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Extracting silver from used X-ray film.

Approximately one-fifth of the silver used each year worldwide ends up in X-ray film, according to Haruo Ishikawa, a professor in the Chemical Engineering Department of Japan's University of Osaka Prefecture. After X-ray film is developed and fixed, it holds about two percent by weight of silver in its emulsion layers, which are made of gelatin.

The most common current process to recover silver from used X-ray film is to burn the film in a furnace and retrieve the precious metal from the ashes. However, this method is "rather expensive because of the cost of maintaining the furnace and treating the soot and foul-smelling smoke. Furthermore ... recovery and purity of silver are low and the re-use of the polyester film is impossible."

For an inexpensive and efficient alternative, Ishikawa and his colleagues have developed a silver-recovery technique using alkaline protease, an enzyme produced by Bacillus sp. B21-2 bacteria. A shredder cuts used X-ray film into strips that are fed into a reactor, where they are mixed with the enzyme under alkaline conditions. The enzyme breaks down the film's gelatin layers, releasing the silver particles into solution. The silver then is precipitated out of the solution by adding a coagulant such as aluminum sulfate. The technique "creates no pollution, [so] we can get high grade pure silver, and furthermore we can reuse the polyester base-film," Ishikawa points out.
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Title Annotation:Univ of Osaka Prefecture in Japan has developed a method for using alkaline protease to extract silver from used X-ray film
Publication:USA Today (Magazine)
Article Type:Brief Article
Date:Jun 1, 1996
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