Method for recovering nutrient salt

Abstract

To provide a technique for efficiently recovering a nutrient salt from a used absorptive article.SOLUTION: A method for recovering a nutrient salt is a method for recovering a nutrient salt from a used absorptive article having a water-absorbing resin and a thermoplastic resin. The method for recovering the nutrient salt includes: a drying step of stirring and drying an object to be treated containing the absorptive article; a heating step of heating the object to be treated at a temperature which is equal to or higher than a softening point of the thermoplastic resin and lower than a thermal decomposition temperature of the water-absorbing resin; a granulation step of stirring the object to be treated and cooling the object to be treated to be lower than the softening point of the thermoplastic resin, and thereby obtaining a granule, after the heating step; and a recovery step of recovering the nutrient salt from the granule.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a recycling technology for used absorbent articles. [Background technology]

[0002] Used absorbent articles such as disposable diapers and sanitary napkins absorb excrement such as urine, feces, and menstrual blood. These excrements contain nutrients (phosphorus, nitrogen, potassium, and / or calcium) that can be used as fertilizer. Therefore, techniques for recovering and reusing nutrients derived from excrement from used absorbent articles are known. For example, Patent Document 1 discloses a method for recovering nutrients from a liquid by immersing a used absorbent article in an aqueous solution containing a calcium compound to inactivate the superabsorbent polymer and separate the liquid into a solid containing the constituent materials of the absorbent article and a liquid containing the calcium compound and nutrients. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-182246 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the method described in Patent Document 1, when the nutrients recovered from the liquid are reused as fertilizer, etc., it is necessary to further recover the nutrients from the liquid, which makes the process complicated. In addition, this method may require a large amount of treatment liquid or a large treatment device, so a more efficient nutrient recycling technology is desired.

[0005] The present invention relates to a technique for efficiently recovering nutrients from used absorbent articles. [Means for solving the problem]

[0006] A method for recovering nutrients according to one embodiment of the present invention is a method for recovering nutrients from a used absorbent article that includes a water-absorbent resin and a thermoplastic resin. The method for recovering nutrients comprises: a drying step of agitating and drying the object to be treated including the absorbent article; a heating step of heating the object to be treated at a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbing resin; a granulation step of stirring the material to be treated after the heating step and cooling it to a temperature lower than the softening point of the thermoplastic resin to obtain granules; and a recovery step of recovering the nutrients from the granules. [Effects of the Invention]

[0007] According to the present invention, nutrients can be efficiently recovered from used absorbent articles. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram of a granulation device that can be used in one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram of the granulation device. [Figure 3] 4 is a flowchart showing a method for recovering nutrients according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Outline of an embodiment of the present invention] A method for recovering nutrients according to one embodiment of the present invention is configured to granulate used absorbent articles and recover nutrients from the granules. This allows nutrients to be recovered efficiently and effectively utilized as fertilizer, etc. Furthermore, in this embodiment, nutrients are separated from the water-absorbent resin during the granulation process, making it possible to produce a recycled material that is easy to handle and has low water absorption and hygroscopicity. Therefore, the recovery method according to this embodiment contributes to promoting the recycling of waste, including used absorbent articles, and ultimately contributes to the realization of a resource-circulating society.

[0010] In this embodiment, the absorbent article refers to an article that contains a water-absorbent resin and a thermoplastic resin and is capable of absorbing body fluids, including excrement. Furthermore, in this embodiment, the used absorbent article refers to an absorbent article that has absorbed or adhered to excrement and / or other body fluids. The absorbent article according to this embodiment includes, for example, one or more items selected from disposable diapers, sanitary napkins, urine absorption pads, panty liners, pet toilet sheets, portable toilets, and drip sheets. The term "drip sheet" refers to a water-absorbent sheet that absorbs excess moisture (body fluids) from food ingredients such as meat and fish.

[0011] In this embodiment, excrement includes liquid excrement such as urine and menstrual blood, and solid excrement such as feces. Furthermore, other body fluids include liquids exuded from animal foods such as meat and fish. These excrements and body fluids contain nutrients. In this embodiment, nutrients refer to salts containing nitrogen, phosphorus, potassium, or calcium that can be used as fertilizer, and more specifically, include ammonium salts, phosphate salts, potassium salts, calcium salts, etc.

[0012] Absorbent articles used to treat bodily waste, such as disposable diapers and sanitary napkins, generally include an absorbent body containing a water-absorbent resin and sheet materials covering the front and back of the absorbent body, with each component bonded together with an adhesive or the like. The sheet materials include, for example, thermoplastic resin and / or cellulose. The absorbent body includes at least an absorbent core containing a water-absorbent resin and may further include a core wrap sheet covering the absorbent core. The core wrap sheet is made of a hydrophilic material, such as cellulose. The absorbent core may also include a hydrophilic material, such as cellulose pulp, in addition to the water-absorbent resin.

[0013] In this embodiment, the water-absorbent resin is a resin having water-absorbing properties, and includes, for example, one or more water-absorbent resins selected from polyacrylic acid, polyacrylic acid salts, partially crosslinked polymeric compounds having carboxyl groups or salts thereof, partially crosslinked polysaccharides, etc. Partially crosslinked polymeric compounds having carboxyl groups or salts thereof include crosslinked polyacrylates, poly(vinyl alcohol / acrylate) copolymers (crosslinked), starch-acrylate graft copolymers (crosslinked), and polyvinyl alcohol-polymaleic anhydride graft copolymers (crosslinked). Partially crosslinked polysaccharides include crosslinked carboxymethyl cellulose salts, etc. Furthermore, the "salt" constituting the water absorbent resin includes, for example, one or more salts selected from alkali metal salts (sodium salts, potassium salts, lithium salts, etc.), alkaline earth metal salts (calcium salts, magnesium salts, barium salts, etc.), ammonium salts (quaternary ammonium salts, quaternary alkylammonium salts, etc.), etc. The water-absorbing resin of the present invention preferably contains one or more water-absorbing resins selected from polyacrylic acid, sodium polyacrylate, or a crosslinked sodium polyacrylate.

[0014] In this embodiment, the thermoplastic resin is not limited to a specific type, and examples thereof include polyolefin, polyester, polystyrene, etc. Examples of polyolefin include polyethylene and polypropylene, etc. Examples of polyester include polyethylene terephthalate, etc. These thermoplastic resins are used, for example, as sheet materials or exterior materials in absorbent articles. Furthermore, thermoplastic resins may also be contained in objects to be treated other than absorbent articles.

[0015] The material to be treated by the recovery method of this embodiment may consist solely of used absorbent articles, but may also include other waste, secondary materials, etc. The other waste is not particularly limited and may include, for example, used packaging containers (food containers, bottles, etc.), marine debris, other waste generated from homes, businesses, etc.

[0016] In this embodiment, the material to be treated preferably contains cellulose. When the material to be treated contains cellulose, it becomes easier to carbonize a portion of the material to be treated in heating step S2, as described below. The cellulose may be contained in an absorbent article, or may be contained in a material to be treated other than an absorbent article.

[0017] In this embodiment, "granular" refers to the shape of each fragment of the crushed material. Furthermore, "granular body" refers to an aggregate of multiple fragments that are separated from each other. Furthermore, "granulation" refers to converting the material to be crushed into granular bodies. In this embodiment, the maximum diameter of the granular body is preferably 5 cm or less, more preferably 1 cm or less, and even more preferably 0.5 cm or less, from the viewpoint of ease of handling.

[0018] In this embodiment, the particle size of a granular material refers to the size of the granular material. In other words, the particle size of a granular material serves as a standard for separating granular materials by size. The size of a granular material here may be defined as a range of particle diameters equivalent to the diameter of each granular material if it were assumed to be a sphere, but it does not have to be strictly defined by particle diameter as long as the size of the granular material can be classified. For example, the particle size of a granular material can be determined by the opening of a sieve or the classification point set in a classifier.

[0019] [Configuration of granulation equipment] First, a granulation apparatus 100 will be described as an example of a processing apparatus that can be used in the recovery method according to this embodiment. FIG. 1 shows the granulation apparatus 100 from the front, and FIG. 2 shows the granulation apparatus 100 from the side. The granulation apparatus 100 has a processing tank 101. The processing tank 101 has a wall 101a and a storage section 101b. The storage section 101b can store materials to be granulated by the granulation apparatus 100, and is configured as an internal space of the processing tank 101 surrounded by the wall 101a. FIGS. 1 and 2 each show a vertical cross section of the wall 101a, showing the interior of the storage section 101b. In the processing tank 101, a portion of the wall 101a is configured to be openable and closable, allowing access from the outside to the storage section 101b for loading and unloading materials.

[0020] The granulation device 100 further includes an agitation shaft 102. The agitation shaft 102 includes a shaft portion 102a and multiple blade portions 102b. The shaft portion 102a is configured as a round bar-shaped member that can rotate around a rotation axis C extending horizontally laterally. Both ends of the shaft portion 102a are supported by the wall portion 101a on the sides of the storage portion 101b, and the portion between the both ends supported by the wall portion 101a is located within the storage portion 101b. The multiple blade portions 102b are provided at intervals along the longitudinal direction, substantially evenly distributed over the entire portion of the shaft portion 102a located within the storage portion 101b. Furthermore, each blade portion 102b protrudes in various radial directions from the outer circumferential surface of the shaft portion 102a.

[0021] The granulation apparatus 100 further includes a heater 103. The heater 103 is attached to the outside of the wall 101a of the treatment tank 101. The heater 103 is configured to be able to increase the temperature inside the storage section 101b through the wall 101a when current is applied to the heater 103. In other words, in the granulation apparatus 100, the heater 103 can heat the object to be treated stored in the storage section 101b. Note that the configuration of the heater 103, such as the heating method and installation position, can be determined in various ways depending on the configuration of the treatment tank 101 and the properties of the object to be treated.

[0022] Additionally, the granulation apparatus 100 further includes a blower mechanism 104. The blower mechanism 104 includes an air intake section 104a and an exhaust section 104b. The air intake section 104a and the exhaust section 104b are respectively provided on the wall section 101a of the treatment tank 101 and laterally face each other across the storage section 101b. The blower mechanism 104 is configured to supply air into the storage section 101b through the air intake section 104a and exhaust air from the treatment tank 101 through the exhaust section 104b, thereby blowing air into the storage section 101b. Therefore, in the granulation apparatus 100, the blower mechanism 104 can promote a decrease in temperature during cooling of the workpiece. Note that the gas used to blow air by the blower mechanism 104 in the granulation apparatus 100 is not limited to air and may be, for example, an inert gas such as nitrogen.

[0023] With the above-described configuration, the granulating device 100 can effectively agitate the material to be processed within the storage section 101b by rotating the shaft section 102a around the rotation axis C through the interaction of gravity and the force applied from the multiple blade sections 102b. Therefore, the granulating device 100 can combine temperature control by the heater 103 and / or the air blowing mechanism 104 with agitation, and can perform the drying step S1, heating step S2, and granulating step S3 described below.

[0024] The configuration of the granulator 100 is not limited to the above and can be modified in various ways. For example, in the granulator 100, the rotation axis C of the shaft 102a does not necessarily have to extend horizontally, and may be tilted relative to the horizontal plane. However, to effectively obtain the stirring effect due to gravity in the granulator 100, it is necessary that at least the rotation axis C of the shaft 102a be tilted relative to the vertical direction, and it is preferable that the angle of the rotation axis C of the shaft 102a with respect to the horizontal plane is small. Specifically, in the granulator 100, it is preferable that the angle of the rotation axis C of the shaft 102a with respect to the horizontal plane is 30° or less.

[0025] [Nutrient recovery method] 3, the nutrient recovery method according to this embodiment includes a drying step S1, a heating step S2, a granulation step S3, and a recovery step S4. In this embodiment, an example will be described in which the granulation device 100 is used in the drying step S1, the heating step S2, and the granulation step S3. In this example, first, the object to be treated, including absorbent articles, is placed in the storage section 101b of the treatment tank 101, and the drying step S1 is performed.

[0026] (Drying step S1) In the drying step S1, the materials to be treated, including the absorbent articles, are agitated and dried. This evaporates the water contained in the absorbent articles, reducing the weight of the materials to be treated, and the agitation crushes the materials to be treated. Additionally, along with the water that seeps out from the water-absorbent resin, crystals of nutrients dissolved in this water precipitate around the water-absorbent resin. Furthermore, the agitation separates the water-absorbent resin from the nutrient crystals. In this embodiment, the agitation shaft 102 is rotated to agitate the materials to be treated contained in the storage section 101b of the treatment tank 101, while the heater 103 of the granulation device 100 adjusts the temperature inside the storage section 101b.

[0027] The drying temperature in this step is not particularly limited, but can be 40°C or higher and lower than the softening point of the thermoplastic resin described below. The drying temperature may be maintained substantially constant, but is preferably set to increase continuously or stepwise from the viewpoint of smoothly transitioning to the heating step S2. When increasing the temperature, the rate of temperature increase is preferably 0.001°C / min or higher and 20°C / min or lower.

[0028] If drying is performed without stirring, when the moisture held in the water-absorbent resin seeps out, the water-absorbent resin and highly hydrophilic materials such as pulp and core wrap sheets are likely to bond via the moisture. If drying is performed in this state, the water-absorbent resin and other materials will adhere to each other and form clumps, making it difficult to separate the water-absorbent resin and nutrients. In contrast, in this embodiment, drying is performed while stirring, so that the amount of moisture in the water-absorbent resin can be reduced while promoting separation of the nutrients, water-absorbent resin, and other materials.

[0029] As described above, in absorbent articles, the water-absorbent resin is often integrated with pulp or sheet material. Therefore, in this step, it is preferable to crush the absorbent article to expose the water-absorbent resin. This can promote precipitation of nutrients from the water-absorbent resin and more efficiently reduce the moisture content of the water-absorbent resin. It can also further promote separation of nutrients, the water-absorbent resin, and other materials.

[0030] (Heating step S2) Next, in heating step S2, the workpiece is heated to a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbent resin. This softens the thermoplastic resin and makes it more deformable, while the water-absorbent resin maintains its molecular structure without thermal decomposition. This allows the physical properties of the thermoplastic resin to be significantly changed relative to the water-absorbent resin. In this embodiment, the temperature of the workpiece is adjusted by, for example, heater 103. Also in this step, it is preferable to stir the workpiece by, for example, rotating stirring shaft 102. This allows the workpiece to be heated uniformly and makes it easier to crush.

[0031] In this embodiment, the term "softening point" refers to the temperature at which the thermoplastic resin contained in the workpiece begins to rapidly soften. When the workpiece contains multiple thermoplastic resins, the softening point of any of the thermoplastic resins can be selected as the "softening point of the thermoplastic resin." Preferably, the softening point of the thermoplastic resin with the highest softening point contained in the workpiece can be selected. When the thermoplastic resin contained in the workpiece can be identified, the softening point can be determined by measuring the Vicat softening temperature of the resin removed from the workpiece. When the thermoplastic resin contained in the workpiece cannot be identified, the softening point of the thermoplastic resin can be determined by measuring the Vicat softening temperature of the resin removed from the workpiece. For example, the softening point of polypropylene is 160 to 200°C, and the softening point of polyethylene terephthalate is 200 to 250°C.

[0032] In the present embodiment, the "thermal decomposition temperature" of a water-absorbent resin is a temperature at which molecular chains are cut by heat to lower the molecular weight. The thermal decomposition temperature is defined as a temperature at which the weight of a sample of the water-absorbent resin decreases by 5% when the sample is heated at a predetermined temperature increase rate (5 to 10°C / min) in an air atmosphere using a thermogravimetric differential thermal analyzer (TG-DTA). For example, in the present embodiment, the thermal decomposition temperature of the water-absorbent resin may be 450 to 500°C. A water-absorbent resin having a crosslinked structure and no softening point may undergo a large change in physical properties at the thermal decomposition temperature.

[0033] The heating temperature in this step may be maintained substantially constant or may vary within a range of not less than the softening point of the thermoplastic resin and not more than the thermal decomposition temperature of the water-absorbent resin. Specifically, the maximum temperature in this step is preferably not less than 150°C, more preferably not less than 200°C, and preferably not more than 450°C, more preferably not more than 300°C. The time period for maintaining the maximum temperature in this step is preferably not less than 60 minutes, more preferably not less than 120 minutes, and preferably not more than 480 minutes, more preferably not more than 300 minutes.

[0034] In this step, a portion of the material to be treated may be carbonized by performing the step under temperature conditions that promote carbonization of the material to be treated. In particular, if the material to be treated contains easily carbonized components such as cellulose, carbonized material is likely to be produced in this step. By carbonizing a portion of the material to be treated, highly brittle carbonized material is produced, which makes it easier to form fine granules in the subsequent granulation step S3.

[0035] (Granulation step S3) The heating step S2 is followed by the granulation step S3. In the granulation step S3, the material is agitated and cooled below the softening point of the thermoplastic resin to obtain granules. This step promotes homogenization, density, and weight reduction of the material, while also producing granules in a separated state. This facilitates maintaining the nutrient salt crystals precipitated during drying separate from the water-absorbent resin, etc., facilitating the recovery of the nutrient salts in the next step. In the granulation step S3 of this embodiment, for example, heating of the material by the heater 103 is stopped, while the material is agitated by the agitation shaft 102. Furthermore, in the granulation step S3 of this embodiment, it is preferable to use the air blowing mechanism 104 to blow air. This allows the temperature of the heated material in the storage section 101b to be rapidly reduced. However, the use of the air blowing mechanism 104 in this step is not essential.

[0036] During the cooling process of the thermoplastic resin softened by heat, hardening due to the drop in temperature and an increase in viscosity due to thixotropy proceed simultaneously. In the granulation step S3 of this embodiment, stirring is performed during cooling, thereby maintaining the viscosity of the thermoplastic resin at a low level due to thixotropy and promoting hardening of the thermoplastic resin. This makes it easier for the thermoplastic resin to be finely crushed to form small-sized granules without forming large, clumped granules. Furthermore, the nutrient salt crystals precipitated in the drying step S1 can also form small-sized granules. Furthermore, if highly brittle charcoal is produced in the heating step S2, the stirring in this step can also finely crush the charcoal, forming small-sized granules.

[0037] On the other hand, a water-absorbent resin that has not been thermally decomposed is less likely to be crushed than a thermoplastic resin or a carbonized material. By being stirred while being cooled in this step, the water-absorbent resin is prevented from bonding with other granules to form a mass. Therefore, by this step, the water-absorbent resin can form granules that are larger in size than the other granules.

[0038] In this way, in this step, granules of different particle sizes are formed. The granules of larger particle sizes contain a large amount of water-absorbent resin. The granules of smaller particle sizes are composed of granules composed of nutrient crystals as well as granules of thermoplastic resin, carbide, etc. In other words, in this embodiment, granules are formed in a form that is easily separated from the material to be treated, making it easier to recover nutrients from the material to be treated.

[0039] (Recovery step S4) Next, in recovery step S4, nutrients are recovered from the granules. The recovery of nutrients in this embodiment is not limited to recovering only granules composed of nutrients, but also includes recovering a portion of the granules so that the recovered granules have a higher nutrient salt content than the homogeneously mixed granules before separation. The nutrient salt content is the total content of salts containing nitrogen, phosphorus, potassium, and calcium.

[0040] In this embodiment, nutrients are preferably recovered by separating nutrients based on the particle size of the granules. In this case, granules with a relatively small particle size can be recovered from granules with a high nutrient content. In this embodiment, for example, the granules are removed from the storage section 101b of the granulation device 100, and the granules are separated by particle size using tools or devices such as a sieve or classifier. While either a dry or wet recovery method may be used, a dry recovery method is preferred because dry granules are used. In this step, a vibrating sieve is preferably used to automatically separate the water-absorbent resin with a relatively simple device configuration. Furthermore, in this step, the granules may be separated into at least two particle sizes, but may also be separated into three or more particle sizes. The nutrient recovery method is not limited to this. Other recovery methods will be described later.

[0041] When nutrients are separated based on the particle size of the granules, for example, first granules having a first particle size and second granules having a second particle size larger than the first particle size can be separated and the first granules can be recovered. The second granules are configured as water-absorbent-polymer-containing granules containing a water-absorbent polymer. The first granules are configured as low-absorbent-polymer granules containing a lower water-absorbent polymer content than the second granules and primarily containing components other than the water-absorbent polymer. The water-absorbent polymer content of the second granules is preferably 31% by mass or more, more preferably 50% by mass or more. The first granules contain one or more components selected from, for example, nutrients, thermoplastic resins, carbides, etc. The water-absorbent polymer content of the first granules is preferably 30% by mass or less, more preferably 25% by mass or less. By recovering the first granules, granules containing nutrients and having a low water-absorbent polymer content can be recovered. This allows for the production of a nutrient salt-containing material with low water absorption and hygroscopicity, which can be easily reused as fertilizer, etc. Furthermore, as will be described later, it also becomes easy to purify the nutrient salt from the first granules using an aqueous solvent.

[0042] Furthermore, in this embodiment, for example, it is preferable that the particle size distribution of the granular material after the granulation step S3 has a first peak and a second peak. The peak in the particle size distribution refers to a peak that forms an upward convex mountain-like portion when a particle size distribution graph is created with particle size on the horizontal axis and frequency (%) on the vertical axis. Among the multiple particle size distributions, two particle sizes are selected in descending order of peak height, with the smaller particle size designated as the first particle size and the other designated as the second particle size. Furthermore, "the first particle size corresponds to the first peak" means that the first particle size is set to include a particle size corresponding to the first peak. "The second particle size corresponds to the second peak" means that the second particle size is set to include a particle size corresponding to the second peak. By having a particle size distribution with two or more peaks, the first granular material can be more reliably separated from the second granular material, which contains a large amount of water-absorbent resin.

[0043] The upper limit of the particle size of the first particle size may be set based on a particle size distribution graph. Specifically, the upper limit of the particle size of the first particle size is preferably set between the particle size corresponding to the first peak and the particle size corresponding to the second peak, and more preferably set to a particle size corresponding to a downwardly convex portion of the particle size distribution graph located between the first and second peaks. The particle size distribution of the granular material can be confirmed by a known particle size distribution measurement method.

[0044] Specifically, the upper limit of the particle size of the first particle size is preferably 106 μm or more, more preferably 150 μm or more, and preferably 600 μm or less, more preferably 300 μm or less. For example, when a sieve is used in this step, the sieve opening is preferably 106 μm or more, more preferably 150 μm or more, and preferably 600 μm or less, more preferably 300 μm or less. When a sieve is used, the granules obtained in the granulation step S3 are passed through a sieve with the above openings, and the granules remaining on the sieve can be called the second granules, and the granules that passed through the sieve can be called the first granules.

[0045] Furthermore, in this step, to further increase the purity of the nutrients, the nutrients may be purified from the granules containing the recovered nutrients. In this case, for example, the granules may be added to an aqueous solvent such as water to extract the nutrients. As described above, the first granules have a low content of water-absorbent resin, and therefore are less likely to swell when an aqueous solvent is added. Therefore, using the first granules makes it easier to purify the nutrients.

[0046] [Fertilizer manufacturing method] In this embodiment, a fertilizer can be produced using the nutrient recovery method described above. Like the nutrient recovery method, the fertilizer production method according to this embodiment also includes the drying step S1, heating step S2, granulation step S3, and nutrient recovery step S4 (see FIG. 3). The fertilizer may be composed of the granules recovered in recovery step S4. Furthermore, if the nutrients are purified in recovery step S4, the fertilizer may be composed of the purified nutrients.

[0047] Alternatively, the fertilizer production method may include, in addition to steps S1 to S4, a step of adding additives to the recovered nutrients. The additives include one or more materials selected from magnesium, sulfur, iron, manganese, boron, zinc, molybdenum, copper, chlorine, and other additives that can be added to fertilizers. This allows the production of a fertilizer containing the desired components.

[0048] [Effects of this embodiment] According to this embodiment, by subjecting the material to be treated, including absorbent articles, to the drying step S1, heating step S2, and granulation step S3 described above, the material can be treated as lightweight, finely divided granules. Furthermore, by setting the temperature in the heating step S2 to a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbent resin, the water-absorbent resin becomes less likely to be crushed into small pieces in the granulation step S3, allowing the formation of second granules with larger particle sizes. This allows the first granules, which are small in particle size and comprise nutrient crystals, to be separated from the second granules, which contain a large amount of water-absorbent resin. Therefore, the nutrients and the water-absorbent resin can be efficiently separated, allowing the nutrients to be recovered in a state where they are less likely to be absorbed into the water-absorbent resin again.

[0049] As described above, the recovered granules containing nutrients have a low content of water-absorbent resin, which prevents swelling and moisture absorption due to water. Therefore, the granules themselves can be reused as fertilizer. Alternatively, it is also easy to purify nutrients from the granules using a solvent. Therefore, according to this embodiment, the nutrients contained in used absorbent articles can be effectively utilized, promoting resource circulation. Furthermore, this embodiment can form granules from which nutrients can be recovered using a dry process. As a result, this embodiment can reduce the volume required to accommodate the treated material compared to a wet process, thereby preventing the device from becoming too large. Furthermore, the dry process can prevent water pollution due to wastewater, etc.

[0050] [Additional Description of This Embodiment] When the granulation device 100 accommodates the workpiece in the drying step S1, it is preferable to accommodate a relatively large amount of workpiece in the storage section 101b. Specifically, the amount of workpiece accommodated in the storage section 101b at the start of the drying step S1 is preferably set so that the volume of the granular material in the storage section 101b at the end of the granulation step S3 exceeds the shaft portion 102a. The volume of the granular material in the storage section 101b is obtained, for example, as shown in FIGS. 1 and 2, as the vertical height L of the surface of the granular material smoothed along a horizontal plane in the storage section 101b. In this way, by determining the amount of workpiece accommodated in the storage section 101b at the start of the drying step S1 based on the state at the end of the granulation step S3, when the volume of the workpiece is at its lowest due to the progress of crushing, the shaft portion 102a is maintained submerged in the workpiece during agitation throughout the entire process of the drying step S1, the heating step S2, and the granulation step S3. This allows the workpiece to be efficiently agitated in the granulation step S3. From the perspective of efficiently agitating the workpiece, it is preferable that the volume of the granular material of the workpiece exceeds the entire shaft portion 102a in the storage unit 101b at the end of the granulation step S3, i.e., the shaft portion 102a is completely buried in the granular material of the workpiece. However, in cases such as those described above where the shaft portion 102a is tilted relative to the horizontal plane, the effect of efficiently agitating the workpiece can be achieved as long as the volume of the granular material of the workpiece exceeds at least a portion of the shaft portion 102a in the storage unit 101b at the end of the granulation step S3.

[0051] The object to be processed in this embodiment is not limited to waste including used absorbent articles, and may also include bulking materials. This ensures a sufficient amount of object to be processed in the storage section 101b. The bulking materials can be selected arbitrarily. For example, objects made of a hard, heat-resistant material such as ceramics that do not change during the drying, heating, and granulation processes (e.g., ceramic balls such as alumina balls, zirconia balls, silicon nitride balls, and silicon carbide balls) can be used as the bulking materials. By leaving such objects in the storage section 101b after the granulation step S3, they can be repeatedly used as bulking materials. In addition, any object other than absorbent articles (e.g., food waste, agricultural waste, etc.) can be used as the bulking materials. Furthermore, when carbonizing the object to be treated in the heating step S2, the carbonization of the object to be treated can be promoted by using, as a secondary material, an object whose main component is cellulose, which is easily carbonized (e.g., wood pellets, paper scraps, selected branches, etc.), or an already carbonized material (e.g., carbonized fuel pellets, activated carbon, etc.).

[0052] Furthermore, the granules of the material obtained in the granulation step S3 of the preceding granulation process can be used as a secondary material in the subsequent granulation process. In particular, when performing multiple granulation processes consecutively, by leaving at least a portion of the granules of the material in the storage section 101b after the previous granulation process in the granulation step S3 and adding new material in the drying step S1, the granules of the material obtained in the previous granulation process can be used as a secondary material. This allows for the amount of material to be kept constant in the storage section 101b, even if there is variation in the amount of material newly added in the drying step S1, by recovering the granules of the material from the storage section 101b in an amount calculated backward from the amount of waste to be newly added. This allows for stable repeated granulation processes.

[0053] Furthermore, if a portion of the material to be treated is carbonized in the heating step S2 and the first granules are recovered based on particle size in the recovery step S4, the first granules contain char in addition to nutrients. Therefore, the first granules after purifying the nutrients as described above and removing them can be used as char. This char can be effectively used as a fuel, for example, a water purification agent, building materials such as insulation, adsorbents, detoxifiers, deodorizers, raw materials for disposable hand warmers, activated carbon, etc. The char derived from the second granules has high homogeneity and a low content of water-absorbent resin, which reduces swelling and moisture absorption due to water. Therefore, the first granules can be an easily usable recycled material.

[0054] Furthermore, when a portion of the workpiece is carbonized in the heating step S2, it is preferable to fix as much of the carbon constituting the carbon-resistant thermoplastic resin as part of the charcoal. This reduces the amount of carbon dioxide emitted from the carbon constituting the thermoplastic resin. From this perspective, it is preferable to add at least one of phosphoric acid and a phosphate as a catalyst to the workpiece in the storage section 101b. Phosphoric acid and a phosphate have the effect of promoting the desorption of hydrogen, water, and other elements from the thermoplastic resin. Therefore, using phosphoric acid or a phosphate as a catalyst promotes the desorption of hydrogen, water, and other elements from the thermoplastic resin contained in the waste during the heating step S2, making it easier for the thermoplastic resin to be fixed integrally as elemental carbon on the surface of the charcoal. The phosphate added to the workpiece as a catalyst can be, for example, at least one of ammonium phosphate, potassium phosphate, and sodium phosphate. Examples of ammonium phosphate include ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and triammonium phosphate. Examples of potassium phosphate include potassium dihydrogen phosphate, dipotassium hydrogen phosphate, and tripotassium phosphate. Examples of sodium phosphate include sodium dihydrogen phosphate, disodium hydrogen phosphate, and trisodium phosphate. Phosphoric acid and phosphate salts condense upon heating to form polyphosphoric acid and polyphosphate salts, which soften in the heating step S2 and have the property of bonding fragments together during the cooling process, similar to thermoplastic resins. However, by stirring in the granulation step S3 according to this embodiment, the material to be treated remains in a granular state composed of separated fragments.

[0055] Furthermore, if the material to be treated contains two or more thermoplastic resins with different softening points, as described above, the heating temperature in heating step S2 can be set based on the softening point of any of the thermoplastic resins. Therefore, the material to be treated may contain a thermoplastic resin (e.g., polyethylene terephthalate) with a softening point higher than the maximum temperature of heating step S2. Such thermoplastic resins that do not reach their softening point in heating step S2 may not form granules after granulation step S3, but may form fibrous or string-like objects. These fibrous or string-like thermoplastic resins can be removed based on their shape differences from the granules, and can be easily separated from the granules. In other words, in this embodiment, nutrients can be recovered in recovery step S4 even if the material to be treated contains a thermoplastic resin with a high softening point.

[0056] [Other embodiments] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made within the scope of the gist of the present invention.

[0057] In the above embodiment, the recovery step S4 is described as recovering nutrients based on particle size, but this is not a limitation. For example, the recovery step S4 can also recover nutrients based on the difference in specific gravity of the granules. That is, in this example, nutrients can be recovered by utilizing the fact that the specific gravity of granules made of nutrient crystals is lighter than that of other granules. Separation based on the difference in specific gravity can be performed by either a wet method or a dry method. Alternatively, as another embodiment of the recovery step S4, nutrients can be recovered by extracting the granules obtained in the granulation step S3 with a liquid solvent. [Example]

[0058] [Examples and Comparative Examples] Examples and comparative examples of the nutrient recovery method according to this embodiment will be described. In each example and comparative example, a sample of actual used absorbent articles or a sample of material to be treated that simulated such absorbent articles was prepared. Specifically, the sample of material to be treated in each of Examples 1 and 2 and Comparative Examples 1 to 4 was prepared by absorbing 7.5 kg of tap water into 15 kg of unused disposable diapers (Merrys Pants, size L, manufactured by Kao Corporation). Furthermore, 0.6 kg of ammonium dihydrogen phosphate was added to the sample to simulate the nutrients contained in urine. Furthermore, the sample of material to be treated in each of Examples 3 and 4 and Comparative Example 5 was prepared by absorbing 15 kg of actual used disposable diapers collected from a nursery school.

[0059] The treatment samples of Examples 1 to 4 were subjected to all of the above-mentioned steps including the drying step S1, the heating step S2, the granulating step S3, and the collecting step S4. On the other hand, as shown in Table 1, at least one of the drying step S1, heating step S2, granulation step S3, and recovery step S4 was not performed on the treatment samples of Comparative Examples 1 to 5. Specifically, in Comparative Example 1, none of the steps were performed. In Comparative Example 2, only the stirring step of the drying step S1 was performed, and no other treatments were performed. In Comparative Example 3, the drying step S1 was performed, but the other steps were not performed. In Comparative Examples 4 and 5, the drying step S1, heating step S2, and granulation step S3 were performed, but the recovery step S4 was not performed.

[0060] In Examples 1 to 4 and Comparative Examples 4 and 5, the drying step S1 and heating step S2 were performed consecutively. In these steps, the temperature was increased from 20°C to 200°C at a rate of 0.5°C / min and maintained at 200°C for 180 minutes. The material to be treated was continuously stirred during these steps. In the granulation step S3 in Examples 1 to 4 and Comparative Examples 4 and 5, the temperature was decreased from 200°C to 30°C at a rate of 0.2°C / min while blowing air, and the material was cooled while stirring.

[0061] [Table 1]

[0062] [Recovery processing] The granular samples obtained after the granulation step S3 in Examples 1 to 4 were subjected to a recovery process (recovery step S4) using a sieve. In Examples 1 and 3, a sieve with 300 μm openings was used, and the granular material that passed through the sieve was recovered. In Examples 2 and 4, a sieve with 150 μm openings was used, and the granular material that passed through the sieve was recovered.

[0063] On the other hand, the sample of Comparative Example 1 maintained the shape of the diaper itself, and it was not possible to recover the nutrients. Furthermore, in the sample of Comparative Example 2, although the disposable diaper was crushed and the absorbent resin was exposed, it was not possible to recover the nutrients. In the sample of Comparative Example 3, the disposable diaper was crushed and dried, and precipitated crystals of nutrients were observed, but these precipitates adhered to the crushed disposable diaper, and it was not possible to recover the nutrients. In Comparative Examples 4 and 5, separation by sieving was not performed, and all the granules were recovered.

[0064] [Element ratio measurement] The elemental ratios of nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca) were measured for the recovered granular samples of Examples 1 to 4 and Comparative Examples 4 and 5. These elements constitute nutrients. The results are shown in Table 1. Note that since K was not contained in Examples 1 and 2 and Comparative Examples 1 to 4, which used unused diapers and ammonium dihydrogen phosphate, K could not be measured for these samples. The elemental ratios were measured as follows: Nitrogen (N) was measured using an elemental analyzer in accordance with the Japanese Industrial Standard "- Coals and cokes - Methods for elemental analysis by instrumental analyzers (JIS M 8819)." Phosphorus (P), potassium (K), and calcium (Ca) were measured using IPC optical emission spectrometry in accordance with "Refuse-derived fuels - Part 5: Testing method for metal content (JIS Z 7302-5:2002)."

[0065] The results of Examples 1 and 2 and Comparative Example 4, which used the same sample of material to be treated, were compared. As shown in Table 1, the element ratios of each element in Examples 1 and 2 were higher than those in Comparative Example 4. These results indicated that the above recovery process enabled the recovery of granules with a higher nutrient salt content than the granules before recovery. Furthermore, Example 2, which used a sieve with a smaller mesh size than Example 1, had a higher element ratio for some elements (e.g., N, Ca). This indicated that using a sieve with a mesh size of less than 300 μm, e.g., 150 μm or less, enabled the recovery of granules with a higher nutrient salt content. Similar results were obtained for Examples 3 and 4 and Comparative Example 5, which used used diapers collected from a nursery school. These results indicated that by performing the drying step S1, heating step S2, and granulation step S3, followed by the recovery step S4 using a sieve, granules with a higher nutrient salt content than the granules after the granulation step S3 could be recovered. [Explanation of symbols]

[0066] 100...granulation device 101... Treatment tank 101b...container 102...Agitator shaft 102a...Shaft part 102b...wing part 103...Heater 104...Ventilation mechanism C...Rotation axis

Claims

1. A method for recovering nutrients from a used absorbent article having a water-absorbent resin and a thermoplastic resin, comprising: A drying step of stirring and drying the object to be treated including the absorbent article; A heating step of heating the object to be treated at a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbing resin; a granulation step of stirring the material to be treated after the heating step and cooling the material to a temperature lower than the softening point of the thermoplastic resin to obtain granules; and a recovery step of recovering the nutrients from the granules. Methods for nutrient recovery.

2. In the recovery step, the nutrients are recovered by separating the nutrients based on the particle size of the granules. The method for recovering nutrients according to claim 1.

3. In the recovery step, a first granular material containing the nutrient salt and having a first particle size is separated from a second granular material containing the water absorbent resin and having a second particle size larger than the first particle size, thereby recovering the first granular material. The method for recovering nutrients according to claim 2.

4. The particle size distribution of the granular material has a first peak corresponding to the first particle size and a second peak corresponding to the second particle size. The method for recovering nutrients according to claim 3.

5. In the drying step, the absorbent article is crushed to expose the water-absorbent resin. The method for recovering nutrients according to claim 1 or 2.

6. In the heating step, a part of the object to be treated is carbonized. The method for recovering nutrients according to claim 1 or 2.

7. The material to be treated includes cellulose. The method for recovering nutrients according to claim 1 or 2.

8. A method for producing a fertilizer containing nutrients from a used absorbent article having a water-absorbent resin and a thermoplastic resin, comprising the steps of: A drying step of stirring and drying the object to be treated including the absorbent article; A heating step of heating the object to be treated at a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbing resin; a granulation step of stirring the material to be treated after the heating step and cooling the material to a temperature lower than the softening point of the thermoplastic resin to obtain granules; and a recovery step of recovering the nutrients from the granules. Fertilizer manufacturing method.

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