Use of Mining Waste Classification in the Context of a Circular Economy—A Review

Abstract

The beneficial use of mining waste aligns with circular economy thinking: saving primary resources can extend their lifetime and maintain availability, reduce the volume of legacy mining waste and its environmental impacts, and develop a resource beneficiation industry that is less energy and water intensive; mining lower grades at larger scale inevitably requires more beneficial reuse. Existing classifications applicable to different types of mine waste were reviewed. These include factors such as the mode of origin during the mining operation, grain size, chemical composition and stability. The result shows that these factors also largely control their civil engineering applications, suitability for end use sectors and potential hazards. Long-term liabilities related to chemical stability were identified as the most difficult challenge. When developing a reuse project, either by the end users or by the mine operator, it is likely that resource screening covering a comprehensive range of factors will be required, as none of the existing schemes individually cover all of the aspects needed to fully assess suitability for beneficial use. In conclusion, there is a need for a systematic and structured approach to classification of mining waste to facilitate reuse as raw materials, such as that presented in our review.

1. Introduction

The mining industry is one of the biggest waste-producing human activities, most of it being left in perpetuity at huge legacy disposal sites. Meanwhile the evolution of recovery technology with lower ore grades leads to continuously increasing waste-to-commodity ratios [1,2], and so to ever larger waste management facilities [3,4]. There is no option to reduce the volume of waste generation by process improvement, contrary to the favored approach in many industries [5,6]. The only possible mitigation scheme is to find secondary uses for new and historic mine waste [7]. Such secondary uses are an opportunity for the mining industry to integrate circular economy thinking, and to reduce its needs for hazardous landfill areas and its global footprint.

Matching a given waste’s characteristics and the requirements of an end user industry (in civil engineering, construction, minerals industries, and others) is usually a tough challenge, based on compromises on the specifications, acceptable volumes, distance between mine and user, and time [8]. Characteristics and specifications are poorly known, and this does not allow the emergence of resource platforms between producers and users, despite scientific efforts [9].

Mining waste is usually classified according to its origin in the mining process:

• Waste rock, which comprises barren rock that has to be excavated to gain access to ore (overburden) and undergrade ore (parts of an orebody which cannot be processed economically at the time of mining). Most of it is coarse-sized (pebble to boulder), stored as dumps [10,11];
• Tailings, or mining residues, which are the waste fraction after ore processing. They are milled down to the same fine grain as valorizable ore, and therefore stored behind dams or other containment facilities [10,11]. They may be used to backfill mining cavities. They contain large volumes of minerals in sand or mud forms, concentrations of the mined commodity below valorization cutoff grade (hereinafter called undergrade), and increased concentrations of other elements including undesirable ones [12,13];
• Metallurgy waste, such as slag, at sites where the mined commodity is further refined. This type of waste is not strictly a mining waste, as it originates from a metallurgy activity, but it is mentioned here nevertheless because such activities were frequently conducted on the mining site, and their waste is often found close to the mining and mineral processing sites which form the supply chain to extractive metallurgy of primary resources, especially for historical sites.

These terms may have equivalents in specific mining activities or sectors, which are summarized in

Table 1. Mining waste terminology by activity or sector.

Mine Typology	Hard Rock and Metal	Coal	Nickel Laterite	Bauxite	Phosphate
Barren terranes (overburden)	waste rock	overburden, strip	overburden, strip	overburden	overburden
Low grade host rocks	waste rock	spoil	waste rock	waste rock	waste rock (often named PMWR, Phosphate Mining Waste rock)
Crushing waste	Fines	Fines	Fines	Fines	Fines
Facility	Waste dumps, waste heaps	Coal tips	Waste dumps, waste heaps	Waste dumps, waste heaps	Waste dumps, waste heaps
Processing	Physical or chemical processing	Coal cleaning (coal preparation plant, CPP)	Acid or heap leaching, ferronickel	Alumina refinery	Physical or chemical processing
Processing waste	Tailings	Washing rejects	Leaching waste	Red mud	Phosphogypsum
Facility	Tailings dams, TSF	Tailings dam	Tailings dams, TSF	Mud basins	Tailings dams, TSF
Mine water waste including acid or neutral drainage	Neutralization mud	Neutralization mud	Decantation basin	Decantation basin	Decantation basin
Minesite metallurgy and pyro processes	Smelting	Spoil combustion	n/a	n/a	n/a
Minesite metallurgy and pyro processes residues	Slag	Ash	n/a	n/a	n/a

.

The aim of the present paper is to integrate existing classification schemes to facilitate beneficial use of mining waste and implement circular economy principles in this traditionally linear industry, so as to reduce as much as possible the volume of mining waste at legacy disposal sites. To achieve this, the available classification approaches or schemes and their relevance for selecting possible uses for a given waste are reviewed and compared, considering the constraints of each possible user industry. These are often limited in their general applicability, by virtue of their focus on recovery of particular types of metal (e.g., copper [14] or critical metals [14,15]), to securing resources for single industrial sectors (e.g., in road construction [8]), or having other specific target objectives such as eliminating acid rock drainage [16] likely to generate further mining waste. The objective is to align the various generic criteria-based schemes to provide a single unified and systematic approach.

Extraction activities, beneficiation and processing are all considered in scope here, as each produce mining waste. The included sectors cover metallic ores, non-metals, coal and industrial minerals. Coal mining is a closely related extractive industry to metal mining; indeed, it is often the source of energy or reductants for metal extraction. It uses different terminology for its wastes and processes, however, often relying on gravity separation alone [17]. The oil sector is not included, but some findings may be applicable to it. The timescale is not limited, due to the relative inalterability of waste, and the author visited waste storage facilities more than 100 years old. Thus, age does not preclude any reuse operation. Though part of the findings is applicable to historic waste on a site which is still owned by the original operator, most of the case studies are constituted by legacy waste left by the original operator, no longer in business, or perhaps no longer identifiable at an “orphan” site. The mining industry is itself included among the end users to include remining of legacy waste for new or previous commodities. Small or niche uses are excluded to maintain a focus on waste reduction.

Various literature sources were used for the present review, with selection criteria based on waste management strategies for each addressed sector rather than a blind bibliometric approach, as the subject is not covered extensively. Public guidelines, professional papers and sustainability stakeholder investigations were included where relevant.

2. Theory and Context

2.1. Reference to Circular Economy Principles

Circular economy is a framework for an array of critical changes in the global economy, aimed at reducing its pressure on the Earth’s resources, on the environment and on the climate [18,19,20]. Its relationships with sustainability were discussed [21]. The most prominent of these changes is a systematic increase in recycling, simultaneously reducing both the need for primary resources and waste generation. Both are critical for the mining sector, which embodies resources extraction and has to manage waste as one of its biggest costs—and burdens [7].

Early works [22,23] connected the recycling loop with the concept of sustainability, in which no human activity should exceed the ability of the Earth to support it. Unsustainable activities are bound to be short-lived when meeting their system boundaries: resource exhaustion, energy or land shortage, or permanent damage to the environment. Mining, according to 20th-century practice, was unsustainable, as it extracted, at faster and faster rates, finite resources, with increasing energy and land needs, and generating ever-increasing waste piles.

Circular economy principles are already applied to metals: “ Recovery and reuse of metals from products is on the increase for some metals. For example, 75 per cent of all aluminium ever produced is still in use” [24]. Case studies on recent developments in this field were summarized by the International Council on Mining and Metals (hereinafter “ICMM”) [5].

2.2. The Size of the Problem

Mining waste (Figure 1) is the biggest waste flow generated by humankind—at least 65,000 Mt/year [25]—it is second only to construction waste in the EU-27 at least (Figure 2) and first in mining-based economies. Due to its inalterability, it accumulates without any significant reduction [26]. The main cause of reduction in existing waste stocks is weathering and erosion, which is more accurately its dispersal in the surface water network (Figure 1) and on the sea bottom [10]. It is highly desirable, therefore, to find uses for mining waste generated by the world’s needs for minerals.

An example of the contribution of mining waste compared to other waste flows can be provided for Europe using Eurostat statistics (Figure 2, [27]). The evolution with time shows the slow reduction in the share of mining waste in the EU (EU: The European Union–EU-27: The 27 European member states from 2007 to 2013 and after 2020). This can be extrapolated to countries with large mining sectors (e.g., Australia, Canada, Chile, China, the USA, South Africa) using their own statistics, or their total mining commodities production as a scaling factor. In most developing countries, mining waste is an even larger part of the total waste [28], due to less developed secondary industries and consumer markets. As mining technology is almost the same everywhere, the use of ore production statistics to estimate mining waste is justified [11].

2.3. Waste Rates of Modern Mining

Since the beginning of mining statistics, a continued decreasing trend has been observed in minable grades for most commodities [1]. This is first due to mining economics: small high-grade deposits rapidly became exhausted, but, more importantly, were unable to cope with the ever-increasing demand. This is also due to the continuous improvement of beneficiation technology, which allowed the profitable recovery from ever lower grade ores [29]. This is exemplified by West [30]: The “massive decrease in copper ore grades was not driven by depletion of higher grade deposits with resulting higher copper prices. It was instead a direct result of innovation that converted massive supplies of previously worthless “waste” rock into valuable ore”. This tendency towards larger, lower grade deposits, coupled with the non-recoverable losses inherent in ore beneficiation implies that the waste production rate increases. For example, a decrease by 10% of the cutoff grade, for a constant commodity production, will typically result in an 11 to 12% increase in the waste produced for a given commodity production.

Energy concerns also play a role in this general trend [2,29]. The higher energy needs of lower grade ores [29] are hidden by the preferential availability of cheap energy and water offered by mining countries as an infrastructure support to mining development. This driver may fade away in the future with climate change mitigation actions (most of mining energy is provided by fossil fuels) and is expected to be the first cause of future commodity shortages [29], even before that of orebody depletion.

Furthermore, mechanization of mining operations allows for mining of deeper orebodies, with larger waste rock amounts. The move from underground mining to open-pit extraction also massively increased the amount of waste rock to be managed and disposed of. More generally, the constant trend to lower grades leads to higher waste amounts for a given quantity of commodity [1,11,29].

Reprocessing existing waste to further recover the commodity will have positive impacts on extracted volumes or energy needs, but it will not significantly affect the volume of residual waste, except where cutoff grades are measured in double-figure percentages (e.g., for Al, Fe, coal). The main benefit of reprocessing, in waste reduction terms, is to reduce the need for new extraction and additional waste generation elsewhere, although hazard reduction may be achieved simultaneously through better management and containment.

3. Profitable Uses of Mine Waste in the Modern/Circular Economy

Three main types of beneficial uses of mine waste can be identified within a waste reduction and circular economy perspective:

• Profitable use by the mining sector itself, for its own needs and benefit. We describe it hereafter as “further recovery of commodities”, “reprocessing” or “remining”. It uses historic waste as a feed for processing by modern technology [11]. It is most often led by a new company, different from the mining company which originally produced and stored the waste. This activity reduces the need for extraction of primary resources, and it may reduce the volume of residual waste.
• Use by the mining sector itself, for its own needs and benefit in site rehabilitation and/or mine closure. Using mine waste with desirable features such as acid neutralization potential also reduces the need for extraction of primary resources,
• Secondary use of mining waste as a raw material in an economic sector other than mining reduces the need for extraction of primary minerals, and it always reduces the volume of residual waste.

The first two options will be described here to allow comparisons with non-mining uses, but the latter will be the main focus of the present paper, as waste reduction is its direct purpose and it offers significant contributions to sustainability Safeguard Standards 1 (Biodiversity, Ecosystems and Sustainable Natural Resource Management), 2 (Climate Change and Disaster Risks), 3 (Pollution Prevention and Resource Efficiency) and 4 (Community Health, Safety and Security) [31]. Reference to sustainability is also explicated by [32].

3.1. Further Recovery of Commodities

3.1.1. Remining

Remining available waste has been an active practice for centuries (see, for instance, [33]). Recovery of valuable resources from mining waste applies mainly to existing waste from past activities, as the business model of current activities implies that all that is economically recoverable will be included in the beneficiation chain. However, undergrade ore can still be considered as waste, though the current practice is to store it separately in the hope of better commodity prices [34].

Recoverable commodities from existing waste include:

• Previously undergrade ore which becomes amenable to beneficiation due to better commodity prices or to improved technology. In this case, the ore can be classified under the same commodity as for previous mining at the site. At still active mines, blending waste ore and primary ore can be applied to streamline process feed;
• Commodities which were not beneficiated at the time of previous mining, either because of a lack of interest in them or because no economic beneficiation technique was then available. This is often the case for critical metals, currently required by new technologies (for instance B, Be, Li, Ga, Ge, Ni, Co, V, Sr, In, Hf, Ta, W, Nb, Y, rare earths, Cd, Sb, Ba, Bi and PGEs). It is possible to reprocess this waste, which will be classified as resource under the new commodity [15]. For instance, copper mining or processing waste containing residual cobalt [35,36] may be classified under “recoverable cobalt”;
• Non-metallic commodities (usually industrial minerals) that may be recovered from existing waste following site closure or remediation work. The largest such commodity supplies are of aggregate from waste rock [37] and of sand from tailings [38,39].

This approach to recovery can be applied to exploration, as an alternative to new deposit discovery, for the evaluation of strategic resources at a country level [9] or as a way to fund site remediation [40,41].

3.1.2. Tailings and Process Waste

Reprocessing of existing tailings for commodities is probably the easiest and fastest route to recovery of secondary resources from waste. The potential targets are the legacy Tailings Storage Facilities (TSFs) left in huge numbers by centuries of mining. It saves on cost, energy and water, compared to primary resource extraction, both for crushing and milling and for extraction. It can be easily scaled up, with hydraulic mining, or down scaled, possibly even using mobile processing units for small TSFs. Further milling of coarser tailings may be needed but at a fraction of the cost. Extraction of tailings from a TSF is also much cheaper than open-cast or underground mining. Last but not least, reprocessing an older TSF and rebuilding a new one allows the improvement of its stability and environmental impacts, saving on future site management costs. The world tailings stock was estimated to be 217 billion m³ stored in 8500 identified large facilities [42], with an annual growth of 11.1 billion m³. A larger estimate of 223 Gt (534 billion m³) based on a wider inventory of 29,000–35,000 facilities, with an annual growth of 8 to 10 Gt was reported by [12].

Many examples are available, both from research [13,42,43,44] and from full-size operations [45,46,47], covering both the original commodities and new elements of interest, including critical elements [15,44] (Figure 3). Hydraulic mining is the most frequently used technology [48]. Limitations and bottlenecks were studied by Kinnunen and Kaksonen [49].

This is applicable to most metal and precious metals mines, but specific applications of tailings beneficial use need to be taken into account for other mine types. For instance, residues from bauxite refineries (red muds) currently have a very low reuse rate, though beneficial use options are actively studied [50]. Coal mining [17] and phosphate mining [51] residues also have specific beneficial use strategies.

3.1.3. Waste Rock and Undergrade Ore

Reprocessing of existing waste rock heaps for commodities is possible for low-grade ores, disposed of as undergrade at the time of initial mining (Figure 4). It saves on cost and energy compared to primary resource extraction by open-cast or underground mining. Although waste rock heaps are usually less of a concern than TSFs, reprocessing an older waste rock heap may allow also the improvement of its stability and reduce environmental impacts.

3.1.4. Slag

Using slag or other metallurgical waste is considered less often than tailings or waste rock for further beneficiation for commodities, as extraction and mechanical processing are usually more complex and energy-intensive. The most commonly used technologies are pyrometallurgical processes (re-smelting the slag to separate metals, common in copper recovery using an electric furnace or a slag cleaning reactor), hydrometallurgical processes (acid or alkaline leaching, followed by electrowinning or precipitation, used especially for zinc, lead or nickel recovery) and bioleaching (use of microorganisms to extract metals). Recovery from slag is described for Cu [45,52], Cr and V [53], and for Zn and Pb [54]. These processes include the recovery of precious and critical elements such as Se, Te, Sb or Bi [48].

3.2. Waste as a Raw Material

3.2.1. Tailings and Process Waste

Tailings and other fine-grained process wastes are minerals whose grain sizes range from sand to clay and may be used as such (Figure 3) when their chemical properties do not preclude this use. Most tailings are deposited by gravity from slurries in tailing dams, otherwise designated as TSFs, but a significant proportion are used by the miners as backfill material, often with cement addition, and can be considered as engineering resources rather than waste.

The easy handling and large available volumes are assets for sand, landscaping material and backfill in civil engineering and road construction, as long as their sulfide, salts or labile metal contents are not excessive. Cement and concrete applications are possible [55], as well as artificial soil applications. The homogeneous grain size of many tailings may be an advantage for sand, concrete and technosoil [56] applications. The matrix chemistry of a large part of tailings is based on silica or alumino-silicate minerals, which is suitable for most applications. The beneficiation process, besides homogenizing grain size, tends also to homogenize the mineral chemistry. More specific applications such as coatings, resin, composite materials, glass and glazes, bricks, floor tiles and cement, soil amendment or water treatment may be considered for clayey tailings or red mud from aluminium refineries [50]. Reactive waste can be used as a raw chemical agent for other mineral industries or for site remediation (i.e., as a source of acids, for red mud neutralization [57], or of alkalis, for acid drainage mitigation [50]), as long as this use does not require long-distance transportation.

3.2.2. Overburden Waste Rock

Waste rock is similar in many aspects to primary rocks quarried for aggregate applications, but also for backfill, landscaping material, aggregate in construction and civil engineering, and source material for cement and concrete (Figure 4; [55]). Waste rock with acid-neutralizing properties [58] is a valuable raw mineral resource for mine closure and acid drainage mitigation.

3.2.3. Undergrade Ore

Though undergrade ore is rather used for reprocessing to extract minerals and metals, it may be used as aggregate as long as its chemical and physical stability does not constitute a significant hazard. The main issue is the content of acid-generating oxidizable sulfides or insoluble salts.

3.2.4. Slag and Other Metallurgical Waste

Slag is also used for road construction and in concrete, due to its toughness, and in cement for its chemical properties. It can be used as a replacement for Portland cement up to 70%, improving long-term strength, permeability and durability [59].

3.3. The Need for a Waste Classification

Cotrina-Teatino and Marquina-Araujo [60] emphasized the link between circular economy strategies for tailings and waste management in mining, and sustainability and optimized resource use in the mining industry, calling for more innovation in mining waste beneficial use.

Specific classifications were developed for specific beneficial uses, for instance construction materials, civil engineering and road construction [8], or for national resources policies [61].

An in-depth study [62] identified five key drivers for mine waste valorization in a circular economy perspective: 1/ Social dimensions, 2/ Geoenvironmental aspects, 3/ Geometallurgy specifications, 4/ Economic drivers and legal implications, and 5/ Circular economy aspirations. As the objective here is to evaluate the feasibility of a mine waste classification based on circular economy abilities, our focus was on 3/, 4/ and 5/, as social and geoenvironmental issues are chiefly site-specific, whereas a classification should be based on material characteristics.

4. Mining Waste Classifications

There are various classification schemes for mining waste, each of them according to a specific purpose. The paper will review them in order to establish how far they can be used to evaluate circular economy opportunities. Each heading in this section deals with potential classification criteria.

4.1. Classification by Mining Activity and Storage Facility Type

The main waste categories are extraction waste (more often named waste rock) and processing waste, with markedly different properties, and, subsequently, of waste forms [63]. Their key properties are summarized in

Table 2. Mining waste categories [63].

Extraction Waste	Processing Waste
Coarse material abundant, large heterogeneity	Mostly fine-grained, sandy or silty, homogeneous
Ore elements in variable amounts	Valorized elements depleted Unused elements concentrated
Mechanically dumped	Slurry decantation

.

Extraction waste can be further subdivided into overburden, barren host rocks, and low-grade mineralized rocks. Most are stored as waste heaps, or dumps, or spoils, usually above the original ground level, which makes further access easier than excavation mining, but in some cases, they can be dumped in previous mining excavations or along slopes.

Milled ore is usually directly processed, and processing waste is mostly referred to as tailings. This is the most familiar category of mining waste. It is usually extremely fine-grained, but on older mine sites, coarser waste from mechanical beneficiation processes can be found. It is stored as reservoirs limited by dams in which it is transferred in fluid or semi-fluid form (slurry) for natural decantation and dehydration (Figure 5), usually named tailing dams, Tailings Storage Facilities or TSFs (Table 1).

They can be built in available valleys, similarly to hydropower reservoirs, or by elevation above the natural ground in flat areas. After complete decantation and dehydration, the result is a compact, fine-grained, sedimented structure, which can be later dug for commodity recovery with minimal comminution [10,11,43]. Originally, these dams were typically constructed with waste rock and then raised with dewatered tailings. To maintain the external slope angles and thickness, such raises were inevitably placed onto earlier dewatered fill, resulting in an inherent instability prone to failure. The construction and management of TSFs is now strictly regulated and subject to risk assessment [64,65,66,67].

When metal recovery is performed at the mine site, metallurgy waste (slag) can be found, mostly as stockpiles, as it is usually granular. A specific category of metallurgy waste, red muds from alumina processing, is similar to tailings.

4.2. Classification by Ore Grade in Waste Rock

During mining (both underground and in open pits), various categories of rock are extracted. Rocks without any ore content may need to be removed to expose ore or to access it: this is usually referred to as overburden but may also be called “steriles”. At the moment they are extracted, the miners know they are barren. They do not direct them to crushing but to waste rock stockpiles (Table 1).

Orebody sections which are known to be significantly lower (this means that the estimated grade of the ore batch is lower than the acceptable minimum grade for the current beneficiation process, including measurement uncertainties, or is too low to be acceptable for blending. To become minable again, it needs a major change in market conditions or in the processing technique. It may nevertheless be profitably stockpiled as such changes may happen during the lifetime of any mine) than cutoff grade may also be directed to waste rock stockpiles if the miners do not expect them to be profitably beneficiated even with better market conditions. However, it may be desirable to manage them separately to allow later repurposing or to reduce storage risks. They are then managed as mineralized waste rock or undergrade ore stockpiles (Table 1).

Ore found to be slightly below the cutoff grade is usually stored for blending and later directed to crushing. In some configurations, crushed ore may be stockpiled rather than directly milled.

Lower grade ore, which may become minable under better market conditions, will be directed to undergrade ore stockpiles, hopefully waiting for future beneficiation.

Undergrade ore stockpiles have to be managed separately if they have AMD (acid mine drainage, or ARD, acid rock drainage) potential or other properties that can make them unsuitable for safe disposal or direct reuse. They are submitted to specific management constraints for safety [11,58,64,65] and, if not used, become waste rock stockpiles when the mine closes.

4.3. Classification by Ore Grade in Tailings

Tailings inherently fit the definition of waste, a substance discarded after primary use as worthless. Tailings are ore which underwent the full beneficiation process and for which no economic valorization option is available at the time of disposal. Beneficiation options may arise later as a result of newly available technologies or of a rise in commodities’ market value. Reprocessing legacy tailings is then a desirable option, as the cost and energy needs of milling will be much lower than for primary ore. When considering the new market conditions and technologies, tailings, which have still no possible economic valorization for commodities, are raw fine-grained minerals.

4.4. Grain Size and Beneficial Use Options

One key separation between potential beneficial uses is between coarse-grained material, used as a substitute for aggregate, and fine-grained material, used as a substitute for sand or as bulk minerals.

4.4.1. Coarse-Grained Material

Waste rock and other coarse material will be evaluated for potential reuse according to the criteria applicable to aggregate. This includes bulk aggregate used for infrastructures by civil engineering and aggregate used in construction, especially by the concrete industry [68]. Physical criteria defining the fitness for purpose are the following [37,64,69],

Table 3. Physical aggregate suitability assessment criteria for mining waste rock.

Criterion	Method	ISO Standard	ASTM Standard	BS Standard
Particle Size Distribution (Grading)	Dry sieve analysis	ISO 20290-5:2023-Aggregates for concrete	C33/C33M Standard specification for concrete aggregates	BS EN 12620:2013
Particle shape, Flakiness index	Petrographic and image analysis		C295	BS 882:1992
Bulk density	Calibrated containers, pycnometer bottle	ISO 20290-1:2021	C 29/C 29M	BS 812
Water Absorption	Pycnometer bottle	NF EN 1097-6	D570	BS EN 1097-6
Strength testing	Aggregate Impact Value (AIV)		D58-74	BS812-112
Strength testing	Aggregate Crushing Value (ACV)-Ten Percent Fines Test	ISO 20290-3:2019–EN 1097-2		BS812-110-BS 812-111
Strength testing	Los Angeles Abrasion Value (LAAV)	ISO 20290-2:2019	C-131-06
Aggregate durability testing: wear	Aggregate Abrasion Value (AAV)		C-131	BS812-113
Aggregate durability testing: soundness	Magnesium Sulphate Soundness Value (MSSV)	EN 1367-2	C 88-05
Aggregate durability testing: soundness	Methylene Blue Absorption Value (MBV)	EN 933-9	C 837-99
Aggregate durability testing: soundness	Alkali Silica Reactivity (ASR)		C289, C1260

:

According to civil engineering properties, it is often the case that coarser waste rock provides better strength, especially for angular blocks, while for concrete applications, a blend of various grain sizes may be designed for the planned use. This implies that a good knowledge of the grain size of waste rock is needed for beneficial use, especially as it was not homogeneous in the waste feed, and tends to evolve with storage time, the finer fragments moving towards the bottom of the form between the larger blocks. Other criteria defining the fitness for purpose, such as matrix chemistry and stability, will be listed further on.

4.4.2. Tailings

Grain size of tailings ranges from sand to clay, with a large proportion of silt-size material. Sand-sized tailings can be used in civil engineering or in concrete production if their chemical composition does not preclude this use [13,70]. They tend to have more homogeneous grain sizes than natural sand. Silt- and clay-sized tailings are suitable for bulk mineral uses, such as cement or glass, subject to the same chemical limitation, or for non-structural engineering (landscaping, for instance). In this case, their grain size has little effect on their suitability for beneficial use. Due to the evolution of ore processing technology, older tailings are coarser, especially those from gravity processes, while chemical processes require finer-grain material for better exchange surfaces.

4.5. Matrix Chemistry and Mineralogy

Rock matrix chemistry and mineralogy are determined by the geology of the ore and gangue and the orebody genetic type. They will define chemical groups with potentially different characteristics for beneficial use and possible applications. Some examples are listed in

Table 4. Matrix chemistry criteria for mining waste in relation to applications.

Main Components	Ore Deposit Type	Possible Applications
Siliceous and quartz	Placers, quartz veins (gold)	Civil engineering, glassworks
Si-Fe	BIF (Banded Iron Formations), supergene (gold)	Civil engineering, concrete, roads
Si-Al-Fe	Lateritic, bauxite	Civil engineering, concrete, ARD remediation
Si-Al-K	VMS (Volcanogenic Massive Sulphide deposit), epithermal, granite-related	Aggregate, concrete, bricks, tiles
Ca-Fe-Mg	Volcano-sedimentary, basalt and diorite	Civil engineering, aggregate, concrete
Ca and Ca-Mg (carbonate)	Sedimentary	Cement, ARD remediation

.

The matrix chemistry determines the possible use as a raw material component (cement, glass, bricks), part of the physical properties and the stability (concrete). This applies also to bauxite refinery residues [46], though with specific criteria.

4.6. Chemical Stability

Leaching tests (

Table 5. Chemical stability aptitude criteria for mining waste.

Criterion	Method	ISO Standard	ASTM Standard	US-EPA Standard
Acid generation potential (AP)	ABA		E-1915
Acid neutralization potential (NP)	ABA		E-1915
Acid base accounting (ABA) (independent determination of AP and NP)	ABA		E-1915
Net acid generation (NAG) procedure	NAG
Paste pH	Paste pH
Synthetic Precipitation Leaching Procedure (SPLP)	Water/acid leach			Method 1312
Toxicity Characteristic Leaching Procedure (TCLP)	Acetic leach			Method 1311
Compliance Test for Leaching of Granular Materials and Sludge	Water/acid leach	EN 12457
Up-flow Percolation Test	Water leach	CEN/TS 14405
Influence of pH on Leaching	Acid/base solutions	CEN/TS 14429, EN 14997
Acid and Base Neutralization Capacity	Acid/base solutions	CEN/TS 15364
Humidity Cell Test (HCT)	Long term Water leach		D5744-96
US Geological Survey Field Leach Test (FLT)	Water leach			USGS

) are at the core of this classification. Potential undesirable properties are solubility (salts), reactivity (alkali-silica, -silicate and -carbonate reactions in concrete) and release of potentially harmful elements (metals or metalloids) during weathering of the secondary use products. Reactions to produce secondary minerals can also result in swelling, such as when low-density gypsum forms as carbonates react with sulfuric acid from the oxidation of pyrite. The key issue pertaining to chemical stability is the possibility of acid mine drainage (AMD) or acid rock drainage (i.e., ARD, which can occur also outside of mining areas). For a full review of ARD issues and prediction, see [52]. AMD/ARD is a critical factor for metal mines with sulfide, and for coal mines. A notable exception is bauxite residue, which is highly alkaline, and which requires neutralization for chemical stability prior to any beneficial use [50].

Acid–base accounting (ABA) is a laboratory-based calculation method for assessing the overall potential of a geological material to produce acid. It is based on the balance between its acid producing and acid neutralizing properties: Acid Generation Potential (AGP) or Maximum Potential Acidity (MPA) is the maximum amount of acid which can be produced from the material, primarily due to the oxidation of pyrite; Acid Neutralization Potential (ANP) or Neutralization Potential (NP) is the total amount of acid neutralizing substances present, such as carbonates. The MPA and NP are determined independently by various chemical tests for sulfur and carbon respectively, then compared by difference or a ratio as shown below. Various other standard chemical and physical tests can be applied to mining wastes to predict their future acid generation and contaminant leaching behavior (Table 5).

4.6.1. Waste Rock

If they do not contain undesirable substances or sulfide, such rocks can be directly reused as aggregate or for civil engineering. Removing sulfide from waste rock to meet engineering or aggregate applications would require crushing and milling. If it has acid neutralization potential, waste rock may be stored separately for future site remediation and AMD mitigation. Testing waste rock for potential release of contaminants is generally based on the quantification of undesirable elements in water or other solutions exposed to waste according to a standardized protocol [71,72,73]. The large size of waste rock fragments, and the subsequently low interaction surface, are a challenge for such tests: the release potential may be underestimated if large blocks are tested, especially as block fragmentation is likely to occur in the long term, beyond test duration, and if blocks are fragmented for the test, the test result will not reflect the real waste rock behavior.

4.6.2. Crushed Rock and Low-Grade Ore

Testing for chemical stability and potential leaching is more critical for lower grain size material, which is expected to be more prone to oxidation and leaching. Due to its potential applications, acid generation and subsequent metal and metalloid leaching are the most critical issues for beneficial uses. Chemical reactivity is also an issue, especially for use in concrete. Applications in mine backfill are less critical, as such waste does not differ much from the host rocks, apart from its fragmentation and permeability to water and oxygen.

4.6.3. Tailings

When used for backfilling [74] or in civil engineering, tailings are less permeable than crushed rocks and may present a more hydro-geochemically favorable behavior. When used as a raw material, such as in cement, bricks or concrete, chemical stability and potential leaching are critical for acceptability as a substitute for primary raw materials. It must be remembered that criteria for material acceptability in civil applications were designed for primary extraction mineral characteristics and not for mining waste, especially tailings. These criteria should ensure the long-term stability and safety of any works built with these materials. This may be difficult to demonstrate with tailings, especially sulfidic ones. Fine-grain milling offers easy access to oxidation and facilitates sulfide decomposition. Long-term stability and leaching behavior needs to be assessed, and kinetic testing may be useful.

Removing sulfide from tailings to make it amenable to civil engineering applications (environmental desulfurization) has been attempted at the laboratory or pilot scale by physical or chemical techniques [75,76] or implemented for site compliance [77] but it is usually too expensive for beneficial use projects. It was used for the preparation of a desulfurized layer [78,79] for use as a safe cover for a TSF. Unfortunately, public operational economic data on global tailings desulfuration processes could not be obtained.

4.7. Risk and Legislation

The main categories defined by the European Mining Waste Directive (2006/21/EC, or MWD) are inert waste, hazardous waste, and non-inert, non-hazardous waste [63,64]. Equivalents can be found in most national regulations or international guidelines.

The European definition of hazardous mining waste is closely derived from the definition of hazardous waste in the Waste Directive (2008/98/EC), while the definition of inert mining waste is specific (MWD, appendix II, and Decision 2009/359/EC). This leaves an important gap between these categories, containing a large part of all mining waste, which is neither hazardous nor inert. The proportion of inert waste, hazardous waste, and other (i.e., non-inert, non-hazardous) waste varies for each site, with often more than 50% falling into the latter category. Non-inert, non-hazardous mining waste has no explicit definition but nevertheless requires appropriate management [64].

Similar definitions can be found worldwide. For Australia, Canada, the USA or Brazil, it is often at the state level rather than the country level. For the USA, the Resource Conservation and Recovery Act (RCRA) governs the hazard classification of waste, but there are many exemptions for mining waste, and state-to-state variations. Reuse strategies are described by ITRC (Interstate Technology & Regulatory Council) [56]. Hazardousness classifications in most countries are based on the same criteria as those of Figure 6 (potential toxicity and ecotoxicity, acid generation, ignition, solubility, particle generation), for instance in Chile, as described in decrees DS 148 and DS 185. Tailings minimization, reprocessing and reuse is mentioned by the Australian Government’s and the Canadian Mining Industry good practice guidance [80,81,82] but without any criteria beyond site-specific requirements.

Definitions in China are included in the National Hazardous Waste Catalogue. In developing countries, national definitions are often derived from those in use by the international mining partners.

This classification is used to define waste management categories, their specifications and monitoring needs, but it does not help to guide the potential future use of the waste. This use is regulated by the dispositions on general waste, including the producer responsibility, despite the specificities of mining waste. Furthermore, the waste regulations are designed with freshly produced waste in mind and not for legacy waste, with no provisions on reuse during or for remediation. This has, until now, hampered most initiatives for considering legacy waste as a possible resource.

4.7.1. Inert Waste

The inert category is based on an extended characterization scheme, intended at demonstrating the innocuity of the waste or minimizing the risk of any further hazard. The European scheme is summarized in Figure 6.

There are marked differences between mining waste characterization schemes, based chiefly on environmental hazardousness, and civil engineering characterization schemes, based chiefly on technical performance, with minor environmental aspects. This is why the former cannot be used in a circular economy perspective.

4.7.2. Hazardous Waste

In Europe, no specific definition of hazardous waste is applicable to mining waste. The general provisions of the waste regulation 2008/98/EC (

Table 6. Hazardousness criteria for mining waste (adapted from [63]).

Hazardousness Criteria Applicable to Usual Mine Waste	Hazardousness Criteria Applicable to Specific Mine Waste	Non Applicable to Mining Waste
(H4) irritant substances (H5) harmful substances (H6) toxic substances (H7) carcinogenic substances (H8) corrosive substances (H10) teratogenic substances (H11) mutagenic substances (H13) substances that may release potentially dangerous leachates (H14) ecotoxic substances	(H1) explosive substances (H2) oxidizing substances (H3-A) highly flammable substances: COAL WASTE (H12) may release toxic gases: CYANIDE PROCESSING WASTE	(H9) infectious substances

) are applicable:

Some of the following substances may also be found in mining waste, due to present or past extraction machinery or processing reagents:

• Residue from substances employed as solvents;
• Halogenated organic substances not employed as solvents, excluding inert polymerized materials;
• Tempering salts containing cyanides;
• Mineral oils and oily substances (e.g., cutting sludges, etc.);
• Oil/water, hydrocarbon/water mixtures, emulsions;
• Substances containing PCBs and/or PCTs (PCBs: Poly-Chlorinated Biphenols; PCTs: Poly-Chlorinated Terphenols) (e.g., dielectrics, etc.);
• Tarry materials arising from pyrolytic treatment (e.g., still bottoms, etc.);
• Pyrotechnics and other explosive materials;
• Spent detonators, electrical cable;
• Other wastes (e.g., timber shoring, rubber or plastic pipes, scrap metal or fragments).

In most other countries, hazardous waste criteria are also generic and independent of the origin of the waste. Hazardousness criteria are often similar to those in Table 6. It may happen that mining waste is presumed to be hazardous in local regulatory practice. In this case, it may be required to demonstrate that a given waste is inert or non-hazardous before considering its beneficial use.

4.7.3. Non-Inert, Non-Hazardous Waste

This category includes by far the greatest quantity of mining wastes. It has no specific definition; waste that does not fit the inert criteria nor meet the hazardous thresholds is implicitly non-inert and non-hazardous. There are no specific rules and regulations for such mining waste in Europe beyond the general waste regulations, which are usually less stringent than rules for hazardous waste. Waste management facilities for this category follow design rules applicable to very large volumes.

4.7.4. The Key Role of Sulfide and Sulfate in Inertness

Hazardous waste definitions are not specific to mine waste. They are based on potential effects. They cannot be directly related to the mining activity classification. However, most ores, and therefore mining waste, contain sulfides, with the exception of Al and many Fe ores. These reactive minerals and their oxidized forms, especially sulfates, are specifically targeted by inertness criteria (Figure 6 and Table 6) and are often the main reason for failing the inert status.

Among the hazardousness criteria, H5 and H13 (Table 4) are the most likely affected by sulfides. This is especially true when sulfides are accompanied by potentially toxic elements (for instance, As, Cd, Pb, Hg), which are likely to be more mobile in the presence of sulfides, sulfates or sulfur-generated acidity.

4.8. Circular Economy Potential

None of the above classifications, which are based on the inherent material properties, can by itself provide a robust framework for circular economy potential evaluation of mining waste, as the constraints for a safe beneficial use vary widely between waste types. Nevertheless, all of them have to be taken into account, together with additional economic factors relating to supply and demand.

4.8.1. Waste Producer Influencing Factors

Research is often carried out by the mining and mineral chemical industries in order to develop applications for their waste flows: coal fly ash [83,84], aluminium waste [85,86], and phospho-gypsum [87], in order to improve their profitability and reduce waste expenditures. Even if this is carried out in-house, such developments belong to the circular economy and improve the sustainability of the producer. The phospho-gypsum example is particularly meaningful due to the very large size of its producing industry, and the need to find an outlet for this waste. The limitations are the concentration of radioactive elements [88] and potentially toxic trace elements (the so-called “heavy metals”).

4.8.2. Use Sector Influencing Factors

Due to the low intrinsic value of mining waste and the high cost of its transportation, having a potential user as close as possible to the waste flow or waste stock is of primary importance. The most widespread use sector, anywhere, is the building materials and civil engineering sector [89]. Road construction or maintenance [8] is often considered as it is needed almost anywhere.

Some more specific mineral transformation industries can use large amounts of specific types of mining waste. An example of this is the aluminate sector for the cement and concrete industry, which is able to use high-Al waste, usually bauxite residues [50,90]. In this case, the user industry may be implemented closer to the waste source in order to ship only higher-value products, and even on the mine site itself. Similarly, plasterboard manufacture can use phospho-gypsum [87,91]. This requires careful monitoring for undesirable substances, including fluoride and radionuclides [92], which could lead to regulatory health issues with the end product.

The concrete sector [8,37] has by itself large needs for raw materials which may be problematic for territories. This is a driver to tap alternative sources such as mineral waste, hence the early consideration of mining waste.

4.8.3. Demand for New Sources of Scarce Substances, Esp. Critical Elements

When a need quickly arises for a substance which was not actively mined before, or which is in short supply, it may be far more cost- and energy-effective to extract it from waste. When the opening of new mines has to face social reluctance and legacy waste is an environmental burden, it is tempting to extract new commodities from old waste while remediating the legacy site. A well-known example is the former Kasese copper mine in Uganda, whose tailings were a serious threat for the catchment but were also a rich untapped stock of cobalt [35,46]. Gold tailings in Witwatersrand (South Africa) are currently being re-examined as a potential uranium resource [93]. Scarcity may affect other mineral resources than critical elements, depending on the needs of the local economy. UNEP [94] identified a rising crisis threat for global sand supply and its potential deleterious effects on the global economy and welfare. This should be considered as a major valorization opportunity for tailings.

5. Discussion and Conclusions

The beneficial use of mining waste should clearly be encouraged, since several key benefits are to be expected:

• Saving primary resources and subsequently extending the availability or lifetime of scarce mineral resources;
• Reducing the volume of legacy mining waste and its environmental impacts;
• Developing a resource beneficiation industry which is less energy- and water-intensive.

In this perspective, beneficial use of mining waste is fully in line with circular economy thinking. Examples of successful mine waste remining for commodities are many and will be even more common in the future. Examples of beneficial use as raw materials or for civil engineering are rarer, however, although they might appear to be easier.

5.1. Beneficial Use for Commodities Recovery

This part of mining waste beneficial use is indeed in the scope of the circular economy, as it saves primary resources without adding to waste generation. It cannot reduce significantly the volume of legacy waste unless raw materials production is a side activity. But, by reworking it to better standards, it contributes to impact reduction and site remediation.

It is, however, still a mining activity, organized with the traditional linear exploration–processing–waste storage and closure cycles and rules. A circular economy classification of existing waste will be of little use, as this waste is still chiefly an ore, and there will still be waste stocks afterwards, even if these are cleaner. The main focus of this paper is therefore limited to the use of mining waste as raw minerals.

5.2. Key Criteria Conditioning Beneficial Use as Raw Minerals

The most important criteria identified by this review are:

• Grain size and homogeneity, which will screen possible large-scale applications, especially for civil engineering and construction;
• Chemical stability and potential contaminant release: in mine waste, the abundance of sulfides is a key criterion, as it controls acid drainage (ARD) and metal leaching, now and in the long term. Sulfide separation in order to produce low-sulfide, potentially neutral tailings is possible using mineral processing techniques and was tested at the laboratory scale [75,76], but did not lead to real beneficial applications because the cost of processing exceeds by far the potential value of the product;
• The local needs in raw minerals and the distance between the waste stock and the end user.

Additionally, the financial cost of large-scale waste storage and sustainability image of the mine operator are directly affected by any circular option with a significant effect on the volumes to be stored, and the land surface required.

The economic value of the available waste is often close to zero, and the comparative costs of primary mineral extraction are usually low. The economic benefit of waste reuse is difficult to demonstrate unless the indirect benefits are identified and quantified. The greatest of these is the contribution of beneficial use to mining site remediation, an expensive activity which is rarely taken fully in charge by the mine operator.

5.3. Constraints of Mining Waste Reuse When Compared with Primary Material

Mining waste reuse for commodities obeys mining economics and requires traditional mining technology. The size of operations is usually smaller than for new mines, and may even be small enough to allow use of mobile technologies. The capital needs and associated risks are not high. The carbon and water footprints are usually smaller. The smaller output compared to a large mine may be a limitation if the commodities’ customers require a dependable resource availability.

Risk factors are more important when the beneficial use is for raw materials. The end user can be a minerals provider or a civil engineering contractor. They need to be sure that no liability will result from the beneficial use, due to the mining waste origin. For instance, the raw materials should not degrade with time, disaggregate, collapse or release contaminants at significant levels. This would be likely for potentially acid-generating materials. Natural primary extraction material requires extensive testing to meet various civil engineering suitability and conservative safety requirements [95]. Meeting these requirements is more difficult for mining waste, which was not extracted with engineering performance in mind and usually contains many more undesirable substances than raw materials [25]. The additional cost of testing and treatments for waste may exceed the cost of primary mineral extraction. Even when the beneficial use is expected to be profitable, the precautionary principle or the lack of public acceptance for waste reuse may preclude any operation.

Mining waste processing so as to reuse part of it is still an extractive industry, and it has to take care of its own waste management and later site cleanup. It cannot be a zero-waste operation, despite many claims from research projects or consultancies: regardless of the process, a part of the waste cannot be repurposed for technical or environmental reasons (the quantification is further explained as a Supplementary Materials Text). Rather, it contributes to the legacy site cleanup, which would otherwise have to be supported only by public money.

5.4. Types of Waste Material Most Promising for Reuse

When reuse is aimed at extracting more commodities from existing waste, the answer is obvious: tailings and other milled waste offer the advantage of much lower energy needs, and easier handling and processing. The answer is not straightforward when the reuse option is as a raw material. It may be dictated by external factors, such as the local needs or the distance between them and the mining waste site. When several reuse options are available, the suitability of the waste depends on the type of use (aggregate, sand or clay), the requirements for performance, such as mechanical stability, and for environmental safety. Aggregate uses will best fit waste rock [8,37,96], especially that with little or no hydrothermal alteration. On the other hand, altered zones may provide sand or clay, such as for kaolin clay [95], from an historic tin mining district. Tailings may be successfully used for agricultural purposes (as technosols or a soil amendment [97]), provided that their potential release of toxic substances is controlled.

5.5. Refining Criteria and Developing Tests

Lottermoser [98] stressed the fact that circular economy applications require innovative protocols beyond what was developed for environmental purposes, such as compliance leaching tests. The present review demonstrated that none of the available criteria is able by itself to give a reliable indication of the viability of a beneficial use application and the suitability of a given waste for this application. Examples in the literature are scarce [61]. All of them have some relevance for this purpose, but the potential user needs to weigh them up according to the specificities of his application and the candidate waste stock.

Would a formal classification of waste, based on the criteria listed in this paper, help? The main purpose would be to identify possible beneficial use options for a given stock of waste. This will be difficult, not least due to the diversity of criteria, of regulations and of the lack of consensus on economic and indirect benefits. Such a classification will be complex to build, due to the wide diversity of waste types and range of possible beneficial uses.

Developing public acceptance and stakeholder confidence is another obstacle which cannot be surmounted by test compliance alone. Compliance is required, and transparency on test results will help, but other benefits need to be demonstrated to gain adhesion.

On the other hand, selecting a shortlist of possible beneficial uses from local material needs or from waste availability and characteristics seems to be a sensible approach, with specific test thresholds.