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Precipitation and calcination of monohydrate alumina from the bayer process liquors

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Objectives and content
The Bayer process is universally applied for the
production of alumina from bauxites. In the current
industrial practice, established for over 100 years, the
process has three main steps: Digestion, where bauxite is
digested with caustic soda at elevated temperature and
pressure to produce aluminate liquor, containing alumina
in the form of A1(OH)4; precipitation, where the
aluminate liquor is hydrolysed at 55 - 60 C to produce
crystalline alumina trihydrate, (gibbsite, Al2O3.3H2O)
and calcination, when gibbsite is calcined at 1000 -
1200 C to produce alumina: Al203.3H2O
Al203 + 3H2O.
The Bayer process is energy intensive, requiring
approximately 12GJ/t alumina. The calcination step
absorbs 3 GJ/t alumina, or 25% of the total Bayer
process.
The present proposal aims at the development of a highly
innovative variation of the Bayer process whereby, at the
precipitation step, alumina monohydrate (Al2O3.H2O)
rather than trihydrate will be precipitated at
atmospheric conditions and subsequently calcined to
produce an hydrous alumina. The proposed process is a
technological breakthrough since for the first time with
this variation of the precipitation step, boehmite can be
produced economically and in substantial quantities under
conditions and with equipment similar to these used for
gibbsite precipitation in the current practice of the
Bayer process.
The developed process will result in significant energy
savings during the calcination stage. The energy saving
in calcination of boehmite is estimated to be 34.8 kg/t
of alumina for fuel oil and 7.2 Kwh/t of alumina for
electricity. This corresponds to 40-50% less fuel or
electricity consumption compared with gibbsite
calcination for the same alumina production. For a
typical Bayer process plant with a capacity of l .000.000
tpa alumina. this represents energy savings of 35000 t of
fuel oil per year. Besides the production of smelter
grade alumina, the boehmite produced will be used for the
production of new special types of non-metallurgical
grade alumina. This is a fast growing industrial sector
consuming approximately 12% of the alumina produced by
the Bayer process.
The development of the new process will be realised
through completion of the following tasks:
Optimisation of boehmite precipitation (grain size of
the precipitate, efficiency of precipitation)
Study of the calcination (energy efficiency, properties
of the calcined alumina)
Identification of new alumina grades for nonmetallurgical applications
Modelling of the boehmite precipitation and calcination
process
Scale-up of the precipitation and calcination process
Development of the process flowsheet
Techno-economic evaluation of the process and the new
materials.
The consortium comprises two academic institutions and
three industrial companies. The first one is a major
alumina and aluminium producer possessing several plants
all over the world of 2,1 million tonnes per year
capacity and will be an end-user of the process. The
second is involved with the construction of a new alumina
plant of 750.000 tpa capacity and will be an end-user of
the process and the third is one of the leading
engineering companies worldwide with extensive experience
and expertise in the design of Bayer process plants; the
company possesses significant know-how in tube digestion
of bauxites as well as in alumina calciners. It will be
involved in the engineering aspects of the development of
the new process and will act as technology promoter.

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Koordinator

NATIONAL TECHNICAL UNIVERSITY OF ATHENS
EU-Beitrag
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Adresse
Zographou Campus
15780 ATHENS
Griechenland

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