Environ. Sci. Technol. 2007, 41, 2950-2956
Vitrification of Chromium Electroplating Sludge C H U N - T E H L I , † W E N - J H Y L E E , †,§ K U O - L I N H U A N G , * ,‡,§ S H E N G - F E N G F U , † A N D Y I - C H I E H L A I †,§ Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan, Department of Environmental Engineering and Science, National Pingtung University of Science and Technology, Neipu, Pingtung 91201, Taiwan, and Sustainable Environmental Research Center, National Cheng Kung University, Tainan, 70101, Taiwan
This work investigated the vitrification of hazardous electroplating sludge containing 140 mg/g Cr with/without bottom ash or cullet conditioning to lower specimens’ basicities to 0.97-1.17 or 0.18-0.23, respectively. The conditioning was found to enhance the smooth/glassy appearance of slags but no ingot was obtained. Cr was >98% retained in the vitrified slags. Cr2O3 dominated in crystalline structure for the slag vitrified from the sludge, but this domination gradually shifted to Fe2SiO4/Fe3O4/SiO2 or SiO2 crystalline with increasing addition of bottom ash or cullet into the sludge, respectively. Compared to the raw sludge, the sludge-vitrified slag displayed lower leaching concentrations for most metals (particularly Cr (2.54 mg/ L)), and smaller leaching ratios for Ag, Cr, and Cu (1.35, 0.02, and 14.7 MPa at the mixing ratios g2/1 and 1/1 for bottom ash/sludge and cullet/sludge, respectively. Fulfilling the criteria of Toxicity Characteristic Leaching Procedure (TCLP), all the slags were recyclable.
1. Introduction Metal plating/finishing processes usually generate wastes with significant potential for environmental impact and many platers still rely heavily on conventional precipitation wastewater treatment systems that often produce considerable amounts of electroplating/galvanic sludge (1). In Taiwan, the annual generation of electroplating sludge has significantly increased recently, and it was approximately 43 500 tons in 2005 (2). Recognized world wide as hazardous waste, electroplating sludge can be incinerated to reduce volume but yields hazardous fly ashes (3, 4). Solidification processes are available to lower the mobilities of the heavy metals in * Corresponding author phone: +886-8-7703202 ext. 7092; fax: +886-8-7740256; e-mail:
[email protected]. † Department of Environmental Engineering, National Cheng Kung University. ‡ Department of Environmental Engineering and Science, National Pingtung University of Science and Technology. § Sustainable Environmental Research Center, National Cheng Kung University. 2950
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galvanic sludge, but the leaching of toxic metals from the solidified products is still a concern. Co-incinerated with clinker raw meal in cement rotary kilns, chromium and nickel oxides commonly found in galvanic sludges may be incorporated into clinkers with low leaching concentrations of Cr and Ni; however, the clinkerization processes are still conducted by in-lab scales with small addition (below 2% weight) of the chromium and nickel oxides into clinkers (4, 5). The electroplating sludge may also be solidified using alkali-activated pulverized fuel ash as cementitious binder; nevertheless, Cr was found to exceed the USEPA limit in some cases in TCLP leachates (6). Alternatively, vitrification of hazardous wastes has been proposed for many years (7) and has received increasing attention for the treatment of electroplating sludge. Vitrification processes not only can achieve a large volume reduction of wastes beneficial for long storage or landfill, but also may homogeneously stabilize heavy metals in a vitrified glassy matrix (7). Both slags and ingots can be obtained in the vitrification of bottom ash or hazardous fly ash from municipal solid waste (MSW) incineration with/without bottom ash or cullet addition, and they may be utilized as recyclable materials (8-11). It has been reported that electroplating sludge treated at 1000 °C in N2-O2 plasma with calcinations might obtain ferrite/chromite concentrated in the collected deposits/powders (12). Another study indicated that 99.6% of Cr could be retained in the slags of chromium electroplating sludge specimens thermally treated at
