From Waste to Resource: Circular Economy Approaches to Valorize Fine Glass, Ceramic, and Plastic Residues in a Glass Recycling Plant

Abstract

Waste glass recycling generates waste streams such as fine glass fraction, waste ceramics containing fine glass, and waste polyethylene plastics. All of the aforementioned streams contain contaminants of organic and inorganic origin that are difficult to remove. This research was conducted to determine technological processes aimed at achieving a circular economy (CE) in the recycling of waste glass. Foam glass was made from the fine-grained, multicolored fraction of contaminated glass, an effective method for recycling glass waste at a low cost. A frothing system based on manganese oxide (MnO₂) and silicon carbide (SiC) was proposed, and an optimum weight ratio of MnO₂/SiC equal to 1.0 was determined. The possibility of controlling the process to achieve the desired foam glass densities was demonstrated. Statistical analysis was used to determine the effect of the MnO₂/SiC ratio and MnO₂ content on the density of the resulting foam glass products. Waste ceramics contaminated with different-colored glass were transformed into ceramic–glass granules. The characteristic temperature curve of the technological process was determined. The metal content in water extracts from ceramic–glass granules and pH value indicate their potential use for alkalizing areas degraded by industry and agriculture. Waste polyethylene-based plastics were converted into polyethylene waxes by thermal treatment carried out in two temperature ranges: low temperature (155–175 °C) and high temperature (optimum in 395 °C). The melting temperature range of the obtained waxes (95–105 °C) and their FTIR spectral characteristics indicate the potential application of these materials in the plastics and rubber industries. The integrated management of all material streams generated in the glass recycling process allowed for the development of a CE model for the glass recycling plant.

1. Introduction

The realization of a circular economy (CE) requires a comprehensive analysis of recycling processes aimed at efficiently transforming material waste streams into full-value marketable products [1]. Central to this is a systems approach, taking into account technological, economic, and environmental aspects, which minimizes the need for landfilling, thereby reducing the negative environmental and social impacts of waste. Transforming waste into secondary raw materials suitable for reuse within the value chain lies at the core of the circular economy, contributing to enhanced resource efficiency and reduced environmental pressure [2,3]. The implementation of such solutions supports the transition toward sustainability and addresses the challenges posed by the limited availability of natural resources. Waste glass cullet recycling plants face a problem in achieving a circular economy due to the waste generated during cullet cleaning [4,5]. These wastes include fine glass fraction, ceramic, and plastic waste [6,7,8,9]. During the treatment of glass cullet, the fine fraction is typically rejected due to its heterogeneous color and contamination with organic matter, ceramic particles, and plastics [10]. This material stream constitutes a significant percentage of the input directed toward recycling and, in many cases, ends up in landfills [11]. However, this waste fraction exhibits considerable potential due to its high glass content (80–95%) and can be converted into foam glass [12,13]. Foam glass is produced by melting fine glass particles with a foaming agent, which generates gas through a chemical reaction at temperatures between 700 and 900 °C [14]. Graphite or soot in combination with dolomite, limestone, and boron sulfate, a combination of silicon carbide and manganese dioxide, and activated soot with blast furnace slag and sulfur compounds are used as pore-forming systems [15,16,17,18]. There are many examples in the literature of studies on making foam glass from a single type of waste. In one of them, it was shown that bottle glass waste could be effectively used to produce foam glass using silicon cutting waste as a foaming agent. The resulting foam glass had high porosity (78%), low density (0.56 g/cm³), and good compressive strength (4.1 MPa), meeting the requirements of thermal insulation materials [19]. Other authors combined waste mineral wool and waste glass from construction to obtain foam glass at (800 °C). The resulting material had a homogeneous porous structure and low density (0.7 g/cm³) [20]. Research on glass waste from the automotive industry has shown that it can be processed into lightweight foam glass with low density (300–400 kg/m³), low thermal conductivity (<0.10 W/(mK)), and good compressive strength (~2 MPa) [21]. These methods are not suitable for glass waste of different origins and colors, contaminated with organic and mineral matter. Therefore, an optimal method for foam glass production was developed for this specific type of waste.

In addition to the fine cullet fraction stream, two other streams are generated: ceramic waste and plastic waste. Ceramic waste in cullet recycling plants is defined as ceramics consisting of porcelain, biscuit, faience, terracotta, and stoneware, as well as fine-grained glass of various colors containing a fraction of organic impurities [7]. There are studies in the literature on the management of porcelain and terracotta, but no studies use all the components of ceramic wastes present in the recycled cullet stream. For example, the use of waste sanitary porcelain and terracotta in cement production was studied, where the maximum substitution of these wastes was 30% [22]. In another study, waste porcelain and red ceramics were used as aggregate substitutes in concrete [23]. Waste from sanitary porcelain production has also been used as a substitute for feldspar or pegmatite in porcelain tile paste [24].

Plastic waste sorted out during glass cullet recycling is mainly polyethylene fractions from both the plastic bags used for selective glass collection and the corks and caps from bottles or jars. Accompanying glass cullet, waste from plastic bags consists of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) [25]. In addition, due to long-term storage in the open air, they are degraded by UV radiation, temperature fluctuations, and moisture. As a result, a reduction in molecular weight, the brittleness of the material, and the formation of carbonyl groups are observed, which negatively affects mechanical properties and prevents their direct mechanical recycling [26]. Additional problems are mineral impurities (sand and fine glass particles) and organic impurities, the presence of which reduces the efficiency of processing and deteriorates the quality of the obtained products. For this reason, the use of chemical or thermal recycling technologies—primarily pyrolysis—is recommended, as it enables the decomposition of polymer chains and the production of value-added compounds such as polyethylene waxes [27]. PE pyrolysis, conducted at 300–550 °C under inert conditions, allows for the production of waxes similar to commercial ones, along with energy recovery and reduced solid residue formation [28,29].

Although the literature includes studies on the pyrolysis of polyethylene waste, it rarely addresses plastic bags, which pose a particular challenge for glass recycling facilities. The pyrolysis of mixed HDPE and LDPE waste at 450 °C, in an autoclave reactor, was studied, resulting in polyethylene wax with an efficiency of 94%. The use of table salt in this case reduced the process temperature [25]. In another study, the authors showed that waste agricultural film can be converted into pyrolytic wax with an efficiency of 95% during pyrolysis in a stirred closed reactor at 380–410 °C [30]. Pyrolytic wax was also obtained from waste high-density polyethylene with a yield of 87.25% during pyrolysis with a semi-batch reactor at an optimal temperature of 600 °C, indicating that temperature has a decisive effect on wax yield, which is critical for industrial applications [31].

To date, the literature has not proposed a comprehensive solution for managing all material streams generated during cullet recycling in a manner that aligns with the principles of the circular economy. Addressing this gap is the primary objective of the present study. Three previously undeveloped waste streams were used in the research: fine glass fraction to produce foam glass, ceramic waste to produce granules, and waste polyethylene plastics to produce waxes is a modern approach to realizing the idea of sustainable development (the streams under study are marked in green in Figure 1). A foaming system based on manganese dioxide (MnO₂) and silicon carbide (SiC) was proposed for the production of foam glass from fine and mixed-color glass fractions, with the MnO₂/SiC weight ratio and process temperature conditions being key parameters. Applied statistical analysis showed the dependence of the MnO₂/SiC ratio and MnO₂ content on the density of the obtained foam glass products. In the study of waste ceramics transformation, the characteristic temperature curve of the process was determined, and the obtained granules were characterized by water absorption and metal content in water extracts derived from the ceramic–glass granules. In the case of waste plastics consisting of polyethylene, studies were conducted to obtain waxes by thermal treatment in two temperature ranges: low temperature (155–175 °C) and high temperature (with an optimum temperature of 395 °C). The melting temperatures of the waxes were investigated, and the material was identified using FTIR analysis. The integrated management of all material streams generated in the glass recycling process allowed for the development of a circular economy model for the glass recycling plant.

2. Materials and Methods

2.1. Materials

The material streams during glass cullet recycling and the final commercial products are shown in Figure 1. Five main streams are generated in the recycling plant: three of them (marked in blue) are processed without issues, while the remaining three streams, fine glass fraction, ceramic waste, and waste polyethylene plastics (marked in green), present challenges for the plant. These problematic streams are the focus of the experimental study described in this article.

Waste material streams generated at the cullet recycling plant: fine, multicolored glass fraction with impurities of <4 mm grain size (Figure 2a), waste ceramics containing glass fines (Figure 2b), and waste LDPE and HDPE plastics (Figure 2c,d), taken from cullet recycling process cycles (Figure 1), were used in the study. Pure manganese dioxide MnO₂ (purchased from Pol-Aura Sp. z o.o. Zawroty, Poland) and waste silicon carbide, which was obtained from glass polishing and grinding processes from the Zawiercie Glassworks with a grain size of 0.1–0.2 mm, were used in the study of obtaining foam glass. Hydrotreated paraffin wax with a melting point of 54–56 °C (purchased from Pol-Aura Sp. z o.o. Zawroty, Poland) was used for the study of obtaining wax by a low-temperature process.

2.2. Sample Preparation

2.2.1. Foam Glass

The waste glass fraction with granulation of less than 4 mm, containing organic and inorganic impurities, was dried at room temperature and then ground in a ring mill. The ground waste was mixed with the reactants silicon carbide SiC and manganese (IV) oxide MnO₂, wetted with water, and samples of the same weight of glass fraction were formed using a cylindrical mold. Molded samples with a height of 0.5 cm and a diameter of 2.5 cm were obtained. The weight ratio of MnO₂/SiC was 0.25, 0.33, 0.5, 0.8, and 1.0, respectively. The moldings were placed in a muffle furnace (model VMK 22, Linn High Therm GmbH, Eschenfelden, Germany). The samples were heated using varying temperature and time conditions for each stage (drying, foaming, annealing, and cooling), which made it possible to determine the optimal thermal treatment conditions for the samples. Thus, the optimal heating curve was determined.

2.2.2. Ceramic–Glass Granules

Ceramic waste is a mixture of porcelain, faience, terracotta, and biscuit. The waste contains an admixture of multi-colored, fine glass and a small amount of organic matter. Samples of ceramic waste were dried at room temperature, averaged, and a sample was separated for laboratory testing. The sample was ground on a ring mill for 15 s to obtain a fine powder. Additives of clay, water, and calcium hydroxide (Ca(OH)₂) in various proportions were introduced into the ground material. Ceramic–glass granules with a diameter of 5–10 mm were formed, which were dried for 12 h under air-dry conditions. The granules were then sintered in a muffle furnace at temperatures of 800–900 °C to determine the optimal sintering curve for the granules. Using the determined temperature conditions, successive granule samples were sintered using a variable proportion of additives.

2.2.3. Polyethylene Waxes

Two types of waste plastics were used in the study: transparent and colored plastic bags, consisting of high-density polyethylene (HDPE) and low-density polyethylene (LDPE), and caps and corks from bottles and jars, mainly composed of HDPE. Both types of waste were washed using an ultrasonic bath and dried under room conditions. The plastic bags were separated into transparent and colored foils (green and black) and shredded. The process of dissolving the prepared waste samples in liquid hydrotreated paraffin at 155–175 °C was then carried out. Under these temperature conditions, the PE decomposition products combine with the paraffin to form a homogeneous substance—polyethylene wax. The resulting waxes were marked as SWPE1 (soft polyethylene wax from colorless bags) and SWPE2 (soft polyethylene wax from colored bags). The hot product was cast in the prepared cylindrical mold to cool. The maximum percentage of waste in paraffin was determined. The second type of waste, cleaned and dried caps and corks, after grinding, were pyrolyzed to obtain polyethylene waxes (HWPE—hard polyethylene wax). The pyrolysis process was carried out in a steel reaction tube with two holes on its surface, which led out the resulting gases (top hole) and liquid (bottom hole). The thermal degradation process was carried out in the temperature range of 375–440 °C with a heating rate of 5 °C/min. Processing time was 20 min for all samples. Additionally, for optimal temperature, samples were foamed for 15, 10, and 5 min. Next, the samples were cooled at room temperature for 24 h, and then the yield of wax, pyrolytic liquid, and gas was determined. The optimal process time and temperature were determined. The obtained products were ground in a ring mill to obtain a powder. The melting point of the obtained waxes was determined. For selected samples, their identification was carried out.

2.3. Methods

The grain size composition of the waste glass fraction with grain size <4 mm was determined by sieve analysis on laboratory sieves. For the smallest fraction, the loss on ignition (LOI) was determined with the standard PN-EN 1744-1+A1:2013-05 [32]. The determination consisted of drying the sample at 105 °C and then calcining it in a muffle furnace at 550 °C for two hours. The loss on ignition was calculated as the percentage difference between the initial and final mass of the sample.

The grain size composition of the ground samples (glass and ceramic waste) was determined using a laser diffraction particle size analyzer, Analysette 22 NeXT (Fritsch, Idar-Oberstein, Germany). The apparatus operates on the principle of laser light scattering, where a dispersed sample passes through a laser beam and a set of detectors measures the resulting scattering pattern. Based on the angular distribution of the scattered light intensity, the particle size distribution is calculated using the Fraunhofer theory.

The apparent density ρ_app was determined using the hydrostatic weighing method based on Archimedes’ principle. This method is suitable for porous and lightweight materials such as foam glass. The mass of the dry sample was first measured in air, followed by immersion in a non-wetting fluid to determine its displaced volume. The apparent density was calculated using the following formula:

$${\rho }_{app}={\displaystyle \frac{m_{air}}{m_{air}-m_{sub}}} {\rho }_w$$

(1)

where m_air is the dry mass measured in air, g; m_sub is the apparent mass when fully immersed in the water, g; ρ_w is the density of water, g/cm³.

Water absorption was determined in accordance with the standard PN-EN 772-21:2011 [33]. Water absorption was calculated based on the ratio of the mass of water absorbed by the sample to the mass of the sample dried at 105 °C to constant mass. The mass of water was calculated based on the difference in the mass of the sample kept for 24 h in water and the mass of the dried sample.

The test for leaching metals from ceramic–glass granules was carried out by preparing water extracts in triplicate, with a liquid-to-solid phase ratio of L/S = 10 dm³/kg. The leaching liquid was deionized water. Then, the extracts were shaken on a laboratory shaker for 24 ± 0.5 h, after which the suspension was filtered. The procedure was carried out based on the standard PN-EN 12457-2:2006 [34]. The contents of Zn, Cr, Pb, Fe, Na, Cd, P, Al, Mg, Ca, K, Cu, and Ni were determined in the water extracts using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Spectro Arcos, Spectro Analytical Instruments GmbH, Kleve, Germany) method with the use of a certified reference material NIST SRM 1643f (Sigma-Aldrich, Saint-Louis, MO, USA, trace elements in water) to validate the results. The following operating conditions were applied: RF power: 1.2 kW; Plasma gas flow: 12 L/min; Auxiliary gas flow: 1.0 L/min; Nebulizer gas flow: 0.7 L/min; Sample uptake rate: 1.0 mL/min; Internal standard-Yttrium. The Limit of Detection (LOD) and Limit of Quantification (LOQ) for selected elements were determined based on calibration curve parameters and signal-to-noise ratios (S/N = 3 for LOD, S/N = 10 for LOQ). All measurements were performed in triplicate, and the precision (RSD) for all elements was below 5%.

The distribution of functional groups in the polyethylene waxes (SWPE, HWPE) was assessed using a Fourier-transform infrared spectroscopy (Nicolet^TM iS50 FTIR Spectrometer/Thermo Fisher Scientific, Madison, WI, USA). FTIR spectra were recorded in the range from 4000 to 400 cm⁻¹. Melting points of the waxes were determined using a thermocouple, a temperature recorder, and a hot plate with a constantly controlled temperature increase (2 °C/min). 5 g of wax were placed in a glass beaker in which a thermocouple connected to the recorder was placed. The sample was heated, and data recording (time vs. temperature) was started. The melting temperature was read at the moment of slowing down the temperature increase when the effect of heat absorption on the phase change occurs.

Macroscopic images of the cross-sections of the foam glass samples, as well as photographs of the ceramic–glass granules, were taken using a high-resolution digital camera. The photos were used to evaluate the pore distribution, morphology, and structural homogeneity of the obtained materials.

Statistical Analysis

Statistical analyses were performed using Statistica 13.3 software. A two-way analysis of variance (ANOVA) was used to evaluate the effect of two independent factors on the density of foam glass, namely:

• MnO₂/SiC weight ratio—5 levels: 0.25, 0.33, 0.5, 0.8, and 1.0;
• MnO₂ content—3 levels: 0.4 g, 0.8 g, and 1.0 g.

The dependent variable was the density of the foam glass (g/cm³). Post hoc comparisons were performed using Tukey’s Honest Significant Difference (HSD) test, which controls for family-wise error rate. Therefore, no additional Bonferroni correction was applied. Statistical significance was considered at p < 0.05.

3. Results

3.1. Foam Glass Characteristics

Foam glass was produced from a stream of fine-grained glass fraction and additives (MnO₂ and SiC) under the conditions shown in the heating curve of the foaming process of the samples (Figure 3). The annealing temperature was 900 °C for 30 min, the stress-relief temperature was 650 °C for 2 h, and the cooling time was 12 h.

The waste glass fraction with a grain size >4 mm constitutes a secondary material stream from the glass cullet recycling process. This fraction is contaminated with organic matter (label residues) and inorganic matter (ceramic and sand particles). This is indicated by the loss on ignition (LOI), which is 3.9%. The share of the fraction contaminating the tested material depends on the origin of the glass cullet, its storage, and the method of transport. Due to economic reasons and the lack of interest in this waste, no processing methods are used. The granulometric analysis of the ground glass was carried out using a laboratory sieve method, which revealed that the fraction smaller than 1 mm constitutes 62% of the sample. The fraction 4 mm > d > 1 mm constitutes 38%, while the fraction <0.063 mm is 4.2% in the tested sample (

Table 1. Granulometric analysis of fine-grained waste glass.

Grain Size, mm	Share, %
>4	0
4–2	7.0
2–1	31.0
1–0.5	29.0
0.5–0.25	18.8
0.25–0.125	8.0
0.125–0.063	2.0
<0.063	4.2

).

As a first stage, the research on foam glass production from waste glass fractions focused on grinding the material and analyzing its granulometric composition, which was performed using a laser particle size analyzer (Figure 4).

The waste had to be ground to obtain a fine-grained fraction of <100 µm. The basic fractions in the tested sample are: 0.1–10 µm (31.2%) and 10–100 µm (52.3%). A material with such a high content of fine fractions, where 90% are grains with a diameter of <132 µm, is suited for the production of foam glass. The fine fraction is crucial for uniform foaming and high-quality foam glass [35]. Fractions with dimensions larger than 300 µm do not occur in the analyzed sample. During the formation of samples sent to the foaming process, an important aspect was the selection of the optimal weight ratio of MnO₂/SiC, because the reaction between SiC and MnO₂ provides oxygen to the reaction environment [36]. The foaming system was as follows: MnO₂/SiC = 0.25; 0.33; 0.5; 0.8; 1.0, with different mass fractions of MnO₂ and SiC.

Table 2. Composition of samples and the influence of the MnO₂/SiC coefficient on the density of foam glass.

Sample No.	MnO2, g	MnO2/SiC	Density g/cm3	Sample Composition wt%
				Glass	MnO2	SiC
1	0.4	0.25	0.5	96.15	0.77	3.08
2		0.33	0.46	96.88	0.78	2.34
3		0.5	0.45	97.66	0.78	1.56
4		0.8	0.43	98.23	0.79	0.98
5		1.0	0.41	98.42	0.79	0.79
6	0.8	0.25	0.4	92.59	1.48	5.93
7		0.33	0.36	93.95	1.50	4.55
8		0.5	0.34	95.42	1.53	3.05
9		0.8	0.30	96.53	1.54	1.93
10		1.0	0.28	96.90	1.55	1.55
11	1.0	0.25	0.26	90.91	1.81	7.27
12		0.33	0.24	92.59	1.85	5.56
13		0.5	0.21	94.34	1.89	3.77
14		0.8	0.19	95.69	1.92	2.39
15		1.0	0.18	96.16	1.92	1.92

presents the composition of individual samples, labeled from 1 to 15, along with the density of the resulting foam glass, where the density was determined by the hydrostatic method using a hydrostatic balance. Selected cross-sections of the obtained foam glass are shown in Figure 5, and the images were taken with a high-resolution digital camera.

The mutual ratio of reducers to oxidizers has the greatest influence on the redox state of glass, especially multi-colored and contaminated cullet. Oxygen released in this reaction performs two functions. The first is the oxidation of impurities accompanying glass cullet (paper, plastics, food residues). The second function of oxygen is the generation of foaming gases resulting from the active decomposition of SiC. Silicon carbide is reactive towards oxygen at high temperatures. The rate of SiC oxidation increases at 900 °C. The kinetics of the oxidation reaction also depend on the substances in contact with the SiC surface. Manganese (IV) oxide, which decomposes to Mn₂O₃ at these temperatures, can be used as an oxidant, supplying oxygen to the reaction. Silicon carbide oxidizes, and at the same time, a passivating layer of SiO₂ is formed on its surface, which dissolves in the glass. Diffusion to the SiC surface is facilitated, and Mn₂O₃ dissolves in the glass mass, affecting its color. The gases produced, mainly CO₂, cause foaming of the glass. The appropriate MnO₂/SiC weight ratio affects the amount of oxygen, strength, specific density, and uniform distribution of pores in the foamed material.

Statistical analysis confirmed a statistically significant effect of both MnO₂ content and the MnO₂/SiC ratio on the foam glass density (p < 0.05, two-way ANOVA). Selected post hoc comparisons are shown in

Table 3. Results of post hoc (different letters indicate significant differences (p < 0.05) according to the Tuckey test (HSD).

Sample Description	Density of Foam Glass, g/cm3	Group
MnO2 = 1.0 g, MnO2/SiC = 1.0	0.180	a
MnO2 = 1.0 g, MnO2/SiC = 0.8	0.190	a
MnO2 = 1.0 g, MnO2/SiC = 0.5	0.210	ab
MnO2 = 1.0 g, MnO2/SiC = 0.33	0.240	abc
MnO2 = 1.0 g, MnO2/SiC = 0.25	0.260	abcd
MnO2 = 0.8 g, MnO2/SiC = 1.0	0.280	abcde
MnO2 = 0.8 g, MnO2/SiC = 0.8	0.300	bcde
MnO2 = 0.8 g, MnO2/SiC = 0.5	0.340	bcdef
MnO2 = 0.8 g, MnO2/SiC = 0.33	0.360	cdef
MnO2 = 0.8 g, MnO2/SiC = 0.25	0.400	defg
MnO2 = 0.4 g, MnO2/SiC = 1.0	0.410	efgh
MnO2 = 0.4 g, MnO2/SiC = 0.8	0.430	fgh
MnO2 = 0.4 g, MnO2/SiC = 0.5	0.450	gh
MnO2 = 0.4 g, MnO2/SiC = 0.33	0.460	gh
MnO2 = 0.4 g, MnO2/SiC = 0.25	0.500	h

. The impact of the MnO₂/SiC weight coefficient and MnO₂ content is clearly illustrated in Figure 6.

Table 3 presents the results of the Tukey HSD post hoc test in a simplified format. Values are sorted from the lowest to the highest density. Identical letters in the ‘Group’ column indicate no statistically significant difference between means (p < 0.05), while different letters denote significant differences.

The two-way ANOVA results demonstrated that the MnO₂ content had the strongest effect on foam glass density (F = 168.48; p < 0.0001), while the MnO₂/SiC weight ratio also had a statistically significant, though less pronounced, impact (F = 10.76; p < 0.0001). No significant interaction between the two factors was observed (F = 0.23; p = 0.98), indicating that their effects are independent.

Post hoc analysis using Tukey’s HSD test revealed that the density of foam glass increased with decreasing MnO₂ content and MnO₂/SiC weight ratio. At the extreme MnO₂ contents tested (0.4 g and 1.0 g), a reduction in the MnO₂/SiC ratio from 1.0 to 0.8 did not significantly affect density values. Similarly, for mixtures containing 0.4 g MnO₂, reducing the MnO₂/SiC ratio generally did not result in statistically significant changes in density.

The lowest density (0.18 g/cm³) was recorded for samples with MnO₂ = 1.0 g and MnO₂/SiC = 1.0, while the highest (0.50 g/cm³) was obtained for MnO₂ = 0.4 g and MnO₂/SiC = 0.25. The difference between these extremes was highly significant (p < 0.001). Full post hoc results are presented in Table 3, with identical letter groupings indicating no statistically significant difference at p < 0.05.

The lowest density was observed in foam glass samples with a MnO₂/SiC weight ratio of 1.0 (Figure 7).

Theoretically, the given value of the MnO₂/SiC coefficient, for mass quantities of 0.4/0.4, 0.8/0.8, and 1.0/1.0, allows for the achievement of the lowest values of foam glass density for each mass set. At the same time, a MnO₂/SiC equilibrium ratio of 1.0 ensures an adequate supply of oxygen to the reaction environment, enabling the complete oxidation of SiC to silicon dioxide (SiO₂) and carbon dioxide (CO₂). Finally, the applied process conditions ensure the production of low-density foam glass, effective foaming within the appropriate time interval, and a uniform pore distribution in the final product.

3.2. Ceramic–Glass Granules Characteristic

Glass–ceramic granules were produced from a stream of ceramic waste. The main process in their production was firing in a furnace. By applying an appropriately selected heating profile, a granular product was obtained. The heating profile is shown in Figure 8.

Ceramic–glass granules from ground ceramic waste, glass, and additives: clay, water, and calcium hydroxide were produced. Granulometric analysis of ground ceramic waste, which was performed using a laser particle size analyzer, showed that the dominant fractions are 0.1–10 μm, 11–30 μm, and 31–50 μm (

Table 4. Granulometric analysis of ceramic waste after grinding.

Grain Size, µm	Share, %
0.1–5	28.2
6–10	19.0
11–30	32.0
31–50	10.5
51–100	1.5
101–300	8.8
>300	0.0

). The finest grain size class (0.1–10 μm) constitutes 47.2%, while 90% of the material is grains <88 μm.

Kaolin or ordinary clay is a common mineral component of ceramic waste. These types of ingredients can be used as binders in forming ceramic–glass granules [37].

Table 5. Composition of raw ceramic–glass granules, wt%.

Component	Granules 1	Granules 2	Granules 3
Ceramic	5.9	25.0	24.7
Glass	35.2	37.5	37.0
Clay	29.4	12.5	12.4
Water	29.4	25.0	24.7
Ca(OH)2	0.1	0	1.2

presents the composition of raw granules of 5 to 10 mm in diameter.

Ceramic–glass granules after calcination according to the sintering curve are hard balls with a low-pore structure (Figure 9). The water absorption is within the range of 18–20%. The photos were taken with a digital camera, while water absorption was determined by immersing the samples in water for 24 h, then weighing, drying, and reweighing the samples.

The results indicate that ceramic waste from glass cullet recycling can be used as ceramic–glass granules for loosening, aeration, drainage, and moisture distribution in degraded and acidic soils. The condition for using such granules is the absence of heavy metals in water extracts. Studies have shown that in water extracts from ceramic–glass granules, heavy metals such as Zn, Cr, Cd, Cu, and Ni occur below the detection threshold (

Table 6. Metal content in water extracts.

Type of Granules	Metal Content in Water Extracts, ppm													pH After 24 h
	Zn	Cr	Pb	Fe	Na	Cd	P	Al	Mg	Ca	K	Cu	Ni
Granules 1	<0.001	<0.001	4.2	8.24	47.8	<0.001	13.6	75.8	7.98	64.4	73.9	<0.001	<0.001	8.9
Granules 2	<0.001	<0.001	4.4	6.1	48.3	<0.001	13.8	84.5	8.0	61.2	90.0	<0.001	<0.001	8.7
Granules 3	<0.001	<0.001	4.3	8.1	46.0	<0.001	14.0	84.5	7.8	58.4	73.4	<0.001	<0.001	9.0

). Another direction may be the use of ceramic–glass granules as a substitute for aggregates in the production of lightweight concrete [38,39]. However, this requires further research into construction aggregates.

The main factor determining the mobility of heavy metals in the soil environment is its pH value [40]. In the case of acidic soils, liming neutralizes soil acidity, removes the physiological effect of mineral fertilizers, and neutralizes trace elements. This neutralization consists of the immobilization of metals by their transformation into insoluble forms [41]. Most heavy metals are easily absorbed by plants when the soil is acidic. The pH value of water extracts was 8.7–9.0 and increased slightly over the next few days (Table 6). It indicates the gradual release of alkali from the glass contained in ceramics. This property of granules affects the limitation of heavy metal mobility in the soil.

3.3. Polyethylene Waxes Characteristic

3.3.1. SWPE Wax

The crushed waste polyethylene bags were dissolved in hydrotreated paraffin at a temperature of 155–175 °C. The selection of the temperature range resulted from the need to achieve the flow temperature (T_f) for HDPE waste (130–160 °C) and to obtain a higher T_f temperature for LDPE waste (110–120 °C) [42,43]. Exceeding the flow temperature of the polymer causes its degradation, and the melted paraffin combines with the products of polyethylene decomposition into a homogeneous substance. The study determined the efficiency of the process by weight substitution of polyethylene in relation to the weight of paraffin. The efficiency of the process was 27–30%. The melting point of the obtained products was 55 °C for the SWPE1 sample (colorless bags) and 60 °C for the SWPE2 sample (green and black bags) (

Table 7. Conditions for SWPE waxes and the melting point.

Sample	Substitution, %	Temperature, °C	Time, min	Melting Point, °C
SWPE1	30.0	155–175	30	55
SWPE2	27.0	155–175	30	60

). The melting points for soft paraffin waxes are in the range of 40–60 °C [44].

The FTIR spectrum of SWPE1 shows characteristic absorption bands that confirm the presence of chemical groups typical for long-chain saturated hydrocarbons (Figure 10).

The most intense bands are located in the range of 2913 cm⁻¹ and 2846 cm⁻¹, which correspond to the asymmetric and symmetric stretching of C–H bonds in CH₂ groups. These are typical signals for aliphatic saturated chains present in the polyethylene structure. The presence of a band at 1472 cm⁻¹ can be attributed to the deformation vibrations of CH₂ groups, while the signal at 1377 cm⁻¹ is associated with the deformation vibration of CH₃ groups. Additionally, the peak around 729 cm⁻¹ corresponds to the pendulum vibrations of the CH₂ group, which is characteristic of long-chain systems such as polyethylene (Figure 10). The above observations indicate that the tested sample is polyethylene wax, which is also confirmed by the FTIR spectrum of the standard [45].

3.3.2. HWPE Wax

Caps and corks of different colors were pyrolyzed after cleaning and grinding. The polymer degradation process was carried out over a temperature range of 375 °C to 440 °C. The processing time at a given temperature was 20 min, (for HWPE1-HWPE8 trials). For the optimal process temperature, trials were performed for 15, 10, and 5 min (for HWPE3_15, HWPE3_10, and HWPE3_5). Thermal degradation of polyethylene led to low-molecular-weight polymers and liquid oligomers with different mass shares. During the pyrolysis of polyethylene, gaseous degradation products containing alkanes and alkenes were also released. The temperature and time of the process affected the yields of the various products [32]. The optimal process temperature was determined based on the wax yield and other pyrolysis products—liquids and volatiles (gases). According to

Table 8. Conditions of the pyrolysis, yield of wax, gas, and liquid, and melting point of the waxes.

Sample	Pyrolysis Temperature, °C	Pyrolysis Time, min	Melting Point of Wax, °C	Yield, %
				Wax	Gas	Liquid
HWPE1	375	20	-	0.00 *	0.77	0.00
HWPE2	385	20	105	97.08	2.92	0.31
HWPE3	395	20	104	93.56	6.44	0.30
HWPE4	400	20	103	89.24	10.76	0.33
HWPE5	410	20	100	85.90	14.10	0.26
HWPE6	420	20	98	58.30	32.90	8.80
HWPE7	430	20	95	34.63	42.11	23.19
HWPE8	440	20	95	14.41	60.10	25.39
HWPE3_15	395	15	104	94.41	5.30	0.29
HWPE3_10	395	10	103	95.62	4.10	0.28
HWPE3_5	395	5	103	96.15	3.60	0.25

* no polymer degradation occurred.

, the optimal temperature of the thermal degradation process was 395 °C, a time of 5 min (sample HWPE3_5), for these conditions the yield of wax was 96.15%, with small proportions of liquids (0.25%) and volatiles (3.6%). For comparison, the yield of polyethylene wax from HDPE waste in the study conducted by Panda et al. was 87.25% [31].

The melting temperature of the obtained products is 95–105 °C. This is a typical temperature for polyethylene waxes [46]. The FTIR spectra of selected polyethylene wax samples (HWPE3, HWPE_5) exhibit characteristic absorption bands, confirming the presence of typical chemical groups associated with long-chain saturated hydrocarbons (Figure 11). The absorption bands coincide with those present in the FTIR standard of polyethylene wax [45].

4. Discussion

The implementation of circular economy principles is a key strategy for addressing the overexploitation of natural resources, which leads to the generation of substantial amounts of waste. In the case of waste glass, which is characterized by high durability and does not undergo biological degradation, its proper storage, selective collection, and recycling help minimize the negative impact on the environment. Recycled glass can be reused as a secondary raw material in metallurgical processes. It should be noted that only glass that meets certain quality requirements—homogeneous in color and free of impurities—is sent for remelting. One of the main challenges in glass recycling is the presence of fine cullet fractions of various colors, which often contain impurities such as ceramics, paper, and plastics. Due to its heterogeneity, this type of material is rejected and is not subject to standard recycling processes. In the presented work, an alternative use of this fraction was proposed by processing it into foam glass using the MnO₂/SiC foaming system. The use of this type of system allows control of the foaming process, enabling the production of material with the desired density. The obtained results may serve as a basis for semi-technical trials, which could exhibit certain deviations due to process scaling—such as the influence of charge mass, temperature distribution, or heat transfer characteristics. Therefore, further large-scale studies are necessary. The literature also indicates that using silicon carbide as a foaming agent may be associated with high costs [47]. In the proposed approach, this problem was partially mitigated by the use of waste SiC, derived from glass grinding and polishing processes, which is recovered in accordance with the procedure described in the patent application PAT.227000 [48]. In the context of using MnO₂ as a foaming agent, the optimal solution could be to use the “black mass” from recycled used batteries. This direction is the subject of planned research in the next stages of work. It is worth noting that during the production of foam glass using the MnO₂/SiC system, the only gases released are carbon dioxide, originating from the thermal decomposition of silicon carbide, and oxygen, released during the decomposition of manganese dioxide. This process does not generate emissions of other potentially harmful gases. The structure of the resulting foam glass contains bubbles containing mainly trapped CO₂ and O₂, as well as trace amounts of gases formed by the oxidation of organic substances present in the waste material.

Ceramic waste generated in waste glass recycling is a heterogeneous mixture of materials such as porcelain, faience, terracotta, glaze, and bisque. A common feature of these components is the presence of kaolinite or clay, and admixtures of multi-colored glass and organic and inorganic impurities. By enriching the chemical composition of this mixture with appropriate additives, ceramic–glass granules were obtained. The potential area of application of ceramic–glass granules is the reclamation of soils degraded by industrial activity or intensive agricultural use. The basic factor affecting the mobility of heavy metals in the soil environment is pH—in soils with alkaline pH (pH > 7), these metals are transformed into poorly soluble forms, limiting their bioavailability. In the case of acidic soils, using the described granules can contribute to their gradual alkalization, which has a positive effect on the stabilization of heavy metals in the long term. Moreover, the analyses of water extracts from ceramic–glass granules did not reveal the presence of heavy metals, indicating the safe use of the obtained granules in soils. The relatively low absorbability of these granules (18–20%) suggests that they can be used as insulation materials, m.in. for insulating ceilings, attics, and floors, as well as an element of band drainage. However, this requires further research.

Unlike the previously discussed material streams, which do not degrade (e.g., glass, ceramics), plastics are characterized by their susceptibility to destruction by environmental factors such as UV radiation, changing temperature conditions, or moisture. These processes lead to an irreversible loss of their mechanical and physicochemical properties, which significantly limits the possibility of reusing them in their original applications, especially in the production of new plastic components, due to the deterioration of the quality of the final product. This study investigated the thermal degradation of waste polyethylene, which resulted in the formation of two distinct products: soft wax produced under low-temperature conditions and hard wax produced under high-temperature conditions. The first is used to a limited extent, m.in. in candle production, shoes, and floor polishes, where its addition extends the burning time and improves the gloss of the surface. Additionally, thanks to the phase change temperature in the range of 45–55 °C, soft wax can be used as a material for heat accumulation in domestic water tanks. Depending on the type of paraffin used, the amount of heat energy adsorbed ranges from 140 to 240 kJ/kg [45]. Hard waxes obtained in high-temperature processes can be used to modify paraffins and waxes, thus enabling changes in their physicochemical properties, such as melting, solidification, or dropping point, and improving performance parameters—viscosity, hardness, and elasticity. Polyethylene technical wax is an effective viscosity modifier in plastic processing. In addition, it is a good slip additive and dispersant, facilitating the homogeneous distribution of pigments and dyes in the polymer matrix [49]. Polyethylene waxes can be subjected to a controlled oxidation process, the purpose of which is to introduce polar functional groups into the polymer chain structure. This preliminary step enables further modification to obtain stable aqueous emulsions. Currently, research is being conducted on the ozone oxidation of polyethylene waxes and on the development of technologies for producing aqueous emulsions based on these materials, which represent a promising direction for industrial applications.

Foam glass plays an important role among thermal insulation materials, particularly as a foundation slab insulation material, even on waterlogged soils. The costs of foam glass insulation are comparable to other solutions. For example, the market price of foam glass is approximately 53–67 EUR/m³, expanded polystyrene (EPS) costs 53–76 EUR/m³, and extruded polystyrene (XPS) costs 98–111 EUR/m³. In the case of foundation slabs, EPS with a thickness of 24 cm costs about 15.6 EUR/m², and XPS with a thickness of 20 cm costs about 20.9/EUR/m². Foam glass, used at a thickness of 50 cm, costs 26.7–33.3 EUR/m² (price data obtained from the local market, 2025). When comparing the total cost of EPS, XPS, and foam glass insulation, it is necessary to include the cost of additional construction layers. EPS insulation requires a drainage layer of aggregate at least 15 cm thick (8.9 EUR/m²), a 10 cm sand bedding layer (3.6 EUR/m²), and a moisture barrier foil layer. XPS insulation requires only a sand bedding layer. Foam glass insulation requires only a geotextile layer (0.67 EUR/m²). After including all costs, EPS foundation insulation totals about 28.0 EUR/m², XPS about 24.4 EUR/m² (110 PLN/m²), and foam glass about 27.3–33.3 EUR/m². Although the initial cost of foam glass is slightly higher than XPS, its lifespan is significantly greater—around 100 years, compared to 20 years for EPS or XPS. Furthermore, EPS and XPS are petroleum-based materials, whereas foam glass can be produced entirely from waste glass that would otherwise be landfilled. In the technology proposed in this study, a product of negative waste value is transformed into a durable, eco-friendly material that is easy to apply, aligning directly with the principles of the circular economy.

Ceramic–glass granules produced from ceramic waste (a mixture of porcelain, faience, terracotta, and biscuit with fine glass) can serve as a material for soil loosening, aeration, drainage, and moisture distribution in degraded and acidic soils, as well as a potential substitute for lightweight aggregates in concrete production. For comparison, commercially available expanded clay aggregate (keramzite) costs about 49–62 EUR/m³, horticultural perlite 78–100 EUR/m³, and drainage gravel 27–36 EUR/m³. (price data obtained from the local market, 2025). While drainage gravel is cheaper, it lacks the lightweight and porous structure beneficial for aeration and moisture retention in soils. Perlite, on the other hand, is more expensive and less durable under long-term soil contact conditions. When considering the circular economy perspective, ceramic–glass granules present a significant advantage: they are manufactured entirely from post-consumer ceramic waste from glass cullet recycling, which would otherwise be landfilled. The environmental benefits are further strengthened by the absence of detectable heavy metals (Zn, Cr, Cd, Cu, Ni) in water extracts, ensuring safe use in agricultural and landscaping applications. As production scales up, costs could become even more competitive with traditional lightweight aggregates, enhancing both the environmental and economic viability of the material. As further validation, a more detailed techno-economic assessment or cost breakdown analysis would be worthwhile in future work to substantiate these figures.

Many aspects indicate that the polyethylene waxes produced in this study from post-consumer polyethylene waste—including HDPE and LDPE bags and HDPE bottle caps—can be cost-competitive compared to commercial PE waxes available on the market. Commercial virgin polyethylene waxes are typically priced in the range of 1.2–2.0 EUR/kg in Europe, depending on purity, hardness, and application grade [50]. Several factors suggest that the product obtained in this study could be priced lower than commercial virgin polyethylene waxes. These include the negative or near-zero acquisition cost of the waste feedstock—HDPE/LDPE films and HDPE caps—and the relatively moderate energy requirements of the production processes (dissolution at 155–175 °C for soft waxes, SWPE1 and SWPE2, and pyrolysis at 375–440 °C for hard wax, HWPE). From a circular economy perspective, the substitution of virgin PE wax with wax derived from waste diverts significant volumes of polyethylene from landfill or incineration while producing a high-value industrial additive used in applications such as masterbatches, hot-melt adhesives, and coatings. The environmental and economic benefits combined make waste-derived PE wax an attractive alternative.

5. Conclusions

Research has shown that glass, ceramic, and polyethylene waste can be effectively processed into materials with high application potential, in line with the principles of the circular economy. The technology for producing foam glass from fine-grained glass cullet enables the manufacture of a durable and safe insulating material with reduced environmental impact, particularly through the use of waste silicon carbide (SiC). Ceramic–glass granulate obtained from mixed ceramic waste is a promising material for the remediation of degraded soils and as a substitute for lightweight aggregates while eliminating the risk of heavy metal release. Polyethylene waxes derived from the thermal degradation of PE waste exhibit properties suitable for use in the polymer industry, energy storage, and consumer products, at potentially lower costs than virgin waxes.

The results confirm both the environmental and economic benefits of the proposed technologies and highlight the need for further research on a technical and industrial scale.