AU6022796A

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

Info

Publication number: AU6022796A
Application number: AU60227/96A
Authority: AU
Inventors: Ronald Lee Bell; Ronald Paul Zapletal
Original assignee: ALUCHEM Inc
Current assignee: ALUCHEM Inc
Priority date: 1995-05-17
Filing date: 1996-05-17
Publication date: 1996-11-29
Also published as: WO1996036563A1

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

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

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.