When Fraunhofer researcher Hans-Jürgen Friedrich wants to get to his underground laboratory, he has to descend more than 100 meters into the Ehrenfriedersdorf tin mine. Once at the bottom, he can sometimes change from the ropeway to the mine train – a rusty remnant of mining in the pre-reunification era.

"A bit cramped, but much more comfortable than on foot," the 1.90-metre man calls out from the cramped cabin, crouching down and tightening his yellow safety helmet. Another one and a half kilometers to his test facilities. The compartments rattle roaringly. It is dark. Only on bends do weak lamps bathe the surroundings in pale light. Friedrich usually walks because the train is then full of tourists. His destination is not open to the public.

Toxic water pollutes nature

The tunnel is barely wider than the track. Mine water flows next to the rails. "It sometimes rises knee-high here, especially now in the wet season," explains Friedrich, as the train finally stops at a concrete barrier, and he steps into the water with his rubber boots. It is precisely this water that the chemist is interested in.

"Thank you, Heiko," he waves a greeting to the miner, who shortly afterwards sets the mine train in motion again with a heavy lever. After a few minutes, nothing can be heard apart from the water dripping from the ceiling onto something metallic and rushing along the floor. Silence. The headlamp casts a cone of light onto the gray marbled rock walls. They shimmer damply. Here, too, the rippling water forms a delicate film. "The white here is tin veins," says Friedrich. He points to the rock structures. "The so-called scored mountains are also clearly visible here." By this he means the many cracks, veins and fissured areas that were created decades ago by tin ore mining. Oxygen attacks here. Together with the rainwater and meltwater that seeps into the ground above and penetrates the mine workings, this is the formula that causes pollutants to dissolve in the rock: The mine water here contains high levels of fluoride and toxic arsenic. Outside, it flows into the Wilisch river, continues into the Elbe and finally into the North Sea – along with the pollutants. According to the scientist, over a ton of arsenic pollutes nature every year.

recomine alliance confronts mining legacy

Since the Middle Ages, tin has been mined in Ehrenfriedersdorf, later blasted, until the "Last Hunt" was extracted in October 1990 and the mine was flooded. Since then, the mine water has reached the surface through a natural overflow.

In 2019, numerous companies, research institutions, universities and environmental and resource technology authorities from the extended Ore Mountains joined forces to form the "recomine – rethinking resources" alliance in order to sustainably rehabilitate sites damaged by mining and contain the resulting ecological damage. The alliance is supported by the Federal Ministry of Education and Research BMBF from the "WIR! – Wandel durch Innovationen in der Region" funding program. Today, recomine has almost 100 partners. Among them is the team led by Hans-Jürgen Friedrich from the Fraunhofer Institute in Dresden, who oversees the recomine sub-project TerZINN.

The underground laboratory

Friedrich trudges a few steps and switches on several construction spotlights with an experienced touch. Pumps, hoses, metal cylinders and measuring devices are lined up on a waist-high table made of gratings. He is currently testing how the toxins can be captured using electrolysis. The process works with electrical voltage. This causes dissolved pollutants in the water to form solid particles that are easier to filter. However, not everything that works in theory works under real mining conditions. The temperatures of around eight degrees and the high humidity are particularly challenging – pumps need more power, the power supply is vulnerable and requires 50 percent more power than in the laboratory, shorter maintenance cycles, corrosion. There is no remote monitoring – every investigation requires trips between Dresden and Ehrenfriedersdorf: "This gives us reliable results under real conditions," says Friedrich.

The project should be completed by the end of this year. If the process is implemented on a large scale, the processing plants would also have to remain underground for legal and energy reasons. Will this happen? It is still unclear how the large-scale toxic separation plant will be financed.

Clean water for agriculture and industry

Reducing environmental pollution is just one aspect that drives Friedrich. His aim is to develop cost-effective processes that can be used to treat the water so that it is suitable for higher-value purposes. This is because the toxic mine water cannot be used for agricultural irrigation, drinking water production or industrial processes. In view of the water shortage in many regions, people are therefore once again looking more closely at such sources.

By “once again” Friedrich is referring to the beginnings of his career in the GDR era. Back then, it was of scientific and economic interest to recover raw materials from mining water and extract metals from mining residues. "The GDR was dependent on its own supply of raw materials as imports were hardly possible. In Rossendorf, we were already making great technological leaps at the time – until a large part was finally scrapped with the fall of communism." Unfortunately, the hope of extracting valuable tin and tungsten from the mine water proved to be unprofitable. The water itself offers great revenue potential. After all, we are talking about two million cubic meters – per year.

Mayor Silke Franzl confirms high level of public interest

Friedrich and his team did everything they could to obtain a permit for a special operating plan in accordance with the German Federal Mining Act so that they could carry out underground research. It took three quarters of a year to obtain all the permits. "It was a long road," says Friedrich. Cooperation with the Saxon Mining Authority in Freiberg and the Lower Water Authority of the Erzgebirgskreis was crucial. Silke Franzl, mayor of the municipality of Ehrenfriedersdorf (non-party), confirms the high level of public interest in a statement from the consortium: "The people of Ehrenfriedersdorf and the surrounding area are hoping for innovations that will ensure clean water. For us, the focus is on the desire to remove pollutants from mining water and soil in order to minimize the negative impact on people and the environment, as well as the enormous financial outlay." For example, the municipality has a high arsenic content in its sewage sludge, which must be disposed of as expensive hazardous waste.

Friedrich opens his backpack. He takes out plastic tubes and a felt-tip pen and marks the sample tubes by hand. Date, number one, expiry. Friedrich takes an electrolytically treated water sample from a thin tube that leads into a kind of rain barrel. Later, at the institute in Dresden, the purified samples are to be analyzed for separation rates. "One liter of untreated mine water here contains around 300 micrograms of arsenic." By way of comparison, the maximum permissible level of arsenic in drinking water in many countries is ten micrograms per liter according to the World Health Organization (WHO).

Technologies tested in Ehrenfriedersdorf also of international interest

Friedrich has already been retired for a year and a half – actually. But he is sticking to his vision of showing that damaged mining sites can be restored to their former value: "Yes, I'm retired. Fortunately, the course has been set in such a way that I can continue my professional work for a while longer." To this end, he is now sitting around a table with the Free State, district, town and site owners to discuss development options for the traditional Ehrenfriedersdorf: Large-scale photovoltaic systems are planned for the stockpile site. Hydrogen is also being discussed. "For this, we need renewable power – solar energy in this case – and very pure water in not inconsiderable quantities." And because this cannot be tapped from the local grid, Friedrich is confident that he can make a big difference with his technologies and the "water treasure" hidden here, as the mine water is hopefully referred to.

The purification processes tested in Ehrenfriedersdorf are also likely to be of international interest, as contaminated mine water is a global problem.

Friedrich stows the sample tubes in his backpack. He heads back. This time on foot, three kilometers. Miner Heiko Wendsche welcomes Friedrich at the ropeway. The basket made of heavy iron bars closes. Wendsche rings the signal bell twice. The men chat. You need a lot of stamina. That's why Hans-Jürgen Friedrich is looking forward to basketball training with his recreational team this evening. "I have to stay fit," he says and, once at the top, takes his ropeway token from the board.