Thermal Beneficiation of Sra Ouertane (Tunisia) Low-Grade Phosphate

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Article Thermal Beneﬁciation of Sra Ouertane (Tunisia) Low-Grade Phosphate Rock

Noureddine Abbes 1,* , Essaid Bilal 2 , Ludwig Hermann 3 , Gerald Steiner 4 and Nils Haneklaus 4,*

1 Groupe Chimique Tunisien, Gabes 6000, Tunisia 2 École Nationale Supérieure des Mines de Saint Etienne, 42023 Saint-Etienne, France; [email protected] 3 Proman Management GmbH, Weingartenstrasse 92, 2214 Auersthal, Austria; [email protected] 4 Td Lab Sustainable Mineral Resources, Danube University Krems, Dr.-Karl-Dorrek-Straße 30, 3500 Krems, Austria; [email protected] * Correspondence: [email protected] (N.A.); [email protected] (N.H.)

Received: 7 August 2020; Accepted: 13 October 2020; Published: 22 October 2020

Abstract: Low-grade phosphate rock from Sra Ouertane (Tunisia) was beneficiated using a thermal treatment consisting of calcination, quenching, and disliming. Untreated phosphate rock samples (group 1), calcined phosphate rock samples (group 2), as well as calcined, quenched, and dislimed (group 3) phosphate rock samples, were investigated using inductively-coupled plasma atomic emission spectroscopy (ICP-AES), inductively-coupled plasma mass spectrometry (ICP-MS), thermogravimetric analysis (TGA), and X-ray powder diffraction (XRD). Besides, the particle size distribution of the aforementioned three groups was determined. The proposed thermal treatment successfully increased the P2O5 content of the untreated phosphate rock from 20.01 wt% (group 1) to 24.24 wt% (group 2) after calcination and, finally, 27.24 wt% (group 3) after calcination, quenching, and disliming. It was further found that the concentration of relevant accompanying rare earth elements (Ce, La, Nd, Pr, Sm, and Y) was increased and that the concentration of Cd could be significantly reduced from 30 mg/kg to 14 mg/kg with the proposed treatment. The resulting phosphate concentrate showed relatively high concentrations in metal oxides: Σ MgO, Fe2O3, Al2O3 = 3.63 wt% and silica (9.81 wt%) so that it did not meet the merchant grade specifications of a minimum P2O5 content of 30 wt% yet. Removal of these elements could be achieved using additional appropriate separation techniques.

Keywords: thermal beneﬁciation; calcination; low-grade phosphate rock; Sra Ouertane; Tunisia

1. Introduction Phosphate rock plays a critical economic role in Tunisia and other countries worldwide. The increasing demand for phosphate rock for fertilizer production and its importance in animal feedstocks, as well as food-grade phosphates and other industrial uses, further solidiﬁes the importance of the phosphate industry in these countries. The demand for phosphate rock is typically fulﬁlled through phosphate rock mining. The phosphate industry mined nearly 250 million tons of phosphate rock in 2018 [1]. In Tunisia, the phosphate rock industry is of considerable importance to the country’s economy. Since the discovery of Tunisian phosphate deposits in the 19th century, phosphate production is controlled and operated by the Gafsa Phosphate Company (CPG, Compagnie des Phosphates de Gafsa, Gafsa, Tunisia). Tunisia has three major phosphate rock deposits: the Northern Basin, the Eastern Basin, and the Gafsa Basin. Figure1 provides a brief overview of the major phosphate rock deposits in Tunisia.

Minerals 2020, 10, 937; doi:10.3390/min10110937 www.mdpi.com/journal/minerals Minerals 2020, 10, 937 2 of 13 Minerals 2020, 10 2 of 13

Figure 1. MajorMajor phosphate phosphate rock rock deposits deposits in in Tunisia.

Currently, more than 90% of the phosphate rock produced in Tunisia is mined from the Gafsa Basin. Tunisia possessespossesses other other large large phosphate phosphate rock rock deposits deposits with with reserves reserves matching matching those those of the of Gafsa the GafsaBasin inBasin quantity in quantity but not but in qualitynot in quality with regards with regards to the phosphorous to the phosphorous (P) content (P) [2content]. These [2] deposits. These depositsare in Sra are Ouertane, in Sra Ouertane which is, which part of is thepart Northern of the Northern Basin. Basin. Up to date,Up to thesedate, reservesthese reserves have nothave been not beenexploited exploited [3]. With [3]. theWith increased the increased interest interest in mining in mining of phosphate of phosphate rock and rock other and monetarily other monetarily valuable valuableaccompanying accompanying elements, elements, such as rare such earth as rare elements earth (REEs)elements and (REEs) uranium and [4 uranium–11], presently [4–11] considered, presently consideredlow P-grade low deposits P-grade are deposits becoming are increasingly becoming increasingly relevant. relevant. Table1 1 provides provides an an overview overview of theof the typical typical P 2O P52,O REEs,5, REEs and, and uranium uranium concentrations concentrations of some of some mine minesites insites the in Northern-, the Northern Eastern-,-, Eastern and-, Gafsaand Gafsa Basin Basin for which for which detailed detailed data data was was made m availableade available for thisfor thisstudy. study. The Sra The Ouertane Sra Ouertane low-grade low phosphate-grade phosphate rock deposit rock in northern deposit Tunisia in northern can be Tunisia characterized can be as characterizeda deposit with as relatively a deposit high with uranium relatively and high REE uranium content comparedand REE content to other compared deposits into Tunisiaother deposits [12–18]. in Tunisia [12–18]. Table 1. Potential P2O5, Uranium, and Rare Earth Element (REE) Concentrations in Some Tunisian

TablePhosphate 1. Potential Rock Deposits P2O5, U [ranium19,20]. , and Rare Earth Element (REE) Concentrations in Some Tunisian Phosphate Rock Deposits [19,20]. P P O (wt%) Uranium (mg/kg) REE (mg/kg) Basin Deposit 2 5 Min. Max. Avg. Min. Max. Avg. Min. Max. Avg. P2O5 (wt%) Uranium (mg/kg) ∑ REE (mg/kg) Basin SekernaDeposit 19.7 23.8 21.8 36.2 55.1 45.6 750 800 775 Northern Basin Min. Max. Avg. Min. Max. Avg. Min. Max. Avg. Sra Ourtane 21.9 26.1 24.0 73.9 112.2 93.6 401 690 546 Sekerna 19.7 23.8 21.8 36.2 55.1 45.6 750 800 775 EasternNorthern Basin Basin Jebel Jebs 27.9 29.5 28.7 40.8 46.7 44.1 935 1020 974 Sra Ourtane 21.9 26.1 24.0 73.9 112.2 93.6 401 690 546 Eastern Basin JellabiaJebel Jebs 16.427.9 30.329.5 26.028.7 19.940.8 53.746.7 44.1 32.7 935 155 1020 617 974 432 Kef Eddour 25.4 28.8 26.5 23.9 41.1 31.9 171 478 320 Gafsa Basin MetlaouiJellabia 24.116.4 28.330.3 26.426.0 20.919.9 36.253.7 32.7 29.9 155 175 617 550 432 326 NaguessKef Eddour 23.9 25.4 2928.8 27.226.5 25.423.9 44.241.1 31.9 33.6 171 204 478 508 320 345 Gafsa Basin Metlaoui 24.1 28.3 26.4 20.9 36.2 29.9 175 550 326 The relatively largeNaguess amount of23.9 impurities 29 found27.2 in the25.4Sra 44.2 Ouertane 33.6 deposit 204 make508 beneﬁciation345 using ﬂotation technically challenging [21–23]. Calcination is, therefore, considered here for P2O5 concentration.The relatively Calcination large amount can largely of impurities remove carbonates found in the and Sra organic Ouertane matter deposit from themake phosphate beneficiation rocks. usingOrganic flotation matter technicallycan lead to increasedchallenging use [21 of– sulfuric23]. Calcination acid for the is, therefore digestion, processconsidered that here consequently for P2O5 concentration.increases overall Calcination production can costs largely [24 –remove26]. In carbonates this basic technical and organic study, matter the economicsfrom the phosphate of such a rocks.calcination Organic system matter were can not leadconsidered to increased yet. The use location of sulfuric in Tunisia acid provides for the excellent digestion conditions process that for consequentlyconcentrated solarincreases power, overall and production we hope that costs phosphate [24–26]. rockIn this from basic Sra technical Ouertane study, could the ultimately economics be of such a calcination system were not considered yet. The location in Tunisia provides excellent conditions for concentrated solar power, and we hope that phosphate rock from Sra Ouertane could ultimately be heat treated using solar-powered calcination systems. Reliable cost estimations of the

Minerals 2020, 10, 937 3 of 13 Minerals 2020, 10 3 of 13 heattotal treated treatment using costs solar-powered with such calcinationsolar-powered systems. systems Reliable can becost provided estimations at ofa higher the total technology treatment costsreadiness with level. such solar-powered systems can be provided at a higher technology readiness level. Currently, approximately 10% of the phosphate rock processed worldwide is is calcined calcined [27] [27].. Further depletion of higher grade phosphate rock resources,resources, as well as technical innovations,innovations, such as solar-drivensolar-driven calcination o off phosphate rocks [28,29] [28,29],, may increase the percentage of phosphate rocks that will be calcinedcalcined inin thethe future.future. Previously thermal beneﬁciationbeneficiation of Tunisian phosphatephosphate rock withwith aluminum silicates [30] [30] and ammonium ammonium sulfate sulfate [[31]31] hashas been been studied. studied. Furthermore, Furthermore, calcination calcination of Gafsa-Metloui:Gafsa-Metloui: Jebel Oum El Khacheb deposit [3 [322]] and Ras-DharaRas-Dhara deposit [[33]33],, as well as M’dhilla [[34]34] phosphate rock, rock has, has been been investigated, investigated and, and Jaballi Jaballi et al. [ et35 ] al. analyzed [35] analyzed the thermal the behavior thermal ofbehavior phosphate of rocksphosphate from rocks the Ypresian from the phosphate Ypresian deposit. phosphate Besides, deposit. Daik Besides, et al. [36 Daik] and et Dabbebial. [36] and et al.Dabbebi [37] looked et al. into[37] thelooked calcination into the of phosphate calcination washing of phosphate waste from washing the Gafsa-Metloui waste from deposit. the Gafsa The-Metloui objective deposit. of this work The wasobjective to examine of this thework technical was to examine feasibility the of technical a thermal feasib beneﬁciationility of a thermal path for beneficiation Sra Ouertane path low-grade for Sra phosphateOuertane low rock-grade using phosphate calcination, rock quenching, using calcination, and disliming. quenching, and disliming.

2. Materials and Methods The Sra Ouertane deposit consists of three phosphate rock layers (A1, A2,A2, and C). Stratigraphic informationinformation of the Sra Ouertane deposit is provided in Figure2 2.. SamplesSamples fromfrom layerslayers A1,A1, A2,A2, andand CC were made available for this study. The usual P2O5 concentrationconcentration of of the the different diﬀerent layers layers is is as as follows follows:: A1 (15–21(15–21 wt% P22O5),), A2 A2 (8 (8–13–13 wt% wt% P P22OO55),), and and C C (10 (10–13–13 wt% wt% P P2O2O5)5. ).

Lithology Log Layer Thickness (m) P2O5 content (wt%)

Ostreas shales, coquinas F 3

Nummulitic limestone E 40 1.6

Alternance marly and limestone D 15 4

Coarse phosphate grain C 10 13

Maris B 1 4 Fine calcareous phosphate A2 12 10

Fine-grained phosphate A1 21 18

Maris (El Haria formation) M 100

White limestone (Abiod formation) Ab3 120

Figure 2. StratigraphicStratigraphic information information of the Sra Ouertane deposit in Northern Tunisia [[16].16].

Samples from layers A1, A2 A2,, and C of the Sra Ouertane phosphate rock deposit were used for the thermogravimetric analysis (TGA).(TGA). Later,Later, a detailed sample analysis was performed with samples from the layer A1. The samples were provided by the National Office of Mines (ONM) of Tunisia and homogenized following the French regulation for the preparation of soil samples (NF X31-101) [38].

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Mineralsfrom the 2020 layer, 10 A1. The samples were provided by the National Office of Mines (ONM) of Tunisia4 of and 13 homogenized following the French regulation for the preparation of soil samples (NF X31-101) [38]. The received samples were washed with water, dried at 110 ◦°CC overnight,overnight, and sieved to give a size fraction less than 250 μmµm using ASTM (American Society for Testing andand Materials)Materials) sieves.sieves. Elemental analysis analysis of of the the samples samples from from layer layer A1 was A1 conducted was conducted using inductively-coupled using inductively-coupled plasma plasmamass spectrometry mass spectrometry (ICP-MS, (ICP Perkin-MS, Elmer, Perkin Woodbridge, Elmer, Woo ON,dbridge Canada), ON Canada to determine) to determine the uranium, the uranium,thorium, andthorium REE, contentand REE and content inductively-coupled and inductively plasma-coupled atomic plasma emission atomic spectroscopyemission spectroscopy (ICP-AES, (ICPHORIBA-AES, Jobin HORIBA Yvon, Jobin France) Yvon, for all France other) constituents. for all other All constituents. chemical analyses All chemical were conducted analyses were with conductedexperimental with errors experimental below 5%. error Thes particlebelow 5%. size The distribution particle size was distribution determined was using determined laser diffraction using lasertechniques diffraction on a techniques Malvern Master on a Malvern Sizer S (Spectris Master Sizer plc, Egham,S (Spectris UK) plc in, Egham the wet, UK mode.) in the X-ray wet powder mode. Xdi-ffrayraction powder (XRD) diffraction analysis was (XRD) performed analysis using was a “Philips performed MPD1880-PW1710” using a “Philips diff ractometer MPD1880-PW1710” (Philipps, diffrEindhoven,actometer The (Philipps Netherlands), Eindhoven that uses, NL CuK) thatα radiation uses CuKα (λ = radiation0.15418 nm) (λ = in 0.15418 the 2–80 nm)◦ interval in the with2–80° a intervalstep size withof 0.02 a◦ and step counting size of time 0.02°of and 20 s/ countingstep at room time temperature. of 20 s/step The at identification room temperature. of the phases The identificationwas determined of the using phases the ICDD-PDF2004 was determined standard using the database ICDD-PDF2004 [39]. standard database [39]. Of particular relevancerelevance for for this this study study was was the the required required calcination calcination temperature temperature and a residencend residence time timeof the of phosphate the phosphate rock thatrock should that should be processed be processed so that so the that suitability the suitability of solar-driven of solar-driven systems systems can be candetermined. be determined. Depending Depending on the givenon the phosphategiven phosphate rock, calcination rock, calcination temperatures temperatures can vary can from vary as from high as high 900 ◦ asC to900 as °C low to as 500low ◦asC[ 50040, 41°C]. [40,41] In this. In first this attempt, first attempt, the thermal the thermal analysis analysis of the of samples the samples from fromlayer layer A1 (9.66 A1 mg(9.66 sample mg sample weight), weight), layer A2layer (16.75 A2 mg(16.75 sample mg sample weight), weight) and layer, and C layer (12.35 C mg (12.35 sample mg sampleweight) weight) (particle (particle distribution: distribution: 1 mm, 1 500 mm,µm 500 and μm, 250 andµm) 250 was μm) conducted. was conducted. The characteristic The characteristic TGA TGAcurves curves for layers for A1,layers A2, A1, and A2 C of, and the SraC of Ouertane the Sra Ouertane phosphate phosphate rock are shown rock inare Figure shown3. Experimentsin Figure 3. Experimentswere conducted were at conducted a temperature at a oftemperature up to 800 ◦ C.of up to 800 °C.

Figure 3. The characteristic thermogravimetric analysis (TGA) curves of the samples from layers A1, FigureA2, and 3. C The from characteristic Sra Ouertane thermogravimetric phosphate rock. analysis (TGA) curves of the samples from layers A1, A2, and C from Sra Ouertane phosphate rock. Three peaks, typical of the calcination of phosphate rock, could be identified. The first mass loss at approximatelyThree peaks, 80typical◦C corresponded of the calcination to the of removalphosphate of adsorbedrock, could water. be identified. A second The weak first peak mass could loss atbe approximately identified at approximately 80 °C corresponded 550 ◦C. to Elgharbi the removal et al. of [33 adsorbed] attributed water. this A peak second to theweak simultaneous peak could beelimination identified of at chemical approximately water in 550 the rock°C. Elgharbi structure et and al. oxidation[33] attributed of organic this matter.peak to As the most simultaneous phosphate eliminationrocks contain of organic chemical matter water rich in in the sulfur, rock sulfur structure gas was and released. oxidation The of most organic pronounced matter. peak As most was phosphate rocks contain organic matter rich in sulfur, sulfur gas was released. The most pronounced found at 650–750 ◦C, where carbonate dissociated, and CO2 was released. Based on the TGA results, 2 peakthe calcination was found temperature at 650–750 °C was, where set to carbonate 720 ◦C. The dissociated weight loss, and at CO 720 ◦wasC was released. 11.88% Based (layer on A1), the 9.72%TGA results(layer A2),, the and calcination 7.90% (layer temperature C), respectively. was set Best to 720 practice °C. The experience weight withloss at conventional 720 °C was kilns 11.88% in Tunisia (layer A1),recommends 9.72% (layer a calcination A2), and time7.90% of (layer 3 h. Thus,C), respectively. calcination Best experiments practice experience were performed with conventional at 720 ◦C for kilns3 h. It in is Tunisia worth notingrecommends that the a calcination thermal dissociation time of 3 h of. Thus, carbonates calcination is an experiments endothermic were reaction performed with a atsignificant 720 °C for energy 3 h. requirement:It is worth noting that the thermal dissociation of carbonates is an endothermic reaction with a significant energy requirement: CaCO3 + Heat CaO + CO2 ∆H = 965 kcal/kg CO2 or 42.9 kcal/mol CO2 (1) CaCO + Heat→ → CaO + CO ΔH = 965 kcal/kg CO2 or 42.9 kcal/mol CO2 (1) The calcine was further quenched in 5% ammonium nitrate solution with 25% solid content. The calcine was kept in the ammonium nitrate solution for 2 h. In a subsequent disliming process, particles of less than 60 μm were removed using a hydro-cyclone. The preliminary flow sheet of the alternative beneficiation process for Sra Ouertane phosphate rock is provided in Figure 4.

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The calcine was further quenched in 5% ammonium nitrate solution with 25% solid content. The calcine was kept in the ammonium nitrate solution for 2 h. In a subsequent disliming process, particles of less than 60 µm were removed using a hydro-cyclone. The preliminary ﬂow sheet of the Mineralsalternative 2020, beneﬁciation10 process for Sra Ouertane phosphate rock is provided in Figure4. 5 of 13

Figure 4. 4. SimplifiedSimplified process process flow flow diagram diagram,, describing describing thermal thermal beneficiation beneficiation of the of Sra the Ouertane Sra Ouertane low- gradelow-grade phosphate phosphate rock rockby calcination, by calcination, quenching quenching,, and anddisliming. disliming. 3. Results and Discussion 3. Results and Discussion The elemental components relevant for further fertilizer processing of samples from layer A1 are The elemental components relevant for further fertilizer processing of samples from layer A1 provided in Table2. The P 2O5 content of the untreated phosphate rock was successfully increased are provided in Table 2. The P2O5 content of the untreated phosphate rock was successfully increased from 20.01 wt% to 24.24 wt% after calcination. Further treatment consisting of quenching and from 20.01 wt% to 24.24 wt% after calcination. Further treatment consisting of quenching and disliming increased the P2O5 content to 27.24 wt%. Merchant grade phosphate rock should show disliming increased the P2O5 content to 27.24 wt%. Merchant grade phosphate rock should show P2O5 P2O5 concentrations of at least 30 wt%. Besides, it is important to mention that the atomic ratio concentrations of at least 30 wt%. Besides, it is important to mention that the atomic ratio CaO/P2O5 CaO/P O = 1.94 of the untreated phosphate rock was still higher than the stoichiometric value of = 1.94 of2 the5 untreated phosphate rock was still higher than the stoichiometric value of francolite with francolite with very low carbonate substitution (CaO/P2O5 = 1.65). Besides, the metal element ratio very low carbonate substitution (CaO/P2O5 = 1.65). Besides, the metal element ratio (MER) of 0.18 = (MER) of 0.18 = (MgO + Fe2O3 + Al2O3)/P2O5 was just slightly below the maximum acceptable (MgO + Fe2O3 + Al2O3)/P2O5 was just slightly below the maximum acceptable value of 0.2. value of 0.2. The calcined and treated phosphate rock, on the other hand, could be associated with the fluorapatiteTable 2. phase.Elemental The Analysis CaO/P2O of5 Untreated,= 1.68 was Calcined,now very as close Well to as that Calcined of francolite. and Treated Besides Sra Ouertane, the MER of 0.13 was(Layer well A1) below Phosphate the acceptable Rock. value of 0.2. A loss of 39 wt% Al2O3 and 30 wt% MgO from untreated phosphate rock to calcined and treated Calcined and Treated phosphateComponent rock Untreatedcan be observed Phosphate in Rock Tab (wt%)le 2. This Calcined loss could Phosphate be associated Rock (wt%) with clays that were in a Phosphate Rock (wt%) fraction of fewer than 60 microns. It is known that Mg is correlated with CO32− apatite, and it could P2O5 20.01 24.24 27.24 2− thus beCaO estimated that a portion 42.93 of MgO was associated 47.01 with the departure of 45.65 CO3 during calcination.SO3 SO3 increased by1.88 18% in the calcined and treated 2.03 sample (Table 2). 2.25Francolite, the phosphoricMgO mineral of almost 1.29 all sedimentary phosphorites, 1.18 has a variable chemical composition 0.92 that Fe2O3 1.52 1.26 1.16 10 4 4 3 6 2–3 32− can beAl represented2O3 by (Ca, Mg,2.56 Sr, Na) (PO , SO , CO ) F 2.14. We hypothesized that a part 1.55 of the CO of francoliteSiO2 was substituted 3.08by SO42− during calcination in 12.4addition to the conventional 9.81 substitution CO2 14.78 1.95 1.95 between PO43− and CO32−. F 2.50 2.45 2.20

TableThe calcined 2. Elemental and A treatednalysis of phosphate Untreated, rock,Calcined on, theas W otherell as C hand,alcined could and T bereated associated Sra Ouertane with the (Layer A1) Phosphate Rock. ﬂuorapatite phase. The CaO/P2O5 = 1.68 was now very close to that of francolite. Besides, the MER of 0.13 was well belowUntreated the acceptable Phosphate value ofCalcined 0.2. Phosphate Calcined and Treated Phosphate Component A loss of 39 wt%R Alock2O (3wt%and) 30 wt% MgO fromRock untreated (wt%) phosphate rock toRock calcined (wt%) and treated phosphateP2O5 rock can be observed20.01 in Table2. This loss24.24 could be associated with clays27.24 that were in a CaO 42.93 47.01 2 45.65 fraction of fewer than 60 microns. It is known that Mg is correlated with CO3 − apatite, and it could SO3 1.88 2.03 2 2.25 thus be estimated that a portion of MgO was associated with the departure of CO3 − during calcination. MgO 1.29 1.18 0.92 SO3 increased by 18% in the calcined and treated sample (Table2). Francolite, the phosphoric mineral Fe2O3 1.52 1.26 1.16 of almost all sedimentary phosphorites, has a variable chemical composition that can be represented by Al2O3 2.56 2.14 1.55 2 (Ca, Mg,SiO2 Sr, Na)10 (PO4, SO3.084, CO3)6 F2–3. We hypothesized12.4 that a part of the CO3 9.81− of francolite was CO2 14.78 1.95 1.95 F 2.50 2.45 2.20

Relevant trace element concentrations are provided in Table 3. During the beneficiation process, the cadmium content was significantly reduced from 30 mg/kg to 14 mg/kg. Many countries have legal limits for the cadmium content in fertilizers [42,43] so that reducing the cadmium content early on in the process is a positive side effect of heat treatment of phosphate rock and can be pursued deliberately by controlling the treatment temperature and the oxygen content within the reactor [10]. At

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2 3 substituted by SO4 − during calcination in addition to the conventional substitution between PO4 − 2 and CO3 −. Relevant trace element concentrations are provided in Table3. During the beneficiation process, the cadmium content was significantly reduced from 30 mg/kg to 14 mg/kg. Many countries have legal limits for the cadmium content in fertilizers [42,43] so that reducing the cadmium content early on in the process is a positive side effect of heat treatment of phosphate rock and can be pursued deliberately by controlling the treatment temperature and the oxygen content within the reactor [10]. At temperatures >750 ◦C, cadmium can be transferred to the gaseous phase and removed through the air pollution control system. Depending on the process parameters and the targeted cadmium removal rate, up to 95% cadmium can be removed from most sedimentary phosphate rock types. Electrostatic precipitation, wet scrubbing, and/or baghouse filters are usually installed for gas cleaning.

Table 3. Relevant Trace Elements in Untreated, Calcined, as Well as Calcined and Treated Sra Ouertane (Layer A1) Phosphate Rock.

Element Untreated Phosphate Rock (mg/kg) Calcined Phosphate Rock (mg/kg) Calcined and Treated Phosphate Rock (mg/kg) Cd 30 26 14 Cr 160 93 74 Mn 67 48 42 V 66 45 43 Zn 214 207 142 U 60 68 79 Th 25 29 34

The uranium and thorium contents were increased since volatile constituents in the phosphate rock were driven off. The uranium content at Sra Ouertane reported in the literature (see Table1) could reach higher concentrations than the uranium concentration that was ultimately measured in the samples here. The uranium concentration measured here would most likely be too low to enable commercially viable uranium recovery [44–47]. Relevant REE concentrations of the Sra Ouertane low-grade phosphate rock are provided in Table4. The Sra Ouertane phosphate rock deposit showed relatively high REE concentrations by Tunisian [48] and international [9,49] standards, and recovering REEs might be attractive when developing the ore [8,50,51]. During the beneficiation process, all REEs were concentrated as volatile constituents were driven off.

Table 4. Relevant Rare Earth Elements (REEs) in Untreated, Calcined, as Well as Calcined and Treated Sra Ouertane (Layer A1) Phosphate Rock.

Untreated Phosphate Rock Calcined Phosphate Rock Calcined and Treated Crustal Abundance Element (mg/kg) (mg/kg) Phosphate Rock (mg/kg) (mg/kg) [52] Ce 220 240 285 67 La 161 175 192 39 Nd 67 76 89 42 Pr 29 37 49 9 Sm 28 34 47 7 Y 66 76 88 33

The particle size distribution was another important parameter for phosphate rock calcination. The particle size of the Sra Ouertane (layer A1) phosphate rock is provided in Figure5 after sieving with 250 µm ASTM sieves. The particle size distribution of the untreated (Figure5), calcined (Figure6), as well as the calcined and treated (Figure7) phosphate rock, was provided. Calcination slightly increased particle diameters. After calcination, less than 25 vol% of the phosphate rock particles had an equivalent particle diameter below 10 µm. The characteristic peak of particles at 100 µm was present in all stages. It is important to emphasize that the lab-scale experiments conducted here might result in very different particle size distributions than the ones obtained from industrial (rotary kiln) calcination that might, for instance, impose larger friction on the material during processing. The effect of disliming or removal of particles with an equivalent diameter of 60 µm in a hydro-cyclone could best be observed by the difference in particle distribution after calcination and after disliming. Minerals 2020, 10, 937 7 of 13

The disliming step conducted on the lab-scale here successfully removed the largest share of small Minerals 2020, 10 7 of 13 particlesMinerals 2020 that, 10were unwanted in further wet acid processing. 7 of 13

Figure 5. Particle size distribution in vol% of untreated Sra Ouertane (layer A1) phosphate rock. Figure 5. Particle size distribution in vol% of untreated Sra Ouertane (layer A1) phosphate rock.

Figure 6. Particle size distribution in vol% of the calcined Sra Ouertane (layer A1) phosphate rock. Figure 6. Particle size distribution in vol% of the calcined Sra Ouertane (layer A1) phosphate rock.

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Figure 7. ParticleParticle size size distribution distribution in in vol% vol% of of the the calcined calcined and and treated treated Sra Sra Ouertane (layer A1) Figure 7. Particle size distribution in vol% of the calcined and treated Sra Ouertane (layer A1) phosphate rock. phosphate rock. XRD analysis was conducted to better understand the effect eﬀect of the concentration procedure on XRD analysis was conducted to better understand the effect of the concentration procedure on the phosphate rock. The diﬀractogram of the XRD analysis was characterized by a number of typical thethe phosphate phosphate rock. rock. The The diffractogram diffractogram of of the the XRDXRD analysis waswas characterizedcharacterized by by a anumber number of of typical typical peaks found in other natural sedimentary phosphate rocks (see for instance [32,33,53]) and revealed the peakspeaks found found in inother other natural natural sedimentary sedimentary phosphatephosphate rocks (see(see forfor instance instance [32,33,53] [32,33,53]) and) and revealed revealed presence of the following phases: Fluorapatite (Ca10(PO10 4)6F42),6 quartz2 (SiO2), and2 carbonates, which are thethe presence presence of of the the following following phases: phases: Fluorapatite Fluorapatite Ca10(PO4))6FF2),), quartz quartz (SiO (SiO2),) and, and carbonates carbonates, which, which mostly present in the form of calcite (CaCO3). Figure8 shows the XRD pattern of the untreated Sra areare mostly mostly present present in in the the form form of of calcite calcite (CaCO(CaCO33).). Figure 88 showsshows the the XRD XRD pattern pattern of of the the untreated untreated SraOuertaneSra Ouertane Ouertane phosphate phosphate phosphate rock. rock. rock.

FigureFigure 8. 8.XRD XRD pattern pattern of of the the untreateduntreated Sra Ouertane Ouertane (layer (layer A1) A1) phosphate phosphate rock. rock. Figure 8. XRD pattern of the untreated Sra Ouertane (layer A1) phosphate rock. CalcinationCalcination reduced reduced the the CO CO22 contentcontent of thethe Sra Ouertane ore ore significantly. significantly. Figure Figure 9 9shows shows the the diffdiffractogramractogramCalcination of of thereduced the XRD XRD analysisthe analysis CO2 of contentof the the SraSra of OuertaneOuertane the Sra phosphateOuertane rockore rock significantly.after after calcination calcination Figure and and without 9 without shows the the diffractogramcharacteristiccharacteristic calciteof calcite the XRD (CaCO (CaCO analysis3)3) peak peak of thatthat the canSracan Ouertane bebe observedobserved phosphate in in Figure Figure rock 8.8. Besides Besides, after calcination, the the CaO CaO peaks peaksand withoutresulting resulting the characteristicfromfrom carbonate carbonate calcite decomposition decomposition (CaCO3) peak couldcould that be can well well-identified be-identif observedied in inin the theFigure calcined calcined 8. Besides phosphate phosphate, the rock CaO rock (Figure peaks (Figure 9). resulting 9). from carbonate decomposition could be well-identified in the calcined phosphate rock (Figure 9).

MineralsMinerals 20202020,, 1010 , 937 99 of of 13 13 Minerals 2020, 10 9 of 13

FigureFigureFigure 9. 9. 9. XRDXRD XRD pattern patternpattern of ofof the the calcinedcalcined Sra Sra Ouertane Ouertane (layer (layer (layer A1) A1) A1) phosphate phosphate phosphate rock. rock. rock.

DuringDuringDuring disliming, disliming disliming, smaller,smaller smaller particles particles particles< 60 <60 <60µm μm μm were were removed removedremoved using using using a hydro a a hydro hydro cyclone. cyclone. cyclone. The XRD The The XRD pattern XRD patternofpattern the calcined of ofthe the calcined andcalcined treated and and Sratreated treated Ouertane Sra Sra Ou Ou phosphateertaneertane phosphate rock is provided rockrock is is provided provided in Figure in in 10Figure Figure. The 10. absence10. The The absence absence of peaks ofrelated of peaks peaks to related CaCO related 3 to and to CaCO CaCO CaO3 3 wasand and noticeable. CaO CaO waswas noticeable.noticeable. The only remaining The The only only remainingmineral remaining species mineral mineral were species species ﬂuorapatite, were were fluorapatitequartz,fluorapatite and, silicates.quartz, quartz, and, and silicates. silicates.

Figure 10. XRD pattern of the calcined and treated Sra Ouertane (layer A1) phosphate rock. FigureFigure 10. 10. XRDXRD pattern pattern of of the the calcined calcined and and treated treated Sra Sra Ouertane Ouertane (la (layeryer A1) A1) phosphate phosphate rock. rock. All three XRD patterns are depicted together in Figure 11. Shown is the diffractogram of the untreatedAllAll three three phosphate XRD XRD patterns patterns rock (SRT are are- DP),depicted the calcined together phosphate in in Figure rock 11.11. (SRTShown Shown-CAL) is is ,the theas welldiffractogram diff ractogramas the calcined of of the the untreateduntreatedand treated phosphate phosphate (SRT-CALT) rock rock (SRT (SRT-DP), Sra - OuertaneDP), the the calcined calcined phosphate phosphate phosphate rock (bottom rock rock (SRT (SRT-CAL), to top).-CAL) The, as as comparison well well as the the ofcalcined calcined the andanddifferent treated XRD (SRT-CALT) (SRT patterns-CALT) Srashowed Sra Ouertane Ouertane a significant phosphate phosp reductionhate rock rock (bottom of calcite (bottom to (CaCO top). to The3 top).) again comparison The and comparison thus ofproved the di that offferent the the process, on a laboratory scale, significantly increased the P2O5 content. differentXRD patterns XRD showedpatterns a showed significant a significant reduction ofreduction calcite (CaCO of calcite3) again (CaCO and3) thusagain proved and thus that proved the process, that theon aprocess, laboratory on a scale, laboratory significantly scale, significantly increased the increased P2O5 content. the P2O5 content.

Minerals 2020, 10, 937 10 of 13 Minerals 2020, 10 10 of 13

FigureFigure 11. 11. XRD patterns of the calcined and treated (SRT-CALT),(SRT-CALT), calcined calcined (SRT-CAL), (SRT-CAL), asas well well asas untreateduntreated (SRT-DP)(SRT-DP) SraSra Ouertane Ouertane (layer (layer A1) A1) phosphate phosphate rock. rock. 4. Conclusions 4. Conclusions In total, the P2O5 content could be increased from some 20.01 wt% (untreated ore) to 27.24 wt% In total, the P2O5 content could be increased from some 20.01 wt% (untreated ore) to 27.24 wt% (calcined and treated ore). To reach merchant grade P2O5 concentrations of at least 30 wt%, further (calcined and treated ore). To reach merchant grade P2O5 concentrations of at least 30 wt%, further processing steps would be required. If, for instance, the relatively high silica content of 9.81 wt% could processing steps would be required. If, for instance, the relatively high silica content of 9.81 wt% be reduced to some 2.5 wt%, phosphate concentrate with 30.2 wt% P2O5 would result as a final product. could be reduced to some 2.5 wt%, phosphate concentrate with 30.2 wt% P2O5 would result as a final It is important to emphasize that even though additional processing steps for further ore concentration product. It is important to emphasize that even though additional processing steps for further ore are required, the first steps described here are valuable and may help promote the use of Sra Ouertane concentration are required, the first steps described here are valuable and may help promote the use low-grade phosphate rock ore in a sustainable manner. of Sra Ouertane low-grade phosphate rock ore in a sustainable manner. Author Contributions: Conceptualization, N.A. and N.H.; methodology, N.A.; writing—original draft preparation, N.H.Author and Contributions: N.A.; writing—review Conceptualization, and editing, N all.A authors.. and N All.H.; authors methodology, have read N and.A.; agreed writing to— theoriginal published draft versionpreparation, of the N manuscript..H. and N.A.; writing—review and editing, all authors. All authors have read and agreed to the Funding:publishedThis version project of the received manuscript. funding from the IAEA in Vienna as part of CRP: T11006 [54]. Acknowledgments:Funding: This projectThe received authors funding are grateful from the for IAEA funding in Vienna from as the part IAEA of CRP: that T11006 made this[54]. research possible. The views expressed may have not been endorsed by the sponsoring agency. Any remaining errors, omissions, orAcknowledgments: inconsistencies are The the authors’authors are alone. grateful for funding from the IAEA that made this research possible. The views expressed may have not been endorsed by the sponsoring agency. Any remaining errors, omissions, or Conflicts of Interest: The authors declare no conflict of interest. inconsistencies are the authors’ alone.

ReferencesConflicts of Interest: The authors declare no conflict of interest.

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