JP3665186B2 - Continuous hydrothermal synthesis of alumina particles - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for continuously producing fine plate-like alumina particles suitable for paint pigments, ceramic raw materials, magnetic tape fillers, abrasives, and the like by hydrothermal treatment.
[0002]
[Prior art]
As a method for producing fine alumina particles, a method is known in which alumina is produced by the Bayer method, and fine powder particles are obtained from the obtained powder by a mechanical fragmentation method. This method has a problem that it is difficult to control each of the resulting particles. In addition, a hydrothermal synthesis method is known as a method for obtaining alumina particles by a chemical growth method, and it is known that alumina particles having a uniform particle shape and particle size distribution are produced. However, this method currently uses an autoclave and requires a relatively long reaction time.
[0003]
[Problems to be solved by the invention]
In the conventional method for producing alumina particles, fine alumina particles which are uniform primary particles are not obtained. Further, since a long time is required for the reaction, an object of the present invention is to continuously produce uniform and fine alumina particles in a relatively short time.
[0004]
[Means for Solving the Problems]
The present invention uses a slurry of aluminum hydroxide or alumina hydrate together with water or an alkaline aqueous solution, and water, while heating the water above the saturated vapor temperature and pressurizing it below the saturated water vapor pressure. On the other hand, the slurry is supplied while being pressurized to a temperature lower than the saturated vapor temperature and higher than the saturated vapor pressure, and spray-mixed while adjusting the mixing ratio so that it does not fall below the saturated vapor temperature. A continuous hydrothermal synthesis method of alumina particles, characterized in that hydrothermal synthesis is performed, and after synthesis, the temperature is reduced and reduced to produce alumina particles.
[0005]
The high-temperature and high-pressure conditions in the reaction section are 350 ° C. or more and 50 kgf / cm ² or more, more preferably 400 to 500 ° C. and 100 to 200 kgf / cm ² , and the flow rate in the pipe after spray mixing is 0 per minute. .2m or more.
In the slurry, the concentration of aluminum hydroxide or alumina hydrate with respect to water or an aqueous alkali solution is 1 to 50 wt%, more preferably 2 to 20 wt%.
[0006]
The present invention uses as a starting material alumina hydrate such as aluminum hydroxide or boehmite, which has been previously pulverized with a ball mill or the like and adjusted in particle size to a submicron order, and this is mixed with water or an alkaline aqueous solution such as caustic soda or sodium carbonate. The slurry is fed into the pipe while being pressurized at 1 to 50 wt%, preferably 2 to 20 wt%, and this is spray mixed with the other superheated steam to simultaneously make the slurry into superheated steam and solids, and a high temperature and high pressure atmosphere It is carried out by continuous hydrothermal synthesis in which the inside is retained, and then the temperature is lowered and the pressure is reduced. Adjusting the particle size of the starting raw material, aluminum hydroxide or alumina hydrate, to the submicron order means that the raw material solids cannot be suspended during spray mixing in continuous hydrothermal synthesis, This is because clogging occurs during mixing, and a fine raw material is required to further synthesize fine particles. Temperature, for the conditions of the output, in Al ₂ O ₃ · H ₂ O phase diagram, must be stable region of α-Al ₂ O _3. The reason why the temperature is limited to 350 ° C. or higher is that α-Al ₂ O ₃ cannot be obtained at a temperature lower than 350 ° C. Although the upper limit about temperature and pressure is not particularly limited, it is related to the apparatus and is preferably within the range in consideration of economy. Preferably, the pressure is 50 atm or higher, preferably 100 at a concentration of 400 to 550 ° C. The condition is 200 kgf / cm ² , and the higher the temperature and the higher the pressure, the higher the production rate of α-Al ₂ O ₃ , and fine particles can be obtained in a short time.
[0007]
The reason why the flow rate after spray mixing is 0.2 m / min or more is that if the flow rate is low, the residence time in the pipe can be increased, and α-Al ₂ O ₃ can be synthesized. It is not possible to obtain uniform fine particles due to variations in the shape and size of each one, and it is necessary to have a synthesis condition that takes into account the flow rate. When the flow rate is 0.2 m / h or more, the size of the particles is fine. Thus, each particle becomes α-Al ₂ O ₃ particles which are uniform and aligned, and when the flow rate is further increased, fine and thin plate-like alumina is synthesized.
[0008]
According to the production method of the present invention, fine α-Al ₂ O ₃ particles having a hexagonal crystal form and a specific crystal plane grown in a flat plate shape can be obtained. Further, the particles may have a particle diameter of 0.2 to 1 μm and an aspect ratio (particle long diameter / thickness) of 1 to 50. In particular, the particle diameter can be less than 0.5 μm, and such α-Al ₂ O ₃ particles can be used as an abrasive or a filler.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is implemented by a continuous hydrothermal synthesizer as shown in FIG. In FIG. 1, 1 is a water quality liquid tank, 2 is a slurry tank, and 3 and 4 are pumps for supplying each. Reference numeral 5 denotes a heating unit, and 6 is a heater. The water quality liquid (water) in the water quality liquid tank 1 supplied by the pump 3 is heated to a temperature equal to or higher than the saturated vapor temperature, particularly the critical temperature, by the heater 6, and the slurry in the slurry tank 2 supplied by the pump 4. Is sent in at room temperature. The heated water fluid and the normal temperature slurry are mixed in the spray mixing unit 8, then heated to near the reaction temperature by the heater 9, and sent to the reaction unit 10. The reaction unit 10 is provided with a heater 11, which is additionally heated as necessary. The reaction product is forcibly cooled by the cooler 12, passes through the throttle valve 20, and then sent to the product discharge unit 13 or the thin tube 21. The throttle valve 20 may be omitted. The product discharger 13 or the thin tube 21 is switched by a cock 22. When switching to the product discharger 13, a shutoff valve 14 is provided upstream of the product discharger 13, and shutoff valves 15 and 23 are provided downstream. The product first contains the product in the product discharger 13 by opening the shut-off valve 14, then closes the shut-off valve 14 and opens the shut-off valves 15 and 23 to send the product to the receiver 16. The product is discharged into a take-out container 18. Reference numeral 19 denotes an auxiliary device provided in consideration of a case where the pressure of the slurry reaction product in the system is insufficient when the shut-off valve 14 is closed and the shut-off valve 15 is opened, and is a compressed gas supplier. When switching to the thin tube 21, the product passes through the thin tube, is reduced to normal pressure by the pressure loss in the thin tube, is sent to the receiver 16, and is discharged to the take-out container 18.
[0010]
In the above, the spray mixing unit 8 sprays the fine particles with a spray device as necessary, and then further reduces the fine particles to 10 μm or less with a pulverizer.
Examples of the present invention implemented using the above apparatus will be described below. The slurry pump capacity was 2 liters / hour, the water pump capacity was 8 liters / hour, and the volume of the reaction part was 32 liters.
[0011]
Example 1
Aluminum hydroxide obtained by the Bayer method, adjusted to a particle size of 0.4 μm with a ball mill using a ball mill, is adjusted to a concentration of 2.5 wt% with ion-exchanged water, and sent at a feed rate of 2 liters per hour. The reaction was carried out at a pressure of 100 kgf / cm ² and a temperature of 450 ° C. using a continuous device adjusted so that the exchanged water was delivered at 8 liters per hour. The flow rate after spray mixing was 1.0 m / min. When the product obtained by washing, filtering and drying the obtained product was identified by X-ray diffraction, 100% α-alumina was produced, and the particle size of this α-alumina was 0.2 μm, The aspect ratio was 20.
[0012]
Example 2
Aluminum hydroxide obtained by the Bayer method, adjusted to a particle size of 0.4 μm with a ball mill, is adjusted to a concentration of 10 wt% with ion-exchanged water, sent at a feed rate of 2 liters per hour, and ion-exchanged water Was reacted at a pressure of 200 kgf / cm ² and a temperature of 430 ° C. in a continuous apparatus adjusted to deliver 8 liters per hour. The flow rate after spray mixing was 0.9 m / min. When the product obtained by washing, filtering and drying the obtained product was identified by X-ray diffraction, 100% α-alumina was produced, the particle diameter was 0.5 μm, and the aspect ratio was It was 20.
[0013]
Example 3
Aluminum hydroxide obtained by the Bayer method, adjusted to a particle size of 0.4 μm with a ball mill, is adjusted to a concentration of 10 wt% with ion-exchanged water, sent at a feed rate of 2 liters per hour, and ion exchange The reaction was carried out at a pressure of 200 kgf / cm ² and a temperature of 500 ° C. using a continuous device adjusted to feed water at 8 liters per hour. The flow rate after spray mixing was 0.5 m / min. The powder obtained by washing, filtering and drying the obtained product was identified by X-ray diffraction. As a result, 100% α-alumina was produced, the particle diameter was 0.4 μm, and the aspect ratio was 10.
[0014]
Example 4
Aluminum hydroxide obtained by the Bayer method, adjusted to a particle size of 0.4 μm with a ball mill, is adjusted to a concentration of 10 wt% with ion-exchanged water, sent at a feed rate of 2 liters per hour, and ion-exchanged water Was reacted at a pressure of 200 kgf / cm ² and a temperature of 550 ° C. in a continuous apparatus adjusted to deliver 8 liters per hour. The flow rate after spray mixing was 0.55 m / min. When the product obtained by washing, filtering and drying the obtained product is identified by X-ray diffraction, 100% α-alumina is produced, the particle diameter is 0.3 μm, and the aspect ratio is It was 5.
[0015]
Example 5
Aluminum hydroxide obtained by the Bayer method, adjusted to a particle size of 0.4 μm with a ball mill, is adjusted to a concentration of 10 wt% with ion-exchanged water, sent at a feed rate of 1 liter per hour, and ion-exchanged water Was reacted at a pressure of 200 kgf / cm ² and a temperature of 500 ° C. with a continuous apparatus adjusted to deliver 4 liters per hour. The flow rate after spray mixing was 0.25 m / min. When the product obtained by washing, filtering and drying the obtained product was identified by X-ray diffraction, 100% α-alumina was produced, the particle diameter was 0.4 μm, and the aspect ratio was 2. Met.
[0016]
According to the above example, the reaction temperature range is 430 to 600 ° C. and the pressure is 200 kgf / cm ² , but in this example, the temperature range and pressure are not limited, and a temperature of 350 ° C. or more is preferable. by employing the temperature of 400 ° C. to 550 ° C. is also to preferably 100 to 50 kgf / cm ² or more pressure range by employing a 200 kgf / cm ^2, the α-alumina powder which can control the particle size and aspect ratio Can be synthesized.
[0017]
【The invention's effect】
According to the present invention, since hydrothermal synthesis can be performed continuously, significant technical and economic effects can be achieved by shortening the time and saving labor of the process. In the conventional batch-type synthesis method, it is necessary to consider the temperature rise and fall times after setting the raw materials in addition to the target temperature and pressure synthesis time. It takes a long time to remove the product. According to the present invention, water quality liquid and slurry are continuously fed from one side, pressurized and rapidly heated, and the cooling is also rapidly cooled by the cooler, so that the time is very short, and the discharge is in the pipe. It is always performed without lowering the pressure, and the synthesis time only needs to take into account the residence time in the piping, the synthesized particles are also refined, and it is very efficient to synthesize fine plate-like alumina powder .
[Brief description of the drawings]
FIG. 1 shows an example of an apparatus suitable for practicing the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Water quality liquid tank 2 Slurry tanks 3 and 4 Pump 5 Heating part 6 Heating heater 8 Spray mixing part 9 Heating heater 10 Reaction part 11 Heating heater 12 Cooler 13 Product discharge part 14, 15 Shut-off valve 16 Receiver 18 Container 19 Compressed gas supply 20 Throttle valve 21 Narrow tube 22 Cock 23 Shut-off valve

Claims (8)

Using aluminum hydroxide or alumina hydrate slurryed with water or an alkaline aqueous solution and water, water is heated to the saturated vapor temperature or higher and supplied while being pressurized to the saturated vapor pressure or lower, The slurry is supplied while being pressurized to below the saturated steam temperature and above the saturated steam pressure, and these are spray mixed while adjusting the mixing ratio so that the temperature does not fall below the saturated steam temperature. A method of continuous hydrothermal synthesis of alumina particles, characterized in that after the synthesis, the alumina particles are produced by reducing the temperature and reducing the pressure. The method for continuous hydrothermal synthesis of alumina particles according to claim 1, wherein the aluminum hydroxide or the alumina hydrate has a particle size adjusted to the sub-micro order. The method for continuous hydrothermal synthesis of alumina particles according to claim 1 or 2, wherein the high-temperature and high-pressure conditions in the reaction section are 350 ° C or higher and 50 kgf / cm ² or higher. The continuous hydrothermal synthesis method of alumina particles according to claim 3, wherein the high-temperature and high-pressure conditions in the reaction section are 100 to 200 kgf / cm ² at 400 to 500 ° C. The continuous hydrothermal synthesis method of alumina particles according to claim 1, wherein the flow velocity in the pipe after spray mixing is 0.2 m / min or more. The continuous hydrothermal synthesis method of alumina particles according to claim 1, wherein the slurry has a concentration of aluminum hydroxide or alumina hydrate with respect to water or an aqueous alkali solution of 1 to 50 wt%. The continuous hydrothermal synthesis method of alumina particles according to claim 6, wherein the concentration is 2 to 20 wt%. The continuous hydrothermal synthesis method according to any one of claims 1 to 7, wherein the alumina particles to be produced have a particle size of 0.2 to 1 µm and an aspect ratio (particle long diameter / thickness) of 1 to 50.

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