Research On Calcification-Carbonation Process For Red Mud

With the high-speed development of Chinese alumina production,domestic high quality bauxite resources are difficult to meet the need of alumina production.The use of low grade bauxite not only declines alumina leaching rate,but also outputs a la ge number of high alkali red mud.The high basicity of red mud makes it difficult to be used comprehensively.The storage of red mud not only takes up a lot of land,but also consumes a large amount of manpower and material resources to manage and maintain the red mud yard,witch has caused a series of economic and environmental problems.Under this condition,"Calcification-Carbonization Method" was put forward to deal with Bayer red mud by Northeastern University.This article mainly investigated thermodynamics and kinetics of calcification and carbonization process.Meanwhile the effect of calcification,carbonization process parameters on dispose result for Bayer red mud was studied.On the basis of above research,the various mechanism of treatment process for red mud forming at different conditions has been explored.The main research content is as follows:(1)The thermodynamic analysis results show that,In the temperature range of 298-473K,the calcification and carbonization reaction for red mud will happen.The free energy of calcification and carbonation reaction will rise with the increase of temperature,the free energy of the carbonation reaction is slightly lower along with the increase of CO2 pressure.(2)The experimental study of gibbsite’s red mud shows that:The alkali content in red mud is as high as 10.15wt.%,alumina silicon ratio of red mud is 0.60,main phase in red mud are hematite,sodium aluminosilicate hydrate and nordstrandite.The calcification process for red mud is controled hybridly by diffusion and chemical reaction in the differential thermal curve of rising and decling stage,but the activation energy is significant lower in declining stage,which shows that high temperature is advantageous to the calcification process.The red mud was treated at following conditions:calcifying temperature was 120 ℃,calcium silicon was 4.2:1,liquid solid ratio was 4:1,holding time was 1h;carbonizing temperature was 120℃,CO2 pressure was 1.0MPa,liquid solid ratio was 10:1,holding time was 2h;leaching temperature was 60℃,Na2O concentration was 100g·L-1,liquid solid ratio was 10:1,holding time was 90 minutes.After treated five times at above conditions,a new structure red mud was obtained,whose alumina silicon ratio can drop to 0.22,alkali content can be reduced to 0.18wt.%.(3)The experimental study of diaspore’s red mud shows that:The alkali content in red mud is 8.44wt.%,alumina silicon ratio of red mud is 1.22,main phase in red mud are calcium nepheline,hydrogrossular,hydroandradite and hematite.The red mud was treated at following conditions:calcifying temperature was 240℃,lime content was 30wt.%,liquid solid ratio was 4:1,holding time was 1h;carbonizing temperature was 120℃ CO2 pressure was 1.0MPa,liquid solid ratio was 10:1,holding time was 2h;leaching temperature was 60℃ Na2O concentration was 100g·L-1,liquid solid ratio was 12:1,holding time was 90 minutes.After treated five times at above conditions,a new structre red mud was obtained,whose alumina silicon ratio can drop to 0.68,alkali content can be reduced to 0.5wt.%.(4)The reaction mechanism of red mud in calcifying and carbonizing process was analyzed by hydroandradite systhesis experiment with pure substance,the results show that:High temperature is advantageous to the synthesis of hydroandradite,which can only generate when the temperature is higher than 200 ℃.While high temperature is not advantageous to the decomposition of hydroandradite,pressure of CO2 has little e:ffect on carbonizing process.The synthetic hydroandradite decomposed thoroughly in carbonizing process and did not affect the subsequent process.High temperature is not advantageous to the synthesis and decomposition of hydrogrossular,pressure of CO2 has little effect on carbonizing process.With temperature rising,the chemical stability of the synthetic hydrogrossular was improved and content of residual hydrogrossular in leaching residue increased,which shows that the high stability of hydrogrossular synthesis at high temperature should accounts for the low alumina recovery efficiency of diaspore’s red mud.