Toward Sustainable Mining: Exploring Alternative Mineral Resources and Innovative Extraction Techniques

Abstract

The relentless pace of industrialisation and globalisation has precipitated the rapid depletion of surface mineral deposits, presenting a formidable challenge to conventional mining operations and exerting a detrimental impact on their profitability. This depletion, coupled with the escalating demand for minerals, has driven prices to unprecedented highs, thereby inflating operating costs across various industries. Traditional surface and underground mining methods, struggling to meet burgeoning demands, contribute significantly to environmental degradation and substantial energy consumption. In response to these challenges, this study advocates for a paradigm shift from conventional mining methods and mineral resources toward untapped alternatives that hold the potential for enhanced economic viability and sustainability. Utilising environmentally friendly techniques and adopting more economical approaches becomes paramount in addressing the pressing demands of the current era and securing resources for future generations. This short review examines potential alternative mineral resources and the associated mining methods, including fluidised mining, deep-sea mining, brine mining, urban mining, in-situ and heap leaching, and space mining. A meticulous evaluation of the state-of-the-art technologies developed for these unconventional methods is conducted, including an assessment of their respective advantages and disadvantages. Finally, the study deliberates on the prospects of each approach, elucidating their potential contributions to alleviating the global metal crisis. This research provides insights that can inform sustainable mining practices and guide the industry toward a more environmentally responsible and economically viable future. The urgency of such a transition is underscored by the need to address the challenges posed by conventional mining and ensure the availability of mineral resources for generations to come.

1. Introduction

With the onset of accelerated globalisation, the usage of natural resources is skyrocketing, thereby placing unprecedented pressure on mineral reserves. According to the United Nations Environment Programme (UNEP), global material consumption is expected to grow substantially by mid-century. Importantly, this growth is driven not only by the exponential increase in world population but also by rising per capita mineral consumption associated with urbanisation, industrialisation, and the energy transition [1]. For example, the World Bank projects that demand for critical minerals such as lithium, cobalt, and graphite will rise by more than 450% by 2050, largely due to the adoption of clean energy technologies. Meanwhile, global per capita steel consumption has increased from ~150 kg in 1950 to over 230 kg by 2020. Thus, both demographic and lifestyle factors underpin the sharp rise in future mineral demand [2,3]. Japan Oil, Gas and Metals National Corporation (JOGMEC) reported that the consumption of copper and lead would increase to 37 and 9.55 million tonnes, respectively, by 2050, highlighting the severity of the mineral deficit [4,5]. As a result of growing demand and extensive market competition, many operating mines are struggling to maintain profitability, reporting low Net Present Values (NPV). Mine economics are highly sensitive to international commodity prices, which are driven by global supply–demand dynamics, technological change and macroeconomic factors. Recent experience in the lithium market illustrates this: a rapid build-out of new supply and a temporary softening in electric-vehicle uptake led to a sharp fall in lithium prices in 2023–2024, reducing margins and forcing producers to cut costs and delay investment. Consequently, while additional mines are needed to meet long-term demand, price cycles can render near-term development financially unviable, thereby slowing the pace of new project approvals and commissioning [6]. Although the global demand for mineral resources is generally increasing, regional patterns differ significantly. In Asia (notably China and India), copper and steel consumption is growing rapidly. Europe focuses more on critical raw materials such as lithium and rare earth elements. North America is a major consumer across a broad range of mineral commodities, while Latin America and Africa continue to act primarily as suppliers of raw minerals rather than major consumers.

Alongside decreasing ore reserves, conventional mining methods incur high production costs to maintain their complex mining infrastructure, safety protocols and low environmental impact. On the other hand, despite significant advances in the mining industry, conventional mining methods have notable negative environmental impacts, both during operation and during the closure stage. Some instances include soil erosion, contamination of the soil profile, deforestation, etc. [7,8,9]. Studies conducted by researchers [10,11] showed that the impact of the ecological damage is retained even in decommissioned sites, where the regional soil and water profile appears to be irreversibly polluted. Furthermore, the remaining infrastructure, such as roads, ports and railways, is also noted to affect the migratory routes of animals and increase habitat destruction [12,13]. Studies also report the detrimental effects on aquatic and groundwater systems following the decommissioning of an acid mine, thus driving current methods out of practice [14].

Despite these well-documented environmental and operational challenges, it is also important to recognise the benefits that traditional mining provides. Conventional mines remain the backbone of global mineral supply and are directly linked to local and regional economic development, providing employment, infrastructure investment, and royalties that support public services. Furthermore, many critical minerals have no known synthetic substitutes; for example, copper, iron, and rare earth elements are irreplaceable in energy, construction, and high-technology applications. Thus, while sustainability concerns drive the search for alternative methods, traditional mining will remain essential for global development and technological progress for the foreseeable future. Owing to the depletion of easily accessible and high-grade natural resources, the extraction of minerals from greater depths has increased significantly worldwide. Deep underground mining has made significant progress since its introduction in the early 1980s. For instance, countries like China have explored depths of more than 1000 m across 47 of its operational innings [15,16]. However, meeting the required efficiency in deep underground operations is very challenging due to technological, environmental and socio-economic limitations [17]. More importantly, deep underground openings are typically subjected to high geostress conditions, which can cause the rockburst phenomenon due to the sudden release of accumulated strain energy in the rock mass [18]. Failure to detect, monitor, and relieve these high-stress zones in underground mines via appropriate monitoring systems and adequate support infrastructure results in catastrophic hazards, fatalities, and significant economic losses [19].

Unconventional/alternative mining methods are a potential solution to the problems mentioned above, as they offer alternative sources of minerals while contributing a low environmental impact. In this regard, recent regulatory and policy developments have accelerated the shift toward alternative practices. For instance, the European Union has introduced the Critical Raw Materials Act (2023), which emphasises secure and sustainable supply chains, while the United States has strengthened its policies on critical minerals under the Energy Act of 2020 [20,21]. Such initiatives highlight the growing regulatory pressure on the industry to adopt innovative and less environmentally damaging extraction methods. Looking ahead, evolving regulations are likely to play a decisive role in shaping the mining industry’s future. By setting stricter environmental standards and prioritising the security of critical mineral supply chains, these policies will not only accelerate the adoption of alternative mining technologies but also gradually phase out environmentally damaging practices. Consequently, regulatory frameworks are expected to act as both a constraint and a driver of innovation, pushing the industry toward safer, cleaner, and more resource-efficient extraction methods.

This study reviews potential future/alternative mining methods, their benefits, limiting factors, and the relevant technologies are discussed in detail. In preparing this review, we conducted a structured search of major scientific databases, including Scopus, Web of Science, and Google Scholar. Keywords and combinations such as “sustainable mining”, “alternative mineral resources”, “fluidised mining”, “deep-sea mining”, “urban mining”, “in-situ leaching”, and “space mining” were applied. We prioritised peer-reviewed journal articles and authoritative reports published mainly after 2010, with an emphasis on studies from 2015 onwards to capture recent technological and regulatory developments. Sources were deemed relevant if they directly addressed the environmental, technological, or economic dimensions of unconventional mining methods. In total, 120 publications were initially identified, including both research and review articles and 102 were included in the final review. Most contributions originated from China, the United States, and Europe, with the majority published after 2015.

2. Fluidised Mining

Deep underground mining has undergone extensive research and development. However, an increase in depth is usually accompanied by critical challenges, such as higher temperatures and high in-situ stresses, which can limit the performance and economic viability of conventional deep mining operations. This challenge could be addressed by remote operation and full automation of underground mining processes. One of the potential techniques is fluidised mining empowered with Tunnel Boring Machine (TBM) technology. TBMs provide continuous excavation, often outperforming conventional drill-and-blast methods in many ground conditions. Reference [22] reports drill-and-blast advance rates of 4–9 m/day (median 7 m/day), while mechanical/TBM systems typically range from 8 to 45 m/day with an average of about 20 m/day, varying with rock quality, machine size, and usage. In very hard, abrasive conditions, rates may reach ~10 m/day for both methods, making the speed advantage site-specific [22]. These higher advance rates are one reason TBMs are attractive for mechanised/continuous extraction and for integration with remote/automated systems; however, the site-dependent caveats above must be considered when evaluating applicability to deep mining.

Adapting TBMs to deep mining projects can provide the following advantages: (1) improving personnel safety by reducing the blast-induced toxic fumes, (2) minimising seismic disturbances and applying lower support systems for ground control, (3) reducing the ventilation requirements by creating the smooth walls by TBM, and (4) providing a continuous excavation process, suitable for automation and remote operations. The application of TBMs in mining projects dates back to the late 1950s, followed by more attempts in the 1960s and 1970s. In the mining industry, the most common variation in TBM employed is the open gripper type. However, earth pressure balance and slurry types have also been reported in some parts of Australia [23]. For highly fractured grounds, TBMs can be modified by equipping them with impact bars to prevent damage to mucking buckets, conveyor belts and cutters. Furthermore, in the case of methane explosions in coal mines caused by the TBM cutters, an integrated drilling and slotting technology that has been successfully employed in a Chinese coal mine for the co-extraction of coal and methane can be considered [24].

The TBM mining concept can be further highlighted when it integrates several stages of the mining operation through the novel approach of the fluidised mining method, another potential alternative to conventional deep underground mining. The conceptual development of fluidised mining technology was conducted by Xie et al., offering in-situ transformation of solid mineral resources to liquid or gas with integrated mining, sorting, and refining mechanisms [25]. Adopting the cutter head from a conventional TBM enables the fluidised mining machinery to break the targeted minerals, which are then fed to the automated separation unit, where the rock is disassociated from the goaf. Backfilling chambers are also installed to store the generated mine wastes, which can later be treated by integrated waste treatment facilities. Post fluidisation, for example, in coal mines, the minerals may be fed to an internal power generation system to convert the mined coal into electricity via pulverisation, chemical conversion, or biodegradation, part of which may be utilised to meet its consumption requirements. Alternatively, they may be transported to above-ground facilities for further processing [26].

The conventional design may be improved by installing microwave radiation at the cutter head (see Figure 1) to improve rock fragmentation, as reported by [27]. Worm-like designs may also be considered, along with separate functional features to cater to tunnelling and mining seam needs. The overall setup must also be fitted with gas extraction and transportation systems to prevent potential gas explosions and coal gas outbursts. The extracted gas may be sent to an underground gas power generation facility, thereby increasing the overall efficiency of the operation. Theoretical calculations and studies demonstrate the ability of fluidised mining methods to significantly lower production costs up to 4.8 times compared to conventional mining operations. Moreover, the numerical analysis and evaluations conducted by Ju et al. reported that greenhouse gases and damage to the geological formations can be dramatically minimised through the fluidised mining approach [28].

Despite technological advancements, deep underground mining continues to face significant challenges. TBMs are often ineffective in highly stressed environments, resulting in operational failures and necessitating the switch to more traditional methods, such as drilling and blasting. Although fluidised mining offers potential benefits, it lacks real-world prototypes, and its applicability remains inconclusive. Furthermore, most studies on fluidised mining focus on coal seams, and further research is needed to assess how this technique could be applied to metal mines and hard rocks. This gap may be due to key technological challenges, including the high energy demands for processing dense hard rocks and converting them into pumpable particle sizes compared to coal. In coal, organic matter is the primary target and can be fluidised relatively selectively. However, in metal ores, gangue minerals often behave physically like valuable minerals, making fluidisation less likely to selectively transport ore versus waste without complicated beneficiation procedures. Additionally, high-density rock fragments in suspension will cause significantly more wear on pipes, pumps, and transport systems than coal slurries. Additionally, installing a fluidised mining system requires altering the entire mining infrastructure, which, in turn, implies substantial capital costs. Therefore, developing new technologies to address these critical issues is essential. As such, this mining method is estimated at a conservative Technology Readiness Level (TRL) 2–4 (concept to lab demonstration). The global metal and mineral crisis is likely to spur more funding and exploratory studies, presenting opportunities for further advancements in digital and remotely controlled deep underground fluidised mining.

3. Deep-Sea Mining

The rising metal price and shortage of supply on land provide stakeholders with the motivation to search for alternative sources of minerals, specifically, the deep-sea environment, which encompasses approximately 50% of the Earth’s surface [29]. Metals such as cobalt are of great economic interest mainly due to their extensive use in producing superalloys for aircraft and batteries. In other key sectors such as steel production, electrical and semiconductor manufacturing and construction, additional valuable metals such as nickel, copper, manganese and iron are used [30,31]. Recent exploratory studies have shown that the deep-sea floor (depth > 200 m) is home to many of these minerals, and the prime target of most marine mining operations is often limited to three types of deposits: poly-metallic nodules or Manganese Nodules (MN), Seafloor Massive Sulfides (SMS) and Cobalt-Rich Crusts (CRC) [30,32,33]. The location and mineral composition of these three sources are shown in Figure 2a.

The selection of an appropriate deep-sea mining (DSM) method depends significantly on numerous factors, including the form and quantity of the targeted reserve and the geotechnical characteristics of the deep-sea environment. Additionally, the type of waste disposal, project lifespan, environmental commitments, and characteristics are significant criteria. Remotely Operated Vehicles (ROVs) are state-of-the-art technology that can accommodate the ever-changing mining environment while minimising project costs. Each ROV operation consists of a lifting system, a Deep-Sea Mining Vehicle (DSMV) and a semi-automatic control system to allow its smooth operation and extraction of samples at depths up to 2000 m [34]. The ultimate purpose of the lifting system is to transport the extracted minerals from the mining site to the support vessel, where they can be processed further. Figure 2b shows the development of deep-sea mining systems. The submarine drag bucket mining systems were utilised in the early 60s; however, their operation was discontinued due to their low efficiency. Similarly, a continuous line bucket was introduced, offering improved manufacturing costs; however, frequent blockages were experienced due to its lower mobility. Shuttle vessel systems offered better mobility but at the expense of higher capital costs. Thus, pipeline lift systems are predominant in modern mining applications as they allow simultaneous extraction and transportation of deep-sea nodules [34,35]. DSMVs are crucial for collecting minerals in complex deep-sea terrain, requiring a tractive analysis system to prevent sinking, slipping, or turnover and ensure smooth manoeuvrability. The control of the DSMV can be monitored and adjusted via path planning control, dynamic control, or motion control. The tracked mining vehicle also needs a dynamic control system focusing on speed maintenance, heading correction and slip control using PI or PID control [36,37,38]. Navigation and positioning of the DSMV are widely achieved using acoustic baseline positioning and a diverse set of transceivers and transponders, details of which are outlined in the works of [34,39,40].

Various institutions developed novel machinery specifically designed for deep-sea mining. One such organisation is Nautilus Minerals Inc., Canada, which constructed the first ROV and conducted licenced deep-sea mining in Papua New Guinea at Solwara. The production plan utilised three Seafloor Mining Tools (SMTs) powered by six electric hydraulic power units. The Auxiliary Cutter (AC) levels the sea surface, while the Bulk Cutter (BC) uses a drum cutter to break nodules into smaller pieces, and the Collection Machine (CM) transports the mined ores to the support vessel using subsea pumps and flexible pipes. These machines are robustly constructed to operate in temperatures up to 400 °C in semi-active thermal vent environments [41,42]. South Korea developed the self-propelled mining robot MineRo I in 2007 to extract deep-seabed manganese nodules. Equipped with multiple operational features, including a dual power pack and pressure compensators, it operates under high hydrostatic pressure, offering remote control and real-time data collection capabilities. Shallow water tests showed that it maintained subsystem integrity at a depth of 100 m but faced issues with acoustic sensing due to sediment plumes. In 2012, the Korean Institute of Ocean Science and Technology (KIOST) introduced MineRo II, which maintained similar hydraulic and mechanical structures. Pilot testing required support from a vessel named MIRAERO and a low-voltage power system. Although a high-voltage converter would be ideal for commercial use, the chosen low-cost power source was deemed sufficient for this application. The control system enabled machine manoeuvrability, monitoring, and video collection [43,44].

Du et al. traced the evolution of deep-sea mining, emphasising resource richness and technological progress. They specifically highlighted the role of automation and standardisation as future solutions in this field [45]. Jones et al. examined a test site 44 years after completing a deep-sea operation [46]. They reported that despite partial species recovery, long-term biological impacts persist in areas that have been disturbed. The findings emphasise the importance of responsible mining practices. In another study, Global Sea Mineral Resources (GSR), in collaboration with partners from different countries, conducted Patania II pre-prototype seabed collector trials in 2021 to test mobility and nodule collection on the abyssal seafloor; an array of ISA exploration contracts supports ongoing baseline and technology studies [47]. While collector prototypes have been sea-tested (TRL ≈ 5–7 for individual components), fully integrated, environmentally acceptable exploitation systems remain constrained by regulatory, environmental and socio-economic issues and therefore sit at a lower integrated TRL (≈4–6).

Considering the above investigations, DSM faces significant challenges that hinder its widespread application. A major issue is the limited understanding of deep-sea environments, as mineral structures and contents vary geographically, and small-scale tests cannot fully capture these variations. This lack of knowledge complicates the development of environmentally friendly technologies. Additionally, DSM can cause extensive ecological damage, disrupt habitats, and affect marine food chains and water quality, with long-term impacts that may be irreversible. Socio-economic factors also pose threats, including conflicts over resource ownership, negative impacts on local communities and tourism, and high costs with uncertain benefits. The failure of Nautilus Inc.’s Solwara I project, which left Papua New Guinea with a substantial debt, exemplifies the financial and social risks associated with DSM. Despite vast mineral reserves on the deep-sea floors, further research is needed to gather baseline data and conduct reliable cost–benefit analyses, particularly quantifying environmental impacts. Government institutions should facilitate marine studies and approve exploratory contracts with reliable companies. Enhanced environmental management systems and updated mining codes from the International Seabed Authority (ISA) have led to several exploratory projects, such as those by China Minmetals Corporation and the Cook Islands Investment Corporation, as well as Korea’s interest in SMS recovery. These projects aim to improve understanding of the deep-sea environment and test emerging technologies. Ultimately, improved product design is crucial for balancing environmental concerns and economic benefits, thereby fostering a sustainable circular economy.

4. Brine Mining

The oceans contribute more than 97% of the world’s total water reserves and are home to many minerals, dissolved gases and specific organic molecules. These mineral reserves are established via several natural causes; one is when geothermal fluids interact with hot rock bodies and saturate the stream with minerals. The degree of mineralisation and the chemical composition of the resultant brine is heavily dependent on multiple factors such as interaction temperature, the fluid’s chemistry, and the rocks’ lithology [48]. Many studies have demonstrated the presence of numerous high-value products, including silica, zinc, lithium, manganese, and uranium, within oceanic waters, as well as dissolved gases such as hydrogen. Seawater mining alleviates the need for energy-intensive extraction and beneficiation processes, as the recovered products do not pertain to mineral grade variance [49]. The abundance of geothermal fields has also been noted across many regions, such as the Salton Sea in the United States, the Milos field in Greece, and the Cheleken Geothermal Field in Russia, and may be regarded as potential brine mining sites [48]. The recovered high-value elements, such as lithium, enable stakeholders to tap into a market worth more than $350 million per year, thus showcasing its profitable prospects. Other brine sources can be found in saline lakes (with salinity greater than that of seawater), sedimentary basins, and industries such as dairy, textile, leather, and oil, which produce brines as byproducts from which valuable materials can be extracted and reused. Despite the existence of multiple technological routes, the principal methods may be classified into five main categories: (1) Solar or vacuum evaporation, (2) Electrodialysis (ED), (3) Membrane distillation (MD)/Membrane Distillation Crystallisation (MDC), and (4) Combined MD-Adsorption. The first three methods are generally employed when the concentrations in the brine solution are substantial (i.e., NaCl and MgSO₄), and the latter is more prominent when the minerals are in lower quantities (e.g., Li, Sr, Rb, and U). Recent advancements have also led to the development of other methods, such as eutectic freeze crystallisation and microbial desalination. However, these techniques are not discussed in this review, and further information can be found in the works of [50].

The solar evaporation method withdraws brine from the sea and utilises solar energy to induce water evaporation, leaving behind a concentrated salt solution (Figure 3a). This process involves a reservoir tank connected to evaporator ponds, with the concentrated liquor fed to a crystallising pan for salt extraction. Thermodynamically, the precipitation hierarchy follows the order CaCO₃, CaSO₄, and NaCl. Despite its simplicity, the method offers operational advantages, such as low maintenance and the incorporation of renewable energy, but requires large land masses for optimal evaporation. Potential drawbacks include time consumption and risks of ground contamination without appropriate bunds. Improvements include conductive pans for heat transfer, heat insulation with black paint, and utilising wind energy to accelerate evaporation kinetics, thus reducing land requirements [49,51]. ED extraction (Figure 3b) involves using an induced electric field to separate minerals through anion and cation-selective membranes. This method is favoured for producing high-purity products with a simple pre-concentration step and generating pure water from brine. This method minimises fouling layer formation and chemical usage but requires rigorous pretreatment to prevent membrane scaling. Commercially explored in desalination and brine treatment, ED has potential applications, such as lithium extraction from seawater. Combining Reverse Electrodialysis (RED) with ED can reduce overall energy consumption by generating electrical power [50].

Membrane distillation (MD) is a thermally driven process that separates pure water from brine using a hydrophobic microporous membrane. Vapour pressure generated by a temperature gradient drives the process, collecting purified water as distillate. MD brine is then fed to a temperature-controlled crystalliser for high-quality crystal formation. Membrane distillation crystallisation (MDC) enables the formation of specific crystal superstructures and the recovery of multivalent ions, such as barium, strontium, and magnesium. It can also treat Reverse Osmosis (RO) brine for high-quality lithium extraction. Despite the efficiency and crystal quality advantages, commercial applications for seawater mining face challenges such as the high water recovery rates required at low salt concentrations and susceptibility to gypsum scaling. Mineral scaling, especially with inorganic salts, reduces membrane lifespan [52,53]. Although studies involving large-scale seawater mining are relatively limited, significant research and development have been conducted to create a process that minimises environmental impact while maintaining high production capacity. Reference [54] explored the effect of coupling MD with absorption to extract a valuable industrial metal, Rubidium (Ru). This bench-scale study used submerged hollow fibre MD and sanctioned various benefits, such as lower heat consumption and process intensification. Their study was conducted at an optimum temperature of 55 °C, with a 0.24 g/L dose rate, which entails a high Ru recovery rate. The chosen adsorbent, granular potassium copper hexacyanoferrate (KCuFC), was also demonstrated to show high affinity and absorption capacity towards Ru, which contributed to its high recovery rate of 97%. Fresh water was also produced as a byproduct. However, some level of Calcium sulfate (CaSO₄) scaling was observed on the membrane surface, significantly impacting the MD performance; hence, further research is required to increase production efficiency [54].

In general, the primary hindrance to the application of seawater mining is the presence of scaling and low selectivity of the adsorbents involved. This, coupled with the accelerated corrosion rates of the equipment involved, renders this method relatively expensive and unsustainable. Reports by Lindner et al. indicated that mining uranium from seawater was not economically competitive, as the costs of deployment and delivery, solid-phase adsorbent, and recovery were too high. Furthermore, the separation of the minerals from the brine solution consumes large amounts of power and requires the rejection of huge volumes of water [55]. Due to the limited power options available in the sea environment, operations often rely on diesel generators, which contribute significantly to total greenhouse gas emissions. Renewable sources such as windmills and solar cells are also not viable options due to the increased corrosion rate and salting of photovoltaic cells [55]. Despite the numerous difficulties, there are still plenty of prospects in brine mining. This mining method enables the extraction of various exotic minerals, such as lithium and uranium, from sustainable sources, thereby contributing to a lower environmental burden compared to conventional mining. The recent onset of technological advancement allows more research and development into marine power options, which would greatly supplement the high energy costs required. The integration of mining technologies with existing desalination plants would enable the utilisation of their delivery and disposal systems, thereby significantly reducing production costs and allowing for a shorter payback period [55].

Beyond its technological processes, brine mining has gained strategic importance in the global transition to clean energy. Lithium extracted from brines currently accounts for more than 60% of global lithium supply, forming the backbone of lithium-ion battery production. With the rapid expansion of electric vehicles and renewable energy storage systems, brine mining is expected to remain a key driver in securing critical raw materials for low-carbon technologies [56].

In conjunction with the increased metal costs, brine mines have demonstrated their operational feasibility in various establishments, such as the Cauchari-Olaroz project in Argentina, which aims to achieve a production target of 25,000 tonnes per annum of lithium within an operational lifespan of 40 years [57]. Another recent development involves Direct Lithium Extraction (DLE) from geothermal brines: Controlled Thermal Resources’ Hell’s Kitchen project and related pilot activities in the Salton Sea region have advanced to drilling, pilot testing, and early construction phases. Several technology vendors, including startups and established firms, are operating pilot or demonstration plants to validate reagent performance and brine management. These projects suggest that DLE for geothermal brines is currently at the pilot or demonstration stage (TRL ≈ 6–8), although regional environmental and scaling challenges persist and must be assessed alongside commercial-scale up.

5. Urban Mining

Urban mining is defined as the process of recovering components and elements from anthropogenic materials, which can be sourced from demolished buildings, industrial products, emissions, or even end-of-life vehicles and electronics [58]. Electronic waste (E-waste) is generally regarded as a source of precious metals, and 95% of the feedstock generated is reported to be recyclable. Micro-integrated circuits, especially their plaques, can contain up to 7% (w/w) iron, 5% (w/w) aluminium, 20% (w/w) copper, 20 ppm gold, and 280 ppm silver [59]. Electronics formed by Economic Value Added (EVA) elements contain high levels of rare earth elements (REEs), including gold, silver, and copper. Other minerals, such as yttrium, europium, cerium, and lanthanum, are also present in fluorescent, LED, plasma and CRT displays. These sources contain valuable plastics, such as polyethylene, polypropylene, epoxy and polyesters, which may be integrated with fibre-reinforced plastics to produce a cheaper product with extended mechanical abilities. The extraction of minerals from e-waste has been found to consume only 10–15% of the energy required for traditional gold mining. 150 g of gold can be extracted from one ton of micro-integrated circuits, which is significantly higher compared to the extraction capacity of 30 g per ton of ore in conventional mining [59]. Urban mining is already extensively practiced in several countries, such as China, Chile, Spain, and Australia and contributes up to 20% and 3% of the world’s copper and gold reserves, respectively. During urban mining (Figure 4), the collected e-waste first undergoes physical separation where mechanical methods are used to disassemble the parts and separate them into various categories (e.g., plastics, steel, aluminium, copper and printed circuit boards, Printed Circuit Boards (PCBs)). This step is generally conducted manually via commonly available tools such as hammers and screwdrivers. However, advanced tools such as crushers and grinders may also be used, followed by the application of a density separation technique. Even though the latter method enables production at a higher efficiency, it also incurs higher capital costs. Following its segregation, the e-waste can be treated through one of the following pathways: pyrometallurgical, hydrometallurgical, or bio-hydrometallurgical processes [60].

The pyrometallurgical method involves incineration and smelting in a blast or plasma furnace. It is the most common and conventional method to extract nonferrous and precious metals from e-waste. The crushed waste is melted in a molten bath, thereby removing plastics and metal oxides through the formation of a slag phase. This method eliminates the need for any pretreatment steps, as it can be applied to any form of electronic waste, regardless of its origin. Additionally, it is associated with high recovery rates for valuable metals, such as copper, gold, and silver, while maintaining a competitive economic edge. However, the recovery of plastics is deemed impossible as they are utilised as a source of energy. Furthermore, this process dramatically complicates the recovery of metals such as iron and aluminium as it promotes oxide formation. Even the extracted elements must undergo subsequent hydrometallurgical or electrochemical purification to establish a product with adequate purity. Lastly, the formation of dioxins and furans is also a source of concern, as they impart adverse effects on the environment [60].

Hydrometallurgical extraction involves dissolving metal contents with leaching solutions, followed by separation to extract and purify metals. Leaching solutions include acid, ammonia, ammonium salt, and chloride, which are chosen for their high solubility and low cost. Elevated-temperature acid leaching is common for rare earth metals, while cyanide is used despite its toxicity for high recovery rates [61]; thiosulphate-based alternatives show promise. Precipitation with oxalic acid concentrates minerals, with ionic liquids explored as solvent alternatives. Electro-refining, cementation, absorption, ion exchange, or solvent extraction are alternative methods. While hydrometallurgy reduces the risk of air pollution, high consumption of leaching agents and their toxicity pose environmental and safety concerns. The Bio-hydrometallurgical process (Figure 5), on the other hand, is a relatively novel urban mining technique that can be conducted through biosorption and bioleaching, where the primary method involves using numerous microorganisms, which tend to accumulate rare/heavy metals through various physical-chemical reactions. Organisms conducting bioleaching convert insoluble solid minerals into soluble components, which can later be extracted via multiple purification technologies. The leaching principles employed are either via intrinsic redox reactions, the formation of organic and inorganic acids or by excreting complexing agents. The advantages of this method are its simplicity, affordability, and environmental friendliness, while maintaining high extraction efficiencies. They also contribute lower capital and operational costs. Unfortunately, these benefits come at the expense of high production time and the release of toxic chemicals (e.g., sulphuric acid and H⁺ ions), which, if not adequately contained, may pose large-scale environmental damage [60,62,63].

Ambrós presented a comprehensive review of gravity concentration techniques in urban mining, emphasising their application in solid waste recovery rather than traditional ore processing. The study focused on three major waste categories: plastics, construction and demolition waste (CDW), and waste electrical and electronic equipment (WEEE). Gravity-based methods were recognised for their simplicity, cost-effectiveness, and environmental compatibility, although limitations such as material variability and regulatory gaps remain. The author recommends future research toward process optimisation and industrial scalability [64]. Vega-Garcia et al. examined the transfer of conventional mining methodologies into urban mining, with a specific focus on WEEE. Their comparative analysis of physical and chemical separation approaches underscored the value of gravity concentration when paired with effective size reduction and pre-treatment. The authors argue that these adaptations could reduce hazardous outputs while enhancing the selective recovery of critical materials [65]. Razmjou and Asadnia explored the integration of artificial intelligence (AI) into modern mining workflows, including urban mining applications. A dedicated chapter addressed the potential of AI-driven control systems to improve the efficiency of gravity-based separation. Techniques such as machine learning, sensor-based classification, and automated feedback loops were highlighted as key enablers for real-time process optimisation and waste valorisation [66]. Zhang et al. introduced the Packed Column Jig (PCJ) as a novel gravity concentration device suited for urban waste processing. Their case studies demonstrated the successful extraction of compounds such as phosphate and gypsum from industrial byproducts, with minimal chemical consumption and high throughput. The PCJ system was proposed as a scalable, eco-efficient technology capable of handling fine particulate waste, positioning it as a strategic tool for future urban mining operations [67]. Currently, battery recycling [68], as part of urban mining, has achieved industrial maturity, with large-scale commercial recycling facilities and campuses operated by companies such as Umicore and Redwood Materials demonstrating industrial TRL (≈8–9) for mechanical and hydrometallurgical battery recycling. However, automation for disassembly and some advanced flowsheet elements remains the focus of active industrial R&D and scale-up.

Urban mining faces multifaceted challenges spanning political, social, institutional, technological, ecological, and financial realms. The lack of dedicated e-waste collection institutions and stringent regulations hinders industry growth. Manual dismantling poses health risks, while the diverse types of devices complicate extraction processes. Insufficient data hinders cost–benefit analysis, and public awareness is lacking, resulting in feedstock instability. Addressing these challenges requires dedicated research, detailed techno-economic studies, and optimisation of extraction technologies. Projects like the ADIR showcased the role of automation in urban mining by developing a novel inverse production scheme capable of fully automating the processes of disassembly, separation and recovery of valuable metals from waste circuit boards, with projected recovery rates of up to 90% [69]. Therefore, the implementation of industrial sensors and Artificial Intelligence (AI)-based techniques (e.g., for image processing and detecting different components of PCBs could significantly improve the efficiency of urban mining methods.

Other recent empirical studies have also demonstrated tangible improvements in recovery and processing efficiency through AI-enabled automation. For instance, Santos et al. showed that a robotic desoldering system achieved nearly 100% success in removing larger PCB components [70]. In another case, a six-degree-of-freedom robotic arm reduced the time to disassemble battery tabs from approximately 220 s (by skilled technicians) to about 112 s. These examples affirm that AI and robotics not only enhance speed but also precision in disassembly and sorting operations [71]. Empirical evidence confirms that automation and AI can materially improve recovery and process performance. For example, the ADIR project (EU H2020) used automated disassembly, lasers and robotic handling to selectively extract components from mobile phones and PCBs; pilot demonstrations yielded very high tantalum recovery (reported ~96–98%), illustrating that selective automated disassembly can recover critical elements effectively [72,73]. Field deployments of AI-powered sorting corroborate these gains at larger scales. A modern plastic-sorting plant using advanced sensor + AI stacks increased capture of targeted plastic fractions from roughly 47% to ~95%, while a commercial AI-first sorting facility (AMP ONE) consistently recovers >90% of the targeted commodities and operates at multi-kton annual throughput. These examples demonstrate both high recovery and industrial throughput [74]. Recent literature reviews further report that AI-based classification and sorting systems can achieve high accuracy and yield operating benefits (fewer manual sorters, higher purity bales, optimised logistics), which together make AI-enabled disassembly/sorting an evidence-based route to improved circularity [75,76].

6. In-Situ Leaching and Heap Leaching

Other potential alternatives to conventional mining methods are In-Situ Leaching (ISL) (Figure 6a) and Heap Leaching (HL) (Figure 6b) techniques, which can produce high-quality metals from low-grade ores. Commercial applications of the in-situ mining method have demonstrated its ability to extract REE, such as uranium, from roll front sandstone deposits, with recoveries up to 70–90%, as well as evaporites, such as soda ash, potash and other salts. According to the study conducted by [77], copper recoveries of up to 70% have been estimated when utilising a mix of sulphuric acid and ferric acid sulphate solutions as the principal lixiviants. ISL has been established in several existing USA-based mines, including San Manuel, Santa Cruz, and Mineral Park, which can produce up to 2000 tonnes of uranium per year, along with other minerals such as scandium, rhenium, yttrium, and selenium [78]. ISL projects offer various benefits, including the elimination of large open pits, exhaust pollution, rock dumps, and tailings, as well as the production of smaller volumes of mining effluents, making them a popular alternative to traditional mining methods. Likewise, HL also began its use in the mid-20th century within the USA, where it was employed to recover rare earth metals using cyanide as the main leaching agent. Further technological advancement also extended its application in the USA, such as the Bluebird copper oxide mine and the Cortex heap leach, for copper and gold extraction, respectively. Each of these establishments reports high recovery rates. Uranium recovery via heap leaching methods has also been explored with significant success in other countries such as Chile. Heap leaching technology can also take higher-grade ores as potential feedstock in remote and politically high-risk locations, as the process incurs low capital costs while maintaining high recovery rates [79].

The mining site chosen for both ISL and HL should ideally have desirable mineralogical and hydrological conditions which facilitate a process with low solvent consumption, quick dissolution of oxide minerals, and sufficient fluid flow. In the event of insufficient existing porosity, resulting in insufficient fluid movement through the mine, artificial fracturing or rubberisation may be applied to increase permeability. Resident meteorological contaminants, especially primary sulphides, should also be minimised as they incur longer reaction times and require further biological treatment. Operational parameters, such as permeability and hydrologic data, fracture spacing, and reaction kinetics, should be tested prior to mining to ensure high fracture density and minimal side reactions. A summary of the pre-mining testing parameters and their respective uses can be found in [78].

The utilisation of a leaching agent as the primary component to extract and contain the targeted minerals is prominent in these techniques. This reagent is regenerated in a separate purification facility. Conventional agents have been reported to be toxic to most ecological systems, although they are associated with high extraction efficiencies and recovery rates. Modern studies permit the use of other non-toxic leaching agents, such as thiosulphate leaching, as an alternative to cyanide. The advantages of thiosulphate derivatives as the primary lixiviant are their low toxicity and high selectivity for gold extraction. However, its incorporation results in high solvent consumption and requires copper II supplementation to increase gold dissolution. Furthermore, the lack of a suitable gold recovery method, incompatibility towards other metals, and inadequate understanding of the underlying solution chemistry hinder its application. Ammonia may also be used to extract copper oxide and zinc oxide deposits. It has demonstrated high selectivity and a simplified decommissioning procedure that is suitable for short-term, low-cost establishments. Ammonia, as a leaching agent, is insoluble against impurity elements, such as iron, calcium and manganese, thereby contributing to the high extraction efficiency of the target minerals.

Additionally, no further neutralisation is necessary, and the requirement for acid mine drainage is virtually eliminated. Despite its numerous benefits, it experiences solvent losses of 5–20%, which in turn increases operating costs. In other studies, chloride-enhanced heap leaching using NaCl or CaCl₂ has been used to extract oxides, mixed oxides and sulphide ores. The inclusion of chloride-based compounds improves the properties of the stack and reacts with sulphuric acid to form gypsum and chloride ions. Chloride ions stabilise copper ions and catalyse their precipitation through the formation of ferric ions. This process reports high recovery rates (up to 65%) and lower leaching times [79,80,81]. The containment of leaching solution in ISL and HL methods is a significant concern due to the potential for extensive environmental pollution from groundwater contamination. Challenges include low selectivity, side reactions, the presence of clay, and extended leaching times, highlighting the need for further research and development. Despite these challenges, advanced simulations, experimental studies, and diagnostic tools offer promise for improvement. Technologies such as lixiviant movement tracking systems, NMR logging, selective encapsulated leaching, and self-healing barrier systems mitigate environmental damage and enhance mining efficiency. Better environmental remediation plans further support the potential of leaching technologies as eco-friendly alternatives to terrestrial mining [82,83].

Estay et al. introduced an empirical leaching ratio to scale heap leaching operations. Derived from residence-time equations, this ratio links irrigation parameters with heap design, allowing simple estimation of irrigation flows and heap lifespan without relying on complex kinetic models [84]. Shumilova et al. demonstrated year-round heap leaching of gold-bearing tailings in Siberian permafrost by using insulated modules and warm-air injection. This setup maintained subzero operating temperatures and achieved an 86.8% gold recovery—27% higher than conventional seasonal leaching [85]. Wang et al. investigated the effects of in situ leaching with 2% ammonium sulfate on ion-adsorption rare-earth clay ore. Triaxial tests and pore-structure analyses revealed anisotropic layering and bond loss, which reduce shear strength, highlighting early permeability decline and slope stability risks [86]. Li and Yao reviewed global uranium in situ leaching methods, categorising acid, alkaline, neutral, and bioleaching approaches. They identified ore permeability, hydrogeology, lixiviant control, and real-time monitoring as critical success factors, and emphasised selective reagents and reactive-transport modelling for future improvements [87]. Wang et al. compared leached and un-leached rare-earth tailings in Fujian, finding that soil total and bioavailable REEs increased by up to 108% and heavy-REE uptake in plants surged. The study highlights the potential of phytoremediation despite residual acidification and nitrogen pollution [88].

Generally, in-situ leaching is commercially established for uranium and is widely Latin used in several producing jurisdictions (e.g., Kazakhstan, USA), representing TRL ≈ 9 for uranium ISL. Application of ISL to other metals (copper, some REE targets) remains at pilot or demonstration scales in many settings. Similarly, heap leaching is a mature commercial technology for gold and copper, and alternative lixiviants (for example, thiosulfate for cyanide-sensitive ores [80]) have progressed through pilot and some commercial deployments, indicating TRL levels that vary between 6 and 9, depending on feedstock and process variant.

7. Space Mining

In response to the dwindling supply of resources and energy on Earth, off-Earth (space) mining has been identified as a potential source of energy and raw materials. Space mining offers an environmentally friendly solution by shifting the burdens (i.e., less erosion, sinkhole creation, habitat destruction, and toxic runoff and contamination of soil, groundwater, and surface water) off-world, thereby providing a better life on Earth. The countless leftover materials from the formation of the solar system, known as asteroids, have been scattered in different orbits around Earth and contain a wealth of minerals, ores and volatile elements that are essential to Earth’s economy [89]. Asteroids can be categorised into three main groups: stony, carbonaceous, and metallic. Although the exact composition varies from asteroid to asteroid, stony types generally present as spherical formations containing a variety of metals and other materials. This is in contrast to the carbonaceous type, which is characterised by high levels of complex organic molecules and ice. Metallic types, on the other hand, are predominantly nickel-iron rich and contain high concentrations of other heavy metals, which can be considered the primary focus for asteroid mining [90]. The unconsolidated, loose and heterogeneous superficial deposits covering the asteroids, known as regolith, are also rich in Platinum Group Metals (PGMs) and volatiles. More importantly, the regolith in near-earth orbits could be used for the construction of fuel tanks and radiation shields for spacecraft, enhancing their thermal stability, and for in-situ 3D printing of the infrastructures, providing a proper platform for long-term and economic space exploration [91]. Water extracted from ice formations can also be used as propellants for spacecraft, requiring minimal processing, to further explore space and sustain a human presence by providing drinking water.

Comprehensive information on over 600,000 asteroids, including their location, name, type, composition, estimated value, etc., can be found on the Asterank website (https://www.asterank.com/, accessed on 21 June 2024). For instance, platinum reserves found on asteroid 2011 UW158 project a potential revenue opportunity worth $ 5.4 trillion [92]. However, the most significant challenge for space mining is determining how to extract it. Prospecting, which involves finding an adequately sized asteroid with the appropriate density, mass, and concentration of the target material, is the first step in operating a space mining system. These characterisations can be conducted via remote telescopic methods or local characterisation, where the primary aim is to determine the surface mineral content, shape and rotation rate by spectroscopic and photometric means. The latter involves the use of a spacecraft for accurate mass measurements and imaging. However, currently available technology is not capable of delivering accurate estimations, thus hindering the development of effective excavation technology [90]. Excavation of minerals can be performed through pneumatic/magnetic methods or using an auger/scoop. Otherwise, the entire asteroid may be captured and sent to the extraction plant. Following its excavation, the minerals can be purified to increase the concentration of targeted compounds through magnetic or electrostatic methods. These processes are generally avoided altogether as they incur high energy consumption, which may not be appropriate in a resource-limited, deep-space environment. Alternatively, chemical or mechanical methods may be employed, although the overall process dynamics may require significant development to conduct operations under weightless conditions. Finally, the storage of the minerals must be within a protective, pressurised, sealed enclosure to prevent any losses [91].

The literature review reveals that potential asteroid mining methods can be categorised into four primary groups: the asteroid-capturing method, the open-cutting method, the underground filling method, and in-situ extraction. The first method, as shown in Figure 7a, involves using a heavy-duty spacecraft with excellent load-bearing capacity, as the entire asteroid is captured and transported back from Solar orbit to low Earth orbit. Chemical thrusters are installed to provide the necessary propulsion. Afterwards, vertically landing heavy-lift rockets will be used to transport the captured asteroid to Earth. The anchorage system is reinforced with a high-power sucker to tightly adhere the captured asteroid to the spacecraft, and solar batteries enable strong flight endurance. The advantage of this method is that potential minerals present in low percentages are not lost as the whole asteroid is utilised. However, they come at the cost of a very power-consuming propulsion system to accommodate the heavy weight of the asteroids [93]. The open-cutting method introduces a concept similar to open-pit terrestrial mining, except the mining equipment is anchored to the surface of the asteroid. The suggested system also comprises a cutting machine, a rock-breaking machine, and an isolation cover (Figure 7b). The cutting and breaking can be conducted by incorporating non-explosive means, such as ultra-high pressure water jets and comprehensive thermal rock fragmenting, and the extracted materials can be captured within the installed isolation cover with appropriate elastic lining [94,95]. Although this method alleviates the need to transport the entire asteroid, technological limitations, including the lack of gravity and scarcity of resources, greatly limit its application. Furthermore, the non-explosive cutting methods cannot attain the same efficiency as those used in terrestrial mining [93].

Lal and Salter explored the legal and institutional landscape of extraterrestrial resource extraction, analysing gaps in current space governance. Their study emphasised the limitations of existing treaties and advocated for the development of regulatory frameworks that can guide responsible mining on the Moon and asteroids, considering both private and state actors [96]. Lewis proposed a technological roadmap for mining and manufacturing beyond Earth, focusing on lunar, Martian, and asteroid environments. By integrating current engineering techniques with in-situ resource utilisation (ISRU) strategies, the study argued for scalable off-world infrastructure to reduce terrestrial environmental pressure and support long-term space colonisation [97]. Rosa et al. introduced the concept of biomining in extraterrestrial settings, demonstrating that selected microorganisms can efficiently extract water, metals, and nutrients from planetary regolith. Their experimental results highlighted the feasibility of low-energy biological recovery systems under reduced gravity, which may be critical for closed-loop life support and material recycling in space habitats [98]. Kilpin and Jahankhani examined the role of artificial intelligence and blockchain technologies in optimising space mining operations. They proposed the use of smart contracts and decentralised data validation to enhance transparency in resource ownership and automate robotic mining processes, suggesting this dual-tech approach can improve economic viability and inter-organisational coordination in off-world mining ventures [99]. Razmjou and Asadnia provided a forward-looking analysis of AI integration in future mining sectors, including space-based extraction. The paper identified machine learning, sensor fusion, and IoT frameworks as key drivers of autonomous mining systems beyond Earth. Challenges in terrain mapping, real-time decision-making, and ethical oversight were also addressed, positioning AI as a pivotal element in next-generation space mining initiatives [66].

As shown in Figure 7c, the underground filling method follows the principles used in TBMs, where large holes are drilled from one side of the asteroid to the other. To compensate for the lack of gravity, the deep space drilling machine should be pressed up against the holes while the mined rocks would float upwards. The waste generated can be used to fill up the areas where mining has taken place. This technology also allows the extraction of minerals on the asteroid itself and does not require the transportation of the asteroid itself. The re-usage of waste generated lowers the treatment expenses while simultaneously stabilising the surrounding rock mass. However, the significant loss of minerals may lead to changes in the previous orbit and the speed of the asteroid, which may further complicate the retrieval of the minerals back to Earth [93]. The In-situ extraction method (Figure 7d) is similar to the Frasch process, which involves melting and extracting liquid sulphur from deep deposits by injecting high-pressure steam and using a circulating solvent for solution mining, as is employed in in-situ leaching mining of uranium. The advantage of this process is the inclusion of a simpler and smaller set of equipment, which significantly lowers the launch cost and fuel consumption for propulsion. Furthermore, the mined goods may be transported directly to an extraction plant without the need for extraterrestrial transportation or any form of crushing or grinding. Despite the various advantages, this process may lead to incomplete separation of solids and utilises high-pressure steam as its primary mode of excavation. Water is already a scarce resource in the deep space environment, and coupled with the need to generate high pressures in a weightless environment, it would incur high production costs. Additionally, plugging may occur due to precipitation along with a significant loss of drilling fluid in subsurface voids, thus requiring regular top-ups [100].

While a broad vision of space mining often includes ambitious projects such as extracting platinum-group metals from near-Earth asteroids, near-term feasible goals are considerably more modest and focus primarily on in-situ resource utilisation (ISRU). Current initiatives led by agencies such as NASA and ESA emphasise mining water ice from the Moon or Mars to provide essential life-support materials (water, oxygen) and spacecraft propellants, thereby enabling long-term exploration missions. In contrast, large-scale asteroid mining for precious metals remains highly speculative, facing unresolved challenges in technology readiness, economic viability, and legal governance. Distinguishing between these near-term practical applications and long-term visionary concepts provides a more realistic roadmap for the future of space mining [101].

Space mining holds potential opportunities, but its development faces significant impediments, including technological limitations and high capital costs. Established mining technologies, which rely on gravity and water, are not readily applicable in space, and the lack of clear legal guidelines on property rights further hinders progress. Despite these challenges, ongoing advancements by major corporations and space agencies, like NASA, offer promise for future expeditions. Feasibility studies suggest that traditional mining methods could be adapted for asteroid mining, and government support, such as Japan’s funding for space startups, indicates a growing interest in commercial space activities. Furthermore, as mentioned earlier, space biomining is at the experimental stage. Microgravity biomining experiments [102] flown to the International Space Station (ESA’s BioRock and follow-on BioAsteroid/BioAsteroid-type experiments) demonstrate that microbes can leach elements from basalt and meteoritic material in low-gravity environments; these laboratory-scale space experiments validate basic principles (TRL ≈ 3–5), but engineering for in-situ industrial extraction remains an early-stage research challenge.

Figure 7. Schematic structure of potential asteroid mining methods: (a) asteroid-capturing method, (b) open-cutting method, (c) underground filling method, and (d) in-situ extraction method. Modified from [93,103].

Figure 7. Schematic structure of potential asteroid mining methods: (a) asteroid-capturing method, (b) open-cutting method, (c) underground filling method, and (d) in-situ extraction method. Modified from [93,103].

8. Discussion

The six alternative mining approaches reviewed in this paper present a diverse spectrum of maturity, applicability, environmental and operational trade-offs. While each has been developed in response to critical sustainability pressures, they vary significantly in their readiness for deployment, the minerals they target, and their capacity to complement or substitute existing mining methods. A central theme across all technologies is that trade-offs shift rather than disappear. For instance, brine mining through direct lithium extraction (DLE) can dramatically reduce land and water footprints compared with evaporation ponds, yet it introduces new challenges in closed-loop brine reinjection and reagent consumption. Deep-sea mining could unlock vast polymetallic resources, but it raises unresolved ecological risks, including sediment plumes and habitat loss. In-situ and heap leaching have proven commercially viable for some metals, but groundwater protection and lixiviant toxicity remain persistent barriers.

Another important dimension is the Technology Readiness Level (TRL). Urban mining is already a mature industry, with large-scale industrial facilities recovering cobalt, nickel, lithium, and precious metals from e-waste streams. In contrast, space biomining and fluidised mining remain at conceptual or experimental stages, requiring substantial engineering breakthroughs before field trials are feasible. The uneven maturity underscores the need for differentiated policy, research investment, and pilot testing strategies rather than a uniform approach across all alternatives. Ultimately, the potential of these methods to replace hazardous conventional mining practices, such as deep-level underground gold mining, is uncertain. Urban mining and in-situ leaching can already help offset some demand for gold and copper, while brine mining contributes to the decarbonization of battery supply chains. However, fluidised mining, space biomining, and deep-sea mining are unlikely to provide near-term substitutes, although they may play a long-term role in diversifying supply. To synthesise these insights,

Table 1. Comparison of six alternative mining methods reviewed in this study.

Method	Target Minerals/Ores	Approx. TRL *	Key Advantages	Main Hurdles (Economic/Environmental)
Fluidised Mining	Coal (concept focus), concept for stratiform ores	2–4 (concept-lab)	Reduced surface disturbance; potential in-situ separation; lower GHG vs. conventional coal mining	Lack of prototypes; high energy demand; geomechanical stability issues
Deep-Sea Mining	Polymetallic nodules, sulfides, cobalt crusts	5–7 (components tested); 4–6 (integrated)	Vast untapped resources; avoids terrestrial deforestation and displacement	Sediment plumes, biodiversity loss, high cost, regulatory uncertainty
Brine Mining	Lithium, potash, borates, magnesium, and other brine salts	6–9 (evaporation ponds mature; selective DLE still pilot/demo)	Smaller land footprint than hard-rock mining; scalable in arid areas; renewable energy integration possible	High water use in evaporation ponds; ecological footprint of large ponds; chemical management; reinjection challenges (for DLE)
Urban Mining	Precious metals, base metals, rare earths from e-waste and end-of-life batteries	8–9 (commercial)	Reduces reliance on primary mining; high metal grades in waste; lower worker exposure if automated	Hazardous components (e.g., Pb, Cd, radioactive elements); logistics of collection; high capital cost for facilities
In-Situ and Heap Leaching	Gold, copper, uranium, REEs	7–9 (mature for gold, uranium; experimental for REEs)	Lower surface disturbance; reduced waste rock; cost-effective for low-grade ores	Risk of groundwater contamination; lixiviant toxicity (e.g., cyanide, sulfuric acid); slow kinetics; need for containment
Space Mining	Fe, REEs, other metals in asteroids/planetary crust	3–5 (lab/ISS experiments)	Enables off-Earth resource utilisation; potential support for space industry	Extreme energy/water constraints; very early stage; unclear governance/legal frameworks

* TRL = Technology Readiness Level (author estimate based on literature and field trials).

compares the six methods across their TRL, target minerals, main advantages, and principal hurdles.

9. Conclusions

Considering the global metal crisis and the high demand from different industries, this study reviews the potential alternative resources, their extraction techniques, and the relevant state-of-the-art technologies available. Fluidised mining, deep-sea mining, brine mining, urban mining, in-situ leaching, and heap leaching, as well as space mining, were identified as alternative/future mining methods that could benefit different industries by providing a vast array of minerals in an endless abundance. The main hindrance to the rapid commercialisation of these methods is the lack of mature technologies, as is the case for deep underground mining and deep-sea mining. Additionally, the sustainability of these methods is in question, either due to a lack of geological information or inadequate environmental regulations. Brine mining presents better prospects from an environmental standpoint; however, the process suffers from low recovery rates and is noted to be extremely capitally intensive. Most existential urban mining operations are conducted manually, which is the major source of concern due to the apparent radioactivity of the components involved. Hence, future research should focus on automated separation and collection strategies to mitigate the potential effects of radioactive poisoning. In-situ leaching mining is an innovative alternative to underground mining and has proven its applicability at numerous sites; however, careful containment strategies must be employed to prevent groundwater pollution. The lixiviant used should also be sourced from a sustainable source, resulting in a low environmental impact. Lastly, space mining enables the extraction of valuable minerals from extraterrestrial sources while minimising environmental impact; however, its exact implications for the Earth’s ecosystem and the development of suitable extraction technology are still in the early stages of research and development. In summary, the existing technologies and techniques are generally premature and do not meet industry standards for most alternative mining methods. Furthermore, the current financial models are not comprehensive enough to project sufficient profitability to support global investments in these fields. Similarly, the exact environmental impacts of these techniques have not yet been quantified. The long-term success of these methods requires concerted, high-level, interdisciplinary collaborative research and development along with global socio-political cooperation.