JP7274289B2 - Method for recovering gypsum dihydrate from waste gypsum board - Google Patents

Description

This invention relates to recovery of gypsum dihydrate from waste gypsum board.

The applicant has succeeded in industrializing the recovery of gypsum dihydrate from waste gypsum boards. A method for recovering gypsum dihydrate will be described according to Patent Document 1 (WO2012/176688A) of the applicant. The waste gypsum board is pulverized, foreign matters such as paper chips are separated from the gypsum dihydrate, and the gypsum dihydrate is calcined to convert it to gypsum hemihydrate. Gypsum hemihydrate is mixed with gypsum slurry in a mixing tank, transferred to a precipitation tank to precipitate gypsum dihydrate particles in the slurry, and the slurry is refluxed to the mixing tank. After removing the remaining paper pieces with a vibrating sieve or the like, the precipitated gypsum dihydrate particles are extracted with a solid-liquid separator such as a filter press. The extracted gypsum dihydrate is used as a raw material for gypsum board.

Explain the role of the mixing tank. When gypsum hemihydrate is mixed with gypsum slurry in a mixing tank, gypsum scale is generated on the vessel walls and the like. It is difficult to completely prevent the generation of scale, and a scale removal operation is required. If gypsum hemihydrate is directly put into the precipitation tank, gypsum scale will be generated in the precipitation tank, and the scale removal operation will be extensive. On the other hand, if a mixing tank is provided upstream of the precipitation tank and the location where scale is generated is limited to the mixing tank, the scale can be easily removed.

The mixing tank is fed with a slurry containing gypsum dihydrate particles, such as the gypsum slurry of the precipitation tank. Since the gypsum slurry contains gypsum dihydrate seed crystals, the gypsum portion in the slurry precipitates on the surface of the seed crystals, and at the same time, the degree of supersaturation of gypsum in the slurry decreases. Therefore, relatively large gypsum dihydrate particles can be obtained. Gypsum dihydrate, which has a large average particle size, is easy to handle because it has a small amount of adhering water, and has a high industrial value.

Prior patents relating to mixing tanks will be described. In patent document 2 (WO2014/141926A) of the applicant, the vicinity of the inlet of gypsum hemihydrate into the mixing tank is heated to prevent the inlet from being clogged with gypsum scale.

In Patent Document 3 (Patent No. 6336385B) of the applicant, a swirling flow of gypsum slurry is generated in a mixing tank, and the gypsum slurry is discharged in the same direction as the swirling flow. This suppresses entrainment of air in the discharged gypsum slurry. Since the gypsum slurry is fluidly pumped from the mixing tank to the deposition tank, reducing air entrainment can prevent cavitation of the pump.

WO2012/176688A WO2014/141926A Patent 6336385B

An object of the present invention is to increase the average particle size of gypsum dihydrate particles when recovering gypsum dihydrate from waste gypsum boards.

In the present invention, gypsum derived from waste gypsum board is calcined to form semi-aqueous and/or anhydrous type III gypsum granules, and the gypsum granules are mixed with an aqueous slurry containing gypsum dihydrate particles in a mixer. to prepare a slurry stock solution, supply the slurry stock solution to a precipitation tank to make a gypsum slurry, precipitate gypsum dihydrate particles in the gypsum slurry in the precipitation tank, and solid-liquid separate a gypsum dihydrate recovery method,
V0 (m ³ ) is the total volume of the undiluted slurry in the mixer and the pipe from the mixer to the precipitation tank, w0 (ton/hr) is the amount of gypsum granules fed into the mixer, and w0 (ton/hr) is the gypsum When the true specific gravity of the granules is d (g/cm ³ ) and the amount of the aqueous slurry fed into the mixer per hour is u0 (m ³ /hr), Tr = V0/(u0 + w0 d ^-1 )×3600 (seconds), the residence time Tr of the undiluted slurry in the mixer and piping is 5 seconds or less and 0.1 seconds or more.

The inventors tried to shorten the residence time Tr from 10.0 seconds and 17.2 seconds to, for example, 2.4 seconds by changing the structure and arrangement of the mixer and piping. Under the same other conditions, the average particle size of gypsum dihydrate after solid-liquid separation was 34 μm at Tr of 10.0 seconds, 33 μm at 17.2 seconds, and increased to 61 μm at Tr of 2.4 seconds. This indicates that a large amount of crystal nuclei of gypsum dihydrate are generated while the slurry undiluted solution stays in the mixer and pipes, which delays crystal growth in the precipitation tank.

From the above data, shortening the residence time Tr of the slurry undiluted solution in the mixer and piping can limit the generation of crystal nuclei, promote the growth of seed crystals in the precipitation tank, and recover gypsum dihydrate with a large average particle size. I know you can. The residence time Tr is preferably as short as possible. For example, 2.4 seconds can sufficiently increase the average particle size of gypsum dihydrate, so it is set to 5 seconds or less, preferably 4 seconds or less. If the residence time Tr is extremely shortened, it becomes difficult to design the mixer and piping. In the present invention, the dwell time Tr ranges from 5 seconds to 0.1 seconds, preferably from 4 seconds to 1.0 seconds.

Preferably, when the precipitation tank consists of a plurality of tanks, the volume of the gypsum slurry in the first-stage precipitation tank, and when it consists of a single-stage tank, the volume of the gypsum slurry in the entire precipitation tank is V,
The time Tc determined by Tc=V/(u0+w0·d ⁻¹ )×60 (minutes) is set to 5 minutes or more and 100 minutes or less. Tc means the ratio of the gypsum slurry in the precipitation tank and the charging speed of the undiluted slurry from the mixer. When Tc is 5 minutes or more, seed crystals are sufficiently present in the precipitation tank, and the degree of supersaturation of gypsum in the slurry can be reduced. If Tc is less than 5 minutes, the capacity of the fluid pump feeding the slurry to the mixer becomes excessive, the seed crystal becomes insufficient, and the reduction of supersaturation becomes slow. On the other hand, if Tc is increased, the volume of the precipitation tank will increase, and if it exceeds 100 minutes, an unnecessarily large precipitation tank will be required. Therefore, Tc is preferably 5 minutes or more and 100 minutes or less, more preferably 10 minutes or more and 30 minutes or less.

Also preferably, the temperature Ts of the aqueous slurry is 60° C.±20° C., and the concentration C of the gypsum content in the aqueous slurry in terms of gypsum dihydrate is 20 mass% or more and 50 mass% or less, more preferably 30 mass% or more and 50 mass%. % or less. Since it is practically preferable to circulate the gypsum dihydrate slurry in the precipitation tank to the mixer, the temperature Ts of the aqueous slurry becomes substantially equal to the slurry temperature in the precipitation tank. When the temperature of the slurry in the precipitation tank is raised, the average particle size of the gypsum dihydrate particles tends to increase, but the energy cost increases. From these considerations, the temperature of the slurry in the precipitation tank is preferably 60°C±20°C, and the temperature of the slurry supplied to the mixer is also preferably 60°C±20°C. The above concentration C is a concentration converted assuming that all the gypsum content in the slurry is gypsum dihydrate. If the concentration C is low, the recovery amount of gypsum dihydrate will decrease, so it is preferably 20 mass% or more, more preferably 30 mass% or more. Also, if the concentration C is too high, the slurry viscosity increases, so it is preferably 50 mass% or less.

Another factor is the ratio u0/w0 between the charging speed u0 of the aqueous slurry and the charging speed w0 of the gypsum granules. The higher the u0/w0 ratio, the lower the concentration of crystal nuclei in the stock slurry and the lower the degree of supersaturation, but a larger fluid pump is required. From these balances, the ratio u0/w0 is preferably 10 m ³ /ton or more and 100 m ³ /ton or less, and particularly preferably 20 m ³ /ton or more and 50 m ³ /ton or less.

The temperature at which the gypsum granules are introduced into the mixer is arbitrary. However, when the temperature of the granules at the time of charging is 90° C. or higher, foaming at the time of charging decreases, so that the granules are uniformly dispersed in the slurry and the average particle size of the gypsum dihydrate particles increases. By the way, the calcination temperature for gypsum hemihydrate or the like is about 130°C or higher and 160°C or lower. The temperature at which the gypsum granules are charged into the mixer is arbitrary, but is preferably 90° C. or higher and 130° C. or lower in order to utilize the residual heat during calcination.

A diagram showing a method for recovering gypsum dihydrate according to an embodiment.

Examples for carrying out the present invention are shown below. The scope of the present invention should be determined according to the understanding of those skilled in the art based on the description of the claims, taking into consideration the description of the specification and well-known techniques in this field.

Fig. 1 shows the recovery method of gypsum dihydrate. The crusher 2 crushes the waste gypsum board to an average particle size of 0.5 to 20 mm, preferably 1 to 10 mm. , hemihydrate gypsum and/or anhydrous type III gypsum.

A mixer 6 mixes the hemihydrate gypsum and/or anhydrous type III gypsum granules with the gypsum slurry from the precipitation tank 11 to obtain a slurry stock solution, which is sent to the first-stage precipitation tank 8 via a pipe 7 . supply. The gypsum slurry supplied to the mixer 6 is slurry in which gypsum dihydrate particles are dispersed in an aqueous medium. In the deposition tank 8, the slurry stock solution is mixed with the gypsum slurry to reduce the supersaturation of the gypsum, and ions such as Ca ²⁺ , SO ₄ ^2- are brought into contact with the gypsum dihydrate seed crystals.

Although the mixer 6 has a cylindrical shape in the embodiment, it may have a gutter shape or the like, and the shape and structure are arbitrary. . Therefore, the internal volume of the mixer 6 is reduced and the pipe 7 is shortened. Furthermore, in order to reduce the volume of the pipe 7, no fluid pump is provided in the pipe 7, and the gypsum slurry is transferred to the precipitation tank 8 by the kinetic energy and gravity of the gypsum slurry when it is fed from the precipitation tank 11 to the mixer 6.

For example, 4 stages of precipitation tanks 8 to 11 are provided downstream of the mixer 6, and the precipitation tanks 8 to 11 are collectively referred to as a precipitation section 12. The precipitation tanks 8 to 11 are provided with stirrers 14 to stir the slurry, and the gypsum slurry overflowing from the upstream precipitation tank is introduced into the downstream precipitation tank via a pipe 15 . In addition, you may transfer between deposition tanks with a fluid pump. The number of stages of the deposition tanks 8 to 11 is arbitrary, and the deposition section 12 may be composed of only one stage of the deposition tank. The gypsum slurry in precipitation tanks 8 to 11 is an aqueous slurry containing gypsum dihydrate particles. The gypsum dihydrate particles grow while reducing the degree of supersaturation.

For example, the gypsum slurry is introduced from the lowermost deposition tank 11 to the mixer 6 through the pipe 18 by the fluid pump P1 and circulated from the mixer 6 to the deposition section 12 . The gypsum slurry may be circulated to the mixer 6 from the precipitation tanks 8 to 10 other than the precipitation tank 11 at the bottom.

A gypsum slurry is extracted from the precipitation tank 11 at the bottom, foreign matters such as paper chips are removed by a sieve S2, and separated into a filtrate and gypsum dihydrate particles by a solid-liquid separator such as a filter press 16. The separated filtrate is replenished with the water lost in the filter press 16 and the like, and is returned to, for example, the first-stage precipitation tank 8 by means of the fluid pump P2 and the pipe 19.

Parameters used in the examples will be described. Let w0 (ton/hour) be the input amount of hemihydrate gypsum and/or anhydrous type III gypsum per hour, and V0 (m ³ ) be the volume of the undiluted slurry in the mixer 6 and pipe 7 . w0 is a parameter directly linked to the amount of gypsum dihydrate collected per hour. Let u0 (m ³ /hr) be the circulation amount of the aqueous slurry to the mixer 6, d (g/cm ³ ) be the true specific gravity of gypsum dihydrate, and V (m ³ ) be the volume of the gypsum slurry in the precipitation tank 8. . d is approximately two. In the embodiment, the deposition tanks 8 to 11 all have the same volume V. The concentration of gypsum in the gypsum slurry (concentration converted to gypsum dihydrate) in the precipitation section 12 is C (mass%), the temperature of the gypsum slurry is Ts, and the temperature of the slurry stock solution in the mixer 6 etc. is also Ts. be.

By reducing the volume V0 of the undiluted slurry in the mixer 6 and the pipe 7 and increasing the transfer speed (u0+w0·d ⁻¹ ) of the undiluted slurry from the pipe 7 to the precipitation tank 8, To shorten the time that the undiluted slurry stays. This reduces the number of crystal nuclei generated in the undiluted slurry and reduces the degree of supersaturation of the undiluted slurry. If Tr is the time in seconds that the undiluted slurry stays in the mixer 6 and the pipe 7, then Tr=V0/(u0+w0·d ⁻¹ )×3600. Tr is preferably 5 seconds or less and 0.1 second or more, and 4 seconds or less and 1 second or less.

An important element other than Tr is the time Tc determined by Tc=V/(u0+w0.d ^-1 )×60 (minutes). This means the time the slurry stays in the first-stage deposition tank 8 . If this time Tc is long, the undiluted slurry is immediately diluted with the gypsum slurry in the precipitation tank 8, and the degree of supersaturation of the slurry and the concentration of crystal nuclei generated in the mixer 6 and the pipe 7 are quickly reduced. When the time Tc is 5 minutes or more, preferably 8 minutes or more, the condition that the undiluted slurry is rapidly diluted with the gypsum slurry is satisfied. If the time Tc is increased, the volume of the deposition portion 12 becomes excessively large, so practically the time Tc is preferably 100 minutes or less.

Many of the factors other than the above are restricted by other conditions in the recovery of gypsum dihydrate, and are difficult to change freely. Increasing the temperature Ts of the gypsum slurry increases the average particle size of the gypsum dihydrate particles, but increases the energy cost due to the heating of the precipitation tank. Therefore, the slurry temperature Ts is preferably 60°C±20°C, more preferably 60°C±10°C. If the slurry concentration C in the precipitation section 12 is low, the amount of gypsum dihydrate will be reduced, so it is preferably 20 mass% or more, more preferably 30 mass% or more. If the concentration C is too high, the viscosity of the slurry will increase excessively. Therefore, it is preferably 50 mass% or less. Since the total volume Vt of the precipitation section 12 is proportional to the aging time of the slurry, the total volume of the precipitation section 12 is also determined by determining the aging time. The ratio Vt/w0 between the total volume Vt (m ³ ) of the precipitation section 12 and the input speed w0 (ton/hr) of the gypsum granules is preferably 20 m ³ ·hr/ton or more and 50 m ³ ·hr/ton or less, particularly 30 m ³ ·h/ton or more and 50 m ³ ·h/ton or less are preferable.

If the ratio u0/w0 of the charging speed u0 of the aqueous slurry and the charging speed w0 of the gypsum granules is increased, the degree of supersaturation of the raw slurry solution is lowered, but a larger fluid pump is required. Therefore, the ratio u0/w0 is preferably 10 m ³ /ton or more and 100 m ³ /ton or less, and particularly preferably 20 m ³ /ton or more and 50 m ³ /ton or less.

The temperature at which the gypsum granules are introduced into the mixer 6 is arbitrary. increases. The temperature at which the gypsum granules are charged into the mixer 6 is preferably 90° C. or higher and 130° C. or lower, since it is preferable to use residual heat from calcination.

Experimental example Influence on the average particle size of precipitated gypsum particles by changing the structure and arrangement of the mixer 6 and the piping 7 in a practical plant and changing the volume V0 of the slurry undiluted solution in the mixer 6 and the piping 7 examined. The feeding speed w0 of the gypsum granules into the mixer 6 was fixed at 4 tons/hr, and the temperature was room temperature. The gypsum slurry was supplied to the mixer 6 at a rate u0 of 120 m ³ /hr, a concentration C of 40 mass%, and a temperature Ts of 60°C. Precipitation tanks 8 to 11 each had a volume of 35 m ³ . Also, the filtrate and water were introduced into the precipitation tank 8 at a total rate of 8 m ³ /hr by the fluid pump P2. The slurry extracted from the precipitation tank 11 was filtered, and the average particle size of the gypsum dihydrate particles was measured by a laser light scattering method.

Experimental example 1
The volume of the undiluted slurry in the mixer 6 and pipe 7 was set to 0.08 ^m3 . The residence time Tr was 2.4 seconds, and the average particle size of the deposited gypsum particles was 61 µm.

Comparative example 1
The amount of the slurry stock solution in the mixer was set to 0.16 m ³ , and the slurry was supplied from the mixer to the deposition tank 8 via a fluid pump. The total amount of the undiluted slurry in the piping from the mixer to the precipitation tank was 0.18 m ³ . The residence time Tr was 10.0 seconds, and the average particle size of the deposited gypsum particles was 34 μm.

Comparative example 2
The amount of undiluted slurry in the mixer was 0.16 m ³ , and the total amount of undiluted slurry in the piping (with fluid pump) from the mixer to the precipitation tank was 0.42 m ³ . The residence time Tr was 17.1 seconds, and the average particle size of the deposited gypsum particles was 33 μm.

When the residence time Tr was 2.4 seconds and 10 seconds or more, the average particle size of the precipitated gypsum dihydrate particles was significantly different, and it was found that it is important to keep the residence time Tr to 5 seconds or less.

Experimental example 2
Under the same conditions as in Experimental Example 1, when the temperature of the gypsum granules was changed to 95° C. at the time of charging into the mixer 6, the average particle size of the precipitated gypsum dihydrate particles was 71 μm.

Discussion The mechanism by which the average particle size of the extracted gypsum dihydrate increases by shortening the residence time Tr of the undiluted slurry in the mixer 6 and the pipe 7 will be examined. The crystal nuclei once formed grow in precipitation tanks 8 to 11 to become seed crystals of gypsum dihydrate, and are finally extracted by a filter press 16 as gypsum dihydrate particles. When the particle size distribution is stabilized with a constant normal distribution, the product of the cube of the average particle size of the gypsum dihydrate particles and the number of particles is constant, and the number of particles is proportional to the rate of seed crystal formation. Therefore, the average particle size of gypsum dihydrate particles is proportional to the -1/3 power of the crystal nucleus generation rate.

It is considered that crystal nuclei of gypsum dihydrate are generated because the undiluted slurry is supersaturated in the mixer 6 and the pipe 7 . Moreover, when the slurry undiluted solution is put into the first-stage precipitation tank 8, the degree of supersaturation drops rapidly. The slurry stock solution has kinetic energy at the outlet of the pipe 7, and the slurry stock solution has a higher specific gravity than the dihydrate gypsum slurry by the gypsum granules, so the slurry stock solution is stirred while settling in the precipitation tank 8. . Therefore, the undiluted slurry is rapidly mixed with the gypsum dihydrate slurry in the precipitation tank 8, and the degree of supersaturation is rapidly lowered. Therefore, it can be considered that the formation of crystal nuclei in the deposition tanks 8 to 11 is slight.

Due to these mechanisms, it is considered that reducing the number of crystal nuclei generated in the mixer 6 and the pipe 7 increases the average particle size of the gypsum dihydrate particles. Since the rate of crystal nucleus generation depends on the residence time of the undiluted slurry in the mixer 6 and the pipe 7, shortening the residence time suppresses the generation of crystal nuclei and reduces the average particle size of the gypsum dihydrate particles. is considered to have succeeded in increasing the

2 Crusher 4 Calcination furnace 6 Mixer 7 Piping 8-11 Precipitation tank 12 Precipitation section 14 Stirrer 15 Piping 16 Filter press 18, 19 Piping
S1,S2 Sieve
P1,P2 fluid pump

Claims (3)

Gypsum derived from waste gypsum boards is calcined to form semi-aqueous and/or anhydrous type III gypsum granules, and the gypsum granules are mixed with an aqueous slurry containing gypsum dihydrate particles in a mixer to form a slurry stock solution. A method for recovering gypsum dihydrate, in which the slurry stock solution is supplied to a precipitation tank to form a gypsum slurry, gypsum dihydrate particles are precipitated in the gypsum slurry in the precipitation tank, and solid-liquid separation is performed,
V0 (m ³ ) is the total volume of the undiluted slurry in the mixer and the pipe from the mixer to the precipitation tank, w0 (ton/hr) is the amount of gypsum granules fed into the mixer, and w0 (ton/hr) is the gypsum When the true specific gravity of the granules is d (g/cm ³ ) and the amount of the aqueous slurry fed into the mixer per hour is u0 (m ³ /hr), Tr = V0/(u0 + w0 d ^-1 )×3600 (seconds), the residence time Tr of the undiluted slurry in the mixer and piping is 5 seconds or less and 0.1 seconds or more. Where V is the volume of the gypsum slurry in the first-stage precipitation tank when the precipitation tank consists of a plurality of tanks, or in the entire precipitation tank when it consists of a single-stage tank, Tc = V / (u + w0 d ^-1 ) The method for recovering gypsum dihydrate from waste gypsum boards according to claim 1, characterized in that the time Tc determined by x60 (minutes) is 5 minutes or more and 100 minutes or less. 3. The method of claim 1 or 2, wherein the temperature Ts of the aqueous slurry is 60° C.±20° C., and the concentration C of the gypsum content in the aqueous slurry in terms of gypsum dihydrate is 20 mass% or more and 50 mass% or less. A method for recovering gypsum dihydrate from waste gypsum board.

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