Manufacturing method for sintered waste by-products

Abstract

To provide a manufacturing method for a waste by-product sintered body that can promote recycling of coal ashes generated in a thermal power plant and Fe sources as waste by-products, as foaming agents, structural materials and the like, by reutilizing the coal ashes and the Fe sources at the same time.SOLUTION: A waste by-product sintered body is sintered by irradiating a mixture, in which Fe sources as waste by-products (for instance garnets as waste products) and Na sources as sintered auxiliary agents (for instance NaCl) are mixed with coal ashes (for instance fly ashes), with a microwave and heating the mixture up to above a sintering temperature. This can achieve the waste by-product sintered body which functions as a foaming agent and has high strength.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for utilizing two waste by-products to produce sintered bodies with useful properties. [Background technology]

[0002] To improve the sustainability of thermal power plants, there is a need to identify viable treatments and uses for by-products. Fly ash is a major by-product of fossil fuel combustion in thermal power plants, with approximately 500 million tons produced annually worldwide. This fly ash contains approximately 20% by mass of unburned carbon. Therefore, many researchers have been exploring ways to reduce the amount of fly ash produced or to remove and convert the residual carbon (see Non-Patent Documents 1 to 3). Garnet is the second hardest mineral after diamond, and because of its high specific gravity, it is widely used as an abrasive for water treatment, generating a large amount of garnet powder as industrial waste. For this reason, there has been great interest in the reuse of garnet powder (see Non-Patent Documents 4 and 5).

[0003] Therefore, if it were possible to simultaneously reuse iron sources such as coal ash generated at thermal power plants and garnet as waste by-products, efficient reuse would become possible, broadening the options for reuse. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Miyake M, Kimura Y, Ohashi T, Matsuda M. Preparation of activated carbon-zeolite composite materials from coal fly ash. Microporous Mesoporous Mater 2008;112:170-7. https: / / doi.org / 10.1016 / j.micromeso.2007.09.028. [Non-patent document 2] Yang L, Li D, Zhu Z, Xu M, Yan X, Zhang H. Effect of the intensification of preconditioning on the separation of unburned carbon from coal fly ash. Fuel 2019;242:174-83. https: / / doi.org / 10.1016 / j.fuel.2019.01.038. [Non-patent document 3] Murayama N, Yamamoto H, Shibata J. Zeolite synthesis from coal fly ash by hydrothermal reaction using various alkali sources. J Chem Technol Biotechnol 2002;77:280-6. https: / / doi.org / 10.1002 / jctb.604. [Non-patent document 4] Babu MK, Chetty OVK. A study on recycling of abrasives in abrasive water jet machining. Wear 2003;254:763-73. https: / / doi.org / 10.1016 / S0043-1648(03)00256-4. [Non-Patent Document 5] Zhou X, Tan H. A study on recycling of supreme garnet in abrasive waterjet machining. Appl Mech Mater 2012;248:499-503. https: / / doi.org / 10.4028 / www.scientific.net / AMM.248.499. Summary of the Invention [Problem to be solved by the invention]

[0005] Fly ash and garnet powder require long sintering times and high temperatures. Fly ash contains primarily SiO2 and Al2O3, resulting in a high melting temperature. From a materials science perspective, sintering proceeds at the Tammann temperature (the minimum temperature at which a solid-state reaction can occur, typically about two-thirds of the material's melting point). However, from a thermodynamic perspective, it is difficult to reach the Tammann temperature in the center of a material like fly ash. This is because heat exchange within a sintered body with few voids is limited by thermal conductivity. As a result, rapid sintering requires a large temperature gradient, which is inconvenient and costly. Therefore, the development of a technology that can quickly heat the center of a material with low thermal conductivity is expected to reduce sintering costs and promote the recycling of waste materials such as fly ash and garnet powder into structural materials.

[0006] The present invention has been made in view of the above circumstances, and has as its main object to provide a method for producing a sintered body of a waste by-product that can simultaneously reuse coal ash produced in a thermal power plant and an Fe source as a waste by-product such as garnet powder generated as waste in a polishing process, and promote recycling of the by-product into a foaming agent, structural material, etc. [Means for solving the problem]

[0007] In order to achieve the above object, the method for producing a sintered body of a waste by-product according to the present invention is to use coal ash (e.g., fly ash) as a waste by-product. Garnet as a source of Feand a Na source (for example, NaCl) as a sintering aid, is mixed, and the mixture is irradiated with microwaves to raise the temperature above the sintering temperature, thereby sintering the mixture. This results in the sintering mixture being filled with fine bubbles (forming a porous structure) and having a significantly increased strength compared to a sintering mixture that does not contain an Fe source.

[0008] It is desirable to use a mixture of coal ashes with different carbon contents. This is useful for adjusting (lowering) the melting point of coal ash, since the carbon content (amount of unburned carbon) of coal ash varies depending on the power plant.

[0009] The mixture can be heated to 800 to 1200°C by microwave irradiation. As the temperature increases, the bubbles become larger, and the apparent volume reduction rate decreases. However, microwave absorption increases with increasing temperature, and the decomposition of the sodium source and the combustion reaction of carbon in the coal ash are promoted with increasing temperature. In this case, it is desirable that the microwaves irradiated onto the mixture have a frequency of about 2.45 GHz and are electric field components.

[0010] In addition, when fly ash is used as coal ash, garnet, a waste by-product, is used as the Fe source, and sodium chloride is used as the Na source, it is desirable to add the Fe source in an amount up to 10 / 6 equivalent to the Na source. Adding the Fe source in this range ensures the effect of adding the Fe source. [Effects of the Invention]

[0011] As described above, according to the present invention, by heating and sintering a mixture of coal ash, an Fe source as a waste by-product, and a Na source as a sintering aid using microwaves, it is possible to obtain a sintered body of the waste by-product that functions as a foam material and has high strength. Therefore, it is possible to simultaneously reuse two waste by-products and obtain a sintered body with properties suitable for structural materials, etc. [Brief explanation of the drawings]

[0012] [Figure 1] Figure 1 is a characteristic diagram showing the relationship between time and temperature when 1.5 g of fly ash containing garnet is heated by microwave at various temperatures (800 to 1200°C, holding time: 10 minutes). [Figure 2] Figure 2 is a characteristic diagram showing the relationship between the relative permittivity (real and imaginary parts) of fly ash mixed with garnet and temperature at various temperatures (25 to 700°C). [Figure 3] Figure 3 is a diagram showing the temperature and volume reduction rate of mixed fly ash heated to 800-1200°C for various holding times (1, 5, and 10 min). [Figure 4] FIG. 4 shows an SEM image (15 kV) of fly ash heated by microwaves at 800 to 1200°C. [Figure 5] Figure 5(a) shows the change in residual carbon and chlorine concentration over time when heated to 1200°C, and Figure 5(b) shows the temperature dependence of residual carbon and chlorine concentration when heated for 10 minutes. [Figure 6] Figure 6(a) shows the stress-strain curve of the test specimen, an image of the compression tester, and an inset showing the sample during the compression test. Figure 6(b) shows the sintered body of fly ash mixed with garnet after microwave heating. Figure 6(c) shows the sintered body of fly ash without garnet mixed after microwave heating. BEST MODE FOR CARRYING OUT THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The method for producing a sintered body of a waste by-product according to the present invention is a method for forming a sintered body with a porous structure and high strength by sintering a mixture of coal ash (fly ash) to which an iron source (e.g., garnet powder as industrial waste) and a Na source (e.g., NaCl) as a sintering aid are added using microwaves.

[0014] The method for producing a sintered body of waste by-products according to the present invention will be described in detail below. 1. Materials and sintering methods The microwave absorption of coal ash (fly ash) is highly dependent on its carbon concentration. However, the amount of unburned carbon in fly ash varies greatly depending on the power plant. Therefore, to adjust (lower) the melting point, we obtained fly ash samples from two power plants (denoted as S and M). S-fly ash and M-fly ash have the carbon contents shown in Table 1. These two types of fly ash were mixed with industrial waste garnet (here, garnet powder from grinding wheels generated when cutting carbon fiber) to create samples. NaCl was also used as a sintering aid. [Table 1]

[0015] The sample was prepared by mixing 40 g of S-fly ash, 8 g of M-fly ash, 0.65 g of NaCl powder, and 0.65 g of garnet for 5 min. The mixture (weight: 1.5 g; standard deviation: 0.001 g) was placed in a quartz sample container, which was tapped 100 times to suppress volume change, and then set in the cavity resonator.

[0016] The heating method employed an isolated microwave field with a frequency of 2.45 GHz. The system consisted of six waveguides (109.1 × 56.4 × 149.3 ± 5 mm) each consisting of a magnetron oscillator, an EH tuner, a plunger, and a dummy load. The microwave was focused by an aperture and TE was generated inside the cavity. 103 The microwave generated by the magnetron is adjusted by an EH tuner to the wave with the best irradiation efficiency and introduced into the cavity resonator. A 52 mm slit parallel to the direction of the electric field is used as the aperture. The plunger is placed at the end of the waveguide. This system makes it possible to spatially separate the electric field and magnetic field of the microwave.

[0017] The sample was then placed at the electric field node (the point where the electric field is maximum (Emax) after eliminating the magnetic field) so that only the electric field was irradiated onto the sample. The temperature of the reactants was monitored using a radiation thermometer (FTZ6-R220-5S22, Japan Sensor Co., Ltd.). To understand the heating behavior, the dielectric constant was investigated at various temperatures by the cavity perturbation method.

[0018] After heating, the volume reduction rate, residual carbon, and strength of the fly ash samples were investigated. These parameters play an important role in the utilization of fly ash. To confirm the heating characteristics of the model samples, the heating behavior and dielectric constant were investigated using the perturbation method. The volume reduction rate and residual carbon content were measured using the Archimedes method and XRF, respectively. Finally, uniaxial compression tests were conducted to measure the strength of the samples. Samples with a diameter of approximately 42 mm and a height of 30 mm were cut from the quartz sample container.

[0019] 2. Results and Evaluation Microwave irradiation can rapidly heat coal fly ash over the entire temperature range, but the microwave absorption of fly ash is temperature dependent, with higher temperatures resulting in greater absorption.

[0020] Figure 1 shows the temperature change over time when a 1.5 g mixture of fly ash + garnet + NaCl was heated to 800, 1000, or 1200 °C with a 10 minute hold time. The figure shows that using relatively low power microwave irradiation (one-fifth the power of a microwave), the mixture can be heated to the specified temperature in 400 seconds.

[0021] Furthermore, in this figure, the slope of the curve indicates the temperature dependence of microwave absorption. The slope is proportional to the difference between microwave absorption and heat removal of fly ash. When the temperature reaches 400 °C, the slope changes significantly, suggesting that the microwave absorption of fly ash is significantly improved at this temperature.

[0022] The microwave absorption of coal fly ash increases with increasing temperature. The difference between microwave absorption and heat removal determines the temperature of the material. The microwave absorption of a material is determined by the following equation:

number

[0023] Therefore, by measuring the dielectric constant, the microwave intensity of fly ash can be directly evaluated. Figure 2 shows the temperature dependence of the relative permittivity (real and imaginary parts) of the mixture. The real part of the relative permittivity is an indicator of how much the wavelength of microwaves shortens as they pass through the fly ash, while the imaginary part is an indicator of microwave absorption by the fly ash per unit volume. At room temperature, the imaginary part of the relative permittivity is approximately 0.1, but at 700°C it increases to approximately 7.3. This means that microwave absorption increases approximately 100-fold with increasing temperature.

[0024] The mixture showed an inflection point in its heating behavior at approximately 400 °C (Fig. 1), and its microwave absorption showed an inflection point at approximately 500 °C (Fig. 2), indicating that the temperature at the center of the mixture increased when heated with microwaves.

[0025] Figure 3 shows the volume reduction of mixed fly ash samples after microwave heating at 800, 1000, or 1200°C with holding times of 1, 5, or 10 minutes. The volumes were measured before and after heating using the Archimedes method. The amount of fly ash was reduced by approximately 15–35% by microwave heating. However, the volume reduction was approximately 5–10% lower than that without garnet (not shown). (The volume reduction with garnet addition was minimal.) To identify the reason for this lower volume reduction, cross sections of sintered compacts at each temperature (800–1200°C) were examined using SEM (Figure 4). Fine bubbles were observed inside the sintered compacts of all the coal ash (fly ash) and garnet mixtures. Therefore, it was found that the addition of garnet triggered a reaction that created closed pores within the mixture during sintering, thereby reducing the volume reduction during sintering.

[0026] After sintering at 800°C, bubbles with diameters of 20–50 μm were observed in the mixture, whereas after sintering at 1200°C, closed pores with diameters of 300 μm were observed. This indicates that closed pores grow with increasing temperature. No time dependence of bubble growth was observed.

[0027] In this study, NaCl was used as a sintering aid, so the amount of residual chlorine in the sintered samples was investigated. Figures 5(a) and (b) show the dependence of residual carbon and chlorine on the holding time and sintering temperature, respectively, for microwave-sintered materials. The amounts of residual carbon and chlorine in the sintered materials remained nearly constant regardless of the holding time. In contrast, the amounts of residual carbon and chlorine decreased as the sintering temperature increased. This indicates that the decomposition of NaCl and the carbon combustion reaction proceed further with increasing temperature. Furthermore, this reaction reached a steady state after approximately one minute.

[0028] Figure 5 shows that NaCl decomposes rapidly. This result is supported by thermodynamic calculations. According to the thermodynamic equilibrium based on Gibbs free energy data, NaCl reacts with Fe2O3 and Al2O3 in the coal ash to produce chlorine gas. The partial pressures of aluminum chloride and iron chloride are thermodynamically 6.1 × 10, respectively, considering that the sodium oxide in equilibrium with NaCl is not pure Na2O but Na2O 2SiO2, and the CO partial pressure from carbon combustion is 1 atmosphere. -3 This can be calculated as atm and 383atm.

[0029] Therefore, the following equation can be proposed for the decomposition of NaCl: (chemical 1) 6NaCl + 10Fe2O3+ 3C → 3Na2O + 6Fe3O4+ 2FeCl3(gas) + 3CO(gas)

[0030] This chemical formula is also consistent with the observation of bubbles in SEM images of sintered materials, the reduction in chlorine in XRF analysis, and the reduced volume loss rate of coal ash during microwave sintering with the addition of garnet. The Fe source (Fe2O3) is specified relative to the Cl content of NaCl, and is effective when added in an amount of up to 10 / 6 equivalents per NaCl equivalent. For example, xg of NaCl is equivalent to x / 58 mol of Cl, so x / 58 * 10 / 6 = 0.028x mol of Fe2O3 is effective. This is equivalent to 4.6xg of Fe2O3 by mass (3.2xg when converted to iron).

[0031] Microwave-sintered compacts with and without garnet were subjected to uniaxial compression tests, as shown in Figure 6. The results showed that the specimens with garnet had higher strength and stiffness than those without garnet. As shown in Figure 6(a), after 10 minutes of microwave heating at 1000°C (a representative temperature between 800 and 1200°C), the average strength of the garnet-containing specimens (sintered compacts) was 5.475 N / mm², while the average strength of the specimens without garnet was 0.3375 N / mm². Furthermore, the specimens with garnet maintained their original shape, while the specimens without garnet fractured during the test. This phenomenon is believed to be due to the specimen manufacturing process. The specimens with garnet contained small bubbles (Figures 4 and 6(b)), while the specimens without garnet had large internal cavities (Figure 6(c)). These large cavities were identified as the cause of the strength reduction of the specimens without garnet.

[0032] Garnet is a raw material that generates iron chloride bubbles when heated. Therefore, in gram-scale desktop tests, bubbles were observed in sintered bodies containing garnet after heating to 1000 °C. In contrast, small bubbles were not observed in specimens without garnet up to 1000 °C. When a large fly ash sample (100 g) was heated with microwave irradiation, the center of the sample reached a higher temperature than the outside due to its high thermal insulation. Since bubbles are difficult to generate in fly ash without garnet, this internal heating rapidly generates bubbles during sintering, resulting in larger cavities. In contrast, bubbles formed throughout the sintered body containing garnet, giving it relatively high strength.

[0033] As described above, coal ash (fly ash) containing NaCl and garnet could be sintered using microwave irradiation. A cavity-resonator-controlled microwave field (TE103) was applied, and the dielectric constant of the fly ash was measured at temperatures ranging from 25 to 700 °C using the cavity perturbation method. The temperature of the fly ash increased rapidly at 400 °C. High-temperature dielectric constant measurements indicated this was due to enhanced microwave absorption. Garnet-mixed fly ash samples contained small bubbles after sintering at 1000 °C, and XRF results revealed that the carbon and chlorine contents decreased at this temperature. Therefore, as predicted by equilibrium thermodynamics, decomposition of NaCl by carbon and iron oxide occurred during microwave heating of coal fly ash, and garnet acted as a foaming agent in the sintering process. The compressive strength of sintered coal ash samples with garnet was approximately 10 times higher than that without garnet.

[0034] Therefore, by using the above-mentioned sintered body formed by microwave irradiation, two waste by-products, namely, fly ash produced in thermal power plants and garnet powder generated as waste in the polishing process, can be simultaneously utilized, and recycling into foaming agents, structural materials, etc. can be facilitated.

[0035] In the above example, garnet, which is an industrial waste, is used as the Fe source, which is a waste by-product. However, scrap iron generated during steelmaking or product processing, or steel slag generated as a by-product in the steel manufacturing process, may also be used. Furthermore, although NaCl is used as the Na source, NaO may also be used. Similar sintered bodies can be produced using these Fe and Na sources.

Claims

1. A method for producing a sintered body of waste by-product, characterized in that a mixture of coal ash, garnet as a waste by-product, which is an Fe source, and a Na source as a sintering aid is mixed with the mixture, and the mixture is sintered by irradiating microwaves to raise the temperature above the sintering temperature.

2. 2. The method for producing a sintered body of waste by-products according to claim 1, wherein the coal ash is a mixture of a plurality of coal ashes having different carbon contents.

3. 3. The method for producing a sintered body of a waste by-product according to claim 1, wherein the mixture is heated to 800 to 1200° C. by irradiating the mixture with microwaves.

4. 4. The method for producing a sintered body of a waste by-product according to claim 1, wherein the frequency of the microwave is about 2.45 GHz, and the microwave irradiated to the mixture is an electric field component.

5. 5. The method for producing a sintered body of a waste by-product according to claim 1, wherein the coal ash is fly ash, the Na source is sodium chloride, and the Fe source contains up to 10 / 6 equivalents relative to the sodium chloride equivalent.

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