Indian Jo urn~l of Chcm i c~ l Technology Voi.IO. March 2003. pp. 159- 165

Articles

Chemical recycling of galvanized steel scrap

M N C Ijomah * & A I Ijomah

Department of M eta llurgica l & Material s Engi neering. Nnamdi Az ikiwe University . P.M.I3. 5025. Awka. Anambra State. Nigeria

Reccil 'l'd 23 Oc/Oher 200 1: revised received 2 Dece/11/Jer 2002: accep!ed 15 Oece111hn 2002

The chemical recyc ling of galvanized steel was investigated by preferential di sso lution of the ga lvani zed coating (dezincing or degalvani zati on) in a variety of ac idic environments compri sing dilute and moderately concentrated nitri c, hydrochloric and sulphuric ac ids. Weight loss immersion tes ts were conducted for ga lvanized stee l in the various acid ic en­vironmen ts. It was genera ll y observed that the more concelllrated ac ids (I 0% or more) reacted or dezinced much faster with greater inherent attack on the steel substrate while the more dilute acids (2% or less) reacted very slow ly with parti al or in­effecti ve dezincing. Optimum resu lts were obtained at illlermed iate (5%) ac id concentrations w ith little or no attack on the underly ing stee l substrate, except in 5% HNO.~ . It was suggested that two competing cathod ic reacti ons. hydrogen evo lution and oxygen reduction, simul taneously occurred in thi s characteristic ox idi zing environment (5 % HNO.~ ) lead ing to high cor­ros ion rates observed, the remedy of which lies in electrolyte deaeration or use of appropriate corrosion inhi bi tors. These find ings were full y supported w ith opti cal microscopy . The various factors favouring or hindering the ga lvanized stee l recy­cling and the scope for possible scrap product utili za tion were discussed.

Scrap is an impo rtant raw material in the steel indu s­try and it has been projected from recent studi es that there may soon be a sho rtage of good quality sc rap, i. e., s teelworks sc rap (internal or prompt scrap) and scrap fro m the stee l-process ing industry (industri al scrap) g iven the hi gh rate of ferro us sc rap consump­ti on. For example, in conventional basic oxygen s teel making (BOS) ferrous sc rap acts both as a source of iro n and as a cooling medium . Hence, there is need to gene rate usab le sc rap from s tee l- intensive co nsumer goods and redundant investment goods. In Europe a lo ne. it is estimated that the automoti ve indu stry con­sumes cl ose to 750.000 tons of cold-rolled stee l each year. The resultant sc rap is basica lly a mixture of sin­g le-s ided and doubl e-s ided ga lvanized o r z inc-coated stee l. Only a sma ll pe rcentage of the sc rap is zinc­free . In Ni geri a (and indeed, mos t developing coun­tri es), ga lvanized s tee l is widely used in numerous industrial and domest ic constructions in c luding pipes, water tanks , storage vesse ls, transportat ion case lin­ings, fences , roofing sheets, buckets, etc . The large vo lume of scrap generated from these app li cations together with sc rap from di sused o r redun ant compo­nen ts pose seri ous probl ems to corros io n and materi­als eng inee rs, parti cul arly in the unde rdeveloped countries where , ow ing to poo r maintenance and in-

* For correspondence

adequate materials rec lamation sche mes, most of these scraps are us ually dumped and a llowed to waste away in the various urban scrap yards, constituting serious enviro nmental and safety hazards.

Various research findings on the recyc ling of ga l­vanized steel scrap 1

-9 mostly invo lve a lkaline dez­

incing, i.e., the disso luti on of zinc by hot caustic soda (NaO H) to fo rm sodium zincates (Na2Zn02 and Na­HZn02). The zi nc is recovered e lectro lytically as den­dritic powder wh ile the z inc-free stee l is readil y uti l­ized by the foundries, Fig. I (b). Spontaneous a lk al ine dissolution of zi nc (w ithout heating or the application o f exte rna l elec tric curren t) is thermodynamica ll y feas ibl e, but kinetica lly limited 11

.13

. Dissoluti on wi ll occu r at a measurable rate onl y if the z inc is in e lec­tri ca l contact with a metal of lower hydrogen overpo­tential such as platinum, iron and nicke l o r if a strong ox idi sing agent is added. However, an acid-dezinc ing recycling technique, Fig. I (a), has recentl y been in­vestigated and duly recommended as an effic ient and viab le alternative means of processing ga lvanized stee l10

. The very low acid concentrat ions usually in­vo lved makes the scheme rathe r safe and the dez­incing reaction occurs spontaneous ly with no need for heating or use of ex ternal e lectric powe r. The essen­tial logic behind the use of di lute ac ids is to com­pletely di ssolve the z inc coating with little o r no in-

Articles Ind ian J. Chem. Tec lmo l. . March 20()]

(a) The Acid Scrap-processing Scheme (h) The Alkaline Scrap-processing Scheme

Fi g . 1- Schem;lli c il lustrations o f the ocid and o/kali11 e che mi ca l recycl ing or ga lva ni zed s teel scrap

herent attack on the underl y ing steel substra te which is ultimate ly recycled fo r va ri ous end uses whi le the zinc is recoveree! from so lution either in th e pure form or as a spec ifi c meta l salt dependent upon the ty pe o f acid usee! for th e stripping.

The present work further evaluates the novel acid­clez i nci ng technique Fis-a- vis the conventional al ka­line-clez incing process and seeks to es tab li sh the op­timum acid concentrati ons for effec ti ve scrap proc­ess ing and. pro vide necessary insights into the corro­sion and fi lming properti es of ga lva ni zed steel.

Experimenta l Procedure Galvani zed steel sheets of various section thick­

nesses . viz. 0.2. 0. 1 and 0.02 em. were used . These were shredded or chopped into convenient dimens ions prior to dezincing in 25, 10, and Y'/o (or less) HC I, H :-SO~ and HN01 and the respective clez inc ing times carefu ll y recorclecl. Specimens meant for co rros ion were cut into small rec tangular pieces measuring 4 x 12 cm. the surraces o f which were partl y masked off w ith araldite adhes ive to expose a spec imen area of ;tbout 80 em: to the elec trol y te. Each spec imen was carel'u ll y we ighed. clamped to a spec imen holder and suspended vert icall y-downwards into a 4 litre capac­ity plas ti c vessel conta ining the appropria te ac id me­dium. Tota l immers ion tes ts vvere generall y under­taken. part immersion being occasiona ll y used for comparat ive mi crost ructural anal ys is or the dezi ncecl specimen surl'ace t'is-a -vis th e or ig inal unco rroded surl'ace. The spec imens were consecutive ly corroded ror 15 min. 30 min, I , 1.5, 2, 2.5, 3, 3.5 h (o r more) af'ter which th ey were thorough ly rin sed wi th d istilled

160

water, dri ed in a stream of co ld air and agai n weighed. T he weight losses were determin ed from th e cl i ller­ence in weights before and al'ter corros ion. W eight loss versus time curves were recorded for galvan ized stee l in th e specific env ironmen ts. Un less otherw ise stated, the respecti ve corrosion med ia were prepared with analar grade reagents.

Results and Discussion

Response to dezincing Consequent upon immersion, the galvani zed stee l

spec imen remains dormant for a small while and then, v igorous chemica l reac ti on (di sso lu tion or the zinc coating) l'o ll ows w ith cop ious evo luti on of gas. The reaction then abrupll y stopped or proceeclecl at a minimal pace indi ca tive o f complete di ssoluti on or the zinc coating. A non-adherent, clark -grey corrosion fi lm is left on the underl y ing steel subs trate. The re­corded dezi nci ng times ror ga lvani zed steel spcci men in the different acid media at vary ing spec imen sec­ti on thi cknesses are shown in Tab le I. I t was observed that the higher th e electrol yte concentrati on and the thinn er the spec imen cross-sec ti on, the smaller th e time req u i reel for specimen dczi nci ng.

Weight loss analysis The we ight loss measurements ror galvanized stee l

in some acid ic env ironments arc g iven in Tab le 2 ancl pl otted time curves in Fig. 2. The weight loss In ­creased w ith increasing HCI concentration. At con­stant ac id concentrati on , the weight is dependent upon the type or nature of elec tro lyte emp loyee!. An unusu-

ljomah & ljomah : Chemical recyc ling of gal vanized stee l scrap

all y hi gh and ever- increasing corrosion rate was re­corded in 5% nitric acid (HN0_1) while low but com­parative corrosion rates were observed in 5% HCI and 5% H2S04. It was also clearly ev ident th at the corro- · sion rate in H 0 3 progressively increased with time while in 5% H2S04 and 5% HCI it declined consid­erably following the initi al fast ri se and tended to sta­bili ze at a low residual threshold (corrosion arrest), a phenomenon which has obv ious implications in the chemi ca l recyc ling of ga lvani zed steel scrap.

Tabl e 1- Dczincing times for galvan ized stee l specimen in acidi c media as a function o f spec imen sec ti on thi ckness and

vary ing acid composi ti ons

Ac id med ium Dezinc in ~ time (mi n)

Sp. thi ckness (em) 0.02 0. 10 ().20

25% HN0.1 1.00 :too 5.00

JO'Yr HN01 3.00 I 0.00 14.00

.~ % HNO, 6.00 25.00 62.00

2.5% HNO, 30.00 75 .00 120.00

25 % H~SO-t 2.00 7.00 I 0.00

10'/r H ~SO-t 7.00 20.00 2!Ul0

y y,., H,SO-t I 0.00 3000 75.00

2.5% H, SO-t 45.00 105.00 150.00

25% HCI 1.50 5.00 8.00

10% HCI 6.00 19.00 24.00

5r;,, HCI I 0 00 25.00 46.00

2Yk HCI 37.00 XO.OO I 05 00

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Specimen microstructures Some microstructures of the ga lvani zed steel

spec imen before and after etching and dez incing in 5% H2S04 are displayed in Fig. 3. It is clearl y ev ident that the gal vanized stee l speci men, although smooth and attractive in appearance, is characterized by some mi cro-surface imperfection s or defec ts and these in ­clude slip and strain bands, en trained particles as well as impurity or dirt stains , Fi gs 3(a), (c) and (c). Micro­etching is obv iou. ly the very first step in the dez­incing process and can be almost instantaneous for very thin spec imens, Fig. 3(f) , but takes con siderab ly longer for thicker sec tion s, Figs 3(b) and (d). Evi­dently, etching dissolved away the strained or high energy regions (microdcfects) to obtain a stab le (regular) zi nc outer structure wh ich in turn was pro­gress ively undermined to revea l the underlyi ng stee l substrate structure. For the thi cker spec imens, this st ru cture comprised of ferrite grai ns (white) in pearlite (dark) background !Fig. 3(h)]. while the thinnes t sec­tions, the most heav il y cold-worked. exc lusive ly por­tray a network of deformation bands [Fi g. 3(g) J. Ap­parent ly , the grain s have been completely deformed into fibres by previous cold ro lling.

Corrosion prr~files The opti ca l micrograph s of some ga lvani zed steel

specimens corroded in the various acid media arc shown in Fig. 4. As expected from Fig. 2, corrosion was most severe in 5% HNO, [Fi gs 4(d), (f) and (h) 1. followed by 5% HCI [Figs 4(c) and (e)] and finall y. 5% H2S04 [Fi gs 4(a), (b) and (g)J. Moreover, inter­granular corrosion was observed on the thi cker sec-

Tab le 2- Weigh t loss versus ti me measurements for ga lvan ized stee l spec imen under changi ng electro ly te (acid) condit ions

Time Weight loss (gem -~ ) (h) 25% HCI 10% HCI 5% HCI 5% II NO, y y,, 1-I, SO-t

0 25 0.0 136 0.0047

0.50 0.0 19 1 (Ul liO 0.0052 0.011 3 0.0065

0 75 0.02 15 0.0 14X 0.0093 0.0225 0.0 I 02

I .00 0.0240 0.01 73 0.01 25 0.0297 0.0 13 1

1.50 0.0296 0.0211 0.01 44 0 O.WO 0 () 1-+4

2.00 0.0343 0.022X 0.0 149 0.0-+21 0.0143

2.50 0.0390 0.0252 o.o 156 0.0453 00 1-+1

:1.00 0.0421 0.0270 0.0 164 0.0475 0 0 1-+-+

3.50 0.0452 0.0286 0.0 I 63 O.O-+X6 0.01-+6

4.00 0.049 1 0.0307 0.0167 0.0507 (l.()J50

24.00 0.01 80 0.1 26 1 0.0 162

16 1

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~E u

"' ., ., 0

..J

£ .r:!' "' ;=

o.os-.-------------------.

o.os

0.0 4

0.03

0.02

o.O\

--a- 25%HQ _._ 10%HCI

--- 5%HCI

Q L---,---,---.---.---.---.---.---~ 0 o.s 1·5 2·5 3 3.5 4

Time (h)

Fig. 2- WeiglH loss versus ti me curves for galvani zed steel spec imen (sec ti on thi ck ness = 0.2 em) under di f ferent ac id envi­ronments

ti ons (x=O. l em, 0.2 em), expos ing a relatively equi ­axed grain stru cture while the corrosion profile of the thinnes t spec imens (x = 0.02 em) seem to foll ow pri or deformation lines [Fi g. 4(b)] and trac ing out the bas ic micros tru cture of heav il y deformed steel substrate. T he said micros tructure v is-a-v is the outer zinc coat­ing is depi cted by part-immersion [Fi g. 4(a) ].

Chemica l recycling of galvani zed steel scrap es­senti all y invo lved the selec tive or preferential di sso­lution of the external zinc coating (dezincing) to iso­late the underl y ing steel substrate which is converted to various end uses while the zinc is electroly ti ca lly recovered in the pure, metal! ic fo rm or as a speci f ie

Indian J. Chem. Techno!. . M arch 2003

chemical salt dependent on the type o f reagent used for its dissolution. Elec trochemicall y. the zinc disso­lution reacti on (dezincing) is under cathod ic con tro l and progress ive ly exposes the under iy ing steel sub­strate (increases the cathode-to-anode area rati o), thereby sustaining or accelerating the dezincing reac­ti on to completi on. The reacti on even tu all y stops or proceeds at a residual pace fo llowing complete di s­so lution of the ex tern al zinc coating and a poss ible tran sition from cathodic to mi xed con trol. The corro­sion resistance of galvani zed steel in aqueous env i­ronments depends primaril y on the stabili ty of the characteri sti c Zn (OH )2 film fo rmed wh ich in turn de­pends upon the elec trolyte concentration or pH va lue. Zn (OHh is thermodynamica ll y stab le onl y in the pH range 8.5-1 0.5 but unstabl e in ac id and very alka line solutions.

The entire dezincing process may be mathemat i­cally depicted as given below.

The magnitude o f surface defects (s lip and strai n lines etc.), the essential charac teri sti cs of a zinc coat­ing supported on a highl y deformed steel base. appar­entl y , played an important role in the dez incing proc­ess, so is the effect of vary ing secti on thickness.

For the acid-dezincing process (the main focus of the present inves tigation), it was generall y fou nd that the more concentrated acids ( I 0% or more) tended to accelerate the rate o f dezincing with seri ous attack on the underl y ing steel substrate while the more dilute acids (2% or less) tended to lower the rate of disso lu -

1. zn-. Zn2+ + 2e (Zinc dissolution) Ol c 'Ci c

';:::j

162

2. 2H+ + 2e ----. H2

(Hydrogen evolution)

Zn 2+ + 2N0·3 + Zn(N03) 2 }

zn z+ + soz·4

+ ZnS04

Formation of

zn z+ + 2CI' _. Zn(C1)2

zrnc salts

3.

4. 02

+ 2Hp + 4e ___. 40H· (Oxygen reduction)

- (]) "0

"0 'Ci <(

5. Zn 2+ + 20H· --. Zn(OH)2

(Formation of zinc hydroxide)

6. Zn(OH)2 + NaOH ---+ NaHZn02 + Hp 1-

(Formation of sodium bi-zincate)

7. Zn(OH)2 + 2NaOH ___. Nal n02 + 2Hp

(Formation of sod ium zincate)

Ol c 'Ci c

';:::j (])

"0 (]) c ro .::,{,

<(

Jjomah & ljo mah: C hemi ca l recyc ling or gal vani zed stee l sc rap

I (a ) 100:\ lJ netchcd ( x = 0.2 rm)

l( c) 2 oox Unctched (x = 0.1

~,...,........, .. ;::oon......,.~"l' ....

l(c ) 200X Unctchcd ( x = 0.02 em)

I (g) 200X Dezinced (x = 0.02 em)

l(b) Etched 3 n11n ( x

200:\ 0.2 em)

l(d) 200X Etched 2 mm (x = 0.1 em)

~: tched 20 sec (x

I (h)

200X 0.02 em)

200X Dezinced ( x = 0 .I em)

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Fig. :l- Opt ical micrographs of ga lva ni zed steel spec imens before and afte r etch ing and dezincing in 5"/c· H 2SO~ (s = spec imen ' ec tio n thick ness)

163

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2(a) Part-i mmcr·sion SOX C onodcd 20 min (5°/., H2SO_.)

2(e) x 0.1 ern lOOX CoiToded 3 hr (S'Yc, 1-ICI)

2(c) x = 0.2 em IOOX Conoded 24 hr (5'% HCI)

2(g) x = 0.1 em 1 OOX Conoded 24 hr· (S'Yo H 2SO-t)

Indian J. Chcm. Tcc hnul.. March :WO:I

2(b) x = 0.02 ern 1 OO X CorTodcd 3 hr· (5°!.> H 2SO-t) .. ~ ..... ~

' ---2(d) X

Corroded 0.1 em I OO X I hr (5°ft, I-IN03)

2(f) x = 0.2 em l OOX Conoded 3 In· (5'Yo HNOJ)

2(h) x 0.1 em l OOX Corroded 24 hr (5o/o HN03 )

Fig. 4- 0pt ica l micrographs o f ga lva ni zed steel spec imens corroded in va rious ac id med ia (x = spec imen section thick ne~~ )

104

Ijomah & Ijomah: Chem ica l recyc ling of galvan ized stee l sc rap

tion with parti al or ineffective dezincing. Not sur­pri singly, optimum results were recorded at interme­diate (5 %) acid concentrations with littl e or no inher­ent attack on the steel substrate, except in 5% HNO, where anomalous results (unusuall y hi gh corrosion rates) were obtained. It was sugges ted that two com­peting cathodic reactions, hydrogen evolution and oxygen reducti on, occurred simultaneously in thi s hi ghl y ox idi zin g medium, the hi gh, ever-increas ing weight loss versus time behaviour indicative of in­crease in both dezincing rate and subseq uent substrate corrosion. On the contrary, low corrosion rates were observed in 5°/e> HCI and 5% H2S04 solutions, the characteristic weight loss versus time curve of which dec lined sharpl y, following the initi al fast ri se, and approached a constant low threshold indicative of cor­ros ion arrest or no sign ifi can t attack on the basis metal (s teel substrate). Yet, the incidence of progres­sive electrolyte depletion whether in ac idic or alkaline sol uti on could ultimately lead to a serious ri se or de­cline in electrolyte pH , both of which, in the long run , favour passivation of the underlying steel. Thus, it is possible that a passive film was formed at so me criti­cal stage followin g the dezincing process. Neverthe­less. all available facts seem to sugges t that the aforementioned ac id-dezincing process is quite versa­tile and economical and would effect ively overcome the energy and safety demands inherent in the alkaline dezincing technique.

Conclusions

I. Chemical recycling of galvani zed steel scrap was successfull y effected by ac id-dezincing using the conventional nitric, sulphuri c and hydrochloric acids (HN03, H2S04, HCI ). Optimum results (di sso lution of zinc with virtually no attack on the steel substrate) were generall y obtained at 5% ac id concentrati ons,

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except in 5% HN03 where anomalous results (unusu­ally hi gh corrosion rates) were recorded.

2. This dev iation was attributed to the occurrence of two competing cathod ic reactions, hyd rogen evo lution and oxygen red uction in this characteristic (ox idi zing) environment the remedy of which apparentl y li es in electrolyte deaerati on, or use of the appropriate corro­sion inhibitor. 3. The aforesaid ac id-dezincin g process is or par­ti cul ar relevance to the under-developed countri es where the basic amenities and infrastructures are seri­ously lacking. Often, there is ep ilepti c power and water supply and very few fo undries can melt, let alone process, steel.

References I Dudek F J, Daniels E J & Morga n W A. flllemalional Snn­

positllll- World Zinc '93. Hobart . Australia (Oct. 199] ). 2 Dudek F J. Daniels E J & Morga n W A. Kellner A W &

Harri son J, Proc. 751ft S!ee/111aking Con{erence. ISS- AIME 75. Toronto. Ontario (A pr. 1992). 743.

3 Dudek F J. Daniels E J. Nagy Z. Za romb S & Yonk o R M. Sep Sci Tecluwl. 25 ( 13- 15) ( 1990) 2 109.

4 Dudek F J. Daniels E J & Morga n W A. ?roc. o/ lite IMM Con( Min erals. Me{([/s and Environ111elll. Manchester. Eng­land. Feb. 1992. 495.

5 Van-Rij P W & Mooij J N. lm1t and Sleet Engineer, 74(4) ( 1997) 32.

6 Barnell C J. Hill iard R V & Lang R S. J Elec/rochelll Soc. 100(4) ( 1953) 165 .

7 St. Pierre J & Pi ron D L, J App Eleclrochelll . 16 ( 1986) 447. R Koros P J. Proc. 751h Sleel111aking Conf'erence. ISS-A IME

75. Toronto. Ontario (Apr. I 992) 679. 9 Boom R P. Reinen A G & Yan-R ij P W. Se111 inar 11 !he Sleet

lndu.,·u y all(/ Recycling (Dusse ldorf, Germ any). Apr. 1995. I 0 ljomah M N C. Br Corr J (to be publi shed). II Pourbaix M. Alias of Eleurochelllical Equilibria in Aqueous

Sollllions (NACE International Press. Houston. Texas). 1974. 12 Fo ntana M G & Grene N D, Corrosion Engineering

(McGraw-H ill Press. N.Y.). 1967. 13 West J M. Elec!rodeposilion wtd Corrosion Processes (va n

Nostrand . Press, N.Y. ). 197 1.

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