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discovered_02_2015 - Recycling - the kidney as an example

WWW.HZDR.DE discovered 02.15 RESEARCH RECYCLING - THE KIDNEY AS AN EXAMPLE // Together with the company Freiberg Compound Materials GmbH and the TU Bergakademie Freiberg, HZDR doctoral candidate Oliver Zeidler has developed a process that could increase the recycling rate of gallium during the production of electronic components by up to 20 percent. Although these days most computer chips and solar cells are made of silicon, there is a semiconducting material that transports electrons more quickly and transforms sunlight into energy much more efficiently: Gallium arsenide. Compounds composed of gallium and arsenic are superior to silicon in regards to the switching speed for transistors as well as the energy output of solar cells. Beyond that, the material is also indispensable for the construction of light-emitting diodes or for use with the lasers that carry information through fiber optic networks. Since gallium arsenide enjoys such high popularity, the need for gallium is growing fast. Unfortunately, the silvery white material with its typical blue sheen is considered a "rare metal". Not just because it is especially rare - in the Earth's crust there is about as much gallium as there is lead - but also because the metal is only present in very low concentrations in ore. So in many cases the technical cost required for extraction stands in the way of great profits. For this reason, and since the metal is somewhat irreplaceable, the search for cost-effective recycling methods is an urgent task. During the production of gallium arsenide wafers alone about 60 percent of the gallium used falls by the wayside as waste. In production cylindrical crystals are grown from this material and cut in millimeter-thin disks using a mechanical cutting process. Today almost half of all waste, such as sawed off crystal ends or defective disks, makes its way back into production without a problem. Things get more difficult, however, if the wafer surfaces have to be cleaned with water or polished with an acidic pickling solution. Then the costly metal will end up in different types of wastewater that have to be reprocessed in a variety of different ways. Poisonous arsine The reprocessing of pickling solutions is especially costly. The metal can be extracted from such wastewater by electrolysis. In order to do this, it first has to be pretreated. Without this pretreatment, the electrolysis process would release an extremely poisonous gas: Arsine, which is made of arsenic- hydrogen that blocks the nerve receptors and impedes the transport of oxygen through the body. If you want to eliminate this poisonous malady at its chemical source, you will have to remove a completely different material from the mix: Arsenic acid. This compound, which contains oxygen atoms as well as hydrogen, is the source material for arsine. It emerges when gallium arsenide dissolves in the acidic pickling solution. Chemists deal with the problem by adding limewater - a calcium hydroxide suspension - to the wastewater in order to increase its pH value. This decreases the solubility of the calcium ions as well as the arsenic acid. _TEXT . Tina Schulz DIFFUSION: Water hits the membrane, the resin swells, the pores are closed. Arsenic acid passes through the membrane unhindered and is thus separated from the gallium (picture taken with a reflected-light microscope at 50 times magnification). RECYCLING - THE KIDNEY AS AN EXAMPLE