Phosphoric acid iron wastewater treatment process based on the recycling of magnesium

Abstract

The present invention discloses a process for treating iron phosphate wastewater based on the recycling of magnesium. An excessive amount of magnesium compound is used to precipitate iron phosphate in the synthesis mother liquor and the concentrated solution of iron phosphate rinsing water, removing heavy metal ions, hydrogen ions, fluoride ions, phosphate ions and ammonium ions at one time. Then, the pH is adjusted to 7-9 with dilute sulfuric acid and then evaporated and crystallized to obtain solid magnesium sulfate. After calcining the solid magnesium sulfate, solid magnesium oxide and sulfur dioxide gas are obtained. The magnesium oxide prepared by the present invention can be continuously used for the treatment of iron phosphate wastewater, and the obtained sulfur dioxide gas can be used for the preparation of sulfuric acid, truly realizing the recycling of magnesium and having no problem of secondary pollution.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron phosphate wastewater treatment, and particularly relates to an iron phosphate wastewater treatment process based on the recycling of magnesium. Background Art

[0002] Iron phosphate is the precursor of the cathode material of lithium iron phosphate batteries. With the continuous expansion of the market scale of lithium iron phosphate batteries, the production scale of iron phosphate has also increased rapidly accordingly. In order to reduce costs, the current iron phosphate production process mainly uses by-product ferrous sulfate of titanium dioxide as the iron source, and synthesizes iron phosphate through the "two-step" process. During the production process of iron phosphate, processes such as synthesis and rinsing will be passed through, and a large amount of synthesis mother liquor and rinsing wastewater will be generated during this process. Through long-term follow-up research, the pH of iron phosphate wastewater is 2 - 2.5, F - 30 - 50 mg / L, total P 2200 - 3000 mg / L, Fe 2+ 30 - 50 mg / L, Mg 2+ 500 - 700 mg / L, NH3-N 6000 - 7000 mg / L, SO4 2- 50000 - 60000 mg / L. Therefore, this wastewater belongs to high-ammonia-nitrogen, high-phosphorus, high-salt, and high-acidity wastewater, and is extremely difficult to treat. Direct discharge will seriously damage the environment.

[0003] At present, the treatment of iron phosphate wastewater mainly includes the "calcium method" and the "ammonia method", but many problems have emerged during the operation process. For example, the treatment device of the "calcium method" is severely fouled and blocked, and the pollutants are not completely removed. The "ammonia method" has a very large ammonia water dosage and incomplete salt separation problems. This makes the treatment of iron phosphate wastewater a worldwide problem, and there is an urgent need to seek a better solution. Summary of the Invention

[0004] In order to overcome the problems in the prior art, the present invention provides an iron phosphate wastewater treatment process based on the recycling of magnesium, which avoids the problems of fouling and blockage of the treatment device and large ammonia water dosage. The treated iron phosphate wastewater will not cause secondary pollution, and a single magnesium sulfate solid is obtained after treatment, without the need for salt separation. The magnesium oxide generated after calcining the magnesium sulfate solid can be reused for the treatment of iron phosphate wastewater, realizing the recycling of magnesium.

[0005] To solve the above technical problems, the technical solutions proposed by the present invention are as follows:

[0006] The first aspect of the present invention provides an iron phosphate wastewater treatment process based on the recycling of magnesium, including the following steps:

[0007] S1. Add an excessive amount of magnesium compound to the iron phosphate wastewater for precipitation reaction, and after solid-liquid separation, obtain a solid residue and a precipitation solution;

[0008] S2. Adjust the pH of the precipitate solution obtained in step S1 to 7 - 9 with dilute sulfuric acid, and then perform evaporation crystallization to obtain solid magnesium sulfate.

[0009] S3. Calcinate the solid magnesium sulfate to obtain solid magnesium oxide and sulfur dioxide gas.

[0010] In order to solve the problems of secondary pollution and incomplete salt separation resulting in impurity salts during the treatment of iron phosphate wastewater, taking the sulfate ion with the highest content in the iron phosphate wastewater as the main recovery substance, an excessive amount of magnesium compound is added to the iron phosphate wastewater, and the following reactions mainly occur, where M n+ represents heavy metal ions:

[0011] MgO + M n+ + H + = M(OH) n + H2O + Mg 2+ ;

[0012] Mg(OH)2 + M n+ + H + = M(OH) n + H2O + Mg 2+ ;

[0013] Mg 2+ + 2F - = MgF2;

[0014] NH4 + + OH - = NH3·H2O;

[0015] HPO4 3- + Mg 2+ + NH3·H2O = NH4MgPO4 + H2O.

[0016] After the above reactions, heavy metal ions, hydrogen ions, fluoride ions, phosphate ions, and ammonium ions in the iron phosphate wastewater are effectively removed, and no new impurities are introduced. The purpose of removing precipitation is achieved at one time, avoiding the use of the coagulation precipitation process, shortening the process time. The main component of the obtained solid slag is magnesium ammonium phosphate, which is commonly known as "struvite" and is a multi-element compound fertilizer containing N, P, and Mg, and can be directly used as fertilizer. The pH value of the obtained precipitate solution is greater than 9. Dilute sulfuric acid is added for neutralization, and the pH value is controlled within the range of 7 - 9, so that the precipitate solution is mainly magnesium sulfate solution. Then, the magnesium sulfate is evaporated and crystallized to obtain a solid, which is calcined to obtain magnesium oxide and sulfur dioxide. Among them, magnesium oxide can be reused for the treatment of other iron phosphate wastewaters, and sulfur dioxide can be used for the preparation of sulfuric acid, truly realizing the thorough treatment of iron phosphate wastewater without secondary pollution, and at the same time realizing the recycling of magnesium, solving the problems existing in the "calcium method" and "ammonia method" in the prior art.

[0017] The mass concentration of salt in the synthetic mother liquor of iron phosphate is generally about 6%, and no concentration treatment is required. The mass concentration of salt in the rinsing water of iron phosphate is generally about 1%. Through concentration treatment, a good reduction effect can be achieved, reducing the amount of wastewater treatment in the subsequent process, which not only reduces the investment cost but also reduces the operating cost.

[0018] As an alternative embodiment, in the treatment process provided by the present invention, in step S1, the magnesium compound is selected from one or more of magnesium oxide, magnesium hydroxide, magnesium phosphate, and magnesium hydrogen phosphate.

[0019] As an alternative embodiment, in the treatment process provided by the present invention, in step S1, the iron phosphate wastewater includes the synthetic mother liquor of iron phosphate and the concentrated solution of iron phosphate rinsing water; the pH of the iron phosphate wastewater is 2 - 2.5, the fluoride ion concentration is 30 - 50 mg / L, the total phosphorus is 2200 - 3000 mg / L, the Fe 2+ concentration is 30 - 50 mg / L, the Mg 2+ concentration is 500 - 700 mg / L, the NH3-N is 6000 - 7000 mg / L, and the SO4 2- concentration is 50000 - 60000 mg / L.

[0020] As an alternative embodiment, in the treatment process provided by the present invention, the concentrated solution of iron phosphate rinsing water is obtained by concentrating the iron phosphate rinsing water by 4 - 6 times, and the fresh water obtained after concentration is recycled for treatment.

[0021] In the present invention, the fresh water obtained after concentrating the iron phosphate rinsing water can be directly recycled for use without causing secondary pollution.

[0022] As an alternative embodiment, in the treatment process provided by the present invention, the addition amount of the magnesium compound is determined by beaker test.

[0023] In the present invention, the addition amount of magnesium is determined according to the ion concentration in the actual wastewater. The specific operation is the beaker test method. First, take the combined solution of the synthetic mother liquor of iron phosphate and the concentrated solution of iron phosphate rinsing water with a 100 - milliliter beaker, add the magnesium compound, stir and test the ion concentration in the wastewater, and then count the amount of medicine added.

[0024] As an alternative embodiment, in the treatment process provided by the present invention, the addition amount of the magnesium compound is 10 - 20% in excess of the theoretical mass.

[0025] As an alternative embodiment, in the treatment process provided by the present invention, in step S1, the solid slag is used as a fertilizer for crop growth.

[0026] In the present invention, the main component of the solid residue obtained after precipitation is magnesium ammonium phosphate, commonly known as struvite, which can be used as a fertilizer. The content of other solid residue impurities is less, and it does not affect the soil properties when used as a fertilizer.

[0027] As an alternative embodiment, in the treatment process provided by the present invention, in step S1, after the phosphoric acid iron rinsing water is cooled to less than 40 °C, it is subjected to concentration treatment, and the phosphoric acid iron synthesis mother liquor is cooled to less than 40 °C for precipitation treatment.

[0028] In the present invention, considering that the temperature of the phosphoric acid iron rinsing water is about 80 °C, and the operating temperature of the current RO membranes used for concentration is below 40 °C, the phosphoric acid iron rinsing water must be cooled first before entering the pure water membrane system.

[0029] As an alternative embodiment, in the treatment process provided by the present invention, in step S2, the precipitation liquid is treated by a plate and frame filter and a precision filter, and then the pH value is adjusted with dilute sulfuric acid.

[0030] After solid-liquid separation in step S1, the obtained precipitation liquid still contains fine precipitates that were not removed during the solid-liquid separation process. Therefore, after being treated by a plate and frame filter and a precision filter to further remove the precipitates, it is beneficial to obtain a single magnesium sulfate product subsequently.

[0031] As an alternative embodiment, in the treatment process provided by the present invention, in step S2, MVR evaporation system is adopted for evaporation crystallization. The MVR evaporation system produces condensed water and concentrated liquid. The condensed water is cooled and then passes through the pure water membrane system to obtain pure water with a conductivity less than 10 us / cm, and the concentrated liquid enters the MVR evaporation system again to produce magnesium sulfate.

[0032] In the present invention, the filtered precipitation liquid is evaporated by using the MVR evaporation system. The temperature of the evaporated condensed water is about 80 - 90 °C and needs to be cooled first. The conductivity of the evaporated condensed water is about 100 us / cm, which does not meet the reuse index requirement (less than 10 us / cm). Therefore, it needs to be treated by the pure water membrane system to reduce the conductivity; the obtained concentrated liquid enters the MVR evaporation system again to produce magnesium sulfate, and the produced condensed water is fresh water and can be directly recovered up to the standard.

[0033] As an alternative embodiment, in the treatment process provided by the present invention, in step S2, the solid magnesium oxide enters the phosphoric acid iron wastewater treatment process again, and the sulfur dioxide gas enters the sulfuric acid production process.

[0034] In the present invention, the products obtained after treating the phosphoric acid iron wastewater are solid magnesium oxide and gaseous sulfur dioxide. Among them, the magnesium oxide can continue to be used in the treatment of phosphoric acid iron wastewater, realizing the recycling of magnesium, and the prepared sulfur dioxide gas can be used in the preparation of sulfuric acid.

[0035] As an alternative embodiment, in the treatment process provided by the present invention, in step S2, in the solution after adjusting the pH value with dilute sulfuric acid, the fluoride ion concentration is less than 5 mg / L, the iron ion concentration is less than 1 mg / L, the ammonia nitrogen concentration is less than 10 mg / L, and the total phosphorus concentration is less than 0.5 mg / L.

[0036] As an alternative embodiment, in the treatment process provided by the present invention, in step S1, a regulating tank is used for precipitation, and the material of the regulating tank is selected from one of PPH (propylene homopolymer), fiberglass reinforced plastic, rubber-lined steel, fiberglass-reinforced plastic-lined steel, or concrete lined with an anti-corrosion coating.

[0037] As an alternative embodiment, in the treatment process provided by the present invention, the precision filter is a microfiltration-level filter with or without an automatic backwashing device.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0039] (1) The present invention uses an excessive amount of magnesium compound to precipitate the synthetic mother liquor of iron phosphate and the concentrated solution of iron phosphate rinsing water. After treatment, the fluoride ion concentration is less than 5 mg / L, the iron ion concentration is less than 1 mg / L, the ammonia nitrogen concentration is less than 10 mg / L, and the total phosphorus concentration is less than 0.5 mg / L. Heavy metal ions, hydrogen ions, fluoride ions, phosphate ions, and ammonium ions are removed at one time, which can not only ensure the removal effect but also retain the sulfate ions in the solution without introducing new impurity ions. This is crucial for improving the purity and grade of magnesium sulfate products. At the same time, it effectively avoids the problem of incomplete salt separation and the generation of miscellaneous salts in the "ammonia method". The magnesium oxide obtained after calcining the magnesium sulfate product can be continuously used for the treatment of iron phosphate wastewater, and the obtained sulfur dioxide gas can be used for the preparation of sulfuric acid, truly realizing the recycling of magnesium without secondary pollution problems.

[0040] (2) In the present invention, defluorination of iron phosphate wastewater can effectively reduce the corrosion of the evaporation system by fluoride ions and can also reduce the material requirements of the evaporation system: from 2205 duplex steel to 316L, thus effectively reducing the investment cost.

[0041] (3) In the present invention, divalent iron ions and other heavy metal ions in the iron phosphate wastewater are removed. Divalent iron ions have color, and if not removed, they will enter the crystalline salt, seriously affecting the quality of the salt. In addition, iron ions will oxidize the reverse osmosis membrane, causing irreversible damage, and strict control of the content is required to ensure its stable operation. At the same time, iron ions will cause scaling and corrosion of the system. The present invention can effectively remove iron ions and other heavy metal ions, thereby improving the purity and grade of magnesium sulfate by-products, protecting the reverse osmosis membrane and enabling it to operate stably. At the same time, it can also effectively reduce the corrosion, scaling, and blockage of the evaporation system, making its operation more stable.

[0042] (4) The process flow of the system of the present invention is short, the removal of pollutants is efficient and thorough, and the investment and operation costs are extremely low. Compared with other technologies, it has incomparable advantages in terms of cost and is more meaningful for popularization and use. Description of the Drawings

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0044] Figure 1 It is the process flow diagram of the iron phosphate wastewater treatment in Embodiment 1 of the present application. Specific Embodiments

[0045] To facilitate the understanding of the present invention, the following will describe the present invention more comprehensively and in detail in combination with the drawings in the specification and the preferred embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0046] Unless otherwise defined, all the technical terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0047] Unless otherwise specifically stated, all kinds of raw materials, reagents, instruments, and equipment used in the present invention can be obtained through the market or can be prepared by existing methods.

[0048] Embodiment 1

[0049] For a 50,000-ton iron phosphate project, 84 m 3 / h of iron phosphate synthesis mother liquor is produced, with a temperature of about 90 °C, a pH of 2 - 2.5, F - 30 - 50 mg / L, total P of 2200 - 3000 mg / L, Fe 2+ 30 - 50 mg / L, Mg 2+ 500 - 700 mg / L, NH3-N of 6000 - 7000 mg / L, SO4 2- 50000 - 60000 mg / L. About 450 m of iron phosphate rinsing water is produced 3 / h, with a temperature of about 90 °C, a pH of 2.2 - 2.5, F - ≤20 mg / L, total P of 200 - 400 mg / L, Fe 2+ ≤30 mg / L, Mg 2+30-50mg / L, NH3-N1500-2000mg / L, SO4 2- 3000-4000mg / L.

[0050] Treatment methods such as Figure 1 As shown:

[0051] (1) The iron phosphate rinse water is first cooled to below 40°C by a plate heat exchanger before entering the membrane concentration system. The membrane concentration system has a water production rate of about 80%, and produces about 100m3 of concentrated water for iron phosphate rinse water. 3 / h enters the pH adjustment tank.

[0052] (2) The mother liquor of ferric phosphate synthesis is first cooled to below 35°C by a plate heat exchanger and then enters the regulating tank to be mixed with the concentrated water of ferric phosphate rinse water for homogenization.

[0053] (3) The volume of mixed wastewater is about 180m 3 / h, temperature about 30℃, pH 2-2.5, F - 50mg / L, total P 2500mg / L, Fe 2+ ≤1mg / L, Mg 2+ 500mg / L, NH3-N 7500mg / L, SO4 2- 50000mg / L.

[0054] (4) Add magnesium phosphate and magnesium hydroxide to the regulating tank. Before adding, use the beaker method to determine the amount to be added. The specific process is: first use a beaker to take 100mL of wastewater, add magnesium phosphate and magnesium hydroxide, stir and test the ammonia nitrogen concentration in the water. When the ammonia nitrogen concentration drops below 10mg / L, calculate the dosage. The test calculation shows that when the water volume of the mixed wastewater is 180m 3 / h, the dosage of reagents is: magnesium phosphate dosage is about 10kg / h, magnesium hydroxide dosage is 50kg / h. In actual production, the reagents are added in excess of 10%, that is, the actual dosage of magnesium phosphate is about 11kg / h, and the actual dosage of magnesium hydroxide is about 55kg / h, for precipitation reaction.

[0055] (5) After sufficient sedimentation, the effluent first passes through a vacuum belt dehydrator to separate the slag from the water, then passes through a two-stage filtration process of plate and frame filtration and a regenerable precision filter to remove fine suspended matter, and then enters the pH adjustment tank.

[0056] (6) Add 30% dilute sulfuric acid to the pH adjustment tank to adjust the pH of the solution to 7. The water quality of the pH adjustment tank outlet is: pH = 7, F - ≤6mg / L, total P≤0.5mg / L, Fe 2+ ≤1mg / L, NH3-N≤10mg / L, SO4 2-50000 mg / L, and the contents of other heavy metals fully meet the requirements of the direct discharge indicators in the "Pollutant Discharge Standards for the Inorganic Chemical Industry" (GB 31573-2015).

[0057] (7) The effluent from the pH adjustment tank enters the MVR evaporation system. The temperature of the condensed water generated by the MVR evaporation system is 80 °C, and it is cooled to below 35 °C through a heat exchanger and then enters the pure water membrane system. The pure water membrane system includes a security filter, ultrafiltration, and reverse osmosis processes to obtain pure water with a conductivity less than 10 μS / cm, and the pure water is recycled for production. The concentrated water is returned to the MVR evaporation system for evaporation and crystallization. The fresh water obtained here meets the standards for recovery, and the magnesium sulfate crystals obtained are reserved for use. The magnesium sulfate production is approximately 12 t / h.

[0058] In this embodiment, the MVR evaporation system, heat exchanger, and pure water membrane system are all existing technologies and will not be elaborated here.

[0059] (8) Using the obtained magnesium sulfate crystals and charcoal powder as raw materials, they are calcined in a high-temperature calcination furnace. The calcination temperature is controlled at 800 °C, the calcination time is 2 h, and the ratio of magnesium sulfate to charcoal powder is controlled at 8:1 to obtain solid magnesium oxide and sulfur dioxide gas. The magnesium oxide can continue to be used for the treatment of iron phosphate wastewater, and the sulfur dioxide gas can be collected and used for the preparation of sulfuric acid.

[0060] Example 2

[0061] The difference from Example 1 is as follows: In step (4), magnesium phosphate, magnesium oxide, and magnesium hydroxide are added to the adjustment tank. Before adding, the addition amount is determined by the beaker method. The specific process is as follows: First, take 100 mL of wastewater in a beaker, add magnesium phosphate, magnesium oxide, and magnesium hydroxide, stir and test the ammonia nitrogen concentration in the water. When the ammonia nitrogen concentration is reduced to below 10 mg / L, the dosing amount can be counted. After testing and calculation, when the water volume of the mixed wastewater is 180 m 3 / h, the dosing amounts of the agents are as follows: the dosing amount of magnesium phosphate is approximately 10 kg / h, the dosing amount of magnesium oxide is approximately 17.5 kg / h, and the dosing amount of magnesium hydroxide is approximately 25 kg / h. In actual production, the agents are dosed with an excess of 20%: that is, the dosing amount of magnesium phosphate is approximately 12 kg / h, the dosing amount of magnesium oxide is approximately 21 kg / h, and the dosing amount of magnesium hydroxide is approximately 30 kg / h for the precipitation reaction. In step (6), 30% dilute sulfuric acid is added to the pH adjustment tank to adjust the pH of the solution to 8. The water quality of the effluent from the pH adjustment tank is: pH = 7, F - ≤ 2.7 mg / L, total P ≤ 0.2 mg / L, Fe 2+ ≤ 0.6 mg / L, NH3-N ≤ 5 mg / L, SO4 2-50,000 mg / L, and the contents of other heavy metals fully meet the requirements of the discharge standards for direct discharge in the "Discharge Standards for Pollutants in the Inorganic Chemical Industry" (GB 31573-2015). The output of magnesium sulfate obtained in step (7) is about 12 t / h.

[0062] The remaining steps are the same as those in Example 1.

[0063] Example 3

[0064] The difference from Example 1 is that in step (4), magnesium phosphate and magnesium oxide are added to the adjustment tank. Before adding, the addition amount is determined by the beaker method. The specific process is as follows: First, take 100 mL of wastewater with a beaker, add magnesium phosphate and magnesium oxide, stir and test the ammonia nitrogen concentration in the water. When the ammonia nitrogen concentration is reduced to less than 10 mg / L, the dosage can be counted. After testing and calculation, when the water volume of the mixed wastewater is 180 m 3 / h, the dosage of the medicament is: the dosage of magnesium phosphate is about 10 kg / h, and the dosage of magnesium oxide is about 35 kg / h. In actual production, the medicament is added at an excess of 15%: that is, the dosage of magnesium phosphate is about 11.5 kg / h, and the dosage of magnesium oxide is about 38.5 kg / h, for precipitation reaction. In step (6), 30% dilute sulfuric acid is added to the pH adjustment tank to adjust the pH of the solution to 9. The water quality of the effluent from the pH adjustment tank: pH = 7, F - ≤1.8 mg / L, total P ≤ 0.3 mg / L, Fe 2+ ≤0.5 mg / L, NH3-N ≤ 3 mg / L, SO4 2- 50,000 mg / L, and the contents of other heavy metals fully meet the requirements of the discharge standards for direct discharge in the "Discharge Standards for Pollutants in the Inorganic Chemical Industry" (GB 31573-2015). The output of magnesium sulfate obtained in step (7) is about 12 t / h.

[0065] The remaining steps are the same as those in Example 1.

[0066] Comparative Example 1

[0067] The difference from Example 1 is that in step (4), magnesium phosphate and magnesium hydroxide are added in equal amounts to the adjustment tank. Before adding, the addition amount is determined by the beaker method. The specific process is as follows: First, take 100 mL of wastewater with a beaker, add magnesium phosphate and magnesium hydroxide, stir and test the ammonia nitrogen concentration in the water. When the ammonia nitrogen concentration is reduced to less than 10 mg / L, the dosage can be counted. After testing and calculation, when the water volume of the mixed wastewater is 180 m 3 / h, the dosage of the medicament is: the dosage of magnesium phosphate is about 10 kg / h, and the dosage of magnesium hydroxide is 50 kg / h. Add in equal amounts.

[0068] After sufficient precipitation, the effluent first passes through a vacuum belt filter press for slag-water separation, and then undergoes two-stage filtration through a plate-and-frame filter and a renewable precision filter to remove fine suspended solids, and then enters the pH adjustment tank.

[0069] (6)Add 30% dilute sulfuric acid to the pH adjustment tank to adjust the pH of the solution to 7. The water quality of the effluent from the pH adjustment tank: pH = 7, F - 10 mg / L, total P 0.8 mg / L, Fe 2+ 5 mg / L, NH3-N 25 mg / L, SO4 2- 50000 mg / L.

[0070] (7)Feed the effluent from the pH adjustment tank into the MVR evaporation system. The temperature of the condensed water generated by the MVR evaporation system is 80 °C. After being cooled to below 35 °C through a heat exchanger, it enters the pure water membrane system. The pure water membrane system includes a security filter, ultrafiltration, and reverse osmosis processes to obtain pure water with a conductivity less than 10 μS / cm, and the pure water is recycled for production. The concentrated water is returned to the MVR evaporation system for evaporation crystallization. The fresh water obtained here contains a small amount of ammonia nitrogen, and the magnesium sulfate crystals obtained contain a small amount of fluoride ions, phosphate radicals, and iron impurities and are slightly yellowish. The magnesium sulfate production is about 12 t / h.

[0071] The remaining treatment steps are the same as those in Example 1.

[0072] The above content is a further detailed description of the present invention in combination with specific preferred embodiments. It cannot be determined that the specific implementation of the present invention is limited to these descriptions. For those of ordinary skill in the art to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can still be made, and all should be regarded as belonging to the protection scope of the present invention.

Claims

1. A ferric phosphate wastewater treatment process based on magnesium recycling, characterized in that: The following steps are involved: S1. Adding an excess amount of magnesium compound to the iron phosphate wastewater for precipitation reaction, and obtaining solid slag and precipitate after solid-liquid separation; S2, adjusting the pH of the precipitate obtained in step S1 to 7-9 with dilute sulfuric acid and then performing evaporation and crystallization to obtain magnesium sulfate solid; S3. Calcining the magnesium sulfate solid to obtain solid magnesium oxide and sulfur dioxide gas.

2. The ferric phosphate wastewater treatment process based on magnesium recycling according to claim 1 is characterized in that: In step S1, the magnesium compound is selected from one or more of magnesium oxide, magnesium hydroxide, magnesium phosphate and magnesium hydrogen phosphate.

3. The ferric phosphate wastewater treatment process based on magnesium recycling according to claim 1 is characterized in that: The ferric phosphate wastewater includes ferric phosphate synthesis mother liquor and ferric phosphate rinsing water concentrate; the ferric phosphate wastewater has a pH of 2-2.5, a fluoride ion concentration of 30-50 mg / L, a total phosphorus of 2200-3000 mg / L, and Fe 2+ The concentration is 30~50mg / L, Mg 2+ The concentration is 500~700mg / L, NH3-N is 6000~7000mg / L, SO4 2- The concentration is 50000~60000mg / L.

4. The ferric phosphate wastewater treatment process based on magnesium recycling according to claim 3 is characterized in that: The concentrated ferric phosphate rinse water is obtained by concentrating the ferric phosphate rinse water 4 to 6 times, and the concentrated fresh water is recovered for treatment.

5. The ferric phosphate wastewater treatment process based on magnesium recycling according to claim 1 is characterized in that: In step S1, the amount of magnesium compound added is 10-20% excess of the theoretical mass.

6. The ferric phosphate wastewater treatment process based on magnesium recycling according to claim 1 is characterized in that: In step S1, the solid residue is used as fertilizer for crops.

7. The ferric phosphate wastewater treatment process based on magnesium recycling according to claim 1 is characterized in that: In step S2, the precipitate is treated by a plate and frame filter and a precision filter and then the pH value is adjusted with dilute sulfuric acid.

8. The ferric phosphate wastewater treatment process based on magnesium recycling according to claim 1 is characterized in that: In step S3, the solid magnesium oxide enters the iron phosphate wastewater treatment process again, and the sulfur dioxide gas enters the sulfuric acid production process.

9. The ferric phosphate wastewater treatment process based on magnesium recycling according to claim 1, characterized in that: In step S2, in the solution after the pH value is adjusted with dilute sulfuric acid, the fluoride ion concentration is less than 5 mg / L, the iron ion concentration is less than 1 mg / L, the ammonia nitrogen concentration is less than 10 mg / L, and the total phosphorus concentration is less than 0.5 mg / L.

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