A method for preparing light calcium carbonate and co-producing sodium sulfate by circulating leaching of phosphogypsum

Abstract

The present application relates to the field of phosphogypsum resource utilization, particularly to a method for preparing light calcium carbonate and co-producing sodium sulfate by cyclic leaching of phosphogypsum, which comprises: adding desilicated phosphogypsum into a leaching reagent, stirring and reacting, after the reaction is completed, performing suction filtration to obtain a leaching solution and a filter cake; transferring the leaching solution into a pressurized reaction kettle, introducing carbon dioxide gas into the leaching solution, after the carbonation reaction is completed, waiting for the reaction solution to return to room temperature, discharging the reaction solution, performing vacuum suction filtration to obtain calcium carbonate solid product and a lower mixed solution; drying the calcium carbonate solid product to obtain light calcium carbonate; weighing and recycling the lower mixed solution for use, after several cycles, the sulfate radical concentration reaches a relative stability, anhydrous sodium sulfate crystals are precipitated, the multiple use mother liquor is obtained by suction filtration, a certain amount of leaching reagent is added, and fresh phosphogypsum is continuously leached. The present application effectively improves the resource utilization efficiency of phosphogypsum, greatly reduces the preparation cost, almost does not produce wastewater, and is suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of phosphogypsum resource utilization, and in particular to a method for preparing light calcium carbonate and sodium sulfate by cyclic leaching of phosphogypsum. Background Technology

[0002] my country is a major producer and consumer of phosphate chemicals. While the wet-process phosphoric acid production line contributes nutrients needed for food security, it also generates a massive amount of industrial solid waste, phosphogypsum. Its main chemical component is calcium sulfate dihydrate (CaSO4·2H2O), and it contains impurities such as undecomposed phosphate rock, residual phosphoric acid, fluorides, organic matter, and heavy metals. With the rapid development of the phosphate chemical industry, the discharge of solid waste such as phosphogypsum has shown a year-on-year increasing trend. According to statistics from the China Phosphate Fertilizer Industry Association, the historical stockpile of phosphogypsum in China now exceeds 600 million tons, with an annual increase of over 70 million tons, but the comprehensive utilization rate is less than 30%.

[0003] The presence of water-soluble phosphorus and fluorine, organic matter, and heavy metals in phosphogypsum is the main reason hindering its resource utilization. At the same time, its narrow application areas and lack of large-scale disposal sites are also major reasons for the slow disposal of phosphogypsum. Large-scale stockpiling of phosphogypsum not only occupies land resources but also poses a long-term potential threat to soil, groundwater, and the surrounding ecological environment, becoming a key problem restricting the green and sustainable development of the phosphate chemical industry.

[0004] Meanwhile, the environmental problems caused by CO2 have attracted the attention of countries around the world, and countries are facing enormous pressure to reduce CO2 emissions. Based on the concept of "treating waste with waste," many researchers have begun to study the use of phosphogypsum as a CO2 fixative.

[0005] Based on the above issues, it is worthwhile to study how to better utilize phosphogypsum waste and CO2 while achieving carbon emission reduction.

[0006] Currently, the main methods for preparing calcium carbonate include: limestone calcination and carbonation, non-limestone (calcium chloride, calcium nitrate, calcium oxalate) chemical metathesis, and preparation of light calcium carbonate from solid waste resources (carbide slag, steel slag, white mud, phosphogypsum). However, the above methods have drawbacks such as high energy consumption, poor economic efficiency, and poor product quality. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing light calcium carbonate and producing sodium sulfate by cyclic leaching of phosphogypsum, which ensures the purity and whiteness of the prepared calcium carbonate; at the same time, the sulfate concentration steadily increases after multiple leachings, and anhydrous sodium sulfate crystals are formed by cooling and crystallization. This method has a short reaction route, mild reaction, simple operation, stable reaction process, and low temperature, making it suitable for industrial production.

[0008] The specific technical solution of the present invention is as follows: A method for preparing light calcium carbonate and sodium sulfate by cyclic leaching of phosphogypsum includes the following steps: Step (1) Add the desilication phosphogypsum to the leaching reagent, stir and react. After the reaction is complete, filter to obtain the leachate and filter cake. Step (2) Transfer the above leachate to a pressurized reactor, introduce carbon dioxide gas into the leachate, and after the carbonization reaction is completed, slowly cool down and depressurize. When the reaction solution returns to room temperature, release the reaction solution and filter it under vacuum to obtain calcium carbonate solid product and lower layer mixture. Step (3) Dry the solid calcium carbonate product, crush and grind it to finally obtain light calcium carbonate; Step (4) Weigh and recycle the lower layer mixture. After several cycles, the sulfate concentration becomes relatively stable. Take out part of the filtrate, cool and crystallize it to precipitate anhydrous sodium sulfate crystals. Filter to obtain the mother liquor from multiple cycles. Add a certain amount of leaching reagent to the mother liquor from multiple cycles and continue to leach fresh phosphogypsum.

[0009] As a preferred option, in step (1), the leaching reagent is sodium nitrite or sodium chloride, with the concentration of sodium nitrite being 3~6 mol / L and the concentration of sodium chloride being 5~10 mol / L.

[0010] As a preferred option, in step (1), the liquid-to-solid ratio of leaching phosphogypsum is 3~10:1, the reaction temperature is 30~70℃, and the reaction time is 2~5h.

[0011] As a preferred option, in step (1), the filter cake is dried, weighed to calculate the extraction efficiency, and the calcium ion and sulfate ion content of the leachate is measured to inversely estimate the extraction efficiency.

[0012] As a preferred option, in step (2), the carbonization reaction pressure is 0.1~0.8 MPa, the reaction temperature is 30~70℃, the reaction time is 1~2 h, and the CO2 gas concentration is above 99%.

[0013] As a preferred option, in step (3), the drying temperature of calcium carbonate is 80~200℃, and the grinding speed of the ball mill is 100~300 r / min.

[0014] As a preferred option, in step (4), the sulfate concentration reaches a relatively stable level when it approaches 15%, the sodium sulfate cooling crystallization temperature is 5~15℃, and the sodium sulfate drying temperature is 90~150℃. Attached Figure Description

[0015] Figure 1 This is a process flow diagram of one embodiment of the present invention; Figure 2 This is a particle size distribution diagram of calcium carbonate obtained in Example 1 of the present invention; Figure 3 This is a sample image of calcium carbonate and sodium sulfate obtained in Example 1 of the present invention. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased.

[0017] Unless otherwise stated, all percentages in this invention represent mass fractions. Ratios are mass percentages, and concentrations are mass concentrations.

[0018] Unless otherwise specified, all materials, instruments, and equipment used below are conventional materials, instruments, and equipment or obtained through commercial channels; all testing methods used are existing methods unless otherwise specified.

[0019] In existing technologies, the utilization of phosphogypsum waste and CO2 involves several methods. The limestone calcination carbonization method has high energy consumption and large carbon emissions, and suffers from over-burning or under-burning, directly affecting the activity and quality of lime slurry. The non-limestone chemical metathesis method has high raw material costs, poor economic efficiency for large-scale production, and residual chloride ions in the product directly affect product quality. Directly preparing light calcium carbonate from solid waste resources (carbide slag, steel slag, white mud, phosphogypsum, etc.) easily results in impurities being deposited in the calcium carbonate, directly affecting the crystal form, whiteness, and purity of the calcium carbonate product, thus limiting its application as a filler in downstream industries such as papermaking, PP, PVC, and rubber.

[0020] Furthermore, this invention provides a method for preparing light calcium carbonate and sodium sulfate by cyclic leaching of phosphogypsum, comprising the following steps: Step (1) Add the desilication phosphogypsum to the leaching reagent, stir and react. After the reaction is complete, filter to obtain the leachate and filter cake. Step (2) Transfer the above leachate to a pressurized reactor, introduce carbon dioxide gas into the leachate, and after the carbonization reaction is completed, slowly cool down and depressurize. When the reaction solution returns to room temperature, release the reaction solution and filter it under vacuum to obtain calcium carbonate solid product and lower layer mixture. Step (3) Dry the solid calcium carbonate product, crush and grind it to finally obtain light calcium carbonate; Step (4) Weigh and recycle the lower layer mixture. After several cycles, the sulfate concentration becomes relatively stable. Take out part of the filtrate, cool and crystallize it to precipitate anhydrous sodium sulfate crystals. Filter to obtain the mother liquor from multiple cycles. Add a certain amount of leaching reagent to the mother liquor from multiple cycles and continue to leach fresh phosphogypsum.

[0021] The method of this invention uses leaching to release effective calcium and sulfate ions, transferring them from the solid phase to the liquid phase. A large number of metal ions and impurities remain in the solid phase, thus improving the calcium enrichment... 2+ The efficiency of the liquid-phase carbonation reaction greatly improves the purity and whiteness of calcium carbonate. In addition, the extraction solvent used can be recycled multiple times, with almost no wastewater generated, which greatly reduces the production cost of calcium carbonate, improves the economic benefits of calcium carbonate products, and is conducive to industrial application.

[0022] Simultaneously, the large amount of sulfur in phosphogypsum is converted into anhydrous sodium sulfate, further improving resource utilization efficiency. After multiple leaching processes, the sulfate concentration steadily increases, and anhydrous sodium sulfate crystals are formed upon cooling and crystallization. This method has a short reaction route, mild reaction, simple operation, stable reaction process, and low temperature, making it suitable for industrial production. In addition, the separation of sodium sulfate greatly avoids the accumulation of sulfate ions during multiple cycles, significantly improving the leaching efficiency of phosphogypsum in multiple cycles.

[0023] This invention uses calcium sulfate from phosphogypsum, a byproduct of phosphate fertilizer production, as a raw material to prepare high-value calcium carbonate and co-produce sodium sulfate. This effectively improves the resource utilization efficiency of phosphogypsum. The selected sodium nitrite solution as the leaching solution can be recycled multiple times, greatly reducing the preparation cost and producing almost no wastewater, making it suitable for industrial production. Simultaneously, the calcium carbonate and anhydrous sodium sulfate produced by this process have a particle size of less than 1000 mesh and a whiteness of over 90, fully meeting the needs of high-whiteness rubber and plastic (PE, PP, PA6, PVC, etc.) fillers. The anhydrous sodium sulfate can be used as a paper filler to improve paper strength and smoothness.

[0024] The advantages of the embodiments of the present invention are also as follows: (1) The raw material cost is extremely low, turning waste into treasure. Using industrial solid waste phosphogypsum and industrial flue gas CO2 as raw materials, the large-scale promotion and industrial application of this process can greatly improve the value of phosphogypsum, broaden the utilization path of phosphogypsum, and ensure the sustainable development of the phosphoric chemical industry. There is no need to mine, and the purchase costs of limestone, calcium chloride, calcium nitrate and calcium oxalate are saved. The raw materials have almost zero cost and can even collect solid waste disposal fees. (2) The traditional high-energy-consuming calcination process of light calcium carbonate is eliminated, and industrial tail gas CO2 is used to reduce carbon emissions, which is in line with the dual carbon policy orientation and significantly reduces energy consumption and fuel consumption.

[0025] (3) Large-scale solid waste disposal solves the problem of environmentally friendly stockpiling.

[0026] (4) The process flow is shorter, the infrastructure and operation investment is lower, and the entire process of mining, crushing, calcining and cooling is eliminated. The development of this process also helps to solve the above-mentioned difficult problems of phosphogypsum.

[0027] It is evident that the method of this invention is a key measure for phosphate chemical enterprises to expand new avenues for solid waste resource utilization and promote the transformation of phosphogypsum from "end-of-pipe treatment" to "process resource utilization." By transforming phosphogypsum from simple stockpiling or low-value-added utilization into utilization as a chemical raw material, the comprehensive utilization level of phosphogypsum is improved, an independent and controllable deep utilization technology is constructed, environmental risks and compliance pressures are reduced, resource utilization efficiency is improved, and the utilization mode of phosphogypsum is changed from the source. This promotes the transformation of enterprises from passive solid waste disposal to active resource utilization, which is in line with the development direction of circular economy and cleaner production.

[0028] In one embodiment, in step (1), the leaching reagent is sodium nitrite or sodium chloride. The concentration of sodium nitrite is 3~6 mol / L and the concentration of sodium chloride is 5~10 mol / L. For example, the concentration of sodium nitrite is 4 mol / L or 5 mol / L, and the concentration of sodium chloride is 5 mol / L, 7 mol / L or 10 mol / L, which can maximize the leaching and serve as a prerequisite for high leaching efficiency.

[0029] In one embodiment, in step (1), the liquid-to-solid ratio of leaching phosphogypsum is 3~10:1, the reaction temperature is 30~70℃, and the reaction time is 2~5h. For example, the liquid-to-solid ratio is 7:1 and the reaction time is 3h, which is set by taking into account the leaching efficiency and carbonization rate.

[0030] In one implementation method, in step (1), the filter cake is dried, weighed and the extraction efficiency is calculated, and the calcium ion and sulfate ion content of the leachate is measured to inversely estimate the extraction efficiency.

[0031] There are two methods to calculate the leaching efficiency here. The first method is to calculate the difference in weight of phosphogypsum before and after leaching, after deducting impurities. The second method is to calculate it by the change in calcium ion concentration or sulfate ion concentration in the leachate. In step (4), the mother liquor is recycled, which involves the cumulative effect of sulfate. When calculating with sulfate, the amount of initial sulfate needs to be deducted. The calculation of calcium ions and the gravimetric method are the same as before.

[0032] In one embodiment, in step (2), the carbonization reaction pressure is 0.1~0.8 MPa, the reaction temperature is 30~70℃, the reaction time is 1~2h, and the CO2 gas concentration is above 99%, for example 99.7%. For example, the carbonization reaction pressure is 0.2 MPa, the temperature is 50℃, and the time is 1.5h.

[0033] The carbonization reaction in this embodiment of the invention can be fully converted under slightly positive pressure and almost room temperature conditions. The reaction process is stable, with low energy and equipment consumption and simple operation.

[0034] In one embodiment, in step (3), the drying temperature of calcium carbonate is 80~200℃, and the grinding speed of the ball mill is 100~300 r / min, for example, 300 r / min. In this embodiment of the invention, the calcium carbonate is pulverized and then ground in a ball mill to obtain light calcium carbonate with a particle size less than 1000 mesh, a purity greater than 98%, and a whiteness greater than 90. Suitable temperature and appropriate grinding speed help reduce the drying and grinding time of calcium carbonate.

[0035] In one implementation, in step (4), the sulfate concentration reaches a relatively stable level when it approaches 15%, and the sodium sulfate cooling crystallization temperature is 5~15℃, for example, 10℃. The sodium sulfate drying temperature is 90~150℃.

[0036] In this embodiment of the invention, after the sulfate content tends to stabilize, the temperature is lowered to below 5~15℃, and the mixture is filtered at low temperature to obtain a large amount of anhydrous sodium sulfate solid. Then, it is dried in an oven at 90~150℃ to obtain anhydrous sodium sulfate crystals. Due to the optimization of the leaching efficiency and carbonization rate of the previous cyclic leaching process, the process conditions here are mild, reducing energy consumption.

[0037] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a method for preparing light calcium carbonate by cyclic leaching of phosphogypsum with sodium nitrite mother liquor provided by the present invention.

[0038] The component contents of the desilication gypsum samples used in the following examples and comparative examples are shown in Table 1 below: Table 1. Element content of desilication gypsum samples.

[0039] Example 1

[0040] like Figure 1 As shown, the method for preparing light calcium carbonate and sodium sulfate by cyclic leaching of phosphogypsum in this embodiment is carried out as follows: Weigh 20g of desilication phosphogypsum from a wet-process phosphoric acid plant, prepare 140g of sodium nitrite solution (5M), and stir and extract at room temperature (25℃) for 3 hours. Filter under vacuum to obtain the upper residual solid and the lower layer of calcium nitrite and sodium sulfate solution of a certain concentration (i.e., the primary filtrate).

[0041] The filter cake was dried at 45℃ for 12 hours, weighed, and the extraction efficiency was calculated. The lower layer solution was taken to determine the calcium ion and sulfate ion content to inversely estimate the extraction efficiency.

[0042] Take the above sodium nitrite extraction solution and add it to a 500 mL pressurized reactor. Introduce 99.7% CO2 gas into the reactor and control the carbonization reaction pressure at 0.2 MPa, rotation speed at 300 r / min, and temperature at 50 °C. React for about 1.5 h, then close the CO2 inlet valve, stop stirring, and wait for the temperature to naturally drop below 30 °C. Slowly release the pressure to atmospheric pressure, open the bottom valve of the pressurized reactor, and release the reaction slurry for vacuum filtration to separate the solid and liquid phases, obtaining a solid phase and a lower layer of sodium nitrite and sodium sulfate mixture.

[0043] The solid phase was dried in an oven at 100℃ for 5 hours to obtain a light calcium carbonate product with a purity of 98% and a whiteness of 90%.

[0044] The lower layer of sodium nitrite and sodium sulfate mixture (i.e., secondary filtrate) is weighed and recycled. After five recycling cycles, the sulfate ion concentration reaches a relatively stable level. The filtrate is partially removed and cooled for crystallization, precipitating anhydrous sodium sulfate crystals. The recycled mother liquor is obtained by filtration. A certain amount of sodium nitrite is added to the recycled mother liquor, and the process of leaching fresh phosphogypsum continues.

[0045] Example 2

[0046] The method for preparing light calcium carbonate and sodium sulfate by cyclic leaching of phosphogypsum in this embodiment is carried out as follows: Weigh 20g of desilication phosphogypsum from a wet-process phosphoric acid plant, prepare a (140g, 10M) sodium chloride solution, and stir and extract for 3 hours at room temperature (25℃). Filter under vacuum to obtain an upper layer of residual solid and a lower layer of calcium chloride solution of a certain concentration (i.e., the primary filtrate).

[0047] The filter cake was dried at 45℃ for 12 hours, weighed, and the extraction efficiency was calculated. The lower layer solution was taken to determine the calcium ion and sulfate content to inversely estimate the extraction efficiency.

[0048] Take the above sodium chloride leaching solution and add it to a 500 mL pressurized reactor. Introduce 99.7% CO2 gas into the reactor and control the carbonization reaction pressure at 0.2 MPa, rotation speed at 300 r / min, and temperature at 50℃. React for about 1.5 h, close the CO2 inlet valve, stop stirring, and wait for the temperature to naturally drop below 30℃. Slowly release the pressure to atmospheric pressure, open the bottom valve of the pressurized reactor, and release the reaction slurry for vacuum filtration to separate the solid and liquid phases, obtaining a solid phase and a lower layer of sodium chloride and sodium sulfate mixture.

[0049] The solid phase was dried in an oven at 100℃ for 5 hours to obtain a light calcium carbonate product with a purity of 98% and a whiteness of 90%.

[0050] The lower layer of sodium chloride and sodium sulfate mixture (i.e., secondary filtrate) is weighed and recycled. After five recycling cycles, the sulfate ion concentration reaches a relatively stable level. The filtrate is partially removed and cooled for crystallization, precipitating anhydrous sodium sulfate crystals. The recycled mother liquor is obtained by filtration. A certain amount of sodium chloride is added to the recycled mother liquor, and the process of leaching fresh phosphogypsum continues.

[0051] Example 3

[0052] The method for preparing light calcium carbonate and sodium sulfate by cyclic leaching of phosphogypsum in this embodiment is carried out as follows: Weigh 20g of desilication phosphogypsum from a wet-process phosphoric acid plant, prepare 100g of sodium nitrite solution (5M), and stir and extract at room temperature (50℃) for 3 hours. Filter under vacuum to obtain the upper residual solid and the lower layer of calcium nitrite and sodium sulfate solution of a certain concentration (i.e., the primary filtrate).

[0053] The filter cake was dried at 45℃ for 12 hours, weighed, and the extraction efficiency was calculated. The lower layer solution was taken to determine the calcium ion and sulfate ion content to inversely estimate the extraction efficiency.

[0054] Take the above sodium nitrite extraction solution and add it to a 500 mL pressurized reactor. Introduce 99.7% CO2 gas into the reactor and control the carbonization reaction pressure at 0.5 MPa, rotation speed at 300 r / min, and temperature at 50 °C. React for about 1 h, close the CO2 inlet valve, stop stirring, and wait for the temperature to naturally drop below 30 °C. Slowly release the pressure to atmospheric pressure, open the bottom valve of the pressurized reactor, and release the reaction slurry for vacuum filtration to separate the solid and liquid phases, obtaining a solid phase and a lower layer of sodium nitrite and sodium sulfate mixture.

[0055] The solid phase was dried in an oven at 100℃ for 5 hours to obtain a light calcium carbonate product with a purity of 98% and a whiteness of 90%.

[0056] The lower layer of sodium nitrite and sodium sulfate mixture (i.e., secondary filtrate) is weighed and recycled. After five recycling cycles, the sulfate ion concentration reaches a relatively stable level. The filtrate is partially removed and cooled for crystallization, precipitating anhydrous sodium sulfate crystals. The recycled mother liquor is obtained by filtration. A certain amount of sodium nitrite is added to the recycled mother liquor, and the process of leaching fresh phosphogypsum continues.

[0057] Comparative Example 1 Unlike Example 1, the liquid-to-solid ratio of the leaching phosphogypsum was 11:1. 20g of desilicationized phosphogypsum from the wet phosphoric acid plant was weighed and a (220g, 5M) sodium nitrite solution was prepared, ultimately yielding a light calcium carbonate product with a purity of 96% and a whiteness of 90%.

[0058] As the leaching liquid-to-solid ratio increases, the concentration of leached calcium ions decreases, leading to a decrease in carbonation reaction efficiency. Consequently, the purity of calcium carbonate obtained in this comparative example is lower than that in the example.

[0059] Comparative Example 2 Unlike Example 1, the leaching time of phosphogypsum was 1 hour. 20g of desilicationized phosphogypsum from the wet phosphoric acid unit was weighed and a sodium nitrite solution of (140g, 5M) was prepared to finally obtain a light calcium carbonate product with a purity of 95% and a whiteness of 92%.

[0060] Because the leaching time was shorter, the concentration of leached calcium ions decreased, resulting in a lower carbonation reaction efficiency. Consequently, the calcium carbonate obtained in this comparative example had a lower purity than that in the example.

[0061] The calcium carbonate product prepared in the example showed no heavy metal residues or free acid insolubles, meeting the requirements for food-grade calcium carbonate.

[0062] Taking Examples 1 and 2 as examples, Tables 1 and 2 show the results of the first to fifth cycles of Example 1 and Example 2, respectively.

[0063] Table 1 Results of cyclic leaching-mineralization with sodium nitrite solution

[0064] Table 2 Results of cyclic leaching-mineralization with sodium chloride solution

[0065] As can be seen from Tables 1 and 2, as the number of leaching cycles increases, the leaching efficiency of phosphogypsum tends to decrease when the sulfate concentration accumulates to about 10%. However, the preparation of light calcium carbonate products by the second step of pressurized carbonation reaction is almost unaffected by the accumulation of sulfate ion concentration. Moreover, the content of each element in the calcium carbonate sample of Example 1 is shown in Table 3, which compares the trace elements of the prepared calcium carbonate.

[0066] As can be seen, the calcium carbonate product prepared by the method of the present invention has no heavy metal residues and no detectable free acid insoluble matter, and can meet the requirements of food-grade calcium carbonate.

[0067] Table 3. Element content of calcium carbonate samples prepared by leaching desilication gypsum.

[0068] from Figure 2 It can be seen that the particle size of the calcium carbonate prepared in Example 1 is mainly distributed between 2 and 10 μm. Figure 3 This is a physical illustration of the calcium carbonate and sodium sulfate prepared in Example 1.

[0069] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing light calcium carbonate and co-producing sodium sulfate by cyclic leaching of phosphogypsum, characterized in that, Includes the following steps: Step (1) Add the desilication phosphogypsum to the leaching reagent, stir and react. After the reaction is complete, filter to obtain the leachate and filter cake. Step (2) Transfer the above leachate to a pressurized reactor, introduce carbon dioxide gas into the leachate, and after the carbonization reaction is completed, slowly cool down and depressurize. When the reaction solution returns to room temperature, release the reaction solution and filter it under vacuum to obtain calcium carbonate solid product and lower layer mixture. Step (3) Dry the solid calcium carbonate product, crush and grind it to finally obtain light calcium carbonate; Step (4) Weigh and recycle the lower layer mixture. After several cycles, the sulfate concentration becomes relatively stable. Take out part of the filtrate, cool and crystallize it to precipitate anhydrous sodium sulfate crystals. Filter to obtain the mother liquor from multiple cycles. Add a certain amount of leaching reagent to the mother liquor from multiple cycles and continue to leach fresh phosphogypsum.

2. The method according to claim 1, characterized in that, In step (1), the leaching reagent is sodium nitrite or sodium chloride, with the concentration of sodium nitrite being 3~6 mol / L and the concentration of sodium chloride being 5~10 mol / L.

3. The method according to claim 1, characterized in that, In step (1), the liquid-to-solid ratio of leaching phosphogypsum is 3~10:1, the reaction temperature is 30~70℃, and the reaction time is 2~5h.

4. The method according to claim 1, characterized in that, In step (1), the filter cake is dried, weighed and the extraction efficiency is calculated. The calcium ion and sulfate ion content of the leachate is measured to inversely estimate the extraction efficiency.

5. The method according to claim 1, characterized in that, In step (2), the carbonization reaction pressure is 0.1~0.8 MPa, the reaction temperature is 30~70℃, the reaction time is 1~2 h, and the CO2 gas concentration is above 99%.

6. The method according to claim 1, characterized in that, In step (3), the drying temperature of calcium carbonate is 80~200℃, and the grinding speed of the ball mill is 100~300 r / min.

7. The method according to claim 1, characterized in that, In step (4), the sulfate concentration reaches a relatively stable level when it approaches 15%, the sodium sulfate cooling crystallization temperature is 5~15℃, and the sodium sulfate drying temperature is 90~150℃.

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