WO2024076226A1 - Method for recovering phosphorus in wastewater laden with orthophosphate ions - Google Patents

Method for recovering phosphorus in wastewater laden with orthophosphate ions Download PDF

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Publication number
WO2024076226A1
WO2024076226A1 PCT/MA2023/050015 MA2023050015W WO2024076226A1 WO 2024076226 A1 WO2024076226 A1 WO 2024076226A1 MA 2023050015 W MA2023050015 W MA 2023050015W WO 2024076226 A1 WO2024076226 A1 WO 2024076226A1
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solid
waste water
calcium
calcium hydroxide
orthophosphate ions
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PCT/MA2023/050015
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French (fr)
Inventor
Hicham HASSOUNE
Adil LACHEHAB
Rachid BOULIF
Rachid BENHIDA
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Ocp Sa
Universite Mohammed VI Polytechnique
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Publication of WO2024076226A1 publication Critical patent/WO2024076226A1/en

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    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/66—Treatment of water, waste water, or sewage by neutralisation; pH adjustment
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F11/00—Compounds of calcium, strontium, or barium
    • C01F11/46—Sulfates
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
    • C02F1/5236—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities using inorganic agents
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2101/00—Nature of the contaminant
    • C02F2101/10—Inorganic compounds
    • C02F2101/105—Phosphorus compounds
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2103/00—Nature of the water, waste water, sewage or sludge to be treated
    • C02F2103/18—Nature of the water, waste water, sewage or sludge to be treated from the purification of gaseous effluents

Definitions

  • the invention relates to a process for recovering phosphorus from waste water loaded with orthophosphate ions.
  • STATE OF THE TECHNIQUE Anthropogenic activities generate very large volumes of industrial effluents which, without adequate treatment, contaminate the receiving environment. Uncontrolled discharges of industrial origin loaded with phosphorus are considered among the most undesirable forms of pollution for the receiving environment. Indeed, they cause a significant ecological imbalance in aquatic ecosystems, known as eutrophication.
  • the phenomenon of eutrophication is mainly due to an enrichment of water in nutrients such as phosphorus and nitrogen which can lead to an increased presence of algae and a depletion of oxygen in the water, with the consequence of a risk increased mortality in certain aquatic organisms.
  • the phosphate chemical industries and particularly the phosphoric workshops discharge effluents rich in phosphorus, particularly from gas washing water. Indeed, gas washing water contains significant phosphorus contents, which can in certain cases reach values of between 450 and 550 mg/L in orthophosphate ions PO ⁇ ⁇ , measured at pHs of 11 and 13 Generally, this water rich in phosphorus does not undergo any treatment and is discharged directly into the receiving environment at the risk of generating environmental problems.
  • An aim of the invention is to design a process which makes it possible to recover phosphorus in wastewater so as to limit the environmental impact of said wastewater while being inexpensive and attractive for manufacturers.
  • the invention proposes a process for recovering phosphorus from waste water loaded with orthophosphate ions comprising at least the following successive steps: - (E1) mixing a source of phosphogypsum and an alkaline hydroxide in an aqueous medium so as to precipitate a solid comprising calcium hydroxide, - (E2) determination of the concentration of orthophosphate ions in waste water loaded with orthophosphate ions, - (E3) dispersion of the solid comprising calcium hydroxide in the wastewater so as to precipitate the calcium with the orthophosphate ions of the wastewater in the form of a solid comprising monetite, the quantity of dispersed calcium hydroxide being chosen as a function of the concentration of orthophosphate ions determined during step (E2), - (E4) separation of the solid comprising monetite and waste water depleted in orthophosphate ions.
  • the process according to the invention makes it possible to recover phosphorus from wastewater by recovering a by-product generated in large quantities by the phosphate chemical industries: phosphogypsum.
  • phosphorus is extracted from wastewater and recovered in an equally recoverable form, monetite, which can be sold as phosphate fertilizer.
  • monetite which can be sold as phosphate fertilizer.
  • the dual recovery of phosphorus present in wastewater and phosphogypsum makes the process particularly interesting in the context of a circular economy with reduced environmental impact.
  • the source of phosphogypsum is gypsum water directly resulting from the treatment of fluorophosphate calcium ores;
  • the alkaline hydroxide comprises calcium hydroxide or potassium hydroxide;
  • the dispersion step (E3) is implemented for a duration of between 10 minutes and 30 minutes, preferably a duration of between 10 minutes and 20 minutes;
  • the pH during the dispersion step (E3) is between 7 and 12, preferably between 10.5 and 12.5; - no strong base or other source of calcium is added during the dispersion step (E3);
  • the temperature during the dispersion step (E3) is between 25°C and 60°C, preferably between 25°C and 40°C, even more preferably of the order of 25°C;
  • the waste water (4) initially comprises between 400 mg/L and 2000 mg/L of orthophosphate ions, preferably between 500 mg/L and 1000 mg/L of orthophosphate ions; - the waste water (4)
  • FIG. 1 represents a diagram of the process according to the invention
  • - Figure 2 represents a result of X-ray diffraction analysis of the solid comprising calcium hydroxide resulting from the mixture of an alkaline hydroxide with a phosphogypsum
  • - Figure 3 represents a result analysis by X-ray diffraction of the solid comprising monetite resulting from the dispersion of the solid comprising calcium hydroxide in the waste water from which we seek to recover the phosphorus
  • DETAILED DESCRIPTION OF EMBODIMENTS In the remainder of the description, phosphates or orthophosphate ions refer to PO 4 3- ions derived from phosphoric acid (H 3 PO 4 ) by loss of three hydrogen atoms.
  • Waste water may initially contain between 400 mg/L and 2000 mg/L of orthophosphate ions, preferably between 500 mg/L and 1000 mg/L of orthophosphate ions.
  • the waste water is gas washing water constituting an industrial waste, preferably gas washing water emitted by phosphoric workshops.
  • the invention uses the property of calcium ions, under certain pH conditions, to precipitate with hydrogen phosphate ions to generate anhydrous dicalcium phosphate or monetite.
  • the process uses a source of calcium hydroxide as a source of calcium ions.
  • the hydroxide ions introduced with the calcium ions make it possible to control the pH so as to generate monetite.
  • the source of calcium hydroxide comes from the transformation of phosphogypsum or hydrated calcium sulfate.
  • Preparation of the source of calcium hydroxide from phosphogypsum With reference to Figure 1, in a step E1, a source of phosphogypsum 1 is dispersed in an aqueous medium and an alkaline hydroxide 2 is added.
  • phosphogypsum from the phosphate chemical industry includes several impurities such as quartz (SiO 2 ), fluorides (F-), phosphates (PO 4 3- ), organic matter, iron and aluminum minerals (Fe 2 O 3 , Al 2 O 3 ) and heavy metals.
  • quartz SiO 2
  • fluorides F-
  • phosphates PO 4 3-
  • organic matter iron and aluminum minerals
  • Fe 2 O 3 , Al 2 O 3 iron and aluminum minerals
  • heavy metals heavy metals.
  • the discharge of phosphogypsum into the sea is a source of pollution and eutrophication of the environment.
  • phosphogypsum is stored in the form of slag heaps or slag heaps. If the soil in the storage areas is not waterproofed, toxic elements contained in said phosphogypsum can diffuse and pollute the soil and water tables.
  • the process according to the invention makes it possible to recover this industrial waste, a source of pollution.
  • the source of phosphogypsum 1 can be gypsum water directly from the operation of phosphoric workshops.
  • alkaline hydroxide 2 can be dissolved directly in gypsum water without additional addition of water.
  • water can be added to homogenize the medium, adjust the pH and/or adjust the molar ratio of calcium ions to hydroxide ions, noted Ca/OH.
  • the source of phosphogypsum 1 can also be phosphogypsum dried in a heap or slag heap. In this case, we dissolve the phosphogypsum and the alkaline hydroxide 2 in water.
  • the alkaline hydroxide 2 is preferably sodium hydroxide or potassium hydroxide.
  • the alkaline hydroxide 2 can be added to the reaction medium so as to obtain a molar ratio of calcium ions to hydroxide ions, denoted Ca/OH, of the order of 2, the calcium concentration of the phosphogypsum source 1 having been previously determined by 150 min and 180 min, so as to precipitate calcium hydroxide.
  • the mixture of phosphogypsum and alkali hydroxide also generates sulfates. For example, if we use sodium hydroxide, we generate sodium sulfate. If we use potassium hydroxide, we generate potassium sulfate.
  • Table 1 presents the chemical composition, in percentages by weight, of the solid comprising calcium hydroxide 3 resulting from the mixture of alkaline hydroxide 2 with the source of phosphogypsum 1, the chemical composition being determined by X-ray fluorescence spectrometry and by ionometry (for fluoride ions) after filtration and drying of said solid 3.
  • LD means limit of detection. It appears from this chemical composition that solid 3 comprises around 65% calcium hydroxide (see element CaO in Table 1).
  • the solid 3 resulting from the mixture of the phosphogypsum source 1 with the alkaline hydroxide 2 constitutes a relatively concentrated source of calcium.
  • Such a concentration will advantageously be used in step E3 of dispersing the solid comprising calcium hydroxide 3 in the waste water of interest 4.
  • the solid comprising calcium hydroxide 3 resulting from the step of mixture between the phosphogypsum source 1 and the alkaline hydroxide 2 is filtered and dried in an oven at a temperature between 50°C and 70°C, preferably at a temperature of 60°C.
  • Said solid 3 can then be directly dispersed in the waste water 4 from which it is desired to recover and valorize the phosphorus without any additional washing or purification step.
  • sulfates for example potassium sulfates or sodium sulfates
  • Potassium sulfates or sodium sulfates are obtained by drying said filtrate at a temperature between 70°C and 90°C, preferably at a temperature of the order of 80°C, so as to precipitate said sulfates.
  • a step E2 Determination of the concentration of orthophosphate ions in the wastewater loaded with orthophosphate ions
  • the invention preferably focuses on the recovery of phosphorus in wastewater 4 highly loaded with orthophosphate ions.
  • the waste water may have a concentration of orthophosphate ions of between 400 mg/L and 2000 mg/L, preferably a concentration of between 500 mg/L and 1000 mg/L.
  • Waste water 4 is for example water for washing gases from phosphoric workshops which is water particularly concentrated in orthophosphate ions.
  • the phosphogypsum source 1 and the gas washing water 4 come from the same phosphoric workshop, so that the process makes it possible to recover the phosphorus from the gas washing water. gas 4 from said workshop while eliminating the source of phosphogypsum 1 produced by this same workshop.
  • the concentration of orthophosphate ions in wastewater 4 can be determined for example by UV visible spectrophotometry.
  • a step E3 the solid comprising calcium hydroxide 3 is then dispersed in the waste water 4 so as to precipitate the calcium ions with the ions orthophosphates contained in wastewater 4 in the form of monetite or anhydrous dicalcium phosphate CaHPO 4 , according to a mechanism represented by the following equations: Ca(OH) 2 + H 2 O ⁇ Ca 2+ + 2 OH- + H 2 O Eq . (3) PO 4 3- + H 2 O ⁇ HPO 4 2- + OH- Eq. (4) HPO 4 2- + Ca 2+ ⁇ CaHPO 4 Eq. (5).
  • the pH is between 7 and 12, preferably between 10.5 and 12.5.
  • a basic pH favors an almost complete reaction between the calcium ions and the orthophosphate ions present in solution, which makes it possible to maximize the conversion efficiency of the orthophosphate ions into monetite.
  • the dispersed quantity of solid comprising calcium hydroxide 3 is chosen as a function of the concentration of orthophosphate ions determined during the previous step.
  • the solid comprising calcium hydroxide 3 is introduced so as to obtain calcium to phosphorus molar ratios, denoted Ca/P, which are high, preferably between 1 and 10, even more preferably between 1 and 5.
  • Phosphogypsum which is an industrial waste with a high calcium content, makes it possible to obtain such Ca/P mass ratios.
  • the solid comprising calcium hydroxide 3 forms a calcium matrix having the particularity of being easily dissociable into calcium Ca 2+ ions and OH- ions which therefore dissolves easily during its subsequent dispersion in water. spent 4.
  • the hydroxide counterion resulting from the dispersion of calcium hydroxide makes it possible to generate a basic to strongly basic medium, with a pH between 11 and 12.5. Obtaining a basic medium, preferably with a pH greater than 8, is necessary for the generation of the hydrogen phosphate ion and the precipitation of phosphorus in the form of monetite or anhydrous dicalcium phosphate.
  • the solid comprising calcium hydroxide therefore not only constitutes a source of calcium, but also makes it possible to avoid the use of a strong base.
  • the dispersion of the solid comprising calcium hydroxide 3 in the waste water 4 of step E3 is carried out in a reactor, for example a stirred reactor, for a period of between 10 min and 30 min, preferably a duration of between 10 min and 20 min, at atmospheric pressure and at a temperature of between 25°C and 60°C, preferably a temperature of between 25°C and 40°C.
  • step E3 of dispersing the solid comprising calcium hydroxide in the waste water and the associated conversion yields are described, the conversion yield being calculated as: Where C 0 is the initial concentration of orthophosphate ions in the waste water 4 and C f the final concentration of orthophosphate ions in the water depleted in orthophosphate ions 6 at the end of the process.
  • Example 1 Masses of between 23 g and 33 g of solid comprising calcium hydroxide prepared from the alkaline treatment of phosphogypsum with sodium hydroxide or potassium hydroxide and sieved at 200 ⁇ m are dispersed in a volume between 1 L and 5 L of gas washing water (real effluent).
  • the concentration of orthophosphate ions in these gas washing waters was previously measured and evaluated as being of the order of 500 mg/L.
  • the solid comprising calcium hydroxide is introduced into the gas washing water so that the Ca/P molar ratio in the dispersed solution is of the order of 1.
  • the dispersion is carried out in a stirred reactor for 10 min at a temperature of 40°C and a pH of 7.
  • the pH is maintained at 7 by acidification of the medium using hydrochloric acid (10%).
  • the yield is between 75% and 85%.
  • Example 2 The gas washing water from Example 1 is replaced by pure water doped with synthetic phosphorus prepared by dissolving sodium hydrogen phosphate in pure water at a concentration of 500 mg/L. All other parameters remain unchanged compared to Example 1.
  • the dispersion of the solid comprising calcium hydroxide in pure water doped with phosphorus is carried out for 10 min at a temperature of 40 ° C and at a pH of the order of 7.
  • the masses of the solid comprising calcium hydroxide are also identical to those of Example 1.
  • the yield is between 75% and 85%
  • Example 3 The dispersion of the solid comprising the calcium hydroxide in the gas washing water of Example 1 (initial concentration of orthophosphate ions of the order of 500 mg/L) is used for 20 min at a temperature of 25 ° C and at a pH of the order of 11 (pH fixed by the solid comprising calcium hydroxide not requiring modification of the pH by the addition of an additional reagent).
  • Example 4 The gas washing water of Examples 1 and 3 is replaced by pure water doped with phosphorus prepared by dissolving sodium hydrogen phosphate in pure water at a concentration of 500 mg/L and identical to that of example 2. The dispersion of the solid comprising calcium hydroxide in pure water doped with phosphorus is carried out for 20 minutes at a temperature of 25 ° C and at a pH of around 11, the other parameters remaining unchanged compared to Example 2. The yield is between 85% and 99.7%.
  • the method according to the invention works in the same way on a model solution as on real effluent.
  • the pH is preferably chosen basic, because the yield is better.
  • Separation of monetite from waste water In a step E4, the solid 5 resulting from the dispersion of the solid comprising calcium hydroxide 3 in the waste water of interest 4 is finally separated from the waste water depleted in phosphorus 6 by filtration. Then, the solid obtained is dried by steaming at a temperature between 50°C and 70°C, preferably at a temperature of 60°C, without any other additional treatment.
  • the X-ray diffraction analysis of said solid 5 includes all the lines of the monetite CaHPO 4 and therefore indicates that said solid 5 comprises monetite.
  • Element Content (%) SO3 1.91 SiO2 0.72 P 2 O 5 2.15 Na2O ⁇ LD MgO 0.006 F- 0.09 Al 2 O 3 0.30 K 2 O ⁇ LD Na2O ⁇ LD CaO 53.05 Fe2O3 0.052
  • Table 2 presents a table of the chemical composition, in percentage by weight, of the solid comprising monetite 5 resulting from the dispersion of the solid comprising calcium hydroxide 3 in the waste water 4 from which we seek to recover phosphorus.
  • the chemical composition of solid 5 is determined by X-ray fluorescence spectrometry and by ionometry (for fluoride ions).
  • the solid comprising monetite is directly revalued as fertilizer, in the plaster industry and/or in the cement industry.
  • the dual recovery of phosphorus present in wastewater and phosphogypsum makes the process particularly interesting in the context of a circular economy with reduced environmental impact.

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  • Life Sciences & Earth Sciences (AREA)
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Abstract

The invention relates to a method for recovering phosphorus in wastewater laden with orthophosphate ions (4), said method comprising at least the following successive steps: - (E1) mixing a source of phosphogypsum (1) and an alkali metal hydroxide (2) in an aqueous medium so as to precipitate a solid comprising calcium hydroxide (3); - (E2) determining the concentration of orthophosphate ions in the wastewater laden with orthophosphate ions (4); - (E3) dispersing the solid comprising calcium hydroxide (3) in the wastewater (4) so as to precipitate the calcium with the orthophosphate ions from the wastewater in the form of a solid comprising monetite (5), the amount of dispersed calcium hydroxide being chosen on the basis of the concentration of orthophosphate ions determined in step (E2); - (E4) separating the solid comprising monetite (5) obtained and the wastewater (6) depleted in orthophosphate ions.

Description

PROCEDE DE RECUPERATION DU PHOSPHORE DANS UNE EAU USEE CHARGEE EN IONS ORTHOPHOSPHATES DOMAINE TECHNIQUE L’invention concerne un procédé de récupération du phosphore dans une eau usée chargée en ions orthophosphates. ETAT DE LA TECHNIQUE Les activités anthropiques génèrent de très gros volumes d'effluents industriels qui, sans traitements adéquats, contaminent le milieu récepteur. Les rejets incontrôlés d’origine industrielle chargés en phosphore sont considérés parmi les formes de pollution les plus indésirables pour le milieu récepteur. En effet, ils provoquent un déséquilibre écologique important dans les écosystèmes aquatiques, connu sous le nom d'eutrophisation. Le phénomène d’eutrophisation est principalement dû à un enrichissement de l’eau en nutriments tels que le phosphore et l’azote qui peut conduire à une présence accrue d'algues et un appauvrissement de l'eau en oxygène, avec pour conséquence un risque accru de mortalité chez certains organismes aquatiques. Les industries chimiques des phosphates et particulièrement les ateliers phosphoriques rejettent des effluents riches en phosphore, issus des eaux de lavage des gaz notamment. En effet, les eaux de lavage des gaz contiennent des teneurs en phosphore non négligeables, et qui peuvent atteindre dans certains cas des valeurs comprises entre 450 et 550 mg/L en ions orthophosphates PO^^ ^ , mesurées à des pH de 11 et 13. Généralement, ces eaux riches en phosphore ne subissent aucun traitement et sont directement déversées dans le milieu récepteur au risque de générer des problèmes environnementaux. En outre, le phosphore contenu dans ces effluents constitue une ressource non renouvelable à forte valeur ajoutée qu’il conviendrait de valoriser. Il existe donc un besoin de valorisation des effluents industriels riches en phosphore, par exemple des eaux de lavage de gaz, pour répondre à des contraintes notamment d’ordre environnemental et économique. BREVE DESCRIPTION DE L’INVENTION Un but de l’invention est de concevoir un procédé qui permette de récupérer le phosphore dans des eaux usées de sorte à limiter l’impact environnemental desdites eaux usées tout en étant peu coûteux et attractif pour les industriels. A cet effet, l’invention propose un procédé de récupération du phosphore dans une eau usée chargée en ions orthophosphates comprenant au moins les étapes successives suivantes : - (E1) mélange d’une source de phosphogypse et d’un hydroxyde alcalin en milieu aqueux de sorte à faire précipiter un solide comprenant de l’hydroxyde de calcium, - (E2) détermination de la concentration en ions orthophosphates dans l’eau usée chargée en ions orthophosphates, - (E3) dispersion du solide comprenant de l’hydroxyde de calcium dans l’eau usée de sorte à faire précipiter le calcium avec les ions orthophosphates de l’eau usée sous forme d’un solide comprenant de la monétite, la quantité d’hydroxyde de calcium dispersée étant choisie en fonction de la concentration en ions orthophosphates déterminée lors de l’étape (E2), - (E4) séparation du solide comprenant de la monétite et de l’eau usée appauvrie en ions orthophosphates. Le procédé selon l’invention permet de récupérer le phosphore dans des eaux usées en valorisant un sous-produit généré en grande quantité par les industries chimiques du phosphate : le phosphogypse. En outre, le phosphore est extrait des eaux usées et récupéré sous une forme également valorisable, la monétite, qui peut être vendue en tant qu’engrais phosphaté. La valorisation double du phosphore présent dans les eaux usées et du phosphogypse rend le procédé particulièrement intéressant dans le cadre d’une économie circulaire dont l’impact environnemental est réduit. Selon d’autres caractéristiques optionnelles de l’invention prises seules ou en combinaison lorsque cela est techniquement possible : - la source de phosphogypse est une eau gypseuse directement issue du traitement de minerais calciques fluorophosphatés ; - l’hydroxyde alcalin comprend de l’hydroxyde de calcium ou de l’hydroxyde de potassium ; - l’étape de dispersion (E3) est mise en œuvre pendant une durée comprise entre 10 minutes et 30 minutes, préférentiellement une durée comprise entre 10 minutes et 20 minutes ; - le pH lors de l’étape de dispersion (E3) est compris entre 7 et 12, préférentiellement compris entre 10,5 et 12,5 ; - aucune base forte ni aucune autre source de calcium n’est ajoutée lors de l’étape de dispersion (E3) ; - la température lors de l’étape de dispersion (E3) est comprise entre 25 °C et 60 °C, préférentiellement entre 25 °C et 40 °C, encore plus préférentiellement de l’ordre de 25 °C ; - l’eau usée (4) comprend initialement entre 400 mg/L et 2000 mg/L d’ions orthophosphates, préférentiellement entre 500 mg/L et 1000 mg/L d’ions orthophosphates ; - l’eau usée (4) est une eau de lavage d’un gaz constituant un rejet industriel, préférentiellement une eau de lavage d’un gaz issu d’un atelier de synthèse d’acide phosphorique, préférentiellement encore une eau de lavage d’un gaz issu de la même installation que la source de phosphogypse ; - le solide comprenant de l’hydroxyde de calcium issu de l’étape (E1) est filtré, séché et dispersé dans l’eau usée lors de l’étape (E3) sans autre étape de traitement ou de lavage préalablement à la dispersion dans l’eau usée ; - le pourcentage massique en hydroxyde de calcium dans le solide issu de l’étape (E1) est supérieur à 60 % ; - le solide comprenant de la monétite est revalorisée en tant qu’engrais, dans l’industrie du plâtre et/ou dans l’industrie des ciments ; - la quantité d’hydroxyde alcalin mélangée à la source de phosphogypse dans l’étape (E1) est choisie en fonction de la concentration en calcium de la source de phosphogypse préalablement déterminée par spectrométrie de fluorescence des rayons X ; - on valorise des sous-produits issus de l’étape (E1), parmi lesquels le sulfate de potassium et le sulfate de sodium, sous la forme d’adjuvants industriels ; - le solide comprenant de l’hydroxyde de calcium issu de l’étape (E1) est filtré, le filtrat étant séchée à une température comprise entre 70 °C et 90 °C, préférentiellement à une température de l’ordre de 80 °C, de sorte à faire précipiter le sulfate de potassium et/ou le sulfate de sodium ; - la quantité de solide comprenant de l’hydroxyde de calcium dispersée dans l’eau usée lors de l’étape (E3) est choisie de sorte que le rapport massique Ca/P soit compris entre 1 et 10, préférentiellement de sorte que le rapport massique Ca/P soit compris entre 1 et 5. BREVE DESCRIPTION DES FIGURES D’autres caractéristiques et avantages de l’invention ressortiront de la description détaillée qui va suivre, en référence aux dessins annexés, sur lesquels : - la figure 1 représente un schéma du procédé selon l’invention, - la figure 2 représente un résultat d’analyse par diffraction des rayons X du solide comprenant de l’hydroxyde de calcium issu du mélange d’un hydroxyde alcalin avec un phosphogypse, - la figure 3 représente un résultat d’analyse par diffraction des rayons X du solide comprenant de la monétite issu de la dispersion du solide comprenant de l’hydroxyde de calcium dans l’eau usée dont on cherche à récupérer le phosphore, DESCRIPTION DETAILLEE DE MODES DE REALISATION Dans la suite de la description, on désigne par phosphates ou ions orthophosphates les ions PO4 3- dérivés de l’acide phosphorique (H3PO4) par perte de trois atomes d’hydrogène. L’invention concerne un procédé de récupération du phosphore dans une eau usée chargée en ions orthophosphates. L’eau usée peut comprendre initialement entre 400 mg/L et 2000 mg/L d’ions orthophosphates, préférentiellement entre 500 mg/L et 1000 mg/L d’ions orthophosphates. Selon un mode de réalisation de l’invention, l’eau usée est une eau de lavage des gaz constituant un rejet industriel, préférentiellement une eau de lavage de gaz émis par des ateliers phosphoriques. L’invention utilise la propriété des ions calcium, dans certaines conditions de pH, à précipiter avec les ions hydrogénophosphate pour générer du phosphate dicalcique anhydre ou monétite. Le procédé utilise une source d’hydroxyde de calcium comme source d’ions calcium. Les ions hydroxydes introduits avec les ions calcium permettent de contrôler le pH de sorte à générer la monétite. Dans l’invention, la source d’hydroxyde de calcium est issue de la transformation du phosphogypse ou sulfate de calcium hydraté. Préparation de la source d’hydroxyde de calcium à partir du phosphogypse En référence à la figure 1, on disperse, dans une étape E1, une source de phosphogypse 1 en milieu aqueux et on ajoute un hydroxyde alcalin 2. Les industries chimiques des phosphates génèrent du phosphogypse ou sulfate de calcium en très grande quantité (de l’ordre de 280 Mt/an) lors de la fabrication de l’acide phosphorique et des engrais phosphatés à partir des minerais calciques fluorophosphatés : L’équation de la réaction de production d’acide phosphorique est la suivante : Ca5(PO4)3F + 5 H2SO4 + 10 H2O → 3 H3PO4 + 5 CaSO4(H2O)2 + HF (Eq.1). Outre le calcium et le sulfate, le phosphogypse issu de l’industrie chimique des phosphates comprend plusieurs impuretés telles que le quartz (SiO2), les fluorures (F-), les phosphates (PO4 3-), de la matière organique, des minéraux de fer et d’aluminium (Fe2O3, Al2O3) et des métaux lourds. Le rejet de phosphogypse en mer est source de pollution et d’eutrophisation des milieux. Alternativement, le phosphogypse est stocké sous forme de terrils ou de crassiers. Si le sol des zones de stockage n’est pas imperméabilisé, des éléments toxiques contenus dans ledit phosphogypse peuvent diffuser et polluer le sol et les nappes. Le procédé selon l’invention permet de valoriser ce déchet industriel source de pollutions. La source de phosphogypse 1 peut être une eau gypseuse directement issue de l’exploitation des ateliers phosphoriques. Dans ce cas, l’hydroxyde alcalin 2 peut être dissout directement dans l’eau gypseuse sans ajout supplémentaire d’eau. Alternativement, de l’eau peut être ajoutée pour homogénéiser le milieu, ajuster le pH et/ou ajuster le rapport molaire des ions calcium sur les ions hydroxyde, noté Ca/OH. La source de phosphogypse 1 peut aussi être du phosphogypse séché en terril ou crassier. Dans ce cas, on dissout le phosphogypse et l’hydroxyde alcalin 2 dans de l’eau. L’hydroxyde alcalin 2 est préférentiellement de l’hydroxyde de sodium ou de l’hydroxyde de potassium. L’hydroxyde alcalin 2 peut être ajouté au milieu réactionnel de sorte à obtenir un rapport molaire des ions calcium sur les ions hydroxyde, noté Ca/OH, de l’ordre de 2, la concentration en calcium de la source de phosphogypse 1 ayant été préalablement déterminée par spectrométrie de fluorescence des rayons X. Dans l’étape E1, la source de phosphogypse 1 est mélangée avec l’hydroxyde alcalin 2, de préférence dans un réacteur agité à température ambiante et à pH supérieur à 10 pendant une durée comprise entre 150 min et 180 min, de sorte à faire précipiter de l’hydroxyde de calcium. Le mélange de phosphogypse et d’hydroxyde alcalin génère également des sulfates. A titre d’exemple, si on utilise de l’hydroxyde de sodium, on génère du sulfate de sodium. Si on utilise de l’hydroxyde de potassium, on génère du sulfate de potassium. L’équation de la réaction entre la source de phosphogypse 1 et l’hydroxyde alcalin 2 est la suivante : CaSO4.2H2O + 2 NaOH/KOH → Ca(OH)2 + Na2SO4/K2SO4 + 2 H2O Eq. (2). La figure 2 représente le diffractogramme des rayons X du solide 3 issu de l’étape E1 de mélange de la source de phosphogypse 1 et de l’hydroxyde alcalin 2. On peut identifier sur le diffractogramme les raies caractéristiques de l’hydroxyde de sodium, ce qui prouve que le solide 3 comprend de l’hydroxyde de calcium. Élément Teneur (%) SO3 3,59 SiO2 2,53 P2O5 <LD Na2O 0,65 MgO 0,11 F- 0,6 Al2O3 0,05 Fe2O3 0,25 K2O 0,10 Na2O 0,65 CaO 65,31 Tableau 1 Le tableau 1 présente la composition chimique, en pourcentages massiques, du solide comprenant de l’hydroxyde de calcium 3 issu du mélange de l’hydroxyde alcalin 2 avec la source de phosphogypse 1, la composition chimique étant déterminée par spectrométrie de fluorescence des rayons X et par ionométrie (pour les ions fluorures) après filtration et séchage dudit solide 3. LD signifie limite de détection. Il apparait de cette composition chimique que le solide 3 comprend de l’ordre de 65 % d’hydroxyde de calcium (voir élément CaO du tableau 1). Les autres éléments compris dans le solide 3 et visibles dans le tableau 1 sont des impuretés qui proviennent du phosphogypse. Ainsi, le solide 3 issu du mélange de la source de phosphogypse 1 avec l’hydroxyde alcalin 2 constitue une source relativement concentrée en calcium. Une telle concentration sera avantageusement mise à profit dans l’étape E3 de dispersion du solide comprenant l’hydroxyde de calcium 3 dans l’eau usée d’intérêt 4. Le solide comprenant de l’hydroxyde de calcium 3 issu de l’étape de mélange entre la source de phosphogypse 1 et l’hydroxyde alcalin 2 est filtré et séché à l’étuve à une température comprise entre 50 °C et 70 °C, préférentiellement à une température de 60 °C. Ledit solide 3 peut ensuite être directement dispersé dans l’eau usée 4 dont on souhaite récupérer et valoriser le phosphore sans étape de lavage ou de purification supplémentaire. Optionnellement, les sulfates, par exemple les sulfates de potassium ou les sulfates de sodium, peuvent également être récupérés du filtrat pour être valorisés en tant qu’adjuvants dans plusieurs secteurs industriels. Les sulfates de potassium ou les sulfates de sodium sont obtenus par séchage dudit filtrat à une température comprise entre 70 °C et 90 °C, préférentiellement à une température de l’ordre de 80 °C, de sorte à faire précipiter lesdits sulfates. Détermination de la concentration en ions orthophosphates dans l’eau usée chargée en ions orthophosphates Dans une étape E2, on détermine ensuite, dans l’eau usée d’intérêt 4, la concentration en ions orthophosphates que l’on cherche à récupérer et valoriser, par précipitation avec des ions calcium, sous forme de monétite. L’invention s’intéresse de préférence à la récupération du phosphore dans des eaux usées 4 très chargées en ions orthophosphates. De par la forte concentration en calcium du solide issu de la transformation du phosphogypse, l’eau usée pourra présenter une concentration en ions orthophosphates comprise entre 400 mg/L et 2000 mg/L, préférentiellement une concentration comprise entre 500 mg/L et 1000 mg/L. L’eau usée 4 est par exemple une eau de lavage des gaz issus des ateliers phosphoriques qui est une eau particulièrement concentrée en ions orthophosphates. Selon un mode de réalisation particulièrement avantageux de l’invention, la source de phosphogypse 1 et l’eau de lavage de gaz 4 sont issus du même atelier phosphorique, de sorte que le procédé permet de valoriser le phosphore de l’eau de lavage de gaz 4 dudit atelier tout en éliminant la source de phosphogypse 1 produit par ce même atelier. La concentration en ions orthophosphates dans l’eau usée 4 peut être déterminée par exemple par spectrophotométrie UV visible. Dispersion du solide comprenant de l’hydroxyde de calcium dans l’eau usée Dans une étape E3, on disperse ensuite le solide comprenant de l’hydroxyde de calcium 3 dans l’eau usée 4 de sorte à faire précipiter les ions calcium avec les ions orthophosphates contenu dans l’eau usée 4 sous forme de monétite ou phosphate dicalcique anhydre CaHPO4, selon un mécanisme représenté par les équations suivantes : Ca(OH)2 + H2O → Ca2+ + 2 OH- + H2O Eq. (3) PO4 3- + H2O → HPO4 2- + OH- Eq. (4) HPO4 2- + Ca2+→ CaHPO4 Eq. (5). Lors de cette étape E3 de dispersion, le pH est compris entre 7 et 12, préférentiellement compris entre 10,5 et 12,5. Un pH basique favorise une réaction quasi-totale entre les ions calcium et les ions orthophosphates présents en solution, ce qui permet de maximiser le rendement de conversion des ions orthophosphates en monétite. La quantité dispersée de solide comprenant de l’hydroxyde de calcium 3 est choisie en fonction de la concentration en ions orthophosphates déterminée lors de l’étape précédente. Le solide comprenant de l’hydroxyde de calcium 3 est introduit de sorte à obtenir des rapports molaires calcium sur phosphore, noté Ca/P, qui sont élevés, préférentiellement compris entre 1 et 10, encore plus préférentiellement entre 1 et 5. Le phosphogypse, qui est un déchet industriel avec une teneur forte en calcium, permet d’obtenir de tels rapport massiques Ca/P. En outre, le solide comprenant de l’hydroxyde de calcium 3 forme une matrice calcique présentant la particularité d’être facilement dissociable en ions calcium Ca2+ et en ions OH- qui se dissout donc facilement lors de sa dispersion ultérieure dans l’eau usée 4. Enfin, le contre-ion hydroxyde issu de la dispersion de l’hydroxyde de calcium permet de générer un milieu basique à fortement basique, avec un pH compris entre 11 et 12,5. L’obtention d’un milieu basique, préférentiellement d’un pH supérieur à 8, est nécessaire pour la génération de l’ion hydrogénophosphate et la précipitation du phosphore sous la forme de monétite ou phosphate dicalcique anhydre. A titre d’exemple, un pH plus acide pourrait entrainer la précipitation du phosphore sous forme de brushite (CaHPO4.2H2O). Le solide comprenant de l’hydroxyde de calcium constitue donc non seulement une source de calcium, mais permet également de s’affranchir de l’utilisation d’une base forte. La dispersion du solide comprenant de l’hydroxyde de calcium 3 dans l’eau usée 4 de l’étape E3 est mise en œuvre dans un réacteur, par exemple un réacteur agité, pendant une durée comprise entre 10 min et 30 min, préférentiellement une durée comprise entre 10 min et 20 min, à pression atmosphérique et à une température comprise entre 25 °C et 60 °C, préférentiellement une température comprise entre 25 °C et 40 °C. Dans la suite, on décrit quatre exemples de mise en œuvre de l’étape E3 de dispersion du solide comprenant de l’hydroxyde de calcium dans l’eau usée et les rendements de conversion associés, le rendement de conversion étant calculé comme :
Figure imgf000010_0001
Où C0 est la concentration initiale en ions orthophosphates de l’eau usée 4 et Cf la concentration finale en ions orthophosphates de l’eau appauvrie en ions orthophosphates 6 à l’issue du procédé. Exemple 1 Des masses comprises entre 23 g et 33 g de solide comprenant de l’hydroxyde de calcium préparé à partir du traitement alcalin de phosphogypse par de l’hydroxyde de sodium ou de l’hydroxyde de potassium et tamisé à 200 µm sont dispersées dans un volume compris entre 1 L et 5 L d’eaux de lavage de gaz (effluents réels). La concentration en ions orthophosphates de ces eaux de lavage de gaz a préalablement été mesurée et évaluée comme étant de l’ordre de 500 mg/L. Le solide comprenant de l’hydroxyde de calcium est introduit dans les eaux de lavage de gaz de sorte que le rapport molaire Ca/P dans la solution dispersée soit de l’ordre de 1. La dispersion est mise en œuvre en réacteur agité pendant 10 min à une température de 40 °C et à un pH de 7. Le pH est maintenu à 7 par acidification du milieu à l’aide d’acide chlorhydrique (10%). Le rendement est compris entre 75 % et 85 %. Exemple 2 L’eau de lavage des gaz de l’exemple 1 est remplacée par une eau pure dopée en phosphore synthétique préparée par dissolution d’hydrogénophosphate de sodium dans de l’eau pure à une concentration de 500 mg/L. Tous les autres paramètres demeurent inchangés par rapport à l’exemple 1. Ainsi, la dispersion du solide comprenant de l’hydroxyde de calcium dans l’eau pure dopée au phosphore est mise en œuvre pendant 10 min à une température de 40 °C et à un pH de l’ordre de 7. Les masses du solide comprenant de l’hydroxyde de calcium sont également identiques à celles de l’exemple 1. Le rendement est compris entre 75 % et 85 % Exemple 3 La dispersion du solide comprenant de l’hydroxyde de calcium dans les eaux de lavage des gaz de l’exemple 1 (concentration initiale en ions orthophosphates de l’ordre de 500 mg/L) est mise en œuvre pendant 20 min à une température de 25 °C et à un pH de l’ordre de 11 (pH fixé par le solide comprenant de l’hydroxyde de calcium ne nécessitant pas de modification du pH par l’ajout d’un réactif supplémentaire). Les autres paramètres demeurent inchangés par rapport à l’exemple 1, notamment les masses de solide comprenant de l’hydroxyde de calcium et volumes d’eaux de lavage des gaz utilisés. Dans ces conditions, les ions calcium réagissent quasi totalement avec le phosphore présent dans la solution. Le rendement est compris entre 85 % et 99,7 %. Exemple 4 L’eau de lavage des gaz des exemples 1 et 3 est remplacée par une eau pure dopée en phosphore préparée par dissolution d’hydrogénophosphate de sodium dans de l’eau pure à une concentration de 500 mg/L et identique à celle de l’exemple 2. La dispersion du solide comprenant de l’hydroxyde de calcium dans l’eau pure dopée au phosphore est mise en œuvre pendant 20 minutes à une température de 25 °C et à un pH de l’ordre de 11, les autres paramètres demeurant inchangés par rapport à l’exemple 2. Le rendement est compris entre 85 % et 99,7 %. Ainsi, le procédé selon l’invention fonctionne de la même façon sur une solution modèle que sur effluent réel. En outre, le pH est choisi préférentiellement basique, car le rendement est meilleur. Séparation de la monétite de l’eau usée Dans une étape E4, le solide 5 issu de la dispersion du solide comprenant de l’hydroxyde de calcium 3 dans l’eau usée d’intérêt 4 est finalement séparé de l’eau usée appauvrie en phosphore 6 par filtration. Puis, le solide obtenu est séché par étuvage à une température comprise entre 50 °C et 70 °C, préférentiellement à une température de 60 °C, sans autre traitement supplémentaire. En référence à la figure 3, l’analyse par diffraction des rayons X dudit solide 5 comprend toutes les raies de la monétite CaHPO4 et indique donc ainsi que ledit solide 5 comprend de la monétite. Élément Teneur (%) SO3 1,91 SiO2 0,72 P2O5 2,15 Na2O <LD MgO 0,006 F- 0,09 Al2O3 0,30 K2O <LD Na2O <LD CaO 53,05 Fe2O3 0,052 Tableau 2 Le tableau 2 présente un tableau de la composition chimique, en pourcentage massique, du solide comprenant de la monétite 5 issu de la dispersion du solide comprenant de l’hydroxyde de calcium 3 dans l’eau usée 4 dont on cherche à récupérer le phosphore. La composition chimique du solide 5 est déterminée par spectrométrie de fluorescence des rayons X et par ionométrie (pour les ions fluorures). Selon un mode de réalisation particulier de l’invention, le solide comprenant de la monétite est directement revalorisé en tant qu’engrais, dans l’industrie du plâtre et/ou dans l’industrie des ciments. La valorisation double du phosphore présent dans les eaux usées et du phosphogypse rend le procédé particulièrement intéressant dans le cadre d’une économie circulaire dont l’impact environnemental est réduit. METHOD FOR RECOVERING PHOSPHORUS FROM WASTE WATER LOADED WITH ORTHOPHOSPHATES IONS TECHNICAL FIELD The invention relates to a process for recovering phosphorus from waste water loaded with orthophosphate ions. STATE OF THE TECHNIQUE Anthropogenic activities generate very large volumes of industrial effluents which, without adequate treatment, contaminate the receiving environment. Uncontrolled discharges of industrial origin loaded with phosphorus are considered among the most undesirable forms of pollution for the receiving environment. Indeed, they cause a significant ecological imbalance in aquatic ecosystems, known as eutrophication. The phenomenon of eutrophication is mainly due to an enrichment of water in nutrients such as phosphorus and nitrogen which can lead to an increased presence of algae and a depletion of oxygen in the water, with the consequence of a risk increased mortality in certain aquatic organisms. The phosphate chemical industries and particularly the phosphoric workshops discharge effluents rich in phosphorus, particularly from gas washing water. Indeed, gas washing water contains significant phosphorus contents, which can in certain cases reach values of between 450 and 550 mg/L in orthophosphate ions PO^^ ^, measured at pHs of 11 and 13 Generally, this water rich in phosphorus does not undergo any treatment and is discharged directly into the receiving environment at the risk of generating environmental problems. In addition, the phosphorus contained in these effluents constitutes a non-renewable resource with high added value which should be exploited. There is therefore a need to recover industrial effluents rich in phosphorus, for example gas washing water, to respond to constraints in particular of an environmental and economic nature. BRIEF DESCRIPTION OF THE INVENTION An aim of the invention is to design a process which makes it possible to recover phosphorus in wastewater so as to limit the environmental impact of said wastewater while being inexpensive and attractive for manufacturers. To this end, the invention proposes a process for recovering phosphorus from waste water loaded with orthophosphate ions comprising at least the following successive steps: - (E1) mixing a source of phosphogypsum and an alkaline hydroxide in an aqueous medium so as to precipitate a solid comprising calcium hydroxide, - (E2) determination of the concentration of orthophosphate ions in waste water loaded with orthophosphate ions, - (E3) dispersion of the solid comprising calcium hydroxide in the wastewater so as to precipitate the calcium with the orthophosphate ions of the wastewater in the form of a solid comprising monetite, the quantity of dispersed calcium hydroxide being chosen as a function of the concentration of orthophosphate ions determined during step (E2), - (E4) separation of the solid comprising monetite and waste water depleted in orthophosphate ions. The process according to the invention makes it possible to recover phosphorus from wastewater by recovering a by-product generated in large quantities by the phosphate chemical industries: phosphogypsum. In addition, phosphorus is extracted from wastewater and recovered in an equally recoverable form, monetite, which can be sold as phosphate fertilizer. The dual recovery of phosphorus present in wastewater and phosphogypsum makes the process particularly interesting in the context of a circular economy with reduced environmental impact. According to other optional characteristics of the invention taken alone or in combination when technically possible: - the source of phosphogypsum is gypsum water directly resulting from the treatment of fluorophosphate calcium ores; - the alkaline hydroxide comprises calcium hydroxide or potassium hydroxide; - the dispersion step (E3) is implemented for a duration of between 10 minutes and 30 minutes, preferably a duration of between 10 minutes and 20 minutes; - the pH during the dispersion step (E3) is between 7 and 12, preferably between 10.5 and 12.5; - no strong base or other source of calcium is added during the dispersion step (E3); - the temperature during the dispersion step (E3) is between 25°C and 60°C, preferably between 25°C and 40°C, even more preferably of the order of 25°C; - the waste water (4) initially comprises between 400 mg/L and 2000 mg/L of orthophosphate ions, preferably between 500 mg/L and 1000 mg/L of orthophosphate ions; - the waste water (4) is water for washing a gas constituting an industrial waste, preferably water for washing a gas coming from a phosphoric acid synthesis workshop, preferably water for washing 'a gas from the same installation as the phosphogypsum source; - the solid comprising calcium hydroxide from step (E1) is filtered, dried and dispersed in the waste water during step (E3) without any other treatment or washing step prior to dispersion in waste water; - the mass percentage of calcium hydroxide in the solid resulting from step (E1) is greater than 60%; - the solid comprising monetite is revalued as fertilizer, in the plaster industry and/or in the cement industry; - the quantity of alkaline hydroxide mixed with the phosphogypsum source in step (E1) is chosen as a function of the calcium concentration of the phosphogypsum source previously determined by X-ray fluorescence spectrometry; - we recycle by-products from step (E1), including potassium sulfate and sodium sulfate, in the form of industrial adjuvants; - the solid comprising calcium hydroxide from step (E1) is filtered, the filtrate being dried at a temperature of between 70°C and 90°C, preferably at a temperature of the order of 80°C , so as to precipitate the potassium sulfate and/or sodium sulfate; - the quantity of solid comprising calcium hydroxide dispersed in the waste water during step (E3) is chosen so that the Ca/P mass ratio is between 1 and 10, preferably so that the ratio mass Ca/P is between 1 and 5. BRIEF DESCRIPTION OF THE FIGURES Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which: - Figure 1 represents a diagram of the process according to the invention, - Figure 2 represents a result of X-ray diffraction analysis of the solid comprising calcium hydroxide resulting from the mixture of an alkaline hydroxide with a phosphogypsum, - Figure 3 represents a result analysis by X-ray diffraction of the solid comprising monetite resulting from the dispersion of the solid comprising calcium hydroxide in the waste water from which we seek to recover the phosphorus, DETAILED DESCRIPTION OF EMBODIMENTS In the remainder of the description, phosphates or orthophosphate ions refer to PO 4 3- ions derived from phosphoric acid (H 3 PO 4 ) by loss of three hydrogen atoms. The invention relates to a process for recovering phosphorus from wastewater loaded with orthophosphate ions. Waste water may initially contain between 400 mg/L and 2000 mg/L of orthophosphate ions, preferably between 500 mg/L and 1000 mg/L of orthophosphate ions. According to one embodiment of the invention, the waste water is gas washing water constituting an industrial waste, preferably gas washing water emitted by phosphoric workshops. The invention uses the property of calcium ions, under certain pH conditions, to precipitate with hydrogen phosphate ions to generate anhydrous dicalcium phosphate or monetite. The process uses a source of calcium hydroxide as a source of calcium ions. The hydroxide ions introduced with the calcium ions make it possible to control the pH so as to generate monetite. In the invention, the source of calcium hydroxide comes from the transformation of phosphogypsum or hydrated calcium sulfate. Preparation of the source of calcium hydroxide from phosphogypsum With reference to Figure 1, in a step E1, a source of phosphogypsum 1 is dispersed in an aqueous medium and an alkaline hydroxide 2 is added. The chemical industries of phosphates generate phosphogypsum or calcium sulfate in very large quantities (of the order of 280 Mt/year) during the manufacture of phosphoric acid and phosphate fertilizers from fluorophosphate calcium ores: The equation for the production reaction of phosphoric acid is as follows: Ca 5 (PO 4 ) 3 F + 5 H 2 SO 4 + 10 H 2 O → 3 H 3 PO 4 + 5 CaSO 4 (H 2 O) 2 + HF (Eq.1). In addition to calcium and sulfate, phosphogypsum from the phosphate chemical industry includes several impurities such as quartz (SiO 2 ), fluorides (F-), phosphates (PO 4 3- ), organic matter, iron and aluminum minerals (Fe 2 O 3 , Al 2 O 3 ) and heavy metals. The discharge of phosphogypsum into the sea is a source of pollution and eutrophication of the environment. Alternatively, phosphogypsum is stored in the form of slag heaps or slag heaps. If the soil in the storage areas is not waterproofed, toxic elements contained in said phosphogypsum can diffuse and pollute the soil and water tables. The process according to the invention makes it possible to recover this industrial waste, a source of pollution. The source of phosphogypsum 1 can be gypsum water directly from the operation of phosphoric workshops. In this case, alkaline hydroxide 2 can be dissolved directly in gypsum water without additional addition of water. Alternatively, water can be added to homogenize the medium, adjust the pH and/or adjust the molar ratio of calcium ions to hydroxide ions, noted Ca/OH. The source of phosphogypsum 1 can also be phosphogypsum dried in a heap or slag heap. In this case, we dissolve the phosphogypsum and the alkaline hydroxide 2 in water. The alkaline hydroxide 2 is preferably sodium hydroxide or potassium hydroxide. The alkaline hydroxide 2 can be added to the reaction medium so as to obtain a molar ratio of calcium ions to hydroxide ions, denoted Ca/OH, of the order of 2, the calcium concentration of the phosphogypsum source 1 having been previously determined by 150 min and 180 min, so as to precipitate calcium hydroxide. The mixture of phosphogypsum and alkali hydroxide also generates sulfates. For example, if we use sodium hydroxide, we generate sodium sulfate. If we use potassium hydroxide, we generate potassium sulfate. The equation for the reaction between the phosphogypsum source 1 and the alkaline hydroxide 2 is as follows: CaSO4.2H 2 O + 2 NaOH/KOH → Ca(OH) 2 + Na 2 SO 4 /K 2 SO 4 + 2 H 2 O Eq. (2). Figure 2 represents the X-ray diffractogram of the solid 3 resulting from step E1 of mixing the phosphogypsum source 1 and the alkaline hydroxide 2. We can identify on the diffractogram shows the characteristic lines of sodium hydroxide, which proves that solid 3 includes calcium hydroxide. Element Content (%) SO3 3.59 SiO2 2.53 P 2 O 5 <LD Na2O 0.65 MgO 0.11 F- 0.6 Al 2 O 3 0.05 Fe 2 O 3 0.25 K2O 0.10 Na 2 O 0.65 CaO 65.31 Table 1 Table 1 presents the chemical composition, in percentages by weight, of the solid comprising calcium hydroxide 3 resulting from the mixture of alkaline hydroxide 2 with the source of phosphogypsum 1, the chemical composition being determined by X-ray fluorescence spectrometry and by ionometry (for fluoride ions) after filtration and drying of said solid 3. LD means limit of detection. It appears from this chemical composition that solid 3 comprises around 65% calcium hydroxide (see element CaO in Table 1). The other elements included in solid 3 and visible in table 1 are impurities which come from phosphogypsum. Thus, the solid 3 resulting from the mixture of the phosphogypsum source 1 with the alkaline hydroxide 2 constitutes a relatively concentrated source of calcium. Such a concentration will advantageously be used in step E3 of dispersing the solid comprising calcium hydroxide 3 in the waste water of interest 4. The solid comprising calcium hydroxide 3 resulting from the step of mixture between the phosphogypsum source 1 and the alkaline hydroxide 2 is filtered and dried in an oven at a temperature between 50°C and 70°C, preferably at a temperature of 60°C. Said solid 3 can then be directly dispersed in the waste water 4 from which it is desired to recover and valorize the phosphorus without any additional washing or purification step. Optionally, sulfates, for example potassium sulfates or sodium sulfates, can also be recovered from the filtrate to be used as adjuvants in several industrial sectors. Potassium sulfates or sodium sulfates are obtained by drying said filtrate at a temperature between 70°C and 90°C, preferably at a temperature of the order of 80°C, so as to precipitate said sulfates. Determination of the concentration of orthophosphate ions in the wastewater loaded with orthophosphate ions In a step E2, we then determine, in the wastewater of interest 4, the concentration of orthophosphate ions that we seek to recover and valorize, by precipitation with calcium ions, in the form of monetite. The invention preferably focuses on the recovery of phosphorus in wastewater 4 highly loaded with orthophosphate ions. Due to the high concentration of calcium in the solid resulting from the transformation of phosphogypsum, the waste water may have a concentration of orthophosphate ions of between 400 mg/L and 2000 mg/L, preferably a concentration of between 500 mg/L and 1000 mg/L. Waste water 4 is for example water for washing gases from phosphoric workshops which is water particularly concentrated in orthophosphate ions. According to a particularly advantageous embodiment of the invention, the phosphogypsum source 1 and the gas washing water 4 come from the same phosphoric workshop, so that the process makes it possible to recover the phosphorus from the gas washing water. gas 4 from said workshop while eliminating the source of phosphogypsum 1 produced by this same workshop. The concentration of orthophosphate ions in wastewater 4 can be determined for example by UV visible spectrophotometry. Dispersion of the solid comprising calcium hydroxide in the waste water In a step E3, the solid comprising calcium hydroxide 3 is then dispersed in the waste water 4 so as to precipitate the calcium ions with the ions orthophosphates contained in wastewater 4 in the form of monetite or anhydrous dicalcium phosphate CaHPO 4 , according to a mechanism represented by the following equations: Ca(OH) 2 + H 2 O → Ca 2+ + 2 OH- + H 2 O Eq . (3) PO 4 3- + H 2 O → HPO 4 2- + OH- Eq. (4) HPO 4 2- + Ca 2+ → CaHPO 4 Eq. (5). During this dispersion step E3, the pH is between 7 and 12, preferably between 10.5 and 12.5. A basic pH favors an almost complete reaction between the calcium ions and the orthophosphate ions present in solution, which makes it possible to maximize the conversion efficiency of the orthophosphate ions into monetite. The dispersed quantity of solid comprising calcium hydroxide 3 is chosen as a function of the concentration of orthophosphate ions determined during the previous step. The solid comprising calcium hydroxide 3 is introduced so as to obtain calcium to phosphorus molar ratios, denoted Ca/P, which are high, preferably between 1 and 10, even more preferably between 1 and 5. Phosphogypsum, which is an industrial waste with a high calcium content, makes it possible to obtain such Ca/P mass ratios. In addition, the solid comprising calcium hydroxide 3 forms a calcium matrix having the particularity of being easily dissociable into calcium Ca 2+ ions and OH- ions which therefore dissolves easily during its subsequent dispersion in water. spent 4. Finally, the hydroxide counterion resulting from the dispersion of calcium hydroxide makes it possible to generate a basic to strongly basic medium, with a pH between 11 and 12.5. Obtaining a basic medium, preferably with a pH greater than 8, is necessary for the generation of the hydrogen phosphate ion and the precipitation of phosphorus in the form of monetite or anhydrous dicalcium phosphate. For example, a more acidic pH could lead to the precipitation of phosphorus in the form of brushite (CaHPO 4 .2H 2 O). The solid comprising calcium hydroxide therefore not only constitutes a source of calcium, but also makes it possible to avoid the use of a strong base. The dispersion of the solid comprising calcium hydroxide 3 in the waste water 4 of step E3 is carried out in a reactor, for example a stirred reactor, for a period of between 10 min and 30 min, preferably a duration of between 10 min and 20 min, at atmospheric pressure and at a temperature of between 25°C and 60°C, preferably a temperature of between 25°C and 40°C. In the following, four examples of implementation of step E3 of dispersing the solid comprising calcium hydroxide in the waste water and the associated conversion yields are described, the conversion yield being calculated as:
Figure imgf000010_0001
Where C 0 is the initial concentration of orthophosphate ions in the waste water 4 and C f the final concentration of orthophosphate ions in the water depleted in orthophosphate ions 6 at the end of the process. Example 1 Masses of between 23 g and 33 g of solid comprising calcium hydroxide prepared from the alkaline treatment of phosphogypsum with sodium hydroxide or potassium hydroxide and sieved at 200 μm are dispersed in a volume between 1 L and 5 L of gas washing water (real effluent). The concentration of orthophosphate ions in these gas washing waters was previously measured and evaluated as being of the order of 500 mg/L. The solid comprising calcium hydroxide is introduced into the gas washing water so that the Ca/P molar ratio in the dispersed solution is of the order of 1. The dispersion is carried out in a stirred reactor for 10 min at a temperature of 40°C and a pH of 7. The pH is maintained at 7 by acidification of the medium using hydrochloric acid (10%). The yield is between 75% and 85%. Example 2 The gas washing water from Example 1 is replaced by pure water doped with synthetic phosphorus prepared by dissolving sodium hydrogen phosphate in pure water at a concentration of 500 mg/L. All other parameters remain unchanged compared to Example 1. Thus, the dispersion of the solid comprising calcium hydroxide in pure water doped with phosphorus is carried out for 10 min at a temperature of 40 ° C and at a pH of the order of 7. The masses of the solid comprising calcium hydroxide are also identical to those of Example 1. The yield is between 75% and 85% Example 3 The dispersion of the solid comprising the calcium hydroxide in the gas washing water of Example 1 (initial concentration of orthophosphate ions of the order of 500 mg/L) is used for 20 min at a temperature of 25 ° C and at a pH of the order of 11 (pH fixed by the solid comprising calcium hydroxide not requiring modification of the pH by the addition of an additional reagent). Other parameters remain unchanged compared to Example 1, in particular the masses of solid comprising calcium hydroxide and volumes of gas washing water used. Under these conditions, the calcium ions react almost completely with the phosphorus present in the solution. The yield is between 85% and 99.7%. Example 4 The gas washing water of Examples 1 and 3 is replaced by pure water doped with phosphorus prepared by dissolving sodium hydrogen phosphate in pure water at a concentration of 500 mg/L and identical to that of example 2. The dispersion of the solid comprising calcium hydroxide in pure water doped with phosphorus is carried out for 20 minutes at a temperature of 25 ° C and at a pH of around 11, the other parameters remaining unchanged compared to Example 2. The yield is between 85% and 99.7%. Thus, the method according to the invention works in the same way on a model solution as on real effluent. In addition, the pH is preferably chosen basic, because the yield is better. Separation of monetite from waste water In a step E4, the solid 5 resulting from the dispersion of the solid comprising calcium hydroxide 3 in the waste water of interest 4 is finally separated from the waste water depleted in phosphorus 6 by filtration. Then, the solid obtained is dried by steaming at a temperature between 50°C and 70°C, preferably at a temperature of 60°C, without any other additional treatment. With reference to Figure 3, the X-ray diffraction analysis of said solid 5 includes all the lines of the monetite CaHPO 4 and therefore indicates that said solid 5 comprises monetite. Element Content (%) SO3 1.91 SiO2 0.72 P 2 O 5 2.15 Na2O <LD MgO 0.006 F- 0.09 Al 2 O 3 0.30 K 2 O <LD Na2O <LD CaO 53.05 Fe2O3 0.052 Table 2 Table 2 presents a table of the chemical composition, in percentage by weight, of the solid comprising monetite 5 resulting from the dispersion of the solid comprising calcium hydroxide 3 in the waste water 4 from which we seek to recover phosphorus. The chemical composition of solid 5 is determined by X-ray fluorescence spectrometry and by ionometry (for fluoride ions). According to a particular embodiment of the invention, the solid comprising monetite is directly revalued as fertilizer, in the plaster industry and/or in the cement industry. The dual recovery of phosphorus present in wastewater and phosphogypsum makes the process particularly interesting in the context of a circular economy with reduced environmental impact.

Claims

REVENDICATIONS 1. Procédé de récupération du phosphore dans une eau usée chargée en ions orthophosphates (4) comprenant au moins les étapes successives suivantes : - (E1) mélange d’une source de phosphogypse (1) et d’un hydroxyde alcalin (2) en milieu aqueux de sorte à faire précipiter un solide comprenant de l’hydroxyde de calcium (3), - (E2) détermination de la concentration en ions orthophosphates dans l’eau usée chargée en ions orthophosphates (4), - (E3) dispersion du solide comprenant de l’hydroxyde de calcium (3) dans l’eau usée (4) de sorte à faire précipiter le calcium avec les ions orthophosphates de l’eau usée sous forme d’un solide comprenant de la monétite (5), la quantité d’hydroxyde de calcium dispersée étant choisie en fonction de la concentration en ions orthophosphates déterminée lors de l’étape (E2), - (E4) séparation du solide comprenant de la monétite (5) et de l’eau usée appauvrie en ions orthophosphates (6). CLAIMS 1. Process for recovering phosphorus from waste water loaded with orthophosphate ions (4) comprising at least the following successive steps: - (E1) mixing a source of phosphogypsum (1) and an alkaline hydroxide (2) in an aqueous medium so as to precipitate a solid comprising calcium hydroxide (3), - (E2) determination of the concentration of orthophosphate ions in waste water loaded with orthophosphate ions (4), - (E3) dispersion a solid comprising calcium hydroxide (3) in the waste water (4) so as to precipitate the calcium with the orthophosphate ions of the waste water in the form of a solid comprising monetite (5), the quantity of dispersed calcium hydroxide being chosen as a function of the concentration of orthophosphate ions determined during step (E2), - (E4) separation of the solid comprising monetite (5) and waste water depleted in orthophosphate ions (6).
2. Procédé selon la revendication 1, dans lequel la source de phosphogypse (1) est une eau gypseuse directement issue du traitement de minerais calciques fluorophosphatés. 2. Method according to claim 1, in which the source of phosphogypsum (1) is gypsum water directly resulting from the treatment of fluorophosphate calcium ores.
3. Procédé selon l’une des revendications 1 ou 2, dans lequel l’hydroxyde alcalin (2) comprend de l’hydroxyde de calcium ou de l’hydroxyde de potassium. 3. Method according to one of claims 1 or 2, in which the alkaline hydroxide (2) comprises calcium hydroxide or potassium hydroxide.
4. Procédé selon l’une des revendications 1 à 3, dans lequel l’étape de dispersion (E3) est mise en œuvre pendant une durée comprise entre 10 minutes et 30 minutes, préférentiellement une durée comprise entre 10 minutes et 20 minutes. 5. Procédé selon l’une des revendications 1 à 4, dans lequel le pH lors de l’étape de dispersion (E3) est compris entre 7 et 12, préférentiellement compris entre 10,5 et 12,4. Method according to one of claims 1 to 3, in which the dispersion step (E3) is implemented for a duration of between 10 minutes and 30 minutes, preferably a duration of between 10 minutes and 20 minutes. 5. Method according to one of claims 1 to 4, in which the pH during the dispersion step (E3) is between 7 and 12, preferably between 10.5 and 12,
5. 5.
6. Procédé selon l’une des revendications 1 à 5, dans lequel aucune base forte ni aucune autre source de calcium n’est ajoutée lors de l’étape de dispersion (E3). 6. Method according to one of claims 1 to 5, in which no strong base or any other source of calcium is added during the dispersion step (E3).
7. Procédé selon l’une des revendications 1 à 6, dans lequel la température lors de l’étape de dispersion (E3) est comprise entre 25 °C et 60 °C, préférentiellement entre 25 °C et 40 °C, encore plus préférentiellement de l’ordre de 25 °C. 7. Method according to one of claims 1 to 6, in which the temperature during the dispersion step (E3) is between 25 °C and 60 °C, preferably between 25 °C and 40 °C, even more preferably of the order of 25°C.
8. Procédé selon l’une des revendications précédentes, dans lequel l’eau usée (4) comprend initialement entre 400 mg/L et 2000 mg/L d’ions orthophosphates, préférentiellement entre 500 mg/L et 1000 mg/L d’ions orthophosphates. 8. Method according to one of the preceding claims, in which the waste water (4) initially comprises between 400 mg/L and 2000 mg/L of orthophosphate ions, preferably between 500 mg/L and 1000 mg/L of orthophosphate ions.
9. Procédé selon l’une des revendications précédentes, dans lequel l’eau usée (4) est une eau de lavage d’un gaz constituant un rejet industriel, préférentiellement une eau de lavage d’un gaz issu d’un atelier de synthèse d’acide phosphorique, préférentiellement encore une eau de lavage d’un gaz issu de la même installation que la source de phosphogypse. 9. Method according to one of the preceding claims, in which the waste water (4) is water for washing a gas constituting an industrial waste, preferably water for washing a gas coming from a synthesis workshop of phosphoric acid, preferably water for washing a gas from the same installation as the source of phosphogypsum.
10. Procédé selon l’une des revendications précédentes, dans lequel le solide comprenant de l’hydroxyde de calcium (3) issu de l’étape (E1) est filtré, séché et dispersé dans l’eau usée (4) lors de l’étape (E3) sans autre étape de traitement ou de lavage préalablement à la dispersion dans l’eau usée (4). 10. Method according to one of the preceding claims, in which the solid comprising calcium hydroxide (3) from step (E1) is filtered, dried and dispersed in the waste water (4) during the step (E3) without any other treatment or washing step prior to dispersion in the waste water (4).
11. Procédé selon l’une des revendications précédentes, dans lequel le pourcentage massique en hydroxyde de calcium dans le solide (3) issu de l’étape (E1) est supérieur à 60 %. 11. Method according to one of the preceding claims, in which the mass percentage of calcium hydroxide in the solid (3) resulting from step (E1) is greater than 60%.
12. Procédé selon l’une des revendications précédentes, dans lequel le solide comprenant de la monétite (5) est revalorisé en tant qu’engrais, dans l’industrie du plâtre et/ou dans l’industrie des ciments. 12. Method according to one of the preceding claims, in which the solid comprising monetite (5) is revalued as fertilizer, in the plaster industry and/or in the cement industry.
13. Procédé selon l’une des revendications précédentes, dans lequel la quantité d’hydroxyde alcalin (2) mélangée à la source de phosphogypse (1) dans l’étape (E1) est choisie en fonction de la concentration en calcium de la source de phosphogypse (1) préalablement déterminée par spectrométrie de fluorescence des rayons X. 13. Method according to one of the preceding claims, in which the quantity of alkaline hydroxide (2) mixed with the source of phosphogypsum (1) in step (E1) is chosen as a function of the calcium concentration of the source of phosphogypsum (1) previously determined by X-ray fluorescence spectrometry.
14. Procédé selon l’une des revendications précédentes dans lequel on valorise des sous-produits issus de l’étape (E1), parmi lesquels le sulfate de potassium et le sulfate de sodium, sous la forme d’adjuvants industriels. 14. Method according to one of the preceding claims in which by-products from step (E1), including potassium sulfate and sodium sulfate, are recovered in the form of industrial adjuvants.
15. Procédé selon l’une des revendications précédentes, dans lequel le solide comprenant de l’hydroxyde de calcium (3) issu de l’étape (E1) est filtré, le filtrat étant séchée à une température comprise entre 70 °C et 90 °C, préférentiellement à une température de l’ordre de 80 °C, de sorte à faire précipiter le sulfate de potassium et/ou le sulfate de sodium. 15. Method according to one of the preceding claims, in which the solid comprising calcium hydroxide (3) resulting from step (E1) is filtered, the filtrate being dried at a temperature between 70 ° C and 90 °C, preferably at a temperature of the order of 80 °C, so as to precipitate the potassium sulfate and/or the sodium sulfate.
16. Procédé selon l’une des revendications précédentes, dans lequel la quantité de solide comprenant de l’hydroxyde de calcium (3) dispersée dans l’eau usée lors de l’étape (E3) est choisie de sorte que le rapport massique Ca/P soit compris entre 1 et 10, préférentiellement de sorte que le rapport massique Ca/P soit compris entre 1 et 5. 16. Method according to one of the preceding claims, in which the quantity of solid comprising calcium hydroxide (3) dispersed in the waste water during step (E3) is chosen so that the mass ratio Ca /P is between 1 and 10, preferably so that the Ca/P mass ratio is between 1 and 5.
PCT/MA2023/050015 2022-10-07 2023-10-04 Method for recovering phosphorus in wastewater laden with orthophosphate ions WO2024076226A1 (en)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3725265A (en) * 1971-01-22 1973-04-03 Grace W R & Co Purification of waste water
US5112499A (en) * 1991-05-22 1992-05-12 Freeport-Mcmoran Resource Partners, Limited Partnership Process for treating pond water
WO1994000391A1 (en) * 1992-06-23 1994-01-06 Water Board Process for the removal of phosphorous

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3725265A (en) * 1971-01-22 1973-04-03 Grace W R & Co Purification of waste water
US5112499A (en) * 1991-05-22 1992-05-12 Freeport-Mcmoran Resource Partners, Limited Partnership Process for treating pond water
WO1994000391A1 (en) * 1992-06-23 1994-01-06 Water Board Process for the removal of phosphorous

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