Extraction of copper by smelting uses silica as an additive to remove.

Smelting is a form of extractive metallurgy to produce a metal from its ore. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gasses or slag and leaving just the metal behind. The reducing agent is commonly a source of carbon such as coke, charcoal, and coal.

$\mathrm{FeO}$is gangue and $\mathrm{S}\mathrm{i}{\mathrm{O}}_2$ is flux to form slag $\mathrm{F}\mathrm{e}\mathrm{S}\mathrm{i}{\mathrm{O}}_3$.

$\left[\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{S}\mathrm{i}{\mathrm{O}}_2\to \mathrm{F}\mathrm{e}\mathrm{S}\mathrm{i}{\mathrm{O}}_3\right]$