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Article

An Investigation of Fly Ash and Slag Processing and Fiber Production Using Plasma Technology

by
Žydrūnas Kavaliauskas
*,
Romualdas Kėželis
,
Viktorija Grigaitienė
,
Mindaugas Milieška
,
Vitas Valinčius
,
Rolandas Uscila
,
Rita Kriūkienė
and
Arūnas Baltušnikas
Plasma Processing Laboratory, Lithuanian Energy Institute, Breslaujos Str. 3, LT-44403 Kaunas, Lithuania
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(11), 4801; https://doi.org/10.3390/app14114801
Submission received: 21 May 2024 / Revised: 29 May 2024 / Accepted: 31 May 2024 / Published: 1 June 2024
Browse Figures
Versions Notes

Abstract

:
As the world’s population grows and resource consumption rises, there will be an increase in household waste and, consequently, the fly ash and slag from after the incineration of this waste need to be properly recycled or used as secondary raw materials in other industries. The authors of this work propose an innovative technology that has not yet been studied much in the literature; the obtained slag and fly ash are processed by plasma technology, and from those, fiber is formed. As shown by SEM studies, the raw material (before plasma–chemical treatment) consists of irregularly shaped granules or rod-shaped microforms. EDS studies show that both the original material and the products obtained after plasma–chemical processing are composed of various metal elements, but they make up a significantly small part, as the predominant elements are C, O, Cl, Si, and Ca. BET studies show that when the fiber is obtained with a plasma torch power of 55.5; 55.6 and 74 kW, respectively, the specific surface area is 11.9; 8.5 and 3 m2/g. It is worth noting that the raw material obtained after plasma–chemical processing is sufficiently clean and harmless to the environment, so it can be used as a secondary raw material, e.g., for the production of thermal insulation, in the production of catalysts, as a component in the production of concrete, or in other industries.

1. Introduction

Increasing amounts of waste of various origins are generated in the world daily, such as various industrial production waste, household waste, etc. Most waste, both organic and inorganic, contains various harmful elements—organic compounds or heavy metals. Various methods, such as incineration, pyrolysis, mechanical shredding, etc., are used for waste processing [1,2,3,4]. Pyrolysis is one of the most common processing methods, relying on the thermal decomposition of a polymer in an inert environment, usually nitrogen or air. In this process, part of the organic material breaks down into molecules that, in certain cases, can be recovered and used. Through pyrolysis, uncured resins can be recycled while maintaining their original morphology. During this process, it is possible to obtain not only filler particles and fibers but also organic components and liquid fuel. Pyrolysis usually takes place in the temperature range of 300 to 500 °C, and catalysts are sometimes used to lower the reaction temperature [1,2,5,6,7,8]. There are different types of pyrolyzers, including stationary reactors, rotary kilns, screw pyrolyzers, fluidized bed reactors, etc. As for the production of fuel from composites during pyrolysis, in some studies, organic liquid compounds have been obtained. However, in many cases, the method of gas and fuel extraction does not prove economically viable due to the low proportion of the extracted amount compared to the cost of the process [9,10,11,12].
Inertization is a method that is designed to reduce the toxicity of fly ash or neutralize hazardous substances left after the incineration of household waste. This process often involves modifying chemical properties by adding certain substances that help to harmlessly stabilize or neutralize hazardous substances [1,2,3,13,14,15]. This can be achieved by adding mineral sorbents or chemical reagents that interact with toxic substances and prevent their spread into the environment. The inertization method is important for reducing the impact of fly ash on the environment and ensuring safer handling and disposal. However, it is important to pay attention to the fact that the effective inertization of fly ash may depend on the composition, conditions, and technological possibilities, so well-thought-out planning and the correct implementation of the process are required [1,2,16,17,18,19].
One of the most popular waste recycling methods is incineration at a temperature of about 800 °C. This method is popular due to its simplicity and relatively low energy costs. However, this is not a suitable method for reprocessing difficult-to-decompose compounds such as dioxins or furans and various heavy metals [20,21,22,23,24]. When the waste is burned, thermal energy is generated, which is used for electricity generation or heating. However, in this case, a lot of solid slag is formed, and a lot of small particles (fly ash) are deposited on the filters. It is not possible to process this waste using only incineration technology. Various types of fly ash are known to be toxic to the environment. It is predicted that, in 2025, globally, 2200 Mt of fly ash will be generated due to urbanization. These ashes are very harmful because they are light enough to be carried into the atmosphere as fine particles. These particles can enter the respiratory or digestive systems of living organisms [1,2,3,25,26,27]. Also, fly ash is dangerous for aquatic life, as waste of this origin can easily pollute water bodies. The mentioned volatile products must also be recycled or properly used. One of the fields to use both obtained slag and fly ash in is construction, e.g., the construction of pavement tiles, concrete, road surfaces, etc. However, this is not a simple process because it is necessary to ensure that these ashes do not affect the environment and do not deteriorate the properties of structural materials [1,2,3,4].
In summary, waste treatment methods have different advantages and disadvantages. Pyrolysis allows for the processing of raw resins while maintaining their original morphology and obtaining filler particles, fibers, organic components, and liquid fuel, but it can be economically inefficient due to the low amount of extracted products and requires an inert environment, which increases the cost of the process. Inertization reduces the toxicity of ash or neutralizes hazardous substances remaining after the incineration of household waste, but its effectiveness depends on the composition of the waste, the conditions, and technological possibilities, requiring careful planning and implementation. Incineration is a simple process with relatively low energy consumption that generates thermal energy, but it is inefficient for the processing of hard-to-degrade compounds and heavy metals, and it also produces a large amount of solid waste (slag and ash) that is highly toxic to the environment, requiring additional waste treatment by different methods. Plasma technology, with a process temperature of 2000 °C and beyond, ensures the complete decomposition of hazardous organic and inorganic substances and allows for the processing of both slag and ash, yielding stable materials such as wollastonite glass ceramics and high-quality fibers but with the potential for high energy expenses. Each method has its own specific advantages and disadvantages, depending on the type of waste and the treatment goals, but plasma technology stands out as the most advanced method, being especially suitable for the neutralization of hazardous substances.
During plasma processing, the process temperature can reach 2000 °C and beyond. At such high temperatures, all dangerous compounds of both organic and inorganic origin are completely broken down. In addition, both fly ash and slag vitrification processes can be applied using the same technology simultaneously [2,3,4,5]. Predominant materials in this process can include SiO2 MgO, Ca(OH)2, CaCO3, etc. It has been found that wollastonite glass ceramics can be obtained from recycled glass, fly ash waste (obtained from an incinerator), monoclinic wollastonite (b-CaSiO3), gehlenite (Ca2Al2SiO7), etc. Such compounds are stable, so their impact on the environment is imperceptible, and they can be safely stored in landfills or used as a secondary raw material in production. Vitrification using plasma pyrolysis has been found to reduce environmental costs [1,2,3,28,29,30].
Fly ash can be made into fiber using plasma technology, which is one of the most advanced ways to recycle fly ash and use it to make new materials. This process takes place under a high-temperature gas environment (2000 °C and beyond) and also provides high kinetic energy, which causes the conversion of fly ash or slag into fiber [1,2,3,4]. The plasma, which is formed at high temperatures, electric fields, and flow rates, partially melts the volatile particles of the ashes and transforms them into a fine micrometer fiber. Plasma technology ensures high-quality fiber production, which can be used in various industries, such as construction and automotive manufacturing, for the production of insulation materials, etc. However, in the scientific literature, there is almost no data on the use of plasma technologies for fiber production from fly ash and slag processing.
The purposes of this study were to process fly ash and slag (formed during the incineration of household waste) using a plasma-based chemical process, to investigate the influence of technological parameters on the properties of the processed waste, and to evaluate the possibility of obtaining fiber during this process.

2. Materials and Methods

Plasma technology was used for the processing of slag and fly ash. A plasma generator (plasma torch) directly connected to a plasma–chemical channel consisting of separate water cooled sections was used to realize the plasma–chemical treatment. Before injecting the slag and ash particles, they were mechanically trimmed using a mechanical grinding tool, ML-1. The primary raw material was crushed to a powder with a diameter of 150–500 µm fraction. The diameter of the plasma–chemical channel was 13 mm. A plasma torch was used to form the plasma, the electric current of which varied between 139 and 180 A, and the power varied between 63 and 71 kW. The plasma-forming gas was air with a total flow rate of 13 g/s, and the ionized gas flow rate at the reactor exit nozzle reached 900 m/s. The average temperature of the plasma flow at the exit from the reactor was about 2300 °C. The fiber tested in this work was obtained using a special plasma–chemical channel (as a plasma–chemical reactor), a schematic of which is shown in Figure 1.
The temperature characteristics of the plasma–chemical process were calculated by evaluating the thermal balance throughout the process. A Hitachi S-3400 N model scanning electron microscope (SEM) was used to evaluate the morphology and geometric characteristics of the primary material and the obtained product after the plasma–chemical process. The energy-dispersive X-ray spectroscopy (EDS) method was used to evaluate the elemental composition of the raw material and post-processing products and was performed in conjunction with surface scanning electron microscopy. X-ray diffraction (XRD) with standard Bragg–Brentano focusing geometry was used to study the raw material and the post-processed material (to determine the crystalline structure and predominant compounds). Thermogravimeter Netzsch STA 449 F3 (TG) was used for thermal analysis of the raw material and the post-processing material. Fourier transform infrared spectroscopy (FTIR) was used to evaluate the functional groups in the raw material (slag and fly ash) and the material after processing. By analyzing the FTIR spectra, the changes in the characteristic peaks of organic compounds were evaluated, revealing the dynamics of substances of organic origin that appear during their formation. The specific surface area and porosity of the obtained microfibers were determined by the BET method. Before BET, the samples were degassed at 200 °C.

3. Results and Discussion

The raw material was processed during the plasma–chemical process, and the fiber was also obtained. The surface morphologies of the original material and the material obtained after the plasma–chemical process were studied using an SEM. Images from the SEM studies are presented in Figure 2.
Figure 2a shows SEM images of wood ash as raw material before plasma treatment. As can be seen, the wood ash consists of various irregularly shaped microparticles with a diameter of up to 10 µm. This material is obtained after the wood combustion process. Since the wood combustion process is variable and heterogeneous, particles of different sizes are formed. Figure 2b shows SEM images from when wood ash was processed in a plasma–chemical reactor. As can be seen, the surface consists of irregularly shaped particles. The particles are non-sharp-edged and round. The diameter of the particles is up to 15 µm. It is likely that the particles develop a rounded structure during plasma–chemical processing, as a melting process takes place, during which the sharp edges disappear. Figure 2c shows SEM surface images of the fly ash primary raw material [1,2,3,4]. In this case, the ash was obtained at the household waste incineration plant of a company operating in Klaipėda (the ash was obtained during the processing/incineration of household waste). These ashes accumulate in air cleaning filters and scrubbers. As can be seen, ashes consist of irregularly shaped microparticles, the diameter of which can reach up to 10 µm. Figure 2d shows the surface of fly ash after plasma–chemical processing. As can be seen from the SEM images, the surface consists of irregular round-shaped structures of various diameters that can reach up to 30 µm and beyond. These round structures are formed during the melting process. Figure 2e shows SEM images of the initial material slag. This slag is formed during the incineration of household waste as a secondary waste. As can be seen, the slag consists of microforms of irregular sharp shapes, the dimensions of which reach up to 30 µm. During the incineration of household waste, various complex processes, both chemical and physical ones, take place, such as the decomposition of various compounds, melting, and evaporation. Figure 2f shows a surface image of the slag after plasma–chemical treatment. As can be seen from the SEM photo, the surface consists of individual large particles that are round in shape, with a diameter of 1–2 µm. These particles are likely formed during chemical reactions and melting processes. An attempt was made to obtain a micrometer fiber during the plasma–chemical process. This fiber was obtained from slag, which is formed during the incineration of household waste. SEM images of this fiber are presented in Figure 2g,h. As we can see, the fiber consists of elongated structures. The diameter of fiber threads ranges from 0.1 to 5 µm. Micrometric fibers are formed as the melt of raw material is flowing on the walls of the plasma–chemical channel, and a high-speed gas flow rips out some droplets of the melt and stretches it into fiber using high kinetic energy [1,2,3,4,5].
While studying the samples using the SEM, the elemental composition of the materials was also studied using the EDS method. The results of the EDS studies are presented in Table 1. In this case, sample 01 corresponds to wood ash before plasma–chemical treatment; sample 02—wood ash after plasma–chemical treatment; sample 03—fly ash obtained during the processing of household waste via incineration (primary raw material); sample 04—fly ash after plasma–chemical treatment; sample 05—slag obtained during the incineration of household waste (primary raw material); sample 06—slag after plasma–chemical treatment; sample 07—fiber obtained from slag during the plasma–chemical process when the power was 55.5 kW; sample 08—fiber produced when the power was 55.6 kW and 09–74 kW. After analyzing the results of the EDS tests, it became clear that in wood ash, in the primary material (sample 01), the predominant elements are C—15.7, O—59.5, and Ca—15.6%, while the remaining elements, such as Na, Al, S, and others, occupy a vanishingly small part. It is likely that most of the oxygen and Ca exist in the formed compounds in the form of oxides. As can be seen, the predominant elements in wood ash after plasma–chemical treatment are basically the same (C—14.4, O—58.6–59.5, and Ca—18.6%). A slight change in the percentage composition is observed after the plasma process. Analysis of the elemental composition of fly ash obtained by burning household waste (taken from air filters) (sample 03) revealed that the predominant elements are C—12, O—45.6, S—4.9, Cl—13.4, and Ca—17.5%. Meanwhile, after the plasma–chemical processing of fly ash (sample 04), the predominant elements are C—11.9, O—55.1, S—1.4, Cl—7.5, and Ca—19.1%. A slight decrease in the contents of the predominant elements is visible. The results of the EDS tests show that the elemental compositions and predominant elements of the primary material of slag obtained by burning household waste (sample 05) are C—12.9, O—61.3, Si—7.6, and Ca—10.6%. Meanwhile, after plasma–chemical slag processing (sample 06), the predominant elements are C—114, O—65.6, Si—9.4, and Ca—5%. As you can see, the remaining elements make up a vanishingly small percentage. Analyzing the EDS results, a tendency that can be observed is that the percentage of metals decreased after plasma–chemical treatment in all cases. This is determined by the fact that at a high plasma temperature (in the plasma–chemical channel), part of the metals are melted and evaporated.
During plasma–chemical processing, fiber was obtained (it was possible to obtain fiber only by processing the slag obtained during the incineration of household waste). The results of the EDS tests showed (samples 07–09) that the predominant elements in the composition of the fiber showed varied percentages—C: 36–39, O: 43–46, Si: 6–7.4, and Ca: 5.2–6.6%. Oxygen exists as oxides forming quartz and CaO.
From the thermal process that took place in the plasma–chemical channel, the process temperature during the plasma–chemical treatment was estimated. These research results are presented in Figure 3. The temperature distribution in the plasma–chemical channel was obtained by calculating the overall heat balance.
As can be seen from the research results, the highest process temperature was used during the wood processing, and it decreased from 2800 to 2500 °C. Regarding the representation of these results, the temperature dependence is presented on the relative length x/d, where d is the entire length of the plasma–chemical channel (25 cm), and x is any selected distance from the inlet of the channel. The temperature decreases most rapidly from 2650 to 2200 °C during fly ash processing. The temperature change process is influenced by the fact that different types of input products consume different amounts of heat released by the plasma flow. As can be seen, the temperature of the process does not fall below 2000 °C throughout the length of the channel, which means that such conditions are sufficient (the process temperature must be at least 1800 °C) for completely neutralizing various dangerous compounds present in both fly ash and slag.
Figure 4 shows the thermogravimetry results of primary and post-plasma–chemical treatment materials and obtained fiber using reactive air gas environment. In this case, sample 1 corresponds to wood ash before plasma–chemical treatment, sample 2—wood ash after plasma–chemical treatment, sample 3—fly ash obtained during the processing of household waste via incineration (primary raw material), sample 4—fly ash after plasma–chemical treatment, sample 5—slag obtained from household waste incineration (primary raw material), sample 6—slag after plasma–chemical treatment, sample 7—fiber obtained from slag during the plasma–chemical process when the power was 55.5 kW, sample 8—fiber when the power was 55.6 kW and 9–74 kW. These tests were carried out at temperatures ranging from 0 to 900 °C. As can be seen from the measurement results, the main weight loss process of the material took place in a temperature range of about 100–900 °C.
In this temperature range, the total mass of wood ash (sample 1) decreased the most. As we can see, in this case, the total mass has decreased from 100 to 70%. Such a mass decrease may be due to the fact that this primary material contains a high relative amount of carbon (this was also confirmed by EDS studies), which was removed in the form of volatile substances as the temperature increased. Meanwhile, the slowest material mass loss process occurred in the case of the fiber (samples 7–9). As can be seen from the TG curves, in this case, the material loss is only from 100 to 98%. This can be explained by the fact that although the fiber contains a lot of carbon, the structure of the fiber material which is formed during the plasma–chemical process is non-reactive. It is believed that a glass-like structure is formed (derived from the vitrification process), so the chemical reactions with the gas used during the TG tests almost do not occur, even when the temperature is increased. A general tendency can be observed (by analyzing the results of the TG tests), suggesting that the original material (untreated in the plasma–chemical channel) loses mass faster than the material after plasma–chemical treatment.
In order to identify the predominant compounds, both the raw material and the treated material were subjected to XRD studies. The results of these studies are presented in Figure 5. In this case, sample 1 corresponds to wood ash before plasma–chemical treatment, sample 2—wood ash after plasma–chemical treatment, sample 3—fly ash obtained during the processing of household waste via incineration (primary raw material), sample 4—fly ash after plasma–chemical treatment, sample 5—slag obtained by burning household waste (primary material), sample 6—slag after plasma–chemical treatment, sample 7—fiber obtained from slag during the plasma–chemical process when the power was 55.5 kW.
In this case, the analysis of XRD spectra becomes very complicated and difficult because, although only present in vanishingly small amounts, there are many very different compounds which cannot be precisely identified because the radiographs and characteristic peaks of each element overlap with each other. Therefore, it is difficult to name the exact values of the peaks in degrees and the crystallographic orientations. It can be noticed that in the case of wood ash (both when we have the base material and after plasma treatment), the predominant compounds (i.e., those with the most intense peaks) are SiO2, Ca(OH)2, CaSO4, CaSiO2, and CaO. These compounds are likely to be formed during combustion and also during plasma processing. In both of these cases, oxidation processes occur when combinations of various oxides are formed. Meanwhile, in the case of fly ash obtained during the incineration of household waste (sample 3) and after plasma–chemical treatment (sample 4), the characteristic compounds are CaSO4, SiO2, and CaO. The reasons for the formation of these compounds are the same as in the case of wood ash. In the case of slag obtained from household waste (sample 5) and material after the plasma–chemical treatment of slag (sample 6), the predominant compounds, according to our XRD studies, are SiO2 and CaO. The reasons for the formation of these compounds are the same as in the first two cases. However, in this case, a smaller variety of metals is observed. Analyzing the XRD studies, it became clear that the resulting fiber has an amorphous structure. It is likely that this happens because the vitrification process takes place during the formation of fibers, during which a disordered structure (resembling a homogeneous melt) and amorphous formations are formed [1,2,3,4,5,6]. This fact was also partially confirmed by the TG studies, since there is almost no reaction with reactive gases and no mass loss is observed, which is characteristic of the structures formed during the vitrification process.
Figure 6 shows the FTIR spectra of wood ash before plasma–chemical treatment and after it; fly ash obtained from the incineration of household waste before plasma–chemical treatment and after; slag primary materials and slag after plasma–chemical treatment; and the produced fiber.
Several broad and high-intensity absorption peaks in the wavenumber range from 400 to 3700 cm−1 were obtained. It has been shown that the stretching vibration of Ca-O bonds occurs at 700, 1050, and 1400 cm−1. Meanwhile, absorption peaks were also obtained for Si=O at 650 and 1400 cm−1. A C=O low-intensity peak at 1900 cm−1 could also be observed. These discussed peaks of compounds are characteristic of wood ash both before and after plasma treatment. After the treatment, wood ash is also characterized by OH at 3600 cm−1. While analyzing the FTIR spectra of fly ash (obtained by the incineration of household waste) both before plasma treatment and after treatment, it was observed that the same peaks are characteristic as in the case of wood ash, except that at 3400 cm−1, for which the high-intensity CaSO4 is observed; this peak basically coincides with the OH peak. Analysis of the FTIR spectra in the case of slag (obtained by burning domestic waste) before plasma treatment and after treatment, showed high-intensity peaks: CaSO4 at 490 and 3400 cm−1; Si=O at 650 and 1400 cm−1; Ca-O bond stretching is obtained at 1400 cm−1. These peaks of the FTIR spectrum are similar for the slag before plasma treatment and after it. The only essential difference is that at 3400 cm−1, there is a peak characteristic of CaSO4 slag before plasma–chemical treatment. It was possible to obtain fiber from slag during plasma–chemical treatment. The FTIR spectra of the fiber (obtained at 55.5 kW plasma torch power) show that the peaks of CaSO4 at 490 and Si=O at 650 cm−1 are characteristic, and there is also a peak at 1400 cm−1 that is characteristic of Ca-O and Si=O. When the fiber was obtained at 74 kW plasma torch power, the obtained peaks of the FTIR spectrum were the same as those of the fiber obtained at 55.5 kW power.
Figure 7 presents the BET results of fiber obtained by processing slag (obtained by burning household waste) by the plasma–chemical method. The fiber was obtained at 55.5; 55.6 and 74 kW, with a specific surface area of 11.9; 8.5 and 3 m2/g. The best-fit computational model for estimating the pore size distribution of the sample was the QSDFT, N2, carbon equilibrium transition core at 77.4 K based on the slot pore model, with a computational fit error of 3.27%. The largest total surface area was occupied by pores with a diameter of 3 nm in all cases when the fiber was obtained at 55.5; 55.6 and 74 kW. Clearly, the predominant pore distribution range was between 1 and 10 nm in all three cases. It is worth noting that the characteristics of the porous structure depended on the chemical and physical characteristics of the particles of the primary material (slag), such as composition, melting process, etc. Fraction and geometric shapes of slag granules also have an influence. It is worth noting that the porosity and specific surface area of the obtained fiber depended on the formation conditions, such as the plasma flow rate, the temperature at the exit of the reactor, the diameter of the plasma–chemical channel, etc.
Microfiber is formed when the primary raw material particles are melted and flow at high speed in the plasma–chemical channel. In this way, a micrometric fiber is formed at the exit of the plasma–chemical channel. In this case, it is not necessary to separate the fiber from the general mixture, since the entire primary production is used to form the fiber. Micron-sized fibers obtained by the plasma–chemical processing of slag can be used in various industrial fields due to their unique properties and potential. Micrometer fibers can be used as a reinforcing material in composites such as polymers or metals, improving their mechanical properties. This may be important in the development of vehicles, structures, or other engineering systems to increase their strength and durability. These fibers can be used as catalysts in chemical processes where a high surface area and activity are required. They can be used both on a production and laboratory scale, speeding up chemical reactions and improving product purification. In addition, micrometer fibers could be applied in the field of nanotechnology, for example, as nanoparticles, due to their small size and special properties. This could lead to the development of new materials and products in the fields of, for example, nanosensors, nanomaterials, and nanoelectronics [1,2,3,4,5]. The final benefits and scope of application depend on the chemical composition, shape, and other properties of micrometer fibers, so their potential is very high, and they can be applied to different fields of industry and science.

4. Conclusions

A plasma–chemical method was used to process wood ash, fly ash, and slag (obtained during the incineration of household waste). A special cooled plasma–chemical channel was designed and manufactured to process these products and obtain the fiber. As shown by the SEM studies, the raw material (before plasma–chemical treatment) consisted of irregularly shaped granules or rod-shaped microforms. The fiber consists of elongated structures whose diameter ranges from 0.1 to 5 µm. As shown by EDS research, although, in all cases, both the primary material and the products obtained after plasma–chemical processing are composed of various metal elements, but they make up a vanishingly small part, as the predominant elements are C, O, Cl, Si, and Ca. The temperature of the plasma–chemical process does not fall below 2000 °C throughout the length of the channel, which means that this condition is sufficient (the process temperature must be at least 1800 °C) for completely neutralizing various dangerous compounds in both the fly ash and the slag. A general tendency can be observed (by analyzing the results of the TG tests): the original material (unprocessed and untreated in the plasma–chemical channel) loses mass faster than the material after plasma–chemical treatment. After analyzing the XRD studies, it became clear that the obtained fiber has an amorphous structure. It is likely that the vitrification process takes place during the formation of fibers, during which a disordered structure (resembling a homogeneous melt) and amorphous formations are formed. The dominant fiber absorption bands were assigned to Ca-O, Si=O, and Ca-O groups for all raw material cases. The BET studies show that when fiber is obtained at 55.5; 55.6 and 74 kW of plasma torch power, respectively, the specific surface area of samples is 11.9; 8.5 and 3 m2/g. The summarized results of this study show that the fibers, fly ash, and slag obtained after the plasma processing were clean enough and harmless to the environment, so they can be widely used as secondary raw materials, e.g., for the production of thermal insulation; in the production of catalysts; as a component of concrete, cement, and asphalt; and in other industries. The high-temperature process (over 2000 °C) ensures that most harmful metals are vaporized or converted to stable oxide forms, reducing potential emissions and damage to the environment.

Author Contributions

Conceptualization, Ž.K., R.K. (Romualdas Kėželis) and V.G.; methodology, M.M.; software, V.V.; validation, R.K. (Rita Kriūkienė) and Ž.K.; formal analysis, M.M.; investigation, Ž.K.; resources, V.V.; data curation, R.U.; writing—original draft preparation, Ž.K.; writing—review and editing, Ž.K.; visualization, A.B. and R.K. (Rita Kriūkienė); supervision, V.V.; project administration, V.V.; funding acquisition, V.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are openly available in Zydrunas at www.lei.lt, reference number LK-109.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic view and photo of the plasma technology used for the processing of fly ash and slag: (a)—plasma torch; (b)—waste inlet; and (c)—plasma–chemical channel.
Figure 1. Schematic view and photo of the plasma technology used for the processing of fly ash and slag: (a)—plasma torch; (b)—waste inlet; and (c)—plasma–chemical channel.
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Figure 2. SEM pictures of fly ash and slag and obtained fiber: (a)—wood ash, primary raw material; (b)—the edge of wood after processing; (c)—fly ash, primary raw material; (d)—fly ash after processing; (e)—slag, primary material; (f)—slag after processing; (g)—fiber with a power of 63 kW; (h)—fiber with a power of 74 kW.
Figure 2. SEM pictures of fly ash and slag and obtained fiber: (a)—wood ash, primary raw material; (b)—the edge of wood after processing; (c)—fly ash, primary raw material; (d)—fly ash after processing; (e)—slag, primary material; (f)—slag after processing; (g)—fiber with a power of 63 kW; (h)—fiber with a power of 74 kW.
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Figure 3. Temperature distribution in the plasma–chemical channel: 1—during wood ash processing; 2—during slag processing; 3—during the processing of fly ash.
Figure 3. Temperature distribution in the plasma–chemical channel: 1—during wood ash processing; 2—during slag processing; 3—during the processing of fly ash.
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Figure 4. TG research results of primary and post-plasma–chemical treatment materials and obtained fiber.
Figure 4. TG research results of primary and post-plasma–chemical treatment materials and obtained fiber.
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Figure 5. The XRD research results of primary and post-plasma–chemical treatment materials.
Figure 5. The XRD research results of primary and post-plasma–chemical treatment materials.
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Figure 6. FTIR results: 1—wood ash before plasma–chemical treatment, 2—wood ash after plasma–chemical treatment, 3—fly ash obtained during the incineration of household waste, 4—fly ash after plasma–chemical treatment, 5—slag obtained from the incineration of household waste (primary material), 6—slag after plasma–chemical treatment, 7—fiber obtained from slag during the plasma–chemical process when the power was 55.5 kW, 8—fiber obtained when the power was 55.6 kW.
Figure 6. FTIR results: 1—wood ash before plasma–chemical treatment, 2—wood ash after plasma–chemical treatment, 3—fly ash obtained during the incineration of household waste, 4—fly ash after plasma–chemical treatment, 5—slag obtained from the incineration of household waste (primary material), 6—slag after plasma–chemical treatment, 7—fiber obtained from slag during the plasma–chemical process when the power was 55.5 kW, 8—fiber obtained when the power was 55.6 kW.
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Figure 7. BET results of fiber obtained after plasma–chemical processing of slag: (a)—55.5 kW; (b)—55.6 kW and (c)—74 kW.
Figure 7. BET results of fiber obtained after plasma–chemical processing of slag: (a)—55.5 kW; (b)—55.6 kW and (c)—74 kW.
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Table 1. Results of EDS tests of primary and post-plasma–chemical treatment materials and obtained fibers.
Table 1. Results of EDS tests of primary and post-plasma–chemical treatment materials and obtained fibers.
No.CONaMgAlSiPSClKCaTiFe
0115.7 ± 0.159.5 ± 0.50.2 ± 0.21.4 ± 0.21.3 ± 0.21.5 ± 030.9 ± 0.30.4 ± 0.10.1 ± 0.053.3 ± 0.315.6 ± 10.1 ± 0.050.1 ± 0.03
0214.4 ± 458.6 ± 50.4 ± 0.12 ± 0.41.1 ± 0.22.3 ± 0.31.1 ± 0.040.05 ± 0.01-1 ± 0.318.8 ± 20.07 ± 0.010.2 ± 0.02
03 12 ± 245.6 ± 12.9 ± 0.30.2 ± 0.050.3 ± 0.050.7 ± 0.06-4.9 ± 0.313.4 ± 11.8 ± 0.217.5 ± 050.1 ± 0.010.3 ± 0.01
04 11.9 ± 155.1 ± 21.7 ± 0.30.41 ± 0.050.5 ± 0.11 ± 0.05-1.4 ± 0.37.5 ± 21 ± 0.319.1 ± 0.4-0.2 ± 0.01
05 12.9 ± 0.261.3 ± 11.9 ± 0.41 ± 0.41.9 ± 0.37.6 ± 0.50.1 ± 0.050.8 ± 0.010.3 ± 0.010.5 ± 0.0310.6 ± 0.70.31 ± 0.70.9 ± 0.1
06 11.4 ± 265.6 ± 23.4 ± 0.11.5 ± 0.12.4 ± 0.29.4 ± 0.50.2 ± 0.010.1 ± 0.01-0.2 ± 0.015 ± 10.3 ± 0.020.6 ± 0.1
07 36.7 ± 544.5 ± 41.5 ± 0.20.7 ± 0.11.4 ± 0.17.4 ± 0.5--0.1 ± 0.020.2 ± 0.036 ± 10.2 ± 0.051 ± 0.2
08 39 ± 743 ± 41.8 ± 0.40.8 ± 0.21.5 ± 0.37.2 ± 20.1 ± 0.01--0.2 ± 0.025.2 ± 0.60.2 ± 0.030.7 ± 01
09 36 ± 546 ± 41 ± 0.10.5 ± 0.11.4 ± 0.26.7 ± 0.70.1 ± 0.01--0.2 ± 0.036.6 ± 10.3 ± 0.051 ± 0.1
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Kavaliauskas, Ž.; Kėželis, R.; Grigaitienė, V.; Milieška, M.; Valinčius, V.; Uscila, R.; Kriūkienė, R.; Baltušnikas, A. An Investigation of Fly Ash and Slag Processing and Fiber Production Using Plasma Technology. Appl. Sci. 2024, 14, 4801. https://doi.org/10.3390/app14114801

AMA Style

Kavaliauskas Ž, Kėželis R, Grigaitienė V, Milieška M, Valinčius V, Uscila R, Kriūkienė R, Baltušnikas A. An Investigation of Fly Ash and Slag Processing and Fiber Production Using Plasma Technology. Applied Sciences. 2024; 14(11):4801. https://doi.org/10.3390/app14114801

Chicago/Turabian Style

Kavaliauskas, Žydrūnas, Romualdas Kėželis, Viktorija Grigaitienė, Mindaugas Milieška, Vitas Valinčius, Rolandas Uscila, Rita Kriūkienė, and Arūnas Baltušnikas. 2024. "An Investigation of Fly Ash and Slag Processing and Fiber Production Using Plasma Technology" Applied Sciences 14, no. 11: 4801. https://doi.org/10.3390/app14114801

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