Process for the manufacture of coarse aluminium hydroxide

Details Process for the manufacture of coarse aluminium hydroxide Process for the manufacture of coarse aluminium hydroxide

1999-10-20

2001-10-02

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Group iiia metal or beryllium

C423S127000, C423S629000

Reexamination Certificate

active

06296819

ABSTRACT:

The invention concerns a process for manufacturing coarse aluminium hydroxide by decomposition of a supersaturated alkaline aluminate liquor in a two stage precipitation process having an agglomeration stage in agglomeration tanks connected in series, where the liquor is seeded with fine aluminium hydroxide to induce precipitation and formation of a suspension, followed by a growth stage, where the suspension is seeded with coarse aluminium hydroxide.
The invention relates to the manufacture of aluminium hydroxide Al(OH)
3
for the purpose of manufacturing metal grade alumina which meets the requirements of the modern aluminium smelters.
In particular the invention relates to a process for the manufacture of coarse aluminium hydroxide via the Bayer Process in which the alumina values of the alumina containing ore are solubilized at a relatively high temperature in an aqueous liquor of caustic soda and sodium aluminate and in which these alumina values are crystallized later on at a lower temperature in the form of aluminium hydroxide. The aluminium hydroxide, also called product hydrate, is calcined to yield a sandy alumina.
This crystallization procedure, hereinafter referred to as precipitation, is enhanced by the seeding of the supersaturated sodium aluminate liquor (also called the pregnant liquor) with aluminium hydroxide (also simply called “hydroxide” or even “hydrate”).
The invention concerns more particularly a mode of application of the Bayer process in which the precipitation is achieved in two stages, namely an agglomeration stage followed by a growth stage.
In the U.S. Pat. No. 4,234,559 an agglomeration stage is characterized by the seeding of the pregnant liquor with a controlled amount of relatively fine hydrate referred to as the fine seed allowing to achieve the control of the product hydrate granulometry. A following growth stage is characterized by the seeding of the suspension leaving the agglomeration stage with a high amount of coarser hydrate referred to as the coarse seed allowing to achieve a high precipitation yield. In order to make it operational, this precipitation process in two stages must be complemented with the classification of the separated hydrate into fine seed, coarse seed and product hydrate, and with the separation of the exhausted liquor (also called the spent liquor) from the hydrate.
This two-stage precipitation process allows the production of a product hydrate with a particle size distribution showing a proportion of particles with the diameter smaller than 45 micrometers not exceeding 15% by weight and which may be as low as 3% by weight.
This two-stage precipitation process also allows to achieve a high liquor productivity i.e. a high yield of precipitation of aluminium hydroxide per unit volume of pregnant liquor. Thus, liquor productivities of typically 70 to 85 g Al
2
O
3
per liter of liquor can be commonly achieved under industrial operating conditions. A yield of 91.7 g Al
2
O
3
was achieved in a test procedure.
As far as the chemical composition of the metallurgical alumina is concerned, the modern smelters are favoring nowadays an alumina with a relatively low soda content.
In the AIME-Report of 1988, pages 125 to 128, “Operation of the Alusuisse precipitation Process at Gove” by S. G. Howard, a precipitation process layout had been presented, showing cooled pregnant liquor seeded in two phases. The pregnant liquor flow is split between the first two precipitation tanks of the agglomeration phase.
It is an objective of the instant invention to reduce the occlusion of soda in the aluminium hydroxide (also called “product hydrate”) produced in a two-stage precipitation process while maintaining a high liquor productivity and a good control of the granulometry and strength of the product.
Under occlusion of soda is meant the incorporation in the aluminium hydroxide crystal lattice of soda values which cannot be removed by the thorough washing with water of the product hydrate. For example an occluded soda content of the product hydrate could be as high as 0.4% or even 0.45% calculated in weight percent Na
2
O on an Al
2
O
3
basis, depending on the liquor purity. With the process according to the instant invention it is possible to reduce the occluded soda content for example by 0.05% to 0.15% Na
2
O, thus allowing to achieve an occluded soda content of for example below 0.35% or even of 0.25% and below.
According to the present invention, the feed of the supersaturated alkaline aluminate liquor to the agglomeration stage is split into a first substream and said first substream is fed to a first agglomeration tank or to the first and more of a series of first agglomeration tanks at a higher liquor temperature of 70 to 100° C. and a second substream and said second substream is fed to a second agglomeration tank or to two or more of second agglomeration tanks at a lower liquor temperature of 50 to 80° C., at the end of precipitation yielding a strong coarse product hydrate, giving after calcination a sandy alumina with low occluded soda content, compatible with a high liquor productivity.
In a preferred embodiment of the instant invention the supersaturated alkaline aluminate liquor is split into a first substream representing 30 to 60% of the total supersaturated alkaline aluminate liquor stream. Said first substream is fed to the first agglomeration tank or a series of first agglomeration tanks of the agglomeration stage. The first substream of the supersaturated alkaline aluminate liquor is fed at a relatively high temperature of for example 70 to 90° C. and preferably 80 to 90° C. One first agglomeration tank can be present or a series of two or more first agglomeration tanks can be present. The first substream of the supersaturated alkaline aluminate liquor can be fed to the one first agglomeration tank or can be fed and distributed to each of two, preferably two, or more of the series of first agglomeration tanks. If a series of two of first agglomeration tanks are present, the first substream of the supersaturated alkaline aluminate liquor can be distributed to the first and the second agglomeration tank of the series of first agglomeration tanks. The fine aluminium hydroxide, also called the fine seed, can be fed into the one first agglomeration tank or can be fed in the first agglomeration tank of the series of two or more first agglomeration tanks or can be fed and distributed over the first two agglomeration tanks of the series of two or more first agglomeration tanks. The supersaturated alkaline aluminate liquor and the fine seed form a suspension with a solids content in the one first agglomeration tank or in the series of first agglomeration tanks of 100 to 500 g/l, preferably 150 to 450 g/l and especially 300 to 400 g/l.
Connected in series with the first agglomeration tank or the series of first agglomeration tanks are the second agglomeration tank or a series of second agglomeration tanks.
The balance of 70 to 40% of the supersaturated alkaline aluminate liquor forms the second substream. Said second substream is typically fed to the one second agglomeration tank or fed to one of the series of second agglomeration tanks or fed and distributed to two or more or all of the series of second agglomeration tanks. The second substream of the supersaturated alkaline aluminate liquor is preferably fed to the second agglomeration tank or to the series of second agglomeration tanks at a relatively low temperature of preferably 60 to 80° C. and especially 60 to 70° C.
For example the second substream of liquor representing 70 to 40% of the total supersaturated alkaline aluminate liquor flow may be sent to the one second agglomeration tank, or to one of two or more of the series of second agglomeration tanks, or to two or more of two or more of the series of second agglomeration tanks. If two or more agglomeration tanks are present in the series of second agglomeration tanks, the second agglomeration tanks are connected in series. The one first agglomeration tank or the last of the series of first agglomeration tanks

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