Characterisation of phosphorus deportment in ferromanganese oxide ores – Implications for P removal

Characterisation of six supergene ferromanganese ore samples revealed that P was hosted by several different Fe and Mn oxides. Goethite had the highest (up to >1% or higher) and highest mean (up to 0.32%) P contents, then cryptomelane-hollandite (mean up to 0.12% P) and nsutite-pyrolusite (mean up to 0.14% P). When present in the same sample, nsutite had a higher mean P content than pyrolusite. Phosphate minerals were not detected in the samples. P deportment was variable with deposit, deposit type and within individual mineral grains. EPMA showed that P content is elevated in cryptomelane with higher Mn/(Fe + Al + Si) ratios and in todorokite with lower Mn/(Fe + Al + Si) ratios. The results show that higher phosphorus supergene ferromanganese ores with considerable cryptomelane and nsutite contents are likely to have multi-phase Fe-P and Mn-P deportment. The propensity for very fine-scale intergrowths of goethite and Mn oxides ± aluminosilicates in some ore types means that reliable characterisation of P deportment is critical in informing potential P-reduction pathways for ferroalloy feedstocks. P was most probably incorporated into Mn oxides via near-surface adsorption via complexation with cations such as Ca or Mg in todorokite, but the presence of structural P cannot be precluded in cryptomelane and nsutite.