Recovery of gallium from light emitting diodes (LEDs) is receiving great attention due to both high content of gallium and potential risk of environmental pollution. In this study, a novel environment-friendly route was proposed for efficient recovery of gallium from surface mounted device (SMD) of LEDs by using organic acid. Reduction of non-metallic components, selection of lixiviants, optimization of leaching parameters, and possible mechanism of gallium were investigated. Results showed that pyrolysis could reduce non-metallic components effectively, and 22% weight loss was achieved at 733 K. Selection of lixiviant experiments indicated that 83.42% gallium could be extracted from SMD LEDs by oxalic acid, which was much higher than that of 79.89% by hydrochloric acid, 70.62% by citric acid, and 71.69% by DL-malic acid. Further investigation revealed that 90 °C of leaching temperature, 10 g L−1 of pulp density, 0.7 M of oxalic acid and 48–75 μm of particle size were the optimum conditions for effective gallium leaching by oxalic acid. After optimization, the gallium recovery efficiency reached as high as 90.36% in 60 min. Such efficient gallium leaching came from the higher dissociation constant of oxalic acid and the formation of ferrous oxalate which would promote the generation and maintenance of H+. Hence, oxalic acid could be a promising lixiviant for efficient recovery of gallium from SMD LEDs.
The global waste electrical and electronic equipment (WEEE) consumption was increasing speedily during the previous decades (Afroz et al., 2013; Cucchiella et al., 2015; Wang and Xu, 2014). As known, WEEE contains various valuable materials, such as metals, glass, plastics and other materials (Chan et al., 2007; Kumar et al., 2017; Sun et al., 2017). The US Environmental Protection Agency has made a list of seven major benefits to recover metals from WEEE in comparison with original ores, such as saving in energy and reduction in pollutions (Cui and Forssberg, 2003). Therefore, WEEE was also called urban mine and could be recognized as one of the main sources of metal resources (Isildar et al., 2018; Van et al., 2016). Previously, electrochemical technology, supercritical technology, vacuum metallurgical separation, bio-metallurgical approach, etc. (Rocchetti et al., 2013; Zhu et al., 2011) were widely used in WEEE recycling. However, the literatures mainly concentrated on recycling of common metals such as copper, iron, aluminum and lead (Xiang et al., 2010; Yazici and Deveci, 2013). There were only few reports on the recycling of precious and rare metals. Light emitting diodes (LEDs), a new kind of electronic device, are widely used as distributed sources of lighting, and the key substances in their chips are gallium nitride (GaN), gallium phosphide (GaP) and indium gallium nitride (InGaN). Surface mounted device LEDs (SMD LEDs) are one of the LEDs which are easy to be recycled due to small size and simple structure so that SMD LEDs are collected in this study. In China, the scale of LED industry reached ¥ 761.5 billion at steady growth of 19% in 2018 and less than 5% LED waste was recycled properly (Kim and Schubert, 2008; Lim et al., 2011). It was considered that the initial content of gallium in waste LEDs was 2.077 mg kg−1 (Zhan et al., 2015; Rahman et al., 2017; Ruiz-Mercado et al., 2017). Gallium existed in LED chips was viewed as a strategic resource in USA, EU, Japan and China (Fröhlich et al., 2017; Poledniok, 2008; Xu et al., 2007). However, significant gallium losses arose in primary production and in waste management.