A Product Diffusion Model for the Extraction of Cerium and Yttrium from Magnetic Coal Fly Ash Using Citric Acid Solution

    The recovery of rare earth elements from coal-related materials, primarily fly ash, has become an emerging topic for the past few years. The availability of fly ash as solid waste from coal combustion and its low radionuclide concentrations benefit its utilization as an alternative source of rare earth elements. Using organic substances like citric acid to extract the elements further helps the environmental aspect. The maximum recovery value of cerium and yttrium was determined by reacting magnetic fly ash of 5 grams with 0.5 M of a citric acid solution with an S/L ratio of 10 for 24 hours at various temperatures. A mathematical model is also suggested to elucidate the leaching phenomenon better. The mechanistic model is developed based on the metal complex's diffusion through the ash layer. The results show that the leaching capacity of either cerium or yttrium rises along with the temperature. The maximum recovery value for leaching at 363 K is 40.21% and 54.90% for cerium and yttrium, respectively. The product diffusion model presents befitting graphs to the experimental data quite well. The effective diffusion coefficient (D_e) for both cerium and yttrium rises exponentially with extraction temperature. It is found that the value of D_e's increases from the order of 10^-10 at 298 K to 10^-8 cm²/s at 363 K. The diffusion activation energy for cerium and yttrium complexes appears to be 62.5 kJ/mole and 58.4 kJ/mole, respectively.

Citric acid; Kinetics; Magnetic fly ash; Product diffusion model; Rare earth elements

Rare earth elements consist of 15 lanthanides, yttrium, and scandium (Trisnawati et al., 2020). These elements possess unique catalytic, electronic, and magnetic properties, thus widespread utilization (Ascenzi et al., 2020). Rare earth is a vital element to be used in modern industries, including automobile catalytic converters, lasers, hybrid car batteries, and energy-efficient lighting (Charalampides et al., 2015; Tuan et al., 2019). The emergence of clean technologies increases the demand for the elements. Zhou et al. (2017) predicted that the demand for lanthanum, cerium, neodymium, europium, terbium, yttrium, and dysprosium oxide would increase to 33,600 tonnes in 2025 and reach 51,900 tonnes in 2030.
    The recovery of rare earth elements from coal-related materials, primarily fly ash, has gained much attention these past few years. The use of coal-related materials as an alternative resource of rare earth elements has its advantage compared to conventional ore as it has much lower radionuclide (uranium and thorium) concentrations (Zhang et al., 2020).
    Since fly ash is available as solid waste, the utilization of fly ash as a rare earth source possesses no mining cost and is economically and environmentally beneficial (Pan et al., 2020).
      Acid leaching is the most common method to extract rare earth elements from fly ash.  Not only inorganic acids like H2SO4 and HCl, some researchers have also recovered rare earth elements by employing organic acids like acetic and citric acid (Manurung et al., 2020; Prihutami et al., 2020; Rosita et al., 2020a; Wen et al., 2020). Many have used citric acid as its utilization gives an excellent performance in extracting rare earth elements, prevents mineral acid leakage, and averts the release of toxic gasses generated by inorganic acid (Tang et al., 2016; Gergoric et al., 2018). Some studies have also reported the kinetics of organic acid in leaching rare earth elements from fly ash (Kashiwakura et al., 2013; Kim et al., 2017; Cao et al., 2018; Handoyo et al., 2019).
     The leaching of rare earth elements from fly ash using a citric acid solution is a heterogeneous solid-liquid reaction. Generally, this reaction follows the shrinking core model and undergoes five steps of mechanisms (Levenspiel, 1999), namely: (1). Diffusion of reactant from bulk solution through the liquid film onto the solid surface, (2). Internal diffusion of reactant to the surface of unreacted solid, (3). Reaction at the surface of unreacted solid, (4). Internal diffusion of the product to the solid surface, and (5). Product diffusion through the liquid film to the bulk solution. The leaching kinetics is based on the slowest step as it has the most significant resistance. Even though the shrinking core model gives a good fit for most experimental data, there are times when the model needs a modification to fit specific leaching cases (Mirwan et al., 2017; Setiawan et al., 2019). This study proposed a modified shrinking core mathematical model to better explain the kinetics phenomenon of cerium and yttrium leaching from magnetic fly ash by citric acid.

    The rate of diffusion can be enhanced by increasing the kinetic energy of molecules via heating. As cerium and yttrium leaching is controlled mainly by diffusion, their leaching capacity depends on temperature. Besides, the leaching capacity is also greatly influenced by the fly ash origin. The value of leaching capacity is an important variable in calculating the kinetics model. The proposed product diffusion model fits very well for cerium and yttrium leaching. The diffusion activation energy calculated from the model shows that cerium complexes have a higher value than yttrium. The result explains the lower recovery value of cerium as it needs more energy to leach.

    This research was supported by the Sustainable Mineral Processing Research Group and Laboratory of Energy Conservation and Pollution Prevention in the Department of Chemical Engineering, Faculty of Engineering, Universitas Gadjah Mada. The authors also offer gratitude to Tanjung Awar-awar power plant, Tuban, Jawa Timur, for the sample supply and Mineral Technology Research Center, Indonesian Institute of Sciences, Lampung, for help characterizing the material.

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