Phosphorus recovery method and phosphorus recovery system for phosphogypsum leachate

Abstract

The application discloses a phosphorus recovery method and system in phosphogypsum leachate, and relates to the technical field of environmental protection. The phosphorus recovery method comprises the following steps: adding calcium oxide into the phosphogypsum leachate, adjusting the pH value to 4, removing filter residues, removing COD through sand filtration and ultrafiltration; adjusting the pH value of the ultrafiltrate to 4, producing water through first nanofiltration, adjusting the pH value of the permeate to 10, collecting reaction generated gas into a second sedimentation tank for dissolution, continuously producing water through second nanofiltration, and obtaining concentrated water as phosphorus recovery liquid; collecting the first nanofiltration concentrated water and the second nanofiltration permeate, adding phosphate into the second sedimentation tank to obtain magnesium ammonium phosphate precipitation, and performing reverse osmosis treatment on the filtrate. The phosphorus recovery method provided by the application is simple in operation, low in cost, and can make the phosphorus recovery rate reach more than 90%, has a high removal rate of impurities such as F and S, and the obtained products in the treatment process can be recycled, and have high economic value.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of environmental protection, and particularly relates to a method for recovering phosphorus in phosphogypsum leachate and a phosphorus recovery system thereof. BACKGROUND

[0002] The phosphogypsum leachate is a byproduct of phosphogypsum produced in the process of wet production of phosphoric acid and formed by stacking and leaching. The leachate mainly comes from two parts, one is the moisture carried by the phosphogypsum when it is placed in the yard, and the other is caused by natural rainfall. The phosphogypsum leachate is an acidic (pH≈1-3) wastewater containing high concentrations of fluorine, phosphorus and sulfur, with a low COD content of about 100, and also contains metal cations such as calcium, magnesium, sodium, potassium, aluminum and zinc.

[0003] At present, the methods for recovering phosphorus in phosphogypsum leachate mainly include chemical precipitation, biological method and ion exchange method. The chemical precipitation method mainly adds a large amount of reagents to adjust the pH of the solution before and after precipitation, and it is difficult to achieve efficient and economical treatment requirements using a single reagent. Moreover, when the phosphorus content in the leachate is low, the precipitation effect is not ideal. The biological method produces a large amount of phosphorus-rich activated sludge containing heavy metals, which cannot be directly used as fertilizer. In addition, it is still very difficult to separate and identify PAOs, and the in-depth study of the EBPR process is greatly limited. The ion exchange method has high cost and complex resin treatment process.

[0004] Chinese patent CN202111646897.1 discloses a method for recovering phosphorus resources in phosphogypsum and producing ammonium magnesium phosphate as a byproduct, which selectively adsorbs, efficiently elutes and induces crystallization by using zirconium oxide beads as an adsorbent material to efficiently recover the soluble phosphorus in phosphogypsum. However, zirconium oxide has high adsorption of fluoride ions, which will affect the adsorption of phosphate by zirconium oxide, resulting in an undesirable adsorption efficiency of phosphorus and increasing the input cost of zirconium oxide beads, which also affects the generation of precipitates in the later stage. Chinese patent CN202221203878.1 discloses a system for treating high-concentration phosphogypsum leachate, which reduces the dosage of chemicals and improves the phosphorus content of the effluent by using a pretreatment and chemical precipitation method. However, this method is mainly aimed at removing phosphorus, and therefore contains a large amount of fluorine, sulfur, calcium and other substances in the phosphorus-containing impurities, which is not conducive to the recovery of phosphorus and causes waste of resources and an increase in the cost of later treatment. SUMMARY

[0005] The present application aims to provide a method and system for recovering phosphorus in phosphogypsum leachate, which has a simple process, low cost, a phosphorus recovery rate of more than 90%, a high removal rate of impurities such as F and S, and recycled products in the treatment process, which have high economic value.

[0006] In order to achieve the object of the present application, the present application provides a method for recovering phosphorus from phosphogypsum leachate, which specifically comprises the following steps:

[0007] S1. A calcium compound is added to the phosphogypsum leachate, the pH value is adjusted to 4-5, and a mixing reaction is performed, followed by removal of filter residue, and the obtained filtrate is further filtered by a sand filter tank, and then treated by an ultrafiltration membrane to remove COD, thereby obtaining an ultrafiltrate.

[0008] S2. Alkali or acid is added to the above ultrafiltrate, the pH value is adjusted to 4-5, and a first nanofiltration is performed to produce water, the obtained permeate is adjusted to a pH value of 9-10 by alkali, the gas generated by the reaction is collected and dissolved in a second sedimentation tank, and the remaining solution is continuously subjected to a second nanofiltration to produce concentrated water, which is sent to a phosphorus recovery system, i.e., a phosphorus recovery liquid.

[0009] S3. The concentrated water obtained by the first nanofiltration and the permeate obtained by the second nanofiltration are collected and sent to the second sedimentation tank, and a phosphate is added to adjust the pH value to 9-10, followed by stirring and reaction, thereby obtaining an ammonium magnesium phosphate precipitate.

[0010] S4. The filtrate from the second sedimentation tank is treated by reverse osmosis to make the ion content in the effluent meet the standard. The concentrated water after reverse osmosis treatment is sent to a salt recovery system for recovery, and the fresh water is sent to a circulating cooling water system.

[0011] Further, in the step S1, the calcium compound is calcium oxide, and the pH value is adjusted to 4, and the mixing reaction is performed for 1-1.5 h.

[0012] Further, in the step S2, the alkali is a NaOH solution, and the acid is a dilute HCl solution.

[0013] Further, in the step S2, the gas generated by the first nanofiltration is NH3, which is collected by a gas concentration collector.

[0014] Further, in the step S2, the concentrated water of the second nanofiltration is concentrated and then adjusted in pH value together with the ultrafiltrate, and the first nanofiltration is continuously performed to produce water.

[0015] Further, in the step S3, the phosphate is disodium hydrogen phosphate, and the addition of disodium hydrogen phosphate makes the molar ratio of PO4 3- to NH4 + in the reaction system of the second sedimentation tank be (1-1.5):1.

[0016] The application further provides a phosphorus recovery system for phosphogypsum leachate, which comprises, in sequence, an adjusting tank, a first sedimentation tank, a sand filter tank, an ultrafiltration device, a first nanofiltration device, a pH adjusting tank, a second nanofiltration device, a water storage tank, a second sedimentation tank, an RO device and a salt recovery system; gas of the pH adjusting tank is collected by a gas concentration collector and then connected to the second sedimentation tank through a conveying device; a concentrated water outlet of the first nanofiltration device is connected to the water storage tank through a conveying device; a concentrated water outlet of the second nanofiltration device is connected to the phosphorus recovery system through a conveying device; and a fresh water outlet of the RO device is connected to a circulating cooling water system through a conveying device.

[0017] Further, the concentrated water outlet of the second nanofiltration device is connected to the first nanofiltration device through a concentration device.

[0018] Further, the first nanofiltration device uses an acid-resistant nanofiltration membrane, and the second nanofiltration device uses an alkali-resistant nanofiltration membrane.

[0019] The application has the following beneficial effects:

[0020] 1. The application adjusts the pH value of the phosphogypsum leachate to 4 by adding calcium oxide, so that the removal rate of fluorine ions in the phosphogypsum leachate reaches 95%, the cost of adding is low, the filter residue obtained can be used as a raw material for generating fluorite, resources are reasonably utilized, and environmental protection and energy saving are achieved.

[0021] 2. The application produces water through twice nanofiltration, i.e., producing water through acid nanofiltration and alkali nanofiltration in sequence, to obtain phosphorus recovery liquid, so that the phosphorus recovery rate is greatly improved and can reach more than 90%.

[0022] 3. The application adjusts the N / P ratio by adding Na2HPO4 and uses the ammonium magnesium phosphate precipitation method to remove nitrogen, so that the nitrogen removal rate of the application can reach more than 90%. The Na2HPO4 can be provided by the phosphorus recovery liquid of the phosphorus recovery system of the application, resources are reasonably utilized, and costs are saved.

[0023] 4. The application can make the final phosphorus recovery rate of the phosphogypsum leachate reach more than 90%, significantly improve the phosphorus recovery utilization rate, save resources, and can recover each component in the phosphogypsum leachate separately, improve the resource recycling rate, achieve the purpose of energy saving and environmental protection, and has high economic value. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a flow chart of the phosphorus recovery system for phosphogypsum leachate of the application. DETAILED DESCRIPTION

[0025] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings. All features disclosed in this specification, or steps in all disclosed methods or processes, may be combined in any manner, except for mutually exclusive features and / or steps.

[0026] The phosphogypsum leachate used in this embodiment of the invention has a pH value between 1.5 and 3, COD ≤ 150 mg / L, TP > 2.5 g / L, and is an acidic wastewater with high concentrations of fluorine, phosphorus, and sulfur. The cations include metal ions such as calcium, magnesium, and sodium.

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0028] Example 1

[0029] like Figure 1 As shown, the method for phosphorus recovery from phosphogypsum leachate provided by the present invention specifically includes the following steps:

[0030] (1) The phosphogypsum permeability is transported to the equalization tank, and CaO is added to adjust the pH value to 4. The mixture is reacted for 1-1.5 hours for defluorination. The reaction product contains CaF2 and Ca2SiF6 precipitates, and the fluoride ion removal rate is about 95%. The reaction product is sent to the first sedimentation tank of the inclined tube for precipitation to remove the filter residue containing Si, SS, CaF2 and Ca2SiF6. This filter residue can be used as a raw material for fluorite production. The filtrate obtained from precipitation is sent to a sand filter for filtration to make the SS content meet the requirements of subsequent ultrafiltration membrane treatment. The filtrate after sand filtration is then sent to an ultrafiltration device to remove COD through an ultrafiltration membrane (which is an acid-resistant membrane) to obtain ultrafiltrate.

[0031] (2) Add NaOH solution or dilute hydrochloric acid to the above ultrafiltrate to adjust the pH to 4 (within this pH range, monovalent phosphate is the main form present after phosphate hydrolysis). The resulting ultrafiltrate contains Ca. 2+ Mg 2+ Na + NH4 + Cl - H2PO4 - SO4 2- The COD is ≤30. The treated ultrafiltrate is then fed into a first nanofiltration unit (the nanofiltration membrane in this unit is an acid-resistant nanofiltration membrane) for the first nanofiltration permeate. The resulting permeate (containing only monovalent ions) is sent to a pH adjustment tank. The concentrate retained from the first nanofiltration permeate (containing retained divalent ions, including Ca2+) is then treated. 2+ Mg 2+ SO4 2- (etc.) are sent to the water storage tank.

[0032] NaOH solution is added to the pH adjustment tank to adjust the pH value to 10, i.e. H2PO4 - is adjusted to HPO4 2- NH3 gas generated in the reaction is blown into the gas concentration collector and then transported to the second precipitation tank for dissolution; the obtained solution is stirred and sent into the second nanofiltration device (the nanofiltration membrane in the nanofiltration device is an alkali-resistant nanofiltration membrane) for second nanofiltration to produce water, and the concentrated water retained is divalent phosphate, i.e. phosphorus recovery liquid, which is sent to the phosphorus recovery system and can be used as the source of Na2HPO4 added in the second precipitation tank.

[0033] However, some domestic nanofiltration membranes also retain a part of monovalent ions in the concentrated water. At this time, in order to improve the purity of the phosphorus recovery liquid, the concentrated water retained in the second nanofiltration to produce water is concentrated and then sent into the first nanofiltration device for acid nanofiltration treatment by adjusting the pH value to 4, and the previous steps are repeated to sequentially improve the recovery rate of the final phosphorus recovery liquid.

[0034] (3) The permeate obtained by the second nanofiltration to produce water is sent to the water storage tank, and then the mixed solution in the water storage tank is sent into the second precipitation tank. Na2HPO4 solution is added to the second precipitation tank to adjust the pH value to 10, and the molar ratio of PO4 3- to NH4 + in the reaction system of the second precipitation tank is (1-1.5):1, and the reaction is stirred to obtain magnesium ammonium phosphate (MAP) precipitation for recycling.

[0035] (4) The filtrate from the second precipitation tank is sent into the RO device for reverse osmosis treatment, so that the ion content in the obtained fresh water (permeate) meets the standard and is sent to the circulating cooling water system. The concentrated water (concentrate) obtained by the reverse osmosis treatment is sent to the salt recovery system.

[0036] Example 2

[0037] The phosphogypsum leachate of a certain site is treated by the phosphorus recovery method in Example 1, and the pH value and components of the raw water of the phosphogypsum leachate are shown in Table 1.

[0038] Table 1

[0039]

[0040] The phosphogypsum leachate containing the data in Table 1 is treated by the phosphorus recovery method in Example 1, and the treatment process is shown in Figure 1 .

[0041] The addition amount of calcium oxide in the adjustment tank is 18-20 g / L, the reaction temperature is normal temperature, and the stirring time is 1-1.5 h. After the first precipitation tank and the sand filter tank, the F -The removal rate of SiF6 2- The removal rate of SiF6

[0042] The water inlet requirement of the first nanofiltration device is: COD < 30 mg / L, SDI < 5, pH = 4, and the removal rate of divalent ions is 92.3%.

[0043] The water inlet requirement of the second nanofiltration device is: COD < 30 mg / L, SDI < 5, pH = 10, and the removal rate of divalent ions is 93.5%.

[0044] The dosage of disodium hydrogen phosphate in the second sedimentation tank is 20 mmol / L.

[0045] Through detection, the final phosphorus recovery rate of the obtained phosphorus recovery liquid (secondary nanofiltration concentrated water) is 92.8%, and the phosphorus recovery can be used as a raw material for preparing disodium hydrogen phosphate.

[0046] The above-described embodiments only express the specific implementation of the present application, and the description is more specific and detailed, but it cannot be understood as a limitation on the scope of the patent of the present application. It should be noted that, for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are all within the scope of protection of the present application.

Claims

1. A method for recovering phosphorus from phosphogypsum leachate, characterized by, Specifically comprising the following steps: S1. Adding calcium compound into the phosphogypsum leachate, adjusting the pH value to 4-5, mixing reaction, removing the filter residue, and obtaining the filtrate which is further filtered by a sand filter tank, and then removing COD by an ultrafiltration membrane to obtain an ultrafiltrate; S2. Adding alkali or acid into the above ultrafiltrate, adjusting the pH value to 4-5, and then producing water by first nanofiltration to obtain permeate, adjusting the pH value of the permeate to 9-10 by alkali, collecting the gas produced by reaction into a second precipitation tank for dissolution, and continuously producing water by second nanofiltration to obtain secondary nanofiltration concentrated water which is sent to a phosphorus recovery system, i.e. phosphorus recovery liquid; S3. Collecting the concentrated water obtained by first nanofiltration and the permeate obtained by second nanofiltration, and sending them to the second precipitation tank, adding phosphate to adjust the pH value to 9-10, and stirring to obtain magnesium ammonium phosphate precipitate; S4. Treating the filtrate from the second precipitation tank by reverse osmosis to make the ion content in the effluent meet the standard.

2. The method of claim 1, wherein the phosphogypsum is obtained from a phosphoric acid production process. In the step S1, the calcium compound is calcium oxide, the pH value is adjusted to 4, and the mixing reaction is performed for 1-1.5 h.

3. The method of claim 1, wherein the phosphogypsum is obtained from a phosphoric acid production process. In the step S2, the alkali is NaOH solution, and the acid is dilute HCl solution.

4. The method of claim 1, wherein the phosphogypsum is obtained from a phosphoric acid production process. In the step S2, the gas produced by first nanofiltration is NH3 which is collected by a gas concentration collector.

5. The method of claim 1, wherein the phosphogypsum is obtained from a phosphoric acid production process. In the step S2, the concentrated liquid of second nanofiltration is adjusted in pH value together with the ultrafiltrate, and then first nanofiltration is continuously performed to produce water.

6. The method of phosphorus recovery from phosphogypsum leachate according to claim 1, characterized in that, In the step S3, the phosphate is disodium hydrogen phosphate, and the disodium hydrogen phosphate is added to make the molar ratio of PO4 3- to NH4 + (1-1.5):

1.

7. A phosphorus recovery system in phosphogypsum leachate, characterized by, The system comprises, in sequence, an adjusting tank, a first precipitation tank, a sand filter tank, an ultrafiltration device, a first nanofiltration device, a pH adjusting tank, a second nanofiltration device, a water storage tank, a second precipitation tank, an RO device, and a salt recovery system; the gas from the pH adjusting tank is collected by a gas concentration collector and then connected to the second precipitation tank by a conveying device; the concentrated water outlet of the first nanofiltration device is connected to the water storage tank by a conveying device; the concentrated water outlet of the second nanofiltration device is connected to the phosphorus recovery system by a conveying device; and the fresh water outlet of the RO device is connected to a circulating cooling water system by a conveying device.

8. The phosphogypsum leachate phosphorus recovery system of claim 7, wherein, The concentrated water outlet of the second nanofiltration device is connected to the first nanofiltration device by a concentration device. 9.The phosphogypsum leachate phosphorus recovery system according to claim 7, characterized in that, The first nanofiltration device uses acid-resistant nanofiltration membrane, and the second nanofiltration device uses alkali-resistant nanofiltration membrane.

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