Treatment methods for purifying residual acid from phosphoric acid extraction

Abstract

This disclosure provides a method for purifying residual phosphate raffinate. First, fluorine is removed from the raffinate. Then, a two-stage neutralization reaction is performed. The primary purpose of the first-stage neutralization is to precipitate iron, aluminum, a small amount of magnesium, and residual fluorine from the raffinate. The purpose of the second-stage neutralization is to precipitate magnesium from the first-stage filtrate, resulting in a purified filtrate with fewer impurities. Phosphate ions are then precipitated from the purified filtrate, resulting in a phosphate precipitate with fewer impurities. This method has no requirements on the composition of the raffinate, has a wide range of applications, and the two-stage neutralization process separates and removes fluorine, magnesium, and other impurities from the raffinate, facilitating the recovery and utilization of these impurities. Simultaneously, it yields high-quality dicalcium phosphate.

Description

Technical Field

[0001] This invention relates to the field of battery raw material technology, and more specifically, to a method for purifying residual acid from phosphoric acid extraction. Background Technology

[0002] Industrial-grade phosphoric acid is produced using two methods: thermal and wet purification. Thermal phosphoric acid uses yellow phosphorus as raw material. The yellow phosphorus is first burned, and then the resulting phosphorus pentoxide is absorbed by pure water to directly produce phosphoric acid. This method produces high-quality phosphoric acid, but it suffers from high cost, high energy consumption, and high pollution. Wet purification uses wet-process phosphoric acid as raw material. After a series of synergistic physical and chemical purification treatments, a phosphoric acid product with lower impurity content and higher purity is produced. The purity of this product can meet the needs of most industrial applications. Compared with thermal industrial phosphoric acid, wet purification phosphoric acid has advantages such as lower energy consumption, less pollution, less waste residue, and lower production cost. Based on these characteristics, wet purification phosphoric acid is gradually replacing thermal industrial phosphoric acid, which has become the general development trend in the phosphoric acid industry. The rapid growth in demand from the power industry, especially for lithium iron phosphate products, has accelerated this replacement process.

[0003] Wet extraction purification, also known as solvent extraction purification, involves the extraction of phosphoric acid molecules into the organic solvent phase through the reaction of the extractant. Impurity ions in the acid react minimally with the extractant and remain in the aqueous phase after phosphoric acid extraction. This aqueous phase is the raffinate, which is crude phosphoric acid enriched with both anionic and cationic impurity ions. For companies with fertilizer processing plants, the raffinate can be returned to the plant for fertilizer production, thus minimizing difficulties in its disposal. However, for companies without fertilizer processing plants, it is necessary to specifically research suitable raffinate treatment methods to address its fate.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for treating purified phosphate raffinate, converting the phosphate ions therein into calcium phosphate, while also facilitating the subsequent recovery and utilization of other elements in the purified phosphate raffinate.

[0006] This invention is implemented as follows:

[0007] In a first aspect, the present invention provides a method for purifying residual acid from phosphoric acid raffinate, comprising:

[0008] After a neutralization process, an oxidant is added to the defluorinated residual acid and the pH is adjusted to 3.0-3.5. After a neutralization reaction, solid-liquid separation is performed to obtain the first-stage precipitate and the first-stage filtrate. The first-stage precipitate contains a complex phosphate containing iron, aluminum, magnesium and fluorine.

[0009] Two-stage neutralization: After adjusting the pH of the first-stage filtrate to 4.5-5.0 and performing a two-stage neutralization reaction, solid-liquid separation is performed to obtain a second-stage precipitate and a purified filtrate. The second-stage precipitate includes magnesium phosphate.

[0010] The impurity-removing filtrate is recovered. A second precipitant is added to the impurity-removing filtrate and the pH is adjusted to 5.5-6.5 to carry out the impurity removal reaction. After solid-liquid separation, phosphate precipitate and dephosphorization mother liquor are obtained.

[0011] In some embodiments, the oxidant is at least one of ozone, oxygen, and hydrogen peroxide.

[0012] In some embodiments, the amount of oxidant used is 0.1% wt to 1% wt of the mass of the residual acid.

[0013] In some embodiments, in the first-stage neutralization reaction step and / or the second-stage neutralization reaction step and / or the impurity removal filtrate recovery step, a neutralizing agent is used to adjust the pH. The neutralizing agent is at least one of ammonia, sodium hydroxide, potassium hydroxide, and carbonates, bicarbonates, orthophosphates, and monohydrogen phosphates of potassium, sodium, and ammonium.

[0014] In some embodiments, the second precipitant is at least one of calcium oxide, calcium hydroxide, and calcium sulfate.

[0015] In some embodiments, the molar ratio of calcium in the second precipitant to phosphorus in the purified filtrate is (0.90 to 0.95):1.

[0016] In some embodiments, after the first-stage and / or second-stage neutralization reaction is completed, the reaction solution is aged at 75°C to 85°C for 45 min to 60 min, and then solid-liquid separation is performed.

[0017] In some embodiments, defluorination is also included, the defluorination step comprising:

[0018] Intermediate acid and activated diatomaceous earth are added to the residual raffinate to convert free fluoride ions into fluorosilicate ions, and then solid-liquid separation is performed to obtain the supernatant.

[0019] A first precipitant is added to the supernatant to carry out a defluorination reaction, resulting in fluorosilicate precipitate and residual acid after defluorination.

[0020] In some embodiments, the first precipitant is a sodium-containing compound or sodium hydroxide.

[0021] In some embodiments, the sodium salt is at least one of sodium carbonate, sodium bicarbonate, sodium phosphate, or sodium sulfate.

[0022] In some embodiments, the molar ratio of sodium in the first precipitant to fluorine in the supernatant is (0.4-0.6):1.

[0023] In some embodiments, the temperature of the defluorination reaction is 40°C to 60°C.

[0024] In some embodiments, the intermediate acid is at least one of sulfuric acid and hydrochloric acid, and the mass ratio of H2SO4 in the intermediate acid to the residual acid is 1%wt to 10%wt.

[0025] In some embodiments, the dephosphorization mother liquor is evaporated and crystallized to obtain sulfate crystals.

[0026] In some embodiments, the recycling of the first-stage precipitate is also included: the first-stage precipitate is pulped, alkali is added to it to carry out a phosphorus extraction reaction, and then solid-liquid separation is performed to obtain phosphorus-containing filtrate and dephosphorized residue;

[0027] Calcium ions are added to the phosphorus-containing filtrate to carry out a dephosphorization reaction, resulting in a calcium salt precipitate including calcium fluorophosphate and calcium hydroxyphosphate.

[0028] In some embodiments, the solid content in the slurry obtained after pulping the first precipitate is 20% wt to 50% wt.

[0029] In some embodiments, the alkali used in the phosphorus extraction reaction is a concentrated alkali solution of 30% wt to 40% wt.

[0030] In some embodiments, the base used is at least one of sodium hydroxide and potassium hydroxide.

[0031] In some embodiments, the alkali further includes sodium aluminate and / or potassium aluminate.

[0032] In some embodiments, the pH of the phosphorus extraction reaction is 14.0 to 14.2, the reaction time is 60 min to 90 min, and the reaction temperature is 20°C to 60°C.

[0033] In some embodiments, during the dephosphorization reaction, the calcium ions are added to the phosphorus-containing filtrate in the form of lime milk, and the solid content of the lime milk is 20% wt to 30% wt.

[0034] In some embodiments, the precipitation of calcium phosphate from the phosphorus-containing filtrate is carried out using a countercurrent precipitation method, with 2 to 4 countercurrent precipitation stages;

[0035] In some embodiments, in the dephosphorization reaction, lime slurry is added from the last stage, phosphorus-containing filtrate is added from the first stage, the effluent from the previous stage of the countercurrent precipitation reaction is used as the raw material for the next stage of the reaction, the precipitate produced in the next stage of the precipitation reaction is used as the dephosphorization precipitant for the previous stage of the precipitation reaction, and the precipitate obtained from the first stage of the reaction is a calcium salt precipitate including calcium fluorophosphate and calcium hydroxyphosphate.

[0036] In some embodiments, during the dephosphorization reaction, the molar ratio of calcium in the lime slurry to phosphorus in the phosphorus-containing filtrate is (1.15–1.55):1.

[0037] In some embodiments, the dephosphorization reaction is carried out at a rotation speed greater than 300 rpm, a reaction temperature of 20°C to 60°C, and a reaction time of 30 min to 60 min.

[0038] In some embodiments, in the dephosphorization reaction, the effluent from the final stage precipitation reaction is concentrated and supplemented with alkali after the solids are separated.

[0039] In some embodiments, the recycling of the second-stage precipitate is also included: the second-stage precipitate is mixed with calcium chloride solution to carry out a conversion reaction to obtain calcium hydrogen phosphate and conversion reaction solution.

[0040] In some embodiments, the calcium chloride solution has a calcium chloride mass fraction of 12% wt to 13% wt and a pH of 4.3 to 4.7.

[0041] In some embodiments, the conversion reaction employs a countercurrent precipitation method, with 2 to 4 countercurrent precipitation stages; in the conversion reaction, calcium chloride solution is added from the first stage and discharged from the last stage, and the second-stage precipitate is added from the last stage and discharged from the first stage, and the precipitate discharged from the first stage is the solid phase containing calcium hydrogen phosphate.

[0042] In some embodiments, the conversion reaction is carried out at a rotation speed greater than 300 rpm and a reaction time of 60 min to 120 min.

[0043] In some embodiments, the molar ratio of calcium in the calcium chloride solution to phosphorus in the second-stage precipitate is (1.5–3.0):1 in the conversion reaction.

[0044] In some embodiments, the method further includes: adding calcium oxide or calcium hydroxide to the conversion reaction solution to carry out a regeneration reaction, and after the regeneration reaction is completed, performing solid-liquid separation on the reaction solution to obtain a solid phase containing magnesium hydroxide.

[0045] In some embodiments, the molar ratio of calcium added to magnesium in the conversion reaction solution during the regeneration reaction is (0.85–0.95):1.

[0046] The present invention has the following beneficial effects:

[0047] This disclosure provides a method for purifying residual phosphate raffinate. First, fluorine is removed from the raffinate. Then, a two-stage neutralization reaction is performed. The primary purpose of the first-stage neutralization is to precipitate iron, aluminum, a small amount of magnesium, and residual fluorine from the raffinate. The purpose of the second-stage neutralization is to precipitate magnesium from the first-stage filtrate, resulting in a purified filtrate with fewer impurities. Phosphate ions are then precipitated from the purified filtrate, resulting in a phosphate precipitate with fewer impurities. This method has no requirements on the composition of the raffinate, has a wide range of applications, and the two-stage neutralization process separates and removes fluorine, magnesium, and other impurities from the raffinate, facilitating the recovery and utilization of these impurities. Simultaneously, it yields high-quality dicalcium phosphate. Attached Figure Description

[0048] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 This is a flowchart of the method for purifying residual acid from phosphoric acid extraction in this disclosure;

[0050] Figure 2 This is a flowchart illustrating the recycling process of the first-stage sediment in this disclosure.

[0051] Figure 3 This is a flowchart illustrating the recycling process of the second-stage sediment in this disclosure. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0053] In a first aspect, the present invention provides a method for purifying residual acid from phosphoric acid raffinate, comprising:

[0054] After a neutralization process, an oxidant is added to the defluorinated residual acid and the pH is adjusted to 3.0-3.5. After a neutralization reaction, solid-liquid separation is performed to obtain the first-stage precipitate and the first-stage filtrate. The first-stage precipitate contains a complex phosphate containing iron, aluminum, magnesium and fluorine.

[0055] Two-stage neutralization: After adjusting the pH of the first-stage filtrate to 4.5-5.0 and performing a two-stage neutralization reaction, solid-liquid separation is performed to obtain a second-stage precipitate and a purified filtrate. The second-stage precipitate includes magnesium phosphate.

[0056] The impurity-removing filtrate is recovered. A second precipitant is added to the impurity-removing filtrate and the pH is adjusted to 5.5-6.5 to carry out the impurity removal reaction. After solid-liquid separation, phosphate precipitate and dephosphorization mother liquor are obtained.

[0057] This disclosure provides a method for purifying residual phosphate raffinate. First, fluorine in the residual raffinate is removed. Then, a two-stage neutralization reaction is carried out. The main purpose of the first-stage neutralization is to precipitate iron, aluminum, a small amount of magnesium, and residual fluorine in the residual raffinate. The purpose of the second-stage neutralization is to precipitate magnesium in the first-stage filtrate to obtain a purified filtrate with fewer impurities. Phosphate ions in the purified filtrate are then precipitated to obtain a phosphate precipitate with fewer impurities.

[0058] In this embodiment, fluorine, magnesium, and other impurity elements in the raffinate are removed separately through two neutralization steps, which facilitates the recovery and utilization of these impurity elements. This embodiment has no requirements regarding the composition of the raffinate and has a wide range of applications.

[0059] In some embodiments, the oxidant is at least one of ozone, oxygen, and hydrogen peroxide. The oxidant is used to oxidize the ferrous ions present in the residual acid after defluorination to ferric ions, so that they can subsequently form a precipitate with phosphate. The choice of ozone, oxygen, and hydrogen peroxide as oxidants will not introduce impurities into the system, which is beneficial to improving the purity of dicalcium phosphate and facilitating the recovery and utilization of other separated metal elements.

[0060] In some embodiments, the amount of oxidant used is 0.1%wt to 1%wt of the mass of the residual raffinate, specifically any value between 0.1%wt, 0.2%wt, 0.4%wt, 0.6%wt, 0.8%wt, 1%wt or 0.1%wt to 1%wt, so that the ferrous ions in the residual raffinate are fully oxidized.

[0061] In some embodiments, in the first and / or second neutralization steps and / or the impurity removal filtrate recovery step, a neutralizing agent is used to adjust the pH. The neutralizing agent is at least one of ammonia, sodium hydroxide, potassium hydroxide, and carbonates, bicarbonates, orthophosphates, and monohydrogen phosphates of potassium, sodium, and ammonium. When selecting a neutralizing agent, cationic options such as potassium, sodium, and ammonia are preferred because they combine with phosphate in the defluorinated residual acid to form salts that do not precipitate; anionic options are preferred because phosphates do not introduce new impurities; carbonates or bicarbonates are easily removed, and the impact on the type and content of impurities in the product is minimal.

[0062] In some embodiments, the second precipitant is at least one of calcium oxide, calcium hydroxide, and calcium sulfate. The calcium in the second precipitant reacts with phosphate to generate calcium hydrogen phosphate, which is then separated from the reaction system as calcium hydrogen phosphate dihydrate precipitate.

[0063] In some embodiments, the molar ratio of calcium in the second precipitant to phosphorus in the purified filtrate is (0.90–0.95):1, specifically, it can be any value between 0.90:1, 0.91:1, 0.92:1, 0.93:1, 0.94:1, 0.95:1, or (0.90–0.95):1. A slight deficiency of calcium ions is avoided to prevent excessive calcium ions from reacting with other anions in the system, such as sulfate, to form sulfate precipitates and increase the impurity content in the dicalcium phosphate.

[0064] In some embodiments, after the first-stage and / or second-stage neutralization reactions are completed, the reaction solution is aged at 75°C–85°C for 45–60 minutes before solid-liquid separation. Specifically, the aging temperature can be any value between 75°C, 80°C, 85°C, or 75°C–85°C; the aging time can be any value between 45 minutes, 50 minutes, 55 minutes, 60 minutes, or 45 minutes–60 minutes. Aging allows the precipitate particles to continuously precipitate, crystallize, and grow, facilitating subsequent solid-liquid separation.

[0065] In some embodiments, defluorination is also included, the defluorination step comprising:

[0066] Intermediate acid and activated diatomaceous earth are added to the residual raffinate to convert free fluoride ions into fluorosilicate ions, and then solid-liquid separation is performed to obtain the supernatant.

[0067] A first precipitant is added to the supernatant to carry out a defluorination reaction, resulting in fluorosilicate precipitate and residual acid after defluorination.

[0068] As the pH increases, the ionization rate of phosphoric acid gradually increases. The ionized phosphate ions combine with metal ions in the solution, such as iron and aluminum, to form phosphate precipitates. In this embodiment, an intermediate acid is added to the residual raffinate to prevent a significant increase in the pH of the residual raffinate during the defluorination reaction. This avoids co-precipitation of iron phosphate, aluminum phosphate, etc., with fluorosilicates, thereby reducing the purity of fluorosilicates and affecting their utilization.

[0069] Activated diatomaceous earth can be added in excess. If a small amount of activated diatomaceous earth can be separated after the defluorination reaction, it is considered an excess. The activated diatomaceous earth obtained by solid-liquid separation after converting free fluoride ions into sodium fluorosilicate can be recycled and reused.

[0070] In some embodiments, the first precipitant is a sodium-containing compound or sodium hydroxide.

[0071] In some embodiments, the sodium salt is at least one of sodium carbonate, sodium bicarbonate, sodium phosphate, or sodium sulfate. Sodium ions react with fluorosilicate to form sodium fluorosilicate, which is sparingly soluble in water, facilitating the separation of fluorosilicate.

[0072] In some embodiments, the molar ratio of sodium in the first precipitant to fluorine in the supernatant is (0.4-0.6):1, specifically any value between 0.4:1, 0.45:1, 0.5:1, 0.55:1, 0.6:1, or (0.4-0.6):1. If the amount of sodium added is too small, the removal effect of fluorine will be poor, while an excessive amount of sodium will increase the cost.

[0073] In some embodiments, the temperature of the defluorination reaction is 40°C to 60°C, specifically any value between 40°C, 45°C, 50°C, 55°C, 60°C, or 40°C to 60°C, which is conducive to the forward reaction.

[0074] In some embodiments, the intermediate acid is at least one of sulfuric acid and hydrochloric acid, and the mass ratio of H2SO4 in the intermediate acid to the residual acid is 1%wt to 10%wt, specifically, it can be any value between 1%wt, 2%wt, 4%wt, 6%wt, 8%wt, 10%wt or 1%wt to 10%wt, to stabilize pH and inhibit phosphate ionization.

[0075] In some embodiments, the dephosphorization mother liquor is evaporated and crystallized to obtain sulfate crystals. The mother liquor after evaporation and crystallization can be combined with the impurity-removed filtrate for the next filtrate recovery cycle.

[0076] In some embodiments, the recycling of the first-stage precipitate is also included: the first-stage precipitate is pulped, alkali is added to it to carry out a phosphorus extraction reaction, and then solid-liquid separation is performed to obtain phosphorus-containing filtrate and dephosphorized residue;

[0077] Calcium ions are added to the phosphorus-containing filtrate to carry out a dephosphorization reaction, resulting in a calcium salt precipitate including calcium fluorophosphate and calcium hydroxyphosphate.

[0078] In the phosphorus removal reaction, iron, aluminum, and other substances in the first-stage precipitate are converted into hydroxides, and phosphate ions are transferred to the phosphorus-containing filtrate, where they combine with calcium ions to form calcium salt precipitates. The calcium salts include calcium fluorophosphate and calcium hydroxyphosphate, and can be used to produce wet-process phosphoric acid. The dephosphorization slag can be washed once with 2 to 4 times its weight of clean water, and the resulting washing liquid is used to prepare lime milk and pulp the first-stage precipitate. The calcium salt precipitate can also be washed once with 2 to 4 times its weight of clean water, and the resulting washing liquid is used to prepare lime milk and pulp the first-stage precipitate.

[0079] The main components of the dephosphorization slag obtained in this embodiment are a mixture of aluminum hydroxide, iron hydroxide and magnesium hydroxide. Therefore, the Bayer process can be used to treat the dephosphorization slag and recover the aluminum element therein, thereby achieving the purpose of reducing the amount of waste slag.

[0080] In some embodiments, the solid content in the slurry obtained after the first precipitate is 20%wt to 50%wt, specifically it can be any value between 20%wt, 30%wt, 40%wt, 50%wt or 20%wt to 50%wt, in a liquid phase environment, which is conducive to the rapid and uniform progress of subsequent reactions.

[0081] In some embodiments, the alkali used in the phosphorus extraction reaction is a concentrated alkali solution of 30% wt to 40% wt, specifically any value between 30% wt to 40% wt or between 30% wt to 40% wt, so as to dissolve the phosphate ions in the first-stage precipitate. If the alkali concentration is too low, the first-stage precipitate cannot be dissolved.

[0082] In some embodiments, the base used is at least one of sodium hydroxide and potassium hydroxide, a strong base is used, and the potassium and sodium in it do not combine with phosphate to form a precipitate.

[0083] In some embodiments, the alkali also includes sodium aluminate and / or potassium aluminate, which can inhibit the dissolution of aluminum from the first-stage precipitate.

[0084] In some embodiments, the pH of the phosphorus extraction reaction is 14.0–14.2, specifically any value between 14.0, 14.1, 14.2, or 14.0–14.2; the reaction time is 60–90 min, specifically any value between 60, 70, 80, 90 min, or 60–90 min; and the reaction temperature is 20°C–60°C, specifically any value between 20°C, 30°C, 40°C, 50°C, 60°C, or 20°C–60°C. Slightly increasing the temperature helps reduce the viscosity of the solution, facilitates sufficient contact of the reactants, and increases the reaction rate. However, excessively high temperatures increase energy consumption, leading to increased costs; therefore, the temperature should not be too high.

[0085] In some embodiments, during the dephosphorization reaction, the calcium ions are added to the phosphorus-containing filtrate in the form of lime milk. The solid content of the lime milk is 20%wt to 30%wt, specifically 20%wt, 22%wt, 24%wt, 26%wt, 28%wt, 30%wt, or any value between 20%wt and 30%wt. The calcium in the solid can gradually dissolve into the phosphorus-containing filtrate, maintaining a high concentration of calcium ions in the reaction solution and promoting the rapid progress of the dephosphorization reaction.

[0086] In some embodiments, the precipitation of calcium phosphate from the phosphorus-containing filtrate is carried out using a countercurrent precipitation method, with 2 to 4 countercurrent precipitation stages, specifically 2, 3, or 4 stages.

[0087] In some embodiments, in the dephosphorization reaction, lime slurry is added from the last stage, phosphorus-containing filtrate is added from the first stage, the effluent from the previous stage of the countercurrent precipitation reaction is used as the raw material for the next stage of the reaction, and the precipitate produced in the next stage of the precipitation reaction is used as the dephosphorization precipitant for the previous stage of the precipitation reaction. The precipitate obtained from the first stage of the reaction is a calcium salt precipitate including calcium fluorophosphate and calcium hydroxyphosphate. The multi-stage precipitation method can improve the separation effect of phosphorus elements from other substances in the phosphorus-containing filtrate.

[0088] In some embodiments, during the dephosphorization reaction, the molar ratio of calcium in the lime slurry to phosphorus in the phosphorus-containing filtrate is (1.15–1.55):1. Specifically, it can be any value between 1.15:1, 1.25:1, 1.35:1, 1.45:1, 1.55:1, or 1.15–1.55:1. A slight deficiency of calcium ions is avoided to prevent excessive calcium ions from reacting with other anions in the system, such as aluminate ions, to form precipitates and increase the impurity content in the calcium phosphate precipitate.

[0089] In some embodiments, during the dephosphorization reaction, the rotation speed is greater than 300 rpm, specifically 300 rpm, 500 rpm, 700 rpm, or any value greater than 300 rpm; the reaction temperature is 20℃~60℃, specifically 20℃, 30℃, 40℃, 50℃, 60℃, or any value between 40℃ and 60℃; and the reaction time is 30min~60min, specifically 30min, 40min, 50min, 60min, or any value between 30min and 60min. Vigorous stirring is beneficial for improving the uniform distribution of components in the reaction system, and for increasing the reaction rate and dephosphorization effect.

[0090] In some embodiments, in the dephosphorization reaction, the effluent from the last stage precipitation reaction is concentrated and supplemented with alkali after the solids are separated, in order to maintain the water balance and alkali balance during the recovery process of the first stage precipitate residue, wherein the amount of alkali supplemented is determined according to the amount of alkali loss.

[0091] In some embodiments, the recycling of the second-stage precipitate is also included: the second-stage precipitate is mixed with calcium chloride solution to carry out a conversion reaction to obtain dicalcium phosphate and conversion reaction liquid. Calcium chloride can convert magnesium and other substances in the second-stage precipitate to obtain a conversion reaction liquid containing magnesium, which is beneficial for the subsequent recycling of magnesium. Under certain conditions, the dicalcium phosphate dihydrate obtained meets the requirements of GB 22549-2017 standard.

[0092] In some embodiments, the mass fraction of calcium chloride in the calcium chloride solution is 12%wt to 13%wt, specifically, it can be any value between 12%wt, 12.5%wt, 13%wt, or 12%wt to 13%wt; the pH is 4.3 to 4.7, specifically, it can be any value between 4.3, 4.4, 4.5, 4.6, 4.7, or 4.3 to 4.7, which is conducive to the conversion of magnesium phosphate to calcium hydrogen phosphate.

[0093] In some embodiments, the conversion reaction employs countercurrent precipitation, with 2 to 4 stages of countercurrent precipitation. In the conversion reaction, calcium chloride solution is added from the first stage and discharged from the last stage, while the second-stage precipitate is added from the last stage and discharged from the first stage. The precipitate discharged from the first stage is the solid phase containing dicalcium phosphate. Countercurrent precipitation is beneficial for increasing the conversion rate of magnesium phosphate to calcium phosphate and reducing the content of magnesium phosphate impurities in dicalcium phosphate.

[0094] In some embodiments, the rotation speed in the conversion reaction is greater than 300 rpm, specifically 300 rpm, 500 rpm, 700 rpm, or any value greater than 300 rpm; the reaction time is 60 min to 120 min, specifically 60 min, 80 min, 100 min, 120 min, or any value between 60 min and 120 min. Vigorous stirring is beneficial for improving the uniform distribution of the components in the reaction system, and for increasing the reaction rate and dephosphorization effect.

[0095] In some embodiments, the molar ratio of calcium in the calcium chloride solution to phosphorus in the second-stage precipitate is (1.5–3.0):1 in the conversion reaction. Specifically, it can be any value between 1.5:1, 2:1, 2.5:1, 3.0:1, or (1.5–3.0):1, which is beneficial to the formation of dicalcium phosphate.

[0096] In some embodiments, the method further includes: adding calcium oxide or calcium hydroxide to the conversion reaction solution to carry out a regeneration reaction; after the regeneration reaction is completed, performing solid-liquid separation on the reaction solution to obtain a solid phase containing magnesium hydroxide, which can be utilized as a byproduct.

[0097] The above steps can also be used to wash the obtained dicalcium phosphate and magnesium hydroxide, and the washing solution can be recycled after treatment.

[0098] In some embodiments, the molar ratio of the added calcium element to the magnesium element in the conversion reaction solution in the regeneration reaction is (0.85-0.95):1, specifically it can be any value between 0.85:1, 0.90:1, 0.95:1 or (0.85-0.95):1, which improves the magnesium ion precipitation rate while avoiding the excessive use of calcium ions.

[0099] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0100] Example 1:

[0101] A method for utilizing residual acid, such as Figures 1-3 As shown, the process includes the following:

[0102] (1) A type of residual acid, the basic information of which is as follows:

[0103]

[0104] (2) Take 1000g of the above-mentioned residual acid, first add 50g of 50%wt dilute sulfuric acid to the acid, then add excess activated diatomaceous earth, stir the reaction thoroughly, filter, and separate the unreacted activated diatomaceous earth.

[0105] (3) Preheat the filtrate from step (2) to 50°C, add 64.42g of soda ash, stir to allow the soda ash to react fully with the filtrate, keep warm and stand for 90 minutes after the reaction is complete, then filter the reaction solution to separate the crude sodium fluorosilicate product, wash and dry to obtain the finished sodium fluorosilicate.

[0106] (4) Add 5g of hydrogen peroxide to the filtrate from step (3), stir to make them fully contact, and then use ammonia to adjust the pH of the filtrate to 3.2. After the reaction is complete, place the filtrate in an environment of 80℃ and keep it warm for 50 minutes. After aging, filter to separate the first-stage precipitate.

[0107] (5) Continue to add ammonia water to the filtrate from step (4) until the pH of the reaction solution reaches 4.8. After the reaction is complete, place the filtrate at 80°C and let it stand for 55 minutes. After aging, filter to separate the second-stage precipitate:

[0108] (6) Add ammonia and calcium sulfate dihydrate to the filtrate of step (5), control the pH of the reaction solution to 6.0, and after the reaction is complete, separate the precipitate in the reaction solution, wash and dry the precipitate to obtain calcium hydrogen phosphate 1. After the filtrate is evaporated and crystallized, filter to separate the generated ammonium sulfate crystals. Collect the mother liquor after separating the crystals and add it to the original residual acid.

[0109] (7) Take the first-stage precipitate separated in step (4), add 600g of process water to it to make slurry, place the slurry in a water bath at 50°C and heat it. Add 35%wt sodium hydroxide solution and 25%wt sodium aluminate (dry matter) to the preheated slurry to react. Maintain the pH of the slurry at 14.1. Stir and react for 75 minutes and then filter while hot to separate the leachate and dephosphorization residue.

[0110] (8) The obtained dephosphorization residue was thoroughly soaked in 235g of water and then filtered. The filtrate was collected and used to make pulp for the first precipitate and to prepare lime milk.

[0111] (9) Weigh 189.75g of calcium hydroxide according to the mass ratio of Ca:P = 1.30:1, add 736g of water (including the washing liquid from step 8) to make a slurry, and then react the lime slurry and the leachate collected in step (7) in a three-stage countercurrent manner to precipitate phosphorus in order to produce calcium phosphate.

[0112] Specifically, the effluent from the previous stage of precipitation reaction is used as the raw material for the next stage of reaction, and the reaction effluent from the third stage of precipitation is recycled and used to extract the next batch of precipitate residue.

[0113] (10) Add 830g of water to the calcium phosphate obtained in step (9) and wash thoroughly. After filtration and drying, calcium phosphate salt 2 is obtained. The filtrate is collected and used to make pulp for the first precipitate and to prepare lime milk.

[0114] (11) Prepare a 12.5% ​​wt calcium chloride solution with pH=4.5. Take 2240g of the solution and the second-stage precipitate separated in step (5). Perform the reaction conversion of the second-stage precipitate in the form of three-stage countercurrent. Separate the conversion product calcium hydrogen phosphate and the conversion reaction liquid. Add 50g of calcium oxide to the conversion reaction liquid to replace the magnesium ions. After the reaction is complete, filter to separate the generated magnesium hydroxide precipitate. The filtrate is recycled after treatment.

[0115] In particular, in a tertiary conversion reaction, the effluent from the previous conversion reaction is used as the feed liquid for the next conversion reaction;

[0116] (12) Add twice the mass of water to the dicalcium phosphate and magnesium hydroxide from step (11) and wash them. After washing and drying, dicalcium phosphate 3 and magnesium hydroxide samples are obtained. The washing solution is collected, desalted and recycled.

[0117] The composition of the sodium fluorosilicate obtained in the above embodiments is as follows:

[0118]

[0119] The above results indicate that the sample meets the quality requirements of Class I products in the national standard, and the free acid is included in the drying loss.

[0120] The composition of the two-stage precipitate in the above embodiments is as follows:

[0121] First stage precipitate sludge 44.94 11.87 3.62 0.07 3.25 1.42 0.75 1.26 Second stage precipitate sludge 38.21 0.02 0.0001 0.01 21.12 0.04 0.37 0.03

[0122] Under the above reaction conditions, the iron, aluminum, and fluoride ions in the residual acid are basically completely precipitated in the first stage of precipitation, and the second stage precipitate is magnesium phosphate.

[0123] The composition of the dephosphorization slag in the above embodiments is as follows:

[0124]

[0125] It is evident that the phosphorus element in the phosphate precipitate residue is almost completely extracted by the alkali solution during the leaching process, with a phosphorus leaching rate >99%. The dephosphorization residue is a mixture of hydroxides of metallic elements.

[0126] The composition of the calcium phosphate obtained in the above examples is as follows:

[0127] Calcium hydrogen phosphate 1 40.76 0.0108 0.0001 35.24 0.0107 0.0001 0.0001 0.2141 0.012 Calcium phosphate salt 2 38.21 0.20 0.12 51.38 0.13 0.0001 2.69 2.32 1.85 Calcium hydrogen phosphate 3 41.02 0.0008 0.0001 34.41 0.5211 0.0081 0.0021 0.0001 0.027

[0128] The content of harmful elements in dicalcium phosphate 1 and dicalcium phosphate 3 was analyzed, and the results are as follows:

[0129] Calcium hydrogen phosphate 1 0.0011 0.0005 0.0001 0.0006 Calcium hydrogen phosphate 3 0.0013 0.0005 0.0001 0.0013

[0130] It is evident that dicalcium phosphate 1 and dicalcium phosphate 3 meet the quality requirements for feed-grade dicalcium phosphate; calcium phosphate salt 2 is a mixture of fluorophosphate and hydroxyapatite, and its composition meets the quality requirements for wet-process phosphoric acid concentrate.

[0131] The magnesium hydroxide obtained in the above examples was analyzed according to the HG / T 3607-2007 standard:

[0132]

[0133] Example 2:

[0134] A method for utilizing residual acid includes the following steps:

[0135] (1) A type of residual acid, the basic information of which is as follows:

[0136]

[0137] (2) Take 1000g of the above-mentioned residual acid, first add 125g of 40%wt dilute sulfuric acid to the acid, then add excess activated diatomaceous earth, stir and react thoroughly, filter, and separate the unreacted activated diatomaceous earth.

[0138] (3) Preheat the filtrate from step (2) to 55°C, add 110g of sodium bicarbonate to it, stir to allow the sodium bicarbonate to react fully with the filtrate, and after the reaction is complete, keep it warm and stand for 105 minutes. Then filter the reaction solution to separate the crude sodium fluorosilicate product, wash and dry it to obtain the finished sodium fluorosilicate.

[0139] (4) Add 6g of hydrogen peroxide to the filtrate from step (3), stir to make the two fully contact, and then use sodium hydroxide to adjust the pH of the filtrate to 3.5. After the reaction is complete, place the filtrate in an environment of 85℃ and keep it in the environment for 45 minutes to age. After aging, filter to separate the first-stage precipitate.

[0140] (5) Add sodium hydroxide to the filtrate from step (4) until the pH of the reaction solution reaches 5.0. After the reaction is complete, place the filtrate at 75°C and let it stand for 60 minutes. After aging, filter to separate the second-stage precipitate:

[0141] (6) Add calcium hydroxide to the filtrate of step (5), control the pH of the reaction solution to 5.6, and after the reaction is complete, separate the precipitate in the reaction solution, wash and dry the precipitate to obtain calcium hydrogen phosphate 1. After the filtrate is evaporated and crystallized, filter to separate the generated sodium sulfate decahydrate crystals. Collect the mother liquor after separating the crystals and add it to the original residual acid.

[0142] (7) Take the first-stage precipitate separated in step (4), add 600g of process water to it to make slurry, place the slurry in a water bath at 45°C for heating, add 40%wt sodium hydroxide solution and 25%wt sodium aluminate (dry matter) to the preheated slurry for reaction, maintain the pH of the slurry ≈ 14.0, stir and react for 60min, filter while hot, and separate the leachate and dephosphorization residue;

[0143] (8) The obtained dephosphorization residue was thoroughly soaked in 260g of water and then filtered. The filtrate was collected and used to make pulp for the first precipitate and to prepare lime milk.

[0144] (9) Weigh 193g of calcium hydroxide according to the mass ratio of Ca:P = 1.25:1, add 800g of water (including the washing liquid from step 8) to make a slurry, and then react the lime slurry and the leachate collected in step (7) in a three-stage countercurrent manner to precipitate phosphorus in order to produce calcium phosphate.

[0145] Specifically, the effluent from the previous stage of precipitation reaction is used as the raw material for the next stage of reaction, and the reaction effluent from the third stage of precipitation is recycled and used to extract the next batch of precipitate residue.

[0146] (10) Add 850g of water to the calcium phosphate obtained in step (9) and wash thoroughly. After filtration and drying, calcium phosphate salt 2 is obtained. The filtrate is collected and used to make pulp for the first precipitate and to prepare lime milk.

[0147] (11) Prepare a 15% wt calcium chloride solution with pH = 4.5. Take 1736g of the solution and the second-stage precipitate separated in step (5). Perform the reaction conversion of the second-stage precipitate in the form of three-stage countercurrent. Separate the conversion product calcium hydrogen phosphate and the conversion reaction liquid. Add 45g of calcium oxide to the conversion reaction liquid to replace the magnesium ions. After the reaction is complete, filter to separate the generated magnesium hydroxide precipitate. The filtrate is recycled after treatment.

[0148] In particular, in a tertiary conversion reaction, the effluent from the previous conversion reaction is used as the feed liquid for the next conversion reaction;

[0149] (12) Add twice the mass of water to the dicalcium phosphate and magnesium hydroxide from step (11) and wash them. After washing and drying, dicalcium phosphate 3 and magnesium hydroxide samples are obtained. The washing solution is collected, desalted and recycled.

[0150] The composition of the sodium fluorosilicate obtained in the above embodiments is as follows:

[0151]

[0152] The above results indicate that the sample meets the quality requirements of Class I products in the national standard.

[0153] The composition of the two-stage precipitate in the above embodiments is shown in the table below.

[0154] First stage precipitate sludge 42.12 11.22 3.43 0.05 3.11 2.57 0.56 1.88 Second stage precipitate sludge 39.20 0.0012 0.0001 0.02 21.88 0.14 0.31 0.003

[0155] Under the above reaction conditions, the iron, aluminum, and fluoride ions in the residual acid are basically completely precipitated in the first stage of precipitation, and the second stage precipitate is magnesium phosphate.

[0156] The composition of the dephosphorization slag in the above embodiments is as follows:

[0157]

[0158] It is evident that the phosphorus element in the phosphate precipitate residue is almost completely extracted by the alkali solution during the leaching process, with a phosphorus leaching rate >99%. The dephosphorization residue is a mixture of hydroxides of metallic elements.

[0159] The composition of the calcium phosphate obtained in the above examples is as follows:

[0160] Calcium hydrogen phosphate 1 40.82 0.0117 0.0001 35.04 0.0092 0.0001 0.0074 0.2343 0.001 Calcium phosphate salt 2 37.96 0.24 0.06 52.31 0.11 0.0001 2.82 1.67 2.11 Calcium hydrogen phosphate 3 40.84 0.0018 0.0001 34.12 0.6246 0.0081 0.0124 0.0001 0.003

[0161] The content of harmful elements in dicalcium phosphate 1 and dicalcium phosphate 3 was analyzed, and the results are as follows:

[0162]

[0163] It is evident that dicalcium phosphate 1 and dicalcium phosphate 3 meet the quality requirements for feed-grade dicalcium phosphate; calcium phosphate salt 2 is a mixture of fluorophosphate and hydroxyapatite, and its composition meets the quality requirements for wet-process phosphoric acid concentrate.

[0164] The magnesium hydroxide obtained in the above examples was analyzed according to the HG / T 3607-2007 standard:

[0165]

[0166] Comparative Example 1:

[0167] A method for utilizing residual acid includes the following steps:

[0168] (1) A type of residual acid, the basic information of which is as follows:

[0169]

[0170] (2) Take 1000g of the above-mentioned residual acid, add it to the excess activated diatomaceous earth, stir and react thoroughly, then filter to separate the unreacted activated diatomaceous earth.

[0171] (3) Preheat the filtrate from step (2) to 50°C, add 110g of sodium bicarbonate to it, stir to allow the sodium bicarbonate to react fully with the filtrate, and after the reaction is complete, keep it warm and stand for 90 minutes. Then filter the reaction solution to separate the crude sodium fluorosilicate product, wash and dry it to obtain the finished sodium fluorosilicate.

[0172] The composition of the sodium fluorosilicate obtained in the above comparative examples is shown below:

[0173]

[0174] If the residual acid contains excessively high levels of metal cation impurities, the acidity of the residual acid will change during the reaction of sodium bicarbonate with fluorosilicic acid in the acid. This will increase the ionization rate of phosphoric acid, and the newly ionized phosphate ions will combine with metal ions (mainly iron and aluminum) in the solution to form phosphate precipitates, which will then co-precipitate with sodium fluorosilicate. This will prevent the production of the desired sodium fluorosilicate product. Therefore, it is necessary to add a portion of strong acid to the residual acid to suppress the ionization of phosphoric acid. Similarly, some fluoride in the residual acid exists as fluoride ions, which need to be pre-converted.

[0175] Comparative Example 2:

[0176] (1) A type of residual acid, the basic information of which is as follows:

[0177]

[0178] (2) Take 1000g of the above-mentioned residual acid, first add 125g of 40%wt dilute sulfuric acid to the acid, then add excess activated diatomaceous earth, stir and react thoroughly, filter, and separate the unreacted activated diatomaceous earth.

[0179] (3) Preheat the filtrate from step (2) to 55°C, add 110g of sodium bicarbonate to it, stir to allow the sodium bicarbonate to react fully with the filtrate, and after the reaction is complete, keep it warm and stand for 105 minutes. Then filter the reaction solution to separate the crude sodium fluorosilicate product, wash and dry it to obtain the finished sodium fluorosilicate.

[0180] (4) Add 6g of hydrogen peroxide to the filtrate from step (3), stir to ensure full contact between the two, and then adjust the pH of the filtrate to 2.8 with ammonia. After the reaction is complete, place the filtrate at 85°C for 45 minutes to keep it warm and stand for aging. After aging, filter to separate the first-stage precipitate.

[0181] (5) Add ammonia water to the filtrate from step (4) until the pH of the reaction solution reaches 5.0. After the reaction is complete, place the filtrate at 75°C for 60 minutes to stand and age. After aging, filter to separate the second-stage precipitate.

[0182] The composition of the two-stage precipitate in the above comparative example is shown below:

[0183] First stage precipitate sludge 40.14 12.01 3.32 0.05 1.21 0.18 0.56 1.13 Second stage precipitate sludge 38.42 1.88 1.09 0.02 19.04 0.28 0.48 0.34

[0184] If the pH of the primary neutralization reaction is too low, iron and aluminum will not precipitate completely. The unprecipitated iron and aluminum will enter the secondary precipitate, preventing the secondary precipitate from being recycled according to the established process. Similarly, if the pH of the primary neutralization reaction is too high, some magnesium phosphate will precipitate prematurely, resulting in a reduction in the amount of secondary precipitate and affecting the recovery of magnesium resources.

[0185] Comparative Example 4:

[0186] A method for utilizing residual acid includes the following steps:

[0187] (1) A type of residual acid, the basic information of which is as follows:

[0188]

[0189] (2) Take 1000g of the above-mentioned residual acid, first add 50g of 50%wt dilute sulfuric acid to the acid, then add excess activated diatomaceous earth, stir and react thoroughly, filter, and separate the unreacted activated diatomaceous earth.

[0190] (3) Preheat the filtrate from step (2) to 50°C, add 64.42g of soda ash, stir to allow the soda ash to react fully with the filtrate, keep warm and stand for 90 minutes after the reaction is complete, then filter the reaction solution to separate the crude sodium fluorosilicate product, wash and dry to obtain the finished sodium fluorosilicate.

[0191] (4) Add 5g of hydrogen peroxide to the filtrate from step (3), stir to make them fully contact, and then use ammonia to adjust the pH of the filtrate to 3.2. After the reaction is complete, place the filtrate in an environment of 80℃ and keep it warm for 50 minutes. After aging, filter to separate the first-stage precipitate.

[0192] (5) Continue to add ammonia water to the filtrate from step (4) until the pH of the reaction solution reaches 4.8. After the reaction is complete, place the filtrate at 80°C and let it stand for 55 minutes. After aging, filter to separate the second-stage precipitate:

[0193] (6) Add ammonia and calcium sulfate dihydrate to the filtrate of step (5), control the pH of the reaction solution to 6.0, and after the reaction is complete, separate the precipitate in the reaction solution, wash and dry the precipitate to obtain calcium hydrogen phosphate 1. After the filtrate is evaporated and crystallized, filter to separate the generated ammonium sulfate crystals. Collect the mother liquor after separating the crystals and add it to the original residual acid.

[0194] (7) Take the first-stage precipitate separated in step (4), add 600g of process water to it to make slurry, place the slurry in a water bath at 50°C and heat it. Then add 35%wt sodium hydroxide solution to the hot slurry and react until the pH of the slurry is ≈14.1. Stir and react for 75 minutes and then filter while hot to separate the leachate and dephosphorization residue.

[0195] (8) The obtained dephosphorization residue was thoroughly soaked in 235g of water and then filtered. The filtrate was collected and used to make pulp for the first precipitate and to prepare lime milk.

[0196] (9) Weigh 189.75g of calcium hydroxide according to the mass ratio of Ca:P = 1.30:1, add 736g of water (including the washing liquid from step 8) to make a slurry, and then react the lime slurry obtained with the leachate collected in step (7) directly. After the reaction is complete, filter to separate the calcium phosphate precipitate and the dephosphorization alkali solution. The dephosphorization alkali solution is recycled and used to extract the next batch of precipitate residue.

[0197] (10) Add 830g of water to the calcium phosphate obtained in step (9) and wash thoroughly. After filtration and drying, calcium phosphate salt 2 is obtained. The filtrate is collected and used to make pulp for the first precipitate and to prepare lime milk.

[0198] (11) Prepare a 12.5% ​​wt, pH=4.5 calcium chloride solution. Take 2240g of the solution and react it directly with the second-stage precipitate separated in step (5). After the reaction is complete, filter to separate the reaction product calcium hydrogen phosphate and the reaction solution. Add 40g of calcium oxide to the reaction solution to regenerate magnesium ions in the reaction solution. After the reaction is complete, filter to separate the generated magnesium hydroxide. The filtrate is recycled and reused.

[0199] (12) Add twice the mass of water to the dicalcium phosphate and magnesium hydroxide from step (11) and wash them. After washing and drying, dicalcium phosphate 3 and magnesium hydroxide samples are obtained. The washing solution is collected, desalted and recycled.

[0200] The composition of the sodium fluorosilicate obtained in the above embodiments is as follows:

[0201]

[0202] The above results indicate that the sample meets the quality requirements of Class I products in the national standard.

[0203] The composition of the two-stage precipitate in the above embodiments is shown in the table below.

[0204]

[0205] Under the above reaction conditions, the iron, aluminum, and fluoride ions in the residual acid are basically completely precipitated in the first stage of precipitation, and the second stage precipitate is magnesium phosphate.

[0206] The composition of the dephosphorization slag in the above embodiments is as follows:

[0207]

[0208] Comparative Example 4 used only sodium hydroxide to extract the first-stage precipitate residue. The composition of the dephosphorized residue after leaching is shown in the table above. Compared with the dephosphorized residue obtained in Example 1, the aluminum content in this dephosphorized residue has decreased, indicating that some aluminum in the precipitate residue was extracted by the alkaline solution during the leaching process. It also indicates that the aluminate in the reaction solution has a certain inhibitory effect on the dissolution of aluminum.

[0209] The composition of the calcium phosphate obtained in the above examples is as follows:

[0210]

[0211]

[0212] The content of harmful elements in dicalcium phosphate 1 and dicalcium phosphate 3 was analyzed, and the results are as follows:

[0213] Calcium hydrogen phosphate 1 0.0012 0.0003 0.0001 0.0011 Calcium hydrogen phosphate 3 0.0012 0.0001 0.0001 0.0008

[0214] Compared with Example 1, the aluminum content in sample 2 of calcium phosphate salt increased significantly, indicating that the single-stage precipitation process is not as effective as the multi-stage precipitation process in separating phosphorus and aluminum in the leachate.

[0215] Compared with Example 1, the magnesium content in sample 3 of dicalcium phosphate increased significantly, indicating that the magnesium conversion rate in a single-stage conversion process is lower than that in a multi-stage conversion process.

[0216] The magnesium hydroxide obtained in the above examples was analyzed according to the HG / T 3607-2007 standard:

[0217]

[0218] Industrial applicability

[0219] This disclosure provides a method for purifying residual phosphate raffinate. First, fluorine is removed from the raffinate. Then, a two-stage neutralization reaction is performed. The primary purpose of the first-stage neutralization is to precipitate iron, aluminum, a small amount of magnesium, and residual fluorine from the raffinate. The purpose of the second-stage neutralization is to precipitate magnesium from the first-stage filtrate, resulting in a purified filtrate with fewer impurities. Phosphate ions are then precipitated from the purified filtrate, resulting in a phosphate precipitate with fewer impurities. This method has no requirements on the composition of the raffinate, has a wide range of applications, and the two-stage neutralization process separates and removes fluorine, magnesium, and other impurities from the raffinate, facilitating the recovery and utilization of these impurities. Simultaneously, it yields high-quality dicalcium phosphate.

[0220] The method disclosed herein is relatively simple, converting insoluble phosphate salts with low utilization value into calcium phosphate salts, and recovering phosphorus, fluorine, and magnesium elements from the raffinate, thereby improving resource utilization.

Claims

1. A method for purifying residual acid from phosphoric acid extraction, characterized in that, include: After a neutralization process, an oxidant is added to the defluorinated residual acid and the pH is adjusted to 3.0-3.

5. After a neutralization reaction, solid-liquid separation is performed to obtain the first-stage precipitate and the first-stage filtrate. The first-stage precipitate contains a complex phosphate containing iron, aluminum, magnesium and fluorine. Two-stage neutralization: After adjusting the pH of the first-stage filtrate to 4.5-5.0 and performing a two-stage neutralization reaction, solid-liquid separation is performed to obtain a second-stage precipitate and a purified filtrate. The second-stage precipitate includes magnesium phosphate. The impurity-removing filtrate is recovered by adding a second precipitant to the filtrate and adjusting the pH to 5.5-6.5 to carry out the impurity removal reaction. After solid-liquid separation, phosphate precipitate and dephosphorization mother liquor are obtained. The second precipitant is at least one of calcium oxide, calcium hydroxide and calcium sulfate. It also includes defluorination, the defluorination step of which includes: An intermediate acid and activated diatomaceous earth are added to the residual raffinate to convert free fluoride ions into fluorosilicate ions. After solid-liquid separation, a supernatant is obtained. The intermediate acid is at least one of sulfuric acid and hydrochloric acid. A first precipitant is added to the supernatant to carry out a defluorination reaction, resulting in fluorosilicate precipitate and residual acid after defluorination. The first precipitant is a sodium-containing compound.

2. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, The oxidant is at least one of ozone, oxygen, and hydrogen peroxide; And / or, the amount of the oxidant used is 0.1%wt to 1%wt of the mass of the residual acid.

3. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, In the first-stage neutralization step and / or the second-stage neutralization step and / or the impurity removal filtrate recovery step, a neutralizing agent is used to adjust the pH. The neutralizing agent is at least one of ammonia, sodium hydroxide, potassium hydroxide, and carbonates, bicarbonates, orthophosphates, and monohydrogen phosphates of potassium, sodium, and ammonium. And / or, after the first-stage and / or second-stage neutralization reaction is completed, the reaction solution is aged at 75℃~85℃ for 45min~60min, and then solid-liquid separation is performed.

4. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, The molar ratio of calcium in the second precipitant to phosphorus in the purified filtrate is (0.90~0.95):

1.

5. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, The first precipitant is sodium hydroxide.

6. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, The first precipitant is at least one of sodium carbonate, sodium bicarbonate, sodium phosphate, or sodium sulfate.

7. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, The molar ratio of sodium in the first precipitant to fluorine in the supernatant is (0.4-0.6):

1.

8. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, The temperature for the defluorination reaction is 40℃~60℃.

9. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, The mass ratio of H2SO4 in the intermediate acid to the residual acid is 1%wt to 10%wt.

10. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, The dephosphorization mother liquor was evaporated and crystallized to obtain sulfate crystals.

11. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, It also includes the recycling of the first-stage sediment: after the first-stage sediment is pulped, alkali is added to it to carry out a phosphorus extraction reaction, and then solid-liquid separation is carried out to obtain phosphorus-containing filtrate and dephosphorized residue; Calcium ions are added to the phosphorus-containing filtrate to carry out a dephosphorization reaction, resulting in a calcium salt precipitate including calcium fluorophosphate and calcium hydroxyphosphate.

12. The method for treating residual acid from phosphoric acid extraction according to claim 11, characterized in that, The solid content of the slurry obtained after the first sedimentation residue is 20%wt~50%wt.

13. The method for treating residual acid from phosphoric acid extraction according to claim 11, characterized in that, The alkali used in the phosphorus extraction reaction is a concentrated alkali solution of 30%wt to 40%wt.

14. The method for treating residual acid from phosphoric acid extraction according to claim 11, characterized in that, The alkali used is at least one of sodium hydroxide and potassium hydroxide.

15. The method for treating residual acid from phosphoric acid extraction according to claim 11, characterized in that, The alkali also includes sodium aluminate and / or potassium aluminate.

16. The method for treating residual acid from phosphoric acid extraction according to claim 11, characterized in that, The phosphorus extraction reaction was carried out at a pH of 14.0-14.2, a reaction time of 60-90 min, and a reaction temperature of 20℃-60℃.

17. The method for treating residual acid from phosphoric acid extraction according to claim 11, characterized in that, In the dephosphorization reaction, the calcium ions are added to the phosphorus-containing filtrate in the form of lime milk, and the solid content of the lime milk is 20%wt~30%wt.

18. The method for treating residual acid from phosphoric acid extraction according to claim 11, characterized in that, The phosphorus-containing filtrate is precipitated using a countercurrent precipitation method, with 2 to 4 countercurrent precipitation stages.

19. The method for treating residual acid from phosphoric acid extraction according to claim 11, characterized in that, In the dephosphorization reaction, lime slurry is added from the last stage, phosphorus-containing filtrate is added from the first stage, the effluent from the previous stage of the countercurrent precipitation reaction is used as the raw material for the next stage of the reaction, and the precipitate produced in the next stage of the precipitation reaction is used as the dephosphorization precipitant for the previous stage of the precipitation reaction. The precipitate obtained from the first stage of the reaction is a calcium salt precipitate including calcium fluorophosphate and calcium hydroxyphosphate.

20. The method for treating residual acid from phosphoric acid extraction according to claim 19, characterized in that, In the dephosphorization reaction, the liquid discharged from the previous precipitation reaction is separated into solids and then concentrated and supplemented with alkali.

21. The method for treating residual acid from phosphoric acid extraction according to claim 17, characterized in that, In the dephosphorization reaction, the molar ratio of calcium in the lime slurry to phosphorus in the phosphorus-containing filtrate is (1.15~1.55):

1.

22. The method for treating residual acid from phosphoric acid extraction according to claim 21, characterized in that, In the dephosphorization reaction, the rotation speed is greater than 300 rpm, the reaction temperature is 20℃~60℃, and the reaction time is 30min~60min.

23. The method for treating residual acid from phosphoric acid extraction according to claim 1, characterized in that, It also includes the recycling of the second-stage precipitate: the second-stage precipitate is mixed with calcium chloride solution to carry out a conversion reaction to obtain calcium hydrogen phosphate and conversion reaction solution.

24. The method for treating residual acid from phosphoric acid extraction according to claim 23, characterized in that, The calcium chloride solution has a calcium chloride mass fraction of 12%wt~13%wt and a pH of 4.3~4.

7.

25. The method for treating residual acid from phosphoric acid extraction according to claim 23, characterized in that, The conversion reaction is carried out using countercurrent precipitation, with 2 to 4 stages of countercurrent precipitation.

26. The method for treating residual acid from phosphoric acid extraction according to claim 23, characterized in that, In the conversion reaction, the molar ratio of calcium in the calcium chloride solution to phosphorus in the second-stage precipitate is (1.5~3.0):

1.

27. The method for treating residual acid from phosphoric acid extraction according to claim 23, characterized in that, In the conversion reaction, the rotation speed is greater than 300 rpm and the reaction time is 60 min to 120 min.

28. The method for treating residual acid from phosphoric acid extraction according to claim 23, characterized in that, Also includes: Calcium oxide or calcium hydroxide is added to the conversion reaction solution to carry out a regeneration reaction. After the regeneration reaction is completed, the reaction solution is subjected to solid-liquid separation to obtain a solid phase containing magnesium hydroxide.

29. The method for treating residual acid from phosphoric acid extraction according to claim 28, characterized in that, In the regeneration reaction, the molar ratio of the added calcium element to the magnesium element in the conversion reaction solution is (0.85~0.95):

1.

30. The method for treating residual acid from phosphoric acid extraction according to claim 23, characterized in that, In the conversion reaction, calcium chloride solution is added from the first stage and discharged from the last stage, and the second-stage precipitate is added from the last stage and discharged from the first stage. The precipitate discharged from the first stage is a solid phase containing calcium hydrogen phosphate.

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