Electric arc furnace flue dusts: characterization and leaching with sulphuric acid

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1 Hydrometallurgy, 31 (1992) Elsevier Science Publishers B.V., Amsterdam Electric arc furnace flue dusts: characterization and leaching with sulphuric acid M. Cruells, A. Roca and C. Nti~ez Department Chemical Engineering and Metallurgy, Faculty Chemistry, University of Barcelona, Barcelona, Spain (Received October 8, 1991; revised version accepted December 3, 1991 ) ABSTRACT Crnells, M., Roca, A. and Nfifiez, C., Electric arc furnace flue dusts: characterization and Leaching with sulphuric acid. Hydrometallurgy, 31: Two electric arc furnace flue dusts, principally iron oxides, containing zinc, lead, cadmium, chromium and other elements in a lesser amount were examined. The non-ferrous metals make these materials hazardous wastes. The direct recycling of the dusts is not possible, which makes it necessary to look for a process to beneficiate these materials to generate a non-hazardous residue that can be used as landfill. This paper contains an electric arc furnace (EAF) flue dusts characterization and a systematic study of sulphuric acid leaching to obtain the reaction rates of dissolution. Results show that it is possible to beneficiate the dusts by leaching the non-magnetic fraction with sulphuric acid at low acid concentration and room temperature, giving high recoveries ofzn ( ~ 80%), recycling of the magnetic fraction to the EAF with partial recoveries of iron, zinc and chromium and leaving a compact, non-pollutant residue. INTRODUCTION Electric arc furnace flue dusts (EAF) are waste oxide materials generated in steelmaking. The majority compound is iron oxide, which also contains zinc and lead, in addition to other elements in lesser amounts. The direct recycling of these by-products is very difficult because they contain undesirable elements (zinc, lead and alkaline metals ) that can cause operating problems in the blast furnace and high energy consumption in the electric arc furnace. Furthermore, the dusts contain toxic elements (zinc, lead, cadmium, chromium and arsenic) that make these materials hazardous wastes unacceptable for landfill. The fact that it is not possible to recycle these dusts directly or to use them as landfill makes it necessary to consider the recovery of the valuable ele- Correspondence to: A. Roca, Department Chemical Engineering and Metallurgy, Faculty Chemistry, University of Barcelona, Diagonal 647, Barcelona, Spain X/92/$ Elsevier Science Publishers B.V. All rights reserved.

2 214 M. CRUELLS ETAL. ments contained in them and to obtain a non-hazardous residue that can be stored without problems. Recycling operations around the world are inconsistent. Whereas in Japan and West Germany all electric arc furnace flue dusts are recycled, the percentage in the USA is only 10% and it is 0% in England [ 1 ]. The total steel production of the European Community in 1988 was 138 X 106 t, of which approximately 30% was in electric arc furnaces. In Spain 60% was recycled with a total steel production of 12X l0 6 t [2]. The EAF dust production is 1-2% of the steel obtained. Thus, about X 106 t of EAF flue dust are generated each year in the European Community alone. Prior to the recycling of tin plate, the scrap is treated by a process to remove the tin layer. Complete removal of all the non-ferrous elements contained in scrap iron, before putting it into an electric arc furnace could reduce the amount of EAF flue dust generated, drastically diminishing the problems of pollution and landfill. However, at this moment, all scrap iron (with the exception of tin plate ) is placed directly into the electric arc furnace. Numerous authors have developed alternatives for recycling EAF flue dust on a commercial scale and at pilot plant level, as well as in the laboratory. These processes may be classified as follows. Dust treatments without recovery of valuable metals The aim of these processes is to immobilize the dust by chemical treatment to make it non-hazardous by impermeabilization, thus rendering it suitable for landfill [ 3,4 ]. Dust treatments with partial or total recovery of valuable metals: pyrometallurgical processes Under this item we can include all the processes in which the valuable metals contained in the dusts (Fe, Zn, Pb, Cu, Cr, Ni and Mo) are partially or totally recovered. We can distingish between processes in which iron is recycled and others in which it is not. Processes with production and recovery of iron: Examples of processes in which the non-ferrous metals and iron are recovered include: ( 1 ) The Waelz process [ 5-7 ], in which the EAF dusts are mixed with carbon and fluxes and injected into a rotary kiln. With this process, a crude zinc oxide with lead and cadmium and an iron-rich slag are obtained. The slag may be processed to obtain iron clinker. (2) Iron clinker is also obtained in an electrothermic shaft furnace after the dusts are mixed with coke and sintered, as in the St Joe plant [ 8 ] or in the Onahamo refinery [ 9 ]. Zinc is obtained in its metallic form. ( 3 ) Metallized iron and new dusts with high zinc content may be obtained in a rotary hearth furnace with coke as reducing agenthinmetco process [ 10 ]hor in a rotary kiln with fine coal and gas--ptc Technologies [ 11 ].

3 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H2SO4 215 (4) Pig iron is obtained by the Tetronics Plasma System [ 12 ] in a transferred arc plasma furnace using anthracite as reducing agent, or by the Plasmadust process [ 13 ] in a plasma shaft furnace with coke. In both cases new dusts with higher zinc and lead content are obtained. ( 5 ) The CENIM process [ 14 ] includes iron, lead and zinc recovery in metal formed by carbothermal reduction and fusion. Processes without iron recovery: These processes make it possible to recover non-ferrous metals without iron extraction. We include as examples: ( 1 ) The flame reactor process [ 15 ], or Boliden Inred process [ 16 ]; these processes mix EAF dusts with fine coal and both are injected into a reactor with oxygen. These processes convert the EAF dusts into crude zinc oxide and a non-hazardous iron slag. If justified by economics, the addition of sufficient coal makes it possible to recover iron from the slag. (2) Greenballing process [ 17 ], which consists of pelletizing the dusts and recycling the pellets to a steelmaking furnace. With this treatment a new dust with higher zinc and lead content can be obtained and iron may be partially recovered. (3) EAF sulphatizing or chloridizing roasting processes: these processes are based on the high stability of non-ferrous sulphates or chlorides. In the first process [ 18 ], soluble zinc sulphate and insoluble lead sulphate are formed and zinc is recovered from these solutions by electrowinning. In the second process [ 19 ], the non-ferrous metals are recovered from the volatile chlorides. Hydrometallurgical processes proposed for recovering non-ferrous metals We include here some processes in which the valuable metals are recovered by hydrometallurgical processes: ( 1 ) Leaching of EAF dusts with sodium hydroxide [20,21 ] has been proposed. Lead is recovered by cementation and zinc is electrowon from the alkaline leach solution. (2) Dreisinger et al. [ 22 ] developed the UBC-Chaparral process, based on the leaching of dusts with CaC12-NaC1-KC1-H20 solutions, the removal of CaO as Ca 2 and subsequent precipitation as gypsum, the recovery of Zn as Zn (OH)2-ZnCO3 and the recovery of Pb, Cd, Cu in the metallic cement. (3) Pearson [23] has studied the leaching of zinc from dusts (6.7-57% Zn) and fumes (45-78% Zn) with sulphuric acid. After leaching and purification of solutions, the metal is recovered by electrowinning. The zinc extraction is 80-99%. Lead is not recovered with the procedure used. (4) Jha and Duyvesteyn [24] have leached steelmaking dusts to recover zinc. In a first stage the dusts are leached with the liquor coming from the second stage, in which the solids from the first stage are treated in an autoclave with fresh sulphuric acid. Zn is recovered from the solutions and Fe is rejected as haematite residue.

4 216 M. CRUELLS ET AL. A process to beneficiate electric arc furnace flue dusts must take into account the following: ( 1 ) Dusts are generated in numerous steel plants, but in relatively small amounts by each plant. (2) These materials present high variability in their composition from the different steelmaking processes and, occasionally, differences in composition have been detected within the same company. ( 3 ) The pollution generated in the processes for recovering the metals contained in the EAF dusts must be less than that produced when landfilling these materials. In line with these points, a process must involve: ( 1 ) Low investment cost to the industrial plant. (2) Use of a minimum number of unitary operations. (3) Recovery of the most valuable metals contained in the flue dust at low costs in reagents and energy. (4) Generation of non-pollutant residue acceptable for landfill. This paper includes an exhaustive characterization of electric arc furnace flue dusts from steelmaking plants, and a systematic acid-leaching study of the reaction rates of the species contained in the non-magnetic fraction obtained from the dusts, as well as the results obtained in the pulp experiments carried out. CHARACTERIZATION OF ELECTRIC ARC FURNACE FLUE DUSTS Samples of electric arc furnace flue dusts named A (from special steelmaking) and B from (from structural steelmaking) were exhaustively characterized prior to the study of reactivity of these materials. The samples were magnetically concentrated with a permanent laboratory magnet of low intensity to obtain two different fractions for each sample: magnetic fraction (M) (52% A and 48% B, by weight) and non-magnetic fraction (NM) (47% for both samples). X-ray fluorescence and quantitative chemical analysis According to the peaks detected and their intensities, the results of X-ray fluorescence were: Sample A: Majority elements: Si, Fe Minority, 1 st order: Ca, Mn, Mg, K, C1, Zr Minority, 2nd order: Zn, Pb, Cu, Mo, Cr, A1 Trace: P, Ba, Na, Ni, Hg, Cd, Rb, Sr Sample B: Majority elements: Fe, Zn, Ca

5 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H2SO TABLEI Chemical analysis of EAF flue dusts Component Sample A AM ANM B BM BNM SiO R.i. ~ Fe Ca C Mg Zn Pb Mo < 0.05 < 0.05 Mn Ni K CI Cu A Cr Ba Na Hg Cd < Se 2 20 < 60 < 60 < 12 < 60 < 60 As 2 < All values in percentages unless otherwise stated. Residual insolubles after leaching with aqua regia, including SIO2. 2 In g/t. Minority, 1 st order: Si, Pb, Mg, Mn, K, C1 Minority, 2nd order: S, Cu, Cr, P, A1, Mo, Zr, Cd Trace: Mg, Na, Sn, Ag, Ba, Sr, Ni A quantitative chemical analysis of all the samples was carried out based on the results obtained by X-ray fluorescence examination. The samples were leached with aqua regia and the elements determined by atomic absorption spectrometry (AAS) and inductively coupled plasma (ICP). These analyses are shown in Table 1. Particle size analysis Figure 1 gives the composition of screened fractions of EAF dusts studied (samples A and B ) whose overall analysis is contained in Table 1. Rectangles of this figure correspond to the proportion by weight of the different fractions. We can see that 92% (sample A) and 12% (sample B ) of the

6 218 M. CRUELLS ET AL ' I 80 [] R i z~ Fe o Ca zn C \~-\ 6 Q ~ /zx\ = --/-Vq uj I / 't d (~m) 8, j ~ o Fig. 1. Granulometric analysis of samples A and B. Ri=the residual insolubles after leaching with hot concentrated HC1. fraction > 100 ~tm and also 48% of the fraction between 100 and 56 ~tm were silica. These values for sample A are unusual and give a global coarse particle size. The fraction > 100 lam can be eliminated from sample A by screening or hydrocyclone. In this sample there were no water-solubilized phases in the screening step, whereas in sample B, 4% by weight was solubilized during this process. In sample B only one third presents a particle size less than 100 ~tm. In this sample zinc is concentrated in sizes less than 36 ~tm. Mineralogical analysis X-ray diffraction The bulk EAF dusts and the M and NM fractions obtained by magnetic concentration were examined by X-ray diffraction (XRD). The results obtained are shown in Table 2. According to the chemical composition of samples A and B, and their X- ray diffraction patterns, it seems that there is a predominance of zinc ferrite in sample B and magnetite in sample A. A new phase, Ca [ Zn (OH) 3 ] 2" 2HzO, appears during the magnetic concentration of sample B. The XRD of the residues obtained after leaching all the samples with aqua

7 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H2S TABLE 2 X-ray diffraction of EAF dusts Sample Majority Minority compounds compounds A co-quartz,spinel ~ haematite, graphite A M spinelt a-quartz, haematite A NM a-quartz spinel 1, haematite, calcite B spinel, zincite haematite, graphite, calcite B M spinep zincite, haematite, graphite B NM zincite, Zn ferrite haematite, graphite Ca[Zn(OH)3]2'2H20 calcite The spinel phase is a solid solution of magnetite and zinc ferrite. ~. ;i', ~ -, Fig. 2. SEM showing morphology of particles from sample AM. 1-5=particles described in the text. regia showed that the principal component was quartz. The residues of samples AM and ANM also contained zircon, microcline and albite. The residues of samples BM and BNM contained maghemite and magnetite, respectively. Scanning electron microscopy and micro-analysis Scanning electron microscopy with X-ray energy dispersive analysis (EDS) was performed to gain further knowledge of the mineralogical species contained in the EAF flue dusts and their morphology. The studies were made using bulk samples A and B and their magnetic and non-magnetic fractions.

8 220 M. CRUELLS ET AL. Fig. 3. SEM showing complex silicate particle, sample A. 1-3 = zones described in the text. Fig. 4. SEM showing morphology of panicles from sample BM. 1-6 = panicles described in the text. Figure 2 shows some individual particles corresponding to the magnetic fraction of sample A. Particles 1,2 and 5 comprise essentially iron and oxygen (magnetite) and are spherical in shape. Particle 3 presents irregular morphology (slag-type) probably formed by silicates, and number 4 is a silica particle. Almost all the particles in this figure are silica. The non-magnetic fraction of sample A consists of irregular silica particles and, to a lesser extent, metallic silicates. Figure 3 is a micrograph of a complex particle from sample A. Zone 1 contains Si, AI and K as majority elements, probably silicate. In zone 2 only iron

9 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H2SO4 221 Fig. 5. SEM showing complex particle from sample B. Fig. 6. SEM showing complex silicate particle, sample B. was detected in the form of oxide. In zone 3 the elements detected were P and Ca, probably calcium phosphate. We found a few particles in sample A presenting a magnetite dendritic phase embedded in silicate-glass with the same morphology as shown later in Fig. 7. Figure 4 shows some particles of the magnetic fraction, sample B. Particle 1 contains exclusively iron and oxygen (magnetite) with a spherical morphology. Particles 4, 5 and 6 are iron, zinc and calcium in approximately the same composition and with irregular morphology. Number 2 is a spherical particle of iron, calcium and oxygen. Particle 3 presents an irregular mor-

10 222 M. CRUELLS ETAL. Fig. 7. SEM showing dendritic phase particle in calcium silicate glass in sample B. Fig. 8. SEM showing dendritic phase particle in calcium-iron silicate glass in sample B. phology and is of the slag-type, probably made up of metallic silicates. Figure 5 is an electron microscope image of details of a complex particle of Ca, Fe and Zn from sample B, formed by the accretion of smaller particles less than 1 ~tm in size. Figure 6 is a micrograph of a complex particle from sample B. The majority elements detected were Ca, Fe and Si, Ca being the most prominent. The polygonal precipitate that appears in this particle contains iron as the main element and chromium is concentrated in these precipitates. Figures 7 and 8 show particles consisting of dendritic phase in which the majority elements detected were iron, calcium and chromium enclosed in a

11 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H2SO4 223 Fig. 9. SEM showing particle formed by accretion of smaller particles, sample B. 1-5 = zones described in the text. calcium-iron silicate glass. Other particles, consisting of a skeletal magnetite phase enclosed in the same silicate glass, have also been observed. Figure 9 shows a particle formed by the accretion of smaller particles. Zone 1 of this particle is formed of Ca and Si: calcium silicate. In zone 2 Fe, Pb, Si and Zn, probably a complex silicate, was found. In zone 3, corresponding to the spherical particles, only iron was detected, an iron oxide. Zone 4 consisted of calcium chloride. In zone 5 only Ca was detected, probably as calcium oxide or carbonate. Numerous particles of zincite, with sizes in the range between 1 and 10 ~tm were also detected in sample B. The magnetite particle sizes were from 10 to 100 ~tm. The chemical and mineralogical analyses carried out on the EAF dust showed that sample A is a-quartz, a spinel phase of magnetite and, in minor amounts, haematite, graphite, some metallic silicates, zircon, calcite and calcium phosphate. Sample B is a spinel phase, essentially zinc ferrite, zincite and, in minor amounts, haematite, graphite, calcite, maghemite and some metallic silicates. EXPERIMENTAL PROCEDURE The material used in this experimental study was the sample BNM characterized above. During the magnetic concentration procedure zinc was concentrated in this sample. Sample A was not considered because its non-ferrous metals content was very low. Leaching studies to determine the reaction

12 224 M. CRUELLS ET AL. rates of dissolution using sample BNM were carried out in a magnetically stirred reactor. The reaction conditions were: stirring rate 1,000 min -1, solid:liquid ratio 1:1,000 by weight, sulphuric acid concentration varying between 0.1 and 2 M (the interval of the activity variation corresponds to and molal, respectively), reaction time 24 h, and temperature 35 C in a reactor of I 1. The zinc and iron conversion was determined by AAS and the reaction behavior by XRD. The temperature was varied between 18 and 51 C to investigate the effect of this variable, in which the sulphuric acid concentration used was 0.1 M. The effects of sulphuric acid activity and temperature were studied. The effect of the solid:liquid ratio was studied under the following conditions: stirring rate 1,000 min - ~, sulphuric acid concentration 2.0 M, temperature 30 C, 24 h and pulp ratio (S:L) between 1:5 and 1:20. Previously, the effect of sulphuric acid concentration was determined at a pulp ratio of 1:i 0,over 24 h and a temperature of 30 C in the range of M (that corresponds to mol H2SO4/kg H20 in the activity values). RESULTS AND DISCUSSION Stoichiometry of the reactions involved According to the results obtained in the characterization of EAF dusts, the reactions of the main species present in sample BNM, and their stoichiometry can be written as follows: ZnO(s) + H2SO4(aq) -*ZnSO4(aq) +H20 ( 1 ) Ca[Zn(OH)3 ]:.2HzO(s) +H2SO4(aq) --*CaSO4(aq) +2ZnSO4(aq) +8H20 (2) CaCO3 ~s) -.I.- H 2 SO4(aq ) --~ CaSO4 (aq) + CO 2 (g) + H20 ( 3 ) ZnO'Fe203~) + 4H2SO4~q) --*ZnSO4(aq) +Fe2(SO4)3(aq) +4H20 (4) Fe3 O4~.q) + 4H2 SO4~aq~ ~ FeSO4~aq~ + Fe2 (SO4) 3 ~aq) + 4H20 ( 5 ) Calcium sulphate has a limited solubility and, according to the solid:liquid ratio used, will be in aqueous solutions or in the solid residue. It is interesting that the reactions ( 1 ), (2) and (4) occur, whereas most of the iron remains in the residue. Reaction (4) occurs slowly at room temperature and runs with a high rate at elevated temperatures [25 ]. Unfortunately, reaction (3) will take place at all sulphuric acid concentrations, increasing the sulphuric acid consumption.

13 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H2SO Leaching studies of sample BNM with sulphuric acid Effect of sulphuric acid activity The effect of this variable was investigated in the activity range mol HzSO4/kg H20 (0.1-2 M). The results obtained are shown in Figures 10 and 11. From these results we can deduce, as could be expected, that for the zinc species the reaction rate was virtually independent of the sulphuric acid activity in the range investigated, whereas for the iron species the reaction rate was a function of the acid activity. Thus, the zinc extraction contained in the most reactive species is a zero order reaction with respect to the sulphuric acid activity. Figure 12 shows the XRD diagrams for the following samples: ( 1 ) BNM sample; (2) the same sample partially leached with 1 M sulphuric acid (approximately 75% Zn is leached); (3) the same sample leached for 24 h. According to these results, the leached species in sample 2 were Ca[Zn (OH)312"2H20, ZnO and CaCO3, whereas zinc ferrite and magnetite remained unaltered. After 24 h of leaching sample 3, these species were partially dissolved and, with the acid treatment, PbSO4 and CaSO4 were formed. Thus, it seems that about 75% of the zinc content is in the form of highly reactive species, whereas the remaining zinc is in the form of zinc ferrite. Thus, for technological purposes we can choose a sulphuric acid concentration in which the highly reactive species of zinc are dissolved and most of the iron remains in the residue. According to Figs. 10 and 11, it seems that the k 0,8 N ZO*6 (n uj > Z 0/4 o U o2 H2S04 M a [] LI J s I I I t rain Fig. 10. Effect of sulphuric acid activity on Zn conversion.

14 226 M. CRUELLS ET AL. A~ 6 Z0, o IIg 0,8 t ~ L ~ ~ - - A. o4. z o (..) O, 2 f / 3 / o / ~ / H=S04 M a E] A 2, O~ _~...- i t I I, t nnin Fig. 11. Effect of sulphuric acid activity on Fe conversion. best sulphuric acid concentration may be 0. I M. This concentration must also be compatible with the stoichiometric amounts of all the reactive species present in the reactor and, consequently, will depend of the solid:liquid ratio chosen. Effect of temperature The effect of temperature was studied at 18, 30, 35 and 51 C. The results obtained are given in Fig. 13. According to these results, the reaction rate for the dissolution of the highly reactive species of zinc is virtually independent of temperature. This is consistent with a process controlled by diffusion. However, the reaction rate of the iron species is dependent on the temperature, which means that it is necessary to work at room temperature to obtain high extraction values of zinc and simultaneously lower extraction values for iron. With this, the energy requirements for the reaction are reduced to a minimum. Effect of solid.'liquid ratio According to the results obtained above, the best sulphuric acid concentration may be 0.1 M. However, experiments carried out to determine the acid concentration compatible with the stoichiometric amount of all the reactive species indicated that the sulphuric acid concentration should be 1 Mor higher. Sulphuric acid concentrations less than 1 M are not sufficient to completely leach these species. Under these conditions ( 1 M, molal activity and

15 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H:SO4 227 BNM-X s' t~'. 3,88 CuRal+2 I! 2, I ,,i ~, i I'.. I ~.~'~;~,.;~,,.I.~. ~ 4.B@1 x " 2theta y ' 2~0. Linem~?O.O01) 5 1 BNN- 2 ss: tm: 3.88 CuXal x ' 2tlwta y : Linoap?O.BI) ]NM-~ ss: tm: 3.99 CuXal i: x : 2theta y.' Linoax, ) Fig. 12. XRD of samples BNM-1, BNM-2 and BNM-3. l=zn ferrite; 2=zincite; 3=calcium zinc hydroxide hydrate; 4=calcite; 5=graphite; 6=anglesite; 7=bassanite; 8= haematites.

16 228 M.CRUELLSET AL ~ : 'tb C D Zn Z 0 w Z 0 11,4 Fe -- e-'-", I, I t I [. I 10 SO fo t m(n Fig. 13. Effect of temperature on Zn and Fe conversion. C)f (~-"" a[ C) t.. t.) a[ n, )n: Ikt GO 40 / / Zn [~ Fe J 1 [ L = o~ o,4 a~2so 4 Fig. 14. Effect of sulphuric acid activity on Zn and Fe extraction: pulp experiments. solid:liquid ratio of 1:10) and with a residence time of 3 h, 80% Zn extraction and 40% Fe extraction were obtained. With a residence time of 24 h, Zn extraction was 90% and Fe extraction was 45%. Figure 14 shows the zinc and iron extraction values against sulphuric acid activity after 24 h of reaction. These experiments showed that the dissolution of the reactive zinc species is

17 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H2SO TABLE 3 Extraction of some elements at different solid/liquid ratios Element Extraction (%) at S/L ratio: 1/5 1/10 1/15 1/20 Zn Fe Ca Cd Cr As virtually independent of the sulphuric acid activity, whereas the iron extraction increases with the acid activity. Table 3 includes the results obtained from the study of the effect of the solid:liquid ratio. From these results we can deduce that, within the range of the solid:liquid ratio studied, the extraction of the different elements is practically independent of the solid:liquid ratio. When the solid:liquid ratio was 1:5 a high-viscosity pulp was obtained and the solid/liquid separation was very difficult. These results also showed that the zinc and iron dissolution is approximately 90% for both metals over 24 h under the experimental conditions used. Other elements partially dissolved during this process were Ca, Cd, Cr and AS. Thus, to obtain the best selectivity and residence time in the leaching step, the process of leaching EAF flue dusts with sulphuric acid must take place with a sulphuric acid concentration of 1 M (molal activity of ) within 3 h, with a solid:liquid ratio of 1:10, at room temperature. CONCLUSIONS According to the results obtained in this study, a process to beneficiate EAF flue dusts leads to: ( 1 ) Reduction of the amount of dusts destined for landfill because the magnetic concentrate can be recycled to the furnace after agglomeration by a cold pelletizing process, with partial recovery of elements such as zinc, iron and chromium. (2) Recovery of the zinc contained in the form of highly reactive species of the samples as BNM, by leaching this fraction with sulphuric acid at room temperature in a process involving little investment and low reagents and energy consumption. ( 3 ) Production of an inert residue suitable for landfill. The presence in this

18 230 M. CRUELLS ET AL. residue of calcium sulphate and iron hydroxides from the removal of iron in the leaching fluid, gives a highly compact residue. (4) This treatment can be applied to electric arc furnace flue dusts of the same type as sample B. Other materials, such as sample A, of low non-ferrous metals content and low pollutant power can be treated with chemicals (for example, lime) and then used for landfill. ACKNOWLEDGEMENTS The authors wish to express their gratitude to the Serveis Cientifico T~cnics de la Universitat de Barcelona (Departments of X-ray Fluorescence and Diffraction, AAS, ICP and Electronic Microscopy) for their assistance in our studies. We also wish to express our gratitude to Mrs. Esther Vilalta for her collaboration in the microscopy studies. REFERENCES 1 Engelleitner, W.H., In: Disposal, Recycling and Recovery of EAF Exhaust Dust. Iron Steel Soc. AIME, Warrendale, Pa. (1987), pp UNESID, La Industria Siderfirgica Espafiola en UNESID, Madrid (1989), p Stone, J.N., Electr. Furn. Conf. Proc., 44 ( 1986): Connor, J.R., USPat. 3,837,872 (1974). 5 Unger, T.W., Electr. Furn. Conf. Proc., 44 (1986): Maczek, H. and Cola, R., TMS Paper Select. 108th Annu. Meet. AIME (New Orleans) (1979), A De Goicoechea, N., Proc. 3rd Congr. Int. Min. Met., Oviedo (Spain) (1988), pp Morris, A.E., O'Keefe, T.J., Cole, E.R. and Neumeier, L.A., Disposal, Recycling and Recovery of EAF Exhaust Dust. Iron Steel Soc. AIME, Warrendale, Pa. (1987), pp Yatsunami, K., Miyashita, T., Tajima, O., Watanabe, H. and Suzawa, Y., Electr. Furn. Conf. Proc., 39 (1981): Bauer, K.H., Lehmktihler, H.J. and Schmauch, H., MPT-MetaI1. Plant Tech. Int., 4 ( 1990): Weiss, F.J., Goksel, M.A., Coburn, J.L. and Metius, G.E., Electr. Furn. Conf. Proc., 44 ( 1986): Cowx, P.M. and Roddis, B., Electr. Furn. Conf. Proc., 44 (1986): Herlitz, H.G. In: G.K. Bhat and L.W. Lherbier (Editors), Vacuum Metall. Conf. (Pittsburg) (1984), Iron Steel Soc., Warrendale, Pa., pp L6pez, F.A., Medina, F. and Medina, J., Rev. Metal., 26 (2) (1990): Pusateri, J.F., Chew, R. and Stanze, A.E., In: R. Kaltenhauser (Editor), Disposal, Recycling and Recovery of EAF Exhaust Dust. Iron Steel Soc. AIME, Warrendale, Pa. (1986), pp Elvander, H.I. and Westman, R.A., Iron Steel Eng., (1982). 17 Hoffman, A.O., Mezey, E.J., Varga, J. Jr., Steedman, W.G. and Tenaglia, R.D., Proc. Symp. Iron Steel Pollution Abatement Techniques for U.S. Environ. Protect. Agency, Washington (1980), pp Chaubal, P.C., Ironmaking Steelmaking, 9 ( 1982): Fumio Ono, Tetsu-to Hagane (Iron and Steel) 64, (8) ( 1978): A95-78, A98.

19 EAF FLUE DUST CHARACTERIZATION AND LEACHING WITH H2SO Eacott, J.G., Robinson, M.C., Busse, E., Burgener, J.E. and Burgener, P.E., CIM Bull., 77 (869) (1984): Frenay, J., Ferlay, S. and Hissel, J., Disposal, Recycling and recovery of Electric Furnace Exhaust Dust. Iron Steel Soc. AIME Warrendale, Pa. ( 1987), pp Dreisinger, D.B., Peters, E. and Morgan, G., The hydrometallurgical treatment of carbon steel electric arc furnace dusts by the UBC-Chaparral process. Hydrometallurgy, 25 ( 1990 ): Pearson, D., In: A.T. Kuhn (Editor), Process and Fundamental Considerations of Selective Hydrometallurgical Systems. SME-AIME, Warrendale, Pa., Chap. 14 (1981), Jha, M.C. and Duyvesteyn, W.P.C., In: P.R. Taylor, H.Y. Sohn and N. Jarret (Editors), Recycle and Selective Recovery of Metals. TMS-AIME, Warrendale, Pa. ( 1985 ), pp Nt]fiez, C. and Vifials, J., Metall. Trans. B, 15B ( 1984):