Page 101 - Chemical and process design handbook

Speight_Part II_A 11/7/01 3:16 PM Page 2.42

ALUMINA

Pure alumina (Al O ) is a dry, snow-white, free-flowing crystalline pow-
2 3
der and may be obtained in a wide range of particle sizes.
There are two main types of alumina (bauxite) ores used as the primary
sources for aluminum metal and aluminum chemicals: aluminum hydrox-
ide [Al(OH) ] (gibbsite) and a mixed aluminum oxide hydroxide
3
[AlO(OH)] (boehmite). Thus, bauxite is a term for a family of ores rather
than a substance of one definite composition. An average composition of
the ores used by industry today would be: alumina (Al O ), 35 to 60%; sil-
2 3
ica (SiO ), 1 to 15%; ferric oxide (Fe O ), 5 to 40%; and titanium dioxide
2 2 3
(TiO ), 1 to 4%.
2
In the process to produce alumina (Fig. 1), bauxite is crushed and wet
ground to 100-mesh, dissolved under pressure and heated in digesters with
concentrated spent caustic soda solution from a previous cycle and sufficient
lime and soda ash. Sodium aluminate is formed, and the dissolved silica is
precipitated as sodium aluminum silicate. The undissolved residue (red
mud) is separated from the alumina solution by filtration and washing and
sent to recovery. Thickeners and Kelly or drum filters are used. The filtered
solution of sodium aluminate is hydrolyzed to precipitate aluminum
hydroxide by cooling. The precipitate is filtered from the liquor, washed,
and heated to 980°C in a rotary kiln to calcine the aluminum hydroxide.
Several other processes for producing alumina based on ores other than
bauxite have been announced. One process uses alunite, a hydrous sulfate
of aluminum and potassium. It is claimed to be capable of producing 99%
pure alumina from alunite containing only 10 to 15% alumina, compared
with bauxite that assays 50% alumina. The alunite is crushed, dehydroxy-
lated by heating to 750°C, ground, and treated with aqueous ammonia.
Filtration removes the alumina hydrate, and potassium and aluminum sul-
fates are recovered from the filtrate (to be used as fertilizer constituents).
The alumina hydrate is treated with sulfur dioxide gas, and the resulting
aluminum sulfate is converted to alumina by heating in a kiln.
Another process for alumina manufacture involves treatment of clay
and shale with concentrated sulfuric acid. Hydrochloric acid is added dur-

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