Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 275

The Bayer Process in Alumina Refining

Alumina (aluminium oxide and aluminium trihydroxide) is the material from which all
of the world's aluminium is produced. Transforming bauxite ore into alumina is based on
the Bayer process and the principles of this process are essentially the same today as when
it was first introduced about 100 years ago. Bauxite ore is ground and mixed with caustic
soda. On average 3 tons of ore is needed to produce 1 ton of alumina ( Figure 6.11 ). The
mixed solution is pumped into high-pressure, heated vessels in which the aluminium tri-
hydroxide in the bauxite is dissolved. The hydrate (Al(OH) 3 ) is precipitated from the solu-
tion after mud separation. Hydrate solids are washed using filters. Calcination converts
the hydrate into alumina oxide, a sugar-like white powder.
Figure 6.12 illustrates these steps. The four basic steps are (1) digestion, (2) clarification,
(3) precipitation and filtration, and (4) calcination.

Alumina (aluminium oxide and
aluminium trihydroxide) is the
material from which all of the
world's aluminium is produced.

Digestion

Mined bauxite is crushed and ground to produce a uniform fine grained material with
a maximum grain diameter of 2 mm to ensure sufficient solid-liquid contact during the
digestion phase. After grinding, the ore is mixed with spent liquor (typically a solution of
sodium hydroxide from the recycled caustic soda liquor circuit) and pumped into a series
of digestion vessels. The prepared slurry ranges from 40 to 50% solids content.
The controlling factors for the digestion step are the concentrations of caustic soda and
bauxite, grain size, pressure, temperature, and retention time. Pressure, temperature, and
retention time vary from plant to plant, but typical figures are as follows: a digester pres-
sure equivalent of about 4 kg/cm 2 , a temperature of about 140 º C, and a retention time of 30
to 45 minutes.
At the digestion process, solid hydrate alumina in the bauxite (mineralogically gibbsite)
reacts with sodium hydroxide:

The controlling factors for
the digestion step are the
concentrations of caustic soda
and bauxite, grain size, pressure,
temperature, and retention time.

(6.17)

Al 2 O 3 3H 2 O 2NaOH → 2NaAlO 2 4H 2 O

The slurry leaving the digestion tanks contains the aluminium ion in solution (in the form
of a sodium aluminate solution), together with undissolved ore solids.

FIGURE 6.11
From Bauxite Ore to Alumina:
A Simplifi ed Mass Balance

Crude Ore from
Mine

3t

On average 3 tons of ore is needed to
produce 1 ton of alumina.

1.5 t

Bauxite Residue

Bayer Process

1.5 t

0.5 t

Calcination Water

1t

Alumina