1997 TMS Annual Meeting: Wednesday Session

ALUMINA & BAUXITE TECHNOLOGY: Session IV

Session Chairman: J.L. Anjier, Kaiser Aluminum and Chemical Corp., Box 337, Gramercy, LA 70052

2:00 pm

FCB RETROFITTING OF ALUMINA ROTARY KILN: WHEN DRASTIC FUEL SAVING MEETS INCREASED CAPACITY: Vincent Giroud, Andre' Pinoncely, FCB- subsidiary of the Fives Lille Group, Mineral Processing and Carbon Plant Division, 32, rue fleury Neuvesel, BP 24-69702 Givors Cedex.

Alumina calcination using rotary kilns is an expensive operation. In 1972 an innovative concept of kiln retrofitting was introduced which greatly enhanced the overall thermal efficiency while preserving most of the existing capital equipment. As a result, nine kilns have been retrofitted over the last eighteen years, gradually highlighting the wide range of possible arrangements and the relevant strong process and mechanical reliability. This paper draws up the main features of these retrofits, namely the substantial energy saving and the correlative increased capacity. Long-term observations also demonstrated that alumina quality remains as consistent as previously with no significant impact on the calcined alumina size distribution. The latest retrofitting pattern including new cyclones and ducting arrangements are presented yielding a fresh look for a still useful technology.

2:25 pm

ALUMINA CALCINATION WITH THE MULTI-PURPOSE CALCINER: Jorgen Ilkjaer, Lars Bastue and Benny E. Raahauge, FLS Minerals A/S, Alumina & Bauxite Technology, Vigerslev Allé 77, DK-2500 Valby Copenhagen, Denmark

The Gas Suspension Calciner (GSC) and the rotary kiln are both well proven technologies for calcination of alumina. The GSC is mainly used for production of smelter grade alumina and the rotary kiln for special grade alumina. A multi-purpose calciner which combines these two technologies has been designed, constructed and commissioned in the summer of 1996. By combining these two calcination technologies it is possible to produce a wide range of alumina qualities only by changing a few parameters in the operation using a Fuzzy II Logic Control System and a CemScanner.

2:50 pm

PREDICTING MOISTURE CONTENT ON ALUMINAS FROM MEASUREMENT OF WATER ISOTHERMS: A.R. Gillespie, Comalco Research Centre, Thomastown, Vic., Australia; M.M. Hyland, J.B. Metson, University of Auckland, Auckland, New Zealand

Water adsorption isotherms on alumina can play an important predictive role in aluminium smelting since the level of moisture on the alumina influences its handling, feeding and dissolution characteristics, along with the potential for generation of emissions. Measurements of adsorption isotherms of water on smelter aluminas indicate moisture pick-up and loss is rapid, with initial rates of change in adsorbed water in excess of 0.1 wt% per minute, meaning the ex-situ measurements are not likely to be accurate. Adsorption and desorption curves, to water pressures in excess of 20 Torr, indicate near complete reversibility. The discrepancy is attributed to the irreversible formation of Al(OH)3. Analysis of the shape of the curves suggest several distinct mechanistic regimes within the adsorption curve.

3:15 pm BREAK

3:35 pm

ALUMINA QUALITY TESTING PROCEDURE: R.G. Haverkamp, B.J. Welch, Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand; S. Bouvet, Pechinay Centre de Recherches de Voreppe, Voreppe, France; P. Homsi, Aluminium Pechiney, St. Jean de Maurienne, France

A laboratory method for comparing characteristics of alumina dissolution in molten cryolite is described. The method uses fast modified linear sweep voltammetry combined with thermal analysis to determine the dissolution rate of alumina in a molten cryolite electrolyte. Practical aspects of the method are discussed. Emphasis is on consistent alumina feeding and stirring and a careful monitoring of the alumina moisture content.

4:00 pm

SINTERING AND HEAT CONDUCTIVITY OF ALUMINA: Terje Østvold, Øyvind T. Gustavsen, Heidi Mediaas, Institute of Inorganic Chemistry, Norwegian University of Science and Technology, N-7034 Trondheim, Norway; Torstein Haarberg, Hydro Aluminium, Technology Centre Årdal, N-5870 Øvre Årdal, Norway

Heat conductivities are measured in alumina powders treated with 0-7 wt% NaAlF4 at varying temperatures up to 900°C. The transition of alumina, triggered by addition of NaAlF4, was found to be essential for the observed increase in heat conductivity. A maximum heat conductivity was measured at 3.5 wt% NaAlF4 for aluminas heated to 700°C and at ~2 wt% NaAlF4 for heat treatments at 800 and 900°C. The heat conductivity was also measured versus temperature after the samples had been sintered. A decrease in heat conductivity with increasing sample temperature was observed. A mathematical model for the thermal conductivity of porous materials was applied and tested against the measured data, confirming the importance of sintering with respect to thermal conductivity. The relevance of the present findings concerning heat balance of industrial smelting cells is discussed.

4:25 pm

RECOVERING ALUMINA, SILICA AND BYPRODUCTS FROM COAL ASH THROUGH THE USE OF PROCESS FOR SILICON PRE-EXTRACTION: Victor L. Rayzman, 933 Regal Road, Encinitas, CA 92024; Solomon A. Shcherban, 110 Bennet Avenue, #3H, New York, N.Y. 10033

The large volume of coal combustion wastes cause a problem of great concern in environmental protection. Only about a quarter of these wastes are being used today, generally for construction materials production. At the same time, many million tons of aluminum and silicon contained in coal ash are lost in landfills annually. Of all the large number of processes for recovering alumina and byproducts from aluminum and silicon-bearing wastes only one, the lime-soda sintering process, has been commercially used on nepheline residue and red mud. To approach ash composition similar to these above-mentioned wastes, a process for silicon pre-extraction has been developed and pilot plant tested. The method reduces the material stream trough the sintering kiln in half and produces high pure silica alongside the alumina and calcium silicate. The heat consumption for the new process is estimated significantly lower than that for the sintering method and approaches the Bayer technology indices.

4:50 pm

FEATURES OF ALUMINA PRODUCTION TECHNOLOGY FROM ALUMINOSILICATE RAW MATERIAL WITH POTASSIUM'S HIGH CONTAIN: V.A. Lipin, N. N. Tikhonov Russian National Aluminium-Magnesium Institute (VAMI), 88, Sredny pr., St. Petersburg, 199026, Russia

Aluminosilicate raw materials have large differences in chemical and mineralogical structure, particularly in the relationship between sodium and potassium. Large deposits of raw materials with high portions of potassium alkalis are known and have been studied for the purpose of the industrial processing by the sintering and hydrometallurgical methods. Technological features of the processing of the high-potassium aluminosilicate raw materials utilize for the most part the processes of sintering, leaching and desilication. The optimum molecular ratios of the main components before sintering were found in each case. Distribution of alkaline metals between liquid and solid phases during leaching and desilicating was determined. Utilizing experimental data, equations have been developed predicting the dependence of desilicating indexes from chemical composition of processing ores. These relationships aid in choosing the most effective conditions for research of promising raw materials.