AU2018359800B2 - Method for processing bauxite - Google Patents

Abstract

A method for producing alumina from a bauxite pre-processed by a method comprising calcination (2020) and leaching (2030), said pre-processed bauxite being characterised in that it has a loss on ignition of less than 2.5% by mass, said method comprising the steps of: (a) Processing (2120) of the pre-processed bauxite with an aqueous sodium hydroxide solution at a temperature of at least 100°C, said aqueous sodium hydroxide solution having a concentration of between 100 g Na

Description

W02019/086792 1 PCT/FR2018/052678

METHOD FOR PROCESSING BAUXITE

Technical field of the invention

The invention relates to the field of processing ore,

and more particularly the physical and chemical

processing of bauxite. The invention relates in

particular to a method for thermal processing and

chemical processing of bauxite with a low alumina

/ silica mass ratio. In this method, first the ore is

depleted of silicon by thermal preprocessing followed by

leaching, then this preprocessed ore is used in the Bayer

process to extract the aluminum therefrom in the form of

aluminum trihydrate, which can be transformed into

alumina.

Prior art

Aluminum is the third most abundant chemical element

in the Earth's crust, after oxygen and silicon.

Associated with oxygen it is found in a very large number

of rocks. The main industrial ore of aluminum is bauxite,

discovered in 1821 in the village Les Baux (France) by

geologist Pierre Berthier. Bauxite represents a complex

mixture of oxides of aluminum, of iron and of silicon

that can comprise various impurities such as titanium,

calcium, magnesium. More precisely, bauxite is an ore

mainly comprising three aluminum minerals, namely

gibbsite, boehmite, and diaspore, mixed with smaller

quantities of iron minerals, namely goethite and hematite

(which gives bauxite its characteristic color), as well

as aluminosilicates (kaolinite, illite...) and titanium

minerals (anatase, rutile, ilmenite).

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The main industrial method used to extract the

aluminum (in the form of oxide) from bauxite is the Bayer

process, developed at the end of the 19th century. It

substantially comprises two steps: a first step of

leaching of the ore under pressure by a solution of

sodium hydroxide (see the patent DE 43 977 from 3 August

1888), and a second step of precipitation of the pure

hydrated alumina from the solution of sodium aluminate

thus obtained, by seeding with crystals of hydrated

alumina (see the patent DE 65604 from 3 November 1892).

This precipitation alumina, hydrated, can then be

subjected to thermal processing to dehydrate it; this

thermal processing also determines the structure and

morphology of the alumina obtained, with a view to its

use (in the Hall-Hroult process to produce aluminum by

electrochemical reduction of the alumina in a molten salt,

or as technical alumina, in particular in the ceramics

industry).

More precisely, the Bayer process mainly involves a

selective attack (digestion) of the hydrates of alumina

contained in the bauxite by a solution (called "liquor")

of hot caustic soda, which is recycled. After separation

by decantation and washing of the bauxite residues (these

residues being called "red mud"), the solution of sodium

hydroxide enriched with sodium aluminate is cooled then

decomposed (crystallization phase) to precipitate and

extract the alumina trihydrate (A1 2 0 3 -3H 2 0); the latter is

then washed then calcined at high temperature to give

alumina (A1 2 0 3 ) . The liquor depleted of sodium aluminate

after the crystallization phase and diluted by the inputs

of water, coming substantially from washing of the

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residues of bauxite, is evaporated and recycled to the

attack.

The composition of bauxite depends on its geographic

origin. This variation in composition relates to both its

content of main elements (Al, 0, Si), its content of

impurities, and its mineralogical structure. For example,

the lateritic bauxites (on an aluminosilicate geological

substrate) coming from mines located in Guinea or from

Australia in general have a high content of gibbsite,

which is a trihydrate, and a lower silicon content than

the karst bauxites (on a carbonate geological substrate)

coming from certain mines located in Iran, in Kazakhstan,

in Azerbaijan and in Turkey, in which the aluminum is

mainly in the form of boehmite and diaspore (two

modifications of monohydrates). Thus, a simple parameter

for representing the quality of a bauxite is the ratio of

alumina to silica, abbreviated as "A/S ratio". For

example, the bauxites from Guinea generally have

A1 2 0 3 /SiO 2 ratios of approximately 20 or more, and the

bauxites from western Australia ratios greater than 15.

In the bauxites of North Queensland the aluminum is

present mainly in the form of boehmite and of gibbsite.

The aluminum content is not, however, the only

criterion: it is also necessary for this aluminum to be

in a form chemically and crystallographically capable of

being extracted from the bauxite by the Bayer process. It

is known that the conventional Bayer process does not

allow to solubilize the aluminum contained in the

aluminosilicates: this part of the aluminum is lost in

the red mud. Moreover, it is known that the

aluminosilicates contained in red mud can carry off a

part of the sodium hydroxide and thus increase the

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overall consumption of sodium hydroxide of the Bayer

process; this is described in the publication "Basic

Research on Calcification Transformation, Process of Low

Grade Bauxite" by Xiaofeng Zhu et al., published in Light

Metals 2013, p. 239-244 (TMS).

Like most ores, bauxite is traded on a world market

and often transported for thousands of kilometers to its

place of use. However, certain countries prefer limiting

their procurement on the world market in favor of the use

of their domestic mining resources, even if the latter

are of lesser quality. This is in particular the case of

China. China, the largest producer of aluminum in the

world, possesses bauxite deposits and mines that do not

(or no longer) have an A/S ratio as high as the

Australian bauxites, for example. More precisely, in

China, the deposits of alumina with a low silicon content

(A/S > 8 or even > 6) are becoming rare, whereas the

bauxite with a high silicon content exists in

considerable quantities. Moreover, in a plurality of

Chinese bauxites with a high silica content, the silicon

is present for the most part in the form of kaolinite, an

aluminosilicate that also encloses a part of the aluminum

present in the bauxite; they also include a small

fraction of quartz (a silicate) and of muscovite (another

aluminosilicate). However, as indicated above, the

conventional Bayer process does not allow to extract the

aluminum present in aluminosilicates.

For decades the production of primary aluminum (and

consequently the consumption of bauxite) has been

increasing regularly by several percent per year. These

last few years the use of bauxites of lesser quality has

been becoming a major economic and technological issue,

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especially in Iran, in Kazakhstan and in China. This

question is also posed in other countries, such as Russia

and Turkey, and it is especially researchers from these

countries that have sought possibilities for using

bauxites having a low A/S ratio. Some of these bauxites

further have a greater iron and/or sulfur content than

the bauxites rich in aluminum.

There are various approaches for preprocessing

bauxite with a view to modifying its mineralogical

structure in such a way as to increase the percentage of

aluminum extractible by the Bayer process, or in such a

way as to facilitate the separation of the silicon

upstream of the Bayer process. There are also methods

aiming to facilitate the separation of the iron upstream

of the Bayer process, with a view to reducing the

formation of red mud and recovering this element in a

usable form.

For example, the article "Pre-beneficiation of low

grade Diasporic bauxite ore by reduction roasting" by K.

Yilmaz et al., published in 2015 in the review Int. J.

Chemical, Molecular, Nuclear, Materials and Metallurgical

Engineering, vol 9(9), p. 1023-1026, describes a method

for calcination (called "roasting") of bauxite of Turkish

origin with a high Si and Fe content to transform the

iron into phases capable of being separated by a magnetic

process. Other methods for calcination in a reducing

medium use CO (CN 103 614 547 - Central South University;

CN 104 163 445 - China Aluminium) or coal (CN 101 875 129

- Central South University).

For the bauxites with a low A/S ratio a method

called "calcification - carbonation" was described (see

for example "Calcification - carbonation method for

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alumina production by using low-grade bauxite" by Ting'an

Zhang et al., K. Yilmaz et al., Light Metals 2013, p.

233-238 (TMS)). This method comprises the processing of

the bauxite by an alkaline liquor in the presence of lime

(leading to the formation of an insoluble calcium

aluminum-silicate phase), and the processing of this

insoluble phase (after decantation) by pressurized C02 to release a part of the aluminum by forming two new

insoluble phases, Ca3SiO 4 and CaC03. A certain number of other methods have been

described, for example the calcination of a solid mixture

of bauxite + Na2CO3 at 600 - 10000C then the dissolution

of the calcined mixture in NaOH at 750C, followed by

seeding to precipitate a phase that carries away the

silicon (CN 102 180 498 and CN 101 767 807, Aifang Pan) ;

in an alternative of this method described in CN

205 603 238 (Hangzhou Jinjiang) the mixture subjected to

the calcination also includes lime; and the Na2CO3 forms

with water a base that allows the leaching of the silica.

Methods are also known in which the bauxite is calcined

without the addition of products. CN 203 408 047

describes such a method (Xi'An University) to desulfurize

bauxites that contain pyrite (FeS2). The idea of a method for thermochemical activation

of bauxite (called "roast-leach process") was described

by Smith and Xu-Parker ("Options for processing of high

silica bauxites", Travaux ICSOBA Vol. 35 (39) , 184-192

(2010)). This method was initially developed for raw

materials of the clay type (see US 2 939 764) and illite

(see T Jiang et al., "Desilication from illite by

thermochemical activation", Trans. Nonferrous Met. Soc.

China, Vol 14(5), p. 1000-1005 (2004)), as well as for

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aluminum ores containing diaspore and not comprising

silicon (Q Zhou et al., "Temperature dependence of

crystal structure and digestibility of roasted diaspore",

Trans. Nonferrous Met. Soc. China, Vol 14(1), p. 180-183

(2004)). Indeed, it comprises two distinct steps, a first

step of calcination of the bauxite at a temperature of

9800C, without additive, during which the kaolin partly

decomposes to form amorphous silica and aluminas called

transition aluminas, and a second step of leaching that

aims to selectively dissolve the amorphous silica in

milder conditions than those of the Bayer process. The

residue (the calcined and leached bauxite) is then

introduced into the conventional Bayer process. This

method presents at first glance the disadvantage of

requiring additional thermal energy for the calcination

of the bauxite, to arrive at a calcined and leached

bauxite that has characteristics comparable to those of a

high-quality bauxite. This overconsumption of energy can

pose an obstacle to the development of the method.

Moreover, the method requires an additional raw material,

namely lime, to precipitate the silicate coming from the

solubilization of the amorphous silica formed during the

calcination of the bauxite.

The roast-leach process has given rise to a rather

large scientific literature, which has focused on the

calcination step, with divergent indications as to the

optimal calcination temperature. The maximum value of the

calcination temperature appears to be given by the

formation of mullite: According to Xu et al. ("Thermal

behaviors of kaolinite-diasporic bauxite and desilication

from it by roasting-alkali leaching process", Light

Metals TMS 2002), the decomposition of meta-kaolinite into amorphous silica and gamma alumina starts at 990°C, but at approximately 1100°C these two phases begin to react to form mullite, insoluble in sodium hydroxide under the conditions of the conventional Bayer process; a temperature between 10000C and 10500C is recommended. Li et al. ("Desilication of bauxite ores bearing multi aluminosilicates by thermochemical activation process", Light Metals TMS 2009, p. 57 - 61) also observe mullite starting from 11000C and note a decrease in the desilication at 1150-1200°C, but conclude that the activation of the bauxite is optimal between 11000C and 11500C. A publication by N. Eremin from 1981 ("Beneficiation of low-grade bauxites by hydrometallurgical methods", ICSOBA 1981, p. 135 - 142) indicates a similar mechanism with an optimal temperature between 9250C and 10000C. The publication by Moazemi et Rezai ("Desilication studies of diasporic bauxite by thermochemical treatment", Proc. XI Int. Seminar on Minerals Processing Technology (MPT-2010), p. 832-838) shows that the curve of the yield of extraction by leaching according to the calcination temperature has a rather narrow maximum at 10000C. Despite this progress in the understanding of the roast-leach process, it is noted that it is hardly used industrially since it adds a significant additional cost of use to the conventional Bayer process. Moreover, this additional cost is related to two items particularly targeted in the establishment of an environmental report, namely energy consumption and water consumption. One or more embodiments of the invention provide a method or an improved and economically viable process for using bauxites with a low alumina content (A/S < 5 and preferably < 4 and even more preferably < 3) in a method of the Bayer process type.

One or more embodiments of the invention provide a method for processing bauxite that comprises a preprocessing of the bauxite, which improves its ability to be used as a raw material in the Bayer process, known per se, and by adapting the Bayer process to this preprocessed bauxite. One aspect of the invention provides a method for manufacturing alumina trihydrate or alumina from a bauxite preprocessed by a method comprising a calcination and a leaching, wherein said preprocessed bauxite has a loss on ignition of less than 2.5% by mass, said method comprising the steps of: (a) Processing (called "digestion") the preprocessed bauxite with an aqueous solution of sodium hydroxide at a temperature of at least 1000C, said aqueous solution of sodium hydroxide having a concentration of between 100g Na20/L and 220g Na20/L; (b) Separating the solid residue from the liquid phase; (c) Crystallizing the aluminum trihydrate by addition of seeds ; (d) Separating the crystallized aluminum trihydrate from the liquid phase;

9A

(e) optionally, calcining the aluminum trihydrate

obtained in step (d) to obtain the alumina.

Said preprocessing of the bauxite comprises a first

step of preprocessing, which is physical preprocessing,

namely thermal. This thermal preprocessing is intended to

provoke a chemical and crystallographic modification of

the bauxite (or at least of some of these mineralogical

phases that constitute it). This first step of

preprocessing the bauxite is advantageously carried out on

a ground bauxite. It leads to a modified bauxite that can

then be introduced into the Bayer process, or be subjected

to a second, chemical, preprocessing step.

More particularly, said thermal preprocessing is

carried out at a temperature and for a time such that at

least a part of the silicates present in the bauxite are

transformed into amorphous silica. For a diasporic bauxite

this temperature is advantageously located between 10000C

and 10500C, preferably between 10150C and 1030C, and even

more preferably between 10150C and 10250C. For boehmitic

bauxites this temperature is approximately 400C lower and

is located between approximately 9600C and approximately

10000C and preferably between 9700C and 990°C.

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This method can be implemented in part on the

industrial site of production of the bauxite or entirely

on the industrial site at which the Bayer process is

installed. It requires dedicated equipment, namely a

furnace. This method is advantageously implemented on

ground bauxite.

According to the invention, the method for

processing the bauxite comprises a second preprocessing

step, which is a chemical step. It comprises the leaching

of the modified bauxite by sodium hydroxide. During this

step, and under the suitable temperature, residence time,

sodium hydroxide concentration and solid/liquid ratio

conditions, the amorphous silica obtained during the

calcination is dissolved while very little of the alumina

goes into solution.

This leaching step must be carried out on a ground

bauxite, and for this reason it is advantageous for the

grinding to be done upstream of the thermal preprocessing

step. The grinding method and the desired particle size

can be similar to those used for the conventional Bayer

process. It is also possible to regrind the processed

bauxite before its introduction into the Bayer process.

The method for preprocessing the natural bauxite by

a method successively comprising a calcination and a

leaching leads to a product called here "preprocessed

bauxite" which differs, both in chemical and

mineralogical terms, from a natural bauxite. A simple

parameter that expresses this particularity of the

preprocessed bauxite is its loss on ignition; this loss

on ignition is much lower (typically by a factor of ten

to twenty) than that of a natural (not preprocessed)

bauxite.

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Thus, a first object of the invention is a method

for manufacturing alumina trihydrate or alumina from a

bauxite preprocessed by a method comprising a calcination

and a leaching, said preprocessed bauxite being

characterized in that it has a loss on ignition of less

than 2.5% by mass, preferably less than 2.0%, and even

more preferably less than 1.5%. Advantageously, this

preprocessed bauxite is also characterized by the absence

of diaspore and the presence of amorphous silica.

Said method comprises the steps of:

(a) Processing ("digestion") the preprocessed

bauxite with an aqueous solution of sodium hydroxide at a

temperature of at least 1000C (typically in an

autoclave), said aqueous solution of sodium hydroxide

having a concentration of between 100g Na20/L and 220g

Na20/L, preferably between 140g Na20/L and 200g Na20/L,

more preferably between 155g Na20/L and 190g Na20/L, and

even more preferably between 160g Na20/L and 180g Na20/L;

(b) Separating the solid residue from the liquid

phase;

(c) Crystallizing the aluminum trihydrate by

addition of seeds;

(d) Separating the crystallized aluminum trihydrate

from the liquid phase;

(e) Calcining the aluminum trihydrate obtained in

step (d) to obtain alumina.

This last step is optional: if the method according

to the invention is aimed at obtaining aluminum

trihydrate, which is a commercial product, it will

suffice to dry the aluminum trihydrate obtained in step

(d). If the method is aimed at alumina, step (e) is

necessary.

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Advantageously, the temperature in step (a) is

between 1500C and 3500C, preferably between 2000C and

3000C, more preferably between 2200C and 2800C, and even

more preferably between 2500C and 2700C.

Said preprocessed bauxite advantageously has a mass

ratio of A1 2 0 3 / SiO 2 greater than 8, and preferably

greater than 9, and even more preferably greater than 10.

Its mass content of alumina is advantageously greater

than 60%, preferably greater than 65%, and even more

preferably greater than 70%. Its mass content of silica

is less than 12%, preferably less than 10%, and even more

preferably less than 8%.

In an advantageous embodiment of the method

according to the invention, the liquid phase coming from

step (d) is reintroduced into the aqueous solution of

sodium hydroxide used in step (a).

Advantageously, said preprocessed bauxite has been

preprocessed by calcination at a temperature between

approximately 9200C and approximately 12000C. This

temperature is preferably between approximately 9500C and

approximately 10700C, and even more preferably between

approximately 10000C and approximately 10500C, in

particular in the case of diasporic bauxites; for

boehmitic bauxites, a lower calcination temperature is

preferred, between approximately 9500C and approximately

11000C, more particularly between approximately 9600C and

approximately 10000C, and even more preferably between

approximately 9700C and approximately 9900C.

This calcination leads to a chemical and

crystallographic transformation of the bauxite. More

particularly, the majority fraction by far (and often the

totality) of the diaspore (which is the form in which the

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vast majority (and often the quasi-totality) of the

alumina in the bauxites with a low A/S ratio is found) is

transformed into alpha alumina. This transformation is

accompanied by the departure of certain volatile matter

present in the bauxite or formed during said chemical and

crystallographic transformation. The loss on ignition is

a parameter that can be easily determined and that

synthetically expresses the state of this chemical and

crystallographic transformation during the calcination.

The calcined bauxite has a better solubility of the

aluminum contained in particular in the aluminosilicates

under the usual conditions of the step called of

digestion of the bauxite of the Bayer process, and a

better solubility of the silicon under milder conditions

than those of the step of digestion of the Bayer process.

Thus, the processing of the calcined bauxite by leaching

with an aqueous solution of sodium hydroxide under milder

reaction conditions than those of the step of digestion

of the Bayer process allows to solubilize the silica.

Because of the chemical and crystallographic

transformation that occurs during the calcination, the

preprocessed bauxite has a mass percentage of diaspore

that is much lower than in bauxite; after calcination

this percentage is preferably less than 5%, more

preferably less than 3%, even more preferably less than

2%, and optimally less than 1%. For the same reason, the

mass percentage of kaolinite in the preprocessed bauxite

is preferably less than 4%, more preferably less than 3%,

even more preferably less than 2%, and optimally less

than 1%. These mass percentages of diaspore and of

kaolinite can be determined by the usual methods of X-ray

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crystallography, implemented on samples of powder of

preprocessed bauxite.

For example, a preprocessed bauxite preferred for

use in the method according to the invention has a loss

on ignition of less than 2%, a percentage of diaspore of

less than 3% and a percentage of kaolinite of less than

3%, these percentages being mass percentages. More

preferably, this preprocessed bauxite has a loss on

ignition of less than 2%, a percentage of diaspore of

less than 2% and a percentage of kaolinite of less than

2%, and even more preferably a loss on ignition of less

than 1.5%, a percentage of diaspore of less than 1% and a

percentage of kaolinite of less than 2%.

Another object of the invention is a method for

manufacturing alumina trihydrate or alumina from a

bauxite comprising the following steps:

(i) Preprocessing a bauxite to obtain said

preprocessed bauxite, said preprocessing successively

comprising:

- a calcination,

- a leaching with an aqueous solution of sodium

hydroxide,

- the separation of the solid from the leaching

aqueous phase, said separated solid representing said

preprocessed bauxite,

(ii) Processing said preprocessed bauxite by the

method according to the first object of the invention.

Said bauxite, capable, after preprocessing by

calcination and leaching with sodium hydroxide, of

entering the method according to the invention,

advantageously has a ratio of A1 2 0 3 / SiO 2 between 1 and 8,

preferably between 1 and 7, even more preferably between

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1 and 5.5, even more preferably between 1 and 4, and even

between 1 and 3 or between 2 and 3. The preprocessing of

the bauxite according to the invention leads to a

significant increase in the A1 2 0 3 / SiO 2 ratio, typically

by a factor of two to three. If the A1 2 0 3 / SiO 2 ratio of

the bauxite that enters the method according to the

invention is greater than 8, the method may not be

economically viable, since using a preprocessed bauxite

rather than a natural bauxite does not lead to a

sufficiently attractive additional yield of alumina, and

the potential for reduction of the consumption of sodium

hydroxide is limited.

Another object of the invention is an alumina

capable of being obtained by the method according to the

invention.

Yet another object of the invention is a facility

for the implementation of the method according to the

invention, comprising:

- a unit for preprocessing the bauxite by

calcination and leaching, allowing to transform a bauxite

into a preprocessed bauxite; and

- a unit for manufacturing alumina from said

preprocessed bauxite for the implementation of the method

according to the invention,

characterized in that:

- said preprocessing unit comprises

-- at least one calcination furnace for calcining

the bauxite, -- at least one leaching unit for leaching the

calcined bauxite with an aqueous solution of sodium

hydroxide (called "leaching solution"), and

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-- at least one unit for solid - liquid separation

for separating the calcined and leached bauxite from said

leaching solution;

- said unit for manufacturing alumina from said

preprocessed bauxite comprises

-- at least one chamber (such as an autoclave or a

tubular device) for processing the preprocessed bauxite

with an aqueous solution of sodium hydroxide (called

"Bayer liquor") at a temperature of at least 100°C,

-- at least one unit for solid - liquid separation

for separating the solid residue (called "red mud") from

said Bayer liquor;

-- at least one crystallization unit for

crystallizing aluminum trihydrate from said Bayer liquor

by addition of seeds of aluminum trihydrate;

-- at least one unit for solid - liquid separation

for separating the crystallized aluminum trihydrate from

said Bayer liquor;

-- optionally at least one calcination unit for

transforming said aluminum trihydrate into alumina.

In this facility said Bayer liquor coming from said

unit for solid - liquid separation used to separate the

crystallized aluminum trihydrate from the liquid phase is

recirculated to the digestion step.

Drawings

In figures 1 and 2, the three-digit references

designate physical objects (devices, components or

products), while the four-digit references designate

method steps. After a step of phase separation the letter

"L" designates the liquid phase, the letter "S" the solid

phase.

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Figure 1 shows a simplified schema of the Bayer

process according to the prior art.

Figure 2 shows a simplified schema of an embodiment

of the method according to the invention.

Figures 3 and 4 refer to example 3 and show curves

of thermogravimetric and differential thermal analysis

(TGA - DTA) of samples of bauxite; the rise in

temperature corresponds to the calcination for obtaining

a calcined bauxite. The curve that refers to the scale on

the left, identical in the two drawings, represents the

mass loss. The curve that refers to the scale on the

right represents the mass loss (ML) per minute (figure 3)

and the heat flow (figure 4).

Figure 5 shows the loss on ignition (measured after

calcination at 10600C) of a diasporic bauxite with a high

silica content that has been calcined at different

temperatures.

Description

1.The conventional Bayer process

The invention will be explained in detail with

respect to the Bayer process according to the prior art

that is shown in figure 1. The bauxite coming from a

bauxite mine is ground (step 1100) in the presence of a

liquid phase, which is aluminate of sodium, as will be

explained in greater detail below. The grinding aims to

increase the specific surface area of the bauxite

accessible to the action of the liquid phase during the

attack with a view to the digestion of the bauxite.

Typically, a particle size of several hundred pm is

targeted. The grinding is carried out with the addition

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of lime (step 1102), in the form of milk or in solid form.

The lime exercises a triple action: (i) During the

digestion of the bauxite the lime reduces the consumption

of sodium hydroxide since it favors the precipitation of

the soluble silicates in the form of calcium silico

aluminates rather than in the form of sodium silico

aluminates (which would otherwise carry away a part of

the sodium of the sodium hydroxide, more expensive that

the lime); (ii) the lime promotes the dissolution of the

aluminum and improves the yield of extraction of the

alumina during the digestion; and (iii) the lime improves

the decantation of the mud after attack, since it favors

the transformation of the goethite, difficult to decant

and filter, into hematite, better crystallized.

The ground bauxite is then attacked by an aqueous

solution of sodium hydroxide (step 1110) under pressure

and at high temperature in autoclaves or tubular

exchangers. This attack (called "digestion") leads to the

partial digestion of the bauxite (step 1120), more

precisely, it is the soluble part of the aluminum

minerals (in particular the alumina, whether it is

present in the form of monohydrate or of trihydrate) that

forms ions of aluminate. In numerous cases the digestion

is carried out at a temperature of between 2500C and

2700C in closed autoclaves or in tubular exchangers. Said

aqueous solution of sodium hydroxide is in practice an

aqueous solution of aluminate of sodium. A concentration

of sodium hydroxide of between 235g Na20/L and 245g

Na20/L is typically used. A person skilled in the art

knows how to finely adapt the parameters of this step, in

particular the temperature, the residence time and the

concentration of the sodium, to the composition of the

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bauxite; the same observation applies to the quantity of

lime added in step 1102. For example, it is known that

the bauxites with a high content of monohydrate aluminas

(boehmite, and especially diaspore) need higher digestion

temperatures than the bauxites with a high content of

trihydrate (gibbsite).

The temperature range between 2500C and 2700C allows

to ensure that all of the soluble alumina contained in

the bauxite (including the fraction of diaspore, which is

the most difficult to dissolve out of the oxides of

aluminum, and the content of which can be highly variable)

is digested. In particular, the karst bauxites, which are

the main field of use of the present invention, require

this temperature range. Some factories using karst

bauxites are even designed to work with a temperature of

up to 2800C, in order to be able to adapt if necessary to

the use of bauxites with a very specific composition.

During this digestion step 1120, the bauxite can be

placed in contact with the preheated liquor (method

called double flow), or the suspension of bauxite in the

Bayer liquor is formed before being heated (method called

single flow). In certain factories the digestion step

1120 is carried out in two steps, each one being carried

out at a different temperature, in order to first

dissolve the easily soluble fractions, then, at a higher

temperature, the solid residue from the first step. This

alternative with double digestion can save energy, but it

supposes a greater investment and complicates the method.

When the suspension is expanded (step 1124), by

successive expansion steps, a part of the water

evaporates (auto-evaporation).

W02019/086792 20 PCT/FR2018/052678

During the decantation at ambient pressure (step

1130) the residue (called "red mud") is separated from

the liquid phase (liquor); flocculating agents that

increase the speed of phase separation and improve the

clarification of the liquors (i.e. the residual quantity

of dry matter in the liquid) are added. The solid residue

is called "red mud"; it contains all the crystalline

phases coming from the bauxite non-reactive to the

digestion 1120 and those formed during the Bayer cycle.

The residue (called "red mud") is recovered (step 1140)

and washed with water (step 1142) in order to recover as

much of the liquor as possible; this washing is carried

out in general with raw water and countercurrent (in

order to minimize the quantity of water used); it is

followed by a step of decantation and/or filtration (not

shown in the drawing). The red mud is a powdery residue,

the reuse of which is not easy, and which still

frequently ends up in special storage.

The liquid phase ("L") coming from the step 1130 of

separation of phases is a solution of sodium aluminate.

After dilution (step 1150) the aluminum trihydrate is

crystallized (step 1170) by cooling of the aluminate and

addition of seeds of aluminum trihydrate (step 1160).

This crystallization step is usually called

"decomposition"; its duration is approximately 40 hours.

The dilution with cold water (step 1150) reuses the water

for washing the red mud.

This step 1170 requires a certain expertise, known

to a person skilled in the art, in order to best adapt

the numerous parameters of the method (saturation of the

liquor at the input of the decomposition, concentration

of Na20 and of impurities of various natures, starting

W02019/086792 21 PCT/FR2018/052678

and final temperatures of decomposition, seed surface

area, crystallization technology, particle size grading)

to the nature of the desired alumina product; the

physico-chemical phenomena that are involved relate

especially to nucleation (spontaneous formation of fine

particles in a suspension), the agglomeration of the fine

particles, the particle size grading by cycloning and/or

decantation.

The trihydrate precipitated is separated by

decantation and filtration (step 1180) by using various

known technologies; it is recovered (step 1190). A

significant part of the trihydrate must be recycled into

the decomposition step 1160, the rest is dried (step 1192)

and calcined (step 1194) into alumina. The latter is

stored (step 1196) with a view to its transport to a

consumer site. The drying step (step 1192) is typically

carried out as the first step of the calcination (step

1194) which takes place in several stages. During this

rise in temperature first the water of impregnation is

eliminated, starting at approximately 1000C, then the

water of constitution of the trihydrate (at approximately

10000C); the rise in temperature is then continued to

obtain the desired crystalline structure. Two

technologies are used for the most part: calcination in a

furnace with a circulating fluidized bed ("Circulating

Fluid Bed", abbreviated as CFB), and calcination in

suspension in hot gases ("Gas suspension calcination",

abbreviated as GSC). Older factories still have rotary

kilns that are also suitable for implementing this step.

The liquid phase coming from the step of phase

separation 1180 is an aqueous solution of sodium

hydroxide, more diluted than that used in step 1110

W02019/086792 22 PCT/FR2018/052678

because of the various inputs of water into the flow

(water for washing the red mud (step 1142) and the

trihydrate, dilution water (step 1150)). For this reason

it must be concentrated by evaporation of water (step

1210) to be recycled (step 1220) in the solution of

sodium hydroxide used for the digestion step (step 1120).

It is also reused (step 1222) in the step of wet grinding

of the bauxite (step 1100).

The trihydrate obtained in step 1190 can be washed

before drying; this washing water can be reused in the

washing of the red mud in step 1142 (not shown in the

drawing).

In the Bayer process according to the prior art,

sodium hydroxide is consumed during the processing of the

bauxite to produce alumina. More precisely, this

consumption is related to three mechanisms: (i) the

formation of insoluble phases of silico-aluminate of

sodium during the attack (digestion step 1120); (ii) the

residual sodium hydroxide carried away with the mud (1140)

despite its washing (step 1142); (iii) the

coprecipitation with the alumina during the

crystallization phase 1170. These losses must be

compensated for by the addition of new sodium hydroxide

(step 1110). As much as possible all the washing liquid

phases including sodium hydroxide (including during the

chemical cleaning of the tanks and piping) are recycled

in the Bayer liquor.

2. The preprocessing of the bauxite according to the

invention

2.1 General overview

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An embodiment of the method according to the

invention is illustrated in figure 2. It comprises a

preprocessing of the bauxite. The preprocessed bauxite is

introduced into the Bayer process. The steps of the Bayer

process noted as llxx and 12xx in figure 1 are designated

in figure 2 by the references 21xx and 22xx, while the

preprocessing steps carry the references 20xx.

According to a very advantageous embodiment of the

invention certain operating conditions of this Bayer

process are adapted to the chemical and mineralogical

composition of the preprocessed bauxite; this will be

explained below in greater detail. The preprocessed

bauxite is a product that does not exist as such in

nature, it is necessarily a product resulting from an

industrial method, namely from the preprocessing method.

Its chemical composition differs from that of the natural

bauxite from which it comes by two essential

characteristics: it has a greater A/S ratio (since it

includes less silicates), and it now includes only very

little crystallization water. Moreover, its

mineralurgical composition is different, after the

transformations that it undergoes during the various

steps of the preprocessing, as will be explained in

greater detail below. For the diasporic bauxites

containing silica for the most part in the form of

kaolinite, the essential differences relate to the

dehydration of the diaspore and its transformation into

alumina which is for the most part alpha alumina, as well

as to the kaolinite which after its dehydration

transforms into meta-kaolinite (as is explained in

section 2.3 below), allowing the solubilization of the

silica by the sodium hydroxide.

W02019/086792 24 PCT/FR2018/052678

The very low content of crystallization water also

distinguishes the chemical composition of the

preprocessed bauxite from that of the natural bauxites

with a similar A/S ratio. The loss of crystallization

water is the main parameter that enters into the loss (of

mass) on ignition. For example, a diasporic natural

bauxite in general has a loss on ignition greater than

approximately 10%, whereas a bauxite preprocessed

according to the invention has a loss on ignition of less

than 2.5%, preferably less than 2.0%, and even more

preferably less than 1.5%. The loss on ignition is a

parameter known to a person skilled in the art;

complementary explanations are given below in section 2.3.

According to the invention, the bauxite coming from

a bauxite mine is ground (step 2000) after the addition

of water (step 2002), filtered (step 2004), and the solid

residue after filtration (step 2004) is calcined (step

2010). Here, this intermediate product is called "calcined bauxite". An aqueous solution of sodium

hydroxide is added (step 2020) and the leaching of the

calcined bauxite is carried out (step 2030). After the

phase separation (step 2040) the solid phase which is

called "leached calcined bauxite" or "preprocessed

bauxite" here is recovered and introduced into the Bayer

process; according to the particle size obtained during

the grinding in step 2000 it may be necessary to regrind

it (step 2100, not shown in figure 2). The liquid phase

coming from the phase separation in step 2040 is

processed with lime to precipitate the silicates (step

2050). After another phase separation (step 2060) the

residue, a white mud, is recovered (step 2070). The

liquid phase coming from the phase separation in step

W02019/086792 25 PCT/FR2018/052678

2060 is an aqueous solution of sodium hydroxide; it is

recovered (step 2080) and partly recycled into the attack

solution of the Bayer process. Advantageously the lime

2052 is introduced in the form of milk of lime.

According to the invention this preprocessing method

can include numerous alternatives. For example, the step

of phase separation 2060 can be followed by an additional

step of filtration of the liquid phase (step not shown in

figure 2, designated here as 2062). It can comprise a

washing of the white mud (step not shown in figure 2,

designated here as 2064) after the step of phase

separation 2060, the washing water being recycled into

step 1150. These two alternatives can be combined. The

bauxite can also be dry ground, and in this case the

calcination step (2010) follows directly.

As will be explained below in detail, the

implementation of the preprocessed bauxite in the Bayer

process can be carried out in the same factory, i.e. with

the same equipment, and according to the same process

flow diagram, as the implementation of the non

preprocessed bauxite (the only exception is the

evaporation step 2210 which can be eliminated in certain

alternatives of the method according to the invention).

However, if the same operating parameters (for example

the duration, the temperature and/or the concentration of

the sodium hydroxide) are used for the implementation of

the preprocessed bauxite a different result than that

which would be obtained with an unprocessed bauxite is

obtained. For this reason, in certain advantageous

embodiments of the invention, for certain steps of the

Bayer process implemented with preprocessed bauxite

according to the invention the operating parameters are

W02019/086792 26 PCT/FR2018/052678

modified with respect to a usual operation of the Bayer

process.

The inventors have found that the temperature of the

calcination (step 2010) of the bauxite strongly

influences the yield of extraction of the alumina from

the leached bauxite. According to the invention this

temperature must be greater than 9800C for a diasporic

bauxite. For a calcination temperature of 9800C or less,

the kaolinite is activated and is not completely

transformed; it reacts to the leaching (step 2030) to

give an insoluble compound of the zeolite type. For this

reason a calcination temperature greater than 9900C is

preferred. According to an advantageous embodiment it is

greater than 10000C. For a temperature between 10100C and

10350C the transformation of the kaolinite is total; a

temperature between 10200C and 10300C is preferred. The

calcination method is followed by a leaching that will be

explained below.

The preprocessed, improved bauxite that results from

this calcination - leaching (roast-leach) method can be

introduced as such into the Bayer process.

According to an advantageous embodiment of the

invention this method is modified. More precisely,

certain operating parameters are modified, which allows

to substantially reduce, inter alia, the energy

consumption.

2.2 Specific embodiments

To illustrate embodiments of the invention, here

certain steps of the preprocessing method are specified.

Grinding (step 2000)

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In step 2000 the grinding can be carried out in a

cylindrical mill containing balls or steel bars. The

quantity of water (step 2002) can be approximately 0.7m 3

per tonne of bauxite, with a bauxite load of

approximately 1000kg per M 3 . A suspension of water and of

bauxite that can be separated by filtration on a filter

press (step 2004) is thus obtained. A residual

impregnation of water of approximately 10% by mass is

acceptable. The target particle size in the grinding can

be the same as that in the case of the conventional Bayer

process, namely several hundred pm.

Calcination (step 2010)

The calcination in step 2010 can be carried out in a

rotary or static kiln. The progressive heating allows to

eliminate the water of impregnation of the ore, then the

water of constitution, crystalline phases present in the

bauxite, then to carry out the transformation of these

phases, at the temperatures indicated above. In these

conditions, it is observed that: - The silica that was present in the form of

silicates is transformed for the most part into amorphous

silica;

- The diaspore and boehmite phases are transformed

into alumina of the "alpha" type;

- The iron that was present in the form of goethite

(FeO(OH)) is, after the calcination, transformed into

hematite (Fe203); - The phases containing carbon, carbonates and

sulfur are thermally dissociated mainly in the form of

C02 and SO 2 for the volatile part.

W02019/086792 28 PCT/FR2018/052678

Leaching (step 2030)

After the calcination the calcined bauxite is

immersed in a solution of sodium hydroxide (step 2020).

This leaching step (step 2030) allows to solubilize the

transformed silica as well as certain impurities. The

sodium hydroxide content of the liquid phase can be

comprised between approximately 70g NaOH/L and

approximately 160g NaOH/L, preferably between

approximately 90g/L and approximately 150g NaOH/L, and

even more preferably between approximately 110g/L and

approximately 140g NaOH/L. For example, a content of 129g

NaOH/L was used successfully. This solution can be

obtained from a mixture of recycled sodium hydroxide and

soda lye at 50%, the quantities of which are adjusted to

obtain the concentration necessary for the leaching.

Below 70g/L a fraction of the leachable silica that is

too small is dissolved, a residence time (i.e. a time of

contact between the solid phase and the liquid phase)

that is too great is needed, and the stock of solution of

sodium hydroxide circulating in the facilities of the

method is diluted too much. Above 150g/L the risk of loss

of aluminum by dissolution of alumina becomes significant.

The temperature of the aqueous solution of sodium

hydroxide is typically between 800C and 1200C; if the

temperature is too low the silica dissolves poorly, if

the temperature is too high alumina tends to be dissolved.

For example, the calcined bauxite and the solution

of sodium hydroxide (2010) can be introduced into a

stirred reactor tank in such a way as to obtain an

initial suspension containing approximately 80kg/m 3 of

solid. A reaction temperature of approximately 1000C is

W02019/086792 29 PCT/FR2018/052678

suitable; the residence time at the reaction temperature

can be approximately 45 min.

Separation of phases (step 2040)

The separation of phases in step 2040 can be carried

out by filtration on a filter, of the filter press type,

supplied by the suspension coming from the reactor of the

leaching 2030. The solid residue is the "leached calcined

bauxite" or "preprocessed bauxite"; a residual

impregnation of leaching liquor of approximately 10% by

mass is acceptable. The liquid phase is a liquor loaded

with dissolved silica coming during the leaching 2030; it

is purified by adding lime 2052. The raw material

advantageously consists of quicklime (CaO) or milk of

lime (Ca(OH)2). It is preferred to use a quicklime with a

fine particle size distribution, containing at least 85%

CaO; typically it contains between 85% and 95% CaO. The

milk of lime can be manufactured by slaking of this lime

(approximately 100kg of CaO/m 3 ) with hot water in a

stirred reactor tank.

Precipitation of the silicates (step 2050) and flow of

the phases

The step of precipitation of the silicates (step

2050) allows to form an insoluble calcium silicate. The

precipitation of the silica can be carried out in a

reactor tank with stirring in the presence of quicklime

or milk of lime (100g CaO/L) at 1000C for 2 hours. The

stoichiometric ratio of CaO SiO 2 is advantageously

between 1.1 and 1.5. A suspension that contains, at the

end of the operation, approximately 35kg/m 3 to 50kg/m 3 of

W02019/086792 30 PCT/FR2018/052678

solid, preferably between 39kg/m 3 and 46kg/m 3 of solid,

is typically obtained.

The phase separation (step 2060) can advantageously

be carried out by decantation of the suspension.

According to the invention the clear liquid phase

("overflow") is advantageously recycled into the sodium

hydroxide circuit of the method, preferably partly to the

leaching 2030, and partly upstream of the digestion step

of the Bayer process 2120.

The thickened suspension ("underflow") consisting of

calcium silicate (typically from 600 to 700kg of

solid/m 3 ), called white mud (2070), is extracted from the

decanter. It can supply a filter, of the belt filter type,

on which a methodical washing (step 2072) with water is

carried out in order to reduce the concentration of the

impregnation liquor. The washed white mud can have a

residual impregnation of diluted liquor of approximately

10%; the concentration of sodium hydroxide in this

impregnation liquor is typically approximately 6 to lOg

NaOH/L. The white mud is composed of a calcium silicate

that is close to that of tobermorite (approximately

Ca4.31Sis.51Alo.5016 (OH)2 x 4 H 2 0) . It can be directed towards

an intermediate storage while awaiting its reuse.

The water for washing the silicate mud (white mud

2070) recovered after step 2072 can join the circuit of

the liquid phase 2080 obtained in step 2060 to be used in

the leaching step (step 2030) and in the Bayer process

(step 2120). In the latter case it requires a

readjustment of its sodium hydroxide content (step 2110)

which will have become lower than the initial content in

step 2030 (for example 129g NaOH/L). This readjustment is

W02019/086792 31 PCT/FR2018/052678

carried out with the addition of an aliquot of soda lye

at 50%.

The preprocessing according to the invention

generates a certain loss of sodium hydroxide, by

occlusion of sodium in the precipitated silicate and by

impregnation of the white mud. This loss must be

compensated for by the addition of soda lye, typically at

50% (step 2110). However, as will be explained below, the

modified Bayer process according to the invention

consumes less sodium hydroxide than the conventional

Bayer process, per tonne of alumina produced.

The Bayer liquor is recirculated (step 2200); it

consists of a mixture of liquor coming from the step of

filtration of the trihydrate (step 2180) having undergone

or not undergone a concentration by evaporation of water

(step 2210) and addition of soda lye (step 2110).

2.3 Meaning of the loss on ignition

It is known that the loss on ignition is a parameter

that is an integral part of the usual characterization of

a bauxite; this value, expressed in mass percent, is

present on the analysis certificate that accompanies any

delivery of bauxite intended for the Bayer process. It is

determined in general by calcination at 10600C for 2

hours, after a previous drying at 1050C. The calcination

of the bauxite always leads to a net loss of mass, which

is provoked by the departure of volatile matter, even if

there can be oxidation reactions which, taken alone, lead

to an increase in mass. This departure of volatile matter

results from physical (in particular from sublimation)

and chemical (in particular thermal decomposition, such

as dehydration, dehydroxylation and thermal dissociation,

W02019/086792 32 PCT/FR2018/052678

and reduction) phenomena. More precisely, the loss on

ignition corresponds mainly to the elimination of the

water of constitution (i.e. of the molecules of water

that are integrated into the crystallographic structure),

of the carbon dioxide coming from the organic matter and

from the mineral carbonates, and of certain other

volatile compounds, in particular of the oxides of sulfur.

The loss on ignition of bauxite depends on its

chemical and mineralogical composition. For a bauxite

intended for the Bayer process its order of magnitude is

typically located between 10% and 30%. It can be

determined by simple weighing before and after

calcination in the conditions indicated. Differential

thermogravimetry, which also allows to characterize the

mineral species present in the bauxite, can also be used.

The loss on ignition of the diasporic bauxites with

a high silica content, which form a raw material usable

in the context of the present invention to prepare the

bauxite called preprocessed, is typically between 10% and

18%, and more often between 12% and 15%, but these

empirical numbers do not limit the scope of the invention.

Figure 5 shows the loss on ignition (determined after

calcination at 10600C for 2 hours) of a diasporic bauxite

calcined according to the invention at various

temperatures ranging from 9800C to 10300C. It is clear

that the loss on ignition after calcination at 10300C is

extremely low (0.10%), or in other words: when this

material is heated beyond 10300C and up to 10600C, the

quantity of volatile matter that is released is extremely

low.

For example, for the kaolinite phase contained in

the bauxite, the preprocessing by calcination according

W02019/086792 33 PCT/FR2018/052678

to the invention schematically leads to the following

reactions (the indications of the temperatures being

approximative):

- Endothermic dehydration between 400C and 200°C:

A1 2 Si 2 O 5 (OH) 4 x n H 2 0 decomposes into A1 2 Si 2 O 5 (OH) 4 + n H 2 0; - Endothermic dehydroxylation between 5300C and

5900C: A1 2 Si 2 O 5 (OH) 4 decomposes into A1 2 0 3 x 2 SiO 2 (meta

kaolinite) + H 2 0; - Exothermic dissociation between 9000C and 1000°C:

A1 2 0 3 x 2 SiO 2 (meta-kaolinite) decomposes into 2 A1 2 0 3 x 3

SiO 2 (pseudo-mullite) + SiO 2 (amorphous) + y A1 2 0 3

. 3. The modified Bayer process according to the

invention, using the preprocessed bauxite

The phase separation (step 2040) allows, preferably

after filtration, to separate the solid from the liquor

to be able to use the preprocessed bauxite in the step of

digestion (step 2120) of the Bayer process. The inventors

made a certain number of observations that led them to

modifying certain steps of the Bayer process; this

modification constitutes an essential feature of the

present invention.

Digestion (step 2120)

The digestion involves solubilizing the aluminous

phases contained in the preprocessed bauxite in a sodium

hydroxide liquor. The alpha alumina contained in the

preprocessed bauxite, which was generated during the

calcination, is solubilized by the Bayer liquor at high

temperature, at the same time as the preexisting soluble

aluminum. This step can be carried out under temperature

and pressure conditions similar to those of the

W02019/086792 34 PCT/FR2018/052678

conventional Bayer process, namely: a temperature

typically between 2500C and 2700C in closed autoclaves or

in a pressurized tubular system (approximately 50 bar to

60 bar). The heating is advantageously carried out by

progressively increasing the temperature up to the

reaction temperature. The residence time at the reaction

temperature is advantageously between 30 min and 60 min,

preferably between 30 min and 50 min, and even more

preferably between 35 min and 45 min.

According to an essential feature of the method

according to the invention a concentration of sodium

hydroxide significantly lower than that used for a normal

bauxite in the conventional Bayer process can be used for

the digestion of the preprocessed bauxite. More

particularly, this concentration is between 140g Na20/L

and 200g Na20/L, preferably between 155g Na20/L and 190g

Na20/L, and even more preferably between 160g Na20/L and

180g Na20/L. This concentration is advantageously

monitored continuously by the measurement of the

electrical conductivity of the liquor; it can also be the

subject of a chemical analysis in a laboratory.

In one embodiment, the inventors have observed that

despite a greater circulating flow rate of the attack

liquor (12.11m3 /t versus 8.52m 3 /t) due to a lower

concentration of caustic soda (162g/L versus 240g/L) in

the digestion liquor the yields of extraction of the

alumina remain very high; they are greater than 96% on

alumina minus silica by the adaptation of the setting

parameters of the workshop for example, the saturation of

the liquor (concentration of alumina and WR of the

addition of lime (8 to 10.4%) and the temperature (2600C).

W02019/086792 35 PCT/FR2018/052678

Evaporation (step 2210)

At the output of the digestion the suspension is

expanded, that is to say that it is brought to

atmospheric pressure by successive expansions; this

operation allows to evaporate a significant quantity of

water (auto-evaporation).

Since the method according to the invention uses a

Bayer liquor with a concentration of sodium hydroxide

significantly lower than the conventional Bayer process,

the quantity of water to be evaporated is much smaller.

Moreover, the preprocessed bauxite generates less red mud

(2140) than the majority of natural bauxites, which

reduces the required quantity of water for washing 2150

the mud, given that this washing water loaded with sodium

hydroxide will be introduced into the circuit of the

Bayer liquor. In certain cases the evaporation during the

expansion from the autoclave (step 2124) to the digestion

2120 is sufficient to maintain the concentration of the

recycled aluminate liquor (2220); the step of evaporation

2210 can thus be omitted. The step of evaporation 2210

consumes thermal energy and requires a significant

investment in terms of evaporators; being able to

minimize or even eliminate this step is of significant

economic interest.

It is noted that while there are factories using the

conventional Bayer process that do not carry out the

evaporation step 1210, namely factories that exclusively

use a bauxite generating a particularly small quantity of

red mud, the elimination of this step would be totally

impossible in the case of bauxites with a low A/S ratio

digested by the conventional Bayer process.

W02019/086792 36 PCT/FR2018/052678

Decantation of the red mud (step 2130)

At the output of the digestion the suspension is

diluted with a liquor of aluminate coming from the first

stage for washing the red mud in step 2140. This dilution

is regulated according to the input of washing water. It

allows to reach a concentration of the liquor compatible

with the solid-liquid separation and the crystallization

in step 2160.

The decantation is carried out in an apparatus

called "decanter", a tank with a large diameter (most of

the time with a flat or conical bottom) provided with

slow stirring allowing the separation. The decantation is

optimized by the use of additives called flocculants in

order to increase the speed of sedimentation of the solid

particles.

In the method according to the invention, the

absence of certain phases such as goethite, which was

transformed during the calcination of the bauxite in step

2010 and which is known for hampering the flocculation,

allows to reduce the quantity of flocculant used.

Typically, the thickened suspension (underflow) is

sent to the first washing stage. The clarified liquor

(overflow) is sent to the "safety" filtration which has

the goal of eliminating the very fine particles of mud in

order to guarantee a liquor free of impurities towards

crystallization.

Washing of the red mud (step 2140)

The washing of the red mud in step 2140 is carried

out preferably countercurrent; the washing water is

introduced in the last stage of the chain of washers. The

chain of washers can be completed by a filtration of the

W02019/086792 37 PCT/FR2018/052678

mud coming from the last washer by using a filter press.

The use of flocculant allows to improve the sedimentation

in order to ensure better washing of the mud.

Moreover, under equal conditions, the smaller

quantity of red mud and its better aptitude for

decantation allows to vey substantially improve the

washing since the quantity of water available for this

operation remains the same for a lesser quantity of solid.

While preserving identical washing efficiencies (raw

bauxite and preprocessed bauxite) it is possible to

reduce the quantity of water, which has the consequence

of greatly reducing the evaporation, as will be explained

below, and to thus save a significant amount of energy.

More precisely, the small quantity of red mud

generated (typically reduced by approximately 60%) by the

digestion 2120 of the preprocessed bauxite requires a

smaller quantity of water (typically reduced by

approximately 50%) than that necessary for the washing of

the mud generated by the digestion of an unprocessed

bauxite.

For example, the quantity of washing water and the

tonnage of mud are the following: - 3.04m 3 /t of water for 1.063t/t of mud after

washing in the case of a bauxite processed by the method

according to the invention, - 5.70m 3 /t of water for 3.040t/t of mud after

washing in the case of a raw bauxite.

The inventors found that with the bauxite

preprocessed by the method according to the invention,

the judicious adjustment of the main parameters of the

Bayer process, in particular in the attack (residence

time, saturation of the liquors, quantity of lime added,

W02019/086792 38 PCT/FR2018/052678

etc.), allows to preserve excellent yields of

solubilization of the alumina (greater than 96% on

alumina minus silica) with attack liquors with a low

concentration of caustic soda (approximately 160 to

170g/L for 238g/L to 240g/L for the conventional attacks).

These results and the improvement in the efficiency of

the washing of the mud allow to carry out the cycle of

the Bayer liquor with a low concentration of caustic soda,

generating significant savings of energy and of

maintenance costs at the evaporation workshop. In

addition, by eliminating the step of evaporation 2210

there are savings in terms of the investment in an

evaporation workshop in the case of a new production line.

Decantation of the white mud (step 2040)

The method according to the invention leads to a

significant reduction in the quantity of mud, even when

taking into account the fact that the step of

desilication 2050 by precipitation of the silicates

generates specific mud ("white mud") that does not appear

in the conventional Bayer process with non-preprocessed

bauxites: in the method according to the invention it is

noted that the reduction in the quantity of red mud is

much greater than the quantity of white mud. Moreover,

the later fate (and the possible use in usable products)

of these types of mud is not the same. The method

according to the invention reduces the quantity of mud to

be decanted (steps 2040, 2060 and 2130), typically by 10

to 40%, and preferably by 20% to 40%. For the red mud

this reduction can reach 65% (step 2130). Moreover this

mud has a better aptitude for decantation because of the

transformation, during the phase of calcination of the

W02019/086792 39 PCT/FR2018/052678

ore, of the goethite into hematite. These two advantages

lead to numerous other positive effects: the ease of

running the workshop (related to the reduction in the

quantities of mud to be managed), the reduction of the

consumption of electric energy, the reduction of the

consumption of flocculant, the optimization of the

maintenance costs of the workshop.

Consumption of sodium hydroxide and energy consumption of

the method according to the invention

The consumption of sodium hydroxide during the Bayer

process is related to the formation of solid compounds

including sodium (silicoaluminate, sodium crystallized

with the alumina) on the one hand and to the loss of

sodium hydroxide in liquid form (liquor of impregnation

of the red mud, and of the trihydrate) on the other hand.

The method for preprocessing by calcination-leaching also

generates losses by occlusion of sodium (occlusion in the

silicate) and by losses of sodium hydroxide in liquid

form (impregnation of the white mud). All these losses re

compensated for by an addition of soda lye at 50%.

The energy consumption of the Bayer process for

diasporic bauxites with a low Al/Si ratio is

approximately 8GJ per tonne of alumina obtained, without

taking into account the calcination of the trihydrate

(step 1194, 2194); insofar as the energy consumption of

the calcination of the trihydrate does not depend on the

origin of the bauxite, here only the method up to the

trihydrate (1190, 2190) is compared. For a preprocessed

bauxite according to the invention, by using a natural

karst bauxite with a low Al/Si ratio this consumption is

reduced by 1.9GJ/t via the elimination of the evaporation

W02019/086792 40 PCT/FR2018/052678

step 2210. Given that the preprocessing method,

comprising the step of calcination 2010 and the step of

leaching 2030, adds a consumption of approximately 2.2GJ

per tonne of alumina obtained, it is noted that the

method according to the invention leads to an

overconsumption of energy of 0.3GJ per tonne of alumina.

This overconsumption corresponds to approximately 4% of

the energy consumption of the method according to the

invention.

This overconsumption is very low, since the method

according to the invention increases the yield of

extraction of the alumina from the bauxite: in the

example used for the estimation of the energy consumption

above, the method according to the invention allows to

lower the quantity of karst bauxite with a low Al/Si

ratio necessary to obtain a tonne of alumina from 3.5t to

approximately 2.6t.

Moreover, as already mentioned, the method according

to the invention consumes less water and less sodium

hydroxide than the conventional Bayer process applied to

a low-grade karst bauxite. In the above example, the

consumption of sodium hydroxide goes from 490kg of NaOH

to 104kg of NaOH per tonne of alumina produced. The

consumption of lime indeed doubles, from approximately

360kg per tonne of alumina to approximately 800kg per

tonne of alumina, but the cost of the lime is

approximately 10% of the cost of the NaOH.

These savings remain of great interest even when

taking into account the investment necessary for the

additional steps of the preprocessing. This investment

involves adding a preprocessing unit to an existing

factory that uses the Bayer process: no modification of

W02019/086792 41 PCT/FR2018/052678

the equipment is necessary in this existing factory

(except for the integration of the flows of Bayer liquor

between the two "preprocessing" and "Bayer" units which

involves piping): the modification that the present

invention makes to the Bayer process is significant, but

it only concerns the operating parameters of the Bayer

process, and not the industrial equipment.

It is thus clear that in terms of economy, the

method is largely gainful. Moreover, the significant

reduction in the quantity of red mud, which tends to have

a negative economic value, also reduces the cost of its

reprocessing and storage. With regard to the white mud

(substantially silicates), it comprises fewer heavy

metals and other potentially toxic substances (if they

ended up going into solution) than the red mud; its

economic value is not negative. Indeed the mineralogy of

the calcium silicate in the form of tobermorite suggests

uses in particular in the field of construction.

The greatest advantage of the method according to

the invention is certainly the possibility of increasing

the deposits usable in terms of bauxite in accounting

(microeconomic and macroeconomic), by allowing to use

mineral resources that cannot be used with the methods

according to the prior art under economically competitive

conditions. The possibility thus offered to certain

factories of using bauxites coming from geographically

close mines leads to related savings in transport costs.

It is of course possible, and this falls within the

context of the present invention, to supply a Bayer

factory, the method of which has been modified according

to the invention, with preprocessed bauxite that has not

been preprocessed at the same site: this preprocessed

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bauxite can come either from a separate factory for

preprocessing bauxite (for example set up near a bauxite

mine, in order to create savings in terms of cost of

transport of the bauxite), or from an integrated factory

(preprocessing unit + Bayer unit) that has excess

capacity in terms of preprocessed bauxite. This first

embodiment with a separate preprocessing unit is not,

however, preferred since the reuse of the liquid phase

loaded with sodium hydroxide coming from the leaching

step 2040 and from the washing of the white mud 2072

cannot in this case be carried out by recirculation in

the Bayer liquor.

4. Other advantages of the method according to the

invention

As indicated above, the method according to the

invention has numerous advantages. Its main advantage is

to allow the implementation, in the context of the Bayer

process, of the bauxites with a low Al/Si ratio that

cannot be used according to the prior art, or only with a

low yield, and with a production cost significantly

higher in comparison to the bauxites with a higher Al/Si

ratio.

Another advantage is that the preprocessing

according to the invention eliminates not only the

silicon, but also the quasi-totality of the organic

carbon and a large part of the sulfur naturally contained

in the bauxite. It is known that organic carbon

accumulates in the liquor of aluminate of soda and a part

can precipitate in the form of oxalate on the trihydrate.

It is known that in the desirable case in which the

concentration of oxalate in the Bayer liquor is low,

W02019/086792 43 PCT/FR2018/052678

there is greater latitude to adapt the parameters of the

step 2160 of decomposition (in particular the temperature,

the residence time and the rate of recycling of the seeds)

to the needs of obtaining a product with a particle size,

a distribution of the size of the particles and a shape

of crystallites that are controlled. Moreover, the yield

of this crystallization step is greater if the

concentration of oxalate in the Bayer liquor is low.

Another advantage is that the quantity of red mud is

reduced (up to 60%). This presents two advantages: in

terms of phase separation, and in terms of its ultimate

processing. More precisely, the method according to the

invention allows to reduce the quantity of red mud; it

does generate a new type of residue, the white mud, but

the sum of the white and red mud is reduced (up to 25%)

with respect to the red mud according to the conventional

Bayer process. In addition to the decrease in the

quantity of mud, it is observed that the red mud

generated in the method according to the invention shows

a better aptitude for decantation (because of the

transformation during the calcination (step 2010) of the

goethite into hematite), which allows to reduce the

quantity of flocculating agent added to the suspensions, for facilitating the management of the decantation

workshop, for reducing the cost of its maintenance, and

for reducing the consumption of electric energy.

Another advantage is the possible decrease in hard

deposits of aluminosilicates on the surfaces of the

various equipment used in the digestion (step 2120) and

downstream of the step of digestion; these hard deposits

tend to hinder the heat exchange and must be removed from

time to time in specific maintenance operations.

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Yet another advantage is that the residual iron and

silica content of the alumina obtained by the method

according to the invention is particularly low.

Another advantage, already mentioned above, is the

reduction in the quantity of water to be evaporated in

the step of evaporation of water (step 2210), which can

be eliminated in numerous cases. This significantly

contributes to saving energy.

The method according to the invention includes an

additional calcination step (step 2010) which consumes

thermal energy and sodium hydroxide. However, this sodium

hydroxide can be recycled for the most part in the Bayer

process and the energy consumption of the calcination

step is almost compensated for by the savings made in the

Bayer process.

The method according to the invention can be used

advantageously with a preprocessed bauxite obtained from

the natural bauxites having an A/S ratio between 1 and 8

and preferably between 1.5 and 7. Above a value of 7 or 8

the benefit in terms of additional alumina made available

by the method for preprocessing the bauxite by calcination and leaching becomes smaller, and the

advantage of the reduction in the consumption of sodium

hydroxide is more limited since there is less silica

capable of carrying away sodium hydroxide. This upper

threshold depends on certain technico-economic parameters

that can vary according to the economic data. For a value

of 1 the mineral resource is especially kaolinite, which

has other technical uses. Natural bauxites rarely have an

A/S ratio of less than approximately 2.5 or 2, and the

preferred lower limit of the method according to the

W02019/086792 45 PCT/FR2018/052678

invention is the use of a preprocessed bauxite obtained

from a natural bauxite having an A/S ratio equal to 2.

Examples

Example 1: Bauxite from Jiaokou

A powder of bauxite from Jiaokou (China), the

chemical analysis of which is described in detail in

table 1 below, was supplied.

Table 1 Chemical composition of the bauxite from Jiaokou A1 2 0 3 SiO 2 Fe203 TiO 2 MgO CaO K 20 Total Total C02 S

61.4 13.46 4.4 2.8 0.25 1.15 0.39 0.82 0.05

This raw ore contains a relatively large quantity of

alumina, but is also rich in silicon (low A/S ratio, of

approximately 4.6), and the silica is mainly present in

the form of kaolinite (88%), the rest (12%) being in the

form of muscovite.

Samples of 200g of this bauxite were calcined at

980°C or at 1030°C. The calcination was carried out in a

muffle kiln preheated to 200°C - 250°C, with a rise in

temperature as quickly as possible. The duration of

calcination was 30 minutes at the target temperature 0 (980 C or 1030 0 C). At the end of the calcination the

bauxite was cooled in a desiccator. Its chemical

composition was analyzed by X-ray fluorescence, its

structure by X-ray diffraction, its content of volatile

matter ("loss on ignition", abbreviated as LOI) by

weighing before and after heating to 1060 0 C.

W02019/086792 46 PCT/FR2018/052678

The leaching was carried out on a suspension at a

rate of 90g/L of calcined bauxite, with a solution of

pure sodium hydroxide at 100g/L of Na20, for a suspension

volume of 500mL. The suspension thus obtained was

maintained for one hour at 1000C, and was then filtered

on a Millipore membrane filter (5pm). The filtrate was

preserved in an oven at 900C; an aliquot was taken for

analysis. The leached bauxite was washed and dried.

Another leaching was tried on the bauxite calcined

at 9800C, with a solution of sodium hydroxide at 260g/L

of Na20.

Since the calcination of the bauxite at 9800C did

not allow to obtain a good desilication, the digestion

trials were only carried out with the bauxite calcined at

10300C. The calcined and leached bauxite was attacked at

2600C with a residence time of 40 min or of 60 min,

according to the following protocol:

(i) Rise in temperature 4 0 C/min until a first

plateau at 1750C, duration of the plateau 5 min;

(ii) Rise in temperature 1.5 0 C/min until a second

plateau at 2550C, duration of the plateau 5 min;

(iii) Rise in temperature 1°C/min until a third

plateau at 2600C, duration of the plateau 40 min or 60

min.

The attack aqueous phase was an industrial Bayer

liquor having the composition (in g/L):

Na 2 0 ctq = 238.0; A1 2 0 3 = 128.6; Wr = 0.540; Na20 cbte = 17.2; SiO 2 = 1.5; CaO = 0.001; Fe203 = 0.008; V 2 05 = 0.020; C org = 2.4; Density at 200C = 1.3777

(The designation "Na20 ctq" refers to the useful

("caustic") fraction of the sodium hydroxide, while the

designation "Na20 cbte" refers to the fraction of the

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Na 2 0 that corresponds to the carbonate ("cbte") residues;

this distinction is possible by analyzing the liquid

phase by pH-metric titration according to a method known

to a person skilled in the art).

The silicon was precipitated by addition of milk of

lime; the quantity of lime added varied between 2.5% and

4%. This is described in greater detail in example 2

below.

The load of bauxite was adjusted to obtain a value

of WR (alumina / sodium hydroxide ratio) at the end of

the attack that varied between 1.00 and 1.28.

Example 2: Desilication trials

Various desilication trials were carried out,

according to two approaches:

In a first series of trials various raw materials of

lime allowing to purify the leachate (i.e. to precipitate

the silica) were tested, and in a second series of trials

the effectiveness of the purification according to the

concentration of the leachate was tested.

a) Trials on the raw material of lime (comparison solid

lime and milk of lime)

The precipitation of the silica was carried out with

milk of lime at 100g/L of CaO and with solid CaO, at a

temperature of 1000C for 2 hours. The milk of lime was

prepared with stirring at 700C for 90 min then at 850C

for 60 min.

After precipitation and filtration, the filtrate was

analyzed (SiO 2 , CaO and A1 2 0 3 ) and the filter cake was

characterized by chemical analyses (SiO 2 , CaO and A1 2 0 3 ), X-ray diffraction and scanning electron microscopy (SEM).

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The precipitation of the silica was carried out on a

volume of 300ml of the solution of first hot filtration

of the trial of leaching a Wanji bauxite calcined at

10200C. Thus, two trials were carried out, one for the

5 precipitation with the milk of lime and the other for the

precipitation with solid lime.

Two preliminary analyses on a leaching solution

showed that the concentration of SiO 2 was 5.79 and

6.10g/L, or an average content of 5.96g/L ([Si] = 0.1M/L).

10 Thus, for a Ca/Si stoichiometry of 1.2, 2.03g of solid

CaO or 20ml of milk of lime must be introduced into the

volume of 300ml of leaching solution.

However, a slightly greater quantity, of 2.53g of

solid lime and of 25ml of milk of lime, was used, which

15 corresponds to a stoichiometry of approximately 1.5.

The results of the analyses in terms of Si0 2 , A1 2 0 3 and CaO, of the solutions before and after precipitation

and of the precipitates, are recorded in table 2 and the

rates of precipitation of Si0 2 and A1 2 0 3 appear in table 3.

20 It is deduced therefrom that the milk of lime

precipitates the silica better (87%) than the solid lime

(74%), but it also precipitates a little more aluminum

(49% instead of 37% for solid CaO). It is also noted that

the milk of lime hardly affects the calcium content of

25 the solution.

Table 2 Analyses of the solutions and of the precipitates of the trials of precipitation of the silica with solid lime and 30 milk of lime; Wanji bauxite calcined at 10200C. Leaching solution Precipitation Precipitate [%] Precipitation

[g/L] filtrate [g/L] with SiO 2 A1 2 0 3 CaO SiO 2 A1 20 3 CaO SiO 2 A1 2 0 3 CaO Solid lime 5.794 1.849 0.012 1.528 1.117 0.035 26.49 3.76 39.05

W02019/086792 49 PCT/FR2018/052678

Milk of lime 6.081 1.830 10.015 10.782 10.937 10.011 126.76 14.03 142.39

Table 3 Rate of precipitation of SiO2 and A1 2 0 3 with solid CaO and the milk of lime, using the solutions for leaching of the 5 Wanji bauxite calcined at 1020°C. ,rcpt o Rate of Precipitation precipitation

% with SiO 2 A1 2 0 3 Solid CaO 74 37 Milk of lime 87 49

The characterization of the precipitates by X-ray

diffraction shows that they are composed of the same

crystallized phases, the tobermorite formed during the

10 precipitation of Si by the portlandite, and the calcite

that was already present in the lime used.

The observations and microanalyses carried out with

the SEM (cf. table 4 below) indicate that the tobermorite

is substituted by Al, Na and sometimes Mg, and that it is

15 in the form of aggregates of particles with a flaky

appearance that recall C-S-Hs (Calcium Silicate Hydrates).

Table 4 Analyses of the lime and of the milk of lime with the SEM Elements Precipitation with

[mass %] Solid lime Milk of lime Na20 3.53 1.21 MgO 0.65 0.83 A1 2 0 3 4.36 3.97 Si0 2 24.66 20.57 CaO 30.48 30.24 20

After leaching, the precipitation of the silica

placed in solution is more effective with the milk of

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lime than with the solid lime. A greater insolubilization

of the sodium hydroxide with the solid lime is noted.

b) Trials on the concentration of lime

To define the optimal concentration for the

desilication of the leachates two concentrations were

tested. These trials were carried out using the liquors

from leachings of the Jiaokou bauxite calcined at 1030°C,

as was described in example 1 above.

The precipitation of the silica was carried out with

milk of lime at 100g/L of CaO (lime coming from a Chinese

factory) at a temperature of 980C for 2 hours. Table 5

below indicates the chemical composition of this lime.

Table 5 Chemical analysis of the lime used to manufacture the milk of lime (in mass %) A1 2 0 3 TiO 2 Fe203 SiO 2 CaO Na20 Loss on Total ignition 1.50 0.06 0.50 1.64 90.71 0.03 5.80 100.26

The trials were carried out with a load of CaO

corresponding to 110% of the SiO 2 +Al 20 3 stoichiometry. The

analyses and methods used were the following:

- Mud: LOI, A1 2 0 3 , TiO 2 , Fe203, CaO, SiO 2 , Na20, total

carbon and XRD

- Liquors: MetrohmTM Thermogravimetry, complete ICP,

complete chromatography and Ph6nix Tm (organic carbon,

mineral carbon).

The conditions of the trials are summarized in table

6 below. Table 7 indicates the result of the chemical

analysis of the liquor before desilication, table 8

indicates the result of the chemical analysis of the

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liquor after desilication. Table 9 gives information on

the yield of precipitation of the silica and of the

alumina.

Table 6 Operating conditions of the trials of desilication of the liquor for leaching the Jiaokou bauxite calcined at 10300C Desilication 980C Temperature Trial N° 1 2 3(*) 4(*) Na20 ctq g/l 96.1 96.1 48.05 48.05 Volume Leachate ml 150 150 75 75 Density Leachate 1.1 1.1 1.1 1.1 Mass Leachate g 168.9 168.9 84.5 84.5 Mass water g 0 0 75 75 Lime g 2.053 2.053 1.027 1.027 Water of the milk of lime g 18.6 18.6 9.3 9.3 Mass suspension input g 189.6 189.6 169.8 169.8 (*) Tests 3 and 4 were carried out with a leachate diluted by half.

Table 7 Analysis of the liquors used for the desilication Trial N° 1 2 3 4 Na20 ctq g/l 93.23 93.23 46.62 46.62 A1 2 0 3 g/l 7.66 7.66 3.83 3.83 Na20 cbte g/l 1.52 1.52 0.76 0.76 Na20 cbte % 1.60 1.60 1.60 1.60 SiO 2 g/l 7.81 7.81 3.91 3.91 CaO g/l 0.005 0.005 0.003 0.003 Fe203 g/l 0.005 0.005 0.003 0.003 V 2 05 g/l 0.08 0.080 0.04 0.04 Na20 total g 14.21 14.21 7.11 7.11 A1 2 0 3 in the liquor g 1.15 1.15 0.57 0.57 SiO 2 in liquor g 1.17 1.17 0.59 0.59

Table 8 Analysis of the liquors after desilication

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Trial N0 1 2 3 4 Mass of liquor g 186.29 186.18 168.38 168.18 calculated Density 1.104 1.118 1.062 1.061 Volume of liquor ml 168.80 166.51 158.56 158.55 Na20 ctq g/l 91.70 95.25 48.04 47.90 A1 2 0 3 g/l 0.00 0.13 0.51 0.19 Na20 cbte g/l 2.70 2.86 1.83 1.59 Na20 cbte % 2.86 2.92 3.67 3.21 SiO 2 g/l 0.64 0.82 0.94 0.56 CaO g/l 0.018 0.018 0.014 0.015 Fe203 g/l 0.00 0.00 0.00 0.00 V 2 05 g/l 0.07 0.07 0.06 0.06 Wr liq 0.000 0.001 0.011 0.004

Table 9 Yields of precipitation of the silica and of the alumina

Precipitation with Precipitation milk of lime rate [%] Al SiO 2 A1 20 3 Leaching at 96g/L 90 99 a20 Leaching at 48g/L 80 90 Na20

It is observed that the reduction by half of the

concentration of the leachate makes the rate of

precipitation of the silica drop by 10%. It is also noted

that the milk of lime hardly affects the CaO content of

the solution.

Table 10 below presents the analysis of the

precipitated silicate (white mud).

Table 10 Analysis of the precipitated silicate Trial N 1 2 3 4 Mass of mud weighed g 3.28 3.39 1.41 1.61 Loss on ignition % 18.90 18.90 19.90 19.90 A1 2 0 3 % 2.5 2.5 2.1 2.1 SiO 2 % 31.96 31.96 30.16 30.16

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Fe203 % 0.4 0.4 0.3 0.3 TiO 2 % 0.05 0.05 0.04 0.04 Na20 % 0.05 0.05 0.05 0.05 CaO % 44.28 44.28 43.95 43.95 Total % 98.14 98.14 96.50 96.50 CaO/SiO2 % 1.39 1.39 1.46 1.46 Na 2 0/SiO 2 % 0.002 0.002 0.002 0.002 Calculated introduced CaO % 55.10 55.10 72.39 72.39 A1 2 0 3 in the mud g 0.08 0.08 0.03 0.03 Na20 in the mud g 0.002 0.002 0.001 0.001 SiO2 in the mud g 1.048 1.083 0.425 0.486

The X-ray diffractogram shows that the precipitated

silicate phase obtained is tobermorite [Ca 5 Si 6 Oi 6 (OH) 2 x 4

H 2 0] (ML=702). Excess portlandite and calcite are also

found.

The precipitation rate is between 87 and 90%, for

the precipitation with milk of lime, and 74% when solid

lime is used. The insolubilization of the sodium

hydroxide is estimated at 0.0017 points per point of

silica dissolved.

Example 3: Bauxite from Xiao Yi

A powder of bauxite from Xiao Yi (Shanxi, China),

the chemical analysis of which is described in detail in

table 11 below, was supplied.

Table 11 Composition of the bauxite from Xiao Yi A1 2 0 3 SiO 2 Fe203 TiO 2 MgO CaO K 20 Total Total F C02 S 51.59 19.27 8.24 2.67 0.15 0.33 0.38 0.5 0.05 0.1

This raw ore has a composition substantially

different from that of the bauxite from Jiaokou. In

particular, its aluminum content is lower, and its

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silicon content is greater: the A/S ratio is very low,

approximately 2.7; the silica is mainly present in the

form of kaolinite. This is a bauxite qualified as "low

grade", the introduction of which into the methods

according to the prior art is avoided. Therefore, it

forms an excellent challenge for the method according to

the invention.

Calcination and leaching: operating conditions

Samples of ore of 30g were placed in crucibles made

of silica; the latter were introduced into a muffle kiln

at 10200C into a muffle kiln with a residence time of 30

min. After calcination the samples were taken out of the

kiln and left to cool in the ambient air; they were then

weighed.

The calcined bauxite was then leached at 1000C for

45 min in a solution at 100g/L of Na20 (corresponding to

130g/L of NaOH), with a solid concentration of 80g/L.

This leaching was carried out in a jacketed reactor

heated with a thermostated oil bath.

After leaching the suspension was filtered on a slow

filter paper (ref. Whatman 589/3) in a Bichner funnel

mounted on a flask connected to a vacuum pump. A first

filtration was carried out while hot, then two other

filtrations were carried out after repulping of the cake

with demineralized water at ambient temperature. The cake

from the second filtration was washed on a filter with

ethanol.

The raw ore, the calcined bauxite and the leaching

residue were characterized by chemical analyses, X-ray

diffraction (XRD), infrared spectroscopy (DRIFTS) and

scanning electron microscopy (SEM). Moreover, for the

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bauxite from Xiao Yi (Shanxi), a thermogravimetric

analysis (TGA) coupled with differential thermal analysis

(DTA) was carried out on the raw ore.

Results after the calcination and the leaching

The characterization by X-ray diffraction showed

that the crystallized phases of the starting bauxite are

in order of decreasing importance: diaspore (AlO(OH)),

kaolinite (Al2 Si 2 O 5 (OH)4 ), magnetite (Fe304), anatase

(TiO2 ), rutile (TiO2 ), hematite (Fe203), goethite

(FeO(OH)), quartz (SiO2 ), a mica (illite, muscovite

(A120 3 AlO1 0 (OH) 2 K), calcite (CaC03), palygorskite

(MgAl) 4 Si 8 O 2 o (OH) 2 -8H 2 0. In this ore the high silica

content thus comes substantially from the kaolinite and

secondarily from the quartz and other phyllosilicates

(micas, palygorskite).

The thermogravimetric analysis (TGA) coupled with

the differential thermal analysis (DTA) substantially

shows the dehydroxylation of the diaspore and of the

kaolinite (see figures 3 and 4). At around 9000C an

aspect of the curve was observed that suggests the

hypothesis of the crystallization of a neoformed compound

that could be mullite. These results lead the inventors

to set the temperature of calcination to 1020°C.

After calcination of the bauxite from Xiao Yi at

this temperature of 10200C, the characterization by XRD

now only revealed the presence of corundum (c'-Al 2 0 3 ), of magnetite, of hematite, of anatase, of rutile and of

quartz. Via IR spectroscopy it is noted that all the

signals relative to the kaolinite disappear and

consequently, the planetary tetrahedral structure also:

W02019/086792 56 PCT/FR2018/052678

only a highly distributed signal corresponding to

amorphous silica remains of it.

The mass loss during the calcination at 10200C of

the bauxite from Xiao Yi, measured with very good

reproductivity over a plurality of trials, was 14.02%, as

shown in table 12 below.

Table 12 Mass loss by calcination at 10200C Crucib 1 2 3 4 5 6 7 8 Tota le N 1 1

Mass. 30.8 31.0 31.5 30.0 32.9 29.1 30.6 246. bauxit 3 7 8 9 30.1 8 7 4 47 e [g] Calcin ed 26.5 6.71 27.1 25.9 25.8 28.3 25.1 26.3 211. mass 2 9 3 4 7 92

[g] Mass 14.0 14.0 14.1 13.9 13.9 14.0 13.9 13.9 14.0 loss 5 3 1 3 9 9 6 5 2

[%]

Tests of leaching of the calcined mineral in a sodic

solution were carried out under the operating conditions

defined at the beginning of this paragraph. During this

experiment, the mass percentages of dissolution observed

were between 16.8% and 17.8%. The results are gathered

together in table 13 below.

Table 13 Mass percentages of dissolution of the trials of leaching of the bauxite from Xiao Yi Trial N° 1 2 3 4 5 Total Mass calcined bauxite 32 24 152

[g] Mass filter cake [g] 26.6 26.5 26.4 26.3 19.8 125.6 Mass percentage of 16.8 17.2 17.5 17.8 17.5 17.36 dissolution [%]

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The percentage of desilication is 74.2%.

The mineralogical characterization by XRD of the

bauxite after leaching reveals the presence of the same

5 crystallized phases as before leaching namely: corundum

(oa-Al2 0 3 ), magnetite, hematite, anatase, rutile and

quartz. However, the IR spectrum indicates that the

product has become globally very hydrophilic, which is

favorable to its suspension, and that the signals

10 relative to the Si-O elongations have considerably

decreased in intensity, which agrees with a dissolution

of the silicate phases.

Table 14 below compares the chemical analyses of the

original bauxite, after calcination and after leaching:

15

Table 14 Chemical analysis of the raw, calcined and calcined leached ("preprocessed") bauxite

A1 2 0 3 SiO 2 Fe 2 0 3 TiO 2 MgO CaO K 20 Na 2 0 Total Total F LOI A/S

[%] [%] [%] [%] [%] [%] [%] [%] C02 S [%] [%]

Bauxite [%] [%]

raw 51.59 19.27 8.24 2.67 0.15 0.33 0.38 0.03 0.5 0.05 0.1 13.69 2.7

calcined 59.46 22.66 9.97 3.1 0.17 0.38 0.45 0.04 0.02 0.05 0.06 0.33 2.6

Leached 71.51 7.08 11.55 3.74 0.2 0.48 0.08 1.85 0.33 0.03 0.07 2.16 10.1

"LOI" refers to the (mass) loss on ignition, "A/S" refers to the alumina to silica mass ratio.

20 The transformation of the ore after processing is

clearly observed: the alumina content goes from 51.59% to

71.51% and the alumina/silica ratio goes from 2.7 to 10.1.

This shows that the method according to the invention is

capable of transforming a "low-grade" bauxite into a

25 good-quality bauxite.

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Attack (digestion) of the bauxite from Xiao Yi carried

out under industrial operating conditions

The leached bauxite was ground (all undersize at

300pm) and dried in an oven at 1100C. The attack was

carried out in a 150-ml autoclave brought to 2600C for 40

min. The chemical composition of the attack liquor was

very close to that of the Chinese industrial liquors

mainly: Na20 ctq = 238g/L, WR = 0.540 and Na20 cbte =

17.2g/L.

The lime added to the attack came from a Chinese

alumina factory; it contains 86.02% CaO. The lime was

slaked and was added in the form of milk. Various

quantities of lime were tried.

At the end of the attack, the suspension was

filtered. The solid was washed and prepared for chemical

and crystallographic analyses (loss on ignition, X-ray

fluorescence and X-ray diffraction). The liquor was

analyzed with a Methrom (Na20, A1 2 0 3 , carbonate) .

The main results are reported in table 15 below.

Table 15 Results of the trials of attack of the bauxite from Xiao Yi on raw ore and ore processed by the method according to the invention (calcination and leaching) Raw bauxite Calcined and leached bauxite (invention) Trial N° 351-7 351-8 360-1 360-2 360-3 360-4 Bauxite analysis Loss on ignition 13.4 13.4 1.3 1.3 1.3 1.3

[%] A1 2 0 3

[%] 53.7 53.7 71.5 71.5 71.5 71.5 SiO 2 [%] 19 19 6.92 6.92 6.92 6.92 Fe203 [%] 8.8 8.8 12.2 12.2 12.2 12.2 Na20 [%] 0.05 0.05 1.44 1.44 1.44 1.44 A/S 2.8 2.8 10.3 10.3 10.3 10.3 Attack

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conditions Temperature [°C] 260 260 260 260 260 260 Time [min] 40 40 40 40 40 40 CaO added 9.2 9.1 5.7 7.9 5.9 7.5 (calculated)[%] WR attack liquor 0.54 0.54 0.54 0.54 0.54 0.54 Na20 ctq [g/L] 238 238 238 238 238 238 Na20 cbte [% 6.7 6.7 6.7 6.7 6.7 6.7 total] Target WR 1.2 1.2 1.11 1.11 1.212 1.212 WR calculated on 1.19 1.184 1.061 1.085 1.106 1.164 the solid Analysis of the mud Loss on ignition 10.2 10.1 7.2 8.2 6.4 8

[%] A1203 [%] 24.8 25.2 25.3 20.1 32 23.2 SiO 2 [%] 23.5 23.4 13.74 14.2 12.57 13.35 Fe203 [%] 10.5 10.7 24.3 25 22.3 25.1 Na20 [%] 13.5 13.4 8.33 7.71 7.57 7.28 Attack yield on A1 2 0 3 - SiO 2 96.6 95.8 91.06 95.56 83.63 92.47

[%] on total A1 2 0 3 61.6 61.5 82.32 86.34 75.65 83.98

[%] The abbreviation WR means: "weight ratio".

Certain analytical results have slight differences

with respect to the table above. This results from

measurements made by different laboratories in order to

confirm the validity of the method.

It is substantially observed that the alumina

contained in the bauxite calcined at 10200C then leached

in a sodic medium is soluble in the operating conditions

of the Chinese factories at high temperature with

absolutely acceptable extraction yields that confirm the

interest of the method according to the invention. This

result clearly shows that the corundum is soluble in a

sodic medium at high temperature according to its degree

of crystallization and its particle size distribution.

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The dispersion noted is without a doubt partly

related to the difference in quantity of lime added to

the attack. This important parameter has an influence on

the yield, the curves of yield according to the alumina

/ caustic soda weight ratio, and the quantity of insoluble

sodium hydroxide.

These results allowed to estimate the specific

consumptions of the method according to the invention for

this bauxite.

Table 16 Specific consumptions of a Bayer process according to the prior art and of a Bayer process according to the invention implementing the technology of calcination leaching Material Unit According to According the prior to the art invention Raw materials and energy Bauxite Tonne per tonne 3.46 2.61 A1 2 0 3 produced Total CaO Tonne per tonne 0.361 0.81 A1 2 0 3 produced Total NaOH Tonne per tonne 0.49 0.148 A1 2 0 3 produced Total GJ per tonne A1 2 0 3 11 11.4 energy produced Byproducts Red mud Tonne per tonne 3.04 1.063 A1 2 0 3 produced White mud Tonne per tonne 0 1.24 A1 2 0 3 produced Total mud Tonne per tonne 3.04 2.303 A1 2 0 3 produced

This example shows several advantages of the

invention. It is clear that the method according to the

invention leads to a better use of the bauxites with a

low aluminum content and high silicon content. A very

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significant reduction in the quantity of residues of the

red mud type and a significant reduction in the overall

quantity of residues are also observed (it is noted that

the precipitation of the silicates, absent in the Bayer

process according to the prior art, generates its own

residue, of a white color, but in the method according to

the invention the sum of the two residues is

significantly less than the red mud according to the

prior art). Given that the residues represent a cost

factor, this environmental aspect represents linked to

the residues of the method represents a very attractive

advantage for operators; the financial value of this

advantage is difficult to evaluate globally since it

depends on numerous factors related to the factory.

Example 4: Trials of digestion of the bauxite from Xiao

Yi, preprocessed by the method of calcination - leaching

according to the invention, with a low concentration of

sodium hydroxide

On the basis of the excellent results of example 3,

the inventors sought to further improve their method.

Given that the method for preprocessing the bauxite

(roast-leach method) greatly reduces the mass of bauxite

at the input of the Bayer process and thus of the red mud

at the output (with a constant mass of alumina produced)

one might think that the washing of the mud would be more

efficient in the context of the method according to the

invention. If an identical washing efficiency is

preserved the quantity of clear water for washing of the

mud can thus be lowered, which allows to reduce the total

quantity of water to be evaporated in the context of the

Bayer process. Thus the energy consumption at this stage

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of the method is greatly reduced. Indeed, in the Bayer

process, the reduction in the evaporation is necessarily

accompanied by a reduction in the concentration of

caustic soda in the liquor used for the digestion. The

inventors thus carried out digestion trials on the

leached bauxite from Xiao Yi at a concentration of Na 2 0 ctq of 171.5g/L which represents a significant reduction

with respect to the first trials (Example 3) carried out

with a concentration of 238g/L. The concentration of

171.5g/L of Na20 ctq corresponds to a Bayer unit (called

a "refinery" in the profession) that would function

without evaporators.

In the present example 4 the bauxite from Xiao Yi

used in example 3 was processed by the same method as

that described in example 3, but its digestion in the

Bayer process was carried out at a reduced concentration

of sodium hydroxide: the concentration of Na20 ctq was

lowered from 238g/L to 171.5g/L. The results are gathered

together in table 17 below.

Table 17 Results of the digestion, at a low concentration of sodium hydroxide, of the leached bauxite from Xiao Yi Raw bauxite Calcined and leached bauxite (invention) Trial N° 351-7 351-8 LOMB3 LOMB4 Bauxite analysis Loss on ignition 13.4 13.4 1.30 1.30

[%] A1 2 0 3

[%] 53.7 53.7 71.50 71.50 SiO 2 [%] 19 19 6.92 6.92 Fe203 [%] 8.8 8.8 12.20 12.20 Na20 [%] 0.05 0.05 1.44 1.44 A/S 2.8 2.8 10.3 10.3 Attack conditions Temperature [°C] 260 260 260 260

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Time [min] 40 40 40 40 CaO added 9.2 9.1 8 10.4 (calculated)[%] WR attack liquor 0.54 0.54 0.54 0.54 Na20 ctq [g/L] 238 238 171.5 171.5 Na20 cbte [% 6.7 6.7 7 7 total] Target WR 1.2 1.2 1.118 1.116 Analysis of the mud Loss on ignition 10.2 10.1 8.1 9 .3

[%] A1 2 0 3[%] 24.8 25.2 16.6 16.5 SiO 2 [%] 23.5 23.4 13.3 12.7 Fe203 [%] 10.5 10.7 22.4 20.5 Na 2 0 [%] 13.5 13.4 7.43 6.68 Attack yield on A1 2 0 3 - SiO 2 [%] 96.6 95.8 97.2 96.6 on total A1 2 0 3 [%] 61.6 61.5 87.5 86.5

It is noted that the yield of solubilization of the

alumina remains very high despite a considerable decrease

in the concentration of caustic soda in the attack liquor.

This result of interest will allow to greatly reduce

the energy consumption and the operating costs of the

alumina refineries that use the technology according to

the invention.

Example 5: Bauxite from Wanji (Henan)

Bauxite from Wanji (Henan province) was supplied.

The calcination and leaching trials as well as the

physico-chemical analyses were carried out under the same

conditions as for the bauxite from Xiao Yi (example 3).

The essential difference between the two ores comes from

the form of the silica in the bauxite. For the ore from

Wanji the characterization by X-ray diffraction indicates

the following crystallized phases:

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Diaspore (AlO (OH)), muscovite ( (K1 -, Nax) (Al 2 -yFey)

(AlSi3 ) (01 o (OH) 2 ), kaolinite (Al 2 Si 2 O5(OH) 4 ), quartz (SiO2),

siderite (FeCO3), goethite (FeO(OH)), hematite (Fe203)

, magnetite (Fe 3 04), anatase and rutile (TiO2 )• The SiO 2 content thus comes substantially from the

muscovite and from the quartz, secondarily from the

kaolinite.

The analyses of the raw, calcined (10200C) and

leached bauxite are reported in table 18 below:

Table 18 Chemical analyses of the bauxite from Wanji before and after processing Elemen A1 20 SiO 2 Fe 2 0 TiO MgO CaO K 20 Na 2 0 Tot Tot F Li L.0. A/ ts 3 % 3 2 % % % % al al % pp I. S % % % C02 S m

% Raw 50. 12. 17. 2.8 0.4 0.2 2.3 0.0 4.0 0.6 0.1 18 13.1 4. bauxit 92 58 62 3 2 3 9 31 8 9 7 4 1 0 e Calcin 59. 14. 20. 3.2 0.4 0.2 2.7 0.0 <0. 0.0 0.0 20 0.2 4. ed 51 29 33 6 8 6 4 37 01 8 7 2 2 bauxit e "Leach 62. 8.5 21. 3.6 0.5 0.2 2.5 0.0 <0. 0.0 0.0 10 0.39 7. ed" 35 8 55 3 6 6 33 01 8 7 0 3 bauxit e

After calcination, the characterization by X-ray

diffraction reveals the disappearance of the kaolinite,

the transformation of the diaspore into corundum (a

A1 2 0 3 ), of the siderite and of the goethite into hematite.

The other phases, muscovite (residual structure), quartz,

magnetite, anatase and rutile are still present. The

detection limit of this X-ray characterization is less

than 1% by mass.

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The leaching in a sodic medium allows to lower the

SiO2 content from 12.58 to 8.58% or a decrease of 4

points, and a change of the alumina/silica ratio from 4.0

to 7.3 (cf. table Z2 above).

The difference observed with the ore from Xiao Yi

(reduction of 12 points of the silica content) comes from

the fact that in the latter ore the SiO 2 content comes

substantially from the kaolinite whereas in the bauxite

from Wanji it comes from the muscovite which is thus

difficult to leach even after calcination.

Nevertheless the digestion tests carried out show a

reduction of 60% in the consumption of sodium hydroxide

and of 25% in the consumption of bauxite per ton of

alumina.

Example 6: Bauxite from Shanxi

Another bauxite from Shanxi (China) was supplied. It

has an alumina/silica ratio of 1.94. The analysis by X

ray diffraction indicates that it is a diasporic bauxite,

the silicates of which consist substantially of kaolinite,

a small proportion of muscovite, and quartz. The iron is

present substantially in the form of goethite.

This bauxite was calcined for 30 minutes at 1030°C.

The leaching of this calcined bauxite was carried out at

1000C with sodium hydroxide at 100g Na20 / liter for 45

minutes.

The analyses of the raw, calcined (10300C) and

leached bauxite are reported in table 19 below:

Table 19 Chemical analyses of the bauxite from Shanxi before and after processing

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Elements A1 2 0 3 SiO 2 Fe 2 0 3 TiO 2 MgO CaO K 20 Na 20 Cr203 MnO P 20S V 20S ZrO 2 L.O.I. A/S

Raw 53.80 27.75 2.20 2.52 0.24 0.35 0.46 0.06 0.03 0.01 0.15 0.05 0.07 13.80 1.94 bauxite

Calcined 61.50 31.30 2.50 2.92 0.22 0.44 0.49 0.07 0.04 0.01 0.18 0.06 0.08 0.10 1.96 bauxite

Leached 73.80 14.49 3.20 3.71 0.35 0.54 0.16 1.18 0.07 0.02 0.12 0.04 0.11 2.20 5.09 bauxite

After calcination, the characterization by X-ray

diffraction reveals the disappearance of the kaolinite,

and the appearance of a siliceous phase, mullite. A bulge

5 in the diffractogram is observed in a 20 zone between 180

and 200, translating the formation of a poorly

crystallized silica phase. The diaspore has disappeared.

The corundum is present. The goethite was transformed

into hematite.

10 After leaching, the characterization by X-ray

diffraction shows that the mullite is still present. The

bulge in the diffractogram has disappeared. Most of the

amorphous silica has dissolved. The corundum, hematite,

rutile and anatase phases are still present.

15 The leaching of the calcined ore in a sodic medium

allowed to the SiO 2 content from 27.75% to 14.49% or an

increase in the alumina/silica ratio from 1.94 to 5.06.

The desilication yield rises to 62.9%. The incorporation

of sodium hydroxide is 0.98%.

Throughout this specification and claims which follow,

unless the context requires otherwise, the word "comprise",

and variations such as "comprises" and "comprising", will

be understood to imply the inclusion of a stated integer

or step or group of integers or steps but not the exclusion

of any other integer or step or group of integers or steps.

The reference in this specification to any prior

publication (or information derived from it), or to any

matter which is known, is not, and should not be taken as

an acknowledgment or admission or any form of suggestion

that that prior publication (or information derived from

it) or known matter forms part of the common general

knowledge in the field of endeavour to which this

specification relates.

Claims (13)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. Method for manufacturing alumina trihydrate or alumina from a bauxite preprocessed by a method comprising a calcination and a leaching, wherein said preprocessed bauxite has a loss on ignition of less than 2.5% by mass, said method comprising the steps of: (a) Processing (called "digestion") the preprocessed bauxite with an aqueous solution of sodium hydroxide at a temperature of at least 1000C, said aqueous solution of sodium hydroxide having a concentration of between 100g Na20/L and 220g Na20/L;

(b) Separating the solid residue from the liquid phase; (c) Crystallizing the aluminum trihydrate by addition of seeds ; (d) Separating the crystallized aluminum trihydrate from the liquid phase; (e) optionally, calcining the aluminum trihydrate obtained in step (d) to obtain the alumina.

2. Method according to claim 1, wherein the temperature in step (a) is between 150°C and 350°C.

3. Method according to claims 1 or 2, wherein said preprocessed bauxite has a mass ratio of A1 2 0 3 / Si0 2

greater than 8

4. Method according to any one of claims 1 to 3, wherein said preprocessed bauxite has a mass content of alumina greater than 60%.

5. Method according to any one of claims 1 to 4, wherein said preprocessed bauxite has a mass content of silica of less than 12%.

6. Method according to any one of claims 1 to 5, wherein said preprocessed bauxite has a loss on ignition of less than 2%, a mass percentage of diaspore of less than 3%, and a mass percentage of kaolinite of less than 3.

7. Method according to any one of claims 1 to 6, wherein the liquid phase coming from step (d) is reintroduced into the aqueous solution of sodium hydroxide used in step (a).

8. Method according to any one of claims 1 to 7, wherein said preprocessed bauxite has been preprocessed by leaching with an aqueous solution of sodium hydroxide.

9. Method according to any one of claims 1 to 8, wherein said preprocessed bauxite has been preprocessed by calcination at a temperature between 9200C and 11200C..

10. Method according to any one of claims 1 to 9, comprising the following steps: (i) Preprocessing a bauxite to obtain said preprocessed bauxite, said preprocessing successively comprising: - a calcination ,

- a leaching with an aqueous solution of sodium hydroxide, - the separation of the solid from the leaching aqueous phase, said separated solid representing said preprocessed bauxite, (ii) Processing said preprocessed bauxite by the method according to any one of claims 1 to 9.

11. Method according to any one of claims 1 to 10, wherein

said bauxite has before preprocessing a ratio of A1 2 0 3

/ SiO 2 between 1 and 7.

12. Method according to any one of claims 1 to 11, wherein: - said preprocessed bauxite has a loss on ignition of

less than 2.0% by mass, and even more preferably less than

1.5; and/or

- said aqueous solution of sodium hydroxide has a

concentration of between 140g Na20/L and 200g Na20/L, more

preferably between 155g Na20/L and 190g Na20/L, and even

more preferably between 160g Na20/L and 180g Na20/L; and/or

- the temperature in step (a) is between 200°C and 300°C,

more preferably between 2200C and 2800C, and even more

preferably between 2500C and 2700C; and/or

- said preprocessed bauxite has a mass ratio of A1 2 0 3

/ SiO 2 greater than 9, and even more preferably greater than 10; and/or

- said preprocessed bauxite has a mass content of

alumina greater than 65%, and even more preferably greater

than 70%; and/or

- said preprocessed bauxite has a mass content of silica

of less than 10%, and even more preferably less than 8%;

and/or

- said preprocessed bauxite has a loss on ignition of

less than 2%, a mass percentage of diaspore of less than

2% and a mass percentage of kaolinite of less than 2%, and

even more preferably a loss on ignition of less than 1.5%,

a mass percentage of diaspore of less than 1%, and a mass

percentage of kaolinite of less than 2%; and/or

- said preprocessed bauxite has been preprocessed by calcination at a temperature between 9500C and 10700C, and even more preferably between 10000C and 10500C; and/or

- said bauxite has before preprocessing a ratio of A1 2 0 3

/ SiO 2 between 1 and 5.5, even more preferably between 1

and 4, and most preferably between 1 and 3.

12. Facility for the implementation of the method

according to any one of claims 10 or 11, comprising:

- a unit for preprocessing the bauxite by calcination and

leaching, allowing to transform a bauxite into

preprocessed bauxite; and

- a unit for manufacturing alumina from said preprocessed

bauxite for the implementation of the method according to

any one of claims 1 to 9,

wherein:

- said preprocessing unit comprises

-- at least one calcination furnace for calcining the

bauxite,

-- at least one leaching unit for leaching the calcined

bauxite with an aqueous solution of sodium hydroxide

(called "leaching solution"), and

-- at least one unit for solid - liquid separation for

separating the calcined and leached bauxite from said

leaching solution; - said unit for manufacturing alumina from said

preprocessed bauxite comprises

-- at least one chamber for processing the preprocessed

bauxite with an aqueous solution of sodium hydroxide

(called "Bayer liquor") at a temperature of at least 1000C,

-- at least one unit for solid - liquid separation for

separating the solid residue from said Bayer liquor;

-- at least one crystallization unit for crystallizing

aluminum trihydrate from said Bayer liquor by addition of

seeds of aluminum trihydrate;

-- at least one unit for solid - liquid separation for

separating the crystallized aluminum trihydrate from said

Bayer liquor;

-- optionally at least one calcination unit for

transforming said aluminum trihydrate into alumina.

13. Facility according to claim 12, wherein said Bayer

liquor coming from said unit for solid - liquid separation

for separating the crystallized aluminum trihydrate from

said Bayer liquor is recirculated to the digestion step.

Bauxite

1100 1102 1222 Grinding Lime 1110 1220 NaOH / H2O 1120 Digestion (pressure, temperature)

1124 Expansion

1150 Dilution

L 1142 1130 1140 Decantation and filtration S Red mud Washing with water of the suspension

L 1170 1210 Crystallization of the Evaporation H2O 1160 Seeds of trihydrate trihydrate

1180 1190 Filtration of the S Aluminum suspension trihydrate

L 1191 1200 Washing NaOH / H2O 1192 Drying

1194 Calcination

1196 Alumina

Figure 1

Bauxite

2000 2004 2010 Water 2002 Grinding Filtration Calcination S 2030 2020 Leaching NaOH / H2O

2040 2050 2052 Decantation of the Precipitation of Lime suspension the silicates

2060 2070 Decantation of S White mud S the suspension

2080 2072 NaOH / H2O Washing with water

2110 L 2220 NaOH / H2O

2120 2122 Digestion (pressure, temperature) Lime

2124 2126 Expansion Dilution 2150 L 2140 2130 2140 Washing Decantation of S Red mud with water 2210 the suspension Evaporation H2O L 2150 Dilution H2O

2160 Crystallization of Seeds of 2170 the trihydrate trihydrate

2180 2190 Decantation and filtration S Aluminum of the suspension trihydrate

L 2192 Drying 2200 NaOH / H2O 2194 Calcination

2196 Alumina

Figure 2