ACID ATTACK METHOD FOR A PHOSPHATE SOURCE

Description

[0001] The present invention relates to a process for the acid attack of a phosphate source comprising calcium, for the production of a purified phosphate-based compound.

[0002] Acid attacks on a phosphate source containing calcium are well known from the prior art.

[0003] A classic process of this type involves reacting phosphate rock with sulfuric acid under conditions that lead to the crystallization of calcium sulfate dihydrate, or gypsum (CaSO₄·2H₂O). The gypsum slurry obtained in a first reactor can then be subjected, in a second reactor, to maturation, allowing the sulfate grains formed to grow larger and thus increasing filterability. The matured slurry is then filtered, yielding phosphoric acid with a free P₂O₅ content of approximately 25 to 35% by weight.

[0004] Phosphoric acid production processes are also known, involving sulfuric acid digestion that, at higher temperatures and concentrations of P₂O₅ and / or SO₃, yields a slurry of calcium sulfate in the form of hemihydrate (CaSO₄·½H₂O) or anhydrite. These processes generally produce concentrated phosphoric acid and a readily filterable sulfate, but the P₂O₅ extraction yield is lower than that of the conventional process. In some cases, the resulting calcium sulfate hemihydrate is also converted to calcium sulfate dihydrate after digestion (Ullman's Encyclopedia of Industrial Chemistry, 2008, pages 8 and 9).

[0005] A further process is known in which the phosphate rock is subjected again to the conditions of the conventional process in order to obtain a first slurry in which the gypsum formed has a grain size that allows for good filtration. A portion of this first slurry is then taken and subjected to conditions in which the gypsum is converted into hemihydrate, thus forming a second slurry. The remainder of the first slurry is then mixed with the second, and the whole is filtered (see WO 2005 / 118470).

[0006] A major problem in phosphoric acid production lies in the depletion of phosphate ore deposits rich in P₂O₅. These deposits have been exploited. It is now necessary to turn to ores with a P₂O₅ concentration considered low, for example, P₂O₅ contents of 25% by weight or less relative to the phosphate rock, and in some cases 20% or less.

[0007] A process for exploiting such ores and extracting high-quality production phosphoric acid has been described in international patent application WO2011 / 067321. The processing conditions involve a substantially stoichiometric reaction between the introduced sulfuric acid and the calcium contained in the phosphate rock, while the free P₂O₅ content in the crystallization slurry is maintained high, between 38 and 50 wt%, and the temperature between 70 and 90°C. Surprisingly, these conditions give rise to very fine crystals of stable dihydrate. This slurry is then subjected to a temperature increase during which the dihydrate grains dissolve and release the unattacked or co-crystallized P₂O₅, while a readily filterable calcium sulfate hemihydrate crystallizes, yielding production phosphoric acid with a very low free SO₃ content.It should be noted that these ores, which are low in P₂O₅, frequently also exhibit increasingly high levels of impurities. The impurity content is commonly expressed as the ratio (Al₂O₃ + Fe₂O₃ + MgO) / P₂O₅ x 100, also denoted MER (Minor Element Ratio). So-called conventional phosphates are characterized by a MER ratio of approximately 5 to 8.

[0008] Above 10, the impurity content is so high that it begins to negatively influence the crystallization of calcium sulfate into gypsum during sulfuric acid treatment of the ore. At these impurity levels, phosphoric acid production becomes problematic, particularly due to difficulties in crystallizing and filtering calcium sulfate dihydrate. This presents a significant drawback in all processes where filtration occurs immediately after the phosphate rock has been treated.

[0009] In a process such as described in patent application WO2011 / 067321, gypsum crystallization is also affected by impurities, but since this gypsum is not intended to be filtered, this is not a problem.

[0010] Document WO2012 / 163425 aims to develop a process for producing phosphoric acid by attacking low-quality phosphate rock with sulfuric acid. This process should yield high-quality production phosphoric acid and a good P₂O₅ extraction yield from the rock. Furthermore, it should be easily applicable to an existing conventional plant and therefore not require costly and economically untenable modifications. According to this document, the process involves adding a fluorine source to the initial slurry during the attack, at a concentration of 1% to 5% by weight of fluorine relative to the P₂O₅ contained in the phosphate rock. The attack conditions are designed to produce a substantially stoichiometric reaction between the introduced sulfuric acid and the calcium in the phosphate rock, primarily in the form of calcium carbonate and calcium phosphate.The acidic aqueous phase of this first slurry resulting from the attack contains little or no free sulfuric acid and its free P2O5 content is quite high.

[0011] As can be seen, the difficulty in producing phosphoric acid from phosphate rocks is always to have a sufficient attack yield, an acceptable acid quality as well as a more or less easily usable calcium sulfate and in this context, it is generally accepted that the attack of phosphate rocks by concentrated sulfuric acid must be carried out at stoichiometry in order to produce crude phosphoric acid and to ensure a sufficient and economically profitable P2O5 extraction rate.

[0012] In the case of phosphoric acid production by attacking phosphate rocks with stoichiometric sulfuric acid, the reaction is written: (I) Ca3(PO4)2 + 3 H2SO4 → 3 CaSO4.2H2O + 2 H3PO4 Where the molar ratio SO4 / Ca = 3 / 3, or = 1

[0013] It is also known that from this phosphoric acid thus produced, a calcium base can be added to produce food-grade dicalcium phosphate (DCP) (for human or animal consumption) or for any other application.

[0014] Thus, we know from document GB-938468 the production of monocalcium phosphate (MCP) or dicalcium phosphate (DCP) from phosphate rocks or natural ore with hydrochloric acid.

[0015] Document WO 2015 / 082468 also describes an attack on phosphate rocks with hydrochloric acid.

[0016] Unfortunately, these hydrochloric acid etching processes require a DCP washing step to remove chloride ions, which may not be present in certain technical grades of DCP. Hydrochloric acid processes generate residual calcium chloride in which some impurities from the raw material accumulate. This solution requires further purification treatments before it can be used. Furthermore, the presence of hydrochloric acid in the etching tank causes corrosion problems in the equipment as soon as the temperature reaches or exceeds 60°C.

[0017] A process for attacking phosphate rocks using sulfuric acid is also known from US patent 3161466. In the process described therein, an initial sulfuric acid attack is carried out in an attack tank, yielding a pasty slurry. This slurry can be left to mature or transferred to a second tank for further acid attack. This process relies on sequential pH control, with incremental pH increases used to selectively precipitate various impurities present in the liquid phase (liquor). The liquor contains high concentrations of MCP and phosphoric acid.

[0018] Unfortunately, this described process is restrictive in that it requires rigorous pH controls at each stage given the selective precipitation taught, but also uneconomical given the numerous steps involved and therefore the time required to treat the phosphate rock.

[0019] Finally, another process is described in document GB793801. In this document, the described process involves the substoichiometric attack of 14 to 62% concentrated sulfuric acid on phosphate rock. The purpose of the described process is the recovery of rare earth elements contained within the phosphate rock. Therefore, one of the critical steps lies in the complete dissolution of the phosphate rock to form a liquid phase from which the contained rare earth elements will be extracted. The process thus includes the addition of reactive silica to keep the rare earth elements in solution and requires an attack time of approximately 24 hours. The disclosed P₂O₅ / Ca ratios range from 10:1 to 4:1.

[0020] As can be seen, this process is time-consuming and involves significant processing costs for the phosphate rock, due to the fact that the process is likely made profitable by the extraction of rare earth elements, which have a high market value. However, in an approach aimed at producing a purified phosphate-based material, the economic viability of this process is questionable.

[0021] The invention aims to overcome the drawbacks of the state of the art by providing an economically profitable process around an optimum between energy cost, production cost, resistance of materials used in production devices and the flexibility of raw materials.

[0022] Indeed, one of the objects of the present invention is to provide a process for treating rocks concentrated in phosphate matter as well as rocks with low concentrations of phosphate matter and secondary phosphate sources.

[0023] To solve this problem, the invention provides a process for the acid attack of a phosphate source comprising calcium for the production of a purified phosphate-based compound as indicated at the beginning, comprising the steps of a) an acid attack using sulfuric acid on said phosphate source, for a predetermined period of time of between 20 and 180 minutes with formation of a first suspension containing a first solid matter and a first liquid phase in which the first solid matter is suspended, said first solid matter comprising at least calcium sulfate and impurities, said first liquid phase comprising phosphoric acid and dissolved monocalcium phosphate, said attack being carried out under conditions at the inlet according to which the molar ratio of sulfate from the sulfuric acid and possibly from the phosphate source to calcium is between 0.6 and 0.8, and the P2O5 content is less than 6%, b) a first filtration of said first slurry with separation of said first solid matter from said first liquid phase,(c) recovery from said first liquid phase of a purified phosphate-based compound.

[0024] Advantageously, said P2O5 content is a P2O5 content dissolved in said first liquid phase.

[0025] The SO4 / Ca molar ratio defines the amount of acid required to attack the Ca-containing phosphate source at the reagent inlet.

[0026] Advantageously, the acid attack takes place in one, two, or more attack tanks. As can be seen, the process according to the present invention is an attack process under strongly substoichiometric conditions, with the sulfate molar ratio from the sulfuric acid and optionally from the calcium phosphate source (SO4 / Ca) present in the phosphate source being between 0.6 and 0.8. This process offers several advantages. First, sulfuric acid consumption is reduced, and a multitude of phosphate sources can be treated by the process according to the present invention to produce various purified phosphate-based compounds. Indeed, the process according to the present invention makes it possible to obtain a liquid phase containing phosphoric acid and monocalcium phosphate, from which dicalcium phosphate can also be obtained, thus offering great flexibility.Indeed, this dicalcium phosphate can be attacked to produce relatively pure phosphoric acid and its derivatives. Furthermore, the attack time is relatively short, thus reducing production costs through the combined effect of flexibility regarding the phosphate source and the wide variety of products obtained. Maintenance costs can also be reduced thanks to the low aggressiveness of the reaction medium.

[0027] To achieve the sulfate molar ratio from sulfuric acid and possibly from the phosphate source to calcium, the calcium content is based mainly on the calcium content present in the phosphate source, but it is possible to add more if necessary.

[0028] Advantageously, the process according to the invention comprises the steps of: an acid attack in 1, 2, or more attack tanks using sulfuric acid from said phosphate source for a predetermined period of time of between 20 and 180 minutes with formation of a first suspension containing a first solid material and a first liquid phase in which the first solid material is suspended, said first solid material comprising at least calcium sulfate and impurities, said first liquid phase comprising phosphoric acid and dissolved monocalcium phosphate, said attack being carried out under inlet conditions according to which the molar ratio of sulfate from the sulfuric acid and possibly from the phosphate source to calcium present in the phosphate source is between 0.6 and 0.8 and the P2O5 content in the attack tank(s) is less than 6%,a first filtration of said first slurry with separation of said first solid matter from said first liquid phase, and recovery from said first liquid phase of a purified phosphate-based compound.

[0029] The SO4 / Ca molar ratio defines the amount of acid required to attack the Ca-containing phosphate source at the reagent inlet in the attack cell(s).

[0030] Advantageously, the process according to the invention comprises the steps of: an acid attack in 1, 2, or more attack tanks using sulfuric acid from said phosphate source for a predetermined period of time of between 20 and 180 minutes with formation of a first suspension containing a first solid matter and a first liquid phase in which the first solid matter is suspended, said first solid matter comprising at least calcium sulfate and impurities, said first liquid phase comprising phosphoric acid and dissolved monocalcium phosphate, said attack being carried out under inlet conditions according to which the molar ratio of sulfate from the sulfuric acid and possibly from the phosphate source to calcium is between 0.6 and 0.8 and the P2O5 content in the attack tank(s) is less than 6%, a first filtration of said first boil with separation of said first solid matter from said first liquid phase,and the recovery from said first liquid phase of a purified phosphate-based compound.

[0031] The SO4 / Ca molar ratio defines the amount of acid required to attack the Ca-containing phosphate source at the reagent inlet in the attack cell(s).

[0032] Advantageously, said step a) of acid attack comprises: an acid attack in an attack tank, or an acid attack in a first attack tank with the addition of sulfuric acid and transfer of said first suspension formed or being formed in the first attack tank to a second attack tank without the addition of sulfuric acid, or an acid attack in two successive attack tanks with the addition of sulfuric acid in both tanks, or an acid attack in three attack tanks, or an acid attack in a first attack tank with the addition of sulfuric acid and transfer of said first suspension formed or being formed in the first tank to a second and a third attack tank without the addition of sulfuric acid,or an acid attack in a first attack tank and a second attack tank with or without the addition of sulfuric acid and transfer of said first suspension formed or being formed in the first attack tank and in the second attack tank to a third attack tank without the addition of sulfuric acid.

[0033] In a particular embodiment, the process according to the invention comprises the steps of: an acid attack in an attack tank using sulfuric acid from said phosphate source for a predetermined period of time between 20 and 180 minutes with formation of a first suspension containing a first solid material and a first liquid phase in which the first solid material is suspended, said first solid material comprising at least calcium sulfate and impurities, said first liquid phase comprising phosphoric acid and dissolved monocalcium phosphate, said attack being carried out under conditions at the inlet according to which the molar ratio of sulfate from the sulfuric acid and possibly from the phosphate source to the calcium present in the phosphate source is between 0.6 and 0.8 and the P2O5 content in the attack tank is less than 6%,a first filtration of said first slurry with separation of said first solid matter from said first liquid phase, and recovery from said first liquid phase of a purified phosphate-based compound.

[0034] The SO4 / Ca molar ratio defines the amount of acid required to attack the Ca-containing phosphate source at the reagent inlet to the reaction vessel.

[0035] It has indeed surprisingly emerged that the combination of highly substoichiometric conditions (low quantity of sulfuric acid available to attack the phosphate source) with a short attack time allows the production of purified phosphate-based compounds that are easily marketable and economically viable, all while the P₂O₅ concentration in the attack tank(s) is low but nonetheless of sufficient purity for use in various subsequent production processes, including, but not limited to, the industrial-scale and food-grade production of DCP. Therefore, in the process according to the present invention, the extraction of P₂O₅ from numerous phosphate sources is optimal, regardless of whether the phosphate sources are phosphate-concentrated or not.This means that once this process is implemented at a production site, the industrialist can then exploit conventional rocks concentrated in calcium phosphate, but also rocks less concentrated in calcium phosphate or any secondary product containing calcium phosphate.

[0036] By using a sulfate molar ratio from sulfuric acid, and possibly from the phosphate source, to calcium present in the phosphate source of between 0.6 and 0.8, the solid-to-liquid ratio is low, resulting in a low suspension density. The sulfate present in the attack tank(s) originates primarily from the rock, but it can also come from the phosphate source and possibly the dilution water. Indeed, the solids content in the attack tank(s) is typically less than 16%, preferably between 4% and 15%, thus providing a suspension rather than a slurry. This low solids content is typically counterintuitive in a phosphate source treatment process where the attack time is short, the sulfuric acid is diluted, and a filtration step is required.

[0037] In the process according to the present invention, the reaction is substoichiometric, according to the reaction Ca 3 (PO 4 ) 2 + 2 H 2 SO 4 + H 2 O → 2 CaSO 4 .2H 2 O + Ca(H 2 PO 4 ) 2 with a theoretical molar ratio SO 4 / Ca around 0.66.

[0038] By implementing a sulfate molar ratio from sulfuric acid and possibly from the phosphate source to calcium present in the phosphate source of between 0.6 and 0.8 as close as possible to the theoretical SO4 / Ca ratio, the attack conditions allow us to remain mostly below the precipitation curve of calcium with the phosphate ion and therefore to produce MCP soluble in the acidic liquid phase, where the P2O5 extraction yield has been measured at over 90%.

[0039] In a particular embodiment, the molar ratio of sulfate from sulfuric acid and possibly from the phosphate source to calcium of between 0.6 and 0.8 can be obtained by adding calcium to the system if the phosphate source does not contain calcium.

[0040] The term "acid attack using a mineral acid, preferably sulfuric acid from a phosphate source for a predetermined period of time" means that the predetermined period of time is the average residence time in one or more attack tanks, whether in batch or continuous attack, possibly with a recycling phase, as indicated later.

[0041] In the process according to the present invention, the first solid material comprises unattacked calcium phosphate, calcium sulfate (calcium sulfate hemihydrate, anhydrite, or gypsum), and impurities. The calcium sulfate is predominantly present in the form of gypsum (calcium sulfate dihydrate).

[0042] The term "monocalcium phosphate" refers to a compound with the formula Ca(H2PO4)2 (MCP) which has several English names such as monocalcium phosphate, monobasic calcium phosphate, calcium biphosphate, calcium acid phosphate, acid calcium phosphate, mono basic calcium phosphate, calcium dihydrogen phosphate.

[0043] Advantageously, the predetermined period of time is less than 120 minutes, preferably less than 90 minutes, more preferably less than 60 minutes, particularly less than 45 minutes and more particularly about equal to 30 minutes.

[0044] As can be seen, the predetermined time period during which the acid attack occurs can be greatly reduced to achieve attack times as short as 60 minutes, in particular 45 minutes, or even 30 minutes.

[0045] In a particular embodiment, the P2O5 content in the attack tank(s) is less than 5%, preferably between 0.5 and 4% in P2O5. Preferably between 1.5 and 3%.

[0046] Indeed, in the process according to the present invention, the P2O5 content is low in the attack medium, but it is ultimately sufficiently pure, contrary to expectations, so that it can then be valorized into purified phosphate-based compounds.

[0047] In another preferred embodiment of the process according to the present invention, said attack is carried out at room temperature.

[0048] In a preferred embodiment of the process according to the present invention, said attack is carried out at a temperature in the attack tank(s) less than or equal to 90°C, preferably less than or equal to 80°C, preferably less than or equal to 75°C, more preferably less than or equal to 60°C, preferably greater than 40°C.

[0049] Indeed, according to the present invention, it has been identified that it is possible to treat the rock by acid attack at a generally low temperature, certainly between 40 and 60°C, which avoids any heat input and further reduces production costs from an energy cost perspective, while using dilute sulfuric acid, also reducing SO3 residues in the purified phosphate-based compound.

[0050] Advantageously, sulfuric acid is a dilute sulfuric acid, especially before addition to the attack tank(s), which reduces the costs of treating phosphate sources, whether concentrated in phosphate or not, while also reducing the SO3 content in the liquid phase.

[0051] Advantageously, in the process according to the present invention, said dilute sulfuric acid has a concentration of H2SO4 of less than 14% by weight, preferably less than or equal to 13%, preferably less than or equal to 10% by weight, more particularly between 0.5 and 9% by weight, preferably between 3 and 7% and more preferably around 5% by weight, relative to the total weight of the dilute sulfuric acid.

[0052] In one variant, the sulfuric acid is concentrated sulfuric acid, in particular it will be diluted in the attack tank(s), the dilution water being either drinking water, river water, seawater, recycled water or water from the production of DCP.

[0053] As mentioned previously, the molar ratio of sulfate from sulfuric acid, and possibly from the phosphate source, to the calcium present in the phosphate source is between 0.6 and 0.8, which is low enough to approach the theoretical optimum and maintain the MCP and phosphoric acid in the liquid phase. The key challenge is to achieve the solubility of calcium sulfate without precipitating calcium phosphate.Therefore, when sulfuric acid is advantageously diluted and has a H₂SO₄ concentration of less than 14%, preferably less than or equal to 10%, or even between 0.5 and 9% by weight, relative to the total weight of the diluted sulfuric acid, the optimum is achieved, in conjunction with the predetermined short duration of the acid attack, by reducing the risk of precipitating calcium with phosphate ions in solution. This promotes the formation of calcium phosphate rock (CPR) and phosphoric acid in the attack vessel(s) in the liquid phase, rather than as a precipitate, since the reaction medium in the attack vessel(s) is sufficiently dilute to prevent the precipitation of calcium phosphate salts. Only calcium sulfate, preferably gypsum, precipitates, and in quantities lower than in conventional processes for attacking phosphate sources.Therefore, in the process according to the present invention, it is possible to use a low-value recovered or recycled acid.

[0054] In the process according to the present invention, the sulfuric acid may be dilute sulfuric acid recycled from existing streams from the phosphate, metallurgy, chemical, and other industries. The liquid phase recovered, for example, after the production of DCP by precipitation, can be recycled to dilute the attacking sulfuric acid solution. Preferably, the attacking sulfuric acid is stored in a storage tank. The attacking sulfuric acid can therefore be obtained from recycling other steps or by diluting concentrated acid, such as concentrated sulfuric acid of 98% or less, which can be diluted with water or with the liquid phase recovered, for example, after the production of DCP by precipitation (second liquid phase). Preferably, the dilution of the sulfuric acid will be carried out in-line during the feeding of the attack tank(s).

[0055] More particularly, in the process according to the present invention, said molar ratio of sulfate from sulfuric acid and possibly from the phosphate source to calcium present in the phosphate source is between 0.68 and 0.78, preferably between 0.7 and 0.75 at the inlet.

[0056] More particularly, in the process according to the present invention, said sulfate molar ratio from sulfuric acid and possibly from the calcium phosphate source is between 0.68 and 0.78, preferably between 0.7 and 0.75 at the inlet.

[0057] Preferably, the process according to the present invention comprises an addition of a base to said first suspension, before filtration.

[0058] Adding the base to the initial suspension allows for the precipitation of calcium fluoride before filtration (pre-neutralization), which can be advantageous depending on the desired final compounds and their intended use. If the added base is a calcium base, such as quicklime or slaked lime, powdered lime or lime milk, or even limestone, the formation of gypsum before filtration is promoted, thus reducing the residual SO3 content in the liquid phase.

[0059] In one variant, the process according to the invention comprises, prior to said recovery step from said first liquid phase of said purified phosphate-based compound, an addition of a base to said first liquid phase after filtration with formation of a second suspension comprising a second solid material suspended in a second liquid phase and a filtration of said second suspension to separate said second solid material suspended from said second liquid phase, said purified phosphate-based compound being thus recovered from said second liquid phase, originating from the first liquid phase depleted in said second solid material, principally calcium fluoride.

[0060] In this embodiment, if a base was added before filtration, the calcium fluoride was removed during filtration and is present in the first solid, regardless of whether the desired purified phosphate compound is DCP or MCP and phosphoric acid. DCP production involves adding a calcium base to MCP (neutralization), which would also cause calcium fluoride to precipitate if it had not been removed beforehand.

[0061] If no base has also been added before the filtration of the first suspension, the first solid material contains essentially calcium sulfate (calcium sulfate hemihydrate, anhydrite or gypsum), impurities and unattacked phosphate, while fluorine is still in the first liquid phase.

[0062] When the base is added in a controlled manner to the first liquid phase, which is significantly depleted in the initial solids but still contains fluorine, the calcium fluoride can then be selectively removed. In such a case, various subsequent steps can be considered.

[0063] If said purified phosphate-based compound thus recovered from said second liquid phase which is to be produced is MCP and / or phosphoric acid, the second liquid phase is recovered and then treated for this purpose.

[0064] If the purified phosphate-based compound thus recovered from the second liquid phase to be produced is DCP, the second liquid phase is treated by the subsequent addition of a calcium base, such as quicklime or slaked lime, powdered or in the form of slurry of lime, or even limestone. In this case, a third suspension is formed following the addition of the calcium base to the second liquid phase, which is depleted of fluoride, and which is then filtered to recover the third solid phase containing the DCP.

[0065] Of course, when the DCP may contain fluorine or when the phosphate source used does not contain fluorine, the second suspension is formed by adding a calcium base, such as quicklime or hydrated lime, powdered or in the form of slurry of lime, or even limestone, to the first liquid phase. This phase is then filtered to separate the second solid matter containing the DCP from the second liquid phase, which forms wastewater. This wastewater can be recycled to prepare the sulfuric acid solution for acid etching the phosphate source or for diluting the etching tank(s). In this case, the controlled addition of a base to selectively precipitate the fluoride is not necessary.

[0066] In a particularly preferred embodiment, a base is added before filtration of the first solid material to precipitate the fluorides and remove them from the first liquid phase along with the unattacked calcium sulfate and calcium phosphate. The first liquid phase is then further treated by the addition of a calcium base, such as quicklime or slaked lime, powdered lime or slurry of lime, or even limestone, to form a second suspension containing precipitated DCP as a second solid material, which is then recovered from the second suspension by filtration, centrifugation, decantation, or any other solid-liquid separation method.

[0067] Whether the DCP is formed from the first liquid phase or from the second liquid phase, a stoichiometric amount of calcium base, as mentioned above, is added to the first liquid phase or the second liquid phase, for example in a neutralization reactor to precipitate the DCP, preferably with pH control up to a value between 5 and 6.

[0068] A preferred method for precipitating DCP is to add finely ground limestone to either the first or second liquid phase to neutralize the liquid phase containing MCP and phosphoric acid. The neutralization process preferably has a residence time of at least 30 minutes to allow the neutralization reaction and the resulting CO₂ release to complete. In a preferred embodiment, to obtain a pH between 5 and 6, lime slurry is further added to ensure complete precipitation of the DCP and thereby extract all of the P₂O₅ from the residual liquid.

[0069] More particularly, in the process according to the present invention, said first solid material separated from said first liquid phase is recycled in whole or in part by introduction into the first suspension.

[0070] Indeed, it may be advantageous to increase the solids content in the first suspension, either in the attack tank(s) or in the filtration device to aid in its filtration or to be able to treat the residual calcium phosphate found in the first solids.

[0071] The first suspension containing calcium sulfate and possibly calcium fluoride is preferably recovered by any liquid / solid separation method such as filtration, including a rotary filter manufactured by the applicant, centrifugation, decantation, hydrocycloning, or a belt filter, to separate the first solid from the first liquid phase. The first liquid phase is a dilute solution of P₂O₅ containing a slight excess of sulfate, for example, between 0.05% and 0.6%, preferably between 0.1% and 0.25%.

[0072] During filtration, the filtration unit can be washed with water to displace interstitial water from the filter cake and recover any remaining traces of P₂O₅ in the calcium sulfate cake. The calcium sulfate is then washed and separated. However, prior to this operation, calcium sulfate can be recycled back into the processing tank(s) to improve the conditions for processing the phosphate source and thus enhance the precipitation of the calcium sulfate that forms the first solid, thereby facilitating its filtration.

[0073] Recycling can be provided either by recycling part of the washed calcium sulfate back to the attack tank(s) and thus increasing the calcium sulfate suspension density well beyond 10% by weight relative to the total weight of the suspension.

[0074] Recycling can alternatively be provided by installing a thickener of the first suspension prior to separation; a portion of the thickened suspension can then be taken and returned to the attack tank(s).

[0075] This recycling process increases the solids density in the suspension within the processing tank(s) and facilitates the removal of calcium sulfate supersaturation from the medium in the processing tank(s). This prevents uncontrolled nucleation of the suspension and results in better-crystallized calcium sulfate particles in the initial suspension. This recycling also prevents blocking reactions in the sulfuric acid attack of the ore.

[0076] In a variant of the present invention, the second solid material separated from the second liquid phase is recycled by introduction into the first or second suspension. Preferably, when the second solid material is calcium fluoride, it will not be recycled.

[0077] Indeed, in some cases, if filtration proves complicated due to low solids content, it may be advantageous to be able to increase the solids content by introducing the second solid into the first suspension or into the second suspension, for example to add nuclei that promote crystallization.

[0078] Indeed, when DCP is produced, the suspension containing it is filtered or centrifuged to separate the DCP from the liquid phase. Since the liquid phase is practically water, there is no need to wash the separated DCP cake, and the liquid phase can be advantageously recycled back into the sulfuric acid tank, either online or in-situ in the etching tank(s). In some cases, when the initial liquid phase is properly purified, the amount of impurities is reduced, which facilitates the recycling of the liquid phase recovered after DCP isolation.

[0079] Preferably, the phosphate source is defined as any phosphate material of organic or mineral origin containing less than 45% by weight of P₂O₅ relative to the total weight of the dry matter (dry at 105°C); preferably less than or equal to 40%, preferably less than or equal to 30%, preferably less than or equal to 20%, preferably less than or equal to 10%. In this phosphate source, the calcium may or may not be bound to the phosphate, hydrogen phosphate, and / or dihydrogen phosphate ion.The said source may be chosen from the group consisting of conventional phosphate rock, low P2O5 phosphate rock, ashes of various mineral or organic origins such as ash from anaerobic digestates of organic waste, such as slurries, sewage sludge, compost, manure, residues from the metallurgical, chemical including phosphate chemistry, agri-food industries, sewage sludge, guano, bone ash, slurries, manure, green waste.

[0080] As a general rule, in cases where a Ca deficiency is present, calcium can be added in the form of lime, milk of lime, calcium carbonate, calcium chloride and possibly phosphate rock containing calcium.

[0081] The term "conventional phosphate rock" means, for the purposes of this invention, a rock which has a typical P2O5 analysis greater than 25%, it may or may not have undergone one or more physico-chemical treatments (crushing, screening, washing, flotation) which may or may not increase the (P2O5) content of the rock.

[0082] For the purposes of this invention, the term "low P2O5 phosphate rock" means a rock which has a typical P2O5 analysis of less than 25%, preferably 20%.

[0083] The terms "ashes, sewage sludge, bone ash, manure and any raw material with a phosphate content of less than or equal to 40% by weight of P2O5 relative to the total weight of the raw material" refer to secondary phosphate sources, generally difficult to valorize, such as ash from sewage sludge, plant matter (wood, wheat bran), ash from rendering yards, co-products of waste or biomass incineration for energy production.

[0084] More particularly, in the process according to the present invention, said purified phosphate-based compound is a monocalcium phosphate MCP, a dicalcium phosphate DCP, more particularly a food-grade (human or animal) dicalcium phosphate DCP, a phosphoric acid and its derivatives, such as, for example, directly from said first liquid phase or a phosphoric acid produced from said DCP.

[0085] The term “dibasic calcium phosphate (DCP)” means dibasic calcium phosphate (DCP) (in English dibasic calcium phosphate or dicalcium phosphate) with the formula CaHPO4, which can be in anhydrous form (DCPA) or dihydrate form (DCPD).

[0086] The term “food grade dicalcium phosphate (DCP)” means any DCP intended for animal feed (particularly in the Feed Grade and Pet Food sectors), human food, and the dental and oral care industry.

[0087] In a preferred embodiment of the process according to the present invention, said second liquid phase is recycled by introduction into said attack tank(s).

[0088] Other embodiments of the process according to the invention are indicated in the attached claims. A DCP dicalcium phosphate in anhydrous or dihydrate form obtained by the process according to the invention has a chloride content less than or equal to 0.025% by weight relative to the total weight of said dicalcium phosphate, and / or a fluoride content less than or equal to 2% by weight relative to the total weight of said dicalcium phosphate, and / or a Na2O content less than or equal to 0.15% by weight relative to the total weight of said dicalcium phosphate.

[0089] More particularly, a DCP dicalcium phosphate in anhydrous or dihydrate form obtained by the process according to the invention has a chloride content less than or equal to 0.02% by weight relative to the total weight of said dicalcium phosphate, and a fluoride content less than or equal to 1% by weight relative to the total weight of said dicalcium phosphate, more particularly, as an additive for animal feed.

[0090] Alternatively, an anhydrous or dihydrated DCP dicalcium phosphate obtained by the process according to the invention has a chloride content less than or equal to 0.02% by weight relative to the total weight of said dicalcium phosphate, more particularly as an ingredient in fertilizers or as a source of phosphate to be attacked in the production of phosphoric acid.

[0091] Other embodiments of the dicalcium phosphate according to the invention are indicated in the attached claims. A use of the anhydrous or dihydrated form of DCP dicalcium phosphate obtained by the process according to the invention, having a chloride content less than or equal to 0.02% by weight relative to the total weight of said dicalcium phosphate, and a fluoride content less than or equal to 1% by weight relative to the total weight of said dicalcium phosphate, is in animal feed, particularly for feed grade (livestock, poultry, aquaculture, pig farming) and domestic animals.A use of DCP dicalcium phosphate in anhydrous form, or dihydrated form obtained by the process according to the invention having a chloride content less than or equal to 0.02% by weight relative to the total weight of said dicalcium phosphate, is more particularly as an ingredient in a fertilizer or as a phosphate source for the production of phosphoric acid.

[0092] Advantageously, the DCP dicalcium phosphate in anhydrous or dihydrate form obtained by the process according to the invention has a chloride content less than or equal to 0.025% by weight relative to the total weight of said dicalcium phosphate, and / or a fluoride content less than or equal to 2% by weight relative to the total weight of said dicalcium phosphate, and / or a Na2O content less than or equal to 0.15% by weight relative to the total weight of said dicalcium phosphate.

[0093] More particularly, the DCP dicalcium phosphate in anhydrous or dihydrate form obtained by the process according to the invention has a chloride content less than or equal to 0.02% by weight relative to the total weight of said dicalcium phosphate.

[0094] Advantageously, the DCP dicalcium phosphate in anhydrous or dihydrate form obtained by the process according to the present invention has a fluoride content less than or equal to 1% by weight relative to the total weight of said dicalcium phosphate.

[0095] Other features, details and advantages of the invention will become apparent from the description given below, by way of non-limitation and with reference to examples.

[0096] The process according to the present invention offers a number of advantages that enable the implementation of a competitive process. Indeed, it allows the use of dilute sulfuric acid with a concentration, for example, less than 14%, preferably between 0.5 and 10%, particularly between 1 and 7%, more particularly between 2 and 5%, and more specifically between 3 and 4% by weight relative to the total weight of the dilute sulfuric acid, or even recycled sulfuric acid, thus reducing the cost of raw materials. It also allows, but is not limited to, the processing of various phosphate sources, such as rocks with low P₂O₅ content or secondary phosphorus sources.

[0097] Working in substoichiometry within the meaning of the present invention with an SO4 / Ca ratio between, for example, 0.68 and 0.8 allows a saving of 20 to 25% in H2SO4 and an extraction yield advantageously greater than 85%, preferably greater than 90%.

[0098] The attack time is relatively short, potentially as short as 90 minutes or less, such as between 30 and 60 minutes.

[0099] The attack temperature is also relatively low compared to a conventional attack process, for example between 40°C and 60°C compared to conventional temperatures between 75 and 95°C, which allows for energy savings.

[0100] The P2O5 content in the liquid phase of the first suspension is preferably between 1 and 5%, in particular between 1.5 and 3.5%, or even 2 to 3% by weight relative to the total weight of the first liquid phase.

[0101] The process according to the present invention also makes it possible to obtain a purification rate of more than 50%, preferably more than 60% by weight in As, Al, U, Th, Na compared to the original weight of these elements contained in the phosphate source.

[0102] More particularly, the present invention relates, but is not limited to, a DCP, for example obtained by the process according to the present invention, having chloride and fluoride contents which make applications in human or animal food possible, namely a chloride content of less than 0.025%, which can go down to contents as low as 1 ppm and a fluoride content of less than 2%, which can go down to contents as low as 0.1% by weight relative to the total weight of the DCP.

[0103] Preferably, the DCP has low levels of residual SO3 due to the attack with very dilute sulfuric acid. The Na2O content is also less than 0.15% by weight relative to the total weight of the DCP in certain embodiments.

[0104] In an advantageous DCP product, the MgO content is also less than 1% by weight relative to the total weight of the DCP.

[0105] More particularly, an advantageous DCP obtained by the process according to the present invention has an Sr content of less than 100 ppm; preferably less than 50 ppm, more particularly less than 10 ppm, more specifically less than 1 ppm compared to the DCP.

[0106] The DCP obtained by the process according to the present invention also has a Th content typically less than 5 ppm compared to the DCP.

[0107] Similarly, the Mn content in the DCP obtained by the process according to the present invention is less than 10 ppm relative to the DCP.

[0108] Typically, the Mo content in the DCP obtained by the process according to the present invention is less than 2 ppm relative to the DCP.

[0109] Finally, the DCP obtained by the process according to the present invention preferably has a U3O8 content of less than 32 ppm.

[0110] Furthermore, the object is a composition of dicalcium phosphate DCP comprising a) a CaO content greater than or equal to 40% by weight relative to the total weight of said dicalcium phosphate; b) a chloride content less than or equal to 0.020% by weight relative to the total weight of said dicalcium phosphate; c) a fluoride content less than or equal to 2% by weight relative to the total weight of said dicalcium phosphate; d) a Na₂O content less than or equal to 0.15% by weight relative to the total weight of said dicalcium phosphate, as can be seen, the DCP obtained by the process according to the present invention It possesses the qualities required for use in human or animal food as well as in technical applications. Examples.- Example 1 - Attack on a laboratory-scale phosphate source

[0111] 100 g of phosphate source (phosphate rock) containing 30.5 g of P2O5, 49.5% CaO equivalent, 3.95% fluorine, 0.308% Fe2O3 equivalent, 0.547% Al2O3 equivalent and 0.303% MgO equivalent, by weight relative to the weight of the phosphate source, is brought into contact with sulfuric acid diluted to a concentration of 2% for an attack time of 30 minutes, an attack temperature of 60°C and according to the SO4 / Ca molar ratio of 0.8. The SO4 / Ca molar ratio defines the quantity of acid necessary to attack the phosphate source containing Ca at the inlet of the attack tank.

[0112] Once all the additions have been made, let it stir for half an hour before filtering.

[0113] The resulting suspension is then filtered under vacuum through a Büchner funnel. The different quantities obtained are recorded, and the calcium sulfate and liquid phase products are analyzed.

[0114] In the laboratory protocol, this is a batch process, without washing. However, the washing was extrapolated and the quantity of P2O5 in the impregnation liquid of the filter cake was calculated.

[0115] The attack yield is calculated using the following formula: (Mass of P₂O₅ in the filtrate + mass of P₂O₅ in the filter cake impregnation liquid) / (total mass of P₂O₅ in the phosphate source). The P₂O₅ content in the impregnation liquid corresponds to the P₂O₅ that can be recovered by washing the filter cake in an industrial process.

[0116] The quantity of diluted sulfuric acid added is 3499 g for an SO4 content of 70.2 g. The SO4 / Ca ratio is 0.8, due to the calcium content of the phosphate source.

[0117] The recovered liquid phase has a volume of 3.09 liters and a mass of 3125 g, with a pH of 2.1. The P₂O₅ content in the liquid phase is 0.83%, and the SO₃ content is 0.16% by weight relative to the weight of the liquid phase. The mass of P₂O₅ in the impregnation liquid is 1.2 g.

[0118] The molar ratio CaO / P2O5 in the first solution is 0.58, while the residual CaO content in the liquid phase is 0.19% by weight relative to the weight of the liquid phase.

[0119] The P2O5 attack yield is 89%. As can be seen, despite the use of low-concentration sulfuric acid at 2% and sub-stoichiometric attack conditions for a total attack time of only 30 minutes, the P2O5 attack yield is significantly high. Example 2 - Attack on a laboratory-scale phosphate source

[0120] 150 g of phosphate source (rock) containing 15.8 g of P₂O₅, 27.6% CaO equivalent, 2.2% fluorine, 2.37% Fe₂O₃ equivalent, 2.88% Al₂O₃ equivalent, and 0.416% MgO equivalent, by weight relative to the weight of the phosphate source, is contacted with 5% diluted sulfuric acid for an attack time of 30 minutes, an attack temperature of 40°C, and according to the SO₄ / Ca ratio of 0.8, as per the protocol of Example 1: The quantity of diluted sulfuric acid added is 1131 g for an SO₄ content of 61.0 g. The SO₄ / Ca ratio is 0.8, due to the calcium content of the phosphate source.

[0121] The recovered liquid phase has a volume of 0.955 liters and a mass of 976 g, with a pH of 1.8. The P₂O₅ content in the liquid phase is 1.97%, and the SO₃ content is 0.27% by weight relative to the weight of the liquid phase. The mass of P₂O₅ in the impregnation liquid is 3.24 g.

[0122] The molar ratio CaO / P2O5 in the first solution is 0.43, while the residual CaO content in the liquid phase is 0.33% by weight relative to the weight of the liquid phase.

[0123] The P2O5 attack yield is 95%. As can be seen, despite the use of low-concentration sulfuric acid at 5% and a phosphate source containing very few phosphates, under sub-stoichiometric attack conditions for a total attack time of only 30 minutes, the P2O5 attack yield is significantly high. Example 3 - Attack on a laboratory-scale phosphate source

[0124] 100 g of phosphate source (phosphate rock) containing 30.5 g of P 2 O 5 , 49.5% CaO equivalent, 3.95% fluorine, 0.308% Fe 2 O 3 equivalent, 0.547% Al 2 O 3 equivalent and 0.303% MgO equivalent, by weight relative to the weight of the phosphate source is brought into contact with sulfuric acid diluted to a concentration of 5% for an attack time of 30 minutes, an attack temperature of 60°C and following the SO 4 / Ca ratio of 0.8 according to the protocol of example 1.

[0125] The quantity of diluted sulfuric acid added is 1398 g for an SO4 content of 70.1 g. The SO4 / Ca ratio is 0.8, due to the calcium content of the phosphate source.

[0126] The recovered liquid phase has a volume of 1.13 liters and a mass of 1161 g, with a pH of 2.2. The P₂O₅ content in the liquid phase is 2%, and the SO₃ content is 0.20% by weight relative to the weight of the liquid phase. The mass of P₂O₅ in the impregnation liquid is 2.6 g.

[0127] The molar ratio CaO / P2O5 in the first solution is 0.38, while the residual CaO content in the liquid phase is 0.30% by weight relative to the weight of the liquid phase.

[0128] The P2O5 attack yield is 85%. As can be seen, despite the use of low-concentration sulfuric acid at 5% and sub-stoichiometric attack conditions for a total attack time of only 30 minutes, the P2O5 attack yield is significantly high. Comparative Example 1 - Attack on a laboratory-scale phosphate source

[0129] 100 g of phosphate source (phosphate rock) containing 30.5 g of P 2 O 5 , 49.5% CaO equivalent, 3.95% fluorine, 0.308% Fe 2 O 3 equivalent, 0.547% Al 2 O 3 equivalent and 0.303% MgO equivalent, by weight relative to the weight of the phosphate source is brought into contact with sulfuric acid diluted to a concentration of 5% for an attack time of 30 minutes, an attack temperature of 60°C but this time following the SO 4 / Ca molar ratio of 1 according to the protocol of example 1.

[0130] The quantity of diluted sulfuric acid added is 1747 g for an SO4 content of 87.2 g. The SO4 / Ca ratio is 1, due to the calcium content of the phosphate source.

[0131] The recovered liquid phase has a volume of 1.4 liters and a mass of 1429 g, with a pH of 2.1. The P₂O₅ content in the liquid phase is 1.63% and the SO₃ content is 0.61% by weight relative to the weight of the liquid phase. The mass of P₂O₅ in the impregnation liquid is 3.5 g.

[0132] The molar ratio CaO / P2O5 in the first solution is 0.26, while the residual CaO content in the liquid phase is 0.17% by weight relative to the weight of the liquid phase.

[0133] The P₂O₅ etching efficiency is 88%. As can be seen in the comparative example under stoichiometric conditions, the specific consumption of sulfuric acid is higher for a similar etching efficiency. The consumption of calcium source required for neutralization will also be greater. Example 4.- Substoichiometric attack of phosphate rock on a pilot scale

[0134] The pilot plant comprises three agitated and thermostatically controlled tanks, each with double jackets heated by oil. The tanks are arranged in a continuous overflow pattern; the first two have a capacity of 20 liters, and the third, with a capacity of 30 liters, serves only as a buffer before filtration.

[0135] 10 liters of water are poured into the first tank and heated to the working temperature. The phosphate source and the dilute sulfuric acid are fed into the first reactor with flow rates corresponding to the desired attack conditions (SO4 / CaO molar ratio, attack time, H2SO4 concentration for the attack of the phosphate source, P2O5 content in the attack tank).

[0136] The resulting suspension overflows into the second reactor. This second reactor is designed to perform neutralization before filtration. However, this neutralization before filtration is not always carried out.

[0137] The suspension finally overflows into the third reactor which is used to feed the filtration cell.

[0138] A quantity of the suspension is filtered every 30 minutes. Two types of filtration are performed alternately: Filtration for recycling in the attack reactor: The filter cake is not washed and is recycled to the first (aggression) reactor to increase the solids content in the reaction medium. The liquid phase (filtrate) is poured into a drum and stored for the neutralization and DCP production step. This filtration step is certainly not required at an industrial scale. It can, of course, be carried out, but is not necessary. At a pilot scale, it is advantageous to perform this step since it is preferable to increase the solids content in the aggression reactor. The filtration process for the production of calcium sulfate:Here, the calcium sulfate cake is washed with a predetermined amount of water to recover the P₂O₅ contained in the impregnation liquid. The liquid phase, along with the wash filtrate, is poured into the filtrate recovery drum. The calcium sulfate is then discharged for disposal.

[0139] The installation is in stable mode, samples of calcium sulfate and liquid phase (filtrates) are taken for analysis and the different products are also analyzed.

[0140] The yield is calculated as follows: mass of P2O5 in the liquid phase (g / h) / mass of P2O5 in the phosphate source (g / h).

[0141] A phosphate source in rock form containing 30.3% by weight of P₂O₅, 47.6% CaO equivalent, 3.68% fluorine, 0.144% Fe₂O₃ equivalent, 0.18% Al₂O₃ equivalent, and 0.542% MgO equivalent, by weight relative to the weight of the phosphate source, is added to the reaction vessel in the presence of sulfuric acid diluted to 10% by weight relative to the weight of the diluted acid, with a SO₄ / Ca molar ratio of 0.8. The reaction temperature is 60°C and the reaction time is approximately 1 hour. The pH in the reaction vessel is 2.04. The rock flow rate is 2.67 kg / h and the acid flow rate is 17.5 liters / h. The P2O5 content in the attack suspension is 4.5% by weight relative to the total weight of the suspension.

[0142] During filtration, the flow rate of recovered liquid phase is 16.13 kg / h.

[0143] The attack efficiency is 93%.

[0144] As can be seen, the laboratory tests are confirmed in pilot studies; the attack efficiency of phosphate rock in the presence of dilute sulfuric acid and under conditions of low P2O5 content in the attack tank (<6%) and short attack time is particularly high, even under sub-stoichiometric conditions. Example 5.- Substoichiometric attack of phosphate rock on a pilot scale

[0145] The driver used is that of example 4.-, the same process as in example 4.- is implemented there.

[0146] The same phosphate source as in Example 4 is added to the etching tank in the presence of 5% (by weight) diluted sulfuric acid, with a SO₄ / Ca molar ratio of 0.7, due to the calcium content of the phosphate source. The etching temperature is 60°C and the etching time is approximately 1 hour. The pH in the etching tank is 2.5. The rock flow rate is 3 kg / h and the acid flow rate is 35.6 liters / h. The P₂O₅ content in the etching suspension is 2.32% (by weight) relative to the total weight of the suspension.

[0147] At filtration, the recovered liquid phase flow rate is 35.44 kg / h.

[0148] The attack efficiency is 94%.

[0149] As can be seen, compared to example 4, despite the presence of a sulfuric acid twice as dilute, the yield in P2O5 is even higher. Example 6.- Substoichiometric attack of phosphate rock on a pilot scale

[0150] The pilot used is that of example 4.-, the same process as in example 4.- is implemented there, except that in the second reactor, i.e. the neutralisation reactor before filtration, the pH has been adjusted to 2.48 by adding milk of lime Ca(OH) 2.

[0151] A phosphate rock containing 34.9% by weight of P₂O₅, 49.8% CaO equivalent, 3.78% fluorine, 0.136% Fe₂O₃ equivalent, 0.386% Al₂O₃ equivalent, and 0.156% MgO equivalent by weight relative to the weight of the phosphate source is added to the reaction vessel in the presence of 5% by weight of diluted sulfuric acid, with a SO₄ / Ca ratio of 0.8, due to the calcium content of the phosphate source. The reaction temperature is 60°C and the reaction time is approximately 1 hour. The pH in the reaction vessel is 2. The rock flow rate is 2.6 kg / h and the acid flow rate is 35.7 liters / h. The P2O5 content in the attack suspension is 2.10% by weight relative to the total weight of the suspension.

[0152] At filtration, the recovered liquid phase flow rate is 38.22 kg / h.

[0153] The attack efficiency is 92%. Example 7.- Substoichiometric attack of phosphate rock on a pilot scale

[0154] The driver used is that of example 4.-, the same process as in example 4.- is implemented there.

[0155] A phosphate rock containing 24.90% by weight of P₂O₅, 40.5% CaO equivalent, 2.54% fluorine, 3.97% Fe₂O₃ equivalent, 1.13% Al₂O₃ equivalent, and 1.88% MgO equivalent, by weight relative to the weight of the phosphate source, is added to the reaction vessel in the presence of sulfuric acid diluted to 5% by weight relative to the weight of the diluted acid, at a SO₄ / Ca ratio of 0.8, due to the calcium content of the phosphate source. The reaction temperature is 60°C and the reaction time is approximately 1 hour. The pH in the reaction vessel is 1.95. The rock flow rate is 3.19 kg / h and the acid flow rate is 34.5 liters / h. The P2O5 content in the attack suspension is 1.82% by weight relative to the total weight of the suspension.

[0156] At filtration, the recovered liquid phase flow rate is 37.91 kg / h.

[0157] The attack efficiency is 90%. Example 8.- Pilot-scale production of DCP from phosphate rock

[0158] For DCP production, the pilot plant used to perform the rock attack is operated independently of this initial attack. The equipment is then used sequentially.

[0159] The pilot used is that of example 4.-, In this example, the liquid phase recovered from the filtration of example 7 is treated to precipitate the DCP by neutralization in the following way: Quicklime (or limestone) is added at nominal flow rate into the reactor into which the liquid phase recovered from example 7 is also introduced, the pH is regularly controlled.

[0160] When the pH reaches 5.5 / 6, the filtrate feed pump is started. The pH is checked regularly, and the limestone or lime feed rate is adjusted to maintain a pH between 5.5 and 6.

[0161] Filtration is carried out every half hour from the buffer reactor. Every other time, the filter cake containing calcium sulfate is recycled to the first attack reactor to increase the solids content of the reaction medium.

[0162] The production cake containing the precipitated DCP is recovered, and the mother liquors are stored in a drum. Samples of the products (DCP and mother liquors) are taken for analysis.

[0163] The neutralization temperature is 60°C. The pH in the first tank is 4.4, while it is 5.55 in the second tank. The quicklime flow rate is 1.05 kg / h.

[0164] The precipitation yield of DCP is calculated by the formula (P2O5 content in DCP / P2O5 content initially present in the MCP and acid solution) P2O5 balance of the operation is 92%. Comparative Example 2 - Sub-stoichiometric attack of phosphate rock on a pilot scale

[0165] The driver used is that of example 4.-, the same process as in example 4.- is implemented there.

[0166] The same phosphate source as in Example 4 is added to the etching tank in the presence of sulfuric acid at 20% by weight relative to the acid, with a SO₄ / Ca ratio of 0.8, due to the calcium content of the phosphate source. The etching temperature is 60°C and the etching time is approximately 1 hour. The pH in the etching tank is 1.73. The rock flow rate is 5 kg / h and the acid flow rate is 15.6 liters / h. The P₂O₅ content in the etching suspension is 7.10% by weight relative to the total weight of the suspension.

[0167] At filtration, the recovered liquid phase flow rate is 13.3 kg / h.

[0168] The attack yield is 65%.

[0169] As can be seen, compared to example 4, the presence of a more concentrated sulfuric acid and a P2O5 content greater than 6% causes the yield to drop to 65%.

Claims

1. A method of acid attack of a phosphate source comprising calcium for the production of a purified phosphate compound comprising the steps of a) an acid attack using sulphuric acid of said phosphate source for a predetermined period of time between 20 and 180 minutes with formation of a first suspension containing a first solid material and a first liquid phase in which the first solid material is in suspension, said first solid material comprising at least calcium sulphate and impurities, said first liquid phase comprising phosphoric acid and dissolved monocalcium phosphate, said attack being carried out under inlet conditions in which the sulphate molar ratio from the sulphuric acid and optionally the phosphate source with calcium is between 0.6 and 0.8, and the content of P2O5 is lower than 6%, b) a first filtration of said first slurry with a separation of said first solid material from said first liquid phase c) recovering from said first liquid phase of a purified phosphate compound.

2. The method according to claim 1, wherein said acid attack takes place in 1, 2 or more attack tanks.

3. The method of claim 1 or claim 2, wherein the predetermined period of time is lower than 120 minutes.

4. The method according to claim 2 or claim 3, wherein the content of P2O5 in the liquid phase in the attack tank or tanks is lower than 5%.

5. The method according to any one of claims 2 to 4, wherein said attack is carried out at a temperature in the attack tank or tanks of less than or equal to 90°C.

6. The method according to any one of claims 2 to 5, wherein the sulphuric acid is a dilute sulphuric acid, in particular before addition to the attack tank or tanks.

7. The method according to claim 6, wherein said dilute sulphuric acid has a H2SO4 concentration of less than or equal to 13% by weight.

8. The method according to any one of the preceding claims, wherein said sulphate molar ratio from the sulphuric acid as well as optionally the phosphate source with calcium present in the phosphate source is between 0.68 and 0.78.

9. The method according to any one of the preceding claims, further comprising adding a base to said first suspension, prior to filtration.

10. The method according to any one of claims 1 to 9, further comprising, prior to said step of recovering from said first liquid phase of said purified phosphate compound, adding a base to said first liquid phase after filtration with formation of a second suspension comprising a second solid material in suspension in a second liquid phase and filtering said second suspension to separate said second solid material in suspension from said second liquid phase, whereby said purified phosphate compound is recovered from said second liquid phase, from the first liquid phase depleted in said second solid material.

11. The method according to any one of the preceding claims, wherein said first solid material separated from said first liquid phase is recycled by introduction into the first suspension.

12. The method according to any one of the preceding claims, wherein said calcium containing phosphate source is selected from the group consisting of conventional phosphate rock, low content of P2O5 phosphate rock, ash, sewage sludge, bone ash, pig manure, chicken manure, sewage sludge ash, sewage sludge, and any raw material having a content of phosphate of less than 30% by weight of P2O5 based on the total weight of the raw material.

13. The method according to any one of the preceding claims, wherein said purified phosphate compound is a monocalcium phosphate MCP, a dicalcium phosphate DCP, more particularly a food grade dicalcium phosphate DCP, a phosphoric acid, as for example directly from said first liquid phase or a phosphoric acid produced from said DCP.

14. The method according to any one of claims 10 to 13, wherein said second liquid phase is recycled by introduction into said attack tank or tanks.