Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 276

Clarification

Clarification refers to the separation of undissolved solids from the aluminium-bearing diges-
tion solution. The hot slurry from digestion is first directed through a series of flash tanks grad-
ually reducing the pressure and recovering heat that can be reused in the digestion process.
Separation is then achieved using large clarification vessels (mud thickeners), which
allow the solids to settle out. A flocculant is added to improve separation by gravitation
and subsequent filtration. Separated solids are passed through a counter-current washing
train (mud washers) using water to recover caustic soda which then is directed to the caus-
tic liquor circuit (step 1).

ALUMINA
REFINERY

Ground and
Washed Ore

Digestion

Heat Recovery

Liquor Heating Heat Recovery

Caustic
Soda
NaOH

Bauxite Residue
Washing

Liquor Surge Clarification

BAYER
PROCESS

Evaporation Filtration Filtration

Heat Recovery

Bauxite Residue
Disposal

Heating Vacuum Cooling

Precipitation

Hydrate
Classification

Hydrate Filtration

Calcination

FIGURE 6.12
The Bayer Process Illustrated

Most of the world's alumina is produced using the Bayer proc-
ess. The principles of this process are essentially the same
today as when it was first introduced about 100 years ago.

Processing/
Shipment