Studying on mineralogical and petrological characteristics of Gara Djebilet oolitic iron ore, Tindouf (Algeria)

: Demand for iron ore worldwide has been steadily increasing which leads to the extraction of iron ore deposits with more complex mineralogies and higher levels of silicon and phosphorus impurities. This is the case in Algeria with the iron ore deposit of Gara Djebilet, Tindouf; where it has recently been exploited to ensure the sufficiency of iron ore required to produce iron and steel products. This deposit has remained unexploited for several decades due to inadequate knowledge of its mineralogy, treatment, and economic assessments. This study aims to find out the microstructure, chemical composition, and mineralogical distribution of valuable minerals and impurities, to understand the efficient processing methods for this specific iron ore. The characterization of representative ironstone samples taken from the studied area was carried out using optical microscopy, X-ray fluorescence spectrometer (XRF), petrographic microscope, X-ray diffractometer (XRD), and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The results of the mineralogical analyses confirmed that it is an oolitic fine-grained ore consisting of gangue minerals principally composed of quartz, apatite, and iron-rich concentric cored structures. Chemical analyses of the ore indicated that it contains 56.58 wt% Fe with 7.98 wt% SiO 2 , 7.09 wt% Al 2 O 3 , and minor amounts of P 2 O 5 , CaO, MgO, and TiO 2 compounds. The phosphorus associated was present in both ooids and groundmass, indicating that the ore has a complex texture with very rich and diverse mineralogy. For that, two conceptual scenarios were potentially proposed for processing the studied iron ore, while further detailed automated mineralogical information was required to make sure about the processing units from a practical perspective.


Introduction
Iron ore is the main raw material used in iron and steel productions confirmed as a significant resource for both the economy and society (Li et al., 2020).Iron and copper ores make up more than half of all metal ores that are extracted globally (Nakajima et al., 2018).In recent years, the world's large infrastructure has increased the demand for iron ore and steel products (Wu et al., 2016), and there are forecasts that by 2023, steel demand will see a 2.3% rebound to reach 1,822.3Mt and will grow by 1.7% to 1,854.0Mt in 2024 (World Steel Association, 2022).Global steel production is expected to increase at the current pace until reaching 2.065 Mt in 2030.After that, it will slow down and reach a maximum of 2.336 Mt in 2040 (Lopez et al., 2022), which means an increased global demand for iron ore over the coming decades.
With these requirements for iron ore, and as the steel industry develops, the use of iron ore, which is of high quality and easy to handle, gradually declines.That leads to the extraction of iron ore deposits with more complex mineralogies and a higher level of impurities (Silva et al., 2020).Many types of iron ore deposits have historically been significant as iron sources containing hematite, magnetite, titanomagnetite, and pisolitic ironstone (Clout and Manuel, 2015).This last type of ores is widely distributed in South China (Li et al., 2021), Germany, Australia, Canada, Colombia, and the United States (Quast, 2018).Characterization of ores is an integral part of mineral processing because it is the key to choosing the most appropriate route for the recovery of ores.Specifically, this type of ore where its peculiar characteristics complicate processing and resist conventional mineral processing methods (Quast 2018;Gholami et al., 2022).Further detailed information regarding various iron processing routes suitable for several iron ore types is presented elsewhere (Ripke et al., 2019).
Oolitic iron ores are formed from ooids comprised of many concentric layers, with 0.4 to 1.1 wt% phosphorus mostly occurring as apatite.Because of their poor liberation and fine distribution of iron particles, these ores have yet to be economically exploited.(Song et al., 2013).The modern steel industry has limited the concentration of phosphorus in steel to less than 0.04 wt% and this is because of their harmful effects on the mechanical properties of steel (Tang et al., 2013).As a result, phosphorus must typically be removed from phosphorus-rich iron ores before they can be used as feedstock for manufacturing iron.(Li et al., 2015).The acceptable levels of phosphorus in a hot metal range from 0.08 to 0.14; therefore, it is desirable to use iron ores with very low phosphorus contents for steelmaking.Iron ores can be divided into low-phosphorus ore (<0.07%P), medium-phosphorus ore (0.07-0.10%P), and high-phosphorus ore (> 0.10% P) categories depending on the amount of phosphorus they contain (Cheng et al., 1999;Fisher-White et al., 2012).For example, it is well known that for DRI (direct iron reduction) processes the pellets should contain the following chemical properties Fe= minimum 67%, SiO2+Al2O3=maximum 3%, S=maximum 0.08%, P=0.03% and TiO2=0.15%(Lu et al., 2015).
Proper knowledge of the chemical and mineralogical characteristics of valuable and gangue minerals is the key to developing the efficient beneficiation of oolitic iron ores.Various analytical techniques can be used to characterize iron ores, among which X-ray Diffraction (XRD), X-ray fluorescence spectrometry (XRF), chemical analyses, and scanning electron microscopy (SEM) are the most common ones.These methods provide critical information on mineralogical distribution, texture, and liberation degree that are key information to elaborate an efficient processing schema (Gholami et al., 2022;Quast, 2018).Clout and Manuel (2015) published a comprehensive review on the mineralogical, chemical, and physical characteristics of different iron ores.They classified mineralogical, petrological, and chemical properties of various iron ores beside some other physico-mechanical characteristics that are necessary for designing a good beneficiation process, including relative hardness, compressive strength, and particle size.Useful research works on the application of XRD characterization of iron ores by de Villiers and Lu (2015), automatic optical image analysis of iron ore by Donskoi et al. (2015), SEM-aided quantitative characterization by Tonzetic (2015), and spectroscopic techniques by Ramanaidou et al. (2015) have been also addressed in the literature.Literature review revealed that most of the research studies on oolitic iron ores deal with particle size and chemical analyses, often accompanied by XRDbased mineralogical studies and there is a big gap in detailed mineralogical information on such iron resources.Sun et al. (2013) and Song et al. (2013) performed simple chemical and XRD analyses to characterize oolitic iron ores from a reservoir in China for the development of a potential processing route.They showed that the contents of Al2O3 (3-6%) and SiO2 (21-22%) respectively in the form of quartz and chamosite, which are detrimental to the efficient recovery of iron, where high phosphorus was the primary harmful element and principally distributed in apatite.Studies on chemical and mineralogical characterization of Australian oolitic iron ores reviewed by Beattie et al. (2017) revealed that there are mild conditions with those ores with relatively high contents of Al2O3 (1.56%) and SiO2 (4.7%) and LOI (Loss on Ignition) value of 9.0%.Novoselov et al. (2018) characterized an oolitic iron ore in Russia using conventional chemical and instrumental methods and showed that the studied ore sample was rich in quartz (32.55%) with relatively high contents of alumina (4.43%) and phosphorous (0.58%).Recently, Junhui et al. (2020) performed a detailed mineralogical study on a refractory highphosphorous oolitic iron ore in China.They showed that the main gangue mineral in the ore was quartz (13.41%), with minor amounts of calcite, dolomite, chlorite, hydromica, and collophanite.The contents of alumina and phosphorous were about 6.5% and 1.5%, respectively.Monzavi and Raygan (2020) beneficiated an oolitic-iron ore (containing 45.46wt% Fe2O3) by magnetization roasting (at 625 °C for 25 min) and dry magnetic separation (2500 G).
Currently, the ooidal ironstone deposits are widespread in the south of Algeria and also compose two of the greatest known giant deposits of ooidal ironstones in North Africa i.e., Gara Djebilet and Mechri Abdel Aziz in Tindouf (ANAM et ASGA, 2019).The ore of these deposits often has an oolitic structure, where the intercalation relationship between iron minerals and gangue minerals is complicated and the phosphorus content is high.However, it is necessary to characterize the ore to determine its grade on the iron and other mineral contents, as well as to know about its texture and the nature of gangues associates.Therefore, this research investigation aims to obtain the mineralogical and morphological characterizations of the Gara Djebilet oolitic iron ore.The studies were implemented by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), and optical microscopy.Then, potential conceptual scenarios were suggested for the recovery of iron from the studied ore based on mineralogical and chemical results.

Site description
The oolitic iron ore deposit in the Gara Djebilet area represents the largest and the major iron ore deposit in Algeria and North Africa (Taib, 2009).As seen in Fig. 1, the research area includes the southwest portion of the Tindouf basin, 130 km southeast of the city of Tindouf, and close to the Algerian-Mauritanian border.The East-West area, which spans a distance of around 60 Km, has three significant distinct deposits that have been identified.The 1.7 billion tons of iron-rich, exploitable deposits in Gara Djebilet are spread across two significant lenses: the so-called "west" lens, with 780 million tons, and the so-called "center" lens, with 900 million tons.(Bersi et al., 2016).These three lenses have different sizes (7-90 km 2 ), and their thickness is not constant and varies up to about 30 m (Guerrak, 1987).The deposits are still present as mesas of mostly argillaceous and sandy sediments interbedded with flat-lying (1.5-2° north dipping) oolitic iron-stone lenses.Three zones may be distinguished within the main body of iron ore: 1-the lower non-magnetitic ore (3-10 m thick, Fe = 54.6%);2-the magnetitic ore (6-10 m thick, Fe = 57.8%),which corresponds to the main economic ore; and 3-the upper non-magnetitic ore (4-12 m thick, Fe = 53%) (Guerrak, 1988).

Sampling procedure
the ironstone samples utilized in this study were collected from the Gara Djebilet deposit in the Algerian province of Tindouf, such that to cover as much of the study region as possible while maintaining an acceptable degree of dependability.These lumps were gathered from the old mine and the pits SP2, SP3, SP4, SP5 at the gara west from the magnetitic ore, which corresponds mainly to the workable ore (cutoff grade at 57%), and from gara center with particle size in the range of 0-170 mm.The iron ore lumps were reduced to finer sizes using a sledgehammer.Chosen samples were taken from the received sample for the preparation of polished sections and thin blades.Sub-samples were further crushed to 5 mm using a jaw laboratory crusher (48-D0530/A model of Controls Group), and then mixed and split by quartering technique to obtain a representative sample.After that it was pulverized using a benchtop ring mill (produced by Rocklabs, New Zealand) to obtain fine powder (<74 µm) which was used for chemical and mineralogical analyses.

Characterization methods
The research methods applied in this work were optical microscopy, petrographic microscopy, X-ray fluorescence spectrometry, X-ray diffraction spectrometry, and scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX).

Textural and petrographic analysis
For the identification of texture, polished sections were prepared and investigated with a Nikon Eclipse LV150N optical microscope.The petrographic microscope Axiolab 5 was used to visually identify the minerals present in the iron ore samples.Thin blades of the samples were formed and examined under the combination of both plane and cross-polarised light of the microscope.The petrographic investigation was performed at the Laboratory of thin sections in the Geology Department, Badji Mokhtar University, Annaba, Algeria.

Chemical analysis
A pressed pellet prepared from a representative sample was analyzed for its major oxides (Fe2O3, SiO2, Al2O3, MnO, MgO, CaO, K2O, Na2O, TiO2, and P2O5) by the XRF technique using a Bruker-Axs: S8 TIGER X-ray spectrometer.Loss on ignition (L.O.I.) was obtained by heating the sample powder to 1000 °C for 2 h (Fozooni et al., 2017).The micro-morphology and the dispersion state of the minerals in the iron ore were observed using a scanning electron microscope (SEM, Quanta 250 model) combined with EDS, and with the advantages of high magnification, clear image, and simple operation.

Mineralogical analysis
X-ray diffractometer (XRD), Bruker D8 ADVANCE model, equipped with Copper Anticathode in configuration Bragg-Brentano optics with fixed slits, Ni filter, and ultrafast linear detector was used for mineralogical analysis.The radiation employed was a Cu K-α with a wavelength of 1.540598 Å within the values of 2θ from 0 -80° Bragg's angle (Khoshdast and Shojaei, 2012).The XRD was used to identify the mineral phases present in the ore.The mass of sample was required to be about 3 g with a particle size of <74 µm.The sample was pressed to obtain a flat and uniform surface and after that it was put into the XRD equipment to start detection (Hasanizadeh et al., 2023).The XRF and XRD investigations were performed at the Technological platform "Development of Materials and Manufacturing", National Polytechnic School, Constantine, Algeria.

X-ray fluorescence spectroscopy
The results of chemical analysis by the X-ray fluorescence (Table 1) indicate that the Gara Djebilet iron ore contains 56.58 wt% of iron.Fe2O3, SiO2, and Al2O3 are the most abundant oxides, making up more than 90% of the oxides and indicating the presence of quartz and hematite in the ore.However, associate elements such as phosphorus are considered to be deleterious since it is higher than the recommended benchmarks.The phosphorus content of Gara Djebilet iron ore is 0.921 wt%, which makes it a highphosphorous iron ore (>0.1% P) and presents a challenge that requires better beneficiation techniques for its reduction.Additionally, as seen in Table 1, this iron ore has a significant concentration of impurities including TiO2, MnO, and MgO whereas other oxides are present at negligible levels.Because of the iron ore's high gangue content and lack of commercially graded blast furnace ore quality, its exploration can only be helpful for the manufacture of direct reduction iron; 4% Al2O3, and 6% SiO2 (Joan et al., 2015).

Optical and petrographic microscopy
Fig. 3 depicts the findings from optical microscope observations of polished cross sections of the ore, which reveal the ore's oolitic structure, which is made up of lighter spherical grains arranged in concentric layers contained in a darker matrix.The ore's appearance is consistent with its oolitic ore structure.The oval-shaped ooids that make up the oolitic texture range in size from 150 μm to 800 μm.
Those ooids are containing hematite, goethite, and magnetite along with impurity elements such as silicon and phosphorus.Gara Djebilet iron ore is brownish red of oolitic texture and semi-hard.The petrographical and mineralogical studies of rocks are shown in Figs. 4 and 5.The photomicrographs of thin sections (Fig. 4) show that the ironstone is dominated by the mineralogic magnetite-rich association, and two iron ore facies types: facies with scattered ooids in a quartz-poor groundmass (non-detrital facies: FOND) as shown in (Fig. 4a), and facies with cemented jointed ooids and pore-filling structures (cemented facies: FOC) as shown in (Fig. 4b).The predominant cement component in the FOC facies type is oxidized (goethite with a brownish-yellow tint).The iron ore minerals can appear in a variety of ways, including as the small automorphic crystals (5-20 μm) known as magnetite that are found within ooids underlining the oolitic structure (Fig. 5a) and as larger automorphic crystals (50-100 μm) in the groundmass that are opaque and has no cleavage under the microscope as shown in (Fig. 4a).Within the shallower parts of the magnetitic ore, maghemite is commonly found.It grows at its cost and occurs inside magnetite.(Fig. 4a).Chamosite is found in ooids and groundmass in various ways; in Fig. 4a, it is visible as fibrous minerals in the groundmass.All types of iron ore contain hematite, which is tightly connected to chamosite or alternates throughout the layers of ooids.It reflects as a vivid red color, as seen in Fig. 4a (hematitic ooids).Non-ferruginous minerals.Apatite is quite scarce that frequently appears as prismatic crystals in both ooids and groundmass.It is bright-yellow emission (Fig. 4b).Quartz as nuclei or detrital grains has weak birefringence (Fig. 5b).
The observation of these photomicrographs and the granulochemical analyses (Table 2) allows the determination of the liberation size of iron and gangue minerals, particularly that of phosphorous containing oolitic particles (Fig. 4b); where the particle size of apatite is about 230 μm and 210 μm.
Table 2 shows a decrease in the content of total iron and Fe2O3 in the fine size classes [-0.25 mm, +0.125 mm], [-0.125 mm, +0.063 mm], and [-0.063 mm] and, in return, an increase in the contents of gangue minerals Si2O, Al2O3 and P2O5.This means that the liberation size starts from -125 μm (Fig. 6).
The results of the sieve analysis (Table 2) show that the mass is distributed between the larger fractions [+2, +1, +0.5 and +0.25 mm], and the finer fractions [+0.125, +0.063 and -0.063 mm], which confirms that the iron ore of Gara Djebilet is semi-hard.

Powder X-ray diffraction results
To find out the types of minerals in the raw material, the mineral phase of the iron ore sample was qualitatively analyzed using an X-ray diffractometer.As shown in Fig. 7, the XRD analysis confirmed the presence of iron oxide (hematite Fe2O3) as the major phase along with magnetite (Fe3O4), goethite (FeO(OH)), chlorite in the form of chamosite ((Mg,Fe,Al)6(Si,Al)4O10(OH)8), siderite (FeCO3) and quartz (SiO2).Due to the low content of other minerals as apatite, they could not be displayed in the XRD spectrum.The semi-quantitative results of XRD show that the mass fractions of hematite, magnetite, goethite and siderite making above of 80%, which hematite is the dominant phase.

Scanning electron microscope with energy dispersive X-ray spectrometry
To confirm the results obtained previously of raw material samples and the thin sections, some SEM images were taken.The results are illustrated in Fig. 8. Fig. 8a confirms that its unique concentric oolitic structure, in which iron and gangue minerals are embedded.There are three phases observed and represented by grey, dark grey and black, where hematite, goethite and magnetite (grey colour), fluorapatite (dark grey colour), chlorite (chamosite) (black colour).These results are in good agreement with XRD and petrographic results.The Fig. 8b and 8c exhibits that phosphorous is distributed in both the ooids and the groundmass.Fig. 8d shows that iron ore contains phosphorus and Fig. 8e shows that Al, Mg and Si distribute in chlorite (chamosite).

Proposed scenarios for processing of Gara Djebilet iron ore
The results proved that the beneficiation of the Gara Djebilet iron ore is potentially expected to be extremely difficult.This is mainly related to the distribution of phosphorous throughout the sample and the dissemination of iron minerals as well as their presence in finer fraction sizes.So far, there have been several studies in the field of processing oolitic iron ores, which were briefly reviewed by Quast (2018).According to the reported processing methods as well as the mineralogical and petrological results presented, two scenarios can be suggested for the processing of Gara Djebilet iron ore: fine comminution-reverse flotation-high gradient magnetic separation (Fig. 9a), and fine comminutionroasting-low intensity magnetic separation (Fig. 9b).The first point in both scenarios is the need to pay attention to the complexity of the mineralogical texture and the necessity to increase the degree of liberation in fine particle size.Therefore, fine comminution and efficient classification will be integral parts of any proposed processing process (Jarkani et al., 2014).The necessity of fine grinding has also been emphasized by other researchers (Hanna and Anazia, 1990) and this makes processing challenging due to the production of high slime (Khoshdast, 2019).
In the first scenario (Fig. 9a), after achieving the desired liberation during fine comminution, using reverse flotation, phosphorus present in the form of apatite compounds in the ore is removed   Khoshdast and Sam, 2012).For this, conventionally used mechanical and column flotation cells combined with a cavitation tube or even intensified flotation equipment which are more effective for finely disseminated particles can be applied (Hassanzadeh, 2023).Apatite has a relatively hydrophobic nature that helps improve flotation efficiency.Then, the flotation concentrate can be directed to an optimized circuit of magnetic separators.Considering that the dominant iron mineral is hematite, the use of high-gradient magnetic separators (HGMS) will be useful.These separators are also significantly efficient for the treatment of fine feeds.At this stage, it is expected that a large proportion of the gangue minerals, the majority of which is quartz, will be removed.If needed, possible residual silica can also be removed with a reverse flotation step of the concentrate (Rao et al., 2013;Quast, 2017).
One of the efficient methods to convert hematite to magnetite is to use reduction roasting.This method may be used in single or in combination with hydrometallurgical methods (Guo et al., 2015;Dey et al., 2017;Pan et al., 2022).Although the development of high-gradient magnetic separation technology has significantly reduced the need for relatively expensive thermal roasting, improving ore magnetic properties can still be of great interest to processing engineers.For this reason, in the second approach, as shown in Fig. 8b, it is possible to convert hematite into magnetite with a roasting pretreatment step and then concentrate the ore by using low-intensity magnetic separation (LIMS).A great amount of phosphorous can be also removed in the roasting stage by mixing the finely ground ore with a roasting additive like Na2Co3 or Na2SO4.Then, if needed, the silica and phosphorus content of the concentrate can be reduced by one or two reverse flotation steps.It should be noted that the above processing methods are proposed based on the characteristics of the investigated iron ore and the available technologies in Algeria.There is no room for doubt that to evaluate their efficiency, laboratory studies and large-scale validations are needed.

Conclusions
Gara Djebilet iron ore, a high-phosphorus iron ore, has been characterized using a variety of analytical techniques (XRF, XRD, SEM-EDX and optical, petrographical microscopies) in an attempt to unravel the presentation and distribution of the various iron minerals and the impurities in the ore.The existence of phosphorus was confirmed by the findings of these investigations, its concentration in the sample studied was determined to be 0.921 wt%.It occurred in higher concentration compared to the limit of P in the steel.Its spatial distribution in the ore was demonstrated to be in both the ooids and the groundmass.Total iron (TFe) existed in the form of hematite, magnetite, goethite, and siderite.Al, Mg, Si were distributed in chamosite, while SiO2 was present as quartz.It was found based on the chemical and mineralogical results that the phosphorus can be removed from iron ore by very different routes of treatment, however with variable dephosphoration rates.According to the mineralogy of the Gara Djebilet iron ore, thermal or mixed processes were closer to reality technical solutions and can remove phosphorus from the ore to meet market specifications of 0.08 wt% P, because the thermal processing disrupts the structure of goethite FeO(OH) is effective in releasing phosphorus associated with this mineral and becoming available to be extracted by a leaching solution.We believe that these data will be extremely helpful in developing successful beneficiation and phosphorus removal strategies.Finally, two conceptual flowsheets were proposed for the recovery of iron from the studied ore based on characterization results.However, further experimental investigations including mineralogical analyses using automated mineralogy as well as systematic ore dressing processes are necessary to confirm the effectiveness of the suggested beneficiation processes.

Fig. 3 .
Fig. 3. Photomicrographs of bulks of Gara Djebilet iron ore sample mounted on epoxy and polished.The largest (ooid) grain measures 800 μm in diameter

Fig. 6
Fig.6 Graphical representation of granulochemical analysis results

Table 1 .
Chemical composition of Gara Djebilet iron ore sample by the XRF method * LOI is the percentage of mass lost on ignition at 1000 °C

Table 2 .
Granulochemical analysis of Gara Djebilet iron ore sample by particle size analysis and XRF method