Rand sixty different metals can contain a modern smartphone. The best-known metals are certainly lithium in batteries, copper in electrical connections and wires, and silicon for processors. All components of a smartphone depend on certain elements for their function. Some of these have only limited geological occurrences; some only occur indirectly as a by-product of the degradation of other materials. In addition, the mining of metals and their ores involves a great deal of effort and a certain degree of environmental pollution.
That is why circular economies for important raw materials are highly desirable, even if they cannot completely replace mining. Copper and lead are role models for well-functioning cycles – at least half of the global production of these metals comes from recycling processes. In this context, the recovery of elements from industrial wastewater or natural waters is becoming increasingly important. A research group from Yale University in New Haven led by Menachem Elimelech has now investigated which elements and processes are most suitable for this.
Since recovery from waste water is very complex, the researchers believe that the process is only worthwhile under certain conditions: the element should be as rare, in demand and difficult to degrade as possible. The scientists report in the journal “Nature Water” that this applies above all to rare earths and materials for battery technologies (lithium and cobalt) as well as to germanium and vanadium. However, such a process would only be worthwhile if the wastewater had a sufficiently high concentration of the respective element. As an example, the researchers cite the recovery of gallium from the waste water from semiconductor production plants.
Filtration in a precious metal recycling plant at Heraeus: filter with solids at the top, filtrate at the bottom and side opening to the vacuum pump on the right.
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Image: Maximilian von Lachner
How to separate metal ions from each other
Electrochemical separation processes are particularly promising. In aqueous solution, metals are usually present as charged ions, which migrate along an applied electric field to the positive or negative pole depending on the charge. The electrodes consist of porous materials that specifically bind or store metal ions as long as a voltage is applied. After the loading phase, the waste water is replaced by a saline solution. For discharging, the polarity of the applied voltage is reversed, whereby the bound ions are released and accumulate in the salt solution in a targeted manner. The greatest difficulty with these methods so far has been that the electrode materials degrade over time.
Another way of enriching or removing ions from solutions are membrane processes, such as those used in the treatment of drinking water. The membranes are mostly made of plastic and have small pores that are permeable to certain ions. Pressure, current or concentration gradients can be used to control which ions pass through the membrane and in which direction. In this way, certain ions can be filtered out of the waste water solution. Selectivity and transport speed are critical for membrane processes. The Yale researchers led by Elimelech hope to make further progress with new membranes made of nanomaterials, liquid crystals or porous framework compounds.
The recovery of metals from solid waste, i.e. scrap, has long been common practice. Recycling systems for precious metals and lithium-ion batteries are currently in vogue. One example is the Hanau-based technology company Heraeus, which operates plants worldwide for the recovery of precious metals from every imaginable branch of industry. Two more plants in the United States and China in cooperation with BASF were announced just last year. In Europe alone, fifty kilotons of lithium-ion batteries are recycled every year, and the trend is rising. According to a study by the Fraunhofer Institute for Systems and Innovation Research in Karlsruhe, forty percent of the cobalt required and fifteen percent each of the lithium, nickel and copper required for the production of new lithium-ion batteries could be covered by recycling by 2040.