PROCESS FOR RECOVERING PHOSPHORUS

Abstract

Process for recovering phosphorus from ashes coming from combustion plants that are fed with renewables and/or waste materials as solid fuel, comprising a phase of mixing said ashes coming from combustion plants that are fed with renewables and/or waste materials as solid fuel with a sulfuric acid solution, in order to extract the phosphorus therein contained; a system for carrying out said process is also described.

Claims

1. A process for recovering phosphorus from ashes coming from combustion plants that are fed with renewables and/or waste materials as solid fuel, comprising the phases of: a) mixing said ashes comprising phosphorus with a sulfuric acid solution, thereby obtaining an acidic slurry comprising phosphoric acid and having a pH with a pH value between 0 and 2; b) mixing said acidic slurry with urea or an aqueous solution thereof until an enriched acidic slurry with a pH value comprised between 2 and 4 is obtained, wherein said enriched acidic slurry comprises a solid dispersed phase and a liquid dispersing phase, said liquid dispersing phase comprising a phosphoric acid-urea adduct; c) separating said dispersing liquid phase from said solid dispersed phase, thereby making said liquid dispersing phase available.

2. The process according to claim 1, wherein said ashes come from combustion plants that are fed with a solid fuel selected from the group consisting of urban or industrial solid waste, vegetal biomass, sewage sludges and any combination thereof.

3. The process according to claim 1, wherein in said phase a) of mixing said ashes with a sulfuric acid solution, the sulfuric acid solution has a sulfuric acid concentration comprised between 20-60% by weight.

4. The process according to claim 1, wherein said phase a) of mixing said ashes with a sulfuric acid solution and/or said phase b) of mixing said acidic slurry with urea are carried out at a temperature comprised between 40-150° C.

5. The process according to claim 1, wherein in said phase b) of mixing said acidic slurry with urea, said urea aqueous solution is in the form of a thick solution with a concentration comprised between 20% and 90% by weight.

6. The process according to claim 1, wherein said phase a) of mixing said ashes with a sulfuric acid solution comprises the following steps: providing a turbo-reactor comprising a cylindrical tubular body having two inlet openings for a flow of ashes and for a flow of aqueous solution of sulfuric acid and a discharge opening and a rotor, arranged in the cylindrical tubular body and comprising a shaft provided with elements radially projecting therefrom; supplying a continuous flow of said ashes and a continuous flow of aqueous solution of sulfuric acid into said turbo-reactor, thereby dispersing, by means of said rotor, said ashes into a continuous flow of particles and said aqueous solution of sulfuric acid into a continuous flow of fine droplets; centrifuging said ashes and the aqueous solution of sulfuric acid against the inner wall of the turbo-reactor, said rotor being rotated at a speed equal to or greater than 100 rpm, with formation of a dynamic, tubular, thin, highly turbulent fluid layer, in which the particles of said ashes and the droplets of said aqueous solution of sulfuric acid are mechanically kept in intimate contact by the radially projecting elements of said rotor, while they advance in substantial contact with said inner wall of the turbo-reactor towards the discharge opening, thereby obtaining an acidic slurry comprising phosphoric acid, having a pH with a pH value between 0 and 2; and, discharging a continuous flow of acidic slurry comprising phosphoric acid from said discharge opening.

7. The process according to claim 6, wherein said phase b) of mixing said acidic slurry with urea comprises the following steps: providing a turbo-mixer comprising a cylindrical tubular body having two inlet openings for a flow of acidic slurry and a flow of urea and a discharge opening and a rotor, disposed in the cylindrical tubular body and comprising a shaft provided with elements radially projecting therefrom; supplying a continuous flow of said acidic slurry together with a continuous flow of urea into said turbo-mixer, the acidic slurry being dispersed by said rotor into a flow of fine droplets; centrifuging urea and said acidic slurry against the inner wall of said turbo-mixer, said rotor being rotated at a speed equal to or greater than 100 rpm, with formation of a dynamic, tubular, thin, highly turbulent fluid layer, in which urea and the acidic slurry droplets are mechanically kept in intimate contact by the radially projecting elements of said rotor, while they advance in substantial contact with said inner wall of the turbo-mixer towards the discharge opening, thereby obtaining a continuous flow of an enriched acidic slurry having a pH value comprised between 2 and 4, wherein said enriched acidic slurry comprises a solid dispersed phase and a liquid dispersing phase, said liquid dispersing phase comprising a phosphoric acid-urea adduct; discharging a continuous flow of enriched acidic slurry from said discharge opening.

8. Process according to claim 1, wherein said phase a) of mixing ashes with a sulfuric acid solution comprises the following steps: providing a reactor comprising a cylindrical tubular body with a horizontal axis having a first and a second opening for the inlet of reagents, arranged in proximity of a first end of said cylindrical tubular body, a third opening for the inlet of reagents, arranged in a position interposed between said first end of said cylindrical tubular body and a second opposite end of said cylindrical tubular body, an opening for discharging the final product, and a rotor, disposed in the cylindrical tubular body and comprising a shaft provided with elements radially projecting therefrom; supplying a continuous flow of said ashes, through said first inlet opening, and a continuous flow of an aqueous solution of sulfuric acid, through said second inlet opening, into said reactor, thereby dispersing, by means of said rotor, said ashes into a continuous flow of particles and said aqueous solution of sulfuric acid into a continuous flow of fine droplets; centrifuging said ashes and the solution of sulfuric acid against the inner wall of the reactor, said rotor being rotated at a speed equal to or greater than 100 rpm, with formation of a dynamic, tubular, thin, highly turbulent fluid layer, in which the particles of said ashes and the droplets of said aqueous solution of sulfuric acid are mechanically kept in intimate contact by the radially projecting elements of said rotor, while they advance in substantial contact with said inner wall of the reactor towards the discharge opening, thereby obtaining a continuous flow of acidic slurry comprising phosphoric acid, having a pH with a pH value between 0 and 2; said phase b) of mixing acidic slurry with urea comprises the following steps: supplying a continuous flow of urea through said third inlet opening into said turbo-mixer, and centrifuging the latter together with said continuous flow of acidic slurry by the action of said radially projecting elements of said rotor, by advancing them in substantial contact with said inner wall of the reactor towards the discharge opening, thereby obtaining a flow of enriched acidic slurry having a pH value comprised between 2 and 4, wherein said enriched acidic slurry comprises a solid dispersed phase and a liquid dispersing phase, said liquid dispersing phase comprising a phosphoric acid-urea adduct; discharging a continuous flow of enriched acidic slurry from said discharge opening.

9. System for carrying out a process for recovering phosphorus from ashes coming from combustion plants that are fed with renewables and/or waste materials as solid fuel according to claim 7, comprising the following units: a turbo-reactor comprising a cylindrical tubular body having two inlet openings for a flow of ashes and for a flow of aqueous solution of sulfuric acid, and a discharge opening for discharging a flow of acidic slurry, and a rotor, disposed in the cylindrical tubular body and comprising a shaft provided with elements radially projecting therefrom; and, a turbo-mixer comprising a cylindrical tubular body having two inlet openings for a flow of acidic slurry and a flow of urea, and a discharge opening for discharging an enriched acidic slurry, and a rotor, disposed in the cylindrical tubular body and comprising a shaft provided with elements radially projecting therefrom.

10. System for carrying out a process for recovering phosphorus from ashes coming from combustion plants that are fed with renewables and/or waste materials as solid fuel according to claim 8, comprising a reactor comprising a cylindrical tubular body with a horizontal axis having a first and a second opening for the inlet of reagents, arranged in proximity of a first end of said cylindrical tubular body, a third opening for the inlet of reagents, arranged in a position interposed between said first end of said cylindrical tubular body and a second opposite end of said cylindrical tubular body, an opening for discharging the final product, which can be an enriched acidic slurry, and a rotor, disposed in the cylindrical tubular body and comprising a shaft provided with elements radially projecting therefrom.

11. System according to claim 9, further comprising a separation unit, capable of operating in continuous or discontinuous, having an inlet opening for a flow of enriched acidic slurry comprising a dispersed phase and a dispersing phase, and a discharge outlet for discharging a continuous or discontinuous flow of said dispersing phase.

12. System according to claim 10, further comprising a separation unit, capable of operating in continuous or discontinuous, having an inlet opening for a flow of enriched acidic slurry comprising a dispersed phase and a dispersing phase, and a discharge outlet for discharging a continuous or discontinuous flow of said dispersing phase.

13. Process according to claim 1, wherein said ashes are bottom ashes.

14. Process according to claim 1, wherein in said phase a) of mixing said ashes with a sulfuric acid solution the acidic slurry comprising phosphoric acid has a pH value equal to about 1.

15. Process according to claim 1, wherein in said phase b) of mixing said acidic slurry with urea or an aqueous solution thereof an enriched acidic slurry with a pH value equal to about 2.5 is obtained.

16. Process according to claim 3, wherein in said phase a) of mixing said ashes with a sulfuric acid solution the sulfuric acid solution has a sulfuric acid concentration comprised between 25-45% by weight.

17. Process according to claim 16, wherein said sulfuric acid solution has a sulfuric acid concentration comprised between 30-40% by weight.

18. Process according to claim 4, wherein said phase a) of mixing said ashes with a sulfuric acid solution and/or said phase b) of mixing said acidic slurry with urea are carried out at a temperature comprised between 45-60° C.

19. Process according to claim 5, wherein in said phase b) of mixing said acidic slurry with urea said urea aqueous solution has a concentration comprised between 40% and 80% by weight.

Description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0087] With reference to FIG. 1, a system 1 used for the process according to the invention, comprising a turbo-reactor 100 and a turbo-mixer 200, is shown.

[0088] The turbo-reactor 100 comprises in turn a tubular cylindrical body 101, closed at the opposite ends by bottoms 102 and 103, and coaxially provided with a heating jacket 104 intended to be crossed by a heat-transfer fluid, for instance diathermic oil, for keeping the inner wall of the tubular cylindrical body 101 at a predefined temperature.

[0089] Said heating jacket 104 has an inlet opening 104a and a discharge opening 104b, generally arranged for inletting the heated heat-transfer fluid and for outletting the cooled heat-transfer fluid, respectively.

[0090] The direction of the arrows drawn at the openings 104 and 104b exemplifies the direction of said incoming flow of heat-transfer fluid and of said outgoing flow of heat-transfer fluid, respectively.

[0091] The tubular body 101 is provided with inletting openings 105 and 106 for a continuous flow of ashes, optionally bottom ashes, coming from combustion plants, and for a continuous flow of a concentrated sulfuric acid solution, respectively, as well as with a discharge opening 107.

[0092] The tubular body 101 rotatably supports therein a rotor comprising a shaft 108 provided with radially projecting elements 109 in the form of blades, said blades 109 being arranged helicoidally and oriented for centrifuging and simultaneously conveying a flow of an acidic slurry resulting from the mixing of the aforementioned two flows to the discharge opening 107. The acidic slurry comprises phosphoric acid extracted from the aforementioned ashes, optionally bottom ashes, and comprises ashes depleted of their intrinsic phosphorus content.

[0093] The direction of the arrows drawn at the openings 105, 106 and 107 exemplifies the direction of said incoming reagent flows and said outgoing acidic slurry flow, respectively.

[0094] In particular, when said continuous flow of ashes coming from combustion plants and said continuous flow of sulfuric acid solution enter the turbo-reactor 100 and there are finely mixed in an acidic slurry, the latter is centrifuged by the blades 109 of the rotor against the inner wall of the cylindrical tubular body 101, which may be heated by means of the heating jacket 104.

[0095] Said turbo-reactor 100 is thus suitably arranged to carry out phase a) of mixing ashes with a sulfuric acid solution.

[0096] In particular, the intense mechanical action exerted by the rotor of the turbo-reactor 100 is such that a significant amount of kinetic energy is transmitted to the substrate, namely to said bottom ashes and to said sulfuric acid solution: the continuous transmission of energy to the substrate causes an intimate interaction between said ashes, finely divided into particles of ash, and the droplets of sulfuric acid solution.

[0097] Said intimate interaction allows a complete extraction of the phosphorus content in said bottom ashes with transformation of the phosphorus oxides (V) and of the metal phosphates contained in said agglomerates of bottom ashes into phosphoric acid.

[0098] The phosphoric acid thus obtained is normally in its various dissociative forms, i.e. orthophosphoric acid, pyrophosphoric acid and metaphosphoric acid. Moreover, further to the interaction between sulfuric acid and the above phosphorus compounds (V), sulfate is released into the acidic slurry.

[0099] In particular, advantageously, said intimate interaction occurs inside a layer of acidic, dynamic, thin, tubular slurry flowing inside the turbo-reactor 101 from said bottom 102 to said bottom 103, in proximity of which the discharging opening 107 is located.

[0100] As a consequence, being phase a) carried out by means of a turbo-reactor 100, the process according to the present invention is undoubtedly effective and absolutely practicable in the context of a continuous industrial application.

[0101] A motor, not shown, is suitably provided for operating the bladed rotor at variable speeds, which may be equal to or greater than 100 rpm.

[0102] Said discharge opening 107 is in fluid communication with a discharge duct 112 into which a continuous flow of acidic slurry exiting from the tubular body 101 is conveyed.

[0103] Once out of said turbo-reactor 100, said flow of acidic slurry is supplied in continuous to the turbo-mixer 200 through the inlet opening 205.

[0104] The turbo-mixer 200 comprises in turn a tubular cylindrical body 201, closed at the opposite ends by bottoms 202 and 203, and coaxially provided with a heating jacket 204 intended to be crossed by a heat-transfer fluid, for instance a diathermic oil, for keeping the inner wall of the tubular cylindrical body 201 at a predefined temperature.

[0105] Said heating jacket 204 has an inlet opening 204a and a discharge opening 204b, generally arranged for inletting the heated heat-transfer fluid and for outletting the cooled heat-transfer fluid, respectively.

[0106] The direction of the arrows drawn at the openings 204a and 204b exemplifies the direction of said incoming flow of heat-transfer fluid and said outgoing flow of heat-transfer fluid, respectively.

[0107] The tubular body 201 is provided with inletting openings 205 and 206 for a continuous flow of said acidic slurry and for a continuous flow of urea, for instance solid urea or urea in aqueous solution, respectively, and with a discharge opening 207.

[0108] The tubular body 201 rotatably supports therein a rotor comprising a shaft 208 provided with radially projecting elements 209 in the form of blades, said blades 209 being arranged helicoidally and oriented for centrifuging and simultaneously conveying a flow of an enriched acidic slurry resulting from the mixing of the above two flows towards the discharge opening 207, wherein said enriched acidic slurry comprises an adduct of phosphoric acid and urea.

[0109] The direction of the arrows drawn at the openings 205, 206 and 207 exemplifies the direction of said incoming flows of acidic slurry and urea and said outgoing flow of enriched acidic slurry, respectively.

[0110] In particular, when said continuous flow of acidic slurry and said continuous flow of urea enter the turbo-mixer 200, they are centrifuged by the blades 209 of the rotor against the inner wall of the cylindrical tubular body 201, which may be heated by means of the heating jacket 204.

[0111] The turbo-mixer 200 is thus arranged for carrying out said phase b) of mixing acidic slurry with urea.

[0112] In particular, the intense mechanical action exerted by the rotor of the turbo-mixer 200 is such that a significant amount of kinetic energy is transmitted to the substrate, i.e. to the acidic slurry and urea, either solid or in aqueous solution: the continuous transmission of energy to the substrate causes an intimate interaction between the droplets of said acidic slurry containing phosphoric acid and urea, finely divided into solid particles or droplets, in case of solid urea or an urea aqueous solution, respectively.

[0113] Said intimate interaction allows an almost complete coupling between the phosphoric acid molecules and the urea molecules with formation of phosphoric acid-urea adducts.

[0114] Specifically, the phosphoric acid, whatever its dissociative form, has marked Lewis acid characteristics and, for this reason, it has great affinity with the urea molecule, having a high-energy occupied orbital that is well willing to share its own electronic pair with an acid molecule, forming said phosphoric acid-urea adduct (LUMO-HOMO interaction).

[0115] Advantageously, said intimate interaction occurs inside a layer of enriched acidic, dynamic, thin, tubular slurry flowing inside the turbo-mixer 201 from said bottom 202 to said bottom 203, in proximity of which the discharge opening 207 is located.

[0116] As a consequence, being said phase b) carried out by means of a turbo-mixer 200, the process according to the present invention is undoubtedly effective and absolutely practicable in the context of a continuous industrial application.

[0117] A motor, not shown, is provided for operating the bladed rotor of the turbo-mixer 200 at variable speeds, which may be equal to or greater than 100 rpm.

[0118] Said discharge opening 207 is in fluid communication with a discharge duct 212 into which a continuous flow of enriched acidic slurry exiting from the tubular body 201 is conveyed.

[0119] Once out of the turbo-mixer 200, the flow of enriched acidic slurry is fed in continuous to the separation unit 400, into which it is introduced through an inletting opening 405.

[0120] Inside the separation unit 400, the dispersing phase of the enriched acidic slurry is suitably separated from the dispersed phase of the same enriched acidic slurry.

[0121] Said separation unit may be a completely traditional container, suitable for instance for centrifuging said enriched acidic slurry.

[0122] Specifically, inside said separation unit 400, the aqueous solution comprising said phosphoric acid-urea adduct (dispersing phase) is easily separated from the residues of said bottom ashes depleted of phosphorus, as well as from metal sulphates (dispersed phase), which suitably formed during phase a) by interaction of the sulfuric acid with the aforementioned metal phosphates (mainly bivalent metal phosphates, for instance calcium phosphates).

[0123] In particular, among these metal sulphates, calcium sulphate and its hydrated forms prevail, which have a natural tendency to precipitate in aqueous solution, even under low pH conditions.

[0124] Said continuous flow of enriched acidic slurry may be cooled at the inlet of said separation unit, for instance by contact with the walls of the separation unit itself, which may be cooled by means of a cooling jacket crossed by a cooling fluid, said cooling jacket not being represented since it is completely conventional.

[0125] At the exit from said separation unit 400, through a discharge opening 407a, a flow of said liquid dispersing phase, namely of an aqueous solution of phosphoric acid-urea adduct, is discharged, which is recovered to be advantageously re-used as fertilizer in the agricultural context, especially in drip fertigation.

[0126] At the exit from said separation unit 400, through a discharge opening 407b, said dispersed phase is also discharged.

[0127] Finally, in FIG. 2 a further embodiment of the system 1 according to the present invention is schematically reported, which comprises a unique reactor 300, in which the first phase a) of mixing ashes with a sulfuric acid solution and then phase b) of mixing acidic slurry with urea, as previously described in the summary, may be carried out.

[0128] Specifically, the reactor 300—which actually is a turbo-reactor - comprises in turn a tubular cylindrical body 301 with a horizontal axis (the horizontal axis is exemplified by a dashed line marked with letter A), closed at the opposite ends by bottoms 302 and 303, and coaxially provided with a heating jacket 304 arranged to be crossed by a heat-transfer fluid, for instance diathermic oil, so as to keep the inner wall of the tubular cylindrical body 301 at a predefined temperature.

[0129] Said heating jacket 304 has an inlet opening 304a and a discharge opening 304b, generally arranged to inlet the heated heat-transfer fluid and to outlet the cooled heat-transfer fluid, respectively.

[0130] The direction of the arrows drawn at the openings 304a and 304b exemplifies the direction of said incoming flow of heat-transfer fluid and of said outgoing flow of heat-transfer fluid, respectively.

[0131] The tubular body 301 is provided with a first inlet opening 305 and a second inlet opening 306, for a continuous flow of ashes, optionally bottom ashes, coming from combustion plants, and for a continuous flow of a concentrated sulfuric acid solution, respectively.

[0132] The tubular body 301 is also provided with a third inlet opening 310 for a continuous flow of urea, arranged in a position that is interposed between the first end 302 of said cylindrical tubular body and the second opposite end 303 of said cylindrical tubular body, as well as with at least one opening 307 for discharging the final product, such as enriched acidic slurry.

[0133] The tubular body 301 rotatably supports therein a rotor comprising a shaft 308 provided with radially projecting elements 309 in the form of blades, said blades 309 being helicoidally arranged and oriented for centrifuging and simultaneously conveying towards the bottom 303 a flow of an acidic slurry resulting from the mixture of said ashes and said aqueous solution of sulfuric acid.

[0134] In particular, when said continuous flow of ashes coming from combustion plants and said continuous flow of sulfuric acid solution enter the reactor 300 and are finely mixed therein into an acidic slurry, the latter is centrifuged by the blades 309 of the rotor against the inner wall of the cylindrical tubular body 301, which may be heated by means of the heating jacket 304.

[0135] Said reactor 300 is thus suitably arranged for carrying out phase a) of mixing ashes with a sulfuric acid solution.

[0136] In particular, the intense mechanical action of the rotor of the reactor 300 is capable of exerting on the substrate an intense mechanical action, as previously described in connection with the turbo-reactor 100.

[0137] Along the fluid path towards the bottom 303, the acidic slurry encounters a flow of urea that is supplied into the cylindrical tubular body 301 through the third inlet opening 310.

[0138] In particular, as soon as it enters the cylindrical tubular body, the flow of urea undergoes an intense mechanical action exerted by the rotor of the reactor 300, then said continuous flow of acidic slurry and said continuous flow of urea are centrifuged together by the blades 309 of the rotor against the inner wall of the cylindrical tubular body 301.

[0139] The blades 309 are also helicoidally arranged and oriented to centrifuge and simultaneously convey a flow of an enriched acidic slurry resulting from the mixture of the above two flows of acidic slurry and urea towards the discharge opening 307, where said enriched acidic slurry comprises an adduct of phosphoric acid and urea.

[0140] Said flows intimately interact inside a layer of enriched acidic slurry, as previously described in connection with the turbo-mixer 200.

[0141] The reactor 300 is thus arranged for carrying out said phase b) of mixing acidic slurry with urea.

[0142] Even when phase a) and phase b) are carried out by means of a reactor 300, the process according to the present invention is undoubtedly effective and absolutely practicable in the context of a continuous industrial application.

[0143] A motor, not shown, is provided for operating the bladed rotor of the reactor 300 at variable speeds, which may be equal to or greater than 100 rpm.

[0144] Said discharge opening 307 is in fluid communication with a discharge duct 312 into which a continuous flow of enriched acidic slurry exiting from the tubular body 301 is conveyed.

[0145] Once out of the reactor 300, the flow of enriched acidic slurry is supplied in continuous to the separation unit 400, into which it is introduced through an inlet opening 405.

[0146] Analogously to what has been previously described, inside the separation unit 400 the enriched acidic slurry is separated into a dispersed phase and into a dispersing phase, the latter comprising a phosphoric acid-urea adduct, entailing all the advantages already described above.

EXAMPLE 1

[0147] A flow of bottom ashes generated by the combustion of urban solid wastes inside a waste-to-energy plant was fed in continuous (150 kg/h), through the opening 105, into the turbo-reactor 100, wherein the bladed rotor 108 was rotated at a speed of 1000 rpm.

[0148] Simultaneously, a flow of aqueous solution of sulfuric acid at 30% w/w was fed in continuous (200 kg/h) through the opening 106.

[0149] Immediately at the inlet of the turbo-reactor 100, the flow of bottom ashes was finely mechanically divided into fine agglomerates which were immediately centrifuged.

[0150] Simultaneously, the aqueous solution of sulfuric acid, fed through the opening 106, was centrifuged by the blades of the rotor 108 in order to intimately mix it with the above flow of bottom ashes, thus forming a flow of acidic slurry.

[0151] Said flow of acidic slurry was immediately centrifuged against the inner wall of the reactor, where a layer of dynamic, tubular and thin fluid was formed.

[0152] Inside the turbo-reactor 100, the wall temperature was kept at a value of about 55° C., while the rotational speed of the bladed rotor 108 was constantly kept at 1000 rpm.

[0153] After an average residence time of about 120 seconds within the reactor, the acidic slurry with a pH equal to 0.95 and containing phosphoric acid was discharged in continuous from the opening 107.

[0154] The acidic slurry thus obtained was fed in continuous into the turbo-mixer 200, through the inlet opening 205 with a flow rate of approximately 120 kg/h, being co-current with a flow of urea aqueous solution with a concentration equal to 50% w/w, fed through the inlet opening 206.

[0155] Immediately at the inlet into said turbo-mixer 200, the continuous flow of acidic slurry and the flow of urea aqueous solution were mixed to produce a flow of enriched acidic slurry, right from the beginning, although partially, comprising a phosphoric acid-urea adduct.

[0156] Said flow of enriched acidic slurry was centrifuged against the inner wall of the mixer, where a layer of dynamic, tubular and thin flow was formed.

[0157] Inside the turbo-mixer 200, the wall temperature was kept at a value of about 45° C., while the rotational speed of the bladed rotor 208 was constantly kept at 800 rpm.

[0158] After an average residence time of 45 seconds inside the turbo-mixer 200, a flow of enriched acidic slurry with a pH equal to 2.5 and containing a phosphoric acid-urea adduct was discharged in continuous from the turbo-mixer 200.

[0159] Afterwards, said flow of enriched acidic slurry was fed in continuous into the separation unit 400, in this case into a horizontal centrifugal decanter for sludge thickening, with a flow rate of 100 kg/h.

[0160] After an average residence time of 5 minutes inside the decanter, said enriched acidic slurry was effectively separated into a solid precipitate containing calcium sulphate, hydrated salts thereof (gypsum) and residues of bottom ashes depleted of phosphorus, and into an aqueous solution of phosphoric acid-urea adduct.

[0161] The aqueous solution of phosphoric acid-urea adduct, having a pH equal to 2.5, was discharged in continuous through a discharge opening 407a. The output temperature recorded in said aqueous solution of phosphoric acid-urea adduct was 25° C.

[0162] Said aqueous solution of phosphoric acid-urea adduct was then collected and stored to be used as fertilizer.

[0163] Meanwhile, said solid precipitate was discharged through a discharge opening 407b and afterwards collected for the disposal thereof.