Bayer process

Schematic representation

Launched by Carl Josef Bayer developed Bayer process is used in the production of aluminum used to bauxite in sufficiently pure alumina to convert.

First, bauxite is separated from the iron oxide hematite and goethite it contains . The process takes advantage of the fact that the aluminum hydroxides of the finely ground bauxite can easily be dissolved in caustic soda at a higher temperature. The finely ground bauxite is mixed with concentrated sodium hydroxide solution at approx. 7  bar and approx. 180 ° C.

${\displaystyle \mathrm {Bauxit+NaOH\ {\xrightarrow[{Druck}]{\triangle }}\ Na\left[Al(OH)_{4}\right]+Fe_{2}O_{3}} }$

The undissolved residue mainly contains insoluble iron compounds, which explains the red color. This mass, known as red mud , poses a disposal problem and is therefore often initially stored in landfills, but in some countries is also simply discharged into rivers.

An elemental analysis by the Austrian Federal Environment Agency showed that the red mud is composed of around 40 different chemical elements . At this point in production, for example, the extraction of the gallium contained as an admixture can be interposed. Pure aluminum hydroxide is precipitated from the dilute aluminate liquor on cooling. The filtrate is diluted with sodium hydroxide solution, the temperature is lowered to 78 ° C. and the pressure is reduced again to normal pressure. By “inoculating” with solid aluminum hydroxide as a crystallization nucleus , the aluminum hydroxide then precipitates and part of the caustic soda can be recovered.

${\displaystyle \mathrm {Na\left[Al(OH)_{4}\right]\ {\xrightarrow {\bigtriangledown }}\ Al(OH)_{3}+NaOH} }$

The resulting solid aluminum hydroxide is burned in rotary kilns at a temperature of 1200 to 1300 ° C, whereby aluminum oxide is formed.

${\displaystyle \mathrm {2\ Al(OH)_{3}\ {\xrightarrow {\triangle }}\ Al_{2}O_{3}+3\ H_{2}O} }$

Metallic aluminum can then be obtained from the aluminum oxide produced by fused- salt electrolysis in the Hall-Héroult process .

literature

• Charles E. Mortimer, Ulrich Müller: Chemie , 11th edition, 2014, Georg Thieme Verlag, ISBN 978-3-13-484311-8 , p. 459

Individual evidence

1. ↑ Toxic sludge: According to Greenpeace, increased fine dust levels. ORF , October 12, 2010, accessed on October 17, 2010 . 
2. ↑ Test report no. 1010/441 "Heavy metal screening and determination of Cr (VI) in red mud" ( Memento from February 2, 2014 in the Internet Archive ). Order A 9928 - project no. 2490, accessed on October 17, 2010 (PDF; 46 kB, created by Umweltbundesamt GmbH on behalf of Greenpeace).
3. ↑ Test report No. 1010/431 "Determination of arsenic, mercury and chromium (total) in red mud" ( Memento of February 2, 2014 in the Internet Archive ). Order A 9928 - project no. 249, accessed on October 17, 2010 (PDF; 42 kB, created by Umweltbundesamt GmbH on behalf of Greenpeace)