Introduction

The phosphoric acid manufacture by the wet process produces high quantities of phosphogypsum (PG) waste. About 15 million tons are produced in Morocco, this quantity will increase in the coming years. PG is commonly stored in heap or dumped in the Atlantic Ocean [1, 2]. It has a negative impact on the environment and human health. Some authors have proposed solutions to recover or to reuse this waste in several fields, for example in agriculture as soil improvement and fertilization or in alkaline biochar production processing acidic PG waste [3, 4]. In cement, PG delay the setting time of the cement, which allows it to replace the natural gypsum in the cement and brick industry [5,6,7,8,9]. In plaster and cement industry, after appropriate treatment with sulfuric acid, PG meets standards that limit P2O5 and F impurities for its use as alternative to natural gypsum [10].

Other researchers have worked on the valuation of PG by converting it with an alkali fluoride (NaF) or ammonia and hydrofluoric acid according to the following reactions [11, 12]:

$${\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + 2{\text{NaF}} \to {\text{CaF}}_{2} + {\text{Na}}_{2} {\text{SO}}_{4} + 2{\text{H}}_{2} {\text{O}}$$
(1)
$${\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + {\text{NH}}_{3} + 2{\text{HF}} \to {\text{CaF}}_{2} + ({\text{NH}}_{4} ){\text{HSO}}_{4} + 2{\text{H}}_{2} {\text{O}}$$
(2)

The valorization of PG by alkaline carbonates (K2CO3, Na2CO3 and Li2CO3) have also been studied, its conversion yields to marketable products, as explained in the following reactions [13, 14]

$${\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + {\text{K}}_{2} {\text{CO}}_{3} \to {\text{K}}_{2} {\text{CO}}_{3} + {\text{K}}_{2} {\text{SO}}_{4} + 2{\text{H}}_{2} {\text{O}}$$
(3)
$${\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + {\text{Na}}_{2} {\text{CO}}_{3} \to {\text{Na}}_{2} {\text{CO}}_{3} + {\text{K}}_{2} {\text{SO}}_{4} + 2{\text{H}}_{2} {\text{O}}$$
(4)
$${\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + {\text{Li}}_{2} {\text{CO}}_{3} \to {\text{Li}}_{2} {\text{CO}}_{3} + {\text{K}}_{2} {\text{SO}}_{4} + 2{\text{H}}_{2} {\text{O}}$$
(5)

Other works used alkaline hydroxide (NaOH) to convert PG. The reaction is expressed as the following [15]:

$${\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + {\text{NaOH}} \to {\text{Na}}_{2} {\text{SO}}_{4} + {\text{K}}_{2} {\text{SO}}_{4} + 2{\text{H}}_{2} {\text{O}}$$
(6)

In these wet conversions. PG is used as raw material for chemical processes. Cited reactions were carried out at room temperature and atmospheric pressure. Specific conditions must be controlled to improve product quality like the attack duration and initial reagents concentrations to avoid syngenite (K2Ca(SO4)2·2H2O) formation. Nevertheless, valued quantity of PG remains lower compared to that produced.

Potassium sulfate K2SO4 is produced from different natural ores such as Kainite KCl·MgSO4·3H2O or Leonite K2SO4·MgSO4·4H2O. It can be also synthetized according to the Mannheim process where the reaction between H2SO4 acid and KCl requires a temperature of 800 °C [16]. Other researchers produced K2SO4 from gypsum, ammonia and KCl in aqueous solution, but chloride content in the final product avoid its use in agriculture [17, 18].

The present work suggests an attractive process allowing total PG conversion by potassium hydroxide KOH. This decomposition leads to potassium sulfate K2SO4 and calcium hydroxide Ca(OH)2 formation. K2SO4 is used as a fertilizer; growers frequently use it for crops where chlorides additions should be avoided. In such cases, K2SO4 makes a very suitable K and S source, whereas Ca(OH)2 is a source of CaO which is the main component of Portland cement manufacturing. Its use in cement makes it possible to control the hydration mechanism of the cement and provides information on the duration of the pozzolanic reaction [19]. It has also been used as adsorbent for radioactive and heavy elements in wastewater, desulfurizing agent and material for thermal energy storage [20, 21]. In addition, mineral carbonation is a mitigation strategy to reduce global anthropogenic CO2 emissions, which is constantly increasing [22,23,24].

Our previous work studied the PG conversion by KOH at several concentrations and various durations, but this reaction was limited at the concentration 0.6 M of PG due to the formation of parasitic phase of syngenite [25].

The proposal discussed in this work is to defy this problem by studying temperature effect on reaction progression especially in a saturated medium and therefore on the quality of obtained products by spending lesser energy compared to other processes. Indeed, diluted filtrates require a lot of energy to recrystallize the salt. This process have also other advantages, it is ecological and enhances a significant environmental benefit that allows the PG conversion, which is otherwise considered as hazardous waste. In addition, it is not difficult and not expensive, In fact, 1 ton of PG react with 0,651 ton of KOH (600 $ per ton [26]) to produce 0.430 ton of portlandite (420 $ per ton [26]) and 1.013 ton of K2SO4 (700 $ per ton [26]). This economical approach permits to benefit of about 500 $ per ton of PG converted, which can largely cover transport cost and the energy used during this process and does not require any difficult operation for recuperating these valuables products.

Material and methods

Phosphogypsum was taken from the fertilizer plant Morocco Phosphorus, located at twenty kilometers from the town of El Jadida. This phosphogypsum was washed to serve as a raw material for all tests. The washing step serves to remove soluble impurities as well as several organic matters in suspension. The potassium hydroxide KOH used in this research was from Fluka, with the purity ≥ 85%.

For each reaction, the mass corresponding to the desired concentration is dissolved in the potassium hydroxide solution of well-known concentration. The reaction is displaced in the direction of K2SO4 and Ca(OH)2 formations. This is because portlandite Ca(OH)2 (Kps = 7.9 × 10−6 at 25 °C) is less soluble than PG (Kps = 3.14 × 10−5 at 25 °C). The mixtures were put under stirring during one hour. A white precipitates Ca(OH)2 are formed. They are separated from the solution by simple filtration and afterward dried in an oven at 60° C. The filtrates, which contain K2SO4 are placed in the oven at 60 °C to recrystallize the salts (Fig. 1). All compounds prepared in this work were identified by X-ray diffraction using Bruker AXS Diffractomètre D8 Advance diffract meter using Cu-Kα radiation. XRD patterns were presented from 2θ = 10°–60°. Infrared spectra were performed on a FTIR Nicolet iS10 Thermo Fisher spectrometer using FTIR iTR. Thermogravimetric measurements were performed with TA Instruments "SDT Q600" combined to a mass spectrometer Hiden 0–200 uma. Small quantity of products was placed in an alumina crucible and heated from 20 to 900 °C at a speed of 5 °C min−1 under dry air flow of 100 mL min−1. Also the obtained precipitate at optimal conditions was characterized at − 196 °C by physisorption of N2 using Micrometrics ASAP2020, which is an automated volumetric apparatus. The specific surface area was calculated using Brunauer–Emmett–Teller (BET) model. Its morphology was observed by scanning electron microscopy X-ray analysis (SEM Environmental FEI Quanta 200). X-ray fluorescence was used to characterize major elements is an XRF spectrometer S4 PIONEER BRUKER aXS, which is equipped with an X-ray tube of 4 kW. Concentrations of trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS Model HP-4500) after acid digestion. Potassium and sodium concentrations are determined by flame photometer (FP JENWAY 500–731 Model PFP7). Measurements are made three times to make sure the determined values.

Fig. 1
figure 1

Diagram of the procedure for converting PG

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Results and discussion

The process consists of dispersing PG in a KOH solution using stoichiometric concentrations of 0.7–1.3 M of PG at ambient pressure and at various temperatures under constant magnetic agitation for 1 h. The basic reaction describing this process is:

$${\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + {\text{2KOH}} \to {\text{K}}_{2} {\text{SO}}_{4} + {\text{Ca(OH)}}_{2} + 2{\text{H}}_{2} {\text{O}}$$
(7)

This process leads to the PG dissolution and the precipitation of Ca(OH)2 as whitish solid phase. After filtration, filtrate was evaporated to dryness in the oven, yielding transparent salts K2SO4. The maximum solubility of K2SO4 at the chosen temperatures of 25, 40, 60 and 80 °C correspond to the concentrations of 0.6, 0.8, 1 and 1.2 M, respectively. We cannot exceed the maximum temperature of 80 °C for the PG conversion. Otherwise, the gypsum can be transformed into plaster, which therefore becomes less reactive. According to reaction process, PG and K2SO4 concentrations are equals, so the mixtures studied in this work are 0.7, 0.9, 1.1 and 1.3 M at 25, 40, 60 and 80 °C, respectively, which are just after saturation concentrations at these temperatures.

Search for optimal conditions of concentration and temperature

For 0.7 M of PG, DRX pattern of obtained precipitate (Fig. 2a) corresponds mostly to portlandite Ca(OH)2 (JCPDS N°: 01–076-0571) which exhibits an hexagonal structure. A very small amount of CaCO3 (JCPDS N°: 01–072-1937) was detected. The presence of calcium carbonate results from carbonation when preparing the samples. It has rhombohedra structure. A low quantity of inert quartz (JCPDS N°: 00–033-1161) is also detected. It comes from impurities included in PG remaining inert and not attacked during this conversion and appears in solid XRD patterns. Existence of unreacted PG (JCPDS N°: 01–074-1433) show that the reaction is not complete even longer than four hours. In addition, syngenite (JCPDS N°: 00–028-0739) is detected, which crystallizes in monoclinic structure, it is a salt of calcium and potassium sulfate (K2Ca(SO4)2·H2O)2 (Kps = 3.54 × 10−8 at 25 °C). The formation of syngenite is due to the oversaturation of the reaction medium by K2SO4 which reaches its maximum solubility of 0.6 M at 25 °C. Thus, K2SO4 partially reacts with PG and forms syngenite according to the following reaction:

$${\text{K}}_{2} {\text{SO}}_{4} + {\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O(s)}} + {\text{H}}_{2} {\text{O}} + {\text{K}}_{2} {\text{Ca(SO}}_{4} {)}_{2} \cdot {\text{H}}_{2} {\text{O(s)}}$$
(8)
Fig. 2
figure 2

XRD patterns of precipitates obtained at different temperatures

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To prevent the formation of this parasitic phase of syngenite, we have studied the decomposition of PG at higher temperatures (40, 60 and 80 °C) which increases K2SO4 solubility and consequently push the reaction to higher concentrations of K2SO4.

XRD pattern of the precipitate corresponding to the concentration 0.9 M of PG at 40 °C (Fig. 2b) is similar to that of 0.7 M. The increase of the concentration to 0.9 M leads to Ca(OH)2 and syngenite which appears also in the precipitate since the medium becomes over saturated by K2SO4 (reaction 8). In addition, the presence of a small amount of PG reveals that its decomposition is not complete in these conditions.

For the same purpose, we studied the decomposition of PG at 60 and at 80 °C. As before, we chose for each temperature concentrations after saturation of the medium by K2SO4. XRD patterns P 1.1 M-60 °C and P 1.3 M-80 °C (Fig. 2c, d) represent the obtained precipitates. These XRD patterns correspond to the formation of a major phase of portlandite Ca(OH)2. This occurs even in media oversaturated by K2SO4. The absence of PG indicates that its decomposition is complete. Moreover, syngenite does not appear. Apparently, this competitive and parasitic phase is not stable at these temperatures.

Salts recovered from the filtrates under these optimal conditions are also characterized by XRD. Figure 3 shows DRX patterns of obtained salts. All filtrates (Fig. 3a–d) are well crystallized and confirm the formation of arkanite K2SO4 (JCPDS No: 00–083-0682) with orthorhombic structure. A small amount of potassium carbonate K2CO3 (JCPDS No: 00–001-1001) is observed in the XRD patterns corresponding to the concentrations 0.7 M and 0.9 M of PG at 25 °C and 40 °C, respectively (Fig. 3b). It is formed when Saturated PG react with saturated K2SO4 instead of KOH, this later react with atmospheric CO2 to form K2CO3. Whereas recovered filtrates corresponding to the concentrations 1.1 M and 1.3 M of PG at 60 °C and 80 °C, respectively, correspond to arkanite K2SO4.

Fig. 3
figure 3

XRD patterns of recovered salts at different temperatures

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By combining XRD analyzes results of obtained precipitates and filtrates, it appears clear that increasing the temperature allowed the total PG conversion with high concentration and avoid the formation of syngenite at lower temperatures. The choice of 80 °C instead of 60 °C is very important because it will be possible to recover a filtrate more concentrated in K2SO4 (1.3 M instead of 1.1 M) which facilitates its recrystallization after cooling.

Therefore, the optimal conditions for the total conversion of PG by KOH to obtain a high concentration of K2SO4 are as follows: stoichiometric mixture using the concentration 1.3 M of PG during 1 h at 80 °C.

Infrared study

The products obtained by conversion of PG have also been identified by FTIR. The infrared spectrum of the PG sample is illustrated in Fig. 3 (PG). We can notice two asymmetric stretching bands at 1100 and 1004 cm−1 and two bending vibration bands at 666 and 596 cm−1 attributed to sulfate group SO42−, it appears also the band at 837 cm−1, it corresponds to HPO42− which is syncrystallized with PG [27]. FTIR spectra evolution from 25 to 80 °C show a decrease of sulfate bands in obtained precipitates P 0.7 M-25 °C and P 0.9 M-40 °C (Fig. 4a, b). Sulfate bands are due to unreacted PG or syngenite formation. For spectra of precipitates P 1.1 M-60 °C and P 1.3 M-80 °C (Fig. 4c, d), characteristic bands corresponding to portlandite Ca(OH)2 appear and those of sulfate disappear, whereas different vibration modes C–O of carbonate groups CO32− are visible at 1417 and 872 cm−1 [28, 29]. The presence of calcium carbonate is explained by the following reaction:

$${\text{CO}}_{2} + {\text{Ca}}({\text{OH}})_{2} \to {\text{H}}_{2} {\text{O}} + {\text{CaCO}}_{3}$$
(9)

Moreover, we observed a band at 1055 cm−1 assigned to Si–O stretching mode of SiO2 [13, 27].

Fig. 4
figure 4

FTIR Spectra of phosphogypsum (PG), precipitates P 0.7 M-25 °C (a), P 0.9 M-40 °C (b), P 1.1 M-60 °C (c), P 1.3 M-80 °C (d) and commercial Ca(OH)2

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The infrared spectra of obtained salts (Fig. 5) show similar characteristic bands of pure arkanite K2SO4 except for bands of carbonate group CO32−. Table 1 summaries vibration band frequencies and their assignments for PG and obtained products at optimum conditions. From these results, we can notice that the conversion of PG waste is a valid means and permits to obtain a portlandite and an arkanite quite pure.

Fig. 5
figure 5

FTIR Spectra of K2SO4 (PG) and salts F 0.7 M-25 °C (a), F 0.9 M-40 °C (b), F 1.1 M-60 °C (c), F 1.3 M-80 °C (d)

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Table. 1 Infrared and RAMAN frequencies of PG, precipitate and salt synthetized at optimal conditions
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Raman study

The Raman spectra of PG and obtained products at optimum conditions are shown in Fig. 6. The principal vibrational modes of PG, precipitate P 1.3 M-80 °C and salt F 1.3 M-80 °C are assigned in Table 1.

Fig. 6
figure 6

Raman spectra of PG, precipitate P 1.3 M-80 °C and salt F 1.3 M-80 °C

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In PG Raman spectrum, the strongest peak was found at 1011 cm−1 and was assigned to υss symmetric stretch vibration mode of SO4 tetrahedra. For pure gypsum, it appears at 1008 cm−1. The other peaks, which are weaker, correspond also to SO4 tetrahedra. The peak at 1139 cm−1 (1135 cm−1 in pure gypsum) corresponds to υ3 antisymmetric stretch vibration mode, and peaks at 619 and 669 cm−1 (620 and 670 cm−1 in gypsum) are attributed to υ4 antisymmetric bending vibration modes. PG shows for υ2 symmetric bending of the SO4 tetrahedra a doublet at 419 and 495 cm−1 (415 and 439 cm−1 in pure gypsum). Generally, the covalence of the bonds in a given structure as in the case of the ionic group SO42− and its chemical environment cause it a systematic displacement of the Raman peaks.

In Raman spectrum of the salt F 1.3 M-80 °C, peaks observed at 1146 cm−1 and 1092 cm−1 (1137 cm−1 and 1096 cm−1 in K2SO4 crystal) were assigned to υ3 asymmetric stretching vibration mode of SO4 groups. Symmetric stretching υ1 of SO4 groups is assigned in the frequency 988 cm−1 (978 cm−1 in K2SO4 crystal). The peaks observed at frequencies 620 cm−1 and 453 cm−1 (448 cm−1 in K2SO4 crystal) were assigned to the asymmetric bending δ4 of SO4 groups. The two small peaks observed at 1108 and 1065 cm−1 correspond to the symmetric stretching vibration mode υ1 of the CO32−, which is degenerate. This degeneration of the vibration mode of υ1 let’s suppose the existence of two different symmetrical stretching modes in two types of sites for CO32− ions [30].

In Raman spectrum of the precipitate P 1.3 M-80 °C, stretching vibration mode of Ca–O appears at 650 cm−1 in addition to a narrow peak observed at 3600 cm−1 typical of stretching vibration mode of OH of calcium hydroxide.

TGA-MS measurements under dry air atmosphere.

Thermal analyzes are useful for the determination of Ca (OH)2 content because it is a relatively simple and fast technique compared to other methods. Ca(OH)2 decomposes approximately at about 350 °C or 550 °C [31]. It depends on various factors, like quantity of the sample, the pressure in the instrument and material fineness or crystallinity. The TGA, Deriv. weight and DTA curves of precipitate (P 1.3 M- 80 °C) are represented in Fig. 7. TGA exhibits two weight losses at 440 °C and 680 °C. Mass spectroscopy measurements (TGA-MS) indicate that there is elimination of water (m/e 18) and carbon dioxide (m/e 44). This evidences the following decomposition reactions:

$${\text{Ca}}({\text{OH}})_{2} \to {\text{CaO}} + {\text{H}}_{2} {\text{O}}\quad {\text{(m/e 18)}}$$
(10)
$${\text{CaCO}}_{3} \to {\text{CaO}} + {\text{CO}}_{2} \quad ({\text{m/e}}\;44)$$
(11)
Fig. 7
figure 7

DTA–TGA and derivative curves of the weight loss of precipitate P 1.3 M-80 °C

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The TGA results confirm that a certain quantity of calcium carbonate (CaCO3) was produced as a result of the carbonation of Ca(OH)2 along the sample preparation process. It is really difficult to perform all sample preparation operations (like, grinding, mixing or drying) in an atmosphere free of CO2.The start of CaCO3 decomposition begins after decomposition of Ca(OH)2. The decomposition of CaCO3 takes place over a much wider temperature range (500–700 °C). From data, we determine the temperatures of the beginning and the end of each decomposition then, we report the corresponding masses in equations below to determine Ca(OH)2 and CaCO3 contents.

$${\text{\% Ca(OH)}}_{2} = { }\frac{74}{{18}} \cdot \frac{{{\text{m}}_{{{\text{start}}}}^{{\text{P}}} - {\text{m}}_{{{\text{end}}}}^{{\text{P}}} }}{{{\text{m}}_{{{\text{start}}}}^{{\text{P}}} }} \cdot 100$$
$${\text{\% CaCO}}_{3} = { }\frac{100}{{44}} \cdot \frac{{{\text{m}}_{{{\text{start}}}}^{{\text{C}}} - {\text{m}}_{{{\text{end}}}}^{{\text{C}}} }}{{{\text{m}}_{{{\text{start}}}}^{{\text{P}}} }} \cdot 100$$

where \({\text{m}}_{{{\text{start}}}}^{{\text{P}}}\), \({\text{m}}_{{{\text{end}}}}^{{\text{P}}}\), \({\text{m}}_{{{\text{start}}}}^{{\text{C}}}\) and \({\text{m}}_{{{\text{end}}}}^{{\text{C}}}\) are masses (g) obtained by the TGA measurements at the start and at the end temperatures of Ca(OH)2 and CaCO3 decompositions, respectively.

According to the formulas above, the precipitate is composed of 70.82% of Ca(OH)2 and 18.25% of CaCO3. Pressler et al. mentioned that X-ray diffraction study methods usually give a lesser CaCO3 value because it allows determining only its crystallized form [32].

The first weight losses bellow 200 °C is small (2.5%), and it is due to the removal of crystallized or adsorbed water of certain chemicals impurities like MgSiF6·6H2O, CaHPO4·2H2O, CaFPO3·2H2O.

TGA curve of salt (F 1.3 M-80 °C) indicates a small weight loss of 1.2%, which justify that this salt is anhydrous (Fig. 8). For K2SO4 prepared by the evaporation method, Anooz et al. detected a weight loss 0.8% [33]. The endothermic peak observed on the DTA curve at 585 °C corresponds to a structural transformation of K2SO4 from orthorhombic (β) to hexagonal (α) form. This transition is observed at 587 °C by M. Miyake et al. for pure K2SO4 [34].

Fig. 8
figure 8

DTA–TGA curves of salt F 1.3 M-80 °C

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Particle size

Analysis by laser granulometry gives a curve (Fig. 9) from which we can visualize the grains distribution as well as the thickness of the sample. We note that there are practically no particles larger than 1000 µm and that 71% of the sample weight results from particle sizes in the range 37–250 µm. The granulometric distribution in the precipitate (P 1.3 M-80 °C) exhibits two populations. The first one has a particle sizes around 75 µm in diameter. The second has grains of about 650 µm in diameter (Fig. 9).

Fig. 9
figure 9

Particle size distribution of precipitate P 1.3 M-80 °C

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Observation by scanning electron microscopy (SEM)

SEM technique investigates morphological structure PG and products synthesized synthetized at optimum conditions. Figure 10a illustrates that PG crystals form is tabular and their size varies from 5 to 30 μm.

Fig. 10
figure 10

SEM–EDX images PG, precipitate P 1.3 M-80 °C and salt F 1.3 M-80 °C

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Morphological structure of obtained precipitate (P 1.3 M-80 °C) indicates two distinctive shapes of particles. The first shape is predominant and slightly rounded. According to the XRD, portlandite is hexagonal, but the phenomenon of dissolution of Ca(OH)2 gives it a rounded rather than hexagonal shape. The second form of minority crystals (as indicated in yellow zones in Fig. 10b) is attributed to calcite, which was also detected by XRD crystallizing in the rhombohedral structure. Similar observations have been cited by Regnault Olivier et al. in the experimental kinetic study of the carbonation of portlandite with CO2 [35].

The obtained translucent salt F 1.3 M-80 °C recrystallized after the drying filtrate, was formed by macro-crystals of length between 3 to 6 mm and has the form of rods (Fig. 10c). This permits to get K2SO4 easily after recrystallization.

According to SEM–EDX (Fig. 10a′), we can say that PG particles are composed principally by Ca, S and O (gypsum) beside a low quantity of Si. The precipitate (P 1.3 M-80 °C) is formed essentially by Ca and O, the presence of Si, F and P is due to insoluble impurities remaining in the solid phase after filtration. The salt (F 1.3 M-80 °C) is manly composed by K, S and O. A presence of C is also detected.

BET measurement

By the nitrogen Adsorption- Desorption isotherm presented in Fig. 11, obtained precipitate (P 1.3 M-80 °C) develops a low BET surface area around 11 m2/g and pore size diameter close to 14 nm which explain that the precipitate have a mesoporous property. This isotherm can be classified as type IV according to International Union of Pure and Applied Chemistry (IUPAC) classification. To explain this result, crystalline size is calculated by Scherer equation:

$$D({\AA}) = \frac{\lambda }{\beta \cos (\theta )}.$$

where D(Å): the crystallite size, \({\uplambda }\): Cu-Kα radiation (1.5418 Å), β: Full Wide at High Middle (FWHM). This value is corrected using LaB6 standard. θ: The most intense peak position. The crystalline size calculated is 80 nm. This large crystallite size calculated is in agreement with the low surface area found by BET.

Fig. 11
figure 11

Nitrogen adsorption–desorption isotherms of precipitate P 1.3 M-80 °C

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Chemical elements distribution

Chemical analysis results of the of precipitate and salt obtained from PG conversion by KOH at 80 °C using 1.3 M of PG during one hour, are collected in Table 2. They give an idea about the existing impurities and their distribution between the powder and the salt during the total conversion reaction.

Table 2 Major elements contents in wt% of PG, precipitate (P 1.3 M-80 °C) and salt (F 1.3 M-80 °C) analyzed by ICP
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The process of PG conversion can be followed through the major elements S, Ca and K. Agreeing to the XRD and XRF results, the majority of Ca was transported from PG to the precipitate. Furthermore, maximum of the S in the PG was found in the filtrate with K coming from potassium hydroxide. Other elements such as P, Si, F and Al were also completely transported to the portlandite. As regards the trace impurities initially contained in PG, most of these elements were also found to have been completely transferred to the precipitate.

The dissolution of phosphogypsum and precipitation of portlandite implies a loss of mass and, therefore, an increase effect in the concentration of contaminants. In the filtrate, most contaminants are in lower concentrations with some values close to or even below the detection limit. Previous studies that we have done show that the distribution of major and trace elements after PG conversion by K2CO3 is similar to our results [14].

Conclusion

The high amount of phosphogypsum waste generated by the fertilizer industry in Morocco, as well as the CO2 emissions generated by the same industry make our work an attractive and environmentally friendly solution. The effect of temperature on phosphogypsum conversion was evidenced reacting a high concentration of phosphogypsum waste (1.3 M) with potassium hydroxide to produce portlandite Ca(OH)2 and potassium sulfate K2SO4. At 80 °C, the conversion is complete since we were able to avoid the formation of syngenite phase which blocked this conversion reaction. In addition, the reaction becomes faster complete after only one hour.

Obtained portlandite can be used for CO2 sequestration from the production plant and can be converted into calcium carbonate which is useful in cement industry and for the treatment of solutions with abnormally high acidity and metal concentrations like in regions of experiencing intense mining activity. Our results support the development of an economically viable carbon sequestration technology based on the reuse of this industrial waste.

In addition, since the main impurities contained in the phosphogypsum are transferred into portlandite, the obtained potassium sulfate is high purity, which allows its marketing. The temperature chosen for the conversion reaction (80 °C) makes it possible to obtain a supersaturated filtrate of K2SO4, which facilitates its recrystallization by cooling at ambient temperature. This economical process permits to profit of about 500 $ per ton of PG converted, which can largely cover transport cost and the energy used during this process and does not require any difficult operation for recuperating these marketable products.