Characterization studies on Agbaja iron ore: a high-phosphorus content ore

Abstract

Characterization of Agbaja iron ore was carried out using optical microscopy, scanning electron microscopy with energydispersive X-ray spectroscopy, X-ray fluorescence spectrometry, powder X-ray diffraction, thermal gravimetry, and differential scanning calorimetry. The ore consists of a matrix of gangue minerals composed principally of aluminosilicates and iron-rich concentric cored structures characteristic of oolitic ores. Chemical analyses of the ore indicate that it is principally composed of 53.1 wt% Fe, 1.395 wt% P, and aluminosilicates. The phosphorus associated with the ore was found to be present in both the iron-rich mineral(s) and the gangue minerals, indicating that beneficiation by elimination of gangue will have little effect on the percentage of phosphorus in the valuable mineral. Based on the results, it is postulated that phosphorus in Agbaja iron ore is most probably present in an amorphous phase and not as a component element of a distinct crystalline phase.

Technological Challenges of Phosphorus Removal in High-Phosphorus Ores: Sustainability Implications and Possibilities for Greener Ore Processing

“…However, looking at the upper limits of phosphorus contents reported for high-phosphorus iron ores in which distinct phosphorus minerals are yet to be identified (0.8% [41,42] to −1.395% [20]), with the maximum plausible substitution of Fe cations with P cations calculated assuming the non-plausible equality of charge, and the maximum hydroxyl ion substitution with phosphate ligand that permits the maintenance of the stability of the respective structures (in P solid solution with goethite and phosphate ligand substitution of hydroxyl groups in goethite), it might be argued that both theories may not fully account for the full range of phosphorus content in iron ores. In light of the known affinity of iron oxide/hydroxide surfaces for phosphorus/phosphate anions which permits their use as adsorbents for phosphorus removal [49,[72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89], it stands to reason that a significant proportion of phosphorus in high-phosphorus iron ores, especially in goethite ores, might be present as adsorbed species on the mineral surface.…”

“…This report suggests that the thermal treatment of iron ores can influence their interaction with phosphate. On the other hand, iron ores are known to be associated with various other phases like kaolinite, as in the Agbaja ore from Nigeria [20], dolomite, clinochlore, and quartz, as in the ore from Hubei province (China) [257], kaolinite, as in the ore from Pilbara area of Western Australia [258], quartz and chamosite as in the ore from Aswan area of Egypt [38], clay-like minerals such as carbonates, chlorite, other aluminosilicates, and quartz as in the ore from Lisakovsk, North Kazakhstan [41,107], and silica and alumina, as in the ore from Moncorvo in north-east Portugal [45,[259][260][261]. Wei et al [234] studied the surface properties and phosphate adsorption of goethite, kaolinite, goethite-kaolinite association (GKA) and goethite-kaolinite mixture (GKM), and established their respective pHs at a point of zero charge (PZC) to be around 8.2, 4.1, 7.0, and 6.1 respectively, and their surface charges at pH 5 to be 0.561, −0.092, 0.097, and 0.041 mmol/g, respectively.…”

“…They concluded that in comparison to the adsorption capacities of goethite and kaolinite, their combinations (goethite-kaolinite association (GKA) and goethite-kaolinite mixture (GKM)) were more effective phosphate adsorbents, with the phosphate adsorption capacity of GKA being markedly higher than that of GKM. Hence, when the associated gangue has similar phosphorus adsorption abilities to the iron-rich phase (as in the Agbaja iron ore [20]), goethite; phosphorus removal can be quite challenging. In such scenarios, repartition of the associated phosphorus from the iron-rich phase to the gangue phase by chemical processing is quite difficult without a prior thermal treatment to manipulate the phases present in the ore.…”

“…Besides the presence of significant quantities of phosphorus in minerals/ores, its presentation, distribution, and interaction with mineral surfaces are of vital importance, as these determine the ease or difficulty with which the ore can be beneficiated. From insights obtained from analyses of phosphorus content and distribution in high-phosphorus iron ores [20], it is posited that phosphorus can be present in ores in four principal forms: as distinct crystalline phase(s) in the iron-rich phase, as distinct crystalline phase(s) in the associated gangue mineral, as amorphous phase(s) or adsorbed phase(s) in the iron-rich fraction of the ore, as an amorphous phase(s) or adsorbed 2 of 38 phase(s) in the gangue, or any combination of these, with implications on the appropriate phosphorus removal approach.…”

“…When present as a distinct crystalline or amorphous phase in iron ores, phosphorus can be finely distributed in the iron-rich phase, in the gangue mineral, or in both [20]. Its predominance in the gangue mineral makes phosphorus removal from ores easier, and amenable to the use of mechanical methods for its removal, while its presence in the iron-rich phase necessitates the application of other/additional methods (often chemical or thermal) to achieve phosphorus removal/reduction with associated economic and environmental costs.…”