AU6022796A - Method and apparatus for making high-grade alumina from low- grade aluminum oxide fines - Google Patents

Method and apparatus for making high-grade alumina from low- grade aluminum oxide fines

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Publication number
AU6022796A
AU6022796A AU60227/96A AU6022796A AU6022796A AU 6022796 A AU6022796 A AU 6022796A AU 60227/96 A AU60227/96 A AU 60227/96A AU 6022796 A AU6022796 A AU 6022796A AU 6022796 A AU6022796 A AU 6022796A
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AU
Australia
Prior art keywords
fines
alumina
grade
treatment chamber
plasma
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Abandoned
Application number
AU60227/96A
Inventor
Ronald Lee Bell
Ronald Paul Zapletal
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
ALUCHEM Inc
Original Assignee
ALUCHEM Inc
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Filing date
Publication date
Application filed by ALUCHEM Inc filed Critical ALUCHEM Inc
Publication of AU6022796A publication Critical patent/AU6022796A/en
Abandoned legal-status Critical Current

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Classifications

    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27FURNACES; KILNS; OVENS; RETORTS
    • F27BFURNACES, KILNS, OVENS, OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
    • F27B7/00Rotary-drum furnaces, i.e. horizontal or slightly inclined
    • F27B7/20Details, accessories, or equipment peculiar to rotary-drum furnaces
    • F27B7/38Arrangements of cooling devices
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01FCOMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00Compounds of aluminium
    • C01F7/02Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/44Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water
    • C01F7/441Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by calcination
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01FCOMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00Compounds of aluminium
    • C01F7/02Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/44Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water
    • C01F7/441Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by calcination
    • C01F7/444Apparatus therefor
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01FCOMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00Compounds of aluminium
    • C01F7/02Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/46Purification of aluminium oxide, aluminium hydroxide or aluminates
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01PINDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00Physical properties of inorganic compounds
    • C01P2006/12Surface area
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27FURNACES; KILNS; OVENS; RETORTS
    • F27BFURNACES, KILNS, OVENS, OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
    • F27B7/00Rotary-drum furnaces, i.e. horizontal or slightly inclined
    • F27B7/20Details, accessories, or equipment peculiar to rotary-drum furnaces
    • F27B7/32Arrangement of devices for charging
    • F27B7/3205Charging
    • F27B2007/3211Charging at the open end of the drum
    • F27B2007/3217Charging at the open end of the drum axially, optionally at some distance in the kiln
    • F27B2007/3241Charging at the open end of the drum axially, optionally at some distance in the kiln in the flame of the burner
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27FURNACES; KILNS; OVENS; RETORTS
    • F27DDETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
    • F27D99/00Subject matter not provided for in other groups of this subclass
    • F27D99/0001Heating elements or systems
    • F27D99/0006Electric heating elements or system
    • F27D2099/0031Plasma-torch heating

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Geology (AREA)
  • Inorganic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)

Description

METHOD AND APPARATUS FOR MAKING HIGH-GRADE ALUMINA FROM LOW-GRADE ALUMINUM OXIDE FINES
Field of the Invention
The present invention relates to method and apparatus for
making high-grade alumina from low-grade aluminum oxide fines
produced as a by-product during calcination of hydrated alumina into metallurgical grade alumina.
Background of the Invention
Hydrated alumina, also referred to as alumina trihydrate or
AI2O3-3H20, is obtained by high-temperature digestion of host bauxite
ore in sodium hydroxide at elevated pressure in the well-known Bayer
process. The hydrated alumina which results from processing of the
host ore is fed into conventional rotary, fluidized bed or flash calciners
where it is converted into anhydrous alumina (Al203 or metallurgical
grade alumina) for aluminum metal manufacture. The hydrated alumina
which is the feed source for making metallurgical grade alumina is also further refined, depending on such factors as calcination degree and
sodium oxide (Na20) content of the starting hydrate, to produce a line
of specialty aluminas, including alpha alumina (σ-AI203), for use in
chemical, abrasive, ceramic, refractory and glass applications. The
calcined alpha alumina may be further treated in traditional arc-type
furnaces where it is converted into chemical grade alumina, commonly referred to as "white" fused alumina, for various end uses.
During calcination of the hydrated alumina, a large amount
of aluminum oxide dust is generated as the alumina trihydrate particles
are thermally and mechanically agitated during passage through the
calciner. As the hydrate particles are calcined in rotary kiln, fluidized
bed or flash calciners of conventional design, fine aluminum oxide
particles or dust become entrained in a high velocity heated air stream
generated by a heat source disposed within the calciner. As a result,
these fines become suspended in and are potentially carried out of the
calciner by the heated air stream. To prevent air pollution and loss of
alumina product, calcination systems are provided with dust collectors, such as electrostatic precipitators, high-efficiency bag houses or the
like, to retain the aluminum oxide dust, commonly referred to as ESP
(Electrostatic Precipitator) dust, within the system.
It is estimated that approximately 90% or more by weight
of the aluminum oxide dust captured during calcination of the hydrated
alumina is characterized by a particle size below 44 microns and the
fines generally have a high soda content. Furthermore, the captured fines generally comprise a mixture of particles calcined to varying
degrees, i.e., calcined, partially calcined, and uncalcined particles, with
a resulting water content of the aluminum oxide dust, as determined by
loss on ignition (LOI) test, varying widely between about 1 and 35% by
weight. With these properties, the aluminum oxide dust is
unacceptable for electrolytic reduction at aluminum smelters and is
therefore treated as a waste by-product of the alumina trihydrate
calcination process. It will be appreciated that handling of the dust by¬
product creates serious problems when it is considered that between
about 3-10% of the total amount of alumina yielded in the calcination
process is aluminum oxide dust. Thus, for example, in the case of an
alumina calcining operation having an annual alumina production
capacity of about 500,000 tons per year, the amount of alumina dust
captured during the calcination process could amount to between about 1 5,000-50,000 tons per year.
To handle disposal of the aluminum oxide dust by-product,
alumina refineries have sent the aluminum oxide fines, with other by¬
products of the Bayer process, to clay bottom settling ponds or
mudlakes as a plant discharge. However, settling ponds are generally
expensive to build and maintain, and disposal of the dust in ponds may
present potential environmental problems in the event the containment
function of the pond should fail.
Other alumina refineries have attempted to reduce the
quantity of dust by-product by blending a part of the aluminum oxide fines with the metallurgical grade alumina before sending the mixed
product to aluminum smelters. With blending, however, only a small
fraction of the overall amount of dust by-product can be utilized as
operators of alumina reduction facilities have established alumina
specifications which limit the acceptable content of 44 micron-or-less
sized particles within the metallurgical grade alumina to reduce
production problems in operating the electrolytic cells. Additionally, the
aluminum oxide dust blended with the metallurgical grade alumina
creates serious handling problems at the aluminum smelter when the
dust is captured by pollution control equipment and is thereafter
contaminated by fluoride laden exhaust fumes from the aluminum metal
production pot lines. When the dust reaches this collection point, it is
no longer pure aluminum oxide but rather aluminum oxide laden with
fluoride contamination, a hazardous waste.
Further attempts have been made to reduce the quantity
of aluminum oxide dust by recycling the dust to the digestion stage of
the Bayer process where it is partially redissolved in caustic soda to
yield sodium aluminate and insoluble fine alumina which is difficult to
filter. This method may reduce the overall productivity of the Bayer
plant to the extent it adds additional steps for reprocessing the
aluminum oxide dust and the undissolved fines cause filtration problems
and resultant capacity constraints in the Bayer process. Additional
methods have been disclosed in the prior art of using the dust, either
processed or directly, as seeding at the precipitation stage of the Bayer process (Gynra, U.S. Patent No. 4,051 ,222 and Anjier et al., U.S.
Patent No. 4,568,527) . However, these methods typically require
improved control of the precipitation process, and further have the
potential of contaminating the alumina trihydrate product with
anhydrous alumina. Moreover, plant operating costs are generally
increased as a result of recycling the dust by-product through
precipitation and calcination.
In U.S. Patent 4,797,270 issued to Alvarado Cendan et
al., a method is disclosed for obtaining specialty alumina from dust by-
product generated during calcination of alumina trihydrate. In this
process, the aluminum oxide dust is washed to reduce its sodium oxide
content and then calcined in a conventional fossil fuel calciner between
1 , 100°C and 1 ,400°C to convert the dust to special alumina (σ-AI2O3 or corundum) . No provision is disclosed, however, to prevent the dust
from being entrained in the heated air stream within the calciner as the
dust is thermally and mechanically agitated during passage through the
calciner. Thus, the aluminum oxide dust is likely to be suspended in
and/or carried out of the calciner by the heated air stream as occurs
during calcination of the alumina trihydrate particles. In those instances
where the fines are not carried out of the calciner, the suspension of
the dust in the heated air stream will typically reduce the amount of
time the fines are in contact with the hot zone of the calciner, thereby
compromising complete calcination of the aluminum oxide fines into
high alpha phase specialty aluminas. Additionally, aluminum oxide dust cannot be used as a raw
material for fusing alumina in conventional arc resistance furnaces.
Low-grade aluminum oxide fines contain chemically combined water of
hydration ranging typically between about 6 and 1 8% by weight, with
individual particles ranging between about 1 and 35% by weight.
Water vapor generated while fusing creates serious safety and
operational problems in arc resistance-type furnaces to such an extent
that aluminum oxide particles selected for fusion in the prior art must
be of a high quality, typically containing less than 0.6% total water,
with less than 0.2% total water the most preferred. Moreover, the low-
grade aluminum oxide fines are typically too fine in particle size
distribution to charge conventional arc resistance furnaces as the fines
are typically characterized by a minus 44 micron content of 90% or
greater. Alumina selected for fusing in arc resistance-type furnaces
must generally be coarse, with the minus 44 micron fraction of alumina
particles amounting to less than 10% of the overall content.
Accordingly, there is a need to reduce or eliminate
potentially hazardous disposal of uncalcined or partially calcined
aluminum oxide fines generated during calcination of hydrated alumina.
There is also a need for economically efficient and technically feasible
method and apparatus for converting uncalcined or partially calcined
aluminum oxide dust by-product (ESP dust) into calcined high-grade
alumina particles, alumina agglomerates, or fused alumina. Summary of the Invention
To these ends, the present invention is directed to method
and apparatus for efficiently converting low-grade aluminum oxide fines
(ESP dust) into high-grade alumina particles, alumina agglomerates, or
fused alumina. The low-grade aluminum oxide fines are produced as a
by-product during calcination of hydrated alumina into metallurgical
grade alumina or specialty aluminas.
In accordance with one embodiment of the invention, a
novel plasma-fired rotary kiln is provided having a rotating treatment
chamber and a plasma torch disposed within the chamber. The plasma
torch directs heat within the treatment chamber to calcine and convert low-grade aluminum oxide fines into intermediate and high alpha phase alumina particles within the chamber.
The treatment chamber of the rotary kiln includes an inlet
end and an outlet end for calcining the low-grade aluminum oxide fines
as the fines are moved or pass through the chamber. The treatment
chamber is preferably inclined from outlet end to inlet end with the
chamber rotating at a speed sufficient to move the alumina through the
chamber and the plasma torch operating at a temperature whereby the
low-grade aluminum oxide fines are converted into high-grade alumina
particles within the chamber before exiting the outlet end. The speed
of rotation depends upon the size of the chamber, amount of fines, and
temperature; thus, it is not considered to be a critical aspect of this
invention as will be understood in light of this description. It will be appreciated that the plasma-fired rotary kiln of
the present invention provides numerous advantages over the fossil fuel
calciners of the prior art. In particular, plasma torches use
approximately 1 /100th of the combustible air needed by fossil fuel
heaters. That is, a fossil fuel heater may typically have a combustible
air to fuel ratio in a range between 10: 1 and 1 5: 1 . On the other hand,
a plasma torch may have a similar ratio in a range between 0.10: 1 and
0.1 5: 1 . Thus, plasma torches provide "massless heat" compared to
fossil fuel heaters as virtually all of the heat generated by the plasma
torch is released with minimal mass. Moreover, plasma torches operate
at temperatures well beyond the operating range of fossil fuel heaters.
For example, plasma torches may operate in a range between 4,000°C
and 7,000°C whereas fossil fuel heaters may operate in the range between 1 ,500°C and 2,000°C.
Accordingly, the plasma-fired rotary kiln of the present
invention has essentially little or negligible heated air flow within the
treatment chamber as compared to fossil fuel calciners of the prior art.
The negligible air flow in the plasma-fired rotary kiln significantly
reduces entrainment and suspension of the aluminum oxide fines in the
heated air stream, thereby increasing the amount of time the fines are
in contact with the hot zone of the treatment chamber and reducing the
amount of dust escaping from the rotary kiln. Due to the high
operating temperature of the plasma torch and the reduced suspension
or containment of the fines in the heated air stream, the plasma-fired rotary kiln provides improved calcination of the aluminum oxide fines
into specialty alumina over the fossil fuel calciners of the prior art.
Moreover, as a lesser amount of fines are carried out of the rotary kiln
of the present invention, less investment in pollution control equipment
to capture the fines is required over the prior art.
According to a method of the invention, the low-grade
aluminum oxide fines produced during calcination of the hydrated
alumina are preferably washed with an acidic aqueous medium selected
from a group consisting of water, acetic acid, hydrochloric acid and
sulfuric acid to reduce sodium oxide content of the fines.
After the fines have been separated from the aqueous
medium to substantially remove the sodium oxide content, the washed
fines are introduced into an inlet end of the plasma-fired rotary kiln.
The washed fines are calcined during passage through the rotating
treatment chamber by exposure to heat generated by the plasma torch.
The treatment chamber rotates at a speed and the plasma torch
operates at a temperature whereby the low-grade aluminum oxide fines are converted into high-grade (high alpha phase) alumina particles
within the chamber before exiting an outlet end of the rotary kiln.
Preferably, the high-grade alumina particles are ground to achieve a
desired particle size for an intended application.
In accordance with another embodiment of the invention,
a novel plasma-fired reactor is provided having a fixed treatment
chamber and a plasma torch disposed within the chamber. The plasma torch directs heat from a plasma arc flame within the treatment
chamber to convert the low-grade aluminum oxide fines into calcined
high-grade alumina particles, alumina agglomerates, or fused alumina
within the chamber. The method of the present invention is achieved
by introducing the low-grade aluminum oxide fines into the plasma-fired
reactor and treating the fines by exposure to the heat generated by the
plasma arc flame to convert the fines into high-grade alumina within the
treatment chamber. Preferably, the fines are mixed with a carrier gas
before being introduced into the plasma-fired reactor.
As with the plasma-fired rotary kiln, the plasma-fired
reactor of the present invention has essentially little or negligible heated
air flow within the treatment chamber as compared to fossil fuel heated
calciners of the prior art. Thus, the plasma-fired reactor provides
improved calcination of the aluminum oxide fines into high-grade alpha
alumina particles over the fossil fuel heated calciners of the prior art.
Moreover, the plasma torch is not adversely affected by
the chemically combined water content or particle size of the fines as
is presently a severe limitation in conventional arc resistance-type
furnaces. Thus, the plasma-fired reactor permits the ESP dust by-
product to be charged directly into the reactor for agglomerating or
fusing the fines without the need to pre-agglomerate or calcine the
fines as is presently required with arc resistance-type furnaces of the
known art. Brief Description of the Drawings
The objectives and features of the present invention will
become more readily apparent when the following detailed description
is taken in conjunction with the accompanying drawings in which:
Fig. 1 is a cross-sectional view of a plasma-fired rotary kiln in accordance with one embodiment of the present invention; and
Fig. 2 is a cross-sectional view of a plasma-fired reactor
in accordance with a second embodiment of the present invention.
Detailed Description of the Invention
The present invention relates to method and apparatus for
making high-grade alumina from low-grade aluminum oxide fines
produced as a by-product during calcination of hydrated alumina into metallurgical grade alumina.
As used herein, the terms "aluminum oxide fines",
"aluminum oxide dust", "ESP dust", or "dust" all refer to an aluminum
oxide by-product produced during calcination of hydrated alumina into
metallurgical grade alumina or specialty aluminas and captured by an
electrostatic precipitator, high-efficiency bag house, cyclone separator
or the like. The term "low-grade aluminum oxide fines" refers to fines
comprising between about 0 and about 20% σ-AI2O3 by weight and a
chemically combined water content between about 6 and 28% by
weight. The term "high-grade alumina" refers to alumina comprising
between about 60 and about 99% σ-AI203 by weight.
"Agglomerating" refers to forming a coherent mass of alumina particles (loose or hard agglomerates) by heating the low-grade aluminum oxide
fines while "fusing" refers to melting the low-grade aluminum oxide
fines into molten alumina.
Referring now to the figures, in one embodiment of the
present invention as shown in Fig. 1 , a plasma-fired rotary kiln 10 in
accordance with the present invention is shown for converting low-
grade aluminum oxide fines 12, captured during calcination of hydrated
alumina, into high-grade alumina particles 14. Plasma-fired kiln 10
includes a cylindrical rotary kiln 1 6 supported on a variable-speed
driving means 1 8 of conventional structure for rotating the kiln about
an axis 20. The rotating kiln 1 6 is provided with an inlet end 22 for
receiving the low-grade aluminum oxide fines 1 2 from a variable speed
volumetric screw feeder 24 and an outlet end 26 for discharging the
high-grade alumina particles 14 into a discharge receptacle 28. The
volumetric screw feeder 24 includes a feed-pipe 29 extending into the
inlet end 22 for delivering the low-grade aluminum oxide fines 12 at a
predetermined rate. It will be appreciated that volumetric screw feeder
24 and discharge receptacle 28 are shown and described for illustrative
purposes only and do not form any part of the present invention.
The rotating kiln 1 6 includes a rotating treatment chamber
30 having a refractory lining 32 for calcining the low grade aluminum
oxide fines 1 2 as the fines pass through the treatment chamber 30
between the inlet end 22 and the outlet end 26 of the kiln as will be
described in more detail below. Preferably, the rotating kiln 1 6 is inclined as shown in Fig. 1 to facilitate travel of the fines 1 2 through
the treatment chamber 30 and includes a nose ring dam 33 for
maintaining the fines 1 2 in a rolling bed 35 within the chamber. In one
embodiment, the rotating treatment chamber 30 includes a series of
inwardly directed vanes 34 (shown in phantom) for cascading the low-
grade aluminum oxide fines 12 as the fines are introduced into the inlet
end 22 of the rotating kiln 1 6. However, it will be understood that
vanes or similar devices may not be necessary.
In accordance with the invention, a non-transferred plasma torch 36 is mounted on a sealed end 38a of the rotating kiln 1 6 for
directing heat within the treatment chamber 30. The treatment
chamber 30 is rotatably mounted within sealed end 38a through
rotating bearings 40 disposed about a peripheral sealing flange 42 of
the sealed end. In this way, the sealed end 38a is held stationary by
suitable means (not shown) while providing a closed seal through flange
42 with the rotating treatment chamber 30. A sealed end 38b of
similar construction is provided at the inlet end 22 of the rotating kiln
1 6 to enclose the treatment chamber 30. It will be appreciated that
while the plasma torch 36 is shown directing heat from a plasma flame 44 toward the inlet end 22 (counter-current), the invention
contemplates reversing the plasma torch 36 such that the plasma flame
44 directs heat toward the outlet end 26 (co-current, not shown) .
The plasma torch 36 of the present invention is preferably
selectively operable in a range between about 4,000°C and about 7,000°C. Known plasma torches capable of providing temperatures
within this range are supplied by Westinghouse Electric Corporation of
Pittsburgh, PA and Plasma Energy Corporation of Raleigh, NC.
In a method of the present invention, the low-grade
aluminum oxide fines 1 2 are introduced into the inlet end 22 of the
rotating treatment chamber 30. The treatment chamber 30 is rotated
at a speed and the plasma torch 36 is operated at a temperature
whereby the low-grade aluminum oxide fines 1 2 are converted into
high-grade alumina particles 14 within the chamber 30 before exiting
the outlet end 26. In one embodiment, the high-grade alumina particles
are ground to achieve a desired particle size for an intended application.
It is estimated that the low-grade aluminum oxide fines
generally have a sodium oxide content in a range between about 0.40
and about 1 .2% Na2O by weight. To reduce the sodium oxide content
of the low-grade aluminum oxide fines before calcination, the fines are
preferably washed with an aqueous solution. In one embodiment, the
aqueous medium is acidic and is preferably selected from a group
consisting of water, acetic acid, hydrochloric acid and sulfuric acid.
Those skilled in the art will understand that the term "washing"
includes spraying or percolating the fines with the aqueous solution and
may further include the additional step of repulping the fines in the
aqueous medium.
The washed fines are then separated from the aqueous solution through filtration, cycloning, centrifuging or decanting, for example, whereby the sodium oxide content of the fines is preferably
reduced to a range selected from a group consisting of between about
0.40 and about 0.70%, between about 0.1 5 and about 0.40%, and
below 0.1 5% Na2O by weight. The washed fines preferably have a
water content in a range between about 10 and about 25% by weight
after the fines have been separated from the aqueous medium.
The washed fines are introduced into the inlet end 22 of
the rotating treatment chamber 30 wherein the fines are calcined by the
plasma torch 36 disposed within the chamber 30. A mineralizing agent,
such as AIF3, is frequently added at .01 to 1 .0% to the incoming feed
or to the kiln via the heat source to further enhance alpha conversion.
Preferably, the fines reach a temperature in a range between about
2, 1 00°F and 2,900°F during the calcining step. In a preferred embodiment, the fines are maintained in the rolling bed 35 as the fines
pass through the chamber 30 between the inlet and outlet ends 22 and
26, respectively. In another embodiment, the fines are cascaded at the
inlet end 22 by the series of inwardly directed vanes 34 disposed within
the treatment chamber 34. In accordance with the method, the
treatment chamber 30 is rotated at a speed and the plasma torch 36 is
operated at a temperature whereby the low-grade aluminum oxide fines
are converted to high-grade alpha alumina particles within the chamber.
In the following examples, the advantages of the present
plasma-fired rotary kiln 1 0 are further illustrated. EXAMPLE I A plasma-fired rotary kiln comprising a rotating kiln having a length of 5'6" and an inner diameter of 30" was constructed with an 8" refractory (high alumina) lining as shown in the figure. A 150 kW
plasma torch manufactured by Plasma Energy Corporation was mounted at the outlet end of the rotating kiln to direct heat toward the inlet end of the kiln (counter-current flow). The rotating kiln was inclined at about 3° from outlet to inlet end and supported on a variable-speed drive mechanism. A variable-speed volumetric screw feeder was used to introduce unwashed ESP dust by-product, captured during calcination of hydrated alumina into metallurgical grade alumina, into the inlet end of the rotary kiln at about 0.5 lbs. /min. through a 2" feed¬ pipe.
The unwashed ESP dust had a sodium oxide content of about 0.8% by weight and a free moisture content of less than about
0.5% by weight. About 99% by weight of the ESP dust was characterized by a particle size below 44 microns. A mineralizing agent comprising about 0.1 % AIF3 was added to the ESP dust before being introduced into the inlet end of the rotary kiln. The kiln was rotated at about 3 RPM and the plasma torch was employed to achieve a material bed temperature at about 2,650°F while the ESP dust was introduced into the inlet end of the rotary kiln. The ESP dust was calcined in a rolling bed within the rotary kiln at these operating parameters. It was observed that the alumina particles discharged at the outlet end of the kiln had a surface area between about 0.3 and about 0.7 m2/g and
were about 99% σ-AI2O3 by weight as determined by microscope
employing Petrographic analysis with 1 .72 refractive index oil.
EXAMPLE II
In this example, ESP dust by-product, captured during
calcination of hydrated alumina into metallurgical grade alumina, was
first washed with acetic acid through repulping, filtration and
displacement steps to reduce the sodium oxide content of the ESP dust
to about 0.1 % by weight. The ESP dust was dried to a free moisture
content of less than about 0.5% by weight. As in the previous
example, about 99% by weight of the ESP dust was characterized by
a particle size below 44 microns. A mineralizing agent comprising
about 0.1 % AIF3 was added to the ESP dust before being introduced into the inlet end of the rotary kiln described in the previous example.
The kiln was rotated at about 3 RPM and the plasma torch
was operated to obtain a material discharge temperature at about
2,650°F while the ESP dust was introduced into the inlet end of the
rotary kiln at about 0.5 lbs. /min. through the variable-speed volumetric
screw feeder described in the previous example. The ESP dust was
calcined in a rolling bed within the rotary kiln at these operating
parameters. It was observed that the alumina particles discharged at
the outlet end of the kiln had a surface area between about 0.3 and
about 0.7 m2/g and were about 99% σ-AI2O3 by weight as determined by microscope employing Petrographic analysis with 1 .72 refractive index oil.
Now referring to a second embodiment of the present
invention as shown in Fig. 2, a plasma-fired reactor 50 in accordance
with the present invention is shown for converting low-grade aluminum
oxide fines into high-grade alumina, shown generally at 52. As
discussed in more detail below, the high-grade alumina 52 may
comprise calcined alpha alumina particles, alumina agglomerates, or
fused alumina (preferably chemical grade "white" fused alumina)
depending on the operating parameters and configuration of the plasma-
fired reactor 50.
Reactor 50 includes a cylindrical treatment chamber 54,
preferably made of steel, having a water cooling jacket 56 surrounding
the chamber. The treatment chamber 54 is provided with an inlet end
58 for receiving the low-grade aluminum oxide fines from a variable
speed volumetric screw feeder 60 and an outlet end 62 for discharging
the high-grade alumina 52 into a discharge receptacle 64. It will be
appreciated that the screw feeder 60 and discharge receptacle 64 as
shown and described are for illustrative purposes only and do not form
any part of the present invention.
The treatment chamber 54 has an alumina "skull" or
refractory lining 66 of generally uniform thickness for maintaining
constant heat within the reactor 50 as the low-grade aluminum oxide
fines pass through and are converted within the treatment chamber 54 as described in more detail below. In one configuration as shown in
Fig. 2, the plasma-fired reactor 50 is inclined from the outlet end 62 to
the inlet end 58 to facilitate travel of the converted alumina through the
treatment chamber 54. In another configuration (not shown), the
reactor 50 is arranged vertically with the outlet end 62 located beneath the inlet end 58.
In accordance with the second embodiment of the present
invention, a non-transferred plasma torch 68 is mounted on a plasma
torch head 70 arranged at one end of the reactor 50 for directing heat within the treatment chamber 54 from a plasma arc flame 72. A
plasma arc gas 74, preferably comprising natural gas, is fed into the
plasma torch 68 to create the plasma arc flame 72. As shown in Fig.
2, the reactor 50 is vented at the inlet end 58 to allow a very small
percentage of the exhaust gases, shown generally as arrows 76, to
escape from the treatment chamber 54. The reactor 50 has a sealed
end 78 at the outlet end 62 of the treatment chamber 54 which
communicates with an exhaust port 80 and a discharge port 82. The
exhaust port 80 is provided to transfer the entrained fines in the
effluent stream within the treatment chamber 54, shown generally at
84, to a fines collector 86 of conventional design before any exhaust
gases are expelled to the environment through exhaust fan 88. The
heated exhaust from the treatment chamber 54 is preferably treated
with cooling air 90 before the fines are collected at the fines collector
86. The discharge port 82 is provided to permit the converted high- grade alumina 52 to exit the treatment chamber 54 as calcined alpha
alumina particles, alumina agglomerates, or fused alumina as will be
described in more detail below. The high-grade alumina 52 may
thereafter be subjected to a grinding operation to achieve a desired
particle size.
In one configuration as shown in Fig. 2, the low-grade
aluminum oxide fines are injected into the plasma torch 68 from a feed line 92 extending from the volumetric screw feeder 60. A carrier gas
94, preferably comprising natural gas or compressed air, is used to
aspirate the fines in the feed line 92 before the fines are injected into
the plasma torch 68. In this way, the fines are treated by the intense
heat generated by the plasma arc flame 72 and are converted into
calcined alpha alumina, alumina agglomerates, or fused alumina within
the treatment chamber 54 before exiting through discharge port 82. It
will be appreciated by those skilled in the art that the operating
parameters and construction of the plasma-fired reactor 50 can be
readily changed to affect conversion of the low-grade aluminum oxide
fines into high-grade alumina. For example, in another configuration
(not shown) the aspirated fines are introduced into the inlet end 58 of
the treatment chamber 54 without being introduced directly into the
plasma arc flame 72. In yet another configuration (not shown) the fines are introduced directly into the inlet end 58 of the treatment chamber
54 without being pre-mixed with a carrier gas or being introduced
directly into the plasma arc flame 72. In still another configuration (not shown), the unmixed fines are introduced directly into the plasma arc
flame 72. Moreover, in each of these disclosed embodiments, the
plasma-fired reactor 50 may be arranged at an incline from the outlet
to inlet ends 62 and 58, respectively, or may be arranged vertically
with the outlet end 62 located beneath the inlet end 58
Thus, the conversion of low-grade aluminum oxide fines
into high-grade alumina, i.e., calcined alpha alumina particles, alumina
agglomerates, orfused alumina, can be selectively controlled by varying
the introduction of the fines to the plasma-fired reactor 50 (e.g., with
or without pre-mixing with a carrier gas or introducing the fines directly
into the plasma arc flame 72), by varying the arrangement of the reactor (e.g., inclined or vertical), and by varying the operating
temperature of the plasma torch 68. It will be appreciated that the
converted form of the high-grade alumina 52 discharged at the outlet
end 62 will be determined predominantly by the reaction time and
intensity of heat between the fines and the plasma arc flame 72.
In accordance with a method of the present invention, the
low-grade aluminum oxide fines are introduced into the inlet end 58 of
the treatment chamber 54 for treatment by exposure to the heat
generated by the plasma torch 68. The plasma torch 68 operates at a
pre-selected temperature to convert the fines into high-grade alumina
within the treatment chamber 54 before exiting the outlet end 62.
Conversion of the fines within the plasma-fired reactor 50 comprises
calcining, agglomerating, or fusing the low-grade aluminum oxide fines within the treatment chamber 54 by exposure to heat generated by the
plasma arc flame 72. The fines may be introduced directly into the inlet
end 58 of the treatment chamber 54 from the screw feeder 60 or may
be pre-mixed with the carrier gas 94 before being introduced at the inlet
end 58. In either embodiment, the mixed or unmixed fines may be
introduced directly into the plasma arc flame 72 or at the inlet end 58
without being introduced directly into the plasma arc flame 72.
To reduce the sodium oxide content of the low-grade
aluminum oxide fines before treating the fines in the plasma-fired reactor 50, the fines are preferably washed with an aqueous solution.
In one embodiment, the aqueous medium is acidic and is preferably
selected from a group consisting of water, acetic acid, hydrochloric acid
and sulfuric acid. Those skilled in the art will understand that the term
"washing" includes spraying or percolating the fines with the aqueous
solution, and may further include the additional step of repulping the
fines in the aqueous medium.
The washed fines are then separated from the aqueous
solution through filtration, cycloning, centrifuging or decanting, for
example, whereby the sodium oxide content of the fines is preferably reduced to a range selected from a group consisting of between about
0.40 and about 0.70%, between about 0.1 5 and about 0.40%, and
below 0.1 5% Na2O by weight. The washed fines preferably have a
water content in a range between about 1 0 and about 25% by weight
after the fines have been separated from the aqueous medium. The washed fines are introduced into the inlet end 58 of the treatment
chamber 54 for conversion into calcined alpha alumina particles,
alumina agglomerates, or fused alumina within the chamber. The fines
preferably reach a temperature in a range between about 2, 100°F and
about 2,900°F during calcination in the plasma-fired reactor 50 to
achieve between about 60% and about 99% conversion to alpha
alumina particles. The calcined alumina particles received at the outlet
end 62 may be recycled into the inlet end 58 of the treatment chamber
54 to increase the alpha content of the particles. During fusion, the
fines preferably reach a temperature in a range between about 1 ,800°C
and 2,200°C.
With further reference to Fig. 2, in another aspect of the
invention the low-grade aluminum oxide fines are introduced into the
inlet end 58 of the treatment chamber 54 without prior washing to
reduce the sodium oxide content of the fines. As the fines are treated
by exposure to heat generated by the plasma arc flame 72, a very small
fraction of the fines will become entrained in effluent gases within the treatment chamber 54 as shown generally at 84. During conversion of
the fines into high-grade alumina, the heat generated by the plasma
torch 68 is used to volatize sodium oxide within the fines. The sodium
oxide content of the resultant high-grade alumina is selectively
controlled and reduced by crystallizing the volatized sodium oxide on
the entrained fines in the effluent stream escaping through exhaust port
80. The amount of crystallization is controlled by regulating, through air balance and temperature within the treatment chamber 54, the
amount of entrained fines recirculating to the plasma torch 68 or
escaping through the exhaust port 80. The level of sodium oxide
content in the converted high-grade alumina is controlled by removing
all or a selected portion of the crystallized entrained fines from the
treatment chamber 54.
Accordingly, those skilled in the art will appreciate that the
present invention provides efficient and technically feasible method and
apparatus for making high-grade alumina from ESP dust by-product
which is heretofore unknown in the prior art. In one embodiment, the
dust by-product is converted into calcined high-grade alumina particles
within the plasma-fired rotary kiln of the present invention without the
inherent problems associated with conventional fossil fuel calciners. In another embodiment, the ESP dust is converted into calcined high-grade
alumina particles, alumina agglomerates, or fused alumina within the
plasma-fired reactor of the present invention without the need to pre-
agglomerate and calcine the fines as is presently required with known
arc resistance-type furnaces. Moreover, the present invention provides
a novel method for reducing sodium oxide content of the converted
high-grade alumina without the need to pre-wash the fines as is
presently required in the known art.
While the present invention has been illustrated by
description of various embodiments and while those embodiments have
been described in considerable detail, it is not the intention of applicants to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made without departing from the spirit or scope of Applicants' invention.
WE CLAIM:

Claims (30)

1. A method for making high-grade alumina from low-grade aluminum oxide fines produced as a by-product during calcination of hydrated alumina, the steps comprising: providing a plasma-fired reactor having a treatment
chamber with an inlet end and an outlet end for treating said fines, said reactor being equipped with a plasma torch inside the treatment chamber for directing heat therein from a plasma arc flame; introducing said fines into the inlet end of said treatment
chamber for passage therethrough and conversion to a form of high- grade alumina for discharge out the outlet end; and converting said fines to said form of high-grade alumina during passage through said treatment chamber by exposure to the heat generated by said plasma torch.
2. The method of claim 1 wherein said converting step
comprises calcining said fines to make calcined high-grade alumina.
3. The method of claim 1 wherein said converting step
comprises agglomerating said fines to make high-grade agglomerated
alumina.
4. The method of claim 1 wherein said converting step
comprises fusing said fines to make high-grade fused alumina.
5. The method of claim 1 further comprising the step of
mixing said fines with a carrier gas before introducing said fines into the
inlet end of said treatment chamber.
6. The method of claim 5 wherein said carrier gas is selected
from the group consisting of natural gas and compressed air.
7. The method of claim 5 wherein said carrier gas and fines
mixture is introduced into said plasma arc flame at the inlet end of said
treatment chamber.
8. The method of claim 1 wherein said reactor comprises a
rotary kiln.
9. The method of claim 1 further comprising the step of
grinding said high-grade alumina to achieve a desired particle size.
10. The method of claim 1 , said steps further comprising:
washing said fines with an aqueous medium to reduce sodium oxide content of said fines; and
separating said washed fines from said aqueous medium
to substantially remove said sodium oxide content before introducing
said fines into the inlet end of said treatment chamber.
1 1 . The method of claim 10 wherein said separating step is
selected from the group consisting essentially of filtering, cycloning,
centrifuging, and decanting.
12. The method of claim 1 0 wherein said separating step
reduces the sodium oxide content of said fines to a range selected from
a group consisting essentially of between about 0.40 and about 0.70%,
between about 0.1 5 and about 0.40%, and below about 0.1 5% by
weight.
1 3. The method of claim 1 0 wherein said aqueous medium is
selected from a group consisting of water, acetic acid, hydrochloric acid
and sulfuric acid.
14. The method of claim 1 0 wherein said washed fines have
a water content in a range between about 10 and about 25% by weight
after said fines have been separated from said aqueous medium.
1 5. The method of claim 1 wherein said fines comprise dust
captured by an electrostatic precipitator, high-efficiency bag house, or cyclone separator.
1 6. The method of claim 1 wherein said fines being introduced
into the inlet end of said treatment chamber comprise between about
0 and about 20% alpha alumina by weight.
1 7. The method of claim 2 wherein said high-grade alumina comprises between about 60 and about 99% alpha alumina particles.
1 8. The method of claim 3 wherein said high-grade alumina
comprises between about 60 and about 99% alpha alumina
agglomerates.
1 9. The method of claim 4 wherein said high-grade alumina comprises white fused alumina.
20. A method for making high-grade alumina with reduced
sodium oxide content from low-grade aluminum oxide fines produced
as a by-product during calcination of hydrated alumina, the steps
comprising:
providing a plasma-fired reactor having a treatment
chamber with an inlet end and an outlet end for treating said fines, said
reactor being equipped with a plasma torch inside the treatment
chamber for directing heat therein from a plasma arc flame;
mixing said fines with a carrier gas before introducing said
fines into the inlet end of said treatment chamber;
introducing said carrier gas and fines mixture into said
plasma arc flame at the inlet end of said treatment chamber for passage
therethrough and conversion to a form of high-grade alumina for discharge out the outlet end; entraining a portion of said fines in an effluent gas stream within said treatment chamber;
converting said fines to said form of high-grade alumina
during passage through said treatment chamber by exposure to the heat
generated by said plasma torch while at the same time volatizing
sodium oxide within said fines;
crystallizing said sodium oxide on said entrained fines; and selectively removing said crystallized entrained fines from
said treatment chamber.
21 . The method of claim 20 wherein said converting step comprises calcining said fines to make calcined high-grade alumina.
22. The method of claim 20 wherein said converting step comprises agglomerating said fines to make high-grade agglomerated alumina.
23. The method of claim 20 wherein said converting step comprises fusing said fines to make high-grade fused alumina.
24. The method of claim 20 wherein said carrier gas is selected from the group consisting of natural gas and compressed air.
25. An apparatus to convert low-grade aluminum oxide fines into high-grade alumina, comprising: a plasma-fired reactor having a treatment chamber with an inlet end and an outlet end for treating said fines, said fines being introduced into the inlet end of said treatment chamber for passage therethrough and conversion to a form of high-grade alumina for discharge out the outlet end; and a plasma torch disposed within said treatment chamber for directing heat therein, said plasma torch operating at a temperature whereby said fines are converted into said form of high-grade alumina within said chamber before exiting said outlet end.
26. The apparatus of claim 25 wherein said reactor comprises a rotary kiln.
27. The apparatus of claim 25 wherein said plasma torch is arranged for directing the heat toward the outlet end of said treatment chamber.
28. The apparatus of claim 25 wherein said plasma torch is arranged for directing heat toward the inlet end of said treatment chamber.
29. The apparatus of claim 25 wherein said plasma-fired reactor is inclined from said outlet end to said inlet end.
30. The apparatus of claim 25 wherein said plasma-fired reactor is arranged vertically with said outlet end located beneath said inlet end.
AU60227/96A 1995-05-17 1996-05-17 Method and apparatus for making high-grade alumina from low- grade aluminum oxide fines Abandoned AU6022796A (en)

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US443600 1995-05-17
US62284696A 1996-03-27 1996-03-27
US622846 1996-03-27
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GB2414729A (en) * 2004-06-01 2005-12-07 Atraverda Ltd Method of producing sub-oxides with a substantially moisture free gas
WO2006106443A2 (en) * 2005-04-06 2006-10-12 Ffe Minerals Denmark A/S Method and plant for manufacturing of alumina
TWI632954B (en) * 2013-05-16 2018-08-21 科學設計股份有限公司 Carrier treatment to improve catalytic performance of an ethylene oxide catalyst
CN112537789B (en) * 2020-11-06 2021-11-12 太原理工大学 Bauxite organic matter removal method and device
CN113600101A (en) * 2021-08-17 2021-11-05 任立民 Roasting reaction generator and production method of nano aluminum oxide

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FR1081584A (en) * 1952-07-24 1954-12-21 Godfrey L Cabot Method and device for the treatment of finely divided solids
NL251687A (en) * 1959-06-13
DE2103949A1 (en) * 1971-01-28 1972-08-17 Kloeckner Humboldt Deutz Ag Device for the continuous feeding of an alumina calcination plant with hydrates of aluminum
ES2004759A6 (en) * 1987-07-17 1989-02-01 Espanola Alumina Sa Method for the obtention of an especial alumina from the powder produced in metallurgical alumina calcination
DE4124581C2 (en) * 1991-07-24 1999-12-16 Martinswerk Gmbh Process for calcining hydrated alumina

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