Treatment of phosphate compounds

Abstract

A method for treating phosphate compounds includes providing a primary liquid (S10), which is an acidic solution containing at least phosphorus. Iron-containing phosphate compounds are precipitated from the primary liquid (S20) by adding a first additive to the primary liquid (S22). The first additive includes iron hydroxide and a solid filter aid. A first solid, including the precipitated phosphate compounds and the solid filter aid, is separated by a first filtration (S28). The separated first solid is exposed to an alkaline solution (S30), causing dissolution of the phosphate compounds (S32) and precipitation of iron hydroxide (S32), thereby obtaining a secondary liquid. A second solid, including the precipitated iron hydroxide and the solid filter aid, is removed by a second filtration (S38). A portion of the second solid is reused as the first additive (S40). A system for treating phosphate compounds is also disclosed.

Description

[Technical Field]

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

[0002] Phosphorus is an important element and essential for life. However, its release into surface waters, where it contributes to eutrophication, has raised concerns about water quality. Consequently, policies have been implemented worldwide to reduce the amount of phosphorus entering surface waters by introducing technologies to remove phosphorus from domestic and industrial wastewater. Consequently, phosphorus accumulates in sewage sludge, a major by-product of wastewater treatment plants.

[0003] Mineral phosphorus resources are limited and considered finite, which has led to growing interest in technologies that facilitate the recycling and beneficial reuse of phosphorus contained in waste materials such as sewage sludge.

[0004] Many technologies for recovering phosphorus have been proposed. Examples of relevant patent literature include Japanese Patent No. 9145038, published European patent application EP2016203A1, published international patent application WO00 / 50343A1, published international patent application WO2008 / 115121A1, and published international patent application WO03 / 000620A1. Various approaches are also described in scientific literature, such as in the conference by Schaum et al. (Conference on the Management of Residues Emanating from Water and Wastewater Treatment, 12.08.2005, Johannesburg, South-Africa), by Franz (Waste Manag. 2008;28(10):1809-18), or the conference by Dittrich et al. (International Conference on Nutrient Recovery from Wastewater Streams, Vancouver, 2009).

[0005] All of these approaches are also discussed in the background section of European Patent EP3623348B. This patent discloses a method for treating materials containing phosphorus and at least one of iron and aluminum. The proposed process has generally proven to work very well. However, it faces minor issues that may primarily affect the economics of industrial implementation. One step that has not been entirely free of additional consideration is the addition of a base containing iron hydroxide and the subsequent removal of precipitated phosphate compounds. For example, when using filtration as a separation technique, frequent treatments have been found to be necessary to ensure a sufficient flow rate for the filtered solids to pass through the filter. Furthermore, different process parameters can affect the efficiency of precipitating phosphate compounds. Summary of the Invention

[0006] It is therefore a general object of the present technology to provide a method and system for treating phosphate compounds that is more efficient in precipitating and separating phosphate compounds.

[0007] The above object is 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 acidic solution containing at least phosphorus. Iron-containing phosphate compounds are precipitated from the primary liquid. Precipitating the iron-containing phosphate compounds includes adding a first additive to the primary liquid. The first additive includes iron hydroxide and a solid filter aid. A first solid, including the precipitated iron-containing phosphate compounds and the solid filter aid, is separated from the primary liquid by a first filtration. The separated first solid is exposed to an alkaline solution, which causes the alkaline solution to dissolve the phosphate compounds and precipitate iron hydroxide, providing a secondary liquid. A second solid, including the precipitated iron hydroxide and the solid filter aid, is removed from the secondary liquid by a second filtration. The removed secondary liquid is an alkaline solution containing phosphorus. At least a first portion of the second solid is reused as at least a portion of the first additive.

[0009] In a second aspect, a phosphate compound treatment system includes a phosphate compound precipitation unit, an iron hydroxide precipitation unit, and a recycling device. The phosphate compound precipitation unit includes a primary liquid inlet for a primary liquid. The primary liquid is an acidic solution containing at least phosphorus. The phosphate compound precipitation unit includes an additive inlet for a first additive. The first additive includes iron hydroxide and a solid filter aid. The phosphate compound precipitation unit includes a first device for mixing the primary liquid and the first additive to cause precipitation of an iron-containing phosphate compound. The phosphate compound precipitation unit includes a first filter configured to separate a first solid of the precipitated iron-containing phosphate compound and the solid filter aid from the primary liquid. The phosphate compound precipitation unit includes a first solid outlet for the first solid. The phosphate compound precipitation unit includes a filtered primary liquid outlet for the filtered primary liquid. The iron hydroxide precipitation unit includes a solid inlet for the first solid connected to the first solid outlet. The iron hydroxide precipitation unit includes an alkaline solution inlet for an alkaline solution. The iron hydroxide precipitation unit includes a second device for mixing the first solids with the primary liquid and the alkaline solution to precipitate iron hydroxide in the secondary liquid. The iron hydroxide precipitation unit includes a second filter for removing second solids, including the precipitated iron hydroxide and solid filter aid, from the secondary liquid. The iron hydroxide precipitation unit includes a second solids outlet for the second solids. The iron hydroxide precipitation unit includes a filtered secondary liquid outlet for the filtered secondary liquid. The recycling unit has a first connection connecting the second solids outlet to the additive material inlet. The first connection is configured to reuse (recycle) at least a first portion of the second solids as at least a part of the first additive. The recycling unit further includes a dividing device configured to divide the second portion of the second solids and an extractor configured to extract iron compounds from the second portion of the second solids. The extraction apparatus includes a reactor for exposing a second portion of the second solids to a hydrochloric acid solution, thereby dissolving iron and producing an iron chloride solution, and a recovery apparatus configured to recover the solid filter aid from the second portion of the second solids.

[0010] One advantage of the proposed technology is that improved filtering is achieved without consuming large amounts of additives, other advantages will be apparent from the detailed description.

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

[0012] [Figure 1] FIG. 1 shows a known example of processing of phosphate compounds. [Figure 2] FIG. 1 is a flow diagram of the steps of one embodiment of a method for treating phosphate compounds. [Figure 3] FIG. 1 shows the filtration time of iron hydroxide using different filter aids. [Figure 4] FIG. 1 is a schematic diagram of components of one embodiment of a system for treating phosphate compounds. [Figure 5] FIG. 2 is a flow diagram of the steps of another embodiment of a method for treating phosphate compounds. [Figure 6] FIG. 2 is a schematic diagram of components of another embodiment of a system for treating phosphate compounds. [Figure 7] FIG. 1 is a flow diagram of the steps of yet another embodiment of a method for treating phosphate compounds. [Figure 8] FIG. 10 is a schematic diagram of components of yet another embodiment of a system for treating phosphate compounds. [Figure 9] FIG. 10 is a flow diagram of the component steps of one embodiment of the step of providing a primary liquid. [Figure 10] FIG. 1 is a schematic diagram of components of one embodiment of a leachate reactor. [Figure 11] FIG. 1 shows an experiment on phosphorus extraction efficiency. [Figure 12A] FIG. 1 shows the time dependence of the reaction between the primary liquid and iron hydroxide. [Figure 12B] FIG. 1 shows the time dependence of the reaction between the primary liquid and iron hydroxide. [Figure 13] FIG. 1 is a flow diagram of the steps of yet another embodiment of a method for treating phosphate compounds. [Figure 14] FIG. 10 is a schematic diagram of components of yet another embodiment of a system for treating phosphate compounds. DETAILED DESCRIPTION OF THE INVENTION

[0013] Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

[0014] To better understand the proposed technology, it will be useful to start with a brief overview of some of the detailed issues regarding the implementation of the system according to patent EP3623348B.

[0015] In one embodiment, as shown schematically in Figure 1, sewage sludge ash was dissolved in an aqueous hydrochloric acid solution. The remaining undissolved residue was separated as ash sand. An acidic solution containing at least phosphorus and iron was mixed with iron hydroxide to precipitate iron phosphate compounds. The precipitated iron phosphate compounds were separated and subsequently exposed to sodium hydroxide, resulting in ferric hydroxide and sodium phosphate precipitated in solution. A portion of the ferric hydroxide could be used in the next batch of precipitation of iron phosphate compounds. Calcium hydroxide was added to the separated sodium hydroxide solution, resulting in calcium phosphate (PCP) precipitated in the sodium hydroxide solution. The sodium hydroxide separated from this could be reused in the next batch of exposure of precipitated iron phosphate to sodium hydroxide.

[0016] When this process was implemented using standard filtration techniques to remove the precipitated ferric hydroxide, several drawbacks were found. Regardless of the filter, the ferric hydroxide formed a relatively thin layer of filter material as a compressible filter cake. When pressure was applied to the filter cake to hasten separation, the filter cake compressed, forming a compact layer that relatively effectively blocked further liquid permeation through the filter. At the same time, this compressed filter cake was not mechanically stable enough by itself, making mechanical removal difficult. Overall, the filtration times were too long to be attractive for efficient large-scale industrial use, even if the process were indeed operable.

[0017] Filtration experiments have shown 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 proven to withstand the high pH of the solution being filtered. A drawback, however, is the relatively large amount of filter aid required. A 10% by weight filter aid relative to the iron hydroxide being filtered showed some improvement in filtration characteristics. However, even higher filter aid contents were required to improve filtration characteristics. Preferably, at least 30% by weight, and more preferably about 50% by weight, of filter aid is added to the solution before filtration.

[0018] The high filter aid content required to provide useful filtration conditions naturally leads to high additional costs. Consuming large amounts of filter aid is economically unfeasible for industrial applications. Furthermore, the filter cake consists of a mixture of filter aid and iron hydroxide, making it less attractive as a commercial product. While it is possible to post-treat the filter cake to separate the filter aid, this is relatively costly and eliminates the possibility of reusing the iron in the form of iron hydroxide.

[0019] However, this filter aid was found to be inert to strongly acidic conditions and to be able to withstand solutions down to at least pH 0. Furthermore, it was found that this filter aid did not affect the chemical reaction between iron hydroxide and phosphate ions. This filter aid also performed well when filtered with precipitated iron-containing phosphorus compounds. This unexpectedly allowed the filter aid to be recycled back into the process along with the iron hydroxide.

[0020] FIG. 2 is a flow diagram of steps in one embodiment of the method for treating phosphate compounds of the present invention. In step S10, a primary liquid is provided. This primary liquid is an acidic solution containing at least phosphorus. This primary liquid can be provided in a variety of ways, but one option is to generate it by leaching sewage sludge ash. This particular embodiment is described further below. However, the concept of the present invention is applicable to many types of strongly acidic liquids containing phosphorus. In step S20, iron-containing phosphate compounds are precipitated from the primary liquid. This step of precipitating iron-containing phosphate compounds S20 is followed by step S22, in which a first additive is added to the primary liquid. The first additive comprises iron hydroxide and a solid filter aid. The pH of the primary liquid is preferably adjusted to a range of 2.5 to 3.5, most preferably 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 the precipitated iron-containing phosphate compounds and the solid filter aid.

[0021] In step S30, the separated first solids are exposed to an alkaline solution, which dissolves the phosphate compounds, as shown in step S31, and precipitates iron hydroxide, as shown in step S32. This dissolution and precipitation results in a remaining secondary liquid. In step S38, second solids are removed from the secondary liquid by a second filtration. The second solids include precipitated iron hydroxide and solid filter aid. The secondary liquid after removal step S38 is thus a phosphorus-containing alkaline solution. This phosphorus-containing alkaline solution may be post-treated, in certain embodiments, as further described below. In step S40, at least a first portion of the second solids is recycled as at least a portion of the first additive material.

[0022] Tests showed that perlite dissolves only to a small extent during cycles of acid-alkaline conditions, i.e., from pH 0-1 to pH 12-14. Long-term tests in which the filter cake was repeatedly cycled through the filter aid showed an improvement in overall filtration characteristics. No deterioration of filtration capacity over time due to filter aid degradation or fouling was observed.

[0023] This method of entraining filter aid in the iron circulation within the process significantly reduces costs. The filter aid is filtered together with iron-containing phosphate compounds in the first filtration and with iron hydroxide in the second filtration. Apart from supplying a large amount of filter aid initially, only minor losses during processing need to be replenished in later stages.

[0024] As will be discussed below, if the primary liquid contains iron, it will accumulate within the circulation process. In such cases, a bleed of iron material must be removed from the process. In the above approach, since the iron compounds are always mixed with the filter aid, the bleed will also contain filter aid, which must be compensated for in the process. This is also discussed in more detail below.

[0025] As mentioned above, it is preferable to add a large amount of filter aid to facilitate handling of the filter cake. In one embodiment, the additive comprises a solid filter aid in an amount that provides at least 15% by weight of the solid filter aid in the second solid material. In a preferred embodiment, the additive comprises a solid filter aid in an amount that provides at least 45% by weight of the solid filter aid in the second solid material, and most preferably, the additive comprises a solid filter aid in an amount that provides at least 70% by weight of the solid filter aid in the second solid material.

[0026] Because the filter aid is exposed to both strong acids and strong bases, it must be at least largely inert in all of these situations. In typical applications, the pH of the primary liquid may be as low as 0.8, while dissolution in alkaline solutions may be as high as pH 12.2. Therefore, in preferred embodiments, the solid filter aid is chemically inert in the pH range of 0.8 to 12.2.

[0027] Furthermore, porous solid filter aids are believed to be more efficient because the co-filtered materials in the first and second materials can be contained within such a porous structure without significantly reducing the permeation rate of the liquid through the filter cake. Thus, in one embodiment, the solid filter aid has a porous structure.

[0028] The solid filter aid may be of many different types. In one embodiment, the solid filter aid is selected as at least one of perlite, diatomaceous earth, and cellulose. Most experiments based on the above principles are conducted using perlite. Perlite 30, perlite 50, and perlite 180 were used, for example, during filtration time tests, the results of which are shown in Figure 3. The average filtration time of iron hydroxide from a solution of iron phosphate dissolved in sodium hydroxide was measured for different qualities and amounts of perlite. The diagram in Figure 3 shows that for all perlite qualities, even relatively low perlite fractions, up to at least about 15 wt% relative to the dry iron hydroxide, had some effect on filtration time. At about 70 wt% perlite, a significant decrease in average filtration time was achieved. Higher perlite fractions did indeed further reduce the filtration rate, but at the expense of significantly greater amounts of perlite. The perlite fraction was initially measured as a weight percent of the dry iron phosphate dissolved in the alkaline solution and then recalculated as a weight percent of the dry iron hydroxide content in the actual iron hydroxide filtration process.

[0029] 4 is a diagram schematically illustrating one embodiment of the phosphate compound treatment system 1. The phosphate compound treatment system 1 includes a phosphate compound precipitating unit 20 and an iron hydroxide precipitating unit 30.

[0030] The phosphate compound precipitation unit 20 includes a primary liquid inlet 21 for a primary liquid 101. The primary liquid 101 is an acidic solution containing at least phosphorus. The phosphate compound precipitation unit 20 further includes an additive inlet 22 for a first additive 102. The first additive 102 includes iron hydroxide and a solid filter aid. The phosphate compound precipitation unit 20 further includes a first arrangement 23 for mixing the primary liquid 101 and the first additive 102. Precipitation of iron-containing phosphate compounds is caused. The phosphate compound precipitation unit 20 further includes a first filter 24 configured to separate first solids 103 from the primary liquid. The first solids include precipitated iron-containing phosphate compounds and a solid filter aid. The phosphate compound precipitation unit 20 also includes a first solids outlet 25 for the first solids 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 the acid used to dissolve the phosphorus in the primary liquid.

[0031] The iron hydroxide precipitation unit 30 includes a solids inlet 31 for the first solids 103. The solids inlet 31 is thereby directly or indirectly connected to the first solids outlet 25 for supplying the first solids 103. The iron hydroxide precipitation unit 30 further includes an alkali inlet 32 ​​for the alkali solution 105. The iron hydroxide precipitation unit 30 includes a second arrangement 33 for mixing the first solids 103 and the alkali solution 105. This results in the precipitation of iron hydroxide in the secondary liquid 107. The iron hydroxide precipitation unit 30 further includes a second filter 34 for removing second solids 106 from the secondary liquid 107. The second solids 106 include precipitated iron hydroxide and solid filter aid. The iron hydroxide precipitation unit 30 further includes a second solids outlet 35 for the second solids 106 and a filtered secondary liquid outlet 36 for the filtered secondary liquid 107. The filtered secondary liquid is an alkaline solution containing phosphorus.

[0032] The phosphoric acid compound treatment system 1 further includes a recycling device 40. The recycling device 40 has a first connection 41 that connects the second solids outlet 35 to the additive inlet 22. The recycling device 40 is thereby configured to recycle at least a first portion of the second solids 106 as at least a part of the first additive 102.

[0033] As mentioned above, the present method is particularly suitable for treating primary liquors resulting from the leaching of sewage sludge ash. Such primary liquors inevitably contain phosphate and iron ions. Since the iron in the basic process described above is circulated within the process, the iron content in the process liquor will build up if it is not removed from the circulating material from time to time.

[0034] In embodiments where the primary liquid contains iron, the method preferably includes additional steps. Figure 5 is a flow diagram of the steps of another embodiment of the method for treating phosphate compounds. Most of the steps are the same as in Figure 2, and will not be described unless they are affected by the additional steps.

[0035] As described in connection with FIG. 2, step S40 recycles a first portion of the second solids. Additionally, step S50 separates a second portion of the second solids. Step S52 extracts iron compounds from the second portion of the second solids. These recovered iron compounds compensate for the amount of iron entrained with the primary liquid. These steps can be performed as a continuous process, with the second portion of the second solids being continuously extracted depending on the iron content of the incoming primary liquid. Alternatively, these steps can be performed intermittently, for example, when the iron concentration in the circulation between steps S20 and S30 exceeds a predetermined level. In this way, excess iron is removed from the recycle portion of the process.

[0036] As a side effect of removing iron from the recycling process, some of the solid filter aid is also removed from the process, and more solid filter aid must be added in step S22 to maintain the required level of solid filter aid.

[0037] In a preferred embodiment, the removed solid filter aid can also be recycled (reused). To this end, step S52 of extracting iron compounds from the second portion of the second solid material includes step S53, in which the second portion of the second solid material is exposed to a hydrochloric acid solution. This dissolves the iron into the solution, producing an iron chloride solution. Iron chloride is a substance that has some commercial value and is used in many industrial processes. The solid filter aid is released from the iron hydroxide by this dissolution of the iron. In step S54, the solid filter aid is recovered from the second portion of the second solid material.

[0038] Preferably, the recovered solid filter aid is reused as at least a part of the first additive, as shown in step S56. In this way, the amount of solid filter aid is kept constant in the process, and only the small losses occurring during different partial processes need to be compensated for.

[0039] 6 schematically illustrates an embodiment of the phosphate compound treatment system 1 configured to recover iron provided in the primary liquid. In this embodiment, the recycling device 40 of the phosphate compound treatment system 1 further includes a dividing device 50 configured to divide the 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.

[0040] Preferably, extractor 51 comprises a reactor 52 in which second portion 106B of the second solids is exposed to a hydrochloric acid solution 108, thereby dissolving the iron and producing an iron chloride solution 110. Extractor 51 further comprises a recovery device 54 configured to recover solid filter aid material 109 from second portion 106B of the second solids.

[0041] Preferably, the recycle device 40 further comprises a second connection 55 connecting the recovery device 54 and the additive inlet 22. The second connection 55 is configured to recirculate the recovered solid filter aid 109 as at least a portion of the first additive material 102.

[0042] In step S30 of Figures 2 and 5, the first solids, i.e., the precipitated iron-containing phosphorus compounds and solid filter aid, are exposed to an alkaline solution to convert the phosphorus compounds into an alkaline solution containing iron hydroxides and phosphates. This can be accomplished with many different alkaline solutions. Preferably, the alkaline solution has a pH >12.

[0043] In a particularly advantageous embodiment, a solution of sodium hydroxide, i.e., caustic soda, is used for this purpose. In other words, the alkaline solution in the exposure comprises sodium hydroxide. Figure 7 is a flow diagram of the steps of yet another embodiment of a method for treating phosphate compounds. Steps that are essentially the same as those in the previous embodiment will not be described in detail again, and some of the legends have been omitted from the figure to make the figure easier to understand. This embodiment may be combined with any of the embodiments described herein above.

[0044] Step S30, in this embodiment, includes step S32, in which the first solid material is exposed to an aqueous sodium hydroxide solution, i.e., an alkaline solution containing caustic soda. The filtered secondary liquid remaining after step S38 contains dissolved sodium phosphate.

[0045] In step S60, lime is added to the secondary liquor after removal step S38. The addition of lime causes the precipitation of calcium phosphate and also causes the regeneration of the sodium hydroxide-containing liquor. The precipitated calcium phosphate is a commercially attractive material and is used in many different industrial and / or agricultural processes. Therefore, as indicated by step S62, the precipitated calcium phosphate is extracted from the sodium hydroxide-containing liquor.

[0046] Also, sodium hydroxide solution is of interest as a commercial product. However, since sodium hydroxide is used in another step S32 of the method, it is very convenient to reuse this sodium hydroxide-containing extract for use in the exposure step S32. That is, in step S64, at least a portion of the sodium hydroxide-containing liquid is reused as at least a portion of the alkaline solution.

[0047] FIG. 8 schematically illustrates yet another embodiment of a system 1 for treating phosphate compounds, configured 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 connected to the second solids outlet 36, through which the alkaline solution 105 is transported from the iron hydroxide precipitation section 30 to the calcium phosphate reactor 60. The calcium phosphate reactor 60 further includes a lime inlet 62 for adding lime 111. If the alkaline solution 105 contains sodium hydroxide, calcium phosphate 112 precipitates. Simultaneously, a sodium hydroxide-containing liquid 113 is regenerated. The calcium phosphate reactor 60 further includes an extractor 63 configured to extract the precipitated calcium phosphate 112 from a calcium phosphate outlet 64, leaving behind a sodium hydroxide-containing liquid 113. The calcium phosphate reactor 60 further includes a filtered secondary liquid outlet 65 for the sodium hydroxide-containing liquid 113.

[0048] As mentioned above, the sodium hydroxide is preferably circulated back into the system, and to this end, in a preferred embodiment, the third connection 66 between the filtered secondary liquid outlet 65 and the alkaline solution inlet 32 ​​is configured to reuse at least a portion of the sodium hydroxide-containing liquid 113 as at least a portion of the alkaline solution 105.

[0049] As noted in the background, the technology was developed for use in the recovery of phosphate from sewage sludge, particularly sewage sludge ash, but more generally, the technology is applicable to different processes for separating phosphorus from different types of acid solutions.

[0050] However, in one particular embodiment, the primary liquid has its origin in sewage sludge ash treatment. Figure 9 is a flow diagram of an embodiment of step S10, which provides a primary liquid of a phosphorus-containing acid solution. 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 obtain the primary liquid. Typically, sewage sludge ash always contains some iron, as iron compounds are used in sewage treatment processes, for example, to precipitate phosphorus. Because iron is already present in the chemical system, the use of iron hydroxide in the remaining steps of the present technology is particularly advantageous.

[0051] In a preferred embodiment, the mineral acid is hydrochloric acid.

[0052] Primary liquors derived from sewage sludge ash typically contain phosphorus and iron, as well as various other substances, such as heavy metals. Therefore, in a preferred embodiment, precipitation of iron-containing phosphorus compounds, such as FePO4, is induced by adjusting the pH to 2.5-3.5, preferably around pH 3. At low pHs, the solubility of FePO4 remains relatively high, reducing the efficiency of phosphorus extraction from the liquor. At higher pHs, the solubility decreases, risking the precipitation of heavy metal phosphates and calcium phosphates, for example. At a pH of around 3, most of the iron phosphate precipitates, while other phosphates precipitate only slightly.

[0053] When Ca(OH)2 is used to raise the pH, the remaining primary liquid filtered after precipitating the iron-containing phosphorus compounds typically contains dissolved salts based on the acid used in the primary liquid, along with heavy metals or calcium.

[0054] Similarly, FIG. 10 schematically illustrates an embodiment of a leachate reactor 10. The leachate reactor 10 is configured to dissolve sewage sludge ash containing phosphorus and iron in mineral acid and remove undissolved sewage sludge ash residue. To this end, the leachate reactor 10 includes an ash inlet 11 for receiving sewage sludge ash 114 and a mineral acid inlet 12 for receiving mineral acid 115. Preferably, the mineral acid is hydrochloric acid. A third mixer 13 is used to mix the sewage sludge ash 114 and the mineral acid 115 to promote dissolution of the phosphorus. A third filter 14 is used to separate the undissolved sewage sludge ash residue 116, which is discharged through a residue outlet 15. The resulting solution is advantageously used as the primary liquid. The primary liquid outlet 16 for the primary liquid 101 from the leachate reactor 10 is directly or indirectly connected to a primary liquid inlet 21 of the phosphorus compound precipitation section.

[0055] Of particular interest in this process is the efficient extraction of phosphorus from the primary liquor. Of course, a high yield would increase the economic benefits of this technology. Also of interest is the low phosphorus content in the residual liquor after this step, which is of particular interest for environmental reasons.

[0056] Therefore, the efficiency of the phosphate removal reaction using ferric hydroxide is of great interest. In the chemical process where phosphoric acid reacts with ferric hydroxide through dissolution and precipitation, the reaction can be described as follows: H3PO4+Fe(OH)3→Fe 3+ +PO4 3- +3H20→FePO4↓+3H20

[0057] In this reaction, each phosphate ion binds with one iron ion, resulting in a relatively high efficiency of extracting phosphate ions from solution.

[0058] However, there are competing processes. One alternative, such as adsorption, can be defined as:

[0059] [ka]

[0060] This process requires three iron atoms to bind two phosphate ions, resulting in low phosphate extraction efficiency.

[0061] Providing a large amount of iron hydroxide in the primary liquor may consistently result in a high phosphate capture efficiency. However, the large amount of iron hydroxide converts the primary liquor into a thick slurry with a relatively high pH, ​​which is difficult to treat industrially and requires a compromise between phosphorus removal efficiency and treatability.

[0062] Figure 11 shows the results of several test experiments. Sewage sludge ash was leached with hydrochloric acid to obtain three different leachates with chloride concentrations ranging from 3 to 5.5%. The pH of these leachates was 0.61, 0.82, and 1.15, respectively. To each leachate, Fe(OH)3 cake was added at different liquid / dry solid ratios. After stirring for approximately 1 hour and adjusting the pH to approximately 3, the resulting pH and phosphorus content were analyzed. The resulting slurries were partially filtered through syringe filters, and the filtrates were analyzed for P content.

[0063] In the figure, the percentage of phosphorus remaining in solution is indicated by the dotted line and the scale on the right. The solid line and the scale on the left represent the final pH of the slurry. It can be seen that essentially complete extraction of phosphorus can be achieved by using a very low liquid / dry solids ratio. However, as shown above, such slurries are difficult to handle in an industrial environment. It can be seen that different original pH values ​​of the leachate result in different phosphorus extraction efficiencies. A lower pH leachate allows for more efficient phosphorus extraction. This is because a lower pH allows for more solubility of Fe(OH)3, releasing free Fe ions and removing more phosphorus than a higher pH leachate. For example, a liquid / dry solids ratio of approximately 5 and an original leachate pH of 0.61 resulted in a reasonably thin slurry, achieving less than 5% phosphorus loss.

[0064] As mentioned above, precipitation of iron-containing phosphorus compounds, such as FePO4, is preferably induced by adjusting the pH to 2.5-3.5, preferably around pH 3. However, at such a pH, the solubility of Fe(OH)3 is low. Thus, in a continuous process where phosphorus extraction is essentially carried out at pH 3, phosphorus extraction is relatively low.

[0065] This can be carried out as a subsequent step in a single batch reactor, which is less attractive commercially, or as a continuous process in two subsequent reactors, which is more commercially important.

[0066] Additional experiments were conducted to investigate the timing of this partitioning. Sewage sludge ash was dissolved in hydrochloric acid at a solid-to-liquid ratio of 5 to obtain an ash leachate. The hydrochloric acid concentration of the leachate was approximately 6.6%, and the pH was approximately 0.5. Ferric hydroxide cake containing perlite was added to the ash leachate. The pH was continuously monitored. Samples were taken during stirring and filtered through a syringe filter. The filtrate was analyzed spectrophotometrically for P and Fe content. Two liquid-to-dry solid ratios were used for the leachate and ferric hydroxide: 5.3 and 4.0, respectively. The results are shown in Figures 12A and 12B.

[0067] In Figure 12A, the original liquid / dry solids ratio was 5.3. After 20 minutes, the iron hydroxide dissolved, increasing the liquid / dry solids ratio to approximately 6.6. The pH can be seen to increase during the first few minutes due to the dissolution of iron hydroxide. The phosphorus content decreases due to the precipitation of iron-containing phosphate compounds. The dissolution of iron hydroxide is also seen as an increase in the iron content. The dissolution of iron hydroxide is seen to be most efficient at a pH below 1. As the pH increases, iron dissolution decreases significantly.

[0068] In Figure 12B, the original liquid / dry solids ratio was 4.0. After 20 minutes, the iron hydroxide dissolved and the liquid / dry solids ratio increased to approximately 4.9. Essentially the same behavior is repeated. Iron dissolution occurs primarily at pH below 1. Therefore, regardless of the amount of iron hydroxide added, iron dissolution is limited.

[0069] In patent EP3623348B, it is reported that gel formation occurs at low molar ratios of [P / (Fe+Al)] ≤ 1. However, dissolved iron usually gives a P / Fe molar ratio of 3 or more, so gel formation was not obtained regardless of the large amount of Fe(OH)3 available for dissolution.

[0070] FIG. 13 is a flow diagram of steps for treating phosphate compounds according to one embodiment. In step S10, a primary liquid containing phosphorus is provided as an acidic solution. In certain embodiments, the primary liquid may be generated from dissolved sewage sludge ash, as described above. Preferably, the primary liquid has a pH of <1, as shown in step S11. In step S20, iron-containing phosphate compounds are precipitated. Step S20 for precipitating iron-containing phosphate compounds from the primary liquid includes a first partial step S21 followed by a second partial step S22. In the first partial step S21, a first additive is added to the primary liquid. The first additive includes iron hydroxide. In a preferred embodiment, the first additive also includes a solid filter aid, as described above. That is, step S21 can include step S22 of FIG. 2. In the second partial step S23, the pH is increased by adding a base. In a preferred embodiment, the base is added in an amount to achieve a pH between 2 and 4, as shown in step S24. In a preferred embodiment, the base added in the second partial step S23 comprises lime.

[0071] As mentioned above, steps S21 and S23 can be carried out as a continuous process in a single reactor. However, to increase the advantages in industrial production, in a preferred embodiment, the first partial step and the second partial step are continuous processes carried out in separate reactors.

[0072] In step S29, a first solid is separated. The first solid includes iron-containing phosphorus compounds. In a preferred embodiment, as described above, the first solid also includes a solid filter aid. That is, step S29 can include step S28 of FIG. 2. In step S30, the first solid is exposed to an alkaline solution. In step S39, a second solid is removed. The second solid includes iron hydroxide. In a preferred embodiment, as described above, the second solid also includes a solid filter aid. That is, step S39 can include step S38 of FIG. 2. In step S40, at least a portion of the second solid is reused as the first solid.

[0073] The reaction in step S21 is rapid, 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 and can take about 20 minutes. To design a continuous system, slightly longer reaction times are preferred, e.g., 20-30 minutes for step S21 and about 40 minutes for step S23.

[0074] Based on the above idea, continuous tests were carried out. In the two-part process system in continuous operation, phosphorus removal efficiencies of over 95% were easily achieved.

[0075] 14 is a schematic diagram of one embodiment of a system 1 for treating phosphate compounds. The system 1 for treating phosphate compounds includes a phosphate compound precipitation unit 20 and an iron hydroxide precipitation unit 30.

[0076] The phosphate compound precipitation unit 20 includes a primary liquid inlet 21 for a primary liquid 101. The primary liquid 101 is an acidic solution containing at least phosphorus. The phosphate compound precipitation unit 20 further includes an additive inlet 22 for a first additive 102. The first additive 102 includes iron hydroxide. Preferably, the first additive also includes a solid filter aid.

[0077] The phosphoric acid compound precipitation section 20 comprises a first reactor 28. The first reactor 28 has a primary liquid inlet 21 and an additive inlet 22, and further comprises a first arrangement for mixing 23. The phosphoric acid compound precipitation section 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 102 and the primary liquid 101. The second reactor further comprises a mixture inlet 80 connected to the mixture outlet 81 for the mixture 117 between the first additive 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 between the first additive 101 and the primary liquid 101. Preferably, an additional arrangement for mixing 82 is used for this purpose.

[0078] Preferably, the first and second reactors are continuous process reactors.

[0079] This causes the precipitation of iron-containing phosphate compounds. The phosphate compound precipitation unit 20 further includes a first filter 24 configured to separate first solids 103 from the primary liquid. The first solids include precipitated iron-containing phosphate compounds. Preferably, the first solids also include a solid filter aid. The phosphate compound precipitation unit 20 also includes a first solids outlet 25 for the first solids 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 the acid used to dissolve phosphorus in the primary liquid.

[0080] The iron hydroxide precipitation unit 30 includes a solids inlet 31 for the first solids 103. The solids inlet 31 is thereby directly or indirectly connected to the first solids outlet 25 for supplying the first solids 103. The iron hydroxide precipitation unit 30 further includes an alkali inlet 32 ​​for an alkali solution 105. The iron hydroxide precipitation unit 30 includes a second arrangement 33 for mixing the first solids 103 and the alkali solution 105. This results in the precipitation of iron hydroxide in the secondary liquid 107. The iron hydroxide precipitation unit 30 also includes a second filter 34 for removing second solids 106 from the secondary liquid 107. The second solids 106 include precipitated iron hydroxide. Preferably, the second solids 106 also include a solid filter aid. The iron hydroxide precipitation unit 30 further includes a second solids outlet 35 for the second solids 106 and a filtered secondary liquid outlet 36 for the filtered secondary liquid 107. The filtered secondary liquid is an alkaline solution containing phosphorus.

[0081] The phosphoric acid compound treatment system 1 further includes a recycling device 40. The recycling device 40 has a first connection part 41 that connects the second solids outlet 35 to the additive inlet 22. As a result, the recycling device 40 is configured to reuse at least a first portion of the second solids 106 as at least a part of the first additive 102.

[0082] The above-described embodiments should be understood as some illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations, and changes can be made to the embodiments without departing from the scope of the present invention. In particular, different partial solutions of different embodiments can be combined in other configurations, where technically possible. However, the scope of the present invention is defined by the appended claims.

Claims

1. A method for treating a phosphate compound, comprising: Providing a primary liquid (101) (S10), which is an acidic solution containing at least phosphorus; A step (S20) of precipitating iron-containing phosphate compounds from the primary liquid (101); The step (S20) of precipitating the iron-containing phosphate compound includes a step (S22) of adding a first additive (102) to the primary liquid (101), the first additive (102) comprises iron hydroxide and a solid filter aid; Separating (S28) a first solid (103) of the precipitated phosphate compounds containing the iron and the solid filter aid from the primary liquid (101) by a first filtration; a step (S30) of exposing the separated first solid (103) to an alkaline solution (105); The alkaline solution (105) thereby causes dissolution of phosphate compounds and precipitation of iron hydroxide to give a secondary liquid (107), removing (S38) the precipitated iron hydroxide and a second solid portion (106) of the solid filter aid from the secondary liquid (107) by a second filtration; Thereby, the secondary liquid (107) after the removing step (S38) is an alkaline solution containing phosphorus, and (S40) reusing at least a first portion of the second solid material (106) as at least a portion of the first additive material (102).

2. 10. The method of claim 1, wherein the additive material (102) comprises the solid filter aid in an amount providing at least 15 wt. %, preferably at least 45 wt. %, and more preferably at least 70 wt. % of the solid filter aid in the second solids.

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

2.

4. 4. The method of claim 3, wherein the solid filter aid is selected as at least one of perlite, diatomaceous earth, and cellulose.

5. A step (S50) of dividing a second portion (106B) of the second solid material (106); and (S52) extracting iron compounds from the second portion (106B) of the second solid material (106), 5. The method of claim 4, whereby the recovered iron compounds compensate for the amount of iron that enters with the primary liquid (101).

6. A step (S52) of extracting iron compounds from the second portion (106B) of the second solid material (106) includes: a substep (S53) of exposing the second portion (106B) of the second solid (106) to a hydrochloric acid solution (108) to dissolve iron and produce an iron chloride solution (110); and a partial step (S54) of recovering solid filter aid (109) from said second portion (106B) of said second solids (106).

7. 7. The method of claim 6, further comprising the step (S56) of reusing the recovered solid filter aid (109) as at least a portion of the first additive material (106).

8. The method according to any one of claims 1 to 7, wherein the alkaline solution (105) has a pH > 12.

9. The method of claim 8, wherein the alkaline solution (105) comprises caustic soda.

10. The step (S20) of precipitating iron-containing phosphate compounds from the primary liquid (101) comprises a first partial step (S21) followed by a second partial step (S23), The first partial step (S21) comprises adding the first additive (102) to the primary liquid (101), The method according to any one of claims 1 to 9, wherein the second partial step (S23) comprises adding a base (118) to increase the pH.

11. 11. The method of claim 10, wherein the pH of the primary liquid (101) is less than 1 and the base (118) in the second partial step (S23) is ignited in an amount that results in a pH in the range of 2 to 4.

12. 12. The method according to claim 10 or 11, wherein the base material (118) added in the second partial step (S23) comprises lime.

13. The method according to any one of claims 10 to 12, wherein the first partial step (S21) and the second partial step (S23) are continuous processes.

14. the alkaline solution (105) contains sodium hydroxide; The method comprises: after the removing step (S38), adding lime (S60) to the secondary liquid (107) to precipitate calcium phosphate (112) and regenerate a liquid containing sodium hydroxide (113); The method of any one of claims 1 to 13, further comprising the step (S62) of extracting the precipitated calcium phosphate (112) from the liquid containing sodium hydroxide (113).

15. 15. The method of claim 14, further comprising the step (S64) of recycling at least a portion of the liquid comprising sodium hydroxide (113) as at least a portion of the alkaline solution (105).

16. The step (S10) of supplying the primary liquid (101), A step (S11) of dissolving sewage sludge ash (114) containing phosphoric acid and iron in a mineral acid (115); removing undissolved sewage sludge ash residue (116) to obtain the primary liquid (101) (S12); The method according to any one of claims 1 to 15, comprising:

17. 17. The method of claim 16, wherein the mineral acid (115) is hydrochloric acid.

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

19. 19. The system of claim 18, wherein the recycling device (40) further comprises a second connection (55) connecting the recovery device (54) and the additive inlet (22) and configured to reuse the recovered solid filter aid (109) as at least a portion of the first additive (102).

20. the phosphoric acid compound precipitation section (20) comprises a first reactor (28) having the primary liquid inlet (21) and the additive inlet (22), and a second reactor (29) having the first solid 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 (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 (102) and the primary liquid (101); 20. The system of claim 18 or 19, wherein the second reactor (29) further comprises a base inlet (27) for a base (118), whereby an increase in pH of the mixture (117) of the first additive (102) and the primary liquid (101) is achieved.

21. 21. The system of claim 20, wherein the first reactor (28) and the second reactor (29) are continuous process reactors.

22. 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, when the alkaline solution (105) contains sodium hydroxide, calcium phosphate (112) precipitates and a liquid containing sodium hydroxide (113) is regenerated; the calcium phosphate reactor (60) further comprises an extractor (63) configured to extract the precipitated calcium phosphate (112) from the liquid containing sodium hydroxide (113) via a calcium phosphate outlet (64); The system of any one of claims 18 to 21, wherein the calcium phosphate reactor (60) further comprises an alkaline solution outlet (65) for the liquid comprising the sodium hydroxide (113).

23. 23. The system of claim 22, further comprising a third connection (66) between the alkaline solution outlet (65) and the alkaline solution inlet (32) configured to reuse at least a portion of the liquid comprising sodium hydroxide (113) as at least a portion of the alkaline solution (105).

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

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