I do not think there is anyway to remove the antimony..
I used to ad it to my lead to make a harder bullet for 30
cal...I got it in a powder form and had to use a special
flux as well to get it to mix in..I would just leave it in there
and add more lead to make it softer..I used a 50-50 mix of
wheel weights and lead to make bullets for my 45-70
sharps rifle and they seemed to work great..Of course
this was for target shooting..