Method for recovering cement paste from waste concrete

Abstract

To provide a method for recovering cement paste from waste concrete in a high yield.SOLUTION: A method for recovering cement paste from waste concrete, in which crushed concrete containing aggregate and cement paste is subjected to a process including steps (1) and (2) of the following: (1) a first step in which the crushed material is separated into heavy products, intermediate products, and light products by thin flow sorting, and the light products are recovered as cement paste. (2) a second step in which the intermediate products separated in the first step are subjected to gravity sorting, which uses a principle different from the thin flow sorting, and the light products are recovered as cement paste.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Waste concrete generated during the demolition work of reinforced concrete buildings and the like has mainly been reused as roadbed materials, backfill materials, etc. However, due to the decreasing demand for roadbed materials and the problem of depletion of river aggregates used in concrete, technologies for recycling concrete aggregates from waste concrete have been studied.

[0003] For example, a method for producing high-quality recycled aggregates has been proposed, in which waste concrete is sieved to select recycled aggregate raw materials with a particle size of 5 mm or less, which are then separated into aggregate components and mortar components by a fine aggregate mill, and the aggregate components are separated into fine aggregates and mortar by a wet specific gravity separator (Patent Document 1). Also, a method for recycling aggregates from waste concrete has been proposed, in which waste concrete is compressed and crushed, sorted by size, rotated to grind by rotating the waste concrete sorted by size, to peel off the mortar adhering to the crushed stone, select the sand-cement component, and separate the aggregate of the crushed stone and mortar components and the sand-cement component into products such as crushed stone, sand, and cement (Patent Document 2). Further, a method for producing recycled fine aggregates is known, in which waste concrete with a size of about 40 mm or less is crushed to a size of about 5 mm or less, the waste concrete crush is polished to remove the mortar, and the object to be treated is cyclone-type specific gravity sorted to remove fine powder (Patent Document 3).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

[0005] As described above, all of the prior arts aim to recover high-quality recycled aggregate by removing the cement paste from the aggregate surface of crushed waste concrete through grinding or polishing, and either do not consider the recovery of cement paste at all, or if they do, they do not take into account the improvement of the quality of the cement paste. As a result, in reality, most of it is not being effectively utilized. The inventors of the present invention, in order to recover high-quality cement paste from waste concrete, subjected concrete crushed material containing aggregate and cement paste to specific gravity separation using a commonly used specific gravity separator. They found that, because most specific gravity separators are susceptible to the influence of particle size, there is a problem in that the recovery rate decreases when trying to improve the quality of the cement paste. Therefore, the object of the present invention is to provide a method for recovering cement paste from waste concrete with high yield. [Means for solving the problem]

[0006] Waste concrete contains two components with different specific gravities: aggregate and cement paste. If these existed as independent particles without adhering to each other, they should be able to be completely separated at a certain specific gravity boundary, as shown in Figure 2(a). However, in actual specific gravity sorting, as shown in Figure 2(b), the principle is such that larger or smaller particles are more easily distributed to the heavy product, leading to problems such as missed cement paste and mixed aggregate. Therefore, the inventors investigated a method that could recover cement paste with good yield while solving the problems of missed cement paste and mixed aggregate. They found that by combining sorting devices with opposing effects on particles, for example, a sorting device that easily distributes coarser particles to the light product and a sorting device that easily distributes finer particles to the light product, the influence of particle size is suppressed, the separation effect between cement paste and aggregate is enhanced, and the recovery rate of cement paste can be improved.

[0007] In other words, the present invention provides the following [1] to [4]. [1] A method for recovering cement paste from waste concrete, wherein the waste concrete crushed material containing aggregate and cement paste is subjected to the following steps (1) and (2). (1) A first step in which the crushed material is separated into heavy products, intermediate products and light products by thin-flow sorting, and the light products are recovered as cement paste. (2) In the second step, the intermediate products separated in the first step are subjected to specific gravity separation using a principle different from thin flow separation, and the light products are recovered as cement paste. [2] The cement paste recovery method according to [1], wherein in the second step, the cement paste is separated by specific gravity based on the difference in settling velocity. [3] The cement paste recovery method according to [1] or [2], wherein the specific gravity separation in the second step is centrifugal separation, jig separation, cyclone separation, or heavy liquid separation. [4] The cement paste recovery method according to any one of [1] to [3] above, wherein the thin flow separation in the first step is wet table type separation. [Effects of the Invention]

[0008] According to the present invention, cement paste can be recovered from waste concrete with high yield. [Brief explanation of the drawing]

[0009] [Figure 1] This flowchart shows an example of the present invention's method for recovering cement paste from waste concrete. [Figure 2] This is a schematic diagram illustrating the principle of cement paste recovery in the present invention. [Figure 3] This flowchart shows the cement paste recovery method from waste concrete used in the examples and comparative examples. [Modes for carrying out the invention]

[0010] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Figure 1 shows a flowchart of a preferred embodiment of the cement paste recovery method from waste concrete of the present invention.

[0011] (First step) The present invention's method for recovering cement paste from waste concrete, as shown in Figure 1, first separates crushed waste concrete containing aggregate and cement paste into heavy products, intermediate products, and light products by thin-flow separation, and recovers the light products. Because there is a difference in specific gravity between cement paste particles and aggregate particles, subjecting the crushed waste concrete containing these to thin-flow separation allows for the removal of aggregate as a heavy product and the recovery of concentrated cement paste as a light product. Here, "heavy products, intermediate products, and light products" refers to differences in relative specific gravity, but generally, the apparent specific gravity of heavy products is usually around 2.4 g / cm³. 3 The above is true, and the bulk density of light products is usually 2.1 g / cm³. 3 The following is true, and the bulk density of the intermediate product is between that of the heavy product and the light product.

[0012] Examples of the waste concrete used in this project include demolished concrete generated by civil engineering work, demolition of structures, etc., and surplus concrete generated during the construction of buildings, etc. In addition, as waste concrete, in terms of work efficiency and improving the quality of cement paste, concrete blocks that have been crushed, magnetically separated, or manually separated for the purpose of separating foreign substances such as steel bars can be used. Their dimensions are usually 200 - 400 mm square, preferably about 300 mm square.

[0013] Waste concrete crushed material can be produced by crushing waste concrete with a crusher. Known crushers can be used, for example, any of jaw crushers, impact crushers, hammer crushers, roll crushers, and rotary crushers may be used. The crushing process may be carried out two or more times. When carried out two or more times, the same or different crushers can be used. When the crushing process is carried out two or more times, it is possible to appropriately select a crusher so that the waste concrete crushed material has a desired particle size. In addition, it is possible to attach a screen with a desired mesh size to the crusher for the purpose of particle size adjustment. When the screen is not attached, the fixed teeth, rotating teeth, inner wall, etc. may be adjusted to a desired clearance.

[0014] From the perspective of improving the quality of cement paste, the maximum particle size of the waste concrete crushed material used in this project is preferably less than 10 mm, more preferably less than 5 mm, and even more preferably less than 2 mm. Here, in this specification, "maximum particle size" refers to the particle size expressed by the minimum mesh opening of the sieve through which all the samples pass. Although the lower limit value of the particle size of the waste concrete crushed material is not particularly limited, from the perspective of sorting efficiency, it is preferably 0.01 mm or more, more preferably 0.05 mm or more, and even more preferably 0.1 mm or more.

[0015] Here, in this specification, "thin-flow separation" refers to an operation of separating particles with different specific gravities by utilizing the thin flow of a fluid flowing horizontally. Since the flow velocity of the thin flow increases as it moves away from the wall surface, particles with a larger particle size and a smaller specific gravity are more likely to be washed away by the thin flow and separated from particles with a smaller particle size and a larger specific gravity. Examples of thin-flow separation include, for example, wet table separation, wet thin-flow separation (water naget), and Reichert cone. Among them, wet table separation, which has many variable conditions and is suitable for continuous processing, is preferred.

[0016] For wet table separation, a wet table separator can be used. A wet table separator is provided with a vibration mechanism that applies vibration to a table moving in a certain direction from a predetermined direction. Typically, vibration is applied in a direction perpendicular to the thin flow of water flowing from above to below the inclined table. Also, for the table, a flat plate provided with riffles (weirs) in a direction perpendicular to the thin flow can be used. The size and shape of the riffles are not particularly limited, but for example, they are about 1 to 5 mm in height, about 5 to 20 mm in width, and about 10 to 40 mm in interval. In wet table separation, for example, when waste concrete crushed materials are put on an inclined vibrating table and water is flowed from the upstream side, the light products are washed downward by the water flow, the heavy products are pushed upward by the friction of the vibration of the inclined surface, and the intermediate products fall to the lower side of the table and are separated.

[0017] In wet table separation, the inclination angle of the table, the amplitude, the flow velocity of the thin flow, and the vibration frequency can be appropriately adjusted according to the raw materials. For example, the table inclination can be 3 to 10°, the amplitude of the table can be 5 to 20 mm, the flow velocity of the thin flow can be 1.0 to 8.0 L / min, and the supply amount can be 50 to 300 g / min, but it is not limited to these.

[0018] (Second step) Next, as shown in Figure 2, the intermediate product separated in the first step is subjected to specific gravity separation using a principle different from thin-flow separation, and the light product is recovered as cement paste. The intermediate product contains aggregate mixed with the cement paste, and by subjecting it to specific gravity separation using a principle different from the thin-flow separation in the first step, the cement paste and aggregate are separated, and high-quality cement paste can be recovered as the light product.

[0019] The specific gravity separation in this process is not particularly limited as long as the principle differs from the thin-flow separation in the first process, but it is preferable to separate the particles by specific gravity based on the difference in their settling velocity. This allows, for example, if light and large particles are separated from heavy and small particles in the first process, to separate the light and small particles and heavy and large particles that could not be separated in the first process in the second process. Since the settling velocity of particles is correlated with their specific gravity, sorting methods that utilize this correlation can be employed. Examples include centrifugal sorting, jig sorting, cyclone sorting, and heavy liquid sorting.

[0020] In centrifugal separation, intermediate products are dropped down a circular incline into a centrifugal separation device, and the centrifugal force increases the difference in settling velocity, thereby separating products with different specific gravities. This allows for the removal of aggregate as a heavy product and the recovery of high-quality cement paste as a light product. In jig sorting, intermediate products are placed in a tank with a water flow that oscillates vertically, and then a periodically oscillating water flow is applied to the tank. This causes the particles on the mesh to stratify in order of specific gravity, forming layers of heavy and light products. As a result, aggregates are removed as heavy products, and high-quality cement paste is recovered as a light product. In cyclone sorting, intermediate products are supplied to a cyclone sorting device that has a swirling airflow or water flow formed inside. The intermediate products are swirled by the airflow or water flow, and while they are swirling in the swirling flow, the heavier products with a higher specific gravity fall out of the swirling flow and are discharged from the lower end opening, while only the lighter products with a lower specific gravity are carried by the rising airflow or water flow formed in the center of the swirling flow and discharged from the upper end opening. This makes it possible to remove aggregate as a heavy product and recover high-grade cement paste as a light product. In heavy liquid separation, intermediate products are placed in a specific gravity liquid, and the density of the liquid acts as a boundary, separating the heavy products with an apparent density greater than the specific gravity liquid's density from the light products with an apparent density less than the specific gravity liquid's density. This allows for the removal of aggregate as a heavy product and the recovery of high-quality cement paste as a light product. The sorting conditions in the sorting method can be set appropriately depending on the raw materials, but for example, in centrifugal sorting, the centrifugal force can be 20 to 100 times that of gravity.

[0021] In this way, cement paste can be recovered from waste concrete with a high yield.

[0022] The present invention has been described in detail above based on its embodiments. However, the present invention is not limited to the above embodiments. The present invention can be modified in various ways without departing from its spirit. In the above embodiments, concrete crushed material containing aggregate and cement paste particles is subjected to a first step and the selected light products are recovered as cement paste. However, the present invention is not limited thereto, and the light products from the first step may not be recovered, and the intermediate products from the first step may be subjected to a second step, and only the obtained light products may be recovered as cement paste. This makes it possible to produce cement paste of even higher quality. Furthermore, although the above embodiments were described based on waste concrete crushing, the present invention is not limited thereto and can be applied to any composition in which there is a difference in specific gravity between the target component to be recovered and other non-target components. Examples of targets other than waste concrete crushing include crushed waste electrical and electronic equipment containing metal and resin, and ore extracted from mines containing gangue. [Examples]

[0023] The embodiments of the present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following embodiments.

[0024] The apparatus used in the example is as follows: (1) Thin flow sorting Wet table-type sorting system (Holman-Wilfley Model 800 Table, manufactured by Holman-Wilfley Ltd., with vibrating feeder) (2) Specific gravity sorting based on a different principle than thin flow sorting Centrifugal sorting device (Sepro Mineral Systems Corp. FALCON semi-batch concentrator laboratory model L-40)

[0025] Example 1 Crushed waste concrete with a particle size of 0.15 to 0.3 mm was processed according to the flow shown in Figure 3(a). (Wet table sorting) Using a vibrating feeder, crushed waste concrete was fed in, and flow water was sprayed to create a slurry, which was then sorted into three categories: light products, intermediate products, and heavy products. Next, each product was collected and dried, with the light products being used as cement paste concentrate and the heavy products as aggregate concentrate. (Centrifugal sorting) 50g of the sample was mixed with water to prepare a 1L slurry, which was then stirred with a stirrer. A centrifugal sorting device was activated, and the slurry was fed in to separate it into heavy and light products. Each product was collected and dried, with the light products being used as cement paste concentrate and the heavy products as aggregate concentrate. Then, the light products obtained from wet table sorting and the light products obtained from centrifugal sorting were mixed and analyzed as cement paste.

[0026] Comparative Example 1 Crushed waste concrete with particle sizes of 0.15 to 0.3 mm was processed according to the flow shown in Figure 3(b). (Wet table sorting) The crushed waste concrete was subjected to wet table sorting using the same procedure as in Example 1 to separate it into three categories: light products, intermediate products, and heavy products, and the light products were recovered. After drying the light product, it was analyzed as cement paste.

[0027] Comparative Example 2 Crushed waste concrete with a particle size of 0.15 to 0.3 mm was processed according to the flow shown in Figure 3(c). (Centrifugal sorting) 50g of crushed waste concrete was mixed with water to prepare a 1L slurry, which was then subjected to centrifugal separation in the same manner as in Example 1 to separate it into heavy and light products, and the light products were recovered. After drying the light product, it was analyzed as cement paste.

[0028] Comparative Example 3 Crushed waste concrete with a particle size of 0.15 to 0.3 mm was processed according to the flow shown in Figure 3(d). (Wet table sorting) 50g of crushed waste concrete was sorted into three categories—light products, intermediate products, and heavy products—by wet table sorting using the same procedure as in Example 1. Next, each product was collected and dried, with the light products being used as cement paste concentrate and the heavy products as aggregate concentrate. (Wet table sorting) Next, the intermediate products were further sorted into three categories—light products, intermediate products, and heavy products—by wet table sorting using the same procedure as in Example 1. Next, each product was collected and dried, with the light products being used as cement paste concentrate and the heavy products as aggregate concentrate. The light products obtained from each wet table sorting process were then mixed and analyzed as cement paste.

[0029] Analysis and evaluation Following the Cement Association's F-18 method, the sample was crushed, and the residual mass was measured by dissolving it in hydrochloric acid (1+100). The mass percentage of insoluble residue (in sol) was then calculated using the following formula and was defined as the aggregate content.

[0030] Aggregate content (in.sol) (%)=w / s×100

[0031] [In the formula, w represents the residual mass (g), and s represents the sample mass (g).]

[0032] Next, assuming that the crushed concrete consists of only two components, cement paste and aggregate, the cement paste content was calculated using the following formula.

[0033] Cement paste content (%) = 100 - Aggregate content (%)

[0034] Furthermore, the cement paste recovery rate can be determined from the material balance of the recovered material. Table 1 shows the mass percentage, cement paste content, and cement paste recovery rate for the light product (a combined product in examples and comparative examples where multiple light products are obtained) which is a cement paste concentrate, with the raw material set to 100.

[0035] [Table 1]

[0036] The results in Table 1 show that by separating waste concrete crushed material containing aggregate and cement paste into heavy products, intermediate products, and light products by thin-flow separation, recovering the light products as cement paste, and further subjecting the intermediate products to specific gravity separation using a different principle than thin-flow separation, and recovering the light products as cement paste, cement paste can be recovered from waste concrete in good yield.

Claims

[Claim 1] A method for recovering cement paste from waste concrete, comprising subjecting crushed concrete containing aggregate and cement paste to the following steps: (1) and (2). (1) A first step in which the crushed concrete is separated into heavy products, intermediate products and light products by wet table sorting, and the light products are recovered as cement paste. (2) A second step in which the intermediate products separated in the first step are subjected to centrifugal separation and the light products are recovered as cement paste.

Images