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Studies on the solidification mechanisms of Ni and Cd in cement clinker during cement kiln co-processing of hazardous wastes
Construction and Building Materials 57 (2014) 138–143
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Studies on the solidification mechanisms of Ni and Cd in cement clinker during cement kiln co-processing of hazardous wastes Yufei Yang a, Jingchuan Xue b, Qifei Huang a,⇑ a
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Science, Beijing 100012, China Wadsworth Center, New York State Department of Health, and Department of Environmental Health Sciences, School of Public Health, State University of New York at Albany, Empire State Plaza, P. O. Box 509, Albany, NY 12210-0509, United States b
h i g h l i g h t s Cd concentrated in Cao phases but distributed mainly in C3S phases. Ni was mainly formed MgNiO2, also present in C4AF and C3S. Cd and Ni solidified by isomorphous replacement and formation of a new or interstitial solid solution. Ca–Cd–O sosoloid and Ni–Mg compound were formed in solidification. Cd and Ni incorporated in C3S by substituting Ca and in C4AF by inter lattice space.
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Article history: Received 28 October 2013 Received in revised form 20 December 2013 Accepted 24 December 2013 Available online 25 February 2014 Keywords: Cd Ni Solidification mechanism Cement clinker Cement kiln co-processing Microanalysis
a b s t r a c t To clarify the solidification mechanisms of heavy metals in cement clinker during the cement kiln co-processing of hazardous wastes, cement clinker samples were produced. EPMA1 and XANES2 spectroscopy were employed to acquire information on the microstructure and the Cd and Ni species formed in the cement system. The average Cd concentrations in CaO, C4AF3 and C3S4 were 3.92%, 1.21% and 0.75%, respectively, but Cd was mostly present in C3S in clinker (71.0%). The solidification methods of Cd during the calcination process included the formation of a Ca–Cd–O sosoloid, substitution for Ca in C3S and C4AF (isomorphous replacement) and inter the space of the lattice (interstitial solid solution). Ni was combined mainly with Mg in the form of a new Ni–Mg compound (distribution ratio of 61.2%), which was MgNiO2, as confirmed by XANES analysis. Ni was also present in C3S and C4AF (distribution ratios of 24.9% and 10.3%, respectively) by replacement of Ca2+ in C3S and substituting Fe in C4AF (isomorphous replacement), and also by inter the spaces of the C4AF lattice (interstitial solid solution). Ó 2014 Published by Elsevier Ltd.
1. Introduction Cement kilns possess inherent features that suit the treatment of hazardous wastes. These features include high temperatures and long residence times (greater than 1200 °C for several
Abbreviations: EPMA, electron probe microanalysis; XANES, X-ray absorption near-edge structure; C4AF, tetracalcium aluminoferrite; C3S, tricalcium silicate; C2S, bicalcium silicate; C–S–H, calcium silicate hydrate. ⇑ Corresponding author at: Chinese Research Academy of Environmental Science, Beijing 100012, China. Tel.: +86 10 84915142; fax: +86 10 8491 3903. E-mail addresses: [email protected] (Y. Yang), [email protected] (J. Xue), [email protected] (Q. Huang). 1 Electron probe microanalysis. 2 X-ray absorption near-edge structure. 3 Tetracalcium aluminoferrite. 4 Tricalcium silicate. http://dx.doi.org/10.1016/j.conbuildmat.2013.12.081 0950-0618/Ó 2014 Published by Elsevier Ltd.
seconds), surplus oxygen during and after combustion, good turbulence and mixing conditions, and no generation of by-products such as slag, ashes, or liquid residues [1–3]. The co-processing of waste using cement kilns has been widely and successfully employed in the United States, Europe, Japan and other developed countries for several decades, and more recently in developing countries [4,5]. As far as wastes containing heavy metals are concerned, heavy metals are discharged to the atmosphere through gaseous emissions, but in the main, the metals reside in the cement clinker. Studies have indicated that the leaching rate of heavy metals in cement derived from the co-processing of waste is low [6–8]. The key reason for this is that the heavy metals in cement clinker and cement products have been solidified and are relatively inert to leaching. The solidification of heavy metals in cement kiln co-processing wastes typically occurs by two processes, cement hydration and
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calcination. There has been much research on the solidification theory of heavy metals as a result of hydration [9]. In the cement hydration process, Cd is solidified by the formation of a mixed Ca/Cd hydroxide or Cd undergoes an exchange with Ca in a C–S– H gel [10,11]. Vespa et al. used unhydrated cement to condition a nitrate solution containing Ni. Results of X-ray absorption spectroscopy and scanning electron microscopy revealed that the main form of Ni in the cement-based solidified/stabilized wastes was a mixture of a Ni–Al layered double hydroxide phase and Ni (OH)2 [9]. Sheidegger et al. also demonstrated that the formation of Niand Al-containing hydrotalcite-like layered double hydroxides is a relevant binding mechanism for Ni in cement [12]. Currently, several solidification effects of heavy metals in cement clinker have been reported. Barros studied the incorporation ratio of ZnO, PbO and CdO when the heavy metals were added to clinker raw material [13]. Additionally, when a galvanic sludge containing Cu and Ni was added to clinker raw material, at a ratio of 2.4 wt% and 1.2 wt%, these metals were totally incorporated into the clinker [14]. There have been relatively few studies on the solidification mechanism of heavy metal in cement clinker for the calcination process. Murat studied the effect of large additions of heavy metals to cement raw material on the composition and properties of the cement clinker. The results showed that Cd2+ enters the C3S lattice by replacing Ca2+ in C3S, Pb forms PbSO4 and dissolves in C3S and C2S, and Cr is mainly distributed in C2S [15]. Investigations by Zhang showed that, in the case of As, Cd, Pb and Zn, for calcinations in a cement rotary kiln and then hydration during cement application, the metals were more effectively fixed than in the case of cement-based solidification [16]. Clearly, the different retention mechanisms for co-processing and cement hydration result in different fixation effects for the heavy metals. As can be seen, the physical and chemical changes for heavy metals that occur in the calcination processes have not been fully characterized. The purpose of this study was to clarify the solidification mechanisms of heavy metals in cement clinker during the cement kiln co-processing of hazardous wastes. Experiments were conducted for the simulation of the high-temperature calcination of cement raw material with the heavy metals of Cd and Ni. Microanalytical methods including electron probe microanalysis (EPMA) and Xray absorption near-edge structure (XANES) analysis were employed to obtain information on microstructure and the species of heavy metals formed in the cement clinker. 2. Materials and methods
requirements for EPMA and XANES analysis. Heavy-metal compounds were thoroughly mixed with the cement raw materials in accordance with the ratios listed in Table 2. The table also gives the total contents of heavy metals in mixed raw materials. Each mixture was calcined in the furnace at 1450 °C for 1 h to prepare the clinker, which was then smashed and milled to a powder having specific surface area of 310 m2/kg. 2.3. EPMA-EDS The prepared clinker powder was compacted to clinker flake (thickness, 10 mm) using a press with pressure of 8 t/cm2. The head of the press was polished to ensure that the clinker flake surface was smooth to facilitate EPMA. Energy dispersive spectrometry (EDS) was employed to determine the elemental content. A JEOL JXA8800R EPMA analyzer and an Oxford ISIS300 X-ray energy spectrometer were used for analyses. The high voltage employed was 15 keV and the electron beam current employed was 2 108 A. Backscattered-electron images and X-ray images (elemental distribution images) were acquired. Standard samples of oxides and silicates were used for quality control of EPMA measurements. The overall features of the clinker mineral phase were first observed with EPMA under low-power magnification (scale of 200 lm), and then typical mineral phases were measured under medium-power magnification (scale of 50–100 lm) in surface, line and point mode. Some mineral phases with fine particles were transferred to the high-power lens (scale of 20 lm) for further measurement. 2.4. XANES measurements Cd and Ni K-edge XANES spectra were collected at the Beijing Synchrotron Radiation Facility. The typical energy of the storage ring was 2.5 GeV with the current decreasing from 250 to 160 mA during runs. Spectra for standard samples of CdO, CdSO4, CdCO3, CdCl2–2.5H2O, NiO, Ni2O3, Ni(OH)2, NiCO3–Ni(OH)2–6H2O, NiSO4– 6H2O and NiCl2–4H2O were recorded in transmission mode. Clinker samples were measured in fluorescence mode. All standards were ground to fine grains and pressed to form wafers (diameter, 1.0 mm). Data processing of the XANES spectra was performed with FEFF8.0 software [17].
3. Results and discussion 3.1. Solidification process The solidification of a heavy metal by cement clinker may be characterized by the fixation ration, which is expressed as
Table 2 Addition of heavy-metal compounds and heavy-metal content in mixture. Items
Cd
Ni
Heavy metal compound Heavy metal addition ratio (%) Amount added (g/kg) Amount of raw material (kg) Heavy metal content (mg/kg)
CdO 1 22.84 1.98 10,012
Ni2O3 1 28.18 1.97 10,027
2.1. Characterization of raw materials The cement raw materials, including limestone, clay and iron powder, were supplied by a local cement plant. The chemical compositions of the raw materials are listed in Table 1.
2.2. Sample preparation The clinker was made in a laboratory furnace using industrial raw materials and heavy-metal compounds. Limestone, clay and iron powder were thoroughly mixed at a weight ratio of 76.5:21.0:2.5 and homogenized. The addition of heavy metals was based on considerations of current industrial practice and measurement
Table 1 Chemical compositions of raw materials (%). Material
SiO2
Fe2O3
Al2O3
CaO
MgO
Cd (mg/kg)
Ni (mg/kg)
Limestone Clay Iron powder
0.18 62.14 34.98
0.04 9.10 50.56
0.04 16.17 4.95
55.64 1.63 1.34
0.05 0.00 0.84
6.13 35.0 9.95
27.9 26.6 27.1 Fig. 1. Morphology of cement clinker with sites for analysis.
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Fig. 2. EDS spectra for mineral phases at the seven locations.
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G ¼ MK
1 Loss 100%; Ms
ð1Þ
where G (%) is the fixation ration; Mk (mg/kg) represents the Cd and Ni contents of the clinker, which are 8636 and 13,359 mg/kg, respectively; Ms is the heavy metal content of the mixture (mg/kg including raw material, shown in Table 2) and Loss is the fraction of mass loss on ignition of the mixture, which is 0.3485. The ratio of fixation of Ni by the clinker in the experiment was 86.8%, which was slightly lower than that in an industrial process, where the fixation ration ranges from 87% to 97% [18]. Compared with the case for Ni, the fixation ratio for Cd was lower at 56.2%. This fixation ratio for Cd was also lower than that in industry processes, which typically ranges from 74% to 88% [18]. The difference in fixation ratio between Cd and Ni mainly depends on element volatility, with Cd being more volatile than Ni. The fixation ratios for Cd and Ni in the tests were lower than those in industrial processes for the following reasons. In an industrial kiln, the convection of gas and raw materials creates a strong turbulent atmosphere, which is conducive to reducing elemental volatility. There are also strong oxidizing conditions in the industrial kiln, and the concentration of dust in the gaseous atmosphere is much higher. All of these factors contribute to an enhanced absorption of acidic gases in the exhaust, thereby reducing the volatility of some elements and improving the condensation temperature. In contrast, for the laboratory experiment, the raw material was static, which reduces the probability of alkaline oxides being in gaseous contact with the volatilized heavy metals, which would otherwise increase element volatility.
Table 3 Cd and Ni concentrations in main mineral phase (%). Analysis site
C3S
C2S
C4AF
CdO
NiO
CdO
NiO
CdO
NiO
1 2 3 4 5 6 7 8 9 10 Average
1.04 0.73 0.77 0.65 0.68 0.33 0.75 0.89 1.02 1.08 0.79
0.51 0.52 0.16 0.31 0.30 0.34 0.52 0.44 0.51 0.28 0.39
0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00
0.00 0.07 0.05 0.00 0.09 0.05 0.05 0.00 0.05 0.08 0.04
1.47 1.24 1.20 1.30 1.19 1.14 0.60 1.47 1.24 1.20 1.21
1.42 0.90 0.72 1.16 1.06 1.29 1.82 1.42 0.90 0.72 1.14
Table 4 Cd and Ni concentrations in minor mineral phase (%). Analysis site
1 2 3 4 5 6 7 8 9 10 Average
CaO
Ni–Mg
CaO
CdO
NiO
MgO
CdO
NiO
78.62 76.26 84.91 73.40 58.41 63.06 63.49 76.52 65.83 80.23 70.1
7.97 6.00 1.43 4.96 2.50 2.45 2.50 6.00 3.55 1.83 3.92
0.32 0.39 0.25 0.27 0.80 0.04 0.80 0.45 0.20 0.33 0.39
50.48 39.40 60.50 53.44 44.05 37.36 51.35 45.20 53.20 55.42 49.0
0.00 0.03 0.08 0.06 0.00 0.00 0.00 0.05 0.05 0.00 0.03
40.54 39.20 31.66 31.97 35.30 33.70 35.46 36.80 33.20 32.70 35.1
3.2. Mode of occurrence of Cd and Ni in cement clinker EPMA was first employed to determine which elements were present in the cement clinker phases, and from the contents and ratios of elements, the specific cement clinker minerals were identified. Typical features of the cement clinker are illustrated in Fig. 1 (scale of 200 lm). A total of 49 points were set in the scanning area, and element contents at each point were determined by EDS. From the results of EDS, seven analysis points (No. 1 to No. 7) were selected (Fig. 1), where the element contents and ratios at each point conformed to the elemental composition characteristics of C3S, C2S, a Fe–Ca phase, C4AF, CaO, SiO2 and Ni–Mg, respectively. The corresponding EDS spectra for the seven points are presented in Fig. 2. To determine the Cd and Ni concentrations in the mineral phases at each location, a point-analysis routine was employed at high magnification (scale of 20 lm) and with scanning of 10 sampling points per location. The results for Cd and Ni in the main minerals and some minor mineral phases are presented in Tables 3 and 4. In contrast to the case for C2S, which was the main mineral, both C3S and C4AF contained Cd. The Cd concentration was much higher in the CaO phase, with the average concentration being 3.92% (range, 1.83–7.97%) (Table 3). The Cd concentration in C4AF (average concentration, 1.21%; range, 0.60–1.47%) was higher than that in C3S (average concentration, 0.75%; range, 0.33–1.08%) (Table 4). These findings differ from those of Karlstuhe [17], where Cd was not incorporated into the main clinker but existed in a minor phase (CaO). The Cd total content and distribution ratios in mineral phase were different (Table 5) as a result of the content of the main mineral phase being much higher than that of the minor mineral phases in cement clinker. The total content of CdO was 0.4835% in C3S, while it was 0.118% in C4AF and 0.0789% in f-CaO, and the corresponding distribution ratios were 71.0%, 17.4% and 11.5%, respectively.
Table 5 Cd and Ni total contents and distribution ratios in mineral phase (%). Material
C3S
C2S
C4AF
C3A
f-CaO
Ni–Mg
Other
Content in clinker Total CdO content Total NiO content Cd
61.2
21.5
9.80
4.00
2.00
1.20
0.30
0.4835
–
0.1186
–
0.0786
0.0004
–
0.1714
0.0172
0.0706
–
0.0078
0.4212
–
distribution ratio71.0–17.4–11.50.1–Ni distribution ratio24.92.5010.3– 1.1061.2–Note: ‘‘–’’means not detected.
There was good correlation between Ni and Mg concentrations, indicating that Ni and Mg formed a new Ni–Mg phase in the clinker. Lan observed a new mineral phase in addition to the main mineral phases of C3S and C2S in Ni-containing clinker [18]. The interplanar spacing (d) of this new mineral phase was 2.0917 and the diffraction angle was 43.217°. These crystal structural parameters are consistent with those of MgNiO2. Other reports have shown that when the contents of Ni and Mg in cement clinker are relatively high, MgNiO2 forms in the clinker process [19–21]. The total content of NiO in the Ni–Mg phase was 0.4212%, and the corresponding distribution ratio was 61.2.0%. Ni existed in C3S and C4AF phases besides the Ni–Mg phase, and the corresponding distribution ratios were 24.9% and 10.3%, respectively. 3.3. Species of heavy metals in cement clinker K-edge XANES spectra for Cd and Ni in cement clinker and for standard samples of Cd and Ni are presented in Figs. 3 and 4, respectively.
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Fig. 3. K-edge XANES spectra of Cd in clinker and in standard samples.
Fig. 4. K-edge XANES spectra of Ni in clinker and in standard samples.
The spectra for Cd in cement clinker were quite consistent with those for CdO samples. It was shown that Cd existed in the +2 form, and the ligand forms of Cd in clinker were similar to those of Cd in CdO. The spectra of Ni in cement clinker were consistent with those for NiO and Ni(OH)2 samples. These findings suggest that chemical changes in Ni in the cement raw materials had taken place during calcination at high temperature and that new chemical compounds formed. The valence of Ni in the new compound was +2, and the ligand forms of Ni in the clinker were similar to those of Ni in NiO and Ni(OH)2. There were six O
atoms around the Ni atoms in NiO and Ni(OH)2, and the Ni–O bond lengths in both NiO and Ni(OH)2 were 2.1 Å. It was established that the coordination number of Ni in the new Ni–Mg compounds was six, and the bond length was 2 Å. It can thus be inferred that the new Ni–Mg compound was MgNiO2, which further validates the EPMA data. 3.4. Solidification mechanisms of Ni and Cd in cement clinker According to the EPMA results, Cd existed mainly in the CaO phases and the spectra for Cd in the cement clinker were consistent
Y. Yang et al. / Construction and Building Materials 57 (2014) 138–143
with those for CdO samples. All these findings indicate that Ca in CaO was substituted by Cd to form a Ca–Cd–O sosoloid during high-temperature calcination. In addition, the ionic radius of Cd2+ (95 Å) is close to that of Ca2+ (100 Å) and the respective electronegativities are also of similar magnitude (Cd, 1.45 versus Ca, 1.05). Therefore, given the similar ionic characteristics of Cd2+ and Ca2+, it would be relatively easy for Cd to replace Ca in CaO to form a Ca–Cd–O sosoloid. In addition, the EPMA results show that Cd was distributed in the minerals C4AF and C3S. According to the theory of Bussem [22], there are many cavities within the C4AF crystal structure as a result of the layered architecture structure of tetrahedral and octahedral elements, and therefore Cd and Ni tend to be interred and located in the spaces of C4AF lattice and form an interstitial solid solution. Furthermore, Fe3+ and Al3+, which occupy the tetrahedral and octahedral positions of the C4AF lattice, respectively, tend to be substituted by ions of a similar valence and structure. Moreover, Cd may replace Ca in C4AF. In addition, a small amount of Cd may be incorporated into C3S, which has a dense structure, by replacement of Ca. The EPMA results demonstrate that a new Ni–Mg compound formed in clinker, and XANES results suggest this compound to be MgNiO2. Thus, Ni was solidified mainly as an inert species of MgNiO2. Ni was also found in the other main mineral phases (C3S and C4AF) of clinker. This result indicates that Ni did not fully react with Mg, as evidenced by Ni in clinker existing partly in the +3 form. The Ni3+ form was incorporated into the clinker lattice during calcination. The Ni3+ incorporated into the clinker lattice may be of two types. One type is where the Fe3+ in C4AF is replaced by Ni3+ because the difference in radius between Ni3+ and Fe3+ is less than 15%, and the respective electronegativities are quite similar (1.91 and 1.83 respectively). The other type is where Ni3+ enters the interspaces of C4AF where many cavities exist within the crystal structure. As the coordination numbers of both Ni3+ and Ca2+ are six, and in the case of a mineral with high calcium content, Ni3+ may replace Ca2+ in C3S. 4. Conclusions In the processing of hazardous wastes using a cement kiln, the volatility of heavy metals and the firing condition of the kiln affect heavy-metal solidification. This study found the solidification effect of Cd in cement clinker to be lower than that of Ni. The solidification mechanisms for Cd and Ni during cement kiln coprocessing of hazardous wastes were of three types. One was the formation of a new sosoloid. Substitution of the Ca in CaO to form a Ca–Cd–O sosoloid was the main mechanism for Cd solidification in cement clinker. In the case of Ni, the metal tended to form a new inert compound, MgNiO2. A second process was the formation of an interstitial solid solution; Cd and Ni could partly inter and be sited in the spaces of the C4AF lattice. The third type was isomorphous replacement, whereby a small number of Cd2+ and Ni2+ ions substitute for Ca and Fe in C4AF, respectively, and are incorporated into the C3S crystal lattice by replacement of Ca2+.
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Acknowledgments This research was jointly supported by the National Natural Science Foundation of China (51178439) and the National High-tech R&D Program of China (SS2012AA063401). We thank the staff on beamline 1W1B at the Beijing Synchrotron Radiation Facility and on beamline 14W1 at the Shanghai Synchrotron Radiation Facility. References [1] Mokrzycki E, Uliasz-Boche czyk A, Sarna M. Use of alternative fuels in the Polish cement industry. Appl Energy 2003;74:101–11. [2] Karstensen KH. Formation, release and control of PCDD/PCDF in cement kilns – a review. Chemosphere 2008;70:543–60. [3] Karstensen KH, Mubarak AM, Gunadasa HN. Test burn with PCB-oil in a local cement kiln in Sri Lanka. Chemosphere 2010;78:717–23. [4] Yan DH, Karstensen KH, Huang QF, Wang Q, Cai ML. Co-processing of industrial and hazardous wastes in cement kilns: a review of current status and future need in China. Environ Eng Sci 2010;27:37–45. [5] Karstensen KH, Kinh NK, Thang LB, Viet PH, Tuan ND, Toi DT, et al. Environmentally sound destruction of obsolete pesticides in developing countries using cement kilns. Environ Sci Policy 2006;9:577–86. [6] Yang YF, Yang Y, Huang QF. Release of heavy metals from concrete made with cement from cement kiln co-processing of hazardous wastes in pavement scenarios. Environ Eng Sci 2011;28:35–42. [7] Van der Sloot HA. Developments in evaluating environmental impact from utilization of bulk inert wastes using laboratory leaching tests and field verification. Waste Manage 1996;16:65–81. [8] Van der Sloot HA. Characterization of the leaching behaviour of concrete mortars and of cement–stabilized wastes with different waste loading for long term environmental assessment. Waste Manage 2002;22:181–6. [9] Vespa M, Dahn R, Grolimund D. Speciation of heavy metals in cementstabilized waste forms: a micro-spectroscopic study. J Geochem Explor 2006;88:77–80. [10] Pomies MP, Lequeux N, Boch P. Speciation of Cadmium in cement. Part I. Cd uptake by C–S–H. Cem Concr Res 2001;31:563–9. [11] Pomies MP, Lequeux N, Boch P. Speciation of Cadmium in cement Part II. C3S hydration with Cd solution. Cem Concr Res 2001;31:571–6. [12] Sheidegger AM, Widland E, Sheinost AC. Spectroscopic evidence for the formation of layered Ni–Al double hydroxides in cement. Environ Sci Technol 2000;34:4545–8. [13] Barros AM, Tenório JAS, Espinosa DCR. Evaluation of the incorporation ratio of ZnO, PbO and CdO into cement clinker. J Hazard Mater 2004;B112:71–8. [14] Ract PG, Espinosa DCR, Tenório JAS. Determination of Cu and Ni incorporation ratios in Portland cement clinker. Waste Manage 2003;23:281–5. [15] Murat M, Sorrentino F. Effect of large additions of Cd, Pb, Cr, Zn to cement raw mental on the composition and the properties of the clinker and the cement. Cem Concr Res 1996;26:377–85. [16] Zhang JL, Liu JG, Li C. Comparison of the fixation effects of heavy metals by cement rotary kiln co-processing and cement based solidification/ stabilization. J Hazard Mater 2009;165:1179–85. [17] Karlstuhe. Heavy metals in cement and concrete resulting from the coincineration of wastes in cement kilns with regard to the legitimacy of waste utilization [R]. Germany: Umwelt Bundes Amt; 2005. [18] Lan MZ. The behavior research of heavy metal in cement clinker burning and cement hydration process. Beijing: China Building Material Academy; 2008. [19] Halim CE, Short SA, Scott JA. Modeling the leaching of Pb, Cd, As, and Cr from cementitious waste using PHREEQC. J Hazard Mater 2005;125:45–61. [20] Omotoso OE, Ivey DG, Mikula R. Hexavalent chromium in tricalcium silicate – part II – effects of Cr–VI on the hydration of tricalcium silicate. J Mater Sci 1998;33:515–22. [21] Richardson IG. The nature of the hydration products in hardened cement pastes. Cem Concr Compos 2000;22:97–113. [22] Bussem W. Die Struktur des Tetracalciumaluminatferrits. Fortschr Min 1937;22:31.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Studies on the solidification mechanisms of Ni and Cd in cement clinker during cement kiln co-processing of hazardous wastes Yufei Yang a, Jingchuan Xue b, Qifei Huang a,⇑ a
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Science, Beijing 100012, China Wadsworth Center, New York State Department of Health, and Department of Environmental Health Sciences, School of Public Health, State University of New York at Albany, Empire State Plaza, P. O. Box 509, Albany, NY 12210-0509, United States b
h i g h l i g h t s Cd concentrated in Cao phases but distributed mainly in C3S phases. Ni was mainly formed MgNiO2, also present in C4AF and C3S. Cd and Ni solidified by isomorphous replacement and formation of a new or interstitial solid solution. Ca–Cd–O sosoloid and Ni–Mg compound were formed in solidification. Cd and Ni incorporated in C3S by substituting Ca and in C4AF by inter lattice space.
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 20 December 2013 Accepted 24 December 2013 Available online 25 February 2014 Keywords: Cd Ni Solidification mechanism Cement clinker Cement kiln co-processing Microanalysis
a b s t r a c t To clarify the solidification mechanisms of heavy metals in cement clinker during the cement kiln co-processing of hazardous wastes, cement clinker samples were produced. EPMA1 and XANES2 spectroscopy were employed to acquire information on the microstructure and the Cd and Ni species formed in the cement system. The average Cd concentrations in CaO, C4AF3 and C3S4 were 3.92%, 1.21% and 0.75%, respectively, but Cd was mostly present in C3S in clinker (71.0%). The solidification methods of Cd during the calcination process included the formation of a Ca–Cd–O sosoloid, substitution for Ca in C3S and C4AF (isomorphous replacement) and inter the space of the lattice (interstitial solid solution). Ni was combined mainly with Mg in the form of a new Ni–Mg compound (distribution ratio of 61.2%), which was MgNiO2, as confirmed by XANES analysis. Ni was also present in C3S and C4AF (distribution ratios of 24.9% and 10.3%, respectively) by replacement of Ca2+ in C3S and substituting Fe in C4AF (isomorphous replacement), and also by inter the spaces of the C4AF lattice (interstitial solid solution). Ó 2014 Published by Elsevier Ltd.
1. Introduction Cement kilns possess inherent features that suit the treatment of hazardous wastes. These features include high temperatures and long residence times (greater than 1200 °C for several
Abbreviations: EPMA, electron probe microanalysis; XANES, X-ray absorption near-edge structure; C4AF, tetracalcium aluminoferrite; C3S, tricalcium silicate; C2S, bicalcium silicate; C–S–H, calcium silicate hydrate. ⇑ Corresponding author at: Chinese Research Academy of Environmental Science, Beijing 100012, China. Tel.: +86 10 84915142; fax: +86 10 8491 3903. E-mail addresses: [email protected] (Y. Yang), [email protected] (J. Xue), [email protected] (Q. Huang). 1 Electron probe microanalysis. 2 X-ray absorption near-edge structure. 3 Tetracalcium aluminoferrite. 4 Tricalcium silicate. http://dx.doi.org/10.1016/j.conbuildmat.2013.12.081 0950-0618/Ó 2014 Published by Elsevier Ltd.
seconds), surplus oxygen during and after combustion, good turbulence and mixing conditions, and no generation of by-products such as slag, ashes, or liquid residues [1–3]. The co-processing of waste using cement kilns has been widely and successfully employed in the United States, Europe, Japan and other developed countries for several decades, and more recently in developing countries [4,5]. As far as wastes containing heavy metals are concerned, heavy metals are discharged to the atmosphere through gaseous emissions, but in the main, the metals reside in the cement clinker. Studies have indicated that the leaching rate of heavy metals in cement derived from the co-processing of waste is low [6–8]. The key reason for this is that the heavy metals in cement clinker and cement products have been solidified and are relatively inert to leaching. The solidification of heavy metals in cement kiln co-processing wastes typically occurs by two processes, cement hydration and
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calcination. There has been much research on the solidification theory of heavy metals as a result of hydration [9]. In the cement hydration process, Cd is solidified by the formation of a mixed Ca/Cd hydroxide or Cd undergoes an exchange with Ca in a C–S– H gel [10,11]. Vespa et al. used unhydrated cement to condition a nitrate solution containing Ni. Results of X-ray absorption spectroscopy and scanning electron microscopy revealed that the main form of Ni in the cement-based solidified/stabilized wastes was a mixture of a Ni–Al layered double hydroxide phase and Ni (OH)2 [9]. Sheidegger et al. also demonstrated that the formation of Niand Al-containing hydrotalcite-like layered double hydroxides is a relevant binding mechanism for Ni in cement [12]. Currently, several solidification effects of heavy metals in cement clinker have been reported. Barros studied the incorporation ratio of ZnO, PbO and CdO when the heavy metals were added to clinker raw material [13]. Additionally, when a galvanic sludge containing Cu and Ni was added to clinker raw material, at a ratio of 2.4 wt% and 1.2 wt%, these metals were totally incorporated into the clinker [14]. There have been relatively few studies on the solidification mechanism of heavy metal in cement clinker for the calcination process. Murat studied the effect of large additions of heavy metals to cement raw material on the composition and properties of the cement clinker. The results showed that Cd2+ enters the C3S lattice by replacing Ca2+ in C3S, Pb forms PbSO4 and dissolves in C3S and C2S, and Cr is mainly distributed in C2S [15]. Investigations by Zhang showed that, in the case of As, Cd, Pb and Zn, for calcinations in a cement rotary kiln and then hydration during cement application, the metals were more effectively fixed than in the case of cement-based solidification [16]. Clearly, the different retention mechanisms for co-processing and cement hydration result in different fixation effects for the heavy metals. As can be seen, the physical and chemical changes for heavy metals that occur in the calcination processes have not been fully characterized. The purpose of this study was to clarify the solidification mechanisms of heavy metals in cement clinker during the cement kiln co-processing of hazardous wastes. Experiments were conducted for the simulation of the high-temperature calcination of cement raw material with the heavy metals of Cd and Ni. Microanalytical methods including electron probe microanalysis (EPMA) and Xray absorption near-edge structure (XANES) analysis were employed to obtain information on microstructure and the species of heavy metals formed in the cement clinker. 2. Materials and methods
requirements for EPMA and XANES analysis. Heavy-metal compounds were thoroughly mixed with the cement raw materials in accordance with the ratios listed in Table 2. The table also gives the total contents of heavy metals in mixed raw materials. Each mixture was calcined in the furnace at 1450 °C for 1 h to prepare the clinker, which was then smashed and milled to a powder having specific surface area of 310 m2/kg. 2.3. EPMA-EDS The prepared clinker powder was compacted to clinker flake (thickness, 10 mm) using a press with pressure of 8 t/cm2. The head of the press was polished to ensure that the clinker flake surface was smooth to facilitate EPMA. Energy dispersive spectrometry (EDS) was employed to determine the elemental content. A JEOL JXA8800R EPMA analyzer and an Oxford ISIS300 X-ray energy spectrometer were used for analyses. The high voltage employed was 15 keV and the electron beam current employed was 2 108 A. Backscattered-electron images and X-ray images (elemental distribution images) were acquired. Standard samples of oxides and silicates were used for quality control of EPMA measurements. The overall features of the clinker mineral phase were first observed with EPMA under low-power magnification (scale of 200 lm), and then typical mineral phases were measured under medium-power magnification (scale of 50–100 lm) in surface, line and point mode. Some mineral phases with fine particles were transferred to the high-power lens (scale of 20 lm) for further measurement. 2.4. XANES measurements Cd and Ni K-edge XANES spectra were collected at the Beijing Synchrotron Radiation Facility. The typical energy of the storage ring was 2.5 GeV with the current decreasing from 250 to 160 mA during runs. Spectra for standard samples of CdO, CdSO4, CdCO3, CdCl2–2.5H2O, NiO, Ni2O3, Ni(OH)2, NiCO3–Ni(OH)2–6H2O, NiSO4– 6H2O and NiCl2–4H2O were recorded in transmission mode. Clinker samples were measured in fluorescence mode. All standards were ground to fine grains and pressed to form wafers (diameter, 1.0 mm). Data processing of the XANES spectra was performed with FEFF8.0 software [17].
3. Results and discussion 3.1. Solidification process The solidification of a heavy metal by cement clinker may be characterized by the fixation ration, which is expressed as
Table 2 Addition of heavy-metal compounds and heavy-metal content in mixture. Items
Cd
Ni
Heavy metal compound Heavy metal addition ratio (%) Amount added (g/kg) Amount of raw material (kg) Heavy metal content (mg/kg)
CdO 1 22.84 1.98 10,012
Ni2O3 1 28.18 1.97 10,027
2.1. Characterization of raw materials The cement raw materials, including limestone, clay and iron powder, were supplied by a local cement plant. The chemical compositions of the raw materials are listed in Table 1.
2.2. Sample preparation The clinker was made in a laboratory furnace using industrial raw materials and heavy-metal compounds. Limestone, clay and iron powder were thoroughly mixed at a weight ratio of 76.5:21.0:2.5 and homogenized. The addition of heavy metals was based on considerations of current industrial practice and measurement
Table 1 Chemical compositions of raw materials (%). Material
SiO2
Fe2O3
Al2O3
CaO
MgO
Cd (mg/kg)
Ni (mg/kg)
Limestone Clay Iron powder
0.18 62.14 34.98
0.04 9.10 50.56
0.04 16.17 4.95
55.64 1.63 1.34
0.05 0.00 0.84
6.13 35.0 9.95
27.9 26.6 27.1 Fig. 1. Morphology of cement clinker with sites for analysis.
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Fig. 2. EDS spectra for mineral phases at the seven locations.
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G ¼ MK
1 Loss 100%; Ms
ð1Þ
where G (%) is the fixation ration; Mk (mg/kg) represents the Cd and Ni contents of the clinker, which are 8636 and 13,359 mg/kg, respectively; Ms is the heavy metal content of the mixture (mg/kg including raw material, shown in Table 2) and Loss is the fraction of mass loss on ignition of the mixture, which is 0.3485. The ratio of fixation of Ni by the clinker in the experiment was 86.8%, which was slightly lower than that in an industrial process, where the fixation ration ranges from 87% to 97% [18]. Compared with the case for Ni, the fixation ratio for Cd was lower at 56.2%. This fixation ratio for Cd was also lower than that in industry processes, which typically ranges from 74% to 88% [18]. The difference in fixation ratio between Cd and Ni mainly depends on element volatility, with Cd being more volatile than Ni. The fixation ratios for Cd and Ni in the tests were lower than those in industrial processes for the following reasons. In an industrial kiln, the convection of gas and raw materials creates a strong turbulent atmosphere, which is conducive to reducing elemental volatility. There are also strong oxidizing conditions in the industrial kiln, and the concentration of dust in the gaseous atmosphere is much higher. All of these factors contribute to an enhanced absorption of acidic gases in the exhaust, thereby reducing the volatility of some elements and improving the condensation temperature. In contrast, for the laboratory experiment, the raw material was static, which reduces the probability of alkaline oxides being in gaseous contact with the volatilized heavy metals, which would otherwise increase element volatility.
Table 3 Cd and Ni concentrations in main mineral phase (%). Analysis site
C3S
C2S
C4AF
CdO
NiO
CdO
NiO
CdO
NiO
1 2 3 4 5 6 7 8 9 10 Average
1.04 0.73 0.77 0.65 0.68 0.33 0.75 0.89 1.02 1.08 0.79
0.51 0.52 0.16 0.31 0.30 0.34 0.52 0.44 0.51 0.28 0.39
0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00
0.00 0.07 0.05 0.00 0.09 0.05 0.05 0.00 0.05 0.08 0.04
1.47 1.24 1.20 1.30 1.19 1.14 0.60 1.47 1.24 1.20 1.21
1.42 0.90 0.72 1.16 1.06 1.29 1.82 1.42 0.90 0.72 1.14
Table 4 Cd and Ni concentrations in minor mineral phase (%). Analysis site
1 2 3 4 5 6 7 8 9 10 Average
CaO
Ni–Mg
CaO
CdO
NiO
MgO
CdO
NiO
78.62 76.26 84.91 73.40 58.41 63.06 63.49 76.52 65.83 80.23 70.1
7.97 6.00 1.43 4.96 2.50 2.45 2.50 6.00 3.55 1.83 3.92
0.32 0.39 0.25 0.27 0.80 0.04 0.80 0.45 0.20 0.33 0.39
50.48 39.40 60.50 53.44 44.05 37.36 51.35 45.20 53.20 55.42 49.0
0.00 0.03 0.08 0.06 0.00 0.00 0.00 0.05 0.05 0.00 0.03
40.54 39.20 31.66 31.97 35.30 33.70 35.46 36.80 33.20 32.70 35.1
3.2. Mode of occurrence of Cd and Ni in cement clinker EPMA was first employed to determine which elements were present in the cement clinker phases, and from the contents and ratios of elements, the specific cement clinker minerals were identified. Typical features of the cement clinker are illustrated in Fig. 1 (scale of 200 lm). A total of 49 points were set in the scanning area, and element contents at each point were determined by EDS. From the results of EDS, seven analysis points (No. 1 to No. 7) were selected (Fig. 1), where the element contents and ratios at each point conformed to the elemental composition characteristics of C3S, C2S, a Fe–Ca phase, C4AF, CaO, SiO2 and Ni–Mg, respectively. The corresponding EDS spectra for the seven points are presented in Fig. 2. To determine the Cd and Ni concentrations in the mineral phases at each location, a point-analysis routine was employed at high magnification (scale of 20 lm) and with scanning of 10 sampling points per location. The results for Cd and Ni in the main minerals and some minor mineral phases are presented in Tables 3 and 4. In contrast to the case for C2S, which was the main mineral, both C3S and C4AF contained Cd. The Cd concentration was much higher in the CaO phase, with the average concentration being 3.92% (range, 1.83–7.97%) (Table 3). The Cd concentration in C4AF (average concentration, 1.21%; range, 0.60–1.47%) was higher than that in C3S (average concentration, 0.75%; range, 0.33–1.08%) (Table 4). These findings differ from those of Karlstuhe [17], where Cd was not incorporated into the main clinker but existed in a minor phase (CaO). The Cd total content and distribution ratios in mineral phase were different (Table 5) as a result of the content of the main mineral phase being much higher than that of the minor mineral phases in cement clinker. The total content of CdO was 0.4835% in C3S, while it was 0.118% in C4AF and 0.0789% in f-CaO, and the corresponding distribution ratios were 71.0%, 17.4% and 11.5%, respectively.
Table 5 Cd and Ni total contents and distribution ratios in mineral phase (%). Material
C3S
C2S
C4AF
C3A
f-CaO
Ni–Mg
Other
Content in clinker Total CdO content Total NiO content Cd
61.2
21.5
9.80
4.00
2.00
1.20
0.30
0.4835
–
0.1186
–
0.0786
0.0004
–
0.1714
0.0172
0.0706
–
0.0078
0.4212
–
distribution ratio71.0–17.4–11.50.1–Ni distribution ratio24.92.5010.3– 1.1061.2–Note: ‘‘–’’means not detected.
There was good correlation between Ni and Mg concentrations, indicating that Ni and Mg formed a new Ni–Mg phase in the clinker. Lan observed a new mineral phase in addition to the main mineral phases of C3S and C2S in Ni-containing clinker [18]. The interplanar spacing (d) of this new mineral phase was 2.0917 and the diffraction angle was 43.217°. These crystal structural parameters are consistent with those of MgNiO2. Other reports have shown that when the contents of Ni and Mg in cement clinker are relatively high, MgNiO2 forms in the clinker process [19–21]. The total content of NiO in the Ni–Mg phase was 0.4212%, and the corresponding distribution ratio was 61.2.0%. Ni existed in C3S and C4AF phases besides the Ni–Mg phase, and the corresponding distribution ratios were 24.9% and 10.3%, respectively. 3.3. Species of heavy metals in cement clinker K-edge XANES spectra for Cd and Ni in cement clinker and for standard samples of Cd and Ni are presented in Figs. 3 and 4, respectively.
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Fig. 3. K-edge XANES spectra of Cd in clinker and in standard samples.
Fig. 4. K-edge XANES spectra of Ni in clinker and in standard samples.
The spectra for Cd in cement clinker were quite consistent with those for CdO samples. It was shown that Cd existed in the +2 form, and the ligand forms of Cd in clinker were similar to those of Cd in CdO. The spectra of Ni in cement clinker were consistent with those for NiO and Ni(OH)2 samples. These findings suggest that chemical changes in Ni in the cement raw materials had taken place during calcination at high temperature and that new chemical compounds formed. The valence of Ni in the new compound was +2, and the ligand forms of Ni in the clinker were similar to those of Ni in NiO and Ni(OH)2. There were six O
atoms around the Ni atoms in NiO and Ni(OH)2, and the Ni–O bond lengths in both NiO and Ni(OH)2 were 2.1 Å. It was established that the coordination number of Ni in the new Ni–Mg compounds was six, and the bond length was 2 Å. It can thus be inferred that the new Ni–Mg compound was MgNiO2, which further validates the EPMA data. 3.4. Solidification mechanisms of Ni and Cd in cement clinker According to the EPMA results, Cd existed mainly in the CaO phases and the spectra for Cd in the cement clinker were consistent
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with those for CdO samples. All these findings indicate that Ca in CaO was substituted by Cd to form a Ca–Cd–O sosoloid during high-temperature calcination. In addition, the ionic radius of Cd2+ (95 Å) is close to that of Ca2+ (100 Å) and the respective electronegativities are also of similar magnitude (Cd, 1.45 versus Ca, 1.05). Therefore, given the similar ionic characteristics of Cd2+ and Ca2+, it would be relatively easy for Cd to replace Ca in CaO to form a Ca–Cd–O sosoloid. In addition, the EPMA results show that Cd was distributed in the minerals C4AF and C3S. According to the theory of Bussem [22], there are many cavities within the C4AF crystal structure as a result of the layered architecture structure of tetrahedral and octahedral elements, and therefore Cd and Ni tend to be interred and located in the spaces of C4AF lattice and form an interstitial solid solution. Furthermore, Fe3+ and Al3+, which occupy the tetrahedral and octahedral positions of the C4AF lattice, respectively, tend to be substituted by ions of a similar valence and structure. Moreover, Cd may replace Ca in C4AF. In addition, a small amount of Cd may be incorporated into C3S, which has a dense structure, by replacement of Ca. The EPMA results demonstrate that a new Ni–Mg compound formed in clinker, and XANES results suggest this compound to be MgNiO2. Thus, Ni was solidified mainly as an inert species of MgNiO2. Ni was also found in the other main mineral phases (C3S and C4AF) of clinker. This result indicates that Ni did not fully react with Mg, as evidenced by Ni in clinker existing partly in the +3 form. The Ni3+ form was incorporated into the clinker lattice during calcination. The Ni3+ incorporated into the clinker lattice may be of two types. One type is where the Fe3+ in C4AF is replaced by Ni3+ because the difference in radius between Ni3+ and Fe3+ is less than 15%, and the respective electronegativities are quite similar (1.91 and 1.83 respectively). The other type is where Ni3+ enters the interspaces of C4AF where many cavities exist within the crystal structure. As the coordination numbers of both Ni3+ and Ca2+ are six, and in the case of a mineral with high calcium content, Ni3+ may replace Ca2+ in C3S. 4. Conclusions In the processing of hazardous wastes using a cement kiln, the volatility of heavy metals and the firing condition of the kiln affect heavy-metal solidification. This study found the solidification effect of Cd in cement clinker to be lower than that of Ni. The solidification mechanisms for Cd and Ni during cement kiln coprocessing of hazardous wastes were of three types. One was the formation of a new sosoloid. Substitution of the Ca in CaO to form a Ca–Cd–O sosoloid was the main mechanism for Cd solidification in cement clinker. In the case of Ni, the metal tended to form a new inert compound, MgNiO2. A second process was the formation of an interstitial solid solution; Cd and Ni could partly inter and be sited in the spaces of the C4AF lattice. The third type was isomorphous replacement, whereby a small number of Cd2+ and Ni2+ ions substitute for Ca and Fe in C4AF, respectively, and are incorporated into the C3S crystal lattice by replacement of Ca2+.
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Acknowledgments This research was jointly supported by the National Natural Science Foundation of China (51178439) and the National High-tech R&D Program of China (SS2012AA063401). We thank the staff on beamline 1W1B at the Beijing Synchrotron Radiation Facility and on beamline 14W1 at the Shanghai Synchrotron Radiation Facility. References [1] Mokrzycki E, Uliasz-Boche czyk A, Sarna M. Use of alternative fuels in the Polish cement industry. Appl Energy 2003;74:101–11. [2] Karstensen KH. Formation, release and control of PCDD/PCDF in cement kilns – a review. Chemosphere 2008;70:543–60. [3] Karstensen KH, Mubarak AM, Gunadasa HN. Test burn with PCB-oil in a local cement kiln in Sri Lanka. Chemosphere 2010;78:717–23. [4] Yan DH, Karstensen KH, Huang QF, Wang Q, Cai ML. Co-processing of industrial and hazardous wastes in cement kilns: a review of current status and future need in China. Environ Eng Sci 2010;27:37–45. [5] Karstensen KH, Kinh NK, Thang LB, Viet PH, Tuan ND, Toi DT, et al. Environmentally sound destruction of obsolete pesticides in developing countries using cement kilns. Environ Sci Policy 2006;9:577–86. [6] Yang YF, Yang Y, Huang QF. Release of heavy metals from concrete made with cement from cement kiln co-processing of hazardous wastes in pavement scenarios. Environ Eng Sci 2011;28:35–42. [7] Van der Sloot HA. Developments in evaluating environmental impact from utilization of bulk inert wastes using laboratory leaching tests and field verification. Waste Manage 1996;16:65–81. [8] Van der Sloot HA. Characterization of the leaching behaviour of concrete mortars and of cement–stabilized wastes with different waste loading for long term environmental assessment. Waste Manage 2002;22:181–6. [9] Vespa M, Dahn R, Grolimund D. Speciation of heavy metals in cementstabilized waste forms: a micro-spectroscopic study. J Geochem Explor 2006;88:77–80. [10] Pomies MP, Lequeux N, Boch P. Speciation of Cadmium in cement. Part I. Cd uptake by C–S–H. Cem Concr Res 2001;31:563–9. [11] Pomies MP, Lequeux N, Boch P. Speciation of Cadmium in cement Part II. C3S hydration with Cd solution. Cem Concr Res 2001;31:571–6. [12] Sheidegger AM, Widland E, Sheinost AC. Spectroscopic evidence for the formation of layered Ni–Al double hydroxides in cement. Environ Sci Technol 2000;34:4545–8. [13] Barros AM, Tenório JAS, Espinosa DCR. Evaluation of the incorporation ratio of ZnO, PbO and CdO into cement clinker. J Hazard Mater 2004;B112:71–8. [14] Ract PG, Espinosa DCR, Tenório JAS. Determination of Cu and Ni incorporation ratios in Portland cement clinker. Waste Manage 2003;23:281–5. [15] Murat M, Sorrentino F. Effect of large additions of Cd, Pb, Cr, Zn to cement raw mental on the composition and the properties of the clinker and the cement. Cem Concr Res 1996;26:377–85. [16] Zhang JL, Liu JG, Li C. Comparison of the fixation effects of heavy metals by cement rotary kiln co-processing and cement based solidification/ stabilization. J Hazard Mater 2009;165:1179–85. [17] Karlstuhe. Heavy metals in cement and concrete resulting from the coincineration of wastes in cement kilns with regard to the legitimacy of waste utilization [R]. Germany: Umwelt Bundes Amt; 2005. [18] Lan MZ. The behavior research of heavy metal in cement clinker burning and cement hydration process. Beijing: China Building Material Academy; 2008. [19] Halim CE, Short SA, Scott JA. Modeling the leaching of Pb, Cd, As, and Cr from cementitious waste using PHREEQC. J Hazard Mater 2005;125:45–61. [20] Omotoso OE, Ivey DG, Mikula R. Hexavalent chromium in tricalcium silicate – part II – effects of Cr–VI on the hydration of tricalcium silicate. J Mater Sci 1998;33:515–22. [21] Richardson IG. The nature of the hydration products in hardened cement pastes. Cem Concr Compos 2000;22:97–113. [22] Bussem W. Die Struktur des Tetracalciumaluminatferrits. Fortschr Min 1937;22:31.