Rare-earth elements (REEs), particularly heavy-REEs (HREEs), are key enablers for rapid transition to a decarbonized world, as they are indispensable raw materials in many low-carbon technologies1, 2. The reliance of low-carbon technologies on HREE has resulted in a growing demand for HREE globally. However, the sourcing of HREE, primarily ion-adsorption deposits (IADs)3, 4, accounts for less than 4% of the total REE resources but supplies more than 95% of the global HREE demand5. The unbalance between the high demand and limited supply poses a significant challenge to the REE mining industry6. However, the historic environmental record of REE mining is disastrous7.
The current mining technique for IADs is predominantly in situ leaching, where highly concentrated ammonium salt solutions are injected into weathering crust soils for ion-exchanging REEs7. To achieve high yields of REE recovery, excessive ammonium salts are consumed in in-situ leaching practices, leading to severe environmental issues. The use of ammonium-salt-based leaching techniques has thus been restricted for new mining sites since 20188, exacerbating the imbalance between HREE supply and demand. Moreover, the current leaching technique has a low recovery rate (40%-60%), due to the leakage of REE fluids in undesirable directions. Hence, new approaches to mine REEs more effectively and sustainably are urgently needed9, 10, 11.
Recently, we presented a novel approach, electrokinetic-mining (EKM) that applies electric fields, for green and efficient recovery of REEs from IADs8. The viability of the EKM technique has been demonstrated at different spatial scales (from grams to 14 tons scale), yet, confirmation of its applicability at the industrial scale is still pending. The transition of the EKM technique from laboratory-scale to industrial-scale poses significant challenges12, 13, delineated by three key distinctions: (1) The reliability of electrodes over extended periods of months in a moist and erosive environment. (2) The potential for subsurface flow leakage in a large-scale mining site. (3) The mining efficiency when operating at a scale of tens of meters. Thus, whether the EKM technique can be a cure for the current mining industry remains a mystery.
Here we present a demonstration of the EKM technique at an industrial-scale IAD ore and assess its environmental impacts and economic costs. The mining site is in Meizhou City, South China, representing a typical IAD (see Sections S1-S2 for detailed characterizations). Results suggest that the spatial scale of the mining ore is 3400 m3 containing 5100 tons of soil weight and 1.54 t of the industrially minable (REE content > 400 mg/kg) REE Oxide (REO). The bedrock layer is funnel-shaped, with the middle zone being lower than the surrounding zones. This type of IAD ore presents difficulties for traditional leaching methods as the REE leachates tend to migrate towards the central zone, complicating their efficient collection.
We design an EKM workflow (Fig. 1a) that involves several key procedures, including the injection of leaching agents, electrokinetic mining, recovery and treatment of REE leachates, and regeneration of leaching agents. The working principle is depicted in Fig. 1b. To address the issue of electrode degradation in erosive environments, we have developed a novel type of electrode called conductive plastic electrodes (CPE) with exceptional conductivity (1.861×104 S/m) and withstand strike current ability (70 A). By utilizing the newly developed CPE, the electro-corrosion is prevented and water electrolysis is reduced (see Section S3 for details).
To overcome the challenge of subsurface flow leakage, we utilized electro-osmosis to control the subsurface flow in a single direction. By applying a high voltage to the three outer edges of the mining site, we established a high voltage gradient (80 V·m-1) along the boundaries, known as the voltage gradient barrier (VGB, Fig. 1c). Results demonstrated that REEs and subsurface flow were contained within the VGB (see Section S4 for detailed VGB mechanism).
In real industrial applications of EKM, the presence of multiple electrodes often leads to interference, resulting in increased energy consumption and decreased mining efficiency. To avoid these adverse effects, we proposed an intermittent power alternation (IPA) strategy, that is, one third of electrodes is powered on in turn (see Section S5 for details). Results suggested that the IPA strategy is promising in reducing electrode impacts and power consumption while ensuring the recovery rate of REE. Note that the whole mining process is automatically controlled by a self-developed smart power supply.
During a 60-day period of EKM implementation, we achieved the recovery of 1.47 t of REO, equating to a recovery rate of 95.5% (Figs. 2a-b). Moreover, the EKM approach offers significant advantages (Section S6): 1) reducing the consumption of ammonium salt by ~80%, 2) shortening the mining time by 80%, 3) increasing the recovery rate by over 30%, and 4) reducing the Aluminum impurity by 90% compared to the conventional leaching technique (Fig. 1c).
For environmental assessments, we placed three environmental monitoring wells along the upstream, midstream, and downstream of the mining ore, respectively, to monitor variations of contaminates (e.g., NH4+) and REE in the leachates, surrounding groundwater, and surface water during the EKM process (Section S7). It is noteworthy that the emission of NH4+ is significantly reduced by over 95% in all the leachate, groundwater, and surface water (Figs. 1d-1f), in comparison to the conventional technique. Moreover, REE concentrations in both the groundwater and surface water changed little after EKM, suggesting that no leakage occurred in the EKM process.
Economically, we operate comparative techno-economic analysis (TEA) to assess the economic feasibility of the EKM technique (Section S8). The production costs ($6,214 vs $7,078) of the two techniques were comparable for obtaining 1 t of REO. Note that, the environmental costs, such as costs for soil remediation and water treatment, have not been considered yet, which are the main costs ($16,477) of the conventional technique8. The production cost of the conventional technique would be three times higher than that of the EKM technique. Thus, we suggest that the EKM technique is economically sustainable with a significant reduction in environmental costs.
The successful application and demonstration of the EKM technique on this 5000-ton-scale real IAD ore sets a precedent for environmentally friendly, efficient, and cost-effective REE mining, laying the groundwork for its practical adoption in sustainable mining practices. Combining the successful applications of electrokinetics in copper and gold mining14, 15, we conclude that the use of electrokinetics opens up a sustainable mining scenario that contributes to a low-carbon economy.