In-situ observation of TiN precipitates at stainless steel/CaO-Al 2 O 3 -MgO-SiO 2 interface
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Hatta Number (Ha)
1 ⫻ 10⫺3 5 ⫻ 10⫺2 0.10 0.20 0.25
0.09 0.6 0.9 1.27 1.42
oxygen reacts in the diffusion film. The process is essentially one of gas-liquid mass transfer followed by reaction in the bulk liquid. In the moderately fast reaction regime, the reaction is fast enough for a substantial amount of oxygen to react in the film rather than be transferred unreacted into the bulk liquid, resulting in a curved concentration profile, Ci Cb , as shown in Figure 2. The net result is gas-liquid mass-transfer enhancement. Hatta numbers for various Fe2+ concentrations were computed using Eq. [7]. The influence of an electrolyte on the diffusivity of oxygen in aqueous solutions is reported to be dependent on the change in viscosity of the solution.[11] Since the change in viscosity of the FeCl2 solution within the range of concentrations employed in the present study is negligible, D0 was taken as the diffusivity of oxygen in water.[12] The value of k0 was taken from a previous study.[7] Hatta numbers computed for various Fe2+ concentrations are listed in Table III. It shows that Fe2+ concentrations of 0.05 mol/L and above correspond to a moderately fast reaction regime. The Fe2+ concentration of 1 ⫻ 10⫺3 mol/L corresponds to a typical case where the reaction was started with NH4Cl as the only electrolyte. A Hatta number corresponding to this concentration is in the slow reaction regime. According to the data reported by Charpentier,[13] the maximum gas-liquid mass transfer enhancement possible in the moderately fast reaction regime is around 1.5. Figure 1 makes it evident that the observed enhancements in the rate of iron removal for various concentrations of ferrous chloride are in the range of 1 to 1.5. This leads to the conclusion that the ferrous oxidation reaction is in the moderately fast reaction regime for the concentrations of ferrous chloride investigated in the present study. Variation of particle size and weight percentage of reduced ilmenite in the slurry will lead to the variation of active surface area for solid-liquid mass transfer. However, since the process is controlled by gas-liquid mass transfer, enhancement is not affected by particle size and weight percentage of reduced ilmenite, as shown in Tables I and II. When the Fe2+ oxidation is in the fast reaction regime, the liquid-phase concentration of oxygen will be negligible and hence oxygen will not be the dominant cathodic reactant at the surface of reduced ilmenite particles. In this case, Fe3+ ions could provide for the cathodic reaction as follows:[14] Fe3⫹ ⫹ e⫺ → Fe2⫹
[8]
Irrespective of the cathodic reactants, the rate of iron removal is proportional to the total amount of oxygen transferred to the liquid phase according to Eq. [1]. Hence, enhancement in the oxygen mass-transfer rate in the presence of ferrous chloride gives rise to an additional pathway for cathodic reaction through the presence of ferric ions. Thus, it can be concluded that the intensification of the oxygen leaching process for iron removal can be achieved under envir
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