Study of the structure of hot-dip galvanizing byproducts

Transcription

1 JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 9, No. 9, Septemer 007, p Study of the structure of hot-dip glvnizing yproducts G. VOURLIAS, N. PISTOFIDIS, EL. PAVLIDOU, G. STERGIOUDIS, E. K. POLYCHRONIADIS Deprtment of Physics, Aristotle University of Thessloniki, GR 544 Thessloniki, Greece In glvnizing process severl yproducts re formed. Among them solid mterils composed y different compounds re grown in the glvnizing kettle. The present work exmines the structure of the ottom dross nd the lowing wste using X-ry nd electron microscopicl techniques. From this reserch it ws deduced tht ottom dross is composed y crystllites of the ζ-phse of the Fe- phse digrm scttered in mtrix of pure, while lowing wste, which is grnulr, is composed y spheres, whose core is of pure covered with lyer of O. Mixed Fe- oxides were lso detected in ottom dross. Hence these yproducts re minly composed y pure nd different zinc compounds, while lrge mount of iron in the form of Fe- intermetllics nd Fe oxides is present in ottom dross. In ny cse, this composition implies tht the exmined mterils re suitle for zinc recovery. (Received Jnury 5, 007; ccepted August 3, 007) Keywords: Zinc, Hot-dip glvnizing, XRD, SEM, TEM. Introduction Hot dip glvnizing is one of the most effective corrosion protection methods for steel nd iron sustrtes []. However, lthough the nticorrosive performnce of the hot dip glvnized products is undisputle nd s result this method is very dvntgeous, the production of glvnized ojects involves the genertion of severl yproducts. Among them, the glvnizing dross, which mounts to 5-7% of the zinc consumed y the coting formtion [], is one of the most importnt. Glvnizing dross is formed inside or on top of the molten zinc []. Therefore, it cn e clssified s floting or ottom dross. The floting dross (lso known s "glvnizing shes") consists minly of zinc nd/or luminum oxides nd chlorides [], while the ottom dross consists exclusively of intermetllic compounds, such s luminum sturted δ (Fe 7 ) phse []. Intermetllic compounds were lso identified in the floting yproduct, ut in this cse the min component, which ws reported, is Fe Al 5 x []. Both floting nd ottom dross contin lso lrge mount of solidified zinc, which is trpped in liquid form in the device used for their removl from the zinc th. The intermetllics of the floting nd ottom dross re formed when the concentrtion of Al nd Fe in the zinc kettle is ove their soluility limits t certin temperture. As result their formtion is unvoidle even in the cse of perfect mngement of the zinc th, ecuse the delierte Al dditions, the Fe dissolution from the immersed ojects nd the kettle wlls nd the temperture uniformity re very difficult to e controlled []. This phenomenon is lso enhnced y the Ni ddition [3-4], which is used s medium to inhiit the Sndelin effect (the formtion of Fe- lloys up to the surfce of the glvnized coting [5]). Also, the formtion of the surfce oxides, which re present in the shes, is lso impossile to e inhiited, since oth nd Al re rective metls nd t the temperture of the zinc th (out 450 o C []) their rectivity is extremely high. Consequently their rection with the tmospheric oxygen is inevitle. The presence of shes nd ottom dross in the molten zinc is undesirle ecuse, s their prticles flot or suspend inside the liquid metl, they could e trpped inside the coting nd cuse common fult known s "coting pimples" [6], resulting to low qulity coting. Therefore it is necessry to e regulrly removed from the kettle. Especilly the oxides formed on top of the th re removed every time n oject is immersed in the molten zinc. Another yproduct, formed only t the tue glvnizing fcilities during the finishing of the glvnized coting, is the lowing wste. After tue emerges from the glvnizing th, its inner surfce is sujected to compressed ir wiping, in order to diminish its roughness nd consequently lower the flow resistnce. This wy, very thin powder is produced, which is minly composed y nd O [7]. However, these yproducts re not useless. By contrst, is possile to e recovered from them thnks to its high concentrtion [, 7-8]. Consequently they re vlule row mteril. Therefore it is very importnt to e fully chrcterized, so s to collect useful dt. Furthermore their chrcteriztion could led to etter understnding of the glvnizing process, resulting to its optimiztion nd the minimiztion of their production. The ove-mentioned reserches [, 7-8] focus only on recovery process. Aprt from rief chrcteriztion consisting of rough chemicl nlysis of ech yproduct, nothing else is reported. On the other hnd, other studies [ nd references herein], tht exmine hot-dip glvnizing very thoroughly, offer only little informtion on the compounds tht compose floting nd ottom dross, without going into detils. A more detiled

2 938 G. Vourlis, N. Pistofidis, El. Pvlidou, G. Stergioudis, E. K. Polychronidis reserch ws recently pulished [9] which ws focused on the floting dross with little reference to its effect on the glvnizing process. In the present work detiled nlysis of the structure of ottom dross nd lowing wste is ttempted. In order to fulfill this work ottom dross smples were collected from locl glvnizing fcility nd lowing wste from locl tue mill. Also smples were tken from the clen zinc th nd the pure zinc ingots, s they re provided to the glvnizing industry, in order to e compred with the yproduct specimens.. Experimentl A rough flow sheet of typicl hot-dip glvnizing line is presented in Fig.. Bottom dross smples were provided y tch hot-dip glvnizing fcility. These smples were collected right fter eing removed from the zinc kettle. A tue mill provided the lowing wste. The smples of the clen zinc th nd the pure zinc ingots were provided y oth industries. The zinc th ws considered "clen" right fter the usul removl of the ottom nd floting dross, s it is performed in the industry (mechniclly with the usge of suitle shovels). The smples of the clen th were collected with specil device from three different depths (0, 00 nd 50 cm elow the zinc surfce). Ferrous Sustrte Degresing (NOH (q) ) Pickling (HCl (q) ) Fluxing (Cl. NH 4 Cl (q) ) Glvnized Product Zinc Melt Drying Fig.. Schemtic digrm of typicl tch hot-dip glv-nizing line. The che-micl sustnces used in every step re pre-sented in the rckets. The index (q) implies queous solution. The exmintion of the smples took plce with X- Ry Diffrction (XRD), Scnning Electron Microscopy (SEM) nd Conventionl Trnsmission Electron Microscopy (CTEM). For the exmintion with XRD two-cycle SEIFERT 3003 TT diffrctometer ws used with FeKα rdition (λ=.936å). The exmintion with SEM took plce in 0 kv JEOL 840A SEM equipped with n EDAX nlyzer (OXFORD ISIS 300) nd the necessry softwre to perform liner micronlysis nd chemicl mpping of the exmined sustrte. For the exmintion with TEM prt of the lowing wste smples, which were powder-like, were cptured etween silicon tiles, polished up to out 40 μm nd fterwrds thinned with ion omrdment. For the ulk smples (ottom dross nd pure zinc th) only polishing nd ion omrdment took plce. The exmintion ws ccomplished in 00 kv JEOL00CX TEM. 3. Results nd discussion 3.. Chrcteriztion of the zinc ingot nd the clen zinc th The first step of the present work ws the chrcteriztion of the pure zinc ingots used in the glvnizing process. Therefore one of them ws cut in four pieces nd out 50 prticles in totl were scrtched from every surfce of these pieces. These prticles contin in verge wt.%, s it ws determined with EDS. No impurities were detected with this method ecuse of its low sensitivity, lthough the chemicl nlysis of the product, which ws provided y its producer, mentions lso the presence of P, Cd, Fe, Cu nd Sn t totl concentrtion of out 0.04 wt.%. In ny cse, impurities of this concentrtion, lthough they re dissolved in the molten th, they do not ffect significntly the structure of the coting [0-]. Hence the zinc ingot composition hs no effect on the formtion of the different yproducts. Afterwrds the clen th ws exmined. For this investigtion out 5 smples (from ech smpling depth s it ws previously explined) were exmined with EDS in order to determine their chemicl composition. The verge results re presented in Tle, from which it is ovious tht the concentrtion of the impurities is much higher thn in the ingot. Al is deliertely dded to the zinc th in order to improve the ppernce of the finl product [], while Fe nd Cl re trnsferred y the ferrous sustrte during its immersion in the liquid metl. Fe is lso dissolved from the kettle wlls. However, s it ws mentioned erlier, impurities of so low concentrtion do not hve significnt effect on the coting structure. Furthermore the concentrtion in the th remins high enough, so s to e close to the necessry concentrtion for the production of

3 Study of the structure of hot-dip glvnizing yproducts 939 good qulity coting []. No nickel ddition tkes plce t the industries tht provided the smples, s it ws stted y their chief engineers nd consequently this metl ws not detected. Other elements such s Sn, Cu nd Cd tht originte from the zinc ores were not detected ecuse of the low sensitivity of EDS. The exmintion with XRD is in ccordnce with the EDS results. As mtter of fct, it verifies the presence of zinc, iron nd morphous zinc nd iron chlorides (Fig. ). TEM lso detected pure (PDF#04083 [3]), s it ws expected. However, the diffrction ptterns of Fig. 3 nd 3 prove tht, prt from pure, Fe- solid solutions nd Fe- phses (PDF#30478 nd PDF#3434 [3]) re present in the exmined smple. However, if we tke into ccount tht TEM is very sensitive method nd the fct tht the XRD ptterns do not include ny peks referring to these phses, it could e deduced tht in this cse their concentrtion is very low nd consequently their effect is seemingly unimportnt, t lest when the melt is clen. In ny cse from the ove exmintion it is ovious tht the zinc th lwys contins smll mount of Fe- phses. Furthermore it is likely to e sturted in Fe, if we tke into ccount the mximum Fe soluility in liquid s it is determined in the Fe- phse digrm []. From this oservtion it could e deduced tht the formtion of ottom or floting is unvoidle, since the formtion of Fe- intermetllics is fvored y the zinc th composition. Tle. Results of EDS micronlysis (verge vlues) of pure zinc th. Element Concentrtion (wt.%) Fe 0.5 Cl 0.4 Al 0.4 The presence of Fe- solid solutions is indicted y the rodness of the zinc diffrction rings reltively to the position of the rings of pure. These compounds re most proly formed during the solidifiction of the liquid smple, which ws tken from the kettle, since the th contins smll concentrtion of dissolved Fe. They re not visile with XRD due to their low concentrtion. INTENSITY (ARBITRARY UNITS) XRD Fe Chrcteriztion of ottom dross As it ws mentioned erlier, ottom dross is usully removed from the kettle y mechnicl mens (specil shovels). This method results to the simultneous removl of n importnt quntity during the clening of this yproduct. This phenomenon is ovious t the SEM microgrphs, which were recorded (Fig. 4 nd 4). Three phses re oserved in these microgrphs. One of them is lighter nd ppers like "islnds" inside the second phse, which looks like drker "se". Mny white spots re lso scttered in the second phse. Also, in oth microgrphs drk spots re present, which refer to voids, proly formed ecuse of the slow solidifiction process of the liquid metl fter its removl from the zinc kettle. Furthermore, in oth microgrphs there re stright lines formed ecuse of the imperfect polishing of the specimens prior to the SEM oservtion. ( 0 ) Γ- phse (3 3 0) θ degrees 0 Fig.. XRD pttern of the pure zinc th (: morphous nd Fe phses, : Cl ). By contrst, Fe- phses re likely to hve een lredy present in the liquid when it ws collected from the kettle nd not formed during its solidifiction, ecuse these phses re insolule in molten nd their melting point is higher thn the temperture of the th []. This fct indictes tht the clening of the zinc th is incomplete since the perfect purifiction of liquid metl in lrge-scle industril fcility is very difficult. So, their presence is not peculir. Fe- phses Fig. 3. Diffrction pttern of the zinc th (: nd Fe- solid solutions, : Γ-phse).

4 940 G. Vourlis, N. Pistofidis, El. Pvlidou, G. Stergioudis, E. K. Polychronidis 00 μm 0 μm 3 Fig. 4. SEM microgrph of the ottom dross (: x450, : x000, focused on the dr-ker re etween the lighter frgments). Tle. Results of EDS micronlysis (verge vlues) of ottom dross. The numers of the phses refer to Fig. 4. Phse Numer Concentrtion (wt. %) Fe P The results of the micronlysis of the ovementioned phses re summrized in Tle, where the verge of the dt recorded from 0 points from ech one of these phses is presented. This tle shows tht the drker re etween the lighter frgments (phse # in Fig. 4) is composed lmost exclusively y. This is the liquid trpped in the device used for the clening of the kettle. The lighter res of the dross (phse # in Fig. 4) re minly composed y, while they contin lso out 6 wt.% Fe. This composition is typicl for the ζ (zet) phse of the Fe- phse digrm []. The presence of nd the ζ-phse ws lso identified with XRD (Fig. 5). In ny cse ζ-phse is insolule in molten nd its melting point (out 530 o C []) is higher thn the kettle temperture (out 450 o C). Consequently, prticles of this phse, formed when the Fe concentrtion in the kettle is ove its soluility, precipitte t the ottom of the liquid. The white spots (phse #3 in Fig. 4) oserved in this phse contin lrge mount of P. Led is dded in the molten in order to diminish the surfce tension of the liquid nd consequently reduce the coting thickness []. However the P soluility in liquid is low, therefore lyer of molten P with n verge thickness of few centimeters precipittes t the ottom of the kettle. It is likely tht this lyer is the source of P in the exmined specimen ecuse, s it ws lredy mentioned, ζ-phse is denser thn liquid zinc nd settles t the ottom of the zinc kettle. However it is not denser thn liquid led nd s result the dross crystls flot on its surfce. Hence during their removl P is lso drifted y the tool used. INTENSITY (ARBITRARY UNITS) XRD θ degrees 0 Fig. 5. XRD pttern of the ottom dross. Peks noted with # refer to the ζ-phse of the Fe- phse digrm nd peks noted with # refer to mixed Fe- oxides. 0 μm 0 μm Fig. 6. SEM micro-grphs of two different grins of the lowing wste. In Fig. 6 the rrow points the needle-like crystls tht were oserved.

5 Study of the structure of hot-dip glvnizing yproducts 94 In ny cse from the ove exmintion it could e deduced tht ζ-phse is the min yproduct, while nd P re removed ecuse of the imperfect method pplied for the clening process. The XRD pttern revels lso the presence of mixed Fe- oxides which were not detected with EDS. Their formtion is proly due to the superficil oxidtion of the ottom dross when removed from the zinc kettle. By tht time zinc is still liquid, while the ζ-prticles re hot. As result they re very vulnerle to corrosion. If we tke into ccount tht these mterils usully remined stored for severl months the oxide formtion is not peculir. EDS did not detected ny oxygen ecuse the cross-section of the smple ws exmined fter polishing. Hence since plin view ws not nlyzed oxygen ws not reported. XRD y contrst exmined the surfce of the s-received smple, where oxide concentrtion ws rther high Chrcteriztion of the lowing wste yproduct formed only in tue glvnizing fter hotdipping. By contrst to the ottom dross whose specimens re ulk, the lowing wste is powder-like nd it is minly composed y spheres s it cn e seen t the SEM microgrphs of Fig. 6. Moreover some needle-like crystls re lso oserved (Fig. 7) in higher mgnifiction. The spheres re formed through the solidifiction of droplets during lowing. As their micronlysis shows (Tle 3) only nd O re present in them. Consequently, they re proly composed y core of pure covered with lyer of O. The dt of Tle 3 is the verge of the micronlysis 0 different spheres. The micronlysis of the needle-like crystls indictes tht these crystls re minly composed lso y nd O with mole proportion equl to :. Hence, these crystls re proly pure O. The presence of nd O ws verified with XRD (Fig. 8, O-PDF#486, - PDF#04083 [3]) nd finlly the presence of O ws lso detected y electron diffrction (Fig. 9). As it ws mentioned efore, the lowing wste is 5 μm 5 μm Fig. 7. SEM microgrphs of the needle-like phse formed on the grins of the lowing wste. INTENSITY (ARBITRARY UNITS) XRD θ degrees Fig. 8. XRD pttern of the lowing wste. Peks # to #7 correspond to (PDF# [3]) nd peks #, #, #4 nd #6 correspond to O (PDF#486 [3]). Fig. 9. ED ptterns referring to O (PDF#3645).

6 94 G. Vourlis, N. Pistofidis, El. Pvlidou, G. Stergioudis, E. K. Polychronidis Tle 3. Results of EDS micronlysis (verge vlues) of lowing wste. Phse Concentrtion wt.% O Spheres Needles Conclusions From the ove investigtion it ws deduced tht the composition of the pure zinc ingot does not contriute to the formtion of glvnizing yproducts, while the zinc th composition renders the formtion of ottom dross unvoidle. Furthermore, oth ottom dross nd lowing wste contin lrge mounts of zinc in different forms. In ottom dross minly crystllites of the ζ-phse of the Fe- phse digrm scttered in mtrix of pure were detected, while in lowing wste, which is grnulr, is composed y spheres whose the core is of pure covered with lyer of O. Bottom dross contins lso smll mount of P. Finlly these compounds could e used s row mterils for zinc recovery or for other processes for the production of different zinc compounds. However the lrge Fe concentrtion could e certin drwck. References [] A.R. Mrder, Prog. in Mt. Sci. 45, 9(000). [] F. C. Shin, B. Derin, O. Yucel, Scn. J. Met. 9, 4(000). [3] G. Reumont, R.S. De Figueiredo, J. Foct, J. Mt. Sci. Let. 8, 879(999). [4] G.P. Lewis, J. Pedersen, 3 rd Asin Pcific Glvnizing Conference, Melourne, Austrli, 996. [5] Glvnizing Rective Steels-A guide for Glvnizers nd Specifiers, ILZRA, N. Crolin (996). [6] D. Horstmnn, Fults in Hot Dip Glvnizing, Sthleisen M..H., Dusseldorf (975). [7] M.K. Jh, V. Kumr, R.J. Singh, Res. Cons. Rec. 33, (00). [8] M.A. Rh, A.S. El-Syed, Hydromet. 37, 3(995). [9] G. Vourlis, N. Pistofidis, G. Stergioudis, E.K. Polychronidis, Sol. St. Sci. 7, 465(005). [0] N. Ktiforis, G. Ppdimitriou, Surf. Cot. Tech. 78, 85(996). [] G. Vourlis, N. Pistofidis, G. Stergioudis, Crys. Res. Tech. 39, 3(004). [] ASTM B6, Stndrd Specifiction for Zinc. [3] PC Powder Diffrction Files, Version.0, JCPDS- ICDD, 000. * Corresponding uthor: gvourlis@uth.gr