Since ancient times, people have extracted salts, such as table salt, from the sea. While table salt is the easiest to obtain, seawater is a rich source of various minerals, and researchers are studying which ones they can pull 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 draw renewed attention to domestic magnesium production. Currently in the US, magnesium is extracted from brine brines, some of which are endangered by drought, using an energy-intensive process. The Department of Energy included magnesium in its recently released List of critical materials for domestic production.
An article published in Letters on Environmental Science and Technology shows how researchers at the Pacific Northwest National Laboratory (PNNL) and the University of Washington (UW) found a simple way to isolate a pure magnesium salt, a precursor to magnesium metal, from seawater. Her new method allows two solutions to flow side by side in one long stream. Called the laminar coflow method, the process exploits the fact that the flowing solutions form a constantly reacting boundary. Fresh solutions flow by, never allowing the system to reach equilibrium.
This method plays a new trick on an old process. mid 20thth In the 19th century, chemical companies successfully made magnesium raw materials from seawater by mixing it with sodium hydroxide, commonly known as lye. The resulting magnesium hydroxide salt, which gives its name to the antacid milk of Magnesia, was then processed into magnesium metal. However, the process results in a complex mixture of magnesium and calcium salts that are difficult and expensive to separate. This recent work produces pure magnesium salt, allowing for more efficient processing.
“Typically, 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 are pursuing a completely different approach. We found a simple process that works. If scaled, this process could help fuel the renaissance of US magnesium production through primary feedstock generation. We are surrounded by a vast, blue, untapped resource.”
From sequim water to solid salt
Subban and the team tested their new method using seawater from the PNNL Sequim campus, allowing the researchers to use PNNL facilities across 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 ice packs and a bottle of chilled Sequim seawater.” This work represents the collaboration that can take place between PNNL’s Richland, Seattle, and Sequim campuses.
Using the laminar coflow method, researchers flow seawater past a solution containing hydroxide. The magnesium-rich seawater reacts quickly and forms a layer of solid magnesium hydroxide. This thin layer acts as a barrier to the solution mixing.
“The flow-through process produces dramatically different results than simply mixing solutions,” said Qingpu Wang, a postdoctoral researcher at PNNL. “The initial barrier of solid magnesium hydroxide 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 effective. By generating pure magnesium hydroxide without calcium contamination, researchers can skip energy-intensive and expensive purification steps.
sustainability for the future
The new and gentle process has the potential to be very sustainable. For example, the sodium hydroxide used to extract the magnesium salt can be generated on-site from seawater and renewable ocean energy. Magnesium removal is a necessary pre-treatment for seawater desalination. Pairing the new process with existing technologies could make it easier and cheaper to convert seawater into freshwater.
The team is particularly 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 opportunities and research directions to advance the blue economy.
“We want to take this work from empiricism to prediction,” said PNNL materials scientist Elias Nakouzi. “It presents an exciting 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 hard-to-separate ions for water treatment and resource reclamation.”
Extraction of high-quality magnesium sulphate from seawater desalination brine
Qingpu Wang et al, Flow-Assisted Selective Mineral Extraction from Seawater, Letters on Environmental Science and Technology (2022). DOI: 10.1021/acs.estlett.2c00229
Citation: 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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