The Use of Coniferous Tree Cone Biomass as an Energy Source and a Reducing Agent in the Recycling of Metals from Oxide Secondary Raw Materials

Abstract

The challenges faced by the metallurgical industry implicate that actions aimed at reducing negative impacts on the environment are becoming extremely important. This is justified both in the search for economically competitive methods of producing basic construction materials, consistent with the circular economy policy, and in improving the efficiency of metal production technology. An essential aspect of biomass use is the introduction of an energy source that naturally reduces the energy supplied to the reactor, thereby reducing the carbon footprint of the metal produced. In this case, the research undertaken aims to determine the possibility of using a bioreductant that will allow for the reduction or elimination of the fossil raw material, which is coal, thus reducing the costs associated with ETS and ETS II (European Union Emissions Trading System). This paper presents the results of research on the reduction process of oxide metal-bearing raw material, the chemical composition of which is similar to slags from the copper industry. The effects of slag reduction time on the degrees of copper and lead removal were examined. The process was carried out at 1300 °C, with the constant addition of a reducing agent, in the form of crushed pine cones. After processing for 1 h, the copper content in the waste slag was 1.30 wt%, whereas extending the process to 5 h reduced the copper content to 0.15 wt%. For lead, at the exact reduction times, the element’s contents in the slag after processing were 1.92 wt% and 0.79 wt%, respectively. The results of the studied process showed that, in the first stage of the slag reduction process, intensive reduction of copper and lead oxides occurs. Research was also conducted to characterize the biomaterial during the high-temperature process. Results show high degrees of removal for basic metals at the following levels: 99% for Cu and 72% for Pb. The waste slag is characterized by low metal content, which allows for safe storage or use in other sectors of the economy. This type of biomaterial is, therefore, recommended for research in large-scale laboratories or on a semi-industrial scale, particularly in relation to the gas phase formed and its possible impacts on the structural elements of industrial installations. It should be noted that there is a lack of data in the literature on the use of forest biomass in the form of pine cones as an alternative to coke as a reducing agent for use in pyrometallurgical processes.

1. Introduction

The growing interest in biomass as a potential biofuel for use in energy processes, observed in recent years, results from the need to mitigate the adverse effects of climate change, as well as the related need to reduce the use of solid fossil fuels. It is assumed that, in recent years, about 60% of the total renewable energy in EU countries came from biomass (mainly forest and agricultural). This applies to energy used in both the professional energy sector and the power industry [1]. In the case of the possibility of using forest biomass for energy purposes, problems are encountered in two areas: The first is the logistics associated with delivering biomass to processing plants, and the second is the need to pre-densify small fractions of this type of biomass, e.g., wood chips, by pelletizing or briquetting [2,3]. The more the raw wood biomass is processed, on the one hand, the higher the purchase costs. On the other hand, due to the higher unit energy value of the pellet or briquette produced, the EOQ (economic order quantity) indicator is reduced.

One of the naturally occurring types of forest biomass that can be a potential source of renewable energy is pine cones. Cones after so-called forest operations, used for the production of seedlings and afforestation, are used as biomass—they can be burned as heating fuel or processed into pellets, briquettes, or compost, where they decompose slowly, but enrich the soil with organic matter. Disposal by combustion should be assessed negatively, as, on the one hand, it contributes directly to the emission of greenhouse gases or VOCs. On the other hand, due to their low mass and rapid combustion, cones do not provide a stable source of heat, like coal or wood. However, due to the high content of lignocellulose, pine cones can be successfully used in the processes of producing various forms of bioenergy [4,5,6,7,8]. For many years, research has been conducted on the use of cones for the production of activated carbon, using various activating agents, as well as for the production of liquid biofuels [9,10,11,12,13,14,15].

Other directions of utilization for this type of biomass include its use as a natural coagulant in the process of industrial wastewater treatment, or as a raw material for obtaining composite foils based on lignocellulose and nanocellulose [16,17,18,19,20,21,22].

Biomass has also been studied for many years in terms of using its chemical properties as an alternative to coke in pyrometallurgical processes for obtaining metals from both primary and secondary raw materials [23,24,25,26,27,28,29,30,31,32,33]. For most of the analyzed cases, biomass acts as a reducing agent. This is due to the fact that cellulose, hemicellulose, and lignin, which are the main components of biomass, release reducing gases, such as CO, H₂, and CH₄, during thermal decomposition, as follows [34]:

$$\mathrm{M}\mathrm{e}\mathrm{O}+\mathrm{C}\mathrm{O}=\mathrm{M}\mathrm{e}+\mathrm{C}{\mathrm{O}}_2$$

(R1)

$$\mathrm{M}\mathrm{e}\mathrm{O}+{\mathrm{H}}_2=\mathrm{M}\mathrm{e}+{\mathrm{H}}_2\mathrm{O}$$

(R2)

$$4\mathrm{M}\mathrm{e}\mathrm{O}+\mathrm{C}{\mathrm{H}}_4=4\mathrm{M}\mathrm{e}+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}$$

(R3)

The research results obtained so far on the use of biomass, in the metal recycling process, as a reducing agent show its huge potential in this area. In industrial technologies, the basic solid reducing agent used in the processes of reducing oxide metal-bearing raw materials (e.g., copper slag) is coke. Data from the literature indicate that research was also conducted on the use of other materials in the types of processes that allow for replacing the standard reducing agent, namely anthracite, charcoal, and fine-grained coal-bearing waste generated during the enrichment of hard coal [35,36,37,38]. The literature data also present the results of research on the possibility of replacing coke breeze with various types of food biomass, e.g., waste oil, sunflower husks, and nut husks [39,40,41,42,43,44,45,46,47].

This paper presents the properties and potential possibilities of using forest biomass in the form of pine cones. This resource has promising prospects to support a closed-loop economy. An essential aspect of biomass use is the introduction of an energy source that naturally reduces the energy supplied to the reactor, thereby reducing the carbon footprint of the metal produced and improving the efficiency of metal production technology. The area of usefulness for this type of biomass was examined for the copper recycling process based on the reductive melting process. The chemical composition of the sample was similar to slag originating from copper production. The aim of this research was not only to recover valuable metals from this material, mainly copper, confirming the reduction potential of the tested biomass, but also to obtain secondary waste slag that can be safely managed, i.e., stored or reused.

2. Materials and Methods

During the high-temperature process of reduction and separation of metal, either from primary or secondary raw materials, a number of key phenomena occur that support the thermodynamic reactions and affect the kinetics of their course. Based on the analysis of these factors, it is possible to determine prospects for the use of metal-bearing raw materials and raw materials themselves, as they play important roles from the point of view of the course of the reduction reaction. On this basis, the main elements of the research program were formulated in the form of the following three basic stages:

• Characterization of the energetic characteristics, i.e., the determination of the heat of combustion of pine cones in calorimetric tests;
• Characterization of physical changes and their courses under high-temperature conditions, i.e., conducting thermogravimetric analysis (TGA) in both inert and oxidizing atmospheres;
• Verification of process suitability by performing reductive melting of metallurgical slags using pine cone biomass as a reductor.

2.1. Research Material

Biomass in the form of pine cones was used for the tests. Its chemical composition comprised the following: C—47.57% by mass, S—0.02% by mass, Cl—0.02% by mass, N—0.76% by mass, H—6.80% by mass, and O—40.11% by mass. The content of the basic components of this biomass was determined using the ELTRA CHS HELIOS (Haan, Germany) analyzer. The sample combustion temperature was 1400 °C (2550 F). The results are the average values of five measurements of the tested material.

For the high-temperature reduction tests, a batch of slag with a chemical composition similar to that of the materials produced in the copper industry was used. The chemical composition of the slag comprised the following: SiO₂—34.50% by mass, CaO—14.12% by mass, Fe—11.11% by mass, Cu—10.30% by mass, and Pb—2.25% by mass.

The qualitative analyses of the phase compositions of the samples were performed based on the interpretation of the diffraction pattern obtained using a Seifert-FPM XRD7 (Freiberg, Germany) X-ray diffractometer. The characteristic Cu Kα X-ray radiation and the Ni filter were used. The analysis was carried out in the 2θ angle range from 10° to 100°, which corresponds to the interplanar distance range dhkl from 0.88455 to 0.10064 nm. The compounds present in the sample were identified based on the Seifert and Match software (version 3) and the ICDD PDF-4+ catalog data from 2021. The following phases, containing basic metals, were identified in the tested sample, i.e., copper, lead, and iron:

• Fe_1.966O_2.963—tetragonal maghemite;
• Cu₂O—regular crystal-structured cuprite;
• Al₂PbSi₂O₈ of monoclinic structure;
• PbO—lead (II) oxide orthorhombic structure (massicot).

2.2. Method

2.2.1. Heat of Combustion

One of the basic energy parameters of fuel is the heat of combustion. It is defined as the heat released during the most complete possible oxidation of a given fuel, i.e., during complete and total combustion, whereby the products are brought to the initial temperature. The determination of the heat of combustion value was performed in accordance with the [48] standard. The test consists of placing a sample of the material in a calorimetric bomb, after previously determining the mass of the sample and the mass of the igniting wire. Oxygen is injected into the container to achieve complete and total combustion of the sample (Figure 1). Since the container is placed in a double water jacket, it is possible to accurately determine the heat of combustion, thus taking into account the heat from the condensation of water vapor generated during the combustion process. The sample is ignited by an electrical system, allowing electric current to flow through the igniting wire. The heat of combustion value is calculated from the following relationship:

$$\mathrm{G}\mathrm{C}\mathrm{V}={\displaystyle \frac{E\cdot \left(T_m-T_i+C\right)-b_i}{m}}$$

(1)

where GCV—gross calorific value, MJ∙kg⁻¹; E—water equivalent of the calorimeter, bomb, and the equipment and water introduced into the bomb, read from the standard ISO 1716, MJ∙K⁻¹; T_m—maximum temperature, °C; T_i—initial temperature, °C; C—temperature correction factor, taking into account the heat exchange with the environment (here: C = 0.0), °C; and b_i—correction factor for the heat of combustion of the ignition wire, MJ, which is the product of the heat of combustion of the wire b (MJ∙kg⁻¹) and its mass m_d, kg; m—mass of the test sample, kg.

Table 1. Combustion heat of fruit stone biomass.

Sample	${\mathit{Q}}_{\mathit{s}\mathit{p}},\mathbf{k}\mathbf{J}/\mathbf{k}\mathbf{g}$	${\mathit{Q}}_{\mathit{ś}\mathit{r}},\mathbf{k}\mathbf{J}/\mathbf{k}\mathbf{g}$	Comments
Pine cones	18,130	19,453	Own research
	20,400
	19,830
Pine cones	-	19,400	[49]
Cherry stones	-	19,540–22,020	[50,51,52]
Coke breeze	-	29,760–32,460	[53]

summarizes the determined calorific values for the tested biomass in the form of cones, for comparison with the literature values, as well as for cherry stones and coke breeze.

Knowing the process parameters in the form of reagent mass, and the specific heat value of biomass, the process temperature at which a given reaction can be exothermic, it is possible to calculate the equivalent of the necessary amount of biomass to heat the feedstock and run the process as an equivalent of a non-renewable energy source.

2.2.2. Thermogravimetric Analysis (TGA)

TG analysis is one of the methods that allows for the characterization of materials subjected to high temperatures. Capturing changes related to mass loss, determining the temperature ranges at which these changes occur and the dynamics of mass changes, allows for the prediction of the behavior of the material under given conditions depending on its intended use. In this case, TGA was performed on a material in the form of biomass, whose organic components are particularly sensitive to temperature. Considering that, in pyrometallurgical processes, the impact of high temperature is significant, it is necessary to characterize the sample so that it fulfills the intended role in the process. In addition, gases released during the thermal decomposition of biomass can also be an important reagent affecting the efficiency of the implemented processes. The thermogravimetric test was carried out in accordance with the current state of knowledge. Since there is no technical standard for testing biomass, the tests were carried out in accordance with the assumptions of the [54] standard dedicated to plastics. This method is coherent with the requirements of the [55] standard.

A device manufactured by TA Instruments (New Castle, DE, USA), marked with the symbol TGA Q500, was used for the test. The test was carried out by preparing a representative sample of biomass. Then, a sample weighing approximately 22 mg was placed on a measuring pan. The tests were carried out in an oxidizing atmosphere (in air) and in an inert atmosphere (in nitrogen). High-purity nitrogen, marked with the symbol N5.0, was used. It was subjected to a quantitative analysis of the contents of water vapor, oxygen, hydrocarbons, carbon monoxide, carbon dioxide, and molecular hydrogen. The following results were obtained:${\sigma }_{{\mathrm{H}}_2\mathrm{O}}=0.9 \mathrm{p}\mathrm{p}\mathrm{m}, {\sigma }_{{\mathrm{O}}_2}=0.8 \mathrm{p}\mathrm{p}\mathrm{m}, {\sigma }_{{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}}<0.1 \mathrm{p}\mathrm{p}\mathrm{m}, {\sigma }_{\mathrm{C}\mathrm{O}}<0.1 \mathrm{p}\mathrm{p}\mathrm{m}, {\sigma }_{\mathrm{C}{\mathrm{O}}_2}<1 \mathrm{p}\mathrm{p}\mathrm{m}, \mathrm{a}\mathrm{n}\mathrm{d} {\sigma }_{{\mathrm{H}}_2}<1 \mathrm{p}\mathrm{p}\mathrm{m}$. According to the device’s operating instructions, the sample mass measurement error is 0.01%, and the measurement resolution is 0.1 μg. The temperature measurement error is 1 K. The tests were carried out for a biomass sample heating rate of 100 K/min.

2.2.3. Reductive Melting Process

Reduction melting of slags was carried out in a PT-40 pit resistance furnace by Czylok (Jastrzebie, Poland), which allowed the process to be carried out at a temperature of 1350 °C. This unit was adapted to safely load the crucible with the charge and, at the same time, allowed for its rapid heating. The variable parameter in this research was the duration of the process (1–5 h). The duration of the process was selected based on the actual duration of operations conducted under industrial conditions. In this research, the target temperature for carrying out the experiments was assumed to be 1300 °C, as this is the temperature used for the industrial conditions of copper smelters. The mass of the slag sample assumed in this study was 80 g. Based on the chemical composition of the slag, the amount of elemental carbon needed for the reduction reactions of copper and lead oxides contained in the slag was estimated, at about 3 g. In practice, however, the biomass weight used in this study was 18 g, which included the necessary excess carbon for the Boudouard reaction, thus allowing for the creation of a reduction atmosphere in the working chamber of the device. After the experiment was completed, the mass of the molten metal and the mass of the secondary slag were determined for each sample. The slag was subjected to chemical analysis for contents of copper, lead, and iron. The analysis of the chemical composition of the slag was carried out using a Primus II X-ray fluorescence spectrometer from Rigaku (Tokyo, Japan). The measurement procedure was based on the [56] standard. Additionally, some of the post-process (secondary) slags were subjected to morphology tests, which were carried out using a Hitachi S-3400N (Tokyo, Japan) scanning electron microscope equipped with a Thermo Noran EDS (Madison, WI, USA) energy dispersive X-ray spectrometer. The imaged structures were made using a BSE backscattered electron detector.

3. Results and Discussion

3.1. Calorimetry

The attempt to test the possibility of using biomass as a reducing agent in pyrometallurgical processes requires the same evaluation criteria as in the evaluation of traditional solid fuels. Therefore, it was considered necessary to determine the value of the combustion heat for the tested biomass, and to determine the chemical composition, taking into account the contents of moisture, ash, and sulfur.

Table 2. Ultimate analysis results of chosen biomass types.

Biomass Type	Content, % by Mass					Source
	C	H	S	O	N
Pine cones	47.57	6.80	0.02	40.11	0.76	Own research
Pine cones	45–55	5.3–6.8	<0.25	38–45	0.1–0.94	[57,58,59,60]
Spruce cones	46–54	5.1–5.7	<0.03	37–41	0.37–0.8
Larch cones	47–52	5.2–5.8	<0.02	37–40	0.24–0.8
Fir cones	54.26	6.3	0.48	35.89	0.37

summarizes the chemical compositions of the tested samples of pine cone biomass and, comparatively, cones of other coniferous trees.

The data in the table show that the contents of chemical elements, i.e., hydrogen, carbon, and oxygen, in pine cones is comparable to the chemical compositions of other types of cones. The data presented in the table are characterized by low sulfur content, which is very beneficial from the point of view of using cones as fuel. Cones, in comparison to wood biomass, have similar carbon content and increased ash content. In the case of pine cones, in the batch of biomass used in this study, the ash content was 1.92% by mass, and, in the case of coniferous wood, the ash content is at the level of 0.5 to 2% by mass.

3.2. Results of Thermogravimetric Analysis

Thermogravimetric analysis of the biomass samples revealed a transient moisture content of 16–17% by mass, which is visible at the beginning of the TG curves presented in Figure 2 (inert atmosphere) and Figure 3 (oxidizing atmosphere). There are local maxima visible at the corresponding DTG curves.

In the oxidizing atmosphere, i.e., during thermal decomposition of the samples, two stages could be distinguished. The first one, occurring in the range of 260–340 °C, with a maximum DTG value at 325 °C, corresponds to the decomposition of hemicellulose and cellulose. In the second stage of thermal decomposition, i.e., in the range of 440–475 °C, with the maximum intensity of sample mass loss in time recorded at 455°, the decomposition of another polymer of natural origin, i.e., lignin, occurs. It should be emphasized that the second phase of thermal decomposition may also be associated with the ignition of volatile combustion products and the oxidation of the produced coke. Hence, there is no second phase of pyrolysis in an inert atmosphere.

Lignin is a material that undergoes thermal decomposition in a much wider temperature range than cellulose and hemicellulose. Therefore, it can be assumed that all three polymers mentioned are decomposed during the only phase of pyrolysis, i.e., in the range of 250–375 °C (in an inert atmosphere). At the same time, the oxidation of the generated volatile products of thermal decomposition of the samples is the cause of the loss of 50% of the sample mass by 10 K, earlier in relation to the same parameter determined for the inert atmosphere. At a temperature above 490 °C, the oxidation of the biomass sample, the mass of which does not change significantly until the end of the test, ends.

The mineral content of the biomass sample from the cones is, therefore, below 12% by mass. In turn, above approx. 380 °C, in the case of an inert atmosphere, a slow loss of sample mass is observed, attributed to probable slow coking of the organic residue in the solid phase, which, due to the lack of oxygen availability, did not undergo flame combustion in the earlier phase of this study. The obtained results are summarized in

Table 3. TGA results for pine cone biomass in both inert and oxidizing atmospheres.

Parameter	Inert Atmosphere	Oxidizing Atmosphere
Beginning of thermal decomposition, °C	266	293
50% mass loss temperature, °C	350	340
1st phase of pyrolysis/thermal decomposition, °C	250–375	260–340
Maximum DTG value in the 1st phase of pyrolysis/thermal decomposition (@ temperature), %/min @°C	8.6 @355	8.3 @325
2nd phase of pyrolysis/thermal decomposition, °C	-	440–475
Maximum DTG value in the 2nd phase of pyrolysis/thermal decomposition (@ temperature), %/min @°C	-	4.2 @455
Ash content, %mass	17.45	11.73

.

3.3. Results of Reductive Melting Process

As previously mentioned, cellulose, hemicellulose, and lignin, which are the main components of biomass, including cones, release gases during pyrolysis that act as reducing agents in the discussed process. These gases include CO, H₂, and CH₄ [34]. In order to assess the possibility of the reduction reactions of copper and lead oxides contained in the tested batch of slag [61,62,63], the values of the free enthalpy changes for example reactions were estimated, as follows:

$$\mathrm{C}\mathrm{u}\mathrm{O}+\mathrm{C}\mathrm{O}=\mathrm{C}\mathrm{u}+\mathrm{C}{\mathrm{O}}_2$$

(R4)

$$\mathrm{C}\mathrm{u}\mathrm{O}+{\mathrm{H}}_2=\mathrm{C}\mathrm{u}+{\mathrm{H}}_2\mathrm{O}$$

(R5)

$$4\mathrm{C}\mathrm{u}\mathrm{O}+\mathrm{C}{\mathrm{H}}_4=4\mathrm{C}\mathrm{u}+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}$$

(R6)

$$\mathrm{P}\mathrm{b}\mathrm{O}+\mathrm{C}\mathrm{O}=\mathrm{P}\mathrm{b}+\mathrm{C}{\mathrm{O}}_2$$

(R7)

$$\mathrm{P}\mathrm{b}\mathrm{O}+{\mathrm{H}}_2=\mathrm{P}\mathrm{b}+{\mathrm{H}}_2\mathrm{O}$$

(R8)

$$4\mathrm{P}\mathrm{b}\mathrm{O}+\mathrm{C}{\mathrm{H}}_4=4\mathrm{P}\mathrm{b}+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}$$

(R9)

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+3\mathrm{C}\mathrm{O}=2\mathrm{F}\mathrm{e}+3\mathrm{C}{\mathrm{O}}_2$$

(R10)

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2=2\mathrm{F}\mathrm{e}+3{\mathrm{H}}_2\mathrm{O}$$

(R11)

$$4\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+3\mathrm{C}{\mathrm{H}}_4=8\mathrm{F}\mathrm{e}+3\mathrm{C}{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}$$

(R12)

The obtained values of the Gibbs free energy change ∆G_T indicating the direction of the reaction, and the values of enthalpy as a thermal effect during the reaction at 1300 °C, are presented in

Table 4. Gibbs free energy ΔG_T and enthalpy ΔH_T at the temperature of 1300 °C for given reduction reactions.

No.	Reaction Number	$∆{\mathit{G}}_{\mathit{T}}$, kJ/mol	ΔHT, kJ/mol
1	(R4)	−126.70	−170.97
2	(R5)	−140.89	−141.32
3	(R6)	−720.69	−370.29
4	(R7)	−72.41	−97.16
5	(R8)	−86.60	−67.50
6	(R9)	−503.52	−75.05
7	(R10)	−21.64	−36.63
8	(R11)	−64.21	52.34
9	(R12)	−728.21	794.26

. The calculations were made using the thermodynamic basis HSC 6.1 [64]

The results of laboratory tests of copper slag reduction using pine cones as a reductant are presented in

Table 5. Summary of laboratory test results of the reduction process, using a bioreductant in the form of pine cone biomass, conducted at a temperature of 1300 °C.

Test no.	Reduction Time, h	Metal Mass, g	Slag Mass, g	Metal Content in Secondary Slag
				Cu	Pb	Fe
1	1	9.84	66.2	1.30	1.92	12.50
2	2	11.04	64.1	0.72	1.11	12.07
3	3	11.75	63.9	0.33	0.91	13.36
4	4	12.57	62.1	0.37	0.86	12.19
5	5	13.05	62.2	0.15	0.79	11.77

. The graphical interpretation of these results is presented in Figure 4, Figure 5 and Figure 6.

The basic parameters determining the efficiency of this process were the degrees of slag decoppering and deleading, determined by the following relationship:

$$S_{Me(Cu,Pb)}={\displaystyle \frac{{m_p·C}_{Me}^0-m_k·C_{Me}^k}{{m_p·C}_{Me}^0}}·100\mathrm{\%}$$

(2)

where $C_{Me (Cu, Pb)}^0$—initial concentrations of copper and lead in primary slag, respectively, % mass; $C_{Me (Cu, Pb)}^k$—final concentrations of copper and lead in primary slag, respectively, % mass; $m_p$—initial slag mass, g; $m_k$—final slag mass (after reduction), g.

To illustrate the microstructure of the secondary slag, microscopic photos were taken, which are shown in Figure 7.

4. Conclusions

Analysis of the results of the reductive slag melting at a temperature of 1300 °C with the addition of a bioreductant in the form of pine cones, presented in Table 5, indicates the possibility of significant reductions in the contents of both copper and lead metals in the waste slag generated after the process. In the case of copper, after 1 h of the reduction process, slag containing less than 1.3% by mass was obtained, and after 5 h, the content was further reduced to less than 0.2% by mass. In the case of the mass share of lead in the slag, for identical times of the reduction process (respectively: 1 and 5 h), the content of this metal decreased in the slag to 1.9% by mass and 0.79% by mass, respectively. During the tests, no significant degree of iron removal from the slag was noted by reducing its oxides. The Fe content in the waste slag for all experiments ranged from 11.77% by mass to 13.3% by mass.

In the copper industry, when the copper slag reduction process of a similar chemical composition is carried out in an electric furnace with the addition of coke breeze, the copper content in the secondary slag is at the level of 0.4–0.6% by mass. The content at this level allows for safe deposition of this material or use in another branch of the economy.

The results of the studied process showed that, in the first stage of the slag reduction process, intensive reduction of copper and lead oxides occurs. This is accompanied by the formation of a metallic alloy, containing mainly these two metals. At the same time, a temporary increase in the iron content in the secondary slag was observed. The analysis of the secondary slag microstructure showed the presence of numerous metallic inclusions (Figure 7). Unfortunately, this is an unfavorable phenomenon, which significantly reduces the efficiency of the process of smelting the metallic alloy, which is the main product in the analyzed process. The available literature data indicate that, in secondary post-reduction slags obtained under industrial conditions, metallic copper may occur as metallic inclusions, with sizes of up to 100 µm. The rate of settling of metal droplets suspended in the slag depends mainly on their diameter, as well as the viscosity and density of the liquid slag. It has also been shown that half of the copper present in the secondary (post-process) slag is in the oxide form, and half is in the form of suspended small metallic copper droplets. The increased efficiency of separation of metallic droplets from the slag can be achieved, among other methods, by increasing their volume as a result of their coagulation. This can be achieved, for example, by intensive mixing, increasing the frequency of collisions of suspended metal droplets. Under industrial conditions, this effect is achieved by obtaining turbulent movement of liquid slag through the addition of calcium carbonate. The gases generated as a result of its thermal decomposition intensify the mixing process.

Considering the energy properties of biomass, including its designated combustion heat, it can be assumed that this raw material will allow for a reduction in the consumption of fossil fuels, such as natural gas and other non-renewable energy sources, contributing to a reduction in the product’s carbon footprint.

If the reduction process produces slag containing less than 0.5% by mass of Cu and 0.6% by mass of Pb, it is not classified as hazardous waste and, for example, in the form of granulate, it can be used to produce aggregates used in civil engineering, e.g., construction. The results obtained allow the consideration of pine cone biomass for further testing on a larger scale, e.g., semi-technical. The results of these studies will enable application to a full-scale industrial process.