Transformation of a dual stream low temperature digestion facility

Abstract

Refineries processing primarily Gibbsitic bauxites require a low temperature digestion circuit to extract the alumina. These circuits typically operate at between 145 °C and 150 °C. Since the 1960’s, many of the world’s low temperature digestion facilities operated as dual stream circuits with the caustic liquor heated progressively through shell and tube heaters using regenerative flash steam. The liquor and bauxite streams were then mixed at the autoclave or digester vessels. With increasing focus on maximizing energy efficiency, process simplicity and plant utilization, single stream digestion technology has been increasingly the technology selected for refineries around the world. The single stream digestion flowsheet preferentially combines the bauxite and caustic liquor streams together in a single stream prior to regenerative heating. This paper provides a brief overview of the considerations required for the transformation of a dual stream low temperature digestion facility to a single stream operation. A review of process performance parameters and design considerations will be discussed.