Mineralogical and chemical characterization of an oolitic iron ore, and sustainable phosphorus removal

Synopsis An oolitic iron ore from Gara Djebilet, Algeria was characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive spectrosopy and the phosphorus distribution between various phases studied. Two ore types were identified – a haematite-dominated, oolitic type and a more massive, magnetite-dominated type. The haematite-dominated ore had a higher phosphorus content, up to 0.6% in the iron oxide minerals, than the magnetite-dominated type (0.3% P). Owing to the very fine-grained texture of the gangue minerals, reducing the phosphorus content to levels suitable for steelmaking (below 0.05% P) by conventiobal means would entail an energy-intensive process with a high carbon footprint. Preliminary leaching test work using a sulphurous geothermal water as lixiviant, assisted by solar thermal acceleration, gave very promising results, removing 0.13% of the residual P (or 21.7% removal of total P) from the high-phosphorus oolitic ore.


Introduction
Algeria holds one of the richest oolitic ironstone deposits in North Africa (Diab et al., 2020).Removal of phosphorus from these ores using conventional beneficiation methods is not possible (Tang et al., 2017), owing to poor selectivity and recoveries, and low economic efficiency (Pownceby et al., 2019).However, the increase in the international iron ore price in recent years has forced steel companies in many countries, such as FERAAL and MANAL in Algeria, to seek domestic sources of iron ore in order to reduce costs (Chen and He, 2014;Chen and Chu, 2014).
Many of the world's current iron ore resources contain over 0.08 wt% P, and are thus not suitable for iron-and steelmaking (Delvasto et al., 2008;Cheng et al.,1999).Phosphorus increases the hardnessof iron and decreases the solubility of carbon in iron at high temperatures.This decreases its usefulness in making blister steel (cementation), where the speed and amount of carbon absorption is the overriding consideration affecting the economics of the ironmaking process and the quality of the produced steel (Song, 2013).
Previous workers have investigated the adsorption mechanisms of phosphorus and other impurities by goethite-rich iron ores in order to identify a possible removal method.Parfitt and Atkinson (1976) found that phosphate adsorption on synthetic goethite results in the formation of a binuclear complex.Omran et al. (2015) investigated phosphorus removal from three different iron ore samples with varying total iron and P2O5 contents and mineralogical textures, using combined microwave pretreatment and ultrasonic treatment.Their findings indicated that microwave pretreatment increases the efficiency of ultrasonic disintegration of gangue particles by about 20%.Depending on the sample texture and phosphorus distribution about 59% phosphorus removal could be obtained.Liu et al. (2017) applied the slag ion theory to calculate the distribution ratio of phosphorus in the smelting reduction of high-phosphorus iron ores.They found that the main factors affecting the distribution ratio of phosphorus are ferrous oxide activity, phosphorus capacity of the slag, and the phosphorus activity coefficient in molten iron.Quast (2018) reviewed the techniques that can be used for economically processing oolitic iron ores containing high levels of impurities, notably phosphorus.Pownceby et al. (2019) investigated the distribution and association of phosphorus within goethite in a high-P Brockman-type iron ore from the Pilbara region of Western Australia.They concluded that limited removal of phosphorus by physical separation techniques is possible, but will most likely result in a significant loss of iron units.Chemical leaching is also possible, but this approach is only successful after thermal pretreatment to recrystallizse the goethite as Fe2O3 and consequent concentration of impurities at grain boundaries.
Da Siva et al. (2020) used a kinetic model of heating using microwave energy in the acid baking/leaching process to reduce particulate phosphorus content in iron ores.Their results indicated that with microwave energy as a heating source, 600°C is the optimum temperature for promoting the diffusion of leaching agent within the iron ore particles.
Yehia (2021) investigated the use of a fungal cellulase enzyme to improve the selective separation of apatite from haematite during reverse anionic flotation of a high-phosphorus iron ore.The results showed that the enzyme was adsorbed on the surfaces of both minerals.However, the depressant effect was significant in the case of haematite, but not apatite.
The Gara Djebilet field (Figures 1 and 2) is an important deposit situated in the North African Palaeozoic ironstone belt in southwestern Algeria.It extendeds from Zemmour to Libya and includes ironstones of Ordovician, Silurian, and Devonian age (Guerrak,1988;Diab et al., 2020).
In this investigation we studied the structural, microstructural, and chemical characteristics of this oolitic iron ore in detail in order to identify the nature of phosphorus inclusions in the ore, and carried out leaching tests with sulphurous geothermal water within a low-temperature solar greenhouse thermal acceleration system.

Sample identification
Representative samples were selected, identified, and classified according to their morphology, weight, and colour.Two types of ore were identified, type A (haematite-dominated) and type B (magnetite-dominated).
The samples were crushed and pelletized in the following steps: ➤ Stagewise crushing in a series of jaw crushers with different slot openings: After quartering, the representative samples were ground to a particle size of 90−70 µm using a disc mill.
Mineralogical and chemical analysis was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to investigate the structural complexity of each sample and to determine the phosphorus content.

Preliminary geothermal water leaching test
Sulphurous thermal water extracted from Hammam Righa, a geothermal spring located in northern Algeria, was used as a leaching solution in order to reduce the phosphorus content.The chemical composition of the water is given in Table I.A 'bottle drip leaching' set-up (Figure 3) was used tor the test, which requires 500 g of material.
Geothermal spring water from the Hammam Righa region is a sulphated-acid water with high concentrations of (SO4) 2− and Cl − ions.
Figure 4 shows the greenhouse solar heating system used to treat sample A (phosphorus content 0.6%), for a period of 6 hours (from 9 a.m. to 3 p.m. the time of maximum solar insolation) in order to accelerate the geothermal water absorption process, and to dry the sample after soaking prior to grinding in a disc mill.The process was repeated six times.

Mineralological analysis
The mineralogical compositions of samples A and B are given in Tables II and III respectively.The polished section of sample A (Figure 7A) is characterized by a massive texture with a nodular or oolitic structure with 60% haematite content, 20% sub-automorphic magnetite, and 5% iron hydroxides in the form of a coating, giving the rock a reddish hue.
Sample B (Figure 7B) is characterized by a massive texture with a granular, nodular, and auto-morphic structure and major mineralogical components consisting of 60% massive automorphic magnetite, 25% oolitic haematite, and traces of xenomorphic pyrite.

Optical microscopy
Observation of the polished section of sample A showed haematite in association with magnetite and goethite.The haematite mineralization (Figure 7A) consists of oolites ranging in size from 0.05 to 10 mm.The oolite cores contain iron hydroxide (goethite) and gangue minerals.
Magnetite crystalline structure is automorphic (Figure 7B) with size ranging from 0.1 to 0.3 mm.Magnetite also occurs as irregularly nodules in the rock.The iron hydroxide is visible as a red coating.
The study of the polished section of sample B shows a rock rich in magnetite and haematite, which are associated with small amounts of pyrite.Figure 8A illustrates magnetite oolite and massive magnetite.Figure 8B shows the magnetite, which represents around 60% of the total volume of the rock.It clearly shows haematite with a goethite core and massive magnetite.
The results give us idea of the intermolecular phosphorus inclusions in haematite, magnetite, and possibly in goethite (Parfitt,1976), as well as in the gangue minerals.In order to determine the location and concentration of the phosphorus inclusions, further analysis using SEM and EDS spectra is required.

SEM analysis
The samples were dehydrated and processed to become electrically conductive.The pressure inside the instrument reaches a value of 2.99 10 -3 Pa (2.99 10 -8 bar).Energy-dispersive X-ray spectroscopy (EDS) analysis was used to determine the elements contained in the iron oxide ore.

Results of sample A
Sample A was examined at magnifications from 500 μm to 10 μm.The thin section of haematite shown in Figure 9A depicts a regular spherical oolitic form consisting of a core around which concentric shells have developed by chemical (or biochemical) precipitation.
The EDS analysis of the oolite grain in Figure 9B is shown in Tables IV, V, and VI.It can be seen that the haematite in zone 1 (Table IV) has a composition of about 65.07%iron and 29.78% oxygen by weight.The main impurities are a mixture of 2.31% C, 1.52% Al, 0.7% Si, and 0.6% P.
The results for zone 2, the core (Table V), show that the iron content is lower than in zone 1, with 14.13% Fe and a very high oxygen content (46.71%), which indicates the presence of goethite (FeO(OH)); This zone also contains 0.5% P, 6.84% Si, 12.03% Al, 18.21% C, and 1.13% Mg.

Results of sample B
SEM observations of sample B reveal a completely different microstructure to sample A, with recrystallization of haematite and apatite within the core.The iron-rich radiating lamellae can be seen in Figure 10A.The three-dimensional microstructure of sample B is illustrated in the SEM image in Figure 10B.The EDS spectra show that sample B is mainly composed of iron (62.08%) and oxygen (26.78%), with 7.21% C, 1.04% Al, 0.93% Si, and less phosphorus (0.3%) than in sample A. There are also fewer inclusions of Na (0.39%), Mg (0.39%), S (0.16%), Cl (0.16%), K (0.1%), Ca (0.23%), and Ti (0.22%).It has been observed that when the oxygen content is lower, the phosphorus content is also lower.

Preliminary results of experimental geothermal water leaching
The geothermal springs host several habitats, including thermal water, biofilms, and sediment (Chen et al., 2023).The sulphurous thermal water used In this investigation contains a variety of dissolved minerals, as shown in Table I, and according to previous studies, it contains elementary sulphur-oxidizing bacteria (Culivicchi., 2005) which facilitate two simultaneous processes: These bacteria proliferate at temperatures between 55°C and 85°C (Culivicchi-, 2005), and metabolize oxygen and inorganic phosphorus.Therefore, these essential elements (oxygen and phosphorus) available in this iron ore can be dissolved and eliminated at the same time.
A low-temperature leaching process using water from Hammam Righa was investigated with the aim of reducing the level of harmful phosphorus in oolitic iron ore in an eco-friendly manner.
A 500 g portion of sample A, with a particle size varying between 70 and 90 µm, and an initial phosphorus content of 0.6% (by weight) was used in the test.The sample was contacted with the thermal water for six consecutive days.On each day, the sample was well mixed and placed in the solar thermal heating system for 6 hours, from 9 am to 3 pm (maximum solar insolation).At the end of the experiment, the sample was dry and the temperature inside the solar heating system was approximately 62.5°C.The sample was then finely ground in a disc mill, and contacted again with the thermal water.This process was repeated six times.Sulphuric acid dissolves phosphorus in the iron-rich minerals and gangue minerals as follows: The Fe in solution is reduced from Fe 3+ to Fe 2+ (Dong et al., 2019): The thermal solution contain iron ions, sulphate ions SO4 2− and dissolved phosphorus as PO4 2− .

2(3H + (PO4
The results are summarized in Table VIII.This experiment was repeated three times, and the same results were obtained with a small margin of error.
These preliminary results are encouraging, with 0.13% P removal, representing a removal ratio of 21.7%.

Conclusion and recommendations
The mineralogy, microstructure, and chemical composition of a high-phosphorus oolitic iron ore from Gara Djebilet in Algeria were investigated.Two types of ore were identified, A (haematitedominated) and B (magnetite-dominated), with distinctly different microstructures.Type A ore had the higher phosphorus content, 0.6% compared with 0.3% for type B. The phosphorus content is proportional to the Fe and O contents, indicating that P is largely substituted in goethite.
Preliminary test work to reduce the phosphorus content of the haematite-dominated ore by solar-assisted leaching in sulphurous geothermal water yielded promising results, wirh 21.7% phosphorus removal.The sulphur in the geothermal water is produced by sulphur-oxidizing bacteria at temperatures of 55-85°C.The use of a bioleaching process to improve the P removal ratio is a possible topic for future investigations.
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Figure 3 -
Figure 3-Set-up for bottle drip leaching

Figure 6 -
Figure 6-XRD spectrum of sample B

Table VII EDS results for sample B Element %mass %atomic Total intensity Error% K-ratio
The Journal of the Southern African Institute of Mining and Metallurgy VOLUME 124 FEBRUARY 2024