Gypsum grain calcining method

Abstract

To reduce a gypsum dihydrate content in gypsum grains after calcining when calcining the gypsum grains derived from a waste gypsum board by using a hot air rotary calcining machine.CONSTITUTION: Gypsum grains comprising gypsum dihydrate made from crushed waste gypsum board is calcined. The gypsum grains are supplied into a rotating cylindrical tank, and are stirred by rotation of the tank. Hot air is supplied into a pipe which is positioned at an axial direction center part of the tank and comprises a plurality of nozzles and hot air is blown into the gypsum grains from the nozzles, thereby changing the gypsum dihydrate in the gypsum grains to gypsum hemihydrate or anhydrous III type gypsum. A circumferential speed of an inner wall by rotation of the tank is so made as to be 2 m / min. or more and 10 m / min. or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[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. [Background technology]

[0002] The inventors have developed a technology for recovering gypsum dihydrate and other gypsum from waste gypsum board. To recover gypsum, the waste gypsum board is crushed, separated into gypsum blocks and paper fragments using a sieve, metals removed using a magnetic separator, and the gypsum is finely crushed to adjust its particle size. The crushed gypsum particles are stored in a silo or other container and heated to, for example, 110°C to 140°C in a calciner to convert gypsum dihydrate to 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 gypsum can recover reusable gypsum powder.

[0003] In Patent Document 1 (JP2023-36205A), the inventors 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 installed 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 scattered by the 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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP2023-36205A Summary of the Invention [Problem to be solved by the invention]

[0006] During test operation, gypsum granules consisting of gypsum dihydrate and anhydrous type III gypsum were successfully extracted 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 suspected 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 gypsum dihydrate content in the gypsum granules after calcination when the gypsum granules derived from waste gypsum board are calcined using a hot air rotary calciner (hot air rotary dryer). An auxiliary object of this invention is to eliminate the need to stop the calciner and remove foreign matter such as metals and gravel that has become mixed in with the gypsum granules when calcining gypsum granules derived from waste gypsum boards. [Means for solving the problem]

[0008] This invention relates to a method for calcining gypsum granules made of gypsum dihydrate obtained by crushing waste gypsum boards, comprising the steps of: Gypsum granules are supplied to a cylindrical rotating tank, and the gypsum granules are agitated by the rotation of the tank. Hot air is supplied to a pipe located at the axial center of the tank and equipped with a plurality of nozzles, and the hot air is blown from the nozzles into the gypsum granules, thereby converting the dihydrate gypsum in the gypsum granules into hemihydrate gypsum and / or anhydrous type III gypsum; Furthermore, the peripheral speed of the inner wall due to the rotation of the tank is set to 2 m / min or more and 10 m / min or less.

[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, which indicates 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. [Brief explanation of the drawings]

[0014] [Figure 1] Process diagram of the method for recovering gypsum from waste gypsum board in the example [Figure 2] A cross-sectional view of a hot air rotary calciner used in the examples, taken along a vertical plane perpendicular to the axial direction. [Figure 3] Axial vertical cross section of the hot air rotary calciner of Figure 2 DETAILED DESCRIPTION OF THE INVENTION

[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. [Example]

[0016] Figures 1 to 3 show an embodiment. Figure 1 shows the process of the embodiment. Crusher 2 crushes waste gypsum board into gypsum blocks and paper chips. Sieve 4, such as a vibrating sieve, separates the gypsum blocks from the paper chips, and magnetic separator 6 separates metal pieces such as nails. Crusher 8 finely crushes the gypsum blocks into gypsum granules, which are then stored in 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 gypsum granules after crushing 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"). 22 denotes a cylindrical rotary vessel, 23 denotes its inner wall, and the space surrounded by inner wall 23 is calcination space 24. A fixed pipe 25 penetrates the center of calcination space 24, and nozzles 26 are arranged parallel to the axial direction of pipe 25 in, for example, one to four rows, and hot air supplied from pipe 25 is blown into gypsum granules 29. A plurality of ribs 28 are provided from inner wall 23 toward 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 mechanism for rotating the rotary vessel 22 is arbitrary.

[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 granules 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 (not shown) or the like. 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 granules, and a gypsum outlet 54 is provided below to discharge the gypsum granules 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 granules are then 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 example, it is set to 300°C. The amount of hot air blown is, for example, 1500 to 2100 Nm per ton of the gypsum granules 29. 3 / hr(Nm 3 is the volume at 0°C and 1 atmosphere, and in this 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, 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, 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, approximately 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 after calcination, when the gypsum granules were observed with an optical microscope.

[0024] Table 1 Rotation speed (rot / min) Peripheral speed (m / min) Gypsum dihydrate content (mass%) Amount of fine powder Example 1 0.8 6 Almost 0 Slight Example 2 1.2 9 Almost 0 Minutes Example 3 0.4 3 Almost 0 Slight Comparative Example 1 2 15 15% More Comparative Example 2 3 22.5 70% More Comparative example 3 4 / 30 1 5% low

[0025] The gypsum dihydrate content after calcination depended on the peripheral speed of the inner wall 23. At a peripheral speed 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 granules 29 and represents the strength of agitation of the gypsum granules 29. When the interior of the rotary tank 22 was observed after five days of operation under the conditions of Comparative Example 2, a large amount of gypsum granules with relatively large particle sizes, approximately 3 to 6 mm, remained. 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 granules with large particle sizes were not discharged from the rotary tank 22, and gypsum granules newly supplied to the rotary tank 22 were discharged as insufficiently calcined gypsum dihydrate. The mechanism by which large gypsum granules 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 portion of the gypsum dihydrate was discharged as it was due to insufficient stirring.

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

[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. [Explanation of symbols]

[0029] 2 Crusher 4 sieve 6 Magnetic separator 8. Crusher 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 nozzles 29 Gypsum granules 28 protrusion 30 Weir plate 32 Movable plate 40 Chain 42 sprockets 44 Motor 46 bearings 50 Hot air inlet 51 Fixed part 52 Waste air outlet 54 Gypsum outlet 56 Plaster inlet 57 holes

Claims

1. A method for calcining gypsum granules made of gypsum dihydrate obtained by crushing waste gypsum board, comprising: The gypsum granules are supplied to a cylindrical rotating tank, and the gypsum granules are agitated by the rotation of the tank. Hot air is supplied to a pipe located at the axial center of the tank and equipped with a plurality of nozzles, and the hot air is blown from the nozzles into the gypsum granules, thereby converting the gypsum dihydrate in the gypsum granules into gypsum hemihydrate and / or anhydrous type III gypsum; The method for calcining gypsum granules is further characterized in that the peripheral speed of the inner wall due to the rotation of the tank is set to 2 m / min or more and 10 m / min or less.

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

3. 2. The method for calcining gypsum granules according to claim 1, wherein the inner diameter of the tank at the position of the inner wall 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.

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

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