Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Reexamination Certificate
1999-06-16
2001-10-02
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
C423S122000, C423S127000
Reexamination Certificate
active
06296818
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the treatment, by alkaline digestion according to the Bayer process, of alumina monohydrate bauxite or boehmite, including the addition of alumina trihydrate or gibbsite.
DESCRIPTION OF RELATED ART
The Bayer process, which is widely described in the literature, constitutes the essential production technique of alumina to be transformed by igneous electrolysis into aluminum, or to be used in the hydrate state or transition, calcinated, sintered or melted alumina states, in numerous applications in the technical alumina field.
According to this process, the bauxite mineral is digested when hot by means of an aqueous liquor of sodium hydroxide at the appropriate concentration, thereby making the alumina soluble and obtaining a slurry made up of particles of non-digested residue (mud) in a sodium aluminate liquor referred to as “aluminate liquor”.
This slurry is then usually diluted so as to separate the mud from the aluminate liquor by settling. Once the liquor is “purged” of this mud, it is cooled to a temperature at which it is in a state of strong supersaturation imbalance. At this stage it is referred to as “pregnant liquor”. Particles of alumina trihydrate are then seeded in order to initiate the “crystallization” of the pregnant liquor, in other words the precipitation of the alumina in the form of alumina trihydrate. Finally, the sodium aluminate liquor, depleted in alumina as a result of the precipitation and referred to as “spent liquor”, is recycled towards the digestion stage after being concentrated by evaporation and, if appropriate, the addition of sodium hydroxide so as to constitute a liquor, known as “digestion liquor”, whose concentration is adapted to the digestion of the mineral.
Those skilled in the art are well aware that treatment conditions must be adapted to the degree of hydration and the crystallographic structure of the alumina contained in the bauxite, and to the nature and level of the impurities which it contains.
Bauxite containing alumina in the monohydrate state (boehmite, diaspore) is more difficult to digest than trihydrate bauxite and is treated according to the Bayer process at temperatures higher than 200° C., usually between 220 and 300° C. Bauxite containing alumina in the trihydrate state (gibbsite) is treated at temperatures below 200° C., usually between 100 and 170° C.
The digestion yield is defined by the ratio between the quantity of alumina dissolved in the digestion liquor and the total quantity of alumina contained in the mineral. At the present time, a digestion yield in the region of 90% is usually reached with monohydrate bauxites containing between 2 and 3% of silica.
The chief factors acting on the digestion yield of bauxite are temperature and the concentration of “free” sodium hydroxide or caustic soda, i.e. capable of dissolving alumina. Caustic soda is usually expressed by the base group Na
2
O and its concentration given in Na
2
O grams per liter (g Na
2
O/1).
In addition, the saturation or stability state of the liquors is characterized by the weight ratio:
A
/
C
=
concentration in dissolved
AL
2
O
3
(
in
g
/
l
)
concentration in
Na
2
O
caustic soda
(
in
g
/
l
)
With regard to bauxites which are rich in monohydrate, the dissolution kinetics falls sharply in the course of dissolution, to such an extent that, in economically viable treatment conditions, the quantity of extracted alumina remains appreciably less than what it could be with the same quantity of caustic soda digesting alumina trihydrate. Thus, the maximum A/C attained with a monohydrate bauxite is of the order of 1.20 while a ratio of 1.40 which can be reached with a trihydrate bauxite.
This A/C weight ratio, characterising the saturation state in alumina dissolved in the liquor of the Bayer cycle, determines the productivity of the liquor during its crystallization. This productivity is defined by the quantity of alumina recovered, in the form of alumina trihydrate, after crystallization of the pregnant liquor, in relation to a given volume of pregnant liquor. The productivity is expressed in kilograms of alumina per cubic meter of liquor (kg Al
2
O
3
/m
3
) and is obtained by multiplying the A/C variation before and after crystallization by the caustic concentration of the pregnant liquor. This concentration is usually higher in European-type Bayer processes than in American-type processes and that is the reason why a productivity at the crystallization of the pregnant liquor is considered as good when it exceeds 70 kg Al
2
O
3
/m
3
for an American-type Bayer process and when it exceeds 80 kg Al
2
O
3
/m
3
for an European-type Bayer process.
In the case of monohydrate bauxite, productivity at crystallization could be increased if it were possible to raise the dissolved alumina level and thus the A/C weight ratio, for example by addition of alumina trihydrate and dissolution during the cooling of the liquor, before separation of the red mud. The trihydrate solubility is in fact good up to approximately 100° C. in alkaline medium and such an operation would increase, at least in the first instance, the quantity of alumina dissolved to the extent that the A/C weight ratio could reach 1.25 and even 1.30.
It has occurred that such a mode of liquor supersaturation, developed during the 1950s and referred to as “sweetening”, is difficult to exploit on an industrial scale. In point of fact, it is known (YAMADA—Jl of Jap. Inst. of Light Metals—vol. 31 pp. 43-48—January 1981) that in these extreme supersaturation conditions, part of the alumina reprecipitates inopportunely before or during the separation of the red mud and is evacuated with it: this reversion is a particularly unwelcome phenomenon which is to be avoided since it reduces the extraction yield of the alumina.
Thus the sweetening implemented according to American patent U.S. Pat. No. 2,701,752, consisting in adding a certain quantity of trihydrate to the slurry during cooling after the high-temperature digestion of the monohydrate, limits the A/C after digestion to less than 1.08 with caustic concentrations not exceeding 120 g Na
2
O/l. While the risk of reversion is certainly minimized in these conditions, the productivity of the liquor suffers as a result and does not exceed 70 kg Al
2
O
3
/m
3
.
In such conditions, the sole interest of the sweetening mode lies in its ability to reduce the energy cost of the treatment, since it provides a trihydrate digestion which costs nothing in energy terms because it uses the heat supplied for the digestion of the monohydrate.
It therefore emerges clearly that the various means of the prior art, designed to limit the unwelcome effects of reversion on the extraction yield of alumina, are implemented at the expense of the productivity of the liquor which A/C weight ratio can barely exceed 1.15.
Thus alumina producers are still concerned by the development of a an effective process for the treatment of bauxites which are rich in monohydrates, particularly in view of the importance of this type of bauxites as a source of supply. Such a process must satisfy the following criteria:
total alumina digestion yield higher than 90%;
productivity of crystallization greater than 70 kg Al
2
O
3
/m
3
of liquor, being capable of exceeding 90 kg Al
2
O
3
/m
3
in a European-type Bayer process;
reduction of the quantity of energy required to dissolve the same quantity of alumina.
SUMMARY OF THE INVENTION
The process according to the invention developed by the applicant satisfies these criteria. For this purpose, a sweetening stage is included in the process but is implemented in an entirely different way.
More specifically, the invention relates to a process for treating alumina monohydrate-based bauxite by caustic and high-temperature digestion of the ground bauxite so as to form a slurry which is settled and filtered in order to separate a sodium aluminate liquor to be crystallized in the presence of seeds and an insolubl
Aluminium Pechiney
Dennison, Scheiner Schultz & Wakeman
Griffin Steven P.
Ildebrando Christina
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