Complex sentence: The "antimony" here was in fact stibnite, antimony-sulfide Sb 2 S 3; the surplus of iron was burned away in this process, reducing the antimony while removing the unwanted sulfur.
Split into simpler sentences: The "antimony" here was in fact stibnite, antimony-sulfide (Sb 2 S 3). The surplus of iron was burned away in this process, reducing the antimony while removing the unwanted sulfur.