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Sodium sulfide is one of the most widely used alkali metal sulfides in the flotation of copper, lead, and zinc minerals in their oxidized form. The sulfidization process, developed in the U.S.A. in 1915-20 for oxide copper minerals flotation, is especially suitable for oxidized lead ores in which the gangue is usually basic. The early users of sulfidization had realized that the reaction of sodium sulfide is dynamic and delicate, and the rate of reaction varied with the temperature, soluble salts, pH, etc.
Chemistry of Sodium Sulfide
Sodium sulfide is a salt of hydrogen sulfide, a weak dibasic acid. Of the 30 plus molecular and ionic sulfur species that exist, only six are thermodynamically stable in aqueous solution at room temperature and one atmospheric pressure: HSO4-, SO4²-, H2S, HS-, and S²-. Other sulfur forms such as polysulfides, polythionates, and thiosulfate also occur in the natural environment but are considered thermodynamically unstable.
In aqueous systems, pH is the controlling variable in the oxidation of hydrogen sulfide. Below pH 6, “molecular hydrogen sulfide” is the predominant reduced sulfur species. At pH 7, the reduced sulfur species in solution are equally divided between “dissolved hydrogen sulfide” and the bisulfide species [HS-] which is the predominant species in the pH range of 8 to 11.
Though the kinetics and mechanisms of the auto-oxidation reaction of reduced sulfur compounds in aqueous systems have been studied extensively, the complexity of the reactions has precluded uniform interpretation and generalization of those results.
Since above pH 8 no polysulfide formation was observed another mechanism is needed to account for this. The authors further examined the kinetics of oxidation of aqueous sulfide by oxygen, demonstrating that oxygenation of sulfide is extremely slow in the absence of a catalyst. This is believed to be a result of the high activation energy needed to break the oxygen-oxygen bond (118.9 Kcal/mol) as well as the low solubility and diffusion of oxygen in water. Therefore, air oxidation alone is not viewed favorably as a process for sulfide consumption.
In aqueous systems the sulfide oxidation is extremely slow; whereas, in the flotation pulps where oxygen is normally present, both S²- and the HS- are oxidized rapidly. The rate of oxidation of the sulfide markedly increases as a result of catalytic action of the mineral surface. Other Ions hydrogen sulfide, sulfite, and sulfate—were present in minor proportion in the presence of all four minerals. The reasons for difference in the sphalerite behavior are not clear.
In the study of the action of sodium sulfide on oxide minerals the nature of heterogeneous reactions that occur between sulfide ions in solution, and the surface of oxide minerals and those of gangue minerals are significant. According to Castro et al the heterogeneous reactions are quite complex and various parallel processes may be taking place in the system. Three steps were identified for copper oxides: 1) adsorption of sulfide ions with the formation of copper sulfide, 2) sulfide oxidation, and 3) desorption of oxidized compounds by ion-exchange.
The adsorption of sulfide ions appears to be a fast reaction, normally completed in less than a few minutes. Subsequent to physical adsorption the sulfide ions may react with copper oxide, to form a copper sulfide layer and thereby activate its surface. This conclusion was derived from their study because 80 percent of the added sulfide was not found in an oxidized form in the solution. Some evidence exists that this process may not be a mere surface reaction but could involve inner layers of the solid.
Factors that affected early attempts of sulfidization of oxide ores were:
- Ore mineralogy diversity—presence of galena, sphalerite, and cerussite together.
- Oxide mineral mineralogy diversity—presence of malachite, azurite, atacamite, and chrysocolla together.
- Sodium sulfide consuming minerals—pyrite, marcasite, talc, cerussite, clays, slimes, alkaline earth salts, gypsum, and chlorides.
- Excessive pH at high sodium sulfide dosage.
- Location and number of stages of sodium sulfide addition.
- Water composition—calcium, magnesium and hardness.
The results indicate that when xanthate addition was low so that the xanthate residual was zero throughout the test a poor recovery was obtained, even though the sulfidizing conditions were excellent. When the xanthate addition was raised so that a residual was present throughout flotation, the copper recovery was good.
In the flotation of malachite even at low concentrations of sodium sulfide, the depression of malachite was observed without an effective oxidation. The oxidizing reaction takes place on the surface of the mineral particles and can be accounted for satisfactorily by assuming a catalytic oxidation mechanism.
Sodium Sulfide in Sulfide Minerals Flotation
Other than mineralogical complexity of ores and activation characteristics of sulfide minerals, oxidation of sulfide minerals is one of the most serious metallurgical problems. The oxidation of complex sulfide ores is especially enhanced with the association of pyrite which has been explained by electrochemical and pure chemical mechanisms. Detailed discussion of these mechanisms is beyond the scope of this paper.
In a study of collectorless flotation, chalcopyrite was floated from ore samples wet ground using iron balls followed by sodium sulfide treatment, and sphalerite was floated from a copper-zinc ore by activating it with cupric tons in the presence of sulfide ions. These results indicated that sodium sulfide has acted as a cleaning agent for the mineral surfaces involved.
This review on the role of sodium sulfide in the flotation of copper, lead, and zinc ores resulted in the following conclusions:
- In mineral pulps though oxygen is one of the oxidants of S²- and HS- ions, the rate of oxidation is markedly increased as a result of catalytic action of mineral and gangue surfaces and ionic species of heavy metals.
- The presence of slimes and precipitates increases the dosage requirement of sodium sulfide.
- Over sulfidization of the pulp appears to be due to high HS- and S²- ions content, which are responsible for depression of the minerals.