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Ind. Eng. Chem. Res. 2003, 42, 318-322
MATERIALS AND INTERFACES Study of the Preparation of Zinc(II) Ferrite and ZnO from Zinc- and Iron-Containing Industrial Wastes Be´ la Kazinczy,† La´ szlo´ Ko´ tai,*,† Istva´ n Ga´ cs,† Istva´ n E. Sajo´ ,† B. Sreedhar,‡ and Ka´ roly La´ za´ r† 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-dip galvanizing sludges at 1000 °C for 5 h. Ammoniacal leaching of this mixture with a concentrated ammonia solution at room temperature by applying a 60-fold molar excess of ammonia over the whole amount of zinc for 24 h (or using a 48-fold excess of ammonia for 72 h) leads to an almost complete 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 slight loss of zinc (0.7%). Heating of the purified solution at 100 °C leads to the precipitation of pure ZnO and regeneration of the ammonia. Introduction
Experimental Section
Pure and metal-ion-doped zinc(II) ferrites, Zn1-xMxFe2O4 (1) as well as the solid solutions with MM′2O4type spinel oxides (where M and M′ are two- and threevalence metals), have been widely used as gas desulfurization absorbents,1,2 anticorrosive electrode materials in alumina electrolysis,3 oxidation catalysts,4 anticorrosive pigments,5,6 or magnetic materials in the electronic industry.7-9 Compound 1 forms in many industrial processes, e.g., in low-temperature precipitation of zinc(II) and iron(III) hydroxides,10,11 in mechanochemical activation of ZnO and Fe2O3,12 or in the hightemperature processes of zinc- and iron-containing waste materials.13-17 Formation of 1 could be observed even at 250 °C in a simple heat-treatment process of hot-dip galvanizing sludges18 containing R- and γ-FeOOH, Zn5(OH)8Cl2‚ 5H2O, and -Zn(OH)2. The formation of 1 may be induced 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 sludges is greater than 1:2, the excess zinc is converted into ZnO.18 Generally, these sludges are submitted to sintering at various temperatures between 100 and 1000 °C, and therefore the preparation of 1 and ZnO from the heat-treated hot-dip galvanizing sludges (with acid or ammoniacal leaching processes) was studied, and the results are reported here. In addition, distributions of calcium and several toxic metals during the ammoniacal and acidic treatments are also established.
Elemental composition of the starting sludge obtained from Dunaferr Steel Works, Dunaujva´ros, Hungary, was determined by ICP with an AtomScan 25 instrument (Thermo Jarrel Ash, USA). The volatile water content was determined by drying to constant weight at 105 ( 1 °C. Powder X-ray phase analysis was performed by a Philips model PWW 1050 Bragg-Brentano parafocusing goniometer equipped with a secondary beam graphite monochromator and proportional counter. The scans were recorded in step mode using Cu KR radiation at 40 kV (the tube power was 35 mA). The sludge samples were heated at 100, 200, 500, and 1000 °C for 1 and 5 h. All samples were analyzed to determine the amount of vaporized ZnCl2 and studied by powder X-ray methods. Leaching experiments were performed with 4-, 6-, 9-, 10-, or 15-fold excess of ammonia (in the form of a 25% aqueous solution) needed to dissolve the zinc content of the waste at 25 °C for 1, 3, or 24 h. To determine the dissolution rate of the toxic metals and calcium, some dissolution experiments were performed using a 50 mL of leaching agent/g of sample ratio with concentrated ammonia, water, and 1.5 M acetic and nitric acid leachants.
* Corresponding author. Tel.: (+36-1)-3257933. Fax: (+361)-3257554. E-mail:
[email protected]. † Hungarian Academy of Sciences. ‡ Indian Institute of Chemical Technology.
Results and Discussion The Fe/Zn molar ratio in the samples was 1.40:1, and the zinc content was distributed in a 81.1:19.9 ratio between Zn5(OH)8Cl2‚5H2O and -Zn(OH)2. The CaCO3 content was found to be 5.4%. The phase relations of the heat-treated sludge samples have changed basically because of the decomposition of the compounds origi-
10.1021/ie020517e CCC: $25.00 © 2003 American Chemical Society Published on Web 12/13/2002
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Figure 1. Phases formed during heat treatment. Table 1. Effect of the Leaching Time and Ammonia Excess on the Ammoniacal Recoverability of Zinc from the Sludge Dried at 100 or 200 °C recovery in weight(%) 1 1 hb
Zn:NH3 molar ratio 1:16 1:24 1:36 1:40 1:60 1:16 1:24 1:36 1:40 1:60
4c 6c 9c 10c 15c 4c 6c 9c 10c 15c
ha 5 hb
48.8 49.7 54.0 62.7 63.7
100 °C 33.0 38.4 42.2 44.0 46.7
41.5 49.5 49.4 54.5 56.5
200 °C 36.3 38.3 38.1 41.1 55.7
3 1 hb 53.5 55.5 56.8 64.3 64.2 43.0 43.8 49.8 58.4 60.9
ha 5 hb 38.5 45.0 41.4 54.1 56.2 38.0 36.7 38.1 58.9 59.2
ha
24 1 hb 5 hb 59.0 58.0 58.0 66.1 68.8 53.7 55.0 56.1 68.4 64.6
43.5 47.9 53.1 56.1 60.1 56.7 62.4 45.4 50.9 53.6
a Leaching time. b Heating time c Quantity of NH in excess of 3 what is needed to form the zinc tetraammine complex.
nally present and because of the formation of the new Zn- and Fe-containing materials.18,19 1. Heat Treatment at 100 and 200 °C. At these temperatures the main decompositions are due to the loss of physisorbed and chemisorbed water. Nevertheless, the consequence of this dehydration process (e.g., to 5 h) at 100 °C is a decreased leachability of the zinc compounds formed. At 200 °C the prolonged heating leads to a further loss of water, and the nucleation processes of R- and γ-FeOOH decrease the zinc-absorbing capability of the iron compounds present in the samples. In the case of samples dried at 100 °C, the recovery of zinc at fixed leaching conditions decreases with increased heat-treatment time. If, however, the leaching time and the amount of the excess ammonia is increased, the recovery of the zinc in the samples heated for various time periods (e.g., 1-5 h) also increases. In the case of samples heat treated at 200 °C, a similar tendency could be observed for 1 and 3 h of leaching time. However, irregular changes in zinc recovery were observed for the long-time leaching experiments (24 h). These changes were attributed to the extension of the hydroxide ion absorption and to the colloidal repeptization processes of FeOOH occurring in large excesses of ammonia. The results of the ammoniacal leaching experiments are shown in Table 1.
Table 2. Effect of the Leaching Time and Ammonia Excess on the Ammoniacal Recoverability of Zinc from the Sludge Treated at 500 or 1000 °C recovery in wt % 1 ha 24 ha
Zn:NH3 molar ratio 500 °C 1:16 1:24 1:36 1:40 1:60
4b 6b 9b 10b 15b
1:16 1:24 1:36 1:40 1:60
4b 6b 9b 10b 15b
42.1 41.9 45.4 46.2 48.5
49.5 53.6 51.7 47.3 41.5
15.5 13.6 19.3 30.1 25.0
19.4 23.9 25.1 40.5 45.7
1000 °C
b Quantity of NH in excess of what is needed to form the zinc 3 tetraammine complex.
Leachability of the zinc in the wet sludge with concentrated 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 composition of the heat-treated sludge are summarized in Figure 1. Recovery of zinc was found to be influenced by the formation of various zinc compounds possessing different reactivity toward ammonia (β-Zn(OH)Cl, basic zinc carbonate, ZnCl2, or ZnO).18 The dissolution rate of ZnO formed in various chemical processes at various temperature intervals18 strongly depends on its crystallinity, surface area, and pore size distribution.18,20 The colloid-forming and zinc-ion-absorbing ability of the intermediate iron compounds formed can also influence the zinc recovery. The Zn-absorbing abilities of the detected hydrohematite, protohematite, and R- and γ-Fe2O3 intermediates depend on the pH. Therefore, the excess of ammonia and the leaching time are important factors. The ammonia-insoluble zinc ferrite formation could be observed even at 250 °C, and the process is found to be completed up to 1000 °C.18 Because the formation of 1 can encapsulate both the zinc and the iron in a nonreactive form, the formation of 1 has dual effect on zinc leaching. To decrease the number of chemical phases,18 two sets of sintering experiments were performed at 500 and 1000 °C for 5 h. The results are presented in Table 2.
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Figure 2. Technological scheme of zinc recovering from hot-dip galvanizing sludge.
Because the dissolution of various zinc compounds and the repeptization of the iron compounds take place at different rates, the extraction time has a significant effect on the zinc recovery. In the case of the samples heated at 500 °C, for 1 h of leaching time an increase in the amount of ammonia excess increases the amount of recovered zinc. This is attributed to the increasing dissolution rate of the zinc, which exceeds the increase in the repeptization rate of the R-iron oxide. A longer (24 h) leaching time, however, is sufficient for completing the repeptization process of R-Fe2O3, especially in the presence of an excess of ammonia (higher pH), and the zinc reabsorption monotonically decreases the amount of recovered zinc. In the case of samples heat treated at 1000 °C, irregularities in the amount of leached zinc were observed at 1 h of leaching time if the amount of the ammonia excess was increased. This phenomenon can be attributed to the fact that at 1000 °C 1 and various types of sintered ZnO form. The dissolution rates of the various types of sintered ZnO are different; therefore, the recovery may change with the level of the ammonia excess. At a longer leaching time (24 h), however, the dissolution of ZnO is equalized, and the yield of the recovered Zn proportionally increases with an increase in the ammonia excess. Complete recovery of ZnO could be reached at a 60-fold molar excess of ammonia over the total amount of zinc. At lower excess of ammonia (48-fold), the recovery was found to be ca. 44% (the leaching time was 24 h) and the leaching was completed in 72 h (Table 3.). Dissolution of ZnO besides 1 leads to solid ZnFe2O4 and tetramminezinc(II) hydroxide. The amount of the recovered ammonia complex forming metals is presented in Table 4. Heating of the aqueous solution of [Zn(NH3)4](OH)2 leads to the recovery of NH3 and to the precipitation of ZnO.20 Recycling the ammonia provides an environmental-friendly method for the separation of
Table 3. Effect of Long-Time Leaching at 25 °C on the Zinc Recoverability during Ammoniacal Leaching of Sintered Hot-Dip Galvanizing Sludges at 1000 °C for 5 h leaching time, h
recovered zinc, wt %
leaching time, h
recovered zinc, wt %
24 72
25.7 43.9
168
42.8
Table 4. Distribution of the Toxic Elements and Calcium during the Ammoniacal and Acidic Leaching (wt %) after 5 h of Heat Treatment at Different Temperatures element Pb Sr Ba Ca Cd Cu As Co Ni Cr
temp
ammonia
1.5 M CH3COOH
1.5 M HNO3
25 500 1000 25 500 1000 25 500 1000 25 500 1000 25 500 1000 25 500 1000 25 500 1000 25 500 1000 25 500 1000 25 500 1000
4.25 0.81 0.85 20.9 89.9 100.0 3.3 9.4 36.1 8.3 25.4 29.4 32.1 39.9 5.30 100 56.3 23.1 91 5.9 0.1 13.4 8.1 0.3 100 4.0 0.2 4.7 0.6 18.0
4.32 0.86 0.85 83.4 100.0 100.0 14.2 30.9 40.3 27.2 49.6 66.6 33.4 40.0 57.5 100 55.7 20.4 100 15.4 17.4 13.4 8.5 29.5 100 4.3 19.7 4.8 1.6 18.0
4.17 0.85 0.85 94.6 100.0 100.0 20.5 34.3 39.5 30.8 52.4 72.3 36.4 40.0 53.1 100 57.6 22.1 100 10.9 17.2 13.5 8.5 23.2 100 4.3 19.1 4.4 2.3 37.0
the ZnO and ZnFe2O4 formed during the heat treatment of the hot-dip galvanizing sludges (Figure 2.).
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By means of the technological processes indicated by Figure 2, the preparation of, e.g., ZnO, ZnCl2, ZnFe2O4, and other various Zn salts, namely, Zn-containing and Zn-free raw materials, becomes possible from such industrial wastes as the hot-dip galvanizing sludges (fresh or sintered). In this way, the environmentally and economically useful utilization of the already sintered sludges can also be realized. The value of the investment, the available energy, the demand for Zn compounds, etc., fix the terms of a planned technological setup. By using acetic acid as a solvent, zinc(II) acetate can be prepared. Acetic acid, as a weak acid, cannot dissolve all compounds; therefore, its use is advantageous. On the other hand, compound 1 is not soluble in cold dilute mineral acids; meanwhile, ZnO dissolves easily. Thus, the preparation of ZnFe2O4 and various zinc salts can also be achieved easily. 3. Distribution of the Toxic Elements during Ammoniacal Leaching. The hot-dip galvanizing sludge contains some toxic elements (Pb, Sr, Ba, Cd, Cr, Cu, Ag, As, Co, and Ni) and calcium. Calcium is present in a large amount. Distribution of the toxic elements and the calcium during the ammoniacal and acidic leaching (see Table 4) indicates that these methods provide a possibility for the encapsulation of some toxic metals (such as As, Cu, and Ni). Both the acidic and the ammoniacal leachability of Pb and Cu decrease with increasing sintering temperature, which indicates their (Pb and Cu) incorporation into the ferrite structure. Trivalent elements (Cr and As), Co, and Ni show decreased ammoniacal and acidic leachability after heating to 500 °C, but after the 1000°C heat treatment, their dissolution increases. This indicates their incorporation into a spinel structure and a change in their position (octahedral or tetrahedral site and inverse or normal spinel structure) in the crystalline lattice. The resistance toward the dissolution depends on the chemical environment around these metals. The acid solubilities of the alkaline-earth metals and Cd increase with the heating temperature. This increase is attributed to the formation of acid-soluble oxide phases. Because of the decreased reactivity of the sintered CdO (formed at 500 °C) toward ammonia (similar to ZnO18,20), recovery of the Cd in the ammoniacal leaching process is decreased. Because compound 1 is insoluble in cold dilute mineral acids, these elements (alkaline-earth metals and Cd) may partially be recovered by means of a nitric acid leaching technique. Some of the toxic elements can form ammonia complexes during the ZnO leaching.22 The separation of these elements from the tetramminezinc(II) hydroxide solutions could be performed by a sulfide treatment.23 Calculation of the metal-ion concentration of the ammonia complex forming elements detected before and after the sulfide treatment (1 and 10 mol % for the Zn content of the solution) is presented in Table 5. The measured results are in good correlation with the calculated values, which indicates that all of these elements can be separated by means of the sulfide treatment (1 mol % sulfide ion content related to the Zn content). In this case, at pH ) 13.7 (only 0.01% of the sulfide content is protonated) ca. 0.70% of zinc precipitates as ZnS. On the basis of these results, it can be stated that ZnO can be converted to a heavy-metalfree quality form by ammoniacal leaching of the
Table 5. Distribution of Heavy Metals between the Liquid (25% Ammonia, pH ) 13.7) and Precipitate Phases during Sulfide Addition calcd values of metal concn, ppm
Ag As Cd Co Cu Ni Pb Zn
metal concn in ammoniacal leachant, ppm
1 mol % sulfide ion related to Zn
10 mol % sulfide ion related to Zn
0.109 0.143 0.099 0.033 4.095 0.399 0.300 1690
5.66 × 10-19 1.13 × 10-4 3.02 × 10-19 7.0 × 10-15 8.6 × 10-30 6.8 × 10-13 2.71 × 10-19 1683
1.77 × 10-19 3.57 × 10-6 3.02 × 10-20 7.0 × 10-16 8.5 × 10-31 6.7 × 10-13 2.71 × 10-20 1530
ZnFe2O4-ZnO mixture prepared by thermal treatment of the hot-dip galvanizing sludges. Conclusion Formation of various iron or zinc compounds during heat treatment of hot-dip galvanizing sludges leads to differences in zinc recoverability. Depending on the ammonia excess (pH), secondary processes can be evolved, e.g., repeptization of the iron compounds. Therefore, both the leaching time and ammonia excess affects the zinc leachability. A mixture of ZnFe2O4 and ZnO can easily be obtained by heating fresh or already sintered hot-dip galvanizing sludges at 1000 °C for 5 h. Ammoniacal leaching with a concentrated ammonia solution at room temperature by applying a 60-fold molar excess of ammonia over the whole amount of zinc for 24 h (or using a 48-fold excess of ammonia for 72 h) leads to an almost complete 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 slight loss of zinc (0.7%). Heating of the purified solution at 100 °C leads to precipitation of ZnO and to regeneration of ammonia.20 Distribution of the toxic metals and calcium in the ammoniacal and the acidic leaching processes of the heat-treated samples indicates encapsulation of some toxic contaminants (Pb and Cu) into the ferrite structure, while other metals (alkaline-earth metals and Cd) are transformed into their oxides, which can be recovered by acidic leaching. Some of the di- and trivalent elements, e.g., Co and Ni or Cr and As, are incorporated into the spinel structure. They can occupy both tetrahedral and octahedral sites of the spinel structure. Their leachability depends on the extent of the spinel-inverse spinel transformations. Because this latter process is temperature dependent,21 so is the leachability of these constituents. Literature Cited (1) Ahmed, M. A.; Alonso, L.; Palacios, J. M.; Cilleruelo, C.; Abanades, J. C. Structural changes in zinc ferrites as regenerable sorbents for hot coal gas desulfurization. Solid State Ionics 2000, 138, 51. (2) Akyurtlu, J. F.; Akyurtlu, A. Hot gas-desulfurization with vanadium-promoted zinc ferrite sorbents. Gas Sep. Purif. 1995, 9, 17. (3) Yu, X.; Qiu, Z.; Jin, S. Corrosion of zinc-ferrite in NaFAlF3-Al2O3 molten salts. Zhongguo Fushi Yu Fanghu Xuebao 2000, 20, 275 (CA: 134.196813).
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Received for review July 15, 2002 Revised manuscript received October 16, 2002 Accepted October 25, 2002 IE020517E
