JP4601539B2 - Method for producing coal ash sintered body using coal ash powder as raw material - Google Patents

Description

  The present invention relates to a method for producing a coal ash sintered body by effectively using coal ash powder.

  In recent years, coal ash emitted from coal combustion at thermal power plants, etc. is used in the cement field (cement raw materials, cement admixtures, concrete admixtures), and civil engineering fields (ground improvement materials, civil engineering works, electric power works, road roadbeds). Materials, asphalt filler material, coal mine filler), construction field (building material board, artificial lightweight aggregate, secondary concrete product), agriculture / forestry / fishery field (fertilizer, snow melting material, soil conditioner), sewage sewage treatment field (sewage) However, some coal ash is landfilled as industrial waste, and there is concern about the shortage of landfill sites. Therefore, development of a technology that can use coal ash more effectively is demanded.

  As a further effective utilization technology of coal ash, a method for producing a coal ash sintered body such as a tile or a ceramic product using coal ash as a raw material is considered (see Non-Patent Document 1). However, in this manufacturing method, since the coal ash is calcined in the atmosphere, carbon dioxide gas is generated by the unburned carbon contained in the coal ash, and as a result, the inside of the roof tile and ceramic product becomes porous, There is a problem that the strength of tiles and ceramic products decreases.

In addition, as a method for preventing the generation of carbon dioxide gas described above, a method of sintering a ceramic product in a non-oxidizing atmosphere such as in a vacuum or an inert gas such as a vacuum heating furnace or a hot press furnace can be considered. In this method, since the entire heating furnace in the vacuum vessel is heated, product removal in a high temperature state causes oxidation of parts in the furnace. For this reason, cooling at a specified temperature is required, so that the cooling time is prolonged and the production efficiency is reduced.
Kazuhiko Igarashi, Current Status and Future Prospects of Coal Ash, Journal of the Institute of Electrical Engineers of Japan, May, 1986, p25-31

  Then, an object of this invention is to provide the method which can manufacture efficiently the sintered compact of the coal ash excellent in compactness.

  As a result of diligent efforts to solve the above-mentioned problems, the present inventors efficiently produce a coal ash sintered body excellent in compactness by conducting and sintering coal ash powder with a pulse current under pressure. As a result, the present invention has been completed.

That is, the manufacturing method of the coal ash sintered body according to the present invention, the coal ash powder heat treated beforehand at 950 ° C. or higher, the pulse current under pressure, the temperature of the heat treatment at a temperature above 830 ° C. It is characterized in that the current sintering is performed at a lower temperature .

The method for producing a coal ash sintered body according to the present invention is characterized in that the electric current sintering is performed by a pulse current of 100 A or more generated under the condition that the maximum voltage is 5 V and the frequency is 10 Hz. . Further, the current sintering is performed under a pressure of 50 MPa.

  Furthermore, the method for producing a coal ash sintered body according to the present invention may further include a step of cooling at a cooling rate in the range of 0.1 to 50 ° C./second after the electric current sintering.

  The coal ash is fly ash, for example.

  ADVANTAGE OF THE INVENTION According to this invention, the method which can manufacture efficiently the sintered compact of the coal ash excellent in the compactness can be provided.

  An embodiment for carrying out the present invention completed based on the above knowledge will be described in detail with reference to examples.

  As described above, a coal ash sintered body excellent in denseness can be efficiently manufactured by conducting current sintering of the coal ash powder with a pulse current under pressure. Further, since the coal ash sintered body produced in this way is also excellent in hardness, it is considered useful for wear-resistant materials and highly practical ceramic-based industrial materials using fly ash as a raw material. Therefore, the method for producing a sintered coal ash according to the present invention enables efficient production of ceramic products such as tiles and bricks, improves the effective utilization rate of coal ash, and alleviates the shortage of landfill disposal sites. At the same time, it will be possible to contribute to the production of new ceramic products and the creation of industries.

  As said coal ash, fly ash, clinker ash, cinder ash etc. can be used, for example. In addition, as the above-mentioned current sintering method, for example, a pressure sintering method in which a direct current pulse current is passed such as a discharge plasma sintering method, a discharge sintering method, a plasma activated sintering method, or the like can be used. The current sintering is desirably performed at 700 ° C. or higher. This is because if the temperature is lower than 700 ° C., the sintering phenomenon of the coal ash powder does not occur and it is considered that a coal ash sintered body cannot be obtained.

  In the present invention, the coal ash powder may be heat-treated in advance before the coal ash powder is energized and sintered with a pulse current under pressure. By previously heat-treating the coal ash powder in this way, it is possible to increase the sintering temperature by burning unburned carbon in the coal ash powder, and to produce a coal ash sintered body having a denser structure. become able to. The heat treatment may be performed at a temperature (for example, 600 ° C. or higher) at which unburned carbon in the coal ash powder burns in a mixed gas containing oxygen gas (for example, in the air) or oxygen gas. It is preferable to carry out at a temperature (for example, 950 ° C. or higher) at which combustion can be performed so as not to substantially contain fuel carbon. When the coal ash powder thus heat-treated so as not to substantially contain unburned carbon is subjected to current sintering under pressure by a pulse current, it is desirable to perform current sintering at 830 ° C. or higher. This is because if the temperature is lower than 830 ° C., the sintered phenomenon of the heat-treated coal ash powder does not occur and it is considered that a coal ash sintered body cannot be obtained.

  In addition, it is preferable that it is in the range of 0.1-50 degree-C / sec as a speed | rate heated to the temperature which carries out the above-mentioned electric sintering. The reason why the heating rate is set to 0.1 ° C./second or more is that when the heating speed is less than 0.1 ° C./second, the manufacturing time of the coal ash sintered body becomes long. The heating rate is set to 50 ° C./sec or less because if the heating rate is faster than 50 ° C./sec, more heat is released to the outside at the time of pulsed heating, which requires more input power. Because it is not right.

  In the present invention, the coal ash powder may be cooled at a cooling rate in the range of 0.1 to 50 ° C./second after the coal ash powder is energized and sintered with a pulse current under pressure. The reason why the cooling rate is set to 0.1 ° C./second or more is that if the cooling rate is less than 0.1 ° C./second, the cooling time becomes long and the hardness of the surface of the coal ash sintered body cannot be increased. Because it is considered. The reason why the cooling rate is set to 50 ° C./second or less is that cracks are considered to occur in the coal ash sintered body due to rapid cooling when the cooling rate exceeds 50 ° C./second.

  Hereinafter, the present invention will be specifically described by way of examples. These examples are for explaining the present invention, and do not limit the scope of the present invention.

[Example 1]
First, fly ash powder (particle size: about 1 to 100 μm, SiO ₂ : 63.5 ± 5.1% as main components, Al ₂ O ₃ : 22.5 ± 3.0%, Fe ₂ O ₃ : 4.3 ± 1.0%, C: 2.5 ± 2.0 %, And CaO: 1.8 ± 1.3%.) Alternatively, the heat-treated fly ash powder was sintered by the discharge plasma sintering method, and the density and strength of the obtained sintered body were examined. The heat-treated fly ash powder was prepared by heat treating the fly ash powder in an electric furnace at 1000 ° C. for 24 hours, and then naturally cooling and pulverizing the fly ash powder in the electric furnace.

  The fly ash powder or heat-treated fly ash powder was weighed with an electronic balance, and 3.0 g was placed in a 10 mmφ graphite die of PLASMAN (manufactured by SS Alloy Co., Ltd.) and sandwiched between graphite punches. Thereafter, the pressure in the apparatus was reduced to 10 Pa, a pressure of 50 MPa was applied to the graphite punch, and then a pulse current was applied under the conditions shown in Table 1 to heat to the temperature shown in Table 1. After heating for 10 minutes, it was cooled to room temperature in 10 minutes to produce each sintered body (bulk body).

  The hardness of each sintered body thus produced was measured according to the method of JIS R1610 (Vickers hardness test method for fine ceramics). The results are shown in Table 1.

  As shown in Table 1, it was found that all the sintered bodies were superior to the Vickers hardness (200 to 300 Hv) of general iron ore materials. Moreover, it turned out that any sintered compact can be manufactured at low temperature and for a short time (usually within 20 minutes). From these, it became clear that the coal ash sintered body excellent in Vickers hardness can be efficiently produced by energizing and sintering coal ash powder with a pulse current under pressure. This suggests that it is useful for wear-resistant materials and highly practical ceramic-based industrial materials made from fly ash.

  In addition, when a structure of each sintered body was observed using a scanning electron microscope (manufactured by JEOL), no porous structure that caused a decrease in strength was observed in any of the sintered bodies. From this, it was found that a dense coal ash sintered body can be produced by conducting current sintering of the coal ash powder with a pulse current under pressure.

Furthermore, concentration analysis by EPMA (Electron probe Micro Analyzer) of each sintered body confirmed that carbon distribution considered to be unburned carbon was confirmed in the sintered body of fly ash powder. It could not be confirmed in the powder sintered body (the figure is not shown). In addition, the coal ash powder started sintering at about 700 ° C, and became a sintered body with a density of 2.3 × 10 ³ kg / m ^{3 at} a sintering temperature of about 830 ° C. Sintering was started and it became clear that the sintered body became a dense sintered body with a density of 2.8 × 10 ³ kg / m ³ at a sintering temperature of about 1070 ° C. From these facts, it has been found that burning the unburned carbon can increase the sintering temperature and produce a denser sintered body.

[Example 2]
Next, in order to investigate the influence of combustion of unburned carbon contained in fly ash, differential thermal analysis (DTA) and thermogravimetric analysis (TG) were performed using the above-described fly ash powder and heat-treated fly ash powder. . Note that alumina (Al ₂ O ₃ ) was used as a standard sample, and the temperature increase and decrease rate was 20 K / min. The results are shown in FIG.

  From the heating curve of TG shown in FIG. 1, it was found that the weight of fly ash powder decreased from around 600 ° C. Further, from the heating curve of DTA shown in FIG. 1, it was found that there was an exothermic peak in which the unburned carbon contained in the fly ash was burned at around 600 ° C., 700 ° C., and 950 ° C. From the above, it is necessary to perform heat treatment at a temperature of at least 600 ° C. in order to burn the unburned carbon, and heat treatment at a temperature of 950 ° C. or more to completely burn the unburned carbon. It was suggested.

[Example 3]
Next, the surface hardness and the hardness of the central portion of the sintered body produced using the fly ash powder described in Example 1 were examined.

  30 g of fly ash powder weighed with an electronic balance was put into a 30 mmφ graphite die of PLASMAN (manufactured by SS Alloy Co., Ltd.) and sandwiched between graphite punches. After that, the pressure inside the device is reduced to 10 Pa, and a pressure of 50 MPa is applied to the graphite punch. Then, a pulse current (current: 100 A, maximum voltage: 5.0 V, frequency: 10 Hz) is applied and room temperature is reached for 10 minutes ( 20 ° C) to 830 ° C. After being held for 5 minutes, it was cooled to room temperature in 10 minutes to produce a sintered body (bulk body) having a diameter of 3 cm and a height of 1.8 cm. Using the obtained sintered body, the Vickers hardness of the surface portion and the center portion of the sintered body was measured in the same manner as in the method described in Example 1 (Example of the present invention).

  Furthermore, in order to investigate the effect on the hardness of the surface or center of the sintered body when the heating rate and the cooling rate are changed in the production of the sintered body, it is heated to 830 ° C. for 140 minutes (current) : 0 to 100 A variable), except that it was cooled to room temperature in 140 minutes, a sintered body was prepared in the same manner as described above, and the Vickers hardness of the surface and center of the sintered body was measured (Comparative Example 1). ). In addition, a sintered body was prepared in the same manner as described above except that it was heated to 830 ° C. for 15 seconds (current: 1000 A) and cooled to room temperature in 15 seconds. Hardness was measured (Comparative Example 2). The cooling for 15 seconds was performed by supplying He gas into the PLASMAN apparatus at 0.8 MPa and a flow rate of 100 L / min. These results are shown in Table 2.

  As shown in Table 2, it became clear that the Vickers hardness of the surface portion of the sintered body of the present invention example was superior to the central portion of the sintered body. This is because the surface portion is hardened earlier than the inside by rapidly cooling the sintered body, and a compressive strain layer is formed on the surface, and a tensile strain layer that balances the layer is formed on the inside. it is conceivable that. From these facts, it became clear that the hardness of the surface portion of the sintered body can be increased by rapidly cooling the sintered body.

In one Example of this invention, it is a figure which shows the result of having performed the differential thermal analysis (DTA) and the thermogravimetric analysis (TG) with respect to the fly ash powder (A) and the heat-treated fly ash powder (B).

Claims (5)

Coal ash powder heat treated beforehand at 950 ° C. or higher, the pulse current under pressure, coal ash, characterized by electric current sintering at a temperature lower than the temperature of the heat treatment at a temperature above 830 ° C. A method for producing a sintered body. 2. The method for producing a coal ash sintered body according to claim 1, wherein the electric current sintering is performed by a pulse current of 100 A or more generated under conditions of a maximum voltage of 5 V and a frequency of 10 Hz . The method for producing a coal ash sintered body according to claim 1 or 2 , wherein the electric current sintering is performed under a pressure of 50 MPa . The method for producing a coal ash sintered body according to any one of claims 1 to 3, wherein after the electric current sintering, cooling is performed at a cooling rate in a range of 0.1 to 50 ° C / second. The method for producing a coal ash sintered body according to any one of claims 1 to 4, wherein the coal ash is fly ash.

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