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Simple process extracts valuable magnesium salt from seawater

Researchers can isolate magnesium feedstocks from the ocean, important for renewable energy applications. Credit: Composite image by Cortland Johnson, Pacific Northwest National Laboratory

Since ancient times, people have extracted salts, such as table salt, from the ocean. Although table salt is the easiest to obtain, seawater is a rich source of several minerals, and researchers are investigating which ones they can get from the ocean. One such mineral, magnesium, is abundant in the sea and is becoming increasingly useful on land.

Magnesium has emerging sustainability-related applications, including in carbon capture, low-carbon cement and potential next-generation batteries. These applications are bringing renewed attention to domestic magnesium production. Currently, magnesium is extracted in the United States through an energy-intensive process from brine from salt lakes, some of which are at risk from drought. The Department of Energy has included magnesium in its recently released list of critical materials for domestic production.

A paper published in Letters on Environmental Science and Technology shows how researchers from Pacific Northwest National Laboratory (PNNL) and the University of Washington (UW) have found a simple way to isolate a pure magnesium salt, a raw material for magnesium metal, from seawater. Their new method allows two solutions to flow side by side in a long stream. Called the laminar coflow method, this process takes advantage of the fact that the flowing solutions create a constantly reacting boundary. Fresh solutions flow by, which means that the system can never reach equilibrium.

This method plays a new trick with an old process. Mid 20e century, chemical companies successfully created magnesium raw materials from seawater by mixing it with sodium hydroxide, better known as lye. The resulting magnesium hydroxide salt, which gives the magnesia antacid milk its name, was then processed to make magnesium metal. However, the process results in a complex mixture of magnesium and calcium salts, which are difficult and expensive to separate. This recent work produces pure magnesium salt, which allows for more efficient processing.

“Normally, people advance separation research by developing more complicated materials,” said PNNL chemist and UW affiliate professor of Materials Science and Engineering Chinmayee Subban. “This work is so exciting because we’re taking a very different approach. We’ve found a simple process that works. If scaled, this process could help fuel the renaissance of U.S. magnesium production by generating primary feedstock. We’re surrounded by a huge, blue, untapped resource.”

Simple process extracts valuable magnesium salt from seawater

Seawater from the PNNL-Sequim campus fueled this research project. Credit: Andrea Starr, Pacific Northwest National Laboratory

From Sequim water to solid salt

Subban and team tested their new method using seawater from the PNNL-Sequim campus, which allowed the researchers to take advantage of PNNL facilities in Washington state.

“As a Coastal Sciences employee, I just called a member of our Sequim chemistry team and asked for a seawater sample,” Subban said. “The next day a cool box was delivered to our lab in Seattle. Inside we found cold packs and a bottle of chilled Sequim seawater.” This work represents the collaboration that can take place at PNNL’s Richland, Seattle, and Sequim campuses.

In the laminar coflow method, the researchers let seawater flow past a solution containing hydroxide. The magnesium-containing seawater reacts quickly and forms a layer of solid magnesium hydroxide. This thin layer acts as a barrier to mixing of the solution.

“The flow process produces dramatically different results than simply mixing solutions,” said PNNL postdoctoral researcher Qingpu Wang. “The initial solid magnesium hydroxide barrier prevents calcium from interacting with the hydroxide. We can selectively produce pure solid magnesium hydroxide without the need for additional purification steps.”

The selectivity of this process makes it particularly powerful. By generating pure magnesium hydroxide, without any calcium contamination, researchers can skip energy-intensive and expensive purification steps.

Simple process extracts valuable magnesium salt from seawater

The laboratory scale flow-through device for extracting magnesium salt. Credit: Qingpu Wang, Pacific Northwest National Laboratory

Sustainability for the future

The new and gentle process has the potential to be very durable. For example, the sodium hydroxide used to extract the magnesium salt can be generated on site using seawater and renewable marine energy. The removal of magnesium is a necessary pre-treatment for the desalination of seawater. Linking the new process to existing technologies can make it easier and cheaper to turn seawater into fresh water.

The team is very excited about the future of the process. Their work is the first demonstration of the laminar coflow method for selective separations. This new approach has many additional potential applications, but more work needs to be done to understand the underlying chemistry of the process. The knowledge gap offers new possibilities and research directions for driving the blue economy.

“We want to take this work from empirical to predictive,” says PNNL materials scientist Elias Nakouzi. “There is a great opportunity to develop a fundamental understanding of how this process works while applying it to important problems such as creating new energy materials and achieving selective separation of difficult-to-separate ions for water treatment and resource recovery. .”

Extracting high-quality magnesium sulfate from seawater desalination brine

More information:
Qingpu Wang et al, Flow-Assisted Selective Mineral Extraction from Seawater, Letters on Environmental Science and Technology (2022). DOI: 10.1021/acs.estlett.2c00229

Provided by Pacific Northwest National Laboratory

Quote: Simple process extracts valuable magnesium salt from seawater (2022, September 23) retrieved September 23, 2022 from https://phys.org/news/2022-09-simple-valuable-magnesium-salt-seawater.html

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