METHOD OF USING FLY ASH

Abstract

A sieve with a mesh opening of 75 to 20 m is provided. A raw fly ash powder is separated into a component remaining on the sieve and a component passing through the sieve by classification using the sieve. A fine fly ash powder, which is the component passing through the sieve, is used as a cement admixture. A coarse fly ash powder, which is the component remaining on the sieve, is used for production of cement clinker.

Claims

1. A method of using fly ash, comprising: providing a sieve with a mesh opening of 75 to 20 m; separating fly ash discharged from a thermal power plant into a component remaining on the sieve and a component passing through the sieve by classification using the sieve; using a fine fly ash powder, which is the component passing through the sieve, in admixture with cement; and using a coarse fly ash powder, which is the component remaining on the sieve, for production of cement clinker.

2. The method of using according to claim 1, wherein the fine fly ash powder is used for production of concrete or mortar through its use as a cement admixture or a concrete admixture.

Description

MODE FOR CARRYING OUT THE INVENTION

<Fly Ash>

[0033] Fly ash is matter which is caught by a dust collector from dust and soot generated during a combustion process. In the present invention, fly ash collected by an electrostatic precipitator of a coal-fired power plant is preferably used, particularly because it occurs in a large amount, it can be industrially utilized, and it has constant quality.

[0034] In the present invention, a raw fly ash powder collected by the electrostatic precipitator generally contains silica (SiO.sub.2) in an amount of 40% by mass or more, especially 45 to 60% by mass, and alumina (Al.sub.2O.sub.3) in an amount of 15% by mass or more, especially 20 to 35% by mass or more, the SiO.sub.2/Al.sub.2O.sub.3 mass ratio being in a range of the order of 1.5 to 2.5, and further contains Fe.sub.2O.sub.3, MgO, and CaO as other oxides. The ignition loss (corresponding to the unburned carbon content) at 1000 C. is of the order of 3 to 6% by mass. The particle size of the raw powder is in a wide range and, on the average, of the order of 10 to 50 m.

<Classification>

[0035] In the present invention, the raw fly ash powder is classified into a coarse powder and a fine powder, and the fine powder is used as an admixture, while the coarse powder is used as a raw material for clinker production. It is important that this classification be performed using a sieve. That is, as will be shown in the working example to be described later, a fine powder deprived of unburned carbon particles can be obtained by classification using a sieve having a certain mesh opening, and such a fine powder can be used as an admixture. The coarse powder has a low Al.sub.2O.sub.3 content as compared with the raw powder, and can be used preferably as a raw material for clinker production.

[0036] Airflow classification, for example, is available as a classification means which is employed industrially. Such a means, however, cannot separate the unburned carbon particles from the fine powder, because the unburned carbon particles, owing to their low specific gravity, are recovered in a form contained in the fine powder used as an admixture. In this case, therefore, the measure of heating the recovered fine powder to remove the unburned carbon particles by combustion is adopted, but such heating lowers the inherent reactivity of the fly ash with cement to impair the aptitude of the fine powder for use as an admixture.

[0037] In the present invention, as noted above, the raw fly ash powder is classified into the coarse powder and the fine powder with the use of the sieve. As the sieve, one with a mesh opening of 75 to 20 m, particularly 63 to 20 m, is used, and more preferably one with a mesh opening of 45 m or more is used.

[0038] The classification using the sieve with such a mesh opening greatly reduces the unburned carbon content in the fine fly ash power which is the component passing through the sieve. Consequently, a fine fly ash powder having a high quality as an admixture without deterioration of its reactivity with cement is obtained. That is, the unburned carbon particles contained in the raw fly ash powder contain a large amount of particles with such a particle size as not to pass through the sieve with the above mesh opening.

[0039] As will be indicated by the working example to be described later, the coarse fly ash powder which is the component remaining on the sieve has an alumina content decreased compared with the raw powder (silica/alumina mass ratio increased), so that a coarse fly ash powder having a high aptitude for use as a raw material for cement clinker production is obtained. That is, the alumina component contained in the raw fly ash powder contains a large amount of particles with such a small particle size as to pass through the sieve with the above mesh opening.

[0040] If a sieve with a greater mesh opening than the above mesh opening is used to perform classification, for example, the amount of the unburned carbon particles in the component passing through the sieve increases, thereby impairing the aptitude of the fine powder, which is the component passing through the sieve, for use as an admixture. Moreover, the recovery rate of the coarse powder lowers, thus decreasing the yield of the raw material for clinker production.

[0041] If a sieve having a smaller mesh opening than the mesh opening mentioned above is used, particles with a high Al.sub.2O.sub.3 content do not pass through the meshes of the sieve. As a result, the SiO.sub.2/Al.sub.2O.sub.3 mass ratio of the coarse powder nearly equals that of the raw powder, and the aptitude of the coarse powder as a raw material for clinker production is impaired. Furthermore, clogging of the sieve meshes is apt to occur, and the durability of the sieve also declines.

[0042] In the present invention, a publicly known classifier, for example, a gyrating airflow sieving machine, a centrifugal airflow sieving machine, a centrifugal dispersing sieving machine, a round vibrating sieving machine, or a shaking sieving machine, can be used as the above classifier, as long as classification by a sieve with the aforementioned mesh opening is performed thereby.

[0043] Of these classifiers, the centrifugal dispersing sieving machine has the advantage that its treating capacity per screen (sieve) unit area is high. Moreover, it has the advantage of being able to effectively classify fly ash (coarse powder) even if the fly ash contains water and agglomerates highly.

[0044] Further, the shaking sieve is inferior to the centrifugal dispersing sieving machine in the treating capacity per screen unit area, but lightens a load on the main body of the machine due to vibrations. Thus, this machine is effective particularly when classifying a large amount of fly ash.

<Fine Fly Ash Powder>

[0045] In the present invention, 80% to 90% of the original fly ash is obtained as a fine fly ash powder by classification using the sieve mentioned above. The fine fly ash powder is effectively deprived of unburned carbon by classification. Compared with the raw fly ash powder, therefore, the fine fly ash powder is lower in the unburned carbon content. For example, its ignition loss at 1,000 C. is 4.0% by mass or less and, depending on the composition of the raw powder, is 3.0% by mass or less.

[0046] Furthermore, such a fine fly ash powder has not been subjected to heating for removing the unburned carbon, and thus its reactivity with cement has not been lowered. For example, in accordance with JIS A 6201, etc., the proportion (%) of compressive strength, as measured in connection with mortar incorporating a predetermined amount of the fine fly ash powder, to the compressive strength of reference mortar is known as an activity index. The activity index of the fine fly ash powder obtained by the present invention is 80% or more after a lapse of 28 days, and 90% or more after a lapse of 91 days.

[0047] As shown above, the fine fly ash powder obtained using the aforementioned sieve satisfies the values of the ignition loss and the activity index required by quality standards such as JIS A 6201 and ASTM C618 (CLASS F).

[0048] In the present invention, therefore, the above-mentioned fine fly ash powder is used in mixture with cement. Concretely, the fine powder is mixed with cement containing gypsum or clinker and used, or the fine powder and other components are simultaneously mixed and used for the preparation of cement. In this case, a composition formed by mixing is usually called cement, depending on the standards of each country. If a large amount of fly ash is mixed, for example, the mixture is called fly ash cement.

[0049] Such cement may further contain other admixtures used where necessary (ground granulated blast furnace slag, fine limestone powder, siliceous admixture, etc.). Any of gypsum dihydrate, gypsum hemihydrate, and anhydrous gypsum can be used as gypsum.

[0050] The fine powder can also be mixed with various cements produced separately (for example, Portland cement, blast furnace cement, and blended cement) for the purpose of use in adjusting the physical properties of the cement. Furthermore, when a cement paste is to be prepared by mixing water with cement, or when a fine aggregate or the like is kneaded with this cement paste to produce concrete or mortar, the fine fly ash powder can be mixed.

<Coarse Fly Ash Powder>

[0051] In the present invention, 10% to 20% of the raw fly ash powder is obtained as a coarse fly ash powder (i.e., component remaining on the sieve) by classification using the sieve mentioned above.

[0052] Compared with the raw powder, the coarse fly ash powder is increased in the unburned carbon content. However, its SiO.sub.2/Al.sub.2O.sub.3 mass ratio is increased, and thus the Al.sub.2O.sub.3 content is decreased. As the mesh opening of the sieve increases, in particular, this tendency becomes higher. When the sieve with a mesh opening of 45 m or more is used on condition that the mesh opening is in the aforementioned range, a great reduction in the Al.sub.2O.sub.3 content is confirmed.

[0053] Hence, the coarse fly ash powder is used as a raw material for clinker production, whereby the amount of fly ash used per unit weight of clinker can be increased. That is, the coarse fly ash powder has a low Al.sub.2O.sub.3 content, so that the amount of the resulting aluminate (C.sub.3A) can be kept down. Thus, the amount of the coarse fly ash powder used can be increased. Assume that the amount of the coarse fly ash powder used per unit weight of clinker is 100, for example. If this coarse powder is used in this case, its amount of use can be set at 110 or more.

[0054] Clinker production using the coarse fly ash powder is performed by mixing the coarse fly ash powder with various inorganic materials, concretely, limestone, clay, silica stone, slag, etc., in such a manner that CaO, SiO.sub.2, Al.sub.2O.sub.3 and Fe.sub.2O.sub.3 necessary for the formation of a cement component are supplied, and then calcining the mixture at a high temperature. By using such a coarse fly ash powder, it becomes possible to reduce the amounts of clay and silica stone used, which serve as SiO.sub.2 and Al.sub.2O.sub.3 sources, thereby achieving cost reduction.

Examples

[0055] Hereinbelow, the present invention will be described more concretely by reference to experimental examples, but the present invention is in no way limited to these experimental examples.

[0056] <Raw Fly Ash Powder>

[0057] Five raw fly ash powders (to be described hereinafter as FA1 to FA5) occurring in different coal-fired power plants in Japan were used.

<Classification>

Sieve:

[0058] A JIS testing sieve (JIS Z 8801-1:2006) made of stainless steel, which had a mesh opening of 75, 45 or 20 m, was used. All of these sieves used were circular sieves with a screen diameter of 200 mm.

[0059] An ultrasonic vibration generator (PNS35-50/100-S/T, a product of Artech) was mounted with the above JIS testing sieve, and classification was performed, with ultrasonic vibrations being applied to the sieve.

<Various Evaluation Methods>

Ignition Loss:

[0060] The ignition loss of the resulting coarse or fine fly ash powder was measured in accordance with the method specified in JIS A 6201:2015.

Activity Index:

[0061] The activity index of the resulting fine fly ash powder was measured in accordance with the method specified in JIS A 6201:2015.

Chemical Composition:

[0062] The contents (% by mass) of SiO.sub.2, Al.sub.2O.sub.3 and other components of the resulting coarse fly ash powder were determined by X-ray fluorescence analysis using an X-ray fluorescence analyzer. The values were calculated so that the total value of the ignition loss (unburned carbon content) and the content of each component would be 100% by mass.

Example

[0063] FA1 to FA3 were classified using the sieve to obtain coarse fly ash powders and fine fly ash powders. The yields of the resulting coarse fly ash powders are shown in Table 1.

TABLE-US-00001 TABLE 1 FA1 FA2 FA3 Average 75 m coarse 4.6 12.7 9.7 9.0 powder 45 m coarse 10.0 22.7 17.9 16.9 powder 20 m coarse 24.2 42.6 36.7 34.5 powder (unit: mass %)

[0064] The above results show that when the raw fly ash powder was classified using the sieve with a mesh opening of 45 m, the coarse fly ash powder was obtained in an amount of 16.9% by mass on the average relative to the raw powder.

[0065] The resulting coarse fly ash powders were each measured for the ignition loss at 1,000 C. and the chemical composition, and the SiO.sub.2/Al.sub.2O.sub.3 mass ratio was calculated. The results are shown in Table 2.

TABLE-US-00002 TABLE 2 Component Ignition loss (mass %) SiO.sub.2/ (mass %) SiO.sub.2 Al.sub.2O.sub.3 others Al.sub.2O.sub.3 FA1 Raw powder 5.3 51.8 25.8 17.1 2.0 75 m coarse 17.3 48.3 20.9 13.5 2.3 powder 45 m coarse 9.2 50.2 22.6 18.0 2.2 powder 20 m coarse 9.0 50.8 24.0 16.2 2.1 powder FA2 Raw powder 4.2 58.8 26.4 10.6 2.2 75 m coarse 29.1 48.5 15.5 7.0 3.1 powder 45 m coarse 17.1 54.3 19.3 9.4 2.8 powder 20 m coarse 11.3 56.8 22.1 9.9 2.6 powder FA3 Raw powder 3.5 48.3 31.1 17.2 1.6 75 m coarse 9.3 45.9 27.6 17.1 1.7 powder 45 m coarse 5.8 47.0 28.5 18.8 1.7 powder 20 m coarse 4.6 48.6 30.0 16.8 1.6 powder

[0066] The above results show that the coarse fly ash powder underwent a great ignition loss. Moreover, the coarse fly ash powder was shown to be low in the Al.sub.2O.sub.3 content, with the result that its SiO.sub.2/Al.sub.2O.sub.3 mass ratio was high.

[0067] The resulting coarse fly ash powder was used as a substitute material for the raw fly ash powder to produce clinker. The amount of each of the raw powders FA1 to FA3 used as the raw material for clinker production was taken as 100%, and the amount of the coarse fly ash powder used was determined. The results are shown in FIG. 3. The amount of use was calculated from the ratio between the Al.sub.2O.sub.3 contents of the raw powder and the coarse powder. For example, the Al.sub.2O.sub.3 content in the 75 m coarse powder from FA1 was 20.9 compared with 25.8 of the raw powder, the ratio being as low as 20.9/25.8=1/1.235. That is, in supplying Al.sub.2O.sub.3 necessary for clinker production with the use of the 75 m coarse powder, the amount of 123.5% is usable in comparison with the raw powder.

TABLE-US-00003 TABLE 3 FA1 FA2 FA3 Average Raw powder 100 100 100 100 75 m coarse 123.5 170.6 112.4 135.5 powder 45 m coarse 113.9 137.0 109.0 119.9 powder 20 m coarse 107.4 119.4 103.7 110.2 powder (unit: mass %)

[0068] The above results demonstrate that when the coarse fly ash powder obtained by classification using the sieve with a mesh opening of 45 m was used, an average of 119.9% by mass of fly ash could be used compared with the use of the raw fly ash powder. The possibility for the increase in the amount of fly ash used would be ascribed to the facts that the SiO.sub.2/Al.sub.2O.sub.3 mass ratio of the coarse fly ash powder was high and that its ignition loss was also great.

[0069] The FA1 to FA3 and the raw fly ash powders thereof were classified using the sieve with a mesh opening of 75, 45 or 20 m. The ignition loss and activity index of each of the resulting fine fly ash powders were measured, and the results of the measurements are shown in Table 4. As a reference, the specified values of JIS Class II fly ash highly versatile as fly ash for an admixture are also shown in Table 4.

TABLE-US-00004 TABLE 4 Activity Activity Ignition index index Example loss 28 days 91 days FA1 Raw powder 5.3 81 94 75 m fine powder 4.0 84 99 45 m fine powder 3.5 82 98 20 m fine powder 3.2 89 105 FA2 Raw powder 4.2 83 99 75 m fine powder 3.0 83 101 45 m fine powder 2.3 84 101 20 m fine powder 2.0 87 103 FA3 Raw powder 3.5 83 97 75 m fine powder 2.6 86 98 45 m fine powder 2.4 87 100 20 m fine powder 2.7 87 100 JIS Class II 5.0 or less 80 or more 90 or more

[0070] The fine fly ash powder was shown to be smaller in the ignition loss and lower in the unburned carbon content than the raw fly ash powder. The fine fly ash powder was also shown to be higher in the activity index than the raw fly ash powder, and thus be superior in the reactivity with a cement composition.

Comparative Example

[0071] In connection with FA4 and FA5, the ignition loss and the activity index after heat treatment were measured. The results of the measurements are shown in FIG. 5. As the heat treatment, FA4 and FA5 were heated for 10 minutes in an electric furnace held at 800 C. or 1,000 C., whereafter they were air-cooled at room temperature to obtain samples. As a reference, the specified values of JIS Class II fly ash highly versatile as fly ash for an admixture are also shown in Table 5.

TABLE-US-00005 TABLE 5 Activity Activity Comparative Ignition index index Example loss 28 days 91 days FA4 raw powder 4.0 81 96 after heating at 800 C. 1.1 80 94 after heating at 1000 C. 0.8 80 94 FA5 raw powder 3.7 80 95 after heating at 800 C. 1.0 79 93 after heating at 1000 C. 0.5 76 87 JIS Class II 5.0 or less 80 or more 90 or more

[0072] Table 5 shows that when heated, fly ash decreased in the ignition loss and had the unburned carbon removed by combustion. On the other hand, the heated fly ash lowered in the activity index, and thus its reactivity with cement was low.