Method for transforming dolomite into phosphate-based compounds

Abstract

The invention relates to a method for transforming dolomite-rich waste into, selectively, dicalcium phosphate, struvite and hydroxyapatite, said method comprising the following steps: 1. Dissolving the dolomite-rich waste using phosphoric acid 2. Precipitating the dicalcium phosphate by increasing the pH 3. Eliminating the calcium by adding ammonium sulfate 4. Precipitating the struvite by increasing the pH 5. Precipitating the hydroxyapatite by increasing the pH and adding calcium

Description

[0001] Process for Transforming Dolomite into Phosphate-Based Compounds FIELD OF THE INVENTION The present invention relates generally to the field of extractive metallurgy. More specifically, the invention concerns a process for valorizing dolomite-rich tailings to selectively produce dicalcium phosphate, struvite, and hydroxyapatite. PRIOR ART The transformation of dolomite can be carried out by two methods: the hydrometallurgical route and the pyrometallurgical route. The hydrometallurgical route generally begins with the leaching stage, which has been extensively studied for (i) the extraction of magnesium and calcium from dolomite ores (Royani et al., 2018) or (ii) the improvement of valuable ores that contain dolomite as a gangue mineral. The process involves the use of various acids, including inorganic (Mubarok et al., 2015) and organic (Gharabaghi ​​et al., 2010) acids.The inorganic acids that have been used so far are HCl (Altiner et al., 2017), H2SO4 (Makarov et al., 2006), and HNO3. (Pultar(et al., 2018). These acids solubilize calcium and magnesium as nitrates or chlorides, which are highly soluble and therefore difficult to recover. Conversely, when these elements are solubilized as sulfates from H₂SO₄, the resulting product is gypsum, which has little economic value. In this context, phosphoric acid has never been used, even though it could serve both as a leaching agent and as a precursor to phosphate ions. For the pyrometallurgical route, dolomite calcination is the first step. This process involves calcining dolomite ore to produce magnesium oxide and calcium oxide, known as dolime. The temperature and duration of calcination have a significant impact on purity and yield. Higher temperatures and longer durations result in higher purity and yield, but require more energy and increase production costs.This calcination (equation 1), which takes place between 700 and 1300 °C, requires a significant amount of energy and results in the emission of carbon dioxide both from the reaction itself and from the combustion of the fuel used to generate the heat (Ramakrishnan et al., 2004). The objective of the present invention is to selectively produce dicalcium phosphate, struvite, and hydroxyapatite by hydrometallurgical means from the dissolution of dolomite using phosphoric acid. The dolomite can come from various sources: natural deposits or solid mine tailings (e.g., phosphate mine tailings). The products of this reaction have already been the subject of independent synthesis methods, but have several disadvantages, including: Dicalcium phosphate is mainly synthesized by a dry reaction between calcite and phosphoric acid (H. 3AFTER 4A major technical problem with this process is the need to use pure calcite. The presence of impurities such as quartz in calcite can lead to their incorporation into the final product, thus reducing the purity of the dicalcium phosphate. Struvite is primarily synthesized from wastewater rich in phosphate and ammonium. Researchers and manufacturers tend to use Mg(NO3)2 or MgCl2, a readily available magnesium precursor. The cost of adding magnesium represents approximately 75% of the total process cost (Kataki et al., 2016). Furthermore, this wastewater does not have high concentrations of phosphate and ammonium, which limits its potential for large-scale production.The aim of the present invention is to provide an integrated process for transforming natural dolomite or dolomite-rich tailings to simultaneously and selectively produce the following three products: dicalcium phosphate, struvite, and hydroxyapatite. The inventiveness of this solution lies in the direct use of a single, abundant raw material (dolomite) while overcoming the aforementioned drawbacks and improving upon prior art processes. BRIEF DESCRIPTION OF THE INVENTION The invention aims to valorize dolomite-rich tailings to selectively produce dicalcium phosphate, struvite, and hydroxyapatite. The DOLPHOS process is a method for treating dolomite-rich tailings to selectively obtain dicalcium phosphate, struvite, and hydroxyapatite.This process takes place in several stages: the dissolution of the waste using phosphoric acid; the precipitation of dicalcium phosphate through an increase in pH; the removal of residual calcium by adding ammonium sulfate; the precipitation of struvite following a further increase in pH; and finally, the precipitation of hydroxyapatite, which requires a further increase in pH as well as the addition of calcium.

[0002] BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the invention will become apparent upon reading the following detailed description, for which reference should be made to the accompanying drawings, in which: - Figure 1 is a diagram of said process. - Figure 2 shows an X-ray diffraction analysis of dolomite-rich tailings. This figure illustrates the mineralogical composition of the material to be treated. - Figure 3 shows an X-ray diffraction analysis of precipitated dicalcium phosphate. - Figure 4 illustrates the X-ray diffraction of the struvite obtained. - Figure 5 illustrates the X-ray diffraction of precipitated hydroxyapatite. DETAILED DESCRIPTION OF THE INVENTION The said invention represents a process for transforming dolomite-rich tailings by a series of hydrometallurgical steps in order to selectively obtain up to three products depending on the initial feed mineralogy.These products are: dicalcium phosphate, struvite, and hydroxyapatite. The process is illustrated in Figure 1. Composition of the raw material to be processed: The material used in this invention comes from dolomite-rich tailings from phosphate extraction. This material is mainly composed of dolomite, but may also contain calcite, quartz, and fluorapatite. The mineralogical composition of this material is detailed in Figure 2. The dolomite-rich material to be processed must have a MgO content greater than or equal to 8%. Dissolution: The first step of the process involves the acid dissolution of the dolomite-rich tailings using phosphoric acid (H3PO4). The process is shown in Equation 2. The molar concentration of this acid varies between 0.5 mol / L and 5 mol / L; the higher the acid concentration, the greater the dissolution.The solid-to-liquid ratio used ranges from 1:50 to 1:5, allowing for flexibility; the higher the amount of RRD, the lower the dissolution rate. This dissolution reaction takes place at a temperature between 18°C ​​and 60°C. The duration of this reaction can vary from 3 to 60 minutes. Furthermore, a stirring speed of between 200 and 800 revolutions per minute (rpm) is used to ensure homogeneous mixing and efficient interaction between the solid particles and the phosphoric acid. The residue from this step consists of minerals that are not leached by the phosphoric acid, notably quartz and clays. This residue is removed by vacuum filtration, yielding a solution S1 rich in calcium, magnesium, and phosphates, as well as a solid rich in silicon. 2+ + ^^^^ 2+ + 2^^^^2+ 2^^2^^ + 2^^^^^^ 4 2−(2) Precipitation of Dicalcium Phosphate The resulting solution S1 from the first step is colorless and has a pH between 0.7 and 1.8. The second step consists of selectively precipitating the dicalcium phosphate, according to equation 3, by raising the pH of solution S1 to a value between 2 and 4, using one of the alkaline agents such as NaOH, Mg(OH)2, or Ca(OH)2. This operation is carried out at room temperature with constant stirring at 400 rpm. The duration of this precipitation is 30 minutes. 2+ + ^^^^^^ 4 2−+ 2^^2^^ → ^^^^^^^^^^4 ∙ 2^^2^^ (3) The precipitated dicalcium phosphate was mainly in the form of brushite [CaHPO₄·2H₂O], while monetite [CaHPO₄] was present in small proportions (Figure 3). Removal of residual calcium Depending on the calcium recovery rate in the previous step, removal of residual calcium from solution S2 may be necessary. In this case, calcium ions can be precipitated as CaSO₄ by adding (NH₄)₂SO₄ according to the precipitation reaction shown in equation 4. This step is essential because a high calcium concentration in the next step could compromise the efficient precipitation of struvite in the following step. ^^^^ 2+ + ^^^^ 4 2−+ 2^^2^^ → ^^^^^^^^4 ∙ 2^^2^^ (4) This step is carried out at room temperature and lasts 55 minutes, with stirring at 400 rpm. Struvite Precipitation Struvite precipitation from solution S3 is carried out at a pH between 7 and 9 by adding NaOH to shift the phosphate ion equilibrium towards the PO4 form 3- The chemical reaction of struvite precipitation is shown in equation 5. This operation is carried out at room temperature, and under constant stirring of 400 revolutions per minute, with a maturation time that can vary from 0 (no maturation) to 7 days. 4.4 The precipitated struvite was characterized by X-ray diffraction (Figure 4), and no phase other than struvite was detected. Hydroxyapatite Precipitation: After struvite precipitation, hydroxyapatite forms when the pH is raised above 10. To achieve this, a calcium source is added according to Equation 6, for example, Ca(NO3)2, CaCl2, or Ca(OH)2, although these options are not limiting. The pH increase is achieved using NaOH or Ca(OH)2. 3.4 3− + 5^^^^ 2+ + ^^^^ −→ ^^^^5(^^^^4)3^^^^ (6) The precipitated hydroxyapatite was characterized by X-rays (Figure 5) and it was shown that there is a small proportion of brucite. INDUSTRIAL APPLICATION The process of which this application is the subject of is designed for the chemical industry, and more particularly for the sectors of fertilizer production and feed additives. The present invention is in no way limited to the embodiments described and illustrated, but those skilled in the art will be able to make any variation in accordance with their intent. REFERENCES (1) Royani, A.; Sulistiyono, E.; Prasetiyo, AB; Subagja, R. Extraction of Magnesium from Calcined Dolomite Ore Using Hydrochloric Acid Leaching; Jakarta, Indonesia, 2018; p 020017. https: / / doi.org / 10.1063 / 1.5038299. (2) Mubarok, MZ; Adi Kurniawan, C. Synthesis of Magnesia Powder from East Java Dolomite through Leaching, Precipitation and Calcination. AMR 2015, 1112, 550–554. https: / / doi.org / 10.4028 / www.scientific.net / AMR.1112.550.(3) Gharabaghi, M.; Irannajad, M.; Noaparast, M. A Review of the Beneficiation of Calcareous Phosphate Ores Using Organic Acid Leaching. Hydrometallurgy 2010, 103 (1–4), 96–107. https: / / doi.org / 10.1016 / j.hydromet.2010.03.002. (4) Altiner, M.; Yildirim, M. Production and Characterization of Synthetic Aragonite Prepared from Dolomite by Eco-Friendly Leaching–Carbonation Process. Advanced Powder Technology 2017, 28 (2), 553–564. https: / / doi.org / 10.1016 / j.apt.2016.10.024. (5) Makarov, V. N.; Makarov, D. V. Reaction of Dolomite with Dilute Solutions of Sulfuric Acid and Iron(II), Copper (II), and Zinc(II) Sulfates. Russ. J. Inorg. Chem. 2006, 51 (3), 347–349. https: / / doi.org / 10.1134 / S0036023606030028. (6) Pultar, M.; Vidensky, J.; Sedlarova, I. Study of the Reaction between Dolomite and Nitric Acid. Physicochemical Problems of Mineral Processing; ISSN 2084-4735 2018. https: / / doi.org / 10.5277 / PPMP18140. (7) Ramakrishnan, S.; Koltun, P.Global Warming Impact of the Magnesium Produced in China Using the Pidgeon Process. Resources, Conservation and Recycling 2004, 42 (1), 49–64. https: / / doi.org / 10.1016 / j.resconrec.2004.02.003. (8) Kataki, S.; West, H.; Clarke, M.; Baruah, D. C. Phosphorus Recovery as Struvite: Recent Concerns for Use of Seed, Alternative Mg Source, Nitrogen Conservation and Fertilizer Potential. Resources, Conservation and Recycling 2016, 107, 142– 156. https: / / doi.org / 10.1016 / j.resconrec.2015.12.009.

Claims

CLAIMS 1. A process for transforming dolomite-rich tailings to selectively obtain three products: dicalcium phosphate, struvite, and hydroxyapatite, comprising the following steps: Acid dissolution of the dolomite-rich tailings with phosphoric acid. Selective precipitation of the dicalcium phosphate. Removal of residual calcium. Precipitation of the struvite. Precipitation of the hydroxyapatite.

2. The process according to claim 1, wherein the raw material to be treated has a MgO content greater than or equal to 8%.

3. The process according to claim 1, wherein the acid dissolution of the dolomite-rich tailings is carried out with a molar concentration of phosphoric acid ranging from 0.5 mol / L to 5 mol / L, with a solid-liquid ratio ranging from 1:50 to 1:

5. 4.The process according to claim 1, wherein the selective precipitation of dicalcium phosphate is carried out by increasing the pH of the solution to a value between 2 and 4, using one of the alkaline agents such as NaOH, Mg(OH)2, or Ca(OH)2.

5. The process according to claim 1, wherein the precipitation of dicalcium phosphate is carried out mainly in the form of brushite.

6. The process according to claim 1, wherein the removal of residual calcium is carried out by precipitating the calcium ions as CaSO4 by adding (NH4)2SO4.

7. The process according to claim 1, wherein the precipitation of struvite is carried out at a pH between 7 and 9.

8. The process according to claim 1, wherein the precipitation of hydroxyapatite is carried out by increasing the pH above 10, with the addition of a calcium source, such as Ca(NO3)2, CaCl2, or Ca(OH)2, but not limited to these. 9.The process according to claim 1, wherein the reaction temperature is: • For step a: between 18 °C and 60 °C • For step b: ambient temperature • For step c: ambient temperature • For step d: ambient temperature.

10. The process according to claim 1, wherein the duration of each reaction is: • For step a: from 3 minutes to 60 minutes • For step b: 30 minutes • For step c: 55 minutes • For step d: instantaneous to 7 days 11. The process according to claim 1, wherein constant stirring is maintained at a speed of: • For step a: between 200 and 800 revolutions per minute (rpm) • For step b: 400 rpm • For step c: 400 rpm • For step d: 400 rpm

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