| As alumina can be separated from silica thoroughly in the sintering process and thus the extraction of alumina can reach 100% theoretically, the sintering process is a very important technology to deal with the medium or even low grade diasporic bauxite in China alumina industry. The sintering operation is the key to the formation of sodium aluminate and calcium silicate, The energy consumption and production cost is high by the traditional sintering process with high mole ratio of calcium oxide to silica and low alumina content. As for the-80% of Chinese diasporic bauxite with high silica, the objective of saving energy greatly and reducing production cost significantly may come into true by controlling the species of calcium silicate, strengthening the stability of calcium silicate, increasing the content of alumina in sinter, So the studies on the reaction mechanism of components and the influencing law among the components in the sintering process are of great importance to improve the sintering technology with low molar ratio of calcium oxide to silica ([C]/[S]).By means of thermodynamic calculation, thermal analysis and XRD analysis, the reaction rule of alumina, silica and iron oxide, as well as the influencing principle of the components were systematically investigated in this work. And the influence of factors on the sintering process was assessed by the extraction of alumina, sodium oxide and silica. Then the formula for the raw material slurry and the sintering technology was proposed. Finally, the proposed sintering technology was optimized based on the results of industrial scale experiments and industrial alumina production. The main conclusions were listed as follows.1. Thermodynamic calculation shows that Na2O-Al2O3 and Na2O·Al203-2Si02, as well as Na2O·Fe203 and 2CaO·Fe2O3, can be formed at elevated temperatures; that calcium sodium silicate can be formed by the reaction of sodium ferrite and calcium silicate, while calcium iron aluminate can not be formed by the reaction of calcium ferrite and sodium aluminate; that all kinds of calcium silicates may be formed in CaO-SiO2 system or Al2O3-CaO-SiO2 system when the lime is sufficient, and [C]/[S] and sintering temperature affect the formation of calcium silicate species; and that 3CaO·2Si02 is the most possible species in CaO-SiO2 system when [C]/[S] is 1.5. In contrast, in addition to 2CaO·Si02,3CaO·2SiO2 may be formed in Na20-Al2O3-CaO-SiO2 system with [C]/[S] of 1.5, and 2CaO·Si02 and 3CaO·2Si02 can not readily react with Na2O·Al203.2. All the reaction kinetic equations of components in Na2CO3-Al2O3, Na2CO3-AlOOH,Na2CO3-Al2O3·2SiO2, Na2CO3-Al2O3-2SiO2·2H2O, Na2CO3-Fe2O3, Na2O-Al2O3-Fe2O3 and Na2O·Al2O3·2SiO2-CaO system correspond with the Jander model, being controlled by two-dimension or three- dimension diffusion, and can be expressed as [1-(1-x)1/3]2=Ae-Ea/RTt or 1-(2/3)x-(1-x)2/3=Ae-Ea/RTt. The apparent activation energy of the reaction of Na2CO3 and Al2O3(268.0kJ/mol) is larger than that of the reaction of Na2CO3 and Fe2O3 (99.20kJ/mol) at the temperature range from 650?to 970?, and the activation energy of the reaction of sodium aluminosilicate and calcium oxide, where calcium silicate can be formed, is 189.29kJ/mol at the temperature range from 780?to 800?. The activation energy of reaction between Na2O·Fe203 and AI2O3 is 246.37kJ/mol. The experiments prove the models above are reasonable.3. The conclusions obtained by the experiment results can be made as following: the reaction rate of calcium oxide can be improved by reducing [C]/[S], increasing the sintering temperature and sintering duration time, and adding mineralizer in two-element systems or mutil-element systems, favoring the formation of calcium silicate with low [C]/[S]. The reaction rate of Na2CO3 with Fe2O3 is larger than that of Na2CO3 with Al2O3 at relatively low sintering temperature. The reaction rate of Na2CO3 and Al2O3 can be accelerated with the present of Fe2O3 at elevated sintering temperature. Based on the experimental research results, the favorable formula for the charge and the detailed sintering technological parameters being suitable for the sintering operations of diasporic bauxite with high silica are determined:molar ratio of Na2O to the sum of A12O3 and Fe2O3 ([N]/([A]+[F])) of 1, [C]/[S] of 1.5, molar ratio of Fe2O3 to Al2O3 ([F]/[A]) of about 0.1, sintering temperature at the range of 1250??1300?, sintering duration time of approximate 30min. The leaching rate of Al2O3 and Na2O exceeds 95% and 98%, respectively, under the above sintering conditions. This technology applies to sintering process of high-silica diasporic bauxite.4. The results of industrial experiments are consistent with those of the laboratorial experiments. The optimized technological parameters are as follows:1) the preparation of raw material slurry:[N]/([A]+[F]) 1.02±0.04, [C]/[S] 1.5±0.05, mass ratio of alumina to silica(A/S) 7.5-7.8, H2O<40%; (2) the sinter:A/S 7.0-7.4, [N]/([A]+[F]) 0.9-0.98, [C]/[S] 1.3±0.05, bulk density 0.9-1.15,?Astandard>95%,?N standard>97%.5. In the industrial applications, the measures to form kiln skin are proposed by increasing the sintering temperature, decreasing the sintering duration time and maintaining the kiln skin at upper temperature limit. Maintaining stability of the sintering operation and improving productivity of the sintering rotary kiln need to control the temperature and regulate the temperature system in the kiln, by regulating the amount of the first and second wind to form long-flame operation, controlling kiln tail temperature, modifying the coal-spraying gun positon to maintain the distance from the flame center to the charge of 1.5 meter.6. After applying the new sintering technology in Zhongzhou Branch, Chalco, comparing with the traditional sintering process, there was a reduction of 39.9% and an increase of 70% in the technological energy consumption and the production capacity of single rotary kiln, respectively. In addition, the lifespan of the liner in rotary kiln increased from 100 days to 200 days. This means that the enterprise can greatly reduce the consumption of raw materials of coal and limestones, and thus obviously reduce the emission of CO2 and the discharge of red mud to the environment with the same annual alumina yields. |
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