Method for treating ash in order to produce phosphoric acid

Abstract

A method for treating ash comprising the following steps: - reacting the ash with phosphoric acid resulting in the formation of a digestion solution which comprises a liquid phase and a solid phase, - separating and treating said liquid phase leading to the formation of the solution P, characterized in that the treatment of said liquid phase comprises at least a first step of depletion of said liquid phase by means of an ion exchange resin, followed by a second step of depletion by means of an ion exchange resin, and in that said solution P comprises between 15% and 60% by weight of phosphorus expressed as P2O5 equivalent and an iron content of ≤ 500 ppm.

Description

[0001] PROCESS FOR TREATING ASHES TO PRODUCE PHOSPHORIC ACID

[0002] The present invention relates to a process for treating ashes to produce phosphoric acid.

[0003] Phosphoric acid can be produced in several ways, generally using phosphorus-containing rocks as raw materials. Various processing methods for these rocks are available on the market.

[0004] Today, all sources containing phosphorus are of interest to phosphate producers for both environmental and economic reasons. Secondary phosphorus-containing products, such as enrichment residues from industrial or agricultural processes, sewage sludge, and ash, are all affected by these issues. Indeed, utilizing such secondary products would prevent their disposal in the environment. Giving them a second life by processing them to extract the valuable material, in this case phosphorus, would allow for the production of products usable in various applications, particularly in agriculture and industry.

[0005] The present invention focuses on the treatment of ash containing inorganic matter and phosphorus to produce phosphoric acid. The process typically involves extracting phosphorus from this secondary material to yield phosphoric acid. This phosphoric acid can then be transformed into a grade of sufficiently high quality for use in various high-value applications.

[0006] Ash typically contains phosphorus, silica, iron, and aluminum. Ash can be generated by various industrial processes; for example, it can be a by-product of the combustion of bio-based residues or biomass, such as wood or agri-food waste.

[0007] They can also originate from the incineration of sewage sludge. In this case, wastewater from homes, industries, and businesses is treated in wastewater treatment plants. This treatment produces sewage sludge as a byproduct. Before incineration, this sewage sludge is often dewatered to reduce its water content, which facilitates combustion. The dewatered sludge is incinerated at high temperatures in incinerators specifically designed to treat sewage sludge. The ash from the incineration of sewage sludge often contains phosphorus, expressed as P₂O₅ equivalent.

[0008] The phosphorus contained in these ashes is the material of interest.

[0009] The challenge lies in the treatment of ash to recover the maximum amount of phosphorus in the form of phosphoric acid while avoiding the presence of impurities such as metallic elements which are undesirable in the phosphoric acid produced.

[0010] Current processes attempt to produce phosphoric acid from ash without adequately considering production yield and impurity removal. Indeed, a high impurity content in the produced phosphoric acid hinders its practical application in the aforementioned fields.

[0011] Some processes use large quantities of strong acids to solubilize phosphorus, which is not economically viable. Furthermore, extraction yields remain low. In addition, known processes involve numerous pretreatment, filtration, and precipitation steps without guaranteeing the quality of the final product.

[0012] There are also processes that carry out a rapid attack (for example of 1 minute) on the ashes in order to solubilize the fewest impurities in the acid, which does not allow sufficient extraction of P2O5.

[0013] For these reasons, there is a real need to provide a process for treating ash from the incineration of sewage sludge, preferably sludge from wastewater treatment, to produce quality phosphoric acid with viable implementation costs for industrialists and which presents an adequate balance between phosphorus extraction and impurity removal.

[0014] The invention aims to overcome the drawbacks of the prior art by providing an efficient process in terms of phosphorus extraction yield and impurity removal to provide phosphoric acid which can be used in various fields, including as a metal treatment agent, production of technical grade salts, fertilizers, flame retardants, etc.

[0015] To solve this problem, the present invention provides a process for treating ash containing phosphorus and impurities to produce a phosphoric acid solution (hereinafter referred to as Solution P), said process comprising the following steps:

[0016] - to supply ash from the incineration of residues and / or sewage sludge, and comprising phosphorus and impurities selected from the group comprising calcium, potassium, magnesium, sodium, iron, aluminum ions and their combinations, - to react the ash with phosphoric acid (hereinafter referred to as Acid R) which has an iron content < 500 ppm, preferably < 250 ppm, more preferably < 100 ppm and an aluminum content < 500 ppm, preferably < 250 ppm, more preferably < 100 ppm, leading to the formation of a digestion solution which comprises a liquid phase containing phosphorus and some of the said impurities, and a solid phase depleted in phosphorus,

[0017] - separation of said liquid phase from said digestion solution,

[0018] - treatment of said liquid phase leading to the formation of Solution P, the quantity of impurities of which is less than that corresponding to the impurities contained in said liquid phase, characterized in that:

[0019] - the treatment of said liquid phase comprises at least a first depletion step, using an ion exchange resin, of said liquid phase of at least one or more impurities selected from the group comprising calcium, potassium, magnesium, sodium ions and their combinations, followed by a second depletion step, using an ion exchange resin, primarily of iron ions from the solution obtained after said first depletion,

[0020] - said Solution P comprises between 15 and 60% by weight of phosphorus expressed as P2O5 equivalent and an iron content < 500 ppm, preferably < 250 ppm, more preferably < 100 ppm.

[0021] As can be seen, the process according to the present invention allows for efficient ash treatment in that the phosphorus production yield is high and the impurity content is greatly reduced compared to the starting material. This makes it possible to produce phosphoric acid that can be used in many technical fields, which is excellent for product diversification.

[0022] The process is easy to implement and can also be applied continuously on an industrial scale while remaining cost-effective.

[0023] Preferably, Solution P has an aluminum content < 200 ppm, more preferably < 100 ppm.

[0024] Advantageously, said Acid R is supplied at a concentration of less than 25% by weight, preferably less than 23% by weight, preferably less than 21% by weight.

[0025] Advantageously, said Acid R is supplied at a concentration greater than 5% by weight of phosphorus, preferably greater than 10% by weight of phosphorus expressed as P2O5 equivalent.

[0026] Advantageously, said Acid R is supplied at a concentration of less than 25% by weight and more than 5% by weight of phosphorus, expressed as P2O5 equivalent.

[0027] Advantageously, this means that phosphoric acid can be supplied at a higher concentration and then adjusted to carry out the reaction with the ash. One of the advantages of the invention lies in the production of phosphoric acid (Solution P) with a low impurity content, making it an ideal starting reagent. Solution P can act as Acid R. Thus, when the process is implemented continuously, a portion of the Solution P generated at the end of the process can be used as Acid R to recirculate a portion of the produced Solution P.

[0028] The ashes according to the invention are preferably derived from the incineration of residues and / or sewage sludge. They may advantageously be biomass residues such as bone ash, wood ash, wheat bran, green waste, industrial waste residues, sludge from urban or industrial wastewater treatment plants, preferably sludge from urban and / or industrial wastewater treatment, or mixtures thereof. Preferably, the ashes are ash from sludge derived from urban and / or industrial wastewater treatment.

[0029] Ashes can be fly ash or bottom ashes, or a mixture of both.

[0030] Preferably, said ashes according to the invention comprise at least 5% by weight, preferably at least 15% by weight, more preferably at least 25% by weight, more preferably still at least 30% by weight of phosphorus, expressed as P2O5 equivalent. More preferably, the ashes comprise between 5 and 30%, preferably between 15 and 25% by weight of phosphorus, expressed as P2O5 equivalent.

[0031] Advantageously, the reaction between said ashes and said Acid R is carried out over a period of time between 5 and 300 minutes, preferably between 5 and 200 minutes, more preferably between 10 and 100 minutes, more preferably still between 15 and 60 minutes, advantageously between 15 and 45 minutes, more advantageously between 15 and 30 minutes.

[0032] More advantageously, the mass ratio between the mass of phosphorus expressed as P2O5 equivalent present in the ash and the mass of phosphorus expressed as P2O5 equivalent present in Acid R is between 0.01 and 1, preferably between 0.05 and 0.5, more preferably between 0.08 and 0.15. Preferably, the weight ratio between said ash and said Acid R is between 0.01 and 1, preferably between 0.05 and 0.5, more preferably between 0.08 and 0.15.

[0033] According to a particularly advantageous method, the final mass percentage of phosphorus expressed as P2O5 equivalent of said solid phase containing phosphorus-depleted material is less than 15%, preferably less than 10%, preferably less than 8%, preferably less than 5%, preferably less than 3% relative to the initial total mass percentage of phosphorus expressed as P2O5 equivalent of said ash.

[0034] The total P₂O₅ yield of the process is greater than 65%, preferably greater than 75%, more preferably greater than 85%, even more preferably greater than 90%, advantageously greater than 95%, and more advantageously greater than 98%. This total yield is defined as 100% – the total P₂O₅ content after washing in the solid residue divided by the total P₂O₅ content in the ash. It is preferably measured based on the analysis of the total P₂O₅ in the solid phase after washing.

[0035] Preferably, the reaction between said ashes and said Acid R is carried out by applying a temperature and / or maintaining a temperature between 25 and 60 °C, preferably between 30 and 50 °C, preferably between 35 and 45 °C.

[0036] In a preferred method, between 0 and 100% of Solution P obtained at the end of the process is mixed with at least a portion of Acid R during the reaction step with the ash in order to continue the reaction step with replenished ash. This allows for the reuse of a portion of Solution P to supply the process with a material acting in the role of Acid R, thus enabling the process to continue continuously. It also reduces the amount of fresh acid used for the reaction.

[0037] According to a preferred method, in steady state, the Solution P obtained at the end of the process is the Acid R introduced during the reaction step with the ash. This allows for the total or partial substitution of the fresh acid (Acid R) used for the reaction.

[0038] Advantageously, the solid phase of the digestion solution contains the majority of the impurities from the ash, compared to the amount of impurities in the liquid phase. The reaction between the ash and Acid R allows for the solubilization of fewer impurities in the liquid phase while optimizing the P2O5 yield.

[0039] Advantageously, the solid phase of the digestion solution is washed, preferably with hot water (approximately between 20 and 40°C) and an aqueous solution to recover the P2O5 soluble in the solid phase. The washing can be carried out in co-current or counter-current flow.

[0040] More advantageously, the addition of sulfuric acid is avoided in the context of the present invention.

[0041] Preferably, none of the steps according to the invention includes an addition of sulfuric acid or one or more steps of the process according to the invention is substantially free from the addition of a quantity of sulfuric acid.

[0042] Preferably, any one of the process steps, preferably the digestion solution or the liquid phase, is substantially free from the addition of sulfuric acid. Even more preferably, the liquid phase is substantially free from the addition of sulfuric acid. "Substantially free" means that the addition of sulfuric acid is not desired; preferably, the free SO4 content measured in the liquid phase to be treated is less than 2% by weight, advantageously less than 1% by weight, preferably less than 0.5% by weight, relative to the total weight of the liquid phase.

[0043] Advantageously, the formation of calcium sulfate is not targeted within the scope of the present invention.

[0044] Preferably, the process according to the invention comprises an addition of at least one heavy metal removal agent, in particular arsenic (As), said removal agent comprising sulfur, and preferably being selected from the group comprising sulfides, NaHS, Na2S, FhS, thiosulfate, thiocarbamate, thiocarbonate and their combinations.

[0045] According to one embodiment, the addition of said at least one heavy metal removal agent may be carried out in said digestion solution.

[0046] Advantageously, the step of depleting said liquid phase of at least one or more impurities is carried out by means of at least one ion exchange resin.

[0047] Advantageously, the aforementioned processing step includes:

[0048] - A first pass which includes the introduction of said liquid phase into a first ion exchange resin A with obtaining a solution depleted of at least one or more metallic cation(s) chosen from the group including calcium, potassium, magnesium, sodium ions and their combinations.

[0049] - Possibly, the addition of a reducing agent to the solution from the first pass, leading in particular to a reduction of ferric ions to ferrous ions,

[0050] - A second pass comprising the introduction of said possible reduced solution or of the solution resulting from the first pass into said first ion exchange resin A or into a second ion exchange resin A with obtaining a solution depleted in iron ions and possibly of at least one or more cations chosen from the group comprising calcium, potassium, magnesium, sodium ions, and their combinations.

[0051] The term ion exchange resin refers to a system comprising one or more columns in series or parallel, allowing continuous operation with charge, discharge and wash cycles.

[0052] Preferably, the first ion exchange resin A is identical or similar to the second ion exchange resin A.

[0053] Each ion exchange resin according to the invention can contain at least 2 columns.

[0054] According to an advantageous mode, the process further includes a third pass which includes an introduction of said iron-depleted solution, which results from said second pass, into a third ion-exchange resin B, different from said first ion-exchange resin A, with formation of a solution mainly depleted in aluminium ions, and preferably in iron ions.

[0055] Preferably, the third ion exchange resin B is different from the first ion exchange resin A. Preferably, the third ion exchange resin B is different from the second ion exchange resin A.

[0056] Preferably, the above-mentioned reducing agent is chosen from the group comprising Fe(0), hydrazine, h, quinone, hydroquinone, CO and their mixtures.

[0057] Preferably, said liquid phase, before introduction into said first ion exchange resin A, has a concentration preferably less than 30%, more preferably less than 25%, more preferably less than 21% by weight of phosphorus expressed as P2O5 equivalent. This facilitates the treatment of the acid and reduces the risk of fouling and destruction of the resin.

[0058] More preferably, said first ion exchange resin A and / or second ion exchange resin A is a strong acid cationic resin, preferably made of a crosslinked polymer comprising sulfonic acid groups; and wherein said polymer is selected from the group consisting of polystyrene, polyacrylate, and combinations thereof.

[0059] More preferably, said third ion-exchange resin B is a chelating resin, preferably comprising at least one cross-linked polymer and phosphonic acid and sulfonic acid groups; and wherein said polymer is selected from the group consisting of polystyrene, polyacrylate, and combinations thereof.

[0060] Advantageously, said first ion exchange resin and / or said second ion exchange resin and / or said third ion exchange resin is washed with at least one acid in solution, preferably HCl, advantageously after each of the above-mentioned passes.

[0061] The process according to the invention advantageously includes further a concentration step, preferably carried out under vacuum, of said Solution P obtained after treatment leading to the formation of a concentrated phosphoric acid solution (hereinafter referred to as Solution C).

[0062] Preferably, nanofiltration is performed on the solution obtained after the 3 ème passage, with obtaining a permeate of a purified phosphoric acid solution. This purified phosphoric acid solution can also be understood as solution P according to the invention.

[0063] Preferably, said Solution P has a reduction rate greater than 70%, preferably greater than 75%, more preferably greater than 85%, more preferably still greater than 95% in calcium ions compared to their content in the ashes supplied before treatment.

[0064] More preferably, said Solution P has a reduction rate greater than 70%, preferably greater than 80%, more preferably greater than 90%, in iron ions compared to their content in the ash supplied before treatment.

[0065] More preferably, said Solution P has a reduction rate of more than 60%, preferably more than 70%, more preferably more than 85%, more preferably more than 90% in aluminium ions compared to their content in the ashes supplied before treatment.

[0066] In an advantageous manner, the Solution P obtained at the end of the process has an iron content < 100 ppm and an aluminum content < 200 ppm, more preferably < 100 ppm. The Solution P thus produced can be partially recycled at the reaction stage with the ash to ensure a sustainable and efficient process for industrial production. This allows the Solution P to be used like Acid R. In this case, the Solution P can have the same characteristics as Acid R.

[0067] Preferably, the solution P generated according to the invention can completely substitute for Acid R.

[0068] Preferably, any one of the process steps is substantially free from any added amount of a precipitation / turbidity reagent, preferably an anionic, cationic, nonionic polymer, or gelatin. "Substantially free" means that the addition of a precipitation / turbidity reagent, preferably an anionic, cationic, nonionic polymer, or gelatin, to any of the process steps is not desired, or its amount is less than 1% by weight, preferably less than 0.5% by weight, and more preferably less than 0.01% by weight, relative to the total weight of the composition to which that reagent is added.

[0069] More preferably, the said ashes are substantially free of organic matter. “Substantially free” means that the ashes do not contain organic matter or the amount of organic matter is less than 1% by weight, preferably less than 0.5% by weight, more preferably less than 0.01% by weight in relation to the total weight of the said ashes.

[0070] Other embodiments of the process according to the invention are indicated in the attached claims.

[0071] The invention also relates to the use of Solution P in the production of: - Purified phosphoric acid of technical grade or fertilizer and / or

[0072] - Purified phosphate salts, preferably of the diammonium phosphate type, and / or

[0073] - Polyphosphoric acid used as is or in salt form (e.g., ammonium polyphosphate) and / or

[0074] - Fertilizers, preferably of the monoammonium or diammonium phosphate type (MAP or DAP), NP fertilizers (phosphate-potassium fertilizer), NPK fertilizers (ammonium phosphate-potassium fertilizer), NPS fertilizers (ammonium phosphate-sulfate fertilizer), MCP (monocalcium phosphate), DCP (dicalcium phosphate), SSP (single superphosphate), TSP (triple superphosphate), monopotassium phosphate (MKP).

[0075] Other embodiments of the use according to the invention are indicated in the attached claims.

[0076] 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 the drawings and examples.

[0077] The ash from sewage sludge incineration, often called sewage sludge incineration ash, is the fine or heavy residue produced when sludge from wastewater treatment plants is incinerated. This ash contains various chemical and mineral elements, some of which can be recovered and reused. Silica, aluminum, and iron can be found in this ash. Aluminum and iron may be present in the form of phosphates.

[0078] Within the scope of the present invention, all weight percentages of P2O5 can be measured by any suitable method known in the art. Preferably, the weight percentage of P2O5 can be measured by phospho-vanado-molybdate colorimetry at 436 nm (yellow) (light absorption spectrophotometry).

[0079] Within the scope of the present invention, all weight percentages of Ca, Al, Fe, K, Na can be measured by any suitable method known in the art. Preferably, the weight percentages of Ca, Al, Fe, K, Na can be measured, after adequate sample preparation, by Inductively Coupled Plasma Optical Emission spectroscopy.

[0080] In the context of the present invention, unless otherwise specified, contents or concentrations are respectively contents or mass concentrations.

[0081] Within the framework of the present invention, the terms "digestion", "leaching" and "attacks" are used interchangeably to refer to the reaction between the ashes and phosphoric acid R.

[0082] The term Solution P can refer to an acid which includes ions derived from a phosphoric acid, said ions being able to be chosen from the group including: dihydrogen phosphate ions, hydrogen phosphate ions, phosphate ions and their combinations.

[0083] The term "Acid R" refers to the acid used to carry out the reaction between an acid, preferably phosphoric acid, and the ash as described in the invention. This Acid R can be mixed with a portion of the Solution P generated during the process, for example, to completely replace Acid R. Thus, all the technical features described in the invention are also applicable to the Solution P, which is recycled within the process to effectively ensure continuous operation. Acid R can therefore be Solution P. Over time, this means that Solution P can completely replace Acid R, if necessary.

[0084] Thus, said Solution P may advantageously be supplied at a concentration of less than 25%, preferably less than 23%, and advantageously greater than 10% by weight of phosphorus expressed as P2O5 equivalent when used in the reaction step with ash, preferably added in mixture with said Acid R or alone.

[0085] Preferably, the reaction between said ashes and said Solution P or said Solution P mixed with said Acid R is carried out by applying or maintaining a temperature between 25 and 60 °C, preferably between 30 and 50 °C, preferably between 35 and 45 °C.

[0086] More preferably, the mass ratio between the mass of phosphorus expressed as P2O5 equivalent present in the ash and the mass of phosphorus expressed as P2O5 equivalent present in Solution P or in the mixture formed from said Solution P and said Acid R is between 0.01 and 1, preferably between 0.05 and 0.5, more preferably between 0.08 and 0.15.

[0087] Advantageously, the reaction between said ashes and said Solution P or said Solution P mixed with said Acid R is carried out over a period of time between 5 and 300 minutes, preferably between 5 and 200 minutes, more preferably between 10 and 100 minutes, more preferably still between 15 and 60 minutes, advantageously between 15 and 45 minutes, more advantageously between 15 and 40 minutes.

[0088] Advantageously, any separation / filtration step indicated within the scope of the invention can be carried out with a filter press or a belt filter or any equivalent known to a person skilled in the art.

[0089] The levels expressed in ppm can be measured by any method known to the person skilled in the art and preferably by ICP-OES (inductively coupled plasma optical emission spectrometry.

[0090] The process according to the invention makes it possible to use ashes as a source of low-quality phosphate to produce technical-grade phosphoric acid, which can then be converted into high-quality phosphoric acid.

[0091] According to a first embodiment, the process comprises the following steps:

[0092] Ashes are digested for 25 minutes at a temperature of 42 °C in phosphoric acid (Acid R, approximately 12% by weight of phosphorus expressed as P₂O₅ equivalent) in a mass ratio (ashes / P from the acid) between 0.08 and 0.2 under reaction conditions that lead to the solubilization of phosphorus (expressed as P₂O₅) while limiting the solubilization of impurities. Most of the rock impurities are removed as a solid residue, separated from the phosphoric acid solution (Solution P) of interest, for example, by pressure filtration.

[0093] A first treatment step is carried out. The phosphoric acid solution (Solution P) obtained after filtration is purified using a first ion-exchange resin A to remove some of the cationic impurities. The targeted impurities are Ca, K, Mg, and Na. This first treatment reduces the Ca, K, Mg, and Na ions by more than 85% compared to their concentration in the ash before treatment. The resin is then regenerated with an acidic solution, for example, HCl, to restore its purification capacity and perform another purification cycle.

[0094] A second treatment step is applied. The phosphoric acid solution obtained after the first step is treated with a second ion-exchange resin A to remove some of the cationic impurities. The main impurity targeted is iron, and this step also polishes Ca, K, Mg, and Na. The resin is then regenerated with an acidic solution (e.g., HCl) to restore its purification capacity and perform another treatment cycle. This second step reduces the Fe ion content by more than 85% compared to its concentration in the ash before treatment.

[0095] According to a second embodiment of the process according to the invention, a third treatment step is applied to the phosphoric acid solution obtained according to the first embodiment. This 3 èmeThis step involves introducing the previously recovered solution into a third ion-exchange resin B, distinct from the first and second ion-exchange resins A, to remove any remaining cationic impurities. The primary impurity targeted is Al, and this step also allows for iron polishing. This second treatment results in an Al ion reduction of over 85% compared to its content in the ash before treatment. The resin is then regenerated with an acidic solution to restore its purification capacity and perform another purification cycle. The resulting phosphoric acid (Solution P) can then be partially recycled back into the ash reaction step to ensure process continuity. The remaining Solution P constitutes the product of interest.

[0096] According to a third embodiment, Solution P from the second embodiment is treated by nanofiltration. This optional step improves the quality of the final phosphoric acid. ème This embodiment also allows the production of a first stream of final acid used as a product of interest and a second stream of residual (less pure) phosphoric acid which is returned to the reaction step with the ash.

[0097] It is understood that the present invention is in no way limited to the embodiments described above and that many modifications can be made to it without departing from the scope of the annexed claims.

Claims

DEMANDS 1. A process for treating ash comprising phosphorus and impurities to produce a phosphoric acid solution (hereinafter referred to as Solution P), said process comprising the following steps: - to supply ash from the incineration of residues or sewage sludge, and comprising phosphorus and impurities selected from the group comprising calcium, potassium, magnesium, sodium, iron, aluminum ions and their combinations, - reacting the ashes with phosphoric acid (hereinafter referred to as Acid R) which has an iron content < 500 ppm, preferably < 250 ppm, more preferably < 100 ppm and an aluminium content < 500 ppm, preferably < 250 ppm, more preferably < 100 ppm, leading to the formation of a digestion solution which comprises a liquid phase containing phosphorus and a portion of said impurities, and a solid phase depleted in phosphorus, in which the mass ratio between the mass of phosphorus expressed as P2O5 equivalent present in the ashes and the mass of phosphorus expressed as P2O5 equivalent present in Acid R is between 0.01 and 1, preferably between 0.05 and 0.5, more preferably between 0.08 and 0.15, - separation of said liquid phase from said digestion solution, - treatment of said liquid phase leading to the formation of Solution P, the quantity of impurities of which is less than that corresponding to the impurities contained in said liquid phase, characterized in that: - the treatment of said liquid phase comprises at least a first depletion step, using an ion exchange resin, of said liquid phase of at least one or more impurities selected from the group comprising calcium, potassium, magnesium, sodium ions and their combinations, followed by a second depletion step, using an ion exchange resin, primarily of iron ions from the solution obtained after said first depletion, - said Solution P comprises between 15 and 60% by weight of phosphorus expressed as P2O5 equivalent and an iron content < 500 ppm, preferably < 250 ppm, more preferably < 100 ppm.

2. A process according to claim 1, wherein said Acid R is supplied at a concentration of less than 25%, preferably less than 23%, and advantageously greater than 10% by weight of phosphorus expressed as P2O5 equivalent.

3. A process according to claim 1 or 2, wherein said ashes comprise at least 5% by weight, preferably at least 15% by weight, more preferably at least 25% by weight, more preferably still at least 30% by weight of phosphorus, expressed as P2O5 equivalent.

4. A process according to any one of the preceding claims, wherein the reaction between said ashes and said Acid R is carried out over a period of time of between 5 and 300 minutes, preferably between 5 and 200 minutes, more preferably between 10 and 100 minutes, more preferably still between 15 and 60 minutes, advantageously between 15 and 45 minutes, more advantageously between 15 and 40 minutes.

5. A process according to any one of the preceding claims, wherein the final mass percentage of phosphorus, expressed as P2O5 equivalent, of said solid phase containing a material depleted in phosphorus is less than 15%, preferably less than 10%, preferably less than 8%, preferably less than 5%, preferably less than 3% relative to the initial total mass percentage of phosphorus expressed as P2O5 equivalent of said ash.

6. A process according to any one of the preceding claims, wherein an amount between 0 and 100% of said Solution P obtained at the end of the process is mixed with at least a part of said Acid R during said reaction step with said ashes in order to continue the reaction step with replenished ashes.

7. A process according to any one of the preceding claims, wherein said solid phase of the digestion solution comprises the majority of the impurities from the ash, relative to the amount of impurities contained in the liquid phase.

8. A process according to any one of the preceding claims, comprising an addition of at least one heavy metal removal agent, in particular arsenic (As), said removal agent comprising sulfur, and preferably being selected from the group comprising sulfides, NaHS, Na2S, FhS, thiosulfate, thiocarbamate, thiocarbonates and combinations thereof.

9. A method according to any one of the preceding claims, wherein said addition of said at least one heavy metal removal agent is carried out in said digestion solution.

10. A method according to any one of the preceding claims, wherein the aforementioned processing step comprises: - A first step which includes the introduction of said liquid phase into a first ion exchange resin A with obtaining a solution depleted of at least one or more metallic cation(s) chosen from the group including calcium, potassium, magnesium, sodium ions and their combinations. - Possibly, the addition of a reducing agent to the solution from the first pass, leading in particular to a reduction of ferric ions to ferrous ions, - A second pass comprising the introduction of said possible reduced solution or of the solution resulting from the first pass into said first ion exchange resin A or into a second ion exchange resin A with obtaining a solution depleted in iron ions and possibly of at least one or more cations chosen from the group comprising calcium, potassium, magnesium, sodium ions, and their combinations. 1 1. A process according to claim 10, including a third pass comprising an introduction of said iron-depleted solution, resulting from said second pass, into a third ion-exchange resin B, different from said first ion-exchange resin A, with formation of a solution mainly depleted in aluminium ions, and preferably in iron ions.

12. A process according to claim 10 or 11, wherein said liquid phase, before introduction into said first ion exchange resin A, has a concentration of less than 30%, preferably less than 25%, more preferably less than 21% by weight of phosphorus expressed as P2O5 equivalent.

13. A method according to any one of claims 10 to 12, wherein said first ion exchange resin A and / or said second ion exchange resin is a strong acid cationic resin.

14. A method according to any one of claims 10 to 13, wherein said third ion exchange resin B is a chelating resin.

15. A process according to any one of the preceding claims, further comprising a concentration step, preferably carried out under vacuum, of said Solution P obtained after treatment leading to the formation of a concentrated phosphoric acid solution (hereinafter referred to as Solution C).

16. A process according to any one of the preceding claims, wherein said Solution P has a reduction rate greater than 70%, preferably greater than 75%, more preferably greater than 85%, more preferably still greater than 95% in calcium ions compared to their content in the ash supplied before treatment.

17. A process according to any one of the preceding claims, wherein said Solution P exhibits a reduction rate greater than 70%, preferably greater than 80%, more preferably greater than 90%, in iron ions relative to their content in the ash supplied before treatment.

18. A process according to any one of the preceding claims, wherein said Solution P has a reduction rate greater than 60%, preferably greater than 70%, more preferably greater than 85%, more preferably still greater than 90% in aluminium ions compared to their content in the ash supplied before treatment.

19. A process according to any one of the preceding claims, wherein the Solution P obtained at the end of the process has an iron content < 100 ppm and an aluminum content < 200 ppm, preferably an aluminum content < 100 ppm.