METALLURGICAL AND MATERIALS TRANSACTIONS B-PROCESS METALLURGY AND MATERIALS PROCESSING SCIENCE, cilt.48, ss.2067-2076, 2017 (SCI İndekslerine Giren Dergi)
In this study, isothermal reaction behavior of loose NiO powder in a flowing undiluted CH4 atmosphere at the temperature range 1000 K to 1300 K (727 degrees C to 1027 degrees C) is investigated. Thermodynamic analyses at this temperature range revealed that single phase Ni forms at the input n(CH4)(o)/(n(CH4)(o) + n(NiO)(o)) mole fractions (X-CH4) between similar to 0.2 and 0.5. It was also predicted that free C co-exists with Ni at values higher than similar to 0.5. The experiments were carried out as a function of temperature, time, and CH4 flow rate. Mass measurement, XRD and SEM-EDX were used to characterize the products at various stages of the reaction. At 1200 K and 1300 K (927 degrees C and 1027 degrees C), the reaction of NiO with undiluted CH4 essentially consisted of two successive distinct stages: NiO reduction and pyrolytic C deposition on pre-reduced Ni particles. At 1200 K (927 degrees C), 1100 K (827 degrees C), and 1000 K (727 degrees C), complete oxide reduction was observed within similar to 7.5, similar to 17.5, and similar to 45 minutes, respectively. It was suggested that NiO was essentially reduced to Ni by a CH4 decomposition product, H-2. Possible reactions leading to NiO reduction were suggested. An attempt was made to describe the NiO reduction kinetics using nucleation-growth and geometrical contraction models. It was observed that the extent of NiO reduction and free C deposition increased with the square root of CH4 flow rate as predicted by a mass transport theory. A mixed controlling mechanism, partly chemical kinetics and partly external gaseous mass transfer, was responsible for the overall reaction rate. The present study demonstrated that the extent of the reduction can be determined quantitatively using the XRD patterns and also using a formula theoretically derived from the basic XRD data.