Process for producing a phosphate containing product from a phosphate source by digestion with sulphuric acid

Abstract

The present invention concerns a process for producing a phosphate containing product from a phosphate source (P0), the process comprising the following steps: feeding a reactor with a raw material (PC1) comprising at least 10 wt.% P2O5, and at least 0.7 wt.% CaO, in a digestion step (D1), adding to the raw material (PC1) a digestion liquor (L) comprising H2SO4 to yield a digested suspension (PC2), in a separation step (S1), separating the digested suspension (PC2) into an aqueous phosphate rich solution (P1) a calcium rich first solid cake (C1). Characterized in that, the digestion liquor (L) is added to the at least one raw material (PC1) in amounts such that 0.80 < SO4 / Ca < 1.7.

Description

PROCESS FOR PRODUCING A PHOSPHATE CONTAINING PRODUCT FROM A PHOSPHATESOURCE BY DIGESTION WITH SULPHURIC ACIDFIELD OF THE INVENTION

[0001] The present invention relates to a process for producing a phosphate containing product from a phosphate source (P0) by digestion with in particular sulphuric acid. The phosphate source can comprise phosphate ores or ashes from different origins, industrial or municipal. The present invention enhances extraction from phosphates source having high values of MER (= (%AI2O3+ %MgO + %Fe2O3) I %P2O5).BACKGROUND OF THE INVENTION

[0002] Phosphate sources can be issued from ore, typically containing apatite. Because of a combination of reduction of reserves of high P2Os contents ores and of limited accessibility for geo-political reasons, phosphate producers have to work with ores of lower quality (such as beneficiation residues). Circular economy pushes industries to (re-) use resources which were previously used. In this context, ashes from the incineration can be an interesting alternative phosphate source. The ashes can be ashes from the incineration of sewage, animal bones, and the like. These phosphate sources comprise numerous other components which need be separated to reach the levels of purity required by the corresponding end applications. To this purpose, raw material issued from the phosphate source can be digested in a first step in a digestion liquor to solubilize the phosphate ions and maintain as many other components as possible insoluble. This digestion step is often followed by a separation step, to separate a solid residue mainly composed of insoluble components from a liquid filtrate containing the dissolved phosphates. The selection of the conditions used in this first step, including the composition of the digestion liquor, the duration of the digestion, the temperature, the pH, etc., is often a trade-off between ensuring that a substantial amount of the phosphates present in the raw material is indeed solubilized and present in the liquid filtrate after separation, and preventing as many other components as possible from solubilizing too and thus remaining in the filtrate, unseparated from the phosphates. If the digestion is too weak, phosphates can be retained in the raw material in the insoluble form and if, on the other hand, digestion is too strong, many impurities will be dissolved too and entrained with the phosphates in the liquid filtrate.

[0003] Most ores and many ashes comprise certain amounts of CaO and I or sulphates (SO42). Undesired calcium can be precipitated as calcium sulphates (= CaSO4.nH2O) by addition of sulphuric acid (= H2SC>4). Digesting the raw material with sulphuric acid in dihydrate I hemi-hydrate process mode can, however, be detrimental to the efficacy of the process, because of co-precipitation of calcium phosphates. In particular, addition of sulphuric acid introduces sulphate ions (SO42) which need beeliminated and may require in some cases addition of Ca2+ions to precipitate calcium sulphates (CaSC .nFW) with different degrees of hydration (i.e., n = 0 to 2). If calcium sulphates co-precipitate with substantial amounts of P2O5 and I or if P2O5 is entrapped in calcium sulphate crystals the recovery in phosphorus is decreased accordingly, as it cannot easily be dissociated after co-precipitation or entrapment. Moreover, the dihydrate-reaction of phosphate rock (e.g., Cas(PO4)2) with sulphuric acid (H2SO4) yields more calcium sulphates than phosphoric acid, i.e., more side products than end-products.

[0004] Digesting raw material with sulphuric acid may also lead to the formation of hemihydrate or anhydride calcium sulphate. Calcium sulphate hemihydrate (= CaSC .^FW) is prompt to rehydrate and set to form dihydrate plaster (= CaSO4.2H2O) which can block the production line, generating extra costs either by stopping the production for the time required to unclog and clean the equipment or by including spare pieces of equipment to switch the production through the spare pieces. Anhydride (= CaSC ) is difficult to filter, thus hindering the separation of liquid from solid phases.

[0005] Because of the decreasing access to high quality ores for the extract phosphates, ores of lower quality, i.e., comprising more other components, which must thus be separated in later steps. The Minor Element Ratio, (MER = (%AbO3 + %MgO + %Fe2C>3) I %P2C>5), is a ratio known in the art which is representative of the total amount of selected impurities (viz., Al, Mg, Fe) expressed as their oxides equivalent relative to the amount of total phosphorus expressed as P2O5 equivalent present in the phosphate source. It is generally more difficult to extract a high proportion of phosphates present in a phosphate source having a high value of the MER than in a phosphate source having a lower value of MER.

[0006] EP4201880 addresses relatively pure sources of phosphates, with a MER of 0.046.WO2019030403 proposes to digest a phosphate source containing less than 6 wt.% P2O5 and having high MER-values with sulphuric acid (= H2SO4) such as to have a SO41 Ca ratio of 0.6 to 0.8 and separate the solid from the liquid filtrate to precipitate calcium sulphates (= CaSC .nFhO). Similarly, GB793801 describes a process for the recovery of values from phosphate rock including the steps of leaching the rock with sulphuric acid and separating by filtration the liquid dissolved filtrate from the undissolved retentate. The foregoing documents do not address the issue of phosphate extraction from phosphate sources having high values of MER. CN20161717263 describes a process for producing phosphoric acid and hemihydrate calcium sulphate powder by a conventional hemihydrate route with a digestion molar SO41 Ca ratio of 0.3 to 0.8 in the solution. The phosphates thus produced can be used in a variety of applications, including the production of fertilizers as described e.g., in Gunnar Kongshaug et al. "Phosphate Fertilizers" In: "Ullmann’s Encyclopedia of Industrial Chemistry", 26 March 2014 (2014-03-26), Wiley- VCH Verlag, The hemihydrate route can, however, not be applied with phosphate sources having a high value of MER, e.g., higher than or equal to 0.051 .

[0007] There is a need in the art for a simple and reliable process for extracting phosphates efficiently from a raw material containing certain amounts of undesired components characterized by high values of MER. The present invention proposes such simple process which allows extracting substantial amounts of phosphorus contained in a phosphate source with high MER. Less separation steps are therefore required for separating the undesired components and less phosphorus remain trapped as solids in the cake forming the solid residue. These and other advantages of the present invention are explained more in detail in the following sections.SUMMARY OF THE INVENTION

[0008] The objectives of the present invention were reached with a process for producing a phosphate containing product from a phosphate source (P0), the process comprising the following steps: a feeding step, a digestion step, and a separation step.

[0009] The feeding step includes feeding one or more reactors with at least one raw material (PC1) issued from the phosphate source (P0), the at least one raw material comprising a phosphorus required amount expressed in equivalent P2O5 amounts (= wt.% P2O5) of at least 10 wt.% P2O5, and a required calcium amount expressed in equivalent CaO amounts (= wt.% CaO) of at least 0.7 wt.% CaO.

[0010] The digestion step includes adding to the at least one raw material in the reactor a digestion liquor for digesting the at least one raw material and yielding a digested suspension, wherein the digestion liquor is an aqueous solution of sulphuric acid (H2SO4) and optionally of one or more mineral acids, wherein a ratio (H+(SA) I H+(L)) x 100% of a mole content of H+issued from the sulphuric acid (= H+(SA)) to a total mole content of H+ions in the digestion liquor (L) (= H+(L)) is comprised between 50 and 100%.

[0011] The separation step includes separating the digested suspension into, on the one hand, an aqueous phosphate rich solution comprising phosphate ions and, on the other hand, a first solid phase (C1) containing calcium sulphate and impurities.

[0012] The present invention is particularly suitable for phosphate sources characterized by a Minor Element Ratio, (MER = (%AbO3 + %MgO + %Fe2C>3) I %P2C>5), representative of an amount of selected impurities present in the phosphate source, which is equal to at least 0.051 (i.e., MER > 0.051), wherein (%AbO3, %MgO, %Fe2Os, and %P2C>5 refer to the weight % of each element expressed in their corresponding oxide forms in the phosphate source (P0)). The gist of the present invention is to leach the raw material issued from such phosphate sources by adding the digestion liquor to the at least one raw material in amounts such that a molar ratio (SO41 Ca) of the total amount of sulphate ions (SO4) in the digestion liquor to a total amount of Ca atoms (= Ca) present in both digestion liquor (L) and at least one raw material is more than 0.80 and is less than 1 .7 (i.e., 0.80 < SO4 / Ca < 1 .7), and is preferably comprised between 0.81 and 0.99 (i.e., 0.81 < SO4 / Ca < 0.99), more preferably between 0.85 and 0.98 (i.e., 0.85 < SO4 / Ca < 0.98), or between 1 .0 and 1 .6 (i.e., 1 .0 < SO4 / Ca < 1 .6).

[0013] The solid phosphate source (P0) can either:• contain calcium in an amount of at least the required calcium amount and thus form the at least one raw material (PC1) (i.e., PC1 = P0), or• contain less than the required calcium amount, and the at least one raw material would be formed in a raw material formation step (MO) by addition of a calcium compound to the phosphate source to form the at least one raw material with the required amounts of calcium (i.e., PC1 c P0 + CO). The calcium compound can be chosen among, o calcium oxide, lime derivatives including quick lime, slaked lime, pulverized lime, lime milk, finely ground limestone o calcium carbonate, o calcium phosphate salts (mono di-; tri-), o kaolin, o calcium hydroxide, o calcium sulphate, o calcium fluoride, e a second phosphate source containing calcium, o recirculated fraction of digested suspension (PC2) or of aqueous phosphate rich solution; o phosphoric acid, or o a mixture thereof.

[0014] The phosphate source can for example be chosen among one or more of the following origins:• phosphate ores,• beneficiation residues including one or more of phosphate tailing and phosphate slimes,• phosphate salts,• ashes, preferably ashes from one or more of incinerated sewage sludge, bones, manure,• black mass.

[0015] The phosphate source can be used alone or admixed with a second phosphate source (P02) of different origin than the phosphate source, wherein the admixing occurs either in the reactor or prior to feeding the phosphate source (P0) into the reactor. The phosphate source is preferably characterized by the Minor Element Ratio (MER) equal to at least 0.060 (i.e., MER > 0.060), and is preferably comprised between 0.100 and 1 .500 (i.e., 0.100 < MER < 1 .500), preferably between 0.140 and 1 .500 (i.e., 0.140 < MER < 1.500), preferably between 0.250 and 1.400, preferably between 0.300 and 1.000; preferably between 0.500 and 0.900.

[0016] The at least one raw material (PC1) preferably comprises phosphorus expressed as P2O5 equivalent in amounts comprised between 10 and 50 wt.% P2O5, preferably between 12 and 45 wt.% P2O5, more preferably between 14 and 40 wt.% P2O5, more preferably between 15 and 38 wt.% P2O5, more preferably between 18 and 35 wt.% P2O5, more preferably between 19 and 30 wt.% P2O5, more preferably between 20 and 27.5 wt.% P2O5, more preferably between22.5 and 25 wt.% P2O5.

[0017] In a preferred embodiment, a portion of the digested suspension and / or a portion of the aqueous phosphate rich solution is recirculated into the digestion step as a component of the digestion liquor or as a calcium source. Each of the portion of the digested suspension and I or of the portion of the aqueous phosphate rich solution being recirculated is preferably comprised between 1 and 50%, more preferably between 5 and 30%, most preferably between 10 and 20% of the total flow of the corresponding digested suspension and I or aqueous phosphate rich solution, respectively.

[0018] The digestion step preferably occurs at a temperature lower than 100°C, preferably lower than 90°C, preferably lower than 80°C, more preferably between 25°C and 75°C; more preferably between 30°C and 72°C; more preferably between 60°C and 71 °C; preferably the temperature is 65°C. The pressure can be atmospheric or with a light depression, of about 0.07 + 0.02 bar. The digestion step) can have a duration lower than 180 min, preferably lower than 90 min, more preferably lower than 60 min, more preferably lower than 50 min or of 45 min + 5 min. The duration is preferably at least 5 min, more preferably at least 10 min.

[0019] The digestion liquor is preferably characterized by a combined concentration of the sulphuric acid and the one or more mineral acids forming the digestion liquor comprised between 3 wt.% and 30 wt.%, preferably between 4 and 25 wt.%; preferably between 5 and 20 wt.%, more preferably between 6 and 15 wt.%; most preferably between 7 and 12 wt.% or is equal to 7 + 2 wt.%, or 10 wt.%. The ratio (H+(SA) I H+(L)) x 100% of the mole content of H+issued from the sulphuric acid (= H+(SA)) to the total amount of H+ions in the digestion liquor (= H+(L)) is comprised between 70 and 99 %, preferably between 80 and 98%, more preferably between 85% and 97%, preferably between 90% and 95%.

[0020] In a preferred embodiment, in particular wherein the phosphate source has an MER lower than 0.250, the molar ratio (SO41 Ca) in the digestion step (D1) is sub-stoichiometric (i.e., SO4 / Ca < 1) and is preferably greater than 0.8 and less than 1 .0, preferably at least 0.81 and less than 0.99; more preferably comprised between 0.85 and 0.98; most preferably comprised between 0.87 and 0.95; and is most preferably 0.90 + 0.02.

[0021] In an alternative embodiment, in particular wherein the phosphate source has an MER higher than 0.250, the molar ratio (SO41 Ca) in the digestion step (D1) is sur-stoichiometric or “in-excess” (i.e., SO4 / Ca > 1) and is preferably comprised between 1 and 1 .6, preferably, between 1 .05 and 1 .5; more preferably comprised between 1 .10 and 1 .20, most preferably comprised between 1 .15 and 1.17.

[0022] In a preferred embodiment, the aqueous phosphate rich solution is neutralized in a neutralizing step to form dicalcium phosphate (DCP). The neutralisation step is preferably performed in presence of a source of calcium to reach a pH preferably comprised between 3 and 6. The source of calcium can be chosen among,• lime derivatives including quick lime, slaked lime, pulverized lime, lime milk,• calcium carbonate or finely ground limestone,• calcium hydroxide,• calcium phosphate, or• a mixture thereof.

[0023] The DCP thus produced can be further processed to produce,• a fertilizer, preferably of type monoammonium phosphate (MAP), diammonium phosphate (DAP), Ammonium-Potassium Phosphate fertilizer (NPK fertilizer),• animal feed,• food grade products,• merchant grade or purified phosphoric acid or technical grade phosphoric acid.BRIEF DESCRIPTION OF THE FIGURES

[0024] On these figures,Fig. 1 shows the process steps of the present invention in its broadest scope.Fig. 2(a) & 2(b) show the process steps of two preferred embodiments of the present invention.Fig. 3 shows the process steps of another preferred embodiment.Fig. 4 shows the process steps of yet another preferred embodiment.Fig. 5 shows a graph plotting the filtering time and P2O5 loss as a function of the ratio H+(SA) I H+(L) (wt.%).Fig. 6(a) & 6(b) plots the yield of P2O5 extracted from phosphate sources as a function of MER with a sulphate to calcium ratio SC>4 / Ca = 0.8 (white circles) and, according to the present invention, SO4 / Ca = 0.9, with sulphuric acid at a concentration (a) of 7 wt.% and (b) of 10 wt.%.Fig. 6(c) plots the P2O5-yield difference between a sulphate to calcium ratio SO4 / Ca = 0.8 vs 0.9 as a function of MER; white circles [H2SO4] = 10 wt.%. and black circles [H2SO4] = 10 wt.%.Fig. 7(a) plots the P2O5-yield of Figures 6(a) and 6(b) at MER = 0.05 with H2SO4 concentrations of 7 and 10 wt.%; white column = SO4 / Ca = 0.8, and black columns = SO4 / Ca = 0.9 (according to the invention).Fig. 7(b) plots the P2O5-yield of Figures 6(a) and 6(b) at MER = 0.82 with H2SO4 concentrations of 7 and 10 wt.%; white column = SO4 / Ca = 0.8, and black columns = SO4 / Ca = 0.9 (according to the invention).Fig.8 plots the P2O5-yield in the aqueous phosphate rich solution (P1) as a function of digestionconditions (SO4 / Ca) for the source S5 at H2SO4 solution concentration of 10wt%.DETAILED DESCRIPTION.

[0025] As shown in Figure 1 , the process of the present invention for producing a phosphate containing product from a phosphate source (P0) comprises the following steps.

[0026] One or more reactors are fed with at least one raw material (PC1) issued from the phosphate source (P0). The at least one raw material (PC1) comprises,• a phosphorus required amount expressed in equivalent P2O5 amounts (= wt.% P2O5) of at least 10 wt.% P2O5, and• a required calcium amount expressed in equivalent CaO amounts (= wt.% CaO) of at least 0.7 wt.% CaO.

[0027] All analyses of compositions performed on phosphate source (P0) and on raw materials (PC1 ) must be carried out on samples of weight comprised between 100 and 250 g and dried in an oven at a temperature of 105°C for a time of 8 h.

[0028] In a digestion step (D1), the at least one raw material (PC1) is digested with a digestion liquor (L) in the one or more reactors to yield a digested suspension (PC2). The digestion liquor (L) is an aqueous solution of sulphuric acid (H2SO4) and optionally of one or more mineral acids (HX). The digestion liquor (L) must contain sulphuric acid in amounts such that a ratio (H+(SA) I H+(L)) x 100% of a mole content of H+issued from the sulphuric acid (= H+(SA)) to a total mole content of H+ions in the digestion liquor (L) (= H+(L)) is comprised between 10 and 100%. For example, it can be comprised between 70 and 99%, preferably between 80 and 98%, more preferably between 85% and 97%, more preferably between 90% and 95%. In a preferred embodiment, the digestion liquor has the ratio H+(SA) / H+(L) = 100%. The expression “digestion step" is herein to be understood as a perfect synonym of the expression “leaching step" and derivations thereof, which can be interchanged indifferently. Sometimes, the expressions “attack", “acidic attack" and derivations thereof are used in the literature to refer to the “digestion step."

[0029] Figure 5 illustrates graphically the filtration times (= black circles, solid line - left-hand ordinate axis) and P2O5 losses (= white circles, dashed line - right-hand ordinate axis) as a function of the amount of sulphuric acid in the digestion liquor expressed in terms of the ratio (H+(SA) / H+(L)) x 100% of the mole content of H+issued from the sulphuric acid (= H+(SA)) to the total mole content of H+ions in the digestion liquor (L) (= H+(L)). The mineral acid (HX) used in the tests of Figure 5 was HCI. It can be seen from Figure 5, that digesting a raw material with a digestion liquor (L) comprising HCI only (i.e., H+(SA) / H+(L) = 0) does not yield any gypsum and has the effect of increasing substantially the filtering time to 740 s versus 12 s when the digestion liquor (L) comprises H2SO4 only (i.e., H+(SA) / H+(L) x 100% = 100%). Digesting the raw material (PC1) with a liquor comprising H2SO4 only (i.e., H+(SA) / H+(L) x 100% = 100%), however, increases the P2O5 losses to 16% versus 2% when thedigestion liquor (L) comprising HCI only (i.e., H+(SA) / H+(L) = 0).

[0030] In the present invention, the presence of at least 50% of H+ coming from H2SO4 in the digestion liquor is essential (cf. shaded area in Figure 5). The proportion of mineral acid (HX) present in the digestion liquor (L) can be varied up to 50% depending on which properties of the digestion and separation steps are prioritized.

[0031] In a separation step (S1), the digested suspension (PC2) is separated into, on the one hand, the aqueous phosphate rich solution (P1) (= filtrate) comprising phosphate ions and, on the other hand, the first solid phase (C1) (= residue) in the form of a cake containing calcium sulphate and impurities. In a preferred embodiment, the first solid phase (C1) is washed with water (counter- or co-current washing) before being stored or used by e.g., recirculating the first solid phase (C1) into the digestion step (D1) to increase the solids contents in the digested suspension (PC2). The washing filtrate after washing the cake can be re-used, such as by recirculating the wash filtrate into the digestion step (D1), or to dilute the digestion liquor, or for transporting the phosphate source to the digestion step (D1).

[0032] According to the present invention, the digestion liquor (L) is added to the at least one raw material (PC1) in amounts such that a molar ratio (SC / Ca) of the total amount of sulphur atoms expressed as sulphate equivalent (SO4) in the digestion liquor (L) to a total amount of Ca atoms (= Ca) present in both digestion liquor (L) and at least one raw material (PC1) is more than 0.80 and is less than 1.7 (i.e., 0.80 < SO4 / Ca < 1 .7), preferably more than 0.82 and less than 1.7 (i.e., 0.82 < SO4 / Ca < 1 .7) or more than 0.85 and lesst than 1 .7 (i.e., 0.85 < SO4 / Ca < 1 .7). In a preferred embodiment the molar ratio (SC / Ca) is comprised between 0.82 and 0.99 (i.e., 0.82 < SO4 / Ca < 0.99), more preferably between 0.85 and 0.98 (i.e., 0.85 < SO4 / Ca < 0.98). A molar ratio SO4 / Ca < 1 is herein referred to as defining sub-stoichiometric conditions. In an alternative embodiment, the molar ratio (SO41 Ca) is comprised between 1 .0 and 1 .6 (i.e., 1 .0 < SO4 / Ca < 1 .6). An SO4 / Ca > 1 is herein referred to as defining sur-stoichiometric conditions or in-excess conditions.PHOSPHATE SOURCE (P0) AND RAW MATERIAL (PC1)

[0033] The phosphate source (P0) can be an ore or rock containing in particular calcium phosphate in the form of apatite (and derivatives thereof including fluoro, chloro, hydroxy, ... - apatite) or tri-calcium phosphate or dicalcium phosphate or whitlockite. It can be a phosphate salt such as ammonium phosphate salts, sodium phosphate salts, aluminium phosphate salts, iron phosphate salts, aluminium and iron phosphates salts, struvite or a mixture of salts and the like. Alternatively, the phosphate source can be a beneficiation residue including one or more of phosphate tailing and phosphate slimes. Beneficiation is a mining process step carried out after ore extraction from the ground. It consists of mechanically separating minerals present in the phosphate ore (or “matrix”) from each other. For example, phosphates can be mechanically separated from clay, and quartz (or sand). This can be achieved, e.g., by separation of the minerals as a function of their grain size or density. Finally, withinthe frame of circular economy, the phosphate source can be a phosphate containing ash obtained by burning in a waste incineration plant phosphate-containing sewage sludges, bio-degradable wastes, bio-wastes and / or animal wastes, in particular animal bones. It can also be “black mass” issued from recycling of electric batteries which contains phosphorus.

[0034] A given phosphate source (P0) can be used alone, as shown in Figure 2(b), or admixed with a second or more phosphate source(s) (P02) of different origin(s), as shown in Figure 2(a). The admixing can occur either prior to feeding the phosphate sources (P0, P02) into the one or more reactors, or in the one or more reactors.

[0035] As illustrated graphically in Figures 6(a) to 6(c) and 7(a) & 7(b) discussed in more detail in continuation, the process of the present invention is particularly suitable for treating phosphate sources (P0) having high contents of other components than phosphates. In a preferred embodiment, the phosphate source has a Minor Element Ratio, (MER) defined as a ratio of the total amounts of Al, Mg, and Fe expressed as equivalent oxides, to the total amount of phosphorus expressed as P2O5 equivalent contained in the phosphate source (P0), MER = (“ / oAhCh + %MgO + %Fe2C>3) I %P2C>5) , equal to at least 0.051 (i.e., MER > 0.051) or at least 0.060 (i.e., MER > 0.060). MER is preferably comprised between 0.100 and 1 .500 (i.e., 0.100 < MER < 1 .500). In a preferred embodiment the MER is comprised between 0.140 and 1.500 (i.e., 0.140 < MER < 1 .500), preferably between 0.200 and 1.400 preferably between 0.400 and 1.000; preferably between 0.500 and 0.900; wherein (%Al2C>3, %MgO, %Fe2C>3, and %P2C>5 refer to the weight % of the total amount of Al, Mg, Fe, P expressed as their equivalent oxides present in the phosphate source (P0)).

[0036] In one embodiment, the phosphate source (P0) contains calcium in an amount of at least the required calcium amount. This can typically be the case if the phosphate source is a phosphate ore. In this case, the phosphate source (P0) forms the at least one raw material (PC1) and needs not be treated by addition of any calcium source (i.e., PC1 = P0).

[0037] In an alternative embodiment, the phosphate source (P0) contains less than the required calcium amount. This can be the case with ashes, depending on their origins and with ammonium or sodium salts. In these cases, the at least one raw material (PC1) is formed in a raw material formation step (M0) illustrated in Figures 2(a), 2(b), 3, and 4 by addition of a calcium compound (CO) to the phosphate source (P0) to form the at least one raw material (PC1) with the required amounts of calcium (i.e., PC1 c P0 + CO). The calcium compound can be chosen among,• lime derivatives including quick lime, slaked lime, pulverized lime, lime milk, finely ground limestone, as a pure product or as a residue,• calcium carbonate,• calcium phosphates (mono-, di-, tri),• kaolin,• calcium hydroxide,• calcium sulphate,• calcium fluoride,• a second phosphate source (P02) containing calcium,• recirculated fraction of digested suspension (PC2) or of aqueous phosphate rich solution (P1), or• a mixture thereof.

[0038] For example, a portion of the digested suspension (PC2) and / or a portion of the aqueous phosphate rich solution (P1) can be recirculated into the digestion step (D1) as a component of the digestion liquor (L) or as a calcium source or, for the digested suspension (PC2), as an input of solid content in the digestion step (D1). The portion of the digested suspension (PC2) and I or of the aqueous phosphate rich solution (P1) which is recirculated is preferably comprised between 1 and 50 wt.%, more preferably between 5 and 30 wt.%, most preferably between 10 and 20 wt.% of the total flow of digested suspension (PC2) and I or aqueous phosphate rich solution (P1).

[0039] In the embodiment wherein the first solid phase (C1) is washed with water (counter- or co-current washing) before being stored or used, the entire wash filtrate quantity or a portion of the wash filtrate of the first solid phase can be recirculated into the digestion step (D1) as a component of the digestion liquor or as a calcium source. The portion of the wash filtrate which is recirculated is preferably comprised between 1 and 100 wt.%, more preferably between 5 and 90 wt.%, more preferably between 10 and 80 wt.%, most preferably between 15 and 50 wt. %.

[0040] Regardless of the origin of the calcium, the at least one raw material (PC1) can comprise calcium in amounts comprised for example between 0.7 wt.% and 60 wt.% of calcium expressed as CaO equivalent, more preferably between 1 and 57.5 wt.% of calcium expressed as CaO equivalent, more preferably between 1 .5 and 55 wt.% of calcium expressed as CaO equivalent, more preferably between 2 and 52,5 wt.% of calcium expressed as CaO equivalent, more preferably between 3 and 50 wt.% of calcium expressed as CaO, more preferably between 6 and 47,5 wt.% of calcium expressed as CaO equivalent, more preferably between 10 and 45 wt.% of calcium expressed as CaO equivalent, more preferably between 15 and 35 wt.% of calcium expressed as CaO equivalent, and most preferably between 17.5 and 30 wt.% of calcium expressed as CaO equivalent.

[0041] The at least one raw material (PC1) can comprise a total amount of phosphorus expressed as P2O5 equivalent preferably comprised between 10 and 50 wt.% P2O5, preferably between 12 and 45 wt.% P2O5, more preferably between 14 and 40 wt.% P2O5, more preferably between 15 and 38 wt.% P2O5, more preferably between 18 and 35 wt.% P2O5, more preferably between 19 and 30 wt.% P2O5, more preferably between 20 and 27.5 wt.% P2O5, more preferably between 22.5 and 25 wt.% P2O5.DIGESTION STEP (D1) AND DIGESTION LIQUOR (L)

[0042] The digestion liquor (L) used in the digestion step (D1) must comprise sulphuric acid (H2SO4). It can optionally contain one or more additional mineral acids (HX). The sulphuric acid must be present in an amount such that a ratio (H+(SA) I H+(L)) x 100% of a mole content of H+issued from the sulphuric acid (= H+(SA)) to a total mole content of H+ions in the digestion liquor (L) (= H+(L)) is comprised between 50 and 100 %, preferably between 70 and 98 %, more preferably between 80 and 97 %. In a preferred embodiment, the molar ratio (H+(SA) I H+(L)) x 100% is comprised between 95 and 100 % and is preferably equal to 100 %.

[0043] The sulphuric acid and one or more mineral acids (HX) of the digestion liquor can be pure or residual, i.e. generated by washing installations or during routine production or maintenance operations in industries such as metallurgy, food processing, pharmaceuticals, chemicals, and particularly sulphuric acid production or phosphoric acid production.

[0044] The one or more mineral acids (HX), if present, can be chosen among hydrochloric acid, nitric acid, fluosilicic acid, hydrofluoric acid, the aqueous phosphate rich solution (P1), the wash filtrate of the first solid phase (C1) or a combination thereof. The one or more mineral acids can be derived from aqueous solutions of acidic calcium chloride. For example, as shown in Figure 4, an aqueous phase (A1) resulting from a DCP separation step (S2) discussed more in detail in continuation, after precipitation of dicalcium phosphate (= DCP) by neutralisation (N1) with a base, can be recirculated from the same process line, or from a separate process line.

[0045] The one or more mineral acids can also be derived from a residual aqueous solution containing PCu-ions (such as PC3-, HPC2-, H2PO4; H3PO4 and mono-calcium-phosphate) from industries or agro-industries, qualified as residual because they contain impurities of the following types: heavy metals, fluorine, carbons, chlorides, and the like or because the phosphorus content of these solution expressed as P2O5 equivalent is too low (such as for example, less than 20 wt %, or less than 15 wt %, less than 10 wt %, preferably less than 5 wt%). Such solutions can generally not be used as such in the production of pure phosphoric acid or in the production of pure phosphate salts.

[0046] The one or more mineral acids can comprise any one of the foregoing acids, and any mixture thereof. In a preferred embodiment, however, the mineral acid (HX) comprises hydrochloric acid, and preferably, hydrochloric acid alone. In an alternative embodiment, the mineral acid (HX) comprises hydrofluoric acid, and preferably, hydrofluoric acid alone. In yet an alternative embodiment, the mineral acid (HX) comprises fluosilicic acid, and preferably, fluosilicic acid alone. In a preferred embodiment, the mineral acid (HX) comprises phosphoric acid or phosphoric acid alone. In an alternative embodiment, the mineral acid (HX) comprises at least a fraction of the wash filtrate of the first solid phase (C1) which is recirculated. In an alternative embodiment, the mineral acid (HX) comprises a fraction of the digested suspension which is recirculated. In yet an alternative embodiment, the mineral acid (HX) comprises a fraction of the aqueous phosphate rich solution (P1), which is recirculated. Theamount of sulphuric acid present in the digestion liquor is conditioned by the value of the molar ratio (SO41 Ca) of the total amount of sulphur atoms expressed as sulphate equivalent (SO4) in the digestion liquor (L) to a total amount of Ca atoms (= Ca) present in both digestion liquor (L) and at least one raw material (PC1) is more than 0.80 and is less than 1 .7 (i.e., 0.80 < SO4 / Ca < 1 .7). As a rule of thumb, it can be said that the calcium ions present in the digestion step (D1) originate predominantly from the one or more raw materials (PC1) and the sulphate ions originate mostly from the digestion liquor. In view of the different origins of raw materials, there can be exceptions to this rule of thumb.

[0047] In one embodiment, the SO41 Ca molar ratio is smaller than 1 .00, such as comprised between 0.81 and 0.99 (i.e., 0.83 < SO4 / Ca < 0.99), preferably between 0.83 and 0.98, more preferably between 0.85 and 0.97, more preferably between 0.89 and 0.96, or between 0.9 and 0.95. A SO41 Ca molar ratio smaller than unity is defined as a “sub-stoichiometric composition." A shown in Figures 6(a) & 6(b) and 7(a) & 7(b), digestion with sulphuric acid (of two different concentrations of 7% and 10%) of different phosphate sources differing by their MER-value with a sub-stoichiometric composition with a molar ratio SO4 / Ca = 0.9 (cf. black circles) according to the present invention yields a P2O5 extraction of the order of 90% over the total P2O5 contents in the phosphate source. By contrast, the same phosphate sources digested with a composition characterized by a SO4 / Ca = 0.8 (cf. white circles), i.e., falling out of the scope of invention, has P205 yields which are between 8 and 14% lower than with the SO4 / Ca = 0.9 (cf. Figure 6(c)).

[0048] In an alternative embodiment, the SO4 / Ca ratio is at least equal to 1 .00, such as comprised between 1 .05 and 1 .50 (i.e., 1 .05 < SO4 / Ca < 1 .5), preferably between 1.10 and 1 .40, more preferably between 1.15 and 1.30, more preferably between 1.17 and 1.20. A SC / Ca ratio larger than unity is defined as an “in-excess composition.” In-excess compositions are advantageous for digesting phosphate sources (P0) with high values of MER, typically MER > 0.25, or MER > 0.5. In particular, the excess of SO4 / Ca ratio (i.e. excess of acid) does not only serve to react with calcium but is also useful to lixiviate other impurities such as silicium oxides and I or iron phosphate, and I or aluminium phosphate. An “in-excess composition" can also be advantageous in cases wherein the phosphate source (P0) contains other forms of phosphates than calcium phosphates.

[0049] In one embodiment, the SO4 / Ca ratio is smaller than 1 .0, such as comprised between 0.83 and 0.99 (i.e., 0.83 < SO4 / Ca < 0.99), preferably between 0.85 and 0.98, more preferably between 0.88 and 0.97, more preferably between 0.90 and 0.95. A SO4 / Ca ratio. A SO4 / Ca ratio smaller than unity is defined as a “sub-stoichiometric condition". A sub-stoichiometric composition is preferably applied to treat phosphate source (P0) of MER < 0.25.

[0050] The digestion step (D1) can advantageously occur at atmospheric pressure or under slight depression. The digestion temperature can be quite low, such as lower than 100°C, preferably lower than 90°C, preferably lower than 80°C, more preferably between 25°C and 75°C; more preferably between 30°C and 72°C; more preferably between 60°C and 71 °C; preferably the temperature is 65°C.

[0051] The digestion step (D1) can be quite rapid with a duration lower than 180 min, preferably lower than 90 min, more preferably lower than 60 min, more preferably lower than 50 min, such as 45 min + 5 min, and wherein the duration is preferably at least 5 min, more preferably at least 10 min.

[0052] The acid concentrations in the digestion liquor (L) needs be controlled. For example, a combined concentration of the sulphuric acid and the one or more mineral acids in the digestion liquor (L) can be comprised between 3 wt.% and 30 wt.%, preferably between 4 and 25 wt.%; preferably between 5 and 20 wt.%, more preferably between 6 and 15 wt.%; most preferably between 7 and 12 wt.% or is equal to 7 + 2 wt.%.

[0053] A digestion liquor comprising also a mineral acid (HX) other than sulphuric acid is referred to as a “mixed digestion liquor." In case of mixed digestion liquors, the one or more reactors can be fed in different manners. In a first embodiment, the mixed digestion liquor is formed by mixing the aqueous solution of sulphuric acid (H2SO4) and the aqueous solution of one or more mineral acids (HX) prior to feeding into the one or more reactors.

[0054] In an alternative embodiment, the mixed digestion liquor can be formed and mixed in the one or more reactors. For example, the aqueous solution of sulphuric acid (H2SO4) can first be fed to the one or more reactors, followed by the aqueous solution of one or more mineral acids (HX). Alternatively, the aqueous solution of one or more mineral acids (HX) can be fed first to the one or more reactors, followed by the aqueous solution of sulphuric acid (H2SO4).

[0055] In a preferred embodiment, a portion of the digested suspension (PC2) and / or a portion of the aqueous phosphate rich solution (P1) is recirculated into the digestion step (D1) as a component of the digestion liquor or as a calcium source or the digested suspension (PC2) can be used as an input of solid content. Each of the portion of the digested suspension (PC2) and I or of the portion of the aqueous phosphate rich solution (P1) being recirculated is preferably comprised between 1 and 50%, more preferably between 5 and 30%, most preferably between 10 and 20% of the total flow of the corresponding digested suspension (PC2) and I or aqueous phosphate rich solution (P1), respectively. Such recirculation has the advantage of reducing the amount of fresh acid to be added in the digestion step, as well as of diluting the digestion mixture formed by the digestion liquor (L) and the raw material (PC1) during the digestion step to reduce the input of fresh water. If, on the other hand, the digestion mixture is too fluid, solids can be added by recirculating a portion of the solid residue (C1) of the first separation step (S1) or by a portion of the digested suspension (PC2).

[0056] More than one reactor can be arranged in series. The raw material (PC1) and digestion liquor (L) can be fed to a first reactor, and the thus formed suspension can be transferred from the first reactor to the following reactors in series. This method increases the digestion time accordingly. In an alternative embodiment, the sulphuric acid or all or a selection of the one or more mineral acids (HX) forming a mixed digestion liquor can be fed in the first reactor with the raw material, and the remainingacid components forming the mixed digestion liquor can be added to successive reactors, as the suspension travels from one reactor to the next one. It is, however, preferred to contact the raw material (PC1) with all the components of the mixed digestion liquor (L) in the first reactor.

[0057] The present invention applies to any raw material (PC1), including raw materials comprising substantial amounts of impurities. In a raw material (PC1) comprising substantial amounts of impurities, it is advantageous that as much P2O5 be dissolved in the aqueous phosphate rich solution (P1), and as much impurities as possible do not dissolve and remain in the first solid phase (C1) separated from the aqueous phosphate rich solution (P1) after the separation step (S1)..

[0058] For example, the efficacy of the P2O5 recovery from the raw material can be quantified by a P2O5-I0SS ratio (m(P2O5)ci I m(P2O5)pci) of a total mass (m(P2O5)ci) of phosphorus in the first solid phase (C1) to a total mass (m(P2O5)pci) of phosphorus in the raw material (PC1). The P2O5-I0SS ratio defines the amount of P2O5 which does not dissolve and remains in the first solid phase (C1). The P2O5-I0SS ratio can be comprised between 1 % and 30 %, preferably between 1.5% and 25 %, more preferably between 2% and 20%, most preferably between 2.5 % and 15 %; most preferably between 3 and 10%; most preferably 5 and 8%, most preferably 6 and 7%. Alternatively, the first solid phase (C1) can comprised between 0.25 wt.% and 10 wt.% P2O5, preferably between 0.3 wt.% and 7 wt.% P2O5, more preferably between 0.5 wt.% and 5 wt.% P2O5, most preferably between 0.7 wt.% and 3 wt.% P2O5.

[0059] Since substantially all CaO present in the raw material which reacts with H2SO4 precipitates as calcium sulphate, the cake, which forms the first solid phase (C1), comprises substantial amounts of calcium sulphates, depending on the amounts of CaO present in the raw material (PC1) relative to the impurities. It follows that a raw material comprising little impurities other than CaO, will form a cake comprising high proportions of calcium sulphates, whilst the proportion of calcium sulphates in the cake decreases with increasing amounts of impurities in the raw material which are successfully separated into the first solid phase (C1). The proportion of calcium sulphates present in the cake (or first solid phase (C1)) can be quantified by a calcium weight ratio (m(CaSC>4) I m(C1)) of a total mass (m(CaSO4) of CaSO4.nH2O (with n = 0 to 2) in the first solid phase (C1) to a total mass (m(C1)) of the first solid phase (C1). For example, the calcium weight ratio (m(CaSO4) / m(C1)) can be less than 97 wt.%, preferably comprised between 5 wt.% and 90 wt.%, between 10 wt.% and 85 wt.%, between 15 wt.% and 75 wt.%, more preferably between 50 wt.% and 70 wt.%.MER AND DIGESTION LIQUOR

[0060] In case of “medium quality P-source” (P0), e.g., MER < 0.25, sub-stoichiometric conditions (i.e. SO4 / Ca < 1) can be used. The digestion liquor mainly lixiviates calcium phosphate present in the phosphate source (P0). Figures 6(a) and 6(b), black circles, plot the P2O5 yield as a function of MER with a sub-stoichiometric digestion according to the present invention (SO4 / Ca) = 0.9) showing amaximum P2O5 yield for compositions of MER at about 0.25.

[0061] In case of “poor quality P-source” (P0) e.g. MER > 0.25, sur-stoichiometric conditions (i.e. SO4 / Ca > 1) can be used instead. The digestion liquor is used to leach phosphorus present in the phosphate source (P0) in different forms of the phosphorus, including calcium phosphate and other forms. Figure 8 plots the P2O5 yield of a phosphate source of MER = 0.28 as a function of the SO41 Ca ratio. It can be seen that higher P2O5 yields are obtained with sur-stoichiometric leaching conditions (i.e. SO4 / Ca > 1).SEPARATION STEP (S1)

[0062] Digestion of the raw material (PC1) forms a digested suspension (PC2) comprising an aqueous phosphate rich solution (P1) comprising phosphate ions (such as PO43-, HPO42-, H2PO4_, H3PO4and mono-calcium-phosphate) and a first solid phase (C1) containing among others calcium sulphate and impurities. The aqueous phosphate rich solution (P1) must be separated from the first solid phase (C1) in a separation step (S1) as shown in Figures 1 to 4. The separation step (S1) can be carried out in a separation unit selected among any one of a decantation unit, a vacuum belt filter, a press filter, a membrane press filter, or a centrifuge.

[0063] The solid phase (C1) can be discarded or further processed for other applications. As mentioned supra, in some applications, when the solid content of digested suspension (PC2) is too low, the solid phase (C1) can be recirculated into the digestion step (D1). Depending on the nature of the phosphate source (P0) and the amounts of impurities present therein, the aqueous phosphate rich solution (P1) can be further processed to yield desired phosphate containing end-products of desired purities. For example, the aqueous phosphate rich solution (P1) can be concentrated and purified by means of well-known phosphoric acid purification steps. For example, the aqueous phosphate rich solution (P1) can be further treated to yield phosphoric acid or phosphate salt of desired degree of purity by one or more of any one of the following techniques: precipitation or selective precipitation, ion exchange, nanofiltration or membrane filtration, or solvent extraction. The foregoing techniques are suitable for reducing the contents of one or more of any one of sulphates, cadmium, arsenic, heavy metals, organics, and the like.

[0064] In an alternative embodiment, the aqueous phosphate rich solution (P1) can be neutralized in a neutralization step (N1) described in continuation.NEUTRALIZATION STEP (N1) OF THE AQUEOUS PHOSPHATE RICH SOLUTION (P1)

[0065] The process of the present invention can be used for the preparation of a dicalcium phosphate (DCP). To this purpose, the process illustrated in Figures 3 and 4 further comprises a neutralization step (N1), wherein the aqueous phosphate rich solution (P1) is neutralized at a pH sufficient to form aDCP-suspension (P2) comprising an aqueous phase (A1) and a DCP-rich solid phase (P3) comprising DCP. The DCP-rich solid phase (P3) is separated from the aqueous phase (A1) of the DCP-suspension (P2) in a DCP separation step (S2).

[0066] The neutralisation step (N1) is performed in presence of a source of calcium (B1) to reach a pH preferably comprised between 2 and 6. The source of calcium (B1) can be chosen among,• Lime derivatives including quick lime, slaked lime, lime, lime milk, as pure product or as residue.• calcium carbonate or limestone,• calcium hydroxide as pure product or as residue,• calcium phosphate,• a mixture thereof.

[0067] In an embodiment of the process, the DCP-solid is washed with water. The DCP - washing filtrate and the mother liquor (A1) of the DCP separation step (S2) can be recycled back to the phosphate source (P0), raw material (PC1), or digestion step (D1) or in the neutralisation step (N1), or into the digestion liquor (L).

[0068] In an embodiment of the process illustrated in Figure 4, the aqueous phosphate rich solution (P1) or the DCP-rich solid phase (P3) is further processed in an end process (EP) to produce an endproduct (P4) selected among,• a fertilizer, preferably of type monoammonium phosphate (MAP), diammonium phosphate (DAP), Ammonium-Potassium Phosphate fertilizer (NPK fertilizer),• animal feed,• food grade product,• merchant grade or purified phosphoric acid or technical grade phosphoric acid.EXAMPLESPhosphate production from a phosphate source:

[0069] 200 g of each of phosphate samples S1 to S4 of raw material (PC1) as defined in Table 1 was mixed in a vessel with corresponding amounts of sulphuric acid of concentrations 7 wt.% and 10 wt.%, to yield the desired values of the SO4 1 Ca ratios. Digestion (D1) was carried out at a temperature of 60°C during 50 min. The thus obtained digested suspension (PC2) was separated using a Buchner filter driven by vacuum. The aqueous phosphate rich solution (P1) was recovered. In these tests, the first solid phase (C1) was not washed with washing water. As some phosphate may remained trapped in the liquid phase still present in the cake, it follows that the P2O5-yields measured in these tests underestimate the actual P2O5-yield obtained in industrial conditions generally including a washing of the cake. With washing, the wash water can either be joined with the aqueous phosphate rich solution(P1) or recirculated into the digestion step (D1). The cake of the first solid phase (C1) is dried at 45°C in an oven.

[0070] Four different phosphate sources (P0) were used to yield the five raw materials (PC1), S1 to S5.• Ashes of calcined animal bones and meat as listed in Table 1 , used as such as raw material S1 ,• Rock (ore) as listed in Table 1 , used as such as raw material S2,• Beneficiation residue as listed in Table 1 , used as such as raw material S4,

[0071] Rock (ore) as listed in Table 1 , used as such as raw material S5.Raw material S3 was obtained by mixing 50 wt.% of rock S2 with 50 wt.% of beneficiation residue S4 to yield an intermediate value of MER.Table 1: Description of the raw materials S1 to S5

[0072] The content of P2O5 (m(P2O5)ci) of the first solid cake (C1) was measured, and the P2O5 yield in the aqueous phosphate rich solution (P1) was determined with Formula (1),P2O5 yield (%) = (1 - (m(P2O5)ci / m(P2O5)pci)) x 100% (1) wherein m(P2O5)pci is the weight of P2O5 present in the raw material (PC1). As discussed supra, since the first solid cake (C1) was not washed with wash water, Formula (1) underestimates of some percentual points the actual P2O5-yield, as it does not include the P2O5 in solution with the liquid phase still present in the first solid cake (C1) which would be entrained in the wash water.

[0073] The results are listed in Table 2 and plotted graphically in Figures 6(a) to 6(c). Figures 6(a) and 6(b) plots the P2O5-yield as a function of MER with a sulphuric acid concentration of (a) 7 wt.% and (b) 10 wt.%. The white circles represent the P2O5-yield obtained with a digestion liquor characterized by a SO4 / Ca ratio of 0.8 according to prior art, and the black circles represent the P2O5-yield obtained with a digestion liquor characterized by a SO4 / Ca ratio of 0.9 according to the present invention. It can be seen that for both H2SO4 concentrations of 7% and 10%, the P2O5-yield is higher with a SO4 / Ca ratio of 0.9 than with SO4 / Ca ratio of 0.8. Figure 6(c) plots the A-yield defined as the differencebetween P2O5-yield at SO4 / Ca = 0.9 and P2O5-yield at SO4 / Ca = 0.8 (white circles = [H2SO4] = 7 wt.% and black circles = [H2SO4] = 10 wt.%). It can be seen that the value of A-yield increases with higher values of MER, with a maximum value of A-yield = 13.4 at MER = 0.8.

[0074] Figures 7(a) and 7(b) show a graphical column-representation of the P2O5-yield at SO4 / Ca = 0.8 (white columns) and 0.9 (black columns - Invention) with [H2SO4] = 7 wt.% and 10 wt.%, for the raw material (a) S1 with MER = 0.050 and for the raw material (b) S4 with MER = 0.821 . This clearly shows that the P2O5-yield is higher with more diluted digestion liquors (L), here with [H2SO4] = 7 wt.%, and for raw materials having higher values of MER, here with MER = 0.821. The latter is an excellent result since, as discussed in the Background Art section supra, phosphate producers will have to work with phosphates sources (P0) of lower quality with higher values of MER. Note that the A-yield is also substantial for raw material (S1) in Figure 7(a) with a low value of MER = 0.050, with A-yield = 9% and 8% depending on the concentration of H2SO4.Table 2: PzOs-yield in the aqueous phosphate rich solution (P1) as a function of digestion conditions

[0075] The phosphates source S5, a rock of MER = 0.280 was leached with digestion liquor comprising sulphuric acid at a concentration of 10 wt.%. The digestion conditions were varied withdifferent SO4 / Ca ratios ranging from 0.7 to 1 .3. The P2O5 weight were measured and P2O5 yield was calculated. The results are listed in Table 3 and plotted in Figure 8.Table 3: P2Os-yield in the aqueous phosphate rich solution (P1) as a function of digestion conditions (SC / Ca) for the source S5 at H2SO4 concentration of 10wt%.

[0076] As shown in table 3, for phosphates source was a rock with a MER > 0.250, leaching with a digestion liquor characterized by a SO4 1 Ca ratio greater than 0.8 according to the present invention, yields better results than with a SO41 Ca ratio smaller than 0.8. Furthermore, the P2O5 yield increases when passing from sub-stoichiometric conditions (i.e., SO4 / Ca < 1) to sur-stoichiometric conditions (i.e., SO4 / Ca > 1), with a P2O5 yield rising from 88.1 % at SO4 / Ca = 0.9 to 91 .1 % withSC / Ca = 1 .3.Neutralizing step (N1) to form dicalcium phosphate (DCP)

[0077] 1504 g of the aqueous phosphate rich solution (P1) was neutralized with a basic source of calcium, Ca(OH)2 at 20 wt.%, until reaching a pH = 5. The mixture was allowed to react for 40 min at 45°C. Precipitated dicalcium phosphate (DCP) was separated from the liquid phase in a DCP separation step (S2) as shown in Figures 3 and 4. 126 g of DCP was recovered in the retentate and the P2O5 contents measured. The liquid phase contained less than 2 wt.% P2O5. The content of P2O5 (m(P2O5)p3) of the precipitated DCP (P3) was measured, and the DCP-yield in the precipitated DCP (P3) was determined with Formula (2),DCP-yield (%)= (m(P2O5)p3 / m(P2O5)pi) x 100%, (2) wherein m(P2O5)pi is the weight of P2O5 in the aqueous phosphate rich solution (P1). The DCP-yield was quite high for all raw materials (S1 to S4) with an average value of 91 %.

Claims

CLAIMS1. A process for producing a phosphate containing product from a phosphate source (PO), the process comprising:• feeding one or more reactors with at least one raw material (PC1) issued from the phosphate source (PO), the at least one raw material comprising a phosphorus required amount expressed in equivalent P2O5 amounts (= wt.% P2O5) of at least 10 wt.% P2O5, and a required calcium amount expressed in equivalent CaO amounts (= wt.% CaO) of at least 0.7 wt.% CaO,• in a digestion step (D1), adding to the at least one raw material (PC1) in the reactor a digestion liquor (L) for digesting the at least one raw material (PC1) and yielding a digested suspension (PC2), wherein the digestion liquor (L) is an aqueous solution of sulphuric acid (H2SO4) and optionally of one or more mineral acids (HX), wherein a ratio (H+(SA) I H+(L)) x 100% of a mole content of H+issued from the sulphuric acid (= H+(SA)) to a total mole content of H+ions in the digestion liquor (L) (= H+(L)) is comprised between 50 and 100%,• in a separation step (S1), separating the digested suspension (PC2) into, on the one hand, an aqueous phosphate rich solution (P1) comprising phosphate ions and, on the other hand, a first solid phase (C1) containing calcium sulphate and impurities.Characterized in that, the phosphate source (P0) is characterized by a Minor Element Ratio, (MER = (%AhO3 + %MgO + %Fe2Os) / %P2O5), representative of an amount of selected impurities present in the phosphate source (P0), which is equal to at least 0.051 (i.e., MER > 0.051), wherein (%AhO3, %MgO, %Fe2C>3, and %P2C>5 refer to the weight % of the corresponding oxides in the phosphate source (P0)). and in that the digestion liquor (L) is added to the at least one raw material (PC1) in amounts such that a molar ratio (SO4 1 Ca) of the total amount of sulphate ions (SO4) in the digestion liquor (L) to a total amount of Ca atoms (= Ca) present in both digestion liquor (L) and at least one raw material (PC1) is more than 0.80 and is less than 1 .7 (i.e., 0.80 < SO4 / Ca < 1 .7), and is preferably comprised between 0.85 and 0.99 (i.e., 0.85 < SO4 / Ca < 0.99), or between 1 .0 and 1 .6.

2. Process according to claim 1 , wherein the solid phosphate source (P0) either:• contains calcium in an amount of at least the required calcium amount and thus forms the at least one raw material (PC1) (i.e., PC1 = P0), or• contains less than the required calcium amount, and the at least one raw material (PC1) is formed in a raw material formation step (M0) by addition of a calcium compound (CO) to the phosphate source (P0) to form the at least one raw material (PC1) with the required amounts of calcium (i.e., PC1 c P0 + CO), wherein the calcium compound is chosen among: o calcium oxide, lime derivatives including quick lime, slaked lime, pulverized lime, lime milk, finely ground limestone o calcium carbonate, o calcium phosphates salts (mono di-; tri-),o kaolin, o calcium hydroxide, o calcium sulphate, o calcium fluoride, e a second phosphate source (P02) containing calcium, o recirculated fraction of digested suspension (PC2) or of aqueous phosphate rich solution (P1), o phosphoric acid, or o a mixture thereof.

3. Process according to claim 1 or 2, wherein the phosphate source (PO) is chosen among one or more of the following origins:• phosphate ores,• beneficiation residues including one or more of phosphate tailing and phosphate slimes,• phosphate salts,• ashes, preferably ashes from one or more of incinerated sewage sludge, bones, manure,• black mass. and wherein the phosphate source (PO) is used alone or admixed with a second phosphate source (P02) of different origin than the phosphate source, wherein the admixing occurs either in the reactor or prior to feeding the phosphate source (PO) into the reactor.

4. Process according to any one of the preceding claims, wherein the phosphate source (PO) is characterized by the Minor Element Ratio (MER) is equal to at least 0.060 (i.e., MER > 0.060), and is preferably comprised between 0.100 and 1.500 (i.e., 0.100 < MER < 1.500), preferably between 0.140 and 1.500 (i.e., 0.14 < MER < 1.500), preferably between 0.200 and 1.400 preferably between 0.400 and 1.000; preferably between 0.500 and 0.900.

5. Process according to any one of the preceding claims, wherein the at least one raw material (PC1) comprises phosphorus expressed as P2O5 equivalent in amounts comprised between 10 and 50 wt.% P2O5, preferably between 12 and 45 wt.% P2O5, more preferably between 14 and 40 wt.% P2O5, more preferably between 15 and 38 wt.% P2O5, more preferably between 18 and 35 wt.% P2O5, more preferably between 19 and 30 wt.% P2O5, more preferably between 20 and 27.5 wt.% P2O5, more preferably between 22.5 and 25 wt.% P2O5.

6. Process according to any one of the preceding claims, wherein a portion of the digested suspension (PC2) and / or a portion of the aqueous phosphate rich solution (P1) is recirculated into the digestion step (D1) as a component of the digestion liquor or as a calcium source, wherein each of the portion of the digested suspension (PC2) and I or of the portion of the aqueous phosphate rich solution (P1) being recirculated is preferably comprised between 1 and 50%, more preferably between 5 and30%, most preferably between 10 and 20% of the total flow of the corresponding digested suspension (PC2) and I or aqueous phosphate rich solution (P1), respectively.

7. Process according to any one of the preceding claims, wherein the digestion step (D1) occurs at a temperature lower than 100°C, preferably lower than 90°C, preferably lower than 80°C, more preferably between 25°C and 75°C; more preferably between 30°C and 72°C; more preferably between 60°C and 71 °C; preferably the temperature is 65°C.

8. Process according to any one of the preceding claims, wherein the digestion step (D1) has a duration lower than 180 min, preferably lower than 90 min, more preferably lower than 60 min, more preferably lower than 50 min or of 45 min + 5 min, , and wherein the duration is preferably at least 5 min, more preferably at least 10 min.

9. Process according to any one of the preceding claims, wherein a combined concentration of the sulphuric acid and the one or more mineral acids in the digestion liquor (L) is comprised between 3 wt.% and 30 wt.%, preferably between 4 and 25 wt.%; preferably between 5 and 20 wt.%, more preferably between 6 and 15 wt.%; most preferably between 7 and 12 wt.% or is equal to 7 + 2 wt.%.

10. Process according to any one of the preceding claims, wherein the ratio (H+(SA) I H+(L)) x 100% of the mole content of H+issued from the sulphuric acid (= H+(SA)) to the total amount of H+ions in the digestion liquor (L) (= H+(L)) is comprised between 70 and 99%, preferably between 80 and 98%, more preferably between 85% and 97%, preferably between 90% and 95%. preferably between11 . Process according to any one of the preceding claims, wherein the MER of the phosphate source (P0) is lower than 0.250 and wherein in the digestion step (D1), the molar ratio (SC / Ca) is at least 0.810 and less than 0.990; preferably comprised between 0.850 and 0.980; more preferably comprised between 0.890 and 0.950; and is more preferably 0.900.

12. Process according to any one of the preceding claims 1 to 10, wherein the MER of the phosphate source (P0) is at least 0.250 and wherein in the digestion (D1) step, the molar ratio (SC / Ca) is comprised between 1 .05 and 1 .5; preferably comprised between 1.10 and 1 .20, preferably comprised between 1.15 and 1.17.

13. Process according to any one of claims 1 to 12, wherein the aqueous phosphate rich solution (P1) is neutralized in a neutralizing step (N1) to form dicalcium phosphate (DCP), wherein the neutralisation step (N1) is performed in presence of a source of calcium (B1) to reach a pH preferably comprised between 3 and 6, wherein the source of calcium (B1) is chosen among,• lime derivatives including quick lime, slaked lime, pulverized lime, lime milk,• calcium carbonate or finely ground limestone,calcium hydroxide, calcium phosphate, or a mixture thereof.

14. Process according to claim 12 or 13, wherein the DCP is further processed to produce,• a fertilizer, preferably of type monoammonium phosphate (MAP), diammonium phosphate (DAP), Ammonium-Potassium Phosphate fertilizer (NPK fertilizer),• animal feed,• food grade products, • merchant grade or purified phosphoric acid or technical grade phosphoric acid.

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