Which of the following species is desirable substance in extraction of copper but not in extraction of iron?

SiO₂

The correct answer is option 3. SiO₂.

Out of the listed species, the one desirable in copper extraction but not necessarily desirable in iron extraction is $SiO_2$ (Silicon Dioxide)

Copper Extraction: In copper extraction processes like froth flotation, silicon dioxide $(SiO_2)$ can act as a gangue mineral (impurity) that needs to be separated from the copper sulfide ore particles. However, small amounts of $SiO_2$ can be tolerable in some copper concentrates depending on the specific process and desired product purity.

Iron Extraction: In iron extraction using blast furnaces, silicon dioxide $(SiO_2)$ plays a crucial role as a flux. Flux reacts with other gangue minerals in the iron ore to form a low melting point slag (typically calcium silicate, $CaSiO_3$). This slag floats on top of the molten iron, allowing for easy separation.

Here is a breakdown of the other options:

1. $CaSiO_3$ (Calcium Silicate): This is the main slag component formed in iron extraction using limestone $(CaCO_3)$ as a flux. It's not particularly desirable in copper extraction, as it would be considered a gangue mineral.

2. $FeSiO_3$ (Iron Silicate): This can be a component of the gangue in both copper and iron ores. While not necessarily desirable, its presence might be tolerated depending on the specific process and the efficiency of removing it.

4. Coke: Coke is a crucial reducing agent in both copper and iron extraction. It reacts with oxygen in the furnace to produce carbon monoxide $(CO)$, which then reduces the metal oxides to their elemental forms ($Cu$ or $Fe$).

While $SiO_2$ can be present in both copper and iron ores, its role differs:

In copper extraction, it's often an unwanted impurity (gangue) to be minimized.

In iron extraction, it's a necessary component for slag formation, facilitating iron separation.