Research On Calcification-Carbonation Process Disposing Diasporic Red Mud By Using Calcium Aluminate As Calcium Source

Bayer alumina production is merely suitable for high-grade bauxite,and red mud has the problem of high alumina and sodium oxide.There is a large amount of bauxite can not be used efficiently.At present,1.2～1.4 tonnes of red mud are produced by Bayer processes for each tonne of alumina.As the bauxite mineral resources and ore grade gradually decrease,the amount of red mud and the loss of alumina and alkali continue to increase.To solve the problems above,our team puts forward a novel "calcification-carbonation process" which can be used to utilize low-grade bauxite and red mud.There are some deficiencies in calcification-carbonation process.In the process of Al(OH)3 leaching,a lot of sodium aluminate solution with high Na2O/Al2O3 mole ratio is generated which has difficulty in the integration with current Bayer process.In this paper,lime is used to precipitate aluminum and produce calcium aluminate as a source of calcium in the calcification step and make aluminum circle in the system.The thermodynamics,process conditions and reaction mechanism of the process of aluminum precipitation,calcification process and carbonization process have been studied and analyzed.The specific research contents and results are as follows:(1)The thermodynamic calculation for the reactions of sodium aluminate and lime show that in 50～100℃ temperature range,reactions can occur spontaneously and more conducive to generate calcium aluminate hydrate in low temperatures.Thermodynamic study on the reaction between calcium aluminate and sodium silicate shows that when the temperature is between 50℃ and 300℃,hydrogarnet with higher silicon saturation coefficient tend to react first.With the increase of temperature,the formation of hydrogarnet is easier.Thermodynamic study on carbonization decomposition process shows that the hydrogarnet with lower silicon saturation coefficient is easier to decompose.(2)The single factor experiments on the calcification of red mud show that optimal results can be achieved in the following conditions:calcifying temperature is 260℃,calcium-silicate ratio is 2.5:1,liquid-solid ratio is 4:1,holding time is 60min.Na content reduces to 0.12%and Al2O3/SiO2 mass ratio reduces to 0.72.The temperature and alkali concentration has a great influence on the silicon saturation coefficient of hydrated garnet,while the ratio of calcium and silica,liquid solid ratio and reaction time has little effect on the silicon saturation coefficient.(3)The results of carbonization and Al(OH)3 leaching show that the saturation coefficient of silicon is closely related to carbonized products.With calcium aluminate as the calcium source calcification process,at a relatively high calcium alkali concentrations(around 200 g/L)and calcification temperature above 200℃,calcium slag silicon saturation coefficient is 0.8～1.0,silicon to calcium aluminate dissolved in full.Carbonization process with no side effects and did not appear too hinder the phenomenon of decomposition.Carbonation effect is good.The Al2O3/SiO2 mass ratio of Al(OH)3 leaching slag is 0.51 and sodium content is 0.3%.Dissolution efficiency of alumina reaches 73.79%.The extraction percent of sodium is 98%.Good transformation has been achieved.