Subsurface drainage material and method for manufacturing the same

Abstract

The present invention provides a drainage channel hydrophobic material that suppresses the leaching of heavy metals contained in coal ash, and a method for producing the same. [Solution] The method for manufacturing a drainage material includes a kneading step of kneading coal ash and clay in a weight ratio of coal ash:clay = 9:1 to 5:5, a granulation step of granulating the kneaded material from the kneading step, and a firing step of firing the granulated material from the granulation step at a firing temperature of 1100°C or higher. In the firing step, the firing temperature of 1100°C or higher may be maintained for a range of 1 to 2 hours.

Description

[[Technical Field]]

[0001] The present invention relates to a underdrain hydrophobic material and a method for producing the same. [[Background Art]]

[0002] Agricultural land may be provided with a facility called underdrain, in which a water-permeable underdrain hydrophobic material is covered over a drainage pipe. In paddy field underdrains, gravel and volcanic scoria are used as underdrain hydrophobic materials, but the production volume of these materials has been decreasing year by year. From such a background and the perspective of promoting the recycling of coal ash, studies are being conducted on whether coal ash can be used in the production of underdrain hydrophobic materials. For example, Patent Document 1 discloses that a soil-improving granule obtained by adding water to coal ash generated from a boiler of a thermal power plant and granulating the mixture is used as an underdrain hydrophobic material. [[Related Art]] [[Patent Literature]]

[0003] [[Patent Document 1]] Japanese Unexamined Patent Publication No. 2007-100402 [[Summary of the Invention]] [[Problem to be Solved by the Invention]]

[0004] The underdrain hydrophobic material of Patent Document 1 uses coal ash obtained by adding limestone to coal and combusting the mixture. For this reason, when water is added to coal ash and kneaded, sulfur oxides contained in the coal react with limestone to generate gypsum, and the mixture hardens by itself when left standing at normal temperature. As a result, heavy metals are prone to elute from the amorphous components of coal ash contained in the underdrain hydrophobic material, and there is a risk that heavy metals will elute from the underdrain hydrophobic material when it is buried in soil for a long time.

[0005] The present invention has been made based on such a background, and an object of the present invention is to provide an underdrain hydrophobic material in which elution of heavy metals contained in coal ash is suppressed, and a method for producing the same. [[Means for Solving the Problem]]

[0006] To achieve the above objective, the method for manufacturing a drainage hydrophobic material according to the present invention is: It mainly consists of coal ash and clay, with additives at 10 wt% or less. Coal ash and clay of weight ratio but Coal ash:Clay = within the range of 9:1 to 5:5 The mixture The mixing process involves kneading the ingredients, A granulation step is performed to granulate the kneaded material obtained in the kneading step, A firing process in which the granulated material produced in the granulation process is fired at a firing temperature of 1100°C or higher, Includes. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a drainage hydrophobic material that suppresses the leaching of heavy metals contained in coal ash, and a method for producing the same. [Brief explanation of the drawing]

[0008] [Figure 1] This is a front view showing the configuration of a culvert according to an embodiment of the present invention. [Figure 2] This flowchart shows the flow of a method for manufacturing a drainage water-repellent material according to an embodiment of the present invention. [Figure 3] This figure shows the results of the arsenic elution test in Example 1. [Figure 4] This figure shows the results of the hexavalent chromium elution test in Example 1. [Figure 5] This figure shows the X-ray absorption measurement test in Example 2 and the sample used in the X-ray absorption measurement test. [Figure 6] This graph shows the results of the X-ray absorption measurement in Example 2. [Figure 7] This graph shows the results of the X-ray diffraction measurement in Example 2. [Modes for carrying out the invention]

[0009] Hereinafter, an embodiment of the present invention, a drainage material and a method for manufacturing the same, will be described in detail with reference to the drawings. In each drawing, the same or equivalent parts are denoted by the same reference numerals.

[0010] The drainage material for underground drainage according to this embodiment is a granular material placed on top of a drainpipe to ensure water permeability to the drainpipe of the underground drainage system. As shown in Figure 1, the underground drainage system 1 is a waterway that takes in water into a drainpipe 2 installed underground in farmland and discharges it to the outside. The drainpipe 2 has numerous through-holes in its walls, allowing it to take in excess water from the soil and discharge it to the outside. The drainpipe 2 is installed in a hole 3 about 40 cm to 50 cm deep, which is excavated in a straight line into the ground of the farmland, and the drainage material for underground drainage is placed on top of it. Soil is placed on top of the drainage material for underground drainage. The drainpipe 2 is, for example, a PVC pipe or a polyethylene pipe, and its diameter is, for example, in the range of 5 cm to 10 cm.

[0011] The drainage hydrophobic material according to the embodiment contains coal ash and clay as its main components. The drainage hydrophobic material according to the embodiment is obtained by adding water to coal ash and clay and kneading the mixture, granulating the kneaded mixture, and firing the granulated material. As a result of the inventor's diligent research, it was found that the leaching of heavy metals from the drainage hydrophobic material can be suppressed by setting the weight ratio of coal ash to clay within the range of coal ash:clay = 9:1 to 5:5 and firing the granulated material at a high temperature of 1100°C or higher. The reason why the leaching of heavy metals can be suppressed is thought to be that the amorphous components of coal ash crystallize when fired at a high temperature of 1100°C or higher.

[0012] Coal ash is ash obtained by burning coal, especially pulverized coal, and its main components are silica (SiO2) and alumina (Al2O3). Coal ash contains clinker and fly ash, but fly ash is preferred as the coal ash for drainage pipe hydrophobic material. Fly ash is spherical fine particles collected by an electrostatic precipitator from coal suspended with combustion gases.

[0013] In the underdrain hydrophobic material, clay functions as a binder to bind coal ash particles together. The clay is preferably a clay suitable for use as a raw material for bricks and pottery, and more preferably has a particle diameter of 5 µm or less. A plurality of types of clay may be kneaded into the coal ash.

[0014] In the underdrain hydrophobic material according to the embodiment, the weight ratio of coal ash to clay is set within the range of coal ash:clay = 9:1 to 5:5 for the following reasons. First, if the proportion of coal ash is higher than that in the ratio of coal ash:clay = 9:1, the moldability required for granulation and the strength required for the underdrain hydrophobic material cannot be obtained due to insufficient clay. On the other hand, if the proportion of coal ash is lower than that in the ratio of coal ash:clay = 5:5, the product cannot receive certification from the prefectural government as a recycled product made from coal ash.

[0015] In the step of kneading coal ash and clay, it is preferable to add water. Although the amount of water added to the kneaded mixture of coal ash and clay depends on the moisture content contained in the clay, a guideline is that the amount falls within the range of 25 wt% to 30 wt% relative to the kneaded mixture.

[0016] Additives other than coal ash and clay may also be added to the underdrain hydrophobic material. As the additive, waste-based additives such as concrete sludge, slag, and desulfurized gypsum may be used. In addition, reagents such as calcium carbonate, sulfates (iron sulfate, calcium sulfate, potassium sulfate, aluminum sulfate), water glass, boric acid, and potassium chloride may be used as additives. The additive may be added, for example, in a range of 1% to 10% by weight ratio.

[0017] The shape of the underdrain hydrophobic material is, for example, spherical or spindle-shaped. In consideration of water permeability, the particle diameter of the underdrain hydrophobic material is, for example, within the range of 5 mm to 20 mm. If the average particle diameter is made too small, the gaps between granular particles become small, thereby reducing the water permeability of the underdrain 1. On the other hand, if the average particle diameter is made too large, the gaps between granular particles become large, and soil enters these gaps, thereby reducing the water permeability of the underdrain 1. The above is the configuration of the underdrain hydrophobic material.

[0018] (Manufacturing method) Next, with reference to Figure 2, a method for manufacturing a drainage hydrophobic material according to an embodiment will be described. First, a mixture is prepared by adding water to the raw materials, coal ash and clay, and kneading them together (Step S1). A mixer with stirring blades is recommended for the kneading process.

[0019] Next, granules are created by granulating the kneaded mixture prepared in step S1 (step S2). For granulation of the kneaded mixture, an extrusion molding machine is preferable. In extrusion molding, the particle size of the granules can be adjusted as appropriate by selecting a perforated mold according to the desired particle size.

[0020] Next, the granules created in step S2 are dried (step S3). For this drying process, for example, natural drying can be performed.

[0021] Next, the granules dried in step S3 are fired at a high temperature (step S4). Specifically, the dried granules are placed in a heating furnace. Then, the temperature inside the heating furnace is gradually raised to a predetermined target firing temperature, and then the firing temperature inside the heating furnace is maintained for a certain period of time. After a certain period of time has elapsed, the heating in the heating furnace is stopped, and the temperature inside the heating furnace is gradually lowered to cool it down. This completes the firing of the granules in the heating passage. The firing temperature is 1100°C or higher, preferably in the range of 1100°C to 1200°C.

[0022] The time required to raise the temperature from room temperature to the firing temperature (heating time), the time required to maintain the firing temperature (holding time), and the time required to cool from the firing temperature to room temperature (cooling time) should all be set according to the size of the granules to prevent cracking. The heating time and holding time are both, for example, within the range of 1 to 2 hours. The above describes the method for manufacturing drainage material for underground drains.

[0023] Because the drainage material according to this embodiment is manufactured with the above composition and manufacturing method, the amorphous components of the coal ash crystallize, making it difficult for heavy metals derived from the coal ash to leach out. Furthermore, because the drainage material according to this embodiment uses coal ash that is continuously discharged from thermal power plants and can be obtained at low cost, it can be supplied to the market at a low price and in a stable manner, and the costs and environmental burden of disposing of coal ash can also be reduced. In addition, because the drainage material according to this embodiment is fired at high temperatures, it has sufficient strength and excellent water permeability and durability when buried underground.

[0024] As described above, the drainage material according to the embodiment includes a kneading step of kneading coal ash and clay in a weight ratio of coal ash:clay = 9:1 to 5:5, a granulation step of granulating the kneaded material from the kneading step, and a firing step of firing the granulated material from the granulation step at a firing temperature of 1100°C or higher. Therefore, the leaching of heavy metals contained in the coal ash of the drainage material can be suppressed.

[0025] The present invention is not limited to the embodiments described above, and the following modifications are also possible.

[0026] (modified version) In the above embodiment, a mixer was used in the kneading process and an extrusion molding machine was used in the molding process, but the present invention is not limited thereto. For example, the kneading process and the molding process may be carried out in one batch using a plow-type mixer. The plow-type mixer is equipped with shovel blades and a crushing chopper, and can efficiently crush and mix lumps of clay containing moisture and dry lime ash.

[0027] In the above embodiment, the firing temperature was maintained at a constant temperature, but the present invention is not limited thereto. The firing temperature may be allowed to vary within a certain range.

[0028] In the above embodiment, the operation of the heating furnace was simply stopped when the fired body was cooled, but the present invention is not limited to this. For example, outside air may be blown into the heating passage.

[0029] The embodiments described above are illustrative, and the present invention is not limited thereto. Various embodiments are possible without departing from the spirit of the invention as described in the claims. The components described in the embodiments and modifications can be freely combined. Furthermore, inventions equivalent to the invention described in the claims are also included in the present invention.

[0030] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these examples.

[0031] (Example 1) In Example 1, samples of several types of hydrophobic materials were prepared by firing at different firing temperatures, and heavy metal elution tests were performed on these samples to verify whether the elution of heavy metals was suppressed.

[0032] The sample was prepared by mixing coal ash and clay in a 1:1 ratio, adding an appropriate amount of water in the range of 10 wt% to 20 wt%, rolling it by hand to a particle size of approximately 20 mm, and firing it in an electric furnace. The coal ash used was fly ash discharged from a thermal power plant, and the clay used was Nopporo clay collected in the Nopporo district of Ebetsu City. The Nopporo clay contains approximately 20% silicon, 10% aluminum, and 5% iron by weight, and its particle size is 5 μm or less. In the firing process, the heating time, holding time, and cooling time were all set to 2 hours each. In the leaching test, the amount of arsenic leached and the pH of the sample were measured. The leaching test was performed after crushing the sample to 2 mm or less in accordance with Environmental Notification No. 46, which sets environmental standards for soil contamination.

[0033] As a result, as shown in Figure 3, although the change in pH with firing temperature was small, all samples became alkaline. Furthermore, the peak arsenic leaching occurred at a firing temperature of 900°C, and as the firing temperature increased beyond this point, the amount of arsenic leaching decreased in roughly inverse proportion to the temperature. At a firing temperature of 1100°C, the arsenic leaching level decreased to a value close to the soil environmental standard value required for waste, indicating that arsenic leaching was significantly suppressed.

[0034] Next, since chromium is more easily oxidized from trivalent to hexavalent in alkaline samples and leached out, an elution test was conducted to measure the amount of hexavalent chromium leached out. The procedure for the elution test was the same as that used for arsenic. As a result, as shown in Figure 4, the amount of hexavalent chromium leached out at a calcination temperature of 1150°C was lower than the soil environmental standard value required for waste, and it is considered that it falls below the standard value at a calcination temperature of 1100°C. From the above, it can be inferred that if the firing temperature is set higher than 1100°C, the amount of arsenic and hexavalent chromium leaching can be suppressed to a level equivalent to that required for waste materials.

[0035] (Example 2) In Example 2, X-ray absorption and X-ray diffraction measurements were performed on the sample to investigate the mechanism of suppression of elution during high-temperature firing.

[0036] In the X-ray absorption measurements, the arsenic content was measured using SPring-8. SPring-8 is a large synchrotron radiation facility that can estimate the content, valency, and chemical structure of trace elements by irradiating a sample with high-intensity X-rays and measuring element-specific absorption and fluorescence. Three types of samples were used for the measurements, as shown in Figure 5. In these samples, the ratio of coal ash to clay was set to coal ash:clay = 8:2 to increase the amount of arsenic eluted. As can be seen from the X-ray intensity of arsenic shown in Figure 6 (area enclosed by the dotted line) in the X-ray absorption measurements, it was found that when the granulated material was calcined at 1180°C, the arsenic content was reduced by about half due to the calcination. From the above, it can be understood that raising the firing temperature to a high temperature of around 1100°C significantly reduces the arsenic content in the fired product.

[0037] Furthermore, in X-ray diffraction measurements, samples fired at 900°C and 1100°C (coal ash:clay = 8:2) were used to analyze the relationship between firing temperature and crystal structure. As shown in Figure 7, mullite was the main component in the unfired raw material and the material fired at 900°C, while cristobalite was the main component in the material fired at 1100°C. This is thought to be mainly due to the crystallization of the amorphous components of the coal ash by firing at 1100°C. From the above, it can be understood that the decrease in arsenic content in the calcined material due to firing is because the main component of the calcined material changes from mullite to cristobalite. [Explanation of symbols]

[0038] 1. Underground culvert 2 Drain pipe 3 holes

Claims

1. A kneading step of kneading a mixture mainly composed of coal ash and clay, with additives of 10 wt% or less, and the weight ratio of coal ash to clay being in the range of coal ash:clay = 9:1 to 5:5, A granulation step is performed to granulate the kneaded material obtained in the kneading step, A firing process in which the granulated material produced in the granulation process is fired at a firing temperature of 1100°C or higher, A method for manufacturing a drainage material containing [the specified substance].

2. In the firing process, a firing temperature of 1100°C or higher is maintained for a period of 1 to 2 hours. A method for producing a drainage hydrophobic material according to claim 1.

3. The drainage material is spherical or spindle-shaped. A method for producing a drainage hydrophobic material according to claim 1.

4. The particle size of the drainage material is in the range of 5 mm to 20 mm. A method for producing a drainage hydrophobic material according to claim 3.

5. A drainage hydrophobic material manufactured by the manufacturing method described in any one of claims 1 to 4.

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