When I get a few hundred pounds of used wheel weights accumulated I wait till late evening or later and then go check out my local car and truck washes until I find one that isn't busy (there's only three in town). I dump the weights along a wall and go at them with the high pressure car wash nozzle. Sometimes I even wash the truck. Valve stems get bounced right out. Soap and rinse. Nearly clean enough to use again.
Take a small scoop shovel to load them back in buckets that have drain holes in the bottom. Let them sit several days to be sure there is no water in them and you're good to melt. Sure beats any other way I've tried to clean them. And, no mess at home.