PROCEDE DE PREPARATION DE MONOHYDROGENOPHOSPHATE DE CALCIUM

Description

[0001] The present invention relates to a process for preparing calcium monohydrogen phosphate or dicalcium phosphate comprising the steps of: a) digestion in aqueous medium, during a first period of time, of a phosphate source by an acid with formation of a pulp comprising an aqueous phase containing calcium phosphate in solution and a solid phase containing impurities, b) a first separation between said aqueous phase containing calcium phosphate in solution and said solid phase containing impurities, during a second period of time, c) a neutralization of said aqueous phase containing calcium phosphate in solution to a pH sufficient to obtain precipitation in an aqueous medium of insoluble calcium phosphate as the aforementioned calcium monohydrogen phosphate, and d) a second separation between said aqueous medium and said calcium monohydrogen phosphate.

[0002] The attack of a phosphate source, such as phosphate ore, by an acid has been known for many years, for example US patent 3304157 and GB 105 15 21.

[0003] Document WO 2004002888 discloses a process for manufacturing calcium monohydrogen phosphate (DCP) and also a process for producing phosphoric acid from the DCP obtained according to the process described in this patent document.

[0004] This document describes an attack on a phosphate ore in a digestion tank by a hydrochloric acid solution, having a concentration of approximately 5% by weight, to form a pulp comprising an aqueous phase containing calcium phosphate and chloride ions in solution and a solid phase containing impurities.

[0005] Passing the aforementioned pulp through a filter press allows the aqueous phase containing calcium phosphate and chloride ions in solution to be separated from the solid phase containing impurities.

[0006] Neutralization of the separated aqueous phase containing calcium phosphate and chloride ions is achieved by adding a calcium compound to precipitate the insoluble DCP in the aqueous phase, following a rise in pH.

[0007] An additional separation is carried out to recover a wet DCP cake at the end of the process.

[0008] Regarding the production of phosphoric acid using the DCP thus obtained, this is carried out by liquid-liquid extraction and requires several implementation steps.

[0009] It is indeed necessary to perform a solubilization step on the wet DCP cake, obtained according to the process described above, by a further attack with a more concentrated hydrochloric acid, up to 20% by weight. This solubilization allows the formation of an aqueous solution containing phosphate ions, calcium ions, and chloride ions, which can then be extracted using an organic extraction agent.

[0010] More specifically, this extraction step is carried out in an extraction column using an organic solvent and produces an aqueous extraction phase containing chloride and calcium ions and an organic extraction phase containing phosphoric acid.

[0011] The organic extraction phase containing phosphoric acid is re-extracted using an aqueous re-extraction agent to isolate an aqueous re-extraction phase containing phosphate ions.

[0012] Finally, concentration of the aqueous re-extraction phase allows the formation of an aqueous solution of phosphoric acid.

[0013] This process for producing phosphoric acid is complex and requires several costly steps to provide phosphoric acid of sufficient quality for the intended applications.

[0014] Furthermore, the use of organic extraction solvents makes such a process unattractive in terms of ecological impact.

[0015] Document WO 2005 066 070 relates to a process for attacking phosphate ore with an aqueous hydrochloric acid solution having an HCl concentration of less than 10% by weight, with formation of a pulp consisting of an aqueous phase containing calcium phosphate in solution and chloride ions and an insoluble solid phase containing impurities.

[0016] A neutralization of the aforementioned aqueous phase is carried out at a first pH at which a significant proportion of calcium phosphate is maintained in the aqueous phase in order to precipitate impurities.

[0017] Next, the aforementioned insoluble solid phase is separated from the aqueous phase while the precipitated impurities are isolated.

[0018] An additional neutralization of the aqueous phase is carried out at a second pH higher than the first pH mentioned above to precipitate DCP which is then separated from the aqueous medium.

[0019] This process is limited by the fact that it requires the use of an aqueous hydrochloric acid solution with a concentration of less than 10% by weight.

[0020] More recently, a process for attacking a phosphate source with an aqueous solution of hydrochloric acid was the subject of patent application WO 2015 082 468.

[0021] According to this disclosure, the digestion of the rock in the presence of the aqueous hydrochloric acid solution allows the formation of a pulp consisting of an aqueous phase containing calcium phosphate and chloride ions in solution, and an insoluble solid phase containing impurities.

[0022] The aqueous phase is then separated from the solid phase by filtration so that the aqueous phase can be neutralized to a sufficient pH to form an aqueous medium containing chloride ions and to precipitate the calcium phosphate as the phosphate salt. A further separation isolates the phosphate salt.

[0023] According to this process, the attack and filtration steps are carried out at a temperature between 50°C and 70°C and the first aqueous hydrochloric acid solution has an HCl concentration less than or equal to 15% by weight.

[0024] Unfortunately, the known processes are not very practical for industrialists since they do not have a sufficient P2O5 extraction yield compared to the amount of P2O5 present in the starting phosphate source and the phosphate salt obtained is not sufficiently pure, which nevertheless determines its potential use in several fields of application, such as agriculture or food and high value added technical applications.

[0025] There is therefore a real need to provide a process for manufacturing calcium monohydrogen phosphate which ultimately solves this problem related to the purity of calcium monohydrogen phosphate, while limiting the losses of P2O5 in the waste generated by the process in order to improve the overall performance of the process.

[0026] The invention aims to provide a simple, reliable, and rapid process for manufacturing calcium monohydrogen phosphate. Digestion and filtration must allow for a high P₂O₅ extraction yield, while duly considering the trade-off between the desired purity of the calcium monohydrogen phosphate obtained at the end of the process and the minimization of P₂O₅ losses.

[0027] To solve this problem, the invention provides a process as indicated at the beginning, characterized in that said first period of time of said step a) of digestion is greater than said second period of time of the aforementioned step b) of first separation.

[0028] Surprisingly, it was found that when the digestion step a) has a longer period of time than the period of time associated with the aforementioned first separation step b), the process has an interesting P2O5 yield, while guaranteeing the DCP an optimal degree of purity for the intended applications in the agricultural and food sectors.

[0029] More specifically, it has been found that the longer time period of step a) digestion compared to step b) first separation significantly reduces the time required for the first separation. This is particularly advantageous because the first separation step is no longer burdensome for the user, as it only requires minimal cleaning of the separation means, such as a filter, which can be used to perform the filtration corresponding to said first separation. In this way, the first separation step is quick and efficient. Consequently, the process according to the present invention is sufficiently cost-effective, as it does not require repeated and extensive washing of the separation means.

[0030] In practice, whether using a continuous or batch process, these advantages significantly reduce the required filtration area. This makes the initial separation step simpler and more efficient, ultimately resulting in a more cost-effective process than existing prior art methods.

[0031] It is also noted that the longer the digestion stage is carried out over a period of time, the shorter the initial separation stage can take place over a period of time.

[0032] As explained, the calcium monohydrogen phosphate obtained according to the process of the present invention can be used in agriculture, food processing, or in compositions intended for agricultural or food use. The agricultural sector includes, in particular, fertilizers. It can also be used for the production of phosphoric acid.

[0033] Thus, it has been observed that the process according to the invention makes it possible to obtain a P2O5 extraction yield for the aforementioned steps a and b which can reach up to 95% and makes it possible to provide a final product which has a degree of purity such that it is suitable for quality use in the food or agricultural sector, such as fertilizers.

[0034] Advantageously, the above steps a) and b) are carried out in a period of less than 2 hours, preferably in a period of between 30 and 100 minutes, preferably between 30 and 70 minutes, more preferably between 40 and 65 minutes.

[0035] The fact that the process according to the invention comprises a combination of parameters whereby steps a) and b) are carried out in a period of less than 2 hours and the time period associated with the aforementioned step a) of digestion is greater than the time period associated with the aforementioned step b) of first separation makes it possible to provide a process that is even more cost-effective and efficient in terms of P2O5 extraction yield compared to the prior art.

[0036] More advantageously, the predetermined period of time of said step a) is between 75 and 100 minutes, preferably between 80 and 95 minutes, or between 20 and 45 minutes, in particular between 24 and 40 minutes, preferably between 30 and 35 minutes.

[0037] According to an advantageous embodiment, said first separation of said step b) is carried out at a filtration rate of at least 0.1 tonne of P 2 O 5 / √ΔP / m 2< / day, preferably between 0.1 and 5 tonnes of P 2 O 5 / √ΔP / m 2< / day, more preferably between 0.15 and 3 tonnes of P 2 O 5 / √ΔP / m 2< / day, more preferably still between 0.3 and 0.9, in particular between 0.4 and 0.7, tonne of P 2 O 5 / √ΔP / m 2< / day, said filtration rate being calculated according to the following equation: Vitesse de filtration = Q P 2 O 5 Δ P Ω T f Or, QP2O5 corresponds to the quantity of P2O5 collected in the filtrate and is expressed in tonnes, Ω is the filter surface area expressed in m2, ΔP is the difference between the filtrate outlet pressure and the pressure applied to the pulp at the time of said first separation and is expressed in bar, and T f is the period of time of said first separation and is expressed in days.

[0038] The filtration rate indicated above is calculated, in a manner known to those skilled in the art, as described in particular in Albert Rushton, Anthony S. Ward and Richard G. Holdich, Solid-Liquid Filtration and Separation Technology, p. 35-93, ed. John Wiley & Sons, 2008.

[0039] It has been found that the process according to the invention also makes it possible to carry out said step b) of first separation at a filtration rate which is particularly advantageous in that it makes it possible, at the same time, to reduce the filtration times, while not increasing the size of the filter, which is required for an industrial application.

[0040] Moreover, surprisingly, it appeared that it was possible to obtain in a simple manner a P2O5 extraction yield for steps a and b of more than 90% by weight, preferably more than 93% by weight, advantageously more than 95%, by applying fast and economical filtration speeds on an industrial scale.

[0041] According to a preferred method, said phosphate source and said acid are introduced into a first reactor comprising said aqueous media simultaneously or successively, in order to carry out said step a) of digestion, and said pulp comprising said aqueous phase containing calcium phosphate in solution and said solid phase containing impurities is transferred from the first reactor to a separation means to carry out said first separation taken up in step b) of first separation above.

[0042] More preferably, said separation means is located between said first reactor and a second reactor.

[0043] Even more preferably, said separation means is present in a second reactor into which said pulp is introduced comprising an aqueous phase containing calcium phosphate in solution and a solid phase containing impurities, to carry out said first separation taken in step b) above.

[0044] Furthermore, the said separation means may preferably be a filter chosen from the group consisting of a rotary filter, preferably with tilting cells, filter press, belt filter and drum filter.

[0045] According to a preferred method, said acid is chosen from the group consisting of hydrochloric acid (HCl), nitric acid, sulfuric acid, phosphoric acid and mixtures thereof.

[0046] Advantageously, said acid is an aqueous solution of acid, in particular hydrochloric acid, having an acid concentration less than or equal to 15% by weight.

[0047] Even more preferably, steps a) and b) are carried out at a temperature between 50°C and 70°C, preferably equal to 60°C.

[0048] Preferably, the said phosphate source is chosen from the group consisting of a phosphate rock, a phosphate ore, secondary phosphate sources, such as ash (for example from sewage sludge or bone or pig manure) or mixtures thereof.

[0049] Advantageously, the neutralization step is carried out using a neutralizing agent selected from the group consisting of calcium-based compounds, such as calcium oxide, hydroxide and carbonate, and water-soluble calcium salts and hydroxide.

[0050] Other embodiments of the process according to the invention are indicated in the attached claims.

[0051] This disclosure also presents the use of calcium monohydrogen phosphate obtained according to the present invention to produce phosphoric acid.

[0052] Such production of phosphoric acid may involve an attack of the calcium monohydrogen phosphate obtained with sulfuric acid.

[0053] Preferably, the calcium monohydrogen phosphate obtained according to the process according to the present invention is used in the food industry or in the agricultural or horticultural field.

[0054] Other embodiments of the use according to the invention are indicated in the attached claims.

[0055] Other features, details and advantages of the process and use will emerge from the description given below, which is not exhaustive.

[0056] Within the framework of the present invention, the expression "digestion is carried out during a first period of time" should be understood as meaning that digestion ends when the first separation step is initiated, which corresponds to the moment when the pulp is introduced into a separation means, such as a filter.

[0057] In the context of the present invention, the expression "first separation carried out during a second period of time" should be understood as meaning that the duration related to this first separation is determined from the moment when the pulp to be filtered is introduced into a separation means, such as a filter.

[0058] According to a practical example, a phosphate ore and an aqueous solution of hydrochloric acid are introduced simultaneously or successively into an aqueous medium contained in a first reactor.

[0059] After digestion for a first period of time, a pulp is obtained in the first reactor and is introduced into a separation means in order to carry out step b) of first separation during a second period of time which is less than that corresponding to step a) of digestion.

[0060] This separation method can be present in the first reactor or in a second reactor.

[0061] When the separation means is present in the first reactor, the latter can be in fluidic communication with that reactor.

[0062] Thus, the predetermined digestion time ends from the moment the pulp is introduced into the separation medium.

[0063] The separation means may also be present in a second reactor possibly in fluidic communication with the first reactor.

[0064] It is also possible to use a first reactor, a second reactor and a separation means which can be arranged between said first and second reactors so as to be in fluidic communication with the latter.

[0065] In all the aforementioned scenarios, the said phosphate source has a residence time in the said first reactor greater than the residence time of the said pulp in the said separation means, the pulp consisting of an aqueous phase comprising calcium phosphate and an insoluble solid phase containing impurities.

[0066] Within the framework of the present invention, the process can be carried out continuously or discontinuously.

[0067] The neutralization step of said aqueous phase comprising calcium phosphate and chloride ions in solution, when the attack is carried out with hydrochloric acid, is performed at a pH sufficient to precipitate the calcium phosphate in the form of said calcium monohydrogen phosphate.

[0068] A second separation is provided between said aqueous medium comprising chloride ions and calcium monohydrogen phosphate so as to provide the calcium monohydrogen phosphate obtained by the process according to the present invention.

[0069] The neutralization and second separation steps are known to those skilled in the art, notably from document WO 2015 082 468. Example 1

[0070] We start with a phosphate ore exhibiting the characteristics shown in Table 1 below: Quantity of phosphate 1000,0 g Humidity 1,93 % 19,3 g CaO 48,90 % 489,0 g P 2 O 5 31,00 % 310,0 g

[0071] A quantity of 120.8 g of demineralized water is introduced into a beaker, and then a quantity of 75 g of phosphate from Table 1 is added to the demineralized water, while stirring, to form a mixture. The beaker is then covered with a watch glass, and the mixture is heated to 60°C.

[0072] To an aqueous solution of hydrochloric acid, which has an HCl concentration of 37%, 120.8 g of demineralized water are mixed, so as to obtain 357.7 g of an aqueous solution of HCl at 12%. The latter is then added to the hot mixture of phosphate and demineralized water.

[0073] The digestion time is measured from the moment the aqueous solution of dilute acid is added to the hot mixture containing the phosphate and demineralized water.

[0074] After a digestion time of 30 minutes, the resulting solution is filtered, at a filtration temperature of 60°C, using a polyester fiber filter with a diameter of 90 mm and a thickness of 0.17 mm placed on a Buchner type equipment previously placed under vacuum.

[0075] The filtration pressure used is 0.4 bar, which represents a driving pressure difference of 0.6 bar compared to the atmospheric pressure of 1 bar.

[0076] The filtration time corresponds to the time required to obtain a moist cake from the pulp formed in the previous steps. After filtration, the cake undergoes a drying step during which ambient air is drawn through it; this drying step lasts 5 minutes. According to this first embodiment, the filtration time is 5 minutes.

[0077] The weight of the resulting wet cake is then measured, as well as the weight of the filtrate. The filtrates and the cake are then subjected to analysis.

[0078] The moist cake is then dried at a temperature of 60°C and its weight, after drying, is also measured. Table 2 Example 1 Starting quantity of phosphate 75 grams Quantity of 12% aqueous HCl solution 357.7 grams Ore digestion temperature 60°C Digestion time 30 minutes Filtration temperature 60°C Filtration time 5 minutes Filtration speed 1.3 tonnes of P₂O₅ / m² / √ΔP / day

[0079] The P₂O₅ yield in the final product obtained after steps a and b at the end of the process is 94.03%. The yield is calculated based on the amount of P₂O₅ present in the phosphate ore. It represents the percentage of P₂O₅ in the filtrate after the first separation step b relative to this amount. Example 2

[0080] This example is carried out under the same operating conditions as those described in example 1, except that the digestion time is 45 minutes and the filtration time is 5.5 minutes, as illustrated in Table 3 below. Table 3 Example 2 Starting quantity of phosphate 75 grams Quantity of 12% aqueous HCl solution 357.7 grams Ore digestion temperature 60°C Digestion time 45 minutes Filtration temperature 60°C Filtration time 5.5 minutes Filtration speed 1.1 tonnes of P₂O₅ / m² / √ΔP / day

[0081] The P2O5 yield after steps a and b of the process is 93.02%. Example 3

[0082] This example is carried out under the same operating conditions as those described in example 1, except that the digestion time is 60 minutes and the filtration time is 4.75 minutes, as illustrated in Table 4 below. Table 4 Example 3 Starting quantity of phosphate 75 grams Quantity of 12% aqueous HCl solution 357.7 grams Ore digestion temperature 60°C Digestion time 60 minutes Filtration temperature 60°C Filtration time 4.75 minutes Filtration speed 1.3 tonnes of P₂O₅ / m² / √ΔP / day

[0083] The P2O5 yield in the product after steps a and b of the process is 93.16%. Example 4

[0084] This example is carried out under the same operating conditions as those described in example 1, except that the digestion time is 90 minutes and the filtration time is 2.33 minutes, as illustrated in Table 5 below. Table 5 Example 4 Starting quantity of phosphate 75 grams Quantity of 12% aqueous HCl solution 357.7 grams Ore digestion temperature 60°C Digestion time 90 minutes Filtration temperature 60°C Filtration time 2.33 minutes Filtration speed 2.7 tonnes of P₂O₅ / m² / √ΔP / day

[0085] The P2O5 yield in the product obtained after steps a and b of the process is 91.96%. Example 5

[0086] This example is carried out under the same operating conditions as those described in example 1, except that the digestion time is 21.5 minutes and the filtration time is 9 minutes, as illustrated in Table 6 below. Table 6 Example 5 Starting quantity of phosphate 75 grams Quantity of 12% aqueous HCl solution 357.7 grams Ore digestion temperature 60°C Digestion time 21.5 minutes Filtration temperature 60°C Filtration time 9 minutes Filtration speed 0.7 tonnes of P₂O₅ / m² / √ΔP / day

[0087] The P2O5 yield in the product obtained after steps a and b of the process is 96.09%. Example 6

[0088] This example is carried out under the same operating conditions as those described in Example 1, except that the digestion time is 26.33 minutes and the filtration time is 7.33 minutes, as illustrated in Table 7 below. Table 7 Example 6 Starting quantity of phosphate 75 grams Quantity of 12% aqueous HCl solution 357.7 grams Ore digestion temperature 60°C Digestion time 26.33 minutes Filtration temperature 60°C Filtration time 7.33 minutes Filtration speed 71.8 tonnes of P₂O₅ / m² / √ΔP / day

[0089] The P2O5 yield in the product obtained after steps a and b of the process is 95.85%.

Claims

1. A method for preparing calcium monohydrogen phosphate comprising the steps of: a) digesting in an aqueous medium, for a first period of time, a source of phosphate using an acid, with a pulp being formed comprising an aqueous phase containing calcium phosphate in solution and a solid phase containing impurities, b) a first separation between said aqueous phase containing calcium phosphate in solution and said solid phase containing impurities, for a second period of time, c) neutralizing said aqueous phase containing calcium phosphate in solution to a pH sufficient to obtain a precipitation in an aqueous medium of calcium phosphate insoluble as said calcium monohydrogen phosphate, and d) a second separation between said aqueous medium and said calcium monohydrogen phosphate, characterised in that said first period of time of said step a) of digestion is greater than said second period of time of said step b) above.

2. The method according to claim 1, wherein the above steps a) and b) are carried out for a duration of less than 2 hours, preferably a duration between 30 and 100 minutes, preferably between 30 and 70 minutes, more preferably between 40 and 65 minutes.

3. The method according to claim 1, wherein said first period of time of said step a) is between 75 and 100 minutes, preferably between 80 and 95 minutes, or between 20 and 45 minutes, in particular 24 and 40 minutes, more preferably between 30 and 35 minutes.

4. The method according to any one of the preceding claims, wherein said first separation of said step b) is carried out at a filtration rate of at least 0.1 tonnes of P2O5√ΔP / m2 / day, preferably between 0.1 and 5 tonnes of P2O5 / √ΔP / m2 / day, more preferably between 0.15 and 3 tonnes of P2O5 / √ΔP / m2 / day, more preferably still between 0.3 and 0.9, in particular between 0.4 and 0.7 tonnes of P2O5 / √ΔP / m2 / day, said filtration rate being calculated according to the following equation: Filtration rate = Q P 2 O 5 Δ P Ω T f wherein QP2O5 corresponds to the quantity of P2O5 collected in the filtrate and is expressed in tonnes, Ω is the surface area of the filter expressed in m2, ΔP is the difference between the outlet pressure of the filtrate and the pressure applied to the pulp at the time of said first separation and is expressed in bar, and Tf is the first period of time of said first separation and is expressed in days.

5. The method according to any one of the preceding claims, wherein said source of phosphate and said acid are introduced into a first reactor comprising said aqueous media simultaneously or successively, in order to carry out said digestion step a) and wherein said pulp comprising said aqueous phase containing calcium phosphate in solution and said solid phase containing impurities is transferred from the first reactor to a separation means for carrying out said first separation taken up in step b) above.

6. The method according to claim 5, wherein said separation means is located between said first reactor and a second reactor.

7. The method according to claim 5, wherein said separation means is present in a second reactor wherein said pulp comprising an aqueous phase containing calcium phosphate in solution and a solid phase containing impurities is introduced, to carry out said first separation in step b) above.

8. The method according to any one of claims 5 to 7, wherein said separation means is a filter selected from the group consisting of a rotary filter, preferably with tilting pan, press filter, band filter and drum filter.

9. The method according to any one of the preceding claims, wherein said acid is selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid and mixtures thereof.

10. The method according to claim 9, wherein said acid is an aqueous acid solution having an acid concentration of less than or equal to 15% by weight, preferably in said first reactor.

11. The method according to any one of the preceding claims, wherein steps a) and b) are carried out at a temperature of between 50°C and 70°C, preferably equal to 60°C.

12. The method according to any one of the preceding claims, wherein said source of phosphate is selected from the group consisting of a phosphate rock, a phosphate ore, sources of secondary phosphate such as ashes, in particular from sewage plant sludge or pig slurry or bones, or mixtures thereof.

13. The method according to any one of the preceding claims, wherein the neutralization step is carried out by means of a neutralizing agent selected from the group consisting of calcium-based compounds such as calcium oxide, hydroxide and carbonate and water-soluble calcium salts, and mixtures thereof.