Effects of Sulfur on the Solidification of Cadmium during Clinkerization

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Effects of sulfur on the solidification of cadmium during clinkerization Bin Zhang, Jiangxiong Wei, Zhengxiang Zeng, Weiting Xu, and Qijun Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01970 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Sustainable Chemistry & Engineering

Effects of sulfur on the solidification of cadmium during clinkerization

Bin Zhang1, Jiangxiong Wei1*, Zhengxiang Zeng1, Weiting Xu1, Qijun Yu1

1. School of Materials Science and Engineering, South China University of Technology, Wushan Road 381, Tianhe District, 510640, Guangzhou, China.

ABSTRACT The solidification of heavy metals during clinkerization is very important to the utilization of solid wastes in cement kiln from the sustainable development and environmental perspective. In this study, sulfur, which commonly contained in solid waste and cement raw materials, was taken into consideration to investigate its effects on the Cd solidification and mineral phases formation during the clinker production. Cement clinkers and tricalcium silicate (C3S) doped with CdO and sulfur were prepared, and sulfur effects on the solidification of Cd, as well as its mechanism, during clinkerization were investigated. The solidification of Cd was determined by atomic absorption spectroscopy and energy dispersive spectroscopy, indicating that the solidification of Cd increased with increasing sulfur content (up to 2 wt.%) at 1450°C. Higher calcination temperature decreased the solidification ratio of Cd. Cd was mainly concentrated in C3S, aluminate (C3A), and ferrite (C4AF). With increasing

*

Corresponding author: [email protected], Tel.: +86 020 8711 4137

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the content of sulfur to 2.0 wt.%, the solidified Cd content decreased by 42% in C3S, and increased by 40% and 26% in C3A and C4AF, respectively. The X-ray photoelectron spectroscopy confirmed the presence of Cd–O bond in the clinker, attributing to the lattice substitution of Cd2+ to Ca2+. The X-ray diffraction analyses proved the transformation of C3S from T to R polymorph, decreasing the solidification ability of C3S to Cd with the addition of sulfur. The results of this study could provide a guideline for decreasing the emission of Cd by adding appropriate amount of sulfate or sulfide in cement raw meal during the production of clinker in cement kiln.

Keywords: cadmium; sulfur; clinkerization; solidification; tricalcium silicate

Introduction Resource conservation, energy savings, and environmental protection without compromising natural resources are the main ways to achieve sustainable development. As a representative example, Portland cement used worldwide is a basic ingredient of concrete, and its current production requires large consumption of natural resources and energy, and therefore seems unsustainable.1 Meanwhile, more than 3.5 billion tons of solid wastes (SW), such as industrial SW, municipal SW, and sewage sludge, are generated annually in China,2 occupying much potential farmland and posing serious environmental problems. Successful application of such types of waste can lead to sustainable development and protect our environment to some


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extent. Therefore, taking the SW as raw fuels and/or cement raw materials in cement kiln, namely the co-processing of SW, is an efficient way of utilizing resources and sustainable development.3–5 Nevertheless, heavy metals (HM) present in the SW should be paid significant attention during the clinkerization process, which is very important to the scalable applications of co-processing of SW in cement industry.6–9 As one of highly toxic HM, Cd is easier to volatilize than other HM (such as Cu and Cr) during the combustion of SW.8,10 The volatilization temperature of Cd depends on the Cd compounds (e.g. CdCl2, ∼560°C; CdO, ∼1050°C). Nowak et al.10 reported that the volatilization ratio of Cd was ∼75% during heat treatment of municipal SW at 900°C for 45 min. Besides, Cd is considered as a carcinogenic substance, which blocks the body’s ability to absorb zinc in the long-term.11 Moreover, Cd is widely used in the manufacturing of batteries and plastics.12 The content of Cd in some battery and plastic wastes is about 0.01–15% and 0.001–0.1%, respectively.12,13 Cement kiln may introduce more Cd primarily through the co-processing of SW such as battery and plastic wastes. During the cement production process, part of Cd not solidified in the clinker may exhaust into the atmosphere, seriously affecting cement production and environment.7 Therefore, studying the solidification of Cd during clinkerization is particularly important for the co-processing of Cd-containing SW in cement industry. The solidification of Cd during clinkerization has been previously studied. Barros et al.14 reported that Cd in the form of CdO completely solidified in clinker with the addition of 0.05 wt.% CdO in the cement raw meal, while the Cd solidification ratio decreased to 50% with



increasing CdO to 1.00 wt.%. Wang et al.15 found that the solidification ratio of CdO reached 25.09% with the addition of 2.00 wt.% Cd in cement raw meal, and the content of Cd in mineral phases was different. In addition to these literature reports, sulfur, which comes from SW (e.g., incinerator ash, industrial SW, tyre, and plastic bag with CaSO4, CaS, Na2SO4, Na2S, elemental sulfur, et al.) combustion,16–20 could strongly affect the Cd emission during the heat treatment of SW.21,22 Besides, some researchers investigated the effects of sulfur on the clinkering process. Kolovos et al.23 reported that the incorporation of sulfur in the raw materials for cement could promote the formation of the liquid phase and accelerate the solid-state reaction. Horkoss et al.24 also confirmed that the content of tricalcium silicate (C3S) decreased during clinkerization with the addition of sulfur. Although the solidification of Cd during clinker formation has been studied, the sulfur effects on the solidification of Cd are still not well understood. Besides, the mechanism of sulfur effects on the solidification of Cd should be addressed further. In our work, sulfur was taken into consideration to investigate its effects on the solidification of Cd and compositions of the clinker, which was more corresponding to the co-processing conditions of SW during the clinker production.

Materials and method Materials Pure chemical reagent of CaCO3, Al2O3, SiO2, and Fe2O3 (Shengtai chemical reagent Co. Ltd., Jiangxi, China) were used as cement raw materials. Cd was


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introduced to the cement raw meal as CdO. CaSO4·2H2O, CaS, and elemental sulfur were introduced as sulfur sources.

Experimental The main mineral compositions of the clinker are C3S, dicalcium silicate (C2S), calcium aluminate (C3A) and alumino-ferrite (C4AF), which were calculated to be 56.1%, 22.6%, 8.6%, and 11.3%, respectively, according to the Bogue method25 (Eq. (1) – (3)) in this work. Cement raw meal with 2.0 wt.% CdO was prepared, with varying sulfur contents such as 0.5%, 1.0%, 1.5%, and 2.0% by the mass of the raw meal. The composition of the formulated mixtures is listed in Table 1. ‫ ݋݅ݐܽݎ ݊݋݅ݐܽݎݑݐܽݏ ݁݉݅ܮ‬ሺ‫ܪܭ‬ሻ =

݈ܵ݅݅ܿܽ ‫ ݋݅ݐܽݎ‬ሺܵ‫ܯ‬ሻ =

‫ ܱܽܥ‬− 1.65‫݈ܣ‬2 ܱ3 − 0.35‫݁ܨ‬2 ܱ3 = 0.9 2.8ܱܵ݅2

ܱܵ݅2 = 2.5 ‫݈ܣ‬2 ܱ3 + ‫݁ܨ‬2 ܱ3

‫ ݋݅ݐܽݎ ܽ݊݅݉ݑ݈ܣ‬ሺ‫ܯܫ‬ሻ =

‫݈ܣ‬2 ܱ3 = 1.6 ‫݁ܨ‬2 ܱ3

(1)

(2)

(3)

Table 1 Mixtures with CdO and different species of sulfur. Sample A0/B0/C0 A0.5/B0.5/C0.5 A1.0/B1.0/C1.0 A1.5/B1.5/C1.5 A2.0/B2.0/C2.0

CdO (wt.%) 2.0 2.0 2.0 2.0 2.0

CaSO4·2H2O/CaS/Elemental sulfur (S: wt.%) 0 0.5 1.0 1.5 2.0

Note: A, B, and C are the samples added with CaSO4·2H2O, CaS, and elemental sulfur, respectively.

The as-prepared raw materials were thoroughly blended in a ball mill to form ACS Paragon Plus Environment


homogeneous mixtures. Then, pellets (50 g, Ø 50 mm) were shaped from the mixtures (added with 8% absolute ethanol) under 40 MPa in a steel mold. The prepared pellets were heated to the target temperature in a high-temperature electric furnace. After reaching the designated temperature, the furnace was maintained at that temperature for 45 min. Finally, the pellets were taken out and quickly cooled with a fan to the room temperature.25 C3S doped with Cd and/or CaSO4·2H2O was prepared as follows: stoichiometric amounts of CaCO3 and SiO2 (mole ratio = 3:1) were ground and passed through an ASTM 180 mesh sieve. CdO and/or CaSO4·2H2O were added at 2.0 wt.% by the mass of the raw material, and the mixtures were thoroughly blended in a ball mill to form homogeneous mixtures. After that, pellets (50 g, Ø 50 mm) were shaped from the mixtures (added with 8% absolute ethanol) under 40 MPa in a steel mold. The prepared pellets were heated to 1500°C in a high-temperature electric furnace and then maintained at 1500°C for 3 h. Finally, the pellets were taken out and quickly cooled with a fan to the room temperature. Each sample was calcined thrice.

Analytical methods The prepared samples were ground and passed through an ASTM 180 mesh sieve. Then, 0.5 g of the powder was mixed with deionized water (2 ml), hydrogen fluoride (3 ml), aqua regia (10 ml), hydrogen peroxide (3 ml), and perchloric acid (5 ml), then digested in a graphite digestion instrument (ZEROM, China) at 180°C for 60 min. After that, the dissolved samples were diluted with nitric acid (5 vol.%) to 200 ml.


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ContrAA 700 high-resolution atomic absorption spectrometer (Analytik Jena AG, Germany) equipped with an acetylene-air burner (10 cm slit) was used for the determination of Cd. The measurement was conducted for twice and the mean value was used to calculate the solidification ratio by Eq. (4): ܴ=

‫ܭ‬ × 100% ܵ/ሺ1 − ‫ܮܱܮ‬ሻ

(4)

where R and LOI represent the Cd solidification ratio and loss on ignition of the sample, respectively. S and K (mg·kg-1) are the content of Cd in cement raw meal and prepared samples, respectively. Thermogravimetric (TG) curves were recorded using a thermogravimetry analyzer of NETZSCH STA 449F3. 20±1.0 mg of the samples were heated up from 30°C to 1250°C (for CaSO4·2H2O and CaS - up to 1400°C) at a heating rate of 10°C/min in 80%N2/20%O2 atmospheres with a flow rate of 25 ml/min. The chemical state of Cd in the clinker was investigated by X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi) instrument equipped with a monochromatic Al K Alpha (1486 eV) X-ray source (40 W, 15 kV). The binding energies in XPS spectra were calibrated using the C (1 s) carbon peak appearing at 284.8 eV. The prepared clinkers were fixed by epoxy resin and then polished by Tegramin-25 polishing instrument (Struers, Denmark) with absolute alcohol. The polishing process was conducted on the following sandpapers: #250-mesh (2min), #800-mesh (5min), #1200-mesh (10min), #2000-mesh (15min), and #4000-mesh (20min). Electron probe micro-analysis (EPMA-1600, Japan) was used to determine



the distributions of Cd, S, Si, Al, and Fe in the mineral phases of clinker. The electron beam diameter of the analysis was 1µm with an acceleration voltage of 15.0 kV. The detection limit of EPMA for Cd was 0.1 wt.%. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS) analyses were conducted using an EVO 18 Carl Zeiss SEM and an Oxford Instrument INCAx-sigth EDS-system to determine the content of Cd in C3S, C2S, C3A and C4AF. The detection limit of EDS for Cd was 0.1 wt.%. Fifty points were analyzed to calculate the average content of Cd in each phase of the clinker. X-ray diffraction (XRD) patterns (2θ, 5–70°) of the clinker and C3S were collected on an X-ray diffractometer (Bruker D8) at a step size of 0.013° with Cu Kα radiation (λ =1.5406 Å, 40 mA, and 40 kV). The peak fitting was conducted by PEAKFIT software. With the background was removed, fitting was performed by a minimum number of Lorentz–Gauss cross-product function bands. The Lorentz– Gauss ratio was maintained at values greater than 0.7 and fitting coefficients of all the corresponding peaks were >0.9998.

Results and discussion Effects of temperature and sulfur on the solidification of Cd The Cd solidification results with increasing content of CaSO4·2H2O, CaS, and element sulfur at each temperature were shown in Fig. 1. Without sulfur addition, Cd solidification ratio decreased from 80.1% at 950°C to 40.6% at 1450°C. Besides, Cd solidification ratio decreased ∼10% with the content of CaSO4·2H2O or CaS increased


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to 2 wt.% at 950, 1050, 1150, 1250, and 1350°C. However, at 1450°C, the solidification ratio increased by 22% with increasing the content of CaSO4·2H2O or CaS to 2 wt.%, compared to that of the reference sample (Fig. 1(a, b)).

Fig. 1. Solidification of Cd in clinker with sulfur at different temperatures: a) CaSO4·2H2O; b) CaS; c) Elemental sulfur.

Sulfur plays the role as mineralizer in the cement raw meal, which decrease the surface tension and viscosity of the liquid phase,24,26,27 and promote the migration of Cd-bearing species. These effects will benefit the reaction between the interstitial phases and Cd. Besides, Kolovos et al.28 investigated the influence of CaSO4 and CaS on the clinkerization process by TGA up to 1500°C. The results showed that main



stage of silicate phase formation was shifted to lower temperatures with the addition of 1.0 wt.% CaSO4 or CaS. This effect will promote the solidification of Cd by silicate phase during clinkerization. Consequently, clinker might solidify more Cd and the solidification ratio of Cd increased at 1450°C. The TG results (Fig. 2) show that CdO starts volatilizing at 1070°C and the volatilization temperature decreased to 1035°C and 1033°C with adding CaSO4·2H2O and CaS, respectively. Elemental sulfur had no effect on the volatilization temperature of CdO. Therefore, CaSO4·2H2O/CaS had a negative effect and elemental sulfur had no effect on the solidification of Cd at lower temperatures, as confirmed from Fig. 1.

Fig. 2. TG curves of CdO with/without sulfur. a) CdO with/without CaSO4·2H2O; b) CdO with/without CaS; c) CdO with/without elemental sulfur (S).


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Besides, CaS had a similar effect on the solidification of Cd to that of CaSO4·2H2O (Fig. 1(b)), probably because CaS will be oxidized to CaSO4 following the chemical reaction of equation (5) at ∼700°C.29,30

‫ ܵܽܥ‬+ 2ܱ2 ሺ݃ሻ = ‫ܱܵܽܥ‬4

(5)

In addition, the addition of elemental sulfur has no effect on the solidification of Cd (Fig. 1(c)). The reason is that elemental sulfur oxidizes to SO2 at ∼180°C, and SO2 produced will volatilize with increasing temperature.31 Moreover, the point chemical analysis result showed that sulfur cannot be detected by EDS in the sample added with elemental sulfur, because almost all the elemental sulfur volatilized with increasing temperature. Accordingly, although CaSO4·2H2O and CaS decreased the solidification ratio of Cd slightly when temperatures

Effects of Sulfur on the Solidification of Cadmium during Clinkerization

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