JP2003320308A - Sieving method for granulated matter of hydraulic powder and apparatus therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform the sieving of a hydraulic powder granulated matter not clogging the meshes of a sieve. <P>SOLUTION: When active alumina granulated matter is classified through the meshes 6 and 7 of the upper and lower stage sieves 4 and 5 of a vibration part 2, hot air is blown from below to prevent the clogging of the meshes 6 and 7 due to scaling. Moisture is removed from the surface of the active alumina granulated matter by hot water and water becomes a binder to prevent generated scaling. If the bottom of a product discharge chute 12, or the like, is heated, the adhesion of a hydraulic powder can be prevented. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method and a device for sieving granules of hydraulic powder such as activated alumina compacts.

[0002]

2. Description of the Related Art Conventionally, activated alumina moldings molded to have a porous surface have been disclosed, for example, in Japanese Examined Patent Publication No. 63-63.
As disclosed in Japanese Patent No. 24932, it is used for various purposes such as a desiccant, an adsorbent, a catalyst and a catalyst carrier.

FIG. 5 schematically shows the manufacturing process of the activated alumina compact, and FIG. 6 shows the shape of the alumina particles which changes in the manufacturing process. Gibbsite (A
1 ₂ O ₃ .3H ₂ O) powder or the like is used. This raw material is
As shown in FIG. 6A, the surface is smooth and the specific surface area based on the BET adsorption isotherm is about 1 m ² / g. When this raw material is heated to 300 ° C. to 1100 ° C., water is removed from the raw material, and a porous, crystalline or amorphous activated alumina powder as shown in FIG. 6B is obtained. The activated alumina is usually hydraulic in that it reacts with water and hardens when heated. This specific surface area is 100 to 300 m ² / g
There are many small holes of about 10Å. When heated to 1200 ° C. or higher, the porosity is lost and α-alumina having a specific surface area of 10 m ² / g or less is obtained.

The obtained powder of activated alumina is a hydraulic powder which hardens in the presence of water, and is granulated by adding water at a low temperature. The granulation is performed, for example, by supplying the raw material powder to a rotating pan that is inclined and spraying 0.4 to 0.6 part by weight of water with respect to 1 part by weight of the powder, and using water as a binder. It is carried out by using a rotating pan type granulator or the like that agglomerates the body. In the granulation process, as shown in FIG. 6 (c), water serves as a binder between the powder particles to bond and aggregate them. The powder of activated alumina has a particle size of about 5 μm to 20 μm, and a granulated product having a particle size selected from the range of about 1 to 15 mm, for example, a particle size of 3 mm is obtained by granulation. The particle size of the granulated product is required to be within a certain range so that the desiccant, the adsorbent, the catalyst carrier and the like can be easily handled according to the purpose of use. Therefore, a sieving device is used to screen granules having a particle size within a certain range. The sieved granules are subjected to indirect aging in which aging is performed in the presence of water vapor without heating and aging at high temperature by heating. Then, by heating and dehydration, FIG. 6 (d)
As shown in Fig. 4, the powder particles constituting the granulated product undergo a crosslinking reaction in the water portion serving as the binder, and are cured to form a mechanically strong porous body. A formed body is obtained.

Granules of activated alumina are aged after being adjusted to a certain range of particle size by using a sieving apparatus similar to the vibrating sieving apparatus disclosed in, for example, JP-A-7-275801. . This is because it is possible to reuse the granulated product that is out of the desired particle size range before aging. However, although the sieve disclosed in Japanese Patent Application Laid-Open No. 7-275801 has one stage, a sieve apparatus having two upper and lower sieves is used for sieving the granules of activated alumina. The upper screen has a larger mesh than the lower screen, and a granulated product having a desired particle size can be taken out between the upper and lower screens. Vibration is applied to the upper and lower sieves to prevent clogging.

[0006]

Granules of activated alumina as shown in FIG. 5 are composed of hydrate particles, and a sieving apparatus having two upper and lower vibrating sieves is used for classification. Since the upper sieve has a large opening and the lower sieve has a small opening, when the activated alumina granules are fed from above the upper sieve, the granules with the target particle size accumulate between both sieves. Then, this can be taken out from the discharge port to obtain a granulated product having a target particle size. However, the granules of activated alumina are composed of activated alumina powder and water, and the powder is detached and agglomerated to cause so-called scaling, which causes clogging of the screen of the sieve. If the upper sieve is clogged, the granules having the target particle size cannot pass through the upper sieve, and the particles are apparently excluded because they are too large. The granules that pass through the upper sieve are smaller than the target particle size. If the lower sieve is clogged, it will not pass through even if the particle size is smaller than that of the granulated product having the target particle size, and it will be apparently taken out as the target particle size. For this reason,
The particle size of the granulated product obtained as the target particle size becomes smaller than the desired range. Further, if the sieve is clogged, the amount of the granulated material to be sieved will be reduced as a whole.

Therefore, in order to prevent the clogging of the sieve, the conventional sieving apparatus stops the operation after a certain period of operation and cleans the sieve to prevent the clogging from progressing periodically. In need. Cleaning the screen interrupts continuous operation and forces work that does not directly contribute to production.

An object of the present invention is to provide a sieving method and apparatus for hydraulic powder which does not clog the screen of the screen.

[0009]

DISCLOSURE OF THE INVENTION The present invention is a method for sieving granules of hydraulic powder containing 25 to 40% by weight of water in a range of a predetermined particle size. This is a method for sieving granules of hydraulic powder, which comprises heating to remove 0.7% to 5% of water relative to the mass of the granules while sieving.

According to the present invention, when the granules of hydraulic powder are sieved in the range of a predetermined particle size, the granules are heated to remove moisture of 0.7% to 5% by mass. Do while removing.
The amount of water to be removed is such an amount that it exists on the surface of the granulated product, and the hydraulic powder that constitutes the granulated product desorbs from the granulated product,
It is possible to prevent these from aggregating with each other and clogging a screen for sieving or the like. Since only a part of water is removed, water remains in the granulated product even after sieving, so that the hydraulic property can be maintained.

Further, in the present invention, the sieving is performed using a vibrating mesh, and the granulated material is heated by applying hot air through the mesh.

According to the present invention, when sieving the granulated product with a vibrating net, hot air is applied to the granulated product through the net.
It is possible to prevent the hydraulic powder from desorbing from the granulated material by heating the surface of the granulated material to evaporate the water, and to prevent these from aggregating with each other using water as a binder to cause clogging.

Further, in the present invention, the mesh is installed in two stages, an upper side with coarse mesh and a lower side with fine mesh, and the granules to be sieved are introduced from above the upper mesh. The granulated material for sieving is taken out between the upper net and the lower net, and the hot air is introduced from below the lower net.

According to the present invention, an open mesh on the upper side,
When the granules for sieving purposes are taken out from between the fine mesh on the lower side, hot air is introduced from below the lower mesh to heat the granules, so moisture from the surface of the granules It is possible to prevent clogging of the net by efficiently removing the granules for the purpose of sieving.

Further, the present invention is characterized in that the bottom of the chute for discharging the granules of the hydraulic powder to be sieved is heated so as to maintain a predetermined temperature.

According to the present invention, since the bottom of the chute for discharging and taking out the granulated product of the hydraulic powder to be sieved is heated so as to maintain a predetermined temperature, it is detached from the granulated product. The hydraulic powders thus formed can be prevented from aggregating with each other at the bottom of the chute and adhering to the chute.

Further, in the present invention, activated alumina powder is used as the hydraulic powder, and the heating is performed by applying hot air to the powder.
It is characterized by heating to ℃ to 105 ℃.

According to the present invention, the activated alumina powder granules are heated to 40 ° C. to 105 ° C., so that the water content on the surface evaporates and the granules are constituted at the time of sieving. It is also possible to prevent the activated alumina powder from desorbing from the granulated product and aggregating with water as a binder, and to prevent clogging of the sieve due to the agglomerate. Since the heating with hot air is 105 ° C. or less, the amount of water removed from the granulated product is reduced as a whole, and physical properties such as hydraulic property can be prevented from being lost after sieving.

The present invention further provides an upper net for passing granules having a predetermined particle size range, and a lower net for preventing passage of the granules having a predetermined particle size range below the upper net. While vibrating and, while introducing the granules of hydraulic powder from above the upper net, hydraulic powder to obtain granules in the range of the predetermined particle size from the middle of the upper net and the lower net Of the granules of hydraulic powder, comprising hot air blowing means for blowing hot air from below the lower net to heat the granules of hydraulic powder. It is a separate device.

According to the present invention, the hydraulic powder sieving apparatus classifies the granulated product in the upper and lower two-stage nets, and obtains the classified granulated product from the middle of the upper net and the lower net. At this time, hot air is blown from below the lower net by hot air blowing means. Since the granulated material of the hydraulic powder is heated by blowing hot air, water is removed from the surface of the granulated material, and the hydraulic powder desorbed from the granulated material is agglomerated with water as a binder. Can be prevented, clogging caused by agglomeration can be prevented, and the operation can be continued continuously.

Further, according to the present invention, the temperature detecting means for detecting the temperature of at least one of the upper net or the lower net, and the temperature detected by the temperature detecting means are set to a predetermined temperature. The temperature control means for controlling the hot air blown from the hot air blowing means is further included.

According to the invention, by the temperature control means,
The temperature and volume of the hot air blown from the hot air blowing means for heating the granules of hydraulic powder are controlled so that the temperature of the net detected by the temperature detecting means becomes a predetermined temperature.
Although it is difficult to directly detect the temperature of the granulated product,
The temperature of the mesh can be detected more easily than that of the granulated product. Since the mesh is in constant contact with the granules to be screened, the temperature of the mesh faithfully reflects the temperature of the surface of the granules, and highly precise control can be performed.

Further, the present invention further comprises heat retaining means for keeping a bottom surface of a chute for discharging granules of hydraulic powder which are sieved by the upper net and the lower net at a predetermined temperature. Characterize.

According to the present invention, since it further includes a heat retaining means for keeping the bottom surface of the chute for discharging the granulated material of the hydraulic powder which is sieved by the upper net and the lower net at a predetermined temperature,
Even when the granules discharged after sieving move while touching the bottom surface of the chute, the water is removed from the surface of the granules, and the hydraulic powders detached from the granules bind water to each other. Can be prevented from aggregating and adhering to the shoot.

[0025]

1 shows a schematic configuration of a sieving machine 1 as a sieving apparatus according to an embodiment of the present invention.
The sieving machine 1 supports an upper vibrating portion 2 by a lower supporting portion 3 so that it can vibrate. The vibrating section 2 includes an upper sieve 4 and a lower sieve 5.
Are vertically stacked and inside the upper screen 4 and the lower screen 5, meshes 6 and 7 for screening are respectively installed. The mesh size of the upper mesh 6 is larger than that of the lower mesh 7. The top of the upper sieve 4 is covered with a hood 8. At the bottom of the lower sieve 5, a conical iron plate 9 having a high center is provided. A raw material inlet 10 is provided at the top of the hood 8, and a granule of activated alumina as a raw material to be sieved is introduced. The vibrating portion 2 and the support portion 3 are substantially cylindrical and have a bottom diameter of, for example, about 1000 mm.

The activated alumina granules charged from the raw material charging port 10 contain 25 to 40% by weight of water, preferably 30 to 40%, and more preferably 30 to 35%. Here, the water includes crystal water. The granulated product is first sieved through the upper mesh 6. The large-diameter activated alumina granules that do not pass through the upper mesh 6 are discharged from the large-size discharge chute 11 to the outside.
The activated alumina granules that pass through the upper mesh 6 are
The mesh 7 on the lower side is screened again. The activated alumina granules that do not pass through the lower mesh 7 are discharged from the product discharge chute 12 to the outside. Lower mesh 7
The activated alumina granules with a small particle size that pass through are discharged from the small particle size discharge chute 13 to the outside. That is, from the product discharge chute 12, passes through the upper mesh 6,
It is possible to take out a granulated product having a particle size range that does not pass through the lower mesh 7.

In the sieving machine 1 of this embodiment, the hood 8 is provided with an exhaust hole, and hot air generated by the hot air generator 15 is blown from below the lower mesh 7 to heat the activated alumina granules. . The hot air generator 15 is arranged outside the vibrating portion 2 and the support portion 3, and supplies the generated hot air to the lower side of the mesh 7 on the lower stage side via the flexible tube 16. By heating the activated alumina granules with hot air and performing sieving while removing water from the surface of the granules, the activated alumina powders desorbed from the granules aggregate with each other using water as a binder. It is possible to prevent clogging of screens for sieving. Since only a part of water is removed, water remains in the granulated product even after sieving, so that the hydraulic property can be maintained. The sieving is performed using vibrating meshes 6 and 7, and the activated alumina granules are heated by meshes 6 and 7.
It can be performed by applying hot air through. Mesh 6,
7 are divided into two stages, an upper mesh side with a coarse mesh and a lower mesh side with a fine mesh, and the activated alumina granules to be screened are introduced from above the upper mesh 6 to form the upper mesh 6
It is possible to take out the granulated material for the purpose of sieving from between the lower mesh 7 and the lower mesh 7 and heat it by introducing hot air from below the lower mesh 7.

Further, in the sieving machine 1 of the present embodiment, the jackets 17, 18, 1 are provided at the bottoms of the large particle size discharging chute 11, the product discharging chute 12 and the small particle size discharging chute 13.
9 is provided, and the bottom portion is heated by the hot air that is supplied to the collecting pipe 20 from the hot air generator 21 and is guided through the flexible pipes 22, 23, and 24, respectively. Large particle discharge chute 1
1. Product discharge chute 12 and small particle size discharge chute 1
Since the bottom of No. 3 is heated by hot air so as to maintain a predetermined temperature, water is removed from the surface of the activated alumina granules, and the activated alumina powder desorbed from the activated alumina granules is the bottom of the chute. Can be prevented from aggregating and sticking to the chute. The hot air generator 21 that heats the chute can be shared with the hot air generator 15 that heats the granulated material, but if it is separate, temperature control and the like can be easily performed. The chute may be heated by using a heater or the like instead of hot air.

Since the sieving machine 1 sifts the granulated product obtained by adding water to activated alumina powder to granulate, heating is carried out at 40 ° C. to 105 ° C. by applying hot air. The temperature of 40 ° C. is the lower limit temperature that is confirmed to have the effect of preventing scaling. The temperature of 105 ° C. is the upper limit temperature that is confirmed not to impair the physical properties. At least one of the meshes 6 and 7, for example, the temperature of the lower mesh 7 is detected by the temperature sensor 25, and this temperature is set to a constant temperature within the range of 40 ° C. to 105 ° C., for example, about 60 ° C. to 70 ° C. Therefore, the controller 26 controls the hot air generator 15. The control is performed on the temperature and the air volume of the hot air. The temperature of the hot air is set to about 350 ° C. in the hot air generator 15.
When hot air hits the granules of activated alumina powder and the granules reach a temperature of about 40 ° C to 105 ° C, the water content on the surface evaporates, and the hydraulic properties that constitute the granules during sieving It is possible to prevent the powders from aggregating with each other using water as a binder, and to prevent the mesh from being clogged with the agglomerates of hydraulic powders. Since the heating temperature by hot air is 105 ° C or less, the amount of water removed from the granulated product is small as a whole, and as described later, it is about 0.7% to 5% of the mass, so that the hydraulic property after sieving, etc. The physical properties of can be prevented.

FIG. 2 shows a simplified shape of the vibrating portion 2 shown in FIG. The height H of the vibrating portion 2 is, for example, about 50 cm. The outer diameter D of the vibrating portion H is, for example, about 90 cm. The meshes 6 and 7 of FIG. 1 installed in the vibrating section 2
Is a net formed of, for example, stainless steel thin wires,
The temperature can be easily detected by the temperature sensor 25.
In the vibrating part 2, the activated alumina granules introduced from the upper part and the hot air blown from the bottom exchange heat in a counter-current manner to efficiently dry only the surface of the activated alumina granules. it can.

FIG. 3 shows the relationship between the amount of water lost from the activated alumina granules by hot air and the required amount of heat. In order to heat the atmosphere of 20 ° C. to 350 ° C. by the hot air generator 15, the heat amount of Q1 is required, and the heat amount Q2 of evaporating the moisture corresponding to the difference in the igross due to the removal of the moisture by the hot air is required. The required heat quantity Q1 is set larger than the heat quantity Q2. It should be noted that the term "igross" refers to activated alumina granules
It represents the decrease in water content when heated to 100 ° C, and also includes water of crystallization. The result obtained by operating the sieving machine 1 of the present embodiment shows that the activated alumina granulated product is 25
With 0 kg added, the igross has decreased from 31.3% to 30.5%, so 0.8% (= 31.3% -30.5%)
%) Of water is reduced by hot air heating with an air volume of 94.8 m ³ / h. When the air volume becomes large, the activated alumina granules are fluidized and do not pass through the meshes 6 and 7, so it is necessary to suppress the volume so that fluidization does not occur.

To calculate the heat quantities Q1 and Q2, assume the following. Assumption: Activated alumina granules heated to a predetermined temperature are introduced into the sieving machine 1, only the surface is dried by hot air, and the activated alumina granules are discharged from the sieving machine 1 at the predetermined temperature. The residence time is short. Assumption: Sensible heat of water and sensible heat of activated alumina granules are not necessary in the sieving machine 1. This is because the activated alumina granules that are usually heated in the pre-aging tank are introduced into the sieving machine 1.

The latent heat for evaporating 0.8% of the water content described above can be calculated as follows. 250 kg / h x (0.313-0.305) x 238
3kJ / kg = 4410kJ / h

This is 18 per unit weight of 1 kg.
It becomes kJ / h / kg. When drying at 5%,
By the same calculation, it was found that a heat quantity of 29800 kJ / h was required, which was 119 kJ / h / kg per unit weight.
Becomes Therefore, the amount of heat required to dry 1 kg of the activated alumina granulated product with hot air at 0.7 to 5% is 18 to 12
It becomes 0 kJ / h / kg.

The hot air generator 15 was used to heat the atmosphere at 20 ° C. to 350 ° C.
The amount of heat for heating to 0.015
(KgH ₂ O / kgAir) and the specific heat of air is 1.0
When kJ / kg / K, Q1 = 1.00 (kJ / kg / K) × 94.8m 3 / h) × 1.29 (kg / m 3) × (350-20) (K) = 40356 ( kJ / h).

FIG. 4 shows the product discharge chute 12 shown in FIG.
The structure for heat retention of is shown. The large particle size discharge chute 11 and the small particle size discharge chute 13 have the same configuration. 4 (a) is a partial front view state, FIG. 4 (b) is a partial plan view state, and FIG. 4 (c) is a cross section taken along the section line AA ′ of FIG. 4 (b). Shown respectively. The hot air is introduced into the jacket 18 from the introduction part 27 and is discharged from the discharge port 28. When hot air flows through the jacket 18,
The bottom surface of the product discharge chute 12 can be heated to prevent scaling to the bottom surface. It is not always necessary to heat such shoots for all shoots. For example, when the amount of the activated aluminum granules screened as a large particle size or a small particle size is small, heating by the jackets 17 and 19 of the large particle size discharge chute 11 or the small particle size discharge chute 13 is not performed. You can also do so.

In the sieving machine of the present embodiment, the hot air is supplied from the hot air generator 15 by the controller 26 with the target of 60 ° C. or higher, though the hot air is introduced and clogging occurs due to scaling after 8 hours of operation. It has been confirmed that scaling can be prevented even in continuous operation for 5 to 6 days or more, for example, 10 days. Note that the scaling prevention effect is 40
It is recognized above ℃.

In the above description, the present invention is applied to the sieving machine 1 for sieving activated alumina granules, but the present invention is similarly applied to powder having hydraulic properties similar to activated alumina granules. Can be applied. For example, the present invention can be applied to powders of other porous ceramics.
Further, the sieve is not limited to the case where two stages are provided, and it goes without saying that the same effect can be obtained even if one stage is provided. Further, even when the sieve is not vibrated, scaling can be similarly prevented.

[0039]

As described above, according to the present invention, when the granules of hydraulic powder are sieved in the range of a predetermined particle size, the water present on the surface of the granules serves as a binder. It is possible to prevent the powders detached from the granulated material from agglomerating with each other and clogging a screen for sieving. Since sufficient water remains in the granulated product even after sieving, hydraulic properties can be maintained.

Further, according to the present invention, when sieving is performed with a vibrating mesh, hot air is blown through the mesh to evaporate water on the surface of the granulated material so that clogging is not caused. it can.

Further, according to the present invention, when the granules for the purpose of sieving are taken out from between the upper and lower nets, hot air is introduced from below the lower net to heat the granules, thereby clogging the nets. It is possible to efficiently remove the granulated product for the purpose of sieving.

Further, according to the present invention, it is possible to prevent the hydraulic powder from adhering to the chute for discharging and taking out the granulated product of the hydraulic powder to be sieved.

Further, according to the present invention, hot air is blown to the granules of activated alumina powder to bring the temperature to 40 ° C. to 105 ° C. Therefore, the powders desorbed from the granules during sieving remove water from each other. It is possible to prevent agglomeration as a binder to cause clogging of the sieve and prevent physical properties from being lost even after sieving.

Further, according to the present invention, by blowing hot air while classifying the granulated product of the hydraulic powder with the upper and lower nets, water is removed from the surface of the granulated product and the granulated product is removed. It is possible to prevent the separated powders from aggregating with each other using water as a binder, to prevent clogging caused by the agglomeration, and to continue the operation continuously.

Further, according to the present invention, the hot air blown from the hot air blowing means is controlled so that the temperature of the screen for sieving the granules of hydraulic powder becomes a predetermined temperature. It is possible to perform highly accurate control more easily than directly detecting the temperature of the object.

Further, according to the present invention, the bottom surface of the chute for discharging the granules of the hydraulic powder to be sieved is kept at a predetermined temperature, and the powders detached from the granules are aggregated with each other to form the chute. Can be prevented from clogging.

[Brief description of drawings]

FIG. 1 is a front view showing a schematic configuration of a sieving machine 1 according to an embodiment of the present invention.

FIG. 2 is a simplified front view of a vibrating section 2 of FIG.

FIG. 3 is a diagram showing conditions for calculating heat input to a vibrating section 2 in FIG.

FIG. 4 is a partial front view, a plan view and a sectional view showing a configuration for heating a product discharge chute 12 of FIG.

FIG. 5 is a process drawing showing a schematic manufacturing process of a conventional activated alumina granulated product.

FIG. 6 is a diagram schematically showing the shape of alumina particles changing along the manufacturing process of FIG.

[Explanation of symbols]

1 sieving machine 2 vibration parts 4 Upper sieve 5 Lower sieve 6,7 mesh 11 Large particle discharge chute 12 Product discharge chute 13 Small particle discharge chute 15,21 Hot air generator 17,18,19 jacket 25 temperature sensor 26 Controller

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C04B 35/10 C04B 35/10 BF term (reference) 4D021 AA02 AB02 CA03 DA01 DA13 DB01 DB03 DC02 EA10 4G030 AA36 BA32 BA34 GA01 GA05 GA11 HA25

Claims (8)

[Claims] 1. A method of sieving a granulated product of hydraulic powder containing 25 to 40% by weight of water in a range of a predetermined particle size, wherein the granulated product is heated to obtain the granulated product. For the mass of.
A method for sieving granules of hydraulic powder, which comprises sieving while removing 7% to 5% of water. 2. The hydraulic powder according to claim 1, wherein the sieving is performed using a vibrating mesh, and the granulated material is heated by applying hot air through the mesh. Method of sieving granules. 3. The mesh is installed in two stages, an upper stage with coarse mesh and a lower stage with fine mesh, and the granules to be sieved are introduced from above the upper mesh to obtain the upper stage. 3. The granulated product of hydraulic powder according to claim 2, wherein the granulated product for sieving purpose is taken out between the net of the lower stage and the net of the lower stage, and the hot air is introduced from the lower side of the net of the lower stage. Screening method. 4. The method according to claim 1, wherein the bottom of the chute for discharging the granules of the hydraulic powder to be sieved is heated so as to maintain a predetermined temperature. A method for sieving granules of hydraulic powder. 5. The activated alumina powder is used as the hydraulic powder, and the heating is performed by applying hot air to 40 ° C. to 105 ° C. Method for sieving granules of hydraulic powder of the above. 6. A vibrating upper net for passing granules having a predetermined particle size range and a lower net for preventing granules having a predetermined particle size range from passing under the upper net. While introducing, the granules of hydraulic powder are introduced from above the upper net, and the granules in the predetermined particle size range are obtained from the middle of the upper net and the lower net. An apparatus for sieving granules of hydraulic powder, comprising hot air blowing means for blowing hot air from below the lower net to heat granules of hydraulic powder. 7. A temperature detecting means for detecting the temperature of at least one of the upper net or the lower net, and the hot air blowing so that the temperature detected by the temperature detecting means becomes a predetermined temperature. The apparatus for sieving granules of hydraulic powder according to claim 6, further comprising temperature control means for controlling hot air blown from the means. 8. The method further comprises heat retaining means for keeping a bottom surface of a chute for discharging a granulated product of hydraulic powder that is sieved by the upper net and the lower net at a predetermined temperature. An apparatus for sieving a granulated product of hydraulic powder according to claim 6 or 7.