GB2173181A - Alpha alumina production in a steam-fluidized reactor - Google Patents
Alpha alumina production in a steam-fluidized reactor Download PDFInfo
- Publication number
- GB2173181A GB2173181A GB08607139A GB8607139A GB2173181A GB 2173181 A GB2173181 A GB 2173181A GB 08607139 A GB08607139 A GB 08607139A GB 8607139 A GB8607139 A GB 8607139A GB 2173181 A GB2173181 A GB 2173181A
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- Prior art keywords
- alumina
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Classifications
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F7/00—Compounds of aluminium
- C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
- C01F7/44—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water
- C01F7/441—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by calcination
- C01F7/445—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by calcination making use of a fluidised bed
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/60—Particles characterised by their size
- C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/12—Surface area
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/22—Rheological behaviour as dispersion, e.g. viscosity, sedimentation stability
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/60—Optical properties, e.g. expressed in CIELAB-values
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Life Sciences & Earth Sciences (AREA)
- Geology (AREA)
- Inorganic Chemistry (AREA)
- Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
Abstract
A process for transforming alumina hydrate into an anhydrous product comprising at least 10 wt% alpha alumina. Alumina hydrate is dehydrated, heated and transferred to a reactor where it is fluidized with steam and maintained at a temperature of about 900-1350 DEG C. Steam fluidization enhances crystal growth and results in a product having uniform quality and superior grinding characteristics.
Description
SPECIFICATION
Alpha alumina production in a steam-fluidized reactor
Field of the invention
The present invention relates to a process involving calcination of alumina in a fluidized bed reactor.
The product comprises alpha alumina and is a useful ceramic or refractory material.
Background of the invention
Processes for making alpha alumina are known in the prior art. The following articles describe the principles of one such process that is closely related to the present invention: William M. Fish, "Alumina
Calcination in the Fluid-Flash Calciner", Light Metals
1974, pages 673-682 and Edward W. Lussky, "Experipence with Operation of the Alcoa Fluid Flash Calciner", Light Metals 1980, pages 69-79. The disclosures of such articles are incorporated herein by reference to the extent that they are not inconsistent with the present invention.
Other processes for making products comprising alpha alumina are described in the following patents: Newsome U.S. Patent No. 2,642,337; Turpin
U.S.Patent No. 3,265,465; DuBellay et al U.S.Patent No.3,336,109; Hrishikesan U.S.Patent No. 3,442,606;
Reh U.S.Patent No. 3,565,408 and Potter U.S.Patent
No. 4,224,288. However, each of these prior art
processes suffers from one or more serious limitations making it less than entirely suitable for its
intended purpose.
Currently, rotary kiln calcination is the principal
method used commercially to produce alumina with
a high alpha content. Rotary kiln calcination overcomes a stickiness and agglomeration effect that
occurs in high temperature (above about 1 220"C) fluidized beds of alumina. This stickiness effect, until
now, has prevented the use of fluidized beds for the continuous production of aluminas with alpha content greater than about 65 wt%.
It is a principal object of the present invention to
provide a process for transforming alumina hydrate
into an anhydrous alumina product comprising alpha alumina wherein transformation to alpha alumina takes place in a reactor containing a bed that is fluidized with steam.
A related object of the invention is to provide a
process for making alpha alumina in a fluidized bed at temperatures which may be greater than 1 220'C, while avoiding the stickiness effect that has heretofore prevented operation at such temperatures.
It is an advantage of the invention that the process will yield alpha alumina without addition of alumi
num trifluoride or other mineralizing agent at an
intermediate step. The alpha alumina product will
not, therefore, be contaminated with residual fluoride or other mineralizing agent. Fluoride is
known to be detrimental for high strength ceramic applications because it promotes crystal growth during sintering.
Another advantage of the invention is that it saves
heat energy compared with prior art alumina calcination processes. Energy usage resulting from practice of the present invention is estimated at about 10-30% less than for rotary kiln calcination.
An additional advantage of the invention is that the alumina produced is more uniformly calcined than with rotary kiln calcination.
A further advantage of the invention is that the process does not require increased pressures and accordingly can be retrofitted to existing fluid flash calcination systems.
Additional objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification and drawings.
Summary of the invention
In accordance with the present invention, alumina hydrate is transformed into an anhydrous alumina product comprising at least about 10 wt% alpha alumina.
The alumina hydrate is initially heated in a dehydrating zone to a sufficiently high temperature and for sufficient time to obtain alumina having a residual water content below about 15 wt%. The alumina is preferably heated to an elevated temperature above about 800"C so that residual water content is reduced below about 10 wt% and typically is about 5 wt%.
The alumina is transferred into heating zone and there heated to an elevated temperature greater than the temperature reached by the alumina in the dehydrating zone. The heating zone is preferably located in a furnace heated by combustion of natural gas at a flame temperature of about 1649-1677"C.
Residence time in the furnace is usually short (about 10 to 100 seconds).
The alumina is next transferred into a reactor separate from the heating zone and there maintained in a fluidized bed at a temperature above about 900"C for a sufficient time to transform the alumina into an anhydrous product comprising at least about 10 wt% alpha alumina. A preferred temperature range is about 900-1350"C. Reactor temperature is preferably above about 1100 C, more preferably above about 1220 C. Residence time varies from about 1 to 45 minutes, depending upon the temperature. Two particularly preferred reactor temperatures are 1250 C and 1275"C. Reactor pressure is generally below about 1.5 atmospheres (gauge).
The reactor is fluidized with a fluidizing gas comprising principally steam. The fluidizing gas preferably comprises at least 90 volume percent steam and is usually substantially all (greater than about 99 volume percent) steam. An advantage of the present invention is that the steam-fluidized bed can be maintained at about 1220-1300 Cfor long periods oftimewithout losing control over temperature or obtaining excessively sticky particles in the product.
The alumina is heated in the reactor for a sufficient time to obtain an anhydrous product comprising at least about 10 wt% alpha alumina. More preferably, the alpha alumina content is at least about 65 wt%, usually at least about 80 wt%. The product may have less than about 10 m2/g surface area, generally less than about 6 m2/g and sometimes less than about 3 m2/g.
The hot alumina product is discharged from the reactor into a series of cyclones where it is partially cooled. Final cooling is accomplished in a two-bed heat exchanger comprising an upper air-cooled bed and a lower water-cooled bed.
Brief description of the drawings
Figure 1 is a flowsheet diagram of a preferred system for carrying out the process of the present invention.
Figure2 is an electron microphotograph of an anhydrous alumina product made in accordance with the present invention.
Detailed description ofa preferred embodiment
A preferred fluid flash calcination system for
carrying out the process of the invention is shown
schematically in Figure 1.
Alumina hydrate 8 from the Bayer process is filtered and washed with wash water 9 on a conventional table filter 10. The filtered alumina hydrate at this stage comprises aluminum hydroxide, Al(OH)3.
The filter product has both free moisture, to the
extent of about 8-16 wt% H2O, and chemically bound
water amounting to about 34.6 wt% on dry Al (OH)3.
As used herein, the term "residual water content"
refers to the sum of free moisture and chemically
bound water. For example, alumina containing 34.6 wt% chemically bound water and about 10 wt% free
moisture has a residual water content of about 44.6 wt%.
The damp hydrate is fed into a flash dryer 20
through a feed screw 21. A gas line 22 feeds a hot
gas stream into the dryer 20, where free water is
driven off from the alumina hydrate. The dried
hydrate is transferred to a first cyclone 30 where it is
separated from hot gases and water vapor and
discharged into a fluidized bed dryer 40. Hot gases
and water vapor from the cyclone 30 are carried to
an electrostatic precipitator 42 where dust is re
moved to a dust bin 43 and a clean off-gas is
released to the atmosphere through a vent 44. The
dryer 40 contains alumina that is fluidized by air
from an air source 46. The flash dryer 20, cyclone 30
and fluidized bed dryer 40 define, in combination, a
dehydrating zone wherein the hydrate is heated to
reduce its residual water content.Heated alumina
hydrate metered out of the dryer 40 through a valve
47 has a residual water content below about 15 wt%,
usually below about 10 wt% and typically about 5 wt%. Off-gas released through the vent 40 contains
steam originating from dehydration and calcination
of the alumina as well as from steam used as a
fluidizing gas.
The valve 47 releases hydrate at a controlled rate
into a reactor or holding vessel 50. The reactor 50
includes a cyclone portion 51 and a lower portion 52
containing a fluidized bed of alumina. A steam
source 53 fluidizes alumina in the reactor 50.
Hydrate released into the cyclone portion 51
contacts a hot gas stream which particularly calcines
the hydrate and carries it to a second cyclone 60.
There, solids are separated from the hot gas and transferred into a furnace 70. Hot gas separated from the solids in the second cyclone 60 may conveniently be returned through a gas line 22 into the flash dryer 20.
In the furnace 70, fuel is burned in a series of peripherally located burners directly into a heating zone 71 containing a suspension of alumina. Residence time in the furnace 70 is short (about 10-100 seconds). The furnace 70 is preferably heated by combustion of natural gas at a flame temperature of about 1649-1677"C (3000-3500 F). The furnace 70 may also be heated by combustion of other fossil fuels or by electric heater means. The furnace 70 heats alumina in the heating zone 71 to an elevated temperature above about 800"C. An alumina-gas suspension passes from the furnace 70 into the cyclone portion 51 of the reactor 50. Solid alumina particles separated in the cyclone portion 51 drop downwardly into the fluidized bed in the lower portion 52.
The fluidized bed is maintained at a temperature above about 900"C, usually above about 1100 C, and preferably above about 1 220 C. An advantage of the present invention is the ability to maintain temperature above about 1 2200C in the fluidized bed without losing control over temperature. Two particularly preferred operating temperatures in the reactor 50 are 1250 C and 1275"C.
A further advantage of the invention is that the reactor 50 does not require increased pressure.
Reactor pressure is less than about 1.5 atmospheres, usually about atmospheric pressure or slightly higher.
Alumina is maintained in the fluidized bed reactor 50 for a sufficient time to transform it into an anhydrous alumina product comprising at least
about 10 wt% alpha alumina. Residence time in the
reactor may be about 1 to 45 minutes, depending
upon the temperature and desired alpha alumina
content ofthe product. The reaction is usually
maintained for a sufficient time to raise alpha
alumina content to at least about 65 wt%, preferably
at least about 80 wt%. The product may have surface area of less than about 10 m2lg, usually less than
about 5 m2/g or even less than about3 m2/g.
Characteristics of the product can be controlled by varying retention time or reaction temperature.
The fluidizing gas introduced through the source
53 is principally steam. The fluidizing gas preferably
comprises at least about 90 volume percent steam,
and is optimally substantially all (greater than about
99 volume percent) steam. Usage of steam as the fluidizing gas permits operation of the reactor 50 at
higher temperatures than might otherwise be maintained for long periods of time and avoids collapse
of the fluidized bed as a result of alumina particle
stickiness at such temperatures. The steam may be
superheated to a temperature above about 105"C, usually not more than about 200 C. The steam
promotes crystal growth and enhances conversion
to alpha alumina.
The exothermic reaction forming alpha alumina
releases heat at a rate of about 133 BTU's per pound
of alpha alumina formed. This heat of formation
maintains alumina in the fluidized bed at an elevated operating temperature.
Hot alumina product is discharged through a valve 54 and pneumatically conveyed through a series of cyclones 80 where the product is partially cooled. An air pump 81 supplies external air to the cyclones 80.
Heated air may be returned from the cyclones 80 through an air duct 82 to the furnace 70 where such heated air provides a major portion of the air required for combustion. A small auxiliary burner 83 adjacentthe air duct 82 ensures proper air temperature for combustion and also provides initial heating on start-up.
Alumina is discharged from the cyclones 80 into a two-bed fluidized cooler 90. The alumina is initially cooled with an air-cooled tubular heat exchanger 91 surrounding an upper bed. Air heated here can be transferred to the air source 46 for the fluidized bed dryer 40. Alumina cooled in the upper bed is dropped into a lower bed for final cooling by a water-cooled heat exchanger 92. An air header 93 supplies air for fluidizing both beds in the cooler 90.
The cooled alumina is shifted into a pneumatic conveyor 100 powered by an air source 101 and then dumped into an alumina storage bin 110.
Examples
The preferred process described above results in an anhydrous alumina product having high alpha alumina content. A typical product is shown in
Figure 2, which is an electron microphotograph taken at 400X magnification. This product was made by treating alumina with steam in the reactor 50 at a temperature of about 1275 C. Alpha alumina content is about 85%, based upon intensity as measured by
X-ray diffraction. Surface area (BET) is about 2 m2/g.
The product is useful in ceramic and refractory applications calling for alumina having high alpha content. The product has superior grinding characteristics and is more uniform than alpha alumina products produced in a rotary kiln.
Alumina heated at 1225"C for 30 minutes in a vessel 50 containing a bed fluidized with steam resulted in a product having an alpha alumina content of about 87% and BET surface area of about 4m21g.
Alumina heated at 12700Cfor 13 minutes in a vessel 50 containing a bed fluidized with steam resulted in a product with an alpha alumina content of about 88% and BET surface area of about 5 m2/g.
While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.
Claims (17)
1. Aprocessfortransforming alumina hydrate into an anhydrous alumina product containing alpha alumina, said process comprising
(a) heating alumina hydrate in a dehydrating zone to a sufficiently high temperature and for a sufficient time to obtain an alumina having a residual water content below 15 wt%.
(b) transferring the alumina from said dehydrating zone into a heating zone and there heating the alumina to an elevated temperature greater than the temperature in said dehydrating zone,
(c) transferring the alumina from said heating zone into a reactor separate from said heating zone and there maintaining the alumina in a fluidized bed at a temperature above 900"C. for a sufficient time to transform the alumina into an anhydrous alumina product comprising at least about 10 wt% alpha alumina, and
(d) fluidizing the alumina in the reactor with a fluidizing gas comprising principally steam.
2. A process according to claim 1, wherein the residual water content of the alumina obtained in step (a) is below about 10 wt%.
3. A process according to claim 1 or 2, wherein the alumina is heated in the heating zone to a temperature above 800 C.
4. A process according to any one ofthe preceding claims, wherein the alumina is heated in the heating zone by combustion of natural gas.
5. A process according to claim 4, wherein the flame temperature of the combustion is 1649 1677"C.
6. A process according to any one of the preceding claims, wherein the alumina is maintained at a temperature of 900-1350"C. in the reactor.
7. A process according to any one of claims 1 to 5, wherein the alumina is maintained at a temperature above 1100 C. in the reactor.
8. A process of claim 7, wherein the alumina is maintained at a temperature above 1220"C. in the reactor.
9. A process according to any one of the preceding claims, wherein the alumina is maintained in the fluidized bed for a sufficient time to obtain a product comprising at least 65 wt% alpha alumina.
10. A process according to claim 8, wherein the alumina is maintained in the fluidized bed for a sufficient time to obtain a product comprising at least 80 wt% alpha alumina.
11. A process according to any one of the preceding claims, wherein the alumina is maintained in the fluidized bed for a sufficient time to obtain a product having a surface area of less than 10 m2/g.
12. A process according to claim 11, wherein the alumina is maintained in the fluidized bed for a sufficient time to obtain a product having a surface area of less than 6 m2/g.
13. A process according to claim 11, wherein the alumina is maintained in the fluidized bed for a sufficient time to obtain a product having a surface area of less than 3 m2/g.
14. A process according to any one of the preceding claims, wherein the fluidizing gas comprises at least 90 volume percent steam.
15. A process according to claim 14, wherein the fluidizing gas is substantially all steam.
16. A process according to any one of the preceding claims, wherein the pressure in the reactor is less than 1.5 atmospheres.
17. A processfortransforming aluminum hydrate into an anhydrous alumina product containing alpha aluminum substantially as hereinbefore described with reference to the Examples.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US06/717,937 US4618374A (en) | 1985-03-29 | 1985-03-29 | High solids processing of kaolinitic clays with post-leaching oxidation |
US06/719,559 US4585645A (en) | 1985-04-03 | 1985-04-03 | Alpha alumina production in a steam-fluidized reactor |
Publications (3)
Publication Number | Publication Date |
---|---|
GB8607139D0 GB8607139D0 (en) | 1986-04-30 |
GB2173181A true GB2173181A (en) | 1986-10-08 |
GB2173181B GB2173181B (en) | 1988-12-21 |
Family
ID=27109801
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
GB08607139A Expired GB2173181B (en) | 1985-03-29 | 1986-03-21 | Alpha alumina production in a steam-fluidized reactor |
Country Status (1)
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GB (1) | GB2173181B (en) |
Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB1143880A (en) * | 1967-06-16 | 1900-01-01 | ||
GB922277A (en) * | 1959-10-28 | 1963-03-27 | Electro Chimie Metal | Process and apparatus for making anhydrous alumina |
EP0094081A2 (en) * | 1982-05-07 | 1983-11-16 | THE UNITED STATES OF AMERICA as represented by the Secretary United States Department of Commerce | Thermal decomposition of aluminum chloride hexahydrate |
-
1986
- 1986-03-21 GB GB08607139A patent/GB2173181B/en not_active Expired
Patent Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB922277A (en) * | 1959-10-28 | 1963-03-27 | Electro Chimie Metal | Process and apparatus for making anhydrous alumina |
GB1143880A (en) * | 1967-06-16 | 1900-01-01 | ||
EP0094081A2 (en) * | 1982-05-07 | 1983-11-16 | THE UNITED STATES OF AMERICA as represented by the Secretary United States Department of Commerce | Thermal decomposition of aluminum chloride hexahydrate |
Also Published As
Publication number | Publication date |
---|---|
GB8607139D0 (en) | 1986-04-30 |
GB2173181B (en) | 1988-12-21 |
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Legal Events
Date | Code | Title | Description |
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PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20020321 |