In chemical engineering, recycling refers to the process of reacting two chemicals in an environment in which one is far more prevalent than the other. For instance, if for some reason you wanted to make some of Auric Oxide, you might sprinkle gold dust into a large chamber filled with concentrated oxygen. Almost all of the gold will react, but there will still be plenty of Oxygen left over. The reason for doing it this way is that by flooding the environment with one chemical, you can greatly speed the rate of reaction. Note also that you're speeding the reaction by flooding the environment with the relatively cheap component.

The 'recycling' part comes at the end of the process, when you take out your auric oxide. You'll still have a lot of unreacted O2 left over. This Oxygen can just be recycled back into the tank for the next reaction.