A method for preparing phosphoric acid using bulk solid waste phosphogypsum

Abstract

The invention discloses a method for preparing phosphoric acid by utilizing bulk solid waste phosphogypsum. High-concentration SO2 gas generated by thermal decomposition of phosphogypsum is introduced into a reactor. Under the joint action of a catalyst and a strong oxidant, SO2 converts phosphate ore into phosphoric acid in one step, thereby achieving the recycling of phosphogypsum sulfur resources and reducing the amount of concentrated sulfuric acid used in a wet phosphoric acid process. The invention eliminates the sulfuric acid production step in the traditional phosphoric acid production process and realizes the recycling resource utilization of phosphogypsum, thereby providing a clean production process method for the industrial production of phosphoric acid and providing a new idea and new approach for the large-scale utilization of bulk solid waste phosphogypsum.

Description

Technical Field

[0001] The invention relates to a method for preparing phosphoric acid by utilizing bulk solid waste phosphogypsum, and belongs to the technical field of phosphorus chemical industry. Background Art

[0002] A large amount of concentrated sulfuric acid is usually required in the process of wet phosphoric acid production, which greatly increases the production cost of phosphoric acid. In addition, phosphogypsum is the largest by-product solid waste produced in the process of wet preparation of phosphoric acid using phosphate rock and sulfuric acid as raw materials. The main component of phosphogypsum is CaSO4·2H2O, accounting for more than 80% of the total mass. Other impurities mainly include Ca(H2PO4)2·H2O, CaHPO4·2H2O, Ca3(PO4)2, heavy metals (Cr, Cu, Zn and Cd), organic matter (aliphatic, amine and ketone compounds), fluoride (NaF, Na2SiF6, Na2AlF6 and CaF2), residual acid, sulfate ions and a small amount of radioactive elements. According to statistics, on average, about 4.5 to 5.0 tons of phosphogypsum are produced for every ton of phosphoric acid (in terms of P2O5). At present, the comprehensive utilization rate of phosphogypsum is less than 40%, and most of the phosphogypsum is disposed of by landfill, which not only wastes resources but also pollutes the environment. Thermal decomposition of phosphogypsum is a treatment method with high added value. At present, phosphogypsum is mainly used for calcination to prepare calcium oxide (CaO). During the calcination process, a large amount of high-concentration SO2 gas will be released, which will have a great impact on the environment. This seriously limits the development of thermal decomposition of phosphogypsum. Summary of the invention

[0003] In order to alleviate the environmental pollution problem caused by a large amount of SO2 generated during the thermal decomposition of phosphogypsum, and the problem that the wet-process phosphoric acid process requires a large amount of concentrated sulfuric acid as a raw material, resulting in excessively high costs, the present invention provides a method for preparing phosphoric acid using bulk solid waste phosphogypsum, wherein high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum is introduced into a reactor, and under the combined action of a catalyst and a strong oxidant, SO2 converts phosphate ore into phosphoric acid in one step, thereby achieving the recycling of phosphogypsum sulfur resources and reducing the amount of concentrated sulfuric acid used in the wet-process phosphoric acid process.

[0004] The technical solution of the present invention is as follows:

[0005] A method for preparing phosphoric acid using bulk solid waste phosphogypsum, the specific steps are as follows:

[0006] (1) crushing, grinding and screening phosphate rock to obtain phosphate rock powder, and mixing the phosphate rock powder with water to obtain phosphate slurry;

[0007] (2) placing the phosphate slurry obtained in step (1) in a reactor, adding a catalyst and a strong oxidant thereto, to obtain a mixture;

[0008] (3) placing the phosphogypsum particles in a horizontal tube furnace, calcining them once in a CH4 atmosphere, and then replacing them with anhydrous air atmosphere for secondary calcination, and collecting the reaction gas during the entire calcination process;

[0009] (4) The reaction gas collected in step (3), i.e., the high-concentration SO2 gas generated by the decomposition of phosphogypsum, is directly introduced into the mixture of the high-temperature and high-pressure reactor in step (2) for reaction. After the reaction is completed, the supernatant is taken as the phosphoric acid solution, and the remaining solid plus gypsum is returned to step (3) as phosphogypsum for recycling.

[0010] The phosphate rock screening in step (1) is through a 40-100 mesh sieve; the mass volume ratio of phosphate rock powder to water is 5-20:100 g:mL.

[0011] The catalyst in step (2) is any one of hematite, red mud and MnO2; the added amount is 2% to 5% of the mass of the phosphate slurry.

[0012] The strong oxidant in step (2) is any one of H2O2, KMnO4, Na2O2 and K2Cr2O7; the concentration of the strong oxidant in the mixture is 400mg / L to 1000mg / L.

[0013] The pyrolysis of phosphogypsum in step (3) is a primary calcination in a CH4 atmosphere, wherein the temperature is raised to 700-1000°C at a heating rate of 10-20°C / min and the pyrolysis is performed at a constant temperature for 1-2 hours; the secondary calcination is performed after replacing the atmosphere with anhydrous air and then pyrolyzing at the same temperature as the primary calcination for 1-2 hours.

[0014] The reaction pressure in step (4) is 0-0.3Mpa and the temperature is 25-55°C.

[0015] The main component of the phosphogypsum of the present invention is CaSO4·2H2O, which will be decomposed at high temperature to produce a large amount of CaO and SO2. The high-concentration SO2 gas produced by the thermal decomposition of the phosphogypsum is introduced into the phosphate ore slurry and a catalyst and a strong oxidant are added, so that the SO2 dissolved in the liquid phase can be directly reacted with Ca5F(PO4)3 to generate phosphoric acid.

[0016] Beneficial effects of the present invention:

[0017] (1) The present invention realizes the recycling of phosphogypsum sulfur resources by using phosphate slurry to absorb the high-concentration SO2 gas generated during the decomposition of phosphogypsum.

[0018] (2) The present invention utilizes SO2 and phosphate slurry to directly react to generate phosphoric acid, which is beneficial to improving the efficiency of the wet phosphoric acid process, reducing the consumption of concentrated sulfuric acid, and thus reducing costs.

[0019] (3) The concentration of phosphoric acid prepared by the present invention is 3177.04 mg / L, and the SO2 absorption rate in the first 3 hours is 100%.

[0020] (4) The present invention can dispose of phosphogypsum to the greatest extent, providing a new idea for the resource utilization of phosphogypsum solid waste and reducing environmental pollution caused by pollutant storage.

[0021] (5) The addition of a catalyst in the present invention can promote the absorption and oxidation of SO2 in the slurry, thereby increasing the yield of phosphoric acid; the addition of a strong oxidant can greatly enhance the oxidation of SO2 in the slurry, thereby increasing the yield of phosphoric acid. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a flow chart of the present invention;

[0023] Figure 2 XRD pattern of phosphate rock particles;

[0024] Figure 3 XRD pattern of phosphogypsum particles;

[0025] Figure 4 Example 1-4 Comparative Example 1-4 Phosphate ion concentration comparison diagram;

[0026] Figure 5 Example 1-4 Comparative Example 1-4 SO2 absorption rate comparison chart. DETAILED DESCRIPTION

[0027] The present invention is further described in detail with reference to the following examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. If 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 is not specified, they are all conventional products that can be purchased commercially.

[0028] Example 1

[0029] A method for preparing phosphoric acid using bulk solid waste phosphogypsum, such as Figure 1 As shown, the following steps are included:

[0030] (1) Phosphate rock from a company in Kunming, Yunnan Province was crushed using a crusher and ground in an agate mortar, then passed through a 40-mesh sieve to obtain phosphate rock powder, and the phosphate rock powder was mixed with water to form 40 mL of phosphate slurry with a mass volume ratio of 5:100 g:mL;

[0031] (2) placing the phosphate slurry obtained in step (1) into a high-temperature and high-pressure reactor, and adding 2 wt% of the mass of the phosphate slurry as catalyst red mud and a strong oxidant H2O2 into the reactor, wherein the concentration of the strong oxidant H2O2 is 400 mg / L, to obtain a mixture;

[0032] (3) The phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried at 60°C for 12 h in an electric constant temperature blast drying oven, ground with an agate mortar to obtain phosphogypsum particles with a mesh size of 40, placed in a horizontal tube furnace, heated to 700°C at a heating rate of 10°C / min and maintained for 1 h in a CH4 atmosphere for a primary calcination, then replaced with air, and calcined at 700°C for 1 h in an anhydrous air atmosphere for a secondary calcination, and the reaction gas was collected during the entire calcination process;

[0033] (4) The reaction gas collected during the entire process of step (3), i.e., the high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum, is directly introduced into the mixture of the high-temperature and high-pressure reactor of step (2), and the temperature of the high-temperature and high-pressure reactor is set to 25°C. The gas flow rates at the inlet and outlet are adjusted so that the pressure of the high-temperature and high-pressure reactor is 0 MPa (the same as the external standard atmospheric pressure). After reacting for 5 hours, the supernatant is taken as phosphoric acid, and the remaining solid plus gypsum are returned to step (3) for use as phosphogypsum.

[0034] Example 2

[0035] A method for preparing phosphoric acid using bulk solid waste phosphogypsum comprises the following steps:

[0036] (1) Phosphate rock from a company in Kunming, Yunnan Province was crushed using a crusher and ground in an agate mortar, then passed through a 40-mesh sieve to obtain phosphate rock powder, and the phosphate rock powder was mixed with water to form 40 mL of phosphate slurry with a mass-to-volume ratio of g:mL of 10:100;

[0037] (2) placing the phosphate slurry obtained in step (1) into a high-temperature and high-pressure reactor, and adding 3 wt% of the catalyst MnO2 and a strong oxidant Na2O2 to the reactor, wherein the concentration of the strong oxidant Na2O2 is 600 mg / L, based on the mass of the phosphate slurry, to obtain a mixture;

[0038] (3) The phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried at 80°C for 24 hours in an electric constant temperature blast drying oven, ground with an agate mortar to obtain phosphogypsum particles with a mesh size of 60, placed in a horizontal tube furnace, introduced into a CH4 atmosphere, heated to 800°C at a heating rate of 15°C / min and maintained for 1.5 hours for a primary calcination, then replaced with air, and calcined at 800°C for 1 hour in an anhydrous air atmosphere for a secondary calcination, and the reaction gas was collected during the entire calcination process;

[0039] (4) The reaction gas collected during the entire process of step (3), i.e., the high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum, is directly introduced into the mixture of the high-temperature and high-pressure reactor of step (2), and the temperature of the high-temperature and high-pressure reactor is set to 35°C. The gas flow rates at the inlet and outlet are adjusted so that the pressure of the high-temperature and high-pressure reactor is 0.1 MPa (relative to the external standard atmospheric pressure, the same below). After reacting for 5 hours, the supernatant is taken as phosphoric acid, and the remaining solid plus gypsum are returned to step (3) for use as phosphogypsum.

[0040] Example 3

[0041] A method for preparing phosphoric acid using bulk solid waste phosphogypsum comprises the following steps:

[0042] (1) Phosphate rock from a company in Kunming, Yunnan Province was crushed using a crusher and ground in an agate mortar, then passed through an 80-mesh sieve to obtain phosphate rock powder, and the phosphate rock powder was mixed with water to form 40 mL of phosphate slurry with a mass volume ratio of g:mL of 15:100;

[0043] (2) placing the phosphate slurry obtained in step (1) into a high-temperature and high-pressure reactor, and adding 4 wt% of the catalyst hematite and a strong oxidant KMnO4 to the reactor, wherein the concentration of the strong oxidant KMnO4 is 800 mg / L, to obtain a mixture;

[0044] (3) The phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried at 100°C for 36 hours in an electric constant temperature blast drying oven, ground with an agate mortar to 80 mesh to obtain phosphogypsum particles, placed in a horizontal tube furnace, introduced into a CH4 atmosphere, heated to 900°C at a heating rate of 20°C / min and maintained for 2 hours for a primary calcination, then replaced with air, and calcined at 900°C for 2 hours in an anhydrous air atmosphere for a secondary calcination, and the reaction gas was collected during the entire calcination process;

[0045] (4) The reaction gas collected during the entire process of step (3), i.e., the high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum, is directly introduced into the mixture of the high-temperature and high-pressure reactor of step (2), and the temperature of the high-temperature and high-pressure reactor is set to 45°C. The gas flow rates at the inlet and outlet are adjusted so that the pressure of the high-temperature and high-pressure reactor is 0.2 MPa. After reacting for 5 hours, the supernatant is taken as phosphoric acid, and the remaining solid plus gypsum are returned to step (3) for use as phosphogypsum.

[0046] Example 4

[0047] A method for preparing phosphoric acid using bulk solid waste phosphogypsum comprises the following steps:

[0048] 1) Phosphate rock from a company in Kunming, Yunnan Province was crushed using a crusher and ground in an agate mortar, then passed through a 100-mesh sieve to obtain phosphate rock powder, and the phosphate rock powder was mixed with water to form 40 mL of phosphate slurry with a mass volume ratio of g:mL of 20:100;

[0049] (2) placing the phosphate slurry obtained in step (1) into a high-temperature and high-pressure reactor, and adding 5 wt% of the catalyst hematite and a strong oxidant KMnO4 to the reactor, wherein the concentration of the strong oxidant KMnO4 is 1000 mg / L, to obtain a mixture;

[0050] (3) The phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 100°C for 24 hours, ground with an agate mortar to obtain phosphogypsum particles, placed in a horizontal tube furnace, introduced into a CH4 atmosphere, heated to 1000°C at a heating rate of 20°C / min and maintained for 2 hours for a primary calcination, then replaced with air, and calcined at 1000°C for 2 hours in an anhydrous air atmosphere for a secondary calcination, and the reaction gas was collected during the entire calcination process;

[0051] (4) The reaction gas collected during the entire process of step (3), i.e., the high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum, is directly introduced into the mixture of the high-temperature and high-pressure reactor of step (2), and the temperature of the high-temperature and high-pressure reactor is set to 55°C. The gas flow rates at the inlet and outlet are adjusted so that the pressure of the high-temperature and high-pressure reactor is 0.3 MPa. After reacting for 5 hours, the supernatant is taken as phosphoric acid, and the remaining solid plus gypsum are returned to step (3) for use as phosphogypsum.

[0052] Comparative Example 1

[0053] The conditions and raw materials of this embodiment are the same as those of embodiment 3, except that no hematite catalyst and KMnO4 strong oxidant are added to the high temperature and high pressure reactor. The specific steps are as follows:

[0054] (1) Phosphate rock from a company in Kunming, Yunnan Province was crushed using a crusher and ground in an agate mortar, then passed through an 80-mesh sieve to obtain phosphate rock powder, the phosphate rock powder was mixed with water to form 40 mL of phosphate slurry with a mass volume ratio of g:mL of 15:100, and the phosphate slurry was loaded into a high temperature and high pressure reactor;

[0055] (2) The phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried at 100°C for 36 hours in an electric constant temperature blast drying oven, ground with an agate mortar to 80 mesh to obtain phosphogypsum particles, placed in a horizontal tube furnace, introduced into a CH4 atmosphere, heated to 900°C at a heating rate of 20°C / min and maintained for 2 hours for a primary calcination, then replaced with air, and calcined at 900°C for 2 hours in an anhydrous air atmosphere for a secondary calcination, and the reaction gas was collected during the entire calcination process;

[0056] (3) The reaction gas collected during the entire process of step (2), i.e., the high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum, is directly introduced into the phosphate slurry of the high-temperature and high-pressure reactor of step (1), and the temperature of the high-temperature and high-pressure reactor is set to 45°C, and the gas flow rates at the inlet and outlet are adjusted so that the pressure of the high-temperature and high-pressure reactor is 0.2 MPa. After reacting for 5 hours, the supernatant is taken, which is phosphoric acid.

[0057] Comparative Example 2

[0058] The conditions and raw materials of this embodiment are the same as those of embodiment 3, except that only the catalyst hematite is added into the high temperature and high pressure reactor without adding the strong oxidant KMnO4. The specific steps are as follows:

[0059] (1) Phosphate rock from a company in Kunming, Yunnan Province was crushed using a crusher and ground in an agate mortar, then passed through an 80-mesh sieve to obtain phosphate rock powder, and the phosphate rock powder was mixed with water to form 40 mL of phosphate slurry with a mass volume ratio of g:mL of 15:100;

[0060] (2) placing the phosphate slurry obtained in step (1) into a high temperature and high pressure reactor and adding 4 wt% of the catalyst hematite by weight of the phosphate slurry into the reactor to obtain a mixture;

[0061] (3) The phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried at 100°C for 36 hours in an electric constant temperature blast drying oven, ground with an agate mortar to 80 mesh to obtain phosphogypsum particles, placed in a horizontal tube furnace, introduced into a CH4 atmosphere, heated to 900°C at a heating rate of 20°C / min and maintained for 2 hours for a primary calcination, then replaced with air, and calcined at 900°C for 2 hours in an anhydrous air atmosphere for a secondary calcination, and the reaction gas was collected during the entire calcination process;

[0062] (4) The reaction gas collected during the entire process of step (3), i.e., the high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum, is directly introduced into the mixture of the high-temperature and high-pressure reactor of step (2), and the temperature of the high-temperature and high-pressure reactor is set to 45°C. The gas flow rates at the inlet and outlet are adjusted so that the pressure of the high-temperature and high-pressure reactor is 0.2 MPa. After reacting for 5 hours, the supernatant is taken, which is phosphoric acid.

[0063] Comparative Example 3

[0064] The conditions and raw materials of this embodiment are the same as those of embodiment 3, except that no catalyst hematite is added to the high temperature and high pressure reactor, and only a strong oxidant KMnO4 is added. The specific steps are:

[0065] (1) Phosphate rock from a company in Kunming, Yunnan Province was crushed using a crusher and ground in an agate mortar, then passed through an 80-mesh sieve to obtain phosphate rock powder, and the phosphate rock powder was mixed with water to form 40 mL of phosphate slurry with a mass volume ratio of g:mL of 15:100;

[0066] (2) placing the phosphate slurry obtained in step (1) into a high-temperature and high-pressure reactor and adding a strong oxidant KMnO4 into the reactor, wherein the concentration of the strong oxidant KMnO4 is 800 mg / L, to obtain a mixture;

[0067] (3) The phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried at 100°C for 36 hours in an electric constant temperature blast drying oven, ground with an agate mortar to 80 mesh to obtain phosphogypsum particles, placed in a horizontal tube furnace, introduced into a CH4 atmosphere, heated to 900°C at a heating rate of 20°C / min and maintained for 2 hours for a primary calcination, then replaced with air, and calcined at 900°C for 2 hours in an anhydrous air atmosphere for a secondary calcination, and the reaction gas was collected during the entire calcination process;

[0068] (4) The reaction gas collected during the entire process of step (3), i.e., the high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum, is directly introduced into the mixture of the high-temperature and high-pressure reactor of step (2), and the temperature of the high-temperature and high-pressure reactor is set to 45°C. The gas flow rates at the inlet and outlet are adjusted so that the pressure of the high-temperature and high-pressure reactor is 0.2 MPa. After reacting for 5 hours, the supernatant is taken, which is phosphoric acid.

[0069] Comparative Example 4

[0070] The conditions and raw materials of this embodiment are the same as those of embodiment 3, except that the gas decomposed from phosphogypsum is first converted into sulfuric acid, and then phosphoric acid is used to react with phosphate rock to generate phosphoric acid. The specific steps are as follows:

[0071] (1) Phosphate rock from a company in Kunming, Yunnan Province was crushed using a crusher, ground in an agate mortar, and then passed through an 80-mesh sieve to obtain phosphate rock powder;

[0072] (2) adding 40 mL of deionized water into a high temperature and high pressure reactor, and adding 4 wt% of the catalyst hematite and a strong oxidant KMnO4 into the reactor, wherein the concentration of the strong oxidant KMnO4 is 800 mg / L after the addition of the deionized water, to obtain a mixture;

[0073] (3) The phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried at 100°C for 36 hours in an electric constant temperature blast drying oven, ground with an agate mortar to 80 mesh to obtain phosphogypsum particles, placed in a horizontal tube furnace, introduced into a CH4 atmosphere, heated to 900°C at a heating rate of 20°C / min and maintained for 2 hours for a primary calcination, then replaced with air, and calcined at 900°C for 2 hours in an anhydrous air atmosphere for a secondary calcination, and the reaction gas was collected during the entire calcination process;

[0074] (4) introducing the reaction gas collected during the entire process of step (3), i.e., the high-concentration SO2 gas generated during the thermal decomposition of phosphogypsum, into the high-temperature and high-pressure reactor of step (2), and setting the temperature of the high-temperature and high-pressure reactor to 45° C., adjusting the gas flow rates at the inlet and outlet so that the pressure of the high-temperature and high-pressure reactor is 0.2 MPa, and reacting for 5 hours to obtain sulfuric acid;

[0075] (5) Add the phosphate rock powder from step (1) to the sulfuric acid obtained from step (4), set the reactor temperature to 45° C. and the pressure to 0.2 MPa, react for 5 hours, and filter to obtain the supernatant, which is phosphoric acid.

[0076] Figure 2 This is the XRD diagram of the phosphate rock used in the examples. It can be seen from the figure that the main minerals of the phosphate rock are hydroxyapatite (Ca5(PO4)3(OH)), dolomite (CaMg(CO3)2), calcium phosphate dihydrate (Ca(HPO4)(H2O)2), silicon dioxide (SiO2), and fluoroapatite (Ca5(PO4)3F). Among them, Ca5(PO4)3(OH), Ca(HPO4)(H2O)2 and Ca5(PO4)3F can all react with sulfuric acid to generate phosphoric acid.

[0077] Figure 3 This is the XRD diagram of the phosphogypsum used in the example. It can be seen from the figure that the main component of phosphogypsum is CaSO4·2H2O, which decomposes at high temperature to produce CaO and SO2, indicating that the experiment is feasible.

[0078] The phosphate ion concentration in the phosphoric acid solution produced by Examples 1 to 4 and Comparative Examples 1 to 4 and the SO2 absorption rate after 5 hours are shown in Table 1:

[0079] Table 1

[0080]

[0081] As shown in Table 1, when Example 3 is compared with Comparative Example 1, it can be seen that when the catalyst and the strong oxidant are added to the phosphate slurry, the concentration of phosphate ions and the SO2 absorption rate in the phosphate slurry are increased. The reason is that due to the Fe 3+The variable valence characteristics can efficiently oxidize and absorb SO2. Therefore, KMnO4 is added as a strong oxidant into the phosphate slurry containing hematite to accelerate the Fe 2+ and Fe 3+ The conversion rate between them can increase the absorption rate of SO2 and the production of phosphoric acid. In addition, KMnO4 can enhance the oxidation and hydration reaction of hematite to SO2. After completing the oxidation and phosphoric acid production, KMnO4 can be absorbed by Fe 3+ The price change effect of Mn 4+ valence state, thereby increasing the overall production of phosphoric acid.

[0082] Comparing Example 3 with Comparative Example 2, it can be seen that adding hematite catalyst to the phosphate slurry greatly promotes the concentration of phosphate ions in the solution and the SO2 absorption rate. The reason is that metal ions (especially Fe 3+ ) can promote the catalytic oxidation of SO2 by dissolved oxygen (DO) in the liquid phase; in addition, Fe 3+ The presence of can reduce the maximum reaction energy barrier of different reaction processes in the reaction pathway (SO2 oxidation, SO2 hydration, SO3 hydration, H2SO3 oxidation). Therefore, adding a catalyst to the phosphate slurry can enhance the absorption and oxidation of SO2, thereby increasing the concentration of phosphate ions in the solution and the SO2 absorption rate.

[0083] Comparing Example 3 with Comparative Example 3, it can be seen that after only adding the strong oxidant KMnO4, the phosphate ion concentration in the solution is greatly increased, while the SO2 absorption rate is reduced compared with Comparative Example 2. The reason is that KMnO4 can improve the migration and conversion of the P element in the phosphate slurry, that is, increase the production of phosphoric acid in the liquid environment. Compared with the catalyst hematite, the strong oxidizing property of KMnO4 is mainly manifested in promoting the formation of phosphoric acid.

[0084] By comparing Example 3 with Comparative Example 4, it can be seen that the gas decomposed from phosphogypsum is first converted into sulfuric acid, and then the sulfuric acid is used to react with phosphate rock to generate phosphoric acid. The yield is much lower than the yield of generating phosphoric acid in one step by passing high-concentration SO2 flue gas into phosphate slurry in Example 3. The reason is that phosphate slurry has a great promoting effect on the absorption of SO2. When phosphoric acid is produced by a one-step method, phosphate slurry can promote the absorption of SO2 in the gas.

[0085] Figure 4-5It is a comparison chart of the phosphate ion concentration and SO2 absorption rate in the solution after the reaction of Example 1-4 and Comparative Example 1-4. Through the comparison, it can be seen that the phosphate ion concentration and SO2 absorption rate are related to the pressure and temperature during the reaction and the heating rate and temperature during the decomposition of phosphogypsum. The reason is that the pressure can affect the increase of mass transfer between gas and liquid, but when the pressure is too high, the required phosphate slurry exceeds the actual use of the phosphate slurry, resulting in a shortened contact reaction time between SO2 and the phosphate slurry, which will reduce the SO2 absorption rate; the temperature can affect the speed of the reaction, but too high a temperature will cause the moisture in the phosphate slurry to precipitate rapidly with the outgassing, increase the solid-liquid ratio of the phosphate slurry, thereby resulting in a decrease in the desulfurization efficiency and the phosphoric acid yield; and the decomposition of phosphogypsum will affect the concentration of SO2 gas introduced into the phosphate slurry.

Claims

1. A method for preparing phosphoric acid using bulk solid waste phosphogypsum, characterized in that: The specific steps are as follows: (1) crushing, grinding and screening phosphate rock to obtain phosphate rock powder, and mixing the phosphate rock powder with water to obtain phosphate slurry; (2) placing the phosphate slurry obtained in step (1) in a reactor, adding a catalyst and a strong oxidant thereto, to obtain a mixture; (3) calcining the phosphogypsum particles once in a CH4 atmosphere, and then replacing it with anhydrous air atmosphere for a second calcination, and collecting the reaction gas during the entire calcination process; (4) the reaction gas collected in step (3) is introduced into the mixture of the high temperature and high pressure reactor in step (2) for reaction. After the reaction is completed, the supernatant is taken as phosphoric acid, and the remaining solid plus gypsum is returned to step (3) as phosphogypsum for recycling; The catalyst in step (2) is any one of hematite, red mud and MnO2; the added amount is 2% to 5% of the mass of the phosphate slurry; The strong oxidant in step (2) is any one of H2O2, KMnO4, Na2O2 and K2Cr2O7; the concentration of the strong oxidant in the mixture is 400 mg / L to 1000 mg / L; The reaction pressure in step (4) is 0-0.3 MPa and the temperature is 25-55°C.

2. The method for preparing phosphoric acid using bulk solid waste phosphogypsum according to claim 1, characterized in that: The phosphate rock screening in step (1) is through a 40-100 mesh sieve; the mass volume ratio of phosphate rock powder to water is 5-20:100 g:mL.

3. The method for preparing phosphoric acid using bulk solid waste phosphogypsum according to claim 1, characterized in that: The first calcination in step (3) is to heat the material to 700-1000°C at a heating rate of 10-20°C / min and then pyrolyze it at a constant temperature for 1-2 hours; the second calcination is to replace the atmosphere with anhydrous air and then pyrolyze it at the same temperature for 1-2 hours.

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