The Use of Copper Slag in the Thermolysis Process for Solar Hydrogen Production—A Novel Alternative for the Circular Economy

Abstract

Copper slag, produced in pyrometallurgical processes, has the potential to generate hydrogen through thermolysis, depending on its composition. This manuscript explores the use of copper slag as a highly abundant and low-cost material for thermochemical water splitting using concentrated solar power. Copper slag can undergo endothermic reactions with water vapor at high temperatures, conditions which are favorable for activating hydrogen evolution reactions which can be a potential resource for metal recovery such as magnetite and hematite in the circular economy. While research on copper slag and its components has primarily focused on the recovery of valuable metals and material reuse, its direct application in hydrogen production remains largely unexplored, partly due to historically low interest in hydrogen as an energy source. The vast deposits of copper slag in the Atacama Desert, combined with the growing demand for renewable energy, present a unique opportunity to develop sustainable and cost-effective hydrogen production technologies.

1. Introduction

From an extensive review of the literature, it has been seen that the use of copper slag is not yet considered to be alternative for the cathode materials that currently exist in the thermolysis processes used in the production of solar hydrogen. Copper slag, due to its physical and mechanical characteristics, can be used to manufacture products such as cement, filler, railway ballast, abrasives, aggregates, roofing granules, glass, tiles, etc. In addition, valuable metals can be recovered through various extractive metallurgical routes, such as reduction by direct high-temperature smelting [1]. Shot-peened copper slag has also been used as a fine aggregate in Portland cement concrete. Wang et al. [2], indicate that the most common management for copper slag is recycling to produce value-added products, such as abrasive tools, and granule cutting tools for roofing, tiles, and road base construction [2,3,4,5]. Research related to copper slag is aimed at finding reuse applications for it as a construction material [3], using high-temperature, direct reduction in slag as a means of waste management [6], as a material for the manufacture of high-strength concrete [7], as a potential catalyst (iron present in the slag) for the depletion of phenol present in water during hydrogen peroxide (H₂O₂) production [8], in the recovery of valuable metals from slag [9,10,11], in the recovery of iron from copper slag by direct reduction based on coal [1], as a bifunctional photocatalyst for the degradation of alcohols and hydrogen production [12,13], in thermal energy storage systems [14,15], and in the recovery of copper and magnetite from copper slag using concentrated solar power (CSP) [16].

Among the research carried out with copper slag, its main use has focused on the recovery of valuable metals such as Co and Cu. Currently, solar thermochemical cycles are based on the use of materials from rare earths such as cerium oxide (CeO₂), which is insufficient and expensive [17,18]. However, there is no evidence in the scientific literature of the direct use of copper slag as a thermo-catalytic material for the generation of gaseous hydrogen through a solar thermochemical process. In addition to the above, the abundance of copper slag in the Atacama Desert, mainly that deposited in abandoned landfills, generates one of the largest environmental liabilities related to the processes of obtaining copper [19], which to date has no solution [20].

The smelting of copper concentrate (20–30% Cu) composed of bornite (Cu₅FeS₄), chalcopyrite (CuFeS₄), pyrite (FeS₂), covellite (CuS) and chalcocite (Cu₂S) [21,22], enters the smelting furnace at temperatures of 1200–1300 °C, releasing gases such as O₂, SO₂, N₂, CO, CO₂, and H₂ due to the compositions of the fluxes used such as natural gas, thermal coal, and metallurgical coke [23], with the resultant (Matte) containing a Cu content between 35 and 75%. The copper slag generated normally contains FeO, Fe₃O₄, SiO₂, Al₂O₃, MgO, CaO, and Cu₂O [24]. Phiri et al. have investigated in-depth the presence of TiO₂, MnO, As, Pb, and other chemical elements [25], which makes it a complex multi-component residual material. However, it is mainly composed of Fe (40 wt%), Si (15–37 wt%), and Cu (0.8–13 wt%) [21].

This communication highlights the importance of copper slag in the thermolysis process for H₂ production, examining its role in the circular economy and the potential benefits of treating slag waste through thermochemical methods. By integrating copper slag into solar-driven thermolysis, it is possible to add economic value to existing applications—improving energy efficiency, enabling hydrogen generation, and recovering valuable metals such as Fe₃O₄ y Fe₂O₃. This approach helps reduce the environmental impact of Cu production by transforming industrial waste into a useful raw material.

2. Production of Copper Slag

The smelting process involves separating Cu, the primary element, from contaminants or impurities that diminish its economic value, to obtain a high degree of purity. The copper smelting process typically comprises five main stages: (i) flotation, (ii) smelting, (iii) conversion, (iv) fire refining, and (v) electrolytic refining [26]. Figure 1 shows the process in which, during the smelting stage, the copper concentrate (20–30% Cu), composed of Cu₅FeS₄, CuFeS₂, FeS₂, and Cu₂S, enters the smelting furnace at approximately 1200 °C, releasing gases such as O₂, SO₂, N₂, CO, CO₂, H₂, and H₂O (v). The resulting slag and matte (Cu₂S and FeS) contain between 35 and 75% Cu. During the conversion stage, sulfur and iron present in the sulfide phase are removed through an oxidation process, yielding relatively pure Cu metal, known as blister Cu (approximately 99% Cu). At this stage, copper slag is formed, primarily composed of FeO, Si, and CaCO₃, resulting in a slag with low Cu content. In the refining stage, the blister copper still contains impurities such as Ag, Au, As, Sb, Bi, and Fe. The final stage, electrorefining, is used to recover Cu with a purity of 99.9% from the initial concentrate (20–30% Cu). Copper slag typically contains components such as FeO, Fe₃O₄, SiO₂, Al₂O₃, MgO, CaO, and Cu₂O [21].

Copper Slag

Copper slag is a by-product generated during the pyrometallurgical process used to extract concentrated copper from sulfide mineral ores. It typically contains materials such as FeO, SiO₂, Al₂O₃, CaO, and residual Cu [3]. The pyrometallurgical stage, known as smelting, involves the use of furnaces operating at very-high temperatures. During this process, the intense heat generates two immiscible liquid phases: a Cu-rich matte and a copper slag referred to as the oxide phase [19]. Generally, copper slag is composed of oxides such as FeO and Fe₃O₄ (30–40%), SiO₂ (35–40%), Al₂O₃ (0–10%), and CaO (0–10%) [3] which vary their presence in the product slag due to (i) the nature of the minerals, (ii) the nature of the fluxes, (iii) the nature of the concentrates, and (iv) the operating conditions, as well as other factors related to the production process [27]. Copper slag is one of the main solid wastes from the pyrometallurgical process that is applied to concentrates in copper industry plants. Chile produces approximately 4.5 million tons of slag per year, and more than 30 million tons of copper slag are produced annually in the world [28], demonstrating the magnitude of the waste from mining mineral processing. Furthermore, copper slag often contains a large amount of valuable metal and is currently considered a secondary metal resource [6]. These metals can be Fe, Cr, Cu, Al, Zn, Co, Ni, Nb, Ta, Au, and Ag, and they can be recovered by units operated in the mineral processing of slags, such as (i) crushing, (ii) grinding, (iii) magnetic separation, (iv) flotation, (v) leaching, and (vi) roasting [9], respectively. Materials used for cathodes have become increasingly important [29]. Therefore, recycling the metals present in copper slag can not only recover strategic metals but also contribute to carbon neutrality. Regarding their appearance and properties, naturally cooled copper slags have a black color and a glassy surface, and are dense, lumpy, hard, and brittle (see

Table 1. Physical and mechanical properties of copper slag [3,31].

Property	Value
Unit weight (T/m3)	2.8–3.8
Absorption	0.13
Bulk density (T/m3)	2.3–2.6
Conductivity (µs/cm)	500
Specific gravity	2.8–3.8
Hardness (Moh)	6–7
Moisture (ppm)	<5
Abrasion loss (%)	2.4–10
Internal friction angle	40–53

). Important properties to consider for use in a thermochemical solar process are the unit weight (transport), bulk density (storage), and hardness (abrasive material). However, depending on the cooling pattern, which can be natural or water quenched, they can differ in appearance, density, and shape (see

Table 2. Visual description of copper smelting slag.

Property	Description	Ref.
Particle shape	Angular	[32,33,34]
	Irregular	[31,35,36]
	Multifaceted	[37]
Surface texture	Glassy	[24,32,34,36]
	Smooth	[6,9,31,33,38,39]
	Granular rough	[34,40,41]
Color	Black	[32,42]
	Blackish gray	[43,44]
	Brown with green, red, or black tint	[45]

) [30].

In recent years (1999–2019), approximately 752 Mt [20] of copper smelting slag have been generated from the 13 main producing countries (see Figure 2). The largest generation of this waste occurs in South America (40%), followed by Asia (13%), USA (8%), Oceania (6%), Europe (6%), Africa (6%), and others (15%).

In Chile there are seven copper smelters, five of them are state-owned and the other two are private. The location, capacity, and production of the Chilean smelters are reported in Figure 3. Among the state-owned smelters, four are under the jurisdiction of the National Copper Corporation (CODELCO), Chuquicamata (Calama), Potrerillos (El Salvador), Ventanas (Puchuncaví), and Caletones (Rancagua), with a smelting capacity of 3880 kt/y (63.08% national capacity), while one operates under the National Mining Company (ENAMI) with a capacity of 450 kt/y (7.33% national capacity), which is the Hernan Videla Lira Smelter Plant (Copiapó). The remaining two smelters, Alto Norte (Xstrata, Antofagasta) and Chagres (Anglo American, Catemu), are privately owned [46]. At Caletones, Potrerillos, and Chuquicamata, they are transitioning from slag treatment furnaces to grinding and flotation processes. It is noteworthy to emphasize that between 1999 and 2019, Chile has accounted for approximately one-third of global copper smelting slag production. This equates to an average annual output of 12 million tons, stemming from the seven copper smelters within the country [6].

3. Discussion

Copper slag is a massive by-product of the mining industry, generated in large volumes annually worldwide. Traditionally considered a waste material, its composition rich in metal oxides makes it attractive for advanced applications, such as its use as a cathodic electrode to promote hydrogen evolution reactions (HERs) during the thermochemical cathodic subprocess. Its implementation as a cathode material for H₂ production represents a disruptive innovation for several reasons: (i) instead of being discarded or underutilized, copper slag can be treated and transformed into functional electrodes, extending its life cycle and reducing the need to extract new resources—thus closing the material use loop, a core principle of the circular economy; (ii) the presence of iron oxides and iron silicate (Fe₂SiO₄) gives the slag a porous structure and sufficient conductivity to act as a redox-supporting conductor in systems for water treatment, due to its tendency to produce H₂O₂, or in alkaline electrolysis for HERs; (iii) reusing copper slag as an electrode prevents its disposal in legal or illegal landfills, reducing the occupation of large volumes of public land. Moreover, by replacing exotic materials like Ti, Pt, or Ni, it reduces reliance on costly and uncommon metals, and lowers the carbon footprint and energy consumption in the fabrication of electrochemical devices; (iv) manufacturing electrodes from copper slag allows a significant reduction in costs, which is crucial in efforts to democratize technologies such as wastewater treatment or distributed hydrogen generation [2,3,4,5].

The elemental characterization of copper slag in Chile has been extensively reported by Nazer et al. [21,24], and the researchers indicate that modern copper slags possess high contents of SiO₂ (10–71%) and FeO (0.7–62%) and the predominant chemical elements correspond to Cu, Pb, Ni, and Zn, with its mineralogical phases being mainly Fe₂SiO₄ and Fe₃O₄ [24,47,48]. The variation in the elemental and mineralogical chemical composition of copper slag is mainly influenced by the type of (i) concentrate, (ii) type of fluxes, (iii) type of furnace, and (iv) cooling rates during and after the smelting and converting processes [25]. Table 3 shows the different compositions of copper slag provided through the scientific literature, in which the high contents of Fe, Fe₂O₃, FeO, and SiO₂ stand out. However, the extensive scientific literature indicates that compounds containing Fe, FeO, Fe₂O₃, and Fe₃O₄ react with water steam at high temperatures to generate H₂ gas [49]. It is reported that at 900 °C, 1.43 mol H₂/kg was produced from high-temperature steam steel slag [50,51]. Nevertheless, the mineralogical study in Chile of the copper slag indicates that it is composed of 50% Fe₂SiO₄, 39% Fe₃O₄, and 10% (Mg,Fe)₂SiO₄ [7]. There is then the possibility that copper slag deposited in Chile reacts with water steam to generate H₂ because its main compounds contain iron oxides.

The XRD analysis of the copper slag from the Chilean smelter indicates that it is composed of, Fe₂SiO₄, Fe₃O₄, KAl₂[AlSi₃O₁₀](OH)₂, Cu₆Si₂S₇, (Mg,Fe)₂SiO₄, and Ca₂Mg₅Si₈O₂₂(OH)₂, show-up in the Figure 4a. Apart from the X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) examination shown in Figure 4, the findings in Figure 5 provide additional support for the chemical and mineralogical description of copper slag from Chile. Fe, Si, and O are the most common components in the sample, as shown by the SEM-EDS image. This is in line with the majority composition of Fe₂SiO₄ found in the earlier investigation.

Additionally, a spatial depiction of the primary components is provided by the elemental mapping). The micrographs’ high concentrations of Fe, Si, and O support the idea that Fe₂SiO₄ is the predominant mineralogical phase. In accordance with the compounds found in the XRD analysis (Figure 4a), such as Cu₆Si₂S₇, trace amounts of Cu are also detected, indicating the potential presence of copper sulfides or silicates in the sample. When these methods are combined, it is confirmed that the examined slag retains a composition rich in Si and Fe, mostly in the form of Fe₃O₄ and Fe₂SiO₄, with (Mg,Fe)₂SiO₄ and KAl₂[AlSi₃O₁₀](OH)₂ as secondary minerals. This information is essential for evaluating the slags possible industrial uses and value as well as its reactivity in particular environmental settings.

The oxidation of a metal or metal oxide at high temperature is influenced by (i) the temperature, (ii) the composition of the gases generated, (iii) the time of exposure to the corrosive environment, and (iv) the pressure of the system, and is characterized by the reduction in the thickness (penetration) of the metal and the rate of formation of corrosion products. The oxidation rate increases significantly with increasing temperature [52], creating a homogeneous medium and improving the thermodynamic equilibrium [53]. When a metal is exposed to oxidation at high temperatures (corroded metal or thermal degradation), considering the reaction xM + yH₂O → MxOy + yH₂ [52], a value of the Gibbs free energy (activation energy) is obtained, as determined by Equations (1) and (2):

∆G° = −RT ln(K)

(1)

∆G° = −yRT ln(PH₂/PH₂O)

(2)

During the oxidation process of a metal (M) exposed to water vapor, H₂ can be produced by the following reaction [54]:

MO_(x−1) + H₂O→MO_x + H₂

(3)

where the expression for the Gibbs free energy is as follows:

$$∆\mathit{G}°=∆{\mathit{H}}^{{\mathit{M}\mathit{O}}_{\mathit{x}}}-∆{\mathit{H}}^{{\mathit{M}\mathit{O}}_{(\mathit{x}-1)}}-∆{\mathit{H}}^{{\mathit{H}}_2\mathit{O}}-\mathit{T}({\mathit{S}}^{{\mathit{H}}_2}-{\mathit{S}}^{{\mathit{H}}_2\mathit{O}})\le 0$$

(4)

where MO_x is the oxidized metal and MO_x−1 is the reduced oxide, respectively.

The gas data published by Johansen et al. in 1970 [55] regarding the behavior of copper in the direct smelting process at 1300 °C considers among its analyses materials such as iron silicate and magnetite. However, specific thermodynamic data for copper slag have been presented in studies focused on metal recovery using the direct reduction smelting method [1]. The lack of information regarding thermodynamic data related to the oxidation of copper slag in water vapor and at high temperatures provides an interesting line of research that would significantly contribute to generating valuable information regarding the reduction in mining pollutants resulting from the use of slag in electrochemical processes, providing unprecedented added value. However, it is known that the generated copper slag normally contains metal oxides (FeO, Fe₃O₄, SiO₂, Al₂O₃, MgO, CaO, and Cu₂O), with its mineralogical phases being mainly Fe₂SiO₄ and Fe₃O₄ [24].

Table 3. Composition of the different copper slags published in scientific journals.

Country	Company	Cu	Fe	S	K	P	Fe2O3	FeO	SiO2	CaO	Al2O3	MgO	P2O5	SO3	CuO	K2O	Na2O	MnO	TiO2	Ref.
Chile	O’Higgins. El Teniente.Converter, Rancagua	4.56	40.30	1.53	0.80	-	-	-	20.30	0.33	2.86	0.75	-	-	-	-	1.20	-	-	[21]
Chile	O’Higgins.El Teniente. Reververo furnace, Rancagua	0.83	33.30	0.50	1.10	-	-	-	37.90	7.70	7.50	1.40	-	-	-	-	2.10	-	-	[21]
Chile	Atacama, Century scoriales XIX Playa Negra, Copiapo	-	-	-	-	-	14.23	18.99	35.00	17.05	6.69	1.41	0.49	0.40	0.77	1.21	0.81	0.17	0.27	[19]
Chile	Atacama, Century scoriales XIX Canto del Agua, Copiapo	-	-	-	-	-	12.87	29.60	39.39	4.32	4.62	1.30	0.75	1.14	1.40	0.32	0.33	0.05	0.14	[19]
Chile	Atacama, Century scoriales XIX Nantoco, Copiapo	-	-	-	-	-	13.58	22.71	38.58	7.66	6.57	1.13	0.70	0.53	1.83	1.01	0.58	0.30	0.13	[19]
China	Tongling Non-ferrous Metals Group Holding Co., Ltd., Tongling, Anhui	0.80	39.10	0.45	-	0.10	-	42.85	31.11	2.44	2.29	1.79	-	-	-	-	-	-	-	[56]
Spain	Cabezo Juré archeological settlement, Alosno, Provincia de Huelva.	20.1	-	-	-	-	50.60	-	13.81	0.20	1.79	0.20	0.058	-	-	0.01	0.26	-	0.307	[57]
Mexico	Company and location unknown	-	-	-	-	-	45.46	-	33.12	11.68	4.67	0.42	0.10	-	-	0.77	2.64	0.39	0.22	[58]
Mexico	Smelter of the Mining-Metallurgical Complex of La Caridad, Sonora	1.05	44.57	-	-	-	-	-	25.12	-	-	-	9.42	-	-	-	-	-	-	[59]
Mexico	Industrias Magnelec SA de CV, Saltillo, Coah	-	-	-	-	-	0.13	-	0.2	1.00	0.15	98.5	-	-	-	-	-	-	-	[60]
Italy	Campigilia Marittima Century scoriales XI-XIII. Capattoli	-	-	-	-	-	-	42.53	20.06	8.26	2.93	0.72	-	-	-	0.74	0.02	-	-	[61]
Italy	Campigilia Marittima Century scoriales XI-XIII. Capattoli	-	-	-	-	-	-	52.17	30.60	15.62	5.06	0.90	-	-	-	1.20	0.07	-	-	[61]
Portugal	District Sao Domingos Mining Centre, Century XX, Moitinha, Achada do Gamo	-	-	-	-	-	52.86	-	37.70	1.14	3.93	0.40	0.20	-	-	1.04	0.25	0.07	0.688	[62,63]
Namibia	Dundee Precious Metals Ltd., Tsumeb smelting complex, Windhoek	-	-	6.51	-	-	32.47	9.49	14.68	4.50	4.70	1.43	0.59	-	-	0.30	0.26	0.08	0.25	[64,65]
India	Sterlite Industries India Limited, Tuticorin, Tamil Nadu	-	-	-	-	-	61.24	-	19.72	5.65	3.03	0.90	0.63	3.31	1.49	1.23	-	0.08	0.44	[10]
Australia	Tropical coastal zone of north Queensland.	0.52	-	-	-	-	-	32.88	34.80	15.53	5.41	0.91	0.27	-	-	0.60	0.26	0.39	-	[3]
Canada	INCO’s flash smelting furnace, Sudbury, Ontario	0.58	38.6	0.93	-	-	45.40	52.20	17.40	1.11	2.51	1.65	-	-	-	-	-	0.03	0.15	[56,66]

Table 3. Composition of the different copper slags published in scientific journals.

Country	Company	Cu	Fe	S	K	P	Fe2O3	FeO	SiO2	CaO	Al2O3	MgO	P2O5	SO3	CuO	K2O	Na2O	MnO	TiO2	Ref.
Chile	O’Higgins. El Teniente.Converter, Rancagua	4.56	40.30	1.53	0.80	-	-	-	20.30	0.33	2.86	0.75	-	-	-	-	1.20	-	-	[21]
Chile	O’Higgins.El Teniente. Reververo furnace, Rancagua	0.83	33.30	0.50	1.10	-	-	-	37.90	7.70	7.50	1.40	-	-	-	-	2.10	-	-	[21]
Chile	Atacama, Century scoriales XIX Playa Negra, Copiapo	-	-	-	-	-	14.23	18.99	35.00	17.05	6.69	1.41	0.49	0.40	0.77	1.21	0.81	0.17	0.27	[19]
Chile	Atacama, Century scoriales XIX Canto del Agua, Copiapo	-	-	-	-	-	12.87	29.60	39.39	4.32	4.62	1.30	0.75	1.14	1.40	0.32	0.33	0.05	0.14	[19]
Chile	Atacama, Century scoriales XIX Nantoco, Copiapo	-	-	-	-	-	13.58	22.71	38.58	7.66	6.57	1.13	0.70	0.53	1.83	1.01	0.58	0.30	0.13	[19]
China	Tongling Non-ferrous Metals Group Holding Co., Ltd., Tongling, Anhui	0.80	39.10	0.45	-	0.10	-	42.85	31.11	2.44	2.29	1.79	-	-	-	-	-	-	-	[56]
Spain	Cabezo Juré archeological settlement, Alosno, Provincia de Huelva.	20.1	-	-	-	-	50.60	-	13.81	0.20	1.79	0.20	0.058	-	-	0.01	0.26	-	0.307	[57]
Mexico	Company and location unknown	-	-	-	-	-	45.46	-	33.12	11.68	4.67	0.42	0.10	-	-	0.77	2.64	0.39	0.22	[58]
Mexico	Smelter of the Mining-Metallurgical Complex of La Caridad, Sonora	1.05	44.57	-	-	-	-	-	25.12	-	-	-	9.42	-	-	-	-	-	-	[59]
Mexico	Industrias Magnelec SA de CV, Saltillo, Coah	-	-	-	-	-	0.13	-	0.2	1.00	0.15	98.5	-	-	-	-	-	-	-	[60]
Italy	Campigilia Marittima Century scoriales XI-XIII. Capattoli	-	-	-	-	-	-	42.53	20.06	8.26	2.93	0.72	-	-	-	0.74	0.02	-	-	[61]
Italy	Campigilia Marittima Century scoriales XI-XIII. Capattoli	-	-	-	-	-	-	52.17	30.60	15.62	5.06	0.90	-	-	-	1.20	0.07	-	-	[61]
Portugal	District Sao Domingos Mining Centre, Century XX, Moitinha, Achada do Gamo	-	-	-	-	-	52.86	-	37.70	1.14	3.93	0.40	0.20	-	-	1.04	0.25	0.07	0.688	[62,63]
Namibia	Dundee Precious Metals Ltd., Tsumeb smelting complex, Windhoek	-	-	6.51	-	-	32.47	9.49	14.68	4.50	4.70	1.43	0.59	-	-	0.30	0.26	0.08	0.25	[64,65]
India	Sterlite Industries India Limited, Tuticorin, Tamil Nadu	-	-	-	-	-	61.24	-	19.72	5.65	3.03	0.90	0.63	3.31	1.49	1.23	-	0.08	0.44	[10]
Australia	Tropical coastal zone of north Queensland.	0.52	-	-	-	-	-	32.88	34.80	15.53	5.41	0.91	0.27	-	-	0.60	0.26	0.39	-	[3]
Canada	INCO’s flash smelting furnace, Sudbury, Ontario	0.58	38.6	0.93	-	-	45.40	52.20	17.40	1.11	2.51	1.65	-	-	-	-	-	0.03	0.15	[56,66]

The analysis of the SEM-EDS image shown in Figure 5 confirms the presence of Fe₂SiO₄ and small traces of Cu in the sample.

The principal minerals and oxides present in the copper slag, including those that represent 1% of the chemical composition, have melting temperatures (Tm) very different from each other, which are detailed below: Fe₂SiO₄ (Tm = 1216.9 °C) < Cu₂O (Tm = 1243.6 °C) < FeO (Tm = 1376.9 °C) < Fe₃O₄ (Tm = 1596.9 °C) < SiO₂ (Tm = 1722.9 °C) < MnO (Tm = 1841.9 °C) < ZnO (Tm = 1701.9 °C) < Al₂O₃ (Tm = 2053.9 °C) < MgO (Tm = 2831.9 °C) < CaO (Tm = 2926.9 °C) [67]. These temperatures allow us to see that metal oxides could volatilize when exposed to high temperatures and in contact with water steam. Opila et al. [68] indicate that the interaction of high-temperature water steam with oxides to form volatile hydroxides leads to material loss. The following reactions are according to the supposed reaction that could occur during the thermolysis process. The thermal decomposition of Fe₂SiO₄ or 2FeO-SiO₂ mainly involves its transformation into Fe₂O₃, Fe₃O₄, and amorphous silica (SiO₂) [3,64,69]. The oxidation of the slag includes the decomposition of Fe₂SiO₄, and the adsorption and diffusion of superoxide ions (O²⁻) and other species, promoting the formation of FeO, Fe₂O₃, Fe₃O₄, and SiO₂ [64]. Chen et al. [70] investigated the oxidation behavior of Fe₂SiO₄ in water steam at 1000 °C, reporting that Fe₂SiO₄ was completely decomposed into Fe₃O₄ (58.8 wt%), Fe₂O₃ (21.6 wt%), and SiO₂ (19.6 wt%) (see Equation (5)) and obtained a hydrogen production capacity of 24.42 mL/g. Equation (5) is as follows:

$$3{\mathrm{Fe}}_2{\mathrm{SiO}}_4+2{\mathrm{H}}_2\mathrm{O} \to 2{\mathrm{Fe}}_3{\mathrm{O}}_4+3{\mathrm{SiO}}_2+2{\mathrm{H}}_2$$

(5)

The use of magnetite has been widely investigated, but the production of H₂ from the use of Fe₃O₄ was proposed by Nakamura et al. in 1977 [69,71,72,73,74,75,76] according to Equation (6), which is as follows:

$$2{\mathrm{Fe}}_3{\mathrm{O}}_4+{ \mathrm{H}}_2\mathrm{O} \to 3{\mathrm{Fe}}_2{\mathrm{O}}_3+{ \mathrm{H}}_2$$

(6)

However, other researchers indicate that when Fe₃O₄ is in contact with water steam it can produce volatile metal oxides (Equation (7)) [77], as can be seen in the following equation:

$${\mathrm{Fe}}_3{\mathrm{O}}_4+2{\mathrm{H}}_2\mathrm{O} \to \mathrm{Fe}{\left(\mathrm{OH}\right)}_2+2\mathrm{FeO}\left(\mathrm{OH}\right)$$

(7)

(Mg_1−x,Fe_x)₂SiO₄ [78], which is also written as 2(Mg_1−x,Fe_x)O–SiO₂, is a mineral formed between two metals (Fe and Mg) and SiO₂ and is found in a solid solution between Fe₂SiO₄ and Mg₂SiO₄ [79]. Freund et al. [80] proposed that, if H₂O is incorporated into the (Mg_1−x,Fe_x)₂SiO₄ structure, its presence can create extrinsic point defects that are associated with H₂ production due to the formation of OH ions. But the structure of (Mg_1−x,Fe_x)₂SiO₄ has mainly been investigated for the influence it has on the production of H₂ during a serpentinization reaction which is accompanied by volume expansion (serpentine (Mg₃Si₂O₅(OH)₄) possibly results from the reaction of water with ferrous minerals rich in Fe that are contained in ultramafic rocks). The generation of H₂ during the serpentinization mechanism is a result of the reaction of water with ferrous iron (Fe²⁺) derived from minerals, mainly olivine and pyroxene, to ferric iron (Fe³⁺), which normally precipitates as magnetite. The process was modeled numerically by McCollom et al. [81], indicating that at high temperatures (>315 °C), the rates of serpentinization reactions are fast, and their reaction is represented by Equation (8) which follows:

$${\mathrm{Mg}}_{1.8}{\mathrm{Fe}}_{0.2}{\mathrm{SiO}}_4+1.37{\mathrm{H}}_2\mathrm{O} \to 0.067{\mathrm{Fe}}_3{\mathrm{O}}_4+0.5{\mathrm{Mg}}_3{\mathrm{Si}}_2{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4+0.3\mathrm{Mg}{\left(\mathrm{OH}\right)}_2+0.067{ \mathrm{H}}_2$$

(8)

Olsson et al. [82] mention that the dissolution of (Mg_1−x,Fe_x)₂SiO₄ is faster in acidic solutions than in alkaline ones. The acidic solution of (Mg_1−x,Fe_x)₂SiO₄ can be written as (Mg,Fe)₂SiO₄ + 4H⁺ ≥ 2(Mg²⁺,Fe²⁺) + H₄SiO₄. As (Mg_1−x,Fe_x)₂SiO₄ dissolves and Si is produced, protons that are consumed cause the pH to increase. Wang et al. [83] also indicate that the dissolution rate of (Mg_1−x,Fe_x)₂SiO₄ is fast relative to other silicates. In the experiments carried out with (Mg_1−x,Fe_x)₂SiO₄ with CO₂/H₂O, they conclude that olivine only reacted with H₂O to produce Mg₃Si₂O₅(OH)₄, (Mg,Fe)(OH)₂), and Fe₃O₄. The latest research is aimed at estimating the influence of the amount of H₂ on the serpentinization process in a temperature range of 50 to 320 °C through numerical modeling [84]. Silica has been investigated in depth by Opila et al. [68,85,86], who studied the oxidation kinetics of chemically vapor deposited (CVD) silicon carbide (SiC) in the temperature range of 1200–1400 °C [85] and 1000–1400 °C [86] in H₂O/O₂ gas mixtures. They observed that the silica scales formed in SiC can be volatilized simultaneously, forming silicon hydroxide (SiO(OH)₂) or silicon oxyhydroxide (Si(OH)₄) at atmospheric pressures (0.1 MPa). The formation of volatile hydroxides depends, in some cases, on the partial pressures of water vapor and oxygen, and for processes at 0.1 MPa and temperatures below 1300 °C the volatility of silica can be attributed to the reaction SiO₂ + 2H₂O → Si(OH)₄. At higher temperatures and at lower partial pressures of water steam and oxygen, the reaction SiO₂ + H₂O → SiO(OH)₂ is more likely [68]. Kendelewicz et al. [87] studied the adsorption of water on the surface of a single crystal of MnO at low water steam pressures (1.33 × 10⁻⁶–1.33 × 10⁻² Pa), in which the reaction with H₂O molecules results in the formation of hydroxyl groups on the surface (MnO + H₂O → Mn(OH)₂). Another thermodynamic and experimental study of thermochemical cycles (three steps) to produce H₂ involving hydroxides (NaOH and KOH) and MnO among others, has concluded that the reaction of hydroxides with MnO is stable, but they did not produce H₂ at 750 °C, even in strong oxidizing media [88]. Finally, Orfila et al. [89] indicate that the Mn-oxide cycle is the most studied, but unfortunately the H₂O cleavage reactions with MnO are not thermodynamically favorable and the H₂ produced is negligible.

Zn is a metal that when placed in contact with water releases H₂ and produces ZnO. Among the research found, Bilgen et al. [90] carried out the direct decomposition of H₂O in a temperature range of 2000–2500 °C and proposed the exothermic reaction of water vapor, 2Zn + 2H₂O → 2ZnO + 2H₂. Steinfeld [91] considered the production of H₂ from H₂O dissociation through a thermochemical solar cycle (two steps) with ZnO/Zn. In the second step (exothermic, non-solar), they propose the hydrolysis of Zn at 427 °C to form H₂ and ZnO through the reaction Zn + H₂O → ZnO + H₂. Another study focused on the production of syngas from H₂O and CO₂ in a two-step cycle through Zn/ZnO redox reactions [92]. And recently, Bhosale [93] has proposed a hybrid solar thermochemical cycle of ZnSO₄/ZnO for water splitting (WS), with an oxidation step through the reaction ZnO + SO₂ + H₂O → ZnSO₄ + H₂ at a temperature of 127 °C. Opila et al. [68] indicate that alumina volatilizes in environments containing water steam at high temperatures through the reaction 1/2 Al₂O₃ + 3/2 O₂ → Al(OH)₃, but they mention that the volatility of alumina due to the formation of Al(OH)₃ is not a problem at temperatures < 1300 °C. Huang et al. [94] indicate that Mg is a good candidate for H₂ generation due to its very low cost, high availability, high theoretical H₂ yield, and the formation of environmentally friendly by-products. The reaction between Mg and H₂O can be described as Mg + 2H₂O → Mg(OH)₂ + H₂, and this reaction is interrupted due to the formation of a passive layer of Mg(OH)₂ on the unreacted Mg particles. Nissen et al. [95] investigated the reaction rate of Ca oxidation with water steam in the temperature range of 25 and 300 °C, obtaining a reaction rate of linear type (at 25 °C) and logarithmic (at 300 °C). In the temperature range of 20–150 °C, Ca reacts according to Ca + 2H₂O → Ca(OH)₂ + H₂, and at temperatures above 250 °C it reacts according to the mechanism Ca + H₂O → CaO + H₂. Bhosale [96] thermodynamically analyzed the CaO/Ca-based WS cycle (Ca-WS) using Metso’s chemical reaction and equilibrium software (HSC Chemistry 9.9). Furthermore, they indicate that during the reoxidation of Ca, the result was the formation of H₂.

The oxidation reaction of metal oxides (see Equation (9)), present in copper slag (2FeO-SiO₂, Fe₂O₃, MgO, CaO, MnO, ZnO, K₂O, TiO₂, etc.) [24], can be applied, with the equation as follows:

$${\mathrm{M}}_{\mathrm{x}}{\mathrm{O}}_{\mathrm{y}}+{ \mathrm{H}}_2\mathrm{O} \to { \mathrm{M}}_{{\mathrm{x}}^{\prime }}{\mathrm{O}}_{{\mathrm{y}}^{\prime }}+{\mathrm{H}}_2$$

(9)

There is abundant scientific information explaining the interaction between water vapor (H₂O(g)) and Fe under high temperature conditions (200 to 1000 °C), where it is specified that H₂ is produced by the oxidation reaction of Fe and FeO according to Equations (10) and (11) [71,72,97], which are as follows:

$$3\mathrm{Fe}+4{\mathrm{H}}_2\mathrm{O} \to {\mathrm{Fe}}_3{\mathrm{O}}_4+4{\mathrm{H}}_2$$

(10)

$$3\mathrm{FeO}+{ \mathrm{H}}_2\mathrm{O} \to {\mathrm{Fe}}_3{\mathrm{O}}_4+{\mathrm{H}}_2$$

(11)

On the other hand, the dissociation reaction of water in contact with FeO is favorable at temperatures below 800 °C, when ΔG° < 0 (the reaction is spontaneous and the theoretical conversion of FeO decreases with increasing temperature) [74,98]. Our expectations are that oxides such as Fe₂SiO₄, Fe₃O₄, and (Mg, Fe)₂SiO₄ will react with water steam at high temperatures (in a range of 200–1000 °C and 1 atm), generating products such as Fe₃O₄, Fe₂O₃, SiO₂, Mg₂SiO₄, H₂, and possible volatile metal oxides (M_xO_y(OH)₂).

The circular economy in mining seeks the recovery, valorization, and reuse of mining waste, transforming it into inputs for new applications instead of disposing of it. According to the above, the use of copper slag in the production of H₂ through thermolysis would represent an important opportunity to transform a waste product that currently plagues the world, especially in desert areas such as the Atacama Desert where mining activity is prevalent, into a high-value-added material, prioritizing its reuse as an alternative application in solar energy. While H₂ production with copper slag is aligned with the principles of the circular economy (see Figure 6), it is essential to assess its environmental impact. The H₂ product of this process can be “reused” as fuel in mining processes, reducing the use of fossil fuels (for example, diesel which is widely used in mining trucks). In addition, by-products such as Fe₃O₄ y Fe₂O₃ can be “recycled” for the recovery of Fe, an element in high demand in Cu smelting.

The release of heavy metals (Pb, As, and Cd) through volatile metal oxides is unlikely because the thermolysis process must be carried out at temperatures between 200 and 1000 °C, and the majority compound of Chilean copper slag is Fe₂SiO₄ (melting temperature is 1217 °C) and Fe₃O₄ (melting temperature is 1597 °C) [67,99]. To ensure a complete circular economy cycle, the waste generated after thermolysis of copper slag (magnetite and hematite) must be analyzed for possible reuse or Fe recovery, rather than being disposed of, thus maximizing material efficiency. The presence of toxic elements such as As, Cd, and Pb in copper slags depends primarily on the mineralogical composition of the copper concentrate, as well as on the conditions of the smelting process. Complex minerals such as enargite (Cu₃AsS₄) [100,101,102,103], tetrahedrite–tennantite ((Cu,Fe)₁₂(Sb,As)₄S₁₃) [102,104,105], galena (PbS) [104], and sphalerite (ZnS) [106] containing traces of Cd are the main sources of toxic elements in copper slag. This contamination is the result of flotation processes applied to sulfide ores without strict control over the hydrometallurgical stages. Such operational shortcomings can cause metallic impurities to be carried over into the copper concentrate used in pyrometallurgical processing. In addition, smelters often process blended concentrates from different mining origins, which increases the presence of difficult-to-manage impurities, such as Pb in the form of PbS. During smelting, factors such as temperature, redox atmosphere, and the type of fluxes used influence the final composition of copper slag. Moreover, the recycling of metallurgical dusts or secondary residues can enrich the slag with trace amounts of heavy metals. Therefore, process control and proper stabilization of the slag are essential to minimize the risk of copper slag contamination with toxic materials. At high temperatures (thermolysis process), toxic compounds may pass into the gas phase if not adequately controlled. Currently, copper smelters implement a combination of physical, chemical, and environmental strategies to eliminate or control contaminants such as As and Cd. It is worth noting that Pb is not commonly found as a by-product in copper mining. The removal or control of contaminants in copper smelting is achieved through a combination of physical, chemical, and operational approaches, in which the toxic elements are stabilized as oxides, sulfides, or silicates within glassy or crystalline matrices, reducing their mobility. Simultaneously, at high temperatures during smelting, some of these elements volatilize and are captured by gas treatment systems, producing metallic dusts that require specialized handling. In addition, hydrometallurgical processes are employed to recover or stabilize impurities, such as the removal of As in the form of insoluble compounds like scorodite (FeAsO₄·H₂O) [105] or calcium arsenate (Ca₃(AsO₄)₂) [107]. The final disposal of slags and dusts typically involves their stabilization and confinement in controlled storage facilities, with physicochemical encapsulation to prevent leaching. At a preventive level, many smelters also apply source minimization strategies, ensuring the quality control of the concentrate and limiting the input of minerals with high levels of penalized elements.

A major challenge for researchers using copper slag for H₂ production is the potential release of impure gases, such as SO₂, SO₃, and volatile metal species (Pb, As, etc.), during high-temperature reactions. These gases can reduce the purity of the H₂, requiring post-treatment processes before its use in fuel cells or specific industrial applications.

In general, the reduction of Fe₂O₃ does not lead directly to metallic Fe. If the reduction temperature is below 570 °C, the reduction to Fe occurs stepwise from Fe₂O₃ to Fe₃O₄, and continues to Fe. The intermediate oxide, like Fe_(1−x)O, is not stable at temperatures below 570 °C.

At reduction temperatures above 570 °C, wüstite must also be considered in the reduction process. In this case, the reduction proceeds from Fe₂O₃ through Fe₃O₄ to Fe_(1−x)O and then continues to Fe. The following equations show the reduction process at different temperatures [108]:

$$3{\mathrm{Fe}}_2{\mathrm{O}}_3+{\mathrm{H}}_2 \to 2{\mathrm{Fe}}_3{\mathrm{O}}_4+{ \mathrm{H}}_2\mathrm{O}, \mathrm{T}<570 °\mathrm{C}$$

(12)

$${\mathrm{Fe}}_3{\mathrm{O}}_4+4{\mathrm{H}}_2 \to 3\mathrm{Fe}+4{\mathrm{H}}_2\mathrm{O}, \mathrm{T}>570 °\mathrm{C}$$

(13)

$$\left(1-\mathrm{x}\right){\mathrm{Fe}}_3{\mathrm{O}}_4+(1-4\mathrm{x}){\mathrm{H}}_2 \to 3{\mathrm{Fe}}_{\left(1-\mathrm{x}\right)}\mathrm{O}+\left(1-4\mathrm{x}\right){\mathrm{H}}_2\mathrm{O}$$

(14)

$${\mathrm{Fe}}_{\left(1-\mathrm{x}\right)}\mathrm{O}+{\mathrm{H}}_2 \to \left(1-\mathrm{x}\right)\mathrm{Fe}+{ \mathrm{H}}_2\mathrm{O}$$

(15)

The presence of sulfur-containing phases such as Cu₂S and FeS₂ would lead to SO₂ emissions, while carbon residues from smelting processes can contribute to CO₂ generation. To mitigate these effects, pretreatment methods or gas scrubbing technologies can be employed to remove impurities after thermolysis. In addition, post-process gas separation techniques, such as selective membranes or pressure swing adsorption (PSA), need to be explored to ensure the high purity of the hydrogen produced.

The efficiency of copper slag in producing H₂ by thermolysis depends on temperature, pressure, and its chemical composition. Studies on iron-based thermochemical cycles have shown that FeO can react with H₂O at temperatures below 1000 °C to generate H₂. However, Fe₂SiO₄ and other silicates in copper slag can reduce the reaction efficiency by stabilizing iron oxides in less reactive forms. To improve the reactivity of copper slag, strategies such as thermal pre-activation (reduction in H₂ or CO atmosphere), the addition of catalytic promoters (Ni, Co), or surface modifications can be explored.

To justify the viability of H₂ production with copper slag, a comparative technical–economic analysis is necessary, evaluating energy efficiency, process scalability, and market demand. In the future, hybrid approaches could be explored, where copper slag is processed in conjunction with other industrial waste to improve overall resource recovery efficiency. However, a substantial advantage is that the copper slag thermolysis process for H₂ production can be integrated with renewable energy, taking advantage of the high solar potential in desert regions such as the Atacama Desert, for example using concentrated solar power (CSP) such as the CSP plant called Cerro Dominador in Chile for high-temperature slag oxidation, medium-temperature concentrated solar power (using parabolic troughs) for steam generation, and photovoltaic energy for electricity consumption. This allows for the design of a completely sustainable system where the slag acts as a reactive material for the production of H₂ without the need for fossil fuels.

This approach allows for closing the resource loop in the mining and energy industries by reusing waste and avoiding dependence on fossil fuels, thereby reducing the carbon footprint of the H₂ production process. Although copper slag is an abundant and inexpensive material, its economic viability in H₂ production must consider the following:

• Costs of collecting and transporting copper slag from smelters to H₂ production plants.
• Pretreatment costs (impurity removal, structural modification).
• Capital expenditures (CAPEXs) and operating costs (OPEXs) for high temperature thermolysis reactors and solar concentrators.
• Costs of purifying water for the process and hydrogen for industrial applications.

A levelized cost of hydrogen (LCOH) analysis should be conducted to assess whether H₂ production using copper slag can compete economically with conventional technologies such as water electrolysis, natural gas reforming (SMR), or biomass gasification. Integration with concentrated solar power (CSP) could significantly improve the process’s profitability and sustainability.

Future research is needed to consolidate and scale the use of copper slag as a thermo-catalytic material for hydrogen production (and other electrochemical processes), considering approaches from materials science, process engineering, sustainability, and the circular economy. Key areas of focus include the following: (i) advanced thermochemical and kinetic studies to develop thermodynamic models capable of predicting the energy required for oxidation processes during the interaction between copper slag and water vapor at high temperatures, and to evaluate reaction rates, corrosion behavior, and the specific mechanism of H₂ production during thermolysis; (ii) investigation into the functionalization and modification of copper slag to enhance its catalytic or electrocatalytic activity, for example, through doping with metals such as Ni, Co, or Cu to improve thermal activation under operational conditions; (iii) development and design of electrodes and solar thermolysis reactors to assess solar–thermochemical performance; (iv) environmental and circularity assessments, including life cycle analysis (LCA), with particular emphasis on evaluating the potential of copper slag to substitute exotic materials such as CeO₂.

4. Conclusions

The use of copper slag for hydrogen production via thermolysis represents a compelling and innovative application of circular economy principles to the energy and mining sectors. Globally, more than 30 million tons of copper slag are generated annually, with major producers including Chile, Mexico, Australia, Spain, Namibia, India, and the United States. In northern Chile, particularly in the Atacama Desert, decades of intensive copper smelting have resulted in vast accumulations of copper slag, often stored in large heaps with limited reuse and increasing environmental concern. Although rich in metal oxides such as magnetite and fayalite, copper slag continues to be primarily used in low-value applications such as construction filling and abrasive blasting. Reimagining copper slag as a raw material for hydrogen production marks a significant paradigm shift. Unlike traditional methods such as water electrolysis, which rely on purified water and often require critical and expensive materials (e.g., platinum-based catalysts), the thermolysis of copper slag enables hydrogen generation using a readily available industrial by-product. This approach benefits from several unique advantages, such as the following: (i) extremely low material cost, as copper slag is abundant and often treated as waste; (ii) no requirement for additional reactants, with hydrogen being generated through redox reactions with water vapor; (iii) integration with concentrated solar power (CSP), allowing for carbon-free energy input and significantly lowering operational emissions; (iv) potential for metal recovery (e.g., magnetite, hematite) as a co-benefit, enhancing overall resource efficiency. In regions like the Atacama Desert, where both slag deposits and solar irradiance are abundant, this approach offers a unique opportunity to combine industrial waste recovery with renewable energy to create a local, sustainable pathway for hydrogen production. Moreover, the hydrogen generated through this method could be reintegrated into mining and metallurgical operations, powering microgrids, fueling equipment, or supporting broader decarbonization strategies. This closes material and energy loops, reinforcing circularity in traditionally linear sectors and contributing to the emergence of a low-carbon, green mining economy. Copper slag valorization through solar thermolysis offers a cost-effective, low-emission, and circular alternative to conventional hydrogen production methods. Its success will depend on the continued optimization of reaction conditions, advances in reactor design, and integration with emerging clean technologies. Positioned at the intersection of waste recovery, renewable energy, and green hydrogen, this approach has the potential to contribute meaningfully to national and global clean energy transitions—particularly in resource-rich regions such as the Atacama Desert.