Purification method for urea

Abstract

The invention belongs to the technical field of urea production, and discloses a purification method for urea, which comprises the following steps: 1) dissolving urea in water, and filtering through a microfiltration membrane to obtain a primary filtrate; (2) adding an activated carbon-based adsorbent into the primary filtrate, stirring and adsorbing, and then filtering through a micro-filtration membrane to obtain secondary filtrate; (3) enabling the secondary filtrate to pass through an exchange column filled with difunctional ion exchange resin to obtain exchange liquid; and 4) performing vacuum concentration on the exchange liquid, performing cooling crystallization, and drying crystals to obtain purified urea. According to the method, volatile impurities, biuret and metal ions in the urea are efficiently and synergistically removed mainly through the treatment step of introducing the activated carbon-based adsorbent and the bifunctional ion exchange resin, and the purity of the urea is increased to 99.9% or above under the conditions of low cost and low energy consumption.

Description

Technical Field

[0001] The invention belongs to the technical field of urea production, and particularly relates to a method for purifying urea. Background Art

[0002] The steel industry generates large amounts of nitrogen oxides (NOx) during production processes (such as sintering, ironmaking, and steelmaking), making it a major source of industrial air pollution. To reduce NOx emissions, selective catalytic reduction (SCR) technology is widely used in the steel industry's denitrification systems, with urea hydrolysis to produce ammonia being a core step. In this process, urea solution undergoes thermal decomposition and hydrolysis at high temperatures to produce ammonia. The ammonia then acts as a reducing agent, reacting with NOx in the exhaust gas to convert it into harmless nitrogen and water.

[0003] However, existing urea raw materials often contain a variety of impurities, which have a negative impact on the urea hydrolysis process. Biuret in urea forms sediments when decomposed at high temperatures, reducing the hydrolysis efficiency and poisoning the catalyst, especially in the high-temperature exhaust gas environment of the steel industry. Metal ions in urea (such as Ca 2+ Mg 2+ 、Fe 3+ ) interferes with the hydrolysis reaction kinetics, easily leading to equipment scaling, and synergistically interacts with alkali metal components in steel exhaust gas to exacerbate catalyst deactivation. Water and volatile substances (such as NH3) in urea affect the stability of the urea solution and easily trigger side reactions in the high-temperature and high-humidity SCR system of the steel industry, reducing ammonia yield. Therefore, it is necessary to purify the urea raw material.

[0004] Existing urea purification methods, such as multi-stage filtration, organic phase extraction, and recrystallization, all exhibit low processing efficiency, particularly in their ability to synergistically remove multiple impurities. While these methods rely on multi-stage processing to improve efficiency, multi-stage filtration requires excessive water resources and consumables, while multi-stage extraction and multi-stage crystallization require excessive equipment investment and energy consumption, making them technically uneconomical. Therefore, a highly efficient and cost-effective method for purifying urea raw materials is needed to meet the demand for high-purity urea in industries such as steel. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to address the shortcomings of the existing technology and provide a method for purifying urea. The method mainly introduces the treatment steps of activated carbon-based adsorbent and bifunctional ion exchange resin to efficiently and synergistically remove volatile impurities, biuret and metal ions in urea, thereby achieving urea purity of more than 99.9% at low cost and low energy consumption.

[0006] In order to solve the technical problem raised by the present invention, the present invention provides a method for purifying urea, comprising the following steps:

[0007] 1) Dissolve urea in water and filter through a microfiltration membrane to obtain a primary filtrate;

[0008] 2) adding an activated carbon-based adsorbent to the primary filtrate, stirring and adsorbing the mixture, and then filtering the mixture again through a microfiltration membrane to obtain a secondary filtrate;

[0009] 3) passing the secondary filtrate through an exchange column filled with a bifunctional ion exchange resin to obtain an exchange liquid;

[0010] 4) The exchange solution is vacuum concentrated, cooled and crystallized, and the crystals are dried to obtain purified urea.

[0011] In the above scheme, the urea is industrial urea with a purity of 98-99.5%, a biuret content of ≥0.5%, a volatile impurity NH3 content of ≥0.01%, and a metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + Total content ≥1000ppm.

[0012] In the above scheme, the water used is preferably deionized water, and the water quality requirements are resistivity ≥ 0.1MΩ·cm, total dissolved solids TDS ≤ 10mg / L, and no metal ions (such as Ca 2+ Mg 2+ 、Na + ) and organic matter interference.

[0013] In the above scheme, the urea is dissolved at 40-50° C. to form a solution with a mass concentration of 30-50%, but a saturated solution is preferred to reduce the energy consumption of concentration.

[0014] In the above solution, the pore size of the microfiltration membrane is 0.1-0.2 μm, and the filtration pressure is ≤0.05 MPa.

[0015] In the above scheme, the turbidity of the primary filtrate is <1NTU.

[0016] In the above scheme, the activated carbon-based adsorbent is prepared by modifying activated carbon with nitric acid and then pyrolyzing it.

[0017] In the above scheme, the pH of the activated carbon-based adsorbent is 6.5-7.5, the particle size is 0.5-1 mm, the pore size is 2-5 nm, and the specific surface area is 600-800 m 2 / g, the total content of carboxyl and hydroxyl groups is 0.5-2 mmol / g, and the adsorption capacity for NH3 is 10-15 mg / g.

[0018] In the above scheme, the specific preparation method of the activated carbon-based adsorbent is: placing the activated carbon in a nitric acid solution, stirring the reaction, washing and drying to obtain nitric acid-modified activated carbon; pyrolyzing the nitric acid-modified activated carbon in an inert atmosphere, taking it out, washing and drying it to obtain the activated carbon-based adsorbent.

[0019] Furthermore, the concentration of the nitric acid solution is 1.5 to 2 mol / L.

[0020] Furthermore, the amount of the activated carbon added to the nitric acid solution is 100-120 g / L.

[0021] Furthermore, the stirring reaction temperature is 50-60° C., and the reaction time is 3-4 hours.

[0022] Furthermore, the inert atmosphere is one of nitrogen, argon and helium.

[0023] Furthermore, the pyrolysis temperature is 600-650° C., the heating rate is 5-8° C. / min, and the pyrolysis time is 2-3 h.

[0024] In the above solution, the added amount of the activated carbon-based adsorbent is 2-3% of the mass of the primary filtrate.

[0025] In the above scheme, the adsorption temperature of the stirred adsorption is 30-35°C, which can not only ensure the adsorption efficiency of the activated carbon but also avoid the decomposition of urea or premature volatilization of NH3. The adsorption time is 30-40 minutes, and the stirring rate is 200-300 rpm. The stirring rate can ensure that the adsorbent is fully in contact with the solution and avoid excessive disturbance that causes adsorbent wear.

[0026] In the above scheme, the NH3 content of the secondary filtrate is less than 0.0005%.

[0027] In the above solution, the bifunctional ion exchange resin is prepared by chloromethylating a polystyrene-divinylbenzene copolymer resin and grafting aminophosphonic acid groups and carboxylic acid groups.

[0028] In the above scheme, the pH of the bifunctional ion exchange resin is 6.5-7.5, the particle size is 0.4-0.8 mm, the aminophosphonic acid group content is 1.0-1.2 mmol / g, the carboxylic acid group content is 1.2-1.5 mmol / g, the adsorption capacity for metal ions is 1-3 mmol / g, and the adsorption capacity for biuret is 0.6-1.2 g / g.

[0029] In the above scheme, the specific preparation method of the bifunctional ion exchange resin is as follows: polystyrene-divinylbenzene copolymer resin, chloroform and chloromethyl methyl ether are mixed, stirred for reaction, the product is washed and dried to obtain a chloromethylated resin; the chloromethylated resin, water and aminomethylphosphonic acid are mixed, stirred for reaction, acrylic acid is added, stirred for reaction, the product is washed and dried to obtain a bifunctional ion exchange resin.

[0030] Furthermore, the mass ratio of the polystyrene-divinylbenzene copolymer resin, chloroform and chloromethyl methyl ether is 1:(5-8):(1-1.5), the reaction temperature is 40-60° C., and the reaction time is 4-8 hours.

[0031] Furthermore, the mass ratio of the chloromethylated resin, water and aminomethylphosphonic acid is 1:(5-8):(0.4-0.8), the reaction temperature is 60-80° C., and the reaction time is 6-10 h.

[0032] Furthermore, the mass ratio of acrylic acid to chloromethylated resin is (0.4-0.8):1, the reaction temperature after addition is 50-70° C., and the reaction time is 4-8 hours.

[0033] In the above scheme, the flow rate of the secondary filtrate through the ion exchange column is 0.6 to 10 BV / h.

[0034] In the above scheme, the metal ion Ca in the exchange solution 2+ Mg 2+ 、Fe 3+ 、Na + , K + Total content <50mg / kg, biuret content <0.01%.

[0035] In the above scheme, the vacuum degree (the difference between the system pressure and the atmospheric pressure) of the vacuum concentration is -0.1 to -0.08 MPa, the temperature is 40 to 50°C, and the volume is concentrated to 50 to 60% of the original volume. This low-temperature and low-pressure design ensures water evaporation while avoiding urea thermal decomposition and biuret formation, and significantly reduces energy consumption.

[0036] In the above scheme, the vacuum concentration is carried out under stirring conditions with a stirring rate of 100 to 300 rpm.

[0037] In the above scheme, the NH3 content in the vacuum concentrated solution is less than 0.005%, and the biuret content is ≤0.05%.

[0038] In the above scheme, the crystallization process is firstly slowly cooled to 5-10°C under stirring conditions of 100-150 rpm, and then the stirring is stopped and the mixture is allowed to stand for 2-4 hours.

[0039] Furthermore, the cooling rate is 0.5-2°C / min.

[0040] In the above scheme, the drying temperature of the crystals is 40-50° C., and the drying time is 4-6 hours.

[0041] In the above scheme, the purity of the purified urea is ≥99.9%, the moisture content is <0.1%, the biuret content is <0.05%, the volatile impurity NH3 content is <0.001%, and the metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + Total content ≤200ppm.

[0042] The main technical concepts of the present invention are as follows:

[0043] The present invention proposes a systematic and synergistically optimized urea purification process flow: first, urea is dissolved and then filtered to remove insoluble impurities; secondly, an activated carbon-based adsorbent with a pore size of 2 to 5 nm is used to remove NH3 in advance through physical adsorption and microporous capture, thereby avoiding the problem of volatile impurities volatilizing and re-dissolving during subsequent vacuum concentration; then, in response to the problem that biuret and metal ions need to be treated separately, a bifunctional ion exchange resin with a polystyrene matrix and loaded with carboxylic acid groups and aminophosphonic acid groups is designed. The resin utilizes the highly selective adsorption of metal ions by carboxylic acid groups and the coordination effect of aminophosphonic acid groups on biuret to achieve simultaneous removal of the two types of impurities, thereby reducing the burden of subsequent crystallization steps; finally, further purification is carried out through vacuum concentration and recrystallization, and moisture is controlled by drying; the above steps are closely connected and interconnected to form an efficient and synergistic process chain, thereby obtaining high-purity urea.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] 1) By introducing a bifunctional ion exchange resin, the present invention achieves the simultaneous and efficient removal of metal ions and biuret for the first time, breaking through the limitation of the existing technology that requires the two types of impurities to be treated separately, reducing the burden of the subsequent crystallization step, and increasing the metal ion removal rate to over 90%, and the biuret removal rate to over 95%, which are significantly better than traditional monofunctional resins.

[0046] 2) The present invention adds a low-temperature pre-adsorption step before vacuum concentration, using an activated carbon-based adsorbent to effectively remove the volatile impurity NH3. This solves the problem of volatile impurities easily volatilizing and re-dissolving during the concentration process in the prior art, and achieves an NH3 removal rate of over 90%, significantly improving the concentration efficiency and final product purity.

[0047] 3) Through the synergistic effect of the above-mentioned innovations, the present invention achieves a urea purity of ≥99.9% under conventional equipment conditions. Compared with existing multi-stage filtration, multi-stage extraction, recrystallization and other processes, energy consumption is reduced by about 20-55%, overall operating costs are reduced by about 30-50%, and process efficiency (urea processing capacity per unit time) is improved by about 20-30%. It is both economical and practical, far exceeding the expected effects of existing technology combinations. DETAILED DESCRIPTION

[0048] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with the examples, but the content of the present invention is not limited to the following examples.

[0049] In the following examples, the polystyrene-divinylbenzene copolymer resin selected is specifically a polystyrene-divinylbenzene copolymer resin of model Amberlite XAD-4 produced by DuPont, with a crosslinking degree of 8%, a particle size of 0.3-0.8 mm, and a specific surface area of ​​750-800 m 2 / g (BET method, N2 adsorption-desorption), pore diameter 4-6nm, pore volume 0.98cm 3 / g.

[0050] Example 1

[0051] Raw materials preparation:

[0052] ①Industrial grade urea, purity 98.5%, biuret content 1.0%, NH3 content 0.012%, metal ion Ca 2+ 220ppm, Mg 2+ 130ppm, Fe 3+ 550ppm, Na + 350ppm, K + 230ppm, total 1480ppm, dry for use.

[0053] ② Deionized water, resistivity 0.5MΩ·cm, TDS 5mg / L, free of metal ion and organic matter interference.

[0054] ③Microfiltration membrane, pore size 0.2μm, filtration pressure set to 0.05MPa.

[0055] ④ Activated carbon-based adsorbent, preparation process: 100g activated carbon is placed in 1L 1.8mol / L nitric acid solution, stirred at 50℃ for 3.5h, washed to neutral, and dried at 60℃ for 12h to obtain nitric acid-modified activated carbon; the nitric acid-modified activated carbon is pyrolyzed at 6℃ / min to 620℃ in a nitrogen atmosphere for 2.5h, taken out, washed, and dried at 60℃ for 12h to obtain an activated carbon-based adsorbent. Its pH is 7, particle size is 0.5-1mm, pore size is 2-5nm, and specific surface area is 700m2 / g, the total content of carboxyl and hydroxyl groups was 1 mmol / g as determined by Boehm titration, and the adsorption capacity of NH3 in a static adsorption test at 25°C and 0.1 MPa was 12 mg / g.

[0056] ⑤ Bifunctional ion exchange resin, preparation process: Polystyrene-divinylbenzene copolymer resin, chloroform, and chloromethyl methyl ether were mixed in a mass ratio of 1:6:1.2, stirred at 50°C for 6 hours, washed, and dried at 60°C for 12 hours to obtain a chloromethylated resin. The chloromethylated resin was then mixed with water and aminomethylphosphonic acid in a mass ratio of 1:6:0.6, stirred at 70°C for 8 hours, and acrylic acid was added at a mass ratio of 0.6:1. The mixture was stirred at 60°C for 6 hours, washed, and dried at 60°C for 12 hours to obtain a bifunctional ion exchange resin. The resin has a pH of 7, a particle size of 0.4-0.8 mm, an aminophosphonic acid group content of 1.1 mmol / g, and a carboxylic acid group content of 1.3 mmol / g. Static adsorption tests at 25°C and 0.1 MPa showed adsorption capacities of 2 mmol / g for metal ions and 1.0 g / g for biuret.

[0057] Purification of urea:

[0058] 1) Take 237.5g of technical grade urea, add 500mL of deionized water, stir at 45℃ and 400rpm for 20min to form a 32.2wt% solution, filter through a microfiltration membrane to remove insoluble impurities, and obtain a primary filtrate with turbidity <1NTU, NH3 content 0.0039wt%, biuret content 0.322wt%, and metal ion content 477mg / kg, of which Ca 2+ 71mg / kg, Mg 2+ 42mg / kg, Fe 3+ 177mg / kg, Na + 113mg / kg, K + 74mg / kg;

[0059] 2) Adding 2.5% of the primary filtrate's mass to the primary filtrate was an activated carbon-based adsorbent, and the mixture was stirred and adsorbed at 35°C and 250 rpm for 35 min. The mixture was then filtered again through a microfiltration membrane to obtain a secondary filtrate: the NH3 content decreased from 0.0039 wt% to 0.0002 wt%, with a removal rate of 94.87%, as tested by acid-base titration; the biuret content decreased from 0.322 wt% to 0.302 wt%, with a removal rate of 6.21%; and the metal ion content decreased from 477 mg / kg to 451 mg / kg, with a removal rate of 5.45%;

[0060] 3) The secondary filtrate was passed through an exchange column containing 100 g of bifunctional ion exchange resin at a flow rate of 1.5 mL / min (0.9 BV / h) to obtain an exchange solution: Ca2+ 10mg / kg, Mg 2+ 3mg / kg, Fe 3+ 18mg / kg, Na + 6mg / kg, K + 5mg / kg, total 42mg / kg, removal rate 90.69%, detection method is ICP-OES; biuret is reduced from 0.302wt% to 0.0051wt%, removal rate 98.31%, detection method is HPLC; NH3 0.0002wt%;

[0061] 4) The exchange solution was vacuum concentrated at 40° C., −0.09 MPa, and 200 rpm. It took 1.5 h to concentrate to 50% of the original volume. The NH3 content in the concentrate was 0.0004 wt%, and the biuret content was 0.0102 wt%. The concentrate was cooled to 5° C. at a rate of 1° C. / min under stirring at 120 rpm, and then allowed to stand for 4 h to crystallize. The crystals were separated by centrifugation and vacuum dried at 50° C. and a system absolute pressure of 0.01 MPa for 6 h to obtain 210 g of urea.

[0062] The purified urea was tested: purity 99.92% (difference method), water content 0.05%, NH3 content 0.0006% (acid-base titration), final removal rate 95.00%, biuret content 0.016% (HPLC detection), final removal rate 98.40%, metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + The total content was 135 mg / kg (ICP-OES), and the final removal rate was 90.88%.

[0063] Example 2

[0064] Raw materials preparation:

[0065] ①Industrial grade urea, purity 98.35%, biuret content 1.3%, NH3 content 0.015%, metal ion Ca 2+ 240ppm, Mg 2+ 110ppm, Fe 3+ 520ppm, Na + 330ppm, K + 250ppm, total 1450ppm, dry for use.

[0066] ② Deionized water, resistivity 0.5MΩ·cm, TDS 5mg / L, free of metal ion and organic matter interference.

[0067] ③Microfiltration membrane, pore size 0.1μm, filtration pressure set to 0.04MPa.

[0068] ④ Activated carbon-based adsorbent, preparation process: 100g activated carbon is placed in 1L 1.9mol / L nitric acid solution, stirred at 55℃ for 3.8h, washed to neutral, and dried at 60℃ for 12h to obtain nitric acid-modified activated carbon; the nitric acid-modified activated carbon is pyrolyzed at 7℃ / min to 630℃ in a nitrogen atmosphere for 2.8h, taken out, washed, and dried at 60℃ for 12h to obtain an activated carbon-based adsorbent. Its pH is 7, particle size is 0.5-1mm, pore size is 2-5nm, and specific surface area is 750m 2 / g, the total content of carboxyl and hydroxyl groups was 1.2mmol / g determined by Boehm titration method, and the adsorption capacity of NH3 in static adsorption test at 25℃ and 0.1MPa was 13mg / g.

[0069] ⑤ Bifunctional ion exchange resin, preparation process: Polystyrene-divinylbenzene copolymer resin, chloroform, and chloromethyl methyl ether were mixed in a mass ratio of 1:6.5:1.3, stirred at 55°C for 6.5 hours, washed, and dried at 60°C for 12 hours to obtain a chloromethylated resin. The chloromethylated resin was then mixed with water and aminomethylphosphonic acid in a mass ratio of 1:6.5:0.65, stirred at 75°C for 8.5 hours, and acrylic acid was added at a mass ratio of 0.65:1. The mixture was stirred at 65°C for 6.5 hours, washed, and dried at 60°C for 12 hours to obtain a bifunctional ion exchange resin. The resin has a pH of 7, a particle size of 0.4-0.8 mm, an aminophosphonic acid group content of 1.2 mmol / g, and a carboxylic acid group content of 1.4 mmol / g. In a static adsorption test at 25°C and 0.1 MPa, the adsorption capacity for metal ions was 2.1 mmol / g, and the adsorption capacity for biuret was 1.1 g / g.

[0070] Purification of urea:

[0071] 1) Take 225g of industrial grade urea, add 500mL of deionized water, stir at 50℃ and 400rpm for 25min to form a solution with a concentration of 31.03wt%. Filter through a microfiltration membrane to remove insoluble impurities and obtain a primary filtrate with turbidity <1NTU, NH3 content 0.0047wt%, biuret content 0.40wt%, metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + Total content 450mg / kg;

[0072] 2) Adding 3% of the mass of the primary filtrate to the primary filtrate, stirring and adsorbing the mixture at 30°C and 300 rpm for 40 minutes, and then filtering the mixture again through a microfiltration membrane to obtain a secondary filtrate: the NH3 content is reduced to 0.0003 wt% as determined by acid-base titration; the biuret content is reduced to 0.388 wt%; and the metal ion content is reduced to 430 mg / kg;

[0073] 3) The secondary filtrate was passed through an exchange column containing 120 g of bifunctional ion exchange resin at a flow rate of 1.8 mL / min (1.1 BV / h) to obtain an exchange solution: Ca 2+ 9mg / kg, Mg 2+ 6mg / kg, Fe 3+ 13mg / kg, Na + 8mg / kg, K + 4mg / kg, total 40mg / kg, removal rate 90.7%, detection method is ICP-OES; biuret is reduced to 0.0025wt%, removal rate 99.36%, detection method is HPLC; NH3: 0.0003wt%;

[0074] 4) The exchange solution was vacuum concentrated at 45°C, -0.08 MPa, and 150 rpm stirring conditions. It took 1.7 h to concentrate to 50% of the original volume. The NH3 content in the concentrate was 0.0006 wt% and the biuret content was 0.005 wt%; the concentrate was cooled to 8°C at a rate of 1.5°C / min under stirring at 100 rpm, and then allowed to stand for 3 h to crystallize. The crystals were separated by centrifugation and vacuum dried at 50°C and a system absolute pressure of 0.01 MPa for 5 h to obtain 198 g of urea.

[0075] The purified urea was tested: purity 99.93% (difference method), water content 0.04%, NH3 content 0.0009% (acid-base titration), final removal rate 94.00%, biuret content 0.0081% (HPLC detection), final removal rate 99.38%, metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + The total content was 128 ppm (ICP-OES), and the final removal rate was 91.17%.

[0076] Comparative Example 1

[0077] The only difference between Comparative Example 1 and Example 1 is that in the urea purification process, the microfiltration membrane filtration process is removed in both step 1) and step 2). The urea purification process is as follows:

[0078] 1) Take 237.5g of industrial grade urea, add 500mL of deionized water, stir at 45℃ and 400rpm for 20min to form a 32.2wt% solution. The resulting solution has a turbidity of 5.5NTU, an NH3 content of 0.0039wt%, a biuret content of 0.322wt%, and a metal ion content of 477mg / kg, of which Ca 2+ 171mg / kg, Mg 2+ 42mg / kg, Fe 3+ 177mg / kg, Na + 113mg / kg, K74mg / kg;

[0079] 2) Activated carbon-based adsorbent was added to the solution at 2.5% by weight of the solution. After stirring and adsorption at 35°C and 250 rpm for 35 minutes, the resulting solution showed: NH3 content decreased from 0.0039 wt% to 0.0010 wt% with a removal rate of 74.36%, as tested by acid-base titration; biuret content decreased from 0.322 wt% to 0.305 wt% with a removal rate of 5.28%; and metal ion content decreased from 477 mg / kg to 458 mg / kg with a removal rate of 3.98%.

[0080] 3) The above solution was passed through an exchange column containing 100 g of bifunctional ion exchange resin at a flow rate of 1.5 mL / min (0.9 BV / h) to obtain an exchange solution: Ca 2+ 25mg / kg, Mg 2+ 5mg / kg, Fe 3+ 32mg / kg, Na + 11mg / kg, K + 8mg / kg, total 81mg / kg, removal rate 82.31%, detection method is ICP-OES; biuret is reduced to 0.036wt%, removal rate 88.20%, detection method is HPLC; NH30.001wt%;

[0081] 4) The exchange solution was vacuum concentrated at 40° C., −0.09 MPa, and 200 rpm stirring conditions. It took 1.5 h to concentrate to 50% of the original volume. The NH3 content in the concentrate was 0.002 wt%, and the biuret content was 0.070 wt%. The concentrate was cooled to 5° C. at a rate of 1° C. / min under stirring at 120 rpm, and then allowed to stand for 8 h to crystallize. The crystals were separated by centrifugation and vacuum dried at 50° C. and a system absolute pressure of 0.01 MPa for 6 h to obtain 212 g of urea.

[0082] The purified urea was tested: purity 99.60% (difference method), water content 0.10%, NH3 content 0.003% (acid-base titration), final removal rate 75.00%, biuret content 0.112% (HPLC detection), final removal rate 88.80%, metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + The total content was 251.55 ppm (ICP-OES), and the final removal rate was 83.0%.

[0083] In this comparative example, due to the lack of microfiltration in steps 1 and 2, the original water-insoluble matter and suspended matter in the raw materials cannot be removed, thereby affecting the removal of NH3, biuret and metal ions in the solution by the modified activated carbon and bifunctional resin.

[0084] Comparative Example 2

[0085] The only difference between Comparative Example 2 and Example 1 is that during the urea purification process, the activated carbon-based adsorbent is replaced by activated carbon that has not been modified with nitric acid and pyrolyzed, and the activated carbon has a pH of 7, a particle size of 0.5 to 1 mm, a pore size of 2 to 10 nm, and a specific surface area of ​​513 m 2 / g, the total content of carboxyl and hydroxyl groups determined by Boehm titration is 0.2mmol / g, and the adsorption capacity of NH3 in static adsorption test at 25℃ and 0.1MPa is only 3.2mg / g. The specific process of urea purification is as follows:

[0086] 1) Take 237.5g of technical grade urea, add 500mL of deionized water, stir at 45℃ and 400rpm for 20min to form a 32.2wt% solution, filter through a microfiltration membrane to remove insoluble impurities, and obtain a primary filtrate with turbidity <1NTU, NH3 content 0.0039wt%, biuret content 0.322wt%, and metal ion content 477mg / kg, of which Ca 2+ 71mg / kg, Mg 2+ 42mg / kg, Fe 3+ 177mg / kg, Na + 113mg / kg, K + 74mg / kg;

[0087] 2) Activated carbon (2.5% by weight of the primary filtrate) was added to the primary filtrate, and the mixture was stirred and adsorbed at 35°C and 250 rpm for 35 min. The mixture was then filtered through a microfiltration membrane to obtain a secondary filtrate: the NH3 content decreased from 0.0039 wt% to 0.0015 wt%, with a removal rate of 61.54%, as tested by acid-base titration; the biuret content decreased from 0.322 wt% to 0.310 wt%, with a removal rate of 3.7%; and the metal ion content decreased from 477 mg / kg to 461 mg / kg, with a removal rate of 3.35%;

[0088] 3) The secondary filtrate was passed through an exchange column containing 100 g of bifunctional ion exchange resin at a flow rate of 1.5 mL / min (0.9 BV / h) to obtain an exchange solution: Ca 2+ 13mg / kg, Mg 2+ 4mg / kg, Fe 3+ 19mg / kg, Na + 8mg / kg, K + 6mg / kg, total 50mg / kg, removal rate 89.15%, detection method is ICP-OES; biuret is reduced from 0.310wt% to 0.0054wt%, removal rate 98.26%, detection method is HPLC; NH3 0.0015wt%;

[0089] 4) The exchange solution was vacuum concentrated at 40° C., −0.09 MPa, and 200 rpm stirring conditions. It took 1.5 h to concentrate to 50% of the original volume. The NH3 content in the concentrate was 0.0030 wt%, and the biuret content was 0.0105 wt%. The concentrate was cooled to 5° C. at a rate of 1° C. / min under stirring at 120 rpm, and then allowed to stand for 8 h to crystallize. The crystals were separated by centrifugation and vacuum dried at 50° C. and a system absolute pressure of 0.01 MPa for 6 h to obtain 209 g of urea.

[0090] The purified urea was tested: purity 99.71% (difference method), water content 0.09%, NH3 content 0.0047% (acid-base titration), final removal rate 60.83%, biuret content 0.017% (HPLC detection), final removal rate 98.30%, metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + The total content was 155 ppm (ICP-OES), and the final removal rate was 89.53%.

[0091] In this comparative example, since commercially available ordinary activated carbon is used, its adsorption capacity for ammonia is relatively low, resulting in a low removal rate of NH3-N in the subsequent treatment process.

[0092] Comparative Example 3

[0093] The difference between Comparative Example 3 and Example 1 is that during the urea purification process, the bifunctional ion exchange resin is replaced with a polystyrene-divinylbenzene copolymer resin that has been chloromethylated and grafted with a carboxylic acid group. The specific preparation process is as follows: polystyrene-divinylbenzene copolymer resin, chloroform and chloromethyl methyl ether are mixed in a mass ratio of 1:6:1.2, stirred at 50°C for 6 hours, washed, and dried at 60°C for 12 hours to obtain a chloromethylated resin; chloromethylated resin, water and acrylic acid are mixed in a mass ratio of 1:6:0.6, stirred at 60°C for 6 hours, washed, and dried at 60°C for 12 hours to obtain the ion exchange resin, which has a pH of 7, a particle size of 0.4-0.8 mm, a carboxylic acid group content of 1.5 mmol / g, and a static adsorption test at 25°C and 0.1 MPa for metal ions and biuret. The adsorption capacity is 1.2 mmol / g and 0.08 g / g, respectively. The urea purification process is as follows:

[0094] 1) Take 237.5g of technical grade urea, add 500mL of deionized water, stir at 45℃ and 400rpm for 20min to form a 32.2wt% solution, filter through a microfiltration membrane to remove insoluble impurities, and obtain a primary filtrate with turbidity <1NTU, NH3 content 0.0039wt%, biuret content 0.322wt%, and metal ion content 477mg / kg, of which Ca 2+ 71mg / kg, Mg 2+ 42mg / kg, Fe 3+ 177mg / kg, Na + 113mg / kg, K + 74mg / kg;

[0095] 2) Adding 2.5% of the mass of the primary filtrate to the primary filtrate was an activated carbon-based adsorbent, and the mixture was stirred and adsorbed at 35°C and 250 rpm for 35 minutes. The mixture was then filtered again through a microfiltration membrane to obtain a secondary filtrate: the NH3 content decreased from 0.0039 wt% to 0.0002 wt%, with a removal rate of 94.87%, as tested by acid-base titration; the biuret content decreased from 0.322 wt% to 0.305 wt%, with a removal rate of 5.28%; and the metal ion content decreased from 477 mg / kg to 449 mg / kg, with a removal rate of 5.87%;

[0096] 3) The secondary filtrate was passed through an exchange column containing 100 g of the above ion exchange resin at a flow rate of 1.5 mL / min (0.9 BV / h) to obtain an exchange solution: Ca 2+ 23mg / kg, Mg 2+ 12mg / kg, Fe 3+ 51mg / kg, Na +14mg / kg, K+12mg / kg, totaling 112mg / kg, with a removal rate of 75.06%, detected by ICP-OES; biuret decreased from 0.305wt% to 0.11wt%, with a removal rate of 63.93%, detected by HPLC; NH30.0002wt%;

[0097] 4) The exchange solution was vacuum concentrated at 40° C., −0.09 MPa, and 200 rpm stirring conditions. It took 1.5 h to concentrate to 50% of the original volume. The NH3 content in the concentrate was 0.004 wt%, and the biuret content was 0.22 wt%. The concentrate was cooled to 5° C. at a rate of 1° C. / min under stirring at 120 rpm, and then allowed to stand for 8 h to crystallize. The crystals were separated by centrifugation and vacuum dried at 50° C. and a system absolute pressure of 0.01 MPa for 6 h to obtain 208 g of urea.

[0098] The purified urea was tested: purity 99.43% (difference method), water content 0.33%, NH3 content 0.0006% (acid-base titration), removal rate 95.00%, biuret content 0.34% (HPLC detection), removal rate 66.00%, metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + The total content is 347.83 (ICP-OES) and the removal rate is 76.50%.

[0099] In this comparative example, since the monofunctional ion exchange resin is only grafted with aminophosphonic acid groups, the removal rate of metal ions is reduced and the removal rate of biuret is significantly reduced.

[0100] The above embodiments are merely examples for clarification and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications may be made based on the above descriptions. It is not necessary and impossible to enumerate all implementation methods here, and any obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for purifying urea, characterized in that: The following steps are involved: 1) Dissolve urea in water and filter through a microfiltration membrane to obtain a primary filtrate; 2) adding an activated carbon-based adsorbent to the primary filtrate, stirring and adsorbing the mixture, and filtering the mixture again through a microfiltration membrane to obtain a secondary filtrate; the activated carbon-based adsorbent is prepared by modifying activated carbon with nitric acid and then pyrolyzing the activated carbon; 3) passing the secondary filtrate through an exchange column filled with a bifunctional ion exchange resin to obtain an exchange solution; the bifunctional ion exchange resin is a polystyrene-divinylbenzene copolymer resin that is chloromethylated and grafted with aminophosphonic acid groups and carboxylic acid groups; 4) The exchange solution is vacuum concentrated, cooled and crystallized, and the crystals are dried to obtain purified urea.

2. The method for purifying urea according to claim 1, wherein The specific preparation method of the bifunctional ion exchange resin comprises the following steps: mixing polystyrene-divinylbenzene copolymer resin, chloroform and chloromethyl methyl ether, stirring and reacting the mixture, washing and drying the product to obtain a chloromethylated resin; and mixing the chloromethylated resin, water and aminomethylphosphonic acid, stirring and reacting the mixture, adding acrylic acid, stirring and reacting the mixture, washing and drying the product to obtain a bifunctional ion exchange resin.

3. The method for purifying urea according to claim 2, wherein The mass ratio of the polystyrene-divinylbenzene copolymer resin, chloroform and chloromethyl methyl ether is 1:(5-8):(1-1.5), the reaction temperature is 40-60°C, and the reaction time is 4-8 hours; the mass ratio of the chloromethylated resin, water and aminomethylphosphonic acid is 1:(5-8):(0.4-0.8), the reaction temperature is 60-80°C, and the reaction time is 6-10 hours; the mass ratio of acrylic acid to chloromethylated resin is (0.4-0.8):1, the reaction temperature after the addition of acrylic acid is 50-70°C, and the reaction time is 4-8 hours.

4. The method for purifying urea according to claim 1, wherein The specific preparation method of the activated carbon-based adsorbent is: placing activated carbon in a nitric acid solution, stirring for reaction, washing, and drying to obtain nitric acid-modified activated carbon; pyrolyzing the nitric acid-modified activated carbon in an inert atmosphere, taking out, washing, and drying to obtain the activated carbon-based adsorbent.

5. The method for purifying urea according to claim 4, wherein: The concentration of the nitric acid solution is 1.5 to 2 mol / L; the amount of activated carbon added to the nitric acid solution is 100 to 120 g / L; the temperature of the stirring reaction is 50 to 60° C., and the reaction time is 3 to 4 hours; the pyrolysis temperature is 600 to 650° C., the heating rate is 5 to 8° C. / min, and the pyrolysis time is 2 to 3 hours.

6. The method for purifying urea according to claim 1, wherein In step 1), urea is dissolved in water to form a solution with a mass concentration of 30-50%; in steps 1) and 2), the pore size of the microfiltration membrane is 0.1-0.2 μm, and the filtration pressure is ≤0.05 MPa; the turbidity of the primary filtrate is <1 NTU.

7. The method for purifying urea according to claim 1, wherein: The activated carbon-based adsorbent has a pH of 6.5-7.5, a particle size of 0.5-1 mm, a pore size of 2-5 nm, and a specific surface area of ​​600-800 m 2 / g, the total content of carboxyl and hydroxyl groups is 0.5-2 mmol / g, and the adsorption capacity for NH3 is 10-15 mg / g; the addition amount of the activated carbon-based adsorbent is 2-3% of the mass of the primary filtrate, the adsorption temperature of the stirred adsorption is 30-35°C, the adsorption time is 30-40 min, the stirring rate is 200-300 rpm, and the NH3 content of the obtained secondary filtrate is less than 0.0005%.

8. The method for purifying urea according to claim 1, wherein The bifunctional ion exchange resin has a pH of 6.5 to 7.5, a particle size of 0.4 to 0.8 mm, an aminophosphonic acid group content of 1.0 to 1.2 mmol / g, a carboxylic acid group content of 1.2 to 1.5 mmol / g, an adsorption capacity for metal ions of 1 to 3 mmol / g, and an adsorption capacity for biuret of 0.6 to 1.2 g / g; the flow rate of the secondary filtrate through the ion exchange column is 0.6 to 10 BV / h, and the metal ion Ca content of the obtained exchange solution is 0. 2+ Mg 2+ 、Fe 3+ 、Na + , K + Total content <50mg / kg, biuret content <0.01%.

9. The method for purifying urea according to claim 1, wherein: The vacuum degree of the vacuum concentration is -0.1 to -0.08 MPa, the temperature is 40 to 50°C, and the concentration is carried out until the volume is reduced to 50 to 60% of the original volume, the NH3 content of the concentrated liquid is less than 0.005%, and the biuret content is ≤0.05%; the crystallization process is first cooled to 5 to 10°C under stirring conditions, and then allowed to stand for 2 to 4 hours; the drying temperature of the crystals is 40 to 50°C, and the drying time is 4 to 6 hours.

10. The method for purifying urea according to claim 1, wherein: The urea is industrial urea with a purity of 98-99.5%, a biuret content of ≥0.5%, a volatile impurity NH3 content of ≥0.01%, and a metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + The total content is ≥1000ppm; the purity of the purified urea is ≥99.9%, the moisture content is <0.1%, the biuret content is <0.05%, the volatile impurity NH3 content is <0.001%, the metal ion Ca 2+ Mg 2+ 、Fe 3+ 、Na + , K + Total content ≤200ppm.