Elsevier

Hydrometallurgy

Volume 205, November 2021, 105732
Hydrometallurgy

Processing of anode slime with deep eutectic solvents as a green leachant

https://doi.org/10.1016/j.hydromet.2021.105732Get rights and content

Highlights

  • A new green solvent (Deep Eutetic Solvents) were used to recover precious metals from anode slime.

  • Leaching conditions for maximizing gold recovery were optimized by Taguchi experimental design method.

  • Composition of deep eutectic solvent is the most effective process parameter for copper and silver recovery.

  • The leaching efficiency of the predicted and the confirmation experiment under optimum condition were compatible.

Abstract

This paper describes an efficient process of recovering valuable metals from a copper anode slime sample using green chemicals such as choline chloride (ChCl) based deep eutectic solvents (DESs). The leaching agents include ChCl-urea, ChCl-ethylene glycol, ChCl-urea-ethylene glycol and ChCl/urea-deionized water in a 1:2, 1:2, 1:0.5:1.5, and 1:1 ratio, respectively. Prior to leaching experiments, chemical and mineralogical characterization of copper anode slime was performed. Considerable amount of precious metals was detected in the copper anode slime by chemical analysis. The main phases of copper anode slime detected by XRD and SEM/EDX analysis were namely Cu2O, SnO2, and, PbSO4. After leaching experiments, 97% of copper was recovered from the copper anode slime using ChCl-urea under an optimum condition at 95 °C of reaction temperature, 4 h reaction duration and, 1/25 g/mL solid/liquid ratio. The results showed that 91% of silver was extracted from the anode slime under the optimum condition with ChCl-urea DES composition, 95 °C reaction temperature, 48 h reaction duration, and 1/10 solid/liquid ratio. Gold was not leachable in the experiment using ChCl based DES with hydrogen bonding reagents such as urea and ethylene glycol. Moreover, the results revealed that PbSO4 was very soluble in DES prepared by ChCl and urea.

Introduction

Copper anode slimes produced during electro-refining step of pyrometallurgical copper production are the most important secondary resources for precious metal recovery (Au, Ag, Se, Te, and platinum group metals) due to their chemical compositions (Singh Randhawa and Hait, 2020). The copper anode slimes contain significant amounts of precious metals, including several heavy metals (As, Cd, and Pb), which are considered hazardous wastes (Dong et al., 2020). Up to now, there have been several studies on the recovery of valuable metals, especially Cu, Au, Ag, Se, and Te or on the disposal of some heavy metals from the anode slime by cyanide, thiocyanate, acid, and alkaline leaching methods (Dönmez et al., 1998; Guo et al., 2017; Kholmogorov et al., 2002; Liu et al., 2014; Liu et al., 2020a; Shouming et al., 2003; Xiao et al., 2018; Xing and Lee, 2017). However, these leaching agents can be hazardous to environment due to their toxicity or corrosion effects. Alternatively, imidazolium-based ionic liquids were used for valuable metal recovery from the anode slime (Rüşen and Topçu, 2017a; Rüşen and Topçu, 2017b; Rüşen and Topçu, 2018). In these studies, optimum leaching conditions were determined by the Taguchi method for Cu, Au, and Ag with aqueous solutions of 1-butly-3-methyl imidazolium hydrogen sulphate (BmimHSO4) and 1-ethyl-methyl imidazolium hydrogen sulphate (EmimHSO4) ionic liquids. The results showed that almost 90% of copper recovery was achieved using 60% (v/v %) BmimHSO4 at 50 °C, and a high amount of gold recovery (97.3%) was obtained with 80% (v/v %) BmimHSO4 at 95 °C, although the silver recovery was very poor.

Unique chemical and physical properties of ionic liquids provide a suitable media for extractive-metallurgical processes, but their usage is restricted due to their poor biodegradability, toxicity, and high cost (Ramón and Guillena, 2019). In order to overcome adverse effects of imidazolium-based ionic liquids, a new type solvent, deep eutectic solvent (DES) was used as leaching agents (Abbott et al., 2003). The DESs are composed of two or more components, providing a low cost, poor toxicity, and high biocompatibility. As a subclass of ionic liquids, these new types of solvents can be formed by reacting with each other via hydrogen bond interaction to form a deep eutectic mixture (Abbott et al., 2003; Ramón and Guillena, 2019). As a raw material, choline chloride (ChCl) was generally used as an organic component to synthesize a eutectic mixture. Hydrogen bond donors such as urea, carboxylic acids, and polyols are cheap and safe materials used to prepare DES with ChCl (Cai and Qiu, 2019).

Currently, the solvating properties of DESs are of significant interest in many fields of technology. Especially, the studies on the solubility of various metal oxides in different DESs have opened a new window in the extractive metallurgy (Abbott et al., 2003). Researchers firstly focused on the novel solvent properties of choline chloride/urea mixtures. Then, this was followed by the solubility of different metal oxides such as CuO, Cu2O, PbO, ZnO, Fe2O3, and Fe3O4 in various DESs formed with ChCl. Thus, different hydrogen bond donors have been investigated in many studies (Abbott et al., 2004, Abbott et al., 2005, Abbott et al., 2006). Despite of its hazardous effect on environment and human health, DESs are expected to protect the water resources since they work in non-aqueous media. And, this process is defined as solvometallurgy by several researchers (Binnemans and Jones, 2017; Rodriguez Rodriguez et al., 2020). Recent studies have been performed on metal recovery with DES from various metal resources such as electric arc furnace (EAF) dust, zinc leach residue or lithium-ion batteries. The results of the studies on EAF dust indicated that zinc and lead could be selectively extracted from EAF dust using ChCl-urea up to 60% of Zn and 39% of Pb (Abbott et al., 2009; Bakkar, 2014). Additionally, Bakkar and Neubert (2019) showed that zinc could be efficiently extracted from cupola furnace dust (including about 30% ZnO) with a negligible amount of Fe dissolution using DES prepared by ChCl-urea-ethylene glycol. In this research, the electrodeposition process was successfully employed for zinc recovery from the leach solution. Rüşen and Topçu (2017c) managed to recover zinc and lead up to 55% and 47%, respectively, from the zinc leach residue using ChCl-urea as a leaching agent. Furthermore, Li and Co elements were recovered from end-of-life lithium-ion batteries with DESs considering as green agents (Peeters et al., 2020; Wang et al., 2020).

Despite the facts that DESs have been recently applied for the extraction of precious metals from a couple of secondary sources, no studies have been found to recover metals from copper anode slime. Therefore, the precious metal recovery (for Cu, Au, and Ag) from copper anode slime was first-ever investigated by using various deep eutectic solvents. In this paper, the optimum leaching conditions for metal recovery were determined by the Taguchi method. Moreover, detailed physical, chemical, and mineralogical characterizations of the copper anode slime were conducted.

Section snippets

Chemicals

Choline chloride (C5H14ClNO, >98% purity) was supplied from Merck (Darmstadt, Germany), urea (CH4N2O, >99%), and ethylene glycol (C2H6O2, >98%) was purchased from VWR Chemicals (Leuven, Belgium) to prepare deep eutectic solvents in different compositions. Hydrochloric acid (≥36.5–38%) was obtained from Sigma Aldrich (Steinheim, Germany) to make all the dilutions. Standard solutions of individual elements (1000 mg/L) for an inductively coupled plasma optical emission spectrometer (ICP-OES,

Characterization

The particle size distribution of the anode slime is shown in Table 2.

According to the sieve analysis, the particle size of >90% of the anode slime was under 33 μm. Therefore, the effect of grain size was not selected as a parameter for leaching process of anode slime.

Table 3 shows the chemical composition of copper anode slime indicating copper, tin and lead as the major elements. The results of chemical analysis revealed that the anode slime used in the leaching experiments contains a small

Conclusions

In this paper, Taguchi method was used to determine the optimum leaching conditions for recovery of precious metals from anode slime by using ChCl based deep eutectic solvents. The leaching parameters selected as DES composition, reaction temperature, reaction duration, and solid/liquid ratio were studied by following orthogonal experimental design L16(44). To maximize the copper recovery, DES composition: ChCl-urea, reaction temperature: 95 °C, reaction duration: 4 h, and solid/liquid ratio:

Declaration of Competing Interest

None.

Acknowledgment

This work is supported by TUBITAK (The Scientific and Technological Research Council of Turkey) under project number 116-M-057.

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