Assessment of copper and zinc recovery from MSWI fly ash in Guangzhou based on a hydrometallurgical process

Highlights

• A new hydro-recycling process of Cu and Zn from MSWI fly ash was evaluated.

• 95% of Cu and 61% of Zn were recovered in this pilot scale system.

• A scenario was developed based on the process for fly ash treatment in Guangzhou.

• Economic analysis shows combing metals recycling and local disposal as best option.

• The hydrometallurgical process could be alternative for industrial application.

Abstract

Fly ash commonly accumulates a significant amount of heavy metals and most of these heavy metals are toxic and easily leached out to the environment, posing risks to human health. Thus, fly ash has been classified as a type of hazardous waste and requires proper treatment before disposal in specific landfill sites for hazardous waste. In this study, a hydrometallurgical process developed to recover copper and zinc performed in pilot scale close to industrial scale followed by a landfill compliance leaching test of the ash residue is evaluated. LIX860N-I and Cyanex 572 gave high selectively for extractions, a yield efficiency of 95% and 61% was achieved for copper and zinc respectively. Results of pilot experiments reveals that the combining metal recovery/recycling and landfill disposal of the ash residue in a local regular landfill was demonstrated to be a technically and economically effective strategy. Specifically, the economic and environmental aspects of a scenario, in which the fly ash generated in Guangzhou is processed were systematically assessed. the assessment results show that a 7.15 million US$ of total expense reduction, a less energy cost of 19k GJ as well as 2100 tons less CO₂ emissions could be achieved annually comparing to the current alternative, direct disposal of the fly ash as hazardous waste. The results reveal that the hydrometallurgical process has industrial application potential on both economic and environmental aspects and further optimization of the process can give more accurate assessment of the cost and environment effect. In addition, leaching tests and evaluation of solid residue according to the regulations specific to the country should be studied in future.

Introduction

Accompanying the rapid economy growth in China, tremendous amounts of municipal solid waste (MSW) are produced. At present, China is the second largest MSW producing country in the world, with approximately 220 million tons of MSW being produced in 2015 and an annual increasing rate of 8–10% (Xin-gang et al., 2016). Without proper treatment, MSW can cause serious environmental pollution (Han et al., 2016, Zhao et al., 2016). However, MSW also offers excellent opportunities for resource recovery, because of rich organic matters, valuable metals, and nutrients embedded (Lederer et al., 2017, Tang and Steenari, 2016).

Incineration is a popular MSW treatment because it can markedly reduce the volume and mass of the waste, accompanied with energy recovery (Narayana, 2009, Song et al., 2017, Wang et al., 2010). Meanwhile, China has been ranked the world’s largest energy consumer, importing a great number of energy (Lo, 2014, Ming et al., 2013, Sharma et al., 2013, Yu and Qu, 2013). Therefore, during the “13th Five-Year Plan” period (2016–2020), MSW was defined as a critical renewable energy resource and WTE is suggested as an important technique for the sustainable management of municipal solid waste (Zheng et al., 2014).

Fly ash is one of the solid residue of MSW incineration. It was reported that 3,950,000 tons of fly ash were generated in China in 2015 (Yuanyuan et al., 2017). Since fly ash generally is enriched with leachable heavy metals, it has been classified as hazardous waste with a category number of HW18 in China (MEP, 2016). However, the management of fly ash in China is still poor. Only a few cities have specific hazardous landfill sites for fly ash in service or under construction (Cheng and Hu, 2010a, Cheng and Hu, 2010b). The disposal of fly ash is generally restricted by cost. It is reported that the expense on the treatment of fly ash is up to US$ 244.5/t (Zhao et al., 2016).

Currently, there are multiple disposal options for fly ash, such as stabilization and vitrification, given that most of the components are hazardous and the rationale behind these methods is to immobilize and stabilize fly ash in a matrix (Chou et al., 2009, Sun et al., 2011, Yu et al., 2016). However, fly ash is also enriched in valuable metals, such as copper and zinc. It was estimated that the contents for copper and zinc are 3000–5000 mg/kg and 9000–70,000 mg/kg, respectively (Chandler, 1997). Therefore a direct disposal in landfill of the fly ash causes a loss of metal resources (Chen et al., 2009, Li et al., 2014). Considering the disposal cost and reclaiming these metals at a later stage will be significantly more difficult, it is of environmental and economic significance to recover the valuable metals in a process attached after the incineration. Till now only a handful investigations are available in the literature; a method for copper recovery was reported by Lassesson (Lassesson et al., 2014), a similar recovery method, so called FLUREC process based on acid leaching, solvent extraction was evaluated by Schlumberger and co-worker (Schlumberger and Bühler, 2012). Other initiatives have been developed to separate metals as chloride (Ko et al., 2013), to remove metals by combining thermal and hydrochemical methods (Kuboňová et al., 2013) and to recover metals by integrating leaching, bioelectrochemical systems and electrolysis (Tao et al., 2014). Even so, few of those methods have been evaluated for industrial application. Efforts have been devoted to evaluate the technical feasibility of recovering copper and zinc from fly ash in batch scale (Tang and Steenari, 2015) and in laboratory pilot scale (Tang et al., 2017) relying on leaching and sequential extraction of the target metals. High recovery efficiency can commonly be achieved in such a recovery/recycling process. In addition to the technical feasibility, the economic and environmental aspects of a recovery/recycling process also need to be evaluated for successful implementation. This work includes a systematically conducted economic aspect and environmental impact analysis based on the hydrometallurgical process applied to a Chinese MSWI fly ash in comparison to the common fly ash disposal route currently used in Guangzhou. The experiments were carried out at pilot scale close to industrial application scale. Life cycle assessment (LCA) was employed to evaluate the environmental impacts of this studied process.