bauxite double digestion

A normal single digestion process

Bauxite is fed into high temperature (HT) digestion, so both Gibbsite and Boehmite are dissolved.

There are following Flash tanks or Blow-off tanks, where liquor drop pressure quickly, for subsequent operations.

In the middle of the Flash banks, there can be better quality bauxite fed into the tank for low temperature (LT)digestion as well.

The good quality bauxite (low temperature) bauxite is normally from a different mine with a higher percentage of Gibbsite (requires low temperature to dissolve) and lower Boehmite (requires high temperature to dissolve).

For double digestion, Bauxite is fed into low temperature (LT) first, where the Gibbsite is dissolved.

A following thickener separates the liquor from the slurry (still with all the Boehmite at this stage).

The slurry goes into a second stage high temperature (HT) digestion, where the Boehmite is dissolved.

Because only part of the bauxite is digested with high temperature, the energy consumption is reduced greatly.

After another thickener separate liquor from slurry. The liquor with dissolved Boehmite joins back to the lower temperature LT digestion, to further improve the Alumina/Caustic soda percentage, which makes it easier to precipitate later on.