| The utilization of high phosphorus iron ore, which has large reserve and cannot be efficiently utilized by current state-of-the-art technology, is of great importance for China to ensure the supply security of iron ore. Among various dephosphorization methods, acid-leaching is getting more and more attention due to its simpleness, low equipment demand and high efficiency. The efficiency of the dephosphorization by acid-leaching is closely related to the nature of phosphorus existed in iron ores. While dephosphorization is easy for phosphorus existed as apatite, collophanite or svanbergite, phosphorus in limonite is difficult to be removed by the acid leaching process. It is therefore necessary to further study the dephosphorization mechanism to facilitate the utilization of high phosphorus iron ore. In the present thesis, the Huiming limonite was employed as an example to explore the dephosphorization mechanism, e.g. the existing state of phosphorus, the process and mechanism of the acid-leaching dephosphorization were investigated in details. The main innovative results were summarized as follows:(1)The influence of sulfuric acid concentration, leaching temperature, leaching time, agitation speed, etc. on dephosphorization rate was investigated for direct leaching of the Huimin high phosphorus iron ore. It was found that the dephosphorization rates were less than 9% in all leaching exmperiments. It has been concluded together with phase analyses that the phosphorus existed as amorphous Fe3PO7 in the Huimin high phosphorus iron ore. Moreover, the Fe3PO7 was synthesized using chemical reagents to clarify the leaching speed, and it was found for the first time that Fe3PO7 showed very low rate of leaching, e.g. less than 1% under the same leaching conditions, and the low rate of phosphorus removal is caused by the Fe3PO7 phase.(2)The phase evolution caused by roasting was investigated in detail to clarify the roasting effect. It was found that Fe3PO7 kept in amorphous state up to 600℃, while the amorphous to crystalline phase transition was observed at 600-800℃. Upon roasting above 800℃, the Fe3PO7 reacts with gangue phases in the iron ore to form acid-soluble Mg3(PO4)2, Ca3(PO4)2 and AIPO4 phases. These investigations clarify the dispute about the existing state of phosphorus associated with limonites. It was further found that high phosphorus removal ratio could only be achieved for the Huimin iron ore when the roasting temperature was greater than 1000℃.(3)To further clarify the enhanced leaching mechanism by roasting, the reaction between Fe3PO7 and SiO2, Al2O3, CaO and MgO was investigated. It was found that the order of reactivity is SiO2<Al2O3MgO<CaO. Moreover, the nearly complete reaction temperatures were roughly determined as 1000℃ for Al2O3; 900℃ for CaO and MgO, while obvious reaction with Fe3PO7 was not detected for SiO2. The reactivity investigations revealed that the leaching effect was enhanced due mainly to the reaction of CaO and MgO in the iron ore with Fe3PO7, not Al2O3, which has been guessed as the main cause. These investigations could provide theoretical base for the dephosphorization process optimization for limonite type high phosphorus iron ores.(4)In order to reduce the roasting temperature, the present thesis explored the addition of CaCl2 and Na2CO3 during the roasting process. It was found that the reaction of Fe3PO7 with CaCl2 to form Ca3(PO4)3Cl could be complete at 850℃, which is ～150℃ lower as compared with direct roasting. Similarly, the reaction of Fe3PO7 with Na2CO3 was found to be complete at 750℃, which is ～250℃ lower as compared with direct roasting. Subsequently, the Huiming iron ore was roasted separately with CaCl2 and Na2CO3 to study the additive effects. It was found that the roasting temperature could indeed be reduced, e.g. after roasting with CaCl2 and Na2CO3 at 850℃ and 750℃, the phosphorus removal ratio reaches 75% and 77%, respectively. |
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