Fundamental Research On Self-reduction Of Basic Synthetic Iron Ore Briquettes By Carbon To Make Iron Nuggets At High Temperature

DRI is one of the important sourses of EAF steelmaking. At present, the DRI produced bycommercial process, no matter through gas-based or coal-based direct reduction process, whoseproducts are sponge iron with acide gangue. The seperation of iron and gangue are complete inEAF finaly. The direct reduction technology researched in this project was named Wcomet directreduction process, which is managed by means of producing iron nuggets by self-reduction ofbacisity coal mixed briquettes at high temperature. Through raising the reduction temperature,carburization reaction of the reducing sponge iron would be developed, and the melting point ofwhich decreased after carburization, and it would aggregated to big iron nuggets by the functionof interfacial tension; because the slag bacisity is designed in2CaO×SiO2phase area of CaO-Al2O3-SiO2-MgO phase diagram, the slag phase whose main mineral is2CaO×SiO2formed bysolid-phase reaction at high temperature, and the polymorphic transformation would occurred,and pulverized due to the volume expansion, and then the seperation of the slag and iron wouldbe achieved.This paper mainly research the aggregation machanism of iron nuggets in the process ofself-reduction of bacisity coal mixed briquettes at high temperature and the effect of additions tothe aggregation behavour of iron nuggets at reduction temperature; and research the iron and slagseperation machnanism of reduced products and its influencing factors; besides, desulfurizationand dephosphorization pattern and the removal pattern of K, Na, Pb, Zn are also studied in thispaper.（1） The aggregation process of iron phase can be seen through laser scanning confocalmicroscope, it reveals that: Slag phase has completely melted and adhered to the reduced nuggetswhen basicity is lower, which results in the bad separation of slag and iron and poor aggregationof iron nuggets. With the increasing of slag bacisity, the porosity of the reduced products ishigher, which is benefit to the diffusion of reducing gas and accelerate the reducing reaction. Theseperation of the iron and slag is very well when slag basicity was2.0, iron and slag can beseperated completely through screening. The seperation of iron and slag also very well whenbasicity is2.4, but leading to small iron nuggets. Which is because that when the slag basicity ishigher, the amount of slag is increased and the diffusion length is enlarged, and this is bad for theaggregation of iron, therefore, the appropriate basicity ranged from2.0to2.4?（2） The whole reduction process consists of gas-solid reaction, solid-solid reaction, gas-liquid reaction, solid-liquid reaction, the thermal analysis study on the direct reduction of coalmixed briquettes in favour of STA449C-QMS403C Thermal analysis mass spectrometry combined analyzer. Raising the temperature of coal mixed briquettes to1330℃and keepconstant for15min,DSC line, DTA line and MS line of the outflowed gas can be obtained in the reductionprocess. From the above lines, we can find that not only direct reduction but also indirectreduction occurred in the reduction process, and with the liquid iron generating.（3） The surface tension of slag and iron would be increased when adding CaF₂to the coalmixed briquettes, it can observe through laser scanning confocal microscope, the iron and slag isnot infiltrate at the moment, thus is beneficial to the aggregation of iron. In addition, addingdefined amount of CaF₂is good for the form of2CaO SiO2and remarkablely promote theseperation of iron and slag.（4） The melting point of iron decreased due to the carburization reaction of iron, thereforethe liquid iron is generate and aggregate into balls in the reduction process. Analysing the finnaliron nuggets, we can find that the carbon content in the rion nuggets ranged from2.0%to4.5%.（5） Parts of the S in the briquette is removed by the means of gasification, and the otherparts is removed in the form of CaS and achieved with the seperation of iron and slag. Analysingthe MS line of S gas obtained through STA449C-QMS403C Thermal analysis massspectrometry combined analyzer, it can find that CS₂，S₂and SO₂generated in the reductionprocess. Analysing the samples with CS-800High Frequency Infrared Carbon and SulfurAnalyzer, S in the iron can be lowest to0.008%, total desulfurization rate is above97%, amongwhich gasification desulphurization rate is9%³1%. The desulphurization reaction is conductedin the reduction atmosphere, so the partial pressure of gas phase reduction products CO plays animportant role to desulphurization. The gasification reaction of C increased with the C/O molarratio increasing, therefore which is in favour of desulphurization. However, when C/O molarratio oversized, S in the briquette is increasing with adding excessive coal; this may lead toincreasing of S in iron nuggets.（6） Parts of P in the nuggets is removed by the means of gasification, and the other partsgets into slag. Analysing the MS line of P gas obtained through STA449C-QMS403C Thermalanalysis mass spectrometry combined analyzer, it can find that P，P2and P4generated in thereduction process. In general, P in the iron is0.12%⁰.14%, while P in slag is0.05%⁰.09%,which illustrate that the reduced P is mainly absorbed by iron. The total dephosphorization rate is27%³7%, gasification dephosphorization rate is less than10%, the unreduced parts of P isremoved by the means of slag in the form of generating phosphate with calcium oxide.（7） It shows that K element in the briquettes exists in the form of K₂O, KO₂and KFe2O4andNa exists in the forms of Na₃PO₄and Na₂O by XRD analysis, analysing the MS line of K gas obtained through STA449C-QMS403C Thermal analysis mass spectrometry combined analyzer,it reveals that K is removed by gasification. In the chemical analysis the existence of K and Nacan not be detected in iron, making us think their total removal rate reaches100%. Part of Kenter into slag, the gasification desulphurization rate of K is nearly80%. But the removal of Namainly relys on the slag phase, the removal rate of Na is far lower than that of K rate bygasification.（8） Zn is reducted in briquettes in the form of steam. The reaction of Zn is endothermicreaction, so rising temperature is good to the removal of Zn by gasification. Through theSTA449C-QMS403C thermal analysis mass spectrometry combined analyzer analysis found that,Zn steam appeasr at300℃, which shows that metal Zn exists in raw materials （steel secondarydust）. The dynamics research indicated that the diffuse outwards of Zn steam is the restrictivestep, with the pass of time, the amount of diffuse increases and the removal rate of Zn steamincreases. The content of Zn is about0.001%in iron nuggets, the removal rate of Zn reache99%,about90%of Zn enter into slag during reduction process, about10%of Zn are excluded frombriquettesin with gas.（9） It finds that Pb exists in the form of Pb₃O₄and PbSO₃by XRD analysis, becausebriquettes in internal is reducing atmosphere, so Pb is reduced and it is endothermic reaction. Bythermodynamic analysis, temperature is good to the removal of Pb, by the dynamics analysis, wecan conclude that the diffuse outwards of Zn steam is the restrictive step, with the reductivereaction going, Pb is removed in the form of steam, the removal rate of Pb is more than90%.（10） It can produce iron particles of5²0mm in diameter by using WCOMET directreduction process with secondary dust as raw material and it does not contain gangue. The yieldof iron is more than97%; sulfur content is0.01%on average and P content is between0.12%⁰.14%; the removal rate of Zn is more than99%, the removal rate of Pb reaches90%.The author gratefully acknowledges the financial support for this work from NationalNatural Science Foundation of China and Baosteel Co.Ltd.（No.50774113）.and National NaturalScience Foundation of China.（No.50804037）.