Extracting Minerals from Seawater

The oceans contain immense amounts of dissolved ions which, in principle, could be extracted without the complex and energy intensive processes of extraction and beneficiation which are typical of land mining. In addition, an important fraction of the minerals which are lost as waste at the end of the economic process end up in the sea as dissolved ions. In this sense, the oceans could be considered an infinite repository of materials that could be used for closing the industrial cycle and attain long term sustainability. Minerals have been extracted from seawater from remote times, the classic example being sodium chloride, the common table salt. Today, the four most concentrated metal ions in seawater (Na, Mg, Ca and K) are commercially recovered. Several attempts have been made to extend this range but, so far, with no success. Mining seawater had become popular during the “oil crisis” of the 1970 and a considerable body of knowledge had been gained with the studies performed at the time. But, with the end of the crisis, these attempts were abandoned. Today, history repeats itself and the great surge in commodity prices of the early years of the 21st century has generated new interest in mining seawater.

This paper examines the perspectives of extracting metals from seawater mainly from the viewpoint of the energy involved. The analysis takes into account two possible strategies of extraction; one is pumping seawater through a selective membrane, the other dropping the membrane into the sea and exploiting the natural marine current to bring metal ions to the active membrane sites. Of the two methods, the first is more costly in terms of energy; the second is more complex and less efficient in terms of the use of the membrane. A specific benchmark in this analysis is that of the extraction of uranium, which can be used as energy source in fission reactors, since the future availability of mineral uranium from land mines has been questioned in several studies. In this case, the critical parameter is EROEI.

 

The results of the energy analysis show that uranium extraction from seawater is an unfeasible process if carried out in connection with the present nuclear technology. For the other ions contained in seawater, energy remains a critical parameter determining the feasibility of extraction. The results show that for most ions dissolved in seawater, the energy involved in extraction is very large in comparison to the present world production. Therefore, using metals from seawater to offset ore depletion does not appear to be feasible, with lithium being a possible exception.

Open ocean water contains dissolved salts in a range of 33 to 37 g/L, corresponding to a total mass of some 5 × 10 16 tons for a total mass of water of about 1.3 × 10 18 tons. Extraction from this huge source is theoretically promising and has been considered in several studies performed in the 1970s. The interest in mining the oceans is especially evident if we compare the amount of dissolved minerals to the total mass of minerals extracted today in the world; estimated to be in the order of a hundred billion tons (1 × 10 11 tons) per year.
In the present work, we will consider only metals which appear normally as positive ions dissolved in seawater. The four most concentrated metal ions, Na + , Mg 2+ , Ca 2+ and K + , are the only commercially extracted today, with the least concentrated of the four being potassium (K) at 400 parts per million (ppm). Below potassium, the next metal ion in order of decreasing concentration is lithium; at a value more than three orders of magnitude lower: 0.17 ppm. Noble metals and refractory metals exist in seawater at the opposite side of the concentration spectrum; in many cases in such minute concentrations that are impossible to determine with certainty. The concentration of ions in seawater depends mainly by two factors: their crustal abundance and the existence of water-soluble species. These two constraints account for the spread in the concentration values. As an example, thorium and uranium are both usable as fuel for fission plants, but uranium exists in seawater at concentrations about four orders of magnitude larger than thorium. This is due to the different chemistry of the two ions. Thorium is present mainly as scarcely soluble hydroxide, whereas uranium exists as the much more soluble uranyl carbonate complexes.

 

Seawater element concentrations are taken from. Oceanic abundance is calculated assuming a total ocean volume of 1.3 × 10 9 km 3 (1.3 × 10 18 tons). Mineral reserves are from USGS data, except for uranium reserves; for which two values are given, the smaller one from and the larger one from. The USGS does not provide data for the world reserves of sodium and calcium. All land reserves are in terms of the pure element, except for aluminium, iron, potassium, thorium and titanium, given in terms of oxides. Thorium is shown as an exception, because of its potential importance as nuclear fuel, although obviously the value reported only suggests an order of magnitude.

Extraction Techniques
Traditionally, the most concentrated ions in seawater (e.g., minerals such as sodium chloride) were concentrated and extracted from seawater by evaporation. Ions such as Mg or K can subsequently be recovered by electrolytical processes. These methods are not practical for low concentration ions, for which the most general extraction method is to pump seawater through a membrane containing functional groups that selectively bind to the species of interest. No known membrane is 100% selective, but it is possible to create membranes that can retain a small number of species. The adsorbates can be extracted from the membrane by flushing it with appropriate chemicals, a process called “elution”. After this stage, the metal ions can be separated and recovered by precipitation or electrodeposition.

We are working on new extraction technologies in Project S2PW3693