| Petrochemical enterprises processing sulfur-containing crude oil or handling other sulfur-containing chemicals can lead to the production of spontaneously combustible active ferrous sulfide（FeS）.In the process of equipment opening,inspection and maintenance of active FeS is prone to spontaneous combustion in the presence of air,resulting in fire and explosion from time to time.The application of FeS vapor phase passivation requires experimental investigation of safe and effective vapor phase passivation process conditions and evaluation of the intrinsic safety of the passivated samples,as well as molecular simulations to reveal the effect of vapor phase passivation.The microscopic surface reaction quantum chemical mechanism of the gas phase passivation method is revealed by molecular simulations.Therefore,in this paper,we used an in-house designed FeS sample synthesis device,advanced experimental instruments such as microcalorimeter（C80）,X-ray diffraction spectroscopy（XRD）,scanning electron microscope（SEM）,simultaneous thermal analyzer（STA）,and quantum chemical calculation software Materials Studio-CASTEP to systematically study the FeS vapor phase passivation using a combination of experimental and simulation,macroscopic and microscopic methods.The experimental conditions,passivation effect evaluation and passivation microscopic mechanism of FeS vapor phase passivation were systematically investigated using a combination of experimental and simulation,macroscopic and microscopic methods.The main findings are as follows.1.The experimental results of gas-phase passivation show that the passivation gas concentration of 7.5%is the critical concentration for the uncontrolled oxidation of FeS,with 35 mL/min as the dividing line,and that the FeS exothermic heat and the maximum exothermic rate show a trend of increasing and then decreasing with the increase of gas flow.The gas phase passivation process can adopt the"low concentration and high flow rate"scheme,and the passivation gas concentration is controlled below 5%.2.The XRD results showed that some Fe2O3and S were produced in the FeS passivated samples,and the electron microscope pictures of the FeS passivated samples showed the phenomenon of"agglomeration",which greatly increased the specific surface area of the particles and produced a rough passivation film on the surface,reducing the probability of contact between the internal active components and oxygen molecules.The results of the thermal analysis showed that the TG-DSC curve of the passivated FeS sample was significantly shifted from 25℃to 175℃,and the peak structure was significantly changed and the exothermic peak I disappeared.The passivation treatment terminated the key spontaneous combustion reaction initiation step of the active FeS and increased the difficulty of spontaneous combustion.3.The apparent activation energies of the active and passivated samples were 27.18 kJ/mol and 90.03 kJ/mol,respectively,calculated by the thermal analysis kinetic Kissinger method,and the activation energy values of the active and passivated samples were 17-58 kJ/mol and40-140 kJ/mol,respectively,calculated by the AKTS-Friedman differential method.140 kJ/mol.It was confirmed that the gas-phase passivation method can significantly increase the activation energy of FeS oxidation reaction,and the gas-phase passivation has an inhibitory effect on FeS activity,which achieves the essential safety control requirements.4.Molecular simulation studies show that FeS is a tetragonal crystalline phase with space group P4/nmm and is layered,and the surface energy of FeS（001）terminated by S atoms is 0.095 J/m2,which is the natural state exposed surface,and FeS（001）is used as the reaction surface in accordance with the reality.Compared to the FeS（001）ideal active surface,the FeS（001）oxygen-doped passivated surface atoms based on the hypothesis have a larger charge density distribution.The Fe-O bond formed by the replacement has reduced Mulliken populaiton value compared to the original Fe-S bond,increased charge number of Fe atoms and enhanced ionicity of Fe-O bond.5.After the addition of adsorbent oxygen molecules,quantum chemical calculations show that the adsorption energy of oxygen molecules on the ideal active surface of FeS（001）is-135.75 kJ/mol and on the oxygen-doped passivated surface of FeS（001）is-327.34 kJ/mol.The preferential site of action of oxygen molecules on both surfaces is Fe atoms rather than S atoms.The charge Mulliken populaiton analysis shows that the electron transfer of the oxygen molecule adsorption process is mainly manifested by the formation of covalent bonds between the 2p orbitals of O atoms and the 3d orbitals of Fe atoms.6.Gas-phase passivation treatment makes the oxygen molecules and surface atoms fully react and replace the singlet sulfur,so that the surface forms a protective oxide film,resulting in a weakened oxygenophilicity of the FeS surface,the activation energy of the occurrence of oxidation reaction is increased,and then achieve the purpose of suppression of spontaneous combustion activity.The low proportion of oxygen-doped surface has stronger reactivity to oxygen compared with the ideal active surface Fe atoms,which cannot truly reflect the oxygenophobic nature of the FeS saturated passivated surface during the experiment,and requires the construction of surface models with different substitution ratios for further calculation and analysis. |
