Method for preparing acid by direct wet process of collophanite

Abstract

The invention provides a method for directly preparing acid by using collophanite through a wet method, which comprises the following steps: S1, grinding collophanite until the collophanite with the particle size of-74 mu m accounts for 40-70%, and entering ore pulp which is qualified in ore grinding into the next step; s2, formic acid or lactic acid is added into the qualified concentrate pulp obtained in the step S1 for acidolysis magnesium removal, and magnesium-removed concentrate and magnesium-containing filtrate are obtained; s3, the filtrate containing the magnesium ions and the organic acid in the S2 is circulated for multiple times and then subjected to organic acid regeneration treatment through oxalic acid, and regenerated acid liquor is returned to the acidolysis magnesium removal step; s4, phosphoric acid is added into the magnesium-removed concentrate obtained in the step S2 for acidolysis desiliconization treatment, and a silicon-containing oxide and a high-concentration monocalcium phosphate solution are obtained; s5, sulfuric acid is added into the monocalcium phosphate solution for crystallization and decalcification, and crude phosphoric acid and dihydrate gypsum are obtained. According to the method, carbonate impurities and silicon-containing impurities are removed sequentially by adopting organic acid and phosphoric acid step by step, then sulfuric acid is added for crystallization and decalcification treatment to obtain crude phosphoric acid and dihydrate gypsum, flotation is not needed, and the quality of phosphoric acid and gypsum can be remarkably improved.

Description

Technical Field

[0001] The invention belongs to the field of wet-process phosphoric acid, and in particular relates to a method for directly producing acid by wet-process using collophanite. Background Art

[0002] Collophanite usually refers to a collection of phosphates with a small amount of impurity elements such as magnesium, aluminum, silicon, and iron. It is the main raw material for humans to obtain phosphorus. There are two processes for producing acid from phosphate rock: thermal and wet processes. The wet process has a production cost 20-30% lower than the thermal process, making it dominant.

[0003] Wet phosphoric acid is divided into sulfuric acid method, nitric acid method and hydrochloric acid method according to the types of inorganic acids in the process. Most of the phosphate ores in my country have high impurity content and cannot be directly used for wet phosphoric acid production. It is necessary to remove most of the impurities through mineral processing technology before wet acid can be produced.

[0004] The impurities in collophanite are mainly composed of two types: carbonates and silicates. Carbonates include minerals such as dolomite and calcite, and silicates include minerals such as feldspar, quartz and mica.

[0005] Excessive impurity content in phosphate concentrate has the following disadvantages: phosphoric acid and sulfuric acid can decompose impurities, increasing the production cost of wet acid production; feldspar and mica acidification releases a large amount of potassium ions and sodium ions that react with fluorosilicic acid to form potassium fluorosilicate and sodium fluorosilicate. During the process of transporting the slurry in pipelines, the temperature drops and causes potassium fluorosilicate and sodium fluorosilicate to cool and crystallize, blocking the pipeline and affecting the continuity of production; acid hydrolysis impurities are prone to produce a large amount of metal cations, which enter phosphoric acid and not only increase the cost of phosphoric acid purification, but also affect the quality of back-end products; a large amount of silicon-containing acid-insoluble matter will affect the whiteness and purity of phosphogypsum.

[0006] Patents CN116374975A ​​and CN114956025A propose the use of phosphoric acid to decompose phosphate rock to prepare monocalcium phosphate. The paper "Research on Wet Phosphoric Acid Decalcification Process" uses sulfuric acid to remove calcium ions from monocalcium phosphate to prepare phosphoric acid and obtain high-quality gypsum. Selective acid demagnesiation with lactic acid can significantly reduce the magnesium content, and its magnesium removal effect is better than that of flotation process, which can effectively reduce the magnesium content in phosphoric acid; phosphoric acid is weaker than sulfuric acid. First, phosphoric acid is used to decompose phosphate rock to reduce the dissolution of impurity elements in aluminosilicate minerals, and then sulfuric acid crystallization decalcification can improve the quality of phosphogypsum.

[0007] The method of the invention does not require flotation to remove impurities, has a wide range of applications, and has high quality phosphoric acid and by-product gypsum. It can significantly reduce the production cost of wet phosphoric acid purification and improve the utilization rate of such medium and low-grade calcium-silicon colloid phosphate ores. Summary of the invention

[0008] The invention provides a method for directly preparing acid from collophanite by wet process, which has simple process flow, is stable and easy to operate, and can significantly improve the quality of phosphoric acid and gypsum.

[0009] The technical solution of the present invention is a method for directly producing acid from collophanite by wet process, comprising the following steps: S1. The phosphate rock is crushed and ground. The grinding process adopts a closed-circuit grinding process. The grinding to -74μm accounts for 40~70%. The grinding fineness meets the requirements and enters the next step.

[0010] The phosphate rock described in S1 has a P2O5 grade of 15-30%, a MgO grade of 1.0-10.0%, and an Al2O3 grade of 1-10%.

[0011] S2, transferring the ore pulp with qualified fineness obtained in S1 to a reactor, heating the ore pulp and adding an organic weak acid to perform acid removal of magnesium, and separating the reaction slurry into solid and liquid. The obtained solid phase is the de-magnesiumized phosphate concentrate, and the liquid phase is the magnesium-containing filtrate.

[0012] The organic acid used in the acid-free magnesium is lactic acid or formic acid. The process of acid-free magnesium is mainly that the organic acid decomposes dolomite and calcite to generate organic acid calcium salt, organic acid magnesium salt, carbon dioxide and water.

[0013] The amount of lactic acid or formic acid in S2 is 5-40% of the mass of the reaction slurry, and the acid solution is added in a concentration of 2-50%.

[0014] Unsaturated acid is also added to the organic acid solution, and the unsaturated acid includes any one or more combinations of hydroxyethylethylenediaminetriacetic acid, tiglic acid, 3,3-dimethylacrylic acid, and 2-(trifluoromethyl)acrylic acid. The amount of the unsaturated acid is 0.1-2wt% of the amount of the organic acid solution.

[0015] The unsaturated acid in the present invention helps to promote formic acid or lactic acid to achieve the technical effect of acid-releasing magnesium.

[0016] The reaction temperature for acid removal of magnesium in S2 is 20-80°C, and the reaction liquid-to-solid ratio is 3:1-10:1.

[0017] The magnesium content of the de-magnesiumized phosphate concentrate obtained by acid-freezing magnesium in S2 is ≤0.2%, more preferably the magnesium content of the de-magnesiumized phosphate concentrate is ≤0.1%, more preferably the magnesium content of the de-magnesiumized phosphate concentrate is ≤0.05%, and more preferably the magnesium content of the de-magnesiumized phosphate concentrate is ≤0.01%.

[0018] For those skilled in the art, it is inevitable to use clean water to perform 4 to 8 countercurrent washings on the demagnesized phosphate concentrate obtained in S2, and the washing water from one washing is used to prepare an organic acid solution or to adjust the liquid-solid ratio of the magnesium leaching operation.

[0019] The filtrate obtained by the solid-liquid separation in S2 is returned to the magnesium leaching operation. The organic acid in the filtrate is not completely consumed and still needs to be returned to the next round of magnesium leaching operation. After 2 to 8 cycles, the magnesium leaching filtrate enters the next step.

[0020] S3, the magnesium leaching filtrate obtained in S2 after 2 to 8 reactions is subjected to organic acid regeneration treatment, and oxalic acid is used for organic acid regeneration. After the reaction is completed, the solid phase and filtrate are obtained by solid-liquid separation, and the filtrate rich in organic acid is returned to the magnesium leaching stage, and the solid phase is countercurrently washed with clean water for 4 to 8 times.

[0021] The acid used in the regeneration treatment of organic acid in S3 is oxalic acid, which can not only produce weak acid to generate lactic acid or formic acid, but also precipitate Ca 2+ and Mg 2+ .

[0022] The solid phase in S3 is counter-currently washed 4 to 8 times with clean water, and the washing water from the first wash is used to prepare the organic acid solution or to adjust the liquid-to-solid ratio of the magnesium leaching operation.

[0023] S4, the de-magnesiumized phosphate concentrate obtained in S2 is subjected to acid desiliconization using phosphoric acid whose acidity is weaker than sulfuric acid. After the acidolysis reaction is completed, acid-insoluble slag and filtrate are obtained through solid-liquid separation, and the filtrate enters the next step.

[0024] Phosphoric acid decomposition of magnesium concentrate in S4 mainly includes the following chemical reactions: Ca5F(PO4)3+ 7H3PO4= 5Ca(H2PO4)2+ HF The phosphoric acid-to-ore ratio in S4 is 4:1-10:1, and the acid concentration of the added phosphoric acid is 20-30%.

[0025] The phosphoric acid hydrolysis reaction temperature in S4 is 40-90° C., the reaction liquid-to-solid ratio is 3:1-10:1, and the reaction time is 30-60 min.

[0026] The acid-insoluble matter S4 is a silicon-containing compound, including minerals such as silicon dioxide, feldspar and mica that cannot be decomposed by phosphoric acid.

[0027] The filtrate obtained by the solid-liquid separation in S4 is a high-concentration monocalcium phosphate solution containing a small amount of impurities, and the filtrate enters the next step.

[0028] S5. Sulfuric acid is added to the high-concentration calcium dihydrogen phosphate solution obtained in S4 to perform crystallization and decalcification treatment. After the reaction is completed, solid-liquid separation is performed. The white solid phase is dihydrate gypsum, and the liquid phase is crude phosphoric acid.

[0029] S5 medium and high concentration calcium dihydrogen phosphate solution sulfuric acid crystallization decalcification mainly has the following chemical reactions: Ca(H2PO4)2+ H2SO4+ H2O = CaSO4·H2O +2H3PO4 The reaction temperature of sulfuric acid crystallization decalcification in S5 is 60-90° C., the amount of sulfuric acid used is 0.85-1.02 times the mass of the demagnesized concentrate, and the crystallization reaction time is about 20 minutes.

[0030] The concentration of phosphoric acid obtained in S5 is ≥38%, more preferably ≥40%, and more preferably ≥42%; Alumina <1%, MgO <0.2%; more preferably, alumina <0.5%, MgO <0.1%; further preferably, alumina <0.1%, MgO <0.05%.

[0031] The grade of dihydrate gypsum obtained in S5 is ≥97% and the whiteness is ≥90%. Phosphoric acid hydrolysis of phosphate concentrate can separate gangue minerals such as feldspar and mica that can be decomposed by sulfuric acid, which can reduce the entry of silicon-containing oxides into gypsum and prevent potassium ions and magnesium ions decomposed by impurities such as feldspar and mica from entering the acid solution, while improving the quality of gypsum and phosphoric acid.

[0032] The phosphogypsum needs to be washed 3-5 times in countercurrent with clean water, and the washing water is returned to the phosphoric acid hydrolysis reaction stage in S4 to achieve full process recycling. DETAILED DESCRIPTION

[0033] The embodiments of the present invention will be described in detail below with reference to examples, but those skilled in the art will appreciate that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention.

[0034] Example 1 The method of the present invention is used to prepare phosphoric acid from calcium siliceous colloidal phosphate with a P2O5 grade of 25.33%, a MgO grade of 2.05% and an Al2O3 grade of 4.13%.

[0035] The above-mentioned phosphate rock is ground to -74μm, accounting for 55-60%; the ground slurry is transferred to the reactor, and lactic acid with a mass concentration of 40% is added according to the acid-ore ratio of 1.8:1, and then water is added to adjust the liquid-solid ratio to 6:1, and the temperature is raised to 50°C for reaction for 60 minutes. The filtrate containing lactic acid obtained by solid-liquid separation is continuously returned to the acid-demagnesium removal, and the magnesium-removed concentrate is countercurrently washed 5 times with clean water, and the washing water is used to adjust the liquid-solid ratio in the acid-demagnesium removal stage; the lactic acid filtrate after the three-cycle reaction is precipitated with oxalic acid Ca 2+ and Mg 2+, use calcium chloride to determine whether oxalic acid is added excessively, and add lactic acid until no white precipitate is produced; add phosphoric acid to the washed demagnesified concentrate to decompose the phosphate rock, with an acid-ore ratio of 5:1, and react at 65°C for 40 minutes, and separate the solid and liquid to obtain a monocalcium phosphate solution and acid-insoluble slag; continue to heat the monocalcium phosphate solution to 80°C, and add sulfuric acid for crystallization and decalcification; then separate the solid and liquid to obtain phosphogypsum and phosphoric acid, and wash the phosphogypsum with clean water three times in countercurrent, and return the washing water to the phosphoric acid acidification desiliconization stage to reduce the loss of phosphoric acid.

[0036] The above steps were repeated for 10 cycles. After all stages became stable, phosphoric acid with an acid concentration of 41.34%, a MgO content of 0.17%, an Al2O3 content of 0.89% and phosphogypsum with a grade of 98.39% and a whiteness of 93.45% were obtained.

[0037] Example 2 A certain calci-silico-phosphite has a P2O5 grade of 20.59%, a MgO grade of 3.45%, and an Al2O3 grade of 5.21%, carbonate impurities include calcite and dolomite, and aluminum-containing impurities are mainly potassium feldspar and sodium feldspar. The phosphate ore is used to prepare phosphoric acid by the method of the present invention.

[0038] First, the phosphate rock is ground to -74μm, accounting for 55-60%; the ground slurry is mixed with lactic acid at an acid-ore ratio of 2:1, and lactic acid is added with a concentration of 40%, and the liquid-solid ratio is adjusted to 6:1. The temperature is raised to 70°C for reaction for 70 minutes, and the solid-liquid separation obtains acid solution to continue acid removal of magnesium. The concentrate is countercurrently washed with clean water for 5 times, and the washing water is used to adjust the liquid-solid ratio; after the lactic acid is circulated for 3 times, oxalic acid is used to regenerate lactic acid, and calcium chloride is used to determine whether the oxalic acid is added excessively; phosphoric acid is added to decompose the washed phosphate rock, the acid-ore ratio is 5.5:1, the reaction temperature is 60°C, and the reaction time is 40 minutes to obtain monocalcium phosphate solution and acid-insoluble slag; the monocalcium phosphate solution is further heated to 80°C, and sulfuric acid is added for crystallization and decalcification to obtain phosphogypsum and phosphoric acid. The phosphogypsum is countercurrently washed with clean water for 3 times, and the washing water is returned to the phosphoric acid acid removal operation.

[0039] The above wet phosphoric acid steps were repeated more than 10 times. After the product was stabilized, phosphoric acid with an acid concentration of 38.58%, a MgO content of 0.27%, and an Al2O3 content of 0.95% was obtained. The by-product phosphogypsum had a grade of 98.52% and a whiteness of 94.11%.

[0040] Example 3 The method of the present invention is used to prepare phosphoric acid from calcium siliceous colloidal phosphate with a P2O5 grade of 28.65%, a MgO grade of 1.06% and an Al2O3 grade of 3.24%.

[0041] The phosphate rock is ground to -74μm, accounting for 50-55%; the ground slurry is mixed with formic acid at an acid-ore ratio of 1.8:1, the formic acid concentration is 40%, the liquid-solid ratio is adjusted to 6:1, the temperature is raised to 60°C for reaction for 50 minutes, the acid is filtered to continue acid removal of magnesium, the concentrate is countercurrently washed with clean water for 5 times, and the washing water is used to adjust the liquid-solid ratio; after the formic acid is circulated 6 times, oxalic acid is used for regeneration, and calcium chloride is used as an indicator; then phosphoric acid is used to decompose the magnesium concentrate, the acid-ore ratio is 5:1, the reaction temperature is 60°C, and the reaction time is 40 minutes to obtain monocalcium phosphate solution and acid-insoluble slag; the monocalcium phosphate solution is heated to 80°C for crystallization and decalcification to obtain phosphogypsum and phosphoric acid, the phosphogypsum is countercurrently washed with clean water for 3 times, and the washing water is returned to the phosphoric acid acid removal operation.

[0042] The above wet phosphoric acid steps were repeated more than 10 times until the process flow and product indicators were stable, and phosphoric acid with an acid concentration of 42.45%, a MgO content of 0.11%, and an Al2O3 content of 0.67% was obtained. The by-product phosphogypsum had a grade of 99.10% and a whiteness of 93.14%.

[0043] Example 4 The method of the present invention is used to prepare phosphoric acid from calcium siliceous colloidal phosphate with a P2O5 grade of 29.44%, a MgO grade of 1.15% and an Al2O3 grade of 2.63%.

[0044] The phosphate rock is ground to -74μm, accounting for 50-55%; the ground slurry is mixed with formic acid at an acid-ore ratio of 1.8:1, the formic acid concentration is 40%, the liquid-solid ratio is adjusted to 6:1, the temperature is raised to 60°C for reaction for 50 minutes, the filtrate is further acidified to remove magnesium, the concentrate is countercurrently washed with clean water for 5 times, and the liquid-solid ratio in the dissolution and magnesium removal stage is adjusted with the first washing water; after the formic acid is circulated 6 times, oxalic acid is used for regeneration, and calcium chloride is used as an indicator; phosphoric acid is used to decompose the concentrate, the acid-ore ratio is 5:1, the reaction temperature is 60°C, the reaction time is 40 minutes, and the solid-liquid separation is performed to obtain monocalcium phosphate solution and acid-insoluble slag; the monocalcium phosphate solution is heated to 80°C for crystallization and decalcification to obtain phosphogypsum and phosphoric acid, the phosphogypsum is countercurrently washed with clean water for 3 times, and the first washing water is returned to the phosphoric acid acid desiliconization operation.

[0045] The above steps are repeated until the process flow and product indicators of the ore are stable, and phosphoric acid with an acid concentration of 43.58%, MgO content of 0.17%, and Al2O3 content of 0.71% is obtained. The by-product phosphogypsum has a grade of 98.78% and a whiteness of 93.25%.

[0046] Example 5 The P2O5 grade of a certain calci-silico-phosphate ore is 18.85%, the MgO grade is 3.23% and the Al2O3 grade is 5.25%. The method of the present invention is used to prepare phosphoric acid from the phosphate ore.

[0047] First, grind the phosphate rock to -74μm, accounting for 55-60%; then add lactic acid with a mass concentration of 40% according to the acid-ore ratio of 2:1, adjust the liquid-solid ratio to 6:1, heat to 70℃ and react for 750min to obtain the de-magnesium feed and filtrate, the filtrate continues to return to the acid to remove the magnesium, the concentrate is washed 5 times with clean water in countercurrent, and the washing water is used to adjust the liquid-solid ratio of the de-magnesium stage; after the lactic acid filtrate is circulated for 3 times, oxalic acid is used to regenerate the lactic acid, and Ca is precipitated. 2+ and Mg 2+ , using calcium chloride as an indicator; adding phosphoric acid to decompose the demagnesium concentrate, with an acid-ore ratio of 5:1, reacting at 65°C for 40 minutes to obtain monocalcium phosphate solution and acid-insoluble slag; heating the monocalcium phosphate solution to 80°C, and using sulfuric acid crystallization to decalcify; finally obtaining phosphogypsum and phosphoric acid, and the phosphogypsum is countercurrent washed 3 times with clean water.

[0048] Repeat the above steps until the process and product indicators are stable, and obtain phosphoric acid with an acid concentration of 38.22%, a MgO content of 0.22%, an Al2O3 content of 0.90% and phosphogypsum with a grade of 99.15% and a whiteness of 93.43%.

[0049] Example 6 The method of the present invention is used to prepare phosphoric acid from calcium siliceous colloidal phosphate with a P2O5 grade of 25.33%, a MgO grade of 2.05% and an Al2O3 grade of 4.13%.

[0050] The above-mentioned phosphate rock is ground to -74μm, accounting for 55-60%; the ground slurry is transferred to a reactor, and lactic acid with a mass concentration of 40% is added according to an acid-ore ratio of 1.8:1, and 0.5wt% of hydroxyethylethylenediaminetriacetic acid accounting for the amount of lactic acid is added, and then water is added to adjust the liquid-solid ratio to 6:1, and the temperature is raised to 50°C for reaction for 60 minutes. The filtrate containing lactic acid obtained by solid-liquid separation is continuously returned to the acid-demagnesium removal, and the magnesium-removed concentrate is countercurrently washed 5 times with clean water, and the liquid-solid ratio is adjusted in the acid-demagnesium removal stage with the washing water; the lactic acid filtrate after the three-cycle reaction is precipitated with oxalic acid Ca 2+ and Mg 2+ , use calcium chloride to determine whether oxalic acid is added excessively, and add lactic acid until no white precipitate is produced; add phosphoric acid to the washed demagnesified concentrate to decompose the phosphate rock, with an acid-ore ratio of 5:1, and react at 65°C for 40 minutes, and separate the solid and liquid to obtain a monocalcium phosphate solution and acid-insoluble slag; continue to heat the monocalcium phosphate solution to 80°C, and add sulfuric acid for crystallization and decalcification; then separate the solid and liquid to obtain phosphogypsum and phosphoric acid, and wash the phosphogypsum with clean water three times in countercurrent, and return the washing water to the phosphoric acid acidification desiliconization stage to reduce the loss of phosphoric acid.

[0051] The above steps were repeated for 10 cycles. After all stages became stable, phosphoric acid with an acid concentration of 43.15%, a MgO content of 0.04%, an Al2O3 content of 0.1% and phosphogypsum with a grade of 99.12% and a whiteness of 95.25% were obtained.

[0052] Example 7 Phosphoric acid is prepared from calcium siliceous colloidal phosphate with a P2O5 grade of 25.33%, a MgO grade of 2.05% and an Al2O3 grade of 4.13% using the method of the present invention.

[0053] The above-mentioned phosphate rock is ground to -74μm, accounting for 55-60%; the ground slurry is transferred to a reactor, and lactic acid with a mass concentration of 40% is added according to an acid-ore ratio of 1.8:1, 3,3-dimethylacrylic acid accounting for 0.5wt% of the lactic acid is added, and then water is added to adjust the liquid-solid ratio to 6:1, and the temperature is raised to 50°C for reaction for 60min. The filtrate containing lactic acid obtained by solid-liquid separation is continuously returned to the acid-demagnesium removal, and the magnesium-removed concentrate is countercurrently washed 5 times with clean water, and the washing water is used to adjust the liquid-solid ratio in the acid-demagnesium removal stage; the lactic acid filtrate after the three-cycle reaction is precipitated with oxalic acid to precipitate Ca 2+ and Mg 2+ , use calcium chloride to determine whether oxalic acid is added excessively, and add lactic acid until no white precipitate is produced; add phosphoric acid to the washed demagnesified concentrate to decompose the phosphate rock, with an acid-ore ratio of 5:1, and react at 65°C for 40 minutes, and separate the solid and liquid to obtain a monocalcium phosphate solution and acid-insoluble slag; continue to heat the monocalcium phosphate solution to 80°C, and add sulfuric acid for crystallization and decalcification; then separate the solid and liquid to obtain phosphogypsum and phosphoric acid, and wash the phosphogypsum with clean water three times in countercurrent, and return the washing water to the phosphoric acid acidification desiliconization stage to reduce the loss of phosphoric acid.

[0054] The above steps were repeated for 10 cycles. After all stages became stable, phosphoric acid with an acid concentration of 42.67%, a MgO content of 0.05%, and an Al2O3 content of 0.2% and phosphogypsum with a grade of 98.63% and a whiteness of 94.30% were obtained.

Claims

1. A method for direct wet acid production from collophanite, characterized in that: The following steps are involved: S1. Crushing and grinding the collophosphate ore to -74μm accounts for 40~70%; S2, transferring the ore pulp with qualified fineness obtained in S1 to a reactor, heating the ore pulp and adding an organic acid solution to perform acid removal of magnesium, and performing solid-liquid separation of the reaction slurry to obtain de-magnesiumized phosphate concentrate and magnesium-containing filtrate; S3, the de-magnesiumized phosphate concentrate obtained in S2 is subjected to acid desiliconization with phosphoric acid, and after the acidolysis reaction, the acid-insoluble slag and the filtrate are obtained by solid-liquid separation, and the high-concentration monocalcium phosphate filtrate enters the next step; S4, adding sulfuric acid to the high-concentration monocalcium phosphate solution obtained in S3 for crystallization and decalcification, and performing solid-liquid separation of dihydrate gypsum and high-grade phosphoric acid after the reaction is completed.

2. The method according to claim 1, characterized in that: The collophanite described in S1 is siliceous-calcium collophanite, with a P2O5 grade of 15-30%, a MgO grade of 1.0-10.0%, and an Al2O3 grade of 1-10%.

3. The method according to claim 1, characterized in that: The organic acid liquid used in the acid removal of magnesium in S2 is one or more of lactic acid and formic acid, and the amount used is 5-40% of the mass of the reaction slurry, and the acid liquid is added at a concentration of 2-20%.

4. The method according to claim 3, characterized in that: Unsaturated acid is also added to the organic acid solution, and the unsaturated acid includes any one or more combinations of hydroxyethylethylenediaminetriacetic acid, tiglic acid, 3,3-dimethylacrylic acid, and 2-(trifluoromethyl)acrylic acid. The amount of the unsaturated acid is 0.1-2wt% of the amount of the organic acid solution.

5. The method according to claim 1, characterized in that: The reaction temperature of the organic acid solution in S2 for acid-releasing magnesium is 20-80° C., and the reaction liquid-to-solid ratio is 3:1-10:

1.

6. The method according to claim 1, characterized in that: The magnesium filtrate obtained in S2 after 2 to 8 reactions is subjected to organic acid regeneration treatment, and oxalic acid is used to regenerate the organic acid. After the reaction is completed, a solid phase and a filtrate rich in organic acid are obtained by solid-liquid separation. The filtrate rich in organic acid is returned to step S2, and the solid phase is countercurrently washed 4 to 8 times with clean water.

7. The method according to claim 1, characterized in that: The acid-ore ratio of phosphoric acid to demagnesized phosphate concentrate in S3 is 4:1 to 10:

1.

8. The method according to claim 1, characterized in that: When the calcium dihydrogen phosphate described in S4 is decalcified by the sulfuric acid method, the amount of sulfuric acid used is 0.85 to 1.02 times the mass of the demagnesized concentrate.

9. The method according to claim 1, characterized in that: The grade of the dihydrate gypsum obtained by the solid-liquid separation in S5 is ≥97%, and the whiteness is ≥90%.

10. The method according to claim 9, characterized in that: The phosphoric acid concentration obtained by the solid-liquid separation in S5 is ≥40%, aluminum oxide is <1%, and magnesium oxide is <0.2%.