Froth floatation process - why CuSO4 is activating floating character of ZnS?

Question

I read the following :

"In a mixture of $\ce{PbS}$, $\ce{ZnS}$ and $\ce{FeS2}$, each component is separated from each other in froth floatation process by using the reagents potassium ethyl xanthate, $\ce{KCN}$, $\ce{NaOH}$, copper sulphate and acid. Here potassium ethyl xanthate acts as a collector. $\ce{KCN}$ and $\ce{NaOH}$ depress the floatation property of $\ce{ZnS}$ and $\ce{FeS2}$ particles. Thus, only $\ce{PbS}$ particles go into the froth. Now copper sulphate is added to the mixture which activates floating character of $\ce{ZnS}$ and this time only $\ce{ZnS}$ comes along with froth. The remaining slurry is acidified and $\ce{FeS2}$ floats along with froth."

Can someone help me in exactly understanding the role of copper sulphate clearly along with its specific property associated?

Answer 1

I'm going to consult the literature later, but for me it seems to be as follows.

In the compounds discussed, there are following cations:

• $\ce{Fe^2+}$. It is a transition metal with a moderate affinity to oxygen, a slightly larger affinity to sulfur and a high affinity to cyanide.
• $\ce{Zn^2+}$. It is a small cation with a high affinity to oxygen, a slightly larger affinity to sulfur and a moderate affinity to cyanide.
• $\ce{Pb^2+}$. It is a large cation with a moderate affinity to oxygen and a slightly larger affinity to sulfur.

Xanthate bind to the surface via the sulfur atoms. So, in a basic cyanide solution the binding sites on the surfaces of pyrite and sphalerite are both completely filled with cyanide and hydroxide respectively (please, note that sulfur in xanthate is a lot less nucleophilic than in sulfide form). Copper has a high affinity to sulfur, a lot larger than zinc or iron. So it binds to xanthate and to the sulfur in either pyrite or sphalerite. However, the surface of pyrite is still occupied with cyanide, which binds really strongly there. And in an acidic environment, cyanide is transformed into $\ce{HCN}$ and thus the pyrite surface is open to binding with xanthate.

Answer 2

I navigated the net and came across the following details.....

The most common collectors for sulfide minerals are the sulfhydryl collectors, such as the various xanthates and dithiophosphates. Xanthates are most commonly used. Xanthates are highly selective collectors for sulfide minerals, as they chemically react with the sulfide surfaces and do not have any affinity for the common non-sulfide gangue minerals.

Sulfhydryl collectors such as xanthate ions compete with $\ce{OH-}$ ions to adsorb on mineral surfaces, and so adsorption is a function of pH. This makes it possible for sulfhydryl collectors to be used to progressively separate specific minerals. The pH where the xanthate ion wins the competition with $\ce{OH-}$ ions depends both on the concentration of xanthate in solution, and on the specific sulfide mineral present.For the same purpose, alkali like $\ce{NaOH}$ may be used and not $\ce{Ca(OH)2}$ because the sodium cation generally does not have any significant effect on the particle surface chemistries unlike the calcium ion.

Activators are specific compounds that make it possible for collectors to adsorb onto surfaces that they could not normally attach to. A classic example of an activator is copper sulfate as an activator for sphalerite ($\ce{ZnS}$) flotation with xanthate collectors. When untreated, xanthate cannot attach to the sphalerite surface because it forms a zinc-xanthate compound that quickly dissolves:

$$\ce{ZnS(s) + Xanthate -> S(s) + ZnXanthate (aq)}$$

The surface of the sphalerite can be activated by reacting it with a metal ion that does not form a soluble xanthate, such as soluble copper from dissolved copper sulfate:

$$\ce{ZnS(s) + CuSO4(aq) -> CuS(s) + ZnSO4(aq)}$$

This forms a thin film of copper sulfide on the sphalerite surface, which allows for stable attachment of the xanthate, rendering the sphalerite particle hydrophobic and floatable. Other metals such as silver and lead can also be used to activate zinc, but copper is cheaper than silver and less toxic than lead.

I read the following in a website which mentions the research as follows :

The role of copper sulphate in the flotation of pyrite has been studied. A survey of a number of pyrite flotation plants using copper sulphate showed that in correct dosages it was able to increase both grades and recoveries. This was confirmed in batch tests. Copper sulphate had no significant effect on the rate of adsorption of potassium n-butyl xanthate (PNBX) onto pyrite. Copper adsorbed irreversibly onto the pyrite when the latter was slurried in a copper sulphate solution. Copper sulphate had a significant effect on the maximum froth height obtained in static three-phase froth tests. Proposals to explain these observation are made.

If the above-mentioned thing is taken into consideration, the role of $\ce{CuSO4}$ is to provide $\ce{Cu}^{2+}$ which will act as an activator in the manner explained above.

Also, the role of cyanide ions may be:

Cyanide ($\ce{CN-}$) is a particularly useful depressant in sufide mineral flotation. Its activity is believed to be due to its ability to complex with, and in some cases dissolve, a number of metal ions, preventing them from attaching to the xanthate molecules. In particular, it is a strong depressant for pyrite ($\ce{FeS2}$), and can be used to “deactivate” sphalerite that has been activated by copper ions in solution. It is interesting to note that flotation of galena ($\ce{PbS}$) is unaffected by the presence of cyanide.

References

1. https://www.researchgate.net/file.PostFileLoader.html?id=567e6a16614325ed178b4589&assetKey=AS%3A310856070434816%401451125270248