• 1School of Environmental Science and
Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou, China 2CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China 3Shanxi Key Laboratory for Gold and Resources, School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an, China
4Department of Environmental
Engineering, University, Bolu, Turkey Bolu Abant Izzet Baysal
5Department of Environmental Engineering Turkey, Ardahan University, Ardahan Received 10 August 2018; Revised 29 September 2018; Accepted 19 October 2018
Science and Technology Planning Project of Guangdong Province, China, Grant/Award Number: 2017B030314175, 2016A050502059, 2017A050501036 and 2017A040403047; Scientific and Technological Planning Project of Guangzhou, China, Grant/Award Number: 2016201604030058 and 201704030109; National Natural Science Foundation of China, Grant/Award Number: 51608129 Correspondence to: Jingyong Liu, School of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou, China.
Emails: www053991@126.com; Liujy@ gdut.edu.cn
Published online 28 January 2019 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/wer.1031
© 2018 Water Environment Federation
Thermodynamic equilibrium predictions of zinc
volatilization, migration, and transformation during
sludge co- incineration
Jingyong Liu,
1Haiming Cai,
1Shijun Wu,
2Xiaoe Dang,
3Musa Buyukada,
4Fatih Evrendilek
4,5 • AbstractThe effects of interactions between and among chlorine (Cl), sulfur (S), phosphorus (P), and minerals on migration, transformation, and volatilization of zinc (Zn) were numerically simulated in sludge co- incineration using the chemical thermodynamic equilibrium method. Our results showed that all the minerals of Fe2O3, Al2O3, Fe2O3,
and TiO2 except for CaO in the sludge co- incineration system reacted with Zn which
inhibited the Zn volatilization. The presence of S and P was beneficial to the formation of ZnSO4(s) and Zn3(PO4)2(s). Cl weakened the chemical reactions between the
min-erals and Zn, thus increasing the Zn volatilization. Changes in Zn transformation and migration induced by the coupling of Cl + S were mainly controlled by Cl, S, and the minerals, while those induced by Cl + P and S + P were mainly controlled by P and S + P. The presence of P + Cl, S + Cl, S + P, S + Cl + P, Cl, and Al2O3 in the coexisting
mineral system controlled the reactions between the minerals and Zn. © 2018 Water Environment Federation
• Key words
chlorine (Cl)–sulfur (S)–phosphorus (P); co-incineration; mineral; sludge; thermodynamic equilibrium analysis; zinc (Zn)
Introduction
With the increasing quantity and deteriorating quality of wastewater, the production of sludge as the by- product of wastewater treatment and its resultant loads of toxic and harmful substances have increased at unprecedented rates (Chen et al., 2017; Zhuo et al., 2017). Thus, seeking the sustainable disposal methods of sludge has grown increasingly vital to public and environmental health (Deng, Yuan, Mei, Liu, & Su, 2017). For example, “Treatment and Pollution Control Technology Policy for Sludge in Municipal Wastewater Treatment Plant” adopted by the Ministry of Environmental Protection of China incentivizes the use of sludge as a fuel in co- incineration by ther-mal power plants, and cement and brick kilns. However, the complexity and variability of fuel components (e.g., sludge, coals, and municipal solid waste—MSW) in co- incineration have led to the emissions of pollutants with strong toxicity such as heavy metals (HMs), organic pollutants, and acid gases (Li, Zhang, Shao, & He, 2017; Li et al., 2014; Liu, Fu, et al., 2015; Yu et al., 2016) whose migration, transformation, and emission patterns have drawn worldwide attention (Han, Hwang, Kim, Park, & Kim, 2015; Liu, Huang, et al., 2015; Liu, Huang, Sun, & Xie, 2016; Liu, Zeng, et al., 2016).
Guo, Chen, Yang, Zheng, and Gao (2014) reported that the average content of zinc (Zn) enrichment in Chinese sludge was estimated at 729.6 mg/kg between 2006 and 2013. Comparatively, the sludge limits as stipulated by “Discharge Standards of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002)” were deter-mined as 5.9% and 10.3% for neutral basic and acid agricultural soils, respectively (Guo et al., 2014). The recent utilization of Cl- containing flocculants and condi-tioners in wastewater treatment and deep dewatering has further increased the Cl content of sludge (Zhang, Liu, Liu, Hu, & Yao, 2014). The high P availability of
sludge and its co- combustion with MSW add Cl, S, and P to the co- incineration systems (Li et al., 2017; Lin & Ma, 2012; Zhang et al., 2016). Also, the bottom slag formed during the co- incineration of sludge, MSW, and coal contains such main minerals as SiO2, Al2O3, and CaO which can act as the
carri-ers of different forms for the adsorption of HMs (Zhou, Sun, Meng, Li, & Zhang, 2014). All these minerals, Cl, S, P, and air pollutants such as HMs in the co- incineration system interact with one another changing the distribution pattern of air pol-lutants (Luan, Li, Zhang, Li, & Zhao, 2013). However, there still exists a knowledge gap in related literature concerning how Zn chemically behaves given the interactions among Cl, S, P, and the minerals (SiO2- Al2O3- CaO).
The migration and transformation patterns of multiple HMs in the co- incineration system depend on incinerator type, operational conditions (e.g., incineration temperature, residence time, and combustion atmosphere) (Peng, Lin, & Wey, 2015; Roy, Dutta, Corscadden, Havard, & Dickie, 2011), initial sludge composition, Cl content of mixed wastes, and S content of coal (Liu, Fu, et al., 2015; Liu, Huang, et al., 2016; Liu, Zeng, et al., 2016; Liu et al., 2018). For example, the Cl- containing compounds are easily reactive with Zn, thus pro-ducing HM chlorides and large Zn emissions (Fraissler, Jöller, Mattenberger, Brunner, & Obernberger, 2009; Saqib & Backstrom, 2014; Struis, Ludwig, Lutz, & Scheidegger, 2004; Stucki & Jakob, 1997). At low temperatures, metal sulfides can replace metal chlorines or metal oxides, thus inhibiting HM volatilization, whereas the presence of volatile metal sulfides at above 800°C can boost Zn volatilization (Verhulst, Buekens, Spencer, & Eriksson, 1996). The P- containing compounds have strong heat stability and inhibit the volatilization and emission of HMs (Aubert, Husson, & Sarramone, 2006, P. Ndiba, Axe, & Boonfueng, 2008). The presence of P, S, and Cl was reported to affect the reactions between the minerals and HMs, and thus, the migration pathways of HMs (Liu, Huang, et al., 2016).
Currently, the tube furnaces and the fluidized bed incin-erators are usually employed in the laboratory to track the distribution patterns of HMs (Lin et al., 2014; Soria, Gauthier, Flamant, Rodriguez, & Mazza, 2015). Due to the limita-tions of the partial detection technologies, the migration,
transformation, and emission of Zn across the entire tempera-ture range of the sludge co- incineration have not been experi-mentally simulated in response to the interactions among Cl, S, P, and the minerals. These simulation results can provide the scientific basis for the accurate predictions for the emission behaviors of Zn. Therefore, this study, for the first time, aims at predicting the migration, transformation, and volatilization of Zn in an interaction with Cl, S, P, and the minerals using the thermodynamic equilibrium software of FACTsage 6.3 and at analyzing effects of the Cl- S- P and SiO2- CaO- Al2O3 couplings
on the thermodynamic equilibrium distribution of Zn.
Methodology
Materials
Sludge samples were collected from four water resource recov-ery facilities (coded as S1, S2, S3, and S4), one paper mill plant (S5) in Guangzhou, and one water resource recovery facility (S6) in Zhaoqing in the Guangdong province. The average ele-mental and mineral contents of the samples were taken as the initial conditions to carry out simulations (Table 1) (Liu & Sun, 2012; Liu, Sun, Xu, Xie, & Chen, 2009). To accurately reflect the Zn content, the average HM content of Chinese sludge between 2006 and 2013 and the geometric average content of Zn (729.6 mg/kg) were adopted as the initial values for the simulations (Table 2; Guo et al., 2014).
Simulation procedure
The equilibrium constant and Gibbs minimum free energy methods are commonly used in dealing with the complex systems of chemical equilibriums in thermodynamics. In this study, the facility for the analysis of chemical thermody-namics (FACT) software and the minimization of total Gibbs energy for a system (MINGSYS) program were utilized in the simulations. The minimum free energy method considers Gibbs free energy of a given system under a constant pres-sure and temperature as the equilibrium criterion to solve the composition and concentration of each component, with the Lagrangian undetermined coefficients. Gibbs free energy represents the minimum value when a system reaches a Table 1. Input values (%) of elements and minerals in sludge samples (Liu & Sun, 2012; Liu et al., 2009)
SLUDGE S1 S2 S3 S4 S5 S6 AVERAGE VALUE
Ultimate analysis (%) N 2.88 4.7 4.56 4.48 1.15 6.09 3.98 C 17.46 30.3 25.95 33.73 22.98 34.04 27.41 H 3.51 4.01 4.29 5.25 3.2 5.03 4.21 O 16.14 21.46 19.17 22.98 23.57 23.48 21.13 S 1.22 2.15 1.68 2.55 0.88 1.61 1.68 Cl 0.21 0.36 0.5 0.21 0.35 0.5 0.35 Minerals analysis (%) SiO2 37.16 30.14 35.41 28.41 21.32 / 30.49 CaO 3.21 3.96 2.82 3.71 18.13 / 6.37 Al2O3 9.16 7.53 6.73 4.14 8.75 / 7.26 Fe2O3 2.27 1.94 2.06 2.19 0.37 / 1.77
chemical equilibrium as a function of the chemical composi-tion, structure, condensed state, quantity, pressure, and tem-perature of a material. The FACTsage covers a compound database of all sludge components where possible products are automatically selected when the chemical symbols of the elements are typed in. One is able to remove some impos-sible products based on prior knowledge. Once the amount of each element or mineral is entered, the other operational conditions such as temperature and pressure are optimized for simulations.
A system was established to represent a sludge com-bustion atmosphere, the composition of which is similar to that of a burner tube. The scheme of equilibrium system in a sludge incinerator by MINGSYS is described in Figure 1. The sludge used in the simulation model was assumed to consist of the following six major elements: C, H, N, O, Cl, and S (Table 1). The equilibrium calculations were performed in the range of 400–1800 K (127–1,527°C), and under the total pressure of 1.013 × 105 Pa. Temperature step and excess air
ratio (λ) were set at 100 K and 1.2, respectively, for all the simulations, as typically set for the sludge combustion. The air was assumed to be composed (mole fractions) of 79% N2
and 21% O2.
The initial input parameters of the simulations include the contents of the four sludge elements (C, H, O, and N), the four minerals (CaO, SiO2, Al2O3, and Fe2O3), Zn (729.6 mg/
kg), and the three elements (Cl, S, and P). The Cl, S, and P contents used in the simulations in this study were 0.3%, 3%, and 3%, respectively, as the commonly used values (Luan et al., 2013). The TiO2 content was set according to
mole ratios (nTiO2: nZn = 1,000:1). The initial conditions and
parameterization of the model simulations are presented in Table 3.
Results and Discussion
Effects of a single mineral on Zn distribution
Figure 2a shows the distribution of Zn from 400 to 1,800 K considering only the existence of the four major elements (C, N, H, and O) and no mineral or a single mineral in the incin-eration system. As can be seen in Figure 2a, Zn mainly existed in the form of ZnO (s) from 400 to 1,400 K. At above 1,400 K, ZnO (s) began to transform into Zn (g). At 1,550 K, the mass fractions of ZnO (s) and Zn (g) were equaled. Zn (g) gradually dominated with the elevated temperature and became the only form at above 1,600 K.
Figure 2b shows the distribution of Zn when SiO2 appeared
in the incineration system. Within the incineration temperature range of 400–1,400 K, Zn mainly existed as Zn2SiO4 (s) which
replaced ZnO (s) in Figure 2a. At above 1,400 K, Zn2SiO4 (s)
gradually decomposed into Zn (g). At above 1,550 K, Zn (g) dominated the system and became the only form at above 1,700 K. Therefore, the presence of SiO2 in the incineration
sys-tem appeared to widen the sys-temperature range of Zn2SiO4 (s)
and to increase the volatilization temperature of Zn (g), which is propitious to the inhibition of Zn volatilization (Fraissler et al., 2009).
Figure 2c shows the distribution of Zn when Al2O3 existed
in the incineration system. Zn mainly appeared in form of ZnAl2O4 (s) from 400 to 1,500 K, which substituted ZnO (s)
in Figure 2a. At above 1,500 K, ZnAl2O4 (s) gradually
decom-posed into Zn (g). At above 1,650 K, Zn (g) dominated and became the only form at above 1,700 K. The presence of Al2O3
in the incineration system formed ZnAl2O4 (s) (which existed
in a wide temperature range) and increased the volatilization temperature of Zn (g), which kept Zn in the bottom slag (Yu et al., 2012). The presence of CaO had no effect on the distribu-tion of Zn across the entire temperature range (see Figure 2d). Table 2. Zn content (mg/kg) of Chinese sludge (Guo et al., 2014)
HEAVY METAL ZN (MG/KG)
Sample size 84
Content range 42.1–3568.3
Geometric average value 729.6
Arithmetical average value 739.5
Figure 1. The equilibrium system of sludge incinerator by MINGSYS.
Air˄N2+O2˅
Minerals
˄SiO2, CaO, Al2O3, Fe2O3, TiO2˅ Incinerator
FACTsage ˄Temperature, Pressure˅
Solid
(ZnO, ZnS, ZnCln, ZnCO3, ZnSO4, etc)
(CO2, H2O, O2, N2, SO2, SO3, CO, H2, H2S, HCl, Cl2, etc)
Gas
Sludge
˄C, H, N, O˅
Figure 2e–f shows the distribution of Zn when Fe2O3 and TiO2
were available in the incineration system, respectively. Zn was mainly in the forms of ZnFe2O4 (s) and Zn2TiO4 (s) from 400
to 1,400 K. At above 140 K, ZnFe2O4 (s) and Zn2TiO4 (s)
grad-ually decomposed into Zn (g).
From the above results, we can infer that the minerals of SiO2,
Al2O3, Fe2O3, and TiO2 reacted with Zn, producing Si- and Al-
containing compounds difficult to decompose even at high tem-peratures. This in turn increased the volatilization temperature
of Zn (g). However, CaO had no effect on the Zn volatilization. Therefore, finding a suitable adsorbent to control Zn emissions during the incineration is of great significance as the Zn distribu-tion differs significantly with the different minerals.
Effects of Cl on reactions between a single mineral and Zn
Figure 3 indicates the effects of a single mineral (Fe2O3, CaO,
Al2O3, SiO2, and TiO2) on the distribution of Zn considering
Table 3. Model parameterization under 79% N2–21% O2 atmosphere (n Zn = 11.16 × 10−3 mol)
FIGURES AIR (N(MOL)2 + O2) SLUDGE (MOL) CL (MOL) S (MOL) P (MOL) MINERALS (MOL)
2 O2 = 32.7107 N2 = 122.9316 C = 22.8416 H = 42.100 O2 = 6.6031 N2 = 1.4214 0 0 0 SiO2 = 5.0817 Al2O3 = 0.5628 CaO = 1.1375 Fe2O3 = 0.2213 TiO2 = 11.16 3 O2 = 32.7107 N2 = 122.9316 C = 22.8416 H = 42.100 O2 = 6.6031 N2 = 1.4214 0.3% Cl = 0.08451 0 0 SiO2 = 5.0817 Al2O3 = 0.5628 CaO = 1.1375 Fe2O3 = 0.2213 TiO2 = 11.16 4 O2 = 32.7107 N2 = 122.9316 C = 22.8416 H = 42.100 O2 = 6.6031 N2 = 1.4214 0 3% S = 0.9375 0 SiO2 = 5.0817 Al2O3 = 0.5628 CaO = 1.1375 Fe2O3 = 0.2213 TiO2 = 11.16 5 O2 = 32.7107 N2 = 122.9316 C = 22.8416 H = 42.100 O2 = 6.6031 N2 = 1.4214 0 0 3% P = 0.9677 SiO2 = 5.0817 Al2O3 = 0.5628 CaO = 1.1375 Fe2O3 = 0.2213 TiO2 = 11.16 6 O2 = 32.7107 N2 = 122.9316 C = 22.8416 H = 42.100 O2 = 6.6031 N2 = 1.4214 0.3% Cl = 0.08451 3% S = 0.9375 0 SiO2 = 5.0817 Al2O3 = 0.5628 CaO = 1.1375 Fe2O3 = 0.2213 TiO2 = 11.16 7 O2 = 32.7107 N2 = 122.9316 C = 22.8416 H = 42.100 O2 = 6.6031 N2 = 1.4214 0.3% Cl = 0.08451 3% P = 0.9677 SiO2 = 5.0817 Al2O3 = 0.5628 CaO = 1.1375 Fe2O3 = 0.2213 TiO2 = 11.16 8 O2 = 32.7107 N2 = 122.9316 C = 22.8416 H = 42.100 O2 = 6.6031 N2 = 1.4214 0 3% S = 0.9375 3% P = 0.9677 SiO2 = 5.0817 Al2O3 = 0.5628 CaO = 1.1375 Fe2O3 = 0.2213 TiO2 = 11.16 9 O2 = 32.7107 N2 = 122.9316 C = 22.8416 H = 42.100 O2 = 6.6031 N2 = 1.4214 0.3% Cl = 0.08451 3% S = 0.9375 3% P = 0.9677 SiO2 = 5.0817 Al2O3 = 0.5628 CaO = 1.1375 Fe2O3 = 0.2213 TiO2 = 11.16
only the existence of the major elements (C, N, H, and O) and Cl in the incineration system. As can be seen in Figure 3a, Zn mainly existed in the forms of ZnCl2 (s), ZnO (s), ZnCl2 (g),
and Zn (g) in the presence of Cl but the minerals. At low tem-peratures (400–550 K), ZnCl2 (s) was the major form, while
ZnCl2 (s) began to transform into ZnO (s) and ZnCl2 (g) with
the risen temperature. At 600–1,400 K, ZnO (s) gradually con-verted into ZnCl2 (g). At above 1,500 K, ZnO (s) and ZnCl2
(g) started to transform into Zn (g). ZnCl2 (g) existed in a
wide range of 500–1,500 K with the presence of Cl that clearly enhanced the Zn volatilization.
When SiO2 existed, Zn was mainly in the form of ZnCl2
(s) from 400 to 500 K but with the mass proportion of ZnCl2 (s)
decreasing with the increased temperature. In a wide tempera-ture range, Zn2SiO4 (s) replaced ZnO (s) and inhibited the
for-mation of ZnCl2 (g) with a slight effect on the Zn volatilization
Figure 2. Effects of the following mineral conditions without Cl, S, and P during co- incineration on thermodynamic equilibrium distribu-tion of Zn: (a) without a mineral; (b) SiO2; (c) Al2O3; (d) CaO; (e) Fe2O3; and (f) TiO2.
at high temperatures. With the presence of Al2O3 from 400 to
1,600 K, Zn was mainly observed in the form of ZnAl2O4 (s)
but ZnCl2 (s), and the proportion of ZnCl2 (g) was below 5%
within the entire temperature range. When Fe2O3 (TiO2) was
considered in the incineration system, ZnCl2 (s) was mainly
produced at low temperatures, while ZnFe2O4 (s) (Zn2TiO4
(s)) became the major form from 600 to 1,500 K. However, CaO had no significant influence on the Zn migration. From the above results, it was evident that Cl easily reacted with Zn
producing ZnCl2 (g) which enhanced the Zn volatilization. Cl
weakened the reactions between the minerals and Zn, whereas CaO had no significant effect on the Zn distribution.
Effects of S on reactions between a single mineral and Zn
The effects of a single mineral (SiO2, Al2O3, CaO, Fe2O3, and
TiO2) on the distribution of Zn are shown in Figure 4, only
when S was available during the sludge co- incineration. Zn was Figure 3. Effects of the following mineral conditions with 0.3% Cl during co- incineration on thermodynamic equilibrium distribution of Zn: (a) without a mineral; (b) SiO2; (c) Al2O3; (d) CaO; (e) Fe2O3; and (f) TiO2.
in the following forms of ZnSO4 (H2O) (s), ZnSO4 (s), and ZnO
(s) in the ranges of 400–500 K, 450–1,050 K, and 1,050–1,550 K, respectively. With the increased temperature, ZnO (s) began to convert into Zn (g) (Figure 4a). The temperature range of ZnSO4 (s) shrank with SiO2 by 100 K, and a small amount of
Zn2SiO4 (s) was generated (Figure 4b). The presence of Al2O3
had no effect on the Zn distribution from 400 to 600 K but nar-rowed the temperature range of ZnSO4 (s) from 600 to 900 K,
with ZnAl2O4 (s) observed from 800 to 1,700 K (Figure 4c). The
temperature range of ZnSO4 (s) shrank with Fe2O3 or TiO2 by
100 K, with ZnFe2O4(s) or Zn2TiO4(s) observed (Figure 4e–f).
No reaction between Zn and CaO was observed which may be attributed to CaO preferring to react with S, thus generating CaSO4. Overall, Zn mainly reacted with S to produce ZnSO4
(s) at low temperatures, whereas Zn reacted with Fe2O3, Al2O3,
SiO2, and TiO2 to produce the difficult- to- volatilize compounds
Figure 4. Effects of the following mineral conditions with 3% S during co- incineration on thermodynamic equilibrium distribution of Zn: (a) without a mineral; (b) SiO2; (c) Al2O3; (d) CaO; (e) Fe2O3; and (f) TiO2.
at high temperatures which helped to inhibit the Zn volatiliza-tion. However, CaO had no effect on the Zn distribuvolatiliza-tion.
Effects of P on reactions between a single mineral and Zn
The effects of a single mineral (SiO2, Al2O3, CaO, Fe2O3, and
TiO2) on the distribution of Zn are shown in Figure 5, under
P only during the sludge incineration. The existence of SiO2,
CaO, Fe2O3, and TiO2 had no influence on the reactions
between P and Zn (Figure 5). Zn reacted with P initially to pro-duce Zn3(PO4)2 (s) difficult to volatilize. However, when Al2O3
appeared, Zn reacted with Al2O3 instead of P, to form ZnAl2O4
(s). Thus, S and P promoted the formation of ZnSO4 (s) and
Zn3(PO4)2 (s), thus inhibiting the Zn volatilization. However,
Cl weakened the reactions between Zn and the minerals which enhanced the Zn volatilization.
Figure 5. Effects of the following mineral conditions with 3% P during co- incineration on thermodynamic equilibrium distribution of Zn: (a) without a mineral; (b) SiO2; (c) Al2O3; (d) CaO; (e) Fe2O3; and (f) TiO2.
Effects of coexisting Cl- S- P on reactions between a single mineral and Zn
The coexisting Cl, S, and P in the complex incineration system complicated the Zn migration and transformation. To simplify, the migration and transformation behaviors of Zn in the co- incineration system were simulated considering only the exist-ence of sludge (C, N, H, and O), a single mineral (SiO2, Al2O3,
CaO, Fe2O3, and TiO2), and the Cl- S- P couplings (Cl + S,
Cl + P, and S + P) (Figures 6–8).
Effects of coexisting Cl- S on reactions between a single mineral and Zn
In the Cl and S coexistence with a single mineral (SiO2, Al2O3,
Fe2O3, and TiO2) (Figure 6), Zn was mainly observed in the
forms of ZnSO4 (H2O) (s) and ZnSO4 (s) at below 1,000 K and
of Zn2SiO4 (s), ZnAl2O4 (s), ZnFe2O4 (s), and Zn2TiO4 (s) at
the higher temperatures. Zn mainly reacted with Cl at the low temperatures, and CaO had no effect on Zn at the high tem-peratures. With the Cl + S system, SiO2, Al2O3, Fe2O3, and
Figure 6. Effects of the following mineral conditions with 0.3% Cl and 3% S during co- incineration on thermodynamic equilibrium distri-bution of Zn: (a) SiO2; (b) Al2O3; (c) CaO; (d) Fe2O3; and (e) TiO2.
TiO2 but CaO had a pronounced inhibition effect on the Zn
volatilization.
Effects of coexisting Cl- P on reactions between a single mineral and Zn
In the Cl- P coexistence system (Figure 7), the reaction between a single mineral (SiO2, TiO2, CaO, and Fe2O3) and Zn was
mainly affected by P. Zn was mainly in the form of Zn3(PO4)2
(s) within a wide temperature range. In the system with Al2O3,
Zn mainly existed as ZnAl2O4 (s). In the entire temperature
range, Cl had no significant influence on the Zn transforma-tion. Therefore, in the Cl+P system, the reactions between a single mineral except for Al2O3 and Zn were mainly controlled
by P but Cl.
Figure 7. Effects of the following mineral conditions with 0.3% Cl and 3% P during co- incineration on thermodynamic equilibrium distri-bution of Zn: (a) without a mineral; (b) SiO2; (c) Al2O3; (d) CaO; (e) Fe2O3; and (f) TiO2.
Effects of coexisting S- P on reactions between a single mineral and Zn
In the S- P coexistence system (Figure 8), the reactions between a single mineral and Zn were greatly affected by S
at low temperatures and by P at high temperatures. Zn trans-formation was also affected by S at low temperatures, while Zn only reacted with Al2O3 at high temperatures (Figure 8).
Therefore, in the S + P system, the reactions between a single Figure 8. Effects of the following mineral conditions with 3% S and 3% P during co- incineration on thermodynamic equilibrium distribu-tion of Zn: (a) TiO2; (b) Al2O3; (c) CaO; (d) Fe2O3; and (e) SiO2.
Figure 9. Effects of the following conditions of the coexisting SiO2, CaO, and Al2O3 with 0.3% Cl, 3% S, and 3% P (Cl + S, S + P, and Cl + S, Cl + P + S) during co- incineration on thermodynamic equilibrium distribution of Zn: (a) (SiO2 + CaO + Al2O3) + Zn; (b) (SiO2 + CaO + Al2O 3) + Zn + 0.3% Cl; (c) (SiO2 + CaO + Al2O3) + Zn + 3% S; (d) (SiO2 + CaO + Al2O3) + Zn + 3% P; (e) (SiO2 + CaO + Al2O3) + Zn + 3% S + 0.3% Cl; (f) (SiO2 + CaO + Al2O3) + Zn + 3% P + 3% S; (g) (SiO2 + CaO + Al2O3) + Zn + 3% P + 0.3% Cl; and (h) (SiO2 + CaO + Al2O3) + Zn + 0.3% Cl + 3% P + 3% S.
mineral (except for Al2O3) and Zn were affected by both S
and P.
Effects of coexisting Cl- S- P on reactions between multiple minerals and Zn
The coexistence of the multiple minerals (SiO2 + CaO + Al2O3)
during the sludge co- incineration renders the migration and transformation paths of Zn more complicated. Therefore, the present study quantified the thermodynamic equilibrium dis-tribution of Zn in the co- incineration system considering only the existence of sludge (C, N, H, and O), the multiple miner-als (SiO2 + CaO + Al2O3), and the Cl- S- P couplings (Cl + S,
Cl + P, S + P, and Cl + S + P) (Figure 9). In the co- incineration system with the coexistence of the multiple minerals and Zn, the Zn distribution was affected only by Al2O3. Zn was mainly
observed in the form of ZnAl2O4 (s) from 400 to 1,500 K, while
ZnAl2O4 (s) began to transform into Zn (g) at above 1,500 K
(Figure 9a).
The presence of Cl influenced the Zn transformation (Figure 9b). With the presence of Cl alone, the Zn distribution was only affected by Al2O3 and Cl. Zn was mainly in the form of
ZnAl2O4 (s) from 400 to 1,500 K, while ZnCl2 (g) was observed
within the range of 1,000 to 1,800 K, with a <5% proportion. However, the presence of S or P alone in the system had no effect on the Zn distribution (Figure 9c,d). The Zn distribution appeared to be only related to Al2O3. Zn was in the form of
ZnAl2O4 (s) from 400 to 1,500 K, with ZnAl2O4 (s)
transform-ing into Zn (g) at above 1,500 K. With the presence of Cl+S, Cl+P, or Cl+P+S in the system, both Cl and Al2O3 played a role
in the concentration distribution of Zn (Figure 9e,g and h). Zn existed in the form of ZnAl2O4 (s) from 400 to 1,500 K, with
ZnCl2 (g) observed from 1,000 to 1,800 K. The effect of the
coexisting P- S on the Zn distribution was constrained mainly by Al2O3 (Figure 9f). Overall, the effects of the simultaneous
presence of Cl, S, and P (P + Cl, S + Cl, S + P, and S + Cl + P) on the distribution of Zn were mainly controlled by Cl and Al2O3.
Al2O3 appeared to play a more influential role than did Cl in
preventing the Zn volatilization, with their overall contribution rates of >95% and <5%, respectively.
Conclusions
In the sludge co- incineration system, without the presence of Cl, S, and P, except for CaO, the minerals of Fe2O3, Al2O3, Fe2O3,
and TiO2 reacted with Zn, thus creating Si- and Al- containing
refractory compounds which postponed the Zn volatilization. The presence of Cl, S, and P affected the reactions between the minerals and Zn. S and P promoted the production of ZnSO4(s)
and Zn3(PO4)2(s) which inhibited the Zn volatilization.
However, Cl weakened the reactions between the minerals and Zn, thus enhancing the Zn volatilization. The presence of Cl + S, S + P, and P + Cl in the co- incineration system differently influ-enced the reactions between the minerals and Zn. In the Cl + S system, the Zn migration and transformation were controlled by Cl, S, and the minerals. In the Cl + P system, P limited the Zn distribution. In the S + P system, both S and P played domi-nant roles in the Zn volatilization. In the co- incineration system
with the coexistence of SiO2 + CaO + Al2O3 in the presence of
P + Cl, S + Cl, S + P, and S + Cl + P, both Cl and Al2O3
pre-dominated the reactions between the minerals and Zn. Al2O3
exerted a more influential impact on the prevention of the Zn volatilization, with a contribution rate of over 95%.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 51608129), the Science and Technology Planning Project of Guangdong Province, China (2017B030314175; 2017A050501036; 2017A040403047; 2016A050502059) and the Scientific and Technological Planning Project of Guangzhou, China (No.2016201604030058, 201704030109). We are grateful to Lv Jun for his enthusiastic help with the numerical simulations.
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