Basic Research On Treating Low-grade Bauxite And Red Mud By Lime-carbonation Process

China is one of the largest producers of alumina, but the reserves of high-quality bauxite resources is not much. At present, most of the alumina are produced from the mid-and low-grade bauxite with A/S between3and5in Chinese alumina plant. Due to the high content silicon in the mineral, both the amount of red mud and the loss of alumina will be quite large by Bayer process. As the increasing capacity of our country’s alumina production and the continuous improvement of production technology, how to use such resources efficiently becomes the main research direction.Aiming at these problems, we propose the lime-carbonation process, including lime-sodium carbonated carbonization process and lime-carbon dioxide carbonization process, to treat low-grade bauxite and Bayer red mud. The effectiveness of extracting alumina in the new processes has been investigated, as well as the key techniques in processes, such as bauxite transformation, leaching in sodium carbonate solution, CO2treatment and dissolution in NaOH solution at low temperature.First, the thermodynamics of main reactions of bauxite transformation and different carbonization processes have been researched. The calculated results show that with the increase of the reaction temperature, the reaction of sodium carbonate and hydrogarnet can be more favorable, but opposite for the reaction of carbon dioxide and hydrogarnet. The increase of the pressure is favorable to the reaction of carbon dioxide and hydrogarnet.Second, the experiments of lime-sodium carbonated carbonization dealing with bauxite were exerted. The proper process parameters for the process of adding lime to synthetize hydrogarnet and leaching in sodium carbonate solution have been obtained. But it is difficult to get the red mud with low value of A/S is this process. Under the most proper condition, the A/S of the red mud can just goes down to1.07, and the content of sodium is about2%.Third, the effect of the main technical aspects of lime-carbon dioxide process on alumina extraction is studied by the way of using pure materials to synthetize hydrogarnet. The results show that the proper conditions of synthetizing hydrogamet are the temperature240℃, theoretical lime addition and holding time of30min. The produced hydrogarnet, is decomposed by carbon dioxide at120℃, under partial pressure of carbon dioxide of1.2MPa for2hours, with liquid-solid ratio of10to1and400r/min stirring speed. The carbonized residue is dissolved in NaOH solution under the conditions of temperature60℃, liquid-solid ratio10:1, reaction time90minutes. The extraction efficiency of alumina could reach71.8%. A/S and sodium content of the final product can go down to0.54and0.76%, respectively, which are much lower than the existing red mud and can be used as the materials in cement and building industry.Finally, the low-grade bauxite and red mud were dealt with by the lime-carbon dioxide carbonization process. The results show that, A/S of the slag of low-grade bauxite is as low as0.44, the dissolution rate of aluminum climbs up to86.6%, content of sodium is only0.12%; A/S of the slag of red mud declines to0.82, the content of sodium is0.73%, and the technical indicators are superior to the existing Bayer production process.