Sponsored by: LMD Aluminum Committee
Program Organizer: C.G. Hendriks, Kaiser Aluminum and Chemical Corp.,9131 Interline Drive, Baton Rouge, LA 70809
Tuesday, AM Room: A6
February 6, 1996 Location: Anaheim Convention Center
Session Chairman: Fred S. Williams, Alcoa, Point Comfort, TX 77978
8:30 am Invited
MASS TRANSFER CONSIDERATIONS DURING THE PRECIPITATION OF DEPOSITS ON ALUMINA REFINERY HEAT EXCHANGER SURFACES: Dr. Victor Rayzman, 933 Regal Road, Encinitas, CA 92024.
A theoretical mass transfer equation was established for the analysis and calculation of the heat exchanger fouling during alumina manufacture as a function of flow velocity, heating pipe diameter, and heated medium temperature. In order to determine the interaction of all hydrodynamic quantities with respect to scale deposit formation, a pilot heat exchanger of the type "tube-in-tube" was designed, built and tested for heating the aluminate-silicate solution. The suitability and reliability of the theoretical and practical results were corroborated by a comparison with known data for the heating various bauxites and nepheline slurries. The data from one of the tests were applied to the design and operation of the aluminum plant. The results showed that lowering the concentration of the deposit forming substances by prior heating of the slurry or aluminate-silicate solution is the only alternative for mitigating the precipitation of deposits on heat exchanger surfaces.
IMPURITIES BUILD UP AND BEHAVIOR OF LOW ORGANIC BAUXITE OF PANCHPATMALI IN THE BAYER PROCESS: R.C. Mohanty, B. K Satpathy, National Aluminum Co. Ltd., IDCO Tower, Janpath, Bhubaneswar- 751 007, Orissa, India
The major portions of the impurity enter the Bayer process through bauxite. Beyond the critical levels the impurities alter the kinetics of the process. Among the various impurities, organic carbon is the most prominent one that affect the precipitation operation and the product quality, depending on the level of organics with respect to the precipitation process adopted. A lower level of less than 0.2% organic carbon in the Panchpatmali bauxite of Nalco although has allowed smooth operation of the plant for the last seven years, the same has been a matter of concern today. The present paper has analyzed the situation at Nalco plant with respect to various impurities build up over the years. The occurrence of organic in bauxite and its behavior in the Bayer process has been analyzed. Even such low level organic has been a point to watch for achieving productivity and quality product. The paper also highlights how the problems of sodium oxalate, emerging from the organics has been resolved at Nalco with the help of Aluminum Pecheney, France. Results are supported with experiments, plant data and characterization of bauxites and hydrates through instrumental techniques.
LAYERED DOUBLE HYDROXIDE FORMATION IN BAYER LIQUOR AND ITS PROMOTIONAL EFFECT ON OXALATE PRECIPITATION: A. J. Perrotta, Alcoa Technical Center, Alcoa Center, PA 15069-0001; F. S. Williams, Alcoa Operations, Pt. Comfort, TX 77978-0101
Enhancing the precipitation of sodium oxalate from Bayer process liquor to improve the quality of alumina product remains an important objective for Bayer refining. The formation of layered double hydroxides by the reaction of alkaline earth oxides, such as lime and magnesia, with Bayer liquor gives a crystal structure which is capable of intercalating anions, both inorganic and organic, within its structure. Both lime and magnesia, with long contact times in Bayer liquor, show layered double hydroxide formation. This layered double hydroxide formation is accompanied with a decrease in the sodium oxalate content in the liquor from about 3 g/l to below 1 g/l. Short contact times lead to a destabilization of the liquor which facilitates sodium oxalate precipitation. Additional work on magnesium hydroxide shows, in comparison to lime and magnesia, much less layered double hydroxide formation with equivalent residence time in the liquor. Destabilization of the liquor also occurs giving enhanced oxalate precipitation with less alumina being consumed in agreement with lower layered double hydroxide formation. Thermal regeneration of these structures followed by in situ recrystallization in Bayer liquor also gives enhanced oxalate precipitation, suggesting that there is an opportunity for a regenerable oxalate reduction system.
10:30 am BREAK
DETERMINATION AND EVALUATION OF ORGANIC BALANCES OF ALUMINA REFINERIES: Dr. Karoly Solymar, Mrs. Marta Gimpl- Kazar, Mrs. Eleonore Molnar, Hungalu Eng. and Dev. Center, ALUTERV-FKI Ltd, H-1502 Budapest, P.O. Box 308, Hungary
The adequate measures to control the organic level and especially that of humates and oxalate need the determination and evaluation of the organic balance of the given alumina plant. The organic materials of the different alumina refineries have special compositions, therefore it is very difficult and limited to draw general and common conclusions for all alumina refineries concerning the role and control of organics. The methodology of the determination of the organic balance of an alumina plant will be discussed. The qualitative and quantitative determination of the organic materials has been performed by means of gas-chromatography of the esterized organic materials. The organic balance of three different alumina refineries will be compared and evaluated relating to the total organic, humate and oxalate contents.
BOUND SODA INCORPORATION DURING HYDRATE PRECIPITATION: EFFECTS OF TEMPERATURE, CAUSTIC, AND ORGANICS: K. McCormick, J. Hunter, H. Warren, L. Armstrong, Senior R & D Chemist, Technology Department, Queensland Alumina Limited (QAL), P.O. Box 1, Gladstone, Queensland 4680, Australia
Soda is incorporated into aluminum hydroxide ("Hydrate") during the
precipitation stage of the Bayer Process. A review of general literature shows
the predominant effect is alumina supersaturation. Our research extends the
literature by quantifying the secondary effects of temperature, caustic and
organics on soda incorporation beyond their effect through alumina
supersaturation. This work advances industry knowledge towards better control
of soda incorporation in the refinery in the pursuit of higher and more
consistent product quality for smelter grade alumina.
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