Unroasted iron pyrites fall under heading 2502. Once the material is roasted — a thermal process that oxidizes the sulfide — the resulting iron oxide residue is no longer classified here and moves to a different chapter covering mineral residues and ashes. The distinction is based on the processing state at time of import.

The main commercial driver is sulfuric acid manufacturing, where pyrites are burned to generate sulfur dioxide as a process gas. Some metallurgical and chemical applications also use pyrites as a sulfur source. Because domestic pyrite mining is limited in the US, imports from China, Finland, and South Africa fill the supply gap.