In the process of recycling aluminum the use of salt is essential. The salt has two main roles: It removes the impurities in the aluminum. Small parts of the cleansed aluminum will cling together into bigger parts, making the recycling process easier and reducing waste of material. But what is the ideal amount of salt?
The salt used is almost the same as the one we put in our food, but it’s chemical properties has been slightly altered. In addition, some other elements are added.
It is almost like cooking a soup: To the pot you add salt, mixed with some other chemical elements, and you heat it until it has formed a liquid. Into the liquid you drop your scrap pieces of aluminum.
Reacting with the liquid salt, the impurities in the aluminum are dissolved, forming a foamy layer on top of the soup. You remove the pot from the oven to cool the soup, and remove the foam.
And then, at the bottom of the pot – you find your clean pieces of aluminum. If the metal drops entrapped in the salt are too small they are difficult to collect, as they will float to the surface and stick with the foamy part on top. So the goal is to have big pieces, that will sink to the bottom of the pot to be collected, instead of being removed with the salty foam and wasted.
Here the salt comes in again: If you have the right amount of salt, compared to the amount of aluminum (and/or the right composition of the salt?) the pieces of aluminum will cling together into large pieces. Or, even better: one single piece.
I am investigating the amount of salt (or the composition of it?) that is ideal for the pieces of aluminum to coagulate, or cling together. When aluminum is recycled, there will often be parts of the metal that is coated. Like soda cans that have colourful outsides. I also investigate whether the coating effects the salt or not.
This blog post is written by Stefano Capuzzi, visiting PhD from the University of Padova in Italy.