Research On Phase Transformation Law And Leaching Property Of Ca, Al, Si Compounds In The Formation Of Calcium Aluminate Slag

Leaching alumina from calcium aluminate slag treating by alkaline solution which is rich in Na2CO3 is an important method to produce Al2O3. The materials of this method are iron-bearing bauxite and fly ash in general. The materials have the characteristics of high silicon and low aluminum, and have a better value of comprehensive utilization. Exploitation and utilization of these resources could relieve the shortage situation of bauxite resource to some extent.At present, almost all of the studies of leaching alumina from calcium aluminate slag were process tests and the related studies of basis theory were not deep enough. For example, the phase transformation law during the formation of calcium aluminate slag was not studied systematically. Material ratio and temperature system were not reasonable and this made the leaching rate low. In addition, the impurities would have a great effect on the self-disintegrating and alumina leaching property of calcium aluminate. But few studies were done on the action mechanism of impurities at present. Therefore, aiming at these problems, phase transformation law and leaching property and mechanism of Ca, Al, Si compounds during the formation of calcium aluminate slag were studied systematically in this paper. The main results were as follows:(1) Mathematical model of calcium aluminate multicomponent systems was established and thermodynamics was calculated based on Gibbs free energy minimization principle. The optimal C/A and A/S were 1.6～1.9 and 1.0～3.0 which were verified by theoretical calculation and experiments if the final compounds were 12CaO·7Al2O3 and y-2CaO·SiO2. The compound 2CaO·Al2O3·SiO2 which is difficult to leach Al2O3 was found when C/A<1.5, and this decreased the leaching and self-disintegrating property of slag. Increase C/A could eliminate the formation of 2CaO·Al2O3·SiO2 and the slag had a better phase composition when C/A=1.6～1.9. 3CaO·Al2O3 which is unfavourable to alumina leaching would be formed if continue increasing C/A of slag. A/S had little effect on phase composition of calcium aluminate slag. A spot of 2CaO·Al2O3·SiO2 was found when A/S=0.8. Slag had a better leaching property and self-disintegrating property when A/S=1.0～3.0. The decrease of γ-2CaO·SiO2’s content caused the decrease of slag’s self-disintegrating rate and alumina leaching rate.(2) Phase transformation law, surface morphology and crystal structure of Ca, Al, Si compounds were studied under the conditions of different smelting temperature, holding time and cooling system by the means of XRD, SEM etc. Self-disintegrating property and leaching property were also studied under these conditions.The self-disintegrating and leaching property of slag was worse when smelting temperature was lower than 1450℃, because the reaction was controlled by solid diffusion, the reaction speed was slow and a certain amount of 2CaO·Al2O3·SiO2 was formed in slag. Liquid phase emerged in slag and the reaction was complete, the main phase were 12CaO·7Al2O3 andγ-2CaO·SiO2, and had a better self-disintegrating and leaching property when smelting temperature was higher than 1450℃. The smelting temperature had no effect on self-disintegrating and leaching property when it was higher than 1500℃.Holding time had no effect on phase composition. But the crystal size of 12CaO·7Al2O3 and crystallization integrity increased with the prolongation of time. This enhanced the stability of 12CaO·7Al2O3, decreased the reaction ability of CO32- and H2O molecule to penetrate the lattice of 12CaO·7Al2O3, thus decreased the alumina leaching property of slag to some extent. The phase composition, self-disintegrating property and leaching property of calcium aluminate slag were better when cooling rate was below 20℃·min-1.2CaO·Al2O3·SiO2 and 3CaO·Al2O3 would be formed and caused a great downtrend of self-disintegrating property and leaching property of calcium aluminate slag if continue accelerated the cooling rate.(3) The effect of MgO on Al2O3 occurrence status, surface morphology, crystal structure, self-disintegrating property and leaching property of calcium aluminate slag were studied. The first action of MgO to calcium aluminate slag was the formation of 20CaO·13Al2O3·3MgO·3SiO2 which is difficult to leach alumina, the second was the solid solution of MgO in 12CaO·7Al2O3 crystal. And these reasons caused the leaching rate of calcium aluminate slag decreased obviously. The saturated solid solubility of MgO in 12CaO·7Al2O3 crystal was 1%. Mg2+ whose ion radius is bigger replaced Al3+ whose ion radius is smaller in unit cell and this enlarged the unit cell and d value. The occurrence status of A12O3 were 12CaO·7Al2O3 and 20CaO·13Al2O3·3MgO·3SiO2 which was accumulated in the form of lath structure when the content of MgO was lower than 2.0%. The occurrence status of Al2O3 was mainly 20CaO·13Al2O3·3MgO·3SiO2 when the content of MgO was higher than 2.0%.20CaO·13Al2O3·3MgO·3SiO2 could react with Na2CO3 solution, but the leaching rate was far lower than that of 12CaO·7Al2O3. MgO had little effect on the self-disintegrating property of calcium aluminate slag. The self-disintegrating property was above 92% even if MgO content reached 4%.Increase C/A could reduce the side effects of MgO to calcium aluminate slag, but could not eliminate completely. The effect of additional CaO were decompounding 20CaO·13Al2O3·3MgO·3SiO2 and stabling Ca and Al compounds. The optimal C/A increased with the increasing of MgO content. MgO crystallized independently in the form of periclase, and didn’t find quaternary compound which is difficult to leach. The side effects of MgO would also be decreased if there was Na2O in slag. 20CaO·13Al2O3·3MgO·3SiO2 had been disappeared in slag when the content of Na2O reached 4%.(4) Effect and action mechanism of TiO2、Na2O and FeO on phase composition, self-disintegrating property and Al2O3 leaching property of calcium aluminate slag were studied. TiO2 would reacted with CaO in slag and formed CaO·TiO2, and influenced the alumina leaching property of slag. The influence of TiO2 on calcium aluminate slag could be eliminate when CaO which was used to form CaO·TiO2 was added, and TiO2 would have an effect of mineralization and promote the formation of calcium and aluminium compounds and then improve the alumina leaching property of calcium aluminate slag. Lattice distortion and the weakness of bond energy which is asymmetry in Molecular were caused by the solid solution of Na2O into12CaO·7Al2O3. And this made Na2CO3 and H2O easy to penetrate into 12CaO·7Al2O3 crystal, and improved the reaction speed and degree. So the existence of Na2O could improve the leaching property of calcium aluminate slag. But the self-disintegrating property of slag would be decreased because a great amount of (3-2CaO·SiO2 was formed when the content of Na2O was higher than 2.5%. Considering from the property of calcium aluminate slag only, the largest allowable content of Na2O was 2.0～2.5% in slag and 1.2～1.5% in materials. Quaternary compound 6CaO·4Al2O3·FeO·SiO2 would be formed and decreased the leaching property of slag obviously when the content of FeO was higher than 0.5%. As a result, the FeO in the slag should be reduced into iron metal as sufficient as possible in blast furnace smelting.(5)Actual calcium aluminate slag was optimized based on the experiments mentioned above and verified by experiment. Surface morphology and composition of slag before and after leaching were studied by the means of SEM and EDS. Sintering ore with high basicity was obtained successfully when carbon content was 5.0%, white lime content was 5.0%, and water content was 9.0%. The reduction property of sintering ore with high basicity was better, the metallization rate was higher than 90%, and could satisfy to the need of smelting process in blast furnace. The smelting result indicated that the optimal C/A of actual calcium aluminate slag was 2.2, and in this C/A the alumina leaching rate could reach 84.29%. Self-disintegrating process was generated from inner of slag and had no law. Most of the large particles were 12CaO·7Al2O3, and most of the small particles were y-2Ca0·SiO2. There was agglomeration phenomenon in the slag after leaching.