Sodium sulphide is the most versatile modifier in a flotation reagent cabinet. In the same plant it can make an oxidised copper ore float, hold a copper mineral back while molybdenite reports to the concentrate, and strip collector off a mixed concentrate ahead of a separation stage. That range comes from a single chemical behaviour — the sulphide ion’s affinity for metal surfaces — but the three duties are controlled in completely different ways. This guide explains what sodium sulphide does in each role, how to dose it, and what to watch when buying and handling it.
What Sodium Sulphide Is and Why It Is Effective
Sodium sulphide (Na2S), usually supplied as the hydrated salt Na2S·9H2O or in anhydrous flake form, dissolves to give a strongly alkaline solution containing sulphide and hydrosulphide ions. Both species adsorb readily on mineral surfaces, and that adsorption is the basis of everything sodium sulphide does in a concentrator:
- On oxidised minerals such as malachite, cerussite or smithsonite, the sulphide ion reacts with surface metal ions to build a thin metal-sulphide film. That film is what a xanthate collector can actually attach to — without it, the collector has nothing to bond with and the valuable mineral stays in the tailings.
- On sulphide minerals, the same ion competes with the collector for surface sites and, at sufficient concentration, displaces it. This is why sodium sulphide both inhibits minerals selectively and can de-sorb collector from a concentrate when a change of chemistry is required.
Nothing else in the reagent cabinet covers all three duties, which is why plants that treat mixed oxide-sulphide ores or run multi-metal separation circuits keep it in constant use.
Role 1: Sulphidisation of Oxidised Ores
Oxide ores are the classic application. A copper circuit treating malachite and chrysocolla, a lead circuit treating cerussite and anglesite, or a zinc circuit treating smithsonite will usually run a sulphidisation stage before flotation. Sodium sulphide is added to the conditioner or to the mill, the pulp is given time to react, and only then is the xanthate collector introduced.
Two operating points decide whether the stage works. The first is contact time and staging: sulphidisation is a surface reaction, and split addition with adequate conditioning generally beats a single large dose. The second is avoiding excess: once the surfaces are sulphidised, additional sodium sulphide begins to act as a depressant on the very minerals you are trying to float, and recovery falls away sharply. Operators therefore treat sulphide addition as a live control variable rather than a fixed recipe, adjusting to ore blend and to the froth they see.
The film formed is thin and fragile, so the sulphidised surface must be floated promptly. Long retention, over-grinding or aggressive agitation can all destroy the coating before the collector has done its work.
Role 2: Depression in Copper-Molybdenum Separation
The second common duty is depression in copper-molybdenum separation. Molybdenite floats readily and needs little help; the difficulty is keeping copper minerals — chalcopyrite in particular — out of the molybdenum concentrate. Sodium sulphide and its close relative sodium hydrosulphide are added to the cleaner circuit for exactly that purpose, holding the copper minerals back while molybdenite reports to the froth.
Dosage here is far lower than in sulphidisation, and control is delicate for the same underlying reason: too little leaves copper in the molybdenum concentrate, too much can start to affect molybdenum recovery as well. Plants often heat the cleaner circuit as well, since depression of chalcopyrite becomes more effective at elevated pulp temperature, and they run the two variables together.
Role 3: De-Sorption of Collector Before Separation
The third duty is less discussed but equally useful: stripping collector off mineral surfaces before a separation stage. A bulk lead-zinc or copper-lead concentrate that has been floated with a strong xanthate carries collector on every particle. Adding sodium sulphide in a conditioning tank desorbs that collector, after which the pulp can be re-conditioned with a fresh, selective reagent scheme. Without the de-sorption step, residual collector from the bulk float disrupts the selective stage, and both concentrates suffer.
Practical Points: Dosage, Handling and Supply
Dosage depends entirely on the duty. Sulphidisation stages typically consume far more sodium sulphide than depression or de-sorption duties, and plants working oxidised ores should expect their consumption to track the proportion of oxide in the feed, which can change from shift to shift.
Three handling and purchasing points deserve attention:
- Solution preparation: sodium sulphide is normally dissolved and added as a dilute solution. Prepared solution ages, loses strength and releases hydrogen sulphide, so it should be made up fresh and kept covered and alkaline.
- Safety and packaging: the solid is strongly alkaline, hygroscopic, and can release hydrogen sulphide if it contacts acid or damp air. Sealed, moisture-proof packaging and compliant documentation for sea freight are not optional.
- Specification: sodium sulphide is bought on Na2S content, with iron and insolubles as the common impurities of concern; grade drift changes the effective dose, so a certificate of analysis per lot is the practical way to keep a sulphidisation stage stable.
Conclusion
Sodium sulphide earns its place in the reagent cabinet because one chemistry does three jobs: it sulphidises oxide minerals so collectors can work, it depresses copper minerals in molybdenum cleaning, and it de-sorbs collector before selective separation. All three depend on tight control of addition — the difference between a productive dose and a harmful one is small — so the reagent is best managed as a live variable with test-backed guidance. For the modifier reagents that work alongside it, see our guide to copper sulphate and sodium hydrosulphide in flotation, and for a related depressing duty, the zinc sulphate depressant guide.