Chemical treatment of sewage sludge ash

Abstract

The method for chemically treating sewage sludge ash comprises dissolving starting material originating from sewage sludge ash in an acid comprising hydrochloric acid (S10). The starting material comprises at least silicon and iron compounds. The undissolved residue is separated (S12), thereby leaving a leachate. The amount of colloidal silica in the dissolved sewage sludge ash is controlled (S15). At least one of iron and phosphorus is extracted from the leachate by liquid-liquid extraction with an organic solvent (20). At least a portion of the raffinate resulting from the iron and phosphorus extraction step, from which at least one of iron and phosphorus has been at least partially removed, is recycled (S90) for dissolving the starting material originating from sewage sludge ash. The recycled portion of the raffinate from which at least one of iron and phosphorus has been at least partially removed comprises chloride ions. A composition for chemically treating sewage sludge ash is also disclosed.

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The present technology relates generally to the chemical treatment of sewage sludge ash, and more particularly to methods and compositions for the recovery of components of sewage sludge ash. [Background technology]

[0002] Phosphorus is an essential nutrient for living organisms and is irreplaceable. Agricultural demand for phosphorus fertilizer has increased significantly in recent decades and is predicted to continue to increase due to global population growth and improving living standards. The raw material for inorganic phosphorus fertilizer production and animal feed production is phosphate rock, which is also a limited resource. In the European Union (EU), both phosphate rock and phosphorus are considered important raw materials. Many countries lack natural phosphate deposits and are completely dependent on imports. Because phosphate rock mining is concentrated in a few regions of the world, many countries are vulnerable to market fluctuations and geopolitical factors. One proposed measure to reduce phosphorus import dependence and overexploitation of natural deposits is the recovery of phosphorus from secondary sources. These secondary sources include municipal and industrial wastewater, sewage sludge, sewage sludge ash, and other phosphorus-containing wastes. Governments and industry have recognized the importance of phosphorus recovery from secondary sources, and some countries have begun to enact legislation to promote this.

[0003] Phosphorus release into surface waters contributes to eutrophication. Therefore, phosphorus must be removed from domestic and industrial wastewater during treatment. This results in phosphorus accumulation in sewage sludge, a major by-product of wastewater treatment plants. Using this sludge as fertilizer is not a viable option due to the large volume generated and the presence of heavy metals, organic pollutants, and pathogens. Some of these drawbacks can be mitigated by incinerating sewage sludge to produce sludge ash. This is done in dedicated incinerators, where the wet sludge is pre-dried using regenerated heat before incineration, reducing its moisture content from 70–80% to approximately 30–40%. Incineration reduces the sludge volume, destroys organic pathogens, and yields a dry product containing approximately 6–14% phosphorus by weight. By 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 fertilizer. Although over 90% of the phosphorus initially present in wastewater is converted to ash, the phosphorus compounds produced during incineration, such as calcium phosphate and aluminum phosphate, are poorly water-soluble. Therefore, the phosphorus is not dissolved and available for plant uptake. Furthermore, heavy metals in the sludge that are harmful to living organisms are concentrated in the ash due to the reduced volume. Co-incineration of sewage sludge with biomass is possible in conventional incinerators, but this results in an 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 treatments are preferred for recovering phosphorus and iron from sludge ash. Several such processes are detailed in the literature and are briefly described below. These use a combination of leaching, precipitation, solvent extraction, and / or ion exchange to recover sludge ash components, although not all of these are necessarily used.

[0005] The book "Phosphorus: Polluter and Resource of the Future - Removal and Recovery from Wastewater," IWA Publishing, chapter 25, describes a phosphorus recovery process from sludge ash, called "LEACHPHOS." The LEACHPHOS process relies on leaching sludge ash with dilute sulfuric acid. The leachate, containing phosphorus, aluminum, iron, and other dissolved impurities, is separated from the undissolved residue using filtration. The leachate is gradually neutralized, and a mixed product of calcium phosphate, aluminum phosphate, and iron phosphate is isolated. Heavy metals precipitate at pH levels above 9, resulting in a mixed-metal hydroxide sludge. These mixed products have low market value, and individual separation of the main components is not achieved. The phosphorus product has low water solubility and cannot be used directly as a fertilizer.

[0006] A phosphorus recovery process, called "The PHOS4LIFE PROCESS," has been developed by Technicas Reunidas. This process uses sulfuric acid to leach sludge ash. Dissolved iron is recovered by solvent extraction. While no information is available regarding the iron extraction process, it is believed that the iron in the sulfuric acid leachate is in cationic form, so the iron is extracted with a solvent suitable for extracting cationic iron, followed by the use of hydrochloric acid to strip the extracted iron from the loading solvent. The iron chloride product is concentrated using steam evaporation. After iron extraction, phosphoric acid is separated using solvent extraction. This is further concentrated using steam evaporation to obtain technical-grade phosphoric acid (75% concentration). After phosphoric acid extraction, the metal ions remaining in solution are precipitated as a mixed product at a high pH achieved by the addition of lime. This process requires the use of both sulfuric acid and hydrochloric acid, has a high energy input during the evaporation step, and separates a mixed metal hydroxide product with limited commercial value. The main drawback of using sulfuric acid to dissolve ash is that it reacts with calcium in the ash to form gypsum, which is filtered out along with the undissolved ash residue. This significantly increases the amount of residue that needs to be disposed of. Another drawback of using sulfuric acid is that all cations leached into solution from the ash are in the form of sulfates (e.g., magnesium sulfate). When a raffinate containing sulfates (e.g., due to precipitation of heavy metals) is neutralized with lime, large amounts of gypsum are produced, which increases disposal costs.

[0007] A phosphorus recovery process from ash, dubbed "The Pasch Process," was presented at BALTIC21, held in Berlin from September 28 to 30. In this process, sludge ash is leached using dilute hydrochloric acid. The undissolved residue is separated using a parallel-plate separator and filter, then dewatered using a centrifuge. Dissolved metals, such as iron, lead, cadmium, copper, and zinc, are extracted as a group using solvent extraction with a mixture of Alamine 336 and TBP. These are then precipitated as mixed hydroxides. Therefore, individual separation of commercially available metal compounds is not achieved. Phosphorus remaining in the raffinate after solvent extraction can be recovered using conventional phosphate precipitation, but aluminum is not extracted by Alamine 336 and TBP and is co-precipitated with the phosphorus product, reducing its value. Therefore, the potential for industrial use of such recovered phosphorus products is low.

[0008] The aforementioned EcoPhos company proposed a phosphorus recovery process from sludge ash based on leaching sludge ash or other raw phosphates with hydrochloric acid. Monocalcium phosphate in the leachate was converted to dicalcium phosphate by adding calcium carbonate. Addition of sulfuric acid precipitated gypsum and produced phosphoric acid. Separation of heavy metals was not described, nor was the leaching of iron and aluminum from the sludge ash considered.

[0009] Another phosphorus recovery process by the aforementioned EcoPhos company is described in WO 2015 / 091946. This process relies on leaching ash from waste incineration, including sewage sludge ash, with phosphoric acid. The leachate is treated with activated carbon. The phosphoric acid is separated from dissolved metals using a cation exchange resin. The resin is regenerated with hydrochloric acid, which leaches a mixture of magnesium chloride and calcium chloride, and aluminum chloride and ferric chloride. The fidelity of the heavy metals is unknown. This process does not recover iron and aluminum as separate commercial products, nor does it separate calcium and magnesium from each other. The use of ion exchange resins is unreliable, especially for streams with high metal content and low phosphorus content. These can result in premature loading of the ion exchange resin, breakthrough of metals in the phosphoric acid stream, shortened use 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 it from contaminating the purified phosphoric acid product.

[0010] WO 2015 / 067328 describes a method for treating ash from waste incineration to obtain aluminum, calcium, phosphorus, and nitrogen compounds. The ash is leached using a mixture of phosphoric acid and nitric acid, dissolving phosphorus as phosphate and metals as metal nitrates. Adding sulfuric acid to the leachate precipitates calcium as gypsum. A mixture of aluminum hydroxide and aluminum phosphate is precipitated by further increasing the solution's pH with lime. The remaining solution, containing phosphoric acid and nitric acid, is concentrated by evaporation. Further addition of lime produces a mixture of calcium nitrate and calcium phosphate, which is recovered in solid form by evaporative crystallization.

[0011] WO 2015 / 165481 describes a method for producing purified phosphoric acid, calcium sulfate, aqueous calcium hydrogen phosphate, and metal salt solutions by treating ash from a waste incineration plant with raw phosphoric acid. The ash is reacted with raw 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] The TetraPhos Process by Remondis is described in Chapter 24 of the book "Phosphorus: Polluter and Resource of the Future - Removal and Recovery from Wastewater" (IWA Publishing, 2018). This process also relies on leaching sludge ash with phosphoric acid and then precipitating gypsum using sulfuric acid. Subsequent ion exchange separates metal ions, such as iron, aluminum, and magnesium, from the phosphoric acid. The metals in the ion exchange resin are eluted with hydrochloric acid, resulting in a mixed metal salt solution. The resulting phosphoric acid is further purified using membranes and concentrated using evaporation. This method has limited applicability for ashes with low phosphorus content and high soluble metal content. This method does not recover iron, aluminum, and other metals as separate commercial products. This method requires the use and handling of both sulfuric acid and hydrochloric acid. Furthermore, the use of ion exchange resins results in the aforementioned drawbacks. Additionally, the use of membranes reduces process reliability, including fouling and a short service life.

[0013] WO 2018 / 046621 describes a process and device for recovering phosphorus from 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 in subsequent leaching of phosphate rock, further increasing the phosphorus concentration in the solution. While this method can concentrate phosphorus with each leaching cycle, it does not separate a pure phosphorus stream because soluble impurities in the sludge ash and phosphate rock also dissolve with the phosphorus.

[0014] European Patent No. 3266742 describes a method for producing phosphoric acid from phosphorus-containing primary and secondary raw materials, including sludge ash, using a combination of acidic chemical digestion and electrodialysis, followed by purification of the crude phosphoric acid. The raw material is leached with monovalent mineral acids, 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% by volume using vacuum evaporation, falling film evaporation, membrane distillation, and / or reverse osmosis. This concentrate is further purified using solvent extraction. Recovery of monovalent mineral acids from the concentrate is achieved using bipolar electrodialysis. Prior to electrodialysis, calcium and magnesium ions are precipitated from the concentrate as calcium hydroxide and magnesium hydroxide. This process requires high energy for the phosphoric acid concentration and electrodialysis steps.

[0015] WO 2020 / 169708 describes a process for recovering phosphoric acid from phosphorus-containing solid materials, such as struvite, sludge ash, meat and bone meal ash, manure ash, calcium phosphate, and phosphate-containing minerals, such as apatite, vivisite, and phosphate rock. The solid material is reacted with a strong acid in a monophasic reaction medium that also contains an organic solvent. This results in the formation of phosphoric acid in the organic solvent, yielding a phosphorus-depleted solid material. The phosphoric acid-containing organic solvent is separated from the remaining solid material, and the phosphoric acid is recovered from the organic phase using conventional stripping. This method has limited industrial applicability to sludge ash due to the high loss of organic solvent in the undissolved ash residue. Complete solvent recovery and residue removal are difficult to achieve. This process also exhibits very low phosphorus yields. The only example of phosphorus recovery from sludge ash describes extraction using a system containing sulfuric acid, sludge ash, water, and n-propanol. The recovery efficiency was 15.8% in 24 hours.

[0016] WO 95 / 06004 describes a method for treating wastewater sludge using sulfuric acid leaching, solvent extraction and stripping of iron and aluminum, sulfide precipitation of heavy metals, precipitation of phosphorus as hydroxyapatite or struvite, and precipitation of aluminum as aluminum hydroxide. Direct treatment of wastewater sludge without incineration results in significant chemical consumption and the generation of large amounts of waste liquid due to the large volume and water content of the unincinerated sludge.

[0017] WO 2014 / 178788 describes several routes for recovering phosphorus from sludge ash using hydrochloric acid leaching and solvent extraction of metals with tributyl phosphate (TBP). All of the solvent extraction steps presented involve pre-treating the sludge ash leachate with a base to precipitate intermediate compounds, primarily mixtures of iron, aluminum, and calcium phosphate. This results in pre-concentration and partial purification of phosphorus. The intermediate precipitate is dissolved in hydrochloric acid and then subjected to solvent extraction. Thus, solvent extraction of metals and phosphorus is not performed immediately after leaching. This intermediate precipitation step of iron, aluminum, and calcium phosphate avoids many of the drawbacks of applying solvent extraction immediately after leaching, such as reduced phosphorus and iron concentrations in solution and the presence of additional impurities. These drawbacks are discussed in more detail herein. However, the intermediate precipitation step makes the processing of sludge ash by solvent extraction more complex and expensive, and increases chemical consumption. For example, a base is required to precipitate an intermediate compound, which is then dissolved again in acid to dissolve the phosphorus, aluminum, calcium, iron and other components.

[0018] A better process is needed for recycling sludge ash, ensuring not only high-purity and high-yield recovery of phosphorus in a form usable, for example, in the fertilizer industry, but also high-purity and high-yield recovery of other individual components. For example, recovery of iron separately from aluminum, recovery of calcium as a marketable product without losses during leaching, such as precipitation as gypsum, or recovery of magnesium and heavy metals are required. Removal of these fractions from sludge ash produces an insoluble residue rich in silicates, which has important applications in the concrete industry, especially if harmful heavy metals are removed. Due to the large amount of sludge ash generated annually, use of this residue in the concrete industry would also result in significant reductions in gas emissions and savings in landfill space. Sludge ash processing should preferably be accomplished with minimal energy and chemical consumption and minimal waste generation. Many of the above processes fail to address several of these aspects. Summary of the Invention

[0019] The overall objective of this technology is to provide a process and system for recycling sludge ash, thereby recovering phosphorus and other valuable components in high purity and yield while limiting energy and chemical consumption.

[0020] The above object is achieved by a method and a device 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 starting materials derived from the sewage sludge ash in an acid, including hydrochloric acid. The starting materials include at least silicon and iron compounds. Undissolved residue is separated, leaving 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 accommodate a silica crud content during the organic solvent extraction that is sufficiently low to make the organic solvent extraction operable. The controlling step is performed by adding a silica coagulant to at least one of the dissolved starting materials derived from the sewage sludge ash and the leachate to promote coagulation for particle growth, and / or by performing the dissolution step at an elevated temperature of at least 50°C. At least a portion of the solution resulting from the iron and phosphorus extraction step, from which at least one of the iron and phosphorus has been at least partially removed, is recycled to dissolve the starting materials derived from the sewage sludge ash. The recycled portion of the solution from which at least one of iron and phosphorus has been at least partially removed comprises chloride ions.

[0022] In a second aspect, an arrangement for chemically treating sewage sludge ash includes a dissolution reactor, a separation device, an extraction section, a device for controlling the amount of colloidal silica present in the leachate provided to the extraction section, and a return pipe. The dissolution reactor is configured to dissolve starting materials derived from sewage sludge ash in an acid containing hydrochloric acid. The starting materials include at least silicon compounds and iron compounds. The separation device is configured to separate the undissolved residue, leaving a leachate. The extraction section is configured to extract at least one of iron and phosphorus from the leachate by liquid-liquid extraction with an organic solvent. The device for controlling the amount of colloidal silica present in the leachate provided to the extraction section is configured to control the amount to match the amount of silica crud during the liquid-liquid extraction with an organic solvent, which is sufficiently small to make the liquid-liquid extraction operable. The device for controlling the amount of colloidal silica comprises a pretreatment chamber configured to receive the leachate and add a silica coagulant to the leachate, the pretreatment chamber optionally comprising a separation device for removing coagulated silica particles from the leachate and / or a heater configured to provide an elevated temperature of at least 50°C within the dissolution reactor. The return pipe is configured to recirculate at least a portion of the solution from which at least one of iron and phosphorus has been at least partially removed, resulting from the step of extracting at least one of iron and phosphorus from sewage sludge ash, to dissolve the starting material. The recirculated portion of the solution from which at least one of iron and phosphorus has been at least partially removed contains chloride ions.

[0023] One advantage of the proposed technology is that valuable sewage sludge ash components can be extracted in high purity and yield with low energy consumption and low consumption of additional chemicals. Other advantages will be apparent from the detailed description.

[0024] The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken together with the accompanying drawings, in which: [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a process flow diagram of one embodiment of a method for chemically treating sewage sludge ash. [Figure 2] 1 is a part of a flow diagram of a pretreatment process according to an embodiment of a method for chemically treating sewage sludge ash. [Figure 3] FIG. 1 is a partial flow diagram of a pretreatment process according to another embodiment of the method for chemically treating sewage sludge ash. [Figure 4] FIG. 1 is a flow diagram of the substeps of one embodiment of a process for reducing the amount of colloidal silica. [Figure 5] FIG. 1 is a flow diagram of a substep of an embodiment of a process for treating sulfur. [Figure 6] FIG. 10 is a partial flow diagram of a pretreatment process according to yet another embodiment of the method for chemically treating sewage sludge ash. [Figure 7] FIG. 10 is a partial flow diagram of a pretreatment process according to yet another embodiment of the method for chemically treating sewage sludge ash. [Figure 8] FIG. 1 is a schematic diagram of one embodiment of an arrangement for chemical treatment of sewage sludge ash. [Figure 9] FIG. 1 is a schematic diagram of one embodiment of a dissolution reactor. [Figure 10] 1 is a schematic diagram of one embodiment of a pre-treatment chamber. [Figure 11] FIG. 1 is a schematic diagram of one embodiment of a generic solvent extractor. [Figure 12] FIG. 1 is a schematic diagram of an embodiment of an iron extraction section having two solvent extractors. [Figure 13] FIG. 1 is a flow diagram of the substeps of an embodiment of a process for extracting iron. [Figure 14] FIG. 1 is a schematic diagram of one embodiment of an embodiment for scrubbing impurities from an iron-loaded organic phase. [Figure 15] FIG. 1 is a schematic diagram of an arrangement for neutralization of ferric chloride products with hydrochloric acid. [Figure 16] FIG. 1 is a flow diagram of a partial process of one embodiment of neutralization of a ferric chloride product with hydrochloric acid. [Figure 17] FIG. 1 is a flow diagram of a partial process for treating iron in one embodiment of a method for chemically treating sewage sludge ash. [Figure 18] FIG. 1 is a schematic diagram of an embodiment of an iron extraction section having one solvent extractor. [Figure 19] FIG. 1 is a schematic diagram of one embodiment of a two-stage arrangement for chemical treatment of sewage sludge ash. [Figure 20] FIG. 1 is a process flow diagram of one embodiment of a two-stage method for chemically treating sewage sludge ash. [Figure 21] FIG. 1 shows phosphate distribution between the aqueous and organic phases. [Figure 22] FIG. 1 is a schematic diagram of one embodiment of a phosphorus extraction section. [Figure 23] FIG. 1 is a schematic diagram of a heavy metal removal arrangement in one embodiment of the phosphorus extraction section. [Figure 24] FIG. 1 is a schematic diagram of one embodiment of a heavy metal removal arrangement. [Figure 25] FIG. 10 is a schematic diagram of another embodiment of a heavy metal removal arrangement. [Figure 26] FIG. 1 is a flow diagram of a substep of an embodiment of a process for recovering aluminum. [Figure 27] FIG. 1 is a schematic diagram of one embodiment of an aluminum isolation structure. [Figure 28] FIG. 1 is a flow diagram of a partial process of one embodiment of a process for recovering heavy metals. [Figure 29] FIG. 1 is a schematic diagram of one embodiment of a heavy metal separation arrangement. [Figure 30] FIG. 1 is a flow diagram of a partial process of one embodiment of a process for recovering magnesium. [Figure 31] FIG. 1 is a schematic diagram of one embodiment of a magnesium isolation arrangement. [Figure 32] FIG. 1 is a schematic diagram illustrating an embodiment of a recycled stream in an arrangement for chemical treatment of sewage sludge ash. Specific Description of the Invention

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

[0027] Sewage sludge ash is composed primarily 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 hazardous waste, requiring either burial in dedicated landfills or appropriate handling of heavy metals if the ash is used for other purposes. Table 1 shows the elemental composition of five different sludge ashes. Ash A, Ash B, and Ash D were obtained from the incineration of sewage sludge alone, while Ash C and Ash E were obtained from the co-incineration of sewage sludge and wood chips. Table 1 shows that the phosphorus concentration of co-incinerated sludge ash (approximately 4%) is lower than that of incinerated sludge ash (approximately 8%). Table 1 also lists the types of chemicals used in WWTPs to precipitate phosphorus. Three different groups of phosphorus precipitation chemicals are typically used: 1) ferric-based precipitation chemicals such as ferric chloride (FeCl3) or ferric sulfate (Fe2(SO4)3), 2) ferrous-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).

[0028] [Table 1]

[0029] Regardless of the chemicals used in WWTPs, phosphorus in sludge ash exists mostly as poorly water-soluble calcium and aluminum phosphates. For this reason, wet chemical processes for recovering phosphorus from ash typically employ acid leaching, which effectively dissolves metal phosphates and captures phosphate ions in solution, from which they can be recovered using hydrometallurgical techniques such as precipitation, solvent extraction, and ion exchange.

[0030] Many of the processes for recovering components from sewage sludge ash use sulfuric acid because it is inexpensive and can precipitate gypsum, resulting in a phosphorus stream with low calcium content. However, gypsum generally has low commercial value. Furthermore, there are concerns about the co-precipitation of other elements in the leachate with the gypsum, which can lead 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, which means that most of the calcium is not recovered from the ash and the amount of undissolved ash increases significantly.

[0031] Some processes use phosphoric acid for leaching. In this case, the solution is enriched with phosphorus dissolved from the ash, ensuring a phosphorus-rich stream. However, this solution is contaminated with heavy metals and other metals such as iron, aluminum, calcium, and magnesium. This necessitates additional purification, which is neither simple nor inexpensive, especially when using ion exchange. Most approaches aim to isolate pure phosphoric acid, producing a mixture of other metals with limited uses and no commercial value.

[0032] There are only a few processes that use hydrochloric acid, leaving room for improvement. Using hydrochloric acid for leaching has several attractive properties. For example, compared to using sulfuric acid, it eliminates the problem of gypsum precipitation during leaching. Furthermore, hydrochloric acid is cheaper than phosphoric acid, and because phosphorus is one of the components targeted for extraction, resources are not spent on recovering a substance intentionally added to the process.

[0033] The approach contemplated by this disclosure is to leach sewage sludge ash with hydrochloric acid, followed by solvent extraction with TBP to separate the individual compounds of iron and phosphorus in solution. Preferably, selective precipitation of aluminum, heavy metals, magnesium, calcium, sodium, and potassium is also performed. Leaching sludge ash with hydrochloric acid followed by solvent extraction has been previously described, for example, in the Pasch process or in EP 3266742 and WO 2014 / 178788.

[0034] However, in the Pasch process, iron and other metals in the leachate are extracted as a group using an organic mixture of Alamine 336 and TBP. The metals extracted into the organic phase are stripped together and precipitated as mixed hydroxides, meaning they are not recovered as individual compounds. The process uses solvent extraction to remove metal contaminants from the phosphorus-containing leachate. 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, energy-intensive water evaporation or conventional precipitation of poorly water-soluble phosphorus compounds. Such poorly water-soluble phosphorus compounds are not suitable for use as fertilizers unless chemically converted to more water-soluble products. EP 3266742 describes solvent extraction using the organic compounds di(2-ethylhexyl)phosphate (DEHPA), Cyanex 923, and Cyanex 272, or mixtures thereof, to extract iron and aluminum. Solvent extraction is used in combination with bipolar electrodialysis to regenerate the acid used in leaching, a process that is energy intensive and sensitive to contaminants in the solution.

[0035] One of the applicant's prior inventions, disclosed in WO 20141 / 78788, describes leaching sewage sludge ash with hydrochloric acid followed by solvent extraction of iron and phosphorus with TBP. However, the solvent extraction step described in WO 2014 / 178788 is not performed immediately after leaching the sewage sludge ash with hydrochloric acid, as is the intent of the present invention. Instead, in WO 2014 / 178788, the leachate is treated with a base to obtain an intermediate precipitate containing a mixture of calcium phosphate, iron phosphate, and aluminum phosphate with other coprecipitated elements. In this way, phosphorus is preconcentrated and purified. This intermediate precipitate is separated from the original leachate and redissolved in hydrochloric acid before solvent extraction with TBP.

[0036] However, the use of hydrochloric acid for leaching sewage sludge ash is not without additional problems. As noted in the Background section, hydrochloric acid leaching 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, hydrochloric acid leaching has been found to result in leachates containing non-negligible amounts of colloidal silica, which precludes the application of direct solvent extraction processes.

[0037] In WO 2014 / 178788, this is overcome by using an intermediate precipitation step. While such a solution generally works well, it consumes disadvantageously high amounts of added chemicals. This is because the process described in WO 2014 / 178788 requires chemicals to precipitate the intermediate phosphorus compounds in a separate step after leaching, a filtration step to separate and wash the intermediate compounds, and an acid dissolution step to redissolve the phosphorus and other metal components of the intermediate precipitate to facilitate the subsequent solvent extraction step.

[0038] However, the present invention can be applied to a wide range of sludge ash materials, or sludge ash materials resulting from prior treatment of sludge ash (e.g., leaching of at least a portion of one or more components), without any intermediate steps. Such ashes include those with very low phosphorus content and high iron, calcium, aluminum, and other impurities. This leads to significant savings in chemicals, a reduction in the number of steps, and less equipment required. To achieve a more efficient process in terms of consumption of added chemicals, the process should be based on direct extraction from a hydrochloric acid-based sewage sludge ash leachate.

[0039] Surprisingly, hydrochloric acid was found to be able to leach more silicon than either nitric or sulfuric acid. Furthermore, dissolution yields increased with lower acid concentrations and temperatures. The solubility limit of silicon species at acidic pH is actually very low. At pH 2, the solubility limit of silicic acid is approximately 150 ppm. Above this concentration, silicic acid polymerizes and forms colloidal particles, precipitates, or gels. Depending on the conditions, these morphologies can gradually change, which is undesirable, especially during solvent extraction. Colloidal silica can lead to emulsification, crud formation, and significant entrainment losses in subsequent solvent extraction. It has been found that even levels as low as 50 ppm of colloidal silica can interfere with solvent extraction of iron and phosphorus with TBP, rendering it virtually impossible.

[0040] Over time, colloidal silica grows into larger silica particles that can be filtered, but this process is too time-consuming for industrial applications. For this approach to work, the amount of colloidal silica present in the leachate that proceeds to solvent extraction must be actively controlled. To this end, the leachate process must actively reduce the amount of colloidal silica in the leachate or actively reduce the formation of colloidal silica. In other words, there must be a process for controlling the amount of colloidal silica present in the leachate that proceeds to the solvent extraction stage.

[0041] Due to their very small size (0.01-0.5 μm), colloidal silica particles are very difficult to remove using conventional methods such as sedimentation, filtration, or centrifugation, however, this step may also be carried out by other methods, which are described in more detail below.

[0042] Another important aspect of hydrochloric acid-based processes is the efficiency of the solvent extraction of iron and phosphorus due to the high ionic strength of the solution, particularly chloride. This salting-out effect during solvent extraction allows for more selective extraction. This can be enhanced by recycling at least a portion of the raffinate containing chloride ions after solvent extraction and using it for leaching sewage sludge ash. Recycling streams from various process steps back into the process is key to ensuring the ionic strength required for the separation of iron and phosphorus using solvent extraction. Recycling also ensures water balance and minimizes the amount of secondary waste generated.

[0043] The basic process, based on these considerations, can be applied to a variety of different extraction processes to extract valuable components of sewage sludge ash in pure fractions and relatively high yields, and typically such extracted compounds are worth further use.

[0044] Thus, this technology provides an improved method for hydrochloric acid leaching of sewage sludge ash-containing materials, followed by solvent extraction and optional precipitation of dissolved components. This technology allows for the recovery of a majority of the iron in the sludge ash as iron chloride compounds. This technology also allows for the recovery of phosphorus as high-value products such as phosphoric acid or ammonium phosphate. This technology also allows for the recovery of aluminum as commercially available compounds, such as aluminum phosphate, aluminum hydroxide, sodium aluminate, and / or sodium aluminum phosphate. This technology also allows for the recovery of heavy metal contaminants as, for example, metal hydroxides and / or metal sulfides. This technology also allows for the recovery of magnesium as commercially available compounds, such as magnesium hydroxide. This technology also allows for the recovery of calcium as commercially available compounds, such as calcium chloride. This technology also allows for the recovery of sodium as commercially available compounds, such as sodium chloride. This technology also allows for the recovery of potassium as commercially available compounds, such as potassium chloride. This technology also allows for the recovery of undissolved, clean silica sand ash residue for commercial applications, such as in the concrete industry. This technique further improves the overall process effectiveness by reusing and recycling side streams, minimizing secondary waste generation and chemical consumption, and as noted above, provides favorable operating conditions at various steps of the process, such as the ionic strength required for selective separation.

[0045] Because sludge ash is a heterogeneous stream that may contain many fractions with different chemical properties and compositions, the present invention can take on a variety of embodiments, distinguished by how the process steps are arranged, omitting steps or inserting additional steps as needed, and operating the process steps under different conditions, e.g., temperature, acidity, pH, etc. The various embodiments are described in detail later in this specification and are all supported by research data.

[0046] Certain terms frequently used in this disclosure shall be interpreted as follows:

[0047] leaching, leaching agent, leachate - Leaching refers to the removal of the soluble fraction contained in the solid phase as a solution. This is achieved by treating the solid phase with a solution called a leachant. The resulting solution, called the leachate, contains the components of the soluble fraction as ions.

[0048] Solvent extraction or liquid-liquid extraction A separation method in which the chemical species to be separated are divided between two immiscible liquid phases, i.e., an aqueous phase (e.g., a leachate) and an organic phase containing, in particular, an extractant (see below). When referring to solvent extraction of a specific element, this refers to solvent extraction of an extractable complex or compound containing said specific element. For example, in the present text, the extraction of phosphoric acid with TBP is often referred to as the extraction of phosphorus.

[0049] Extractant -An active ingredient, typically organic, that binds to the target species and allows its extraction.

[0050] Diluent - the liquid in which the extractant and modifier dissolve to form the organic phase, in this context an organic liquid.

[0051] Modifier - Substances added to the organic phase to increase the solubility of the extractant, extractant salt, or ionic species resulting from extraction or stripping, and to suppress emulsion formation.

[0052] Stripping or back-extraction - The reverse of extraction; eluting extracted compounds from the loaded organic phase with an aqueous solution to form an aqueous solution containing the extracted species.

[0053] Scrubbing - Similar to stripping, but typically refers to the selective removal of unwanted extracted species in the stripped product, e.g., co-extracted impurities. Usually performed prior to stripping.

[0054] Raffinate-The aqueous phase from which solutes (dissolved species) have been removed by extraction. In other words, the aqueous phase after solvent extraction.

[0055] Clad -Materials resulting from the agitation of organic phase, aqueous phase, and particulates to form a stable mixture, which may result in high organic losses and / or inability to perform solvent extraction.

[0056] Salting out - Facilitating extraction of species into the organic phase by increasing the ionic strength of the aqueous phase, for example by adding acids, salts or other chemicals capable of doing this.

[0057] Figure 1 shows a process flow diagram of one 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, or, for example, undissolved residue from a previous dissolution of sewage sludge ash, which still contains the components desired to be recovered. The starting material includes at least silica and iron. In other words, this initial step involves leaching of sewage sludge ash or sewage sludge ash-containing material.

[0058] Thus, the sewage sludge ash or sewage sludge ash-containing material is contacted with a leach solution. The leach solution comprises hydrochloric acid and a suitable aqueous diluent, typically water, although feed streams from various process steps may also be included. Additionally, as discussed in more detail below, other compounds may be added during leaching, such as oxidizing or reducing agents, colloidal silica coagulants, antifoaming agents, other acids, bases, pH buffers, and / or salts. The presence of all of these components is not necessary, depending, for example, 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 discussed in more detail below.

[0059] The dissolution step S10 can be controlled to leach only a portion of the components of the sludge ash material. Thus, by controlling the amount of acid in the leach solution and process parameters such as leaching time, temperature, mixing conditions, atmosphere, addition of oxidizing or reducing agents, liquid-to-solid ratio, particle size, and ionic strength, it is possible to control the amount of dissolution of one or more components. For example, most of the iron in the sludge ash can be leached using concentrated hydrochloric acid and / or elevated temperatures, while a smaller proportion of the iron can be leached at ambient temperatures using dilute acid.

[0060] Because sludge ash typically contains a large amount of insoluble silicates, the leaching process does not completely dissolve the sludge ash, resulting in a leachate and an undissolved solid residue. In step S12, the undissolved residue is separated, leaving behind the leachate. This can also be described as the recovery of the leachate from the undissolved sewage sludge ash residue, typically using a solid-liquid separation process such as filtration, sedimentation, or centrifugation. Due to its high silicate content and removal of soluble components, the residue is suitable for use as a material in, for example, the concrete industry.

[0061] Step S12 may optionally also include a substep S13 of washing the undissolved residue to produce a silicate product. Washing the undissolved ash residue, e.g., rinsing off the residual leachate, can be carried out using a suitable washing agent, typically water, with or without additional additives. Such additives can be, for example, compounds for neutralizing residual acids and / or compounds for stabilizing one or more components. The washing agent used, e.g., wash water containing the washed residual leachate, can be discarded. Alternatively, it can be treated separately to recover its components, reused for further washing of the residue, and / or used at other points in the process, e.g., mixed with the leachate and / or leach solution.

[0062] In step S15, the amount of colloidal silica in the dissolved sewage sludge ash is controlled, i.e., reduced, relative to the starting material composition shown above. The amount of colloidal silica present in the leachate provided to the extraction step described herein below is controlled to match the amount of silica crud during the organic solvent liquid-liquid extraction, which is sufficiently low to make the organic solvent liquid-liquid extraction operable. This can be performed as a separate process, as shown in the flow diagram, or as a process at least partially shared with steps S10 and / or S12. For example, in different embodiments, the dissolution process in S10 can be affected to reduce the colloidal silica content, or the colloidal silica that forms larger silica particles can be separated in step S12. Step S15 can also be completely separated from steps S10 and S12 in other embodiments.

[0063] Step S15 can be considered a substep of step S14 of pretreating the leachate. This step S14 of pretreating the leachate may optionally include other types of pretreatment of the leachate. In addition to colloidal silica, sulfur, if present, is also typically leached with hydrochloric acid. Sulfur can precipitate as gypsum, at least upon aging of the leachate. In optional step S16, the leachate is pretreated with respect to sulfur to hasten gypsum precipitation, thereby typically separating it with undissolved residue, or to prevent gypsum formation, thereby utilizing the sulfur for subsequent processing. In the case of colloidal silica, such pretreatment of the sewage sludge ash leachate may be performed separately and / or simultaneously with leaching.

[0064] As discussed in more detail later in this disclosure, it has been found that ferrous iron is extracted less efficiently by liquid-liquid extraction than ferric iron. In optional step S17, the ferric iron in the leachate is reduced to ferrous iron, resulting in lower iron extraction efficiency later in the process. In optional step S18, the ferrous iron in the leachate is instead oxidized to ferric iron, resulting in higher extraction efficiency later in the process. Which of these steps, if any, are performed depends on the actual approach for iron extraction selected later in the process, as discussed in more detail below.

[0065] The process 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 with an organic solvent. Different embodiments of the sub-processes are discussed further below, describing sub-step S21, separation, purification, and recovery of iron, and / or sub-step S30, separation, purification, and recovery of phosphorus.

[0066] In optional step S40, the raffinate from step S20 may undergo different types of post-treatment. Typically, such post-treatment may include further separation of valuable elements resulting from the sewage sludge ash. This may include, for example, step S41 for the separation and recovery of aluminum, step S50 for the separation and recovery of heavy metals, step S60 for the separation and recovery of magnesium, step S65 for the separation and recovery of sodium, step S70 for the separation and recovery of potassium, and / or step S75 for the separation and recovery of calcium. Some of these steps may be performed in combination in different embodiments. Various types of such processes are discussed in more detail below.

[0067] In step S90, at least a portion of the iron- and / or phosphorus-removed solution is recycled. The iron- and / or phosphorus-removed solution originates from step S20, where iron and / or phosphorus are extracted, possibly via an optional post-treatment step S40. The recycled portion of the iron- and / or phosphorus-removed solution contains chloride ions. The recycled solution is used to dissolve starting materials derived from sewage sludge ash. In one embodiment, dissolution using this recycled solution is the subsequent step S10. Recycling, reuse, and / or recycling process streams contributes to minimizing secondary waste generation and chemical consumption. This may also ensure that some of the process conditions required to perform some of the aforementioned steps are met.

[0068] The step S90 of recycling at least a portion of the raffinate is preferably controlled in accordance with the findings discussed in the Examples so that the concentration of chloride salts and / or hydrochloric acid in the leach solution is at least 1M.

[0069] In different embodiments of the present technology, the ash leachate undergoes several steps to selectively separate, purify, and convert the different components in the solution into commercial compounds. This is preferably done using solvent extraction with a suitable solvent, such as TBP, and precipitation. However, as mentioned above, pretreatment steps are required before the actual extraction can begin.

[0070] Figure 2 discloses a flow diagram of a partial embodiment of step S14 of the pretreatment of sewage sludge ash leachate. As indicated above, this step includes a partial step S15 of controlling the amount of colloidal silica in the dissolved sewage sludge ash.

[0071] None of the above-mentioned prior art documents the occurrence of colloidal silica and its problematic effect on extraction. However, it has been discovered that colloidal silica does exist and adversely affects the solvent extraction process to the extent that it becomes impossible to carry out due to irreversible crud formation. Due to the large amount of silicates in sewage sludge ash, silicon species are present in the leachate at concentrations of at least several hundred ppm, despite their low solubility.

[0072] When different types of sewage sludge ash and sludge ash materials were leached with hydrochloric acid at room temperature and solvent extraction was immediately performed after solid-liquid separation of the leachate from the undissolved ash residue, irreversible crud formation prevented the process from operating. This highly stable emulsion adversely affected phase separation, preventing the organic phase from being reused for stripping and extraction. The cause of crud formation was silica present in the leachate. Silicon is a major component of sewage sludge ash, and it has been found that some is 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, disrupting the steric silicate structure and essentially depolymerizing to silicic acid. Silicon in solution reacts via two pathways, starting with silicic acid: dissociation and polymerization. As previously mentioned, 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 very small colloidal silica particles present in the leachate can stabilize oil-in-water emulsions, preventing phase separation, which is the basis for silica crud formation in solvent extraction.

[0073] 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 are discussed in more detail later, but here we focus on silica. The leachates in Table 2 had a pH <0. The data show that when leaching is performed with higher concentrations of hydrochloric acid, less silicon is leached, and the higher the liquid-solid ratio (L:S), the less silicon is leached.

[0074] [Table 2]

[0075] The leachates in Table 2 were used in solvent extraction experiments using organic phases containing organophosphorus extractants, e.g., trialkyl phosphate (TBP), kerosene, and long-chain organic alcohols as phase modifiers, as well as trialkylphosphine oxide (Cyanex 923), kerosene, and long-chain organic alcohols as phase modifiers. Crud formation occurred in all systems. Analysis of the raffinate after solvent extraction confirmed a decrease in silica concentration in the solution, contributing to crud formation. As the organic-to-acid ratio (O:A) decreased, crud became more pronounced. The crud was associated with the formation of a stirred-egg-white emulsion at the interface with the organic phase, which remained stable even several days after the end of the experiment, rendering the organic phase unusable. At O:A ratios below 1:1, crud was widespread throughout the organic phase. Several types of kerosene, including pure aliphatic kerosene, 1-dodecanol, and 1-decanol, were tested as phase modifiers. Various concentrations of extractant, diluent, and phase modifier in the organic phase consistently resulted in crud formation. Furthermore, crud formation occurred when the leachate was mixed with only the diluent and / or modifier. This occurred when using a low-ionic strength (low acidity and / or low calcium chloride content) leachate, which results in low iron and phosphorus extraction efficiency, as well as when using a high-ionic strength (high hydrochloric acid and / or high calcium chloride content) leachate, which results in efficient iron and phosphorus extraction. Filtration of the leachate through a 0.45 μm PVDF filter prior to solvent extraction did not prevent crud formation due to the very small size of the colloidal silica particles (0.01–0.5 μm). Removing these particles cost-effectively using conventional filtration or centrifugation on an industrial scale is not feasible.

[0076] In further experiments, the leaching behavior depending on the hydrochloric acid concentration during leaching was further investigated.

[0077] The sludge ash was leached with hydrochloric acid solution at ambient temperature (21°C) at a liquid-to-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 the undissolved residue by filtration (pore size 1 μm). Immediately after filtration, the filtrate samples were diluted with deionized water to stabilize the solution and prevent unwanted precipitation of silicon species. The elemental concentrations in the solution were quantified using ICP-MS. The results are shown in Table 3 below.

[0078] [Table 3]

[0079] The results show that by controlling the hydrochloric acid concentration, it is possible to control the amount of silicon (including colloidal silica) remaining in the leachate, as well as the amounts of other elements such as iron, sulfur, chromium, and nickel. This allows for control of the recovery rate of ash components. When a higher concentration of hydrochloric acid was used, less silicon was present in the solution, and more iron and heavy metals were dissolved.

[0080] This example shows that the total silicon concentration in solution (including colloidal silica) and the leaching behavior of the sludge ash components can be controlled by controlling the hydrochloric acid concentration during leaching, which affects, for example, the stability of the solution after calcium sulfate precipitation and the amount of crud formation during solvent extraction.

[0081] The hydrochloric acid concentration also affects the efficiency of solvent extraction of phosphorus and iron.

[0082] Three leachates were subjected to batch solvent extraction tests immediately after preparation. Samples of each leachate were mixed for 5 minutes with an organic solution containing TBP (organic / aqueous ratio 1:1). 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. Irreversible crud formed every time, regardless of the leachate used, the amount of TBP and 1-decanol in the organic phase, or the presence or absence of kerosene. However, when the sludge ash was leached with 9 M hydrochloric acid, less crud formed, which correlates with the lower silicon concentration (115.5 mg / L) quantified in this leachate. The extraction yields of iron and phosphorus were significantly higher when the sludge ash was leached with a higher concentration of acid, confirming that the extraction process can be controlled by controlling the concentration of acid used to leach the ash. In fact, the iron and phosphorus extraction yields from 3M acid leachates were very low, approximately 2% for a 30% by volume TBP solution. However, efficiency improved significantly when the chloride content of the solution was increased, for example, by internally recycling chloride-containing streams (chloride salts and / or hydrochloric acid). In contrast, the iron extraction yield increased to approximately 90% for a 6M hydrochloric acid leachate due to its higher acidity and chloride concentration. Higher extraction yields could also be achieved by adding chloride salts to a 3M ash leachate to increase its ionic strength and by recycling an internal stream to maintain a sufficiently high chloride ion concentration. For example, the addition of calcium chloride significantly improved the iron and phosphorus recovery efficiency. Adding 1M calcium chloride to the leachate increased the iron extraction yield from 2% to 25% per stage, 53% at 1.5M calcium chloride, and 78% at 2M calcium chloride.

[0083] Colloidal silica particles grow slowly, and it has been observed that some eventually precipitate when the leachate is aged for a sufficient period of time, ranging from a few hours to even several weeks, depending on the pH, initial silicon content, and other parameters. However, even with leachate aged for several weeks, crud still formed and precipitated, which was removed by 0.45 μm filtration before solvent extraction. This is because, despite the long aging time, the colloidal silica does not completely precipitate, and the solvent extraction process is sensitive to very small amounts of total suspended solids. While a total suspended solids concentration of less than 20 ppm is considered ideal for solvent extraction, many treatment plants operate with average total suspended solids concentrations above 40 ppm. It has been found that even low levels of silica (as little as 50 ppm present as colloidal silica) can cause crud to form.

[0084] Further experiments have shown that aging can reduce the amount of silicon in the leachate. Aging promotes the growth of colloidal silica particles, which initially have a very small submicron particle size and do not settle. The rate of this process depends on several factors, including but not limited to the solution pH, other elements present in the solution, temperature, and initial silicon concentration. Allowing the silica particles to grow to a sufficient size allows for filtration and / or sedimentation. The leachate prepared in the examples referenced in Table 3 was aged for 48 hours by leaving it in a sealed container. At various time points, the aged leachate was filtered through 1 μm pores to remove particles that may have grown larger than 1 μm. Table 4 below shows the total silicon concentration measured at various time points. The results show a slow decrease in silicon concentration. This decrease is greater with higher hydrochloric acid concentrations; for example, 9M acid was more effective than 6M acid, which in turn was more effective than 3M acid. Furthermore, in solvent extraction tests carried out in the same manner as in the above examples, crud was formed even at the lowest silicon concentration, i.e., 95.2 ppm or mg / L.

[0085] [Table 4]

[0086] Thus, aging has been shown to reduce silicon concentration, however, it is currently believed that aging must be combined with other colloidal silica control actions to obtain a leachate suitable for solvent extraction.

[0087] Further experiments were carried out to investigate the role of temperature in leaching of silica. These experiments surprisingly reveal that control of temperature and time during leaching, optionally in combination with selection of acid concentration, can be effectively used to minimize crud formation during solvent extraction processing of sludge ash.

[0088] Sludge ash was leached with 6 M hydrochloric acid at a liquid-solid ratio of 3 / 1 (v / w) but at higher temperatures for longer times: i) 60 °C for 5 h, and ii) 90 °C for 1 h. A significant decrease in the silicon content in the solution was observed in both cases compared to leaching at ambient temperature (Table 5).

[0089] [Table 5]

[0090] This finding was quite surprising, since prior art literature suggested otherwise. According to literature, e.g., Lenher and Merrill, 1917, the solubility of silicon in hydrochloric acid increases significantly at higher temperatures. According to this literature, if leaching of sludge ash was carried out at 60°C or 90°C, one would expect more silicon to be in solution, but surprisingly, this was not the case in the present case, where starting material derived from sewage sludge ash was used. When solvent extraction batch tests were performed with the leachates obtained at 60°C and 90°C, no significant crud was observed (organic:aqueous ratio 1:1, 80% by volume of TBP).

[0091] The above observations proved valid when sewage sludge ash, which had already been leached with hydrochloric acid to remove most of the phosphorus and some, but not all, of the iron, was used instead. This starting material was leached at temperatures above ambient temperature using at least 6 M HCl. This was intended to dissolve the iron and produce an iron-rich solution containing small amounts of phosphorus. This solution was then used for solvent extraction tests. As in the previous example, the leaching process resulted in lower amounts of silicon in the solution compared to ambient temperature leaching, and no crud was formed when using 80% TBP in a 1:1 organic / aqueous phase. Subsequent treatment of the leachate with a silica coagulant and subsequent filtration reduced the silicon concentration from several tens of mg / L to approximately 6 mg / L. This solution was then subjected to solvent extraction in a countercurrent separation device (multiple mixer-settler units), and the process was carried out continuously for several days without silica crud.

[0092] From these surprising findings, it was concluded that the dissolution step, performed at an elevated temperature of at least 50°C, could yield a leachate compatible for operable liquid-liquid extraction with organic solvents.

[0093] Surprisingly, while publications such as "Dealing with a siliceous crud problem in solvent extraction," Hydrometallurgy 15 (1985) discuss treating silica-containing leachates with activated carbon as a means of mitigating silica crud during solvent extraction, this was unsuccessful. Treating sludge ash leachates with activated carbon followed by filtration through a 0.45 μm PVDF filter did not prevent TBP crud formation. Publications such as "Coagulation of colloidal silica by calcium ions, mechanism, and effect of particle size," Journal of Colloid and Interface Science, volume 53, issue 3, pp. 576-588, 1975, also mention that colloidal silica may be adsorbed by calcium ions at high calcium concentrations in solution. Surprisingly, this was ineffective for the sludge ash leachates tested here, up to calcium concentrations of 2.7M. Adding a flocculant to the leachate did not solve the problem, as the colloidal silica particles were too small to be affected by the flocculant. Diluting the leachate with water, hydrochloric acid, or salt solutions has been found to minimize crud formation because dilution reduces the silica concentration in the solution. However, this is not feasible because it increases the amount of aqueous phase in the process, consumes chemicals or water, and requires greater processing capacity.

[0094] However, the formation of crud was avoided by adding a fluorine source, in this case ammonium fluoride, to the leachate, which converts the insoluble colloidal silica particles into ammonium fluorosilicate, which has a solubility limit in water of approximately 185 g / L at 25°C. In one embodiment, the addition of NHF was used to form (HN)SiF. In other words, in one embodiment, the step of reducing the colloidal silica involves adding a fluorine source, such as NHF, HF, NaF, or CaF, to convert the colloidal silica to a fluorosilicate species, such as (HN)SiF.

[0095] Additionally, crud was further avoided by using coagulants designed to bond and grow colloidal silica particles, facilitating their removal from solution via conventional solid-liquid separation techniques, such as clarifiers, filtration, or centrifugation. This method is an alternative to adding fluoride ions when fluorine is not desired downstream. Adding several hundred ppm of coagulant to the leachate instantly precipitates colloidal silica, allowing removal via filtration and avoiding crud during solvent extraction. Silicon in solution can exist as both soluble silicon species, such as silicic acid, and insoluble colloidal silica formed from the dissociation and polymerization of silicic acid. Coagulation precipitates only colloidal silica.

[0096] Adding a silica coagulant to the leaching reactor during ash leaching, followed by a single filtration to remove both the undissolved ash residue and the coagulated silica, proved effective in experiments in facilitating subsequent processing of the resulting leachate using a mixer-settler without a silica crud. A silicon concentration of 36.4 mg / L was determined in the solution. Adding a silica coagulant to the leachate and removing the coagulated silica by filtration after separating it from the undissolved ash residue also made it possible to operate without a silica crud. This was achieved regardless of whether a TBP solution diluted with 1-decanol and / or kerosene, e.g., 40% or 75% by volume of TBP, or neat TBP, was used.

[0097] This example demonstrates that i) silica coagulation is effective when sludge ash is leached with 3 M aqueous hydrochloric acid at ambient temperature (21°C), regardless of whether a coagulant is added during or after ash leaching, and ii) after removal of colloidal silica by coagulation, the solvent extraction process can be carried out continuously in a countercurrent separation unit (multiple mixer-settlers) without silica crud. Such leachates, if uncoagulated, can contain more than 300 mg / L of silicon (see Table 3), and crud occurred during the solvent extraction of iron and phosphorus with TBP, even when only a single organic-aqueous mixing stage was used. Multiple separation stages (e.g., mixer-settlers), common in the industry, could accentuate the problem and render the process inoperable.

[0098] While the former may not generate crud directly, it can be extracted and stripped by the organic phase. When extracted and stripped, silicic acid or other ionic silicon species can be concentrated in the strip product if the O:A ratio is changed, as is the case in the present invention. Colloidal silica can form in the strip product due to ionic silicon concentrations exceeding the stability threshold and / or due to changes in pH, which can generate crud. To prevent this, a silica coagulant or fluoride source can be added to the aqueous phase, i.e., the strip solution and / or the strip product, to reduce crud formation and promote phase separation in later stages of the solvent extraction process. The presence of a silica coagulant in solution has been found to aid in the precipitation of newly formed colloidal silica. This was also observed in sewage sludge ash leachate that was left to stand for several days after the colloidal silica had been removed by coagulation and filtration and then aged. Therefore, it is generally recommended to perform the solvent extraction process as soon as possible after the colloidal silica has been coagulated and removed from the leachate.

[0099] According to the technology presented herein, colloidal silica is controlled in a pretreatment step, which can be carried out at different points in the process, preferably before contacting the sewage sludge ash leachate with the organic phase. In one embodiment shown in Figure 2, step S15 of reducing the amount of colloidal silica is carried out after the sewage sludge ash has been leached with hydrochloric acid and the leachate has been separated from the undissolved residue. In other words, in one embodiment, colloidal silica control is at least partially carried out in the leachate after the step of separating the undissolved residue, whereby the method includes an additional step S19 of removing coagulated silica particles from the leachate.

[0100] In another embodiment, shown in Figure 3, pretreatment occurs 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 occurs at elevated temperatures, preferably above 50°C, thereby reducing the amount of colloidal silica in the leachate. In other words, in some embodiments, control of colloidal silica occurs at least partially simultaneously with the dissolution process.

[0101] These two embodiments may of course be combined, in which case part of step S15 controlling the amount of colloidal silica is carried out simultaneously with dissolution and another part after separation of the residue.

[0102] Thus, the inventors have surprisingly demonstrated that crud formation is avoided if the coagulant is either: 1) added first to the leachant, then mixed with the sludge ash, and the solution is filtered after leaching is complete, 2) added to the leachant-sludge ash slurry during mixing but before filtration, 3) added to the leachate after filtration, and then the precipitated silica fraction is removed, or 4) a combination of the above (only a portion of the silica is reduced in each step).

[0103] Although not optimal, it has been found possible to precipitate colloidal silica by adding a coagulant to the filtered leachate and then immediately follow with solvent extraction without removing the precipitated silica from solution. While this approach mitigates much of the crud caused by silica particle growth due to coagulation, operating under such conditions is not considered ideal and can still lead to organic loss due to crud.

[0104] Experiments have demonstrated that colloidal silica cannot be completely removed by conventional filtration or centrifugation, but can be removed by using a specially formulated coagulant for colloidal silica, which bonds and grows colloidal silica particles, allowing removal by precipitation, clarification, or filtration, and allowing solvent extraction operations with significantly reduced organic phase losses, even without a removal step.

[0105] In addition to the option of converting colloidal silica to ionic silicon by promoting coagulation to grow particles and then removing the ionic silicon, other options are feasible. If very fast processes are used, the ionic silicon present in the leachate may be removed before it is converted to colloidal silica. The silicon reduction step is therefore part of the normal dissolution and separation steps.

[0106] Alternatively, the reverse time schedule may be used, where the dissolved sewage sludge ash is aged to allow time for silica growth, which typically occurs after residue separation, but may also occur at least partially before.

[0107] Experiments have also shown that the amount of colloidal silica can be controlled by performing a dissolution process using a strong acid solution, so that the control process becomes part of the normal dissolution and separation process.

[0108] From the experiments, it was concluded that by carrying out the dissolution process at an elevated temperature of at least 50°C, it is possible to obtain a leachate suitable for operable liquid-liquid extraction using organic solvents.

[0109] In other words, in one embodiment shown in FIG. 4, the step of reducing colloidal silica comprises: - promoting coagulation to grow particles, as in step S151; - converting the colloidal silica to ionic silicon, as in step S152, and optionally removing the ionic silicon; - Rapidly removing ionic silicon before it is converted to colloidal silica, as in step S153; - carrying out a dissolution step using a strong acid solution as in step S154; - Aging the dissolved sewage sludge ash to promote at least one of the conversion of ionic silicon to colloidal silica and the growth of colloidal silica particles, as in step S155; and - Leaching at elevated temperatures, as in step S156 It includes at least one of the following:

[0110] As mentioned above, the pretreatment step S14 and the colloidal silica reduction step S15 can be carried out after the dissolving step S10 or, surprisingly, simultaneously with the dissolving step S10. In the latter case, the colloidal silica coagulant is added during leaching. The advantage of this is that the coagulated silica can be removed together with the undissolved ash residue, eliminating the need for a second solid-liquid separation step S19 to remove the precipitated silica.

[0111] 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. For example, according to the paper "Composite Fouling of Calcium Sulfate and Calcium Carbonate in a Dynamic Seawater Reverse Osmosis Unit," MSc. Thesis, The University of New South Wales, Sydney, Australia, 2005, the induction period of gypsum (the appearance of gypsum crystals) in aqueous solutions of inorganic salts is particularly increased by the presence of iron ions.

[0112] For leachates similar to those in Table 2, the inventors observed that sulfur precipitation occurred slowly after filtration, sometimes over several days. The presence of gypsum particles in the leachate is undesirable because it can cause crud during solvent extraction and complicate the process.

[0113] Of course, further aging can be combined with a pretreatment step to ensure that gypsum is formed and precipitated prior to solvent extraction.

[0114] Alternatively, the induction period can be extended to allow operation before gypsum is formed, by adding substances conventionally known to those skilled in the art for increasing the induction period, such as sodium carboxymethylcellulose, sodium hexametaphosphate or polyacrylamide.

[0115] Furthermore, it was found that sulfur precipitation was faster at high background calcium concentrations, so ensuring a high calcium concentration in the leach solution would avoid slow precipitation.

[0116] This is summarized in FIG. 5, where different embodiments of step S16 for treating sulfur are illustrated. In step S161, the leachate resulting from the dissolution of the starting materials is aged. In step S162, a gypsum inhibitor is added to the leachate resulting from the dissolution of the starting materials. In step S163, gypsum formation is promoted. This is preferably carried out 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.

[0117] In other words, in one embodiment, the starting material contains sulfur. The method for chemically treating sewage sludge ash further comprises the step of adding a CaSO4 inhibitor prior to the step of extracting at least one of iron and phosphorus, thereby preventing calcium sulfate precipitation from occurring immediately before or during the step of extracting at least one of iron and phosphorus. The term "immediately before" should be interpreted as a period that does not allow any solid-liquid separation process.

[0118] In another embodiment, the starting material comprises sulfur. The method for chemically treating sewage sludge ash comprises, prior to the step of extracting at least one of iron and phosphorus, the further step of carrying out leachate aging and / or accelerating CaSO precipitation to prevent calcium sulfate precipitation from occurring immediately prior to or during said step of extracting at least one of iron and phosphorus.

[0119] Precipitation of sulfur as gypsum is facilitated in preferred embodiments by increasing the calcium chloride content of the leachate, which is also advantageous for subsequent solvent extraction. Increasing the ionic strength of the leachate facilitates solvent extraction of iron and phosphorus, as will be discussed in more detail below. Calcium provision is preferably achieved by adding calcium chloride to the leachate, returning the calcium chloride-rich aqueous raffinate after phosphoric acid extraction to the leach step in place of water, or a combination of both.

[0120] Various timing-related embodiments can be identified to achieve a high calcium concentration in the leachate and thus a more rapid precipitation of gypsum. Figure 6 shows an embodiment in which step S16 of sulfur treatment is performed after the actual leaching is completed. A calcium chloride salting reagent is added to the leachate after leaching. This is followed by step S169 of gypsum removal. Preferably, this step can be performed in conjunction with the addition of a silica coagulant, for example, as described above, to promote gypsum precipitation and filtration together with the silica. Since gypsum is a good filter aid, gypsum precipitation has a positive effect on silica filtration. Step S19 of Figure 2 and step S169 of Figure 6 can then be combined.

[0121] Alternatively, the addition of the substance that increases the induction period of gypsum may occur after the actual leaching is complete, in which case the sulfur will remain in solution and removal step S169 will not be necessary.

[0122] Aging the leachate for a period of time sufficient to precipitate gypsum prior to solvent extraction can also be done after the actual leaching is complete. This can be combined with the silica treatment step, if any, and can be done in settlers, clarifiers, and / or similar equipment.

[0123] Alternatively, as shown in Figure 7, step S16 can be carried out, at least in part, together with dissolution step S10. In one embodiment, a high calcium background is ensured during leaching while promoting gypsum precipitation. This can be done by, for example, adding calcium chloride during leaching, by returning the calcium chloride-rich aqueous raffinate to the leaching step after iron and / or phosphate extraction, thereby replacing water, or a combination of both. This results in a higher gypsum precipitation yield during leaching. This also allows the precipitated gypsum to be separated, e.g., filtered, together with the undissolved ash residue in step S12.

[0124] Alternatively, the addition of the substance that increases the induction period of gypsum may be performed together with the dissolution step S10.

[0125] Furthermore, as indicated above, the rate at which gypsum precipitates may also be increased by the addition of CaSO4 seed particles. Such CaSO4 seed particles may 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.

[0126] In other words, in one embodiment, the acceleration of CaSO precipitation is achieved by: adding calcium chloride at the latest during the dissolution step; adding calcium chloride to the leachate after the step of separating the undissolved residue, - returning, at the latest during said dissolution step, the aqueous calcium chloride-rich raffinate obtained after the preceding iron and / or phosphorus extraction step, - returning the aqueous calcium chloride-rich raffinate obtained after the preceding iron and / or phosphorus extraction step after the step of separating the undissolved residue, - Adding CaSO4 seed particles at the latest during the dissolving step (S10), and / or - adding CaSO4 seed particles to the leachate (206) after the separation step (S12) of the undissolved residue (204); Includes.

[0127] Iron recovery from sludge ash leachate may be achieved using an organic phase containing TBP as an extractant. The form in which the iron ion is present in the leachate, e.g., ferrous or ferric, also known as Fe(II) and Fe(III), respectively, determines whether it can be effectively extracted by TBP. Iron extraction by TBP has been shown to be significantly reduced if ferric iron is reduced to ferrous iron by the use of a reducing agent (e.g., adding metallic iron to the leachate). The reverse is also true. To facilitate the extraction of ferrous iron by TBP, oxidation to ferric iron can be achieved using an oxidizing agent.

[0128] This allows for various approaches to iron extraction. For example, if unextractable ferrous iron is present in the sludge ash or leachate, oxidation can be used to maximize iron recovery. Referring to Figure 1, step S18, which oxidizes Fe(II) to Fe(III), can be combined with the pretreatment step S14. Oxidation can occur during and / or after the dissolution step S10. The oxidizing agent can be, for example, hydrogen peroxide, ozone, chlorine gas, etc.

[0129] In other words, in one embodiment, the method for chemically treating sewage sludge ash includes a further step prior to the extracting step S20 of oxidizing Fe(II) in the leachate to Fe(III).

[0130] Conversely, reduction can be used to minimize iron extraction, for example, by avoiding iron extraction and proceeding to phosphorus recovery. This may be preferable for ash and / or leachate with very low iron content, where the benefits of recovering iron do not compensate for the increased complexity of recovering both iron and phosphorus. This reduction of Fe(III) to Fe(II), step S17, can be combined with pretreatment step S14. Reduction can occur during and / or after the dissolution step. Metallic iron or other reducing species reduces ferric iron, Fe(III), in 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 precipitates.

[0131] In other words, in one embodiment, the method for chemically treating sewage sludge ash includes the additional step of reducing Fe(III) in the leachate to Fe(II) prior to the extraction step S20. In such a case, step S20 may, in certain embodiments, include a phosphorus extraction step S30, but may not include an iron extraction step.

[0132] FIG. 8 shows a schematic diagram of an arrangement 1 for chemically treating sewage sludge ash. The dissolution reactor 10 has an inlet for a starting material 201, which originates from sewage sludge ash and contains at least silica and iron. The dissolution reactor 10 also has an inlet for an acid 202, which contains hydrochloric acid. The starting material originating from sewage sludge ash is at least partially dissolved in the dissolution reactor. A separation device 12, e.g., a filter, separates the undissolved residue 204 from the leachate 206. Preferably, the arrangement 1 for chemically treating sewage sludge ash further comprises a washing chamber 13 configured to wash the remaining acid from the separated undissolved residue 204 to obtain a washed residue product 204X. The used washing liquid 205 may be returned to the appropriate stage of the process via a return line 131. Alternatively, the wash solution 205 may be fed to the extraction section 20 (described further below) and mixed with, for example, the percolation solution 206 or the percolation solution 207 (described below).

[0133] The arrangement 1 for the chemical treatment of sewage sludge ash further comprises devices 14A, 14B, 14C for controlling the amount of colloidal silica present in the leachate 207 fed to the extraction section 20. These devices may comprise an inlet 14B for the additive 203 to the dissolution reactor, and / or at least one pretreatment chamber 14A into which the additive 203 may be introduced, and / or a heater 14C arranged to provide a temperature increase of at least 50° C. to the dissolution reactor 10, as will be described in more detail below. A leachate 207 containing little or no colloidal silica is obtained.

[0134] The arrangement 1 for chemical treatment of sewage sludge ash further comprises an extraction section 20 having equipment 22 for liquid-liquid extraction with an organic solvent. The equipment 22 for liquid-liquid extraction is configured to extract at least one of iron and phosphorus from a leachate 207 containing little or no colloidal silica. A stripping liquid 209 is added, thereby providing at least one liquid 210 containing iron and / or phosphorus. A raffinate 211 from which iron and / or phosphorus has been at least partially removed is provided from the extraction section 20.

[0135] Preferably, the arrangement 1 for chemical treatment of sewage sludge ash further comprises a post-treatment section 40, in which components remaining in the raffinate 211, such as aluminum 213, heavy metals 214, magnesium 215, sodium 216, potassium 217, and / or calcium 218, may be separated and recovered by using different post-treatment additives 212.

[0136] The optionally post-treated raffinate is at least partially recycled. The at least partially iron- and / or phosphorus-free raffinate 211 from the extraction section 20 can be returned to the inlet of the dissolution reactor 10 via the return line 90 for dissolving the starting material originating from the sewage sludge ash. The recycled portion 219 of the at least partially iron- and / or phosphorus-free raffinate 211 contains chloride ions.

[0137] Preferably, the return line 90 is equipped with means for controlling the amount of raffinate recycled to obtain a concentration of at least 1M chloride salts and / or hydrochloric acid in the leach solution.

[0138] FIG. 9 shows a schematic diagram of one embodiment of a dissolution reactor 10 incorporating a device for reducing the amount of colloidal silica in dissolved sewage sludge ash. In this embodiment, an inlet 14B is provided through which additives 203A can be introduced into the dissolution reactor 10. These additives 203A may promote coagulation to grow silica particles from the colloidal silica, as further described above, or may convert the colloidal silica to ionic silicon, which can remain in solution if reasonably soluble or, if its solubility limit is reached, can at least partially precipitate into the undissolved residue 204. The silica particles grow until they can be separated by the separation device 12. Thus, control of the amount of colloidal silica can surprisingly be achieved within the same dissolution reactor 10, simultaneously with the dissolution itself.

[0139] Similarly, an additive 203B may be included to prevent calcium sulfate precipitation during or upon transport to the extraction section 20. For example, a CaSO inhibitor may be introduced to mitigate the precipitation of gypsum, so that the sulfur passes through this constituent in solution. Alternatively, an additive may be added to accelerate the precipitation of CaSO, so that the gypsum precipitates and is separated by the separation device 12 together with the undissolved residue 204. Thus, the treatment of sulfur can surprisingly be carried out in the same dissolution reactor 10, simultaneously with the dissolution itself.

[0140] Additionally, oxidizing or reducing additives can be added to affect the oxidation state of the iron ions, and possibly other metals as well. A reducing agent 203C, such as iron metal, may be used to reduce Fe(III) to Fe(II). Alternatively, an oxidizing agent 203D may be used to oxidize Fe(II) to Fe(III). Thus, the manipulation of the oxidation state of iron can surprisingly be carried out in the same dissolution reactor 10, simultaneously with the dissolution itself.

[0141] 9 also shows a heater 14C, which is positioned to provide elevated temperature within the dissolution reactor 10. This may be done by heating the acid 202 prior to or in conjunction with entering the dissolution reactor and / or by heating the acid during the leaching process in the dissolution reactor.

[0142] FIG. 10 illustrates one embodiment of a pretreatment chamber 14A. This pretreatment chamber 14A can be used as a complement or alternative to the configuration for reducing the amount of colloidal silica in dissolved sewage sludge ash shown in FIG. 9. In the first mixing chamber 141, additives 203A can be added through an inlet 143. These additives 203A can promote coagulation to grow silica particles from the colloidal silica or convert the colloidal silica to ionic silicon, as described further above. The silica particles grow until they reach a separable size. In this embodiment, the pretreatment chamber 14A includes an aging chamber 142 where precipitate can settle and silica particles can grow. Any precipitate and silica particles 204C can be separated by a separation device 144, resulting in a pretreated leachate 207. In alternative embodiments, either the mixing chamber 141 or the aging chamber 142 can be omitted, and in yet other embodiments, the mixing chamber 141 and the aging chamber 142 can be combined.

[0143] Similarly, additives 203B may be included to prevent calcium sulfate precipitation occurring at or during feeding to extraction section 20. For example, a CaSO inhibitor may be introduced to mitigate gypsum precipitation, allowing sulfur to pass through this component in solution. Alternatively, additives may be added to accelerate CaSO precipitation, causing gypsum to precipitate 204C and be separated by separation device 144.

[0144] Additionally, oxidizing or reducing agents can be added to affect the oxidation state of the iron ions and possibly other metals. For example, a reducing agent 203C, such as iron metal, may be used to reduce Fe(III) to Fe(II). Alternatively, an oxidizing agent 203D may be used to oxidize Fe(II) to Fe(III).

[0145] In one embodiment, extracting at least one of iron and phosphorus from the leachate by liquid-liquid extraction with an organic solvent comprises extracting iron from the leachate by liquid-liquid extraction with an organic solvent.

[0146] As mentioned above, iron recovery from sludge ash leachate can be achieved as iron chloride using an organic phase containing TBP as the extractant. Long-chain alcohols, such as decanol, can be used as modifiers, and kerosene can be used as a diluent, if necessary. Other organic compounds and mixtures may also be suitable for this purpose. TBP preferentially extracts iron over phosphorus and most other impurities in the leachate, although coextraction of phosphorus, calcium, and other trace elements has been observed.

[0147] The amount of elements co-extracted with iron by TBP can be reduced if the extraction is performed at approximately the loading capacity of the iron-containing organic phase. This situation may be achieved by controlling the organic-to-aqueous (O:A) ratio and / or the TBP content in the organic phase. Thus, iron can be controlled to be concentrated in the loaded organic phase by performing the extraction at a low O:A ratio. Conversely, co-extraction of, for example, iron and phosphorus can be achieved by performing the extraction at a high O:A ratio. This allows for the possibility of adapting the characteristics of the extraction process to, for example, the content of the leachate.

[0148] However, as briefly mentioned above, the content of the leachate can also be controlled to some extent by varying the operating conditions during the leaching procedure, which allows for different approaches to recover the iron and phosphorus, as well as other components, of the leachate.

[0149] Therefore, another aspect of preferred embodiments 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 duration, presence of oxidizing or reducing species, ash particle size, L:S ratio, O:A ratio, etc. This allows for control of the amounts of sewage sludge ash components dissolved and extracted at various stages of the process, thereby enabling individual separation of components, e.g., iron from aluminum, calcium from magnesium, or a pure phosphorus stream, ensuring high purity of the separated compounds.

[0150] For example, the dissolution yields of some components can be controlled during leaching, resulting in a leachate rich in both iron and phosphorus, or a leachate rich in phosphorus but low in iron. Table 6 shows the leaching yields of iron and phosphorus when sewage sludge ash is leached using hydrochloric acid solutions of different concentrations, leaching times, temperatures, and L:S ratios. The leaching yields of calcium were similar to those of phosphorus. At ambient temperature and different acid concentrations, the leaching yields of aluminum ranged from 30 to 35%. As shown in Table 2, increasing the hydrochloric acid concentration and / or decreasing the L:S ratio resulted in less silica leaching. These parameters strongly influence the leaching process and can be selected to achieve the desired leaching yields for iron and phosphorus. These affect both the element concentrations in the leachate and the undissolved residue after leaching.

[0151] For example, in one embodiment, leaching can be carried out using a dilute hydrochloric acid solution at a low L:S ratio, e.g., at ambient conditions, to produce a leachate with a low iron:phosphorus ratio, and the corresponding undissolved silicate residue will then contain most of the iron.

[0152] In another embodiment, a more concentrated hydrochloric acid solution and / or a higher temperature and / or a longer leaching time can be used to obtain a leachate with a higher iron:phosphorus ratio than previously, which can result in a higher recovery of these elements from the sludge ash and produce a cleaner, iron-depleted, undissolved silicate residue.

[0153] [Table 6]

[0154] In yet another embodiment, sequential leaching can be performed to obtain leachates with various iron:phosphorus ratios. For example, in a first stage, leaching is performed to leach most of the phosphorus but not completely dissolve the iron. Preferably, iron dissolution is minimized. This is typically achieved by using lower concentrations of hydrochloric acid and / or shorter leaching times and / or lower temperatures. If iron dissolution is minimized, a significant amount of iron remains in the undissolved silicate ash residue. In a subsequent second stage, this residue is treated to recover the iron and / or other undissolved components, resulting in a silicate product with reduced iron and impurity content. To achieve this, leaching is performed with concentrated hydrochloric acid and / or elevated temperatures and / or longer leaching times. Such leachates typically have a higher iron:phosphorus ratio than leaching sewage sludge ash in a single step. The raffinate after iron extraction can be efficiently used to leach new sewage sludge ash and / or reused in other steps of the process and / or processed according to embodiments described herein.

[0155] The above three embodiments will be described in detail below.

[0156] In various embodiments of the present technology, solvent extraction is used to extract iron from other components of the leachate. One embodiment of the general solvent extraction principle in the form of a solvent extractor 220 is shown schematically in FIG. 11. An input aqueous solution 221 is fed to a contact vessel 80, where iron is extracted by an organic phase 222, resulting in an iron-loaded organic phase 222B. A raffinate 227 is passed from the contact vessel 80 to a subsequent recovery process, such as a phosphorus extraction process. The iron-loaded organic phase typically also contains co-extracted phosphorus, calcium, and other trace impurities. Optionally, scrubbing 81 of the co-extracted impurities from the loaded organic phase 222B can be performed using a scrubbing liquor 223, thereby forming a loaded scrubbing liquor 223B and a scrubbed organic phase 222C. The scrubbed organic phase 222C may still contain, for example, some phosphorus and calcium, but typically at low concentrations. In stripping vessel 82, iron is stripped from loaded organic phase 222B or scrubbed organic phase 222C into strip solution 225 to obtain iron-loaded strip solution 226. Organic phase 222 is recovered and may be reused for the next extraction.

[0157] As described below, the iron-loaded strip solution 226 may be further processed to remove impurities, concentrate the iron, and / or neutralize the hydrochloric acid that was extracted and stripped with the iron.

[0158] A wide variety of organic extractants have been proposed in the literature for the extraction of iron ions from chloride media: solvating agents (trialkyl phosphates, e.g., tributyl phosphate (TBP), alkyl phosphine oxides, e.g., 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 various extractants, e.g., solvating and acidic extractants, can be used to enhance extractability and / or selectivity. Tributyl phosphate is a commercially available solvating extractant commonly used in the phosphate industry for the purification of phosphoric acid by solvent extraction. Tributyl phosphate is non-flammable, has low toxicity, and has a low solubility in water, approximately 0.4 grams per liter at room temperature. Furthermore, solubility decreases with increasing temperature and with increasing phosphoric acid concentration in the aqueous phase. Because TBP has a relatively high density (approximately 0.98 kg / L), it is commonly mixed with a diluent such as aliphatic kerosene to improve the physical separation of TBP from the aqueous phase.

[0159] Experiments performed have shown that when in contact with concentrated salt solutions such as calcium chloride, the density difference can be sufficient to avoid the use of a diluent, provided that stripping is carried out in a manner that allows for efficient phase separation. However, the ability of TBP to extract both iron and phosphate from hydrochloric acid media also poses a problem, as it means that the iron and / or phosphate streams produced by solvent extraction with TBP run the risk of being contaminated with phosphorus and / or iron, respectively.

[0160] Extraction of ferric iron with TBP can be carried out according to equation (1), which indicates that for every mole of iron, one mole of hydrochloric acid is extracted. This means that sufficient hydrochloric acid must be present in the leachate to extract the iron. The extracted iron can be stripped using water or other solutions, such as dilute acid, salt, or basic solutions, as long as the pH after stripping is low enough to prevent iron precipitation. The hydrochloric acid extracted by TBP is also stripped, resulting in an acidic stripped product, a mixture of hydrochloric acid and iron chloride, when stripping is carried out with water. Because TBP prefers more acidic solutions, if the strip solution is too acidic, the stripping process becomes ineffective. In other words, re-extraction of iron from the stripped product can occur simultaneously with stripping. Therefore, there is a limit to the O:A ratio that can be used for water stripping, which determines how concentrated the stripped product can be.

number

[0161] Controlling the uptake of iron and phosphorus in the solvent extraction process is an important aspect of this technology, as TBP can extract phosphate with high yields (this is further detailed below when discussing phosphorus recovery). Such control allows for the production of high-purity iron and phosphorus streams. Leaching sewage sludge ash with 3M hydrochloric acid solution at a 3:1 L:S ratio (L / kg) followed by solvent extraction with TBP significantly reduced the extraction efficiency of iron and phosphorus due to the low ionic strength of the leachate. Significantly better extraction yields for iron and phosphorus were obtained by increasing the ionic strength of the leachate by adding approximately 2M solid metal chloride salts to the leachate and / or by leaching the sludge ash with concentrated hydrochloric acid, e.g., 6M hydrochloric acid. Preferably, the ionic strength should be controlled to be high.

[0162] Ionic strength control can be achieved in at least three different ways. To improve or optimize iron and phosphorus extraction yields, sludge ash may be leached using a leaching agent with an initial hydrochloric acid and / or salt content selected so that the resulting leachate under particular operating conditions, e.g., leaching time, L:S ratio, temperature, etc., contains a sufficiently high ionic strength of hydrochloric acid and chlorides to ensure the desired extraction yield. Referring to Figure 8, the leaching agent can thus be included in stream 201 or stream 202.

[0163] In an alternative or supplemental embodiment, the addition of hydrochloric acid and / or salts can be performed during leaching by one or more compounds (e.g., stream 203). A recycled process stream 219 may also be used, which can increase the ionic strength of the leachate to ensure the desired extraction yield.

[0164] As a further option, 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 can be carried out after leaching to ensure the desired extraction yield.

[0165] Of course, the above options can be combined in various ways, with two or three of them being used simultaneously or at different times during the production session. For example, at the start of a production session, if no recycled process stream is used, salt can be introduced after leaching, while at later stages, recycled process stream added during leaching may be sufficient to reach the desired ionic strength.

[0166] One of the applicant's previous inventions, disclosed in WO 2014 / 178788, describes the preferential extraction of iron over phosphorus from an aqueous solution containing 21.42 g / L of iron and 26.69 g / L of phosphorus at a 1:1 O:A ratio using an organic phase containing 30% TBP. In this particular case, the raffinate after solvent extraction contained 0.74 g / L of iron and 27.53 g / L of phosphorus, resulting in an organic phase containing more than 20 g / L of iron and no phosphorus. However, a typical sludge ash leachate obtained at ambient temperature using a mild hydrochloric acid solution contains significantly less iron (3-5 g / L), as seen in Table 2. This is a result of the low iron leaching yield under these conditions (Table 5). The high iron content (21.42 g / L) in the aqueous phase of WO 2014 / 178788 was achieved after adding base to a similar sludge ash leachate to precipitate an intermediate calcium phosphate-iron-aluminum compound, which was then dissolved in hydrochloric acid. Thus, the aqueous solution was not obtained directly from sewage sludge ash but was produced by additional chemical processing by adding base to the ash leachate. This approach also allowed for the preparation of an aqueous phase with sufficient ionic strength to ensure efficient extraction of iron with TBP, producing a concentrated, relatively pure, loaded organic phase that could be stripped to yield low-phosphorus iron chloride.

[0167] Using the same extraction conditions and sewage sludge ash leachate in Table 2, it was not possible to obtain the same iron extraction results without this intermediate precipitation step. To obtain a concentrated iron product from such a sewage sludge ash leachate after colloidal silica removal, extraction with TBP must preferably be carried out at a low O:A ratio to concentrate the iron in the organic phase. Furthermore, if further enrichment of iron in the stripped product is desired, stripping is preferably carried out at a high O:A ratio. Furthermore, without the addition of an appropriate salting-out reagent to the 3M hydrochloric acid leachate, low iron extraction yields were observed. For such low-iron leachates, characteristic of ash with very low iron content, and / or when leaching is carried out with a mild hydrochloric acid solution at ambient temperature, it is not feasible to extract iron at a high O:A ratio, as this results in an overly dilute organic phase that also contains significant amounts of phosphoric acid. Although iron is preferentially extracted over phosphorus, surprisingly, even at O:A ratios below 1:1, it was not possible to obtain a relatively purely loaded organic phase or iron-stripped product due to the co-extraction of phosphorus along with the iron. Increasing the amount of TBP in the organic phase to above 30% by volume and / or increasing the O:A ratio increased phosphorus co-extraction. Reducing the amount of TBP adversely affected iron extraction. In the embodiments described herein, experiments established that to minimize phosphorus extraction and achieve good iron extraction, the amount of TBP in the organic phase and the extraction O:A ratio should be adjusted to match the amount of iron present in the leachate, thereby ensuring that the organic phase is loaded with as much iron as possible, preferably at approximately the TBP loading capacity. Overloading the organic phase with iron prevents phosphorus co-extraction, with most of the phosphorus remaining in the aqueous phase, e.g., the leachate. Nevertheless, phosphorus co-extraction could not be completely avoided. Due to the low O:A ratio required to extract and concentrate the iron in the organic phase and the high O:A ratio required to further concentrate the iron in the stripped product in the stripping step, significant amounts of co-extracted phosphorus (13.0 g / L) and calcium (8.95 g / L) remained in the stripped product along with the iron (30.4 g / L) (see Table 7). However, the majority of the phosphorus was not extracted and remained in the raffinate where it may be recovered in a subsequent phosphorus solvent extraction step.Most of the other impurity elements in the leachate did not remain in the iron strip product.

[0168] [Table 7]

[0169] In summary, the amount of elements co-extracted with iron by TBP can be reduced by performing extraction at approximately the loading capacity of the iron-containing organic phase. This can be achieved by controlling the organic-to-aqueous (O:A) ratio and / or the TBP content in the organic phase. Iron can be concentrated in the loaded organic phase by performing extraction at a low O:A ratio. Stripping of iron from the loaded organic phase is performed using a suitable stripping solution, typically water, although other solutions can also be used. Stripping can be performed at a high O:A ratio to further concentrate iron in the stripped product. Stripping results in a stripped organic phase, which is recycled and reused in the solvent extraction process. Additional fresh organic phase can be added to compensate for losses. Furthermore, additional treatment of the organic phase is possible, such as washing with sodium carbonate or sodium hydroxide solution to remove decomposition products of dibutyl phosphate and / or monobutyl phosphate that may be formed during extraction and / or stripping.

[0170] Because the O:A ratio used during iron extraction and stripping can vary, phosphorus, calcium, and other co-extracted impurities may be concentrated in the stripped product along with the iron, especially if the ash leachate contains little iron relative to phosphorus, calcium, and other extractable species. To further purify the iron stripped product, it is subjected to a similar solvent extraction and stripping cycle again. If necessary, the ionic strength of the aqueous feed solution can be increased to improve iron extraction. This can be achieved by adding a compound or mixture of compounds, such as hydrochloric acid and / or metal chloride salts, or by mixing the solution with a high ionic strength stream from another step in the process, such as a calcium chloride-rich solution after precipitation of magnesium, sodium, and potassium (i.e., after step S70 in Figure 1). The composition of the organic phase in this second extraction step can be tailored to the amount of iron in the aqueous phase to promote iron extraction at approximately the loading capacity of the organic phase (where selectivity is maximized). Another advantage of subsequent solvent extraction-stripping cycles is further enrichment of iron in the iron chloride product, typically achieved by performing the extraction at a low O:A ratio and the stripping at a high O:A ratio. It has been observed that two solvent extraction-stripping cycles to extract iron are sufficient to obtain a relatively concentrated iron chloride solution low in phosphorus and other impurities. If desired, a third solvent extraction-stripping cycle can be performed, and this can be repeated, to obtain an even higher purity iron chloride product. The raffinate after the second iron solvent extraction step contains unextracted impurities, primarily phosphorus and calcium. Because the initial iron extraction is performed at a low O:A ratio and the stripping is performed at a high O:A ratio, the raffinate volume is sufficiently small to facilitate adjustments at the start of various processes, such as leaching, solvent extraction, or other processes. It has been determined that phosphorus can be effectively scrubbed from the iron-loaded organic phase using concentrated hydrochloric acid. This leaves the iron in the organic phase, but the phosphorus can be back-extracted in hydrochloric acid to obtain a higher purity iron chloride product with reduced phosphorus content.

[0171] Therefore, a particular aspect of this embodiment of the present technology is the subsequent treatment of the stripped product after iron solvent extraction with the same extractant to obtain high-purity iron chloride while simultaneously increasing the iron concentration in the final product. Experiments have shown that by adjusting the TBP concentration and extraction O:A ratio to match the iron content in the aqueous phase and performing extraction at approximately 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 performed.

[0172] FIG. 12 shows a general embodiment for the subsequent treatment of the strip product after the first solvent extraction of iron using solvent extraction to obtain high-purity iron chloride and simultaneously increase the iron concentration in the final product. The iron solvent extraction configuration includes two solvent extractors 220A and 220B connected in series. The colloidal silica-free leachate 207 enters the contact vessel 80 of the first solvent extractor 220A, where iron is loaded into the organic phase 222 along with at least one of phosphorus, calcium, and possibly other contaminants, resulting in 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 affect the extraction selectivity. The raffinate 211 is output to a section for further processing and / or 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 the stripping liquid 209 to obtain an intermediate strip solution 228 loaded with iron along with at least one of phosphorus, calcium, and possibly other contaminants.

[0173] The intermediate strip solution 228 is used as the input aqueous solution for the second solvent extractor 220B. The intermediate strip solution 228 is placed in the contact vessel 80 of the second solvent extractor 220B, and iron is loaded into the organic phase 222, resulting in an iron-loaded organic phase 222B. The composition of the organic phase 222 and / or process parameters can preferably be adjusted to match the composition of the intermediate strip solution 228. For example, additives 230, such as hydrochloric acid or other compounds, such as salts, can be optionally added. In this way, the ionic strength can be adjusted to improve extraction. A second-stage raffinate 229 containing at least one of phosphorus, calcium, and other contaminants is output and can preferably be reused in the process, such as a leaching process or a pretreatment process. In this way, phosphorus is reintroduced into the process. In the stripping vessel 82 of the second solvent extractor 220B, iron is stripped from the loaded organic phase 222B into the stripping liquid 209, resulting in a stripping solution in the form of an iron-containing liquid 210A.

[0174] Note that an optional scrubbing step can be performed on the loaded organic phase 222B, similar to FIG. 11 above.

[0175] Figure 13 shows one embodiment of step S21 for Fe extraction. In step S22, a first extraction stage is carried out to liquid-liquid extract preferably most of the iron from the leachate. In step S23, a first stripping is carried out to an intermediate strip solution. In step S24, a second extraction stage is carried out to liquid-liquid extract preferably essentially only iron from the intermediate strip solution. In step S25, a second stripping is carried out to a strip solution.

[0176] More specifically, the aqueous feed for a subsequent solvent extraction step for iron, e.g., intermediate strip solution 228, is thus the iron strip product obtained in a previous solvent extraction step for iron. 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 low O:A ratios. This is preferred because it allows for further concentration of iron in the organic phase. At the same time, stripping at O:A ratios greater than 1 allows for further concentration of iron.

[0177] Intermediate strip solution 228 is acidic and contains a minimum amount of hydrochloric acid in mol / L equal to the iron concentration in mol / L when iron extraction with TBP is carried out according to Equation 1. The hydrochloric acid, along with calcium and phosphorus compounds in the solution, contributes to the ionic strength required to extract iron into the organic phase. If necessary, the ionic strength can be increased by adding fresh hydrochloric acid and / or other salting agents, as indicated by additives 230. These additives can also be a full or partial recycle of one or more high ionic strength streams obtained from other process steps.

[0178] For example, it has been determined experimentally that the extraction yield of iron is favorably affected by the addition of hydrochloric acid solution and / or chloride salts to the aqueous phase. Table 8 shows how the stripped product after the initial solvent extraction and stripping step can be further concentrated and purified in subsequent solvent extraction steps. The data demonstrate that the amounts of calcium, phosphorus, and trace impurities such as aluminum, copper, and zinc are significantly reduced using subsequent extraction-stripping steps. While two extraction-stripping steps are shown here, the number of steps can be adjusted depending on the nature and composition of the sludge ash; for example, three or more steps can be considered for ash with low iron content and / or high content of co-extractable impurities.

[0179] [Table 8]

[0180] In other words, in one embodiment, the process for extracting iron from the leachate comprises at least two extraction stages. The first extraction stage comprises selective liquid-liquid extraction of at least a portion of the iron content, followed by stripping into an intermediate strip solution. Preferably, the majority of the iron content is extracted, preferably as FeCl3. The second extraction stage comprises selective liquid-liquid extraction of primarily iron ions from the intermediate strip solution, followed by stripping into a second strip solution. Preferably, essentially only iron is extracted, typically in the form of FeCl3. In this way, higher iron purity and / or higher iron concentration in the recovered iron product can be obtained.

[0181] This principle can be further exploited by adding further extraction steps, in other words, the subsequent extraction steps now include at least one further extraction step to achieve higher iron purity and / or higher iron concentration in the recovered iron product compared to the second strip solution.

[0182] This is advantageous when using a solution with a lower 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 carried out at ambient temperature using an acid with a hydrochloric acid concentration of less than 3 M. Preferably, the first extraction step involves controlling the relative amount of organic solvent to the leachate to match the loading capacity of the organic solvent to the Fe(III) content in the leachate. Even more preferably, the second extraction step and / or any further extraction step involves controlling the relative amount of organic solvent to the first stripping solution to match the loading capacity of the organic solvent to approximately the Fe(III) content in the first stripping solution or less. If the loading capacity is slightly higher than Fe(III), some contaminants may be co-extracted in greater amounts. In many cases, such contaminants may be tolerated or can be removed in a later step. However, the amount of contaminants generally increases as the loading capacity increases. The raffinate from the liquid-liquid extraction of the second extraction stage and / or any further extraction stages is preferably recycled for use in a subsequent step to dissolve the starting material.

[0183] Experiments have shown that co-extracted phosphorus can also be selectively scrubbed from iron by treating the loaded organic phase in a liquid-liquid extraction with at least slightly concentrated hydrochloric acid. Figure 14 shows a general embodiment for scrubbing impurities from an iron-loaded organic phase. Scrubber 81 is configured to scrub co-extracted impurities from loaded organic phase 222B with scrubbing liquor 223, thereby forming loaded scrubbing liquor 223B and scrubbed organic phase 222C.

[0184] Higher selectivity was found when the loaded organic phase 222B was scrubbed with hydrochloric acid instead of water. Selectivity increased with increasing hydrochloric acid concentration. For example, a load organic containing 16.6 g / L iron and 2.90 g / L phosphorus was scrubbed with a 15% hydrochloric acid solution at an O:A ratio of 10:1. This removed 0.5% iron and 54% phosphorus in a single step. Calcium and copper were also efficiently scrubbed, along with some of the co-extracted arsenic, cadmium, and zinc. Scrubbing at a 20:1 O:A ratio instead of 10:1 also improved selectivity, but increasing the hydrochloric acid concentration proved more important.

[0185] However, stripping after scrubbing with 15% hydrochloric acid is slightly less efficient than stripping without scrubbing because the hydrochloric acid is extracted during scrubbing. As mentioned above, the efficiency of the stripping process depends, for example, on the amount of acid in the stripping solution. High acidity and / or ionic strength promotes the extraction of iron by TBP, which means that if the stripped product has sufficiently high acidity and / or ionic strength, the stripping effect is reduced. Therefore, scrubbing impurities with hydrochloric acid produces a more acidic stripped product due to the aforementioned extraction of hydrochloric acid. The scrubbed organic phase 222C may still contain some phosphorus and calcium, for example, but at typically low concentrations.

[0186] The loaded scrubbing liquor 223B contains primarily Ca and P, but as noted above, typically also contains other impurities. Because the phosphorus content is typically considered valuable as such, and the loaded scrubbing liquor 223B preferably also contains hydrochloric acid, it is preferred that the loaded scrubbing liquor 223B be returned to the process at some stage. For example, the loaded scrubbing liquor 223B may be used during leaching or to increase the ionic strength in the leachate.

[0187] Experiments have shown that water stripping of iron is preferable to hydrochloric acid solutions because it results in higher stripping yields. Basic solutions such as sodium hydroxide, potassium hydroxide, or other soluble bases can also be used. We have found that stripping with 0.5 M aqueous sodium hydroxide solution results in slightly better stripping yields than with water. However, the O:A ratio and base concentration must be appropriately selected to prevent the resulting stripped product from having an excessively high pH, ​​which could lead to the precipitation of iron hydroxide. Literature (e.g., Sato T., 2002. Liquid-liquid extraction of iron(III) from hydrochloric acid solutions by tributyl phosphate, Shingen-to-Sozai 118, 612-616) has reported that the extraction efficiency of iron from hydrochloric acid using TBP decreases with increasing temperature. This suggests that a higher temperature during stripping may result in a higher iron concentration in the stripped product. However, surprisingly, experiments have shown that the iron concentration in the water strip product decreases with increasing temperature, and the trend is linear over the temperatures tested (0-70°C). Preferably, stripping is performed at lower temperatures to ensure a higher iron concentration in the strip product.

[0188] One of the primary uses of iron chloride is as a coagulant in wastewater treatment plants. According to equation (1), the ferric chloride product also contains a significant amount of free hydrochloric acid, and considering that TBP can also extract hydrochloric acid without iron, the acid:iron molar ratio is at least 1:1. Typically, the free hydrochloric acid content in commercially available ferric chloride coagulants ranges from 0 to 2%. For these reasons, neutralization of some of the free acid in the iron product may be desirable. According to embodiments of the present technology, this can be accomplished in several ways. Neutralization can be carried out using conventional bases such as sodium hydroxide, calcium hydroxide, or equivalents. However, this can result in contamination of the final iron solution with sodium, calcium, or other elements, which may or may not be acceptable to the end user.

[0189] Therefore, for the production of high-purity iron chloride coagulant, neutralization is preferably carried out with iron oxide, iron hydroxide, iron oxyhydroxide, and / or metallic iron, or a material containing at least one of these, such as magnetite. Ferric oxide, ferrous oxide, or mixed oxides can be used. Depending on the chemicals used for neutralization, the final iron chloride product can contain ferric chloride, ferrous chloride, or a mixture of the two. Experiments have shown that neutralization of iron oxide with metallic iron powder occurs significantly slower at ambient temperatures than at elevated temperatures. When metallic iron is added to a hydrochloric acid solution containing dissolved ferric and other impurities, two primary chemical processes occur. The first process is the reduction of species, such as ferric to ferrous or divalent copper to metallic copper (which precipitates). The second process is the reaction of hydrochloric acid with metallic iron, accompanied by the evolution of hydrogen, i.e., neutralization. The inventors have observed that when reduced metallic iron powder is added to an iron-containing acid strip product, the reduction of ferric to ferrous occurs in preference to the neutralization of hydrochloric acid. This was most pronounced at low temperatures, which did not favor the latter process. These processes were studied at ambient (21 °C) and elevated temperatures (60–95 °C) using the acidic iron chloride product obtained after solvent extraction with TBP and hydrochloric acid. The theoretical amount of metallic iron required for complete reduction of the iron and neutralization of the hydrochloric acid was added to the solution. In both cases, reduction was significantly faster than neutralization. Reduction occurred in just a few minutes, as indicated by a color change from the characteristic yellow of ferric iron to the pale blue-green color characteristic of ferrous iron. At ambient conditions, hydrogen gas evolution was not noticeable, even though the metallic iron was stirred with the leachate for several hours. Much of the added metallic iron remained undissolved even after 24 hours. Only small gas bubbles were observed to slowly form. The pH remained acidic even after 68 hours, at which point much of the added metallic iron was still undissolved. At elevated temperatures, vigorous effervescence, i.e., hydrogen evolution, was observed, indicating that acid neutralization was more effective. Nevertheless, at 60 °C, neutralization was significantly slower than reduction. The pH of the solution was still below 0 and there was iron powder in the solution after 1.5 hours.At 95°C, the reaction was significantly faster, with most of the iron dissolving after 4 hours when the pH of the solution rose to pH 2.

[0190] Neutralization of free hydrochloric acid with metallic iron is also an effective method for minimizing the amount of impurities in the iron product, especially impurities that can be reduced and are insoluble in their reduced form, such as copper. Neutralization with ferric hydroxide or oxyhydroxide has the advantage, among other things, of reacting quickly at ambient temperature without generating hydrogen. Neutralization of free acid with iron compounds has the advantage of increasing the iron content in the final product due to the chemical reaction of these compounds with the components of the solution.

[0191] One embodiment of a structure 24 for neutralizing a ferric chloride product containing hydrochloric acid is shown schematically in FIG. 15. The ferric chloride product containing hydrochloric acid can be, for example, the iron-containing liquid 210A of FIG. 12, but can also be the result of other embodiments described further below. The iron-containing liquid 210A is used here as a representative example. The iron-containing liquid 210A is introduced into a neutralization chamber 83 along with a neutralizing agent 231, e.g., metallic iron, iron oxide, iron hydroxide, and / or iron oxyhydroxide. The neutralizing agent 231 neutralizes at least a portion of the hydrochloric acid in the iron-containing liquid 210A and, if reducing conditions are met, typically reduces at least a portion of the ferric iron to ferrous iron. A heater 85 is preferably present to enable neutralization at elevated temperatures. Separation equipment 84 is provided to separate residual solid components 232, e.g., undissolved metallic iron or metallic Cu contaminants. Such residual solid components 232 may be reintroduced into a process, e.g., a dissolution reactor, or disposed of. The resulting liquid 233 is at least partially neutralized, i.e., has a reduced concentration of hydrochloric acid.

[0192] Optionally, if a ferric iron product is desired, the resulting liquid 233 can be placed in an oxidation vessel 86, which also adds an oxidizing agent 234. The oxidizing agent 234 can be, for example, hydrogen peroxide, ozone, chlorine gas, etc. Ultimately, a ferric iron product 235 is produced.

[0193] 16, one embodiment of a method for chemically treating sewage sludge ash may include step S26, in which the ferric chloride product containing hydrochloric acid is at least partially neutralized by adding a neutralizing agent. Step S26 follows step S21. In optional step S27, an oxidizing agent is used to oxidize the ferrous iron to ferric iron.

[0194] In summary, the neutralization chemistry and / or conditions can be selected to obtain iron in a desired chemical form, such as reduced, oxidized, or a mixture of the two. The chemical form of the iron can then be changed. For example, if ferric chloride is desired, an oxidizing agent such as chlorine gas can be used to oxidize the ferrous iron present. Reduction of ferric to ferrous iron can be achieved using a reducing agent such as metallic iron.

[0195] As discussed further above, dissolution of sewage sludge ash, also known as leaching, may be carried out under different conditions to obtain leachate of different compositions. When low concentrations of hydrochloric acid are used at ambient temperatures, the level of dissolved iron is generally very low. As noted above, this low iron content was extracted by purifying and concentrating the iron, preferably through two sequential liquid-liquid extraction stages. An alternative approach could instead ignore the amount of leached iron and focus only on extracting phosphorus and, potentially, additional components. One way to control the extraction of iron in the organic phase is to utilize different oxidation states of iron.

[0196] All of the iron-related recovery processes presented herein can be controlled by controlling the chemical form of iron in solution, e.g., ferric, ferrous, or a mixture of the two. Experiments have shown that controlling iron speciation controls the amount of iron rejected or extracted by TBP during the iron and phosphorus extraction processes, as well as the pH at which iron compounds precipitate. For example, ferrous species, e.g., ferrous hydroxide and ferrous phosphate, precipitate at significantly higher pHs (above 6) than their ferric equivalents (below 3). Control and / or variation of the chemical form of iron in solution can occur at various points in the process, such as dissolution, pretreatment, different solvent extraction blocks (extraction, stripping, scrubbing), recycle, and recycle loops, and is not limited to a single step. For example, oxidation or reduction of iron can occur several times throughout the process and / or alternate between process loops. For example, ferrous iron can be concentrated in aqueous solution during several loops, followed by oxidation and extraction in subsequent loops. This allows for efficient iron recovery from ash or leachate with low iron content.

[0197] We have found 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 metallic iron powder (0.1 mol) equivalent to 5.6 g / L at 21°C for 10 minutes. The leachate changed color from deep yellow to pale blue-green within minutes, indicating the reduction of ferric iron to ferrous iron. During this time, no gas evolution was observed, and more than 60% of the added iron powder remained undissolved. The solution was filtered. Solvent experiments were performed using a TBP stock solution with a 1:1 organic to aqueous phase ratio. In parallel, identical solvent extraction experiments were performed with the non-reduced leachate. Analysis of the aqueous raffinate from the reduced sample revealed that the majority of the iron (more than 92%) remained in solution and was not extracted by TBP. In contrast, 85% of the ferric iron in the non-reduced sample was extracted. Phosphate extraction was similar for both solutions. Approximately 45% of the acid was extracted by TBP. The traces of iron extracted from the reduced samples were due to incomplete reduction of ferric iron or reoxidation of ferrous iron to ferric iron during the solvent extraction batch test. To ensure good mixing of the aqueous and organic phases, the solutions were vigorously mixed for 5 minutes at ambient conditions. The presence of atmospheric oxygen and vigorous mixing would have caused reoxidation of ferrous iron. In practice, this does not occur because mixing can be performed for significantly shorter times and atmospheric oxygen levels can be controlled, such as by performing the extraction under a nitrogen blanket.

[0198] Iron reduction can be used to minimize the amount of iron co-extracted with phosphate in the phosphorus extraction step. It can even be used to avoid the iron extraction step altogether. This is shown in Figure 17. In this embodiment, iron is reduced in step S17 of pretreatment step S14, which is carried out during and / or after dissolution, step S10, but before phosphate extraction, step S30. In this embodiment, step S20 does not involve significant iron extraction, but does involve phosphorus extraction S30.

[0199] Iron in the raffinate after phosphorus recovery in step S30 can be reoxidized and further recovered, such as by solvent extraction with TBP. This can be done at various stages of the process after phosphoric acid extraction, such as before or after aluminum precipitation, as described below. Because ferrous hydroxide precipitates at higher pHs than ferric hydroxide, which would otherwise precipitate along with aluminum, ferrous hydroxide can selectively precipitate aluminum at pHs below 3 (leaving the iron in solution).

[0200] Similarly, oxidation of iron can be performed using oxidizing agents such as hydrogen peroxide, ozone, or chlorine gas to facilitate extraction with TBP and / or precipitation at low pH, e.g., as ferric hydroxide or ferric phosphate. This can be performed at different stages of the process, as described above. In one embodiment, this is performed as step S18 (Figure 1) of pretreatment step S14 (Figure 1). For example, it has been found that with ash and leachate containing ferrous iron, extraction with TBP is ineffective, with the ferrous iron remaining in the raffinate. Iron recovery can be maximized by oxidizing the ferrous iron to ferric iron, which can then be extracted with TBP. In an actual experiment using sludge ash that also contains ferrous iron, some of the iron in the leachate (0.6 g / L) was not fully extracted with TBP and remained in the raffinate. Subsequent extraction with fresh TBP failed to extract this iron. Adding a small amount of hydrogen peroxide oxidized the ferrous iron to ferric iron, which could then be fully recovered with TBP, maximizing the iron recovery efficiency of the process.

[0201] As discussed further above, dissolution, also called leaching, of sewage sludge ash may be carried out under different conditions, resulting in leachate of different compositions. When more concentrated hydrochloric acid and / or elevated temperatures and / or longer leaching times are used, the extent of iron dissolution generally increases, resulting in a higher iron:phosphorus ratio than, for example, in a more dilute hydrochloric acid leach. This allows for a higher recovery of these elements from the sludge ash and the production of a cleaner, iron-depleted silicate residue.

[0202] Due to the high iron content in these leach streams, it has been found that a first solvent extraction step, using a high O:A ratio to extract iron from the organic phase at near-loading capacity, can directly produce a relatively pure iron chloride product, resulting in low phosphorus and other impurities in the strip product. A second extraction step to purify the iron from phosphorus and concentrate the iron is often not necessary.

[0203] 18 shows an embodiment of the extraction section 20 with one solvent extractor 220A. The resulting iron and / or phosphorus-bearing liquid 210, in this embodiment, can contain more or less only iron, even after a single extraction, with or without scrubbing, if the organic to aqueous phase (O:A) ratio and / or the content of TBP in the organic phase are controlled to match the composition of the available leachate.

[0204] In other words, in one embodiment, the step of dissolving the starting material is carried out using 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 relative amount of organic solvent to the leachate so that the loading capacity of the organic solvent is about or less than the content of Fe(III) in the leachate.

[0205] In a preferred embodiment, the step of dissolving the starting materials is carried out at an elevated temperature to facilitate dissolution of the iron.

[0206] As mentioned above, sequential leaching can also be performed to obtain leachates with various iron:phosphorus ratios. Figure 19 shows a schematic diagram of such a sequential two-phase solution. A first-stage chemical treatment system 1A for sewage sludge ash is supplied with starting material 201A consisting of sewage sludge ash. Low-concentration hydrochloric acid 202A is used to perform the leaching, i.e., dissolution. The first-stage chemical treatment system 1A for sewage sludge ash outputs a phosphorus-containing liquid 210A and possibly other liquids containing recovered elements 213A-218A. An undissolved residue 204A is output. A second-stage chemical treatment system 1B for sewage sludge ash is supplied with starting material 201B consisting of undissolved residue 204A from the first-stage chemical treatment system 1A, which is derived from the sewage sludge ash. High-concentration hydrochloric acid 202B, preferably at elevated temperature, is used to perform the leaching, i.e., dissolution. The second stage component 1B for the chemical treatment of sewage sludge ash outputs an iron-containing liquid 210B and possibly other liquids containing recovered elements 213A-218A. An undissolved residue 204B is output. Preferably, at least a portion of the two-stage raffinates 219A and 219B are recycled to be used in a different part of the process, preferably for example for dissolution of starting materials in the first stage component 1A. Preferably, a portion of the two-stage raffinates 219A and 219B may even be used in the second stage component 1B, as indicated by the dotted line.

[0207] Each stage thereby constitutes a complete processing entity in itself. Two stages may be performed in a time- and / or space-related relationship, but may also be separated. For example, one and the same device may be used for both stages, but in that case they will naturally be separated in time.

[0208] Figure 20 shows a flow diagram of one embodiment of the two-stage process. In the first stage S98, sewage sludge ash is treated using low-concentration HCl leaching. This treatment results in a solution of recovered phosphorus and a quantity of a first 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 the starting material and is dissolved using high-concentration HCl. This treatment results in a solution of recovered iron and a quantity of a second undissolved residue.

[0209] Because phosphorus is a valuable component, a preferred next step using the techniques presented here is to separate it from the other components of the leachate, typically calcium, aluminum, magnesium, sodium, potassium, and heavy metals. Several extractants suitable for the extraction of phosphate have been proposed in the literature. These can generally be divided into the following groups: 1) alkyl phosphates, e.g., tributyl phosphate (TBP), 2) amines, e.g., tri-n-octylamine, 3) alcohols, e.g., isoamyl alcohol, n-amyl alcohol, cyclohexanol, methylcyclohexanol, tertiary amyl alcohol, isobutanol, n-butanol, and heptanol, 4) ketones, e.g., methyl isobutyl ketone, methyl propyl ketone, diethyl ketone, methyl ethyl ketone, and methyl-n-butyl ketone, 5) amides, e.g., butylacetamide, 6) aldehydes, e.g., benzaldehyde, 7) esters, e.g., ethyl acetate, butyl acetate, amyl acetate, and cyclohexanone, and 8) ethers, e.g., diethyl ether, di-n-amyl ether, and glycol ethers, e.g., diethylene glycol. Many of these extractants are sparingly soluble in aqueous solutions, particularly in the presence of acids, salts, and / or other chemical species, some more soluble than others. As a result, trace amounts of extractant may be present in the aqueous stream and final product. This is undesirable, especially considering the goal of utilizing the separated phosphorus as a fertilizer. High water solubility typically necessitates recovery of the dissolved extractant from the aqueous stream by distillation, which is costly and complicated. For example, n-butanol has a water solubility of approximately 90 g / L at room temperature. Some of the solvents listed, such as diisopropyl ether, have other drawbacks in addition to their high solubility, such as flammability and / or explosion hazard. Furthermore, some of the solvents listed exhibit limited extraction capacity for phosphoric acid below a certain threshold concentration. This means that the feed solution must contain a high concentration of phosphoric acid, which typically requires concentration of the acid by evaporation of water. Additionally, such solvents only allow partial extraction of phosphoric acid. A typical example of a solvent with a significant threshold concentration for phosphoric acid extraction is methyl isobutyl ketone.In general, ethers, esters, and selected ketones have significant threshold concentrations for phosphate extraction and are therefore not suitable for extracting phosphate from fairly low concentrations (approximately 1.5 M phosphate), which is a concern for many sludge ash leachates. Alcohols with relatively long carbon chains have relatively low water solubility. Therefore, alternative solvents for extracting phosphate are long-carbon chain alcohols, such as heptanol, which have a water solubility of less than 2%. Below this level, the amount of solvent in the aqueous stream is reasonably small enough to allow processing using relatively inexpensive and complex configurations. While TBP has been a standard solvent used on a large scale for over 40 years, the industry has very limited experience with the use of long-chain alcohols, so TBP is the preferred solvent in the present invention.

[0210] In other words, in one embodiment, when the starting material is sewage sludge ash, the method comprises extracting phosphorus in the form of phosphoric acid by liquid-liquid extraction using an organic solvent, preferably TBP.

[0211] In one embodiment, phosphorus extraction is performed from the raffinate of an 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 that is low in iron, so that this iron level is acceptable in the final phosphorus product, or the iron contamination can be removed at a later stage.

[0212] There is very limited data in the literature regarding the use of TBP for phosphate extraction from chloride systems. The main literature includes the following papers: 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." J.Chem.Eng.Data 55,3196-3199, 4)Fernando Pereira,2013 Doctoral Thesis. Ecole Nationale Superie Desure Mines De Saint-Etienne. "Production d'acide phosphorique par attaquechlorhydrique de minerais phosphates avec reduction desnuisances 5)Pereira and Bilal, 2013, "Phosphoric acid extraction and rare earth recovery from apatites of the Brazilian phosphatic ores." 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. .

[0213] Tributyl phosphate allows for the extraction of phosphate preferentially over dissolved salts, such as chlorides, and over dissolved acids, such as hydrochloric or sulfuric acid. The presence of dissolved salts or acids promotes the extraction of phosphate through a salting-out mechanism. Because tributyl phosphate preferentially extracts nitric acid over phosphoric acid, selective extraction of phosphate from nitric acid is not possible. Therefore, leaching of sludge ash with hydrochloric acid is preferred over nitric acid. In general, solvents with low water solubility preferentially extract nitric acid over phosphoric acid. Solvents that are highly selective for phosphate, such as amyl alcohol, are highly soluble in water but still co-extract significant amounts of nitric acid. The main advantage of using TBP as a solvent to extract phosphate is its low water solubility, allowing for operations that do not require costly and complicated distillation of the solvent from the aqueous stream.

[0214] As previously mentioned, extraction of phosphate with TBP is typically performed after iron extraction. However, it can also be performed with iron present in solution. If ferric iron is present in solution, it is extracted with TBP along with the phosphoric acid. This is a suitable option if the amount of iron extracted into the phosphorus product is deemed acceptable and / or if the iron is subsequently removed from the phosphorus stream by means such as solvent extraction, ion exchange, or selective precipitation. As noted above, if the iron is reduced to ferrous iron prior to phosphorus extraction, the efficiency of the extraction will be reduced, and the ferrous iron will remain in the raffinate that proceeds to the next process step.

[0215] It has been stated in the literature that nearly complete extraction of phosphate is possible using TBP and a background calcium chloride content of approximately 3 M. For example, Habashi et al. (1987) stated that at an O:A ratio of 1:1, more than 99% of the phosphate can be extracted in three steps. The same statement was repeated in Pereira (2013).

[0216] However, the present applicant surprisingly found that complete extraction of phosphate from sewage sludge ash leachate using TBP was not possible under the above conditions. As can be seen in Figure 21, the phosphate concentration could only be reduced from about 0.7 M to about 0.15 M, corresponding to about 78.5% of the maximum extraction yield. The diagram in the figure shows the extraction of phosphate from a solution obtained by leaching sewage sludge ash after iron extraction with 3 M hydrochloric acid. Approximately 2.7 M calcium chloride was present in the background. The phosphorus content of the solution was 21 g / L P, i.e., about 0.7 M phosphate. The organic phase was TBP. Furthermore, the TBP solvent loading capacity reported, for example, by Habashi et al., was not possible. Habashi et al. reported a 3 M phosphate loading in TBP in contact with 0.7 M phosphate in an apatite leachate. This is clearly not possible, as can be seen from Figure 21, where the TBP was loaded with only about 0.6 M phosphate (slightly more than 18 g / L phosphorus). This complicates the recovery of phosphate from the loaded organic phase using conventional water stripping. In this case, the resulting stripped product will be relatively low in concentration. To produce a concentrated phosphate solution, a large amount of water must be evaporated.

[0217] While technically feasible, this would adversely affect the economics of the process. Therefore, the approach described in one of the applicant's previous inventions, disclosed in WO 2010 / 138045, is preferably used to mitigate this drawback of phosphorus extraction. Figure 22 shows an extraction section 20 having a device 22D for liquid-liquid extraction of phosphorus using an organic solvent. The device 22D for liquid-liquid extraction of phosphorus is configured to extract phosphorus from the leachate or raffinate of the device for liquid-liquid extraction of iron. A stripping liquid 209D of saturated monoammonium phosphate (MAP) solution is added, resulting in a liquid 210D containing phosphorus in the form of MAP and phosphoric acid. A raffinate 211, from which phosphorus has been at least partially removed, is provided from the extraction section 20.

[0218] More specifically, according to WO 2010 / 138045, the TBP organic phase containing phosphoric acid can be scrubbed, for example, with water at a high O:A ratio to remove some of the co-extracted calcium and other unwanted impurities. The scrubbed product is returned to one of the previous process steps. The phosphoric acid-containing organic phase is stripped with saturated MAP solution 209D as described above. 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 to MAP, which can then be precipitated from the now supersaturated solution. The volume of saturated MAP solution 209D increases with the addition of DAP. The 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.

[0219] In other words, in one preferred embodiment, the method for chemically treating sewage sludge ash includes the further step of stripping the organic solvent in the step of extracting phosphorus using a saturated aqueous solution of monoammonium phosphate.

[0220] In a preferred embodiment, the method for chemically treating sewage sludge ash comprises, in the phosphorus extraction step, the further step of scrubbing the organic solvent with water to form an aqueous scrub solution prior to the step of stripping the organic solvent in the phosphorus extraction step, and the further step of returning the aqueous scrub solution to the raffinate of the iron extraction step.

[0221] In another embodiment, the method for chemically treating sewage sludge ash comprises the further step of stripping the organic solvent in the step of extracting phosphorus with water to obtain a phosphoric acid solution.

[0222] By using MAP as the strip solution, phosphorus can be directly recovered as solid MAP, the most desirable phosphorus fertilizer product. This avoids the need for conventional aqueous phosphate stripping, dilute phosphate concentration by evaporation, and conversion of concentrated phosphate to MAP. Furthermore, this method allows for the effective treatment of organic phases with low phosphate loads, a key advantage that allows for the treatment of low-phosphorus sludge ash.

[0223] However, the technology presented in WO 2010 / 138045 is preferably further improved. One aspect is the handling of co-extracted impurities, particularly heavy metals. Co-extracted metal ions are stripped along with the phosphoric acid and remain in the MAP strip solution. Some of these impurities have also been found to 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 in relatively large amounts. It is desirable to remove such impurities from the system before MAP precipitation. This is important because the saturated MAP solution is recycled within the system and used for subsequent stripping, contributing to the accumulation of impurities with each loop.

[0224] Impurities can be handled at various stages of the aqueous MAP-DAP loop, as can be seen in Figure 23. Impurity removal components 34 may be implemented at one or several of the indicated locations, for example, after stripping, after treatment with DAP in the resulting saturated MAP solution, in the common flow, or in either the divided flow, or in the DAP solution 240. This is confirmed by the experimental results described below. One embodiment of such an impurity removal component 34 is shown in Figure 24. Heavy metals can be precipitated as low-soluble sulfides by adding an appropriate sulfur-containing compound 242 to the stream, for example, in the mixing chamber 35. The sulfur-containing compound 242 can be, for example, metal hydrogen sulfide, metal sulfide, organic sulfide, or any mixture thereof. The heavy metal precipitate 243 is removed from the stream using solid-liquid separation equipment 36, such as filtration, centrifugation, sedimentation, clarification, etc. This results in a stream 244 with reduced heavy metal concentrations. Sulfur species remaining in solution can optionally be removed using sulfur scavenger compound 245, which is added to sulfur removal chamber 37. Sulfur scavenger compound 245 binds the sulfur in an easy-to-remove phase 246, for example, a solid that can be removed using separation equipment 38 according to one of the solid-liquid separation techniques described above, leaving a heavy metal and sulfur-depleted solution 247. Phase 246 can also be a separate top or bottom layer that can be isolated from the stream by an appropriate separation technique.

[0225] [Table 9]

[0226] Additionally, or as an alternative, heavy metals can be removed using ion exchange, preferably using ion exchange resins capable of selectively binding heavy metals, such as thiol-based ion exchange resins or other sulfur-containing resins with a high affinity for heavy metal ions. Heavy metal removal using ion exchange is preferably performed according to the embodiment presented in Figure 23. The contaminated MAP or DAP stream enters the first ion exchange resin unit 39A. Added ion exchange resin 248 removes heavy metals from the original MAP or DAP stream, which is discharged as a heavy metal-depleted solution 250. Because some of the streams in the MAP-DAP loop have a high pH, ​​thiol resins can be used without significant degradation, which could oxidize the thiol functional groups to disulfides, which would interfere with the process. Furthermore, because the heavy metal content in these streams is typically low (ppm levels), loading capacity is not quickly reached, making saturation a minimal concern. Saturated resins containing small amounts of heavy metals can be appropriately disposed of. Alternatively, the resin can be regenerated with an appropriate solution, e.g., an acidic solution, to recover small amounts of the heavy metals in a highly concentrated form. The ion exchange resin loaded with heavy metals 249 is then placed in a second ion exchange resin unit 39B. Eluate 253 is added to remove the heavy metals from the ion exchange resin, and the removed ion exchange resin 251 can be returned for a new ion exchange operation. This is preferably done after the eluate 253 has been completely removed from the resin, e.g., by employing a water wash. The eluate loaded with heavy metals 252 is removed.

[0227] Table 9 shows the impurity concentrations of the low-purity MAP strip product obtained after phosphoric acid extraction and stripping. This strip product had an acidic pH of approximately 1.8. Some impurity elements precipitated along with the solid MAP product when it was generated by adding DAP to the stream (Table 9). Impurity precipitation with sodium hydrogen sulfide was tested at different points in the MAP-DAP loop: the phosphoric acid-loaded MAP, the MAP filtrate after MAP precipitation, and the DAP solution. Precipitating copper, molybdenum, and arsenic as metal sulfides directly from the phosphoric acid-loaded MAP strip product was effective, but much of the zinc remained in solution. Sulfide precipitation of the heavy metals remaining in solution after precipitation of the solid MAP product and before DAP generation, followed by DAP generation and filtration, resulted in the precipitation of most impurities, including zinc, copper, and arsenic, and relatively rapid filtration (Table 10). Sulfide precipitation at pH 3.8 prevented residual sulfides from remaining in solution. Although zinc and copper could be precipitated by sulfide from DAP solutions above pH 8, arsenic precipitated with significantly less efficiency (Table 10). Filtration was more difficult. In addition, residual sulfide remained dissolved in solution, potentially causing problems during the MAP precipitation step. Zinc, calcium, and cadmium could be precipitated in the DAP production process without the addition of sulfide by raising the pH of the MAP filtrate with ammonia, but arsenic and copper could not be precipitated in this manner.

[0228] When precipitation is performed at low pH using metal hydrogen sulfide or metal sulfides, the generation of hydrogen sulfide gas is a concern. The use of organic sulfides can prevent this. Precipitation of impurity metals was also tested using two commercially available organic sulfides. This was performed on the acid stream, the loaded MAP strip product, and the filtrate after solid MAP precipitation, and compared with precipitation with sodium hydrogen sulfide. Precipitation of heavy metals, particularly arsenic, zinc, and copper, was more efficient from the filtrate after solid MAP precipitation than from the phosphate-loaded MAP solution. Precipitation with one of the tested organic sulfides was slightly more efficient than with sodium hydrogen sulfide. However, residual sulfide remained in solution when using the organic sulfide, whereas this was not observed with sodium hydrogen sulfide. In this case, residual sulfide could be removed from the solution using a residual sulfide scavenger.

[0229] [Table 10]

[0230] In summary, phosphorus recovery is preferably achieved using solvent extraction with TBP, either mixed with an appropriate diluent and / or modifier or used neat. Phosphorus is extracted as phosphate by TBP. Scrubbing of the organic phase to remove co-extracted impurities such as calcium can be achieved using a suitable scrubbing agent (water, dilute acid solution, etc.) at a high O:A ratio. The scrubbed product can be reused in the process, for example, at the beginning of a leaching step, a solvent extraction step, or other steps. Conventional industrial operations strip the loaded TBP with water to produce a dilute phosphate solution, which is then concentrated using evaporation, which is energy-intensive and expensive. However, the process described herein favors the production of solid monoammonium phosphate (MAP) directly from the loaded TBP phase. This is achieved by stripping the phosphate with a saturated MAP solution, followed by treatment of the MAP phosphate-stripped product. Several additional preferred processes have been developed to remove impurities, including heavy metals, that are co-extracted and stripped with the phosphate. These involve the selective precipitation of impurities using metal sulfides and / or organic sulfides at various points in the MAP-DAP process. Removal of impurities ensures that the MAP product recovered from sewage sludge ash is of high purity and that impurities do not accumulate in the process stream during MAP production.

[0231] Three specific aspects of the extraction process of this technology are interrelated: providing the high ionic strength necessary for effective extraction of iron and phosphorus, ensuring a good water balance in the process, and maximizing product recovery while minimizing waste. To effectively extract iron and phosphorus with TBP, the aqueous stream, or leachate, must have a sufficiently high ionic strength. This is achieved by having sufficient hydrochloric acid in solution and / or other sources of chloride ions with a salting-out effect. While the process of this invention relies on the presence of free hydrochloric acid and / or dissolved calcium chloride in the leachate, the process is not limited to the use of only these two chemicals. Ionic strength can be provided by any chloride salt or mixture of salts, preferably soluble components of the ash not extracted by TBP (e.g., calcium, aluminum, magnesium, sodium, potassium, etc.). Even salts of elements not found in sludge ash can be used. The use of calcium chloride has several advantages. Calcium is a major component of sewage sludge ash and is easily leached even with dilute hydrochloric acid. Because calcium is poorly extracted by TBP, most of the calcium present in the initial leachate remains in the raffinate after phosphorus extraction. A significant portion of this raffinate is returned to the leaching process, where further calcium is leached. Recycling and recirculation of the raffinate with continuous calcium leaching ensures the ionic strength required for iron and phosphorus extraction, minimizing the need for additional external calcium sources. To maintain water balance and provide an outlet for calcium and other leached elements, an aqueous bleed is removed from the recirculation loop for further processing. The remainder of the stream is recycled to the leaching process, where sludge ash components are leached, further enriching the elements already present in the solution. The stage at which the bleed is removed can vary, for example, after phosphorus recovery, after aluminum recovery, or any subsequent stage. The bleed is then processed to recover the components in solution, typically as individual solid compounds. A typical example of processing the bleed removed after iron and phosphorus recovery is as follows:

[0232] After iron and phosphorus extraction, the raffinate typically contains, in addition to hydrochloric acid, some of the phosphorus not extracted in the previous step, as well as calcium, aluminum, magnesium, heavy metals, sodium, and potassium. The pH of the raffinate is typically low, less than pH 1.

[0233] Aluminum is typically present in sewage sludge ash and may be gradually increased in concentration due to recirculation of the treatment liquor. Preferably, the method for chemically treating sewage sludge ash includes the further step of recovering aluminum from at least one of the at least partially iron-depleted solution and a bleed solution from the at least partially iron-depleted solution.

[0234] Aluminum recovery is preferably carried out by increasing the pH of the solution after the phosphorus recovery step using a suitable compound, such as a metal carbonate, metal oxide, ammonia, metal hydroxide, or a mixture thereof. Figure 26 shows a process flow diagram of an embodiment of step S41 in which aluminum is recovered. In this embodiment, in step S42, the pH is increased using a compound that allows the introduced metal to be recovered later in the process, for example, using calcium carbonate or lime, and then the calcium compound is recovered later in the process.

[0235] Typically, due to the presence of hydrochloric and phosphoric acids, the pH of the feed solution is low, for example less than 1. The nature of the previous process steps affects the recovery of aluminium.

[0236] The aluminum:phosphorus ratio in solution, the precipitant used, the precipitation pH, and the temperature determine the chemical form in which aluminum precipitates: aluminum phosphate, aluminum hydroxide, calcium aluminum phosphate, or a mixture of these. Temperature plays a key role, affecting both the kinetics of the precipitation process and the filtration and washing of the aluminum precipitate. Higher temperatures are preferable because they have a positive effect on both. Tests have shown that aluminum phosphate precipitates completely along with lime at pH 1.1–1.6 when the aluminum:phosphorus molar ratio is less than 1. A low aluminum:phosphorus molar ratio, e.g., 0.5, has been found to be advantageous in terms of filtration rate compared to higher aluminum:phosphorus molar ratios, such as 1 and 1.5. Aluminum and phosphorus precipitated at higher aluminum:phosphorus molar ratios in the raffinate and higher pH.

[0237] Precipitation of aluminum with calcium carbonate was effective even at pH levels below 2, and filtration of the precipitate was faster than with lime. Phosphorus deficiency leads to the precipitation of aluminum hydroxide. Precipitation of a combination of aluminum phosphate and aluminum hydroxide occurs at high aluminum:phosphorus ratios.

[0238] Thus, it has been observed that aluminum compounds can be selectively precipitated from magnesium, calcium, sodium, potassium, and other impurities, and that precipitation can be induced even at pH levels below 2. Aluminum precipitates as aluminum phosphate if there is sufficient phosphorus in the solution, and / or as aluminum hydroxide if there is a phosphorus deficiency. This can be used to obtain specific chemical forms of aluminum.

[0239] If necessary, the phosphorus content of the solution can be adjusted to match the aluminum content to precipitate only aluminum phosphate or to produce a specific mixture containing aluminum phosphate. For example, by adding calcium carbonate to a raffinate with an aluminum:phosphorus molar ratio of less than 1, all of the phosphorus in the solution could be precipitated as a mixture of aluminum phosphate and calcium phosphate at a pH greater than 3. Selectivity can be controlled by controlling the pH; in this case, at a pH less than 2, only aluminum phosphate precipitated, with the excess phosphorus remaining in solution.

[0240] 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 option, shown in optional step S43, is to dose the raffinate with a phosphorus compound to increase the phosphorus concentration in solution if it is insufficient.

[0241] In other words, in one embodiment, recovering aluminum comprises adjusting the pH of at least one of the iron-removed solution and a bleed solution from the iron-removed solution to a pH value at which at least a portion of the aluminum precipitate precipitates as at least one of aluminum phosphate, calcium aluminum phosphate, and aluminum hydroxide.

[0242] In step S44, the aluminum precipitate is separated from the solution using a solid-liquid separation process. The aluminum precipitate can preferably be further processed. In optional intermediate step S45, a calcium source, e.g., lime, is added to the aluminum precipitate. This is done if there is a calcium deficiency in the aluminum precipitate and promotes calcium phosphate precipitation 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 leaves the aluminum in solution and precipitates phosphorus, 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 small volume and can be contained within part of the process described above, typically the leaching / dissolution of sludge ash material.

[0243] FIG. 27 schematically illustrates an embodiment of an aluminum separation arrangement 41 included in post-treatment section 40. Raffinate 211 from iron / phosphorus extraction is added to aluminum precipitation vessel 42 along with a pH-increasing additive 255 and an optional phosphorus compound. Aluminum precipitates as at least one of aluminum phosphate, calcium aluminum phosphate, and aluminum hydroxide 256, which is separated by solid-liquid separation device 43 to produce an at least partially removed aluminum-removed solution 254. Preferably, the separated aluminum precipitate is added to aluminum reactor 44, where sodium hydroxide 257 and an optional calcium source are added. A calcium source, such as lime, can be added if the precipitate is calcium-deficient. In reactor 44, the aluminum precipitate is converted into sodium aluminate solution 213 and a phosphorus-containing solid fraction 258 containing 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 sludge ash material.

[0244] Preferably, the heavy metal ions in the resulting solution are then recovered at a higher pH. This recovery is performed on the mainstream or bleed solution resulting from the raffinate from the iron / phosphorus extraction, optionally with aluminum removed. Removal of heavy metal ions is primarily done to avoid contamination of other recovered fractions. However, the recovered heavy metal compounds can also be used as feedstock for further purification or processing to utilize the actual value of the metal components. One approach, shown in Figure 28, involves step S51, further increasing the pH of the solution. This pH increase promotes the formation of insoluble metal hydroxides and / or insoluble metal phosphates, if phosphorus is present in the solution. As with aluminum precipitation, the amount of phosphorus present in the solution can determine which compounds precipitate, which can be used to control the chemical form of the heavy metal products. In optional step S52, a phosphorus compound is added for this purpose. Heavy metal ions precipitate over a wide pH range, typically between 3 and 10. It was observed that heavy metal precipitation could occur up to pH 9.3, selectively from magnesium, calcium, sodium, potassium, and to some extent strontium.

[0245] Another approach is to precipitate heavy metals as sulfides using metal hydrogen sulfides, metal sulfides, organic sulfides, or mixtures thereof, as shown in step S53. NaHS is a typical candidate. Sulfide precipitation by sulfide addition can also be combined with increasing the pH of the solution. Precipitation of heavy metals by sulfide at a higher pH avoids the generation of hydrogen sulfide gas.

[0246] In step S54, the precipitated heavy metals are separated using a solid-liquid separation process.

[0247] A third approach, represented by step S55, is to use ion exchange to purify the solution from heavy metals. Thiol-based resins have a high affinity for various heavy metals. This can be done similarly to the separation of heavy metals from the extracted ammonium phosphate solution described above.

[0248] In other words, in one embodiment, the method for chemically treating sewage sludge ash includes the further step of recovering heavy metals from at least one of the solution resulting from the at least partially removed solution of iron and / or phosphorus and the bleed solution derived from the solution resulting from the at least partially removed solution of iron and / or phosphorus by adding at least one of a pH-increasing additive and a sulfur-containing compound to the solution and separating the precipitated heavy metal compounds.

[0249] 29 shows a schematic diagram of an embodiment of a heavy metal separation arrangement 50 included in post-treatment section 40. Aluminum-removing solution 254 from the aluminum separation arrangement is added to a heavy metal precipitation vessel 51. Alternatively, if aluminum removal is not performed, the raffinate from the iron / phosphorus extraction is used. A pH-increasing additive 255 and / or a sulfur-containing compound 242, and optionally a phosphate compound, are also added to cause precipitation of a heavy metal precipitate 214. The heavy metal precipitate 214 is separated by solid-liquid separation equipment 52, leaving a heavy metal-removed solution 259.

[0250] Alternatively, a structure similar to that of FIG. 23 can be used.

[0251] Once the heavy metals are removed, a solution containing primarily magnesium, calcium, sodium, and potassium is obtained.

[0252] In one embodiment, the method for chemically treating sewage sludge ash includes the further step of recovering magnesium, calcium, sodium, and potassium from at least one of the solution resulting from the iron-removed solution and a bleed solution derived from the solution resulting from the iron-removed solution. Preferably, the magnesium, calcium, sodium, and potassium are individually separated.

[0253] As shown by the embodiment of Figure 30, magnesium can be selectively precipitated as magnesium hydroxide in step S61 by further increasing the pH of the solution to a pH level that causes precipitation of magnesium as magnesium hydroxide. Typically, this level is above pH 9.3. This allows, for example, the use of an alkali or alkaline earth base. It has been found that most of the magnesium can be selectively precipitated as magnesium hydroxide from the other components using lime before the pH reaches 10. The magnesium precipitate is recovered from the solution in step S62 using a solid-liquid separation process.

[0254] Figure 31 shows a schematic of an embodiment of the magnesium separation arrangement 60 included in the post-treatment section 40. The heavy metal-removed solution 259 from the heavy metal separation arrangement is added to the magnesium precipitation vessel 61. Alternatively, if heavy metal removal is not performed, the aluminum-removed solution or the raffinate from the iron / phosphorus extraction is used. A pH-increasing additive 255 is also added to cause the precipitation of magnesium hydroxide 215. The magnesium hydroxide 215 is separated by solid-liquid separation equipment 62, leaving a magnesium-removed solution 260.

[0255] The compound(s) previously used to increase the pH of the solution can be preferably selected to enrich one or more of the remaining components. For example, sodium hydroxide can be used before the sodium removal step to enrich the solution in sodium. Similarly, calcium hydroxide can be used to ensure a higher calcium content in the solution. Alternatively, a different compound can be used to increase the content of one or more species. If an appropriate calcium compound is used to increase the pH and the sewage sludge ash contains small amounts of sodium and potassium, the resulting solution will contain mostly calcium chloride, i.e., a calcium chloride-rich solution. This can be partially evaporated to obtain a calcium chloride concentrate or completely evaporated to produce solid calcium chloride.

[0256] If the solution contains large amounts of sodium and / or potassium in addition to calcium chloride, the separation of the individual components is preferably carried out by taking advantage of the difference in solubility of sodium and potassium chloride at high temperatures in the presence of a large calcium chloride background, e.g. according to WO 2017 / 111685.

[0257] The present invention is not limited to the specific embodiments and examples described above and may be modified within the scope of the claims. For example, sludge ash leaching can be carried out under various conditions, with or without integrating some pretreatment methods into this process. Side streams such as raffinate, scrub solution, and wash water, as well as treated water, can be recycled in different steps to achieve the same results. For example, returning two streams to the leaching reactor and one to the filtered leachate can achieve the same results as returning all three streams to the leaching reactor. Regardless of how the ionic strength required in the solvent extraction process is ensured, for example, by sufficiently high acidity and / or chloride salt content, a key aspect of the process is the recycling and recirculation of these ionic species in the background until a steady state is reached. The bleed is treated to ensure water balance and separate the product. The location of the return and bleed points can be varied depending on the desired results, the chemical nature and composition of the feedstock, the chemicals used, etc.

[0258] Typically, after phosphorus solvent extraction, the raffinate contains hydrochloric acid, unextracted phosphate, calcium, aluminum, heavy metals, magnesium, sodium, and potassium. Ferrous iron will be present in solution if not handled as described above. Depending on how the ionic strength was controlled, this raffinate may contain significant amounts of hydrochloric acid and / or chloride salts. At any point after phosphorus extraction, the raffinate is split into two streams: a bleed stream that is treated separately to recover its components, and a stream that is recycled to an earlier step in the process, preferably a leaching step.

[0259] 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 a starting material comprising sewage sludge ash.

[0260] Figure 32 shows a schematic representation of the recycling concept. Downstream of the extraction section 20, several possible stream branches exist: one stream of at least partially iron- and / or phosphorus-depleted solution from the extraction section 20 is recycled to an earlier stage of the process as solution 219, while a bleed stream 270 is sent to a further component recovery stage. This can be done at one or more of the locations shown. The recycled portion 219 of the raffinate, at least partially iron- and / or phosphorus-depleted from a possible outlet, constitutes the return stream. Multiple return outlets are possible. Returning the stream to the leaching / dissolution step means that the resulting leachate contains not only the elements in the return solution but also newly leached sludge ash components. Thus, in addition to the leached iron and phosphorus, the leachate will contain increased amounts of non-extracted components and hydrochloric acid. After a subsequent extraction step, returning the stream to the leaching / dissolution step to leach new sewage sludge ash again increases the elemental concentrations and acidity of the leachate. This occurs until a steady state is reached, i.e., the concentrations of non-extractable / under-extractable species remain relatively stable.Since calcium is one of the elements present in significant amounts in sludge ash, increasing the calcium chloride concentration in solution partially or completely ensures the ionic strength required for effective extraction of iron and phosphorus.

[0261] In other words, in one embodiment, the recycled portion of the at least partially iron and / or phosphorus-depleted solution further comprises chloride ions.

[0262] 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 a starting material comprising sewage sludge ash.

[0263] Other streams are also returned within the system. As mentioned above, the wash liquid 205 can be re-entered into the dissolution reactor 10. The second stage raffinate 229 from the extraction section 20, which contains phosphorus and calcium, and possibly also some other contaminants, can be recycled in the process, for example in the dissolution reactor 10 or the pre-treatment section 14. The phosphorus stream 258 from the aluminum recovery can also be returned to the dissolution reactor 10, the pre-treatment section 14, or the extraction section 20. These streams contribute to the process, for example, ensuring a higher overall yield of phosphorus extraction, but also maintaining ionic strength.

[0264] Tests have shown that adequate extraction of iron and phosphorus by TBP can be achieved by providing an additional 2M calcium chloride background when leaching sludge ash with a mild hydrochloric acid solution, e.g., 3M hydrochloric acid. To ensure this, salting reagent must be added during or after leaching in the first loop. For example, calcium chloride is added to achieve the desired background chloride concentration. The ionic strength is then preferably maintained by recycling the process stream back to the dissolution step. This is done by adjusting the amount of bleed removed from the system and / or the amount returned to the leaching step. If necessary, the return stream can be concentrated to mitigate losses of salting reagent. This can be done, for example, using partial evaporation. Make-up salting reagent can also be added to the return stream, leach solution, or leachate from an external source or from a later stage in the process. For example, calcium chloride solution or solid chloride salts separated from calcium chloride solution can be used for this purpose.

[0265] Of course, the ionic strength required for extraction can be partially or completely supplied by a background concentration of hydrochloric acid. In this particular embodiment, leaching is performed using a hydrochloric acid solution of sufficient concentration to ensure a high hydrochloric acid / chloride background during extraction. The background acidity / chloride content is recycled in a manner similar to that described above. For example, the inventors tested leaching of sludge ash with a 6 M hydrochloric acid solution. Iron and phosphorus could be effectively recovered from the resulting leachate without the need for adding salting reagents such as calcium chloride. A portion of the acid raffinate after phosphoric acid extraction, which still contains a significant amount of background hydrochloric acid, was reused in the leaching step of new ash, and this alone was sufficient to provide the necessary ionic strength in subsequent extraction cycles. Furthermore, in a modification of this embodiment, leaching was performed at elevated temperatures, resulting in nearly complete leaching of iron from the sludge ash (greater than 90% iron leaching efficiency). In this particular case, the amount of iron in the solution was 5-6 times that typically leached at ambient temperature (approximately 5 g / L). Because the iron content (approximately 25-30 g / L) is significantly higher than the phosphorus (also approximately 30 g / L), solvent extraction of iron could be performed near the loading capacity of the organic phase without the need to use very low O:A ratios. In this case, extraction could be performed at O:A ratios of 0.5-1, provided the system showed improved selectivity for iron compared to using a leachate with a much lower iron content under the same conditions.

[0266] As previously mentioned, a bleed is required from the system to maintain water balance and recover other components. If the phosphoric acid is not completely extracted with TBP, some will still be present in the raffinate. A significant portion of this stream is returned to the leaching step, ensuring that no phosphoric acid is lost. The phosphoric acid removed in the bleed allows for complete or partial recovery of aluminum as precipitated aluminum phosphate, which, as further noted above, was achieved at a pH below 2.

[0267] The subsequent treatment of the solution to recover heavy metals and magnesium has already been described above. This is achieved by gradually increasing the pH using suitable compounds capable of achieving this, such as metal carbonates, metal oxides, ammonia, metal hydroxides, or mixtures thereof. Further recovery of sodium, potassium, and calcium from the brine obtained after magnesium recovery is achieved by taking advantage of the difference in solubility of sodium and potassium at different temperatures and high calcium chloride concentrations, according to processes known in the prior art, for example, from WO 2017 / 111685.

[0268] The above-described embodiments should be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations, and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different partial solutions in different embodiments may be combined in other configurations, where technically possible. However, the scope of the present invention is defined by the appended claims.

Claims

1. A method for chemically treating sewage sludge ash, comprising: A step (S10) of dissolving a starting material (201) derived from sewage sludge ash in an acid (202) containing hydrochloric acid, A step (S10) in which the starting material (201) contains at least a silicon compound and an iron compound; a step (S12) of separating the undissolved residue (204), thereby leaving a leachate (206); a step (S20) of extracting at least one of iron and phosphorus from the leachate (206) by liquid-liquid extraction using an organic solvent (222); a step (S15) of controlling the amount of colloidal silica present in the leachate (206) provided to the extracting step (S20) to match the amount of silica crud during the liquid-liquid extraction with organic solvent (222) to be sufficiently low to make the liquid-liquid extraction with organic solvent (222) operable; A step (S90) of recycling at least a portion (219) of the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed, resulting from the step (S20) of extracting at least one of iron and phosphorus, for dissolving a starting material (201) derived from sewage sludge ash, a step (S90) in which a recycled portion of the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed comprises chloride ions; Including, The controlling step (S15) Adding a silica coagulant to at least one of the starting material (201), which is a dissolved starting material derived from sewage sludge ash, and the leachate (206) to promote coagulation for particle growth (S151); The dissolving step (S10) is carried out at an elevated temperature of at least 50°C (S156); performed by at least one of method.

2. 2. The method according to claim 1, wherein the controlling step (S15) is carried out by at least adding a silica coagulant to at least one of the starting material (201), which is a dissolved starting material derived from sewage sludge ash, and the leachate (206), thereby promoting coagulation (S151) to grow particles.

3. 3. The method of claim 1 or 2, wherein promoting coagulation to grow the particles (S151) is performed at least in part by adding a silica coagulant to the starting material (201).

4. The method of any one of claims 1 to 3, wherein promoting solidification to grow particles (S151) is performed at least partially simultaneously with the dissolving step (S10).

5. 5. The method according to any one of claims 1 to 4, wherein the promoting coagulation (S151) to grow the particles is performed at least in part in the leachate (206) after the step (S12) of separating undissolved residue (204), whereby the method optionally comprises an additional step (S19) of removing coagulated silica particles from the leachate (206).

6. a step (S154) of performing the dissolving step (S10) using a hydrochloric acid solution having a concentration of more than 3M; and Aging the dissolved sewage sludge ash (S155) to promote at least one of the conversion of ionic silicon to colloidal silica and the growth of colloidal silica particles; The method according to any one of claims 1 to 5, comprising at least one of:

7. 7. The method of any one of claims 1 to 6, wherein the step (S90) of recycling at least a portion (219) of the raffinate (211) is controlled so that the concentration of chloride salts and / or hydrochloric acid in the leach solution (206) is at least 1 M.

8. The starting material (201) comprises sulfur, and the method further comprises, before the step (S20) of extracting at least one of iron and phosphorus, A process (S161) for aging the leachate; and CaSO ４ a process of accelerating precipitation (S163); to prevent calcium sulfate precipitation from occurring immediately before or during the step (S20) of extracting at least one of iron and phosphorus, by carrying out at least one of the processes CaSO ４ The precipitation acceleration process (S163) 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 calcium chloride-rich aqueous raffinate obtained after the step (S20) of extracting at least one of iron and phosphorus at the latest during the step (S10) of dissolving, returning, after said step (S12) of separating the undissolved residue (204), the aqueous raffinate rich in calcium chloride obtained after said step (S20) of extracting at least one of iron and phosphorus, At the latest, during the dissolving step (S10), CaSO ４ adding seed particles; After the step (S12) of separating the undissolved residue (204), the leachate (206) is added with CaSO ４ adding seed particles; and The dissolving step (S10) is carried out at an elevated temperature of at least 50°C; at least one of: The method according to any one of claims 1 to 7.

9. 9. The method of claim 1, wherein the step (S20) of extracting at least one of iron and phosphorus comprises extracting iron, and wherein extracting iron comprises extracting iron from the leachate (206) by liquid-liquid extraction using an organic solvent (222) (S21).

10. Extracting (S21) the iron from the leachate (206) comprises a subsequent extraction step; a first extraction stage (220A) comprising selective liquid-liquid extraction of iron contents from said leachate (206) followed by stripping into an intermediate strip solution (228); 10. The method of claim 9, wherein a second extraction stage (220B) comprises selective liquid-liquid extraction of primarily iron ions from the intermediate strip solution (228) followed by stripping into a second strip solution (210A) to achieve at least one of higher iron purity and higher iron concentration in the recovered iron product.

11. the first extraction step (220A) comprising adjusting the loading capacity of the organic solvent (222) to the Fe(III) content in the leachate (206) by controlling the relative amount of the organic solvent (222) to the leachate (206); 11. The method of claim 10, wherein at least one of the second extraction stage (220B) and the further extraction stage, if any, comprises controlling the relative amount of the organic solvent (222) to the intermediate strip solution (228) to control a loading capacity of the organic solvent (222) to be about or less than the content of Fe(III) in the intermediate strip solution (228).

12. 12. The method of claim 11, wherein the step (S90) of recycling at least a portion (219) of the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed comprises recycling a raffinate (229) from the liquid-liquid extraction of at least one of the second extraction stage (220B) and the further extraction stages, if any, for use in the subsequent step (S10) of dissolving the starting material (201).

13. 10. The method of claim 9, wherein the step (S10) of dissolving the starting material (201) is carried out using an acid having a hydrochloric acid concentration greater than 3M, and wherein extracting (S21) the iron from the leachate (206) comprises controlling the relative amount of the organic solvent (222) to the leachate (206) so that the loading capacity of the organic solvent (222) is about or less than the content of Fe(III) in the leachate (206).

14. 14. The method of claim 13, wherein the step (S10) of dissolving the starting material (201) is performed at elevated temperature.

15. 15. The method according to any one of claims 1 to 14, wherein the starting material (201) is sewage sludge ash, and the step (S20) of extracting at least one of iron and phosphorus further comprises extracting phosphorus (S22) from the raffinate of extracting iron (S21) by liquid-liquid extraction using an organic solvent (222).

16. 10. The method of claim 9, wherein the starting material (201) comprises undissolved residue (204) from phosphorus leaching of sewage sludge ash, the step of dissolving (S10) the starting material (201) is carried out using an acid having a hydrochloric acid (202) concentration greater than 3M, and extracting (S21) the iron from the leachate comprises controlling the relative amount of the organic solvent (222) to the leachate (206) so that the loading capacity of the organic solvent (222) is less than the content of Fe(III) in the leachate (206).

17. 17. The method of claim 16, wherein the step (S10) of dissolving the starting material (201) is performed at an elevated temperature.

18. 18. The method according to claim 16 or 17, wherein the step (S90) of recycling the at least a portion (219) of the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed comprises recycling the at least a portion (219) of the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed to the step (S10) of dissolving a starting material (201) comprising sewage sludge ash.

19. 19. The method according to any one of claims 1 to 18, comprising a further step (S41) of recovering aluminum from at least one of the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed and a bleed solution (270) from the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed.

20. 20. The method according to any one of claims 1 to 19, comprising a further step (S50) of extracting heavy metals from at least one of the solution resulting from the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed and a bleed solution (270) resulting from the solution resulting from the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed by adding a pH-increasing additive and a sulfur-containing compound to at least one of the solution resulting from the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed and a bleed solution (270) resulting from the solution resulting from the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed, and separating precipitated heavy metal compounds.

21. 21. The method according to any one of claims 1 to 20, comprising the further steps (S60, S65, S70, S75) of recovering Mg, Ca, Na, and K from at least one of a solution resulting from the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed and a bleed solution (270) from the solution resulting from the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed.

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