Journal of Environmental Management 156 (2015) 218e224
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CO2 sequestration using accelerated gas-solid carbonation of pre-treated EAF steel-making bag house dust Muftah H. El-Naas a, *, Maisa El Gamal a, Suhaib Hameedi a, Abdel-Mohsen O. Mohamed b a b
Chemical and Petroleum Engineering Department, UAE University, P.O. Box 15551, Al-Ain, United Arab Emirates Zayed University, P.O. Box 144534, Abu Dhabi, United Arab Emirates
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2014 Received in revised form 24 March 2015 Accepted 25 March 2015 Available online 4 April 2015
Mineral CO2 sequestration is a promising process for the reduction of carbon dioxide emissions to the atmosphere. In this paper, alkaline calcium-rich dust particles collected from bag filters of electric arc furnaces (EAF) for steel making were utilized as a viable raw material for mineral CO2 sequestration. The dust particles were pre-treated through hydration, drying and screening. The pre-treated particles were then subjected to direct gasesolid carbonation reaction in a fluidized-bed reactor. The carbonated products were characterized to determine the overall sequestration capacity and the mineralogical structures. Leaching tests were also performed to measure the extracted minerals from the carbonated dust and evaluate the carbonation process on dust stabilization. The experimental results indicated that CO2 could be sequestered using the pre-treated bag house dust. The maximum sequestration of CO2 was 0.657 kg/kg of dust, based on the total calcium content. The highest degree of carbonation achieved was 42.5% and the carbonation efficiency was 69% at room temperature. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon sequestration Gasesolid Electric arc furnace Bag house dust Leaching Fluidized-bed reactor
1. Introduction Carbonation of alkali metal oxides simulates natural chemical transformations of carbon dioxide. In weathering procedures, eroded rock surfaces come into contact with rainwater saturated with dissolved atmospheric CO2. As a result, alkali and alkaline earth elements dissolve into the water and precipitate as carbonate minerals. These weathering processes, reduce high CO2 concentrations present in the earth atmosphere; about 80% of all carbon in the world are present in the form of carbonate rocks (Seifritz, 1990; Dunsmore, 1992). This carbonation process could be improved and industrially applied to fix gaseous CO2 into a solid carbonate and thus prevent the emission of CO2 into the atmosphere. The process offers a safe and permanent method of CO2 disposal, since there is no possibility of any release of CO2 from the disposal site for long times. The carbonation process may be proceeding through direct or indirect methods. In the direct method, minerals and other waste materials go through the direct carbonation process. One of the main advantages of gasesolid mineralization is the possibility of
* Corresponding author. E-mail address: [email protected] (M.H. El-Naas). http://dx.doi.org/10.1016/j.jenvman.2015.03.040 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
recovering the heat released by the exothermic carbonation reaction in a beneficial form (Zevenhoven et al., 2013). The direct carbonation of calcium-silicates in an aqueous suspension at elevated temperature and CO2 pressure is considered to be the most promising process route. In the indirect method, mineral components are separated from the waste material, then reacted with CO2 (Huijgen and Comans, 2003). Other routes include pretreatment of the solid feedstock like size reduction and thermal activation (Huijgen, 2007). Steel making by-products are waste materials, produced during iron and crude steel production. These by-products include slag, dust, sludge, and mill scale. A large amount of bag house dust (BHD) is usually produced as a byproduct of the steel-making process. BHD contains water leachable heavy metals such as lead, chromium and cadmium. It was categorized as hazardous waste K061 under EPA regulations in 1980 (Sofilic et al., 2004). It is also produced in large quantities, due to the dramatic population increase and the growth of industrialization. The management of such wastes, therefore, is a major concern of steelmakers due to the environmental impact. The high content of CaO in such by-products is the main reason of its utilization in CO2 sequestration (El-Naas et al., 2013; Johnson, 2000; Huijgen et al., 2004; Stolaroff et al., 2005; Kodama et al., 2006). The use of these byproducts in mineral carbonation is
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resulting in a reduction of pH and leached metals, leading to solid stabilization (Baciocchi et al., 2010). The degree of carbonation depends on the temperature, pressure and surface area of the waste materials. In this work, BHD residue, recovered from EAF steel-making, was evaluated as a possible raw material for mineral CO2 sequestration. The direct gasesolid carbonation reaction was applied to avoid the leaching of heavy metals in water. Furthermore, the acid extraction process is not recommended, since the pH of the dust is greater than 12 and will consume excessive amount of acid for extraction. The chemical compositions of dust vary from one plant to another, depending on the type and quality of scrap used, charging method, oxygen-blowing rates, furnace-operating practices, and the type of steel produced (Al-Sugair et al., 1996; Idris and Siong, 1997). The main objective of this work is to evaluate the performance of the BHD for CO2 sequestration in a specially designed fluidized bed reactor. 2. Experimental methods 2.1. Materials Bag house dust (BHD) samples were collected from Emirates Steel in Abu Dhabi, The United Arab Emirates (UAE). The samples were collected from the out storage yard. A mechanical sieve shaker was used for sieving analysis. Carbon dioxide CO2 was provided by Sharjah Oxygen Company, UAE with 99.99% chemical purity. 2.2. Pre-treatment process The reactions of alkaline earth metal oxides and silicates in BHD with water were modeled using chemical reaction and equilibrium software HSC 6.1. Hydration process was performed at different water temperatures (25, 40, 60 C and steam water). A specific amount of 1 kg BHD was mixed with water using mechanical stirrer at 400 rpm for 2 h to form the hydrated products. Subsequently, the hydrated dust was air dried at room temperature. Screening of the dust using tray shaker is finally performed to separate the agglomerated particles and consequently increase surface area. 2.3. Carbonation process Carbonation experiments were carried out in a fluidized bed reactor, which was made of Acrylic glass with 123 mm in diameter and 500 mm in length. The reactor had a gas feed tube, a flue gas outlet and a perforated distributor plate. CO2 gas was forced upward through the distributor to the solid particles with an input pressure of 3 bars and output pressure of 1 bar. The bed was operated slightly above minimum fluidization velocity at a flow rate of 12 L/min, using different moisture percentage ranging from 1 to 7 mass%.
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dissolved solids (TDS) and electrical conductivity. TGA analysis and mineralogical compositions were performed using TA Q500 thermal gravimetric analyzer and Philips PW/1840 x-ray diffractometer (XRD), respectively. Microstructure characterization was carried out using JOEL JSM-5600 scanning electron microscope. VISTAMPX CCD inductively coupled plasma-atomic emission spectrometry (ICP-AES) was utilized for alkali and heavy metals analysis, while DIONX IC-ion-chromatography (IC) was used for anions determination. 2.6. Degree of carbonation and sequestration efficiency A CO2 uptake, based on the weight fraction of the TGA curve (Dm550e850 C) and dry weight (m105 C), was used as the carbon content, expressed in terms of CO2 (wt %), Eq. (1):
CO2 wt ð%Þ ¼
Dm550850 C 100 m105 C
(1)
The theoretical total carbon content based on basic metal oxides present in the fresh samples was calculated using Eq. (2) (Steinour, 1959):
%CO2 ¼ 0:785ð%CaO 0:56% CaCO3 0:7% SO3 Þ þ 1:091% MgO þ 0:71% Na2 O þ 0:468% K2 O
(2)
The CO2 sequestration efficiency was determined using the maximum CO2 sequestration capacity for TGA data and the theoretical maximum CO2 sequestration capacity, Eq. (3).
CO2 sequestration efficiency ð%Þ ¼
Maximum CO2 sequestration capacity 100 Theortical CO2 sequestration capacity
(3)
3. Results and discussion 3.1. Characterization of the BHD The chemical composition of the examined BHD specimen is shown in Table 1. The predominant material is iron oxide and the second major component is calcium oxide. It is a highly dense material (approximate density is 2.89 g/cm3), which is compatible with the chemical and mineralogical content. The dust has been classified as a hazardous waste due to the relatively high lead, cadmium and hexavalent chromium contents. The order of the major elements is Fe, Ca, Mg, Si, Na, K, S, Mn, Al, Zn, Pb, Sr, Cr, Cu and Cd. This consequence was in a good agreement with data reported in Twilley (2002). The sieve analysis of the BHD indicated that the dust particle ranged in size from less than 38 mm to more than 250 mm, with more than 80% of the particles are less than
2.4. Leaching test Metal ions leaching test was carried out in accordance to British Standard BS (EN12457 2002), which is designed to examine the short-term and the long-term leaching behavior of the uncarbonated and the carbonated materials. The concentration of the major and minor cations were measured using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), whereas anions were measured using Ion Chromatography (IC). 2.5. Laboratory analyses Accumet 925 pH/ion meter was used for measuring pH, total
Table 1 Chemical composition of BHD uses an Inductively Coupled Plasma. Composition
Concentration (weight percentage)
Composition
Concentration (part per million)
FeO (total) CaO MgO Na2O K2O MnO SiO2 Al2O3 SO3
42.8 40.2 4.96 2.52 1.98 0.68 4.49 0.28 1.108
Zn Pb Sr Cr Cu Cd
1973.2 399.4 268.4 148.1 53.6 30.1
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Table 2 Sieve analysis of BHD. U.S. sieve no.
Sieve size (mm)
Percent passing
40 60 70 100 140 200 400
425 250 212 150 106 75 38
100 97.68 96.22 86.79 60.65 32.15 3.08
150 mm (Table 2). The morphology of BHD was studied by SEM-EDS analysis; the majority of BHD particles were spheres with the presence of some angular fragments. The fine spheres can be observed as discrete or agglomerated particles (Fig. 1). XRD analysis revealed that the common minerals of BHD were magnetite (Fe2O3), Franklinite (ZnO.Fe2O3), Larnite (Ca2SiO4) with other silicate phases, metal oxides such as CaO, MgO, Na2O, K2O, MnO, calcite (CaCO3) and dolomite CaMg(CO3)2. A considerable amount of salts like arcanite (K2SO4), sylvite (KCl), gypsum (CaSO4$H2O) and syngenite (K2Ca(SO4)2$H2O) could also be detected (Fig. 2). The chemical complexity of the dust is enhanced by the amphoteric nature of iron, the reducing or oxidizing conditions in the furnace and the presence of large quantities of metal oxides, such as calcium oxide and magnesium oxide.
Fig. 2. XRD analyses of fresh BHD. Where, C: Calcite, V: Vaterite A: Aragonite D: Dolomite, G: Gypsum P: Portlandite, Sy: Sylvite, F: Franklinite, H: Hematite and L: Larnite.
by the HSC chemistry software, Eq. (6). The thermodynamic data for the free energy of formation of CeSeH obtained by HSC are comparable with previously reported results (Fujii and Kondo, 1983).
3Ca2 SiO4 ðsÞ þ 3:5 H2 OðlÞ /4CaO$3SiO2 $1:5H2 OðsÞ þ 2CaðOHÞ2 ðsÞ DG ¼ 38:85 kJmol1 (6)
3.2. Pre-treatment process The composition of BHD is basically metal oxides and silicates; calcium and magnesium oxides react with water and form the corresponding hydroxides. These reactions are highly spontaneous and exothermic (Eqs. (4) and (5)).
CaOðsÞ þ H2 OðlÞ/CaðOHÞ2 ðsÞ
DG ¼ 57:9 kJmol1 (4)
MgOðsÞ þ H2 OðlÞ/MgðOHÞ2 ðsÞ
DG ¼ 27:3 kJmol1 (5)
Actually, the hydration process of BHD is so complicated due to the presence of silicates and other complex oxides associated with calcium oxide. Since the secondary mineral phases are mainly silicate minerals, the preparation of calcium silicate hydrate (CeSeH) from dicalcium silicate (C2S) may vary in composition as modeled
In the presence of water, silicates and aluminates are hydrated into stiff and hard products with very low solubility in water. These products are mainly tri-calcium silicate hydrate (C3S2H3), tricalcium aluminate hydrate (C3AH6) and calcium alumino-ferrite hydrate (C6AFH12) (Mohamed and El Gamal, 2014; Cizer, 2009; Massazza, 1998; Taylor, 1990). Laboratory scale experiments indicated that the hydration process was very slow at room temperature. The formation of the calcium hydroxide and CeSeH crystals provide a nucleus for more calcium silicate hydrate. It is likely that impermeable coatings of these reaction products formed on the surface of the grains prevent further reactions. The CeSeH crystals grow thicker, making it so difficult for water molecules to reach the non-hydrated calcium silicate. The speed of the reaction is then controlled by the rate at which water molecules diffuse through the CeSeH coating. Hydration reaction was found to be accelerated by increasing the hydration temperature. Based on the degree of carbonation, more than 10% increase in carbonate content was achieved upon applying steam water, over that obtained by hydration at room temperature. Mechanical mixing and steam can increase the diffusivity of water and consequently, increase the hydration reactions. 3.3. Carbonation process In this work, direct gasesolid carbonation of the pre-treated BHD was applied to achieve the chemical reaction. Since the mineral carbonation depends on the reaction of CO2 with metal hydroxides and metal silicate hydrates, the amount of heat produced by these reactions depends on the amounts of bivalent metal contained like Ca and Mg. Thermodynamic analysis was performed using HSC to study the effects of different variables on the carbonation process. The analysis data indicated that the carbonation reactions of calcium hydroxide and calcium silicate hydrate were spontaneous and exothermic, Eqs. (7) and (8).
CaðOHÞ2ðsÞ þ CO2ðgÞ /CaCO3ðsÞ þ H2 OðlÞ Fig. 1. SEM image of fresh BHD.
DH ¼ 113 kJmol1 (7)
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4CaO$3SiO2 $1:5H2 OðsÞ þ 4CO2ðgÞ /4CaCO3ðsÞ þ 3SiO2ðsÞ þ 1:5H2 OðlÞ
DH ¼ 393 kJmol1 (8)
It was found that the moisture content affected both carbonation and fluidization processes; carbonation does not completely occur in totally dried samples and the penetrability of CO2 gas increases within the hydrated dust, but too much water limits the reaction due to the blockage of the pores. The effect of moisture content on the carbonation of BHD was evaluated through the measurements of the pH, total dissolved solids (TDS) and electrical conductivity of the carbonated samples. It was found that moisture content ranging from 3 to 4% enhanced the gasesolid carbonation reaction of BHD. The reported mechanism of this fact suggested that the water adsorbed on hydrophilic and basic surface sites allowed carbonation formation at the gasesolid interface followed by the formation of solid carbonate around reacting particles (Beruto and Botter, 2000). The major hydrated chemical compounds that react with carbon dioxide are calcium hydroxide, calcium silicate hydrate, calcium aluminate hydrate, Ettringite and calcium aluminoferrite hydrate. The calcium silicate grains are covered with a loose layer of CeSeH gel, which is quickly dissolved, releasing Ca2þ and SiO44 ions. The gel is progressively decalcified leading to the formation of CaCO3 intermixed with silicate hydrate gel. The reaction of calcium aluminate hydrates is minimal, due to the tight aluminum-sulfate/ carbonate coating, nucleation of CaCO3 and CeSeH, which is promoted by increasing the temperature and solid phase precipitates. It is worth noting that the above reactions have three aspects in 2þ common: (I) they all involve the interaction of ions CO2 3 and Ca to form highly insoluble calcium carbonate, (II) they are all exothermic, and (III) they may happen simultaneously but at different rates, as previously reported (Young et al., 1974; Moorehead, 1986). 3.4. Performance of carbonation 3.4.1. Thermal Gravimetric Analysis (TGA) TGA results indicated that CO2 sequestration is favored in the pre-treated BHD samples as shown in Fig. 3. The different mass loss regions are 25e105 C for the moisture content, 130e200 C for the dehydration of the CeSeH, 205e500 C for organic carbon compounds, and 550e850 C for inorganic carbon compounds which occurred due to the thermal decomposition of calcium carbonate (Chang and Chen, 2006). At 750 C, dolomite structure is changed to calcite. Calcium hydroxide begins to decompose at 325 C and has a
Fig. 3. TGA of BHD as fresh, untreated carbonated and pre-treated carbonated samples, at a heating rate of 20 C/min.
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quoted dehydration temperature of 518 C (Mu and Perlmutter, 1981). 3.4.2. Carbon uptake Fig. 4 represents the average CO2 uptake in the overall sample under each condition, at the same fluidization conditions (input pressure of 3 bar, output pressure of 1 bar, and flow rate of 12 L/ min). It appears that the CO2 uptake in the untreated carbonated sample increased slightly with time, whereas a higher increase in the CO2 uptake was observed in the pre-treated samples. Further increase in time did not increase the CO2 uptake substantially. Generally, pre-treated samples showed higher carbon contents ndez Bertos et al., compared with those of untreated ones (Ferna 2004). Thermodynamically, the reaction of calcium oxide with CO2 is more favorable than that of calcium hydroxide (where DG are 130.4 kJ and 72.6 kJ, respectively). The reaction of hydrated metal oxide with CO2 produces initially a layer of a metal carbonate, which limits the diffusion of CO2 through the hydroxide particles, Eq. (9).
CaðOHÞ2ðsÞ þ nH2 OðlÞ þ CO2ðgÞ /CaCO3ðsÞ þ ðn 1ÞH2 OðlÞ (9) However, under the applied pressure, the formation of relatively soluble calcium bicarbonate facilitates CO2 diffusion to the bulk of the calcium hydroxide particles, Eqs. (10) and (11).
CaCO3ðsÞ þ CO2ðgÞ þ H2 OðlÞ/CaðHCO3 Þ2ðaqÞ
(10)
CaðOHÞ2ðsÞ þ CaðHCO3 Þ2ðaqÞ /2CaCO3ðsÞ þ 2H2 OðlÞ
(11)
A necessary condition for the carbonation reaction is the presence of nano-drops of water on the surface of Ca(OH)2 crystals, which leads to the formation of Ca(HCO3)2 and CaCO3, upon interaction with CO2. The water resulting from the reaction between Ca(OH)2 and CO2 accelerates the reaction, and favors crystallization of the formed carbonate under powerful fluidizing conditions as well. It is also reported that the increased rate of Ca(OH)2 carbonation was due to its high specific surface compared to CaO (Kalinkin et al., 2005). 3.4.3. CO2 sequestration effectiveness The theoretical total carbon content of BHD was calculated as a function of chemical composition. All alkali metal species like CaO, MgO, Na2O and K2O are converted to the corresponding carbonate. The maximum CO2 uptake by BHD was found to be
Fig. 4. CO2 uptakes for the untreated and pre-treated carbonated BHD.
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18.8 wt%. The high carbon content of 12.5 wt% in the fresh dust is attributed to the weathering interaction in the open waste stores. The physical properties of fresh and carbonated BHD are shown in (Table 3). The pH, electrical conductivity and TDS were reduced after carbonation due to the conversion of alkaline metal oxides and silicates into insoluble carbonates. Carbon uptake, degree of carbonation and carbonation efficiency of the pre-treated BHD are clearly increased compared with the untreated one. The highest degree of carbonation achieved was 42.5% and the carbonation efficiency was 69% at room temperature. The maximum sequestration of CO2 in the pre-treated carbonated dust was calculated as 0.657 kg/kg of dust, based on the total calcium content. This result is notably high compared with that reported by Gupta and Nema (2012), where bag house dust residue, containing 48 wt % CaO, was able to sequester about 0.28 kg of CO2 per kg of dust, with maximum degree of carbonation of 7.56% using gasliquid-carbonation method at different water-to-solid ratios. They found that a lower water-to-solids ratio produced a higher degree of carbonation. They attributed their findings to the hindrance caused by water to the diffusion of CO2 and the reduction of gaspermeability of the dust. 3.4.4. Mineralogical composition and microstructure Diffraction peaks presented on (Fig. 5) showed that the absolute intensities of calcite peaks are increased, whereas, Portlandite, Larnite, Sylvite and Gypsum peaks was disappeared after carbonation due to the consumption of calcium ion in the formation of calcium carbonate. SEM observations highlighted that the carbonation products of BHD are calcium carbonates with different morphologies, welldefined rhombohedra calcite and vaterite particles (Fig. 6a). Additionally, wheat-sheaf-like bundles aragonite particles was found in the carbonated dust (Fig. 6b). It was reported that phase of vaterite can occur during the formation of calcite and aragonite (Carlson, 1983). It was also reported that aragonite was formed due to the presence of Mg2þ ions in the dust composition (Tai and Chen, 1998; Sawada, 1997; Fernandez-Diaz et al., 1996; Nishino et al., 2009).
Fig. 5. XRD analyses of the carbonated pre-treated BHD. Where, C: Calcite V: Vaterite, A: Aragonite, D: Dolomite and F: Franklinite.
concentration was increased because the specific surface area of ferric carbonate is higher than that of its hydroxide form. The slight increase of Mg concentration seems to be a result of the presence of Cl ions, which combine with Mg, and hence increase the solubility of magnesium salts. This explanation is matched with the anion results obtained and similar previously reported works (O'Connor et al., 2001). The Cu, Cd and Pb concentrations decreased upon carbonation due to the formation of insoluble of metal carbonates.
3.5. Leaching properties Table 4 indicates that the leaching behavior of BHD is affected after carbonation, due to the stabilization of a metal carbonate through CO2 uptake. At the same time, as there is an increase in the leaching rate of some leached species, there are noticeably decreases in other leached species. Leaching test results displayed in (Table 4) specified that Ca, Na, K, Cr, and Al concentrations largely decreased due to the formation of stable carbonates. The solubility of producing calcium carbonate is much lower than that of free lime (un-carbonated form). Additionally, Ba and Sr concentrations are extremely decreased upon carbonation. Fe
Table 3 Physical properties of fresh and carbonated BHD. Properties
Unit
Fresh Carbonated Carbonated (un-treated) (pre-treated)
pH Electrical conductivity Total dissolved solids CO2 Carbonate content Carbonation efficiency CO2 Sequestration
unitless millisiemens part per thousand Weight percent Weight percent Percent (kg CO2/kg dust)
12.2 29.3 14.7 12.5 28.5 e e
10.5 15.6 8.1 14.4 32.7 52.0 0.5034
9.3 5.6 2.8 18.8 42.8 69 0.657
Fig. 6. SEM images of the carbonated BHD show the formation of calcium carbonates as (a) Rhombohedral and Vaterite structures, (b) Aragonite structures.
M.H. El-Naas et al. / Journal of Environmental Management 156 (2015) 218e224 Table 4 Short and long leaching test of BHD in part per million. Parameter
Time (hour) Liquid/Solid K Na Ca Mg Ba Al Fe Mn Cr Sr Zn Cu Cd Pb SO4 Cl
Non-carbonated
Carbonated
Short
Long
Short
Long
6 2 10,609 8932 961 13.00 3.10 9.03 0.84 0.04 19.90 0.12 1.89 0.12 0.001 0.15 6580 39.33
18 8 642 666 355 9.20 1.50 4.54 0.59 0.04 1.33 0.05 0.03 0.02 0.001 0.05 339 12.12
6 2 7753 7320 273 18 0.29 1.44 4.61 0.27 1.25 0.01 0.07 0.07 0.00 0.01 4926 17.24
18 8 357 424 76 10 0.00 1.36 2.03 0.14 0.07 0.00 0.04 0.02 0.00 0.011 161 4.70
In the fresh BHD, most of the heavy metals are bound to organic matter and sulfides; hence it can be expected that a similar behavior of immobilization takes place due to the carbonation process (Meima and Comas, 1998). Additionally, after carbonation, the pH decreases to a range of 9e9.5, reducing the metal mobility. Furthermore, the produced calcite encapsulates and immobilizes heavy metals (Lange et al., 1996). The concentration of sulfate and chloride were notably decreased in the leaching solution of the carbonated BHD. This could be attributed to the formation of insoluble forms of chloride and sulfate compounds and the restricted mobility of the anions in the solution (Li et al., 2007). 4. Conclusions This study focused on the utilization of EAF steel making bag house dust in CO2 mineral sequestration, since it does not have such wide economic application as steel slag. The main feature of this study is the pre-treatment method that was conducted through different steps; hydration, drying and screening. The carbonation reaction was then carried out in a fluidized bed reactor. The experimental results clearly indicated that the direct gasesolid carbonation reaction of the pre-treated bag house dust is an appropriate method for CO2 mineral carbonation. The maximum CO2 sequestration capacity was 0.657 kg of CO2/kg of dust, based on the total calcium content. Such results emphasize the potential use of EAF bag house dust in CO2 mineral carbonation, as an effective tool for managing these hazardous wastes and, at the same time, contributing to the reduction of CO2 emission. Acknowledgment The authors would like to acknowledge the financial support of the National Research Foundation of the United Arab Emirates. Special thanks are extended to the Emirates Steel Industries (Abu Dhabi, UAE) for their cooperation and support. References Al-Sugair, F.H., Al-Negheimish, A.I., Al-Zaid, R.Z., 1996. Use of electric arc furnace byproducts in concrete US patent No. 5557031 A. Baciocchi, R., Costa, G., Di Bartolomeo, E., Polettini, A., Pomi, R., 2010. Carbonation of stainless steel slag as a process for CO2 storage and slag valorization. Waste Biomass Valoriz. 1, 467e477. Beruto, D.T., Botter, R., 2000. Liquid-like H2O adsorption layers catalyze the Ca(OH)2/CO2 solid-gas reaction and to form a non-protective solid product layer
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Application of surface complexation precipitation modelling to contaminant leaching from weathered municipal solid waste incinerator bottom ash. Environ. Sci. Technol. 32, 688e693. Mohamed, A.M.O, El Gamal, M., 2014. Method for treating particulate material, US patent No. 8721785 B2. Moorehead, D.R., 1986. Cementation by the carbonation of hydrated lime. Cem. Concr. Res. 16 (5), 700e708. Mu, J., Perlmutter, D.D., 1981. Thermal decomposition of carbonates, carboxylates, oxalates, acetates, formates and hydroxides. Thermochim. Acta 49, 207e218. Nishino, Y., Oaki, Y., Imai, H., 2009. Magnesium-mediated nanocrystalline mosaics of calcite. Cryst. Growth Des. 9, 223e226. O'Connor, W.K., Walters, R.P., Dahlin, D.C., Rush, G.E., Nilsen, D.N., Turner, P.C., 2001. Carbon dioxide sequestration by direct aqueous mineral carbonation. In: Sakkestad, B.A. (Ed.), Proceedings of the 26th International Technical Conference on Coal Utilization & Fuel Systems; 4e8 March 2001; Clearwater, FL, United States. Coal Technology Association, Gaithersburg, MD, pp. 765e776. Sawada, K., 1997. The mechanisms of crystallization and transformation of calcium carbonates. Pure Appl. Chem. 69, 921e928. Seifritz, W., 1990. CO2 disposal by means of silicates. Nature 345, 486. Sofilic, T., Rastovcan-Mioc, A., Cerjan-Stefanovic, S., Novosel-Radovic, V., Jenko, M., 2004. Characterization of steel mill electric-arc furnace dust. J. Hazard. Mater. 109, 59e70. Steinour, H.H., 1959. Some effects of carbon dioxide on mortars and concrete-discussion. J. Am. Concr. Inst. 30, 905. Stolaroff, J.K., Lowry, G.V., Keith, D.W., 2005. Using CaO and MgO rich industrial waste streams for carbon sequestration. Energy Convers. Manag. 46, 687e699. Tai, C.Y., Chen, F.B., 1998. Polymorphism of CaCO3, precipitated in a constantcomposition environment. 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Taylor, H.F.W., 1990. Cement Chemistry. Academic Press Limited, London. Twilley, R.K., 2002. Hardened particle comprising a reaction product of metal baghouse dust, US patent No. 6451083 B1. Young, J.F., Berger, R.L., Breese, J., 1974. Accelerated curing of compacted calcium silicate mortars on exposure to CO2. J. Am. Ceram. Soc. 57 (9), 394e397.
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
CO2 sequestration using accelerated gas-solid carbonation of pre-treated EAF steel-making bag house dust Muftah H. El-Naas a, *, Maisa El Gamal a, Suhaib Hameedi a, Abdel-Mohsen O. Mohamed b a b
Chemical and Petroleum Engineering Department, UAE University, P.O. Box 15551, Al-Ain, United Arab Emirates Zayed University, P.O. Box 144534, Abu Dhabi, United Arab Emirates
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2014 Received in revised form 24 March 2015 Accepted 25 March 2015 Available online 4 April 2015
Mineral CO2 sequestration is a promising process for the reduction of carbon dioxide emissions to the atmosphere. In this paper, alkaline calcium-rich dust particles collected from bag filters of electric arc furnaces (EAF) for steel making were utilized as a viable raw material for mineral CO2 sequestration. The dust particles were pre-treated through hydration, drying and screening. The pre-treated particles were then subjected to direct gasesolid carbonation reaction in a fluidized-bed reactor. The carbonated products were characterized to determine the overall sequestration capacity and the mineralogical structures. Leaching tests were also performed to measure the extracted minerals from the carbonated dust and evaluate the carbonation process on dust stabilization. The experimental results indicated that CO2 could be sequestered using the pre-treated bag house dust. The maximum sequestration of CO2 was 0.657 kg/kg of dust, based on the total calcium content. The highest degree of carbonation achieved was 42.5% and the carbonation efficiency was 69% at room temperature. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon sequestration Gasesolid Electric arc furnace Bag house dust Leaching Fluidized-bed reactor
1. Introduction Carbonation of alkali metal oxides simulates natural chemical transformations of carbon dioxide. In weathering procedures, eroded rock surfaces come into contact with rainwater saturated with dissolved atmospheric CO2. As a result, alkali and alkaline earth elements dissolve into the water and precipitate as carbonate minerals. These weathering processes, reduce high CO2 concentrations present in the earth atmosphere; about 80% of all carbon in the world are present in the form of carbonate rocks (Seifritz, 1990; Dunsmore, 1992). This carbonation process could be improved and industrially applied to fix gaseous CO2 into a solid carbonate and thus prevent the emission of CO2 into the atmosphere. The process offers a safe and permanent method of CO2 disposal, since there is no possibility of any release of CO2 from the disposal site for long times. The carbonation process may be proceeding through direct or indirect methods. In the direct method, minerals and other waste materials go through the direct carbonation process. One of the main advantages of gasesolid mineralization is the possibility of
* Corresponding author. E-mail address: [email protected] (M.H. El-Naas). http://dx.doi.org/10.1016/j.jenvman.2015.03.040 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
recovering the heat released by the exothermic carbonation reaction in a beneficial form (Zevenhoven et al., 2013). The direct carbonation of calcium-silicates in an aqueous suspension at elevated temperature and CO2 pressure is considered to be the most promising process route. In the indirect method, mineral components are separated from the waste material, then reacted with CO2 (Huijgen and Comans, 2003). Other routes include pretreatment of the solid feedstock like size reduction and thermal activation (Huijgen, 2007). Steel making by-products are waste materials, produced during iron and crude steel production. These by-products include slag, dust, sludge, and mill scale. A large amount of bag house dust (BHD) is usually produced as a byproduct of the steel-making process. BHD contains water leachable heavy metals such as lead, chromium and cadmium. It was categorized as hazardous waste K061 under EPA regulations in 1980 (Sofilic et al., 2004). It is also produced in large quantities, due to the dramatic population increase and the growth of industrialization. The management of such wastes, therefore, is a major concern of steelmakers due to the environmental impact. The high content of CaO in such by-products is the main reason of its utilization in CO2 sequestration (El-Naas et al., 2013; Johnson, 2000; Huijgen et al., 2004; Stolaroff et al., 2005; Kodama et al., 2006). The use of these byproducts in mineral carbonation is
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resulting in a reduction of pH and leached metals, leading to solid stabilization (Baciocchi et al., 2010). The degree of carbonation depends on the temperature, pressure and surface area of the waste materials. In this work, BHD residue, recovered from EAF steel-making, was evaluated as a possible raw material for mineral CO2 sequestration. The direct gasesolid carbonation reaction was applied to avoid the leaching of heavy metals in water. Furthermore, the acid extraction process is not recommended, since the pH of the dust is greater than 12 and will consume excessive amount of acid for extraction. The chemical compositions of dust vary from one plant to another, depending on the type and quality of scrap used, charging method, oxygen-blowing rates, furnace-operating practices, and the type of steel produced (Al-Sugair et al., 1996; Idris and Siong, 1997). The main objective of this work is to evaluate the performance of the BHD for CO2 sequestration in a specially designed fluidized bed reactor. 2. Experimental methods 2.1. Materials Bag house dust (BHD) samples were collected from Emirates Steel in Abu Dhabi, The United Arab Emirates (UAE). The samples were collected from the out storage yard. A mechanical sieve shaker was used for sieving analysis. Carbon dioxide CO2 was provided by Sharjah Oxygen Company, UAE with 99.99% chemical purity. 2.2. Pre-treatment process The reactions of alkaline earth metal oxides and silicates in BHD with water were modeled using chemical reaction and equilibrium software HSC 6.1. Hydration process was performed at different water temperatures (25, 40, 60 C and steam water). A specific amount of 1 kg BHD was mixed with water using mechanical stirrer at 400 rpm for 2 h to form the hydrated products. Subsequently, the hydrated dust was air dried at room temperature. Screening of the dust using tray shaker is finally performed to separate the agglomerated particles and consequently increase surface area. 2.3. Carbonation process Carbonation experiments were carried out in a fluidized bed reactor, which was made of Acrylic glass with 123 mm in diameter and 500 mm in length. The reactor had a gas feed tube, a flue gas outlet and a perforated distributor plate. CO2 gas was forced upward through the distributor to the solid particles with an input pressure of 3 bars and output pressure of 1 bar. The bed was operated slightly above minimum fluidization velocity at a flow rate of 12 L/min, using different moisture percentage ranging from 1 to 7 mass%.
219
dissolved solids (TDS) and electrical conductivity. TGA analysis and mineralogical compositions were performed using TA Q500 thermal gravimetric analyzer and Philips PW/1840 x-ray diffractometer (XRD), respectively. Microstructure characterization was carried out using JOEL JSM-5600 scanning electron microscope. VISTAMPX CCD inductively coupled plasma-atomic emission spectrometry (ICP-AES) was utilized for alkali and heavy metals analysis, while DIONX IC-ion-chromatography (IC) was used for anions determination. 2.6. Degree of carbonation and sequestration efficiency A CO2 uptake, based on the weight fraction of the TGA curve (Dm550e850 C) and dry weight (m105 C), was used as the carbon content, expressed in terms of CO2 (wt %), Eq. (1):
CO2 wt ð%Þ ¼
Dm550850 C 100 m105 C
(1)
The theoretical total carbon content based on basic metal oxides present in the fresh samples was calculated using Eq. (2) (Steinour, 1959):
%CO2 ¼ 0:785ð%CaO 0:56% CaCO3 0:7% SO3 Þ þ 1:091% MgO þ 0:71% Na2 O þ 0:468% K2 O
(2)
The CO2 sequestration efficiency was determined using the maximum CO2 sequestration capacity for TGA data and the theoretical maximum CO2 sequestration capacity, Eq. (3).
CO2 sequestration efficiency ð%Þ ¼
Maximum CO2 sequestration capacity 100 Theortical CO2 sequestration capacity
(3)
3. Results and discussion 3.1. Characterization of the BHD The chemical composition of the examined BHD specimen is shown in Table 1. The predominant material is iron oxide and the second major component is calcium oxide. It is a highly dense material (approximate density is 2.89 g/cm3), which is compatible with the chemical and mineralogical content. The dust has been classified as a hazardous waste due to the relatively high lead, cadmium and hexavalent chromium contents. The order of the major elements is Fe, Ca, Mg, Si, Na, K, S, Mn, Al, Zn, Pb, Sr, Cr, Cu and Cd. This consequence was in a good agreement with data reported in Twilley (2002). The sieve analysis of the BHD indicated that the dust particle ranged in size from less than 38 mm to more than 250 mm, with more than 80% of the particles are less than
2.4. Leaching test Metal ions leaching test was carried out in accordance to British Standard BS (EN12457 2002), which is designed to examine the short-term and the long-term leaching behavior of the uncarbonated and the carbonated materials. The concentration of the major and minor cations were measured using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), whereas anions were measured using Ion Chromatography (IC). 2.5. Laboratory analyses Accumet 925 pH/ion meter was used for measuring pH, total
Table 1 Chemical composition of BHD uses an Inductively Coupled Plasma. Composition
Concentration (weight percentage)
Composition
Concentration (part per million)
FeO (total) CaO MgO Na2O K2O MnO SiO2 Al2O3 SO3
42.8 40.2 4.96 2.52 1.98 0.68 4.49 0.28 1.108
Zn Pb Sr Cr Cu Cd
1973.2 399.4 268.4 148.1 53.6 30.1
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Table 2 Sieve analysis of BHD. U.S. sieve no.
Sieve size (mm)
Percent passing
40 60 70 100 140 200 400
425 250 212 150 106 75 38
100 97.68 96.22 86.79 60.65 32.15 3.08
150 mm (Table 2). The morphology of BHD was studied by SEM-EDS analysis; the majority of BHD particles were spheres with the presence of some angular fragments. The fine spheres can be observed as discrete or agglomerated particles (Fig. 1). XRD analysis revealed that the common minerals of BHD were magnetite (Fe2O3), Franklinite (ZnO.Fe2O3), Larnite (Ca2SiO4) with other silicate phases, metal oxides such as CaO, MgO, Na2O, K2O, MnO, calcite (CaCO3) and dolomite CaMg(CO3)2. A considerable amount of salts like arcanite (K2SO4), sylvite (KCl), gypsum (CaSO4$H2O) and syngenite (K2Ca(SO4)2$H2O) could also be detected (Fig. 2). The chemical complexity of the dust is enhanced by the amphoteric nature of iron, the reducing or oxidizing conditions in the furnace and the presence of large quantities of metal oxides, such as calcium oxide and magnesium oxide.
Fig. 2. XRD analyses of fresh BHD. Where, C: Calcite, V: Vaterite A: Aragonite D: Dolomite, G: Gypsum P: Portlandite, Sy: Sylvite, F: Franklinite, H: Hematite and L: Larnite.
by the HSC chemistry software, Eq. (6). The thermodynamic data for the free energy of formation of CeSeH obtained by HSC are comparable with previously reported results (Fujii and Kondo, 1983).
3Ca2 SiO4 ðsÞ þ 3:5 H2 OðlÞ /4CaO$3SiO2 $1:5H2 OðsÞ þ 2CaðOHÞ2 ðsÞ DG ¼ 38:85 kJmol1 (6)
3.2. Pre-treatment process The composition of BHD is basically metal oxides and silicates; calcium and magnesium oxides react with water and form the corresponding hydroxides. These reactions are highly spontaneous and exothermic (Eqs. (4) and (5)).
CaOðsÞ þ H2 OðlÞ/CaðOHÞ2 ðsÞ
DG ¼ 57:9 kJmol1 (4)
MgOðsÞ þ H2 OðlÞ/MgðOHÞ2 ðsÞ
DG ¼ 27:3 kJmol1 (5)
Actually, the hydration process of BHD is so complicated due to the presence of silicates and other complex oxides associated with calcium oxide. Since the secondary mineral phases are mainly silicate minerals, the preparation of calcium silicate hydrate (CeSeH) from dicalcium silicate (C2S) may vary in composition as modeled
In the presence of water, silicates and aluminates are hydrated into stiff and hard products with very low solubility in water. These products are mainly tri-calcium silicate hydrate (C3S2H3), tricalcium aluminate hydrate (C3AH6) and calcium alumino-ferrite hydrate (C6AFH12) (Mohamed and El Gamal, 2014; Cizer, 2009; Massazza, 1998; Taylor, 1990). Laboratory scale experiments indicated that the hydration process was very slow at room temperature. The formation of the calcium hydroxide and CeSeH crystals provide a nucleus for more calcium silicate hydrate. It is likely that impermeable coatings of these reaction products formed on the surface of the grains prevent further reactions. The CeSeH crystals grow thicker, making it so difficult for water molecules to reach the non-hydrated calcium silicate. The speed of the reaction is then controlled by the rate at which water molecules diffuse through the CeSeH coating. Hydration reaction was found to be accelerated by increasing the hydration temperature. Based on the degree of carbonation, more than 10% increase in carbonate content was achieved upon applying steam water, over that obtained by hydration at room temperature. Mechanical mixing and steam can increase the diffusivity of water and consequently, increase the hydration reactions. 3.3. Carbonation process In this work, direct gasesolid carbonation of the pre-treated BHD was applied to achieve the chemical reaction. Since the mineral carbonation depends on the reaction of CO2 with metal hydroxides and metal silicate hydrates, the amount of heat produced by these reactions depends on the amounts of bivalent metal contained like Ca and Mg. Thermodynamic analysis was performed using HSC to study the effects of different variables on the carbonation process. The analysis data indicated that the carbonation reactions of calcium hydroxide and calcium silicate hydrate were spontaneous and exothermic, Eqs. (7) and (8).
CaðOHÞ2ðsÞ þ CO2ðgÞ /CaCO3ðsÞ þ H2 OðlÞ Fig. 1. SEM image of fresh BHD.
DH ¼ 113 kJmol1 (7)
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4CaO$3SiO2 $1:5H2 OðsÞ þ 4CO2ðgÞ /4CaCO3ðsÞ þ 3SiO2ðsÞ þ 1:5H2 OðlÞ
DH ¼ 393 kJmol1 (8)
It was found that the moisture content affected both carbonation and fluidization processes; carbonation does not completely occur in totally dried samples and the penetrability of CO2 gas increases within the hydrated dust, but too much water limits the reaction due to the blockage of the pores. The effect of moisture content on the carbonation of BHD was evaluated through the measurements of the pH, total dissolved solids (TDS) and electrical conductivity of the carbonated samples. It was found that moisture content ranging from 3 to 4% enhanced the gasesolid carbonation reaction of BHD. The reported mechanism of this fact suggested that the water adsorbed on hydrophilic and basic surface sites allowed carbonation formation at the gasesolid interface followed by the formation of solid carbonate around reacting particles (Beruto and Botter, 2000). The major hydrated chemical compounds that react with carbon dioxide are calcium hydroxide, calcium silicate hydrate, calcium aluminate hydrate, Ettringite and calcium aluminoferrite hydrate. The calcium silicate grains are covered with a loose layer of CeSeH gel, which is quickly dissolved, releasing Ca2þ and SiO44 ions. The gel is progressively decalcified leading to the formation of CaCO3 intermixed with silicate hydrate gel. The reaction of calcium aluminate hydrates is minimal, due to the tight aluminum-sulfate/ carbonate coating, nucleation of CaCO3 and CeSeH, which is promoted by increasing the temperature and solid phase precipitates. It is worth noting that the above reactions have three aspects in 2þ common: (I) they all involve the interaction of ions CO2 3 and Ca to form highly insoluble calcium carbonate, (II) they are all exothermic, and (III) they may happen simultaneously but at different rates, as previously reported (Young et al., 1974; Moorehead, 1986). 3.4. Performance of carbonation 3.4.1. Thermal Gravimetric Analysis (TGA) TGA results indicated that CO2 sequestration is favored in the pre-treated BHD samples as shown in Fig. 3. The different mass loss regions are 25e105 C for the moisture content, 130e200 C for the dehydration of the CeSeH, 205e500 C for organic carbon compounds, and 550e850 C for inorganic carbon compounds which occurred due to the thermal decomposition of calcium carbonate (Chang and Chen, 2006). At 750 C, dolomite structure is changed to calcite. Calcium hydroxide begins to decompose at 325 C and has a
Fig. 3. TGA of BHD as fresh, untreated carbonated and pre-treated carbonated samples, at a heating rate of 20 C/min.
221
quoted dehydration temperature of 518 C (Mu and Perlmutter, 1981). 3.4.2. Carbon uptake Fig. 4 represents the average CO2 uptake in the overall sample under each condition, at the same fluidization conditions (input pressure of 3 bar, output pressure of 1 bar, and flow rate of 12 L/ min). It appears that the CO2 uptake in the untreated carbonated sample increased slightly with time, whereas a higher increase in the CO2 uptake was observed in the pre-treated samples. Further increase in time did not increase the CO2 uptake substantially. Generally, pre-treated samples showed higher carbon contents ndez Bertos et al., compared with those of untreated ones (Ferna 2004). Thermodynamically, the reaction of calcium oxide with CO2 is more favorable than that of calcium hydroxide (where DG are 130.4 kJ and 72.6 kJ, respectively). The reaction of hydrated metal oxide with CO2 produces initially a layer of a metal carbonate, which limits the diffusion of CO2 through the hydroxide particles, Eq. (9).
CaðOHÞ2ðsÞ þ nH2 OðlÞ þ CO2ðgÞ /CaCO3ðsÞ þ ðn 1ÞH2 OðlÞ (9) However, under the applied pressure, the formation of relatively soluble calcium bicarbonate facilitates CO2 diffusion to the bulk of the calcium hydroxide particles, Eqs. (10) and (11).
CaCO3ðsÞ þ CO2ðgÞ þ H2 OðlÞ/CaðHCO3 Þ2ðaqÞ
(10)
CaðOHÞ2ðsÞ þ CaðHCO3 Þ2ðaqÞ /2CaCO3ðsÞ þ 2H2 OðlÞ
(11)
A necessary condition for the carbonation reaction is the presence of nano-drops of water on the surface of Ca(OH)2 crystals, which leads to the formation of Ca(HCO3)2 and CaCO3, upon interaction with CO2. The water resulting from the reaction between Ca(OH)2 and CO2 accelerates the reaction, and favors crystallization of the formed carbonate under powerful fluidizing conditions as well. It is also reported that the increased rate of Ca(OH)2 carbonation was due to its high specific surface compared to CaO (Kalinkin et al., 2005). 3.4.3. CO2 sequestration effectiveness The theoretical total carbon content of BHD was calculated as a function of chemical composition. All alkali metal species like CaO, MgO, Na2O and K2O are converted to the corresponding carbonate. The maximum CO2 uptake by BHD was found to be
Fig. 4. CO2 uptakes for the untreated and pre-treated carbonated BHD.
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18.8 wt%. The high carbon content of 12.5 wt% in the fresh dust is attributed to the weathering interaction in the open waste stores. The physical properties of fresh and carbonated BHD are shown in (Table 3). The pH, electrical conductivity and TDS were reduced after carbonation due to the conversion of alkaline metal oxides and silicates into insoluble carbonates. Carbon uptake, degree of carbonation and carbonation efficiency of the pre-treated BHD are clearly increased compared with the untreated one. The highest degree of carbonation achieved was 42.5% and the carbonation efficiency was 69% at room temperature. The maximum sequestration of CO2 in the pre-treated carbonated dust was calculated as 0.657 kg/kg of dust, based on the total calcium content. This result is notably high compared with that reported by Gupta and Nema (2012), where bag house dust residue, containing 48 wt % CaO, was able to sequester about 0.28 kg of CO2 per kg of dust, with maximum degree of carbonation of 7.56% using gasliquid-carbonation method at different water-to-solid ratios. They found that a lower water-to-solids ratio produced a higher degree of carbonation. They attributed their findings to the hindrance caused by water to the diffusion of CO2 and the reduction of gaspermeability of the dust. 3.4.4. Mineralogical composition and microstructure Diffraction peaks presented on (Fig. 5) showed that the absolute intensities of calcite peaks are increased, whereas, Portlandite, Larnite, Sylvite and Gypsum peaks was disappeared after carbonation due to the consumption of calcium ion in the formation of calcium carbonate. SEM observations highlighted that the carbonation products of BHD are calcium carbonates with different morphologies, welldefined rhombohedra calcite and vaterite particles (Fig. 6a). Additionally, wheat-sheaf-like bundles aragonite particles was found in the carbonated dust (Fig. 6b). It was reported that phase of vaterite can occur during the formation of calcite and aragonite (Carlson, 1983). It was also reported that aragonite was formed due to the presence of Mg2þ ions in the dust composition (Tai and Chen, 1998; Sawada, 1997; Fernandez-Diaz et al., 1996; Nishino et al., 2009).
Fig. 5. XRD analyses of the carbonated pre-treated BHD. Where, C: Calcite V: Vaterite, A: Aragonite, D: Dolomite and F: Franklinite.
concentration was increased because the specific surface area of ferric carbonate is higher than that of its hydroxide form. The slight increase of Mg concentration seems to be a result of the presence of Cl ions, which combine with Mg, and hence increase the solubility of magnesium salts. This explanation is matched with the anion results obtained and similar previously reported works (O'Connor et al., 2001). The Cu, Cd and Pb concentrations decreased upon carbonation due to the formation of insoluble of metal carbonates.
3.5. Leaching properties Table 4 indicates that the leaching behavior of BHD is affected after carbonation, due to the stabilization of a metal carbonate through CO2 uptake. At the same time, as there is an increase in the leaching rate of some leached species, there are noticeably decreases in other leached species. Leaching test results displayed in (Table 4) specified that Ca, Na, K, Cr, and Al concentrations largely decreased due to the formation of stable carbonates. The solubility of producing calcium carbonate is much lower than that of free lime (un-carbonated form). Additionally, Ba and Sr concentrations are extremely decreased upon carbonation. Fe
Table 3 Physical properties of fresh and carbonated BHD. Properties
Unit
Fresh Carbonated Carbonated (un-treated) (pre-treated)
pH Electrical conductivity Total dissolved solids CO2 Carbonate content Carbonation efficiency CO2 Sequestration
unitless millisiemens part per thousand Weight percent Weight percent Percent (kg CO2/kg dust)
12.2 29.3 14.7 12.5 28.5 e e
10.5 15.6 8.1 14.4 32.7 52.0 0.5034
9.3 5.6 2.8 18.8 42.8 69 0.657
Fig. 6. SEM images of the carbonated BHD show the formation of calcium carbonates as (a) Rhombohedral and Vaterite structures, (b) Aragonite structures.
M.H. El-Naas et al. / Journal of Environmental Management 156 (2015) 218e224 Table 4 Short and long leaching test of BHD in part per million. Parameter
Time (hour) Liquid/Solid K Na Ca Mg Ba Al Fe Mn Cr Sr Zn Cu Cd Pb SO4 Cl
Non-carbonated
Carbonated
Short
Long
Short
Long
6 2 10,609 8932 961 13.00 3.10 9.03 0.84 0.04 19.90 0.12 1.89 0.12 0.001 0.15 6580 39.33
18 8 642 666 355 9.20 1.50 4.54 0.59 0.04 1.33 0.05 0.03 0.02 0.001 0.05 339 12.12
6 2 7753 7320 273 18 0.29 1.44 4.61 0.27 1.25 0.01 0.07 0.07 0.00 0.01 4926 17.24
18 8 357 424 76 10 0.00 1.36 2.03 0.14 0.07 0.00 0.04 0.02 0.00 0.011 161 4.70
In the fresh BHD, most of the heavy metals are bound to organic matter and sulfides; hence it can be expected that a similar behavior of immobilization takes place due to the carbonation process (Meima and Comas, 1998). Additionally, after carbonation, the pH decreases to a range of 9e9.5, reducing the metal mobility. Furthermore, the produced calcite encapsulates and immobilizes heavy metals (Lange et al., 1996). The concentration of sulfate and chloride were notably decreased in the leaching solution of the carbonated BHD. This could be attributed to the formation of insoluble forms of chloride and sulfate compounds and the restricted mobility of the anions in the solution (Li et al., 2007). 4. Conclusions This study focused on the utilization of EAF steel making bag house dust in CO2 mineral sequestration, since it does not have such wide economic application as steel slag. The main feature of this study is the pre-treatment method that was conducted through different steps; hydration, drying and screening. The carbonation reaction was then carried out in a fluidized bed reactor. The experimental results clearly indicated that the direct gasesolid carbonation reaction of the pre-treated bag house dust is an appropriate method for CO2 mineral carbonation. The maximum CO2 sequestration capacity was 0.657 kg of CO2/kg of dust, based on the total calcium content. Such results emphasize the potential use of EAF bag house dust in CO2 mineral carbonation, as an effective tool for managing these hazardous wastes and, at the same time, contributing to the reduction of CO2 emission. Acknowledgment The authors would like to acknowledge the financial support of the National Research Foundation of the United Arab Emirates. Special thanks are extended to the Emirates Steel Industries (Abu Dhabi, UAE) for their cooperation and support. References Al-Sugair, F.H., Al-Negheimish, A.I., Al-Zaid, R.Z., 1996. Use of electric arc furnace byproducts in concrete US patent No. 5557031 A. Baciocchi, R., Costa, G., Di Bartolomeo, E., Polettini, A., Pomi, R., 2010. Carbonation of stainless steel slag as a process for CO2 storage and slag valorization. Waste Biomass Valoriz. 1, 467e477. Beruto, D.T., Botter, R., 2000. Liquid-like H2O adsorption layers catalyze the Ca(OH)2/CO2 solid-gas reaction and to form a non-protective solid product layer
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