This is a version of a publication in Please cite the publication as follows: DOI: Copyright of the original publication: This is a parallel published version of an original publication. This version can differ from the original published article. published by Towards mine tailings valorization: Recovery of critical materials from Chilean mine tailings Araya Natalia, Kraslawski Andrzej, Cisternas Luis Araya, N., Kraslawski, A., Cisternas, L. (2020). Towards mine tailings valorization: Recovery of critical materials from Chilean mine tailings. Journal of Cleaner Production, vol. 263. DOI: 10.1016/j.jclepro.2020.121555 Post-print Elsevier Journal of Cleaner Production 10.1016/j.jclepro.2020.121555 © 2020 Elsevier Journal Pre-proof Towards mine tailings valorization: Recovery of critical materials from Chilean mine tailings Natalia Araya, Andrzej Kraslawski, Luis A. Cisternas PII: S0959-6526(20)31602-4 DOI: https://doi.org/10.1016/j.jclepro.2020.121555 Reference: JCLP 121555 To appear in: Journal of Cleaner Production Received Date: 16 June 2019 Revised Date: 6 March 2020 Accepted Date: 5 April 2020 Please cite this article as: Araya N, Kraslawski A, Cisternas LA, Towards mine tailings valorization: Recovery of critical materials from Chilean mine tailings, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.121555. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd. Credit author statement Natalia Araya: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing- original draft, Visualization, Andrzej Kraslawski: Conceptualization, Methodology, Writing- review and editing, Supervision. Luis A. Cisternas: Conceptualization, Validation, Formal analysis, Writing- review and editing, Visualization, Supervision, Funding acquisition. Towards mine tailings valorization: Recovery of critical materials from Chilean mine tailings Natalia Arayaa b *, Andrzej Kraslawskib , Luis A. Cisternasa a Depto de Ing. Química y Procesos de Minerales, Universidad de Antofagasta, Antofagasta 1240000, Chile bLUT School of Engineering Science, LUT University, Lappeenranta FI-53851, Finland * Corresponding author: School of Engineering Sciences, Lut University, FI-53851, Lappeenranta, Finland. Email addresses: natalia.araya..gomez@lut.fi (N. Araya); andrzej.Kraslawski@lut.fi (A. Kraslawski); luis.cisternas@uantof.cl (L. A. Cisternas) 1 Words: 10060 1 Towards mine tailings valorization: Recovery of critical materials from Chilean mine 2 tailings 3 Abstract 4 The mining industry produces large volumes of mine tailings – a mix of crushed rocks 5 and process effluents from the processing of mineral ores. Mine tailings are a major 6 environmental issue due to implications related to their handling and storage. 7 Depending on the mined ore and the process used, it may be possible to recover 8 valuable elements from mine tailings, among other critical raw materials (CRMs) like 9 rare earths, vanadium, and antimony. 10 The aim of this study was to investigate the techno-economic feasibility of producing 11 critical raw materials (CRMs) from mine tailings. Data from 477 Chilean tailings facilities 12 were analyzed and used in the techno-economic assessment of the valorization of mine 13 tailings in the form of CRMs recovery. A review of applicable technologies was 14 performed to identify suitable technologies for mine tailings processing. To assess the 15 economic feasibility of CRMs production, net present value (NPV) was calculated using 16 the discounted cash flow (DCF) method. Sensitivity analysis and design of experiments 17 were performed to analyze the influence of independent variables on NPV. Two options 18 were assessed, rare earth oxides (REOs) production and vanadium pentoxide (V2O5) 19 production. The results show that it is possible to produce V2O5 with an NPV of 76 20 million US$. In the case of REOs, NPV is positive but rather low, which indicates that 21 the investment is risky. Sensitivity analysis and the ANOVA run using the design of 22 2 experiments indicated that the NPV of REOs is highly sensitive to the price of REOs 23 and to the discount rate. 24 Keywords: mine tailings; critical raw materials; techno-economic assessment; 25 discounted cash flow; sensitivity analysis. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 3 1. Introduction 42 Mine tailings are waste from the processing of mineral ores. They are a mixture of 43 ground rocks and process effluents generated during processing of the ores, and their 44 composition depends on the nature of the mined rock and the recovery process used. In 45 copper mining, tailings can account for 95-99% of crushed and ground ores (Edraki et 46 al., 2014). Worldwide, mine tailings are produced at a rate of anywhere from five to 47 fourteen billion tons per year (Adiansyah et al., 2015; Edraki et al., 2014; Schoenberger, 48 2016). 49 In view of the volumes of mine waste produced and the nature of the chemicals 50 involved, the storage and handling of mine tailings is a significant environmental 51 problem. Mine tailings are a source of serious contamination of soils and groundwater 52 with nearby communities particularly badly affected by the results of eolian and water 53 erosion of tailing disposal sites (Mendez and Maier, 2008). Another cause of 54 environmental pollution from mine tailings is acid mine drainage (AMD) (Larsson et al., 55 2018; Moodley et al., 2017). AMD is formed from the exposure of sulfide ores and 56 minerals to water and oxygen, once the ore is exposed, sulfate and heavy metals are 57 released into the water (Moodley et al., 2017). AMD is considered one of the most 58 significant forms of water pollution and the USA Environmental Protection Agency (US-59 EPA) considers it to be the second only to global warming and ozone depletion in terms 60 of ecological risk (Moodley et al., 2017). 61 Tailing storage facilities (TSF), also called tailing deposits, are the source of most 62 mining-related disasters (Schoenberger, 2016). Approaches to the handling and storage 63 of mine tailings include riverine disposal, wetland retention, backfilling, dry stacking and 64 4 storage behind damned impoundments (Kossoff et al., 2014). Mine tailings dam failures 65 can have catastrophic consequences. 237 cases of significant tailings accidents were 66 reported for the period 1971 to 2009 (Adiansyah et al., 2015). More recently, in January 67 2019, an accident at the Córrego do Feijão mine in Brumadinho in the metropolitan 68 region of Belo Horizonte in southeastern Brazil killed at least 65 people with about 280 69 people were missing (De Sá, 2019). 70 To achieve a circular economy model, the valorization of mine tailings is crucial for the 71 mining industry, which needs to improve its processes to minimize its environmental 72 impact and close the loops (Kinnunen and Kaksonen, 2019). Different approaches to 73 tailings valorization can be taken, such as reprocessing to extract metals and minerals, 74 tailings as backfill material, tailings as construction material, energy recovery and 75 carbon dioxide sequestration (Lottermoser, 2011). 76 Challenges that the mining industry needs to face to achieve the valorization of tailings 77 aligned with circular economy principles include improving the rather limited knowledge 78 about mineralogy, impurities concentration, and the quantity of tailings; developing new 79 business models that take account of price development, lower disposal costs, and 80 market demand; providing institutional impulse indispensable to encourage the 81 transformation from a linear to a circular economy; technology development to make 82 processes economically feasible since most mine tailings have low grades of different 83 elements mixed with residues of previous processes (Kinnunen and Kaksonen, 2019; 84 Lottermoser, 2011). 85 Due to the geological heterogeneity of the rocks mined and the continuous flow 86 processes used in mineral processing, tailings deposits contain large quantities of 87 5 valuable elements whose recovery could bring potential economic benefits. A number of 88 studies have investigated the recovery of valuable elements from mine tailings (Ahmadi 89 et al., 2015; Alcalde et al., 2018; Andersson et al., 2018; Ceniceros-Gómez et al., 2018; 90 Falagán et al., 2017; Figueiredo et al., 2018; Khalil et al., 2019; Khorasanipour, 2015; 91 Mohamed et al., 2017; Sracek, O., Mihaljevič, M. Kříbek, B., Majer, V. Veselovský, 92 2010). 93 As shown by recent studies (Ceniceros-Gómez et al., 2018; Markovaara-Koivisto et al., 94 2018; Moran-Palacios et al., 2019; Tunsu et al., 2019), among elements contained in 95 mine tailings, there are many critical raw materials (CRMs) . Raw materials have 96 significant economic importance and are utilized in the manufacture of a wide range of 97 goods. In particular, critical raw materials can be applied in areas such as alternative 98 energy production and communications devices, and they play a significant role in the 99 development of globally competitive and eco-friendly innovations. Securing access to a 100 stable supply of many raw materials has become a major challenge for national and 101 regional economies with a limited production, which relies on imports of numerous 102 minerals and metals (European Commission, 2017a). 103 Many studies have examined the criticality of raw materials. This study utilizes the list 104 compiled by the European Commission (EC), where raw materials are considered 105 critical when they are both of high economic importance for the European Union (EU) 106 and vulnerable to supply disruptions (European Commission, 2017b). The term 107 “vulnerable to supply disruption” means that their supply is associated with a high risk of 108 not meeting the demand of the EU industry. High economic importance means that the 109 raw material is of fundamental importance to industry sectors that create added value 110 6 and jobs, which may be lost in the case of inadequate supply and if adequate 111 substitutes cannot be found (Blengini et al., 2017). The most critical metals are those for 112 which supply constraints result from the fact that they are largely or entirely mined as 113 by-products, generate environmental impacts during production, have no effective 114 substitutes, and are mined in areas prone to geopolitical conflict (Graedel et al., 2015). 115 In 2011, the European Commission (EC) published a list of 14 raw materials that are 116 critical for emerging technologies of European industries, so-called critical raw materials 117 (CRMs) (European Commission, 2017a, 2014, 2011). The list has been updated twice 118 since 2011, the last update was in 2017, and it currently contains twenty-seven CRMs 119 including 3 element groups: light rare earth elements (LREEs), heavy rare earth 120 elements (HREES) and platinum group elements. 121 According to the International Union for Pure and Applied Chemistry (IUPAC), rare earth 122 elements (REEs) are a group of 17elements that includes lanthanides, composed of 15 123 elements, and yttrium and scandium, which are included in this group due to the 124 similarity in chemical characteristics. REEs can be found in over 250 different minerals 125 (Jordens et al., 2013; Sadri et al., 2017). REEs have an important role in the transition 126 to green technologies because of their use in crucial components such as permanent 127 magnets and rechargeable batteries, and their use as catalysts (Koen Binnemans et al., 128 2013). China is responsible for almost 80% of the global supply of REEs, such 129 monopoly has raised concerns about a possible shortage of supply, (Hornby and 130 Sanderson, 2019; Vekasi and Hunnewell, 2019). 131 Other elements on the list of CRMs are platinum group elements (PGEs), which include 132 ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum 133 7 (Pt). These metals are very rare in the Earth’s continental crust, ranging from 0.022 ppb 134 for iridium to 0.52 to Pd (Mudd et al., 2018). 135 Nowadays, due to the increasing demand for CRMs, new sources are being sought, 136 and secondary sources such as metal scrap and industrial waste are attracting more 137 attention. The use of the hitherto unexploited secondary sources can reduce demand 138 for virgin materials and, in consequence, contribute to a decrease in mining production. 139 One of the core principles of the circular economy is the reduction and minimization of 140 resource use, and ways to achieve that goal include recycling and reuse of wastes 141 (Kirchherr et al., 2017). Mine tailings from mineral processing of a certain branch of the 142 metal industry could be used as a source in a process designed to obtain one or more 143 critical raw materials, a simplified flowsheet of this idea is shown in Fig. 1. 144 145 Fig. 1: Simplified mining processes flowsheet featuring conventional processes to obtain 146 metal and the re-processing of tailings to obtain CRMs. 147 Chile has a long history of mining and large-scale mining started in the first decade of 148 the twentieth century. In 2016, Chilean mining exports were valued at 30,379 million 149 USD according to the National Service of Geology and Mining (SERNAGEOMIN), 90% 150 8 of which came from copper mining (SERNAGEOMIN, 2017). Chile is the world’s leading 151 producer of copper. Currently, a decrease in the grade of mined copper ores is being 152 observed, which increases the amount of processed ore and, consequently, leads to 153 greater tailings deposits for the same level of copper production. Currently, Chile 154 produces 1,400,000 tons of mine tailings daily and there are 696 tailings storage 155 facilities (TSF) (SERNAGEOMIN, 2018). 156 The objective of this study is to conduct a technical and economic assessment of the 157 valorization of mine tailings of Chile as a source of CRMs. Therefore, the research 158 questions addressed in this paper are: 159 What critical materials can be recovered from mine tailings? 160 What are the challenges in the production of critical materials using mine tailings as a 161 source? 162 In recent years, the use of secondary sources for obtaining raw materials has gained 163 growing importance. This research supplements these works with a techno-economic 164 feasibility study for producing critical raw materials from mine tailings. 165 The data used in the study refer to mine tailings samples of 477 Chilean copper mining 166 industrial deposits. These data have not been previously used to assess the economic 167 potential of the recovery of critical materials. 168 2. Methodology 169 The first step to evaluate the recovery of CRMs from mine tailings is the calculation of 170 the amount of each CRM present in tailings. The feasibility of recovery is next assessed 171 for critical materials found in larger quantities. 172 9 In the technological assessment, technologies for processing mine tailings are first 173 examined. If no technologies are available, technologies for processing ore, as an 174 analogous process, are considered taking into account differences between the 175 processing of ore and processing of waste. 176 In the economic assessment, the discounted cash flow (DCF) method is used to assess 177 the feasibility of the options for the recovery CRMs from mine tailings. This method has 178 been widely used for valuation projects (De Reyck et al., 2008; Kodukula and 179 Papudesu, 2006; Žižlavský, 2014). DCF is a commonly adopted economic valuation 180 technique and consists of discounting expected cash flow of a future project at a given 181 discount rate and then summing all the cash flows of a determined period of time 182 (Ibáñez-Forés et al., 2014; Žižlavský, 2014). 183 Sensitivity analysis is performed to assess the impact of various parameters on the NPV 184 of CRMs recovery from mining tailings. Sensitivity analysis is a tool used to analyze how 185 different values of a set of independent variables affect a dependent variable. The sale 186 price of critical materials, operating costs, capital costs, and discount rate are the main 187 inputs in the DCF method, then these variables are studied in the sensitivity analysis. 188 These variables and interactions among them were also tested using a design of 189 experiments with response surface methodology. 190 3. Mine tailings assessment 191 Mining is one of the main economic activities in Chile due to the country’s favorable 192 geochemical and mineralogical characteristics. Chile is the world’s leading producer of 193 copper, producing 5,552.6 thousand tons of copper in 2016 (SERNAGEOMIN, 2017), 194 the world’s second supplier of molybdenum, producing 62,746.1 tons in 2017, and the 195 10 second producer of lithium, producing 77,284 tons of lithium carbonate in 2017 196 (SERNAGEOMIN, 2017). For some regions in Chile, mining is the main economic 197 activity; most mining activity is found in the Atacama Desert in northern Chile. 198 The Atacama Desert is the driest non-polar desert on the earth, and its copper ore 199 deposits are world-class porphyry copper deposits (Oyarzún et al., 2016; Tapia et al., 200 2018). Porphyry deposits are the principal sources of copper and molybdenum 201 (Khorasanipour and Jafari, 2017). Porphyry deposits consist of distributed and 202 stockwork sulfide mineralization located in various host rocks that have been altered by 203 hydrothermal solutions into roughly concentric zonal patterns (Dold and Fontboté, 204 2001). 205 Chilean mining processing plants produce large quantities of waste every year. Tailings 206 dams are the most common type of tailing deposit in the country (Ghorbani and Kuan, 207 2017). Previously, prior to the adoption of appropriate regulations, tailings were 208 abandoned in deposits and no efforts were made to ensure the safety of the nearby 209 communities but nowadays the handling and storage of tailings are strictly regulated. In 210 2011, the Law 22.551 was promulgated. It regulates the closing of mining facilities and 211 specifies that tailings must be physically and chemically stabilized (Ministerio de 212 Minería, 2011; SERNAGEOMIN, 2011). 213 In Chile, there are 696 mine tailings deposits registered in a national registry, compiled 214 between 2016 and 2018. The registry is expected to be updated as new mine tailings 215 facilities are opened and old abandoned tailing deposits are discovered. Antofagasta 216 Region hosts larger mine tailings deposits (SERNAGEOMIN, 2018) because of the size 217 of the mining sector in this region, which accounts for 47% of the contribution to Chilean 218 11 mining activity. The most serious problems associated with tailings and handling and 219 storage of tailings are related to the seismic nature of the country, and risks associated 220 with tailings dam failure include fatalities, serious water contamination, and destruction 221 of the land. 222 3.1. Characterization of mine tailings 223 The chemical composition of tailings in 477 mine tailings deposits is available on the 224 website of the National Service of Geology and Mining of Chile (SERNAGEOMIN) 225 (SERNAGEOMIN, 2018). This database contains values for concentrations of 56 226 elements, including 22 CRMs featuring on the latest EC list. The CRMs analyzed in the 227 SERNAGEOMIN database are vanadium, cobalt, yttrium, niobium, scandium, hafnium, 228 tantalum, antimony, bismuth, tungsten, lanthanum, cerium, praseodymium, neodymium, 229 samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, 230 ytterbium and lutetium (SERNAGEOMIN, 2018). 231 Chemical composition in each mine tailings deposit is different and it depends on the 232 type of mineral rocks mined and the processes used in the plant. In the geochemical 233 characterization of Chilean tailings, it can be noticed that most tailings deposits have a 234 high percentage of silicon oxide or ferric oxide due to the type of minerals processed 235 (SERNAGEOMIN, 2018). 236 Data in the SERNAGEOMIN database are classified by the current status of the tailings 237 deposits: active, inactive, and abandoned. In the methodology used in this study, only 238 inactive and abandoned tailings were analyzed, because their volume and chemical 239 composition do not change over time. In the case of active tailings, although their 240 volume is greatest, their chemical composition may change over the course of years, 241 12 which is why they have not been considered in this study. Mine tailings of the 242 Antofagasta Region are examined because the tailing volume storage is greater in this 243 region than in other regions. The TSFs analyzed cover 16 inactive deposits. The 244 location of mine tailings of the Antofagasta Region can be seen in Fig. 2. 245 246 Fig. 2: Tailings storage facilities in Antofagasta Region, blue represents inactive or 247 abandoned deposits and red is for active deposits. 248 CRMs found in larger quantities are given in Table 1. The sum of REEs was also 249 calculated, to produce REE concentrate or mischmetal, which is an alloy of REEs. The 250 sum of REEs does not consider scandium because it is separated in a different process. 251 Table 1: Total tonnage and uses of CRMs present in inactive tailings deposits of the 252 Antofagasta Region (16 deposits). 253 CRMs Tons Uses Vanadium (V) 46,110 Most of the vanadium produced is used in ferrovanadium or as a steel additive. Another use is as vanadium pentoxide. Cerium (Ce) 22,886 Cerium is used as a catalyst converter for carbon monoxide emissions, as an additive in glass for reducing UV transmission, and in carbon-arc lighting. Cobalt (Co) 16,940 The main uses of cobalt are in battery chemicals for Ni-Cd, Ni-metal hydride and Li-ion battery types, superalloys, hard materials, catalysts, and magnets. Yttrium (Y) 16,039 Yttrium is used for energy-efficient fluorescent lamps, in the treatment of 13 various cancers, in aerospace surface and barriers, as a superconductor, in aluminum and magnesium alloys, and in-camera lenses. Neodymium (Nd) 14,880 Neodymium is used to create high-strength magnets for computers, cell phones, medical equipment, electric cars, wind turbines, and audio systems. It is also used in the glass and ceramic industries. Lanthanum (La) 10,253 Lanthanum is used in nickel metal hydride rechargeable batteries for hybrid automobiles, in high-quality camera and telescope lenses, and in petroleum cracking catalysts in oil refineries. Scandium (Sc) 9,359 Scandium is used to increase strength and corrosion resistance in aluminum alloys, in high-intensity discharge lamps, and in fuel cells to increase efficiency at lower temperatures. Niobium (Nb) 4,823 Niobium is used in high strength low alloy (HSLA) steels as ferroniobium and in superconducting magnets. Antimony (Sb) 3,751 Principal uses for antimony are in alloys with lead and tin, and in lead-acid batteries. Samarium (Sm) 3,456 The main use of samarium is in cobalt-samarium alloy magnets for small motors, quartz watches, and camera shutters. Samarium is also used in lasers. Gadolinium (Gd) 3,357 Gadolinium is mainly used for NdFeB permanent magnets, lightning applications and in metallurgy. Praseodymium (Pr) 3,245 Praseodymium is used in NdFeB magnets, ceramics, batteries, catalysts, glass polishing and fiber amplifiers. Dysprosium (Dy) 2,705 Dysprosium is used mainly and almost inclusively in NdFeB magnets. REEs (total) 82,254 3.2. Technology Assessment 254 A literature review was conducted to investigate the available technologies for the 255 recovery of critical raw materials from mine tailings. If no technologies are available for 256 tailings processing, then those used for processing of primary ores are considered as a 257 reference. It is important to notice that mine tailings are already in the form of slurry or 258 paste, depending on the percentage of water present, so there are no mining costs, 259 which represent approximately 43% of operating cost in a mine (Curry et al., 2014). 260 Existing technologies for CRMs production are briefly described in Table 2. Most of 261 these technologies are for primary ores. Some applications for secondary sources such 262 as industrial waste and mine tailings exist (Abisheva et al., 2017; Binnemans et al., 263 2015; Figueiredo et al., 2018; Innocenzi et al., 2014; Jorjani and Shahbazi, 2016; 264 14 Peelman et al., 2016), but they should be treated as emerging technologies. Significant 265 further development of these new technologies is required before they are suitable for 266 industrial-scale usage (Kinnunen and Kaksonen, 2019). 267 In spite of the low concentration of REEs in comparison to end-of-life consumer goods, 268 mine tailings are a potential source of REEs because of the large volumes of mine 269 tailings, which mean that the total amount of recoverable REEs could be high 270 (Binnemans et al., 2015). Several processes have been proposed for the recovery of 271 REEs from mine tailings. Peelman et al. (2018) have proposed a method for the 272 recovery of REEs from mine tailings from apatite mineral with an REE content of 1200-273 1500 ppm using acidic leaching followed by cryogenic crystallization and solvent 274 extraction. They achieved a 70-100% recovery of REE. 275 Table 2: Available and emerging technologies for CRMs processing. 276 CRMs Production process Rare earth elements -Acidic leaching-cryogenic crystallization-solvent extraction from mine tailings with apatite and monazite. (Peelman et al., 2016). -Bioleaching for REEs extraction from low-grade sources. (Peelman et al., 2014). -Solvent extraction to recover REEs from mine tailings of gold and tellurium mining (Tunsu et al., 2019). -Use of solvent impregnated resins (SIR) to recover REEs from low concentration solutions (Onishi et al., 2010; SUN et al., 2009; Yoon et al., 2016). Antimony -Crushing and pyrometallurgical methods for primary ores (Anderson, 2012). -Crushing and hydrometallurgical methods like leaching and electrodeposition (Anderson, 2012). Cobalt -Bioleaching of sulfidic tailings of iron mines. (Ahmadi et al., 2015). -Mineral beneficiation, comminution, flotation, smelting, leaching or refining for sulfide ores (European Commission, 2017b). -Calcination, pyrometallurgical process, hydrometallurgical methods for lanthanides ores (European Commission, 2017b). 15 Niobium -Gravity separation, froth flotation, magnetic and electrostatic separation, and acid leaching depending on the ore (European Commission, 2017b). Vanadium -Extraction of vanadium as a co-product to iron from vanadium slag includes bearing, roasting, acid leaching solvent extraction, ion exchange, and precipitation (Xiang et al., 2018). -Desliming-flotation from low-grade stone coal (European Commission, 2017b). -Preform reduction process (PRP) based on a metallothermic reduction of vanadium pentoxide (V2O5). (Miyauchi and Okabe, 2010). 277 There are no processing plants using copper mine tailings as a source of CRMs. 278 Therefore, technologies used for primary sources are assumed to be also applicable to 279 the processing of mine tailings. Based on the content of the mine tailings analyzed, two 280 feasibility studies are conducted; the first for producing rare earth oxides and the 281 second for vanadium recovery, using mine tailings as a source. 282 The extraction process for REEs, in a general form, includes three steps: mining and 283 comminution; ore beneficiation processes consisting of flotation, gravity and magnetic 284 techniques to generate REE concentrate; and hydrometallurgical methods to extract 285 REE compounds (Sadri et al., 2017). Hydrometallurgical methods include cracking of 286 REE concentrate; leaching, neutralization and precipitation processes; and separation 287 and purification techniques such as solvent extraction. Solvent extraction allows 288 recovering REEs with a high degree of purity, moreover, a variety of solvent extraction 289 reagents is available. For secondary waste, selective extraction of REEs is required 290 from solutions with a high content of other species (Tunsu et al., 2019). 291 A life cycle inventory and impact assessment of the production of RE oxides from 292 primary bastnasite and monazite has been presented for the Bayan Obo mine in Inner 293 16 Mongolia, China, in (Koltun and Tharumarajah, 2014). The study found out the mining 294 and beneficiation stage accounts for 6.98% of energy consumption and 6.51% of water 295 consumption. When processing mine tailings, there is no mining stage, so the values 296 were adapted. Adapted values of energy and water consumption to obtain RE oxides 297 from waste material are included in the supplementary material. 298 Primary ores of REEs are usually treated with alkaline pressure leaching or sulfuric acid 299 roasting. However, mine tailings are a low-grade source of REEs, so these technologies 300 may not be economically feasible. Chloride-based hydrometallurgical processes may be 301 a potential alternative to traditional capital intensive hydrometallurgical processes based 302 on high temperature and pressure (Onyedika et al., 2012) and they could be a suitable 303 option for REE recovery from tailings at economically viable capital and operating cost. 304 In the case of vanadium, it is mainly produced as a co-product from the vanadium slag 305 before the steel converter. The main vanadium products are vanadium pentoxide (V2O5) 306 and ferrovanadium (FeV) (European Commission, 2017b). Other sources of vanadium 307 are stone coal, steel scrap, and fossil fuels. 308 The mine tailings analyzed in this study have a CRMs content that varies between 80-309 214,000 grams per ton of tailing. In Chile, there are currently no projects providing for 310 the use of mine tailings as a source of CRMs, nor approved initiatives for the production 311 of CRMs from primary ores. 312 3.3. Economic Assessment 313 The economic assessment is done in two main steps. The first step focuses on the 314 economic potential of CRMs found in inactive mine tailings as an in-situ value, 315 17 considering the monetary value of the CRMs to assess the feasibility of CRMs 316 production. The second stage concentrates on the analysis of the feasibility of CRMs 317 production using mine tailings as a source. 318 Prices of critical materials may differ from one source to another. In addition, the prices 319 of some critical materials are not publicly available as they are traded privately. To 320 calculate the economic potential of inactive mine tailings deposits, the following prices 321 were used, see Table 3. 322 Table 3: CRMs prices in July 2018 323 Critical material Price ($US/kg)* Critical material Price (US$/kg)* Antimony 8.51 Neodymium metal ≥ 99.5% 68.0 Cerium metal ≥ 99.5% 7.00 Neodymium oxide ≥ 99.5% 66.7 Cerium oxide ≥ 99.5% 5.59 Praseodymium metal ≥ 99% 125.00 Cobalt 87.5 Praseodymium oxide ≥ 99.5% 81.6 Dysprosium metal ≥ 99% 268.57 Samarium metal ≥ 99.9% 15 Dysprosium oxide ≥ 99.5% 226.80 Scandium metal ≥ 99.9% 3,458 Gadolinium metal ≥ 99.9% 44.00 Scandium oxide ≥ 99.95% 1,079 Gadolinium oxide ≥ 99.5% 20.94 Vanadium (as V2O5 80%) 40.00 Lanthanum metal ≥ 99% 7.00 Yttrium metal ≥ 99.9% 36.5 Lanthanum oxide ≥ 99.5% 7.80 Yttrium oxide ≥ 99.99% 4.60 * Sources: (Mineralprices.com, 2018),(Thenorthernminer.com, 2018), (LME, 2018). 324 The economic potential of CRMs recovery was calculated as the fraction of each CRM 325 in the tailings multiplied by the mass of each TSF for the 16 TSFs studied. The 326 economic potential is a reference value for the total REE value of the mine tailings. The 327 economic potential of these TSFs is shown as supplementary material. 328 To assess the feasibility of CRMs recovery, the DFC method was used to calculate the 329 NPV and IRR for REOs production and V2O5 production using mine tailings. The NPV is 330 the difference between the present value of cash inflows and the present value of cash 331 18 outflows in a particular period of time. IRR is the discount rate at which the NPV of 332 future cash flows is equal to the initial investment. NPV and IRR are metrics used in 333 capital budgeting and decision-making. The calculation does not include external factors 334 such as inflation. To obtain the NPV and IRR for the options assessed, capital costs and 335 operating costs of projects with similar characteristics were used. 336 Capital costs, also referred to as capital expenses or CAPEX, represent the investment 337 made for the project, which includes costs of the development phase which, among 338 other costs, comprises the purchase of the equipment, building a manufacturing plant 339 and the cost of product launch. The investment represents the first cash flow in the DFC 340 method. 341 Operating costs, operating expenses or OPEX, are expenses incurred during the 342 lifetime of the project. In the case of a mining project, these would include the cost of 343 labor, water, and energy, maintenance, spare parts, and indirect costs (Bhojwani et al., 344 2019). 345 The first option assessed is the production, using mine tailings as a source, of the 346 following rare earth oxides (REOs): cerium, lanthanum, neodymium, yttrium, samarium, 347 gadolinium, praseodymium and dysprosium. Scandium is also considered as REE but it 348 has different properties and a different production process, which is why it was not 349 assessed together with the above mentioned REEs. 350 The second option assessed is vanadium as the production of vanadium pentoxide 351 (V2O5). It is due to the fact that vanadium is the main CRM found TSFs in the 352 Antofagasta Region (see Table 1). 353 19 3.3.1. Feasibility of Producing rare earth elements using mine tailings as a source 354 For REOs production, we have considered only REEs found in larger quantities. Due to 355 the lack of data about similar projects that use mine tailings or industrial waste as 356 source material, we used data from a Canadian project that produces rare earth oxides 357 (Hudson Resources Inc, 2013) from primary sources to produce of neodymium, 358 praseodymium, lanthanum, and cerium. Data used for NVP calculation are shown in 359 Table 4. 360 Table 4: Data for REOs project 361 Data Value Unit Capital cost 342,514,448 US$ Life of mine 20 years Operating cost 13,080 US$/ton REOs REOs Price 22,000 US$/ton REOs Production capacity 4,000 tons REOs/year Annual increase (OPEX) 1.5 % Annual increase (PRICE) 1.5 % Discount rate 10 % The price used to calculate NPV corresponds to the weighted average for REOs; 362 cerium, lanthanum, samarium, gadolinium, praseodymium, dysprosium, and yttrium 363 oxide, which is 37 USD/kg of REOs produced, 40% was discounted to reflect the 364 difference between REO concentrate and separated individual rare earth oxide prices, 365 so the price used for NPV calculations is 22 USD/kg, as in the report it was used as a 366 reference price. The grade of REEs corresponds to the average REEs grade in all the 367 deposits analyzed. In the mine tailings covered by the analysis, the average grade is 368 lower than in most primary ore processing projects, so the production was reduced 369 accordingly. 370 20 It is important to note that operating costs and capital costs are referential values. In the 371 case of mine tailings, costs related to extracting mineral ores should not be considered 372 since tailings are materials that have already been mined and processed. 373 The NPV is 672,987 USD which means that the projected earnings generated for this 374 proposed REOs production exceed the anticipated costs and the overall value for the 375 project is positive. However, even though the NPV is positive, its value is too low to 376 invest in a project of such a magnitude. The IRR is 10.03% which is almost the same as 377 the discount rate chosen for the project, this confirms that the project is not highly 378 profitable. Cash inflows and outflows are included as supplementary material. 379 3.3.2. Feasibility of producing vanadium using mine tailings as a source 380 Vanadium is the main CRM found in mine tailings in the Antofagasta Region. There are 381 46,110 tons of vanadium in inactive TSFs, but active tailings in this area have the 382 potential for ca. 900, 000 tons of vanadium. 383 Capital and operating costs for vanadium production are taken from a preliminary 384 economic assessment study for the Gibellini vanadium project (Lee, 2018). This 385 project has been designed as an open pit heap leaching operation to obtain vanadium 386 pentoxide (V2O5). The Gibellini project is designed for processing of low-grade minerals, 387 so it is suitable for mine tailings, but in this study, production is reduced because the 388 grade in mine tailings is lower in mine tailings. The values used for the calculation of 389 NPV and IRR are given in Table 5. The values of NPV and IRR for vanadium production 390 from Chilean mine tailings are shown in the supplementary material. 391 Table 5: Data for vanadium project 392 21 Data Value Unit Capital cost including 25% contingency 116,760,000 US$ Life of mine 14 years Operating cost 14,767 US$/ton V2O5 Vanadium pentoxide price 40,000 US$/ton V2O5 Production capacity 1,000 tons V2O5/year Annual increase (OPEX) 1.5 % Annual increase (PRICE) 1.5 % Discount rate 10 % 393 The NPV is 76 million US$ and the IRR is 21%, these values indicate that the project is 394 profitable as the NPV is positive and the IRR is higher than the discount rate. Cash 395 inflows and outflows are shown as supplementary material. 396 3.5. Sensitivity analysis 397 In this study, a sensitivity analysis was performed on four parameters: capital cost, 398 operating cost, critical materials price, and the effect of the discount rate on NPV for the 399 examined options. The objective of the sensitivity analysis is to understand the 400 uncertainty in the NPV for the examined parameters. These parameters were chosen 401 because they are the key components in the DCF method. 402 Sensitivity analysis determines how different values of one or more independent 403 variables affect a dependent variable under a given set of assumptions. Sensitivity 404 analysis is the last stage of the process of assessing and selecting a technological 405 alternative (Ibáñez-Forés et al., 2014). Sensitivity analysis studies how several sources 406 of uncertainty contribute to the entire uncertainty of a mathematical model. 407 In the DCF method, the discount rate is the rate used to convert the future value of a 408 project cash flows to today’s value. The discount rate is adjusted to the risk associated 409 22 with a project. Therefore, the higher the risk, the higher the discount rate (Kodukula and 410 Papudesu, 2006). Risk is associated with the uncertainty of a project. In business, risks 411 may have a positive or negative effect. The discount rate was varied to acknowledge 412 that mining projects deal with uncertainties that can be included in the model by 413 choosing a higher discount rate. 414 Mining commodity prices always show greater volatility than those of any other primary 415 products (Foo et al., 2018). Prices of critical materials may experience price spikes due 416 to their instability caused by the risk of supply disruption. Critical materials have 417 inelasticity element in their prices, this means that the demand for these materials is not 418 highly affected by the price (K. Binnemans et al., 2013; Leader et al., 2019). Critical 419 materials are needed in technologies, such as clean energy technologies, in which there 420 are not substitutes for the critical materials needed (Leader et al., 2019). 421 The price of each critical material assessed was considered as an important parameter 422 that contributes to the overall uncertainty of the project. 423 Since capital costs and operating costs used in this study are referential values, and 424 they are further used as inputs in the DCF method, it was necessary to address the 425 variability of the real values of these parameters vis-a-vis the values used here. 426 Capital cost, operating costs, and prices varied between -30 and 30% of the original 427 value. The discount rate varied between 0.05 and 0.3. 428 The results of the sensitivity analysis for the REOs price are shown in Figure 3. It can 429 be seen that for every 5% increase in the price of the REOs, the NPV increases by 38 430 23 million US$. NPV is highly sensitive to changes in REO prices. NPV becomes negative 431 when the price of REOs is below 22 US$/kg, making the project financially unviable. 432 The NPV is less sensitive to changes in operating costs than price; NPV decreases to 433 21 million US$ with an increase of 5% in operating costs. The results of the sensitivity 434 analysis of the NPV to the capital cost show that as the investment cost increases by 435 5%, the NPV decreases by ca. 15 million US$. 436 The discount rate varied between 0.05 and 0.3. The NPV is not a linear function of the 437 discount rate, the value considered was 0.1. When the discount rate is 0.11, NPV 438 decreases by approximately 21 million US$. With a discount rate higher than 0.1, NPV 439 becomes negative, making the project unviable. 440 Results of sensitivity analysis of NPV for vanadium pentoxide production are shown in 441 Fig. 3. When the price increases by 5%, NPV increased by ca. 14 million US$. When 442 the price drops by 26%, NPV becomes negative and the project unviable. 443 Results of the sensitivity analysis of NPV to operating costs show that NPV is slightly 444 sensitive to changes in operating costs. When operating costs increase by 5%, the NPV 445 decreases by ca. 5 million US$. Sensitivity analysis of the NPV to changes in capital 446 cost shows that with an increase of 5% in the capital cost, the NPV decreases by ca. 5 447 million US$. The values of NPV are very similar for both operating costs and capital 448 costs. 449 The sensitivity analysis of NPV to changes in the discount rate shows that if the 450 discount rate increases by 0.01 from the value of 0.1 used to 0.11, the NPV decreases 451 24 by 10 million US$ approximately. When the discount rate is higher than 0.21, NPV 452 becomes negative. 453 454 Fig. 3: Sensitivity analysis, a) Sensitivity of the NPV (REOs project) to the price of 455 REOs, operating costs and capital costs; b) Sensitivity of NPV to discount rate in REOs 456 project; c) sensitivity of NPV (vanadium project) to the price of V2O5, operating costs and 457 capital costs; d) Sensitivity of NPV to discount rate in vanadium project. 458 Results show that under certain prices, operating costs and capital costs, it is possible 459 to invest in producing CRMs using a secondary source such as mine tailings. 460 25 The parameters analyzed in the sensitivity study may change simultaneously. 461 Therefore, their interactions were analyzed using design-of-experiments together with 462 response surface methodology. In the analysis of the NPV of both projects, REOs 463 production and V2O5 production, four factors and three levels were considered. The 464 factors are: the price, capital costs (CAPEX), operating costs (OPEX), and the discount 465 rate (). The levels correspond to the value used in the economic assessment, then low 466 and high levels for the same value were multiplied by 0.85 and 1.15, respectively, which 467 means the experimental design results are valid in the range between -15% and +15%. 468 A percentage of 15% was chosen to ensure a good adjustment. The values tested for 469 the discount rate are 0.05, 0.1, and 0.15. ANOVA results show which parameters and 470 interactions influence the NPV by analyzing the p-value. For the p-value < 0.01 all linear 471 parameters and the interaction with the discount rate were significant. Also, the 472 statistical analysis confirms that price and the discount rate are the parameters exerting 473 greater influence. Regression models obtained have the following form: 474  =  +   +    +   +   +   +     + ℎ    +    The values for , , , , , , , ℎ and  are -17.6, -0.9103, -59.55, 67.1, -1766, 14323, 475 0.0, 242.9, and -299.5 for REOs project, and 47.64, -0.9932, -12.572, 12.572, -1143.3, 476 5717, 0.828, 49.63, -49.625 for V2O5 project, respectively. The units for  and 477 CAPEX are MUS$, OPEX and price are kUS$/ton, and the discount rate is 478 dimensionless. The R-squared values or the coefficient of the regressions were 479 2 = 98.17%, 2 = 97.99%, and 2 = 97.79% for REOs project, and 2 = 99.95%, 480 2 = 99.95%, and 2 = 99.94% for the V2O5 project. The 2 for both projects are 481 over 98% which means that at least 98% of the variation of the NPV can be explained 482 26 by the model. Also, excellent values of adjusted 2 and predicted 2 were observed 483 which suggests that the number of parameters is the model is correct and that the 484 model is able to produce high quality predictions. The ANOVA results and Pareto 485 graphics are included in the supplementary material. Also, supplementary material gives 486 the results of the design-of-experiment and response surface methodology for the IRR 487 which behaves differently from the NPV. 488 4. Discussion 489 Mine tailings are waste obtained from the processing of a rock with a view to obtain one 490 or more products that will be refined to finally get a metal(s) that is needed. Tailings 491 should be stored in facilities where they are disposed in accordance with the regulations 492 binding in each region, otherwise, the consequences to the environment can be 493 devastating. 494 The lack of a long-term consideration of the entire life-cycle of a mine and the instability 495 of mine projects contribute to irreversible mineral losses and resource sterilization. With 496 this knowledge in mind, further research should address new strategies to anticipate the 497 future use of material beyond the closing of a mine (Lèbre et al., 2017). Mine waste 498 hierarchy goes from prevention as the most favorable option to treatment and disposal 499 as the least favorable options; if waste cannot be prevented then reuse and recycling 500 are needed (Lottermoser, 2011). Nowadays most mine tailings go to the treatment and 501 disposal phase. In the Sustainable Development Goals, the World Economic Forum 502 suggests the re-use of tailings, these goals are meant to be achieved by 2030 (World 503 Economic Forum, 2016). The reprocessing of mine tailings is also an element of the 504 transformation from a linear to a circular economy that the mining industry must face. 505 27 Reprocessing mine tailings to obtain critical materials reduces the dependency on 506 reserve extraction (El Wali et al., 2019). 507 Other approaches to mine tailings management from a circular economy point of view 508 include recovering water from mine tailings, which helps to reduce the reliance on 509 seawater (Cisternas and Gálvez, 2018). Recovering water or reducing the amount of 510 water in tailing diminish the need to pump water, which decreases energy consumption 511 and greenhouse gas emissions involved in pumping water to high altitudes, where 512 mines are usually located in Chile (Araya et al., 2018; Herrera-León et al., 2019; 513 Ramírez et al., 2019). Another approach is to use mine tailings as cementitious 514 materials and pigment for sustainable paints (Barros et al., 2018; Vargas and Lopez, 515 2018). 516 There have been conducted several studies on new technologies or processes to 517 recover CRMs from secondary sources such as mine waste (Alcalde et al., 2018; 518 Andersson et al., 2018; Figueiredo et al., 2018; Khalil et al., 2019; Markovaara-Koivisto 519 et al., 2018; Peelman et al., 2018). Most of these studies are carried out at laboratory 520 and pilot plant scale. Nevertheless, the literature on the recovery of CRMs from mine 521 tailings is constantly growing. It is due to the fact that new sources of CRMs are 522 urgently needed as their importance in the global economy is constantly growing. 523 Moreover, the utilization of wastes such as mine tailings, instead of mineral deposits, is 524 essential from a circular economy point of view. Therefore, extrapolation of the potential 525 of these technologies is immensely needed. 526 Results show that mine tailings facilities of the copper industry in Chile store valuable 527 elements such as CRMs. Therefore, the early evaluation of geochemical content, 528 28 identification of suitable technologies, and an economic analysis will help to find more 529 sustainable alternatives to CRMs production. 530 The DCF is a widely used method of financial assessment, but it is not a decisive 531 metrics for making a final decision on real investment. In order to ensure the robustness 532 of assessment, sensitivity analysis was performed to analyze the effect of the possible 533 fluctuations of market prices, capital and operating costs on the analyzed options of 534 CRMs production. It has been found out that the discount rate and both capital and 535 operating costs play critical roles in economic decisions in different areas (Choi et al., 536 2018; Cisternas et al., 2014; Santander et al., 2014). 537 Reprocessing mine tailings will also have an impact on the environment. Due to the 538 nature of chemical and physical processes, mineral processing is water and energy 539 intensive, some quantities of solvents and reagents are used and at the end of the 540 process, there will still be waste that should be stored in a tailing facility. The mining 541 waste obtained after the reprocessing of tailings should be stored in a tailing facility 542 complying with the regulations designed to protect people and the environment. 543 5. Conclusions 544 There are 696 tailings storage facilities in Chile, mainly from copper mining, which is the 545 biggest mining industry in the country. The biggest TSF has the capacity to store 546 4,500,000,000 tons of tailings. Currently, there are some initiatives for recovering metals 547 of interest from mine tailings, but such initiatives are all in the early stages of feasibility 548 assessment. This study provides valuable information for the assessment of the techno-549 economic feasibility of industrial-scale critical materials recovery from copper industry 550 tailings. 551 29 Copper production will continue to grow as the copper grade decrease. Therefore, the 552 volume of mine tailings that are produced every year will increase as well. Mine tailings 553 are a worldwide environmental problem as they can generate acid drainage, and cause 554 air pollution and soil contamination. Yet, mine tailings contain several valuable 555 elements, among them critical raw materials. Therefore, the use of mine tailings as a 556 secondary source would help mitigate shortages in critical raw materials by minimizing 557 the reliance on primary sources. 558 Chilean copper mine tailings have substantial economic potential as a source of critical 559 materials such as vanadium, cobalt, rare earth elements and antimony. Minerals 560 contained in Chilean mine tailings from copper production are mostly silicates with a low 561 grade of CRMs; currently, no approved projects exist that consider mine tailings as a 562 source of CRMs. Although mine tailings have a low grade of CRMs, their already stored 563 quantity is enormous. In addition, prices of critical raw materials can be very high, and 564 these factors could make a future production of CRMs from mine tailings feasible. 565 Two options of producing CRMs using mine tailings were assessed; production of rare 566 earth oxides (REOs) and production of vanadium pentoxide (V2O5). The DFC method 567 was used to evaluate the economic feasibility of both operations. The NPV and IRR for 568 the production of REOs are positive, which means that the project is feasible. 569 Nevertheless, the NPV is low for an investment of this scale and the IRR is close to the 570 discount rate value. The sensitivity analysis of the NPV of REOs production from mine 571 tailings showed that NPV is highly sensitive to the discount rate and REO prices. 572 Results of the ANOVA confirm that the discount rate and price are the most significant 573 variables influencing the NPV behavior. 574 30 Vanadium pentoxide production is feasible for an investment of 14 years, as the NPV is 575 76 million US$ and the IRR IS 21% for V2O5 production. Vanadium is the main CRMs 576 found in tailings in the Second Region in Chile. It is concluded that producing CRMs 577 using inactive tailings and later tailings from the active mining processes may be a 578 feasible option to ensure profitable use of mine tailings and to diversify CRMs supply. 579 Acknowledgments 580 This publication was supported by Anillo–Grant ACM 170005 (PIA-CONICYT). N. Araya 581 thanks the National Council for Scientific and Technological Research (CONICYT, 582 PFCHA/Doctorado Nacional/2017-21170815) for a scholarship in support of her 583 doctoral studies. L.A.C. thanks the supported of MINEDUCUA project, code ANT1856 584 and Fondecyt program grant number 1180826. The authors are grateful to Peter Jones 585 for his help in editing the paper. 586 References 587 Abisheva, Z.S., Karshigina, Z.B., Bochevskaya, Y.G., Akcil, A., Sargelova, E.A., 588 Kvyatkovskaya, M.N., Silachyov, I.Y., 2017. Recovery of rare earth metals as 589 critical raw materials from phosphorus slag of long-term storage. Hydrometallurgy 590 173, 271–282. https://doi.org/10.1016/j.hydromet.2017.08.022 591 Adiansyah, J.S., Rosano, M., Vink, S., Keir, G., 2015. A framework for a sustainable 592 approach to mine tailings management: Disposal strategies. J. Clean. Prod. 108, 593 1050–1062. https://doi.org/10.1016/j.jclepro.2015.07.139 594 Ahmadi, A., Khezri, M., Abdollahzadeh, A.A., Askari, M., 2015. Bioleaching of copper, 595 nickel and cobalt from the low grade sulfidic tailing of Golgohar Iron Mine, Iran. 596 Hydrometallurgy 154, 1–8. https://doi.org/10.1016/j.hydromet.2015.03.006 597 Alcalde, J., Kelm, U., Vergara, D., 2018. Historical assessment of metal recovery 598 potential from old mine tailings: A study case for porphyry copper tailings, Chile. 599 Miner. Eng. 127, 334–338. https://doi.org/10.1016/j.mineng.2018.04.022 600 Anderson, C.G., 2012. The metallurgy of antimony. Chemie der Erde 72, 3–8. 601 https://doi.org/10.1016/j.chemer.2012.04.001 602 Andersson, M., Finne, T.E., Jensen, L.K., Eggen, O.A., 2018. Geochemistry of a copper 603 31 mine tailings deposit in Repparfjorden, northern Norway. Sci. Total Environ. 644, 604 1219–1231. https://doi.org/10.1016/j.scitotenv.2018.06.385 605 Araya, N., Lucay, F.A., Cisternas, L.A., Gálvez, E.D., 2018. Design of Desalinated 606 Water Distribution Networks: Complex Topography, Energy Production, and 607 Parallel Pipelines. Ind. Eng. Chem. Res. 57. 608 https://doi.org/10.1021/acs.iecr.7b05247 609 Barros, L., Andrade, H.D., Brigolini, G.J., Peixoto, F., Mendes, J.C., Andr, R., 2018. 610 Reuse of iron ore tailings from tailings dams as pigment for sustainable paints Iron 611 Ore Tailings Dams Failures in Brazil 200, 412–422. 612 https://doi.org/10.1016/j.jclepro.2018.07.313 613 Bhojwani, S., Topolski, K., Mukherjee, R., Sengupta, D., El-Halwagi, M.M., 2019. 614 Technology review and data analysis for cost assessment of water treatment 615 systems. Sci. Total Environ. https://doi.org/10.1016/j.scitotenv.2018.09.363 616 Binnemans, K., Jones, P.T., Blanpain, B., Van Gerven, T., Pontikes, Y., 2015. Towards 617 zero-waste valorisation of rare-earth-containing industrial process residues: A 618 critical review. J. Clean. Prod. 99, 17–38. 619 https://doi.org/10.1016/j.jclepro.2015.02.089 620 Binnemans, Koen, Jones, P.T., Blanpain, B., Van Gerven, T., Yang, Y., Walton, A., 621 Buchert, M., 2013. Recycling of rare earths: A critical review. J. Clean. Prod. 51, 1–622 22. https://doi.org/10.1016/j.jclepro.2012.12.037 623 Binnemans, K., Jones, P.T., Van Acker, K., Blanpain, B., Mishra, B., Apelian, D., 2013. 624 Rare-earth economics: The balance problem. Jom 65, 846–848. 625 https://doi.org/10.1007/s11837-013-0639-7 626 Blengini, G.A., Nuss, P., Dewulf, J., Nita, V., Peirò, L.T., Vidal-Legaz, B., Latunussa, C., 627 Mancini, L., Blagoeva, D., Pennington, D., Pellegrini, M., Van Maercke, A., Solar, 628 S., Grohol, M., Ciupagea, C., 2017. EU methodology for critical raw materials 629 assessment: Policy needs and proposed solutions for incremental improvements. 630 Resour. Policy 53, 12–19. https://doi.org/10.1016/j.resourpol.2017.05.008 631 Ceniceros-Gómez, A.E., Macías-Macías, K.Y., de la Cruz-Moreno, J.E., Gutiérrez-Ruiz, 632 M.E., Martínez-Jardines, L.G., 2018. Characterization of mining tailings in México 633 for the possible recovery of strategic elements. J. South Am. Earth Sci. 88, 72–79. 634 https://doi.org/10.1016/j.jsames.2018.08.013 635 Choi, C.H., Eun, J., Cao, J., Lee, S., Zhao, F., 2018. Global strategic level supply 636 planning of materials critical to clean energy technologies – A case study on 637 indium. Energy 147, 950–964. https://doi.org/10.1016/j.energy.2018.01.063 638 Cisternas, L.A., Gálvez, E.D., 2018. The use of seawater in mining. Miner. Process. 639 Extr. Metall. Rev. 39, 18–33. https://doi.org/10.1080/08827508.2017.1389729 640 Cisternas, L.A., Lucay, F., Gálvez, E.D., 2014. Effect of the objective function in the 641 design of concentration plants. Miner. Eng. 63, 16–24. 642 https://doi.org/http://dx.doi.org/10.1016/j.mineng.2013.10.007 643 32 Curry, J.A., Ismay, M.J.L., Jameson, G.J., 2014. Mine operating costs and the potential 644 impacts of energy and grinding. Miner. Eng. 56, 70–80. 645 https://doi.org/10.1016/j.mineng.2013.10.020 646 De Reyck, B., Degraeve, Z., Vandenborre, R., 2008. Project options valuation with net 647 present value and decision tree analysis. Eur. J. Oper. Res. 184, 341–355. 648 https://doi.org/10.1016/j.ejor.2006.07.047 649 De Sá, G., 2019. Brazil’s deadly dam disaster may have been preventable [WWW 650 Document]. January 29. URL 651 https://www.nationalgeographic.com/environment/2019/01/brazil-brumadinho-mine-652 tailings-dam-disaster-could-have-been-avoided-say-environmentalists/ (accessed 653 3.27.19). 654 Dold, B., Fontboté, L., 2001. Element cycling and secondary mineralogy in porphyry 655 copper tailings as a function of climate, primary mineralogy, and mineral 656 processing. J. Geochemical Explor. 74, 3–55. https://doi.org/10.1016/S0375-657 6742(01)00174-1 658 Edraki, M., Baumgartl, T., Manlapig, E., Bradshaw, D., Franks, D.M., Moran, C.J., 2014. 659 Designing mine tailings for better environmental, social and economic outcomes: A 660 review of alternative approaches. J. Clean. Prod. 84, 411–420. 661 https://doi.org/10.1016/j.jclepro.2014.04.079 662 El Wali, M., Golroudbary, S.R., Kraslawski, A., 2019. Impact of recycling improvement 663 on the life cycle of phosphorus. Chinese J. Chem. Eng. 27, 1219–1229. 664 https://doi.org/10.1016/j.cjche.2018.09.004 665 European Commission, 2017a. Communication from the Commission to the European 666 Parliament, the Council, the European Economic and Social Committee and the 667 Committee of the Regions on the 2017 list of Critical Raw Materials for the EU. 668 European Commission, 2017b. Study on the review of the list of critical raw materials - 669 Publications Office of the EU [WWW Document]. Publ. Off. EU. URL 670 https://publications.europa.eu/en/publication-detail/-/publication/08fdab5f-9766-671 11e7-b92d-01aa75ed71a1/language-en (accessed 6.10.18). 672 European Commission, 2014. Communication from the Commission to the European 673 Parliament, The Council, The European Economic and Social Committee and The 674 Committee of the regions on the review of the list of critical raw materials for the EU 675 and the implementation of the Raw Materia [WWW Document]. URL https://eur-676 lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:52014DC0297 (accessed 677 6.12.18). 678 European Commission, 2011. Communication from the Commission to the European 679 Parliament, The Council, the European Economic and Social Committee and the 680 Committee of the Regions teckling the challenges in commodity markets and on 681 raw materials [WWW Document]. URL https://eur-lex.europa.eu/legal-682 content/EN/TXT/?uri=CELEX:52011DC0025 (accessed 6.3.18). 683 Falagán, C., Grail, B.M., Johnson, D.B., 2017. New approaches for extracting and 684 33 recovering metals from mine tailings. Miner. Eng. 106, 71–78. 685 https://doi.org/10.1016/j.mineng.2016.10.008 686 Figueiredo, J., Vila, M.C., Matos, K., Martins, D., Futuro, A., Dinis, M. de L., Góis, J., 687 Leite, A., Fiúza, A., 2018. Tailings reprocessing from Cabeço do Pião dam in 688 Central Portugal: A kinetic approach of experimental data. J. Sustain. Min. 17, 139–689 144. https://doi.org/10.1016/j.jsm.2018.07.001 690 Foo, N., Bloch, H., Salim, R., 2018. The optimisation rule for investment in mining 691 projects. Resour. Policy 55, 123–132. 692 https://doi.org/10.1016/j.resourpol.2017.11.005 693 Ghorbani, Y., Kuan, S.H., 2017. A review of sustainable development in the Chilean 694 mining sector: past, present and future. Int. J. Mining, Reclam. Environ. 31, 137–695 165. https://doi.org/10.1080/17480930.2015.1128799 696 Graedel, T.E., Harper, E.M., Nassar, N.T., Nuss, P., Reck, B.K., Turner, B.L., 2015. 697 Criticality of metals and metalloids. Proc. Natl. Acad. Sci. U. S. A. 112, 4257–4262. 698 https://doi.org/10.1073/pnas.1500415112 699 Herrera-León, S., Lucay, F.A., Cisternas, L.A., Kraslawski, A., 2019. Applying a multi-700 objective optimization approach in designing water supply systems for mining 701 industries. The case of Chile. J. Clean. Prod. 210. 702 https://doi.org/10.1016/j.jclepro.2018.11.081 703 Hornby, L., Sanderson, H., 2019. Rare earths: Beijing threatens a new front in the trade 704 war [WWW Document]. Financ. Times. URL https://www.ft.com/content/3cd18372-705 85e0-11e9-a028-86cea8523dc2 (accessed 2.5.20). 706 Hudson Resources Inc, 2013. Sarfartoq Rare Earth Elements Project [WWW 707 Document]. URL https://hudsonresourcesinc.com/projects/sarfartoq-rare-earth-708 element-project/ (accessed 6.6.18). 709 Ibáñez-Forés, V., Bovea, M.D., Pérez-Belis, V., 2014. A holistic review of applied 710 methodologies for assessing and selecting the optimal technological alternative 711 from a sustainability perspective. J. Clean. Prod. 70, 259–281. 712 https://doi.org/10.1016/j.jclepro.2014.01.082 713 Innocenzi, V., De Michelis, I., Kopacek, B., Vegliò, F., 2014. Yttrium recovery from 714 primary and secondary sources: A review of main hydrometallurgical processes. 715 Waste Manag. 34, 1237–1250. https://doi.org/10.1016/j.wasman.2014.02.010 716 Jordens, A., Cheng, Y.P., Waters, K.E., 2013. A review of the beneficiation of rare earth 717 element bearing minerals. Miner. Eng. 41, 97–114. 718 https://doi.org/10.1016/j.mineng.2012.10.017 719 Jorjani, E., Shahbazi, M., 2016. The production of rare earth elements group via tributyl 720 phosphate extraction and precipitation stripping using oxalic acid. Arab. J. Chem. 9, 721 S1532–S1539. https://doi.org/10.1016/j.arabjc.2012.04.002 722 Khalil, A., Argane, R., Benzaazoua, M., Bouzahzah, H., Taha, Y., Hakkou, R., 2019. 723 Pb–Zn mine tailings reprocessing using centrifugal dense media separation. Miner. 724 34 Eng. 131, 28–37. https://doi.org/10.1016/j.mineng.2018.10.023 725 Khorasanipour, M., 2015. Environmental mineralogy of Cu-porphyry mine tailings, a 726 case study of semi-arid climate conditions, sarcheshmeh mine, SE Iran. J. 727 Geochemical Explor. 153, 40–52. https://doi.org/10.1016/j.gexplo.2015.03.001 728 Khorasanipour, M., Jafari, Z., 2017. Environmental geochemistry of rare earth elements 729 in Cu-porphyry mine tailings in the semiarid climate conditions of Sarcheshmeh 730 mine in southeastern Iran. Chem. Geol. 477, 58–72. 731 https://doi.org/10.1016/j.chemgeo.2017.12.005 732 Kinnunen, P.H.M., Kaksonen, A.H., 2019. Towards circular economy in mining: 733 Opportunities and bottlenecks for tailings valorization. J. Clean. Prod. 228, 153–734 160. https://doi.org/10.1016/j.jclepro.2019.04.171 735 Kirchherr, J., Reike, D., Hekkert, M., 2017. Conceptualizing the circular economy: An 736 analysis of 114 definitions. Resour. Conserv. Recycl. 127, 221–232. 737 https://doi.org/10.1016/j.resconrec.2017.09.005 738 Kodukula, P., Papudesu, C., 2006. Project Valuation Using Real Options - A 739 practitioner’s guide. J. Ross Publishing, Fort Lauderdale, Florida. 740 Koltun, P., Tharumarajah, A., 2014. Life Cycle Impact of Rare Earth Elements. ISRN 741 Metall. 2014, 1–10. https://doi.org/10.1155/2014/907536 742 Kossoff, D., Dubbin, W.E., Alfredsson, M., Edwards, S.J., Macklin, M.G., Hudson-743 Edwards, K.A., 2014. Mine tailings dams: Characteristics, failure, environmental 744 impacts, and remediation. Appl. Geochemistry 51, 229–245. 745 https://doi.org/10.1016/j.apgeochem.2014.09.010 746 Larsson, M., Nosrati, A., Kaur, S., Wagner, J., Baus, U., Nydén, M., 2018. Copper 747 removal from acid mine drainage-polluted water using glutaraldehyde-748 polyethyleneimine modified diatomaceous earth particles. Heliyon 4, e00520. 749 https://doi.org/10.1016/j.heliyon.2018.e00520 750 Leader, A., Gaustad, G., Babbitt, C., 2019. The effect of critical material prices on the 751 competitiveness of clean energy technologies. Mater. Renew. Sustain. Energy 8, 752 1–17. https://doi.org/10.1007/s40243-019-0146-z 753 Lèbre, É., Corder, G., Golev, A., 2017. The Role of the Mining Industry in a Circular 754 Economy: A Framework for Resource Management at the Mine Site Level. J. Ind. 755 Ecol. 21, 662–672. https://doi.org/10.1111/jiec.12596 756 Lee, J., 2018. Prophecy Announces Positive Preliminary Economic Assessment Study 757 for the Gibellini Vanadium Project - Junior Mining Network [WWW Document]. URL 758 https://www.juniorminingnetwork.com/junior- miner-news/press-releases/693-759 tsx/pcy/47430-prophecy-announces-positive-preliminary-economic-assessment-760 study-for-the-gibellini-vanadium-project.html (accessed 6.5.18). 761 LME, 2018. London Metal Exchange: LME Cobalt [WWW Document]. URL 762 https://www.lme.com/en-GB/Metals/Minor-metals/Cobalt#tabIndex=0 (accessed 763 1.7.18). 764 35 Lottermoser, B.G., 2011. Recycling, reuse and rehabilitation of mine wastes. Elements 765 7, 405–410. https://doi.org/10.2113/gselements.7.6.405 766 Markovaara-Koivisto, M., Valjus, T., Tarvainen, T., Huotari, T., Lerssi, J., Eklund, M., 767 2018. Preliminary volume and concentration estimation of the Aijala tailings pond – 768 Evaluation of geophysical methods. Resour. Policy 59, 7–16. 769 https://doi.org/10.1016/j.resourpol.2018.08.016 770 Mendez, M.O., Maier, R.M., 2008. Phytostabilization of mine tailings in arid and 771 semiarid environments - An emerging remediation technology. Environ. Health 772 Perspect. 116, 278–283. https://doi.org/10.1289/ehp.10608 773 Mineralprices.com, 2018. Rare Earth Metals [WWW Document]. URL 774 http://mineralprices.com/rare-earth-metals/ (accessed 7.3.18). 775 Ministerio de Minería, 2011. Ley 20.551 - Ministerio de Minería [WWW Document]. URL 776 http://www.minmineria.gob.cl/leyes-sectoriales/ley-20551/ (accessed 2.5.20). 777 Miyauchi, A., Okabe, T.H., 2010. Production of Metallic Vanadium by Preform 778 Reduction Process. Mater. Trans. 51, 1102–1108. 779 https://doi.org/10.2320/matertrans.M2010027 780 Mohamed, S., van der Merwe, E.M., Altermann, W., Doucet, F.J., 2017. Process 781 development for elemental recovery from PGM tailings by thermochemical 782 treatment: Preliminary major element extraction studies using ammonium sulphate 783 as extracting agent. Waste Manag. 66, 222–224. 784 https://doi.org/10.1016/j.wasman.2017.04.009 785 Moodley, I., Sheridan, C.M., Kappelmeyer, U., Akcil, A., 2017. Environmentally 786 sustainable acid mine drainage remediation: Research developments with a focus 787 on waste/by-products. Miner. Eng. 126, 1–14. 788 https://doi.org/10.1016/j.mineng.2017.08.008 789 Moran-Palacios, H., Ortega-Fernandez, F., Lopez-Castaño, R., Alvarez-Cabal, J. V., 790 2019. The Potential of Iron Ore Tailings as Secondary Deposits of Rare Earths. 791 Appl. Sci. 9, 2913. https://doi.org/10.3390/app9142913 792 Mudd, G.M., Jowitt, S.M., Werner, T.T., 2018. Global platinum group element 793 resources, reserves and mining – A critical assessment. Sci. Total Environ. 622–794 623, 614–625. https://doi.org/10.1016/j.scitotenv.2017.11.350 795 Onishi, K., Nakamura, T., Nishihama, S., Yoshizuka, K., 2010. Synergistic solvent 796 impregnated resin for adsorptive separation of lithium ion. Ind. Eng. Chem. Res. 49, 797 6554–6558. https://doi.org/10.1021/ie100145d 798 Onyedika, G.O., Achusim-Udenko, A.C., Nwoko, C.I.A., Ogwuegbu, M.O.C., 2012. 799 Chemistry, processes and problems of complex ores utilization: Hydrometallurgical 800 options. Int. J. Chem. Sci. 10, 112–130. 801 Oyarzún, J., Oyarzun, R., Lillo, J., Higueras, P., Maturana, H., Oyarzún, R., 2016. 802 Distribution of chemical elements in calc-alkaline igneous rocks, soils, sediments 803 and tailings deposits in northern central Chile. J. South Am. Earth Sci. 69, 25–42. 804 36 https://doi.org/10.1016/j.jsames.2016.03.004 805 Peelman, S., Kooijman, D., Sietsma, J., Yang, Y., 2018. Hydrometallurgical Recovery of 806 Rare Earth Elements from Mine Tailings and WEEE. J. Sustain. Metall. 4, 367–377. 807 https://doi.org/10.1007/s40831-018-0178-0 808 Peelman, S., Sun, Z.H.., Sietsma, J., Yang, Y., 2016. Hydrometallurgical Extraction of 809 Rare Earth Elements From Low Grade Mine Tailings. Rare Met. Technol. 2016 17–810 29. https://doi.org/10.1007/978-3-319-48135-7_2 811 Peelman, S., Sun, Z.H.I., Sietsma, J., Yang, Y., 2014. Leaching of Rare Earth 812 Elements : Past and Present. ERES2014 1st Eur. Rare Earth Resour. Conf. 446–813 456. https://doi.org/10.1016/B978-0-12-802328-0.00021-8 814 Ramírez, Y., Kraslawski, A., Cisternas, L.A., 2019. Decision-support framework for the 815 environmental assessment of water treatment systems. J. Clean. Prod. 225, 599–816 609. https://doi.org/10.1016/j.jclepro.2019.03.319 817 Sadri, F., Nazari, A.M., Ghahreman, A., 2017. A review on the cracking, baking and 818 leaching processes of rare earth element concentrates. J. Rare Earths 35, 739–819 752. https://doi.org/10.1016/S1002-0721(17)60971-2 820 Santander, C., Robles, P.A., Cisternas, L.A., Rivas, M., 2014. Technical-economic 821 feasibility study of the installation of biodiesel from microalgae crops in the 822 Atacama Desert of Chile. Fuel Process. Technol. 125, 267–276. 823 https://doi.org/10.1016/j.fuproc.2014.03.038 824 Schoenberger, E., 2016. Environmentally sustainable mining: The case of tailings 825 storage facilities. Resour. Policy 49, 119–128. 826 https://doi.org/10.1016/j.resourpol.2016.04.009 827 SERNAGEOMIN, 2018. Datos Públicos Depósito de Relaves [WWW Document]. URL 828 https://www.sernageomin.cl/datos-publicos-deposito-de-relaves/ (accessed 829 5.10.18). 830 SERNAGEOMIN, 2017. Anuario de la Minería de Chile [WWW Document]. URL 831 https://www.sernageomin.cl/anuario-de-la-mineria-de-chile/ (accessed 9.10.18). 832 SERNAGEOMIN, 2011. Ley 20.551 - Cierre de Faenas Mineras [WWW Document]. 833 URL https://www.sernageomin.cl/cierre-de-faenas-mineras/ (accessed 2.5.20). 834 Sracek, O., Mihaljevič, M. Kříbek, B., Majer, V. Veselovský, F., 2010. Geochemistry and 835 mineralogy of Cu and Co in mine tailings at Copperbelt, Zambia. J. African Earth 836 Sci. 57, 14–30. https://doi.org/10.1016/j.jafrearsci.2009.07.008 837 SUN, X., JI, Y., CHEN, J., MA, J., 2009. Solvent impregnated resin prepared using task-838 specific ionic liquids for rare earth separation. J. Rare Earths 27, 932–936. 839 https://doi.org/10.1016/S1002-0721(08)60365-8 840 Tapia, J., Davenport, J., Townley, B., Dorador, C., Schneider, B., Tolorza, V., von 841 Tümpling, W., 2018. Sources, enrichment, and redistribution of As, Cd, Cu, Li, Mo, 842 and Sb in the Northern Atacama Region, Chile: Implications for arid watersheds 843 37 affected by mining. J. Geochemical Explor. 185, 33–51. 844 https://doi.org/10.1016/j.gexplo.2017.10.021 845 Thenorthernminer.com, 2018. The Northern Miner – Mining News Since 1915 [WWW 846 Document]. URL https://www.northernminer.com/ (accessed 1.7.18). 847 Tunsu, C., Menard, Y., Eriksen, D.Ø., Ekberg, C., Petranikova, M., 2019. Recovery of 848 critical materials from mine tailings: A comparative study of the solvent extraction of 849 rare earths using acidic, solvating and mixed extractant systems. J. Clean. Prod. 850 218, 425–437. https://doi.org/10.1016/j.jclepro.2019.01.312 851 Vargas, F., Lopez, M., 2018. Development of a new supplementary cementitious 852 material from the activation of copper tailings: Mechanical performance and 853 analysis of factors. J. Clean. Prod. 182, 427–436. 854 https://doi.org/10.1016/j.jclepro.2018.01.223 855 Vekasi, K., Hunnewell, N.L., 2019. China’s Control of Rare Earth Metals [WWW 856 Document]. Natl. Bur. Asian Res. URL https://www.nbr.org/publication/chinas-857 control-of-rare-earth-metals/ (accessed 2.5.20). 858 World Economic Forum, 2016. Mapping Mining to the Sustainable Development Goals: 859 An atlas [WWW Document]. URL 860 https://www.undp.org/content/dam/undp/library/Sustainable 861 Development/Extractives/Mapping_Mining_SDGs_An_Atlas_Executive_Summary_862 FINAL.pdf (accessed 2.7.20). 863 Xiang, J., Huang, Q., Lv, X., Bai, C., 2018. Extraction of vanadium from converter slag 864 by two-step sulfuric acid leaching process. J. Clean. Prod. 170, 1089–1101. 865 https://doi.org/10.1016/j.jclepro.2017.09.255 866 Yoon, H.S., Kim, C.J., Chung, K.W., Kim, S.D., Lee, J.Y., Kumar, J.R., 2016. Solvent 867 extraction, separation and recovery of dysprosium (Dy) and neodymium (Nd) from 868 aqueous solutions: Waste recycling strategies for permanent magnet processing. 869 Hydrometallurgy 165, 27–43. https://doi.org/10.1016/j.hydromet.2016.01.028 870 Žižlavský, O., 2014. Net Present Value Approach: Method for Economic Assessment of 871 Innovation Projects. Procedia - Soc. Behav. Sci. 156, 506–512. 872 https://doi.org/10.1016/j.sbspro.2014.11.230 873 874 875 876 877 878 879 880 38 881 List of figures 882 Fig. 1: Simplified mining processes flowsheet featuring conventional process to obtain 883 metal and the re-processing of tailings to obtain CRMs. 884 Fig. 2: Tailings storage facilities in Region II, blue represents inactive or abandoned 885 deposits and red is for active deposits. 886 Fig. 3: Sensitivity analysis, a) Sensitivity of the NPV (REOs project) to the price of 887 REOs, operating costs and capital costs; b) Sensitivity of NPV to discount rate in REOs 888 project; c) sensitivity of NPV (vanadium project) to the price of V2O5, operating costs and 889 capital costs; d) Sensitivity of NPV to discount rate in vanadium project. 890 891 Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: