Pyrophoric Nature of Iron Sulfides

Hydrogen sulfide, often present in crude oil tankers, can react with rust to form various sulfides including mackinawite (FeS), greigite (Fe 3 S 4 ), and pyrite (FeS 2 ). The tendency for these compounds to react with oxygen in air to form potentially explosive mixtures depends upon their morphology and the environmental conditions. The experimentally determined heat of oxidation of finely divided mackinawite was −7.45 kJ g -1 . For samples with a larger particle size and smaller surface area the values measured were lower due to incomplete oxidation of the sulfide. All the sulfides produced, whether from magnetite or acicular, prismatic or spherical geothite, were approximately spherical in form. The heat of oxidation of greigite was found to be approximately −2100 kJ mol -1, and the heat of formation of greigite is approximately −320 kJ mol -1 .