WO2002090605A1

WO2002090605A1 - Acid digestion process for treating alumina-bearing ores to recover metal values therefrom

Info

Publication number: WO2002090605A1
Application number: PCT/US2002/013960
Authority: WO
Inventors: Robert J. Barnett; Michael B. Mezner
Original assignee: Goldendale Aluminum Company
Priority date: 2001-05-03
Filing date: 2002-05-02
Publication date: 2002-11-14
Also published as: WO2002090605A8

Abstract

A method for processing alumina-bearing ores such as bauxite to recover iron, aluminum, silicon and titanium metal values therefrom the method comprising the steps of adding the alumina-bearing ores to a digester containing an acid to provide a mixture of acid and alumina-bearing ores and heating the mixture to dissolve soluble compounds of at least one of iron, aluminum, silicon and titanium to provide a digest containing dissolved salts of the soluble compounds and to provide a gas component. Thereafter, the digest is treated with water to dissolve water soluble salts therein to provide a slurry comprised of a liquid containing water and the dissolved soluble salts and a solid component comprised of silica. The solid component is separated from the liquid and the pH of the liquid is adjusted to form an aluminate and an iron-containing precipitate. The iron-containing precipitate is separated from the liquid to provide an iron-depleted liquid whose pH is adjusted to precipitate aluminum trihydrate which is separated from the iron-depleted liquid to provide an aluminum trihydrate-depleted liquid. The pH of the aluminum trihydrate-depleted liquid is adjusted to form a precipitate comprised of at least one remaining salt in the aluminum trihydrate-depleted liquid to provide a salt-depleted liquid. The salt precipitate is separated from the salt-depleted liquid thereby recovering the metal values from the alumina-bearing ores.

Description

ACID DIGESTION PROCESS FOR TREATING ALUMINA-BEARING ORES TO RECOVER METAL VALUES THEREFROM

Background of the Invention

[0002] This invention relates to alumina-bearing ores such as bauxite, and more

particularly, it relates to a process for treating alumina-bearing ores to recover the metal

values therein.

[0003] The Bayer process is used for recovering substantially pure alumina from

bauxite and uses NaOH to dissolve the alumina in the bauxite. Most of the other

components of the bauxite are inert in the process, except silica, some of which reacts

with the hydroxide. The inert components and the silica and silica compounds are

rejected and are referred to as red mud or red sludge. This waste product from the Bayer

process has presented serious disposal problems. Red mud can contain 1 1 to 45 wt.%

A1₂0₃, 5 to 66 wt.% Fe₂0₃, 1 to 16 wt.% Si0₂, 6 to 17 wt.% Ti0₂ and 3 to 8 wt.% Na₂0.

[0004] Many attempts have been made to recover the metal values from red mud.

For example, U.S. Patent 3,574,537 discloses a process for the treatment of red-mud to

extract Fe₂0₃, A1₂0₃, Si0₂ and Na₂0, in which S0₂ is passed into a solution of red-mud

and the Fe₂0₃ separated therefrom. The liquor is heated until a pH of 4.5 to 5.0 is reached

forming a precipitate of Si0₂ and Al(OH)S0₃. The precipitate is separated from the

liquor which is concentrated to crystallize out Na₂S0₃. Sulphuric acid is added to the separated precipitate forming water-soluble aluminum salts. Si0₂ remains as a residue

and is removed from solution. Water and a potassium or ammonium salt is added to the

liquor from which the corresponding alum may be crystallized out.

[0005] U.S. Patent 4,119,698 discloses that red mud is digested with concentrated

sulfuric acid or with sulfur trioxide gas to produce sulfates that can be leached out to the

resulting mass with water. The solution is then heated at a pH of 1 to precipitate titanium

oxide hydrate by hydrolysis. The remaining sulfates of the solution are then obtained in

solid form by evaporation, or by precipitation with acetone, and the solid is then roasted

to convert the aluminum and iron to the oxide. After leaching out the sodium sulfate with

water, the aluminum and iron oxide are separated by the Bayer process, which works in

this case even though x-ray diffusion patterns show that the aluminum oxide is mainly

αAl₂0₃.

[0006] U.S. Patent 4,017,425 discloses a method for activating the red mud formed

in the Bayer alumina producing process for allowing its use as adsorbent, catalyst, ion-

exchanging substance and clarifying substance, comprising digesting red mud and

dispersing the metal oxide compound particles in the compound of metal hydroxides and

silica gel.

[0007] U.S. Patent 5,043,077 discloses a method of treating Bayer process red mud

slurries to improve or facilitate the handling thereof, comprising adding to such a slurry a

minor proportion of humic acids or humates effective to reduce the viscosity of the slurry.

[0008] Australian patent 223,794 discloses treating red mud by first calcining then

forming a slag in an electric furnace followed by treating the slag with sulfuric acid. Alumina can be recovered from aluminum sulfate by heating.

[0009] German Patent 26 53 762 discloses a treatment for red mud which

comprises heating to 250° to 350°C (pref. 280° to 320°C), adding concentrated H₂S0₄ or

SO₃ gas, leaching the sulphates formed with water and separating the solution. Ti oxide

is precipitated after adjusting the pH to 1 and the solid Ti oxide is separated by filtering.

Acetone is added or the solution is evaporated to crystallize the sulphates which are

roasted at 900 to 1000°C. The remaining Na sulphate is leached with water and the

oxides of Al and Fe remaining are treated by the Bayer process.

[0010] Japanese reference J 54137-499 discloses that silica, titania and alumina can

be separated from red mud by adding sulfuric acid and then ammonia, reducing trivalent

iron to divalent, and hydrolyzing by adjusting pH.

[0011] It will be seen from the above that great effort is expended to recover the

values from red mud. Thus, there is still a great need for a process which can extract the

metal values from alumina-bearing ores in an economical manner which does not

generate red mud. The instant invention provides such a process.

Summary of the Invention

[0012] It is an object of the present invention to provide a process to recover metal

values from alumina-bearing ores.

[0013] It is another object of the present invention to provide a process for

recovering alumina, iron oxide, silica and titania values from alumina-bearing ores.

[0014] It is a further object of the present invention to provide an acid process for

recovering the metal values from alumina-bearing ores. [0015] It is still a further object of the present invention to provide an acid

digesting process for recovering the metal values from alumina-bearing ores in an

economical manner.

[0016] These and other objects will become apparent from a reading of the

specification and claims appended hereto.

[0017] In accordance with these objects, there is provided a method for processing

alumina-bearing ores to recover iron, aluminum, silicon and titanium metal values

therefrom, the method comprising the steps of adding the alumina-bearing ores to a

digester containing an acid to provide a mixture of acid and alumina-bearing ores and

heating the mixture to dissolve soluble compounds of at least one of iron, aluminum,

silicon and titanium to provide a digest containing dissolved salts of the soluble

compounds and to provide a gas component. Thereafter, the digest is treated with water

to dissolve water soluble salts therein to provide a slurry comprised of a liquid containing

water and the dissolved soluble salts and a solid component comprised of silica. The

solid component is separated from the liquid and the pH of the liquid is adjusted to form

an iron-containing precipitate. The iron-containing precipitate is separated from the

liquid to provide an iron-depleted liquid whose pH is adjusted to precipitate aluminum

trihydrate which is separated from the iron-depleted liquid to provide an aluminum

trihydrate-depleted liquid. The pH of the aluminum trihydrate-depleted liquid may be

adjusted to form a precipitate comprised of at least one remaining salt in the aluminum

trihydrate-depleted liquid to provide a salt-depleted liquid. The salt precipitate is

separated from the salt-depleted liquid thereby recovering the metal values from the alumina-bearing ores.

Brief Description of the Drawing

[0018] Figure 1 is a flow diagram showing the process of recovering metal values

from alumina-bearing ores in accordance with the invention.

Detailed Description of the Preferred Embodiments

[0019] In Figure 1, there are shown process steps of the present invention for

recovering metal values from alumina-bearing ores such as bauxite. It should be

understood that the composition of bauxite can vary, depending to some extent on the

source of the bauxite. Generally, bauxite can comprise 30 to 60 wt.% A1₂0₃, 3 to 23.5

wt.% Fe₂0₃, 1.5 to 26.5 wt.%> Si0₂, 1 to 18 wt.% Ti0₂ and other minor ingredients.

Preferably, free water is removed. That is, it is preferred to use a high solids content for

use in the digester. Thus, the alumina-bearing ores for use in the digester can comprise

about 25 to 75 wt.% solids or higher, the balance water, with the higher level of solids

being preferred.

[0020] In the process, alumina-bearing ores 5 is added to an acid digester 10 for

providing a mix for purposes of digesting the major components such as soluble

compounds of iron, aluminum, and titanium. Minor components will also be digested.

[0021] The preferred acid used in the digester is concentrated sulfuric acid (e.g.,

approximately 93%> by weight). However, other strong acids such as HClO₄, HCl, HN0₃

and H₃(P0₄) may be used singly or in combination with H₂S0₄ acid, for example. The

different acids may produce different salts, however, the process may be adjusted to

accommodate the different materials. [0022] Preferably, alumina-bearing ores, e.g., bauxite, is added to the digester to

provide a concentration which is just in excess of a stoichiometric ratio. The ratio can

range from about 0.8 to 2, preferably about 1 to 1.1. For example, the alumina-bearing

ores can be added to provide about 2831 lbs of H₂S0₄ to 2000 lbs of alumina-bearing ores

and preferably 2973 lbs of H₂S0₄ to 2000 lbs of alumina-bearing ores, depending to some

extent on the composition of the alumina-bearing ores or bauxite. The amount of acid

provided in the mix is that which is effective in digesting the alumina-bearing ores

efficiently.

[0023] In the preferred embodiment, digester 10 is maintained at an elevated

temperature, for example, up to 300°C and typically in the range of about 160° to 200°C,

e.g., about 180°C. Heat is added along line 12. The ranges provided herein are inclusive

of the numbers within the range as if specifically set forth. Gases generated can be

removed along line 14 on a continuous basis and directed to a scrubber 170. For purposes

of removing gases, preferably the digester is maintained at a slight negative pressure to

remove gases such as S0₂, S0₃, H₂0 and C0₂.

[0024] The period of time for digesting an alumina-bearing ore such as bauxite

with acid or time for contacting with acid can range from 15 to 90 minutes or even longer

with longer times not known to be detrimental. Shorter times can be employed,

especially at the higher temperatures. If a continuous feeder is employed, then the feed

rate is controlled to permit a retention time for digesting, e.g., 15 to 90 minutes or longer

as suitable.

[0025] When sulfuric acid is employed in digester 10, the principle metals in the alumina-bearing ores (aluminum, iron, titanium, sodium and calcium) are converted to

sulfates. Another acid that could be used is hydrochloric acid which would convert the

principle metals to chlorides.

[0026] After the digesting step, the digested mix is treated with water 20 for

purposes of recovering water soluble sulfates from the mix. The treatment can be at room

temperature but preferably is carried out at about 80° to 100°C to promote dissolution of

the water soluble salts. Preferably, the water used for purposes of dissolution is

maintained acidic, e.g., about a pH in the range of 1 to 2. The soluble sulfates are leached

from the digested mix with water 20 to provide a liquid comprised of water containing

soluble salts and solids. In a preferred operation, the water for the leaching step includes

the water from scrubber 170. By using water from scrubber 170, acids used in the

process are recovered. The use of water from scrubber 170 eliminates the need to dispose

of a waste stream and provides for more favorable economics in the process. The amount

of water added depends on the concentration of soluble salts, but generally water is added

to maintain about 5 to 25% salts in solution.

[0027] The solids that remain after digesting and leaching of the soluble salts are

comprised of silica. As will be seen from Figure 1, the silica and any other solids

remaining, e.g., trace amounts of aluminum and iron, are separated from the liquid by

filtration 30. The silica solids can be rinsed with water to recover any residual salts. The

solids may be removed along line 34 and dried by dryer 40 before being removed from

the system as a first product 42. The silica can be used in the cement industry or may be

disposed of as backfill without presenting environmental problems. [00281 The liquid resulting after the digesting step and the leaching step may be

subjected to a hydrolysis step 50. This step is optional and is only necessary if titanium is

present in the alumina-bearing ores in substantial amounts, e.g., greater than 3%. Thus, it

will be seen from Figure 1 that the liquid resulting from the filtration step is removed

along line 32 to a hydrolysis tank where sufficient water is added to form a titanium oxide

precipitate. For purposes of precipitating titanium oxide, the solution should be

maintained at a pH of about 1 to 1.5 and a temperature of 90° to 95°C for about two hours.

[0029] After the titanium oxide precipitate is formed, the precipitate is separated by

filtration, leaving a titanium-depleted solution. The precipitate may be rinsed several

times to remove residual liquid and the rinse water added to the titanium-depleted

solution. The pH of the titanium-depleted liquid is acidic, e.g., in the range of 1 to 1.5.

The titanium oxide precipitate is removed along line 62 and heated in a dryer 70 which

removes the water of hydration thereby forming titanium oxide. This is the second useful

product which may be recovered from the alumina-bearing ores.

[0030] The titanium-depleted liquid is removed along line 64 to a first pH

correction tank 80 (see Fig. 1). The pH is adjusted upwardly to a pH sufficiently high to

form an aluminate, e.g., sodium aluminate and to form a precipitate containing iron such

as Fe(OH)₂ or Fe(OH)₃. Generally, the pH for forming sodium aluminate and

precipitating iron compounds is in the range of about 11 to 13. The pH can be adjusted

upwardly by adding sodium hydroxide in which case sodium aluminate and iron

hydroxide are formed. However, the pH may be adjusted upwardly by addition of any

suitable alkali or alkaline earth compound such as an hydroxide which forms a suitable compound such as aluminate, e.g., sodium aluminate and a precipitate of iron hydroxide

which can be readily removed from the aluminate-containing solution. As the pH is

increased, typically the iron hydroxide forms in a pH range of about 3 to 6, and typically

the sodium aluminate forms in a pH range of about 11 to 13.

[0031] The iron hydroxide can be removed by filtration 90, as shown in Figure 1,

to provide an iron hydroxide-depleted solution. Typically, upon removal, the iron

hydroxide precipitate is washed several times to remove residual sodium sulfate and

sodium aluminate. The wash water from the iron hydroxide precipitate is returned to the

iron hydroxide-depleted solution. It should be noted that precipitation of other elements

which are present in minor amounts may occur. The iron hydroxide is removed along

line 92 to be dried in drying step 100, as shown in Figure 1, and recovered as the third

useful product from the alumina-bearing ore.

[0032] The iron hydroxide-depleted solution, removed along 94, is subjected to a

further pH adjustment 110, as shown in Figure 1. That is, the pH is lowered sufficiently

to form alumina trihydrate precipitate, i.e., alumina trihydrate crystals. This provides a

slurry of alumina trihydrate crystals and water. Alumina trihydrate as used herein is

meant to include A1₂0₃ ^■ 3H₂0 or Al(OH)₃ and may be referred to as aluminum

hydroxide, aluminum hydrate, aluminum trihydrate, hydrated alumina or hydrated

aluminum oxide. For purposes of forming alumina trihydrate, the pH of the iron-depleted

solution is lowered to a pH in the range of about 11 to 12, preferably pH in the range of

11.3 to 11.8. The pH can be lowered by the addition of an acid. Preferably, the acid is

selected from the group comprising H₂S0₄, HCl, HN0₃, H₂SO₃, H₃(P0₄) or combinations of such as acids with H₂S0₄ acid being preferred. A 10 to 20 wt.% H₂S0₄ solution can be

used for purposes of lowering the pH. Acid may have to be added several times until the

pH stabilizes between 11.3 to 11.8. That is, after the addition of acid, the pH may rise

again and require further additions.

[00331 The slurry of alumina trihydrate crystals and liquid are filtered, step 120, to

separate the crystals from the liquid. The crystals may be washed or rinsed several times

to remove residual liquid containing soluble salts to ensure high purity alumina trihydrate

crystals. The wash water can be returned to the residual liquid. The alumina trihydrate

crystals may be dried and sold as a fourth product recovered from the alumina-bearing

ores. Or, the alumina trihydrate crystals may be forwarded along line 122 to the

crystalizers 130 similar to or the same as used in the Bayer process. Of the alumina

trihydrate crystals formed, 95% have a particle size in the range of 30 to 50 μm with the

remainder having a size less than 30 μm. However, the crystal size can be larger or

smaller, depending to some extent on the time and temperature and acid used for

precipitation. Typically, the alumina trihydrate crystals will have Na₂0 content in the

range of 0.05 to 0.15 wt.%. Further, the crystals are substantially free of organic

compounds and only contain traces of calcium, e.g., less than 0.01 wt.% and less than

0.005 wt.% Fe.

[0034] One of the last operations in obtaining alumina is calcination. In this

operation, the temperature of the alumina trihydrate is raised sufficiently to effect the

following reaction: 2A1(0H)₃ — » A1₂0₃ + 3H₂0. If the temperature of the reaction is approximately 1100°C, alumina obtained is mostly α-phase. However, lower

temperatures produce a less α-phase.

[0035] The alumina trihydrate of the invention is particularly suited for producing

aluminum by electrolysis or for producing activated aluminas because of the high level of

purity, e.g., less than 0.1 wt.% Na₂0 and substantially free of organics and iron oxide. In

producing activated alumina, the alumina trihydrate can be ground to the desired particle

size, e.g., 0.1 to 50 μm before or after activation. Flash calcination or fast dehydration of

the alumina trihydrate can occur either by vacuum or by exposure to high temperature gas

(780°-1000°C) for a few seconds. This material may then be used for forming rehydrated

agglomerations or bodies of activated alumina using a rotating pan or the oil-drop

method. The bodies are activated by heating to 400°-500°C and can provide surface areas

of 250 to 450 m²/g.

[0036] It will be appreciated that barium aluminate can be produced in this process

by using barium hydroxide to adjust the titanium-depleted liquid upwardly to a pH of

about 11 to 13 instead of sodium hydroxide.

[0037] The residual liquid separated from the alumina trihydrate crystals can be

removed along line 124 or can be further treated to recover remaining salts. That is, the

pH of the residual liquid is further lowered in step 140 to precipitate out the remaining

dissolved salts. The pH is preferably lowered to a pH in the range of about 6 to 7. This

pH range may be obtained with the addition of an acid such as H₂S0₄, HCl, HN0₃, H₂S0₃

and H₃(P0₄) with H₂S0₄ acid being preferred. The solids are removed by filtering. Such solids may be washed as desired. Preferably, the solids recovered are returned along line

152 and added to the titanium-depleted liquid at step 80 for reprocessing. The liquid

remaining is forwarded long line 154 to step 160 and subjected to acid and base recovery

operations. Thus, substantially all of the ingredients are re-used.

[0038] In the acid-base recovery process, the solution can be processed in several

ways. When sulfuric acid is used for digesting, the liquid directed to step 160 is high

purity sodium sulfate (about 99.9%). Thus, the solution can be crystallized or extracted

with 1 : 1 methanol and sodium sulfate recovered. Or the solution can be subject to

electrolysis to form ammonium sulfate and sodium hydroxide. The ammonium sulfate

may be converted to ammonia and sulfuric acid by heating at 400° to 500°C. Depending

on which products are produced, they may be sold or reused in the process to improve

efficiency.

[0039] The process may be adjusted as needed to suit the bauxite being used for

highest efficiency. For example, alumina removal efficiency can be as high as 97% and

no red mud is produced. High efficiency for the Bayer process is about 89%; however, as

noted, the Bayer process produces red mud residue.

[0040] It should be noted that digestion of alumina-bearing ores with sulfuric acid

in digester 10 will liberate gases such as S0₂, S0₃ and C0₂. These gases can be removed

along line 14 to scrubber 170 where the gas is converted to Na₂S0₄ and Na₂C0₃ which

may be redirected along with water along line 172 to step 20 and used for leaching the

water soluble salts from the digest. This eliminates a waste stream in the overall process.

[0041] An alternate method may be employed for treating the titanium-depleted liquid removed along line 64 (Fig 1), depending on the amount of sodium oxide contained

therein. If the titanium-depleted solution contains less than about one (1) wt.% sodium

oxide, the titanium-depleted solution may be evaporated to crystallize the aluminum

sulfate and iron sulfate component contained in the solution. After evaporation, the

aluminum sulfate and iron sulfate component is heated to a temperature of about 400° to

600°C. In this temperature range, the iron sulfate is decomposed to iron oxide and sulfur

dioxide and sulfur trioxide gas. The sulfur dioxide and sulfur trioxide may be recovered

in an oleum scrubber such as scrubber 170 as sulfuric acid for re-use in the process.

[0042] The aluminum sulfate is not decomposed in the temperature range of 400°

to 600°C and thus may be washed or separated from the iron oxide. That is, water is used

to dissolve the aluminum sulfate from the iron oxide. Preferably, water at temperature in

the range of about 60° to 100°C is used to dissolve the aluminum sulfate. It is preferred to

maintain the aluminum sulfate in solution at a high concentration, e.g., 60 to 80 wt.%.

Thus, in dissolving aluminum sulfate from the iron oxide, the amount of water used

should be minimized to maintain a high concentration of aluminum sulfate in solution.

Thereafter, the aluminum sulfate solution is chilled to a temperature range of about 0 to

15°C which crystallizes out a portion of the aluminum sulfate leaving about 28 to 35 wt.%

aluminum sulfate in solution. The crystallized aluminum sulfate is recovered from the

solution and heated, e.g., to about 700° to 850°C. This decomposes the aluminum sulfate

crystals into alumina and releases sulfur dioxide and sulfur trioxide which may be

captured in the oleum scrubber as sulfuric acid for re-use in the process. The alumina is

recovered and used without further modification. [0043] When the digest residue or alumina-containing ore contains more than

about 1 wt.% sodium oxide, it should be noted that the liquid leaving filter 150 (Fig. 1) is

very high in sodium sulfate. This liquid or sodium sulfate solution may be treated or

reacted with barium hydroxide at about stoichiometric or slightly less. This reaction

forms barium sulfate which precipitates leaving sodium hydroxide in solution. The

barium sulfate is removed from the sodium hydroxide solution which can be re-used in

the process. The barium sulfate can be heated to a temperature range of 1450° to 1570°C

and decomposed to form barium oxide, sulfur dioxide and sulfur trioxide gases. The

sulfur-containing gases can be converted to sulfuric acid in oleum scrubber 170 and re¬

used in the process. The barium oxide can be dissolved in warm water, e.g., 60° to 90°C,

to form barium hydroxide which can be re-used in the process.

[0044] Alternatively, the barium sulfate may be mixed with silica and reacted to

form a mixture containing barium silicate. The mixture can be heated to about 1250° to

1350°C to liberate sulfur dioxide and sulfur trioxide which are recovered in scrubber 170.

The remainder of the mixture is added to hot water as noted to produce barium hydroxide

and silica. The silica can be recovered and sold and the barium hydroxide re-used in the

process.

[0045] While the process of the invention has been described with respect to

alumina-bearing ores or bauxite, it has application to other alumina-bearing ores such as

alunite, kaolinite, and other kaolin-type ore. Normally, these sources of alumina would

be washed to remove clays, etc., prior to processing in accordance with the invention.

Typically, for processing in accordance with the invention, the alumina-bearing ores are ground to about less than 16 mesh. Thereafter, the alumina-bearing ores are digested as

described herein and has the advantage that no residues such as red mud are produced.

[0046] While the invention has been described in terms of preferred embodiments,

the claims appended hereto are intended to encompass other embodiments which fall

within the spirit of the invention.

Claims

1. A method for processing alumina-bearing ores to recover iron,

aluminum, silicon and titanium metal values therefrom, the method comprising the steps

of:

(a) adding said alumina-bearing ores to a digester containing an acid

to provide a mixture of acid and alumina-bearing ores;

(b) heating said mixture to dissolve soluble compounds of at least

one of iron, aluminum and titanium to provide a digest containing dissolved

salts of said soluble compounds and to provide a gas component;

(c) treating said digest with water to dissolve water soluble salts

therein to provide a slurry comprised of a liquid containing water and said

dissolved soluble salts and a solid component comprised of silica;

(d) separating said solid component from said liquid;

(e) adjusting the pH of said liquid upwardly to form an iron-

containing precipitate;

(f) separating said iron-containing precipitate from said liquid to

provide an iron-depleted liquid;

(g) adjusting the pH of said iron-depleted liquid to precipitate

aluminum trihydrate; and

(h) separating said aluminum trihydrate from said iron-depleted

liquid to provide an aluminum trihydrate-depleted liquid.

2. The method in accordance with claim 1 wherein the acid is selected from

the group consisting of H₂S0₄, HCl, HNO₃, HC10₄, and H₃(PO₄) or mixtures thereof.

3. The method in accordance with claim 1 wherein said mixture contains

alumina-bearing ores to acid in a ratio of about 0.8 to 2.

4. The method in accordance with claim 1 wherein said heating of said

mixture includes heating to a temperature in the range of ambient up to 300°C.

5. The method in accordance with claim 1 including heating of said

mixture to a temperature in the range of 160° to 200°C.

6. The method in accordance with claim 1 wherein said mixture is heated

for a period in the range of 15 to 90 minutes.

7. The method in accordance with claim 1 including adjusting the pH of the

liquid in step (e) upwardly after separating said solid component by adding a base

material.

8. The method in accordance with claim 1 including adjusting the pH of

said liquid in step (e) upwardly by adding a base material selected from NaOH, Ca(OH)₂

and KOH.

9. The method in accordance with claim 1 wherein NaOH is added to

adjust the pH of the liquid in step (e) upwardly to form sodium aluminate.

10. The method in accordance with claim 1 including adjusting the pH of

the liquid in step (e) to a pH in the range of 3 to 6 to form an iron-containing precipitate.

11. The method in accordance with claim 1 wherein the iron-containing

precipitate is iron hydroxide.

12. The method in accordance with claim 1 including adjusting the pH of

the iron-depleted liquid to a pH in the range of 11 to 13 to form aluminum trihydrate.

13. The method in accordance with claim 1 including adding an acid to the

iron-depleted liquid to adjust said pH to precipitate said aluminum trihydrate.

14. The method in accordance with claim 1 including adding H₂S0₄ acid to

the iron-depleted liquid to adjust said pH to precipitate said aluminum trihydrate.

15. The method in accordance with claim 1 including adjusting the pH of

the aluminum trihydrate-depleted liquid to a pH in the range of about 6 to 7 to precipitate

remaining dissolved salts.

16. The method in accordance with claim 1 including adding water to said

liquid from step (d) to hydrolyze titanium contained therein to form a titanium containing

precipitate.

17. The method in accordance with claim 16 including the step of

separating said titanium containing precipitate from said liquid.

18. The method in accordance with claim 1 wherein the alumina-bearing

ore is bauxite.

19. The method in accordance with claim 1 including adjusting the pH of

the aluminum trihydrate-depleted liquid to form a precipitate comprised of at least one

remaining salt in said aluminum trihydrate-depleted liquid to provide a salt-depleted

liquid; and separating said salt precipitate from said salt-depleted liquid thereby

recovering said metal values from said alumina-bearing ores.

20. The method in accordance with claim 1 including calcining the alumina

trihydrate to recover alumina.

21. The method in accordance with claim 1 including calcining the alumina

trihydrate and recovering alumina having less than 0.15 wt.% sodium oxide.

22. A method for processing bauxite to recover iron, aluminum, silicon and

titanium metal values therefrom, the method comprising the steps of:

(a) adding said bauxite to a digester containing sulfuric acid to

provide a mixture of acid and bauxite;

(b) heating said mixture to a temperature in the range of 120° to

200°C to dissolve soluble compounds of at least one of iron, aluminum and

titanium to provide a digest containing dissolved salts of said soluble

compounds and to provide a gas component, said heating being for a period

of about 15 to 90 minutes;

(c) treating said digest with water to dissolve water soluble salts

therein to provide a slurry comprised of a liquid containing water and said

dissolved soluble salts and a solid component comprised of silica;

(d) separating said solid component from said liquid;

(e) adjusting the pH of said liquid to form an iron-containing

precipitate;

(f) separating said iron-containing precipitate from said liquid to

provide an iron-depleted liquid; (g) adjusting the pH of said iron-depleted liquid to a pH range of 11

to 13 to precipitate aluminum trihydrate;

(h) separating said aluminum trihydrate from said iron-depleted

liquid to provide an aluminum trihydrate-depleted liquid;

(i) adjusting the pH of the aluminum trihydrate-depleted liquid to a

pH range of about 6 to 7 to form a salt precipitate of salts remaining in said

aluminum trihydrate-depleted liquid to provide a salt-depleted liquid; and

(j) separating said salt precipitate from said salt-depleted liquid

thereby recovering said metal values from said bauxite.