Research On Kaolinite Transformation Under Calcification-Carbonation Process

According to the resource characteristics of bauxite resources in China,the low-grade bauxite accounts for more than 90%of China’s bauxite resources.The traditional Bayer process for alumina production has a higher requirement for the grade of raw material,and the tailings(red mud)will be a kind of harmful slag,entrained with a large amount of alumina and sodium alkali,causing huge economic losses and environmental pollution problems.With the development of China’s aluminum industry,the quality of resources cannot meet the needs of current industrial technology.Therefore,in order to solve the above problems,our group proposed a new process for the production of alumina by the "calcification-carbonization method",and it can be used as a promising red mud detoxification process,and effectively recovers alumina and alkali in the process.The new red mud produced,due to the lower alkali content,provide potential for subsequent use and utilization.As the core process of calcification-carbonization method,the calcification process may be affected by more factors,which may have a great influence on the subsequent process.The silicon-containing phase in low-grade bauxite is represented by kaolinite.After calcification,converted into hydrated garnet phase,the silicon saturation coefficient of hydrated garnet has a great influence on the later aluminum dissolution process.According to this study,natural kaolinite mineral was used as a representative of silicon mineral in bauxite.The dissolution process of hydrogarnet was investigated and its dissolution mechanism was analyzed.The subsequent crystallization process of hydrogarnet phase was investigated.The results are as follows:(1)The thermodynamic calculation showed that the hydrogarnet formation reaction is more difficult to proceed with the increase of the reaction temperature;during the carbonization process,the low silicon saturation coefficient carbonization decomposition process is easier to carry out,and the pressure has little effect on this process.(2)Through the kinetics of kaolinite dissolution in the calcification process,it was determined that the kaolinite was controlled by surface chemical reaction in the calcification reaction,and the activation energy reached 95.13 kJ/mol;when the reaction temperature was high,kaolinite Dissolved rapidly,at 180℃,kaolinite dissolved more than 90%after 10 minutes.Kaolinite dissolution mechanism:during the reaction,the silicon-oxygen bond and the aluminum-oxygen bond are broken.Water molecules and SiO44-ion sites combine with water molecules to form activated molecules,while aluminum ion sites combine with hydroxide to form active Al(OH)4-complexes.(3)The silicon saturation coefficient of the calcification reaction slag shows that the silicon saturation coefficient of the hydrated garnet phase increases with the reaction time;and the silicon saturation coefficient is greatly affected by the reaction temperature,and the hydrated garnet particles are formed.The hydrated garnet layer is wrapped with different silicon saturation coefficients,and the silicon saturation coefficient increases from the inside to the outside.In the calcification mechanism,the aluminum ion complex in the mother liquor and the instantaneous calcium hydroxide occur due to the addition of the mother liquor in the high calcium environment.The reaction produces water and a calcium aluminate phase.Since calcium hydroxide is present in the system in the early stage of the reaction,the calcium aluminate formed during the reaction of the system is coated on the surface of the calcium hydroxide,and along with the dissolution process of the kaolinite by the alkali.The Al(OH)4complex is continuously released and reacts with calcium hydroxide.At the same time,the content of active SiO44-ion complex in the kaolinite dissolution system gradually increases,and the structure is replaced with calcium aluminate to form water.The hydrogarnet phase is 3CaO·Al2O3·xSiO2·(6-x)H2O.(4)Carbonation and aluminum dissolution experiments show that the larger the silicon saturation coefficient is,the more unfavorable the carbonization decomposition reaction is.The reaction rate of carbonization decomposition process is obviously divided into two stages.The carbonized slag in the later stage of the reaction indicates the velvet product,which will affect the diffusion of the whole reaction.Therefore,the late reaction is affected by the diffusion of the product layer;the hydrated garnet formed by the kaolinite phase can be completely decomposed by carbonization after being reacted at 120℃ for 90 min;carbonization mechanism:carbon dioxide is dissolved in the solution to form carbonate ions,and carbonate is destroyed by hydration.The garnet phase produces hydrated calcium silicate,calcium carbonate and amorphous aluminum hydroxide.The generated water and calcium silicate are decomposed by carbonate to form the orthosilicate phase and calcium carbonate.The orthosilicate phase is easy to lose water to form silicic acid.It is thermally decomposed to finally form an amorphous silica gel.The aluminum support should be in the form of amorphous aluminum hydroxide,which is then efficiently recovered through the low alkali aluminum dissolution process.