Method for dehydrating used sanitary products and dehydrating agent

Abstract

To solve the problem of ions derived from a calcium chloride, particularly chloride ions which facilitate generation of rust, regarding a conventional method for immersing moisture held by a high water absorption polymer in an aqueous solution including calcium ions such as a calcium chloride solution beforehand in order to reduce the moisture through mechanical dehydration such as centrifugal separation when the high water absorption polymer or a pulp is recycled from a used sanitary article such as a paper diaper.SOLUTION: Propionate ions are contained in addition to calcium ions in an aqueous solution used for immersion. A water-soluble propionate such as a calcium propionate can be used as a propionate ion supply source. Further, since generation of rust is greatly suppressed through the presence of the propionate ions even if there are chloride ions, a calcium chloride can be used as a part of a calcium ion source.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for treating used sanitary products such as used disposable diapers, and a treatment liquid used in the method. [Background technology]

[0002] There are sanitary products such as disposable diapers and urine absorption pads that contain superabsorbent polymers as their main component in order to absorb and retain large amounts of moisture.

[0003] Such sanitary products are disposable and are discarded after use, but they contain a large amount of moisture and are difficult to burn, which creates a problem of placing a heavy load on incinerators.

[0004] Meanwhile, with growing environmental awareness in recent years, there has been a movement to collect, recycle, and reuse the superabsorbent polymers and pulp that make up disposable diapers and other products. In other words, in facilities where large quantities of used sanitary products are disposed of, such as hospitals and nursing care facilities, these products are collected separately from other waste and subjected to various processes to recover the superabsorbent polymers and pulp. Even with this collection and recycling process, the large amount of water retained by superabsorbent polymers can be a hindrance. Therefore, dehydration is required for recycling, but the high water retention capacity of superabsorbent polymers makes simple mechanical processes such as squeezing and centrifugation ineffective at removing water.

[0005] To solve these problems, a method has been proposed in which used sanitary products are brought into contact with an aqueous solution containing dissolved alkaline earth metal salts prior to mechanical dehydration (e.g., Patent Document 1). Furthermore, various methods for treating used sanitary products such as disposable diapers that include this method as part of the process have been proposed (e.g., Patent Documents 2 to 5).

[0006] In these treatment methods, calcium salts, particularly calcium chloride, are generally used as the alkaline earth metal salt in view of cost, solubility in water, ease of handling, and the like. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 06-007765 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-150976 [Patent Document 3] Japanese Patent Application Publication No. 2013-198862 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-004034 [Patent Document 5] International Publication No. 2015 / 064209 Brochure Summary of the Invention [Problem to be solved by the invention]

[0008] However, the method of contacting the material with an aqueous calcium salt solution prior to mechanical dehydration has the problem of the mechanical dehydration equipment being prone to rust. Rust generation is particularly pronounced when calcium chloride, which dissociates to produce chloride ions, is used. The method of using a rust-resistant material, such as resin, for the wetted parts of the mechanical dehydration equipment still has problems in terms of cost and durability.

[0009] Therefore, an object of the present invention is to provide a method for mechanically dehydrating used sanitary products that is less likely to cause rust, even when a method with excellent dehydration efficiency, such as contacting the products with an aqueous calcium salt solution prior to the dehydration process, is adopted, even when a mechanical dehydration device made of the same material as conventional devices is used. [Means for solving the problem]

[0010] The present inventors have conducted extensive research in view of the above-mentioned problems, and as a result have found that the use of calcium propionate as a calcium salt can improve dewatering while significantly reducing the occurrence of rust. As a result of further research, they have completed the present invention.

[0011] That is, the present invention provides a method for treating used sanitary products containing a superabsorbent polymer, which comprises a step of contacting the sanitary product with an aqueous solution containing calcium ions and then mechanically dehydrating the sanitary product when the sanitary product is to be incinerated or recycled, the method comprising the steps of: The method for treating sanitary products is characterized in that the aqueous salt solution containing calcium ions is an aqueous solution that also contains propion ions.

[0012] Furthermore, through investigations by the present inventors, it was found that even in an aqueous solution containing calcium chloride, which is a chloride ion source that is particularly prone to rusting, the coexistence of propionate ions significantly suppresses the generation of rust, and therefore it is possible to suppress the generation of rust even when inexpensive calcium chloride is used as a calcium ion source.

[0013] That is, another aspect of the present invention is the method for treating sanitary products, wherein the aqueous solution containing calcium ions is an aqueous solution further containing chloride ions. [Effects of the Invention]

[0014] According to the present invention, when treating used sanitary products containing superabsorbent polymers, the efficiency of mechanical dehydration can be improved by contacting the used sanitary products with an aqueous solution containing a calcium salt, while the occurrence of rust can be significantly reduced even when a mechanical dehydration device made of the same material as conventional devices is used. Therefore, this invention is of great benefit to those who wish to efficiently dehydrate large amounts of used sanitary products. DETAILED DESCRIPTION OF THE INVENTION

[0015] In the present invention, sanitary products containing superabsorbent polymers include, but are not limited to, disposable diapers (disposable diapers), urine absorption pads, incontinence pads, feminine hygiene products, bed pads, etc., as long as they are incinerated or recycled after use (hereinafter, collectively referred to as "disposable diapers, etc.").

[0016] In the present invention, used disposable diapers, etc. are brought into contact with an aqueous solution containing calcium ions, then subjected to mechanical dehydration, and then incinerated or the superabsorbent polymer and / or pulp is recovered.

[0017] In carrying out the present invention, the step of contacting the calcium ion-containing aqueous solution, the step of mechanical dehydration, and the various steps carried out before and after these steps may be carried out by conventional methods, except that an aqueous solution containing both calcium ions and propionate ions is used as the calcium ion-containing aqueous solution. For example, in the methods described in Patent Documents 1 to 5, an aqueous solution containing both calcium ions and propionate ions may be used in place of the calcium chloride (or other polyvalent metal salt) aqueous solution.

[0018] As is well known, steel materials and the like are prone to rust when they come into contact with water, particularly with an aqueous solution containing dissolved chloride ions. In contrast, when the propionate ions used in the present invention are contained in water, rust formation is significantly suppressed compared to water that does not substantially contain chloride ions.

[0019] Propionate ions can be contained in the aqueous solution by dissolving a water-soluble propionate salt such as calcium propionate, sodium propionate, or potassium propionate. Calcium propionate is preferred because it can also provide a calcium salt.

[0020] The ease of rust formation is affected by other ions, so the amount of propionate ions that should be included cannot be determined in general, but usually, a content of 0.01 mol / kg or more will be effective, and 0.05 mol / kg or more will be reliable. Since the effect plateaus even if a large amount is added, 1.5 mol / kg or less is sufficient, and even 1.0 mol / kg or less, or even 0.5 mol / kg or less is sufficient.

[0021] In mechanical dehydration, the effect of improving water-release properties from superabsorbent polymers is mainly exerted by calcium ions, and up to a certain point, the higher the calcium ion concentration, the better the water-release properties. From this perspective, the calcium ion concentration in the aqueous solution used in the present invention is preferably 0.05 mol / kg or more, and more preferably 0.07 mol / kg or more. Although there is no particular upper limit from the viewpoint of water-release properties, taking into consideration the amount of calcium salt that can be dissolved and various effects of counter ions, a concentration of 1.0 mol / kg or less is sufficient, and sufficient performance can be obtained even at 0.5 mol / kg or less.

[0022] Even in an aqueous solution containing dissolved compounds such as calcium chloride, which act as a source of chloride ions and therefore strongly promote rust formation, if the aqueous solution also contains propionate ions, it is possible to suppress rust formation more effectively than in the case of simple water.

[0023] In general, calcium propionate is more expensive than calcium chloride. Therefore, by using inexpensive calcium chloride as part of the calcium ion supply source, it is possible to reduce raw material costs while maintaining high water-repellent properties and rust-preventive properties.

[0024] Considering the amount of propionate ions, which have an inhibitory effect on chloride ions that promote rust formation, and the cost of propionate salts, when calcium chloride is used as part of the calcium source, it is preferable to prepare the calcium chloride so that the amount of propionate ions is 0.06 to 12.0 moles per mole of calcium chloride. From the viewpoint of rust prevention, 0.1 moles or more is more preferable, and 0.6 moles or more is particularly preferable. From the viewpoint of cost, 3.0 moles or less is preferable, 2.0 moles or less is more preferable, and 1.0 mole or less is particularly preferable.

[0025] For example, when preparing an aqueous solution of the above proportions using calcium chloride, the least expensive calcium source, and calcium propionate, which also serves as a calcium source when dissolved, the amount of calcium propionate can be 5 to 1,000 parts by mass per 100 parts by mass of calcium chloride. The amount of calcium propionate is preferably 10 parts by mass or more, and particularly preferably 50 parts by mass or more. The amount is preferably 300 parts by mass or less, more preferably 200 parts by mass or less, and particularly preferably 100 parts by mass or less.

[0026] The wastewater generated by mechanical dehydration also contains various ions contained in the aqueous solution. Considering the environmental impact of the wastewater, it is undesirable for the wastewater to contain a large amount of chloride ions. Therefore, when chloride ions are contained in an aqueous solution containing calcium ions, for example, by using calcium chloride as part of the calcium source, the chloride ion concentration is preferably 0.1 to 1.0 mol / kg, more preferably 0.54 mol / kg or less, and particularly preferably 0.27 mol / kg or less. When the chlorine source is solely calcium chloride, the calcium chloride concentration in the aqueous solution is preferably 5.6% by mass or less, more preferably 3% by mass or less, and particularly preferably 1.5% by mass or less. To use calcium chloride, which is as inexpensive as possible, as a calcium ion source and to facilitate the attainment of the aforementioned calcium ion concentration, the lower limit is preferably 0.5% by mass or more, and particularly preferably 0.7% by mass or more.

[0027] In addition, from the viewpoint of the environmental impact of wastewater, it is preferable that the upper limit of the amount of propionate ions is set to the above value in order to prevent the chemical oxygen demand from becoming too high.

[0028] The aqueous solution containing calcium ions used in the present invention may contain, in addition to calcium ions, propionate ions, and optionally chloride ions derived from calcium chloride or the like, other components that may be contained in an aqueous solution used in treating disposable diapers, etc. Examples of such components include fragrances, silica particles, polyphenols, chitosan, etc.

[0029] The aqueous solution having the above composition may be prepared on-site at the site of use, or may be prepared at another location and then transported for use.

[0030] To explain this process in more detail, let us take the example of using inexpensive calcium chloride as the aqueous solution containing calcium ions and dissolving it in water with a water-soluble propionate salt. One method involves preparing a powder mixture in the aforementioned ratio, i.e., 0.06 to 12.0 moles of propionate ions per mole of calcium chloride, and then dissolving this in water at the time of use. It should be noted that in the case of a divalent metal salt such as calcium, 2 moles of propionate ions are produced from 1 mole of propionate salt.

[0031] The water used in the above method can be tap water or well water, which is relatively easy to procure in Japan, and has the advantage that only the powder or granular material needs to be transported. On the other hand, powder or granular material needs to be dissolved, and if handled by an unfamiliar worker, it tends to be difficult to work with, as it is prone to spillage when being transferred to a container.

[0032] Preparing the solution as a liquid at a factory and transporting it would eliminate the above-mentioned problems at the site of use, but would increase transportation costs due to the amount of water. Therefore, considering the relatively easy availability of water as mentioned above, one approach is to prepare a more concentrated solution than the one to be used and dilute it with water at the site of use. While higher concentrations of calcium chloride and other compounds reduce transportation costs, considering solubility limits, it is preferable to prepare an aqueous solution containing chloride ions at concentrations of 1.8 to 6.3 mol / kg and propionate ions at concentrations of 0.06 to 2.0 mol / kg. The chloride ion concentration is preferably 2.7 to 4.5 mol / kg, even more preferably 3.2 to 4.0 mol / kg, and the propionate ion concentration is preferably 0.43 to 1.5 mol / kg, even more preferably 0.75 to 1.2 mol / kg. Since 1 mole of calcium chloride produces 1 mole of calcium ions and 2 moles of chloride ions, the calcium ion concentration is usually at least 1 / 2 mole of the chloride ions.

[0033] To prepare such a concentrated aqueous solution from calcium chloride and calcium propionate, simply dissolve calcium chloride in water in an amount that results in a concentration of 10 to 35% by mass and calcium propionate in an amount that results in a concentration of 0.56 to 18.6% by mass. The upper limit of the calcium propionate content is lower than in the case of powdered or granular solutions, allowing for stable production, storage, and transportation as a homogeneous aqueous solution. Taking into consideration factors such as long-term storage and a composition that is less susceptible to salt precipitation even in cold climates, the calcium chloride concentration is preferably 15 to 25% by mass (chloride ion concentration of 2.7 to 4.5 mol / kg), more preferably 18 to 22% by mass, and the calcium propionate concentration is 4 to 14% by mass, more preferably 7 to 11% by mass, and more preferably the total content of calcium chloride and calcium propionate is 29% by mass or less. The most preferred combination is one in which the concentration of calcium chloride is 19 to 21 mass %, the concentration of calcium propionate is 7 to 9 mass %, and the total of both is 28 mass % or less.

[0034] When used, such powder or concentrated solution may be diluted with tap water or the like so that the chloride ion concentration falls within the above-mentioned range (0.1 to 1.0 mol / kg).

[0035] In the present invention, the amount of aqueous solution containing the calcium salt as described above cannot be determined in general because used disposable diapers, etc. do not necessarily have essentially the same properties. However, it should be at least an amount sufficient to immerse the entire amount of used disposable diapers, etc. to be treated (contacted) in the aqueous solution. On the other hand, the larger the amount used, the greater the amount of wastewater, and a larger container such as a tank for contacting the used disposable diapers, etc. is required. Taking these factors into consideration, the amount of aqueous solution to be used is approximately 3 to 50 L per 1 kg of used disposable diapers, etc.

[0036] The contact time between used disposable diapers and the aqueous solution containing dissolved calcium salts is approximately 10 minutes or more, but 1 hour or less is sufficient. Prior to contact, the used disposable diapers may be torn or cut. In the present invention, items that have undergone such pretreatment, such as torn or cut, are also included in the "sanitary goods" category.

[0037] The mechanical dehydration method may be any known method, such as centrifugation or squeezing, with centrifugation being particularly common.

[0038] Used disposable diapers and the like that have been mechanically dehydrated are subjected to further processing to recover (recycle) the superabsorbent polymer, pulp, etc. Any applicable method may be used for the processing after dehydration. The dehydrated material can also be incinerated. [Example]

[0039] The present invention will be further explained with reference to the following examples and comparative examples, but the present invention is not limited thereto. The methods for measuring the corrosion rate, water retention amount, and water separation rate are described below.

[0040] <Method for measuring corrosion level> 1. Preparation of test specimen: A metal plate (30 mm × 50 mm × 2 mm thick: total area 0.33 dm) was used as the test specimen. 2 Cold-rolled steel plate (SPCC) was used for the test. The test specimens were polished using sandpaper (#150 → #240 → #400) to reduce surface variations among the specimens.

[0041] Next, excess oil and dirt were removed from the surface of the test piece with acetone. After drying, the mass (w1) of the test piece was measured to the nearest 0.1 mg.

[0042] 2. Preparation of water-release promoting liquid: The water-release promoting liquid was prepared as an aqueous solution by dissolving the raw material powder in ion-exchanged water (tap water passed through a G-10D cartridge manufactured by Organo Corporation) so as to have the composition shown in Table 1. The citric acid aqueous solution was prepared by dissolving citric acid powder in ion-exchanged water to give a 5% by mass aqueous solution.

[0043] 3. Evaluation of the amount of rust formation: The polished test piece was immersed in the water-releasing agent for 1 minute, then stood upright and drained for 10 minutes. The test piece was then exposed to air for 24 hours at 25°C ± 2°C. The test piece was then immersed in a 5% by weight aqueous solution of citric acid for 15 minutes to remove the surface product. Any remaining portions were then scraped off with Kimwipes (manufactured by Nippon Paper Crecia Co., Ltd.) to simultaneously remove moisture, and the mass (w2) of the test piece was measured. The liquid temperature was kept at 25°C ± 2°C throughout all procedures.

[0044] The mass loss of the metal plate was calculated from w1 and w2 obtained in the above measurement (actual test) using the following formula (1).

[0045] Mass loss of metal plate in actual test (mg) = w1 - w2 (1)

[0046] On the other hand, in order to correct for the effect of the 5 mass % citric acid aqueous solution dissolving the (non-rusted) surface of the metal plate, the same procedure as above was carried out except that the operation of immersing the metal plate in the water-releasing agent for 1 minute was not carried out, and the masses (w3 and w4) were measured (blank test), and the mass loss of the metal plate in the blank test was calculated using the following formula (2).

[0047] Mass loss of metal plate in blank test (mg) = w3 - w4 (2)

[0048] The mass loss in each test was calculated based on the area of ​​the metal plate (0.33 dm 2 ), the amount of corrosion per unit area in both tests can be calculated, and by taking the difference, the amount of corrosion per unit area due to the influence of the water-repellent promoting liquid can be calculated. The value obtained by dividing this amount of corrosion per unit area by the number of days required (1 day) is the corrosion rate (mg / dm 2 / d: (hereinafter sometimes abbreviated as "mdd").

[0049] That is, the corrosion rate due to the influence of the water-repelling liquid can be calculated from w1 to w4 using the following formula:

[0050] Corrosion level = [(mass loss of metal plate in actual test) / (surface area of ​​metal plate) / (number of days of air exposure)] - [(mass loss of metal plate in blank test) / (surface area of ​​metal plate) / (number of days of air exposure)] =[(w1-w2)-(w3-w4)]mg / 0.33dm 2 / 1d

[0051] Measurements were carried out three times for each type of water-releasing solution, and the above values ​​were calculated for each. The average value was taken as the corrosion degree for the water-releasing solution used.

[0052] <Evaluation method for syneresis rate> 1. Measurement of initial water retention capacity 1.00 g of acrylate-based superabsorbent polymer (hereinafter referred to as "SAP", manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was placed in a 20 cm x 10 cm bag made of nylon mesh with 63 μm openings, and immersed in 1000 mL of physiological saline (salt concentration 0.9% by mass) adjusted to 25°C ± 2°C for 1 hour without stirring. After that, the bag was removed and hung for 15 minutes to drain. The nylon bag was then placed in a centrifuge (medium-sized centrifuge, H-122, manufactured by Kokusan Co., Ltd.) and centrifuged at 150 G for 90 seconds to dehydrate.

[0053] After the centrifugal dehydration process, the mass of the entire bag (w5) was measured. In addition, to correct for the effect of the amount of water held by the nylon bag itself, a nylon bag of the same size and weight was created separately, and after carrying out the same procedure as above without putting SAP in it, the mass of the nylon bag (w6) was measured. Note that these masses were measured to the nearest 0.01g. Using these values, the initial water retention capacity was calculated using the following formula. Note that "1.00" in the formula below is the mass (g) of SAP.

[0054] Initial water retention amount (g / g)=(w5-w6-1.00) / 1.00

[0055] 2. Measurement of syneresis rate Next, the nylon bag after the centrifugation was immersed for 5 minutes without stirring in 500 mL of water-release promoting solution adjusted to 25°C ± 2°C. This was then placed back into the centrifuge and centrifuged for 90 seconds at 150 G for spin-drying. The mass (w7) of the nylon bag was measured, and the water retention capacity after treatment with the water-release promoting solution was calculated using the formula below. (w8) is the mass of the nylon bag measured using the same procedure as above but without SAP.

[0056] Water retention capacity after treatment with water-releasing agent (g / g) = (w7-w8-1.00) / 1.00 Then, the water separation rate was calculated using the following formula.

[0057] Syneresis rate (%) = (1 - (water retention amount after syneresis promoting liquid treatment) / (initial water retention amount)) x 100

[0058] The measurement was carried out three times for each type of water-release promoting liquid, and the above values ​​were calculated for each, and the average value was taken as the water-release rate for the water-release promoting liquid used.

[0059] <Reference example> When the corrosion rate was measured using tap water instead of the water-repellent solution, the result was 26 mg dm -2 ·d -1 It was.

[0060] <Comparative Example 1> When tests were carried out using a 1% by mass aqueous solution of calcium chloride as a water-release promoting solution, the water-release rate was good at about 80%, as shown in Table 1, but the corrosion rate was 55 mg dm -2 ·d -1 and had deteriorated significantly.

[0061] Example 1 As shown in Table 1, a 1% by mass aqueous solution of calcium propionate was prepared as a water-release promoting solution and tests were conducted. The amount of rust produced (mg), corrosion rate (mdd), and water-release rate (%) were determined. The molar concentration of each ion and the test results are also shown in Table 1.

[0062] <Comparative Example 2> As shown in Table 1, each test was carried out using a sodium propionate aqueous solution instead of a calcium propionate aqueous solution. The molar concentrations of each ion and the results are shown in Table 1. Although the sodium salt exhibited rust-preventing properties, no improvement in water-repellency was observed.

[0063] <Examples 2 to 8> A water-releasing agent was prepared with the composition shown in Table 1 and various tests were carried out. The test results are also shown in Table 1.

[0064] [Table 1]

[0065] Reference example The stability of concentrated aqueous solutions containing calcium chloride and propionate at the following concentrations was evaluated. Aqueous solutions were prepared: (1) 29% by weight calcium chloride and 3% by weight calcium propionate, (2) 18% by weight calcium chloride and 8% by weight calcium propionate, (3) 20% by weight calcium chloride and 9% by weight sodium propionate, and (4) 20% by weight calcium chloride and 9% by weight calcium propionate. When dissolved at 50°C, a homogeneous solution was obtained for all compositions. However, composition (1) became cloudy when stored at room temperature. A slight precipitation was also observed for composition (4) at room temperature.

[0066] Example 9 The concentrated solution of the composition (2) above was diluted with water to prepare an aqueous solution containing 1 mass% calcium chloride and 0.45 mass% calcium propionate. This solution was used to evaluate the corrosion rate and water separation rate, and the results were 19 mg dm -2 ·d -1 , 80%.

Claims

1. A method for treating used sanitary products containing a superabsorbent polymer, when the sanitary products are to be incinerated or recycled, comprising the steps of contacting the sanitary products with an aqueous solution containing calcium ions and then mechanically dehydrating the sanitary products, The method for treating sanitary products as described above, wherein the aqueous solution containing calcium ions is an aqueous solution that also contains propionate ions.

2. 2. The method for treating sanitary products according to claim 1, wherein the aqueous solution containing calcium ions further contains chloride ions.

3. 3. The aqueous solution used in the method for treating sanitary products according to claim 2, which can be obtained by dissolving calcium chloride and a water-soluble propionate in water in an amount such that the amount of propionate ions is 0.06 to 12.0 moles per mole of calcium chloride.

4. 4. A powdery or granular mixture of calcium chloride and a water-soluble propionate for dissolving in water to prepare the aqueous solution of claim 3, wherein the water-soluble propionate is mixed in a proportion such that the propionic acid residue moiety constituting the water-soluble propionate is equivalent to 0.06 to 12.0 moles per mole of calcium chloride.

5. an aqueous solution containing chloride ions at a concentration of 1.8 to 6.3 mol / kg, propionate ions at a concentration of 0.06 to 2.0 mol / kg, and calcium ions at a molar concentration of at least 1 / 2 of the chloride ions; 3. A concentrated aqueous solution which is diluted to a concentration of the chloride ions of 0.1 to 1.0 mol / kg and used as the aqueous solution containing calcium ions in the method for treating sanitary products according to claim 2.

6. 6. The method for producing a concentrated aqueous solution according to claim 5, wherein calcium chloride is dissolved in water in an amount to give a concentration of 10 to 35% by mass and calcium propionate in an amount to give a concentration of 0.56 to 18.6% by mass.

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