Removing heavy glue/adhesives from aluminum sign frames etc..

Can you just scrape it off with a steel putty knife?

You could try oven cleaner, that might just do the trick. Just regular, cheap oven cleaner.

I have never personally used this next method, and I'm not sure if it will work or create a problem so you want to test it first and do so in a well ventilated area, and make sure you wear eye protection and gloves, etc.

Sodium Hydroxide or as it's commonly called, lye, will actually eat Aluminum, so do NOT use lye/Sodium Hydroxide. However, Drano that has Sodium Hydroxide also contains inhibitors which are suppose to prevent it from eating metal, namely Aluminum and I have heard some people use it to eat the carbon buildup on car parts.

I would try cheap oven cleaner first, it's much safer and might just do the trick. Aluminum has a relatively low melting point, not like lead or tin but it's still low enough to make it very easy to soften or melt, unless it doesn't matter.

There is also another method that you might consider. If you are selling the aluminum as scrap, you might ask your buyer if they will accept melting aluminum. If they will you could melt the aluminum, throw a bit of flux in while it's melting, then pour the melted aluminum into a cast iron mold, or muffin mold so you get round billets, thus cleaning the aluminum in the process. That will take more energy so I don't think it would be profitable if you are only doing small amounts. You might be able to actually sell aluminum billets like this to people who cast aluminum, which might actually make you more money than selling for scrap. You can check with the casting forums to see if anyone would be interested and maybe use this melting method at a later time.

Anyway, hope the ideas help, and remember if you do choose to use anything that might be dangerous, protect yourself.

Scott