Summary
The 4 metals currently extracted (in combination with other materials (eg "salt") or as ions in an extraction process are the only ones commercially viable at present, and may be the only ones readily able to be extracted for the forseeable future. The "big 4" are -
- Sodium
- Magnesium
- Calcium
- Potassium,
Energy considerations make the task of extraction non-viable in all other cases except possibly for Lithium and Uranium, due to their high energy content when used as nuclear 'fuels'. Lithium is substantially more viable in this respect but has not as yet been extracted commercially. For all other materials the need to process extremely large amounts of water mean that the energy required almost invariably greatly exceeds either that used for extraction from other sources and the "energy value" of the recovered materials.
Development of unconventional extraction systems using eg bacteria or sea plants or animals to concentrate materials selectively may make some other materials viable in future.
Detail:
The four most concentrated metals in sea water are Sodium, Magnesium, Calcium and Potassium, and these are the only ones commercially extractable today. The least concentrated is Potassium at 400 parts per million.
Materials below the level of concentration of the above 4 are not economically extractable by direct removal by filtering, membranes and similar due to energy considerations. The energy required to move the vast quantities of water through a filtration system typically greatly exceeds the energy needed for extraction by other means on land.
Two possible exceptions are Lithium and Uranium due to the large amount of energy released when they are used as 'fuel' in nuclear reactions.
The Japanese ran a Uranium extraction experimental system for some years but practical activity on the project seems to have ceased (see below).
Available materials can be surprisingly low. All the dissolved copper in seawater (20th in occurrence) amounts to about 50 years of use worldwide at current rates. Getting the Copper out would be a humungous task and clearly not practical.
(1) Other means: As an alternative to filtering other means have been and are being investigated which MAY have application for a range of minerals.
(2) Good discussion of energy issues
Extracting Minerals from Seawater: An Energy Analysis
Abstract here
Full free paper here {PDF}
Ugo Bardi, Dipartimento di Chimica, Università di Firenze, ... 2010
Abstract: ... Seawater contains large amounts of dissolved ions and the four most concentrated metal ones (Na, Mg, Ca, K) are being commercially extracted today.
However, all the other metal ions exist at much lower concentrations. This paper reports an estimate of the feasibility of the extraction of these metal ions on the basis of the energy needed. In most cases, the result is that extraction in amounts comparable to the present production from land mines would be impossible because of the very large amount of energy needed.
This conclusion holds also for uranium as fuel for the present generation of nuclear fission plants. Nevertheless, in a few cases, mainly lithium, extraction from seawater could provide amounts of metals sufficient for closing the cycle of metal use in the economy, provided that an increased level of recycling can be attained.
(3) Ref1: This excellent paper discusses the issues involved and provides details of attempts to extract Uranium and Lithium.
An image in Ref1 shows Japanese researchers investigating Uranium extraction!
They say -
The four most concentrated metal ions, Na+, Mg2+, Ca2+, and K+, are the only ones commercially extractable today, with the the least concentrated of the four being potassium (K) at 400 parts per million (ppm).
Below potassium, we go down to lithium, which has never been extracted in commercial amounts from seawater, with a concentration of 0.17 ppm.
Other dissolved metal ions exist at lower concentrations, sometimes several orders of magnitude lower.
None has ever been commercially extracted.
A company set up in New Zealand a decade + ago to do Lithium extraction. It SOUNDED plausible and much money was spent. The promotrer turned out to be an internationally known con-man! :-(.
The main problem - energy cost and return.
From Ref 1:
Table 2. (see paper). Elements are ordered as a function of the mass of seawater that would need to be filtered in order to obtain the same amount of materials that we obtain today from traditional mining. That value is calculated in the optimistic assumption of 100% efficiency of the filtering membrane. For data sources, see note (1) at the end of the text
The table shows that, even for the best case listed, lithium, in order to recover the same amount we get today from conventional mining we would need to set gigantic facilities. We'd need to process at least ten times as much water as it is processed by desalination plants today. All the other metals would require to process amounts of water orders of magnitude larger.
Moving these gigantic amounts of water is not just a practical problem:
it involves energy; a critical parameter especially if we consider the extraction of two elements that are to be used as energy sources: lithium and uranium.
**In both cases, the feasibility of extraction is determined by the energy needed according to the well known concept of "EROEI" (energy returned for energy investment)
Related:
(4) The geochemistry of seawater
Table of 84 minerals in seawater here {jpg image} Copied below - order inobvious.
