Calcination method for gypsum particles

Abstract

The present invention, when gypsum particles derived from waste gypsum boards are calcined using a hot-air rotary calcinator, makes the calcined gypsum particles have a reduced gypsum dihydrate content. Gypsum particles obtained by crushing waste gypsum boards and made of gypsum dihydrate are calcined. The gypsum particles are supplied to a cylindrical rotating tank, and the gypsum particles are stirred by the rotation of the tank. Hot air is supplied to a pipe positioned at the axial-direction center of the tank and provided with a plurality of nozzles, and is blown from the nozzles into the gypsum particles, thereby converting the gypsum dihydrate in the gypsum particles into gypsum hemihydrate and / or III-form anhydrous gypsum. The peripheral speed of the inner wall of the rotating tank is in the range of 2-10 m / min.　

Description

Method for calcining gypsum granules

[0001] The present invention relates to a method for calcining gypsum granules, and more particularly to a method for calcining gypsum granules obtained by crushing waste gypsum boards.

[0002] The inventors have developed a technology for recovering gypsum dihydrate and other gypsum materials from waste gypsum board. To recover gypsum, the waste gypsum board is crushed, sieved into gypsum blocks and paper fragments, and then metals are removed using a magnetic separator or similar device. The gypsum is then finely crushed to adjust the particle size of the gypsum. The crushed gypsum particles are stored in a silo or similar device and heated to, for example, 110°C to 140°C in a calciner, converting the gypsum dihydrate into hemihydrate and / or anhydrous gypsum III. While there are other types of anhydrous gypsum, such as type I and type II, these have low reactivity with water and are not suitable for processes that utilize crystallization. Reacting hemihydrate and / or anhydrous gypsum III with water to crystallize gypsum dihydrate and other materials from the water allows for the recovery of reusable gypsum powder.

[0003] In Patent Document 1 (JP2023-36205A), the inventor proposed a fluidized bed calciner suitable for calcining gypsum granules derived from waste gypsum board. In this fluidized bed calciner, a dam plate is not provided at the outlet of the fluidized bed, and the gypsum granules are discharged from the outlet using a rotary valve or the like. The gypsum granules are also introduced into the inlet of the fluidized bed using a rotary valve or the like. The amount of gypsum granules introduced and discharged is controlled to maintain a constant amount of gypsum granules in the fluidized bed. The width of the top of the fluidized bed (the width perpendicular to the horizontal direction connecting the inlet and outlet) is also increased to facilitate the settling of gypsum fines dispersed by hot air.

[0004] The inventors investigated the use of a hot air rotary dryer to calcinate gypsum granules derived from waste gypsum boards. A hot air rotary dryer is equipped with a rotating tank and a fixed nozzle that blows hot air onto the granules in the tank, heating the granules by blowing hot air from the nozzle. The rotation of the tank then agitates the granules, heating them evenly.

[0005] JP2023-36205A

[0006] In test runs, gypsum granules consisting of gypsum dihydrate and anhydrous type III gypsum could be extracted without any problems at the normal rotation speed. However, about five days after the start of actual operation, the proportion of gypsum dihydrate in the gypsum granules after calcination increased, making the gypsum granules unsuitable for crystallization. The inventors assumed that the reason gypsum dihydrate was mixed into the gypsum granules after calcination was because the gypsum granules were not being stirred sufficiently, and increased the rotation speed of the tank. As a result, the gypsum dihydrate content in the gypsum granules after calcination further increased.

[0007] An object of the present invention is to reduce the content of gypsum dihydrate in gypsum granules after calcination when gypsum granules derived from waste gypsum boards are calcined using a hot air rotary calciner (hot air rotary dryer).An auxiliary object of the present invention is to eliminate the need to stop the calciner and remove foreign matter such as metal and gravel that has become mixed in the gypsum granules when calcining gypsum granules derived from waste gypsum boards.

[0008] This invention is a method for calcining gypsum granules made of gypsum dihydrate obtained by crushing waste gypsum board, which comprises supplying the gypsum granules to a rotating cylindrical tank and agitating the gypsum granules by rotating the tank, supplying hot air to a pipe equipped with multiple nozzles located at the axial center of the tank, and blowing the hot air from the nozzles into the gypsum granules to convert the gypsum dihydrate in the gypsum granules into gypsum hemihydrate and / or anhydrous type III gypsum, and further setting the peripheral speed of the inner wall of the rotating tank to between 2 m / min and 10 m / min.

[0009] Preferably, the peripheral speed of the inner wall is set to 3 m / min or more and 9 m / min or less.

[0010] The experiment was conducted under conditions of an inner diameter of the tank of 2.4 m and a length of 6 m, so preferably the inner diameter of the tank at the inner wall position is 1.5 m or more and 4 m or less, and the axial length of the tank is 4 m or more and 8 m or less.

[0011] Preferably, the temperature of the hot air is set to 250°C or higher and 350°C or lower. If the temperature exceeds 350°C, anhydrous type II gypsum may be produced, and if the temperature of the hot air is lower than 250°C, it is inefficient. The amount of hot air blown in is set to 1500 to 2100 Nm per ton of gypsum granules in order to sufficiently convert the gypsum granules into hemihydrate and / or anhydrous type III gypsum. ３ is preferred.

[0012] In this invention, the gypsum granules are agitated by rotating the tank, and hot air is blown into the gypsum granules from a nozzle to convert the gypsum dihydrate into gypsum hemihydrate and / or anhydrous gypsum III. In this case, if the tank peripheral speed is not selected appropriately, gypsum dihydrate will be mixed into the gypsum after calcination (Table 1). The mixing of gypsum dihydrate occurs when the tank peripheral speed is excessively high, indicating that increasing the peripheral speed to strengthen agitation actually makes gypsum dihydrate more likely to remain. Furthermore, when the inside of the tank was observed after operation under conditions that resulted in gypsum dihydrate remaining, it was found that gypsum granules with relatively large particle sizes remained in the tank. Furthermore, the gypsum dihydrate in the gypsum after calcination was mainly in the form of fine powder. These results indicate that if agitation is too strong, the fine gypsum dihydrate powder will migrate to the tank outlet before being fully calcined and will be discharged as gypsum dihydrate.

[0013] In this invention, by appropriately setting the peripheral speed of the inner wall of the tank, the content of gypsum dihydrate in the gypsum granules after calcination can be kept low. Furthermore, when a rotating tank is used, foreign matter contained in the gypsum granules, such as metals and gravel, is tumbled by the rotation of the tank and discharged from the outlet. Therefore, there is no need to periodically stop calcination and clean the inside of the tank.

[0014] A process diagram of a method for recovering gypsum from waste gypsum boards in the examples. A cross-sectional view of a hot air rotary calciner used in the examples, taken along a vertical plane perpendicular to the axial direction. A vertical cross-sectional view of the hot air rotary calciner in FIG. 2.

[0015] Examples for carrying out the present invention are shown below. The scope of the present invention should be determined based on the claims, taking into account the description in the specification and well-known techniques in this field, and in accordance with the understanding of those skilled in the art. The scope of the present invention is not limited by the examples.

[0016] Figures 1 to 3 show an example. Figure 1 shows the process of the example. A crusher 2 crushes waste gypsum board into gypsum blocks and paper chips. A sieve 4, such as a vibrating sieve, separates the gypsum blocks from the paper chips, and a magnetic separator 6 separates metal pieces such as nails. A crusher 8 finely crushes the gypsum blocks into gypsum granules, which are then stored in a silo 10. If necessary, foreign matter such as gravel and sand is further separated, and the paper chips are further separated from the gypsum granules. The crushed gypsum granules have a wide particle size distribution, ranging from particles of a few millimeters to particles on the order of μm, which corresponds to the thickness of the needle-like crystals of gypsum dihydrate in the waste gypsum board.

[0017] The hot air rotary calciner 12 heats the gypsum granules to a temperature of, for example, 110°C to 140°C, converting the gypsum dihydrate into gypsum hemihydrate and / or anhydrous gypsum III. In this specification, ranges indicated by symbols such as "to" are intended to include both the upper and lower limits. The calcined gypsum granules are mixed with a gypsum slurry or the like in a mixing tank 14, and gypsum dihydrate, for example, is precipitated in a crystallization tank 16. The form of the precipitated gypsum can be controlled by the slurry temperature; for example, gypsum hemihydrate may be precipitated. The slurry from which gypsum dihydrate has precipitated is treated with a sieve 18, such as a vibrating sieve, to further remove paper fragments and the like. The slurry is then filtered with a filter 20 to separate gypsum powder such as gypsum dihydrate. The filtrate is returned to, for example, the mixing tank 14, the crystallization tank 16, or the like.

[0018] 2 and 3 show a hot air rotary calciner 12 (hereinafter referred to as "calciner 12"). Reference numeral 22 denotes a cylindrical rotary tank, 23 denotes its inner wall, and the space surrounded by the inner wall 23 is a calcination space 24. A fixed pipe 25 penetrates the center of the calcination space 24, and nozzles 26 are arranged in one to four rows parallel to the axial direction of the pipe 25, for example, and hot air supplied from the pipe 25 is sent into gypsum granules 29. A plurality of ribs 28 are provided from the inner wall 23 toward the calcination space 24, and a dam plate 30 and a movable plate 32 are provided on the gypsum granule outlet side to regulate the discharge of the gypsum granules. The ribs 28 and dam plate 30 are not necessarily provided.

[0019] The rotary vessel 22 is rotated by, for example, a chain 40, 42 is a sprocket that rotates the chain 40, 44 is a motor, and 46 is a bearing that supports the rotary vessel 22. The rotary vessel 22 may be rotated by any mechanism.

[0020] As shown in Fig. 3, hot air is supplied from a hot air inlet 50 at one end of a pipe 25, and gypsum particles are supplied, for example, from a gypsum inlet 56 at the other end of the pipe 25 through a hole 57 by a screw conveyor or the like (not shown). For example, a waste air outlet 52 is provided above a fixed part 51 on the hot air inlet 50 side to discharge the waste air that has exchanged heat with the gypsum particles, and a gypsum outlet 54 is provided below to discharge the gypsum particles after calcination. The waste air contains fine gypsum particles (mainly gypsum dihydrate), which are collected by a bag filter or the like and returned to the calcination space 25 together with the hot air or the like. The discharged gypsum particles are also supplied to the mixing tank 14 of Fig. 1 by a belt conveyor or the like.

[0021] Returning to Figure 2, the operation of the calciner 12 is shown. The rotation of the rotary tank 22 agitates the gypsum layer 29 inside the tank. As such, it is the peripheral speed of the inner wall that determines the agitating force on the gypsum granules in the tank 22, not the rotation speed of the tank 22 itself. The hot air from the nozzles 29 heats the gypsum granules 29 and also contributes to agitating the gypsum granules 29. As the rotary tank 22 rotates, the ribs 28 lift the gypsum granules 29 and drop them from a slightly higher position. This contributes slightly to agitating the gypsum granules 29. The weir plate 30 regulates the amount of gypsum granules in the calcination space 24.

[0022] The temperature of the hot air (measured at the hot air inlet 50, for example) is, for example, 250 to 350°C, and in this embodiment, it is set to 300°C. The amount of hot air blown is, for example, 1500 to 2100 Nm per ton / hr of input amount of gypsum granules 29. ３ / hr (Nm ３ is the volume at 0°C and 1 atmosphere), and in the example, it is 1800 Nm ３ / hr. The rotary tank 22 has a diameter at the inner wall 23 of, for example, 1.5 m to 4 m, preferably 2 m to 3 m, and in this embodiment, it was 2.4 m. The axial length of the rotary tank 22 from the weir plate 30 to the end on the gypsum inlet 56 side is, for example, 4 m to 8 m, and in this embodiment, it was 6 m. The target heating temperature of the gypsum granules 29 at a position closer to the weir plate 30 is, for example, 110 to 140°C, and in this embodiment, it is about 130°C, and it is preferable that the gypsum granules remain in the calcination space 24 for, for example, 20 to 40 minutes on average. The supply rate of the waste gypsum granules is, for example, 1.0 ton / hr to 10 ton / hr depending on the size of the tank 22, and in this embodiment, it was 2.5 ton / hr.

[0023] The supply rate of waste gypsum granules was fixed at 2.5 ton / hr, and the supply rate of 300°C hot air was set at 4500 Nm ３ / hr. The rotation speed of the rotary tank 22 was changed, and the peripheral speed of the inner wall 23 was varied in the range of 1 m / min to 22.5 m / min. Other conditions were the same as above. Table 1 shows the content of gypsum dihydrate in the gypsum granules after calcination at the gypsum outlet 54 on the fifth day after the start of operation at each peripheral speed. Table 1 also shows the amount of fine powder in the gypsum granules when the gypsum granules after calcination were observed with an optical microscope.

[0024] Table 1 Rotational speed Peripheral speed Gypsum dihydrate content Amount of fine powder (rot / min) (m / min) (mass%) Example 1 0.8 6 Almost 0 Very small Example 2 1.2 9 Almost 0 Very small Example 3 0.4 3 Almost 0 Very small Comparative Example 1 2 15 15% Large Comparative Example 2 3 22.5 70% Large Comparative Example 3 4 / 30 1 5% Small

[0025] The gypsum dihydrate content after calcination depended on the peripheral speed of the inner wall 23. At peripheral speeds of 3 m / min to 9 m / min, almost no gypsum dihydrate was present, but the gypsum dihydrate content increased when the peripheral speed was excessively high. The peripheral speed is the speed at which the inner wall 23 lifts the gypsum particles 29 and represents the strength of agitation of the gypsum particles 29. Observation of the interior of the rotary tank 22 after five days of operation under the conditions of Comparative Example 2 revealed a large amount of gypsum particles with relatively large particle sizes, approximately 3 to 6 mm, remaining. Considering this and the extremely high gypsum dihydrate content after calcination in Comparative Example 2, it can be assumed that in Comparative Example 2, gypsum particles with large particle sizes were not discharged from the rotary tank 22, and gypsum particles newly supplied to the rotary tank 22 were discharged as insufficiently calcined gypsum dihydrate. The mechanism by which large gypsum particles remain in the tank due to excessive agitation is unknown. Comparative Example 3 is an example in which the peripheral speed was excessively low, and shows that a part of the gypsum dihydrate was discharged as it was due to insufficient stirring.

[0026] From Table 1, it can be seen that the peripheral speed of the inner wall is preferably 2 m / min to 10 m / min, and particularly preferably 3 m / min to 9 m / min.

[0027] The calciner 12 is originally intended for drying food waste, coal, coffee grounds, chips, etc. In the calciner 12, the rotation of the tank 22 causes the gypsum granules to roll, so heavy foreign matter such as metal pieces and gravel also moves to the gypsum outlet 54 and is discharged. This eliminates the need to periodically stop the calciner 12 and clean the tank 22. In contrast, in a fluidized-tank calciner, metals and gravel are not discharged from the fluidized-tank, so calcination must be periodically stopped and the inside of the tank cleaned. Note that, although hot air is supplied from one end of the rotary tank 22 in the embodiment, it may be supplied from both ends.

[0028] In the examples, the factor determining the amount of gypsum dihydrate in the gypsum particles after calcination is the peripheral speed of the inner wall of the vessel, and other factors can be determined appropriately.

[0029] 2 Crusher 4 Sieve 6 Magnetic separator 8 Pulverizer 10 Silo 12 Hot air rotary calciner 14 Mixing tank 16 Crystallization tank 18 Sieve 20 Filter 22 Rotary tank 23 Inner wall 24 Calcination space 25 Pipe 26 Nozzle 29 Gypsum granules 28 Ridge 30 Dam plate 32 Movable plate 40 Chain 42 Sprocket 44 Motor 46 Bearing 50 Hot air inlet 51 Fixed part 52 Waste air outlet 54 Gypsum outlet 56 Gypsum inlet 57 Hole

Claims

1. A method for calcining gypsum granules made of gypsum dihydrate obtained by crushing waste gypsum board, comprising: supplying the gypsum granules to a rotating cylindrical tank; agitating the gypsum granules by rotating the tank; supplying hot air to a pipe equipped with a plurality of nozzles located at the axial center of the tank; and blowing the hot air from the nozzles into the gypsum granules to convert the gypsum dihydrate in the gypsum granules into gypsum hemihydrate and / or anhydrous type III gypsum; and setting the peripheral speed of the inner wall of the rotating tank to between 2 m / min and 10 m / min.

2. The method for calcining gypsum granules according to claim 1, characterized in that the peripheral speed of the inner wall is set to 3 m / min or more and 9 m / min or less.

3. The method for calcining gypsum granules according to claim 1, characterized in that the inner diameter of the vessel at the position of the inner wall is 1.5 m or more and 4 m or less, and the axial length of the vessel is 4 m or more and 8 m or less.

4. The temperature of the hot air is 250°C or more and 350°C or less, and the amount of hot air blown is 1500 to 2100 Nm per ton of the gypsum granules. ３ 4. The method for calcining gypsum granules according to claim 3, wherein the following is performed:

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