Influence of Pulsed Mechanical Activationof Hematite- Graphite-Aluminum powder Mixtures

on the Reduction of Iron OxidesL. E. Bodrova, N. A. Vatolin, E, A. pastukhov, S. A. petrova, E. A. popova, and R. G. Za.kharov

It)ltitute af ltletallurg), Ltral Brunch, Russian Academ! of Scie ces, yektterinburg, Russio

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Abstract-Jb d€crease tl'le tenlperature ofdirect iron reduction by carbon and aluminum, shon-term pulsedmechanical activatidl (PMA) oian Fc.Or + C*. + AI powder mlxture is perfumed during sound_frequencyshock loading by a flat aclivating plunger. The PMA efliciency lor powders in comparable with mechanicalactivation in high-cncrgy ball mills in a decreasc in the activatioo lirire and retaining the chemical purity ofapowder composition.

DOI: I 0.1 l34ils0036029s I I I 10024

INTRODUCTIONOne ofthe aspects ol improvement ofiron produc-

tion by direct reduction ofits oxides with carbon-con-taining nraterials is rclated to scarching for optimalways to decrease the tentpelatures of leducing reac_tions, which is prontoted, in particular. by an additionofmore intensive reducing agents (Al, Ca, Si, Mg) andby mechanical activation ofreagents nrainly by nti ingjn ball mills over a long period of time Il-3]. Whenanalyzing all previous studies ofmechanical activationintended for the reduction of metal oxides with car-bon, the authors of [3] used the exothermic eflect ofaluminothermic reduction in order to decrease thetemperature of the endothermio carbotherlnjc reduc-tion ofhenlatite. Tlte iniluence of alun.tinum addition(0 l0%) on the hematite reduction by graphite wasstudied on heating to I i00.C aftcr mechanical activa-tion of their nrixture in a Frjtsch pulverisette high-energy planetary nriil in 2, 5 and l0 h, and the possi-bility of a signiilcaltt decrease in the initial tempera-tules ofreducing processes was denonsrrareo.

In this work, instead of long-term grinding in ballmill, we used short-ten.n pulsed mcchanical activation(P\4A) oi le,O, - C,. I Aj powder rr)i\tures durinBrmpacl sound-lr(quenc) loading in ord(r ro \tud) thepossibility of decreasing the temperarure of pllasetranstonnations in reducing annealil.lg.

EXPERIM ENTALThe Institute of Metallurgy has an installation for

p[ocessing of powders and Iiquids by elastic ]ow-iie-quency vib[ations. All prevjous expe riments using thisinstallation were carried out under condition ofgener_ation ofair flows [4] or liquid flows [5] in the opCrating

space under and around a vibrating plunger. In thiswork, a new approach was applied to this installation:mechanical activation ofpowdered reagents by pulsedblows witlr a flat plunger on the total powder volume.The initial stage of prelirr.rinary mjxing (PM) of pow-ders with air was perfornted at the ratio olthe plungerbottom-to-powder mixture suface distance to thepowder layer height of I : I . The plunger was then low-€red onto the powder surfaca to achieve the ma{imumpossiblc contact, and thc mixture was proccssed for5 and I 0 nrin by P\4A a plunSer intpact frequcncy of40 s . an accelerarion ol 240 m/s:. and a pu.hinglorce of I200 N.

Fe2Or + Cg. + Al powder miKures were prepared ina weight ratio of 8 : 2 : l. Reagent grade commercialhematite powder was used as initial Fe,O, annealed at1000"C in a CO - A.renvironnlent and n tijlcd ro a par-ticle size of <50 pnt. According to X-ray diffractiondata, the reagent alter amrealing consisted only ofFe2O.r. Powderod Alwas obtained by washing of pAp-2aluminum powder (GOST Srandard 5494-95) fronrfatty ojl in sevcral solvcnts. High-purity graphite witha maxinrum pafiicle size of5 pm was used as Cr,.

The rnixtures processed by PM and PMA were usecllbf f'ablication of I,g pellets at a pressure of 40 M pa;they were llren placed into small gr.aphite crucibleswith lids. The small crucibles we re placed into a widegraphile cruciblc. fillcd wirh a graphit. powdcr, andheatcd to ll00'C at a ratc of -1O.C/nin in a resis-lance furna(c in a constant argon ilou In reducingannealing, the small crucibles were seouentiallvfe movcd t -ol) the turnace staflirrg li.on r 200.C at a srepoi 100'C. The pellers in rlte snrall elucibles wericooled in air and weighed. The phasc conteltt wasdetermined by seni-quantitativc X-ray diffraction

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Fig. l. lnfluencc ol reducing anncaling tlnrpcraturc I onthe phasc composition olan Fc.or + Csr + Al powdcrnlirturc aficf (dashcd li'rcs) PM and (solid lincs) PMA.

analysis (XRD) using a DRON-3M difiiactometerand Cul(c( radiation.

RESULTS AND DISCUSSION

Thc XRD anabsis of thc spccimens dcmonstrateslhat, during thcir annealing. the content of the initialplrases decreases and the following newphases are gener-ated: magnetite Fero4. wristite FeO. spinel FeO Al2Or

Influence of preliminary preparation of a powder n'rixtureon thc tcnlpctatures ol the onsct and cnd ol acductjon ol'iron oxides (r.r. /\., and rr) and thc metallic iron content(Fe,rct) at ll00"C

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Fig.2. lhriarion oflhe chan8c in rhe pcllel q,eight A/l, li)l( /) PM and (2) Pl\4A nii\tures as a llnclior of the rdduc-iDg anncaling tcmpcrature

and metallic iron Fen,",. The XRD data are illusrratedI

in Fig. I .

The chalacter of thc Fe,Oi curves. which describea gradual decrease in the lremattitc content duringheating fbr the pellcts after PM and PMA. varicsequally with tlre tellperature: there are even sintilalproc€ss delay steps a1 600-700oC in a PM specimenand at 3-50-500"C in a PMA specimen. In this case.the magnetite Fe ]Oo content curves contain delay step,which can be related to an incubation pcriod of struc-tural rearrangements in hentatite. in tlte same tentper-ature range. The hentatite content in the PM ntixtures(dashed curves) begins to decrease alter 500'C.whereas aboul one-third of FC.OJ is reduced lo mag-netite in the PMA nrixture (solid curves) already at_r50"c.

At the sta8e ofSeneration ofwiistite FeO, the cffectof PMA nlanifests itselfalso in a decrease in the tentper-aturc ofonset ofthe process by >2000C. Here , the nrag-netite and wiistite content curyes reach nraxima, at 600and 90{)'C for a PMA rnixture and at 800 and 1000'Cfbr a PM mixture, respectively- Spinel FeO . Al,Orfbrms in the nixtures almost simultaneously with \\'0s-Iite. Before the ternrination ofthe redustion of Fer- toFe2*. nretallic iron appears starting liont tentperaturcs>800 and >1000'C in PMA and PM nrixtures.resPectivelY.

Thus. the activation of PMA |rixtures ntanif'estsitself in a decrease in the ternperature of the onset ofhematite reduction by >350oC and the tentperaturc ofthe appearance of metallic iron by 200"C. The influ-ence of powder prepantion conditions on the sontentof nrelallic iron after rcducing h€aling to I 100'C canbe lmcked tron1 the data sunlrltarized iu the table.

The pellet weight variation as a lunctiou of thereducir)g heating tenrperature is illustlated in Fig. 2-The observed insignificant scattcring of the points in

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RUSSIAN METALLURGY (MITALLY) Vol. 2r)ll

INFLUENCE OF PULSED

the cutves is relatsd to nonideal homogeneity ofpow-der nixing and does not inlluence the general charac-ter of the cun,es. The inorease in the pellet weight by2-3% at the beginning of reducjng heating can b-attributed to aluminum oxidation by adsorbed oxygenretained on the particle sufaces ofthe initjalpowden.

It can be seen that the weight decrease related tothe carbon loss during carbotltermic reduction staftsin PMA mixtures from 700 800.C, thar is, by 200oCearlier tltan in PM mixtures. For a PMA mix(ure, tltereductiolt intensity also incleases: at ll00"C, theweight foss is l9%, whereas it is no higher than 7.5%for a PM mixture.

The table summarizes the data on the tempelaturesofthe onset oi'lrematite reduction from the beginningof appearance of magnetite (l,r) and il.om the temper,ature of beginning a decrease in the pellet weight (/sr).The tempelature olthe end of aluminothermic redut-tion (/f) is determined fron] the tempetature ofdisap-pearance of aluntinum using XRD data. lt followsfrom a comparison ofthe data in tlre table and in Fig. Iconcerning tite relative pltase content that tsr is thetemperaturc of starting of aluminothcrmic reductionand t r is thlr lemperature of star-ting of carbothermicreduction.

With the aim ofrevealing the jnfluence ofthe pMAtinre, we carried out an additional experiment with anincrease in the powder actjvation tinle to l0 min. XRDanalysis of this powder alter reduction heating toI l00'C and holding in 3 min demonstrates that theprocess€s of reduction are completed and iron-con-taining phases are only present by metallic iron Fe",.r.ln pcllets of the same con'tposition obtained aftcrmechanioal activation in I 0 h after holding at t 100.Cin 30 min, there exists non-reduced iron in spinelFeO.Al2O.r in addjrion to metallic iron [3].

The highcr efficiency of powdered reagents alierPMA in contparison with activation in ball ntills canbe explained by the fact that the kinetic energy ofacti-vating plungeI is tt.anslerred to all particles ofthe pro-cessed material simultaneously, whereas the small(point) contact area between activated powde$ andmilling balls rcquires a longer timo for obtaining ahomogenously activated powder volume. TIle latterresults in longer tinte intervals of all chemical reac-tions during subsequent heating. as mentioned in [3].

MECHANICAL ACTIVATION t029

One more impoftant advantage ol application ofthe PMA method is the absence of contact betweenthe activating plunger and the crucible walls, whichprovides preser'/ation of chemical purity of the acti-vated powden, whereas a conmon problom ofmechanical chemistry is the contantination of pro-cessed powden with the materials ofthe milling bodies(balls) and drum as a consequence oftheir liacture dueto a high lbrce concentration at collisions.

CONCLUSIONS(l) Our cxperimcnts dcmonstrated that the

sequence of phase formation in the FelOr C",-Alsystem upon reducing heating corresponds to a grad-ual decrease in tl're iron vale nce: FerO, -..* FeqO4 -*(FeO, FeO . Al,O, -* Fe","r.

(2) Short-term (5 min) pulsed mechanical activa-tion ( PMA) of powder mixtures by sound-frequencyimpact loads using a flat aotivating plunger beforeleducing anrlealing decreases the temperatures of theonset and end ofall reducing stages by 200 300"C.

(3) The high efliciency of PMA processing is likelyto be causcd by the lal€e contact area ofthe contactingsufaces during an impact.

REFERENCESl. I- F. Kurunoy "Direct lron Manufacture and Direct

MetallurSy of Cast Iron in the XXI Century" Metal-lurg, No. 6. 27-32 (2010).

2. A. I. Ancharov ct al., Mechanical CompL)sites- Precursot sfbr Developtnent of M1terials vttll Innovative hopenies(Siberian Branch, Russian Academy of Sciences,Novosibirsk, 2010).

3. J. V Khaki, M. Panjehpouf, Y Kashiwaya, K. Ishii. andM. S.-S. Bafghi, 'A Study ofrhe Eflect ofAluminiumon the Mechanocl'tentical Reduction of Hematite byGraphitc 'Steel Rcs. lnrern. 75 (3),169-177 (20041.

4. L V Frjshberg. L L Telitsin, S. V Zhidovinova.E. A. Pastukhov, aDd N. A. Vatolit], 'lnfluence of LowFrequency Sound Prooessing on Reactive Ability ofMctal Powdem," Dokl. Ross. Al(ad. Nauk 317 (4),879-883 ( l99 t).

5. E. A. Pastukhov. E. A. Popova, L. E. Bodrova, andN-A.\htolin, 'Peculiar Features ol Cavitation Processcs during the Action on Liquid MedjuDt by LowFrequency Vibrations in Caviation Mode," Raspla$,.No. 3, 7 l3 (1998).

RUSSIAN METALLURCY (METALLI Vol. 201 I No. I I