Characterization and Optical Study of Phosphogypsum Industrial Waste

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Characterization and Optical Study of Phosphogypsum Industrial Waste Ines Hammas1, Karima Horchani-Naifer2, Mokhtar Férid3 Laboratory of Physical Chemistry of Mineral Materials and their Applications/National Research Center in Materials Sciences Technopole Borj Cedria B.P. 73- 8027 Soliman, Tunisia. Tel.: +216 – 7932 – 5250 Fax: +216 – 7932 – 5314 ines_hammas@yahoo.fr; 2karima_horchani@yahoo.com; 3mokhtar.ferid@gmail.com

1

Abstract Industrial wet phosphoric acid production in Tunisia has led to the accumulation of high amounts of waste phosphogypsum by-products which are stocked mainly in piles. This harmful waste has still caused much anxiety and problems for the living especially in Sfax region (South-East of Tunisia) where it is rejected.

In this paper two samples of Tunisian phosphogypsum, differing in their age, were analyzed by different analytical methods in order to compare and study the effect of impurities presence on gypsum’s properties. Infrared spectroscopy allowed the observation of phosphoric acid and fluorosilicates absorption bands as well as the determination of the P/S ratio. X-ray powder diffraction confirmed the presence of impurities of HPO42- (crystallized in ardealite and brushite phases) and SiF62- (crystallized in malladrite phase) in phosphogypsum samples. A shapedneedles phase observed only in old sample micrographs corresponded to the brushite phase. Differential Scanning Calorimetry analysis showed the influence of impurities on the thermal behavior of dehydrated calcium sulfate.

Elemental analysis was provided with respect to scanning electron microscopy coupled with energy dispersive X-ray technique, X-ray fluorescence, ultraviolet-visible spectroscopy, flame atomic absorption spectroscopy and selective electrode analysis. Optical properties investigated by photoluminescence measurements allowed not only the identification of the matrix emissions but also the observation of Cd2+, Zn2+ and Mn2+ impurities showing thus their presence in the studied phosphogypsum samples.

Keywords Phosphogypsum; Elemental Analysis; Optical Properties

Analysis;

Impurities;

Thermal

Introduction Phosphogypsum (PG) is the main waste as a byproduct of phosphoric acid factories generated from the chemical attack of phosphate rock ore with sulfuric acid, and represents one of the most serious problems the phosphate industry today confronts. Large

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quantities have been produced worldwide because of the high demand for phosphate fertilizer, and because the wet process produces about 5 tons of phosphogypsum for every ton of phosphoric acid manufactured. Annual world production has been estimated to be about 100-280 million tons. Only a small part (15%) is applied to industrial or commercial use (as building material, soil amendment for the remediation of soils, agricultural fertilizers or set controller in the manufacture of Portland cement), while the remaining (85%) is stockpiled near the factory, occupying considerable land areas and causing serious environmental damage.

In Tunisia, the phosphoric acid industry has been developed since the early 1950’s. The annual Tunisian production of phosphogypsum is around 10 million tons which are stocked in non protected sites near the SIAPE (Société Industrielle d’Acide Phosphorique et d’Engrais) in Sfax. Several important environmental concerns underlying the disposal of this waste are related to the presence of several impurities such as P2O5, fluorides, heavy metals and radionuclides.

Concentrations of these hazardous elements depend on the origin of the rock phosphate sources, the type, and the efficiency of the wet process used.

The main objective of this investigation is to present and discuss the results of the analysis of two phosphogypsum samples taken from Sfaxien dumps, as one is of fresh production and the other is older than 40 years. After an elemental analysis carried out using different analytical techniques, thermal behavior and optical properties of these phosphogypsum samples (fresh and old) were compared with those of a commercial gypsum CaSO4.2H2O in order to study the effect of impurities presence on such properties.

Materials and Methods Infrared spectra were performed on a FT-IR System

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Perkin Elmer spectrometer using KBr pellets in the region of 4000–400 cm–1.

Raman spectra were recorded using a LabRam HR – Horiba Scientific system (Jobin Yvon) in the range from 200 to 1400 cm-1.

The X-ray powder diffraction patterns of all samples were recorded on an X’PERT Pro PANAnalytical diffractometer with Cu Kα radiation (λ=0.15418 nm).

Scanning Electron Microscopy with Energy-Dispersive X-ray analysis was performed by a QUANTA 200 microscope.

A Differential Scanning Calorimetry Mettler Toledo Model 823 was used for the thermoanalysis of the samples with a heating rate of 20°C/min.

Photoluminescence measurements were recorded on samples pellets and solutions with Perkin Elmer spectrometer LS55.

Results and Discussion Infrared Spectroscopy Analysis Infrared spectra of the studied phosphogypsum samples superposed with that of the commercial gypsum CaSO4.2H2O (Flucka) are depicted on Fig.1.

Similarities of these spectra are underlined by the presence of the characteristic bands of water vibrations at the 3600-3200 cm-1 region and 1684-1622 cm-1 region (stretching and bending vibration modes respectively) as well as the characteristic bands of sulfate vibrations.

The two bands observed at 1684 cm-1 and 1622 cm-1 are due to the presence of two types of water molecules in the samples. Bands observed at 2248 – 2118 cm-1 , 1143 – 1117 cm-1 and 658 – 602 cm-1 are assigned, respectively, to νO-S-O (ν3 and ν1) and δO-S-O (ν4).

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exist in the phosphogypsum as well as to quantify them using the infrared spectroscopy technique. Water molecules vibrations are significantly influenced by the ratio phosphorus/sulfur (P/S) such that the presence of the band at 3244 cm-1 indicates a P/S < 0.15 %. Similarly, the bands at 836 cm-1 and 1015 cm-1 (clearly observed for the fresh production sample) further confirm the presence of HPO42- in the sulfate matrix. According to the same work of Bourgier, for 0<P/S<0.01 a double band appears at 834-840 cm-1 when HPO42- ions begin to be introduced in the gypsum lattice. Whereas for P/S > 0.01%, the intensity of the double bands increases progressively until a single absorption band forms at 836 cm-1 and the intensity continues to increase. This band was recorded on the spectra of the two studied phosphogypsum. Then it can be deduced that these phosphogypsum samples are characterized with a P/S ratio in between 0.01% and 0.15%. The band at 875 cm-1 confirms the presence of the brushite phase CaHPO4.2H2O in the studied phosphogypsum samples.

(a)

(b)

FIG.2. RAMAN SPECTRA OF A) THE FRESH PHOSPHOGYPSUM AND B) THE OLD PHOSPHOGYPSUM. TABLE 1. RAMAN BANDS POSITIONS OF THE SAMPLES OF PHOSPHOGYPSUM AT ROOM TEMPERATURE AND CALCINED AT 900°C AND THEIR ASSIGNMENT.

(A) (B) FIG.1. SUPERPOSITION OF THE INFRARED SPECTRA OF THE COMMERCIAL CASO4.2H2O, OLD PHOSPHOGYPSUM AND FRESH PHOSPHOGYPSUM ((A): 4000-2000 CM-1, (B): 2000-400 CM1).

The investigation made by Bourgier. showed that it is possible to identify the phase in which HPO42- ions

The bands observed at 748 cm-1 and 480 cm-1 are attributed to the νas (Si-F) and δas (Si-F) modes 31

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respectively. Their higher intensities observed in the spectrum of the fresh sample indicate the higher amount of fluorosilicates in this latter.

Raman Spectroscopy Analysis FIG 2 shows the Raman spectra of the studied phosphogypsum samples at room temperature and after calcination at 900째C. The Raman spectra of SO42and H2O are well known. The vibrational bands and their assignment are summarized in Table 1.

X-Ray Diffraction Analysis

FIG.3. X-RAY DIFFRACTION PATTERN OF THE FRESH TUNISIAN PHOSPHOGYPSUM.

malladrite impurities than the old sample which contains an additional phase corresponding to brushite. The XRD results correlate with those of the infrared spectroscopy and confirm the existence of HPO42- and SiF62- ions in the studied samples.

Scanning Electron Microscopy-Energy Dispersive XRay Characterization (Sem-Edx) The morphology of phosphogypsum particles was determined using scanning electron microscopy (SEM).

The observation of the fresh sample showed the presence of agglomerates in tabular form and other in hexagonal form (see FIG 5). Its semi quantitative composition analyzed by SEM-EDX showed high contents of Ca, S, O and impurities such as P, Si, Na and F. The SEM micrographs as well as the elemental composition of the old phosphogypsum, depicted on FIG 6, show that the particles exhibit different shapes: hexagonal, tabular and needles. They are composed mainly of Ca and S with the presence of P, Si, Na and F impurities. By comparing the micrographs of the phosphogypsum samples, we note the presence of well-shaped needles in the old sample. This shape corresponds most probably to the brushite phase.

Previous works, studying the synthesis and the formation mechanisms of CaHPO4.2H2O, showed the presence of needle-shaped for this phase. This is in good agreement with DRX results which showed the presence of the brushite phase more clearly for the old phosphogypsum.

FIG.4. X-RAY DIFFRACTION PATTERN OF THE OLD TUNISIAN PHOSPHOGYPSUM.

The two samples of phosphogypsum were analyzed through X-ray diffraction (XRD) to determine their crystalline composition. They were identified mainly to the gypsum CaSO4.2H2O (file PDF N째00-033-0311), which crystallizes in the monoclinic system with C2/c as space group, with the coexistence of other impurities phases which correspond mainly to ardealite Ca2(HPO4)(SO4)(H2O)4, brushite CaHPO4.2H2O and malladrite Na2SiF6 (see figures 3 and 4). The obtained patterns showed that the fresher phosphogypsum contains more of ardealite and

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FIG.5. SEM IMAGES AND ELEMENTAL COMPOSITION OF THE FRESH PHOSPHOGYPSUM.

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The phosphogypsum samples were characterized with X-ray fluorescence, UV-visible spectroscopy, flame atomic absorption spectroscopy and selective electrode in order to determine the amounts of their major elements. From the composition of the studied samples, summarized in Table 2, we can confirm the result of the P/S ratio (framed between 0.01 % and 0.15 %) which was determined by infrared spectroscopy analysis. The P/S ratio for the fresh and the old Tunisian phosphogypsum are respectively 0.0125% and 0.06%. The table provides also a comparison of those same components with their extreme amounts in a phosphogypsum sample.

Thermal Analysis (DSC)

FIG.6. SEM IMAGES AND ELEMENTAL COMPOSITION OF THE OLD PHOSPHOGYPSUM.

The thermal behavior study of the Tunisian phosphogypsum samples was performed by Differential Scanning Calorimetry (DSC) (with a heating rate of 20°C/min up to 600°C). The effect of impurities presence on phase transformation temperatures of the gypsum was studied after the thermogram of the commercial CaSO4.2H2O was superposed with those of the phosphogypsum samples (see FIG 7).

Chemical Composition TABLE 2. MEAN CONTENT (% WEIGHT) OF MAJOR SPECIES AND IMPURITIES IN THE TUNISIAN PHOSPHOGYPSUM SAMPLES. RESULTS ARE COMPARED WITH EXTREME AMOUNTS OF DIFFERENT ELEMENTS IN A GENERAL PHOSPHOGYPSUM SAMPLE (COLOMBEL, 1978; MANGIN, 1978; GORLÉ, 1985; SIMON, 1990; LAVALIN, 1993; SINGH ET AL., 1996; SINGH,

2000; SFAR FELFOUL ET AL., 2002; SINGH, 2002).

FIG.7. SUPERPOSITION OF THE THERMOGRAMS OF THE COMMERCIAL GYPSUM, THE OLD TUNISIAN PHOSPHOGYPSUM AND THE FRESH TUNISIAN PHOSPHOGYPSUM.

* measured by UV-visible absorption ** measured by flame atomic absorption spectroscopy *** measured by selective electrode a Loss on ignition

FIG 7 shows that the decomposition temperature of the starting material to hemihydrate decreases from gypsum, old phosphogypsum and fresh phosphogypsum (156°C, 140°C and 112°C respectively). This is due especially to the presence of residual acid. Indeed, the work provided by Sfar et.al showed that the presence of phosphoric acid decreases the decomposition temperature of gypsum to hemihydrate, while it does not have a great influence 33

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on the transformation temperature characteristic of the transition from hemihydrate to anhydrite III (hexagonal CaSO4). This was confirmed in our study since this phase transition temperature remains near to 170°C for the three samples ( 170°C for the gypsum, 177°C for the old sample and 166°C for the fresh sample). In addition, when the exothermic pic (relative to the transformation from the anhydrite III (hexagonal CaSO4) to the anhydrite II (orthorhombic CaSO4)) is spread and strongly shifted to higher temperatures (386°C, 519°C and 547°C for the commercial gypsum, old phosphogypsum and fresh phosphogypsum respectively) (see FIG 7), the calcium sulfate sample contains syncrystallized HPO42- ions in its structure. The intensity of this exothermic peak decreases as the percentage of co-crystallized ions in the gypsum lattice increases. Thus, this technique further confirms the presence of a higher content of P2O5 in the fresh phosphogypsum compared to that in the old one.

The emission spectra of phosphogypsum samples (λexc = 406 nm) (Fig.10(a)) show emission bands at 607 nm and 614 nm for the fresh and the old studied samples respectively. These photoluminescence bands are 6A1 (6S) transition of Mn2+ in ascribed to the 4T1 (4G) the sulfate matrix (Manam and Das, 2010). Fig.10 (b) shows excitation spectra of the studied samples monitoring the emissions obtained at 607 and 614 nm.

The absorption bands obtained at 406 nm for both samples correspond to the 4A1g, 4Eg (4G) transition of Mn2+.

(a)

(b)

FIG.8. EMISSION (A) AND EXCITATION (B) SPECTRA OF THE COMMERCIAL GYPSUM CASO4.2H2O.

Study of the Optical Properties of the Studied Samples The optical properties of the phosphogypsum samples were studied by photoluminescence. In order to identify the matrix emissions, the commercial gypsum was firstly analysed by photoluminescence and then the obtained emission bands were compared with those of the studied samples (see FIG 8 and FIG 9).

Emission bands were observed at 300 nm and 342 nm after excitation of the commercial CaSO4.2H2O at 220 nm. Excitation spectra of these emissions (300 nm and 342 nm) show bands at 222 nm, 241 nm and 259 nm.

Referring to previous research these bands are attributed to SO4- and SO3-.

The emission spectra of the phosphogypsum samples (λexc = 220 nm), depicted on FIG 9, show in addition to the matrix emission bands, other emissions attributed to luminescent impurities. Based on previous researchs, the photoluminescence bands observed at 373 nm and 367 nm for the fresh and the old phosphogypsum respectively are assigned to Cd2+ emissions and correspond precisely to the recombination between electrons in the conduction band and holes in the valence band. The emission bands observed at 391 nm and 410 nm for the fresh and the old phosphogypsum samples respectively (FIG 9) are attributed to electron transition from the shallow donor level of Zn interstitials to the valence band.

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(a)

(b)

FIG.9. EMISSION (A) AND EXCITATION (B) SPECTRA OF TUNISIAN PHOSPHOGYPSUM SAMPLES.

(a)

(b)

FIG.10. EMISSION SPECTRA AT ΛEXC = 406 NM (A) AND EXCITATION SPECTRA (B) OF TUNISIAN PHOSPHOGYPSUM SAMPLES.

Conclusions Various analyses carried out on Tunisian phosphogypsum samples are coherent and show the presence of residual acids (HPO42-) and fluorosilicates (SiF62-) with higher amounts in the fresh produced

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sample. The P/S ratio determined by infrared spectroscopy (between 0.01% and 0.15%) was verified after an elemental analysis provided by different techniques. SEM and XRD results showed the presence of the brushite phase (as fine needles) in the old phosphogypsum. Thermal analysis by DSC showed the decrease of the phase transition temperature for dehydrated–hemihydrated transformation and the increase of the phase transition temperature for anhydrite III–anhydrite II transformation with the increase of acid impurities amount in the sample. Whereas, the temperature corresponding to the transformation from the hemihydrate to the anhydrite III was not influenced by this type of impurities. Optical analysis performed by photoluminescence measurements allowed the observation of the sulfate matrix emissions as well as those of impurities ions identified to Cd2+, Zn2+ and Mn2+ ions.

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