"Smelt" is a verb just for melting metal. "Melt" is for other materials.

Smelt is specifically used to describe the process of melting metal using extreme heat. Melt is used in every day English. For example, my ice-cream melted in the sun.

Melt means turning something solid into liquid or soft. Smelt is past and past participle of verb "smell" (smell means something is noticed by your nose)