Treatment of phosphate compounds

Abstract

A method for treating a phosphate compound, the method comprising providing (S10) a primary liquid, the primary liquid being an acid solution comprising at least phosphorus. An iron-containing phosphate compound is precipitated (S20) from the primary liquid by adding (S22) a first additive material to the primary liquid. The first additive material comprises ferric hydroxide and a solid filter aid material. A first solids comprising the precipitated phosphate compound and the solid filter aid material is separated (S28) by a first filtration. The separated first solids are exposed (S30) to an alkaline solution, causing dissolution (S32) of the phosphate compound and precipitation (S32) of iron hydroxide, giving a secondary liquid. A second solids comprising precipitated iron hydroxide and solid filter aid material is removed (S38) by a second filtration. A portion of the second solids is recycled (S40) as the first additive material. A system for treating a phosphate compound is also disclosed.

Description

Technical Field

[0001] The present technology relates generally to the processing of phosphate compounds, and particularly to methods and systems for separating iron and phosphorus. Background Art

[0002] Phosphorus is an essential element and indeed essential for life. However, the release of phosphorus into surface waters and its consequent contribution to eutrophication has led to increasing concerns about water quality. As a result, policies have been implemented around the world to reduce phosphorus levels entering surface waters by implementing technologies to remove phosphorus from domestic and industrial wastewater. As a result, phosphorus accumulates in sewage sludge, a major by-product of wastewater treatment plants.

[0003] Mineral phosphorus resources are considered to be finite and expendable. Therefore, there is growing interest in technologies that can facilitate the recycling and beneficial reuse of phosphorus present in wastes such as sewage sludge.

[0004] Many technologies for recovering phosphorus have been proposed. Some examples of relevant patent publications are Japanese Patent 9145038, published European patent application EP 2016203 A1, published international patent application WO00 / 50343A1, published international patent application WO 2008 / 115121 A1, and published international patent application WO 03 / 000620A1. Different approaches are also described in the scientific literature, for example by Schaum et al. at a conference (Conference on the Management of Residues Emanating from Water and Wastewater Treatment, August 12, 2005, Johannesburg, South Africa), by Franz in Waste Manag. 2008; 28(10): 1809-18, or by Dittrich et al. at a conference (International Conference on Nutrient Recovery from Wastewater Streams, Vancouver, 2009).

[0005] All these approaches are also discussed in the background section of European patent EP 3623348 B. In this patent, a method for treating materials containing phosphorus and at least one of iron and aluminum is disclosed. It has been proven that the proposed process works well in general. However, some minor problems have been encountered, which may mainly affect the economic aspects of industrial implementation. One part of the process that cannot be completely exempted from additional considerations is the addition of an alkali containing iron hydroxide and the subsequent removal of the precipitated phosphate compounds. For example, it has been found that the use of filtration as a separation technique requires frequent handling of the filtered solid material to ensure that the flow through the filter is satisfactory. In addition, the efficiency of precipitating the phosphate compounds varies with the process parameters. Summary of the invention

[0006]

[0013] It is therefore a general object of the present technology to provide methods and systems for processing phosphate compounds that are more efficient in phosphate compound precipitation and separation.

[0007] The above objects are achieved by a method and an apparatus according to the independent claims. Preferred embodiments are defined in the dependent claims.

[0008] In general, in a first aspect, a method for treating phosphate compounds includes providing a primary liquid. The primary liquid is an acid solution containing at least phosphorus. Iron-containing phosphate compounds are precipitated from the primary liquid. The precipitation of the iron-containing phosphate compounds includes adding a first additive material to the primary liquid. The first additive material contains iron hydroxide and a solid filter aid material. A first solid (containing the precipitated iron-containing phosphate compound and the solid filter aid material) is separated from the primary liquid by a first filtration. The separated first solid is exposed to an alkaline solution. The alkaline solution thereby causes the dissolution of the phosphate compound and the precipitation of the iron hydroxide, giving a secondary liquid. A second solid containing the precipitated iron hydroxide and the solid filter aid material is removed from the secondary liquid by a second filtration. The secondary liquid after removal is an alkaline liquid solution containing phosphorus. At least a first portion of the second solid is recycled as at least a portion of the first additive material.

[0009] In a second aspect, a system for treating phosphate compounds includes a phosphate compound precipitation section, an iron hydroxide precipitation section, and a recirculation device. The phosphate compound precipitation section includes a primary liquid inlet for the primary liquid. The primary liquid is an acid solution containing at least phosphorus. The phosphate compound precipitation section includes an additive material inlet for the first additive material. The first additive material contains iron hydroxide and a solid filter aid material. The phosphate compound precipitation section includes a first device for mixing the primary liquid and the first additive material, thereby causing precipitation of iron-containing phosphate compounds. The phosphate compound precipitation section includes a first filter, which is configured to separate the precipitated iron-containing phosphate compound and the first solid of the solid filter aid material from the primary liquid. The phosphate compound precipitation section includes a first solid outlet, which is used for the first solid. The phosphate compound precipitation section includes a filtered primary liquid outlet, which is used for the filtered primary liquid. The iron hydroxide precipitation section includes a solid inlet for the first solid, which is connected to the first solid outlet. The iron hydroxide precipitation section includes an alkaline solution inlet for the alkaline solution. The iron hydroxide precipitation section includes a second device for mixing the first solid with the primary liquid and the alkaline solution to cause the iron hydroxide to precipitate in the secondary liquid. The iron hydroxide precipitation section includes a second filter for removing the precipitated iron hydroxide and the second solid of the solid filter aid material from the secondary liquid. The iron hydroxide precipitation section includes a second solid outlet for the second solid. The iron hydroxide precipitation section includes a filtered secondary liquid outlet for the filtered secondary liquid. The secondary liquid is a phosphorus-containing alkaline liquid solution. The recycling device has a first connector that connects the second solid outlet to the additive material inlet. The first connector is configured to recycle at least a first portion of the second solid as at least a portion of the first additive material. The recycling device further includes a distribution device configured to distribute a second portion of the second solid and an extractor configured to extract iron compounds from the second portion of the second solid. The extractor includes a reactor in which a second portion of the second solids is exposed to a hydrochloric acid solution to dissolve iron to produce a ferric chloride solution, and a recovery device configured to recover solid filter aid material from the second portion of the second solids.

[0010] One advantage of the proposed technology is that improved filtration is achieved without significant consumption of additives. Other advantages will be appreciated upon reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

[0012] Figure 1 Known examples of treatment of phosphate compounds are shown;

[0013] Figure 2 is a flow chart of steps of an embodiment of a method for treating phosphate compounds;

[0014] Figure 3 is a graph showing filtration time of iron hydroxide when using different filter aid materials;

[0015] Figure 4 is a schematic diagram of a portion of an embodiment of a system for treating phosphate compounds;

[0016] Figure 5 is a flow chart of steps of another embodiment of a method for treating phosphate compounds;

[0017] Figure 6 is a schematic diagram of a portion of another embodiment of a system for treating phosphate compounds;

[0018] Figure 7 is a flow chart of steps of yet another embodiment of a method for treating phosphate compounds;

[0019] Figure 8 is a schematic diagram of a portion of yet another embodiment of a system for treating phosphate compounds;

[0020] Fig. 9 is a flowchart of sub-steps of an embodiment of the step of providing a primary liquid;

[0021] Fig.10 is a schematic diagram of a portion of an embodiment of a leachate reactor;

[0022] Fig.11 is a graph of the phosphorus extraction efficiency experiment;

[0023] Fig. 12A - B is a graph showing the time dependence of the reaction between the primary liquid and iron hydroxide;

[0024] Fig.13 is a flow chart of steps of yet another embodiment of a method for treating phosphate compounds; and

[0025] Fig.14 is a schematic diagram of a portion of yet another embodiment of a system for processing phosphate compounds. DETAILED DESCRIPTION

[0026] Throughout the drawings, the same reference numerals are used for similar or corresponding elements.

[0027] In order to better understand the proposed technique, it may be useful first to briefly outline some detailed issues in the implementation of the system according to patent EP 3623348 B.

[0028] In one embodiment, if Figure 1 As schematically shown in , sewage sludge ash is dissolved in an aqueous hydrochloric acid solution. The remaining undissolved residue is separated as ash sand. An acid solution containing at least phosphorus and iron is mixed with iron hydroxide, and an iron phosphate compound is precipitated. The precipitated iron phosphate compound is separated and subsequently exposed to sodium hydroxide to produce precipitated iron hydroxide and sodium phosphate in solution. A portion of the iron hydroxide can be used in subsequent batches of precipitated iron phosphate compounds. Calcium hydroxide is added to the separated sodium hydroxide solution to produce precipitated calcium phosphate (PCP) in the sodium hydroxide solution. The sodium hydroxide separated therefrom can be reused in subsequent batches of precipitated iron phosphates exposed to sodium hydroxide.

[0029] When this process is implemented with standard filtration techniques to remove the precipitated iron hydroxide, some disadvantages are found. Regardless of the filter, the iron hydroxide forms a relatively thin layer of filter material in the form of a compressible filter cake. When pressure is applied to the filter cake to accelerate separation, the filter cake is compressed and forms a compacted layer that relatively efficiently blocks further penetration of the liquid through the filter. At the same time, this compressed filter cake itself is not mechanically stable enough, and therefore mechanical removal is difficult. In short, even if this process can indeed be operated in this way, the filtration time is too long and is not attractive in efficient large-scale industrial applications.

[0030] In filtration experiments, it was found that the filtration characteristics of iron hydroxide can be improved by adding an inert filter aid to the solution before filtration. One example of such an inert filter aid is perlite, which has been shown to tolerate the high pH of the solution to be filtered. However, a disadvantage is that a relatively high amount of filter aid is required. At 10% by weight (compared to the iron hydroxide to be filtered) of filter aid, some improvement in the filtration characteristics was found. In order to improve the filtration characteristics, even high levels of filter aid are required. Preferably, more than 30% by weight and more preferably about 50% by weight of filter aid are added to the solution before filtration.

[0031] The high content of filter aid required to give useful filtering conditions of course causes high additional costs. The consumption of large amounts of filter aid is economically not feasible in industrial applications. In addition, the filter cake contains a mixture of filter aid and iron hydroxide and is not very attractive as a commercial product itself. The filter cake can be post-treated to separate the filter aid, but this is relatively expensive and eliminates the possibility of reusing the iron content in the form of iron hydroxide.

[0032] However, the filter aid was found to be inert to highly acidic conditions as well and could tolerate solutions at least as low as pH 0. Furthermore, the filter aid was found to not affect the chemical reaction between the iron hydroxide and the phosphate ions. The filter aid also worked well for filtering with the precipitated iron-containing phosphorus compounds. This unexpectedly opens avenues to allow the filter aid to be recycled in the process along with the iron hydroxide.

[0033] Figure 2 is a flow chart of the steps of an embodiment of a method for treating phosphate compounds. In step S10, a primary liquid is provided. This primary liquid is an acid solution containing at least phosphorus. This primary liquid can be provided in different ways, but one option is to produce it by leaching sewage sludge ash. This specific embodiment will be discussed further below. However, the idea is applicable to many types of highly acidic liquids containing phosphorus. In step S20, iron-containing phosphate compounds are precipitated from the primary liquid. The step S20 of precipitating the iron-containing phosphate compounds further includes a step S22 of adding a first additive material to the primary liquid. The first additive material comprises iron hydroxide and a solid filter aid material. Preferably, the pH of the primary liquid is adjusted to a range of 2.5–3.5, and most preferably to about 3, to reduce the solubility of the iron-containing phosphate compounds and minimize the precipitation of impurities. In step S28, a first solid is separated from the primary liquid by a first filtration. The first solid comprises precipitated iron-containing phosphate compounds and a solid filter aid material.

[0034] In step S30, the separated first solid is exposed to an alkaline solution. The alkaline solution thereby causes the dissolution of the phosphate compound (as shown in step S31) and the precipitation of iron hydroxide (as shown in step S32). This dissolution and precipitation give a remaining secondary liquid. In step S38, the second solid is removed from the secondary liquid by a second filtration. The second solid contains precipitated iron hydroxide and solid filter aid material. Therefore, the secondary liquid after the removal step S38 is a phosphorus-containing alkaline liquid solution. In a specific embodiment, this phosphorus-containing alkaline liquid solution can be post-treated as further discussed below. In step S40, at least a first portion of the second solid is recycled as at least a portion of the first additive material.

[0035] In tests, it was found that perlite dissolves only to a very limited extent during cycling under acid-alkaline conditions (i.e., from pH 0-1 up to pH 12-14). Long-term testing of repeated cycling of the filter cake with filter aids showed generally improved filtration characteristics. No deterioration in filtration capacity over time due to, for example, degradation or scaling of the filter aid material was observed.

[0036] This approach allows the filter aid to circulate within the process along with the iron, significantly reducing costs. The filter aid is filtered together with the iron-containing phosphate compounds in the first filtration and together with the iron hydroxide in the second filtration. Apart from the initial supply of large amounts of filter aid, only a replacement for minor losses during operation has to be added at a later stage.

[0037] As will be discussed further below, if the primary liquid includes iron, the iron content will accumulate within the circulating process. In such cases, the effluent of iron substances must be removed from the process. In the approach proposed above, the iron compounds are always mixed with the filter aid, which means that any effluent will also contain the filter aid, which must then be compensated for in the process. This will also be discussed in more detail further below.

[0038] As described above, a large amount of filter aid is preferably added to make the filter cake easy to handle. In one embodiment, the additive material comprises solid filter aid material in an amount of at least 15% by weight of the solid filter aid material in the second solid. In a preferred embodiment, the additive material comprises solid filter aid material in an amount of at least 45% by weight of the solid filter aid material in the second solid, and most preferably, the additive material comprises solid filter aid material in an amount of at least 70% by weight of the solid filter aid material in the second solid.

[0039] The filter aid material will be exposed to both strong acids and strong bases, and in all of these cases, at least the predominant portion should be inert. In typical applications, the primary solution may have a pH as low as 0.8, and dissolution in alkaline solutions may occur at a pH as high as 12.2. Therefore, in a preferred embodiment, the solid filter aid material is chemically inert in the pH range of 0.8 to 12.2.

[0040] Furthermore, porous solid filter aid materials are believed to be more effective because co-filtered materials in the first and second substances can be contained within such porous structures without significantly reducing the permeation rate of the liquid through the filter cake. Thus, in one embodiment, the solid filter aid material has a porous structure.

[0041] The solid filter aid material can be of many different kinds. In one embodiment, the solid filter aid material is selected from at least one of perlite, diatomaceous earth, and cellulose. Most experiments according to the above principles were conducted using perlite. For example, perlite 30, perlite 50, and perlite 180 were used during the filtration time test, and the results are shown in Figure 3 The average filtration time for ferric hydroxide from a solution of ferric phosphate dissolved in sodium hydroxide was measured for different qualities of perlite and different amounts of perlite. Figure 3The graph shows that for all perlite qualities, some effect on the filtration time is obtained even at relatively low perlite proportions (at least down to about 15% by weight compared to dry ferric hydroxide). At about 70% by weight of perlite, a significant reduction in the average filtration time is achieved. At even higher proportions of perlite, the filtration speed does decrease even further, however at the expense of very large amounts of perlite. The perlite proportions were initially measured in weight-% relative to the dry ferric phosphate dissolved in the alkaline solution and then recalculated in weight-% relative to the dry ferric hydroxide content in the actual ferric hydroxide filtration process.

[0042] Figure 4 An embodiment of a system 1 for treating phosphate compounds is schematically shown. The system 1 for treating phosphate compounds comprises a phosphate compound precipitation stage 20 and an iron hydroxide precipitation stage 30.

[0043] The phosphate compound precipitation section 20 comprises a primary liquid inlet 21 for a primary liquid 101. The primary liquid 101 is an acid solution containing at least phosphorus. The phosphate compound precipitation section 20 further comprises an additive material inlet 22 for a first additive material 102. The first additive material 102 comprises iron hydroxide and a solid filter aid material. The phosphate compound precipitation section 20 further comprises a first device 23 for mixing the primary liquid 101 and the first additive material 102. This causes precipitation of iron-containing phosphate compounds. The phosphate compound precipitation section 20 further comprises a first filter 24 configured to separate the first solid matter 103 from the primary liquid. The first solid matter comprises precipitated iron-containing phosphate compounds and a solid filter aid material. The phosphate compound precipitation section 20 further comprises a first solid matter outlet 25 for the first solid matter 103 and a filtered primary liquid outlet 26 for the filtered primary liquid 104. The filtered primary liquid 104 is typically a solution of a salt of an acid used to dissolve phosphorus into the primary liquid.

[0044] The iron hydroxide precipitation stage 30 comprises a solids inlet 31 for the first solids 103. The solids inlet 31 is thus directly or indirectly connected to the first solids outlet 25 for providing the first solids 103. The iron hydroxide precipitation stage 30 further comprises an alkaline inlet 32 ​​for the alkaline solution 105. The iron hydroxide precipitation stage 30 comprises a second device 33 for mixing the first solids 103 and the alkaline solution 105. This causes the iron hydroxide to precipitate in the secondary liquid 107. The iron hydroxide precipitation stage 30 further comprises a second filter 34 for removing the second solids 106 from the secondary liquid 107. The second solids 106 comprise precipitated iron hydroxide and solid filter aid material. The iron hydroxide precipitation stage 30 further comprises a second solids outlet 35 for the second solids 106 and a filtered secondary liquid outlet 36 for the filtered secondary liquid 107. The secondary liquid filtered is an alkaline liquid solution containing phosphorus.

[0045] The system 1 for treating phosphate compounds further comprises a recycling device 40. The recycling device 40 has a first connection 41 which connects the second solids outlet 35 to the additive material inlet 22. The recycling device 40 is thus configured for recycling at least a first portion of the second solids 106 as at least a portion of the first additive material 102.

[0046] As indicated above, the present process is particularly well suited to treating the primary liquor resulting from the leaching of sewage sludge ash. Such primary liquors inevitably contain phosphate ions as well as iron ions. Since the iron in the above basic process circulates within the process, the iron content will accumulate in these process liquids if it is not occasionally or continuously removed from the recycle.

[0047] In embodiments wherein the primary liquid comprises iron, the method preferably comprises an additional step. Figure 5 is a flow chart of steps for another embodiment of a method for treating phosphate compounds. Most of the steps are similar to Figure 2 , and if not affected by the additional steps, they will not be discussed.

[0048] As combined Figure 2As described, the first part of the second solid is recycled by step S40. In addition, in step S50, the second part of the second solid is distributed. In step S52, iron compounds are extracted from the second part of the second solid. These recovered iron compounds compensate for any amount of iron added with the primary liquid. These steps can be carried out as a continuous process, and the second part of the second solid is continuously extracted according to the iron content of the added primary liquid. Alternatively, these steps can be carried out intermittently, for example when the concentration of iron in the circulation between steps S20 and S30 exceeds a predetermined level. In this way, excess iron is removed from the recycled part of the process.

[0049] A side effect of removing iron from the circulating process is that a portion of the solid filter aid material is also removed from the process. In order to maintain the required level of solid filter aid material, more solid filter aid material must be added in step S22.

[0050] In a preferred embodiment, the removed solid filter aid material can also be recycled. To this end, the step S52 of extracting the iron compound from the second portion of the second solid object further includes a sub-step S53, in which the second portion of the second solid object is exposed to a hydrochloric acid solution. This will cause the iron to dissolve into the solution, thereby producing a ferric chloride solution. Ferric chloride is a substance with certain commercial value and is used in many industrial processes. By this dissolution of iron, the solid filter aid material is released from the ferric hydroxide. In step S54, the solid filter aid material is recovered from the second portion of the second solid object.

[0051] Preferably, as shown in step S56, the recovered solid filter aid material is recycled as at least a part of the first additive material. In this way, the amount of solid filter aid material remains constant in the process, and only the marginal losses caused in the different sub-processes have to be compensated.

[0052] Figure 6 An embodiment of a system 1 for treating phosphate compounds is schematically shown, the system being configured to recover iron provided in a primary liquid. In this embodiment, the recycling device 40 of the system 1 for treating phosphate compounds further comprises a distribution device 50 configured to distribute a second portion 106B of the second solids and an extractor 51 configured to extract iron compounds from the second portion 106B of the second solids.

[0053] Preferably, the extractor 51 comprises a reactor 52 in which the second portion 106B of the second solids is exposed to a hydrochloric acid solution 108. Iron is thereby dissolved, producing a ferric chloride solution 110. The extractor 51 further comprises a recovery device 54 configured to recover solid filter aid material 109 from the second portion 106B of the second solids.

[0054] Preferably, the recovery device 40 further comprises a second connection 55 connecting the recovery device 54 with the additive material inlet 22. The second connection 55 is configured to recycle the recovered solid filter aid material 109 as at least a portion of the first additive material 102.

[0055] exist Figure 2 and Figure 5 In step S30, the first solid (i.e., the precipitated iron-containing phosphorus compound and the solid filter aid material) is exposed to an alkaline solution to convert the phosphorus compound into an alkaline solution containing iron hydroxide and phosphate. This can be achieved by many different alkaline solutions. Preferably, the pH of the alkaline solution is > 12.

[0056] In a particularly advantageous embodiment, a sodium hydroxide solution, ie caustic soda, is used for this purpose. In other words, the alkaline solution in the exposure comprises sodium hydroxide. Figure 7 Flow chart of the steps of yet another embodiment of a method for treating phosphate compounds. Steps that are substantially the same as the previous embodiment are not discussed in detail, and some explanatory text is removed from the figure to make the figure easier to understand. This embodiment can be combined with any of the embodiments described above.

[0057] In this embodiment, step S30 includes step S32, in which the first solid is exposed to a sodium hydroxide solution. In other words, the alkaline solution contains caustic soda. The remaining filtered secondary liquid after step S38 contains dissolved sodium phosphate.

[0058] In step S60, lime is added to the secondary liquid after the removal step S38. The addition of lime causes the precipitation of calcium phosphate and also causes the regeneration of the liquid containing sodium hydroxide. The precipitated calcium phosphate is a commercially attractive substance and is used in many different industrial and / or agricultural processes. Therefore, as shown in step S62, the precipitated calcium phosphate is extracted from the liquid containing sodium hydroxide.

[0059] Sodium hydroxide liquid is also of interest as a commercial product. However, since sodium hydroxide is used in another step S32 of the method, it is very convenient to recycle this extracted sodium hydroxide-containing liquid for use in the exposure step S32. In other words, at least a portion of the sodium hydroxide-containing liquid is recycled as at least a portion of the alkaline solution in step S64.

[0060] Figure 8 Yet another embodiment of a system 1 for treating phosphate compounds is schematically shown, which is configured to be able to recover phosphorus in a more valuable form. The system 1 for treating phosphate compounds includes a calcium phosphate reactor 60. The calcium phosphate reactor 60 has a filtered secondary liquid inlet 61, which is connected to the second solids outlet 36, whereby the alkaline solution 105 is transported from the iron hydroxide precipitation stage 30 to the calcium phosphate reactor 60. The calcium phosphate reactor 60 further includes a lime inlet 62, which is used to add lime 111. If the alkaline solution 105 contains sodium hydroxide, calcium phosphate 112 is precipitated. At the same time, the liquid 113 containing sodium hydroxide is regenerated. The calcium phosphate reactor 60 further includes an extractor 63, which is configured to extract the precipitated calcium phosphate 112 through the calcium phosphate outlet 64, leaving the liquid 113 containing sodium hydroxide. The calcium phosphate reactor 60 further includes a filtered secondary liquid outlet 65, which is used for the liquid 113 containing sodium hydroxide.

[0061] As mentioned above, it is preferred that the sodium hydroxide is circulated back in the system. To this end, in a preferred embodiment, a third connection 66 is provided between the filtered secondary liquid outlet 65 and the alkaline solution inlet 32 ​​for recycling at least a portion of the liquid 113 containing sodium hydroxide as at least a portion of the alkaline solution 105.

[0062] As noted in the background, the present technology was developed with the intent to use it for the recovery of phosphates from sewage sludge, and in particular from sewage sludge ash. However, the technology is more generally applicable to different processes in which phosphorus is to be separated from different types of acid solutions.

[0063] However, in one embodiment, the primary liquid is derived from sewage sludge ash treatment. Fig. 9 A flow chart of an embodiment of step S10 (providing a primary liquor containing a phosphorus acid solution) is shown. In step S11, sewage sludge ash containing phosphorus and iron is dissolved in a mineral acid. In step S12, the undissolved sewage sludge ash residue is removed to give a primary liquor. Since iron compounds are typically used in sewage treatment processes to, for example, precipitate phosphorus, sewage sludge ash always contains some iron. Since iron is already present in the chemical system, it becomes particularly advantageous to use iron hydroxide in the remaining process of the present technology.

[0064] In a preferred embodiment, the inorganic acid is hydrochloric acid.

[0065] The primary liquid produced from the sludge ash typically contains phosphorus and iron and various other substances, such as heavy metals. Therefore, in a preferred embodiment, the precipitation of iron-containing phosphorus compounds (e.g., FePO4) is caused by adjusting the pH to 2.5-3.5, preferably about pH 3. At lower pH, the solubility of FePO4 is still relatively high, resulting in low efficiency of extracting phosphorus from the liquid. Instead, there is a risk of precipitation of heavy metal phosphates and calcium phosphates, for example, at higher pH, because the solubility of these substances decreases. At a pH of about 3, most of the iron phosphate will precipitate, but only a small amount of other phosphates will precipitate.

[0066] If Ca(OH)2 is used to raise the pH, the filtered primary liquor remaining after precipitation of the iron-containing phosphorus compounds typically contains dissolved salts based on the acid used in the primary liquor as well as heavy metals or calcium.

[0067] Similarly, Fig.10 An embodiment of a leachate reactor 10 is schematically shown. The leachate reactor 10 is configured for dissolving sewage sludge ash containing phosphorus and iron in an inorganic acid and for removing undissolved sewage sludge ash residues. To this end, the leachate reactor 10 comprises an ash inlet 11 for allowing sewage sludge ash 114 to enter and an inorganic acid inlet 12 for allowing the addition of an inorganic acid 115. Preferably, the inorganic acid is hydrochloric acid. A third mixing device 13 is used to mix the sewage sludge ash 114 and the inorganic acid 115 to promote the dissolution of phosphorus. A third filter 14 is used to separate undissolved sewage sludge ash residues 116. The undissolved sewage sludge ash residues 116 are discharged through a residue outlet 15. This produces a solution that is advantageously used as a primary liquid. The primary liquid outlet 16 for the primary liquid 101 from the leachate reactor 10 is directly or indirectly connected to the primary liquid inlet 21 of the phosphate compound precipitation stage.

[0068] A particularly interesting part of this process is the efficient extraction of phosphorus from the primary liquor. Of course, the high yield increases the economic advantage of this technology. Moreover, the low phosphorus content in the remaining liquor after this step is also interesting, for example for environmental reasons.

[0069] Therefore, the efficiency of the reaction of removing phosphorus by iron hydroxide is of great interest. In the chemical process of reacting phosphorous acid and iron hydroxide by dissolution and precipitation, the reaction can be described as:

[0070] H3PO4+Fe(OH)3→Fe 3+ +PO4 3-+3H2O→FePO4↓+3H2O

[0071] In this reaction, each phosphate ion is matched with one iron ion, and thus the efficiency of extracting phosphate ions from the solution is relatively high.

[0072] However, there are competing processes. A substitution of adsorbent species can be defined as:

[0073]

[0074] In this process, three iron atoms are required to bind two phosphate ions, making this route less efficient in phosphate extraction.

[0075] High efficiency of phosphorus capture can always be obtained by providing large amounts of iron hydroxide to the primary liquor. However, large amounts of iron hydroxide will cause the primary liquor to become a thick slurry with a relatively high pH. This is difficult to process in an industrial manner and there must be some compromise between phosphorus removal efficiency and processability.

[0076] Fig.11 is a graph showing the results of some test experiments. Three different leach solutions were obtained by leaching sewage sludge ash with hydrochloric acid (with different chloride concentrations between 3% and 5.5%). The corresponding pH of these leachates were 0.61, 0.82 and 1.15, respectively. For each leach solution, Fe(OH)3 cake was added at different liquid / dry solid ratios. After stirring for about 1 hour and then adjusting the pH to about 3, the resulting pH and the phosphorus content therein were analyzed. The analysis was carried out by partially filtering the resulting slurry on a syringe filter and analyzing the filtrate for P content.

[0077] In the figure, the fraction of phosphorus remaining in solution is shown by the dotted line and the scale on the right hand side. The solid line and the left hand scale represent the final pH of the slurry. Here, it can be seen that essentially complete phosphorus extraction can be obtained with very low liquid / dry solid ratios. However, as mentioned above, such slurries are more difficult to operate in an industrial environment. It can also be seen that different initial pH of the leachate leads to different efficiencies in extracting phosphorus. Lower pH leachates allow more efficient phosphorus extraction. The explanation is that more Fe(OH)3 dissolves at lower pH, thereby releasing free Fe ions and removing more phosphorus, compared to conditions at higher pH. Less than 5% phosphorus loss can be achieved, for example, at a liquid / dry solid ratio of about 5, which will give a reasonably dilute slurry with an initial leachate pH of 0.61.

[0078] As mentioned above, the precipitation of iron-containing phosphorus compounds (e.g., FePO4) is preferably induced by adjusting the pH to 2.5-3.5, preferably about pH 3. However, at such a pH, the solubility of Fe(OH)3 is low. Therefore, a continuous process in which the phosphorus extraction is carried out substantially at pH 3 will result in a relatively low phosphorus extraction.

[0079] In a preferred embodiment of the present technology, it is instead proposed to first ensure high solubility of iron before adjusting the pH to cause precipitation of the iron-containing phosphorus compound. In other words, the precipitation of the iron-containing phosphorus compound is divided into two parts. This can be carried out as a subsequent process step in a single batch reactor (not very attractive from an industrial point of view) or as a continuous process in two subsequent reactors (of greater industrial importance).

[0080] Additional experiments were performed to investigate the temporal aspect of this partitioning. An ash leach solution was obtained by dissolving sewage sludge ash in hydrochloric acid (liquid / dry solid ratio of 5). The leach solution had an HCl concentration of about 6.6%, corresponding to a pH of about 0.5. Iron hydroxide cake containing perlite was added to the ash leach solution. The pH was continuously monitored. Samples were taken during stirring, filtered on a syringe filter, and the filtrate was analyzed for P and Fe content by spectrophotometry. Two different liquid / dry solid ratios between the leach solution and iron hydroxide were used, 5.3 and 4.0, respectively. The results are shown in Figure 2. Fig. 12A and 12B shown.

[0081] exist Fig. 12A In the embodiment of the present invention, the initial liquid / dry solid ratio is 5.3. After 20 minutes, the iron hydroxide is dissolved and the liquid / dry solid ratio increases to about 6.6. It can be seen that the pH increases in the first few minutes due to the dissolution of the iron hydroxide. The phosphorus content decreases due to the precipitation of iron-containing phosphate compounds. The dissolution of the iron hydroxide is also seen as an increase in the iron content. It can be noted that the dissolution of the iron hydroxide is most efficient when the pH is below 1. When the pH is increased to higher values, the dissolution of iron decreases significantly.

[0082] exist Fig. 12B In the example above, the initial liquid / dry solid ratio was 4.0. After 20 minutes, the iron hydroxide dissolved and the liquid / dry solid ratio increased to about 4.9. Essentially the same behavior was repeated. Iron dissolution occurred primarily at pH below 1. Therefore, there was limited dissolution of iron, regardless of the amount of iron hydroxide added to the process.

[0083] In patent EP 3623348 B, it is reported that low molar ratios of [P / (Fe+Al)] ≤ 1 lead to gel formation. However, since the dissolved iron gives a P / Fe molar ratio typically above 3, no gel is formed, regardless of the large amount of Fe(OH)3 available for dissolution.

[0084] Fig.13 is a flow chart of steps of an embodiment for treating phosphate compounds. In step S10, a primary liquid of a phosphorus-containing acid solution is provided. In a specific embodiment, the primary liquid can be produced from dissolved sewage sludge ash, as described above. Preferably, as shown in step S11, the primary liquid has a pH<1. In step S20, iron-containing phosphate compounds are precipitated. The step S20 of precipitating iron-containing phosphate compounds from the primary liquid includes a first sub-step S21, followed by a second sub-step S22. In the first sub-step S21, a first additive material is added to the initial liquid. The first additive material comprises iron hydroxide. In a preferred embodiment, and as described above, the first additive material also comprises a solid filter aid material, that is, step S21 may include Figure 2 In the second sub-step S23, the pH is increased by adding a base. In a preferred embodiment, as shown in step S24, the base is added in an amount such that the pH is in the range of 2 to 4. In a preferred embodiment, the base added in the second sub-step S23 comprises lime.

[0085] As described above, steps S21 and S23 can be performed as continuous steps in a single reactor. However, in order to increase the advantages in industrial production, in a preferred embodiment, the first sub-step and the second sub-step are continuous processes performed in separate reactors.

[0086] In step S29, the first solid material is separated. The first solid material comprises iron-containing phosphorus compounds. In a preferred embodiment, and as described above, the first solid material also comprises solid filter aid material, i.e., step S29 may comprise Figure 2 In step S28, the first solid material is exposed to an alkaline solution. In step S30, the second solid material is removed. The second solid material comprises iron hydroxide. In a preferred embodiment, and as described above, the second solid material also comprises a solid filter aid material, i.e., step S39 may include Figure 2 In step S38, at least a portion of the second solid matter is recycled as the first solid matter.

[0087] The reaction in step S21 is fast, as shown in the above experiments, and is typically completed in about 10 minutes. The second reaction in step S23 is limited by the reactivity of lime dissolution, which may take about 20 minutes. In order to design a continuous system, a slightly longer reaction time is preferred, such as 20-30 minutes for step S21, and about 40 minutes for step S23.

[0088] Based on the ideas proposed above, continuous testing was conducted. In continuous operation, the dual-section process system easily achieved a phosphorus removal efficiency greater than 95%.

[0089] Fig.14 An embodiment of a system 1 for treating phosphate compounds is schematically shown. The system 1 for treating phosphate compounds comprises a phosphate compound precipitation stage 20 and an iron hydroxide precipitation stage 30.

[0090] The phosphate compound precipitation section 20 comprises a primary liquid inlet 21 for a primary liquid 101. The primary liquid 101 is an acid solution containing at least phosphorus. The phosphate compound precipitation section 20 further comprises an additive material inlet 22 for a first additive material 102. The first additive material 102 comprises iron hydroxide. Preferably, the first additive material further comprises a solid filter aid material.

[0091] The phosphate compound precipitation stage 20 comprises a first reactor 28. The first reactor 28 has a primary liquid inlet 21 and an additive material inlet 22, and also a first means for mixing 23. The phosphate compound precipitation stage 20 further comprises a second reactor 29. The second reactor 29 has a first solids outlet 25 and a filtered primary liquid outlet 26. The first reactor 28 further comprises a mixture outlet 81 for a mixture 117 of the first additive material 102 and the primary liquid 101. The second reactor further comprises a mixture inlet 80 connected to the mixture outlet 81 for a mixture 117 of the first additive material 102 and the primary liquid 101. The second reactor 29 further comprises an alkali inlet 27 for an alkali 118, thereby achieving an increase in the pH of the mixture 117 of the first additive material 101 and the primary liquid 101. Preferably, an additional means for mixing 82 is used for this purpose.

[0092] Preferably, the first reactor and the second reactor are continuous process reactors.

[0093] This causes precipitation of iron-containing phosphate compounds. The phosphate compound precipitation section 20 further includes a first filter 24, which is configured to separate the first solid 103 from the primary liquid. The first solid contains precipitated iron-containing phosphate compounds. Preferably, the first solid also contains solid filter aid material. The phosphate compound precipitation section 20 also includes a first solid outlet 25 and a filtered primary liquid outlet 26, the first solid outlet is for the first solid 103, and the filtered primary liquid outlet is for the filtered primary liquid 104. The filtered primary liquid 104 is typically a solution of a salt of an acid for dissolving phosphorus into the primary liquid.

[0094] The iron hydroxide precipitation stage 30 comprises a solids inlet 31 for the first solids 103. The solids inlet 31 is thus directly or indirectly connected to the first solids outlet 25 for providing the first solids 103. The iron hydroxide precipitation stage 30 further comprises an alkaline inlet 32 ​​for the alkaline solution 105. The iron hydroxide precipitation stage 30 comprises a second device 33 for mixing the first solids 103 and the alkaline solution 105. This causes the iron hydroxide to precipitate in the secondary liquid 107. The iron hydroxide precipitation stage 30 further comprises a second filter 34 for removing the second solids 106 from the secondary liquid 107. The second solids 106 comprise precipitated iron hydroxide. Preferably, the second solids 106 also comprise solid filter aid material. The iron hydroxide precipitation stage 30 further comprises a second solids outlet 35 for the second solids 106 and a filtered secondary liquid outlet 36 for the filtered secondary liquid 107. The secondary liquid filtered is an alkaline liquid solution containing phosphorus.

[0095] The system 1 for treating phosphate compounds further comprises a recycling device 40. The recycling device 40 has a first connection 41 which connects the second solids outlet 35 to the additive material inlet 22. The recycling device 40 is thus configured for recycling at least a first portion of the second solids 106 as at least a portion of the first additive material 102.

[0096] The above embodiments should be understood as some illustrative embodiments of the present invention. It will be appreciated by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. Specifically, where technically possible, different partial solutions in different embodiments may be combined in other configurations. However, the scope of the present invention is limited by the appended claims.

Claims

1. A method for treating phosphate compounds, the method comprising the following steps: - providing (S10) a primary liquid (101), wherein the primary liquid (101) is an acid solution containing at least phosphorus; - precipitating (S20) iron-containing phosphate compounds from the primary liquid (101); The step of precipitating (S20) the iron-containing phosphate compound comprises adding (S22) a first additive material (102) to the primary liquid (101); The first additive material (102) comprises iron hydroxide and a solid filter aid material; - separating the precipitated iron-containing phosphate compounds and the first solid matter (103) of the solid filter aid material from the primary liquid (101) by a first filtration (S28); - exposing (S30) the separated first solid object (103) to an alkaline solution (105); The alkaline solution (105) thereby causes dissolution of the phosphate compounds and precipitation of iron hydroxide, giving a secondary liquid (107); - removing (S38) the precipitated iron hydroxide and the second solid matter (106) of the solid filter aid material from the secondary liquid (107) by a second filtration; Thus, the secondary liquid (107) after the removal step (S38) is an alkaline liquid solution containing phosphorus; and - recycling (S40) at least a first portion of the second solid matter (106) as at least a portion of the first additive material (102).

2. The method according to claim 1, characterized in that The additive material (102) comprises the solid filter aid material in an amount of at least 15% by weight, preferably at least 45% by weight, and more preferably at least 70% by weight of the solid filter aid material in the second solid matter.

3. The method according to claim 1 or 2, characterized in that: The solid filter aid material has a porous structure and is chemically inert in the pH range of 0.8 to 12.

2.

4. The method according to claim 3, characterized in that The solid filter aid material is selected from at least one of the following: Perlite Diatomaceous earth, and Cellulose.

5. The method according to claim 4, characterized in that The following additional steps: - distributing (S50) a second portion (106B) of the second solid object (106); - extracting (S52) iron compounds from the second portion (106B) of the second solid object (106); Thus, the recovered iron compounds compensate for the amount of iron added with the primary liquid (101).

6. The method according to claim 5, characterized in that The step of extracting (S52) the iron compound from the second portion (106B) of the second solid (106) further comprises the following sub-steps: - exposing (S53) the second portion (106B) of the second solid object (106) to a hydrochloric acid solution (108) to dissolve iron, thereby producing a ferric chloride solution (110); and - Recovering (S54) solid filter aid material (109) from said second portion (106B) of said second solid matter (106).

7. The method according to claim 6, characterized in that The following additional steps: - Recycling (S56) the recovered solid filter aid material (109) as at least a part of the first additive material (106).

8. The method according to any one of claims 1 to 7, characterized in that The pH of the alkaline solution (105) is >12.

9. The method according to claim 8, characterized in that The alkaline solution (105) contains caustic soda.

10. The method according to any one of claims 1 to 9, characterized in that The step of precipitating (S20) the iron-containing phosphate compound from the primary liquid (101) comprises a first sub-step (S21) and then a second sub-step (S23); wherein the first sub-step (S21) comprises adding the first additive material (102) to the primary liquid (101); and The second sub-step (S23) includes a step of increasing the pH by adding a base (118).

11. The method according to claim 10, characterized in that The pH of the primary liquid (101) is less than 1, and the base (118) is added in an amount in the second sub-step (S23) such that the pH is in the range of 2 to 4.

12. The method according to claim 10 or 11, characterized in that: The alkali (118) added in the second sub-step (S23) comprises lime.

13. The method according to any one of claims 10 to 12, characterized in that The first sub-step (S21) and the second sub-step (S23) are continuous processes.

14. The method according to any one of claims 1 to 13, characterized in that The alkaline solution (105) comprises sodium hydroxide, whereby the method comprises the following further steps: - after said removal step (S38), adding (S60) lime to said secondary liquid (107), causing precipitation of calcium phosphate (112) and regeneration of a liquid (113) containing sodium hydroxide; and - Extracting (S62) the precipitated calcium phosphate (112) from the sodium hydroxide-containing liquid (113).

15. The method according to claim 14, characterized in that The following additional steps: - Recirculating (S64) at least a portion of the sodium hydroxide-containing liquid (113) as at least a portion of the alkaline solution (105).

16. The method according to any one of claims 1 to 15, characterized in that The step (S10) of providing the primary liquid (101) further comprises the following steps: - dissolving (S11) sewage sludge ash (114) containing phosphorus and iron in a mineral acid (115); and - Removing (S12) undissolved sewage sludge ash residue (116) to give said primary liquid (101).

17. The method according to claim 16, characterized in that The inorganic acid (115) is hydrochloric acid.

18. A system (1) for treating phosphate compounds, the system comprising: - phosphate compound precipitation section (20); - ferric hydroxide precipitation section (30); The phosphate compound precipitation section (20) comprises a primary liquid inlet (21), the primary liquid inlet is for a primary liquid (101), the primary liquid (101) is an acid solution containing at least phosphorus; The phosphate compound precipitation stage (20) includes an additive material inlet (22) for a first additive material (102) comprising iron hydroxide and a solid filter aid material; The phosphate compound precipitation section (20) comprises a first device (23) for mixing the primary liquid (101) and the first additive material (102) to cause precipitation of the iron-containing phosphate compound; The phosphate compound precipitation stage (20) comprises a first filter (24) configured to separate the precipitated iron-containing phosphate compound and the first solid matter (103) of the solid filter aid material from the primary liquid (101); The phosphate compound precipitation section (20) comprises a first solids outlet (25), the first solids outlet being used for the first solids (103); The phosphate compound precipitation section (20) comprises a filtered primary liquid outlet (26) for the filtered primary liquid (104); The iron hydroxide precipitation section (30) comprises a solid inlet (31), the solid inlet is used for the first solid (103), and the solid inlet is connected to the first solid outlet (25); The iron hydroxide precipitation section (30) comprises an alkaline solution inlet (32), wherein the alkaline solution inlet is used for the alkaline solution (105); The iron hydroxide precipitation stage (30) comprises a second device (33) for mixing the first solid (103) and the alkaline solution (105) to cause precipitation of iron hydroxide in the secondary liquid (107); The iron hydroxide precipitation stage (30) comprises a second filter (34) for removing the precipitated iron hydroxide and the second solid matter (106) of the solid filter aid material from the secondary liquid (107); The iron hydroxide precipitation stage (30) comprises a second solids outlet (35), the second solids outlet is for the second solids (106); The iron hydroxide precipitation section (30) comprises a filtered secondary liquid outlet (36), wherein the filtered secondary liquid outlet is used for the filtered secondary liquid (107), wherein the filtered secondary liquid is an alkaline liquid solution containing phosphorus; as well as a recycling device (40) having a first connection (41) connecting the second solids outlet (35) to the additive material inlet (22) and configured to recycle at least a first portion of the second solids (106) as at least a portion of the first additive material (102); wherein the recycling device (40) further comprises a distribution device (50) and an extractor (51), the distribution device being configured to distribute the second portion (106B) of the second solid (106), and the extractor being configured to extract iron compounds from the second portion (106B) of the second solid (106); and The extractor (51) comprises a reactor (52) in which the second portion (106B) of the second solid (106) is exposed to a hydrochloric acid solution (108) to dissolve iron to produce a ferric chloride solution (110) and a recovery device (54) configured to recover solid filter aid material (109) from the second portion (106B) of the second solid (106).

19. The system according to claim 18, characterized in that The recycling device (40) further comprises a second connection (55) connecting the recovery device (54) with the additive material inlet (22), and the second connection is configured to recycle the recovered solid filter aid material (109) as at least a part of the first additive material (102).

20. The system according to claim 18 or 19, characterized in that The phosphate compound precipitation stage (20) comprises a first reactor (28) having the primary liquid inlet (21) and the additive material inlet (22) and a second reactor (29) having the first solids outlet (25) and the filtered primary liquid outlet (26); The first reactor (28) further comprises a mixture outlet (81) for a mixture (117) of the first additive material (102) and the primary liquid (101); The second reactor (29) further comprises a mixture inlet (80) connected to the mixture outlet (81) for the mixture (117) of the first additive material (102) and the primary liquid (101); The second reactor (29) further comprises a base inlet (27) for a base (118), thereby achieving an increase in the pH of the mixture (117) of the first additive material (102) and the primary liquid (101).

21. The system according to claim 20, characterized in that The first reactor (28) and the second reactor (29) are continuous process reactors.

22. A system according to any one of claims 18 to 21, characterised in that a calcium phosphate reactor (60) having a filtered secondary liquid inlet (61) connected to the filtered secondary liquid outlet (36) and a lime inlet (62) for adding lime (111), whereby, if the alkaline solution (105) contains sodium hydroxide, calcium phosphate (112) precipitates and the sodium hydroxide-containing liquid (113) is regenerated; The calcium phosphate reactor (60) further comprises an extractor (63) configured to extract the precipitated calcium phosphate (112) from the sodium hydroxide-containing liquid (113) through a calcium phosphate outlet (64); and The calcium phosphate reactor (60) further comprises an alkaline solution outlet (65) for the liquid (113) containing sodium hydroxide.

23. The system according to claim 22, characterized in that A third connection (66) is provided between the alkaline solution outlet (65) and the alkaline solution inlet (32), the third connection being configured to recirculate at least a portion of the liquid containing sodium hydroxide (113) as at least a portion of the alkaline solution (105).

24. A system according to any one of claims 18 to 23, characterised in that A leachate reactor (10) is configured for dissolving phosphorus- and iron-containing sewage sludge ash (114) in a mineral acid (115) and for removing undissolved sewage sludge ash residue (116), thereby giving said primary liquid (101); wherein a primary liquid outlet (15) for said primary liquid (101) from said leachate reactor (10) is connected to said primary liquid inlet (21).

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