Scholarly article on topic 'The Effects of Water Washing on Cement-based Stabilization of MWSI Fly Ash'

The Effects of Water Washing on Cement-based Stabilization of MWSI Fly Ash Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Procedia Environmental Sciences
OECD Field of science
Keywords
{"MSWI fly ash" / "heavy metal" / "water washing" / "cement-based stabilization"}

Abstract of research paper on Materials engineering, author of scientific article — Xuexue Wang, Aimin Li, Zhikun Zhang

Abstract Municipal solid waste incineration (MSWI) fly ash is classified as a hazardous waste due to its high content of leachable heavy metals. Currently, cement-based stabilization process before landfill has attracted much attention since it can transform the hazardous waste into non-hazardous waste. It is known that MSWI fly ash in China has high-level of soluble chlorides, which has a negative effect on the stabilization of the solidified matrix. In this study, therefore, a water washing pre-treatment process was applied to remove chlorides as much as possible. Experiments were conducted at the conditions of liquid-to-solid (L/S) ratio of 10, 20, 30, 40 and 50 and washing time of 0.5, 1, 2, 3 and 4h for the washing process. Based on the analysis, the MSWI fly ash with L/S ratio of 10 and washing time of 2h was selected for the subsequent experiment. The washed fly ash and raw fly ash were then mixed with different amounts of cement and placed into the mould for 7, 14 and 28 days, respectively. The results showed that cement-based solidification process exhibited the maximum compressive strength of 7.87MPa when the addition of cement was 67% and the maintenance period was 7 days. Besides, toxic characteristic leaching procedure (TCLP) tests showed that the solidified matrix can meet the requirement of landfill.

Academic research paper on topic "The Effects of Water Washing on Cement-based Stabilization of MWSI Fly Ash"

CrossMark

Available online at www.sciencedirect.com

ScienceDirect

Procedía Environmental Sciences 31 (2016) 440 - 446

The Tenth International Conference on Waste Management and Technology (ICWMT)

The effects of water washing on cement-based stabilization of

MWSI fly ash

Xuexue Wanga, Aimin Lia'*, Zhikun Zhanga

aKey Laboratory of Industrial Ecology and Environmental Engineering, School ofEnvironmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, People 's Republic of China

Abstract

Municipal solid waste incineration (MSWI) fly ash is classified as a hazardous waste due to its high content of leachable heavy metals. Currently, cement-based stabilization process before landfill has attracted much attention since it can transform the hazardous waste into non-hazardous waste. It is known that MSWI fly ash in China has high-level of soluble chlorides, which has a negative effect on the stabilization of the solidified matrix. In this study, therefore, a water washing pre-treatment process was applied to remove chlorides as much as possible. Experiments were conducted at the conditions of liquid-to-solid (L/S) ratio of 10, 20, 30, 40 and 50 and washing time of 0.5, 1, 2, 3 and 4 h for the washing process. Based on the analysis, the MSWI fly ash with L/S ratio of 10 and washing time of 2 h was selected for the subsequent experiment. The washed fly ash and raw fly ash were then mixed with different amounts of cement and placed into the mould for 7, 14 and 28 days, respectively. The results showed that cement-based solidification process exhibited the maximum compressive strength of 7.87 MPa when the addition of cement was 67% and the maintenance period was 7 days. Besides, toxic characteristic leaching procedure (TCLP) tests showed that the solidified matrix can meet the requirement of landfill.

© 2016 The Authors. PublishedbyElsevierB.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific Keywords: MSWI fly ash; heavy metal; water washing; cement-based stabilization

1. Introduction

Incineration method has been widely used to treat municipal solid wastes in the world due to its obvious advantages of reducing volume greatly and heat recovery, etc. However, a large amount of fly ash was generated in China per year. The data shows that more than 50,000 tons of MSWI fly ash is produced annually in Shanghai1.

* Corresponding author. Tel.: +86-411-8470-7448 ; fax: +86-411-8470-7448 . E-mail address: leeam@dlut.edu.cn

1878-0296 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific doi: 10.1016/j.proenv.2016.02.095

Meanwhile, MSWI fly ash is classified as a hazardous waste for its high levels of leachable heavy metals and soluble salts, which can transfer into the environment and cause underwater contamination. Therefore, it is necessary to find a method for safer disposal of MSWI fly ash before landfill.

At present, cement-based stabilization treatment is one of the main methods to reduce the mobility of toxic materials in fly ash2. Researches have shown that chlorides have a negative effect on the stabilization process, which exists mainly in the form of potassium chloride (KCl), sodium chloride (NaCl), and calcium chloride (CaCl2), respectively3,4. Hence, removing soluble chlorides prior to cement solidification by water washing pre-treatment is a necessary step5,6,7. Kung-Yuh Chiang found that soluble salts can be eliminated by dissolution with water8. Water-soluble chlorides such as NaCl, KCl and CaCh in fly ash were easily washed away9. R. Cioffi found that a two-stage washing process was able to remove chlorides using less water as much as possible to promote the cement hydration10. On the other hand, a relevant reduction of cement addition in the stabilization process can be achieved through a preliminary water washing treatment of MSWI fly ash11,12. In many cases, the composition of fly ash is complex enough so it is necessary to discuss the feasibility of stabilization process.

The main purpose of this study is to stabilize/solidify MSWI fly ash by the cement-based stabilization process, as well as removing chlorides as much as possible by water washing pre-treatment. Firstly, raw fly ash was submitted to water washing process at the conditions of different L/S ratios and different washing time, and optimal washing condition was determined by testing the content of chlorides in leachate. Then, different percentage of cement was added into the washed fly ash, mixed with deionized water and put into the mould for 7, 14 and 28 days. Finally, the compressive strength test and heavy metal leaching test were conducted to evaluate the safety of solidified matrixes. The entire work of this study aimed at the hazard-free treatment before landfill, namely the transformation of hazardous fly ash into nonhazardous solidified matrixes.

2. Materials and methods

2.1. Materials

Fly ash used in this research was collected from the filtration unit of MSWI plant located in Dalian, China. It was dried at 105 oC until its weight remained constant. Different fly ash-cement ratios (weight-to-weight) of 0.5:1, 1:1, 2:1, 3:1, 4:1 and 5:1 of the cement were added to act as a binder of stabilization process.

2.2. Characteristics and methods

For the purpose of determining the characteristics of raw fly ash and cement, two samples were submitted to chemical composition analysis through X-ray fluorescence spectrum (XRF, Bruck SRS-3400, Germany).

The water washing experiment was conducted at the conditions of liquid-to-solid (L/S) ratios of 10, 20, 30, 40 and 50 for 0.5 h, 1 h, 2 h, 3 h at room temperature, using deionized water as extraction, in which case L/S ratio was kept at 10 first to change washing time, then washing time was kept without change to adjust L/S ratios in turn to optimize washing condition. The washed fly ash was used for subsequent stabilization process, and the chloride contents in the extraction liquid were test by using an ion chromatography (IC, Shimadzu LC-10A, Japan).

The washed fly ash and raw fly ash were mixed with cement at different fly ash-cement ratios (weight-to-weight) of 0.5:1, 1:1, 2:1, 3:1, 4:1 and 5:1. The mixtures were maintained in the mould with a size of 10 mm*10 mm*40 mm for 7, 14, and 28 days at room condition, respectively.

Compressive strength of the stabilized matrix was measured by a universal test machine (CSS-2205, Changchun Testing Machine Institute, China) with a crosshead speed of 1 mm/min. For each test, three samples were tested and the results were averaged.

The stabilized matrixes with particle size below 5 mm were used for the investigation of heavy metals leaching test by TCLP according to the US Environmental Protection Agency (EPA)13. The samples were immersed in the leaching liquid for 18 h at room temperature. The preparation work involved adding 5.7 mL acetic acid to a 1 L volumetric flask while the rest was filled with deionized water. The liquid to solid ratio was kept at 20 (L/S=20) in the leaching tests. The concentrations of heavy metals of Cu, Zn, Cd, Cr and Pb were determined by using inductively coupled plasma optical emission spectroscopy (ICP-OES).

3. Results and discussion

3.1. Characterization of fly ash and cement

The chemical compositions of fly ash and cement determined by XRF were shown in Table 1. The main components of fly ash were CaO (40.3 wt.%), Na2O (8.69 wt.%), K2O (5.46 wt.%), SO3 (5.45 wt.%), SiO2 (3.10 wt.%), and the major elements were Cl, Ca, S, Na, K and Si. It can be found that the content of CaO and chlorides was high in the Chinese MSWI fly ash due to its lime pretreatment and plastic14,15.

Table 1. Chemical composition of fly ash and cement.

Chemical composition (wt%) Fly ash Cement

CaO 40.30 58.90

Cl 30.50 --

NaiO 8.69 0.32

SO3 5.45 3.26

SiO2 3.10 20.70

K2O 5.46 1.26

MgO 1.35 1.31

Fe2Oj 1.17 5.53

AI2O3 1.04 7.60

ZnO 0.87 --

TiO2 0.76 0.58

P2O5 0.65 --

PbO 0.36 --

CuO 0.10 --

C2O3 0.20 --

Total 100 100

--: the content is not detected by XRF.

Water washing pretreatment was carried out to remove chlorides from fly ash as much as possible because they mainly existed in the form of soluble salts. The contents of chlorides in the solutions are shown in Table 2 and Table 3. It can be seen clearly that the removal ratios of chlorides decreased from 75.33% to 64.80% as L/S changing from 10 to 50 when the washing time was 2 h, as shown in Table 2. The data indicated that removal ratio of Cl was affected by L/S ratio to some extent.

In addition, the removal ratio of Cl reached optimum value of 75.33% when the L/S was 10 washing 2 h, as washing time was more than 2 h, the removal ratio increased 1.37% and 0.31%, respectively. From the view of economical efficiency and environmental benefits, washing 2 h, as a result, was chosen to be the optimal washing time. And then the washed fly ash under this condition was used as the experimental sample for the cement solidification.

Table 2. Removal ratio of Cl at different L/S (washing time=2 h).

L/S 10 20 30 40 50

Removal ratio of Cl/% 75.33 69.48 67.25 68.48 64.80

Table 3. Removal ratio of Cl at different washing time (L/S=10).

Washing time/h 0.5 1 2 3 4

Removal ratio of Cl/% 66.21 66.71 75.33 76.70 77.01

After drying, the chemical compositions of the washed fly ash are shown in Table 4. The contents of chemical composition in washed fly ash increased compared with raw fly ash except those of Cl, Na and K. The reason was that chlorides were removed during water washing process. The chlorides in washed fly ash decreased from 30.50% to 16.20% as L/S increasing, the result was corresponding to the conclusion as Table 3 shown.

Table 4. Chemical composition of raw fly ash and washed fly ash.

Chemical composition (wt%) Raw fly ash Washed fly ash

CaO 40.30 51.90

Cl 30.50 16.20

NaiO 8.69 0.87

SO3 5.45 10.30

SiO2 3.10 6.10

K2O 5.46 1.76

MgO 1.35 2.65

Fe2O3 1.17 2.46

M2O3 1.04 1.75

ZnO 0.87 2.15

TiO2 0.76 1.64

P2O5 0.65 1.06

PbO 0.36 0.62

CuO 0.10 0.09

Cr2O3 0.20 0.19

Total 100 100

3.2. Compressive strength

The compressive strength tests of raw fly ash with different amounts of cement were not conducted since they cannot form solidified body. Figure 1 shows the results of compressive strength measurements of the washed fly ash solidified matrixes after maintaining 7, 14, and 28 days, respectively. Cement-based solidification samples demonstrated the compressive strength of 1.08-7.87 MPa, which decreased along with the percentage of cement addition was reduced. Besides, it was seen that the compressive strength was 7.87-4.72 MPa when fly ash-cement ratio was 0.5:1 as the maintenance period changed from 7 to 28 days. The compressive strength were reduced to 7.81-5.18 MPa when the fly ash-cement ratio was 1:1 and to 5.37-3.11 MPa when the mix ratio was 2:1. When fly ash-cement ratio was 3:1, the values reduced to 4.73-2.13 MPa, while the ratio was 4:1, the compressive strength turned to 4.31 to 1.19 MPa, and 3.57-1.08 MPa was under the condition of the ratio was 5:1. All compressive strength values of the samples exceed the standard for landfills, which was 0.414 MPa after 28 days.

Fly ash/Cement (kg/kg)

Figure 1. Compressive strength of washed fly ash-cement solidified matrixes.

The compressive strength values of the solidified body decreased as the ratio of cement reduced. The compressive strength of 7 days changed from 7.87 MPa to 3.57 Mpa and when the maintenance period were 14 days and 28 days, the strength values showed the same decrease trend as 7 days. The stabilization effect increased with the increase of cement addition, thus cement hydration played a significant role in the solidification of fly ash16,17. It can also be seen that calcium chloride formed during the hydration process, reacted with aluminate in cement, and thus strongly affected the strength of solidified matrixes18. Compressive strengths of all the tested samples were in compliance with the regulatory limits of landfill in China.

3.3. Leaching test of heavy metals

Fly ash is classified as hazardous waste for its leachable heavy metals, and the leaching performance was conducted to evaluate the effect of cement-based stabilization using the TCLP (EPA Test Method 1311)19,20,21. The results revealed that the cement addition had positive effect on the immobilization of heavy metals as shown in Table 5-722,23,24. Pb was not detected when the samples were maintained for 7 days and the contents of heavy metals increased as cement percentage decreasing. The content of Cu, Cr and Zn increased as fly ash-cement ratios going up when 14 days and 28 days maintenance period were provided. In addition, the concentration of heavy metals decreased as maintenance period changed from 7 days to 28 days. As shown in Table 5-7, the content of Zn increased from 0.15 to 0.81 mg/L as the fly ash-to-cement being 0.5:1-5:1 when the maintenance period was 7 days, and that of Cu content was 0.39-0.89 mg/L and Cr content 0.12 to 0.28 mg/L. When the maintenance period was 14 days, as the fly ash-to-cement ratio varied from 0.5:1 to 5:1, the contents of Pb, Zn, Cu, and Cr increased to 0.23 mg/L, 5.07-16.21 mg/L, 0.39-2.31 mg/L and 0.13-0.29 mg/L, respectively. When maintenance increased to 28 days, the contents of Pb, Zn, Cu, and Cr were in the range of 0.10-0.24 mg/L, 9.74-12.02 mg/L, 0.96-3.94 mg/L and 0.15-0.31 mg/L, respectively. No statistically significant difference was realized in these leaching test data, however, the results indicated that all of the data was much lower than the regulatory standard limits, which reflected that cement-based stabilization process was stable enough to convert hazardous MSWI fly ash into nonhazardous material for safer disposal. As shown in Table 5-7, the content of heavy metals in tested samples with fly ash-cement ratios of 0.5:1, 1:1, 2:1, 3:1 increased, but that of the samples with ratios of 4:1 to 5:1 decreased.

It should be noted that, the leached heavy metals concentration presented an increasing tendency when the maintenance period extended from 7 days to 28 days, and it reached the maximum value when the fly ash-cement ratio was 5:1 and the maintenance period was 28 days. The leaching concentrations of heavy metals were below the limits suggested by the US EPA.

Table 5. Heavy metal leaching test of solidified matrixes for 7 days (mg/L).

Heavy Fly ash-cement ratios

metals 0.5:1 1:1 2:1 3:1 4:1 5:1

Pb -- -- -- -- -- --

Zn 0.15 0.16 0.34 0.56 0.62 0.81

Cu 0.39 0.47 0.49 0.63 0.70 0.89

Cr 0.12 0.13 0.12 0.15 0.18 0.28

--): Not detected

Table 6. Heavy metal leaching test of solidified matrixes for 14 days (mg/L).

Heavy Fly ash-cement ratios

metals 0.5:1 1:1 2:1 3:1 4:1 5:1

Pb -- 0.08 0.10 0.12 0.15 0.23

Zn 5.07 5.06 14.27 15.34 15.6 16.21

Cu 0.39 0.49 0.54 0.67 1.21 2.31

Cr 0.13 0.14 0.16 0.21 0.22 0.29

Table 7. Heavy metal leaching test of solidified matrixes for 28 days (mg/L).

Heavy Fly ash-cement ratios

metals 0.5:1 1:1 2:1 3:1 4:1 5:1

Pb 0.10 0.19 0.16 0.20 0.18 0.24

Zn 9.74 10.07 10.82 10.96 10.88 12.02

Cu 0.96 1.06 1.41 2.56 3.08 3.94

Cr 0.15 0.14 0.15 0.16 0.21 0.31

4. Conclusion

In this study, the treatment of MSWI fly ash by cement solidification was studied. The effect of water washing pre-treatment, including L/S ratio and washing time, were carried out to remove chlorides. XRF tests were conducted to determine the chemical composition of fly ash and cement. The compressive strength test and heavy metal leaching test revealed that the effect of cement-based stabilization was highly affected by the percentage of cement addition and maintenance period. The condition of fly ash-cement ratio 0.5:1 maintaining 7 days exhibited the best performance, and the leaching concentration of heavy metals in solidified matrixes meet the requirement of landfill.

References

1. Xinying Li, Quanyuan Chen, Yasu Zhou, et al. Stabilization of heavy metals in MSWI fly ash using silica fume. J Waste Management

2014;34:2494-2504.

2. Fenfen Zhu, Masaki Takaok, Kazuyuki Oshita, et al.Chlorides behavior in raw fly ash washing experiments. J Journal of Hazardous Materials

2010;178:547-552.

3. A.Polettini, R.Pomi, P Sirini,et al. Properties of Portland cement-stabilised MSWI fly ashes. J Journal of Hazardous Materials 2001;B88:123-138.

4. Qunhui Wang, Jie Yang, Qi Wang, et al. Effects of water-washing pretreatment on bioleaching of heavy metals from municipal solid waste

incinerator fly ash. J Journal of Hazardous Materials 2009;162:812-818.

5. Wei-Sheng Chen, Fang-Chih Chang, Yun-Hwei Shen, et al. Removal of chloride from MSWI fly ash. J Journal of Hazardous Materials

2012;237-238:116-120.

6. Watcharapong Wongkeo, Pailyn Thongsanitgarn, Athipong Ngamjarurojana, et al. Compressive strength and chloride resistance of self-compacting concrete containing high level fly ash and silica fume. J Materials & Design 2014;64:261-269.

7. Jian Geng, Dave Easterbrook, Long-yuan Li, et al. The stability of bound chlorides in cement paste with sulfate attack. J Cement and

Concrete Research 2015;68:211-222.

8. Kung-Yuh Chiang, Yu-Hsin Hu. Water washing effects on metals emission reduction during municipal solid waste incinerator (MSWI) fly ash

melting process. J Waste Management 2010;30:831-838.

9. Renbo Yang, Wing-Ping Liao, Pin-Han Wu. Basic characteristics of leachate produced by various washing processes for MSWI ashes

in Taiwan. J Journal of Environmental Management 2012;104:67-76.

10. F.Colangelo, R.Cioffi, F.Montagnaro. Soluble salt removal from MSWI fly ash and its stabilization for safer disposal and recovery as road basement material. J Waste Management 2012;32:1179-1185.

11. T.Mangialardi. Disposal of MSWI fly ash through a combined washing-immobilisation process. J Journal of Hazardous Materials 2003;B98:225-240.

12. Lei Zheng, Chengwen Wang, Wei Wang, et al. Immobilization of MSWI fly ash through geopolymerization: Effects of water-wash. J Waste Management 2011;31:311-318.

13. USEPA Method 1311-TCLP. Test methods for evaluating solid waste, physical/chemical methods. 3rd ed. Washington, DC: US Environmental Protection Agency, Office of Solid Waste; 1992.

14. Jiang, Y. Effect of water-extraction on characteristics of melting and solidification of fly ash from municipal solid waste incinerator. J Journal of Hazardous Materials 2009;161: 871-877.

15. Yun Pan, Zhiming Wu, Jizhi Zhou, et al. Chemical characteristics and risk assessment of typical municipal solid waste incineration (MSWI) fly ash in China. J Journal of Hazardous Materials 2013;261:269-276.

16. Pawe! T, Mohsen Ben Haha, Cyrille F. Dunant, et al. A new quantification method based on SEM-EDS to assess fly ash composition and study the reaction of its individual components in hydrating cement paste. J Cement and Concrete Research 2015:73:111-122.

17. Faiz U.A. Shaikh, Steve W.M. Supit. Compressive strength and durability properties of high volume fly ash (HVFA) concretes containing ultrafine fly ash (UFFA). J Construction and Building Materials 2015;82:192-205.

18. Ali Kazemian, Asghar Gholizadeh Vayghan, Farshad Rajabipour. Quantitative assessment of parameters that affect strength development in alkali activated fly ash binders. J Construction and Building Materials 2015;93:869-876.

19. Yilmaz Kocak, Suna Nas. The effect of using fly ash on the strength and hydration characteristics of blended cements. J Construction and Building Materials 2014;73:25-32.

20. Kalliopi Anastasiadou, Konstantinos Christopoulos, Epameinontas Mousios. Solidification/stabilization of fly and bottom ash from medical waste incineration facility. J Journal of Hazardous Materials 2012;207-208:165-170.

21. Y. Luna Galiano, C. Fernández Pereira, J. Vale. Stabilization/solidification of a municipal solid waste incineration residue using fly ash-based geopolymers. J Journal of Hazardous Materials 2011;185:373-381.

22. Hatice Yilmaz. Characterization and comparison of leaching behaviors of fly ash samples from three different power plants in Turkey. J Fuel Processing Technology 2015;137:240-249.

23. Kanchinadham Sri Bala Kameswari, L.M. Narasimman, Vihita Pedaballe. Diffusion and leachability index studies on stabilization of chromium contaminated soil using fly ash. J Journal of Hazardous Materials 2015;297:52-58.

24. Francesco Colangelo, Francesco Messina, Raffaele Cioffi. Recycling of MSWI fly ash by means of cementitious double step cold bonding pelletization: Technological assessment for the production of lightweight artificial aggregates. J Journal of Hazardous Materials 2015;299:181-191.