Green process for wet-process phosphoric acid production by means of mixed crystallization and dihydrate process

Abstract

Disclosed is a green process for wet-process phosphoric acid production by means of mixed crystallization and a dihydrate process. The green process at least comprises a reaction step and a filtration step, wherein in the reaction step, a phosphate rock slurry prepared from raw phosphorite, sulfuric acid, and return acid from the filtration step are added to a circulating slurry in multiple reaction tanks in series for a reaction to obtain a reaction slurry; part of the reaction slurry discharged from the last reaction tank is delivered to multiple digestion tanks in series to eliminate supersaturation, and is then delivered to the subsequent filtration step; the remaining reaction slurry is returned as a circulating slurry to the first reaction tank; and the reaction step is divided into two stages. By controlling the ionic product Q of Ca2+ and SO4 2- and eliminating supersaturation in the reaction slurry, the timing for adding the phosphate ore slurry and sulfuric acid and the amounts thereof can be well controlled, thereby reducing the consumption of sulfuric acid while effectively improving the product quality.

Description

A green mixed crystallization dihydrate wet process for phosphoric acid production Technical Field

[0001] This invention relates to the field of phosphogypsum production, specifically a green mixed crystallization dihydrate wet process for producing phosphoric acid. Background Technology

[0002] Phosphoric acid is an important chemical raw material. It can be used directly in the production of phosphate compound fertilizers, and after further processing, it can be used to produce various phosphate products, including food-grade and electronic-grade phosphoric acid. The basic method for producing phosphoric acid using the wet process is by decomposing phosphate rock with sulfuric acid. The main chemical component of phosphate rock is Ca5F(PO4)3. When it reacts with sulfuric acid, it produces phosphoric acid and insoluble calcium sulfate crystals. The chemical reaction is as follows: Ca5F(PO4)3 + 5H2SO4 + mH2O = 3H3PO4 + 5CaSO4·m / 5H2O↓ + HF↑

[0003] Depending on factors such as reaction temperature and the amount of sulfuric acid added, calcium sulfate can precipitate in three different crystalline forms: calcium sulfate dihydrate (CaSO4·2H2O), calcium sulfate hemihydrate (CaSO4·1 / 2H2O), and anhydrous calcium sulfate (CaSO4). The process of crystallizing calcium sulfate dihydrate by controlling the reaction conditions is called the dihydrate phosphoric acid process.

[0004] Currently, China has a wet-process phosphoric acid production capacity of nearly 20 million tons / year of P2O5, of which the dihydrate wet-process phosphoric acid accounts for more than 80%. A typical dihydrate phosphoric acid process includes a reaction step and a filtration step. For example, patent number CN01138286.4 discloses a process for producing dihydrate wet-process phosphoric acid from phosphate rock using sulfuric acid extraction, including extraction reaction, phosphoric acid slurry filtration and separation, and tail gas washing. Sulfuric acid and phosphoric acid react in a multi-chamber reaction unit integrating reaction and digestion functions. The heat released during the reaction is removed through a low-level flash cooling circulation loop. A key feature is that the overflow of the reaction slurry from the reaction chamber to the digestion chamber is achieved within the equipment.

[0005] In existing technologies, the wet-process phosphoric acid dihydrate mainly adjusts the upstream reaction and crystallization process parameters by adjusting the ease of filtrate removal from the end filter and the residual phosphorus (P2O5) content in the phosphogypsum (calcium sulfate dihydrate (CaSO4·2H2O)) filter cake. This results in low phosphorus yield (generally around 94%) and high SO4 content in the product acid during the current wet-process phosphoric acid dihydrate production. 2- Problems such as high root concentration and high solid content, for example, SO4 in 28% P2O5 finished phosphoric acid. 2- Phosphoric acid with a root concentration of over 3.5% (equivalent to 12.5% ​​sulfate in 100% P2O5) and a solid content of 2.5% can only be used for the production of agricultural fertilizers.

[0006] To meet the national requirements for high-end processing and utilization of phosphorus resources, it is necessary to remove SO4 from wet-process phosphoric acid as much as possible. 2- Ionic impurities and solid impurities are used to produce higher purity electronic-grade phosphoric acid, food-grade phosphoric acid, and industrial phosphoric acid. The excessive sulfate ions in the wet-process phosphoric acid produced by the current dihydrate process not only generate solid waste during the removal process, but also lead to a large waste of sulfuric acid resources (approximately 2.5 million tons of sulfuric acid resources worth about 2 billion RMB are wasted annually).

[0007] Publication No. CN 112299386 discloses a method for removing sulfate from wet-process dilute phosphoric acid using phosphate rock slurry. This method utilizes phosphate rock slurry with a water content of 33%-35 wt% as raw material. The slurry is added to the seventh, eighth, and ninth digestion zones of a traditional wet-process phosphoric acid extraction tank. Utilizing the phosphoric acid extraction reaction temperature, the phosphate rock slurry reacts with excess sulfate in the dilute phosphoric acid, reducing the sulfate content. Because sulfate has the best reactivity with dolomite in the phosphate rock slurry, it reacts preferentially, effectively improving the desulfurization effect. Although this technical solution aims to remove sulfate and improve phosphoric acid quality, it still has the following problems: 1) Adding the phosphate rock slurry to the digestion zone introduces additional Ca... 2+ 1) The system, which was originally free of supersaturation, re-enters a supersaturated state, resulting in a significant increase in the amount of CaSO4·2H2O dissolved in the product acid, affecting the operation of the entire system; 2) Unreacted phosphate rock slurry enters the phosphogypsum system, causing the total phosphorus in the phosphogypsum to exceed the standard; 3) The water in the phosphate rock slurry dilutes the phosphoric acid concentration to more than 1% P2O5; therefore, the above measures are not widely used in actual production.

[0008] To date, research on the dihydrate wet-process phosphoric acid production has been based on the process of passing sulfate (SO4) ions from the finished phosphoric acid. 2- The crude method of adjusting the parameters of the front-end reaction and crystallization process by adjusting the amount of phosphorus root and the residual phosphorus (P2O5) in the filter cake of phosphogypsum (calcium sulfate dihydrate (CaSO4·2H2O)) not only wastes resources, but also results in the sulfate content and impurity content in the phosphoric acid product having limits that are difficult to break. Summary of the Invention

[0009] The purpose of this invention is to solve the above-mentioned technical problems and provide a green mixed crystallization dihydrate wet phosphoric acid production process that produces high-quality wet phosphoric acid through a two-step mixed crystallization process based on existing processes and through in-depth research and meticulous control methods.

[0010] This invention discloses a green mixed crystallization dihydrate wet process for producing phosphoric acid, comprising at least a reaction step and a filtration step. In the reaction step, phosphate rock slurry prepared from raw phosphate rock, sulfuric acid, and back acid from the filtration step are added to a circulating slurry in a series of multi-cell reaction tanks to obtain a reaction slurry. A portion of the reaction slurry exiting the last reaction tank is sent to a series of multi-cell digestion tanks to eliminate supersaturation before being sent to the subsequent filtration step. The remaining portion of the reaction slurry is returned to the first reaction tank as a circulating slurry. The invention is characterized by the reaction step being divided into two stages:

[0011] In the first stage, most of the phosphate rock slurry is added to the circulating slurry for reaction, and the Ca content in the reaction slurry is controlled. 2+ and SO4 2- The ion product Q of the radical is slightly greater than that of SO4 in the reaction slurry. 2- Roots and Ca 2+ The saturation solubility product Ks;

[0012] In the second stage, when Ca in the reaction slurry 2+ and SO4 2- The ion product Q of the radical after reaction decreases to SO4 in the reaction slurry. 2- Roots and Ca 2+ When the saturation solubility product Ks is reached, the remaining phosphate rock slurry is added first and reacted for 15-20 minutes. Then, sulfuric acid and back acid from the filtration process are added in multiple batches to reduce the SO4 content in the reaction slurry. 2- Roots and Ca 2+ The ion product Q is always less than that of SO4 in the reaction slurry. 2- Roots and Ca 2+ The critical supersaturation solubility product Kcss.

[0013] In the first stage, the Ca content in the reaction slurry is controlled. 2+ and SO4 2- The ion product Q of the radical is greater than that of SO4 in the reaction slurry. 2- Roots and Ca 2+ The amount of the saturation solubility product Ks is (Q-Ks) / Ks*100% = 10-20%.

[0014] Part of the reaction slurry exiting the last reaction tank is cooled to 60-65°C by the first flash cooler and sent to the multi-cell digester connected in series. The remaining part of the reaction slurry is cooled to 75-76°C by the second flash cooler and then returned to the first reaction tank as circulating slurry.

[0015] The multi-cell reaction tanks connected in series are each equipped with multiple sampling points for detecting SO4 in the reaction slurry. 2- Concentration and Ca 2+ The concentration was calculated to obtain the SO4 concentration in the corresponding reaction slurry in the reaction tank. 2- Roots and Ca 2+The ion product Q, Q = [SO4] 2- ]*[Ca 2+ ].

[0016] SO4 in the reaction slurry 2- Roots and Ca 2+ The method for calculating the saturation solubility product Ks is as follows:

[0017] First calculate the SO4 content in the reaction slurry. 2- Roots and Ca 2+ Saturation solubility product Kd:

[0018] According to publicly available literature, when the phosphoric acid concentration is 20–30% P₂O₅wt, the sulfuric acid concentration is 0–10%wt, and the temperature is 70–80℃, the product of the saturated concentrations of dissolved CaO and H₂SO₄ in a saturated solution of calcium sulfate dihydrate (CaSO₄·2H₂O) in a mixed acid mixture of phosphoric acid and sulfuric acid is a constant.

[0019] [CaO]*[H2SO4]=0.5

[0020] [H2SO4]---Total SO4 in mixed acids 2- Root concentration, expressed as H2SO4 wt%, molecular weight 98;

[0021] [CaO]---Ca dissolved in the mixed acid 2+ Root concentration, expressed as CaO wt%, with a molecular weight of 56;

[0022] At this point, the formula for the saturation solubility product Kd of calcium sulfate dihydrate (CaSO4·2H2O) in the mixed acid is as follows:

[0023] Kd = [SO4] 2- ]*[Ca 2+ ]

[0024] SO4 2- --- Total SO4 in mixed acids 2- Root concentration, in terms of SO4 2- wt% indicates a molecular weight of 96;

[0025] [Ca 2+ ---Ca dissolved in the mixed acid 2+ Root concentration, in terms of Ca 2+ wt% indicates a molecular weight of 40;

[0026] Then, calculate the SO4 content in the reaction slurry. 2- Roots and Ca 2+ Saturated solubility Ks:

[0027] Ks = m * Kd / S, where m is a constant, which is 0.42;

[0028] S---The volume percentage of solid calcium sulfate dihydrate (CaSO4·2H2O) in the reaction slurry, ranging from 22% to 28% vol; the SO4 content in the reaction slurry 2- Roots and Ca 2+ The critical supersaturation solubility Kcss is calculated as follows:

[0029] Kcss = n * Ks * T, where n is a constant, which is 0.21;

[0030] T---Temperature of the reaction slurry (°C); range: 70~80°C.

[0031] In response to the problems existing in the background technology, the inventor changed the conventional thinking and started from the core of the reaction process of the dihydrate wet phosphoric acid process, recognizing that SO4 2- Roots and Ca 2+ The concentration control, i.e., the actual ion product Q < Q = [SO4] 2- ]*[Ca 2+ [The following text appears to be unrelated and possibly from a different source:] SO4 in the reaction slurry is an important control method, and in-depth research is conducted on it. First, the SO4 content in the reaction slurry is calculated using formulas. 2- Roots and Ca 2+ The corresponding saturation solubility product Ks and critical supersaturation solubility product Kcss remain constant throughout the reaction process of the reaction slurry. Then, SO4 levels in the reaction slurry in different reaction tanks were measured through multiple sampling tests. 2- Concentration and Ca 2+ The concentration is calculated, and the ion product Q in the current reaction slurry is adjusted by controlling the timing and amount of addition of phosphate rock slurry, sulfuric acid, and back acid at different reaction stages to regulate the relationship between the ion product Q and the saturation solubility product Ks, as well as the critical supersaturation solubility product Kcss. Specifically, in the first stage, the Ca concentration in the reaction slurry is controlled. 2+ Roots and SO4 2- The ion product Q is greater than that of SO4 in the reaction slurry. 2- Roots and Ca 2+ The saturation solubility product Ks is obtained, and the reaction process is as follows: Ca5F(PO4)3 + 7H3PO4 → 5Ca 2+ +10H2PO4 - +HF↑; (1) Ca 2+ +SO4 2- +2H₂O→CaSO₄·2H₂O↓ (2)

[0032] Although the reaction rate constant k1 of reaction (1) is not large, the concentration of H3PO4 in the reaction slurry exceeds the equilibrium concentration by orders of magnitude, and reaction (1) is completed within a few minutes; due to the formation of CaSO4·2H2O precipitate, the reaction rate constant k2 of reaction (2) is large, but the ion product Q in the slurry < Q = [SO4 2- ]*[Ca 2+ The solubility product was only slightly greater than the saturation solubility product Ks, and reaction (2) was partially completed, ultimately causing SO4 to... 2- Roots and Ca 2+ The calcium sulfate precipitate largely adheres to the existing phosphogypsum crystals, agglomerating into clusters of spherical crystals, reducing phosphate rock encapsulation losses and improving phosphate yield. This can be achieved by controlling the timing and flow rate of phosphate rock addition. Specifically, most of the phosphate rock slurry should be added to the circulating slurry for reaction. The amount added should ideally be such that the ion product Q in the reaction slurry is slightly greater than the saturation solubility product Ks, preferably an excess of 10-20%. Too much excess will cause adjustment lag and poor results, while too little will require frequent adjustments, leading to production chaos.

[0033] As the reaction proceeds, the actual ion product in the slurry, Q < Q = [SO4], decreases. 2- ]*[Ca 2+ The solubility product will decrease to the saturation solubility product Ks, indicating a saturated state. The crystallization rate will slow down. At this point, adding the remaining phosphate rock slurry and sulfuric acid will reduce the SO4 content. 2- Roots and Ca 2+ Calcium sulfate precipitate mainly adheres to existing aggregated spherical phosphogypsum crystals, with crystal diameters growing to between 80 and 140 μm. A small amount of new calcium sulfate crystal nuclei are formed, reacting as follows: H₂SO₄ → 2H₂SO₄ + +SO4 2- (3) CaSO4·2H2O≒Ca 2+ +2SO4 2- +2H₂O→CaSO₄·2H₂O↓+SO₄ 2- (4)

[0034] The dissociation rate constant k3 of sulfuric acid is very large, and reaction (3) proceeds very quickly, reaching 100% completion in just a few minutes; although CaSO4·2H2O precipitate is formed, the reaction rate constant k2 of reaction (2) is very large, but the ion product of the slurry Q < Q = [SO4 2- ]*[Ca 2+ The concentration of Kcss is less than the critical supersaturation solubility product, so reaction (4) proceeds slowly, taking about 1 hour.

[0035] Preferably, after adding the remaining phosphate rock slurry, the reaction continues for 15-20 minutes to further promote the formation of aggregated spherical crystals. Sulfuric acid is added in multiple batches, and the timing and amount of addition should ensure that the ion product Q in the reaction slurry is always less than the critical supersaturation solubility product Kcss. This ensures the crystallization rate while reducing the generation of a large number of new crystal nuclei and improving the product quality of phosphogypsum.

[0036] Preferably, the temperature of the reaction slurry exiting the last reaction tank is around 77°C, which is still in a supersaturated state. The value of the saturation solubility product Ks is still relatively large, and there is still a lot of dissolved CaSO4·2H2O. At this time, the reaction slurry is partially cooled to 60-65°C by a low-level flash cooler. This can reduce the saturation solubility product Ks in the slurry, and all the dissolved CaSO4·2H2O and other impurities will crystallize out and further grow into aggregated cluster spherical crystals, with the crystal diameter further increasing. Beneficial effects:

[0037] This invention is based on the dihydrate wet phosphoric acid process, by controlling the Ca in the reaction slurry 2+ Roots and SO4 2- By controlling the ion product Q and eliminating supersaturation, the timing and amount of phosphate rock slurry and sulfuric acid can be effectively controlled. This reduces sulfuric acid consumption and significantly improves product quality through simple feed adjustment, without requiring major modifications to the existing process. After filtration, the final phosphogypsum crystals have a diameter of 85–150 μm or more, and solid impurities in the product acid are reduced to below 1%, with SO4 content significantly reduced. 2- The root concentration was reduced to below 1.5%, the P2O5 concentration in the reaction slurry was increased by more than 0.5%, and the P2O5 concentration in the product acid reached more than 28.5%, demonstrating significant technical effects. Attached Figure Description

[0038] Figure 1 is a flowchart of an embodiment of the reaction process of the present invention.

[0039] In the diagram, 1-8 are reaction tanks, 9 and 10 are digestion tanks, 11 is the first flash cooler, 12 is the second flash cooler, and 13 is the filter feed pump. Detailed Implementation

[0040] The invention will be further explained below with reference to the accompanying drawings:

[0041] The dihydrate wet phosphoric acid process includes at least a reaction step and a filtration step. The reaction conditions are existing technologies. The filtration step is existing technology and will not be described in detail in this invention.

[0042] SO4 in the reaction slurry 2- Roots and Ca 2+ The method for calculating the saturation solubility product Ks is as follows:

[0043] First calculate the SO4 content in the reaction slurry. 2- Roots and Ca 2+ Saturation solubility product Kd:

[0044] When the phosphoric acid concentration is 20–30% P₂O₅wt, the sulfuric acid concentration is 0–10%wt, and the temperature is 70–80℃, the product of the saturated concentrations of dissolved CaO and H₂SO₄ in a saturated solution of calcium sulfate dihydrate (CaSO₄·2H₂O) in a mixed acid mixture of phosphoric acid and sulfuric acid is a constant: [CaO]*[H₂SO₄]=0.5

[0045] [H2SO4]---Total SO4 in mixed acids 2- Root concentration, expressed as H2SO4 wt%, molecular weight 98;

[0046] [CaO]---Ca dissolved in the mixed acid 2+ Root concentration, expressed as CaO wt%, with a molecular weight of 56;

[0047] At this point, the saturation solubility product Kd for calcium sulfate dihydrate (CaSO4·2H2O) in the mixed acid is as follows: Kd=[SO4·2H2O] 2- ]*[Ca 2+ ]

[0048] SO4 2- --- Total SO4 in mixed acids 2- Root concentration, in terms of SO4 2- wt% indicates a molecular weight of 96;

[0049] [Ca 2+ ---Ca dissolved in the mixed acid 2+ Root concentration, in terms of Ca 2+ wt% indicates a molecular weight of 40;

[0050] Then, calculate the SO4 content in the reaction slurry. 2- Roots and Ca 2+ Saturated solubility Ks:

[0051] Ks = m * Kd / S, where m is a constant, which is 0.42;

[0052] S---The volume content of solids (CaSO4·2H2O) in the reaction slurry, ranging from 22 to 28% vol;

[0053] SO4 in the reaction slurry 2- Roots and Ca 2+ The critical supersaturation solubility Kcss is calculated as follows:

[0054] Kcss = n * Ks * T, where n is a constant, which is 0.21;

[0055] T---Temperature of the reaction slurry (°C), ranging from 70 to 80°C.

[0056] Under the condition that the solid content, P2O5 concentration and reaction temperature of the phosphate rock raw material and reaction slurry remain unchanged, the saturated solubility Ks and the critical supersaturated solubility Kcss calculated in the same reaction process are constant values.

[0057] Example 1: In the reaction process, a phosphate rock slurry with a solid content of 65-70% wt, prepared from raw phosphate rock, sulfuric acid, and back acid from the filtration process are added to the circulating slurry in a series of multi-cell reaction tanks (reaction tanks 1-8 in Figure 1) to obtain a reaction slurry. A portion of the reaction slurry exiting the last reaction tank is cooled to 60-65°C by the first flash cooler 11 and then sent to a series of multi-cell digestion tanks (digestion tanks 9 and 10 in Figure 1) to eliminate supersaturation before being sent to the subsequent filtration process. The remaining portion of the reaction slurry is cooled to 75-76°C by the second flash cooler 12 and returned to the first reaction tank 1 as a circulating slurry. The reaction process is divided into two stages:

[0058] In the first stage, most of the phosphate rock slurry is added to the circulating slurry for reaction, and the SO4 content in the reaction slurry is controlled. 2- Roots and Ca 2+ The ion product Q is greater than that of SO4 in the reaction slurry. 2- Roots and Ca 2+ The amount of the saturation solubility product Ks is (Ks=0.588): (Q-Ks) / Ks*100%=10-20%.

[0059] In the second stage, when SO4 in the reaction slurry 2- Roots and Ca 2+ The ion product Q decreases with the SO4 in the reaction slurry. 2- Roots and Ca 2+ When the saturation solubility product Ks is equal, first add the remaining phosphate rock slurry and react for 15-20 minutes, then add sulfuric acid in multiple batches to reduce the SO4 content in the reaction slurry. 2- Roots and Ca 2+ The ion product Q is always kept less than that of SO4 in the reaction slurry. 2- Roots and Ca 2+ The critical supersaturation solubility product Kcss.

[0060] The multi-cell reactor connected in series may be equipped with multiple sampling points to detect the actual SO4 in the reaction slurry at different stages (in different reactors). 2- Concentration and Ca 2+ After obtaining the relevant data, the concentration of SO4 in the corresponding reaction slurry in the reaction tank can be calculated.2- Roots and Ca 2+ The ion product Q.

[0061] The following example uses domestic phosphate rock as raw material to produce phosphoric acid via a wet process using dihydrate, and Figure 1 illustrates the method of improving phosphoric acid quality according to this invention.

[0062] The raw material phosphate composition is as follows:

[0063] Main chemical components (%) of phosphate concentrate

[0064] In the reaction process of this embodiment, the reaction temperature is set to 77°C, the P2O5 concentration in the reaction slurry is 28.5 wt%, the sulfuric acid concentration is 1.5 wt%, and the volume content of solid calcium sulfate dihydrate (CaSO4·2H2O) is 25%.

[0065] Calculate SO4 in the reaction slurry 2- Roots and Ca 2+ The saturation solubility product Ks is calculated as follows:

[0066] In a saturated solution of calcium sulfate dihydrate (CaSO4·2H2O) in a mixed acid mixture of phosphoric acid and sulfuric acid, the product of the saturated concentrations of dissolved CaO and H2SO4 is a constant: [CaO]*[H2SO4]=0.5

[0067] [H2SO4]---Total SO4 in mixed acids 2- Root concentration, expressed as H2SO4 wt%, molecular weight 98;

[0068] [CaO]---Ca dissolved in the mixed acid 2+ Root concentration, expressed as CaO wt%, with a molecular weight of 56;

[0069] At this point, the saturation solubility product Kd for calcium sulfate dihydrate (CaSO4·2H2O) in the mixed acid is as follows: Kd=[SO4·2H2O] 2- ]*[Ca 2+ ]

[0070] SO4 2- --- Total SO4 in mixed acids 2- Root concentration, in terms of SO4 2- wt% indicates a molecular weight of 96;

[0071] [Ca 2+ ---Ca dissolved in the mixed acid 2+ Root concentration, in terms of Ca 2+ wt% indicates a molecular weight of 40;

[0072] Therefore, Kd = [SO4 2-]*[Ca 2+ =0.5*96*40 / (98*56) = 0.35.

[0073] Then, calculate the SO4 content in the reaction slurry. 2- Roots and Ca 2+ The saturated solubility Ks = 0.42 * Kd / S

[0074] S---The volume content of solids (CaSO4·2H2O) in the reaction slurry, ranging from 22 to 28% vol;

[0075] In this case, the solid volume ratio of the reaction slurry is 25%. Substituting this into the above formula: Ks = 0.35 * 0.42 / 0.25 = 0.588

[0076] Calculate SO4 in the reaction slurry 2- Roots and Ca 2+ The critical supersaturation solubility product Kcss is calculated as follows: Kcss = 0.021 * Ks * T

[0077] T---Temperature of the reaction slurry (°C); range: 70~80°C;

[0078] In this case, the temperature of the reaction slurry is 77℃. Substituting this into the above formula: Kcss=0.021*0.588*77=0.951

[0079] In summary, we obtain SO4. 2- Roots and Ca 2+ The saturation solubility product Ks (Ks=[SO4) 2- ]*[Ca 2+ The value is 0.588, SO4 2- Roots and Ca 2+ The critical supersaturation solubility product Kcss (Kcss = [SO4]) 2- ]*[Ca 2+ The value is 0.951.

[0080] Referring to Figure 1, in the reaction process of this embodiment, reaction tanks 1 to 8 are connected in series, and each reaction tank is equipped with a stirrer.

[0081] First, add most of the phosphate rock slurry to the circulating slurry in the first reaction tank 1 to react, controlling the SO4 content in the reaction slurry. 2- Roots and Ca 2+ The actual ion product Q is greater than that of SO4 in the reaction slurry. 2- Roots and Ca 2+The saturation solubility product Ks is (Ks = 0.588): (Q - Ks) / Ks * 100% = 10 - 20%, that is, the Q value range is between 0.65 and 0.71; In reaction tank 1, phosphate rock decomposes with phosphoric acid in the circulating slurry to generate soluble Ca. 2+ and H2PO 4- Due to SO4 2- Roots and Ca 2+ The ion product Q exceeds the saturation solubility product Ks, SO4 2- Roots and Ca 2+ Calcium sulfate precipitate is formed. In this embodiment, calculations based on samples taken from reaction tank 1 show that the SO4 content in the reaction slurry decreases after adding most of the phosphate rock slurry. 2- Roots and Ca 2+ The ion product Q is 0.71.

[0082] As the reaction proceeds, SO4 precipitates and overflows into the slurry in reaction tanks 2 and 3. 2- Roots and Ca 2+ As the concentration gradually decreases, in this embodiment, after sampling in reaction tank 3, it is calculated that the SO4 concentration in the reaction slurry is... 2- Roots and Ca 2+ The ion product Q drops to 0.6, which is equal to the saturation solubility product Ks (Ks = 0.588) at this time (rounded to one decimal place). At this time, the remaining phosphate rock slurry is added to reaction tank 3 and the reaction continues for 15-20 minutes.

[0083] Then, sulfuric acid and back acid from the filtration process are added to reaction tanks 4-6 in multiple batches. The sulfuric acid dissociates into H+ in the back acid. + and SO4 2- The amount of sulfuric acid added is controlled to ensure that the SO4 content in the reaction slurry of reaction tanks 4, 5, and 6 is within a certain range. 2- Roots and Ca 2+ The ion product Q always remains less than the critical supersaturation solubility product Kcss, which promotes the concentration of most SO42- ions. 2- Roots and Ca 2+ Calcium sulfate precipitates on existing aggregated spherical phosphogypsum crystals, with crystal diameters growing to between 80 and 140 μm, and a small number of new crystal nuclei are formed. In this embodiment, calculations based on samples taken from reaction tanks 4-6 indicate that the SO4 content in the reaction slurry is... 2- Roots and Ca 2+ The ion product Q remained at 0.85, always less than the critical supersaturation solubility product Kcss of 0.951. After passing through reaction tanks 7 and 8, the ion product Q in the reaction slurry decreased to 0.62.

[0084] The reaction slurry overflows sequentially from reaction tank 1 to reaction tank 8. The temperature of the reaction slurry in the last reaction tank 8 is 77℃, which is still in a supersaturated state. At this time, the ion product Q in the reaction slurry drops to 0.62, and there is still a relatively large amount of dissolved CaSO4·2H2O. After part of the reaction slurry is cooled to 60-65℃ by the first flash cooler 11, the saturation solubility product Ks in the slurry drops to 0.52-0.56. All the dissolved CaSO4·2H2O and other impurities crystallize out and then enter the digestion tanks 9 and 10 (both equipped with agitators) in series by gravity for digestion. Then, it is sent to the rotary table filter in the filtration process by the filter feed pump 13.

[0085] The remaining portion of the reaction slurry is pumped to the second flash cooler 12 to be cooled to 75-76°C to remove excess reaction heat, and then returned to the first reaction tank 1 as a circulating slurry, thereby maintaining the reaction temperature in the reaction tank within the stable zone of calcium sulfate dihydrate (CaSO4·2H2O).

[0086] The filtration process described is existing technology and will not be described in detail. For example, the two-water filtration process (such as a two-water filter machine) described in publication number 105036101A can be used.

[0087] Finally, the quality of the prepared product is shown in Table 1.

[0088] Table 1 Performance Evaluation Results of a 500,000 Tons / Year Phosphoric Acid Unit

[0089] Table 1 Performance Evaluation Results of a 500,000 Tons / Year Phosphoric Acid Unit

Claims

1. A green mixed crystallization dihydrate wet process for producing phosphoric acid, comprising at least a reaction step and a filtration step, wherein, In the reaction process, phosphate rock slurry prepared from raw phosphate rock, sulfuric acid, and back acid from the filtration process are added to a circulating slurry in a series of multi-cell reaction tanks to obtain a reaction slurry. A portion of the reaction slurry exiting the last reaction tank is sent to a series of multi-cell digestion tanks to eliminate supersaturation before being sent to the subsequent filtration process. The remaining portion of the reaction slurry is returned to the first reaction tank as a circulating slurry. The reaction process is characterized by being divided into two stages: In the first stage, most of the phosphate rock slurry is added to the circulating slurry to carry out the phosphate rock decomposition reaction, and the Ca content in the reaction slurry is controlled. 2+ and SO4 2- The ion product Q of the radical is slightly greater than that of Ca in the reaction slurry. 2+ and SO4 2- The saturation solubility product of the root, Ks; In the second stage, when SO4 in the reaction slurry 2- Roots and Ca 2+ The ion product Q decreases to Ca in the reaction slurry 2+ and SO4 2- When the saturation solubility product of the root is Ks, first add the remaining phosphate rock slurry and react for 15-20 minutes, then add sulfuric acid and back acid from the filtration process in multiple batches to reduce the SO4 content in the reaction slurry. 2- Roots and Ca 2+ The ion product Q is always less than that of SO4 in the reaction slurry. 2- Roots and Ca 2+ The critical supersaturation solubility product Kcss.

2. The green mixed crystallization dihydrate wet process for producing phosphoric acid as described in claim 1, characterized in that, In the first stage, the Ca content in the reaction slurry is controlled. 2+ and SO4 2- The ion product Q of the radical is greater than that of Ca in the reaction slurry. 2+ and SO4 2- The amount of the saturation solubility product Ks of the root is (Q-Ks) / Ks*100% = 10-20%.

3. The green mixed crystallization dihydrate wet process for phosphoric acid production as described in claim 1 or 2, characterized in that, Part of the reaction slurry exiting the last reaction tank is cooled to 60-65°C by the first flash cooler and sent to the multi-cell digester connected in series. The remaining part of the reaction slurry is cooled to 75-76°C by the second flash cooler and then returned to the first reaction tank as circulating slurry.

4. The green mixed crystallization dihydrate wet process for phosphoric acid production as described in claim 1 or 2, characterized in that, The multi-cell reaction tanks connected in series are each equipped with multiple sampling points for detecting SO4 in the reaction slurry. 2- Concentration and Ca 2+ The concentration was calculated to obtain the SO4 concentration in the corresponding reaction slurry in the reaction tank. 2- Roots and Ca 2+ The ion product Q, Q = [SO4] 2- ]*[Ca 2+ ].

5. The green mixed crystallization dihydrate wet process for phosphoric acid production as described in claim 1, characterized in that, SO4 in the reaction slurry 2- Roots and Ca 2+ The method for calculating the saturation solubility product Ks is as follows: When the phosphoric acid concentration is 20–30% wt P₂O₅, the sulfuric acid concentration is 0–10% wt, and the temperature is 70–80℃, the product of the saturated concentrations of dissolved CaO and H₂SO₄ in a saturated solution of calcium sulfate dihydrate in a mixed acid of phosphoric acid and sulfuric acid is a constant. [CaO]*[H2SO4]=0.5 [H2SO4]---Total SO4 in mixed acids 2- Root concentration, expressed as H2SO4 wt%, molecular weight 98; [CaO]---Ca dissolved in the mixed acid 2+ Root concentration, expressed as CaO wt%, with a molecular weight of 56; At this point, the formula for the saturation solubility product Kd of calcium sulfate dihydrate in the mixed acid is as follows: Kd=[SO4 2- ]*[Ca 2+ ] SO4 2- --- Total SO4 in mixed acids 2- Root concentration, in terms of SO4 2- wt% indicates a molecular weight of 96; [Ca 2+ ---Ca dissolved in the mixed acid 2+ Root concentration, in terms of Ca 2+ wt% indicates a molecular weight of 40; Then, calculate the SO4 content in the reaction slurry. 2- Roots and Ca 2+ Saturated solubility Ks: Ks = m * Kd / S, where m is a constant, which is 0.42; S---The volume percentage of solid calcium sulfate dihydrate in the reaction slurry, ranging from 22% to 28% vol.

6. The green mixed crystallization dihydrate wet process for phosphoric acid production as described in claim 5, characterized in that, SO4 in the reaction slurry 2- Roots and Ca 2+ The critical supersaturation solubility Kcss is calculated as follows: Kcss = n * Ks * T, where n is a constant, which is 0.21; T---Temperature of the reaction slurry (°C); Range: 70~80°C.

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