Hot liquid tin strips water from brine and absorbs its metals; vacuum degassing and slow cooling then concentrate magnesium 280-fold, recovering more than 99 per cent of what the tin had taken up.

Desalination plants produce about 50 trillion litres of brine a year worldwide — more waste brine, by volume, than the fresh water they deliver. That brine is usually returned to the sea, where its salinity and temperature cause local ecological damage, and it is thrown away despite containing sodium and magnesium at higher concentrations than seawater does. It is, in a literal sense, a concentrated ore in liquid form.

Researchers at the Institute of Science Tokyo have reported a route to mining it. Writing in Desalination, Toranosuke Horikawa, a doctoral student in the School of Engineering, and Masatoshi Kondo, an associate professor in the Laboratory for Zero-Carbon Energy, describe a process they label “mining without digging,” built around a bath of molten tin.

Tin as both evaporator and sponge

The group’s earlier work established the first step. Brine fed onto hot liquid tin flash-evaporates; the water vapour can be condensed and collected as distilled water, which is the point of a desalination plant in the first place. What remains does not stay on the surface — magnesium, sodium, calcium and potassium are absorbed into the tin itself.

That is an unusual arrangement. In conventional thermal desalination, heat drives off water and leaves an increasingly concentrated salt solution behind, which eventually has to be dealt with as a slurry. Here the metals move into a liquid metal, which can be handled, heated and cooled as a single phase.

The complication is that chlorine and sulphur come along too, and neither is welcome in a metal bath — both are corrosive and both interfere with recovering a clean product. The new work adds a vacuum-degassing step to get them out.

The experiment, in numbers

The scale is laboratory-small and the conditions are specific. About 8 grams of tin were melted at 573 to 673 kelvin — roughly 300 to 400 °C — and artificial brine, prepared with heavy water so that the hydrogen could be tracked, was fed in at 0.1 millilitres per minute.

The loaded tin was then heated under a vacuum of about 10⁻³ pascals to 973 K, near 700 °C, at 2.5 kelvin per minute, and held for about ten minutes. The gases released were identified as hydrogen chloride, its deuterated counterpart, chlorine, sulphur dioxide and hydrogen sulphide. Between roughly 59 and 67 per cent of the brine-derived gaseous components were estimated to have been removed.

Sixty-odd per cent is the figure a sceptical reader should hold onto. It means a third or more of the chlorine and sulphur stayed in the tin, and in a process intended to run continuously with the same tin inventory, residual halogens and sulphur would be expected to accumulate. The researchers list long-term stability and material compatibility among the next things to evaluate, which is the right list.

Letting the metals sort themselves

The separation step is the elegant part, and it requires no reagents at all. Cooled slowly under vacuum, the tin can no longer hold the dissolved metals, and they precipitate — but not together. Magnesium-rich and sodium-rich regions formed in different locations within the solidified tin, because the two metals have different solubility behaviour as the temperature falls.

The resulting concentration is striking. With 20 millilitres of brine processed at 673 K, magnesium in the precipitate reached about 280 times its concentration in the original brine, and the magnesium-to-sodium ratio rose roughly 5,300-fold. Separating magnesium from sodium is the central difficulty in extracting anything useful from seawater or brine, since sodium is present in far greater quantity; a five-thousand-fold shift in that ratio is the result that makes the process interesting.

More than 99 per cent of the magnesium that the tin had absorbed was recovered, and the dominant crystalline phase in the magnesium-rich precipitate was magnesium oxide. It is worth being precise about what the 99 per cent describes: it is recovery of the magnesium taken up by the tin, not of the magnesium present in the brine. How much of the latter the tin absorbs in the first place is a separate figure the release does not give.

Where the heat would come from

A process running at 400 °C and then 700 °C invites the obvious objection about energy. The researchers propose concentrated solar thermal energy or high-temperature industrial waste heat, which is a reasonable answer: both are heat sources rather than electricity, and the process wants heat.

A conceptual design estimates about 1.7 kilowatt-hours of electricity per kilogram of magnesium recovered for the vacuum-degassing step, described explicitly as under idealised conditions. For comparison, conventional magnesium production is energy-intensive by any measure, so the figure is not obviously uncompetitive — but an idealised estimate from a conceptual design is not a cost.

The illustrative case is a solar module in a sunny, water-stressed region such as Egypt: a concentrator 15 metres in diameter was estimated to yield about 970 kilograms of fresh water and about 2.8 kilograms of magnesium a day. Those two numbers together are the honest summary of the concept’s economics. The water is the product; the magnesium is a by-product that might improve the arithmetic of a desalination plant rather than justify a mine.

Disclosure and what comes next

Funding is credited to the New Energy and Industrial Technology Development Organization and the Japan Science and Technology Agency. The conflict-of-interest statement is oddly worded — Kondo reports that equipment, drugs or supplies were provided by the Institute of Science Tokyo, which appears to be a disclosure form’s boilerplate rather than a meaningful competing interest, given that the institute is the authors’ own employer.

The release contains no direct quotations from either researcher, so none appear in this article. It also carries a date spread worth noting: the release is dated 6 October 2026, the paper went online on 31 July 2026, and its formal publication date is given as 1 December 2026.

The researchers’ own list of next steps — a continuous circulating system, long-term stability, material compatibility, magnesium purification, energy use and economics — amounts to an acknowledgement that essentially everything separating this from a process remains to be done. What has been demonstrated is that the chemistry works: brine in, distilled water and separated magnesium out, with no solvents, no reagents and nothing but heat and vacuum.

---

Notes

1. “Recovery of magnesium from seawater brine using liquid tin direct-contact desalination assisted by vacuum degassing.” Desalination, vol. 639, online 31 July 2026. DOI: 10.1016/j.desal.2026.120582.
2. Institute of Science Tokyo. “Mining without digging: liquid metal tin turns desalination brine into a magnesium resource.” EurekAlert!, 6 October 2026. https://www.eurekalert.org/news-releases/1146623.