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Gillespie, A.R.

A paper presented by QAL at AQW 2002, examined the viability of nano-filtration (NF) membranes for the purification of Bayer liquor. In flat-sheet, cross-flow configuration, the durability, flux and rejection characteristics of the membranes were indicative of potential economic value. The current paper reports recent work to further assess the benefits and risks associated with a potential NF membrane process for use in a Bayer refinery.

Pilot trials were conducted using spiral-wound NF elements, this configuration commonly offering an acceptable compromise between membrane density (capital cost) and fouling rate (operating and capital cost). This enabled development of a flow sheet for a membrane plant that would produce retentate for feed to a downstream organics-removal process, and permeate for return to the washer train.

A low level of permeate flux was found to necessitate relatively high membrane area, while poor recovery necessitated a high level of recycle within the membrane plant. Both factors imposed significant additional capital and operating cost when compared to conventional applications (eg water treatment).

Due to fouling of the membrane, permeate flux declined at a rate of 0.1% per hour. Flushing with water, dilute caustic or dilute acid, returned the permeate flux to approximately 90% of its level at the start of each cycle, thus partially arresting the overall fouling rate. In terms of a full-scale plant, the high fouling rate imposed significant additional capital and operating costs.