Manufacturing method of recycled fine aggregate

Abstract

To provide a manufacturing method of a recycled fine aggregate capable of producing a recycled fine aggregate, while simultaneously decarbonizing waste concrete.SOLUTION: A manufacturing method of a recycled fine aggregate includes: separating a coarse aggregate from waste concrete to obtain a residue 1; introducing a fluid containing carbon dioxide into the residue 1 and separating at least part of the fine aggregate to obtain a residue 2; and applying an electrochemical decarbonization acid treatment to the residue 2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing recycled fine aggregate. [Background technology]

[0002] In recent years, growing interest in global warming has led to calls for reducing carbon dioxide emissions into the atmosphere. At various facilities, such as power plants, incinerators, cement plants, steel mills, and other industrial facilities, efforts are being made to reduce and capture the carbon dioxide-containing exhaust gases emitted during their operations. Steel mills and cement plants, in particular, are known to emit large amounts of carbon dioxide. For example, cement plants require raw materials to be fired at temperatures of over 1,400°C, generating carbon dioxide when fossil fuels are burned. Carbon dioxide is also generated when limestone, the main raw material, is fired to produce calcium oxide. For this reason, reducing carbon dioxide emissions is seen as an urgent issue at steel mills and cement plants.

[0003] Various efforts have been made to reduce carbon dioxide emissions in cement firing facilities. For example, Patent Document 1 proposes a carbon dioxide immobilization method in which materials obtained by crushing waste concrete are accumulated and exposed to conditions in which dry and wet conditions are alternated for a predetermined period of time to capture and immobilize carbon dioxide in the air. Patent Document 2 proposes a method in which a carbon dioxide-containing gas having a temperature of 350°C or higher is brought into contact with a cementitious hardened body to immobilize carbon dioxide in the hardened body. Furthermore, Patent Document 3 discloses a method in which carbon dioxide is supplied to a slurry containing a calcium-containing powder and water before carbonation to perform a carbonation treatment, and the carbonated slurry is supplied to a cement production mill to produce a cement powder composition. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-028581 [Patent Document 2] Japanese Patent Publication No. 2020-131074 [Patent Document 3] Japanese Patent Publication No. 2020-152631 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made in view of the above circumstances, and aims to provide a method for producing recycled fine aggregate from waste concrete, which can also obtain a carbon-free calcium raw material. [Means for solving the problem]

[0006] In order to solve the above problems, the present invention provides the following method for producing recycled fine aggregate. 1. Separating coarse aggregate from waste concrete; introducing a fluid containing carbon dioxide into the residue 1 obtained by the separation to separate fine aggregate; Electrochemically decarbonating the residue 2 obtained by separating the fine aggregate; A method for producing recycled fine aggregate, comprising:

[0007] Furthermore, the present invention provides the following method for producing recycled fine aggregate as a preferred embodiment. 2. The method for producing recycled fine aggregate according to the above item 1, wherein carbon dioxide is introduced into the waste concrete in separating the coarse aggregate. 3. The method for producing recycled fine aggregate according to 2 above, wherein the carbon dioxide is in a gaseous state. 4. The method for producing recycled fine aggregate according to 2 above, wherein the carbon dioxide includes carbon dioxide generated in the electrochemical decarbonation treatment. 5. The method for producing recycled fine aggregate according to the above item 1, wherein the separation of the coarse aggregate is carried out while heating. 6. The method for producing recycled fine aggregate according to 1 above, wherein the fluid containing carbon dioxide is a gas. 7. The method for producing recycled fine aggregate according to the above item 1, wherein the carbon dioxide-containing fluid contains carbon dioxide generated in the electrochemical decarbonation treatment. 8. The method for producing recycled fine aggregate according to the above item 1, wherein alumina silica gel is obtained in the electrochemical decarbonation treatment. 9. The method for producing recycled fine aggregate according to the above item 8, wherein the alumina silica gel is used as a cement additive. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method for producing recycled fine aggregate from waste concrete, which can also provide a carbon-free calcium raw material. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of the present invention (hereinafter, sometimes referred to as "the present embodiment") will be described below. The present invention is not limited to the following embodiment, and can be implemented with any modifications within the scope that does not impair the effects of the invention. Note that the numerical values associated with "greater than or equal to," "less than or equal to," and "to" in the description of numerical ranges in this specification are values that can be arbitrarily combined. For example, when a certain numerical range is described as "greater than or equal to AA and less than or equal to BB" and "greater than or equal to CC and less than or equal to DD," the numerical ranges "greater than or equal to AA and less than or equal to DD" and "greater than or equal to CC and less than or equal to BB" are also included.

[0010] [Method for producing recycled fine aggregate] The method for producing recycled fine aggregate of this embodiment is as follows: Separating coarse aggregate from waste concrete to obtain residue 1; introducing a fluid containing carbon dioxide into the residue 1 and separating at least a portion of the fine aggregate to obtain a residue 2; subjecting the residue 2 to electrochemical decarboxylation treatment; It includes:

[0011] The methods described in Patent Documents 1 and 2 can both be said to be methods of immobilizing carbon dioxide by utilizing calcium contained in the cement paste portion of waste concrete. In addition, the method described in Patent Document 3 uses carbon dioxide to carbonate pre-carbonated calcium-containing powder (calcium oxide) contained in the cement paste portion of concrete waste or the like, thereby immobilizing carbon dioxide and using the carbonated calcium-containing powder in a cement powder composition.

[0012] The methods described in Patent Documents 1 and 2 focus only on immobilizing carbon dioxide using calcium contained in the cement paste, and do not consider at all how to treat the calcium content (calcium carbonate) that has immobilized carbon dioxide. Furthermore, even if the calcium content (calcium carbonate) that has immobilized carbon dioxide is reused as a raw material in a cement factory, it is not possible to reduce the amount of carbon dioxide emitted from the cement factory. In the method described in Patent Document 3, the calcium content that has immobilized carbon dioxide is also used as an admixture for cement. Therefore, it cannot be said that it is highly effective in suppressing the source of carbon dioxide circulating in cement factories. In other words, all of the methods described in the above patent documents only focus on immobilizing carbon dioxide and reusing waste (waste concrete) from cement factories, and do not focus at all on reducing the amount of carbon dioxide emitted from cement factories.

[0013] In contrast, the method for producing recycled fine aggregate of this embodiment not only produces recycled fine aggregate using waste (waste concrete) from cement factories, but also recovers the calcium contained in the waste concrete as calcium hydroxide through electrochemical decarbonation. That is, the calcium contained in the waste concrete (mainly calcium carbonate) can be converted into a carbon-free state (calcium hydroxide). By using this as a substitute for at least a portion of the limestone that is the main raw material in cement factories, that is, by using raw materials that do not emit carbon dioxide as at least a portion of the main raw materials, the amount of carbon dioxide generated by calcining the calcium can be reduced.

[0014] As described above, the method for producing recycled fine aggregate according to the present embodiment makes it possible to produce not only recycled fine aggregate but also calcium hydroxide. Furthermore, by supplying calcium hydroxide to cement plants as a carbon-free calcium raw material, it becomes possible to reduce carbon dioxide emissions from the cement plants.

[0015] Furthermore, according to the manufacturing method of this embodiment, the electrochemical decarbonation treatment is employed to obtain the recycled fine aggregate and calcium hydroxide, but by-products are also naturally produced. In addition to calcium, waste concrete also contains aluminum and silicon components. According to the manufacturing method of this embodiment, by employing electrochemical decarbonation treatment, the aluminum and silicon components contained in the waste concrete can be recovered as alumina silica gel. Furthermore, it is also possible to produce highly pure oxygen, carbon dioxide, and hydrogen. These highly pure gases, such as carbon dioxide, can be used in the manufacturing method of this embodiment, and can also be used in cement factories and various other applications, not limited to the manufacturing method of this embodiment.

[0016] Electrochemical decarbonation is a processing method that consumes electricity. However, as mentioned above, it not only produces recycled fine aggregate, but also has the great advantage of rendering the calcium contained in waste concrete carbon-free. Furthermore, if carbon-free calcium hydroxide is used in cement plants, that is, if non-carbon dioxide-emitting raw materials are used as at least part of the main raw materials in cement plants, it is possible to obtain the advantage of reducing carbon dioxide emissions from the cement plant. Furthermore, it is possible to obtain gases such as oxygen, carbon dioxide and hydrogen with high purity, as well as alumina silica gel which is useful as an adsorbent, etc. Thus, the advantages of employing electrochemical decarbonation treatment are extremely great.

[0017] [Separating coarse aggregate from waste concrete to obtain residue 1] The method for producing recycled fine aggregate of this embodiment includes obtaining a residue 1 by separating coarse aggregate from waste concrete.

[0018] (Waste concrete) The waste concrete used in the manufacturing method of this embodiment can be any waste material containing concrete, for example, waste material generated during the demolition of a building that uses concrete, such as a reinforced concrete building. The waste concrete is preferably generated during the demolition work and treated to remove foreign matter other than concrete, such as reinforcing bars. The waste concrete from which foreign matter such as reinforcing bars has been removed contains cement paste (hereinafter simply referred to as "paste" or "paste portion"), as well as aggregates such as fine aggregate and coarse aggregate. The mixture of cement paste and fine aggregate obtained after the coarse aggregate has been separated from the waste concrete may be referred to as the mortar portion below.

[0019] (Separation of coarse aggregate) The method for separating coarse aggregate from waste concrete can be any method that can separate coarse aggregate, and conventional methods such as rubbing can be used. The rubbing process is a process in which waste concrete is rubbed together, and the paste portion adhering to the surface of the aggregate (fine aggregate and coarse aggregate) is peeled off by the rubbing process, thereby separating the coarse aggregate from the mortar portion (paste portion and fine aggregate). The rubbing process is an established technology, and is therefore preferable in that it can more easily and reliably separate the coarse aggregate contained in the waste concrete.

[0020] The rubbing treatment can be carried out by a rubbing device or the like. As the rubbing device, for example, a double drum type device or the like can be adopted, and in separating the coarse aggregate, the rubbing device is preferably equipped with a jig such as a sieve or a net capable of classifying the coarse aggregate.

[0021] In separating the coarse aggregate, carbon dioxide may be introduced into the waste concrete. By introducing carbon dioxide, a reaction between the paste portion contained in the waste concrete and the carbon dioxide occurs, causing a change in the volume of the paste portion, which promotes separation of the coarse aggregate from the mortar portion (paste portion and fine aggregate).

[0022] Considering ease of introduction, carbon dioxide is preferably introduced as a gas. Also, considering reduction of environmental load, ease of availability, cost, etc., carbon dioxide is preferably supplied as a gas containing carbon dioxide, and as the gas containing carbon dioxide, at least one of combustion exhaust gas emitted from various factories having combustion facilities such as cement calcination facilities and carbon dioxide generated in the electrochemical decarbonation treatment described below can be used.

[0023] More specifically, when carbon dioxide is introduced into waste concrete in separating coarse aggregate, it is preferable that the carbon dioxide contains carbon dioxide generated in the electrochemical decarbonation treatment. Alternatively, the total amount of carbon dioxide may be carbon dioxide generated in the electrochemical decarbonation treatment.

[0024] The separation of the coarse aggregate is preferably carried out while heating, and the combustion exhaust gas is preferably used as the heat source. When the combustion exhaust gas is used as the heat source, heating and introduction of carbon dioxide can be carried out simultaneously. For example, when separating coarse aggregate by the above-mentioned rubbing treatment, it is preferable to perform a heating rubbing treatment that combines heating and rubbing treatment. Heating the waste concrete can promote the dehydration reaction of the paste portion and embrittle it. Therefore, by subjecting the heat-treated waste concrete to a rubbing treatment, the paste portion and aggregate can be easily separated from the waste concrete. Furthermore, embrittlement of the paste portion can reduce the energy required for rubbing and also suppress deterioration of the aggregate.

[0025] When the heat-rubbing treatment is carried out, the heating temperature is preferably 200° C. or higher, more preferably 250° C. or higher, and the upper limit is preferably 400° C. or lower, more preferably 350° C. or lower. When the heating temperature is within the above range, deterioration of the aggregate can be suppressed, and the coarse aggregate can be separated more efficiently by the rubbing treatment.

[0026] The coarse aggregate obtained by separating the coarse aggregate from the waste concrete may contain 85% by mass or more of particles with a particle size exceeding 5 mm (i.e., particles that do not pass through a 5 mm sieve). For example, when using the above-mentioned rubbing device, the particles that do not pass through may be classified using a tool such as a mesh with a predetermined size (e.g., 5 mm sieve openings) to obtain coarse aggregate. In addition, by adjusting the sieve openings, it is possible to produce coarse aggregate as desired.

[0027] [Obtaining residue 2 by separating at least a portion of fine aggregate] The manufacturing method of this embodiment includes introducing a fluid containing carbon dioxide into residue 1 obtained by the above separation to separate at least a portion of the fine aggregate. This results in residue 2 being obtained together with the fine aggregate. Residue 1 is the remainder obtained after separating and removing coarse aggregate from waste concrete in the separation, and mainly contains a paste portion and fine aggregate, i.e., it can be called a mortar portion.

[0028] Residue 2 is obtained by introducing a fluid containing carbon dioxide into residue 1 obtained by the above separation. More specifically, it is a composition containing products generated from the calcium content contained in the paste portion of residue 1 through a reaction between the paste portion and carbon dioxide, as well as fine aggregate. By introducing the fluid containing carbon dioxide (described below), the calcium content contained in the paste portion of residue 1 becomes a product resulting from this, making it easier to separate the fine aggregate. On the other hand, if there is calcium content that does not become a product through the reaction with carbon dioxide and remains, the fine aggregate tends to remain. Therefore, as described above, residue 2 is obtained by separating at least a portion of the fine aggregate from residue 1, and is a composition containing the remaining fine aggregate as well as products generated from the calcium content.

[0029] (Introduction of fluid containing carbon dioxide) The fluid containing carbon dioxide is preferably a gas, and it is preferable to use one of the examples of carbon dioxide that can be used in separating the coarse aggregate, such as the carbon dioxide produced in the electrochemical decarbonation treatment described below.

[0030] The amount of carbon dioxide introduced depends on the concrete mix and water content of the waste concrete. More specifically, it depends on the cement content and calcium content of the cement used in the concrete. Therefore, the amount of carbon dioxide introduced can be determined appropriately depending on the concrete mix and water content of the waste concrete.

[0031] The water content of waste concrete can be measured by methods such as thermal dehydration. 3 If the concrete mix with the unit cement amount, unit coarse aggregate amount, and unit fine aggregate amount per unit is known, the ratios of cement, water, coarse aggregate, and fine aggregate in the waste concrete can be calculated from these ratios and the ratio of cement amount to coarse aggregate amount to fine aggregate amount. It is also possible to estimate the concrete mix proportion by taking a portion of the waste concrete in advance and using a known concrete mix proportion estimation method.

[0032] The residue obtained by sufficiently removing the coarse aggregate from the waste concrete can be approximated as a mortar portion (paste portion and fine aggregate).The cement content and calcium content per kg of the mortar portion obtained by separating the coarse aggregate can be determined from the relationship between the unit cement content, unit fine aggregate content, and water content. Therefore, the amount of carbon dioxide supplied is preferably either an amount that allows the paste portion on the surface of the fine aggregate to be carbonated to form sparingly soluble calcium carbonate, or an amount that allows further carbon dioxide to be supplied to form soluble calcium bicarbonate, causing the paste portion on the surface of the fine aggregate to dissolve and be released.

[0033] For 1 kg of calcium, 1.1 kg of carbon dioxide is equivalent to calcium carbonate, and 2.2 kg is equivalent to calcium bicarbonate. Supplying more carbon dioxide than this will not leave any calcium to react, so it will have little effect on the efficiency of fine aggregate recovery. Therefore, the above supply amount can be used as a guide for the amount of carbon dioxide to be supplied per kg of calcium. Keeping the amount of carbon dioxide supplied within the above range can efficiently promote the reaction between the paste portion contained in the residue and the fluid containing carbon dioxide, and can also promote separation of the paste portion contained in residue 1 from the fine aggregate.

[0034] (Rubbing treatment) In separating at least a portion of the fine aggregate, it is preferable to perform a rubbing treatment, similar to the separation of the coarse aggregate. By rubbing the residue 1 obtained by separating the coarse aggregate, the paste portion adhering to the fine aggregate can be efficiently removed, thereby obtaining a fine aggregate with high purity. In addition, by performing the rubbing treatment, it becomes easier to separate the fine aggregate from the residue 1, making it easier to obtain the fine aggregate.

[0035] The rubbing treatment may be performed on the residue 1 obtained by separating at least the coarse aggregate, or may be performed on a product obtained by introducing a fluid containing carbon dioxide into the residue 1, that is, a product generated due to calcium contained in the paste portion through a reaction between the paste portion in the residue and carbon dioxide, or a composition containing fine aggregate, etc. The rubbing treatment may also be performed while introducing a fluid containing carbon dioxide into the residue 1. Regardless of the type of rubbing treatment performed, the paste portion (or the product) adhering to the fine aggregate can be efficiently peeled off. The rubbing treatment can be carried out in the same manner as the separation of the coarse aggregate described above, and the fine aggregate can be separated by using a device equipped with tools such as sieves and nets that can classify the fine aggregate, and it is also preferable to carry out the treatment while heating.

[0036] Residue 2 contains a reaction product of the paste portion contained in the residue and carbon dioxide. The paste portion usually contains unhydrated cement minerals containing calcium, aluminum, silicon, and iron, such as tricalcium silicate (CS), dicalcium silicate (CS), tricalcium aluminate (CA), and tetracalcium aluminoferrate (CA), as well as cement hydrates produced by hydration reactions. Therefore, it also contains calcium compounds such as calcium carbonate, which are produced by the reaction of the paste portion with carbon dioxide due to the calcium oxide contained in the above compounds.

[0037] Residue 2 contains not only the calcium compounds but also silica, alumina, and cement hydrates containing various metal atoms such as iron, which are produced due to the silicic acid and aluminic acid contained in the compounds.

[0038] [Electrochemical decarboxylation treatment] The method for producing recycled fine aggregate of this embodiment includes subjecting the residue 2 obtained by separating the fine aggregate to electrochemical decarbonation treatment. This simultaneously produces calcium hydroxide, alumina silica gel, and gases such as highly pure oxygen, hydrogen, and carbon dioxide. In this way, the method for producing recycled fine aggregate of this embodiment can also be considered a method for producing coarse aggregate, calcium hydroxide, alumina silica gel, and oxygen, hydrogen, and carbon dioxide.

[0039] The electrochemical decarbonation treatment can be carried out, for example, using an electrolytic cell. Residue 2 is supplied to the positive electrode side, water is supplied to the negative electrode side, and a voltage is applied to generate a pH gradient due to water electrolysis. The reaction that occurs on the acidic positive electrode side is as follows: On the positive electrode side, the calcium compound, i.e., calcium carbonate, dissolves (reaction formula (1) below), while oxygen gas and carbon dioxide gas are simultaneously generated (reaction formulas (2) and (3) below). 2CaCO3 → 2CO3 2- +2Ca 2+ (1) 4H + +2CO3 2- →2H2O+2CO2(2) 2H2O→4H + +O2+4e - (3)

[0040] The reaction occurring at the negative electrode is as follows: A reduction reaction of water occurs at the negative electrode. 4H2O+4e - →2H2+4OH - (4)

[0041] The calcium produced by the reaction (1) reacts with the hydroxide ions produced by the reaction (4) on the negative electrode side to produce calcium hydroxide. Simultaneously with the production of calcium hydroxide, oxygen, carbon dioxide, and hydrogen gases are produced by the reactions (2) to (4). Alumina silica gel is then obtained from the silica hydrate and alumina hydrate.

[0042] In the production method of this embodiment, the electrochemical decarbonation treatment may be carried out using an apparatus capable of carrying out the reactions (1) to (4) above. For example, the electrochemical decarbonation treatment may be carried out using an electrochemical decarbonation apparatus including at least a reaction tank, positive and negative electrodes, and a voltage application tool. The electrochemical decarbonation apparatus may also be equipped with various tools as needed, such as a porous separator (including a permeable membrane having ion selectivity), a positive electrode tank having a positive electrode, a negative electrode tank having a negative electrode, and a connecting tank connecting these tanks and partitioned by a porous separator (including a permeable membrane having ion selectivity). By providing such a connecting tank, calcium produced in the positive electrode tank and hydroxide ions produced in the negative electrode tank can be reacted in the connecting tank, making it easier to recover calcium hydroxide. In addition, when performing the electrochemical decarbonation treatment, water may be supplied as needed.

[0043] (Products and Applications) The fine aggregate obtained by the manufacturing method of this embodiment is sufficient if all of it passes through a 10 mm sieve and contains 85 mass % or more of particles with a particle size of 5 mm or less (i.e., particles that pass through a 5 mm sieve).Fine aggregate can also be manufactured as desired by adjusting the sieve size to obtain the desired particle size. The obtained fine aggregate can be suitably used as recycled fine aggregate, for example, as aggregate for roadbeds, asphalt, etc., or as a cement raw material.

[0044] The coarse aggregate obtained by the manufacturing method of this embodiment may contain 85% by mass or more of particles having a particle size exceeding 5 mm (i.e., particles that do not pass through a 5 mm sieve), as described above. As with the fine aggregate, desired coarse aggregate can be manufactured by adjusting the sieve openings to obtain the desired particle size. The obtained coarse aggregate can be suitably used as recycled coarse aggregate, for example, as aggregate for roadbeds, asphalt, etc., or as a cement raw material.

[0045] The calcium hydroxide obtained by the production method of this embodiment can be suitably used as a cement raw material, a river and soil neutralizer, a flocculant, a reagent, a pH adjuster for foods and cosmetics, a raw material in chemical plants, etc. When used as a cement raw material, it can be used as a substitute for calcium carbonate as described above, and therefore can contribute to reducing the amount of carbon dioxide emitted from cement plants as raw materials.

[0046] The alumina silica gel obtained by the production method of this embodiment can be used as an adsorbent, a dehumidifier, a catalyst carrier, and an additive for cement.

[0047] The hydrogen obtained by the production method of this embodiment is highly pure and of high quality, and is therefore suitable for use as a raw material for various chemical products, as fuel for fuel cells, and in various other fields such as metals and glass.

[0048] Since oxygen and carbon dioxide are obtained as a mixed gas from the positive electrode side, it is preferable to separate the oxygen and carbon dioxide before use. Carbon dioxide can be used to separate the coarse aggregate and the fine aggregate in the manufacturing method of this embodiment. It can also be used as an industrial gas in foods such as soft drinks, welding of steel materials, as a refrigerant, etc. Oxygen can also be used as a combustion gas in various facilities, for welding steel materials, and as a raw material for oxidation reactions in chemical plants.

Claims

1. Separating coarse aggregate from waste concrete to obtain residue 1; introducing a fluid containing carbon dioxide into the residue 1 and separating at least a portion of the fine aggregate to obtain a residue 2; subjecting the residue 2 to electrochemical decarboxylation treatment; A method for producing recycled fine aggregate, comprising:

2. The method for producing recycled fine aggregate according to claim 1, wherein carbon dioxide is introduced into the waste concrete in separating the coarse aggregate.

3. The method for producing recycled fine aggregate according to claim 2, wherein the carbon dioxide is in a gaseous state.

4. The method for producing recycled fine aggregate according to claim 2, wherein the carbon dioxide includes carbon dioxide generated in the electrochemical decarbonation treatment.

5. The method for producing recycled fine aggregate according to claim 1 , wherein the separation of the coarse aggregate is carried out while heating.

6. The method for producing recycled fine aggregate according to claim 1, wherein the fluid containing carbon dioxide is a gas.

7. The method for producing recycled fine aggregate according to claim 1, wherein the fluid containing carbon dioxide contains carbon dioxide generated in the electrochemical decarbonation treatment.

8. 2. The method for producing recycled fine aggregate according to claim 1, wherein alumina silica gel is obtained in the electrochemical decarbonation treatment.

9. The method for producing recycled fine aggregate according to claim 8, wherein the alumina silica gel is used as a cement additive.