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Ind. Eng. Chem. Res. 2002, 41, 720-725
MATERIALS AND INTERFACES Phase Relations and Heat-Induced Chemical Processes in Sludges of Hot-Dip Galvanization Be´ la Kazinczy, La´ szlo´ Ko´ tai,* Istva´ n E. Sajo´ , Sa´ ndor Holly, Ka´ roly La´ za´ r, Emma Jakab, Istva´ n Ga´ cs, and Kla´ ra Szentmiha´ lyi Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri u. 59-67, H-1025 Budapest, Hungary
The hot-dip galvanizing sludges have been studied with the aim of establishing phase relations and chemical processes taking place during the thermal treatment. Phase relations in the sludge heated for 1-5 h between 100 and 1000 °C were studied by infrared, powder X-ray, Mo¨ssbauer, and scanning electron microscopy methods. Dehydration and decomposition processes were also monitored by a thermogravimetry-mass spectrometry technique. Results of our investigations indicate that during the thermal treatment ZnFe2O4 and ZnO were formed, while R- and γ-FeOOH (goethite) as well as the basic zinc chlorides were decomposed and finally cannot be detected in the 1000 °C heat-treated samples. Because the major part of the zinc is present in the form of aqueous ammonia-insoluble ferrite, instead of ammoniacal leaching, another technique using, e.g., iron(II) sulfate, has to be used for recovering the zinc from these types of heat-treated samples. 1. Introduction Sludge forms via neutralization of the hydrochloric acid exhausted during pickling of galvanized scraps.1 Generally lime is used for its neutralization, and the conditions of the neutralization (temperature, pH, amount of dissolved oxygen, oxygen/iron(II) concentrations, etc.) have an influence on phase composition. The sludge is a mixture of modified oxides, hydrated oxides, hydroxides, and basic chlorides of iron and zinc as well as water and calcium compounds (mainly carbonate). Its moisture, iron, and zinc contents were found to be 63.1%, 5.7%, and 8.7%, respectively.2 This sludge contains large amounts of iron that could be recycled in the blast furnace. Because the Zn input to the blast furnace must be limited, the Zn in the sludge has to be removed. In this course, this zinccontaining waste may also be utilized as a Zn raw material. For zinc recovery, a cost-effective and environmentally friendly ammoniacal leaching procedure has recently been developed.2 Thermal shrinking of zinc-containing sludges is widely applied for decreasing their volume prior to disposal. In view of zinc leaching, however, the knowledge of the change in composition taking place in the course of the thermal treatment is important; e.g., formation of various iron oxides and hydrated oxides, due to their zinc-absorbing capacity,3-7 has to be taken into consideration if the leaching is performed on iron-containing wastes. Therefore, our experiments were directed toward the investigation of hot-dip galvanizing sludges with the * Corresponding author. E-mail:
[email protected]. Tel.: +36-1-3257933. Fax: +36-1-3257554.
aim of establishing phase relations and chemical processes taking place during the thermal treatment. Phase relations in the sludge heated for 1-5 h between 100 and 1000 °C were studied by infrared (IR), powder X-ray, Mo¨ssbauer, and scanning electron microscopy (SEM) methods. Dehydration and decomposition processes were also monitored by a thermogravimetrymass spectrometry (TG-MS) technique. 2. Experimental Section Elemental composition of the sludge (Dunaferr Steel Works, Hungary) was determined by inductively coupled plasma (ICP) spectroscopy (instrument: Atom Scan 25, Thermo Jarrel Ash). The water content was measured by drying (105 °C). Thermal decomposition was studied by TG-MS (instrument: Perkin-Elmer TGS-2 thermobalance associated with a Hiden’s HAL 3F/PIC mass spectrometer equipped with a fast ion counter). The heating rate in the decomposition studies was 10 °C min-1, and high-purity argon was used as an ambient gas (flow rate: 140 cm3 min-1). The evolved gases were transferred from the sample pan to the mass spectrometer through a heated capillary within approximately 1 secundum. The ion source was operated at 70 eV electron energy. Selected mass spectrometric intensities were normalized by the initial sample mass and by the sensitivity factors calculated from the observed isotope intensity 38Ar+ of the ambient gas. The differential thermogravimetry (DTG) curves were calculated by spline smoothing. IR spectra were recorded in diffuse reflectance mode at room temperature in KBr (instrument: Nicolet 205 FT-IR). X-ray powder diffraction scans were obtained by a Philips model PWW 1050 Bragg-Brentano parafo-
10.1021/ie010313i CCC: $22.00 © 2002 American Chemical Society Published on Web 01/26/2002
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cusing goniometer equipped with a secondary beam graphite monochromator and a proportional counter. The scans were recorded in step mode using the Cu KR radiation at 40 kV (tube power: 35 mA). Quantitative phase composition was obtained via the evaluation of the diffraction patterns by means of a full profile fitting technique. Mo¨ssbauer spectra were obtained at 300 and 77 K in the constant-acceleration mode (instrument: KFKI standard spectrometer equipped with a 57Co/Rh source). Electron microscopic measurements were performed with a Hitachi S-570 scanning electron microscope equipped with a Rontec EDR 288 detector. The degree of zinc loss due to evaporation of ZnCl2 from the sample heated at 100, 250, 500, 750, and 1000 °C for 1-5 h, respectively, was obtained via the determination of zinc in the samples. 3. Results and Discussion 3.1. Neutralization of the Exhausted Pickling Solution. The main components of the exhausted pickling solution are zinc(II), iron(II), and iron(III) chlorides. Slaked lime was used for the neutralization. As a consequence of increasing pH, iron(II), iron(III), and zinc(II) hydroxides may form; however, because of the complex chemical system, formation of other products has also been induced: (i) aging of Fe(OH)3 results in the formation of FeOOH; (ii) during coprecipitation of Fe(OH)2 and Zn(OH)2, mixed (Zn,Fe)Fe2O4 ferrites might form;8 (iii) Fe(OH)2 could be immediately oxidized and transformed into Fe3O4 (FeFe2O4 spinel) or γ-FeOOH; (iv) in the presence of Zn2+ ions, Fe(OH)2 can form (Fe,Zn)Fe2O4. These processes are well-studied.8,9 It has already been established that the relative amounts of Fe3O4, R-FeOOH, and δ-FeOOH highly depend on the reaction conditions.10 The phase diagram of the Zn2+-Cl--OH--H2O system11 shows that at higher Zn(II) concentrations and lower pH values (initial conditions in exhausted pickling solutions) ZnCl2‚4Zn(OH)2‚5H2O forms. It is also shown that simultaneous with the progressing neutralization the value of the pH is increasing, the Zn(II) content is decreasing (via isomorphous substitution of the Cl ions for OH ions), and ZnCl0.206(OH)1.794 and -Zn(OH)2, respectively, precipitate. The reaction of ZnCl2 with Ca(OH)2 leads to the formation of Zn(OH)2, basic zinc chlorides, and Ca4O3Cl2‚15H2O-type basic calcium chloride.12 Because the calcium hydroxide may be contaminated with calcium carbonate, interaction of calcium carbonate with zinc(II) chloride can also be considered. Although the smithsonite formation is generally negligible,12 its presence beside the basic zinc chlorides (e.g., 4Zn(OH)2‚ZnCl2‚5H2O) can also be presumed.13 3.2. Phase Relations in Sludges Dried at Room Temperature and 100 °C. Because of the small particle size and crystal defects,14 the freshly formed iron and zinc oxides are amorphous.15,16 We identified noncrystalline iron- and zinc-containing phases by their IR spectra,17,18 and IR data on iron oxides and hydrated oxides14,19-21 show that modified forms of Fe2O3 and Fe(2-x)/3O3-x(OH)x (protohematite and hydrohematite) can be excluded. Although the band position of hydrated iron oxides depends on the particle size, the surface conditions, and the water content of the (hydrated iron oxides containing) sludge, the presence of only R- and γ-FeOOH, ZnCl2‚4Zn(OH)2‚5H2O, and -Zn(OH)2 could be confirmed in the sample dried at room temperature. The IR spectra of the materials dried at room temper-
Figure 1. IR spectra of dried (A) and heat-treated sludge samples (heating time was 5 h; heating temperatures are b ) 100 °C, c ) 250 °C, d ) 500 °C, e ) 750 °C, and f ) 1000 °C).
Figure 2. TG-MS plot of air-dried sludge using a 10 °C/min heating rate: s, TG and DTG; b, water (m/z 18); ×, carbon dioxide (m/z 44).
ature and 100 °C respectively for 5 h are shown in parts a and b of Figure 1. The most significant difference between the two spectra is that the band at 997 cm-1 disappears if the sample is dried at 100 °C. This band belongs to the hydrated form of the basic zinc chloride. The increase in the intensity of the band at 874 cm-1 indicates the appearance of partly dehydrated basic zinc chloride.23,24 The presence of a relatively narrow νOH band above 3000 cm-1 and the disappearance of δOH (free water) bands at around 1619 cm-1 suggest the loss of free physisorbed water and the loss of water in -Zn(OH)2,2 respectively. This was confirmed by the TG-MS measurements showing the loss of water around 100 °C (Figure 2). The X-ray study of the dried sludge showed almost the same peaks as ZnCl2‚4Zn-
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Figure 3. X-ray diffractogram of heat-treated samples (a ) 100 °C, b ) 250 °C, c ) 500 °C, d ) 750 °C, and e ) 1000 °C; the heating time was 5 h). Table 1. Mo1 ssbauer Data Obtained from 300 and 77 K Spectra of Dried and 750 °C Heated Samplesa 300 K Fe3+
dried Fe3+magn 750 °C Fe3+ Fe3+
77 K
IS
QS
RI
IS
QS
MHF
RI
0.35
0.65
100
0.44 0.38 0.43
0.70 0.20 0.45
46
55 45 100
0.35 0.35
0.63 0.40
31 69
a IS: isomer shift, related to R-iron. QS: quadrupole splitting, mm/s. MHF: magnetic hyperfine field, T. RI: relative spectral area, %.
(OH)2‚5H2O.23 The slight shifts related to the d values of ZnCl2‚4Zn(OH)2‚5H2O are attributed to the decrease of the water content (Figure 3). Further shifts occurring in the d value may be accounted for isomorphic substitution of Cl- ions for hydroxide ions in the lattice of ZnCl2‚4Zn(OH)2‚xH2O. Results of the Mo¨ssbauer measurements are shown in Table 1. In the case of the dried sample, the data obtained are in agreement with the results of the IR spectroscopy. The spectrum of the raw sludge at 300 K exhibits an Fe3+ doublet on a slightly curved background (Figure 4a). At 77 K, however, a magnetic structure develops (Figure 4b), indicating a superparamagnetic behavior transferred along the Fe-O(OH)Fe chains of the hydrated iron oxides. The appearance of the magnetic splitting via cooling of the sample to 77 K indicates that the particle size of this component is 4-8 nm. The value of the derived internal field in the magnetic component is 46 T. Thus, this constituent may be attributed to the presence of isomorphically substituted γ-FeOOH for zinc in a small scale.25 No Mo¨ssbauer signals of ZnFe2O418,22 could be observed. 3.3. Heat-Induced Transformations and Phase Relations in Hot-Sintered Sludges. The constituents
Figure 4. Mo¨ssbauer spectra of the air-dried sludge at 300 K (a) and 77 K (b) temperatures.
of the sludge decompose upon heating,12,26-28 and various reactions between individual compounds and/or their decomposition products take place. For example, ZnCl2‚4Zn(OH)2‚5H2O starts losing water at about 100 °C; then at about 220 °C, structural water is released and amorphous ZnO (3 mol) and Zn(OH)Cl (2 mol)
Ind. Eng. Chem. Res., Vol. 41, No. 4, 2002 723
form.28 These compounds have also been formulated as Zn5O3Cl2(OH)2.12 Because the peaks assigned to calcium carbonate in the diffractogram recorded at 100 °C disappear at 250 °C, the calcium carbonate reacts with one of the Zn compounds up to 250 °C. The change in the carbonate content was found to be only approximately 30% (from the TG-MS spectrum); therefore, the reaction products may be basic zinc carbonate or smithsonite. The basic zinc chloride decomposes into ZnCl2, ZnO, and H2O at about 250 °C.24,28 At higher temperatures the partial hydrolysis of ZnCl2 leads to a reaction between the HCl formed and the carbonate compounds of the zinc with an intensive evolution of CO2. Hydrolysis of ZnCl2 or Zn5O3Cl2(OH)2 together with the evolution of HCl could be observed in the temperature range of 400-450 °C.13 The evolution of CO2 as a consequence of these reactions is detected by TG-MS (Figure 2). The basic calcium chloride (formed previously via the reaction between ZnCl2 and Ca(OH)2) decomposed to CaO and HCl in the temperature range of 650-800 °C.12 HCl reacts with the residual carbonate compounds, while CO2 is evolved. Above 850 °C carbonate could not be detected in the sample. The decomposition temperature of R-FeOOH depends on the crystallinity29 and other physical characteristics of the sample.27,29 If foreign (e.g., organic) materials are not present, the decomposition product is R-Fe2O3. The R-FeOOH in the sludge lost physically sorbed (capillary or layer) water at a lower temperature (100 °C) than expected (120-130 °C) (Figure 2). Release of the structural water started at about 210 °C with the formation of protohematite or hydrohematite intermediates. Hematite formation set in at about 400 °C,26,27,29 and intensive dewatering could be observed at 450 °C. A relatively small amount (1-2%) of water was released upon further heating, and residual -OH groups could be detected by IR spectroscopy in all of the heat-treated samples, even if the sample was sintered at 1000 °C for 5 h (Figure 1b-f, around 1620 and 3350 cm-1). Because γ-FeOOH was found in the sample dried at 100 °C, γ-Fe2O3 may be formed between 280 and 380 °C.30,31 γ-Fe2O3, however, transforms into R-Fe2O3 above 400 °C;27 therefore, γ-Fe2O3 could not be found either in the samples heated at 250 °C or in the samples heated at 500 °C. The amount of the basic zinc chloride decreased during the heat treatment (Figure 1a-e, bands at 997 and 874 cm-1), and it could not be detected at 1000 °C. As a product of the reaction between the basic zinc chloride and R-FeOOH, ZnFe2O4 appeared at 250 °C (band at 550 cm-1), and its amount was found to be simultaneously increasing with the increasing temperature. The Mo¨ssbauer spectra of the sample heated at 750 °C are significantly different from those recorded on the dried sample. The 77 K spectrum does not exhibit any magnetic component (Figure 5b), a feature which demonstrates the absence of extended Fe-O-Fe chains.25 On the other hand, the major part of the 300 K spectrum (Figure 5a) belongs to an Fe3+ component with a low (0.40 mm/s) quadrupole splitting which is not present in the spectrum of the dried sample (Table 1) and two types of Fe3+ chemical environments could be registered (Table 1). The IR band around 550 cm-1 supports the formation of ZnFe2O417,18 as the main Fe-containing phase; therefore, Fe has to be present mainly in the
Figure 5. Mo¨ssbauer spectra of the sludge heated at 750 °C for 5 h at 300 K (a) and 77 K (b) temperatures.
ferrite phase. ZnFe2O4 was the sole Fe-containing phase in the sample heated at 1000 °C. Comparison of the Zn-Fe ratios in the 750 or 1000 °C heat-treated and the nontreated samples shows that the Zn content has decreased at increased temperature. This type of loss may be due to evaporation of ZnCl2 above 698 °C.28 ZnCl2 can be formed from the basic zinc chloride above 600 °C. The degrees of crystallinity of the zinc ferrite and ZnO are increasing with the increase of the heat-treatment temperature. The amorphous state of the raw sludge and crystallinity of heat-treated samples are illustrated in Figures 6a,b. 3.4. Retention of Zinc in Raw and Heat-Treated Sludges in the Ammoniacal Leaching Experiment. All types of iron(III) oxides and hydroxides can absorb zinc(II) ions by occlusion or absorption.3-7 At the same pH level, the zinc absorption capacity of Fe(OH)3 is 10 times higher than that of FeOOH.5 In the case of R-Fe2O3, R-FeOOH, and freshly prepared Fe(OH)3, absorption maxima occur at pH values of 9, 8, and 6, respectively.3 Therefore, to improve zinc recovery, the samples have to be washed with water to decrease their pH values below these levels. A minimum amount of washing water was required, and the highest yields of leached zinc were obtained for iron(III) oxide. Namely, the leachability of zinc was found to be 1 order of magnitude higher for iron(III) oxide than for hydrated iron oxides. According to the results of our experiments, the ammoniacal leachability of zinc decreased with increasing sintering temperature. The leaching efficiency of untreated samples was found to be 67%.2 In the case of samples heated to 1000 °C for 5 h, this value, however, decreased to 30%. The decrease in leaching efficiency is mainly attributed to the formation of zinc ferrite;
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Literature Cited
Figure 6. SEM photographs of air-dried raw sludge (a) and the heat-treated sludge (1000 °C for 5 h) (b).
however, the evaporation of ZnCl2 may also contribute (ca. 5-10%) to the decreased recoverability. It is also known that ZnO sinters at high temperature. Consequently, the maximal surface area (ca. 30 m2 g-1) and reactivity attainable at 300-400 °C32 are also significantly decreased. 4. Conclusions Depending on the temperature of the thermal treatment, the components of the raw sludge undergo various chemical transformations. The R- and γ-FeOOH as well as the basic zinc chlorides are decomposed, and formation of zinc(II) ferrite takes place via solid-phase reactions (between zinc- and iron-containing compounds) above 250 °C. Because the Zn-Fe ratio is higher than 1:2, besides the ZnFe2O4 formed at 1000 °C, part of the zinc is present as ZnO. It can be stated that a decrease in the zinc absorption capacity is attainable by the hightemperature heat treatment; however, because of the formation of ZnFe2O4 insoluble in aqueous ammonia, this advantageous phenomenon cannot be utilized by ammoniacal leaching. Although leaching of zinc from ZnFe2O4 can be accomplished with sulfuric acid,33 the formation of jarozite-type wastes is incident to increased environmental risk. Conclusively, the results of the present investigations have established the bases of a more favorable process34 for recovering zinc from heattreated hot-dip galvanizing wastes.
(1) Iron Compounds. Ullman’s Encyclopedia of Industrial Chemistry; Verlag Chemie: Weinheim, Vol. A14, pp 591-610. (2) Kazincy, B.; Ko´tai, L.; Ga´cs, I.; Szentmiha´lyi, K.; Sa´ndor, Z.; Holly, S. Study on ammoniacal leaching of zinc from sludges containing iron and zinc hydroxides. Hung. J. Ind. Chem. 2000, 28, 207. (3) Music, S.; Ristic, M. Adsorption of zinc(II) on hydrous iron oxides. J. Radioanal. Nucl. Chem. 1992, 162, 351. (4) Srivastava, A.; Srivastava, P. C. Adsorption-desorption behaviour of zinc(II) at iron(III) hydroxide-aqueous solution interface as influenced by pH and temperature. Environ. Pollut. 1990, 68, 171. (5) Shuman, L. M. Adsorption of zinc by iron and aluminium hydrous oxides as influenced by aging and pH. Soil Sci. Soc. Am. J. 1977, 41, 703. (6) Woitke, P.; Henrion, G. Dynamic zinc-exchange on environmentally significant inorganic solid phases. Acta Hydrochim. Hydrobiol. 1991, 19 (1), 85. (7) Peneva, N.; Paskaleva, K. Zinc absorption by iron hydroxides. Pochvozn. Agrokhim. 1981, 16 (1), 32-37. (8) Krasnobai, N. G.; Raspopov, Yu. G.; Starikova, T. V.; Sheinkman, A. N.; Kleshchev, D. G.; Bubnov, A. A. Phase relations in the Fe(OH)2-H2O-O2 system. Zh. Neorg. Khim. 1986, 31 (9), 2215. (9) Przepiera, A.; Przepiera, K.; Wisniewski, M.; Dabrowski, W. Investigations of the thermal transformations of precipitated mixed transition metal hydroxides. J. Therm. Anal. 1993, 40, 1131. (10) Ueda, M.; Shimada, S.; Inagaki, M. Synthesis of crystalline ferrites below 60 °C. J. Eur. Ceram. Soc. 1996, 16 (6), 685. (11) Grauer, R.; Schindler, P. Solubilities product of the zinc hydroxychloridessCorrosion products of zinc. Corr. Sci. 1972, 12, 405. (12) Vian, A.; Tijero, J.; Rodriguez, J. J.; Guardiola, E. Characterization of the solid precipitates obtained from alcaline treatment of zinc chloride solutions. An. Quim. 1984, 80, 724. (13) Kaushansky, P.; Yariv, S. The interaction between calcite particles and aqueous solutions of magnesium, barium and zinc chlorides. Appl. Geochem. 1986, 1, 607. (14) Ishikawa, T.; Cai, W. Y.; Kandori, K. Characterization of the thermal decomposition products of δ-FeOOH by Fourier transform infrared spectroscopy and N2 adsorption. J. Chem. Soc., Faraday Trans. 1992, 88 (8), 1173. (15) Karpinchik, E. V.; Kuznetsov, S. V. Phase and chemical composition of zinc-containing sludges and their heat-treated products. Vestsi Akad. Navuk BSSR, Ser. Khim. Navuk 1987, (3), 100. (16) Landa, E. R.; Gast, R. G. Evaluation of crystallinity in hydrated ferric oxides. Clays Clay Miner. 1973, 21, 121. (17) El Hiti, M. A.; El Shabashy, M. Spectral studies of the spinel ferrite system NixZn(1-x)Fe2O4. Fizika A2 1993, 3, 163. (18) Kanzaki, T. Moessbauer spectra at 77 K of products formed during transformation of iron(II) hydroxide containing zinc(II)ion to Zn-bearing ferrite in aqueous suspension by air oxidation. J. Electron Spectrosc. Relat. Phenom. 1990, 50, 309. (19) Tugushev, R. E.; Il’ina, L. A.; Gamayunova, I. M. IR spectroscopic study of the phase composition of iron hydroxide. Zh. Neorg. Khim. 1988, 33 (10), 2453. (20) Wolska, E. Relations between the existence of hydroxyl ions in the anionic sublattice of hematite and its infrared and X-ray characteristics. Solid State Ionics 1988, 28-30, 1349. (21) Ishikawa, T.; Nitta, S.; Kondo, S. Fourier transform infrared spectroscopy of colloidal R, β, γ-, and δ-ferric oxide hydroxides. J. Chem. Soc., Faraday Trans. 1986, 82, 2401. (22) Goya, G. F.; Stewart, S. J.; Mercader, R. C. Moessbauer study of Fe-Zn-O phases. Solid State Commun. 1995, 96, 485. (23) Li, Chung-Lee; Tsai, Min-Shing A crystal phase study of zinc hydroxide chloride in electric-arc-furnace dust. J. Mater. Sci. 1993, 28, 4562. (24) Garcia-Martinez, O.; Vila, E.; de Vidales, J. L. M.; Rojas, R. M.; Petrov, K. On the thermal decomposition of the zinc(II) hydroxide chlorides Zn5(OH)8Cl2‚H2O and β-Zn(OH)Cl. J. Mater. Sci. 1994, 29, 5429. (25) Murad, E.; Johnston, J. H. In Mo¨ ssbauer Spectroscopy Applied to Inorganic Chemistry; Long, G. J., Ed.; Plenum Press: New York, 1987; Vol. 2, p 507. (26) Fenerty, J.; Hegarty, E.; Pearce, J. The characterization of lattice transformations during thermal treatment of iron(III)
Ind. Eng. Chem. Res., Vol. 41, No. 4, 2002 725 oxide/hydroxide systems. 2nd European Symposium on Thermal Analysis, Aberdeen, 1981; Dollimore, D., Ed.; Heyden: London, 1981; pp 395-399. (27) Balek, V.; Subrt, J. Thermal behaviour of iron(III) oxide hydroxides. Pure Appl. Chem. 1995, 67, 1839-1842. (28) Rasines, I.; Morales de Setien, J. I. Thermal analysis of β-Co2(OH)3Cl and Zn5(OH)8Cl2‚H2O. Thermochim. Acta 1980, 37, 239. (29) Goni-Elizalde, S.; Garcia-Clavel, M. E. Thermal behavior in air of iron oxyhydroxides obtained from the method of homogeneous precipitation I. Goethite samples of varying crystallinity. Thermochim. Acta 1988, 124, 359. (30) Froemming, W. Investigations on transition points of iron oxide. Therm. Anal., Proc. Int. Conf. 4th 1974, 751-761. (31) Naono, H.; Nakai, K. Thermal decomposition of γ-FeOOH
fine particles. J. Colloid Interface Sci. 1989, 128 (1), 146-156. (32) Karpinchik, E. V.; Komarov, V. S. Structural and adsorption properties of the heat-treated zinc-containing sludges. Vestsi Akad. Navuk BSSR, Ser. Khim. Navuk 1987, (1), 19-21. (33) Li, H.; Li, Y.; Sun, P.; Zhou, J.; Su, P.; Li, M. Method for recovery of zinc from leaching dregs containing zinc ferrite. Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1,170,044A, 1998. (34) Boyanov, B. S. Interaction of (NH4)2SO4 and FeSO4 with metal oxides and ferrites. Thermochim. Acta 1994, 240, 225.
Received for review April 6, 2001 Revised manuscript received November 13, 2001 Accepted November 14, 2001 IE010313I
