Continuous process for selective metal extraction with an ionic liquid

Abstract

This work describes for the first time a continuous process for selective metal extraction with an ionic liquid (IL) at room temperature. The hydrophobic fatty acid based IL tetraoctylphosphonium oleate ([P8888][oleate]) was specifically chosen for its low viscosity and high selectivity towards transition metals. Applying [P8888][oleate] for continuous metal ion extraction with 0.1 M sodium oxalate for regeneration resulted in a process with good and stable extraction efficiencies over time. The selectivity of the IL resulted in a process in which cobalt was selectively removed from two mixed salt solutions (Co/Na, Ca/Co/K) to obtain a pure cobalt stream after stripping the IL. The performed experiments showed that the contact time of the IL for extraction and stripping strongly influenced the achieved efficiencies. The stability of the IL was tested and it was shown that the fatty acid based IL was stable for the duration of the experiment. Liposome tests showed that the IL is very hydrophobic, which limits its leakage towards the water phase, but also results in a higher toxicity towards cell membranes. Economic analysis shows that the IL based process is not (yet) economical compared to ion-exchange resins, in case demineralised water is the only product. However, if the recovery of valuable metals is also taken into account and/or if brine disposal is an issue, then continuous IL metal extraction systems must be regarded as promising alternatives.

Ionic Liquid-Based Process Development for Cobalt Recovery from Aqueous Streams

“…Tetraoctylphosphonium oleate was synthesized according to the procedure previously reported, albeit with some modifications [9]. At first, 0.319 mol (12.74 g) of NaOH was dissolved in 800 ml Milli-Q water before adding 0.231 mol (65.24 g) oleic acid.…”

“…After that, 0.177 mol (100 g) of [P 8888 + ][Br -] was added to the mixture to be stirred for 8 h at 75°C. Following Parmentier et al [9], an excess of oleic acid was applied to shift the equilibrium towards IL formation. The IL was washed nine times with, in total, 9 L of Milli Q-water using a separatory funnel in order to remove the impurities such as bromide and sodium.…”

“…However, the main disadvantage of LLX is the reliance of the use of toxic, flammable, water-miscible, volatile organic solvents (VOCs) such as gasoline, benzene, formaldehyde, toluene, chloroform and tetrachloroethylenes [8]. In order to improve the sustainability of the LLX process, hydrophobic ionic liquids (HILs) can be used instead since they offer an alternative using non-volatile extractants [9].…”

“…In earlier studies, using phosphonium and ammonium-based ionic liquids with the same functionalized anion oleate, sodium oxalate was used as a regeneration solution in both cyclic and continuous experiments. The regeneration efficiency was below 25% in the cyclic experiments (5 cycles) and below 80% in continuous mode (180 minutes) [9,18]. Huang et al also used sodium oxalate for the regeneration of their acid-base coupling bifunctional ionic liquid, applied for rare earth element separation [31].…”

Recovery of metals from waste electrical and electronic equipment (WEEE) using unconventional solvents based on ionic liquids

“…(Kozonoi & Ikeda, 2007). Regarding metals found in WEEE, these ILs were found to be efficient at extracting precious metals such as Au(III), Pt(IV) or Ir(IV) from acidic aqueous solutions containing large amounts of HCl (Génand-Pinaz et al, 2013;Papaiconomou et al, 2012Papaiconomou et al, & 2014Yang et al, 2014;Tong et al, 2015;Kubota et al, 2017 (Parmentier et al, 2015a) and was employed in a continuous selective metal extraction process for the recovery of cobalt from two mixed salt solutions (Co/Na, Ca/Co/K) (Parmentier et al, 2015b Figure 9. This process clearly shows the potential of ILs to recover metals from complex aqueous solutions using careful selection of the ILs and understanding their influence on the metal extracted complex (chloride vs. nitrate, etc).…”