Oxidised lead‑zinc minerals (cerussite, anglesite, smithsonite, hemimorphite, etc.) have hydrophilic surfaces and cannot be directly floated with xanthate collectors. The addition of sodium sulphide sulphidises the ore, enabling subsequent adsorption of xanthates. Beyond sulphidisation and activation, sodium sulphide also provides selective depression, pulp pH adjustment, and reagent removal functions, making it a critical reagent in oxidised lead‑zinc processing.
I. Sulphidisation and activation
- Reaction mechanism
Sodium sulphide hydrolyses in water to generate S²⁻ and HS⁻ ions, which react with surface metal ions on oxidised lead‑zinc minerals via dissolution‑precipitation, forming a hydrophobic sulphide film:
Cerussite:
PbCO₃ + 2HS⁻ → PbS + CO₂ + H₂O (illustrative reaction; image placeholder)
Smithsonite:
ZnCO₃ + 2HS⁻ → ZnS + CO₂ + H₂O (illustrative reaction; image placeholder)
The PbS film formed is dense and stable, firmly adsorbing xanthate and endowing oxidised lead with sulphide‑like floatability. The ZnS film is loose and prone to oxidation and detachment; sodium sulphide alone is insufficient for floating oxidised zinc, which requires secondary activation with amine collectors later.
- Application value
Without sodium sulphide, overall recovery of oxidised lead‑zinc is low. With proper sulphidisation, lead recovery increases significantly. Sulphidisation is a prerequisite for oxidised ore flotation. In practice, sodium sulphide is added first for sulphidisation, followed by xanthate for selective lead flotation.
II. Selective depression
- Depression of impurities such as sphalerite and pyrite
Excess S²⁻ in the pulp coats galena, sphalerite, and pyrite surfaces, hindering xanthate adsorption and producing a depressive effect.
In the bulk lead flotation stage, a moderate sodium sulphide dosage is used to activate only oxidised lead; excess depresses lead while strongly depressing primary sphalerite, thus achieving zinc‑lead separation.
For the zinc flotation stage, the lead tailings must be thoroughly washed to reduce residual sodium sulphide concentration and eliminate depression, before using amine collectors to recover zinc. - Dosage rules
Insufficient dosage: thin and incomplete sulphide film, low lead recovery.
Moderate dosage: full activation of oxidised lead and moderate depression of sphalerite, optimal separation.
Excessive dosage: surplus S²⁻ depresses both oxidised lead and sphalerite, causing lead and zinc recoveries to drop sharply.
III. Pulp conditioning
- pH adjustment
Hydrolysis of sodium sulphide yields a strongly alkaline solution, naturally maintaining pulp pH in the favourable range of 8.5–11. The alkaline environment prevents oxidative decomposition of the sulphide film, enhancing sulphidisation stability. - Eh control
The reducing sulphide ions lower the oxidation potential of the pulp, preventing oxidative deactivation of the PbS film. For highly oxidised ores, a certain excess of sodium sulphide must be maintained as a reducing agent. - Precipitation of harmful heavy metal ions
Free Pb²⁺, Cu²⁺, Zn²⁺, and Fe³⁺ ions in the pulp non‑selectively activate gangue minerals, reducing selectivity. S²⁻ precipitates these ions as sulphides, eliminating interference and reducing unnecessary reagent consumption. - Slime dispersion
Alkaline hydrolysis products mildly disperse fine slimes, alleviating the coating of target mineral surfaces by slimes.
IV. Depressant/desorption action
When a mixed lead‑zinc oxidised concentrate is produced, a large amount of sodium sulphide can be added at high concentration. S²⁻ competitively displaces adsorbed xanthate on mineral surfaces, thoroughly removing the collector. After washing and repulping, staged sulphidisation and separation are applied to resolve locked lead‑zinc intergrowths. This is often used in complex polymetallic oxygen‑sulphide mixed ore flotation circuits.
V. Behavioural differences of sodium sulphide on lead vs. zinc minerals
- Oxidised lead (cerussite) – forms a dense PbS film; moderate sodium sulphide suffices for stable flotation; only excess causes depression. Suitable for xanthate‑based bulk lead flotation.
- Oxidised zinc (smithsonite) – forms a loose ZnS film; sodium sulphide alone cannot float it. Sodium sulphide serves only as a pretreatment, and cationic collectors such as octadecylamine or dodecylamine are required. Residual sodium sulphide strongly depresses oxidised zinc flotation.
VI. Key industrial practice tips
- Split addition: add in both the grinding stage and roughing flotation to extend sulphidisation contact time.
- Lead flotation tailings need dilution and washing to reduce residual sodium sulphide before zinc flotation.
- For highly oxidised, high‑clay ores, combine with reducing agents (iron powder) and dispersants (sodium hexametaphosphate) to stabilise the sulphide film.
Side effects: generation of toxic rotten‑egg gas (H₂S) – ventilation required; excessive reagent adds cost; wastewater sulphide levels exceed limits.
Special note: NaHS, if used, decomposes readily.
Post time: Jul-13-2026
