A Study on the Potential for the Application of Peanut Shells as a Reducer in the Process of Metal Recovery from Metallurgical Slags

Abstract

Copper production technology is a complex process consisting of many stages. The combination of pyrometallurgical and hydrometallurgical stages, on the one hand, complicates production while, on the other hand, allowing for a relatively selective separation of intermediate or waste materials that can be subjected to the process of recovery of useful components. Materials of this type are characterised by a much higher copper content relative to the ore material. On the other hand, due to the oxide form, reduction processes are used in which coke is mainly applied. Reduction of the unfavourable phenomenon of CO₂ emissions, in this case, can be performed through the use of bioreducers, which are characterised by an inert carbon footprint since the generation of carbon dioxides is the same as its absorption at the stage of vegetation and growth. In this paper, the topic of determining the feasibility of using selected bioreducers, such as peanut shells, to verify their suitability in the process of reducing copper oxides as well as the impact on the working components of the laboratory reactor in which the process is carried out are discussed. In this case, raw materials with a composition similar to the that of slags produced at the copper production stage in a flash furnace were tested for reduction. The results referring to reducing lead and copper contents above 88% Pb and 98% Cu indicate the great potential of this type of bioreducer. An additional advantage is the relatively wide availability of peanut resources. The effects of the copper reduction time on the degree of decopperisation performed with a constant reducer addition at 1300 °C were studied in this paper. Following 1 h of the process, the copper content in the slag was 0.78 wt%, while the longer process duration resulted in a copper fraction of 0.19 wt%. Considering lead, its content was 0.33 wt% after the reduction process.

1. Introduction

The first decades of the twenty-first century have seen a huge increase in technical and technological potential in the field of electromobility. Car corporations are looking for diverse solutions at optimal costs to allow them to find the widest possible market, allowing them to maintain the trend of production growth. On the other hand, attention should be paid to the increasing pressure in the area of environmental impact, especially in the area of consumption of natural resources and emissions. The created principles of the closed-loop economy, emphasizing the reduction of the exploitation of natural raw materials and replacing them with renewable energy raw materials, reducing the carbon footprint, and secondary raw materials, which after the period of their exploitation are put to reuse, strongly fit into this theme. In the group of secondary raw materials, one can distinguish materials that are created as waste in the production of primary products, and sometimes the content of useful elements exceeds their natural content, for example, in ores. In the case of technologies used for the production of metals, reducing agents are used to carry out the process of taking away oxygen. Taking into account the impact of carbon dioxide on the changing climate and restrictions in the form of fees for the emission of this gas into the atmosphere, the owners of the technology using the reducing process will, in the near future, be forced to look for technological solutions that allow production on competitive terms with producers from regions of the world not subject to such environmental emission costs. Taking into account that biomaterials/bioreducers have a neutral character in terms of CO₂ emissions, it can be predicted that in the nearer future, metal producers in the European Union area, in the first place, will look for such solutions. The applicable ETS fees will steadily increase, and it is estimated that in 2050 the fee will be around USD 150/ton [1].

The presented subject matter of the article fits into the principles of the closed-loop economy as defined in the EU by reducing the consumption of natural raw materials and using renewable auxiliary materials. However, this is such a difficult topic that it is necessary, first of all, to present to manufacturers the potential use of such materials as well as their impact on the process and components of production equipment. Changing the components used in pyrometallurgical processes results in the production of compounds that are not present in standard processes. On the one hand, this can affect the productivity/efficiency of the process; on the other hand, it can affect the degradation of working components. A lack of information on this subject may limit openness to such changes, and given the lengthy investment and contractual time involved, the entry into force of environmental restrictions will result in losses in the branch of the economy responsible for supplying high-quality and environmentally neutral metals. Sustainable production in metallurgical technology is a necessity that determines its prospects in terms of development and competition with products from outside the European Union.

The apparent depletion of fossil fuel sources and the environmental policy adopted by European Union countries have resulted in the implementation of a new approach to a wider application of renewable energy sources in these countries for the purposes of industrial processes [2,3,4]. The processing of such natural resources as coal, oil or natural gas is associated with gas emissions (mainly CO₂), which is an adverse effect due to constantly more demanding climate targets. Many activities are conducted with the aim of applying net-zero resources in the energy industry or technological processes involved in various industries to limit these emissions or even eliminate them [5,6,7,8,9,10]. These raw materials include biomass perceived as a substitute for fossil fuels. The essential biomass advantage is its photosynthesis-based growth, where necessary carbohydrates are sourced through absorption of CO₂ and H₂O from the atmosphere. It is assumed that the amounts of CO₂ emitted during biomass combustion are equivalent to those used during its generation [11,12]. Other benefits of the biomass are its wide availability and relatively low price.

One of the potential methods of biomass utilisation is its application as a coke substitute in the pyrometallurgical process aimed at metal production. This refers both to secondary raw material processing (recycling of various kinds of technological waste) and to processes of metal production and refining based on natural resources. Basically, these processes involve the use of coke, which mostly functions as an energy source or, indirectly, as a chemical agent, i.e., a reducer [13]. A special issue is the process of copper production, where the products are refined copper and slag, characterised by relatively high contents of useful chemical elements. The slag is subjected to decopperisation using a dedicated device, i.e., an electric arc furnace. The process is based on reduction smelting where the coke addition ensures (the Boudouard reaction) the generation of gaseous CO₂, which is a substrate for the intermediate reduction. The reduction reaction also involves elemental carbon, but its importance is far less considerable. The proposed implementation of biomass as an alternative reducer is associated with many unknown issues basically related to the process’s effectiveness and the resulting reaction products. Regarding high amounts of volatile components in the biomass materials, it will be important to define the stages which determine reactions between the substrates and to identify the products and their interactions also in terms of generation of gases which are emitted into the atmosphere.

This paper presents the comprehensive results of the research on the potential application of peanut shell biomass in the reduction process based on the example of metal reduction using copper slag with the chemical composition characteristic of a one-stage process of copper production. The process products are blister copper containing approx. 98% Cu and slag with 12 wt% to 15 wt% Cu. The literature data show that copper in this type of material may be present in the forms of copper oxide (I), copper cations (Cu⁺) and small metal inclusions [14]. The form of copper in the slag is highly affected by the oxygen potential, which depends on the chemical composition of the slag and its resulting structure. Slag decopperisation under industrial conditions is conducted in an electric arc furnace in a reducing atmosphere using coke and calcium carbonate (CaCO₃). The course of reactions in the process of copper slag reduction smelting may be as follows:

Cu₂O + C = 2Cu + CO

(1)

2Cu₂O + C = 4Cu + CO₂

(2)

Cu₂O + CO = 2Cu + CO₂

(3)

CaCO₃ = CaO + CO₂

(4)

C + CO₂ = 2CO

(5)

The main reduction product is a metal alloy in the form of spheroid droplets suspended in the liquid slag. Due to its higher specific gravity, the metal separates from the slag and accumulates in the furnace hearth. When the metal precipitates are very small, they float on the liquid slag surface, which is an unfavourable effect that inhibits their separation [15]. A method used to limit this effect is intense metal stirring to ensure coagulation of the particles and better sedimentation of the metal droplets. Under industrial conditions, this is achieved by, e.g., the addition of calcium carbonate (CaCO₃) to the decopperisation process. The size of copper loss in this process depends on many factors. The most important issues are the temperature and physicochemical properties of the slag (its viscosity and surface tension) [16].

Peanuts (also called groundnuts or monkey nuts), which were applied in this research, are one of the most popular nuts worldwide. However, this term is misleading, as they are not actually nuts but the seeds of a legume from the family Fabaceae. Peanuts are characterised by a high content of oil, so they also belong to the group of oil plants. They are commonly used in the food industry. Their chemical composition is typical of legumes, which mainly contain proteins, amino acids, sucrose, starch, saturated fatty acids and vitamins from the B and E groups. Peanuts constitute a rich source of antioxidants and fibre. Moreover, they contain potassium, phosphorus and magnesium [17].

While peanuts are processed for food production purposes, their shells (constituting over 20% of the legume dry weight) are removed. It is estimated that more than 11 million tons of this biomass type are generated worldwide yearly [18]. The available literature data regarding the chemical composition of peanut shells are summarised in

Table 1. The contents of basic peanut shell components (literature data) [8].

Component	Carbon	Oxygen	Hydrogen	Nitrogen	Sulphur	Ash
Fraction, wt%	46.41–46.43	39.32–44.22	6.59–6.62	0.49–0.51	0.53–0.55	4.11–4.41

.

Due to the relatively low biodegradability of the shells (waste), research into their wider economic use has been conducted for many years to avoid direct combustion or storage in landfills. The poor level of shell biodegradability results from high lignin contents. Currently, this type of biomass is mostly applied as an addition to garden mulches or peat-based substrates [19]. Peanut shells are also used as the components of livestock feed or as a fibre dietary supplement [20,21,22], in bioethanol production [23,24,25,26,27,28,29,30,31,32,33,34] and in the pulp and paper industry to produce cellulose [35,36,37]. Moreover, there are literature data containing the results of research on the potential of peanut shell application as a fuel in domestic boilers [38,39], for the production of nanofibres [40] as well as in the furniture and construction industries [18,41,42,43,44].

In this work, research was undertaken to determine the suitability of biomass in the form of peanut shells in industrial metal extraction processes. Positive results will not only allow the application work of the implementation of the solution to continue, but they will also allow extractive metallurgy processes to be included in a circular economy with minimization of the carbon footprint of the produced products.

2. Research Section

To determine the potential for the effective application of peanut shells in the process of metal oxide reduction, a research programme was designed including a thermogravimetric analysis of the bioreducer in inert and oxidising atmospheres to identify its characteristic temperature parameters as well as the reduction smelting of copper slag in the electric pit furnace.

2.1. Research Materials

A batch of peanut shells was applied in this research. Their chemical composition is presented in

Table 2. The contents of essential peanut shell components applied in this research.

Component	Carbon	Oxygen	Hydrogen	Nitrogen	Heat of Combustion, kJ/kg
Component content, wt%	47.2	40.0	6.17	3.11	18,005

. The contents of carbon, oxygen, hydrogen and nitrogen in peanut shells were determined using the ELTRA CHS HELIOS analyser (Eltra GmbH, Haan, Germany). The sample combustion temperature was 1400 °C. The results are mean values from five measurements of the tested material.

The comparison of the data presented in Table 1 and Table 2 shows that the chemical composition of the tested peanut shell biomass does not differ from that mentioned in the literature [18]. The table also contains the values of the heat of shell combustion determined in the initial tests. The values of the heat of combustion were determined using a high-pressure calorimetric bomb in accordance with the PN-EN ISO 1716, 2010 standard [45].

The subject of the tests was copper slag, which underwent the reduction process in the presence of peanut shells used as a reducer. The slag chemical composition is presented in

Table 3. The composition of the tested copper slag.

Slag Component	Cu	Pb	Fe	SiO2	CaO	Al2O3
Component content, wt%	10.3	2.25	11.1	34.5	14.1	8.3

.

A phase analysis of the initial slag was performed using the XRD7 X-ray diffractometer (manufactured by Seifert-FPM, Rich. Seifert & Co. Freiberger Präzisionsmechanik GmbH & Co. KG, Freiberg, Germany). Figure 1 shows a diffractogram presenting the phase composition of the tested slag. The following phases were identified in the tested sample:

• Maghemite Fe_1.966O_2.963 (tetragonal structure).
• Cuprite Cu₂O (regular structure).
• Al₂PbSi₂O₈ (monoclinic structure).
• Massicot PbO (rhombic structure).
• Diopside CaMgAl_0.5Si_1.5O₆ (monoclinic structure).
• Leucite K(AlSi₂O₆) (tetragonal structure).
• Akermanite Ca₂Mg_0.54Al_0.92Si_1.54O₇ (tetragonal structure).
• Potassium oxide KO₂ (tetragonal structure).
• Hardystonite Ca₂Zn(Si₂O₇) (tetragonal structure).

The analysis revealed the presence of copper, lead and iron oxides in the slag undergoing the reduction process.

The analysis of the slag microstructures showed their complex and diverse morphologies [46] related both to the sizes of the slag particles that changed from several nanometres to several micrometres and to the particle shapes (particles with clearly flat, rounded or spherical surfaces). An additional diffractometric analysis demonstrated the presence of metals, i.e., copper, lead and iron, mainly as oxides. Metallic copper in the slag was identified in the form of small inclusions [47].

2.2. Research Equipment and Methods

Thermogravimetric analysis is one of the methods that ensures the characterisation of materials in temperature-related processes. Detection of changes related to weight loss as well as determination of temperature ranges and dynamics of weight alterations are useful for predicting material behaviour under specific conditions depending on its intended use. In this case, the thermogravimetric analysis was carried out for the biomass material with organic components which are particularly sensitive to temperature. Considering high temperatures in pyrometallurgical processes, it is necessary to characterise the material so that it fulfils its assigned role in the process. It is an issue that should be addressed, as gases being released that contain hydrocarbons may also be important reagents influencing the effectiveness of specific processes. Thus, the most effective method of adding this type of reducer to the system where reactions occur and the duration if its potential impact can be determined. For these types of biomass material, thermogravimetric analysis was necessary. Moreover, the tests were planned to be performed in the inert and oxidising atmospheres. Their results will enable identification of weight changes for aerobic and anaerobic conditions which may be present in the working chamber of the reactor where the process occurs.

The thermogravimetric analysis was performed with the use of the TGA Q500 device manufactured by TA Instruments (New Castle, UK), where up to 1000 mg samples can be tested. The accuracy of the built-in scale is ±0.01%, and its resolution is 0.1 μg. The tests were performed both in nitrogen and the oxidising (the air) atmospheres. The contents of the particular contaminants in the applied nitrogen (N5.0) were as follows: 0.9 ppm H₂O, 0.8 ppm O₂, <0.1 ppm of hydrocarbons, <0.01 ppm CO, <1 ppm CO₂ and <1 ppm H₂. The gas flow rate was 90 mL min⁻¹. The tests were carried out on the samples weighing 20 mg to 22 mg at a sample heating rate of 100 K/min.

The slag reduction process was conducted in an electric resistance furnace. The target temperature was 1300 °C in the tests, as this is applied in Polish copper smelting plants under industrial conditions. The schematic diagram of the device is presented in Figure 2 [38]. The variable parameter in the tests was the process duration. The variable parameter in the tests was the process duration. The weight of each slag sample in the tests was 80 g. Considering the course of the stoichiometric reactions in the tested sample for copper oxide and lead oxide reduction, the theoretical carbon weight is approximately 3 g, according with the following reactions:

MeO (CuO; Cu₂O; PbO) + C = Me (Cu; Pb) + CO₂

(6)

In addition, the 18 g amount of biomass provides the carbon excess necessary for the Boudouard reaction (5) to occur and to generate the reducing atmosphere in the working chamber of the device (Figure 2). The process duration was selected based on the real time of the processes under industrial conditions, i.e., approximately four to five hours.

Following the experiments, the products (the resulting secondary slag and copper alloy) were weighed. In addition, the slag was analysed for the contents of its basic chemical elements, i.e., copper, lead and iron. The concentrations of the elements in the slags after microwave digestion were determined using the inductively coupled plasma atomic emission spectrometry (ICP-AES) method. A Varian 710-ES spectrometer (Varian Inc., Palo Alto, CA, USA) equipped with a glass SeaSpray nebulizer and a double-pass glass cyclonic spray chamber were applied.

The results of the laboratory slag decopperisation tests using peanut shells demonstrated a potential for a significant reduction in copper and lead contents in the slag. The effectiveness of the process was defined based on the degrees of their removal from the primary slag. This parameter was determined in the following equation:

$$\alpha ={\displaystyle \frac{C_{Cu}^0-C_{Cu}^t}{C_{Cu}^0}}·100\%$$

(7)

where:

$C_{Cu}^0$—the initial concentration of copper (lead) in the slag, wt%.

$C_{Cu}^t$—the final concentration of copper (lead) in the slag, wt%.

3. Results

The test results are presented as the changes in the sample weight for the experiments carried out in the inert (Figure 3) and the oxidising (Figure 4) atmospheres. Based on the analysis of the TG and DTG curves, the following characteristic process parameters were determined:

(a)

The temperature of the theoretical start of thermal decomposition.

(b)

The temperature of the 50% sample weight loss.

(c)

The weight of the solid sample residue.

(d)

The temperatures at which the particular peaks of the derivative of the weight loss over time are observed.

The TG-DTG curves for the thermogravimetric analysis of the peanut shells regarding the measurements in the nitrogen and the air atmospheres demonstrated large weight losses associated first with the moisture loss and then with the process of pyrolysis and combustion of the tested material. The particular ranges of the TG analysis for the tested samples are summarised in

Table 4. Summarised results of the thermogravimetric analysis.

No	Atmosphere	Initial Weight Loss Temperature, °C	50% Weight Loss Temperature, °C	Sample Residue, %
1	Nitrogen	269.46	352	19.67
2	Air	259.6	313.4	1.20

. The first, slight (approx. 5% of the sample weight) weight loss regarding both samples was observed at up to approx. 100 °C. It was related to moisture loss from the material. Considering the measurement performed in the inert atmosphere, the major weight loss was about 60% of the sample weight, and it started at 269 °C. For comparison, the major sample weight loss for the measurement performed in the air was 98.8%, and it started at approx. 259 °C. Such a meaningful difference in the results between both samples is associated with different types of processes. In the case of the measurements in the nitrogen atmosphere, hemicellulose and cellulose (main components of the peanut shells) pyrolysis is observed. The measurement in the air atmosphere is related to the process of material combustion. While comparing the 50% weight loss temperatures for both samples, it was seen that it was reached earlier (at approx. 313 °C) for the measurement in the air atmosphere than for the measurement in the inert atmosphere (352 °C).

The experiments in the inert atmosphere showed the presence of approx. 5% transient moisture in the peanut shell sample and one clear pyrolysis stage beginning at approx. 269 °C. The maximum value of the derivative of the sample weight loss over time was 0.71%/°C at approx. 352 °C. The sample residue weight following the experiment constituted up to 20% of the initial sample weight. The total weight loss observed in the range of 200 °C to 400 °C may have been caused by hemicellulose and cellulose degradation and cellulose decomposition through depolymerisation [48,49,50].

The peanut shells were used as a reducer in the laboratory copper slag smelting experiments as scheduled in the research programme. Their results are summarised in

Table 5. Summarised results of the copper slag smelting (average of three samples).

No	Temp., °C	Initial Slag Weight, g	Time, h	Reducer Weight, g	Secondary Slag Weight, g	Metal Weight, g	Degree of Decopperisation, %	Degree of Lead Removal, %
1	1300	80.0	1	18.0	66.18	10.15	92.59	44.0
2			2		61.69	12.33	98.39	42.6
3			3		61.29	12.04	98.58	79.5
4			4		60.93	12.34	98.20	85.3

and

Table 6. Contents of copper, lead and iron in the secondary slag.

No	Time, h	Contents of Metals in the Secondary Slag, wt%
		Cu	Pb	Fe
1	1	0.78	1.25	14.38
2	2	0.17	1,29	15.07
3	3	0.15	0.46	16.36
4	4	0.19	0.33	17.53

. In addition to the basic data of experimental parameters, the resulting data on the changes in smelted alloy weight, secondary (post-reduction) slag weight and the contents of Cu, Pb and Fe in the slag are presented in Table 6.

Graphical representations of the slag reduction smelting results are presented in Figure 5.

The changes in the copper and lead degrees of removal during the reduction process are presented graphically in Figure 6.

The analysis of the results of the slag smelting experiments in terms of the changes in their compositions showed high degrees of copper and lead removal. The slag containing less than 0.78 wt% and 0.19 wt% of copper was yielded following one hour and four hours of the reduction process, respectively. The content of lead in the slag decreased to 1.25 wt% after 1 h and to 0.33 wt% in the experiments conducted for 4 h. The degrees of copper and lead removal from the slag were over 98% and 85.3%, respectively, in the experiments lasting for longer than two hours. No meaningful changes in the iron contents in the slag were observed in the process. It should be noted that the determination of the reduction order for the particular oxides in the slag is very difficult. In the first instance, a comparison between the chemical affinities of the metals for oxygen may help identify the first slag component to undergo reduction. Knowing that the chemical affinity of a metal for oxygen determines its ability to form oxides, a relatively obvious conclusion is that the metal with a high affinity for oxygen will not be reduced easily. Thus, a metal with a low chemical affinity for oxygen will readily undergo reduction. Based on the results of the studies and analyses presented in the papers by Kucharski, Śmieszek and Madej [15,16,51] regarding the copper slag of comparable composition to that studied in the present paper, it can be demonstrated that the process of iron oxide reduction does not occur until the copper content in the slag decreases to a value below 0.6 wt%.

Similar findings were presented by Buscolic et al. [52] who studied the process of reducing copper slag derived from a standard flash smelting process and proved that the reduction order regarding the particular metals was in accordance with the thermodynamic predictions. At the beginning of the process, the first compounds that undergo reduction are copper oxides and, to a very small extent, iron oxides. When the Cu content in the slag decreases to very small values and its changes are meaningless, the iron oxide reduction begins (Fe₂O₃→FeO→Fe). Iron silicates are directly reduced to Fe.

4. Conclusions

The paper presents the results and analysis of the copper slag decopperisation process using peanut shells as a reducing agent. The results of the experiments showed the potential for a replacement of conventional carboniferous materials such as coke, coke breeze or anthracite. The degrees of removal of the main metals from the tested slag reached values up to 98.5% and 85.3% for Cu and Pb, respectively. The contents of copper and lead in the secondary slag following the reduction process decreased from 10.3 wt% to 0.19 wt% and from 2.25 wt% to 0.33 wt%, respectively. It was assumed in the research that a necessary condition to complete the reduction process was to obtain a fraction of copper in the secondary slag below 0.5 wt%. In our tests, this value was observed as early as following 2 h of the reduction process. The longer duration of this process, i.e., over 4 h, was found ineffective. Due to a higher lead content, the yielded alloy should be subjected to the converting process that is performed under industrial conditions, e.g., in the KGHM Polska Miedź where blister copper is produced in the Peirce–Smith process.

The secondary slag containing less than 0.5 wt% of copper and lead meets environmental standards and can be considered waste slag, which is applied, e.g., for road construction.

Contrary to conventional fossil fuels, the application of peanut shells for copper slag reduction may be considered an environmentally neutral method in terms of carbon offsetting. In addition, the resulting secondary slag is waste which can be stored and utilised without harmful effects on the natural environment due to its minimal lead contents. Further problems that should be solved are the form of the biomass and its significant loss at a relatively early stage of the charge material heating in the industrial process. This requires large-scale laboratory studies or studies on a semi-technical scale.