Removal of Copper Ions from Aqueous Solutions by a Steel-Making By-Product F.A

ARTICLE IN PRESS

Water Research 37 (2003) 3883–3890

Removal of copper ions from aqueous solutions by a steel-making by-product F.A. Lopez*,! M.I. Mart!ın, C. Perez,! A. Lopez-Delgado,! F.J. Alguacil

Department of Primary Metallurgy and Materials Recycling, National Centre for Metallurgical Research (CENIM), CSIC, Avda. Gregorio del Amo, 8, 28040 Madrid, Spain

Received 11 October 2002; received in revised form 10 April 2003; accepted 14 April 2003

Abstract

A study is made of the use of a steel-making by-product (rolling mill scale) as a material for removing Cu2+ ions from aqueous solutions. The inﬂuence of contact time, initial copper ion concentration and temperature on removal capability is considered. The removal of Cu2+ ions from an aqueous solution involves two processes: on the one hand, the adsorption of Cu2+ ions on the surface of mill scale due to the iron oxides present in the latter; and on the other hand, the cementation of Cu2+ onto metallic iron contained in the mill scale. Rolling mill scale is seen to be an effective material for the removal of copper ions from aqueous solutions. r 2003 Elsevier Ltd. All rights reserved.

Keywords: Copper; Steel-making by-product; Rolling mill scale; Cementation; Adsorption

1. Introduction membrane processing and electrolytic methods, etc. [3–10,24–26]. The effluents generated by modern industries (petro- Rolling mill scale is a steel-making by-product from steel leum refineries, non-ferrous metal works, motor vehicles, hot rolling processes and is basically composed of iron aircraft plating and finishing, etc.) generally have a oxides and metallic iron with variable oil and grease complex composition which includes metals (ions or contents [11,12]. Its specific production is about 35–40 kg/t complexes), suspended solids and other components [1,2]. of hot rolled product and Spain generates 0.044 Mt/year of According to the more stringent environmental laws, mill scale [11]. The oil component in rolling mill scale these effluents must be decontaminated because of their makes its recycling difficult, and its direct reuse in sintering hazardousness to humans, animals and plants. Metals are may lead to environmental pollution problems. Mill scale non-biodegradable and can accumulate in living tissues, with a high oil content is recycled after extracting the oil in thus becoming concentrated throughout the food chain. a pre-treatment stage or is dumped. Coarse scale with a With regard to the removal of inorganic pollutants particle size of 0.5–5 mm and an oil content of less than (e.g. metals), several techniques have been proposed for 1% can be returned to the sinter strand without any pre- their processing, such as precipitation, flotation, ion treatment. High oil contents (>3%) result in increased exchange, solvent extraction, adsorption onto different emissions of volatile organic compounds and potentially adsorbents—activated carbons, agricultural by-pro- also of dioxins and can lead to problems in waste gas ducts, natural zeolites, clays, manganese nodules, purification systems (e.g. glowfires in electrostatic pre- materials containing carbon, etc.—cementation on iron, cipitators). Because of this, mill scale needs to be pre- treated before it can be reused. Fine scale sludge consists mainly of very small scale particles (o0.1 mm). Because *Corresponding author. Tel.: +34-91-553-89-00; fax: +34- the fine particles adsorb oil to a very high degree (5–20%), 91-534-74-25. this scale cannot normally be returned to the sinter strand E-mail address: fl[email protected] (F.A. Lopez).! without pre-treatment [13].

The use of different industrial by-products as adsor- The surface composition was studied by means of bent materials in processes involving the removal of X-ray photoelectron spectroscopy (XPS) using a multi- metals from aqueous effluents has recently begun to be technique surface analysis unit, XPS-AES, with a VG developed, with the aim of seeking alternative ways of Microtech MT500 pumping station with double Mg/Al recycling certain by-products and at the same time anode, operating at 15 kV and 20 mA. The Mg Ka1,2 line finding cheaper replacements for expensive conventional (1253.6 eV) was used for all analyses. The working sorbent materials in different situations [14–18]. With pressure in the analysis chamber was o108 mbar. Peak specific regard to by-products from the steel-making positions were corrected for surface charging using the industry, blast furnace sludge has been studied as an C1s peakat 284.8 eV as a reference. Prior to analysis, adsorbent material for certain metallic ions in aqueous samples were cleaned in an Ar stream. solution [19,20,3], given that its iron oxide and carbon Morphological analysis of the mill scale samples contents confer it a high adsorption/absorption capa- before and after the adsorption tests was carried out city. Other steel-making by-products, such as blast by scanning electron microscopy (SEM), using a Jeol furnace slag [21] and electric furnace slag, have more JXA-840 unit equipped with an energy dispersive X-ray recently been studied as sorbing materials for the analyser (EDAX) with which the element composition removal of metallic ions from aqueous solutions [7]. of the studied samples was determined. An X-ray This workinvestigates the capacity of rolling mill fluorescence study by wavelength dispersion was also scale to remove Cu2+ ions from aqueous solutions. carried out, mapping the Fe and Cu contents in order to see their distribution in the mill scale. The samples were prepared by embedding the powder in a polymer resin and then polishing its surface and metallising it with graphite. 2. Materials and methods For the adsorption experiments the mill scale was dried at 80C for 24 h and then passed through a screen, using The mill scale used in this workwas supplied from a only the o0.5mmfractioninthetests.Cu2+ solutions hot rolling mill in an electric steel shop in northern were prepared by dissolving Cu(NO3)2 2.5H2Oin0.01M Spain. The mill scale was dried at 80 C for 24 h, NaNO3, in order to maintain a constant ionic strength of revealing an initial moisture content of 5%. the dissolution. 100 mL of the metal solution was added to For the purpose of analysis, the mill scale was crushed a fixed amount of mill scale (10 g). The initial pH was to obtain a grain size of o40 mm. The chemical pHi=5.0070.01. To determine the rate constant, tests composition of the ground sample was determined by were carried out at several temperatures (between 20C X-ray fluorescence analysis by wavelength dispersion and 80C) for different reaction times (0–6 h) with an (WDXRF) using a Philips PW-1404 spectrometer. aqueous solution of 50 mg/L Cu (II). Removal tests were Carbon and sulphur analyses were carried out by performed at different temperatures (between 20Cand combustion in a Leco CS-244 oven and infrared 80C) for an equilibrium time of 5 h, using aqueous detection. solutions whose concentration varied between 5 and The analysis of iron in its different states of oxidation 8000 mg/L Cu (II). The equilibration time was established (Fe3+,Fe2+ and Fe0) was performed by titrating with a prior to conducting equilibration experiments. The equili- 2+ 0.1 N K2Cr2O7 solution. For the analysis of Fe and brium pH was pHeq=(6.00–6.50)70.01. The samples were Fetotal the mill scale was dissolved in HCl with a few stirred constantly by means of a Hucoa-Erloss Lauda MS- drops of HF. Barium diphenylamine-sulphonate was 20 B thermostatically controlled. The pH was controlled used as indicator. For the determination of metallic iron using a Crison 517 pH-meter. The resulting suspensions (Fe0) the sample was previously dissolved in a bromo- were filtered and the solutions analysed by atomic methanol solution, separating the dissolved metallic iron absorption spectrophotometry (AAS) in a Perkin-Elmer (soluble in bromo-methanol) by filtration. Filtering was 1100B spectrophotometer. The amount of copper ad- carried out with a filtering crucible of 20 mm, with the sorbed on the mill scale was determined by the difference solubilised Fe0 passing into the filtrate and the Fe2+ and between the initial concentration and the equilibrium Fe3+ (insoluble ions in bromo-methanol) being retained concentration (or the concentration at each time). in the filter [22]. The crystalline mineralogical composition was determined by X-ray diffraction (XRD) using a Siemens 3. Results and discussion D-5000 diffractometer (Cu Ka radiation). The N2 adsorption isotherm was determined at 77 K 3.1. Characterisation of rolling mill scale for a mill scale sample previously degasified at 60C and 105 Torr for 120 min, using a Coulter SA-3100 unit. Rolling mill scale is a material of a laminar morphol- 2 The isotherm data was used to determine the value of ogy and low specific surface area (SBET ¼ 0:43 m /g). the BET specific area. It is comprised mainly by a mixture of iron oxides: ARTICLE IN PRESS F.A. Lopez! et al. / Water Research 37 (2003) 3883–3890 3885

100

90 80 °C ° 80 60 C 40 °C 70 20 °C

Copper adsorption (%) 30

0 0123456 Reaction time (h)

Fig. 1. Variation in copper adsorption percentage on mill scale with time at several temperatures. C0 ¼ 50 mg/L Cu (II), mill scale concentration 100 g/L.

Table 1 Rate constants for different temperatures and copper concentration on the mill scale and in the equilibrium solution (t ¼ 5h)

2 2+ T ( C) k (cm/s) R Cu(ads) (mg/g) [Cu ]aq (mg/L) 20 1.6 Â 108 0.971 0.09 39.9 40 6.8 Â 108 0.975 0.24 2.2 60 4.2 Â 107 0.971 0.49 1.2 80 8.0 Â 107 0.973 0.50 0.1

[Cu](aq) (mg/L) 0.00 40.00 80.00 120.00 160.00 200.00 50.00 1.40

1.20 40.00

1.00 [Cu](ads) (mg/g)

30.00 0.80

0.60 20.00 80 °C 60 °C [Cu](ads) (mg/g) 40 °C 0.40 20 °C 10.00 0.20

0.00 0.00 0.00 1000.00 2000.00 3000.00 4000.00 [Cu](aq) (mg/L) Fig. 2. Mass of Cu2+ adsorbed per unit of mass of mill scale as a function of the copper equilibrium concentration in solution. Equilibrium time 5 h, mill scale concentration 100 g/L (the points of 20C and 40C correspond to the scale of the right and top axis). ARTICLE IN PRESS 3886 F.A. Lopez! et al. / Water Research 37 (2003) 3883–3890

Table 2 wustite (FeO); hematites (a-Fe2O3); and magnetite Amount of metallic ion (Cu2+) removed (adsorbed) per unit of (Fe3O4), and metallic iron. The iron composition of the mass of mill scale as a function of temperature mill scale was found to be 51.4% Fe2+,10.2%Fe3+ and 0 7.2% Fe ( 0.5 mm fraction). It also contains small T ( C) Cu(ads) (mg/g) o amounts of other metals: Cu (0.54%), Mn (0.47%), Zn 20 0.13 (0.01%) and Pb (o0.005%), as well as 0.19% of C and 40 0.72 0.026% of S. Approximately 2% of mill scale consists of 60 40.50 oils and greases originating from the lubrication of the 80 41.09 rolling machines.

[Cu](aq) (mg/L) 0.00 20.00 40.00 60.00 80.00 100.00 3500.00 12.00

80 °C 3000.00 60 °C 10.00 40 °C 20 °C 2500.00

8.00 [Fe](aq) (mg/L)

2000.00

6.00

1500.00

[Fe](aq) (mg/L) 4.00 1000.00

2.00 500.00

0.00 0.00 0.00 1000.00 2000.00 3000.00 4000.00 (a) [Cu](aq) (mg/L)

8.00

Cu Fe

6.00

4.00 % Weight

2.00

0.00 0.00 1000.00 2000.00 3000.00 4000.00 (b) [Cu](aq) (mg/L) Fig. 3. (a) Iron concentration in equilibrium solutions as a function of the copper equilibrium concentration in solution. Equilibrium time 5 h, mill scale concentration 100 g/L (the points of 20C and 40C correspond to the scale of the right and top axis). (b) Percentages of metallic iron and copper contained in the mill scale at equilibrium as a function of the copper equilibrium concentration in solution. Equilibrium time 5 h, temperature 60C, mill scale concentration 100 g/L. ARTICLE IN PRESS F.A. Lopez! et al. / Water Research 37 (2003) 3883–3890 3887

3.2. Kinetics of copper adsorption onto mill scale mill scale decreases as the amount of adsorbed copper increases (Fig. 3b). These results indicate that, during Fig. 1 displays the percentage of copper adsorbed (or the removal process of copper from aqueous solutions, removed) by the mill scale after different reaction times an oxidation–reduction (or cementation) process occurs at several temperatures. It is observed that copper between the Cu2+ ions in solution and the metallic iron removal increases with increasing the temperature. contained in the mill scale, in such a way that as the Many of the processes of metal removal from aqueous Cu2+ becomes fixed, the Fe0 in the mill scale oxidises effluents that use solid–liquid systems obey a first-order and passes into solution according the reaction: kinetic law. The removal of copper can be evaluated by 0 2þ 0 Fe "Fe þ 2e ðE 2þ 0 ¼0:44 VÞ: ð2Þ the following expression [23]: Fe =Fe

0 lnðCt=C0Þ¼kðA=VÞt; ð1Þ This would explain the decrease in the Fe content of the mill scale and the increase in the iron concentration where Ct is the copper concentration at time t; C0 is the in solution. This supposes that as the Cu2+ becomes initial copper concentration (at t ¼ 0), k is a rate 0 constant, A is the surface area of mill scale, V is the ﬁxed, it becomes reduced to Cu according to the volume of test solution placed in contact with the mill equation: scale, and t is the reaction time. 2þ 0 0 Cu þ 2e "Cu ðE 2þ 0 ¼ 0:34 VÞ: ð3Þ Table 1 shows the rate constant ðkÞ values obtained Cu =Cu 2 from the plot slopes ðR > 0:971Þ corresponding to The cementation of Cu2+ onto the metallic iron Eq. (1), and the copper concentration on the mill scale contained in the mill scale may be expressed by the and in the equilibrium solution (t ¼ 5 h). It can be seen following reaction: that the value of the rate constants and the amount of 0 2þ 2þ 0 Cu2+ adsorbed per unit of mass of mill scale increase Fe þ Cu -Fe þ Cu : ð4Þ with increasing temperature. The equilibrium concen- In the removal process of copper from aqueous tration of copper ions decreases with increasing solutions, a process of adsorption on the surface of the temperature. mill scale also takes place. The Cu2+ ions contained in the aqueous solution are adsorbed on the mill scale 3.3. Removal tests surface due to the iron oxides present in this by-product. The adsorption process is predominant at 20C Fig. 2 shows the relationship between the amount of and 40C with low initial copper concentrations Cu2+ removed per unit of mass of mill scale and the (50–100 mg/L Cu (II)) where [Cu] /[Fe] >1, equilibrium concentration of copper ions in solution for removed solution and the cementation process predominates at 60C and different temperatures. The removal capacity of mill 80C with high initial copper concentrations (1000– scale is observed to increase with temperature, and is 8000 mg/L Cu (II)), where [Cu] /[Fe] E1. especially favoured at temperatures above 60C, main- removed solution According to redox potentials, it may be expected that taining itself stable from this temperature on. The mill scale could also be used in oxidation–reduction amount of copper removed per unit of mass of mill processes to remove other metallic ions from aqueous scale is greater for the higher copper concentrations. At solutions (Table 3) and even certain inorganic anions. 20C the amount of Cu2+ adsorbed per unit of mass of mill scale remains practically constant from an equilibrium concentration of 40 mg/L Cu (II). The results obtained indicate that mill scale removes Table 3 Standard electrode potentials up to 40 mg Cu (II)/g mill scale at TX60C(Table 2). This value is considerably higher than other published Semireaction E0 (V) values for copper adsorption on other steel-making by- + MnO4 +4H +2e "MnO2k+2H2O 2.26 products; such as blast furnace sludge, which adsorbs a 2 + " 3+ Cr2O7 +14H +6e 2Cr +7H2O 1.33 maximum of 22.58 mg/g at 60 C [3]. Pd2++2e"Pd0 0.99 + 2+ MnO4 +8H +5e "Mn +4H2O 0.95 2+ 2+ 3.3.1. Study of iron contained in solution and on 2Hg +2e "Hg2 0.92 the mill scale NO3 +2e "NO2 0.84 2 + 0 The amount of copper removed depends on the SO4 +8H +6e "S +4H2O 0.36 temperature, increasing with increasing temperature. Pb2++2e"Pb0 0.13 2 " Analysis of the Fe content in the equilibrium solutions CrO4 +2H2O+3e CrO2 +4OH 0.16 2+ " 0 indicates that the amount of iron in solution increases Co +2e Co 0.28 Cd2++2e"Cd0 0.40 with the temperature (Fig. 3a). At the same time, it has Cu2++2e"Cu0 0.34 been determined that the amount of Fe0 existing in the ARTICLE IN PRESS 3888 F.A. Lopez! et al. / Water Research 37 (2003) 3883–3890

Bearing in mind that the mill scale used in this work 3.4. XPS and SEM analysis of reacted mill-scales has an Fe0 content of 7.2%, the amount of copper ions removed is the equivalent of about 0.6 mg Cu (II)/mg Fe0. Fig. 4 shows the distribution of copper and iron on This amount may be much greater if mill scales with a the mill scale surface after the removal process. In Fig. higher metallic iron content are used, i.e. less oxidised mill 4b the metallic copper distribution on the mill scale scales obtained immediately after the rolling process. surface is observed, appearing mainly as a central cloud

(a)

12000 (b) 11000

CuKβ 10000

9000 CuKα Counts

1000

500 OKα FeKα CuKα

0 26104 8 Energy (keV)

5000 (c)

4000 OKα FeKα

3000

Counts FeKρ 2000

1000 FeKα

0 26104 8 Energy (keV)

Fig. 4. Mapping of Cu and Fe in the mill scale treated with a solution of Cu (II) nitrate (C0=850 mg/L) at 60 C for 5 h and EDAX spectra: (a) image of secondary electrons, (b) distribution map of X-rays of the Cu Ka line, (c) distribution map of X-rays of the Fe Ka line. ARTICLE IN PRESS F.A. Lopez! et al. / Water Research 37 (2003) 3883–3890 3889 of points. Fig. 4c shows that the metallic copper particle experimental data obtained. Fig. 6 shows the high- is surrounded by iron oxides which constitute the sample resolution XPS spectrum of the Fe2p3/2 peak. The peak matrix. SEM studies carried out for different mill scale is seen to split into two components with binding samples treated with Cu2+ solutions of different energies of 711.6 and 709.5 eV, corresponding to Fe3+ concentrations gave identical results. and Fe2+ (in the form of oxides) respectively. The peak Fig. 5 shows the high-resolution XPS spectrum of the corresponding to Fe0 (706.7 eV) was not observed. 2+ 0 Cu2p3/2 peakfor a mill scale sample treated with a Cu The presence of Cu on the mill scale surface has been solution. It is seen that the peakis split into two conﬁrmed by XPS, and thus it can be considered that, in components with binding energies of 933.6 and 932.6 eV, the removal process of Cu2+ ions by means of mill scale, which correspond to the binding energies of Cu2+ (in part of the Cu2+ is adsorbed on the mill scale surface the form of oxide) and Cu0 respectively. The existence of and another part is reduced to Cu0, which is deposited copper in these states of oxidation on the mill scale on the mill scale. The metallic iron is oxidised to Fe2+ surface indicates that the copper removal process and these ions are transferred into the solution. includes a cementation process between Cu2+ and the Fe0 present in the mill scale, as has been indicated above. At the same time the oxidation of Fe0 occurs, with Fe2+ 4. Conclusions ions passing into the solution, as has been veriﬁed by the The results obtained indicate that rolling mill scale is 80000 an effective material for the removal of copper ions from Cu0 aqueous solutions, showing a removal capacity of up to Cu2+ 40 mg Cu (II)/g mill scale ðTX60 CÞ: Bearing in mind that the mill scale used in this workhas an Fe 0 content 74000 of 7.2%, the amount of copper ions removed is the equivalent of about 0.6 mg Cu (II)/mg Fe0 ðTX60CÞ:

Counts The removal process of copper ions depends on the 68000 temperature, with the amount of removed Cu2+ increasing with increasing temperature. The XPS and EDAX spectra and chemical analyses 2+ 62000 carried out suggest that during the removal of Cu 937 935 933 931 929 ions, one part of the Cu2+ is adsorbed on the mill scale Binding energy (eV) surface due to the iron oxides present in this residue, and 0 Fig. 5. High-resolution XPS spectrum of the Cu2p3/2 peakof another part is reduced to Cu (by cementation), being the mill scale treated with a solution of Cu (II) nitrate deposited on the mill scale. The metallic iron is oxidised 2+ (C0=850 mg/L) at 60 C for 5 h and tAr ¼ 10 min. to Fe and these ions are transferred into the solution.

50000 Fe3+ Fe2+

45000 Counts 40000

35000 716 714 712 710 708 706 Binding energy (eV)

Fig. 6. High-resolution XPS spectrum of the Fe2p3/2 peakof the mill scale treated with a solution of Cu (II) nitrate (C 0=850 mg/L) at 60 C for 5 h and tAr ¼ 10 min. ARTICLE IN PRESS 3890 F.A. Lopez! et al. / Water Research 37 (2003) 3883–3890

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