Mineral beneficiation from seawater: development and optimization of selective extraction techniques for essential minerals from seawater

Abstract

The growing demand for essential minerals such as lithium and magnesium has underscored the need for sustainable extraction methods. Lithium plays a significant role in various industries since it is a promising metal for energy storage in electric vehicles as well as in electric devices. Magnesium is commercially used in the automotive industry. The governments of developed countries such as those in Europe, have imposed strict laws when it comes to vehicle emissions and have made the use of electric vehicles an alternative for more environmentally sustainable transportation. Traditional mining such as minerals in rock ore demands large amounts of water and energy, which is known to pose substantial environmental and health risks to the miners. Therefore, seawater mining has been reported as one of the strategies to mitigate the depletion of high-grade ores while offering reduced waste generation. This research contributes to finding technologies that align with the blue economy and addresses the environmental challenges of traditional mining. This study focuses on the synthesis and optimization of polymer inclusion membranes (PIMs) to selectively extract essential minerals from seawater. However, the challenge is that lithium is present in extremely low levels approximately 0,17 mg/L in seawater. The approach was to synthesize PIM that will selectively extract the targeted analytes, leaving the non-targets behind when applied to real seawater samples. The research was conducted in three phases. The first phase involved synthesizing PIMs and optimizing parameters such as membrane composition, stripping solution concentration, the effect of the pH and extraction time. The selectivity of the synthesized PIMs was tested in ultrapure water spiked with 15 mg/L of mineral salts such as magnesium carbonate, calcium carbonate, sodium carbonate and potassium carbonate, yielding a selectivity order of Mg2+ > Ca2+ > Na+ > K+ > Li+. In real seawater samples, the selectivity was Mg2+ > Ca2+ >Na+ > K+ and lithium was not detected. The density functional theory (DFT) studies were also conducted to investigate the binding ability of the carriers towards the targeted metal ions. The obtained selectivity was Mg2+ > Ca2+ > Li+  Na+ > K+. The selectivity of the metal ions obtained from the experiments slightly differs from DFT. However, the computational study contributes to finding suitable technologies that will take advantage of the blue economy. The method was optimised successfully and further applied to real seawater samples. The second part of the study involved the synthesis of a PIM with different membrane compositions. The optimized PIMs demonstrated excellent selectivity for lithium which varied with the concentration of the HCl receiver solution. The selectivity obtained for the PIM that was in a 1:1 ratio utilising 0,05 M HCl of the receiver solution was Li+ > Na+ > Ca2+ > Mg2+. As the receiver solution was increased to 0,1 M HCl and 1 M HCl, the selectivity shifted to Li+ > Ca2+ > Na+ > K+ > Mg2+ and Li+ > Ca2+ > Na+ > K+, respectively. The selectivity obtained for the 2:1 ratio was Li+ > Ca2+ > Na+ > Mg2+. When the concentration of the receiver solution was increased to 0,1 M HCl and 1 M HCl, the selectivity was Li+ > Ca2+ > Na+ > K+ for both concentrations. Furthermore, the optimal parameters were further tested on the real seawater. The selectivity obtained was the same for the PIM at a 1:1 ratio for 0,05 M HCl, 0,1 M HCl and 1 M HCl receiver solution which was Ca2+ > Na+ > K+. When the second PIM in a 2:1 ratio was applied in seawater, the selectivity obtained for 0,05 M HCl of the receiver solution was Ca2+ > Na+ > K+ > Mg2+ whereas for 0,1 M HCl the selectivity was Ca2+ > Na+ > K+ and for 1 M HCl the selectivity was Na+ > K+. The third part of this study was a continuation of the first part. The optimised PIM was further employed for the extraction of minerals in seawater using a semi upscaled approach. A much bigger flat sheet membrane of approximately 270 mm (width) and 370 mm (length) was synthesized based on the optimised membrane composition. The other optimised parameters such as the concentration of the receiver solution and the effect of extraction time were tested in 0,05 M HCl and 0,1 M HCl receiver solutions for 39 days, respectively. The volume of the receiver solution was also investigated between 1 L and 2 L. The application was done in real seawater and the selectivity obtained for both volumes of the receiver solutions at 0,05 M HCl was found to be the same: Na+ > Mg2+ > Ca2+ > K+. However, when the concentration of the receiver solution was increased to 0,1 M HCl the selectivity changed to Mg2+ > Ca2+ > K+ for both 1 L and 2 L respectively. The concentration of sodium at 0,1 M HCl in the receiver solution was not clear, thus it was eliminated from the results. Despite some deviations in selectivity compared to smaller-scale experiments, the study demonstrated the feasibility of using PIMs for mineral extraction from seawater on a larger scale. Future work will focus on understanding the reasons for selectivity deviations in upscaled applications and further refining the PIM method to achieve consistent results. This research contributes to developing sustainable technologies for extracting valuable minerals from seawater aligning with the blue economy and addressing the environmental challenges of traditional mining.