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Follow-up: Interview with Thomas Helle on new developments in bioleaching

Some time ago, thanks to BIOPRO Baden-Württemberg, the biotechnology company Novis GmbH met Prof. Dr. Andreas Kappler, a renowned geomicrobiologist at the University of Tübingen. With financial support from the Baden-Württemberg Ministry of the Environment, Climate Protection and the Energy Sector, the two partners went on to test bioleaching methods for their ability to recover metals from slag using bacteria. In an interview with Dr. Thomas Helle, CEO of Novis GmbH, Dr. Ursula Göttert, on behalf of BIOPRO, asked what has become of the project.

In late 2014, BIOPRO published an article about your initial attempts to extract valuable metals from slag. Any news about this project Dr. Helle?

First of all, I'd like to mention that the BIOPRO article published in 2014 had a number of interesting results. At least five people in the field found me thanks to the article. A Fraunhofer Institute even contacted me yesterday to find out whether we could provide material for an internal expert presentation on bioleaching.

That's great! We're delighted to hear this.

I'd also like to point out that Novis now has two full-time employees focusing on this particular topic as well as a team of around seven people at the University of Tübingen. The researchers at the University of Tübingen are carrying out investigations related to our project. In the meantime, at Novis we have developed a concept of what a bioleaching plant might look like. We have just set up the first, large-scale demonstration plant in our laboratory.

What materials are you using as substrate?

The researchers who are working together on bioleaching (from left to right): Prof. Dr. Andreas Kappler, University of Tübingen, Dr. Thomas Helle, Novis GmbH, and Prof. Dr.-Ing. Harald Thorwarth, Rottenburg University of Applied Sciences © Novis GmbH

On the one hand, we are still using slag from the waste incineration plants of a big thermal waste recycling company in Baden-Württemberg. We use bioleaching to remove metals from the slag. The mineral residues can then be used as aggregates in concrete. We are in contact with companies in Baden-Württemberg that have expressed an interest in the process if it works.

On the other hand, we are now also working with Rottenburg University of Applied Sciences in a project using wood ash as substrate. We will remove the metal from the ash and use it as a fertiliser or for forest liming. In parallel, we are continuing our work on individual metal flows with the aim of treating them further. On this particular issue, for example, we're working with Basel University who are experts in the field.

Our ultimate aim is to release the slag into the environment again, and thus contribute to a circular economy.

Is this not yet possible?

No, because the slag contains too many heavy metals. The heavy metal content is too high even in natural wood and the ash from it cannot be spread in the forest after combustion.

Does this mean that your primary goal at the moment is not so much to extract the metals because of their material value, but because you want to clean the slag?

Yes, that is really the major problem. Slag contains huge quantities of minerals and the question is what to do with it. At the moment, slag needs to be disposed of, which costs money. And this is equally true for slag from waste incineration plants and from wood incineration. For this reason, the thermal waste recycling company that I mentioned before has decided to prioritise slag detoxification. If the metal recovered can help reduce costs, that’s great. But it is not necessary. In the long run, the thermal waste recycling companies want to carry out the slag treatment themselves. At present, a large portion of the metal is mechanically removed from the slag. The fraction with the smallest particles remains and is dumped. And this is the type of waste we are currently treating with our bioleaching method.

Which metals does the slag mainly contain?

Aluminium, zinc, copper and a small amount of strontium – slag contains quantities of these metals that exceed the legal limit. Wood is a strong aluminium accumulator. As soon as the latest cocoa regulation comes into force, we will also be faced with the problem that cocoa contains too much aluminium. Although it's really very tiny quantities, the fact is that the legal limits have been considerably decreased.

Which organisms do your partners from the University of Tübingen use to recover these metals from slag?

They use organisms that have been isolated from the environment, for example from the Spanish river, Rio Tinto. Life is no longer possible in this river as it has a pH of two as a result of the acidic water that has entered the river. This pH is perfect for bacteria of the genus Acidithiobacillus or Bacillus sulfooxidans species, which can bring metals in solution. The bacteria are isolated and cultivated in Professor Kappler's laboratory before being added to the slag. The effect is due the acid that bacteria produce when they metabolise sulphur, which they use for growth. Detailed experiments have shown that the metal-dissolving effect is greater with biotic leaching than with abiotic leaching involving only acid.

A completely new method has also emerged recently. A post-doc at the University of Stuttgart is doing research on ciliates. And ciliates have two properties that make them very interesting. They can store and then release rare metals. They are like small waste collectors. Interestingly, they can also collect phosphorus. So this is another area we will also focus on in the future.

You have also received funding from the German Federal Ministry for Economic Affairs and Energy, haven't you?

Yes, we have received funding for a ZIM project on bioleaching that will run until mid-2018. As a company, we receive 40% of the funding we need, and have to supply the remaining 60% ourselves. The University of Tübingen is a research institution and receives 100% of the sum granted. We are also delighted to have a brand new cooperation partner - Prof. Dr. Thorwarth from the Rottenburg University of Applied Sciences. He is now working with the University of Tübingen and Novis GmbH, in particular on the treatment and utilisation of biomass power plant slag, known as incineration bottom ash that accumulates during the incineration of wood. Professor Thorwarth is a well-known expert in combustion processes.

And what do you plan to do next?

Two Novis GmbH employees in front of the bioleaching demonstration plant: Benjamin Gann, a process engineer, and Thomas Rüschenpöhler, a biotechnologist. © BIOPRO Baden-Württemberg GmbH

Well, we have just built the demonstration plant on our premises. As soon as we have the slag, we will start the first large-scale tests. Then we will apply as quickly as possible for CE certification for the use of metal-free slag as aggregate for the concrete industry. On behalf of another partner, a biomass power plant in Switzerland, we are working on the possibility of using bioleaching treatment for reducing wood ash disposal costs.

In addition to all this, we’ve just been contacted by a mine in Brazil that mines scheelite, which is a calcium tungsten mineral. The mine has already invested two million euros in tungsten extraction, but the procedure they are using is not yet profitable enough. The operators have now pinned their hopes on bioleaching. Unfortunately, bioleaching does not work for tungsten. What bioleaching is, however, able to do is remove undesired metals and produce tungsten of higher purity. In addition, the mine also has some ores and metals, including a vein with traces of gold. And if we could extract the gold using thiourea instead of toxic cyanide, I am sure we would be doing good business. One of our post-docs at the University of Tübingen is working on this project until the end of this year.

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