Since their commercial launch in 1991, lithium-ion batteries (LIBs) have become the dominant power source technology for a variety of electronic devices, from electric vehicles (EVs) to laptops, due to their superior electrochemical properties such as low self-discharge rate and high energy density [1
]. The prospected demand of LIBs is expected to grow annually by 25% from 180 GWh in 2018 to 2600 GWh in 2030 [2
]. The major driving factor for the growing demand is attributed to the transportation sector shifting to a low-emission fleet [2
]. Consequently, an increasing demand of LIBs sets pressure on both the upstream processes (e.g., mining and refining) to extract raw materials and manufacture components, and downstream processes (e.g., second life and recycling) to maximize the recovery of secondary raw materials. Depending on the vehicle model, the LIB in an EV can make up 40% of the total costs, making it the most valuable component [3
]. Therefore, the development of cost-efficient EVs is strongly focused on the value chain of batteries [3
]. This in turn advocates for integrating recyclability as a design feature in the development phase [4
The estimated lifetime of EV batteries is eight years during first life and five years for second life [5
]. In consumer electronics, the lifespan is generally less than three years [6
]. In 2035, 104 GWh of battery capacity is expected to reach end-of-life (EoL) [5
], thus making the relatively large amount of metal reserves in EoL batteries attractive for recycling [7
]. The benefits of recycling are multifold in terms of economics, regulatory perspective, securing raw material supply, new business creation opportunities [7
], and environmental protection. Developing cost-efficient technologies and processes for LIB recycling is thus a necessity.
Despite this, only a small portion of the EoL LIBs are currently properly collected [8
], while the vast majority are either hoarded in households, or end up in landfills [9
]. This not only raises environmental concerns but is also a waste of valuable resources [10
]. Additionally, the current EU Battery Directive 2006/66/EC in place does not sufficiently reflect the integration of the life cycle concept or the growing importance of LIBs as the recycling efficiencies are not defined for specific components or elements (e.g., Li and Co) [4
The major components of a LIB include its casing, separator membrane, electrolyte, current collectors, a polymeric binder, and the active materials found in the electrodes. The active material usually comprises of graphite (anode) and some of various lithium metal oxides (cathode), and the mixture of the two is commonly referred to as black mass in industrial jargon [13
]. The different components, relative amounts, and currently the most commonly applied materials were recently summarized by Velazquez-Martinez et al. [14
]. Due to cost savings, supply risks [15
], and increasing energy density requirements, cathode chemistries such as NMC 622 and 811, with less cobalt compared to NMC 111 or LCO chemistries, have entered the market. [7
] In waste LIBs, the black mass hosts the elements with the highest economic value, and recovering these elements has, consequently, been the main focus of the industrial LIB recycling operations. [13
] In the current state-of-the-art industrial LIB recycling processes, mechanical unit operations are first performed to recover the macroscopic components, and to separate the black mass fraction. The black mass is then further treated with either hydro- or pyrometallurgical processes (or a combination of both), and the valuables are recovered as either alloys or salts [16
The main goal of the initial mechanical processing is to produce a sufficiently pure stream of black mass for the subsequent chemical purification, while providing a high enough throughput to ensure the economic profitability of the process. Therefore, a trade-off exists between the throughput and the grade/recovery of the valuables [17
]. For example, it has been found that the different LIB components have distinctive particle size distributions after crushing, with the black mass consisting of considerably finer particles than the other components [17
]. Consequently, after crushing the waste LIBs, screening is usually applied to separate the black mass in the underflow, while retaining the majority of the coarser components (Cu, Al, plastics) in the overflow [14
]. However, due to incomplete liberation of the active particles [18
], in order to increase the black mass recovery, the industrially applied sieve opening sizes are relatively large (~500 μm) [14
]. This, however, results in quantifiable amounts of Al, Cu, and plastics reporting in the underflow. In addition, the Cu-Al rich sieve overflow has been shown to retain a considerable amount of the black mass [18
Generally, it is seen that one of the advantages of applying pyrometallurgy in battery recycling is to minimize mechanical pre-treatment [19
]. However, a relatively high fraction of metals is lost to the slag, which in some cases is further refined to recover metals or utilized by the construction industry [11
]. These include metals, such as Li and Mn, with high oxide stability at high temperatures [21
]. The industrial Umicore ValÉas™ and Sumitomo-Sony recycling concepts include pre-processing steps, such as dismantling and sorting, but no mechanical treatment [14
]. In laboratory scale, Ren et al. [22
] utilized two waste streams, namely (1) LIBs with Al cans and (2) copper slag from an industrial electric arc furnace cleaning process to produce an Fe-Co-Ni-Cu alloy and slag with fayalite (Fe2
) and hernycite (FeAl2
). Guoxing et al. [23
] proposed a smelting reduction process at 1475 °C based on a MnO-SiO2
slag system resulting in a Co-Ni-Cu-Fe alloy and manganese rich slag to recover valuable metals from LIBs.
The EoL LIBs will start gradually returning to metals production as their amount increases and the recycling efficiency improves. Currently, the volumes are rather low and the most feasible way to recycle them is to feed them to existing metals production processes instead of developing and building new production plants. In this approach, the waste LIB would be partially functioning as a chemical reagent, applied as a secondary feed for the industrial process, and ideally eliminating the need for new chemical reagents. Consequently, decreasing the loss of valuable metals to the slag must be researched by introducing suitable mechanical pre-treatment. However, at the same time, new flexible processes should also be developed as the chemistries and compositions of the LIBs (or other batteries) are continuously developing.
In the present investigation, the aim is to integrate existing unit operations (sieving, grinding, froth flotation) and a unit process (EF slag cleaning) for recovering Co and Ni from the black mass fraction of LIB scrap and replacing the conventional reducing agent coke with graphite. Figure 1
shows a flowchart of the proposed process investigated in this article. In previous studies by the authors, the integration of battery recycling to nickel slag cleaning was reported successful for the first time [24
]. Unlike these previous studies and many industrial pyrometallurgical processes for battery recycling, this approach emphasizes the importance of mechanical pre-treatment to minimize the loss of valuable metals to the slag. Coupling froth flotation to a pyrometallurgical unit process is introduced as a novel method for improving the LIB recycling efficiency, by allowing the selective recovery of leftover active materials from the Cu-Al rich sieve overflow, which is produced as a side stream during the initial mechanical processing of the waste LIBs. This approach is fundamentally different compared to the conventional black mass flotation studies [25
] that have targeted to separate the anodic graphite in the froth phase, and the cathode active components in the tailings. Furthermore, the recovered froth flotation products are utilized as the only source of reducing agent in the nickel slag cleaning process replacing the fossil coke.
According to the results, the integration of industrial nickel slag cleaning and LIB recycling was successful. This study presents a novel multifold approach in which:
The laboratory-scale flotation experiments show that when the black mass is not pretreated for the PVDF binder removal, both the anode and the cathode display a hydrophobic response and can be recovered in the froth phase. This allows the recovery of the left-over black mass from the Cu-Al rich sieve overflow in a purity comparable to the underflow of the sieve, thus increasing the overall black mass recovery of the initial mechanical processing. Low-energy milling was demonstrated to improve the black mass separation, and the optimal separation efficiency was achieved with 20-min milling time.
A proof-of-concept for the processing of industrial nickel slag using solely graphite obtained from the flotation of LIB waste as a reductant was reported for the first time. When the 0–1 min flotation fraction was mixed with industrial nickel slag in a 1:4 ratio, the metals deportment into the matte/alloy phase started immediately when the mixture reached the process temperature. The results indicated that the distribution coefficient of valuable metals reached maximum within 5 min reduction time.
The reason for a fast reduction of metals might be caused by the relatively small particle size of graphite and its even distribution in the mixture prior to melting. In conventional nickel slag cleaning, the coke is charged on top of the slag as the coke is lighter than the slag, resulting in the top parts of the slag to be reduced first. In practice, various measures are taken for improving phase contact and slag mixing in the electric furnace in order to enhance reduction of ferric oxide to ferrous after which the valuable metals can start to reduce. Instead, in this approach, the reduction appears to proceed quite evenly in the molten slag. Thus, further analysis on the optimum charging procedure e.g., whether the reductant should be charged mixed, on the bottom or on the top of the slag should be conducted. It should be noted, that in industrial slag cleaning, the slag is tapped molten from the nickel smelting furnace and in this approach, it was not the case.
For optimizing the process parameters further, the ratio of Ni-slag and battery scrap is of importance as the reductant concentration has a great impact on the forming metal alloy and its metal concentration. Therefore, mixing varying flotation fractions with the nickel slag should be studied further. Additionally, the elemental concentrations in the metal alloy should be optimized keeping in mind the further refining steps. This study suggests preliminarily that any excess carbon available after the reduction of Co, Ni, and Cu will increase the concentration of iron in the metal alloy. Due to thermodynamic constraints, it is not possible to avoid the co-reduction of iron when recovering valuable metals. However, for further refining requirements, the possibility of decreasing Fe concentration in metal alloy should be investigated. A thorough comparison with graphite and other reductants, such as coke, biochar, and methane, used in nickel slag cleaning should be conducted as well.