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J. Environ. Eng. Manage., 19(4), 221-226 (2009) 221

ASPHALT CONCRETE AND PERMEABLE BRICK PRODUCED FROM INCINERATION ASH USING THERMAL PLASMA TECHNOLOGY

Sheng-Fu Yang,* Wen-Tung Chiu, To-Mai Wang, Wen-Cheng Lee,

Ching-Liang Chen and Chin-Ching Tzeng

Institute of Nuclear Energy Research Atomic Energy Council Taoyuan 325, Taiwan

Key Words: Plasma melting technology, water-quenched vitrified slag, asphalt concrete, permeable brick

ABSTRACT

Ash residue wastes from municipal solid waste incinerators (MSWI), containing considerable amount of hazardous materials such as heavy metals and dioxins, may deteriorate the ecological environment without further treatment. Plasma melting technology can convert the hazardous ashes into environment benign and recyclable materials. In this study, a pilot-scale plasma furnace (6 t d-1) is used to convert the MSWI ashes into water-quenched vitrified slag for making recycled green products. Samples of asphalt cement and permeable brick have been made and characterized. The results of asphalt concrete pavement density, stiffness and British pendulum number indicated that it was equal to natural construction materials for gradation of road. It also enhanced the strength of asphalt concrete when water-quenched vitrified slag was added in the asphalt concrete layer. For water permeable brick the water permeability (permeation index > 0.01 cm s-1), compressive strength (30.6 MPa), and flexural strength (8.6 MPa) meet the product standards. In addition, it is verified by experiments of toxic characteristic leaching procedure that heavy metals released from the eco-product samples are negligible.

*Corresponding author Email: [email protected]

INTRODUCTION There are 19 larger-scale municipal solid waste

incinerators (MSWI) being operated in Taiwan and six more are to be built within two years. The total treat-ment capacity of these incinerators is 25,400 t d-1. Ac-cordingly, 1,026,500 t ash residues (858,300 t bottom ashes and 168,200 t fly ashes) will be generated annu-ally in Taiwan [1]. Fly ashes, containing high concen-trations of heavy metals and dioxins, are classified as hazardous wastes by the Environmental Protection Administration of Taiwan (Taiwan EPA) and must be stabilized and/or solidified to meet the toxicity charac-teristic leaching procedure (TCLP) regulation limits before final disposal. Currently, the MSWI fly ashes are generally cemented and then sent to disposal sites. Due to the shortage of available disposal sites and lim-ited stability of cement solidified waste forms which may potentially release heavy metals and dioxins to contaminate the environment in the long-term future, Taiwan EPA and the Institute of Nuclear Energy Re-

search (INER) cooperated to study the plasma melting process for reclamation of the MSWI ash residues.

Vitrification for treating incinerator ash has been received considerable attention [2-7], particularly in countries with high population densities in metropoli-tan areas and very limited space available for waste disposal. Thermal plasma treatment is one of the best vitrification methods. In thermal plasma vitrification, the heat generated by the plasma is utilized to treat hazardous wastes containing heavy metals, inorganic and/or organic chemicals at temperatures of 1400–1600 °C. During treatment, inorganic substances are melted and organic contaminants are thermally de-stroyed. The first DC-generated plasma system (1.2 MW; capacity, 250 kg h-1) was established in Taiwan at the INER; this system was used in this study for treating MSWI ashes.

The plasma furnace at the INER has been pro-ducing water-quenched vitrified slag from MSWI ashes via a plasma melting process. Generally, water-quenched vitrified slag can be deposited in landfills,

222 J. Environ. Eng. Manage., 19(4), 221-226 (2009)

and used as construction materials. Jimbo [8] demon-strated that using plasma ash melting furnace system could possibly melt MSWI ash residues and remove hazardous elements as well. Granular water-cooled slag formed had sufficient strength for using as con-struction materials. Nishigaki [9] showed that the mol-ten slag from surface-melting furnace and plasma-melting furnace to melt MSWI ash residues did not leach heavy metals and was suitable for concrete gravel and roadbed aggregate. In addition, Cheng et al. [10,11] used vitrified slag from thermal plasma to manufacture microstructure materials and colored glass-ceramics. Both materials had great potential to serve as a viable alternative for construction applica-tions.

The aims of this study were to dispose MWSI ashes by plasma vitrification, utilize vitrified slag and promote its reutilization by means of manufacturing asphalt concrete and permeable bricks for constructive applications. In this study, trial production of asphalt concrete and permeable bricks was made from water-quenched vitrified slag. The results of characteristic test of water-quenched vitrified slag, asphalt concrete and permeable bricks are also described.

MATERIALS AND METHODS

1. Melting Device

The 250 kg h-1 pilot-scale plasma melting system

built in the INER was used as the melting device for this study. A 1.2 MW transferred plasma torch devel-oped by INER was used as the heat source in the plasma system [12]. Argon was employed for plasma ignition and nitrogen was served as the working gas during the treatment. The fly ash and bottom ash were separately fed into the furnace through different feed-ers for controlling the slag basicity (i.e. the weight ra-tio of CaO and SiO2), and melted into vitreous slag at 1450 °C by the conduction heat transfer and the elec-tric arc produce joule heat from the high-temperature plasma arc. Molten slag then overflowed through the slag tap and continuously discharged into the cooling unit to produce water-quenched vitrified slag. Off-gas was processed by means of the off-gas treatment sys-tem to meet the regulation standards.

Table 1. Major chemical compositions (in wt%) of fly ash and bottom ash

CaO SiO2 Al2O3 Fe2O3 Na2O K2O Cl Fly ash 46.1 12.3 8.1 1.6 5.8 2.9 15.9 Bottom ash 25.6 34.0 15.8 19.6 1.7 0.7 0.6

2. Incinerator Ashes

The incinerator ashes used in this study were ob-tained from one of the MSWIs in Taipei. Table 1 pre-sent the major chemical compositions of the incinera-tor ashes. The major chemical compositions of fly ash and bottom ash were CaO, SiO2, Al2O3. The Zn, Pb, Cd, Cr, Cu, Hg and As in TCLP test of incinerator ashes and water-quenched slag were determined by Taiwan EPA standard method of NIEA R201.13C. TCLP test results of incinerator ashes are shown in Table 2. The leaching concentration of Pb was 5.5 mg L-1 and higher than TCLP criteria in Taiwan. Accord-ing to Taiwan EPA regulations, fly ash was a hazard-ous waste in Taiwan and it requires to be detoxified before final disposal.

3. Characterization Analysis

The true densities of water-quenched vitrified

slag were determined by an UltrapycnoMeter-1000 (Quantachrome, USA). The chemical compositions of MSWI ashes and water-quenched vitrified slag were analyzed by inductively coupled plasma mass spec-trometry (SP1000, Teledyne Leeman, USA). The wa-ter-quenched vitrified slag surface characteristics were analyzed by scanning electron microscopy (S-4800, Hitachi, Japan). Pavement measurements such as pavement density, stiffness and British pendulum number (BPN) were determined by Pave Tracker (2701, Troxler, USA), GeoGauge (H-4140, Humboldt, USA) and British Pendulum Surface Friction Tester (HM-602W, Gilson, USA), respectively.

4. Trial Production of Asphalt Concrete

Figure 1 shows the producing process of asphalt

concrete by using water-quenched vitrified slag as one of the based materials. According to ASTM D-1559

Table 2. TCLP (Toxicity characteristic leaching procedure) analysis results (in mg L-1) of MSWI ashes and water-

quenched vitrified slag

Zn Pb Cd Cr Cu Hg As Fly ash 1.5 5.5 ＜0.01 0.19 ＜0.05 ＜0.05 ＜0.1 Bottom ash ＜0.05 ＜0.1 ＜0.01 0.11 0.42 ＜0.05 ＜0.1 Water-quenched slag 0.2 ＜0.1 ＜0.01 0.05 0.39 ＜0.05 ＜0.1 Asphalt concrete 0.14 ＜0.1 ＜0.01 ＜0.01 0.35 ＜0.05 ＜0.1 Permeable brick 0.36 0.21 ＜0.01 ＜0.01 0.94 ＜0.05 ＜0.1 TCLP criteria － 5.0 1.0 5.0 － 0.2 5.0

Symbol “<” represents below detection limits

Yang et al.: Asphalt Concrete and Permeable Brick from Slag 223

Asphalt Aggregate

Trial paving

Water-quenched vitrified slag

Selecting trial gradation

Mixing

Desired gradation

Fig. 1. Producing process of the asphalt concrete.

Binder Waste ceramic

Sintering at 900

Water-quenched vitrified slag

Granulating and Molding

Mixing

Drying at room temperature

oC

Fig. 2. Producing process of the permeable brick. [13] and American Association of State Highway and Transportation Officials T-245 [14], the best gradation of asphalt concrete can be chosen if the proportion of water-quenched vitrified slag (as fine aggregate), as-phalt and natural aggregate is suitably adjusted. After then the processed trial paving was performed to ap-ply this asphalt concrete in three layers of the road in-cluding surfacing, base course and sub-base course.

5. Trial Production of Water Permeable Bricks

Figure 2 shows the producing process of the

permeable bricks. A water permeable brick consists of three major components including water-quenched vitrified slag, waste ceramic and binder. As a raw ma-terial, water-quenched vitrified slag was grinded and sieved to select the slag particles that pass through a 12-mesh sieve (1.68 mm) and be retained by a 20-mesh sieve (0.84 mm). Then the three types of raw materials were mixed thoroughly at room temperature. After that, the mixture was undergoing a series of pro-cedures such as granulating, molding and drying. Fi-nally, after sintering at 900 oC in a tunnel brick kiln, the water permeable bricks were produced.

RESULTS AND DISCUSSION

1. Water-quenched Vitrified Slag

Water-quenched vitrified slag from MSWI fly ash and bottom ash with the weight ratio of 1:4 was glass-like and leaching-resistant. Figure 3 shows the appearance of water-quenched vitrified slag and Fig. 4 is microphotograph of water-quenched vitrified slag. The color of water-quenched vitrified slag was deep black and the particle size was in the range of 0.4 to 18.0 mm. Figure 5 shows the distribution of water-

Fig. 3. Photograph of water-quenched vitrified slag.

Fig. 4. Microphotograph of water-quenched vitrified slag.

0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100Particle size (mm)

Perc

ent o

f ac

cum

ulat

ed w

eigh

t (%

)

Fig. 5. Particle size distribution in water-quenched vitrified

slag.

224 J. Environ. Eng. Manage., 19(4), 221-226 (2009)

Table 3. The major chemical compositions of water-quenched vitrified slag

Chemical composition wt.% CaO 28.2 SiO2 38.3 Al2O3 14.3 MgO 2.0 Fe2O3 9.0 Na2O 2.7 Cl 0.001 Ignition Loss 0.37

quenched vitrified slag and the d50 is about 3 mm. The average true density and hardness (Mohs) of water-quenched slag are 2.85 g cm-3 and 3-4, respectively. The Zn, Pb, Cd, Cr, Cu, Hg and As in TCLP test and the major chemical compositions of water-quenched vitrified slag are shown in Tables 2 and 3. The main components left are non-hazardous minerals, includ-ing CaO, SiO2, Al2O3, Fe2O3 and MgO. TCLP test re-sults of water-quenched vitrified slag reveal that the leachate concentrations of heavy metals are far below the regulated limits. The low leachability was related to the superior slag structures.

2. Water Quality Analysis of the Quenching Unit

In the experiment, the stream of lava formed dur-

ing the plasma melting of MSWI ashes flowed down to a water-quenching unit and converted it into the form of granulated vitrified slag. In order to know the leaching properties of toxic heavy metals (Pb, Cd, Hg, Cr, Fe, Cu, Zn, Ni) in the quenching unit, water sam-ples were sampled for element analysis. Table 4 shows that the toxic heavy metals concentrations of water in the quenching unit are very low and pH is about 6.4. The results prove that heavy metals do not transport to quenching unit and contaminate the water when the granulated vitrified slag is formed. The wa-ter quality meets the effluent criteria of Taiwan EPA.

3. Pavement of Asphalt Concrete

Figures 6-8 are shown the pavement of asphalt

Fig. 6. Photograph of trial pavement of asphalt.

A (With natural construction aggregate )

0 m 10 m

B (With water-quenched vitrified slag )

3.5 m

3.5 m

Fig. 7. Paving area of asphalt concrete pavement.

Base (Additive of water-quenched vitrified slag , 20%)

Subbase (Additive of water-quenched vitrified slag , 10%)

Surfacing, Asphalt concrete (Additive of water-quenched vitrified slag , 5%)5 cm

10 cm

20 cm

Fig. 8. Paving depth and additive of water-quenched

vitrified slag. concrete, disposition chart of pavement test spot in INER, paving depth and additive of water-quenched vitrified slag, respectively. Test-road with a distance of 10 m was paved at INER using water-quenched vit-rified slag as additives in three layers of the road in-cluding asphalt concrete layer, base course and sub-base course. Each layer has different quantity of wa-

Table 4. Water quality analysis of water-quenched unit

Pb Cd Hg Cr Fe Cu Zn Ni pH 1 ＜0.02 ＜0.01 ＜0.18 ＜0.03 0.12 ＜0.01 0.02 0.10 6.5 2 ＜0.02 ＜0.01 ＜0.18 ＜0.03 0.01 ＜0.01 0.07 0.03 6.5 3 ＜0.02 ＜0.01 ＜0.18 ＜0.03 0.03 0.10 0.13 0.03 6.4 4 0.08 ＜0.01 ＜0.18 ＜0.03 ＜0.01 0.03 1.72 0.07 6.4 5 ＜0.02 ＜0.01 ＜0.18 ＜0.02 0.14 ＜0.01 0.07 0.07 6.4 6 ＜0.02 ＜0.01 ＜0.18 ＜0.03 0.06 0.05 0.23 0.11 6.4 7 ＜0.02 ＜0.01 ＜0.18 ＜0.03 0.05 0.06 0.15 0.04 6.4 Effluent criteria 1.0 0.03 5 2.0 10.0 3.0 5.0 1.0 6.0-9.0

All in mg L-1, except for Hg in μg L-1 and pH. Symbol “<” represents below detection limits.

Yang et al.: Asphalt Concrete and Permeable Brick from Slag 225

Table 5. Properties of the asphalt concrete pavement

Location A* B** Pavement density (%) 92.5 92.3 Stiffness (MN mm-1) 32.8 36.6 British pendulum number (BPN)*** 75 75

* Asphalt concrete with natural construction aggregate ** Asphalt concrete with water-quenched vitrified slag *** Test method base on ASTM E303-93[15] ter-quenched vitrified slag with 20, 10 and 5 wt% for asphalt concrete layer, base course and sub-base course, respectively. Table 2 shows results of asphalt concrete (with 20 wt.% slag added) leaching test; it indicates that asphalt concrete passes the regulation and can be reutilized as construction materials.

The analyzing results of asphalt concrete layer are shown in Table 5. The measurements (Pavement with water-quenched vitrified slag) of asphalt concrete layer such as pavement density (92.4%), stiffness (33-37 MN mm-1) and BPN (75) were equal to pavement with natural construction aggregate (pavement density, 92.5%; stiffness, 32.8 MN mm-1; BPN, 75). The re-sults indicate that water-quenched vitrified slag can substitute for natural construction materials and reuti-lized to asphalt concrete layer of road.

4. Producing Water Permeable Brick

Figures 9-11 show the appearance of the water

permeable brick, vertical face of pavement, pavement of permeable bricks at INER, respectively. The per-meable brick dimension is 200 (L) × 100 (W) × 60 (H) mm. The additive proportion of water-quenched vitri-fied slag in permeable brick was 66 wt.%. The paving depths of aggregate, sand, permeable brick were 15, 3 and 6 cm, respectively. The paving area was 90 m2. Table 2 shows results of water permeable brick leach-ing test for verifying the brick without hazardous properties. It indicates that asphalt concrete passes the regulation and can be reutilized as raw materials to produce water permeable brick.

The results of various physical parameters are listed in Table 6. The average bulk density of the brick samples is 1.67 g cm-3, while the average flexural, compressive strength and permeation index are 8.6

Fig. 10. Vertical face of water permeable brick pavement.

Fig. 11. Photograph of water permeable brick pavement. Table 6. Properties of permeable bricks

Item Sample 1 Sample 2 Average Bulk density (g cm-3) 1.66 1.68 1.67 Bending strength (MPa) 8.4 8.8 8.6 Compressive strength (MPa) 30.0 31.1 30.6 Permeation index (cm s-1) 0.016 0.016 0.016

MPa, 30.55 MPa, and 0.016 cm s-1, respectively. Ac-cording to Chinese National Standard (CNS) 13289, CNS1232 and CNS1233 the permeable brick was be-long to C class and suitable for sidewalk and bicycle lane.

CONCLUSIONS

Hazardous MSWI ash residues were successfully

vitrified into water-quenched slag by thermal plasma torch. TCLP results verified that the leachate concen-trations of all regulated metals from water-quenched vitrified slag were far below the regulation limits of Taiwan EPA. Producing green product such as perme-able brick and substitute for natural aggregate were demonstrated a feasible way for recycling water-quenched vitrified slag from incinerator ashes. The gross plasma processing cost for treatment of MSWI ash residues will be significantly reduced if the water-quenched vitrified slag can be reutilized for making permeable bricks and asphalt cement in construction application. Therefore, the prevention of MSWI ash residues as secondary pollutant and the recycling of them as construction raw material can be optimisti-

Fig. 9. Photograph of water permeable brick.

200 × 100 × 60 mm, permeable brick

Coarse sand, 3-5 cm (thick), ramming

Soil, ramming

Crushed gravel, 25-30 cm (thick), 3-5 cm (particle size), ramming

Curb stone

Draining system

226 J. Environ. Eng. Manage., 19(4), 221-226 (2009)

cally expected.

ACKNOWLEDGMENTS The authors would like to acknowledge the fund-

ing and administrative supports for this study provided by the Taiwan EPA through a cooperative project be-tween the INER and Taiwan EPA for recycling the MSWI ash residues by plasma melting technology.

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Discussions of this paper may appear in the discus-sion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication.

Manuscript Received: March 3, 2009 Revision Received: May 24, 2009

and Accepted: May 27, 2009