A Case Study of the Microwave Sintering for the Stabilization of Mswi Fly Ash

Municipal solid waste incineration fly ash is generated in a significant amount; it is classified as hazardous waste in Taiwan. In order to reduce the volume of treated fly ash, a sintering technology can be considered. However, a traditional sintering consumes too much energy because of the limit of heat transfer. Hence, a microwave process which can provide quick, uniform, and selective heating was considered to substitute a traditional thermal process in the sintering technology. In this study a traditional sintering with electro-furnace at 800, 900, 1000, 1100 and 1200 °C for 30 min and the microwave sintering at 1000 W for 15, 20, 25 and 30 min were performed. The results indicated that the microwave sintering had better sintering efficiency than a traditional sintering, and the former could stabilize and transform washed fly ash with calcium carbonate into blocks in a short time at 1 kW. *Corresponding author Email: [email protected] INTRODUCTION Municipal solid waste (MSW) incineration (MSWI) fly ash is generated in a significant amount; it is classified as hazardous waste in Taiwan. The usual treatment method is cement solidification, but it has a disadvantage of increase in waste volume. Hence, other treatment methods, such as traditional fusion and sintering, should be considered. Wainwright and Cresswell [1] indicate that it is possible to successfully manufacture synthetic lightweight aggregates from the combustion ashes derived from the incineration of sewage sludge, MSW and pulverised coal. Karamanov et al. [2] indicate that a mixture, which consists of MSW ashes and waste from feldspar production, can be transformed into non porous glassceramics at a 30 °C min heating rate and a 40 min isothermal step at 1120 °C, near the liquidus temperature. Aloisi et al. [3] indicate that a glass ceramic composite can be obtained by sinter-crystallisation of vitrified MSW bottom ashes with the addition of alumina waste. Chiou et al. [4] use sewage sludge ash as the principal material and sewage sludge as the admixture to sinter lightweight aggregate. Wu et al. [5,6] transform water treatment sludge into Al-containing adsorbents by sintering. The above-mentioned studies show that traditional fusion and sintering technologies can not only avoid the shortcoming of cement solidification but also transform powdery waste into all kinds of products like aggregate, glass ceramics and adsorbent. However, traditional fusion and sintering technologies are not so perfect, they consume much energy because of limit of heat transfer. Tai and Jou [7] indicate that it is technically feasible to stabilize chromium in soil by the application of granular activated carbon or iron weirs with microwave radiation energy. Gan [8] indicates that microwave radiation can be used for detoxication of the sediment sludge through microwave heating, drying and metal ion immobilization within the sediment solids. Menéndez et al. [9] indicate that the temperature of sewage sludge with a small amount of microwave absorber, which is the char produced in the pyrolysis itself, can be raised up to 900 °C by microwave heating in short tome. This situation results in quick drying and pyrolysis for sewage sludge. Others [10-13] have used various microwave processes with some additives to achieve the drying and stabilization of the 382 Sustain. Environ. Res., 20(6), 381-385 (2010) acid-extracted industrial sludge. These studies show that microwave energy can be used for the treatments of swage sludge, contaminated soil and industrial sludge because microwave can provide quick, uniform and selective heating. Hence, a microwave process was considered to substitute traditional heating for a sintering technology, and this microwave sintering could stabilize and transform fly ash into blocks at the same time. This study aims to establish a microwave sintering which stabilizes and transforms MSWI fly ash into blocks. The stabilization effect of a sinter was evaluated from the results of the Toxicity Characteristic Leaching Procedure (TCLP) test and the modified TCLP test. And a traditional sintering with an electrothermal furnace was also performed to compare with the microwave sintering for sintering efficiency. EXPERIMENTAL METHODS AND MATERIALS The fly ash was gathered from a MSWI in north Taiwan. Dried fly ash of 150 g was mixed with water of 1500 mL and the mixture was rotated at 30 rpm for 40 min. Afterward, fly ash and the above solution were separated. The washed fly ash underwent mixing, rotation, and separation processes again, which resulted in that the ratio of total solution to solid was 20:1. Two batches of the solution were collected together and the pH of the collective solution was adjusted to 7.0 ± 0.2 with nitric acid. Afterwards, sodium carbonate of 160 g was added to the adjusted solution to recover calcium ions in the form of calcium carbonate. Washed fly ash and recovered calcium carbonate were dried at 105 °C and then mixed well. 8 g washed fly ash with calcium carbonate (called CFA) was pressed by hands with a set of simple equipment (a stainless steel bar and ring) into a pellet, of which the diameter and height were 2 cm and about 4 cm, respectively. CFA was undergone a microwave digestion with 3 mL H2O2, 1.5 mL, HCl, 4.5 mL HNO3 and 3 mL HF with the following conditions: (1) the temperature of the digestion solution rose from room temperature to 150 °C at 600 W for 20 min; (2) temperature rose again from 150 to 180 °C at 800 W for 10 min; and (3) temperature kept at 180 °C at 800 W for 20 min. 1. Microwave Sintering A piece of graphite was laid on an aluminum oxide cake (18 g aluminum oxide and 2 g gypsum) on the bottom of a crucible (15 mL). Then, a lining of the mixture of powdered activated carbon (PAC) and gypsum with the ratio of 3:1 was placed on the crucible wall. After a CFA pellet was put into a modified crucible, a piece of fireproof material was set at the position about 0.3 cm away from the crucible opening (Fig. 1). A series of microwave processes at 1 kW for 15, 20, 25 and 30 min were performed with air purging into microwave oven. At the end of pre-set time, the crucible was cooled in the microwave oven, and then the crucible was moved to a ventilation system to cool down completely. The sinter was crashed in a mortar and sieved with mesh No. 14 (1 mm). 1.0 g powder passed through the sieve were taken for TCLP test with the extraction solution B (20 mL 0.1 N acetic acid solution, pH = 2.88 ± 0.05). Another 1 g powder was subject to the modified TCLP test as: (i) 16 mL 0.125 N acetic acid solution of 16 mL was added and the mixture of powder and acetic acid solution was rotated at 30 rpm for 24 h; (ii) during the rotation period, the pH of the mixture was controlled at 5.0 ± 0.2 with nitric acid; (iii) accumulated volume of the mixture was increased to 20 mL with deionized water after 1 h; and (iv) mixture was filtered after rotation. CFA with no treatment also was subject to the TCLP test and the modified TCLP test for comparison. The digestion solution and all filtrates were analyzed with an inductively coupled plasma, JY24. 2. Traditional Sintering with an Electro-thermal