From Longman Dictionary of Contemporary Englishmelt something ↔ down phrasal verbLIQUIDto heat a metal object until it becomes a liquid, especially so that you can use the metal again A lot of the gold was melted down and used for making jewellery. → melt→ See Verb table
Examples from the Corpus
melt down• As if anyone could melt them down.• Tony asks him whether he dug a hole, but he says no need, the stuff just melts its way down.• Back home, he melted them down and added ground - up coloured chalk - if available - to make crayons. 