Aspects of the reaction of CO and CO₂ with iron oxide-containing slags
Creators: name: Chaskar, Vinay D.
Publisher: name: University of British Columbia
Year: 2008

description: A kinetic study of reduction and oxidation slag reactions of importance to the non-ferrous smelting industry was undertaken in the laboratory by employing a thermogravimetry technique. The reduction study included ferrous-to-iron and ferric-to-ferrous reactions whereas a limited amount of data on ferrous-to-ferric and ferrous-to-magnetite reactions was also obtained for comparison. The heterogeneous gas-slag reactions were investigated using unstirred slag melts and the synthetic slags covered a wide range of melt compositions from Fex0-Al203 toFex0-CaO-SiO2-Al203. The results revealed that the rate of the iron formation reaction was highest during the initial10 minutes period and then dropped progressively with the passage of time. The melt silica content influenced the rate values to a degree and it was observed that in silica-free melts the above trend was reversed at high reaction driving force values. The latter trend is attributed to the melt movement induced by the sinking iron metal. A detailed mathematical analysis showed that during the initial period ferrous reduction rates were limited by a mixed-control regime involving gas phase mass transport and interfacial reaction. By using intrinsic rate constant, kc, as a fitting parameter the predicted rates were matched with the measured data and the procedure yielded an average value of 11 x 10'5 g/cm2.s.atm at 1400 °C which is in agreement with previous work. The results of the ferric-to-ferrous reaction showed approximately constant rate during the initial period and subsequently the rate decreased with time. Therefore, the results were analyzed using two separate mathematical models. It is proposed that this reaction operates under a gas and interfacial control regime during the initial period and subsequent to this the rate is controlled by combined gas, liquid and interfacial reaction. The intrinsic rate constant value at 1400 °C is approximately 200 times greater compared to the Fe' Fe reaction. The apparent activation energy value of about 44 kcal/mol is derived for the Fe3+ Fe2+ reaction. By choosing the liquid phase mass-transfer coefficient, kL, as a fitting parameter its values were obtained for various melts and this information was used in conjunction with the boundary layer value of 500 obtained by MOssbauer spectroscopy to generate oxygen anion diffusivity data. The value of apparent activation energy for diffusion, ED, for lime-containing melts was found to be 53 kcal/mole. The results obtained for both reduction reactions revealed the significance of surface active species in the melts and accommodation of this effect in the formulation and development of the mathematical models led to accurate prediction of the rate and weight loss data. In the various melts studied in this investigation silica and ferric oxide were surface active species and their individual proportion in the melts altered the available reaction area. MOssbauer spectroscopic analyses were performed on a limited number of slag samples. A special sample holder was designed to probe an entire cross-section of the quenched slag to identify the variation in iron cations with depth. It was demonstrated for the first time that MOssabuer spectroscopy can be used to study iron distribution as a function of depth. The limited data obtained on ferrous-to-ferric and ferrous-to-magnetite reactions revealed that the rates decreased with time in a manner similar to the reduction reactions. The weight gain-time curves for magnetite formation reaction in both simple and complex melts indicated that solid magnetite covered the melt surface and caused reduction in the rate values. The data on the ferrous-to-ferric reaction at 1300 °C indicated that the mechanisms involved in ferric-ferrous reduction and oxidation reactions are similar.