A promising physical method for recovery of LiCoO₂ and graphite from spent lithium-ion batteries: Grinding flotation

Highlights

• Grinding flotation is creatively proposed for recovery of electrode materials.

• The optimized concentrate grade of LiCoO2 is 97.19% under 5 min grinding.

• The physical morphological and chemical properties of particles are presented.

• The mechanism of dry surface modification by mechanical grinding is revealed.

Abstract

Due to the limitation of secondary pollution and high equipment investment, the industrial-scale recycling technology for electrode materials from spent lithium-ion batteries (LIBs) needs urgent breakthrough. In this paper, a physical recycling method, grinding flotation, is creatively proposed for the separation and recovery of LiCoO₂ and graphite from spent LIBs. According to the exploratory experiments, if the mixed electrode materials is ground in the hardgrove machine for 5 min before reverse flotation, the concentrate grade of LiCoO₂ sinks and graphite floats can reach 97.13% and 73.56%, respectively. Moreover, with the help of advanced analytical technologies, the surface morphology, elemental chemical states and element distribution on the very surface of electrode particles before and after grinding were systematically analyzed to reveal the mechanism of dry surface modification. Results indicate that the mechanical grinding destroys the lamellar structure of graphite, exposing massive newborn hydrophobic surfaces. Meanwhile, the abrasion of organic film coating the LiCoO₂ particles causes its original hydrophilic surface partially regained. Hence, the great wettability difference between LiCoO₂ and graphite contributes to an excellent flotation separation. This grinding flotation method is a promising separation method without any toxic emissions or introducing other impurities in industrial application.

Introduction

The attractive electrochemical characteristics (e.g. high energy density, high battery voltage, low self-discharge and no memory effect) help lithium-ion batteries (LIBs) substitute nickel-metal hydride batteries as power source in portable electronic equipment and even electric vehicle [1], [2], [3], [4]. It is forecasted by Navigant Consulting, Inc. that in term of electric vehicle alone, the global demand for LIBs will reach $221 billion from 2015 to 2024 [5], [6]. However, due to the limitation of service life (the average life is 1000 cycles), a huge quantity of discarded LIBs have been generated annually [7]. For instance in China, there will be more than 25 billion units and 500 thousand ton LIBs into waste stream in 2020 [8]. Besides, the heavy metal and hazardous organic chemicals in these end-life LIBs will pose a serious threat to the ecological system and human health [9], [10]. On the other hand, spent LIBs contain a large amount of valuable metals, such as 21.60 wt% cobalt, 7.17 wt% copper, and 15.13 wt% aluminum [11]. Manufacturing the cathode materials with the cobalt and nickel from spent LIBs can save 45.3% fossil fuel, 51.3% nature ore, and 57.2% nuclear energy demand [12]. Therefore, on the basis of resource preservation and environmental protection, the sustainable recycling of high value-add components from spent LIBs is imperative and significant. Furthermore, the intrinsic material value held in 1 ton of spent LIBs is $7708 and the value of each component is as follows: cathode materials ($6101), graphite ($170), copper ($654), aluminum ($103) and others ($680) [13]. This indicates that positive and negative active materials account for 81.36% of the total value of the battery. Accordingly, separation and recovery of cathode and anode active materials can promise great economic benefits.

The recycling techniques of electrode materials (LiCoO₂ and graphite) from waste lithium-ion batteries are mainly divided into pyro-metallurgy [14], [15], hydrometallurgy [16], [17] and flotation [18], [19]. In recent years, the pyro-metallurgical technology has developed rapidly. Li et al. [20] applied oxygen-free roasting to in situ recycle of cobalt, graphite and lithium carbonate (Li₂CO₃) from mixed electrode materials. The recovery rate of cobalt, lithium and graphite reach 95.72%, 98.93% and 91.05%, respectively. But the immature technology and high-risk investment make it still at the stage of theoretical research. Hydrometallurgy has a long history of development and is widely used as a necessary method to extract noble metals with high purity [21]. Unfortunately, its industrialization scale is restricted as the shortcomings of slow dissolution rate, expensive solvent and secondary pollution. Flotation is a physical separation method based on the wettability difference of particle surface [22]. Cathode material (LiCoO₂) belongs to the ionic crystal with strong polarity and good hydrophilicity. In the flotation process, LiCoO₂ is completely wetted by water and then sinks into the bottom of the flotation cell. Anode material (graphite) is a nonpolar mineral and has a good hydrophobic behavior. In flotation, graphite is attached to the bubbles and then enters the foam layer of the flotation cell. Therefore, flotation method is theoretically achievable for separating and enriching LiCoO₂ and graphite. However, Li et al. [23] and Zhang et al. [24] find that both the surfaces of LiCoO₂ and graphite are covered with organic composite coatings, which are consisted of polyvinylidene fluoride (PVDF) binder, carbon black conductor and CC/CH structure. It is difficult for conventional flotation to separate LiCoO₂ and graphite because their similar surface properties lead to the same wettability. Jin et al. [25] report that the organic film on the particle surface could be removed by roasting for 2 h at 773 K, thereby effectively recovering the hydrophobicity difference and improving the flotation effect. Nevertheless, thermal treatment will make HF, P₂O₅, aldehydes and other toxic gases volatilized, resulting in serious environmental pollution [26]. Therefore, roasting modified flotation is hard to realize industrial-scale production. He et al. [18] propose that the flotation results could be improved after the pretreatment of Fenton modified method, which break the macromolecular material, such as PVDF, into small molecules and degrade organic impurities into H₂O and CO₂. Although the method of removing organic film is environmentally friendly, Fenton will introduce iron impurities [19], which decreases the economic value of LiCoO₂ concentrate. Hence, in order to realize the industrial-scale separation and recovery of the electrode materials, highly effective and environmental-friendly modification flotation method remains to be further explored.

In recent decades, considerable researchers have focused on grinding flotation based on mechanical attrition [27], [28], [29]. Rabieh et al. proposed that the removal of pyrite from refractory gold ores could be achieved by grinding flotation [30]. Bruckard et al. studied the effect of different grinding media on the flotation behavior of copper sulfide minerals [31]. Guern et al. presented the effects of different grinding methods on the floatability of PVC and PET plastics from waste plastic bottles [32], [33]. Xia et al. showed that a large amount of newborn surface of oxidized coal with good hydrophobicity could be obtained by grinding [34]. More importantly, the impact of external oxidative surface was weakened to improve the flotation results [35]. These researches have revealed that the mechanical attrition can not only expose the primary surface of the particles, but also enhance the original wettability of the materials without introducing impurities or releasing toxic substances. As far as graphite and LiCoO₂ are concerned, mechanical grinding does not have a functional damage to their structure. In the literatures, the crystallinity of graphite will not be destroyed if it is ground in a low-pressure attrition system with a short period of time [36]. The factual results are the separation of weakly bonded graphite layers and the generation of newborn hydrophobic surface. Correspondingly, cathode active materials could be renovated with some degree by grinding through mechanical energy transfer [37]. Therefore, it is feasible to modify the surface properties of electrode materials from spent LIBs by mechanical attrition.

In this paper, a promising modified flotation for recovery of electrode materials from spent LIBs with high purity is reported. It is the first time that grinding flotation is utilized to separate and recover the mixed electrode powder. Through laboratory experiments, the grinding time is investigated and the optimized results of grinding flotation are obtained. Furthermore, the surface properties of the particles, including surface morphology, elemental composition and element distribution, are systematically discussed before and after grinding with the help of advanced analytical techniques. Eventually, the mechanism of dry surface modification of electrode materials is revealed to establish theoretical and experimental basis for grinding flotation. Without environmental pollution or any impurity, grinding flotation technology has broad prospects for the industrialization.