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WWW.HZDR.DE 28 29 RESEARCH // THE HZDR RESEARCH MAGAZINE Arsenic can be safely removed from the liquid with the resulting solid matter. The disadvantage of this method lies in the fact that the volume of the liquid is significantly increased once the chemicals necessary for this treatment have been added. The concentration of gallium in the solution drops so dramatically that any attempt to extract it becomes uneconomical. A completely different solution is, therefore, needed in order to separate arsenic acid from gallium. An award-winning idea In December of 2014 this idea was recognized with the German Raw Materials Efficiency Award and 10,000 euros in prize money. The most efficient aspect of this research is as follows: Different types of production wastewater can be processed in just one step and without the costly use of chemicals. This increases the recycling quota for gallium waste from the previous 45 percent to up to 65 percent. HZDR engineer Oliver Zeidler, who has been working on his doctorate degree since 2012 at the Helmholtz Institute Freiberg for Resource Technology, has developed a novel dialysis process for this. It was conceived in collaboration with the company Freiberger Compound Materials GmbH (FCM) and the TU Bergakademie Freiberg. The tall Leipzig native modeled his concept after a very specific organ: "Similarly to how a kidney filters uric acid out of the bloodstream through a membrane, arsenic acid can be removed from process waste water." The process operates according to the simple principle of diffusion: Particles tend to distribute themselves equally throughout a space. If there is a higher concentration of particles in a particular area, then they move to balance out this difference. This occurs without the addition of energy and is therefore especially efficient. But how exactly is arsenic acid separated out from gallium? The solution, as with a kidney, is a selective membrane that only allows certain compounds from the original solution to pass. For this process to work, the membrane has to be acid-resistant. This is because in order for all of the wastewater to be treated at the same time, it is necessary to dissolve the residue with aqua regia – a mixture of several strong acids. Opposites attract Oliver Zeidler uses ion-selective membranes with a layer of positively charged functional groups for the dialysis system. That way only negatively charged ions such as those in the arsenic acid are able to pass through the membrane unhindered. And the positively charged gallium ions? The aqua regia transforms them into negatively charged gallium chloride complexes. The trick to this is that such complexes aren't especially stable. If the concentration of chlorine compounds in the environment decreases, then they will disintegrate, thus releasing the positively charged gallium ions. This occurs in the moment when the complexes pass out of the chloridic starting solution through the membrane and into the chlorine-free wash solution. It is in this way that the gallium and arsenic acid are separated while also preventing the formation of poisonous arsine gas. The materials scientist was able to demonstrate his results on a test system with the help of transport models capable of predicting the processes that will take place at the membrane. The process is very promising: Once put into practice, it could succeed in saving up to two and a half tons of gallium during wafer production with a market value of approximately a half a million euros. DIALYSIS: In 2014 this recycling method received the "German Raw Materials Efficiency Award" of the Federal Ministry for Economic Affairs and Energy. Photo: BGR | Uppenkamp _Helmholtz Institute Freiberg for Resource Technology at HZDR Prof. Christiane Scharf | Oliver Zeidler | CONTACT