Method for recovering cement paste from waste concrete

Abstract

To provide a method for recovering high-quality cement paste from waste concrete.SOLUTION: A method for recovering cement paste from waste concrete has: a first step for bringing waste concrete into contact with a carbon dioxide-containing gas to fix carbon dioxide thereon; a second step for applying an electric pulse to the waste concrete after the first step to pulverize it; and a third step for sorting the waste concrete after the second step through a sieve, to recover the component having passed through the sieve.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for recovering cement paste from waste concrete. [Background technology]

[0002] Waste concrete generated during the demolition of reinforced concrete buildings, etc., is crushed after removing foreign objects other than concrete, such as reinforcing bars, and is used as roadbed material, backfill material, etc. However, while the amount of waste concrete is increasing, mainly in large cities, the demand for roadbed material, etc. is on the decline, so attempts are being made to separate the cement paste attached to the aggregate in the waste concrete from the aggregate itself, and to reuse this aggregate as recycled aggregate.

[0003] For example, a technology has been proposed in which crushed waste concrete is heated and rubbed to remove the cement paste on the aggregate surface as fine powder, thereby producing recycled aggregate (Patent Document 1). Also, a method has been reported in which waste concrete is placed in a container filled with water and a pulse voltage is applied to the waste concrete so that the electrical circuit parameters satisfy a certain relationship, thereby crushing or pulverizing the waste concrete (Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2003-26459 A [Patent Document 2] WO 96 / 26010 Summary of the Invention [Problem to be solved by the invention]

[0005] However, according to the study by the present inventors, it was found that the methods described in Patent Documents 1 and 2 have the following problems. That is, in the method described in Patent Document 1, it is desirable to reuse the recovered fine powder as cement paste from the viewpoint of effective use of resources, but due to over-pulverization to improve the quality of the recycled aggregate, the fine powder contains a large amount of aggregate, so that reuse as cement paste is limited. In addition, in the method described in Patent Document 2, when waste concrete is pulverized by applying an electric pulse in water, the pH and conductivity of the water gradually increase, making it easier for electricity to flow in the water, so that insulation breakdown occurs and the number of times the pulse can be applied is limited. An object of the present invention is to provide a method for recovering high-quality cement paste from waste concrete. [Means for solving the problem]

[0006] The inventors have conducted studies to improve the quality of cement paste recovered from waste concrete, and have discovered that high-quality cement paste rich in calcium can be efficiently recovered by fixing carbon dioxide in the waste concrete, pulverizing it by applying an electric pulse, and then sieving the concrete to recover the under-sieve fraction.

[0007] That is, the present invention provides the following [1] to [8]. [1] A first step of contacting waste concrete with a carbon dioxide-containing gas to fix carbon dioxide; A second step of applying an electric pulse to the waste concrete after the first step to crush it; The third process is to sieve the waste concrete after the second process and collect the under-sieve concrete. A method for recovering cement paste from waste concrete comprising the steps of: [2] The method for recovering waste concrete according to [1] above, wherein in the first step, carbon dioxide is fixed until the mass of the waste concrete increases by 0.5 mass% or more. [3] The method for recovering waste concrete according to [1] or [2], wherein in the second step, an electric pulse is applied to the waste concrete from the first step while it is immersed in water. [4] The method according to any one of [1] to [3] above, wherein the voltage of the electric pulse is 50 to 300 kV. [5] The method according to any one of [1] to [4] above, wherein the pulse period of the electric pulse is 0.5 to 80 Hz. [6] The method according to any one of [1] to [5] above, wherein the electric pulse is applied 30 or more times. [7] The method for recovering the material described in any one of [1] to [6] above, wherein the mesh size of the sieve used for sieving is 1 mm or less. [8] The method for recovering cement according to any one of [1] to [7] above, wherein the cement paste is a cement raw material or an admixture. Effect of the Invention

[0008] According to the present invention, it is possible to efficiently recover cement paste from waste concrete. Furthermore, since the recovered cement paste is rich in calcium and has high quality, the amount that can be used as a cement raw material increases, and the amount of limestone used as a raw material can be reduced. Furthermore, according to the present invention, the sieved remaining material can be reused as recycled roadbed material or recycled aggregate. Therefore, the method of the present invention is also useful as a method for recycling waste concrete. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The method for recovering cement paste according to the present invention includes a first step, a second step, and a third step. Each step will be described below.

[0010] (First step) This process is a process of fixing carbon dioxide by contacting the waste concrete with a carbon dioxide-containing gas. By contacting the waste concrete with the carbon dioxide-containing gas, calcium compounds contained in the cement paste in the waste concrete, such as calcium oxide, calcium hydroxide, and calcium silicate compounds, can be converted into calcium carbonate.

[0011] In this process, first, waste concrete is prepared. Examples of waste concrete include dismantled concrete generated during civil engineering works or the demolition of structures, and surplus concrete generated during the construction of buildings. The waste concrete may be concrete blocks that have been broken into small pieces, magnetically separated, or hand-sorted for the purpose of separating foreign objects such as reinforcing bars. The size of the concrete blocks is usually 100 to 400 mm square, and preferably about 300 mm square.

[0012] Furthermore, in this step, in order to increase the contact area with the carbon dioxide-containing gas and increase the amount of carbon dioxide that is fixed, the particle size of the waste concrete may be adjusted using various crushers and classifiers. Known crushers can be used, including, for example, a jaw crusher that compresses and crushes using a fixed blade and a moving blade fixed to a frame, an impact crusher that crushes by impacting a striking plate rotated at high speed with a repulsion plate fixed to the device, a gyratory crusher or a cone crusher that compresses and crushes using a cone-shaped cone cape and an eccentric mantle, and a double roll crusher that rotates two rolls and compresses and crushes between them. The crushing process may be carried out two or more times, and when it is carried out two or more times, the same or different crushers can be used, and can be appropriately selected depending on the size of the waste concrete. As the classifier, any suitable classification device can be selected and used, for example, a vibrating sieve such as a ripple flow type or a low head type, a rotary sieve such as a trommel, a dry classifier such as an air separator or a cyclone, or a wet classifier such as a spiral classifier.

[0013] The particle size of the waste concrete with adjusted particle size is preferably less than 40 mm, more preferably less than 30 mm, even more preferably less than 25 mm, and even more preferably less than 20 mm, from the viewpoint of increasing the amount of carbon dioxide immobilized and improving the quality of the cement paste. The lower limit of the particle size of the waste concrete is not particularly limited, but from the viewpoint of production efficiency, it is preferably 0.25 mm or more, more preferably 0.5 m or more, and even more preferably 1 mm or more. The "particle size" here refers to the maximum dimension of the crushed material (for example, the dimension of the major axis when the cross section is elliptical).

[0014] Next, the waste concrete is brought into contact with a carbon dioxide-containing gas to fix the carbon dioxide. The carbon dioxide-containing gas is not particularly limited as long as it contains carbon dioxide, and examples thereof include carbon dioxide filled in a cylinder and exhaust gas from a factory. Examples of factory exhaust gas include exhaust gas from a cement factory and exhaust gas from a thermal power plant. The factory exhaust gas may be treated with a separation membrane or an adsorbent to increase the carbon dioxide concentration.

[0015] The carbon dioxide-containing gas may contain gases other than carbon dioxide, such as carbon monoxide, hydrocarbons, nitrogen oxides, sulfur oxides, etc. These gases are usually contained in the exhaust gas from factories. Furthermore, the carbon dioxide-containing gas may contain an inert gas, such as nitrogen gas or argon gas. The total amount of these other gases, in terms of volume fraction, in the carbon dioxide-containing gas is preferably 30% or less, more preferably 20% or less, and further preferably 10% or less.

[0016] From the viewpoint of increasing the amount of fixed carbon dioxide, the ratio of carbon dioxide in the carbon dioxide-containing gas is preferably 5% or more, more preferably 10% or more, even more preferably 15% or more, and even more preferably 20% or more, as a volume fraction value. The upper limit of the ratio of carbon dioxide is not particularly limited, and may be 100%. From the viewpoint of promoting fixation of carbon dioxide, the relative humidity of the carbon dioxide-containing gas is preferably 40% or less, more preferably 35% or less, and even more preferably 20 to 35%. The moisture content can be measured by the method described in "7. Measurement of moisture content in exhaust gas" of "JIS Z 8808:2013 Measurement method of dust concentration in exhaust gas."

[0017] Methods for contacting the waste concrete with a carbon dioxide-containing gas include, for example, a method of injecting a carbon dioxide-containing gas into a device containing the waste concrete, and a method of injecting a carbon dioxide-containing gas into water in which the waste concrete is immersed, but are not limited to these as long as they can promote the carbonation of calcium compounds in the waste concrete. In the method of injecting carbon dioxide gas into a device containing waste concrete, the waste concrete may be placed in a carbonation treatment curing tank in which the atmosphere, such as temperature and humidity, can be controlled and carbon dioxide gas may be injected into it, or the hydrated waste concrete may be placed in a carbonation treatment tank and carbon dioxide gas may be injected into it. In the method of injecting carbon dioxide gas into water, the waste concrete may be put into water and then carbon dioxide gas may be injected into it.

[0018] Furthermore, as a treatment method capable of immobilizing carbon dicarbonate in a short period of time, for example, there can be mentioned a method in which water vapor and a gas containing carbon dioxide are blown into a tubular electric furnace. The proportion of water vapor, as a volume fraction value, relative to carbon dioxide is preferably 10% or more, more preferably 20% or more, even more preferably 30% or more, even more preferably 35% or more, and is preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less.

[0019] From the viewpoint of increasing the fixation of carbon dioxide, the fixation of carbon dioxide is carried out until the mass of the waste concrete increases by preferably 0.5% by mass or more, more preferably 0.8% by mass or more, and even more preferably 1% by mass or more. Note that the upper limit of the fixation amount of carbon dioxide is not particularly limited, but from the viewpoint of production efficiency, it is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, based on the mass of the waste concrete.

[0020] The contact temperature and contact time with the carbon dioxide-containing gas can be appropriately selected as long as the amount of carbon dioxide fixed falls within the above-mentioned ranges, and are, for example, as follows. From the viewpoint of promoting fixation of carbon dioxide, the contact temperature with the carbon dioxide-containing gas is preferably 20°C or higher, more preferably 50°C or higher, even more preferably 75°C or higher, even more preferably 90°C or higher, and is preferably 300°C or lower, more preferably 200°C or lower, even more preferably 150°C or lower, and even more preferably 110°C or lower. Furthermore, from the viewpoint of promoting fixation of carbon dioxide, the contact time with the carbon dioxide-containing gas is preferably 5 minutes or more, more preferably 10 minutes or more, even more preferably 20 minutes or more, even more preferably 30 minutes or more, and especially preferably 40 minutes or more, and from the viewpoint of treatment efficiency, it is preferably 4 hours or less, more preferably 3 hours or less, and even more preferably 2 hours or less.

[0021] (Second step) This process applies an electric pulse to the waste concrete after the first process to crush the waste concrete. This allows the cement paste to be selectively peeled off from the aggregate surface. In addition, this process has the advantage that no dust is generated from the waste concrete because it is a wet crushing process. In this process, for example, a liquid medium is placed in a reactor equipped with a pair of electrodes, and the waste concrete after the first process is entirely immersed below the liquid surface of the liquid medium, and then a voltage pulse is applied between the electrodes to impart discharge energy between the electrodes to the waste concrete after the first process, thereby pulverizing it. If the entire waste concrete is not immersed below the liquid surface, the discharge energy is not sufficiently imparted to the waste concrete, which may result in insufficient pulverization.

[0022] Examples of the liquid medium include insulating liquids such as transformer oil, alcohol, and liquid paraffin, and water. Of these, water is preferred. The water preferably has a conductivity of 10,000 mS / m or less, more preferably 0.1 to 8,000 mS / m, and even more preferably 10 to 1,000 mS / m. With such a conductivity, it becomes easier to pass electricity through the waste concrete immersed in the water, so that waste concrete with smaller particle size can be obtained with less voltage or fewer voltage applications. Among them, the conductivity of tap water is about 10 to 20 mS / m, and the conductivity of pure water is about 0.1 to 0.2 mS / m, so it is preferable to use tap water or pure water. The conductivity here is a value when the temperature of the water is 25°C. It is also possible to adjust the average particle size of the pulverized material obtained by adjusting the electrical conductivity of the water. Specifically, when water-soluble salts such as potassium chloride and sodium chloride are added to water (e.g., tap water) with an electrical conductivity of about 10 to 20 mS / m, the electrical conductivity is increased, and the particle size of the pulverized material obtained tends to become smaller. It is also possible to reduce the electrical conductivity of tap water, industrial water, or other water (containing impurities other than water) by passing it through an ion exchange resin.

[0023] The amount of liquid medium used is not particularly limited as long as it is a sufficient amount to immerse the entire waste concrete in the reactor below the liquid surface, but for example, the mass ratio of the liquid medium to the waste concrete (liquid medium / waste concrete) is preferably 2 or more, more preferably 4 or more, and even more preferably 6 or more. In addition, the upper limit of the mass ratio of the liquid medium to the waste concrete is not particularly limited, but from the viewpoint of production efficiency, it is preferably 200 or less, and more preferably 100 or less.

[0024] In this process, the particle diameter D of the waste concrete before the second process is 50 (A) and the particle size D of the waste concrete after the second process 50 The grinding is continued until the ratio of (A) to (B) [(A) / (B)] (hereinafter referred to as the "grinding ratio") is preferably 1.5 or more, more preferably 2 or more, and even more preferably 2.5 or more. 50 " is the particle diameter (D ) that corresponds to 50% of the cumulative distribution curve when the particle size distribution of the sample is created on a volume basis in accordance with JIS R 1629. 50 The particle size distribution can be measured, for example, in accordance with JIS A 1102 and evaluated by sieving using a sieve specified in JIS Z 8801-1.

[0025] Grinding ratio = A / B [Wherein, A is the particle diameter D of the waste concrete before the second process. 50 B is the particle diameter D of the waste concrete after the second process. 50 Indicates the following.

[0026] The voltage and number of times of application of the electric pulse can be appropriately selected as long as the crushing ratio [(A) / (B)] of the waste concrete after the second process is within the above range. For example, it is as follows, and by applying the electric pulse under such conditions, the waste concrete can be sufficiently crushed and can be crushed with a smaller number of applications. The voltage of the electric pulse is preferably 50 to 300 kV, and more preferably 100 to 200 kV. The pulse cycle of the electric pulse is preferably 0.5 to 80 Hz, more preferably 0.5 to 40 Hz, and even more preferably 1 to 20 Hz. The electrode interval is preferably 1 to 50 mm, and more preferably 20 to 40 mm.

[0027] The number of times that the electronic pulse is applied is preferably 30 times or more, more preferably 50 times or more, even more preferably 70 times or more, and even more preferably 80 times or more. By applying the pulse in this manner, the waste concrete can be sufficiently crushed, and the cost of crushing can be reduced. The upper limit of the number of times that the pulse is applied is not particularly limited, but from the viewpoint of production efficiency, it is preferably 1000 times or less, more preferably 700 times or less, and even more preferably 500 times or less.

[0028] In this process, the size of the resulting pulverized waste concrete can be adjusted by adjusting the voltage and number of times of application of the electric pulse. Specifically, the higher the voltage of the electric pulse and the more times it is applied, the smaller the particle size of the resulting pulverized waste concrete. For example, when the voltage of the electric pulse is 150 kV and the number of times it is applied is 40 or more, the pulverization ratio [(A) / (B)] can be preferably 1.5 or more, more preferably 2 or more, and even more preferably 2.5 or more.

[0029] (Third step) This process is a process in which the waste concrete after the second process is screened and the under-sieve concrete is collected. This allows the recovery of high-quality cement paste. For the sieve separation, a known classifier can be used, and specifically, the classifier described above can be mentioned. From the viewpoint of recovering calcium-rich cement paste, the mesh size of the sieve is preferably 1 mm or less, more preferably 0.5 mm or less, and even more preferably 0.3 mm or less.

[0030] The cement paste recovered by the method of the present invention is rich in calcium, while the alkali content is reduced. For example, the separation efficiency of calcium from alkali content calculated by the following formula can be preferably 16% or more, more preferably 18% or more, and even more preferably 20% or more. In this specification, the "alkali content" is the total amount of sodium and potassium. The calcium recovery rate can be calculated based on the calcium content and mass fraction measured by X-ray fluorescence analysis (XRF) of the sample. The alkali content mixing rate can be calculated based on the sodium and potassium content by XRF and the mass fraction of the sample. As the X-ray fluorescence analyzer, ZSX PrimusII (manufactured by Rigaku Corporation) can be used.

[0031] Separation efficiency of calcium from alkali (%) = [Calcium recovery rate (%)] - [Alkaline contamination rate (%)]

[0032] In this way, the cement paste recovered by the present invention is rich in calcium and has a reduced alkali content, and can be reused as a cement raw material. In addition, the cement paste recovered by the present invention is obtained from carbonated waste concrete and has a pozzolanic reaction, so it can also be reused as a mineral admixture. Furthermore, since the remaining concrete from the sieve in the third process is waste concrete from which the cement paste has been removed, it can be reused as recycled roadbed material or recycled aggregate.

[0033] Although the present invention has been described in detail based on the embodiments, the present invention is not limited to the above embodiments. The present invention can be modified in various ways without departing from the gist of the invention. For example, the waste concrete after the second step can be subjected to gravity separation to recover cement paste, and this may be combined with sieve separation. As the gravity separator, for example, a jig separator, a Wilfrey table, a multigravity separator, or a centrifugal separator can be used. EXAMPLES

[0034] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to the following examples.

[0035] (1) Conductivity and pH measurement The measurements were made using a pH / conductivity meter (D-210PC, Horiba, Ltd.) with the temperature adjusted to 25°C.

[0036] (2) Calculation of crushing ratio Particle size D of waste concrete after the second process 50 and the particle diameter D of the waste concrete before the second process 50 The grinding ratio was calculated from the following formula: 50 The particle size distribution was measured, for example, in accordance with JIS A 1102 and evaluated by sieving using a sieve specified in JIS Z 8801-1.

[0037] Grinding ratio = A / B [Wherein, A is the particle diameter D of the waste concrete before the second process. 50 B is the particle diameter D of the waste concrete after the second process. 50 Indicates the following.

[0038] (3) Calculation of separation efficiency of calcium from alkali The separation efficiency of calcium from alkali was calculated by the following formula. The recovery rate of calcium was calculated based on the calcium content and weight fraction measured by X-ray fluorescence analysis (XRF) of the target sample. The contamination rate of alkali was calculated based on the sodium and potassium content measured by XRF of the target sample and the weight fraction of the target sample. The X-ray fluorescence analyzer used was ZSX PrimusII (manufactured by Rigaku Corporation).

[0039] Separation efficiency of calcium from alkali (%) = [Calcium recovery rate (%)] - [Alkaline contamination rate (%)]

[0040] Example 1 (First step) The waste concrete used as raw material was a dismantled concrete block with a maximum diameter of approximately 200 mm, which did not contain limestone aggregate. Next, the raw waste concrete was crushed to 100 mm or less using heavy machinery (nibbler) and fed into a jaw crusher. Crushing was performed using a jaw crusher with a blade width of 40 mm. Next, sieving was performed using a sieve with a mesh size of 10 mm. For sieving, a vibrating sieve was used, which vibrates the sieve to give it vertical and horizontal motion to shake the sample. Next, the classified material that fell below the 10 mm mesh size was carbonation-treated in an externally heated rotary kiln (φ300 mm × 2,400 mm). The treatment conditions were a treatment amount of 10-20 kg / h, a treatment temperature (material temperature) of 100°C, a filling rate of 8.8%, and a treatment atmosphere of 40 vol% water vapor to carbon dioxide. It was confirmed that the mass of the waste concrete after the first process increased by 1.2 mass% compared to the waste concrete before the first process.

[0041] (Second step) Next, the waste concrete after carbonation was sieved using a sieve with 2 mm openings. A vibrating sieve was used for the sieving, which vibrates the sieve to give it vertical and horizontal motion to shake the sample. Next, the oversieved and undersieved concrete were crushed by applying electric pulses using an electric pulse crushing device (SELFRAG ​​Lab S2.0 manufactured by SELFRAG). Crushing was performed by placing 340g of the waste concrete under the water surface in a water tank so that the entire concrete was immersed, and applying a pulse voltage of 150kV from a minimum of 10 times to a maximum of 320 times. The application period was 5Hz, and the distance between the electrodes was 30mm. The pH and electrical conductivity of the water in the tank were measured after each electrical pulse was applied. The particle size distribution of the waste concrete by volume after the second process was measured to obtain the particle diameter D 50 The particle diameter D of the waste concrete before the second process is calculated. 50 From this, the grinding ratio was calculated.

[0042] (Third step) The waste concrete from the second process was sieved through a wet sieve with 0.25mm openings, and the cement paste was collected below the sieve. The cement paste below the sieve and the residue above the sieve were evaluated for their components using XRF. In addition, the separation efficiency of calcium from alkali (Na·K) content was calculated for the cement paste. The results are shown in Tables 1 and 2. It was confirmed that the residue above the sieve meets the standards for recycled aggregate H and can also be used as recycled roadbed material.

[0043] Comparative Example 1 In the first step, the waste concrete that passed through the 10 mm mesh was not carbonated, and the cement paste that fell under the sieve and the residue on the sieve were collected and analyzed in the same manner as in Example 1. The results are shown in Tables 1 and 2. It was confirmed that the residue on the sieve met the standards for recycled aggregate H and could be used as a recycled roadbed material.

[0044] In Table 1, "+2 mm particle group" refers to the fraction on the 2 mm sieve obtained in the second step, and in Table 2, "-2 mm particle group" refers to the fraction on the 2 mm sieve obtained in the second step. In addition, "-" in Tables 1 and 2 indicates that no measurement was performed because pulse pulverization was not possible.

[0045] [Table 1]

[0046] [Table 2]

[0047] The following can be seen from Tables 1 and 2: (1) In Example 1, pulse pulverization was possible 80 times or more, but in Comparative Example 1, the pH and conductivity of the water increased with pulse pulverization, causing a large current to flow through the water, resulting in dielectric breakdown and making pulse pulverization impossible. This shows that the carbonation treatment of waste concrete contributes to suppressing the increase in the pH and conductivity of the water in the second process. (2) Since the crushing ratio of Example 1 is higher than that of Comparative Example 1, it is understood that the waste concrete can be crushed into smaller particle sizes. (3) Since Example 1 has a higher separation efficiency of calcium from alkali than Comparative Example 1, it is understood that a cement paste rich in calcium and with reduced alkali can be recovered. (4) From the above, it can be seen that the method of the present invention makes it possible to recover high-quality cement paste by combining carbonation of waste concrete with pulse crushing.

Claims

1. A first step of contacting waste concrete with a carbon dioxide-containing gas; A second step of applying an electric pulse to the waste concrete of the first step to pulverize it; A third process of sieving the waste concrete after the second process and recovering the under-sieve concrete. Equipped with In the first step, a carbon dioxide-containing gas containing 30% by volume or more of water vapor relative to carbon dioxide is contacted with the waste concrete until the mass of the waste concrete increases by 0.5% by mass or more relative to the mass of the waste concrete before the first step. A method for recovering cement paste from waste concrete.

2. 2. The method according to claim 1, wherein in the second step, an electric pulse is applied to the waste concrete from the first step while the waste concrete is immersed in water.

3. The method according to claim 1 or 2, wherein the voltage of the electric pulse is 50 to 300 kV.

4. The method according to any one of claims 1 to 3, wherein the pulse period of the electric pulse is 0.5 to 80 Hz.

5. The method according to any one of claims 1 to 4, wherein the electric pulse is applied 30 or more times.

6. The method according to any one of claims 1 to 5, wherein the mesh size of the sieve used for sieving is 1 mm or less.

7. The method according to any one of claims 1 to 6, wherein the cement paste is a cement raw material or an admixture.

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