Recycling Incinerator Fly Ash and Production of Geopolymer Green Cement

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1 Recycling Incinerator Fly Ash and Production of Geopolymer Green Cement Wei-Hao Lee 1a, Ke-Chih Chang 1b, Ta-Wui Cheng 1, Yung-Chin Ding 1 1 Institute of Mineral Resources Engineering, National Taipei University of Technology, Taipei City, Da an Dist., 106, Taiwan (R.O.C.) a glowing955146@hotmail.com, b p @hotmail.com ABSTRACT Incineration has become an important treating method for combustible solid wastes, especially in municipal areas due to the increasing difficulty to get suitable sites for traditional landfill. It is estimated that there are about 2800 tons of incinerator fly ashes generated annually in Taiwan. However, these incinerator fly ashes contain large amount of hazardous materials such as chloride and heavy metals. If these hazardous materials cannot be carefully treated, it will cause detrimental secondary contamination. The objective of this research work is trying to use a saving water attrition scrubbing method to wash incinerator fly ashes in order to removal chloride and heavy metals. The washed fly ashes will be the raw material for making geopolymer green cement. Experimental results show that during washing process, a two-stage attrition scrubbing technology with additional Na 2 CO 3 and CO 2, the washed incinerator fly ash can be achieved the standard limitation of TCLP and the total chloride can be removed 80%. The washed incinerator fly ash reused as raw material for geopolymer green cement show great properties. Keywords: Incinerator Fly ash, Attrition Scrubbing, Geopolymer, Green Cement 1. Introduction Municipal solid waste in Taiwan and in many other industrialized countries is normally burned in incinerators. According to the Taiwan Environmental Protection Administration (EPA) statistics for 2014, incineration is the primary method for treating municipal waste, the success of waste treatment is as high as 97%. In 2014, operating municipal solid waste incinerator (MSWI) produced over 290,000 metric tonnes of fly ash in Taiwan. MSWI fly ash usually contains many hazardous heavy metals and chloride, thus MSWI fly ash is usually placed in a landfill after solidification with cement and the addition of chelating agents. However, toxic components still can leach from landfill solids and pollute groundwater [1,2]. In

2 addition, shortage of landfill sites is a serious problem in Taiwan. Therefore, recycling and reutilization MSWI fly ash could be a potential way to alleviating the problem. Previous studies have presented various methods for recycling MSWI fly ash, including using water washing and acid leaching to remove chloride and heavy metal from MSWI fly ash [3,4], bubbling CO 2 during water washing to remove insoluble chloride from MSWI bottom ash [5]. However, these methods required further solutions to be applicable such as the massive volume of water used for water washing and waste acid handling. The proposes of this study is to use attrition scrubbing and geopolymeric technology to remove water-soluble/ insoluble chlorides and heavy metal from MSWI fly ash and reutilize it in non-structural constructions and related fields. 2. Experimental The MSWI fly ash samples used in this study were collected from an incinerator in northern Taiwan. The chemical composition of the MSWI fly ash sample was analyzed by ICP-AES and the chloride content was examined according to the CNS standard method. The chemical compositions of the as-received MSWI fly ash samples are listed in Table 1. As shown in the result, the major element in MSWI fly ash is calcium, and the fly ash was also containing large amount of water-soluble/insoluble chloride and heavy metals, such as lead and zinc. In this study, attrition scrubbing was used for removing chloride from MSWI fly ash. Attrition scrubbing is a mineral processing technology generally used for separating clay or silts when they are tightly stuck to the silica grains, or when clay particles are similar in size to the silica sand grains. For appropriate attrition scrubbing, it is important that the solid content of the slurry reach a high solid content, i.e %. At this proportion, there is a good particle-to-particle collision and it is easier for the fine particles to separate from the surface of the coarse particle. Normally, chlorides and heavy metals are on the surface of MSWI fly ash. Therefore, by using attrition scrubbing technology, chlorides and heavy metals can be more easily and efficiently removed from the MSWI fly ash surface with a very short attrition time and low water consumption. Attrition scrubbing experiments were divided into two stage treatment, experiment parameters were shown in Table 2. Attrition scrubbing speed was set at 1500 RPM.

3 After MSWI fly ash samples were treated in different conditions, the washed samples were filtrated and dried at 100 for 24hr. The water-soluble/insoluble chloride content in the samples, mineral phases and TCLP results were determined. Table 1 MSWI Fly Ash Chemical Composition Element Content Content Composition Composition (wt %) (wt.%) CaO 38.7 Na 2 O 10.1 SiO K 2 O 6.1 Al 2 O MgO 1.2 Fe 2 O ZnO 1.2 L.O.I. 5.0 soluble-cl PbO 0.2 insoluble-cl Table 2 Attrition Scrubbing Experiment Parameters Experiment No. 1st stage 2nd stage F1 L/S = 1, 3min - F2 L/S = 1, 3min L/S = 2, 3min F3 L/S = 1, 3min, 2wt% Na 2 CO 3 L/S = 2, 3min F4 L/S = 1, 3min L/S = 2, 3min, 2wt% F5 F6 L/S = 1, 3min, CO 2 ( 3L / min ) L/S = 1, 3min Na 2 CO 3 L/S = 2, 3min, CO 2 ( 3L / min ) L/S = 2, 3min, CO 2 ( 3L / min ) This study also used geopolymeric technology by mixing washed MSWI fly ash, blast furnace slag and alkali solution ( SiO 2 /Na 2 O = 1.28) to form geopolymer, then comparing with the samples strength which was mixed with cement and tap water. In this experiment, addition amount of blast furnace slag and cement, curing days, effect of chloride content on compressive strength were also discussed. The size of compressive test samples was Φ3 x 6 cm. The experimental procedure was shown in Figure 1.

4 MSWI Fly Ash Tap Water None / CO 2 / 2% Na 2 CO 3 None / CO 2 / 2% Na 2 CO 3 Attrition Scrubbing Attrition Scrubbing Tap Water Clean MSWI Fly Ash Alkali solution Mix Curing Geopolymer Fig. 1 Overall experiment procedures 3. Results and Discussion 3.1 Water-Soluble/insoluble chloride removal efficiency Experiment results were shown in Figure 2. As the results shown, Only for F1 attrition scrubbing treatment, the total chloride removal efficiency can be reached around 67%. When the 1st stage attrition scrubbing (F1) extended to the 2nd stage (F2), water-soluble chloride removal efficiency only increased 10% due to more water can be dissolved more chloride. The optimal parameter of attrition scrubbing was adding Na 2 CO 3 in the 1st stage, after the 2nd stage attrition scrubbing, more than 75% of water-soluble chloride can be removal, over 70% of insoluble chloride can be remove as shown in F3. After calculation, using optimal parameter of attrition scrubbing on MSWI fly ash, total chloride removal rate was over 80%. According to XRD analysis, adding Na 2 CO 3 or bubbling CO 2 during attrition scrubbing could turn CaCl(OH) into CaCO 3, and this results could be related to insoluble chloride removal efficiency.

5 (a) (b) (c) Fig. 2 Results of removal water-soluble/insoluble chloride (a) water-soluble chloride (b) insoluble chloride (c) total chloride (d) XRD analysis, Calcium Chloride Hydroxide, Sylvite, Halite, Gypsum, Calcite. 3.2 TCLP analysis results- TCLP Experiment results were shown in Table 3. It was shown that the amount of dissolved lead from the original MSWI fly ash was higher than limitation. After attrition scrubbing, the amount of dissolved lead decreased. After adding Na 2 CO 3 during attrition scrubbing, the amount of dissolved lead dropped to below limitation, and complied with legal regulations.

6 Table 3 TCLP analysis results Heavy metals Ba Cu Pb Cr Cd Zn MSWI Fly Ash (mg/l) N.D F1 (mg/l) N.D. N.D. 0.8 F2 (mg/l) N.D F3 (mg/l) N.D. N.D F4 (mg/l) N.D. N.D F5 (mg/l) N.D. N.D. N.D. F6 (mg/l) N.D. N.D Limitation (mg/l) Reutilization washed MSWI fly ash Compressive strength experiment results for washed MSWI fly ash based geopolymer samples were shown in Figure 3. As shown, using unwashed MSWI fly ash to form geopolymer, the compressive strength was higher than that of using cement samples. Using washed MWSI fly ash and blast furnace slag at the wt% ratio of 7:3 to form geopolymer, after curing for 28 days, the geopolymer compressive strength can be reached 15MPa. However, in the same case, the compressive strength of cement sample only reached 2-3MPa.

7 (a) (b) Fig. 3 Results of reutilization (a) Original MSWI fly ash (b)washed MSWI fly ash. G:geopolymer, C: cement. 37:slag(cement):fly ash = 30%:70%, 55:slag(cement):fly ash = 50%:50% After 28 days curing, all the samples using XRD analyze crystal phase. XRD analysis results as shown in Figure 3. As shown, CF-37 still remain a lot of insoluble chloride in the sample, it cause the compressive strength of cement sample always lower than geopolymer sample. According to the XRD analysis results also shown in geopolymer system, insoluble chloride will turn into hydrocalumite, and it made the compressive strength of geopolymer sample higher than cement sample.

8 Fig. 4 Results of XRD analysis Geopolymer and Cement sample Calcium Chloride Hydroxide, Sylvite, Halite, Gypsum, Calcite, Hydrocalumite. 4. Conclusions Using attrition scrubbing to remove chloride from MSWI fly ash was achieved and a removal rate of 75% water-soluble chloride, 90% of insoluble chloride can be reached. The washed MSWI fly ash complies with TCLP regulatory. Compared with traditional water-washing, the advantage of attrition scrubbing was low water consumption and short treatment time. It was found that reutilizing washed MSWI fly ash to fabricate geopolymer, the compressive strength can be reached over 15MPa. Therefore, using attrition scrubbing to removal chloride and geopolymeric technology to reutilize MSWI fly ash have great potential for engineering application, such as non-structural constructions and related fields. 5. Acknowledgements Authors would like to express gratitude for [NSC E MY2] project, supporting the funds for this research. References [1] V. H. Peter, V. D. B. Bart, V. Godfried D, V. Carlo H A, Application of computer modelling to predict the leaching behavior of heavy metals from MSWI fly-ash and comparison with a sequential extraction method, Waste Management, 20, (2000)p [2] M. Svensson, M. Berg, K. Ifwer, R. Sjoblom, H. Ecke, The effect of

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