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Alkaline leaching of zinc from electric arc furnace steel dust
Minerals Engineering 19 (2006) 478–485 This article is also available online at: www.elsevier.com/locate/mineng
Alkaline leaching of zinc from electric arc furnace steel dust A.J.B. Dutra *, P.R.P. Paiva, L.M. Tavares Department of Metallurgical and Materials Engineering, Federal University of Rio de Janeiro, COPPE/UFRJ, P.O. Box 68505, Rio de Janeiro, RJ, Brazil Received 28 June 2005; accepted 24 August 2005 Available online 14 October 2005
Abstract Electric arc furnaces (EAF) generate about 10–20 kg of dust per metric ton of steel, which constitute a hazardous waste, known as EAF dust. This dust contains a remarkable amount of non-ferrous metals, which include zinc, cadmium, lead, chromium and nickel that could be recovered, reducing the environmental impact of the leachable toxic metals, and generating revenue. In this paper, different alkaline leaching techniques were tested in order to dissolve the zinc present in an EAF dust: (i) conventional agitation leaching; (ii) pressure leaching; (iii) conventional leaching following a microwave pretreatment and (iv) leaching with agitation provided by an ultra-sonic probe. Temperature and sodium hydroxide concentration were the variables tested. The highest zinc recovery from the EAF dust, containing about 12% of zinc, was about 74%. This was achieved after 4 h of leaching in a temperature of 90 °C and with a sodium hydroxide concentration of 6 M of the leaching agent. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Leaching; Flue dust; Waste processing
1. Introduction Steel production is associated with a significant accumulation of wastes, as slag, sludge, flue dust and gases. Some of these are recyclable, others are toxic, constituting hazardous wastes, which should be processed, in order to be reutilized or discarded properly in order to avoid environmental impact (Hagni et al., 1991). At present, one of the major concerns related to steel production in electric arc furnaces is the generation of a considerable amount of flue dust, about 15–20 kg per ton of steel (Huber, 1999), which contains a variety of heavy metals and, consequently, is classified by the environmental control agencies as a hazardous waste class I, code K061. Lead, cadmium and chromium (VI) are the most hazardous species in the dust, whereas zinc, due to the relatively large amount present, the most valuable component. Many processes have been or are being developed worldwide to recover values from EAF dust or render them non*
Corresponding author. Tel.: +55 21 25628512. E-mail address: [email protected] (A.J.B. Dutra).
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.08.013
hazardous through stabilization or vitrification (Mikhail et al., 1996; Ionescu et al., 1997; Pereira et al., 2001; Vin˜als et al., 2002; Cheng, 2003). These can be pyrometallurgical (Donald and Pickles, 1996; Pesl et al., 1999; Assis et al., 1999), hydrometallurgical (Cruells et al., 1992; Nyirenda and Lugtmeijer, 1993; Caravaca et al., 1994; Xia and Pickles, 2000; Leclerc et al., 2003; Havlik et al., 2005; Kelebek et al., 2004) or a hybridization of both (Barrera-Godinez et al., 1992; Xia and Pickles, 1999; Youcai and Stanforth, 2000a,b; Menad et al., 2003). However, finding a cost-effective and environmental-friendly process remains the major challenge. According to Nyirenda (1991), the pyrometallurgical process based on the Waelz kiln is capable of processing the EAF dust economically only when the zinc content is higher than about 15–20%. Additionally the plant must process at least 50,000 tons of feed per year. From this perspective, simpler processes, which allow separation of zinc and iron, should prevail (Zunkel, 1999). The choice between pyrometallurgical or hydrometallurgical processing routes strongly depends on the dust characteristics, including particle size and the number of valuable elements and mineralogical phases, which in turn, may indicate the amount of
A.J.B. Dutra et al. / Minerals Engineering 19 (2006) 478–485
2. Experimental 2.1. Sample characterization A 200 kg sample of EAF dust was collected from the silo of a Brazilian steel plant located in the State of Rio de Janeiro. The sample was submitted to chemical and mineralogical analyses by atomic absorption spectrometry (AAS), X-ray diffraction, scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS), and particle size analysis by laser diffraction with a Malvern MastersizerÒ. 2.2. Leaching tests The tests were carried out using sodium hydroxide as leaching agent at a solid/liquid ratio of 1:10 in four different ways: (i) Conventional leaching in a 1-liter closed flask, heated by a thermostatic mantle, and provided with a mechanical stirrer and a water-cooled condenser in order to avoid solution losses by evaporation. The sodium hydroxide concentration varied from 2 to 6 M, at temperatures of 25 °C and 90 °C, and the duration of up to 240 min. (ii) Pressure leaching in a 0.6-liter PARR autoclave. The sodium hydroxide concentration was 6 M, the temperature ranged from 120 °C to 200 °C and the duration was 240 min. (iii) Pretreatment of the sample in a 1 kW, 2.45 GHz microwave oven for 2 min, followed by conventional leaching following the same procedure described in
(i). The sodium hydroxide concentration was 6 M, the temperature 90 °C and the duration 240 min. (iv) Leaching under ultra-sonic agitation, provided by a probe (Thornton-Inpec), in a 0.5-liter beaker. The sodium hydroxide concentration was 6 M, the temperature 55 °C and the duration 60 min. Liquor samples were collected after selected periods of time and analyzed for zinc by AAS. The solid residue after leaching was also analyzed for zinc, as well as iron, by AAS. Selected solid residues were also analyzed by X-ray diffraction and SEM/EDS. 3. Results and discussion 3.1. Characterization of the EAF dust The particle size distribution of the EAF dust is presented in Fig. 1. It shows that only about 15% of the particles are coarser than 10 lm and the median particle size (d50) is around 0.5 lm. This fine size distribution indicates that physical concentration methods, such as gravity or magnetic separation, are not suitable to treat such a material. Further, its size distribution suggests that the material will be difficult to handle dry, if no previous agglomeration is used. On the other hand, reaction kinetics involving this dust should be fast, due to the prevailing fine particle sizes, which suggest that leaching may be an attractive route to treat this material, when zinc recovery is of interest. Results from chemical analyses of the EAF dust are presented in Table 1. It can be observed that iron and zinc are the main components of the dust, which also presents small amounts of very toxic heavy metals, which include chromium, cadmium and lead. The zinc content in this dust, around 12.2%, is considered low for a successful pyrometallurgical treatment for zinc recovery (Nyirenda, 1991), which leads to the greater interest in the hydrometallurgical treatment.
100
Cumulative mass percent passing
leachable constituents. Therefore, a detailed characterization of the dust becomes an important tool for defining the most appropriate recycling strategy (Hagni et al., 1991; Lindblom et al., 2002). A variety of processes have been attempted for dust recycling, but most have never reached pilot scale, and investigations were discontinued because of both metallurgical and economical inefficiencies (Antrekowitsch and Antrekowitsch, 2001). While pyrometallurgical processes face problems such as high energy consumption and generation of worthless residues, hydrometallurgical processes are still only a promise for the future, despite the vast research done in this field. Still, hydrometallurgical process could offer an interesting alternative for zinc recycling, if iron dissolution is controlled and the structure of zinc ferrites—which are normally difficult to leach—is broken, favoring a high recovery of zinc. In the present paper, different techniques for zinc alkaline leaching from an EAF dust are investigated, aiming at the recovery of zinc. Sodium hydroxide was chosen as leaching agent as it is effective in the dissolution of zinc, cadmium and other toxic heavy metals, without significant dissolution of iron, thus reducing the hazardous character of the solid residue.
479
90 80 70 60 50 40 30 20 10 0 0.1
1
10
Particle size (µm)
Fig. 1. Particle size distribution of the EAF dust.
100
480
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Table 1 Chemical analysis of the main elements present in the EAF dust Element
Fe
Zn
Cr
Cd
Pb
Cu
Ca
Al
Weight (%)
37.08
12.20
0.22
0.01
1.72
0.17
2.19
0.41
The X-ray diffraction pattern of the as-received EAF dust is shown in Fig. 2. Four major phases were identified: franklinite, magnetite, zincite and quartz. The other phases
Fig. 2. X-ray diffraction pattern of the as-received EAF dust.
also typically found (Menad et al., 2003; Hagni et al., 1991) in the dust may also be present, but in insufficient amounts for an appropriate detection. A typical aspect of the EAF dust obtained by SEM, with EDS analysis of selected particles, is presented in Fig. 3. The presence of smooth spherical particles (3) rich in iron, oxygen, aluminum and calcium, but poorer in zinc, than particles (1), (2) and (4) was observed. In particle (2), a relatively high amount of sulfur and a smaller content of chlorine than in particle (1) were detected. In particle (4), the presence of carbon is evident. The SEM/EDS analysis would suggest that the majority of the zinc is associated with particles with a rougher surface. The EDS analysis also indicates that the majority of the zinc is associated with the finer dust particles, probably as zincite and ferrites. However, the composition and morphology of the dust strongly depend on the quality of the scrap and on the mode of operation of the electric furnace (Huber et al., 1999). Gold peaks in the spectra are due to sample preparation for SEM.
Fig. 3. SEM micrograph of the EAF dust with EDS of selected particles.
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3.2. Equilibrium calculations The solubility of zinc oxide in water, at 25 °C, can be described by the reactions (Stumm and Morgan, 1981) ZnO þ 2Hþ ¢ Zn2þ þ H2 O þ
ZnO þ H ¢ ZnOH ZnO þ ZnO þ
þ
K s0 ¼ 1:58 1011
K s1 ¼ 1:58 10
þ 2H2 O ¢ ZnðOHÞ 3 þH þ 3H2 O ¢ ZnðOHÞ2 4 þ 2H
ð1Þ
2
ð2Þ
K s3 ¼ 1:26 10
17
ð3Þ
K s4 ¼ 2:00 1030 ð4Þ
where Ksi are the constants for the solubility equilibria. Eqs. (1)–(4) allow to determine the total concentration of zinc in an aqueous solution as function of pH, according to Eq. (5): 2
½ZnT ¼ K s0 ½Hþ þ K s1 ½Hþ þ K s3 ½Hþ
1
2
þ K s4 ½Hþ
ð5Þ
Eqs. (1)–(5) are used to calculate the equilibrium diagram of solid zinc oxide and the zinc ionic species, as shown in Fig. 4. Similarly, the solubility of iron (II) and (III) in equilibrium with their respective hydroxides is presented in Fig. 5.
481
The equilibrium diagrams presented in Figs. 4 and 5 indicate that zinc can be dissolved in either acidic or alkaline media, whereas iron, which is the metal that predominates in the dust, is more readily soluble in acidic media. Therefore, alkaline leaching becomes the more attractive alternative, as no additional purification step for iron removal is necessary. 3.3. Conventional alkaline agitation leaching of the EAF dust The influence of sodium hydroxide concentration on zinc recovery from leaching the EAF dust at 90 °C is presented in Fig. 6. It shows that the increase in NaOH concentration results in an increase is zinc recovery. However, a maximum concentration of 6 M was used, considering that above this concentration the solution becomes very viscous and difficult to manipulate. During the initial instants of leaching the dissolution of zinc is very fast, for all concentrations investigated, tending to a maximum constant value of zinc extraction, that depends on the NaOH
80 0
70 2+
Zn
6M
Zinc recovery (%)
60
-2 +
ZnOH
-
-3
Zn(OH)3
-4
ZnO
4M 50 2M
40 30
-5
20 2-
Zn(OH)4
-6
10
-7
0
0
50
100
150
200
250
-8 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Time (min)
16
pH
Fig. 6. Influence of NaOH concentration on zinc recovery, with leaching at 90 °C.
Fig. 4. Solubility of ZnO as a function of pH, at 25 °C.
0
-2 -3
-1 FeOH+
-2 -3
Fe(OH)2
-4 -5 -6
Fe(OH)3-
-7
Fe
2+
Fe(OH)42-
log (Concentration, molar)
0
log (Concentration, molar)
log (Concentration, molar)
-1
-4 -5 -6 -7
Fe(OH)4-
-8
Fe(OH)3
-9 Fe2(OH)24+
-10
Fe3+ FeOH2+
Fe(OH)2+
-11 -12
-8 2
3
4
5
6
7
8
9
10
11 12 13
14 15 16
0
1
2
3
4
5
6
7
8
pH
pH
(a)
(b)
9
10 11 12 13 14 15 16
Fig. 5. Solubility of ferrous (a) and ferric (b) hydroxides as a function of pH, at 25 °C.
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90 oC
Zinc recovery (%)
60 25 oC 50 40 30 20 10 0
0
50
100
150
200
250
Time (min)
Fig. 7. Influence of temperature on zinc recovery, for a NaOH concentration of 6 M.
concentration, at longer leaching times. Indeed, in about 10 min of leaching, more than 50% of the recovered zinc was already dissolved. The additional extraction obtained after 2 h of leaching, achieved for NaOH concentrations of 2 and 6 M, is likely to be associated to the dissolution of some entrapped zinc or, alternatively, to the destruction of zinc ferrites. On the other hand, only 0.3% of the iron in the dust was dissolved, confirming the alkaline leaching selectivity. The influence of temperature on zinc recovery, for a NaOH concentration of 6 M, is presented in Fig. 7. It can be observed that, as expected, the increase in temperature results in an increase in zinc extraction. For longer leaching times this increase is more significant, probably due to the increase in the solubility of the metal that is associated with the higher temperature. The solid residue obtained after leaching for 4 h, at 90 °C, with a 6 M solution of NaOH was analyzed by SEM/EDS and X-ray diffraction. The results are presented in Figs. 8 and 9, respectively. It can be observed that the
surface of the nearly spherical particles that are rich in iron became cleaner (absence of fine dust attached to them) than those presented in Fig. 3, and the EDS detected only a very small quantity of zinc. Additionally, elements such as calcium, potassium, aluminum, magnesium, copper, lead, silicon and chlorine were no longer detected by the EDS, indicating a nearly pure iron oxide particle, with only a small amount of zinc. Therefore, cadmium and part of the lead present were also leached along with zinc. The X-ray diffraction pattern no longer indicated the presence of zincite, leading to the conclusion that it was completely leached, whereas recovery of zinc that remained in solid phase as franklinite, would likely require more severe leaching conditions. The X-ray diffraction pattern presented other evident peaks related to magnetite and quartz, and a small peak, which can be related to the presence of lead oxide. Iron oxides are known to be insoluble in very caustic environments, even at elevated temperatures, such as those used in the Bayer process, used for aluminum processing. The same analogy is valid for quartz, whose dissolution kinetics is very slow. The presence of zinc ferrites (franklinite) in the residue indicates the need for some treatment prior to leaching or to more severe leaching conditions in order to improve the zinc recovery. In any case, the low iron content in the solution makes the process attractive, so that it is possible to recover the metal (zinc) by electrolysis, after a simple purification process. 3.4. Pressure leaching Results of zinc recovery after 4 h of leaching in a 6 M NaOH solution in an autoclave at 120, 150 and 200 °C (rendering pressures of 2.0, 5.8 and 15.8 atm, respectively) are presented in Fig. 10. It can be observed that the highest recovery of zinc occurs after 30–60 min, and the temperature does not have a significant effect on the recovery of zinc. A drop, of about 10%, on zinc recovery can be observed at a temperature of 120 °C at longer leaching
Fig. 8. SEM micrograph and EDS of the EAF dust after NaOH leaching at 90 °C.
A.J.B. Dutra et al. / Minerals Engineering 19 (2006) 478–485
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The general appearance of the solid residue, analyzed by SEM with EDS, is presented in Fig. 12. The presence of large spherical particles (1) containing only iron and oxygen and its composition is evident. A large quantity of fine particles (2), rich in iron, oxygen, zinc, calcium and magnesium, which indicate the presence of ferrites in these particles, was also observed. The large spherical particles shown in figure present a much rougher surface than those presented in Fig. 8. This suggests the occurrence of incipient iron dissolution under pressure leaching, which, in turn, can lead to a reaction with the dissolved zinc, once again explaining the formation of zinc ferrites. Fig. 9. X-ray diffraction pattern of the EAF dust after NaOH leaching at 90 °C.
80 70
Zinc recovery (%)
60 200 oC
50
150 oC
40
120 oC
30 20 10 0
0
50
100
150
200
250
Time (min)
Fig. 10. Influence of temperature on zinc recovery in pressure leaching, for a NaOH concentration of 6 M.
times. A possible explanation for this behavior is the precipitation of the dissolved zinc as zinc ferrite. Evidence in favor of this hypothesis is found in the X-ray diffraction pattern of the solid residue after leaching, presented in Fig. 11, which identifies the presence of franklinite, magnetite and quartz.
3.5. EAF dust pretreatment in a microwave oven Some materials such as ferrites and other metallic oxides absorb a significant amount of radiation in microwave frequencies. This behavior allows the fast heating of such materials under microwaves, which can induce the nucleation and propagation of cracks in the particles, making them more amenable to leaching. Xia and Pickles (2000) made use of microwave radiation to heat the pulp of EAF dust and sodium hydroxide solution in a teflon reactor. They observed a small increase in the recovery of zinc, but only in a shorter period of time. This improvement in zinc recovery was attributed to the partial dissolution of zinc ferrites. In the case of the present study, microwave radiation was used as a treatment stage prior to conventional caustic leaching. The EAF dust sample, placed inside a ceramic crucible, was heated in a microwave oven (1 kW) for about 2 min, achieving temperatures in the range of 600–700 °C. Longer heating times were found, by visual inspection, to cause sintering, which was undesirable. Immediately after heating, the sample was transferred to the leaching reactor and mixed with a 6 M NaOH solution. Results from these experiments are summarized in Table 2. It can be observed that the microwave treatment was not able to improve zinc recovery. One of the explanations could be the heating time, which was probably long enough to cause an incipient sintering of the low melting-point phases, but not sufficient to induce the formation of cracks in the particles. Since zinc recovery in this series of experiments was smaller than that obtained by conventional leaching, no further analyses were carried out with this technique. 3.6. Leaching with ultra-sonic agitation
Fig. 11. X-ray diffraction pattern of the solid residue after NaOH leaching in autoclave at 120 °C.
Barrera-Godinez et al. (1992) suggested that ultrasound was beneficial in the selective chloride acid leaching of zinc from pellets produced by double-kiln treatment of EAF dust. This effect was attributed to a combination of improved pore penetration capacity of the leaching agent, boundary and product layer breakdown, and localized temperature increases. In the present investigation, it was expected that the ultra-sonic probe would also improve the dispersion of fine and agglomerated particles,
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Fig. 12. SEM micrograph and EDS of the EAF dust after NaOH leaching in autoclave at 120 °C for 240 min. Table 2 Zinc recovery from EAF dust treated in a microwave oven and leached in a 6 M NaOH solution at 90 °C Time (min)
Zinc recovery (%)
5 30 60 120 180 240
49.7 53.7 55.0 56.3 60.4 60.4
Table 3 Zinc recovery from EAF dust leached in a 6 M NaOH solution, at 55 °C, with ultra-sonic agitation Time (min)
Zinc recovery (%)
5 30 60
50.3 53.1 55.9
enhancing zinc recovery. As a result, a few preliminary experiments were conducted with this technique at room temperature, with results presented in Table 3 for different leaching times. It can be observed that no improvements in zinc recovery were achieved as a result of ultra-sonic agitation, as is demonstrated by a comparison of Table 3 with results from Figs. 6 and 7. The temperature of 55 °C during leaching was consequence of the energy dissipation due to the ultra-sonic agitation, since no additional heating was used. Despite the different temperature from the tests of Fig. 7, the results are comparable, since during the first hour of leaching the effect of temperature was found to be very limited. 4. Conclusions The leaching results, combined with the SEM/EDS and X-ray diffraction analyses, indicated that zinc present in an
EAF dust as zincite is readily dissolved, while most of the zinc ferrites remain in the solid phase. The most significant result obtained was a 74% zinc recovery after conventional leaching for 4 h with a 6 M NaOH solution. The alkaline leaching routes investigated present some distinct advantages over either the acid leaching route or the pyrometallurgical processes: the solid residues are considerably less toxic than the original EAF dust, since most of the heavy metals (such as cadmium and lead) are leached and the solid phase is rich in iron oxides and quartz, presenting also some zinc ferrites which are very difficult to leach. Further, the low iron content in the solution makes the process attractive, so that the recovery of zinc as a metal in a downstream electrolysis after a simple purification process is a viable alternative. Acknowledgements The authors would like to acknowledge the financial support from CNPq of Brazil to this investigation. The assistance of Dr. Arnaldo Alcover Neto in the SEM analyses is also deeply acknowledged. References Antrekowitsch, J., Antrekowitsch, H., 2001. Hydrometallurgical recovering zinc from electric arc furnace dusts. JOM 53 (12), 26–28. Assis, G., Barcza, N.A., Freeman, M.J., 1999. Recent advances in the environplas process for the treatment of secondary lead and zincbearing materials. In: Proc. Rewas’99—Global Symposium on Recycling, Waste Treatment and Clean Technology, vol. II. TMS, San Sebastian, pp. 1473–1482. Barrera-Godinez, J.A., O’Keefe, T.J., Watson, J.L., 1992. Effect of ultrasound on acidified brine leaching of double-kiln treated EAF dust. Minerals Engineering 5 (10–12), 1365–1373. Caravaca, C., Cobo, A., Alguacil, F.J., 1994. Considerations about the recycling of EAF flue dusts as source for the recovery of valuable metals by hydrometallurgical processes. Resources, Conservation and Recycling 10 (1–2), 35–41.
A.J.B. Dutra et al. / Minerals Engineering 19 (2006) 478–485 Cheng, T.W., 2003. Combined glassification of EAF dust and incinerator fly ash. Chemosphere 50 (1), 47–51. Cruells, M., Roca, A., Nu´n˜ez, C., 1992. Electric arc furnace flue dusts: characterization and leaching with sulphuric acid. Hydrometallurgy 31 (3), 213–231. Donald, J.R., Pickles, C.A., 1996. Reduction of electric arc furnace dust with solid iron powder. Canadian Metallurgical Quarterly 35 (3), 255– 267. Hagni, A.M., Hagni, R.D., Demars, C., 1991. Mineralogical characteristics of electric arc furnace dusts. JOM 43 (4), 28–30. Havlik, T., Turzakova, M., Stopic, S., Friedrich, B., 2005. Atmospheric leaching of EAF dust with diluted sulphuric acid. Hydrometallurgy 77 (1–2), 41–50. Huber, J.-C., Patisson, F., Rocabois, P., Birat, J.-C., Ablitzer, D., 1999. Some means to reduce emissions and improve the recovery of electric arc furnace dust by controlling the formation mechanisms. In: Proc. Rewas’99—Global Symposium on Recycling, Waste Treatment and Clean Technology, vol. II. TMS, San Sebastian, pp. 1483–1492. Ionescu, D., Meadowcroft, T.R., Barr, P.V., 1997. Glassification of EAF dust: the limits for Fe2O3 and ZnO content and an assessment of leach performance. Canadian Metallurgical Quarterly 36 (4), 269– 281. Kelebek, S., Yo¨ru¨k, S., Davis, B., 2004. Characterization of basic oxygen furnace dust and zinc removal by acid leaching. Minerals Engineering 17 (2), 285–291. Leclerc, N., Meux, E., Lecuire, J.-M., 2003. Hydrometallurgical extraction of zinc from zinc ferrites. Hydrometallurgy 70 (1–3), 175–183. Lindblom, B., Samuelsson, C., Sandstro¨m, A., Ye, G., 2002. Fine-particle characterization—an important recycling tool. JOM 54 (12), 35–38. Menad, N., Ayala, J.N., Garcia-Carcedo, F., Ruiz-Ayu´car, E., Herna´ndez, A., 2003. Study of the presence of fluorine in the recycled fractions during carbothermal treatment of EAF dust. Waste Management 23 (6), 483–491.
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Mikhail, S.A., Turcotte, A.M., Aota, J., 1996. Thermoanalytical study of EAF dust and its vitrification product. Themochimica Acta 287 (1), 71–79. Nyirenda, R.L., 1991. The processing of steelmaking flue-dust: a review. Minerals Engineering 4 (7–11), 1003–1025. Nyirenda, R.L., Lugtmeijer, A.D., 1993. Ammonium carbonate leaching of carbon steelmaking dust. Detoxification potential and economic feasibility of a conceptual process. Minerals Engineering 6 (7), 785–797. Pereira, C.F., Rodrı´guez-Pin˜ero, M., Vale, J., 2001. Solidification/stabilization of electric arc furnace dust using fly ash. Analysis of the stabilization process. Journal of Hazardous Materials B82 (2), 183–195. Pesl, J., Lehner, J., Fleischander, A., 1999. Zinc management and recycling within the iron and steel industry. In: Proc. Rewas’99— Global Symposium on Recycling, Waste Treatment and Clean Technology, vol. II. TMS, San Sebastian, pp. 1413–1422. Stumm, W., Morgan, J.J., 1981. Aquatic Chemistry, second ed. John Wiley & Sons, New York. Vin˜als, J., Balart, M.J., Roca, A., 2002. Inertization of pyrite cinders and co-inertization with electric arc furnace flue dusts by pyroconsolidation at solid state. Waste Management 22 (7), 773–782. Xia, D.K., Pickles, C.A., 1999. Caustic roasting and leaching of electric arc furnace dust. Canadian Metallurgical Quarterly 38 (3), 175–186. Xia, D.K., Pickles, C.A., 2000. Microwave caustic leaching of electric arc furnace dust. Minerals Engineering 13 (1), 79–94. Youcai, Z., Stanforth, R., 2000a. Extraction of zinc from zinc ferrites by fusion with caustic soda. Minerals Engineering 13 (13), 1417–1421. Youcai, Z., Stanforth, R., 2000b. Integrated hydrometallurgical process for production of zinc from electric arc furnace dust in alkaline medium. Journal of Hazardous Materials 80B (1–3), 223–240. Zunkel, A.D., 1999. EAFD treatment technologies and process: a status report. In: Proc. Rewas’99—Global Symposium on Recycling, Waste Treatment and Clean Technology, vol. II. TMS, San Sebastian, pp. 1453–1461.
Alkaline leaching of zinc from electric arc furnace steel dust A.J.B. Dutra *, P.R.P. Paiva, L.M. Tavares Department of Metallurgical and Materials Engineering, Federal University of Rio de Janeiro, COPPE/UFRJ, P.O. Box 68505, Rio de Janeiro, RJ, Brazil Received 28 June 2005; accepted 24 August 2005 Available online 14 October 2005
Abstract Electric arc furnaces (EAF) generate about 10–20 kg of dust per metric ton of steel, which constitute a hazardous waste, known as EAF dust. This dust contains a remarkable amount of non-ferrous metals, which include zinc, cadmium, lead, chromium and nickel that could be recovered, reducing the environmental impact of the leachable toxic metals, and generating revenue. In this paper, different alkaline leaching techniques were tested in order to dissolve the zinc present in an EAF dust: (i) conventional agitation leaching; (ii) pressure leaching; (iii) conventional leaching following a microwave pretreatment and (iv) leaching with agitation provided by an ultra-sonic probe. Temperature and sodium hydroxide concentration were the variables tested. The highest zinc recovery from the EAF dust, containing about 12% of zinc, was about 74%. This was achieved after 4 h of leaching in a temperature of 90 °C and with a sodium hydroxide concentration of 6 M of the leaching agent. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Leaching; Flue dust; Waste processing
1. Introduction Steel production is associated with a significant accumulation of wastes, as slag, sludge, flue dust and gases. Some of these are recyclable, others are toxic, constituting hazardous wastes, which should be processed, in order to be reutilized or discarded properly in order to avoid environmental impact (Hagni et al., 1991). At present, one of the major concerns related to steel production in electric arc furnaces is the generation of a considerable amount of flue dust, about 15–20 kg per ton of steel (Huber, 1999), which contains a variety of heavy metals and, consequently, is classified by the environmental control agencies as a hazardous waste class I, code K061. Lead, cadmium and chromium (VI) are the most hazardous species in the dust, whereas zinc, due to the relatively large amount present, the most valuable component. Many processes have been or are being developed worldwide to recover values from EAF dust or render them non*
Corresponding author. Tel.: +55 21 25628512. E-mail address: [email protected] (A.J.B. Dutra).
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.08.013
hazardous through stabilization or vitrification (Mikhail et al., 1996; Ionescu et al., 1997; Pereira et al., 2001; Vin˜als et al., 2002; Cheng, 2003). These can be pyrometallurgical (Donald and Pickles, 1996; Pesl et al., 1999; Assis et al., 1999), hydrometallurgical (Cruells et al., 1992; Nyirenda and Lugtmeijer, 1993; Caravaca et al., 1994; Xia and Pickles, 2000; Leclerc et al., 2003; Havlik et al., 2005; Kelebek et al., 2004) or a hybridization of both (Barrera-Godinez et al., 1992; Xia and Pickles, 1999; Youcai and Stanforth, 2000a,b; Menad et al., 2003). However, finding a cost-effective and environmental-friendly process remains the major challenge. According to Nyirenda (1991), the pyrometallurgical process based on the Waelz kiln is capable of processing the EAF dust economically only when the zinc content is higher than about 15–20%. Additionally the plant must process at least 50,000 tons of feed per year. From this perspective, simpler processes, which allow separation of zinc and iron, should prevail (Zunkel, 1999). The choice between pyrometallurgical or hydrometallurgical processing routes strongly depends on the dust characteristics, including particle size and the number of valuable elements and mineralogical phases, which in turn, may indicate the amount of
A.J.B. Dutra et al. / Minerals Engineering 19 (2006) 478–485
2. Experimental 2.1. Sample characterization A 200 kg sample of EAF dust was collected from the silo of a Brazilian steel plant located in the State of Rio de Janeiro. The sample was submitted to chemical and mineralogical analyses by atomic absorption spectrometry (AAS), X-ray diffraction, scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS), and particle size analysis by laser diffraction with a Malvern MastersizerÒ. 2.2. Leaching tests The tests were carried out using sodium hydroxide as leaching agent at a solid/liquid ratio of 1:10 in four different ways: (i) Conventional leaching in a 1-liter closed flask, heated by a thermostatic mantle, and provided with a mechanical stirrer and a water-cooled condenser in order to avoid solution losses by evaporation. The sodium hydroxide concentration varied from 2 to 6 M, at temperatures of 25 °C and 90 °C, and the duration of up to 240 min. (ii) Pressure leaching in a 0.6-liter PARR autoclave. The sodium hydroxide concentration was 6 M, the temperature ranged from 120 °C to 200 °C and the duration was 240 min. (iii) Pretreatment of the sample in a 1 kW, 2.45 GHz microwave oven for 2 min, followed by conventional leaching following the same procedure described in
(i). The sodium hydroxide concentration was 6 M, the temperature 90 °C and the duration 240 min. (iv) Leaching under ultra-sonic agitation, provided by a probe (Thornton-Inpec), in a 0.5-liter beaker. The sodium hydroxide concentration was 6 M, the temperature 55 °C and the duration 60 min. Liquor samples were collected after selected periods of time and analyzed for zinc by AAS. The solid residue after leaching was also analyzed for zinc, as well as iron, by AAS. Selected solid residues were also analyzed by X-ray diffraction and SEM/EDS. 3. Results and discussion 3.1. Characterization of the EAF dust The particle size distribution of the EAF dust is presented in Fig. 1. It shows that only about 15% of the particles are coarser than 10 lm and the median particle size (d50) is around 0.5 lm. This fine size distribution indicates that physical concentration methods, such as gravity or magnetic separation, are not suitable to treat such a material. Further, its size distribution suggests that the material will be difficult to handle dry, if no previous agglomeration is used. On the other hand, reaction kinetics involving this dust should be fast, due to the prevailing fine particle sizes, which suggest that leaching may be an attractive route to treat this material, when zinc recovery is of interest. Results from chemical analyses of the EAF dust are presented in Table 1. It can be observed that iron and zinc are the main components of the dust, which also presents small amounts of very toxic heavy metals, which include chromium, cadmium and lead. The zinc content in this dust, around 12.2%, is considered low for a successful pyrometallurgical treatment for zinc recovery (Nyirenda, 1991), which leads to the greater interest in the hydrometallurgical treatment.
100
Cumulative mass percent passing
leachable constituents. Therefore, a detailed characterization of the dust becomes an important tool for defining the most appropriate recycling strategy (Hagni et al., 1991; Lindblom et al., 2002). A variety of processes have been attempted for dust recycling, but most have never reached pilot scale, and investigations were discontinued because of both metallurgical and economical inefficiencies (Antrekowitsch and Antrekowitsch, 2001). While pyrometallurgical processes face problems such as high energy consumption and generation of worthless residues, hydrometallurgical processes are still only a promise for the future, despite the vast research done in this field. Still, hydrometallurgical process could offer an interesting alternative for zinc recycling, if iron dissolution is controlled and the structure of zinc ferrites—which are normally difficult to leach—is broken, favoring a high recovery of zinc. In the present paper, different techniques for zinc alkaline leaching from an EAF dust are investigated, aiming at the recovery of zinc. Sodium hydroxide was chosen as leaching agent as it is effective in the dissolution of zinc, cadmium and other toxic heavy metals, without significant dissolution of iron, thus reducing the hazardous character of the solid residue.
479
90 80 70 60 50 40 30 20 10 0 0.1
1
10
Particle size (µm)
Fig. 1. Particle size distribution of the EAF dust.
100
480
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Table 1 Chemical analysis of the main elements present in the EAF dust Element
Fe
Zn
Cr
Cd
Pb
Cu
Ca
Al
Weight (%)
37.08
12.20
0.22
0.01
1.72
0.17
2.19
0.41
The X-ray diffraction pattern of the as-received EAF dust is shown in Fig. 2. Four major phases were identified: franklinite, magnetite, zincite and quartz. The other phases
Fig. 2. X-ray diffraction pattern of the as-received EAF dust.
also typically found (Menad et al., 2003; Hagni et al., 1991) in the dust may also be present, but in insufficient amounts for an appropriate detection. A typical aspect of the EAF dust obtained by SEM, with EDS analysis of selected particles, is presented in Fig. 3. The presence of smooth spherical particles (3) rich in iron, oxygen, aluminum and calcium, but poorer in zinc, than particles (1), (2) and (4) was observed. In particle (2), a relatively high amount of sulfur and a smaller content of chlorine than in particle (1) were detected. In particle (4), the presence of carbon is evident. The SEM/EDS analysis would suggest that the majority of the zinc is associated with particles with a rougher surface. The EDS analysis also indicates that the majority of the zinc is associated with the finer dust particles, probably as zincite and ferrites. However, the composition and morphology of the dust strongly depend on the quality of the scrap and on the mode of operation of the electric furnace (Huber et al., 1999). Gold peaks in the spectra are due to sample preparation for SEM.
Fig. 3. SEM micrograph of the EAF dust with EDS of selected particles.
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3.2. Equilibrium calculations The solubility of zinc oxide in water, at 25 °C, can be described by the reactions (Stumm and Morgan, 1981) ZnO þ 2Hþ ¢ Zn2þ þ H2 O þ
ZnO þ H ¢ ZnOH ZnO þ ZnO þ
þ
K s0 ¼ 1:58 1011
K s1 ¼ 1:58 10
þ 2H2 O ¢ ZnðOHÞ 3 þH þ 3H2 O ¢ ZnðOHÞ2 4 þ 2H
ð1Þ
2
ð2Þ
K s3 ¼ 1:26 10
17
ð3Þ
K s4 ¼ 2:00 1030 ð4Þ
where Ksi are the constants for the solubility equilibria. Eqs. (1)–(4) allow to determine the total concentration of zinc in an aqueous solution as function of pH, according to Eq. (5): 2
½ZnT ¼ K s0 ½Hþ þ K s1 ½Hþ þ K s3 ½Hþ
1
2
þ K s4 ½Hþ
ð5Þ
Eqs. (1)–(5) are used to calculate the equilibrium diagram of solid zinc oxide and the zinc ionic species, as shown in Fig. 4. Similarly, the solubility of iron (II) and (III) in equilibrium with their respective hydroxides is presented in Fig. 5.
481
The equilibrium diagrams presented in Figs. 4 and 5 indicate that zinc can be dissolved in either acidic or alkaline media, whereas iron, which is the metal that predominates in the dust, is more readily soluble in acidic media. Therefore, alkaline leaching becomes the more attractive alternative, as no additional purification step for iron removal is necessary. 3.3. Conventional alkaline agitation leaching of the EAF dust The influence of sodium hydroxide concentration on zinc recovery from leaching the EAF dust at 90 °C is presented in Fig. 6. It shows that the increase in NaOH concentration results in an increase is zinc recovery. However, a maximum concentration of 6 M was used, considering that above this concentration the solution becomes very viscous and difficult to manipulate. During the initial instants of leaching the dissolution of zinc is very fast, for all concentrations investigated, tending to a maximum constant value of zinc extraction, that depends on the NaOH
80 0
70 2+
Zn
6M
Zinc recovery (%)
60
-2 +
ZnOH
-
-3
Zn(OH)3
-4
ZnO
4M 50 2M
40 30
-5
20 2-
Zn(OH)4
-6
10
-7
0
0
50
100
150
200
250
-8 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Time (min)
16
pH
Fig. 6. Influence of NaOH concentration on zinc recovery, with leaching at 90 °C.
Fig. 4. Solubility of ZnO as a function of pH, at 25 °C.
0
-2 -3
-1 FeOH+
-2 -3
Fe(OH)2
-4 -5 -6
Fe(OH)3-
-7
Fe
2+
Fe(OH)42-
log (Concentration, molar)
0
log (Concentration, molar)
log (Concentration, molar)
-1
-4 -5 -6 -7
Fe(OH)4-
-8
Fe(OH)3
-9 Fe2(OH)24+
-10
Fe3+ FeOH2+
Fe(OH)2+
-11 -12
-8 2
3
4
5
6
7
8
9
10
11 12 13
14 15 16
0
1
2
3
4
5
6
7
8
pH
pH
(a)
(b)
9
10 11 12 13 14 15 16
Fig. 5. Solubility of ferrous (a) and ferric (b) hydroxides as a function of pH, at 25 °C.
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90 oC
Zinc recovery (%)
60 25 oC 50 40 30 20 10 0
0
50
100
150
200
250
Time (min)
Fig. 7. Influence of temperature on zinc recovery, for a NaOH concentration of 6 M.
concentration, at longer leaching times. Indeed, in about 10 min of leaching, more than 50% of the recovered zinc was already dissolved. The additional extraction obtained after 2 h of leaching, achieved for NaOH concentrations of 2 and 6 M, is likely to be associated to the dissolution of some entrapped zinc or, alternatively, to the destruction of zinc ferrites. On the other hand, only 0.3% of the iron in the dust was dissolved, confirming the alkaline leaching selectivity. The influence of temperature on zinc recovery, for a NaOH concentration of 6 M, is presented in Fig. 7. It can be observed that, as expected, the increase in temperature results in an increase in zinc extraction. For longer leaching times this increase is more significant, probably due to the increase in the solubility of the metal that is associated with the higher temperature. The solid residue obtained after leaching for 4 h, at 90 °C, with a 6 M solution of NaOH was analyzed by SEM/EDS and X-ray diffraction. The results are presented in Figs. 8 and 9, respectively. It can be observed that the
surface of the nearly spherical particles that are rich in iron became cleaner (absence of fine dust attached to them) than those presented in Fig. 3, and the EDS detected only a very small quantity of zinc. Additionally, elements such as calcium, potassium, aluminum, magnesium, copper, lead, silicon and chlorine were no longer detected by the EDS, indicating a nearly pure iron oxide particle, with only a small amount of zinc. Therefore, cadmium and part of the lead present were also leached along with zinc. The X-ray diffraction pattern no longer indicated the presence of zincite, leading to the conclusion that it was completely leached, whereas recovery of zinc that remained in solid phase as franklinite, would likely require more severe leaching conditions. The X-ray diffraction pattern presented other evident peaks related to magnetite and quartz, and a small peak, which can be related to the presence of lead oxide. Iron oxides are known to be insoluble in very caustic environments, even at elevated temperatures, such as those used in the Bayer process, used for aluminum processing. The same analogy is valid for quartz, whose dissolution kinetics is very slow. The presence of zinc ferrites (franklinite) in the residue indicates the need for some treatment prior to leaching or to more severe leaching conditions in order to improve the zinc recovery. In any case, the low iron content in the solution makes the process attractive, so that it is possible to recover the metal (zinc) by electrolysis, after a simple purification process. 3.4. Pressure leaching Results of zinc recovery after 4 h of leaching in a 6 M NaOH solution in an autoclave at 120, 150 and 200 °C (rendering pressures of 2.0, 5.8 and 15.8 atm, respectively) are presented in Fig. 10. It can be observed that the highest recovery of zinc occurs after 30–60 min, and the temperature does not have a significant effect on the recovery of zinc. A drop, of about 10%, on zinc recovery can be observed at a temperature of 120 °C at longer leaching
Fig. 8. SEM micrograph and EDS of the EAF dust after NaOH leaching at 90 °C.
A.J.B. Dutra et al. / Minerals Engineering 19 (2006) 478–485
483
The general appearance of the solid residue, analyzed by SEM with EDS, is presented in Fig. 12. The presence of large spherical particles (1) containing only iron and oxygen and its composition is evident. A large quantity of fine particles (2), rich in iron, oxygen, zinc, calcium and magnesium, which indicate the presence of ferrites in these particles, was also observed. The large spherical particles shown in figure present a much rougher surface than those presented in Fig. 8. This suggests the occurrence of incipient iron dissolution under pressure leaching, which, in turn, can lead to a reaction with the dissolved zinc, once again explaining the formation of zinc ferrites. Fig. 9. X-ray diffraction pattern of the EAF dust after NaOH leaching at 90 °C.
80 70
Zinc recovery (%)
60 200 oC
50
150 oC
40
120 oC
30 20 10 0
0
50
100
150
200
250
Time (min)
Fig. 10. Influence of temperature on zinc recovery in pressure leaching, for a NaOH concentration of 6 M.
times. A possible explanation for this behavior is the precipitation of the dissolved zinc as zinc ferrite. Evidence in favor of this hypothesis is found in the X-ray diffraction pattern of the solid residue after leaching, presented in Fig. 11, which identifies the presence of franklinite, magnetite and quartz.
3.5. EAF dust pretreatment in a microwave oven Some materials such as ferrites and other metallic oxides absorb a significant amount of radiation in microwave frequencies. This behavior allows the fast heating of such materials under microwaves, which can induce the nucleation and propagation of cracks in the particles, making them more amenable to leaching. Xia and Pickles (2000) made use of microwave radiation to heat the pulp of EAF dust and sodium hydroxide solution in a teflon reactor. They observed a small increase in the recovery of zinc, but only in a shorter period of time. This improvement in zinc recovery was attributed to the partial dissolution of zinc ferrites. In the case of the present study, microwave radiation was used as a treatment stage prior to conventional caustic leaching. The EAF dust sample, placed inside a ceramic crucible, was heated in a microwave oven (1 kW) for about 2 min, achieving temperatures in the range of 600–700 °C. Longer heating times were found, by visual inspection, to cause sintering, which was undesirable. Immediately after heating, the sample was transferred to the leaching reactor and mixed with a 6 M NaOH solution. Results from these experiments are summarized in Table 2. It can be observed that the microwave treatment was not able to improve zinc recovery. One of the explanations could be the heating time, which was probably long enough to cause an incipient sintering of the low melting-point phases, but not sufficient to induce the formation of cracks in the particles. Since zinc recovery in this series of experiments was smaller than that obtained by conventional leaching, no further analyses were carried out with this technique. 3.6. Leaching with ultra-sonic agitation
Fig. 11. X-ray diffraction pattern of the solid residue after NaOH leaching in autoclave at 120 °C.
Barrera-Godinez et al. (1992) suggested that ultrasound was beneficial in the selective chloride acid leaching of zinc from pellets produced by double-kiln treatment of EAF dust. This effect was attributed to a combination of improved pore penetration capacity of the leaching agent, boundary and product layer breakdown, and localized temperature increases. In the present investigation, it was expected that the ultra-sonic probe would also improve the dispersion of fine and agglomerated particles,
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Fig. 12. SEM micrograph and EDS of the EAF dust after NaOH leaching in autoclave at 120 °C for 240 min. Table 2 Zinc recovery from EAF dust treated in a microwave oven and leached in a 6 M NaOH solution at 90 °C Time (min)
Zinc recovery (%)
5 30 60 120 180 240
49.7 53.7 55.0 56.3 60.4 60.4
Table 3 Zinc recovery from EAF dust leached in a 6 M NaOH solution, at 55 °C, with ultra-sonic agitation Time (min)
Zinc recovery (%)
5 30 60
50.3 53.1 55.9
enhancing zinc recovery. As a result, a few preliminary experiments were conducted with this technique at room temperature, with results presented in Table 3 for different leaching times. It can be observed that no improvements in zinc recovery were achieved as a result of ultra-sonic agitation, as is demonstrated by a comparison of Table 3 with results from Figs. 6 and 7. The temperature of 55 °C during leaching was consequence of the energy dissipation due to the ultra-sonic agitation, since no additional heating was used. Despite the different temperature from the tests of Fig. 7, the results are comparable, since during the first hour of leaching the effect of temperature was found to be very limited. 4. Conclusions The leaching results, combined with the SEM/EDS and X-ray diffraction analyses, indicated that zinc present in an
EAF dust as zincite is readily dissolved, while most of the zinc ferrites remain in the solid phase. The most significant result obtained was a 74% zinc recovery after conventional leaching for 4 h with a 6 M NaOH solution. The alkaline leaching routes investigated present some distinct advantages over either the acid leaching route or the pyrometallurgical processes: the solid residues are considerably less toxic than the original EAF dust, since most of the heavy metals (such as cadmium and lead) are leached and the solid phase is rich in iron oxides and quartz, presenting also some zinc ferrites which are very difficult to leach. Further, the low iron content in the solution makes the process attractive, so that the recovery of zinc as a metal in a downstream electrolysis after a simple purification process is a viable alternative. Acknowledgements The authors would like to acknowledge the financial support from CNPq of Brazil to this investigation. The assistance of Dr. Arnaldo Alcover Neto in the SEM analyses is also deeply acknowledged. References Antrekowitsch, J., Antrekowitsch, H., 2001. Hydrometallurgical recovering zinc from electric arc furnace dusts. JOM 53 (12), 26–28. Assis, G., Barcza, N.A., Freeman, M.J., 1999. Recent advances in the environplas process for the treatment of secondary lead and zincbearing materials. In: Proc. Rewas’99—Global Symposium on Recycling, Waste Treatment and Clean Technology, vol. II. TMS, San Sebastian, pp. 1473–1482. Barrera-Godinez, J.A., O’Keefe, T.J., Watson, J.L., 1992. Effect of ultrasound on acidified brine leaching of double-kiln treated EAF dust. Minerals Engineering 5 (10–12), 1365–1373. Caravaca, C., Cobo, A., Alguacil, F.J., 1994. Considerations about the recycling of EAF flue dusts as source for the recovery of valuable metals by hydrometallurgical processes. Resources, Conservation and Recycling 10 (1–2), 35–41.
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