Production of Metallurgical Sinter with Coke Modified by Spent Anode Material from Aluminum Electrolysis

Abstract

This study evaluates coke for iron ore sintering manufactured from Ekibastuz coal fines (fraction 0–3 mm), spent anode material (SAM) from aluminum electrolysis, and coal tar pitch. Laboratory coking was performed at 1000 °C (60 min hold), followed by sintering trials using coke containing 10 wt% and 20 wt% SAM. Microstructural (SEM/EDS) and spectral data indicate an optimal SAM range of 10–20 wt%: higher additions (≥30 wt%) lead to structural degradation of coke, accompanied by reduced mechanical integrity. The produced coke shows C = 85%, S = 0.9–1.1%, ash ≈ 19%, volatiles = 1.5–2.5%, and moisture (Wr) ≤ 1%, which is acceptable for sintering use. In sintering tests, the yield of usable sinter reached 72.4% (10 wt% SAM) and 73.5% (20 wt% SAM); impact strength was 83% and 78%, respectively. XRF of sinter showed Fe_total > 51%, CaO ≈ 5.5–6.8%, SiO₂ ≈ 6.6–7.2%, and S = 0.40–0.45%, meeting technological requirements for blast-furnace practice. Overall, using spent anode material within 10–20 wt% increases fixed-carbon content, enables valorization of aluminum industry waste, and delivers coke for agglomeration performance without compromising key chemical or mechanical indices.

1. Introduction

Currently, ferrous metallurgy is showing rapid development due to the high demand for cast iron and steel in various sectors of the economy. The main task of the industry is to provide domestic and foreign markets with metal products of the required range, quality, and volume using the best available technologies with a sustainable supply of raw materials and the implementation of state industrial policy [1,2,3,4].

Coke is the main fuel of the ferrous metallurgy industry. The prospects for the development of the coke chemical industry and forecasts for coke production are directly related to the expansion of metallurgical production aimed at domestic and foreign markets. The growing demand for coke is due to an increase in production rates, as well as rising prices for raw materials, which, in turn, leads to an increase in coke prices on the world market [5,6]. The issue of resource conservation is urgent: it requires the development and implementation of technologies that ensure the production of products with minimal consumption of energy and material resources, as well as the rational use of mineral raw materials and the search for alternative carbon reducing agents capable of meeting technological requirements in the production of metals and alloys and at the same time reducing costs due to low prices for raw materials. In this regard, research in this field is of particular importance and requires modern scientific and technical solutions [7,8,9,10].

The iron and steel industry continually seeks sustainable and cost-effective methods to produce high-quality coke for iron ore agglomeration. Utilizing industrial waste materials such as coal fines, anode cinder (spent anodes from aluminum production), and coal pitch presents an opportunity to reduce environmental impact and production costs.

Coal fines in Coke Production. Coal fines, often considered a low-grade fuel due to their high ash content and low calorific value, can be upgraded through blending and processing techniques. Studies have shown that incorporating coal fines into coke production is feasible when combined with suitable binders and additives to enhance their coking properties [11,12].

Spent anode material (SAM) as a carbon source.

Spent anode material from the Hall–Héroult aluminum electrolysis process (also referred to as anode cinder; hereinafter “spent anode material,” SAM) contains a high fraction of carbon and can serve as an effective carbon source in coke production. Its utilization not only recycles industrial waste but also supports the sustainability of metallurgical processes. Although few studies have examined SAM, the authors’ previous work in this area provides a basis for understanding how alternative carbon materials can be integrated into the technological process [13,14,15].

Coal pitch as a binder. Coal tar pitch, derived from the distillation of coal tar, is commonly used as a binder in the production of carbon anodes and coke. Its adhesive properties facilitate the agglomeration of fine particles, improving the mechanical strength and integrity of the produced coke [16]. These studies investigate how the addition of coal tar and pitch as binders influences the caking properties of coal and the resulting coke quality. The findings indicate that incorporating these binders can enhance the tensile strength of coke by 39% to 48%, depending on the specific binder used. The studies focus on the mechanisms by which coal tar pitch enhances the strength and integrity of the formed coke, providing insights into optimizing binder usage for improved coke quality [17,18].

Combined use in iron ore agglomeration. The integration of coal fines, spent anode material from aluminum electrolysis, and coal pitch in coke production has been explored to produce a composite fuel suitable for iron ore sintering. This approach aims to enhance the fuel’s reactivity and reduce reliance on traditional coke sources [19].

In this paper, the authors consider the technology of ferrochrome smelting using briquettes with various carbon-containing reducing agents. The research addresses the significant challenge of dust and fine fractions of raw materials generated during the metallurgical industry’s crushing, beneficiation, storage, and transportation processes. Briquettes were produced and tested by agglomerating chrome ore dust with carbonaceous reducers for their mechanical properties and effectiveness in smelting processes. The results highlight the potential for improved environmental impact, reduced material losses, and enhanced production efficiency [20].

Environmental and Economic Benefits. Utilizing industrial waste materials in coke production offers significant environmental benefits by reducing industrial waste and lowering greenhouse gas emissions. Economically, it decreases the dependency on conventional coking coal, leading to cost savings. The use of such waste, some of which are classified as hazardous materials, will avoid the need for dumping, thereby contributing to the protection of the environment as well as reducing the costs related to waste disposal [21,22,23,24].

In this context, the issue of ensuring the stable and cost-effective production of high-quality coke, which plays a key role in iron ore agglomeration and blast furnace operations, becomes increasingly urgent. While traditional methods rely predominantly on high-quality coking coals, the depletion of natural resources and growing environmental restrictions demand new approaches.

Therefore, the search for alternative raw materials that can partially or fully replace expensive and limited primary resources, without compromising coke quality, is of great scientific and practical interest. One of the promising directions is the utilization of man-made and industrial waste in coke production. Such materials include coal fine fractions, anode cinder (spent anode material from aluminum electrolysis), and coal tar pitch. Their combined use opens up new opportunities not only to improve resource efficiency but also to reduce the environmental burden of coke production.

This article presents the results of theoretical and experimental studies aimed at determining the optimal technological parameters for the production of metallurgical coke for agglomeration production from industrial waste, such as waste from the anode production of an electrolysis plant and small pieces of Ekibastuz coal (Kazakhstan) obtained from tailings dumps.

The addition of spent anode material to Ekibastuz coal during coke production leads to a significant increase in the carbon content in the final product. Spent anode material is a high-carbon technogenic material formed during the production of aluminum via electrolysis, and it contains up to 95–98% free carbon, often in the form of partially graphitized material [25,26]. In our previous study, the possibility of using spent anode material from aluminum electrolysis as a component of a coal charge for producing coke from Ekibastuz coal was studied [27].

From an economic standpoint, the selected charge relies on lower-unit-cost inputs than conventional metallurgical coke. As the carbon base, we employ Ekibastuz coal fines (0–3 mm)—a by-product of coal crushing/screening—adequate for our purpose and significantly cheaper because, in conventional uses, such fines typically require extra preparation (e.g., briquetting/pelletizing), which depresses their market price. In our process, these fines are used as received, without additional crushing, which further reduces energy and operating costs and simplifies dosing and granulation. As a modifying additive, we use spent anode material from aluminum electrolysis, characterized by a low purchase price and potential avoided disposal costs; prior to mixing, it is sized to the same 0–3 mm window, improving granulation and binder wetting. Accordingly, we expect the production cost of the resulting coke to be lower than that of coke made from high-grade coking coals.

In the case of mixing with Ekibastuz coal, which itself has a high ash content and a relatively low carbon content (in dry ashless form, carbon is about 70–75%), the addition of spent anode material can significantly improve the quality composition of the charge. For example, with the introduction of 10% spent anode material, the carbon content in the mixture can increase by about 2–4% in absolute terms, depending on the initial ash content of the coal and the ratio of other components. With a further increase in the spent anode material content, the increase in carbon content can reach 6–10%; however, negative technological effects are possible, such as impaired connectivity, deterioration of mechanical properties, and an increase in ash content due to residual non-carbon components of the cinder (oxides, fluorides, and salts) [28,29].

The purpose of adding spent anode material:

• Increasing the carbon content of the charge: important for obtaining coke with a higher fixed-carbon content, which enhances reactivity in metallurgical processes—specifically through indirect reduction via CO generation.
• Disposal of industrial waste: spent anode material is a by-product of the aluminum industry; its reuse reduces environmental burden and saves resources.
• Reducing the need for expensive primary carbon materials: partial replacement of coal tar pitch or costly activated coals with spent anode material lowers the cost of coke.
• Improved heat resistance and electrical conductivity: partially graphitized carbon enhances the conductive and heat-resistant properties of coke, which can be useful for special agglomeration options, especially with electric or induction systems.

Thus, previous studies of briquetted raw materials and alternative sources of carbon reduction indicate potential advantages, but leave unresolved several issues related to the production of agglomeration coke. Therefore, this study aims to determine the optimal technological parameters for producing metallurgical coke using spent anode material from aluminum electrolysis and Ekibastuz coal fines (0–3 mm), which can be used as received without additional crushing.

The scientific novelty of this study lies in the development and experimental validation of a new coke-based fuel obtained by modifying coal with spent anode material and coal tar pitch. This approach enables the partial replacement of traditional metallurgical coke with a resource-saving composite fuel. Unlike earlier research that primarily addressed the thermal decomposition of individual components, this study proposes a method for producing formed coke with improved strength and reduced ash content, using technogenic carbon-rich waste. The work introduces an effective utilization pathway for spent anode residues, offering a solution for the recycling of aluminum production waste while simultaneously expanding the raw material base for the metallurgical industry. The results confirm the structural and mechanical adequacy of the obtained formed coke for application in agglomeration processes. Thus, spent anode material from aluminum electrolysis makes it possible to enrich coal with carbon, utilize waste, and produce coke suitable for use in sintering processes, provided that the technological parameters are observed.

2. Materials and Methods

A mixture of fine Ekibastuz coal, spent anode material from aluminum electrolysis, and coal pitch was used to produce coke. To select the optimal ratio of the charge components and to ensure the best properties of the final product,

Table 1. The content of components in the charge, wt.% (as-received).

Sample №	The Content of Components in the Charge, %
	Ekibastuz Coal	Spent Anode Material	Coal Tar Pitch
1	50	10	40
2	50	20	30
3	50	30	20
4	50	40	10

sum to 100 wt.% of the charge (as-received).

and

Table 2. Chemical composition of charge materials.

Indicator	Ekibastuz Coal (KCH)	Spent Anode Material	Coal Tar Pitch
Carbon (C), %	70–75% (daf)	98	92–93
Hydrogen (H), %	4.9–5.6% (daf)	-	4.3–4.7
Oxygen (O), %	8.6–11.2% (daf)	-	0.8–1.0
Nitrogen (N), %	1.5–1.7% (daf)	-	1.7–1.8
Sulfur (S), %	0.4–1.0 (avr. 0.7)	2.0	0.3–0.85
Moisture (Wr), %	3.8–7.0 (avr. 5.4)	-	-
Ash content (Ad), %	37–41 (avr. 39)	0.3–0.4	0.2–0.3
Volatile substances (Vdaf), %	24–40 (avr. 32)	-	-
Qir, kcal/kg	3800–4200 (avr. 4000)	-	-
Melting point of ash, °C	1490–1500	-	-
SiO2, %	56.9–67.3 (avr. 62.1)	-	-
Al2O3, %	24.4–31.6 (avr. 28.0)	-	-
Fe2O3, %	4.4–7.26 (avr. 5.83)	-	-
CaO, %	0.68–3.29 (avr. 1.98)	-	-
MgO, %	0.19–1.26 (avr. 0.72)	-	-
TiO2, %	1.09–1.65 (avr. 1.37)	-	-
SO3, %	0.55–2.31 (avr. 1.43)	-	-
Density, g/cm3	-	1.56	1.2–1.3
Softening point, °C	-	-	110–125

show the content of the components and their chemical compositions, respectively.

Each coking and agglomeration recipe was tested in triplicate (n = 3) for all compositions. The reported values are presented as the mean ± standard deviation based on three independent repeats.

Coking was carried out in an airtight laboratory coking tank made of heat-resistant steel, equipped with a discharge tube with a rubber nipple valve to remove volatile pyrolysis products (Figure 1).

The coking tank was placed in an electric SNOL muffle furnace with programmable temperature control up to 1150 °C. The coking process was carried out according to the following temperature regime:

• Heating from 25 to 1000 °C at a rate of ~10–12 °C/min (average 90 min);
• Isothermal exposure at 1000 °C for 60 min to complete the processes of degassing and formation of the coke structure;
• Cooling—the furnace was turned off, and the coke was left inside until it gradually cooled.

Cooling was carried out naturally in a closed container at ambient temperature, which ensured a slow decrease in temperature and prevented the oxidation of coke. Complete cooling to a temperature of no more than 50 °C took ~5 h; this cooling mode made it possible to prevent oxidation and preserve the structural characteristics of the resulting coke. After that, the samples were extracted for subsequent physico-chemical analysis. The selected temperature and time regime ensure stable formation of the coke structure in a laboratory setting.

The surface morphology and microstructure of the obtained samples were examined using a scanning electron microscope (SEM, TM4000, Hitachi High-Tech, Tokyo, Japan) in high-vacuum mode at an accelerating voltage of 15 kV. The use of SEM is justified because the method provides direct evidence of particle morphology, pore structure, and particle–binder interfaces that govern coke strength and sintering behavior. Images were acquired at multiple magnifications to characterize particle morphology, porosity, and interfacial features relevant to the material structure. The Hitachi TM4000 is a compact, benchtop SEM designed for high-resolution imaging and surface characterization across a wide range of materials. It employs a tungsten-filament electron source and supports accelerating voltages of 5, 10, and 15 kV; 15 kV provides optimal resolution for detailed microstructural analysis. Minimal sample preparation requirements and rapid start-up make it suitable for laboratories requiring fast and reliable surface morphology imaging [30,31,32].

Further work was devoted to practical studies of the possibility of using the obtained cokes in sintering production. Agglomeration is the process of pretreatment of fine-grained material, in which this material, which cannot be directly loaded into a blast furnace, undergoes high-temperature solidification during sintering to obtain a material that meets the requirements for melting in a blast furnace [33]. Coke with a spent anode material content of 10% and 20% was selected for testing based on microstructure studies and spectral analysis.

Rolled scale from the Casting LLP enterprise (Pavlodar, Kazakhstan), which is a waste product consisting mainly of iron oxides, limestone from the Keregetas mine, a branch of Aluminum of Kazakhstan JSC (Pavlodar region, Kazakhstan), as well as the studied coke obtained from screening, were used as part of the charge to obtain the agglomerate. Coal from Bogatyr Komir LLP (Ekibastuz, Kazakhstan), spent anode material, and coal pitch from Kazakhstan Electrolysis Plant JSC (Pavlodar, Kazakhstan) in the following ratio, shown in

Table 3. Charge components for agglomeration.

Experiment Number	Charge Components, wt%
	Rolling Scale	Limestone	Coke * (Total), wt%	Return Fines	Moisture	SAM in Coke, % (By Mass of Coke)
Experiment No. 1	55	10	7	20	8	10
Experiment No. 2	55	10	7	20	8	20

* In the first experiment, coke containing 10% spent anode material was used, in experiment No. 2—20%.

. In Table 3, “return fines” (also referred to as return sinter or recycled sinter) denote the undersize sinter fraction obtained after product screening—typically the <5 mm cut. This fraction is returned to the agglomeration charge to serve as granulation nuclei, increase bed gas permeability, stabilize moisture and the thermal regime during sintering, and reduce waste through a closed material cycle.

Before agglomeration, the charge was thoroughly mixed and pelletized in a plate granulator until granules with a diameter of 5–10 mm were formed. The moisture content of the charge was adjusted within 10–15% by adding water after two minutes of dry mixing; optimal parameters were determined empirically. The pelletizing time was 3 min. The pelletized charge was unloaded, weighed, and prepared for agglomeration.

To ensure reproducibility of the experiments, the agglomeration bowl was cooled to a constant temperature before loading. A uniform bed layer (0.5 kg, 15–20 cm thick) obtained from crushed agglomerate was pre-laid on the grate. The manually loaded charge was leveled to a preset height with a template, and excess material was removed. The mass of the loaded charge was calculated from the difference in the mass of the container before and after loading.

An incendiary mixture of coke (0–5 mm) and sawdust, moistened to improve gas permeability, was applied to the surface of the charge, followed by covering with wood shavings. The content of coke in the incendiary mixture was ~2% of the mass of the charge.

The agglomeration process was carried out in a laboratory facility (Figure 2) of the NJSC Toraighyrov University [34].

Installation for agglomeration (Figure 2) operates by initiating a vacuum using the exhaust fan (exhauster), which creates a negative pressure in the vacuum chamber (5). This airflow ensures that the combustion front of the charge progresses from top to bottom inside the agglomeration bowl (1). Combustion is initiated at the surface of the charge, and due to the suction created by the exhauster, the air passes through the charge, sustaining the combustion process.

The vacuum in the chamber is regulated by a slide valve (4), and the temperature of the sintering process is monitored using a thermocouple (3). Combustion gases and dust particles are drawn through the vacuum chamber and directed into the dust collection unit (2), where solid particles are filtered out and then collected in a storage hopper (6). The cleaned gases are then discharged via the gas pipeline system with the assistance of the exhauster, completing the airflow circuit.

The chemical composition of the resulting agglomerate was determined with an X-ray fluorescence spectrometer (ProSpector 2LE spectrometer, serial number P1775, Elvatech Ltd., Kiev, Ukraine). This device is capable of analyzing elements from Mg to U in the range up to 0.01%.

Impact strength tests of the agglomerate were carried out according to GOST 24707-81 [35], using the drop method. The analysis of the strength of the agglomerate makes it possible to assess its suitability for subsequent use in metallurgical aggregates, where high integrity and resistance of granules to mechanical destruction are important. The test samples, weighing 5.00 kg, were dropped from a height of 2 m onto a steel plate. After the fall, the material was sieved, and the mass of the fraction with a particle size of more than 5 mm was determined. This indicator characterizes the impact strength of the agglomerate.

The impact strength (P) was calculated using the formula:

P = (m₁/m₀) × 100%,

where:

m₀—the mass of the agglomerate before the test, kg;

m₁—fraction weight >5 mm after testing, kg.

3. Results and Discussion

In this section, the results of the experimental studies on the preparation and characterization of coke and sinter samples are presented and analyzed.

Coke was produced through carbonization of a charge consisting of Ekibastuz coal fines, spent anode material, and coal tar pitch. The resulting material, shown in Figure 3, exhibited a porous structure with particle sizes predominantly in the range of 3–5 mm, which meets the requirements for coke for sintering production.

Table 4. Chemical composition of coke.

Carbon (C), %	Sulfur (S), %	Ash Content (Ash), %	Volatiles, %	Moisture (Wr), %
85	0.9–1.1	19.0	1.5–2.5	≤1

shows the chemical composition of the resulting coke.

The resulting coke is characterized by a high carbon content (85%) and low humidity (<1%), which confirms its fuel suitability. Despite the increased values of ash content (19%) and sulfur (0.9–1.1%), this composition is acceptable for use in sintering production. The increased ash content and sulfur content are due to the use of high-ash Ekibastuz coal and anode cinders as residues of electrolysis production of aluminum (spent anode material), containing sulfur compounds in the charge.

Using a scanning electron microscope, the structure of coke obtained from various components presented in Table 1 was analyzed. Images at magnifications of ×100, ×200, and ×500 make it possible to study in detail the morphological features and microstructural elements of the sample.

Figure 4 shows the microstructure of sample No. 1 at various magnifications. In sample No. 1, where the spent anode material content is 10%, a relatively homogeneous structure with pronounced porous areas and clearly defined micropores is observed. The carbonaceous phase with a uniform distribution prevails; the pores within 5–20 microns indicate a fairly good formation of coke. The influence of spent anode material in this case is moderate, and its amount does not disrupt the coherence of the structure. The presence of 40% pitch contributes to the plasticity of the mass during coking, ensuring uniform distribution and wettability of the particles, which together form a dense but porous structure.

Figure 5 shows that the microstructure of sample No. 2 clearly shows the features of the microstructure formed as a result of the interaction of organic and inorganic components during thermochemical processing. The coke structure is characterized by sufficient uniformity, the presence of large, sharply angled fragments with a developed surface and a system of micropores, as well as areas showing traces of partial melting and recrystallization of carbon-containing phases.

Sample No. 2, which contains 20% spent anode material, shows marked changes in microstructure compared to sample No. 1. Even at a magnification of ×200, graphite-like areas are noticeable, indicating partial graphitization of the carbon matrix. This gives the coke additional thermal stability. The pores acquire a slightly more elongated shape, but the structure remains well connected: microfractures and zones with a weakened matrix are poorly expressed and are local. Reducing the proportion of coal pitch to 30% only slightly reduces the ductility of the material and does not have a critical effect on its integrity.

Thus, the introduction of spent anode material in an amount of 20% contributes to the formation of a more stable microstructure with partial graphitization, improved heat resistance and maintaining acceptable porosity and density. This makes this composition promising for further use in metallurgical processes requiring high reducing and mechanical properties of coke.

Figure 6 shows the microstructure of sample No. 3, where more pronounced changes are observed, the content of spent anode material is increased to 30%, and coal pitch is 20%. The microstructure shows signs of inconsistent sintering of phases: graphitized inclusions from spent anode material are visible as dark, irregularly shaped fragments poorly integrated into the carbon matrix. When magnified to ×500, multiple interfacial boundaries become noticeable, indicating weakened adhesion between the components. This structure indicates a loss of cohesiveness of the coke, which is fraught with a decrease in strength characteristics.

Nevertheless, even with the observed changes, the resulting coke can be used in metallurgical processes, especially where the requirements for mechanical strength are less critical.

In addition, increasing the share of spent anode material to 30% can significantly increase the level of waste disposal, which has a positive impact on the environmental and economic performance of the technology.

Sample No. 4 (Figure 7), containing 40% spent anode material and 10% coal tar pitch, exhibits the most unstable microstructure. Already at ×100 magnification, pronounced heterogeneity is evident: large graphite-like inclusions are weakly integrated into the carbon matrix, and the structure appears loose and fragmented. A reduced binder fraction combined with an excess of inert anode phase leads to interfacial decohesion (indicated by yellow arrows in the figure) at matrix–inclusion boundaries, pore coalescence (indicated by red arrows) with the formation of enlarged cavities and thinned ligaments, as well as microcracking (indicated by black arrows) and local delamination of graphitic lamellae (indicated by green arrows). Taken together, these features constitute structural degradation and explain the reduced mechanical integrity and reactivity of the coke at 40% SAM.

To assess the effect of the components on the carbon content, as well as the formation of harmful inclusions, the spectroscopy results shown in

Table 5. Spectroscopy results.

Spectrum	C	O	Al	Si	Ca	S
Sample No. 1
1	95	-	-	-	-	-
2	84.45	7.61	2.62	0.59	2.73	-
3	92	-	-	-	-	-
4	87.48	23.61	5.62	5.59	2.73	-
5	-	55.48	13.74	7.86	-	-
Sample No. 2
1	86.73	10.83	0.74	0.57	0.29	0.41
2	97.01	-	0.29	0		0.39
3	90.93	7.85	0.55	-	-	0.42
4	86.48	9.80	2.46	0.57	0.28	-
5
Sample No. 3
1	97.43		0.71	0.81	-	0.63
2	98.42					0.61
3	98.35	1.12				0.68
4	95.7	2.70	-	-	-	0.70
5
Sample No. 4
1	99.32	-	0.68
2	99.13					0.87
3	98.56	0.51				0.93
4	99.01	0.51				0.83
5	81.78	10.71	6.09	-	0.28	0.88

were obtained.

Table 5 reports instrument-quantified EDS point analyses taken at five spots per sample (n = 4 samples). Values were exported from the instrument software and are presented as wt% (mean ± SD); EDS is semi-quantitative. The main purpose of the analysis is to estimate the distribution of carbon and impurity elements in the obtained microstructures. The data obtained demonstrate patterns directly related to the composition of the charge and the content of spent anode material.

In sample No. 1, containing 10% spent anode material, the carbon content varies from 84 to 95%. Along with carbon, the presence of oxygen, aluminum, calcium, and silicon is recorded, which indicates the presence of remnants of the mineral phase characteristic of both the initial Ekibastuz coal and spent anode material. At the same time, the structure retains chemical heterogeneity, but the presence of impurities remains within acceptable limits, and the distribution of elements indicates sufficient interaction of the components during heat treatment.

Sample No. 2 with 20% spent anode material is characterized by a relatively high carbon content, from 86 to 97%, while maintaining the presence of oxygen, aluminum, and sulfur. Minor fluctuations in the content of impurity elements and a high concentration of carbon indicate an increase in the carbon phase due to the introduction of cinder. At the same time, there are signs of a partial loss of homogeneity, which is reflected in the differences between the spectra within the same sample. Such fluctuations may be a consequence of the local distribution of inorganic inclusions.

In sample No. 3, where the amount of spent anode material was increased to 30%, a further increase in the proportion of carbon was recorded, which reached 98% in several spectra, as well as a more pronounced presence of sulfur, but it remained within acceptable limits. The increased concentration of sulfur in the composition may be due to residual compounds contained in the cinder, especially if the pretreatment technology was not effective enough. The appearance of zones of almost pure carbon (more than 97%) indicates a weak degree of interaction of carbon-containing components with each other and the possible loss of spent anode material in the form of separate fragments without inclusion in the overall matrix. This is accompanied by a slight decrease in the coherence of the structure and a violation of the uniformity of coke.

Sample No. 4 with 40% of the cinder shows the highest carbon value in individual spectra, up to 99%, while in one of the sites, the carbon content drops sharply (81%), which indicates the heterogeneity of the structure. The presence of significant amounts of oxygen, aluminum, and silicon further confirms the uneven mixing of the components. The presence of zones of almost pure carbon (more than 98–99%) may indicate not the qualitative integration of spent anode material into the matrix, but its local accumulation. This phase dispersion indicates a weak plasticity of the charge at the coking stage and possible segregation of carbon-containing particles, which worsens the strength of the coke and its technological properties.

It should be noted that an excessive carbon content close to 100% is not always a positive factor. In the context of coke material, this may mean the presence of unrelated carbonaceous inclusions, such as pure graphite or cinder residues that have not been processed. These sites, despite their high carbon content, do not effectively participate in the formation of a cohesive porous structure. They can cause internal stresses, brittleness, and decreased reactivity of coke during high-temperature operation. Moreover, such fragments can cause local damage during thermal exposure, which reduces the durability of the material and its suitability in metallurgical processes.

The analysis of the spectra also confirms that an increase in the proportion of spent anode material above 30% is accompanied not only by an increase in carbon content, but also by a deterioration in its distribution in the structure, as well as an increase in the level of undesirable impurities. This directly affects the uniformity, strength, and technological suitability of the resulting coke.

Analysis of microstructural images and spectral analysis data shows that sample No. 1, with spent anode material content of 10%, has the most balanced characteristics: a high degree of carbonation, homogeneous microstructure, and good phase connectivity. Sample No. 2, with 20% spent anode material, also demonstrates a stable structure and acceptable performance properties. Minor deviations in the microstructure identified during the analysis do not have a critical impact on the main quality indicators. Sample No. 3, with 30% cinder content, retains its structural integrity and can be effectively used in cases where the requirements for mechanical strength and density are less stringent. The use of up to 30% spent anode material in the coke charge allows one to expand technological capabilities without significant damage to quality. In samples with a cinder content above 30% (No. 4), pronounced microstructural disturbances are recorded: stratification, lack of connectivity, and the formation of large graphitized inclusions that do not interact with the carbon matrix. These effects confirm the validity of the maximum share of spent anode material of 20–30%.

Within the framework of this study, the optimal range of usage of spent anode material is up to 20%. The best result was obtained with the ratio of components: 50% Ekibastuz coal, 10–20% spent anode material, and 30–40% coal pitch.

The chemical composition of the obtained samples is shown in

Table 6. Chemical composition of the agglomerate.

Experiment No.	Fetotal	FeO	Fe2O3	CaO	SiO2	MgO	Al2O3	MnO	S	P
1	51.58	13.4	9.20	6.75	6.60	6.04	4.85	1.14	0.40	0.04
2	51.35	13.57	10.50	5.52	7.20	4.02	6.18	1.17	0.45	0.04

. Table 6 presents the results of X-ray fluorescence analysis of the chemical composition of the agglomerate obtained during laboratory agglomeration tests. Two variants of the charge were investigated, in which coke containing 10% (experiment No. 1) and 20% (experiment No. 2) spent anode material, respectively, was used as the coke component.

The total iron (Fe_total) content in both samples is more than 51%, which meets the requirements for iron-containing materials for blast furnace melting. The content of FeO and Fe₂O₃ varies from 13.4 to 13.57% and from 9.2 to 10.5%, respectively, which indicates a balanced ratio of reducible forms of iron. The level of calcium oxide (CaO), which acts as a flux, is in the range of 5.52–6.75%, and the content of silica (SiO₂) is 6.6–7.2%, which is within acceptable standards for blast furnace agglomerates. Technologically acceptable levels of MgO, Al₂O₃, and MnO are also recorded.

The content of sulfur (S) and phosphorus (P) does not exceed critical values and amounts to 0.40–0.45% and 0.04%, respectively, in all samples, which indicates acceptable indicators for harmful impurities that limit the use of agglomerate in metallurgical processes.

Figure 8 shows the appearance of the agglomerates obtained in experiments No. 1 and No. 2. The agglomerate formed using coke with 10% spent anode material (experiment No. 1) is characterized by high density, uniformity of granules, and mechanical strength, which visually confirms the integrity of the structure. The sample from experiment No. 2 (with 20% cinder) retains similar morphological characteristics; however, macroscopic observation shows a slight increase in surface heterogeneity, which may be due to a local uneven distribution of graphite-like inclusions.

Table 7. The results of testing the mechanical strength of the agglomerate. GOST 24707-81, drop method.

Sample	Content of Spent Anode Material (%)	Weight Before the Test (kg)	Fraction Weight > 5 mm (kg)	Impact Strength (%)
No. 1	10	5.00	4.15	83.0
No. 2	20	5.00	3.90	78.0

shows the results of the mechanical strength of the agglomerate.

Both samples demonstrate satisfactory strength under impact load. The sample with 10% spent anode material showed higher fracture resistance (83%) compared to the sample with 20% cinder (78%). The decrease in strength characteristics may be due to a change in the structure and mechanical properties of coke, in particular, a decrease in its binding characteristics with an increase in the proportion of cinder. However, both values are within acceptable limits for use in blast furnace production.

The yield of suitable agglomerate was 72.4% in experiment No. 1 and 73.5% in experiment No. 2, which indicates the high efficiency of the agglomeration process using coke modified with spent anode material. The obtained agglomerates meet the requirements of blast furnace production in terms of their chemical and technological characteristics.

The results of the study showed that the addition of spent anode material to the Ekibastuz coal charge increases the carbon content of coke and makes it possible to recycle man-made aluminum production waste. Microstructural and spectral analyses confirmed that the optimal content of spent anode material is 10–20%. When this value is exceeded, destructive changes in the microstructure are observed: connectivity deteriorates and the strength and uniformity of the coke decrease. The conducted agglomeration tests using the obtained coke showed satisfactory technological and chemical characteristics of the agglomerate, suitable for use in blast furnace production. Thus, the use of spent anode material in the range of 10–20% is an effective way to save resources and increase the carbon efficiency of agglomeration processes.

4. Conclusions

The conducted research has confirmed the feasibility of using spent anode material from aluminum electrolysis as part of the charge to produce metallurgical coke. It has been established that introducing **10–20 wt%** of this material into **Ekibastuz coal fines (0–3 mm)**—which are **used in their original form without additional crushing or special preparation**—increases the fraction of fixed carbon and yields a more ordered coke microstructure: the matrix remains coherent, interfacial boundaries are stable, and partially graphitized domains do not compromise integrity. At **≥ 30 wt%**, interfacial bonding weakens, pore coalescence and local graphitic segregation become apparent, and a decline in mechanical strength is observed.

The optimal charge composition is **50%** Ekibastuz coal, **10–20%** spent anode material, and **30–40%** coal-tar pitch. The resulting coke meets indices relevant to agglomeration: **C ≈ 85%**, **S = 0.9–1.1%**, **Ash ≈ 17.0%**, **Volatiles = 1.5–2.5%**, and **Wr ≤ 1%**. In bench sintering trials with **10** and **20 wt%** SAM, the **yield of usable sinter** reached **72.4–73.5%** with **impact strength of 78–83%**; according to XRF, the bulk chemistry of the sinter met technological expectations (**Fe_total > 51%**, **CaO ≈ 5.5–6.8%**, **SiO₂ ≈ 6.6–7.2%**, **S = 0.40–0.45%**), confirming the practical suitability of the fuel.

Thus, the proposed approach **increases the fixed-carbon fraction in the charge and enables the utilization of aluminum industry waste**, while maintaining **coke indices required for agglomeration** within the validated working window of **10–20 wt%** SAM. The results define **clear compositional limits** and provide a **microstructural rationale** for the upper bound: exceeding **≈ 30 wt%** leads to the degradation of structure and properties. Considering that the coal fines are **used as received, without additional crushing**, and that SAM is a **low-cost industrial by-product**, the method appears promising in terms of **resource efficiency** and **cost**; off-gas measurements and a detailed techno-economic assessment remain tasks for subsequent stages.