Cleaning used carbon

We mentioned earlier that the adsorption process could be reversed with heat. You can use this desorption process to re-activate used carbon. A significant amount of heat is required; re-boiling the carbon will not do the job. Industrially, activated carbon is cleaned with steam, or by baking in an oxygen-free oven at temperatures as high as 800°C (1472° F).

One of the best ways to re-activate carbon is with steam. You can pack the carbon into a metal tube and blow hot steam through it under pressure, which is very efficient because the flow of steam carries the volatile contaminants away. However, you need at least one atmosphere of over-pressure and temperatures of 120°C (250°F), which makes this a dangerous process to try at home.

Alternatively, you can put the carbon into a pressure cooker with a little water. Boiling under pressure, and therefore temperatures higher than 100oC, for half an hour will ensure that the deepest pores of the carbon are washed to remove most of the volatiles, and what few remain can be baked out further by an hour or so in a very hot oven (260°C, 500°F). The water remaining in the carbon when you take it from the pressure cooker will help carry out the majority of the volatiles when it's first put in the hot oven, as the presence of water will depress the combined boiling point. After that, the high temperature does the rest.

Alternatively, you can leave the pressure cooker alone and simply spread the carbon out in a thin layer on a baking sheet and heat it in the oven. As noted above, it helps if you first make sure the carbon has been thoroughly washed in water and is still wet when first put in the oven. This is usually good enough for most purposes. However, do be sure that your oven is reasonably clean, or you may end up with carbon that smells like the last Sunday roast!

The strong smell coming from the pressure cooker and/or the oven is your proof that the process works, and an indication of what you will not be drinking!

These processes all work very well, removing the majority of contaminants and allowing you to reactivate your carbon as many times as you like. Over time, the carbon granules will physically break up and become dust, but if you don't mind the additional time it takes to filter off these small particles, just remember what was said above about carbon powder - pulverized carbon is used to clean liquids.

Before using the re-activated carbon, boil it in fresh water to degas it and make sure that the pores are all filled with liquid. Just wetting the carbon is not enough. Gas retention will greatly diminish the effectiveness of the carbon as the molecules in the liquid will be unable to diffuse into the deepest pores.

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