J. Chern. T w h . Biotechnol. zyxwvutsrqpo 1994, 60, 381-386 Liquid Phase Oxidation of 3,4-Dihydroxybenzoic Acid and the Effect of the Products on Trihydroxy Aluminium zy - Precipitation Yields Barry I. Whittington,* Tam Tran, David zyxw L. Trimm School of Chemical Engineering and Industrial Chemistry, University of New South Wales, PO Box 1, Kensington 2033, Australia & Lyndon G. Armstrong Queensland Alumina Ltd, Parsons Point, Gladstone 4680, Australia (Received 22 October 1993; revised version received 7 February 1994; accepted 24 February 1994) zyx Abstract: Organic compounds are known to interfere with the precipitation of trihydroxy aluminium (gibbsite) in the Bayer process. Studies have been carried out of the effect of 3,4-dihydroxybenzoic acid on precipitation and of the beneficial effect that can be achieved by oxidising the acid. Measurements of precipitate yields both in the presence of the acid and of products obtained by oxidising the acid were made. Oxidation in the presence of various metal ions was also examined. The yield of precipitate was found to decrease in the order Cu2+(1.9 mM) > Fe3+(2.2 mM) > blank (no added organics) > Mn4+(44.3 and 2.3 mM) > Mn2+(2.3 mM) > Mn2+(44.3 mM) % uncatalyset oxidation > Fe3+(43.5 mM) > Cu2+(38.2 mM) > untreated sample. This relative ordering was generally un- affected by reaction temperature or oxygen partial pressure. Soluble manganese salts were formed by partial dissolution of Mn02 in the alkaline solutions. These salts were required for oxidation of the 3,4-dihydroxybenzoic acid to products which, collectively, did not poison the trihydroxy aluminium precipitation process. Key words: liquid phase oxidation, trihydroxy aluminium, gibbsite, precipitation, poisoning. INTRODUCTION Organics in Bayer liquors generally have a detrimental affect on the yield from the trihydroxy aluminium (gibbsite) crystallisation process, as well as causing colouring of the liquor and gibbsite, lowering of the red mud settling rates or loss of caustic. Methods for the removal of these organics include ozonation,' high temperature and transition metal ion- * Present address: CSIRO Division of Minerals Products, Private Bag, PO, Wembley 6014, Australia. t To whom correspondence should be addressed. catalysed oxidation."* In particular, oxidation using MnO, has been extensively However, the complexity of Bayer liquor solutions makes it difficult to determine which organic species are degraded and which are unaffected by these processes. 3,4-Dihydroxybenzoic (3,4-DHBA) acid is an effective poison for the trihydroxy aluminium precipitation process,' making it a useful model for examining the removal of organics from Bayer liquors. This paper examines the effectiveness of various systems for improv- ing the trihydroxy aluminium precipitate yields by oxidation of 3,4-DHBA under basic conditions (3.75 M NaOH dm-3). These studies were conducted in the 38 1 zyxwvut J. Chrm. Tech. Biotechnol. 0268-2575/9?/$09.00 zyxwvu 0 199? SCI. Printed in Great Britain