Treatment of phosphate solutions

Abstract

A method for producing pure phosphate includes the step (S10) of supplying a strip solution, which is an aqueous solution of one of monoammonium phosphate and monopotassium phosphate, on which stripped phosphate is carried from a feed solution containing phosphoric acid contaminated with heavy metals and iron. A base is added to the separated strip solution (S20) to raise the pH to 2-5.5 and form a heated solution to precipitate compounds containing iron and phosphate. A precipitating agent is added to precipitate heavy metals (S30). The precipitated compounds containing iron and phosphate and the precipitated heavy metal compounds are separated (S40). The heated filtration solution is then cooled to a cooled solution (S50) to precipitate the phosphate compound. The precipitated phosphate compound is removed from the cooled solution (S60). The cooled solution is recycled for use as the input strip solution in the stripping operation (S70). A system for producing pure phosphate is also disclosed.

Description

Technical Field

[0001] The present technology generally relates to the treatment of phosphate solutions, and more particularly, to a method and system for producing pure phosphate from a feed solution containing phosphoric acid contaminated with heavy metals and iron.

Background Art

[0002] Liquid-liquid extraction is a very effective method for purifying phosphoric acid that has been conventionally produced using sulfuric acid from impure raw materials such as phosphate ore and sewage sludge ash. Phosphate can be extracted from such leachates. The reason for using this method is that many extractants are selective for phosphoric acid and do not extract large amounts of metal contaminants such as Cu, Cd, Fe, Zn, etc. However, generally, As and F may be co-extracted with P and must be treated in subsequent processes.

[0003] However, the extraction of phosphoric acid from water is inefficient due to unfavourable distribution equilibria. Therefore, many extraction stages are required to achieve a reasonable yield, resulting in high capital and operating costs. An easy way to improve this efficiency is to add chloride to the phosphoric acid solution to change the distribution equilibrium to a more favourable one.

[0004] By replacing the dissolution of phosphorus from sulfuric acid with hydrochloric acid, the Ca content contained in phosphate ore and sewage sludge ash forms soluble CaCl2 instead of insoluble gypsum, resulting in a phosphoric acid solution with a high chloride concentration and improved extraction.

[0005] International patent applications published in International Publication 2022 / 173349 and International Publication 2022 / 115021 disclose methods for recovering phosphate from sewage sludge ash and apatite by hydrochloric acid leaching, respectively. These methods result in relatively low concentrations of supplied phosphoric acid compared to commercial methods for purifying phosphoric acid. Despite high chloride background, this results in relatively dilute strip solutions (less than 3.5 M), which are very costly to concentrate to commercial concentrations using evaporation.

[0006] International patent applications published in International Publication No. 2013 / 191639 and International Publication No. 2021 / 251891 describe the production of ammonium phosphate and potassium phosphate by liquid-liquid extraction of a phosphorus-containing feed solution, respectively. Here, phosphoric acid is extracted using a concentrated solution of MAP or MKP, and the solution is supersaturated by reacting it with ammonia or KOH to precipitate solid MAP or MKP. This can be recovered by filtration instead of evaporation.

[0007] However, the addition of chlorides has a major drawback: it reduces the selectivity of phosphate extraction, leading to large amounts of problematic impurities such as As, Cd, Cr, Cu, Fe, Ni, and Zn being co-extracted. The permissible limits for these impurities are restricted by both fertilizer regulations and industry standards, and some of them form insoluble phosphates, reducing the solubility of the final product below the level desired for high-grade MAP or MKP products. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International Publication No. 2022 / 173349 [Patent Document 2] International Publication No. 2022 / 115021 [Patent Document 3] International Publication No. 2013 / 191639 [Patent Document 4] International Publication No. 2021 / 251891 [Overview of the project] [Problems that the invention aims to solve]

[0009] Therefore, the general objective is to find a method for producing phosphate compounds with minimal contamination from iron and heavy metals.

[0010] The above objectives are achieved by the methods and apparatus according to the independent claims. Preferred embodiments are defined in the dependent claims. [Means for solving the problem]

[0011] In general, in a first embodiment, a method for producing pure phosphate includes the step of providing a stripped phosphate-loaded strip solution. The stripped phosphate-loaded strip solution is formed by liquid-liquid extraction from a feed solution and a loading strip solution containing phosphate contaminated with heavy metals and iron. The loading strip solution is an aqueous solution of one of monoammonium phosphate and monopotassium phosphate. A base is added to the stripped phosphate-loaded strip solution. Thereafter, the pH of the stripped phosphate-loaded strip solution is raised to a range of 2 to 5.5, preferably 3 to 5, most preferably 4 to 5. If the loading strip solution is an aqueous solution of monoammonium phosphate, the base contains ammonia and / or an ammonium salt. If the loading strip solution is an aqueous solution of monopotassium phosphate, the base contains a potassium salt instead. The addition of the base is exothermic, and a heated solution is formed from the separated stripped phosphate-loaded strip solution. The addition of the base causes compounds containing iron and phosphate to precipitate from the heated solution, thereby obtaining an iron-deficient heated solution. A precipitating agent is added to the heated solution to precipitate heavy metal compounds, thereby forming a heavy metal-deficient heated solution. The precipitated compounds containing iron and phosphate, along with the precipitated heavy metal compounds, are separated from the heavy metal-deficient heated solution to form a heated filtration solution. The heated filtration solution is then cooled to form a cooled solution. This cooling causes the phosphate compounds to precipitate. If the input strip solution is an aqueous solution of monoammonium phosphate, the phosphate compounds will contain monoammonium phosphate. If the input strip solution is an aqueous solution of monopotassium phosphate, the phosphate compounds will contain monopotassium phosphate. The precipitated phosphate compounds are removed from the cooled solution. After the step of removing the precipitated phosphate compounds, the cooled solution is recycled and used as the input strip solution for liquid-liquid extraction.

[0012] In a second embodiment, a system for producing pure phosphate includes a liquid-liquid extraction setup, a contaminant precipitation reactor, and a refrigerated precipitation setup. The liquid-liquid extraction setup is operated by the use of a recirculating solvent. The liquid-liquid extraction setup has a first input for the feed solution, a second input for the input strip solution, a first output for the strip solution containing the stripped phosphate, and a second output for the phosphorus-deficient feed solution. The feed solution contains phosphoric acid contaminated with heavy metals and iron. The strip solution is an aqueous solution of one of monoammonium phosphate and monopotassium phosphate. The contaminant precipitation reactor has a first input connected to the first output of the liquid-liquid extraction setup for receiving the strip solution containing the stripped phosphate, and a second input for receiving a base. If the input strip solution is an aqueous solution of monoammonium phosphate, the base contains ammonia or an ammonium salt. If the input strip solution is an aqueous solution of monopotassium phosphate, the base contains a potassium salt. The contaminant precipitation reactor is configured to add a base to a separation strip solution containing stripped phosphate, and the pH of the strip solution is set to a range of 2 to 5.5, preferably 3 to 5, and most preferably 4 to 5. Adding the base to the strip solution containing the stripped phosphate causes an exothermic reaction, forming a heated solution from the strip solution containing the stripped phosphate. Further addition of the base to the strip solution containing the stripped phosphate causes compounds containing iron and phosphate to precipitate from the heated solution, yielding an iron-deficient heated solution. The contaminant precipitation reactor has a third inlet for receiving a precipitant to precipitate heavy metal compounds. The contaminant precipitation reactor is configured to add the precipitant to the iron-deficient heated solution after the addition of the base, thereby forming a heavy metal-deficient heated solution. The contaminant precipitation reactor includes a solid-liquid separator configured to warm-separate the precipitated compounds containing iron and phosphate, as well as the precipitated heavy metal compounds, from the heavy metal-deficient heated solution. This forms a heated-filtered solution.The contaminant precipitation reactor has a first output for filtered precipitated compounds containing iron and phosphate, as well as precipitated heavy metal compounds, and a second output for the heated filtered solution. The refrigerated precipitation setup has an input connected to the second output of the contaminant precipitation reactor to receive the heated filtered solution. The refrigerated precipitation setup includes a device for cooling the heated filtered solution to a cooled solution. This causes the phosphate compound to precipitate. If the strip solution is an aqueous solution of monoammonium phosphate, the phosphate compound contains monoammonium phosphate. If the strip solution is an aqueous solution of monopotassium phosphate, the phosphate compound contains monopotassium phosphate. The refrigerated precipitation setup includes a solid-liquid separator for removing the precipitated phosphate compound from the cooled solution. The refrigerated precipitation setup has a first output for the precipitated phosphate compound and a second output for the cooled solution. The second output of the refrigerated precipitation setup is connected to the second input of the liquid-liquid extraction setup, using the cooled solution as the input strip solution for the liquid-liquid setup. [Effects of the Invention]

[0013] One of the advantages of the proposed technology is that it can produce monoammonium phosphate or monopotassium phosphate with very low levels of contamination. Other advantages will become clear upon reading the detailed explanation.

[0014] The present invention, along with its further objectives and advantages, can be best understood by referring to the following description accompanied by the accompanying drawings: [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 shows the extraction efficiency of phosphates in tributyl phosphate (TBP). [Figure 2] Figure 2 is a flowchart of the steps in one embodiment of a method for producing pure phosphate. [Figure 3] Figure 3 shows the solubility of Fe and Al phosphate at different pH levels. [Figure 4]Figure 4 is a diagram showing the solubility of iron hydroxide at different temperatures. [Figure 5] Figure 5 is a diagram showing the solubility of different metal hydroxides at different pH values. [Figure 6] Figure 6 is a diagram showing the solubility of different metal sulfides at different pH values. [Figure 7] Figure 7 is a schematic diagram of an embodiment of a system for producing pure phosphate.

[0016] Throughout the drawings, the same reference numerals are used for similar or corresponding elements.

Best Mode for Carrying Out the Invention

[0017] To better understand the proposed technology, it may be a good start to briefly explain various approaches to avoid contaminants.

[0018] As described in the background, the extraction of phosphoric acid from water is inefficient due to unfavorable distribution equilibrium. This is shown in Figure 1 by the extraction curve of phosphoric acid using 3M CaCl2 (artificial leachate) and the extraction curve without background CaCl2. That is, when sewage sludge ash, phosphate ore, apatite, etc. are dissolved in sulfuric acid, many contaminants precipitate as different sulfur compounds, and an aqueous solution with a low chloride content can be obtained. The precipitation of contaminants is of course beneficial. However, as shown by the extraction curve 100, many extraction steps are required to reach a reasonable yield of phosphorus extraction, resulting in high capital costs and operating costs.

[0019] By replacing sulfuric acid with hydrochloric acid, the Ca content in phosphate rock or sewage sludge ash forms soluble CaCl2 instead of insoluble gypsum. This results in a phosphoric acid solution with a relatively high chloride concentration, improving extraction, for example, according to extraction curve 102. As mentioned in the background, despite the high chloride background, the liquid-liquid extraction method may yield a relatively dilute strip solution (<3.5 M). A concentrated solution of MAP or MKP can be used as the strip solution and then reacted with ammonia or KOH to supersaturate the solution, thereby obtaining a precipitate of solid MAP or MKP, which can then be separated by a simple filtration method.

[0020] However, as mentioned above, significant co-extraction of problematic impurities can occur. Impurities in the stream may precipitate when the pH rises due to the addition of ammonia or KOH, or they may remain in the solution and exist in the product cake water as inclusions or blockages in the formed product crystals. To reduce the concentration of impurities in the product, impurities must be removed from the strip solution before the product precipitates.

[0021] To reduce the level of Fe, prior art has proposed removing Fe(III) by selective extraction before phosphorus extraction and / or by reduction of Fe(III) to Fe(II), which is not extracted by commonly used solvents such as TBP. The efficiency of liquid-liquid extraction solutions is limited by the limited selectivity of TBP for Fe(III) compared to phosphates, while reduction to Fe(II) requires processing of Fe(II) later in the process.

[0022] Regarding other co-extracted impurities, the published international patent application, International Publication No. 2022 / 173349, proposes the addition of inorganic or organic sulfides, or the use of ion exchange resins, at one of several points in the process. However, larger-scale practical experiments have shown that removing impurities at only one of these points does not allow for the achievement of the desired product quality. Instead, several processing steps are required at different parts of the process, which is costly. Furthermore, if impurities precipitate in the DAP production step of International Publication No. 2022 / 173349, the heat exchanger becomes fouled, negating the significant advantage of adding ammonia to the side stream.

[0023] International Patent Application No. 2022 / 115021, which has been published, suggests that Cd can be removed by either selective liquid-liquid extraction or sulfide precipitation in the main flow or loaded strip solution. Furthermore, it has been suggested that As can be removed from the main flow, loaded strip solution, or MAP solution by sulfide precipitation using inorganic or organic sulfides, such as TMT-15 or Accophos 800, although the pH of the loaded strip solution is typically 0-1, which are acidic conditions in which Accophos 800 does not precipitate heavy metals such as Zn.

[0024] At low pH, high concentrations of H + This is advantageous for many metal ions, and for example, sulfides of Cd, Fe(II), Ni, and Zn are known to be soluble even at low pH. When inorganic sulfides such as NaHS are used, most of the sulfide forms hydrogen sulfide gas, which poses safety problems and reduces efficiency.

[0025] Furthermore, theoretically, iron can also be removed by sulfide precipitation. However, a very large amount of sulfide is required to obtain the required level of Fe. In tests using commercially available organic precipitants, Accophos 800 had difficulty removing sufficient Zn and As from the MAP solution due to competition with residual Fe, while both NaHS and TMT-15 had difficulty removing Zn.

[0026] This technology proposes a method for more efficiently removing both iron and other co-extracted impurities.

[0027] Figure 2 is a flowchart of the steps in one embodiment of a method for producing pure phosphate. In step S10, a strip solution containing stripped phosphate is supplied. The input strip solution is an aqueous solution of one of monoammonium phosphate and monopotassium phosphate. The input strip solution further contains phosphate stripped from a feed solution containing phosphoric acid contaminated with heavy metals and iron, obtained by liquid-liquid extraction.

[0028] In step S20, a base is added to the separation strip solution containing the stripped phosphate. The addition is continued until the pH of the strip solution containing the stripped phosphate is in the range of 2 to 5.5. Preferably, the pH is in the range of 3 to 5, and most preferably, the pH is in the range of 4 to 5. If the input strip solution is an aqueous solution of monoammonium phosphate, the base contains ammonia and / or an ammonium salt. If the input strip solution is an aqueous solution of monopotassium phosphate, the base contains a potassium salt. In a preferred embodiment, and if the input strip solution is an aqueous solution of monoammonium phosphate, the base contains ammonia and / or a basic ammonium salt. More preferably, the base contains ammonia and / or ammonium carbonate, and most preferably, the base contains ammonia. In a preferred embodiment, and if the input strip solution is an aqueous solution of monopotassium phosphate, the base contains a basic potassium salt. More preferably, the base contains potassium hydroxide and / or potassium carbonate, and most preferably, the base contains potassium hydroxide.

[0029] The chemical process involving the addition of a base is exothermic. This generates heat, forming a heated solution from the separation strip solution containing the stripped phosphate. The addition of the base also precipitates compounds containing iron and phosphate from the heated solution, yielding an iron-deficient heated solution. These compounds containing iron and phosphate are typically different types of iron phosphate.

[0030] In step S30, a precipitating agent is added to the heated solution. The precipitating agent has the activity to precipitate heavy metal compounds. This forms a heavy metal-deficient heated solution. In step S40, the precipitated compounds containing iron and phosphate, and the precipitated heavy metal compounds, are separated from the heavy metal-deficient heated solution. The temperature is maintained at a high temperature to keep the phosphate compounds in solution. Separation can be carried out by various methods known in the prior art, such as hot filtration and centrifugation. This forms a heated filtration solution.

[0031] In step S50, the heated filtered solution is cooled to a cooled solution. As the temperature decreases, solubility decreases, and this temperature drop causes the phosphate compound to precipitate. If the input strip solution is an aqueous solution of monoammonium phosphate, the phosphate compound contains monoammonium phosphate. If the input strip solution is an aqueous solution of monopotassium phosphate, the phosphate compound contains monopotassium phosphate. In step S60, the precipitated phosphate compound is removed from the cooled solution. These precipitated phosphate compounds may be the final product of this process or may be further processed. The precipitated phosphate compounds have very low levels of heavy metal contamination. Furthermore, the precipitated phosphate compounds also contain very small amounts of iron. This makes it possible to use the precipitated phosphate compounds as an efficient fertilizer.

[0032] The cooling solution after the step of removing the precipitated phosphate compound is a saturated liquid solution of monoammonium phosphate or monopotassium phosphate. In step S70, the cooling solution is recycled for use as the input strip solution in the stripping process, which supplies the stripped phosphate-laden strip solution.

[0033] In a preferred embodiment, as shown in Figure 2, step S10, which supplies the strip solution, sequentially includes several sub-steps. In step S12, the phosphate is extracted from the feed solution by liquid-liquid extraction into the solvent. In step S14, the solvent is stripped of at least a portion of the phosphate by liquid-liquid extraction, becoming the feed strip solution. In step S16, the strip solution containing the stripped phosphate is separated from the solvent which is at least partially deficient in phosphate. Preferably, the solvent which is at least partially deficient in phosphate is reused in the phosphate extraction step, as indicated by the dotted arrow. Thus, the recirculation of the cooling solution in step S70 is preferably used in step S14 as at least a portion of the strip solution.

[0034] In preferred embodiments, particularly in the case of a feed solution containing a non-negligible amount of calcium, these calcium ions can be removed, at least partially, during the extraction process. For this purpose, in step S13, the solvent is washed with water. This is done after step S12, in which the phosphate is extracted, and before step S14, in which the solvent is stripped.

[0035] As shown in Figure 1, a high chloride background in the feed solution improved the extraction of phosphate ions into the solvent. Therefore, in a preferred embodiment, the feed solution is considered to be a phosphoric acid solution with a chloride concentration of 2 M or higher. Preferably, the chloride concentration is 3 M or higher.

[0036] In preferred embodiments, the solvent includes tributyl phosphate (TBP). However, other types of solvents are also operable; see, for example, the published international patent applications, International Publication 2022 / 173349 and International Publication 2022 / 115021.

[0037] One key component of this technology is the sequential precipitation of contaminants in two steps while retaining phosphate ions in the solution. This yields a purified phosphate solution. The first precipitation step is induced by the S2O step, where a base is added to the strip solution, increasing its pH. The pH of the strip solution obtained from solvent stripping is typically 0–1. Typical phosphate ion concentrations are less than 5M, and less than 3.5M in most setups. This solution also contains iron. Increasing the pH of the strip solution can cause some impurities to precipitate as hydroxides. For example, increasing the pH rapidly decreases the solubility of iron hydroxide and iron phosphate, i.e., strongite. The same behavior is observed with Al, where the solubility of aluminum phosphate, i.e., variscite, decreases with increasing pH. This is reasonable up to at least pH 5.

[0038] Figure 3 shows the solubility of strengthite (curve 104) and variscite (curve 106) as functions of pH. Solubility already drops significantly at pH 2, with much of the Fe and Al precipitating. Above pH 3, and even above pH 4, solubility becomes very low. Adding enough base to convert all the phosphoric acid into monoammonium phosphate or monopotassium phosphate results in a pH typically below pH 5.5, more often below pH 5. The pH typically rises above pH 4 in most cases. Therefore, there is a primary synergistic effect: the amount of base added simultaneously to produce monoammonium phosphate or monopotassium phosphate gives a pH suitable for precipitation of Fe and Al compounds. At the pH where all the phosphoric acid reacts to form MAP or MKP, most of the Fe precipitates as iron hydroxide or iron phosphate, i.e., strengthite, and most of the Al precipitates as aluminum phosphate, i.e., variscite.

[0039] The neutralization reaction between phosphoric acid and a base is an exothermic reaction. Therefore, heat is generated when KOH, and especially ammonia, is added. In other words, an increase in the pH of the strip solution is accompanied by an increase in temperature. For most substances, solubility generally increases with increasing temperature. However, iron phosphate is an exception. Figure 4 shows the solubility of iron phosphate as a function of temperature, represented by curve 132. The H3PO4 concentration in the solution is 1.13 mass%. Therefore, as the temperature increases, the solubility of iron phosphate decreases, the amount of iron compound that precipitates increases, and the remaining iron concentration in the solution rises to an even lower level.

[0040] Simultaneously, increasing the temperature increases the solubility of MAP and MKP, respectively. Raising the temperature reduces potential losses due to precipitation of MAP or MKP. In preferred embodiments, the final temperature is above 40°C. In other words, the step of adding a base to a separation strip solution containing stripped phosphates includes maintaining the temperature of the heated solution above 40°C.

[0041] In prior art for producing fertilizer from phosphate, to increase yields, the process typically begins with phosphate concentrations far exceeding those achievable in current types of systems. In such conventional processes performed at high phosphate concentrations, the heat generated is sufficient to produce a solution above its boiling point. Consequently, many processes involve adding all or part of the base under high pressure, using the excess heat for the drying process. For this reason, pressure chambers that allow operation at temperatures above 100°C are commonly used.

[0042] However, because the phosphoric acid concentration in this system using liquid-liquid exchange technology is much lower, the final temperature after base addition is usually maintained below 80°C, allowing operation without a pressure chamber, and in most cases, no additional cooling is required. Using the onboard MAP strip solution obtained from steady-state pilot-scale tests with sewage sludge ash as raw material, the temperature rose by 40-42°C upon ammoniaization to pH 3.8 and by 48-52°C upon ammoniaization to pH 4.4. Using an artificial onboard MKP strip solution containing approximately 1.2 M phosphoric acid and 221 g / L MKP, with a slight excess of dry KOH added, the temperature rose by 23°C when terminated at a final pH of 4.8.

[0043] In other words, in one embodiment, the step of adding a base to a separation strip solution containing stripped phosphate includes maintaining the temperature of the heated solution below 80°C.

[0044] As described above, the equilibrium temperature depends on the amount of base added. Therefore, in one embodiment, the temperature of the heated solution is maintained by selecting the amount of base in the strip solution, depending on the phosphoric acid concentration, such that the exothermic energy at equilibrium is lower than the energy required to heat the strip solution to 80°C.

[0045] If the phosphoric acid concentration is too high and the temperature may become too high depending on the amount of base required to produce MAP or MKP, it is certainly possible to actively cool the strip solution to keep the temperature sufficiently low. In other words, maintaining the temperature of the heated solution can be done by measuring the temperature of the strip solution and cooling the heated solution if necessary to keep it below 80°C.

[0046] The apparent disadvantage of the relatively low phosphoric acid concentration in the strip solution has now, surprisingly, turned into an advantage. The low concentration is ideal for providing a pH and temperature range that is highly efficient at removing iron from the solution. Therefore, in a preferred embodiment, the strip solution has a phosphoric acid concentration of less than 5 M. In a more preferred embodiment, the strip solution has a phosphoric acid concentration of less than 3.5 M.

[0047] As mentioned above, Fe and Al, if present, are efficiently removed as phosphates and / or hydroxides by a combination of increasing pH and increasing temperature. However, heavy metal phosphates have high solubility, and so do most heavy metal hydroxides, at least within the pH range described above. Figure 5 shows the solubility curves for various metal hydroxides. As can be seen, at pH < 5.5, only Fe (curve 108) and Al (curve 110) have low solubility, potentially leading to precipitation. However, all the remaining contaminants remain in the solution. Curve 112 corresponds to Cu, curve 114 to Ni, curve 116 to Zn, and curve 118 to Cd. In other words, several regulated and potentially toxic elements such as As, Cd, Cu, Cr, Ni, and Zn maintain high solubility at this pH.

[0048] However, since Fe is almost completely removed from the solution at this stage, precipitating agents that compete with Fe precipitates can usually be used either before or in combination with other ions. These impurities can be removed by adding selective precipitating agents, with NaHS being a typical example.

[0049] Figure 6 shows the pH dependence of the solubility of various metal sulfides. Curve 120 corresponds to Cu, curve 122 to As, curve 124 to Pb, curve 126 to Cd, curve 128 to Ni, and curve 130 to Zn. Cr also forms compounds with sulfides that have low solubility. Therefore, using precipitating agents containing various forms of sulfides appears to be a good way to remove such elements from solutions, even when the pH is in the range of 2 to 5.5. In other words, this method is operable to remove contamination if the contaminating heavy metal is at least one of As, Cd, Cr, Cu, Ni, Pb, or Zn.

[0050] Other precipitating agents, or combinations of precipitating agents, can of course be used if their properties or combinations of properties can remove problematic impurities present in the given raw material. Some examples include other inorganic sulfides, organic sulfides, polysulfides, other organic sulfur-based precipitating agents such as Accophos 800, chelate compounds such as phosphines and oxalates, ion exchange resins, and other compounds that preferentially bind to the present impurities. Preferably, the precipitating agent contains a dithiophosphine.

[0051] At pH levels of approximately 4-5, where phosphoric acid completely reacts to form MAP or MKP, at least the sulfides of Cu, Cd, Ni, and Zn become insoluble. Furthermore, the higher the pH, the less likely sulfur gas formation becomes.

[0052] Each precipitate can be filtered separately or together. Filtering both simultaneously is generally preferred because it reduces the number of steps and improves both the capture of small precipitate particles such as inorganic sulfides and the filterability.

[0053] One objective of the current method is to enable a method for recovering phosphates from impurity sources such as phosphate minerals (e.g., calcium phosphate, apatite, struvite, vivienite) and phosphate-containing waste (e.g., sewage sludge ash, meat and bone ash, human waste ash, dried sewage sludge, flame retardant powder, fire extinguisher powder, LiFePO4 battery waste) using hydrochloric acid. Due to the high chloride background, this phosphate is efficiently recovered as phosphoric acid by liquid-liquid extraction. The relatively dilute phosphoric acid strip solution is then processed to produce MAP or MKP without evaporation, by separating impurities such as As, Cd, Cu, Fe, and Zn, while minimizing the use of precipitating agents and other chemicals. [Examples]

[0054] The first column in Table 1 below shows the product quality in a pilot-scale batch test of a modification of the process similar to that described in the published international patent application, International Publication No. 2022 / 173349, in which a commercially available precipitant was added to the side stream of the MAP filtrate before it was ammoniated to produce DAP. After ammoniation, the side stream was filtered. The raw material for this test was sewage sludge ash from a process using an iron-based coagulant, and several cycles were performed with the solution being recirculated until the process reached a steady state.

[0055] Table 1 shows, in the right-hand column, experimental results from a pilot-scale test, where the proposed method was tested in a laboratory setting using the onboard MAP stripping solution. The improvement is remarkable.

[0056] [Table 1]

[0057] Table 1. Test methods of prior art and the quality of the final product tested using this technology.

[0058] The solubility values ​​shown are only for when the pH is increased. This means that since sulfur compounds are also likely to precipitate many residual iron ions, the values ​​after adding a precipitating agent will further increase solubility. The term "negligible" is understood to mean that the contamination level has been reduced to a concentration below the detection limit.

[0059] In one embodiment, the strip solution is an aqueous solution of monoammonium phosphate. The added base thereby contains ammonia and / or an ammonium salt, preferably ammonia and / or a basic ammonium salt, more preferably ammonia and / or ammonium carbonate, and most preferably ammonia. As a result, the phosphate compound contains monoammonium phosphate.

[0060] In another embodiment, the strip solution is an aqueous solution of monopotassium phosphate. The added base thereafter contains a potassium salt, preferably a basic potassium salt, more preferably potassium hydroxide and / or potassium carbonate, most preferably potassium hydroxide. As a result, the phosphate compound contains monopotassium phosphate.

[0061] As described above, one objective of the current method is to enable a method for recovering phosphate from impure sources using hydrochloric acid. Thus, in one embodiment, a method for producing pure phosphate includes a further step of dissolving a phosphorus-containing starting material in hydrochloric acid to obtain an leachate. Undissolved residue is then removed from the leachate, so that the leachate is used as at least part of the feed solution.

[0062] In some embodiments, the starting material includes at least one of sewage sludge ash and phosphate rock.

[0063] In one embodiment, the starting material includes phosphate rock. The contaminating heavy metals typically include at least As and / or Cd.

[0064] In one embodiment, the starting material includes sewage sludge ash. The contaminating heavy metals typically include at least Cu.

[0065] Figure 7 schematically shows one embodiment of System 1 for producing pure phosphate. The liquid-liquid extraction setup 10 uses recirculating solvents 202 and 206. The liquid-liquid extraction setup 10 has a first input 12 for feed solution 200. The feed solution 200 contains phosphoric acid contaminated with heavy metals and iron. A second input 14 is provided for the input of feed strip solution 208. The feed strip solution 208 is an aqueous solution of one of monoammonium phosphate and monopotassium phosphate. A first output 18 is supplied to a strip solution 210 on which the stripped phosphate is loaded. A second output 16 is supplied to a phosphorus-deficient feed solution 204.

[0066] In one embodiment, the liquid-liquid extraction setup 10 includes an extraction unit 20 in which a feed solution 200 from a first input 12 of the liquid-liquid extraction setup 10 comes into contact with a solvent 202 supplied by an extraction unit input 24. The solvent may be of a different type and may be intrinsically insoluble in water and have an affinity for phosphate. In a preferred embodiment, the solvent includes tributyl phosphate. Phosphate ions are thereby extracted from the feed solution 200 into the solvent 202, thereby forming a phosphorus-deficient feed solution 204 and a phosphate-loaded solvent 206, which are output by an extraction unit output 28. The liquid-liquid extraction setup 10 further includes a stripping unit 22 in which the phosphate-loaded solvent 206 is supplied by a stripping unit input 26 connected to the extraction unit output 28 and comes into contact with an input strip solution 208 from a second input 14 of the liquid-liquid extraction setup 10. The phosphate ions are then stripped from solvent 206 into input strip solution 208, thereby forming a strip solution 210 containing the stripped phosphate and a phosphate-deficient solvent. The phosphate-deficient solvent is recycled to the extraction unit 20 as input solvent 202 by the stripping unit output 29.

[0067] According to an embodiment of Figure 1, System 1 for producing pure phosphate further includes a contaminant precipitation reactor 30. The contaminant precipitation reactor 30 further includes a first input 32 connected to a first output 18 of a liquid-liquid extraction arrangement 10 for receiving a strip solution 210 containing stripped phosphate, and a second input 34 for receiving a base 212. If the input strip solution 208 is an aqueous solution of monoammonium phosphate, the base 212 contains ammonia and / or an ammonium salt, and if the input strip solution 208 is an aqueous solution of monopotassium phosphate, the base 212 contains a potassium salt. The contaminant precipitation reactor is configured to add the base 212 to the separation strip solution 210 containing stripped phosphate, so that the pH of the strip solution 210 containing stripped phosphate is in the range of 2 to 5.5, preferably 3 to 5, and most preferably 4 to 5. The addition of the base 212 to the strip solution 210 containing stripped phosphate causes an exothermic reaction. As a result, a heated solution 213 is formed from the strip solution 210 containing the stripped phosphate, and further, a compound 221 containing iron and phosphate precipitates from the heated solution 213, yielding an iron-deficient heated solution 214.

[0068] The contaminant precipitation reactor 30 further has a third input 36 connected to receive a precipitant 216 for precipitating heavy metal compounds 222. The contaminant precipitation reactor 30 is configured to add the precipitant 36 to the heated solution after the addition of the base 34, thereby forming a heavy metal-deficient heated solution 218. This timing of the addition of the precipitant is depicted as a dashed line 35 in the figure. This can be achieved, for example, by having different compartments within the contaminant precipitation reactor 30, where the addition of the base is performed in one compartment and the iron-deficient heated solution 214 is moved to a subsequent compartment within the contaminant precipitation reactor 30 for the addition of the precipitant 216.

[0069] Alternatively, the addition of base 212 and the addition of pre-precipitant 216 can be carried out in the same compartment, in chronological order.

[0070] The contaminant precipitation reactor 30 further includes a solid-liquid separator 37. The solid-liquid separator 37 is configured to warm-separate a precipitated compound 221 consisting of iron and phosphate and the precipitated heavy metal compound 222 from a heavy metal-deficient heated solution 218, forming a heated-filtered solution 220. In different embodiments, the solid-liquid separator may be, for example, a warm-filtered device or a centrifuge. The contaminant precipitation reactor 30 has a first output 38 for the filtered precipitated compound 221 containing iron and phosphate and the precipitated heavy metal compound 222, and a second output 39 for the heated-filtered solution 220.

[0071] System 1 for the production of pure phosphate further includes a refrigerated precipitater configuration 40. The refrigerated precipitater configuration 40 has an input 41 connected to a second output 39 of a contaminant precipitater reactor 30 to receive a heated filtration solution 220. The refrigerated precipitater configuration 40 includes a device 42 for cooling the heated filtration solution 220 to a coolant solution 22 and causing the phosphate compound 226 to precipitate. The phosphate compound 226 contains monoammonium phosphate if the strip solution 208 is an aqueous solution of monoammonium phosphate, and monopotassium phosphate if the strip solution 208 is an aqueous solution of monopotassium phosphate. The refrigerated precipitater configuration 40 includes a solid-liquid separator 46 for removing the precipitated phosphate compound 226 from the coolant solution 224. The refrigerated precipitater configuration 40 has a first output 48 for the precipitated phosphate compound 226 and a second output 49 for the coolant solution 224.

[0072] Various types of apparatus 42 are possible for cooling the heated filtered solution 220. The illustration shows a cooling pipe 44 provided within the compartment of the cooling precipitate arrangement 40. However, other cooling approaches such as bubbling of cold gas or electrically driven cold plates are also applicable, although they are not limited to these. Preferably, there is a cooling control unit 42 that tracks the temperature of the cooled solution 224 to a level low enough to cause precipitation. Also, various types of solid-liquid separators 46 are possible. The illustration shows a filtered solution. However, alternative methods such as centrifugal motion-based methods are also applicable, although they are not limited to these.

[0073] Numerous prior art applications of this type of cooling crystallization technology exist and are applicable to this chemical system. Various types of batch crystallizers can be used. Surface cooling crystallizers, Oslo surface cooling crystallizers, or scrape-type surface crystallizers are examples of prior art applications applicable in the context of this application. Double-tube scrape-type surface crystallizers, also known as Votator or Armstrong crystallizers, are also applicable. Applicable prior art devices for cooling crystallization can be found, for example, in Chinese Patent Application Publication No. 105731407 or Chinese Utility Model No. 203048601, Chinese Utility Model No. 209679546, or Chinese Utility Model No. 203196371.

[0074] The second output 49 of the cooling sedimentation unit 40 is connected to the second input 14 of the liquid-liquid extraction unit 10 in order to use the cooling solution 224 as the input strip solution 208 in the stripping process of the liquid-liquid extraction unit 10.

[0075] The embodiments described above should be understood as some illustrative examples of the present invention. Those skilled in the art will understand that various modifications, combinations, and changes can be made to the embodiments without departing from the scope of the invention. In particular, solutions from different parts of different embodiments can be combined in other configurations, where technically possible. However, the scope of the invention is defined by the appended claims.

Claims

1. A method for producing pure phosphate (226), Step (S10) of supplying a strip solution (210) on which stripped phosphates are carried by liquid-liquid extraction from a feed solution (200) containing phosphoric acid contaminated with heavy metals and iron, and an input strip solution (208) which is an aqueous solution of one of monoammonium phosphate and monopotassium phosphate, The step (S20) is to add a base (212) to the strip solution (210) containing the stripped phosphate, thereby adjusting the pH of the strip solution (210) containing the stripped phosphate to a range of 2 to 5.5, preferably 3 to 5, and most preferably 4 to 5. The base (212) comprises at least one of ammonia and an ammonium salt if the input strip solution (208) is an aqueous solution of monoammonium phosphate, and the base (212) comprises a potassium salt if the input strip solution (208) is an aqueous solution of monopotassium phosphate. The step of adding the base (212) (S20) is exothermic, thereby forming a heated solution (213) from the strip solution (210) on which the stripped phosphate is carried. The step of adding the base (S20) includes precipitating a compound (221) containing iron and phosphate from the heated solution (213) to give an iron-deficient heated solution (214), The process (S30) involves adding a precipitating agent (216) to the iron-deficient heated solution (214) to precipitate a heavy metal compound (222), thereby forming a heavy metal-deficient heated solution (218), The process (S40) involves separating the precipitated compound (221) containing iron and phosphate, and the precipitated heavy metal compound (222) from the heavy metal-deficient heated solution (218) to form a heated filtered solution (220), The heated filtration solution (220) is cooled to a cooled solution (224), and a precipitate of the phosphate compound (226) is formed (S50). If the input strip solution (208) is an aqueous solution of monoammonium phosphate, the phosphate compound (226) contains monoammonium phosphate, and if the input strip solution (208) is an aqueous solution of monopotassium phosphate, the phosphate compound contains monopotassium phosphate, in a step, The step (S60) is to remove the precipitated phosphate compound (226) from the cooling solution (224), The following steps (S60) are taken to remove the precipitated phosphate compound (226): The cooling solution (224) is then recycled (S70) for use as the input strip solution (208) in liquid-liquid extraction. Methods that include...

2. The step (S10) of supplying the strip solution (210) on which the stripped phosphate is mounted is performed in order. Step (S12) involves extracting phosphate from the supply liquid (200) into a solvent (202) by liquid-liquid extraction to form a solvent (206) containing the phosphate, Step (S14) involves stripping at least a portion of the phosphate from the solvent (206) containing the phosphate by liquid-liquid extraction into the input strip solution (208) to form a strip solution (210) containing the stripped phosphate, A step (S16) to separate the strip solution (210) on which the stripped phosphate is carried from the solvent (202) in which at least a portion of the phosphate is deficient, The method according to claim 1, characterized by including

3. The method according to claim 2, characterized in that the solvent (202,206) contains tributyl phosphate.

4. The supply liquid (200) contains calcium ions, The method further includes, after the step of extracting the phosphate (S12) and before the step of stripping the solvent (206) (S14), scrubbing the solvent containing the phosphate (206) with water (S13). The method according to claim 2 or 3, characterized by including the following:

5. The precipitating agent (216) is inorganic sulfide, organic sulfides, polysulfide, Organic sulfur precipitating agent, Phosphines, Chelate compounds such as oxalates, and Ion exchange resin The method according to any one of claims 1 to 4, characterized in that it is selected as at least one of the following.

6. The method according to claim 5, characterized in that the precipitating agent (216) contains a dithiophosphine.

7. The method according to any one of claims 1 to 6, characterized in that the step (S20) of adding a base (212) to the separation strip solution (210) on which the stripped phosphate is carried is maintained at a temperature of less than 80°C.

8. The method according to claim 7, characterized in that the temperature of the heated solution is maintained by selecting an amount of base (212) depending on the phosphoric acid concentration in the strip solution (210) containing the stripped phosphate, and providing an equilibrium exothermic energy lower than the energy required to heat the strip solution (210) containing the stripped phosphate to 80°C.

9. The method according to claim 7, characterized in that maintaining the temperature of the heated solution (213) is done by measuring the temperature of the strip solution (210) on which the stripped phosphate is mounted, and cooling the heated solution (213) if necessary to maintain the heated solution (213) below 80°C.

10. The method according to any one of claims 1 to 9, characterized in that the step (S20) of adding a base (212) to the separation strip solution (210) on which the stripped phosphate is carried is maintained at a temperature of over 40°C.

11. The method according to any one of claims 1 to 10, characterized in that, when the input strip solution (208) is an aqueous solution of monoammonium phosphate, the base (212) comprises at least one of ammonia and a basic ammonium salt, preferably at least one of ammonia and ammonium carbonate, more preferably ammonia, and when the input strip solution (208) is an aqueous solution of monopotassium phosphate, the base (212) comprises a basic potassium salt, preferably at least one of potassium hydroxide and potassium carbonate, more preferably potassium hydroxide.

12. The method according to any one of claims 1 to 11, characterized in that the strip solution (210) on which the stripped phosphate is carried has a phosphoric acid concentration of less than 5 M.

13. The method according to claim 12, characterized in that the strip solution (210) on which the stripped phosphate is carried has a phosphoric acid concentration of less than 3.5 M.

14. The method according to any one of claims 1 to 13, characterized in that the contaminating heavy metal is at least one of As, Cd, Cr, Cu, Ni, Pb, and Zn.

15. The method according to any one of claims 1 to 14, characterized in that the supply liquid (200) is a phosphoric acid solution with a chloride concentration of more than 2 M, preferably more than 3 M.

16. Further: A step of dissolving a phosphorus-containing starting material in hydrochloric acid and supplying the leachate, A step of removing insoluble residue from the leachate, thereby using the leachate as at least a part of the supply liquid (200), The method according to any one of claims 1 to 15, characterized by including

17. The method according to claim 16, characterized in that the starting material includes at least one of sewage sludge ash and phosphate rock.

18. The method according to claim 17, characterized in that the initial material includes phosphate rock and the contaminating heavy metal includes at least one of As and Cd.

19. The method according to claim 17, characterized in that the initial material includes sewage sludge ash and the contaminating heavy metals include at least Cu.

20. The method according to any one of claims 1 to 17, characterized in that the input strip solution (208) is an aqueous solution of monoammonium phosphate, the base (212) comprises at least one of ammonia and an ammonium salt, and the phosphate compound (226) comprises monoammonium phosphate.

21. The method according to any one of claims 1 to 17, characterized in that the input strip solution (208) is an aqueous solution of monopotassium phosphate, the base (212) contains a potassium salt, and the phosphate compound (226) contains monopotassium phosphate.

22. A system (1) for producing pure phosphate, A liquid-liquid extraction setup (10) using recirculating solvents (202, 206), the liquid-liquid extraction setup (10) having a first input (12) for a feed solution (200), a second input (14) for a feed strip solution (208), a first output (18) for a strip solution (210) containing stripped phosphate, and a second output (16) for a phosphorus-deficient feed solution (204), The supply liquid (200) contains phosphoric acid contaminated with heavy metals and iron, The strip solution (208) is an aqueous solution of one of monoammonium phosphate and monopotassium phosphate, in a liquid-liquid extraction configuration (10), A contaminant precipitation reactor (30) having a first input (32) connected to the first output (18) of the liquid-liquid extraction configuration (10) for receiving a strip solution (210) on which the stripped phosphate is loaded, and a second input (34) for receiving a base (212), The base (212) comprises at least one of ammonia and an ammonium salt when the input strip solution (208) is an aqueous solution of monoammonium phosphate, and the base (212) comprises a potassium salt when the input strip solution (208) is an aqueous solution of monopotassium phosphate. The contaminant precipitation reactor (30) is configured to add the base (212) to the separation strip solution (210) on which the stripped phosphate is carried, and to set the pH of the strip solution to a range of 2 to 5.5, preferably 3 to 5, and most preferably 4 to 5. The addition of the base (212) to the strip solution (210) containing the stripped phosphate triggers an exothermic reaction, thereby forming a heated solution (213) from the strip solution (210) containing the stripped phosphate, and further, a compound (221) containing iron and phosphate precipitates from the heated solution (213), giving an iron-deficient heated solution (214). The contaminant precipitation reactor (30) has a third inlet (36) for receiving a precipitant (216) for precipitating heavy metal compounds (222), and the contaminant precipitation reactor (30) is configured to add the precipitant (216) to the iron-deficient heated solution (214) after the addition of the base (212), thereby forming a heavy metal-deficient heated solution (218). The contaminant precipitation reactor (30) is configured to warm-separate the precipitated compound (221) containing iron and phosphate and the precipitated heavy metal compound (222) from the heavy metal-deficient heated solution (218), and includes a solid-liquid separator (37) that forms a heated filtered solution (220). The contaminant precipitation reactor (30) has a first output (38) for the filtration precipitate compound (221) containing iron and phosphate and the precipitate heavy metal compound (222), and a second output (39) for the heated filtration solution (220), and A refrigerated sedimentation unit (40) having an input (41) connected to the second output (39) of the contaminant precipitation reactor (30) for receiving the heated filtered solution (220), The cooling precipitate arrangement (40) includes a device (42) for cooling the heated filtration solution (220) to a cooled solution (224) and causing the precipitation of the phosphate compound (226), The phosphate compound (226) includes monoammonium phosphate when the strip solution (208) is an aqueous solution of monoammonium phosphate, and the phosphate compound (226) includes monopotassium phosphate when the strip solution (208) is an aqueous solution of monopotassium phosphate. The cooling sedimentation unit configuration (40) includes a solid-liquid separator (46) for removing the precipitated phosphate compound (226) from the cooling solution (224), The cooling precipitate configuration (40) has a first output (48) for the precipitated phosphate compound (226) and a second output (49) for the cooling solution (224), The second output (49) of the cooling sedimentation unit (40) is connected to the second input (14) of the liquid-liquid extraction unit (10) in order to use the cooling solution (224) as the input strip solution (208) in the liquid-liquid extraction unit (10). A system that includes this.

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