Animated Solution for Chemistry - Metallurgy: Which of the following ore is concentrated using group 1 cyanide salt ?

Whenever we encounter a question about the concentration of ores, our first instinct should be to identify the chemical nature of the ores given in the options. Let's break them down one by one.

Calamine is a well-known ore of zinc, and its chemical formula is $\mathrm{ZnC}{\mathrm{O}}_3$. This makes it a carbonate ore.

Next, we have Malachite, which is an ore of copper. Its formula is $\mathrm{CuC}{\mathrm{O}}_3\cdot \mathrm{Cu}(\mathrm{OH}{)}_2$, making it a basic carbonate ore.

Siderite is an ore of iron, represented by the formula $\mathrm{FeC}{\mathrm{O}}_3$. Again, this is a carbonate ore.

Finally, we look at Sphalerite. This is the primary ore of zinc, and its chemical formula is $\mathrm{ZnS}$. This makes it a sulphide ore.

Froth Flotation

The question specifically mentions the use of a "group 1 cyanide salt" for concentration. The most common group 1 cyanide salts used in metallurgy are sodium cyanide ($\mathrm{NaCN}$) and potassium cyanide ($\mathrm{KCN}$).

Where do we use these salts? They are famously employed as depressants in the froth flotation process.

Froth flotation is a highly effective method used almost exclusively for the concentration of sulphide ores. The principle relies on the difference in wetting properties of the ore and the gangue particles. Sulphide ore particles are preferentially wetted by oil and rise to the surface with the froth, while gangue particles are wetted by water and settle at the bottom.

Now, let's connect the dots. We know that froth flotation is used for sulphide ores, and cyanide salts are used as depressants in this very process.

Looking back at our list of ores, we found that Calamine, Malachite, and Siderite are all carbonate ores. They are typically concentrated by other methods, such as calcination or gravity separation.

Sphalerite ($\mathrm{ZnS}$), however, is the only sulphide ore among the choices. Therefore, it is the ore that is concentrated using the froth flotation method, where a group 1 cyanide salt can be used as a depressant.

This leads us directly to our final answer: Sphalerite.

You might be wondering, how exactly does sodium cyanide act as a depressant? This is a beautiful piece of chemistry!

In a mixture containing both Sphalerite ($\mathrm{ZnS}$) and Galena ($\mathrm{PbS}$), we want to separate them. If we just use froth flotation, both might come up with the froth.

To prevent this, we add $\mathrm{NaCN}$. The cyanide ions selectively react with the zinc sulphide on the surface of the Sphalerite particles to form a water-soluble complex:

This complex, sodium tetracyanozincate, washes away the hydrophobic collector coating from the Sphalerite. As a result, the Sphalerite becomes hydrophilic and stays in the water, while the Galena ($\mathrm{PbS}$) remains hydrophobic and rises with the froth.

Understanding this underlying mechanism not only helps you solve this specific question but also prepares you for more complex problems involving selective precipitation and complexation!