Development and optimization of a novel gold recovery process from LED waste using eco-friendly chitosan beads

Abstract

Owing to technological advancements and increased demand for electronics, the exponential growth of electronic waste (e-waste) has presented notable environmental and economic challenges globally. This has prompted the emergence of urban mining as a sector dedicated to the extraction and recycling of valuable components from e-waste, particularly precious metals. Current methods, such as pyrometallurgy, hydrometallurgy, and bio-metallurgy, are employed for precious metal recovery, but they suffer from drawbacks such as high costs and environmental pollution. Consequently, there is growing interest in advancing the adsorption processes within hydrometallurgy to effectively overcome these challenges. The aim of this doctoral dissertation was to develop a process to recover high-purity gold from e-waste composed of various metals, especially surface-mounted on device light emitting diode (SMD LED) waste, using eco-friendly chitosan beads. This study focuses on the following four sub-topics: i) understanding the effects of chitosan solution, collection distance, and solidification solution on the fabrication of chitosan beads with uniform size and spherical shape; ii) developing a process to recover high-purity gold from real SMD LED waste using the fabricated chitosan beads; iii) developing a continuous recovery process using fixed-bed adsorption columns and employing an engineering approach for the scale-up of these columns; and iv) conducting an economic evaluation of the gold recovery process using chitosan beads. In the first part, a study was conducted to investigate factors affecting the process for manufacturing uniform size and spherical glutaraldehyde-crosslinked chitosan (GCC) beads. The initial stage involved the preparation of chitosan solutions of different concentrations, which were subsequently dropped into sodium hydroxide (NaOH) solutions of various concentrations, allowing the chitosan droplets to solidify over time. It was found that the shape of the GCC beads was largely dictated during the dropping phase, depending on factors such as the concentration of chitosan and NaOH and the collection distance between the needle and NaOH solution. The chitosan concentration of 3.5% resulted in the highest bead yield, while lower concentrations did not form the bead properly and higher concentrations prevented the solution from passing through the needle. The optimal concentration of NaOH was 2 M, as NaOH concentrations lower than 2 M slowed down the solidification process, while NaOH concentrations higher than 2 M did not increase the solidification speed. At a collecting distance of 7 cm, the shape of beads was close to spherical, while shorter distances resulted in irregular shapes and longer distances resulted in disruptive air bubbles. The GCC beads prepared under the optimal conditions (i.e., 3.5% chitosan solution concentration, 7 cm collecting distance, and 2 M sodium hydroxide) showed a circularity of 0.93 (0.02), an aspect ratio of 1.1 (0.08), and a diameter of 0.94 (0.05) mm. Finally, a GCC bead preparation system with a production capacity of 190 g/batch was established, which could be useful for industrial-scale applications. In the second part, a novel approach for the recovery of gold from SMD LED waste was investigated. Conventional leaching processes using aqua regia generate a leaching solution with a high copper concentration, which could negatively affect the production of high-purity gold. To mitigate this challenge, a nitric acid pretreatment step was proposed and tested to selectively remove copper from LED waste prior to aqua regia leaching. Through optimization of the pretreatment process, most of the copper was removed using 2 M nitric acid at 50 °C for 210 min. With this optimal pretreatment step, the leaching solution exhibited an approximately 97% reduction in the copper concentration compared to the control without nitric acid pretreatment. Subsequent adsorption tests using GCC beads were conducted for the leaching solution, in which the gold adsorption rate of 99.6% was achieved, whereas the removal of copper was limited to only 0.4%. After the adsorption, the GCC beads were combusted, from which the recovered gold showed a purity of 99.1%. In the third part, a continuous gold recovery process utilizing a fixed-bed column with GCC beads was investigated, focusing on finding design factors for column scaling up. The gold breakthrough curves were determined in the GCC column with different column diameters (15, 20, 25, and 30 mm), flow rates (1–4 ml/min), and bed heights (3.6 and 7.2 cm). The gold breakthrough curve in the GCC column could be well described using the Yan model. The gold breakthrough times were similar at ~960 h, indicating a consistent gold elution behavior across all tested conditions. Various column operations resulted in a consistent empty bed contact time (EBCT) and similar breakthrough curves across all columns. This was due to the maintenance of key properties (geometric, kinetic, and dynamic similarities) between the columns. Consequently, by maintaining the key properties of the columns and a constant EBCT, it was demonstrated that the breakthrough curve for the large-scale column closely resembled that of the small-scale columns. This could be quantitatively described as: Q2/Q1=(D2/D1)2*(H2/H1), where Q represents the flow rate, D is the column diameter, H is the bead height, subscripts 1 and 2 denote small- and large-scale columns, respectively. This equation could be used to design large-scale GCC adsorption column. The desorption process within the column was optimized, achieving desorption rate of 95%. This was accomplished by operating the adsorption process on the column for 12 h, considering the ionic state of adsorbed gold. Two operational strategies were suggested depending on the gold concentration in the feed: i) for low gold concentration - operate the column until saturation time and burn the adsorbent; ii) for high gold concentration, repeat adsorption/desorption for 12 h, and recover a concentrated gold solution. Based on these experimental results, a pilot-scale gold adsorption column with the leachate treatment capacity of 1 ton per day could be constructed. Finally, an economic evaluation was conducted on the entire gold recovery process from SMD LEDs using GCC beads. This assessment comprised three key steps: the leaching of gold from the SMD LEDs, the preparation of GCC beads, and the recovery of gold from the leaching solution. Input costs included the cost of reagents, electricity, and equipment needed for each step, whereas revenue was calculated by subtracting the input costs to obtain 1 kg of gold from the selling price of 1 kg of gold (average annual price in 2023). Comparisons of the quantities of LEDs and GCC beads required for 1 kg of gold recovery from the two SMD LEDs (3020 and 7020) indicated that fewer LEDs (321 kg) and GCC beads (41.4 kg) were required for the 7020 SMD LED. Therefore, gold recovery from 7020 SMD LEDs was selected as the best scenario. Profitability assessment against total costs revealed revenues of $27,898.03 and $33,248.79 from the first and second gold recovery processes respectively, the latter excluding equipment purchase costs. Notably, reagent costs had the greatest impact on profits, constituting approximately 41% of the price of 1 kg of gold. Changes in profit due to price fluctuations of SMD LED waste were also explored and discussed. Potential risks, such as insufficient LED collection and gold price fluctuations, were assessed, along with possible mitigation strategies. Overall, this analysis underscores the potential profitability and economic feasibility of gold recovery from SMD LEDs using GCC beads on an industrial scale. Altogether, the PhD work provides valuable insights and novel methodologies for efficient and economically viable recovery of gold from SMD LED waste, paving the way for large-scale implementation and contributing to the sustainability of e-waste management.