A surface reaction kinetic model to compare the reductive leaching of iron from goethite, magnetite, and limonitic nicke
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9/12/03
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m, n reaction order with respect to [H] and [HSO3] in equilibrium with SO2:
Communications A Surface Reaction Kinetic Model to Compare the Reductive Leaching of Iron from Goethite, Magnetite, and Limonitic Nickel Laterite Ores by Acidic Sulfur Dioxide G. SENANAYAKE The kinetic modeling of iron dissolution at low pH in the presence of SO2 or Na2SO3 is of particular interest due to the fact that iron oxides are major components in limonitic nickel laterite ores,[1] deep sea nodules,[2] weathered rare earth (RE) mud,[3] and manganiferrous ores,[4] and are common to many soils.[5] Table I lists the conditions under which the leaching experiments have been carried out by previous researchers and the pct Fe extracted from different types of iron oxide materials. The reactivity of hematite is 1/10th that of goethite, magnetite, and laterite, as shown by the pct Fe leached in 1 hour. Despite the different iron contents and the leaching conditions in the case of laterite and RE mud, the pct Fe extracted in 2 hours is similar. The relatively high reactivity of the mud at low temperature is most likely attributed to finely divided iron oxide in the particle. Nevertheless, the low rates of iron dissolution from hematite in RE mud, particularly at low pH and low SO2 concentrations, show the selective leaching of manganese.[3] Figure 1 compares the fraction of iron leached from variously sized particles of goethite and magnetite as a function of time; the slope represents the rate of leaching. Despite the lower temperature of 20 °C, the leaching of the ultra fine (0.1 m) goethite is much faster than the leaching of coarse (150 m) goethite at 110 °C, which is nearly the same as that of 75-m magnetite at 50 °C. Previous attempts to rationalize the kinetics of reductive leaching of iron oxides were based on the adsorption theory[5,6,7] and electrochemical studies[8,9] with limited success in some investigations.[10] Little or no attempt has been made to compare the available kinetic data for the reduction of goethite,[6] magnetite,[7] and laterite[1] by SO2 under a range of leaching conditions, which gave the results shown in Table I and Figure 1. The effect of some of these parameters has been successfully incorporated into the well-known shrinking sphere model, which is useful for kinetic modeling and reactor design:[11] 1 (1 X)1/3 k [H ]m [HSO3 ]n r1r1t ksst
[1]
where X fraction of Fe reacted at time t (s), k intrinsic rate constant of the surface reaction (cm s1), r initial particle radius of solid (cm), molar density of Fe in the solid (mol cm3), kss apparent rate constant (s1), and
SO2 (aq) H2O H HSO3
[2]
The shrinking sphere model takes into account the decrease in particle size and thus the surface area with time. In general, the value of kss is obtained from the slope of a plot of 1 (1 X)1/3 against t. This is used in the Arrhenius equation, kss A exp (E/RT), to examine the effect of temperature for leaching under a given set of conditions
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