Ind. Eng. Chem. Res. 2004, 43, 847-851
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APPLIED CHEMISTRY Interactions between Specific Organic Compounds during Catalytic Wet Oxidation of Bayer Liquor James Tardio,† Suresh Bhargava,*,† Sharon Eyer,‡ Matt Sumich,‡ and Deepak B. Akolekar† Department of Applied Chemistry, RMIT University, Melbourne, Victoria 3001, Australia, and Technology Delivery Group, Alcoa World Alumina, P.O. Box 161, Kwinana, Western Australia 6167, Australia
The catalytic wet oxidation (WO) mechanisms for the salts of organics, particularly carboxylic acids, in highly alkaline solution are of prime importance from the viewpoint of alumina producers because organic contaminants have adverse effects on the alumina refining process. Four organic compounds of interest, which are found in several worldwide alumina process liquors (Bayer liquor), are sodium acetate, oxalate, malonate, and succinate. CuO-catalyzed WO of the aforementioned organic salts was studied in an alumina process liquor (Bayer liquor) and synthetic Bayer liquor. CuO was found to catalyze the WO of only one of the organic salts, sodium malonate, in isolation in synthetic Bayer liquor at low temperature (165 °C). In Bayer liquor, however, both sodium malonate and sodium succinate underwent CuO-catalyzed WO at 165 °C. Sodium succinate was found to undergo CuO-catalyzed WO at 165 °C in Bayer liquor via a co-oxidation mechanism. Sodium malonate was identified as a compound capable of co-oxidizing sodium succinate under Bayer-like (highly alkaline) conditions. Sodium malonate was also found to be capable of co-oxidizing sodium acetate under Bayer-like conditions, indicating that highly reactive free-radical intermediates are formed during CuO-catalyzed WO of sodium malonate. Introduction Organic compounds contaminate a number of process liquors and wastewaters from the alumina,1-3 petrochemical,4,5 pulp/paper,6,7 and chemical8,9 industries. These compounds sometimes hinder productivity and/ or cause difficulties with wastewater disposal. The alumina refining industry suffers from both of the above-mentioned problems because of unwanted organic compounds in the process liquor. Hence, ways of removing such compounds are of great interest to the alumina refining industry. Wet oxidation (WO)/catalytic wet oxidation (CWO) is a promising organics removal process being considered by some alumina companies. The WO process, originally developed by Zimmermann10 over 50 years ago, removes organic compounds via complete oxidation (in the liquid phase) to carbon dioxide (which is subsequently converted to carbonate in an alkaline solution) and water using an oxidizing gas such as oxygen or air. Although WO/CWO has been the subject of considerable study over the last 3 decades, very little information has been published on the chemistry of WO/CWO in industrial process liquors or wastewaters. This is due to these solutions containing complex mixtures of organic compounds, which in turn has led to a large * To whom correspondence should be addressed. Tel.: +61 3 9925 3365. Fax: +61 3 9639 1321. E-mail: suresh.bhargava@ rmit.edu.au. † RMIT University. ‡ Alcoa World Alumina.
amount of research being conducted on individual compounds in simple solutions. In this study, the WO/ CWO of four organic compounds (sodium acetate, oxalate, malonate, and succinate) has been studied in an alumina process liquor (Bayer liquor, LIQ) and simple alkaline solutions. The main aim of this study was to determine the predominant CWO reaction mechanisms (free radical or ionic) occurring in a highly alkaline complex industrial solution (LIQ). Materials and Methods The WO and CWO experiments were carried out in a continuously stirred, 3.75 L nickel (Inconel) autoclave (Parr Autoclave, Moline, IL), which included attachments for gas addition into the liquid phase and kinetic subsampling of the liquid phase. Two different highly alkaline solutions were used in this study. The composition of these solutions is given in Table 1. The chemicals NaOH, Na2CO3, Al(OH)3, NaCl, and Na2SO4 were used to prepare the synthetic Bayer liquor (SBL). CH3COONa (sodium acetate), CH2(COONa)2 (disodium malonate), (COONa)2 (disodium oxalate), and (CH2)2(COONa)2 (disodium succinate) were used as purchased. The LIQ was collected from an Alcoa alumina refinery located in Western Australia and used without further purification. Commercial copper(II) oxide, CuO (99+%,
