A method for removing and fixing heavy metal impurities in phosphogypsum

Abstract

The present invention relates to the field of wastewater treatment, and specifically discloses a method for removing impurities and fixing heavy metals in phosphogypsum. The method of the present invention uses distilled water to wash phosphogypsum at a specific liquid-to-solid ratio, and the phosphogypsum solid after washing can be directly used; then, modified bentonite is used to coordinate with two auxiliary agents, polyaluminum chloride and carbide slag, to adsorb the washing liquid of the phosphogypsum, and the secondary washing liquid after adsorption treatment can be directly discharged. The method for removing impurities and fixing heavy metals in phosphogypsum of the present invention provides a feasible solution to the current accumulation of phosphogypsum and the industrial preparation of soil conditioners using phosphogypsum as raw material, and also realizes the recycling of carbide slag waste.

Description

Technical Field

[0001] The invention relates to the field of wastewater treatment, and in particular to a method for removing and fixing heavy metals in phosphogypsum. Background Art

[0002] Phosphogypsum (PG) is a solid byproduct produced by phosphate fertilizer enterprises in the process of producing phosphoric acid using the wet phosphoric acid process. The main component of phosphogypsum is calcium sulfate dihydrate crystals, and it also contains undecomposed phosphate rock, quartz, phosphorus, fluorine, aluminum, magnesium, iron and other impurities. At present, my country's annual output of phosphogypsum has exceeded 80 million tons, and the wet phosphoric acid process is mainly based on the dihydrate method. Phosphate rock is leached with sulfuric acid, and phosphogypsum is precipitated while phosphoric acid is obtained. And every ton of phosphoric acid produced will produce 4 to 5 tons of phosphogypsum.

[0003] The resource utilization of phosphogypsum is a difficult problem faced by the whole world. The effective utilization rate of phosphogypsum in the world is less than 10%. Due to the low comprehensive utilization rate of phosphogypsum, a large amount of phosphogypsum is stockpiled, and the accumulated stockpiled amount exceeds 500 million tons. The large amount of stockpiled phosphogypsum has caused waste of land resources and environmental threats. The main utilization ways of phosphogypsum include production of building materials, extraction of rare earth raw materials, production of chemical raw materials, soil improvement, etc. However, phosphogypsum contains impurities such as soluble phosphorus, fluorine and organic matter, which seriously hinders its resource utilization.

[0004] At present, the commonly used impurity removal methods in the phosphogypsum industry include water washing, flotation, calcination, etc. Among them, the water washing method is the most commonly used method for removing soluble impurities in phosphogypsum in the industry. It has the advantages of simple operation and good treatment effect. After water washing, gray-white phosphogypsum crystals can be obtained, and the crystals have the characteristics of clear and regular structural outlines. However, at the same time, the post-treatment of heavy metals in the washing liquid after water washing is still a major problem. The post-treatment of the washing liquid is mainly aimed at the fixation of four types of toxic and harmful heavy metal ions, Hg, Pb, Cr, and As. At present, the treatment method for these four heavy metal ions in the industry is activated carbon adsorption. The impurity removal rate of activated carbon for Hg is about 90%; the impurity removal rate for Cr is only 80%; the impurity removal rate for Pb is about 80-90%; the impurity removal rate for As is 80%, and there is still a lot of room for improvement in impurity removal efficiency.

[0005] One of the effective uses of phosphogypsum is as a soil conditioner, which can supplement phosphorus and adjust soil pH. However, many technical problems have not been effectively solved, such as the removal of harmful heavy metal elements, excessive calcium ions, and pH adjustment. If phosphogypsum is simply applied directly to the soil, while beneficial trace elements are added, harmful heavy metal elements will penetrate into the soil, leading to soil and groundwater pollution. Therefore, the resource utilization of phosphogypsum has been limited. Summary of the invention

[0006] In view of the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for removing and fixing heavy metals in phosphogypsum. The method of the present invention washes phosphogypsum with distilled water, and the phosphogypsum solid after washing can be directly used; then, the water washing liquid of phosphogypsum is adsorbed by using modified bentonite in combination with two auxiliary agents, polyaluminium chloride and carbide slag, and the secondary water washing liquid after adsorption treatment can be directly discharged.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for removing and fixing heavy metal impurities in phosphogypsum comprises the following steps:

[0009] (1) Pretreatment of phosphogypsum: drying the phosphogypsum and grinding it through a 0.3 mm sieve;

[0010] (2) Water washing: add the ground and sieved phosphogypsum obtained in step (1) into distilled water at a solid-liquid ratio of 1 g:20 mL, oscillate in a constant temperature oscillator for 8 to 12 h, take out and let stand for 12 to 24 h, and the supernatant is the water washing solution;

[0011] (3) Preparation of modified bentonite:

[0012] Placing commercially available bentonite in a muffle furnace and calcining it once in an air atmosphere to obtain a primary calcined product; introducing calcium chloride and concentrated sulfuric acid into the primary calcined product and vigorously stirring it for 6 to 12 hours to obtain a precursor; placing the precursor in a muffle furnace and calcining it twice in an air atmosphere to obtain a secondary calcined product; grinding the secondary calcined product through a 0.3 mm sieve to obtain the modified bentonite;

[0013] (4) Adsorption: adding polyaluminium chloride, carbide slag and the modified bentonite obtained in step (3) to the whole water washing liquid obtained in step (2), oscillating in a constant temperature oscillator for 8 to 12 hours, centrifuging to obtain a secondary water washing liquid, and completing the removal and fixation of heavy metals in phosphogypsum.

[0014] Furthermore, in step (1), the phosphogypsum is placed in an oven at 100 to 120° C. and dried for 4 to 8 hours.

[0015] Furthermore, in step (2), the mixture is oscillated at room temperature at a speed of 120 r / min.

[0016] Furthermore, in step (3), the calcination temperature of the first calcination and the second calcination is 300-500°C, the heating rate is 2-6°C / min, and the calcination time is 4-6h; preferably, the calcination temperature of the first calcination and the second calcination is 400°C, the heating rate is 5°C / min, and the calcination time is 4h.

[0017] Furthermore, in step (3), the amount of calcium chloride is 3% to 10% of the mass of commercially available bentonite, the concentrated H2SO4 is 98wt% sulfuric acid, and the amount of concentrated H2SO4 is 2% to 10% of the mass of commercially available bentonite; preferably, the amount of calcium chloride is 3% of the mass of commercially available bentonite, the amount of concentrated H2SO4 is 5% of the mass of commercially available bentonite, and the stirring time is 8 to 10 hours.

[0018] Furthermore, in step (4), the amount of polyaluminium chloride is 0.1% to 1.0% of the mass of the modified bentonite, and the amount of carbide slag is 0.1% to 0.5% of the mass of the modified bentonite; preferably, the amount of polyaluminium chloride is 0.3% of the mass of the modified bentonite, and the amount of carbide slag is 0.25% of the mass of the modified bentonite.

[0019] Furthermore, in step (4), the carbide slag is ground and passed through a 1 mm sieve.

[0020] Furthermore, in step (4), the mixture is oscillated at room temperature with a rotation speed of 120 r / min.

[0021] Furthermore, in step (4), the secondary water washing liquid after adsorption treatment complies with the emission standard of "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] 1. The present invention improves the existing water washing process and improves the efficiency of fixing and removing impurities of heavy metal ions. The present invention uses distilled water to wash phosphogypsum at a liquid-solid ratio of 20:1 (mL / g). The washed phosphogypsum solid can be directly used for co-production of cement, as a roadbed material and soil conditioner; then the water washing liquid of phosphogypsum is adsorbed by using modified bentonite in conjunction with two additives, polyaluminium chloride and carbide slag. The secondary water washing liquid after adsorption treatment meets the emission standard of "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

[0024] 2. In the present invention, polyaluminium chloride, carbide slag and modified bentonite are used to adsorb the washing liquid of phosphogypsum. Polyaluminium chloride, carbide slag and modified bentonite have a synergistic effect. Polyaluminium chloride, as a coagulant, forms floccules in water. These floccules can produce physical adsorption or chemical adsorption with modified bentonite particles, thereby enhancing the adsorption effect of heavy metal ions and effectively improving the adsorption of heavy metal ions. Carbide slag, as a waste material in the production of carbide, can be used as a pH regulator to adjust the pH of the washing agent and realize waste recycling at the same time; on the other hand, the introduction of polyaluminium chloride and carbide slag can simultaneously form calcium sulfonate (3CaO·Al2O3·3CaSO4·32H2O), and the gelling activity of calcium sulfonate can also promote the coagulation and flocculation of some harmful heavy metal elements.

[0025] 3. In the present invention, the auxiliary agent CaCl2 and concentrated H2SO4 are used to synergistically modify the bentonite, which can significantly promote the adsorption effect of heavy metals As and Cr in the water washing liquid of phosphogypsum compared with the unmodified bentonite.

[0026] 4. The method for removing and fixing heavy metals from phosphogypsum of the present invention provides a feasible solution to the current accumulation of phosphogypsum and the industrial preparation of soil conditioners using phosphogypsum as raw material, and also realizes the recycling of carbide slag waste. DETAILED DESCRIPTION

[0027] The technical solution of the present application will be described in detail below in conjunction with embodiments, but the scope of protection requested by the claims of the present invention is not limited to these embodiments.

[0028] Example 1 A method for removing and fixing heavy metal impurities in phosphogypsum

[0029] (1) Pretreatment of phosphogypsum: Take an appropriate amount of phosphogypsum (from Garsili (Jingzhou) Fertilizer Co., Ltd.) and dry it in an oven at 105°C for 6 h. After cooling to room temperature, grind it through a 0.3 mm sieve.

[0030] (2) Water washing: Take 1 g of the ground and sieved phosphogypsum sample and add 20 mL of distilled water. Oscillate in a constant temperature oscillator at room temperature (25°C) at a speed of 120 r / min for 8 hours. After taking it out, let it stand for 16 hours (let it stand overnight) to separate the solid and liquid. The supernatant is the water washing liquid. Put the solid into an oven and dry it at 110°C for 4 hours to obtain the washed phosphogypsum.

[0031] The washed phosphogypsum was tested by ICP-MS, and the test results showed that the heavy metal contents in the washed phosphogypsum solid were As: 35.52 mg / kg, Cr: 13.15 mg / kg, Hg: 0.00 mg / kg, and Pb: 32.44 mg / kg. In this method, the liquid-solid ratio (mL / g) in the washing and leaching process was increased to 20:1, and the washed phosphogypsum solid can be directly used for co-production of cement, as a roadbed material, and as a soil conditioner.

[0032] The pH of the washing liquid was tested with a pH meter, and the pH was 1.31. At the same time, the washing liquid was tested by ICP-MS. The test results showed that the content of heavy metals in the washing liquid was As: 0.486 mg / L, Cr: 25.08 mg / L, Hg: 0.21 μg / L, Pb: 1.55 μg / L. The concentration of As and Cr in the washing liquid exceeded the requirements of the "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015 (total mercury: 0.005 mg / L, total chromium: 0.5 mg / L, total arsenic: 0.3 mg / L, total lead: 0.5 mg / L) and the acidity and alkalinity did not meet the standards, so it could not be discharged directly. The results also show that the Cr content of phosphogypsum is seriously exceeded without water washing, and it cannot be directly used for co-production of cement, as a roadbed material, or as a soil conditioner.

[0033] (3) Preparation of modified bentonite: 10.00 g of commercially available bentonite was placed in a muffle furnace, heated to 400° C. at a heating rate of 5° C. / min, and then calcined for 4 h to obtain precursor 1; 0.3 g (3% of the mass of bentonite) of CaCl2 and 0.5 g (5% of the mass of bentonite) of H2SO4 with a mass concentration of 98% were added to precursor 1, and the mixture was vigorously stirred for 8 h to obtain precursor 2; precursor 2 was placed in a muffle furnace, heated to 400° C. at a heating rate of 5° C. / min, and then calcined for 4 h to obtain modified bentonite;

[0034] (4) Weigh 0.003 g of polyaluminium chloride, 0.0025 g of carbide slag (ground and passed through a 1 mm sieve) and 1.000 g of the modified bentonite obtained in step (3) and add them to the water washing liquid (about 20 mL) obtained in step (2). Use a constant temperature oscillator to oscillate at a speed of 120 r / min at room temperature (25° C.) for 12 h, and centrifuge to obtain a secondary water washing liquid.

[0035] The pH of the secondary washing liquid was tested with a pH meter, and the pH was 7.20. At the same time, the secondary washing liquid was tested by ICP-MS. The test results showed that the content of heavy metals in the secondary washing liquid was As: 0.0208mg / L, Cr: 0.2319mg / L, Hg: 0.00mg / L, Pb: 0.00mg / L. The concentrations of As and Cr in the secondary washing liquid meet the requirements of the "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015 and can be directly discharged.

[0036] The results show that when polyaluminium chloride, carbide slag and modified bentonite are used together to treat the washing liquid of phosphogypsum, the absorption rates of Hg, Pb, As and Cr are 100%, 100%, 95.72% and 99.07%, respectively.

[0037] Example 2 A method for removing and fixing heavy metal impurities in phosphogypsum

[0038] The method of this embodiment, the first three steps are the same as those of embodiment 1, and further includes the following steps:

[0039] (4) 1.000 g of the modified bentonite obtained in step (3) was added to the water washing solution (20 mL) obtained in step (2), and the mixture was oscillated at 120 r / min for 12 h at room temperature (25° C.) using a constant temperature oscillator, and centrifuged to obtain a secondary water washing solution.

[0040] The pH of the secondary washing liquid was tested with a pH meter, and the pH was 1.32. At the same time, ICP-MS was performed on As and Cr in the secondary washing liquid. The test results showed that the content of each heavy metal in the secondary washing liquid was As: 4.219μg / L, Cr: 7.9328mg / L. The Cr element concentration in the secondary washing liquid did not meet the requirements of the "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

[0041] The results show that when only modified bentonite is used to treat the water washing liquid of phosphogypsum, the absorption rate of As is 99.13% and the absorption rate of Cr is 68.37%.

[0042] Example 3 A method for removing and fixing heavy metal impurities in phosphogypsum

[0043] The method of this embodiment, the first two steps are the same as those of embodiment 1, and further includes the following steps:

[0044] (3) Weigh 0.003 g of polyaluminium chloride and 0.0025 g of carbide slag and add them to the water washing liquid (20 mL) obtained in step (2), and oscillate for 12 h at room temperature (25° C.) at a speed of 120 r / min using a constant temperature oscillator, and centrifuge to obtain a secondary water washing liquid.

[0045] The pH of the secondary washing liquid was tested with a pH meter, and the pH was 7.20. At the same time, ICP-MS was performed on As and Cr in the secondary washing liquid. The test results showed that the content of each heavy metal in the secondary washing liquid was As: 98.69μg / L, Cr: 2.802mg / L. The Cr element concentration in the secondary washing liquid did not meet the requirements of the "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

[0046] The results show that when polyaluminium chloride and carbide slag are used to treat the washing liquid of phosphogypsum, the absorption rate of As is 79.69% and the absorption rate of Cr is 88.83%.

[0047] As can be seen from Examples 1-3, the water washing liquid of phosphogypsum is treated with polyaluminium chloride, carbide slag and modified bentonite at the same time, and the water washing liquid meets the emission standard of "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015. Adding carbide slag to the water washing liquid of phosphogypsum can adjust the pH to near neutrality, and the use of polyaluminium chloride, carbide slag and modified bentonite can significantly promote the adsorption effect of heavy metals As and Cr in the water washing liquid. Polyaluminium chloride, as a coagulant, forms floccules in water, and these floccules can produce physical adsorption or chemical adsorption with modified bentonite particles, thereby enhancing the adsorption effect of heavy metal ions, and can effectively improve the adsorption of heavy metal ions. As a waste material in the production of calcium carbide, carbide slag can be used as a pH regulator to adjust the pH of the water washing agent and realize waste recycling at the same time; on the other hand, the introduction of polyaluminium chloride and carbide slag can simultaneously form ettringite (3CaO·Al2O3·3CaSO4·32H2O), and the gelling activity of ettringite can also promote the solidification and precipitation of some harmful heavy metal elements.

[0048] Example 4 A method for removing and fixing heavy metal impurities in phosphogypsum

[0049] (1) Pretreatment of phosphogypsum: The phosphogypsum (from another batch from Garsili (Jingzhou) Fertilizer Co., Ltd.) was dried in an oven at 105°C for 6 h and ground to pass through a 0.3 mm sieve.

[0050] (2) Water washing: Take 1 g of the ground and sieved phosphogypsum sample and add 20 mL of distilled water. Oscillate in a constant temperature oscillator at room temperature (25°C) at a speed of 120 r / min for 8 hours. After taking it out, let it stand for 16 hours (let it stand overnight) to separate the solid and liquid. The supernatant is the water washing liquid. Put the solid into an oven and dry it at 110°C for 4 hours to obtain the washed phosphogypsum.

[0051] The pH of the washing liquid was tested with a pH meter, and the pH was 1.42. At the same time, the washing liquid was tested by ICP-MS. The test results showed that the heavy metal content in the washing liquid was As: 0.603mg / L, Cr: 191.4mg / L, Hg: 0.22μg / L, Pb: 1.75μg / L. The concentration of As and Cr in the washing liquid exceeded the requirements of the "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015 (total mercury: 0.005mg / L, total chromium: 0.5mg / L, total arsenic: 0.3mg / L, total lead: 0.5mg / L) and the acidity and alkalinity did not meet the standards, so it could not be discharged directly.

[0052] (3) Calcination of bentonite: 10.00 g of commercially available bentonite was placed in a muffle furnace, heated to 400 °C at a heating rate of 5 °C / min, and then calcined for 4 h. After calcination, the mixture was naturally cooled to room temperature.

[0053] (4) 1.000 g of the calcined bentonite obtained in step (3) was added to the water washing solution (20 mL) obtained in step (2), and the mixture was oscillated at 120 r / min for 12 h at room temperature (25° C.) using a constant temperature oscillator, and centrifuged to obtain a secondary water washing solution.

[0054] The pH of the secondary washing liquid was tested with a pH meter, and the pH was 1.34. At the same time, ICP-MS was performed on As and Cr in the secondary washing liquid. The test results showed that the content of each heavy metal in the secondary washing liquid was As: 0.518 mg / L, Cr: 43.30 mg / L. The concentration of As and Cr in the secondary washing liquid did not meet the requirements of the "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

[0055] The results show that when only calcined bentonite is used to treat the water washing liquid of phosphogypsum, the absorption rate of As is 14.10% and the absorption rate of Cr is 77.38%.

[0056] Example 5 A method for removing and fixing heavy metal impurities in phosphogypsum

[0057] The method of this embodiment, the first two steps are the same as those of embodiment 4, and further includes the following steps:

[0058] (3) 1.00 g of commercially available bentonite was added to the water washing solution (20 mL) obtained in step (2), and the mixture was oscillated at 120 r / min for 12 h at room temperature (25° C.) using a constant temperature oscillator, and centrifuged to obtain a secondary water washing solution.

[0059] The pH of the secondary washing liquid was tested with a pH meter, and the pH was 1.32. At the same time, ICP-MS was performed on As and Cr in the secondary washing liquid. The test results showed that the content of each heavy metal in the secondary washing liquid was As: 0.559 mg / L, Cr: 39.06 mg / L. The concentration of As and Cr in the secondary washing liquid did not meet the requirements of the "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

[0060] The results show that when only commercial bentonite is used to treat the water washing liquid of phosphogypsum, the absorption rate of As is 7.30% and the absorption rate of Cr is 79.59%.

[0061] Example 6 A method for removing and fixing heavy metal impurities in phosphogypsum

[0062] The method of this embodiment, the first two steps are the same as those of embodiment 4, and further includes the following steps:

[0063] (3) Preparation of acid-modified bentonite: 10.00 g of commercially available bentonite was placed in a muffle furnace, heated to 400° C. at a heating rate of 5° C. / min, and then calcined for 4 h to obtain precursor 1; 0.5 g (5% of the mass of bentonite) of 98% H2SO4 was added to precursor 1, and stirred vigorously for 8 h to obtain precursor 2; the precursor was then placed in a muffle furnace and calcined at 400° C. at a heating rate of 5° C. / min for 4 h to obtain acid-modified bentonite;

[0064] (4) Weigh 0.003 g of polyaluminium chloride, 0.0025 g of carbide slag (ground and passed through a 1 mm sieve) and 1.000 g of the acid-modified bentonite obtained in step (3) and add them to the water washing liquid (20 mL) obtained in step (2). Use a constant temperature oscillator to oscillate at a speed of 120 r / min at room temperature (25° C.) for 12 h, and centrifuge to obtain a secondary water washing liquid.

[0065] The pH of the secondary washing liquid was tested with a pH meter, and the pH was 1.32. At the same time, ICP-MS was performed on As and Cr in the secondary washing liquid. The test results showed that the content of each heavy metal in the secondary washing liquid was As: 0.479 mg / L, Cr: 40.79 mg / L. The concentrations of As and Cr in the secondary washing liquid did not meet the requirements of the "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

[0066] The results show that when polyaluminium chloride, carbide slag and acid-modified bentonite are used to treat the washing liquid of phosphogypsum, the absorption rate of As is 20.56% and the absorption rate of Cr is 78.69%.

[0067] It can be seen from Examples 2, 4, 5, and 6 that the modified bentonite of the present invention can significantly promote the adsorption effect of heavy metals As and Cr in the water washing liquid relative to the unmodified bentonite. The performance improvement of the modified bentonite lies in the use of CaCl2 and concentrated H2SO4 to synergistically modify the bentonite. However, when the water washing liquid of phosphogypsum is treated with only modified bentonite, the water washing liquid still does not meet the emission standard of "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

[0068] Example 7 A method for removing and fixing heavy metal impurities in phosphogypsum

[0069] The method of this embodiment is the same as that of embodiment 1, the only difference being that the mass of bentonite, modifier CaCl2 and concentrated H2SO4 (H2SO4 with a mass concentration of 98%) is adjusted in step (3), as shown in Table 1:

[0070] Table 1

[0071] Bentonite Quality <![CDATA[Mass percentage of CaCl2]]> <![CDATA[Mass percentage of concentrated H2SO4]]> Group 1 2.00g 0.1g(5％) 0.1g(5％) Group 2 2.00g 0.2g(10％) 0.1g(5％) Group 3 2.00g 0.06g(3％) 0.04g(2％) Group 4 2.00g 0.1g(5％) 0.04g(2％) Group 5 2.00g 0.2g(10％) 0.04g(2％) Group 6 2.00g 0.06g(3％) 0.2g(10％) Group 7 2.00g 0.1g(5％) 0.2g(10％)

[0072] The test results of ICP-MS of each group of secondary washing liquid are shown in Table 2:

[0073] Table 2

[0074]

[0075]

[0076] As can be seen from Example 7, CaCl2 and concentrated H2SO4 are used to modify bentonite in a coordinated manner, with the amount of CaCl2 being 3% to 10% of the mass of bentonite and the amount of concentrated H2SO4 being 2% to 10% of the mass of bentonite. A series of modified bentonites are prepared to simultaneously treat the washing liquid of phosphogypsum with polyaluminium chloride and carbide slag, and the washing liquids all meet the emission standard of GB31573-2015, the emission standard of pollutant emission of inorganic chemical industry.

Claims

1. A method for removing and fixing heavy metal impurities in phosphogypsum, characterized in that: The steps include: (1) Pretreatment of phosphogypsum: dry the phosphogypsum and grind it through a 0.3 mm sieve; (2) Water washing: add the ground and sieved phosphogypsum from step (1) into distilled water at a solid-liquid ratio of 1 g:20 mL, oscillate in a constant temperature oscillator for 8 to 12 h, take it out and let it stand for 12 to 24 h. The supernatant is the water washing solution; (3) Preparation of modified bentonite: placing commercially available bentonite in a muffle furnace and calcining it once in an air atmosphere to obtain a primary calcined product; introducing calcium chloride and concentrated sulfuric acid into the primary calcined product and vigorously stirring it for 6 to 12 hours to obtain a precursor; placing the precursor in a muffle furnace and calcining it twice in an air atmosphere to obtain a secondary calcined product; grinding the secondary calcined product through a 0.3 mm sieve to obtain the modified bentonite; the calcination temperatures of the primary calcination and the secondary calcination are both 300 to 500°C; (4) Adsorption: adding polyaluminium chloride, carbide slag and the modified bentonite obtained in step (3) to the whole water washing liquid obtained in step (2), oscillating in a constant temperature oscillator for 8 to 12 hours, centrifuging to obtain a secondary water washing liquid, and completing the removal and fixation of heavy metals in phosphogypsum.

2. The method for removing impurities and fixing heavy metals in phosphogypsum according to claim 1, characterized in that: In step (3), the amount of calcium chloride used is 3% to 10% of the mass of commercially available bentonite, the concentrated H2SO4 is 98wt% sulfuric acid, and the amount of concentrated H2SO4 used is 2% to 10% of the mass of commercially available bentonite; and / or In step (4), the amount of polyaluminium chloride used is 0.1% to 1.0% of the mass of the modified bentonite, and the amount of carbide slag used is 0.1% to 0.5% of the mass of the modified bentonite.

3. The method for removing impurities and fixing heavy metals in phosphogypsum according to claim 2, characterized in that: In step (3), the amount of calcium chloride is 3% of the mass of commercially available bentonite, the concentrated H2SO4 is 98wt% sulfuric acid, the amount of concentrated H2SO4 is 5% of the mass of commercially available bentonite, and the stirring time is 8-10h; and / or In step (4), the amount of polyaluminium chloride used is 0.3% of the mass of the modified bentonite, and the amount of carbide slag used is 0.25% of the mass of the modified bentonite.

4. The method for removing impurities and fixing heavy metals in phosphogypsum according to claim 1, characterized in that: In step (3), the heating rates of the first calcination and the second calcination are both 2-6°C / min, and the calcination time is 4-6h.

5. The method for removing impurities and fixing heavy metals in phosphogypsum according to claim 4, characterized in that: In step (3), the calcination temperature of the first calcination and the second calcination is 400°C, the heating rate is 5°C / min, and the calcination time is 4h.

6. The method for removing and fixing heavy metal impurities in phosphogypsum according to claim 1, characterized in that: In steps (2) and (4), oscillation is performed at room temperature, and the oscillation speed is 120 r / min.

7. The method for removing impurities and fixing heavy metals in phosphogypsum according to claim 1, characterized in that: In step (4), the carbide slag is ground and passed through a 1 mm sieve.

8. The method for removing impurities and fixing heavy metals in phosphogypsum according to claim 1, characterized in that: In step (4), the secondary water washing liquid after adsorption treatment complies with the emission standard of "Inorganic Chemical Industry Pollutant Emission Standard" GB31573-2015.

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