JP7582611B2 - Method for determining mechanochemical state and method for producing mechanochemical fly ash - Google Patents

Description

The present invention relates to a method for determining the mechanochemical state of a self-hardening material made from coal ash (fly ash).

As a conventional technique, a method for producing a self-hardening material using mechanochemically treated fly ash as a raw material has been proposed (Patent Document 1).

Patent No. 6347512

Geotechnical Society of Japan "Soil Testing Methods and Commentary" Revision Editorial Committee: Soil Testing Methods and Commentary (First Revised Edition), 2000

When mechanochemical treatment was performed using conventional technology, it was difficult to determine whether the fly ash was in a mechanochemical state.

In view of the above problem, the present invention provides a method for easily determining whether fly ash is in a mechanochemical state.

To solve the above problems, one aspect of the present invention provides a method for determining whether fly ash is in a mechanochemical state by measuring the electrical conductivity of a suspension of fly ash.

The present invention provides a method for easily determining whether fly ash is in a mechanochemical state.

1 shows the results of X-ray diffraction analysis of fly ash in Test 1. 1 is a particle size distribution of fly ash in Test 1. 1 shows (a) the amounts of Al and Si dissolved in Test 1, and (b) a summary of the test results. 1 is a particle size distribution of fly ash in Test 2. 1 is a chemical composition of fly ash in Test 2. 1 shows (a) the results of X-ray diffraction analysis, and (b) the relationship between the grinding time and the peak intensity in Test 2. 1 shows the relationship between the grinding time and the amorphous ratio in Test 2. 1 shows the results of grinding time, pH and EC measurement in Test 2. 1 shows (a) the results of measurement of the grinding time and pH (30 minutes after mixing), and (b) the results of measurement of the grinding time and EC (30 minutes after mixing) in Test 2. 1 shows the relationship between the grinding time and the amount of dissolved Si and dissolved Al in Test 2. 1 is a summary of (a) the residual electrical conductivity, (b) the decrease rate of the residual electrical conductivity, and (c) the measured values in Test 2. The EC residual rate and the reduction rate of the EC residual rate of ash types B to F. This shows the change over time in the EC residual rate for ash types B to F. This is the standard value for ending mechanochemical treatment for each ash type, taking into account a 10% likelihood.

The method for determining the mechanochemical state of fly ash according to an embodiment of the present invention is described below with reference to Figures 1 to 14.

＜Overview＞
Mechanochemical treatment of coal ash (fly ash) is generally carried out by grinding the fly ash in a ball mill or the like. When the fly ash is subjected to the grinding treatment, the ash particles are pulverized, and a mechanochemical phenomenon occurs in which the surface is activated. When this ground ash is mixed with a highly alkaline silicon slurry, it hardens in a short time, so it can be used as cement-free concrete.

During the mechanochemical treatment process, it is important for process management and quality control to accurately determine whether the surface of the fly ash has reached an activated mechanochemical state, i.e., whether it has been mechanochemicalized.

Conventional methods for determining the mechanochemical transformation of fly ash include measuring changes in constituent minerals using X-ray diffraction and the amount of Al and Si dissolved in highly alkaline solutions. However, both types of measurements require time and effort.

Therefore, below we propose an alternative method to the conventional one, which involves measuring the electrical conductivity (EC) of fly ash.

<Test 1: Characteristics of mechanochemically treated fly ash>
In order to understand the physical and chemical properties of the mechanochemically treated fly ash, a mechanochemical treatment using a ball mill (Test 1) was carried out as follows.

The mechanochemical treatment was carried out by grinding the fly ash. The fly ash used was ash type A. The grinding device used was a planetary ball mill (Fritsch, P-5).

The mechanochemical treatment was carried out by placing 500 g of balls in a pot for 50 g of fly ash and grinding the fly ash for 6 hours. Six types of treatment were carried out by changing the rotation speed of the planetary ball mill in increments of 50 rpm from 150 (rpm, revolutions per minute) to 400 rpm. In this way, in Test 1, the grinding time was kept constant and the rotation speed of the ball mill was changed, so the higher the rotation speed, the greater the degree of mechanochemical treatment.

X-ray diffraction analysis and particle size distribution measurements were performed on the ground fly ash and untreated fly ash (raw ash). In addition, the amount of Al and Si (aluminum and silicon) dissolved in the alkaline solution, as well as the pH and EC of the ground ash were measured.

The results of the X-ray diffraction analysis are shown in Figure 1. Peaks for quartz and mullite as crystalline minerals were detected in the fly ash. At a rotation speed of 200 rpm, the quartz peak was smaller than in the untreated case, suggesting that the quartz crystal structure had been destroyed. As the rotation speed increased further, the mullite peak became smaller, suggesting that the mullite crystal structure had been destroyed.

Figure 2 shows the particle size distribution measurements of raw ash and ground fly ash. A Microtrac Bell particle size distribution analyzer MT3300 EXII was used for the measurements. Raw ash has a wide distribution shape, with a peak at a particle size of around 70 μm (micrometers). When ground, the particle size becomes smaller, with a peak at around 2 μm. Up to a rotation speed of 200 to 350 rpm, the average particle size became smaller as the rotation speed increased, but at 400 rpm the average particle size was larger than at 200 rpm, which is thought to be due to re-aggregation of the particles.

When the milled ash comes into contact with a highly alkaline silicon slurry, Al and Si are dissolved from the ash, which then form a hardened body. In other words, it is important that Al and Si are dissolved from the ash in order to form a hardened body. The milled ash was mixed with an alkaline solution, and the concentration of the dissolved elements was measured using an ICP emission spectrometer. The measurement procedure is as follows.
1. Mix 0.2 g of powder with 20 ml (milliliters) of 3M KOH solution. 2. React in a shaking bath for 24 hours. 3. Ultracentrifuge at 15,000 rpm for 20 minutes. 4. Filter the supernatant with a 200 nm hydrophilic PTFE membrane filter. 5. Dilute 30 times with pure water. 6. Measure.

The measurement results for Al and Si are shown in Figure 3 (a). The amount of Al dissolved increased with grinding, but the amount dissolved at rotation speeds of 150, 200, and 250 rpm was almost the same. Above 300 rpm, the amount dissolved increased with increasing rotation speed. It is presumed that the amount of Al dissolved increased due to the destruction of the mullite crystal structure.

The amount of eluted silicon also increased with grinding, and was almost the same at rotation speeds of 150, 200, and 250 rpm. At 300 rpm or higher, the amount of eluted increased with increasing rotation speed. When compared with the results of X-ray diffraction analysis, it is estimated that elution occurred mainly from quartz (SiO ₂ ) up to 250 rpm, and mainly from mullite (Al ₆ O ₁₃ Si ₂ ) from 300 rpm.

The pH and EC were measured based on the Geotechnical Society of Japan standards "pH test method for soil suspension" (JGS0211-2000) and "Electrical conductivity test method for soil suspension" (JGS0212-2000) (Non-Patent Document 1). Specifically, the sample (fly ash) and water were mixed and stirred at a mass ratio of 1:5, and the suspension was left to stand for 30 minutes before measurement. The measurement results after 30 minutes of standing are shown in Figure 3 (b). Both the pH and EC decreased significantly from 1 hour to 3 hours after grinding, and then decreased gradually thereafter. The decrease in pH suggests a decrease in hydroxide ions OH- ^, and the decrease in EC suggests a decrease in the amount of water-soluble ions.

The EC decreased as the rotation speed increased. Specifically, the EC decreased significantly up to 200 rpm, and a considerable decrease was also observed between 250 and 300 rpm. There was no significant decrease above 300 rpm.

As summarized in FIG. 3(b), the following characteristics were observed when the rotation speed was increased beyond the boundary of 250 to 300 rpm.
The color of the powder turned black. The crystal peak of mullite dropped significantly. The amount of Al and Si dissolved in the highly alkaline solution started to increase. The EC dropped significantly.

As shown in Figures 3(a) and (b), when the rpm exceeds 300 rpm, the amount of Al and Si dissolved increases, and in all test results at 300 rpm or more, it is clear that the fly ash has undergone mechanochemical transformation.

It can also be seen that the EC decreases as the fly ash becomes mechanochemical and the amount of Al and Si dissolved increases. Judging from the fact that the change point at which the amount of Al and Si dissolved from the fly ash begins to increase and the change point of EC almost coincide, it is thought that EC can be an indicator for determining whether the fly ash has become mechanochemical, i.e., whether it has reached a mechanochemical state.

<Test 2: Relationship between EC and mechanochemical conversion>
In order to investigate in more detail the relationship between EC and the state of fly ash, particularly the relationship with the treatment time, the following test (Test 2) was conducted.

In this test, the rotation speed was kept constant and the grinding time was changed to examine the relationship between the amount of Al and Si eluted and EC. Specifically, the coal ash used was ash type A, and a planetary ball mill (Fritsch, P-5) was used for the grinding process, with the rotation speed kept constant at 300 rpm. Five treatment times were used: 1, 3, 6, 12, and 24 hours (test names: MC-1h, MC-3h, MC-6h, MC-12h, MC-24h).

The raw ash was yellowish gray, but after one hour of grinding it became darker gray. After further grinding it turned black-gray in three hours.

The particle size distribution was measured using a laser diffraction particle size distribution measuring device (Microtrac MT3000II). The results are shown in Figure 4. The raw ash had a peak at a particle size of approximately 40 μm, but after grinding the ash was refined and a peak was observed at a particle size of 2 to 3 μm. With MC-3h, a peak also appeared at a particle size of approximately 40 μm, and the particle size distribution began to show two peaks. With MC-24h, the peak at a particle size of approximately 40 μm became larger, suggesting that the finely ground powder had agglomerated.

The chemical composition of the raw ash and the ground fly ash was determined using a wavelength-dispersive X-ray fluorescence analyzer (Shimadzu XRF-1800). Figure 5 shows the proportion of each element by grinding time. The chemical composition was approximately 61% silicon, approximately 22% aluminum, followed by 6% iron, 4% calcium, and approximately 1% titanium, potassium, and sodium. Magnesium was 1.15% for the raw ash, and increased slightly to about 1.5% after 1 to 24 hours of grinding. The main chemical composition ratios of the raw ash and MC-1h to MC-24h were almost the same, with almost no change due to the grinding process.

The X-ray diffraction patterns of the raw ash and the milled ash are shown in Figure 6(a). An X-ray diffraction analyzer (Rigaku SmartLab) was used for the analysis. As shown in Figure 6(a), quartz and mullite were detected as crystalline minerals. It can be seen that the milling process reduced the peak intensity at all processing times (Figure 6(b)).

The proportion of amorphous matter was calculated based on the quantitative analysis results. The relationship between grinding time and the proportion of amorphous matter is shown in Figure 7. The amorphous proportion of the raw ash was 69%, but increased to 77% for MC-3h, and the amorphous proportion gradually increased thereafter.

The pH and EC were measured according to the Geotechnical Society of Japan standards "pH test method for soil suspension" (JGS0211-2000) and "Electrical conductivity test method for soil suspension" (JGS0212-2000) (Non-Patent Document 1). Specifically, as in Test 1, the sample (fly ash) and water were mixed and stirred in a mass ratio of 1:5, and the suspension was left to stand for 30 minutes or 1 hour before measurement.

The measurement results of the suspension are shown in Figures 8 and 9. Both the pH and EC decreased significantly from 1 hour to 3 hours after grinding, and then decreased gradually thereafter. The decrease in pH suggests a decrease in hydroxide ions ^OH-, and the decrease in EC suggests a decrease in the amount of water-soluble ions.

The amount of eluted Si and Al was measured using the same procedure as in Test 1. As shown in Figure 10, the amount of eluted Si increased rapidly after 1 hour of grinding time (MC-1h) and reached a maximum after 12 hours (MC-12h). It also decreased slightly after 24 hours (MC-24h).

Al also increased rapidly after 1 hour of grinding (MC-1h), reached a maximum after 12 hours (MC-12h), and then decreased slightly after 24 hours (MC-24h). This trend was similar to that of Si.

The amount of eluted Si and Al increased sharply in MC-1h. This is presumably because, as shown in the X-ray diffraction analysis described above, the peaks of quartz (SiO ₂ ) and mullite (Al ₆ O ₁₃ Si ₂ ) dropped sharply, that is, their crystal structures collapsed, making it easier for Si and Al to be eluted in the alkaline solution. The fact that the amount of eluted Si and Al after 6 hours of grinding was slightly lower is thought to be related to the fact that the second peak after 6 hours in the particle size analysis described above became slightly smaller.

The amount of eluted Si and Al both increased significantly after 1 hour of grinding, reaching a peak after 3 hours. The tendency for peak strength to decrease also showed the same characteristic of mechanochemical fly ash as confirmed in Test 1. The color of the fly ash also turned dark gray after 3 hours. From this, it can be concluded that when using the ball mill grinding device "Fritsch P-5," ash type A reaches a mechanochemical state after 3 hours of processing at 300 rpm.

In response to the decrease in the amount of eluted Si and Al, EC drops significantly after 1 hour of grinding time, converging to a constant value after 3 hours. It can be seen that EC decreases during the mechanochemical treatment process. It can also be seen that when fly ash is mechanochemicalized, the rate of decrease in EC decreases, and EC converges to a constant value.

Figure 11 shows the time change in EC, the ratio to the EC of the raw ash (EC residual rate), and the rate of decrease per hour in the EC residual rate (decrease rate of the EC residual rate). In Figure 11 (c), the rate of decrease in the EC residual rate is shown as the rate of change per hour from the previous measurement point to that point. As mentioned above, the rate of decrease in the EC residual rate is very small after 3 hours, which shows that the fly ash has become mechanochemical after 3 hours of treatment and the decrease in EC has almost converged.

As shown by the shaded area in Figure 11 (c), the EC residual rate after 3 hours of grinding treatment, when the fly ash was in a mechanochemical state, was 8.8%. In addition, the rate of decrease in the EC residual rate after 1 to 3 hours of treatment was 5.13%/h (hours), whereas the rate of decrease in the EC residual rate after 3 to 6 hours of treatment dropped to 0.28%/h.

In summary, it is possible to use the convergence of the EC residual rate below a certain level as the standard for determining whether fly ash has reached a mechanochemical state. Specifically, it is conceivable that the EC residual rate when the rate of decrease in the EC residual rate has converged can be used as the standard value used to determine whether mechanochemicalization has occurred, or as the standard value for ending the mechanochemical treatment.

When using the results of Test 2, the rate of decrease in the EC residual rate from 3 to 6 hours of treatment converges to 2%/h or less, so it is possible to set the EC residual rate at 3 hours of treatment as the standard value. Since the EC residual rate at 3 hours of treatment is 8.8%, the standard value for the end of mechanochemical treatment will be an EC residual rate of 8.8%.

In addition, taking into consideration that excessive mechanochemical treatment can have negative effects, a certain degree of likelihood can be added to the above criteria. Using the results of Test 2, the EC residual rate after 3 hours of treatment is 8.8%, so a 10% likelihood can be added to this EC residual rate of 8.8%, taking into consideration measurement error, etc., and the standard value for the end of treatment can be set at an EC residual rate of 9.7%.

When using such standard values, when mechanochemical treatment is performed on fly ash of ash type A, the treatment should be terminated when the EC residual rate falls to 9.7% or less.

<Confirmation of condition settings>
The physical and chemical properties of coal ash vary depending on the origin of the raw coal and the combustion method of the boiler. Therefore, the knowledge gained from the results of Test 2 was applied to different ash types, specific numerical values for the standard values were set, and their validity was confirmed.

In order to establish standards for determining mechanochemical conversion even when other ash types are used, a mechanochemical treatment similar to that in Test 2 (ball mill grinding treatment, 300 rpm) was carried out using fly ash other than ash type A, and the measurement results of EC, etc. were compared. Five types of fly ash were used: ash type B to ash type F.

The test results are shown in Figures 12 and 13. In Figure 12, the rate of decrease in the EC residual rate when it becomes 2%/h or less is underlined. Also, the EC residual rate when the rate of decrease in the EC residual rate converges is shaded.

Referring to Figures 12 and 13, it can be seen that for all ash types, as with ash type A, after 3 or 6 hours, the EC residual rate decrease rate falls to 2%/h or less, and the EC residual rate converges. Referring to Figure 13 in particular, it can be seen that for all ash types, when the EC residual rate decrease rate falls to 2%/h or less, the graph bends significantly, and the electrical conductivity value and residual rate converge. The maximum EC residual rate at convergence was 28.9% (ash type F, 3 hours of treatment).

Considering the test results of ash type A in tests 1 and 2, and the test results of ash types B to F, it can be seen that the standard value for determining mechanochemical conversion can be set as the "EC residual rate when the rate of decrease in the EC residual rate falls to a specified value or less." However, taking into account the test results of ash type F, it is preferable to set the upper limit of this standard value at 30%.

The "predetermined value" for determining the rate of decrease in the EC residual rate can be set to 2%/h when grinding at 300 rpm using a ball mill based on the test results for ash type A and ash types B to F.

This standard value may take likelihood into account. Figure 14 shows the standard values set by obtaining the EC residual rate for each ash type when the rate of decrease in the EC residual rate falls to 2%/h or less, and then adding a likelihood of 10%.

As an example, consider the case where mechanochemical treatment is performed on fly ash of ash type B using the reference values in Figure 14. In this case, sampling can be performed each time during the mechanochemical treatment process to measure the EC. When the EC residual rate falls to 16.2% or less, it is possible to determine that the fly ash has been mechanochemically converted, and to terminate the mechanochemical treatment.

By adopting this method, there is no risk of extending the treatment time more than necessary, making it possible to efficiently mechanochemically convert fly ash.

＜Effects＞
In the above embodiment, a method for determining whether the fly ash is in a mechanochemical state is used by measuring the electrical conductivity of a suspension of the fly ash.

By using the above method, it is possible to easily and quickly determine whether or not fly ash is in a mechanochemical state, compared to conventional methods such as measuring changes in constituent minerals using X-ray diffraction or the amount of Al and Si dissolved in a highly alkaline solution.

In the above embodiment, a method is used in which the fly ash is determined to be in a mechanochemical state when the residual rate of electrical conductivity is equal to or lower than a reference value. This reference value is set as the residual rate of electrical conductivity when the rate of decrease in the residual rate of electrical conductivity reaches a predetermined value or lower during the process of carrying out mechanochemical treatment on the fly ash. This predetermined value is, for example, 2%/hour.

By using such a reference value, it is possible to accurately determine whether or not a mechanochemical state is present.

In the above embodiment, a method for producing mechanochemically treated fly ash is proposed, which includes a process for performing mechanochemical treatment on the fly ash and a determination process for determining whether the fly ash has reached a mechanochemical state, and the process is terminated when it is determined in the determination process that the mechanochemical state has been reached.

By using this method, it is possible to efficiently produce mechanochemical fly ash without extending the mechanochemical treatment time more than necessary.

Claims (4)

The electrical conductivity of the fly ash suspension was measured .
The method determines that the fly ash is in a mechanochemical state when the residual rate of the electrical conductivity is equal to or lower than a reference value . The method according to claim 1 , wherein the reference value is set based on the remaining ratio when a rate of decrease in the remaining ratio becomes equal to or less than a predetermined value during the process of performing the mechanochemical treatment on the fly ash. The method according to claim 1 or 2 , wherein the reference value is 30% or less. A treatment step of performing a mechanochemical treatment on the fly ash;
and determining whether the fly ash has reached a mechanochemical state using the method according to any one of claims 1 to 3 ;
A method for producing mechanochemical fly ash, the treatment step being terminated when it is determined in the determination step that a mechanochemical state has been reached.

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