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Technology update
Success after 100 years...
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DaimlerChrysler's plastic membrane filters metallic ions.
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DaimlerChrysler is introducing a plastic membrane at its Bremen Airbus plant to overcome an environmental concern that has been part of its galvanization process. Panels are dipped in the company's galvanization basin for surface sealing before being moved to the rinsing basin. The surface sealing consists almost wholly of aluminum ions. After rinsing, the surplus ions remained in the water "as an environmentally hazardous pollutant," said DaimlerChrysler. However, the use of a plastic membrane separates the active substances from the water, and both are being re-used for the galvanization process.
Water consumption is reduced by some 90% and less aluminum is required. In 2001, this scenario will become reality in large-scale production at the Bremen plant, reported the company.
The plastic membrane in the waste rinse water tank acts like a sieve, separating the minute water molecules from the relatively large aluminum ions. The two substances—water and aluminum ions—can be re-used separately for the galvanization process: the purified water is used for rinsing once more, and the ions are returned via a pipe to the reaction basin. Although the principle of separating ions and water on a microscopic scale via ceramic sieve has been known for more than 100 years, it has taken until the present time to develop a sufficiently robust plastic membrane to endure large-scale manufacturing processes.
Researchers at Daimler-
Chrysler explained that thanks to a physical process known as reversed osmosis, the two substances can now be re-used for further galvanization. By means of a plastic membrane, a cylinder located downstream from the galvanization process is divided into two superimposed channels. Waste water from the rinsing basins is pumped into the upper channel. This applies sufficient pressure on the membrane to allow the water molecules to permeate it. On the other hand, the tiny pores in the plastic membrane are too small for the considerably larger aluminum ions to pass. The purified water accumulates in the lower channel and is re-used for the rinsing process. Meanwhile, the concentration of aluminum ions in the upper channel increases; these are returned to the reaction basin via a pipe. The material cycle is thus complete and the waste water is transformed into a raw material.
Stuart Birch
Aerospace Engineering April 2000
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