Study On Influence Factors For The Formation Of Iron Sulfides And Its Pyrophorisity

In addition to causing the sulfur corrosion and leakage of refining and transportation equipment, fire and explosion accidents caused by spontaneous combustion of the corrosion products of sour crude oil have occurred from time to time, and it is a serious threat to the petroleum refining and transportation safety. Taking iron oxide red, iron oxide black and iron hydroxide as model compounds, the formation of iron sulfides at high temperatures and its propensity of spontaneous combustion, the hazard of dry hydrogen sulfide gas, oxidation process of the iron sulfides, and the role of water in the formation of iron sulfides and in process of spontaneous combustion were systematically studied in the paper.The reaction products formed from reactions between element sulfur, hydrogen sulfide gas and model compounds at high temperatures were analyzed by XRD and the compositions of iron sulfides were pyrite and ferrous sulfide respectively. The corrosion product caused by hydrogen sulfide gas has high propensity of spontaneous combustion. The propensity of spontaneous combustion of corrosion product formed from element sulfur is small and it will increase fast with the temperature increment.The experimental results show that hydrogen sulfide gas and hydrosulfuric acid formed from hydrogen sulfide gas dissolved in water react with iron forming ferrous sulfide at different sulfuration temperatures, and react with iron oxides forming ferrous sulfide at ambient temperature and pyrite at higher temperature (more than or equal to 50℃). The sulfuration products have higher pyrophorisity. Dry hydrogen sulfide gas has no corrosion at low temperature and can not react with dry iron powder, but can react with iron oxides and its products have pyrophorisity.The water-content in model compounds has influence to the sulfuration reaction rate and the spontaneous combustion of corrosion products. During the same sulfuration time, the content of iron sulfides in sulfuration products changes with the different water-content in model compounds and the pyrophorisity of its sulfuration products has significant differences. The sulfuration products formed from iron oxide red or iron oxide black at 5%～15% water content have the highest pyrophorisity. The pyrophorisity of iron sulfides formed from iron oxide red or iron oxide black is much greater than the pyrophorisity of iron sulfides formed from iron hydroxide. The pyrophorisity of sulfuration products depends on micro-surface structure of iron sulfides rather than the content of iron sulfides in sulfuration products.The adsorption strength of rust model compounds to hydrogen sulfide gas is affected by iron oxidation state and iron bond collection. The experimental results show that the affinity of bivalent iron to hydrogen sulfide gas is greater than the affinity of trivalentiron, and the affinity of trivalent iron in iron hydroxide to hydrogen sulfide gas is greater than the affinity of trivalent iron in iron oxide red The calculating results also show that the affinity of ferrous iron to hydrogen sulfide is relatively strong. The sequence of chemical adsorption specific activity of model compounds to hydrogen sulfide is: (FeO)> Fe(OH)₃> Fe₃O₄> Fe₂O₃ The experimental results also show that water or temperature-rising can boost the adsorption-reaction of hydrogen sulfide gas on the surface of model compounds.The oxidation process of iron sulfides is divided into primary oxidation process and advanced oxidation process according to the occurrence of sulfur dioxide gas. Sample temperature is less than about 70 degree Centigrade and it does not generate sulfur dioxide gas in primary oxidation process. Sample temperature is greater than about 70 degree Centigrade and it generates sulfur dioxide gas in advanced oxidation process. The occurrence of sulfur dioxide gas is the symbol that oxidation of the iron sulfides enters advanced process.Water on the oxidation of iron sulfides has significant influence. On one hand, it is directly involved in oxidation reactions and changes the mechanism. The apparent activation energy, which is calculated according to the DTA curves with Kissinger method, of the main dry and wet iron sulfide oxidation reaction is 147.88kJ/mol and 41.87kJ/mol respectively. This is intrinsic reason that their propensity of spontaneous combustion has the significant difference. On the other hand, dioxide sulfur gas produced in oxidation process dissolves in water to form hydro sulfuric acid, which reacts, with ferrous sulfide in sulfuration products to give ferrous sulfate and hydrogen sulfide gas. The hydrogen sulfide gas reacts both with sulfur dioxide gas to give water and element sulfur and with iron oxides to give iron sulfides. Water makes the oxidation process of iron sulfides become much complicated and this is one of the reasons why water-content iron sulfides have greater pyrophorisity.