FILTRATION OF DIGESTER BLOW-OFF – AN OPTION FOR THE REFINERY DESIGN OF THE FUTURE

Rotary vacuum disc filters are successfully used in Alumina refineries since decades in seed deliquoring and wash applications. However, the limitation of the acting pressure difference to the conditions generated by vacuum pumps have restricted the field of applications. With the installation of these rotary disc filters in pressure vessels and the development of reliable solids discharge systems, the so-called hyperbaric filters have made their way into the Alumina refining process. At the moment they are used for Bauxite filtration if the bauxite is sent to the refinery through a pipeline as water/bauxite mix. This water must be removed as much as possible to keep the water balance in the process and to minimize the energy impact. But this may not be the only suitable filtration step for this filter technology. Bauxite residue is certainly a vital option although filter presses are state of the art technology at the moment. However, the discussion about the refinery design of the future always includes options for direct filtration of the digestor discharge that may replace the decanter and washers either partially or in total. The most appropriate filter technology for this application would be the hyperbaric filtration. It would allow to filter at high temperature of up to 200 °C and high pressure of up to 10 bar. Backpressure operation could avoid flashing and moderate cake wash promises to eliminate mud washers in total. As the bauxite residue could be discharged with a moisture of 30 w/w-% or less, there would as well be no need for extra bauxite residue filters. The footprint of the Alumina refinery could be drastically reduced.