Liquid–liquid extraction of yttrium from the sulfate leach liquor of waste fluorescent lamp powder: Process parameters and analysis

Highlights

• Effects of extractants D2EHPA, Versatic Acid 10, TOPO, and Alamine 336 were studied.

• Fe from the leaching solution was completely eliminated using the acidity control method.

• Higher stripping rate was observed for HCl (78.12%) than H₂SO₄ and HNO₃.

• D2EHPA showed a 97.94% extraction rate forming complex compound at 1:1 ratio.

Abstract

Global demand for rare earth metals (REMs), including yttrium, has motivated the scientific community to focus on the recovery of such metals from electronic waste materials. Herein, a solvent extraction method was used to isolate and recover yttrium from the original leaching solution from the fluorescent lamp waste powder dissolved by sulfate. The operating parameters were systematically investigated, including pH, equilibrium time, concentration of extractants, and organic/aqueous ratio using Versatic Acid 10, TOPO, D2EHPA, and Alamine 336. The extracting capacities were in the order of D2EHPA > Versatic Acid 10 > TOPO > Alamine 336. The reaction mechanism of yttrium with each extractant demonstrated the formation of complex compounds with concentration ratios of 1:3, 1:1, and 1:2 with Versatic Acid 10, D2EHPA, and TOPO, respectively. On investigating the extraction mode for yttrium and impurities in the range of equilibrium pH (pH_eq) values from 0.95 to 2.25 using D2EHPA, pH_e 2.02 (initial pH 2.53) was found to be the most suitable for extraction. Fe in the original leaching solution could be utterly eradicated through the acidity control method. Upon calculating the theoretical number of mixer–settler plates, more than 99% of yttrium was extracted in solution with only two plates as the organic phase. Finally, the stripping test showed favorable stripping rates and followed the order HCl (78.12%) > H₂SO₄ (76.36%) > HNO₃ (74.86%) within 10 min. This study is a first step toward developing large-scale operations for extracting REMs from fluorescent lamp waste powder.

Introduction

Rare earth metals (REMs) have been used as the main raw materials in the high-tech industry for items such as optical glasses, electronics, metallic additives, and catalysts due to their unique characteristics of chemical stability and ability to conduct heat well; hence, demand for such metals has increased drastically in the last ten years (Hidayah and Abidin, 2018, Tyler, 2004, Resende and Morais, 2010). The term “rare” earth is a contradiction as they are moderately available and found concentrated rarely in the Earth’s crust but are discrete and thus utilizable as economical minerals (Feng et al., 2014). REMs are unevenly distributed all over the world; they are found mainly in China (55 million tons), USA (13 million tons), India (3.1 million tons), Australia (2.1 million tons), and Brazil (2.2 million tons) (Tunsu et al., 2015, Tunsu et al., 2016). Specifically, 97% of rare earth minerals are produced in China (Du and Graedel, 2011). Because of this, the price of REMs has fluctuated a lot based on the policies of the supplier countries, and supplier prices have been increasing since the implementation of the export quota system under the policy on resources as weapons in China (USGS, 2015). As a result, the European Commission (EC) and UN Environment Program and UN University have decided to secure REMs stably as the core component of economic development and announced the importance of securing a seamless and stable supply of REMs (UN Environment Programme and UN University, 2009, European Commission, 2010, Banda et al., 2019).

Currently, attention is being paid worldwide to solve this problem by collecting REMs from urban mining. Together with industrial development in the world, the amount of daily waste or industrial waste such as electric/electronic products and automobiles has been exponentially increasing (Binnemans et al., 2013). However, waste such as electronic goods and fluorescent lamp powder contain vast amounts of expensive REMs; these waste products have attracted considerable attention as new energy sources in view of REM shortages and waste control (Binnemans and Jones, 2015, Sethurajan et al., 2019).

Among REMs, yttrium (Y) is a heavy rare earth element that is used in a variety of fields, including as an additive to enhance the strength of an alloy (Fan et al., 2012), as a laser to cut metal (Xiao et al., 1999, Mishra Bibhuti Bhusan, June 2019), and in the productions of camera lenses and superconductors (Permyakov, 2009). A total of 76.7% of yttrium production goes toward making the main component of fluorescent lamps, phosphor (Tan et al., 2015, Tanvar and Dhawan, 2019). Because of socioeconomic development, the advanced lifestyles and mindset of people for convenience, and the single use of products, huge amounts of electronic waste are generated in South Korea. Korea is an electronics hub, and its REM market is about $29,658 million. For example, yttrium and europium are required for phosphor screens of cathode ray tubes in TVs (Lee and Kim, 2014). Utilization of electronic waste, particularly fluorescent lamp waste, which is composed of yttrium and europium, will not only address the need for REMs but also diminish the waste disposal problem. Considering these perspectives, Korea is devoting huge research efforts to develop technologies to assure viable resource recovery from waste resources.

Various methods, such as supercritical CO₂ extraction (Yang et al., 2016, Liu et al., 2009), ionic liquids (Larsson and Binnemans, 2015), and solvent extraction (Banda et al., 2019, Innocenzi et al., 2017), are employed to recover and separate REMs from waste. Among these methods, the most popular process for collecting high-purity REMs is solvent extraction. Solvent extraction is generally considered the most suitable and commercial technology for separating REMs because a large volume of dilute liquor can be easily processed and REMs can be extracted from different groups of leachate (Belova, 2017, Hidayah and Abidin, 2018). It has been observed that single or specific elements are very difficult to separate from the mixture because of their similar physical and structural functions. In such cases, solvent extraction methods have been found to be suitable owing to the ease in terms of the equipment and the conditions associated with them and their efficiency for obtaining high-purity compounds (Quinn et al., 2015). The commonly used extractants to extract REMs during solvent extraction are carboxylic acids, phosphorous acids, solvating extractants, and anion exchangers. These extractants make complex compounds with REMs through different extraction reaction mechanisms (Nguyen et al., 2017, Balaram, 2019). Hence, in order to effectively extract yttrium, the best extractant must be chosen by investigating the extraction mechanism of each extractant and REMs. However, few comparison studies for REM extraction have been conducted with reported extractants (Feng et al., 2014).

In this study, a solvent extraction process was developed using four extractants (Versatic Acid 10, TOPO, D2EHPA, and Alamine 336) for the selective extraction of yttrium utilizing a sulfate leach liquor of fluorescent lamp powder, and the extraction reaction mechanism using each extractant and yttrium was investigated. The extraction features were investigated by changing the experimental variables for the solvent extraction, including pH, equilibrium time, concentration of extractant, and organic/aqueous (O/A) ratio; the ideal number of plates was suggested by the preparation of a McCabe–Thiele diagram to apply to the mixer–settler based on the results. The purpose of the study was to remove metals other than the target ones from the solution to make high-purity concentrates. The elimination features of Fe, Mg, and Ca, which exist in large amounts in waste fluorescent lamp powder, were also investigated by pH control during the precipitation and equilibrium pH control during the extraction. Finally, the stripping mechanism of yttrium was investigated in terms of the type, concentration, and reaction time of the acids used as the hydraulic-phase solution in the case of stripping to achieve the optimum stripping rate.