Physicochemical and structural factors in the sulfuric acid leaching of nickel- and copper-bearing synthetic birnessites

A large number of nickel- and copper-doped samples of birnessite (0.7 nm phase), a layered-structure manganese mineral, were synthesized by dehydration of respective buserites (1 nm phase). The samples were characterized in terms of chemical composition, specific surface area, phase constituents, crystallinity, strain, morphological features, and structural complexity, in order to study the influence of the physicochemical characteristics of the samples on the leachability of doped elements and manganese in sulfuric acid. In contrast to manganese, the leaching behavior of doped nickel and copper is found to be more sensitive to the structural characteristics of the host birnessite phase. The leachability of the doped elements does not show any correlation with the specific surface area of the samples. Significant parameters affecting leachability are the interlayer spacing of the parent buserite phase used in the synthesis and the microcrystalline dimension and strain in the 〈001〉 crystallographic direction of the birnessite phase. In addition, leachability is also controlled by the crystal field stabilization energy (CFSE) of the doped metal ion. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies on leach residues indicated the transformation of birnessite phase into other minerals such as nsutite (γ-MnO2). A significant fraction of the doped nickel and copper (20 to 40 pct) remains unleached, even after prolonged leaching up to 6 days, and this is attributed to the compact structure of the newly formed phases during leaching.