Atmospheric leaching of EAF dust with diluted sulphuric acid

42T. Havlik et al. / Hydrometallurgy 77 (2005) 41–50The mineralogical analysis indicates the presence ofiron in an oxide form, mostly as hematite Fe 2 O 3 ,magnetite Fe 3 O 4 and ferrite as franklinite ZnFe 2 O 4 .Zinc is present mostly in two basic compounds, namelyas oxide ZnO and ferrite ZnFe 2 O 4 , and possibly as acomplex ferrite, e.g. (ZnMnFe) 2 O 4 . The mineralogicalform of the zinc presence seems to be the basicindicator for the effectiveness of the EAF dust treatment.ZnO in principle is an easily workable form forboth the pyro- and the hydrometallurgical method, butthe ferrite form is considerably complex and difficult.Treatment technologies of zinc-bearing wastesrepresent various levels of development both ofpyrometallurgical and hydrometallurgical processes.A brief overview of the methods, which have beenused for the treatment of this material, was presentedin our previous work (Havlik et al., 2004). Thesea100Table 1Chemical analysis of EAF dust by Paul Wurth S.A.Element Fe Zn Pb Si Ca Mn Cu Cr[%] 27.8 20.9 2.7 2.1 4.3 3.2 0.36 0.77Element Ni Ti Cd Sb As Cl2SO 4 Loss ofignition[%] 0.06 ~0.04 0.027 0.069 b0.1 0.89 0 12.18works were made by several investigators (Frenay etal., 1985, 1986; Jha and Duyvesteyn, 1985; Dreissingeret al., 1990; Barrett et al., 1992; McElroy andMcClaren, 1994; McElroy and Murray, 1996; Lupi etal., 1996; Jandová et al., 2002), etc.In study (Havlik et al., 2004), the hydrometallurgicalpressure processing of EAF steelmaking dust,provided by Paul Wurth S.A., Luxembourg, wasinvestigated on a laboratory scale in the temperaturerange of 100 to 250 8C. The behaviour of zinc, ironMetal extraction [%]80604020Fe extraction; a/d = 1.2Zn extraction; a/d = 1.2b0100100 120 140 160 180 200 220 240 260Temperature [ o C]Metal extraction [%]80604020Fe extraction; a/d = 0.4Zn extraction; a/d = 0.40100 120 140 160 180 200 220 240 260Temperature [ o C]Fig. 1. Metal extraction vs. leaching temperature dependence(Havlik et al., 2004). (a) a/d ratio=1.2; (b) a/d ratio=0.4 (a/d=gramsof concentrated H 2 SO 4 to grams of dust).Fig. 2. Schematic view of the leaching apparatus. 1—Stirrer engine;2—propeller; 3—pulp; 4—sampler; 5—thermometer; 6—feeder;7—water seal; 8—water thermostat; 9—EAF sample.

T. Havlik et al. / Hydrometallurgy 77 (2005) 41–50 43and calcium under influence of sulphuric acid as theleaching agent was discussed.According to the results obtained in this work,the high pressure hydrometallurgical recovery of zincfrom EAF dust is feasible at a reasonable recoveryyield, while iron mostly remains in the solid phase.The latter is due to the use of the low concentration ofsulphuric acid of approximately 0.4 mol H 2 SO 4 ddm 3 . Three ratios of the free acid weight to solid,with the value of a/d (acid/dust) of 1.2, 0.6, and 0.4,were used. The maximal value of selectivity wasobtained by the a/d ratio of 0.4. Higher temperaturesand pressures moderately increase the zinc yield.Fig. 1a to b show the results for the metalextraction depending on temperature/pressure anda/d ratios obtained in study (Havlik et al., 2004).These results show promise. However, it isnecessary to compare such results with those obtainedby the same hydrometallurgical route but undernormal conditions concerning both temperature andpressure. This is the concept of the present study. Themain factors investigated are the effects of temperatureon the dissolution of Zn, Fe and Ca by use ofsulphuric acid as the extractant at an atmosphericpressure of 0.1 MPa.2. Experimental2.1. MaterialThe materials used in these experiments are thesame as in study (Havlik et al., 2004), i.e. EAF dustgiven by Paul Wurth S.A., Luxembourg. Its chemicalanalysis, given in Table 1, was determined by the ICPmethod.aZn extraction [%]706050t = 20 • C; C H2S04 = 0.4 Ma/d = 1.2a/d = 0.6a/d = 0.4bZn extraction [%]706050t = 40 • C; C H2S04 = 0.4 Ma/d = 1.2a/d = 0.6a/d = 0.44040cZn extraction [%]706050400 20 40 60Leaching time [min]a/d = 1.2a/d = 0.6a/d = 0.4t = 60 • C; C H2S04 = 0.4 M0 20 40 60Leaching time [min]dZn extraction [%]706050400 10 20 30 40 50 60 70Leaching time [min]t = 80 • C; C H2S04 = 0.4 MLeaching time [min]a/d = 1.2a/d = 0.6a/d = 0.40 20 40 60Fig. 3. (a) Extraction of Zn by sulphuric acid at a temperature of 20 8C. (b) Extraction of Zn by sulphuric acid at a temperature of 40 8C. (c)Extraction of Zn by sulphuric acid at a temperature of 60 8C. (d) Extraction of Zn by sulphuric acid at a temperature of 80 8C.

44T. Havlik et al. / Hydrometallurgy 77 (2005) 41–50The results of the X-ray diffraction qualitativeanalysis show the presence of compounds such asfranklinite Zn 2 FeO 4 , zincite ZnO, magnetite Fe 3 O 4 ,cristobalite SiO 2 . Other phases, which, on the basis ofthe chemical analysis, could be present, are probablybelow the detection limit.2.2. Experimental set-up and procedureLeaching tests are performed in the apparatus, ofwhich a scheme is given in Fig. 2.The leaching experiments are performed in a glassreactor of 600 mL provided with a Teflon cap in thecentral hole in which the axis of a glass stirrer foradjustable revolutions is placed. Openings for amercury thermometer, a sampling liquid specimen,and an input of the powdered sample are situated inthe perimeter of the cap. The reactor is placed in awater bath controlled by a thermostat. This allows toconduct the leaching at the desired temperature.Leaching tests are performed with a solution of 300mL of 0.4 M H 2 SO 4 . The sample weight is in therange of 10 to 30 g. A solution with an acid-to-EAFdust ratio a/d of between 0.4 and 1.2 is used. Thetemperatures being used are 20, 40, 60, and 80 8C,under normal atmospheric pressure.The samples for the chemical analysis are takenaccording to a fixed-schedule after 2, 15, 30, 45and 60 min. Then the samples are filtered into testtubes and, thus, a clear solution is obtained. Likeduring the pressure leaching described in work(Havlik et al., 2004), the solution is colourless,weakly yellow or weakly emerald depending on theleaching conditions. In some samples, again theaZn extraction [%]70605040a/d = 1.2, C H2S04 = 0.4 M20 • C40 • C60 • C80 • CbZn extraction [%]70605040a/d = 0.6, C H2S04 = 0.4 M20 • C40 • C60 • C80 • C0 20 40 60Leaching time [min]0 10 20 30 40 50 60 70Leaching time [min]cZn extraction [%]706050a/d = 0.4, C H2S04 = 0.4 M20 • C40 • C60 • C80 • C400 10 20 30 40 50 60 70Leaching time [min]Fig. 4. (a) Extraction of Zn by sulphuric acid for a 10 g charge under various temperatures. (b) Extraction of Zn by sulphuric acid for a 20 gcharge under various temperatures. (c) Extraction of Zn by sulphuric acid for a 30 g charge under various temperatures.

T. Havlik et al. / Hydrometallurgy 77 (2005) 41–50 45white precipitate of calcium sulphate appears after 1or 2 days.The chemical analysis, carried out in order todetermine Zn, Fe, and Ca, was made by means ofan AAS Varian Spectrometer AA 20+. Moreover,the amount of free acid as well as the pH ismeasured. All results are recalculated because ofthe change of the pulp volume due to the samplingand evaporation.3. Results and discussion3.1. Leaching reactionsAs already described in Havlik et al. (2004), thereactions of the main species occurring in thesample and their stoichiometry can be stated asfollows:ZnO+H 2 SO 4(aq) YZnSO 4(aq) +H 2 O (1)ZnFe 2 O 4 +4H 2 SO 4(aq) YZnSO 4(aq) +Fe 2 (SO 4 ) 3 +4H 2 O(2)ZnFe 2 O 4 +H 2 SO 4(aq) YZnSO 4(aq) +Fe 2 O 3 +H 2 O (3)CaCO 3 +H 2 SO 4(aq) YCaSO 4 +CO 2 +H 2 O (4)CaO+H 2 SO 4(aq) YCaSO 4 +H 2 O (5)Reaction (2) occurs slowly at room temperature,but runs at a high rate at elevated temperatures.a14 a/d = 1.2t = 20 • C; C a/d = 0.612H2S04 = 0.4 Ma/d = 0.4b1412t = 40 • C; C H2S04 = 0.4 Ma/d = 1.2a/d = 0.6a/d = 0.4Fe extraction [%]10864Fe extraction [%]1086422cFe extraction [%]00 20 40 60Leaching time [min]1412108642t = 60 • C; C H2S04 = 0.4 MLeaching time [min]a/d = 1.2a/d = 0.6a/d = 0.400 20 40 60dFe extraction [%]00 10 20 30 40 50 60 701412108642Leaching time [min]t = 80 • C; C H2S04 = 0.4 MLeaching time [min]a/d = 1.2a/d = 0.6a/d = 0.400 10 20 30 40 50 60 70Fig. 5. (a) Extraction of Fe by sulphuric acid at a temperature of 20 8C. (b) Extraction of Fe by sulphuric acid at a temperature of 40 8C. (c)Extraction of Fe by sulphuric acid at a temperature of 60 8C. (d) Extraction of Fe by sulphuric acid at a temperature of 80 8C.

46T. Havlik et al. / Hydrometallurgy 77 (2005) 41–503.2. Leaching studies with sulphuric acidIn Fig. 3a to d the extraction curves at 0.4 MH 2 SO 4 and various acid/dust (a/d) ratios for Zn atdifferent temperatures are presented.The temperature has a considerable effect on theyield of the zinc extraction. This is valid for eachsolid/liquid ratio being studied, as demonstrated inFig. 4a to c. Since an increase of the quantity of thecharge causes the reduction of the zinc yield, theconcentration of sulphuric acid (or the a/d ratio)seems to be the second important factor for the zincextraction. The maximum value for the zincextraction of 67% is obtained at a temperature of80 8C, in a time of 60 min and with a concentrationof sulphuric acid of 0.4 mold dm 3 (Fig. 3d).However, the curves in general show a progressiverun. This means that the prolongation of theleaching time should result in an increase of zincextraction.Fig. 5a to d illustrate the extraction curves for ironunder the same conditions. In these cases, the increaseof temperature causes the increase of the iron yield,although the total amount of iron in the solution doesnot exceed 10%.The temperature effect on iron extraction is shownin Fig. 6a to c at various a/d ratio. It was observed inthe high temperature/pressure leaching of EAF dust(Havlik et al., 2004) that calcium sulphate wasprecipitated from the same solution after the leachingand cooling of the leaching solution. In the presentexperiments, under normal conditions concerningboth temperature and pressure the white precipitateof calcium sulphate rarely occurs. Fig. 7a to ddemonstrate the extraction of the calcium during theleaching under various conditions.aFe extraction [%]14 20 • C12a/d = 1.2, C H2S04 = 0.4 M20 • C20 • C20 • C10864200 10 20 30 40 50 60 70Leaching time [min]cFe extraction [%]1412108642bFe extraction [%]14121000 20 40 60864220 • C20 • C20 • C20 • C20 • C20 • C20 • C20 • C00 10 20 30 40 50 60 70Leaching time [min]a/d = 0.4, C H2S04 = 0.4 MLeaching time [min]a/d = 0.6, C H2S04 = 0.4 MFig. 6. (a) Extraction of Fe by sulphuric acid for 10 g charge under various temperatures. (b) Extraction of Fe by sulphuric acid for 20 g chargeunder various temperatures. (c) Extraction of Fe by sulphuric acid for a 30 g charge under various temperatures.

T. Havlik et al. / Hydrometallurgy 77 (2005) 41–50 47a40b40Ca extraction [%]302010t = 20 • C; C H2S04 = 0.4 Ma/d = 1.2a/d = 0.6a/d = 0.4Ca extraction [%]302010a/d = 1.2a/d = 0.6a/d = 0.4cCa extraction [%]00 10 20 30 40 50 60 70Leaching time [min]40302010a/d = 1.2a/d = 0.6a/d = 0.4dCa extraction [%]t = 40 • C; C H2S04 = 0.4 M00 10 20 30 40 50 60 7040302010Leaching time [min]t = 80 • C; C H2S04 = 0.4 Ma/d = 1.2a/d = 0.6a/d = 0.4t = 60 • C; C H2S04 = 0.4 M00 20 40 60Leaching time [min]00 10 20 30 40 50 60 70Leaching time [min]Fig. 7. (a) Extraction of Ca by sulphuric acid at a temperature of 20 8C. (b) Extraction of Ca by sulphuric acid at a temperature of 40 8C. (c)Extraction of Ca by sulphuric acid at a temperature of 60 8C. (d) Extraction of Ca by sulphuric acid at a temperature of 80 8C.Although the amount of dissolved calcium is ratherhigh, it is considerably decreased by the increase ofthe charge, as it is demonstrated in Fig. 8a toc.The concentration of H 2 SO 4 used for these experiments,i.e. 0.4 mold dm 3 , results from previousexperiments described in study (Havlik et al., 2004).The thermodynamic considerations and calculationsreveal that this concentration should mostly be spentfor the dissolution of zinc, but less for iron andcalcium. Fig. 9 shows the acid consumption duringleaching, determined as the dependence of the pHvalueon the charge amount at various temperatures.The results reveal that the a/d ratio, i.e. the chargeamount, plays an important role in the metalextraction. pH values were increased according tothe increase of the a/d ratio due to a higherconsumption of sulphuric acid during leaching. Sincesmall charges are used, the zinc extraction is considerablyhigh. The maximum value for zinc extractiona=67% is obtained at a temperature of 80 8C, in a timeof 60 min and at a used concentration of sulphuricacid of 0.4 mold dm 3 , Figs. 3d and 4a. A furtherincrease of the leaching time would probably result insame zinc extraction increasing as follows from Figs.3d and 4a.The increase of the charge amount results in thedecreasing of the zinc extraction. In these cases,however, temperature plays a less important role. Thisprobably means that the sulphuric acid concentrationis a limiting factor for the dissolution of zinc.This applies to iron and calcium, too. Themaximum of iron dissolved is less than 10%, (Figs.5d and 6a), but in case of calcium it is up to 40%(Figs. 7d and 8a).Comparing of the results obtained in this work withthose obtained in the study of high temperature/

48T. Havlik et al. / Hydrometallurgy 77 (2005) 41–50aCa extraction [%]4030202010• C40 • Ca/d = 1.2, C H2S04 = 0.4 M60 • C80 • C00 10 20 30 40 50 60 70Leaching time [min]cCa extraction [%]40302010bCa extraction [%]40302010a/d = 0.4, C H2S04 = 0.4 Ma/d = 0.6, C H2S04 = 0.4 M00 10 20 30 40 50 60 70Leaching time [min]20 • C40 • C60 • C80 • C20 • C40 • C60 • C80 • C0010 20 30 40 50 60 70Leaching time [min]Fig. 8. (a) Extraction of Ca by sulphuric acid for a 10 g charge under various temperatures. (b) Extraction of Ca by sulphuric acid for a 20 gcharge under various temperatures. (c) Extraction of Ca by sulphuric acid for a 30 g charge under various temperatures.pressure leaching (Havlik et al., 2004) under similarconditions shows that the leaching under normalconditions results in the lower extraction of zinc, evenpH1.81.61.41.21.00.80.620 • C40 • C0.460 • C80 • C0.25 10 15 20 25 30 35grams of charge [g]Fig. 9. pH vs. charge amount dependence for various temperatures.though there is the tendency for the further dissolutionof zinc. In high temperature/pressure leaching theleaching ceases after a short time at a fixed value. Themaximum value is, at best, at a=84%.Furthermore, in regard to iron, the study showsthat the maximum value of iron dissolved undernormal leaching conditions is about 10%, in contrastto high temperature/pressure leaching in which, atthe worst, more than 20% is achieved. In hightemperature/pressure leaching at best around 1% ofdissolved iron is achieved. Under normal leachingconditions, regarding both temperature and pressure,the minimum value for the iron dissolution is about2%.The quantity of dissolved calcium is rather high.With a sulphuric acid concentration of 0.4 mold dm 3 ,only in one case the white precipitate of calciumsulphate is formed. However, this only applies fortemperatures of 100 and 150 8C (concerning high

T. Havlik et al. / Hydrometallurgy 77 (2005) 41–50 49aMetal extraction [%]80604020Zn extraction; a/d = 1.2Fe extraction; a/d = 1.2bMetal extraction [%]80604020Zn extraction; a/d = 0.4Fe extraction; a/d = 0.4020 40 60 80Temperature [ • C]020 40 60 80Temperature [ • C]Fig. 10. (a) Dependency of metal extraction vs. leaching temperature at a/d=1.2. (b) Dependency of metal extraction vs. leaching temperature ata/d=0.4.temperature/pressure leaching). This is the reason thatonly a small amount of calcium remains in the solution.Fig. 10a to b show the dependencies of metalextraction on the leaching temperature for the highestand lowest dust charge, each in relation to theamount of acid available for the leaching. For thelowest charge amount of a/d=1.2, the zinc extractionis higher having the tendency to increase at a highertemperature. Yet, when using the lowest chargeamount, also the quantity of the dissolved iron ishigher compared to a/d=0.4. The comparison withthe experiment at 100 8C, presented in work (Havliket al., 2004), reveals that this, under normal temperature/pressureconditions, is the optimum temperaturefor the maximal zinc extraction. However, theamount of extracted iron is several times higher at100 8C than at 80 8C. It seems that higher values ofselectivity are obtained by lower temperature and,perhaps, a/d ratios.4. ConclusionAccording to the results obtained in this work, thehydrometallurgical recovery of zinc from EAF dust isfeasible with a relatively high recovery yield, whileiron mostly stays in solid phase. A similar conclusioncan be drawn in the case of high temperature/pressureleaching under equivalent experimental conditions.Comparing the results of this work with thoseobtained by high temperature/pressure leaching manifestthat, concerning the zinc extraction, the last ismore effective. Yet, this does not relate to iron.Additionally, calcium permanently remains in thesolution. Although in high temperature/pressureleaching calcium is dissolved first, it later precipitatesas calcium sulphate. This does only relate to temperaturesup to 150 8C and pressures up to 4.1 bar. Evenin higher temperatures/pressures leaching calciumpermanently remains in the solution. This behaviourresults from using a low concentration of sulphuricacid. This also causes in fact a decrease in the zincyield, yet the amount of dissolved iron is low. In sucha way, it can be possible to set up the conditions for anoptimum zinc yield into the solution with a minimisediron dissolution. However, the problem persists thatthe chemical and mineralogical composition of eachsteelmaking dust is individual and that for each onethe conditions of processing have to be studied.AcknowledgementThe authors wish to express their gratitude toSlovak grant agency VEGA (Grant 1/9379/02) forfinancial support.ReferencesBarrett, E.C., Nenniger, E.H., Dziewinski, J., 1992. A hydrometallurgicalprocess to treat carbon steel electric arc furnacedust. Hydrometallurgy (30), 59–68.

50T. Havlik et al. / Hydrometallurgy 77 (2005) 41–50Cruells, M., Roca, A., Nunez, C., 1992. Electric arc furnace fluedusts: characterization and leaching with sulphuric acid. Hydrometallurgy31, 213–231.Dreissinger, D.B., Peters, E., Morgan, G., 1990. The hydrometallurgicaltreatment of carbon steel electric arc dust by theUBC-Chaparral process. Hydrometallurgy (25), 137–152.Frenay, J., Hissel, J., Ferlay, S., 1985. Recovery of lead and zincfrom electric steelmaking dusts by the Cebedeau process. Met.Soc. AIME, 195–208.Frenay, J., Ferlay, S., Hissel, J., 1986. Zinc and lead recovery fromEAF dusts by caustic soda process. Electric Furnace Proc.,Treatment Options for Carbon Steel Electric Arc Furnace Dust,Iron and Steel Soc., vol. 43, pp. 171–175.Havlik, T., Friedrich, B., Stopić, S., 2004. Pressure leaching of EAFdust with sulphuric acid. Erzmetall 57 (2), 113–120.Jandová, J., Dvorak, P., Vu, H., 2002. Recovery of magnetite andzinc from steelmaking dust by H 2 SO 4 leaching. Recycling andWaste Treatment in Mineral and Metal Processing, June 2002,Lulea Sweden, pp. 251–259.Jha, M.C., Duyvesteyn, W.P.C., 1985. A two-stage leaching processfor selective recovery of zinc from steel plant dusts. In: Taylor,P.R., Sohn, H.Y., Jarret, N. (Eds.), Proc. Recycling andSecondary Recovery of Metals, Met. Soc. AIME, pp. 143–157.Lupi, C., Pilone, D., Cavallini, M., 1996. Hydrometallurgicalprocessing of EAF steelmaking fumes. In: Ramachandran, V.,Nesbitt, C.C. (Eds.), Proc. Extraction and Processing for theTreatment and Minimisation of Wastes, TMS, pp. 711 – 718.McElroy, R.O., McClaren, M., 1994. Processing of electric arcfurnace dust via chloride hydrometallurgy. Proc. Hydrometallurgy’94, IMM. Chapman & Hall, pp. 993–1010.McElroy, R.O., Murray, W., 1996. Developments in the Terra Gaiaprocess for the treatment of EAF dust. In: Dutrizac, J.E., Harris,G.B. (Eds.), Iron Control and Disposal. Canadian Inst. of Min.,Met., and Petroleum, Montreal, Canada, pp. 39–44.