Method for producing zinc phosphate

Abstract

The present invention relates to a method for preparing zinc phosphate with a controlled grain size.

Description

[0001] DESCRIPTION

[0002] TITLE: Process for manufacturing zinc phosphate

[0003] FIELD OF INVENTION

[0004] The present invention relates to a process for manufacturing zinc phosphate.

[0005] STATE OF THE ART

[0006] Zinc phosphate (Zn3(PO4)2) is the most widely used non-toxic pigment in paint formulations. It is generally found as a white solid, insoluble in water. It occurs naturally as a mineral and can be found in various forms, more or less hydrated and / or complexed with other metals, forming hopeite, parahopeite, tarbuttite, phosphophyllite, etc.

[0007] It is primarily used for its anti-corrosion properties. It gives paints a glossy appearance and a white color. Zinc phosphate also reduces reactions between the paint and the metal. In addition, it forms a barrier between the substrate and the paint. It also reduces surface corrosion on metallic substrates and minimizes undercoat delamination in the event of paint film damage. 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, aerospace, or marine paints (CN 1164508 A).

[0008] Zinc phosphate is a phosphate derivative. It is formed by reacting zinc oxide in an aqueous slurry with purified phosphoric acid. After the reaction, the water is extracted by filtration, and the zinc phosphate 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.

[0009] Various methods have been proposed for preparing zinc phosphate.

[0010] The first method is based on treating zinc oxide with phosphoric acid in an aqueous medium at a temperature of 70°C to 80°C in the presence of metallic zinc in an amount ranging from 1 to 10 wt% of zinc oxide, followed by separating the resulting residue of zinc phosphate. The yield of zinc phosphate is 98% (US 4207301 A). Zinc phosphate has been synthesized by another method involving 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, 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 (Ramesh et al., Synthesis and characterization of zinc phosphate nanoparticles by precipitation method. ISSN 2347-3207).

[0011] Another process for the production of zinc phosphate has been described. In this process, the zinc oxide used is of a lower quality (purity ranging from 50% to 95%), and the phosphoric acid is obtained by liquid-liquid extraction. The reaction temperature ranges from 70 to 90 °C, the reaction time from 40 to 60 minutes, and the stirring speed from 4 to 6 rpm (CN 1164508 A).

[0012] Document EP 0009175 A1 describes a process for manufacturing zinc phosphate, with the formula Zn3(RO4)2,nH2O (n=0 to 4), by reacting zinc oxide (20-85 wt.) and phosphoric acid (10-50 wt.) at a temperature ranging from 10 to 100°C and a stirring speed ranging from 3000 to 10,000 rpm. High-purity zinc oxide and 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 to achieve a good yield.

[0013] W02022 / 108434 proposes to prepare zinc phosphate by reacting zinc oxide with phosphoric acid under the action of ultrasound with a frequency greater than 100 kHz. This process leads to the preparation of zinc phosphate in the form of particles generally on the order of 45 µm.

[0014] However, the manufacture of paints imposes particular requirements on several characteristics of zinc phosphate, and in particular the particle size which must be relatively small (less than or equal to 5pm) and homogeneous.

[0015] Thus, even though various processes are known for preparing zinc phosphate, a need remains for a simple, efficient, and economical process (low investment and operating costs) to manufacture zinc phosphate with controlled particle size, particularly in the form of particles 5 µm or smaller. Such zinc phosphate particles are especially useful for the preparation of paints. FIGURES

[0016] Figure 1: Illustrative diagram of one embodiment of the process of the invention.

[0017] SUMMARY OF THE INVENTION

[0018] Thus, the invention relates to a process for preparing zinc phosphate, in particular zinc phosphate particles having a median size (d50) less than or equal to 5 pm, comprising the following steps: a) preparing a zinc oxide slurry by mixing water and a zinc oxide; b) adding phosphoric acid to the zinc oxide slurry obtained at the end of step a); c) placing the slurry to which the phosphoric acid has been added under the action of ultrasound having a frequency less than or equal to 20 kHz; d) placing the slurry obtained at the end of step c) under agitation at a speed ranging from 800 to 6000 revolutions per minute; e) collecting the zinc phosphate obtained at the end of step d).

[0019] Other aspects of the invention are as described below.

[0020] Definitions

[0021] By "approximately", it is understood that the value concerned may be 10% lower or higher, in particular 5%, especially 1%, than the value indicated.

[0022] The term "zinc phosphate" refers to the compound with the formula Zn3(PO4)2 in its anhydrous form or one of its hydrated forms. Thus, this compound has the following formula: Zn3(PO4)2,nH2O, where n is an integer from 0 to 4, specifically n = 4.

[0023] For the purposes of the present invention, a range of values ​​designated by the expression "from a to b" represents the range of values ​​from a to b, that is to say including the strict bounds a and b.

[0024] DETAILED DESCRIPTION OF THE INVENTION

[0025] The inventors have developed a process that meets the stated needs. In particular, the inventors have surprisingly discovered that the preparation of zinc phosphate by a process that combines the action of low-frequency ultrasound followed by high-speed agitation makes it possible, efficiently and economically, to obtain zinc phosphate with a controlled particle size, in particular having a particle size less than or equal to 5 pm, preferably ranging from 1 to 3 pm.

[0026] The process for preparing zinc phosphate, in particular zinc phosphate particles having a median size (d50) less than or equal to 5 pm, comprises the following steps: a) preparing a zinc oxide slurry by mixing water and a zinc oxide, generally under stirring at a speed of 800 to 6000 revolutions per minute; b) adding phosphoric acid to the zinc oxide slurry obtained at the end of step a); c) placing the slurry to which the phosphoric acid has been added under the action of ultrasound having a frequency less than or equal to 20 kHz; d) placing the slurry obtained at the end of step c) under stirring at a speed of 800 to 6000 revolutions per minute; e) collecting the zinc phosphate obtained at the end of step d).

[0027] The reaction is typically carried out in a reactor.

[0028] The process can be carried out at ambient temperature. The term "ambient temperature" refers to a temperature ranging from 15°C to 40°C, preferably from 20°C to 30°C, and in particular around 25°C.

[0029] Step a)

[0030] Zinc oxide slurry (or zinc oxide suspension) is typically prepared by adding zinc oxide to water.

[0031] The slurry generally contains 10 to 50% by weight of zinc oxide.

[0032] Zinc oxide can be either technical grade or purified. Technical grade zinc oxide can have the following contents: less than 0.1 ppm of cadmium (Cd), less than 0.1 ppm of chromium (Cr), less than 15 ppm of copper (Cu), and less than 0.1 ppm of arsenic (As). For example, the contents of technical grade zinc oxide could be 0.03 ppm of cadmium (Cd), 0 ppm of chromium (Cr), 9.59 ppm of copper (Cu), and 0 ppm of arsenic (As). Analytical grade zinc oxide, which is purified, can have the following contents: less than 0.1 ppm of cadmium (Cd), less than 0.1 ppm of chromium (Cr), less than 0.2 ppm of copper (Cu), and less than 0.1 ppm of arsenic (As). For example, the contents of analytical grade zinc oxide could be 0 ppm of cadmium (Cd), 0 ppm of chromium (Cr), 0.03 ppm of copper (Cu), and 0 ppm of arsenic (As).

[0033] The zinc oxide used in the process of the present invention is typically in the form of particles. The size of the zinc oxide particles can range from 0.1 µm to 5 µm. Such particles can result from grinding the zinc oxide. Thus, in certain embodiments, the zinc phosphate preparation process of the present invention may include a step of grinding the zinc oxide to obtain particles ranging in size from 0.1 µm to 5 µm. The grinding can be carried out by high-pressure crushing. Refining using a sieve then allows the selection of particles ranging in size from 0.1 µm to 5 µm. The particle size can be measured by laser granulometry.

[0034] The preparation of the zinc oxide slurry is generally carried out under agitation at a speed ranging from 800 to 6000 rotations per minute, preferably ranging from 3000 to 6000 rotations per minute.

[0035] Agitation can be maintained for a period of 5 min to 60 min, preferably 5 min to 20 min, typically about 10 min.

[0036] The resulting zinc oxide slurry can be subjected to ultrasound at a frequency of 20 kHz or less, preferably from 5 to 20 kHz, before proceeding to step b). Preferably, the zinc oxide slurry is subjected to ultrasound after stirring. The ultrasound power typically ranges from 50 to 1000 W, preferably from 500 to 1000 W, and is typically 1000 W.

[0037] Ultrasound can be applied by means of an ultrasonic bath, for example by immersing the slurry, more precisely the reactor containing the slurry, in an ultrasonic bath or by means of an ultrasonic probe, typically by immersing an ultrasonic probe in the slurry.

[0038] The slurry can be subjected to the action of ultrasound for a period of 5 min to 20 min, preferably 5 min to 15 min, typically about 10 min.

[0039] Phosphoric acid is added to the zinc oxide slurry obtained at the end of step a).

[0040] The phosphoric acid used in the process of the present invention is typically pure phosphoric acid, that is, phosphoric acid with a purity of 99.99%, such as purified industrial phosphoric acid. This level of phosphoric acid purity allows for the preparation of very high-purity zinc phosphate. The purified industrial phosphoric acid can be obtained by wet extraction via liquid-liquid extraction or membrane filtration, or by thermal extraction.

[0041] Phosphoric acid may comprise from 20 to 61% by weight of P2O5 (equivalent to 29% to 85% of H3PO4), preferably from 23% to 61% of P2O5 (equivalent to 30% to 85% of H3PO4), typically about 61% of P2O5 (equivalent to 85% of H3PO4).

[0042] The phosphoric acid used can be obtained from the action of a strong acid, such as hydrochloric acid, nitric acid or sulfuric acid, on natural phosphate followed by purification by membrane process.

[0043] Phosphoric acid can be produced as such by the OCP Group's Jorf Lasfar industrial site in Morocco. The concentration of phosphoric acid in P2O5 is 61.6% (equivalent to 85% H3PO4).

[0044] The proportions of zinc oxide and phosphoric acid used in the process of the present invention are typically such that the zinc oxide:phosphoric acid molar ratio (ZnO:H3PO4) is greater than or equal to 1, preferably greater than or equal to 1.2, typically greater than or equal to 1.5. For example, the zinc oxide:phosphoric acid molar ratio can vary from 1 to 2, preferably from 1.2 to 1.7. Such zinc oxide:phosphoric acid molar ratios make it possible to maintain the pH of the reaction medium at slightly acidic values.

[0045] In step b), phosphoric acid is typically added gradually to the zinc oxide slurry. Such a gradual addition helps prevent the formation of agglomerates.

[0046] Step c)

[0047] The slurry to which phosphoric acid has been added is then subjected to ultrasound with a frequency of 20 kHz or less, preferably from 5 to 20 kHz. The power of the ultrasound generally varies from 50 to 1000 W, preferably from 500 to 1000 W, and is typically 1000 W.

[0048] The duration of step c) can range from 5 min to 20 min, preferably from 5 min to 15 min, typically around 10 min. However, it is understood that ultrasound may be applied for a period exceeding 15 min. Ultrasound may be applied using an ultrasonic bath, for example by immersing the slurry to which phosphoric acid has been added, more precisely the reactor containing the slurry to which phosphoric acid has been added, in an ultrasonic bath, or using an ultrasonic probe, typically by immersing an ultrasonic probe in the slurry to which phosphoric acid has been added.

[0049] The slurry obtained at the end of step c) is placed under agitation at a speed of 800 to 6000 rotations per minute, preferably of 1000 to 6000 rotations per minute, for example 3000 to 6000 rotations per minute.

[0050] Agitation can be maintained for a period of 5 min to 20 min, preferably 5 min to 15 min, typically about 10 min.

[0051] Step e)

[0052] The zinc phosphate obtained at the end of step d) is collected.

[0053] The zinc phosphate obtained at the end of step d) can be collected by any method known to those skilled in the art. In particular, the zinc phosphate can be collected by centrifugation or filtration, such as by vacuum filtration.

[0054] The collected zinc phosphate can also be washed with a solvent, preferably water, particularly distilled water. This operation can be repeated several times. Washing notably removes residual phosphoric acid. Thus, the process of the invention may include a step (f) of washing the zinc phosphate.

[0055] When water is the washing solvent, the collected wash water can be used to prepare zinc oxide slurry. In such a case, the wash water can have a pH ranging from 2 to 3, typically around 2.2.

[0056] Zinc phosphate can also be dried. Drying is typically carried out in an oven. The process according to the invention may therefore further comprise a step g) of drying the zinc phosphate, preferably at a temperature ranging from 40 °C to 100 °C, preferably from 50 °C to 80 °C, typically about 60 °C.

[0057] The duration of step g) can range from 30 min to 12 h, preferably from 3 h to 6 h, typically about 6 h. The zinc phosphate obtained at the end of the process is in the form of particles having a median size (d50) less than or equal to 5 pm, preferably ranging from 1 to 3 pm, typically about 2 pm.

[0058] The term "size" refers to the diameter of the particles. In the case of non-spherical particles, it refers to the length of the largest dimension.

[0059] The particle size is determined by laser granulometry.

[0060] The zinc phosphate obtained at the end of the process is advantageously in the form of particles having a d90 ranging from 20 to 40 pm, preferably ranging from 25 to 29 pm.

[0061] The zinc phosphate obtained at the end of the process is advantageously in the form of particles having a d10 ranging from 0.090 to 0.200 pm, preferably ranging from 0.090 to 0.150 pm.

[0062] The parameter “d50” designates the median size / diameter of the particles: 50% by volume of the particles have a diameter less than d50.

[0063] The parameter "d90" indicates that 90% of the particles by volume have a diameter less than d90.

[0064] The parameter "d10" indicates that 10% of the particles by volume have a diameter less than d10.

[0065] The zinc phosphate obtained by the process of the present invention can in particular be used as a paint pigment.

[0066] At the end of the process according to the invention, the zinc phosphate produced can be stored and possibly processed for subsequent uses by techniques known to those skilled in the art.

[0067] Figure 1 is a diagram of an embodiment of the process of the invention. The illustrated process comprises the following steps: a) preparing a zinc oxide slurry by mixing water and a zinc oxide under stirring in a reactor; b) adding phosphoric acid to the zinc oxide slurry obtained at the end of step a); c) placing the slurry to which the phosphoric acid has been added under the action of ultrasound having a frequency less than or equal to 20 kHz in an ultrasonic bath; d) placing the slurry obtained at the end of step c) under stirring at a speed ranging from 800 to 6000 revolutions per minute; e) collecting the zinc phosphate obtained at the end of step d) by filtration; f) washing the zinc phosphate with water; g) drying the zinc phosphate.

[0068] In the illustrated process, the wash waters are reintroduced in step a) to prepare the zinc oxide slurry.

[0069] The following examples illustrate particular embodiments of the invention without limiting its scope.

[0070] EXAMPLES

[0071] Materials and methods

[0072] The particle size distribution is measured by laser granulometry. Effect of high agitation followed by high-frequency ultrasound

[0073] A slurry of 200 g of ZnO and 800 L of water was prepared under stirring at 1200 rpm (rotations per minute) for 10 min. The slurry was then sent to a high-frequency ultrasonic bath at approximately 200 kHz for 10 min. 200 g of purified or technical-grade phosphoric acid (61% P2O5) was then added to the slurry, and the mixture was immersed in the ultrasonic bath at a frequency of 20 kHz, 1000 W for 10 min. The mixture was then sent to the reactor and stirred at 6000 rpm for 10 min. The zinc phosphate was separated by centrifugation and dried at 80 °C for 6 h. The zinc phosphate obtained had the following particle size distribution: d(10) = 14.116 pm; d(50) = 64.067 pm; d(90) = 201.993 pm. Effect of gentle agitation followed by low-frequency ultrasound

[0074] Example 2 was carried out using the experimental protocol of Example 1, varying the stirring and ultrasonic frequency as follows. The ZnO slurry in water was stirred at 300 rpm at a frequency of 20 kHz. After the phosphoric acid was added, the mixture was immersed in the ultrasonic bath at a frequency of 20 kHz, and then the mixture was sent to the reactor, stirred at 1000 rpm.

[0075] Zinc phosphate is obtained with the following particle size distribution: d(10) = 4.324 pm; d(50) = 31.905 pm; d(90) = 302.136 pm.

[0076] Example 3: Effect of high agitation followed by low-frequency ultrasound

[0077] Example 3 was carried out using the experimental protocol of Example 1, varying the agitation and ultrasound frequency. The agitation was high (6000 rpm) and the ultrasound frequency was 20 kHz. Zinc phosphate was obtained with a purity >99% and a yield >98%, as well as the following particle size distribution: d(10) = 0.105 pm; d(50) = 1.856 pm; d(90) = 28.900 pm.

[0078] It is thus demonstrated that the process according to the invention makes it possible to prepare zinc phosphate from zinc oxide and phosphoric acid. Example 3 demonstrates that it is particularly advantageous to combine a high stirring speed and low-frequency ultrasound both before and after the addition of phosphoric acid. Indeed, at the end of Example 3, a zinc phosphate with a fine and controlled particle size is obtained.

Claims

DEMANDS 1. A process for preparing zinc phosphate, in particular zinc phosphate particles having a median size (d50) less than or equal to 5 pm, comprising the following steps: a) preparing a zinc oxide slurry by mixing water and a zinc oxide; b) adding phosphoric acid to the zinc oxide slurry obtained at the end of step a); c) placing the slurry to which the phosphoric acid has been added under the action of ultrasound having a frequency less than or equal to 20 kHz; d) placing the slurry obtained at the end of step c) under stirring at a speed from 800 to 6000 revolutions per minute; e) collecting the zinc phosphate obtained at the end of step d).

2. Method according to claim 1, wherein ultrasound is applied at a power of 50 to 1000 W, preferably 500 to 1000 W.

3. A method according to claim 1 or 2, wherein the zinc oxide slurry from step a) is placed under the action of ultrasound having a frequency less than or equal to 20 kHz.

4. A method according to any one of claims 1 to 3, wherein the duration of step c) is from 5 min to 20 min, preferably from 5 min to 15 min.

5. A method according to any one of claims 1 to 4, wherein the zinc oxide:phosphoric acid molar ratio is greater than or equal to 1, preferably greater than or equal to 1.2, typically greater than or equal to 1.

5.

6. A method according to any one of claims 1 to 5, wherein the stirring speed of step d) is from 1000 rpm to 6000 rpm.

7. A method according to any one of claims 1 to 6, wherein the collection of zinc phosphate is carried out by filtration.

8. A process according to any one of claims 1 to 7, further comprising a step of washing the collected zinc phosphate, preferably with water.

9. A process according to any one of claims 1 to 8, further comprising a step of drying the collected zinc phosphate, preferably at a temperature from 40°C to 100°C, preferably from 50°C to 80°C.

10. A process according to claim 9, wherein the washing is carried out with water and the collected wash water is used in step a) of the process.

11. A method according to any one of claims 1 to 10, wherein the zinc oxide slurry of step a) is prepared under stirring at a speed of 800 to 6000 revolutions per minute.

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