Magnesium from seawater

Materials World magazine
2 Nov 2013

If only magnesium were less expensive. According to researchers at the Pacific Northwest National Laboratory (PNNL), based in Washington State, USA, the magnesium used in the production of lightweight alloys for cars and aeroplanes is seven times more costly than the steel employed in the same applications.

The team at PNNL is developing a process to extract magnesium from seawater and deep geothermal brine sources that is potentially far less energy-intensive than existing methods.

One of the most commonly used techniques involves heating the magnesium chloride salt to 900°C and performing a chemical reaction called electrolysis to extract the magnesium metal from the molten salt. By contrast, the PNNL team has developed a new concept to turn the salt into a metal organic compound, which can be decomposed to produce magnesium metal at less than 300°C.

Lead researcher Pete McGrail explains, ‘We first pass seawater through an ion exchange resin to selectively remove the magnesium chloride from seawater. Then a technique called azeotropic distillation is applied to strip water from the magnesium chloride salt – without having to heat the salt to a very high temperature. We use that technique to get a completely dry magnesium chloride. We then take the magnesium chloride salt and react it with an organic chemical compound. Through this chemical reaction we can produce an intermediate magnesium hydride that we press and sinter. This will decompose into magnesium metal at about 300°C. However, regenerating the organic reactant compound is key to the overall process and will require the development of a new catalyst.’

McGrail and his colleagues believe they can halve the current production price of magnesium using this technique, and they are not the only ones. The group has been given US$2.7m in US Department of Energy funding to develop the process over a three-year period.

The hope is that private industry, such as project partners Global Seawater Extraction Technologies, based in Arizona, USA, will use the process to produce magnesium once the project has finished. McGrail says, ‘There’s nothing particularly exotic about the way the process will work. I’m optimistic that, provided we succeed with the catalyst development and a couple of the other steps, it will be pretty straightforward chemical engineering.’

The project’s success could have broader implications for industry. McGrail gives the automobile industry as an example of a sector that could benefit from magnesium price reduction. ‘Once the price [of magnesium] comes down, these lightweight alloys can be used more extensively. This will have an impact on the cost of the vehicles and the mileage we can get out of a car.’

The project is scheduled to start on 1 January 2014. Let’s hope it’s worth its salt.