Research On Leaching Caracteristics And Mechanism Of Alumina In The System Of 12CaO·7Al₂O₃ And γ-2CaO·SiO₂

Bauxite and iron ore resources are short in China, and this problem become increasingly serious. The reasonable and comprehensive utilization of high-iron bauxite will relief the problem greatly. The reasonable process to deal with the high-iron bauxite is "sintering-blast furnace smelting-alumina leaching"process, but there are some problems need to be improved, such as lowing alumina leaching ratio, long leaching time and high concentration of sodium carbonate. So this thesis studied the main phases of calcium aluminate slag,12CaO·7Al2O3 and y-2CaO·SiO2. The results are as follows.Synthesizing and leaching property of 12CaO·7Al2O3 were studied, and the results showed that 12CaO·7Al2O3 which doesn’t have other phases but has high leaching alumina ratio can be obtained under 1773K for 120min from the mixture of CaO:Al2O3=12:7(mol ratio). The main factor which affect alumina leaching ratio is leaching temperature and the next two are leaching time and sodium carbonate concentration, what’s more, the degree of the two factors’ effects are similar. The optimum conditions for leaching 12CaO·7Al2O3 are leaching temperature 363K, leaching time 120min and sodium carbonate concentration 120 g·L-1, the alumina leaching ratio that obtained under these conditions is 98.69%.Micro-structure and surface morphology of 12CaO·7Al2O3 synthesized under 1773K during more than 60min were researched. Bragg equation and calculating formula of lattice constant in cubic system were used to calculate the lattice constants. Related formula was used to calculate crystal’s preferred orientation factor. Scherrer formula was used to calculate the average grain size. SEM was used to study crystal’s surface morphology. The results are: lattice constants increase, preferred orientation factors of 420 crystal face decrease and surfaces appear more and bigger lacunal structure as holding time become longer. But the average grain sizes don’t change with holding time.The self-disintegrating property and stability ofγ-2CaO·SiO2 are important to leaching property of calcium aluminate slag, so the synthesizing and stability of it were studied. The results showed thatγ-2CaO·SiO2 which has better self-disintegrating property can be obtained under 1773K for more than 60min from mixture of CaO:SiO2=2:1(mol ratio). y-2CaO·SiO2 will react in sodium aluminate solution which is rich in sodium carbonate and the relationship between decomposition ratio and sodium carbonate concentration, temperature, time and alumina concentration was obtained.The mixture of 12CaO·7Al2O3 and y-2CaO·SiO2 were obtained from CaO-Al2O3-SiO2 system. The optimal conditions for synthesizing are CaO:Al2O3:SiO2=2.38:1:0.77(mol ratio), synthesizing temperature 1773K, tapping temperature 373K and holding time 30min. The product doesn’t have other phases except 12CaO·7Al2O3 and y-2CaO·SiO2, and this mixture is called CAS sinter for short.12CaO·7Al2O3 and y-2CaO·SiO2 in CAS sinter were proved to be self-existent in basically by comparison of mechanically mixed mixture of 12CaO·7Al2O3 and y-2CaO·SiO2 and CAS sinter with the assistant of EDS. Relationship equation between alumina leaching ratio and leaching temperature, sodium carbonate concentration, leaching time and solid to liquid ratio was obtained by quadratic regression orthogonal method to study the leaching property of CAS sinter. Secondary reaction mechanism was also preliminary discussed by the phase research of slag after leaching. The research showed that the products are hydro-garnet and sodium-silicon residue when leaching time is short, leaching temperature and Na2Oc are low and that the hydro-garnet will transform to sodium-silicon residue and hydrate-calcium-silicate as leaching time become longer, temperature and Na2Oc become higher.Ultrasonic wave and two-stage leaching method were used to enhance leaching property of 12CaO·7Al2O3 and CAS sinter, and the results showed that both the two method are effective. When leaching time in the two-stage leaching method are 15min and 20min respectively and Na2Oc 60 g·L-1, the alumina leaching ratio of both 12CaO·7Al2O3 and CAS sinter can reach above 95% and alumina concentration in solution after leaching will be increased by about 7g·L-1and 16g·L-1. Na2Oc in the solution to leach 12CaO·7Al2O3 can be reduce by 40 g·L-1 and the temperature can be reduced by 10K with the action of single frequency ultrasonic wave, but 60 g·L-1 and 40K will be reduced by under double frequency collaborative ultrasonic wave when alumina leaching ratio is the same with the traditional method.Leaching dynamics of 12CaO·7Al2O3 and CAS sinter both with and without ultrasonic wave were researched. Macro-kinetics equation and apparent active energy were obtained. As to CAS sinter’s leaching dynamics both with and without ultrasonic wave, frequency factors, diffusion equations and effective diffusion coefficients were also obtained. The results showed that 12CaO·7Al2O3’s leaching is controlled by surface chemical reaction, CAS sinter’s leaching both with and without ultrasonic wave are controlled by solid film inter diffusion.The conclusion was validated by calculation of apparent active energy and derivation of leaching ratio and temperature.The mechanism of enhancement of ultrasonic wave was concluded with the assistant of Raman spectra, conductivity research before and after ultrasonic wave and surface appearance (SEM) both with and without ultrasonic wave. The mechanism can be described as follows: irons’ movement and CO32-’s stretching vibration and stretch vibration are enhanced, mechanical stirring was strengthened so that agglomeration of grains was hindered to promote transformation between liquid and solid, effective diffusion coefficient was increased and apparent active energy was reduced to facilitate solid diffusion in dynamics. Besides, decomposing of y-2CaO·SiO2 in CAS sinter is inhibited by little degree.