KR101178397B1 - Low soda boehmite and preparation method thereof using hydrothermal synthesis - Google Patents

Abstract

PURPOSE: A low soda boehmite and a manufacturing method thereof by hydrothermal synthesis are provided to obtain boehmite having a low soda content and excellent thermal stability. CONSTITUTION: A low soda boehmite and a manufacturing method thereof by hydrothermal synthesis comprises the following steps: adding oxalic acid in a raw material aluminium hydroxide aqueous solution and controlling the pH as 3.0-8.0; and processing hydrothermal synthesis reaction and obtain boehmite having soda (Na2O) content of 0.12 weight% or less. The concentration of the raw material aluminium hydroxide aqueous solution is 1.5-5M. The addition amount for the raw material aluminium hydroxide aqueous solution of the oxalic acid is 0.01-0.1M. The hydrothermal synthesis reaction is processed at 120-250 deg. Celsius and a pressure condition of 4-25 bar for 30-150 minutes. The low soda boehmite has an average particle size of 0.5-50 micro meters, soda (Na2O) content of 0.12 weight% or less, and a specific surface area(BET) of 1-5m^2/g.

Description

LOW SODA BOEHMITE AND PREPARATION METHOD THEREOF USING HYDROTHERMAL SYNTHESIS}

The present invention relates to a low soda boehmite and a method for preparing the same, which can be variously used as a flame retardant filler and a thermal conductivity enhancing filler. Specifically, the present invention has a low content of soda (Na ₂ O). It relates to a manufacturing method.

Aluminum hydroxide is a general term for the hydroxide of aluminum, the typical chemical formula is Al ₂ O _{3 ~} 3H ₂ O, and the naturally occurring form of Gibbsite (Al (OH) ₃ ), boehmite (AlOOH) or diaspore (α-AlOOH). Of these, gibsite loses water molecules when heated to a temperature of 200 ° C. or higher and is converted into AlO · OH form, and when heated to a temperature of 500 ° C. or higher, it is converted into a transition type alumina (γ, η-Al ₂ O ₃ ).

Since boehmite exists in the natural phase but there are many impurities, it is commonly used industrially by producing crystals such as boehmite and transitional alumina using Gibbsite as in Scheme 1. However, depending on the purpose of use, physical properties may be demandingly demanded, and in the case of boehmite prepared by heat treatment, a thermal decomposition initiation temperature starts at 100 ° C. or lower, and a complicated process is added to remove impurities soda.

Scheme 1

As a general method for industrially mass production of aluminum hydroxide (Al ₂ O _{3 to} 3H ₂ O), bauxite is dissolved in sodium hydroxide solution to form a mother liquor, and aluminum hydroxide seed is added to the mother liquor to precipitate aluminum hydroxide. Bayer process is mainly used. Aluminum hydroxide (Al ₂ O ₃ ˜3H ₂ O) prepared by this process is used as a basic raw material which is very important industrially because of its excellent physical and chemical properties and various uses.

However, the aluminum hydroxide produced by the Bayer process contains about 0.4% of soda, which is not suitable for use as a raw material for electricity and electronics. Therefore, the aluminum hydroxide prepared by the Bayer process is recently removed to remove functional alumina such as activated alumina, low soda alumina, and high purity alumina. Manufacturing methods are being developed.

Conventional methods for removing the soda contained in the alumina include Cl ₂ gas reduction, SiO ₂ addition reaction, treatment of activated alumina, etc. (Japanese Patent Laid-Open No. 55-140719, US Pat. No. 2,774,774, US Pat. 3,628,914, US Patent No. 3,092,452, UK Patent No. 2,108,949, and Korean Patent Publication No. 2001-0046015). However, these are related to the method for producing alumina and are not suitable for the production of boehmite.

Accordingly, the present inventors have found that the low soda boehmite can be produced by a convenient and simple environment-friendly process, while being differentiated from the conventional activated alumina production method, alkoxide production method, or precipitation method. The invention was completed.

Japanese Patent Laid-Open No. 55-140719 U.S. Patent No. 2,774,774 U.S. Patent No. 3,628,914 U.S. Patent No. 3,092,452 British Patent No. 2,108,949 Korean Patent Publication No. 2001-0046015

Accordingly, an object of the present invention is to provide a boehmite having a lower content of soda (Na ₂ O) and a method for preparing the same by a hydrothermal synthesis reaction different from the conventional method.

In order to achieve the above object, the present invention adjusts the pH to 3.0 to 8.0 by adding oxalic acid to the aqueous solution of raw aluminum hydroxide, and then performs a hydrothermal synthesis reaction so that the content of soda (Na ₂ O) is 0.12 wt% or less. It provides a method for producing low soda boehmite, comprising the step of obtaining a mite.

The present invention also provides a low soda boehmite having an average particle size of 0.5 to 50 µm, BET 1 to 5 m 2 / g, and a soda (Na ₂ O) content of 0.12% by weight or less.

Boehmite according to the present invention has a very low soda content and excellent thermal stability, it can be variously used as a flame retardant filling and thermal conductivity enhancing fillers in the fields of electrical, electronic, textile, paper, plastic manufacturing, and different from the conventional It can be manufactured in a convenient, simple and environmentally friendly way.

1 and 2 are XRD results and electron micrographs of the boehmite crystals obtained by varying the hydrothermal synthesis reaction temperature and reaction pressure in the manufacturing process of boehmite according to the present invention, respectively.
Figure 3 is an XRD result of the boehmite crystals obtained by varying the hydrothermal synthesis reaction time of the process for producing boehmite according to the present invention.
Figure 4 is a thermal analysis showing the thermal decomposition characteristics of boehmite and raw material aluminum hydroxide according to the present invention.
Figure 5 is an XRD result of the boehmite crystals obtained by varying the particle size of the raw aluminum hydroxide powder of the process for producing boehmite according to the present invention.

Hereinafter, the method for producing boehmite according to the present invention will be described in detail step by step.

Preparation of Boehmite by Hydrothermal Synthesis

In this step, aluminum hydroxide, a raw material, is placed in a hydrothermal synthesis reactor and pyrolyzed at high temperature and steam pressure to prepare boehmite, which is a transition type alumina.

The aluminum hydroxide used as a raw material may be prepared by a conventional Bayer process, and thus the aluminum hydroxide produced by the Bayer process generally contains a trace amount of impurities.

The raw material aluminum hydroxide may be used in the form of a powder, may be used having a particle size of 0.1 to 50 ㎛, preferably one having a particle size of 0.2 to 30 ㎛.

Distilled or industrial water may be used for the water used for the hydrothermal reaction, and the amount of water added to the aluminum hydroxide is added so that the concentration of the aluminum hydroxide solution is 1 to 10 M, more preferably 1.5 to 5 M. It is desirable to. Stirring can be made more smoothly within the above range, there is an advantage that the slurry in a faster time.

Stirring conditions in the hydrothermal reaction tank is suitable within 150 times per minute, it may be less interference with hydrothermal synthesis without inhibiting the occurrence of the slurry within the above range.

In the process of converting aluminum hydroxide to transitional alumina under high temperature pressurization conditions in the hydrothermal synthesis reactor, trihydrate is converted to monohydrate as in Scheme 2. In addition, Na ions present in the aluminum hydroxide crystals react with the activated water molecules to react with NaOH or Na ₂ CO ₃ to de-ionize the aluminum hydroxide.

Scheme 2

Al ₂ O ₃ ˜3H ₂ O → Al ₂ O ₃ ? H ₂ O + 2H ₂ O

The raw aluminum hydroxide contains about 34.5% of crystalline water. The boehmite obtained by crystalline water exiting from pyrolysis under the conditions of high temperature and high pressure of hydrothermal synthesis contains about 16 to 18% by weight of water in the form of crystalline water. In this process, Na ⁺ ions also escape from the aluminum hydroxide crystals, increasing the activity in contact with the aqueous solution, and the Na ⁺ ion removal efficiency is improved.

Preferable conditions of the hydrothermal reaction are as follows.

The reaction temperature can be adjusted in the range of 120 to 250 ° C., more preferably 190 to 230 ° C.

The reaction pressure can be adjusted in the range of 4 to 25 bar, more preferably 15 to 20 bar.

The reaction time can be adjusted to 30 to 150 minutes, more preferably 60 to 120 minutes.

The range of the initial pH of the reaction can be adjusted to 3.0 to 8.0, more preferably pH 5.5 to 7.5.

Within this range, it is easy for the production of boehmite crystals and deionization of Na ⁺ , and the prepared boehmite is suitable for exhibiting transitional alumina properties with high pyrolysis temperature.

According to one embodiment of the invention, the hydrothermal reaction can be carried out for 30 to 150 minutes at a temperature of 120 to 250 ° C and a pressure of 4 to 25 bar at an initial pH of 3.0 to 8.0.

According to another embodiment of the present invention, the hydrothermal reaction may be performed at an initial pH of 5.5 to 7.5 for 60 to 120 minutes at a temperature of 190 to 230 ° C. and a pressure of 15 to 20 bar.

In order to maximize the soda removal efficiency and maintain the stabilized salt state of the removed Na ions in the hydrothermal synthesis reaction of this step, by adding a hydroxide (C ₂ H ₂ O ₄ ) as a pH adjuster to control the acidity of the reaction solution As shown in 3, sodium oxalate (Na ₂ C ₂ O ₄ ) can be promoted to rapidly remove Na ions in aqueous solution.

Scheme 3

2NaOH (ℓ) + C ₂ H ₂ O ₄ (ℓ) → Na ₂ C ₂ O ₄ (ℓ) + 2H ₂ O

At this time, the amount of the hydroxide added to the reaction solution is preferably 0.01 to 0.1 M, it is possible to further activate the reaction of soda by inhibiting the formation of the complex compound by dissolution of aluminum in the concentration range. More preferably, the addition amount may be adjusted within the above range in consideration of the concentration of Na ₂ O in the reaction solution, for example, it may be added at a ratio of 1 to 3 moles of hydroxyl per mole of Na ₂ O in the reaction solution.

Filtration and drying process

This step is to prepare a granular powder by filtration and drying the obtained transitional alumina.

In this step, the boehmite slurry prepared in advance can be filtered and washed using a vacuum filter of 0.4 to 10 cc / ㎠ filtration method.

After washing, the filter cake may be dried by a direct hot air drying method, which may be used as a spray dryer or a flash dryer. In the case of the spray dryer, in order to dry the filter cake, it is preferable to slurry the filter cake again and use a slurry having a solid solution concentration of 10 to 50%. In the case of a flash dryer, the filter cake itself can be used directly. Both methods do not change the basic physical properties of the product, but the spray drying method is more preferable in terms of utilization of the product.

The supply hot air temperature of the dryer can be used at 180 to 300 ℃, it can be applied in the same manner as the usual drying method.

According to the method of the present invention, boehmite is very low and thermally stable in a convenient and simple environmentally friendly manner differentiated from the conventional method of producing activated alumina, alkoxide production, or precipitation method. It can manufacture.

Hereinafter, the low soda boehmite of the present invention that can be produced in the above manner will be described in detail.

The low soda boehmite according to the present invention has a very low content of impurities, in particular a content of soda (Na ₂ O) is 0.12% by weight or less, more preferably 0.06 to 0.1% by weight. Also preferably other impurities are very high purity, such as Fe ₂ O ₃ 0.02 wt% or less, SiO ₂ 0.01 wt% or less, CaO 0.01 wt% or less.

In addition, the average particle size is 0.5 to 50 ㎛ and may have a specific surface area (BET) of 1 to 5 m 2 / g, and may contain 16 to 18% by weight of water as crystal water.

The boehmite of the present invention has very good physicochemical properties, in particular thermal stability, for example, the initiation temperature of pyrolysis may be higher than 320 ° C, for example 320-400 ° C, more preferably 340-380 ° C. have.

Low soda boehmite according to the present invention can be variously applied depending on the intended use. For example, small particle size products can be used for functional paints and cosmetics, as well as flame retardant fillers such as CCL, MMCL, PCB, etc., and large particle size products can be used as catalyst raw materials for catalysts and pharmaceuticals, moisture absorbents, and the like. In case of high temperature firing, it can be used as high value-added functional material such as high voltage rubber and plastic filler, insulator, spark plug, IC package, medicine, cosmetics and high melting point ceramic raw material.

Hereinafter, the present invention will be described in more detail by way of examples. The following examples are examples of the present invention and the scope of the present invention is not limited only to the following examples.

Example 1 Preparation of Boehmite by Hydrothermal Synthesis (Change of Reaction Temperature and Pressure)

Raw material aluminum hydroxide (average: Al ₂ O ₃ ˜3H ₂ O 99.7%, Na ₂ O 0.25 ~ 0.35%, SiO ₂ 0.01%, Fe ₂ O ₃ 0.01%, LOI Thermal loss): 33.4%, moisture: 0.5%, product name: KH-101LC, manufacturer: KC Corporation.

As a hydrothermal reactor, a stainless steel autoclave capable of temperature control and pressure measurement was used.

300 g of the sample was placed in a 2 L stainless steel autoclave, and 1 L of water was added to the reaction solution, and the reaction solution was added with a concentration of 0.06 M (H ₂ C ₂ O ₄ ˜2H ₂ O, manufacturer: Duksan Chemical Co., Ltd.). PH was adjusted to 5.5 by addition.

The reaction temperature and pressure conditions were then raised to (1) 160 ° C and 10 bar, (2) 180 ° C and 12 bar, (3) 200 ° C and 16 bar, or (4) 220 ° C and 19 bar, respectively, followed by hydrothermal synthesis for 60 minutes. It was then cooled slowly.

Thereafter, the prepared boehmite slurry was filtered using a vacuum filter of 0.6 cc / cm 2 cloth filtration method. The slurry was washed twice with industrial water corresponding to the amount of sample used for the reaction after slurry filtration.

After washing, the filter cake was dried by hot air drying, and the dryer used was a spray dryer. In the case of the spray dryer, in order to dry the filter cake, the filter cake was slurried again, and a slurry having a high liquid concentration of 40% was used. The supply hot air temperature of the dryer was used at 180 to 250 ° C., which was the same as the usual drying method. Applied.

Boehmite was prepared by varying the reaction temperature according to the above procedure, and the specific surface area (BET) and soda content thereof were measured and shown in Table 1 below.

Characteristics of Boehmite with Hydrothermal Synthesis Temperature Temperature (℃) 160 180 200 220 BET (㎡ / g) 4.3 4.0 4.2 4.5 Na ₂ O (%) 0.12 0.076 0.035 0.034 Pressure (bar) 10 12 16 19

As shown in Table 1, the influence of the reaction temperature showed the most ideal reaction at around 200 ℃ reaction temperature, the lower the temperature showed a tendency to decrease the phase change to bohemite as the water vapor pressure is lowered . Therefore, in order to obtain the desired boehmite, it was most preferable to set the reaction temperature within 190 to 230 ° C and the reaction pressure within 15 to 20 bar.

In addition, Figure 1 is a graph showing the XRD analysis results of the phase change of aluminum hydroxide according to the hydrothermal synthesis reaction temperature and reaction pressure carried out in Example 1 (Dp 50: 2㎛, reaction holding time 60 minutes). The unreacted gibbsite peak is partially mixed with the boehmite peak at the reaction temperature of 200 ° C. and the reaction pressure below 16 bar. However, only the boehmite peak is clearly grown and present at the conditions of 200 ° C. and 16 bar. Similar results can be seen at the reaction temperature of 220 ° C. and the reaction pressure of 19 bar, which means that the gibbsite is completely changed to boehmite.

In addition, Figure 2 shows an electron micrograph of the boehmite crystals prepared according to the hydrothermal synthesis reaction temperature. It can be seen that the shape of the electron micrographs of the particle surface varies depending on the reaction temperature.When the reaction temperature is low, the gibbsite of hexagonal structure is dominant. It can be confirmed that the phase change. In particular, it can be seen that the reaction conditions of 200 ℃ and 16 bar shows a different crystal structure from the reaction conditions of 160 ℃ and 10 bar, which can be confirmed that the results are consistent with the results of XRD or chemical analysis.

Example 2: Preparation of Boehmite by Hydrothermal Synthesis (Change of Reaction Time)

The same procedure as in Example 1 was carried out, but the reaction temperature was fixed to 200 ℃ and the reaction pressure 16bar, the reaction time was changed to 30, 60, 90 and 120 minutes to prepare boehmite, its specific surface area (BET) And soda content is measured and shown in Table 2 below.

Characteristics of Boehmite with Hydrothermal Synthesis Reaction Time Reaction time 30 minutes 60 minutes 90 minutes 120 minutes BET (/ g) 3.8 4.2 4.5 4.5 Na ₂ O (%) 0.045 0.035 0.035 0.036

As shown in Table 2, when the reaction time is 60 minutes at the reaction temperature of 200 ℃ and the reaction pressure 16 bar was sufficiently converted to boehmite, when the reaction time is less than 60 minutes, unreacted gibbsite Some peaks of coexisted. In addition, even if the reaction time was maintained for more than 60 minutes, there was no significant change in soda content or BET.

In addition, Figure 3 shows the phase change according to the reaction time of the boehmite prepared at the reaction temperature 200 ℃ and reaction pressure 16bar. Influence of the reaction time was sufficiently transferred to boehmite when the reaction time was 60 minutes at the reaction temperature of 200 ° C. and the reaction pressure of 16bar, and when the reaction time was shorter than 60 minutes, the phase change was lowered, resulting in unreacted gibbsite. Some peaks of coexisted.

Example 3: Preparation of Boehmite by Hydrothermal Synthesis (Initial Reaction pH Change)

The same procedure as in Example 1 was carried out, but the reaction temperature was fixed at 200 ° C. and the reaction pressure was fixed at 16 bar. The specific surface area (BET) and soda content thereof were measured and shown in Table 3 below.

Characteristics of Boehmite with pH Change at Initial Hydrothermal Synthesis Fisheries addition amount (M) 0.05 0.06 0.063 0.08 Initial pH 3.5 5.5 7.5 8.5 BET (㎡ / g) 4.2 4.2 4.5 4.3 Na ₂ O (%) 0.030 0.034 0.038 0.45

As shown in Table 3, in the conditions according to the change of the initial pH, the reaction showed the maximum in the weakly acidic or neutral region. This result indicates that the hydroxyl used as pH regulator reacts appropriately with Na ions, and when pH is lower than 5.5, some dissolution of aluminum ions occurs, which tends to suppress sodium oxalate production. . Therefore, the pH was found to show a tendency to maximize the sodium oxalate reaction while minimizing the dissolution of aluminum ions in the 5.5 to 7.5 range.

Example 4 Preparation of Boehmite by Hydrothermal Synthesis (Change of Particle Size of Aluminum Hydroxide)

The same procedure as in Example 1 was carried out, but the reaction temperature was fixed to 200 ℃ and the reaction pressure 16bar, the average particle size of the raw material aluminum hydroxide was changed to 1.5, 8, 25 and 50 ㎛ to prepare boehmite, the ratio thereof Surface area (BET) and soda content were measured and shown in Table 4 below.

Characteristics of Boehmite by Change of Particle Size of Starting Material Average particle size (㎛) 1.5 8 25 50 BET (㎡ / g) 4.5 3.4 1.9 1.2 Na ₂ O (%) 0.035 0.045 0.052 0.051

As shown in Table 4, the reaction rate of boehmite according to the change of the average particle size of the raw material aluminum hydroxide powder was hardly changed under the reaction conditions of 200 ℃ and 16 bar.

FIG. 5 shows the production of boehmite using raw aluminum hydroxide powders (product names: KH-101LC, KH-101LP and KH-25R, manufactured by KC Corporation) having an average particle size (D50) of 1.7 μm, 2.0 μm, and 25 μm, respectively. After that, the phase change is measured. From this, it can be seen that as the average particle size increases, the Gibbsite peak slightly coexists as the boehmite conversion decreases.

Meanwhile, thermal analysis data of boehmite (reaction conditions: 200 ° C. and 16 bar) and raw material aluminum hydroxide prepared in Example 1 are shown in FIG. 4. In the case of boehmite prepared by the method of the present invention, it was confirmed that the crystal water came out while pyrolysis occurred at 320 to 400 ° C. higher than the decomposition temperature of aluminum hydroxide.

On the other hand, when the phase change does not occur completely with boehmite, that is, when aluminum hydroxide and boehmite coexist, it was confirmed that the crystal water escapes in the vicinity of 220 ~ 240 ℃ and 280 ~ 320 ℃.

Comparative Example 1 Preparation of Boehmite, Transitional Alumina by Pyrolysis in a Kiln Process

The phase change of aluminum hydroxide was measured using XRD while changing the reaction temperature to 200-500 ° C. in the same raw material aluminum hydroxide as in Example 1 in a laboratory box kiln. The results are shown in Table 5 below.

Temperature (℃) 200 300 400 500 Phase change G + B B + λ B + λ λ

* G: Gibbsite, B: Boehmite, λ: Gamma Alumina

As shown in Table 5, it was confirmed that the Gibbsite and boehmite peaks coexist in the 200 ℃ region, when the reaction temperature rises to 300 ℃ it could be seen that the boehmite and gamma alumina is generated simultaneously. In addition, when the reaction temperature was increased above, it was confirmed that many gamma peaks existed and transferred to the transition-type alumina, which means that the preparation of boehmite by the kiln process is more difficult to control than the hydrothermal synthesis method.

In addition, as a result of performing a pyrolysis test on boehmite prepared through a kiln heat treatment of aluminum hydroxide, it was confirmed that the pyrolysis started at 100 ° C. or lower and continued to proceed.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, It is to be understood that the invention may be practiced within the scope of the appended claims.

Claims (9)

Adding an oxalic acid to an aqueous solution of the raw aluminum hydroxide, adjusting the pH to 3.0 to 8.0, and performing hydrothermal synthesis to obtain boehmite having a soda (Na ₂ O) content of 0.12 wt% or less. ,
The concentration of the said aqueous solution of aluminum hydroxide is 1.5-5 M, The addition amount with respect to the said raw material aluminum hydroxide aqueous solution of the said hydroxide is 0.01-0.1 M, The manufacturing method of the low soda boehmite.
The method of claim 1,
The hydrothermal synthesis reaction, characterized in that carried out for 30 to 150 minutes at a temperature of 120 to 250 ℃ and a pressure condition of 4 to 25 bar, low soda boehmite production method.
The method of claim 1,
The hydrothermal synthesis reaction, characterized in that the initial pH 5.5 to 7.5 carried out for 60 to 120 minutes at a temperature of 190 to 230 ℃ and pressure conditions of 15 to 20 bar, low soda boehmite production method.
delete The method of claim 1,
The raw material aluminum hydroxide is produced by a Bayer process, characterized in that the low soda boehmite manufacturing method.
The method of claim 1,
The method for producing low soda boehmite, further comprising the step of preparing a granular powder by filtering and drying the boehmite prepared in the hydrothermal synthesis.
As low soda boehmite produced by the method according to any one of claims 1 to 3, 5 and 6,
Low soda boehmite with an average particle size of 0.5 to 50 μm, a soda (Na ₂ O) content of 0.12% by weight or less, and a specific surface area (BET) of 1 to 5 m 2 / g.
The method of claim 7, wherein
The low soda boehmite has a thermal decomposition initiation temperature of 320 ° C. or higher and contains 16 to 18% by weight of water as crystal water.
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