Oxide Reduction Treatment with a Thermal Plasma Torch: A Case Study

Abstract

This article presents the findings of a study on oxide reduction utilizing a novel reducing plasma torch, employing greenhouse gases such as CO₂ and CH₄ as plasma gases. The primary aim of this investigation is to establish the viability of this approach. The innovative plasma torch was employed to reduce various oxides, including aluminum oxide, iron oxide, and titanium oxide, as well as a mixed oxide composition, employing a CO₂/CH₄ molar ratio of 1:1 within a spouted bed reactor. Following plasma treatment, X-ray diffraction (XRD) analysis was conducted to examine the metallic phases, notably titanium, iron, and aluminum. SEM–EDS observations were carried out to assess microstructural changes and identify elemental compositions pre- and post-plasma treatment. The results demonstrate that within the conical section of the reactor, titanium oxide experiences partial reduction, resulting in limited titanium production, while aluminum oxide and iron oxides (magnetite and hematite) undergo reduction to yield aluminum and iron, respectively. Thermodynamic calculations, performed using Factsage software version 8.3, were utilized to predict stable-phase formations following plasma treatment for each material.

1. Introduction

Metals in nature are generally found in oxidized form, except gold, which is often found in its native metallic form. When referring to ore, it generally means a complex mixture containing various metal oxides, sometimes combined with other components. This chemical diversity makes the process of extracting metals, known as extractive metallurgy, a complex and meticulous undertaking [1]. The main aim of extractive metallurgy is to reduce these metal oxides to recover pure metals. This process generally involves several stages, each designed to separate the metals from the other elements present in the ore and return them to their metallic form. Oxides can be reduced by various means, such as chemical reactions with reducing agents or by heating at high temperatures in a controlled environment [1].

Worldwide crude steel production currently stands at 1500 million tons per year, followed by other base metals and alloys. Most of these metals, including iron, aluminum, silicon, manganese, chromium, nickel, titanium, vanadium, and many others, are extracted from oxidized minerals. This extraction process typically utilizes carbon as a reducing agent, posing challenges in terms of future supply and environmental pollution. For instance, steel production alone contributes to 5% of total CO₂ emissions [2].

Thermal plasma represents a versatile decarbonization solution for the large-scale manufacture of various advanced materials required in highly specialized industrial sectors. The growing demand for materials with improved physical and mechanical properties to meet the stringent requirements of cutting-edge industries such as aerospace, chemicals, electronics, semiconductors, transport, and nuclear energy is steadily increasing [3].

Moreover, plasmas are recognized for their superior efficiency compared to conventional heating methods and could eventually supplant them: the torch used in these experiments boasts an efficiency of 75% [4]. In addition, the plasma used in this study offers further advantages by consuming polluting gases such as CO₂ and CH_4.

Industrial plasmas can operate at pressures ranging from near vacuum to several atmospheres. Several significant industrial applications occur at or near atmospheric pressure, sometimes even in the air. These applications utilize non-equilibrium plasmas (non-LTE) with electron temperatures many orders of magnitude higher than the temperatures of the heavy components of the plasma, as well as thermal plasmas approaching local thermal equilibrium (LTE) [5]. For instance, one study examined the effects of low-temperature, non-equilibrium plasma generated by a dielectric barrier discharge in ambient air at atmospheric pressure on the acid–base, adsorption, and flotation properties of natural iron sulfides such as pyrite and arsenopyrite [6]. In another study, the application of the plasma–chemical ore treatment method using nanosecond pulsed dielectric barrier discharge (DBD) plasma at low temperatures in the air under atmospheric pressure showed promising prospects for selective separation processes of semiconducting ores, particularly sulfides and oxides [7].

When the temperature of a plasma is high enough, a local thermodynamic equilibrium (LTE) is reached. For plasma species in motion, LTE is the thermodynamic state achieved when collisions prevail in regions with modest spatial variations (where the plasma does not absorb its own radiation). High-density arc discharges are the only means of creating the plasmas required for industrial-scale pyrometallurgical processes [8]. These heated plasma arcs are created between a cathode and an anode. There is a wide variety of electrodes, each with its own shape and composition. The most common types of electrodes are rods, buttons, tubes, and rings. Rod and button electrodes are often made from thoriated tungsten (2%–3% ThO₂) or graphite. Currently, plasma tools are either transferred arc systems or non-transferred arc devices. The process calls for electricity, which may be provided by the device’s operating current and voltage [9].

There are now two primary areas where thermal plasma technology is being applied: materials processing and waste treatment. Many high-tech companies rely on the large-scale, high-purity manufacturing of novel materials, and thermal plasma synthesis provides a flexible and cost-effective approach for this purpose. High-tech sectors, including aircraft, chemicals, electronics, semiconductors, transportation, and nuclear power, are increasingly aware of the need for materials with enhanced physical and mechanical qualities for demanding applications [10]. Over the past decade, plasma waste treatment has emerged as a crucial technology in the face of growing waste disposal challenges. This method has gained importance, not least due to a growing awareness of the potential for waste recovery and the production of valuable co-products [11]. For example, thermal plasma represents an essential tool in the transformation of waste-derived fuels, providing the optimum conditions for efficient pyrolysis and gasification processes, thus contributing to more sustainable waste management and the production of renewable energy [12]. A recent study brilliantly illustrates the successful integration of hybrid thermal plasma technology with carbon capture, utilization, and storage (CCUS) for sewage sludge treatment. This innovative approach offers the ability to transform sewage sludge into syngas while capturing a proportion of the carbon in solid form and converting CO₂ to CO within the syngas [13]. A study has been carried out into the gasification of various organic materials in a steam plasma generated in a special plasma torch equipped with a water-stabilized arc. This unique configuration produces a thermal plasma characterized by very high enthalpy and low mass flow thanks to an arc discharge in direct contact with water [14]. In conjunction with this, a new process has been proposed and investigated using an equilibrium thermochemical modeling study to assess the effectiveness of an innovative technology that merges thermal plasma and thermochemical looping. This innovative approach aims to use hydrogen in the iron reduction process, opening up new prospects for the steel industry [15]. Recently, thermal plasma has gained worldwide popularity for the treatment of complex waste streams due to its many intrinsic advantages. These include the ability to produce hydrogen-enriched synthesis gas (H₂) [16].

In comparison to existing DC thermal plasma torches, the newly developed DC torch represents a significant breakthrough in technology. By utilizing molecular gases, it provides both high plasma enthalpy (50 MJ/kg at 7000 K) and high thermal conductivity (≈4 W/m·K at 7000 K) [17]. One disadvantage of using argon as a plasma gas is its low thermal conductivity. This reduces the heat transfer rate to the treated materials. To overcome this, a small percentage of hydrogen or helium is typically added. While this addition improves heat transfer, it also accelerates electrodes’ erosion. During arc burning, hydrocarbons completely dissociate into free carbon and hydrogen. Under appropriate conditions, carbon ions from the gas phase diffuse to the cathode surface, establishing a dynamic equilibrium between carbon sublimation and precipitation [18]. The electrode is preserved and has a longer service life due to the carbon coating. For combustion, this torch utilizes a gas similar to carbon dioxide, which can be obtained in near-pure form from certain chemical firms as waste gas [19].

Potential uses for CO₂/CH₄ plasma torches are mainly unexplored. However, it is critical to emphasize their prospective capacity to reduce greenhouse gas (GHG) emissions, such as CO₂, as well as other harmful gases, such as NOx [20]. Furthermore, these plasma torches might increase the consumption of the most harmful gases found across several industrial sectors. In this study, the CO₂/CH₄ torch utilized up to 3.6 m³·h⁻¹ of gas, which is much less than gas burners, which may burn over 2000 m³·h⁻¹ of fossil fuels [21]. However, using oxygen-enriched air may significantly decrease consumption by up to 25% [22].

The main purpose of this study is to demonstrate the viability of the concept of reducing oxides using a plasma torch powered by greenhouse gases such as CO₂ and CH₄. What particularly sets this research apart is its innovative exploration of the use of this type of plasma torch to reduce specific oxides, an approach that has not been attempted yet. Alumina, hematite, magnetite, and titanium oxide are only a few of the oxides that were studied for their reducing potential in this report. Metallic phases have been produced by adopting this approach. The reducing power of the plasma torch was increased, and high temperatures were attained by using a CO₂/CH₄ molar ratio of 1:1. After plasma treatment, metal phases including titanium, iron, and aluminum were detected using XRD analysis. SEM–EDS observations were performed to analyze the microstructure and identify the elemental composition of materials before and after plasma treatment, proving the technology’s efficacy in the field of material processing.

2. Materials and Methods

2.1. Chemicals

Argon gas (99 wt.% purity) was used to cool down the experiment, while pure CO₂ (99 wt.% purity) and CH₄ (99 wt.% purity) were used as plasma gases to generate a stable plasma arc. The experiment employed a molar gas ratio CO₂/CH₄ of 1:1. Each experiment was fed with Al₂O₃ (98 wt.%), Fe₂O₃ (98 wt.%), Fe₃O₄ (98 wt.%), and TiO₂ (98 wt.%).

2.2. How a Plasma Torch Works

New-generation plasma torches can be easily connected to gas or electricity sources using a specially designed connector plate, reducing the amount of handling required. Within their metal casing, these torches contain three critical components (see Figure 1): a positively charged upstream electrode, a negatively charged downstream electrode, and a plasma gas injection chamber. When two electrodes come into contact, a short circuit is formed. An electric arc forms between the two electrodes when the upstream electrode is pushed back and the current increases. When plasma gas is introduced into the injection chamber, it creates a vortex around the arc [23]. A magnetic field generated by a specific coil causes the arc to rotate. This process helps to slow electrode wear, extending its useful life. When the introduced gas contacts the electric arc, it ionizes and transforms into plasma. Plasma, also known as the fourth state of matter after solids, liquids, and gases, is an ionized, electrically conductive medium. Its energy density is so high that temperatures in the heart of the arc can reach 5000 °C [24]. To protect the torch’s external environment from these extremely high temperatures, a water circulation system cools the metal casing, keeping the torch’s external temperature below 40 °C [25].

2.3. Experimental Procedure

A new direct current (DC) plasma torch (P60 with input power up to 60 kW) working with a gas combination of carbon dioxide (CO₂) and hydrocarbon (methane (CH₄)) has been employed for oxide reduction processing. Several steps were taken to prepare the equipment for the experiment. First, the torch was connected directly to the conical part of the reactor. Next, the feeding inlet was removed to allow access to the inside of the reactor. The material was dried at 105 °C overnight and then poured into the reactor through a tube to reach the conical section, while a flow of argon was injected to prevent contamination by unwanted particles inside the torch. Experiments were carried out using a feedstock consisting of particles of various sizes, ranging from 50 µm to 100 µm. In a spouting bed reactor, metal oxides such as aluminum oxide, titanium oxide, and iron oxide were reduced. The plasma torch was operated using a CO₂/CH₄ mixture with a molar ratio of 1:1, and the power applied was 38 kW. At the start of each experiment, a plasma jet was generated at the torch outlet, coming into direct contact with the material to be treated. The temperatures reached were extremely high, ranging from 4000 °C to 5000 °C, melting the material and forming a liquid phase. This liquid phase was then cooled using a flow of argon, giving rise to a molten material known as the "solidified liquid phase", which took on the same shape as the conical part of the reactor. The plasma reactor used for the experiments is shown in Figure 1 (spouted-bed reactor). The CO₂/CH₄ combination at high temperatures has a complex composition where the presence of CO might arise and decrease oxidation. The molar gas ratio of CO₂ to CH₄ was adjusted to 1 to produce a low-oxygen atmosphere. A stable plasma jet was achieved by maintaining the current at 252 A, and the voltage was set at 150 V. Although the pressure changed slightly, it remained close to 1 atm. The duration of each experiment was restricted to two minutes because of overheating of the conical part in direct contact with the torch and the emergence of hot spots throughout the trials.

2.4. Characterization Techniques

2.4.1. XRD Analysis

All phases were determined before and after plasma treatment using a Panalytical’s X’Pert Pro MPD diffractometer. CuKα radiation was used to measure angles ranging from 10° to 70° (angle 2θ) at a rate of 1° per minute. The angle may be adjusted as needed. The collected data were submitted to Rietveld analysis using HighScore Plus version 4.9 to determine the proportion of phases present in the samples. High crystallinity was required for accurate analysis. This approach revealed a significant detection limit of 0.2% and good accuracy in measuring phase proportions [26]. In this work, Rietveld analysis was used to associate peaks with crystalline phases detected in XRD diagrams, and simulation settings were changed until the estimated error was less than 2%.

2.4.2. SEM–EDS Analysis

Microstructure observation was performed with a Hitachi S-4700 scanning electron microscope. This scanning electron microscope, which has a cold cathode and a field-effect gun, can produce very sharp images. The advanced technology of the Hitachi S-4700 scanning electron microscope allows for high-resolution pictures to be acquired from a broad range of materials. EDS examined the composition of materials to identify their elemental composition.

2.5. Thermodynamic Study

The plasma treatment of each oxide was analyzed thermodynamically to determine which phases would be produced when metallic oxide was reduced by a thermal plasma torch working with CO₂ and CH₄. Two systems were examined. The first illustrates the reaction of CO₂ and CH₄ (molar ratio 1:1) in the plasma torch, which produces additional CO and H₂. The second step is to assess the reaction between the oxide being studied and the composition of the gases released by the torch.

Table 1. The gas quantity at the outlet of the plasma torch (mass spectrometer analysis).

	H2	CO	CH4	CO2	C2H2	H2O	O2
Quantity (mole)	0.4	0.5	0.4	0.4	0.4	0.4	0.006

illustrates the composition of the gases that exit the plasma torch. The temperature varied from 500 to 3000 °C, but the pressure remained at 1 atmosphere. All thermodynamic calculations were performed using Factsage 8.3. It relies on minimizing the Gibbs free energy of all thermodynamic processes involving specific chemical species to achieve maximum efficiency (ΔG = ΔH − TΔS). The equilibrium concentrations of a heterogeneous mixture were estimated by minimizing the system’s Gibbs free energy at a given temperature, assuming that the condensed species were mutually insoluble [27]. The thermodynamic approach to the study of chemical reactions enabled us to determine the feasibility of reactions, predict equilibrium compositions, and assess the stability of reaction products [28]. However, this method has limitations, notably its limited applicability to equilibrium conditions and its tendency to simplify reaction mechanisms, as well as the assumption of ideal component behavior and dependence on the availability and quality of experimental data [29].

Three databases, namely FactPS, FToxid, and FSstel, were selected to encompass the solid and gas species produced in the total reaction of the torch gases outlet and oxides (Al₂O₃, TiO₂, Fe₂O₃, and Fe₃O₄). All gaseous, liquid, and solid species were carefully chosen for thermodynamic calculations, including all potential solutions. Overall, 167 species and 9 solutions were applied for Al₂O₃, 168 species and 16 solutions for TiO₂, and only 147 species and 11 solutions for Fe₂O₃ and Fe₃O₄. The figures below emphasize species with a large mass fraction. Only species with a log (mass fraction) ≥ −11 are included in the figures.

3. Results and Discussion

3.1. Heat Transfer Mode

In a plasma-assisted process, heat transfer from the plasma to the ore can occur in several ways, including conduction, convection, and radiation. The general equations for each type of transfer are shown below:

Conduction: thermal conduction occurs when heat is transferred through a solid medium, such as ore, due to collisions between particles [30]. The heat conduction equation can be represented by Fourier’s law [31]:

$$Q=-k·A·\frac{dT}{dx}$$

(1)

where Q is the heat flux (W), k is the thermal conductivity of the material (W/m·K), A is the heat transfer area (m²), and dT/dx is the temperature gradient across the material (K/m).

Convection: thermal convection occurs when heat is transferred between a fluid (in this case, plasma) and a solid surface (the ore) due to the motion of the fluid [32]. The general equation for thermal convection is [32]:

$$Q=h·A· \mathsf{\Delta }T$$

(2)

where Q is the heat flux (W), h is the convective heat transfer coefficient (W/m²·k), A is the heat transfer surface (m²), and ΔT is the temperature difference between the plasma and the ore (K).

Radiation: thermal radiation occurs when heat is transferred in the form of electromagnetic radiation between surfaces at different temperatures [33]. The equation for heat transfer by radiation is given by Stefan Boltzmann’s law:

$$Q=\epsilon ·\sigma ·A·\left(T_P^4-T_o^4\right)$$

(3)

where Q is the heat flux (W), ε is the surface emissivity, σ is the Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴), A is the heat transfer area (m²), Tp is the plasma temperature (K), and T_O is the ore temperature (K).

These equations describe the main heat transfer mechanisms from plasma to ore in a plasma-assisted process.

3.2. Thermodynamic Study

3.2.1. Aluminum Oxide (Al₂O₃)

The thermodynamic equilibrium of the plasma formed by the CO₂/CH₄ gas combination at the torch exit and in the presence of alumina was investigated as a function of temperature. At temperatures over 1000 °C, the CO₂/CH₄ gas mixture principally produces CO, H₂, H₂O, C₂H₂, and O₂ in the gas phase. It is important to note, however, that ion formation happens at temperatures higher than 3000 °C due to electron generation [34]. The decomposition of CH₄ aims to produce carbon ions (C⁻ and C⁺). These carbon ions are subsequently attracted and deposited on the negatively charged graphite cathode, completely compensating for the loss of cathode material during the plasma discharge process, hence increasing the cathode’s life [35].

Figure 2 illustrates the thermodynamic equilibrium composition diagrams of the chemical species produced by the plasma mixture and Al₂O₃ at varying temperatures under atmospheric pressure. As the temperature increases, the equilibrium produces CO(g) and H₂(g), whose concentrations remain constant until reaching 3000 °C. CH₄(g) degrades, resulting in a lower mass fraction. Simultaneously, the mass fractions of H₂O(g) and CO₂(g) decrease between 500 and 2000 °C after reaching their maximum. The decomposition of hydrocarbons (CH₄) produces C₂H₂(g), with its mass fraction increasing up to 2000 °C before decreasing. Additionally, the phases Al(s) and Al(liq) form at low temperatures, at approximately 800 °C, with mass fractions rapidly rising until stabilizing around 1500 °C. Similarly, Al(g) and Al₂O₃(g) are generated at around 1200 °C and 1900 °C, respectively, while aluminum ions such as Al⁻ and Al⁺ are produced at temperatures exceeding 1500 °C. Finally, the thermodynamic study of the equilibrium in the plasma mixture resulting from the interaction between the gases exiting the torch (H₂, CO, CH₄, CO₂, etc.) and Al₂O₃ indicates that the CO₂/CH₄ plasma torch may be used to reduce aluminum oxide and generate metallic aluminum.

3.2.2. Titanium Oxide (TiO₂)

Figure 3 depicts the thermodynamic equilibrium of the plasmagenic mixture at the torch exit and TiO₂ as the temperature varies under atmospheric pressure. As the temperature increases, various chemical species form. The mass fractions of CO(g) and H₂(g) remain constant, while those of CO₂(g) and H₂O(g) decrease up to 1000 °C, after which they experience a minor rise. The mass fraction of CH₄(g) decreases with temperature due to decomposition. At lower temperatures, a Ti₂O₃(s) phase forms, with its fraction increasing as the temperature rises. Additionally, a TiC(s) phase forms, increasing until it reaches a plateau at 1000 °C and then declining rapidly at 1500 °C. Ti(liq) and Ti(s) are present at low temperatures, with their fractions increasing and stabilizing at 1200 °C. Moreover, TiO₂(g), TiO(g), and Ti(g) form above 1000 °C, with their mass fractions increasing with temperature. In summary, the examination of the thermodynamic equilibrium of the plasma mixture and titanium oxide demonstrates the feasibility of reducing TiO₂ using a CO₂/CH₄ plasma torch to produce titanium metal. This investigation also highlights the advantages associated with thermal plasma torches for metallurgical applications [36].

3.2.3. Iron Oxide

a.

Magnetite (Fe₃O₄)

Figure 4 depicts the thermodynamic equilibrium of the plasmageneous mixture at the torch outlet and the magnetite (Fe₃O₄), highlighting the reaction’s production of CO(g), H₂(g), H₂O(g), and CO₂(g), with fractions that are slightly stable as a function of temperature. The concentration of CH₄(g) drops with increasing temperature, while ferrous hydroxide Fe(OH)₂(g) emerges first and increases in mass fraction as temperature rises. At around 1300 °C, oxygen O₂(g) is created, and its proportion grows as the temperature rises. Fe(s) and Fe(liq) phases are also formed, with constant mass fractions. Above 700 °C, Fe(g) and FeO(g) phases are formed, with fractions increasing with temperature. Fe- and Fe+ ions appear at temperatures over 1500 °C. This demonstrates that magnetite reduction may be achieved by utilizing CO₂/CH₄ as a plasma gas, resulting in the formation of a highly reactive gas mixture with a carbon deposit. This carbon deposit also reacts with Fe₃O₄, producing Fe, FeO, and CO(g) [37].

b.

Hematite (Fe₂O₃)

Figure 5 exhibits the thermodynamic equilibrium of the plasmageneous mixture at the torch exit and its interaction with hematite (Fe₂O₃). This reaction system highlights the production of CO(g), H₂(g), H₂O(g), and CO₂(g) because of the reaction, and these fractions show slight stability variations with temperature. As the temperature rises, the concentration of CH₄(g) decreases, while ferrous hydroxide Fe(OH)₂(g) improves prominence and experiences an increasing mass fraction. At approximately 1300 °C, oxygen O₂(g) is produced, and its percentage grows with increasing temperature. Furthermore, solid and liquid iron phases, i.e., Fe(s) and Fe(liq), are produced with similar mass fractions. Beyond 700 °C, Fe(g) and FeO(g) phases and their fractions increase with temperature. Fe- and Fe+ ions emerge at temperatures over 1500 °C. Consequently, hematite can be reduced using CO₂/CH₄ as the plasma gas, inducing the formation of a highly reactive gas mixture composed mainly of CO and H₂. Fe₂O₃ reacts with CO to generate Fe₃O₄, which is then transformed into metallic iron [38].

3.3. XRD Analysis

3.3.1. Aluminum Oxide (Al₂O₃)

As illustrated in Figure 6, the cone component is a metallic part that includes aluminum metal and aluminum oxide surrounding the cone. For this objective, the XRD analysis was concentrated on this unique area of the set-up because of the high levels of temperatures attained in this zone and the low oxygen concentration that allows the reduction of Al₂O₃.

Figure 7 shows the XRD diffractograms of aluminum oxide before and after plasma treatment. Initially, the XRD analysis of the raw material displays only alumina peaks. After plasma treatment, the raw material was reduced in the plasma reactor, leaving some alumina peaks. Furthermore, metallic aluminum peaks appear near the alumina peaks, indicating that aluminum was produced at temperatures higher than 2000 °C, mainly in the cone section [39].

As the XRD spectrum demonstrates, the application of the CO₂/CH₄ plasma torch reduced aluminum oxide Al₂O₃ to its metallic state (Al metal). After plasma treatment, the material underwent a Rietveld analysis to determine the mass proportion of each phase. The results of the Rietveld analysis of Al₂O₃ after plasma treatment are shown in

Table 2. Rietveld analysis of Al₂O₃ after plasma treatment.

Phase	Wt.% (±2)
Al2O3	45.4
Al	54.6

.

3.3.2. Titanium Oxide (TiO₂)

TiO₂ is a complicated phase, requiring high temperatures and an oxygen-depleted atmosphere to be converted into titanium metal. The conical part, which is directly exposed to the plasma arc, reaches relatively high temperatures. Figure 8 shows the metallic cone formed because of plasma treatment.

Figure 9 shows an XRD diffractogram of TiO₂ before and after plasma treatment. Diffraction peaks at 27°, 36°, and 55° in XRD patterns indicate the existence of TiO₂’s rutile phase, as proven by complete agreement with the reference spectrum [40]. However, plasma exposure resulted in the formation of other oxidized forms of rutile, such as Ti₃O₅ and Ti₅O₉. This would indicate a partial reduction in TiO₂ within the plasma reactor. Particular regions showed a further reduction, resulting in the production of titanium metal, as shown by the presence of matching peaks [41].

Following plasma treatment, several titanium oxide phases emerged, indicating that plasma treatment affected the rutile concentrate. Furthermore, the treated concentrate exhibited lower crystallinity than the original concentrate. A minor quantity of titanium was identified on the XRD spectrum, indicating the importance of the material’s exposure time to plasma treatment. Two minutes is insufficient time to reduce titanium oxide and produce further titanium metal.

Table 3. Rietveld analysis of TiO₂ after plasma treatment.

Phase	Wt.% (±2)
TiO2	8.4
Ti3O5	35.4
Ti5O9	54.8
Ti	1.4

shows the results of the Rietveld analysis, which enabled us to quantify the TiO₂ phases after plasma treatment.

3.3.3. Iron Oxide

a.

Magnetite (Fe₃O₄)

Figure 10 depicts the conical section of the reactor after magnetite plasma treatment. It can be observed that the cone’s base is made of a metallic substance from which iron metal may be formed, as well as iron oxide, which covers the whole back of the cone.

Figure 11 exhibits the X-ray diffraction pattern of magnetite, which shows six diffraction peaks corresponding to the (220), (311), (400), (422), (511), and (440) crystal planes of the magnetite phase before plasma treatment [42]. Furthermore, the magnetite’s initial XRD pattern shows a few FeO peaks. After plasma treatment, Fe₃O₄ was reduced, which produced FeO, hematite, and metallic iron. The phase change is indicated by the presence of prominent FeO peaks in the diffraction pattern. The observed presence of peaks at 45° and 65° indicates the production of metallic iron [43]. A Rietveld analysis was performed to determine the proportion of each phase after plasma treatment.

Table 4. Rietveld analysis of Fe₃O₄ after plasma treatment.

Phase	Wt.% (±2)
FeO	45.7
Fe	49.7
Fe2O3	4.3
Fe3O4	0.3

depicts the Rietveld analysis of magnetite after plasma treatment.

b.

Hematite (Fe₂O₃)

Iron metal was formed mainly in the conical part of the reactor, where the material was in direct contact with the plasma jet. Figure 12 illustrates the configuration of the material in the cone section of the reactor.

According to the XRD spectrum shown in Figure 13, the peaks associated with the raw material correspond to the hematite structure [44]. Following plasma treatment, hematite was reduced to metallic iron in the conical section of the reactor, where the partial pressure of oxygen was very low and the temperature reached over 3000 °C [45]. As in the previous example with magnetite, the observation of peaks at 45° and 65° indicates the production of metallic iron [46].

3.3.4. Oxide Mixture (Iron Oxide + Titanium Oxide)

Figure 14 depicts the XRD diffractogram of the oxide mixture. During the thermal plasma treatment of a rutile–magnetite combination in a spouted bed reactor, local production of Ti and Fe metals occurred. The solidification of the metal cone led to a noticeable separation of iron and titanium. While the purity of the iron was confirmed, the presence of oxygen and other impurities in the reactor suggests that the reaction was incomplete due to the experiment’s short duration, as indicated by the hot spots generated on the reactor’s conical section. Although pseudobrookite predominates in the oxide mixture, this implies that only a portion of the entire mixture was reduced.

The capability to reduce a combination of oxides remains a significant advantage of the CO₂/CH₄ plasma torch. The results presented below are preliminary and are still in the development stage. A new reactor design is required to extend exposure duration and achieve higher temperatures within the reactor chamber. A Rietveld analysis was employed to ascertain the proportion of each phase in the oxide mixture following plasma treatment. The findings are detailed in

Table 5. Rietveld analysis of an oxide mixture after plasma treatment.

Phase	Wt.% (±2)
Pseudobrookite (FeTi2O5)	60.2
Ilmenite (FeTiO3)	26.0
TiO2	1.0
Fe	5.6
Ti	7.2

.

3.4. SEM–EDS Analysis

Figure 15 illustrates the elemental distribution of Al₂O₃ before and after plasma treatment. After plasma treatment, aluminum metal production was seen in the reactor’s conical part, as shown by the XRD analysis. SEM–EDS analysis indicated that the elements present before plasma treatment were aluminum and oxygen, with a high concentration of the latter. However, after plasma treatment, the oxygen content decreased significantly, indicating the existence of a distinct aluminum phase.

Thus, plasma has a promising future as an efficient method for alumina reduction. However, it is critical to emphasize that further research and development efforts are necessary in this sector before this technology can be fully realized [35].

The CO₂/CH₄ plasma torch was used for reducing titanium oxide into titanium metal, producing relatively little oxygen throughout the treatment process. The total time of the treatment was also significant since the rate of reduction increased while the material remained in contact with the plasma. Treatment duration is thus critical. Figure 16 depicts the elemental composition of titanium oxide before and after plasma treatment.

Figure 17 shows the elemental distribution of iron oxide before and after treatment. After plasma treatment, the oxygen concentration was reduced because the cone part might have gotten quite hot and the iron might have melted and separated from the other components. Plasma treatment may easily produce metallic iron as long as the quantity of oxygen in the reactor is kept low.

3.5. Mass Balance

A mass balance serves as a crucial tool for determining and documenting the mass of inputs and outputs within a system, as well as any changes in these masses over time [35]. This concept is grounded in the principle of “conservation of mass”, which states that the total quantity of mass within a closed system remains constant. Maintaining careful attention to mass balance is essential for enhancing output and meeting relevant criteria [47].

Table 6. Mass balance summary.

Raw Material	M0 (g)	Reactor + Cone Part	Filter Part	Recovery (%)
Al2O3	300	285	6	97
TiO2	300	276	2	92
Fe2O3	300	250	6	85
Fe3O4	300	253	4	86
TiO2 + Fe3O4	300	230	8.5	80

provides a summary of the preliminary mass balance findings from the research.

4. Conclusions

In various sectors, the quest for tools that combine high efficiency with low cost has become crucial. Plasma technology stands out as one such tool, developed for a multitude of applications, particularly in material processing. This study took an innovative approach, using a new DC plasma torch for reducing oxides such as alumina, iron oxide, and titanium oxide. The fundamental aim of this research is to demonstrate the feasibility of this oxide reduction method using a plasma torch powered by greenhouse gases such as CO₂ and CH₄. This study stands out for its innovative exploration of the use of this specific type of plasma torch for the reduction of specific oxides, an approach that had never been attempted before. The conical portion of the plasma reactor proved critical in preventing overheating, thus reducing processing time. This innovative approach successfully reduced a wide range of oxides, including alumina, hematite, magnetite, and rutile, and their mixtures. The thermodynamic investigation confirmed the production of many chemical species, indicating that pure metals might be produced utilizing the CO₂/CH₄ mixture as a plasmagenic gas. XRD and SEM–EDS studies were used to investigate the reduction of alumina, hematite, magnetite, and rutile. The results show that in the reactor’s conical section, titanium oxide is partially reduced, and a small amount of titanium is produced, while aluminum oxide and iron oxide (magnetite and hematite) are reduced to produce aluminum and iron metal, respectively. The significant variation in oxygen levels before and after treatment confirms the XRD studies, achieved by using the plasma torch’s power as a successful tool for oxide reduction. The oxide mixtures also showed encouraging results, allowing the separation of pure metals such as iron and titanium. Regarding plasma composition, a detailed analysis utilizing mass spectrometry and thermodynamic calculations showed that CH₄ and CO₂ decompose into H₂ and CO. This process paves the way for an innovative procedure that promises to significantly reduce the environmental footprint associated with the thermal treatment of oxides. Using these greenhouse gases for reduction avoids generating new emissions, thereby mitigating adverse environmental impacts. Despite these hopeful findings, further research and development in this area are needed before this technology can realize its full potential.