WO2025010530A1 - Method for recycling raffinate acid in process of wet-process purification phosphoric acid production - Google Patents

Abstract

The present disclosure belongs to the technical field of battery raw materials. Disclosed is a method for recycling raffinate acid in a process of wet-process purification phosphoric acid production. The method comprises: removing sulfur, fluorine, arsenic and heavy metals in raffinate acid to be treated, so as to obtain pretreated acid; pre-neutralizing the pretreated acid; then performing purification to obtain a purified liquid and precipitate residues containing iron, aluminum and magnesium; and rectifying the purified liquid to obtain industrial-grade phosphoric acid. The method can achieve effective recycling on the phosphorus resource in the raffinate acid, and the produced industrial-grade phosphoric acid can be used as a raw material to prepare battery-grade phosphates.

Description

A method for recycling raffinate in the process of wet purification of phosphoric acid Technical Field

The present disclosure relates to the technical field of battery raw materials, and in particular to a method for recycling raffinate in a process of producing phosphoric acid by wet purification.

Background Art

There are two methods for producing industrial-grade phosphoric acid: thermal method and wet phosphoric acid solvent extraction method (referred to as "wet purification method"). Among them, the thermal method, also known as the yellow phosphorus method, uses yellow phosphorus as the raw material, first burns the yellow phosphorus and then uses pure water to absorb the generated phosphorus pentoxide to directly generate phosphoric acid. The wet phosphoric acid solvent extraction method uses wet phosphoric acid as the raw material, and produces phosphoric acid with higher purity after a series of chemical-organic solvent coordinated purification treatments.

Phosphoric acid produced by the wet phosphoric acid solvent extraction method can meet the requirements of most industrial application scenarios. The rapid development of the new energy industry has greatly boosted the demand for battery-grade high-purity phosphates represented by iron phosphate. As the main raw material for the production of such phosphates, the demand for industrial-grade phosphoric acid has also grown rapidly along with this battery-grade phosphate.

Raffinate acid is the main byproduct in the solvent extraction purification process of wet phosphoric acid, and its production accounts for 40%-45% of the mass of the raw phosphoric acid, which is a large amount. In the solvent extraction process, the phosphoric acid molecules in the crude phosphoric acid interact with the extractant and are extracted by the extractant. The impurity ions in the acid (such as iron, aluminum, magnesium, sulfate, fluorine, arsenic and heavy metal ions, etc.) are partially or completely retained in the raffinate acid because they are less or do not interact with the extractant at all, so that the impurity ion content in the raffinate acid is doubled compared with the original phosphoric acid, which greatly limits the application scope of the raffinate acid. In addition, at present, the traditional raffinate acid treatment method is usually to mix the raffinate acid with ordinary wet phosphoric acid in a mass ratio of 1: (2-5) to produce traditional fertilizers represented by monoammonium phosphate, diammonium phosphate, and compound fertilizer. This method will produce a large amount of low-grade fertilizers, which is not conducive to the sustainable development of the phosphorus chemical industry.

In view of this, the present disclosure is proposed.

Summary of the invention

The purpose of the present disclosure includes providing a method for recycling raffinate in the process of producing phosphoric acid by wet purification.

The present disclosure can be implemented as follows:

The present invention provides a method for recycling raffinate acid in a wet purification process for producing phosphoric acid, comprising the following steps: removing sulfur, fluorine, arsenic and heavy metals in the raffinate acid to be treated to obtain pretreated acid; pre-neutralizing the pretreated acid, followed by purification to obtain purified liquid and precipitated residue containing iron, aluminum and magnesium, and distilling the purified liquid to obtain industrial-grade phosphoric acid.

In an optional embodiment, the raffinate acid to be treated is first subjected to rough desulfurization to obtain a first intermediate acid; fluorine, arsenic and heavy metals in the first intermediate acid are removed to obtain a second intermediate acid; and the second intermediate acid is subjected to fine desulfurization to obtain a pretreated acid.

In an optional embodiment, the raffinate acid to be treated is mixed with a crude desulfurization agent to perform crude desulfurization;

The crude desulfurizing agent includes at least one of phosphate concentrate, calcium carbonate, calcium hydroxide and calcium oxide.

In an optional embodiment, the molar ratio of calcium in the crude desulfurizer to sulfate in the raffinate acid is (0.8:1)-(1.0:1).

In an alternative embodiment, the first intermediate acid is mixed with a defluorinating agent, a sulfide, and a filter aid to remove fluorine, arsenic, and heavy metals.

In an optional embodiment, the defluorination agent includes at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium phosphate, disodium hydrogen phosphate and sodium dihydrogen phosphate, and activated diatomaceous earth;

or, the sulfide comprises at least one of sodium sulfide, calcium sulfide, potassium sulfide and phosphorus pentasulfide;

Alternatively, the filter aid comprises activated carbon.

In an optional embodiment, the molar ratio of Na in the defluorinating agent to F in the first intermediate acid is (0.4:1)-(0.8:1);

Or, the molar ratio of As in the first intermediate acid to S in the sulfide is (1:15)-(1:75);

Alternatively, the filter aid is used in an amount of 0.5 wt % to 5 wt % of the first intermediate acid.

In an optional embodiment, the second intermediate acid is mixed with an oxidant and a fine desulfurization agent to perform fine desulfurization to obtain a pretreated acid and fine desulfurization slag;

The fine desulfurizing agent includes at least one of barium hydroxide and barium carbonate.

In an optional embodiment, the oxidant is used in an amount of 0.5 wt% to 1.5 wt% of the second intermediate acid;

Alternatively, the molar ratio of the barium in the fine desulfurizing agent to the sulfate in the second intermediate acid is (1.5:1)-(2.5:1).

In an optional embodiment, the fine desulfurization slag is returned to the rough desulfurization process to be used as a rough desulfurization agent.

In an optional embodiment, before pre-neutralization, the pre-treated acid is concentrated and solid-liquid separated to obtain concentrated pre-treated acid and a first precipitated residue containing iron phosphate and aluminum phosphate;

The content of _P2O5 in the concentrated pre-treated _acid is not less than 40 wt%.

In an optional embodiment, the concentrated pretreatment acid is mixed with a neutralizing agent for pre-neutralization to obtain a pre-neutralized reaction solution;

The neutralizing agent includes salts of potassium, sodium, ammonium, and at least one of ammonia, sodium hydroxide, and potassium hydroxide.

In an alternative embodiment, the potassium, sodium, and ammonium salts are in the form of at least one of carbonates, bicarbonates, and phosphates;

Alternatively, the molar ratio of M in the neutralizing agent to phosphorus in the concentrated pretreatment acid is (0.2:1)-(0.4:1), where M corresponds to ammonia, sodium and/or potassium contained in the neutralizing agent.

In an optional embodiment, the purification process includes: mixing the pre-neutralization reaction liquid with a first precipitant to perform a first purification, and separating the solid and the liquid to obtain a first purified liquid and a second precipitated residue containing iron phosphate, aluminum phosphate, and magnesium phosphate;

The first purification process includes at least one of the following characteristics:

Feature 1: The first precipitant includes at least one of methanol, ethanol, propanol, butanol and acetone;

Feature 2: The usage amount of the first precipitant is 1 to 2.5 times of the pre-neutralization reaction solution;

Feature 3: The temperature of the first purification is 25℃-65℃.

In an optional embodiment, the purification process further includes: mixing the first purification liquid with a detergent to perform a second purification, and separating the solid and the liquid to obtain a second purification liquid and a first washing liquid;

The second purification process includes at least one of the following characteristics:

Feature 1: The solute in the detergent includes at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, potassium dihydrogen phosphate, potassium monohydrogen phosphate and potassium phosphate;

Feature 2: The mass fraction of solute in the detergent is not less than 10%;

Feature 3: The molar ratio of M in the detergent to P in the first purification solution is (0.15:1)-(0.3:1), corresponding to the sodium and/or potassium contained in the detergent.

In an optional embodiment, the first washing liquid is returned to the pre-neutralization process to be used as a neutralizing agent.

In an optional embodiment, the purification process further comprises: mixing the second purified liquid with the second precipitant to perform a third purification, and performing solid-liquid separation to obtain a third purified liquid and a third precipitated residue containing a small amount of magnesium phosphate and alkali metal phosphate;

The third purification process includes at least one of the following characteristics:

Feature 1: The second precipitant includes at least one of methanol, ethanol, propanol, butanol and acetone;

Feature 2: The usage amount of the second precipitant is 20wt%-100wt% of the second purified liquid.

In an optional embodiment, the third precipitated slag is returned to the first purification process, or the third precipitated slag is mixed with the first precipitated slag and the second precipitated slag and then pulped and recovered.

In an optional embodiment, after the third purification liquid is distilled, industrial-grade phosphoric acid is obtained.

In an optional embodiment, the third precipitated residue is mixed with the first precipitated residue and the second precipitated residue and then pulped and recovered, comprising:

The first precipitated residue, the second precipitated residue and the third precipitated residue are mixed with water and a first alkaline substance, neutralized and precipitated to obtain a first slurry; the first slurry is subjected to solid-liquid separation to obtain a separation liquid and phosphate precipitated residue.

In an optional embodiment, the pulp recovery includes at least one of the following features:

Feature 1: the water used for mixing with the first precipitated slag, the second precipitated slag and the third precipitated slag is desalted water;

Feature 2: The mass ratio of water to the total amount of the first precipitated residue, the second precipitated residue and the third precipitated residue is (2:1)-(5:1);

Feature 3: The first alkaline substance includes at least one of sodium hydroxide, potassium hydroxide, sodium orthophosphate and potassium orthophosphate;

Feature 4: The pH value of the first slurry is 5.5-8.0;

Feature 5: The separated liquid is reused as a detergent in the second purification process, or as a neutralizing agent in the pre-neutralization process, or as a defluorinating agent in the defluorinating process.

In an optional embodiment, the phosphate precipitation residue is mixed with water to obtain a second slurry; the second slurry is mixed with a second alkaline substance and subjected to secondary cross-current leaching and solid-liquid separation to obtain dephosphorization residue and orthophosphate mother liquor.

In an optional embodiment, the mass ratio of phosphate precipitate residue to water is (1:4)-(1:9);

Alternatively, the second alkaline substance includes at least one of sodium hydroxide and potassium hydroxide.

In an optional embodiment, the secondary cross-flow leaching includes: performing a first-stage leaching on the second slurry and the second alkaline substance to obtain a first-stage leachate and a first sediment residue;

The first stage leaching process includes at least one of the following features:

Feature 1: The pH value of the reaction solution during the leaching process is 13-14;

Feature 2: Leaching temperature is 50℃-85℃;

Feature 3: The residence time of the reaction slurry is 25min-45min;

Feature 4: The mass ratio of the first-stage leachate to the first-stage sediment is (3:1)-(9:1).

In an optional embodiment, part of the orthophosphate mother liquor obtained from the first stage leaching is recycled to be mixed with the first precipitated slag, the second precipitated slag and the third precipitated slag.

In an optional embodiment, the secondary cross-flow leaching further comprises: performing a second-stage leaching on the first sediment and the second alkaline substance to obtain a second-stage leachate and a second sediment;

The second stage leaching process includes at least one of the following features:

Feature 1: The pH value of the reaction solution during the leaching process is 14.0-14.3;

Feature 2: Leaching temperature is 45℃-55℃;

Feature 3: The residence time of the reaction slurry is 60min-90min;

Feature 4: The mass ratio of the second stage leachate to the second sediment is (5:1)-(9:1).

In an optional embodiment, the second precipitated slag is subjected to alkali washing, and after solid-liquid separation, a first washing liquid and a first slag slurry are obtained;

The first washing liquid is recycled to the second stage leaching process; the first slurry is washed with water, and the solid and liquid are separated to obtain the second washing liquid and the dephosphorization slag.

In an optional embodiment, the second washing liquid is recycled to the alkali washing process of the second slag.

In an optional embodiment, lime milk and the second-stage leachate are subjected to a first double decomposition reaction to precipitate phosphate in the secondary leachate, and the solid and liquid are separated to obtain a second slurry; the second slurry is subjected to a second double decomposition reaction with the first-stage leachate, and the solid and liquid are separated to obtain an alkali solution and calcium hydroxyphosphate.

In an alternative embodiment, the milk of lime is obtained by mixing a precipitant with water;

The precipitating agent includes at least one of calcium oxide and calcium hydroxide.

In an optional embodiment, the mass ratio of the precipitant to water is (1:3)-(1:5).

In an optional embodiment, the molar ratio of calcium in the precipitant to phosphorus in the second stage leachate is (1.5:1)-(1.6:1).

In an alternative embodiment, the temperature of the first metathesis reaction and the second metathesis reaction is independently 40° C.-60° C., or the time of the first metathesis reaction and the second metathesis reaction is independently 30 min-60 min.

In an optional embodiment, the calcium hydroxyphosphate is washed; and the second washing liquid obtained after washing the calcium hydroxyphosphate is collected and returned to the precipitant slurrying and milking process.

In an optional embodiment, the suspended solids in the alkali solution after the double decomposition and dephosphorization are removed and concentrated to obtain a concentrated alkali solution; the concentrated alkali solution is reused to perform leaching treatment on the second slurry.

The beneficial effects of the present disclosure include:

The method for recycling raffinate provided by the present invention does not need to rely on a fertilizer processing plant. Through reasonable process design, phosphorus is selectively extracted from complex phosphate precipitates containing iron, aluminum, and magnesium, and the phosphorus in the raffinate is converted into industrial-grade phosphoric acid to the maximum extent, thereby achieving effective recycling of the phosphorus resources in the raffinate. The obtained industrial-grade phosphoric acid can be used as a raw material to prepare battery-grade phosphate.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

FIG1 is a process flow chart of the pretreatment and purification process of raffinate in Example 1 of the present disclosure;

FIG2 is a flow chart of the recycling process of the first precipitated residue, the second precipitated residue and the third precipitated residue in Example 1 of the present disclosure.

DETAILED DESCRIPTION

In order to make the purpose, technical scheme and advantages of the embodiments of the present disclosure clearer, the technical scheme in the embodiments of the present disclosure will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

The following is a detailed description of the method for recycling the raffinate in the wet purification process for producing phosphoric acid provided by the present invention.

The present invention discloses a method for recycling raffinate acid in a wet purification process for producing phosphoric acid, comprising the following steps: removing sulfur, fluorine, arsenic and heavy metals in the raffinate acid to be treated to obtain pretreated acid; pre-neutralizing the pretreated acid, followed by purification to obtain purified liquid and precipitated residue containing iron, aluminum and magnesium, and distilling the purified liquid to obtain industrial-grade phosphoric acid.

It should be noted that if the method of first using a precipitant for purification and then removing sulfur, fluorine and arsenic is adopted, in order to avoid pollution and purification of phosphoric acid, only barium desulfurization, phosphorus pentasulfide dearsenic and steam stripping defluorination can be used. The purification process is complicated and has high requirements on the purity, dosage and environment of the chemical agents in the purification process. If the pretreatment and purification process are combined into one step, the impurity removal residue and phosphate precipitation residue will be mixed together, complicating the composition of the residue and making it difficult for the phosphate precipitation residue separated in the purification process to be reused.

Therefore, the present invention adopts a method of first removing sulfur, fluorine and arsenic, and then pre-neutralizing and purifying. Heavy metal impurity ions (such as lead, cadmium, etc.) can form sulfide precipitation under acidic conditions and are removed together in the process of arsenic removal.

In some embodiments, the raffinate acid to be treated may be subjected to rough desulfurization to obtain a first intermediate acid; fluorine, arsenic and heavy metals in the first intermediate acid may be removed to obtain a second intermediate acid; and the second intermediate acid may be subjected to fine desulfurization to obtain a pretreated acid.

It should be noted that for phosphoric acid purification enterprises, phosphate concentrate (main component calcium fluorophosphate) is the most easily available desulfurizer, but when crude phosphoric acid is desulfurized with phosphate concentrate, impurity ions such as fluorine and arsenic in the ore will be released into the phosphoric acid. In order to avoid secondary pollution, the present disclosure adopts a rough desulfurization method first. Based on the fact that the product of calcium salt desulfurization is calcium sulfate, which has a certain solubility in aqueous solution, calcium salt can only be used to preliminarily remove sulfate impurities; if the calcium salt and barium salt are mixed and the solution is desulfurized, the two desulfurization products produced will interfere with each other, and the purpose of deep desulfurization cannot be achieved. Therefore, the present disclosure adopts a two-stage method of calcium salt plus barium salt for desulfurization, so as to achieve deep removal of sulfate impurities, while taking into account the desulfurization cost and obtaining a better desulfurization effect.

Fluorine and arsenic will not interfere with each other during the removal process, and the impurity removal products are all hazardous wastes. Combining them can simplify the treatment process, and heavy metal impurities will be removed together during arsenic removal. The acid solution after arsenic removal also contains a certain amount of sulfide in the form of non-sulfate, which also needs to be removed. In the present disclosure, it is selected to oxidize it into sulfate before removal.

In accordance with the above, the present disclosure adopts the method of first performing rough desulfurization, then removing fluorine, arsenic and heavy metals, and then oxidizing non-sulfate sulfides to perform deep desulfurization.

For reference, the raffinate to be treated is mixed with a crude desulfurizing agent for crude desulfurization, and solid-liquid separation is performed to obtain a first intermediate acid and a crude desulfurization slag (calcium sulfate). The crude desulfurizing agent may illustratively include at least one of phosphate concentrate, calcium carbonate, calcium hydroxide and calcium oxide. Phosphate concentrate contains some calcium phosphate. In some optional embodiments, the crude desulfurizing agent selects phosphate concentrate.

The molar ratio of calcium in the crude desulfurizer to sulfate in the raffinate can be (0.8:1) (1.0:1), such as 0.8:1, 0.85:1, 0.9:1, 0.95:1 or 1.0:1, etc., or any other value within the range of (0.8:1)-(1.0:1).

The crude desulfurization slag obtained above can be used to return to the wet phosphoric acid production system to recover the phosphorus element entrained therein.

The first intermediate acid is mixed with a defluorinating agent, a sulfide and a filter aid to remove fluorine, arsenic and heavy metals.

In some embodiments, the first intermediate acid can be mixed with the defluorinating agent, sulfide and filter aid at the same time; in other embodiments, the first intermediate acid can be mixed with the defluorinating agent first for defluorination treatment, and the reaction liquid after defluorination is then mixed with the sulfide and filter aid to carry out arsenic removal and heavy metal removal reactions, followed by solid-liquid separation to obtain the second intermediate acid and defluorination, dearsenicization and heavy metal removal slag.

For reference, the defluorination agent may exemplarily include a sodium source and activated diatomaceous earth, wherein the sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate. In some embodiments, the mass ratio of the sodium source to the activated diatomaceous earth may be (1:0.75)-(1:1.5). If the mass ratio of the sodium source to the activated diatomaceous earth is too low, the expected effect cannot be achieved; if the mass ratio of the sodium source to the activated diatomaceous earth is too high, it is not very helpful for defluorination.

The molar ratio of Na in the defluorinating agent to F in the first intermediate acid can be (0.4:1)-(0.8:1), such as 0.4:1, 0.45:1, 0.5:1, 0.55:1, 0.6:1, 0.65:1, 0.7:1, 0.75:1 or 0.8:1, or any other value within the range of (0.4:1)-(0.8:1).

The sulfide may illustratively include at least one of sodium sulfide, calcium sulfide, potassium sulfide and phosphorus pentasulfide. The molar ratio of As in the first intermediate acid to S in the sulfide may be (1:15)-(1:75), such as 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45, 1:50, 1:55, 1:60, 1:65, 1:70 or 1:75, or any other value within the range of (1:15)-(1:75).

The filter aid may exemplarily include activated carbon. The amount of the filter aid used may be 0.5wt%-5wt% of the first intermediate acid, such as 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt% or 5wt%, etc., or any other value within the range of 0.5wt%-5wt%.

The second intermediate acid is mixed with an oxidant and a fine desulfurizing agent to perform fine desulfurization, and the reaction liquid obtained by the fine desulfurization is subjected to solid-liquid separation to obtain a pretreated acid and fine desulfurization slag.

The oxidant may include hydrogen peroxide or ozone. The hydrogen peroxide may be, for example, industrial-grade hydrogen peroxide, and its usage may be 0.5wt%-1.5wt% of the second intermediate acid, such as 0.5wt%, 0.8wt%, 1wt%, 1.2wt% or 1.5wt%, or any other value within the range of 0.5wt%-1.5wt%.

The fine desulfurizing agent is a barium salt, which may include at least one of barium hydroxide and barium carbonate. The molar ratio of the barium in the fine desulfurizing agent to the sulfate in the second intermediate acid may be (1.5:1)-(2.5:1), such as 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2.0:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1 or 2.5:1, or any other value within the range of (1.5:1)-(2.5:1).

The above-mentioned fine desulfurization slag is mainly barium slag, which can be returned to the rough desulfurization process to be used as a rough desulfurization agent.

It should be noted that the above-mentioned desulfurization, defluorination, dearsenicization and heavy metal removal cannot be considered to remove 100% of all sulfur, fluorine, arsenic and heavy metals in the raffinate acid, but at least most of the sulfur, fluorine, arsenic and heavy metals can be removed.

In the present disclosure, before pre-neutralization, the pre-treated acid may be concentrated and solid-liquid separated to obtain concentrated pre-treated acid and a first precipitated residue whose main components are iron phosphate and aluminum phosphate.

In some embodiments, the concentrated pretreated _acid may be concentrated to a _P2O5 content of not less than 40 wt%, such as 40 wt%, 50 wt%, 60 wt%, 70 wt% or 80 wt%, or any other value not less than 40 wt%.

After the concentrated pretreatment acid is obtained, the concentrated pretreatment acid is mixed with a neutralizing agent for pre-neutralization to obtain a pre-neutralization reaction solution.

The above-mentioned pre-neutralization can transform the impurity cations into phosphate complex salt precipitates with lower solubility and higher crystallinity, which is beneficial to improving the removal rate of cationic impurities and improving the separation performance of precipitated slag.

For reference, the neutralizing agent may exemplarily include salts of potassium, sodium, ammonium and at least one of ammonia, sodium hydroxide and potassium hydroxide. Among them, the salts of potassium, sodium and ammonium may include at least one of carbonate, bicarbonate and phosphate. The molar ratio of M (M includes ammonia, sodium and potassium) in the above-mentioned neutralizing agent to phosphorus in the concentrated pretreatment acid can be (0.2:1)-(0.4:1), such as 0.2:1, 0.25:1, 0.3:1, 0.35:1 or 0.4:1, etc., or any other value within the range of (0.2:1)-(0.4:1).

In the present disclosure, the purification process may include: mixing the pre-neutralization reaction liquid with the first precipitant to perform a first purification, and separating the solid and liquid to obtain a first purified liquid and a second precipitated residue whose main components are iron phosphate, aluminum phosphate and magnesium phosphate.

In the first purification process, the first precipitant is an organic precipitant, which may include at least one of methanol, ethanol, propanol, butanol and acetone. The amount of the first precipitant used may be 1 to 2.5 times of the pre-neutralization reaction solution, such as 1, 1.5, 2 or 2.5 times. It can also be any other value within the range of 1 to 2.5 times.

The temperature of the first purification can be 25°C-65°C, such as 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C or 65°C, etc., or it can be any other value within the range of 25°C-65°C.

Furthermore, the purification process also includes: mixing the first purification liquid with a detergent for a second purification, and separating the solid and the liquid to obtain the second purification liquid and the first washing liquid.

The solute in the detergent used in the second purification process may illustratively include at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, potassium dihydrogen phosphate, potassium monohydrogen phosphate and potassium phosphate. The mass fraction of the solute in the detergent is not less than 10%, such as 10%, 20%, 30%, 40% or 50%, etc., or other values not less than 10%.

The molar ratio of M (including sodium and potassium) in the detergent to P in the first purification liquid can be (0.15:1)-(0.3:1), such as 0.15:1, 0.2:1, 0.25:1 or 0.3:1, etc., or any other value within the range of (0.15:1)-(0.3:1).

The first washing liquid can be returned to the pre-neutralization process to be used as a neutralizing agent.

Furthermore, the purification process also includes: mixing the second purified liquid with the second precipitant for third purification, and separating the solid and liquid to obtain the third purified liquid and the third precipitated residue containing a small amount of magnesium phosphate and alkali metal phosphate.

In the third purification process, the second precipitant is an organic precipitant, which may include at least one of methanol, ethanol, propanol, butanol and acetone. The amount of the second precipitant used may be 20wt%-100wt% of the second purification liquid, such as 20wt%, 30wt%, 40wt%, 50wt%, 60wt%, 70wt%, 80wt%, 90wt% or 100wt%, etc., or any other value within the range of 20wt%-100wt%.

In some embodiments, the third precipitated residue can be returned to the first purification process. In other embodiments, the third precipitated residue can be mixed with the first precipitated residue and the second precipitated residue for pulping and recovery.

In the present disclosure, after the third purification liquid is distilled, industrial-grade phosphoric acid, water and a regenerated precipitant can be obtained.

The above-mentioned industrial-grade phosphoric acid can be used as a raw material to prepare battery-grade phosphate (such as iron phosphate), the regenerated precipitant can be reused in the first purification and/or third purification process, and the water can also be recycled as needed.

As mentioned above, the present invention adopts a three-stage purification method of solvent precipitation-washing-reprecipitation to purify the raffinate acid in view of the high impurity content of the raffinate acid. The staged addition of the solvent can significantly reduce the amount of detergent used, and can also perform secondary purification on the acid-containing solvent after washing to remove cationic impurities that enter the solvent during the washing process. The acid solution after the three-stage purification meets the requirements of industrial-grade phosphoric acid.

In the present disclosure, the third precipitated residue is mixed with the first precipitated residue and the second precipitated residue and then pulped and recovered, which may include: mixing the first precipitated residue, the second precipitated residue and the third precipitated residue with water and a first alkaline substance, neutralizing and precipitating to obtain a first slurry, and performing solid-liquid separation on the first slurry to obtain a separated liquid and phosphate precipitated residue.

In some embodiments, the first precipitated residue, the second precipitated residue and the third precipitated residue are mixed and then water is added to prepare the slurry, and then the first alkaline substance is added to the obtained slurry to adjust the pH value and then the solid-liquid separation is performed to separate the generated phosphate precipitated residue.

For reference, the water used to mix with the first precipitated slag, the second precipitated slag and the third precipitated slag is desalted water. The mass ratio of water to the total amount of the first precipitated slag, the second precipitated slag and the third precipitated slag can be (2:1)-(5:1), such as 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1 or 5:1, or any other value within the range of (2:1)-(5:1).

The first alkaline substance illustratively may include at least one of sodium hydroxide, potassium hydroxide, sodium orthophosphate, and potassium orthophosphate.

The pH value of the first slurry may be 5.5-8.0, such as 5.5, 6, 6.5, 7, 7.5 or 8, or any other value within the range of 5.5-8.0.

In some embodiments, the separated liquid can be reused as a detergent in the second purification process. In other embodiments, the separated liquid can be reused as a neutralizing agent in the pre-neutralization process. In other embodiments, the separated liquid can be reused as a defluorinating agent in the defluorination process.

Furthermore, the obtained phosphate precipitated residue is mixed with water to obtain a second slurry. The second slurry is mixed with a second alkaline substance and subjected to secondary cross-current leaching to extract phosphorus from the phosphate precipitated residue, and the reaction liquid after leaching is subjected to solid-liquid separation to obtain dephosphorized residue and orthophosphate mother liquor.

The mass ratio of phosphate precipitate to water may be (1:4)-(1:9), such as 1:4, 1:5, 1:6, 1:7, 1:8 or 1:9, or any other value within the range of (1:4) (1:9).

The second alkaline substance may illustratively include at least one of sodium hydroxide and potassium hydroxide.

In the present disclosure, the secondary cross-flow leaching may include: subjecting the second slurry and the second alkaline substance to a first-stage leaching to obtain a first-stage leachate and a first precipitated residue.

In the first stage leaching, the pH value of the reaction solution can be 13-14, such as 13, 13.5 or 14.

The first stage leaching process can be carried out at a temperature of 50°C-85°C (such as 55°C, 60°C, 65°C, 75°C, 78°C, 80°C or 85°C, etc.).

During the first stage leaching process, the residence time of the reaction slurry can be 25 min-45 min, such as 25 min, 30 min, 35 min, 40 min or 45 min.

The mass ratio of the first-stage leachate to the first sediment can be (3:1)-(9:1), such as 3:1, 4:1, 5:1, 6:1, 7:1, 8:1 or 9:1.

The orthophosphate mother liquor obtained by the first stage leaching (which can be understood as the liquid in the first slurry) can be partially reused in the first precipitation slag, The second precipitated residue and the third precipitated residue are mixed.

Furthermore, the above-mentioned secondary cross-flow leaching also includes: performing second-stage leaching on the first sediment and the second alkaline substance to obtain a second-stage leachate and a second sediment.

In the second stage leaching, the pH value of the reaction solution can be 14.0-14.3 (such as 14.0, 14.1, 14.2 or 14.3, etc.).

The second stage leaching process can be carried out at a temperature of 45°C-55°C (such as 45°C, 48°C, 50°C, 52°C or 55°C, etc.).

During the second stage leaching process, the residence time of the reaction slurry can be 60 min-90 min, such as 60 min, 70 min, 80 min or 90 min.

The mass ratio of the second stage leachate to the second sediment can be (5:1)-(9:1), such as 5:1, 6:1, 7:1, 8:1 or 9:1.

Furthermore, the second precipitated slag is subjected to alkali washing, and after solid-liquid separation, a first washing liquid and a first slag slurry are obtained.

In the above-mentioned alkali washing process, the second sediment residue can be leached with a dilute alkali solution (pH value is about 14), and the mass ratio of the dilute alkali solution to the second sediment residue can be (3:1)-(5:1), such as 3:1, 4:1 or 5:1. The separated first washing liquid can be reused in the second stage leaching process.

Furthermore, the first slurry is washed with water, and the solid and liquid are separated to obtain a second washing liquid and dephosphorized slag.

The water used for washing can be clean water, and its mass ratio to the first slurry can be (3:1)-(5:1), such as 3:1, 4:1 or 5:1. The separated second washing liquid can be recycled as dilute alkali liquid to the alkali washing process of the second sediment.

Furthermore, the lime milk and the second-stage leachate can be subjected to a first double decomposition reaction (corresponding to the first-stage precipitation) to precipitate the phosphate in the secondary leachate, and the solid-liquid separation is performed to obtain a second slurry; the second slurry and the first-stage leachate are subjected to a second double decomposition reaction (corresponding to the second-stage precipitation), and the solid-liquid separation is performed to obtain an alkali solution and calcium hydroxyphosphate.

For reference, lime milk may be obtained by mixing a precipitant with water, wherein the precipitant illustratively may include at least one of calcium oxide and calcium hydroxide.

The mass ratio of the precipitant to water can be (1:3)-(1:5), such as 1:3, 1:4 or 1:5.

The molar ratio of calcium in the precipitant to phosphorus in the second-stage leachate can be (1.5:1)-(1.6:1), such as 1.5:1, 1.55:1 or 1.6:1.

In the present disclosure, the temperature of the first metathesis reaction and the second metathesis reaction is independently 40°C-60°C, such as 40°C, 45°C, 50°C, 55°C or 60°C, etc. The time of the first metathesis reaction and the second metathesis reaction is independently 30min-60min, such as 30min, 35min, 40min, 45min, 50min, 55min or 60min, etc. Both of the above-mentioned metathesis reactions can be carried out under strong stirring conditions, and the stirring speed can be illustratively above 270rpm.

As mentioned above, the solubility of calcium hydroxyphosphate is much smaller than that of calcium aluminate. When using the leachate to prepare calcium hydroxyphosphate, the phosphorus and aluminum elements in the mother liquor can be separated by controlling the amount of calcium oxide or calcium hydroxide added. The calcium aluminate generated in the first double decomposition reaction can be converted into calcium hydroxyphosphate in the second double decomposition reaction by adopting a two-stage cross-current precipitation method, and the phosphorus precipitation product can be purified for the second time. At the same time, the two-stage double decomposition reaction can effectively improve the conversion rate of soda lime.

In some embodiments, the calcium hydroxyphosphate obtained by the double decomposition reaction can be washed, and the washed calcium hydroxyphosphate can be dried to obtain calcium hydroxyphosphate dry powder. The calcium hydroxyphosphate dry powder can be used as a raw material in the wet process phosphoric acid production process to produce crude phosphoric acid. In addition, the calcium hydroxyphosphate can also be used as a defluorinating agent to defluorinate fluoride-containing wastewater. The principle is that calcium hydroxyphosphate is very easy to exchange ions with fluoride ions in the solution, absorb fluoride ions to generate calcium fluorophosphate, and thus can deeply defluorinate weakly acidic, neutral and weakly alkaline fluoride-containing wastewater.

The mass ratio of the above-mentioned calcium hydroxyphosphate to water can be (2:1)-(4:1), such as 2:1, 2.5:1, 3:1, 3.5:1 or 4:1.

The second washing liquid obtained after washing is collected and can be returned to the precipitant pulping and milking process.

In some embodiments, the alkali liquor after double decomposition and dephosphorization can be subjected to solid suspended matter removal, and then concentrated to obtain concentrated alkali liquor.

The causticity of the dilute alkali solution after dephosphorization is more than 10 times that of the leaching solution. At a higher causticity ratio, aluminate ions are not easy to self-decompose, so the dilute alkali solution after dephosphorization can be directly concentrated.

The concentrated alkali solution obtained can be returned to the second slurry for leaching after replenishing the lost alkali as needed. The above replenishment of alkali to the concentrated alkali solution can maintain the alkali balance and water balance of the system.

When the concentrated alkali solution is used to leach the second slurry again, the aluminate present in the alkali solution has a certain inhibitory effect on the dissolution of aluminum hydroxide. Therefore, in the closed-loop circulation process, the aluminum in the circulating alkali solution does not need to be treated separately.

In addition, the main component of the dephosphorization slag produced in the present disclosure is a mixture of iron, aluminum and magnesium hydroxides. The dephosphorization slag can be further extracted as needed to recover the valuable elements therein, which is helpful to achieve the purpose of comprehensive utilization of the associated resources of wet-process phosphoric acid.

In continuation of the above, the present disclosure proposes a new method for utilizing raffinate acid, which cleverly solves the technical dilemma that the utilization of raffinate acid must rely on fertilizer processing plants, and ensures the integrity and independence of the phosphoric acid purification process. The method proposes a new phosphate precipitate leaching scheme, which achieves the purpose of selectively extracting phosphorus from complex phosphate precipitates containing iron, aluminum, and magnesium. In the open-circuit process, the vast majority of phosphorus (more than 98.5%) and a small proportion of aluminum (less than 30%) in the phosphate precipitate slag can be transferred to the leachate; in the closed-circuit cycle process, phosphorus can be extracted separately from complex phosphate precipitates containing iron, aluminum, and magnesium. The method converts the phosphorus in the raffinate acid into industrial-grade purified phosphoric acid to the maximum extent through reasonable process design, and at the same time converts the by-product low-utilization value and insoluble phosphate salts with basically no practical use into hydroxy calcium phosphate with high value and wide application, realizing the comprehensive recovery and efficient utilization of phosphorus resources in the raffinate acid.

The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

Example 1

This embodiment provides a method for recycling raffinate in the process of producing phosphoric acid by wet purification, referring to FIG. 1 and FIG. 2 , which comprises the following steps:

S1: As shown in FIG1 , 2000 mL of the raffinate acid to be treated is taken, and 280 g of phosphate concentrate powder is added thereto (the molar ratio of calcium in the phosphate concentrate powder to sulfate in the raffinate acid is 0.95:1. After the reaction is complete, the generated calcium sulfate dihydrate is filtered off to obtain the first intermediate acid.

The basic information of the above-mentioned raffinate acid to be treated is as follows:

The water content of the phosphate concentrate powder used is 10wt%, and its chemical composition (anhydrous basis) is as follows:

S2: Add 50 g of soda ash (sodium carbonate) and 50 g of activated diatomaceous earth to the first intermediate acid of S1, stir to make the acid solution fully contact with the diatomaceous earth, and add 12.5 g of sodium sulfide nonahydrate and 56 g of activated carbon powder to the mixed acid solution after no bubbles are generated. After the reaction is complete, filter out the generated precipitate (defluorination, dearsenicization and deheavy metal slag) to obtain the second intermediate acid.

The molar ratio of the total amount of Na in sodium carbonate and activated diatomaceous earth to F in the first intermediate acid is 0.56:1; the molar ratio of As in the first intermediate acid to S in sodium sulfide nonahydrate is 1:50; and the amount of activated carbon powder used is 2wt% of the first intermediate acid.

S3: Add 29g of hydrogen peroxide to the second intermediate acid of S2, stir to make the two fully contact, and then add 115g of barium carbonate to the acid for fine desulfurization. After the reaction is complete, filter the reaction liquid to separate the pretreated acid (fine desulfurization clear liquid) and fine desulfurization slag (barium slag).

The amount of hydrogen peroxide used is 1wt% of the second intermediate acid; and the molar ratio of barium in barium carbonate to sulfate in the second intermediate acid is 2.25:1.

S4: The pretreated acid obtained in S3 is concentrated, filtered, and separated to obtain concentrated pretreated acid and a first precipitated residue containing iron and aluminum.

Among them, the concentrated pretreated acid is composed as follows:

S5: Weigh 500 g of the concentrated pretreated acid in S4, add 50 g of concentrated aqueous ammonia thereto for a pre-neutralization reaction, then place the pre-neutralization liquid in a water bath at 55°C with stirring to obtain a pre-neutralization reaction liquid.

The molar ratio of M (ammonia) in the concentrated ammonia water to phosphorus in the concentrated pretreatment acid is 0.25:1.

S6: Add 1000g of anhydrous ethanol to the pre-neutralization reaction liquid (536.12g) of S5, stir to make it fully react (reaction temperature is 55°C), filter and separate, and obtain the first purified liquid and the second precipitate residue containing iron, aluminum, magnesium and manganese. Add 130g of sodium carbonate aqueous solution with a mass fraction of 15% (the molar ratio of M in the sodium carbonate aqueous solution to P in the first purified liquid is 0.2:1) to the first purified liquid for washing, and stand for separation after sufficient reaction to obtain the second purified liquid and the first washing liquid.

S7: Add 800 g of anhydrous ethanol to the second purified liquid (1359.82 g) in S6, stir to make it fully react, filter and separate, and obtain the third purified liquid and the third precipitate residue containing magnesium and alkali metals.

S8: distilling the third purified liquid in S7, collecting high-concentration ethanol and water, and the remaining liquid is purified phosphoric acid; concentrating the purified phosphoric acid so that the mass fraction of H ₃ PO ₄ in the acid is ≥85%, and obtaining industrial-grade phosphoric acid.

The industrial grade purified phosphoric acid is composed as follows:

Meet the quality requirements of industrial-grade phosphoric acid.

S9: As shown in FIG2 , the first precipitated residue, the second precipitated residue and the third precipitated residue are mixed to obtain a mixed precipitated residue. 1000 g of deionized water is added to the mixed precipitated residue (380.34 g) to prepare a slurry, and the prepared slurry is strongly acidic. Sodium hydroxide is added thereto to react, and the pH value of the first slurry is adjusted to 6.0, and filtered to obtain a phosphate precipitated residue and a separated liquid.

S10: Add 1500g of deionized water to the phosphate precipitate residue (315.67g) obtained in S9, stir and slurry to obtain a second slurry. Heat the second slurry in a water bath at 80°C, then add sodium hydroxide (221g) to the hot slurry for a first-stage leaching reaction. During the first-stage leaching reaction, the pH value of the reaction slurry is about 13.5. Stir the reaction for about 35 minutes, and separate by sedimentation to obtain a first-stage leachate and a first sediment residue.

S11: Add 1500 g of deionized water to the first slag (370.34 g) obtained in S10, stir evenly and heat the prepared slurry in a water bath at 50°C, then add sodium hydroxide (100 g) to the preheated slurry to carry out a second-stage leaching reaction. The pH value of the reaction liquid during the second-stage leaching reaction is approximately equal to 14.1. Stir the reaction for 75 minutes, filter and separate, and obtain a second-stage leachate and a second slag.

S12: The second sedimentation residue (202.32 g) in S11 is washed with 800 g of 4 wt% sodium hydroxide aqueous solution and filtered to obtain a first washing liquid and a first slurry. The first washing liquid is recovered and mixed with the second-stage leaching liquid, and the first slurry is washed with 800 g of deionized water. The solid and liquid are separated to obtain the second washing liquid and the dephosphorization slag, and the dephosphorization slag is dried and its composition is analyzed. The second washing liquid can be recycled as a dilute alkali solution to the alkali washing process of the second sedimentation slag.

The composition of the above dephosphorization slag is as follows:

It can be seen that the phosphorus element in the phosphate precipitation slag is completely extracted by the alkali solution during the leaching process, the phosphorus leaching rate is >99%, and the dephosphorization slag is a mixture of hydroxides of metal elements.

S13: According to Ca:P=1.50:1 (molar ratio), 227.16g of calcium hydroxide was weighed, and 700g of deionized water was added thereto to prepare a slurry to obtain lime slurry. The lime slurry was added to the second leachate of step S11 to precipitate the phosphate ions in the solution, and stirred in a water bath at 50°C to perform a first double decomposition reaction. After the reaction for 55 minutes, the reaction solution was allowed to stand and settle, and the lower layer of slurry was separated and added to the first-stage leachate for a second double decomposition reaction, and stirred in a water bath at 55°C for 40 minutes. After the reaction was completed, the generated calcium hydroxyphosphate was separated by filtration.

S14: 1300 g of deionized water is added to the hydroxy calcium phosphate obtained in S13 for washing. After washing, the mixture is filtered and dried to obtain hydroxy calcium phosphate powder.

The composition of the resulting calcium hydroxyphosphate is as follows:

According to the results, the separation of phosphorus and aluminum is relatively thorough.

Example 2

This embodiment provides a method for recycling raffinate in the process of wet purification of phosphoric acid, wherein S1 to S4 are the same as those in Embodiment 1, except that:

S5: Weigh 500 g of the concentrated pretreated acid in S4, add 50 g of sodium carbonate thereto for a pre-neutralization reaction, then place the pre-neutralization liquid in a water bath at 55°C with stirring to obtain a pre-neutralization reaction liquid.

The molar ratio of M (sodium) in sodium carbonate to phosphorus in concentrated pretreatment acid is 0.31:1.

S6: Add 900g of mixed alcohol (ethanol: isopropanol = 4:1, mass ratio) to the pre-neutralization reaction liquid (527g) of S5, stir to make it fully react (reaction temperature is 60°C), filter and separate, and obtain the first purified liquid and the second precipitate residue containing iron, aluminum, magnesium and manganese. Add 160g of sodium dihydrogen phosphate solution with a mass fraction of 30% (the molar ratio of M in the sodium dihydrogen phosphate solution to P in the first purified liquid is 0.2:1) to the first purified liquid for washing, and stand for separation after sufficient reaction to obtain the second purified liquid and the first washing liquid.

S7: Add 700 g of mixed alcohol (ethanol: n-butanol = 4:1, mass ratio) to the second purified liquid (1250 g) in S6, stir to make it fully react, filter and separate, and obtain a third purified liquid and a third precipitate residue containing manganese and alkali metals.

S8: distilling the third purified liquid in S7, collecting high-concentration alcohol liquid and water, and the remaining liquid is purified phosphoric acid; concentrating the purified phosphoric acid so that the mass fraction of H ₃ PO ₄ in the acid is ≥85%, and obtaining industrial-grade phosphoric acid.

The composition of the resulting technical grade phosphoric acid is as follows:

Compared with Example 1, the usage ratio of the mixed alcohol is lower than that of the single ethanol, which indicates that the mixed alcohol has a better purification effect.

Comparative Example 1

This comparative example provides a method for recycling raffinate in the process of wet purification of phosphoric acid, wherein S1 to S4 are the same as those in Example 1, except that:

S5: Weigh 500 g of the concentrated pretreated acid in S4, add 50 g of concentrated aqueous ammonia thereto for a pre-neutralization reaction, then place the pre-neutralization liquid in a water bath at 55°C with stirring to obtain a pre-neutralization reaction liquid.

The molar ratio of M (sodium) in sodium carbonate to phosphorus in concentrated pretreatment acid is 0.25:1.

S6: Add 1000g of anhydrous ethanol to the pre-neutralization reaction liquid (535.84g) of S5, stir to make it fully react (reaction temperature is 60°C), filter and separate, and obtain the first purified liquid and the second precipitate residue containing iron, aluminum, magnesium and manganese. Add 160g of sodium dihydrogen phosphate solution with a mass fraction of 30% (the molar ratio of M in the sodium dihydrogen phosphate solution to P in the first purified liquid is 0.2:1) to the first purified liquid for washing, and stand for separation after sufficient reaction to obtain the second purified liquid and the first washing liquid.

S7: Add 900 g of anhydrous ethanol to the second purified liquid (1360.13 g) in S6, stir to make it fully react, filter and separate, and obtain the third purified liquid and the third precipitate residue containing magnesium and alkali metals.

S8: distilling the third purified liquid in S7, collecting high-concentration ethanol and water, and the remaining liquid is purified phosphoric acid; concentrating the purified phosphoric acid so that the mass fraction of H ₃ PO ₄ in the acid is ≥85%, and obtaining industrial-grade phosphoric acid.

The industrial grade purified phosphoric acid is composed as follows:

The acid meets the quality requirements of industrial grade phosphoric acid.

S9: Mix the first precipitated residue, the second precipitated residue and the third precipitated residue to obtain a mixed precipitated residue. Add 1000 g of deionized water to the mixed precipitated residue (383.24 g) to make a slurry. The obtained slurry is strongly acidic. Sodium hydroxide is added to the slurry to react, and the pH value of the first slurry is adjusted to 6.5. Filter and separate to obtain phosphate precipitated residue and a separated liquid.

S10: Add 1500g of deionized water to the phosphate precipitate residue (316.11g) obtained in S9, stir and slurry to obtain a second slurry. Heat the second slurry in a water bath at 85°C, then add sodium hydroxide (230g) to the hot slurry for a first-stage leaching reaction. During the first-stage leaching reaction, the pH value of the reaction slurry is about 13.7. Stir and react for about 40 minutes, and separate by sedimentation to obtain a first-stage leachate and a first sediment residue.

S11: Add 1500 g of deionized water to the first slag (368.87 g) obtained in S10, stir evenly and heat the prepared slurry in a water bath at 50°C, then add sodium hydroxide (96 g) to the preheated slurry to carry out a second-stage leaching reaction. The pH value of the reaction liquid during the second-stage leaching reaction is approximately equal to 14.1. Stir the reaction for 75 minutes, filter and separate, and obtain a second-stage leachate and a second slag.

S12: The second sedimentation residue (204.44 g) in S11 is leached with 800 g of 4 wt% sodium hydroxide aqueous solution and filtered to obtain a first washing liquid and a first slurry. The first washing liquid is recovered and mixed with the second-stage leaching liquid, and the first slurry is washed with 800 g of deionized water, solid-liquid separation is performed to obtain a second washing liquid and dephosphorization residue, and the dephosphorization residue is dried and its composition is analyzed. The second washing liquid can be recovered as a dilute alkali solution to the alkali washing process of the second sedimentation residue.

The composition of dephosphorization slag is as follows:

It can be seen that the phosphorus element in the phosphate precipitation residue is completely extracted by the alkali solution during the leaching process, the phosphorus leaching extraction rate is >99%, and the dephosphorization residue is a mixture of hydroxides of metal elements.

S13: According to the molar ratio of Ca:P=1.50:1, 226.75g of calcium hydroxide was weighed, and 700g of deionized water was added thereto to prepare a slurry to obtain lime slurry. The first-stage leachate, the second-stage leachate and the above-mentioned alkali washing solution were first mixed, and then the prepared lime slurry was added to the mixed leachate, and the reaction mixture was stirred in a water bath at 55°C for 70 minutes, and then the reaction solution was filtered to separate the generated calcium hydroxyphosphate.

S14: 1300 g of deionized water is added to the hydroxy calcium phosphate obtained in S13 for washing. After washing, the mixture is filtered and dried to obtain hydroxy calcium phosphate powder.

The composition of the resulting calcium hydroxyphosphate is as follows:

According to the above test results, single-stage precipitation reduces the conversion rate of soda lime and the separation effect of phosphorus and aluminum elements.

Comparative Example 2

This comparative example provides a method for recycling raffinate in the process of wet purification of phosphoric acid, wherein S1 to S4 are the same as those in Example 1, except that:

S5: Weigh 500 g of the concentrated pretreated acid in S4, add 50 g of concentrated aqueous ammonia thereto for a pre-neutralization reaction, then place the pre-neutralization liquid in a water bath at 55°C with stirring to obtain a pre-neutralization reaction liquid.

The molar ratio of M (ammonia) in the sodium carbonate to phosphorus in the concentrated pretreatment acid is 0.25:1.

S6: Add 1800 g of anhydrous ethanol to the pre-neutralization reaction liquid (536.51 g) of S5, stir to make it fully react (reaction temperature is 55° C.), filter and separate to obtain the first purified liquid and the second precipitate residue containing iron, aluminum, magnesium and manganese.

S7: distilling the first purified liquid obtained in S6, collecting high-concentration ethanol and water, and the remaining liquid is purified phosphoric acid; concentrating the purified phosphoric acid so that the mass fraction of H ₃ PO ₄ in the acid is ≥85%, thereby obtaining purified acid.

The purified acid is composed as follows:

S7: Take the second precipitated residue separated in S6, add 700g of deionized water to the residue (210.84g) to make pulp. The obtained slurry is strongly acidic. Add sodium hydroxide to it to react, adjust the pH value of the first slurry to 5.5, and obtain phosphate precipitated residue and separation liquid.

S8: Add 2000g of deionized water to the phosphate precipitate residue (201.63g) obtained in S7, stir and slurry to obtain slurry. Heat the prepared slurry in a water bath at 75°C, then add sodium hydroxide (210g) to the hot slurry for a first-stage leaching reaction. During the first-stage leaching reaction, the pH value of the reaction slurry is about 14.1. Stir the reaction for about 120min, and separate by sedimentation to obtain a leachate and precipitated residue.

S9: The precipitated residue in S8 is washed with 1000 g of 4 wt % sodium hydroxide aqueous solution and then filtered. The filter residue is then washed with 1000 g of deionized water. After washing, the filter residue (dephosphorization residue) is dried and its composition is analyzed.

The composition of dephosphorization slag is as follows:

S10: According to Ca:P=1.50:1 (molar ratio), 131.12g of calcium hydroxide was weighed, and 450g of deionized water was added thereto to prepare a slurry to obtain lime slurry. The lime slurry was added to the above alkaline washing liquid to precipitate the phosphate in the alkaline washing liquid, and stirred in a water bath at 50°C to perform a first double decomposition reaction. After the reaction for 45 minutes, the reaction liquid was allowed to stand and settle, the lower layer of slag slurry was separated and added to the first-stage leachate for a second double decomposition reaction, and stirred in a water bath at 55°C for 45 minutes. After the reaction was completed, the generated calcium hydroxyphosphate was separated by filtration.

S11: Add 600 g of deionized water to the hydroxy calcium phosphate obtained in S10 for washing. After washing, filter and dry to obtain hydroxy calcium phosphate powder.

The composition of the resulting calcium hydroxyphosphate sample is as follows:

By comparing with Example 1, it can be seen that:

① After removing the acid-containing solvent phase washing step, even if a sufficient amount of organic precipitant is added to the purified acid, it is still impossible to effectively remove the magnesium and calcium ions in the acid. In addition, the magnesium ion content in the raffinate acid is too high, and the demand for detergent in the washing process also increases. At the same time, it is necessary to remove the metal ions that enter the acid-containing solvent phase during the washing process by adding organic precipitants in steps.

②. During single-stage leaching, the leaching rate of aluminum in phosphate precipitate slag increased, and the leaching extraction rate of phosphorus was around 96%, which was much lower than that of two-stage cross-flow leaching.

③. The aluminum-phosphorus ratio of the leachate increases, which increases the aluminum content of calcium hydroxyphosphate, the phosphorus precipitation product of the leachate.

Comparative Example 3

This comparative example provides a method for recycling raffinate in the process of wet purification of phosphoric acid, wherein S1 to S4 are the same as those in Example 1, except that:

S5: Weigh 500 g of the concentrated pretreated acid in S4, add 50 g of sodium carbonate thereto for a pre-neutralization reaction, then place the pre-neutralization liquid in a water bath at 55°C with stirring to obtain a pre-neutralization reaction liquid.

The molar ratio of M (sodium) in sodium carbonate to phosphorus in concentrated pretreatment acid is 0.31:1.

S6: Add 1800g of ethanol to the pre-neutralization reaction liquid (527g) of S5, stir to make it fully react (reaction temperature is 60°C), filter and separate, and obtain the first purified liquid and the second precipitate residue containing iron, aluminum, magnesium and manganese. Add 200g of sodium dihydrogen phosphate solution with a mass fraction of 30% (the molar ratio of Na in the sodium dihydrogen phosphate solution to P in the first purified liquid is 0.27:1) to the first purified liquid for washing, and stand for separation after sufficient reaction to obtain the second purified liquid and the first washing liquid.

S7: distilling the second purified liquid obtained in S6, collecting high-concentration ethanol and water, and the remaining liquid is purified phosphoric acid; concentrating the purified phosphoric acid so that the mass fraction of H ₃ PO ₄ in the acid is ≥85%, thereby obtaining purified acid.

The resulting purified acid has the following composition:

By comparing with Example 1, it can be seen that although the use of one-stage purification plus one-stage washing can effectively remove most of the cationic impurities in the raffinate acid. However, in the process of washing the solvent phase, due to the low mass fraction of water in the solvent phase, a large amount of detergent is required to form a relatively stable solvent-salt solution two-liquid phase system to achieve the purpose of washing the acid-containing solvent. In addition, in this process, the sodium ions in the detergent are transferred to the solvent phase in large quantities due to the potential difference between the two phases. After the solvent phase is distilled, the sodium ions entering the solvent will all remain in the purified acid, resulting in the sodium ion content of the purified acid exceeding the standard.

Application Examples

This application example provides a method for recycling raffinate in a pilot wet purification process for producing phosphoric acid, which comprises the following steps:

S1: Phosphate concentrate powder is added with water at a mass ratio of 2:1 to prepare a concentrate slurry with a solid content of 60%, and then the two materials are added into a crude desulfurization reactor with stirring at a mass ratio of concentrate slurry: raffinate = 1:7 (Ca ²⁺ :SO ₄ ^2- =0.90:1, molar ratio) to carry out a crude desulfurization reaction. The residence time of the material in the reactor is controlled at 30-40min. The reaction liquid after sufficient reaction is discharged from the bottom of the reactor and sent to the pretreatment sedimentation tank for sedimentation separation.

The above-mentioned raffinate acid and phosphate concentrate powder to be treated are the same as those in Example 1.

S2: The clear liquid from the upper layer of the sedimentation tank is sent to the defluorination tank, and the slurry discharged from the lower layer is sent to the reaction tank of the wet phosphoric acid device to recover the phosphorus entrained therein.

S3: Add defluorinating agent sodium carbonate and activated diatomaceous earth powder into the defluorinating tank according to 4% of the mass of the clear liquid (Na:F=0.5:1, molar ratio), wherein the mass ratio of sodium carbonate to activated diatomaceous earth is 1:1, stir to make the defluorinating agent and the acid solution evenly mixed, and then send the mixed slurry to the dearsenicating tank.

S4: Prepare a 5% mass fraction of sodium sulfide solution for standby use. First, add the prepared sodium sulfide solution (As:S = 1:50, molar ratio) to the arsenic removal tank at a ratio of 2.8% of the acid mass. Then, add 80% of activated carbon powder passing through an 80-mesh sieve to the acid at a ratio of 2.5% of the acid mass. After stirring for 45 minutes, send the reaction solution to a filter press for filtration. Collect the filtrate in a dearsenic acid collection tank. Collect the filter residue and treat it uniformly.

S5: The hydrogen sulfide gas generated during the arsenic removal process is introduced into the scrubbing tower through a forced ventilation system, absorbed by a 5% mass fraction sodium hydroxide solution, and then reused.

S6: Add dearsenic acid, industrial-grade hydrogen peroxide and barium hydroxide into the fine desulfurization reaction tank according to the mass ratio of dearsenic acid: hydrogen peroxide: barium hydroxide = 100: 1: 2.5 (Ba: SO ₄ ^2- = 2: 1, molar ratio), stir and react for 45 minutes, then send the reaction liquid to the filter press for filtration to separate the fine desulfurization slag (barium slag) and pretreatment acid.

S7: After the barium slag is collected, it is added to the phosphate concentrate slurry to carry out rough desulfurization of the residual acid together with the phosphate concentrate. The amount of phosphate concentrate does not need to be adjusted.

S8: Concentrate the pretreated acid, the P ₂ O ₅ content of the concentrated acid is>40wt%, and the concentrated acid solution is filtered to separate the solid suspended matter in the acid to obtain concentrated pretreated acid and the first precipitated residue containing iron and aluminum. The concentrated pretreated acid is sent to the pre-neutralization tank, and the first precipitated residue is sent to the phosphoric acid residue slurry tank.

S9: The concentrated pre-treated acid is heated to 55° C. in a pre-neutralization tank, and then sodium carbonate powder is added to the pre-neutralization tank at a molar ratio of Na:P=0.3:1. After the reaction is complete, the obtained pre-neutralization reaction liquid is sent to the first-stage solvent precipitation tank.

S10: adding mixed alcohol to the pre-neutralization reaction liquid in a mass ratio of pre-neutralization reaction liquid: mixed alcohol precipitant (ethanol: isopropanol: n-butanol = 8:1:1) = 1:2.5, stirring the reaction for 30 minutes, centrifuging the reaction liquid to obtain a first purified liquid and a second precipitated residue containing iron, aluminum, magnesium and manganese, the second precipitated residue is sent to a phosphate slag slurry mixing tank, and the first purified liquid is sent to the middle of a washing and clarifying tank.

S11: Add 15% sodium carbonate solution to the middle of the washing and clarifying tank at a molar ratio of Na:P=0.18:1, stir to react, and obtain a second purified liquid and a first washing liquid. The first washing liquid after washing is discharged from the bottom of the washing and clarifying tank, and the second purified liquid is all recycled to the pre-neutralization reaction tank. The amount of sodium carbonate is deducted according to the amount of sodium ions in the washing liquid, and 0.5 mol of sodium carbonate is deducted for 1 mol of sodium ions.

S12: The second purified liquid is discharged from the upper part of the washing and clarifying tank and sent to the second-stage solvent precipitation tank. A mixed alcohol precipitant (ethanol: isopropanol: n-butanol = 8:1:1) with a solvent mass of 40% is added to the bottom of the second-stage solvent precipitation tank, and the solvent is stirred to fully react. The solvent after the reaction is filtered with a precision filter to obtain a third purified liquid and a third precipitate residue containing manganese and alkali metals; the third precipitate residue discharged from the precision filter is regularly transported to the first-stage solvent precipitation tank, and the separated third purified liquid is separated and purified from phosphoric acid, water, and precipitant after distillation treatment. The precipitant is collected and recycled, and the phosphoric acid is concentrated and purified so that the mass fraction of _H3PO4 in the acid is ≥85%, _thereby obtaining industrial-grade purified phosphoric acid.

The obtained industrial grade purified phosphoric acid has the following composition (the cycle is performed 7 times, and the industrial grade phosphoric acid composition corresponding to each cycle):

The acid meets the quality requirements of industrial grade phosphoric acid.

S13: adding water to the phosphoric acid slag slurry mixing tank at a liquid-solid ratio of 3:1 to prepare slurry, then sending the prepared slurry to a primary neutralization tank, and adding sodium hydroxide solution to the primary neutralization tank to adjust the pH value of the slurry to 6.0;

S14: the slurry after alkali adjustment is sent to the primary leaching sedimentation tank for static sedimentation, the upper phosphate solution is filtered to remove a small amount of suspended matter and then reused in the solvent washing stage, the pre-neutralization stage and the pre-treatment acid defluorination stage, completely replacing the use of sodium carbonate, and the excess phosphate solution is sent to the secondary phosphorus precipitation tank;

S15: the phosphate precipitation slurry discharged from the lower layer of the sedimentation tank is introduced into the secondary neutralization tank, and is diluted with hot water 5 times the mass of the slurry, and then sodium hydroxide is added to the secondary neutralization tank to maintain the pH of the reaction solution at 13.6-13.8 and the temperature fluctuates between 78-82°C. After stirring and reacting for 45 minutes, the alkaline reaction solution is transferred to the secondary leaching sedimentation tank for sedimentation and separation of the primary leachate and the primary leach slurry, and the primary leachate is sent to the secondary phosphorus precipitation tank and the primary neutralization tank;

S16: the primary leaching slurry is sent to the tertiary neutralization tank, 5 times the mass of the slurry is added to dilute it, and then sodium hydroxide is added to the tertiary neutralization tank to maintain the pH of the reaction solution at 14.1-14.3 and the temperature fluctuates between 48-52°C. After stirring and reacting for 70 minutes, the alkaline reaction solution is transferred to the tertiary leaching sedimentation tank for sedimentation and separation of the secondary leaching solution and the secondary leaching slurry, and the secondary leaching solution is sent to the primary phosphorus precipitation tank;

S17: The secondary leaching slurry is sent to the alkali washing tank, and a 4-6% sodium hydroxide solution with a concentration of 4 times the mass of the slurry is added thereto to rinse the slurry. The rinsing liquid is sent to the fourth-stage leaching sedimentation tank for sedimentation and separation of the alkali washing liquid and the alkali washing slag, and the alkali washing liquid is all returned to the third-stage neutralization tank;

S18: the alkali-washed residue is sent to a water washing tank, and clean water 3 times the mass of the residue slurry is added thereto to rinse the residue slurry. The rinse liquid is filtered by a filter press to separate the dephosphorization residue and the water washing liquid. A small amount of concentrated alkali liquid is added to the water washing liquid to prepare a 4-6% concentration of sodium hydroxide solution, which is then returned to the alkali washing tank. The separated dephosphorization residue is collected and processed uniformly;

The composition of the dephosphorization slag after leaching is as follows:

S19: preparing a lime slurry with a mass fraction of 25%, adding the prepared lime slurry to a primary phosphorus precipitation tank, stirring to make the lime slurry fully contact with the secondary leaching solution to perform a phosphorus precipitation reaction, transferring the primary phosphorus precipitation reaction liquid to a primary phosphorus precipitation settling tank for sedimentation and separation of dilute alkali solution and primary phosphorus precipitation slag; sending the primary phosphorus precipitation slag to a secondary phosphorus precipitation tank to fully contact with the secondary leaching solution for phosphorus precipitation reaction, and separating the dilute alkali solution and the secondary phosphorus precipitation slag after the secondary phosphorus precipitation reaction liquid is treated in the secondary phosphorus precipitation settling tank;

S20: The amount of lime slurry is determined according to the molar ratio of calcium oxide in lime slurry: total phosphorus in the two-stage leaching solution = 1.50-1.60:1, and is appropriately increased or decreased according to the phosphorus and aluminum contents in the secondary phosphorus precipitation slag to ensure that the phosphorus content in the secondary phosphorus precipitation slag is greater than 14.0% and the aluminum content is less than 0.1%;

S21: The secondary phosphorus precipitate slag that does not meet the requirements is returned to the secondary phosphorus precipitate tank for re-conversion, and the secondary phosphorus precipitate slag that meets the requirements is sent to the calcium phosphate washing tank and rinsed with clean water 3 times the mass of the phosphorus precipitate slag. The rinsing liquid is filtered and then the hydroxy calcium phosphate is recovered. The washing water is collected and used to prepare lime milk, and part of the hydroxy calcium phosphate is returned to the rough desulfurization stage to fully replace the phosphate concentrate powder;

S22: The dilute alkali solution separated from the primary phosphorus settling tank and the secondary phosphorus settling tank is sent to the alkali solution collecting tank, and a small amount of alkali lost in the process is added to it and then the alkali solution is appropriately concentrated. The concentrated alkali solution is returned to the secondary neutralization tank and the tertiary neutralization tank for next use.

The composition of calcium hydroxyphosphate is as follows:

The above results show that the phosphorus in the phosphate precipitate residue is completely extracted by the alkali solution during the leaching process, the phosphorus leaching rate is >99%, and the dephosphorization residue is a mixture of hydroxides of metal elements. After the alkali solution has been circulated for many times, the amount of aluminate loaded in the solution no longer changes significantly, that is, the circulating alkali solution no longer dissolves the aluminum in the phosphate precipitate residue, and the dephosphorization residue composition becomes stable.

During the leaching process, the sodium hydroxide solution reacts with carbon dioxide in the air to form sodium carbonate. The ionized carbonate ions are converted into calcium carbonate and enter the hydroxycalcium phosphate product during the lime milk precipitation stage. Therefore, the preparation of hydroxycalcium phosphate is mainly used for the production of wet phosphoric acid and defluorination of fluoride wastewater.

In summary, the method provided by the present disclosure can effectively recycle phosphorus resources in the raffinate acid, and the obtained industrial-grade phosphoric acid can be used as a raw material to prepare battery-grade phosphate.

Industrial Applicability

The method for utilizing raffinate acid proposed in the present disclosure cleverly solves the technical dilemma that the utilization of raffinate acid must rely on fertilizer processing plants, and ensures the integrity and independence of the phosphoric acid purification process. The method proposes a new leaching scheme for phosphate precipitate residues, and achieves the purpose of selectively extracting phosphorus from complex phosphate precipitates containing iron, aluminum, and magnesium. In the open-circuit process, the vast majority of phosphorus (about 98.5% or more) and a small proportion of aluminum (about 30% or less) in the phosphate precipitate residue can be transferred to the leachate; in the closed-circuit cycle process, phosphorus can be extracted separately from complex phosphate precipitates containing iron, aluminum, and magnesium. The method converts the phosphorus in the raffinate acid into industrial-grade purified phosphoric acid to the maximum extent through reasonable process design, and at the same time converts the by-product of low utilization value and insoluble phosphate salts with basically no practical use into hydroxy calcium phosphate with high value and wide application, realizing the comprehensive recovery and efficient utilization of phosphorus resources in the raffinate acid.

Claims (35)

A method for recycling raffinate acid in a wet purification process for producing phosphoric acid, characterized in that it comprises the following steps: removing sulfur, fluorine, arsenic and heavy metals in the raffinate acid to be treated to obtain pretreated acid; pre-neutralizing the pretreated acid, and then purifying it to obtain a purified liquid and a precipitated residue containing iron, aluminum and magnesium, and distilling the purified liquid to obtain industrial-grade phosphoric acid. The recycling method according to claim 1 is characterized in that the raffinate acid to be treated is first subjected to rough desulfurization to obtain a first intermediate acid; fluorine, arsenic and heavy metals in the first intermediate acid are removed to obtain a second intermediate acid; and the second intermediate acid is subjected to fine desulfurization to obtain the pretreated acid. The recycling method according to claim 2, characterized in that the raffinate acid to be treated is mixed with a crude desulfurization agent to perform crude desulfurization; The crude desulfurizing agent includes at least one of phosphate concentrate calcium carbonate, calcium hydroxide and calcium oxide. The recycling method according to claim 3 is characterized in that the molar ratio of calcium in the crude desulfurizer to sulfate in the raffinate acid is (0.8:1)-(1.0:1). The recycling method according to any one of claims 2 to 4, characterized in that the first intermediate acid is mixed with a defluorinating agent, a sulfide and a filter aid to remove fluorine, arsenic and heavy metals. The recycling method according to claim 5, characterized in that the defluorination agent comprises at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium phosphate, disodium hydrogen phosphate and sodium dihydrogen phosphate, and activated diatomaceous earth; Or, the sulfide includes at least one of sodium sulfide, calcium sulfide, potassium sulfide and phosphorus pentasulfide; Alternatively, the filter aid comprises activated carbon. The recycling method according to claim 6, characterized in that the molar ratio of Na in the defluorinating agent to F in the first intermediate acid is (0.4:1)-(0.8:1); Or, the molar ratio of As in the first intermediate acid to S in the sulfide is (1:15)-(1:75); Alternatively, the filter aid is used in an amount of 0.5 wt % to 5 wt % of the first intermediate acid. The recycling method according to any one of claims 2 to 7, characterized in that the second intermediate acid is mixed with an oxidant and a fine desulfurization agent to perform fine desulfurization to obtain the pretreated acid and fine desulfurization slag; The fine desulfurizing agent includes at least one of barium hydroxide and barium carbonate. The recycling method according to claim 8, characterized in that the amount of the oxidant used is 0.5wt%-1.5wt% of the second intermediate acid; Alternatively, the molar ratio of the barium in the fine desulfurizing agent to the sulfate in the second intermediate acid is (1.5:1)-(2.5:1). The recycling method according to claim 8 is characterized in that the fine desulfurization slag is returned to the rough desulfurization process to be used as a rough desulfurization agent. The recycling method according to any one of claims 1 to 10, characterized in that, before pre-neutralization, the pre-treated acid is concentrated and solid-liquid separated to obtain concentrated pre-treated acid and a first precipitated residue containing iron phosphate and aluminum phosphate; The content of P ₂ O ₅ in the concentrated pre-treatment acid is not less than 40 wt %. The recycling method according to claim 11, characterized in that the concentrated pretreatment acid is mixed with a neutralizing agent for pre-neutralization to obtain a pre-neutralized reaction liquid; The neutralizing agent includes salts of potassium, sodium, ammonium and at least one of ammonia, sodium hydroxide and potassium hydroxide. The recycling method according to claim 12, characterized in that the salts of potassium, sodium and ammonium are in the form of at least one of carbonate, bicarbonate and phosphate; Alternatively, the molar ratio of M in the neutralizing agent to phosphorus in the concentrated pretreatment acid is (0.2:1)-(0.4:1), and M corresponds to ammonia, sodium and/or potassium contained in the neutralizing agent. The recycling method according to claim 12 or 13 is characterized in that the purification process comprises: mixing the pre-neutralization reaction liquid with a first precipitant to perform a first purification, and separating the solid from the liquid to obtain a first purified liquid and a second precipitated residue containing iron phosphate, aluminum phosphate and magnesium phosphate; The first purification process includes at least one of the following characteristics: Feature 1: the first precipitant includes at least one of methanol, ethanol, propanol, butanol and acetone; Feature 2: The usage amount of the first precipitant is 1 to 2.5 times of the pre-neutralization reaction solution; Feature 3: The temperature of the first purification is 25℃-65℃. The recycling method according to claim 14 is characterized in that the purification process further comprises: mixing the first purification liquid with a detergent to perform a second purification, and separating the solid and the liquid to obtain a second purification liquid and a first washing liquid; The second purification process includes at least one of the following characteristics: Feature 1: The solute in the detergent includes at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, potassium dihydrogen phosphate, potassium monohydrogen phosphate and potassium phosphate; Feature 2: The mass fraction of the solute in the detergent is not less than 10%; Feature 3: The molar ratio of M in the detergent to P in the first purification solution is (0.15:1)-(0.3:1), and M corresponds to the sodium and/or potassium contained in the detergent. The recycling method according to claim 15 is characterized in that the first washing liquid is returned to the pre-neutralization process to be used as a neutralizing agent. The recycling method according to claim 15 or 16 is characterized in that the purification process further comprises: mixing the second purified liquid with a second precipitant to perform a third purification, and separating the solid and the liquid to obtain a third purified liquid and a third precipitated residue containing a small amount of magnesium phosphate and alkali metal phosphate; The third purification process includes at least one of the following characteristics: Feature 1: The second precipitant includes at least one of methanol, ethanol, propanol, butanol and acetone; Feature 2: The usage amount of the second precipitant is 20wt%-100wt% of the second purified liquid. The recycling method according to claim 17 is characterized in that the third precipitated residue is returned to the first purification process, or the third precipitated residue is mixed with the first precipitated residue and the second precipitated residue and then pulped and recovered. The recycling method according to claim 17 is characterized in that industrial-grade phosphoric acid is obtained after the third purification liquid is distilled. The recycling method according to claim 18, characterized in that the third precipitated slag is mixed with the first precipitated slag and the second precipitated slag to make pulp for recycling, comprising: The first precipitated residue, the second precipitated residue and the third precipitated residue are mixed with water and a first alkaline substance, neutralized and precipitated to obtain a first slurry; the first slurry is subjected to solid-liquid separation to obtain a separated liquid and phosphate precipitated residue. The recycling method according to claim 20, characterized in that the pulping recovery includes at least one of the following characteristics: Feature 1: the water used to mix with the first precipitated slag, the second precipitated slag and the third precipitated slag is desalted water; Feature 2: The mass ratio of water to the total amount of the first precipitated residue, the second precipitated residue and the third precipitated residue is (2:1)-(5:1); Feature 3: The first alkaline substance includes at least one of sodium hydroxide, potassium hydroxide, sodium orthophosphate and potassium orthophosphate; Feature 4: The pH value of the first slurry is 5.5-8.0; Feature 5: The separated liquid is recycled as a detergent in the second purification process, or the separated liquid is recycled as a neutralizing agent in the pre-neutralization process, or the separated liquid is recycled as a defluorinating agent in the defluorinating process. The recycling method according to claim 20 or 21 is characterized in that the phosphate precipitation slag is mixed with water to obtain a second slurry; the second slurry is mixed with a second alkaline substance and subjected to secondary cross-current leaching, solid-liquid separation, to obtain dephosphorization slag and orthophosphate mother liquor. The recycling method according to claim 22, characterized in that the mass ratio of the phosphate precipitate residue to water is (1:4)-(1:9); Alternatively, the second alkaline substance includes at least one of sodium hydroxide and potassium hydroxide. The recycling method according to claim 22 or 23, characterized in that the secondary cross-flow leaching comprises: subjecting the second slurry and the second alkaline substance to a first-stage leaching to obtain a first-stage leachate and a first sediment residue; The first stage leaching process includes at least one of the following features: Feature 1: The pH value of the reaction solution during the leaching process is 13-14; Feature 2: Leaching temperature is 50℃-85℃; Feature 3: The residence time of the reaction slurry is 25min-45min; Feature 4: The mass ratio of the first-stage leachate to the first sediment is (3:1)-(9:1). The recycling method according to claim 24 is characterized in that part of the orthophosphate mother liquor obtained from the first stage leaching is reused to mix with the first precipitated slag, the second precipitated slag and the third precipitated slag. The recycling method according to claim 24 or 25, characterized in that the secondary cross-flow leaching further comprises: performing a second-stage leaching on the first slag and the second alkaline substance to obtain a second-stage leachate and a second slag; The second stage leaching process includes at least one of the following features: Feature 1: The pH value of the reaction solution during the leaching process is 14.0-14.3; Feature 2: Leaching temperature is 45℃-55℃; Feature 3: The residence time of the reaction slurry is 60min-90min; Feature 4: The mass ratio of the second-stage leachate to the second sediment is (5:1)-(9:1). The recycling method according to claim 26, characterized in that the second precipitated slag is subjected to alkali washing, and after solid-liquid separation, a first washing liquid and a first slag slurry are obtained; The first washing liquid is recycled to the second stage leaching process; the first slurry is washed with water, and the solid and liquid are separated to obtain the second washing liquid and the dephosphorization slag. The recycling method according to claim 27 is characterized in that the second washing liquid is recovered to the alkaline washing process of the second slag. The recycling method according to claim 26 is characterized in that lime milk and the second-stage leachate are subjected to a first double decomposition reaction to precipitate phosphate in the secondary leachate, and the solid-liquid is separated to obtain a second slurry; the second slurry is subjected to a second double decomposition reaction with the first-stage leachate, and the solid-liquid is separated to obtain an alkali solution and calcium hydroxyphosphate. The recycling method according to claim 29, characterized in that the lime milk is obtained by mixing a precipitant with water; The precipitating agent includes at least one of calcium oxide and calcium hydroxide. The recycling method according to claim 30 is characterized in that the mass ratio of the precipitant to water is (1:3)-(1:5). The recycling method according to claim 30 or 31 is characterized in that the molar ratio of calcium in the precipitant to phosphorus in the second-stage leachate is (1.5:1)-(1.6:1). The recycling method according to any one of claims 29 to 32 is characterized in that the temperature of the first metathesis reaction and the second metathesis reaction is independently 40° C. to 60° C., or the time of the first metathesis reaction and the second metathesis reaction is independently 30 min to 60 min. The recycling method according to any one of claims 29 to 33 is characterized in that the hydroxy calcium phosphate is washed; and the second washing liquid obtained after washing the hydroxy calcium phosphate is collected and returned to the precipitant slurrying and milking process. The recycling method according to claim 29 is characterized in that the suspended solids in the alkali solution after the double decomposition and dephosphorization are removed and concentrated to obtain a concentrated alkali solution; and the concentrated alkali solution is reused to carry out leaching treatment on the second slurry.