Kinetics of Direct Iron Ore Reduction with CO-H2 Gas Mixtures

Transcription

1 Kinetics f Direct Irn Ore Reductin with CO-H Gas Mixtures Emmanuel Nyanksn Department f Materials Science and Engineering University f Ghana Accra, Ghana Leiv Klbeinsen Materials Science and Engineering Department Nrwegian University f Science and Technlgy Trndheim, Nrway Abstract Thugh the reductin f irn xide pellets by reducing gases is a well-studied phenmenn that has fund a wide technlgical applicatin, many prly understd factrs such as the effect f the water gas shift reactin n the reductin prcess and whether the reductin prcess is diffusin r interfacial chemical reactin cntrlled r mixed cntrlled still exist. The effect f diffusin and interfacial chemical reactin n the reductin f CVRD and KPRS pellets were investigated by varying the prcess temperature, and gas cmpsitin (H-CO mixture), and using diffusin and chemical reactin mdels t calculate the effective diffusin cefficient (De) and the chemical rate cnstant (Kr). The results frm the study shwed that the reductin prcess is bth diffusin and interfacial reactin cntrlled. That is the initial stage f the reductin prcess is interfacial chemical reactin cntrlled while the later stage is diffusin cntrlled. By examining the phases present after the reductin prcess with ptical micrscpe, it was bserved that the phases present were mainly metallic Irn and Wustite since all the experiments recrded a reductin degree greater than 5%. The presence f cracks especially in the pellets reduced with H was bserved and it was attributed t the rapid diffusin f H thrugh the pellets during the reductin prcess. Keywrds Irn re pellets, Direct Reduced Irn, Diffusin, Chemical Reactin Cntrlled I. INTRODUCTION The glbal steel prductin has increased fr mre than 500 % ver the last 5 decades. This can be attributed t the fast increasing steel prductin in China, currently the number ne steel prducer in the wrld []. The steel industry is faced with enrmus envirnmental prblems which are related t energy reuirement, material usage and the by-prduct assciated with the prductin prcess []. The cnventinal steel making prcess reuires the use f cke. One majr prblem assciated with the cke prductin is the emissins f hydrcarbns which are hazardus t the envirnment. A new technlgy which has emerged as result f the attempts being made t reduce the envirnmental prblem assciated with steel prductin is the direct reduced irn (DRI) technlgy. A lt f research has been cnducted s far as the DRI technlgy is cncerned and as a result is currently being used in the steel prductin industry. DRI prcess des nt reuire cke making and sintering, therefre it is mre envirnmentally friendly and less capital intensive than the cnventinal steel prductin rutes []. DRI technlgy invlves the direct reductin f irn re mainly hematite (Fe O ) in the frm f pellets and lumps by a reducing gas which cnsist f Hydrgen (H ) and Carbn Mnxide (CO). As already mentined, DRI technlgy which is an alternative rute fr irn making has been develped t vercme sme f limitatins f the cnventinal rute. This prcess is mstly carried ut in a shaft furnace with the reductin ccurring in stages. The reductin prcess ccurs abve 570 and invlves the fllwing steps: the reductin f Hematite t Magnetite (Fe O 4), which is further reduced t wustite (FeO), and the reductin f wustite t Irn (Fe). Due t the instability f the wustite phase at temperatures belw 570, the magnetite is reduced directly t Irn at thse temperatures [4]. There has been a rapid increase in the prductin f irn via the direct reduced prcess ver the past 5 years. DRI technlgy is dminated by the MIDREX and HYL prcesses. In the MIDREX prcess, the irn xide feed descends in a cylindrical shaft furnace and is heated and reduced by rising ht reducing gas. The reducing gas used in this case is nearly 95 % H and CO with H t CO rati f.5-.6 [4]. The basic HYL prcess emplys a cnventinal natural gas-steam refrmer fr reducing gas generatin. In this prcess, xygen is remved frm the irn re by chemical reactins based n H and CO frm the gas-steam refrmer. The refrmed gas is typically 7 % H and 7 % CO [4]. Mst f the DRI technlgies develped s far make use f CO as the majr reducing gas. Reductin f irn re pellets with CO leads t the generatin f enrmus amunt f CO which affects ur envirnment negatively. The glbal anthrpgenic emissin f CO is expected t increase t 7 Gt by 00 [,5,6]. It is well knwn that, the irn and steel industry is ne f the largest prducers f CO in the metallurgical prcessing field, with a Mt blast furnace emitting as much as 4Mt f CO per year. This value is cmparable t CO emitted by a 500MW cal fired pwer plant. In 996, the irn and steel industry was respnsible fr 4.6 % f the ttal glbal CO emissin f.9gt. With the increasing wrldwide awareness f the effects f CO emissin n the glbal climate, the use f alternative cleaner fuels and reductants in the irn and steel industry can be seen as an inevitable step [7]. The use f H alne as a reducing gas results in a higher reductin f the irn re pellets t irn, hwever the whle reductin prcess is thermally disadvantageus. Therefre a prpsed CO-H therwise knwn as syngas (synthetic gas) is 94

2 being investigated fr use as a reducing gas fr the DRI prcess. The use f irn re pellets as the main re fr the DRI prcess reuires the need t investigate the kinetics f the reductin prcess. As will be shwn in the thery sectin, such a reductin prcess fllws the shrinking cre cncept with reducing gases diffusing thrugh the pellets and reacting with the irn re with subseuent diffusing f prduct gases ut f the pellets. It can therefre be pstulated that tw prcesses may be cntrlling the reductin prcess; that is the diffusin f the reducing gases int the pellets and the reactin f the reducing gases with the re. That is a lt f uestins need t be answered s far the DRI technlgy is cncerned. Sme f these are: is the reductin prcess diffusin cntrlled? Is it interfacial chemical reactin cntrlled? and is it mixed cntrlled? The main bjective f this wrk is therefre t carry ut labratry investigatins f the effect f diffusin and interfacial chemical reactin n the DRI prcess using mixed gases and then carrying ut ptical micrscpy analysis t determine the manner in which the reductin ccurred and the phases frmed after the reductin prcess. II. KINETICS OF THE REDUCTION PROCESS The reductin f irn re passes thrugh a number f stages. That is the hematite is reduced t magnetite, then the magnetite is reduced t wustite and the wustite is then reduced t Irn as represented in Figure. The remval f xygen at the irn-wustite interface f a dense particle must prceed thrugh the fllwing 5 steps ccurring during the reactin [8]: The Transprt f gaseus reactant (hydrgen r carbn mnxide r bth) frm the bulk gas phase t uter surface f irn (Fe); (gas-film resistance) Diffusin f hydrgen r carbn mnxide thrugh the prus irn layer t the surface f the un-reacted xide cre, in this case the irn-wustite interface; (shell-layer resistance) Chemical reactin f the gas with slid xide (Wustite) t frm gaseus prduct, water and r carbn dixide; (interface resistance) Outward diffusin f the prduct gas thrugh the irn layer; (shell-layer resistance). And transfer f prduct species frm the uter surface f irn layer t the bulk gas (gas film resistance) This mdel is generated based n the shrinking cre cncept that is the unreacted shrinking cre mdel. Hwever, this suffers frm tw majr setbacks when it is specifically used fr mdelling gas-slid reactins in a prus pellet; The existence f a sharp bundary between the reacted and unreacted znes which cntained bth ttally reduced and nly partially reduced grains is nt generally supprted by experimental evidence. In this types f mdel, the rle played by structural parameters (like prsity, grain size etc) in determining the verall reactin rate des nt appear explicitly [9, 0]. Figure Shrinking Cre Cncept The cmplexity f the prblem can be reduced by cnsidering the reductin f wustite t irn. Figure Shrinking cre cncept fr reductin f Wustite t irn These steps ffer resistance in series t the verall chemical reactin. If ne is cnsiderably slwer than the thers, it may be identified as the rate cntrlling step. The mdel fr predicting the degree f reductin r xidatin can derived by cnsidering the rate f gas film resistance, shell-layer resistance and the interfacial resistance [5, 9]. III. CHARACTERISING THE KINETICS OF THE REDUCTION PROCESS Based n previus studies, it can be cncluded that, the reductin f the pellets prceeded tpchemically. This kind f reactin includes the prcess f diffusin f gaseus species and that f intrinsic chemical reactin. The ttal reductin time is expressed in E (), and is valid under cnditins f slid sphere being reacted with a gas phase, the effect f gas phase mass transfer being negligible and the reactin rate cntrlled by interfacial chemical reactin and the diffusin f reactant and prduct gas species thrugh the slid prduct layer. 95

3 r t k C C D e r C O O C R R R (). Where t is reductin time (, C and C are reductin gas cncentratin at granule surface and in euilibrium respectively (ml/cm ), r is characteristic initial radius f the pellet (cm), ρ is initial xygen cncentratin in the pellet (ml/cm ), k is the reactin rate cnstant (cm/, D e is the effective diffusin cefficient (cm / and R is the reductin degree. The first part f the euatin represents the reactin cntrlled part f the prcess and the secnd part represents the diffusin cntrlled part f the prcess []. IV. MATERIALS AND METHODS A. Irn res investigated The tw res investigated in this experiment were KPRS pellets frm LKAB and CVRD pellets frm Arcelr-Mittal. In terms f the pellets sizes, the KPRS pellets are uite smaller than the CVRD pellets. B. Reducing the Irn re pellets in TGA euipment The reductin f the pellets was carried ut in a TGA furnace and the detailed descriptin f the prcess is reprted in ur previus article []. Briefly, different samples f the pellets (KPRS and CVRS) weighing abut ~ 50g was used fr the analysis. Half f the samples were first placed in the crucible, the thermcuple detecting the temperature f the samples was placed in the middle f the sample and the remaining pellets were added. This was dne t imprve the accuracy f the temperature taken by the thermcuple. The crucible was then cnnected and placed in the furnace. The varius prcess parameters were prgrammed int the LabVIEW sftware s that, the degree f reductin, and sample temperatures culd be examined. C. CO-H gas mixture Cmpsitin variatin The sample (KPRS) pellets were heated t a temperature f ~8 in the furnace within a time interval f 45 minutes in Ar. The temperature was held at ~8 fr tw hurs during which the sample was subjected t different gas cmpsitin and the degree f reductin mnitred. The different gas cmpsitins examined at this temperature include: 00 % CO, 00 % H, 0 % CO and 80 % H, 40 % CO and 60 % H, 60 % CO and 40 % H and 80 % CO and 0 % H with 00 % crrespnding t 5 l/min flw rate f the reducing gas. Befre and after the experiment, the furnace was purged with Ar fr 5 minutes. This was dne t ensure that, n traces f the ther gases are left in the gas tubes. The first experiment was carried ut at the said temperature (~8 ) because the euilibrium cnstant fr the water gas shift reactin (WGSR) is Apprximately at this temperature. D. Temperature variatin The effect f temperature was investigated by carrying ut the reductin prcess fr a fixed cmpsitin f the KPRS pellets at different temperatures; that is 8 and 8. E. Optical micrscpy analysis The reduced CVRD and KPRS pellets were analysed with an ptical micrscpe t examine the manner and degree f reductin since the shrinking cre cncept fr the reductin f the irn re pellets t irn can easily be seen under an ptical micrscpe. The pellets frm the TGA euipment were imbedded in EpFix resin and were left fr eight hurs t harden. The pellets imbedded were remved and divided int tw halves. The cut samples were grinded with SiC paper n a rtr and images f the surface taken. F. Etching In rder t see clearly the different phases in the plished pellets, the samples were etched in 5 % HF. This was achieved by putting the plished pellets in 5 % HF slutin fr 0 secnds. After which they were washed thrughly using distilled water t remve traces f the etchant frm the plished sample. Images f the etched dried samples were taken after which the micrstructure f the varius revealed phases were bserved in ptical micrscpe and pictures f the varius micrstructures taken V. RESULTS AND DISCUSSION A. Micrstructural analysis f the reduced pellets Frm Figure it can be seen that the reductin f the irn re pellets fllwed the shrinking cre cncept. That is the reductin f the irn xide (hematite) ccurred in cncentric layers r rings. The uter ring is likely t be made up f mainly reduced irn while the inner ring is made up f unreduced hematite, magnetite and wustite. Hence the nature f the reductin prcess can be well explained with the shrinking cre mdel. Figure Picture f the cut etched sample reduced in 5 l/min H fr CVRD pellets The irn re pellets made up f mainly hematite was reduced t magnetite, which was then, reduced t wustite and the wustite was reduced t metallic Irn. Frm the images in Figure, three layers can be bserved: the first utermst layer can be speculated t be mstly metallic Irn. This is due t the fact that, a micrscpic bservatin revealed shinny grain structure which is a representatin f metallic Irn. The next layer (intermediate) is mstly a cmbinatin f the metallic Irn and an unreduced xide (hematite, magnetite and 96

4 wustite). The reductin f hematite t magnetite and frm magnetite t wustite has a reductin degree f.% and 5 % respectively [7]. Since the ttal reductin degree recrded fr all the experiments in this study as reprted in ur previus article exceeded 5 %, the unreduced xide was mainly wustite. The third layer, which is mainly in the center f the reduced pellets, is made up f unreduced wustite. Crack FeO FeO Fe Fe Figure 5 Optical micrscpe images f the; (Left) centre and (Right) edge f a CVRD pellets reduced in 5 l/min H. Figure 4 A picture f a cut etched CVRD pellet reduced in 5 l/min CO Different crack sizes and lengths were bserved in the reduced pellets upn examinatin. Larger and visible cracks were bserved in the pellets reduced with 00 % H (Figure ) cmpared with thse reduced with 00 % CO (Figure 4). The cracks may be related t a large degree in swelling assciated with pellets reduced in H. Pellets reduced in H, swells easily due t the rapid phase transfrmatin (Hematite-Magnetite- Wustite-Irn), which is assciated with the higher reductin degree bserved with H. It can als be attributed t the faster rate f H diffusin thrugh the reactants and prduct layers in the pellets []. Cmparing the images in Figures and 4, it can be deduced that, the nature f the shrinking cre differs with respect t the kind and cmpsitin f the reducing gas used. A vivid bservatin f the varius images depicts that, the pellets reduced frm a gas cmpsitin f 00 % H has a very visible reduced cncentric layers. This can be assciated with the higher rate f reductin assciated with using H as a reducing gas. Hwever an almst invisible cncentric rings were bserved in the case where pure CO was used as the reducing gas. By varying the cmpsitin f CO-H gas mixture, it was bserved that, the gas mixture with higher H cntent recrded a much visible ring cmpared t thse with higher CO cntent. The bserved trend is due t the faster diffusin rate f H cmpared t CO and the fact that xygen remval rate by H is higher than CO as we reprted earlier []. The etched samples were munted n a light micrscpe and images f varius sectins taken t examine the mrphlgy f the reduced pellets. The images were taken frm the centre and edge f the cut samples using a magnificatin f 50X. Bth the CVRD and KPRS pellets were used in this analysis. Different micrstructural phases were bserved frm the reduced pellets as can be seen in Figure 5. The ptical micrscpic images f metallic irn shwed a shiny metallic surface, which can be speculated t be metallic irn. The light grey strips can be assciated with the unreduced xide. As already stated, cnsidering the verall degree f reductin, the unreduced irn was wustite since all the hematite and magnetite has been reduced. The reduced phases were prnunced at the edges f the pellets than the inner regin. This is nt surprising since the pellet edges were expsed t the reducing gases at the entire duratin f the reductin prcess. There exist sme pres (cracks) in the pellets after the reductin prcess. The phases present in Figure 5 wuld have been best identified by using Electrn Micr-Prbe Analyzer (EMPA), but it was nt used due t it unavailability. Figure 6 Micrstructural images f the; (left) centre and (right) edges f the reduced pellets in l/min H - l/min CO gas mixture A similar mrphlgy f the reduced pellets was bserved by cmbing the reducing gases in different ratis. Referring t Figure 6, it was bserved that, the edge f the sample was mainly made up f a shiny structure which is metallic Irn while the centre f the pellets was made up f light greyish grains which can be attributed t the unreduced xide (wustite) with sme shiny metallic irn. Sme f the phases identified resulted frm the EpFix used in the imbedding. There were als sme cracks that resulted frm the rapid diffusin f the reducing gasses thrugh the pellets. B. Kinetics f the reductin prcess Frm E (), tw different graphs can be drawn t investigate whether the prcess is chemical reactin 97

5 cntrlled, diffusin cntrlled r mixed cntrlled. Fr thereactin cntrlled part,a plt f R R against reducing time ( D gave a straight line whseslpeis while fr the plt f k C r diffusin R C e C r against time ( is C cntrlled part, theslpe f the R Figure 8 A plt f vs time ( fr the reductin f KPRS pellets by 60 % CO -40 % H gas mixture. R R Figure 7 A plt f vs time ( fr the reductin f KPRS pellets by 60 % CO - 40 % H gas mixture. Different bservatins were made frm the plts btained with a reducing gas cmpsitin f 60 % CO and 40 % H. Frm Figure 7, it can be seen that the plt is nt perfectly straight. The early stages f the plt curved slightly while the later part is fairly straight. The calculated slpes fr the straight prtin f the curves are summarized in Table. By cnsidering the same gas cmpsitin in Figure 8, it was bserved that the early stages f the curve was fairly straight while the later part f the plt started curving earlier than that f Figure 7 and 9. The slpes f the straight prtin f the curves were calculated and summarized in Table. R R Figure 9 A plt f vs time fr the reductin f KPRS pellets by 0 % CO -80 % H gas mixture. 98

6 R Figure 0 A plt f vs time fr the reductin f KPRS pellets by 0 % CO -80 % H mixture. The features f Figures 9 and 0 were the same as thse f Figures 7 and 8, respectively. Their slpes were summarized in Table. Table Slpe f the straight prtins in Figures 7-0 Figure Temperature (K) Slpe (min - ) The kinetics f the reductin prcess can best be analysed by taken int cnsideratin the factrs that cntrl the reductin prcess as stated in the thery. In rder t achieve this, different plts were made with euatin and analysed. Examining Figures 7 t 0, it can be seen that, the chemical reactin cntrlled curves shw straight line in the initial stages f the curve while thse f the diffusin cntrlled shw straight line at the later part f the curve. This implies that, the initial stages f the reductin prcess were interfacial chemical reactin cntrlled while the later part f the prcess was diffusin cntrlled. The initial stages f the reductin prcess were characterized with the diffusin f reducing gas thrugh the pellet; therefre there was enugh cncentratin build up in the pellet t allw fr the diffusin prcess t ccur. The nly prcess which can slw dwn the reductin prcess is the reactin between the reducing gasses and the irn re pellets. That is the rate limiting step is the interfacial chemical reactin between the reducing gasses and the irn re. At the later stages f the reductin prcess, the rate limiting step is the diffusin f the reducing gasses thrugh the prduct and reactants layers. The rate f diffusin is reduced as a result f the decreased driving frce, which is assciated with reductin in cncentratin difference between the reducing gases at the surface and at the centre f the pellets. The rate f diffusin is further reduced with the frmatin f mre metallic irns. As a result, the later stages f the reductin prcess were diffusin cntrlled. Frm the abve analysis, it can be deduced that, the reductin prcess f the Irn pellets is bth diffusin and chemical reactin cntrlled, that is the prcess is mixed cntrlled. The calculated reactin rate cnstants (Kr) and Effective diffusin cefficient (De) are summarized in Table Gas cmpsiti n Table Calculated kinetic parameters Temperature ( ) High Lw De High temperature (cm / K (cm/ Lw temperature De (cm / Kr (cm/ 00 % H % H, 0 % CO 60 % H, 40 % CO 40 % H, 60 % CO 0 % H, 80 % CO % CO Since the reductin prcess is bth diffusin and chemical reactin cntrlled, the kinetics f the reactin can best be studied by calculating the effective diffusin cefficient (De) and the rate cnstant (Kr) fr the different experiments carried ut. The effect f the varius gas cmpsitins n the reductin prcess can be analyzed by calculating De and Kr. Frm Table, the effect f the varius gas cmpsitin can be clearly seen by taken a clser lk at the De and Kr values. It was bserved that, as the H cmpsitin increased, Kr and De became larger. Hwever a significant reductin in De and Kr ccurred when the CO cmpsitin was increased. This can be cnnected with the easy diffusin f H thrugh the pellets than CO as already mentined in the early part f the discussin. The reductin prcess was therefre characterized with higher reductin degree when H was used as the reducing gas and als when the H cntent in the reducing CO-H gas cmpsitin was increased. 99

7 VI. CONCLUSIONS The results frm the study shwed a prnunced reductin f irn re pellets t metallic irn at the edges f the pellet and the unreduced irn re was mainly wustite. Frm the kinetics analysis, we cncluded that the entire reductin prcess was mixed cntrlled since the initial and later stages were interfacial chemical reactin and diffusin cntrlled, respectively. ACKNOWLEDGMENT Our prfund gratitude ges t LKAB (Swedish mining cmpany) fr supplying us with the KPRS pellets and Arcelrmittal (a multinatinal steel manufacturing crpratin) fr supplying us with the CVRD pellets. REFERENCES [] Chunba XU, C.D.-., A Brief Overview f Lw CO Emissin Technlgies fr Irn and Steel Making. Jurnal f Irn and Steel Research Internatinal, 00. 7(): p. -7. [] Manning, C. and R. Fruehan, Emerging technlgies fr irn and steelmaking. Jm, 00. 5(0): p [] Liu, G.-s., et al., Thermal investigatins f direct irn re reductin with cal. Thermchimica Acta, (): p [4] Feinman, J. and D.R. Mac Rae, Direct reduced irn: technlgy and ecnmics f prductin and use. Irn and Steel Sciety/AIME, 40 Cmmnwealth Dr, P. O. Bx 4, Warrendale, PA , USA, , 999. [5] Agency, I.E., Bifuels fr transprt: an internatinal perspective004: Internatinal Energy Agency, OECD. [6] Outlk, A.E., Energy Infrmatin Administratin. Department f Energy, 00. [7] Gergalli, G.A., The use f Hydrgen/natural gas mixtures as a fuel r reductant in the metallurgical industry,. 00. [8] Spitzer, R., F. Manning, and W. Philbrk, Mixed-cntrl reactin kinetics in the gaseus reductin f hematite. AIME MET SOC TRANS, (5): p [9] Valipur, M., M.M. Hashemi, and Y. Sabhi, Mathematical mdeling f the reactin in an irn re pellet using a mixture f hydrgen, water vapr, carbn mnxide and carbn dixide: an isthermal study. Advanced Pwder Technlgy, (): p [0] Klbeinsen L, L.R.; Irn re reductin with CO and H gas-mixturesthermdynamics and kinetic study. in 4th Ulcs Seminar [] Wang, Y., et al., Reductin extractin kinetics f titania and irn frm an ilmenite by H-Ar gas mixtures. ISIJ Internatinal, (): p [] Emmanuel Nyanksn, L.K., Investigating the Effect f Water Gas Shift Reactin and Other Parameters n the Direct Reductin f Irn Ore pellets. Internatinal Jurnal f Engineering Research and Technlgy. ESRSA Publicatins 05. 4(): p