Aluminium metal production comes from the
electrolysis of alumina : a white powder with the following
chemical formula Al2O3. The most common process for alumina production consists
of dissolving alumina from bauxite ore through a chemical attack with caustic
soda. Concentrated caustic liquor is mixed up with the crushed ore, under high
pressure and temperature. After elimination of insoluble matter through a
filtration stage, the caustic liquor is cooled down to allow precipitation of
alumina hydrate. The alumina hydrate is collected and dehydrated in a very high
temperature calciner. The alumina powder can then be exported to the smelter
for final aluminium production. On the other side, the caustic liquor is
concentrated through evaporation and purified before being recycled.
Although Kestner is specialised in evaporation and crystallisation
technologies, its know-how in the alumina industry covers many aspects,
including :
- Caustic liquor concentration
- Bayer liquor purification
According to the quality of bauxite used, some caustic liquors need to be
purified before being recycled to the digestion stage. Kestner proposes an
evapo-crystallisation unit, in order to remove mineral impurities (‘’
salting-out’’ units) as well as organic impurities (super concentration for ‘’
liquor burning’’, oxalate precipitation).
In 2004, GEA acquired
the subsidiary MESSO Switzerland (former Sulzer Escher Wyss), also
well known from the Alumina Producers, especially in multi flash technology.
Kestner and Messo AG have installed
more than 80 spent liquor evaporation plants and are able today, as world
market leaders, to offer all evaporation and crystallisation technologies for
the alumina industry.