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Metallurgy
Tuesday | 17 December, 2013 | 11:04 am

Seawater’s solution

By Gretchen Salois

Scientists uncover a cheaper way to extract lightweight magnesium

December 2013 - Sometimes a material that could be useful simply isn’t practical. Either the process is too expensive or time-consuming and therefore, not ideal. With those in the auto, aerospace and construction industries looking to lighten up the final product, other metal alloys, such as aluminum and high-strength steel, seem more feasible. Until now, lighter weight magnesium alloys were not considered a viable option. New research could change that.

Researchers at the Pacific Northwest National Laboratory, part of the U.S. Department of Energy, have uncovered a method to economically extract magnesium from seawater. It’s a process that up until now, was limited to remote locations and only possible at great expense. 

“Over the last few decades, we’ve seen magnesium production plants in the U.S. go out of business leaving the sole operating facility in Salt Lake City,” says Pete McGrail, laboratory fellow, PNNL. The Great Salt Lake provides PNNL with a nearly unlimited source of concentrated saltwater, that can be stored in large evaporation ponds where it is concentrated further by taking advantage of the arid climate. 

The electrolysis process used today involves a number of steps and can’t be done just anywhere. “Solar evaporation ponds take a lot of space and the plant needs access to natural gas and cheap electrical power. Much of the heat needed is to form a molten salt of the magnesium chloride but it’s a complicated process,” he says.

“Just heating magnesium chloride obtained from seawater causes it to decompose into forms you don’t want and would destroy the electrolysis cells,” McGrail explains. “So chemical purification steps are needed including adding carbon and heating the salt up to 900 degrees Celsius to get rid of contaminants and water. Once you’ve done that, you can put the molten salt into an electrolysis cell and the electric current breaks the chemical bond between the magnesium and chloride, resulting in magnesium metal and chlorine gas.”

McGrail says the big energy-intensive aspects of the setup have to do with the amount of heat needed to initially dry out the brine and bring it to 900 degrees C. 

A different approach

Instead, researchers have invented a new approach. “It’s a radical way on how to do this,” McGrail says. “We had to break the dependency on the massive solar evaporation ponds that require large land area and the right kind of climate.” To break that dependency, PNNL partnered with Global Seawater Extraction Technologies LLC. 

“They have an ion exchange system that lets them pull components from seawater, producing calcium sulfate salts and one of the byproducts turns out to be magnesium chloride,” he says. “In that process, all you have to do is pump water through an ion exchange resin.

“With this innovation, we can produce magnesium chloride without reliance on massive solar evaporation ponds and eliminate the need to use so much energy to dry out the concentrated brine solution,” McGrail continues. “That alone saves 25 percent of the total amount of energy you would need.”

The next step involves an organometallic chemical reaction, resulting in magnesium compounds with various organic components attached to them that are used in all kinds of chemical processes. “Through a proprietary process, we only need to heat this compound to 250 to 300 degrees C, and it will decompose into magnesium  metal. “Sounds great but the problem is that chemical manufacturers form these organometallic reagents by starting with magnesium metal, which obviously doesn’t help us a heck of a lot,” he says. 

But that is where high-tech R&D comes in. “The goal of the project is to develop and test a new catalyst that will enable generation of the organometallic reactant we need from magnesium chloride instead of magnesium metal. It’s a big challenge but something national laboratories are particularly skilled at doing. If successful, we can eliminate electrolysis of magnesium chloride salt completely.”

The resulting magnesium metal is created using about 50 percent less total energy. And while the metal is the main goal, the process can also yield other products, including magnesium hydride, used in thermal storage applications.

McGrail says research on the new process is slated to start in January 2014. As energy costs for the process drop 50 percent, the overall cost of production declines by 60 percent. “Just the ability to deploy a plant like this now at any location where you have a reliable source of seawater or brine solutions—it could be located by a coast or in an area where there is a deep geological resource like geothermal brine. Maybe that brine was being produced for another reason or was even considered a waste product, someone’s problem they wanted to get rid of—now it’s a viable resource.” 

Interest is growing and McGrail has already been in talks with companies interested in learning more about the process. “If you have costs come down dramatically, you’ll see a ripple effect through the economy, affecting a number of industries down the road that can’t afford to use magnesium today.” MM

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