001.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 83, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš Copyright © 2021, AIDIC Servizi S.r.l. ISBN 978-88-95608-81-5; ISSN 2283-9216 Environmentally Friendly Suspension Electrolysis Technology for Regenerating Lithium Cobalt Oxide Jianbo Wang, Juan Lv, Miqi Tang, Qi Lu, Qingyun Nie, Yan Lu*, Bo Yu Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education, Sichuan Engineering Lab of Non- metallic Mineral Powder Modification and High-value Utilization, and School of Environment and Resource, Southwest University of Science and Technology, 59 Qinglong Road, Mianyang, Sichuan 621010, People’s Republic of China luyan@swust.edu.cn The extensive usage of portable electronic products and the acceleration of their replacement have led to the discarding of a growing amount of LiCoO2 batteries. It is necessary to recycle the batteries from the both viewpoints of environment and economic benefits. Current recycling approaches for LiCoO2 are dominantly based on hydrometallurgy and pyrometallurgy, which usually require multiple complicated steps and involve the use of high temperature or harmful chemicals, like acids and alkalis. There remains an urgent need for a green and simple process. In this paper, suspension electrolysis technology is proposed to recycle spent batteries, in order to achieve the leaching, purification and regeneration of LiCoO2 in one step. The most advantageous of the present technology are that reacting in one reactor at atmospheric condition and no use of highly corrosive and harmful reagents. The reaction of LiCoO2 leaching in the anode region and simultaneously re-synthesizing in the cathode region in ammonia electrochemical system is established for the first time. Three chemical reaction systems of NH4HCO3, NH4HCO3–NH3, and NH4HCO3–(NH4)2SO3, are explored. The results of the laboratory scale experiments show that LiCoO2 can be regenerated in NH4HCO3– (NH4)2SO3 reaction system. Under the investigated conditions of this study, 66.3 % of Li and 75.4 % of Co were leached out in the anode region and 53.2 % of LiCoO2 was regenerated in the cathode region under the conditions of 0.5 mol/L (NH4)2SO3 concentration, 2.5 mol/L NH4HCO3 concentration, 1.5 A current, 2 g/L solid to liquid ratio, 50 °C, and 1.5 h. The reaction system for realizing LiCoO2 leaching in the anode region and simultaneously re-synthesizing in the cathode region is established. This work provides a promising method for recovering LiCoO2 efficiently and environmentally friendly, promoting the sustainable utilization of the resources. 1. Introduction Lithium-ion battery (LIB) is acting a more and more important role in the development of electronic industry (Yang et al., 2017). LiCoO2 is the most widely used cathode active material in portable electronic products (Santana et al., 2017), such as mobile phones, cameras, notebooks , since it was first commercialized by SONY in 1991 (Xiao et al., 2020) due to its competitive advantages of high specific energy, small self- discharge, excellent cycle life and high electronic conductivity (Chen et al., 2015). The increasing demand and the rapid replacement of the electronics have led to the continuous discard of LIBs in considerable quantities (Li et al., 2013). The dispose and recycling of LIBs is getting more and more attention from governments, companies, and researchers worldwide (Lv et al., 2018). LIBs contain large amounts of valuable elements, especially Li and Co, indicating that LIBs are worth recycling from the viewpoints of economic benefits (Gu et al., 2017). LIBs also contain a lot of harmful substances, such as heavy metals and toxic electrolytes (Zeng et al., 2015). Once the LIBs cannot be handled properly, the above harmful substances will leak out into the environment, causing great harm to the environment and human health (Jha et al., 2013). Cobalt has been defined as strategic metal by many countries, such as China, European Union, and South Korea, etc (Zeng and Li, 2015). The demand for cobalt continues to grow (Li et al., 2017), although the natural deposits of cobalt are increasingly being exhausted (Zeng et al., 2020). The continuous supply of resources is a problem facing the world that must be resolved in order to guarantee the DOI: 10.3303/CET2183016 Paper Received: 29/06/2020; Revised: 29/08/2020; Accepted: 31/08/2020 Please cite this article as: Wang J., Lv J., Tang M., Lu Q., Nie Q., Lu Y., Yu B., 2021, Environmentally Friendly Suspension Electrolysis Technology for Regenerating Lithium Cobalt Oxide, Chemical Engineering Transactions, 83, 91-96 DOI:10.3303/CET2183016 91 sustainable development of electronic industry. Regarding spent LIBs as secondary resources for recycling is becoming a worldwide consensus to prevent pollution, conserve mineral resource, and sustainably utilize resource (Zeng and Li, 2016). A great number of studies have been carried out focusing on the recovery of lithium and cobalt from spent LIBs (Wang et al., 2016). Several methods, which are mainly based on pyrometallurgy (Xiao et al., 2017) and hydrometallurgy (Li et al., 2009), have been developed. Pyrometallurgical processes have advantage like less additive usage and less loss of valuable elements (Li, 2016). Hazardous gas emission, particulate matter release, and high energy consumption are the main limitations of pyrometallurgical process applications (Santana et al., 2017). Hydrometallurgical process includes the following steps: (1) leaching by acid (Huang et al., 2016) or alkali (Ku et al., 2016) to total release of metal elements, such as Li, Co, Al, Cu, etc. (2) selective recycling different metal elements according to their unique physicochemical properties by fractional chemical precipitation (Nan et al. 2005) or multistage organic solvent extraction (Provazi et al., 2011) or electrochemistry (Freitas and Carcia, 2007). Although the hydrometallurgical method is more favoured and widely applied due to its advantages of high recovery efficiency, high purity of recovered product, and low energy consumption (Joulie et al., 2017), there are also several drawbacks, such as long process, large dosage of chemicals, and long period (Chen et al, 2017). A greener and shorter process is still urgently needed. Sulfuric acid (Zou et al., 2013), hydrochloric acid (Li et al., 2009), and nitric acid (Li et al., 2011) are the most commonly used reagents for leaching Li and Co due to their natures of high efficiency, strong adaptability to the complex system, and mature operations (Qi et al., 2020). These inorganic acid solutions produce leachates of low pH, posing a threat to the environment and making them unfavourable for subsequent waste water treatment (Zheng et al., 2017). Ammoniacal agents were investigated for leaching Co from cathode active materials. The results showed that at least 94 % of Co can be selectively leached out into the solution, achieving separation from metals such as Mn and Al (Wang et al., 2017). Electrochemical technologies have also been investigated for metals recovery from spent LIBs. Prabaharan (2017) developed an electrochemical process to recycle Co from cathode active materials. The leaching and deposition of Co were processed in two different electrolytic cells, and sulphuric acid was used as electrolyte in the former process. Above 96 % of Co was recovered. Li (2011) developed a process to electrodeposit lithium cobalt oxides directly from the nitric acid leaching solution of lithium cobalt oxides. The result pointed out that the alkaline environment is a necessary condition to achieve the above reaction. An idea is proposed in the present work that can the leaching, purification and regeneration of lithium cobalt oxide be achieved in one reactor in an electrochemical system? A suspension electrolysis technology is put forward to testify the above hypothesis. The composition of the electrolyte that can realize the above reaction was investigated and selected. The factors that would affect the leaching and re-synthesis of lithium cobalt oxide during the electrolysis process were examined in detail. The as-synthesized products were analyzed. The objective of this article is to provide a perspective approach for recovering Li and Co from spent lithium cobalt oxide in an environmentally friendly, easy operation, and cost-effective way. 2. Experimental 2.1 Materials and chemicals The core of the proposed technology for recycling spent LIBs is the regeneration of lithium cobalt oxide. Whether lithium cobalt oxide can be regenerated determines the effectiveness of this technology for the recovery of spent lithium ion batteries. Pure lithium cobalt oxide (6.74 % of Li concentration and 57.2 % of Co concentration, Aladdin) was used as representative of spent cathode active material to avoid the influence of impurities to verify the feasibility of this technology. Chemical reagents used in the experiments were all analytical reagents unless otherwise mentioned. 2.2 Apparatus Electrochemical experiments were conducted in a custom-made rectangular cell (length × width × height = 15 × 7 × 6 cm) made of Teflon. The cell was divided into the anode zone and the cathode zone by an installed acrylic fabric. The anode was graphite plate (7 × 6 × 0.3 cm) and the cathode was platinum-plated titanium base electrode plate (7 × 6 × 0.3 cm). The electrodes were connected to a DC constant current source (R- 3005, Kingrang, China) through a copper wire. For each run, 250 mL electrolyte was added into the cell, and then heated to a desired temperature. A proper mass of pure lithium cobalt oxide was added into the anode zone of the cell. A proper current was introduced for a certain time. The deposited product in the cathode region of the cell was separately collected by filtering through a 0.45-μm membrane filter, washed with distilled water for several times to remove impurities, and dried at 105 °C for 12 h. 92 2.3 Analytical techniques X-ray diffraction (XRD, X’Pert Pro, PANalytical, Netherlands) was used to identify the phase of the regenerated product. Scanning electron microscopy (SEM, SUPRA 55, Zeiss) was used to examine the morphology of the regenerated product. Field-emission transmission electron microscope (TEM, Tecnai G2F20 S-Twin, FEI, U.S.) was used to detect the selected area electron diffraction (SAED) of the regenerated product. The metallic elemental concentrations of the regenerated product were analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 106 7700, Thermo, U.S.) after being digested by HNO3 method. 3. Results and discussion 3.1 Regeneration performance of lithium cobalt oxide in NH4HCO3 reaction system Generally, LiCoO2 must be dissolved by the ammoniacal method in the presence of a reducing agent (Zheng et al., 2017). This study attempts to verify whether the electric field can strengthen the complex of Co and NH3, reducing the use of reducing agents. With the principle of using as few and environmentally friendly reagents as possible, NH4HCO3 was selected in this work as electrolyte due to two reasons: (1) NH3, which could complex with Co3+ to form Co(NH3)n3+ (Zheng et al., 2017), promoting the dissolve of LiCoO2, can be provided via hydrolysis reaction of NH4+ (Qi et al., 2020); (2) NH4HCO3 can act a pH buffer (Zheng et al., 2017) so that the pH of leaching solution changes little during leaching. Figure 1a shows the effect of current on the regeneration performance of LiCoO2 under the conditions of 2.5 mol/L NH4HCO3 concentration, 6 g/L solid to liquid ratio, 50 °C, and 3 h. Figure 1b shows the effect of NH4HCO3 concentration on the regeneration performance of LiCoO2 under the conditions of 1.5 A current, 6 g/L solid to liquid ratio, 50 °C, and 3 h. Figure 1c shows the effect of temperature on the regeneration performance of LiCoO2 under the conditions of 1.5 A current, 2.5 mol/L NH4HCO3 concentration, 6 g/L solid to liquid ratio, and 3 h. Figure 1d shows the effect of solid to liquid ratio on the regeneration performance of LiCoO2 under the conditions of 1.5 A current, 2.5 mol/L NH4HCO3 concentration, 50 °C, and 3 h. Figure 1e shows the effect of holding time on the regeneration performance of LiCoO2 under the conditions of 1.5 A current, 2.5 mol/L NH4HCO3 concentration, 6 g/L solid to liquid ratio , and 50 °C. Figure 1: Effects of (a) current, (b) NH4HCO3 concentration, (c) temperature, (d) solid to liquid ratio, and (e) time on leaching rates of Li and Co and regeneration rate of LiCoO2. From Figure 1, three conclusions could be concluded: (1) the dissolving of LiCoO2 in the electrolytic system was occurred, indicating that the electric field can strengthen the complex of Co and NH3; (2) the leaching rate of Li are overall higher than the leaching rate of Co, indicating that the dissolving of LiCoO2 in the electrolytic system was a step-by-step process; (3) the leaching rates of Li (<9 %) and Co (<2%) were both low; (4) the regeneration rate of LiCoO2 is 0, indicating that LiCoO2 is cannot be synthesised in the above reaction system. The improvement of the composition of the electrolyte for the suspension electrolysis technology to regenerate LiCoO2 is needed. 93 3.2 Regeneration performance of lithium cobalt oxide in NH4HCO3-NH3 reaction system The above experiments demonstrated that in NH4HCO3 electrochemical system, the dissolution of LiCoO2 was thermodynamically feasible but the reaction kinetic was extremely low, which is consistent with the results of LiCoO2 leaching with ammoniacal agents under non-electrochemical system (Zheng et al., 2017), and LiCoO2 was failed to be re-synthesised. The efficient dissolution of Li and Co is the necessary prerequisite for the re- synthesis of LiCoO2. NH3 acts an important role for dissolving LiCoO2 because Co is mainly dissolved in the form of complex with NH3 in this process (Zheng et al., 2017). Previous study (Ku et al., 2016) pointed out that the increase of NH3 concentration can significantly promote the dissolve of LiCoO2. More NH3 in the solution can no longer be obtained by increasing the concentration of NH4HCO3 because 2.5 mol/L is close to the saturated solubility of NH4HCO3. Ammonia water was added to investigate the influence of NH3 concentration on the regeneration performance of LiCoO2 in the present electrolysis system. Table 1 shows the effect of ammonia water concentration on the regeneration performance of LiCoO2 under the conditions of 2.5 mol/L NH4HCO3 concentration, 1.5 A current, 2 g/L solid to liquid ratio, 50 °C, and 3 h. The leaching rates of Li and Co were unaffected by changes in the ammonia concentration from 0.5 to 3 mol/L. LiCoO2 was still failed to be re-synthesised in the cathode region by the addition of ammonia water during the electrolysis process. The leaching rate of Li and Co was even lower when ammonia water was added than when ammonia water was not added, indicating that excessive NH3 would negatively affect the leaching of LiCoO2. The above tests demonstrated that the concentration of NH3 is not the key for LiCoO2 dissolving in the electrochemical process. The composition of the electrolyte for the suspension electrolysis technology to regenerate LiCoO2 is still needed to be improved. Table 1: Influence of ammonia water concentration on the regeneration performance of LiCoO2 Ammonia water concentration (mol/L) Li leaching rate (%) Co leaching rate (%) LiCoO2 regeneration rate (%) 0.5 4.0 0.7 0 1 3.6 0.5 0 3 3.3 0.6 0 3.3 Regeneration performance of lithium cobalt oxide in NH4HCO3-(NH4)2SO3 reaction system According to a study by Zheng et al. (2017), Co(NH3)63+, Co(NH3)52+, Co(NH3)42+ are the major soluble species in the solution over the pH range of 9–11 in the Co-NH3 -H2O system. Co is easier to be dissolved from LiCoO2 by complex with NH3 in lower valence. Co3+ was usually reduced to Co2+ by adding reducing agent to promote the dissolution of LiCoO2 in chemical leaching processes. In this study, (NH4)2SO3 was selected as the reducing agent to avoid the introduction of metal impurities to investigate whether it would contribute to the leaching and re-synthesis of LiCoO2 during the present suspension electrolysis process. Table 2 presents the effect of (NH4)2SO3 concentration on the regeneration performance of LiCoO2 under the conditions of 2.5 mol/L NH4HCO3 concentration, 1.5 A current, 2 g/L solid to liquid ratio, 50 °C, and 1.5 h. The addition of (NH4)2SO3 significantly promoted the leaching of LiCoO2. LiCoO2 was successfully re-synthesised in the cathode region, as shown in Figure 2c. The results also showed that higher (NH4)2SO3 concentration could promote the leaching of LiCoO2 in the anode region and the re-synthesis of LiCoO2 in the cathode region, while excessive (NH4)2SO3 hindered the regeneration of LiCoO2. The above results proved that LiCoO2 can be regenerated in one reactor by suspension electrolysis technology, and the reaction system was established for the first time. The detailed investigation of the factors which might affect LiCoO2 regeneration performance will be conducted in the next study. Table 2: Influence of (NH3)2SO3 concentration on the regeneration performance of LiCoO2 (NH4)2SO3 concentration (mol/L) Li leaching rate (%) Co leaching rate (%) LiCoO2 regeneration rate (%) 0.1 55.4 64.1 42.2 0.5 66.3 75.4 53.2 1 61.5 68.9 51.3 Figure 2a shows that Co was leached out in the form of Co(NH3)63+ (yellow) in the NH4HCO3-(NH4)2SO3 electrolysis process. Figure 2e and 2f show that the regenerated LiCoO2 is different from the raw material LiCoO2 in molecular structure. Figure 2g shows the pH value changes of the electrolyte in the anode region and cathode region under different (NH4)2SO3 concentration with the increase of time. The electrochemical characteristics of the regenerated LiCoO2 will be studied in the future work. This work only focused on the core cause of recycling Li and Co from spent LIBs, which is potential to instead the current hydrometallurgy and pyrometallurgy, to make the recycling process more efficient and 94 environmentally friendly. The expected flow-sheet of the present technology applied to the recycling of spent LIBs is shown in Figure 3. Figure 2: Products analysis of LiCoO2 regeneration test in NH4HCO3-(NH4)2SO3 system at 0.5 mol/L (NH4)2SO3: (a) electrolyte after electrolysis, (b) cathodic deposition product, (c) XRD, (d) SEM of cathodic product, (e) SAED of LiCoO2, (f) SAED of cathodic deposition product, and (g) pH values of electrolyte at different (NH4)2SO3 concentrations in anode region and cathode region. Figure 3: Expected flow-sheet of the present technology applied for the recovery of Li and Co from spent LIBs. 4. Conclusions This paper proposes a novel technology of suspension electrolysis for regenerating LiCoO2 in one step, and the reaction system was established for the first time. The main findings can be concluded as follows: (1) high concentration of NH3 in the electrolyte is not the key to its complexion with Co3+; (2) LiCoO2 can be regenerated by electrolysis in NH4HCO3-(NH4)2SO3 electrolyte system; (3) Co was leached out from LiCoO2 in the form of Co(NH3)63+ in NH4HCO3-(NH4)2SO3 electrolyte system. This work is an exploratory study at the laboratory scale. The reaction mechanisms of anode and cathode are not yet clear. Further study will be conducted in future work to better understand and develop this technology. Acknowledgments This work was supported by the National Natural Science Foundation of China (21808190) and Doctoral Fund of Southwest University of Science and Technology (18zx7116, 18zx7153). The authors are grateful to the reviewers who helped them improve the paper by many pertinent comments and suggestions. References Chen X., Chen Y., Zhou T., Liu D., Hu H., Fan S., 2015, Hydrometallurgical recovery of metal values from sulfuric acid leaching liquor of spent lithium-ion batteries, Waste Management, 38, 349–356. Chen X., Ma H., Luo C., Zhou T., 2017, Recovery of valuable metals from waste cathode materials of spent lithium-ion batteries using mild phosphoric acid, Journal of Hazardous Materials, 326, 77–86. Freitas M.B.J.G., Garcia E.M., 2007, Electrochemical recycling of cobalt from cathodes of spent lithium-ion batteries, Journal of Power Sources, 171(2), 953–959. Jha M.K., Kumari A., Jha A.K., Kumar V., Hait J., B.D., 2013, Recovery of lithium and cobalt from waste lithium ion batteries of mobile phone, Waste Management, 33(9), 1890–1897. Joulie, M., Billy, E., Laucournet, R., Meyer, D., 2017, Current collectors as reducing agent to dissolve active materials of positive electrodes from Li-ion battery wastes, Hydrometallurgy, 169, 426–432. 95 Huang Y.F., Han G.H., Liu J.T., Chai W.C., Wang W.J., Yang S.Z., Su S.P., 2016, A stepwise recovery of metals from hybrid cathodes of spent Li-ion batteries with leaching-flotation-precipitation process. Journal of Power Sources, 325, 555–564. Gu F., Guo J., Yao X., Summers P.A., Widijatmoko S.D., Hall P., 2017, An investigation of the current status of recycling spent lithium-ion batteries from consumer electronics in China, Journal of Cleaner Production, 161, 765–780. Ku H., Jung Y., Jo M., Park S., Kim S., Yang D., Rhee K., An E.M., Sohn J., Kwon K., 2016, Recycling of spent lithium-ion battery cathode materials by ammoniacal leaching, Journal of Hazardous Matererials, 313, 138–46. Li J., Shi P., Wang Z., Chen Y., Chang C.C., 2009, A combined recovery process of metals in spent lithium-ion batteries, Chemosphere, 77(8), 1132–1136. Li L., Chen R., Sun F., Wu F., Liu J., 2011, Preparation of LiCoO2 films from spent lithium-ion batteries by a combined recycling process, Hydrometallurgy, 108(3-4), 220–225. Li L., Dunn J.B., Zhang X., Gaines L., Chen R., Wu F., Amine K., 2013, Recovery of metals from spent lithium- ion batteries with organic acids as leaching reagents and environmental assessment, Journal of Power Sources, 233, 180–189. Li J., Wang G., Xu Z., 2016, Environmentally-friendly oxygen-free roasting/wet magnetic separation technology for in situ recycling cobalt, lithium carbonate and graphite from spent LiCoO2/graphite lithium batteries, Journal of Hazardous Materials, 302, 97–104. Li L., Fan E., Guan Y., Zhang X., Xue Q., Wei L., Wu F., Chen R., 2017, Sustainable recovery of cathode materials from spent lithium-ion batteries using lactic acid leaching system, ACS Sustainable Chemistry and Engineering, 5(6), 5224–5233. Lv W., Wang Z., Cao H., Sun Y., Zhang Y., Sun Z., 2018, A critical review and analysis on the recycling of spent lithium-ion batteries, 6(2), 1504–1521. Nan J., Han D., Zuo X., 2005, Recovery of metal values from spent lithium-ion batteries with chemical deposition and solvent extraction, Journal of Power Sources, 152, 278–284. Provazi K., Campos B.A., Espinosa D.C., Tenorio J.A., 2011, Metal separation from mixed types of batteries using selective precipitation and liquid-liquid extraction techniques, Waste Management, 31(1), 59–64. Prabaharan G., Barik S.P., Kumar N., Kumar L., 2017, Electrochemical process for electrode material of spent lithium ion batteries. Waste Management, 68, 527−533. Qi Y., Meng F., Yi X., Shu J., Chen M., Sun Z., Sun S., Xiu F., 2020, A novel and efficient ammonia leaching method for recycling waste lithium ion batteries. Journal of Cleaner Production, 251, 119665. Santana I.L., Moreira T.F.M., Lelis M.F.F., Freitas M.B.J.G., 2017, Photocatalytic properties of Co3O4/LiCoO2 recycled from spent lithium-ion batteries using citric acid as leaching agent, Materials Chemistry and Physics, 190, 38–44. Wang X., Gaustad G., Babbitt C.W., 2016, Targeting high value metals in lithium-ion battery recycling via shredding and size-based separation, Waste Management, 51, 204–213. Wang H., Huang K., Zhang Y., Chen X., Jin W., Zheng S., Zhang Y., Li P., 2017, Recovery of lithium, nickel, and cobalt from spent lithium-ion battery powders by selective ammonia leaching and an adsorption separation system, ACS Sustainable Chemistry and Engineering, 5(12), 11489−11495. Xiao J., Li J., Xu Z., 2017, Novel Approach for in Situ Recovery of lithium carbonate from spent lithium ion batteries using vacuum metallurgy, Environmental Science and Technology, 51(20), 11960-11966. Xiao, J., Li, J., Xu, Z., 2020, Challenges to future development of spent lithium ion batteries recovery from environmental and technological perspectives. Environmental Science and Technology, 54(1), 9–25. Yang Y., Zheng X., Cao H., Zhao C., Lin X., Ning P., Zhang Y., Jin W., Sun Z., 2017, A closed-loop process for selective metal recovery from spent lithium iron phosphate batteries through mechanochemical activation, ACS Sustainable Chemistry and Engineering, 5(11), 9972–9980. Zou H., Gratz E., Apelian D., Wang Y., 2013, A novel method to recycle mixed cathode materials for lithium ion batteries, Green Chemistry, 15(5), 1183–1191. Zeng, X., Li, J., Shen, B., 2015. Novel approach to recover cobalt and lithium from spent lithium-ion battery using oxalic acid, Journal of Hazardous Materials, 295, 112–118. Zeng, X.; Li, J., 2015, On the sustainability of cobalt utilization in China, Resources Conservation and Recycling, 104(A), 12–18. Zeng X., Li J., 2016, Measuring the recyclability of e-waste: an innovative method and its implications, Journal of Cleaner Production, 131, 156–162. Zheng X., Gao W., Zhang X., He M., Lin X., Cao H., Zhang Y., Sun Z., 2017, Spent lithium-ion battery recycling – Reductive ammonia leaching of metals from cathode scrap by sodium sulphite, Waste Management, 60, 680−688.Zeng X., Ali S.H., Tian J., Li J., 2020, Mapping anthropogenic mineral generation in China and its implications for a circular economy, Nature Communication, 11(1), 1544. 96