Manufacturing method of modified fly ash

Abstract

To provide efficient means with a small environmental load, for obtaining modified fly ash having a stably small amount of unburned carbon when an amount of unburned carbon of raw fly ash varies.SOLUTION: Modified fly ash is obtained by: manufacturing heated fly ash by subjecting raw fly ash having an unburned carbon amount larger than a previously set threshold to modification treatment by a heating method; manufacturing classified fly ash by subjecting raw fly ash having an unburned carbon amount equal to or less than the threshold to modification treatment by a sieve method; manufacturing heated fly ash by singly using residues separated as coarse-grained fraction by a sieve method or mixing with raw fly ash larger than the threshold, and heat-treating the product by a heating method; and mixing the heated fly ash and classified fly ash.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing modified fly ash with a reduced amount of unburned carbon. [Background technology]

[0002] Fly ash is used as a cement admixture, concrete admixture, mortar admixture, etc., and generally, fly ash with a low amount of unburned carbon, which is the carbon residue, is considered to be preferable. For example, if the amount of unburned carbon in fly ash is high, the unburned carbon may appear on the surface of mortar or concrete, causing black spots. In addition, there is a problem that chemical admixtures and other agents added to mortar or concrete may be adsorbed by the unburned carbon.

[0003] The quality of fly ash usable in Japan for the above-mentioned applications is specified in JIS A 6201. Grades I to IV are specified as quality grades, with Type II being the most commonly used. Since the loss on ignition for Type II is regulated to be 5.0% by mass or less, a range of 0 to 5.0% by mass is acceptable to meet the standard. However, in reality, fly ash with a loss on ignition of 1.5 to 3.5% by mass is widely distributed, while fly ash with a loss on ignition of, for example, 0.5% or 4.5% by mass is less common. This is because significant variations in loss on ignition, even within the 0 to 5.0% by mass range, can cause problems with uniformity when used for the above-mentioned applications. For this reason, fly ash used for the above-mentioned applications is required to keep the variation within a range of preferably 1.5 to 3.5% by mass, and more preferably 2.0 to 3.5% by mass.

[0004] However, the amount of unburned carbon in fly ash generated from thermal power plants is generally not uniform, with some containing as much as 15% by mass. As a result, only a limited amount of fly ash is suitable as a minor cement component or as a cement / concrete admixture.

[0005] Under these circumstances, various methods have been proposed for reducing the amount of unburned carbon contained in fly ash, such as a heating method and a sieving method.

[0006] The heating method is one of the methods that can efficiently remove unburned carbon from fly ash. The unburned carbon is removed from the fly ash by burning it in a heating furnace, and this method is a means that can sufficiently reduce the amount of unburned carbon in the fly ash.

[0007] The sieving method is a technique that concentrates unburned carbon in the coarse particles of fly ash, because unburned carbon particles are relatively large among fly ash particles, and produces fly ash with less unburned carbon in the fine particles by sieving. However, the sieving method is characterized by a lower unburned carbon removal rate than other methods, making it difficult to sufficiently reduce the amount of unburned carbon in raw fly ash, which has a high unburned carbon content. In addition, the coarse particles containing a large amount of unburned carbon are generated as residue, which poses a problem for disposal. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2019-130507 [Patent Document 2] International Publication No. 2018 / 180680 Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned above, the heating method can significantly reduce unburned carbon, but it requires the use of fossil fuels, which results in the emission of carbon dioxide. For example, Patent Document 1 describes a fly ash thermal reforming device using a rotary kiln. It also describes a method of reducing unburned carbon by controlling the temperature of a heating furnace to a high temperature using an auxiliary burner or an external heat burner to heat the fly ash. Here, the heat sources for maintaining the heating furnace at a high temperature include fossil fuels supplied from the burner and the combustion of unburned carbon itself. Therefore, to maintain a constant temperature inside the furnace, it is necessary to control the heat sources supplied from both sources at a constant level. In other words, when reforming fly ash with a small amount of unburned carbon using the heating method, the overall calorific value of the unburned carbon itself is small, and a large amount of fossil fuel must be used. Therefore, in addition to the problem of emitting a large amount of carbon dioxide, reforming fly ash with a small amount of unburned carbon is not desirable from the perspective of protecting fossil resources.

[0010] On the other hand, the sieving method has the problem of not being able to sufficiently reduce the amount of unburned carbon. Patent Document 2 shows the change in the amount of unburned carbon when sieving using meshes with 90 μm and 45 μm openings. Even when using a 45 μm mesh, which has a greater effect on reducing unburned carbon, the unburned carbon content of the raw fly ash must be approximately 5% by mass or less in order to reduce the amount of unburned carbon to 3% by mass or less. In other words, the sieving method is not applicable to fly ash with a large amount of unburned carbon. Another problem with the sieving method is the inevitable generation of residue. Patent Document 2 describes that when sieving through a 90 μm mesh, approximately 10% by mass of residue is generated, and when sieving through a 45 μm mesh, approximately 20% by mass of residue is generated. In other words, the amount of target modified fly ash is small compared to the amount of raw fly ash, which further creates problems with the disposal of the residue.

[0011] Therefore, an object of the present invention is to provide a method for obtaining modified fly ash with a low amount of unburned carbon, which can reduce the amount of carbon dioxide emitted and the amount of fossil fuel used compared to conventional methods, and which does not generate residues, when the amount of unburned carbon in the raw fly ash varies. [Means for solving the problem]

[0012] In view of the above problems, the inventors of the present invention have conducted extensive research and have found that by using two methods, namely heating and sieving, to reduce the amount of unburned carbon, and determining the reduction method depending on the amount of unburned carbon in the raw fly ash, and producing fly ash with reduced unburned carbon using each method and then mixing the produced fly ash, it is possible to reduce carbon dioxide emissions and fossil fuel consumption compared to conventional methods, even when the amount of unburned carbon in the raw fly ash varies, and to obtain modified fly ash with a low and minimally variable amount of unburned carbon without generating residue, thereby completing the present invention.

[0013] That is, the present invention is a method for producing modified fly ash having a small amount of unburned carbon by reducing unburned carbon contained in raw fly ash, Raw fly ash having an unburned carbon content greater than a predetermined threshold is modified by a heating method to produce heated fly ash, raw fly ash having an unburned carbon content equal to or less than the threshold is modified by a sieving method to produce classified fly ash, the residue separated as a coarse particle by the sieving method is either alone or mixed with raw fly ash having an unburned carbon content greater than the threshold, and then heated by a heating method to produce heated fly ash, and the resulting heated fly ash and classified fly ash are mixed to produce modified fly ash. This is a method for producing modified fly ash.

[0014] In the method for producing modified fly ash of the present invention, it is preferable to preset a target range for the amount of unburned carbon contained in the modified fly ash, measure the amount of unburned carbon contained in each of the heated fly ash and the classified fly ash, and mix them in a ratio such that the amount of unburned carbon in the modified fly ash falls within the target range. Furthermore, it is preferable that the amount of unburned carbon contained in the modified fly ash is 1.5 to 3.5 mass%. In the sieving method, it is preferable that the mesh size of the sieve be 75 to 106 μm. In the heating method, it is preferable that the heating means be a fluidized bed furnace, and the heating temperature in the fluidized bed furnace is preferably in the range of 800 to 950°C. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a manufacturing method for obtaining modified fly ash that can handle fly ash having a variety of unburned carbon amounts in raw fly ash, can reduce carbon dioxide emissions and fossil fuel consumption compared to conventional methods, does not generate residues, and has a small amount of unburned carbon and little fluctuation. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention is applicable to a method for producing modified fly ash with a low amount of unburned carbon by reducing unburned carbon in raw fly ash. Here, the raw fly ash can be a variety of fly ash, including general fly ash generated in facilities that use coal as fuel, such as coal-fired power plants, as well as fly ash generated by co-firing coal with fuels other than coal (e.g., ammonia, biomass fuel, etc.) or waste (e.g., waste plastic, sludge, etc.). The present invention will be described in detail below.

[0017] The present invention uses two methods for reducing unburned carbon: a heating method and a sieving method. Either the heating method or the sieving method is used depending on the amount of unburned carbon in the raw fly ash.

[0018] The heating method uses a heating furnace to produce heated fly ash with reduced unburned carbon content. The type of heating furnace is not particularly limited, and known heating furnaces can be used. Examples of industrially applicable heating furnaces include fluidized bed heating furnaces and rotary kilns. The heating furnace can be either continuous or batch-type. The heating temperature is preferably set to 700–1000°C, more preferably 800–950°C. If the temperature is too low, the unburned carbon content will not be sufficiently reduced. If the temperature is too high, the fly ash may fuse inside the heating furnace or crystallize, potentially reducing its usefulness as a cement or concrete admixture.

[0019] The sieving method uses a conventional industrial sieving machine to produce classified fly ash with reduced unburned carbon content. Fine particles that pass through the sieve become classified fly ash, while coarse particles that do not pass through become residue. Known sieving machines used in the sieving method include, for example, vibrating sieves, oscillating sieves, and rotating sieves. Sieves can be either continuous or batch-type. The mesh size of the sieve installed in the sieve is preferably in the range of 45 to 150 μm, more preferably 75 to 106 μm. If the mesh size is too small, the recovery rate of the modified fly ash decreases, resulting in increased residue generation and increased susceptibility to problems due to clogging of the mesh. If the mesh size is too large, the amount of residue generated decreases, but the effect of reducing unburned carbon is almost nonexistent. Unburned carbon is concentrated in the residue. As described below, the present invention makes it possible to suppress residue generation by treating the residue using a heating method.

[0020] In the present invention, raw fly ash having an unburned carbon content greater than a predetermined threshold is modified by a heating method to produce heated fly ash, and raw fly ash having an unburned carbon content equal to or less than the threshold is modified by a sieving method to produce classified fly ash.

[0021] If the amount of unburned carbon contained in the raw fly ash is known in advance, there is no need to measure it. However, if the amount of unburned carbon is unknown, it should first be measured. The method for measuring the amount of unburned carbon is not particularly limited, and any known method can be used. For example, the most common method for measuring the amount of unburned carbon in fly ash is the ignition loss measurement method described in JIS A 6201. Other measurement methods include infrared detection of CO₂ and CO gases generated by combustion, calculation based on methylene blue adsorption, and microwave irradiation to estimate the amount of unburned carbon. The "ignition loss" described in JIS A 6201 can be treated as a numerical value equivalent to the "amount of unburned carbon" in this invention. Among these, the method of estimating the amount of unburned carbon using microwave irradiation is particularly recommended. Because the microwave irradiation method requires a short measurement time, it is useful for continuously monitoring the amount of unburned carbon. For example, it is possible to measure the amount of unburned carbon in raw fly ash removed from a storage facility at any time and continuously distribute it to either the heating reforming treatment side or the sieving reforming treatment side depending on the measurement value. To measure, the required amount is automatically removed and introduced into a measuring device, and measurements of the amount of unburned carbon can be obtained at intervals of several minutes to several tens of minutes. Note that the amount of unburned carbon contained in heated fly ash, classified fly ash, and reformed fly ash can also be measured using a similar method.

[0022] Then, a predetermined threshold is used to determine whether the raw fly ash should be reformed by the heating method or the sieving method. Raw fly ash with an unburned carbon content greater than the threshold is assigned to the heating method, while raw fly ash with an unburned carbon content less than the threshold is assigned to the sieving method. By assigning raw fly ash based on the threshold in this way, the heating method reforms raw fly ash with a high unburned carbon content, while the sieving method reforms raw fly ash with a low unburned carbon content. In the heating method, a high amount of unburned carbon increases the amount of heat generated in the heating furnace, making it possible to reduce the amount of fossil fuel used. In the sieving method, selective use of raw fly ash with a low unburned carbon content suppresses variation in the unburned carbon content after reforming, thereby achieving a consistently low value. The threshold for the unburned carbon content of the raw fly ash can be changed as appropriate depending on the situation.

[0023] The threshold value is not particularly limited and can be set appropriately, taking into consideration factors such as the degree of variation in the amount of unburned carbon in the raw fly ash used, the processing capacity of the heating method and the sieving method for the amount of raw fly ash, and the desired level of unburned carbon in the modified fly ash to be finally produced.

[0024] The heating method and the sieving method can also be performed simultaneously. To perform these methods simultaneously, it is preferable to store raw fly ash with an unburned carbon content greater than a threshold value and raw fly ash with an unburned carbon content less than the threshold value in separate storage facilities. By preparing a storage facility for storing raw fly ash with an unburned carbon content greater than the threshold value and a storage facility for storing raw fly ash with an unburned carbon content less than the threshold value and separating and preparing a fixed amount of raw fly ash according to the amount of unburned carbon, it is possible to perform the heating method and the sieving method treatments continuously.

[0025] The amount of unburned carbon in the raw fly ash, which determines which storage tank to allocate to, can be determined by collecting and analyzing a portion of the raw fly ash when it is received, or by using the analytical value from the fly ash emitter, such as a thermal power plant. Alternatively, once the raw fly ash is received into one storage facility, the amount of unburned carbon in the storage facility can be measured and the raw fly ash can be distributed to multiple storage facilities.

[0026] The raw fly ash introduced into the heating furnace or sieving machine is then modified to reduce unburned carbon, producing heated fly ash and classified fly ash with reduced unburned carbon content, respectively. The residue generated by the sieving method, which contains concentrated unburned carbon, is either mixed alone or with raw fly ash greater than the threshold value, and then heated to produce heated fly ash. The resulting heated fly ash and classified fly ash are then mixed to produce modified fly ash. The method for mixing the heated fly ash and classified fly ash is not particularly limited. They may be mixed using a powder mixer, or they may be mixed by joining them together in a transport facility or storage facility.

[0027] The unburned carbon content of the modified fly ash is preferably 1.5 to 3.5% by mass, more preferably 2.0 to 3.5% by mass. Generally, the unburned carbon content of fly ash for concrete distributed in Japan is 1.5 to 3.5% by mass, and within this range, it can be handled in the same way as other commercially available products. Furthermore, by setting the range to 2.0 to 3.5% by mass, the stability of quality can be improved.

[0028] To stabilize the unburned carbon content of the reformed fly ash, it is preferable to determine a target range for the unburned carbon content of the reformed fly ash in advance and then determine and mix the heated fly ash and classified fly ash at a ratio that falls within the target range. For example, if the target range for the unburned carbon content of the reformed fly ash is 1.5 to 3.5 mass%, and if 2.0 mass% of heated fly ash is produced by the heating method and 5.5 mass% of classified fly ash is produced by the sieving method, mixing them in equal amounts will result in a calculated unburned carbon content of 3.8 mass%, which falls outside the target range. In such a case, mixing heated fly ash and classified fly ash at a ratio of 7:3 will result in an unburned carbon content of 3.0 mass%, which falls within the target range. In this case, the mixing ratio can be adjusted appropriately to fall within the target range of 1.5 to 3.5 mass%, without any problems. In actual operation, it is expected that there will be situations where either heated fly ash or classified fly ash must be consumed preferentially due to production and inventory relationships. For example, if you want to use heated fly ash preferentially, you can mix heated fly ash and classified fly ash in a 9:1 ratio to make 2.3 mass%.

[0029] Here, the amount of unburned carbon in each of the heated fly ash and the classified fly ash is measured to determine the mixing ratio of the heated fly ash and the classified fly ash. The amount of unburned carbon in each of the heated fly ash and the classified fly ash can be measured each time during the process. The measurement method can be the method for measuring the amount of unburned carbon in the raw fly ash described above. The heated fly ash and the classified fly ash produced in each reforming process can be stored in separate storage facilities, and the required amount can be extracted and measured. Alternatively, the required amount can be periodically extracted and measured from the transport piping from the outlet of the storage facility to the inlet of the mixing facility. After obtaining the measurements, the mixing ratio of each is determined, and the mixture is sent to the subsequent mixing process.

[0030] Regarding the residue generated in the screening process, the residue alone or mixed with raw fly ash larger than the threshold value can be heat-treated by a heating method, thereby reducing the generation of residue throughout the entire process to zero.

[0031] When attempting to reform raw fly ash with various unburned carbon contents using a single reforming method, as described above, the heating method can produce reformed fly ash with a sufficiently low unburned carbon content, but if the raw fly ash does not have a high unburned carbon content, a large amount of fossil fuel must be used. Furthermore, the sieving method has difficulty dealing with raw fly ash with a high unburned carbon content, and residue processing is also required. Furthermore, the fluctuations in the unburned carbon content of the raw fly ash cannot be counteracted, resulting in inconsistent unburned carbon content in the reformed fly ash. However, by using the present invention, it is possible to reduce the amount of fossil fuel used, suppress the generation of residue, and obtain reformed fly ash with a stable unburned carbon content. [Example]

[0032] Examples and comparative examples are shown below, but the technical scope of the present invention is not limited thereto. In these examples, the measured value of ignition loss is shown as the amount of unburned carbon in the fly ash. The ignition loss was measured according to the method for measuring ignition loss specified in JIS A 6201.

[0033] The raw fly ash was modified by heating and sieving under the following conditions.

[0034] (heating method) Heating furnace: Fluidized bed heating furnace (inner diameter φ400mm) Heating medium: silica sand Heating temperature: 900℃ Fuel used: Propane gas (1.5Nm 3 / h fixed), A heavy oil (variable) (sieving method) Sieve: Swinging sieve (inner diameter 600 mm) Mesh opening: 90 μm

[0035] Example 1 Four types of raw fly ash, A to D, were prepared as shown in Table 1. The threshold value for the amount of unburned carbon was set to 5.0 mass%, and first, raw fly ash A and B, each with an unburned carbon amount of 5.0 mass% or less, were modified by a sieving method.

[0036] Table 2 shows the change in the amount of unburned carbon and the weight balance at this time.

[0037] In the sieving method, raw fly ash was supplied at 86.0 kg / h, and raw fly ash A produced 8.7 kg / h of sieve residue (unburned carbon content 11.5 mass%), while raw fly ash B produced 7.8 kg / h of sieve residue (unburned carbon content 17.9 mass%).

[0038] Next, raw fly ash C and D, each with an unburned carbon content of more than 5.0 mass%, were modified by heating. Raw fly ash C was mixed with the sieve residue (residue A) derived from raw fly ash A, and raw fly ash D was mixed with the sieve residue (residue B) derived from raw fly ash B, and both were modified by heating. For the heat treatment of raw fly ash C, 86.0 kg of raw fly ash was mixed with 8.7 kg of residue A, and the mixture was supplied at a rate of 94.7 kg / h. For the heat treatment of raw fly ash D, 86.0 kg of raw fly ash was mixed with 7.8 kg of residue B, and the mixture was supplied at a rate of 93.8 kg / h.

[0039] Table 3 shows the change in the amount of unburned carbon, the weight balance, and the amount of heavy oil A used at this time.

[0040] [Table 1]

[0041] [Table 2]

[0042] [Table 3]

[0043] The amount of unburned carbon in the classified or heated fly ash obtained by the above modification treatment was 2.1 to 3.6 mass %.

[0044] Then, the entire amounts of the classified fly ash A and heated fly ash C, and the classified fly ash B and heated fly ash D recovered from the one-hour modification treatment were mixed to obtain modified fly ash. Table 4 shows the amount of unburned carbon in each modified fly ash (Example 1-1).

[0045] In addition, modified fly ash was obtained by mixing the entire amounts of classified fly ash A and heated fly ash D, and classified fly ash B and heated fly ash C, which were recovered for one hour of modification treatment. Table 4 shows the amount of unburned carbon in each modified fly ash (Examples 1-2).

[0046] [Table 4]

[0047] In both cases, a stable reformed fly ash with an unburned carbon content of around 2% was obtained. Since the residue on the sieve was treated by heating, no residue was generated. The amount of heavy oil A used in the heat treatment was 1.0 L / h for treating raw fly ash C and 0.6 L / h for treating raw fly ash D. In other words, the amount of heavy oil A used to reform raw fly ash A to D prepared in Example 1 was 1.6 L / h.

[0048] Example 2 Using classified or heated fly ash obtained in Example 1 with an unburned carbon content of 2.1 to 3.6 mass% shown in Tables 2 and 3, and setting the target range of unburned carbon content of the modified fly ash to 2.5 to 3.0 mass%, the classified fly ash A or B and the heated fly ash C or D were mixed in the ratio (by mass) shown in Table 5 to obtain modified fly ash. The unburned carbon content of the obtained modified fly ash is shown in Table 5.

[0049] It can be seen that by adjusting the mixing ratio to match the target range, the amount of unburned carbon in the modified fly ash can be made more stable and less fluctuating.

[0050] [Table 5]

[0051] (Comparative Example 1) The raw fly ashes A to D were modified using only the heating method. The unburned carbon content of the resulting heated fly ash was 1.1 to 2.2 mass%, a stable and low value, as shown in Table 6. Furthermore, because all of the fly ash was modified using the heating method, no residue was generated.

[0052] On the other hand, the total amount of heavy oil A used in this comparative example was 6.4 L / h, which is significantly higher than the 1.6 L / h in Example 1. Furthermore, compared with only raw fly ash C or D, which were heated in Example 1, the consumption of raw fly ash C was 1.0 L / h in Example 1 and 1.3 L / h in Comparative Example 1, and the consumption of raw fly ash D was 0.6 L / h in Example 1 and 0.7 L / h in Comparative Example 1, with Comparative Example 1 showing a higher consumption. This is because in Example 1, the sieve residue, in which unburned carbon is concentrated, is mixed with the raw fly ash used in the heating method, and the heat input from the unburned carbon contributes to a reduction in the amount of heavy oil A used.

[0053] [Table 6]

[0054] (Comparative Example 2) Raw fly ash A to D were modified using only the sieving method. Unlike the combustion method, no fuel is required to burn the unburned carbon. However, the unburned carbon content of the resulting classified fly ash ranged from 2.6 to 6.7 mass%, as shown in Table 7. This shows a large variation and is significantly outside the range of unburned carbon content of fly ash for concrete generally distributed in Japan. Furthermore, because modification was performed using only the sieving method, residue remained unprocessed.

[0055] [Table 7]

[0056] (Comparative Example 3) The classified fly ashes A and B and the heated fly ashes C and D obtained in Example 1 were each used as modified fly ashes without mixing them (Table 8). The unburned carbon content of the modified fly ashes obtained in this case was 2.1 to 3.6 mass%, which is slightly outside the range of unburned carbon content of fly ashes for concrete generally distributed in Japan. Furthermore, compared with the modified fly ashes finally obtained in Example 1 and shown in Table 4, the unburned carbon content varied widely, indicating poor quality stability.

[0057] [Table 8]

[0058] Comparative Example 4 Modified fly ash was obtained in the same manner as in Example 1, except that the sieve residues derived from raw fly ash A and raw fly ash B were not treated by the heating method. The change in the amount of unburned carbon, the weight balance, and the amount of heavy oil A used at this time are shown in Table 9. The amount of unburned carbon in the obtained modified fly ash is also shown in Table 10 (Comparative Examples 4-1 and 4-2).

[0059] [Table 9]

[0060] [Table 10]

[0061] In both cases, it was possible to obtain a stable modified fly ash with an unburned carbon content of around 2%.

[0062] However, because the sieve residue was not treated by the heating method, residue was generated. The amount of heavy oil A used in the heating treatment was 1.3 L / h for treating raw fly ash C and 0.7 L / h for treating raw fly ash D. That is, the amount of heavy oil A used to modify raw fly ashes A to D in Comparative Example 4 was 2.0 L / h, which is larger than the amount used in Example 1, 1.6 L / h. This is for the same reason as in Comparative Example 1: in this Comparative Example, the sieve residue, in which unburned carbon is concentrated, was not mixed with the raw fly ash used in the heating method, and therefore the heat input derived from unburned carbon was smaller than in Example 1.

[0063] From the above examples and comparative examples, it can be seen that raw fly ash with an unburned carbon content greater than a predetermined threshold is modified by a heating method to produce heated fly ash, raw fly ash with an unburned carbon content equal to or less than the threshold is modified by a sieving method to produce classified fly ash, the residue separated as a coarse particle by the sieving method is mixed alone or with raw fly ash with an unburned carbon content greater than the threshold and heat-treated by a heating method to produce heated fly ash, and then the resulting heated fly ash and classified fly ash are mixed to produce modified fly ash. This method stably reduces the unburned carbon content of the modified fly ash, minimizes the amount of fossil fuel used, and enables processing without generating residue.

Claims

1. A method for producing modified fly ash having a low amount of unburned carbon by reducing unburned carbon contained in raw fly ash, comprising: Raw fly ash having an unburned carbon content greater than a predetermined threshold is modified by a heating method to produce heated fly ash, raw fly ash having an unburned carbon content equal to or less than the threshold is modified by a sieving method to produce classified fly ash, the residue separated as a coarse particle by the sieving method is either alone or mixed with raw fly ash having an unburned carbon content greater than the threshold, and then heated by a heating method to produce heated fly ash, and the resulting heated fly ash and classified fly ash are mixed to produce modified fly ash. A method for producing modified fly ash.

2. A method for producing modified fly ash having a low amount of unburned carbon by reducing unburned carbon contained in raw fly ash, comprising: The amount of unburned carbon contained in the raw fly ash is measured. Raw fly ash having an unburned carbon content greater than a predetermined threshold is modified by a heating method to produce heated fly ash, raw fly ash having an unburned carbon content equal to or less than the threshold is modified by a sieving method to produce classified fly ash, a residue separated as a coarse particle by the sieving method is mixed with the residue alone or with raw fly ash having an unburned carbon content greater than the threshold, and the residue is heat-treated by a heating method to produce heated fly ash, and the resulting heated fly ash and classified fly ash are mixed to produce modified fly ash. A method for producing modified fly ash.

3. A target range of the amount of unburned carbon contained in the modified fly ash is set in advance, 3. The method for producing modified fly ash according to claim 1, wherein the amounts of unburned carbon contained in the heated fly ash and the classified fly ash are measured, and the heated fly ash and the classified fly ash are mixed in a ratio such that the amount of unburned carbon in the modified fly ash falls within a target range.

4. The method for producing modified fly ash according to claim 1 or 2, wherein the amount of unburned carbon contained in the modified fly ash is 1.5 to 3.5 mass%.

5. The method for producing modified fly ash according to claim 1 or 2, characterized in that the mesh opening of the sieve is 75 to 106 μm.

6. 3. The method for producing modified fly ash according to claim 1, wherein the heating means is a fluidized bed heating furnace.

7. 7. The method for producing modified fly ash according to claim 6, wherein the heating temperature in the fluidized bed heating furnace is in the range of 800 to 950°C.

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