PROCESS FOR THE PRODUCTION OF ZINC PHOSPHATE

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a process for the manufacture of zinc phosphate catalyzed by ultrasound. BACKGROUND OF THE INVENTION

[0002] Zinc phosphate (Zn 3 (PO 4 ) 2 ) generally occurs as a white solid, insoluble in water. It exists in nature as an ore and can occur in different forms, more or less hydrated and / or complexed with other metals, forming hopeite, parahopeite, tarbuttite, phosphophyllite, etc.

[0003] Zinc phosphate has been widely used for many years in cement and dentistry, as well as in several other industrial applications. One of the major advantages of zinc phosphate is that it provides materials with extremely effective bonding and sealing properties. It is also known for its performance and its rapid setting. It is also used for its anti-corrosion properties in the protection of metal surfaces, thanks, among other things, to its excellent adhesion properties. Thus, zinc phosphate is widely used as an anti-corrosion mineral pigment in protective coating systems. Paint manufacturers use it to produce industrial anti-corrosion paints, for example in automotive, aeronautical, or marine paints [1].

[0004] Zinc phosphate is a phosphate derivative. It is formed by the reaction of zinc oxide in aqueous slurry with purified phosphoric acid. At the end of the reaction, the water is filtered off, and the zinc phosphate in the filtrate is dried before being ground and packaged. The setting reaction is a chemical reaction between a liquid and a solid. The action of phosphoric acid on zinc oxide results in the formation of various hydrated phosphates. Thus, there are four types of reactions: ZnO + 2 H 3 PO 4 → Zn(H 2 PO 4 ) 2 , H 2 O ZnO + H 3 PO 4 + 2 H 2 O → ZnHPO 4 , 3 H 2 O 3ZnO + 2 H 3 PO 4 → Zn 3 (PO 4 ) 2 , 2H 2 O + H 2 O ZnO + 2 H 3 PO 4 + H 2 O → Zn 3 (PO 4 ) 2, 4 H 2 O

[0005] The compound Zn 3 (PO 4 ) 2 .4H 2 O, also called hopeite, exists in two allotropic forms α and B of space group Pnma, with extremely close lattice parameters and crystal structures, the only differences being the position of the hydrogen atoms which induces birefringence and distinct surface charges for the two compounds [1, 6]. However, whatever the allotropic form, hopeite has limited stability as a function of temperature. Indeed, the material begins to dehydrate as early as 70°C [2]. Hopeites of α and B form have exactly the same dehydration mechanisms at the same temperatures. The first stage begins at around 70°C and has a critical temperature T1 / 2 of 120°C. It results in the loss of two water molecules to obtain a product of formula Zn 3 (PO 4 ) 2 .2H 2 O. The last stage around 300°C is due to the loss of the last two water molecules to obtain an anhydrous product of formula Zn 3 (PO 4 ) 2 [2].The structure of hopeite and details of the zinc coordination polyhedra in the structure are shown in . figure 1 .

[0006] Hopeite is made up of a chain of [ZnO 4 ] and [PO 4 ] tetrahedra and [ZnO 2 (H 2 O) 4 octahedra. The structure can be represented as the alternation of three planes respectively made up of [ZnO 2 (H 2 O) 4 ] octahedra (denoted O) of [ZnO 4 ] tetrahedra (denoted Z) and [PO 4 ] (denoted P) along the b direction, by repetition of the OPZP motif. The lattice parameters of α and B hopeites and of the dihydrate product obtained after the first dehydration are presented in Table 1. Table 1 : Lattice parameters of α and B hopeite and the compound Zn 3 (PO 4 ) 2 .2H 2 O [2] Settings αZn 3 (PO 4 )H 2 O βZn 3 (PO 4 ) 2 .4H 2 O Zn 3 (PO 4 ) 2 .2H 2 O GE First Pnma P2 1 / c 10.629(2) 10.6060(4) 10.451(7) b (Å) 18.339(3) 18.2946(5) 5.036(3) c (Å) 5.040(1) 5.0266(2) 31.437(15) Angle β 90° 90° 92.46°

[0007] For temperatures above 300°C, hopeite dehydrates completely and gives an anhydrous orthophosphate of formula Zn 3 (PO 4 ) 2 . Zinc orthophosphate has, according to the literature, three allotropic varieties, all monoclinic α, B and γ [2]. At room temperature, two phases are stable: the α and γ phases. The B phase transitions reversibly to the B phase at 965°C and it should be noted that the B phase is only observable at room temperature after rapid quenching. The space group and the lattice parameters of the three structures α, B and γ Zn 3 (PO 4 ) 2 are presented in Table 1. The crystal structures of the phases are presented in figure 2 The coordination polyhedra of zinc are shown in green, those of phosphorus in purple.

[0008] Different methods have been proposed for preparing zinc phosphate.

[0009] US 2407301 [3] describes a process based on the treatment of zinc oxide with phosphoric acid in an aqueous medium at a temperature ranging from 70 °C to 80 °C in the presence of metallic zinc taken in an amount ranging from 1 to 10% by weight of zinc oxide, followed by the separation of the resulting residue of zinc phosphate. The yield of zinc phosphate is 98%. The process makes it possible to produce zinc phosphate ensuring high protective and physico-mechanical properties of coatings.

[0010] Zinc phosphate was synthesized by S. Rameshet et al.[4] by another method comprising the addition of orthophosphoric acid to a zinc acetate solution under constant stirring followed by the addition of hydrazine hydrate to the resulting solution under stirring for 3 hours. The resulting white precipitate is filtered and washed with water and then with ethanol to remove organic impurities. The precipitate is then calcined in a muffle furnace for 24 hours at 300°C. The morphology and optical properties were studied by XRD, SEM, TEM, FT-IR and Raman spectroscopy.

[0011] CN 1164508 [5] proposes another process for the production of zinc phosphate. The zinc oxide used has a lower quality degree (purity varying from 50% to 95%) and the phosphoric acid is obtained by liquid-liquid extraction. The reaction temperature varies from 70 to 90°C, the reaction time varies from 40 to 60 min and the stirring speed varies from 4 to 6 rpm.

[0012] EP0009175 A1 proposes a process for the production of zinc phosphate, of formula Zn 3 (PO 4 ) 2 ,nH 2 O (n=0 to 4), by a reaction between zinc oxide (20-85% by weight) and phosphoric acid (10 to 50% by weight) at a temperature ranging from 10 to 100°C, at a stirring speed ranging from 3000 to 10,000 rpm. Zinc oxide and high purity phosphoric acid (membrane process) were used in a molar ratio of 1.5 to avoid an acidic pH in the reaction mixture, this reaction was followed by separation and drying of the final product. This process made it possible to produce zinc phosphate having high protective, physicochemical and mechanical properties as a component of paint coating based on synthetic binders. The yield obtained is ≥ 98%.

[0013] EP0009717 illustrates a process for producing zinc phosphate from zinc oxide, phosphoric acid and a diluent under the influence of ultrasound at a frequency of 20 or 40 kHz.

[0014] Jung et al. (Bull. Korean Chem. Soc. 2009. Vol.30, No.10) proposes a method for preparing zinc phosphate nanospheres from zinc phosphate tetrahydrate in the presence of ammonia using ultrasound at a frequency of 20 kHz.

[0015] Onoda et al. (International Journal of Cosmetic Science, 2014, 36, 321-326) propose a process for preparing zinc phosphate from zinc nitrate and phosphoric acid in the presence or absence of sodium lactate under the influence of ultrasound at a power of 26W.

[0016] Yu et al. (Rare Met. (2016) 35(3):211-222) reports on the advances in the field of preparation of nanostructured photocatalysts by sonochemical route. The ultrasound frequency ranges from 10 to 1000 kHz. A process for the preparation of zinc phosphate is not reported.

[0017] Thus, even though various processes are known for preparing zinc phosphate, there remains a need for the provision of a simple, efficient and economical process (low investment and operating costs) for manufacturing high purity zinc phosphate at room temperature. BRIEF DESCRIPTION OF THE INVENTION

[0018] The present invention relates to a process for preparing zinc phosphate comprising a step of reacting zinc oxide with phosphoric acid, preferably purified industrial phosphoric acid, under the action of ultrasound having a frequency greater than 100 kHz.

[0019] Other aspects of the invention are as described below and in the claims. FIGURES

[0020] Figure 1 : Structure of hopeite α [1]. Figure 2 : Crystal structure of the α, B and γ Zn 3 (PO 4 ) 2 phases. Figure 3 : X-ray diffractogram, (Zn 3 (PO 4 ) 2 , 4H 2 O) at 20°C obtained according to example 2. Figure 4 : SEM observation of zinc phosphate (Zn 3 (PO 4 ) 2 ) obtained according to example 2 (A: magnification X259; magnification B: X1000; C: magnification X2000; D: magnification X3084). Figure 5 : X-ray diffractogram, Zn 3 (PO 4 ) 2 , 2H 2 O, Zn 3 (PO 4 ) 2 , 4H 2 O, ZnO at 20°C obtained according to comparative example 1. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention relates to a process for preparing zinc phosphate, particularly zinc phosphate having a high purity (≥ 98%), from phosphoric acid, particularly from a purified industrial phosphoric acid, under the action of high frequency ultrasound.

[0022] Ultrasound is a sinusoidal acoustic wave with a frequency range between 16 kHz and 10 MHz. The unit of measurement for ultrasound is the Hertz (Hz). The use of ultrasound in chemistry was a mere curiosity a few decades ago. The concept of acoustic cavitation was little known in the field of applied chemistry. With the high cost of organic reagents and their often toxic characteristics, the use of ultrasound has become an interesting avenue for reducing reagent usage and reaction times.

[0023] The inventors of the present invention, who benefit from long experience in the activation of chemical reactions by ultrasound, demonstrated by a series of articles in this field [7-11], have thus developed a simple and effective process for preparing zinc phosphate from phosphoric acid, in particular purified industrial phosphoric acid, by a reaction, typically at room temperature, under ultrasonic activation.

[0024] Thus, the process for preparing zinc phosphate of the present invention comprises a step of reacting zinc oxide with phosphoric acid under the action of ultrasound whose frequency is greater than 100 kHz and typically less than 1 MHz.

[0025] Ultrasound with a frequency greater than 100 kHz and less than 1 MHz is referred to in this description as "high frequency ultrasound".

[0026] The zinc oxide used in the process of the present invention is typically in the form of particles. The size of the particles is not limiting and will typically be chosen according to the intended applications. In certain embodiments, the size of the zinc oxide particles varies from 0.1 to 5 µm. In certain embodiments, 90% by number of the particles have a size varying from 0.1 to 5 µm. Particles having a size varying from 0.1 to 5 µm make it possible to increase the chemical yield of the reaction. Such particles can result from the grinding of the zinc oxide. Thus, in certain embodiments, the process for preparing zinc phosphate of the present invention can first comprise a step of grinding the zinc oxide to obtain particles having a size varying from 0.1 to 5 µm. The grinding can be carried out by strong crushing.Refining using a sieve then selects particles ranging in size from 0.1 to 5 µm. Particle size can be measured by laser granulometry.

[0027] Zinc oxide can be a technical grade zinc oxide (purity ranging from 97% to 98%). Its physicochemical characteristics are as follows: Zn 77.14%, Pb 5 ppm, Cu 5 ppm, Cd 20 ppm, Fe 50 ppm and Mn 5 ppm.

[0028] The zinc oxide useful in the context of the present invention can be extracted from ores or from industrial waste.

[0029] The phosphoric acid used in the process of the present invention is typically a pure phosphoric acid, i.e. a phosphoric acid having a purity of 99.99%, such as a purified industrial phosphoric acid. Such a level of purity of phosphoric acid allows the preparation of zinc phosphate of very high purity. The purified industrial phosphoric acid can be obtained from the wet process by liquid-liquid extraction or membrane filtration or from the thermal process. The phosphoric acid is generally a phosphoric acid comprising from 5 to 65% of P 2 O 5 (equivalent to from 6.9 to 89.7% of H 3 PO 4 ), preferably from 10 to 61% P 2 O 5 (equivalent to from 13.8 to 84.18% of H 3 PO 4 ). In some embodiments, the phosphoric acid comprises from 10 to 61% P 2 O 5 , preferably from 45 to 61% P 2 O 5 .

[0030] The phosphoric acid used can be obtained by the action of a strong acid, such as hydrochloric acid, nitric acid or sulfuric acid, on natural phosphate followed by purification by membrane process.

[0031] Phosphoric acid can be as produced by the Jorf Lasfar industrial site of the OCP group, Morocco. The concentration of phosphoric acid in P 2 O 5 is 61.6%, equivalent to 85% in H 3 PO 4 .

[0032] The proportions of zinc oxide and phosphoric acid used in the process of the present invention are typically such that the mass ratio of phosphoric acid: zinc oxide (H 3 PO 4 : ZnO) is greater than or equal to 0.3. Advantageously, the phosphoric acid: zinc oxide ratio varies from 0.3 to 2, preferably from 0.5 to 2, even more preferably from 0.8 to 1.5.

[0033] Zinc oxide is typically added to the phosphoric acid gradually, i.e. progressively so as to maintain a homogeneous mixture and avoid the formation of agglomerates. The whole is stirred to homogenize the mixture. The homogenization time typically varies from 1 min to 90 min, preferably from 5 min to 60 min or even from 5 min to 35 min.

[0034] The reaction is typically carried out in a reactor.

[0035] High-frequency ultrasound catalyzes the reaction. The use of high-frequency ultrasound is beneficial because it allows for homogenization of the solution: it allows the zinc oxide to be completely dissolved and a homogeneous, sandy gel to be obtained. The use of high-frequency ultrasound also allows for an increase in the reaction rate and consequently a reduction in reaction time, a reduction in energy consumption, with the possibility of reducing the reagent input. High-frequency ultrasound is typically applied for a period of less than 30 minutes. The duration of ultrasound application typically varies from 1 to 30 minutes, preferably from 5 to 15 minutes. However, it is understood that high-frequency ultrasound can be applied for a period of more than 30 minutes.

[0036] Advantageously, high-frequency ultrasound allows the reaction time to be reduced by a factor of 6 to 10 for an identical yield.

[0037] High-frequency ultrasound is a sinusoidal acoustic wave with a frequency range between 100 kHz and 1 MHz. The waves are characterized by a frequency expressed in Hz and a power expressed in W.

[0038] The frequency of the high-frequency ultrasound useful in the present invention preferably varies from 100 to 500 kHz, or even from 100 to 200 kHz, typically it is 170 kHz.

[0039] The power density of the high frequency ultrasound useful in the present invention typically varies from 100 to 2000 W, preferably from 100 to 1000 W, typically it is 1000 W. The optimum power density is 1000 W at 170 kHz.

[0040] High-frequency ultrasound can be applied by means of an ultrasonic bath, for example by immersing the reaction medium, more precisely the reactor containing the zinc oxide and the phosphoric acid, in an ultrasonic bath or by means of an ultrasonic probe, typically by immersing an ultrasonic probe in the reaction medium.

[0041] In some embodiments, high-frequency ultrasound is applied in an ultrasonic bath at 170 kHz with a power density of 1000 W. The ultrasonic waves thus activate the reaction.

[0042] It has been shown that the higher the frequencies, the higher the production rate of zinc phosphate for the same power. The frequency of the ultrasound therefore has a significant influence and is adjusted according to the quality of the desired product.

[0043] The resulting product is typically washed to remove residual phosphoric acid and dried. Washing is generally carried out with distilled water. The drying step is typically carried out at a temperature ranging from 40 to 100°C, preferably from 50 to 80°C. Drying is typically carried out in an oven.

[0044] Oven drying is carried out at a temperature that allows for the fastest possible drying while preserving the quality of the crystallized compound. After this drying, zinc phosphate is obtained in solid form. It has a gray color. Its quality can be analyzed by standard chemical analysis methods, such as X-ray diffractometry (XRD), scanning electron microscopy (SEM), etc.

[0045] Scanning electron microscope (SEM) observation coupled with energy dispersive X-ray analysis (EDAX) showed that the zinc phosphate Zn 3 (PO 4 ) 2 , 4H 2 O obtained by the process of the present invention consists essentially of phosphated particles of square, rectangular shape or sheet-shaped crystals of irregular shape ( figure 4 ).

[0046] Chemical analyses have shown that the product obtained by the process of the present invention (Zn 3 (PO 4 ) 2 , 4H 2 O) comprises 65% by mass of Zn and 31% by mass of PO 4 .

[0047] The product is obtained with a good yield, typically greater than 98%, and has a high purity (≥ 98%). High frequencies can even allow the product to be obtained with a yield and purity greater than 99%.

[0048] The zinc phosphate obtained by the process of the present invention can be used as paint pigment, adhesive agent, anti-corrosion agent, trace element, additive in various fertilizer matrices, cement and dental applications, etc.

[0049] A method comprising preparing a solid phase with a liquid phase is described. The method comprises the following steps: (a) preparing a solid phase by grinding a solid to 0.1-5 µm; (b) introducing a liquid phase at a first flow rate into a mixing reactor; (c) introducing the solid phase obtained in step (a) at a second flow rate into said reactor; (d) mixing the solid phase with the liquid phase under mechanical stirring in said reactor; (e) conveying the mixture into an ultrasonic bath having a power of 1000 W; (f) applying ultrasound at 170 kHz in the ultrasonic bath; (g) washing the precipitate obtained with distilled water and drying at 60°C for 4 hours.

[0050] Such a process may be useful in the preparation of phosphate salts. EXAMPLES Material

[0051] An ultrasonic bath (1000 W, model BT90H) from Power Corporation Ultrasound of Freeport, Illinois (USA) combined with a 170 kHz ultrasonic generator (Power Corporation Ultrasound) was used in Examples 2 and 3 below. Comparative example 1 : Production of zinc phosphate without ultrasound

[0052] In this example, the preparation of zinc phosphate was carried out according to the following steps: Preparation of zinc oxide by grinding to 0.1-5 mm (solid phase); Sending phosphoric acid 61% P 2 O 5 into a mixing reactor (liquid phase); Sending zinc oxide at a second flow rate into the reactor in a ZnO / H 3 PO 4 mass ratio equal to 1; Mixing with stirring in the reactor of the solid phase with the liquid phase, for 30 min; Washing of the white precipitate obtained with distilled water and drying at 60°C for 4 hours.

[0053] The product obtained is in the form of a gray powder. This powder is not a pure product: it contains a certain proportion of the other compounds in the initial mixture. The results of the physical analysis by XRD presented in the Figure 5show that the final product contains a mixture of three phases (Zn 3 (PO 4 ) 2 , 2H 2 O, Zn 3 (PO 4 ) 2 , 4H 2 O, ZnO). The final product is impure. Example 2: Process for the manufacture of zinc phosphate including an ultrasonic activation step

[0054] Zinc phosphate was prepared by a process similar to that of Example 1 but including a step of applying high-frequency ultrasound. The process comprises the following steps: Grinding of zinc oxide to 0.1-5 µm (solid phase); Sending of phosphoric acid 61% P 2 O 5 into a mixing reactor (liquid phase); Sending of zinc oxide at a second flow rate into the reactor in a ZnO / H 3 PO 4 mass ratio equal to 1; Mixing under stirring in the reactor of the solid phase with the liquid phase, for 30 min; Activation of the mixture with ultrasound, with optimal power used at 170 kHz for 10 min (1000 W); Washing of the white precipitate obtained with distilled water and drying at 60°C for 4 hours.

[0055] The mixture is homogeneous, it appears uniform to the eye and to the hand (no clumping thanks to ultrasound, uniform particle size). Microscopic observations show a homogeneous compound with separate grains.

[0056] Physical analyses by DRX, titration and chemical analyses showed that the product obtained by including an ultrasonic activation step is pure and that in the composition of Zn 3 (PO 4 ) 2 , nH 2 O, n depends on the temperature and the crystallization time ( figure 3 ).

[0057] The X-ray diffractogram ( figure 3 ) presents a result obtained for the ultrasonically catalyzed mixture of phosphoric acid and technical grade zinc oxide. This mixture was dried at 60°C until the final mass was stable. The final product is zinc phosphate with the chemical formula: Zn 3 (PO 4 ) 2 , 4H 2 O.

[0058] We confirmed the good reproducibility of this method to produce Zn 3 (PO 4 ) 2 , 4H 2 O.

[0059] XRD analyses showed that the product obtained is pure. The reaction yield is greater than 99%. Example 3 : Influence of stirring time

[0060] Zinc phosphate was prepared by a method similar to that of Example 2 in which the stirring time was reduced. The method comprises the following steps: Grinding of zinc oxide to 0.1-5 µm (solid phase); Sending of phosphoric acid 61% P 2 O 5 into a mixing reactor (liquid phase); Sending of zinc oxide at a second flow rate into the reactor in a ZnO / H 3 PO 4 mass ratio equal to 1; Mixing under stirring in the solid phase reactor with the liquid phase, for 10 min; Activation of the mixing with ultrasound, with optimal power used at 170 kHz for 10 min (1000 W); Washing of the gray precipitate obtained with distilled water and drying at 60°C for 4 hours. Physical analyses by DRX, complexometric titration and chemical analyses showed that a pure product is obtained by reducing the stirring time. REFERENCES

[0061] [1] Zhang Jun, Jiang Fengzhi, (preparation of zinc phosphate by liquid-liquid heterogeneous reaction), patent No CN 1164508 A. [2] F. Rodriguez, D. Hernandez, J. Garcia-Jaca, H. Ehrenberg and H. Weitzel, (Optical study of the piezochromic transition in CuMoO4 by pressure spectroscopy), Physical Review B, Vol. 61, Number 24, 2000, 16497 - 16501 [3] Natalia E Danjushevskaya, Pocess for producing zinc phosphate. Patent N°US 4207301A.1980. [4] S. Ramesh and V. Narayanan, synthesis and characterisation of zinc phosphate nanoparticles by precipitation method. Print version ISSN 2347-3207. [5] Zhang Jun, Jiang Fengzhi,preparation of zinc phosphate by liquid-liquid heterogeneous reaction.patent No CN 1164508 A. [6] S. Mallouk, K. Bougrin, H. Doua, R. Benhida, M. Soufiaoui. Ultraound-accelerated aromatisation of trans- and cis-pyrazolines under hetrerogeneous conditions using claycop. Tetrahedron Lett. 2004, 45, 4143-4148 [7] M. Driowya, A. Puissant, G. Robert, P. Auberger, R. Benhida, K.Bougrin. Ultrasound-assisted one-pot synthesis of anti-CML nucleosides featuring 1,2,3-triazole nucleobase under iron-copper catalysis. Ultrasonics Sonochem. 2012, 19, 1132-1138 [8] H. Marzag, G. Robert, M. Dufies, K. Bougrin, P. Auberger, R. Benhida. FeCl3-promoted and ultrasound-assisted synthesis of resveratrol O-derived glycoside analogs. Ultrason. Sonochem. 2015, 22, 15-21. [9] H. Marzag, S. Alaoui, H. Amdouni, A. Martin, K. Bougrin, R. Benhida. Efficient and selective azidation of per-O-acetylated sugars using ultrasound activation: Application to the one-pot synthesis of 1,2,3-triazole glycosides. New. J. Chem. 2015, 5437-5444

[10] S Alaoui, M Driowya, L Demange, R Benhida, K Bougrin. Ultrasound-assisted facile one-pot sequential synthesis of novel sulfonamide-isoxazoles using cerium (IV) ammonium nitrate (CAN) as an efficient oxidant in aqueous medium. Ultrason Sonochem. 2018 ;40, 289-297.

Claims

1. A method for producing zinc phosphate comprising a step consisting of reacting zinc oxide with phosphoric acid, preferably purified industrial phosphoric acid, under the action of ultrasound having a frequency greater than 100 kHz.

2. The method according to claim 1, wherein zinc oxide is added gradually to phosphoric acid under stirring, before applying ultrasound.

3. The method according to claim 2, wherein the stirring duration before applying ultrasound ranges from 1 to 90 minutes, preferably from 5 to 35 minutes.

4. The method according to one of the preceding claims, further comprising a step of washing and drying the product obtained at the end of the reaction of zinc oxide with phosphoric acid.

5. The method according to claim 4, wherein the drying is carried out at a temperature ranging from 40 to 100°C, preferably 50 to 80°C.

6. The method according to one of the preceding claims, wherein the mass ratio of phosphoric acid:zinc oxide is greater than or equal to 0.3.

7. The method according to one of the preceding claims, wherein the zinc oxide is in the form of particles, the size of which ranges from 0.1 to 5 µm as measured by laser particle size analysis.

8. The method according to one of the preceding claims comprising, beforehand, a step of grinding zinc oxide in order to obtain particles, the size of which ranges from 0.1 to 5 µm as measured by laser particle size analysis.

9. The method according to one of the preceding claims, wherein the phosphoric acid comprises 5 to 65% P2O5, preferably between 10 and 61% P2O5.

10. The method according to one of the preceding claims, wherein the ultrasound frequency ranges from 100 to 500 kHz, preferably from 100 to 200 kHz.

11. The method according to one of the preceding claims, wherein the ultrasound is applied at a power density ranging from 100 to 2000 W, preferably from 100 to 1000 W.

12. The method according to one of the preceding claims, wherein the ultrasound frequency is 170 kHz at a power density of 1000 W.

13. The method according to one of the preceding claims, wherein the ultrasound is applied by means of an ultrasound bath or an ultrasound probe.

14. The method according to one of the preceding claims, wherein the zinc phosphate has a purity greater than or equal to 98%.

15. The method according to one of the preceding claims, wherein the duration of application of ultrasound is less than 30 minutes.