(613d) Circular Economy Approach in the Valorization of Rare Earths Generated in Alluvial Mining By Quantifying Environmental Impacts through Life Cycle Analysis

Despite the great efforts of society in relation to the efficient use of resources, circular economy and dematerialization itself, the growing demand in the consumption of goods and services continues to rise, which translate into the increase of extraction and production of primary metals. The availability and access to these resources are fundamental conditions to guarantee human welfare and global economies functioning [1]–[4]. Circular economy aim is to keep resources in a useful state for as long as possible [5], [6], under strategies such as diminishing virgin resources; mainly non-renewable resources; and avoiding the waste generation or implementing reuse and recycling strategies to maximizing the use of renewable resources and reduce the environmental impacts (Eco-efficiency) [7], [8].

The mining sector supplies vital raw materials and energy to a large number of industries. Its activities are still commonly considered to be a threat to the environment, especially because of the effects they have on the air, water and soil, such as greenhouse gas emissions, destruction of ecosystems, damage to protected areas, pollution and affectation to availability of renewable and non-renewable resources [9][10] [11]. These impacts are expected to increase exponentially because the ore grade (metal content) has been falling globally for some time as presented by the Hubber Peak model: The minerals have an increasing speed of production until reaching their maximum peak, and then decline as fast as it grew [12]. This involves processing more rock for an equivalent amount of metal [13], leading to higher consumption of energy resources, water, chemicals and other operating costs; thus, more waste/emissions are generated, mainly in mining and processing stages [14], [15] [16], [17], besides fluctuation in the price of minerals. That is why it is absolutely urgent to make adequate assessments of mineral resources and mining operations to enable better management of mineral capital on Earth to face these challenges [13]. Therefore, mining companies face unprecedented social pressure to assume their commitment to seek competitive advantages in the long term through responsible management of environmental and social problems in response to the economic profits obtained (Botín & Vergara, 2015). Sustainability is being used more and more to describe a paradigm that supports the configuration of social and economic future of humanity (Kharrazi, Kraines, Hoang, & Yarime, 2014)

Life Cycle Assessment (LCA) has been accepted by the European waste policy as a useful tool to measure the impact of products and services on the environment and reduce it by circular economy implementation. Results on the implementation of the Life cycle analysis on gold alluvial mining to assess how sustainable it is this extraction technique. Alluvial mining is the extractive method that presents lowest environmental impacts compared with open-pit and underground mining [5]. However, alluvial technique is less conventional because require geomorphological characteristics more specific; deposits of the flood plain. Residual tails from mining alluvial extraction, presents a high potential of add economic value.

Research carried out in the region of the Bagre-Nechí mining district in Colombia, indicate that this alluvial gold deposit contains mineral resources (monazite) with the potential to be used to obtain REEs in the tailings of mining operations (Luver ref and 9 y 10 from Luver). Rare-Earth Elements (REEs) are a group of chemical elements that include all of the lanthanides (Ln), yttrium, and scandium, often divided into two categories: light rare-earth elements (LREEs), comprising La, Ce, Nd and Pr, and heavy rare earth elements (HREEs) that involve Sm to Lu, Sc and Y [1]. Due to their unique properties, REEs are widely used in applications such as permanent magnets, energy storage systems, superconductors, electronics, and metal alloys. The importance of REEs is growing every day due to their applications in modern technology and their consequent role in the fourth industrial revolution [2]. With the growing global demand for REEs in recent years, traditional prospects for rare earth mining have begun to be reassessed and new extraction possibilities considered. Tails from alluvial mining operations in the Bagre-Nechí mining district in Colombia indicate the presence of monazite in concentrations ranging from 1.1 to 2.0% by weight, with concentrations of rare earth oxides between 55 and 63% [3, 4]. Monazite [(Ce, La, Nd, Th)PO4] is a phosphate of rare-earth elements (REEs) [9] and is one of the most critical rare earth minerals in the world, serving as the main source of thorium and light rare earth elements (LREE) such as lanthanum, cerium, neodymium and praseodymium [10]. Industrial monazite concentrates typically present concentrations between 55 and 65% of rare earth oxides [5, 6]. In this sense it is necessary to carry out concentration studies of the mining tailings produced in this place. Although the REE content in tailings may be lower than in primary sources, their processing may be justified considering environmental benefits, for example mine site remediation and land reclamation. Furthermore, future REE supplies will likely depend on numerous unconventional resources other than classical ore deposits. Among those unconventional resources, low-grade and tailings deposits represent the next logical step for the mining industry, as shown by the decline in minimum cut-off grades for all metals over time [7, 8]. In the year 2011 as a consequence of socio-political situations, and export restriction from China, REEs price peak [30], [31]. Therefore, the rest of the world is motivated to find alternative sources of REEs, and many initiatives to increase resource efficiency, find substitutes, increase recycling, and explore new mining areas have been launched[31], [32]. However the price decrese substanciality (in 80%), due to the production initiatives was not profitable [30].

To date, no studies on the environmental analysis of alluvial mining tails valorization are have done. Regarding the implementation of the LCA on the monazite concentration process as the principal supply of the REEs production whether from virgin mineral raw material extraction or recycling sources neither has been carried out. For REEs and heavy rare earth oxides (HREOs) synthesis process, few studies about the environmental impact under LCA have been done, most of which are based on the largest REE-producing mine—the Bayan Obo deposit in China [33]–[36]. Deng and Kendall (2019) collected primary data did the most recent research from sites in China producing HREOs from ion-adsorption clays, conducting an LCA. Results show that 1 kg of mixed HREOs emit 258–408 kg CO2e, and consume 270–443 MJ primary energy [37], but the authors have not focused on the environmental impact categories but on the cope with the gap life cycle inventory data. Other previous studies as Vahidi et al. (2016) and Schulze et al. (2017) were the first to publish an LCA of REO production from ion-adsorption clays [38], [39]. Zapp et al. (2018) analyzed the production of dysprosium (REE), from ion adsorption clays and bastnaesite/monazite, but they did not report results in terms of mixed REO [40]. Other studies have considered the process route for heavy REO and environmental impacts caused from producing heavy REO, but not from a LCA perspective [41]–[43]. Adibi et al., (2018) calculate some characterization factors for REEs. Characterization factors allow calculate the resources use as category damage in the life cycle assessment and impact categories as Abiotic Depletion Potential under ReCiPe method [44].

In this research is evaluate the environmental impacts from alluvial mining from cradle to gate; that it is, from prospecting stage, extraction, refining process until the gold ingot produced. Additionally, is evaluate routes for recovery of mining residues (rare earths) from the black sands of alluvial gold mining in environmental terms. The Life Cycle Analysis is implemented too as a tool for evaluating the environmental sustainability of production processes to identify the environmental impacts of the selected black lands valorization route in order to propose more sustainable production patterns that allow a better disposal, use and recovery of the waste generated.