Process for manufacturing calcium sulfate hemihydrate

Abstract

The present invention relates to a process for manufacturing calcium sulfate hemihydrate. The present invention further relates to a process for manufacturing phosphoric acid.

Description

[0001] Process for manufacturing calcium sulfate hemihydrate

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a process for manufacturing calcium sulfate hemihydrate.

[0004] The present invention further relates to a process for manufacturing phosphoric acid.

[0005] BACKGROUND OF THE INVENTION

[0006] Calcium sulfate is widely used in various industries, especially in construction, where it is a key starting material for plasters and other building materials. It also finds applications in agriculture, medicine, and chemical manufacturing.

[0007] Calcium sulfate may originate from wet processes of manufacturing phosphoric acid, where it is typically found as end by-products. Said wet processes generally involve reacting a phosphorus source comprising calcium, such as phosphate ores, with sulfuric acid. This typically results in a slurry comprising a phosphoric acid solution and a solid mainly comprising calcium sulfate. Said slurry is usually filtered to separate the solution of phosphoric acid solution from the solid mainly comprising calcium sulfate. Phosphoric acid is thereby obtained, and the calcium sulfate end by-product is, in general, stored, e.g. stockpiled. Given the large quantity of calcium sulfate produced in said manufacturing of phosphoric acid, its valorization is highly desirable.

[0008] Phosphoric acid production processes are typically classified based on the forms of calcium sulfate produced, as described by S. Havelange et al. in “Phosphoric acid and phosphates” in Ullmann’s Encyclopedia of Industrial Chemistry, (2022), DOI: 10.1002 / 14356007.a19_465.pub4. The dihydrate (DH) process of manufacturing phosphoric acid results in theproduction of dihydrate calcium sulfate (CaSC. FW, also known as gypsum) as end by-product. While the DH process is widespread, it suffers from low recovery of phosphorus from the source of phosphorus, and from the presence of valuable phosphorus in the resulting dihydrate calcium sulfate end-product. Furthermore, dihydrate calcium sulfate resulting from said processes does not possess self-drying properties during storage, thus leading to a non-readily available form of calcium sulphate due to its high moisture content. Indeed, dihydrate calcium sulfate crystals are fully hydrated, and are thus unable to capture free water which is present at the end of the process.

[0009] To address these issues, alternative processes, namely the hemihydrate (HH) process, which produces calcium sulfate hemihydrate (CaSO4.1 / 2H2O) as end by-products, and the dihydrate / hemihydrate (DH / HH) process, in which gypsum is formed as intermediary product and converted to is hemihydrate form as end by-products, have been developed, typical DH / HH processes being for example the DA-HF process or the Central Prayon Process (CPP).

[0010] While the HH and DH / HH processes offer definite advantages over the DH process, they are prone to clogging in the installation for manufacturing phosphoric acid. Indeed, it is known that in the installation, the calcium sulfate hemihydrate may be spontaneously converted to dihydrate calcium sulfate crystals, and / or the calcium sulfate may nucleate and grow on the installation surfaces. To mitigate this issue, it is required to use anti-clogging additives, such as condensed phosphates during the process of manufacturing phosphoric acid. For example, WO 2019 / 178118 A1 discloses a method for reducing formation of scale in phosphoric acid production, in which, notably, a scale inhibitor comprising an inorganic phosphate of formula (XPO3)m and / or an inorganic phosphate according to formula Yn+2PnO3n+i, wherein X and Y are selected from Na, K, H or combinations thereof and m and n are integers having a value of at least 6is added in the phosphate-containing ore previously combined with an acid (acidic slurry) or in the processed acidic slurry (named acidic stream).

[0011] However, the use of anti-clogging additives during the HH or the DH / HH process causes, when stored, the prevention or retardation of the hemihydrate to spontaneously hydrate into its dihydrate form. Thus, no or delayed self-drying properties during storage are observed, resulting in a non-readily by-product for end users of calcium sulfate.

[0012] There is thus a need for an improved process for the production of calcium sulfate hemihydrate from phosphoric acid wet processes, said process allowing an efficient recovery of phosphorus without clogging the phosphoric acid production installation, while, when said calcium sulfate hemihydrate is stored, still furnishing in a cost-effective, energy saving and rapid calcium sulfate in a dihydrate form to its end-users.

[0013] SUMMARY OF THE INVENTION

[0014] The inventors have now surprisingly found that it is possible to obtain a process for manufacturing calcium sulfate hemihydrate that meets the above-mentioned needs.

[0015] Therefore, there is now provided a process for manufacturing calcium sulfate hemihydrate, comprising at least the steps of:

[0016] (a) contacting at least one phosphorus source comprising calcium, with a first sulfuric acid solution (SAi), thereby obtaining a first slurry (Si) comprising a first solid residue (SRi) and a first liquid phase (LPi), wherein the first solid residue (SRi) comprises at least one calcium sulfate, wherein said calcium sulfate is calcium sulfate hemihydrate, or dihydrate calcium sulfate, or a mixture thereof;

[0017] (b) removing at least part of the first liquid phase (LPi) from the first solid residue (SRi) obtained in step (a), provided that the first solid residue (SRi) consists essentially of calcium sulfate hemihydrate, thereby forming a first solid-rich phase (PSR1)having a first solid residue (SRi) content higher than 65.0 wt.%; or

[0018] adding a second sulfuric acid solution (SA2) to the first slurry (Si) obtained in step (a), provided that the first solid residue (SR1) comprises dihydrate calcium sulfate, thereby forming a second slurry (S2) comprising a second solid residue (SR2) and a second liquid phase (LP2), wherein the second solid residue (SR2) consists essentially of calcium sulfate hemihydrate; or removing at least part of the first liquid phase (LP1) from the first solid residue (SR1) obtained in step (a), provided that the first solid residue (SR1) comprises dihydrate calcium sulfate, thereby forming a second solid-rich phase (PSR2) having a first solid residue (SR1) content higher than 35.0 wt.%, followed by adding a third sulfuric acid solution (SA3) to the second solid-rich phase (PSR2), thereby forming a third slurry (S3) comprising a third solid residue (SR3) and a third liquid phase (LP3), wherein the third solid residue (SR3) consists essentially of calcium sulfate hemihydrate;

[0019] (c) washing the first solid residue (SR1) comprised in the first solid-rich phase (PSR1) obtained in step (b), with a solution (P); or removing at least part of the second liquid phase (LP2) from the second solid residue obtained in step (b) (SR2), thereby forming a third solid-rich phase (PSRS) having a second solid residue (SR2) content higher than 65.0 wt.%, followed by washing the second solid residue (SR2) comprised in the third solid-rich phase (PSRS) with a solution (P); or

[0020] removing at least part of the third liquid phase (LP3) from the third solid residue (SRs) obtained in step (b), thereby forming a fourth solid-rich phase (PSR4) having a third solid residue (SRs) content higher than 65.0 wt.%, followed by washing the thirdsolid residue (SR3) comprised in the fourth solid-rich phase (PSR4) with a solution (P);

[0021] wherein the solution (P) comprises inorganic condensed phosphates having an average chain length of between 9 and 30 phosphate units; and wherein the solution (P) provides with inorganic condensed phosphates in an amount of between 0.10 wt.% and 0.50 wt.% relative to the total weight of the dry first solid residue (SR1) or the dry second residue (SR2) or the dry third solid residue (SR3), obtained in step (b).

[0022] DETAILED DESCRIPTION OF THE INVENTION

[0023] The term “comprising”, as used in the claims, should not be interpreted as being limited to the means mentioned thereafter; such a term does not exclude other elements or steps. The term should be read as specifying the presence of the stated features, integers, steps, or components, without, however, precluding the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a process comprising steps A and B” should not be limited to the process consisting only of steps A and B. This means that, with respect to the present invention, the only relevant steps of the process are A and B. Accordingly, the terms “comprising” and “including” encompass the more limiting terms “consisting mainly of” and “consisting of “.

[0024] Within the context of the present invention, the term “x consists essentially of y”, as used hereinafter, is intended to denote that x comprises at least 95.0 wt.%, preferably at least 97.0 wt.%, more preferably at least 98.0 wt.% of y, relative to the total dry weight of x.

[0025] As used herein, the terms "optional" or "optionally" means that a subsequently described event or circumstance can or cannot occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.As used herein, the terms “one”, “a”, “an” encompass the term “at least one”, “at least a” and “at least an” respectively. Similarly, the term “said” encompasses “said at least one”.

[0026] Furthermore, the various embodiments, although referred to as “preferred” are to be construed as exemplary manners in which the invention may be implemented rather than as limiting the scope of the invention.

[0027] Within the context of the present invention, the term “solution”, as used hereinafter, is intended to denote an aqueous solution, and the term “aqueous solution”, as used hereinafter refers to a solution in water, demineralized water, or any aqueous solution comprising mineral salts, such as, for example, seawater, city water, or water comprising phosphorous containing salts or phosphoric acid.

[0028] Within the context of the present invention, the term “process water”, as used hereinafter, is intended to denote an aqueous solution originating from at least one step of the present process of the present invention, and / or from an another process being selected from a process of manufacturing sulfuric acid, a process of manufacturing phosphorous containing salts, a process for manufacturing polyphosphoric acid, or combination thereof.

[0029] In particular, the process water according to the present invention may originate from the condensation of water in one or more of the condensers which may be used in the process according to the present invention, and / or from an another process being selected from a process of manufacturing sulfuric acid, a process of manufacturing phosphorous containing salts, a process for manufacturing polyphosphoric acid, or combination thereof.

[0030] The process water according to the present invention may also originate from the washing of flue gases, or from the cleaning (e.g. the periodical cleaning) of installations which may be carried out in the processaccording to the present invention, and / or from an another process being selected from a process of manufacturing sulfuric acid, a process of manufacturing phosphorous containing salts, a process for manufacturing polyphosphoric acid, or combination thereof.

[0031] Within the context of the present invention, the term “recycled phosphoric acid” is intended to denote an aqueous solution comprising phosphoric acid, wherein said aqueous solution originate from at least one step of removal of a liquid from a solid residue according to the present invention and / or from at least one washing step according to the present invention.

[0032] Within the context of the present invention, the term “acidic water”, as used hereinafter, is intended to denote an aqueous solution comprising at least one inorganic acid selected from the group consisting of sulfuric acid, phosphoric, hydrochloric acid, nitric acid and their combinations thereof.

[0033] Within the context of the present invention, the term “concentrated sulfuric acid solution”, as used hereinafter, is intended to denote a solution comprising sulfuric acid in an amount of at least 96 wt.%, preferably of at least 98 wt.%, relative to the total weight of the concentrated sulfuric acid solution.

[0034] Within the context of the present invention, the term “sulfo¬ phosphoric acidic solution” is intended to denote an aqueous solution, as detailed above, comprising sulfuric acid in an amount of between 0.1 wt.% and 10.0 wt.%, expressed in SO3 equivalent, relative to the total weight of the sulfo-phosphoric acid solution, and phosphoric acid, in an amount of between from 2.0 wt.% to 54 wt.% of phosphorous, expressed in P2O5 equivalent, relative to the total weight of the sulfo-phosphoric acid solution.

[0035] Provided that the solution is a sulfo-phosphoric acid solution, as detailed above, the percentage of sulfuric acid, expressed in SO3 equivalents, is preferably measured by ICP-OES (Inductively CoupledPlasma Optical Emission Spectroscopy) or by turbidimetry, in particular using a photometer (e.g. Metrohm), said photometer being previously calibrated with a solution of barium chloride, said photometer measuring the attenuation of the intensity of a light beam of known wavelength passing through the sample.

[0036] Preferably, provided that the solution is a sulfo-phosphoric acid solution, the percentage of sulfuric acid, expressed in SO3 equivalents, is measured by turbidimetry, using a photometer (e.g. Metrohm), said photometer being previously calibrated with a solution of barium chloride, said photometer measuring the attenuation of the intensity of a light beam of known wavelength passing through the sample.

[0037] Within the context of the present invention, the term “acidity”, as used hereafter, is intended to denote the sum of the weight percentage of phosphoric acid, expressed as P2O5 equivalent, and the weight percentage of sulfuric acid, expressed as SO3 equivalent, in the liquid phase of the slurry, relative to the total weight of the liquid phase.

[0038] It will be understood that the term “acidity”, as detailed above, is used by the skilled in the art in said conditions, as the pH, although being capable of being measured, is irrelevant in the conditions of the process of the present invention.

[0039] Typically, the acidity of the liquid phase of the slurry, as detailed above, is determined by measuring the content of phosphoric acid, expressed in P2O5 equivalents, and the content of sulfuric acid, expressed in SO3 equivalents, using the methods as detailed herein-below and as detailed hereinafter.

[0040] Within the context of the present invention, the percentage of sulfuric acid, expressed in SO3 equivalents, as used hereinafter, can be measured by any means known by the skilled person in the art, preferably by turbidimetry, in particular using a photometer (e.g. Metrohm), said photometer being previously calibrated with a solution of barium chloride,said photometer measuring the attenuation of the intensity of a light beam of known wavelength passing through the sample.

[0041] Within the content of the present invention, the term “dry solid” is intended to denote that the water content of the solid, is of at most 5.0 wt.%, relative to the total weight of solid.

[0042] Within the context of the present invention, the expression “at least one phosphorus source”, as used hereinafter, is intended to denote one phosphorus source or more than one phosphorus source. Mixtures of phosphorus sources may be used.

[0043] In the remainder of the text, the term “phosphorus source”, as used hereinafter, is understood, for the purposes of the present invention, both in the plural and the singular form, that is to say that step (a) of the process according to the present invention may be carried out with one or more than one phosphorus source.

[0044] Within the context of the present invention the phosphorus source may be any material known to the skilled person in the art which comprises phosphorus atoms.

[0045] Said phosphorus source may be of natural origin, or be synthetically prepared by a variety of methods known in the art, or may be a by-product of industrial processes.

[0046] Non-limiting examples of phosphorus sources of natural origin may be phosphate ores. Non-limiting examples of phosphate ores may be sedimentary phosphate rocks or igneous phosphate rocks. Non¬ limiting example of sedimentary phosphate rocks is phosphorite. Non¬ limiting example of igneous phosphate rocks is carbonatite. The main phosphate mineral present in both sedimentary and igneous rocks is apatite, non-limiting examples of apatites notably include hydroxyapatite, fluorapatite, and chlorapatite.

[0047] Non-limiting examples of phosphorus sources synthetically prepared may be phosphate salts such as tricalcium phosphate salts,dicalcium phosphate salts, ammonium phosphate salts, sodium phosphate salts, aluminum phosphate salts, iron phosphate salts, aluminum-iron phosphate salts, magnesium-ammonium phosphate salts and mixture thereof. Another non-limiting example is hydroxyapatite, although it is a naturally occurring mineral, it is known by the skilled in the art that it can be synthetically prepared as well.

[0048] Non-limiting examples of phosphorus sources which are by¬ products of industrial processes notably include bone ashes.

[0049] Preferably, the phosphorus source is a phosphorus source of natural origin, more preferably the phosphorus source is a sedimentary phosphate rock or igneous phosphate rock, more preferably the phosphorus source is an apatite.

[0050] Preferably, the phosphorus source comprises at least 15.0 wt.%, preferably at least 20.0 wt.%, even more preferably at least 25.0 wt.%, of phosphorus, expressed as P2O5 equivalents, relative to the total weight of the phosphorus source.

[0051] It is further understood that the phosphorus source preferably comprises at most 50.0 wt.%, preferably at most 45.0 wt.%, even more preferably at most 40.0 wt.%, preferably at most 35.0 wt.%, even more preferably at most 30.0 wt.%, of phosphorus, expressed as P2O5 equivalents, relative to the total weight of the phosphorus source.

[0052] According to a preferred embodiment of the present invention, the phosphorous source comprises from 15.0 wt.% to 50.0 wt.%, preferably from 15.0 wt.% to 45.0 wt.%, even more preferably from 20.0 wt.% to 35.0 wt.%, even more preferably from 25.0 wt.% and 30.0 wt.% of phosphorous, expressed as P2O5 equivalents, relative to the total weight of the phosphorus source.

[0053] According to a preferred embodiment of the process of the present invention, the phosphorus source may further comprise iron, in an amount of between 0.5 wt.% and 3.0 wt.%, preferably between 1.0 wt.%and 2.5 wt.%, more preferably between 1.5 wt.% and 2.0 wt.%, wherein the iron content is expressed as FesOs equivalents relative to the total weight of the phosphorus source.

[0054] According to a preferred embodiment of the process of the present invention, the phosphorus source may further comprise aluminum, in an amount of between 0.1 wt.% and 2.5 wt.%, preferably between 0.2 wt.% and 2.0 wt.%, more preferably between 0.5 wt.% and 1.5 wt.%, wherein the aluminum content is expressed as AI2O3 equivalents relative to the total weight of the phosphorus source.

[0055] According to a preferred embodiment of the process of the present invention, the phosphorus source may further comprise fluorine, in an amount of between 0.1 wt.% and 6.0 wt.%, preferably between 0.5 wt.% and 5.0 wt.%, more preferably between 1.0 wt.% and 4.0 wt.%, relative to the total weight of the phosphorus source.

[0056] The phosphorus source, as detailed above, may also be characterized by determining the Minor Element Ratio (MER) of said source, which is calculated according to the following equation (1):

[0057] A12O3(wt. %) + Fe2O3(wt. %) + MgO (wt. %)MER =- P2O5(wt. %) - (1) wherein AI2O3 (wt.%) is the amount of aluminum, expressed as AI2O3 equivalent, relative to the total weight of the phosphorus source; wherein Fe2Os (wt.%) is the amount of iron, expressed as Fe2Os equivalent, relative to the total weight of the phosphorus source; wherein MgO (wt.%) is the amount of magnesium, expressed as MgO equivalent, relative to the total weight of the phosphorus source; wherein P2O5 (wt.%) is the amount of phosphorus, expressed as P2O5 equivalent, relative to the total weight of the phosphorus source.

[0058] According to a preferred embodiment of the process of the present invention, the phosphorus source, as detailed above, has a MERof between 2 and 20, preferably of between 3 and 18, more preferably of between 4 and 14.

[0059] Within the context of the present invention, the percentage of phosphorus in the phosphorus source, expressed in P2O5 equivalents, as detailed above, can be measured by any means known by the skilled person in the art, preferably the percentage of phosphorus in the phosphorus source, expressed in P2O5 equivalent, is measured by colorimetry of phospho-vanado-molybdate at 436 nm by light absorption colorimetry.

[0060] Within the context of the present invention, the percentage of calcium, expressed in CaO equivalents, as detailed above, can be measured by any means known by the skilled person in the art, preferably by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy).

[0061] Within the context of the present invention, the percentage of iron, expressed in Fe2Os equivalents, as detailed above, can be measured by any means known by the skilled person in the art, preferably by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy).

[0062] Within the context of the present invention, the percentage of aluminum, expressed in AI2O3 equivalents, as detailed above, can be measured by any means known by the skilled person in the art, preferably by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy).

[0063] Within the context of the present invention, the percentage of aluminum, expressed in AI2O3 equivalents, as detailed above, can be measured by any means known by the skilled person in the art, preferably by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy).Within the context of the present invention, the percentage of magnesium, expressed in MgO equivalents, as detailed above, can be measured by any means known by the skilled person in the art, preferably by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy).

[0064] Within the context of the present invention, the term “inorganic condensed phosphates”, as used hereinafter, is intended to denote a mixture obtained by the dehydration and condensation of orthophosphoric acid or its salts thereof, said mixture being composed of condensed phosphate molecules having varying chain length, and structurally having at least two ionic phosphate units (PO43-) and counter- ions, said ionic phosphate units linked together by sharing one oxygen atom.

[0065] It will be thus understood that, in the context of the present invention, the inorganic condensed phosphates are intended to denote a mixture of inorganic condensed phosphate molecules.

[0066] Typically, the average chain length of the inorganic condensed phosphates, as detailed above is determined by potentiometric titration.

[0067] Within the context of the present invention, the average chain length of the inorganic condensed phosphate is determined by potentiometric titration, preferably with a standardized NaOH solution and with a standardized HCI solution.

[0068] The inorganic condensed phosphates may be commercially available, or may be synthetically prepared according to any known methods to the skilled in the art.

[0069] Preferably, the inorganic condensed phosphates comprise as counter-ions, a counter-ion selected from the group consisting of H+, Na+, K+, or NH4+and mixtures thereof, more preferably the inorganic condensed phosphates comprise as counter-ions, a counter-ion being H+,Na+or mixtures thereof, even more preferably the inorganic condensed phosphates comprises as counter-ions Na+.

[0070] The inorganic condensed phosphates according to the present invention may comprise pyrophosphate, at least one linear inorganic polyphosphate molecule, at least one inorganic metaphosphate molecule, at least one inorganic ultraphosphate molecule, or mixtures thereof.

[0071] The terms “pyrophosphates”, “inorganic polyphosphate”, “inorganic metaphosphate” and “ultraphosphates”, as used hereinafter, are notably defined in E. Thilo, Angew. Chem. Int. Ed., Vol.4 (1965), 1061 - 1071.

[0072] Within the context of the present invention, the term “ linear inorganic polyphosphate molecule”, as used hereinafter, is intended to denote an inorganic condensed phosphate molecule having a linear structure, preferably a non-ramified linear structure, said linear inorganic polyphosphate molecule having as general formula Xn+2PnO3n+1, wherein X is selected from the group consisting of H, Na, K and mixtures thereof, preferably X is H or Na, more preferably X is Na, and wherein n is the number of phosphate unit, and wherein n > 1.

[0073] Non-limiting examples of a linear inorganic polyphosphate molecule mention may be made of pyrophosphoric acid, i.e. a linear inorganic condensed polyphosphate having two phosphate units.

[0074] Non-limiting examples of a linear inorganic polyphosphate molecule mention may be made of tripolyphosphate, i.e. a linear inorganic condensed polyphosphate having three phosphate units.

[0075] Within the context of the present invention, the term “inorganic metaphosphate molecule”, as used hereinafter, is intended to denote an inorganic condensed phosphate molecule having a cyclic structure, said inorganic condensed metaphosphate molecule having as general formula Ym(PO3)m, wherein Y is selected from the group consistingof H, Na, K and mixtures thereof, preferably Y is H or Na, more preferably Y is Na, and m is the number of phosphate unit, and wherein m is higher than or equal to 3, preferably m is higher than or equal to 6.

[0076] Non-limiting examples of an inorganic metaphosphate molecule mention may be made of trimetaphosphate i.e. a condensed cyclic phosphate having three phosphate units and hexametaphosphate i.e. a condensed cyclic phosphate having six phosphate units.

[0077] Within the context of the present invention, the term “inorganic ultraphosphate molecule”, as used hereinafter, is intended to denote an inorganic condensed phosphate molecule having cross-linked or ramified units of linear inorganic polyphosphates and / or inorganic metaphosphate, as detailed above.

[0078] Typically, the relative content of each of the molecules / species comprised in the inorganic condensed phosphates, as detailed above, may be measured by any means known by the skilled person in the art, preferably by subjecting the inorganic condensed phosphates to liquid chromatography, using the conductance as a mean of detection, with the assumption that the conductance of each of the condensed phosphate molecules within the inorganic condensed phosphates is sensibly the same. The content of each of the inorganic condensed phosphate molecules is determined by comparison with standards of each of the inorganic condensed phosphates.

[0079] Typically, inorganic condensed phosphates, as the one described in the present invention, may also be denominated as “short chain inorganic condensed phosphates”, “medium chain inorganic condensed phosphates” or “long chain inorganic condensed phosphates”.

[0080] Within the context of the present invention, the term “short chain inorganic condensed phosphates”, as used hereinafter is intended to denote inorganic condensed phosphates, as defined above, having an average chain length of between 11 and 15 phosphate units and having aphosphorous content of between 66.5 wt.% and 68.7 wt.%, expressed as P2O5 equivalent, based on the total weight of the short chain inorganic condensed phosphates.

[0081] Within the context of the present invention, the term “medium chain inorganic condensed phosphates”, as used hereinafter is intended to denote inorganic condensed phosphates, as defined above, having an average chain length of between 16 and 20 phosphate units and having a phosphorous content of between 67.5 wt.% and 69.7 wt.%, expressed as P2O5 equivalent, based on the total weight of the short chain inorganic condensed phosphates.

[0082] Within the context of the present invention, the term “long chain inorganic condensed phosphates”, as used hereinafter, is intended to denote inorganic condensed phosphates, as defined above, having an average chain length of between 21 and 30 phosphate units and having a phosphorous content of between 68.5 wt.% and 71.0 wt.%, expressed as P2O5 equivalent, based on the total weight of the short chain inorganic condensed phosphates.

[0083] Step (a)

[0084] As said above, according to step (a) of the process according to the present invention, at least one phosphorus source, as detailed above, comprising calcium is contacted with a first sulfuric acid solution (SA1), thereby obtaining a first slurry (Si) comprising a first solid residue (SR1) and a first liquid phase (LP1), wherein the first solid residue (SR1) comprises at least one calcium sulfate, wherein said calcium sulfate is calcium sulfate hemihydrate or dihydrate calcium sulfate or a mixture thereof.

[0085] As mentioned above, it is required that the phosphorus source, as detailed above, comprises calcium.

[0086] Preferably, the phosphorus source comprises at least 15.0 wt.%, preferably at least 20.0 wt.%, even more preferably at least 30.0wt.%, even more preferably at least 40.0 wt.% of calcium, expressed as CaO equivalents, relative to the total weight of the phosphorus source.

[0087] It is further understood that the phosphorus source preferably comprises at most 60.0 wt.%, preferably at most 58.0 wt.%, even more preferably at most 55.0 wt.%, of calcium, expressed as CaO equivalents, relative to the total weight of the phosphorus source.

[0088] According to a preferred embodiment of the present invention, the phosphorus source comprises from 15.0 wt.% to 60.0 wt.%, preferably from 20.0 wt.% to 60.0 wt.%, even more preferably from 30.0 wt.% to 58.0 wt.%, even more preferably from 40.0 wt.% and 55.0 wt.% of calcium, expressed as CaO equivalents, relative to the total weight of the phosphorus source.

[0089] Preferably, the phosphorus source comprises at least 15.0 wt.% of phosphorus and at least 15.0 wt.% of calcium, preferably at least 20.0 wt.% of phosphorus and at least 20.0 wt.% of calcium, even more preferably at least 25.0 wt.% of phosphorus and at least 30.0 wt.% of calcium, wherein the phosphorus content is expressed as P2O5 equivalents relative to the total weight of the phosphorus source, and wherein the calcium content is expressed as CaO equivalents relative to the total weight of the phosphorus source.

[0090] It is further understood that the phosphorus source preferably comprises at most 50.0 wt.% of phosphorus and at most 60.0 wt.% of calcium, preferably at most 45.0 wt.% of phosphorus and at most 58.0 wt.% of calcium, even more preferably at most 40.0 wt.% of phosphorus and at most 55.0 wt.% of calcium, wherein the phosphorus content is expressed as P2O5 equivalents relative to the total weight of the phosphorus source, and wherein the calcium content is expressed as CaO equivalents relative to the total weight of the phosphorus source.

[0091] According to a preferred embodiment of the present invention, the phosphorus source comprises from 15.0 wt.% to 50.0 wt.%of phosphorus and from 15.0 wt.% to 60.0 wt.% of calcium, preferably from 20.0 wt.% to 45.0 wt.% of phosphorus and from 30.0 wt.% to 58.0 wt.% of calcium, even more preferably from 25.0 to 40.0 wt.% of phosphorus and from 40.0 wt.% to 55.0 wt.% of calcium, wherein the phosphorus content is expressed as P2O5 equivalents relative to the total weight of the phosphorus source, and wherein the calcium content is expressed as CaO equivalents relative to the total weight of the phosphorus source.

[0092] According to a preferred embodiment of the present invention, the phosphorus source comprises:

[0093] - from 15.0 wt.% to 50.0 wt.% of phosphorus, preferably from 20.0 wt.% to 45.0 wt.% of phosphorus, more preferably from 25.0 wt.% to 40.0 wt.% of phosphorus;

[0094] - from 15.0 wt.% to 60.0 wt.% of calcium, preferably from 20.0 wt.% to 60.0 wt.% of calcium, more preferably from 40.0 wt.% to 55.0 wt.% of calcium; and

[0095] - from 0.1 wt.% to 6.0 wt.% of fluorine, preferably from 0.5 wt.% to 5.0 wt.% of fluorine, more preferably from 1.0 wt.% to 4.0 wt.% of fluorine;

[0096] wherein the phosphorus content is expressed as P2O5 equivalents wherein the calcium content is expressed as CaO equivalents; said contents being expressed relative to the total weight of the phosphorus source.

[0097] According to another embodiment of the present invention, the phosphorus source comprises:

[0098] - from 15.0 wt.% to 50.0 wt.% of phosphorus, preferably from 20.0 wt.% to 45.0 wt.% of phosphorus, more preferably from 25.0 wt.% to 40.0 wt.% of phosphorus;

[0099] - from 15.0 wt.% to 60.0 wt.% of calcium, preferably from 10.0 wt.% to 60.0 wt.% of calcium, more preferably from 40.0 wt.% to 55.0 wt.% of calcium; and- from 0.5 wt.% to 3.0 wt.% of iron, preferably from 1.0 wt.% to 2.5 wt.% of iron, more preferably from 1.5 wt.% to 2.0 wt.% of iron;

[0100] wherein the phosphorus content is expressed as P2O5 equivalents wherein the calcium content is expressed as CaO equivalents; and wherein the iron content is expressed as FesOs equivalents, said contents being expressed relative to the total weight of the phosphorus source.

[0101] According to preferred embodiment of the present invention, the phosphorus source comprises:

[0102] - from 15.0 wt.% to 50.0 wt.% of phosphorus, preferably from 20.0 wt.% to 45.0 wt.% of phosphorus, more preferably from 25.0 wt.% to 40.0 wt.% of phosphorus;

[0103] - from 15.0 wt.% to 60.0 wt.% of calcium, preferably from 20.0 wt.% to 60.0 wt.% of calcium, more preferably from 40.0 wt.% to 55.0 wt.% of calcium; and

[0104] - from 0.5 wt.% to 3.0 wt.% of iron, preferably from 1.0 wt.% to 2.5 wt.% of iron, more preferably from 1.5 wt.% to 2.0 wt.% of iron; and - from 0.1 wt.% to 2.5 wt.% of aluminum, preferably from 0.2 wt.% to 2.0 wt.% of aluminum, more preferably from 0.5 wt.% to 1.5 wt.% of aluminum;

[0105] wherein the phosphorus content is expressed as P2O5 equivalents; wherein the calcium content is expressed as CaO equivalents; and wherein the iron content is expressed as Fe2Os equivalents, and wherein the aluminum content is expressed as AI2O3 equivalents, said contents being expressed relative to the total weight of the phosphorus source.

[0106] According to preferred embodiment of the present invention, the phosphorus source comprises:- from 15.0 wt.% to 50.0 wt.% of phosphorus, preferably from 20.0 wt.% to 45.0 wt.% of phosphorus, more preferably from 25.0 wt.% to 40.0 wt.% of phosphorus;

[0107] - from 15.0 wt.% to 60.0 wt.% of calcium, preferably from 20.0 wt.% to 60.0 wt.% of calcium, more preferably from 40.0 wt.% to 55.0 wt.% of calcium; and

[0108] - from 0.5 wt.% to 3.0 wt.% of iron, preferably from 1.0 wt.% to 2.5 wt.% of iron, more preferably from 1.5 wt.% to 2.0 wt.% of iron;

[0109] - from 0.1 wt.% to 2.5 wt.% of aluminum, preferably from 0.2 wt.% to 2.0 wt.% of aluminum, more preferably from 0.5 wt.% to 1.5 wt.% of aluminum; and

[0110] - from 0.1 wt.% to 6.0 wt.% of fluorine, preferably from 0.5 wt.% to 5.0 wt.% of fluorine, more preferably from 1.0 wt.% to 4.0 wt.% of fluorine;

[0111] wherein the phosphorus content is expressed as P2O5 equivalents; wherein the calcium content is expressed as CaO equivalents; and wherein the iron content is expressed as FesOs equivalents, and wherein the aluminum content is expressed as AI2O3 equivalents, said contents being expressed relative to the total weight of the phosphorus source.

[0112] The phosphorus source, as detailed above, may be crushed, grinded, chunked milled and / or powdered before step (a). Thus, the phosphorus source may be in solid form.

[0113] The phosphorus source, as detailed above, may further be mixed with an aqueous solution, or with process water, or with acidic water, as detailed above, before step (a), thereby obtaining a dispersion having a solid content ranging from 5.0 wt.% to 70.0 wt.%, or from 10.0 wt.% to 60.0 wt.%, or from 20.0 wt.% to 40.0 wt.%, or from 25.0 wt.% to 30.0 wt.%, relative to the total weight of the dispersion.As said, the phosphorus source, as detailed above, is contacted with a first sulfuric acid solution (SA₁).

[0114] According to certain embodiments of step (a) of the process the present invention, the first sulfuric acid solution (SA₁) comprises sulfuric acid in an amount of between 90.0 and 98.0 wt.%, relative to the total weight of the sulfuric acid solution.

[0115] Preferably, the first sulfuric acid solution (SA₁) may comprise phosphoric acid. Thus, the first sulfuric acid solution (SA₁) is preferably a sulfo-phosphoric acid solution, as detailed above.

[0116] Alternatively, the first sulfuric acid solution (SA₁) may comprise sulfuric acid and phosphoric acid, wherein the concentration of sulfuric acid in the first sulfuric acid solution (SA₁) is ranging from 15 to 35 wt.%, preferably from 15 to 35 wt.%, preferably from 15 to 25 wt.%, based on the total weight of the first sulfuric acid solution (SA₁).

[0117] Desirably, the first sulfuric acid solution (SA₁), as detailed above, may be obtained by mixing, prior to step (a) or during step (a), concentrated sulfuric acid, as defined above, with recycled phosphoric acid, as defined above, in a mixing tee.

[0118] Said mixing tee may be any mixing tee known to the skilled in the art.

[0119] In particular, the mixing tee may comprise:

[0120] a) an outer pipe having a beveled end and a tee end, wherein a tee structure is formed at the tee end and at least one, preferably one, recycled phosphoric acid inlet for a recycled phosphoric acid stream, as detailed above, to flow through the at least one tee pipe into the outer pipe;

[0121] b) an inner pipe comprising a beveled end and a concentrated sulfuric acid inlet opposite the beveled end;

[0122] c) said inner pipe is lined with a corrosion-resistant material on its inside surface, preferably the corrosion-resistant material is a nickel alloy;d) said inner pipe is placed concentrically within the outer pipe with a welded clamp fixed to the outer pipe and placed between the beveled end of the inner pipe and the concentrated sulfuric acid inlet, to fix the inner pipe with the outer pipe

[0123] e) the beveled end of outer pipe extending beyond the beveled end of the inner pipe.

[0124] According to a preferred embodiment of the present invention, the first sulfuric acid solution (SA₁) is obtained, prior to step (a), by mixing in a mixing tee, concentrated sulfuric acid, as defined above, by:

[0125] - streaming a concentrated sulfuric acid solution, as defined above, in an inner pipe comprising a beveled end and a concentrated sulfuric acid inlet opposite the beveled end; and

[0126] - streaming recycled phosphoric acid, as detailed above, in an outer pipe having a beveled end and a tee end, wherein a tee structure is formed at the tee end and a recycled phosphoric acid inlet for a return acid stream to flow through the at least one tee pipe into the outer pipe;

[0127] wherein the inner pipe is lined with a corrosion-resistant material on its inside surface, preferably the corrosion-resistant material is an nickel alloy; wherein the inner pipe is placed concentrically within the outer pipe with a welded clamp fixed to the outer pipe and placed between the beveled end of the inner pipe and the concentrated sulfuric acid inlet, to fix the inner pipe with the outer pipe and wherein the beveled end of outer pipe extends beyond the beveled end of the inner pipe.

[0128] It is understood that the contacting according to step (a) of the process of the present invention can be carried out by any method known to the skilled in the art and according to general practice.

[0129] The contacting may be carried out using one or more than one traditional mixers, blenders or tanks, wherein said traditional mixers,blenders or tanks may be of any shape known to the skilled in the art, such as a square shape or a cylindrical shape.

[0130] According to a preferred embodiment of step (a) of the process of the present invention, the phosphorus source, as detailed above, is intimately admixed with the first sulfuric acid solution (SA₁), as detailed above.

[0131] Furthermore, it is understood that any order of contacting or admixing of the phosphorus source and the first sulfuric acid solution (SA₁), as detailed above, is acceptable. It is understood that the skilled person in the art will carry out said intimate admixing according to general practices, such as notably using optimal times, speed, weight, and volume quantities.

[0132] As said, step (a) of contacting of the at least one phosphorous source, as detailed above, with the first sulfuric acid solution (SA₁), as detailed above, allows to obtain a first slurry (S₁) comprising a first solid residue (SR₁) and a first liquid phase (LP₁), wherein the first solid residue (SR₁) comprises at least one calcium sulfate, wherein said calcium sulfate is calcium sulfate hemihydrate or dihydrate calcium sulfate or a mixture thereof.

[0133] In general, in order to obtain the desired form of calcium sulfate, the temperature of the slurry obtained during and after step (a) of the process of the present invention, and the acidity, as detailed above, of the liquid phase comprised in the slurry are crucial parameters. In general, said parameters determine the favored form of calcium sulfate (i.e. calcium sulfate in hemihydrate form, calcium sulfate in dihydrate form or anhydrous calcium sulfate), as for example described in Schrödter et al. (Phosphoric Acid and Phosphates, vol.26, in Ullmann's Encyclopedia of Industrial Chemistry, (2012), page 685, Figure 10).

[0134] According to one embodiment of step (a) of the process of the present invention, the contacting of the at least one phosphoroussource, as detailed above, with a first sulfuric acid solution (SA₁), as detailed above, allows to obtain a first slurry (S₁) comprising a first solid residue (SR₁) and a first liquid phase (LP₁), wherein the first solid residue (SR₁) consists essentially of calcium sulfate hemihydrate.

[0135] It will be understood to the skilled in the art that step (a) according to this embodiment is ascribed to an hemihydrate (HH) process.

[0136] According to this embodiment, step (a) is preferably carried out at a temperature of between 90°C and 110°C, and the first liquid phase (LP₁) obtained in step (a) preferably has an acidity of between 37.5 wt.% and 52.5 wt.%, relative to the total weight of the first liquid phase (LP₁).

[0137] According to this embodiment, step (a) is more preferably carried out at a temperature of between 95°C and 105°C, more preferably at a temperature of 98°C and 102°C, and the first liquid phase (LP₁) obtained in step (a) preferably has an acidity of between 40.0 wt.% and 50.0 wt.%, more preferably of between 41.5 wt.% and 47.5 wt.%, relative to the total weight of the first liquid phase (LP₁).

[0138] It will also be understood to the skilled person in the art that the first liquid phase (LP₁) obtained in step (a) according to this embodiment comprises phosphoric acid, and may further contain residual sulfuric acid which may have been added of excess, relative to the phosphorus content of the phosphate source, as detailed above, or unreacted sulfuric acid.

[0139] The first liquid phase (LP₁) may be retrieved, and may be further purified and concentrated to obtain phosphoric acid.

[0140] When the first liquid phase (LP₁) is retrieved, purified and concentrated, said purification may be carried out to reduce the level of cadmium, sulfate, and / or arsenic in said first liquid phase (LP₁), and / or may be carried out to reduce the level of solid content by decantation.It will be understood that the reduction of level of cadmium, sulfate, and / or arsenic may be carried out by any methods known to the skilled in the art.

[0141] In this embodiment, step (a) is preferably carried out in such a way that the first liquid phase (LP₁) obtained in step (a) has a phosphoric acid content, expressed in P₂O₅ equivalent, of between 35.0 wt.% and 50.0 wt.%, more preferably of between 37.5 wt.% and 47.5 wt.%, more preferably of between 40.0 wt.% and 45.0 wt.%, relative to the total weight of the first liquid phase (LP₁), and a sulfuric acid content, expressed in SO₃ equivalent of between 0.4 wt.% and 3.0 wt.%, relative to the total weight of the first liquid phase (LP₁).

[0142] According to this embodiment, the contacting of the at least one phosphorous source, as detailed above, with a first sulfuric acid solution, as detailed above, is carried out for a period of between 1 hour to 24 hours, more preferably between 2 hours and 12 hours, even more preferably between 2 hours and 6 hours, even more preferably between 3 hours and 5 hours.

[0143] According to another embodiment of step (a) of the process of the present invention, the contacting of the phosphorous source, as detailed above, with a first sulfuric acid solution (SA₁), as detailed above, allows to obtain a first slurry (S₁) comprising a first solid residue (SR₁) and a first liquid phase (LP₁), wherein the first solid residue comprises dihydrate calcium sulfate or a mixture of dihydrate calcium sulfate and calcium sulfate hemihydrate.

[0144] It will be understood that the definition and each of the preference of the “first acid solution (SA1)”, as defined above, equally applies to this embodiment.

[0145] According to this embodiment, step (a) is preferably carried out at a temperature of between 70°C and 85°C, and the first liquid phase(LP₁) obtained in step (a) preferably has an acidity of between 27.0 wt.% and 40.0 wt.%, relative to the total weight of the first liquid phase (LP₁).

[0146] It will be understood to the skilled in the art that step (a) according to this embodiment may be a dihydrate / hemihydrate (DH / HH) process. In other words, step (a) according to this embodiment, may be ascribed to one of the step of a dihydrate / hemihydrate (DH / HH) process for manufacturing calcium sulfate hemihydrate and phosphoric acid according to the present invention.

[0147] According to this embodiment, step (a) is more preferably carried out at a temperature of between 72 °C and 82 °C, more preferably at a temperature of 76 °C and 80 °C and the first liquid phase (LP₁) preferably has an acidity of between 28.0 wt.% and 39.0 wt.%, more preferably of between 29.0 wt.% and 38.0 wt.% relative to the total weight of the first liquid phase (LP₁).

[0148] It will also be understood to the skilled person in the art that the first liquid phase (LP₁) obtained in step (a) according to this embodiment comprises phosphoric acid, and may further contain residual sulfuric acid which may have been added of excess, relative to the phosphorus content of the phosphate source, as detailed above, or unreacted sulfuric acid.

[0149] A part of the first liquid phase (LP₁) may be retrieved, and may be further purified and concentrated to obtain phosphoric acid.

[0150] When part of the first liquid phase (LP₁) is retrieved, purified and concentrated, said purification may be carried out to reduce the level of cadmium, sulfate, and / or arsenic in said first liquid phase (LP₁), and / or may be carried out to reduce the level of solid content by decantation.

[0151] It will be understood that the reduction of level of cadmium, sulfate, and / or arsenic may be carried out by any methods known to the skilled in the art.According to this embodiment, step (a) is preferably carried out in such a way that the first liquid phase (LP₁) obtained in step (a) has a phosphoric acid content, expressed in P₂O₅ equivalent, of 35.0 wt.% and 50.0 wt.%, more preferably of between 37.5 wt.% and 47.5 wt.%, more preferably of between 30.0 wt.% and 35.0 wt.%, relative to the total weight of the first liquid phase (LP₁); and in such a way that the first liquid phase (LP₁) obtained in step (a) has a sulfuric acid content, expressed in SO₃ equivalent, of between 0.2 wt.% and 2.5 wt.%, more preferably of between 0.4 wt.% and 2.2 wt.%, more preferably of between 0.6 wt.% and 2.1 wt.%, relative to the total weight of the first liquid phase (LP₁).

[0152] Step (b)

[0153] As said, according to step (b) of the process of the present invention:

[0154] - provided that the first solid residue (SR₁) consists essentially of calcium sulfate hemihydrate, at least part of the first liquid phase (LP₁) is removed from the first solid residue (SR₁) obtained in step (a), thereby forming a first solid-rich phase (PSR1) having a first solid residue (SR₁) content higher than 65.0 wt.%; or - provided that the first solid residue comprises dihydrate calcium sulfate, a second sulfuric acid solution (SA2) is added to the first slurry (Si) obtained in step (a), thereby forming a second slurry (S2) comprising a second solid residue (SR2) and a second liquid phase (LP2), wherein the second solid residue (SR2) consists essentially of calcium sulfate hemihydrate; or

[0155] - provided that the first solid residue (SR₁) comprises dihydrate calcium sulfate, at least part of the first liquid phase (LP₁) is removed from the first solid residue (SR₁) obtained in step (a), thereby forming a second solid-rich phase (PSR2) having a first solid residue (SR₁) content higher than 35.0 wt.%, followed by adding a third sulfuric acid solution (SA₃) to the second solid-richphase (PSR2), thereby forming a third slurry (S₃) comprising a third solid residue (SR₃) and a third liquid phase (LP₃), wherein the third solid residue (SR₃) consists essentially of calcium sulfate hemihydrate.

[0156] As said, according to one embodiment of step (b) of the present invention, provided that the first solid residue (SR₁) consists essentially of calcium sulfate hemihydrate, as detailed above, at least part of the first liquid phase (LP₁) is removed from the first solid residue (SR₁) obtained in step (a), thereby forming a first solid-rich phase (PSR1) having a first solid residue (SR₁) content higher than 65.0 wt.%.

[0157] Desirably, the first solid-rich phase (PSR1) according to this embodiment has a first solid residue (SR₁) content higher than 67.0 wt.%, more preferably higher than 70.0 wt.%, more preferably higher than 75.0 wt.% relative to the total weight of the first solid-rich phase (PSR1).

[0158] Preferably, the first solid-rich phase (PSR1) according to this embodiment has a first solid residue (SR₁) content of between 65.0 wt.% and 100 wt.%, more preferably of between 67.0 wt.% and 90.0 wt.%, even more preferably of between 70.0 wt.% and 80.0 wt.%, more preferably of between 75.0 wt.% and 80.0 wt.% relative to the total weight of the first solid-rich phase (PSR1).

[0159] The solid content of the first solid-rich phase (PSR1) may be evaluated by calculating the density of the first solid-rich phase (PSR1) thereby obtained, and comparing it with the density of the first liquid phase (LP₁) of the first slurry (S₁), and with the theoretical density of solid calcium sulfate hemihydrate.

[0160] According to this embodiment, the removal of the at least part of the first liquid phase (LP₁) from the first solid residue (SR₁) may be carried out by any means known to the skilled in the art, such as notably, by filtration, through settling or by decantation. Thus, said removal may be carried out using, for example, a drum filter, a disc filter, a rotary table, abelt filter, a press filter, a vacuum filter, a rotary tilting pan filter, a centrifuge decanter, a centrifuge filter, a hydrocyclone or a decanter.

[0161] According to this embodiment, the removal of the at least part of the first liquid phase (LP₁) from the first solid residue (SR₁) is preferably carried out by filtration.

[0162] The filtration may be carried out under pressure, under depression or at atmospheric pressure.

[0163] The filtration, as detailed above, may be carried out by any filter known to the skilled person in the art, such as for example by using a band filter, a drum filter, a press filter, a rotary tilting pan filter, a table filter, or a belt filter.

[0164] According to this embodiment, the removal of at least part of the first liquid phase (LP₁) from the first solid residue (SR₁) is preferably carried out using a belt filter, a tilting pan filter, a rotary tilting pan filter, or a table filter more preferably using a belt filter or a rotary tilting pan filter, more preferably using a rotary tilting pan filter.

[0165] Typically, when a belt filter or a rotary tilting pan filter is used, said filters comprise a series of filtering cells or filtering sectors. Typically, a slurry to be filtrated in a belt filter or a rotary pan filter is charged on a filtering cell or filtering sector, said filtering cell or filtering sector moving towards a discharge zone. As the filtering cell or filtering sector travels to the discharge zone, the filtering cell or filtering sector may undergo a series of drainages and or washings in order to successively remove part of the liquid comprised in the slurry and / or to wash the solid phase found in the filtering cell or filtering sector. The part of the liquid which has been drained may be recovered and reused, preferably at step a.

[0166] According to this embodiment, when the removal of at least part of the first liquid phase (LP₁) from the first solid residue (SR₁) is carried out using a belt filter or a rotary tilting pan filter, the removal of at least partof the first liquid phase (LP₁) from the first solid residue (SR₁) comprises at least the steps of:

[0167] 1) charging the first slurry (S₁) comprising the first liquid phase (LP₁) and the first solid residue (SR₁) onto a filtering cell or filtering sector;

[0168] 2) draining from the filtering cell or from the filtering sector the first liquid phase (LP₁) from the first solid residue (SR₁), in order to obtain the first solid-rich phase (PSR1), as detailed above;

[0169] 3) discharging the first solid-rich phase (PSR1) as detailed above, preferably the first solid-rich phase is discharged into a tank or directly to a discharge.

[0170] According to this embodiment, the filtration of the first slurry (S₁) is preferably carried out at a temperature of between 90.0°C and 110°C, more preferably of 95°C and 105°C, even more preferably of between 98°C and 102°C. Said temperatures are particularly advantageous in order to prevent the conversion of the first solid residue (SR₁) consisting essentially of calcium sulfate hemihydrate, to spontaneously convert to dihydrate calcium sulfate in the filter, thereby preventing the clogging of the filtering cell or filtering sector.

[0171] According to another embodiment of step (b) of the process of the present invention, provided that the first solid residue (SR₁) comprises dihydrate calcium sulfate, a second sulfuric acid solution is added to the first slurry (S₁) obtained in step (a), thereby forming a second slurry (S₂) comprising a second solid residue (SR₂) and a second liquid phase (LP₂), wherein the second solid residue (SR₂) consists essentially of calcium sulfate hemihydrate.

[0172] According to this embodiment, the second sulfuric acid solution (SA2) preferably comprises sulfuric acid in an amount of at least 90.0 wt.%, more preferably at least 95.0 wt.%, even more preferably atleast 96.0 wt.%; even more preferably at least 98.0 wt.% relative to the total weight of the second sulfuric acid solution (SA2).

[0173] According to this embodiment, the step of adding the second sulfuric acid solution (SA2) is carried out at a temperature of between 90°C and 110°C, thereby forming the second slurry (S2) comprising the second solid residue and the second liquid phase, as detailed above, wherein the acidity of the second liquid phase (LP2) of the second slurry (S2), is between 30.0 and 45.0 wt.% relative to the total weight of the second liquid phase (LP2).

[0174] According to this embodiment, the step of adding the second sulfuric acid solution (SA2) is carried out at a temperature of between 95 °C and 105 °C, even more preferably at a temperature of 98 °C and 102 °C, and the second liquid phase (LP2) of the second slurry (S2) more preferably has an acidity of between 30.0 wt.% and 50.0 wt.%, even more preferably of between 32.5 wt.% and 42.5 wt.%, even more preferably of between 35.0 wt.% and 40.0 wt.%, relative to the total weight of the second liquid phase (LP2).

[0175] According to this embodiment, steam is preferably concomitantly injected with the second sulfuric acid solution (SA2). This is particularly preferred to reach the desired temperature.

[0176] According to another embodiment of step (b) of the process of the present invention, provided that the first solid residue (SR1) comprises dihydrate calcium sulfate, at least part of the first liquid phase (LP1) is removed from the first solid residue (SR1) obtained in step (a), thereby forming a second solid-rich phase (PSR2) having a first solid residue (SR1) content higher than 35.0 wt.%, followed by adding a third sulfuric acid solution (SA3), thereby forming a third slurry (S3) comprising a third solid residue (SR3) and a third liquid phase (LP3), wherein the third solid residue (SR3) consists essentially of calcium sulfate hemihydrate.Desirably, the second solid-rich phase (PSR2) according to this embodiment have a first solid residue (SR1) content higher than 40.0 wt.%, more preferably higher than 45.0 wt.%, more preferably higher than 50.0 wt.%, relative to the total weight of the second solid-rich phase (PSR2).

[0177] Preferably, the second solid-rich phase (PSR2) according to this embodiment have a first solid residue (SR1) content of between 35.0 wt.% and 100 wt.%, more preferably of between 40.0 wt.% and 75.0 wt.%, even more preferably of between 45.0 wt.% and 60.0 wt.%, preferably of between 50.0 wt.% and 60.0 wt.%, relative to the total weight of the second solid-rich phase (PSR2).

[0178] The solid content of the second solid-rich phase (PSR2) may be measured by evaluating the density of the second solid-rich phase (PSR2) thereby obtained, and comparing it with (1 ) the density of the second liquid phase (LP2) of the second slurry (S2) and (2) the theoretical density of solid calcium sulfate hemihydrate.

[0179] It will be understood that the definitions and preferences as detailed above for the removal of the at least part of the first liquid phase (LP1) from the first solid residue (SR1), equally applies to this embodiment.

[0180] Thus, according to this embodiment, the removal of the at least part of the first liquid phase (LP1) from the first solid residue (SR1) may be carried out by any means known to the skilled in the art, such as notably, by filtration, through settling or by decantation. Thus, said removal may be carried out using, for example, a drum filter, a disc filter, a rotary table, a belt filter, a press filter, a vacuum filter, a tilting pan filter, a centrifuge decanter, a centrifuge filter, a hydrocyclone or a decanter.

[0181] According to this embodiment, the removal of the at least part of the first liquid phase (LP1) from the first solid residue (SR1) is preferably carried out by filtration.

[0182] The filtration may be carried out under pressure, under depression or at atmospheric pressure.The filtration, as detailed above, may be carried out by any filter known to the skilled person in the art, such as for example by using a band filter, a drum filter, a press filter, a tilting pan filter, a rotary tilting pan filter, a table filter, or a belt filter.

[0183] According to this embodiment, the removal of at least part of the first liquid phase (LP₁) from the first solid residue (SR₁) is preferably carried out using a belt filter, a tilting pan filter, a rotary tilting pan filter, or a table filter more preferably using a belt filter or a rotary tilting pan filter, more preferably using a rotary tilting pan filter.

[0184] Typically, when a belt filter or a rotary tilting pan filter is used, said filters comprise a series of filtering cells or filtering sectors. Typically, a slurry to be filtrated in a belt filter or a rotary pan filter is charged on a filtering cell or filtering sector, said filtering cell or filtering sector is moving to a discharge zone. As the filtering cell travels to the discharge zone, the filtering cell or filtering sector may undergo a series of drainages and or washings in order to successively remove part of the liquid comprised in the slurry and / or to wash the solid phase found in the filtering cell or filtering sector. The part of the liquid which has been drained may be recovered and reused, preferably in the step (a) as detailed above.

[0185] According to this embodiment, when the removal of at least part of the first liquid phase (LP₁) from the first solid residue (SR₁) is carried out using a belt filter or a rotary tilting pan filter, the removal of at least part of the first liquid phase (LPi) from the first solid residue (SRi) comprises the step of:

[0186] 1) charging the first slurry (S₁) comprising the first liquid phase (LP₁) and the first solid residue (SR₁) onto a filtering cell or filtering sector;

[0187] 2) draining from the filtering cell or from the filtering sector the first liquid phase (LP1) from the first solid residue, inorder to obtain the second solid-rich phase (PSR2) as detailed above; and

[0188] 3) discharging the second solid-rich phase (PSR2) as detailed above, preferably the first solid-rich phase is discharged into a tank.

[0189] As said, a third sulfuric acid solution (SA3) is added to the second solid-rich phase (PSR2), thereby forming a third slurry (S₃) comprising a third solid residue (SR₃) and a third liquid phase (LP₃), wherein the third solid residue (SR₃) consists essentially of calcium sulfate hemihydrate.

[0190] According to this embodiment, prior to the step of adding the third sulfuric acid solution to the second solid-rich phase (PSR2), the second solid-rich phase (PSR2) is preferably mixed with an aqueous solution, as detailed above. More preferably, prior to the step of adding the third sulfuric acid solution to the second solid-rich phase (PSR2), the second solid-rich phase (PSR2) is mixed with part of the first slurry (S1) from step (a) according to the process of the present invention.

[0191] Preferably, the third sulfuric acid solution (SA3) is identical to the second sulfuric acid solution (SA2), as detailed above.

[0192] Thus, according to this embodiment, the third sulfuric acid solution (SA3), preferably comprises sulfuric acid in an amount of at least 90.0 wt.%, more preferably at least 95.0 wt.%, even more preferably at least 96.0 wt.%; even more preferably at least 98.0 wt.% relative to the total weight of the third sulfuric acid solution (SA3).

[0193] According to this embodiment, the step of adding the third sulfuric acid solution (SA3) is carried out at a temperature of between 90°C and 110°C, thereby forming the third slurry (S3) comprising the third solid residue and the third liquid phase, as detailed above, wherein the acidity of the third liquid phase (LP3) of the third slurry (S3), is between30.0 and 45.0 wt.% relative to the total weight of the third liquid phase (LP3).

[0194] According to this embodiment, the step of adding the third sulfuric acid solution (SA3) is carried out at a temperature of between 95 °C and 105 °C, even more preferably at a temperature of 98 °C and 102 °C, and the second liquid phase (LP3) of the second slurry (S2) more preferably has an acidity of between 30.0 wt.% and 50.0 wt.%, even more preferably of between 32.5 wt.% and 42.5 wt.%, even more preferably of between 35.0 wt.% and 40.0 wt.%, relative to the total weight of the third liquid phase (LP3).

[0195] According to this embodiment, steam is preferably concomitantly injected to the third sulfuric acid solution (SA3). This is particularly preferred to reach the desired temperature.

[0196] Step (c)

[0197] As said, according to step (c) of the process of the present invention:

[0198] - the first solid residue (SSR1) comprised in the first solid-rich phase (PSR1) obtained in step (b), as detailed above, is washed, as detailed above, with a solution (P); or

[0199] - at least part of the second liquid phase (LP2) is removed from the second solid residue (SR2) obtained in step (b), thereby forming a third solid-rich phase (PSR3) having a second solid residue (SR2) content higher than 65.0 wt.%, followed by washing the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) with a solution (P); or

[0200] - at least part of the third liquid phase (LP3) is removed from the third solid residue (SR3) obtained in step (b), thereby forming a fourth solid-rich phase (PSR4) having a third solid residue (SR3) content higher than 65.0 wt.%, followed by washing the thirdsolid residue (SR3) comprised in fourth solid-rich phase (PSR4) with a solution (P).

[0201] It is required by the present invention that the solution (P) comprises inorganic condensed phosphates having an average chain length of between 9 and 30 phosphate units; and wherein the solution (P) provides with inorganic condensed phosphates in an amount of between 0.10 wt.% and 0.50 wt.% relative to the total weight of the dry first solid residue (SR1) or the dry second residue (SR2) or the dry third solid residue (SR3), obtained in step (b);

[0202] The inventors have surprisingly found that the washing of, respectively, the first solid residue (SR1), the second solid residue (SR2), or the third solid residue (SR3), with the solution (P), as detailed above, allows to:

[0203] - prevent the clogging of the installation due to the deposition of calcium sulfate on the installation’s surface and / or on the separation unit’s surfaces. Thus, an economically viable process is obtained, and this process reduces the occurrence of the periodic shut-down of the installation for cleaning. In particular, the washing of the first solid residue (SR1), the second solid residue (SR2), or the third solid residue (SR3) retards, for a given time, the hydration of calcium sulfate hemihydrate to dihydrate calcium sulfate, the latter being responsible of the clogging;

[0204] - while the clogging is prevented, the process allows to obtain calcium sulfate hemihydrate, which, downstream of the process, in particular when stored, spontaneously convert to dihydrate calcium sulfate. Thus, the process thereby benefits from the self-drying properties of the calcium sulfate hemihydrate and allows to afford dihydrate calcium sulfate in a readily form, to be used by end-users.

[0205] In other words, the inventors have surprisingly found that the process according to the present invention, and more in particular thewashing of the solid residue with the solution (P) in step (c), as detailed above, allows to achieve a desirable balance between not hydrating the calcium sulfate hemihydrate during the process of the present invention, while still allowing the calcium sulfate hemihydrate to hydrate downstream of the process.

[0206] As said above, it is required that, in step (c) of the process of the present invention, that the inorganic condensed phosphates, as detailed above, have an average chain length of between 9 and 30 phosphate units.

[0207] Typically, inorganic condensed phosphates, when in solution, and more in particular when in acidic solution, are prone to hydrolytic cleavage, and ultimately decompose into their respective monophosphates salts or acid (orthophosphate). The inventors have surprisingly found that using inorganic condensed phosphates having an average chain length of between 9 and 30 phosphate units, allows to obtain a desired balance between (1) prevent the clogging of the installation due to the deposition of dihydrate calcium sulfate on the installation’s surface and / or on the separation unit’s surfaces; and (2) allowing to obtain calcium sulfate hemihydrate, to spontaneously convert to dihydrate calcium sulfate downstream of the process, in particular when stored.

[0208] Preferably, the inorganic condensed phosphates, as detailed above, have an average chain length of between 9.5 and 25, more preferably of between 10.0 and 20.0, more preferably of between 10.5 and 17.5, even more preferably of between 10.8 and 15.0.

[0209] Preferably, the inorganic condensed phosphates, as detailed above, comprise at most 1.0 wt.%, or at most 0.5 wt.%, or at most 0.3 wt.% of orthophosphate, relative to the total weight of the inorganic condensed phosphates.Preferably, the inorganic condensed phosphates, as detailed above, comprise at least 55.0 wt.%, or at least 60.0 wt.%, or at least 65.0 wt.%, or at least 70.0 wt.%, or at least 73 wt.% of condensed phosphates having a chain length higher than or equal to 4, relative to the total weight of the inorganic condensed phosphates.

[0210] According to a preferred embodiment of the present invention, the inorganic condensed phosphates comprise:

[0211] - at most 1.0 wt.%, or at most 0.5 wt.%, or at most 0.3 wt.% of orthophosphate, relative to the total weight of the inorganic condensed phosphates; and

[0212] - at least 55.0 wt.%, or at least 60.0 wt.%, or at least 65.0 wt.%, or at least 70.0 wt.%, or at least 73 wt.% of condensed phosphates having a chain length higher than or equal to 4, relative to the total weight of the inorganic condensed phosphates.

[0213] As said above, it is required that the solution (P) provides with inorganic condensed phosphates, as detailed above, in an amount of between 0.10 wt.% and 0.50 wt.% relative to the total weight of the dry first solid residue (SRi), or the dry second residue (SR2), or the dry third solid residue (SR3), obtained in step (b).

[0214] The inventors have surprisingly found that using the solution (P) providing with inorganic condensed phosphates, as detailed above, in an amount of between 0.10 wt.% and 0.50 wt.% relative to the total weight of the solid residue obtained after step (b) allows to obtain a desired balance between allows to obtain a desired balance between (1) preventing the clogging of the installation due to the deposition and / or recrystallisation of dihydrate calcium sulfate on the installation’s surface and / or on the separation unit’s surfaces; and (2) allows to obtain calcium sulfate hemihydrate, to spontaneously convert to dihydrate calcium sulfate downstream of the process, in particular when stored.Preferably, the solution (P) as detailed above, provides with inorganic condensed phosphates, as detailed above, in an amount of at least 0.12 wt.%, preferably with at least 0.15 wt.%, even more preferably with at least 0.17 wt.%, even more preferably with at least 0.20 wt.%, even more preferably with at least 0.22 wt.%, even more preferably with at least 0.25 wt.%, relative to the total weight of the dry first solid residue (SR1) or the dry second residue (SR2), or the dry third solid residue (SR3), obtained in step (b).

[0215] It is further understood that the solution (P) as detailed above, preferably provides with inorganic condensed phosphates, in an amount of at most 0.47 wt.%, more preferably with at most 0.45 wt.%, even more preferably with at most 0.42 wt.%, even more preferably with at most 0.40 wt.%, even more preferably with at most 0.37 wt.%, even more preferably with at most 0.35 wt.% relative to the total weight of the dry first solid residue (SR1) or the dry second residue (SR2) or the dry third solid residue (SR3), obtained in step (b).

[0216] According to a preferred embodiment, the solution (P), as detailed above, provides with inorganic condensed phosphates, in an amount of between 0.12 wt.% and 0.47 wt.%, preferably of between 0.15 wt.% and 0.45 wt.%, more preferably of between 0.17 wt.% and 0.42 wt.%, even more preferably of between 0.20 wt.% and 0.40 wt.%, even more preferably of between 0.22 wt.% and 0.37 wt.%, even more preferably of between 0.25 wt.% and 0.35 wt.% relative to the total weight of the dry first solid residue (SR1) or the dry second residue (SR2) or the dry third solid residue (SR3), obtained in step (b).

[0217] Preferably, the solution (P), as detailed above is an aqueous solution.

[0218] Preferably, the solution (P), as detailed above, is an aqueous solution comprising the condensed inorganic phosphate, as detailed above, and process water, as detailed above. Even more preferably, thesolution (P), as detailed above, is an aqueous solution comprising the condensed inorganic phosphate, as detailed above, and process water, as detailed above, wherein said process water comprises less than 5.0 wt.% of phosphorous, expressed in P2O5 equivalent, relative to the total weight of the process water.

[0219] Preferably, the solution (P) comprises the inorganic condensed phosphates, as detailed above, in an amount of at least 250.0 ppm, preferably of at least 300.0 ppm, more preferably of at least 400.0 ppm, even more preferably of at least 500.0, even more preferably 600.0 ppm, even more preferably 700.00 ppm relative to the total weight of the solution (P).

[0220] It will be understood that the solution (P) preferably comprises the inorganic condensed phosphates, as detailed above, in an amount of at most 1200.0 ppm, more preferably of at most 1100.0 ppm, even more preferably of at most 1000.0 ppm, even more preferably of at most 900.0 ppm, even more preferably of at most 850.0 ppm, even more preferably of at most 800.0 ppm, relative to the total weight of the solution (P).

[0221] According to a preferred embodiment of step (c) of the process of the present invention the solution (P) comprises the inorganic condensed phosphates, as detailed above, in an amount of between 250.0 ppm and 1200.0 ppm, preferably of between 300.0 ppm and 1100.0 ppm, more preferably in an amount of between 400.0 ppm and 1000.0 ppm, even more preferably in an amount of between 500.0 ppm and 900.0 ppm, even more preferably in an amount of between 600.0 ppm and 850.0 ppm, even more preferably in an amount of between 700.0 ppm and 800.0 ppm, relative to the total weight of the solution (P).

[0222] It will be understood that the ratio between the solution (P) and the solid residue obtained after step (b), as detailed above, is calculated in such a way that to ensure a homogeneous contacting of thesolution (P) with the first, second of third solid residue (respectively SR1, SR2and SR3).

[0223] Preferably, the solid residue is washed with the solution (P) in a weight ratio dry solid residue: solution (P) of at least 1:0.4, more preferably of at least 1:0.5, even more preferably of at least 1:0.6.

[0224] It is understood that the solid residue is washed with the solution (P) in a weight ratio dry solid residue: solution (P) of at most 1:3, preferably of at most 1:2, preferably of at most 1:1.

[0225] According to a preferred embodiment of step (c) of the process of the present invention, the solid residue is washed with the solution (P) in a weight ratio dry solid residue: solution (P) of between 1:0.4 and 1:3, preferably of between 1:0.5 and 1:2, more preferably of between 1:0.6 and 1:1.

[0226] Preferably, the washing of the first solid residue (SR1), or the second solid residue (SR2), or the third solid residue (SR3), is carried out with the solution (P), as detailed above, wherein the solution (P) is at a temperature of between 40°C and 65°C, more preferably at a temperature of between 50°C and 62°C, even more preferably at a temperature of between 55°C and 60°C, to allow an efficient contact of the condensed inorganic phosphate, as detailed above, with, respectively the first solid residue, the second solid residue or the third solid residue, and an efficient washing of said residues.

[0227] Preferably, the washing of the first solid residue (SR1), or the second solid residue (SR2), or the third solid residue (SR3), with the solution (P), as detailed above, is carried out at a temperature of between 40°C and 65°C, more preferably at a temperature of between 50°C and 62°C, even more preferably at a temperature of between 55°C and 60°C, to allow an efficient contact of the condensed inorganic phosphate, as detailed above, with, respectively the first solid residue, the second solid residue or the third solid residue, and an efficient washing of said residues.As said, according to one embodiment of step (c) of the present invention, the first solid residue (SRi) comprised in the first solid-rich phase (PSR1) and obtained in step (b) is washed with the solution (P), as detailed above.

[0228] The washing of the first solid residue (SR1) comprised in the first solid-rich phase (PSR1) with the solution (P) may be carried out by any means known to the skilled in the art.

[0229] In particular, the washing of the first solid residue (SR1) comprised in the first solid-rich phase (PSR1) with the solution (P) may be carried out on a filtration mean.

[0230] According to this embodiment, the washing of the first solid residue (SR1) comprised in the first solid-rich phase (PSR1) with the solution (P) is preferably carried out on a filtration mean.

[0231] It will be understood that, when the step of washing is preceded by a step of removal of a liquid phase from a solid residue by filtration, the washing is preferably carried out on the same filtration mean as the step of removal.

[0232] The filtration may be carried out under pressure, under depression or at atmospheric pressure.

[0233] The filtration, as detailed above, may be carried out by any filter known to the skilled person in the art, such as for example by using a band filter, a drum filter, a press filter, a tilting pan filter, a rotary tilting pan filter, a table filter, or a belt filter.

[0234] According to this embodiment, the washing of the first solid residue (SR1) comprised in the first solid-rich phase (PSR1) with the solution (P) is preferably carried out using a belt filter, a tilting pan filter, a rotary tilting pan filter, or a table filter more preferably using a belt filter or a rotary tilting pan filter, more preferably using a rotary tilting pan filter.According to this embodiment, the washing of the first solid residue (SR1) comprised in the first solid-rich phase (PSR1) with the solution (P) is preferably a counter-current wash.

[0235] Typically, a counter-current wash is a washing process during which a wash solution flows in opposite direction to the solid residue being washed. A counter-current wash system typically comprises a plurality of washing zones arranged in series, each of said washing zones being typically separated by a plurality of draining zones. The cleanest wash solution typically enters at the final (i.e. the last) washing zone, relative to the direction of the solid residue, where the cleanest wash solution is contacted with a solid residue which has already undergone at least one prior wash. As the washing solution progresses through the counter-current wash system, said washing solution becomes typically more contaminated at it moves towards the initial (i.e. the first) washing zone, relative to the direction of the solid residue. Meanwhile, the solid residue moves in the opposite direction, continuously being in contact with cleaner wash solution at each washing zone. A counter-current wash is particularly desired to provide a cost-effective, resource-efficient approach by reducing water consumption and by reusing the wash solution multiple times.

[0236] In particular, a counter-current wash may comprise the following step, as illustrated in Figure 2:

[0237] (1) conveying the solid-rich phase (PSR) comprising the solid residue (SR) through n zones (Z1,..., Zn-1, Zn) from Z1to Zn, wherein the n zones are arranged in series, and wherein each of the n zones comprises a washing zone (W1,..., Wn-1, Wn) and a draining zone (D1,..., Dn-1, Dn) arranged in series;

[0238] (2) introducing the solution (P) at the nthwashing zone (Wn) of the nthzone (Zn);(3) collecting a filtrate (F1, Fn-1, Fn) in each of the draining zone (D1, Dn-1, Dn) comprised in the n zones (Z1, Zn-1, Zn);

[0239] (4) introducing the filtrate (Fn) obtained from the nthdraining zone (Dn) to the (n-1)thwashing zone (Wn-1) of the (n-1)thzone (Zn-1) and successively introducing the filtrate collected in each of the draining zone to the washing zone of the directly preceding zone. Thus, according to this embodiment, the washing of the first solid residue (SR1) comprised in the first solid-rich phase (PSR1) with the solution (P) by counter-current wash preferably comprises the steps of:

[0240] (1) conveying the first solid-rich phase (PSR1) comprising the first solid residue (SR1) through n zones, preferably at least 2 zones, preferably 2 zones or 3 zones (Z1,..., Zn-1, Zn), from Z1to Zn, wherein the n zones are arranged in series, and wherein each of the n zones comprises a washing zone (W1,..., Wn-1, Wn) and a draining zone (D1,..., Dn-1, Dn) arranged in series;

[0241] (2) introducing the solution (P), as detailed above, at the nthwashing zone (Wn) of the nthzone (Zn);

[0242] (3) collecting a filtrate (F1,..., Fn-1, Fn) in each of the draining zone (D1,..., Dn-1, Dn) comprised in the n zones (Z1,..., Zn-1, Zn);

[0243] (4) introducing the filtrate (Fn) obtained from the nthdraining zone (Dn) to the (n-1)thwashing zone (Wn-1) of the (n-1)thzone (Zn-1) and successively introducing the filtrate collected in each of the draining zone to the washing zone of the directly preceding zone. Each of the filtrate (F1,..., Fn-1, Fn) which has been collected in step (3) may, independently, be further mixed with a solution of the inorganic condensed phosphate, as detailed above, before step (4). The filtrate F1may be reintroduced in step (a) of the process according to the present invention.

[0244] According to another embodiment of step (c) of the present invention, at least part of the second liquid phase (LP2) is removed fromthe second solid residue (SR2) obtained in step (b), thereby forming a third solid-rich phase (PSR3) having a second solid residue (SR2) content higher than 65.0 wt.%, followed by washing, the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) with a solution (P), as detailed above.

[0245] Desirably, the third solid-rich phase (PSR3) according to this embodiment has a second solid residue (SR2) content higher than 67.0 wt.%, more preferably higher than 70.0 wt.%, relative to the total weight of the third solid-rich phase (PSR3).

[0246] Preferably, the third solid-rich phase (PSR3) according to this embodiment has a second solid residue (SR2) content of between 65.0 wt.% and 100 wt.%, more preferably of between 67.0 wt.% and 90.0 wt.%, even more preferably of between 70.0 wt.% and 80.0 wt.%, relative to the total weight of the third solid-rich phase (PSR3).

[0247] The solid content of the third solid-rich phase (PSR3) may be evaluated by calculating the density of the third solid-rich phase (PSR1) thereby obtained, and comparing it with (1) the density of the first liquid phase (LP1) of the second slurry (S2) and (2) the theoretical density of solid calcium sulfate hemihydrate.

[0248] It will be understood that the definitions and preferences as detailed above for the removal of the at least part of the first liquid phase (LP1) from the first solid residue, equally applies to this embodiment.

[0249] Thus, according to this embodiment, the removal of the at least part of the second liquid phase (LP2) from the second solid residue (SR2) may be carried out by any means known to the skilled in the art, such as notably, by filtration, through settling or by decantation. Thus, said removal may be carried out using, for example, a drum filter, a disc filter, a rotary table, a belt filter, a press filter, a vacuum filter, a tilting pan filter, a centrifuge decanter, a centrifuge filter, a hydrocyclone or a decanter.According to this embodiment, the removal of the at least part of the second liquid phase (LP2) from the second solid residue (SR2) is preferably carried out by filtration.

[0250] The filtration may be carried out under pressure, under depression or at atmospheric pressure.

[0251] The filtration, as detailed above, may be carried out by any filter known to the skilled person in the art, such as for example by using a band filter, a drum filter, a press filter, a tilting pan filter, a rotary tilting pan filter, a table filter, or a belt filter.

[0252] According to this embodiment, the removal of at least part of the second liquid phase (LP1) from the second solid residue (SR1) is carried out using a belt filter, a tilting pan filter, a rotary tilting pan filter, or a table filter more preferably using a belt filter or a rotary tilting pan filter, more preferably using a rotary tilting pan filter.

[0253] Typically, when a belt filter or a rotary tilting pan filter is used, said filters comprise a series of filtering cells or filtering sectors. Typically, a slurry to be filtrated in a belt filter or a rotary pan filter is charged on a filtering cell or filtering sector, said filtering cell or filtering sector is moving to a discharge zone. As the filtering cell or filtering sector travels to the discharge zone, the filtering cell or filtering sector may undergo a series of drainages and or washings in order to successively remove part of the liquid comprised in the slurry and / or to wash the solid phase found in the filtering cell or filtering sector. The part of the liquid which has been drained may be recovered and reused.

[0254] According to this embodiment, when the removal of at least part of the second liquid phase (LP2) from the second solid residue (SR2) is carried out using a belt filter or a rotary tilting pan filter, the removal of at least part of the second liquid phase (LP2) from the second solid residue (SR1) comprises the step of:1) charging the second slurry (S2) comprising the second liquid phase (LP2) and the second solid residue (SR2) onto a filtering cell or filtering sector;

[0255] 2) draining from the filtering cell or from the filtering sector the second liquid phase (LP2) from the second solid residue (SR2), in order to obtain the third solid-rich phase (PSR3) as detailed above.

[0256] As said, according to this embodiment, of step (c) of the present invention, the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) is washed with the solution (P), as detailed above.

[0257] The washing of the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) with the solution (P) may be carried out by any means known to the skilled in the art.

[0258] In particular, the washing of the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) with the solution (P) may be carried out on a filtration mean.

[0259] According to this embodiment, the washing of the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) with the solution (P) is preferably carried out on a filtration mean.

[0260] It will be understood that, when the step of washing is preceded by a step of removal of a liquid phase from a solid residue by filtration, the washing is preferably carried out on the same filtration mean as the step of removal.

[0261] The filtration may be carried out under pressure, under depression or at atmospheric pressure.

[0262] The filtration, as detailed above, may be carried out by any filter known to the skilled person in the art, such as for example by using a band filter, a drum filter, a press filter, a tilting pan filter, a rotary tilting pan filter, a table filter, or a belt filter.According to this embodiment, the washing of the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) with the solution (P) is preferably carried out using a belt filter, a tilting pan filter, a rotary tilting pan filter, or a table filter more preferably using a belt filter or a rotary tilting pan filter, more preferably using a rotary tilting pan filter.

[0263] According to this embodiment, the washing of the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) with the solution (P) is preferably a counter-current wash.

[0264] Typically, a counter-current wash is a washing process during which a wash solution flows in opposite direction to the solid residue being washed. A counter-current wash system typically comprises a plurality of washing zones arranged in series, each of said washing zones being typically separated by a plurality of draining zones. The cleanest wash solution typically enters at the final (i.e. the last) washing zone, relative to the direction of the solid residue, where the cleanest wash solution is contacted with a solid residue which has already undergone at least one prior wash. As the washing solution progresses through the counter-current wash system, said washing solution becomes typically more contaminated as it moves towards the initial (i.e. the first) washing zone, relative to the direction of the solid residue. Meanwhile, the solid residue moves in the opposite direction, continuously being in contact with cleaner wash solution at each washing zone. A counter-current wash is particularly desired to provide a cost-effective, resource-efficient approach by reducing water consumption and by reusing the wash solution multiple times.

[0265] In particular, a counter-current wash may comprise the following step, as illustrated in Figure 2:

[0266] (1) conveying the solid-rich phase (PSR) comprising the solid residue (SR) through n zones (Zi,..., Zn-i, Zn) from Zi to Zn, wherein the n zones are arranged in series, and wherein each of the n zonescomprises a washing zone (W1, Wn-1, Wn) and a draining zone (D1, Dn-1, Dn) arranged in series;

[0267] (2) introducing the solution (P) at the nthwashing zone (Wn) of the nthzone (Zn);

[0268] (3) collecting a filtrate (F1, Fn-1, Fn) in each of the draining zone (D1, Dn-1, Dn) comprised in the n zones (Z1, Zn-1, Zn);

[0269] (4) introducing the filtrate (Fn) obtained from the nthdraining zone (Dn) to the (n-1)thwashing zone (Wn-1) of the (n-1)thzone (Zn-1) and successively introducing the filtrate collected in each of the draining zone to the washing zone of the directly preceding zone. Thus, according to this embodiment, the washing of the second solid residue (SR2) comprised in the third solid-rich phase (PSR3) with the solution (P) by counter-current wash preferably comprises the steps of:

[0270] (1) conveying the third solid-rich phase (PSR3) comprising the second solid residue (SR2) through n zones, preferably at least 2 zones, preferably 2 zones or 3 zones (Z1,..., Zn-1, Zn), from Z1to Zn, wherein the n zones are arranged in series, and wherein each of the n zones comprises a washing zone (W1,..., Wn-1, Wn) and a draining zone (D1,..., Dn-1, Dn) arranged in series;

[0271] (2) introducing the solution (P), as detailed above, at the nthwashing zone (Wn) of the nthzone (Zn);

[0272] (3) collecting a filtrate (F1,..., Fn-1, Fn) in each of the draining zone (D1,..., Dn-1, Dn) comprised in the n zones (Z1,..., Zn-1, Zn);

[0273] (4) introducing the filtrate (Fn) obtained from the nthdraining zone (Dn) to the (n-1)thwashing zone (Wn-1) of the (n-1)thzone (Zn-1) and successively introducing the filtrate collected in each of the draining zone to the washing zone of the directly preceding zone. Each of the filtrate (F1, Fn-1, Fn) which has been collected in step (3) may, independently, be further mixed with a solution of the inorganic condensed phosphate, as detailed above, before step (4). The filtrate F1may be reintroduced in step (a) of the process according to the present invention.

[0274] According to another embodiment of step (c) the present invention, at least part of the third liquid phase (LP3) is removed from the third solid residue (SR3) obtained in step (b), thereby forming a fourth solid-rich phase (PSF4) having a third solid residue (SR3) content higher than 65.0 wt.%, followed by washing the third solid residue (SR3) comprised in fourth solid-rich phase (PSR4) with a solution (P).

[0275] Desirably, the fourth solid-rich phase (PSF4) according to this embodiment has a third solid residue (SR3) content higher than 67.0 wt.%, more preferably higher than 70.0 wt.%, relative to the total weight of the fourth solid-rich phase (PSF4).

[0276] Preferably, the fourth solid-rich phase (PSF4) according to this embodiment has a third solid residue (SR3) content of between 65.0 wt.% and 100 wt.%, more preferably of between 67.0 wt.% and 90.0 wt.%, even more preferably of between 70.0 wt.% and 80.0 wt.%, relative to the total weight of the fourth solid-rich phase (PSF4).

[0277] The solid content of the fourth solid-rich phase (PSF4) may be evaluated by calculating the density of the fourth solid-rich phase (PSF4) thereby obtained, and comparing it with (1) the density of the third liquid phase (LP3) of the third slurry (S3) and (2) the theoretical density of solid calcium sulfate hemihydrate.

[0278] It will be understood that the definitions and preferences, as detailed above for the removal of the at least part of the first liquid phase (LP1) from the first solid residue, equally applies to this embodiment.

[0279] Thus, according to this embodiment, the removal of the at least part of the third liquid phase (LP3) from the third solid residue (SR3)may be carried out by any means known to the skilled in the art, such as notably, by filtration, through settling or by decantation. Thus, said removal may be carried out using, for example, a drum filter, a disc filter, a rotary table, a belt filter, a press filter, a vacuum filter, a tilting pan filter, a centrifuge decanter, a centrifuge filter, a hydrocyclone or a decanter.

[0280] According to this embodiment, the removal of the at least part of the third liquid phase (LP3) from the third solid residue (SR3) is preferably carried out by filtration.

[0281] The filtration may be carried out under pressure, under depression or at atmospheric pressure.

[0282] The filtration, as detailed above, may be carried out by any filter known to the skilled person in the art, such as for example by using a band filter, a drum filter, a press filter, a tilting pan filter, a rotary tilting pan filter, a table filter, or a belt filter.

[0283] According to this embodiment, the removal of at least part of the third liquid phase (LP3) from the third solid residue (SR3) is carried out using a belt filter, a tilting pan filter, a rotary tilting pan filter, or a table filter more preferably using a belt filter or a rotary tilting pan filter, more preferably using a rotary tilting pan filter.

[0284] Typically, when a belt filter or a rotary tilting pan filter is used, said filters comprise a series of filtering cells or filtering sectors. Typically, a slurry to be filtrated in a belt filter or a rotary pan filter is charged on a filtering cell or filtering sector, said filtering cell or filtering sector is moving to a discharge zone. As the filtering cell or filtering sector travels to the discharge zone, the filtering cell or filtering sector may undergo a series of drainages and or washings in order to successively remove part of the liquid comprised in the slurry and / or to wash the solid phase found in the filtering cell or filtering sector. The part of the liquid which has been drained may be recovered and reused.According to this embodiment, when the removal of at least part of the third liquid phase (LP3) from the third solid residue (SR3) is carried out using a belt filter or a rotary tilting pan filter, the removal of at least part of the third liquid phase (LP3) from the third solid residue (SR3) comprises the step of:

[0285] 1) charging the third slurry (S3) comprising the third liquid phase (LP3) and the third solid residue (SR3) onto a filtering cell or filtering sector;

[0286] 2) draining from the filtering cell or from the filtering sector the third liquid phase (LP3) from the third solid residue (SR3), in order to obtain the fourth solid-rich phase (PSFM) as detailed above.

[0287] As said, according to this embodiment, of step (c) of the present invention, the third solid residue (SR3) comprised in the fourth solid-rich phase (PSFM) is washed with the solution (P), as detailed above.

[0288] The washing of the third solid residue (SR3) comprised in the fourth solid-rich phase (PSFM) with the solution (P) may be carried out by any means known to the skilled in the art.

[0289] In particular, the washing of the third solid residue (SR3) comprised in the fourth solid-rich phase (PSFM) with the solution (P) may be carried out on a filtration mean.

[0290] According to this embodiment, the washing of the third solid residue (SR3) comprised in the fourth solid-rich phase (PSFM) with the solution (P) is preferably carried out on a filtration mean.

[0291] It will be understood that, when the step of washing is preceded by a step of removal of a liquid phase from a solid residue by filtration, the washing is preferably carried out on the same filtration mean as the step of removal.

[0292] The filtration may be carried out under pressure, under depression or at atmospheric pressure.The filtration, as detailed above, may be carried out by any filter known to the skilled person in the art, such as for example by using a band filter, a drum filter, a press filter, a tilting pan filter, a rotary tilting pan filter, a table filter, or a belt filter.

[0293] According to this embodiment, the washing of the third solid residue (SR3) comprised in the fourth solid-rich phase (PSFM) with the solution (P) is preferably carried out using a belt filter, a tilting pan filter, a rotary tilting pan filter, or a table filter more preferably using a belt filter or a rotary tilting pan filter, more preferably using a rotary tilting pan filter.

[0294] According to this embodiment, the washing of the third solid residue (SR3) comprised in the fourth solid-rich phase (PSFM) with the solution (P) is preferably a counter-current wash.

[0295] Typically, a counter-current wash is a washing process during which a wash solution flows in opposite direction to the solid residue being washed. A counter-current wash system typically comprises a plurality of washing zones arranged in series, each of said washing zones being typically separated by a plurality of draining zones. The cleanest wash solution typically enters at the final (i.e. the last) washing zone, relative to the direction of the solid residue, where the cleanest wash solution is contacted with a solid residue which has already undergone at least one prior wash. As the washing solution progresses through the counter-current wash system, said washing solution becomes typically more contaminated at it moves towards the initial (i.e. the first) washing zone, relative to the direction of the solid residue. Meanwhile, the solid residue moves in the opposite direction, continuously being in contact with cleaner wash solution at each washing zone. A counter-current wash is particularly desired to provide a cost-effective, resource-efficient approach by reducing water consumption and by reusing the wash solution multiple times.In particular, a counter-current wash may comprise the following step, as illustrated in Figure 2:

[0296] (1) conveying the solid-rich phase (PSR) comprising the solid residue (SR) through n zones (Z1,..., Zn-1, Zn) from Z1to Zn, wherein the n zones are arranged in series, and wherein each of the n zones comprises a washing zone (W1,..., Wn-1, Wn) and a draining zone (D1,..., Dn-1, Dn) arranged in series;

[0297] (2) introducing the solution (P) at the nthwashing zone (Wn) of the nthzone (Zn);

[0298] (3) collecting a filtrate (F1,..., Fn-1, Fn) in each of the draining zone (D1,..., Dn-1, Dn) comprised in the n zones (Z1,..., Zn-1, Zn);

[0299] (4) introducing the filtrate (Fn) obtained from the nthdraining zone (Dn) to the (n-1)thwashing zone (Wn-i) of the (n-1)thzone (Zn-i) and successively introducing the filtrate collected in each of the draining zone to the washing zone of the directly preceding zone.

[0300] Thus, according to this embodiment, the washing of third solid residue (SR3) comprised in the fourth solid-rich phase (PSFM) with the solution (P) by counter-current wash preferably comprises the steps of:

[0301] (1) conveying the fourth solid-rich phase (PSFM) comprising the third solid residue (SR3) through n zones, preferably at least 2 zones, preferably 2 zones or 3 zones (Zi,..., Zn-1, Zn), from Zi to Zn, wherein the n zones are arranged in series, and wherein each of the n zones comprises a washing zone (W1,..., Wn-1, Wn) and a draining zone (Di,..., Dn-1, Dn) arranged in series;

[0302] (2) introducing the solution (P), as detailed above, at the nthwashing zone (Wn) of the nthzone (Zn);

[0303] (3) collecting a filtrate (F1,..., Fn-1, Fn) in each of the draining zone (Di,..., Dn-1, Dn) comprised in the n zones (Zi,..., Zn-1, Zn);

[0304] (4) introducing the filtrate (Fn) obtained from the nthdraining zone (Dn) to the (n-1)thwashing zone (Wn-1) of the (n-1)thzone (Zn-1) andsuccessively introducing the filtrate collected in each of the draining zone to the washing zone of the directly preceding zone.

[0305] Each of the filtrate (Fi, Fn-i, Fn) which has been collected in step (3) may, independently, be further mixed with a solution of the inorganic condensed phosphate, as detailed above, before step (4).

[0306] The filtrate Fi may be reintroduced in step (a) of the process according to the present invention.

[0307] Step (d)

[0308] According to certain embodiments of the process of the present invention, the process further comprises a step (d) of discharging and storing at least part of the first solid residue (SR1) obtained in step (c), or at least part of second solid residue (SR2) obtained in step (c), or at least part of the third solid residue (SR3) obtained in step (c).

[0309] The discharge, as detailed above, may be carried out by any means known to the skilled in the art, such as for example by using a conveyor or a two-way conveyor or a pipe.

[0310] According to certain embodiments of the process of the present invention, prior to step (d), process water, as detailed above, or the solution (P), as detailed above, may be further added to first solid residue (SR1) obtained in step (c), or at least part of second solid residue (SR2) obtained in step (c), or at least part of the third solid residue (SR3) obtained in step (c).

[0311] Preferably, the discharge, as detailed above, is carried out by pouring at least part of the first solid residue (SR1) obtained in step (c), or at least part of second solid residue (SR2) obtained in step (c), or at least part of the third solid residue (SR3) obtained in step (c) in a hopper, followed by the transporting of said at least part of second solid residue obtained in step (c), or at least part of the third solid residue obtained in step (c), to a storage location.According to certain embodiments of step (d) of the present invention, the discharge by pouring at least part of the first solid residue (SR1) obtained in step (c), or at least part of second solid residue (SR2) obtained in step (c), or at least part of the third solid residue (SR3) obtained in step (c) in a hopper, as detailed above, is carried out by further adding process water, as detailed above, or water, or the solution (P), as detailed above to the said at least part of the first solid residue, or at least part of second solid residue, or at least part of the third solid residue, with the solution (P) as detailed above. This is particularly desired in order to avoid clogging of the installation during the transport and / or the discharge.

[0312] It is a further an object of the present invention to provide a process for manufacturing phosphoric acid.

[0313] Typically, upon contacting of a phosphorous source with a sulfuric acid solution allows to obtain a slurry comprising a liquid phase and a solid residue, wherein the liquid phase comprises phosphoric acid.

[0314] Thus, another aspect of the present invention is a process for producing phosphoric acid, wherein said process comprises the steps (a) to (c), as detailed above, and wherein the process comprises a further step of:

[0315] - isolating the first liquid phase (LP1) obtained in step (b);

[0316] as detailed above, or

[0317] - isolating the second liquid phase (LP2) obtained in step (b), as detailed above.

[0318] It will be understood that all definitions and preferences as described above also apply to the process of manufacturing phosphoric acid, as described above.

[0319] FIGURES

[0320] Other advantages and particularities of the preset invention will become apparent from the following description of some particular embodiments of the process according to the present invention. Thedescription is only given by way of example and is not intended to limit the scope of the invention. The reference and reference numerals used in the description relate to the annexed drawings wherein:

[0321] Figure 1 is a diagram of an installation for carrying out the process according to a particular embodiment of the present invention.

[0322] Figure 2 is a diagram of an installation for carrying out washing step according to a particular embodiment of step (c) of the process of the present invention.

[0323] Figure 3 is a diagram representing a process for manufacturing calcium sulfate hemihydrate according to a particular embodiment of the present invention and according to Example E1-E5 and comparatives examples CE1-CE3.

[0324] Figure 4 is a graph of the water of crystallization content of samples obtained according to Examples E1-E5 and comparative examples CE1- CE3, taken before washing step (c) according to the present invention, 3 hours after washing with the solution (P) according to step (c) of the present invention, and 24 hours after washing with the solution (P) according to the present invention, as a function of the concentration of the inorganic condensed phosphates in the solution (P) used for the washing step (c) according to the present invention

[0325] Figure 5 is a column chart representing the content of dihydrate calcium sulfate, in wt.% relative to the total weight of the sample, said samples having been taken 3 hours after washing with the solution (P) according to step (c) of the present invention, and 24 hours after washing with the solution (P) according to the present invention, and according to Examples E1-E5 and comparative examples CE1-CE3.EXAMPLES

[0326] The invention will be now described in more details with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of invention.

[0327] Starting materials

[0328] The following materials are commercial and used for the following experiments without purification:

[0329] - Phosphorus source 1 being a sedimentary rock having the following composition as detailed in Table 1 hereinbelow. Table 1

[0330] Phosphate

[0331] source

[0332] P2O5 (wt.%) 38.5

[0333] CaO (wt.%) 48.3

[0334] Fe2O3 (wt.%) 0.62

[0335] Al2O3 (wt.%) 0.78

[0336]

[0337] - Condensed inorganic phosphate 1 having an average chain length of 11 phosphate units.

[0338] - Solution (P)

[0339] Solution (Pcontrol), and (P1)-(P7) were prepared from the condensed inorganic phosphate 1 by dissolving said condensed inorganic phosphate 1 in process water, as detailed above, according to Table 2 hereinbelow.

[0340] Table 2

[0341] P Pi P2P3 P4P5P6P7control

[0342] Water (m3) 1 0.983 0.975 0.967 0.950 0.925 0.900 0.850 Condensed 0 0.250 0.375 0.500 0.750 1.125 1.500 2.250 phosphate (CP1)

[0343] (kg)

[0344]

[0345] Concentration 0 0.003 0.004 0.005 0.008 0.011 0.015 0.023 CP1 in solution

[0346] (P) (wt.%)

[0347]

[0348] General procedure for the determination of the P2O5 content of the phosphorous source:

[0349] The phosphorus content of the phosphorous source, expressed as wt.% P2O5 equivalent, relative to the total weight of the phosphorous source, was determined according to standard EN156959:2023 or is measured by colorimetry of phospho-vanado- molybdate at 436 nm by light absorption colorimetry.

[0350] General procedure for the determination of the calcium, iron and aluminum content of the phosphorous source

[0351] The calcium, iron and aluminum content, expressed as, respectively, wt.% CaO equivalent, wt.% Fe2O3 equivalent and wt.% Al2O3 equivalent, relative to the total weight of the sample to be analyzed was measured by ICP-OES (Inductively coupled Plasma Optical Emission Spectroscopy).

[0352] General procedure for the determination of the average chain length of the inorganic condensed phosphates:

[0353] The average chain length of the inorganic condensed phosphates was determined by potentiometric titration of said inorganic condensed phosphates using a standardized 0.1 mol / L solution of NaOH and a standardized 0.1 mol / L solution of HCI.

[0354] General procedure for the determination of the water of crystallization content:

[0355] An aliquot of the sample to be analyzed was taken and weighted (maliquot). The aliquot was dried in an oven overnight at a temperature of 50°C and the resulting aliquot dried at 50°C was weighted (maliquot - 50 °C). Said aliquot dried at 50°C was further dried at overnight ata temperature of 250°C. The resulting aliquot further dried at 250°C was weighted (maliquot-250°C).

[0356] The water of crystallization (WC), expressed as wt.% is calculated according to the following formula:

[0357] WC (wt. %) = (maliquot-50°C - maliquot-250°C) / maliquot-250°C

[0358] WC (wt. %) =

[0359]

[0360] maliquot-250°C

[0361] General procedure for the calculation of the percentages of CaSO4. V2H2O and CaSO4.2H2O

[0362] Theoretically, the water of crystallization content of CaSO4.

[0363] ½H2O, WC(CaSO4.½H2O)th is 6.2 wt.%, relative to the total weight of CaSO4.½H2O.

[0364] Furthermore, theoretically, the crystal water content of CaSO4.2H2O, WC(CaS04. 2H2O)th is 20.9 wt.%, relative to the total weight of CaSO4.2H2O.

[0365] For the purpose of the present examples, it was inferred that the percentage of CaSO4.½H2O obtained before the washing step according to step (c) the present invention is of 100 wt.%. Thus, the water of crystallization content determined by the general procedure as detailed above and obtained before the washing step according to the present invention corresponds to the experimental water of crystallization content value of CaSO4.½H2O (WC(CaSO4.½H2O)exp).

[0366] Based on the experimental water of crystallization content value of CaSO4.½H2O and by ponderation with the theoretical crystal water content of CaSO4.½H2O and CaSO4.2H2O, the experimental water of crystallization content value of CaSO4.2H2O (WC(CaSO4.2H2O)exp) is obtained as follows:

[0367] WC(CaS04. y2H20)thWC(CaSO4.2H2O)exp= WC(CaS04.1 / 2H2O)ezp

[0368] WC(CaS04.2H20)thBased on the WC(CaSO4. ½H2O)exp, and based on the calculated WC(CaSO4. 2H2O)exp, it is possible to calculate the percentage of each CaSO4. ½H2O and CaSO4. 2H2O, as follows:

[0369] calcualted WC — WC(CaSO4. ½H2O)expwt. % (CaS04.2H20) =

[0370] WC(CaSO4. 2H2O)exp- WC(CaSO4. ½H2O)exp

[0371] Example 1 (El )

[0372] Step (a)

[0373] As illustrated in Figure 3, the phosphorous source 1, as detailed above, whose composition is detailed in Table 1, is provided into a digester 1 via inlet 2. The phosphorous source 1 was further contacted with a sulfuric acid solution.

[0374] The sulfuric acid solution was formed in the mixing tee 3 by the mixing of a sulfuric acid solution having a sulfuric acid content of 98.5 wt.%, inserted via inner inlet 4 of mixing tee 3 and a recycled phosphoric acid, as detailed above via outer inlet 5.

[0375] The temperature in the digester 1 is of around 78°C. A first slurry comprising a first solid residue comprising dihydrate calcium sulfate, and a first liquid phase having an acidity of 32.8-33.8 wt.% (32-33 wt.% P2O5 and 0.8 wt.% SO3) is obtained. The first solid residue comprises dihydrate calcium sulfate.

[0376] Step (b)

[0377] As illustrated in Figure 3, the first slurry was conveyed via line 6 to Prayon’s rotary tilting pan filter 7, in which 50 wt.% the liquid phase is removed from the first solid residue, in order to obtain a first solid-rich phase having a first solid content of around 50 wt.%. No washing was performed.The first solid-rich phase was discharged via line 8 and placed into a conversion tank 9.

[0378] A third sulfuric acid solution comprising 98.0 wt.% of sulfuric acid, expressed in SO3 equivalent relative to the total weight of the third sulfuric acid solution was added to the conversion tank 8 via line 10, at a temperature of 100°C, thereby forming a third slurry comprising a third solid residue and a third liquid phase, in order to obtain a third liquid phase having an acidity of 36-38 wt.% (30 wt.% P2O5 and 6-8 wt.% SO3). The third solid residue consists essentially of calcium sulfate hemihydrate, as detailed above, relative to the total weight of the dry third solid residue.

[0379] Step (c)

[0380] As illustrated in Figure 3, the resulting third slurry was conveyed via line 11 to Prayon’s rotary tilting pan filter 12 in which around 75 wt.% the liquid phase is removed from the third solid residue, in order to obtain a third solid-rich phase having a third solid residue content higher of around 75.0 wt.%.

[0381] The third solid-rich phase, found on the filter cloth of the rotary tilting pan filter 12 was further washed with solution (Pi ) via line 13, as defined in Table 2, in an amount as detailed in Table 3 and at a temperature of 65 °C.

[0382] The thereby obtained washed third solid residue was further discharged from the filter cloth via line 14 and stored.

[0383] Determination of the water of crystallization content and calculated percentages of CaSO4.½H2O and CaSO4.2H2O

[0384] An aliquot of the second solid residue was taken from line 14 and analyzed for its water of crystallization content according to the general procedure described hereinabove, before the washing (t=0), 3 hours after the washing with the solution (Pi ) (t=3h) and 24 hours after thewashing with the solution (Pi) (t=24h). Results are shown in Table 3 and illustrated in Figure 4.

[0385] With the proviso that at t=0, the aliquot comprises 100 wt.% of CaSO4.1 / 2H2O, the percentages of CaSO4.1 / 2H2O and CaSO4.2H2O, 3 hours after the washing with the solution (Pi) and 24 hours after the washing with the solution (Pi) were calculated according to the general procedure described hereinabove. Results are shown in Table 3 and illustrated in Figure 5.

[0386] It was observed that 3 hours after the washing with the solution (Pi), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 10 wt.% of calcium sulfate hemihydrate.

[0387] It was also observed that 24 hours after the washing with the solution (Pi), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 21 wt.% of calcium sulfate hemihydrate.

[0388] Example 2 (E2)

[0389] Example 2 was realized in the same manner as Example 1 described herein-above, except that the second solid residue, was washed with solution (P2). The water of crystallization content before the washing (t=0), 3 hours after the washing with the solution ( P2) (t=3h), and 24 hours after the washing with the solution (P2) (t=24h) are shown in Table 3 and illustrated in Figure 4.

[0390] The percentages of CaSO4.1 / 2H2O and CaSO4.2H2O, 3 hours after the washing with the solution ( P2) and 24 hours with the solution ( P2) are shown in Table 3 and illustrated in Figure 5.

[0391] It was observed that 3 hours after the washing with the solution (P2), calcium sulfate hemihydrate has spontaneously convertedto dihydrate calcium sulfate to obtain 12 wt.% of calcium sulfate hemihydrate.

[0392] It was also observed that 24 hours after the washing with the solution (P2), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 24 wt.% of calcium sulfate hemihydrate.

[0393] Thus, the conversion of calcium sulfate hemihydrate to dihydrate calcium sulfate occurs at a substantially similar rate as example 1.

[0394] Example 3 (E3)

[0395] Example 3 was realized in the same manner as Example 1 described herein-above, except that the second solid residue, was washed with solution (Pa). The water of crystallization content before the washing (t=0), 3 hours after the washing with the solution (Pa) (t=3h), and 24 hours after the washing with the solution (Pa) (t=24h) are shown in Table 3 and illustrated in Figure 4.

[0396] The percentages of CaSO4.1 / 2H2O and CaSO4.2H2O, 3 hours after the washing with the solution (Pa) and 24 hours with the solution (Pa) are shown in Table 3 and illustrated in Figure 5.

[0397] It was observed that 3 hours after the washing with the solution (Pa), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 12 wt.% of calcium sulfate hemihydrate.

[0398] It was also observed that 24 hours after the washing with the solution (Pa), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 22 wt.% of calcium sulfate hemihydrate.

[0399] Thus, the conversion of calcium sulfate hemihydrate to dihydrate calcium sulfate occurs at a substantially similar rate as example 1 and example 2.Example 4 (E4)

[0400] Example 4 was realized in the same manner as Example 1 described herein-above, except that the second solid residue, was washed with solution (P4). The water of crystallization content before the washing (t=0), 3 hours after the washing with the solution ( P4) (t=3h), and 24 hours after the washing with the solution (P4) (t=24h) are shown in Table 3 and illustrated in Figure 4.

[0401] The percentages of CaSO4.1 / 2H2O and CaSO4.2H2O, 3 hours after the washing with the solution ( P4) and 24 hours with the solution ( P4) are shown in Table 3 and illustrated in Figure 5.

[0402] It was observed that 3 hours after the washing with the solution (P4), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 11 wt.% of calcium sulfate hemihydrate.

[0403] It was also observed that 24 hours after the washing with the solution (Pa), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 20 wt.% of calcium sulfate hemihydrate.

[0404] Thus, the conversion of calcium sulfate hemihydrate to dihydrate calcium sulfate occurs at a substantially similar rate as example 1, example 2 and example 3.

[0405] Example 5 (E5)

[0406] Example 5 was realized in the same manner as Example 1 described herein-above, except that the second solid residue, was washed with solution (P5). The water of crystallization content before the washing (t=0), 3 hours after the washing with the solution ( Ps) (t=3h), and 24 hours after the washing with the solution (Ps) (t=24h) are shown in Table 3 and illustrated in Figure 4.The percentages of CaSO4.½H2O and CaSO4.2H2O, 3 hours after the washing with the solution (P5) and 24 hours with the solution (P5) are shown in Table 3 and illustrated in Figure 5. It was observed that 3 hours after the washing with the solution (P5), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 8 wt.% of calcium sulfate hemihydrate.

[0407] It was also observed that 24 hours after the washing with the solution (Pcontroi), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 20 wt.% of calcium sulfate hemihydrate.

[0408] Thus, the conversion of calcium sulfate hemihydrate to dihydrate calcium sulfate occurs at a substantially similar rate as example 1, example 2, example 3, and example 4.

[0409] Comparative example 1 (CE1)

[0410] Comparative example 1 was realized in the same manner as Example 1 described herein-above, except that the second solid residue, was washed with solution (Pcontroi). The water of crystallization content before the washing (t=0), 3 hours after the washing with the solution (Pcontroi) (t=3h), and 24 hours after the washing with the solution (Pcontroi) (t=24h) are shown in Table 3 and illustrated in Figure 4.

[0411] The percentages of CaSO4.½H2O and CaSO4.2H2O, 3 hours after the washing with the solution (Pcontrol) and 24 hours with the solution (Pcontrol) are shown in Table 3 and illustrated in Figure 5.

[0412] It was observed that 3 hours after the washing with the solution (Pcontroi), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 22 wt.% of calcium sulfate hemihydrate.It was also observed that 24 hours after the washing with the solution (Pcontroi), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 37 wt.% of calcium sulfate hemihydrate.

[0413] Thus, the conversion of calcium sulfate hemihydrate to dihydrate calcium sulfate occurs at a substantially higher rate as example 1, example 2 and example 3.

[0414] Comparative example 2 (CE2)

[0415] Comparative example 2 was realized in the same manner as Example 1 described herein-above, except that the second solid residue, was washed with solution (Pe). The water of crystallization content before the washing (t=0), 3 hours after the washing with the solution (Pe) (t=3h), and 24 hours after the washing with the solution (Pe) (t=24h) are shown in Table 3 and illustrated in Figure 4.

[0416] The percentages of CaSO4.1 / 2H2O and CaSO4.2H2O, 3 hours after the washing with the solution ( Pe) and 24 hours with the solution ( Pe) are shown in Table 3 and illustrated in Figure 5.

[0417] It was observed that 3 hours after the washing with the solution (Pe), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 5 wt.% of calcium sulfate hemihydrate.

[0418] It was also observed that 24 hours after the washing with the solution (Pe), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 13 wt.% of calcium sulfate hemihydrate.

[0419] Thus, the conversion of calcium sulfate hemihydrate to dihydrate calcium sulfate occurs at a substantially slower rate as example 1, example 2, example 3 and example 4.Comparative example 3 (CE3)

[0420] Comparative example 3 was realized in the same manner as Example 1 described herein-above, except that the second solid residue, was washed with solution (P7). The water of crystallization content before the washing (t=0), 3 hours after the washing with the solution (P7) (t=3h), and 24 hours after the washing with the solution (P7) (t=24h) are shown in Table 3 and illustrated in Figure 4.

[0421] The percentages of CaSO4.1 / 2H2O and CaSO4.2H2O, 3 hours after the washing with the solution (P7) and 24 hours with the solution (P7) are shown in Table 3 and illustrated in Figure 5.

[0422] It was observed that 3 hours after the washing with the solution (P7), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 5 wt.% of calcium sulfate hemihydrate.

[0423] It was also observed that 24 hours after the washing with the solution (P7), calcium sulfate hemihydrate has spontaneously converted to dihydrate calcium sulfate to obtain 13 wt.% of calcium sulfate hemihydrate. Thus, the conversion of calcium sulfate hemihydrate to dihydrate calcium sulfate occurs at a substantially slower rate as example 1, example 2, example 3 and example 4, and at a substantially similar rate as comparative example 2.

[0424]

[0425] Table 3

[0426] CE1 E1 E2 E3 E4 E5 CE2 CE3 Solution (P) used Pcontrol Pi P2P3P4P5P6P7Mass of inorganic condensed 0 1 1.5 2 3 4.5 6 9 phosphates 1 (g) per tons of dry

[0427] third solid residue

[0428] Results

[0429] WC at t=0 (%) 5.4 5.4 5.4 5.4 5.4 5.4 5.4 5.4 t=0 % (CaSO4.1 / 2H2O)exp(wt.%) 100 100 100 100 100 100 100 100 % (CaSO4. 2H2O)exp(wt.%) 0 0 0 0 0 0 0 0 WC at t=3h (%) 8.2 6.7 6.9 6.9 6.8 6.4 6.0 6.1 t=3h % (CaSO4.1 / 2H2O)exp(wt.%) 78 90 88 88 89 92 95 95 % (CaSO4. 2H2O)exp(wt.%) 22 10 12 12 11 8 5 5 WC at t=24h (%) 10.1 8.0 8.4 8.2 8.0 8.0 7.1 7.0 t=24h % (CaSO4.1 / 2H2O)exp(wt.%) 63 79 76 78 80 80 87 87

[0430] % (CaSO4. 2H2O)exp(wt.%) 37 21 24 22 20 20 13 13

[0431]

Claims

CLAIMS1. A process for manufacturing calcium sulfate hemihydrate, comprising at least the steps of:(a) contacting at least one phosphorus source comprising calcium, with a first sulfuric acid solution (SAi), thereby obtaining a first slurry (Si) comprising a first solid residue (SRi) and a first liquid phase (LPi), wherein the first solid residue (SRi) comprises at least one calcium sulfate, wherein said calcium sulfate is calcium sulfate hemihydrate, or calcium sulfate dihydrate, or a mixture thereof;(b) removing at least part of the first liquid phase (LPi) from the first solid residue (SRi) obtained in step (a), provided that the first solid residue (SRi) consists essentially of calcium sulfate hemihydrate, thereby forming a first solid-rich phase (PSR1) having a first solid residue (SRi) content higher than 65.0 wt.%; oradding a second sulfuric acid solution (SA2) to the first slurry (Si) obtained in step (a), provided that the first solid residue (SRi) comprises calcium sulfate dihydrate, thereby forming a second slurry (S2) comprising a second solid residue (SR2) and a second liquid phase (LP2), wherein the second solid residue (SR2) consists essentially of calcium sulfate hemihydrate; or removing at least part of the first liquid phase (LPi) from the first solid residue (SRi) obtained in step (a), provided that the first solid residue (SRi) comprises calcium sulfate dihydrate, thereby forming a second solid-rich phase (PSR2) having a first solid residue (SRi) content higher than 35.0 wt.%, followed by adding a third sulfuric acid solution (SA3) to the second solid-rich phase (PSR2), thereby forming a third slurry (S3) comprising a third solid residue (SR3) and a third liquid phase (LP3), wherein the thirdsolid residue (SR3) consists essentially of calcium sulfate hemihydrate;(c) washing the first solid residue (SSR1) comprised in the first solid-rich phase (PSR1) obtained in step (b) with a solution (P); or removing at least part of the second liquid phase (LP2) from the second solid residue (SR2) obtained in step (b), thereby forming a third solid-rich Phase (PSR3) having a second solid residue (SR2) content higher than 65.0 wt.%, followed by washing the second solid residue (SR2) comprised in in the third solid-rich phase (PSR3) with a solution (P); orremoving at least part of the third liquid phase (LP3) from the third solid residue (SR3) obtained in step (b), thereby forming a fourth solid-rich phase (PSR4) having a third solid residue (SR3) content higher than 65.0 wt.%, followed by washing the third solid residue (SR3) comprised in in the third solid-rich phase (PSR3) with a solution (P);wherein the solution (P) comprises inorganic condensed phosphates having an average chain length of between 9 and 30 phosphate units; and wherein the solution (P) provides with inorganic condensed phosphates in an amount of between 0.10 wt.% and 0.50 wt.% relative to the total weight of the dry first solid residue (SR1) or the dry second residue (SR2) or the dry third solid residue (SRs) obtained in step (b).

2. The process according to claim 1, wherein the first solid residue (SR1) consists essentially of calcium sulfate hemihydrate; and wherein step (a) is carried out at a temperature of between 90°C and 110°C and wherein the first liquid phase (LP1) obtained in step (a) has an acidity of between 37.5 wt.% and 52.5 wt.%, relative to the total weight of the first liquid phase (LPi); wherein the acidity is defined as being the sum of the weight percentage of phosphoric acid, expressed as P2O5equivalent, and the weight percentage of sulfuric acid, expressed as SO3equivalent, in the first liquid phase (LP1) of the first slurry (S1).

3. The process according to claim 1, wherein the first solid residue (SR1) comprises calcium sulfate dihydrate; wherein step (a) is carried out at a temperature of between 70°C and 85°C; and wherein the first liquid phase (LP1) obtained in step (a) has an acidity of between 27.0 wt.% and 40.0 wt.%, relative to the total weight of the first liquid phase (LP1); wherein the acidity is defined as being the sum of the weight percentage of phosphoric acid, expressed as P2O5 equivalent, and the weight percentage of sulfuric acid, expressed as SO3 equivalent, in the first liquid phase (LP1) of the first slurry (S1).

4. The process according any one of claims 1 to 3, wherein step the steps of removing at least part of the first liquid phase (LP1) from the first solid residue (SR1) obtained in step (a), or the step of removing at least part of the second liquid phase (LP2) from the second solid residue obtained in step (b) (SR2), or the step of removing at least part of the third liquid phase (LP3) from the third solid residue obtained in step (b) (SR3), is carried out by filtration, preferably with a rotary tilting pan filter.

5. The process according to claim 1 or claim 3, wherein the step of adding the second sulfuric acid solution (SA2) is carried out at a temperature of between 90°C and 110°C, thereby forming the second slurry (S2) and wherein the acidity of the second liquid phase (LP2) of the second slurry (S2), is between 30.0 and 45.0 wt.% relative to the total weight of the second liquid phase (LP2).

6. The process according to claim 1 or any one of claims 3 to 5, wherein the step of adding the third sulfuric acid solution (SA3) is carried out at a temperature of between 90°C and 110°C, thereby forming the third slurry (S3) comprising the third solid residue and the third liquid phase, wherein the acidity of the third liquid phase (LP3) of the third slurry(S3), is between 30.0 and 45.0 wt.% relative to the total weight of the third liquid phase (LP3) relative to the total weight of the third liquid phase (LP3).

7. The process according to any one of claims 1 to 6, wherein the inorganic condensed phosphates have an average chain length of between 9.5 and 25, more preferably of between 10.0 and 20.0, more preferably of between 10.5 and 17.5, even more preferably of between 10.8 and 15.0.

8. The process according to any one of claims 1 to 7, wherein the inorganic condensed phosphates comprise at most 1.0 wt.%, or at most 0.5 wt.%, or at most 0.3 wt.% of orthophosphate, relative to the total weight of the inorganic condensed phosphates.

9. The process according to any one of claims 1 to 8, wherein the inorganic condensed phosphates comprise at least 55.0 wt.%, or at least 60.0 wt.%, or at least 65.0 wt.%, or at least 70.0 wt.%, or at least 73 wt.% of condensed phosphates having a chain length higher than or equal to 4, relative to the total weight of the inorganic condensed phosphates.

10. The process according to any one of claims 1 to 9, wherein the solution (P), provides with inorganic condensed phosphates, in an amount of between 0.12 wt.% and 0.47 wt.%, preferably of between 0.15 wt.% and 0.45 wt.%, more preferably of between 0.17 wt.% and 0.42 wt.%, even more preferably of between 0.20 wt.% and 0.40 wt.%, even more preferably of between 0.22 wt.% and 0.37 wt.%, even more preferably of between 0.25 wt.% and 0.35 wt.% relative to the total weight of the dry first solid residue (SR1) or the dry second residue (SR2) or the dry third solid residue (SR3), obtained in step (b).

11. The process according to any one of claims 1 to 10, wherein the solution (P) comprises the inorganic condensed phosphates, in an amount of between 250.0 ppm and 1200.0 ppm, preferably of between 300.0 ppm and 1100.0 ppm, more preferably in an amount ofbetween 400.0 ppm and 1000.0 ppm, even more preferably in an amount of between 500.0 ppm and 900.0 ppm, even more preferably in an amount of between 600.0 ppm and 850.0 ppm, even more preferably in an amount of between 700.0 ppm and 800.0 ppm, relative to the total weight of the solution (P).

12. The process according to any one of claims 1 to 13, wherein the washing of the first solid residue (SRi) comprised in the first solid-rich phase (PSR1) with the solution (P), or the washing of the second solid residue (SR2) comprised in the third solid-rich phase (PSRS) with the solution (P), or the washing of the third solid residue (SR3) comprised in the fourth solid-rich phase (PSR4) with the solution (P) is carried out by filtration.

13. The process according to any one of claims 1 to 14, wherein the washing of the first solid residue (SR1) comprised in the first solid-rich phase (PSR1) with the solution (P), or the washing of the second solid residue (SR2) comprised in the third solid-rich phase (PSRS) with the solution (P), or the washing of the third solid residue (SR3) comprised in the fourth solid-rich phase (PSR4) with the solution (P) is a counter-current wash.

14. The process according to any one of claims 1 to 12, wherein the at least part of the first solid residue (SR1) obtained in step (c), or at least part of second solid residue (SR2) obtained in step (c), or at least part of the third solid residue (SR3) obtained in step (c) is further discharged and stored.

15. A process for manufacturing phosphoric acid comprising at least step (a)-(c) as defined in any one of claims 1 to 14, and wherein the process comprises a further step of:- isolating the first liquid phase (LP1) obtained in step (b); or isolating the second liquid phase (LP2) obtained in step (b).

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