Selective Enrichment And Phase Separation Of Phosphate In Steelmaking Slags

Although the high-grade phosphate resource is in a short supply all over the word, a large amount of steelmaking slags containing not negligible content of phosphate has been generated and accumulated with the rapid expansion of steelnaking industry in China. Recycling steelmaking slags has become one of the effective method to reuse phosphate in steelmaking slags according to the existence state of phosphate. Extracting phosphate from steelmaking slags has been investigated in this dissertation using the principles of selective enrichment and phase separation through considering selective crystallization, grain growth and phase separation. The aim of separation of phosphates from iron-enriched phase has been primarily realized based on this study. The main conclusions can be summarized as follows.In this dissertation, the phosphate enrichment behavior has been experimentally investigated in CaO-SiO₂-FeO-Fe₂O₃-P₂O₅ steelmaking slags over a range of binary basicity B from 1.0 to 3.5. The reaction ability of structural units in the slags has been represented the mass action concentration N_t from the developed IMCT-N_i, model based on the ion and molecule coexistence theory ?IMCT?. The enrichment possibility N_ci-cj and enrichment degree R_ci-cj of solid solutions containing P₂O₅ were first defined and verified to be valid from the experimental results. The results show that P₂O₅ component can be easily bonded by CaO to form tricalcium phosphate 3CaO-P₂O₅ ?C₃P?, and the formed C₃P can react with the produced dicalcium silicate 2CaOSiO₂ ?C₂S? to generate solid solution 2CaOSiO₂-3CaO-P₂O₅ ?C₂S-C₃P? under the fixed cooling conditions. The maximum value of the defined enrichment degree R_C2S-C3P P of solid solution C₂S-C₃P is obtained about 85% under conditions of binary basicity as 2.5 and the mass percentage ratio ?%Fe,O?/?%CaO? as 0.955 at the fixed cooling conditions in CaO-SiO₂-FeO-Fe₂O₃-P₂O₅ steelmaking slags with higher binary basicity ?2.0<B<3.5?. However, lower binary basicity ?1.0<B<2.0? can affectively promote phosphate enrichment in the quenched slag samples. The asymmetrically inverse V-shape relationship of phosphate enrichment against binary basicity B can be correlated for the slags under the applied two-stage cooling condition. The maximum content of P₂O₅ in C₂S-C₃P solid solution can reach about 30.0% when the binary basicity B is controlled at 1.3.Moreover, the comprehensive effect of Al₂O₃ content and binary basicity B has also been studied through CaO-SiO₂-FeO-Fe₂O₃-Al₂O₃-P₂O₅ and CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ steelmaking slags. The results indicate that adding Al₂O₃ in the slags can decrease the amount of free C₂S ?f-C₂S? in the slags, which affected phosphate enrichment. The added Al₂O₃ as dilute agent for decreasing viscosity and melting point of the slags can be easily bonded with the free C₂S to form gehlenite ?C₂AS? solid solution. Thus, adding Al₂O₃ in the slags can decrease the amount of f-C₂S in the slags The coupling relationship between binary basicity B and mass percentage of Al₂O₃ in the initial slags should be kept as ?%Al₂O₃?=-27.70+21.625 and the maximum content of P₂O₅ in C₂S-C₃P solid solution can reach about 30.0% under conditions of ?%Al₂O₃?< 20.0% and binary basicity 5>1.3. The maximum content of P₂O₅ in C₂S-C₃P solid solution of CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ steelmaking slags can reach about 24.23% with the quaternary basicity Q as 1.23.The crystallization kinetics of phosphate-enriched phase has been investigated through Avrami equation and other various equations of rate constant k?T? in the rapidly quenched CaO-SiO₂-FeO-Fe₂O₃-P₂O₅ and CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ slags with different cooling processes, meanwhile, the apparent activation energy E are also determined. The results show that the average of Avrami exponent n is about 0.472 for CaO-SiO₂-FeO-Fe₂O₃-P₂O₅ and about 0.402 for CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ steelmaking slags during the crystallization of phosphate-enriched phase. The crystalline mechanism of phosphate for CaO-SiO₂-FeO-Fe₂O₃-P₂O₅ steelmaking slags, can be summarized as 1) diffusion controls the crystallization process; 2) phosphate-enriched phase exists as one-dimensional rods; 3) increase the hold time can decrease the crystallizing rate of phosphate-enriched phase. The determined rate constant equations k?T? can be described as Ink=53.80+11163.69/T-7.491nT-0.0057 and the apparent activation energy E for crystallization is about-278.44 KJ/mol for the rapidly quenched CaO-SiO₂-FeO-Fe₂O₃-P₂O₅ steelmaking slags, while the determined rate constant equations k?T? can be described as Ink=40.5+1966.61/T-5.61 InT-0.004T and the apparent activation energy E for crystallization is about-189.84 KJ/mol for the rapidly quenched CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ steelmaking slags.According to the aforementioned chemical composition of steelmaking slags and optimized cooling system, the higher concentration phosphate can be obtained through magnetic separation with magnetic intensity as 3.0 KOe under condition of crushing quenched slag samples below minus 200 mesh. The magnetic separation ratio of phosphate-enriched slags can be reached 74% accompanying with magnetic separation ratio of the iron component as 69% for CaO-SiO₂-FeO-Fe₂O₃-Al₂O₃-P₂O₅ steelmaking slags. Similarly, magnetic separation ratio of phosphate-enriched slags can be reached 69.84% accompanying with magnetic separation ratio of the iron component as 67.74% for CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ steelmaking slags.