I don't know of any way to separate the tin from a lead/tin alloy.
You could "dilute" the alloy by melting with more lead, but that might take a lot of pure lead to do, and you could be shooting the pure lead.
I spent the weekend rendering a bucket an a half of wheel weights. This batch was pretty good in that most of them were actually lead. I had a few zinc ones and a few ones marked Fe that did not melt.
I run my pot at around 700 degrees F so I do not have to worry about the zinc melting in.