In sulfidization flotation, sodium sulfide or sodium hydrosulfide (NaHS) is added to oxide
copper, lead or zinc ore so its surface behaves like a sulfide, and a xanthate collector can
then float it. The key to success is dose control: too little leaves oxide surfaces unfloatable,
while too much depresses the minerals you want. Most plants add the sulfidizer in stages or
control it by electrode potential rather than in a single large slug.
Oxide minerals such as malachite, cerussite and smithsonite do not respond to standard
xanthate flotation on their own. Sulfidization is the most widely used way to bring them into
the flotation circuit, and sodium sulfide and sodium hydrosulfide are the reagents that do it.
Using them well is mostly a matter of controlling how much sulfide is in the pulp, at what pH,
and at which point in the circuit. This guide covers how the two reagents differ, how to dose
them, what published cases show, and how to handle them safely.
dissolved salts and pH, and early industrial attempts struggled with excessive pH at high sodium
sulfide dosage and with where, and how many times, the reagent was added.
What Sulfidization Does
The process was developed in the United States around 1915–1920 for oxide copper flotation and
is especially suited to oxidized lead ores with basic gangue. Sulfide ions react with the surface
of the oxide mineral and build a sulfide-like layer, which gives collectors such as xanthate
something to attach to. For copper oxide ores, the two main alternatives are fatty acid flotation
and sulfidization followed by a sulfhydryl collector.
The reaction chemistry differs by mineral. On cerussite the sulfide layer resembles galena, which
suits anionic collectors such as xanthate. On smithsonite sulfidization increases the negative
surface charge rather than forming a clear zinc sulfide layer, which is why amine collectors are
often used for zinc oxide ores.
Sodium Sulfide vs. Sodium Hydrosulfide
Both reagents supply sulfide species to the pulp. Sodium sulfide hydrolyzes in water to form NaHS
and sodium hydroxide, so it is the more alkaline of the two, and the pH your circuit needs is often
what decides between them.
| Factor | Sodium sulfide (Na₂S) | Sodium hydrosulfide (NaHS) |
|---|---|---|
| Typical forms | Flakes, commonly around 60% Na₂S | Flakes (around 70–72%) or solution |
| Effect on pulp pH | More alkaline | Less alkaline |
| Species adsorbing on mineral | Studies show hydrosulfide ions are the species adsorbed even when sodium sulfide is used | |
| UN numbers (check current SDS) | UN 1385 (anhydrous), UN 1849 (hydrated) | UN 2949 (hydrated flakes), UN 2922 (solution) |
| Storage behavior | Hygroscopic; keep dry and away from acids | Hygroscopic flakes; solutions often kept in closed tanks under nitrogen |
NaHS is also used at different dose levels for different jobs. One supplier’s guidance describes
low doses (below about 500 g/t) depressing sphalerite and iron sulfides, medium doses activating
oxidized or tarnished minerals, and high doses depressing all sulfides in reverse flotation. Treat
that as vendor guidance, not a universal rule, since real thresholds depend on the ore.
How to Dose: Slug, Staged, or Controlled Potential
1. Slug dosing (manual)
The traditional approach adds sulfidizer in large manual slugs. It is simple, but both over- and
under-addition are known to hurt recovery, and plant feed is rarely constant enough for a fixed rate
to stay correct for long.
2. Staged addition
Splitting the sulfidizer across several points improves results. In a study on a mixed oxide-sulfide
copper ore (about 62% oxide copper), staged addition lifted total copper recovery to 81%, against
76% copper oxide recovery under the best single-condition test at 53 µm and pH 10.5. Distributing
the sulfidizer between the grinding mill and the flotation cell balanced activation of oxide and
sulfide minerals. A caution from older laboratory work: multiple additions can give acceptable
metallurgy but with high sodium sulfide consumption.
3. Controlled potential sulfidization (CPS)
CPS monitors and controls the sulfide level in the pulp using an electrode, usually a sulfide
ion-selective electrode, so dosing follows what the pulp actually needs. Published results include:
- A laboratory study found an optimum sulfidizing potential of about −500 mV (versus SCE), using roughly 3-minute sulfidization and 5-minute flotation steps over three to five stages. On previously untested ores it gave good results at first attempt, even though the ores’ sodium sulfide requirements differed by a factor of three.
- In a 2024 IMPC case on a copper ore, adding NaHS to a potential of about −300 to −400 mV improved both oxide and sulfide copper recovery, while going beyond that range began to depress copper sulfides. The rougher protocol used SIBX at 10 g/t, fuel oil at 20 g/t, natural pH, P80 around 150 µm and 30–35% solids.
Plants without sulfide electrodes often use an ORP probe as a more practical indicator, with target
values established in testwork. Because the ideal potential shifts with pH, targets set at one pH
should not be copied to a circuit running at a very different pH.
pH, Slimes and Other Variables
- pH: high pH combined with high sodium sulfide dosage was one of the main reasons early sulfidization failed. Keep pH within the range proven in tests for your ore.
- Slimes and precipitates: fines and dissolved precipitates raise sulfide demand, so ores with heavy slime content need more careful testing.
- Grind size: in the mixed copper ore study above, 53 µm gave the best oxide recovery, though a separate CPS study found grinding finer than 150 µm did not significantly change rougher performance.
- Flotation gas: practitioners report that nitrogen can reduce sulfidizer consumption and stabilize potential, since air continuously oxidizes the sulfide.
Collectors After Sulfidization
Once the surface is sulfidized, a sulfhydryl collector floats it. Potassium amyl xanthate (PAX) is
commonly used on oxide copper, and the mixed ore study found that PAX performed notably better with
a co-collector. Butyl-type xanthates such as SBX
and SIBX are also used, particularly in CPS circuits. See our
potassium amyl xanthate page for
grade details.
Published Case Examples
These figures come from published studies on specific ores. They illustrate the range of reagent
use, not a recipe for your ore.
| Case | Ore | Conditions | Reported result |
|---|---|---|---|
| Mixed oxide-sulfide copper study (2026) | ~62% copper oxide, ~38% copper sulfide | Na₂S and NaHS tested; PAX with co-collector; pH 10.5; 53 µm; sulfidizer split between mill and cell | Copper oxide recovery 76%; total copper recovery 81% with staged addition |
| Copper ore CPS rougher (IMPC 2024) | Oxide-sulfide copper | NaHS to −300 to −400 mV; SIBX 10 g/t; fuel oil 20 g/t; natural pH; P80 150 µm | Improved oxide and sulfide Cu recovery; further NaHS depressed Cu sulfides |
| Turkish carbonate ore (2016 thesis) | Cerussite-rich lead oxide | Na₂S 2,000 g/t with 10 min conditioning; potassium amyl xanthate 400 g/t; frother 53 g/t; pH 9.5 | 63.25% Pb concentrate at 82.76% recovery |
| Fankou Lead-Zinc Mine (2023) | Highly oxidized lead-zinc, bulk flotation | Na₂S 500 g/t; copper sulfate 500 g/t; butyl xanthate 200 g/t; terpineol oil 21 g/t; plus depressants | Bulk concentrate 16.25% Pb, 27.69% Zn; Pb recovery 65.05%, Zn recovery 92.49% |
Note how widely sodium sulfide dosage varies, from about 500 g/t to 2,000 g/t, between ores. That
spread is exactly why a fixed “standard dose” is unreliable.
Troubleshooting Sulfidization Problems
Concentrate grade drops or recovery falls after raising sulfidizer
Over-sulfidation is the likely cause. Excess sulfide can depress sulfide minerals in mixed ores and
reduce the flotation response of over-sulfidized oxide minerals. Lower the dose or move to staged or
potential-controlled addition.
Oxide minerals will not float
Check for under-addition and for sulfide being consumed by slimes or precipitates. Confirm that pH
is within the proven window and that the collector is added after enough sulfidization time.
Results swing as ore changes
A fixed dose rarely survives changing mineralogy or feed rate. Moving to ORP or electrode-based
control is the usual fix.
Safe Handling and Storage
Never let sulfides contact acid
Contact with acids releases toxic, flammable hydrogen sulfide. The U.S. Chemical Safety Board has
reported that NaHS-related incidents caused 32 deaths and 176 injuries, mostly linked to hydrogen
sulfide released when NaHS met acidic material. Keep sulfide reagents physically separated from
acids and from acid-handling areas.
- Keep it dry: sodium sulfide and NaHS flakes are hygroscopic and release hydrogen sulfide in moist air.
- NaHS solutions: manufacturers recommend closed storage tanks under nitrogen.
- No dry grinding: safety data sheets warn that dry grinding of NaHS flakes can cause fires.
- Make solutions fresh: sodium sulfide solution slowly converts to sodium hydroxide and sodium thiosulfate in air, so old solution is not the same reagent you started with.
- Spills and waste: supplier SDS guidance describes oxidizing dissolved sulfide with hydrogen peroxide or hypochlorite to remove the hydrogen sulfide hazard. Follow your site procedures and the current SDS.
Test Before You Scale
Because sulfide demand can differ threefold between ores, run bench tests that compare slug, staged
and potential-controlled addition, and record the sulfide potential or ORP that gives your best
result. Then confirm the collector type and dose at that condition. Testing also gives you actual
consumption figures for comparing sulfidizer suppliers on cost per tonne of ore, not just price per
tonne of reagent.
Lead-Zinc Flotation Reagent Scheme: Typical Setups by Ore Type
Flotation Reagent Regime: Preparation, Dosing Order and Combinations
Xanthate vs Dithiophosphate: 5 Key Flotation Differences
Sodium Sulfide Flakes ·
Sodium Hydrosulfide
Need Sodium Sulfide or NaHS for Oxide Ore Trials?
Tell us your ore type and target pH. We can supply sodium sulfide and sodium hydrosulfide with COA, SDS and samples for your tests.
Frequently Asked Questions
What is sulfidization in flotation?
It is the treatment of oxide minerals with a sulfide source, usually sodium sulfide or NaHS, so their surfaces take on sulfide character and xanthate collectors can adsorb and float them.
Should I use sodium sulfide or sodium hydrosulfide?
Both work. The choice often depends on the pH your circuit needs, because sodium sulfide is more alkaline. Handling form, availability and cost also matter, and the best option should be confirmed in tests on your ore.
What happens if too much sodium sulfide or NaHS is added?
Excess sulfide can depress sulfide minerals and over-sulfidized oxide minerals, lowering recovery or concentrate grade. Under-addition leaves oxide surfaces insufficiently sulfidized.
What is controlled potential sulfidization (CPS)?
CPS uses an electrode to monitor and control the sulfide level in the pulp continuously, instead of adding reagent in manual slugs. It helps avoid over- and under-dosing when ore and feed rate vary.
How should sodium sulfide and NaHS be stored?
Dry, ventilated and away from acids, which release toxic hydrogen sulfide. NaHS solutions are commonly stored in closed tanks under nitrogen. Always follow the current SDS and local regulations.
Conclusion
Sulfidization works when the sulfide level in the pulp is kept in a narrow, ore-specific window.
Sodium sulfide and NaHS can both deliver it, with pH and handling form usually deciding between
them. Staged addition and controlled potential dosing give more consistent results than manual
slugs, and the published cases show that dosage varies enormously between ores, so testing on your
own material is essential. Because both reagents release hydrogen sulfide on contact with acid,
storage and handling discipline matters as much as metallurgy.
YuXing supplies sodium sulfide, sodium hydrosulfide, xanthates and other flotation reagents with
batch COA and sample support for trials.
Dosages, potentials and results quoted here come from published studies and supplier or regulator
documents on specific ores and materials, and are provided for general reference only. They are not
guaranteed performance or a recommended recipe. Confirm reagent selection and dosage through
laboratory testing, follow the current safety data sheet for every reagent, and comply with local
transport, storage and workplace exposure regulations.
