With depleting gibbsitic or tri-hydrate bauxite reserves and the recent step increase in energy costs, alumina plant operators are looking for ways to process boehmitic or mono-hydrate bauxites that require higher digestion temperatures while keeping the energy costs low. The Double digestion (DD) process, as developed by Alcan provides an economic alternative to standard high temperature digestion process, normally used for treating boehmitic bauxites in Greenfield and Brownfield installations.
The DD process, involves extraction of gibbsite and boehmite in two distinct steps from the bauxite feed at different optimum temperatures with a pressure decanter as a link between the two digestion stages. Pressure decantation is designed to separate the unflashed mud residue containing unextracted boehmite after gibbsitic digestion, and provides feed to high temperature digestion. Depending on the boehmite content in the bauxite feedstock and the equipment layout in a brownfield installation, different flowsheets of DD process are available. The present paper will review some of these flowsheets and their possible application to different bauxite feed and plant equipment layout scenarios.