MATERIALS AND INTERFACES

Study of the Preparation of Zinc(II) Ferrite and ZnO from Zinc- andIron-Containing Industrial Wastes

Bela Kazinczy,† Laszlo Kotai,*,† Istvan Gacs,† Istvan E. Sajo,† B. Sreedhar,‡ andKaroly Lazar†

Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri u. 59-67, H-1025 Budapest,Hungary, and Inorganic Chemistry Division, Indian Institute of Chemical Technology,Hyderabad 500 007, India

A mixture of ZnFe2O4 and ZnO can easily be produced by heating fresh or sintered hot-dipgalvanizing sludges at 1000 °C for 5 h. Ammoniacal leaching of this mixture with a concentratedammonia solution at room temperature by applying a 60-fold molar excess of ammonia over thewhole amount of zinc for 24 h (or using a 48-fold excess of ammonia for 72 h) leads to an almostcomplete recovery of ZnO (about 44% of the total zinc content) upon separation from ZnFe2O4.Treatment of the formed ammoniacal tetraamminezinc(II) hydroxide solution with sulfide ion(1 mol % related to the Zn content) precipitates all of the dissolved heavy metals with a slightloss of zinc (0.7%). Heating of the purified solution at 100 °C leads to the precipitation of pureZnO and regeneration of the ammonia.

Introduction

Pure and metal-ion-doped zinc(II) ferrites, Zn1-x-MxFe2O4 (1) as well as the solid solutions with MM′2O4-type spinel oxides (where M and M′ are two- and three-valence metals), have been widely used as gas desulfu-rization absorbents,1,2 anticorrosive electrode materialsin alumina electrolysis,3 oxidation catalysts,4 anticor-rosive pigments,5,6 or magnetic materials in the elec-tronic industry.7-9 Compound 1 forms in many indus-trial processes, e.g., in low-temperature precipitation ofzinc(II) and iron(III) hydroxides,10,11 in mechanochem-ical activation of ZnO and Fe2O3,12 or in the high-temperature processes of zinc- and iron-containingwaste materials.13-17

Formation of 1 could be observed even at 250 °C in asimple heat-treatment process of hot-dip galvanizingsludges18 containing R- and γ-FeOOH, Zn5(OH)8Cl2‚5H2O, and ε-Zn(OH)2. The formation of 1 may beinduced by iron(II) ions13 and the fine grain size of iron-(III) oxide hydroxide.15

Because the ratio of Zn:Fe in these types of sludgesis greater than 1:2, the excess zinc is converted intoZnO.18 Generally, these sludges are submitted to sin-tering at various temperatures between 100 and 1000°C, and therefore the preparation of 1 and ZnO fromthe heat-treated hot-dip galvanizing sludges (with acidor ammoniacal leaching processes) was studied, and theresults are reported here. In addition, distributions ofcalcium and several toxic metals during the ammoniacaland acidic treatments are also established.

Experimental Section

Elemental composition of the starting sludge obtainedfrom Dunaferr Steel Works, Dunaujvaros, Hungary, wasdetermined by ICP with an AtomScan 25 instrument(Thermo Jarrel Ash, USA). The volatile water contentwas determined by drying to constant weight at 105 (1 °C.

Powder X-ray phase analysis was performed by aPhilips model PWW 1050 Bragg-Brentano parafocusinggoniometer equipped with a secondary beam graphitemonochromator and proportional counter. The scanswere recorded in step mode using Cu KR radiation at40 kV (the tube power was 35 mA).

The sludge samples were heated at 100, 200, 500, and1000 °C for 1 and 5 h. All samples were analyzed todetermine the amount of vaporized ZnCl2 and studiedby powder X-ray methods. Leaching experiments wereperformed with 4-, 6-, 9-, 10-, or 15-fold excess ofammonia (in the form of a 25% aqueous solution) neededto dissolve the zinc content of the waste at 25 °C for 1,3, or 24 h.

To determine the dissolution rate of the toxic metalsand calcium, some dissolution experiments were per-formed using a 50 mL of leaching agent/g of sample ratiowith concentrated ammonia, water, and 1.5 M aceticand nitric acid leachants.

Results and Discussion

The Fe/Zn molar ratio in the samples was 1.40:1, andthe zinc content was distributed in a 81.1:19.9 ratiobetween Zn5(OH)8Cl2‚5H2O and ε-Zn(OH)2. The CaCO3content was found to be 5.4%. The phase relations ofthe heat-treated sludge samples have changed basicallybecause of the decomposition of the compounds origi-

* Corresponding author. Tel.: (+36-1)-3257933. Fax: (+36-1)-3257554. E-mail: kotail@chemres.hu.

† Hungarian Academy of Sciences.‡ Indian Institute of Chemical Technology.

318 Ind. Eng. Chem. Res. 2003, 42, 318-322

10.1021/ie020517e CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 12/13/2002

nally present and because of the formation of the newZn- and Fe-containing materials.18,19

1. Heat Treatment at 100 and 200 °C. At thesetemperatures the main decompositions are due to theloss of physisorbed and chemisorbed water. Neverthe-less, the consequence of this dehydration process (e.g.,to 5 h) at 100 °C is a decreased leachability of the zinccompounds formed. At 200 °C the prolonged heatingleads to a further loss of water, and the nucleationprocesses of R- and γ-FeOOH decrease the zinc-absorb-ing capability of the iron compounds present in thesamples.

In the case of samples dried at 100 °C, the recoveryof zinc at fixed leaching conditions decreases withincreased heat-treatment time. If, however, the leachingtime and the amount of the excess ammonia is in-creased, the recovery of the zinc in the samples heatedfor various time periods (e.g., 1-5 h) also increases.

In the case of samples heat treated at 200 °C, asimilar tendency could be observed for 1 and 3 h ofleaching time. However, irregular changes in zincrecovery were observed for the long-time leachingexperiments (24 h). These changes were attributed tothe extension of the hydroxide ion absorption and to thecolloidal repeptization processes of FeOOH occurring inlarge excesses of ammonia. The results of the ammo-niacal leaching experiments are shown in Table 1.

Leachability of the zinc in the wet sludge with concen-trated ammonia has already been published.20

2. Heat Treatment at 250, 500, 750, and 1000 °C.The sintering process of this sludge was studied at 250,500, 750, and 1000 C°.18 Changes in the phase composi-tion of the heat-treated sludge are summarized inFigure 1. Recovery of zinc was found to be influencedby the formation of various zinc compounds possessingdifferent reactivity toward ammonia (â-Zn(OH)Cl, basiczinc carbonate, ZnCl2, or ZnO).18 The dissolution rateof ZnO formed in various chemical processes at varioustemperature intervals18 strongly depends on its crystal-linity, surface area, and pore size distribution.18,20 Thecolloid-forming and zinc-ion-absorbing ability of theintermediate iron compounds formed can also influencethe zinc recovery. The Zn-absorbing abilities of thedetected hydrohematite, protohematite, and R- andγ-Fe2O3 intermediates depend on the pH. Therefore, theexcess of ammonia and the leaching time are importantfactors. The ammonia-insoluble zinc ferrite formationcould be observed even at 250 °C, and the process isfound to be completed up to 1000 °C.18 Because theformation of 1 can encapsulate both the zinc and theiron in a nonreactive form, the formation of 1 has dualeffect on zinc leaching. To decrease the number ofchemical phases,18 two sets of sintering experimentswere performed at 500 and 1000 °C for 5 h. The resultsare presented in Table 2.

Figure 1. Phases formed during heat treatment.

Table 1. Effect of the Leaching Time and AmmoniaExcess on the Ammoniacal Recoverability of Zinc fromthe Sludge Dried at 100 or 200 °C

recovery in weight(%)

1 ha 3 ha 24 haZn:NH3

molar ratio 1 hb 5 hb 1 hb 5 hb 1 hb 5 hb

100 °C1:16 4c 48.8 33.0 53.5 38.5 59.0 43.51:24 6c 49.7 38.4 55.5 45.0 58.0 47.91:36 9c 54.0 42.2 56.8 41.4 58.0 53.11:40 10c 62.7 44.0 64.3 54.1 66.1 56.11:60 15c 63.7 46.7 64.2 56.2 68.8 60.1

200 °C1:16 4c 41.5 36.3 43.0 38.0 53.7 56.71:24 6c 49.5 38.3 43.8 36.7 55.0 62.41:36 9c 49.4 38.1 49.8 38.1 56.1 45.41:40 10c 54.5 41.1 58.4 58.9 68.4 50.91:60 15c 56.5 55.7 60.9 59.2 64.6 53.6

a Leaching time. b Heating time c Quantity of NH3 in excess ofwhat is needed to form the zinc tetraammine complex.

Table 2. Effect of the Leaching Time and AmmoniaExcess on the Ammoniacal Recoverability of Zinc fromthe Sludge Treated at 500 or 1000 °C

recovery in wt %Zn:NH3molar ratio 1 ha 24 ha

500 °C1:16 4b 42.1 49.51:24 6b 41.9 53.61:36 9b 45.4 51.71:40 10b 46.2 47.31:60 15b 48.5 41.5

1000 °C1:16 4b 15.5 19.41:24 6b 13.6 23.91:36 9b 19.3 25.11:40 10b 30.1 40.51:60 15b 25.0 45.7

b Quantity of NH3 in excess of what is needed to form the zinctetraammine complex.

Ind. Eng. Chem. Res., Vol. 42, No. 2, 2003 319

Because the dissolution of various zinc compoundsand the repeptization of the iron compounds take placeat different rates, the extraction time has a significanteffect on the zinc recovery. In the case of the samplesheated at 500 °C, for 1 h of leaching time an increasein the amount of ammonia excess increases the amountof recovered zinc. This is attributed to the increasingdissolution rate of the zinc, which exceeds the increasein the repeptization rate of the R-iron oxide. A longer(24 h) leaching time, however, is sufficient for complet-ing the repeptization process of R-Fe2O3, especially inthe presence of an excess of ammonia (higher pH), andthe zinc reabsorption monotonically decreases the amountof recovered zinc.

In the case of samples heat treated at 1000 °C,irregularities in the amount of leached zinc wereobserved at 1 h of leaching time if the amount of theammonia excess was increased. This phenomenon canbe attributed to the fact that at 1000 °C 1 and varioustypes of sintered ZnO form. The dissolution rates of thevarious types of sintered ZnO are different; therefore,the recovery may change with the level of the ammoniaexcess. At a longer leaching time (24 h), however, thedissolution of ZnO is equalized, and the yield of therecovered Zn proportionally increases with an increasein the ammonia excess. Complete recovery of ZnO couldbe reached at a 60-fold molar excess of ammonia overthe total amount of zinc. At lower excess of ammonia(48-fold), the recovery was found to be ca. 44% (theleaching time was 24 h) and the leaching was completedin 72 h (Table 3.).

Dissolution of ZnO besides 1 leads to solid ZnFe2O4and tetramminezinc(II) hydroxide. The amount of therecovered ammonia complex forming metals is pre-sented in Table 4. Heating of the aqueous solution of[Zn(NH3)4](OH)2 leads to the recovery of NH3 and to theprecipitation of ZnO.20 Recycling the ammonia providesan environmental-friendly method for the separation of

the ZnO and ZnFe2O4 formed during the heat treatmentof the hot-dip galvanizing sludges (Figure 2.).

Table 4. Distribution of the Toxic Elements and Calciumduring the Ammoniacal and Acidic Leaching (wt %) after5 h of Heat Treatment at Different Temperatures

element temp ammonia 1.5 M CH3COOH 1.5 M HNO3

Pb 25 4.25 4.32 4.17500 0.81 0.86 0.85

1000 0.85 0.85 0.85Sr 25 20.9 83.4 94.6

500 89.9 100.0 100.01000 100.0 100.0 100.0

Ba 25 3.3 14.2 20.5500 9.4 30.9 34.3

1000 36.1 40.3 39.5Ca 25 8.3 27.2 30.8

500 25.4 49.6 52.41000 29.4 66.6 72.3

Cd 25 32.1 33.4 36.4500 39.9 40.0 40.0

1000 5.30 57.5 53.1Cu 25 100 100 100

500 56.3 55.7 57.61000 23.1 20.4 22.1

As 25 91 100 100500 5.9 15.4 10.9

1000 0.1 17.4 17.2Co 25 13.4 13.4 13.5

500 8.1 8.5 8.51000 0.3 29.5 23.2

Ni 25 100 100 100500 4.0 4.3 4.3

1000 0.2 19.7 19.1Cr 25 4.7 4.8 4.4

500 0.6 1.6 2.31000 18.0 18.0 37.0

Figure 2. Technological scheme of zinc recovering from hot-dip galvanizing sludge.

Table 3. Effect of Long-Time Leaching at 25 °C on theZinc Recoverability during Ammoniacal Leaching ofSintered Hot-Dip Galvanizing Sludges at 1000 °C for 5 h

leachingtime, h

recoveredzinc, wt %

leachingtime, h

recoveredzinc, wt %

24 25.7 168 42.872 43.9

320 Ind. Eng. Chem. Res., Vol. 42, No. 2, 2003

By means of the technological processes indicated byFigure 2, the preparation of, e.g., ZnO, ZnCl2, ZnFe2O4,and other various Zn salts, namely, Zn-containing andZn-free raw materials, becomes possible from suchindustrial wastes as the hot-dip galvanizing sludges(fresh or sintered). In this way, the environmentally andeconomically useful utilization of the already sinteredsludges can also be realized. The value of the invest-ment, the available energy, the demand for Zn com-pounds, etc., fix the terms of a planned technologicalsetup.

By using acetic acid as a solvent, zinc(II) acetate canbe prepared. Acetic acid, as a weak acid, cannot dissolveall compounds; therefore, its use is advantageous. Onthe other hand, compound 1 is not soluble in cold dilutemineral acids; meanwhile, ZnO dissolves easily. Thus,the preparation of ZnFe2O4 and various zinc salts canalso be achieved easily.

3. Distribution of the Toxic Elements duringAmmoniacal Leaching. The hot-dip galvanizing sludgecontains some toxic elements (Pb, Sr, Ba, Cd, Cr, Cu,Ag, As, Co, and Ni) and calcium. Calcium is present ina large amount. Distribution of the toxic elements andthe calcium during the ammoniacal and acidic leaching(see Table 4) indicates that these methods provide apossibility for the encapsulation of some toxic metals(such as As, Cu, and Ni).

Both the acidic and the ammoniacal leachability ofPb and Cu decrease with increasing sintering temper-ature, which indicates their (Pb and Cu) incorporationinto the ferrite structure. Trivalent elements (Cr andAs), Co, and Ni show decreased ammoniacal and acidicleachability after heating to 500 °C, but after the 1000°Cheat treatment, their dissolution increases. This indi-cates their incorporation into a spinel structure and achange in their position (octahedral or tetrahedral siteand inverse or normal spinel structure) in the crystallinelattice. The resistance toward the dissolution dependson the chemical environment around these metals.

The acid solubilities of the alkaline-earth metals andCd increase with the heating temperature. This increaseis attributed to the formation of acid-soluble oxidephases. Because of the decreased reactivity of thesintered CdO (formed at 500 °C) toward ammonia(similar to ZnO18,20), recovery of the Cd in the ammo-niacal leaching process is decreased.

Because compound 1 is insoluble in cold dilutemineral acids, these elements (alkaline-earth metalsand Cd) may partially be recovered by means of a nitricacid leaching technique.

Some of the toxic elements can form ammonia com-plexes during the ZnO leaching.22 The separation ofthese elements from the tetramminezinc(II) hydroxidesolutions could be performed by a sulfide treatment.23

Calculation of the metal-ion concentration of the am-monia complex forming elements detected before andafter the sulfide treatment (1 and 10 mol % for the Zncontent of the solution) is presented in Table 5.

The measured results are in good correlation with thecalculated values, which indicates that all of theseelements can be separated by means of the sulfidetreatment (1 mol % sulfide ion content related to theZn content). In this case, at pH ) 13.7 (only 0.01% ofthe sulfide content is protonated) ca. 0.70% of zincprecipitates as ZnS. On the basis of these results, it canbe stated that ZnO can be converted to a heavy-metal-free quality form by ammoniacal leaching of the

ZnFe2O4-ZnO mixture prepared by thermal treatmentof the hot-dip galvanizing sludges.

Conclusion

Formation of various iron or zinc compounds duringheat treatment of hot-dip galvanizing sludges leads todifferences in zinc recoverability. Depending on theammonia excess (pH), secondary processes can beevolved, e.g., repeptization of the iron compounds.Therefore, both the leaching time and ammonia excessaffects the zinc leachability.

A mixture of ZnFe2O4 and ZnO can easily be obtainedby heating fresh or already sintered hot-dip galvanizingsludges at 1000 °C for 5 h. Ammoniacal leaching witha concentrated ammonia solution at room temperatureby applying a 60-fold molar excess of ammonia over thewhole amount of zinc for 24 h (or using a 48-fold excessof ammonia for 72 h) leads to an almost completerecovery of ZnO (about 44% of the total zinc content)upon separation from ZnFe2O4. Treatment of the formedammoniacal tetraamminezinc(II) hydroxide solutionwith sulfide ion (1 mol % related to the Zn content)precipitates all of the dissolved heavy metals with aslight loss of zinc (0.7%). Heating of the purified solutionat 100 °C leads to precipitation of ZnO and to regenera-tion of ammonia.20

Distribution of the toxic metals and calcium in theammoniacal and the acidic leaching processes of theheat-treated samples indicates encapsulation of sometoxic contaminants (Pb and Cu) into the ferrite struc-ture, while other metals (alkaline-earth metals and Cd)are transformed into their oxides, which can be recov-ered by acidic leaching. Some of the di- and trivalentelements, e.g., Co and Ni or Cr and As, are incorporatedinto the spinel structure. They can occupy both tetra-hedral and octahedral sites of the spinel structure. Theirleachability depends on the extent of the spinel-inversespinel transformations. Because this latter process istemperature dependent,21 so is the leachability of theseconstituents.

Literature Cited

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Table 5. Distribution of Heavy Metals between theLiquid (25% Ammonia, pH ) 13.7) and Precipitate Phasesduring Sulfide Addition

calcd values of metal concn, ppm

metal concn inammoniacal

leachant, ppm

1 mol %sulfide ion

related to Zn

10 mol %sulfide ion

related to Zn

Ag 0.109 5.66 × 10-19 1.77 × 10-19

As 0.143 1.13 × 10-4 3.57 × 10-6

Cd 0.099 3.02 × 10-19 3.02 × 10-20

Co 0.033 7.0 × 10-15 7.0 × 10-16

Cu 4.095 8.6 × 10-30 8.5 × 10-31

Ni 0.399 6.8 × 10-13 6.7 × 10-13

Pb 0.300 2.71 × 10-19 2.71 × 10-20

Zn 1690 1683 1530

Ind. Eng. Chem. Res., Vol. 42, No. 2, 2003 321

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Received for review July 15, 2002Revised manuscript received October 16, 2002

Accepted October 25, 2002

IE020517E

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