Waste Valorization Technologies in Tannery Sludge, Chromite, and Magnesite Mining

Abstract

Waste valorization involves reusing and recycling waste materials to create useful products such as materials, chemicals, fuels, or energy. The primary goal is the transition to a circular economy model while minimizing the impacts of hazardous waste. Adopting such policies appears to be a one-way path due to the continuous increase in the consumption of raw materials. According to recent projections, by 2050, 180 billion tonnes of materials will be consumed annually. Since natural resources cannot meet these requirements, new sources must be explored. Waste can serve as an alternative source and cover at least part of the needs that arise. In this work, good practices regarding waste valorization are presented. The case studies examined include the waste/by-products of ultrabasic rocks resulting in chromite and magnesite mining, as well as the tannery sludge produced after the corresponding wastewater treatment.

1. Introduction

The United Nations Environment Programme reports that the growing middle class has led to a tripling of primary materials extraction over the past four decades. This increase in consumption will enhance air pollution, decrease biodiversity, accelerate climate change, and eventually result in the loss of natural resources, leading to shortages of essential materials and increased risks of local conflicts [1]. In 1970, there were 22 billion tons; by 2010, there were 70 billion tons. Earth’s primary materials extraction has grown significantly, with the richest nations consuming 10 times more materials than the poorest and twice as much as the global average. By 2050, the nine billion people on Earth will require 180 billion tonnes of material annually, and the current resources cannot meet this demand [2].

Nowadays, a well-planned approach, an accurate methodology, and an adequate execution may lead to the creation of products with marketable value through waste valorization. The European Commission has introduced a new trend called “Closing the Loop” to emphasize the importance of the circular economy, seeking to reduce waste production while preserving the value of goods, materials, and resources for as long as feasible [3]. Reusing waste as a resource in social, economic, and spatial policymaking can be facilitated by the circular economic model. Additionally, it can encourage mutually beneficial relationships between industries and manufacturers, paving the way for successful material recovery procedures and efficient industrial design [4].

According to management theories, including environmental principles in company operations can lead to integrated value creation and long-term competitive advantages. Economic development has led to resource depletion and environmental degradation, making resource recovery crucial for the preservation of ecosystems, a sustainable economy, and less reliance on limited resources [5]. It entails sorting waste materials into those that may be recycled into new products or utilized to replace fossil fuels as an energy source. This process is essential when total waste avoidance is not possible, allowing for the efficient recovery of materials for recycling and re-use without additional processing [6]. Effective resource recovery, such as transforming waste streams into chemicals, energy, and other materials, is crucial for changing waste management systems [7]. Waste incineration is a common treatment method in developed countries for energy recovery [8], while numerous material recovery facilities are employed to increase resource extraction from waste streams, such as noble/precious metals [9].

Mineral processing, a major industrial activity nowadays, is a key process in extractive metallurgy that converts ore into a marketable product by separating valuable minerals. The profitability of a mine depends on the maximum concentration of desired minerals, and it is designed to yield the maximum amount of concentrate before products reach the market [10]. On the other hand, large volumes of waste can be produced by mining operations, which need to be handled and managed carefully to prevent environmental pollution. Approximately 28% of the EU’s total waste stream originates from mining waste [11]. Mineral products such as mine waste, ore processing tailings, leach residues, fly ash, and slags, as well as wastes like gases, dust, and sludges, are being treated to recover valuable metals and other substances, such as gold, titanium, uranium, and fluorine. However, large volumes of these are discarded annually, resulting in significant loss of valuable materials and imposing considerable financial and ecological costs on both the public and industrial sectors [12].

Making the shift to a circular economy requires a thorough understanding of the challenges, and research is essential for identifying these in the mining industry to ensure a smooth process and promote sustainable practices [13]. The study of Upadhyay et al. [14] examined circular economy initiatives in the mining sector, identifying common themes across three major companies, and evaluating barriers, drivers, and triggers through content analysis. Mateus and Martins [15] presented the challenges and opportunities in the mining industry, proposing constructive solutions to address weaknesses and threats, aiming to maintain a long-term balance between supply and demand of mineral products. Moreover, Kaźmierczak et al. [16] explored the economic use of waste from active mining plants, focusing on reduce-recycle-reuse concepts and employing a multicriteria analysis to evaluate waste production and processing. Tayebi-Khorami et al. [17] proposed a “re-thinking” approach of mining waste management, emphasizing social, geoenvironmental, geometallurgical, economic, and legal aspects for improved environmentally friendly outcomes and circular economy applications.

The International Council of Mining and Metals [18] highlights various uses of mining waste, including waste rock for backfill/landscaping and road construction, manganese tailings for agroforestry, clay-rich tailings for bricks, floor tiles, and cement, slag for road construction, and red mud for soil amendment, wastewater treatment, and the production of glass, ceramics, and bricks. Water derived from mining activities is used for dust suppression, mineral processing, industrial and agricultural purposes, and as a coolant. Sludge, originating from acid rock/mine drainage treatment, is used in pigments, and useful industrial chemicals may also be produced through these treatment processes, e.g., sulfur dioxide is converted into sulfuric acid in smelters.

This work aims to highlight and report recent research advances on the application of circular economy and waste valorization. For this purpose, the waste resulting from the mining of chromite and magnesite, as well as that from the tanneries, was used as case studies.

2. Mining Waste—The Chromite Case Study

A poorly managed chromite mining waste may create significant environmental pollution, which is reported in various regions around the world, such as in India [19], Pakistan [20], Iran [21], and South Africa [22]. This type of onsite waste includes soil and rocks that are removed to gain access to the main ore deposits in open pit mines. They are usually accumulated on the surface of mining areas, where further mining activities are not prevented, since transporting large volumes of material is costly. In general, they present low potential for environmental contamination, i.e., they are considered inert material and, often, they are used at mine sites for landscape restoration [23]. However, there are also cases of such inert waste that present significant amounts of chromium. In this case, if not properly protected or managed, the drainage from the surface water may contaminate the nearby surfaces as well as groundwater bodies [24].

In addition, during chromite mining activities, mainly through drilling, blasting, and transportation, a large amount of dust is generated. The stacking and loading of both the ores and the overlying chromite surfaces also generate a large number of particles, with the corresponding dust in this case being attributed to chromite particles [25].

The chromite, after mining, is subject to the enrichment process once it is transferred to the corresponding facility. Through beneficiation, it is separated from the overlying bedrock, namely the ultrabasic rocks, which are considered waste for the facility. The beneficiation residues are alkaline (pH > 12) and contain soluble metal salts, which is why they are regarded as hazardous solid waste [26]. This solid waste mainly contains serpentine and its modified products, such as antigorite, lizardite, and chrysotile, while in smaller quantities carbon-containing minerals such as olivine, limestone, dolomite, and other calcareous minerals [27].

All the aforementioned types of chromite mining (especially opencast mining) wastes are very likely to contain unrecovered chromite. The huge volumes of this waste are significant. Despite the lack of a trustworthy estimate currently, it is likely millions of tons annually. Since chromite is rather inert, the majority of the Cr in these wastes is found as Cr(III). They are generally collected in open storage areas (stockpiles) or landfills without further treatment because landfilling is the simplest and most economical method of waste disposal. Storing the waste in a disposal facility minimizes the environmental impact [28].

Their use as filler material has been prohibited due to their classification as hazardous waste. This is because of the oxidation of Cr(III) to the highly toxic form Cr(VI), which has high solubility. However, old-filled sites still pose a major risk to the environment and human health due to the continuous and long-lasting leaching of Cr. The disposal of waste in landfills has caused contamination of underground water resources. Therefore, they require long-term monitoring as Cr(VI) is one of the leading pollutants that pose a significant threat to human health and the environment [29].

The following general categories can be used to group wastes from chromite mining that include Cr(VI): (i) be recycled in the process (e.g., retention of dust mainly containing raw materials), (ii) reused in other applications (e.g., ultrabasic rocks) and (iii) those considered hazardous (e.g., tailings/mining wastes prone to hexavalent chromium formation via atmospheric oxidation). The last category, i.e., the hazardous waste, should be stored in properly designed waste facilities. It is also important to observe any potential harmful metal leaks from such waste sites [30].

To date, various methods or techniques have been developed to limit the pollution caused by chromium. Some commonly used techniques for treating Cr(VI) wastes include precipitation, membranes, ion-exchange membranes, biosorption, and reduction. Factors such as mobility, distribution, concentration, and form of chromium in the sources are known to govern the selection of the most appropriate method. Physicochemical remediation processes require a high capital investment and result in the production of a large volume of secondary waste. Interest has shifted to biological technologies, as more environmentally friendly, such as bioaccumulation and phytoremediation. However, it is still a technical challenge, and the majority of research still occurs at a laboratory level [31,32].

2.1. Valorization of Ultrabasic Rocks

Chromite occurs in ophiolitic complexes and within ultrabasic rocks, as previously mentioned. Therefore, during the mining and beneficiation of chromite, significant amounts of ultrabasic rocks are produced. Their composition consists of various structures of magnesium, silicon, and iron. The ultrabasic rocks may be regarded as by-products rather than waste, even though the mineral chromite is the primary output of the process. The scientific community is interested in them, and their use in other applications aligns with the circular economy’s tenets.

According to the literature, the cases studied at a laboratory level for the utilization of ultrabasic rocks, as derived from chromite mining, are as a refractory raw material, as a neutralizing reagent of acid mine drainage, and for carbon dioxide storage. Similar case studies of the specific exploitation of rocks from non-waste sources have also been considered [33,34]. In addition, studies regarding the pre-treatment of ultrabasic rocks are limited only to the case of their use as a refractory material. Their heat treatment may improve the application-related properties. Serpentine [(Mg,Fe)₃Si₂O₅(OH)₄], the dominant structure of ultrabasic rocks, decomposes at temperatures above 700 °C to respective dehydroxylated structures, such as olivine [(Mg,Fe)₂SiO₄] [35]. In contrast, the inertness or leachability of the metals it contains has yet to be studied in depth, particularly concerning the chromium presence.

2.1.1. Refractory Raw Material

Taking into account the limited supply of high-grade magnesite deposits and the substantial expenses associated with its beneficiation, research into a less expensive substance with acceptable refractoriness is essential. One of the most prevalent minerals in the Earth’s crust is olivine [(Mg,Fe)₂SiO₄], which is a naturally occurring solid combination of forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). These two structures determine the chemical composition of an olivine mineral and thus the melting temperature associated with it. Olivine is primarily used as a slag improver in the steelmaking industry, while it also finds numerous applications in metallurgy, ceramics, and refractory industries, as well as in the thermal process and energy sectors. For most of these applications, forsterite content is crucial because of its properties, such as high melting point (1890 °C), low thermal expansion, good chemical stability, low thermal conductivity, excellent insulating properties, low electrical conductivity, and dielectric permeability. In addition, the presence of low-melting-temperature iron in fayalite (1205 °C) negatively affects its refractoriness. In general, olivine that contains more than 15% fayalite is unsuitable for use in the refractory industry [36].

Acar [36] investigated the refractory properties of the chromite mining by-product, where olivine was obtained at 79 wt%, and chromite, remaining in the rock after beneficiation, at 1.59 wt%. Cylindrical specimens were produced by using the olivine sample and binding reagents. Sintering in the temperature range 1300–1500 °C was followed. According to the results, the thermally treated sample at the highest temperature exhibited promising physical and mechanical properties. High strength and acceptable shrinkage were the main advantages, while the potential formation of enstatite was considered a significant disadvantage. Enstatite may form from free silicon and iron oxides, reducing the refractory properties.

Nemat et al. [37] also studied the refractory properties of ultrabasic rocks. The original sample of chromite mining by-product in this case consisted mainly of serpentine, which includes hydrous structures of olivine, such as antigorite, while the residual chromite was at 3.5 wt%. High refractory properties were observed for temperatures > 1730 °C, where the dominant structure was that of forsterite. In Emami et al. [38], the same sample was studied, with the aim of its reuse in the ceramic industry. According to the results, its heat treatment at 1300 °C led to the formation of enstatite and cordierite. Their common finding was that specific by-products, i.e., mainly serpentine, can be used in refractory and high temperature ceramics. However, more research is needed for several applications, such as forsterite-chromite refractory raw materials, in firebricks and amorphous refractory materials, forsterite insulating firebricks, cordierite ceramics, and other furnace systems, forsterite-spinel catalyst bed, etc.

2.1.2. Neutralization of Acidic Wastewater

Compared to conventional methods for acid waste neutralization, the use of olivine has the advantage of also producing valuable by-products, such as precipitated silicon and magnetic ferrites, without creating new waste streams [39]. On the contrary, the dissolution rate is considered a disadvantage, as under normal conditions it is too low for industrial application. Therefore, increasing the temperature or applying higher acid concentration, or reducing the grain size of the olivine are factors that help to deal with the problem [40]. In addition, the use of alkaline materials for the remediation of acidic wastewater (e.g., acid mine drainage—AMD), it is possible to significantly reduce the concentration of the metals it contains (

Table 1. Case studies on the application of natural ultrabasic rocks for increasing or adjusting pH.

Material	Equilibrium pH	Acidic Reagent	Neutralization Medium	Highlight	Ref.
Dunite	4	Oxalic acid	Soil	CO2 release prevention	[39]
Natural carbonates	6.5	Field AMD sample	AMD	Toxic metal removal	[41]
Serpentine	8	Simulated AMD	AMD	Toxic metal removal Magnesium dissolution	[43]
Dunite	8	Simulated AMD	AMD	Toxic metal removal	[44]
Olivine	2.3	Sulfuric acid	AMD	Nickel dissolution	[45]

), such as Fe, Al, Mg, Zn, Mn, Cu, Pb, etc. [41]. As the pH of the waste increases, the solubility of the metals decreases resulting in their precipitation [42]. On the contrary, a major drawback was reported by Kokkinos et al. [43]. The initial material was a field serpentine/olivine (both structures were determined in it) sample, which was applied for a simulated AMD neutralization. The treated AMD solution (pH > 7) was enriched with manganese since it was leached/dissolved from the sample to the liquid phase due to the metal’s high solubility at the alkaline pH values.

Daval et al. [46] and Hänchen et al. [47] studied the dissolution kinetics at acidic pH of lizardite and fosterite, respectively. Solubilization was observed to occur at a higher rate with decreasing initial application pH and increasing temperature. For pH values > 5, the rate reaches its lowest value.

When olivine was used to neutralize an acidic wastewater field sample (pH ~ 4.1) by Gerogianni et al. [44], they concluded that dunite is a much more effective toxic metal removal and neutralization agent compared to other ultrabasic materials. An example of neutralizing acid wastewater through its interaction with olivine can be observed in Ballangen, Norway, where the high olivine content around a mined nickel ore body results in a slightly alkaline effluent pH [45].

2.1.3. Carbon Dioxide Storage

Mineral carbonation, in general, is a natural process, and it is based on the formation of stable carbonates (i.e., environmentally friendly products) when CO₂ reacts with alkaline minerals, according to Reaction 1 [48,49]. These rocks offer the potential of CO₂ sequestration for long periods, even permanently [50]. Olivine and serpentinite, the main structures of the ultrabasic rocks, exhibit presented with the highest carbonation efficiency [51], and the corresponding mechanism follows Reaction 2 and 3, respectively [52].

CO₂ + Alkaline metal → Carbonated mineral + Heat

(1)

(Mg/Fe)₂SiO₄ + 2CO₂ → 2MgCO₃ + SiO₂

(2)

(Mg/Fe)₃Si₂OH₅(OH)₄ + 3CO₂ → 3MgCO₃ + 2SiO₂ + 2H₂O

(3)

Mineral carbonation can be a direct or an indirect process (Figure 1). Direct mineral carbonation may occur either in gaseous or aqueous media. Depending on this, it is referred to as gas–solid (dry) or aqueous (wet) mineral carbonation. Dry carbonation is a simple process in which gaseous CO₂ reacts directly with solid alkaline minerals. However, it is relatively slow, and pressure and/or heating should be applied to improve its efficiency [53]. Moreover, when a serpentine sample was used, as a chromite mining by-product, an efficiency of 0.07 g CO₂/g was obtained by applying 200 °C, 25 bar, and 10% v/v H₂O in 1 h. It was also reported that up to 10% v/v H₂O presence in the CO₂ stream may increase the sequestration yield. Water forms CO₂ reactive intermediates that favor the formation of carbonates. A higher vapor percentage leads to competitive adsorption mechanisms [54].

As for direct wet carbonation, minerals react with gaseous CO₂ in aqueous media. Therefore, the dissolution of minerals is considered to be mandatory to obtain an aqueous solution rich in metals (i.e., Mg and Fe). Dissolution and carbonation in the direct wet process occur in a single step [52]. As a result, the dissolution rate should be controlled and optimized according to the carbonation/precipitation rate. Kinetic restrictions may arise on the carbonation rate if large amounts of soluble metals occur in the aqueous phase without carbonates beginning to precipitate [55]. The pH of the aquatic phase mainly determines the dissolution rate, which is affected by the addition of ultrabasic rock (which tends to increase pH) and CO₂ dissolution (which tends to decrease pH) [48]. Toward that end, in most cases, the aquatic phase consists of a solution rich in salts and/or organic acids, e.g., NaHCO₃, NaCl, oxalic acid, ascorbic acid, instead of a typical inorganic acid [52,56]. Various factors that enhance the efficiency of carbon dioxide sequestration are summarized in

Table 2. Enhancing factors on the application of natural ultrabasic rocks for CO₂ sequestration.

Method	Efficiency (Kg CO2/Kg)	Enhancing Factor	Ref.
Wet direct carbonation	-	NaCl addition	[52]
Dry direct carbonation	0.07	Water vapor (10 vol.%)	[54]
Wet direct carbonation	-	NaHCO3, oxalic, and ascorbic acid	[56]
Wet direct carbonation	0.28	Pressure (10.2 bar)	[57]

. In addition, pressure may affect the desired equilibrium of the system since it increases CO₂ dissolution and, consequently, the concentration of carbonates. This leads to an increase in pH, causing ultrabasic rock dissolution to decrease, while carbonate precipitation increases [52]. By applying moderate experimental conditions of temperature (22 ± 3 °C) and pressure (10.5 bar), when an ultrabasic chromite mining residue was tested, the CO₂ sequestration capacity was equal to 0.28 g/g [57]. It is worth mentioning that all the above references were used chromite related residues, but the respective capacities are not provided, nor can they be calculated from the presented data.

On the other hand, in the case of the indirect wet process, dissolution and carbonation occur in different reactors. Wet carbonation, especially indirect, presents the highest efficiency among the mentioned methods [58]. The dissolution rate depends on crystallinity, surface area, and particle size. When a chromite related sample was used, it was concluded that the milling process may optimize these physical characteristics. By applying a planetary mill for 1 h, the surface area was at 4 m²/g, but with the addition of 10 wt% H₂O, the value reached its maximum value at 47 m²/g. A 0.01 M HCl solution was used as a dissolution reagent [59].

3. Mining Waste—The Magnesite Case Study

In the mines producing magnesite, the initial magnesite extraction is followed by several procedures to separate it from the host rock. The parental ultramafic rock, separated by the magnesite, is considered to be the waste/by-products of the separation process, and it should be managed by the respective mining companies. Due to serpentinization processes that have occurred (metamorphosis of ultramafic rocks and formation of secondary serpentine from the primary olivine), the possibility of economic and commercial exploitation is currently limited to nonexistent.

The total quantity of mining waste produced using enrichment methods of processed minerals at the magnesite mine of “Grecian Magnesites SA” (Gerakini, Chalkidiki, N. Greece) is now projected to be above 35 × 10⁶ tons. Along with the important dunite mineral that was present in the mining location during the previous operational years, this mining waste has not yet been effectively utilized [60]. Grecian Magnesites SA, Aristotle University, North Aegean Slops, and Mathios Refractories SA collaborated in the framework of MagWasteVal Research Project to investigate the main geochemical and thermodynamic variables affecting the serpentinization processes and the development-implementation of a reverse process, representing as efficiently as possible the conditions prevailing during the initial serpentinization. The reverse serpentinization process reduces the serpentine v/v percentage in the industrial solid waste, so the amount of olivine rose above 80% v/v, and the altered rock was reconverted to (useful) dunite. This extremely adaptable and carbon-neutral rock is useful for a variety of applications, such as the production of refractory magnesium dunite masses, firebricks production, EBT filling sand, slag conditioner, cement and fire protection, foundry sand, sandblasting material, CO₂ absorber, etc.

Through the use of different (low-cost) solid additives, including Run-Of-Mine (ROM), alumina, chromite ore (CO), iron oxide, and caustic calcined magnesia, the MagWasteVal project has investigated how appropriate thermal treatment can improve the refractory properties of waste samples taken from the mineral mine site (

Table 3. Impact of temperature and additives on mineralogical transformations and refractory properties of magnesite mining waste. [61].

Temperature	Process	Additive	Impact on Mineralogy and Refractory Properties
650–680 °C	Serpentine decomposition	-	Nearly complete decomposition of serpentine
850 °C	Recrystallization in olivine and pyroxenes	-	Initial formation of olivine and pyroxenes
1300 °C	Enhanced pyroxene formation due to excess Si	-	Olivine formation reduced—more pyroxenes due to available Si
>1300 °C	Depending on the additive	Chromite	Promotes forsterite formation—improves refractory behavior
		Alumina	Forms MgAl2O4 spinel—reduces forsterite and refractory properties
		Maghemite	Increases bulk density—facilitates sintering at 5 wt%
		Chromite + Maghemite	Forms spinels Mg(Cr,Fe,Al)2O4 and MgFe2O4—enhances refractory properties

) [61]. Samples of the analyzed magnesite mining waste were gathered from various locations and levels within the mining area. The primary components of samples that may be used as raw materials to produce refractories, along with their corresponding concentrations, are SiO₂ (33.5–46.4%) and MgO (32.49–43.0%). After the samples were thermally treated, it was shown that serpentine decomposed in the range 650–680 °C and completely recrystallized into olivine and pyroxenes at 850 °C. Because of the excess Si available from the initial decomposition of serpentine, it was found that at higher temperatures (1300 °C), a recrystallization process favors the deformation of olivine and the subsequent formation of pyroxenes, whereas the partial capture of available Si caused by the presence of magnesite limited the deformation of olivine. Aiming to increase the proportion of olivine and enhance this material’s refractory qualities at temperatures above 1300 °C, the type and solid additive dose were studied. Additives of chromite increased the formation of forsterite at 1300 °C. Alumina resulted in the formation of MgAl₂O₄, reducing the percentages of forsterite and refractoriness. The addition of maghemite increased bulk density and facilitated sintering at 5 wt%. The addition of chromite and maghemite favored the formation of Mg(Cr,Fe,Al)₂O₄ and MgFe₂O₄ spinel groups. Since chromite additives have a high MgO content, which maximizes olivine formation at 1300 °C, they yielded the best results and improved refractory properties.

Since the accumulated utilizable waste can be converted into a material with a wide range of refractory applications (

Table 4. Comparison of proposed technologies in the literature for the utilization of ultramafic/ultrabasic rocks produced as waste from the mining (chromite and magnesite) sector.

Application	Positives	Negatives
Acidic medium neutralization	Simple and low-cost process Easy to apply in the field Significant metal removal Production of valuable by-products	Low kinetic reaction rate Low liquid-solid ratio Metal leaching from solid Inability to reach a pH above neutral was observed
CO2 sequestration	High kinetic reaction rate Permanent storage In situ carbonation	Low capacity Carbonation favored under high-cost conditions (heat and pressure) Ultrabasic rock grinding Water requirements (wet carbonation)
Refractories and insulating material	Limited effect on mechanical properties Cost-effective additive Corrosion resistance Thermal stability	High sintering temperature Presence of impurities Adding a small amount to maintain the final product quality
Binder in cement production	Lower CO2 emissions Reduction of the higher cost clinker usage Similar hardening and binding properties to cement	Need for thermal pretreatment Ultrabasic rock grinding Presence of impurities Constant quality control monitoring
SOx absorption-	Low-cost absorbent High reactivity Production of stable sulfates	Need for thermal pretreatment Low capacity Ultrabasic rock grinding Metal leaching from solid

), the economic benefits of the relatively simple thermal technology developed and the exploitation of vast volumes of the by-products are considered to be of considerable importance. The utilization of waste also extends the lifespan of the mine by converting the low-quality waste or by-products into a profitable raw material. The MagWasteVal project has the potential to establish a model for the local and global recycling and repurposing of mining wastes as secondary raw materials through appropriate management. It is also important to note that the acquired (dunite-based) products have previously been used in various industrial-scale applications by the cooperating magnesite mine firm, Grecian Magnesite SA.

3.1. Sustainable Binder

Vembu and Ammasi [62] investigated the viability of substituting mine waste for cement in two phases: binary blend (cement + mine waste) and ternary blend (cement + fly ash + mine waste). Shanmugasundaram and Shanmugam [63] assessed the potential use of Magnesite Mine Tailings (MMT) as a subgrade in road construction after collecting them from the Salem (Tamil Nadu, India) magnesite waste site.

Ordinary Portland Cement (OPC) was added to MMT by various % concentrations, and the characteristics of new materials were studied by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), and Energy-Dispersive X-ray spectroscopy (EDX) to explore the stabilization mechanisms. According to this study, MMT that has been treated with cement may be utilized as a subgrade in road construction, which not only lessens the harm caused by open MMT dumping to the environment but also substitutes natural soil in subgrade construction.

The use of Mine Tailings (MTs) in construction is often linked to Portland cement, which is known for its high energy consumption and significant CO₂ emissions. It is estimated that cement production contributes to about 5–7% of total carbon dioxide emissions [64]. To address the environmental impact of the cement industry, integrating MTs into geopolymer matrices shows great promise for effectively immobilizing MTs. Geopolymers (GPs) have emerged as a rapidly evolving area for creating eco-friendly building materials, offering an energy- and resource-efficient alternative to Portland cement-based materials. Alkaline agents, such as solutions of sodium or potassium hydroxides or silicates, activate GPs, which are binders based on amorphous or crystalline aluminosilicate minerals.

3.2. Neutralizing of Acid Mine Drainage

When sulfide containing minerals like iron, gold, copper, lead, zinc, mercury, silver, etc., are exposed to oxygen and water, acid mine drainage is created. Water becomes metalliferous when metals are released into effluent waters. Acid mine drainage (AMD) favors the leaching of heavy metals from nearby geologies due to its acidity. Several active and passive treatment systems have been developed to treat acid mine drainage, but their use in AMD treatment is limited by their high cost, low treatment effectiveness, and production of secondary sludge that is hazardous and costly to dispose of. Masindi [65] studied magnesite tailings to remediate AMD, which is the by-product of gold mining. A dose of 1 g of magnesite tailings and an optimum equilibration duration of 30 min were used when AMD interacted with them. AMD that came into contact with magnesite tailings had a significant decline in inorganic pollutants (>99%) and an increase in pH (pH > 10), except for sulfate, which had a removal efficiency of >80%.

3.3. SOx Absorption

The European Commission is encouraging the companies that produce cement, lime, and magnesium oxide to recycle collected waste or particulate matter to manage SO₂ emissions with 100% removal efficiency. Following this directive, Del Valle-Zermeno et al. [66] investigated the desulfurization potential of three distinct by-products from the calcination of natural magnesite. They revealed that it is practical and sustainable to repurpose these by-products in a wet flue gas desulfurization process, allowing for an extension of their life cycle.

The power sector makes extensive use of carbonate rocks, which serve as sorbents in flue gas desulfurization systems. Limestones are mainly utilized in fluidized bed combustion and wet and dry or semi-dry desulfurization methods. This is due to both the high desulfurization efficiency and the high availability of these rocks, which results in inexpensive purchasing costs. Utilizing a desulfurization product in the form of gypsum, a desirable raw material for a variety of uses, is also beneficial. Compared to limestone, fewer industrially viable desulfurization techniques have been established for carbonate rocks that include magnesium. Magnesites and their calcined derivative (MgO) are used as sorbents in the wet desulfurization process. Similar to the wet limestone process, this approach produces no waste. In this case study, the desulfurization process yields MgSO₄·7H₂O, a fertilizer used in gardening and agriculture, and MgCl₂·6H₂O, a road salt [67].

3.4. Refractories and Insulating Material

The production of refractories, which are materials with rising demand, requires the synthesis of doloma, spinel, and periclase. Magnesia (MgO) is a desirable material due to its high melting point (2800 °C), exceptional resistance to iron oxides and alkalis, while high lime content of flakes was created at the steel melting furnace’s operating temperature. Additionally, it is non-toxic and does not have the same hydration problems as dolomite and lime. Magnesia used in refractory manufacturing comes from three main sources: natural magnesite, seawater extraction, and inland brine extraction [68].

Magnesite refractory has become increasingly popular for high temperature applications over the years due to its many beneficial high temperature characteristics, such as its high softening point and high chemical endurance under normal conditions. High-temperature furnace linings frequently employ materials based on magnesium because of their superior qualities, which include high refractoriness, strong thermal insulation, and resistance to corrosion from alkaline gases and molten slag [69]. Porous MgO-based ceramics combine the benefits of porous ceramics with magnesia materials. They often feature better qualities such as a high melting point, good mechanical properties, and a greater Mohs hardness, which have sparked a lot of interest in the industry recently [70].

Solid-phase content (porosity), pore size, and pore type are the three primary factors that determine a material’s effective thermal conductivity. It has been demonstrated that a higher solid-phase content (lower porosity), a smaller pore size, and a more closed-pore ratio all contribute to a lower effective thermal conductivity [71]. In order to prepare lightweight refractories, it is crucial to regulate and optimize the pore structure to achieve a lower open-pore size and a larger closed porosity.

Ren et al. [70] investigated the partial or complete substitution of lightweight refractory aggregates for conventional dense refractory aggregates (for furnace lining), which is considered a viable and effective method for reducing emissions and conserving energy. One-step sintering at 1600 °C produced lightweight magnesia refractory ceramics with customized closed porosity utilizing high-purity magnesite and silicon kerf waste in varying proportions, focusing on the development of their phase compositions, micro-morphologies, and other characteristics. The viability of employing magnesite tailings as a magnesium source to create porous magnesia ceramics was examined by Ma et al. [72]. By preventing the formation of MgO grains and further encouraging the discharge of pores, the Mg₂SiO₄ played a fixing role. The cryptocrystalline magnesite MgCO₃ used by Terna Mag S.A. is white, contains very little silica and iron, and has a low heavy metal concentration. Ceramics (tiles, etc.) are produced from milling products with 90% w/w MgO composition on an ignited basis. To enhance mechanical qualities, reduce water absorption, and lower the necessary firing peak temperature, magnesium carbonate, also known as raw magnesium, is added to the ceramic batch formula in Grecian Magnesite S.A. Magnesium carbonate can also be used as a component in panel engobe to provide refractoriness and prevent tiles from adhering to kiln rollers at high temperatures. In the early 1990s, magnesium carbonate grades from Grecian Magnesite were successfully introduced into the high-end Italian tile market. Since then, their sales have expanded to include additional countries that produce a significant amount of ceramic tiles.

4. Industrial Waste—The Tannery Sludge Case Study

Tanning is the process in which raw animal skins or hides are treated to present specific properties, such as stability, appearance, water resistance, temperature resistance, elasticity, etc., in terms of becoming leather. The most widespread reagent that is used in tanning, accounting for over 80% of leather production worldwide, is the trivalent chromium sulfate (7–10% w/w) [73]. Skins and hides are added to a chromium solution bath, and the tanning process takes place. The poly-nuclear complexes that chromium produces can promote the coordination of covalent connections with the carboxyl groups of collagen. This is a multistage mechanism, and the reactions based on it present a 60% efficiency in reagent consumption. As a result, the remaining 40% that remains in the liquid phase ends up in the wastewater stream [74].

Tannery sludge is produced after the physicochemical and biological treatment of the aforementioned wastewater stream of the corresponding industrial plants. Its composition mainly includes organic matter, trivalent chromium, and calcium. The chromium content does not exceed 10% w/w in most cases, but without excluding the bibliographic references for even higher percentages [75,76]. The content of organic matter and chromium in tannery sludge has attracted the attention of researchers for energy and metal recovery, respectively (Figure 2).

4.1. Energy Recovery

The combustion of tannery sludge for energy production is based on its high organic matter content. According to Kokkinos et al. [77], the initial sample had an organic matter content equal to 22 wt%, while in the case of Beshah et al. [78] was 35.6 wt%. As a result, in the first case study, a limited higher heating value (HHV) of 2 MJ/Kg was obtained, compared with the 3 MJ/Kg in the second case study. In

Table 5. Percentage of organic matter content in comparison with the higher heating value.

Organic Carbon (wt%)	Total Carbon (wt%)	Ca (wt%)	CaCO3 (wt%)	HHV (MJ/Kg)	Ref.
12.2	-	14.8	26	2	[77]
20.7	18.8	1.2	-	3	[78]
-	15.3	-	-	5.9	[82]
-	44.2	0.6	-	7.25	[83]
-	39.6	15.1 1	-	8.5	[84]
-	21	-	-	9.12	[85]
-	35.2	18.9	-	9.27	[86]
-	33.6	-	Verified, no quantified	14.9	[79]
-	21.9	-	Verified, no quantified	15.1	[80]
-	54.6	-	Verified, no quantified	21.9	[81]

¹ Calculated by the presented data.

, various total carbon content values are presented in comparison with the obtained HHV. The total carbon value includes both organic and inorganic carbon. Unfortunately, in the majority of works, the separation of the two quantities is not calculated/determined, and the values are not presented. Inorganic carbon also affects the HHV of tannery sludge, as carbonates may be contained in it, mainly as CaCO₃ [79,80,81]. In fact, CaCO₃ is decomposed at temperatures above 400 °C, which is an endothermic reaction [77]. Therefore, the HHV value of a tannery sludge, based on the exothermic reaction of organic matter combustion, decreases proportionally to the presence of carbonates in its composition. This leads to increased HHV in samples with slightly lower organic matter content than others, e.g., Di Lauro et al. [79] and Dong et al. [80].

According to Beshah et al. [78], energy recovery from tannery sludge was achieved by using it as an additive in brick production. The addition of 20% sludge can reduce energy requirements during heat treatment by 50%, while the quality characteristics of the final product were not affected. An alternative approach to recover energy from tannery sludge involved its co-combustion with another material. In these case studies, a material with higher HHV than the reference sludge was tested, and the corresponding results are presented in

Table 6. Tannery sludge co-combustion materials and their higher heating value.

Initial HHV (MJ/Kg)	Co-Combustion Material	Proportion of Materials	Additive’s HHV (MJ/Kg)	HHV (MJ/Kg)	Ref.
10.6	Coal	1:1	31.8	21.8	[87]
10.6	Rice husk	1:1	15.7	12.4	[87]
16.6	Hardwood pellets	1:1	19.6	18.1	[88]

. Results refer to a 1:1 proportion concerning sludge to additive material, resulting in final HHVs that are approximately the average of the initial ones [87,88]. Generally, the co-combustion of tannery sludge with other materials, that exhibit higher HHV (e.g., coal), is recommended for energy recovery [77,82,83]. In other case studies, the co-combustion of tannery sludge was also examined, but these works aimed to determine the gaseous emissions and/or chromium’s fate during the process [89,90].

4.2. Chromium Recovery

As previously mentioned, during the tanning process, 40% of the chromium added to the respective tanning bath does not react with the skins/hides and ends up in the wastewater stream. This results in the production of a sludge containing a significant percentage of chromium after the treatment of the liquid waste. Since waste containing chromium is considered hazardous, even if it is in its trivalent form, as it can potentially oxidize to its highly toxic hexavalent form, its recovery has attracted the interest of researchers as an alternative solution for the tanning sludge management [91].

The process applied for the recovery of trivalent chromium from tannery sludge is leaching with an acidic medium. According to

Table 7. Chromium recovery from tannery sludge.

Initial Cr Content (wt%)	Pre-Treatment	Leaching Reagent	Recovery (%)	Recovered Cr Valence	Final Step/Product	Ref.
8.6	No	H2SO4 (pH 1)	97	III	Precipitation (NaOH-pH 8)/Cr(OH)3 (59 wt% Cr)	[92]
~0.9	No	H2SO4 (4% v/v)	90	III	Cr(III)oxidation to Cr(IV) by H2O2 + UV Cr(VI) reduction to Cr(III) by Na2SO3/Cr2(SO4)3	[93]
14.1	No	H2SO4 (1 N)	93	III	Precipitation (NaOH-pH 8)/Cr(OH)3 (36 wt% Cr)	[76]
29	Thermal (700 °C)	H2SO4 (1 N)	90	VI	Electrochemical Cr(VI) reduction to Cr(III)	[94]
18.9	Thermal (600 °C) and 50 wt% Na2CO3	H2SO4 (42 wt%)	99.7	VI	-	[95]

, sulfuric acid proves to be the optimal acid for chromium solubilization, with relative recovery yielding results above 90%. The main advantage of sulfuric acid over others, i.e., hydrochloric and nitric, is the formation of insoluble sulfates with other metals contained in the sludge, such as calcium and magnesium, because of which they precipitate and do not remain in the liquid phase [76]. Due to the low solubility of Cr(III), by applying mild alkaline conditions (pH 8), it is possible to precipitate it from the aqueous phase to produce a high chromium content solid Cr(OH)_3, which can then be fed back into the tanning process [92].

On the other hand, in some cases, a thermal pretreatment stage is applied to the sludge to oxidize the insoluble trivalent form of chromium to the highly soluble hexavalent form. In this case, two factors shape the performance of the process: the temperature and other oxides, e.g., carbonates and/or oxides of calcium and magnesium. Specifically, the oxidation of chromium as a reaction starts to take place at 300 °C in oxic conditions (Reaction 4), while for values above 800 °C the process is reversed, i.e., Cr(VI) is reduced to its trivalent form (Reaction 7) [96]. The optimum, at which the oxidation efficiency is maximized, temperature is determined by the content of the oxides, which catalyze the process (Reaction 6). The oxides, since they are not present in the initial tannery sludge, may be produced either from the decomposition of the corresponding carbonates (Reaction 5) [94] or may be added as reagents during the thermal treatment [95]. The recovered Cr(VI), also by H₂SO₄, may be reduced electrochemically to its trivalent form to be precipitated and fed back into the tanning process [94]. In any case, the presence of toxic hexavalent chromium in the raw tanning sludge or after its thermal treatment requires effective management. Initially, it would be preferable to avoid the natural oxidation of trivalent chromium in the sludge by preventing high pH values and oxygen-rich conditions. Oxidation, if deemed necessary and advantageous, is suggested to be carried out under controlled conditions/processes, along with regular monitoring through leaching tests, with the aim of reducing environmental and health risks. In addition, any unrecovered quantity should be chemically, using agents like ferrous sulfate, sodium metabisulfite, or sulfur dioxide, or electrochemically reduced to its trivalent form for safer management or utilization of the residual sludge.

2Cr₂O₃ + 3O₂→ 4CrO₃ (>300 °C)

(4)

CaCO₃→ CaO + CO₂ (>400 °C)

(5)

Cr₂O₃ + 2CaO + 3/2O₂→ 2CaCrO₄ (>400 °C)

(6)

4CaCrO₄ + 2MgO → 2MgCr₂O₄ + 4CaO + 3O₂ (>800 °C)

(7)

4.3. Other Valorization Methods

In addition to energy and chromium recovery, tannery sludge has been studied in various implementations aimed at its valorization (

Table 8. Comparison of proposed technologies in the literature for the utilization of tannery sludge.

Application	Positives	Negatives
Energy recovery	Waste volume reduction Reduction in fossil fuel use Sludge disinfection	Co-combustion only Cr-rich toxic residue ash
Chromium recovery	Cr(III) re-use Less hazardous sludge	Cost-effective for large quantities of sludge Quality of the recovered Cr(III)
Building materials	Reduction of production cost Energy saving during combustion Stabilization of contaminants during sintering	Cr(VI) formation risk Contaminants leaching Quality of the product Market/public acceptance
Absorption-	Increase in capacity after chemical and thermal modification Notable dye absorption capacity Increased efficiency in acidic pH	Heat treatment requirement (biochar) Limited capacity Saturated material management Modification cost
Fertilizer	Nutrient Content Improves microbial activity Enhances degraded soils	Immobilization and migration of toxic substances Reduced crop productivity in long-term usage

). As mentioned before, it was used as an additive in brick production, which poses two advantages: energy savings and waste recycling. However, as the process produces building material, the quality of the final product should not be affected by the addition of sludge. It has been proved that a content of 10 wt% tannery sludge in clay-based bricks may produce a product that fulfills the quality standards [78]. Another factor that was taken into consideration, since tannery sludge is a waste, was the immobilization of its pollutants. According to the leaching tests (e.g., TCLP), the concentration of the extracted pollutants, mainly metals, was well below the regulation limits [97].

Besides brick manufacturing, tannery sludge was tested as an additive in cement production. By adding up to 0.7 wt% in cement, a change in quality was not observed, its composition was not affected, and chromium leaching met the regulatory limit (5 mg/L) [98]. Moreover, even higher percentage additions (up to 6 wt%) may enhance its quality characteristics, such as porosity, water absorption, strength, density, and ultrasonic pulse velocity. According to the respective leaching tests, the produced cement in this case study was classified in the non-hazardous waste category [99].

Tannery sludge was also added to another construction material, namely geopolymer, to immobilize its pollutants, primarily chromium. A geopolymer containing 20 wt% tannery sludge was synthesized, and the chromium concentration was below the regulation limit during leaching tests. Additionally, the produced material presented high durability, good high-temperature resistance, strong resistance to acids and alkalis, and excellent resistance to acid rain erosion [100].

Incorporating hazardous waste into construction materials (e.g., concrete, bricks) presents several safety challenges related to health, environmental impact, and structural performance. While it offers sustainability benefits, without proper management, it can pose serious risks. The percentage of waste added depends on the quality of the final product, which must meet market specifications. Leaching tests (e.g., TCLP, SPLP, EN 12457) should be applied in the resulting building material and solidification/stabilization techniques must be implemented in the waste if deemed necessary. The migration of contaminants is the main concern and, in addition to constant supervision, this is facilitated by their encapsulation in construction binders such as cement or geopolymer [101].

Biochar was also synthesized using tannery sludge through its pyrolysis and tested as an absorbent for the tanning related pollutants removal from waste streams. On the tannery sludge-based biochar (an initial pyrolysis step), new active groups were introduced (during a secondary pyrolysis step) by the addition of KOH/melanin [102] and CO₂ supply [103]. In the first case study, the synthesized biochar had a maximum absorption capacity of 24.8 ± 0.2 mg/g for both cationic and anionic dyes [102]. In the second case study, malachite green was used as a reference dye, and the corresponding maximum capacity was equal to 231.34 mg/g. Furthermore, the CO₂ activated biochar was tested in Cr(VI) removal from wastewater, and 86.26 mg/g maximum capacity was obtained. The above-mentioned capacities were achieved for pH values above 5 for the dye and at a pH equal to 2 for Cr(VI) [103]. Tannery sludge-based biochar, obtained through pyrolysis, was also tested for tannery air emissions, namely volatile organic compounds. When n-butyl acetate was used as the reference pollutant, the maximum capacity was equal to 73.8 mg/g [104].

Composting of tannery sludge is another method of its valorization, aiming at soil enhancement and fertility increase. The sludge was mixed with sugarcane straw and cattle manure (ratio 1:3:1 v:v:v) while the duration of the process was 90 days under aerobic conditions. Initially, the soil’s fertility and plant growth were improved since composted tannery sludge was rich in organic matter and nutrients [105]. However, over time, the immobilization of toxic substances decreased, mainly due to chromium, contaminating the soil and groundwater. As a result, composted tannery sludge is not recommended as a soil conditioner due to the environmental issues that arise and the adverse effects on plant growth [106]. On the other hand, specific bacterial groups may thrive under these various chemical properties of the soil. These bacterial groups exhibit resistance to tanning pollutants, and they potentially biodegrade them as an alternative method for restoring contaminated sites [107].

5. Conclusions

The need to adopt circular industrial and mining operating models is now widely accepted. This concerns the re-feeding of the production process with useful reagents/raw materials that are, however, recovered from waste or by-products. In the same context, promoting these materials to other industries should be included, either for direct use in their production process or for the treatment of their waste. All these cases are pillars of sustainable development in the primary sectors. Emphasis, however, should be placed by the research community on the development of technologies and methodologies capable of practical application, a parameter that is not a prerequisite in all cases. Crossing the valley of death is a phrase that has been attributed to bridging research with practical application to highlight the difficulty of the undertaking. Some of these challenges are insufficient funding, ambiguous market demand, technical difficulties, limited commercial knowledge, and regulatory compliance obstacles.

This article mainly presents research papers that propose technologies capable of practical application, while some of them have already been implemented by the respective industries. In particular, the utilization of ultrabasic rocks that are obtained as by-products from chromite mining are capable of neutralizing acidic solutions, which may be liquid waste streams from other industries or acid mine drainage. The corresponding ultrabasic rocks produced by the mining of magnesite were also examined and eventually valorized through their application as a refractory material. Moreover, significant amounts of chromium can be recovered from tannery waste, which can be fed back into the tanning process without affecting the quality of the final product (leather). In all the aforementioned cases, the term “good practices” is attributed, and they prove that laboratory experiments lead to field application when included in the research objectives.

As demonstrated in this work, the listed wastes can be utilized in various environmental and non-environmental applications. Therefore, future research proposals of particular interest, taking into account the emerging needs, for ultrabasic rocks include carbon dioxide sequestration. Current research is limited and challenging to apply in the field. On the other hand, its high availability, combined with process optimization, can provide significant solutions in specialized applications. Furthermore, regarding tannery sludge, optimizing chromium recovery should be a priority, as it can be reused in the same facilities. The removal of chromium will also be a key factor for the utilization of the solid residue. Life Cycle Assessment (LCA) and Life Cycle Costing (LCC) are also recommended in all the aforementioned case studies to evaluate sustainability, covering waste processing, material formulation, application, and end-of-life, quantifying trade-offs and benefits against conventional alternatives.