The rates of oxidation of galena and sphalerite in acidic ferric chloride solutions

Abstract

When sulfide minerals are exposed to the oxidizing conditions of the earth's surface, their metal ions are released into solution and the S²⁻ is oxidized to either elemental sulfur or sulfate. The experiments described here used a mixed flow reactor system to determine the oxidation rates of galena and sphalerite under conditions similar to that expected in a weathering ore deposit . The specific surface area of the run solids was determined by N₂ BET procedure and the surface textures observed by SEM. The amount of Fe³⁺ converted to Fe²⁺ by the oxidation reaction was determined using an Eh electrode. Solid reaction products include, orthorhombic S(s) and anglesite (PbSO₄) from the galena oxidation and minor orthorhombic S(s) from the sphalerite oxidation. The rate equations describing the 25°C data are:

dn_Fe³⁺/dt = -5.5 ± 1.1 × 10⁻³ (A)(a<sub>Fe³⁺</sub)^{1.06 ± 0.16} for galena, and dn_Fe³⁺/dt = -1.8 ± 0.3 × 10⁻⁶ (A)(a_Fe³⁺)^{0.47 ± 0.08} for sphalerite.

Where dn_Fe³⁺/dt is the rate of reduction of Fe³⁺ (moles sec⁻¹), and A is the surface area of the solid (m²). The calculated E_a for galena oxidation is 48 kJ mol⁻¹ (25 - 40°C) and is 84 kJ mol⁻¹ (25 -60°C) for sphalerite oxidation. Although galena and sphalerite are both simple, cubic, monosulfides their reaction rate with ferric iron differs by about 2.5 orders of magnitude for m_Fe³⁺ = 10⁻³.