The kinetics of reduction of iron oxides at moderate temperatures

Abstract

The rates of reduction of wustite with hydrogen were measured in the temperature range 511 to 690 K. The wustite specimens were prepared by oxidizing thin iron foils, 50μm thick. These were reduced in a Cahn electrobalance under isothermal conditions and the weight loss sustained was continuously monitored. The effect of the phase transformation of wustite on the reduction kinetics was investigated. By using thin oxide specimens and a rapid flow of reducing gas, the respective influences of pore diffusion and “film” mass transfer were rendered insignificant. The reduction of the oxides, in the temperature range investigated, appears to be chemically controlled. The fractional reduction (α)time(t) plots are sigmoid shaped and exhibit three distinct features: incubation, acceleratory and decaying periods. The rate constantk s was evaluated from the middle region of the α-t plots. Its temperature dependence may be expressed by: % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabm4Aayaara% WaaSbaaSqaaiaadohaaeqaaOGaeyypa0JaaGimaiaac6cacaaIYaGa% aGOnaiaaiodacaGGOaGaeyySaeRaaGimaiaac6cacaaIWaGaaGinai% aaiEdacaGGPaGaciyzaiaacIhacaGGWbWaamWaaeaadaWcaaqaaiab% gkHiTiaaiEdacaaIXaGaaiilaiaaiwdacaaI1aGaaGimaiabgglaXk% aaiMdacaaIWaGaaGimaaqaaiaadkfacaWGubaaaaGaay5waiaaw2fa% aGqaaiaa-DgacaWFGaGaeS4JPFMaiGjG-fgacGaMa+hDaiacyc4FVb% GaiGjG-1gacaWFGaGaa83taiaa-9cacaWFJbGaa8xBamaaCaaaleqa% baGaa8Nmaaaakiabl+y6NjacWc4FZbGamqnGl+y6Njaa-bcacaWFGa% Gaa8xyaiaa-rhacaWFTbaaaa!7140! $$\bar k_s = 0.263( \pm 0.047)\exp \left[ {\frac{{ - 71,550 \pm 900}}{{RT}}} \right]g atom O/cm^2 s atm$$ g· atom o/cm2 · s · atm. The experimental data was interpreted by means of a nucleation and growth model.