| Bayer process is currently the main alumina production method both in domestic and abroad.With the rapid increase of China's alumina production,the bauxite resources,which can be used for alumina production,particularly high-grade bauxite,have been on the verge of exhaustion and were difficult to meet the quality requirement for Bayer process.At the same time,a large number of alumina industrial highly alkaline red mud was discharged and stocked,due to the lack of effective large-scale and low-cost process,causing a lot of land taken up and serious environmental hazards.During the Bayer process,the insoluble hydrated aluminum silicate appears when the silica-contain phase in bauxite was dissolved,causing the alkaline nature of the red mud.When the grade of bauxite becomes lower and the silica content increases,the alumina and caustic loss will increase.It causes this kind of bauxite suitable no more for the Bayer process.Therefore,we propose a novel calcification-carbonation process for treating the low-grade bauxite and red mud.This process changes the combination of sodium,alumina and silica in Bayer red mud.It can be used in alumina production from low-grade bauxite,which also contributes the fact that low alkali red mud can be applied to produce cement and other building materials.For this novel process,both the two key processes,calcification transformation and carbonation decomposition,were focused in this article.It aims at revealing the mechanism of the reaction process,providing a reference for the development of alumina production and treatment of Bayer red mud.(1)The standard Gibbs free energy of calcified product hydrogarnet was calculated based on the homologous linear rule and the thermodynamic behavior of bauxite and red mud was analyzed in the calcification-carbonization process.The results show that the calcified products tend to form hydrogarnet with larger saturated coefficient of silicon(x)from 323 K to 573 K.When x is greater than 0.5,the positive reaction goes;When x is greater than 1.5,the elevated temperature is beneficial to the calcification reaction.When the temperature ranges from 298 K to 473 K,all the calcification reactions can conduct and the increase of temperature is of no advantage to the calcification process.With the increase of x from 323 K to 573 K,the hydro garnet tends to be stable and when x is equal or greater than 2.5,the carbonization can't make hydrogarnet decomposed forward.With the increase of CO2 pressure in the carbonation process,the Gibbs free energy of carbonization reaction decreases slightly.Therefore,CO2 pressure has a little effect on the carbonization process in terms of thermodynamics.(2)The calcification transformation and carbonation reaction kinetics were researched with pure synthetic hydrated calcium aluminate and hydrogarnet as the raw materials.The unreacted layer model can be used to describe the process of calcification due to the complex ions substitute hydrated calcium aluminate tetrahedral.The results indicate that the limiting process of the reaction is chemical reaction,when the temperature is below 140 ?.However,the limiting process of the reaction changes into both chemical reaction and diffusion layer of a solid product controlled,when the temperature is higher than 140 ?.The latter controlling process will increase when the temperature increases.The hydro garnets were synthesized at different temperatures,where the x is variety from 0.27,0.36,0.70 and 0.73.The kinetics of carbonation process of decomposition for the hydrogarnet can be divided into two stages,of which the cut-off point is located between the carbonation time 20?30min,and the apparent reaction rate of the first stage was significantly higher than that of the second stage.The kinetic mechanism functions R3 and D3 can be applied to describe the two stages.The apparent activation energies of the four hydrogarnet in 1st stage and 2nd stage are 41.96?81.64 k7/mol and 14.80?34.84kJ/mol,respectively.The carbonation reaction rate in the 1st stage was controlled by chemical reaction,and the 2nd stage is controlled by both chemical reactions and diffusion processes.(3)The experimental results processing gibbsite bauxite with calcification-carbonation method show that under the calcification conditions(the additive CaO 50%,temperature 180 ?,initial concentration of sodium aluminate solution Na2Ok 140 g/L,ax 3.1)and the carbonation conditions(temperature 100 ?,CO2 pressure 1.0 MPa,liquid-solid ratio 7:1,time 120 min),the recovery rate of alumina reaches 81.2%and Na2O content decreases to 0.57%after two carbonation trials.The main phases of final red mud products are calcite,aragonite calcium carbonate and hematite.The silicon phase is in amorphous form for that no diffraction peaks were found in the XRD patterns.(4)The Bayer red mud after digested gibbsite was processed,and the results showed that when the calcification reaction was conducted at the temperature of 120 ?,the CaO-to-SiO2 mass ratio of 1.5,the reaction time of 120 min,the liquid-to-so lid ratio of 4:1,and carbonation reaction was conducted at the temperature of 80 ?,the CO2 pressure of 1.2 MPa,the liquid-to-so lid ratio of 5:1 and the reaction time of 120 min,75.66%alumina in the Bayer red mud can be recovered.The Na2O content in the processed red mud were all less than 0.5 wt%.which met the requirement of alkali content of raw material in cement industry.(5)The 200 L scale additional experiments were conducted in the internally designed novel folding tube agitation reactor and fluidic carbonation reactor.The results showed that the Al2O3-to-SiO2 mass ratio in twice digestion residue was 0.89 and the Na2O content was 1.01%,after the gibbsite was processed with the Al2O3-to-SiO2 mass ratio of 4.24.For the Bayer red mud,the Al2O3-to-SiO2 mass ratio and Na2O content were decreased from 1.47 and 10.27 wt%to 0.76 and 0.35%,respectively.Through the process,48.3%alumina and 96.6%alkali were recovered,respectively. |
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