Recovery of LiCoO₂ and graphite from spent lithium-ion batteries by Fenton reagent-assisted flotation

Highlights

• Flotation process was applied in the recycling of LiBs.

• Recovered electrode materials regained their original wettability by Fenton reaction.

• The mechanism of the improvement of floatability was presented.

• PVDF was oxidized and decomposed by Fenton reagent.

Abstract

In this paper, a Fenton reagent assisted flotation process is developed to recover valuable electrode materials LiCoO₂ and graphite from spent lithium-ion batteries (LiBs). At room temperature, effect of key parameters for Fenton reaction such as the ratios of H₂O₂/Fe²⁺ (40–280) and liquid-solid (25–100) are investigated to determine the most efficient conditions of surface modification of electrode materials by Fenton reagent. The modified electrode materials are separated by flotation operation to recover the cathode material and anode materials respectively. The results show that in the optimum conditions that the Fe²⁺/H₂O₂ ratio is 1:120, and the liquid-solid ratio is 75:1, most of the organic outer layer coated on the surface of electrode materials can be removed. After modified by Fenton reagent, the original wettability of LiCoO₂ and graphite is regained. The −0.25 mm crushed products of spent LiBs can be separated into LiCoO₂ concentrate and graphite concentrate by flotation process efficiently.

Introduction

Because of the advantages such as high energy density, high voltage, light weight, safe, etc., Lithium-ion batteries (LiBs) are widely used in modern electronic applications such as tablet computer, mobile phones, camera, laptops and other electronic devices (Joulié et al., 2014). LiBs have been chosen to provide power for electric automobiles, too (Hu et al.,, Hu et al., 2016). Therefore, the worldwide consumption of LiBs grows rapidly (Ordoñez et al., 2016). However, large amount of spent LiBs with hazardous materials and high value metals are produced annually (Zeng et al., 2015). In spent LiBs, contents of Cu, Al and Co were fairly high, their grades were 7.17 wt%, 17.62 wt% and 21.60 wt% respectively, even higher than those found in processing concentrates of natural ores, and also have a potential hazard to the ecosystem and human health (Zhang et al., 2014a, Zhang et al., 2014b). So, with the purpose for preventing environmental pollution and relieving the shortage of metal resources, recycling of the spent LiBs plays a significant role in the sustainable development of resources, environment and economy (Gratz et al., 2014, Meshram et al., 2015).

By now, there has been a lot of research achievements developed on the recycling technologies which can be divided into chemical processes and physical processes. The chemical processes are mainly including acid leaching (Joo et al., 2016a, Joo et al., 2016b, Li et al., 2015, Nayaka et al., 2016, Takacova et al., 2016), bioleaching (Horeh et al., 2016, Ijadi Bajestani et al., 2014, Xin et al., 2016, Zeng et al., 2012), solvent extraction (Jha et al., 2013, Joo et al., 2016a, Joo et al., 2016b), chemical precipitation (Huang et al., 2016, Weng et al., 2013) and electrochemical processes (Barbieri et al., 2014, Freitas et al., 2010, Freitas and Garcia, 2007). Mechanical separation processes are usually applied as a pretreatment to treat the outer cases and shells and to concentrate the metallic fraction, which will be conducted to a hydrometallurgical or a pyrometallurgical recycling process in recycling of spent LiBs (Barik et al., 2016, Xu et al., 2008). From the mechanical processing, it was possible to separate the materials as: Cu/Al (foil), Al (external casing), LiCoO₂ and graphite (mixed electrode materials), polymers (Bertuol et al., 2015). How to separate the mixed electrode materials is still a problem.

Spent LiBs are complexes of different materials with good selective crushing property (Zhang et al., 2013), after crushing, the electrode materials including LiCoO₂ and graphite are concentrated in the −0.25 mm size fraction, and the yield is as high as 56.21wt %. What's more, LiCoO₂ is hydrophilic for it is ionic crystal with strong polarity; however, graphite is hydrophobic for it is nonpolar. Therefore, LiCoO₂ and graphite have opposite surface wettability. In theory, flotation may be a useful method for the separation of LiCoO₂ and graphite. But, the LiCoO₂ and graphite cores are coated by organic compounds outer layer which are made of over 75% organic compounds, about 2% phosphates, 5% metal oxides and 6% metal fluorides (Zhang et al., 2014a, Zhang et al., 2014b). The LiCoO₂ and graphite cores were coated by organic compounds outer layer. With this organic outer layer, the recovered LiCoO₂ and graphite have the same wettability, to separate LiCoO₂ and graphite by flotation become impossible. Therefore, in order to recovery both LiCoO₂ and graphite by flotation, the organic outer layer must be removed.

In this paper, we reported a Fenton reagent assisted flotation process for recovery of electrode materials from spent lithium-ion batteries. Effect of the parameters for Fenton reaction such as the ratios of H₂O₂/Fe²⁺ and liquid-solid were investigated to determine the most efficient conditions for surface modification by Fenton reagent, and the reaction mechanism was discussed. Changes of the surface properties of the electrode materials were presented in detail by X-ray photoelectron spectroscopy (XPS). The mechanism of floatability improvement by Fenton reagent was discussed. And then, flotation was carried out to separate the electrode materials treated by Fenton reagent in the best condition to show the separation efficiency. This work presented a new method for mixed electrode materials’ separation.