In the laboratory, pyrrhotite was broken to fine particles. Pyrrhotite particles were added in 0.5 mol/L sulphuric acid solution for 2 h, and washed with deionized water repeatedly until water was clear. The clean pyrrhotite particles were dried in a draught cupboard. The dried pyrrhotite particles were then stored in airtight bags, which were flushed with nitrogen gas to prevent oxidation of pyrrhotite.