Liquid Tin Turns Desalination Waste Brine Into a Magnesium Mine

Every year, the world’s desalination plants produce roughly 50 trillion liters of concentrated brine, a salty byproduct that is usually diluted and dumped back into the ocean. That waste stream, however, contains sodium, magnesium, calcium, and potassium at concentrations even higher than those found in seawater itself. Now, researchers at Institute of Science Tokyo have demonstrated a process that treats this brine not as a disposal problem but as a resource, using molten tin to simultaneously extract freshwater and recover magnesium with remarkable efficiency. The work, published in the journal Desalination, describes a liquid-metal route to magnesium recovery that achieved yields exceeding 99 percent of the magnesium taken up by the metal.

The research team, led by doctoral student Toranosuke Horikawa of the School of Engineering and Associate Professor Masatoshi Kondo of the Laboratory for Zero-Carbon Energy, has been developing what they call an alternative approach to resource extraction based on liquid tin. Their earlier work established the core principle: when desalination brine is brought into direct contact with hot liquid tin, water evaporates and can be collected as distilled water, while dissolved elements such as magnesium, sodium, calcium, and potassium are absorbed into the liquid metal. In effect, the molten tin acts as both a heat-transfer medium and a chemical sponge, separating the valuable components of brine in a single integrated operation.

But the earlier experiments left a critical problem unresolved. Alongside the useful metals, chlorine and sulfur from the brine were also incorporated into the liquid tin. These nonmetallic contaminants react with magnesium to form compounds such as magnesium sulfate, which would sharply limit the industrial usefulness of any recovered material. Magnesium is a strategically important element, used in automobiles, aerospace components, batteries, cement, and refractory materials, and conventional recovery methods typically demand large quantities of chemicals or electricity while generating secondary waste. A product contaminated with sulfur and chlorine would undermine the entire premise of the process.

The team’s solution was to add a vacuum-degassing step before magnesium recovery. In the experiments, approximately 8 grams of tin was melted and held at temperatures between 573 and 673 kelvin, roughly 300 to 400 degrees Celsius. Artificial brine, prepared with heavy water to enable isotope tracing during gas analysis, was supplied onto the liquid surface at a rate of 0.1 milliliters per minute. The water evaporated and was collected separately as distilled product, while magnesium, sodium, chlorine, sulfur, and other brine-derived components were taken up into the liquid metal below.

The brine-loaded tin was then heated under a high vacuum of about 10 to the power of minus 3 pascals, ramping from 573 or 673 kelvin up to 973 kelvin, approximately 700 degrees Celsius, at a controlled rate of 2.5 kelvin per minute, and held at the peak temperature for roughly ten minutes. Gas analysis revealed that the unwanted chlorine- and sulfur-containing species were released as volatile gases, including hydrogen chloride, deuterium chloride, molecular chlorine, sulfur dioxide, and hydrogen sulfide. By measuring the change in the mass of the liquid tin before and after treatment, the researchers estimated that about 59 to 67 percent of the brine-derived gaseous components had been removed from the metal.

With the contaminants largely stripped away, the next stage exploited a simple thermodynamic principle: the solubility of dissolved elements in liquid tin falls as the temperature drops. After degassing, the tin was slowly cooled while the vacuum was maintained, and as the solubility declined, dissolved elements precipitated out of the metal in a process the researchers describe as recovery by cooling, or solubility-driven precipitation. Microscopic observations showed that magnesium-rich and sodium-rich regions formed at different locations within the solidified tin, indicating that the elements could be spatially separated from one another during the cooling process itself.

The quantitative results were striking. When 20 milliliters of brine was processed at 673 kelvin, the magnesium concentration in the recovered precipitate reached up to about 280 times that of the original brine, and the ratio of magnesium to sodium increased by roughly a factor of 5,300. The amount of magnesium recovered was estimated to be equivalent to more than 99 percent of the magnesium that had been absorbed into the liquid tin. Electron microscopy and electron backscatter diffraction analysis further confirmed that the major crystalline phase in the magnesium-rich precipitate was magnesium oxide, MgO, a chemically simple and industrially relevant compound rather than a complex contaminated salt.

One of the most significant advantages of the approach is what it avoids. Because the process separates and concentrates magnesium without relying on large additions of alkaline chemicals, it may substantially reduce chemical consumption and the secondary waste streams associated with conventional precipitation methods. The thermal energy required to keep the tin molten could, in principle, be supplied by concentrated solar thermal energy or by high-temperature industrial waste heat, opening a pathway to low-carbon operation. A conceptual design developed by the team estimated that, under idealized conditions, the electricity required for the vacuum-degassing step could be as low as about 1.7 kilowatt-hours per kilogram of magnesium recovered.

To illustrate the potential scale, the researchers modeled a solar-driven module designed for a sunny, water-stressed region such as Egypt. In this scenario, a solar concentrator with a diameter of 15 meters was estimated to have the potential to produce about 970 kilograms of freshwater and recover approximately 2.8 kilograms of magnesium per day. While these figures come from an idealized design study rather than an operating plant, they sketch the outline of a technology that could turn the economics of desalination on its head, converting the largest liability of the industry into a co-product revenue stream while easing the environmental burden of brine discharge.

The researchers describe the broader concept as mining without digging: obtaining valuable resources not from newly excavated ores, but from water streams that are currently treated as waste. The vision is compelling, but the path from laboratory demonstration to practical deployment still requires substantial engineering. The next step, according to the team, is to develop a continuous system in which liquid tin circulates through the full cycle of freshwater production, vacuum degassing, magnesium precipitation and recovery, and tin reuse. Long-term stability of the molten metal, compatibility with structural materials at operating temperatures, purification of the recovered magnesium product, overall energy consumption, and economic feasibility will all need careful evaluation. If those hurdles can be cleared, the humble brine pipe at the back of a desalination plant may one day rank among the world’s more sustainable magnesium mines.

Subject of Research: Magnesium recovery from seawater desalination brine using liquid tin direct-contact distillation and vacuum degassing

Article Title: Mining without digging: liquid metal tin turns desalination brine into a magnesium resource

Article References: Mining without digging: liquid metal tin turns desalination brine into a magnesium resource. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: desalination, brine, magnesium recovery, liquid tin, vacuum degassing, magnesium oxide, water scarcity, solubility-driven precipitation, solar thermal energy, resource recovery, Science Tokyo, Distalination journal