Sodium leaching equilibrium

The removal of silica from a siliceous iron ore, such as the taconites found in Minnesota and Wisconsin, has been studied by Tiemann (T7, T9). Caustic concentrations from 25-500 gm/liter were used to digest the ore in a bomb at temperatures from 312 to 408°F. The leaching pressures in the bomb correspond closely to the equilibrium vapor pressures of the sodium hydroxide solutions used. A residual concentrate containing around 65% iron was obtained with —200 mesh material in 60 min of contact time. The high rate of dissolution of the silica was attributed to its occurrence in the form of microcrystalline (chalcedonic) varieties with high specific surface. The dissolution rate of pure quartz is directly proportional to the surface area and an average rate of 17 X 10 gm moles/cm sec was obtained for a 100 gm/liter NaOH solution at 312°F for the —400 mesh fraction. 

To leach the more acid-resistant minerals containing tetravalent uranium, steam is fed to the second tank to bring the temperature to 49 to 60°C, and sodium chlorate NaQOj is added to bring the oxidation-reduction potential e, measured relative to the calomel electrode, to from —0.47 to —0.51 V. At —0.51 V, the equilibrium ratio of ferric iron to ferrous iron in the solution is 0.52. Ferric iron catalyzes the oxidation of insoluble tetravalent uranium to the soluble hexavalent uranyl form ... 

The solubility of lead in brine solutions is much higher than in water alone, due to the formation of strong chloro complexes. Commonly, sodium chloride concentrations of the order of 250 g/L are used, together with iron concentrations for ferric iron leaching of the order of 25 g/L. This also applies to the leaching of silver, which can be readily solubilised in brine solutions, whereas AgQ is quite insoluble in water alone. Table 9.3 shows the equilibrium chloride concentrations at which particular complexes form. 

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