How to recycle waste gypsum

Abstract

To provide a recycling method for waste gypsum that can produce type II anhydrous gypsum from waste gypsum contained, e.g., in a waste gypsum board at a temperature lower than that in conventional methods, and can separate paper and other materials with a small number of steps.SOLUTION: A method for recycling waste gypsum, in which an aqueous medium is added to waste gypsum containing paper and the like, and the mixture is heated and hydrothermally treated at a temperature of 140°C or higher, and then the water is separated to recover type II anhydrous gypsum. In cases where the hydrothermal treatment is performed for 5 hours or more, the temperature of the treatment is preferably 160°C or higher and 180°C or lower. When the hydrothermal treatment is performed for 24 hours or more, its temperature is preferably 140°C or higher and 160°C or lower.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for recycling waste gypsum, which can efficiently produce type II anhydrite from waste gypsum containing paper and the like. [Background technology]

[0002] Patent Documents 1 to 3 disclose, for example, the prior art relating to methods for producing gypsum and anhydrous gypsum from waste gypsum.

[0003] Patent Document 1 discloses a method for obtaining a gypsum-tobermorite material by heat-treating waste gypsum board at a temperature of 200°C to 1200°C to obtain anhydrous gypsum, and then hydrothermally treating the solidified material at a temperature of 120°C to 350°C to obtain a gypsum-tobermorite material, as an effective method for processing rubble, including waste building materials, most of which have traditionally been treated as waste, without using any means to separate the required materials, and turning the processed material itself into a useful composition that can be reused, using a simple and easy process.

[0004] Patent Document 2 discloses a method for producing anhydrous gypsum, in which waste gypsum is supplied toward a combustion flame in a heating step and heated, as a method for efficiently producing anhydrous gypsum from waste gypsum while sufficiently suppressing the generation of SOx.

[0005] Patent Document 3 discloses a method for regenerating gypsum from waste gypsum boards, in which the gypsum in the waste gypsum boards is crushed and then dissolved and reprecipitated in an aqueous medium containing gypsum seed crystals. When type II anhydrous gypsum is regenerated, the method describes stirring the gypsum in an aqueous medium containing type II anhydrous gypsum as seed crystals at a temperature of preferably 110°C or higher, more preferably 140 to 200°C, for preferably 0.2 to 6 hours, more preferably 0.5 to 2 hours.

[0006] According to Non-Patent Document 1, when gypsum dihydrate is dehydrated and decomposed, hemihydrate is generally produced at 140°C or higher, and type III anhydrous gypsum is produced by heating at 180°C or higher in the atmosphere. The temperature at which type III anhydrous is produced increases due to water vapor pressure, and type III anhydrous is produced by heating at about 215°C in saturated steam. Type II anhydrous is produced from type III anhydrous at 330°C or higher. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2019-006672 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-149619 [Patent Document 3] Patent No. 5553491 [Non-patent literature]

[0008] [Non-Patent Document 1] "Cement, Gypsum, and Lime Handbook," edited by the Society of Inorganic Materials, Gihodo Publishing, November 1995 Summary of the Invention [Problem to be solved by the invention]

[0009] In the invention described in Patent Document 1, the temperature in the heating process is set to between 200°C and 1200°C, both inclusive, which is an extremely high temperature, in order to decompose the paper contained in the waste gypsum board. Thus, conventional methods require elevated temperatures to produce anhydrous gypsum from the waste gypsum and to decompose the paper contained in the waste gypsum.

[0010] In addition, the invention described in Patent Document 2 has the effect of increasing the temperature rise rate of the waste gypsum by supplying the waste gypsum toward the combustion flame, but such a heating process requires an apparatus that has a burner and can supply the waste gypsum toward the combustion flame from the burner.

[0011] The waste gypsum provided in the invention described in Patent Document 3 is gypsum dihydrate obtained from gypsum boats, and is preferably crushed to an appropriate particle size and the board base paper removed. However, because crushing and grinding alone do not allow for complete separation of the gypsum and paper from the waste gypsum, a process is required to produce the waste gypsum used in the method described in Patent Document 3. Furthermore, when regenerating type II anhydrous gypsum, type II anhydrous gypsum is required as seed crystals.

[0012] The present invention aims to provide a method for recycling waste gypsum, which can produce type II anhydrous gypsum from waste gypsum such as waste gypsum board at a lower temperature than conventional methods and can separate paper and other materials with fewer manufacturing steps. [Means for solving the problem]

[0013] The method for recycling waste gypsum of the present invention involves adding water to waste gypsum obtained from waste gypsum boards, heating the waste gypsum, subjecting it to hydrothermal treatment at a temperature of 140°C or higher and 160°C or lower for 24 hours or longer, cooling the waste gypsum, filtering the waste gypsum to separate the residue from the water, and drying the residue. D99 is 13.6μm or less The method is characterized by recovering type II anhydrous gypsum.

[0014] By hydrothermally treating waste gypsum containing paper, etc., organic acids are produced from the paper contained in the waste gypsum, and it is thought that the organic acids produced react with the gypsum dihydrate contained in the waste gypsum to produce type II anhydrous gypsum at low temperatures.

[0015] In the method for recycling waste gypsum of the present invention, the temperature of the hydrothermal treatment is 140° C. or higher. By performing hydrothermal treatment at a temperature of 140° C. or higher for 24 hours or longer, it is possible to obtain type II anhydrous gypsum that is substantially free of gypsum dihydrate and gypsum hemihydrate.

[0016] Furthermore, in the method for recycling waste gypsum of the present invention, type II anhydrous gypsum that is substantially free of gypsum dihydrate and gypsum hemihydrate can also be obtained by carrying out hydrothermal treatment at a temperature of 160°C or higher for 5 hours or more.

[0017] When hydrothermal treatment is carried out for 5 hours or more, type II anhydrous gypsum can be sufficiently recovered at 180°C or less. Therefore, from the standpoint of efficiency and economy, the temperature of the hydrothermal treatment is preferably 160°C or more and 180°C or less.

[0018] Furthermore, when hydrothermal treatment is carried out for 24 hours or more, type II anhydrous gypsum can be sufficiently recovered at a temperature of 140°C or higher and 160°C or lower, so from the standpoint of efficiency and economy, the temperature of the hydrothermal treatment is preferably 140°C or higher and 160°C or lower. [Effects of the Invention]

[0019] According to the method for recycling waste gypsum of the present invention, waste gypsum containing paper and the like is subjected to hydrothermal treatment at a relatively low temperature, whereby type II anhydrous gypsum can be easily produced with a small number of steps. DETAILED DESCRIPTION OF THE INVENTION

[0020] Tests conducted to confirm the effects of the present invention will now be described.

[0021] Example 1 30g of waste gypsum and 120g of distilled water were placed in a 300ml heat-resistant container and heated for a given time and temperature. After heating, the container was confirmed to have cooled sufficiently, and the sample was separated by filtration. The residue on the filter paper was then dried in a dryer at 45°C and collected. X-ray diffraction analysis was performed on the collected sample, and the formation of Type II anhydrous gypsum was confirmed.

[0022] The test conditions and results are shown in Table 1.

[0023] [Table 1]

[0024] It was confirmed that type II anhydrous gypsum was synthesized from dihydrate gypsum by hydrothermal treatment. In Table 1, ◎ indicates only type II anhydrous gypsum, and × indicates the presence of type II anhydrous, gypsum dihydrate, and gypsum hemihydrate.

[0025] From Table 1, it was confirmed that in No. 1, No. 6, No. 9, and No. 10, only Type II anhydrite was produced from the waste gypsum by hydrothermal treatment.

[0026] Comparing Nos. 1 to 6, which were heated for 24 hours, type II anhydrous was not produced exclusively at heating temperatures of 100°C, 120°C, 130°C, and 135°C, but type II anhydrous was produced exclusively at 140°C and 180°C. These results demonstrate that reaction at 140°C or higher for 24 hours can produce essentially only type II anhydrous.

[0027] Comparing Nos. 7 to 12, which were heated for 6 hours, type II anhydrous was not produced exclusively at heating temperatures of 120°C, 140°C, 150°C, and 155°C, but type II anhydrous was produced exclusively at 160°C and 180°C. These results demonstrate that reaction at 160°C or higher for 6 hours can produce essentially only type II anhydrous.

[0028] The pH of the filtrate from No. 1, which essentially synthesized only type II anhydrous gypsum, was 6.4, Nos. 6 and 9 were 6.5, and No. 10 was 6.4, indicating that they were nearly neutral.

[0029] Furthermore, looking at the particle size distribution results, the cumulative 50% volume particle diameter (D50) of No. 1 was 3.6 μm, and the cumulative 99% volume particle diameter (D99) was 10.9 μm. The D50 of No. 6 was 4.3 μm, and the D99 was 14.0 μm. The D50 of No. 9 was 3.7 μm, and the D99 was 11.5 μm. The D50 of No. 10 was 3.9 μm, and the D99 was 13.6 μm. From the above, it can be seen that the particle size distribution of the synthesized Type II anhydrite is fine, and there is a possibility that it can be effectively utilized as high-quality Type II anhydrite. Therefore, it was confirmed that this is an effective method for recycling waste gypsum.

[0030] Example 2 The hydrothermal synthesis apparatus used was a portable reactor (TRP1-VS2-500) manufactured by Taiatsu Glass Industries Co., Ltd. The sample and water were placed in a pressure-resistant container, sealed, and the hydrothermal reaction was carried out at the desired temperature and time while stirring the contents. The reaction started when the target temperature was reached (approximately 20 minutes after heating began).

[0031] When the reaction was complete, the internal pressure was released (valve opened) into a trap container filled with cooling water to rapidly cool the pressure vessel. However, if the pressure was not released, the vessel was left to cool until the temperature had completely dropped. After confirming that the temperature inside the vessel had completely dropped (approximately 2 hours after the reaction completion time until it had dropped to about 30°C), the sample was removed and separated into a residue and filtrate by suction filtration.

[0032] The residue was dried in a dryer at 45°C, and after recovery, it was measured by XRD and particle size distribution analyzer. The test conditions and results are shown in Table 2.

[0033] [Table 2]

[0034] It was confirmed that type II anhydrous gypsum was synthesized from gypsum dihydrate through hydrothermal treatment. In Table 2, ◎ indicates only type II anhydrous gypsum, × indicates when type II anhydrous gypsum, gypsum dihydrate, and gypsum hemihydrate are included, and △ indicates when hemihydrate and type II anhydrous gypsum are included.

[0035] From Table 2, it was confirmed that in Nos. 1, 2, 5 to 10, only Type II anhydrite was produced from the waste gypsum by hydrothermal treatment.

[0036] Comparing Nos. 1 to 3, in which the hydrothermal treatment holding time was varied between 4, 5, and 6 hours, No. 3, which had a holding time of 4 hours, did not produce only Type II anhydrous gypsum, but Nos. 1 and 2 produced only Type II anhydrous gypsum. These results show that by reacting at 160°C or higher for 5 hours or more, it is possible to produce essentially only Type II anhydrous gypsum.

[0037] Comparing Nos. 4 to 6, in which the stirring speed was changed, No. 4, which had a low stirring speed of 70 rpm, resulted in not only Type II anhydrous but also hemihydrate and Type II anhydrous, but Nos. 5 and 6, which had a stirring speed of 300 rpm or more, were able to produce only Type II anhydrous. These results show that if the stirring speed is significantly low, it is not possible to produce only Type II anhydrous.

[0038] In Nos. 7 to 10, the ratio of sample to solvent was changed, but in all cases only Type II anhydrous gypsum was obtained.

[0039] Furthermore, looking at the particle size distribution results for Nos. 1, 2, and 5-10, which were able to produce only Type II anhydrite, the cumulative 50% volume particle diameter (D50) of No. 1 was 5.3 μm and the cumulative 95% volume particle diameter (D95) was 13.0 μm. The cumulative 50% volume particle diameter (D50) of No. 2 was 4.8 μm and the cumulative 95% volume particle diameter (D95) was 11.9 μm.

[0040] The particle diameter at 50% cumulative volume (D50) of No. 5 was 5.0 μm and the particle diameter at 95% cumulative volume (D95) was 9.6 μm. The particle diameter at 50% cumulative volume (D50) of No. 6 was 4.8 μm and the particle diameter at 95% cumulative volume (D95) was 8.9 μm.

[0041] The particle diameter at 50% cumulative volume (D50) of No. 7 was 4.5 μm and the particle diameter at 95% cumulative volume (D95) was 8.7 μm. The particle diameter at 50% cumulative volume (D50) of No. 8 was 5.0 μm and the particle diameter at 95% cumulative volume (D95) was 9.6 μm.

[0042] The particle diameter at 50% cumulative volume (D50) of No. 9 was 4.5 μm and the particle diameter at 95% cumulative volume (D95) was 9.7 μm. The particle diameter at 50% cumulative volume (D50) of No. 10 was 4.9 μm and the particle diameter at 95% cumulative volume (D95) was 11.2 μm.

[0043] From the above, it was found that the particle size distribution of the synthesized Type II anhydrous gypsum was fine, and it has the potential to be effectively utilized as high-quality Type II anhydrous gypsum. Therefore, it was confirmed that this method is effective as a method for recycling waste gypsum.

Claims

1. This method for recycling waste gypsum comprises adding water to waste gypsum obtained from waste gypsum board, heating the mixture, subjecting it to hydrothermal treatment at a temperature of 140°C or higher and 160°C or lower for 24 hours or longer, cooling the mixture, filtering the mixture to separate the residue and water, and drying the residue to recover type II anhydrous gypsum having a D99 of 13.6 μm or less.

2. This method for recycling waste gypsum comprises adding water to waste gypsum obtained from waste gypsum board, heating the mixture, subjecting the mixture to hydrothermal treatment at a temperature of 160°C or higher and 180°C or lower for 5 hours or longer, cooling the mixture, filtering the mixture to separate the residue and water, and drying the residue to recover type II anhydrous gypsum having a D99 of 13.6 μm or less.

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