Sodium sulfide (Na₂S) is a commonly used reagent in the flotation of sulfide ores. Its main functions are as follows:

(1) It is a depressant for most sulfide ores. When used in large quantities, it can suppress the vast majority of sulfide ores. The order of sulfide ores it suppresses in decreasing order is approximately: galena, sphalerite, chalcopyrite, bornite, covellite, pyrite, and chalcocite. Because molybdenite has excellent natural floatability, sodium sulfide cannot suppress it. Utilizing this, when flotating molybdenite, Na₂S can be used to suppress other sulfide ores.

(2) It is a sulfiding agent for non-ferrous metal oxide ores. Non-ferrous metal oxide ores cannot be directly collected by xanthate. However, if sodium sulfide is added before xanthate flotation to react with the non-ferrous metal oxide ores, a thin film of sulfide can be formed on the surface of the mineral, which the xanthate can then collect. This effect of sodium sulfide is called sulfidation, so it can be used as a sulfiding agent for non-ferrous metal oxide ores.

After reacting with sodium sulfide, the surface color of cerussite changes from white to dark. After reacting with sodium sulfide, the surface color of malachite changes from green to dark black, indicating that after sulfidation, a sulfide film different from the mineral’s surface is formed.

(3) It is a de-reagent for mixed sulfide concentrates. When a large amount of sodium sulfide is used, it can desorb xanthate collectors adsorbed on the mineral surface. Therefore, sodium sulfide can be used as a de-reagent before separating mixed concentrates. For example, before separating lead-zinc or copper-lead mixed concentrates, the slurry can be concentrated, a large amount of sodium sulfide added for de-reagenting, then washed, and fresh water added again for slurry preparation before separation and flotation.

In addition to the above three main functions, because sodium sulfide can react with many metal ions to form insoluble sulfide precipitates, sodium sulfide also has the effect of eliminating certain ions in the slurry that are harmful to flotation.