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    • 专利摘要:
    In a process for phosphate removal from process water, phosphate ions in an aqueous solution are subjected to a precipitation reaction. The phosphate-containing precipitate thus formed and a process water stream are separated from each other. For the precipitation reaction, a reaction with calcium ions is used to form hydroxyapatite (Ca5 (PO4) 3OH, Ca10 (PO4) 60H2) as a phosphate-containing precipitate. The process water flow is separated from the precipitate by means of cross-flow ultrafiltration in a continuous flow-through process.
    公开号:NL2022860A
    申请号:NL2022860
    申请日:2019-04-04
    公开日:2019-10-09
    发明作者:Jozef Maria Van Kan Paulus
    申请人:Meulenaars Bv；
    IPC主号:
    专利说明:


    © 2022860 © Α PATENT APPLICATION © Application number: 2022860 © Application submitted: April 4, 2019 © Int. CL:
    C01B 25/32 (2019.01) C02F 1/52 (2019.01) © Priority:
    April 2018 NL 2020711 © Applicant (s):
    Meulenaars BV in Liempde.
    © Application registered:
    October 2019 (43) Request published:
    October 2019 © Inventor (s):
    Paulus Joseph Maria van Kan in Nijmegen.
    © Authorized representative:
    A.A. Jilderda in Eindhoven.
    54) Method for at least partial phosphate removal from process water as well as a method for recovering a phosphate-containing compound from process water
    57) In a process for phosphate removal from process water, phosphate ions in an aqueous solution are subjected to a precipitation reaction. The phosphate-containing precipitate thus formed and a process water stream are separated from each other. For the precipitation reaction, a reaction with calcium ions is used to form hydroxyapatite (Ca 5 (PO 4) 3 OH, Ca 10 (PO 4) 60 H 2 ) as a phosphate-containing precipitate. The process water flow is separated from the precipitate by means of cross-flow ultrafiltration in a continuous flow-through process.
    NL A 2022860
    This publication corresponds to the documents originally submitted.
    Method for an at least partial phosphate removal from process water as well as a method for recovering a phosphate-containing compound from process water
    The present invention relates to a method for at least partial phosphate removal from process water, in particular from waste water, wherein phosphate ions in an aqueous solution are subjected to a precipitation reaction with calcium ions from a calcium source to form a phosphate-containing precipitate, in particular as hydroxyapatite (Ca 5 (PO 4) 3 OH, Ca 10 (PO 4) 6 OH 2 ).
    Process water can contain significant concentrations of minerals. This mainly occurs with residual flows of animal or vegetable origin, in particular with an origin in the food industry. Manure processing can also yield a mineral-rich liquid fraction. Process water from (meat processing) agro-industry such as slaughterhouses can also contain minerals. In addition to these waste water flows, (residual) products from the food industry are also processed in liquid form. With these water flows, with a high solid content, it is increasingly desirable to reduce the mineral content (salinity) before the process water can be discharged or reused. This relates in particular to a reduction of the nitrogen, potassium and phosphate content in the aqueous fraction. Different purification methods exist for each of these contaminations.
    To remove, or at least reduce, the phosphate content, a known method is based on a precipitation reaction in which a phosphate salt is deposited as precipitate from the liquid. Such a method is known, for example, from an international patent application WO 2017/108933. In this known method, one or more metal salts are added to the aqueous phase to form macroscopic phosphate-containing flakes therein, the salt being selected from a group of iron, magnesium, calcium and aluminum salts. In the known method, an alkali metal or alkaline earth metal hydroxide is supplied to this flocculating liquid. The metal phosphate flakes then react with the hydroxide to form the corresponding alkali metal or alkaline earth metal phosphate which is separated as a liquid or in powder form. In particular, this state of the art involves a reaction with an iron or aluminum salt to form iron or aluminum phosphate flakes to ultimately obtain potassium or sodium phosphate.
    A drawback of this known method, however, is a relatively low economic yield, in addition to which the phosphate ions in the solution give way to metal hydroxide of the metal salt used for the flocculation. Like metal phosphate, such metal ions are not desirable in the process water stream. In particular, iron-phosphate complexes can only be made suitable for reuse, for example as fertilizer, with great difficulty.
    From two scientific publications, namely:
    Water Science and Technology 17.2-3 (1985): 121-132 by Joko, Isao, entitled Phosphorus Removal from Wastewater by the Crystallization Method; and from Chemical Engineering Journal 267 (2015): 142-152 by Hermassi, M et al., entitled Evaluation of Hydroxyapatite Crystallization in a Batch Reactor for Valorization or Alkaline Phosphate Concentrates from Waste Water Treatment Plants using Calcium Chloride, is known for using a calcium source to precipitate hydroxyapatite from waste water as a valuable phosphate-containing precipitate. Unlike the aforementioned metal salts, hydroxya pathite, for example as a fertilizer, can be introduced more or less directly into the front of an agricultural production chain, as a result of which this precipitate indeed represents an economic value. A drawback of the processes described in these scientific publications, however, is that such batch processes generally do not lend themselves well to application on an industrial scale, so that their practical utility is still limited.
    The present invention has for its object, inter alia, to provide a method with which a phosphate content in a process water stream can also be drastically reduced on an industrial scale in a particularly effective and efficient manner. In addition, the invention has for its object to recover phosphate from a process water stream in a usable form, for example for (re) use as fertilizer.
    In order to achieve the intended object, a method of the type mentioned in the preamble according to the invention is characterized in that the calcium source is added to a process water stream, that a basic (alkaline) environment is imposed on the process water stream downstream of the calcium source (at least a basic one) ( alkaline environment is maintained therein, and that the process water flow in a flow through process is at least substantially of
    The phosphate-containing precipitate is separated by ultra-filtration or microfiltration in a cross-flow filtration process, in particular via a membrane filter comprising a bundle of hollow fibers with a nano-porous fiber wall.
    The invention is based on the insight that hydroxyapatite is thermodynamically the most stable form of calcium phosphate and that its precipitation reaction therefore proceeds particularly quickly and with a high efficiency, especially if a basic environment is thereby applied. In accordance with the invention, this precipitation reaction is thereby carried out in a continuous flow process of the waste water stream, whereby the process water is ultimately stripped of phosphate-containing crystals in a also continuous cross-flow ultrafiltration. It is important here that the process water flow is maintained at least substantially unchanged during this process, which means that a process water flow through the pre-filter is of at least substantially the same size as a filtrate / permeate drains from the cross-flow filtration. As a result, the process lends itself to (up) scaling to a desired size to an industrial scale.
    By depositing phosphate in this form, whether or not stoichiometrically, in accordance with the invention, the phosphate content of the process water can be reduced to at least substantially zero by removal of the precipitated hydroxyapatite. In addition, this precipitate manifests itself in the form of micro-crystals which, after they have been obtained as such in powder form, lend themselves particularly well to various applications, such as, in particular, as a phosphate source in fertilizer or, in a pure state, for example, as a calcium phosphate source in medical applications.
    In view of an intended efficiency of the precipitation reaction according to the invention in which hydroxyapatite is formed, the acidity of the environment in which this reaction is carried out plays a major role. It has been found that the precipitation reaction according to the invention leads, in particular in a basic environment, to a recoverable precipitate. More in particular, the method is characterized in that an acidity (pH) above approximately pH = 8 is maintained in the basic environment. From this lower limit of the pH value, the precipitation reaction can proceed further, so that afterwards virtually no phosphate (ions) remains in solution.
    In principle, both the addition of calcium ions to the process water and an increase in the pH thereof can be achieved from a common source, for example by adding calcium hydroxide to the process water. A further preferred embodiment of the method according to the invention, however, is characterized in that the basic environment is applied with a base which is independent of a source of the calcium ions and which is added, whether or not together, and more particularly that the base has a hydroxide, in particular sodium hydroxide. The acidity can thus be imposed independently of the amount of calcium introduced. That amount of calcium can, for example, be determined stoichiometrically and optionally include a (slight) excess, while the base is supplied separately from it in such an amount to create an optimum reaction climate. In particular, this is a climate with an acidity pH> 8.
    In principle, the calcium source can be chosen relatively freely. However, particularly good experiences have been gained with a special embodiment of the method according to the invention, characterized in that a water-soluble calcium salt is used as a calcium source, in particular a calcium halide, more particularly calcium chloride. A preferred embodiment has the feature that the calcium source is used in the form of an aqueous solution with a neutral acidity.
    The precipitation and separation of phosphate in the form of hydroxyapatite has the great advantage that finely divided micro and / or nanocrystals are formed with it which can be isolated particularly practically and are directly suitable for reuse, for example as fertilizer. To this end, an embodiment of the method for recovering a phosphate-containing compound from process water using the method according to the invention is characterized in that the phosphate-containing precipitate is formed at least substantially in microcrystalline and / or nanocrystalline form and upstream of the crossflow filtration as a depot is separated from the process water stream. The phosphate crystals are separated from the process water stream (permeate) by means of cross-flow filtration, after which the phosphate crystals in the concentrate are returned to a settling reservoir in which they grow and settle.
    A particularly practical embodiment of the method according to the invention, which can thereby also be scaled particularly well to industrial proportions, is characterized in that the ultrafiltration is based on a cross-flow filtration process, in particular via a membrane filter comprising a bundle of hollow fibers with a nano-porous fiber wall. Crossflow filtration is a continuous flow process. Use can in particular be made here of a tangential crossflow process, wherein an at least substantially phosphate-free permeate is withdrawn from a circulation of the treated process water. With this technique, the permeate production is considerably higher than with many other filtration methods. This is an important factor when scaling up to larger quantities on an industrial scale. The retentate contains an increasing concentration of hydroxyapatite crystals, which can be separated therefrom in a conventional and economical manner. A preferred embodiment of such a process has the feature according to the invention that the ultrafiltration is carried out with a membrane filter with a maximum pore size between 10 and 20 nanometers. This choice ensures an almost complete retention of even the smallest apatite crystals.
    The method according to the invention can be used in particular in a form in which the process water comprises waste water, in particular a residual stream of animal or vegetable origin, more in particular one from a food industry. The invention will be explained in more detail below with reference to an exemplary embodiment and an accompanying drawing, in which:
    Figure 1 is a schematic representation of an example installation for carrying out the method according to the invention;
    Figure 2 graphically depicts the deprotonation of phosphoric acid against acidity (pH);
    Figure 3 shows the concentration of phosphate in solution against the acidity pH; and
    Figure 4 shows the pH as a function of the amount of NaOH added. It should be noted, incidentally, that Figure 1 is drawn purely schematically and not always on (the same) scale. In particular, for the sake of clarity, some dimensions may be exaggerated to a greater or lesser extent.
    Wastewater can contain significant concentrations of minerals. This mainly occurs with waste water that contains urine and / or faeces. This concerns, for example, sewage, but also the processing of manure in (intensive) animal husbandry leads to a mineral-rich
    -6 liquid fraction. In wastewater from a (meat-processing) agro-industry, such as in slaughterhouses, a high content of minerals is often found. In the (agro) industry there is a great need for minerals that are traditionally extracted from non-sustainable sources. A sustainable alternative involves the recovery of such minerals from process water flows. The invention makes it possible to recover phosphate from process water for reuse as a raw material. With all these waste water streams, often with a high solid content, it is increasingly desirable to reduce a mineral content (salts) therein.
    Control of the content of mineral salts in process water and feed streams is becoming increasingly important from an ecological point of view. For example, manure often contains so many minerals that spread on land must be limited. Animal manure is also produced in such quantities that make a local animal husbandry and arable cycle impossible. On the other hand, for some fertilizers, such as phosphate, there is a threat of global scarcity in addition to such a local surplus, which can already manifest itself in the near future.
    The dairy industry strives for an efficient reuse of residual flows. However, for a residual product such as milk whey, the usability is limited by the mineral content. As a result, the sugars and proteins present cannot be optimally recovered. In all cases, i.e. with process water and manure as well as with feed streams, reduction of the mineral content is therefore of great importance. This applies primarily to the removal of nitrogen, potassium and phosphate-containing minerals, preferably by recovering these components in a form that makes reuse as fertilizer possible. Separate separation and isolation of these minerals in solid form is preferred. This offers the possibility to subsequently mix the different minerals in an optimal mutual ratio and makes the transport of minerals to other geographical locations more efficient.
    The method according to the present invention makes it possible to drastically reduce the phosphate content from the above-mentioned water streams and thereby extract phosphate as a solid (fertilizer) raw material therefrom. Phosphate is bound for this purpose and isolated from the liquid fraction by means of a precipitation reaction. A residual amount of phosphate that
    -7 being discharged can therefore be kept to a minimum. This will be explained below on the basis of an example based on a process water flow from an existing water purification plant, more particularly one from a meat processing industry.
    This process water contains virtually no more ammonium. Due to the use of denitrifying bacteria, the content of nitrite is very low and a complete conversion to gaseous nitrogen and nitrate can be assumed. Phosphate is still present in the process water. This phosphate is almost completely in the (unbound) ortho-phosphate form. The concentration in this example is approximately 36 mg-P (ortho-P) per liter. However, the method according to the invention can be used in a wide concentration range, ranging from concentrations of 0.3 mg to 6 g per liter. Table 1 gives a typical picture of the chemical analysis of an effluent from a water treatment:
    Table 1Process water analysis Component content (mg / l) COD (incl humic acids) 51.7 N total 43.7 nh; <0.1 no; <0.1 no; 44.8 P total 36 sof- 60 cr 255 PH 7.27
    With existing techniques it is possible to remove almost all ammonium (nitrogen) and only a residual nitrogen in the form of nitrate is present. With the method according to the invention, the phosphate content of the process water can then be reduced to a minimum or even reduced to zero by precipitation and separation of hydroxyapatite. In accordance with the invention, this takes place in a continuous flow process. A suitable installation for this is shown in Figure 1. A process water stream 1 becomes
    8 received therewith at a pre-filter 10 to remove macroscopic contaminants therefrom.
    The process water stream is then passed into a pipe system 100. Starting from the composition of the process water as shown in Table 1, Table 2 gives the molar ratios of the main components:
    Table 2
    Relationships in process water
    Component degree(mg / fy MW g / mol degree(mol / l) COD 517 I - i N total 43.7 14 0.0031 NH 4 -N <0.1 : - i NO2-N <0.1 i - i MO3-M 44, S 62 0.00072
    P-Waai W12 SO4 ' 60 I 96 0.00053 Cl 255 35.5 0.0072 pH 7.210 Ca 2 * i - j 40 0.002
    The phosphate content is around 1 mmol / l. For a precipitate of phosphate ions in the form of hydroxyapatite, the following reaction comparison applies:
    5Ca 2+ + 3HPO4 2 + H 2 O * Ca 5 (PO 4 ) 3 OH + 4H *
    This comparison describes the crystallization reaction in which hydroxyapatite precipitates in the form of fine crystals. The hydroxyapatite precipitates by reaction of calcium ions and divalent phosphate ions. To initiate this reaction, a suitable calcium source is used in accordance with the invention, with which calcium ions are introduced downstream of the pre-filter 10 into the process water stream. To this end, the installation comprises a reservoir with the calcium source used from which calcium ions can be accurately dosed into the pipe system 100 in order to be supplied to the process water stream. Calcium chloride (CaCl 2 ) is used for this in this example. For this purpose, the choice was made to add a 0.1 - 1 M CaCl 2 solution with a neutral pH. Addition of typical solid CaCl 2 -2H 2 O is also possible.
    In addition, another well-soluble and relatively harmless calcium salt can be used as a calcium source within the scope of the invention. In the present example, 500 ml of 0.1 M CaCl 2 solution is added per 10 liters of process water. The initial concentration [Ca 2+] is therefore 5 mmol / l.
    The divalent calcium ions thus introduced into the solution react with HPO 4 2 1 This source of divalent phosphate depends on the dissociation of monovalent phosphate according to the following equilibrium reaction:
    h 2 po; H + + HPCV
    The deprotonation of H 2 PO 4 'thus leads to an increase in the H + concentration and thus to a decrease in the pH of the solution. The precipitation of hydroxyapatite is thereby counteracted, as can be seen from figure 1. The concentration of HPO4 2 'in the solution appears to be highly dependent on the acidity (pH). Figure 2 graphically shows the deprotonation of phosphoric acid against acidity (pH). The block line (1) is the relative concentration of monovalent phosphate; the triangle line (2) indicates the divalent phosphate concentration. These forms of phosphate are balanced around pH = 7.21. From this balance there is an excess of divalent HPO4 2 '. To promote this, a base is therefore preferably added to the process water. To this end, the installation comprises downstream of the calcium chloride reservoir 20 a reservoir 30 from which a base can be accurately dosed to be led into the process water stream.
    For this purpose, a sodium hydroxide is used as a separate source for OH ions in this example. It is possible to opt for the addition of base in liquid form, as in this example, or in the form of solid NaOH (pellets). Addition is also possible in the form of a 0.1 -1 M NaOH solution. Other hydroxides or other bases are also suitable for raising the pH, provided that this does not result in any adverse contamination of the process water.
    Figure 3 shows the precipitation of hydroxyapatite with an increase in pH, assuming an initial concentration of 1.2 mmol / l. The phosphate concentration is split into monovalent phosphate (3) (block) and divalent phosphate (4) (triangle). It can be clearly seen that not all phosphate can precipitate at pH 7.2. At pH> 8, the precipitation reaction can proceed much further, resulting in a lower phosphate content in solution. Figure 4 clearly shows that the
    -10 titration curve with NaOH runs very steeply between pH 8 and 9.7. Addition of 0.1 mmol NaOH causes the pH value to increase by more than 1.5 units. In the present example, the pH is thereby maintained above a value of pH = 8, whereby virtually any phosphate can precipitate in the form of hydroxyapatite. To this end, NaOH in solid form was added to the process water until the process water showed a stable pH of 8.2.
    In order to accurately control and control the pH value of the process water flow, pH sensors 110, 120, 130 are provided at different positions, with which in particular current upstream and downstream of the NaOH reservoir 30 can accurately display the current acidity of the process water and if desired with an autonomous control system based on measured pH values, feedback can be made to the NaOH release from the reservoir. To this end, the dosing devices 20,30 and the pH sensors are coupled to a control unit (not shown) that monitors and controls the entire process.
    The addition of calcium ions and a base to the process water leads to instantaneous cloudiness due to the formation and precipitation of crystalline apatite. The precipitate is at the nanocrystalline stage, typically between 20 and 2000 nm in size. The apatite also partially forms in the form of smaller, nano crystals, typically between 20 and 200 nm in size. This fraction of the apatite formed can be recovered from the solution in the form of fine crystals by filtration as a useful fertilizer. For this, the choice was made for a tangential crossflow filtration process, in which retention of the apatite crystals takes place in a hollow-fiber membrane. With this method, the permeate production is higher than with other filter methods. This is an important factor when scaling up to large quantities on an industrial scale.
    The process water stream successively passes through a settling tank 40 and an ultrafiltration step 60 to be excreted as phosphate-free permeate stream. The concentrate is returned to the settling tank 40 in which an increased concentration of nano crystals is formed. These nanocrystals are collected after sedimentation and as a deposit on a bottom. This concentrate 5 can be separated at the bottom with typically more than 5-50% dry matter. This apatite slurry can, for example, be further dewatered by means of centrifugation, pressing or thermal and / or vacuum drying, to a dry matter content of 40-95% and typically
    -11 more than 20% in large-scale industrial applications. By further (freeze) drying, final crystalline hydroxyapatite can be isolated in the form of a finely divided powder. This can then serve as a phosphate-rich raw material or, for example, as a fertilizer and can thus be reused. For many forms of reuse, a less far-reaching form of drying is sufficient, in particular from the point of view of a reduction in transport weight. This is an important factor when scaling up to large quantities on an industrial scale.
    An ultra-filtration filter 60 with an MCWO of 75 kDa with an estimated pore size between 10 and 20 nm was chosen. The process water is led through a pump 50 and recirculated to the tank 40. A pump 50 with a controllable speed has been selected which draws in the liquid from the storage tank 40 and then presses it through a hollow-fiber filter (straws) filter 60. This filter 60 is mounted vertically and the flow through the filter is from bottom to top. On a discharge side of the filter 60, the permeate 1 'is collected which was pressed out via the filter fibers. The retentate acquires an increasing crystalline hydroxyapatite content during the filtration process.
    This ultra-filtration step ensures complete retention of even the smallest apatite crystals, allowing the permeate stream 1 'to be discharged directly into the surface water or returned to the process as clean process water for reuse. It is important that this filtration is also carried out in a continuous flow process. In particular, all this is matched and dimensioned such that the permeate stream 1 'is of the same magnitude (flow) as the process water stream 1 supplied to the input 10.
    An acidity and phosphate content of the permeate 1 'are determined for checking. The result is shown in Table 3 below:
    Table 3Phosphate content before and after treatment Component pH POP content(mg / l) degree(mmol / l) Fraction in filtrate Process water 7.2 36 1.2Process water with apatite precipitation 9.7 30 1.0
    UF75k filtrate 97 0.09 0.003 3
    This shows that the permeate obtained is almost completely phosphate-free. It is important that the apatite precipitate is formed immediately when CaCl 2 is added to the process water. This means that the nucleation of apatite crystals occurred instantaneously. The turbidity (light scattering) that occurs in this case indicates that crystals grow at least a few tens of nm on nucleation and may then undergo a growth phase. This means that with a UF filter almost complete retention of apatite crystals is possible very shortly after nucleation (crystal formation).
    Although the invention has been further elucidated above on the basis of merely a single exemplary embodiment, it will be apparent that the invention is by no means limited thereto. On the contrary, many variations and manifestations are still possible for the average person skilled in the art within the scope of the invention.
    In general, the process according to the invention enables the effective treatment of liquid streams with a dissolved phosphate content in a concentration of typically up to 2500 mg / l on an industrial scale. In addition, the phosphate content in the liquid stream can be precisely regulated and almost completely reduced with an efficiency of more than 9598%. All this takes place in a fully continuous flow process and can be controlled and controlled fully automatically.
    权利要求:
    Claims (8)
    [1]
    Conclusions:
    Method for at least partial phosphate removal from process water, in particular from waste water, wherein phosphate ions in an aqueous solution are subjected to a precipitation reaction with calcium ions from a calcium source to form a phosphate-containing precipitate, in particular as hydroxy apatite (Ca 5 OH, Ca 10 (PO 4) 6 OH 2 ), characterized in that the calcium source is added to a process water stream, that a basic (alkaline) environment is imposed downstream of the calcium source on the process water stream, at least a basic (alkaline) ) environment is maintained therein, and in a flow-through process the process water stream is at least substantially separated from the phosphate-containing precipitate by ultra- or microfiltration in a cross-flow filtration process, in particular via a membrane filter comprising a bundle of hollow fibers with a nano-porous fiber wall.
    [2]
    Method according to claim 1, characterized in that an acidity (pH) above approximately pH = 8 is maintained in the basic medium.
    [3]
    Method according to claim 1 or 2, characterized in that the basic medium is applied with a base that is independent of the source of the calcium ions and is added, whether or not together with it.
    [4]
    Method according to claim 3, characterized in that the base comprises a hydroxide, in particular sodium hydroxide.
    [5]
    Process according to one or more of the preceding claims, characterized in that a water-soluble calcium salt is used as a calcium source, in particular a calcium halide, more in particular calcium chloride.
    [6]
    A method according to claim 5, characterized in that the calcium source is used in the form of an aqueous solution with a neutral acidity.
    [7]
    Method according to one or more of the preceding claims, characterized in that the ultrafiltration is carried out with a membrane filter with a maximum pore size between 10 and 20 nanometers.
    8. Method according to one or more of the preceding claims, characterized in that the phosphate-containing precipitate is formed at least substantially in nanocrystalline and / or microcrystalline form and is separated from the process water stream upstream of the cross-flow filtration.
    [8]
    9. Method according to one or more of the preceding claims, characterized in that the process water stream was passed through a pre-filter upstream of the addition of the calcium source.
    Method according to one or more of the preceding claims, characterized in that the process water comprises waste water, in particular a residual stream of animal or vegetable origin, more in particular one from a food industry.
    Rave


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    同族专利:
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    NL2020711A|NL2020711B1|2018-04-04|2018-04-04|Method for an at least partial phosphate removal from process water as well as a method for recovering a phosphate-containing compound from process water|
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