Synthesis and characterisation of ettringite derived from phosphogypsum

Abstract

The invention relates to an unconventional synthesis method for producing ettringite from phosphogypsum, with the aim of upgrading said industrial byproduct. The method transforms a waste product from the phosphate industry into a material that can be used for applications such as thermal energy storage and construction. By using advanced analysis techniques such as XRD, SEM, FTIR, TGA and DSC, the method ensures the effective formation of ettringite, thereby providing an economical alternative to conventional gypsum.

Description

[0001] SYNTHESIS AND CHARACTERIZATION OF ETTRINGITE DERIVED FROM PHOSPHOGYPSE

[0002] FIELD OF INVENTION

[0003] The present invention relates to an unconventional synthesis process for the production of ettringite from phosphogypsum, with the aim of valorizing this by-product. This innovative process transforms an industrial waste product from the phosphate industry into a useful material, offering potential applications in thermal energy storage and construction, among other sectors, thanks to the unique properties of ettringite.

[0004] EARLIER ART

[0005] Ettringite, whose chemical formula is Ca6Al2(SO4)3(OH)i2-26H2O, or C6AS3H32 as known in cement chemistry notation, is also called calcium trisulfoaluminate hydrate or AFt and is an important member of the group called ettringite.

[0006] In nature, ettringite is considered a rare mineral phase. However, it is commonly and abundantly present in cement paste. Various methods have been explored for the synthesis of ettringite, including methods based on solution conversion and those based on solid-state conversion.

[0007] Solution-based conversion

[0008] Several studies have used this procedure because it is simple, inexpensive, and rapid

[0001] -[5]. In the solution-based ettringite conversion, calcium hydroxide (Ca(OH)2) and aluminum sulfate octadecahydrate (Al2(SO4)3·18H2O) were used as primary reagents with the addition of NaOH to control the pH of the solution. The initial step consisted of dissolving the NaOH in boiled distilled water to remove CO2 before use. The pH must be between 11 and 12.5 to ensure complete ettringite formation. Subsequently, the mixture was stirred for one hour in a closed reactor under nitrogen gas saturation to minimize CO2 contamination.After this, the initial precursors were introduced, and the suspension was thoroughly stirred for several days for complete formation, as shown in reaction 1 [6]. Once the compound was produced, it was vacuum-filtered and washed using filtered and demineralized distilled water (FDDW) and ethanol. The resulting powder sample was then carefully dried and stored.

[0009] Reaction 1: A12(SO4)3• XH2O + 6Ca(OH)23CaO • A12O3• 3CaSO4• 32H2O Where X = 14-18. Ca(OH)2 and Al2(SO4)3- XH2O are used respectively as sources of CaO and Al2O3-SO3.

[0010] Solid-based conversion

[0011] Ettrinitis maintained by CA or C3A

[0012] C3A or CA are prepared from the calcination of a stoichiometric mixture of Al2O3 and CaO (or CaCO3). They are calcined together above 1300 °C [7], [8]. Ettringite crystallizes in an aluminate-rich gel. In a typical ettringite synthesis, C3A and commercial CaSO42H2O are mechanically mixed and then stirred in filtered, demineralized distilled water (FDDW). After 10 days of gentle stirring at room temperature in a closed container, to exclude CO2, the precipitate is filtered and dried in CO2-free air. Several studies have used this procedure

[0001] , [4], [9]-

[0011] . The formation of ettringite is shown in reaction 2

[0001] , Reaction 2 3CaO • Al2O3 + 3CaSO4 • 2H2O + 26H2O 3CaO • Al2O3 • 3CaSO4 • 32H2O

[0013] Ettrinitis conferred by CAC

[0014] The synthesis of ettringite from sulfoaluminate cement (CAC) is a well-documented and patented method. According to several studies and patents

[0012] –

[0016] , the process begins by mixing sulfoaluminate cement with water. Sulfoaluminate cement typically consists of sulfoaluminate clinker and anhydrous gypsum. The mixture is then gently stirred at room temperature to promote ettringite crystallization. After a stirring period that can last several weeks, the precipitate is filtered and air-dried.

[0015] In a recent patent (FR3044082B1)

[0016] Ettringite was synthesized as described above by mixing sulfoaluminate cement with water. In another patent (CN114804790A)

[0015] , an ettringite-based energy storage material is prepared using sulfoaluminate cement and water, with the addition of a foaming agent to create a porous structure. This lightweight calcium aluminate and aluminum sulfate foam is then formed and cured, thereby improving its mechanical and thermal properties for applications such as road de-icing. Furthermore, another patent (CN116375436A)

[0014] describes a method for preparing a cement-stabilized macadam mixture. This mixture consists of Portland cement, macadam, ceramsite, slag, water, phosphogypsum, carbide slag, and basalt fiber. The added phosphogypsum reacts with tricalcium aluminate (C3A) to generate traces of ettringite.In this way, the solidification rate of the cement can be slowed down. Furthermore, this prevents the surface layer of cement from clumping together as it solidifies.

[0016] In this study, we focus on using an unconventional method to synthesize ettringite using phosphogypsum, a by-product generated by various industries involved in the production of phosphoric acid and phosphate fertilizers, as a viable substitute for gypsum. This approach departs from traditional methods and highlights the potential of using alternative materials such as phosphogypsum.

[0017] BRIEF DESCRIPTION OF THE INVENTION

[0018] The invention presents an innovative approach to synthesizing ettringite from phosphogypsum, an industrial by-product, using an unconventional synthesis process. This method offers an economical alternative to traditional gypsum by valorizing a waste product containing heavy metals and high levels of naturally occurring radioactive materials. This valorization of phosphogypsum reduces material costs. To confirm the formation of the ettringite structure, analytical techniques such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), and Differential Scanning Calorimetry were used.

[0019] (DSC) were used. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other features and advantages of the invention will become apparent upon reading the detailed description that follows, for understanding of which reference should be made to the accompanying drawings, in which:

[0021] - Figure 1 illustrates the stability and formation of ettringite in an alkaline environment.

[0022] - Figure 2 shows the results of X-ray diffraction (XRD) analyses of all synthesized materials (Ett-Ca(OH)2, Ett-PG1 & Ett-PG2, and C3A-PG).

[0023] - Figure 3 shows the characteristic vibrational bands of ettringite obtained through FTIR analysis.

[0024] - Figure 4 is a micrograph of the developed ettringite materials, revealing the shape and size distribution of the particles.

[0025] - Figure 5 shows the results of the TGA / DTG analyses observing mass losses from 25 °C up to 1000 °C with a rate of 10 K.min' 1 .

[0026] - Figure 6 shows the DSC analysis for measuring enthalpy during material dehydration via the integration of the heat flux released by the endothermic reaction.

[0027] - Table 1 presents the results of the enthalpy related to the dehydration of the materials.

[0028] DETAILED DESCRIPTION OF THE INVENTION

[0029] The initial phase of this research consisted of synthesizing ettringite from calcium hydroxide and aluminum sulfate, as a fundamental step for conducting a comparative study. Subsequently, we carried out two separate syntheses of ettringite, with different configurations and precursors, which allowed for a comprehensive comparison between the resulting materials.

[0030] Synthesis 1 (Reference Ett-Ca(OH)2)

[0031] In the synthesis of ettringite, we followed the synthesis protocol cited in the literature [11-[5]. We used calcium hydroxide (Ca(OH)2) and aluminum sulfate octadecahydrate (Al2(SO4)3·18H2O) as primary reagents and adjusted the pH of the solution by adding NaOH to maintain it in the range of 11 to 12.5, thus ensuring complete ettringite formation (Fig. 1). The mixture was then stirred for one hour in a sealed reactor under nitrogen gas saturation to prevent CO2 contamination. The resulting wet ettringite sample was washed, filtered, and dried.

[0032] Summary 2 (Ett-PG1 & Ett-PG2)

[0033] In the synthesis of ettringite, two materials were prepared using PG. Initially, PG dissolved in water at room temperature was introduced into a sealed reactor filled with nitrogen gas to prevent CO2 contamination. Subsequently, Al(OH)3 dissolved in water was added to the reactor. Finally, NaOH was introduced to regulate the pH of the solution, which was maintained at 12.2. Maintaining the pH in the range of 11 to 12.5 was crucial for complete ettringite formation (Fig. 1). All precursors were then thoroughly stirred for 2 h and 10 h to synthesize Ett-PG1 and Ett-PG2, respectively, and the resulting precipitates were washed, filtered, and dried.

[0034] Synthesis 3 (C3A-PG)

[0035] C3A is obtained by calcining a stoichiometric combination of Al2O3 and CaO (or CaCO3) at temperatures exceeding 1300°C. In a standard ettringite synthesis, C3A and PG were first mechanically mixed and then blended in distilled water. After three days of gentle stirring at room temperature in a sealed container to prevent CO2 contamination, the precipitate was filtered and dried.

[0036] Results

[0037] Referring to Figure 2, X-ray diffraction analyses were used to evaluate the crystalline phases in the synthesized materials. For the reference material synthesized with Ca(OH)₂, the XRD results confirmed the successful formation of ettringite, consistent with the reference JCDP file (COD 9011576). The ettringite crystallized within a hexagonal structure characterized by the space group (P₃1c). Furthermore, minor traces of calcium sulfate were observed, attributable to the high sulfate concentration at high pH, ​​as illustrated in Figure 1, which is consistent with literature results.

[0038] XRD results for samples Ett-PG1 and Ett-PG2 (Fig. 2) also confirmed the presence of ettringite. However, persistent traces of calcium sulfate were observed in Ett-PG1, likely due to the increased sulfate concentration, similar to the reference sample. In contrast, Ett-PG2 showed greater purity, without any peaks belonging to calcium sulfate. Significantly, XRD analysis of the material synthesized with C3A also indicated the formation of ettringite with a high degree of purity.

[0039] In the comparative analysis of ettringite synthesis from phosphogypsum versus commercial calcium sulfate, our results favored phosphogypsum, which exhibited a higher level of purity. This quality can be attributed to the presence of other impurities in phosphogypsum, which may enhance the reaction kinetics. Therefore, using phosphogypsum as a precursor in the synthesis process shows promise for producing a purer form of ettringite.

[0040] As shown in Figure 3, the FTIR spectra exhibit distinct peaks, providing valuable information on the molecular structure of the material, with all major peaks unequivocally attributed to ettringite

[0017] , thus confirming its successful synthesis. The spectra showed two discernible regions: the high-frequency range (4000–2500 cm⁻¹) 1) mainly consisted of OH stretch vibrations, characterized by the asymmetric mode of 3629 cm' 1 and the symmetrical mode of 3407 cm' 1 originating from [AI(OH)e] 3 ', which is consistent with the earlier assignments of Eva et al.

[0017] , to 3713 and 3672 cm' 1 In the low-frequency region below 2000 cm' 1 The interpretation was complicated by the presence of overlapping vibration bands. The OH bending vibration of water molecules was distinctly represented by the 1663 cm' band. 1 , as also documented by Myneni et al

[0018] at 1650 cm' 1 The observed splitting in the HOH bending mode has been attributed to "non-equivalent" water molecules in the crystal structure, originating from different hydrogen bond strengths and structural environments. The band at ~1445 cm⁻¹ 1band is attributed to a carbonate impurity

[0019] , While a prominent band at 1108 cm' 1 probably corresponds to the antisymmetric stretching mode (SCU) 2 '. Furthermore, the band between 851 and 853 cm' 1 is indicative of the hydroxyl deformation mode associated with the AIOH units

[0018] . Figure 4 presents the micrographs that were recorded for the synthesized ettringite materials (Fig. 4). The results show that the materials exhibited an abundance of needle-like ettringite crystals, arranged in a complex manner on the samples with an average grain size on the order of 10 µm. This needle-like morphology has been well documented in the literature as a characteristic feature of ettringite

[0020] , further validating the identification of this mineral formation for all the materials.

[0041] Referring to Figure 5, TGA / DTG analyses were used to explore the thermal behavior of the materials. The reference material Ett-Ca(OH)2 revealed two distinct mass losses, each linked to specific thermal events. The primary mass loss of 31.62% corresponds to the dehydration of ettringite, involving the removal of 22 water molecules from its crystalline structure, starting at 68 °C. The second mass loss, representing 3.74% of the sample mass, was associated with the decomposition of calcite; interaction with air during synthesis likely influenced its formation. These results are consistent with those reported in the literature

[0014] , indicating a comparable mass loss of approximately 32.83% relative to the initial sample weight, which is very similar to the previous mass loss of 29.3%.

[0042] In the case of Ett-PG1 and Ett-PG2, the two ettringite forms exhibited distinct thermal behaviors. For Ett-PG1, TGA / DTG analysis also revealed three distinct mass losses. The primary mass loss of 25.57% indicated ettringite dehydration (removal of 18 water molecules), and a further mass loss of 2.61% was due to the dehydration of trace gypsum, starting at 74.2 °C. The second mass loss, representing 7.55% of the sample mass, was related to the dehydroxylation of Al(OH)3

[0015] , while the third mass loss, constituting 4.01% of the sample mass, was associated with calcite decomposition. On the other hand, Ett-PG2 revealed that a mass loss belongs to the ettringite, which confirms the purity of the material with a loss of about 29.7% by removing 21 water molecules.

[0043] Similarly, in the case of C3A-PG, the analysis also revealed three distinct mass losses. The primary mass loss of 23.78% indicates dehydration of ettringite (removal of 18 water molecules) at a slightly lower temperature of 67.9 °C. The second mass loss (9.3% of the sample mass) was attributed to the dehydroxylation of Al(OH)3

[0015] , and the third mass loss (4.33% of the sample mass) was associated with the decomposition of calcite.

[0044] Comparison between the reference material, Ett-Ca(OH)2, and the materials synthesized using phosphogypsum (Ett-PG1, Ett-PG2, and C3A-PG) in TGA / DTG analysis reveals similar results. However, differences are observed in the temperature of appearance and the percentage of mass loss. Although the primary mass loss corresponding to the dehydration of ettringite is constant among these materials, the variations in the temperature of appearance and mass loss are apparent due to the use of different synthesis protocols and precursors. These results are generally similar to those reported in the literature.

[0045] Regarding the DSC analysis (Fig. 6), it provides values ​​for the energy released during the first dehydration of the developed materials, from room temperature up to 300°C using a heating rate of 10°C / min. The analysis shows promising results, particularly for the phosphogypsum-based ettringite, where the dehydration enthalpy ranges from 435 J / g to 466.2 J / g for Ett-PG2 and Ett-PG1, respectively. Table 1 presents a summary of the enthalpy results compared to conventionally synthesized ettringite reported in the literature.

[0046] The phosphogypsum-derived samples (Ett-PG1, Ett-PG2 and C3A-PG) exhibit similar enthalpy values, compared to 600 J / g for ettringite synthesized from conventional precursors

[0016] . This presents great potential for phosphogypsum-derived materials to be used for thermochemical energy storage applications.

[0047] INDUSTRIAL APPLICATION

[0048] Ettringite has various applications, particularly in thermal energy storage and the construction sector, thanks to its unique properties. However, the primary purpose of ettringite is the storage of thermal energy in the form of chemical bonds, which could have numerous industrial applications, such as:

[0049] - The storage of thermal energy generated in concentrated solar power (CSP) plants with a storage density much higher than that of molten salts.

[0050] - Storing excess electricity using the "Power to Heat" concept.

[0051] - Recovery of residual heat in industrial processes such as the steel, cement and glass industries.

[0052] - To provide heat for the thermal desalination of seawater.

[0053] - La production d'eau chaude sanitaire.

[0054] - Le stockage saisonnier de la chaleur.

[0055] REFERENCES

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Claims

DEMANDS 1. A process for the synthesis of ettringite (ETT-PG1 and ETT-PG2) using phosphogypsum comprising the following steps: a) Dissolve 1.8 g of phosphogypsum in 1000 mL of water at room temperature and then introduce the resulting solution into a sealed reactor filled with nitrogen gas to prevent contamination by carbon dioxide (CO2); b) Add 0.186 g of aluminum hydroxide (Al(OH)3) dissolved in 250 mL of water to the phosphogypsum solution in the reactor; c) Introduce 0.6 g of NaOH dissolved in 250 mL of water to regulate the pH of the solution; d) Stir the precursors for a specified time to synthesize different types of ettringite, namely: i. Stir for 2 hours to synthesize a first type of ettringite (Ett-PG1). ii. Shake for 10 hours to synthesize a second type of ettringite (Ett-PG2); e) Subject the resulting precipitates to washing, filtration and drying steps to obtain the final products.

2. The process according to claim 1, characterized in that the synthesis reaction takes place in a solution whose pH is maintained between 11 and 12.5, thus facilitating the complete formation of ettringite.

3. The process according to claims 1 and 2, characterized in that the synthesis of ettringite uses aluminium hydroxide (Al(OH)3) and phosphogypsum (PG) and PG as basic reagents, with pH adjustment by addition of NaOH.

4. The process according to claims 1 to 3, characterized in that the synthesis of ettringite is carried out from phosphogypsum which has not undergone any preliminary treatment to remove potential impurities, thus directly using the crude phosphogypsum to explore its natural reactive potential.

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