utilization of industrial waste electric arc furnace dust as iron oxide sorbent for hydrogen sulfide...

Research article

Utilization of industrial waste electric arc furnace dust asiron oxide sorbent for hydrogen sulfide removal

Danlin Zeng,* Shenglan Liu, Guanghui Wang, Jianghua Qiu and Hongxiang Chen

College of Chemical Engineering and Technology, Hubei Key Laboratory of Coal Conversion and New CarbonMaterial, Wuhan University of Science andTechnology, Wuhan 430081, China

Received 3 June 2013; Revised 27 November 2013; Accepted 27 February 2014

ABSTRACT: Leaching iron from the electric arc furnace dust by using sulfuric acid to prepare iron oxide sorbent for H2Sremoval was studied in this paper. The influences of sulfuric acid concentration, reaction temperature and the dosage ofhydrochloric acid on the iron leaching yield were investigated in the leaching process. The iron oxide sorbent was preparedby the filtrate from the leaching process. The experimental results show that the iron species in the dust were transferred intoγ-Fe2O3 · H2O crystal of higher activity for H2S removal in the sorbent. The desulfurization test revealed that thebreakthrough sulfur capacity of the sorbent is similar to that of the commercial active carbon on the same test conditions.© 2014 Curtin University of Technology and John Wiley & Sons, Ltd.

KEYWORDS: electric arc furnace dust; iron oxide; sorbent; H2S removal

INTRODUCTION

Each year, millions of tons of electric arc furnace dustwere collected by the steel-making corporations inChina. The electric arc furnace process generates about10–20 kg of dust per metric ton of steel produced.[1]

Most of the waste dust is stored in the landfills.Because of its metal leaching potential, the waste isfrequently classified as hazardous wastes under theUS Environmental Protection Agency classification.[2]

Classification as hazardous wastes greatly increasesthe cost of disposal of dusts for the treatment to renderthe wastes that are nonhazardous, as well as highertransportation and disposal costs. In fact, the bulkwaste generally is iron oxide. Some of the waste dustis recycled as furnaces feed,[3,4] red ceramic,[5] iron-containing glass[6,7] and iron oxide pigment.[8,9]

The electric arc furnace dust contains high ironspecies content mainly in the form of FeO, α-Fe2O3

and Fe3O4. However, the traditional recycling as redceramic or iron oxide pigment is hard to be appliedbecause of its complicated preparation and purificationprocess. The removal of H2S with iron oxide sorbent isa classic and effective desulfurization method andis widely used because of its advantages of high sulfurcapacity, low cost and easy operation.[10,11] Up to now,

the demand of iron oxide sorbent is continuouslyincreasing in the industrial gas desulfurization. If theiron species in the dust could be used as secondaryraw material to prepare the sorbent for gasdesulfurization, then the waste dust can be the valuablematerials. The iron oxides in the electric arc furnacedust show the very low activity on the H2S removalreaction, thus, the transformation of the iron speciesin the dust into the iron oxide crystal of high H2Sremoval activity is the key question of its applicationfor iron oxide sorbent preparation. However,unfortunately, almost no report about this significantissue is found in the field of the electric arc furnace dustrecycling.In the present study, the electric arc furnace dust was

firstly leached with sulfuric acid, and then, a mixedsolution of ferrous and ferric sulfate was obtainedand used as raw material to prepare iron oxide sorbent.Effects of different parameters such as sulfuric acidconcentration, reaction temperature and the dosage ofhydrochloric acid on the iron leaching yield havebeen investigated. The result shows that the sorbentcan be prepared with electric arc furnace dust byblending the mixed solution with an addition of thepore-forming agent (NH4HCO3), cocatalyst (clay)under suitable conditions. The samples of electric arcfurnace dust and the iron oxide sorbent were alsocharacterized by X-ray diffraction (XRD), scanningelectron microscopy (SEM) and differential thermalgravity (DTG).

*Correspondence to: Danlin Zeng, College of ChemicalEngineering and Technology, Wuhan University of Science andTechnology, Heping Road 947, Wuhan 430081, China. E-mail:[email protected]

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd.Curtin University is a trademark of Curtin University of Technology

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2014; 9: 737–742Published online 14 April 2014 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/apj.1819

MATERIALS AND METHODS

Materials

All reagents used in this study were of analytical grade.The electric arc furnace dust was obtained from WuhanIron and Steel Corporation. The chemical compositionof the material is shown in Table 1. The dust samplecontains predominantly of Fe2O3 (30.50%), FeO(22.18%), CaO (13.23%) and MgO (5.80%) with smallamounts of other metal oxides, as measured by flameatomic adsorption spectrophotometry.

Leaching process

The leaching experimental was performed as follows:10 g of dry electric arc furnace dust with 100-mLdesigned concentration of H2SO4 solution andconcentrated HCl solution (37wt% HCl, 12.0mol/L)was placed into a flask, and then, the mixture wasstirred at the designed temperature for the needed time.The iron species will be leached into the acid solution inthe form of ferric and ferrous ions (Eqns (1) and (2)).When the leaching was over, the mixture was cooledto room temperature, filtered and washed thoroughlywith deionized water. The iron concentration in thefiltrate was titrated by the K2Cr2O7 standard solution.Then, the iron leaching yield was calculated based onthe iron concentration, the filtrate volume and the drywaste dust mass.

FeO sð Þ þ H2SO4 aqð Þ→FeSO4 aqð Þ þ H2O aqð Þ (1)

Fe2O3 sð Þ þ 3H2SO4 aqð Þ →Fe2 SO4ð Þ3 aqð Þþ3H2O aqð Þ

(2)

Preparation of the sorbent

The filtrate from the iron leaching process, theprecipitator NH4HCO3 solution and the mixed clay areplaced into a beaker and followed by stirring, agingand filtrating. Then, the paste was extruded in a syringe.The cylindrical sorbent was approximately 3–5mm indiameter and 5–10mm in height. At last, the sorbent isdried at 120 °C for 12 h. The exhausted sorbent andactive carbon (AC) were obtained by desulfurization test.For comparison, the experiments were performed usingthe as-received AC (wood-based, H3PO4 activation)manufactured by Shanghai Guoyao Groups.

Characterization of the sorbents

XRD was performed with a Philips X’Pert Pro multi-purpose diffractometer, operating with Cu Kα radiation(40 kV, 30mA) and Ni filter.The SEM results were obtained on a Nova 400 Nano

scanning electron microscope.Surface area and porosity of samples were evaluated

by N2 adsorption/desorption isotherms carried out at77K on a Micromeritics ASAP 2020 sorption analyzer.Prior to the adsorption–desorption measurements, allthe samples were degassed at 150 °C in N2 flow for 12 h.Thermal analysis was carried out using TA Instruments

thermal analyzer. The heating rate was 10 °C/min in anitrogen atmosphere at 100mL/min flow rate.Desulfurization test was evaluated by the fixed-bed

continuous reaction apparatus with a fixed-bed glassreactor of 30mm in diameter and 300mm in length atroom temperature and normal pressure. The gas usedin this work is the mixture of 1000mg/m3 H2S andthe balance N2. The gas flow rate was controlled byflow meters. The H2S vapor and N2 were mixed afterthe flow meters. The space velocity of the mixed gaswas 1000 h�1. The exhausted tail gas was washed bya tail gas absorber with 0.1-M NaOH solution. Whenthe H2S concentration in the exit gas was higher than100mg/m3, the test was stopped, and then, thebreakthrough sulfur capacity was calculated based onthe test time, H2S concentration and mass of thesorbent. The concentration of the hydrogen sulfidewas determined by the HP6890 gas chromatography.When the desulfurization test was over, the exhaustedsorbent was collected for further characterization. Theschematic diagram of the desulfurization test is shownin Scheme 1.

RESULTS AND DISCUSSION

Effect of sulfuric acid concentration on the ironleaching yield

To investigate the effect of sulfuric acid concentration onthe iron leaching yield, the results of the iron leachingyield with different sulfuric acid concentration arepresented in Fig. 1. It was shown from Fig. 1 that theiron leaching yield increases as the sulfuric acidconcentration increases from 50wt% (7.1mol/L) to70wt% (11.4mol/L) at which 93.60% iron was extractedin 120min, and then, the iron leaching yield droppeddown at 80 and 90% sulfuric acid concentration. The

Table 1. Chemical composition of the dust (wt%).

Compositions Fe2O3 FeO CaO MgO ZnO SiO2 Al2O3 C

Content (wt %) 30.50 22.18 13.23 5.80 2.22 1.21 0.65 12.22

D. ZENG ET AL. Asia-Pacific Journal of Chemical Engineering738

© 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2014; 9: 737–742DOI: 10.1002/apj

higher sulfuric acid concentration significantly increasedthe viscosity of the solution. It would tend to reduce thediffusion rate of the ions. In fact, we find that the wholemixture will be hard to stir as the sulfuric acidconcentration further increases up to 95wt%. Therefore,the suitable sulfuric acid concentration is suggested at70wt% (11.4mol/L).

Effect of reaction temperature on the ironleaching yield

The results of the iron leaching yield at differentreaction temperatures are presented in Fig. 2. The ironleaching yield increases significantly as the reactiontemperature increases from 40 to 120 °C. As thereaction temperature prolongs from 40 to 120 °C, the

iron yield increases and reaches the peak value of93.87% at 120min. The mixture is boiling in atmospherewhen the reaction temperature increases to 120 °C. It wasinferred that the suitable reaction temperature was 120 °C,at which iron leaching process was mostly completedduring 120min.

Effect of the dosage of hydrochloric acid on theiron leaching yield

The results of the iron leaching yield using differentdosage of concentrated HCl (37wt% HCl, 12.0mol/L)are presented in Fig. 3. It is reported that Cl� ion caninduce the pore corrosion and break the passivation filmover the dust surface.[12] Therefore, as an effectivelixiviant, HCl will accelerate the dissolvability ofH2SO4 on the dust. The iron leaching yield increases to96.20% as the dosage of HCl increases from 0 to 3mL.When the dosage of HCl is higher than 3mL, the furtherincrease in the iron leaching yield is marginal. Therefore,the suitable dosage of concentrated HCl is 3mL.It is noteworthy that some calcium, magnesium and

zinc in the dust will also be leached as CaSO4, MgSO4

and ZnSO4, respectively. CaSO4 can be eliminatedfrom the leaching solution by filtration operation.MgSO4 and ZnSO4 can react with the precipitatorNH4HCO3 in solution to form Mg(OH)2 and Zn(OH)2and further to form MgO and ZnO by thermaldecomposition during the iron oxide sorbentpreparation. The MgO and ZnO component in thesorbent is favorable to H2S removal reaction asreported in the literature.[13,14] So, in the present case,it is not necessary to eliminate MgSO4 and ZnSO4 inthe leaching solution.We also found that some residual solids will be

generated after the leaching process of the electric arcfurnace dust. The weight of the residual solids is about

Scheme 1. Schematic diagram of the desulfurization test.

Figure 1. Effect of sulfuric acid concentration on the ironleaching yield. Reaction temperature: 120 °C, no HCl.

Figure 2. Effect of reaction temperature on the iron leachingyield. Sulfuric acid concentration: 70wt%, no HCl.

Asia-Pacific Journal of Chemical Engineering UTILIZATION OF WASTE DUST AS IRON OXIDE SORBENT 739


5wt% of the total electric arc furnace dust. However,the main chemical composition of these residual solidsis CaSO4 and SiO2. Consequently, almost no toxicmetal ions will be leached from the residual solidswhen these waste solids are disposed in landfills. Thus,the possibility of the residual solid pollution on theenvironment will be greatly decreased. As for theremaining solutions after precipitation, they can bereused for another leaching process. Therefore, thepollution of this method can be neglected in thepractical applications.The filtrate solution from the iron leaching process

was mixed with the precipitator NH4HCO3 solutionand the clay and followed by stirring, aging andfiltrating. Then, the paste was extruded in a syringeand then dried at 120 °C for 12 h. The cylindricalsorbent was approximately 3–5mm in diameter and5–10mm in height with the color of yellow brown.Pore structures of electric arc furnace dust and thesorbent are listed in Table 2. The excess NH4HCO3 inthe paste will decompose into the gas (NH3, CO2 andH2O) during the sorbent drying process (Eqn (3)), thenplenty of small pores will generate in the sorbent; as aresult, the surface area will increase, and it will behelpful to increase the sorbent activity.

NH4HCO3 sð Þ→Δ NH3 gð ÞþCO2 gð Þ þ H2O gð Þ (3)

The sorbent characterization

The active ferric oxide crystals in the ambientdesulfurization are mainly α-Fe2O3 ·H2O, γ-Fe2O3 ·H2Oand γ-Fe2O3.

[15] The XRD spectroscopy was employedto characterize the transformation of the ferric oxidecrystals from the dust to the sorbent. By comparing thesignals in the XRD patterns of the two samples (Fig. 4),it can be found that iron species in the waste dust aremainly α-Fe2O3 (33.19°, 35.73°, 24.18°, 39.30° and62.59°) and Fe3O4 (30.07°, 43.06°, 56.94° and 62.53°).Those iron species exhibit lower efficiency for H2Sremoval.[15] Meanwhile, the corresponding ferric oxidespecies in the sorbent is the γ-Fe2O3 · H2O (14.25°,36.50°, 47.12°, 27.13° and 46.90°).[2] This kind of ferricoxide crystal is the active crystalline phase fordesulfurization.[16] It can be concluded that afterconversion, the iron species in the dust were transferredinto γ-Fe2O3 ·H2O with higher activity for H2S removal.Scanning electron microscope (SEM) photograph for

the electric arc furnace dust and iron oxide sorbent isshown in Fig. 5. It can be seen that electric arc furnacedust is a kind of compacted particles. While for thesorbent (Fig. 5b), some rodlike crystals appear in thephotograph. According to the XRD characterization,those rodlike crystals are mainly γ-Fe2O3 · H2O thatshows high reaction activity in desulfurizationprocess.[16] It also can be seen that in the sorbent, lotsof smaller pores exist than those of the dust and theexhausted sorbent. So, from the SEM experimental

Figure 3. Effect of the dosage of hydrochloric acid on theiron leaching yield. Sulfuric acid concentration: 70wt%,reaction temperature: 120 °C.

Table 2. Pore structures of electric arc furnace dustand the sorbent (fresh, spent and regenerated).

SampleSBET(m2/g)

Vtot(cm3/g)

D(nm)

Dust 29 0.023 1.28Iron oxide sorbent (fresh) 166 0.356 2.12Iron oxide sorbent (exhausted) 103 0.146 2.06

SBET, specific surface area from Brunauer–Emmett–Teller method;Vtot, total pore volume; D, average pore diameter.

Figure 4. XRDpatterns of (a) electric arc furnace dust dust and (b)iron oxide sorbent. ↓, ●, ▽ and ▼denote peaks of -Fe2O3 ·H2O,clay, -Fe2O3 and Fe3O4, respectively.



results, it also can be concluded that the sorbent showsthe potential for H2S removal.It is clearly seen that, with acid leaching and

preparation, the capacity of the dust-derived adsorbentsis significantly increased (Fig. 6). In fact, based on the

desulfurization test, we find that the breakthroughsulfur capacity of the sorbent and dust is 121 and26mg/g, respectively. The sulfur capacity of thesorbent is similar to that of the commercial AC that is138mg/g under the same test conditions. Figure 6 alsoillustrates the stability of all the three samples. As it canbe seen, the breakthrough time of the iron oxide is80min (nearly 90min for the AC) under the testreaction conditions, which suggested that the sorbentshows excellent stability for desulfurization reaction.Therefore, the sorbent prepared from the electric arcfurnace dust can be a low-cost material for removal ofH2S from waste gas because its raw material is anindustrial waste and available free of cost.As discussed by other researchers,[17,18] hydrogen

sulfide can be selectively oxidized to either sulfur orsulfuric acid on the adsorbent surface. By carefulcomparison of the DTG curves of the exhaustedsorbent and the AC, we find that the relatively more

Figure 5. SEM photographs of (a) electric arcfurnace dust, (b) fresh iron oxide sorbent and (c)exhausted iron oxide sorbent. A, rodlike crystal.

Figure 6. H2S breakthrough curves for the electric arc furnacedust (dust), iron oxide sorbent (sorbent) and activate carbon.

Figure 7. DTG curves in nitrogen for the fresh active carbon(AC), fresh iron oxide sorbent and their exhausted counterparts.

Asia-Pacific Journal of Chemical Engineering UTILIZATION OF WASTE DUST AS IRON OXIDE SORBENT 741


sulfur (the peak centered at about 400 °C), the lesssulfuric acid (the peak centered at about 200 °C) existson the surface of the iron oxide sorbent than that of theAC (Fig. 7). Teresa[19] reported that a more acidicenvironment promotes the formation of sulfur oxidesand sulfuric acid, while a basic environment favorsthe formation of elemental sulfur. In the present case,during the iron oxide sorbent preparation process,the NH4HCO3 solution was used as the precipitator;the precipitator provided more basic surface to thesorbent than the H3PO4-activated AC. Therefore, thedifferent selective oxidation product of hydrogensulfide is because of the different surface properties ofthe two sorbents.

CONCLUSIONS

The methods that we studied can efficiently leach ironfrom steel-making waste dust. The optimum conditionsare suggested as follows: reaction temperature of 120 °C,reaction time of 120min, 100-mL sulfuric acid solutionwith 70% wt concentration (11.4mol/L), 3-mLconcentrated hydrochloric acid and 10-g dry dustsample. Under such conditions, the iron leaching yieldreaches 96.20%. The experiment results show that theelectric arc furnace dust is a compacted particleconsisting of mainly α-Fe2O3 and Fe3O4. Meanwhile,the iron species in the H2S sorbents is γ-Fe2O3 · H2Oof higher activity for H2S removal. In addition,hydrogen sulfide is immobilized mainly in the form ofelemental sulfur on the surface of the iron oxide sorbentresulting from its basic surface environment. Thedesulfurization test revealed that the breakthroughsulfur capacity of the sorbent is similar to that of thecommercial AC at the same reaction conditions. Hence,this low-cost sorbent prepared by electric arc furnacedust can be used for the removal of H2S from waste gas.

Acknowledgements

Financial supports from the Science and TechnologyPlan of Wuhan (201060723317) and the Open ResearchFund of Key Laboratory for Ferrous Metallurgyand Resources Utilization of Ministry of Education(FMRU201209) are gratefully acknowledged.

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utilization of industrial waste electric arc furnace dust as iron oxide sorbent for hydrogen sulfide...

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