i.e. lixiviant), and 2) non-aqueous solvent extraction (NASX) where metals are extracted from the non-aqueous solution with an organic phase and later on stripped from the loaded organic phase with an aqueous phase. Solvometallurgy is often considered as a greener alternative to hydrometallurgy, since it circumvents the generation of highly saline aqueous waste streams and reduces the emission of toxic or dangerous gasses. In this work, non-aqueous media (deep-eutectic solvents (DESs) and molecular organic solvents) are used for the recovery of valuable metals. A first case study consisted of the recovery of zinc, iron and lead from jarosite. Large amounts of jarosite waste are produced by the zinc industry and these are often stockpiled at the metallurgical facility, creating a high risk for environmental pollution. Processing of jarosite waste as secondary resource could reduce the pollution risk and avoid the loss of valuable metals. In this PhD thesis, zinc, iron and lead-containing DESs were used as a feed for a NASX process. DESs composed of choline chloride-ethylene glycol (ChCl:EG) and choline chloride-lactic acid were evaluated. Iron was selectively extracted by Cyanex 923 (a commercial mixture of alkylphosphine oxides) from ChCl:EG. Zinc was then extracted from the ChCl:EG-raffinate by Aliquat 336 (a commercial mixture of quaternary ammonium salts), while lead spontaneously precipitated from the DES. Iron and zinc were stripped by oxalic acid and aqueous ammonia solution, respectively. The NASX process was tested in continuous counter-current mode using mixer-settlers and mutual solubility studies were carried out. In a second case study, cobalt was recovered from lithium ion battery (LIB) cathode materials. These batteries are frequently used in electronic devices such as laptops and electrical vehicles. As the use of these devices is increasing, a supply risk of cobalt is expected. Hence, recycling of cobalt from LIBs as secondary resources becomes essential. In this case study, DESs were used to recover cobalt from lithium cobalt oxide (LiCoO2, LCO) in the presence of the current collectors aluminum and copper. Several DESs were evaluated to solvoleach cobalt, with choline chloride-citric acid (ChCl:CA) being the most effective. Optimization ensured quantitative solvoleaching of cobalt, copper and lithium; with copper reducing cobalt(III) from the LCO to the more soluble cobalt(II). Solvoleaching with ChCl:CA was more selective and greener than the traditional hydrochloric acid leaching, since it leached less aluminum and avoided the production of chlorine gas. ChCl:CA was then used as feed for NASX of copper and cobalt with LIX 984 (mixture of oximes and aldoximes) and Aliquat 336, respectively. Both metals were stripped with oxalic acid. As DESs showed stability issues during the investigations, the third part of this PhD thesis questions the use of DESs for the recovery of cobalt from LIBs at high temperatures. Special attention was paid to the mixture ChCl:EG since it is frequently reported to solvoleach LCO at 180 °C. Thermal analysis studies confirmed that ChCl:EG decomposes to toxic products such as 2-chloroethanol and trimethylamine. These observations pose serious doubts on the generally touted green nature of DESs. Finally, molecular organic solvents were used to leach cobalt from LCO in the presence of metallic aluminum and copper. Acidic extractants were tested on their ability to solvoleach cobalt, with di-(2-ethylhexyl)phosphoric acid (D2EHPA) being most efficient. D2EHPA successfully solvoleached cobalt, copper and lithium;, while copper was reducing again cobalt(III) to cobalt(II). Aluminum was left unaffected in the residue. Solvoleaching by D2EHPA was compared with conventional sulfuric acid leaching. Contrary to sulfuric acid, no hydrogen gas is produced and a higher selectivity is achieved. Copper, cobalt and lithium were here directly loaded into the organic D2EHPA phase and afterwards selectively stripped by sulfuric acid. This approached merged solvoleaching with solvent extraction into one single step and is therefore a form of process intensification.

Jaar van publicatie:2022

Toegankelijkheid:Open