Wet-process phosphoric acid purification and preparation system

Abstract

The wet-process phosphoric acid purification and preparation system comprises a purification section, the purification section comprises a second membrane filtration unit, a membrane separation unit, a fine desulfurization unit and a third membrane filtration unit which are connected in sequence, and the second membrane filtration unit is used for completely removing suspended solids in wet-process phosphoric acid; the membrane separation unit is used for removing foreign ions in the phosphoric acid solution and producing a phosphate fertilizer; the fine desulfurization unit is used for deeply desulfurizing the clear liquid produced by the membrane separation unit; the third membrane filtration unit is used for completely removing precipitates generated by deep desulfurization. According to the utility model, suspended matters in wet-process phosphoric acid are thoroughly intercepted in a membrane filtration manner, so that the utilization rate of phosphoric acid is increased; membrane separation is used for replacing an original wet-process phosphoric acid purification extraction process, so that the product quality of wet-process phosphoric acid is improved, the technological process is simplified, the system operation energy consumption is reduced, and cost increase, extraction agent consumption and environmental pollution caused by extraction agent introduction are avoided.

Description

Technical Field

[0001] The utility model belongs to the technical field of wet-process phosphoric acid, in particular to a wet-process phosphoric acid purification and preparation system. Background Art

[0002] There are two main methods for producing phosphoric acid: wet process and thermal process. The wet process uses inorganic acid to decompose phosphate rock to prepare phosphoric acid. Affected by the characteristics of raw materials and process, wet phosphoric acid contains a lot of impurities. At present, the main process route of wet phosphoric acid is: acid hydrolysis → filtration → concentration → pre-desulfurization → defluorination → dearsenication → filtration → extraction → fine desulfurization / arsenication → washing → stripping → concentration → decolorization → distillation → bleaching → refined phosphoric acid; among which "pre-desulfurization → defluorination → dearsenication" is the pretreatment section of phosphoric acid refining; "filtration → extraction → fine desulfurization / arsenication → washing → stripping" is the purification section of phosphoric acid refining; "decolorization → distillation → bleaching" is the post-processing section of phosphoric acid refining.

[0003] The common process routes of wet-process phosphoric acid are detailed in the attached Figure 1 , phosphate rock and sulfuric acid enter the acid hydrolysis reaction tank, and after the acid hydrolysis reaction, the solid matter in the reaction liquid is removed by thickening / filtration. The removed solid matter is phosphogypsum, and the material after the solid is removed is the original acid. The original acid is filtered again after pretreatment. The pretreatment is performed by adding phosphate slurry, sodium hydroxide, sodium sulfide and other purification treatment agents to preliminarily purify the original acid. The filtered original acid enters the extraction section, the main phase of the extraction is finely desulfurized, and the extracted dispersed phase (raffinate acid) enters the phosphate fertilizer plant to make phosphate fertilizer. The desulfurization agent for fine desulfurization generally uses a removal agent such as barium carbonate and barium chloride. The organic phase after fine desulfurization is washed (washing is equivalent to one stripping), two strippings, and three strippings to produce relatively clean low-concentration phosphoric acid. The phosphoric acid after stripping is concentrated and post-treated to obtain the finished acid. Post-treatment generally includes decolorization, distillation, bleaching and other processes.

[0004] The current phosphoric acid preparation process has the following defects:

[0005] 1. Phosphoric acid refining is limited by the insufficient filtration accuracy of phosphoric acid, which causes phosphoric acid to carry some suspended matter into the extraction section, resulting in reduced phosphoric acid extraction efficiency and low quality of the prepared phosphoric acid;

[0006] 2. Limited by the low separation accuracy of the extractant and the raffinate, a large amount of extractant is lost, making the production cost of the traditional method high. Utility Model Content

[0007] In view of the above technical problems, the purpose of the utility model is to provide a wet-process phosphoric acid purification and preparation system, which adopts membrane filtration to completely intercept suspended matter in wet-process phosphoric acid and improve the utilization rate of phosphoric acid; and uses membrane separation to replace the original extraction process of wet-process phosphoric acid purification, so as to improve the product quality of wet-process phosphoric acid, simplify the process flow, reduce the energy consumption of system operation, avoid the cost increase and extraction agent consumption caused by the introduction of extraction agent and the pollution of the environment by the extraction agent.

[0008] In order to achieve the above purpose, the technical solution adopted by the utility model is as follows:

[0009] According to one aspect of the utility model, a wet phosphoric acid purification system is provided, comprising a purification section,

[0010] The purification section includes a second membrane filtration unit, a membrane separation unit, a fine desulfurization unit and a third membrane filtration unit connected in sequence.

[0011] The second membrane filtration unit is used to completely remove suspended matter in wet-process phosphoric acid;

[0012] The membrane separation unit is used to remove impurity ions in the phosphoric acid solution and produce phosphate fertilizer;

[0013] The fine desulfurization unit is used to perform deep desulfurization on the output clear liquid of the membrane separation unit;

[0014] The third membrane filtration unit is used to completely remove the precipitates produced by deep desulfurization.

[0015] In some technical solutions, the second membrane filtration unit is provided with an ultrafiltration membrane element.

[0016] The ultrafiltration membrane element is selected from one or a combination of PTFE, PVDF, PS, silicon carbide, zirconium oxide and aluminum oxide corrosion-resistant materials; and / or,

[0017] The filtration accuracy of the ultrafiltration membrane element is between 10nm and 1000nm.

[0018] In some technical solutions, the membrane separation unit is provided with a nanofiltration membrane element.

[0019] The nanofiltration membrane element is used to remove magnesium, calcium, manganese, aluminum, chromium and zinc impurity cations in the phosphoric acid solution.

[0020] In some technical solutions, the membrane separation unit is arranged in multiple stages, and between two adjacent membrane separation units, the clear liquid outlet of the upper membrane separation unit is connected to the liquid inlet of the lower membrane separation unit, and the clear liquid outlet of the final membrane separation unit is connected to the liquid inlet of the fine desulfurization unit.

[0021] In some technical solutions, the membrane separation unit is set to 1 to 3 levels; and / or,

[0022] The concentrated solution outlet of the membrane separation unit is connected to the arsenic removal unit, and the arsenic removal unit is connected to a phosphate fertilizer outlet pipe.

[0023] In some technical solutions, the fine desulfurization unit and the third membrane filtration unit are used to remove the remaining sulfate in the phosphoric acid solution;

[0024] The clear liquid outlet of the third membrane filtration unit is connected to the concentration unit, and the concentration unit is connected to a finished acid outlet pipe.

[0025] In some technical solutions, the purification system further includes a pretreatment section.

[0026] The pretreatment section includes a pre-desulfurization unit and a defluorination unit connected in sequence, which are used to remove corresponding sulfate and fluoride ions by chemical precipitation.

[0027] According to another aspect of the present invention, a wet-process phosphoric acid preparation system is provided, comprising an acidolysis section, a pretreatment section and a purification section connected in sequence,

[0028] The acidolysis section comprises an acidolysis unit, a dehydration unit and a first membrane filtration unit connected in sequence.

[0029] The acidolysis unit is provided with an organic acid inlet pipe;

[0030] The dehydration unit is connected to a phosphogypsum outlet pipe;

[0031] The concentrated liquid outlet of the first membrane filtration unit is connected back to the acid hydrolysis unit, and the clear liquid outlet is connected to the pretreatment section.

[0032] The utility model adopts the above technical solution to have at least the following beneficial effects:

[0033] 1. The utility model provides a wet-process phosphoric acid purification and preparation system, which uses membrane filtration to completely intercept suspended matter in wet-process phosphoric acid, thereby improving the utilization rate of phosphoric acid; and uses membrane separation to replace the original extraction process of wet-process phosphoric acid purification, thereby improving the product quality of wet-process phosphoric acid, simplifying the process flow, reducing the energy consumption of system operation, and avoiding the cost increase and extraction agent consumption caused by the introduction of extraction agents and the pollution of the extraction agent to the environment; especially during the extraction and back-extraction process, phosphoric acid is diluted, so that the traditional wet-process phosphoric acid preparation process needs to be evaporated and concentrated once before pretreatment, and needs to be concentrated twice before post-treatment; while the wet-process phosphoric acid purification system of the present application does not need to be evaporated and concentrated twice, and only needs to be evaporated and concentrated once after the purification section; at the same time, due to the cancellation of the extraction process, organic matter will no longer be introduced into phosphoric acid, and the post-treatment process can be cancelled, saving the investment and energy consumption of decolorization, distillation, bleaching and other processes;

[0034] 2. The utility model provides a wet-process phosphoric acid purification and preparation system, which uses ultrafiltration membrane elements to filter suspended matter and sediment in wet-process phosphoric acid, and the turbidity of the filtered clear liquid is less than 1NTU; at the same time, nanofiltration membrane elements are used to separate magnesium, calcium, manganese, aluminum, chromium and zinc impurity cations in phosphoric acid, and the single retention rate reaches more than 75%;

[0035] 3. The utility model provides a wet-process phosphoric acid purification and preparation system, which can design the number of membrane separation units according to the specific cation content in phosphoric acid produced by different phosphate rock reactions, implement 1 to 3 levels of membrane separation treatment, and ensure the purification effect of phosphoric acid; the concentrated solution produced during the membrane separation process is used as phosphate fertilizer after arsenic removal treatment;

[0036] 4. The utility model provides a wet-process phosphoric acid purification and preparation system. The clear liquid produced by the membrane separation unit is treated by the fine desulfurization unit and the third membrane filtration unit to remove the remaining sulfate in the phosphoric acid. The slurry produced by the third membrane filtration unit is used to make barium sulfate. The clear liquid produced by the third membrane filtration unit is the purified phosphoric acid. The purified phosphoric acid is evaporated and concentrated by the concentration unit to obtain the finished acid. The concentration of the finished acid can be concentrated to 40%, 55%, 75% or 85% according to the needs of downstream customers and can be sold;

[0037] 5. The utility model provides a wet-process phosphoric acid purification and preparation system, which sequentially performs dehydration treatment and primary membrane filtration treatment on the acid solution, and can completely remove the phosphogypsum impurities in the acid solution; and the phosphogypsum intercepted by the primary membrane filtration is returned to the acid hydrolysis unit for pre-acid hydrolysis, which improves the efficiency of the acid hydrolysis reaction, and at the same time makes the phosphogypsum crystals produced by the acid hydrolysis reaction more regular, and reduces the clogging of the filter;

[0038] 6. The utility model provides a wet-process phosphoric acid purification and preparation system, which sequentially performs pre-desulfurization and defluorination pretreatments on the wet-process phosphoric acid, thereby improving the processing efficiency of subsequent membrane filtration and membrane separation and extending the service life of the membrane elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings and their marks required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0040] Figure 1 It is a structural diagram of a wet-process phosphoric acid system involved in the background technology;

[0041] Figure 2 This is a structural diagram of the wet-process phosphoric acid purification and preparation system described in an embodiment of the utility model. DETAILED DESCRIPTION

[0042] In order to more clearly illustrate the embodiments of the utility model or the technical solutions in the prior art, the specific implementation methods of the utility model will be described below with reference to the accompanying drawings. Obviously, the accompanying drawings described below are only some embodiments of the utility model. For ordinary technicians in this field, other drawings and other implementation methods can be obtained based on these drawings without creative work.

[0043] In order to simplify the drawings, only the parts related to the utility model are schematically shown in each figure, and they do not represent the actual structure of the product. In addition, in order to simplify the drawings and facilitate understanding, in some figures, only one of the parts with the same structure or function is schematically drawn or marked. In this article, "one" not only means "only one", but also means "more than one".

[0044] It should be further understood that the term “and / or” used in the specification and appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0045] In this article, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this utility model can be understood according to specific circumstances.

[0046] In addition, in the description of the present application, the terms "first", "second", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0047] In one embodiment, a wet phosphoric acid purification system is provided, referring to Figure 2 , including a purification section, which includes a second membrane filtration unit, a membrane separation unit, a fine desulfurization unit and a third membrane filtration unit connected in sequence, wherein the second membrane filtration unit is used to completely remove suspended matter in wet-process phosphoric acid; the membrane separation unit is used to remove impurity ions in the phosphoric acid solution and produce phosphate fertilizer; the fine desulfurization unit is used to deeply desulfurize the output clear liquid of the membrane separation unit; the third membrane filtration unit is used to completely remove the precipitate produced by the deep desulfurization.

[0048] This embodiment uses membrane filtration to completely intercept suspended matter in wet-process phosphoric acid, thereby improving the utilization rate of phosphoric acid; and uses membrane separation to replace the original extraction process for purifying wet-process phosphoric acid, thereby improving the product quality of wet-process phosphoric acid, simplifying the process flow, reducing system operation energy consumption, and avoiding the cost increase and extraction agent consumption caused by the introduction of the extraction agent and the pollution of the extraction agent to the environment.

[0049] In a preferred embodiment, the second membrane filtration unit is provided with an ultrafiltration membrane element, which is a corrosion-resistant organic material such as PTFE, PVDF, PS, or a corrosion-resistant inorganic material such as silicon carbide, zirconium oxide, aluminum oxide, etc.

[0050] Specifically, the filtration accuracy of the ultrafiltration membrane element is 10nm~1000nm; the optimal operating temperature of the ultrafiltration membrane element is 20~70℃; the optimal operating pressure of the ultrafiltration membrane element is 0.1~0.8MPa; the optimal operating membrane surface flow rate of the ultrafiltration membrane element is 2~6m / s.

[0051] In this embodiment, the wet-process phosphoric acid is treated by using an ultrafiltration membrane element, which can ensure that the suspended matter and the precipitate in the material are completely intercepted, and the turbidity of the membrane filtered clear liquid after treatment is less than 1NTU.

[0052] In a preferred embodiment, the membrane separation unit is provided with a nanofiltration membrane element, and the nanofiltration membrane element is used to remove impurity cations of magnesium, calcium, manganese, aluminum, chromium and zinc in the phosphoric acid solution.

[0053] In this embodiment, the single interception rate of the membrane separation unit for cations in phosphoric acid reaches more than 75%. The number of stages of the membrane separation unit can be designed according to the specific cation content in phosphoric acid produced by different phosphate rock reactions, and 1 to 3 stages of membrane separation treatment are implemented to ensure the purification effect of phosphoric acid; the concentrated solution produced during the membrane separation process is used as phosphate fertilizer after arsenic removal treatment.

[0054] Specifically, the operating pressure of the membrane element of the membrane separation unit is 2 to 10 MPa, and the membrane material uses a modified composite organic material to ensure the effective service life of the membrane element under high-concentration phosphoric acid conditions.

[0055] In the above embodiments, the membrane separation unit is mainly composed of a membrane separation feed tank, a safety filter, a booster pump, a high-pressure membrane assembly, and supporting pipes, valves, instruments, etc., and a dynamic disc-type membrane separation element is preferably used. Several membranes can be evenly distributed on a hollow shaft, and the hollow shaft is driven by an external motor or other power equipment. Under a certain operating pressure, impurities are retained on the surface of the membrane, and the clear liquid is gathered inside the hollow shaft and output, thereby realizing efficient separation of impurity ions and acid solution.

[0056] In a preferred embodiment, the fine desulfurization unit and the third membrane filtration unit are used to remove residual sulfate in the phosphoric acid solution; the clear liquid outlet of the third membrane filtration unit is connected to the concentration unit, and the concentration unit is connected to a finished acid outlet pipe.

[0057] In this embodiment, the wet phosphoric acid purification system does not need to perform two evaporation concentrations, but only needs to perform one evaporation concentration after the purification stage; and since the extraction process is cancelled, no organic matter will be introduced into the phosphoric acid, and the post-treatment process can be cancelled, saving the investment and energy consumption of the decolorization, distillation, bleaching and other processes.

[0058] In the above embodiment, the wet phosphoric acid purification system also includes a pretreatment section, which includes a pre-desulfurization unit and a defluorination unit connected in sequence, which is used to remove corresponding sulfate and fluoride ions by chemical precipitation, improve the processing efficiency of subsequent membrane filtration and membrane separation, and extend the service life of the membrane element.

[0059] In another embodiment, a system for preparing wet phosphoric acid is provided. Figure 2 , including an acidolysis section, a pretreatment section and a purification section connected in sequence as described in the above embodiment, wherein the acidolysis section includes an acidolysis unit, a dehydration unit and a first membrane filtration unit connected in sequence, the acidolysis unit is provided with an organic acid inlet pipe; the dehydration unit is connected with a phosphogypsum outlet pipe; the concentrated liquid outlet of the first membrane filtration unit is back-connected to the acidolysis unit, and the clear liquid outlet is connected to the pretreatment section.

[0060] In the above embodiments, the first membrane filtration unit, the second membrane filtration unit and the third membrane filtration unit are mainly composed of a membrane filtration circulation tank, a membrane filtration circulation pump, a membrane assembly and supporting pipes, valves, instruments, etc., and the membrane filtration elements can be static or dynamic, which is not specifically limited in this application.

[0061] In this embodiment, the acid hydrolysis solution is sequentially subjected to a dehydration treatment and a membrane filtration treatment, which can completely remove the phosphogypsum impurities in the acid solution; and the phosphogypsum retained by the primary membrane filtration is returned to the acid hydrolysis unit for pre-acid hydrolysis, which improves the efficiency of the acid hydrolysis reaction and makes the phosphogypsum crystals produced by the acid hydrolysis reaction more regular, thereby reducing clogging of the filtration.

[0062] In another embodiment, a wet phosphoric acid purification and preparation process is provided, referring to Figure 2 , including the following steps:

[0063] After the phosphate rock is hydrolyzed with sulfuric acid (or nitric acid, hydrochloric acid), the acidolysis reaction produces phosphoric acid and phosphogypsum. After the material produced by the acidolysis reaction is processed by the dehydration unit, the solid matter produced by the dehydration unit is phosphogypsum. The dehydrated phosphoric acid enters the first membrane filtration unit, which can absolutely intercept the phosphogypsum in the phosphoric acid. The phosphogypsum intercepted by the first membrane filtration unit is returned to the acidolysis section for pre-acidolysis, which improves the efficiency of the acidolysis reaction and makes the phosphogypsum crystals produced by the acidolysis reaction more regular, which reduces the clogging of the filter. The clear liquid produced by the first membrane filtration unit enters the pre-desulfurization unit and the defluorination unit for treatment;

[0064] The clear liquid produced by the first membrane filtration unit enters the second membrane filtration unit after pre-desulfurization and defluorination treatment. The second membrane filtration unit completely removes the precipitate produced by the pre-desulfurization and defluorination treatment. The removed precipitate (i.e., slurry) is used to make sulfate. The clear liquid produced by the second membrane filtration unit enters the membrane separation unit.

[0065] Phosphoric acid entering the membrane separation unit is fully freed of impurity ions in the phosphoric acid under the action of the membrane assembly of the membrane separation unit. The impurity ions include cations such as aluminum, iron, magnesium, calcium, and arsenic. The concentrated solution produced by the membrane separation unit is used as phosphate fertilizer after arsenic removal, and the clear solution produced by the membrane separation unit is the phosphoric acid after the cations are removed. The clear solution produced by the membrane separation unit is treated by the fine desulfurization unit and the third membrane filtration unit to remove the remaining sulfate in the phosphoric acid. The slurry produced by the third membrane filtration unit is used to make barium sulfate. The clear solution produced by the third membrane filtration unit is the purified phosphoric acid.

[0066] The purified phosphoric acid is evaporated and concentrated in a concentration unit to become the finished acid (the concentration of the finished acid is concentrated to the required concentration according to downstream demand).

[0067] In a preferred embodiment, the filtration accuracy of the above-mentioned membrane filtration unit is 10nm~1000nm, the operating temperature is 20~70°C, the operating pressure is 0.1~0.8MPa and the membrane surface flow rate is 2~6m / s; the turbidity of the clear liquid after membrane filtration is <1NTU; the single retention rate of the membrane separation unit for cations in phosphoric acid is more than 75%, and the operating pressure is 2~10MPa.

[0068] In order to have a clearer understanding of the technical solution and technical effects of the present application, a specific example is provided below for illustration:

[0069] A phosphoric acid solution with a mass concentration of 35% is produced after acid hydrolysis reaction of an appropriate amount of phosphate rock and sulfuric acid. The obtained phosphoric acid solution contains a large amount of undissolved slag, mainly phosphogypsum. The 35% mass content phosphoric acid solution is firstly removed from most of the phosphogypsum by a dehydration unit. The clear liquid produced by the dehydration unit enters the first membrane filtration unit. The solid content of the clear liquid obtained by the dehydration unit is 0.02%. The first membrane filtration unit can completely remove the phosphogypsum impurities in the phosphoric acid solution without adding any flocculant. The turbidity of the clear liquid of the first membrane filtration unit is less than 1NTU. The concentrated liquid of the first membrane filtration unit, i.e., the phosphogypsum slurry, is returned to the acid hydrolysis unit.

[0070] The clear liquid of the first membrane filtration unit is removed from most of the sulfate and fluoride impurities by the pre-desulfurization unit and the defluorination unit. Then it enters the second membrane filtration unit, which can completely remove the sulfate produced by desulfurization. The turbidity of the clear liquid of the second membrane filtration unit is less than 1NTU, which meets the water inlet requirements of the membrane separation unit. The mass concentration of phosphoric acid in the clear liquid of the second membrane filtration unit after treatment is 35%, the concentration of magnesium ions is 5000mg / L, the concentration of calcium ions is 700mg / L, the concentration of manganese ions is 200mg / L, the concentration of aluminum ions is 4000mg / L, the concentration of chromium ions is 20mg / L, and the concentration of zinc ions is 15mg / L. The mass concentration of phosphoric acid in the clear liquid after the primary membrane separation treatment of the membrane separation unit is 34%, the concentration of magnesium ions is 800mg / L, the concentration of calcium ions is 150mg / L, the concentration of manganese ions is 45mg / L, the concentration of aluminum ions is 900mg / L, the concentration of chromium ions is 4mg / L, and the concentration of zinc ions is 3mg / L. The phosphoric acid mass concentration in the clear liquid after the secondary membrane separation treatment of the membrane separation unit is 33%, the magnesium ion concentration is 190 mg / L, the calcium ion concentration is 33 mg / L, the manganese ion concentration is 11 mg / L, the aluminum ion concentration is 210 mg / L, the chromium ion concentration is 1 mg / L and the zinc ion concentration is 0.5 mg / L. The phosphoric acid mass concentration in the clear liquid after the tertiary membrane separation treatment of the membrane separation unit is 32%, the magnesium ion concentration is 40 mg / L, the calcium ion concentration is 6 mg / L, the manganese ion concentration is 2 mg / L, the aluminum ion concentration is 45 mg / L, the chromium ion is undetectable and the zinc ion is undetectable. The overall recovery rate of the membrane separation unit is not less than 60%, and the membrane separation stage and recovery rate of the membrane separation unit can be controlled according to the quality requirements of the final product when used.

[0071] The concentrated liquid produced by the membrane separation unit is used as phosphate fertilizer after dearsenicization with hydrogen sulfide. The clear liquid after the multi-stage membrane separation treatment of the membrane separation unit enters the fine desulfurization unit for further desulfurization treatment. The phosphoric acid solution after fine desulfurization enters the third membrane filtration unit to completely intercept and recover the barium sulfate in the phosphoric acid solution. The clear liquid produced by the third membrane filtration unit is directly concentrated to finally form the finished acid. The concentrated concentration can be concentrated to 40%, 55%, 75%, and 85% according to the needs of downstream customers for sale.

[0072] The above-mentioned embodiments only express several implementation methods of the utility model, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the patent of the utility model. It should be pointed out that, for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the utility model, and these all belong to the protection scope of the utility model. Therefore, the protection scope of the utility model shall be based on the attached claims.

Claims

1. A wet phosphoric acid purification system, characterized in that: Including purification section, The purification section includes a second membrane filtration unit, a membrane separation unit, a fine desulfurization unit and a third membrane filtration unit connected in sequence. The second membrane filtration unit is used to completely remove suspended matter in wet-process phosphoric acid; The membrane separation unit is used to remove impurity ions in the phosphoric acid solution and produce phosphate fertilizer; The fine desulfurization unit is used to perform deep desulfurization on the output clear liquid of the membrane separation unit; The third membrane filtration unit is used to completely remove the precipitates produced by deep desulfurization.

2. The wet-process phosphoric acid purification system according to claim 1, characterized in that: The second membrane filtration unit is provided with an ultrafiltration membrane element, The ultrafiltration membrane element is selected from one or a combination of PTFE, PVDF, PS, silicon carbide, zirconium oxide and aluminum oxide corrosion-resistant materials; and / or, The filtration accuracy of the ultrafiltration membrane element is between 10nm and 1000nm.

3. The wet-process phosphoric acid purification system according to claim 1, characterized in that: The membrane separation unit is provided with a nanofiltration membrane element, The nanofiltration membrane element is used to remove magnesium, calcium, manganese, aluminum, chromium and zinc impurity cations in the phosphoric acid solution.

4. The wet phosphoric acid purification system according to claim 1 or 3, characterized in that: The membrane separation unit is arranged in multiple stages. Between two adjacent membrane separation units, the clear liquid outlet of the upper membrane separation unit is connected to the liquid inlet of the lower membrane separation unit, and the clear liquid outlet of the final membrane separation unit is connected to the liquid inlet of the fine desulfurization unit.

5. The wet phosphoric acid purification system according to claim 1 or 3, characterized in that: The membrane separation unit is set to 1 to 3 levels; and / or, The concentrated solution outlet of the membrane separation unit is connected to the arsenic removal unit, and the arsenic removal unit is connected to a phosphate fertilizer outlet pipe.

6. The wet-process phosphoric acid purification system according to claim 1, characterized in that: The fine desulfurization unit and the third membrane filtration unit are used to remove the remaining sulfate in the phosphoric acid solution; The clear liquid outlet of the third membrane filtration unit is connected to the concentration unit, and the concentration unit is connected to a finished acid outlet pipe.

7. The wet-process phosphoric acid purification system according to claim 1, characterized in that: The purification system also includes a pretreatment section, The pretreatment section includes a pre-desulfurization unit and a defluorination unit connected in sequence, which are used to remove corresponding sulfate and fluoride ions by chemical precipitation.

8. A system for preparing wet-process phosphoric acid, characterized in that: It comprises an acid hydrolysis section, a pretreatment section as claimed in claim 7 and a purification section as claimed in any one of claims 1 to 6, which are connected in sequence. The acidolysis section comprises an acidolysis unit, a dehydration unit and a first membrane filtration unit connected in sequence. The acidolysis unit is provided with an organic acid inlet pipe; The dehydration unit is connected to a phosphogypsum outlet pipe; The concentrated liquid outlet of the first membrane filtration unit is connected back to the acid hydrolysis unit, and the clear liquid outlet is connected to the pretreatment section.

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