Moisture absorbent and method for manufacturing moisture absorbent

Abstract

To provide a moisture absorbent capable of exhibiting good moisture absorption characteristics.SOLUTION: A moisture absorbent comprises composite particles containing calcium oxide as a main component and calcium hydroxide, wherein the mass ratio of calcium oxide to calcium hydroxide in the composite particles (calcium oxide / calcium hydroxide) is 2.0 or more, and the effective specific surface area represented by the product of the mass ratio of the calcium oxide in the composite particles and the BET specific surface area (m2 / g) is 16 m2 / g or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a moisture absorbent used for electronic devices, vacuum heat insulating materials, etc., and a method for producing the moisture absorbent.

Background Art

[0002] In order to protect electronic devices such as organic EL (Electro Luminescence) devices and solar cells from moisture, the electronic devices are sealed using a polymer composition layer. As a resin composition layer suitable for sealing electronic devices, those containing a hygroscopic filler are known, and calcium oxide is used as this hygroscopic filler. As characteristics of this calcium oxide, it is required to have high water absorption characteristics in order to protect devices that deteriorate due to moisture absorption, and to be fine powder with the miniaturization and thinning of electronic devices.

[0003] For example, Patent Document 1 discloses an organic electroluminescence element in which a laminate including an organic light emitting material layer and a chemical adsorbent for carbon dioxide are separately accommodated in a hermetically sealed container. Examples of this chemical adsorbent include a mixture of CaO as a moisture absorbent (desiccant) and Ca(OH)2 as a chemical adsorbent for carbon dioxide.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, Patent Document 1 does not evaluate the hygroscopicity of the chemical adsorbent, and its practicality is unclear. The present invention has been made in view of the above, and an object thereof is to provide a moisture absorbent that can exhibit good moisture absorption characteristics. [Means for Solving the Problems]

[0006] As a result of intensive studies to solve the above problems, the present inventors have found that by using composite particles containing calcium hydroxide as a main component and containing calcium hydroxide as a moisture absorbent, and setting the composition and effective specific surface area of the composite particles within a specific range, the above problems can be solved, and thus the present invention has been conceived. That is, the present invention is as follows.

[0007] [1] A moisture absorbent comprising composite particles containing calcium oxide as a main component and containing calcium hydroxide, wherein the mass ratio of calcium oxide to calcium hydroxide (calcium oxide / calcium hydroxide) in the composite particles is 2.0 or more, and the effective specific surface area represented by the product of the mass ratio of calcium oxide in the composite particles and the BET specific surface area (m 2 / g) is 16 m 2 / g or more. [2] The moisture absorbent according to [1], wherein the basicity obtained by dividing the carbon dioxide adsorption amount (μmol / g) by the BET specific surface area (m 2 / g) is 22 μmol / m 2 or less. [3] The moisture absorbent according to [1] or [2], wherein the content ratio of calcium oxide in the moisture absorbent is 60% by mass or more. [4] The moisture absorbent according to any one of [1] to [3], wherein the BET specific surface area is 10 to 30 m 2 / g. [5] The moisture absorbent according to any one of [1] to [4], wherein the 50% particle diameter (d50) in the volume-based cumulative particle size distribution is 0.5 to 5 μm, and the 97% particle diameter (d97) in the volume-based cumulative particle size distribution is 2 to 10 μm. [6] A method for producing the moisture absorbent according to any one of [1] to [5], comprising a firing step of using a cylindrical rotary furnace and firing a raw material containing calcium hydroxide at 600 to 1000°C, wherein the area ratio occupied by the raw material to the axial cross-sectional area inside the cylindrical furnace core tube is 3 to 10%. [7] The method for producing a moisture absorbent according to [6], including a particle size adjustment step of performing pulverization or classification after the firing step to adjust the particle size.

Advantages of the Invention

[0008] According to the present invention, it is possible to provide a moisture absorbent that can exhibit good moisture absorption characteristics. In particular, by using it in an electronic device, it is possible to expect an extended service life and improved performance of the electronic device.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0010] Hereinafter, an embodiment (this embodiment) of the present invention will be described in detail, but the present invention is not limited thereto.

[0011] [Moisture Absorbent] The moisture absorbent according to this embodiment is composite particles mainly composed of calcium oxide and containing calcium hydroxide, and the mass ratio of calcium oxide to calcium hydroxide (calcium oxide / calcium hydroxide) in the composite particles is 2.0 or more. If the mass ratio is less than 2.0, the moisture absorption characteristics will deteriorate. The mass ratio is preferably 2.0 to 122.0, and more preferably 4.0 to 33.0.

[0012] In order to make the mass ratio (content ratio) of calcium oxide to calcium hydroxide in the composite particles within the above range, for example, the firing conditions described later, particularly the firing temperature, supply amount (residence time), etc. may be appropriately changed.

[0013] The effective specific surface area represented by the product of the mass ratio of calcium oxide in the composite particles and the BET specific surface area (m 2 / g) is 16 m 2 / g or more. Here, the effective specific surface area is obtained by the following formula. Since it is calcium oxide that participates in the moisture absorption, it is an important factor in evaluating the moisture absorption characteristics. Formula: BET specific surface area of the moisture absorbent (m 2 / g) × Ratio of calcium oxide in the moisture absorbent If the effective specific surface area is less than 16 m 2 / g, the moisture absorption characteristics will deteriorate. The effective specific surface area is preferably 16 to 19 m 2 / g, and more preferably 18 to 19 m 2 / g. To make the effective specific surface area within the above range, for example, the specific surface area of calcium hydroxide, which is the raw material described later, the firing conditions, particularly the firing temperature and the supply amount (residence time), etc. may be appropriately changed.

[0014] The moisture absorbent according to this embodiment has a basicity obtained by dividing the carbon dioxide adsorption amount (μmol / g) by the BET specific surface area (m 2 / g) of 22 μmol / m 2 or less, preferably 0 to 20 μmol / m 2 , more preferably 1 to 15 μmol / m 2 . To make the basicity within the above range, for example, the firing conditions described later, particularly the firing temperature and the supply amount (residence time), etc. may be appropriately changed.

[0015] Here, the method for measuring the carbon dioxide adsorption amount is shown below. The device is not particularly limited as long as it can recognize the gas adsorption and desorption processes and measure the gas desorption amount while performing gas flow and temperature increase. Specific devices include the catalyst analysis device (BEL-CAT―BASIC) and the quadrupole mass spectrometer (BEL-Mass) manufactured by Microtrac BEL Corporation. Similar models, old models, or successor models of these may also be used. The measurement conditions are as shown in the following table.

[0016]

Table 1

[0017] Even if the pretreatment in Step 1 in the above table is omitted, it does not matter. The initial temperature during the temperature increase in the pretreatment of Step 1 and the pretreatment of Step 2 may not be 30°C, but may be the room temperature at which normal life is possible. In the holding in Step 7, it is waiting for the stabilization of the mass spectrometer, and the holding time is not limited.

[0018] Also, the value obtained by dividing the carbon dioxide adsorption amount (μmol / g) per 1 g of the sample measured by the above method by the BET specific surface area (m 2 / g) is the basicity (μmol / m 2 ).

[0019] The content ratio of calcium oxide in the desiccant according to this embodiment is preferably 60% by mass or more, more preferably 80 to 100% by mass, and even more preferably 89 to 98% by mass. By having the content ratio of 60% by mass or more, calcium oxide / hydroxycalcium can be made 2.0 or more.

[0020] The BET specific surface area of the desiccant according to this embodiment is preferably 10 to 30 m 2 / g, more preferably 19 to 30 m 2 / g, and even more preferably 20 to 25 m 2 / g. By having the BET specific surface area of 10 to 30 m 2 / g, a desired effective specific surface area can be obtained.

[0021] In the desiccant according to this embodiment, the 50% particle size (d50) in the cumulative particle size distribution based on volume is preferably 0.3 to 15 μm, and more preferably 0.5 to 5 μm. Also, the 97% particle size (d97) in the cumulative particle size distribution based on volume is preferably 1 to 260 μm, and more preferably 2 to 10 μm. In particular, when d50 is 0.5 to 5 μm, when processing into a sheet or the like, white streaks and white spots are less likely to occur, and the appearance and performance can be kept good. Also, by preventing the reactivity of calcium oxide with water from increasing due to the BET specific surface area becoming too large, it is possible to suppress the decrease in the calcium oxide content by absorbing moisture in the air during the grinding and classification processes.

[0022] d50 is more preferably 1 to 5 μm, and even more preferably 1 to 3 μm. d97 is more preferably 2 to 9 μm, and even more preferably 3 to 7 μm.

[0023] To make the particle size distribution within the above range, for example, a grinding or classification process described later may be provided. The particle size distribution can be measured by a laser diffraction particle size distribution measuring device. Also, although the refractive index varies depending on the device conditions, it is preferably 1.80 - 0.50i.

[0024] The moisture absorbent according to this embodiment is substantially composed of calcium oxide and calcium hydroxide, but calcium carbonate, magnesium oxide, silicon dioxide, aluminum oxide, iron oxide, etc. may be present as impurities. The total content of calcium oxide and calcium hydroxide is preferably 92% by mass or more, and more preferably 94% by mass or more.

[0025] The moisture absorbent as described above is suitable, for example, as an additive to a resin composition for protecting electronic devices such as organic EL devices and solar cells from moisture.

[0026] [Method for manufacturing moisture absorbent] The method for manufacturing the moisture absorbent according to this embodiment includes a firing step of using a cylindrical rotary kiln to fire a raw material containing calcium hydroxide at 600 to 1000°C.

[0027] As the cylindrical rotary kiln, for example, a rotary kiln as described in Japanese Patent No. 4052930 or Japanese Patent No. 6074540 can be used. However, the rotary kiln is not limited thereto. By using the cylindrical rotary kiln, the powder material can be heat-treated uniformly. The supply quantity of calcium hydroxide per hour may be appropriately adjusted according to the size and firing capacity of the rotary kiln.

[0028] The firing temperature is set to 600 to 1000 °C. This is because if it is less than 600 °C, the thermal decomposition of calcium hydroxide does not proceed and the production ratio of calcium oxide decreases. If it exceeds 1000 °C, the BET specific surface area of calcium oxide becomes small. The firing temperature is preferably 700 to 1000 °C, and more preferably 800 to 1000 °C.

[0029] Calcium hydroxide is used as the raw material. As the quality of calcium hydroxide, a quality equivalent to No. 1 slaked lime described in Japanese Industrial Standard (JIS) R9001 is desirable, and a quality equivalent to special grade slaked lime is more desirable. A low purity of calcium oxide in terms of quality means a large amount of calcium carbonate and other impurities. If the content ratio of calcium carbonate in calcium hydroxide is high, the thermal decomposition of calcium carbonate is not performed in the firing process, and the purity of calcium oxide in the fired product decreases.

[0030] The maximum particle size of calcium hydroxide is preferably 1 mm or less, more preferably 0.6 mm or less, and still more preferably 0.15 mm or less. In the case of particles exceeding 1 mm, the particle size of the calcium oxide obtained after firing will also exceed 1 mm. When a powder is required as the calcium oxide desiccant, an arbitrary pulverization process is required, resulting in high production costs.

[0031] The larger the BET specific surface area of calcium hydroxide, the better. Among them, 10 m 2 / g or more is preferable, and 15 m 2 / g or more is more preferable. 10 m 2When calcium hydroxide of less than / g is used as a raw material, the BET specific surface area of calcium oxide obtained by firing becomes small and the moisture absorption characteristics deteriorate.

[0032] The residence time of calcium hydroxide is preferably 5 to 10 minutes. The residence time can be appropriately set by the inclination of the core tube and the rotation speed of the core tube. The greater the inclination of the core tube, the faster the flow rate of the raw material, and the shorter the residence time. Usually, the range of 0.5 degrees to 2.0 degrees is acceptable. The higher the rotation speed of the core tube, the faster the moving speed of the fired raw material, so the residence time becomes shorter. When the residence time is 5 minutes or more, the production ratio of calcium oxide can be made good and excellent moisture absorption characteristics can be obtained. On the other hand, by setting it to 10 minutes or less, while increasing the production ratio of calcium oxide, it is possible to prevent the BET specific surface area from becoming too small and improve the moisture absorption characteristics.

[0033] In this embodiment, in the firing step, the area ratio occupied by the raw material with respect to the axial cross-sectional area inside the cylindrical core tube is set to 3 to 10%. The area occupied by calcium hydroxide with respect to the cross-sectional area inside the cylindrical core tube is an important factor for manufacturing calcium oxide with higher moisture absorption characteristics.

[0034] When the area ratio is 3% or more, it is possible to prevent the BET specific surface area from becoming small while increasing the production ratio of calcium oxide without excessive heat being applied to calcium hydroxide. Also, when it is 10% or less, the amount of heat for thermally decomposing calcium hydroxide does not decrease, and the production ratio of calcium oxide can be made good. In any case, excellent moisture absorption characteristics can be obtained.

[0035] The calculation method of the exclusive area of the raw material is as follows. First, the mass of the raw material supplied into the furnace can be known from the supply quantity (kg / min.) of the raw material and the supplied time (min.). By measuring the bulk density (kg / m 3 ), the volume V1 (m 3 ) of the raw material staying in the furnace is calculated. From the diameter and length inside the furnace, the volume V2 (m 3) can be understood. Assuming that the raw materials are evenly staying at the bottom of the furnace, V1 / V2 is equal to the area S1 occupied by the raw materials / the cross-sectional area S2 inside the core tube.

[0036] Depending on the firing conditions, the firing may progress and the BET specific surface area may decrease. In this case, even if the calcium oxide content ratio is high, the effective specific surface area may decrease, resulting in a small weight gain rate and the target moisture absorption characteristics may not be obtained. In such cases, a method of making the particle size finer and increasing the BET specific surface area by adding a pulverization and classification process is effective. It is also effective as a treatment when an appropriate particle size is required depending on the intended use.

[0037] Chemical analysis of the fired product obtained by the firing furnace etc. conforms to Japanese Industrial Standard JIS R 9011 (Test methods for lime). Furthermore, the detailed content ratio is calculated as follows.

[0038] (Calculation of content ratio) The following table shows example analysis values of special grade slaked lime for industrial use and special grade quicklime for industrial use (JIS R 9001). Using this value, the calculation method is shown.

[0039]

Table 2

[0040] 1) Calculation of bound water (mol per 100 g) Ig.Loss (ignition loss) is assumed to be the CO2 gas generated by thermal decomposition and the bound water of calcium hydroxide. Subtracting CO2 (mass%) from Ig.Loss (mass%) calculates the bound water (mass%). Dividing the bound water (mass%) by the molecular weight of water (18 g / mol) gives the hydroxyl group (mol per 100 g) (the following table). 2) Calculation of CO2 (mol per 100 g) CO2 is assumed to be the CO2 of calcium carbonate contained in the fired product. Dividing CO2 (mass%) by the molecular weight of CO2 (44 g / mol) gives CO2 (mol per 100 g) (the following table). 3) Calculation of mol per 100 g of CaO Dividing the mass percentage of CaO by the formula weight of CaO (56 g / mol) gives the mol of CaO per 100 g (see the table below). 4) Calculation of calcium oxide (mass percentage) Subtracting the mol of bound water per 100 g and the mol of CO₂ per 100 g from the mol of CaO per 100 g gives the mol of calcium oxide per 100 g contained in the fired product. 5) Calculation of calcium hydroxide (mass percentage) Multiplying the mol of bound water per 100 g by the formula weight of calcium hydroxide (74 g / mol) gives the mass percentage of calcium hydroxide. 6) Calculation of calcium carbonate (mass percentage) Multiplying the mol of CO₂ per 100 g by the formula weight of calcium carbonate (100 g / mol) gives the mass percentage of calcium carbonate (CaCO₃).

[0041]

Table 3

[0042] After the firing step, it is preferable to include a particle size adjustment step of pulverizing or classifying the fired raw material to adjust the particle size. By including this step, a desired particle size can be obtained.

[0043] The method of pulverization is not particularly limited, but dry pulverization apparatuses such as jet mills, roller mills, hammer mills, pin mills, rotary mills, vibration mills, planetary mills, attritors, and bead mills can be used. A jet mill is preferable because a powder with a particularly fine and sharp particle size distribution can be obtained. As the pulverization conditions for the jet mill, it is preferable to use an inert gas such as nitrogen or argon, or compressed air with the contained moisture reduced as much as possible through an air dryer to reduce deterioration due to moisture in the gas. Also, the pulverization pressure is preferably 0.5 - 1.5 MPa, and more preferably 0.3 - 0.9 MPa.

[0044] The classification method can be carried out using sieves or air classifiers (such as inertial force type classifiers, free vortex type classifiers, semi-free vortex centrifugal type, forced vortex type centrifugal classifiers, etc.). Since powders with fine and sharp particle size distributions can be obtained, air classifiers, especially semi-free vortex centrifugal type and forced vortex type centrifugal classifiers, are desirable. Similar to the pulverizer, it is preferable to use inert gases such as nitrogen and argon, or compressed air with the contained moisture reduced as much as possible through an air dryer.

[0045] If necessary, the pulverization process and the classification process may be combined. A classification process may be carried out to further cut the coarse fraction after the pulverization process, or a pulverization process may be carried out after cutting the coarse fraction by the classification process. As the number of processes increases, the chance of contact with air increases, so the possibility of moisture absorption by moisture in the air becomes higher. Furthermore, since the particle size becomes finer, the BET specific surface area increases, and the reactivity between calcium oxide and water becomes higher. In any case, since the content ratio of calcium oxide may decrease and the effective specific surface area may decrease, it should be appropriately selected.

[0046] Prior to the pulverization and classification processes, surface treatment may be carried out with an organic or inorganic agent if necessary. The surface treatment can suppress the moisture absorption of calcium oxide with moisture in the air and reduce the adhesion to the pulverizer and classifier.

Examples

[0047] Next, the present invention will be specifically described by experimental examples and comparative examples, but the present invention is not limited thereto.

[0048] First, it was considered what factors cause the basicity to increase or decrease. It was considered that there is a relationship between the calcium hydroxide content ratio and the basicity, and the relationship between calcium hydroxide and the basicity was examined. Industrial special grade slaked lime and calcium oxide produced in the present invention of the application were mixed at an arbitrary ratio to prepare measurement samples with different calcium hydroxide content ratios. For the prepared samples, the carbon dioxide adsorption amount was measured under the above-mentioned conditions.

[0049] The following table and Figure 1 show the relationships between the calcium hydroxide content ratio, carbon dioxide adsorption amount, BET specific surface area, and basicity. There is a positive correlation between the calcium hydroxide content ratio and the carbon dioxide adsorption amount. Therefore, when the content ratio of calcium hydroxide is high, the carbon dioxide adsorption amount increases. Therefore, it is assumed that when the basicity is high, the moisture absorption characteristics decrease. This study clarified the relationship between the calcium hydroxide content ratio and the carbon dioxide adsorption amount.

[0050]

Table 4

[0051] (Evaluation of moisture absorption) 0.6 g of the sample was placed in a stainless steel petri dish with a diameter of 50 mm and a height of 15 mm (the mass was measured in advance). It was placed in a thermo-hygrostat at a temperature of 24 °C and a relative humidity of 55%, and the total mass after 2 hours was measured. The mass increase rate was calculated as follows. Moisture absorption amount (g) = (Total mass after 2 hours, (g)) - (Initial total mass, (g)) Mass increase rate (%) = Moisture absorption amount (g) × 100% / Sample mass (g)

[0052] (Manufacture of moisture absorbent mainly composed of calcium oxide) Special grade industrial slaked lime shown in Table 1 was used as the raw material. The bulk density is 0.4 g / cm 3 is. Using a rotary furnace, the firing temperature, supply quantity, and filling ratio of calcium hydroxide as the raw material were appropriately set to manufacture moisture absorbents mainly composed of calcium oxide with different content ratios (Table 5). The core tube rotation speed is 24 rpm and the core angle is 1.5 degrees.

[0053]

Table 5

[0054] The analysis values of the obtained fired products are shown in Table 6 below.

[0055]

Table 6

[0056] Based on the analytical values in Table 6, the content ratio was calculated by the aforementioned calculation method. The calculation results of the content ratio are shown in Table 7 below.

[0057]

Table 7

[0058] The effective specific surface area, mass increase rate due to moisture absorption, carbon dioxide adsorption amount, and basicity are shown in Table 8 below. The relationship between the effective specific surface area and the mass increase rate is shown in Figure 2, and the relationship between the basicity and the mass increase rate when the effective specific surface area is stratified at 16 m 2 / g or more is shown in Figure 3.

[0059]

Table 8

[0060] (Description of Examples and Comparative Examples) In Comparative Examples 1 and 2, since the firing temperature was as low as 550 °C, the thermal decomposition of slaked lime did not proceed, and the production ratio of calcium oxide was small. Therefore, the mass increase rate due to moisture absorption was small. The effective specific surface area was less than 16 m 2 / g, and the basicity exceeded 22 μmol / m 2 .

[0061] In Examples 1 to 6, the mass increase rate was 21% or more, and a moisture absorbent exceeding the moisture absorption characteristics of the prior art was obtained. The effective specific surface area was 16 m 2 / or more, and the basicity was 22 μmol / m 2 or less.

[0062] In Comparative Example 3, the filling ratio was 2%. In this case, although a large amount of heat was required and the thermal decomposition of slaked lime proceeded, the BET specific surface area decreased. Therefore, it is presumed that the effective specific surface area decreased and the mass increase rate also decreased. Further, in Comparative Example 4, the filling ratio was 15%. In this case, the heat was insufficient and the thermal decomposition of slaked lime did not proceed, and the content ratio of calcium hydroxide increased. As a result, it is presumed that the effective specific surface area decreased and the mass increase rate also decreased.

[0063] In the present invention, the content ratio of calcium hydroxide was identified as a factor affecting the increase or decrease of the basicity, which was not clear in the prior art. Further, the relationship between the effective specific surface area derived from the content ratio of calcium oxide and the BET specific surface area and the moisture absorption characteristics was clarified. By controlling the effective specific surface area, the content ratio of calcium oxide and calcium hydroxide, and the basicity within a certain range, a moisture absorbent mainly composed of calcium oxide having moisture absorption characteristics exceeding those of the prior art and a method for producing the same were established.

[0064] The particle sizes of the calcium oxide obtained in Example 6, Comparative Example 3, and Example 7 were measured with a laser diffraction particle size distribution measuring device, and the results are shown in the following table. Using these calcium oxide powders, a pulverization and classification process was performed.

[0065]

Table 9

[0066] Examples 11, 13, and 14 and Comparative Example 6 were pulverized with a jet mill, and Examples 12 and 15 were classified with a vortex centrifugal classifier. Further, Comparative Example 5 performed a classification process after the pulverization process.

[0067] Examples 11, 13 and Comparative Example 5 that performed the pulverization process used air with a dew point temperature of -40 ° C through a refrigeration type and an adsorption type dryer. Example 14 used 4N nitrogen gas, and Comparative Example 6 used compressor air as it was. In both cases, the pulverization pressure was 0.85 MPa, and the pulverized product was recovered with a cyclone.

[0068] In Example 13, by pulverizing the calcium oxide of Comparative Example 3, the effective specific surface area could be increased to 19.2 m 2 / g and the weight increase rate could be raised to 24%. On the other hand, in Comparative Example 5, by combining the pulverization process and the classification process, calcium oxide powder with the finest particle size, having a d50 of 0.8 μm and a d97 of 2.7 μm, was obtained. However, due to the absorption of moisture in the air, the calcium hydroxide content increased to 37.8%, and the effective specific surface area decreased to 13.6 m 2 / g. In Comparative Example 6, since the removal of moisture in the air was insufficient, the calcium hydroxide content increased by 37.8%, and the effective specific surface area decreased to 11.9 m 2 / g.

[0069] For Example 12 and Comparative Example 5 where the classification process was carried out, the air passed through the aforementioned dryer, and for Example 15, 4N nitrogen gas was circulated. The raw material supply rate and the suction air volume were all kept constant, and classification was carried out by changing the rotational speed of the rotor to 4,000 rpm for Comparative Example 5 and 3,000 rpm for Example 12 and Example 15. Example 12 and Example 15 were able to obtain fine calcium oxide powder while maintaining the content range, effective specific surface area, and basicity within the scope described in the claims regarding calcium hydroxide.

[0070]

Table 10

[0071]

Table 11

Industrial Applicability

[0072] From the above, a moisture absorbent mainly composed of highly hygroscopic calcium oxide can be used in electronic devices such as organic EL devices and solar cells, and the creation of further high-performance devices is expected.

Claims

1. A moisture absorbent comprising composite particles mainly composed of calcium oxide and containing calcium hydroxide, wherein the mass ratio of calcium oxide to calcium hydroxide (calcium oxide / calcium hydroxide) in the composite particles is 2.0 or more, and the effective specific surface area represented by the product of the mass ratio of calcium oxide in the composite particles and the BET specific surface area (m ２ / g) is 16 m ２ / g or more.

2. The moisture absorbent according to claim 1, wherein the basicity obtained by dividing the carbon dioxide adsorption amount (μmol / g) by the BET specific surface area (m ２ / g) is 22 μmol / m ２ or less.

3. The moisture absorbent according to claim 1 or 2, wherein the content ratio of calcium oxide in the moisture absorbent is 60% by mass or more.

4. The moisture absorbent according to claim 1 or 2, wherein the BET specific surface area is 10 to 30 m ２ / g.

5. The moisture absorbent according to claim 1 or 2, wherein the 50% particle diameter (d50) in the cumulative particle size distribution based on volume is 0.5 to 5 μm, and the 97% particle diameter (d97) in the cumulative particle size distribution based on volume is 2 to 10 μm.

6. A method for producing the moisture absorbent according to claim 1 or 2, comprising a firing step of using a cylindrical rotary furnace and firing a raw material containing calcium hydroxide at 600 to 1000°C, and the area ratio occupied by the raw material with respect to the axial cross-sectional area inside the cylindrical furnace core tube is 3 to 10%.

7. The method for producing the moisture absorbent according to claim 6, further comprising a particle size adjustment step of performing pulverization or classification to adjust the particle size after the firing step.

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