Q.State the role of silica in the metallurgy of copper.

Copper pyrites contains iron as well as copper. During smelting (and again in the converter), any FeO present reacts with added silica: $FeO + SiO_2 \rightarrow FeSiO_3$. This low-melting slag is immiscible with, and lighter than, the molten copper matte, so it can be skimmed off, effectively removing the iron impurity and leaving copper (as matte, later blister copper). …

Step 1 – Why iron needs removing

Copper pyrites ($CuFeS_2$) contains substantial iron along with copper. During roasting, part of the iron sulphide is converted to iron(II) oxide (FeO), which must be removed before pure copper can be obtained; otherwise it would remain mixed in with the copper.

Step 2 – Silica as an acidic flux

Silica ($SiO_2$) is deliberately added during smelting (and lines the converter used in the next stage) because it is an acidic oxide that readily combines with the basic iron(II) oxide impurity to form a low-melting, fusible slag:

Step 3 – Physical separation

This slag is molten at furnace temperature, is immiscible with the molten copper matte ($Cu_2S$ + FeS), and — being less dense — floats above it. It can therefore simply be skimmed/poured off, physically removing the iron impurity from the copper-bearing material.

Step 4 – Where this happens …

Recognise silica's role as an acidic FLUX that reacts specifically with the basic FeO impurity (not with copper) to form a removable slag …

• Saying silica 'removes impurities' vaguely without naming which impurity (iron oxide) it specifically reacts with, or without the slag-forming equation. …