Studies On The Foundamental And Novel Technology Of Al-Fe Separation Of High-aluminium Content Iron Ores Based On The Reduction Method

High-aluminium content iron ore is a typical refractory ore, traditional processes are found invalid to separate aluminium and iron. To utilize such resources, a high-aluminium content iron ore, taken from Indonesia, is used in the thesis. The physicochemical properties and mineralogy characters are systematically studied. Considering the close distribution relationship between iron and aluminium, the thermodynamic behaviours and conditions of ferreous, aluminiferous and silaceous oxides in the process of reduction roasting with addition of sodium salts are analysed. The reaction processes of ferreous, aluminiferous and silaceous oxides are studied by using pure materials, and the fundamental of separation behaviour is set up. The micromechanism of ferreous, aluminiferous and silaceous minerals in the process of reduction roasting with addition of sodium salts is revealed, and the action mechanism of different sodium salts on the Al-Fe separation is also illuminated, based on the studies of the microstructure of roasted pellets, smelting characters of sodium salts in reduction atmosphere and reduction dynamics of iron oxides with addition of sodium salts. On this basis, the new process of reduction roasting is developed, and the Al-Fe separation technological prototype for hig-aluminium content iron ore is formed.1. Research on the physicochemical properties of materialsIt is indicated that major iron minerals in the ore, with 48.92% TFe, 8.16% Al2O3 and 4.24% SiO2, are hematite and goethite, and gangue minerals are gibbsite, quartz and silicate-clay. There are 40.44% aluminium of total Al2O3 content exist in iron minerals in the form of isomorphism, and silicate minerals also form complex relationship with iron minerals. It is shown that the existential relationship among ferreous, aluminiferous and silaceous minerals is very complex, resulting in the difficult removal of aluminiferous and silaceous minerals from ferreous minerals by using normal physical beneficiation methods.2. Thermodynamics investigation(1) When iron oxide is reduced by carbon, iron oxide is reduced to metallic iron when temperature exceeds 968K and CO concentration exceeds 59.1%. FeO, SiO2 and Al2O3 will react with each other and form FeO·SiO2, FeO·Al2O3 and Al2O3·SiO2 in the reduction process. The lowest reduction temperature of FeO·Al2O3 is found to be 1190K by carbon reduction, while the lowest temperature is 1010K when sodium sulfate added, and the temperature is 1080K when sodium carbonate added in the reduction process.(2) It is difficult for the reaction of Al2O3 and SiO2 with Na2SO4 in air atmosphere, while sodium aluminosilicate will be formed preferentially in reducing atmosphere among Al2O3, SiO2 and sodium salts. The lowest temperature for reaction of Al2O3 and SiO2 with Na2CO3 is found to be 275K, and for Al2O3 and SiO2 with Na2SO4, the temperature is 500K. On the other hand, FeS and Na2S will be formed spontaneously in the reduction system with sodium sulfate exited, and the lowest temperature for formation of S is 690K.(3) It is known that, according to the values ofΔG, when 1000KNa2O·Al2O3·4SiO2>Na2O·2 SiO2> Na2O·SiO2>NaAlO2>2Na2O·SiO2.And in reduction system with sodium sulfate exited is as the following:Na2O·Al2O3·6SiO2>Na2O·Al2O3·4SiO2>Na2O·2SiO2>Na2O·SiO2>2N a2O·SiO2>NaAlO2.3. Research on the phase transformation of components in the reduction roasting process in the pure materials system(1) For Fe2O3-Al203-SiO2-Na2SO4 system, Fe3C, Fe1-xS and NaFeS2 are found as main iron phases in the temperatures of 600℃-700℃, and Fe and NaFeS2 will be formed when temperature is between 800℃-1000℃. Iron phases are FeO, Fe, NaFeS2 and NaFe3Si2O6 at 1150℃, and when temperature is 1250℃, iron phases are FeO, Fe, NaFeS2 and (Fe0.88Al0.12)(Al1.88Fe0.12)O4. SiO2 and Al2O3 react with Na2SO4 at 700℃and form sodium aluminosilicate.(2) For Fe2O3-Al2O3-SiO2-Na2CO3 system, iron phases are Fe3C, FeO and Fe3O4 in the temperatures of 400℃-700℃, and when temperature is between 800℃-1100℃, iron phase is metallic Fe mainly. SiO2 and Al2O3 react with each other at 550℃and forms Al2O3·SiO2, then sodium aluminosilicate will be formed simultaneously.(3) For Fe2O3-SiO2-Na2SO4 system, iron phases are Fe3C, Fe, FeS and NaFeS2 at 600℃, FeO, Fe and FeS at 740℃, and FeO, Fe, FeS and NaFeO2.35Si0.175 at 800℃. FeO and Fe are main iron phases in roasted ore when temperature is between 900℃-1000℃. FeO, Fe, FeO·SiO2 and Fe2Si04 are main iron phases at 1100℃. SiO2 reacts with FeO and Na2O at 800℃and forms NaFeO2.35Si0.175, and Na2Si205 is formed at 900℃, and iron silicate is formed at 1100℃.(4) For Fe2O3-Al2O3-Na2SO4 system, iron phases are Fe3C, Fe3O4 and NaFeS2 when temperature is between 600℃-700℃, and iron phases are Fe and NaFeS2 when temperature is between 800℃-1100℃. Al2O3 reacts with Na2SO4 at 740℃and forms sodium aluminate.4. The micromechanism of separation of ferreous, aluminiferous and silaceous minerals in the reduction roasting process(1) Some iron oxides are reduced to non-magnetic ironγ-Fe, which is unable to be recovered through magnetic separation, when high-aluminium content iron ore is reduced without sodium salts. The reduction of iron oxides gets improvement when sodium salts are added, and SiO2, Al2O3 will react with Na2CO3 or Na2SO4 to produce sodium aluminosilicate.(2) It is indicated by the results of microstructure of reduced pellets that, most ferrous minerals combine with aluminiferous and silaceous minerals tightly without addition of sodium salts in the roasting process. Sodium carbonate can be able to promote the reduction of iron oxides, and many iron grains with small size are found in the pellet, which combine with sodium aluminosilicate closely. While the boundary between iron grains and gangue minerals is clear as sodium sulfate is added. Addition of borax promotes iron grains to separate out along borderline of gangue minerals and grow big, making iron grains join together.(3) The smelting properties of sodium salts show that, S, Na2S and FeS will be formed spontaneously in reduction system with addition of sodium sulfate, which become liquid phase in local area, providing migrating passage for iron ions. While in reduction system with sodium carbonate existed, iron ions only diffuse in solid phase, and the migrating resistance is relatively big, so the boundary between iron grains and gangue minerals is not clear as sodium sulfate exited.(4) Sodium sulfate and sodium carbonate can both accelerate the reduction of high-aluminium content iron ore. While the reduction rate is slow in reduction system with sodium carbonate exited compared with sodium sulfate exited during early stage. On the other hand, sodium carbonate reacts with aluminium and silicon oxide quickly, and the formation of sodium aluminosilicate becomes move holdback of iron ions, and the diffusion barrier of iron ions increases, which goes against the separation iron grains and gangue minerals.5. New process of sodium-added reduction for Al-Fe separation of high-aluminium content iron ore(1) The process of sodium-added reduction followed by grinding-magnetic separation has been developed to separate iron and aluminium effectively. A metallic iron powder with total iron grade of 91.00% and Al2O3 content of 1.36% is obtained at the sodium sulfate dosage of 12%, the roasting temperature of 1050℃and time of 60min, the grinding fineness of 98% less than 0.074mm and the magnetic field intensity of 675mT, then the iron recovery is 91.58%, and the removal of Al2O3 is 90.35%.(2) In the sodium-added reduction process, iron oxides are transformed into metallic iron, and most aluminiferous and silaceous minerals react with sodium sulfate and form no-nmagnetic sodium aluminosilicates, which enter into non-magnetic materials during magnetic separation, then Al-Fe separation is realized successfully.(3) The metallic iron powder obtained from the sodium-added reduction process can be used for steel-making in electric furnace by further treatment. Some valuable elements such as alimiunium, silicon, sodium and chrome can be recovered from non-magnetic materials, then the utilization of the resources is realized.