Diversity of manganese oxy-hydroxides and their sorption capacity for Co and Ni in lateritic deposits worldwide

Manganese oxy-hydroxides are ubiquitous in soils and sediments where they occur as fine-grained aggregates and coatings on other mineral particles. Owing to large surface areas these minerals are very reactive and have long been known for their enormous adsorption capacity for Co and Ni. This is now of great relevance for Co and Ni extraction from lateritic ores, where Co and Ni bearing Mn oxy-hydroxides can be found in the most highly oxidised parts of weathering profiles. Detection and characterisation of these minerals however is very challenging, as they present with low bulk concentrations often within mineralogically complex, fine-grained mixtures of poorly crystalline phases.

In this study we identified and characterised a number of Mn oxy-hydroxides in samples from a variety of laterite deposits: Shevchenko (Khazakstan), Acoje (Philippines), Nkamouna (Cameroon), Piauí (Brazil), and Penamax and Tiebaghi (New Caledonia). Bulk chemical and mineralogical characterisation was undertaken with ICP-OES/MS and XRD, followed by spatially resolved imaging at the micron scale using µXRD, EPMA, SEM, µRaman and synchrotron-based µXRF. The chemical state and local environment of Co and Ni were determined using X ray spectroscopy (μXANES and μEXAFS).

The total concentrations of Co and Ni in the bulk samples ranged from 420 mg/kg (Piaui) to 1.245 wt% (Tiebaghi) and 0.5 wt% (Nkamouna) to 1.74 wt% (Piaui) respectively. The low abundance in addition to the poorly crystalline nature of the manganese oxy-hydroxides made them undetectable with XRD with the exception of the Cameroon and New Caledonia samples, where lithiophorite was detected. Following spatially resolved electron microscopy, Mn-rich grains were localised in the bulk samples and further studied with µRaman spectroscopy, µXRD and EPMA. In Shevchenko, asbolane was identified containing Co with concentrations varying from 0.25 to 12.4 wt% (average 6.3%) and Ni from 3.2 to 16.9 wt% (average 11.7 wt%) In samples from Nkamouna Co varied widely from below 1 wt% in romanechite and pyrolusite, average of 5.5% in lithiophorite and up to 21 wt% in lithiophorite-asbolane intermediates. In the Piaui samples asbolane and asbolane-lithiophorite intermediates were identified and found to carry from 0.35 to 14.2 wt% (average 3.4 wt%) of Co and 0.35 to 18.8 wt% of Ni (average 8.5 wt%). In addition, unusually Co-rich barium manganese oxide was found with Co varying from 1.5 to 10.4 wt% (average 3.0 wt%) and Ni from 0.3 to 1.98 wt% (average 0.6 wt%). In the New Caledonia samples asbolane-lithiophorite intermediates were identified with 1.1 to 9.6 wt% Co (average 5.3 wt%) and 1.9-11.4 wt % of Ni (average 6.9 wt%).

X-ray spectroscopy revealed that Co is bound in a range of Mn oxide minerals as Co³⁺ while Ni is present as Ni²⁺. The crystal chemistry of Co was very similar in the various minerals with Co structurally incorporated by substituting Mn in the manganiferous layer. The crystal chemistry of Ni was more variable. In asbolane Ni was found to build Ni(OH)₂ layers, in lithiophorite it was structurally incorporated in the Al(OH)₃ layer while in the lithiophorite-asbolane intermediates it was found partly in the Al(OH)₃ layer and partly adsorbed.