Method for carbonating material of hydraulic compound

Abstract

To more efficiently perform carbonation treatment of at least one of aggregate and powder which are material substances of a hydraulic compound.SOLUTION: The method includes a water addition process S4 for adding liquid water to at least one of aggregates and powder which are materials of the hydraulic compound, and carbonation processes S5, S6, and S8 for placing the materials in a gas atmosphere containing carbon dioxide as a main component while maintaining a state in which water films of water adhere to surfaces of the materials and fixing the carbon dioxide to the materials.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for carbonating hydraulic compound materials. [Background technology]

[0002] Various studies have been conducted on the carbonation treatment of waste concrete in order to reuse waste concrete and recover carbon dioxide. Non-Patent Document 1 describes the carbonation of waste concrete particles for three or seven days in an environment with a relative humidity of 60%, a CO2 concentration of 5%, and a temperature of 20°C. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Karen Midori MASUNAGA, Takeshi Iyoda: Study on the effectiveness of various carbonation methods for waste concrete granules towards carbon neutrality, Proceedings of the Annual Meeting of the Japan Concrete Institute, Vol. 44, pp. 1240-1245, 2022 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the method described in Non-Patent Document 1, carbonation occurs through a chemical reaction between carbonate ions generated from water and carbon dioxide and calcium ions eluted from concrete particles into the water. Therefore, if there is no water in the voids or on the surface of the concrete particles, carbonation is difficult to promote. The same problem applies not only to waste concrete particles, but also to the aggregates and powders that make up hydraulic compounds.

[0005] An object of the present invention is to provide a method for more efficiently carrying out a carbonation treatment of at least one of aggregate and powder, which are raw materials for hydraulic compounds. [Means for solving the problem]

[0006] The method for carbonating a hydraulic compound material of the present invention comprises a water addition step of adding liquid water to at least one of aggregate and powder, which are the hydraulic compound material, and a carbonation step of placing the material in a gas atmosphere containing carbon dioxide as a main component while maintaining a state in which a water film is attached to the surface of the material, thereby fixing carbon dioxide to the material. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a method for more efficiently carrying out a carbonation treatment of at least one of aggregate and powder, which are raw materials for hydraulic compounds. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing a schematic procedure of a carbonation method according to one embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing the schematic configuration of a carbonation vessel. [Figure 3] FIG. 1 is a graph showing the mass change rate (relative humidity 90%) of ferronickel slag fine aggregate (Pamcosand (registered trademark)) during carbonation treatment. [Figure 4] FIG. 1 is a graph showing the mass change rate (relative humidity 90%) of ferronickel slag fine aggregate (green sand) during carbonation treatment. [Figure 5] FIG. 1 is a graph showing the mass change rate (relative humidity 90%) of recycled aggregate in a carbonation treatment. [Figure 6] FIG. 1 is a graph showing the mass change rate (relative humidity 60%) of ferronickel slag fine aggregate (Pamcosand (registered trademark)) during carbonation treatment. [Figure 7] FIG. 1 is a graph showing the mass change rate (relative humidity 60%) of ferronickel slag fine aggregate (green sand) during carbonation treatment. [Figure 8] FIG. 1 is a graph showing the mass change rate (relative humidity 60%) of recycled aggregate in a carbonation treatment. DETAILED DESCRIPTION OF THE INVENTION

[0009] The method for carbonating a material for a hydraulic compound of the present invention will be described using embodiments and examples. The material for the hydraulic compound in this embodiment is at least one of aggregate and powder, and specifically includes slag aggregate, a by-product of metal smelting, recycled concrete aggregate (aggregate obtained during concrete recycling), and recycled concrete fine powder (fine powder obtained during concrete recycling). The aggregate has a particle size of approximately 300 μm or more, and the powder has a particle size of approximately less than 300 μm. In the following description, recycled concrete aggregate will be referred to as recycled aggregate, and recycled concrete fine powder will be referred to as recycled fine powder.

[0010] First, we will explain the mechanism of carbonation of aggregates and powders. The carbonation mechanism differs slightly depending on the material, but in all cases, carbon dioxide (CO2) gas used in the carbonation process is dissolved in water, and bicarbonate ions (HCO3 - ) and carbonate ions (CO3 2- ) is generated (Equations 1 to 3). CO2+H2O→H2CO3 (formula 1) H2CO3→H + +HCO3 - (Formula 2) H2CO3→2H + +CO3 2- (Formula 3)

[0011] When slag aggregate comes into contact with water, its own components (CaO, MgO, etc.) produce calcium ions (Ca 2+ ) and magnesium ions (Mg 2+ ) is dissolved. In slag aggregate, Mg 2+The ratio of Calcium Carbonate (CaCO3) and Magnesium Carbonate (MgCO3) is often high. The dissolved ions react with carbonate ions to form calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). Calcium carbonate and magnesium carbonate precipitate as crystals, forming a film that covers the aggregate. This fixes carbon dioxide to the aggregate. When recycled aggregate comes into contact with water, calcium ions are dissolved from calcium hydroxide (Ca(OH)2), a hydrate (paste component) attached to the aggregate surface. The dissolved calcium ions react with carbonate ions to form calcium carbonate. Calcium carbonate precipitates as crystals, forming a film that covers the aggregate. Recycled fine powder is thought to be mostly composed of calcium hydroxide, a hydrate. Therefore, in this case, calcium ions are dissolved from the recycled fine powder itself. The dissolved calcium ions react with carbonate ions to form calcium carbonate. Calcium carbonate precipitates as crystals, forming a film that covers the fine powder. Note that the pore water contained in recycled aggregate and recycled fine powder is alkaline, so more carbonate ions, which are more stable in alkalis than bicarbonate ions, are produced. Thus, water present as a liquid (liquid water) is essential for carbonation, and therefore, in order to promote carbonation, it is desirable that liquid water be present for at least part of the carbonation treatment period, preferably for the majority of the period.

[0012] Here, referring to Figure 1, an example of the carbonation process procedure at a factory or on-site is described. As mentioned above, the carbonation mechanism differs depending on the material, but the carbonation process procedure is generally the same regardless of the material. First, the material is prepared (Step S1). Commercially available slag aggregate can be used. For recycled aggregate and recycled fine powder, concrete waste generated during the demolition of reinforced concrete buildings is crushed to a certain size and then further crushed to a uniform shape. If rebar is present, it is removed, and the remaining waste material is separated into coarse aggregate, fine aggregate, and fine powder. As mentioned above, hydrates such as calcium hydroxide adhere to the surfaces of the coarse aggregate and fine aggregate, and the fine powder is mostly composed of hydrates. Next, material A is transferred to carbonation vessel 1. Figure 2 shows a conceptual diagram of carbonation vessel 1. Water supply pipe 2 is connected to carbonation vessel 1.

[0013] Next, the moisture content of the material is confirmed. Specifically, the amount of water adhering to the surface and internal voids of the material is measured (Step S2). The moisture content α is used as an indicator of the moisture content. The moisture content α is defined as M1 / M0, where M1 is the mass of water adhering to the surface and internal voids of the material, and M0 is the bone-dry mass of the material. In other words, the moisture content α is the ratio of the mass of water adhering to the material to the mass of the material in a bone-dry state. The moisture content α can be evaluated, for example, by testing a sample of the material. Specifically, if the mass of the material sample is MS1 and the mass of the sample after drying is MS0, it can be calculated as α = (MS1 - MS0) / MS0.

[0014] Next, it is determined whether the moisture content α is smaller than a predetermined moisture content range (Step S3). If the moisture content α is smaller than the predetermined moisture content range, water is supplied to the carbonation vessel 1 from the water supply pipe 2 so that the moisture content α falls within the predetermined moisture content range, and water is added to the material (water addition step, Step S4). It is preferable to thoroughly stir the material A, and although not shown, the carbonation vessel 1 is preferably equipped with a stirring means for the material A. The stirring means may be a movable stirrer or a drive device that vibrates or rotates the entire carbonation vessel 1. There are no particular restrictions on the water as long as it is clean water, and tap water, for example, can be used.

[0015] The specified moisture content range varies depending on the material. For slag aggregate, a moisture content of 20% to 30% is preferred. This corresponds to an amount sufficient to almost completely or completely cover the aggregate surface with a water film. For recycled aggregate and recycled fine powder, a moisture content of 5% to 20% is preferred. This corresponds to an amount sufficient to cover the aggregate or fine powder surface with a water film, or to partially cover the aggregate or fine powder surface with a water film. In summary, the specified moisture content range is preferably sufficient to cover at least a portion of the material surface with a water film. Because the water film is only a few millimeters thick, it cannot be formed by immersion, and the specified moisture content range cannot be achieved. In other words, the material will not be immersed in water during the subsequent carbonation process, and a water surface of a certain height will not form above the material. As described in the examples, the presence of water in the internal voids of slag aggregate or recycled aggregate is not necessarily required. While it is possible to saturate the internal voids with water, this requires the material to be immersed in water for a long period of time, which is less effective and therefore less efficient as an industrial process. Furthermore, the moisture content of the material before the water addition step S4 is not particularly limited and may be any state between bone dry and saturated. If the moisture content α exceeds a predetermined moisture content range, the sample may be dried so that the moisture content α falls within the predetermined moisture content range.

[0016] (carbonation process) Next, with the material A placed in the carbonation vessel 1, a carbonation process is carried out in a gaseous atmosphere to fix carbon dioxide to the material (step S5). The carbonation vessel 1 is a sealed vessel to which a carbon dioxide gas supply pipe 3 and an exhaust pipe 4 are connected. To efficiently carry out the carbonation process, the interior of the carbonation vessel 1 is preferably a gaseous atmosphere containing carbon dioxide as the main component (i.e., a carbon dioxide concentration (molar ratio) of 50% or more), and more preferably, the carbon dioxide concentration in the gaseous atmosphere is 90% or more but less than 100%. To maintain the carbon dioxide gas concentration in the carbonation vessel 1, it is preferable to supply carbon dioxide gas from the supply pipe 3 while exhausting it from the exhaust pipe 5. The carbonation process is preferably carried out at a temperature of 0°C to 60°C, preferably 5°C to 40°C, and more preferably 15°C to 30°C. For example, the internal temperature of the carbonation vessel 1 may be approximately 60°C in summer and approximately 0°C in winter. In this embodiment, the carbonation treatment is carried out at room temperature or near room temperature, so there is little need for special temperature control, and it is possible to omit equipment for temperature control and save energy.

[0017] The interior of the carbonation vessel 1 contains water vapor, and the carbonation process is preferably carried out in an environment with a predetermined relative humidity or higher. For this purpose, the carbonation vessel 1 is equipped with a humidity regulator 5. The humidity regulator 5 can be any device capable of adjusting the relative humidity of the carbonation vessel 1. Examples include a steam supply port connected to an external steam source, a humidifier, and a temperature regulator. This suppresses evaporation of water from the surface of the material (or internal pore water, if present) and allows the carbonation process to be carried out while maintaining water adherence to the material. A carbon dioxide concentration meter 5 and a hygrometer 6 may be installed in the exhaust pipe 4 to automatically adjust the carbon dioxide concentration and relative humidity within the carbonation vessel 1. The predetermined relative humidity varies depending on the material. It is preferable to set it between 90% and 100% for slag aggregate and between 60% and 100% for recycled aggregate and recycled fine powder. This is because the surface water retention capacity of slag aggregate is lower than that of recycled aggregate and recycled fine powder. The carbonation step is preferably carried out in a wet state for slag aggregate, but the carbonation step can be suitably carried out in either a dry or wet state for recycled aggregate or recycled fine powder.

[0018] As an alternative to step S5 (steps S6-S8), a water addition step may be performed intermittently during the carbonation step. Specifically, while carbon dioxide gas is supplied to the carbonation vessel 1 through the carbon dioxide gas supply pipe 3 to perform the carbonation step (steps S6 and S8), water is added to the raw material at least once, preferably multiple times, during the carbonation step (step S7). The raw material may be stirred during this process. This alternative step replenishes the moisture that evaporates from the surface of the raw material during the carbonation step, achieving the same effect as step S5. However, the predetermined relative humidity is preferably 90% to 100% for slag aggregate and 60% to 100% for recycled aggregate and recycled fine powder. Generally, in industrial processes, the raw material is unlikely to be bone dry before carbonation, so the retained water contributes to carbonation even without the initial water addition step. In other words, there is a high degree of flexibility in the timing of the water addition step, which facilitates operational management. However, in this alternative process, the water addition step may be performed before the carbonation step.

[0019] (Example) Carbonation treatment was carried out using two types of ferronickel slag fine aggregate (Pamco Sand (registered trademark) and Green Sand) and recycled aggregate (Tests 1 to 3). Ferronickel slag fine aggregate is a fine aggregate that is a by-product when smelting ferronickel. First, samples of the ferronickel slag fine aggregate and recycled aggregate were placed in a petri dish, dried to an absolutely dry state, and their masses were measured. The mass of the sample at this time was designated m1.

[0020] Next, water was added to the dish and the sample was immersed in water for over 48 hours, after which the water was removed and the surface was dried to a surface-dried state. The surface-dried state of the sample was confirmed using the flow cone method specified in JIS A1109:2020, "Test Method for Density and Water Absorption of Fine Aggregate." Next, water was added again to the dish, and the sample was thoroughly stirred with a glass rod to ensure that the water was evenly distributed on the sample's surface, and the mass was measured. Because the added water adhered to the sample through stirring, there was almost no free water remaining on the sample. The mass of the sample at this point was defined as m2. m2 - m1 is the mass of water added to the sample, and the moisture content was calculated as (m2 - m1) / m1.

[0021] Next, the petri dish was placed in a desiccator and CO2 gas was injected to carry out the carbonation treatment for a specified time (this time is called the carbonation time Tc). During this, the atmosphere inside the desiccator was kept humid with a temperature of approximately 20°C and a relative humidity of 90% or higher, the CO2 concentration was nearly 100%, and the flow rate of the injected CO2 gas was 5 mL / min. After the specified carbonation time Tc had elapsed, the carbonation treatment was terminated, and the sample was dried to bone dryness and its mass was measured. The mass of the sample at this time was taken as m3. m3 - m1 corresponds to the mass of carbon dioxide absorbed during the carbonation treatment. (m3 - m1) / m1 is called the mass change rate. The mass change rate is an indicator of the amount of carbon dioxide fixed during the carbonation treatment.

[0022] Next, carbonation treatment was performed using Pamco sand, green sand, and recycled aggregate in a dry environment at 60% relative humidity (Tests 4–6). The CO2 concentration, CO2 gas flow rate, and temperature were the same as in Tests 1–3. This test aimed not only to evaluate the effects of relative humidity but also to evaluate the extent of carbonation under low moisture content conditions. Specifically, as in Tests 1–3, the samples were dried to bone dryness and their mass m1 was measured. The immersion and surface drying steps in Tests 1–3 were omitted, and the water addition step was then performed. The maximum amount of water added (maximum water content α) was equivalent to the water absorption rate of each sample, sufficient to partially cover the surface with a water film. In Tests 4–6, there was almost no internal pore water, and water is believed to be attached only to the surface of the sample. After the specified carbonation time Tc had elapsed, the carbonation treatment was terminated, and the samples were dried to bone dryness and their mass m3 was measured. The mass change rate was calculated as (m3 − m1) / m1. In addition, the carbonation process was started at a moisture content of 0% (i.e., the water addition process in Tests 4 to 6 was omitted), and tests were also conducted in which water was added and stirred at 30 and 90 minutes (Tests 4A to 6A). The amount of water added at each time point was approximately half the water absorption rate of the sample.

[0023] The mass change rate was calculated using carbonation time and moisture content as parameters. Table 1 and Figure 3 show the test results using Pamco Sand (relative humidity 90%), Table 2 and Figure 4 show the test results using green sand (relative humidity 90%), and Table 3 and Figure 5 show the test results using recycled aggregate (relative humidity 90%). Figure 3(a) shows the relationship between carbonation time and mass change rate when using Pamco Sand, and Figure 3(b) shows the relationship between moisture content and mass change rate when using Pamco Sand. Figure 4(a) shows the relationship between carbonation time and mass change rate when using green sand, and Figure 4(b) shows the relationship between moisture content and mass change rate when using green sand. Figure 5 shows the relationship between moisture content and mass change rate when using recycled aggregate. Table 4 and Figure 6 show the test results using Pamco Sand (relative humidity 60%), Table 5 and Figure 7 show the test results using green sand (relative humidity 60%), and Table 6 and Figure 8 show the test results using recycled aggregate (relative humidity 60%). Figures 6 to 8 show the relationship between moisture content and mass change rate when Pamco sand, green sand, and recycled aggregate were used, respectively. Figures 6 to 8 also show the results of Tests 4A to 6A.

[0024] [Table 1]

[0025] [Table 2]

[0026] [Table 3]

[0027] [Table 4]

[0028] [Table 5]

[0029] [Table 6]

[0030] At a relative humidity of 90%, the mass change rate for both Pamco Sand and Green Sand increased with increasing carbonation time, but tended to saturate at approximately 180 minutes. This is likely due to the calcium and magnesium ions present near the aggregate surface being fully dissolved, preventing further carbonation. The mass change rate also increased with increasing moisture content, reaching a peak at around 20–30% for both Pamco Sand and Green Sand, and tending to decrease at 35% and 50%. One possible explanation is that eluted ions are generated near the surface of the fine aggregate, while carbonate ions are generated near the surface of the water film. Therefore, excessive water film thickness may reduce the efficiency of the reaction between eluted ions and carbonate ions. On the other hand, a low moisture content results in insufficient production of eluted ions and carbonate ions, resulting in insufficient carbonation. This confirms the importance of adjusting the moisture content within an appropriate range for efficient carbonation. In Comparative Examples 1 and 2, the moisture content was 0%, resulting in nearly zero mass change.

[0031] For example, the mass change rate of Pamco Sand in Example 1-15 was 0.64%, more than 60 times that of Comparative Example 1. The mass change rate of Green Sand in Example 2-12 was 0.28%, approximately 10 times that of Comparative Example 2. Pamco Sand has a higher water absorption rate than Green Sand, i.e., it has more internal voids and a larger specific surface area than Green Sand. Therefore, Pamco Sand has a larger surface area in contact with water than Green Sand, which is thought to have facilitated the reaction. It is thought that more water was absorbed into the internal voids. While no clear trend was observed between the carbonation time and the mass change rate for recycled aggregate, a moisture content of 5% to 20% is preferred. The density and water absorption rate of each sample were measured before and after the carbonation treatment, but no significant differences were found.

[0032] At a relative humidity of 60%, the mass change rate was small for green sand and Pamco sand, while the mass change rate was large for recycled aggregate. Green sand and Pamco sand have a lower water retention capacity than recycled aggregate, and it is thought that in low relative humidity environments, water on the surface of the aggregate is more likely to evaporate and dissipate. As a result, carbonation did not progress sufficiently, and the mass change rate remained small. In contrast, recycled aggregate achieved a mass change rate at the same level as when the relative humidity was 90%. This shows that in the case of recycled aggregate, the effects of humidity conditions can be mitigated by adding the necessary and sufficient amount of water before the carbonation process.

[0033] A comparison of Figures 6 and 8 also shows that the influence of internal pore water is small. In Figure 6, the internal pores were filled with water beforehand, and it is thought that carbonation also progressed in the internal pores. In Figure 8, water was added to an absolutely dry sample, and the carbonation treatment was carried out in a dry state with a relative humidity of 60%, so it is thought that there was not much water in the internal pores. However, since there was no significant difference in the mass change rate, it is thought that the influence of internal pore water was small.

[0034] Tests 4A to 6A achieved mass change rates equivalent to those of Tests 4 to 6, respectively. In other words, there was no significant difference in the timing of water addition, whether it was before or during the carbonation process. Generally, in industrial processes, it is unlikely that the raw material will be in an absolutely dry state before carbonation, so it is thought that the retained water will contribute to carbonation even without an initial water addition process.

Claims

1. a water addition step of adding liquid water to at least one of aggregate and powder, which are materials for the hydraulic compound; a carbonation step of placing the material in a gas atmosphere containing carbon dioxide as a main component while maintaining the water film attached to the surface of the material, and fixing the carbon dioxide to the material.

2. 10. The carbonation method of claim 1, wherein the material comprises recycled concrete aggregate.

3. 10. The carbonation method of claim 1, wherein the material comprises recycled concrete fines.

4. 4. The carbonation method according to claim 2 or 3, wherein the mass of the added water is 5 to 20% of the mass of the material in an bone-dry state.

5. The carbonation method according to claim 4, wherein the carbonation step is carried out in an environment with a relative humidity of 60% or more.

6. The carbonation method according to claim 5, wherein the water addition step is carried out between the carbonation steps.

7. The carbonation method of claim 1 , wherein the material comprises slag aggregate.

8. 8. The carbonation method according to claim 7, wherein the mass of the added water is 20 to 30% of the mass of the material in an oven-dry state.

9. The carbonation method according to claim 8, wherein the carbonation step is carried out in an environment with a relative humidity of 90% or more.

10. 2. The carbonation method according to claim 1, wherein the concentration of carbon dioxide in the gas atmosphere is 90% or more.

11. 2. The carbonation method of claim 1, wherein the material is in any state between bone dry and saturated prior to the water addition step.

12. 2. The carbonation method of claim 1, wherein the material is not immersed in the water during the carbonation step.

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