Loganathan and two co-authors reported in 2016 that seawater contains large quantities of valuable minerals, some of which are very scarce and expensive in their land-based form. However, only a few minerals, the ones in high concentrations, are currently mined from the sea. Due to recent problems associated with land-based mining industries as a result of depletion of high-grade ores, sustainable water and energy demand and environmental issues, seawater mining is becoming an attractive option.
The authors conducted a comprehensive and critical review of the current methods of extracting valuable minerals from seawater and seawater brines generated in desalination plants, and suggests ways to overcome some of the limitations and challenges associated with the extraction process. The extraction methods discussed are solar evaporation, electrodialysis (ED), membrane distillation crystallization (MDC), and adsorption/desorption.
The authors concluded that there are much more minerals in the sea compared to those in land-based reserves. Given the difficulties facing land-based mining industries such as sustainable energy and water demands, depletion of easily available high-grade ores and environmental issues related to waste disposal and miners' health, mining minerals from the sea is becoming more attractive. The increasing demand for clean water has led to the installation more desalination plants worldwide.
This process generates enormous amounts of brine. The brine contains all the minerals present in the sea at nearly twice the concentration of that in seawater. Mining minerals from these brines can offset part of the desalination cost as well as solve the brine disposal problem. Of these, the first three can recover only minerals which are found at high concentrations (Na, Mg, Ca). The centuries-old solar evaporation method is limited in its use, in that it is mainly employed in arid regions requiring high solar evaporation and where land is available at low cost. The new WAIV is a promising method that may overcome these limitations. The application of ED for mineral recovery has increased with the developments of monovalent cationic and anionic permeable membranes to separate Na, Cl, and Br from Ca, Mg and SO4.
Further, new research is in progress to develop membranes permeable to specific individual metals such as Li. MDC is a relatively new method which has been shown to recover NaCl and MgSO4 in laboratory studies. New membrane developments with anti-scaling and other beneficial properties and pre-treatment of the feed water are expected to increase the applicability of this method to other mineral recoveries. Studies have shown that using MDC has the potential to recover Li, Sr, Ba, and Ni.
Unlike the other methods, the adsorption/desorption method can concentrate minerals that exist at low concentrations by selectively adsorbing a mineral and quantitatively desorbing it for evaporative crystallization. Though adsorbents with high adsorption capacities have been developed (MnO2-based materials for Li, potassium metal hexacyanoferrate for Rb, and amidoxima-based materials for U), they are not completely selective to the mineral of interest. This has led to the use of several other adsorbents to specifically remove the minerals competing for adsorption with the mineral of interest and removal of the competing mineral by selective precipitation. Only Li appears to have been recovered in the pure crystalline form using the adsorption/desorption method.
The technology advancements of each method show promising potential for its application in mining minerals from seawater brine. Nevertheless, it is still a challenge to use a single method to selectively extract valuable minerals from complex brine matrices. An integrated approach of combining a number of methods may be necessary to extract a specific valuable mineral of seawater brine. A similar observation was made by Jeppesen et al. and Le Dirach et al. on the potential of mineral extraction from seawater brine and nuclear desalination brine.80,81 In this regard, MDC appears to show promising potential in its capacity to simultaneously produce fresh water while recovering major salts from the brine. In an integrated approach, selective valuable minerals, even those present at very low concentrations in seawater, could be extracted from concentrated MDC brine with a post treatment of adsorption/desorption method.
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