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1997 TMS Annual Meeting: Tuesday Session


Sponsored by: LMD Aluminum Committee
Program Organizer: F. S. Williams, Alcoa Alumina & Chemicals L.L.C., Point Comfort, TX 77978-0101

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Room: 230D

Session Chairman: Jean Doucet, Alcan International Limited, 1188 Sherbrooke Street West, Montreal, Quebec, Canada H3A 3G2

8:30 am

REDUCTION OF THE MOISTURE CONTENT IN MRN BAUXITE, BY TREATMENT WITH DEWATERING AID: Carlos Estevam da Silva, Marcos Fernando Dias Moreira, Mineracão Rio do Norte S.A. , Porto Trombetas, PA 68275-000 Brazil; Luis Gustavo Esteves Pereira, Mauro Pinto dos Reis, Nalco Brasil Ltda, Av. das Nacões Unidas, 17891 - 11o. Andar, Sao Paulo, - SP - 04795-100 - Brazil

MRN must sell their bauxite at a maximum of 12% moisture content to avoid paying a penalty. To meet this target it is necessary to mix dried and wet bauxite; however, that mixing is very costly because of the high drying cost. Thus MRN and NALCO BRASH have worked together to develop a chemical program to reduce the moisture content of wet bauxite. A large number of dewatering aid products were tested on a lab scale. Two products were selected and tried in pilot plant scale. Finally the product 95DW121 was selected and tried on an industrial scale. The industrial trial was run for three months in the fine and superfine plants with the results that the bauxite moisture was reduced around three percentage points with a dosage rate of 80 g/t.

8:55 am

BQUANT: COST-EFFECTIVE CALCULATIONS OF BAUXITE MINERALOGY: Frank M. Kimmerle, Frank Feret, Alcan International Ltd., Arvida Research and Development Centre, P. O. Box 1250, Jonquière, QC, G7S-4K8; Barbara Feret, BF Simulation, 3099 St. Patrick St., Jonquière

Routine elemental analysis of the major and minor oxides in alumina-bearing minerals, using modern XRF instrumentation yields better than 0.1% reproducibility at a rate exceeding 100 samples per day. However, the mineralogy required to evaluate the amount of recoverable alumina or other Bayer plant parameters has been far less readily accessible. Empirical fits between elemental composition and simulated plant digests are limited to narrow composition ranges in particular geological deposits. Interpretation of X-ray diffractograms is notoriously slow, influenced by crystallite size, amorphous phases, and preferred orientation effects. Instead, using non-linear programming, the BQUANT software provides quantitative phase analysis based on an elemental mass balance. It can even deal with overburden, clay bottoms and is independent of the exact nature of the geological deposit. Given its rapidity, it is particularly useful for estimating the potential of new deposits. Given its accuracy, it greatly facilitates mine exploitation control.

9:20 am

MONITORING OF HEAT EXCHANGER PERFORMANCE THROUGH THE SCALING FACTOR: Eugènio C. G. Azevedo Jr., Carlos Eduardo A. Batista, Alumar-Consórcio de Alumínio, BR 135 - Km 18, Distrito Industrial de Pedrinhas CEP 65095.050 - São Luis/MA - Brasil

The ALUMAR Refinery was designed to maximize heat recovery, and therefore minimize process steam requirements. Since the increase of alumina production from 0.5 to 1.2 million tons per year, the heat exchangers became a strategic point in the refinery performance. An objective has been to create a tool to improve the performance of the heaters. A continuous method of monitoring individual heaters was developed of calculating the scaling factor using the principles of the heat transfer theory. This paper describes the method and the steps to achieve a significant improvement in the identification and troubleshooting of heat transfer related problems. This method also proved to have a positive effect on the operation and maintenance planning of the heat exchangers in the refining.

9:45 am

THE PRECIPITATION MECHANISM OF SODIUM ALUMINOSILICATE SCALE IN BAYER PLANTS: J. Addai-Mensah, A.R. Gerson, K. Zheng, A. O'dea, R. St. C. Smart, Ian Wark Research Institute, University of South Australia, The Levels, Adelaide, 5095, Australia

During the dissolution of bauxite ores by caustic soda, a serious and unwanted downstream precipitation of sodium aluminosilicate may occur in heat exchangers, due to the presence of silica impurity dissolved in the Bayer liquor. To date, the mechanism of nucleation and growth of scale occurring on heat transfer surfaces is not fully known. This paper presents the results from studies carried out to investigate the mechanism of formation of sodium aluminosilicate scale on metallic surfaces under conditions close to those in alumina refineries. It is shown that the scale formation may involve two aluminosilicate dimorphs: sodalite and cancrinite. The extent of solution desilication and scale growth was observed to be strongly temperature and carbonate impurity concentration dependent.

10:10 am BREAK

10:30 am

ASPECT OF DEEP DESILICATION WITH LIME AGENT: Qingwei Wang, Gengyou Tian Zhengzhou, Light Metals Research Institute, Shangjie District, Zhengzhou City, Henan, 450041, China; Chongyu Yang, Central South University of Technology, Changsha City, Hunan, 410083, China

The formation of hydrogarnet (CAS) with low solubility is the essence of desilication of the sodium aluminate solutions with lime addition. The mechanisms of formation of hydrogarnet were studied. It is indicated that desilication with formation CAS is in a form of SCM-type model, i.e. the unreacted core model for spherical particles of unchanging size. The reaction is controlled by diffusion through outer layer. A mathematical model of the kinetics of the process has been developed.

10:55 am

A STUDY OF DESILICATION OF ALUMINATE SOLUTION WITH HCAC : Peng Zhihong, Li Xiaobin, Department of Metallurgy, Central South University of Technology, Changsha, Hunan, 410083, China

Because the sintering process is very important in the Chinese alumina industry, it is important to understand and improve on desilication of aluminate solutions from the sintering process. This paper presents a thorough study on a new method of desilication utilizing HCAC (4CaO·Al2O3·mCO2·mH2O) both in theory and practice. The reaction thermodynamics and kinetics study reveal the reaction laws during synthesis of HCAC and desilication with HCAC. All levels of tests and applications indicate that the desilication effect of HCAC is superior to lime. It can achieve lower silica concentrations in the purified solution under the same amount of lime addition. At the same target silica concentration it can decrease the amount of lime addition about 44% and reduce the loss of alumina about 30%. The new method has achieved satisfactory results in the plant after replacing the old method of desilication with lime in Shangdong Aluminium Corporation. It sets a new path for sintering processes in China by improving alumina product quality, decreasing energy consumption, and raising the target of technology and economy.

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