International Journal of Scientific & Engineering Research, Volume 4, Issue 5, May-2013 ISSN

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1 Interntionl Journl of Scientific & Engineering Reserch, Volume 4, Issue 5, My- ISSN Preprtion of iron Powders y Reduction of Rolling Mill Scle Sid Mechchti, Omr Benchiheu, Slim Serri, Mohmed E.H.Shli Astrct The ojective of this work is the recycling of mill scle formed during the steel hot rolling process with reducing gs (cron monoxide) in order to produce iron powder hving chrcteristics required y powder metllurgy. The reduction ws crried out t vrious tempertures (75-5 C) during different times rnging etween 4 nd 8 min in n tmosphere of pure CO. The produced iron powder ws chrcterized y chemicl nlysis, x-rys diffrction, opticl microscopy nd scnning electron microscopy. These methods of investigtion confirm the presence of iron, grphite nd iron cride (e C) s the products of rections. The mximum iron content (98.4% e) in the iron powder ws otined y reduction of mill scle t 5 C for 8 min. A reduction nneling under hydrogen mkes it possile to decrese cron nd oxygen content of the reduced iron powder up to cceptle vlues. Index Terms Cron monoxide, Iron powder, Mill scle, Recycling, Reduction. INTRODUCTION HE steelmking y-products such s dust nd mill T scle, very rich in iron ( 7% e), re currently produced in lrge quntities nd represent potentil of lmost 5 million tons in the world []. Generlly, these y-products re recycled y the metllurgicl processes such s the lst furnce or the direct reduction rectors tht uses col s reducing gent to produce pre-reduced pellets intended for the remelt in electric steel plnt. Besides the steelmking, recycling prt of these yproducts is lredy supported y the powder metllurgy where the economic recovery is more fvorle. During the lst twenty yers, powder metllurgy hs presented continued expnsion in ll its spects nd in ll of its pplictions to the industry. Powder metllurgy comprises set of processes of forming hving for common denomintor rw mteril in powder form. The reduced iron powder is the most widely used mteril in powder metllurgy industry. The direct reduction process hs commonly een used y mny compnies (such s Hoegns in Sweden, Kwski in Jpn nd Pyron in US) to otin metllic iron powder y the rection of iron oxide (mgnetite, hemtite ore or mill scle) nd reducing gses (CO/H) under high tempertures (> C) []. The reduction of iron oxides. oundry Lortory, Deprtment of metllurgy nd mterils, Bdji- Mokhtr Univrsity, BP -Ann- Algeri. Author's Emil: sidmechchti@yhoo.fr. Centrl Metllurgicl Reserch nd Development Institute (CMRDI), Ciro, Egypt. nd vrious ores contining iron oxides hve een studied in the pst [-8]. Their reserches re conducted using s reducing gents solid cron nd reducing gs (CO nd/or H). hen the reduction is crried out y solid cron or cron monoxide the finl process of reduction is lwys written s follows: enom + mco(g) = ne + mco(g) () The necessry CO is provided directly or y the Boudourd rection: mc(s) + mco(g) = mco(g) () The Boudourd rection, regenerting reducing gs CO, keeps the PCO/PCO rtio lwys gret nd there for the reduction process ccording to () will e mintined t constnt level. In the event of reduction occurring y circultion gs over or through ed of prticles of iron oxides (fixed or moile) the CO formed y the rection () will e crried wy y gs flow, which llows to rech the qusi-complete rection of ed of oxide prticles [9].In ddition to the reduction to metllic iron, disintegrtion of cron monoxide y the reverse Boudourd rection () nd cruriztion re expected to occur simultneously. This is confirmed y thermodynmic studies which indicte tht reltively low tempertures (less thn C) nd high concentrtions of CO long with the presence of metllic iron, which cts s ctlyst, led to cron deposition []. The increse in temperture nd CO concentrtion leds to reduction in iron oxides without crurizing.

2 Interntionl Journl of Scientific & Engineering Reserch, Volume 4, Issue 5, My- ISSN Other prmeters cn gretly ffect the kinetics of reduction of iron oxides. The rte of reduction of iron oxides depends on severl fctors tht cn vry from one process to nother, especilly the grin size, porosity nd specific surfce, minerlogy (eo nd eo4 re not reduced in the sme wy), pressure, gs flow nd the gngue constituents such s silic, lumin nd silictes which my lter the equilirium []. The im of this work is to study: - The optiml conditions for the reduction of mill scle to otin iron powder with cceptle purity for the use in powder metllurgy industry. - The effect of cron deposition on the reduction of iron oxides y pure CO. EXPERIMENTAL. Mterils Used Mill scle generted in hot rolling step ws heted to 4 C to remove the oil nd crushed to the desired prticle size distriution. Three frctions of prticles were used in this study: -.5, 6.- nd -6 mm. Sieves were stndrdized. Chemicl nlysis of the mill scle is given in Tle. Anlysis y X-ry diffrction (ig..i) nd microstructure of the mill scle prticles (ig..ii) shows the presence of hemtite (eo), mgnetite (eo4) nd wustite (eo) phses. Oxidtion t C converts the iron oxides tht re present in the mill scle, eo nd eo4, to ferric oxide (eo). Ann. The reduction device nd the method used to perform these tests re descried in severl works [, ]. In this method, where reduction ws done in fixed ed, the smple ws plced in verticl reduction shft which ws introduced into n electric furnce mintined t fixed temperture of up to 6 C. As long s the smple hd not reched test temperture, it ws kept under nitrogen to prevent oxidtion. hen the temperture ws stilized, purified CO ws pssed t fixed rte during fixed time. Preliminry reduction tests were crried out t C to determine the optiml gs flow rte which does not significntly ffect the rte of reduction. As result of these tests, l/h gs flow of CO gs ws used in ll the reduction experiments. The reduced smples were cooled to room temperture in nitrogen gs tmosphere for susequent exmintion nd chemicl nlysis. The tests were performed in two steps: () To study the effect of prticle size, the CO/N rtio nd oxidtion of mill scle on its reduciility t C nd 6 min. () Extending the study to investigte the influence of vrious tempertures (75-5 C) nd different reduction times (4-8 min) under n tmosphere of pure CO. The evolution of the reduction rte ws clculted on the sis of dt otined y chemicl nlysis. The scnning electron microscope (SEM), light microscope (LM) nd X-ry technique were used to chrcterize the structure nd the different phses formed in ech reduced smple. The totl mounts of cron deposited in the reduced smples were determined ccording to ISO 47 [4].The oxygen content ws lso mesured in ccordnce with ISO 449- [5]. TABLE CHEMICAL COMPOSITIONS O MILL SCALE Compounds etotl eo CO SiO MnO % I.u Thet ig.. Oxide scle formed on the mill scle: (I) XRD spectr (II) cross-section microstructure. () Hémtite () Mgnétite () ustite. Reduction System nd Procedure The reduction tests were performed t the center of pplied reserch in metllurgy nd steel (URASM), II c. RESULTS AND DISCUSSION. Effect of Some Prmeters on the Rte of Reduction Preliminry tests were crried out in order to study the effect of CO rte in the CO/N mixture, prticle size nd oxidtion of mill scle on the evolution of reduction t fixed time (6 min) nd C. The results re presented in Tle. The mill scle smple reduced in the conditions of test N hs low totl iron content nd low rte of reduction, nd 4.48% respectively. These two indexes evolve in the sme direction (8. nd 4.8%) when the frction 6.- mm ws used, keeping the sme CO/N rtio. or the sme frction (6.- mm), incresing the CO/N rtio from 4 to % (corresponding to pure CO), while mintining the other prmeters unchnged, increses significntly the rte of reduction (4.8 to 68.6%) nd totl iron content (8. to 88.59%). The

3 Interntionl Journl of Scientific & Engineering Reserch, Volume 4, Issue 5, My- ISSN mount of metllic iron (e) incresed nd tht of e + decresed with decresing prticle size nd incresing TABLE EECT O CO/N RATIO, PARTICLE SIZE AND MILL SCALE OXIDATION ON THE RATE O REDUCTION AND CHEMICAL COMPOSITION O REDUCED IRON PODER AT C AND 6 min Tests Size mm Gs CO/N Rte of reduction % Chemicl composition, % etot e + e e + N -6 4/ N 6.- 4/ N 6.- 6/ N N N CO/N rtio. Test N 5 shows the effect of prior oxidtion of the mill scle on its reduciility. The mill scle hs undergone n oxidtion t C. The rte of reduction nd the totl iron content of reduced iron powder increses to 86. nd 9.96% respectively. The mximum iron content (94.74%) nd rte of reduction (9.78%) ws Otined when we used smple 8 of prticle size -.5 mm (test N 6). The most importnt result to e drwn from these tests is the strong influence of the rte of CO in the CO/N mixture, the oxidtion of the mill scle nd the prticle size on the rte of reduction nd the totl iron content of reduced iron powder. The totl mount of iron reches its mximum vlue (94.74%) when oxidized mill scle with prticle size -.5 mm is reduced with pure CO t C nd 6 min... Effects of Temperture nd Reduction Time Another series of tests were performed y vrying the temperture (75 to 5 C) nd reduction time (4 to 8 min). Mill scle with prticle size -.5 mm undergoes n oxidtion t C. The reducing gs used ws pure CO. The results of reduction tests re presented in ig.. At 75 C it ws found tht with the increse reduction time from 4 to 8 min the totl iron content of reduced iron powder decreses from 88.5 to 75.% respectively. At the sme time intervl, the mount of metllic iron increses (4.6 to 56.9%) nd tht of e + decreses (47.8 to 9%). The decrese in totl iron content cn e explined s follows: long the reduction of iron oxides y pure cron monoxide to wustite nd metllic iron disintegrtion of cron monoxide tkes plce 8 e t, e nd e + (%) 6 4 e t, e nd e + (%) C C e t, e nd e + (%) 6 4 e t, e nd e + (%) C C ig.. Evolution of e totl, e nd e + content in reduced iron powders with temperture nd reduction time.

4 Interntionl Journl of Scientific & Engineering Reserch, Volume 4, Issue 5, My- ISSN monoxide tkes plce ccording to the reverse Boudourd rection []: (CO)ds (CO)g + C () nd ccordingly the cron deposition nd its rection with metllic iron nd wustite leds to the formtion of iron cride. Cruriztion of the reduced iron or wustite grins cn tke plce ccording to the following rections: (4) Totl cron (%) e + C ec e + CO ec + CO (5) eo + 5CO ec + 4CO (6) Cron monoxide, dsored (rection ) onto the surfce of metllic iron grins disintegrtes more esily thn when it is free. The metllic iron cts s ctlyst [6]. Thus, s shown in ig. t 75 C, the decrese in totl iron content is ttriuted to the increse of mount of cron deposition with the increse of reduction time nd mount of metllic iron. This cron increses the weight of the smple nd consequently the totl iron content is incresed. The totl mount of cron in reduced iron powder t vrious tempertures nd reduction times is presented in ig. G G G G C C G Reduction 75 C 85 C 95 C 5 C Reduction time, ig.. Evolution of totl cron content in reduced iron powders with temperture nd reduction time. It shows tht the cron content in the reduced iron powders decresed with rise in temperture nd with decrese in the reduction time. At 85 C, s shown in ig., the mount of totl iron, metllic iron nd e + evolve in the sme direction s the tests t 75 C. However, the decrese of mount of totl iron content with reduction time is less pronounced. As result, the mount of cron deposition with reduction time is lower t 85 C. or reduction tests t 95 C nd 5 C there is lredy significnt improvement in the content of totl iron nd metllic iron with incresing reduction time. At 8 min, the content of totl iron nd metllic iron increses up to 98. nd 95.5% t 95 C nd up to 98.4 nd 97.% t 5 C respectively. As shown in ig, the cron content in the reduced smple decresed up to % t 5 C nd 8 min. The different phses formed in the reduced iron powders t 75-5 C nd t 8 min, s identified y X- ry diffrction technique, re shown in igure 4. The diffrction pttern of the reduced iron powder t 75 C shows the presence of metllic iron (eo), wustite, grphite nd iron cride (ec). At 85 nd 95 C, it ws oserved the presence of metllic iron nd grphite nd t 5 C only metllic iron phse ws otined..u Thet - Iron - ustite G- Grphite C- ec 5 C 95 C 85 C 7 5 C ig. 4. X-Ry spectr of the reduced iron powders t vrious tempertures nd 8 min. Microscopic exmintion of polished sections of reduced smples t vrious tempertures nd t 4 min nd 8 min re given in igures 5 nd 6 respectively. igures 5 to 5d, shows the photomicrogrphs of prtilly reduced smples t 4 min nd 75 to 5 C. The four photo-microgrphs show frontl reduction of oxides prticles into mgnetite (igs. 5, ) or wustite (ig. 5c, d) in n envelope of metllic iron. At 8 min nd t lower tempertures (75 nd 85 C), metllic iron dominted with minor phse of wustite were oserved s shown in igs. 6 nd 6. or reduced smples t 95 nd 5 C, iron oxides re completely converted to metllic iron s shown in igures 6c nd 6d.

5 Interntionl Journl of Scientific & Engineering Reserch, Volume 4, Issue 5, My- ISSN SEM photogrphs of reduced smples t 5 C nd 8 min re presented in igures 7 nd 7. Irregulr shpe of the prticles surfces nd prticle porosity re visile. This morphology is the typicl form of prticles of iron powder otined y reduction with cron monoxide [7, 8]. The results presented in igures to 7 shows tht the est reduction conditions for reduced iron powder re 5 C nd 8 min. Chemicl nlysis of these powders re presented in the Tle. M M 47 TABLE CHEMICAL COMPOSITIONS O REDUCED IRON PODER AT 5 C AND 8 min Compounds et % e C Si Mn P S O To reduce the cron nd oxygen content, the iron powder is nneled in n tmosphere of hydrogen t 85 C for hr. During nneling the cron nd oxygen content is considerly reduced nd the strin hrdening of prticles is removed [6, 7]. After nneling, the contents of cron nd oxygen in iron powder increses to. nd.8% respectively. 4 CONCLUSION cc The results presented nd discussed in this study cn e summrized in the following points: ()- Different prmeters for the reduction of the mill scle with cron monoxide were studied to otin the est conditions for the production of iron powder with physico-chemicl properties required y powder metllurgy. ()- The est reduction ws otined t 5 C nd 8 min in pure CO tmosphere. This is confirmed y chemicl nlysis, X-ry diffrction nd light microscope. The content of totl iron nd metllic iron in the reduced iron powder is 98.4 nd 97.% respectively. ()- Anneling y hydrogen reduces considerly cron nd oxygen content in the iron powder up to the desired vlues. (4)- The iron powders otined y the process of reduction of mill scle cn e used in the powder metllurgy industry. d ig. 5. Photomicrogrphs of reduced iron powders (x) t 4 min nd t () 75 C, () 85 C, (c) 95 C nd (d) 5 C. M: mgnetite, : wustite, : iron REERENCES c d [] ig. 6. Photomicrogrphs of reduced iron powders (x) t 8 min nd t () 75 C, () 85 C, (c) 95 C nd (d) 5 C. : wustite : iron [] [] [4] [5] ig. 7. SEM photomicrogrphs of completely reduced iron powder t 8 min nd t 5 C. [6] Bienvenu, Y., Rodrigues, S. Mnufcture of Metl Powders from Pulverulent ste, ENSMP, Centre des mtériux, CNRS UMR 76, rnce (7). Bouvrd, D. Métllurgie des poudres, Pris, Hermès Science Pulictions, (). Pyron Expnds Across the Powder Rnge. MPR, (995). Pineu, A., Knrind, N., Gllh, I. Kinetics of Reduction of Iron Oxides y H, Prt I: Low Temperture Reduction of Hemtite, Thermochimic Act 89, 447 (6). Pineu, A., Knrind, N., Gllh, I. Kinetics of Reduction of Iron Oxides y H, Prt I : Low Temperture Reduction of Mgnetite, Thermochimic Act 75, 456 (7). Kohl, H.K., Mrincek, B. Kinetics of the Reduction of Iron Oxides with Grphite, Archiv ur ds Eisen 6, 85 (965).

6 Interntionl Journl of Scientific & Engineering Reserch, Volume 4, Issue 5, My- ISSN [7] El-Gesy, A.A., Nsr, M.I. Influence of Originl Structure on the Kinetics nd Mechnisms of Cron Monoxide Reduction of Hemtite Compcts. ISIJ Interntionl, 47 (99). [8] Mrtín, M.I., López,.A. Otinment of Sponge Iron y Reduction of Steel king By- Product, st Spnish Ntionl Conference on Advnces in Mterils Recycling nd Eco- energy, Mdrid, (9). [9] Philier, J., Vignes, A. Métllurgie du mineri u mtériu, Dunod, (). [] Mondl, K. Reduction of Iron Oxide in Cron Monoxide Atmosphere, Rection Controlled Kinetics, uel Processing Technology, 86 (4). [] Coudurier, L., Hopkins, D.., ilkomirsky, I.undmentls of Metllurgicl Processes, Pergmon press, (978). [] Mesurement of the Reduciility of Iron Ore y C.R.M. Method, Liege, (98). [] Berg, B., Hnkrt, J., Poos, A. Reduciility of the Sinters. Universl Review of Mines, 9th Series. T.XVII,84 (96). [4] Normes ISO 449-, Metllic Powders, Determintion of Oxygen Content y Reduction Method, Second Edition (997). [5] Normes ISO 47, Determintion of Totl Cron Content, Comustion grvimetric Method, irst Edition, (98). [6] Akiminko,V.B. Iron Powder, Technology, Properties nd Economics,Moscow(98). [7] Aslnoglu, Z. Direct Reduction of Mechniclly Actived Speculr Iron Oxides, Miner Processing nd Extrctive Metllurgy (Trns. Inst. Min. Metll. C) 4 4 (5).