EP3687942A1 - Récupération de phosphate de calcium de haute pureté - Google Patents

Récupération de phosphate de calcium de haute pureté

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Publication number
EP3687942A1
EP3687942A1 EP18778466.5A EP18778466A EP3687942A1 EP 3687942 A1 EP3687942 A1 EP 3687942A1 EP 18778466 A EP18778466 A EP 18778466A EP 3687942 A1 EP3687942 A1 EP 3687942A1
Authority
EP
European Patent Office
Prior art keywords
phosphate
process according
oxidation
recovery
aqueous solution
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP18778466.5A
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German (de)
English (en)
Inventor
Haiyan QU
Liubov VASENKO
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Syddansk Universitet
Original Assignee
Syddansk Universitet
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Filing date
Publication date
Application filed by Syddansk Universitet filed Critical Syddansk Universitet
Publication of EP3687942A1 publication Critical patent/EP3687942A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B25/00—Phosphorus; Compounds thereof
    • C01B25/16—Oxyacids of phosphorus; Salts thereof
    • C01B25/26—Phosphates
    • C01B25/32—Phosphates of magnesium, calcium, strontium, or barium
    • 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
    • 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
    • C02F1/72—Treatment of water, waste water, or sewage by oxidation
    • C02F1/722—Oxidation by peroxides

Definitions

  • the present invention relates to the technical field of crystallization of calcium phosphate.
  • the present invention relates to a method for recovery of high purity calcium phosphate from waste waters comprising organic matter. More particular, the present invention relates to crystallization of dibasic calcium phosphate dihydrate (DCPD) or hydroxyapatite (HAp).
  • DCPD dibasic calcium phosphate dihydrate
  • HAp hydroxyapatite
  • Crystallization of phosphorus compounds from digester supernatant is one of the common ways to recover phosphorus at wastewater treatment plants.
  • Digester supernatant is a liquid phase, which is produced during sludge digestion.
  • the phosphorus concentration in the digester supernatant is the highest compared to all the other streams in the process.
  • the digester supernatant can contain up to 1800-1900 mg phosphate (P04 3 ⁇ ) per liter and all of the phosphate is in the dissolved state.
  • P04 3 ⁇ mg phosphate
  • the presence of a high phosphate concentration in a digester supernatant makes phosphate-recycling from the digester supernatant one of the preferable ways for achieving phosphorus sustainability.
  • An object of the present invention is to provide a two-stage method for recovery of high purity calcium phosphate from waste waters comprising phosphate but also organic matter, such as humic substances, biopolymers, low molecular weight organic compounds etc.
  • organic matter such as humic substances, biopolymers, low molecular weight organic compounds etc.
  • organic matters such as humic substances, have an inhibitory effect on crystallization of calcium phosphate.
  • the inventors have found a method where organic matter (especially humic substances) efficiently can be oxidized such that the calcium phosphate more efficiently can be recovered, both with regard to amount but also the quality of the calcium phosphate.
  • one aspect of the invention relates to a two-stage process for recovery and/or purification of calcium phosphate from an aqueous solution compring organic matter and phosphate, comprising the steps of;
  • step b) adding a calcium salt to the oxidized solution of step a) and adjusting pH to be in the range of 5.5 to 8.5 to form a precipitate comprising calcium phosphate c) separate said precipitate from said solution
  • Figure 1 shows Raman spectra of the different products obtained together with a reference sample of DCPD and HAp.
  • Figure 2 shows PXRD patterns of different products obtained.
  • Figure 3 shows SEM images from the samples obtained in Example 1 with pH 3.9 in the oxidation, a) Sample obtained from experiment No. 5; b) Sample obtained from experiment No. 6; c) Sample obtained from experiment No. 7; d) Sample obtained from experiment No. 8.
  • Figure 4 shows SEM images from the samples obtained in Example 1 with pH 7.9 in the oxidation, a) Sample obtained from experiment No. 1; b) Sample obtained from experiment No. 2; c) Sample obtained from experiment No. 3; d) Sample obtained from experiment No. 4.
  • Figure 5 shows the influence of the pH value at the oxidation stage on phosphorus recovery (P-recovery) during crystallization when oxidation with hydrogen peroxide and ultrasound treatment lasted for 24 hours.
  • Figure 6 shows the influence of the pH value at the oxidation stage on the color of obtained products when oxidation with hydrogen peroxide and ultrasound treatment lasted for 24 hours.
  • Figure 7 shows the influence of ozonation conditions such as dose of ozone and ozonation pH on the reduction of Color436, chemical oxygen demant (COD) and total organic carbon (TOC) concentrations.
  • Figure 8 shows the influence of ozonation conditions such as dose of ozone and ozonation pH on phosphorus recovery (P-recovery) rate durind calcium phosphate precipitation from ozonated digester supernatant
  • Figure 9A shows the effect of pH of ozonation on total alkalinity (TAC) of digester supernatant.
  • Figure 9B shows the effect of pH of ozonation on the color of solid products.
  • Figure 10 Shows the effect of crystallization conditions such as crystallization pH and seeding addition on phosphorus recovery rate.
  • one aspect of the invention relates to a two-stage process for recovery and/or purification of calcium phosphate from an aqueous solution comprising organic matter and phosphate comprising the steps of;
  • step b) adding a calcium salt to the oxidized solution of step a) and adjusting pH to be in the range of 5.5 to 8.5 to form a precipitate comprising calcium phosphate c) separate said precipitate from said solution.
  • two-stage process refers to separately conducting a first stage of oxidation followed by a second stage of precipitation.
  • the term “recovery” refers to the percentage of a particular substance that is isolated or removed from the aqueous solution.
  • aqueous solution refers to a solution wherein the majority of solvent is water.
  • major refers to at least 95 % volume by volume.
  • purification has the usual meaning in the art and refers to removal of one or more unwanted contaminant(s) to obtain a product of higher purity. Purity may be determined by a range of methods known by the skilled person chosen from but not limited to, HPLC methods such as ion chromatography, LC-MS, ICP-MS, FT-Raman or NMR.
  • the term "calcium phosphate” refers to a family of materials and minerals comprising calcium ions (Ca 2+ ) together with inorganic phosphate anions. Hence, this group consists of materials and minerals comprising Ca 2+ ions together with orthophosphate (P0 4 3" ), monohydrogen phosphate (HP0 4 2” ) or dihydrogen phosphate (H 2 P0 4 " ), such as but not limited to monocalcium phosphate (Ca(H 2 P0 4 )2 or Ca(H 2 P0 4 ) 2 H 2 0), dicalcium phosphate (dibasic calcium phosphate, CaHP0 4 or CaHP0 4 2H 2 0), tricalcium phosphate (Ca 3 (P0 4 ) 2 ), octacalcium phosphate (Ca 8 H 2 (P0 4 ) 5 5H 2 0), hydroxyapatite
  • phosphorus refers to any molecule comprising the chemical element phosphorus (P).
  • P chemical element phosphorus
  • phosphorus may be present in any known oxidation state such as 5, 4, 3, 2, 1, - 1, -2, -3 in the aqueous solution .
  • the phosphorus is present as phosphate (P0 4 3" ), HP0 4 2" , H 2 P0 4 " or hbPC depending of the pH of the aqueous solution .
  • the phosphorus concentration of the aqueous solution in step a) may vary depending on the origin . In an embodiment of the invention, the phosphorus concentration may be up to 600 mg elementary phosphorus (P) present as phosphate.
  • the phosphate concentration in the aqueous solution may be as high as 1840 mg phosphate (P0 4 3 ⁇ ) per liter.
  • the oxidation step is crusial, since the oxidation step provides oxidation and degradation of the organic matter in order to prevent the inhibitory effect of organic matter in crystallization of calcium phosphate .
  • Oxidant any substance capable of oxid ising a substrate in a reaction, the oxidant itself in turn being red uced .
  • Oxidation has the usual meaning to the skilled person in the art being the complete, net removal of one or more electrons from a molecular entity and an increase in the oxidation state of any atom within the substrate.
  • the oxidant is selected from the group consisting of peroxides or alkali metal salts thereof, ozone, oxygen, hypochlorites, perchlorates, peroxy acids or alkali metal salts thereof, ammonium cerium(IV) nitrate, sulphur based oxidants, or mixtures thereof.
  • a peroxide has the usual meaning in the art as a compound containing an oxygen-oxygen single bond .
  • a peroxide may be an unbranched or branched monoalkyl peroxide (RO- OH) such as tert-butyl peroxide, an unbranched or branched dialkyi peroxide (RO- OR) such as di-tert- butyl peroxide, hydrogen peroxide (HO-OH) or a peroxide anion ( ⁇ -0 " ) .
  • RO- OH unbranched or branched monoalkyl peroxide
  • RO- OR unbranched or branched dialkyi peroxide
  • HO-OH hydrogen peroxide
  • ⁇ -0 " peroxide anion
  • alkali metal salts of peroxides in the present context should be understood as a peroxide that forms a salt with an alkali metal such as but not limited to (RO-ONa) or (HO-ONa) .
  • sulphur based oxidants may be chosen from but are not limited to sulphur trioxide (SO3), persulfates such as ammonium persulfate (NH 4 )2S208 and monopersulfate compound (KHSOs ⁇ 0.5KHSO 4 ⁇ 0.5K 2 SO 4 ) known as Oxone.
  • the oxidant is selected from the g roup of from peroxides or alkali metal salts thereof, ozone, oxygen or mixtures thereof.
  • the oxidant may for example be oxygen .
  • the oxidant is selected from peroxides or alkali metal salts thereof, ozone or mixtures thereof.
  • the oxidant is peroxides or alkali metal salts thereof, and may be but are not limited to hydrogen peroxide (H 2 0 2 ) or sod ium hydrogen peroxide (NaOOH) or a combination thereof.
  • the peroxide is H 2 0 2 .
  • the oxidant may be added in different concentrations depending on the desired time period used in the oxidation in step a) . Thus, if a short oxidation time period in step a) is desired the oxidant may be added in a hig her concentration and vice versa .
  • the oxidant may be added to obtain a final concentration of 0.01 to 2.5 mol/L, such as 0.05 to 2.0 mol/L, preferably 0.1 to 1.8 mol/L, preferably 0.5 to 1.6 mol/L.
  • final concentration is meant the concentration of the oxidant after addition i .e. the initial concentration of the oxidant in the aq ueous solution in step a) .
  • the concentration may be from 0.75 to 3 mg 03/mg Total Organic Carbon (TOC), preferably, the concentraton of ozone is 1.0 to 2.5 mg 03/mg TOC, such as 1.0 to 2.0 mg 03/mg TOC, most preferably 1.25 mg
  • TOC Total Organic Carbon
  • the calcium (Ca 2+ ) concentration in the aq ueous solution is no higher than 300 mg/L .
  • a too high Ca 2+ concentration may initiate precipitation of calcium phosphate during the oxidation stage and result in lower purity of the calcium phosphate.
  • the pH in step a) is in the range of 4 to 9 such as 5.0 to 8.8, such as 6.0 to 8.6, preferably 7.0 to 8.4, more preferably 7.5 to 8.2, most preferably 7.7 to 8.0.
  • the inventors found that phosphorus recovery in form of calcium phosphate was higher when the pH during oxidation was from 4 to 9 as compared to when a lower or higher pH was used .
  • the phosphorus recovery was found to be highest when the pH in step 5 a) was around 7.7 to 8.0.
  • the oxidation in step a) is performed for a time period of at least 15 min.
  • the oxidation time varies dependent on the oxidant used, For example, if ozone is used as oxidant, the oxidation time could be lower than when for example hydrogen peroxide is used as oxidant.
  • the time periode for oxidation may for example be from 15 minutes to 90 minutes, such as 15 from 20 minutes to 60 minutes, preferably form 30 minutes to 50 minutes.
  • the time periode for oxidation is at least 20 minutes, such as at least 30 min, such as at least 1 h, preferably at least 2 h, such as at least 3 h, more preferably at least 4h, such as at least 5 h, even more preferably at least 10 h, such as at least 20 h.
  • the oxidation in step a) is performed for a time period of at least 25 h, such as at least 30 h, such as at least 35 h, preferably at least 40 h, such as at least 45 h, more preferably at least 50 h, such as at least 24 h, even more preferably at least 48 h.
  • the time period of the oxidation in step a) is from 15 min to 70 h, such as from 30 min to 65 h, such as from 2 to 60 h, such as from 4 to 55 h, such as from 6 to 50 h, such as from 8 to 45 h, such as from 10 to 40 h, such as from 12 to 35 h, such as from 14 to 30 h, such as from 16 to 30 25 h.
  • the time period for oxidation when hydrogen peroxide is used as oxidant is from 20 to 28 h, in an even more preferred embodiment the time period is from 22 to 26 h.
  • the inventors of the present invention has found out that the oxidation in step a) with hydrogen peroxide in the laboratory should be performed for about 20 to 28 hours with the amount of oxidant used in the examples of the present application. In industrial scale, it might be relevant to increase the time of oxidation and this will still provide a process with high calcium phosphate recovery and high purity.
  • the oxidant is added to the aqueous solution comprising phosphorus in step a) under ultrasound treatment.
  • the ultrasound treatment aids in the dissolution of the oxidant if added as a solid to the aqueous solution in step a).
  • the ultrasound treatment is used to decompose H2O2 to OH- radicals. Ultrasound treatment can also decrease contact time of the oxydation step.
  • the ultrasound treatment aids in the formation of the highly reactive hydroxyl radical ( ⁇ ) from the H2O2.
  • HO * is formed due to the energy involved during cavitation bubble collapse when water is treated with ultrasound.
  • the ultrasound treatment is performed for a period of at least 5 minutes such as at least 10 min., such as at least 20 min., such as at least 30 min., such as at least 40 min., such as at least 50 min., such as at least 60 min.
  • the ultrasound treatment is performed for 5 min. to 48 hour, such as 10 min. to 40 hour, such as 15 min. to 32 hour, such as 20 min. to 24 hour, such as 25 min. to 16 hour, such as 30 min. to 8 hour.
  • the ultra sound treatment is performed throughout the oxidation.
  • the process comprises no addition of FeCl2.
  • FeCl2 will have a negative effect on the calcium phosphate recovery products, since the iron can be accumulated in the solid products and high amounts of iron are not suitable for food products and will not be allowed by the EU regulations on food and feed additives.
  • a pH adjusting agent is added to the oxidized solution after step a) and before step b) to adjust the pH to be 5 or below, such as pH 1 to 5, preferably pH 1.5 to 2.5, even more preferably pH 2.
  • the pH is adjusted to obtain an undersaturated state before the addition of the calcium salt in step b).
  • undersaturated state is meant any state wherein the phosphate is soluble in presence of calcium (Ca 2+ ) such that crystallization of calcium
  • the calcium salt in step b) is added in an amount to obtain a Ca/P molar ratio of 2.
  • the calcium salt may be added as a solid or dissolved in an aqueous solution. In a preferred embodiment, the calcium salt is added in an aqueous solution.
  • the calcium salt is added to obtain a Ca/P ratio in the range of from 1 to 5, preferably from 1 to 2, even more preferably from 1 to 1.7 depending on which product is aimed to be crystallized.
  • Ca/P is meant the molar ratio between calcium and phosphorus, 1 mol phosphate comprises 1 mol phosphorus.
  • the pH is adjusted to be in the range of 5.5 to 8.5.
  • the pH is increased to be in the range of 5.5 to 8.5 by addition of a base selected from the list consisting of alkali metal hydroxides or alkali metal carbonates.
  • the base is an alkali metal hydroxide such as sodium hydroxide (NaOH).
  • the base may be added as a solid or predissolved in an aqueous solution.
  • the pH in step b) is adjusted to be in the range of 5.5 to 8.5 such that the nucleation point is reached .
  • Nucleation has the typical meaning in the art and is to be understood as the first step in the formation of a new thermodynamic phase or a new structure via self-assembly or self-organization. Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears, in this case a precipitate. Nucleation is reached when the change in turbidity can be detected.
  • the pH during precipitation in step b) is adjusted to pH 6.0 to 8.2, such as 6.2 to 8.2, preferably 6.5 to 8.0, even more preferably 7.0 to 8.0.
  • the pH during precipitation in step b) is adjusted to pH 6.5 to 8.2, such as 7.2 to 8.2, preferably 7.7 to 8.0 for precipitation of HAp. If DCPD is to be crystallized, pH should be adjusted to 5.5 - 7.0, for example 5.5 to 6.5, such as 5.7 - 6.3, preferably 5.8 - 6.0.
  • the pH of the nucleation is selected as the operation pH for the crystallization and is kept constant during the crystallization by addition of a base such as NaOH .
  • a slight increase of the pH value after nucleation was observed . Then pH was then kept stable at the value to which it increased .
  • pH can be adjusted to a slightly higher value than the nucleation pH in order to increase phosphorus recovery in the form of calcium phosphate, but no higher than 8.5. pH values at the end of the stage b) above 8.5 is not desired, as the supersaturation will be too hig h, causing a to fast
  • the pH at stage b) should be at least pH 5.5, since at a lower pH the system will be in the undersaturated state and no crystallization will happen, or the system will be very slightly supersaturated and the supersaturation will not be enough to start crystallization, i .e no precipitation .
  • step a) and/or, b) is performed under stirring .
  • Stirring should be understood in the broadest sense as any means of causing a movement in a liquid resulting in mixing .
  • Stirring may be performed by any means known to the person skilled in the art such as but not limited to mag netic stirring, mechanical stirring, using a static mixer, using a shaker, using a fluidized bed reactor or using a bubble column reactor.
  • stirring is performed using a fluidized bed reactor.
  • separation in step c) is performed using filtration, centrifugation and/or decantation or any other means for separation of a solid from a liquid known to the skilled person .
  • the filtration may be performed with or without vacuum .
  • separation is performed using vacuum filtration .
  • step b) furthermore comprises add ition of dibasic calcium phosphate dihydrate (DCPD) crystals, hydroxyapatite (HAp) crystals or octacalcium phosphate (Ca 8 H2(P04)6-5H 2 0; OCP) crystals for seeding .
  • DCPD dibasic calcium phosphate dihydrate
  • HAp hydroxyapatite
  • OCP octacalcium phosphate
  • the seeding crystal is added after the addition and dissolution (if added as a solid) of the calcium salt in step b) and after the pH adjustment to the range of 5.5 to 8.5.
  • the seed crystals is added right before the nucleation point is reached.
  • the precipitate formed in step b) comprises dibasic calcium phosphate dihydrate (DCPD), hydroxyapatite (HAp), octacalcium phosphate (OCP), amorphous calcium phosphate (ACP), dibasic calcium
  • phosphate anhydrous or mixtures thereof.
  • Several factors may influence the composition of the precipitate such as but not limited to the pH in the oxidation in step a), the time period for which oxidation is performed in step a), the oxidant used in step a), whether or not seeding in step b) is performed, the seed crystal used such as DCPD, OCP or HAp, the calcium salt added in step b) and the pH used at precipitation in step b).
  • Precipitation should be understood in the broadest sense as the formation of a solid from a solution the solid being either crystalline or amorphous. Thus, precipitation may also encompass crystallization.
  • the precipitate is a crystalline solid.
  • the precipitate is an amorphous solid.
  • the precipitate is a mixture of crystalline and amorphous solid .
  • the precipitate is a crystalline solid.
  • Crystalline and amorphous solid has the usual meaning in the art.
  • a crystalline solid thus means any solid material whose constituents are arranged in a highly ordered microscopic structure forming a crystal lattice, i.e. it is the presence of three-dimensional order on the level of atomic dimensions.
  • Crystalline solid may either be single crystals or polycrystals composed of many microscopic crystals also known as crystallites.
  • the calcium salt added in step b) is selected from the group consisting of CaCI 2 , CaC03, Ca(OH) 2 or mixtures thereof.
  • the calcium salt is CaCI 2 .
  • the process is conducted at a temperature of 5 to 40 C, such as 7 to 38 C, such as 9 to 36 C, preferably 11 to 34 C, such as 13 to 32 C, even more preferably 15 to 30 C, such as 17 to 28 ° , such as 19 to 26 C or at room temperature.
  • the temperature may be the same or different in step a), b) and c). Heating and/or cooling may be performed with any conventional methods known to the person skilled in the art depending on scale on which the process is performed. In a preferred embodiment, the process is conducted at room temperature such that no heating and cooling is required. This allows for a more energy sufficient process.
  • the pH is maintained constant during step b) after the pH adjustment. As calcium phosphate precipitates during step b), the pH of the solution decreases. In order to achieve maximum recovery of calcium phosphate, the pH is kept constant by addition of a base.
  • the base added is selected from the list consisting of alkali metal hydroxides.
  • the base is NaOH.
  • the base may be added as a solid or predissolved in an aqueous solution. In a preferred embodiment the base is predissolved in water.
  • the precipitation in step b) is for a period of 1 to 24 h, such as 2 to 22 h, such as 3 to 20 h, 4 to 18 h, preferably 5 to 16 h, such as 6 to 14 h, more preferably 8 to 12 h, preferably 1 to 3 h, even more preferably 2 h.
  • the aqueous solution comprises organic matter.
  • organic matter is to be
  • organic matter may for example include one or more humic substances, biopolymers, and low molecular organic compounds.
  • the organic matter is present in he aqueous solution in small amounts and therefore the term "organic matter" does not include "organic solvents".
  • organic matter comprises humic substances.
  • Humic substances are the major organic constituents of soil (humus) and humus refers to a fraction of soil organic matter.
  • Humic substances encompass a complex diverse class of molecules with humic acids being the principal component of humic substances.
  • Humic substances can be found in water such as waste water from treatment plants.
  • the oxidation is performed to degrade organic matter, such as humic substances.
  • the quality of the calcium phosphate increases and the color of the precipitate isolated in step c) is becoming more white.
  • the aqueous solution is waste water from a waste water treatment plant, waste water from industry or household waste water.
  • any aqueous solution comprising sufficient phosphorus in the form of phosphate to be precipitated may be used .
  • sufficient phosphorus is meant phosphorus in a minimum concentration 5 mg of phosphorus (P) per liter.
  • the aqueous solution is a digester supernatant from a waste water treatment plant. When sludge is allowed to settle in a digester, a supernatant develops. Anaerobic digester supernatant is commonly returned to the head of wastewater treatment plants and mixed with the influent. Although the supernatant is relatively small in volume, it contains dissolved and suspended organic and inorganic materials such as phosphorus in the form of phosphate . These materials add suspended solids, nutrients (nitrogen and phosphorus), and organic compounds to the influent.
  • the aqueous solution according to the present invention comprises phosphate (P0 4 ), but the aqueous solution does not comprise phosphorous acid (H PO3) .
  • the process of the invention does not only recover valuable high purity calcium phosphate products but also prevents eutrophication if the waste water is returned into the eco system . Eutrophication is to be understood as the
  • the aqueous solution is filtered prior to 5 oxidation to remove suspended solids. Filtration should be understood in the
  • Filtration may be performed with any method known by the skilled person such as but not limited to pressure driven filtrations. Filtration may be performed with a filter with mean
  • pore size 10 pore size of 20 - 25 Mm. Other pore sizes may be applicable depending on the size of the suspended solids in the aqueous solution to be treated. In another embodiment of the invention, no filtration is needed prior to oxidation when the aqueous solution contains low amount of suspended solids. In yet an embodiment of the invention, methods for removing suspended solids may include but are not
  • the pH is adjusted with a pH adjusting agent selected from the group of acids or bases such as NaOH, and/or HCI.
  • Suitable acids include but are not limited to HCI, HBR, HNO3, H3BO3, CH3COOH, H2CO3 and 20 H3PO4.
  • Suitable bases include but are not limited to LiOH, KOH, NaOH, NH3,
  • the acid is HCI and the base is NaOH.
  • the acid or base may be added as a solid, in an aqueous solution or as a gas depending on the acid or base used.
  • the precipitate obtained by the process of the present invention is of food grade quality.
  • food grade quality is to be understood as a product complying with the European Commission Regulation No 231/2012 laying down specifications for food additives in terms of heavy metals content listed in Annexes II and III to Regulation No 1333/2008 of the European
  • the precipitate comprises at least 65% by weight of DCPD, HAp, OCP and/or DCPA or their mixture, such as at least 70%, such as at least 75%, such as at least 80%, such as at least 85%, such as at least 35 90%, such as at least 95%.
  • DCPA or OCP can also be obtained in the precipitate obtained by the present process.
  • the precipitate obtained by the process of the present invention will comprise no or neglible amounts of struvite.
  • the phosphorus recovery from the aqueous solution of step a), when using the two-stage process of the present invention, is at least 70% .
  • the phosphorus recovery of the aqueous solution of step a) when using the process of the present invention is at least 75%, such as at least 80%, in particular at least 85% . It is possible to further improve the process by amending the process parameters, for example by cooling/heating, improve filtration, amend pressure etc, and thus obtain an even higher phosphorus recovery, i .e. a phosphorus recovery of at least 90%, such as at least 92%, for example at least 97% .
  • phosphorus recovery is meant the percentage of phosphorus recovered from the aq ueous solution in step a) .
  • the phosphorus recovery is calculated as defined below and is refered to as P-recovery later on in the text
  • P rec is the % of phosphorus (P) recovered from the aq ueous solution
  • P in is the initial phosphorus (P) concentration in the aqueous solution
  • P fin is the phosphorus (P) concentration in the aqueous solution after step c) .
  • the calcium phosphate recovery obtained in the process according to the present invention will be measured by measuring phosphorus recovery, since all phosphorus present in the digester supernatant is in the form of phosphate.
  • the process is conducted under ambient atmosphere and/or ambient pressure.
  • ambient atmosphere and/or pressure is meant the atmosphere and pressure of the immediate surroundings.
  • the process is conducted under and artificial atmosphere and/or pressure.
  • the artificial atmosphere may be chosen from but is not limited to oxygen and/or ozone.
  • the pressure may be in the range of 0.5 to 20 bar, such as 0.6 to 17 bar, such as 0.7 to 14 bar, such as 0.8 to 11 bar, such as 0.9 to 8 bar, such as 1.0 to 5 bar, such as 1.1 to 2 bar.
  • the process is conducted at ambient atmosphere and/or pressure.
  • Aaby Wastewater treatment plant located in Aarhus, Denmark, provided a digester supernatant with initial total phosphate (P0 4 3" ) concentration of 0.01 mol/L and calcium concentration 0.0002 mol/L.
  • Munktell filter paper with the mean pore size of 20 - 25 Mm was used for the filtration of the digester supernatant before all experiments in order to remove suspended solids.
  • Hydrogen peroxide solution (30 % (w/w) in H2O) was used as oxidant for the oxidation of the digester supernatant. pH adjustment was done using 2M standard NaOH, 23% w/w NaOH, or 37% w/w HCI solutions.
  • 1M CaCI 2 solution for dosing Ca during the crystallization stage was prepared from reagent grade CaCI 2 .
  • Reagent grade DCPD and HAp were used as the control samples for the FT-Raman analysis. Reagent grade DCPD was used for the seeding experiment. pH electrodes were calibrated using pH 4 and pH 7 IUPAC buffer solutions before each experiment and were stored in a 3M KCI solution between measurements.
  • Advantec Glass Fiber filter with the pore size of 0.45 Mm were used for vacuum filtration of solid and liquid phases after crystallization.
  • a 761 SD Compact IC with an 813 Compact Autosampler by Metrohm is used for determination of calcium and phosphate ions in the solution.
  • Phosphate's concentration is analyzed with Metrosep A Supp 5 - 150 (6.1006.520) using 3.2 mM Na 2 CO3/1.0 mM NaHC03 + 10 % acetone as an eluent.
  • Metrosep Cation 4 (6.1010.000) column and a 1.7 nM nitric acid/0.70 mM dipicolinic acid as an eluent is used for calcium determination.
  • All phosphorus (P) was present in the form of phosphate (P0 4 3" ) in the digester supernatant provided by Aaby Wastewater treatment plant, located in Aarhus, Denmark. Hence, P-recovery could be calculated using the phosphate (P0 4 3" ) concentration.
  • P rec is the % of phosphorus (P) recovered from the aqueous solution; is the initial phosphate-(P0 4 3" ) concentration in the aqueous solution;
  • [PO ⁇ " ] in is the phosphate-(P0 4 3" ) concentration in the aqueous solution after step c). Analysis of solid phase
  • a Bruker MultiRAM FT-Raman Spectrometer was used with 1064 nm laser at 100 - 400 mW laser power depending on the color of the samples. Baseline correction of spectra was done using CrystalSleuth software.
  • the Powder X-ray Diffraction (PXRD) analysis was performed using A Rigaku Miniflex 600 X-ray diffractometer Cu source of X-rays and a Ni filter operating at voltage - 40 kV, current - 15 mA, speed -10°/min. The patterns were recorded at 2 ⁇ range of 2 - 70° and a step size 0.02 °.
  • a COD-Inorg database with the demo version of Match! Ver.3. were used to interpret the PXRD patterns.
  • EI Quanta 200E Scanning Electron Microscope was used for imaging of the obtained crystalline products. It was operated at the low vacuum mode (at the pressure of 30 Torr) in order to prevent charging.
  • the selected solid samples were analyzed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to determine the metal contents. Oxidation of the digester supernatant
  • the initial pH of the digested supernatant was 7.9, therefore for the oxidation at the pH 7.9 no adjustment was needed.
  • the solution was adjusted by addition of 37 % HCI. During addition of HCI the formation of gas was observed.
  • pH was adjusted by adding NaOH. When the pH value increased to above pH 9, a brown sludge started to coagulate. This sludge was further destroyed during oxidation. The change of the solution's volume during pH adjustments was less than 1%.
  • Crystallization was performed in the 1 L glass reactor with overhead magnetic agitator set up to 100 rpm.
  • the nucleation point was pre-determined for the different solutions in order to add the seeding crystals right before the nucleation point was reached.
  • Table 1 shows the nucleation point of the different solutions.
  • DCPD was preferably added right before nucleation, because if added too early, the seeds can dissolve and if added after nucleation, the seeding will not be effective.
  • the crystallization stage lasted for 2 hours and after this time crystalline products were separated from solution using vacuum filtrations. Solid samples were weighted and analyzed with FT-Raman, PXRD and SEM as described above. Liquid samples were analyzed for total phosphate content as described above.
  • Example 1 Effect of DCPD seeding, pH and oxidation time on P-recovery and the nature of product obtained.
  • the oxidation was performed at pH 3.9 and pH 7.9 for either 24 h or 48 h with or without DCPD seeding.
  • a digester supernatant from Aaby Wastewater treatment plant was used and the conditions of oxidation and crystaiisation are as mentioned under "Methods" above.
  • the oxidant was added to the system the formation cavitation (bubbles formation) was observed.
  • a significant improvement of color was noticed after the solution was left for oxidation for 24 h or 48 h.
  • the color of the solutions changed from dark brown to light yellow.
  • the color improvement of the liquid was more significant for oxidation at the higher pH values.
  • DCPD seeding promotes crystallization of DCPD at all experimental conditions. Whenever DCPD crystals were added to the solution the only obtained product was DCPD. Mixtures of DCPD and HAp were obtained for the experiments without seeding (experiments No. 2, 4, 6, 8). For the experiments No. l, No.5 and No.7 in both runs DCPD were crystallized as a major product, but some minor amounts of HAp present in the sample. The major and minor products were determined by analyzing the SEM imaged of the samples.
  • Example 2 Effect of pH of the oxidation on P-recovery and the nature of product obtained.
  • Example 1 the highest P-recovery was obtained for the oxidation pH 7.9 with 24 hours of prolonged oxidation and crystallization without seeding.
  • Figure 5 and table 3 shows that the P-recovery is highest when a pH of 7.9 is used during the oxidation step.
  • picture of the 4 samples is shown in order to show the influence of the pH value during oxidation step on the color of the obtained products.
  • the oxidation was performed with hydrogen peroxide and ultrasound treatment for 24 hours. From figure 6 it is shown that calcium phosphate obtained when having a pH during oxidation at 2.0, 3.9 and 10.0 is more colored than when pH 7.9 was used. When pH 7.9 was used, a lower content of impurities is present.
  • Example 1 Two samples from Example 1 (No. l and No.3) were chosen for ICP-MS analysis.
  • Table 4 the results of the metal content determinations are presented compared with the requirements for DCPD and HAp as food grade additives according to the commission regulation No 231/2012.
  • the amount of DCPD and HAp in the samples that were analyzed by ICP-MS were calculated.
  • a digester supernatant from Lille Marquettee WWTP France was used which was produced during mesophilic digestion of sewage sludge.
  • the supernatant and sludge was separated by centrifugation at the WWTP. Concentrations of the selected components in the supernatant is given in table 6.
  • ozone for oxidation of humic substances in the digester supernatant was analysed by using two different doses of ozone, 1.25 and 2.5 mg ozone per mg total organic carbon (TOC).
  • the dose of ozone was regulated by adjusting the flow and concentration of the ozone/oxygen mixture.
  • Ozonation was performed at different initial pH values: 8.1, 6.2 and 2.8, in order to test the efficiency of degradation of organic matter by different mechanisms of ozonation.
  • oxidation is performed at highy reactive OH radicals, while at acidic pH, direct oxidation by O3 takes place. Conditions for the ozonation experiments are shown below in table 7.
  • a glass column was filled with 2.5 L of a filtrated digester supernatant from WWTP and the pH was adjusted to the value mentioned in table 7. Recirculation of the supernatant in the column was done using the pump. Change of pH during ozonation was monitored using the pH probe inserted into the recirculation cirle. O2 was supplied to the ozone generator. In the beginning of the experiment, the flow of O2 and concentration og ozone in the produced ⁇ 2/ ⁇ 3 mixtures were set to the values mentioned in table 7.
  • Concentration of ozone in the produced ⁇ 2/ ⁇ 3 mxture was analysed using the intel ozone analyser. During the setting up period ⁇ 2/ ⁇ 3 mixture was discharged to the fume hood.
  • TOC Total Organic Carbon
  • Color436 Color436, Chemical Oxygen Demand (COD) and dissolved ozone.
  • Concentration of dissolved ozone was determined by the Indigo method.
  • TOC was determined using the total organic carbon analyzer TOC-VCPN equipped with ASI-V auto-sampler from Shimadzu using Non-Purgeable Organic Carbon (NPOC) method.
  • NPOC Non-Purgeable Organic Carbon
  • Spectrophotometer HACH DR 5000 with 10 mm quartz cuvettes were used for determination of Color436.
  • COD of the collected samples was measured using COD cuvettes test LCK 514 from HACH.
  • Crystallization was performed in a 1 L beaker with an overhead stirrer driven by motor at 100 rpm. pH of supernatant was adjusted to the required pH of crystallization using NaOH/HCI standard solutions. 1 M CaCI 2 solution was dosed into the reactor with a peristaltic pump at the rate of 10 ml/min. pH of solution was monitored and kept stable during crystallization. Crystallization lasted for 2 hours. Thereafter, solid and liquid phases were separated using vacuum filtration. Collected solid product was dried at the room temperature and analyzed .
  • a Bruker MultiRAM FT-Raman Spectrometer equipped with 1064 nm laser was used for collection of FT-Raman spectra of the solid samples.
  • Bruker Alpha FTIR Spectrometer was used for collection of FTIR Spectra.
  • Rigaku Miniflex 600 X-ray diffractometer with Cu source of X-rays and a Ni filter was used for Powder X-ray Diffraction (PXRD) analysis. Analysis of the liquid phase
  • Digester supernatant was analyzed for phosphate (P0 4 ) and calcium content before ozonation, after ozonation/before crystallization and after crystallization.
  • HACH cuvettes test LCK 350 was used for determination of phosphate
  • Calcium concentration was determined by using Water Hardness cuvette test LCK 327.
  • figure 8 is the effect of ozonation conditions on P-recovery shown.
  • the P-recovery rate slightly increases when crystallization is performed after ozonation of organic matter comparing to crystallization from a non-ozonated supernatant.
  • Figure 8 shows that the lower the pH of ozonation was, the higher was the P-recovery rate. This can be attributed to pH dependence of carbonate speciation in supernatant. At acidic pH (2.8) all carbonate is present as CO2, which can be stripped from the supernatant during ozonation. Therefore, during crystallization, less calcium is consumed on the reaction with carbonate and more of it is available for the reaction with phosphate. This hypothesis can be supported by the values of alkalinity of supernatant before and after ozonation at different pH ( Figure 9).
  • Example 4C Effect of ozonation conditions on the total alkalinity (TAC) of the digester supernatant.
  • FIG 9B A photo of the solid samples are shown in figure 9B. It illustrates the effect of pH values during ozonation from table 7 with the dose of 1.25 03/mg TOC on the color of the crystallized samples. It was compared with a sample crystallized from digester supernatant without ozonation.
  • sample (I) showing the solid product obtained without ozonation
  • sample (II) is with ozonation at pH 8.1
  • sample (III) is with ozonation at pH 6.2
  • sample (IV) is with ozonation at pH 2.8.
  • Figure 9B shows that the color of the sample with no ozonation is much darker than the other samples. Furthermore, the samples with ozonation at pH 8.1 and pH 6.2 appears much more white as compared to the sample with ozonation at pH 2.8 which is colored.
  • Figure 10A shows that for all tested conditions (no ozonation, ozonation at pH 8.1 and at pH 6.2), the P-recovery rate decreased with the decrease of crystallization pH from 8 to 7, due to lower supersaturation rate. No differences in the products nature or color were observed with the change of pH of crystallization.
  • crystallization pH is supposed to be slightly acidic.

Abstract

La présente invention concerne une méthode de récupération de phosphate de calcium de haute pureté à partir d'eaux usées. En particulier, la présente invention concerne la précipitation de dihydrate de phosphate de calcium dibasique (DCPD) ou d'hydroxyapatite (HAp). Le DCPD et le HAp obtenus par la méthode sont conformes à la Réglementation de la Commission Européenne No 231/2012 établissant des spécifications pour les additifs alimentaires. Ainsi, le phosphate de calcium de grande pureté obtenu par la méthode de l'invention peut être approprié pour une utilisation dans l'industrie alimentaire.
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