Tailings contain variable amounts of different elements that can cause damage to human health and the environment [4
]; this includes arsenic, cadmium, mercury, and others. The author of [4
], moreover, notes that “The risk of harm can be even higher for mining waste, ore processing tailings or metallurgical residues from older operations, as metal recovery rates were generally quite lower than modern operations.”
The copper production process begins with extraction, during which the copper can be present in different mineralogical species. Oxidized and sulfide minerals are the most commonly processed species. Copper sulfides are increasingly abundant, while copper oxides, which are processed by hydrometallurgy, are increasingly scarce, decreasing from 30.8% in 2015 to a projected 12.0% in 2027 [5
]. After extraction, both types of minerals must undergo a crushing process using crushers. However, in the case of sulfide minerals, after being crushed, they must be sent to the grinding stage before they are able to enter the process of mineral concentration. This last process takes advantage of the buoyant properties of copper minerals after they are incorporated into a series of chemical reagents and water. Froth flotation is one of the most important and widespread concentration methods. It is based on the exploitation of the differential properties of valuable and gangue minerals, also known as wettability [6
]. This process is carried out in flotation cells or tank reactors that are agitated by air bubbles injected into the pulp. Foam (which is hydrophobic) forms on the surface and is collected as a concentrate rich in copper ore; the pulp from this process feeds the next cell, and so on. Finally, the discards from this process, called “tailings”, are transported to a tank far from the facilities. These dams are built in uninhabited areas which are close to the mine and take advantage of the geography of the area.
1.1. Risks Involving Tailings Dams
Tailings deposits are a form of mining waste collection that have generated the most problems for the mining industry [7
]. In order to point out the impacts that have occurred due to catastrophes originating from tailings dam failures, a brief description of these events is presented. The WISE Uranium Project database [8
] presents a chronology of the main tailings dam failures in the world from 1961 to January 2023 (the date of the literature review). This organization notes that the availability of data is limited because the compilation is not complete; however, according to the evaluation made by the authors when cross-checking information with local sources, this information is reliable, reporting data such as date, location, company involved, type of mineral, type of incident, amount released, and impacts of dam failures. In the case of Chile, ten incidents involving copper ore were recorded during this period. From the most recent to the oldest, the first incident occurred in 2016, in the town of Ujina, Pica, Tamarugal Province, Tarapacá Region, in which 4500 cubic meters of tailings were spilled. The incident was caused by a rupture in a tailings transport chute. As a result, the material flowed toward an ancestral grazing area, threatening four vicuña specimens and a protected camelid species and ultimately reaching the groundwater. Since 2016, there have been no incidents in the country, which is a sign of the success of the regulations enacted by authorities in the last decade, together with the public and private sectors comprising the mining industry. The second episode was in 2003, in the town of Cerro Negro, Petorca Province, Valparaíso Region, which involved a failure of a tailings dam that spilled 50,000 tons of tailings; the material flowed 20 km downstream in the La Ligua river. The third and fourth episodes took place in 1985, located in Veta de Agua No. 1 and Cerro Negro No. 4. The first event occurred due to the failure of a dam wall, which was caused by liquefaction during an earthquake in 1985 and discharged 280,000 cubic meters. The tailings flowed five kilometers downstream. The second event also occurred due to the failure of a dam wall that was caused by liquefaction during the earthquake. In this event, 500,000 cubic meters of waste were released, and the tailings flowed eight kilometers downstream. Previously, another six episodes of spills occurred in 1965, in locations such as Bellavista, Cerro Negro No. 3, Cobre Nueva Dam, Cobre Vieja Dam, Nueva Patagua Dam, and Los Maquis. All these dam failures occurred during an earthquake that occurred that year. These six episodes, which were all specified as resulting from liquefaction, spilled 70,000, 85,000, 350,000, 1.9 million, 35,000, and 21,000 cubic meters, respectively. The tailings flowed 800 m, 5 km, 12 km (destroying the town of El Cobre and killing more than 200 people), 5 km, and 5 km in the first, second, third, fifth, and sixth cases, respectively; the fourth case lacks information. These catastrophes occurred due to various factors, including operational causes, material fatigue, and earthquakes, among others.
Despite the fact that a given landfill may be well-designed and may have approval from the relevant agencies, or the fact that tailings storage areas are considered to be “low-risk facilities”, it does not mean that such facilities can withstand inadequate management or operation over time. In [9
], the author mentions that “Risk assessments must be carried out before operations begin, and then periodically updated to duly eliminate all failures or deficiencies that may have occurred during construction or operation, in order to prevent potential accidents from occurring in an early stage”. In [10
], the authors discuss the causes of tailings dam failures, including seepage, foundation failure, overflow, and earthquakes, and provide a reference for tailings dam design and construction in order to reduce the occurrence of negative risks. The investments necessary to protect the environment and human health will be rewarded by avoiding the possible costs of remediating the consequences of accidents, which could be many times higher than the initial investment [11
]. Moreover, disasters may occur in sites cataloged as being “abandoned” and left in a worse condition, as was the case in the town of Chañaral, Chile, in March 2015 [12
], in which an alluvium-carrying material from abandoned tailings swept away the town. During the investigation [13
], the worrying situation in Chile was pointed out with respect to the abandonment of tailings fills due to the lack of environmental regulations prior to the enaction of the law on general environmental bases in 1930. These authors of [13
] indicate that waste was discharged into the soil, rivers, and seas for decades, thus pointing out the consequences of contamination on human health and the environment for several generations.
The Ministry of Mining of Chile supervises the public “National Geology and Mining Service”, known as SERNAGEOMIN, whose mission is to be a “technical body in charge of generating, maintaining and disseminating information on basic geology and resources and hazards geology of the national territory, for the well-being of the community and at the service of the country, and to regulate and/or supervise compliance with mining regulations in terms of safety, property and closure plans, to contribute to the development of national mining” [14
]. This service, in addition to other high-level organizations related to environmental regulations, such as the Ministry of the Environment (MMA), the Superintendence of the Environment (SMA), and the Environmental Evaluation Service (SEA), among others, provide the country with a system of government in environmental matters at the national level. In 2018, the Ministry of Mining announced a series of measures to provide security and information to the communities that are close to the presence of tailings dams. These tailings storage facilities can be classified as active tailings, inactive tailings, or abandoned. According to SERNAGEOMIN, “active tailings deposits” are those that have a known owner and are in an operating condition; those classified as “inactive” are deposits with known owners but are not currently being exploited and are still without legal closure; and those classified as “abandoned” are deposits that are not in operation and have an unknown owner or resolution of origin.
Chile ranks first in the world in copper production, with 5,588,084 fine metric tons produced during 2021, which is equivalent to 26.6% of the world’s production [7
]. Ore processing capacity, both in concentration and leaching, is concentrated in the center and north of the country, mainly in the Antofagasta Region, with 35% of the flotation capacity and 75% of the national leaching capacity [5
]. As shown in Table 1
, the largest amounts of tailings deposits are found in the Coquimbo and Atacama Regions, which are directly linked to sites with many scattered deposits, although this does not necessarily imply a greater mass of waste material.
After the disaster of the dam collapse in the town of Brumadinho, Brazil, which occurred on 25 January 2019, many countries became aware of the need to review their status to help prevent catastrophes. Chile was no stranger to this scenario, and in response, the government accelerated the announcement of the National Tailings Deposits Plan policy. This policy is based on three pillars: the safety of people, the protection of the environment, and the circular economy and innovation. In addition, a pilot plan for the direct monitoring of tailings dams and emergency management measures was initiated; the Ministry of Mining indicated that “this (policy) will have a direct relationship with SERNAGEOMIN and in turn with ONEMI (National Emergency Office), so that, as in the case of tsunami alerts, people downriver receive an alert to the cell phone in case of emergency to evacuate the area that could be affected” [15
]. As of 1 January 2023, ONEMI was transformed into the new National Disaster Prevention and Response Service (SENAPRED), an entity that will have the responsibility of advising, coordinating, organizing, planning, and supervising the activities related to Disaster Risk Management in the country.
1.2. Possible Use of Tailings Deposits
Mining is a fundamental industry for the development of modern society, but it is also the largest producer of waste in the world [16
]. There has been an increase in concentration processes, which means a growth in the amount of tailings produced since copper grades are decreasing; this, in turn, means processing a greater quantity of material in order to obtain the same amount of copper production, implying a greater amount of tailings as well. Sulfide residues usually represent between 80 and 98% of the weight and between 150 and 170% of the volume of the extracted ore [17
], as most of the metallic ores are processed using foam in the flotation process. The copper concentration in flotation tailings is reported to be relatively high, especially in northern Chile [2
]. It must also be added that the costs of the process of obtaining the copper concentrate represent 77% of the total cost [18
]. An opportunity to use tailings is presented in [19
], in which it is stated that the cost of extracting residual metals from waste can be more economically attractive than extracting it from a deep deposit. The authors present an experimental study in a copper mine in Las Cruces, Spain, which produces more than 1.5 million tons of mineral waste each year. An extraction rate of copper of up to an 85% (the maximum reached in bioleaching tests) could be reached using a bioleaching technique. They report that with that amount of waste, they could produce 9.2 kilotons of metal (with a value of ~USD 44 million as of 2016). Additional revenue could come from zinc, which could also be recovered by bioleaching (~USD 1.3 million per year). They also mention how much silver could be potentially extracted and recovered, although more research is needed for exploitation at an industrial scale. This study suggests that chemical extraction is a feasible option, but that the copper bioleaching process produces a mineral residue that is poor in silver. In these experiments, an efficient extraction (>90%) was achieved by bioleaching, and the authors state that the estimated total value of the copper that could be extracted and recovered from old waste using this method ranges from USD 225 to 337 million. Again, bioleaching and zinc recovery would have additional value (estimated at USD 30–45 million) [19
]. The production of tailings is expected to increase in coming years due to the intensive mining of low-grade copper ores [20
]. This study contributes to the promotion of a circular economy in abandoned and inactive tailings, demonstrating in a novel way the use of multi-criteria decision models which could be applied to any similar case, with a particular application to the Coquimbo region in Northern Chile.