| As a typical refractory iron ore,the resource reserves of oolitic hematite are very considerable,but its characteristics,such as complex internal structure,wide variety of minerals,embedded fine-grained useful minerals and lots of associated clay minerals included,etc,greatly limit the efficient use of this type of ore.And as a kind of politic hematite,because of its high content of harmful element phosphorus,the extensive development and efficient utilization of high phosphorus oolitic hematite becomes more and more difficult.The separation process with high temperature pretreatment,magnetic roasting,magnetic separation and leaching as main processes has achieved good results in the treatment of GARA oolitic hematite,meanwhile,it was found that high temperature pretreatment was crucial to the reduction of phosphorus content in ore.However,existing work could not give a reasonable expanation for its mechanism,needless to say related micro-evolution,phase transformation,thermodynamic and kinetic mechanisms.In response to the above issues,with GARA oolitic hematite taken as the research object,its process mineralogy was studied in detail.Moreover,high temperature pretreatmentmagnetic roasting-magnetic separation-leaching(HMML)was taken as the main process,and with the aid of some detection means,such as XRD,SEM and BET,etc,combined with research methods such as image analysis,thermodynamics and kinetic,related experimental investigation and theoretical analysis work have been launched,and some corresponding results have been obtained.Process mineralogy revealed that hematite,maghemite and magnetite were the main useful minerals in the ore,and iron mainly occurred in the first two,while the gangue minerals were oolitic chlorite and apatite,and more than 50%of phosphorus was found in apatite,secondly,most of it occurred in hematite and maghemite.Embedded grain sizes of most minerals were relatively fine,which was not conducive to the full dissociation of useful minerals and gangue minerals.Hematite/maghemite and limonite mainly existed in the form of oolites,cements and fine particles,closely combined with iron chlorite,apatite,quartz and carbonate minerals,etc;iron content of magnetite varied greatly,with the average value about 67%;oolitic chlorite mainly existed in the form of cements,presenting the appearances of fine scales,fine fibers and their aggregates;apatite was mainly combined with limonite and iron chlorite to form an oolitic structure,and part of it showed as hypidiomorphic or euhedral column,irregularly wrapped in limonite particles.Because varieties of minerals in the ore were abundant,there would be many conplex chemical reactions between useful minerals and gangue minerals in the ore during high temperature pretreatment,roughly including oxidation reactions of iron minerals,gangue minerals,and reactions between them.High temperature pretreatment test results demonstrated that pretreatment could indeed reduce the phosphorus content of the final leaching products,after pretreatment at 1050℃,the phosphorus content of the final leaching product could be reduced from 0.33%(without pretreatmnt)to about 0.15%,moreover,the influence of temperature was significantly stronger than that of time.After high temperature pretreatment,maghemite and magnetite were transformed into hematite,and cracks in the ore increased,besides,with the increase of temperature,the cracks obviously expanded and extended a lot,and both of the number and width of the cracks increased significantly,however,prolonging the pretreatment time had less effect;ore porosity changed drastically with the change of pretreatment temperature,mainly manifesting as the increase in sample pore size,and the decrease in specific surface area and pore volume,relatively speaking,the change was more stable and slow with time increasing.At the same time,it was revealed that phosphorus would not be removed from the ore during pretreatment,neither would it migrate or accumulate to the cracks,but evenly distributed in apatite and iron minerals.Avrami-Erofeev equation f(α)=1/4(1-α)[-ln(1-α)]-3 was the appropriate mechanism function model of the leaching process,and the corresponding kinetic equation could be obtained as dα/dt=1.33×104 exp(-29040/RT)1/4(1-α)[-ln(1-α)]-3,with leaching tayperature the main influencing factor in the leaching process.There were many factors,mainly pretreatment temperature,that worked together to affect the final leaching process during the high temperature pretreatment process,and what’s more,the joint kinetic equation of high temperature pretreatment and leaching process could be expressed as follow:dα/dt=k(Y)f(α)=Aexp(-Ea/RY)1/4(1-α)[-ln(1-α)]-3.At the same time,it was discovered that the change of leaching conditions,especially the temperature,had a stronger influence on the leaching process than the change of pretreatment conditions.Without pretreatment or with pretreatment at lower temperature,there were only a few of native cracks inside the ore,and fmally,most of the phosphorus in apatite could be removed through leaching,but the phosphorus inside iron mineral crystals was difficult to remove;nevertheless,when the pretreatment temperature gradually riseed above 1000℃,the native cracks in the ore would have got a full development,and a large number of secondary cracks would have been generated at the same tirne,then the phosphorus in iron mineral crystals could be removed in large quantities with leaching agent.Single traditional beneficiation methods are difficult to achieve effective removal of phosphorus from GARA oolitic hematite,thus there should be a variety of means combined to form joint processes,so that the purpose of comprehensive recycling and efficient utilization could be achieved.This work has clarified the mechanism of high temperature pretreatment affecting the dephosphorization of this kind of ore,enriched the theoretical system of high phosphorus oolitic hematite dephosphorization process,and provided a good reference for the separation of similar types of refractory ores at the same time. |
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