Note: A good portion of the MnO2 from these batteries is actually carbon. It still worked for my bromine synthesis, but it is something to keep in mind.
In this video, I get manganese dioxide, zinc and carbon from Zinc-Carbon batteries.
There is a significant amount of carbon in the MnO2. It is not worth the time to try to separate it out though, unless you need pure MnO2.
I used the manganese dioxide to make some bromine. This was filmed in 4K and will be uploaded eventually. The MnO2 can also be used to generate oxygen from hydrogen peroxide. I go over this in my liquid oxygen video and oxygen generator video.