Extracting valuable metals from waste incineration plants using bacteria
Raw materials such as rare earths or noble metals are precious because they are not common in nature and because natural resources are visibly running out. Such raw materials are urgently needed to produce many of our everyday objects, but our throwaway society has tended to put little thought into conserving and recycling them. In collaboration with researchers from the University of Tübingen, scientists from the Tübingen-based biotechnology company Novis GmbH have now evaluated a biological leaching method that can be used to recycle valuable resources from incineration slag using bacteria. Funded by the Baden-Württemberg Ministry of the Environment, Climate Protection and the Energy Sector, the experts from Tübingen are studying the possibility of using this method in cogeneration plants.
Dr. Thomas Helle is the managing director of the biotechnology company Novis GmbH which he founded in Tübingen in 2009. Novis GmbH’s main activity is the generation of energy and raw materials from biogenic residues. Dr. Helle holds a PhD in education, not at all the kind of qualification you would expect an energy generation expert to have. However, as he did not enjoy his original profession as much as he thought he would, Helle decided to do something else instead. So after a 25-year career as a management consultant, five years ago Helle decided to do something completely different, and has since been working with teams from Germany and Africa developing technologies for the effective use of industrial and agricultural waste.
Bioleaching - efficient metal recycling
The use of bacteria for extracting metals from their ores, also known as bioleaching, is playing an increasing role in waste recycling. Bioleaching involves the use of microorganisms for extracting and recycling metals such as gold, aluminium or rare earths from ores. Novis GmbH has spent the past few years assessing the efficiency and suitability of such methods for extracting valuable materials from incineration slag. Together with MVV Umwelt, a subsidiary of the Mannheim-based energy producer MVV Energie, Novis will now run a pilot project with three waste incineration plants to assess new methods of recycling metals from combusted waste. “In principle, these methods can be used for any type of waste incineration plants,” says Helle.
The slag from waste incineration accounts for about ten percent of the volume and around a quarter of the weight of the total material combusted and it contains all the inorganic residues. Up until now, all the slag has been sold to recyclers, who break it up and remove the metals using mechanical means. However, only metal pieces larger than around two millimetres can be recycled. The remainder of the slag is put on a large heap and is often used as a foundation for road construction. The small metal parts, which are often present as metal salts, are therefore simply thrown away. “So we started thinking about ways to extract the metal fraction,” says Helle. “And setting up research projects that specifically focused on bioleaching.”
Multibacterial approach that follows the model of nature
The biotechnologists from Tübingen initially determined which elements were present in the slag before going on to use acids to extract the precious metals. They found that the slag from which metals had been mechanically extracted still contained some interesting treasures that could be further exploited. “Although a recycling procedure can be very interesting in economic terms, you have to bear in mind that a huge amount of investment is required before any kind of return can be expected,” said Helle. “The material must be constantly sprinkled, requiring the installation of complex and expensive equipment.”
In order to increase yield, the Tübingen scientists tried using bacteria to extract the metals: “Bacteria always accumulate in places where they can grow well. We therefore used different mixtures of bacteria, following nature’s model,” said Helle. The result was quite good: after a maximum of one to four days, the scientists had recycled 80 to 100 percent of the metals. “We were thus able to show that multibacterial approaches are far more successful than those that use a single specialized type of bacteria,” said Helle. Groups of researchers have exchanged their bacteria with one another. Novis GmbH works mainly with researchers from the Department of Geomicrobiology at the University of Tübingen who have been looking into the interactions between microorganisms and metals in nature for quite some time.
Upscaling tests are carried out in the laboratory and pilot cogeneration plants
After the bacteria have withdrawn “their” metals from the solution, the metals can be precipitated or separated by electrolysis. Both the solution and the recycled metals can potentially be turned into marketable products and the scientists are currently working on a practical way of achieving this. The initial tests involved 50 to 200 ml samples of waste produced by participating cogeneration plants. The Novis engineers are currently focused on upscaling the approach to an aquarium scale of 50 to 100 litres.
“These upscaling tests will then show whether we can recover the metals or whether we will sell the precipitated salt,” said the raw materials recycling expert. The researchers already know which elements can be recovered from standard slag using bioleaching: in addition to larger amounts of aluminium, they include rare earths such as scandium, neodymium, europium and thulium.
In parallel to the upscaling tests, the researchers designed the basic industrial methods for application in pilot cogeneration plants. The researchers envisage keeping the slag in large containers and sprinkling it with bacteria-acid mixtures. The Baden-Württemberg Ministry of the Environment, Climate Protection and the Energy Sector is interested in the method and is planning a second project to test its viability. This project plans to use all types of slag present in Baden-Württemberg.
Slag from waste incineration plants is not the only waste that Novis’ expert process engineers and economists are working on. They are also assessing the suitability of cocoa and rice husks, mushroom compost and window insulation systems, all of which have an excellent recycling ratio of 99 percent.
Dr. Thomas Helle
Vor dem Kreuzberg 17
Phone: +49 (0)7071 795-25 00