Copper stabilization in beneficial use of waterworks sludge and copper-laden electroplating sludge for ceramic materials

Highlights

• Identify Cu-hosting phases by sintering Cu sludge with waterworks sludge ash.

• Quantify phase compositions in the sintered product.

• Compare product leaching behavior to suggest the preferred metal hosting phases.

• Provide a feasible strategy to safely and reliably recycle metal-containing waste.

Abstract

A promising strategy for effectively incorporating metal-containing waste materials into a variety of ceramic products was devised in this study. Elemental analysis confirmed that copper was the predominant metal component in the collected electroplating sludge, and aluminum was the predominant constituent of waterworks sludge collected in Hong Kong. The use of waterworks sludge as an aluminum-rich precursor material to facilitate copper stabilization under thermal conditions provides a promising waste-to-resource strategy. When sintering the mixture of copper sludge and the 900 °C calcined waterworks sludge, the CuAl₂O₄ spinel phase was first detected at 650 °C and became the predominant product phase at temperatures higher than 850 °C. Quantification of the XRD pattern using the Rietveld refinement method revealed that the weight of the CuAl₂O₄ spinel phase reached over 50% at 850 °C. The strong signals of the CuAl₂O₄ phase continued until the temperature reached 1150 °C, and further sintering initiated the generation of the other copper-hosting phases (CuAlO₂, Cu₂O, and CuO). The copper stabilization effect was evaluated by the copper leachability of the CuAl₂O₄ and CuO via the prolonged leaching experiments at a pH value of 4.9. The leaching results showed that the CuAl₂O₄ phase was superior to the CuAlO₂ and CuO phases for immobilizing hazardous copper over longer leaching periods. The findings clearly indicate that spinel formation is the most crucial metal stabilization mechanism when sintering multiphase copper sludge with aluminum-rich waterworks sludge, and suggest a promising and reliable technique for reusing both types of sludge waste for ceramic materials.

Introduction

Pollution caused by hazardous metals has become an increasingly significant concern. Industrial processes such as electroplating produce solid sludge containing significant amounts of toxic metal-laden compounds (Peng and Tian, 2010). The electroplating sludge discarded each year in China contains more than 100,000 tons of hazardous metals (Wang, 2006), and about 1.3 million tons of wet electroplating sludge is generated each year in the United States, according to the US Environmental Protection Agency (US EPA, 1998). A widely accepted treatment process is the stabilization/solidification (S/S) method, which can convert hazardous wastes into chemically stable solids (Chen et al., 2011, Sophia and Swaminathan, 2005). The S/S method achieves pollutant encapsulation through an interlocking framework of hydrated minerals (Sophia and Swaminathan, 2005, Zhou et al., 2006). In addition to the severe environmental problems caused by industrial wastewater treatment, waterworks processes also generate treatment sludge containing the residues of treatment chemicals used as coagulants (usually aluminum based) (Vicenzi et al., 2005). The costs of handling the constantly generated waterworks sludge from drinking water treatment systems are significant and continually increasing as more stringent regulations are introduced (Babatunde and Zhao, 2007). The ceramic products may include a wide variety of interior and exterior tiles for walls and floors, decoration objects, partitioning materials, soundproofing and fire-resistant materials, which are with a continuous and increasing demand for built environment. In general, the products of solidification/stabilization (S/S) and waterworks sludge are disposed of in landfills (Babatunde and Zhao, 2007, Chen et al., 2011). However, the limited land available for waste disposal, and the adverse environmental impact (Malviya and Chaudhary, 2006), has made the development of effective and financially viable treatment technologies essential. Recent studies have evaluated the feasibility of using waterworks sludge as a raw material for ceramic production, and suggested that incorporating harmful metals into crystal structures can successfully reduce the risks to the environment (Lin and Weng, 2001, Vicenzi et al., 2005, Xu et al., 2008). In our previous studies (Tang et al., 2010, Tang et al., 2011), we found that simulated copper sludge reacted with the aluminum-rich precursors to form CuAl₂O₄ spinel phases after a 3-h sintering process. The metal leachability after thermal treatment was also significantly reduced due to the formation of spinel phases (Tang et al., 2010, Tang et al., 2011).

As a waste-to-resource technology, the use of waste sludge resulting from water and wastewater treatment processes has attracted attention (Babatunde and Zhao, 2007). Recycling industrial and waterworks sludge for ceramic production can encourage a more sustainable use of natural resources and provide economic incentives. Moreover, if the application of waste waterworks sludge in ceramic systems can further benefit the stabilization of hazardous metals, it will simultaneously reduce the burden of waste management and environmental hazards. Therefore, in this study we evaluated the potential use of waterworks sludge and industrial metal sludge for ceramic materials, and explored the feasibility of metal immobilization during the sintering process. The work reported in this study is different from the previous studies (Tang et al., 2010, Tang et al., 2011), although similar copper reaction mechanisms were found to involve in the process. This study first used two real sludge samples derived from the waste streams (waterworks sludge and copper electroplating sludge) to observe the copper stabilization mechanisms under ceramic processing conditions.

The X-ray diffraction (XRD) technique is a basic material characterization method that has been successfully used for many decades to provide accurate information about the structure of materials (Pecharsky and Zavalij, 2009). The technique is often used as a qualitative technique to monitor and identify toxic metal phase(s) contained in samples (Dermatas and Meng, 2003, Malliou et al., 2007). However, when combined with the Rietveld refinement method, XRD can also be used to quantify the phase compositions of natural or industrial materials. It has been proven to be a reliable, precise, and very reproducible technique for quantifying the relative phase abundances (Walenta and Füllmann, 2004, Young, 1995), such as in the cement industry, where it is needed for accurate phase compositions. Therefore, the qualitative and quantitative functions of the XRD technique were adopted in this study to examine potential copper-hosting phases and to explore copper incorporation through ceramic sintering. A prolonged leaching procedure was also carried out under a pH value (4.9) which was much closer to the environmental condition to examine the stabilization result of copper in the sintered ceramic products.