| High iron content gibbsite-type bauxite, located in Guigang, Binyang and Hengxian of Guangxi is the largest gibbsite-type bauxite as known in China. According to chemical composition and properties of this ore, it can be identified as a gibbsite-type bauxite of high iron, high silicon, low aluminum and low A/S ratio. It is economically unviable to develop the resources as a single component mineral because total iron grade and alumina content are less than the requirement for an industrial utilization. Thus, it is necessary to develop efficient technologies in order to utilize aluminiferous minerals and ferrous minerals comprehensively.In order to resolve this problem, several different processes have been studied aimed at separation and utilization of aluminum and iron. Three of them, including beneficiation followed by smelting process, prior extraction alumina process and prior smelting iron process, are recommended finally. However, these processes have not been applied to industry on account of economic or technologic reasons. Therefore, the resoures leaves undevelop-ed until now.In recent years, many research and development have been carried out by researchers from Central South University on comprehensive utilization of the high iron content gibbsite-type bauxite. A novel process named "process of reduction roasting with addition of sodium salts and magnetic separation" has been proposed, which can effectively realize the separation of iron and aluminum in this ore. By the process, a metallic iron concentrate with more than90%total iron grade and a non-magnetic product with40%alumina content were obtained. Metallic iron concentrate can be used as steelmaking burden, and alumina can be extracted from the high alumina content non-magnetic product further in which alumina exists mainly as sodium aluminosilicate. The process has several significant advantages, including effective separation of aluminum and iron, high iron recovery, low reduction temperature due to using bituminous coal as a reductant, short and simple flowsheet, and good prospect of industrial application. Therefore, the research on extraction of alumina from high alumina content non-magnetic product based on the process of reduction roasting with addition of sodium salts and magnetic separation is of great significance to development and utilization of Guangxi high iron content gibbsite-type bauxite.In the thesis, the fundamental on extraction of alumina from Guangxi high iron content gibbsite-type bauxite was investigated. Following conclusions are achieved:(1) The difficulty in separation of iron, alumina and silicon was explained mineralogically by systematical investigation on physicochemical properties of Guangxi high iron content gibbsite-type bauxite. The total iron grade of the bauxite reaches31.22%, much higher than general bauxite. Al2O3and SiO2contents are26.35%and8.32%respectively. It is a bauxite of high iron, high silicon, low alumina, and low A/S ratio. Major minerals in the ore are goethite, gibbsite, hematite and Kaolinite. Aluminum is scattered in gibbsite, goethite, hematite and silicates. Aluminum in goethite and hematite exists in the form of isomorphism. Nearly100%of total iron exists in goethite and hematite. Silicon presents mainly in silicates. Structurally, ferrous minerals occur inside oolite-bean-shaped, bean-shaped, renal-shaped and bulk particles. Aluminiferous minerals present on the surfaces of iron minerals particles, within the cement or in the cracks among iron minerals particles. Silaceous minerals dispersed in various parts of the ore. What is more, aluminiferous, ferrous and silaceous minerals are fine in size, conjoint and substituted with each other. In conclusion, the existential relationship among ferreous, aluminiferous and silaceous minerals is complex. Therefore, physical beneficiation is not feasible due to extremely difficulty in liberation of minerals.(2) Dissolution characteristics in alkali and acid solution of pure sodium aluminosilicate artificially synthesized in laboratory were studied. Dissolubility in sodium aluminate solution indicates that it is technically feasible to extract alumina from sodium aluminosilicate by Bayer process. At the temperature of200℃, caustic sodium concentration of220g/L caustic ratio of3, and digestion time of60min, the relative leaching ratio of Al2O3reached89.51%. However, due to the low A/S ratio of sodium aluminosilicate, absolute leaching ratio of Al2O3was only about60%. Therefore, it is economically unfeasible to extract alumina from sodium aluminosilicate of low A/S ratio by Bayer process. Results of dissolution experiments in sulfuric acid solution show that the components of sodium aluminosilicate can be dissolved into solution easily, which makes it possible for extraction and recovery of them. At temperature of30℃, sulfuric acid concentration of30%, liquid-solid ratio of10, leaching time of30min, leaching ratios of Al2O3, SiO2and Na2O were all over90%. On condition of higher sulfuric acid concentration, or higher temperature, or longer leaching time, SiO2dissolved in the acid leaching solution was polymerized to be silica gel and removed from acid leaching solution by solid-liquid separation.(3) Because of good dissolubility of sodium aluminosilicate in sulfuric acid solution, separation of sodium, aluminum and silicon in acid leaching solution was studied further, and a process of "aging-neutralizing" was proposed. Removal ratio of SiO2reached99.56%on the optimum condition of temperature80℃and aging time30min. Extraction ratio of Al2O3from acid leaching solution reached98.63%at pH of5.5in neutralization experiments using NaOH as neutralizing agent. Content of Al2O3and Na2O in aluminum hydroxide precipitation abtained were33.29%and0.16%, respectively.(4) Aimed at alumina extraction, technical study was carried out by using Guangxi high iron content gibbsite-type bauxite as raw material, and a novel process of "reduction roasting with addition of sodium salts magnetic separation-sulfuric acid leaching-aging-neutralizing" was proposed. At Na2CO3dosage of25%, Na2SO4dosage of15%, Na2B4O7·10H2O dosage of2.5%, roasting temperature of1050℃, time of60min, grinding time of15min, grinding fineness of96%-97%less than0.074mm and magnetic field intensity of1000Gs, a metallic iron concentrate with total iron grade of92.71%and a non-magnetic product with39.29%alumina content were obtained. By magnetic separation,91.17%of Fe in bauxite was separated into metallic iron concentrate, and92.95%of Al2O3into non-magnetic product. Dissolution characteristic of high alumina content non-magnetic product in sulfuric acid solution was studied. According to the optimization of leaching parameters, at temperature of30℃, sulfuric acid concentration of30%, liquid-solid ratio of10, leaching time of30min, leaching ratios of Al2O3, SiO2, Na2O and Fe reached91.72%,94.37%,96.81%and94.92%, respectively. By aging, at temperature of80℃and aging time of30min, the removal ratio of SiO2reached98.82%. Amorphous silica gel was obtained, the SiO2and impurities content in which were53.78%and0.63%. It can be used to produce white carbon black by calcinating. Using NaOH as neutralizing agent, extraction ratio of Al2O3from acid leaching solution reached95.38%at pH of5.5. Content of Al2O3and impurities of the aluminum hydroxide precipitation were34.94%and0.73%, respectively. |
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