I have several hundred pounds of lead from scuba weights, sailboat ballast, medical containers, etc. It's not pure as it does not shrink as much when cast and "wins" a crush test with pure lead. Is there any (at home, non-foundry) way to remove all or some of the unwanted metals?

From what I have been able to find out sailboat ballast is the nastiest and most unknown alloy of all the scrap lead. It could have anything in it from wheel wieghts (good stuff usually) to battery cores (total crap).
Once a metal has alloyed itself (desolved) in lead, its there forever. In the case of mechanical mixtures like excess antimony or zinc it can be skimmed off while the alloy is at a lower temp than required to melt the excess metals.

The best way to find out is to try small batches and see if you get proper fillout in a mould. If you get decent boolits, you're good to go and can probably make an educated guess as to the alloy by comparing the boolit's weight to one cast from a known alloy. The medical containers are your best bet for a usable alloy, the others could be anything but it's worthwile to check them out. You could have a gold mine.
If you cannot get them to cast well even pushing 800*, you probably have something undesirable in the alloy and it's useless trying to use it for boolits. Sell or give it to someone who casts fishing weights. As Rusty said, there's no practical way that you can uncontaminate it at home.

Thanks for the comments.
I actually get pretty good bullets from this lead. They are about .001" dia. larger than the same bullet cast with "known" 30:1 alloy. They have a "dull" (comparred to the shinney 30:1 bullets) look to them (not from excess heat) but they fill the mould out and shoot just fine. The bullet is the 525g Lyman #457125 45/70.
I'm just getting into BP ctg. silhouette shooting and need to standardize my loads for best accuracy. The "unknowns" with the scrap lead is a problem but so is shooting a LOTS of expensive, 1/2oz., 30:1 alloy bullets. Maybe the thing to do is compare the two alloys for accuracy at 200 yds. and, if they are comparable, use the scrap stuff for practice.
Jon

I suspect your unknown alloy may contain a little antimony. No big deal. It means that if you wanted to you could heat treat them after sizing if you want them harder, but for your intended use it is unnecessary. The alloy is more versatile, in that you could effectively cast boolits of smaller diameter and push them to higher pressures/velocities. I'd just see which are more accurate out of your rifle, and if it is the unknown alloy, then use it sparingly until it's used up. For sils competition, bullseye accuracy isn't vital. You should be getting decent enough accuracy even with scrap wheelweights in a .45-70. The confidence factor you get by using the most accurate possible combination doesn't hurt, though. (I hunt big game with loads that are subMOA for that reason, even though I know a 3 or 4 MOA load will fill the freezer just as well. I know what the load can do, so I have that much more confidence in it and in myself.)

My favorite casting metal was linotype could drive 405 gr 45-70 bullets w/o leading the bore using sr 4759. I used wheel wts,pure lead,tin and marvelux

Jon:

As long as it casts nice full bullets and they're hard enough. Don't worry about it, cast 'em and shoot 'em up.

I got into a "clean out the basement and you can have all you want" deal a few yrs ago and ended up with several ton's of scrap bullets.

This stuff is the perfect hardness. I've shot over fifteen hundred thru a couple revolvers and never leaded the least bit.

I really think you're trying to find a problem where there isn't one.

I do understand the idea you have. But, unless there's some rule, or you catch bs over it, just use the same stuff for the big slugs and matches too. Will keep the barrel from leading and give you better shots. Won't hurt the gongs, or metal targets either. Only way they'll be hurt is velocity enough and heavy wt,

Good luck and wish you well,

George