Theoretical And Technological Research On Exacting Valuable Elements From High-iron Bauxite

The high-iron gibbsitic bauxite resource in Guangxi is the largest gibbsitic bauxite resource of China. The bauxite features high-iron content, low-silicon content and low-A/S content. The content of both iron and aluminum fail to meet the requirements of blast furnace smelt. Furthermore, the composition and microstructure are also very complicated, making one of the most refractory ore resources. It is of great significance to study the comprehensive utilization of high-iron bauxite since it is an important resource reserve of our country.It is economically unreasonable and technically difficult to exploit high-iron bauxite merely as iron ore or aluminum ore. Therefore, it is necessary to consider a comprehensive extracting process of aluminum, iron and silicon. In this paper, a novel process for treating high-iron bauxite is proposed. Firstly, the sulphation of aluminum and iron is achieved through sulphating roasting of bauxite with sulfuric acid or ammonium bisulfate. Then the silicon can be extracted from the sulfate solution. Most iron in sulfate solution can be extracted through jarosite process. Then, jarosite is calcined or hydrolysised in order to obtain iron byproducts. After being refined by the jarosite process, the solution is used as a raw material for preparation of sulphation product, sandy alumina and ultrafine alumina.The following studies were conducted on the procedures of the above process:1. Study on the effect of aluminum and iron extraction by two-stage roasting of the bauxite with concentrated sulphuric acid.Results showed that 83.6% of aluminum and 78.6% of iron were extracted with bauxite size of -74μm and suphuric acid 1.1 times of the theoretic value at a roasting temperature of 250℃ for 3h. In order to improve the extraction rate, the material was further roasted under higher temperature. Extraction rates of 97.4% and 97.9% respectively for aluminum and iron were obtained after roasting at 350℃ for 20min.2. Study on critical conditions and kinetics reaction process during ammonium bisulfate roasting process and the solution process.Results showed that extraction rates of over 93% and 95% respectively for aluminum and iron were obtained after roasting at 450℃ for 60min when the weight of ammonium bisulfate is 3.5 times as the weight of the bauxite. The whole reaction process between ammonium bisulfate and bauxite is controlled by the internal diffusion. The activation energies of aluminum and iron were respectively calculated to be 4.15kJ·mol-1 and 2.49kJ·mol-t at 150-250, and 7.48kJ·mol-1 and 11.64kJ·mol-1 at 300-450℃. Studies have shown that the extraction rates of both aluminum and iron could exceed 98% after dissolution at 70℃ for 60min with a liquid-solid ratio of 7:1. The dissolution of sulfates is controlled by the external diffusion.3. Study on separation of iron from ammonium sulfate solution by the jarosite process.Iron was removed by natrojarosite process for sulfate solution and by ammonioiarosite process for ammonium sulfate solution. Results showed that:for sulfuric acid process, precipitation of 94.9% and 4.9% respectively for iron and aluminum could be achieved with pH value kept from 1.5 to 2.0 for 1.5h at 95℃ and a final pH value of 3.0; for ammonium bisulfate process, precipitation of 94.7% and less than 5% respectively for iron and aluminum could be achieved with pH value kept from 1.5 to 2.0 for 2h at 95℃ and a final pH value of 3.0. The iron contents were 59.34% and 57.51% respectively for sodium jarosite hydrolyzate and ammonium jarosite hydrolyzate.4. Preparation of aluminum products from the solution purified by ammonium jarosite: sulfate, sandy alumina and ultrafine alumina.Sulfate products are prepared by crystallization process. Results showed that: crystallization rate of 84.1% for aluminum sulfate was obtained after crystallization at 20℃ for 8h, and crystallization rate of 88.3% for aluminum ammonium sulfate was obtained after crystallization at 20℃ for 6h. Aluminum sulfate and aluminum ammonium sulfate for industrial use were obtained by recrystallization.A mixture of aluminium hydroxide and ferric hydroxide was obtained from crystallized solution and sodium hydroxide. Sand alumina is prepared by Bayer Process. Studies showed that co-precipitation rates of more than 98% for both aluminum and iron were obtained at 60℃ for 40min with a final pH value of 6.5. The dissolution rate was more than 86% for aluminium hydroxide by Bayer Process.Aluminum ammonium carbonate was prepared from the reaction of the recrystallized aluminum ammonium sulfate with ammonium carbonate solution. Ultrafine alumina powders were prepared by pyrolysis of the aluminum ammonium carbonate. Results showed that a precipitation rate of 99.5% from aluminum were obtained at 45℃ with ammonium carbonate concentration of 1.75mol·L-1, aluminum ammonium sulfate concentration of 0.15mol·L-1 and the mole ratio of ammonium carbonate and aluminum ammonium sulfate at 2.75.The results of this work will be helpful for comprehensive utilization of high-iron gibbsitic bauxite.