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SDSU research could improve leather tanning
Aug '10
South Dakota State University research funded in part by the Environmental Protection Agency could lead to a cleaner, greener leather tanning industry. SDSU research associate professor Douglas Raynie has a grant of $350,000 from the EPA to study an alternative method for processing leather. The project uses liquid carbon dioxide instead of water as a carrier for the chemicals used in different stages of tanning leather.

The EPA is interested because conventional leather processing generates a lot of pollutants. The SDSU project is funded under a program to develop green manufacturing processes.

Raynie, in SDSU's Department of Chemistry and Biochemistry, said there hasn't been a great deal of change through history in the steps involved in leather tanning.

“Processors take the animal hide and put it in a solution that will remove the hair and the grease from the hides. Then they salt the hides and add, in olden days, vegetable tanning — that is, the tannins from tree bark and other plant material. In more recent times, they use chromium salts, and chromium is a heavy metal that is considered a toxic waste. This process is very lengthy, it can take anywhere from several days to a month or more, and it generates a lot of wastewater and associated wastes.”

The SDSU research uses liquid carbon dioxide — that is, carbon dioxide under a high pressure. At room temperature, as much as 800 pounds per square inch of pressure is needed. Under such conditions carbon dioxide shows some properties of a gas and some properties of a liquid. It would permeate more like a gas, and could be used as a carrier like water, for example, Raynie said.

“One of the properties of liquid carbon dioxide that is attractive for this process is that it will permeate into the leather much faster than water, so we replaced the water and use the carbon dioxide as the carrier. Now, it will allow for a much faster process and hopefully a more continuous process.”

Additionally, the carbon dioxide can be vented away as a gas once the process is complete, which means the pollutants will fall out of solution so that they can be recovered and treated appropriately. Or alternately, if the carbon dioxide is to be reused, it can be strained through a membrane to remove pollutants.

“It's much easier to remove the waste such as chromium with the CO2 process,” Raynie said. In a water solution, by contrast, removing chromium and other wastes is more difficult.

Raynie said regulatory concerns have been so great in the U.S. that most of the leather-tanning industry is now based in other countries. Raynie said a new method for tanning leather could bring some of those jobs back.

“There is still a lot of infrastructure in the U.S., as well as the growing trend for more of the cottage industries or local industries. So I could see industries on a small scale, a regional leather tanner providing material for a regional shoe company or interactions of that nature.”

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