Simple process enables extraction of magnesium from seawater

mechanics

Technological Innovation Website Editor – 10/06/2022

Simple process enables extra

Today, magnesium is extracted from salt ponds using electrolysis, an energy-intensive process.
[Imagem: Cortland Johnson/PNNL]

magnesium extract

Engineers from the Pacific Northwest National Laboratory (PNNL) and the University of Washington, both in the US, have discovered a simple way to recover a pure magnesium salt from seawater, the raw material for arriving at the metal magnesium.

Magnesium is abundant at sea and increasingly useful on land, including carbon capture, low-carbon cement and next-generation batteries.

Today, however, magnesium is obtained primarily through an energy-intensive process from brines from salt lakes.

In the mid-20th century, chemical industries were able to extract magnesium salts from seawater by mixing it with sodium hydroxide, or caustic soda. The resulting magnesium hydroxide salt, which gives the milk antacid of magnesia its name, was then processed to produce metallic magnesium. However, the process results in a complex mixture of magnesium and calcium salts, which are difficult and expensive to separate.

What Qingpu Wang and his colleagues did was to modify this process to produce pure magnesium salt, allowing for more efficient processing. And, interestingly, they achieved this by simplifying the process.

“Typically, people advance separation research by developing more complicated materials,” said Professor Chinmayee Subban. “This work is so exciting because we’re taking a completely different approach: We’ve found a simpler process that works.”

Simple process enables extra

Prototype of laminar coflow magnesium extraction equipment.
[Imagem: Qingpu Wang et al. – 10.1021/acs.estlett.2c00229]

laminar coflow

The new extraction technique works by flowing two solutions side by side. Dubbed the laminar co-flow method, the process takes advantage of the fact that flowing solutions create a constant reaction threshold: “fresh” solutions, which are entering the flow, never allow the system to reach equilibrium.

The materials are the same as in the original process, sea water and sodium hydroxide. Magnesium-containing sea water reacts quickly, forming a layer of solid magnesium hydroxide. This thin layer acts as a barrier, preventing the liquids from mixing.

“The flow process produces dramatically different results than simply mixing solutions,” said Qingpu Wang. “The initial barrier of solid magnesium hydroxide prevents the 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. The generation of pure magnesium hydroxide, without any contamination by calcium, makes it possible to eliminate the purification steps, and skipping these steps means saving energy and lowering the cost of manufacturing the metal.

In addition to opening, the technique opens up to the industry a source of raw material the size of all oceans.

Now the team intends to test its laminar coflow technique in other applications, such as in water desalination, in which the removal of magnesium is an essential part.

Bibliography:

Article: Flow-Assisted Selective Mineral Extraction from Seawater
Authors: Qingpu Wang, Elias Nakouzi, Elisabeth A. Ryan, Chinmayee V. Subban
Magazine: Environmental Science & Technology Letters
DOI: 10.1021/acs.estlett.2c00229

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