Characterization of Kef Shfeir phosphate sludge (Gafsa, Tunisia) and optimization of its dewatering

Journal of Environmental Management 254 (2020) 109801

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Research article

Characterization of Kef Shfeir phosphate sludge (Gafsa, Tunisia) and optimization of its dewatering M. Ettoumi a, M. Jouini b, C.M. Neculita b, *, S. Bouhlel a, L. Coudert b, I. Haouech c, M. Benzaazoua b, d a

Mineralogy and Geochemistry Research Group, Department of Geology, Faculty of Sciences of Tunis, University of Tunis El Manar, Tunis, Tunisia Research Institute on Mines and Environment (RIME), University of Qu�ebec in Abitibi-T�emiscamingue (UQAT), Rouyn-Noranda, Qu�ebec, Canada Environment Department, Gafsa Phosphate Company, Gafsa, Tunisia d Mohammed VI Polytechnic University (UM6P), Benguerir, Morocco b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Phosphate sludge Flocculant Sludge settling Thickening Water recovery

Water separation and recovery through thickening require adapted flocculants and densification processes. This study aimed to maximize water recovery from phosphate sludge (PS) at Kef Shfeir mine operation, Gafsa Phosphate Company, Tunisia. Representative samples of PS, PS treated with flocculant (F-PS), raw water, and recycled water were collected on the mine site. Solid samples (PS and F-PS) were characterized physically, chemically and mineralogically. To maximize water recovery, thickening tests were performed on the PS using different flocculants to optimize flocculant concentration, the agitation speed and the settling time. Results showed that PS had positive surface charge since its paste pH (7.3) was lower than pHPZC (8.0), whereas the tested flocculant (Slim Floc used by the company) showed negative surface charge. Solid samples contained coarse medium and fine particles of carbonates, silicates and residual hydroxyapatite. The cumulative fractions þ32 μm of PS contained a promising residual potential of fluorapatite (up to 39.2%). Water recovery was about 58.1%, when the anionic Slim Floc was used, for a consumption rate of 1200 g/t of dry solids. Best efficiency (84%) of water recovery was obtained with the anionic flocculant E24 for a consumption rate of 360 g/t of dry solids, which is 3 times lower than actual flocculant consumption.

1. Introduction Phosphorus (P), organic and/or mineral, is an essential component of all living cells. P is mainly used in the production of phosphoric acid and as fertilizer in agriculture (Rtib, 2013). Geologically, there are two types of phosphate ores: 1) apatitic, of magmatic and metamorphic origin, and 2) phosphoritic, continental and maritime (Rtib, 2013; Hiatt et al., 2015). Continental phosphorites are biogenic phosphate deposits, partly resulting from the interaction of bird excrements, rich in P, discharged over thousands of years (Rtib, 2013). Maritime phosphorites, which result from the long-period active sedimentation of phosphate in the sea basins, are known as sedimentary phosphates, rocks that are weakly consolidated and easily friable. The exploitation of sedimentary P is a worldwide problem, throughout Africa and elsewhere (Asia, Oceania), especially because the apatite beneficiation is highly water intensive. Nowadays, the sedi­ mentary P deposits account for more than 80% of the P world

production (Kawatra and Carlson, 2013), with at least half proven re­ serves in North Africa (Morocco and Tunisia), and other large producers such as China, Russia, and USA (Hocking, 2005; Walan et al., 2014; Samreen and Kausar, 2019). As a leading example, in Tunisia, sedimentary P start from the SouthWest Gafsa mountain range, crossing Algeria and sub-Saharian Africa to settle at the front of the Atlantic Ocean in Morocco (Brahmi et al., 2013). The production decreased to reach 2.8 million tons in 2015 (USGS, 2018) with P2O5 grade of the deposit about 29.3% (Reta et al., 2018). From the north to the south of Tunisia, there are three phosphate basins. The first basin, in the north, is an unreclaimed mine site that has been exploited during the last decade. The second one is the Maknessy’s basin, which is now in project for a future exploitation. Finally, the third basin is currently an active operating mine, located in the governorate of Gafsa. The extraction of phosphate ores in this last deposit feeds the concentrator that allows the processing step in the Gafsa Phosphate Company (GPC) located nearby the mine site.

* Corresponding author. RIME-UQAT, 445 Boul. de l’Universit�e, Rouyn-Noranda, QC, J9X5E4, Canada. E-mail address: [email protected] (C.M. Neculita). https://doi.org/10.1016/j.jenvman.2019.109801 Received 3 June 2019; Received in revised form 16 October 2019; Accepted 28 October 2019 Available online 13 November 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.

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Journal of Environmental Management 254 (2020) 109801

Beneficiation of phosphates from sedimentary deposits is done through wet processing. GPC allows the production of 29.5% of com­ mercial phosphate (Rtib, 2013). The treatment process is water intensive consumer (1.5 m3 per ton of phosphate produced) and generates large amounts of low-density sludge (200 kg per ton of phosphate produced) at the end-of-pipe of the processing plant. Considered as waste, this phosphate sludge (PS) contains important quantities of water (120 kg per ton of produced sludge). In the period 1979–2000, more than 1.4 � 107 t of PS was generated and deposited in Kef Shfeir location. The PS discharged from the outlet of laundries is treated by flocculation-decantation using thickeners for water recovery and reuse in phosphate ores washing. The sludge is discharged directly through a channel, built by the GPC, to alleviate it to the main pond located 9 km far from the mine site. Uncontrolled discharges, inappropriate slopes and differences of PS density lead to frequent accidental spills out of the channel tracing at certain levels. The presence of l’Erg plant (Fig. S1, Supplementary Material) in this zone entails the obstruction of the flow to the dyke, leading to sludge inappropriate dispersion and to important water losses into the environment. In the processing plant of GPC, the extraction of phosphate ore via wet processing is one of the most important consumers of water in Tunisia. Its consumption was estimated around 16 Mm3, in 2012 (Chraiti et al., 2016). In southern Tunisia, wastewater recovery from the produced low-density sludge is lasting important because of the nature of the climate (arid and semi-arid conditions) as well as the restricted resources of tap water. Noteworthy, the consumption of deep and groundwater water in Tunisia increased from about 350 Mm3 in 1980 to nearly 740 Mm3 in 1995, an increase of more than 100% within a 15-year-period (Genivar, 2001). Therefore, wastewater recovery and reuse from the extraction of phosphate is responsibly considered by the GPC to prevent the exhaustion of underground resources. Efficient management of the recovered wastewater through the entire extraction and beneficiation chain is a lasting interesting because of the lack of water resources in this region. The PS discharges from GPC are very fine (<71 μm) and considered as suspended matter. The suspended matter is constituted of fine parti­ cles that could occur naturally in surface waters. Indeed, during phos­ phate’s treatment, discharges are highly loaded with suspended matter. This leads to PS accumulation and consolidation in the basement of the channels before reaching the pond. Therefore, treatment of the dis­ charges is lasting compulsory to produce more densified sludge during phosphate production. Treatment of water contaminated by suspended matter enables its further reuse. Suspended solids treatment includes wetlands (Manios et al., 2003), coagulation (Song et al., 2003) and electro-coagulation (Bukhari, 2008). To enhance water recovery, treatment of suspended solids using flocculation can also be used as process widely used in sludge thickening and dewatering (Du et al., 2017). Flocculation con­ sists of adding a flocculant to the wastewater to improve particles’ ag­ gregation. Flocculant is used to break colloidal stability and artificially increase of the size of particles before settling. It was also previously reported that water recovery from PS in the treatment plant of Yous­ soufia (Morocco) did not exceed 50% using 500 g/tds (tons of dry sludge) of flocculant (0.76 g/L) at a speed of 120 rpm (Khoubane, 2015). In this context, the present study aims to thoroughly characterize the sludge from phosphate production plant of GPC, to test and optimize various flocculant dosages to maximize water recovery and reuse during the treatment process.

governorate of Gafsa is in the south-western region of the south subSaharan. The Gafsa phosphate basin covers about 5000 to 6000 km2. The Gafsa region and the mine site locations, including their average P extraction in 2008, are showed in Fig. 1B and C. This Gafsa area includes the town of Metlaoui, in the center, the towns of Redyef and Moulares (called also Om El Araies), in the north-west, the town of Gafsa, in the north-east and the town of Mdhilla, in the east. This region is drained by the valleys of Thelja, Sebseb, El Melah and El Gouifla and their respec­ tive catchment areas up to the level of Chott. 2.2. Sample collection and preservation Samples were collected from Kef Shfeir mine site, at the end-pipe of ore processing in GPC. The Kef Shfeir tailings facility deposit is located about 12 km north-east of Metlaoui and about 35 km west of Gafsa. Five samples (3 solids ¼ sludge and 2 liquids ¼ process waters) including raw phosphate ore feed (RP), PS, phosphate sludge treated with an anionic flocculant (F-PS), raw water (RAW) and recycled water (RYW), were collected from different locations. The RP, PS and RYW were sampled in the processing plant at GPC, whereas F-PS was sampled (five subsamples) from the channel that carries sludge from the GPC to the tail­ ings pond. The RAW was sampled from the tap water that is used in GPC to wash phosphate ore. The flow sheet of the phosphate’s treatment process in the GPC is presented in Fig. 2. Noteworthy, boxes in bleu indicate the locations of the samples collected in this study. The RP sample was obtained by size classification of phosphate ore using a 120 mm-sieve to separate fine particles from the coarse ones (>120 mm) that are disposed of in the waste rock piles. For the phos­ phate (apatite) concentration process, RP is then wet-sieved and the particles between 10 and 40 mm are mixed with water to form the phosphate’s pulp (40 wt %), which is then conveyed to a second sieve (2 mm). The particles less than 2 mm are then separated using a hydrocyclone to get two fractions: > 71 μm and <71 μm. The fraction higher than 71 μm, that represents the valuable fraction or the enriched phosphate, undergoes filtration and drying processes. Fraction <71 μm is then allowed to settle through flocculation/decantation by mixing with an anionic flocculant to get the washed F-PS sample. After decan­ tation, the F-PS is in then disposed and the RYW is recycled in the process. The F-PS is carried through a natural channel to a tailings storage facility where it settles. Because of the long distance of the channel (about 9 km), F-PS sub-samples were collected from five different points (Fig. 3). Then, they were homogenized with the Slurry Sample Splitter to get a uniform F-PS sample. The collected solid samples were oven-dried for 24 h at 105 � C and then stored in sealed containers. The RYW sample was collected after the forced settling process of the PS by Slim floc. Liquid samples (RAW and RYW) were filtered (0.45 μm), acidified with concentrated nitric acid (HNO3), at pH lower than 2 and stored at 4 � C to serve for metal content analysis. However, another quantity of RYW was stored at 4 � C without any filtration or preservation that was served for solid suspended ma­ terial determination. 2.3. Physicochemical and mineralogical characterization 2.3.1. Liquid samples (process water) The RAW and RYW samples were characterized by measuring the pH, oxidation-reduction potential (ORP), electrical conductivity (EC), turbidity and total metal concentrations. The pH was measured using the electrode Orion GD9156BNWP double junction coupled to a multi­ meter (accuracy: � 0.01, model: VWR SympHony SB90M5). The ORP was determined with a potentiometer Sension1 ORP HACH 51939-00 coupled with an internal Pt/Ag/AgCl electrode (accuracy: � 0.01 mV). The EC was measured using a portable Oakton Acorn CON 6 (accuracy: � 0.001 mS/cm). The turbidity was measured using the Kit Hach DR890. This test measures the clearness of the solution, which results from dispersion and absorption of light by the suspended particulate matter

2. Materials and methods 2.1. Study mine site The mining basin corresponding to the present study is in the governorate of Gafsa, about 350 km south of Tunisia, 15 km north of the town of Tozeur and close to the Algerian border (Fig. 1A). The 2

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Journal of Environmental Management 254 (2020) 109801

Fig. 1. Location of (A) the governorate of Gafsa, (B) valleys in the mining basin of Gafsa and (C) mines and phosphate extraction areas (Adapted from Salhi, 2017; MTE, 2011).

present in the sample. The readings are in terms of Formazin Attenua­ tion Units (FAU). Total metal concentrations were determined using an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES -Perkin Elmer OPTIMA 3100 RL) on filtered (0.45 μm) and acidified (with 2% (v/v) of HNO3) samples. Finally, RAW and RYW metal con­ centrations were compared to the Tunisian standards set (NT 106-02), i. e. maximum allowed concentrations of pollutants in wastewater before

its discharge into the environment. Solid suspended material (SSM) was determined by filtering a sample portion through a previously weighed Whatman 1827-047 934-AH Glass Microfiber Filters (1.5 μm). Once the filtration was completed, the residue was dried at 105 � C and weighed again. The mass of suspended matter was obtained by weight difference, before and after drying (CEAEQ, 2015).

3

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Fig. 2. Phosphate’s treatment process and location of samples collected.

Fig. 3. Location of the five sub-samples of F-PS taken from Google earth.

2.3.2. Solid samples The PS and F-PS samples were characterized to determine their elemental composition as well as physicochemical and mineralogical properties. The PS (0–2 mm) was firstly separated to three fractions: cumulative 32 μm (fraction < 32 μm), cumulative þ32 μm (fraction between 32 μm and <75 μm) and finally þ75 μm (fraction between 75 μm and 2 mm that correspond to phosphate fraction: P-fraction) using wet sieving. Fractions cumulative 32 μm and cumulative þ32 μm correspond to the PS. The separation of the both fractions ( 32 μm and þ32 μm) aimed to determine which fraction could be considered for a

further potential valorization. Paste pH was determined in deionized water using a solid/liquid ratio of 1/10 (ASTM, 1995) with an electrode Orion 3 Star Thermo (GENEQ Inc.), after being centrifuged for 5 min at 4500 rpm. The chemical composition was determined by ICP-AES using a Perkin Elmer OPTIMA 3100 RL following an acid digestion (HNO3–Br2–HF–HCl). Crystalline phases of minerals were analyzed by an X-ray diffractometer (XRD). The XRD semi-quantitative mineralog­ ical analysis used Bruker AXS D8 advance diffractometer equipped with a Cu anticathode and a scintillation counter. Before the XRD analyzes, samples were dried at 60 � C for 48 h and micronized in alcohol 4

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(isopropanol) using a McCrone Micronizing tool. The DiffracPlus EVA software was used for the identification of mineral species while the TOPAS software (Rietveld modeling) was used for their quantification. Finally, the grain-size distribution was determined by laser diffraction using a Malvern Instruments Mastersizer S particle size analyzer. X-ray fluorescence (XRF) analysis was performed using the method of solid sample fusion into borated beads as optimal sample preparation tech­ nique for accurate XRF analysis (Claisse and Blanchette, 2016). The point of zero charge (PZC) measurements were performed on the flocculant Slim Floc (unknown origin) used by the GPC, PS and F-PS using the salt addition method. The pHPZC was investigated for a better understanding of the surface charge of solids and hence the settlement behavior of particles in water. The principle consisted of adding iden­ tical amounts of substrate to a set of solutions of the same ionic strength at different pH values (Cristiano et al., 2011). Precisely, in a series of 50-mL centrifuge tubes, 0.2 g of sample was added to 40 mL of 0.1 M KNO3 solution. The pH was then adjusted with solutions of 0.1 M HNO3 and 0.1 M NaOH, as needed, to obtain the targeted pH: 2, 4, 6, 8, 10, and 12 (�0.1 pH units). The pH values of the supernatant in each tube were denoted as pHi. The samples were shaken for 2 h using a shaker (PLATFORM SHAKER Innova, 2000) at 200 rpm. Then, samples were centrifuged using polypropylene tube of 50 mL (VWR ultra-high per­ formance) in the MSE Mistral 2000 of SANYO Centrifuge at 4500 rpm during 10 min. Afterwards, the pH of the supernatant was measured in each tube and was denoted as pHf. The PZC was obtained from the plot of ΔpH (ΔpH ¼ pHf – pHi) in function of pHi. The PZC corresponds to the inflexion point of the curve.

supplied by CCC Chemicals (Percol 7363, Magnafloc 10, Zetag 727, Magnafloc 1011, Percol 919, Flomin 905, Zetag 7654, E24, Percol 336 and AN 923) were tested, under the same conditions except for the initial concentration of the flocculant solution which was fixed at 0.1 g/ L. The test aimed to compare the capacity of these flocculants to increase water recovery relative to Slim Floc, currently used by the GPC. 3. Results and discussion 3.1. Physicochemical and mineralogical properties of the flocculant, PS and F-PS 3.1.1. Particle size distribution of the sludge Particle size distribution showed similar for F-PS and the fraction cumulative 32 μm, whereas the fraction cumulative þ32 μm showed different pattern with the presence of coarser particles (Fig. 4). Overall, PS represents 20% of raw phosphate. Precisely, the fractions cumulative þ32 μm, and cumulative 32 μm represent 70% and 30% of PS, respectively. The uniformity coefficient (Uc; Table 1) had different ranges: i) 2 < Uc � 5 for F-PS and the fraction cumulative 32 μm. Hence, the gran­ ulometry of both solids is considered narrow. These findings suggest that the samples are mainly composed of coarse, medium to fine, silt ac­ cording to the classification system ASTM D-2487 and ii) 5 < Uc � 20 for the fraction cumulative þ32 μm, which was more than a two-magnitude order of high regarding F-PS and cumulative 32 μm. These results indicate that the granulometry of cumulative þ32 μm is a semi-spread. The F-PS consisted of clay (6.9%), silt (92.2%) and sand (0.9%). Fraction cumulative þ32 μm was composed of clay (2.8%), silt (79.7%) and sand (17.5%). This distribution is like previous studies on PS, which was reported as mainly composed of clay (12%), silt (72.5%) and sand (15.5%) (Hakkou et al., 2009, 2016; Boss�e et al., 2015). Fraction cu­ mulative 32 μm was composed mainly of silt (92.4%) and some clay

2.4. Flocculant dosage optimization To dewater the PS, anionic flocculant Slim Floc, used by the GPC, was tested. The optimal consumption was determined, by calculating the volume of flocculant (in mL) added, using the following formula:

Volume of flocculant added ðmLÞ ¼

Phosphate sludge ðg=LÞ � Flocculant consumption ðg=tdsÞ Flocculant concentration ðg=LÞ

where tds represents tons of dry sludge. To optimize flocculant consumption (in g/tds) that enables to recover the maximum amount of water from the PS in short time, the volume added (in mL) for each consumption was firstly determined. To do so, flocculant consumption (g/tds) has been started with 100 g/tds with increment of 100 g/tds: 200 g/tds, 300 g/tds, 400 g/tds and so on until the optimal consumption in g/tds was obtained. The flocculant solution (concentration 0.3 g/L) was prepared and the optimization dose was performed using a Jar Test (Captair) at an agitation speed of 200 rpm. To do so, 15 decantation tests were performed in 1 L graduated cylinders that contain sludge with a solid concentration of 60 g/L (like the concentration of the sludge exiting from the laundry). The necessary volume (in mL) was added to the sludge according to the required flocculant consumption that have been already supposed. The stopwatch was started, and the sludge height drop was estimated as a function of time (15, 30, 45, 60, 90, 120, 150, 180, 220, 280, 340, 400, 640, 900 and 1200 s) to determine the rate (and the volume) of water recovered from the wet sludge. The curves of the settling tests were plotted according to the Kynch curve (sludge’s height as a function of time). Kynch traced in the diagram (h, t) the lines of isoconcentration corresponding to the levels where the concentration has a given value C and to their shift during time (Blazy et al., 1999). Once optimal operating conditions have been set using the Slim Floc (used by the GPC), other commercial anionic and cationic flocculants

Fig. 4. Particle size distribution of F-PS and PS (cumulative þ32 μm and μm fractions). 5

(1)

32

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Journal of Environmental Management 254 (2020) 109801

Table 1 Physical characteristics of F-PS and PS with fractions (þ32 μm,

Table 3 Chemical composition (%) by XRF of F-PS and PS (fractions þ32 μm and μm).

32 μm).

Samples

D10 (μm)

D30 (μm)

D50 (μm)

D60 (μm)

D80 (μm)

Uc

Cc

F-PS PS þ32 μm 32 μm

2.4 5.2

5.0 20

8.3 41

10 50

17 73

4.2 9.6

1.0 1.6

2.4

4.6

7.0

8.4

12.5

3.5

1.0

(6.8%) and sand (0.8%). The F-PS and fraction cumulative 32 μm consisted of more than 99 vol % fine particles (<74 μm), with F-PS slightly coarser than the fraction cumulative 32 μm. These results could probably be explained by the flocculant addition to F-PS. The coefficient of curvature (Cc) was low for all the samples (1.0–1.6), with similar values (Cc ¼ 1.0) for both F-PS and the fraction 32 μm.

(Detection limit: DL)

(g/kg)

PS

Al (0.06) As (0.005) Ca (0.06) Cd (0.005) Cr (0.005) Fe (0.01) K (0.001) Mg (0.015) Na (0.001) Ni (0.005) Pb (0.005) Zn (0.055)

35.6 0.03 135.6 0.06 0.6 18.7 5.8 17.2 11.4 0.05 0.05 0.2

39.4 0.01 112.5 0.05 0.7 21.5 4.8 17.6 2.9 0.06 0.07 0.2

22.3 0.01 211.4 0.1 0.2 9.9 5.5 9.0 9.1 0.02 0.05 0.5

(DL) C (0.05) S (0.009)

3.7 1.7

(%) 3.8 1.9

3.2 1.2

32 μm

30 42.6 7.4 3.0 2.9 15.6 0.6 0.5 0.3 6.1 0.0

þ32 μm 70 27.5 5.1 1.9 1.7 28.4 0.7 1.2 0.1 14.3 0.0

3.1.3. Mineralogical characterization Chemical analysis of F-PS and PS (cumulative þ32 μm and cumula­ tive 32 μm) confirmed the mineral composition found with XRD analysis (Table 4). Sludge samples were mainly composed of silicates, carbonates, zeolites, clays and fluorapatite. Significant fluorapatite content was found in the fraction cumulative þ32 μm (39.2%) but lower than PS (44%) from phosphate ore pro­ cessing in Morocco (Hakkou et al., 2016). Residual fluorapatite suggested high potential for P2O5 valorization. High calcite content (14.1%–26.7%) were found with the highest value in the fraction cumulative 32 μm. Calcite content in the fraction cu­ mulative 32 μm was almost twice as much comparing to another study where its content was about 15% (Hakkou et al., 2016). In addition, no significant difference of dolomite content (5.8–7.3%) was observed be­ tween fractions. These findings are consistent with a previous study where dolomite content was ~7% (Hakkou et al., 2016). Similar

32 μm).

F-PS

– 39.1 7.3 2.6 2.9 16.9 0.8 1.5 0.3 6.5 0.1

form of P2O5 (6.5 wt %)] and small amounts of iron oxides [in the form of Fe2O3 (2.6 wt %)] (Table 3). For both fractions cumulative þ32 μm and cumulative 32 μm, re­ sults showed high contents of CaO (28.4 and 15.6 wt %, respectively), SiO2 (27.5 and 42.6 wt %, respectively), P2O5 (14.3 and 6.1 wt %, respectively) and Al2O3 (5.1 and 7.4 wt %, respectively). These oxides contribute to more than 70 wt % for both fractions. Noteworthy, there is an important and promising residual potential of P2O5 in F-PS (6.5%) and PS (6.1%–14.3%) that should be recovered. The content of P2O5 in the fraction cumulative þ32 μm was similar relative to a previous study on sludge from Morocco (14.3 vs. ~17 wt %; Hakkou et al., 2016). Non-negligible content of alkaline oxides (Na2O and K2O) occurred in all fractions. The MgO content was similar for both F-PS and the fraction cummulative 32 μm (2.9 wt %), but lower in the fraction cumulative þ32 μm (1.7 wt %). The Mg could originate from carbonates (e.g. dolomite) and magnesium clays (e.g. palygorskite).

3.1.2. Chemical characterization of RP and F-PS Chemical analysis of F-PS and PS (fractions cumulative þ32 μm and 32 μm) showed high content of Ca, Al, Fe, Mg, Na and K (Table 2). Higher Al content was found in the fraction cumulative 32 μm comparing to F-PS and the fraction cumulative þ32 μm. Moreover, significant Ca concentrations were found in all samples particularly in the fraction cumulative þ32 μm, which were twice as much comparing to the fraction cumulative 32 μm. This suggests the presence of high Ca-bearing minerals in the sludge (e.g. carbonates). For PS, high Fe content was found in the fraction 32 μm relative to the fraction þ32 μm. The presence of high Fe content was not surprising since Fe-bearing minerals were present in the Metlaoui Gafsa basin (Galfati et al., 2014). High Mg content was found in F-PS and the fraction 32 μm (17.2 g/kg and 17.6 g/kg) relative the fraction þ32 μm (9.0 g/kg). All samples had similar K content (~5.0–6.0%). These significant contents in Mg, Na and K are probably due to the presence of clay and zeolites minerals in all solids. The Zn content in FPS and the fraction cumulative 32 μm was almost the same (0.2 g/kg) and lower than the fraction cumulative þ32 μm (0.5 g/kg). Potential contaminants such as As, Cd, Cr and Pb were also present but in lower concentrations. No difference of C% was found between the fraction cumulative 32 μm and F-PS relative to the fraction cumulative þ32 μm. Same applied to the S%, present in lower content. Whole rock XRF analysis for F-PS showed high content of SiO2 (39.1 wt %), CaO (16.9 wt %), Al2O3 (7.3 wt %), orthophosphate [in the

Elements

PS 32 μm

Weight (%) SiO₂ Al₂O₃ Fe₂O₃ MgO CaO K₂O Na₂O TiO₂ P₂O₅ Cr2O3

Uc: uniformity coefficient ¼(D60/D10); Cc: curvature coefficient ¼ (D302/ (D60*D10)).

Table 2 Results of the chemical assays for F-PS and PS (fractions þ32 μm and

F-PS

32

þ32 μm

Table 4 XRD analysis for the F-PS and PS (cumulative fractions þ32 μm and Samples (%)

Fluorapatite Calcite Dolomite Bassanite HeulanditeCa Palygorskite Hematite Vermiculite Quartz

6

Chemical formula

Ca5(PO4,CO3)3F CaCO3 CaMg(CO3)2 2CaSO4.(H2O) (Ca,Na)2–3Al3(Al, Si)2Si13O36⋅12H2O (Mg,Al)2Si4O10(OH).4(H2O) Fe2O3 (Mg,Feþþ,Al)3(Al,Si)4O10(OH)2.4 (H2O) SiO2

F-PS

32 μm).

PS

μm

32

þ32 μm

15.6 23.1 6.9 9.0 1.0

14.6 26.7 5.8 8.6 1.4

39.2 14.1 7.3 7.9 1.0

9.3 0.9 11.9

0.8 1.5 10.6

2.0 1.6 –

22.3

30.0

28.9

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bassanite content (7.9–9.0%) was also found in all fractions, as it was reported in the Algerian phosphate waste (Bezzi et al., 2001). However, bassanite was not detected in the Morocco phosphate waste (Hakkou et al., 2016). This could probably be explained by the difference of the deposit medium. Low tectosilicates (<1.4%) content in the form of heulandite-Ca were also detected. Palygorskite, a magnesium aluminum phyllosili­ cate, was lower in the fraction cumulative 32 μm (0.8%) relative to the fraction cumulative þ32 μm (2.0%). Vermiculite, a hydrous phyllosili­ cate mineral, was only found in the fraction cumulative 32 μm and its content was almost similar relative to F-PS (10.6% and 11.9%, respec­ tively). The absence of vermiculite in the fraction cumulative þ32 μm could be explained by its low hardness according to Mohs scale (1.5–2). Therefore, due to easily crushing, it is subsequently found in the fine fraction. Noteworthy, the presence of palygorskite and vermiculite suggests high sorption capacity (H2O and ions) of PS. Low hematite content (<1.6%) was found in all solids. Despite the sedimentary context, the presence of hematite could occur since it was previously found in low-phosphorus ore (Clout and Manuel, 2015). Additionally, the presence of Fe could probably have a secondary origin in relation with clay minerals. Contrary to the study of Hakkou et al. (2016), smectite was not detected in the studied solids as an indication of dif­ ferences in the phosphate ores.

Table 5 Calculation of water recovery. Flocculant consumption (g/ tds)

Sludge volume with flocculant addition (mL)

Volume of water recovered after 20 minsettling (mL)

Recovery rate (%)

1200

1000 þ 240 ¼ 1240

1240–520 ¼ 720

58.1%

Where g/tds is gram of flocculant/ton of dry sludge. Recovery rate (%) ¼ 100 � (Volume of water recovered after (20min)/Volume of sludge in mL with flocculant intervention).

1200 g/tds, a sludge concentration of 60 g/L and a flocculant concen­ tration of 0.3 g/L. The added volume was therefore 240 mL. To recover 58.1% of water by thickening (Table 5), the optimal Slim Floc consumption (initial solution of 3 g/L) was 1200 g/tds at a speed rate of 200 rpm. The findings were consistent with a study conducted at the Youssoufia processing plant, OCP, Morocco where water recovery from PS was about ~34–~50% using 500 g/tds of floculant (solution of 0.76 g/L) at a speed rate of 120 rpm (Khoubane, 2015). Despite the optimization of Slim Floc consumption, low water re­ covery (58.1%; Table 5) was obtained. This could probably be explained by the formation of small, weak, dispersed and non-settleable floc while using Slim Floc. In addition, low water recovery using Slim Floc could be explained by several factors. Since PS consisted of more than 99 vol % fine particles and silty material (>90%), it was difficult to achieve high amount of water recovery and sludge dewatering becomes more diffi­ cult. In addition, the presence of clay minerals in the form of phyllosi­ licates (i.e. vermiculite and palygorskite) play an important role in water retention. To better understand the interaction between particles (their mobility) and flocculant, the pHPZC was determined. Indeed, the sta­ bility of the sludge is high at pH ¼ pHPZC and thus, settling of flocs is likely to occur (Dempsey and Jeon, 2001). The pHPZC for the Sim Floc flocculant, PS and F-PS are 6.6, 8.0, and 7.5, respectively (Fig. 6). The PS was positively charged, since its paste pH (7.3) was lower than pHPZC (8.0). However, the flocculant was negatively charged since its pHPZC (6.6) was lower than its paste pH (7.42). Finally, the F-PS was neutral as the paste pH (7.3) was very close to pHPZC (7.5). Therefore, the F-PS could be considered stable (i.e. uncharged) and thus able to settle. Despite this fact, sludge dewatering by Slim Floc was low (58.1%). The limited capacity of this flocculant to adsorb particles could probably be the main cause of this low water recovery. Therefore, to enhance water recovery from PS, other types of flocculant were tested.

3.2. Optimization of Slim Floc dosing Based on sludge settling, the optimization of Slim Floc consumption, which is used by GPC, was investigated to enhance water recovery. Four settling areas were well described according to the Kynch curve as fol­ lows (Fig. 5): 1) domain I that corresponds to the initial flocculation time; 2) domain II where flocs start to collect in flakes and the settling rate is remaining almost constant; 3) domain III that shows disruptive actions that occur between flakes and particles; this point is often poorly defined on the curve; and, finally 4) domain IV that exhibits isolated solids and flakes that are in contact and form pseudo-rigid networks. Based on Table S1 (Supplementary Material), which presents Slim Floc consumption as a function of time, the optimum flocculant con­ sumption after 1200 s (20 min) corresponds to the addition of 1200 g/ tds. For flocculant consumption higher than 1200 g/tds, it was observed that the sludge became more viscous and difficult to settle. For floccu­ lant consumption lower that 400 g/tds, no settling was observed. The flocculant dosage within the sludge was calculated using the following parameters in formula (1): a flocculant rate consumption of

Fig. 5. Best settling of sludge treated with Slim Floc (flocculant concentration 3 g/L, pH ¼ 7.4, T ¼ 23 � C and agitation speed ¼ 200 rpm). 7

M. Ettoumi et al.

Journal of Environmental Management 254 (2020) 109801

for water recovery. The lowest and highest turbidity was found for Maganafloc 10 (70 FAU) and Zetag 7654 (131 FAU), respectively (Table 6). Despite its lowest turbidity, the pH of the supernatant (pH ¼ 6) was slightly acidic and below the limit of the Tunisian standards set NT 10602 (6.5 < pH < 8.5). For all the other flocculants tested, the final pH of the supernatant meets the Tunisian discharge criteria (Tunisian stan­ dards set NT 106-02). For all the supernatants, the suspended matters (3.53–5.52 mg/L; Table 6) were low and below the limit of the Tunisian standards set NT 106-02 (30 mg/L). Water recovery using the flocculant E24 could be considered rela­ tively high compared to the other existing methods. Indeed, the gravity thickening method, considered as low energy consumption and low operational costs method, allowed low reduction volume of sludge only between 5% and 8% (Stefanakis et al., 2014). Dissolved air flotation, which is another method used for low density solids dehydration, gave total solid content of the sludge between 3.5 and 4.5% (Stefanakis et al., 2014). The gravity belt thickening method provides a final total solid content of the sludge between 5 and 12%. However, this last method needs additional chemicals and has high-energy consumption, which raise operational costs (Stefanakis et al., 2014). Finally, belt filter presses method provided sludge with solid content up to 30–40% (Ste­ fanakis et al., 2014). To be noted that, it was also previously reported that the combination of coagulant (alum) and flocculant (anionic poly­ acrylamide) was efficient to reduce about 91.12% of the suspended matter and to enhance water recovery from wastewater (Burhani et al., 2017).

Fig. 6. Point of Zero Charge of the flocculant Slim Floc, PS and F-PS (T ¼ 23 � C and agitation speed ¼ 200 rpm).

3.3. Influence of the nature of flocculant on water recovery improvement Based on the parameters optimized using Slim Floc (1200 g/tds), the added volume of flocculant was fixed at 72 mL for 1000 mL of solution to be treated. Among 10 flocculants tested, the flocculant Flomin 905 displayed the fastest settling (81.3%) after 15 s (Table S2, Supplemen­ tary Material). On the contrary, Percol 7363 had the lowest settling (69.2%) at the same time laps. After 45 s, the flocculant AN 923 had the fastest settling rate and remained at 83.2% until the end of the test. At about 120 s, Flomin 905 had achieved its maximum efficiency (83.2%) in decreasing the sludge height. At 360 s, E24 achieved the lowest sludge level (84.1%) comparing to all the tested flocculants. Therefore, in term of settling time and efficiency, AN 923 could be the best flocculant to decrease the height of the sludge in this study. The efficiency of water recovery for each flocculant is present in Table 6. The results showed that all tested flocculants allowed higher water recovery (79.4%–84%) relative to Slim Floc (58.1%), which is used by the GPC mine. The results showed that all tested flocculants allowed higher water recovery (79.4%–84%) relative to Slim Floc (58.1%), which is used by the GPC mine. The anionic flocculant E24 gave the best performance for water recovery (84%) compared all the tested flocculant. Flomin 905, Magnafloc 1011 and AN 923 had similar efficiency for water recovery (83.2%). Furthermore, Percol 363 and Percol 7363 gave the same water recovery (80.3% and 80.4%, respectively). The flocculant Zetag 7654 gave lower efficiency (79.4%). Nevertheless, Zetag 7654 gave higher efficiency comparing to Slim Floc (58.1%), presently used by GPC. Ac­ cording to the efficiency of water recovery, flocculants could be classi­ fied as follows: E24 > Flomin 905 ¼ Magnafloc 1011 ¼ AN 923 > Percol 919 > Magnafloc 10 > Zetag 727 > Percol 7363 > Percol 336 > Zetag 7654. At the same time, as the AN 923 enabled to faster decrease the PS height up to 83.2% in just 45 s, it could be considered as the best choice

3.4. Physicochemical characterization of RAW and RYW For both samples, RYW and RAW, pH was circumneutral (6.8 and 7.1, respectively) and complied with the Tunisian standard NT 106-02 (6.5–8.5). Because of its recycling, the RYW had higher EC relative to RAW (7.75 vs 3.49 mS/cm) (Table 7), as an indication that RYW is increasingly loading, with respect to the concentrations and the nature of dissolved substances. The ORP was positive and high (474–483 mV) for both RYW and RAW, as an indication of oxidizing conditions. RYW was turbid (2800 FAU) and loaded on suspended matter whereas RAW was free of sus­ pended matter since its turbidity was zero. Metal concentrations showed high Ca (555 mg/L), Mg (242 mg/L) and Na (961 mg/L) concentrations that exceed the Tunisian standards set NT 106-02 (500 mg/L, 200 mg/L and 500 mg/L for Ca, Mg and Na, respectively) in RYW (Table 8). The high contents of Ca2þ, Mg2þ and Naþ probably indicate an in­ crease in the salinity of the water while recycling. This recycled water must be treated before its release into the environment to meet stan­ dards criteria. Table 7 Physical characteristics of RAW and RYW. Parameters

RYW

RAW

Turbidity (FAU) EC (mS/cm) ORP (mv) pH

2800 7.75 474 6.80

0 3.49 483 7.10

Table 6 The characteristics of optimal settling condition for the tested flocculants. Parameters

Percol 7363

Magna-floc 10

Zetag 727

Magna-floc 1011

Percol 919

Flomin 905

Zetag 7654

E 24

Percol 336

AN 923

SSM (mg/L) pH Turbidity (FAU) Water recovery (%)

5.2 6.7 114 80 .4

5.0 6.0 70 81.3

4.7 6.6 83 81.1

5.3 7.6 87 83.2

4.5 7.5 81 82.2

5.0 7.6 78 83.2

5.5 7.8 131 79.4

5.5 7.7 106 84 .1

5.2 7.5 119 80.3

3.5 7.6 147 83.2

8

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Table 8 Chemical characterization of RAW and RYW. Elements (DL)

RAW (mg/L)

RYW (mg/L)

NT 106-02 (mg/L)

Al (0.010) Ba (0.001) Ca (0.030) Co (0.004) Cr (0.003) Fe (0.006) K (n/d) Mg (0.001) Mn (0.002) Mo (0.009) Na (n/d) Ni (0.004) Se (0.100) Zn (0.005)

0.048 0.015 240 0.007

0.047 0.014 555 0.016 0.003 0.070 23.1 242 0.095 0.028 961 0.094 0.119 0.169

5 0.5 500 0.1 2.5 1 50 200 0.5 0.5 500 0.2 0.5 5

DL: Detection limit.

4. Conclusion To enhance water recovery and reuse in the phosphate industry, this study focused on optimizing the densification process (using floccula­ tion) of the PS. The PS, considered as a non-profitable material, had high quantities of residual P₂O₅ (6.1–14%) in the form of fluorapatite, concentrated in the coarser fraction, for potential commercial valori­ zation. Satisfactory water recovery (79.4–84.1%) was found for com­ mercial flocculants relative to Slim Floc (58.1%) used by the Gafsa Phosphate Company in the Kef Shfeir laundry. Additional recovery was reached using the anionic flocculant E24 that gave the best performance (84.1%). The recycled water, resulting from F-PS settling, showed high concentrations of Ca (555 mg/L), Mg (242 mg/L) and Na (961 mg/L) that exceed the Tunisian discharge criteria. The recycled water must be treated before its release into the environment. Further studies should be undertaken to: 1) optimize the E24 flocculant consumption, 2) test the effect of coagulant addition to the PS for a further water recovery and 3) characterize the final PS for potential recovery of value. Acknowledgements This study was funded by the Mineral Resources and Environment Laboratory of the Faculty of Sciences of Tunis, Gafsa Phosphate Com­ pany, and Ministry of Higher Education and Scientific Research, Tunisia, as well as the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Research Chairs Program, and RIME UQATPolytechnique Montreal, Canada. The authors want to sincerely thank Abidi Ghozlen, Ncib Nafti, Sahbi Harakati and Rhili Houcin for their contribution to the study. University of Tunis El Manar, Tunis, Tunisia, which paid a stipend to the first author, as PhD Candidate., Tunisia, which paid a stipend to the first author, as PhD Candidate. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvman.2019.109801. References American Society for Testing and Materials (ASTM), 1995. Standard test method for pH of soils. In: Annual Book of ASTM Standards D4972-95a, Washington, DC. Bezzi, N., Merabet, D., Benabdeslem, N., Arkoub, H., 2001. Caracterisation physicochimique du minerai de phosphate de Bled El Hadba - Tebessa. Ann. Chim. Sci. Mat. 26 (6), 5–23. Blazy, P., Jdid, E.A., Bersillon, J.L., 1999. CTDI4, 3450 Techniques de l’Ing� enieur 7. https ://www.techniques-ingenieur.fr/base-documentaire/procedes-chimie-bio-agro-th 2/operations-unitaires-separation-de-phases-decantation-et-filtration42484210/decantation-j3450/theorie-de-la-sedimentation-j3450niv10004.html. (Accessed 31 May 2019).

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