# Sulfur from pyrite roasting process

I'm solving a quantitative problem involving the process of making oleum from pyrite.

The problem specifies that, at the end of the reaction, we find sulfur which does not come from the impurities present in pyrite.

Where is that sulfur coming from? Is it from the thermal decomposition of $\ce{FeS2}$?

$$\ce{FeS2 -> Fe + 2S}$$

• What reaction? There is more than one, if I remember correctly. – Ivan Neretin Nov 29 '15 at 14:46
• @Ivan Well, I didn't include the reactions used for obtaining sulfur trioxide (which is used for obtaining oleum) because the products ($SO_2,\ SO_3, \ H_{2}SO_4$) are unlikely to decompose into sulfur. – L3ul Nov 29 '15 at 15:31