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 DISCLOSURE 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 has an irreplaceable role. In recent decades, agricultural demand for phosphorus fertilizers has increased significantly. This is expected to continue to increase due to global population growth and improved living standards. The raw material for inorganic phosphorus fertilizer production and 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 have no natural phosphate deposits and are completely dependent on imports. As phosphate rock mining is concentrated in a few areas of the world, many countries are vulnerable to market fluctuations and geopolitical factors. One of the measures proposed 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 wastes containing phosphorus. Governments and industries have recognized the importance of recovering phosphorus from secondary sources, and some countries have begun to enact legislation to advance this.

[0003] Release of phosphorus into surface waters contributes to eutrophication. For this reason, phosphorus needs to be removed from domestic and industrial wastewater during its treatment. This results in phosphorus accumulating in sewage sludge, the main by-product of sewage treatment plants. Using this sludge as fertilizer is not a suitable option due to the large volume generated and the presence of heavy metals, organic pollutants and pathogenic bacteria. Some of these drawbacks are mitigated by incinerating the sewage sludge to produce sludge ash. This is done in dedicated incinerators, where the wet sludge is pre-dried using regenerative heat before incineration, reducing its moisture content from 70-80% to about 30-40%. Incineration reduces the sludge volume, destroys organic pathogens and results in a dry product containing about 6-14% phosphorus by weight. In comparison, phosphate rock contains about 12-16% phosphorus by weight. However, despite its relatively high phosphorus content, sludge ash is not suitable for use as fertilizer. More than 90% of the phosphorus initially present in the wastewater ends up in ash, but the phosphorus compounds produced during incineration, such as calcium and aluminum phosphates, are poorly water-soluble. Therefore, phosphorus is not dissolved and available for uptake by plants. 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 also possible in conventional incinerators, but this results in an ash with a lower phosphorus content, usually less than 5% by weight.

[0004] Due to the low water solubility and high thermal stability of calcium and aluminum phosphates, and iron oxide, 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 need to be used.

[0005] In the book "Phosphorus: Polluter and Resource of the Future - Removal and Recovery from Wastewater", IWA Publishing, chapter 25, a phosphorus recovery process from sludge ash named "LEACHPHOS" is described. The LEACHPHOS process relies on leaching of 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 neutralized in stages and mixed products of calcium phosphate, aluminum phosphate and iron phosphate are separated. Heavy metals become mixed metal hydroxide sludge, which precipitates at pH >9. These mixed products have low market value and individual separation of the main components is not achieved. The phosphorus products cannot be used directly as fertilizer due to their low water solubility.

[0006] A phosphorus recovery process named "The PHOS4LIFE PROCESS" has been developed by Technicas Reunidas. The process uses sulfuric acid to leach the sludge ash. The recovery of dissolved iron is done by solvent extraction. No information is available on the iron extraction process, but since the iron in the sulfuric acid leachate is in cationic form, it is believed that the iron is extracted with a solvent suitable for extracting the cationic form of iron, and then hydrochloric acid is used to strip the extracted iron from the load 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 industrial grade phosphoric acid (75% concentration). After phosphoric acid extraction, the metal ions remaining in the solution are precipitated as a mixed product at a high pH obtained by the addition of lime. The process requires the use of both sulfuric and hydrochloric acids, a high energy input in the evaporation stage, and separation of a mixed metal hydroxide product of limited commercial value. The main drawback of using sulfuric acid for dissolving ash is that it reacts with calcium in the ash to form gypsum, which is filtered out together 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, named "The Pasch PROCESS", was presented during a presentation at BALTIC21, held in Berlin on 28-30 September. In this process, dilute hydrochloric acid is used to leach the sludge ash. The undissolved residue is separated using parallel plate separators and filters and dewatered using a centrifuge. Dissolved metals, e.g. iron, lead, cadmium, copper and zinc, are extracted as a group using solvent extraction using a mixture of Alamine 336 and TBP. These are then precipitated as mixed hydroxides. Individual separation of the commercially available metal compounds is therefore not achieved. Phosphorus remaining in the raffinate after solvent extraction can be recovered by conventional phosphate precipitation, but aluminium is not extracted by Alamine 336 and TBP and is co-precipitated with the phosphorus product, reducing its value. The plant availability of such recovered phosphorus products is therefore low.

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

[0009] Another phosphorus recovery process by EcoPhos, mentioned above, is described in WO 2015 / 091946. This process relies on leaching ash from the incineration of waste, including sewage sludge ash, with phosphoric acid. The leachate is treated with activated carbon. The phosphoric acid is separated from the dissolved metals using a cation exchange resin. The resin is regenerated with hydrochloric acid, which elutes a mixture of magnesium chloride and calcium chloride, and a mixture of aluminium chloride and ferric chloride. The faith of the heavy metals is unknown. The process does not recover iron and aluminium 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 contamination of the purified phosphoric acid product with hydrochloric acid.

[0010] WO 2015 / 067328 describes a method for treating ash derived from waste incineration to obtain compounds of aluminium, calcium, phosphorus and nitrogen. The ash is leached using a mixture of phosphoric and nitric acids, dissolving phosphorus as phosphate and metals as metal nitrates. By adding sulphuric acid to the leachate, calcium is precipitated as gypsum. A mixture of aluminium hydroxide and aluminium phosphate is precipitated by further increasing the pH of the solution with lime. The remaining solution containing phosphoric and nitric acids is concentrated by evaporation. Further addition of lime results in a mixture of calcium nitrate and calcium phosphate, which is recovered in solid form by evaporation crystallisation.

[0011] WO 2015 / 165481 describes a method for producing purified phosphoric acid, calcium sulfate, water-soluble calcium hydrogen phosphate and metal salt solutions by treating ash originating 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 may be precipitated from the leachate by adding sulfuric acid. Calcium hydrogen phosphate may 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 leached 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. The method does not recover iron, aluminum and other metals as separate commercial products. The method requires the use and handling of both sulfuric acid and hydrochloric acid. Furthermore, the method uses ion exchange resins, which suffer from the aforementioned disadvantages. In addition, the use of membranes makes the process less reliable, e.g., subject to fouling and 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 for subsequent leaching of phosphate rock to further increase the phosphorus concentration in the solution. This method allows for the concentration of phosphorus after each leaching cycle, but does not separate a pure phosphorus stream, since soluble impurities in the sludge ash and phosphate rock also dissolve together with the phosphorus.

[0014] EP 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, and 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% 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 performed using bipolar electrodialysis. Prior to electrodialysis, calcium and magnesium ions are precipitated from the concentrate as calcium hydroxide and magnesium hydroxide. The method requires high energy for the phosphoric acid concentration and electrodialysis steps.

[0015] WO 2020 / 169708 describes a process for recovering phosphoric acid from solid materials containing phosphorus, such as struvite, sludge ash, meat and bone meal ash, manure ash, calcium phosphate, and phosphate-containing minerals, such as apatite, viburnum 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 and a solid material from which phosphorus has been removed. The organic solvent containing the phosphoric acid is separated from the remaining solid material, and the phosphoric acid is recovered from the organic phase using conventional stripping. This method has limited industrial applicability for sludge ash due to the high loss of organic solvent in the undissolved ash residue. Complete recovery of the solvent and removal of the residue are difficult to achieve. The process also exhibits very low phosphorus yields. The only example of phosphorus recovery from sludge ash describes extraction with 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 process for treating wastewater sludge using leaching with sulfuric acid, solvent extraction and stripping of iron and aluminium, sulphide precipitation of heavy metals, precipitation of phosphorus as hydroxyapatite or struvite and precipitation of aluminium as aluminium hydroxide. Direct treatment of wastewater sludge without incineration results in significant chemical consumption and large volumes of effluent due to the large volume and water content of the non-incinerated sludge.

[0017] WO 2014 / 178788 describes several routes for recovering phosphorus from sludge ash using leaching with hydrochloric acid and solvent extraction of metals with tributyl phosphate (TBP). All presented solvent extraction steps involve a pretreatment of the sludge ash leachate with a base to precipitate intermediate compounds, mainly a mixture of iron, aluminum and calcium phosphates. In this way, a preconcentration and partial purification of phosphorus is achieved. The intermediate precipitate is dissolved in hydrochloric acid and then solvent extraction is performed. In this way, the solvent extraction of metals and phosphorus is not performed immediately after leaching. This intermediate precipitation step of iron, aluminum and calcium phosphates avoids many of the disadvantages of applying solvent extraction immediately after leaching, such as the reduction in the concentration of phosphorus and iron in the solution and the presence of further impurities. Such disadvantages are discussed in more detail herein. However, the intermediate precipitation step makes the treatment of sludge ash by solvent extraction more complex and expensive, and increases the consumption of chemicals. For example, a base is needed to precipitate intermediate compounds, which are then dissolved again in acid to dissolve the phosphorus, aluminum, calcium, iron and other components.

[0018] There is a need for better processes for recycling sludge ash that guarantee the recovery of phosphorus in a form that can be used, for example, in the fertilizer industry, in high purity and yield, as well as the recovery of other individual components in high purity and yield. For example, recovery of iron separately from aluminum, recovery of calcium as a market-acceptable product without losses during leaching, such as precipitation as gypsum, or recovery of magnesium and heavy metals are required. Removal of these fractions from the sludge ash produces an insoluble residue rich in silicates, which has important applications in the concrete industry, especially if the harmful heavy metals have been removed. Due to the large amount of sludge ash generated each year, the use of this residue in the concrete industry would also lead to a significant reduction in gas emissions, as well as a saving of landfill space. The treatment of sludge ash should preferably be achieved with minimal consumption of energy and chemicals, as well as minimal generation of waste. Many of the above processes fail to address more than one of these aspects. Summary of the Invention

[0019] The overall objective of the present 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 the methods and devices according to the independent claims. Preferred embodiments are defined in the dependent claims.

[0021] Generally speaking, in a first embodiment, the method for chemically treating sewage sludge ash comprises dissolving starting materials originating from sewage sludge ash in an acid, including hydrochloric acid. The starting materials comprise at least silicon and iron compounds. The 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 match the amount of silica crud during the liquid-liquid extraction with an organic solvent, which is sufficiently low to make the liquid-liquid extraction with an organic solvent operable. The controlling step is carried out by adding a silica coagulant to the starting materials, the dissolved starting materials originating from sewage sludge ash, and / or the leachate, to promote coagulation to grow particles, and / or by carrying out the dissolving step at an elevated temperature of at least 50°C. At least a portion of the solution resulting from the extraction step of iron and / or phosphorus, from which at least one of iron and / or phosphorus has been at least partially removed, is recycled to dissolve the starting materials originating from 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 embodiment, an arrangement for chemical treatment of sewage sludge ash comprises 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 originating from the sewage sludge ash in an acid comprising 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 an amount of silica crud during the liquid-liquid extraction with an organic solvent that is sufficiently small to make the liquid-liquid extraction with an organic solvent operable. The device for controlling the amount of colloidal silica comprises a pretreatment chamber configured to receive the leachate and add a silica coagulant into the leachate, the pretreatment chamber optionally comprising a separation device for removing coagulated silica particles from the leachate and / or a heater arranged to provide an elevated temperature of at least 50° C. in the dissolution reactor. The return pipe is configured to recirculate at least a portion of the at least partially depleted solution of iron and / or phosphorus from the step of extracting at least one of iron and / or phosphorus from the sewage sludge ash for dissolving the starting material. The recirculated portion of the at least partially depleted solution of iron and / or phosphorus comprises 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 understood upon reading 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 description 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. [Diagram 2] FIG. 2 is a partial flow diagram of a pretreatment process according to an embodiment of a method for chemically treating sewage sludge ash. [Diagram 3] FIG. 2 is a partial flow diagram of a pretreatment process according to another embodiment of a method for chemically treating sewage sludge ash. [Figure 4] FIG. 2 is a flow diagram of the partial steps of one embodiment of a process for reducing the amount of colloidal silica. [Diagram 5] FIG. 2 is a flow diagram of a sub-step of an embodiment of a process for treating sulfur. [Figure 6] FIG. 1 is a partial flow diagram of a pretreatment process according to yet another embodiment of a method for chemically treating sewage sludge ash. [Figure 7] FIG. 1 is a partial flow diagram of a pretreatment process according to yet another embodiment of a 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] FIG. 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 the iron extraction section having two solvent extractors. [Figure 13] FIG. 2 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 of 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. 2 is a flow diagram of a partial process of one embodiment of neutralization of a ferric chloride product with hydrochloric acid. [Figure 17] FIG. 2 is a flow diagram of a partial process for treating iron in one embodiment of the method for chemically treating sewage sludge ash. [Figure 18] FIG. 1 is a schematic diagram of an embodiment of the 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. 2 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. 2 is a schematic diagram of one embodiment of a heavy metal removal arrangement. [Diagram 25] FIG. 2 is a schematic diagram of another embodiment of a heavy metal removal arrangement. [Figure 26] FIG. 1 is a flow diagram of a portion of an embodiment of a process for recovering aluminum. [Figure 27] FIG. 2 is a schematic diagram of one embodiment of an aluminum isolation structure. [Figure 28] FIG. 2 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. [Diagram 30] FIG. 1 is a flow diagram of a partial process of one embodiment of a process for recovering magnesium. [Diagram 31] FIG. 1 is a schematic diagram of one embodiment of a magnesium isolation arrangement. [Diagram 32] FIG. 1 is a schematic diagram showing an embodiment of a recycled stream in an arrangement for chemical treatment of sewage sludge ash. 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, have contributed to classifying this stream as a hazardous waste that requires burial in dedicated landfills or proper 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. From Table 1, it can be seen that the phosphorus concentration of the co-incinerated sludge ash (about 4%) is lower than that of the singly incinerated sludge ash (about 8%). Table 1 also lists the types of chemicals used in the WWTP to precipitate phosphorus. Typically, three different groups of phosphorus precipitation chemicals are 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 chemistry 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 the metal phosphates and captures the 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 above processes for recovering components from sewage sludge ash use sulfuric acid because it is cheap and can precipitate gypsum, resulting in a phosphorus stream with low calcium content. However, gypsum generally has low commercial value. In addition, there are concerns that other elements may co-precipitate in the leachate along with the gypsum, which can lead to contamination of the gypsum product and / or loss of a potentially recoverable fraction. 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 stream with high phosphorus content. However, this solution is contaminated with heavy metals, as well as other metals such as iron, aluminum, calcium, and magnesium. This requires additional purification, which is neither simple nor cheap, especially when ion exchange is used. Most approaches aim to isolate pure phosphoric acid, producing a mixture of other metals with limited applications and no commercial value.

[0032] There are only a few processes that use hydrochloric acid, leaving room for improvement. The use of hydrochloric acid for leaching has several attractive properties. For example, compared to the use of 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 the recovery of a substance that is intentionally added to the process.

[0033] The approach contemplated by the present 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 described previously, 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 that 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 the raffinate in a diluted form, and this solution requires further processing to recover the phosphorus. This can be, for example, the energy-intensive evaporation of water, or the conventional precipitation of poorly water-soluble phosphorus compounds. Such poorly water-soluble phosphorus compounds are not suitable as fertilizers unless they are chemically converted to more water-soluble products. EP 3266742 describes solvent extraction with 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 solution.

[0035] One of the applicant's prior inventions disclosed in WO 20141 / 78788 describes leaching of 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 carried out immediately after leaching of the sewage sludge ash with hydrochloric acid, which, in contrast, is the intention 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 coprecipitating elements. In this way, phosphorus preconcentration and purification is performed. This intermediate precipitate is separated from the original leachate and redissolved again in hydrochloric acid before solvent extraction with TBP.

[0036] However, the use of hydrochloric acid for leaching of sewage sludge ash is not without further problems. As mentioned in the background, leaching with hydrochloric acid typically results in relatively low concentrations of phosphorus and / or iron in solution, and further impurities are also present, which may make solvent extraction difficult. For example, leaching with hydrochloric acid has been found to result in leachates containing non-negligible amounts of colloidal silica, which makes the application of direct solvent extraction processes unfeasible.

[0037] In WO 2014 / 178788, this is overcome by using an intermediate precipitation step. Although such a solution generally works well, the consumption of added chemicals is disadvantageously high. This is because the method 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 a subsequent solvent extraction step.

[0038] However, the present invention can be applied to a wide range of sludge ashes, 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, it was found that hydrochloric acid can leach more silicon than nitric or sulfuric acid. Furthermore, the dissolution yield is higher at lower acid concentrations and lower temperatures. The solubility limit of silicon species at acidic pH is indeed very low. The solubility limit of silicic acid at pH 2 is about 150 ppm. Above this concentration, silicic acid polymerizes and forms colloidal particles, precipitates, or gels. Under certain conditions, these forms can change over time, which is undesirable, especially if it occurs during solvent extraction. Colloidal silica can lead to emulsification, crud generation, and large entrainment losses in subsequent solvent extraction. It has been found that even colloidal silica at levels as low as 50 ppm can interfere with solvent extraction of iron and phosphorus by TBP, making it virtually impossible.

[0040] Over time, colloidal silica will grow into larger silica particles that can be filtered, but such a process is too time-consuming for industrial applications. For such an approach to work, the amount of colloidal silica present in the leachate that passes to the 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 should be a step that controls the amount of colloidal silica present in the leachate that passes to the solvent extraction stage.

[0041] Colloidal silica particles are very difficult to remove using traditional methods such as precipitation, filtration, or centrifugation due to their very small size (0.01-0.5 μm), however, this step may be carried out in other ways, 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 in the solution, especially of the chloride. This salting out effect during the solvent extraction allows the extraction to be more selective. This can be enhanced by recycling at least a portion of the raffinate containing chloride ions after the solvent extraction and using it for leaching of the sewage sludge ash. Recycling back streams from the various steps of the process is key to guarantee the ionic strength required for the separation of iron and phosphorus using solvent extraction. Recycling also guarantees the 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 in relatively high yields, and typically such extracted compounds are worthy of subsequent use.

[0044] Thus, the present technology provides an improved method for operating the hydrochloric acid leaching of sewage sludge ash-containing material, followed by solvent extraction, and optionally precipitation of dissolved components. The technology allows for the recovery of most of the iron in the sludge ash as iron chloride compounds. The technology also allows for the recovery of phosphorus as high-value products such as phosphoric acid or ammonium phosphate. The 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. The technology also allows for the recovery of heavy metal contaminants, such as metal hydroxides and / or metal sulfides. The technology also allows for the recovery of magnesium as commercially available compounds, such as magnesium hydroxide. The technology also allows for the recovery of calcium as commercially available compounds, such as calcium chloride. The technology also allows for the recovery of sodium as commercially available compounds, such as sodium chloride. The technology also allows for the recovery of potassium as commercially available compounds, such as potassium chloride. The technology also allows for the recovery of undissolved clean silica sand ash residues 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 also provides favorable operating conditions, such as the ionic strength required for selective separation, at various steps of the process, as shown above.

[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, which are distinguished by how the process steps are arranged, omitting steps or inserting additional steps as necessary, and operating the process steps at 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 used frequently in this disclosure shall be interpreted as follows:

[0047] leaching, leaching agent, leachate - Leaching means removing 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 solution obtained after leaching is called the leachate and contains the components of the soluble fraction as ions.

[0048] Solvent extraction or liquid-liquid extraction - a separation method in which the species to be separated are divided between two immiscible liquid phases, i.e. an aqueous phase (e.g. a leachate) and an organic phase, which in particular contains an extractant (see below). When referring to the solvent extraction of a particular element, this means the solvent extraction of an extractable complex or compound containing said particular element. For example, in the present text, the extraction of phosphoric acid by 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 are added to suppress the formation of emulsions.

[0052] Stripping or back-extraction - The reverse of extraction; eluting the 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 extracted species that are not desired 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] FIG. 1 shows a flow diagram of the steps of one embodiment of a method for chemically treating sewage sludge ash. In step S10, the starting material is dissolved in an acid, including hydrochloric acid. The starting material originates from sewage sludge ash. The starting material can thus be sewage sludge ash obtained from an incineration process, or it can be, for example, an undissolved residue from a previous dissolution of sewage sludge ash, which still contains the components that it is desired to recover. The starting material includes at least silica and iron. In other words, this initial step involves the 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, but there may be feed from return streams from various steps of the process. In addition, other compounds may be added during leaching, such as oxidizing or reducing agents, coagulants for colloidal silica, antifoaming agents, other acids, bases, pH buffers and / or salts, as discussed in more detail below. The presence of all of these components is not necessary, depending, for example, on the type of ash being leached, the leach and separation yields desired 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 leach time, temperature, mixing conditions, atmosphere, addition of oxidizing or reducing agents, liquid-solid ratio, particle size, 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 is leached at ambient temperatures using dilute acid.

[0060] Since the sludge ash typically contains a large amount of insoluble silicates, the leaching step does not completely dissolve the sludge ash, resulting in a leachate and an undissolved solid residue. In step S12, the undissolved residue is separated, thereby leaving 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 step, such as filtration, sedimentation, centrifugation, etc. Due to its high silicate content and removal of the soluble components, the residue is suitable for use as a material, for example in the concrete industry.

[0061] Step S12 may also optionally include a partial step S13 of washing the undissolved residue to produce a silicate product. Washing of the undissolved ash residue, e.g. washing off the residual leachate, can be carried out with 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. the washing 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 leachate solution.

[0062] In step S15, the amount of colloidal silica in the dissolved sewage sludge ash is controlled, i.e., reduced, in the composition of the starting material 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 in the liquid-liquid extraction with organic solvent, which is low enough to make the liquid-liquid extraction with organic solvent operable. This can be carried out as a separate process, as shown in the flow diagram, or as a process at least partially in common with steps S10 and / or S12. For example, in different embodiments, the dissolution process of S10 can be affected to have a lower content of colloidal silica, or the colloidal silica formed 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 as one substep of step S14 of the pretreatment of the leachate, which may optionally also include other types of pretreatment of the leachate. In addition to colloidal silica, sulfur, if present, is also typically leached with hydrochloric acid. Sulfur may precipitate as gypsum, at least upon aging of the leachate. In optional step S16, the leachate is pretreated with respect to sulfur in order to hasten the precipitation of gypsum, thereby typically separating it with the undissolved residue, or to prevent the formation of gypsum, thereby making the sulfur available for subsequent processes. In the case of colloidal silica, such pretreatment of the sewage sludge ash leachate may be carried out separately and / or simultaneously with the leaching.

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

[0065] The process then proceeds with 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 partial processes are discussed further below, describing partial step S21, separation, purification and recovery of iron, and / or partial 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-free solution is recycled. The iron and / or phosphorus-free solution originates from step S20 of extracting iron and / or phosphorus, possibly via an optional step S40 of post-treatment. The recycled portion of the iron and / or phosphorus-free solution contains chloride ions. The recycled solution is used to dissolve the starting material originating from sewage sludge ash. In one embodiment, dissolution with this recycled solution is the subsequent step S10. Recycling, reuse and / or recycling of process streams contributes to minimizing the generation of secondary waste and the consumption of chemicals. This also ensures that some of the process conditions necessary to carry out 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 such that the concentration of chloride salts and / or hydrochloric acid in the leachate 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 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 operations are necessary before the actual extraction can begin.

[0070] Figure 2 discloses a flow diagram of a partial embodiment of step S14 of the pretreatment of the 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 arts describe the occurrence of colloidal silica and its problematic effect on extraction.However, it has been discovered that colloidal silica does exist and that it adversely affects the solvent extraction process to such an extent that it becomes impossible to carry out the process due to irreversible crud formation.Due to the large amount of silicates in sewage sludge ash, silicon species are present in the leachate at a concentration 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, the process could not be operated due to irreversible crud formation. This very stable emulsion adversely affects the phase separation and makes the organic phase unable to be reused for stripping and extraction. The cause of the crud formation was silica present in the leachate. Silicon is a major component of sewage sludge ash and it was found that some of it is released into solution during leaching. Acids do not easily attack silicates such as quartz. However, many silicate minerals contain metal ions, typically sodium, potassium, iron, magnesium, and aluminum. These dissolve during leaching and disrupt the steric silicate structure, essentially depolymerizing it to silicic acid. Silicon in solution reacts in two ways, starting with silicic acid, dissociating and polymerizing. As mentioned before, the solubility limit of silicic acid in acidic solutions is very low, about 150 ppm at pH 2. Above this limit, silicic acid polymerizes to form colloidal particles, precipitates, or gels. Very small colloidal silica particles present in the leachate can stabilize oil-in-water emulsions and prevent phase separation, which is the basis for silica crud formation in solvent extraction.

[0073] Table 2 shows the composition of the various leachates obtained after leaching of sewage sludge ash with hydrochloric acid. The leaching process and leaching behavior of other sewage sludge ash components are described 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 more concentrated 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 with organic phases containing organophosphorus extractants, e.g., trialkyl phosphate (TBP), kerosene, and long-chain organic alcohols as phase modifiers, as well as trialkyl phosphine oxide (Cyanex 923), kerosene, and long-chain organic alcohols as phase modifiers. Crud formed in all systems. Analysis of the raffinates after solvent extraction confirmed the reduced silica concentration in solution, which was responsible for the crud formation. As the organic to acid ratio (O:A) decreased, the crud became more pronounced. The crud was responsible for the formation of a stirred egg white-like emulsion at the organic phase and interface, which remained stable for several days after the end of the test, rendering the organic phase unusable. At O:A ratios below 1:1, the crud was widespread throughout the organic phase. Pure aliphatic kerosene, several types of kerosene including 1-dodecanol or 1-decanol were tested as phase modifiers, and various concentrations of extractant, diluent and phase modifier in the organic phase were tested, always resulting in crud. Furthermore, crud formed when the leachate was mixed with diluent and / or modifier only. Crud occurred when using leachates with low ionic strength (low acidity, and / or low calcium chloride content), which have low iron and phosphorus extraction efficiency, as well as when using leachates with high ionic strength (high hydrochloric acid content, and / or high calcium chloride content), which have efficient iron and phosphorus extraction. Filtration of the leachate through a 0.45 μm PVDF filter prior to solvent extraction did not prevent the formation of crud due to the very small size of the colloidal silica particles (0.01-0.5 μm). It is not feasible to remove such particles on an industrial scale in a cost-effective manner using conventional filtration or centrifugation.

[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) for 30 minutes at a liquid-solid ratio of 3 / 1 (v / w). 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 the recovery of the ash components to be controlled. 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 precipitation of calcium sulfate, and the amount of crud formation during solvent extraction.

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

[0082] The three leachates were subjected to batch solvent extraction tests immediately after preparation. A sample of each leachate was 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 was 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, less crud was formed when the sludge ash was leached with 9M hydrochloric acid, which correlates with the lower concentration of silicon quantified in this leachate (115.5 mg / L). The extraction yields of iron and phosphorus were significantly higher when the sludge ash was leached with the higher concentration of acid, confirming that the extraction process can be controlled by controlling the concentration of the acid used to leach the ash. In fact, the extraction yields of iron and phosphorus from the 3M acid leachate were very low, about 2% for a 30% by volume TBP solution. However, the efficiency increased significantly when the chloride content in the solution was increased, for example, by internal recirculation of chloride-containing streams (chloride salts and / or hydrochloric acid). In contrast, for the 6M hydrochloric acid leachate, the iron extraction yield increased to about 90% due to the higher acidity and chloride concentration. Higher extraction yields can also be achieved by adding chloride salts to the 3M ash leachate to increase the ionic strength and by recirculating internal streams to maintain a sufficiently high concentration of chloride ions. For example, the addition of calcium chloride significantly improved the iron and phosphorus recovery efficiency. The addition of 1M calcium chloride to the leachate increased the iron extraction yield from 2% to 25% per stage, to 53% at 1.5M calcium chloride and to 78% at 2M calcium chloride.

[0083] It was observed that colloidal silica particles grow slowly and some eventually precipitate when the leachate is allowed to age for a sufficient period of time, ranging from a few hours to even a few weeks depending on the pH, initial silicon content and other parameters. However, even with leachate aged for several weeks, crud still formed and precipitation was observed, which was removed by 0.45 μm filtration prior to 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. Although 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 of more than 40 ppm. It was 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 are initially very small submicron particles and do not settle. The rate of this process depends on several factors, including but not limited to the pH of the solution, other elements present in the solution, temperature, and initial silicon concentration. The silica particles are grown to a sufficient size to allow filtration and / or settling. The leachate made in the examples mentioned in Table 3 was aged for 48 hours by leaving it in a closed container. At various times, the aged leachate was filtered through a 1 μm pore size to remove particles that may have grown larger than 1 μm. Table 4 below shows the total concentration of silicon measured at various times. 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 was more effective than 3M acid. Furthermore, in solvent extraction tests carried out similarly to 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 reduction in 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 other behavior was suggested in the prior art literature. 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 the sludge ash leaching was carried out at 60°C or 90°C, more silicon would be expected to be in solution, which was surprisingly not the case in the present case, where starting material derived from sewage sludge ash was used. When solvent extraction batch tests were carried out with the leachates obtained at 60°C and 90°C, no significant crud was observed (1:1 organic:aqueous ratio, 80% by volume of TBP).

[0091] The observations made above proved to be valid when instead of sewage sludge ash, sewage sludge ash that had already been leached with hydrochloric acid solution to remove most of the phosphorus and some (but not most) of the iron was used. With this starting material, leaching was carried out at temperatures above ambient temperature with at least 6M HCl. This was aimed at dissolving the iron and producing an iron-rich solution with small amounts of phosphorus, which was then used for the solvent extraction tests. As in the previous example, the leaching process resulted in a lower amount of silicon in the solution compared to leaching at ambient temperature, and no crud was formed when using 80% TBP in organic / aqueous phase 1:1. Subsequent treatment of the leachate with a silica coagulant, followed by filtration, reduced the silicon concentration from several tens of mg / L to about 6 mg / L. This solution was 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 by carrying out the dissolution step at an elevated temperature of at least 50°C, a leachate compatible with operable liquid-liquid extraction with organic solvents could be obtained.

[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 charcoal as a means of mitigating silica crud during solvent extraction, this was unsuccessful here. Treatment of the sludge ash leachate with activated charcoal followed by filtration through a 0.45 μm PVDF filter did not prevent the formation of crud by TBP. 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, 576-588, 1975 also mention that colloidal silica may be adsorbed by calcium ions at high calcium concentrations in solution, but surprisingly, this was ineffective for the sludge ash leachates tested here up to calcium concentrations of 2.7M in solution. Addition of a flocculant to the leachate did not solve the problem, as the colloidal silica particles are 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 solution, however this is not feasible as it increases the amount of aqueous phase in the process, consuming chemicals or water and requiring greater processing capacity.

[0094] However, the formation of crud was avoided by adding a fluorine source to the leachate, in this case ammonium fluoride, which converts the insoluble colloidal silica particles into ammonium fluorosilicate, which has a solubility limit in water of about 185 g / L at 25° C. In one embodiment, the addition of NHF was used to form (HN4)2SiF6. In other words, in one embodiment, the step of reducing the colloidal silica includes the addition of a fluorine source, such as NHF, HF, NaF, or CaF2, to convert the colloidal silica into a fluorosilicate species, such as (HN4)2SiF6.

[0095] Crud was further avoided by using coagulants designed to bond and grow colloidal silica particles and facilitate their removal from solution by 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 in the stream. Adding a few hundred ppm of coagulant to the leachate instantly precipitates colloidal silica, allowing its removal by 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] Addition of a silica coagulant to the leaching reactor during leaching of the ash, followed by a single filtration to remove both the undissolved ash residue and the coagulated silica, was effective during the experiments in facilitating the subsequent processing of the resulting leachate using a mixer-settler without silica crud. A silicon concentration of 36.4 mg / L was determined in the solution. Addition of a silica coagulant to the leachate, already separated from the undissolved ash residue, and removal of the coagulated silica using filtration also made it possible to operate without silica crud. This was obtained regardless of whether TBP solutions diluted with 1-decanol and / or kerosene, e.g. 40% TBP or 75% TBP by volume, or undiluted TBP, were used.

[0097] This example shows that i) when sludge ash is leached with 3M aqueous hydrochloric acid at ambient temperature (21°C), silica coagulation is effective whether or not a coagulant is added during or after leaching of the ash, 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 not coagulated, can contain more than 300 mg / L silicon (see Table 3), and crud occurred during the solvent extraction of iron and phosphorus with TBP, even when only one 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] The former may not generate crud directly, but 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 can be the case in the present invention. Colloidal silica can form in the strip product due to the concentration of ionic silicon exceeding the stability threshold and / or due to changes in pH, which can generate crud. To prevent this, a silica coagulant or a fluoride source can be added to the aqueous phase, i.e., the strip solution and / or the strip product, to reduce the formation of crud and promote phase separation in the later stages of the solvent extraction process. It has been found that the presence of a silica coagulant in the solution aids in the precipitation of the newly formed colloidal silica. This was also observed in sewage sludge ash leachates that were left to stand and aged for several days after the colloidal silica had been removed by coagulation and filtration. It is therefore generally recommended to carry out 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 the sewage sludge ash leachate is contacted 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 is leached with hydrochloric acid and the leachate is separated from the undissolved residue. In other words, in one embodiment, the control of colloidal silica is at least partially carried out in the leachate after the step of separating the undissolved residue, whereby the method comprises 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 is performed at elevated temperatures, preferably above 50°C, which results in a lower 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 also be combined, in which case part of step S15 controlling the amount of colloidal silica is carried out simultaneously with the dissolution and another part after separation of the residue.

[0102] Thus, the inventors have surprisingly demonstrated that the formation of crud is avoided if the coagulant is either: 1) added to the leachant first, 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 leachant after filtration, and then the precipitated silica fraction is removed; or 4) a combination of the above (where 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 even follow immediately with a solvent extraction without removing the precipitated silica from solution. This approach mitigates much of the crud caused by silica particle growth due to coagulation, but operation under such conditions is not considered ideal and can still lead to losses of organic matter 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 coagulant specially formulated for colloidal silica, which bonds and grows the colloidal silica particles, allowing removal by precipitation, clarification, or filtration, and allows for a solvent extraction operation with significantly reduced organic phase losses, even without a removal step.

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

[0106] The reverse time schedule may also be used, where the dissolved sewage sludge ash can be aged to allow time for silica growth, which typically occurs after separation of the residue, 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 with 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 step at an elevated temperature of at least 50°C it was possible to obtain a leachate suitable for operable liquid-liquid extraction with organic solvents.

[0109] In other words, in one embodiment shown in FIG. 4, the step of reducing colloidal silica includes the following steps: - 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 temperature, as in step S156 Includes at least one of the following:

[0110] As mentioned above, the pretreatment step S14 and the step S15 of reducing the amount of colloidal silica can be carried out after the dissolving step S10 or, surprisingly, simultaneously with the dissolving step S10. In the latter case, a 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, making a second solid-liquid separation step S19 to remove the precipitated silica unnecessary.

[0111] In addition to colloidal silica, sulfur is typically also leached with hydrochloric acid, as seen in Table 2. This can precipitate from the leachate as calcium sulfate, e.g., gypsum. For example, according to the article "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 (appearance of gypsum crystals) in aqueous solutions of inorganic salts is particularly increased by the presence of iron ions.

[0112] We have observed that for leachates similar to those in Table 2, sulfur precipitation occurs slowly after filtration, sometimes over several days. The presence of gypsum particles in the leachate is undesirable because it will 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 the operation to take place before gypsum is formed, by the addition of 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 high background calcium concentrations lead to faster sulphur precipitation, therefore, by ensuring high calcium concentrations in the leach solution, slow precipitation could be avoided.

[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 the addition of gypsum seed particles to promote the precipitation of gypsum. Steps S161 and S163 may be combined.

[0117] In other words, in one embodiment, the starting material comprises sulfur. The method for chemically treating sewage sludge ash includes a further step of adding a CaSO4 inhibitor prior to the step of extracting iron and / or phosphorus, thereby preventing calcium sulfate precipitation from occurring immediately prior to or during said step of extracting iron and / or phosphorus. The term "immediately prior" 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 iron and / or phosphorus, a further step of carrying out aging of the leachate and / or accelerating CaSO4 precipitation to prevent calcium sulfate precipitation immediately prior to or during said step of extracting iron and / or phosphorus.

[0119] Precipitation of sulfur as gypsum is promoted in a preferred embodiment by increasing the calcium chloride content in the leachate, which is also advantageous for subsequent solvent extraction. Solvent extraction of iron and phosphorus is promoted by increasing the ionic strength of the leachate, as will be described in more detail below. Calcium supply 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 instead of water, or a combination of both.

[0120] Various embodiments can be identified in terms of timing in order to obtain a high calcium concentration in the leachate and thus a faster precipitation of gypsum. In FIG. 6, an embodiment is shown in which step S16 of treating sulfur is carried out after the actual leaching is finished. A calcium chloride salting reagent is added to the leachate after the leaching. Then, a step S169 of removing gypsum is carried out. Preferably, this measure can be carried out together with the addition of a silica coagulant, for example as mentioned above, in order to promote the precipitation and filtration of gypsum together with the silica. The precipitation of gypsum has a positive effect on the filtration of silica, since gypsum is a good filter aid. Then, step S19 of FIG. 2 and step S169 of FIG. 6 can be combined.

[0121] Alternatively, the addition of the substance that increases the induction period of gypsum may occur after the actual leaching is completed, in which case the sulfur will remain in solution and the 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 the dissolution step S10. In one embodiment, a high calcium background during leaching is ensured while promoting the precipitation of gypsum. This can be done by adding, for example, calcium chloride during leaching, by returning the aqueous raffinate rich in calcium chloride after the extraction of iron and / or phosphates to the leaching step, thereby replacing the water, or by a combination of both. This results in a higher precipitation of gypsum during leaching. This also allows the precipitated gypsum to be separated, for example filtered, together with the undissolved ash residue in step S12.

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

[0125] Furthermore, as indicated above, the addition of CaSO4 seed particles may also enhance the rate at which gypsum precipitates. 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, after the step of separating off the undissolved residue, the aqueous raffinate rich in calcium chloride obtained after the preceding step of extraction of iron and / or phosphorus, - 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] The recovery of iron from sludge ash leachate may be carried out 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 called Fe(II) and Fe(III), respectively, determines whether it can be effectively extracted by TBP. It has been found that the extraction of iron by TBP is significantly reduced if the ferric ion is reduced to ferrous ion by the use of a reducing agent (e.g., addition of metallic iron to the leachate). The reverse is also true. To facilitate the extraction of ferrous ion by TBP, oxidation to ferric ion can be carried out using an oxidizing agent.

[0128] This allows for various approaches to iron extraction. For example, if non-extractable ferrous iron is present in the sludge ash or leachate, oxidation can be utilized to maximize iron recovery. With reference to Figure 1, step S18 of oxidizing 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 extraction step S20 of oxidizing Fe(II) in the leachate to Fe(III).

[0130] Conversely, reduction can be used to minimize the extraction of iron, e.g., avoiding iron extraction and proceeding to phosphorus recovery. This may be preferred for ashes and / or leachates with very low iron content, where the benefits of recovering iron do not compensate for the increased complexity of recovering both iron and phosphorus. Such a reduction of Fe(III) to Fe(II) step S17 can be combined with the pretreatment step S14. Reduction can take place 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, e.g. Cu(II) reduces to metallic copper, which precipitates.

[0131] In other words, in one embodiment, the method for chemically treating sewage sludge ash includes a further 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 step S30 of extracting phosphorus, but without the step of extracting iron.

[0132] FIG. 8 shows a schematic diagram of an arrangement 1 for chemical treatment of 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 originates from sewage sludge ash and is at least partially dissolved in the dissolution reactor. A separation device 12, for example a filter, separates the undissolved residue 204 from the leachate 206. Preferably, the arrangement 1 for chemical treatment of sewage sludge ash further comprises a washing chamber 13, which is configured to wash the remaining acid from the separated undissolved residue 204 and obtain a washed residue product 204X. The used washing liquid 205 may be returned to the appropriate stage of the process by a return line 131. Alternatively, the washing liquid 205 may be supplied to the extraction section 20 (described further below) and mixed with, for example, the percolation liquid 206 or the percolation liquid 207 (described below).

[0133] The arrangement 1 for 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 in which the additive 203 may be introduced, and / or a heater 14C arranged to provide an elevated temperature 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 an apparatus 22 for liquid-liquid extraction with an organic solvent. The apparatus 22 for liquid-liquid extraction is configured to extract at least one of iron and phosphorus from a leachate 207 that is low in or free of 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 have been at least partially removed is provided from the extraction section 20.

[0135] Preferably, the arrangement 1 for the chemical treatment of sewage sludge ash further comprises a post-treatment section 40 in which, by using different post-treatment additives 212, 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.

[0136] The optionally post-treated raffinate is at least partially recycled. The at least partially iron- and / or phosphorus-free raffinate 211 coming from the extraction section 20 can be returned to the inlet of the dissolution reactor 10 by the return pipe 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 representation of an embodiment of a dissolution reactor 10 incorporating a device for reducing the amount of colloidal silica in the 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 further promote coagulation to grow silica particles from the colloidal silica, as described above, or may convert the colloidal silica into ionic silicon, which may remain in solution if moderately soluble, or may precipitate at least partially into the undissolved residue 204 if the solubility limit is reached. The silica particles grow until they can be separated by the separation device 12. Thus, the control of the amount of colloidal silica can surprisingly be carried out in the same dissolution reactor 10, simultaneously with the dissolution itself.

[0139] In the same way, an additive 203B may be included to prevent calcium sulfate precipitation occurring during or during transport to the extraction section 20. For example, a CaSO4 inhibitor may be introduced to mitigate the precipitation of gypsum, so that the sulfur passes through this formation in solution. Alternatively, an additive may be added to accelerate the precipitation of CaSO4, so that the gypsum precipitates and is separated by the separation device 12 together with the undissolved residue 204. Thus, the treatment of the sulfur can surprisingly be carried out in the same dissolution reactor 10, simultaneously with the dissolution itself.

[0140] Also, oxidizing or reducing additives can be added to affect the oxidation state of the iron ions and possibly even other metals. A reducing agent 203C, for example 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 treatment 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 shows an embodiment of the 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 of FIG. 9. In the first mixing chamber 141, additives 203A can be added through the 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 further described above. The silica particles grow until they reach a size that can be separated. The pretreatment chamber 14A, in this embodiment, includes an aging chamber 142 in which precipitates can settle and silica particles can grow. Any precipitates and silica particles 204C can be separated by a separation device 144 to obtain 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 the extraction section 20. For example, a CaSO4 inhibitor may be introduced to mitigate gypsum precipitation, so that the sulfur passes through this formation in solution. Alternatively, additives may be added to accelerate the precipitation of CaSO4, so that the gypsum precipitates 204C and is separated by the separation device 144.

[0144] Also, oxidizing or reducing agents 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).

[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, recovery of iron from sludge ash leachate may be carried out as ferric chloride using an organic phase containing TBP as the extractant. Long chain alcohols, e.g. decanol, may be used as modifiers and kerosene may be used as a diluent if necessary. Other organic compounds and mixtures may also be suitable for this purpose. Iron is preferentially extracted by TBP over phosphorus and most of the other impurities in the leachate, although it has been determined that co-extraction of phosphorus, calcium, and other trace elements does occur.

[0147] The amount of elements co-extracted by TBP together with iron can be reduced if the extraction is carried out at approximately the loading capacity of the organic phase containing iron. This situation may be achieved by controlling the organic to aqueous phase (O:A) ratio and / or the TBP content in the organic phase. Iron can thus be controlled to be concentrated in the loaded organic phase by carrying out the extraction at a low O:A ratio. Conversely, co-extraction of e.g. iron and phosphorus can be achieved by carrying out the extraction at a high O:A ratio. In this way, there is the possibility to adapt the characteristics of the extraction process to e.g. the content of the leachate.

[0148] However, as briefly mentioned above, the leachate content can also be controlled to some extent by different 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] Thus, another aspect of the preferred embodiment of the present technology focuses on controlling the leaching and extraction selectivity of sewage sludge ash components by controlling process conditions such as temperature, pH, pressure, time, mixing speed and duration, presence of oxidized or reduced species, ash particle size, L:S ratio, O:A ratio, etc. This allows control of the amounts of sewage sludge ash components dissolved and extracted at various stages of the process, thereby allowing individual separation of components, e.g., iron from aluminum, calcium from magnesium, separation of pure phosphorus streams, and ensuring that the separated compounds are of high purity.

[0150] For example, the dissolution yields of some components can be controlled during leaching, allowing to obtain 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 leaching sewage sludge ash with hydrochloric acid solutions of different concentrations and / or different leaching times and / or different temperatures and / or different 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%. It has already been shown in Table 2 that with increasing hydrochloric acid concentration and / or decreasing L:S ratio, less silica is leached. These parameters can also be selected to achieve the desired leaching yields for iron and phosphorus, since they strongly influence the leaching process, and they affect both the elemental concentrations in the leachate and in the undissolved residue after leaching.

[0151] For example, in one embodiment, leaching can be carried out with a dilute hydrochloric acid solution, e.g., at ambient conditions, at a low L:S ratio 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 clean, iron-depleted, undissolved silicate residue.

[0153] [Table 6]

[0154] In yet another embodiment, sequential leaching can be performed to obtain leachates with different 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 limited, 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 to obtain a silicate product with a lower iron content and a lower 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 only one step. The raffinate after extraction of iron can be efficiently used to leach new sewage sludge ash and / or can be reused in other steps of the process and / or can be treated according to the embodiments described herein.

[0155] The above three embodiments are 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 diagrammatically in FIG. 11. An input aqueous solution 221 is fed to a contact vessel 80 where iron is extracted by an organic phase 222 to obtain an iron-loaded organic phase 222B. The raffinate 227 is passed from the contact vessel 80 to a subsequent recovery process, for example a phosphorus extraction process. The iron-loaded organic phase typically also contains co-extracted phosphorus, calcium and other trace impurities. Scrubbing 81 of the co-extracted impurities from the loaded organic phase 222B can optionally 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 in low concentrations. In stripping vessel 82, iron is stripped from either the loaded organic phase 222B or the scrubbed organic phase 222C into a strip solution 225 to obtain an iron-loaded strip solution 226. The organic phase 222 is recovered and may be reused in 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 the 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 low solubility in water, about 0.4 grams per liter at room temperature. Moreover, the 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 typically mixed with a diluent, such as aliphatic kerosene, to improve the physical separation of TBP from the aqueous phase.

[0159] Experiments carried out have shown that when in contact with a concentrated salt solution such as calcium chloride, the difference in density can be sufficient to avoid the use of a diluent, provided that stripping can be carried out in a way that allows 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 iron and / or phosphate streams produced by solvent extraction with TBP run the risk of being contaminated with phosphorus and / or iron, respectively.

[0160] The extraction of ferric iron by TBP can be carried out according to equation (1), which shows that for every mole of iron, one mole of hydrochloric acid is extracted along with it. This means that there must be enough hydrochloric acid 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, and if stripping is carried out with water, an acidic stripped product is obtained, which is a mixture of hydrochloric acid and ferric chloride. Since the extraction of iron by TBP is favored by high acidity, 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 stripping with water. This determines how concentrated the stripped product can be.

number

[0161] Controlling the incorporation of iron and phosphorus in the solvent extraction process is an important aspect of this technology, since TBP can extract phosphate with high yields (this is further detailed below when phosphorus recovery is discussed). Such control allows for high purity iron and phosphorus streams to be obtained. Leaching of sewage sludge ash with 3M hydrochloric acid solution at 3:1 L:S ratio units (L / kg) followed by solvent extraction with TBP resulted in a significant reduction in 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 about 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 the iron and phosphorus extraction yields, leaching of the sludge ash may be carried out with a leaching agent with an initial hydrochloric acid and / or salt content selected such that the leachate obtained under certain operating conditions, e.g. leaching time, L:S ratio, temperature, etc., contains a sufficiently high ionic strength of hydrochloric acid and chloride 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 complementary embodiment, 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 that can increase the ionic strength of the leachate to ensure the desired extraction yield.

[0164] As yet another option, the addition of hydrochloric acid and / or salts and / or one or more compounds capable of increasing the ionic strength of the leachate and / or recycled process streams can be carried out after leaching to ensure the desired extraction yield.

[0165] Of course, the above options can be combined in various ways, two or three of them can be used simultaneously or at different times during the production session. For example, at the start of a production session, when no recycled process streams are used, salt can be introduced after leaching, whereas at later stages, recycled process streams added during leaching may be sufficient to reach the preferred 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 of 21.42 g / L iron and 26.69 g / L phosphorus at an O:A ratio of 1:1, using an organic phase containing 30% TBP. In this particular case, the raffinate after solvent extraction contained 0.74 g / L iron and 27.53 g / L phosphorus, meaning that an organic phase containing more than 20 g / L iron and no phosphorus was obtained. However, it can be observed that a typical sludge ash leachate obtained at ambient temperature using a mild hydrochloric acid solution contains significantly less iron (3-5 g / L), as can be seen in Table 2. This is a consequence of the low leaching yield of iron in these conditions (Table 5). The high iron content in the aqueous phase of WO 2014 / 178788 (21.42 g / L) was achieved after adding base to a similar sludge ash leachate, precipitating an intermediate calcium phosphate-iron-aluminium 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 treatment by adding base to the ash leachate. This approach also allowed 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, which could be stripped to obtain ferric chloride with a low phosphorus content.

[0167] Using the same extraction conditions and sewage sludge ash leachate of Table 2, it was not possible to obtain the same iron extraction results without this intermediate precipitation step. In order to obtain a concentrated iron product from such a sewage sludge ash leachate after removal of colloidal silica, the extraction with TBP must be carried out preferably at a low O:A ratio in order to concentrate the iron in the organic phase. Furthermore, if further concentration of iron in the stripped product is desired, it is preferable to carry out the stripping at a high O:A ratio. Furthermore, without adding a suitable salting reagent to the 3M hydrochloric acid leachate, the iron extraction yield was low. In the case of such a low iron content leachate, characteristic of ashes with very low iron content, and / or when leaching is carried out with mild hydrochloric acid solutions at ambient temperature, it is not feasible to carry out the extraction of iron at a high O:A ratio, since this leads to a too dilute organic phase that also contains significant amounts of phosphoric acid. Although iron is preferentially extracted over phosphorus, it was surprisingly not possible to obtain a relatively pure loaded organic phase or iron strip product, even at O:A ratios lower than 1:1, due to the co-extraction of phosphorus along with the iron. Increasing the amount of TBP in the organic phase to more than 30% by volume and / or increasing the O:A ratio increased the co-extraction of phosphorus. Reducing the amount of TBP had a negative effect on the extraction of iron. In the embodiments described herein, it was established by experiment that to achieve good iron extraction with minimal phosphorus extraction, the amount of TBP in the organic phase and the extraction O:A ratio should be adjusted to the amount of iron present in the leachate, so that the organic phase is loaded with as much iron as possible, preferably at approximately the loading capacity of TBP. Overloading the organic phase with iron prevents the co-extraction of phosphorus, with most of the phosphorus remaining in the aqueous phase, e.g., the leachate. Nevertheless, the co-extraction of phosphorus 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 strip product in the stripping step, significant amounts of co-extracted phosphorus (13.0 g / L) and calcium (8.95 g / L) remained in the strip 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 and 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 by TBP together with iron can be reduced if the extraction is carried out at approximately the loading capacity of the iron-containing organic phase. This situation can be achieved by controlling the organic to aqueous phase (O:A) ratio and / or the TBP content in the organic phase. Iron can be concentrated in the loaded organic phase by carrying out the extraction at a low O:A ratio. Stripping of iron from the loaded organic phase is carried out with a suitable stripping solution, typically water, although other solutions can also be used. Stripping can be carried out 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 also be added to compensate for losses. In addition, additional treatment of the organic phase is possible, such as washing with sodium carbonate or sodium hydroxide solutions 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 compared to phosphorus, calcium, and other extractable species. To further purify the iron stripped product, it is again subjected to a similar solvent extraction and stripping cycle. If necessary, the ionic strength of the aqueous feed can be increased to improve the extraction of iron. 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 stream having a high ionic strength from another step of 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 extraction of iron 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 with low amounts of phosphorus and other impurities. If desired, a third solvent extraction-stripping can be performed and repeated to obtain an even higher purity iron chloride product. The raffinate after the second solvent extraction step of iron contains unextracted impurities, mainly phosphorus and calcium. Because the initial extraction of iron is performed at a low O:A ratio and the stripping is performed at a high O:A ratio, the volume of the raffinate is sufficiently low to facilitate adjustments at the start of various steps, e.g., leaching, solvent extraction, or other steps. It has been determined that phosphorus can be effectively scrubbed from the iron-loaded organic phase with concentrated hydrochloric acid. This leaves the iron in the organic phase, but the phosphorus can be back-extracted in hydrochloric acid and used to obtain a higher purity iron chloride product with reduced phosphorus content.

[0171] A particular aspect according to this embodiment of the technology is therefore the subsequent treatment of the stripped product after the iron solvent extraction with the same extractant in order to obtain high purity iron chloride and at the same time increase the concentration of iron in the final product. Experiments have shown that by tailoring the TBP concentration and extraction O:A ratio to the iron content in the aqueous phase and performing the extraction at approximately the iron loading capacity of the organic phase, it is possible to control and reduce the amount of impurities co-extracted with the iron. Therefore, a subsequent iron extraction step operating under these conditions was carried out.

[0172] FIG. 12 shows a general embodiment for the subsequent processing of the stripped product after the first solvent extraction of iron using solvent extraction in order to obtain high purity iron chloride and at the same time increase the concentration of iron in the final product. The iron solvent extraction configuration comprises 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 together 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 adapted to the composition of the aqueous phase to affect the extraction selectivity. The raffinate 211 is output to a portion to be post-processed and / or used for the recycle of 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 that is 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 to the second solvent extractor 220B. The intermediate strip solution 228 is fed into 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 the process parameters can preferably be adapted to 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 that way, the ionic strength can be adapted to improve the 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 re-entered 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 solution 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 of the extraction of Fe. 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 the 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 aimed at 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 as it allows further concentration of iron in the organic phase. At the same time, stripping at O:A ratios above 1 allows further concentration of iron.

[0177] The 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 the extraction of iron with TBP is carried out according to Equation 1. The hydrochloric acid, together with the calcium and phosphorus compounds in the solution, contributes to the ionic strength required to extract the iron into the organic phase. If necessary, the ionic strength can be increased by adding fresh hydrochloric acid and / or other salting agents, as indicated by additives 230. These additives may 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 through experimentation that the extraction yield of iron is positively 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 a subsequent solvent extraction step. The data shows that the amount of calcium, phosphorus, and trace impurities such as aluminum, copper, and zinc are significantly reduced using a subsequent extraction-stripping step. Here, two extraction-stripping steps are shown, but the number of steps can be adjusted depending on the nature and composition of the sludge ash, e.g., three or more steps can be considered for ashes 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 the iron is extracted as FeCl3. The second extraction stage comprises selective liquid-liquid extraction of mainly 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, a higher iron purity and / or a 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 later extraction step now includes at least one further extraction step to achieve a higher iron purity and / or a higher iron concentration in the recovered iron product compared to the second strip solution.

[0182] This is advantageous to use a solution with a smaller iron:phosphorus ratio, for example 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 with an acid having a hydrochloric acid concentration of less than 3M. Preferably, the first extraction stage comprises matching the loading capacity of the organic solvent to the content of Fe(III) in the leachate by controlling the relative amount of organic solvent to the leachate. More preferably, the second extraction stage and / or any further extraction stage comprises controlling the loading capacity of the organic solvent to be approximately or less than the content of Fe(III) in the first stripping solution by controlling the relative amount of organic solvent to the first stripping solution. If the loading capacity is somewhat 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 at a later stage. 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 of dissolving the starting material.

[0183] Experiments have shown that the co-extracted phosphorus can also be selectively scrubbed from the 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 the iron-loaded organic phase. Scrubber 81 is configured to scrub the co-extracted impurities from the loaded organic phase 222B with scrubbing liquor 223. This results in the formation of loaded scrubbing liquor 223B and scrubbed organic phase 222C.

[0184] It was found that selectivity was higher when the loaded organic phase 222B was scrubbed with hydrochloric acid instead of water. Selectivity increased with increasing hydrochloric acid concentration. For example, the loaded organics containing 16.6 g / L iron and 2.90 g / L phosphorus were scrubbed with a 15% hydrochloric acid solution with an O:A ratio of 10:1. This removed 0.5% iron and 54% phosphorus in one step. Calcium and copper were also scrubbed efficiently along with some of the co-extracted arsenic, cadmium, and zinc. Selectivity was also improved when scrubbing with an O:A ratio of 20:1 instead of 10:1, but increasing the hydrochloric acid concentration proved to be 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. It was previously noted that 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 the effectiveness of stripping is reduced if there is sufficiently high acidity and / or ionic strength in the stripped product. Thus, scrubbing of 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 typically in low concentrations.

[0186] The loaded scrubbing liquor 223B contains primarily Ca and P, but typically also contains other impurities, as discussed above. Since 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 is 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 preferred over hydrochloric acid solution due to the higher stripping yield. Basic solutions such as sodium hydroxide, potassium hydroxide, or other soluble bases can also be used. We have found that stripping with 0.5M aqueous sodium hydroxide solution gives slightly better stripping yield compared to water. However, the O:A ratio and base concentration need to be selected appropriately to prevent the precipitation of iron hydroxide due to too high pH of the resulting stripped product. 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 for the temperatures tested (0-70°C). Preferably, stripping is carried out at lower temperatures to ensure a higher iron concentration in the stripped product.

[0188] One of the main uses of ferric 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 desired. According to embodiments of the present technology, this can be done in several ways. Neutralization can be carried out with conventional bases such as sodium hydroxide, calcium hydroxide, or equivalents. However, this can result in contamination of the final iron solution with sodium, calcium, etc., which may or may not be acceptable to the end user.

[0189] Thus, for the production of high purity iron chloride coagulant, neutralization is preferably performed with iron oxide, iron hydroxide, iron oxyhydroxide, and / or metallic iron, or materials 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 is significantly slower at ambient temperature than at elevated temperatures. When metallic iron is added to a hydrochloric acid solution containing dissolved ferric and other impurities, two main 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. It has been observed by the inventors that when reduced metallic iron powder is added to the iron-containing acid strip product, the reduction of ferric to ferrous occurs in preference to the neutralization of hydrochloric acid. This was most evident 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 yellow characteristic of ferric iron to the pale blue-green 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 seen 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 less than 0 and after 1.5 hours there was iron powder in the solution.At 95°C, the reaction was significantly faster and most of the iron was dissolved after 4 hours when the pH of the solution was raised to pH 2.

[0190] Neutralization of free hydrochloric acid with metallic iron is also an effective method to minimize the amount of impurities in the iron product, especially impurities that can also be reduced and are insoluble in their reduced form, e.g. copper. Neutralization with ferric hydroxide or ferric oxyhydroxide has the advantage, among others, of reacting quickly at ambient temperature without generating hydrogen. Neutralization of free acids 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 an arrangement 24 for neutralizing a ferric chloride product with hydrochloric acid is shown diagrammatically in FIG. 15. The ferric chloride product with hydrochloric acid can be, for example, the ferrous liquid 210A of FIG. 12, but can also be the result of other embodiments as further described below. Here, the ferrous liquid 210A is used as a representative example. The ferrous liquid 210A is introduced into a neutralization chamber 83 together with a neutralizing agent 231, for example, metallic iron, iron oxide, iron hydroxide, and / or iron oxyhydroxide. The neutralizing agent 231 neutralizes at least a portion of the hydrochloric acid of the ferrous liquid 210A and, if reducing conditions are present, typically reduces at least a portion of the ferric iron to ferrous iron. Preferably, a heater 85 is present, allowing neutralization at elevated temperature. A separation device 84 is provided to separate residual solid components 232, for example undissolved metallic iron or metallic Cu contaminants. Such residual solid components 232 may be reintroduced into the process, for example, into 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 product is desired, the resulting liquid 233 can be placed in an oxidation vessel 86, which also has an oxidizing agent 234 added thereto. The oxidizing agent 234 can be, for example, hydrogen peroxide, ozone, chlorine gas, etc. Finally, a ferric product 235 is produced.

[0193] Represented as a flow diagram in Figure 16, one embodiment of a method for chemically treating sewage sludge ash may include step S26 of at least partially neutralizing the ferric chloride product with hydrochloric acid 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 altered. 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 with a reducing agent such as metallic iron.

[0195] As further discussed above, dissolution of sewage sludge ash, also called leaching, may be carried out under different conditions to obtain leachates of different compositions. When low concentrations of hydrochloric acid are used at ambient temperature, the degree of dissolved iron is generally very low. Above, this low iron content was extracted by purifying and concentrating the iron, preferably by two consecutive liquid-liquid extraction stages. An alternative approach could instead be to ignore the amount of iron leached and focus only on the extraction of phosphorus and possibly further components. One way to control the extraction of iron in the organic phase is to take advantage of different oxidation states of iron.

[0196] All of the recovery steps involving iron 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 the speciation of iron controls the amount of iron rejected or extracted by TBP in the iron and phosphorus extraction steps, as well as the pH at which the iron compounds precipitate. For example, ferrous species, e.g., ferrous hydroxide and ferrous phosphate, precipitate at significantly higher pH (above 6) than their ferric equivalents (below pH 3). Control and / or change of the chemical form of iron in solution can be done at various points in the process, e.g., dissolution, pretreatment, different solvent extraction blocks (extraction, stripping, scrubbing), recycle, and recycle loops, and is not limited to only one step. For example, oxidation or reduction of iron can be done several times throughout the process and / or alternated during the process loop. For example, ferrous iron can be concentrated in aqueous solution during several loops, followed by oxidation and extraction in subsequent loops. This allows efficient recovery of iron from ash or leachate with low iron content.

[0197] The inventors have found that reduction of ferric iron in the aqueous stream is a feasible method to reduce 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) at the equivalent of 5.6 g / L at 21° C. for 10 min. The leachate changed color from deep yellow to pale blue-green within minutes, indicating 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 with a TBP stock solution with a 1:1 organic to aqueous phase ratio. In parallel, the same solvent extraction experiments were performed with the non-reduced leachate. Analysis of the aqueous raffinate of 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. 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. It is believed that the presence of oxygen in the air and the 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 the oxygen level in the atmosphere can be controlled, e.g., 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 prior to the extraction of phosphate, step S30. In this embodiment, step S20 does not include significant iron extraction and does include phosphorus extraction S30.

[0199] The 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 phosphorus extraction, such as before or after aluminum precipitation, as described below. Because ferrous hydroxide precipitates at a higher pH compared to ferric hydroxide, which would otherwise precipitate along with the aluminum, ferrous hydroxide can selectively precipitate aluminum at a pH below 3 (leaving the iron in solution).

[0200] In a similar manner, oxidation of iron can be performed using oxidizing agents such as hydrogen peroxide, ozone, chlorine gas, etc. to facilitate extraction by 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, similar to above. In one embodiment, this is performed as step S18 (FIG. 1) of pretreatment step S14 (FIG. 1). For example, it has been found that for ash and leachate containing ferrous iron, extraction by TBP is not effective and the ferrous iron remains in the raffinate. Iron recovery can be maximized by oxidizing the ferrous iron to ferric iron, which can be extracted by TBP. In actual experiments with sludge ash, which also contains ferrous iron, some of the iron in the leachate (0.6 g / L) could not be fully extracted by TBP and remained in the raffinate. It was then extracted with fresh TBP, which could not be extracted. Addition of a small amount of hydrogen peroxide oxidized the ferrous iron to ferric iron, which can be fully recovered by 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 leachates of different compositions. When more concentrated hydrochloric acid is used and / or the temperature is increased and / or a longer leaching time is employed, the extent of iron dissolution will generally be greater, 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, producing a cleaner, iron-depleted silicate residue.

[0202] Due to the high iron content in such leach streams, it has been found that a first solvent extraction step, at a high O:A ratio, results in near-loading capacity extraction of the iron in the organic phase, directly producing a relatively pure iron chloride product, with 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, may contain more or less only iron, even after one 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 with an acid having a hydrochloric acid concentration greater than 3 M. Preferably, the hydrochloric acid concentration is greater than 6 M. The step of extracting iron from the leachate further comprises controlling the relative amount of organic solvent to the leachate such 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, successive leaching can also be carried out to obtain leachates with different iron:phosphorus ratios. Figure 19 shows such a sequential two-phase solution diagrammatically. A first stage arrangement 1A for the chemical treatment of sewage sludge ash is supplied with a starting material 201A consisting of sewage sludge ash. Low-concentration hydrochloric acid 202A is used to carry out the leaching, i.e. dissolution. The first stage arrangement 1A for the chemical treatment of sewage sludge ash outputs a phosphorus-containing liquid 210A and possibly other liquids containing elements 213A-218A to be recovered. An undissolved residue 204A is output. A second stage arrangement 1B for the chemical treatment of sewage sludge ash is supplied with a starting material 201B consisting of the undissolved residue 204A from the first stage arrangement 1A for the chemical treatment of sewage sludge ash, which arises from the sewage sludge ash. High-concentration hydrochloric acid 202B, preferably at elevated temperature, is used to carry out the leaching, i.e. dissolution. The second stage arrangement 1B for chemical treatment of sewage sludge ash outputs an iron-containing liquid 210B and possibly other liquids containing elements 213A-218A to be recovered. An undissolved residue 204B is output. Preferably, at least a part of the two stage raffinates 219A and 219B is recycled to be used in a different part of the process, preferably for example for dissolution of starting materials in the first stage arrangement 1A. Preferably, a part of the two stage raffinates 219A and 219B may even be used in the second stage arrangement 1B, indicated by a dotted line.

[0207] Each stage thereby constitutes a complete processing entity in itself. The two stages may be performed in a time and / or space related manner, 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 process 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 a starting material and is dissolved using high-concentration HCl. This process results in a solution of recovered iron and a quantity of a second undissolved residue.

[0209] Since phosphorus is a valuable component, a preferred next step according to the technique 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, heptanol, 4) ketones, e.g., methyl isobutyl ketone, methyl propyl ketone, diethyl ketone, methyl ethyl ketone, methyl-n-butyl ketone, 5) amides, e.g., butylacetamide, 6) aldehydes, e.g., benzaldehyde, 7) esters, e.g., ethyl acetate, butyl acetate, amyl acetate, cyclohexanone, 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, especially in the presence of acids, salts, and / or other chemical species, some more soluble than others. As a result, traces of the extractant may be present in the aqueous stream and in the final product. These are undesirable, especially considering the objective of utilizing the separated phosphorus as a fertilizer. The high water solubility usually requires the 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 about 90 g / L at room temperature. Some of the listed solvents, such as diisopropyl ether, have other drawbacks in addition to their high solubility, such as flammability and / or explosion hazard. Furthermore, some of the listed solvents show limited extraction capacity of phosphoric acid below a certain threshold concentration. This means that the feed solution must contain a high concentration of phosphoric acid, which usually requires concentration of the acid by evaporation of water. In addition, 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 extraction of phosphate from fairly low concentrations (about 1.5M phosphate), which is a concern for many sludge ash leachates. Alcohols with relatively long carbon chains have relatively low water solubility. Thus, alternative solvents for extracting phosphate are long carbon chain alcohols, such as heptanol, which have a water solubility of less than 2%. If the solubility is below such a level, the amount of solvent in the aqueous stream is reasonably small enough to be treated using relatively inexpensive and complex configurations. Although TBP is a standard solvent that has been used on a large scale for over 40 years, the preferred solvent according to the present invention is TBP, since the industry has very limited experience with the use of long chain alcohols.

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

[0211] In one embodiment, phosphorus is extracted from the raffinate of the iron extraction step. In another embodiment, phosphorus is extracted from a leachate in which ferrous iron is present, for example after a pretreatment step. In yet another embodiment, phosphorus is extracted from a leachate with a low iron content, whereby this iron content 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 phosphoric acid in preference to dissolved salts such as chlorides and to dissolved acids such as hydrochloric or sulfuric acid. The presence of dissolved salts or acids enhances the extraction of phosphoric acid by a salting-out mechanism. Tributyl phosphate preferentially extracts nitric acid over phosphoric acid, making it impossible to selectively extract phosphoric acid from nitric acid. Thus, leaching of sludge ash with hydrochloric acid is preferred over nitric acid. In general, solvents with low solubility in water preferentially extract nitric acid over phosphoric acid. Solvents that are highly selective for phosphoric acid, such as amyl alcohol, are highly soluble in water, yet still co-extract significant amounts of nitric acid. The main advantage of using TBP as a solvent to extract phosphoric acid is its low water solubility, allowing for an operation that does not require costly and complicated distillation of the solvent from the aqueous stream.

[0214] As mentioned above, 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 by TBP along with the phosphoric acid. This is a suitable option if the amount of iron extracted in 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, selective precipitation, etc. As mentioned above, if the iron is reduced to ferrous iron prior to phosphorus extraction, the efficiency of its 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 almost complete extraction of phosphate is possible using TBP and a background content of calcium chloride of about 3 M. For example, Habashi et al., 1987, state that with an O:A ratio of 1:1, more than 99% of the phosphate can be extracted in three steps. The same statement is repeated in Pereira, 2013.

[0216] However, the applicant surprisingly found that under the above conditions, complete extraction of phosphate from the sewage sludge ash leachate with TBP was not possible. As can be seen in FIG. 21, the phosphate concentration could only be reduced from about 0.7 M to about 0.15 M, which corresponds 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 the sewage sludge ash after iron extraction with 3 M hydrochloric acid. About 2.7 M calcium chloride was present in the background. The phosphorus content in the solution was 21 g / L P, i.e. about 0.7 M phosphate. The organic phase was TBP. Also, the loading capacity of the TBP solvent reported, for example, by Habashi et al. was not possible. Habashi et al. reported a loading of 3 M phosphate in TBP in contact with 0.7 M phosphate in the 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] Although technically possible, this has a negative impact on 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 an apparatus 22D for liquid-liquid extraction of phosphorus with an organic solvent. The apparatus 22D for liquid-liquid extraction of phosphorus is configured to extract phosphorus from the leachate or raffinate of the apparatus for liquid-liquid extraction of iron. A stripping liquid 209D of saturated monoammonium phosphate (MAP) solution is added, thereby providing a liquid 210D containing phosphorus in the form of MAP and phosphoric acid. A raffinate 211, at least partially depleted of phosphorus, 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 organic phase containing phosphoric acid 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 solution, which is now a supersaturated solution. The volume of the saturated MAP solution 209D is increased by 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 comprises the further step of stripping the organic solvent in the step of extracting phosphorus with 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 stripping step of the organic solvent in the phosphorus extraction step, and 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 includes 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. The need for conventional stripping of phosphate with water, concentrating dilute phosphate by evaporation, and converting concentrated phosphate to MAP is avoided. Furthermore, the method allows for the effective treatment of organic phases with low phosphate loadings, an important advantage that allows for the treatment of sludge ash with low phosphorus content.

[0223] However, the technology presented in WO 2010 / 138045 is preferably further improved. One aspect is the handling of co-extracted impurities, especially heavy metals. The co-extracted metal ions are stripped together with the phosphoric acid and remain in the MAP strip solution. It has been found that some of these impurities also 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 relatively more. It is desirable to remove such impurities from the system before MAP precipitation. This is important because the saturated MAP solution is recycled in 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 from FIG. 23. Impurity removal components 34 may be implemented in one or several of the indicated positions, for example after stripping, after treatment with DAP in the resulting saturated MAP solution, in either the common flow or the divided flow, or in the DAP solution 240. This is confirmed by experimental results described below. One embodiment of such an impurity removal component 34 is shown in FIG. 24. Heavy metals can be precipitated as low-soluble sulfides by adding a suitable 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 sulfides, metal sulfides, organic sulfides, or any mixture of these. Heavy metal precipitates 243 are removed from the stream using solid-liquid separation equipment 36, for example, filtration, centrifugation, sedimentation, clarification, etc. This results in a stream 244 with a low concentration of heavy metals. Sulfur species remaining in solution can optionally be removed using a sulfur scavenger compound 245, which is added to sulfur removal chamber 37. The sulfur scavenger compound 245 binds the sulfur in an easy to remove phase 246, e.g., 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 layer, either top or bottom, that can be isolated from the stream by an appropriate separation technique.

[0225] [Table 9]

[0226] Additionally or as an alternative treatment, heavy metals can be removed using ion exchange, preferably using ion exchange resins that can selectively bind heavy metals, such as thiol-based ion exchange resins or other sulfur-containing resins with high affinity for heavy metal ions. Removal of heavy metals using ion exchange treatment is preferably performed according to the embodiment presented in FIG. 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 heavy metal-removed solution 250. Due to the high pH of some of the streams in the MAP-DAP loop, thiol resins can be used without serious degradation that could oxidize the thiol functional groups to disulfides, which would interfere with the process. Furthermore, due to the generally low heavy metal content (ppm level) in these streams, saturation is a very small concern since loading capacity is not reached quickly. Saturated resins with small amounts of heavy metals can be appropriately disposed of. Alternatively, the resin can be regenerated with a suitable 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 about 1.8. Some of the impurity elements precipitated along with the solid MAP product when it was produced by adding DAP to the stream (Table 9). Impurity precipitation with sodium hydrogen sulfide was tested at different points in the MAP-DAP loop (MAP loaded with phosphoric acid, MAP filtrate after MAP precipitation, and DAP solution). Precipitation of copper, molybdenum, and arsenic as metal sulfides directly from the MAP strip product loaded with phosphoric acid 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 production, followed by DAP production and filtration, resulted in the precipitation of most of the impurities, including zinc, copper, and arsenic, and a relatively fast 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 was precipitated with significantly less efficiency (Table 10). Filtration was more difficult. In addition, residual sulfide remained dissolved in solution, which could cause problems at the MAP precipitation stage. 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 way.

[0228] When precipitation is carried out at low pH with 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 with two commercially available organic sulfides. This was done 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, especially arsenic, zinc, and copper, was more efficient from the filtrate after solid MAP precipitation than from the MAP solution loaded with phosphate. Precipitation with one of the organic sulfides tested was slightly more efficient than with sodium hydrogen sulfide. However, residual sulfide remained in solution when the organic sulfide was used, which was not the case with sodium hydrogen sulfide. In this case, a scavenger for the residual sulfide can be used to remove it from the solution.

[0229] [Table 10]

[0230] In summary, phosphorus recovery is preferably performed using solvent extraction with TBP mixed with a suitable 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 performed at high O:A ratios with suitable scrubbing agents (water, dilute acid solutions, etc.). The scrubbed product can be reused in the process, for example at the start of the leaching step, solvent extraction step, or other steps. In conventional industrial operations, the loaded TBP is stripped with water to produce a dilute phosphoric acid solution, which is then concentrated using evaporation, which is energy intensive and expensive. However, in the process described herein, production of solid monoammonium phosphate (MAP) directly from the loaded TBP phase is preferred. This is done by stripping the phosphoric acid with a saturated MAP solution, followed by treatment of the MAP phosphoric acid stripped product. Several further preferred steps have been developed to remove impurities including heavy metals that are co-extracted and stripped together with the phosphoric acid. These involve 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 the present technology are interrelated: providing the high ionic strength required for effective extraction of iron and phosphorus, ensuring a good water balance in the process, and maximizing product recovery while minimizing waste. To effectively extract iron and phosphorus with TBP, a sufficiently high ionic strength is required in the aqueous stream, i.e., the leachate. This is achieved by having sufficient hydrochloric acid in solution, and / or other sources of chloride ions that have a salting-out effect. Although the process of the present invention relies on having free hydrochloric acid, and / or dissolved calcium chloride in the leachate, the process is not limited to the use of only these two chemicals. The ionic strength can be provided by any chloride salt or mixture of salts, preferably soluble components of the ash that are not extracted by TBP (calcium, aluminum, magnesium, sodium, potassium, etc.). Even salts of elements not contained in sludge ash can be used. There are several advantages to using calcium chloride. Calcium is the major component of sewage sludge ash and is easily leached even with dilute hydrochloric acid. Since calcium is poorly extracted by TBP, it is ensured that 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 step where further calcium is leached. The reuse and recirculation of the raffinate with continuous leaching of calcium ensures the necessary ionic strength for iron and phosphorus extraction and minimizes the need to add an external calcium source to ensure this. 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 step where the sludge ash components are leached and the elements already present in the solution are further concentrated. The step at which the bleed is removed can be varied, for example after phosphorus recovery, after aluminum recovery, or any subsequent step. The bleed is treated to recover the components in solution, typically as individual compounds in solid form. A typical case for 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 usually present in sewage sludge ash and may increase in concentration gradually due to recirculation of the treatment liquor. Preferably, the method for chemically treating sewage sludge ash comprises 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] The recovery of aluminum is preferably carried out by increasing the pH of the solution after the phosphorus recovery step using a suitable compound, for example, a metal carbonate, a metal oxide, ammonia, a 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 the solution and the precipitating agent used, as well as the precipitation pH and temperature, determine the chemical form in which aluminum is precipitated, i.e. aluminum phosphate, aluminum hydroxide, calcium aluminum phosphate, or mixtures of these. Temperature plays an important role, influencing both the kinetics of the precipitation process and the filtration and washing of the aluminum precipitate. Higher temperatures have a positive effect on both and are therefore preferred. Tests have shown that aluminum phosphate is completely precipitated together with the lime at pH 1.1-1.6 when the aluminum:phosphorus molar ratio is less than 1. A low aluminum:phosphorus molar ratio, for example 0.5, was found to be advantageous in terms of filtration rate compared to high aluminum:phosphorus molar ratios such as 1 and 1.5. Aluminum and phosphorus precipitated at higher aluminum:phosphorus molar ratios in the raffinate and at higher pHs.

[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. A 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 a pH 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 exploited to obtain specific chemical forms of aluminum.

[0239] If necessary, the phosphorus content in the solution can be adjusted to 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 less than 1, all 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, and the excess phosphorus remained 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 deficient.

[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 step. The aluminum precipitate can preferably be further processed. In an optional intermediate step S45, a calcium source, for example lime, is added to the aluminum precipitate. This is done if there is a lack of calcium in the aluminum precipitate, to facilitate the precipitation of calcium phosphate in the next step. In step S46, the aluminum precipitate is reacted with sodium hydroxide. This produces, for example, a sodium aluminate solution and a precipitate consisting of calcium phosphate, which is separated in step S47. The production of sodium aluminate leaves the aluminum in solution and precipitates the phosphorus, so that the phosphorus can be returned to the previous process step, ensuring that no phosphorus is lost. Thus, in step S48, the phosphorus is recycled to the previous process step. This stream is small in volume and can be accommodated in part of the steps described above, typically the leaching / dissolution of the sludge ash material.

[0243] FIG. 27 shows a schematic of an embodiment of the aluminum separation arrangement 41 included in the post-treatment section 40. The raffinate 211 from the iron / phosphorus extraction is added to the aluminum precipitation vessel 42 together with a pH increasing additive 255 and an optional phosphorus compound. Aluminum is precipitated as at least one of aluminum phosphate, calcium aluminum phosphate, and aluminum hydroxide 256, which is separated by a solid-liquid separation device 43 to obtain an at least partially removed aluminum removal solution 254. Preferably, the separated aluminum precipitate is added to an 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 the reactor 44, the aluminum precipitate is converted into a sodium aluminate solution 213 and a phosphorus-containing solid fraction 258 that includes calcium phosphate. A 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. The recovery is carried out on the mainstream resulting from the raffinate from the iron / phosphorus extraction, possibly with the aluminum removed, or on a bleed solution therefrom. The removal of the heavy metal ions is mainly 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 exploit the actual value of the metal components. One approach, shown in FIG. 28, includes 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 the precipitation of aluminum, the amount of phosphorus present in the solution can determine which compounds precipitate, and this can be used to control the chemical form of the heavy metal products. In optional step S52, a phosphorus compound is added for this purpose. Heavy metal ions precipitate over a wide pH range, typically 3-10. It was observed that precipitation of heavy metals could be achieved up to pH 9.3, selectively from magnesium, calcium, sodium, potassium, and to some extent strontium.

[0245] Another approach is to precipitate the 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 sulfides 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, illustrated by step S55, is to use ion exchange to purify the solution from heavy metals. Thiol-based resins have a high affinity for a variety of heavy metals. This may 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 solution from which at least one of iron and phosphorus has been at least partially removed and the bleed solution derived from the solution from which at least one of iron and phosphorus has been at least partially removed, 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 of an embodiment of a heavy metal separation arrangement 50 included in the post-treatment section 40. The aluminum removal solution 254 from the aluminum separation arrangement is added to the heavy metal precipitation vessel 51. Alternatively, if no aluminum removal is 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 the heavy metal precipitate 214. The heavy metal precipitate 214 is separated by a solid-liquid separation device 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 comprises the further step of recovering magnesium, calcium, sodium and potassium from at least one of the solution resulting from the iron-removed solution and the bleed solution derived from the solution resulting from the iron-removed solution. Preferably, 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 with lime prior to a pH of 10, from the other components. 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 a magnesium separation arrangement 60 included in the post-treatment section 40. Heavy metal removed solution 259 from the heavy metal separation arrangement is added to a 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 preferably be selected so that one or more of the remaining components are enriched. For example, sodium hydroxide can be used before the sodium removal step to make the solution sodium-rich. Similarly, calcium hydroxide can be used to ensure a higher content of calcium in the solution. Alternatively, different compounds can be used to increase the content of one or more species. If a suitable calcium compound is used to increase the pH and the sewage sludge ash contains small amounts of sodium and potassium, the resulting solution contains mostly calcium chloride, i.e. a calcium chloride-rich solution is obtained. 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 exploiting the difference in solubility of sodium chloride and potassium chloride at high temperature in the presence of a large calcium chloride background, e.g. according to WO 2017 / 111685.

[0257] The invention is not limited to the specific embodiments and examples mentioned above, but can be modified within the scope of the claims. For example, the leaching of sludge ash can be carried out under various conditions, with or without the integration of some of the pretreatment methods in this step. Side streams such as raffinate, scrub solution, wash water, and treated water can be recycled in different steps to obtain the same result. For example, returning two streams to the leaching reactor and one to the leachate after filtration can obtain the same result as returning all three streams to the leaching reactor. Regardless of how the ionic strength required in the solvent extraction step is guaranteed, for example by a sufficiently high acidity and / or chloride salt content, an important aspect of the process is to reuse and recycle these ionic species in the background until a steady state is reached. The bleed is treated to guarantee the water balance and to separate the products. The location of the return and bleed points can be varied depending on the desired result, the chemical nature and composition of the feed material, 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 in solution if not handled as previously described. Depending on how the ionic strength was controlled, this raffinate will 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 the leaching step.

[0259] In other words, in one embodiment, the step of recycling at least a portion of the iron-removed solution comprises recycling at least a portion of the iron-removed solution to the step of dissolving a starting material comprising sewage sludge ash.

[0260] Figure 32 shows the recycling concept diagrammatically. Downstream of the extraction section 20, there are several possible stream branchings, one stream of the at least partially iron and / or phosphorus-free solution coming from the extraction section 20 is recycled to the previous stage of the process as solution 219, and a bleed stream 270 is sent to the further component recovery stage. This can be done at one or more of the locations shown. The recycled part 219 of the raffinate at least partially freed of iron and / or phosphorus from the possible outlets constitutes the return stream. Several return outlets are possible. Returning the stream to the leaching / dissolving step means that the resulting leachate contains not only the elements in the return solution, but also the newly leached sludge ash components. The leachate will therefore contain, in addition to the leached iron and phosphorus, an increased amount of non-extracted components and hydrochloric acid. Returning the stream to the leaching / dissolving step after the subsequent extraction step and leaching new sewage sludge ash, increases again the elemental concentrations and acidity in the leachate. This occurs until a steady state is reached, i.e. the concentrations of non-extractable / poorly extracted species remain somewhat stable.Since calcium is one of the elements present in significant amounts in sludge ash, the ionic strength required for effective extraction of iron and phosphorus can be partially or completely ensured by increasing the calcium chloride concentration in solution.

[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-removed solution comprises recycling at least a portion of the iron-removed solution to the step of dissolving a starting material comprising sewage sludge ash.

[0263] Other streams are also returned in the system. As mentioned before, the wash liquid 205 can be re-entered into the dissolution reactor 10. The second stage raffinate 229 from the extraction section 20, containing phosphorus and calcium and possibly also some other contaminants, can be reused in the process, for example in the dissolution reactor 10 or in 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 to ensure a higher overall yield of phosphorus extraction, but also to maintain the ionic strength.

[0264] Tests have shown that adequate extraction of iron and phosphorus by TBP can be obtained by providing an additional 2M calcium chloride to the background when leaching the sludge ash with a mild hydrochloric acid solution, for example 3M hydrochloric acid. In the first loop, to ensure this, salting reagent must be added during or after leaching, for example calcium chloride is added so that the background chloride concentration reaches the desired value. The ionic strength is then preferably maintained by recirculation of the process stream back to the dissolution step. This is done by adjusting the amount of bleed taken from the system and / or the amount returned to the leaching step. If necessary, the return stream can be concentrated to reduce losses of salting reagent. This can be done, for example, using partial evaporation. A make-up of 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 the 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 the background concentration of hydrochloric acid. In such a particular embodiment, leaching is performed with a hydrochloric acid solution of sufficient concentration to guarantee 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, leaching of sludge ash with a 6M hydrochloric acid solution was tested by the inventors. Iron and phosphorus could be effectively recovered from the resulting leachate without the need to add salting reagents such as calcium chloride. A portion of the acidic raffinate after phosphoric acid extraction, still containing a large amount of background hydrochloric acid, was reused in the leaching step of the new ash, which was sufficient to supply the ionic strength required in the later extraction cycle. Furthermore, in a refinement of this embodiment, leaching was performed at elevated temperature, resulting in almost complete leaching of iron from the sludge ash (>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 (~5 g / L). Because the iron content (approximately 25-30 g / L) is significantly higher than the phosphorus (also approximately 30 g / L), the solvent extraction of iron could be carried out near the loading capacity of the organic phase without the need to use very low O:A ratios. In this case, the extraction could be carried out at O:A ratios of 0.5-1, provided that the system showed improved selectivity for iron compared to the same conditions but using a leachate with a much lower iron content.

[0266] As mentioned above, a bleed is required from the system to maintain the 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 was achieved at a pH below 2, as further described above.

[0267] The subsequent treatment of the solution to recover heavy metals and magnesium has already been described further above. This is achieved by a stepwise increase in 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 recovery of magnesium 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 as known in the prior art, for example in WO 2017 / 111685.

[0268] The above embodiments should be understood as some illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes 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, if 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 the steps of: 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; separating (S12) the undissolved residue (204) and 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 extraction step (S20) in order 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, which is derived from the step (S20) of extracting at least one of iron and phosphorus, for dissolving a starting material (201) originating from sewage sludge ash, a step (S90) in which the recycled portion (219) of the raffinate (211) from which at least one of iron and phosphorus has been at least partially removed comprises chloride ions; Including, The control 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 to grow particles (S151); The dissolving step (S10) is carried out at an elevated temperature of at least 50° C. (S156); by at least one of method.

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

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

4. The method according to any one of claims 1 to 3, wherein the step of promoting coagulation for particle growth (S151) is carried out at least partially simultaneously with the dissolving step (S10).

5. 5. The method according to any one of claims 1 to 4, wherein the coagulation promotion (S151) for the growth of the particles is at least partly carried out in the leachate (206) after the separation step (S12) of the 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 carrying out the dissolving step (S10) with 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 according to any one of claims 1 to 6, wherein the recycle step (S90) of at least a portion (219) of the raffinate (211) is controlled such that the concentration of chloride salts and / or hydrochloric acid in the leach solution (206) is at least 1 M.

8. The method according to claim 1, wherein the starting material (201) comprises sulfur, and the method further comprises, prior to the step (S20) of extracting at least one of iron and phosphorus, A process for aging the leachate (S161); and CaSO ４ A process for accelerating precipitation (S163); and preventing calcium sulfate precipitation from occurring immediately before or during said step (S20) of extracting at least one of iron and phosphorus, by carrying out at least one of the processes The accelerated CaSO 4 precipitation process (S163) comprises: 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, at the latest during said dissolution step (S10), the aqueous calcium chloride-rich raffinate obtained after said extraction step (S20) of at least one of iron and phosphorus, returning, after said step (S12) of separation of the undissolved residue (204), the aqueous raffinate rich in calcium chloride obtained after said step (S20) of extraction of at least one of iron and phosphorus, Adding CaSO 4 seed particles at the latest during the dissolving step (S10); adding CaSO 4 seed particles to the leachate (206) after the step (S12) of separating the undissolved residue (204); and Carrying out the dissolving step (S10) 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. A method according to any one of claims 1 to 8, wherein the extraction step (S20) of at least one of iron and phosphorus includes extracting iron from the leachate (206) by liquid-liquid extraction using an organic solvent (222) (S21).

10. The step (S21) of extracting the iron from the leachate (206) comprises a subsequent extraction stage, a first extraction stage (220A) comprising selective liquid-liquid extraction of iron contents from said leach solution (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 a higher iron purity and a higher iron concentration in the recovered iron product.

11. The method of claim 10, wherein the first extraction stage (220A) comprises 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 stages, if any, comprises controlling 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) by controlling the relative amount of the organic solvent (222) to the intermediate strip solution (228).

12. 12. The method according to claim 11, wherein the recycling step (S90) of 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, used in the subsequent dissolution step (S10) of the starting material (201).

13. 10. The method of claim 9, wherein the step (S10) of dissolving the starting material (201) is carried out with an acid having a hydrochloric acid concentration higher than 3M, preferably higher than 6M, and the step (S21) of extracting 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 carried out at an 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 (S22) phosphorus from the raffinate of the iron extraction step (S21) by liquid-liquid extraction with an organic solvent (222).

16. A method according to claim 9, wherein the starting material (201) comprises undissolved residue (204) from phosphorus leaching of sewage sludge ash, the step (S10) of dissolving the starting material (201) is carried out using an acid having a hydrochloric acid (202) concentration higher than 3M, preferably higher than 6M, and the step (S21) of extracting the iron from the leachate comprises controlling the loading capacity of the organic solvent (222) to be less than the content of Fe(III) in the leachate (206) by controlling the relative amount of the organic solvent (222) to the leachate (206).

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

18. 18. The method according to claim 16 or 17, wherein the step of recycling (S90) 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 of dissolving (S10) a starting material (201) comprising sewage sludge ash.

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

20. The method according to claim 1, 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) 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 separating precipitated heavy metal compounds.

21. The method according to any one of the preceding claims, 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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