So, it’s unfortunate that sulphuric acid often contains mercury, one of the most toxic substances known. Researchers at Chalmers University of Technology, Sweden, said they have now developed a method that can reduce the levels of mercury in sulphuric acid by more than 90%.

“Until now, there has been no viable method for purifying finished sulphuric acid at all. With such a radical reduction in the mercury content, we come well below the current limit values. Such pure high quality sulphuric acid is in high demand in industrial applications and an important step in reducing environmental impact,” said research leader Björn Wickman, associate professor at the Department of Physics at Chalmers.

Five years ago, his research team at Chalmers presented a method for removing mercury from water using electrochemical processes. The method is based on a metal electrode taking up the toxic metal and forming an alloy. The mercury can then be safely removed, and the electrode reused. Now the researchers have taken this technology one step further, and in a new study they have shown how mercury can be removed from concentrated sulphuric acid.

The experiments with sulphuric acid were done in collaboration with Boliden and Atium, a spin-off from the Chalmers School of Entrepreneurship with the aim of commercializing the removal of mercury from water and chemicals. The researchers now hope to be able to move forward with their partners and develop a type of reactor through which sulphuric acid can flow and be purified at the same time.

Currently, mercury is mostly removed at an earlier stage – from the concentrates and recycled streams at the smelter before sulphuric acid is produced. This is an established process but leaves trace amounts of mercury in final products.

The Chalmers-developed purification method removes mercury from the acid by first reducing mercury ions to metallic form and then binding the metallic mercury into an alloy with platinum, which is coated onto the electrode surface. It is then possible to remove the mercury and regenerate the electrode in a controlled process. Thus, the electrode can be reused, and the toxic substance can be safely disposed of.

“Purifying the sulphuric acid prevents additional mercury emissions, while allowing industry to operate more cost-effectively and produce a high-purity, non-toxic product. The next step will be to scale up the method into a pilot process that is closer to real-world volumes of thousands of tons,” said Vera Roth, doctoral student at Chalmers and first author of the recently published article in the journal ACS ES&T Engineering.

The lower the mercury content of the sulphuric acid, the more valuable it is. Sulphuric acid for commercial purposes is considered to be of acceptable quality when its mercury content is below 0.30 milligrams per kilogram. If the content is below 0.08 milligrams per kilogram, the sulphuric acid is considered to have a high purity. With the new method, the researchers have reduced the level of mercury to 0.02 milligrams per kilogram of sulphuric acid in their pilot study.

“The limit values for how much mercury sulphuric acid may contain are based on the technology available today. With the new method for purifying sulphuric acid, our hope is that the legislation around the limit values will be tightened in a global perspective where mercury levels are generally much higher,” said Wickman.