Chemical treatment of sewage sludge ash

Abstract

A method for chemically treating sewage sludge ash includes dissolving (S10) a starting material derived from sewage sludge ash in an acid comprising hydrochloric acid. The starting material comprises at least silicon and iron compounds. A non-dissolved residue is separated (S12), thereby retaining a leachate. An amount of colloidal silicon dioxide in the dissolved sewage sludge ash is controlled (S15). At least one of iron and phosphorus is extracted (20) from the leachate by liquid-liquid extraction with an organic solvent. At least a portion of a raffinate at least partially depleted in at least one of iron and phosphorus resulting from the step of extracting at least one of iron and phosphorus is recycled (S90) for dissolving the starting material derived from sewage sludge ash. The recycled portion of the raffinate at least partially depleted in at least one of iron and phosphorus comprises chloride ions. An apparatus for chemically treating sewage sludge ash is also disclosed.

Description

Technical Field

[0001] The present technology generally relates to the chemical treatment of sewage sludge ash, and more particularly to methods and apparatus for recovering components of sewage sludge ash. Background Art

[0002] Phosphorus is an essential nutrient for living organisms and plays an irreplaceable role. In recent decades, the demand for phosphate fertilizers in agriculture has increased significantly. It is also predicted that this will continue to increase due to population growth and rising living standards worldwide. The raw material for the production of mineral phosphate fertilizers and feed production is phosphate rock, which is a limited resource. In the European Union, both phosphate rock and phosphorus are considered key raw materials. Many countries lack natural phosphate deposits and are completely dependent on imports. Because phosphate rock mining is concentrated in only a few regions of the world, many countries are vulnerable to market fluctuations and geopolitical factors. One of the measures proposed to reduce dependence on phosphorus imports and over-exploitation of natural deposits is to recover phosphorus from secondary sources. These include municipal and industrial wastewater, sewage sludge, sewage sludge ash and other phosphorus-containing wastes. Governments and industry have recognized the importance of recovering phosphorus from secondary sources, and some countries have begun to implement legislation to do so.

[0003] Phosphorus released into surface waters contributes to eutrophication. Therefore, it is necessary to remove phosphorus from domestic and industrial wastewater during its treatment. This leads to the accumulation of phosphorus in sewage sludge, a major byproduct of wastewater treatment plants. Due to its large volume and the presence of heavy metals, organic pollutants, and pathogens, using this sludge as fertilizer is not a suitable option. Incinerating sewage sludge to produce sludge ash mitigates some of these drawbacks. This is done in specialized incinerators, where the wet sludge is pre-dried using recycled heat before incineration to reduce its water content from 70%-80% to approximately 30%-40%. Incineration reduces the sludge's volume, destroys organic pathogens, and produces a dry product containing approximately 6%-14% phosphorus by weight. In comparison, phosphate rock contains approximately 12%-16% phosphorus by weight. However, despite its relatively high phosphorus content, sludge ash is not suitable for use as a fertilizer. Although more than 90% of the phosphorus initially present in the wastewater ultimately ends up in the ash, the phosphorus compounds formed during incineration (e.g., calcium and aluminum phosphates) have low water solubility. Consequently, phosphorus cannot dissolve and be absorbed by plants. Furthermore, due to the reduced volume, heavy metals in the sludge (which are harmful to living organisms) are concentrated in the ash. Co-incineration of sewage sludge with biomass can be carried out in conventional waste incineration plants, but this produces ash with a lower phosphorus content (typically less than 5% by weight).

[0004] Due to the low water solubility and high thermal stability of calcium and aluminum phosphates and iron oxides, wet chemical processing is preferred for recovering phosphorus and iron from sewage sludge ash. Several such methods are well described in the literature and are briefly described below. These methods use a combination of leaching, precipitation, solvent extraction, and / or ion exchange to recover the components of the sewage sludge ash, but not necessarily all of them.

[0005] In the book "Phosphorus: Polluter and Resource of the Future-Removal and Recovery from Wastewater", IWA Publishing, Chapter 25, a process for recovering phosphorus from sewage sludge ash, named "LEACHPHOS", is described. The LEACHPHOS process relies on leaching sewage sludge ash with dilute sulfuric acid. The leachate, which contains phosphorus, aluminum, iron and other dissolved impurities, is separated from the undissolved residue using filtration. The leachate undergoes a stepwise neutralization to separate out mixed calcium phosphate, aluminum phosphate and iron phosphate products. The heavy metals end up in mixed metal hydroxide sludge, which precipitate at a pH above 9. These mixed products have a low market value and individual separation of the main components is not possible. The phosphorus products cannot be used directly as fertilizers due to their low water solubility.

[0006] A phosphorus recovery process, developed by Technicas Reunidas, is named the "PHOS4LIFE process." This process uses sulfuric acid to leach sludge ash. Dissolved iron is recovered using solvent extraction. No information is available regarding the iron extraction step, but since iron is in cationic form in the sulfuric acid leachate, it is believed that the iron is extracted with a solvent suitable for extracting the cationic form of iron, and the extracted iron is then stripped from the loaded solvent using hydrochloric acid. Steam evaporation is used to concentrate the ferric chloride product. After iron extraction, phosphoric acid is isolated using solvent extraction. This is further concentrated using steam evaporation to obtain technical-grade phosphoric acid (75% concentration). The metal ions remaining in solution after phosphoric acid extraction are precipitated as mixed products at a higher pH, which is achieved by adding lime. This process requires the use of both sulfuric and hydrochloric acid, resulting in a high energy input for the evaporation stage; and a mixed metal hydroxide product, which has limited commercial value, is isolated. The main disadvantage of using sulfuric acid to dissolve the ash is that the sulfuric acid reacts with the calcium content in the ash to form gypsum, which is filtered along with the undissolved ash residue. This significantly increases the amount of residue requiring disposal. Another disadvantage of using sulfuric acid is that all cations leached into solution from the ash are in the form of sulfates (e.g., magnesium sulfate, etc.) When the sulfate-containing raffinate is neutralized with lime (e.g., for precipitation of heavy metals), it produces large amounts of gypsum, which makes disposal costly.

[0007] At BALTIC 21, held in Berlin from September 28 to 30, a method for recovering phosphorus from ash, designated the "Pasch process," was presented. This method uses dilute hydrochloric acid to leach sludge ash. The undissolved residue is separated using parallel plate separators and filters and dewatered using a centrifuge. Dissolved metals (e.g., iron, lead, cadmium, copper, zinc) are extracted as a set using solvent extraction with a mixture of Alamine 336 and TBP. These are then precipitated as mixed hydroxides. Therefore, separate commercial metal compounds are not isolated. The phosphorus remaining in the raffinate after solvent extraction can be recovered using conventional phosphate precipitation. However, aluminum is not extracted by Alamine 336 and TBP and co-precipitates with the phosphorus product, making it less valuable. Consequently, the plant availability of this recovered phosphorus product is low.

[0008] The former company, EcoPhos, proposed a method for recovering phosphorus from sewage sludge ash based on leaching sludge ash or other crude phosphates with hydrochloric acid. Monocalcium phosphate in the leachate was converted to dicalcium phosphate by adding calcium carbonate. The addition of sulfuric acid resulted in the precipitation of gypsum and the production of phosphoric acid. Heavy metal separation was not described, and iron and aluminum dissolved from the sewage sludge ash were not considered.

[0009] Another method for phosphorus recovery from the predecessor company Ecofors is described in published international patent application WO2015091946A1. This method relies on leaching ash from waste incineration (including sewage sludge ash) with phosphoric acid. The leachate is treated with activated carbon. A cation exchanger is used to separate the phosphoric acid from dissolved metals. The resin is regenerated with hydrochloric acid, which elutes a mixture of magnesium chloride and calcium chloride, as well as a mixture of aluminum chloride and ferric chloride. Heavy metal content is uncertain. The method does not recover iron and aluminum as separate commercial products; nor does it separate calcium and magnesium from each other. The use of ion exchangers is less reliable, particularly for streams with high metal content and low phosphorus content. This can lead to premature loading of the ion exchanger and metal breakthrough in the phosphoric acid stream; a short service life; the need to elute the loaded resin with hydrochloric acid, which dilutes the metal solution and consumes chemicals; and the need to wash residual hydrochloric acid from the ion exchange resin before processing additional metal-contaminated phosphoric acid to prevent contamination of the purified phosphoric acid product with hydrochloric acid.

[0010] Published international patent application WO 2015067328 A1 describes a method for processing ash from waste incineration to obtain compounds of aluminum, calcium, phosphorus, and nitrogen. The ash is leached with a mixture of phosphoric acid and nitric acid, dissolving phosphorus as phosphoric acid and metals as metal nitrates. Calcium is precipitated as gypsum by adding sulfuric acid to the leachate. The pH of the solution is further raised with lime, precipitating a mixture of aluminum hydroxide and aluminum phosphate. The remaining solution, containing phosphoric acid and nitric acid, is concentrated via evaporation. Further addition of lime produces a mixture of calcium nitrate and calcium phosphate, which is recovered as a solid using evaporation and crystallization.

[0011] Published international patent application WO 2015165481 A1 describes a method for producing pure phosphoric acid, calcium sulfate, water-soluble calcium hydrogen phosphate, and a metal salt solution by treating ash from a waste incineration plant with crude phosphoric acid. The ash is reacted with crude phosphoric acid, lime, and at least one sulfide, and the leachate is separated from precipitated heavy metal sulfides and undissolved residues in a solid-liquid separation step. Gypsum can be precipitated from the leachate by adding sulfuric acid. Calcium hydrogen phosphate can be precipitated by adding tricalcium phosphate, calcium carbonate, and / or calcium oxide.

[0012] Remondis' TetraPhos process is described in the book "Phosphorus: Polluter and Resource of the Future-Removal and Recovery from Wastewater," IWA Publishing, 2018, Chapter 24. This process also relies on leaching sludge ash with phosphoric acid, followed by precipitation of gypsum using sulfuric acid. Subsequent ion exchange separates metal ions (e.g., iron, aluminum, and magnesium) from the phosphoric acid. The metals in the ion exchange resin are eluted with hydrochloric acid to produce a mixed metal salt solution. The resulting phosphoric acid is further purified using membranes and concentrated using evaporation. This process has limited applicability for ashes with low phosphorus content and high soluble metal content. It does not recover iron, aluminum, and other metals as separate commercial products. It requires the use and handling of both sulfuric and hydrochloric acids. Furthermore, it uses ion exchangers, which have the aforementioned disadvantages. Furthermore, the use of membranes results in a less reliable process, such as scaling and a short service life.

[0013] Published international patent application WO 2018046621 A1 describes a method and apparatus for recovering phosphorus from sewage sludge ash. The ash is leached with dilute phosphoric acid (7%-14%) to produce a phosphorus-rich solution. Calcium in the leachate can be precipitated as gypsum by adding sulfuric acid. The resulting solution can be used for subsequent leaching of phosphate rock to further increase the phosphorus concentration in the solution. While this method allows for the concentration of phosphorus in each leaching cycle, it does not isolate a pure phosphorus stream because soluble impurities in the sewage sludge ash and phosphate rock also dissolve along with the phosphorus.

[0014] European Patent EP 3266742 B1 describes a process for producing phosphoric acid from phosphorus-containing primary and secondary raw materials (including sewage sludge ash) using a combination of acidic chemical digestion and electrodialysis, along with crude phosphoric acid purification. The material is leached with a monovalent mineral acid, hydrochloric acid, and / or nitric acid. Aluminum and iron are separated using solvent extraction with one or more organic extractants selected from di-(2-ethylhexyl)phosphoric acid (DEHPA), Cyanex 923, Cyanex 272, or mixtures thereof. The crude phosphoric acid is then concentrated to 40%-70% using vacuum evaporation, falling film evaporation, membrane distillation, and / or reverse osmosis. This concentrate is further purified using solvent extraction. The monovalent mineral acid is recovered from the concentrate using bipolar electrodialysis. Prior to electrodialysis, calcium and magnesium ions are precipitated from the concentrate as calcium hydroxide and magnesium hydroxide. This process has a high energy requirement for the phosphoric acid concentration and electrodialysis steps.

[0015] Published international patent application WO 2020169708 A1 describes a method for recovering phosphoric acid from phosphorus-containing solid materials, such as struvite, sewage sludge ash, meat and bone meal ash, manure ash, calcium phosphate, and phosphate-containing minerals such as apatite, violet iron ore, and phosphate rock. The solid material is reacted with a strong acid in a single-phase reaction medium also containing an organic solvent. This results in the formation of phosphoric acid in the organic solvent and a phosphorus-depleted solid material. The organic solvent containing the phosphoric acid is separated from the remaining solid material, and the phosphoric acid is recovered from the organic phase using conventional stripping. Due to the significant loss of organic solvent in the undissolved ash, the industrial applicability of this method for sewage sludge ash is limited. Complete solvent recovery and residual disposal are difficult to achieve. This method also exhibits very low phosphorus recovery rates. The only example of phosphorus recovery from sewage sludge ash describes extraction using a system consisting of sulfuric acid, sewage sludge ash, water, and n-propanol. The recovery efficiency over a 24-hour period was 15.8%.

[0016] Published international patent application WO 9506004 A1 describes a process for treating wastewater sludge using sulfuric acid leaching, solvent extraction and stripping of iron and aluminum, precipitation of heavy metal sulfides, precipitation of phosphorus as hydroxyapatite or struvite, and precipitation of aluminum as aluminum hydroxide. Direct treatment of wastewater sludge without incineration results in significantly greater chemical consumption and the generation of large amounts of effluent due to the large volume and high water content of the unincinerated sludge.

[0017] Published international patent application WO 2014178788 A1 describes several approaches for recovering phosphorus from sewage sludge ash using hydrochloric acid leaching and tributyl phosphate (TBP) solvent extraction for metals. All presented solvent extraction steps involve pre-treating the sewage sludge ash leachate with alkali to precipitate an intermediate mixture consisting primarily of a mixture of ferric, aluminum, and calcium phosphates. In this manner, initial concentration and partial purification of the phosphorus are achieved. The intermediate precipitate is dissolved in hydrochloric acid before solvent extraction is performed. Consequently, solvent extraction of metals and phosphorus is not performed directly after leaching. This intermediate precipitation step for ferric, aluminum, and calcium phosphates avoids many of the drawbacks of applying solvent extraction directly after leaching, such as lower concentrations of phosphorus and iron in the solution and the presence of additional impurities. These drawbacks are discussed in further detail later in this document. However, the intermediate precipitation step makes solvent extraction of sewage sludge ash more complex, expensive, and increases chemical consumption. For example, alkali is required to precipitate the intermediate mixture, which is then redissolved in acid to dissolve phosphorus, aluminum, calcium, iron, and other components.

[0018] There is a need for better methods for recycling sewage sludge ash which ensure not only the recovery of phosphorus in a form which can be used, for example, in the fertilizer industry, but also the recovery of other individual components in a high purity and high yield. For example, it is desirable to recover iron separately from aluminum, to recover calcium in the form of a saleable product without losses during leaching, for example precipitated in the form of gypsum, or to recover magnesium and heavy metals. Removing these fractions from sewage sludge ash produces an insoluble residue rich in silicates. This has important applications in the concrete industry, especially if the harmful heavy metals are removed. Due to the large amounts of sewage sludge ash produced each year, the use of this residue in the concrete industry results in significant savings in gas emissions, as well as in landfill space. The treatment of sewage sludge ash must preferably be achieved with minimal energy and chemical consumption and minimal waste generation. Many of the above-mentioned methods do not address more than one of these aspects. Summary of the Invention

[0019] A general object of the present technology is to provide a method and system for recycling sewage sludge ash that recovers phosphorus and other valuable components in high purity and high yield in an energy and chemical consumption limited manner.

[0020] The above objects are achieved by a method and an apparatus according to the independent claims. Preferred embodiments are defined in the dependent claims.

[0021] Generally speaking, in a first aspect, a method for chemically treating sewage sludge ash comprises dissolving a starting material derived from sewage sludge ash in an acid comprising hydrochloric acid. The starting material comprises at least silicon and iron compounds. An undissolved residue is separated, thereby retaining a leachate. At least one of iron and phosphorus is extracted from the leachate by liquid-liquid extraction with an organic solvent. The amount of colloidal silica present in the leachate provided to the extraction step is controlled to be compatible with the amount of silica crud in the liquid-liquid extraction with the organic solvent, the silica crud being sufficiently low to enable liquid-liquid extraction with the organic solvent. The controlling step is performed by adding a silica coagulant to at least one of the starting material, the dissolved starting material derived from sewage sludge ash, and the leachate to promote coagulation to allow particle growth, and / or by performing the dissolving step at an elevated temperature of at least 50° C. At least a portion of the at least partially iron- and phosphorus-depleted solution derived from the step of extracting at least one of iron and phosphorus is recycled to dissolve the starting material derived from sewage sludge ash. At least a portion of the recycled portion of the solution depleted in at least one of iron and phosphorus comprises chloride ions.

[0022] In a second aspect, an apparatus for chemically treating sewage sludge ash includes a dissolution reactor, a separation device, an extractor section, a device for controlling the amount of colloidal silica present in a leachate supplied to the extractor section, and a return line. The dissolution reactor is configured to dissolve starting material derived from sewage sludge ash in an acid comprising hydrochloric acid. The starting material comprises at least silicon and iron compounds. The separation device is configured to separate undissolved residue, thereby retaining the leachate. The extractor section is configured to extract at least one of iron and phosphorus from the leachate via liquid-liquid extraction with an organic solvent. The device for controlling the amount of colloidal silica present in the leachate supplied to the extractor section is configured to control the amount to be compatible with the amount of silica contaminant present in the liquid-liquid extraction with the organic solvent, the silica contaminant amount being sufficiently low to make liquid-liquid extraction with the organic solvent feasible. The apparatus for controlling the amount of colloidal silica includes a pretreatment chamber configured to receive a leachate and add a silica coagulant to the leachate, wherein the pretreatment chamber optionally includes a separation device for removing coagulated silica particles from the leachate and / or includes a heater arranged to provide an elevated temperature of at least 50° C. in the dissolution reactor. A return conduit is configured to recirculate at least a portion of the at least partially iron- and phosphorus-depleted solution resulting from the step of extracting at least one of the iron and phosphorus for use in dissolving the sewage sludge ash-derived starting material. The recycled portion of the at least partially iron- and phosphorus-depleted solution includes chloride ions.

[0023] One advantage of the proposed technology is that valuable sewage sludge ash components can be extracted with high purity and high yield at low energy consumption and at low consumption of additional chemicals. Other advantages will be understood upon reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] 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:

[0025] Figure 1 is a flow chart of steps of an embodiment of a method for chemically treating sewage sludge ash;

[0026] Figure 2 is a portion of a flow diagram of a pretreatment step of an embodiment of a method for chemically treating sewage sludge ash;

[0027] Figure 3 is a portion of a flow diagram of a pretreatment step of another embodiment of a method for chemically treating sewage sludge ash;

[0028] Figure 4 is a flow chart of some steps of an embodiment of the steps for reducing the amount of colloidal silica;

[0029] Figure 5 is a flow chart of some steps of an embodiment of the steps for treating sulfur;

[0030] Figure 6 is a portion of a flow diagram of a pretreatment step of yet another embodiment of a method for chemically treating sewage sludge ash;

[0031] Figure 7 is a portion of a flow diagram of a pretreatment step of yet another embodiment of a method for chemically treating sewage sludge ash;

[0032] Figure 8 is a schematic diagram of an embodiment of an apparatus for chemically treating sewage sludge ash;

[0033] Figure 9 is a schematic diagram of an embodiment of a dissolution reactor;

[0034] Figure 10 is a schematic diagram of an embodiment of a pretreatment chamber;

[0035] Figure 11 is a schematic diagram of an embodiment of a general solvent extractor;

[0036] Figure 12 is a schematic diagram of an embodiment of an iron extractor section having two solvent extractors;

[0037] Figure 13 is a flow chart of some steps of an embodiment of the steps for extracting iron;

[0038] Figure 14 is a schematic diagram of an embodiment of an embodiment for washing impurities from an iron-laden organic phase;

[0039] Figure 15 is a schematic diagram of an apparatus for neutralizing a ferric chloride product containing hydrochloric acid;

[0040] Figure 16 is a flow chart of some of the steps of an embodiment for neutralizing a ferric chloride product containing hydrochloric acid;

[0041] Figure 17 is a flow chart of some of the steps for treating iron of an embodiment of a method for chemically treating sewage sludge ash;

[0042] Figure 18 is a schematic diagram of an embodiment of an iron extractor section having one solvent extractor;

[0043] Figure 19 is a schematic diagram of an embodiment of a two-stage apparatus for chemically treating sewage sludge ash;

[0044] Figure 20 is a flow chart outlining the steps of an embodiment of a two-stage process for chemically treating sewage sludge ash;

[0045] Figure 21 is a diagram showing the distribution of phosphate between the aqueous and organic phases;

[0046] Figure 22 is a schematic diagram of an embodiment of a phosphorus extractor portion;

[0047] Figure 23 is a schematic diagram of the arrangement of heavy metal removal equipment in an embodiment of the phosphorus extractor section;

[0048] Figure 24 is a schematic diagram of an embodiment of a heavy metal removal apparatus;

[0049] Figure 25 is a schematic diagram of another embodiment of a heavy metal removal apparatus;

[0050] Figure 26 is a flow chart of some steps of an embodiment of the steps for recycling aluminum;

[0051] Figure 27 is a schematic diagram of an embodiment of an aluminum separation device;

[0052] Figure 28 is a flow chart of some steps of an embodiment of the step of recovering heavy metals;

[0053] Figure 29 is a schematic diagram of an implementation scheme of a heavy metal separation device;

[0054] Figure 30 is a flow chart of some steps of an embodiment of the step of recovering magnesium;

[0055] Figure 31 is a schematic diagram of an embodiment of a magnesium separation apparatus; and

[0056] Figure 32 is a schematic diagram showing an embodiment of a recycling flow in a plant for chemically treating sewage sludge ash. DETAILED DESCRIPTION

[0057] Throughout the drawings, the same reference numerals are used for similar or corresponding elements.

[0058] Sewage sludge ash is primarily composed of oxygen, silicon, phosphorus, calcium, iron, and aluminum, but typically also contains small amounts of other elements, including magnesium, sodium, potassium, and heavy metals such as cadmium, copper, manganese, nickel, lead, and zinc. The presence of arsenic and heavy metals in sludge ash, as well as the leachability of hazardous compounds, contributes to the classification of this stream as a hazardous waste that requires deposition in a dedicated landfill or appropriate treatment of heavy metals if the ash is used in other applications. Table 1 shows the elemental composition of five different sludge ashes. Ashes A, B, and D were obtained from the mono-incineration of sewage sludge, while ashes C and E were obtained from the co-incineration of sewage sludge with wood chips. As can be seen in Table 1, the phosphorus concentration of the co-incinerated sludge ash (about 4%) is lower than the phosphorus concentration of the mono-incinerated sludge ash (about 8%). Table 1 also describes the types of chemicals used to precipitate phosphorus in wastewater treatment plants. Three different groups of phosphorus precipitation chemicals are commonly used: 1) ferric iron based precipitation chemicals, such as ferric chloride (FeCl3) or ferric sulfate (Fe2(SO4)3); 2) ferrous iron based precipitation chemicals, such as ferrous sulfate (FeSO4) or ferrous chloride (FeCl2); and 3) aluminum based precipitation chemicals, such as aluminum chloride (AlCl3) or aluminum sulfate (Al2(SO4)3).

[0059]

[0060]

[0061] Table 1. Elemental composition of ash from incinerated sewage sludge (wt%)

[0062] Regardless of the chemicals used in wastewater treatment plants, phosphorus in sludge ash exists primarily in the form of calcium and aluminum phosphates, which have low water solubility. Therefore, wet chemical methods for recovering phosphorus from ash typically employ acid leaching. This effectively dissolves the metallic phosphates and brings phosphate ions into solution, from which they are recovered using hydrometallurgical techniques like precipitation, solvent extraction, and ion exchange.

[0063] Many previously mentioned methods for recovering components from sewage sludge ash use sulfuric acid due to its low cost and ability to precipitate gypsum, which produces a phosphorus stream with a lower calcium content. However, gypsum generally has low commercial value. Furthermore, co-precipitation of other elements in the leachate with the gypsum is problematic, potentially leading to contamination of the gypsum product and / or loss of potentially recoverable fractions. If gypsum precipitates during leaching, it is lost in the undissolved ash residue, meaning that significant amounts of calcium are not recovered from the ash and the volume of undissolved ash increases significantly.

[0064] Some methods use phosphoric acid for leaching. In this case, the solution is enriched with phosphorus dissolved from the ash, ensuring a stream with a high phosphorus content. However, this solution will be contaminated with heavy metals and other metals such as iron, aluminum, calcium, and magnesium. This requires additional purification, which is neither simple nor inexpensive, especially if ion exchange is used. Most methods aim to isolate pure phosphoric acid and produce a mixture of other metals that has limited uses and no commercial value.

[0065] Methods using hydrochloric acid are in the minority, leaving room for improvement. Using hydrochloric acid for leaching offers several attractive properties. For example, compared to using sulfuric acid, it eliminates any issues associated with gypsum precipitation during leaching. Furthermore, hydrochloric acid is less expensive than phosphoric acid, and since phosphorus is one of the components intended for extraction, resources are not spent on recovering substances intentionally added to the process.

[0066] The method contemplated by the present disclosure involves leaching sewage sludge ash with hydrochloric acid and subsequently separating the individual compounds of the elements iron and phosphorus from the solution by solvent extraction using TBP. Preferably, selective precipitation of aluminum, heavy metals, magnesium, calcium, sodium, and potassium is also performed. The hydrochloric acid leaching of sewage sludge ash followed by solvent extraction has been previously described, for example, in the Pasch process or in European Patent EP 3266742 B1 and International Patent Publication WO 2014178788A1.

[0067] However, in the Pasch process, an organic mixture of Alamine 336 and TBP is used to extract the iron and other metals from the leachate as a group. The extracted metals in the organic phase are stripped together and precipitated as mixed hydroxides, meaning they are not recovered as individual compounds. This method uses solvent extraction to remove metal contaminants from phosphorus-containing leachates. After solvent extraction, phosphorus remains in a diluted form in the raffinate, and this solution requires further processing to recover the phosphorus. This can be, for example, by evaporating large amounts of water (which is energy-intensive) or conventionally precipitating phosphorus compounds with low water solubility. Therefore, such phosphorus compounds with low water solubility are not suitable as fertilizers without chemical conversion to more soluble products. European Patent EP 3266742 B1 describes solvent extraction using the organic compounds di-(2-ethylhexyl)phosphoric acid (DEHPA), Cyanex 923, and Cyanex 272, or mixtures of these, to extract iron and aluminum. Solvent extraction is combined with bipolar electrodialysis to regenerate the acid used for leaching, a process that is energy-intensive and sensitive to contaminants in the solution.

[0068] One of the applicant's prior inventions, disclosed in International Patent Application Publication No. WO 2014178788 A1, describes leaching sewage sludge ash with hydrochloric acid and subsequent extraction of iron and phosphorus with TBP solvent. However, the solvent extraction step described in WO 2014178788 A1 is not performed immediately after leaching the sewage sludge ash with hydrochloric acid, as is the intent of the present invention. Instead, in WO 2014178788 A1, the leachate is treated with alkali to obtain an intermediate precipitate containing a mixture of calcium phosphate, iron phosphate, and aluminum phosphate, as well as other co-precipitated elements. This achieves preliminary concentration and purification of phosphorus. This intermediate precipitate is separated from the original leachate and redissolved in hydrochloric acid, after which solvent extraction with TBP is performed.

[0069] However, the use of hydrochloric acid to leach sewage sludge ash is not without its own challenges. As mentioned in the background, leaching with hydrochloric acid typically results in relatively low concentrations of phosphorus and / or iron in solution, and the presence of additional impurities can make solvent extraction difficult. For example, leaching with hydrochloric acid has been found to produce a leachate containing non-negligible amounts of colloidal silica, which renders direct solvent extraction unsuitable.

[0070] In the published international patent application WO 2014178788 A1, this is overcome by using an intermediate precipitation step. This solution generally works well, but involves disadvantageously high consumption of added chemicals. This is because the method described in WO 2014178788 A1 requires chemicals to precipitate the intermediate phosphorus compounds in a separate step after leaching, a filtration step to separate and clean the intermediate compounds, and a subsequent dissolution step with acid to redissolve the phosphorus and other metal components of the intermediate precipitate to facilitate the subsequent solvent extraction step.

[0071] However, the present invention does not involve any intermediate steps and can be applied to a wide range of sewage sludge ashes or sewage sludge ash materials that have been previously treated (e.g., to leach at least a portion of one or more components). Such ashes include those with very low levels of phosphorus and high levels of iron, calcium, aluminum, and other impurities. This results in significant chemical savings, fewer process steps, and less equipment required. In order to achieve a more efficient process in terms of consumption of added chemicals, the process should be based on extraction from a direct sewage sludge ash leachate based on hydrochloric acid.

[0072] Surprisingly, it has been seen that hydrochloric acid is able to leach more silicon than nitric or sulfuric acid. In addition, the dissolution yield is higher at lower acid concentrations and lower temperatures. The solubility limit of silicon materials at acidic pH is actually very low. The solubility limit of silicic acid at pH 2 is about 150 ppm. Above this concentration, the silicic acid polymerizes and forms colloidal particles, precipitates or gels. Depending on the conditions, these forms can slowly precipitate, which is undesirable, especially if they occur during solvent extraction. Colloidal silica can lead to emulsification, dirt formation and high entrainment losses in any subsequent solvent extraction. It has been found that even low levels of colloidal silica, as low as 50 ppm, can interfere with solvent extraction of, for example, iron and phosphorus with TBP and make it virtually impossible.

[0073] Colloidal silica will grow into larger, filterable silica particles over time, but this process is very slow and therefore unsuitable for any industrial use. In order for such a process to be viable, the amount of colloidal silica present in the leachate entering the solvent extraction must be actively controlled. To do this, the amount of colloidal silica in the leachate must be actively reduced, or the formation of colloidal silica should be actively reduced during the leachate process. In other words, there is a step to control the amount of colloidal silica present in the leachate entering the solvent extraction stage.

[0074] Since the size of colloidal silica particles is very small (0.01-0.5 μm), it is difficult to remove the colloidal silica particles using conventional methods such as sedimentation, filtration or centrifugation. However, this step can be performed in other ways, which will be described in more detail further below.

[0075] Another important aspect of the method based on hydrochloric acid is that the high ionic strength (and then particularly chloride ion strength) in the solution is conducive to the efficiency of solvent extraction of iron and phosphorus. This salting-out effect during solvent extraction enables the extraction operation to be more selective. This can be promoted by recycling at least a portion of the raffinate containing chloride ions for leaching sewage sludge ash after solvent extraction. The return and reuse of the streams from each step of the method are the key to ensuring the ionic strength required for separation of iron and phosphorus using solvent extraction. Recirculation also ensures water balance and minimizes the amount of secondary waste generated.

[0076] The basic method based on these considerations opens up a number of different extraction steps, in which valuable components of sewage sludge ash can be extracted in pure fractions and in relatively high yields, which typically makes such extracted compounds valuable for further use.

[0077] Therefore, the present technology provides an improved method for subjecting materials containing sewage sludge ash to hydrochloric acid leaching, followed by solvent extraction, and optionally precipitation of dissolved components. This technology enables the recovery of most of the iron in the sludge ash as ferric chloride compounds. It also enables the recovery of phosphorus as high-value products, such as phosphoric acid or ammonium phosphate. It also enables the recovery of aluminum as commercial compounds, such as aluminum phosphate, aluminum hydroxide, sodium aluminate, and / or sodium aluminum phosphate. It also enables the recovery of heavy metal pollutants, such as metal hydroxides and / or sulfides. It also enables the recovery of magnesium as commercial compounds, such as magnesium hydroxide. It also enables the recovery of calcium as commercial compounds, such as calcium chloride. It also enables the recovery of sodium as commercial compounds, such as sodium chloride. It also enables the recovery of potassium as commercial compounds, such as potassium chloride. It also enables the recovery of clean, undissolved silicate sand ash for commercial use, such as in the concrete industry. This technology also paves the way for improving the overall effectiveness of the process and minimizing any generation of secondary waste and chemical consumption by reusing and recycling side streams. As indicated above, this will also provide favorable operating conditions in the various steps of the process, such as the ionic strength required for selective separation.

[0078] Since sewage sludge ash is a heterogeneous stream that can contain many fractions with different chemical properties and compositions, the present invention can take different embodiments that distinguish themselves by the way the process steps are arranged, steps are omitted, or additional steps are inserted if necessary, and the operation of the process steps under different conditions (e.g., temperature, acidity, pH, etc.) Various embodiments are described in detail later in this specification, all of which are supported by research data.

[0079] Some commonly used terms in this disclosure should be explained as follows:

[0080] Leaching, Leaching Agent, Leachate - Leaching is the removal of the soluble fraction contained in a solid phase in the form of a solution. This is achieved by treating the solid phase with a solution called a leachate. The solution obtained after leaching is called the leachate and contains the components of the soluble fraction in ionic form.

[0081] Solvent extraction or liquid-liquid extraction - a separation process in which the chemical species to be separated are partitioned between two immiscible liquid phases: an aqueous phase (e.g., a leachate) and an organic phase, typically containing an extractant (see below). When referring to solvent extraction of a specific element, this means solvent extraction of extractable complexes or compounds containing that specific element. For example, in this context, the extraction of phosphoric acid with TBP is generally referred to as phosphorus extraction.

[0082] Extractant - an active ingredient, typically an organic ingredient, which binds substances of interest, enabling their extraction.

[0083] Diluent - a liquid, in this context an organic liquid, in which the extractant and modifier are dissolved to form the organic phase.

[0084] Modifier - A substance added to the organic phase to increase the solubility of the extractant, its salts, or ionic species derived from extraction or back extraction. It is also added to inhibit emulsion formation.

[0085] Stripping or back extraction - the opposite of extraction; eluting the extracted compounds from the loaded organic phase with an aqueous solution to form an aqueous solution containing the extracted material.

[0086] Washing - Similar to stripping, but typically refers to the selective removal of undesirable extracted materials in the stripped product, e.g., co-extracted impurities. Usually performed before stripping.

[0087] Raffinate - The aqueous phase from which solutes (dissolved substances) have been removed by extraction. In other words, the aqueous phase after solvent extraction.

[0088] Sludge - material resulting from the agitation of the organic phase, aqueous phase and fine particles (forming a stable mixture). This may result in significant loss of organic material and / or inability to perform solvent extraction.

[0089] Salting out - The enhancement of the extraction of a substance into the organic phase by increasing the ionic strength of the aqueous phase, for example by adding acids, salts, or other chemicals capable of achieving this.

[0090] Figure 1 A flow chart illustrating the steps of an embodiment of a method for chemically treating sewage sludge ash. In step S10, a starting material is dissolved in an acid including hydrochloric acid. The starting material originates from sewage sludge ash. Thus, the starting material can be sewage sludge ash obtained from an incineration process, but can also be, for example, undissolved residues from a previous dissolution of sewage sludge ash, which still contain components that are desired to be recovered. The starting material contains at least silicon dioxide and iron. In other words, this initial step involves leaching of the sewage sludge ash or a material containing sewage sludge ash.

[0091] The sewage sludge ash or material containing sewage sludge ash is thus contacted with a leaching solution. The leaching solution comprises hydrochloric acid and a suitable aqueous diluent (typically water), but may also include contributions from return streams from various steps of the process. Furthermore, other compounds may be added during leaching, such as oxidizing or reducing agents, coagulants for colloidal silica, defoamers, other acids, bases, pH buffers, and / or salts, as will be discussed in more detail further below. The presence of all of these ingredients is not always necessary, and this depends, inter alia, on the type of ash being leached, the desired leaching and separation yields in subsequent steps, or whether pretreatment of the leachate is performed simultaneously with leaching or as a separate step, as will be discussed further below.

[0092] The dissolution step S10 can be controlled to leach only a portion of the components of the sludge ash material. Thus, the amount of one or more components dissolved can be controlled by, for example, controlling the amount of acid in the leaching solution and process parameters such as leaching time, temperature, mixing conditions, atmosphere, addition of oxidizing or reducing agents, liquid-to-solid ratio, particle size, ionic strength, etc. For example, a high concentration of hydrochloric acid and / or high temperature can be used to leach most of the iron in the sludge ash, while a smaller percentage of iron can be leached using dilute acid at ambient temperature.

[0093] Because sewage sludge ash typically contains a large amount of insoluble silicates, the leaching step will not completely dissolve the sludge ash and will result in a leachate and an undissolved solid residue. In step S12, the undissolved residue is separated, thereby retaining the leachate. This can also be described as typically recovering the leachate from the undissolved sewage sludge ash using a solid / liquid separation step, such as filtration, sedimentation, or centrifugation. Due to its high silicate content and the removal of soluble components, this residue is suitable for use as a material in, for example, the concrete industry.

[0094] Step S12 may also optionally include a partial step S13 of cleaning the undissolved residue to produce a silicate product. Cleaning of the undissolved ash (e.g., washing away residual leachate) can be performed with a suitable cleaning agent (typically water) with or without additional additives. Such additives may be, for example, compounds that neutralize residual acid and / or stabilize one or more components. The spent cleaning agent, such as the wash water with the washed residual leachate, may be discarded. Alternatively, it may be processed separately to recover its components, reused for further cleaning of the residue, and / or used elsewhere in the process, such as by mixing with leachate and / or leaching solution.

[0095] In step S15, the amount of colloidal silica in the dissolved sewage sludge ash is controlled (i.e., reduced, given the composition of the starting materials indicated above). The amount of colloidal silica present in the leachate provided to the extraction step described below is controlled to be compatible with the amount of silica contaminants in the liquid-liquid extraction with an organic solvent, which is sufficiently low to make liquid-liquid extraction with an organic solvent feasible. This can be performed as a separate process or as a process at least partially shared with steps S10 and / or S12, as indicated in the flow chart. For example, in different embodiments, the dissolution process of S10 can be influenced to result in a lower content of colloidal silica, or the colloidal silica that results in the formation of larger silica particles can be separated in step S12. In other embodiments, step S15 can also be completely separate from steps S10 and S12.

[0096] Step S15 can be considered a partial step of the leachate pretreatment step S14. This leachate pretreatment step S14 may also optionally include other types of leachate pretreatment. In addition to colloidal silica, sulfur, if present, is typically leached with hydrochloric acid. The sulfur can precipitate as gypsum, at least if the leachate is allowed to age. In optional step S16, the leachate is pretreated with respect to sulfur to accelerate the precipitation of gypsum, which is then typically separated from undissolved residues, or to prevent gypsum formation, thereby allowing the sulfur to enter subsequent processes. As with the case of colloidal silica, such pretreatment of the sewage sludge ash leachate can be performed separately and / or simultaneously with leaching.

[0097] As will be discussed in further detail later in this disclosure, it was found that ferrous iron was extracted less efficiently than ferric iron by liquid-liquid extraction. In optional step S17, the ferric iron in the leachate is reduced to ferrous iron, which will result in a lower extraction efficiency of iron later in the process. In optional step S18, the ferrous iron in the leachate is instead oxidized to ferric iron, which will result in a higher extraction efficiency later in the process. Which of these steps to perform (if any) depends on the actual method selected for extracting iron in a later step of the process, as will be discussed further below.

[0098] The method then proceeds with the separation, purification, and recovery of the leached elements. In step S20, at least one of iron and phosphorus is extracted from the leachate by liquid-liquid extraction using an organic solvent. Various embodiments of portions of the process are further discussed below, describing the separation, purification, and recovery of iron, portion S21, and / or the separation, purification, and recovery of phosphorus, portion S30.

[0099] In optional step S40, the raffinate from step S20 may be subjected to various types of post-processing. Typically, such post-processing may include further separation of valuable elements from the sewage sludge ash. This may include, for example, a step S41 for separating and recovering aluminum, a step S50 for separating and recovering heavy metals, a step S60 for separating and recovering magnesium, a step S65 for separating and recovering sodium, a step S70 for separating and recovering potassium, and / or a step S75 for separating and recovering calcium. Some of these steps may also be combined in different embodiments. Different types of such processes are discussed in further detail below.

[0100] In step S90, at least a portion of the solution of at least one of the poor iron and phosphorus is recycled. The solution of at least one of the poor iron and phosphorus originates from the step S20 of extracting at least one of the iron and phosphorus, possibly via an optional post-processing step S40. The recycled portion of the solution of at least one of the poor iron and phosphorus comprises chloride ions. The recycled solution is used to dissolve the starting material derived from sewage sludge ash. In one embodiment, the dissolving performed using the recycled solution is a subsequent step S10. The recycling, reuse and / or recycling of process streams helps to minimize the generation of secondary waste and chemical consumption. It can also ensure that some of the method conditions required for performing several aforementioned steps are met.

[0101] According to the results discussed in the examples, the step S90 of recycling at least a portion of the raffinate is preferably controlled to provide a chloride salt and / or hydrochloric acid concentration in the leachate of at least 1 M.

[0102] In various embodiments of the present technology, the ash leachate undergoes several steps in which the various components of the solution are selectively separated, purified, and converted into commercial compounds. This is preferably performed using solvent extraction and precipitation using a suitable solvent such as TBP. However, as mentioned above, pretreatment steps are required before the actual extraction can begin.

[0103] Figure 2 A flow chart of a partial embodiment of a pre-treatment step S14 of the sewage sludge ash leachate is disclosed. As indicated above, this step includes a partial step S15, in which the amount of colloidal silica in the dissolved sewage sludge ash is controlled.

[0104] None of the prior art mentioned above describes the presence of colloidal silica and its problematic effects on extraction. However, it has been found that colloidal silica is in fact present and negatively impacts the solvent extraction steps to the point where these steps are impossible to perform due to irreversible foulant formation. Due to the large amount of silicates in sewage sludge ash, and despite their low solubility, silicon species will also be present in the leachate at concentrations of at least several hundred ppm.

[0105] When different types of sewage sludge ash and sludge ash materials are leached with hydrochloric acid at room temperature and solvent extraction is performed directly after solid-liquid separation of the leachate from the undissolved ash, the process is impossible to operate due to irreversible foulant formation. This very stable emulsion negatively impacts phase separation and renders the organic phase unusable for stripping and reuse. The cause of foulant formation is the silica present in the leachate. Silicon is a major component of sewage sludge ash, and some silicon is found to be released into solution during leaching. Acids do not readily attack silicates such as quartz. However, many silicate minerals contain metal ions, typically sodium, potassium, iron, magnesium, and aluminum. These dissolve during leaching, causing the three-dimensional silicate structure to disintegrate and depolymerize essentially into silicic acid. The silicon in solution initially exists as silicic acid and then reacts along two separate pathways: dissociation and polymerization. As mentioned earlier, the solubility limit of silicic acid in acidic solutions is very low, approximately 150 ppm at pH 2. Above this limit, silicic acid polymerizes to form colloidal particles, precipitates, or gels. The presence of very small colloidal silica particles in the leachate stabilizes the oil-in-water emulsion and hinders phase separation, which is the basis for the formation of silica fouling in solvent extraction.

[0106] Table 2 shows the composition of various leachates obtained after leaching sewage sludge ash with hydrochloric acid. The leaching process and leaching behavior of other sewage sludge ash components will be described in more detail later, but the focus now is on silica. The leachates in Table 2 have a pH < 0. The data shows that the amount of leached silicon is lower when leaching is performed with more concentrated hydrochloric acid, and the amount of leached silicon is lower at higher liquid:solid (L:S) ratios.

[0107]

[0108]

[0109] Table 2 - Concentrations of elements in the sludge ash leachate after leaching the sludge ash with hydrochloric acid solutions of different concentrations at various volume / weight L:S ratios.

[0110] The leachates listed in Table 2 were used in solvent extraction experiments using an organic phase containing an organophosphorus extractant (e.g., trialkyl phosphate (TBP)), kerosene, and a long-chain organic alcohol as a phase modifier, specifically trialkylphosphine oxide (Cyanex 923), kerosene, and a long-chain organic alcohol as a phase modifier. Stain formation occurred in all systems. Analysis of the raffinate after solvent extraction showed a decrease in silica concentration in the solution, confirming its involvement in sludge formation. Sludge became more prominent as the organic:acid (O:A) ratio decreased. Sludge formation involved the formation of a whipped egg white-like emulsion in the organic phase and at the interface, which remained stable for several days after the test, rendering the organic phase unusable. At O:A ratios of 1:1 and lower, sludge spread throughout the organic phase. Testing several types of kerosene (including pure aliphatic kerosene), 1-dodecanol or 1-decanol as phase modifiers, and various concentrations of extractants, diluents, and modifiers in the organic phase consistently resulted in contamination. Furthermore, contamination formed when the leachate was mixed with the diluent and / or modifier alone. Contamination occurred when using a leachate with low ionic strength (low acidity and / or low calcium chloride content), where the extraction efficiency of iron and phosphorus was low, and when using a leachate with increased ionic strength (higher hydrochloric acid and / or higher calcium chloride content), where the extraction efficiency of iron and phosphorus was high. Filtering the leachate through a 0.45 μm PVDF filter before solvent extraction did not prevent contamination formation because the colloidal silica particles are very small (0.01-0.5 μm). It is not feasible to remove such particles cost-effectively using conventional filtration or centrifugation on an industrial scale.

[0111] In further experiments, the leaching behavior was further investigated in dependence on the HCl concentration during leaching.

[0112] Sludge ash was leached with hydrochloric acid solution at ambient temperature (21°C) and a liquid / solid ratio of 3 / 1 v / w for 30 minutes. The hydrochloric acid concentrations used were 3M, 6M, and 9M. The leachate was separated from undissolved residues by filtration (1 μm pore size). Following filtration, a filtrate sample was immediately diluted with deionized water to stabilize the solution and prevent undesirable precipitation of silicon species. The elemental concentrations in the solution were quantified using ICP-MS. The results are presented in Table 3 below.

[0113]

[0114]

[0115] Table 3. Analysis of sludge ash leachates prepared by leaching sludge ash with 3M, 6M and 9M hydrochloric acid solutions at ambient temperature (21°C) and a liquid / solid ratio of 3 / 1 v / w for 30 minutes.

[0116] The results show that by controlling the hydrochloric acid concentration, the amount of silicon (including colloidal silica) ultimately present in the leachate, as well as the amount of other elements such as iron, sulfur, chromium, nickel, etc., can be controlled. This allows the recovery of ash components to be controlled. When more concentrated hydrochloric acid is used, less silicon is present in the solution, and more iron and heavy metals are dissolved.

[0117] This example shows that by controlling the hydrochloric acid concentration during leaching, the total silicon concentration in the solution (including colloidal silica) and the leaching behavior of the sludge ash component can be controlled. This affects the stability of the solution, such as the post-precipitation of calcium sulfate, and the amount of foulants formed during solvent extraction.

[0118] The concentration of hydrochloric acid also affects the solvent extraction efficiency of phosphorus and iron.

[0119] Batch solvent extraction tests were performed on three leachates immediately after preparation. A sample from each leachate was mixed with an organic solution containing TBP (organic / aqueous ratio 1 / 1) for 5 minutes. Two organic phases were tested: i) 30% by volume TBP, 30% by volume 1-decanol, and 40% by volume kerosene; and ii) 80% by volume TBP and 20% by volume 1-decanol. Regardless of the leachate used, the amounts of TBP and 1-decanol in the organic phase, or the presence / absence of kerosene, irreversible foulant formation occurred in each case. However, less foulant was formed when the sludge ash was leached with 9M hydrochloric acid, which is related to the lower silicon concentration (115.5 mg / L) quantified in this leachate. Iron and phosphorus extraction yields were significantly higher when the sludge ash was leached with the more concentrated acid, demonstrating that the extraction process can be controlled by controlling the concentration of the acid used to leach the ash. In fact, the extraction yields of iron and phosphorus from 3M acid leachate are very low—about 2% with a 30% by volume TBP solution. However, when the chloride content of the solution is increased, for example by internal recirculation of a chloride-containing stream (chloride salts and / or hydrochloric acid), the efficiency improves significantly. In contrast, for 6M HCl leachate, iron extraction increases to about 90% due to the higher acidity and chloride concentration. Increased extraction yields can also be achieved by quantitatively adding chloride salts to the 3M ash leachate to increase the ionic strength and by recirculating the internal stream to maintain a sufficiently high chloride ion concentration. For example, quantitative addition of calcium chloride significantly increases the recovery efficiency of iron and phosphorus. Adding 1M calcium chloride to the leachate increases iron extraction efficiency from 2% to 25% per step; adding 1.5M calcium chloride increases iron extraction to 53%; and adding 2M calcium chloride increases iron extraction to 78%.

[0120] It was observed that if the leachate was aged for a sufficient period of time, ranging from hours to even weeks depending on pH, initial silicon content, and other parameters, the colloidal silica particles grew slowly, and some eventually precipitated. However, when leachate was used that had been aged for several weeks, fouling still occurred, with the precipitate being seen before solvent extraction and removed via 0.45 μm filtration. This is because the colloidal silica does not completely precipitate despite the long aging time, and the solvent extraction process is sensitive to very low levels of total suspended solids. For solvent extraction, a total suspended solids concentration of less than 20 ppm is considered ideal, although many plants operate at average total suspended solids above 40 ppm. We found that low levels of silica (as little as 50 ppm present as colloidal silica) can lead to fouling.

[0121] Further experiments have shown that aging can reduce the amount of silicon in the leachate. Aging promotes the growth of colloidal silica particles, which are initially very small, submicron in size, and do not settle. The speed of this process depends on several factors, not limited to the solution pH, other elements present in the solution, temperature, and the initial silicon concentration. The silica particles are allowed to grow to a sufficient size to allow them to be filtered and / or settled. The leachates produced in the examples mentioned in Table 3 were aged for 48 hours by allowing them to stand in a sealed container. At various time points, the aged leachates were filtered through a 1 μm pore size to remove particles that may have grown to larger than 1 μm. Table 4 below shows the total silicon concentration measured at various time points. The results show that the silicon concentration slowly decreases. The decrease is greater at higher hydrochloric acid concentrations; for example, 9M acid is more effective than 6M, and the latter is more effective than 3M. Even at the lowest silicon concentration (i.e., 95.2 ppm or mg / L), solvent extraction tests performed similarly to those in the further examples above resulted in the formation of contaminants.

[0122]

[0123] Table 4. Silicon concentration (mg / L) in the fresh and aged leachates produced in Example 1 above after additional filtration (1 μm).

[0124] Thus it is demonstrated that aging reduces the silicon concentration.However, it is currently believed that aging must be combined with other colloidal silica control actions in order to obtain a leachate suitable for solvent extraction.

[0125] Further experiments were conducted to investigate the effect of temperature on silica leaching.These experiments surprisingly showed that temperature and time control during leaching (optionally combined with selection of hydrochloric acid concentration) can be effectively used to minimize foulant formation during solvent extraction treatment of sewage sludge ash.

[0126] The sludge ash was leached with 6M hydrochloric acid at a liquid / solid ratio of 3 / 1 v / w but at higher temperatures for longer periods of time: i) 5 hours at 60° C. and ii) 1 hour at 90° C. In both cases, a significant reduction in the silicon content of the solution was observed compared to the leaching carried out at ambient temperature (Table 5).

[0127] 21℃，0.5h 60℃，5h 90℃，1h Total Si, mg / L 168.0 51.8 37.1

[0128] Table 5. Amount of silicon in solution when ash leaching was performed with 6M hydrochloric acid (liquid / solid ratio 3 / 1 v / w) at different temperatures and leaching times.

[0129] The results were highly surprising, as prior art literature suggested otherwise. According to literature such as Lenher and Merrill (1917), the solubility of silicon in hydrochloric acid is significantly higher at higher temperatures. According to the literature, when leaching sewage sludge ash at 60°C or 90°C was performed, more silicon would be expected in solution. Surprisingly, this was not the case here, using starting material derived from sewage sludge ash. When solvent extraction batch tests were performed with leachates obtained at 60°C and 90°C (1:1 organic:aqueous phase ratio, 80% by volume TBP), no significant fouling was observed.

[0130] The above observations were also found to be valid when the sewage sludge ash was replaced with sewage sludge ash that had previously been leached with a hydrochloric acid solution to remove most of the phosphorus and some (but not most) of the iron. Using this starting material, leaching was performed using at least 6M HCl at a temperature above ambient temperature. This was intended to dissolve the iron to produce an iron-rich solution with a small amount of phosphorus, which was then used for solvent extraction tests. Similar to the previous example, the leaching step resulted in less silicon in the solution compared to leaching performed at ambient temperature, and no fouling was formed at a 1:1 organic / aqueous phase ratio with 80% TBP. Subsequent treatment of the leachate with a silica coagulant and subsequent filtration reduced the silicon concentration from a few tenths of mg / L to approximately 6 mg / L. This solution was subjected to solvent extraction in a countercurrent separation apparatus (multiple mixer-settler units), and the process was operated continuously for several days without silica fouling.

[0131] From these surprising results it was concluded that carrying out the dissolution step at an elevated temperature of at least 50°C can provide a leachate that is compatible with viable liquid-liquid extraction with an organic solvent.

[0132] Surprisingly, although literature such as "Dealing with a siliceous crud problem in solvent extraction," Hydrometallurgy 15 (1985) mentions treating silica-containing leachates with activated carbon as a means of mitigating silica contamination during solvent extraction, this was unsuccessful in this case. Treatment of the sludge ash leachate with activated carbon, followed by filtration through a 0.45 μm PVDF filter, did not prevent contamination when using TBP. References such as "Coagulation of colloidal silica by calcium ions, mechanism, and effect of particle size," Journal of Colloid and Interface Science, Vol. 53, No. 3, pp. 576-588, 1975, also mention that colloidal silica can adsorb onto calcium ions at high calcium concentrations in solution. However, this surprisingly was ineffective for the sludge ash leachate tested here and for calcium concentrations up to 2.7 M in solution. Adding flocculants to the leachate also did not solve the problem, as the colloidal silica particles were too small to be affected by the flocculants. Diluting the leachate with water, hydrochloric acid, or saline solution did minimize foulant formation, but this was due to the reduced silica concentration in solution due to dilution. However, this is not feasible because it increases the aqueous volume in the process, consumes chemicals or water, and requires greater processing capacity.

[0133] However, foulant formation is avoided by adding a fluorine source (in this case, ammonium fluoride) to the leachate. This converts the insoluble colloidal silica particles into ammonium fluorosilicate, which has a solubility limit of about 185 g / L in water at 25°C. In one embodiment, NH4F is added to form (HN4)2SiF6. In other words, in one embodiment, the step of reducing the colloidal silica includes adding a fluorine source, such as NH4F, HF, NaF, or CaF2, to convert the colloidal silica into a fluorosilicate species, such as (HN4)2SiF6.

[0134] By using a coagulant designed to bind and grow colloidal silica particles, fouling is also avoided, which facilitates removal of the colloidal silica particles from the solution using conventional solid-liquid separation techniques (e.g., clarifiers, filtration, or centrifugation). This approach is an alternative to adding fluoride ions if fluorine is not desired further downstream in the stream. Adding hundreds of ppm of coagulant to the leachate immediately precipitates the colloidal silica, enabling its removal using filtration and avoiding fouling during solvent extraction. The silicon in solution can exist as both a soluble silicon species (e.g., silicic acid) and an insoluble colloidal silica formed by the dissociation and polymerization of silicic acid. Coagulation will only precipitate the latter.

[0135] During ash leaching, adding a silica coagulant to the leaching reactor, followed by a primary filtration to remove both undissolved ash and coagulated silica, effectively facilitated subsequent processing of the resulting leachate in a mixer-settler without silica contamination in the experiment. The quantitative silicon concentration in the solution was 36.4 mg / L. Adding a silica coagulant to the leachate after it had been separated from the undissolved ash and using filtration to remove coagulated silica also allowed operation without silica contamination. This result was achieved regardless of whether TBP solutions diluted with 1-decanol and / or kerosene, such as 40% or 75% by volume, were used, or whether undiluted TBP was used.

[0136] This example demonstrates that: i) when leaching sludge ash with a 3M hydrochloric acid solution at ambient temperature (21°C), silica coagulation is effective regardless of whether a coagulant is added during or after leaching the ash; and ii) after removal of colloidal silica by coagulation, the solvent extraction process can be run continuously in a countercurrent separation unit (multiple mixer-settlers) without silica contamination. Without coagulation, such leachate can contain over 300 mg / L of silica (see Table 3), which can lead to contamination during the extraction of iron and phosphorus with TBP solvent even when only a single organic-aqueous phase mixing stage is used. Multiple separation stages (e.g., mixer-settlers) commonly used in the industry exacerbate this problem and can render this process unfeasible.

[0137] The former may not directly result in contaminants, but may be extracted and stripped from the organic phase. If extracted and stripped, silicic acid or other ionic silicon species may concentrate in the stripped product if different O:A ratios are used, as may be the case in the present invention. Due to ionic silicon concentrations above a stability threshold and / or due to pH changes, colloidal silica may form in the stripped product and may result in contaminants. To prevent this, a silica coagulant or a fluoride source may be added to the aqueous phase, i.e., the stripping solution and / or the stripped product, to mitigate contaminant formation and promote phase separation in the later stages of the solvent extraction process. It has been shown that the presence of a silica coagulant in the solution helps to precipitate newly formed colloidal silica. This has also been observed with sewage sludge ash leachate that was aged for several days after the colloidal silica was removed via coagulation and filtration. It is therefore generally recommended that the solvent extraction process be performed as soon as possible after the colloidal silica has been coagulated and removed from the leachate.

[0138] According to the technology proposed by the present invention, the colloidal silica is controlled in a pre-treatment step. This can be done at different points in the process, preferably before the sewage sludge ash leachate is contacted with the organic phase. In one embodiment, as Figure 2 As shown in , after leaching the sewage sludge ash with hydrochloric acid and separating the leachate from the undissolved residue, a step of reducing the amount of colloidal silica is performed S15. In other words, in one embodiment, the control of the colloidal silica is at least partially performed in the leachate after the step of separating the undissolved residue, whereby the method comprises a further step of removing agglomerated silica particles from the leachate S19.

[0139] In another embodiment, Figure 3 As shown in , pretreatment is performed during leaching, i.e., simultaneously with step S10. In this case, in one embodiment, a silica coagulant or fluorine source is added to the leaching reactor along with the ash and leaching agent. In another embodiment, leaching is performed at an elevated temperature, preferably above 50°C, thereby reducing the amount of colloidal silica in the leachate. In other words, in some embodiments, control of the colloidal silica is performed at least partially concurrently with the dissolution step.

[0140] Of course, these two embodiments can also be combined, wherein a part of step S15 of controlling the amount of colloidal silica is performed simultaneously with the dissolution, and another part is performed after separation of the residue.

[0141] Thus, the present inventors have surprisingly demonstrated that foulant formation is avoided if the coagulant is either: 1) first added to the leachant and then mixed with the sludge ash, followed by filtering the solution after leaching is complete; 2) added to the leachant-sludge ash slurry during mixing but before filtration; 3) added to the leachate after filtration, followed by removal of the precipitated silica fraction; or 4) a combination of the above, wherein only a portion of the silica is reduced in each step.

[0142] It has been shown that it is even possible, although not optimal, to precipitate colloidal silica by adding a coagulant to the leachate after filtration and then proceeding directly to solvent extraction without removing the precipitated silica from the solution. This approach mitigates much of the fouling due to the growth of silica particles via coagulation, but operation under such conditions is not considered ideal and may still result in organic matter losses due to fouling.

[0143] In experiments, it was not possible to completely remove the colloidal silica using ordinary filtration or centrifugation, but removal was demonstrated using a coagulant specifically formulated for colloidal silica. The coagulant binds to the colloidal silica particles and causes them to grow, enabling their removal using sedimentation, clarification, or filtration, and even without a removal step, allowing solvent extraction to be performed with significantly lower organic phase losses.

[0144] In addition to the alternative of promoting coagulation to allow particle growth and converting colloidal silica to ionic silicon and removing the ionic silicon, other alternatives are also possible. If a very rapid process is used, the ionic silicon present in the leachate can be removed before the ionic silicon is converted to colloidal silica. The silicon reduction step is thus part of the common dissolution and separation steps.

[0145] Furthermore, the reverse schedule may be useful, allowing the dissolved sewage sludge ash to age to provide time for silica growth. This would typically be performed after separation of the residue, but could also be performed at least partially before separation of the residue.

[0146] It has also been shown from the experiments that the amount of colloidal silica can be controlled by carrying out the dissolution step with a highly acidic solution. The control step is thus part of the common dissolution and separation steps.

[0147] From the experiments it was concluded that performing the dissolution step at an elevated temperature of at least 50°C can provide a leachate that is compatible with viable liquid-liquid extraction with an organic solvent.

[0148] In other words, in Figure 4 In one embodiment shown in , the step of reducing colloidal silica includes at least one of the following:

[0149] - as in step S151, promoting coagulation to allow particle growth;

[0150] - as in step S152, converting the colloidal silica into ionic silicon, wherein the ionic silicon is optionally removed;

[0151] - As in step S153, quickly removing ionic silicon before it is converted into colloidal silicon dioxide;

[0152] - As in step S154, performing a dissolution step using a high acidity solution;

[0153] - aging the dissolved sewage sludge ash to promote at least one of conversion of ionic silicon to colloidal silica and growth of colloidal silica particles, as in step S155; and

[0154] - Leaching at high temperature as in step S156.

[0155] As mentioned above, the pretreatment step S14 and the step of reducing the amount of colloidal silica S15 can be performed after the dissolution step S10, or surprisingly, simultaneously with the dissolution step S10. In the latter case, the colloidal silica coagulant is added during leaching. This has the advantage that the removal of the condensed silica can be carried out together with the undissolved ash, making the second solid-liquid separation step S19 to remove the precipitated silica unnecessary.

[0156] In addition to colloidal silica, sulfur is typically leached with hydrochloric acid, as shown in Table 2. This can precipitate from the leachate as calcium sulfate (e.g., gypsum). References such as "Composite Fouling of Calcium Sulfate and Calcium Carbonate in a Dynamic Seawater Reverse Osmosis Unit," MSc. Thesis, University of New South Wales, Sydney, Australia, 2005, note that the presence of iron ions, in particular, increases the induction period (appearance of gypsum crystals) of gypsum in aqueous inorganic salt solutions.

[0157] The present inventors have observed that for leachates similar to those in Table 2, sulfur precipitation occurs slowly after filtration, sometimes over several days. This is undesirable because the presence of gypsum particles in the leachate can cause contamination during solvent extraction and complicate the process.

[0158] Of course, additional aging can be combined with the pretreatment steps to ensure gypsum formation and precipitation prior to solvent extraction.

[0159] Alternatively, the induction period can be extended to allow operations to be performed before the gypsum is formed. This can be done by adding substances that increase the induction period, such as sodium carboxymethylcellulose, sodium hexametaphosphate or polyacrylamide, which are previously known to those skilled in the art.

[0160] Furthermore, it was found that sulfur precipitation was faster when the background calcium concentration was high. Therefore, by ensuring a high calcium concentration in the leach solution, slow precipitation can be avoided.

[0161] This is summarized in Figure 5 , which illustrates various embodiments of step S16 for treating sulfur. In step S161, the leachate from the dissolution of the starting material is allowed to age. In step S162, a gypsum inhibitor is added to the leachate from the dissolution of the starting material. In step S163, gypsum formation is promoted. This is preferably performed by ensuring a high calcium concentration in the leachate solution and / or by adding gypsum seed particles to promote gypsum precipitation. Steps S161 and S163 may also be combined.

[0162] In other words, in one embodiment, the starting material comprises sulfur. The method for chemically treating sewage sludge ash includes an additional step of preventing the precipitation of calcium sulfate immediately before or during the step of extracting at least one of iron and phosphorus by adding a CaSO4 inhibitor prior to the step of extracting at least one of iron and phosphorus. The term "immediately before" should be interpreted as a time period that does not allow for any solid / liquid separation process to be performed.

[0163] In another embodiment, the starting material comprises sulfur. The method for chemically treating sewage sludge ash comprises the additional step of preventing precipitation of calcium sulfate from occurring just before or during the step of extracting at least one of iron and phosphorus by performing aging of the leachate and / or accelerating CaSO4 precipitation prior to the step of extracting at least one of iron and phosphorus.

[0164] In a preferred embodiment, the precipitation of sulfur as gypsum is accelerated by increasing the calcium chloride content of the leachate. This also facilitates subsequent solvent extraction. The ionic strength of the leachate is increased, which results in enhanced solvent extraction of iron and phosphorus. This will be described in further detail below. The calcium supply is preferably achieved by adding calcium chloride to the leachate, by returning the calcium chloride-rich aqueous raffinate after the phosphoric acid extraction to the leaching step in place of water, or by a combination of both.

[0165] Regarding timing, different embodiments can be identified to achieve a high concentration of calcium in the leachate and to achieve a faster precipitation of gypsum. Figure 6In the embodiment, a sulfur treatment step S16 is performed after the actual leaching is completed. A quantitative addition of a calcium chloride salting-out agent is performed on the leachate after leaching. Then a gypsum removal step S169 is performed. Preferably, this measure can be performed together with the addition of a silica coagulant, for example as discussed above, to promote the precipitation and filtration of gypsum together with silica. The precipitation of gypsum positively affects the filtration of silica, since gypsum is a good filter aid. Then, Figure 2 Step S19 and Figure 6 Step S169 combination.

[0166] Alternatively, the addition of the substance that increases the gypsum induction period can be performed after the actual leaching is finished. Since the sulphur then remains in solution, there is no need for removal step S169.

[0167] It is also possible to age the leachate after the actual leaching is complete for a sufficient time to allow the gypsum to precipitate before solvent extraction. This can be combined with the silica treatment step (if any) and can be carried out in settling tanks, clarifiers and / or similar equipment.

[0168] Alternatively, as Figure 7 As shown in , step S16 can be performed at least partially together with the dissolution step S10. In one embodiment, a high calcium background is maintained during leaching to promote gypsum precipitation. This can be achieved by adding, for example, calcium chloride during leaching, by returning the calcium chloride-rich aqueous raffinate to the leaching step after iron and / or phosphate extraction to replace water, or by a combination of both. This allows gypsum to precipitate at a higher yield during leaching. This also allows the precipitated gypsum to be separated from the undissolved ash in step S12, for example by filtration.

[0169] Alternatively, the addition of a substance that increases the gypsum induction period may be performed together with the dissolving step S10.

[0170] Furthermore, as indicated above, the addition of CaSO4 seed particles can also improve the rate of gypsum precipitation. Such CaSO4 seed particles can be provided as seed particles to the leachate at the latest during the dissolution step S10 and / or after the step S12 of separating the undissolved residue.

[0171] In other words, in one embodiment, the acceleration of CaSO4 precipitation comprises:

[0172] - calcium chloride is added at the latest during the dissolution step;

[0173] - adding calcium chloride to the leachate after the step of separating the undissolved residue;

[0174] - recycling, at the latest during the dissolution step, the aqueous raffinate rich in calcium chloride resulting from the preceding step of extraction of iron and / or phosphorus;

[0175] - recycling, after the step of separating the undissolved residue, the aqueous raffinate rich in calcium chloride produced after the previous step of extracting iron and / or phosphorus;

[0176] - adding CaSO4 seed particles at the latest during said dissolving step (S10); and / or

[0177] - After the step (S12) of separating the undissolved residue (204), CaSO4 seed particles are added to the leaching solution (206).

[0178] Iron recovery from sludge ash leachate can be performed using an organic phase containing TBP as an extraction agent. The form in which the iron ions exist in the leachate (e.g., ferrous iron or ferric iron, also referred to as Fe(II) and Fe(III) respectively) determines whether they are effectively extracted by TBP. It has been found that if ferric iron is reduced to ferrous iron using a reducing agent, such as by adding metallic iron to the leachate, the extraction of iron by TBP is significantly reduced. The opposite is also true. In order to promote the extraction of ferrous iron by TBP, it can be oxidized to ferric iron with an oxidizing agent.

[0179] This opens the way to different approaches to iron extraction. For example, if non-extractable ferrous iron is present in the sludge ash or leachate, oxidation can be used to maximize iron recovery. Figure 1 The step S18 of oxidizing Fe(II) to Fe(III) can be combined with the pretreatment step S14. The oxidation can be performed during and / or after the dissolution step S10. The oxidizing agent can be, for example, hydrogen peroxide, ozone, chlorine, etc.

[0180] In other words, in one embodiment, the method for chemically treating sewage sludge ash comprises an additional step of oxidizing any Fe(II) in the leachate to Fe(III) prior to the extraction step S20.

[0181] On the contrary, reduction can be used to minimize the extraction of iron, for example, bypassing iron extraction and continuing the recovery of phosphorus. This may be preferred for ash and / or leachate with very low iron content, where the benefit of recovering iron cannot compensate for the increased complexity of recovering both iron and phosphorus. This step S17 of reducing Fe (III) to Fe (II) can be combined with the pre-treatment step S14. Reduction can be performed during and / or after the dissolving step. Metallic iron or other reducing substances reduce the ferric iron Fe (III) in the solution to ferrous iron Fe (II). One advantage is that impurity ions can also be reduced, for example Cu (II) is reduced to metallic copper, which will precipitate.

[0182] In other words, in one embodiment, the method for chemically treating sewage sludge ash includes an additional step of reducing any Fe(III) in the leachate to Fe(II) prior to the extraction step S20. In this case, in a particular embodiment, step S20 may include a step S30 of extracting phosphorus, but without a procedure for iron extraction.

[0183] Figure 8 An apparatus 1 for chemically treating sewage sludge ash is schematically illustrated. A dissolution reactor 10 has an inlet for a starting material 201. This starting material 201 is derived from sewage sludge ash and contains at least silicon dioxide and iron. The dissolution reactor 10 also has an inlet for an acid 202, including hydrochloric acid. The starting material derived from sewage sludge ash is at least partially dissolved in the dissolution reactor. A separation device 12 (e.g., a filter) separates undissolved residue 204 from a leachate 206. Preferably, the apparatus 1 for chemically treating sewage sludge ash further comprises a washing chamber 13 configured to wash the separated undissolved residue 204 from the remaining acid, producing a washed residue product 204X. Spent washing liquid 205 can be returned to a suitable stage in the process via a return line 131. Alternatively, the washing liquid 205 can be provided to an extractor section 20 (described further below), for example, to be mixed into a leachate 206 or a leachate 207 (described below).

[0184] The apparatus 1 for chemically treating sewage sludge ash further comprises means 14A, 14B, 14C for controlling the amount of colloidal silica present in the leachate 207 provided to the extractor section 20. These means may include an inlet 14B for the additive 203 to enter the dissolution reactor and / or at least one pre-treatment chamber 14A into which the additive 203 can be introduced and / or a heater 14C arranged to provide an elevated temperature of at least 50° C. in the dissolution reactor 10. This will be discussed in more detail further below, resulting in a leachate 207 having less or no colloidal silica.

[0185] The apparatus 1 for chemically treating sewage sludge ash further comprises an extractor section 20 having an apparatus 22 for liquid-liquid extraction with an organic solvent. The apparatus 22 for liquid-liquid extraction is configured to extract at least one of iron and phosphorus from a leachate 207 having little or no colloidal silica. A stripping liquid 209 is added, and a supply of at least one liquid 210 containing iron and / or phosphorus is generated. A raffinate 211 at least partially depleted in iron and / or phosphorus is provided from the extractor section 20.

[0186] Preferably, the apparatus 1 for chemical treatment of sewage sludge ash further comprises a post-treatment section 40 , wherein the remaining components in the raffinate 211 , such as aluminum 213 , heavy metals 214 , magnesium 215 , sodium 216 , potassium 217 and / or calcium 218 , can be separated and recovered by using different post-treatment additives 212 .

[0187] The raffinate (possibly post-treated) is at least partially recycled. At least part of the raffinate 211 depleted in iron and / or phosphorus from the extractor section 20 can be returned via the return line 90 to the inlet of the dissolution reactor 10 for dissolving the starting material derived from sewage sludge ash. The recycled portion 219 of the at least partly iron and / or phosphorus depleted raffinate 211 contains chloride ions.

[0188] Preferably, the return conduit 90 comprises means for controlling the amount of raffinate recycled so as to obtain a chloride salt and / or hydrochloric acid concentration of at least 1 M in the leachate.

[0189] Figure 9 An embodiment of a dissolution reactor 10 is schematically illustrated, in which a device for reducing the amount of colloidal silica in the dissolved sewage sludge ash has been integrated. In this embodiment, an inlet 14B is provided through which additives 203A can be introduced into the dissolution reactor 10. As further described above, these additives 203A can promote coagulation to grow silica particles from the colloidal silica, or can convert the colloidal silica into ionic silicon, which, if sufficiently soluble, can remain in solution or, if the solubility limit is reached, at least partially precipitate into any undissolved residue 204. The silica particles grow until they can be separated by the separation device 12. Thus, surprisingly, the control of the amount of colloidal silica can be performed simultaneously with the dissolution itself and within the same dissolution reactor 10.

[0190] In a similar manner, additives 203B can be introduced to prevent the precipitation of calcium sulfate during or during transport to the extractor section 20. For example, a CaSO4 inhibitor can be introduced to mitigate gypsum precipitation. Thus, sulfur continues to flow through the system in solution. Alternatively, additives can be added to accelerate CaSO4 precipitation, causing gypsum to precipitate and be separated by the separation device 12 along with the undissolved residue 204. Thus, surprisingly, sulfur treatment can be performed simultaneously with the dissolution itself and within the same dissolution reactor 10.

[0191] Alternatively, an oxidizing or reducing agent 203D may be used to oxidize Fe(II) to Fe(III). Thus, the manipulation of the iron oxidation state can be unexpectedly performed simultaneously with the dissolution itself and performed within the same dissolution reactor 10.

[0192] Figure 9 A heater 14C is also shown. The heater is arranged to provide an elevated temperature in the dissolution reactor 10. This may be performed by heating the acid 202 prior to or in connection with entry into the dissolution reactor and / or by heating the acid during the leaching process in the dissolution reactor.

[0193] Figure 10 An embodiment of a pre-treatment chamber 14A is shown. The pre-treatment chamber 14A can be used to reduce Figure 9 In addition to or in place of a device for reducing the amount of colloidal silica in the dissolved sewage sludge ash, an additive 203A can be added to the first mixing chamber 141 through the inlet 143. As further described above, these additives 203A can promote coagulation to grow silica particles from the colloidal silica, or can convert the colloidal silica into ionic silicon. The silica particles grow until they reach a separable size. In this embodiment, the pretreatment chamber 14A includes an aging chamber 142, in which the precipitate can settle and the silica particles are allowed to grow. Any precipitate and silica particles 204C can be separated by the separation device 144 to obtain a pretreated leachate 207. In alternative embodiments, the mixing chamber 141 or the aging chamber 142 can be omitted, and in further other embodiments, the mixing chamber 141 and the aging chamber 142 can be combined.

[0194] In a similar manner, additives 203B may be introduced to prevent the precipitation of calcium sulfate during or during the feed to extractor section 20. For example, a CaSO inhibitor may be introduced to mitigate gypsum precipitation. Thus, sulfur will continue to flow through the apparatus in solution. Alternatively, additives may be added to accelerate CaSO precipitation, causing gypsum to precipitate 204C and be separated by separation device 144.

[0195] Oxidizing or reducing additives can also be added to affect the oxidation state of the iron ion, and this is also possible for other metals. A reducing agent 203C (such as iron metal) can be used to reduce Fe(III) to Fe(II). Alternatively, an oxidizing agent 203D can be used to oxidize Fe(II) to Fe(III).

[0196] In one embodiment, the step of extracting at least one of iron and phosphorus from the leachate by liquid-liquid extraction with an organic solvent comprises the step of extracting iron from the leachate by liquid-liquid extraction with an organic solvent.

[0197] As mentioned above, the recovery of iron from the sludge ash leachate in the form of ferric chloride can be performed using an organic phase containing TBP as an extractant. If desired, a long-chain alcohol (e.g., decanol) can be used as a modifier, and kerosene can be used as a diluent. Other organic compounds and mixtures may also be suitable for this purpose. It has been determined that although iron is extracted by TBP in preference to phosphorus and most other impurities in the leachate, some co-extraction of phosphorus, calcium, and other trace elements does occur.

[0198] If the extraction is performed at a temperature close to the iron loading capacity of the organic phase, the amount of elements co-extracted with TBP along with the iron can be reduced. This can be achieved by controlling the organic phase:aqueous phase (O:A) ratio and / or the TBP content in the organic phase. Thus, by performing the extraction at a low O:A ratio, the concentration of iron in the loaded organic phase can be controlled. Conversely, for example, the co-extraction of iron and phosphorus can be achieved by performing the extraction at a high O:A ratio. Thus, it is possible to adjust the characteristics of the extraction process to adapt it to, for example, the contents of the leachate.

[0199] However, as briefly mentioned above, the content of the leachate can also be controlled to a certain extent during the leaching process by varying the operating conditions. This opens the way to different methods of recovering iron and phosphorus as well as other components of the leachate.

[0200] Therefore, another aspect of a preferred embodiment of the present technology focuses on controlling the leaching and extraction selectivity of sewage sludge ash components by controlling process conditions, such as temperature, pH, pressure, time, mixing speed and mixing time, presence of oxidizing or reducing species, ash particle size, L:S ratio, O:A ratio, etc. This allows controlling the amount of sewage sludge ash components dissolved and extracted in the different stages of the process, thus enabling individual separation of components, such as separation of iron from aluminum, separation of calcium from magnesium, separation of pure phosphorus streams, and ensuring high purity of the separated compounds.

[0201] For example, the dissolution yields of several components can be controlled during leaching, allowing for the production of a leachate rich in both iron and phosphorus, or a leachate rich in phosphorus but poor in iron. Table 6 shows the leaching rates of iron and phosphorus when sewage sludge ash is leached using hydrochloric acid solutions of varying concentrations, leaching times, temperatures, and / or L:S ratios. The leaching rate of calcium is similar to that of phosphorus. At ambient temperature and varying acid concentrations, the leaching rate of aluminum ranges from 30% to 35%. As shown in Table 2, increasing the hydrochloric acid concentration and / or decreasing the L:S ratio results in less silica being leached. Because these parameters strongly influence the leaching process, they can also be selected to achieve the desired iron and phosphorus leaching rates, which in turn influences the concentrations of the elements in the leachate and in the undissolved residue after leaching.

[0202] For example, in one embodiment, leaching can be carried out, for example, under ambient conditions and at a low L:S ratio using a dilute hydrochloric acid solution to produce a leachate having a low iron:phosphorus ratio. The corresponding undissolved silicate residue thus contains a significant amount of iron.

[0203] In another embodiment, a leachate having a higher iron:phosphorus ratio than the previous case can be obtained by using a more concentrated hydrochloric acid solution and / or a higher temperature and / or a longer leaching time. This allows for a higher recovery of these elements from the sludge ash and the production of a cleaner, iron-depleted, undissolved silicate residue.

[0204]

[0205] Table 6 - Leaching rates of iron and phosphorus when leaching sludge ash with hydrochloric acid solution using different process parameters such as temperature, leaching time and L:S ratio.

[0206] In yet another embodiment, continuous leaching can be performed to obtain leachates with different iron:phosphorus ratios. For example, in the first stage, leaching is performed to leach out most of the phosphorus, while the iron is not completely dissolved. Preferably, the dissolution of the iron is then kept to a minimum. This is typically achieved by using relatively low concentrations of hydrochloric acid and / or short leaching times and / or low temperatures. If the iron dissolution is kept low, a large amount of iron remains in the undissolved silicate ash. In a subsequent second stage, this residue is processed to recover iron and / or other undissolved components and obtain a silicate product with low iron and low impurity content. To achieve this, leaching is performed with concentrated hydrochloric acid and / or high temperatures and / or longer leaching times. Such leachates typically will have a high iron:phosphorus ratio, which is higher than when the sewage sludge ash is leached in only one step. The raffinate after the iron extraction can be effectively used to leach fresh sewage sludge ash and / or can be reused in other steps of the method and / or can be processed according to the embodiments described herein.

[0207] The three aforementioned embodiments will be discussed in more detail below.

[0208] In various embodiments of the present technology, solvent extraction is used to extract iron from other components of the leachate. An embodiment of the general solvent extraction principle in the shape of the solvent extractor 220 is schematically shown in FIG. Figure 11 The input aqueous solution 221 is provided to the contact vessel 80, where iron is extracted by the organic phase 222 to obtain an iron-loaded organic phase 222B. The raffinate 227 is output from the contact vessel 80 to a subsequent recovery process, such as a phosphorus extraction step. The iron-loaded organic phase typically also contains co-extracted phosphorus, calcium, and other small amounts of impurities. Washing away 81 co-extracted impurities from the loaded organic phase 222B can optionally be performed using a washing solution 223, thereby forming a loaded washing solution 223B and a washed organic phase 222C. The washed organic phase 222C may still contain, for example, some phosphorus and calcium, but typically at a lower concentration. In the stripping vessel 82, iron is stripped from the loaded or washed organic phase 222B or 222C to a stripping solution 225 to obtain an iron-loaded stripping solution 226. The organic phase 222 is recovered and can be reused for the next extraction.

[0209] As will be discussed later, the iron-laden strip solution 226 may be further processed to remove impurities, concentrate the iron, and / or neutralize any hydrochloric acid that was extracted and stripped with the iron.

[0210] A variety of organic extractants have been proposed in the literature for extracting iron ions from chloride media: solvating agents (trialkyl phosphates, such as tributyl phosphate (TBP), alkyl phosphine oxides, such as trioctylphosphine oxide or Cyanex 923), amines (Alamine 336, Aliquat 336), acidic extractants (Cyanex 272, DEHPA, aliphatic monocarboxylic acids) and alcohols (decanol or other long-chain aliphatic alcohols). Combinations of different extractants (e.g., a combination of solvating extractants and acidic extractants) can be used to enhance extractability and / or selectivity. Tributyl phosphate is a commercial solvating extractant reagent commonly used in the phosphate industry for purifying phosphoric acid by solvent extraction. Tributyl phosphate is non-flammable, has low toxicity, and has a solubility in water of as low as about 0.4 g / L at room temperature. In addition, the solubility decreases with increasing temperature and increasing phosphoric acid concentration in the aqueous phase. Due to the high density of TBP (about 0.98 kg / L), it is usually mixed with a diluent such as aliphatic kerosene in order to improve the physical separation between TBP and the aqueous phase.

[0211] According to the experiments performed, in contact with concentrated salt solutions such as calcium chloride, the density difference may be sufficient to avoid the use of diluents if the stripping can be carried out in a manner that allows for efficient phase separation. However, the ability of TBP to extract both iron and phosphoric acid from a hydrochloric acid medium is also problematic, as this means that there is a risk that the iron and / or phosphoric acid streams generated by the TBP solvent extraction will be contaminated with phosphorus and / or iron, respectively.

[0212] Extraction of ferric iron with TBP can be carried out according to equation (1), which shows that one mole of hydrochloric acid is extracted with each mole of iron. This means that in order to extract the iron, sufficient hydrochloric acid needs to be present in the leachate. If the pH after stripping is low enough to prevent iron precipitation, the extracted iron can be stripped using water or other solutions (e.g., dilute acid solution, saline solution or alkaline solution). The hydrochloric acid extracted by TBP is also stripped, producing an acidic stripping product. If stripping is performed with water, the product will be a mixture of hydrochloric acid and ferric chloride. Since it is advantageous to extract iron with TBP at higher acidity, if the stripping solution is too acidic, the stripping process stops working. In other words, re-extraction of iron from the stripping product can occur simultaneously with stripping. Therefore, there is a limit to the O:A ratio that can be used for stripping with water. This determines the degree of concentration of the stripping product.

[0213] Fe 3+ + 2 TBP + HCl = HFeCl4 * 2TBP (1)

[0214] Because TBP can extract phosphoric acid with high yield, which will be further explained below when discussing the recovery of phosphorus, controlling the absorption of iron and phosphorus during the solvent extraction process is a key aspect of this technology. This control enables the acquisition of high-purity iron and phosphorus streams. Leaching sewage sludge ash with a 3M hydrochloric acid solution at an L:S ratio of 3:1 units (L / kg) and subsequent TBP solvent extraction results in relatively poor extraction efficiency for iron and phosphorus due to the low ionic strength of the leachate. Increasing the ionic strength of the leachate by adding approximately 2M solid metal chloride salts to the leachate and / or leaching the sludge ash with more concentrated hydrochloric acid (e.g., 6M) results in significantly better extraction rates of iron and phosphorus. Preferably, the ionic strength should be controlled to be high.

[0215] Ionic strength control can be achieved in at least three different ways. To enhance or optimize the extraction yield of iron and phosphorus, the leaching of the sludge ash can be performed with a leachant containing an initial hydrochloric acid and / or salt content, which is selected so that under certain operating conditions (e.g., leaching time, L:S ratio, temperature, etc.), the leachate obtained will contain hydrochloric acid and chloride of sufficiently high ionic strength to ensure the desired extraction yield. If reference is made to Figure 8 , the leachant may therefore be contained in either stream 201 or 202 .

[0216] In an alternative or supplemental embodiment, the addition of hydrochloric acid and / or salts may be performed during leaching by one or more compounds, such as via stream 203. Process stream 219 may also be used to increase the ionic strength of the leachate to ensure recycling of the desired extraction yield.

[0217] In yet another alternative, the addition of hydrochloric acid and / or salts and / or one or more compounds and / or recycled process streams capable of increasing the ionic strength of the leachate to ensure the desired extraction rate may be performed after leaching.

[0218] Of course, the above alternatives can be combined in different ways, and two or three of them can be used at the same time or at different times during the production period. For example, at the beginning of the production period, when no recycled process stream is used, salt can be introduced after leaching, while at a later stage, the recycled process stream added during leaching may be sufficient to achieve the preferred ionic strength.

[0219] In one of the applicant's previous inventions, disclosed in International Patent Application Publication No. WO 2014178788 A1, the extraction of iron in preference to phosphorus from an aqueous solution containing 21.42 g / L iron and 26.69 g / L phosphorus was described using an organic phase containing 30% TBP at an O:A ratio of 1:1. In this particular case, the raffinate after solvent extraction contained 0.74 g / L iron and 27.53 g / L phosphorus, meaning an organic phase with over 20 g / L iron and no phosphorus was obtained. However, as can be observed in Table 2, a typical sludge ash leachate obtained at ambient temperature using a mild hydrochloric acid solution contains significantly less iron (3-5 g / L). This is a result of the low iron leaching rate under these conditions (Table 5). In International Patent Application Publication No. WO 2014178788 A1, a high iron content (21.42 g / L) in the aqueous phase was achieved after adding alkali to a similar sewage sludge ash leachate to precipitate an intermediate calcium-iron-aluminum phosphate compound, which was then dissolved in hydrochloric acid. The aqueous solution was therefore not obtained directly from sewage sludge ash, but rather was produced by adding alkali to the ash leachate followed by additional chemical treatment. This method also allows for the preparation of an aqueous phase with sufficient ionic strength to ensure efficient iron extraction with TBP and produces a concentrated and relatively pure loaded organic phase that can be stripped to obtain ferric chloride with a low phosphorus content.

[0220] Using the same extraction conditions and the sewage sludge ash leachate in Table 2, but without this intermediate precipitation step, it is impossible to obtain the same iron extraction results. In order to obtain a concentrated iron product from such sewage sludge ash leachate after removing the colloidal silica, extraction with TBP is preferably performed at a low O:A ratio to concentrate the iron in the organic phase. In addition, if it is desired to further concentrate the iron in the stripped product, it is preferably performed at a high O:A ratio. In addition, without adding a suitable salting-out agent to the 3M hydrochloric acid leachate, the iron extraction rate is low. It is not feasible to perform iron extraction for leachates with such low iron contents at a high O:A ratio (such low iron contents are characteristic of ashes with very low iron contents and / or when leached with a mild hydrochloric acid solution at ambient temperature) because this produces an organic phase that is too dilute and also contains a large amount of phosphoric acid. Although iron is extracted preferentially over phosphorus, surprisingly, even at an O:A ratio below 1:1, it is impossible to obtain a relatively pure loaded organic phase or iron stripped product due to the co-extraction of phosphorus and iron. Increasing the amount of TBP in the organic phase to above 30 volume % and / or increasing the O:A ratio causes an increase in the co-extraction of phosphorus. Reducing the amount of TBP negatively impacts the extraction of iron. For the embodiments described herein, it has been experimentally established that the advantageous iron extraction with minimal phosphorus extraction requires adjusting the amount of TBP in the organic phase and the extraction O:A ratio for the amount of iron present in the leachate so that the organic phase is loaded with as much iron as possible, preferably close to the loading capacity of TBP. Overloading the organic phase with iron hinders the co-extraction of phosphorus, and phosphorus mainly remains in the aqueous phase (e.g., leachate). However, the co-extraction of phosphorus cannot be completely avoided. Due to the low O:A ratio required for extracting and concentrating the iron in the organic phase and the high O:A ratio required for further concentrating the iron in the strip product in the strip step, a large amount of co-extracted phosphorus (13.0 g / L) and calcium (8.95 g / L) are ultimately present in the strip product together with iron (30.4 g / L) (see Table 7). However, most of the phosphorus remains in the raffinate unextracted, which can be recovered in a subsequent phosphorus solvent extraction step. Most of the other impurity elements in the leachate do not end up in the iron stripping product.

[0221]

[0222]

[0223] Table 7. Solvent extraction of elements in the leachate obtained by leaching sewage sludge ash with 3M hydrochloric acid solution at ambient temperature. Calcium chloride was added to the leachate to increase its ionic strength. Extraction was performed with 40% TBP at a low O:A ratio and stripping was performed with water at a high O:A ratio to maximize the iron concentration in the stripping product and minimize co-extraction of phosphorus.

[0224] In short, if extraction is performed under the iron loading capacity close to the organic phase, the amount of the element extracted with TBP together with iron can be reduced. This situation can be achieved by controlling the organic phase: the content of TBP in the aqueous phase (O: A) ratio and / or the organic phase. By performing extraction under low O: A ratio, the iron in the organic phase of the load can be concentrated. With suitable stripping solution (typically, water) from the organic phase of the load, iron can be stripped, but other solutions can also be used. Stripping can be carried out under high O: A ratio to further concentrate the iron in the stripped product. Stripping produces the organic phase of depletion, so that it is recycled and reused in the solvent extraction process. Other fresh organic phase can be added to make up for any loss. In addition, organic phase can be processed otherwise, for example, washed with sodium carbonate or sodium hydroxide solution to remove the dibutyl phosphate and / or monobutyl phosphate decomposition products that may be formed during extraction and / or stripping.

[0225] Due to the different O:A ratios used during the extraction and stripping of iron, phosphorus, calcium and other co-extracted impurities can be concentrated in the stripping product along with the iron, especially if the ash leachate contains little iron compared to phosphorus, calcium and other extractable species. To further purify the iron stripping product, it is again subjected to a similar solvent extraction and stripping cycle. If necessary, the ionic strength of the aqueous feed can be increased to enhance the extraction of iron. This can be achieved by adding a compound or mixture of compounds (e.g., hydrochloric acid and / or metal chloride salts), or by mixing the solution with a stream having a high ionic strength from another step of the process (e.g., after the precipitation of magnesium, sodium and potassium (i.e., in the presence of a solvent). Figure 1 The aqueous phase is then mixed with the calcium chloride-rich solution (after step S70 in step S71). In this second extraction step, the composition of the organic phase can be tailored to the amount of iron in the aqueous phase to facilitate extraction of iron close to the loading capacity of the organic phase, thereby maximizing selectivity. Another advantage of subsequent solvent extraction-strip extraction cycles is that the iron in the ferric chloride product is further concentrated, typically by performing extraction at a lower O:A ratio and stripping at a higher O:A ratio. It has been observed that two solvent extraction-strip extraction cycles to extract iron are sufficient to obtain a relatively concentrated ferric chloride solution with small amounts of phosphorus and other impurities. If desired, a third subsequent solvent extraction-strip extraction can be performed to obtain an even higher purity ferric chloride product; and this can be repeated. The raffinate after the second solvent extraction step for iron contains unextracted impurities, primarily phosphorus and calcium. Since the first extraction for iron is performed at a low O:A ratio and the stripping is performed at a high O:A ratio, the volume of the raffinate will be low enough to facilitate its adaptation to various steps, such as the leaching step, the start of the solvent extraction step, or other steps. It was observed that the use of concentrated hydrochloric acid effectively washed phosphorus from the iron-laden organic phase. This left the iron in the organic phase while the phosphorus was back-extracted in the hydrochloric acid; and could be used to obtain a purer ferric chloride product with a reduced phosphorus content.

[0226] Therefore, a particular aspect of this embodiment of the present technology is to perform subsequent processing of the stripped product after solvent extraction of iron using the same extractant to obtain high-purity ferric chloride while simultaneously increasing the iron concentration in the final product. Experimental results indicate that by adjusting the TBP concentration and extraction O:A ratio to the iron content of the aqueous phase and performing the extraction at a temperature close to the iron loading capacity of the organic phase, the amount of impurities co-extracted with the iron can be controlled and reduced. Therefore, a subsequent iron extraction step operating under these conditions was implemented.

[0227] Figure 12 A general embodiment is shown for subsequent processing of the stripped product after a first iron solvent extraction using solvent extraction to obtain high-purity ferric chloride and simultaneously increase the iron concentration in the final product. The iron solvent extraction apparatus comprises two solvent extractors 220A and 220B connected in series. A leachate 207 depleted in colloidal silica is passed into the contact vessel 80 of the first solvent extractor 220A, and iron is loaded into an organic phase 222 along with at least one of phosphorus, calcium, and possibly other contaminants, to produce an iron-loaded organic phase 222B. The composition and / or process parameters of the organic phase 222 can preferably be tailored to the composition of the aqueous phase to influence the extraction selectivity. The raffinate 211 is output for post-processing and / or to a section for recycling chloride ions and / or calcium ions. In the stripping vessel 82 of the first solvent extractor 220A, iron is stripped from the loaded organic phase 222B into a stripping solution 209, producing an intermediate stripping solution 228 loaded with iron, phosphorus, calcium, and possibly other contaminants.

[0228] The intermediate stripping solution 228 is used as the input aqueous solution of the second solvent extractor 220B. The intermediate stripping solution 228 is introduced into the contact vessel 80 of the second solvent extractor 220B, and iron is loaded into the organic phase 222 to obtain an organic phase 222B loaded with iron. The composition and / or process parameters of the organic phase 222 can preferably be customized for the composition of the intermediate stripping solution 228. For example, an additive 230 such as hydrochloric acid or other compounds such as salts can be optionally added. In this way, the ionic strength can be suitable for enhancing extraction. The second-stage raffinate 229 (comprising at least one of phosphorus, calcium, and some other pollutants) is output and can preferably be reused in the method, such as in a leaching or pretreatment process. In this way, phosphorus is re-entered into the method. In the stripping vessel 82 of the second solvent extractor 220B, iron is stripped from the loaded organic phase 222B into the stripping solution 209 to obtain a stripping solution 210A in liquid form containing iron.

[0229] Note that Figure 11Similarly, optional washing stages may be performed on the loaded organic phase 222B.

[0230] Figure 13 An embodiment of step S21 for extracting Fe is shown. In step S22, a first extraction stage is performed, wherein a liquid-liquid extraction of preferably a majority of the iron is carried out from the leachate. In step S23, a first stripping step is performed into an intermediate stripping solution. In step S24, a second extraction stage is performed, wherein a liquid-liquid extraction of preferably substantially only the iron is carried out from the intermediate stripping solution. In step S25, a second stripping step is performed into the stripping solution.

[0231] In slightly more detail, the aqueous feed to the subsequent iron solvent extraction step (e.g., intermediate stripping solution 228) is thus the iron stripping product obtained in the previous iron solvent extraction step. This feed is typically contaminated with at least one of calcium, phosphorus, and other trace elements (see Table 7). Preferably, a more concentrated organic phase is used in this step to avoid premature loading and to facilitate extraction at a low O:A ratio. This is preferred because it allows for further concentration of the iron in the organic phase. At the same time, performing stripping at an O:A ratio above 1 allows for further concentration of iron.

[0232] The intermediate stripping solution 228 is acidic and contains a minimum amount of hydrochloric acid (in mol / L) that is equal to the iron concentration (in mol / L) if the iron extraction with TBP occurs according to Equation 1. The hydrochloric acid, together with the calcium and phosphorus compounds in the solution, contributes to the ionic strength required to extract the iron into the organic phase. If necessary, the ionic strength can be increased by adding fresh hydrochloric acid and / or other salting-out agents, as indicated by additives 230. These additives can also be a complete or partial recycling of one or more streams with high ionic strength obtained from other process steps.

[0233] For example, it was determined experimentally that the iron extraction rate was positively influenced by adding hydrochloric acid solution and / or chloride salts to the aqueous phase. Table 8 shows how the stripping product after the first solvent extraction and stripping steps can be further concentrated and purified in a subsequent solvent extraction step. The data show that the amount of calcium, phosphorus, and minor impurities such as aluminum, copper, and zinc is significantly reduced using the subsequent extraction-stripping step. Here, two extraction-stripping steps are presented, but the number of steps can be adjusted to the properties and composition of the sludge ash; for example, for ash with a low iron content and / or a high content of co-extractable impurities, more than two steps can be considered.

[0234]

[0235] Table 8 - Concentration and purification of iron from phosphorus, calcium and other co-extracted impurities in a subsequent solvent extraction-stripping step.

[0236] In other words, in one embodiment, the step of extracting iron from the leachate comprises at least two extraction stages. The first extraction stage comprises at least partially selective liquid-liquid extraction of the iron content and subsequent stripping into an intermediate stripping solution. Preferably, a majority of the iron content is extracted, preferably as FeCl. The second extraction stage comprises selective liquid-liquid extraction of a majority of the iron ions from the intermediate stripping solution and subsequent stripping into a second stripping solution. Preferably, substantially only iron is extracted, typically as FeCl. In this manner, a higher iron purity and / or a higher iron concentration in the recovered iron product can be achieved.

[0237] This principle can be further exploited by adding further extraction stages. In other words, the subsequent extraction stages thus comprise at least one further extraction stage to achieve an even higher iron purity and / or a higher iron concentration in the recovered iron product compared to the second stripping solution.

[0238] This is advantageous for solutions with a relatively low iron:phosphorus ratio (e.g., less than 1). This may be the case, for example, in applications where the step of dissolving the starting material is performed at ambient temperature using an acid having a hydrochloric acid concentration of less than 3M. Preferably, the first extraction stage comprises matching the loading capacity of the organic solvent to the Fe(III) content in the leachate by controlling the amount of the organic solvent relative to the leachate. Also preferably, the second extraction stage and / or any additional extraction stage comprises controlling the loading capacity of the organic solvent to be near or below the Fe(III) content in the first stripping solution by controlling the amount of the organic solvent relative to the first stripping solution. If the loading capacity is slightly higher than the Fe(III), some contaminants may be co-extracted in higher amounts. In many cases, such contamination is acceptable, or they can be removed at a later stage. However, the amount of contaminants generally increases with increasing loading capacity. Preferably, the raffinate from the liquid-liquid extraction of the second extraction stage and / or any additional extraction stage is recycled for use in a subsequent step of dissolving the starting material.

[0239] It was determined experimentally that co-extracted phosphorus can also be selectively washed from the iron by treating the loaded organic phase in a liquid-liquid extraction with at least slightly concentrated hydrochloric acid. Figure 14 A general embodiment for washing impurities from an iron-laden organic phase is shown. Scrubber 81 is configured to wash co-extracted impurities from a loaded organic phase 222B using a wash solution 223. Loaded wash solution 223B and a washed organic phase 222C are thereby formed.

[0240] Selectivity was found to be higher when hydrochloric acid was used instead of water to wash the loaded organic phase 222B. Selectivity increased with increasing hydrochloric acid concentration. For example, a loaded organic phase containing 16.6 g / L iron and 2.90 g / L phosphorus was washed with a 15% hydrochloric acid solution at an O:A ratio of 10:1. This removed 0.5% of the iron and 54% of the phosphorus in one stage. Calcium and copper, as well as some co-extracted arsenic, cadmium, and zinc, were also efficiently washed away. Washing at an O:A ratio of 20:1 instead of 10:1 also improved selectivity, but increasing the hydrochloric acid concentration was determined to be more important.

[0241] However, due to the extraction of hydrochloric acid during the washing, stripping with 15% hydrochloric acid after washing becomes less efficient than stripping without washing. As mentioned earlier, the efficiency of the stripping process is determined, among other things, by the amount of acid in the stripping solution. High acidity and / or ionic strength promotes the extraction of iron by TBP, which means that once sufficiently high acidity and / or ionic strength are present in the stripping product, stripping becomes less effective. Therefore, due to the aforementioned extraction with hydrochloric acid, washing away impurities with hydrochloric acid will produce a more acidic stripping product. The washed organic phase 222C may still contain, for example, some phosphorus and calcium, but typically at lower concentrations.

[0242] The loaded wash liquor 223B contains mostly Ca and P, but as mentioned above, typically also contains other impurities. The phosphorus content itself is typically considered valuable, and since the loaded wash liquor 223B preferably also contains hydrochloric acid, it is preferred to return the loaded wash liquor 223B to the process at some stage. For example, the loaded wash liquor 223B can be used during leaching or it can be used to increase the ionic strength of the leachate.

[0243] Determine by experiment that water stripping of iron is more preferred than hydrochloric acid solution because it causes higher stripping rate.Also can use alkaline solution, as sodium hydroxide, potassium hydroxide or other soluble alkali.The inventors determine that stripping with 0.5M sodium hydroxide solution produces slightly better stripping rate than water.However, need to select O: A ratio and alkali concentration accordingly, to prevent that the pH value of gained stripping product is too high and cause ferric hydroxide precipitation.Document (for example Sato T., 2002.Liquid-liquid extraction of iron (III) from hydrochloric acid solutions bytributyl phosphate [using tributyl phosphate from hydrochloric acid solution liquid-liquid extraction iron (III)], Shingen-to-Sozai118,612-616) reports that the efficiency of using TBP to extract iron from hydrochloric acid decreases with increasing temperature.This shows that during stripping, higher temperature can cause the concentration of iron in stripping product to be higher. However, surprisingly, it was experimentally determined that the iron concentration in the aqueous stripping product decreases with increasing temperature, and this trend is linear for the temperatures studied (0° C.-70° C.) Preferably, stripping is performed at lower temperatures to ensure higher iron concentrations in the stripping product.

[0244] One of the main applications of ferric chloride is as a coagulant in wastewater treatment plants. According to equation (1), the ferric chloride product will also contain a large amount of free hydrochloric acid, at least a 1:1 molar ratio of acid: iron (taking into account that TBP can also extract hydrochloric acid in the absence of iron). Typically, the free hydrochloric acid content in commercial ferric chloride coagulants ranges from 0% to 2%. For these reasons, it may be desirable to neutralize some of the free acid in the iron product. According to embodiments of the present technology, this can be done in several ways. Neutralization can be done using conventional bases like sodium hydroxide, calcium hydroxide, or equivalents. However, this will cause the final iron solution to be contaminated with sodium, calcium, etc., which may or may not be accepted by the end user.

[0245] Therefore, in order to produce a high-purity ferric chloride coagulant, it is preferred to neutralize with ferric oxide, ferric hydroxide, ferric oxyhydroxide and / or metallic iron or a material containing at least one of these (e.g., magnetite). Ferric oxide, ferrous oxide, or mixed oxides can be used. Depending on the chemical used for neutralization, the final ferric chloride product may contain ferric chloride, ferrous chloride, or a mixture of the two. Experiments have shown that neutralization with metallic iron powder or ferric oxide is significantly slower at ambient temperature than at high temperatures. Adding metallic iron to a hydrochloric acid solution containing dissolved ferric iron and other impurities results in two main chemical processes. The first process is the reduction of the substance, for example, ferric iron is reduced to ferrous iron, or divalent copper is reduced to metallic copper, which precipitates the metallic copper. The second process is the reaction of hydrochloric acid with the metallic iron, wherein hydrogen is produced, i.e., neutralization. When the reduced metallic iron powder is added to the iron-containing acidic stripping product, the inventors observed that the reduction of ferric iron to ferrous iron occurs prior to neutralization with hydrochloric acid. This is most evident at lower temperatures, which are not conducive to the latter process. These processes were studied at ambient temperature (21°C) and high temperature (60°C-95°C) using an acidic ferric chloride product with hydrochloric acid obtained after solvent extraction with TBP. The stoichiometric amount of metallic iron required to completely reduce the iron and neutralize the hydrochloric acid was added to the solution. In all cases, reduction was significantly faster than neutralization. Reduction occurred in just a few minutes, as indicated by a color change from the yellow characteristic of trivalent iron to the light green-blue characteristic of divalent iron. Under ambient conditions, hydrogen gas production was not significant despite the metallic iron being stirred with the leachate for several hours. Even after 24 hours, much of the metallic iron added remained undissolved. Only small bubbles were seen to slowly form. Even after 68 hours, the pH remained acidic, with much of the metallic iron added still undissolved. At high temperatures, intense bubbling (i.e., hydrogen production) was evident, indicating that the neutralization of the acid was more effective. However, at 60°C, neutralization was much slower than reduction. The pH of the solution was still below 0, and iron powder was present in the solution after 1.5 hours. At 95°C, the reaction was significantly faster and most of the iron was dissolved within 4 hours, at which time the pH of the solution increased to pH 2.

[0246] Neutralizing free hydrochloric acid with metallic iron is also an effective method for minimizing the amount of impurities in the iron product, particularly those that can also be reduced and are insoluble in their reduced form, such as copper. Neutralization with iron hydroxide or iron oxyhydroxide has advantages, particularly the rapid reaction at ambient temperature without the generation of hydrogen. The advantage of using iron compounds to neutralize the free acid is that the chemical reaction of these compounds with the solution components increases the iron content in the final product.

[0247] One embodiment of an apparatus 24 for neutralizing a ferric chloride product comprising hydrochloric acid is schematically shown in FIG. Figure 15 The ferric chloride product containing hydrochloric acid can be, for example Figure 12The liquid 210A comprising iron, but can also be the product of other embodiments described further below. The liquid 210A comprising iron is used as a typical example here. The liquid 210A comprising iron is introduced into the neutralization chamber 83 together with a neutralizing agent 231 (such as metallic iron, iron oxide, iron hydroxide and / or ferric oxyhydroxide). Neutralizing agent 231 neutralizes at least a portion of the hydrochloric acid of the liquid 210A comprising iron, and if reducing conditions are produced, then typically at least a portion of ferric iron is also reduced to ferrous iron. Preferably, there is a heater 85 so that it can be neutralized at high temperatures. Separating device 84 is provided to separate any remaining solid component 232, such as undissolved metallic iron or metal Cu pollutants. Such remaining solid component 232 can re-enter the method, such as enter a dissolution reactor, or be disposed of. Gained liquid 233 is at least partially neutralized, and the concentration of hydrochloric acid is reduced.

[0248] Optionally, if a ferric iron product is desired, the resulting liquid 233 can be passed into an oxidation vessel 86, into which an oxidant 234 is also passed. The oxidant 234 can be, for example, hydrogen peroxide, ozone, chlorine, etc. A final ferric iron product 235 is produced.

[0249] Represented by a flow chart, such as Figure 16

[0014] Embodiments of the method for chemically treating sewage sludge ash may include step S26, wherein the ferric chloride product comprising hydrochloric acid is at least partially neutralized by adding a neutralizing agent. Step S26 follows step S21. In optional step S27, any ferrous iron is oxidized to ferric iron using an oxidizing agent.

[0250] In short, neutralization chemicals and / or conditions can be selected to obtain iron in a desired chemical form, such as reduced, oxidized, or a mixture of these two forms. The chemical form of the iron can then be changed. For example, if ferric chloride is desired, any ferrous iron present can be oxidized using an oxidizing agent such as chlorine. The reduction of ferric iron to ferrous iron can be carried out using a reducing agent such as metallic iron.

[0251] As further considered above, the dissolution (also referred to as leaching) of sewage sludge ash can be performed under different conditions, resulting in leachates of different compositions. If low concentration hydrochloric acid is used at ambient temperature, the degree of solubilization of iron is generally very low. As mentioned above, this low iron content has been extracted by purifying and concentrating the iron via a preferably continuous two-stage liquid-liquid extraction. Another approach can alternatively ignore the amount of leached iron and focus only on extracting phosphorus and possibly other components. One way to control the extraction of iron from the organic phase is to utilize the different oxidation states of iron.

[0252] All recovery steps relating to iron presented here can be controlled by the chemical form of iron in the control solution (for example, ferric iron, ferrous iron or the mixture of the two).Determine by experiment, control iron species to form the amount of the iron that has been controlled in the iron extraction step and in the phosphorus extraction step, repelled or extracted by TBP, and the pH that precipitates the iron compound.For example, ferrous iron species (for example, ferrous hydroxide and ferrous phosphate) precipitate under pH (higher than 6) significantly higher than their iron equivalents (pH lower than 3).Control and or change the chemical form of iron in the solution and can be carried out at the different points of the method, for example, dissolving, pre-treatment, different solvent extraction sections (extraction, stripping, washing), recycling and recirculation loop etc., and are not limited to only one step.For example, the oxidation or reduction of iron can be carried out several times and / or can alternately be carried out during the process loop in the whole method.For example, during several loops, ferrous iron can be more highly concentrated in the aqueous solution, is subsequently oxidized and extracted in subsequent loops.This makes it possible to efficiently reclaim iron from ash or leachate with low iron content.

[0253] The present inventors recognized that reducing ferric iron in aqueous streams is a viable method for reducing the amount of iron extracted by TBP. For example, a hydrochloric acid sludge ash leachate containing 0.1 M ferric iron, 0.7 M phosphoric acid, and 2.7 M calcium chloride was stirred with an equivalent of 5.6 g / L metallic iron powder (0.1 mol) at 21°C for 10 minutes. The leachate changed color from intense yellow to light bluish-green within a few minutes, indicating the reduction of ferric iron to ferrous iron. Gas evolution was not visible during this period, and over 60% of the added iron powder remained undissolved. The solution was filtered. Solvent experiments were performed with undiluted TBP at a 1:1 organic:water ratio. In parallel, identical solvent extraction experiments were performed with the unreduced leachate. Analysis of the aqueous raffinate of the reduced sample revealed that the majority of the iron (>92%) remained in solution and was not extracted by TBP. In contrast, 85% of the ferric iron in the unreduced sample was extracted. Extraction with phosphoric acid was similar for both solutions. About 45% of the acid was extracted by TBP. The small amount of iron extracted from the reduced sample was due to incomplete reduction of the ferric iron or reoxidation of the ferrous iron back to ferric iron during the solvent extraction batch test. In order to achieve good mixing of the aqueous and organic phases, the solution was vigorously mixed for 5 minutes under ambient conditions. It is believed that the presence of oxygen in the air and the vigorous mixing may lead to reoxidation of the ferrous iron. In practice, this would not be the case, as mixing can be performed for significantly less time and the oxygen level in the atmosphere can be controlled, for example, by performing the extraction under a nitrogen blanket.

[0254] Iron reduction can be used to minimize the amount of iron that is co-extracted with phosphoric acid in the phosphorus extraction step. Additionally, this can be used to bypass the iron extraction step entirely. Figure 17In this embodiment, iron is reduced in step S17 of the pretreatment step S14, which is performed during and / or after the dissolution (i.e., step S10), but before the phosphoric acid extraction step S30. In this embodiment, step S20 does not include significant iron extraction, but does include phosphorus extraction S30.

[0255] The iron in the raffinate after the phosphorus recovery step S30 can be reoxidized and further recovered, for example, using solvent extraction with TBP. This can be done at various steps in the process after the phosphoric acid extraction, for example, before or after aluminum precipitation, as will be discussed further below. Ferrous iron allows for selective precipitation of aluminum (leaving the iron in solution) at pH below 3 because ferrous hydroxide precipitates at a higher pH than ferric hydroxide, which would otherwise precipitate with the aluminum.

[0256] In a similar manner, oxidation of the iron can be performed with an oxidizing agent such as hydrogen peroxide, ozone, chlorine or other substances to promote its extraction by TBP and / or its precipitation at a lower pH in the form of, for example, iron hydroxide or iron phosphate. This can be performed at different stages of the process, similar to those described above. In one embodiment, it is performed as a pre-treatment step S14 ( Figure 1 ) in step S18( Figure 1 ) is performed. For example, what is found out is that for ash and leachate with ferrous iron, it is invalid to extract with TBP, and ferrous iron is finally present in the raffinate. By oxidizing ferrous iron to ferric iron, making it possible to extract with TBP, the iron recovery rate can be maximized. In the actual experiment of the sludge ash also containing ferrous iron, some iron (0.6g / L) in the leachate can not be fully extracted by TBP and retained in the raffinate. Follow-up can not be extracted with fresh TBP extraction. Add a small amount of hydrogen peroxide that ferrous iron is oxidized to ferric iron, and make it possible to fully reclaim with TBP, maximize the iron recovery efficiency in the method.

[0257] As further discussed above, the dissolution (also known as leaching) of sewage sludge ash can be performed under different conditions, resulting in leachates of varying compositions. The degree of iron solubilization is generally higher if more concentrated hydrochloric acid is used and / or the temperature is increased and / or a longer leaching time is employed. Consequently, the iron:phosphorus ratio becomes higher than, for example, that obtained with more dilute hydrochloric acid leaching. This allows for a higher recovery of these elements from the sludge ash and the production of a cleaner, iron-depleted silicate residue.

[0258] It has been found that due to the high iron content in such leachate streams, a first solvent extraction step can directly produce a relatively pure ferric chloride product because the extraction is performed at a higher O:A ratio at a temperature close to the iron loading capacity of the organic phase, which results in less phosphorus and other impurities in the stripped product. Generally, no second extraction step is required to purify the iron from the phosphorus and concentrate the iron.

[0259] Figure 18 An embodiment of the extractor section 20 is shown that includes one solvent extractor 220A. If the organic phase:aqueous phase (O:A) ratio and / or the content of TBP in the organic phase is controlled to suit the available leachate composition, then in this embodiment, the resulting liquor 210 comprising iron and / or phosphorus can comprise more or less only iron, even after a single extraction with or without washing.

[0260] In other words, in one embodiment, the step of dissolving the starting material is performed with an acid having a hydrochloric acid concentration greater than 3 M. Preferably, the hydrochloric acid concentration is greater than 6 M. The step of extracting iron from the leachate further comprises controlling the loading capacity of the organic solvent to be around or below the Fe(III) content in the leachate by controlling the amount of the organic solvent relative to the leachate.

[0261] In a preferred embodiment, the step of dissolving the starting material is performed at an elevated temperature to enhance the dissolution of the iron.

[0262] As mentioned above, it is also possible to perform continuous leaching to obtain leachates with different iron:phosphorus ratios. Figure 19 This continuous two-stage solution is schematically illustrated. A first-stage facility 1A for chemically treating sewage sludge ash is supplied with a starting material 201A consisting of sewage sludge ash. Leaching, i.e., dissolution, is performed using low-concentration hydrochloric acid 202A. The first-stage facility 1A for chemically treating sewage sludge ash outputs a liquid 210A containing phosphorus and possibly other liquids 213A-218A containing recovered elements. An undissolved residue 204A is output. A second-stage facility 1B for chemically treating sewage sludge ash is supplied with a starting material 201B consisting of the undissolved residue 204A from the first-stage facility 1A for chemically treating sewage sludge ash, which is derived from sewage sludge ash. Leaching, i.e., dissolution, is performed using high-concentration hydrochloric acid 202B, preferably at high temperature. The second-stage facility 1B for chemically treating sewage sludge ash outputs a liquid 210B containing iron and possibly other liquids 213A-218A containing recovered elements. The undissolved residue 204B is output. Preferably, at least a portion of the raffinates 219A and 219B of the two stages is recycled for use in a different part of the process, preferably for dissolving the starting material, for example in the first stage apparatus 1 A. It is even possible to use some portion of the raffinates 219A and 219B of the two stages in the second stage apparatus 1 B, as indicated by the dotted line.

[0263] Thus, each stage constitutes a complete processing entity in itself. The two stages can be performed in time and / or space relative to each other, but can also be separate. For example, the same device can be used for both stages, but of course separated in time.

[0264] Figure 20 A flow chart illustrating an embodiment of a two-stage process is shown. In the first stage (S98), sewage sludge ash is treated by leaching using low-concentration HCl. This treatment produces a solution of recovered phosphorus and a first amount of undissolved residue. The first undissolved residue contains a non-negligible amount of iron. In the second stage (S99), the first undissolved residue is used as starting material and dissolved using high-concentration HCl. This treatment produces a solution of recovered iron and a second amount of undissolved residue.

[0265] Since phosphorus is a valuable component, the preferred next step according to the technology presented here is to separate the phosphorus from the other components of the leachate, which are typically calcium, aluminum, magnesium, sodium, potassium and heavy metals. Several extractants suitable for extracting phosphoric acid have been proposed in the literature. These can generally be divided into the following groups: 1) alkyl phosphates, such as tributyl phosphate (TBP), 2) amines, such as tri-n-octylamine, 3) alcohols, such as isopentanol, n-pentanol, cyclohexanol, methylcyclohexanol, tert-pentanol, isobutanol, n-butanol, heptanol, 4) ketones, such as methyl isobutyl ketone, methyl propyl ketone, diethyl ketone, methyl ethyl ketone, methyl n-butyl ketone, 5) amides, such as butylacetamide, 6) aldehydes, such as benzaldehyde, 7) esters, such as ethyl acetate, butyl acetate, amyl acetate, cyclohexanone, 8) ethers, such as diethyl ether, di-n-pentyl ether, and glycol ethers, such as diethylene glycol. Many of these extractants are slightly soluble in aqueous solution, particularly in the presence of acids, salts and / or other chemicals, and some are more soluble than others. This can lead to contamination of the aqueous stream and the final product with trace amounts of extractant. These are undesirable, especially considering the purpose of using separated phosphorus as fertilizer. High water solubility generally requires distillation to recover the dissolved extractant from the aqueous stream, which is expensive and complicated. For example, n-butanol has a water solubility of about 90 g / L at room temperature. Several of the mentioned solvents (e.g., diisopropyl ether) have other disadvantages besides high solubility, such as flammability and / or explosion risk. In addition, several of the mentioned solvents show limited extraction capacity for phosphoric acid when below a certain threshold concentration. This means that the feed solution must have a high concentration of phosphoric acid, which generally requires the acid to be concentrated by water evaporation. In addition, phosphoric acid can only be partially extracted with such solvents. A typical example of a solvent with a significant threshold concentration for phosphoric acid extraction is methyl isobutyl ketone. Typically, ethers, esters, and selected ketones have a significant threshold concentration for phosphoric acid extraction and are therefore unsuitable for extracting phosphates from relatively low concentrations (approximately 1.5 M phosphoric acid), which is a problem for many sludge ash leachates. Alcohols with relatively long carbon chains have relatively low water solubility. Therefore, alternative solvents for extracting phosphoric acid are long carbon chain alcohols, such as heptanol, which have a solubility in water of less than 2%. If the solubility falls below this level, the amount of solvent that follows the aqueous stream becomes quite low (for processing with relatively inexpensive and complex equipment). Since industrial experience with long chain alcohols is very limited, and TBP is a standard solvent that has been used on a large scale for over 40 years, the preferred solvent according to the present invention is TBP.

[0266] In other words, in one embodiment wherein the starting material is sewage sludge ash, the process comprises the step of extracting phosphorus in the form of phosphoric acid by liquid-liquid extraction with an organic solvent. Preferably TBP is used as the organic solvent.

[0267] In one embodiment, phosphorus extraction is performed from the raffinate of the iron extraction step. In another embodiment, phosphorus extraction is performed from a leachate in which ferrous iron is present, for example, after a pretreatment step. In yet another embodiment, phosphorus extraction is performed from a leachate having a low iron content, whereby this level of iron can be accepted in the final phosphorus product, or the iron contaminant can be removed in a later stage.

[0268] There is very limited data in the literature on the extraction of phosphoric acid from chloride systems using TBP. The main references include the following articles: 1) Habashi et al., 1987. “The hydrochloric acid route for phosphate rock.” Journal of Chemical Technology and Biotechnology, 1987, Vol. 38; 2) Naito and Suzuki, 1961. “The mechanism of the extraction of several proton acids by tri-n-butyl phosphate.” Japan Atomic Energy Research Institute; 3) Jin et al., 2010. “Liquid-Liquid Equilibrium in the System Phosphoric Acid / Water / Tri-n-butyl Phosphate / Calcium Chloride[Liquid-liquid equilibrium of the system phosphoric acid / water / tri-n-butyl phosphate / calcium chloride]." J.Chem.Eng.Data[Journal of Chemical Engineering Data] 55, 3196-3199; 4) Fernando Pereira, 2013Doctoral Thesis.[Doctoral Thesis] Ecole Nationale S uperieure Des Minees De S aint-Etienne.[Saint-Étienne National Higher School of Mines] "Production of acid phosphoriquep ar a ttaquechlorhydrique de minerais phosphatés a vec réductiondes nuisances en environmentalnementales et récupération des terresrares in tant quesous-products"; 5) Pereira and Bilal, 2013, "Phosphoric acid extraction and rare earth recovery from apatites of the Brazilian phosphatic ores.[Extraction of phosphoric acid and recovery of rare earth elements from apatite in Brazilian phosphate ore]” Romanian Journal of Mineral Deposits, 2012, 85(2), pp. 49-52; and 6) Jin et al., 2015. “Extraction kinetics of phosphoric acid from the phosphoric acid-calcium chloride solution by tri-n-butyl phosphate.” Industrial & Engineering Chemistry Research. January 2015.

[0269] Tributyl phosphate enables the extraction of phosphoric acid in preference to dissolved salts such as chlorides and in preference to dissolved acids such as hydrochloric acid or sulfuric acid. The presence of dissolved salts or acids enhances the extraction of phosphoric acid by a salting-out mechanism. Tributyl phosphate extracts nitric acid in preference to phosphoric acid, which makes it impossible to selectively extract phosphoric acid from nitric acid. Therefore, leaching sludge ash with hydrochloric acid is preferred over nitric acid. Generally, solvents with low solubility in water extract nitric acid in preference to phosphoric acid. Solvents with increased selectivity for phosphoric acid, such as amyl alcohol, have high water solubility and still co-extract considerable amounts of nitric acid. The main advantage of using TBP as a solvent for extracting phosphoric acid is the low water solubility, which enables operation without the need for distillation of the solvent from the aqueous stream, which is expensive and complicated.

[0270] As mentioned above, the extraction of phosphoric acid with TBP is typically carried out after the extraction of iron. However, it can also be carried out when there is iron in the solution. If there is ferric iron in the solution, then it will be extracted by TBP together with phosphoric acid. If it is believed that the amount of the iron extracted in the phosphorus product is acceptable and / or if subsequent removal of iron from the phosphorus stream by solvent extraction, ion exchange, selective precipitation or other methods, then this is suitable selection. If as mentioned above before the extraction of phosphorus iron is reduced to ferrous iron, then its extraction efficiency is lower, and ferrous iron will eventually be present in the raffinate that enters the next process step.

[0271] The literature describes the near-complete extraction of phosphoric acid using TBP and a background level of approximately 3 M calcium chloride. For example, Habashi et al. (1987) stated that over 99% of phosphoric acid could be extracted at a 1:1 O:A ratio using a 3-stage extraction. Pereira (2013) repeated the same statement.

[0272] However, the present applicants have surprisingly found that it is not possible to completely extract phosphoric acid from sewage sludge ash leachate using TBP under the above conditions. The phosphoric acid concentration can only be reduced from about 0.7M to about 0.15M. Figure 21 This corresponds to a maximum extraction rate of about 78.5%. The graph in the figure shows the extraction of phosphoric acid from a solution obtained by leaching sewage sludge ash with 3 M hydrochloric acid after the extraction of iron. About 2.7 M calcium chloride was present in the background. The phosphorus content in the solution was 21 g / LP, i.e. about 0.7 M phosphoric acid. The organic phase was TBP. Moreover, the loading capacity of the TBP solvent reported by, for example, Habashi et al. is not possible. Habashi et al. reported the loading of 3 M phosphoric acid in TBP in contact with 0.7 M phosphoric acid in apatite leachate. As shown from Figure 21 As can be seen in Figure 2, this is obviously impossible, wherein TBP only loads about 0.6M phosphoric acid (slightly higher than 18g / L phosphorus). This makes it complicated to use conventional water back extraction to reclaim phosphoric acid from the organic phase of the load. In this case, the back extraction product obtained will have a relatively low concentration. A large amount of water will have to be evaporated to produce concentrated phosphoric acid solution.

[0273] Although technically possible, this negatively impacts the economics of the process. It is therefore preferred to mitigate this disadvantage of phosphorus extraction using the method described in one of the applicant's prior inventions disclosed in International Patent Application Publication No. WO 2010138045 A1. Figure 22 An extractor section 20 is shown having an apparatus 22D for liquid-liquid extraction of phosphorus with an organic solvent. Apparatus 22D for liquid-liquid extraction of phosphorus is configured to extract phosphorus from a leachate or from a raffinate from an apparatus for liquid-liquid extraction of iron. A stripping solution 209D containing saturated monoammonium phosphate (MAP) solution is added, producing a supply of liquid 210D containing phosphorus and phosphoric acid in the form of MAP. At least a portion of the phosphorus-depleted raffinate 211 is provided from extractor section 20.

[0274] In further detail, according to International Patent Application Publication No. WO 2010138045 A1, the TBP organic phase containing phosphoric acid can be washed with, for example, water at a high O:A ratio to remove some of the co-extracted calcium and other unwanted impurities. The washed product is returned to one of the previous process steps. The organic phase containing phosphoric acid is stripped with saturated MAP solution 209D as mentioned. The stripped product 210D, containing phosphoric acid in saturated MAP, is treated with diammonium phosphate (DAP) solution 240 in reactor 30. This converts the phosphoric acid into MAP, allowing it to precipitate from the now supersaturated solution. The volume of saturated MAP solution 209D is increased by the addition of DAP. Excess MAP solution is removed and converted to DAP 240 with ammonia 241 in chamber 32 and reused in the process. The remaining saturated MAP solution 209D is reused for stripping.

[0275] In other words, in a preferred embodiment, the method for chemically treating sewage sludge ash comprises an additional step of back-extracting the organic solvent in the step of extracting phosphorus with a saturated aqueous monoammonium phosphate solution.

[0276] In a preferred embodiment, the method for chemically treating sewage sludge ash comprises, prior to the step of stripping the organic solvent in the step of extracting phosphorus, the additional steps of washing the organic solvent in the step of extracting phosphorus with water into an aqueous washing solution, and returning the aqueous washing solution to the raffinate of the step of extracting iron.

[0277] In another embodiment, the process for chemically treating sewage sludge ash includes the additional step of stripping the organic solvent in the step of extracting phosphorus with water to produce a phosphoric acid solution.

[0278] Using MAP as the stripping solution allows for direct recovery of phosphorus as solid MAP, the most desirable phosphate fertilizer product. This bypasses the need for conventional water stripping of phosphoric acid, which requires concentrating the dilute phosphoric acid via evaporation and converting the concentrated phosphoric acid to MAP. Furthermore, the process allows for efficient processing of organic phases laden with low levels of phosphoric acid, a significant advantage that enables the processing of sewage sludge ash with low phosphorus content.

[0279] However, further improvements to the technology presented in International Patent Application Publication No. WO 2010138045 A1 are preferred. One aspect is the treatment of co-extracted impurities, particularly heavy metals. Co-extracted metal ions are stripped with phosphoric acid and ultimately present in the MAP strip solution. It was determined that several of these impurities also co-precipitate with the MAP product, reducing its quality (see Table 9). Iron, aluminum, barium, chromium, and molybdenum precipitate effectively with the solid MAP product. Calcium and strontium precipitate to a relatively large extent. It is desirable to remove such impurities from the system before MAP precipitation. This is also important because saturated MAP solution is recirculated in the system and used for subsequent stripping, which contributes to the accumulation of impurities in each circuit.

[0280] Impurities can be handled at various steps in the aqueous MAP-DAP circuit, such as Figure 23 Impurity removal device 34 can be implemented at one or more of the designated locations, for example, after stripping, after treatment with DAP in the resulting saturated MAP solution, in the common stream or in any of the split streams or in the DAP solution 240. This is demonstrated by the experimental results described below. One embodiment of such impurity removal device 34 is shown in FIG. Figure 24 By adding a suitable sulfur-containing compound 242 to the stream, for example, into a mixing chamber 35, heavy metals can be precipitated in the form of low-soluble sulfides. The sulfur-containing compound 242 can be, for example, a metal hydrosulfide, a metal sulfide, an organic sulfide, or any mixture of these. Using a solid / liquid separation device 36, such as filtration, centrifugation, sedimentation, clarification, or equivalent treatment, the heavy metal precipitate 243 is removed from the stream. This produces a stream 244 with a lower heavy metal concentration. If desired, a sulfur scavenger compound 245 can be used to remove any residual sulfur species remaining in the solution. The sulfur scavenger compound is added to a desulfurization chamber 37. The sulfur scavenger compound 245 combines with the sulfur to produce a phase 246 that is easily removed, such as a solid that can be removed using a separator device 38, such as according to one of the aforementioned solid-liquid separation techniques, leaving a solution 247 that is depleted in heavy metals and sulfur. Phase 246 can also be a separate layer at the top or bottom that can be separated from the stream by a suitable separation technique.

[0281]

[0282] Table 9 - Precipitation of impurity elements from phosphoric acid loaded MAP strip product during precipitation of solid MAP by addition of DAP to the stream

[0283] Additionally or alternatively, heavy metals may be removed using ion exchange, preferably using an ion exchange resin that is capable of selectively binding heavy metals, such as a mercaptan-based ion exchanger or other sulfur-containing resin with a high affinity for heavy metal ions. Figure 23The embodiment presented in the present invention performs heavy metal removal using ion exchange treatment. A contaminated MAP or DAP stream enters a first ion exchange unit 39A. An added ion exchange resin 248 removes heavy metals from the raw MAP or DAP stream, which then exits as a heavy metal-depleted solution 250. Because some streams in the MAP-DAP loop have a high pH, ​​mercaptan resins can be used without the risk of severe degradation by oxidation of mercaptan functional groups to disulfides, which would hinder the process. Furthermore, since the content of heavy metals in these streams is typically low (ppm levels), saturation is a very small problem because the loading capacity is not reached quickly. Saturated resins containing small amounts of heavy metals can be disposed of in an appropriate manner. Alternatively, the resin can be regenerated using an appropriate solution (e.g., an acidic solution) to recover small amounts of heavy metals in a very concentrated form. The heavy metal-laden ion exchange resin 249 then enters a second ion exchange unit 39B. Eluent 253 is added to remove heavy metals from the ion exchange resin, and the depleted ion exchange resin 251 can be returned to a new ion exchange operation. This is preferably done after the eluent 253 has been completely removed from the resin, for example by washing with water. The heavy metal-laden eluate 252 is removed.

[0284] Table 9 shows the impurity concentrations in the relatively low-purity MAP strip product obtained after extraction and stripping of phosphoric acid. The strip product had an acidic pH of approximately 1.8. When a solid MAP product was produced by adding DAP to the stream, several impurity elements precipitated with the solid MAP product (Table 9). Impurity precipitation with sodium hydrosulfide was tested at different points in the MAP-DAP circuit (MAP loaded with phosphoric acid, MAP filtrate after MAP precipitation, and DAP solution). Direct precipitation of copper, molybdenum, and arsenic as metal sulfides from the phosphoric acid-loaded MAP strip product was effective, but most zinc remained in solution. Sulfide precipitation of heavy metals remaining in solution after precipitation of the solid MAP product but before DAP production, followed by DAP production and filtration, resulted in precipitation of most impurities, including zinc, copper, and arsenic, and relatively rapid filtration (Table 10). Sulfide precipitation at pH 3.8 prevented residual sulfide from remaining in solution. At pH above 8, precipitation of zinc and copper from the DAP solution using sulfide was possible, but arsenic precipitation was significantly less efficient (Table 10). Filtration is more difficult. In addition, residual sulfide remains dissolved in solution, which can cause problems in the MAP precipitation stage. By raising the pH of the MAP filtrate with ammonia, zinc, calcium, and cadmium can be precipitated in the DAP production step without adding sulfide, but arsenic and copper cannot be precipitated in this way.

[0285] The generation of hydrogen sulfide gas is a problem when precipitation is carried out with metal hydrosulfides or metal sulfides at lower pH. This can be prevented by using organic sulfides. The precipitation of impurity metals was also tested with two commercial organic sulfides. This was carried out in the acid stream, the loaded MAP strip product and the filtrate after solid MAP precipitation and compared with precipitation with sodium hydrosulfide. Heavy metals (especially arsenic, zinc and copper) were precipitated more efficiently from the filtrate after solid MAP precipitation than from a MAP solution loaded with phosphoric acid. The precipitation with one of the tested organic sulfides was slightly more efficient than the precipitation with sodium hydrosulfide. However, when using organic sulfides, residual sulfide remained in the solution, which was not seen when using sodium hydrosulfide. In this case, a scavenger for the residual sulfide can be used to remove it from the solution.

[0286]

[0287]

[0288] Table 10 - Precipitation of impurity elements from MAP filtrate with sodium hydrosulfide under the following circumstances: a) after precipitation of solid MAP product (pH 3.8); b) after precipitation of solid MAP product (pH 3.8), but then raising the pH to produce DAP; c) sodium hydrosulfide was added to the solution after DAP production (pH 8.1).

[0289] In summary, phosphorus recovery is preferably carried out using TBP solvent extraction, which is mixed with a suitable diluent and / or modifier, or used undiluted. Phosphorus is extracted by TBP in the form of phosphoric acid. Washing the organic phase to remove co-extracted impurities such as calcium can be carried out with a suitable detergent (water, dilute acid solution, etc.) at a high O:A ratio. The washed product can be reused in the method, for example in a leaching step, at the beginning of a solvent extraction step, or in other steps. In conventional industrial operations, the loaded TBP is stripped with water to produce a dilute phosphoric acid solution, which is then concentrated using evaporation, which is energy-consuming and expensive. However, for the method described herein, it is preferred to produce solid monoammonium phosphate (MAP) directly from the loaded TBP phase. This is performed by stripping the phosphoric acid with a saturated MAP solution and subsequently treating the MAP-phosphoric acid stripping product. Several further preferred steps have been developed to remove impurities, including heavy metals that are co-extracted and stripped with the phosphoric acid. These steps include using metals and / or organic sulfides at different points in the MAP-DAP method to selectively precipitate the impurities. The removal of impurities ensures that the MAP product recovered from the sewage sludge ash is of high purity and that impurities do not accumulate in the process stream during MAP production.

[0290] Three specific aspects of the extraction process of the present invention are interrelated: providing the high ionic strength required for effective extraction of iron and phosphorus; ensuring a good water balance in the process; and maximizing product recovery while minimizing emissions. To effectively extract iron and phosphorus with TBP, a sufficiently high ionic strength is required in the aqueous stream (i.e., the leachate). This is achieved by having sufficient hydrochloric acid and / or other chloride ion sources with a salting-out effect in solution. The present method relies on having free hydrochloric acid and / or dissolved calcium chloride in the leachate, but the method is not limited to the use of only these two chemicals. The ionic strength can be provided by any chloride salt or mixture of salts, preferably soluble components of the ash that are not extracted by TBP (calcium, aluminum, magnesium, sodium, potassium, etc.). Even salts of elements not found in sewage sludge ash can be used. Using calcium chloride has several advantages. Calcium is a major component of sewage sludge ash and is easily leached even with dilute hydrochloric acid. TBP extracts very little calcium, which ensures that most of the calcium present in the first leachate is ultimately present in the raffinate after phosphorus extraction. The majority of this raffinate is returned to the leaching step where additional calcium is leached. The reuse and recycling of the raffinate and the continuous leaching of calcium ensure the required ionic strength for the extraction of iron and phosphorus, minimizing the need to add an external calcium source to ensure this. In order to maintain water balance and have an outlet for calcium and other leached elements, an aqueous effluent is taken from the recirculation loop for further processing. The remainder of the stream is recycled back to the leaching step where the sludge ash component is leached, further increasing the concentration of elements already present in this solution. The step of taking the effluent can vary; for example, it can be after phosphorus recovery, after aluminum recovery, or at any later step. The effluent is treated to recover the components in the solution, typically in the form of solid individual compounds. The following is a general description of the treatment of the effluent taken after iron and phosphorus recovery.

[0291] The raffinate after extraction of iron and phosphorus will typically contain, in addition to hydrochloric acid, some phosphorus that was not extracted in the previous steps as well as calcium, aluminum, magnesium, heavy metals, sodium and potassium. The pH of the raffinate is typically low, below pH 1.

[0292] Aluminium is typically present in sewage sludge ash and its concentration may gradually increase due to the recycling of process liquids. Preferably, the method for chemically treating sewage sludge ash comprises the further step of recovering aluminium from at least one of the at least partially iron-depleted solution and the bleed solution from the at least partially iron-depleted solution.

[0293] Recovery of the aluminium is preferably carried out by increasing the pH of the solution following the phosphorus recovery step with a suitable compound (eg metal carbonates, metal oxides, ammonia, metal hydroxides or mixtures of these). Figure 26A flow chart of the steps of an embodiment of step S41 for recovering aluminum is shown. In this embodiment, in step S42 the pH is raised using a compound that allows the introduced metal to be recovered later in the process, for example using calcium carbonate or lime, and the calcium compound is subsequently recovered later in the process.

[0294] Typically, due to the presence of hydrochloric and phosphoric acid, the pH of the feed solution will be low, for example below 1. The nature of the previous process steps affects the recovery of the aluminium.

[0295] The aluminum:phosphorus ratio in the solution and the precipitant used, as well as the precipitation pH and temperature, determine the chemical form of the aluminum precipitate: aluminum phosphate, aluminum hydroxide, calcium aluminum phosphate, or a mixture of these. Temperature plays an important role, influencing the kinetics of the precipitation process as well as both the filtration and washing of the aluminum precipitate. Higher temperatures are preferred because they positively influence both. Tests have shown that aluminum phosphate is completely precipitated with lime at an aluminum:phosphorus molar ratio below 1 at a pH between 1.1 and 1.6. A low aluminum:phosphorus molar ratio (e.g., 0.5) has been found to be beneficial in terms of filtration rate compared to higher aluminum:phosphorus molar ratios (e.g., 1 and 1.5). Aluminum and phosphorus precipitate in the raffinate at higher pH and higher aluminum:phosphorus molar ratios.

[0296] Aluminum precipitation with calcium carbonate is also effective at pH below 2; and filtration of the precipitate is faster than with lime precipitation. A lack of phosphorus leads to the precipitation of aluminum hydroxide. Precipitation of a combination of aluminum phosphate and aluminum hydroxide occurs at high aluminum:phosphorus ratios.

[0297] It has been observed that aluminum compounds can be selectively precipitated from magnesium, calcium, sodium, potassium, and other impurities, and that precipitation can be triggered even at a pH below 2. If sufficient phosphorus is present in the solution, aluminum precipitates as aluminum phosphate, and / or if phosphorus is deficient, aluminum precipitates as aluminum hydroxide. This can be exploited to obtain certain chemical forms of aluminum.

[0298] If desired, the phosphorus content in the solution can be tailored to the aluminum content so that only aluminum phosphate is precipitated or a mixture of aluminum phosphates is produced. For example, by adding calcium carbonate to a raffinate containing an aluminum:phosphorus molar ratio below 1, all of the phosphorus in the solution can be precipitated as a mixture of aluminum and calcium phosphates at a pH above 3. Selectivity can be controlled by controlling the pH. In this case, at a pH below 2, only aluminum phosphate is precipitated, leaving excess phosphorus in solution.

[0299] The phosphorus content in the raffinate can be controlled by controlling the efficiency of the previous solvent extraction step, for example, extracting only the desired fraction of phosphoric acid with TBP. Another alternative, represented by optional step S43, is to meter a phosphorus compound into the raffinate in order to increase the phosphorus concentration in the solution in the event of phosphorus deficiency.

[0300] In other words, in one embodiment, the recovery of aluminum comprises adjusting the pH of at least one of the iron-depleted solution and the effluent solution from the iron-depleted solution to a pH value wherein at least a portion of the aluminum precipitates as at least one of aluminum phosphate, calcium aluminum phosphate, and aluminum hydroxide.

[0301] In step S44, the aluminum precipitate is separated from the solution using a solid-liquid separation step. The aluminum precipitate can preferably be further processed. In an optional intermediate step S45, a calcium source (e.g., lime) is added to the aluminum precipitate. If there is a calcium deficiency in the aluminum precipitate, this step is performed to promote the precipitation of calcium phosphate in the next step. In step S46, the aluminum precipitate is reacted with sodium hydroxide. This produces, for example, a sodium aluminate solution and a precipitate composed of calcium phosphate, which is separated in step S47. The production of sodium aluminate precipitates phosphorus, leaving the aluminum in solution and thus allowing the phosphorus to be returned to the previous process step, ensuring that it is not lost. Therefore, in step S48, the phosphorus is recycled to the previous process step. This stream has a low volume and can be suitable for some of the steps previously described, typically the leaching / dissolution of the sludge ash material.

[0302] Figure 27 An embodiment of an aluminum separation device 41 included in post-processing section 40 is schematically illustrated. Raffinate 211 from iron / phosphorus extraction is passed to aluminum precipitation vessel 42 along with a pH-increasing additive 255 and, optionally, a phosphorus compound. Aluminum precipitates in the form of at least one of aluminum phosphate, calcium aluminum phosphate, and aluminum hydroxide 256, which is separated by solid-liquid separation device 43 to yield an aluminum-depleted solution 254 that is at least partially depleted. The separated aluminum precipitate is preferably passed to an aluminum reactor 44, into which sodium hydroxide 257 and, optionally, a calcium source, is passed. If the precipitate is calcium-deficient, a calcium source, such as lime, may be added. In reactor 44, the aluminum precipitate is converted into a sodium aluminate solution 213 and a phosphorus-containing solid fraction 258 comprising calcium phosphate. Solid-liquid separation device 45 removes the phosphorus-containing solid fraction 258, which is recycled to a previous process step, typically the dissolution of the sludge ash material.

[0303] Preferably, the heavy metal ions are subsequently recovered in the resulting solution at a higher pH. Their recovery is carried out on the main stream originating from the raffinate from the iron / phosphorus extraction (possibly with the aluminum removed) or on the effluent solution therefrom. The removal of heavy metal ions is mainly carried out to avoid contamination in other recovered fractions. However, the recovered heavy metal compounds can also be used as feed material for further purification or treatment (utilizing the actual value of the metal components). A method (such as Figure 28) includes step S51, wherein the pH of the solution is further increased. If phosphorus is present in the solution, this increase in pH promotes the formation of insoluble metal hydroxides and / or phosphates. Similar to the precipitation of aluminum, the amount of phosphorus present in the solution can determine which compounds precipitate, and this can be used to control the chemical form of the heavy metal product. In the optional step S52, a phosphorus compound is added for this purpose. Heavy metal ions precipitate over a wide pH range (typically 3-10). It has been observed that when a pH of 9.3 is reached, heavy metals can be selectively precipitated from magnesium, calcium, sodium, potassium, and to some extent strontium.

[0304] Another method is to use metal hydrosulfides, metal sulfides, organic sulfides, or mixtures thereof to precipitate the heavy metals as sulfides (as shown in step S53). NaHS is a typical candidate. Sulfide precipitation by adding sulfides can also be combined with increasing the pH of the solution. If the precipitation of heavy metals with sulfides is carried out at a higher pH, the formation of hydrogen sulfide gas is avoided.

[0305] In step S54, the precipitated heavy metals are separated using a solid-liquid separation step.

[0306] The third method (indicated by step S55) is to use ion exchange to purify the heavy metals in the solution. Thiol-based resins have a high affinity for various heavy metals. This can be performed similarly to the separation of heavy metals from the extracted ammonium phosphate solution described above.

[0307] In other words, in one embodiment, the method for chemically treating sewage sludge ash comprises the additional step of recovering heavy metals from at least one of the solution derived from the solution at least partially depleted in at least one of iron and phosphorus and the discharge solution from the solution derived from the solution at least partially depleted in at least one of iron and phosphorus. The recovery is caused by adding at least one of a pH-increasing additive and a sulfur-containing compound to the solution and by separating the precipitated heavy metal compounds.

[0308] Figure 29 An embodiment of a heavy metal separation device 50 included in post-processing section 40 is schematically illustrated. Aluminum-depleted solution 254 from the aluminum separation device is passed to a heavy metal precipitation vessel 51. Alternatively, if aluminum removal is not performed, raffinate from an iron / phosphorus extraction is used. A pH-increasing additive 255 and / or a sulfur-containing compound 242, and optionally a phosphate compound, are also introduced to induce precipitation of a heavy metal precipitate 214. The heavy metal precipitate 214 is separated by a solid-liquid separation device 52, leaving a heavy metal-depleted solution 259.

[0309] Alternatively, one can use Figure 23 Similar equipment.

[0310] The removal of heavy metals produces a solution containing primarily magnesium, calcium, sodium and potassium.

[0311] In one embodiment, the process for chemically treating sewage sludge ash comprises the further step of recovering magnesium, calcium, sodium and potassium from at least one of the solution derived from the iron-depleted solution and the effluent solution from the solution derived from the iron-depleted solution. Preferably, magnesium, calcium, sodium and potassium are separated separately.

[0312] like Figure 30 As shown in the embodiment of FIG, in step S61, magnesium can be selectively precipitated as magnesium hydroxide by further increasing the pH of the solution to a pH level that causes magnesium to precipitate as magnesium hydroxide. Typically, this level is above pH 9.3. Therefore, an alkali metal or alkaline earth metal base, for example, can be used. It can be seen that before pH 10, the use of lime can selectively precipitate most of the magnesium as magnesium hydroxide from other components. The magnesium precipitate is recovered from the solution using a solid-liquid separation step in step S62.

[0313] Figure 31 An embodiment of a magnesium separation device 60 included in post-processing section 40 is schematically illustrated. Heavy metal-depleted solution 259 from the heavy metal separation device is passed to a magnesium precipitation vessel 61. Alternatively, if heavy metal removal is not performed, an aluminum-depleted solution or raffinate from an iron / phosphorus extraction is used. A pH-increasing additive 255 is also passed to induce precipitation of magnesium hydroxide 215. The magnesium hydroxide 215 is separated by solid-liquid separation device 62, leaving a magnesium-depleted solution 260.

[0314] It is preferred to select one or more compounds that have been used to improve the pH of the solution so far, thereby concentrating one or more of the remaining components. For example, sodium hydroxide can be used before the sodium removal step to enrich the solution with sodium. Similarly, calcium hydroxide can be used to ensure a higher calcium content in the solution. Alternatively, different compounds can be used to increase the content of one or more substances. If a suitable calcium compound is used to improve the pH, and the sewage sludge ash contains a small amount of sodium and potassium, the resulting solution will mainly contain calcium chloride, that is, a solution rich in calcium chloride is obtained. It can be partially evaporated to obtain a calcium chloride concentrate or completely evaporated to produce solid calcium chloride.

[0315] If the solution contains significant amounts of sodium and / or potassium in addition to calcium chloride, separation of the individual components is preferably performed by exploiting the solubility differences between sodium chloride and potassium chloride at high temperatures in the presence of a high calcium chloride background, e.g. according to International Patent Application Publication No. WO 2017 / 111685 A1.

[0316] The present invention is not limited to the specific embodiments and examples described above and may vary within the limits of the claims. For example, leaching of the sludge ash may be carried out under various conditions, with or without integration of some pretreatment methods into this step. Side streams and process waters (such as raffinate, wash solution, wash water, etc.) may be recycled at different steps of the process to achieve the same result. For example, returning two streams to the leaching reactor and returning one stream to the leachate after filtration may achieve the same result as returning all three streams to the leaching reactor. Regardless of how the required ionic strength in the solvent extraction step is ensured, for example by sufficiently high acidity and / or chloride salt content, a key aspect of the process is the reuse and recycling of these ionic species in the background until a steady state is reached. The effluent is treated to ensure water balance and separation of the products. The location of the return and discharge points may vary depending on the desired results, the chemical nature and composition of the feed, the chemicals used, etc.

[0317] Typically, after solvent extraction of phosphorus, the raffinate will contain hydrochloric acid, unextracted phosphoric acid, calcium, aluminum, heavy metals, magnesium, sodium, and potassium. If not treated as previously mentioned, ferrous iron will remain in solution. Depending on how the ionic strength is controlled, this raffinate will contain significant amounts of hydrochloric acid and / or chloride salts. At any point after the phosphoric acid is extracted, the raffinate is separated into two streams: a bleed stream that is processed separately to recover its components, and a stream that is recycled in a previous step of the process, preferably a leaching step.

[0318] In other words, in one embodiment, the step of recycling at least a portion of the iron-depleted solution comprises recycling at least a portion of the iron-depleted solution to the step of dissolving the starting material comprising sewage sludge ash.

[0319] Figure 32The recycling concept is schematically illustrated. Downstream of the extractor section 20, there are numerous possible splits, including one stream of the at least partially iron- and / or phosphorus-depleted solution originating from the extractor section 20 being recycled as solution 219 back to a previous stage in the process, and the effluent stream 270 being sent to additional component recovery stages. This can be performed at one or more of the illustrated locations. The recycled portion 219 of the at least partially iron- and / or phosphorus-depleted raffinate from a possible outlet constitutes the return stream. Several return outlets are possible. Returning the stream to the leaching / dissolution step means that the resulting leachate will contain the elements from the return solution, but also newly leached sewage sludge ash components. Therefore, in addition to the leached iron and phosphorus, the leachate will also contain increased amounts of unextractable components and hydrochloric acid. Returning the stream to the leaching / dissolution step after the subsequent extraction step and leaching new sewage sludge ash further increases the element concentration and acidity in the leachate. This continues until a steady state is reached, i.e., the concentration of unextractable / difficult-to-extract substances remains somewhat stable. Since calcium is one of the elements present in large quantities in sludge ash, the increase in the concentration of calcium chloride in the solution partially or completely ensures the ionic strength required for effective extraction of iron and phosphorus.

[0320] In other words, in one embodiment, at least part of the recycled portion of the solution depleted in iron and / or phosphorus further comprises chloride ions.

[0321] In other words, in one embodiment, the step of recycling at least a portion of the iron-depleted solution comprises recycling at least a portion of the iron-depleted solution to the step of dissolving the starting material comprising sewage sludge ash.

[0322] Other streams are also returned within the system. As previously mentioned, the scrubber 205 can be re-entered into the dissolution reactor 10. The second-stage raffinate 229 from the extractor section 20 (containing phosphorus and calcium, and possibly some other contaminants) can be reused in the process, for example, in the dissolution reactor 10 or the pretreatment section 14. The phosphorus stream 258 from aluminum recovery can also be returned to the dissolution reactor 10, the pretreatment section 14, or the extractor section 20. These streams ensure a higher overall yield, for example, of phosphorus extraction, but also help maintain the ionic strength of the process.

[0323] Test shows, when with gentle hydrochloric acid solution (for example, 3M hydrochloric acid) leaching sludge ash, by supplying other 2M calcium chloride in background, obtained sufficient iron and phosphorus extraction with TBP.During the first loop, in order to ensure this, need to add salting-out reagent during or after leaching, for example, add calcium chloride, make the chloride concentration in background reach desired value.Then, preferably by making process stream recirculate back to dissolving step and maintain ionic strength.This is carried out by the volume of the effluent taken from system and / or the volume that returns to leaching step by customizing.If necessary, then can concentrate return flow to alleviate the loss of salting-out reagent.This can be carried out using for example partial evaporation.Also can the salting-out reagent of supplementation be added in return flow, leaching solution or leachate from external source or from the stage later of this method.For example, can be used for this purpose by calcium chloride solution or the solid chloride salt separated from calcium chloride solution.

[0324] Of course, the ionic strength required for extraction can be partially or fully supplied by background hydrochloric acid concentrations. In this particular embodiment, leaching is performed with a sufficiently concentrated hydrochloric acid solution to ensure a high hydrochloric acid / chloride background during extraction. Similar to the above, the background acidity / chloride content is recycled. For example, the present inventors tested leaching of sludge ash with a 6M hydrochloric acid solution. Iron and phosphorus can be effectively recovered from the resulting leachate without the addition of any salting-out reagents, such as calcium chloride. A portion of the acidic raffinate, which still contains significant background hydrochloric acid after phosphoric acid extraction, is reused in the fresh ash leaching step and is sufficient alone to supply the required ionic strength in subsequent extraction cycles. Furthermore, in a modification of this embodiment, leaching is performed at elevated temperature, resulting in nearly complete leaching of iron from the sludge ash (iron leaching efficiency exceeding 90%). In this particular case, the amount of iron in the solution is 5-6 times the amount typically leached at ambient temperature (approximately 5 g / L). Due to the significantly higher iron content (about 25-30 g / L) relative to phosphorus (also about 30 g / L), solvent extraction of iron can be performed at near the loading capacity of the organic phase without the need to use very low O: A ratios. In this case, extractions can be performed with O: A ratios between 0.5 and 1, with the system showing increased selectivity for iron compared to operating under the same conditions but using a leachate with a much lower iron content.

[0325] As mentioned previously, a bleed stream is required from the system to maintain water balance and recover other components. If the phosphoric acid is not completely extracted by the TBP, some phosphoric acid will still remain in the raffinate. Recirculating most of this stream back to the leaching step ensures that phosphoric acid is not lost. The removal of phosphoric acid with the bleed stream enables full or partial recovery of aluminum in the form of precipitated aluminum phosphate, and this is achieved at a pH below 2, as discussed further above.

[0326] The subsequent treatment of the solution has been further described above to recover heavy metals and magnesium. This is achieved by gradually increasing the pH using a suitable compound (e.g., metal carbonate, metal oxide, ammonia, metal hydroxide, or a mixture thereof) capable of achieving a gradual increase in pH. According to methods known in the prior art, such as those disclosed in International Patent Application Publication WO 2017 / 111685A1, by utilizing the solubility differences of sodium and potassium at different temperatures and high calcium chloride concentrations, further recovery of sodium, potassium, and calcium from the brine obtained after magnesium recovery is achieved.

[0327] The above embodiments are to be understood as several illustrative examples of the present invention. Those skilled in the art will appreciate 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 defined by the appended claims.

Claims

1. A method for chemically treating sewage sludge ash, the method comprising the following steps: - dissolving (S10) a starting material (201) originating from sewage sludge ash in an acid (202) comprising hydrochloric acid; The starting material (201) comprises at least silicon and an iron compound; - separating (S12) the undissolved residue (204), thereby retaining the leachate (206); - extracting (S20) at least one of iron and phosphorus from the leachate (206) by liquid-liquid extraction with an organic solvent (222); - controlling (S15) the amount of colloidal silica present in the leachate (206) provided to the extraction step (S20) to be compatible with the amount of silica contamination in the liquid-liquid extraction with the organic solvent (222), the silica contamination amount being sufficiently low to make the liquid-liquid extraction with the organic solvent (222) feasible; - recycling (S90) at least a portion (219) of at least part of the raffinate (211) depleted in at least one of iron and phosphorus resulting from said step (S20) of extracting at least one of iron and phosphorus for dissolving the starting material (201) derived from sewage sludge ash; the recycled portion (219) of the at least partially raffinate (211) depleted in at least one of iron and phosphorus comprises chloride ions; The controlling step (S15) is performed by at least one of the following: promoting (S151) coagulation to allow particle growth by adding a silica coagulant to at least one of the starting material (201), the dissolved starting material derived from sewage sludge ash, and the leachate (206); as well as The dissolving step (S10) is performed (S156) at a high temperature of at least 50°C.

2. The method according to claim 1, characterized in that wherein the controlling step (S15) is performed by promoting (S151) coagulation to allow particle growth via at least adding a silica coagulant to at least one of the starting material (201), the dissolved starting material derived from sewage sludge ash, and the leachate (206).

3. The method according to claim 1, characterized in that The promoting (S151) coagulation to allow particle growth is performed at least in part by adding a silica coagulant to the starting material (201).

4. The method according to claim 1, characterized in that The promoting (S151) coagulation to allow particle growth is performed at least partially simultaneously with the dissolving step (S10).

5. The method according to claim 1, characterized in that The promoting (S151) coagulation to allow particle growth is at least partially performed in the leachate (206) after the step (S12) of separating the undissolved residue (204), whereby the method optionally comprises a further step (S19) of removing coagulated silica particles from the leachate (206).

6. The method according to claim 1, characterized in that At least one of the following steps: - performing (S154) the dissolving step (S10) by a hydrochloric acid solution having a concentration greater than 3M; and - aging (S155) the dissolved sewage sludge ash to promote at least one of conversion of ionic silicon to colloidal silica and growth of colloidal silica particles.

7. The method according to claim 1, characterized in that The step (S90) of recycling at least a portion (219) of the raffinate (211) is controlled to provide a chloride salt and / or hydrochloric acid concentration of at least 1 M in the leachate (206).

8. The method according to claim 1, characterized in that The starting material (201) contains sulfur, and wherein the method comprises the further step of preventing precipitation of calcium sulfate from occurring just before or during the step (S20) of extracting at least one of iron and phosphorus by adding (S162) a CaSO4 inhibitor before the step (S20) of extracting at least one of iron and phosphorus.

9. The method according to claim 1, characterized in that The starting material (201) contains sulfur, and wherein the method comprises the further step of preventing precipitation of calcium sulfate from occurring just before or during the step (S20) of extracting at least one of iron and phosphorus by performing at least one of the following processes before the step (S20) of extracting at least one of iron and phosphorus: - aging (S161) the leachate, and - Accelerate (S163) CaSO4 precipitation.

10. The method according to claim 9, characterized in that The accelerating (S163) CaSO4 precipitation comprises at least one of the following: - adding calcium chloride at the latest during the dissolving step (S10); - adding calcium chloride to the leachate (206) after the step (S12) of separating the undissolved residue (204); - returning the aqueous raffinate rich in calcium chloride produced after the previous step (S20) of extracting at least one of iron and phosphorus at the latest during the dissolving step (S10); - returning the aqueous raffinate rich in calcium chloride produced after the previous step (S20) of extracting at least one of iron and phosphorus after the step (S12) of separating the undissolved residue (204); - adding CaSO4 seed particles at the latest during the dissolution step (S10); - adding CaSO4 seed particles to the leaching solution (206) after the step (S12) of separating the undissolved residue (204); as well as - said dissolving step (S10) is performed at a temperature of at least 50°C.

11. The method according to claim 1, characterized in that Additional steps: - oxidizing (S18) any Fe(II) in the leachate to Fe(III) prior to the step (S20) of extracting at least one of iron and phosphorus.

12. The method according to claim 1, characterized in that The step (S20) of extracting at least one of iron and phosphorus includes extracting iron (S21) from the leachate (206) by liquid-liquid extraction using an organic solvent (222).

13. The method according to claim 12, characterized in that The step (S21) of extracting iron from the leachate (206) comprises subsequent extraction stages, wherein: - a first extraction stage (220A) comprising selective liquid-liquid extraction of the iron content from said leachate (206) and subsequent stripping into an intermediate stripping solution (228); and - The second extraction stage (220B) comprises selective liquid-liquid extraction of a majority of the iron ions from the intermediate strip solution (228) and subsequent stripping into a second strip solution (210A) to achieve at least one of a higher iron purity and a higher iron concentration in the recovered iron product.

14. The method according to claim 13, characterized in that The subsequent extraction stage further comprises at least one additional extraction stage to achieve at least one of a higher iron purity and a higher iron concentration in the recovered iron product compared to the second strip solution (210A).

15. The method according to claim 13, characterized in that The step (S10) of dissolving the starting material (201) is performed at ambient temperature using an acid having a hydrochloric acid concentration lower than 3M.

16. The method according to claim 13, characterized in that The first extraction stage (220A) includes matching the loading capacity of the organic solvent (222) to the Fe(III) content in the leachate (206) by controlling the amount of the organic solvent (222) relative to the leachate (206); and At least one of the second extraction stage (220B) and the additional extraction stages, if any, includes controlling the loading capacity of the organic solvent (222) to be near or below the Fe(III) content in the intermediate strip solution (228) by controlling the amount of the organic solvent (222) relative to the intermediate strip solution (228).

17. The method according to claim 15, characterized in that The step (S90) of recycling at least a portion (219) of the raffinate (211) at least partially depleted in at least one of iron and phosphorus comprises recycling the raffinate (229) of the liquid-liquid extraction from at least one of the second extraction stage (220B) and the additional extraction stage, if any, for use in the subsequent step (S10) of dissolving the starting material (201).

18. The method according to claim 12, characterized in that The step (S10) of dissolving the starting material (201) is performed with an acid having a hydrochloric acid concentration higher than 3M, and wherein the step (S21) of extracting iron from the leachate (206) comprises controlling the loading capacity of the organic solvent (222) to be around or below the Fe(III) content in the leachate (206) by controlling the amount of the organic solvent (222) relative to the leachate (206).

19. The method according to claim 18, characterized in that The step (S10) of dissolving the starting material (201) is performed at a high temperature.

20. The method according to claim 1, characterized in that The starting material (201) is sewage sludge ash, and wherein the step (S20) of extracting at least one of iron and phosphorus further comprises extracting phosphorus (S22) from the raffinate of the step (S21) of extracting iron by liquid-liquid extraction with an organic solvent (222).

21. The method according to claim 1, characterized in that Additional steps: - reducing (S17) any Fe(III) in the leachate (206) to Fe(II) prior to the step (S20) of extracting at least one of iron and phosphorus.

22. The method according to claim 21, characterized in that The step (S20) of extracting at least one of iron and phosphorus includes extracting phosphorus (S22) from the leachate (206) by liquid-liquid extraction using an organic solvent (222).

23. The method according to claim 20, characterized in that Another step of back-extracting the organic solvent (222) in the phosphorus extraction step (S22) with a saturated monoammonium phosphate aqueous solution (209D).

24. The method according to claim 23, characterized in that Additional steps: - washing the organic solvent (222) in the phosphorus extraction step (S22) into an aqueous washing solution with water before the step of stripping the organic solvent (222) in the phosphorus extraction step (S22); as well as - returning the aqueous washing solution to the step preceding the step of extracting phosphorus (S22).

25. The method according to claim 12, characterized in that The starting material (201) comprises undissolved residues (204) from phosphorus leaching of sewage sludge ash, the step (S10) of dissolving the starting material (201) is performed with an acid (202) having a hydrochloric acid concentration greater than 3M, and wherein the step (S21) of extracting iron from the leachate comprises controlling the loading capacity of the organic solvent (222) to be lower than the Fe(III) content in the leachate (206) by controlling the amount of the organic solvent (222) relative to the leachate (206).

26. The method according to claim 25, characterized in that The step (S10) of dissolving the starting material (201) is performed at a high temperature.

27. The method according to claim 25, characterized in that The step (S90) of recycling at least a portion (219) of the at least partially depleted raffinate (211) containing at least one of iron and phosphorus comprises recycling at least a portion (219) of the at least partially depleted raffinate (211) containing at least one of iron and phosphorus to the step (S10) of dissolving the starting material (201) containing sewage sludge ash.

28. The method according to claim 1, characterized in that A further step (S41) of recovering aluminum from at least one of the at least partially raffinate (211) depleted in at least one of iron and phosphorus and a discharge solution (270) from the at least partially raffinate (211) depleted in at least one of iron and phosphorus.

29. The method according to claim 28, characterized in that The recycled aluminum includes: - increasing (S42) the pH of at least one of the at least partially raffinate (211) depleted in at least one of iron and phosphorus and the effluent solution (270) from the at least partially raffinate (211) depleted in at least one of iron and phosphorus to a pH level that causes precipitation of at least one of aluminum phosphate, calcium aluminum phosphate, and aluminum hydroxide; and - separating said precipitate of at least one of aluminium phosphate, calcium aluminium phosphate and aluminium hydroxide.

30. The method according to claim 29, characterized in that Additional steps: - reacting the separated precipitate of at least one of aluminum phosphate, calcium aluminum phosphate and aluminum hydroxide with sodium hydroxide (S46) to obtain a sodium aluminate solution and a precipitate containing calcium phosphate; - separating (S47) the precipitated calcium phosphate; as well as - Recycling (S48) the separated precipitated calcium phosphate to a previous process step.

31. The method according to claim 30, characterized in that Additional steps: - adding (S45) a calcium compound together with sodium hydroxide to said separated precipitate of at least one of aluminium phosphate, calcium aluminium phosphate and aluminium hydroxide.

32. The method according to claim 1, characterized in that The further step (S50) of: extracting heavy metals from at least one of the solution derived from the at least partially raffinate (211) depleted in at least one of iron and phosphorus and the discharge solution (270) from the solution derived from the at least partially raffinate (211) depleted in at least one of iron and phosphorus by adding at least one of a pH-increasing additive and a sulfur-containing compound and the separated precipitated heavy metal compound to at least one of the solution derived from the at least partially raffinate (211) depleted in at least one of iron and phosphorus and the discharge solution (270) from the solution derived from the at least partially raffinate (211) depleted in at least one of iron and phosphorus.

33. The method according to any one of claims 1 to 32, characterized in that and recovering at least one of Mg, Ca, Na and K from at least one of a solution derived from the at least partially raffinate (211) depleted in at least one of iron and phosphorus and a discharge solution (270) from the solution derived from the at least partially raffinate (211) depleted in at least one of iron and phosphorus (S60; S65; S70; S75).

34. The method according to claim 33, characterized in that Separate Mg, Ca, Na and K individually.

35. The method according to claim 33, characterized in that Additional steps: - increasing (S61) the pH of at least one of the solution derived from the at least partially raffinate (211) depleted in at least one of iron and phosphorus and the effluent solution (270) from the solution derived from the at least partially raffinate (211) depleted in at least one of iron and phosphorus to a pH level that causes precipitation of magnesium in the form of magnesium hydroxide; as well as The magnesium hydroxide precipitate is separated (S62).

36. The method according to claim 35, characterized in that Using a calcium compound to increase the pH during the recovery of at least one of aluminum, heavy metals and magnesium, a solution rich in calcium chloride is obtained after the step (S62) of separating the magnesium hydroxide precipitate, whereby the method comprises the following further steps: - Partially or completely evaporating water from said calcium chloride-rich solution to obtain a calcium chloride concentrate or a solid calcium chloride product, respectively.

37. An apparatus (1) for chemically treating sewage sludge ash, the apparatus comprising: a dissolution reactor (10) configured to dissolve a starting material (201) originating from sewage sludge ash in an acid (202) comprising hydrochloric acid; The starting material (201) comprises at least silicon and an iron compound; - a separation device (12) for separating the undissolved residue (204), thereby retaining the leachate (206); - an extractor section (20) configured to extract at least one of iron and phosphorus from the leachate (206) by liquid-liquid extraction with an organic solvent (222); - means (14A; 14B; 14C) for controlling the amount of colloidal silica present in the leachate (206) supplied to the extractor section (20) to be compatible with the amount of silica contamination in the liquid-liquid extraction with the organic solvent (222), the silica contamination being sufficiently low to make the liquid-liquid extraction with the organic solvent (222) feasible; a return pipe (90) configured to recirculate at least a portion (219) of the at least partially raffinate (211) depleted in at least one of iron and phosphorus originating from the extractor section (20) for dissolving the starting material (201) originating from sewage sludge ash; the recycled portion (219) of the at least partially raffinate (211) depleted in at least one of iron and phosphorus comprises chloride ions; The means for controlling the amount of colloidal silica comprises at least one of the following: a pretreatment chamber (14A) configured to receive the leachate (206) and add a silica coagulant to the leachate (206), wherein the pretreatment chamber (14A) optionally comprises a separation device (144) for removing (S19) coagulated silica particles from the leachate (206); and a heater (14C) arranged to provide an elevated temperature of at least 50°C in the dissolution reactor (10); wherein the means for controlling the amount of colloidal silica comprises at least a pre-treatment chamber (14A) configured to receive the leachate (206) and add the silica coagulant to the leachate (206), wherein the pre-treatment chamber (14A) comprises a separation device (144) for removing (S19) coagulated silica particles from the leachate (206).

38. The apparatus according to claim 37, characterized in that The apparatus for reducing the amount of colloidal silica in dissolved sewage sludge ash further comprises an inlet (14B) into the dissolution reactor (10) for introducing the silica coagulant.

39. The device according to claim 37 or 38, characterized in that The return pipe (90) comprises means for controlling the amount of recycled raffinate (211) to obtain a chloride salt and / or hydrochloric acid concentration of at least 1 M in the leachate (206).

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