Method for producing technical-grade phosphoric acid from sewage sludge ash

Abstract

The present invention relates to an improved process for recovering phosphoric acid from sewage sludge ash, said process comprising a leaching step; a conversion step for converting metal phosphates to phosphoric acid; solvent extraction of phosphoric acid from the leach solution using an organic solvent; a washing step for removing sulfuric acid; and stripping of the phosphoric acid to obtain an aqueous solution containing phosphoric acid.

Description

[Technical Field]

[0001] This invention relates to the recovery of reusable materials by extraction techniques, more specifically, to the recovery of technical-grade phosphoric acid from waste incineration, i.e., from sewage sludge ash. [Background technology]

[0002] The recovery and reuse of sewage sludge has increased in recent years, primarily due to environmental and political considerations.

[0003] Incinerating dewatered sludge in incinerators is a common practice at many large wastewater treatment plants. The resulting ash is primarily an inorganic residue containing various metals such as P, Fe, Al, Ca, and Si. Disposing of this ash without further treatment can cause serious environmental pollution, mainly due to the presence of heavy metals and toxic substances combined with the aforementioned metals. Furthermore, untreated ash is lightweight, pulverizes into dust, and as a result is difficult to handle, transport, and dispose of. Therefore, the disposal of such ash is a serious problem in many parts of the world.

[0004] Considering this issue, it has become necessary to develop different procedures for processing these residues with the aim of recovering the elements contained in them for reuse in different applications.

[0005] In fact, prior art has seen numerous different attempts and methods of using sewage sludge as a valuable source of raw materials. The recovery of reusable materials from sewage sludge is becoming increasingly important, particularly in relation to the reuse of phosphorus materials, which are in high global demand but have limited availability.

[0006] Depending on the operation method of the sewage treatment plant, the phosphorus concentration in sewage sludge ash (SSA) is 4-8 wt% and 10-22 wt% P2O5. Furthermore, the main components of SSA are CaO, SiO2, Al2O3, and Fe2O3.

[0007] Various approaches for obtaining phosphorus from sewage sludge ash obtained from incineration processes have been disclosed in the prior art. Reported studies on recovering phosphorus from sewage sludge ash relate to either thermochemical treatment or acid leaching processes.

[0008] In the thermochemical treatment, sewage sludge ash is mixed with chloride salts, and the resulting mixture is subjected to temperatures of 900-1100°C. Under these conditions, the metal chlorides volatilize, and the phosphorus is converted into a more bioavailable form, which can then be used, for example, as fertilizer.

[0009] WO2015 / 189333 describes a method for obtaining citrate-soluble phosphate compounds from sewage sludge ash, the method also based on the calcination of a mixture of phosphorus-containing raw materials with an alkaline sulfur compound. The resulting product readily releases phosphate ions, yielding fertilizer.

[0010] However, concerns exist regarding the operating costs of thermochemical treatments and the lifespan of the equipment due to the occurrence of highly corrosive conditions.

[0011] On the other hand, the acid cleaning process (also known as the acid leaching process) has the advantage of potentially lower energy consumption compared to many other methods.

[0012] EP0004778 describes a method for treating sewage sludge ash, including leaching the ash in an acidic solution, more specifically in a sulfuric acid solution, at a temperature of 100°C or less to recover phosphoric acid, and for further treatment of undissolved residues for the recovery of precious metals.

[0013] US2014 / 0056796 describes a first leaching step in the presence of an acidic solution at approximately 20-40°C, and a method for recovering phosphate by further isolation of phosphate by reprecipitation.

[0014] EP2792949 discloses a method for producing phosphorus-containing compounds from sewage sludge ash, which includes adding an acid such as HCl to leach out the ash, and a subsequent precipitation step to obtain and separate calcium phosphate.

[0015] EP2602013 also refers to a method for recovering phosphorus compounds from sewage sludge ash, which comprises first acid decomposition or leaching of the raw material using dilute mineral acid, and then adding an aluminum salt to precipitate and separate the phosphorus compounds.

[0016] In these cases, the resulting phosphorus-containing products are of low grade, and the majority of them are used in agriculture as phosphate fertilizers.

[0017] While the majority of global demand for phosphates is for fertilizer production, other specialized applications exist that require technical-grade phosphate (85% concentration), which attract significantly higher profits than the commercial-grade phosphate (50% concentration) commonly used in the aforementioned fertilizer production.

[0018] In practice, the advantage derived from the acid leaching process is the potential to produce phosphoric acid products that can be matched to niche products with potentially higher market prices.

[0019] US2016 / 0312333 refers to a method for treating ash obtained from sludge incineration, the method comprising a hot immersion of the ash with a liqueur attack containing phosphate ions in a solution, thereby obtaining a first liquid phase containing phosphate ions separated from a solid phase containing several impurities. The first liquid phase is subjected to a purification step by means of liquid-liquid extraction using an organic solvent or by application to an ion exchange resin, resulting in a second liquid phase having phosphate ions but with low content of other metals. This second liquid phase can be further subjected to a re-extraction step using an aqueous re-extracting agent, thereby obtaining an aqueous phase containing phosphate ions separated from the organic phase. The liquid phase obtained after this purification step is a purified phosphoric acid solution containing a low proportion of metal ions.

[0020] Donatello et al. (Waste Management, 2010, 30, 1634-1642) describe recovering phosphorus from sewage sludge ash to obtain technical-grade phosphoric acid by using a sulfuric acid washing procedure. The effects of reaction time, sulfuric acid concentration, liquid-to-solid ratio, and the origin of the sewage sludge ash on phosphorus recovery are investigated. This method involves a leaching step using sulfuric acid, subsequent filtration, and further purification and concentration of the resulting liquid phase. The purification step is carried out by using a cation exchange resin to remove or at least reduce the content of metal cations, while the concentration of the purified filtrate is carried out by evaporating excess water.

[0021] JP07251141 describes the recovery of phosphorus components from incinerated sewage sludge ash without any prior treatment by using an acidic solution to elute phosphorus components from the ash, separating the eluted material and insoluble residue, and extracting the phosphorus components using an organic solvent to form a two-phase system of eluted material and water.

[0022] Despite the procedures described in the prior art for producing refined phosphoric acid, there remains a need to optimize the experimental parameters and operating steps that enable the extraction of phosphates from sewage sludge ash in a very efficient and selective manner to obtain technical grade phosphoric acid.

Summary of the Invention

[0023] The inventors have developed a new method for treating incinerated sewage sludge that enables the production of technical grade phosphoric acid and allows the separation of other metals contained in the ash, such as Fe, by standard procedures known in the art for use in different applications.

[0024] The present invention is based on the development of a method that enables the extraction of phosphoric acid in a very efficient manner at a high purity level. This is essentially a closed-loop process that enables the almost quantitative recovery of phosphorus contained in the sewage sludge material in the form of phosphoric acid.

[0025] In particular, by the method of the present invention, at least 80% of the phosphorus present in the ash is leached and recovered. Furthermore, the method provides phosphoric acid that meets the criteria required to be considered technical grade, with a minimum level of impurities.

[0026] Thus, a main aspect of the present invention is a method for recovering phosphoric acid from a solid phosphorus-containing raw material, comprising: a) a step of leaching the phosphorus-containing solid raw material using an aqueous sulfuric acid solution having a pH of 1 to 2 and at a temperature of 40°C or lower, wherein: i) a first leaching solution containing a solubilized main fraction of the phosphorus contained in the solid raw material in the form of phosphoric acid and metal phosphates, and ii) the remaining fraction of the phosphorus contained in the solid raw material, as well as a non-leached solid residue containing most of the iron, calcium, and aluminum contained in the solid raw material are obtained; b) A conversion step, in which sulfuric acid is added to the first leaching solution obtained in step a) so that the metal phosphate salt is converted to phosphoric acid, thereby obtaining a second leaching solution containing metal impurities in the form of phosphoric acid and sulfate; c) A first solvent extraction of phosphoric acid from the second leachate obtained in step b) using an organic solvent, thereby obtaining a first phosphoric acid-containing organic liquid phase, as well as a first aqueous raffinate containing phosphoric acid and impurities; d) The first phosphoric acid-retaining organic liquid phase obtained in step c) is washed with a diluted aqueous phosphoric acid solution, thereby obtaining a second phosphoric acid-retaining organic liquid phase and a second aqueous raffinate containing sulfuric acid and metal impurities; e) A first stripping of phosphoric acid from the second phosphoric acid-retaining organic liquid phase obtained in step d) using water at a temperature of 50-70°C, thereby obtaining a first aqueous solution containing phosphoric acid; f) The first aqueous raffinate obtained in step c) - A step of adding sulfuric acid to the first aqueous raffinate to form a mixture; - Phosphoric acid is extracted from the mixture using an organic solvent, and in this way, ○ A third phosphoric acid-containing organic liquid phase also containing sulfuric acid, and ○ A second aqueous raffinate containing impurities and unextracted sulfuric acid The process of obtaining The process involves subjecting the sample to a second solvent extraction containing; g) A second stripping of phosphoric acid from the third phosphoric acid-retaining organic liquid phase obtained in step f) using water at a temperature of 50-70°C, wherein a second aqueous solution containing phosphoric acid and sulfuric acid is obtained, wherein the second aqueous solution reaches at least partially the leaching step a). This includes mentioning methods. [Brief explanation of the drawing]

[0027] [Figure 1] Figure 1 shows a flowchart illustrating the steps of the method of the present invention. [Figure 2] Figure 2 shows a flowchart of the additional steps included in the method of the present invention. [Modes for carrying out the invention]

[0028] The method of the present invention is applicable to the processing of phosphate or phosphorus-containing raw materials, also referred to as solid residues. In the gist of the present invention, the term "phosphate or phosphorus-containing raw material" refers to a solid residue containing at least one phosphate as defined herein.

[0029] In certain embodiments, the phosphate or phosphorus-containing raw material is ash derived from the incineration of organic materials, such as sewage sludge and biological waste. This ash is an oxide of various metals, such as Al2O3, Fe2O3, MgO, P2O5, P4O 10 It is mainly composed of K2O, SiO2, and other components such as Ca9Al(PO4)7, Ca2Si, and Al2(SiO4)O.

[0030] More preferably, the ash content originates from the incineration of sewage sludge, and is therefore referred to as sewage sludge ash.

[0031] In the context of the present invention, the term "sewage sludge" refers to any suspension of finely dispersed particles of a solid substance in a liquid. In a preferred embodiment, the liquid in which the particles are suspended is wastewater as defined herein.

[0032] The term "wastewater" refers to all aqueous and / or organic liquids, or mixtures thereof, which do not possess the quality of drinking water within the meaning of drinking water quality standards.

[0033] In certain embodiments, sewage sludge exists as primary sludge, raw sludge, excess sludge, and treated and / or stabilized sewage sludge (aerobic / anaerobic).

[0034] The term "biological waste" refers to all organic waste of animal or plant origin generated in homes or factories that can be broken down by microorganisms, soil organisms, or enzymes.

[0035] The term "phosphate" refers to P2O5 and P4O 10 This relates to... Furthermore, the term "phosphate" refers to salts and esters of orthophosphate (H3PO4), and explicitly includes condensates (polymers) of orthophosphate and their esters. In particular, the term "phosphate" refers to the general formula X(Y) m (PO4) n (wherein X, and optionally Y, are metals selected from the group consisting of aluminum, beryllium, bismuth, lead, cadmium, chromium, iron, gallium, indium, potassium, cobalt, copper, magnesium, manganese, molybdenum, sodium, nickel, osmium, palladium, rhodium, ruthenium, strontium, titanium, vanadium, tungsten, zinc, and tin) refers to a metal salt of phosphoric acid.

[0036] Preferably, the ash processed by this method has a phosphorus content expressed as a weight percentage of phosphates in the ash, at least 5%, ideally at least 7%, and preferably 7-20%.

[0037] The phosphorus-containing raw material may be finely ground to a suitable particle size with a maximum size of 2 mm before being used as a feed in the method of the present invention.

[0038] Different steps of the method of the present invention are described in detail below in this specification.

[0039] Leaching process (step a) The first step of the method of the present invention consists of leaching a phosphorus-containing solid raw material using an acidic aqueous solution, i.e., an aqueous sulfuric acid solution having a pH of 1 to 2. The supply of the aqueous sulfuric acid solution to the leaching is controlled to maintain the resulting acidic aqueous solution at a pH of 1 to 1.5, more preferably at a pH of 1.

[0040] Unlike other strong acids, such as chloride acids, or especially phosphoric acid, the use of sulfuric acid is more selective because it can dissolve most of the phosphorus contained in phosphorus-containing solid raw materials, while significantly reducing the dissolution of metallic impurities such as iron, aluminum, and calcium.

[0041] Under the conditions described above, the main fraction of phosphorus contained in the solid raw material is gradually leached out, mainly in the form of phosphoric acid, and in much less the form of metal phosphate salts such as iron, aluminum, calcium, and magnesium. As a result, an aqueous phase is obtained containing the main fraction of phosphorus in the form of phosphoric acid and metal phosphate salts, while the remaining fraction of phosphorus from the solid raw material remains undissolved as a non-leached solid residue, along with most of the metal impurities contained in the solid raw material.

[0042] In certain embodiments, the phosphorus-containing raw material is leached in an aqueous sulfuric acid solution at a temperature of 40°C or lower, more preferably 20-40°C, even more preferably 30-40°C, and most preferably 40°C.

[0043] As a result of these conditions, the leaching process preferentially dissolves phosphorus over other metal components. In this process, 80-90 wt% of the phosphorus contained in the raw material is dissolved in the leaching acid solution.

[0044] The leaching step using an aqueous sulfuric acid solution is preferably carried out with a residence time of 10 minutes to 1 hour, more preferably 10 to 50 minutes.

[0045] The sulfuric acid aqueous solution used in the leaching process is preferably an aqueous medium containing sulfuric acid by weight, and may be combined with other acids such as phosphoric acid.

[0046] Advantageously, this sulfuric acid aqueous solution consists at least in part of a recycled solution obtained from a second stripping step of phosphoric acid by step g) of the method of the present invention, i.e., a second aqueous solution obtained from step g) containing sulfuric acid and a lower amount of phosphoric acid as detailed below.

[0047] Therefore, after the leaching process, a phosphorus-rich solution (also called the first leaching solution) is obtained, mainly containing solubilized phosphoric acid, as well as metal phosphate salts of iron, aluminum, calcium, and magnesium, and metal sulfate salts obtained from the reaction of sulfuric acid with metal oxides present in the solid raw material. In addition, a non-leached solid residue is also obtained, which further contains trace fractions of phosphorus contained in the solid raw material, mainly in the form of sulfates, combined with most of the metal impurities.

[0048] Therefore, a solid-liquid separation treatment may be performed to separate the first leachate containing phosphoric acid along with other impurities from the non-leached solid residue.

[0049] After separating the undissolved residue from the leachate, the first leachate containing phosphoric acid is subjected to conversion step b), while the non-leached solid residue is removed from the system and can be used for other industries such as the cement industry.

[0050] In certain embodiments, a first leachate containing phosphoric acid may be pre-supplied to the sulfidation region to remove any traces of Cu that may be present in the leachate in the form of CuSO4. To do this, Na2S is added to the leachate under controlled redox conditions, where the following reaction takes place. CuSO 4(l) +Na2S (l) →CuS (s) +Na2SO 4(l)

[0051] In a preferred embodiment, the amount of Na2S used in the sulfidation step is in the range of 2 to 3 g per 1 kg of solid residue, and more preferably, the amount is 2.5 g Na2S / kg solid residue. Under these conditions, the oxidation-reduction potential reaches 50 to 300 mV, preferably 250 to 270 mV (Ag / AgCl).

[0052] This sulfidation process can be carried out at room temperature to a maximum of 50°C for a residence time of 5 to 50 minutes, preferably 7 to 20 minutes.

[0053] As a result, a solid containing Cu in the form of CuS is obtained with a precipitation efficiency of over 85%. The CuS is removed from the system, but the liquid phase containing phosphoric acid is supplied to the conversion step.

[0054] Conversion process (process b) As described above, the first leaching solution obtained in step a) contains phosphoric acid and phosphorus in the form of metal phosphate salts. Therefore, it is desirable to convert the metal phosphate salts to phosphoric acid in order to increase the concentration of phosphorus in the form of phosphoric acid.

[0055] In step b), sulfuric acid is added to the first leaching solution obtained in step a), thereby obtaining a concentrated solution containing all the leached phosphorus from the phosphorus-containing raw material in the form of phosphoric acid, along with some metal impurities in the form of sulfates.

[0056] In certain embodiments, the sulfuric acid used in this process is 98% sulfuric acid.

[0057] Advantageously, the sulfuric acid added in this process is at least partially, (i) a recycled aqueous solution obtained from the second stripping step of phosphoric acid by step g) of the method of the present invention, i.e., the second aqueous solution obtained from step g), and / or (ii) A recycled aqueous solution obtained from the washing step of the first organic extraction phase by step d) of the method of the present invention, i.e., an acidic aqueous solution obtained from step d). This corresponds to a sulfuric acid solution consisting of [the specified components].

[0058] In another specific embodiment, sulfuric acid is used in various doses of 20 to 50 g, preferably 30 to 40 g, per liter of leaching solution, under conditions that the resulting solution has a pH of 1.5 to 1.

[0059] Advantageously, step b) of the method of the present invention may be carried out at a temperature of 40°C or lower, preferably 30 to 40°C.

[0060] In certain embodiments, step b) is performed with a residence time of at least 10 minutes, preferably 10 minutes to 1 hour, and more preferably 15 minutes.

[0061] As previously mentioned, this process allows for the conversion of phosphorus, which exists in the form of metal phosphate salts, to phosphate, while metal impurities are in the form of sulfates, mainly iron, aluminum, and magnesium sulfates.

[0062] Therefore, this step provides a high concentration of phosphoric acid in the resulting concentrated solution. In particular, it provides an increase in the concentration of phosphoric acid of up to 30% compared to the phosphoric acid contained in the first leaching solution obtained from step a).

[0063] Therefore, as a result of step b), a H3PO4-rich leachate, also called a second leachate, is obtained which is supplied to step c) for solvent extraction.

[0064] In certain embodiments, the H3PO4-rich leachate may be subjected to an evaporation step to concentrate the phosphoric acid stream before proceeding to extraction step c). To do this, the H3PO4-rich leachate is subjected to a heating process to evaporate the water, thus obtaining not only a liquid stream with a higher concentration of phosphoric acid, but also a solid residue consisting mainly of CaSO4 that can be removed from the liquid stream.

[0065] Preferably, the evaporation process is carried out so that the evaporation ratio is at most 5 / 1v / v inlet / outlet, in other words, the evaporation increases the phosphoric acid concentration by up to 5 times, more preferably up to 4 times.

[0066] The heating process may be carried out at a temperature lower than the boiling point of phosphoric acid, preferably lower than 120°C but higher than 100°C.

[0067] Solvent extraction (step c) The H3PO4-rich leaching solution obtained in step b) is then subjected to a liquid-liquid extraction step (step c)) either as is or after evaporation to separate the metal impurities present in the solution, more specifically Al, Mg, and Fe, from the phosphoric acid, thus contributing to obtaining high-purity phosphoric acid.

[0068] This step involves extracting phosphoric acid by contacting the H3PO4-rich leaching solution obtained in step b) with an organic solvent, preferably under countercurrent conditions. Thus, an organic extract phase containing phosphoric acid and an aqueous phase containing unextracted phosphoric acid and impurities are obtained. The organic solvent captures the phosphoric acid molecules through a solvation process.

[0069] The organic solvent is preferably selected from methyl isobutyl ketone (MIBK), tributyl phosphate (TBP), 1-heptanol, diisopropyl ether (DIPE), and mixtures thereof, for example, a mixture of MIBK and TBP, a mixture of MIBK and 1-heptanol, or a mixture of TBP and 1-heptanol, and more preferably the organic solvent is TBP. The organic solvent is used without being dissolved in any other organic compound.

[0070] Preferably, this extraction step is carried out at room temperature, preferably 20-30°C, more preferably 25°C, with a residence time in the range of 5-10 minutes. This extraction step can be carried out several times, for example, two or three times, four or five times.

[0071] During this process, phosphoric acid is selectively and gradually retained in the organic liquid phase. Thus, after the extraction process, a phosphorus-retaining organic phase is formed, which also contains very small amounts of impurities, mainly sulfuric acid and some metals such as arsenic. In addition, an acidic aqueous extraction phase, also called aqueous raffinate, is obtained, which contains most of the metal impurities that were previously present in the H3PO4-rich leaching solution, and this aqueous raffinate is then supplied to the depletion step of the method of the present invention, which is described below.

[0072] Cleaning process (process d) As mentioned above, and since sulfuric acid is used in the leaching process as it is in the conversion process, some trace amounts of the aforementioned acid are extracted into the organic phase along with phosphoric acid, and it is prudent to remove the phosphoric acid from them in order to avoid such impurities being present in the phosphoric acid.

[0073] Accordingly, the method of the present invention further comprises a washing step, which includes washing the organic extract phase obtained from step c) with a diluted aqueous solution of an acid such as phosphoric acid, thereby providing a washed organic phase containing phosphoric acid, as well as an aqueous phase containing sulfuric acid and other trace amounts of co-extracted metal cation impurities, such as zinc and iron, if any. The aqueous phase containing the extracted sulfuric acid can also be recycled to a conversion step for the purpose of reducing the incorporation of such acid.

[0074] In certain embodiments, the concentration of phosphoric acid in the aqueous solution is approximately 120 g / L.

[0075] The washing process may be carried out at a temperature of 20 to 60°C, preferably 40 to 60°C, with a residence time of 5 to 15 minutes.

[0076] Stripping process (process e) The phosphorus-retaining organic phase obtained after step d) is then subjected to a stripping step (step e) in the presence of water to recover the phosphoric acid.

[0077] Preferably, this stripping step is carried out at a temperature below 70 °C, preferably between 40 and 65 °C, with a residence time in the range of 5 to 15 minutes.

[0078] As a result of this step, a purified aqueous solution of phosphoric acid having a phosphoric acid concentration in the range of 20 - 30% is obtained, and an organic stream containing an organic extract that can be recycled to the extraction step c) is also obtained.

[0079] Thereafter, the purified aqueous solution of phosphoric acid is subjected to evaporation concentration, and thus, a concentration of at least 75% required for technical grade phosphoric acid can be reached.

[0080] In certain embodiments, a portion of the purified aqueous solution may be recycled for use in the washing step d).

[0081] The washing step d) described above enables efficient removal of sulfuric acid co-extracted into the organic phase, but a very small amount may still remain in the organic solvent (ppm), which then appears and is concentrated in the final product.

[0082] Therefore, in a preferred embodiment and to remove residual sulfuric acid, the method of the present invention adds BaCO3 to the aqueous solution of phosphoric acid obtained from step e) or to the concentrated aqueous solution of phosphoric acid, and thus, by the following reaction, SO4 ions are precipitated in the form of BaSO4, and a precipitation step is also included. 2- including a precipitation step that includes precipitating ions. BaCO 3(s) +H2SO 4(l) →BaSO 4(s) +CO 2(g) +H2O (l)

[0083] In certain embodiments, this precipitation step is carried out at a temperature of 60 - 90 °C, preferably 70 - 85 °C, with a residence time in the range of 5 to 15 minutes.

[0084] If the concentration / evaporation of the purified aqueous solution of phosphoric acid obtained from step e) has not been previously achieved, the resulting aqueous solution obtained after the removal of sulfuric acid by precipitation in the form of BaSO4 is then subjected to evaporation to reach a concentration of at least 75%, which is required for technical grade phosphoric acid.

[0085] In another specific embodiment, the method of the present invention further comprises the step of removing As cations from the purified aqueous solution of phosphoric acid obtained after the stripping step e), or from the purified aqueous solution of phosphoric acid and / or concentrated aqueous solution obtained after the removal of sulfuric acid. This step comprises adding Na2S or H2S to the aqueous solution, thereby obtaining a concentrated solution of phosphoric acid free of As cation impurities, and a solid containing precipitated As in the form of As2S3 by the following reaction. 2H3AsO 3(l) +3Na2S (l) →As2S 3(s) +6NaOH (l)

[0086] The precipitated solid containing As can be removed by filtration. This process achieves a precipitation effectiveness of nearly 99% for As.

[0087] In certain embodiments, this precipitation step of As may be carried out at a temperature of 20–40°C, preferably 25–35°C, with a residence time ranging from 5–15 minutes.

[0088] On the other hand, in order to recover the phosphorus contained in the first aqueous raffinate obtained from step c) solvent extraction, and thus to increase the overall phosphorus recovery efficiency, the method of the present invention further includes a depletion step comprising two main steps detailed below herein: a second solvent extraction step and a second stripping step.

[0089] Second solvent extraction (step f) In this step, concentrated sulfuric acid is first added to the first aqueous raffinate obtained from step c), thus resulting in a mixture containing unextracted phosphoric acid and sulfuric acid. In certain embodiments, sulfuric acid is added so that the sulfuric acid concentration in the mixture reaches a maximum of 400 g / L.

[0090] The resulting mixture containing phosphoric acid and sulfuric acid is then subjected to a second solvent extraction step, which allows for further separation of phosphoric acid from metal impurities contained in the first aqueous raffinate.

[0091] As in the first solvent extraction step, this step preferably also includes the extraction of phosphoric acid along with sulfuric acid by contacting the mixture obtained above with an organic solvent under countercurrent conditions. The organic solvent captures the molecules of phosphoric acid and sulfuric acid through a solvation process. Thus, a third phosphoric acid-retaining organic liquid phase, also containing sulfuric acid, is obtained together with a second aqueous raffinate containing metal impurities and unextracted sulfuric acid.

[0092] The organic solvent used in this second solvent extraction step is also preferably selected from methyl isobutyl ketone (MIBK), tributyl phosphate (TBP), 1-heptanol, diisopropyl ether (DIPE), and mixtures thereof, for example, a mixture of MIBK and TBP, a mixture of MIBK and 1-heptanol, or a mixture of TBP and 1-heptanol, and more preferably the organic solvent is TBP. The organic solvent is used without being dissolved in any other organic compound.

[0093] Preferably, this second extraction step is carried out at room temperature, preferably 20-30°C, more preferably 25°C, with a residence time in the range of 5-15 minutes. This extraction step can be carried out several times, for example, two or three times.

[0094] During this process, phosphoric acid and sulfuric acid are selectively and gradually retained in the organic liquid phase. Thus, after the second extraction step, a third phosphorus-retaining organic phase is formed, which also contains sulfuric acid. In addition, an acidic aqueous extract phase, also called a second aqueous raffinate, is obtained, which contains most of the metal impurities derived from the H3PO4-rich leaching solution. The second aqueous raffinate can be used for wastewater treatment.

[0095] The wastewater treatment step is performed for the purpose of removing impurities present in the second aqueous raffinate and to satisfy the fluidity requirements. This wastewater treatment may be carried out by precipitation of impurities in the form of carbonates or hydroxides.

[0096] In certain embodiments, the second aqueous raffinate reacts with a precipitating agent such as calcium carbonate or calcium hydroxide, resulting in the formation of solid carbonates and hydroxides of metal impurities present in the aqueous raffinate.

[0097] Preferably, this wastewater treatment is carried out at room temperature, preferably 20-30°C, with a pH of 6-9.

[0098] Second stripping process (process g) The third phosphorus-retaining organic phase obtained after step f) is then subjected to a second stripping step (step g)) in the presence of water to recover phosphoric acid and sulfuric acid in their pure forms.

[0099] Preferably, this stripping process is carried out at a temperature of less than 70°C, preferably 40 to 65°C, with a residence time in the range of 5 to 15 minutes.

[0100] This extraction process can be carried out several times, for example, two or three times.

[0101] As a result of this process, purified aqueous solutions of phosphoric acid and sulfuric acid are obtained, as well as an organic stream containing organic extracts that can be recycled to the second extraction step f).

[0102] The resulting purified aqueous solutions of phosphoric acid and sulfuric acid are supplied to leaching step a), or alternatively, the resulting purified aqueous solutions of phosphoric acid and sulfuric acid are divided into two separate flows, one of which is supplied to leaching step a) and the other to conversion step b), thus providing an essentially closed-loop process that enables the recovery of phosphorus contained in the solid raw material in the form of phosphoric acid with near-quantitative accuracy.

[0103] Figure 1 shows a flowchart of the setup for producing phosphoric acid, which is applied to the process according to Example 1 below. [Examples]

[0104] Example 1: Production of technical grade phosphoric acid Solid sewage ash (SSA) was supplied to the leaching region (R1). The SSA was then leached with a 98% sulfuric acid solution at a rate of 300 g per kg of SSA, so that the final pH of the reaction would be 1. Leaching was carried out at a temperature of 40°C for a residence time of 20 minutes. The sulfuric acid solution consisted partly of an aqueous solution (A5) originating from the stripping region (RE2), which is detailed below.

[0105] Under these conditions, a phosphorus-rich leachate (L1) containing solubilized phosphoric acid, as well as trace amounts of Fe, Al, Ca, and other impurities in the form of phosphates and sulfates, was obtained together with a non-leached solid residue (S1) containing the remaining phosphorus that was not leached.

[0106] Table 1 shows the composition of the leaching solution (L1) and the non-leaching solid residue (S1) after the leaching process.

[0107] [Table 1]

[0108] As described above, most of the phosphorus contained in the solid sewage ash is transferred to the leachate in the form of phosphoric acid and phosphates, while most of the Fe and Ca, and a large amount of Al, remain in the solid residue, thus significantly reducing the amount of impurities in the leachate.

[0109] Subsequently, a solid-liquid separation treatment was performed to separate the leachate (L1) from the non-leachable solid residue (S1).

[0110] The resulting leachate (L1) was supplied to a conversion region (R2) to convert the phosphorus present in the form of metal phosphate salts into phosphoric acid. 35 g of 98% sulfuric acid per liter of leachate (L1) was added to the leachate (L1) at a temperature of 30-40°C for 15 minutes. An aqueous concentrate (L2) was obtained containing all of the leached phosphorus from the SSA in the form of phosphoric acid, along with several metal impurities in the form of sulfates.

[0111] The composition of the leachate (L2) at the outlet of the conversion region, relating to phosphoric acid, phosphates, sulfuric acid, and sulfates, mainly contained the components shown in Table 2 below.

[0112] [Table 2]

[0113] As shown, this process made it possible to provide leached phosphorus from the SSA, essentially in the form of phosphoric acid.

[0114] The resulting solution (L2) was then subjected to an evaporation process in the evaporation region (R3) to concentrate it before entering the extraction region (E1). To do this, the solution (L2) was heated to evaporate the water, thus obtaining a liquid stream (L3) with a higher concentration of phosphoric acid and a solid residue consisting mainly of CaSO4. The evaporation ratio was 4 / 1v / v (inlet / outlet) to concentrate the phosphoric acid from 90-110 g / L to approximately 400 g / L.

[0115] The concentrated solution (L3) obtained after this process had the following main components, as shown in Table 3.

[0116] [Table 3]

[0117] Once the phosphoric acid solution was concentrated to obtain a concentrated solution (L3), this was supplied to an extraction region (E1) subjected to countercurrent liquid-liquid extraction using tributyl phosphate (TBP) as an organic extractant, without the use of any carrier solvent. The volume ratio of the organic phase (O) to the aqueous phase (A) was 9 / 1. This step was carried out at 25°C for a residence time of 10 minutes. Thus, an organic phase (O1) containing phosphoric acid, an organic extractant, and small amounts of impurities, mainly sulfuric acid, and several metals such as Cu, Zn, and As was obtained together with an acidic aqueous raffinate (A1) containing most of the metal impurities present in the concentrated solution (L3) of trace amounts of unextracted phosphoric acid. The aqueous raffinate (A1) was supplied to a depletion region as detailed below herein.

[0118] The organic extractant (TBP) extracted not only phosphoric acid but also trace amounts of sulfuric acid and other metal impurities such as chloride, zinc, and iron. To remove these impurities and avoid their presence in the final product, the organic phase (O1) was then washed in a sulfuric acid washing area (W1) at a temperature of 50°C for a residence time of 10 minutes with an aqueous solution of phosphoric acid (the concentration of phosphoric acid in the aqueous solution was 120 g / L, and the volume ratio O / A was 10 / 1), resulting in an aqueous phase (A2) containing sulfuric acid and other co-extracted metal cation impurities along with the washed organic phase (O2) containing phosphoric acid. This aqueous phase (A2) was recycled to a conversion area (R2), while the washed organic phase (O2) was subjected to a stripping process in an H3PO4 stripping area (RE1).

[0119] This stripping process was carried out in the presence of water, at a temperature of 65°C, with a residence time of 10 minutes and a volume ratio of O / A of 8 / 1. As a result, not only was a purified aqueous solution of phosphoric acid (A3) obtained, but the organic phase (O3) was recycled to the extraction region (E1). A portion of the purified aqueous solution (A3') was supplied to the sulfur washing region (W1).

[0120] Table 4 shows the composition of the purified aqueous solution (A3) and the organic phase (O3) containing phosphoric acid.

[0121] [Table 4]

[0122] On the other hand, in order to recover the phosphorus contained in the aqueous raffinate (A1) obtained from the extraction region (E1), and therefore to increase the overall phosphorus recovery efficiency, the aqueous raffinate (A1) was supplied to a depleted region consisting of the extraction region (E2) and the stripping region (RE2).

[0123] First, concentrated sulfuric acid was added to aqueous raffinate (A1) containing unextracted phosphoric acid until a sulfuric acid concentration of 400 g / L was reached in the resulting mixture.

[0124] The mixture containing phosphoric acid and sulfuric acid was then subjected to a solvent extraction step in an extraction region (E2) under countercurrent conditions, without the use of any carrier solvent, using tributyl phosphate (TBP) as the organic extractant. The volume ratio of the organic phase (O) to the aqueous phase (A) was 4 / 1. This step was carried out at 25°C for a residence time of 10 minutes to further separate phosphoric acid from metal impurities contained in the aqueous raffinate (A1). As a result, an organic phase (O4) mainly containing phosphoric acid, sulfuric acid, and the organic extractant was obtained together with aqueous raffinate (A4) containing metal impurities and unextracted sulfuric acid. The aqueous raffinate (A4) was subjected to wastewater treatment.

[0125] Next, the organic phase (O4) was subjected to a stripping process in the stripping region (RE2) to recover the phosphoric acid and sulfuric acid in their pure forms.

[0126] This stripping process was carried out in the presence of water, at a temperature of 65°C, with a residence time of 10 minutes and a volume ratio of O / A of 4 / 1. As a result, not only were purified aqueous solutions of phosphoric acid and sulfuric acid (A5) obtained, but the organic phase (O5) was recycled into the extraction region (E2), the components of which are shown in Table 4 below.

[0127] [Table 5]

[0128] The resulting purified aqueous solutions of phosphoric acid and sulfuric acid (A5) were then supplied to a partial leaching region (R1), and a trace amount (A5') was supplied to a conversion region (R2), thereby recovering any phosphorus not extracted in the extraction region (E1) and returning it to the process. In this way, the yield of the process was increased, and therefore the amount of phosphorus recovered in the form of phosphoric acid from what was initially present in the solid raw material.

[0129] Therefore, as can be derived from this embodiment, the method of the present invention makes it possible to extract phosphoric acid in a highly efficient manner with a high level of purity that further meets the criteria required to be considered technical grade.

[0130] Example 2: Production of technical grade phosphoric acid Example 1 was repeated, but with the introduction of one or more additional steps detailed below. These steps provide technical-grade phosphoric acid with minimal impurities.

[0131] Cu removal The leached solution (L1) obtained after the leaching process contained 120-150 mg / L of Cu in the form of CuSO4. The leached solution (L1) was supplied to the sulfidation region (R2) with the aim of selectively precipitating the Cu present in the leached solution (L1) and thus reducing the contamination of this element in the final product. To remove it, 2.5 g of Na2S per 1 kg of SSA was added to the leached solution (L1), thus achieving a redox potential of 225-270 mV (Ag / AcCl), a temperature of 40°C, and a residence time of 10 minutes. As a result, a solid containing Cu in the form of removed CuS was obtained, and the Cu-free liquid phase (L1') containing phosphoric acid was supplied to the conversion region (R2).

[0132] In particular, Cu was precipitated in the form of CuS with 86% effectiveness, thus providing a Cu content of less than 30 mg / L in the final purified aqueous solution.

[0133] Sulfur removal The aqueous phosphoric acid solution (A3) contained 50-90 mg of sulfuric acid / L as an impurity present in the organic phase (O2) that was subsequently washed, as the impurity could not be removed in the washing process. Therefore, the purpose of this process was to remove the aforementioned sulfuric acid impurity. A stoichiometric amount of BaCO3 was added to the aqueous solution (A3) to precipitate the residual sulfuric acid contained therein, and in this way, SO4 was precipitated in the form of BaSO4 with a precipitation effectiveness of 75%. 2- The ions were precipitated. The reaction was carried out at a temperature of 80°C and a residence time of 20 minutes. As a result, a sulfuric acid-free aqueous solution of phosphoric acid was obtained, along with the BaSO4 solid residue that was separated and removed (A6).

[0134] Concentration of phosphoric acid The aqueous solution (A6) obtained from the sulfate removal process was subjected to a heating process to evaporate the water from the solution, yielding a concentrated phosphoric acid solution (A7) with a concentration of 75%, thus meeting the requirements for a technical grade phosphoric acid concentration. Specifically, the phosphoric acid content in the concentrated solution (A7) was 1170 g / L.

[0135] As removal To precipitate the As cations contained in the aqueous solution (A7) obtained from the concentration of phosphoric acid, 100 times the stoichiometric amount of Na2S (or H2S as an alternative) was added to the aqueous solution (A7) at a temperature of 30°C and a residence time of 10 minutes. As a result, a purified concentrated solution of phosphoric acid satisfying the requirements for being considered technical grade phosphoric acid is obtained, along with a solid residue containing precipitated As in the form of As2S3, which is removed by filtration.

[0136] Figure 2 shows a flowchart illustrating these additional steps included in the method of the present invention.

Claims

1. A method for recovering phosphoric acid from a solid phosphorus-containing raw material, a) A step of leaching a phosphorus-containing solid raw material using an aqueous sulfuric acid solution with a pH of 1 to 2 and at a temperature of 40°C or lower, a. 1) A first leachate containing a solubilized main fraction of phosphorus contained in the solid raw material in the form of phosphoric acid and metal phosphate salts, and a. 2) Non-leached solid residue containing the remaining fraction of phosphorus contained in the solid raw material To obtain, the process; b) Add sulfuric acid to the first leaching solution obtained in step a) to convert the metal phosphate salt into phosphoric acid, thereby obtaining a second leaching solution containing metal impurities in the form of phosphoric acid and sulfates; c) Using an organic solvent, extract phosphoric acid from the second leachate obtained in step b) to obtain a first aqueous raffinate containing a first phosphoric acid-retaining organic liquid phase, as well as unextracted phosphoric acid and impurities; d) Washing the first phosphoric acid-retaining organic liquid phase obtained in step c) with a diluted aqueous phosphoric acid solution to obtain a second phosphoric acid-retaining organic liquid phase and a second aqueous raffinate containing sulfuric acid and metal impurities; e) Using water at a temperature of 50 to 70°C, the phosphoric acid is (first) stripped from the second phosphoric acid-retaining organic liquid phase obtained in step d), thereby obtaining a first aqueous solution containing phosphoric acid and an organic phase containing the organic solvent; f) A step of subjecting the first aqueous raffinate obtained in step c) to a second solvent extraction, - A step of adding sulfuric acid to the first aqueous raffinate to form a mixture; - Using an organic solvent, phosphoric acid is extracted from the mixture, thereby, ○ A third phosphoric acid-containing organic liquid phase also containing sulfuric acid, and ○ A second aqueous raffinate containing impurities and unextracted sulfuric acid To obtain, the process; g) Using water at a temperature of 50 to 70°C, the phosphoric acid is (second) stripped from the third phosphoric acid-retaining organic liquid phase obtained in step f), thereby obtaining a second aqueous solution containing phosphoric acid and sulfuric acid, and an organic phase containing the organic solvent, wherein the second aqueous solution is at least partially subjected to step a) and Methods that include...

2. The method according to claim 1, wherein the solid phosphorus-containing raw material is sewage sludge ash having a phosphorus content of at least 5%, where the % is expressed as the weight percentage of phosphate in the ash.

3. The method according to claim 1 or 2, wherein the addition of sulfuric acid in step a) is controlled to maintain the resulting leached solution at pH 1.

4. The sulfuric acid aqueous solution in step a) consists at least partially of the second aqueous solution obtained from step g), or alternatively, at least partially of The second aqueous solution obtained from step g), and Acidic aqueous solution obtained from step d) The method according to any one of claims 1 to 3.

5. In step a), the leaching solution obtained is added to Na ２ The method according to any one of claims 1 to 4, further comprising the step of adding S and precipitating the copper contained therein in the form of CuS.

6. The method according to any one of claims 1 to 5, wherein the sulfuric acid in step b) is used in an amount ranging from 20 to 50 g per liter of leaching solution.

7. The method according to any one of claims 1 to 6, further comprising the step of subjecting the second leaching solution obtained in step b) to an evaporation step to obtain a liquid stream having a higher concentration of phosphoric acid before proceeding to step c).

8. The method according to any one of claims 1 to 7, wherein the organic solvent used in step c) and step f) is tributyl phosphate.

9. The method according to any one of claims 1 to 8, wherein the organic phase obtained from step e) is recycled to step c).

10. The method according to any one of claims 1 to 9, further comprising the step of subjecting the first aqueous solution containing the phosphoric acid obtained in step e) to an evaporation process to obtain technical grade phosphoric acid.

11. The first aqueous solution of phosphoric acid obtained from step e) is dissolved in BaCO2. ３ Add BaSO ４ SO in form ４ ２－ The method according to any one of claims 1 to 10, further comprising the step of precipitating ions.

12. The method according to any one of claims 1 to 11, further comprising the step of subjecting the first aqueous solution containing phosphoric acid obtained from step e) or from the method of claim 11 to an evaporation process to reach a concentration of at least 75% phosphoric acid.

13. The first aqueous solution of phosphoric acid obtained from step e), or the aqueous solution of phosphoric acid obtained from the method of claim 11, or the concentrated solution of phosphoric acid obtained from the method of claim 12, are mixed with Na ２ S is added, and a concentrated solution of phosphoric acid free of As cation impurities and As ２ S ３ The method according to any one of claims 1 to 12, further comprising the step of obtaining a solid containing As precipitated in the form of As.

14. The method according to any one of claims 1 to 13, wherein the organic phase obtained from step g) is recycled to step f).

15. The method according to any one of claims 1 to 14, wherein the second aqueous raffinate obtained from step d) is recycled to step b).

Images