A preparation method of a low-temperature type selective catalytic reduction agent solution for vehicle

Abstract

The application discloses a preparation method of a low-temperature type selective catalytic reduction solution for vehicles, and belongs to the technical field of catalytic reduction solutions. The preparation method is composed of the following steps: purifying urea, purifying ammonium formate, dissolving, filtering, fine filtering and ultrafiltrating. The purified ammonium formate is obtained through the following steps: mixing ammonium formate and tertiary water, stirring at 78-80 DEG C, performing first-stage cooling, adding treated montmorillonite, performing second-stage cooling and third-stage cooling, centrifuging, vacuum drying and the like. The preparation method of the treated montmorillonite is as follows: mixing dry polyethylene glycol 400, dry polyethylene glycol 2000 and anhydrous ethanol, stirring at 60-65 DEG C, adding sodium-based montmorillonite, stirring, centrifuging, taking the precipitate, vacuum drying, grinding, sieving and the like. The selective catalytic reduction solution for vehicles prepared by the method has low freezing point and low metal ion content.

Description

Technical Field

[0001] This invention relates to the field of catalytic reducing agent solution technology, and specifically to a method for preparing a low-temperature selective catalytic reducing agent solution for automotive applications. Background Technology

[0002] Diesel engine exhaust emissions are a significant source of nitrogen oxide pollution. Currently, there are various measures to reduce nitrogen oxide emissions from diesel engine exhaust, among which Selective Catalytic Reduction (SCR) technology is a primary method. SCR technology involves injecting a reducing agent into the exhaust pipe at a specific temperature. Under the action of a catalyst, the reducing agent reacts with nitrogen oxides in the engine exhaust to produce nitrogen and water. Because the ammonia produced by the reducing agent reacts preferentially with nitrogen oxides compared to oxygen in the exhaust pipe, it is called selective catalytic reduction (SCR). The most commonly used reducing agent in SCR is a 32.5% urea aqueous solution. Ideally, the urea aqueous solution injected into the high-temperature engine exhaust decomposes into ammonia through evaporation, pyrolysis, and hydrolysis. Under complete decomposition, 1 mol of urea can produce 2 mol of ammonia. However, when using a 32.5% urea aqueous solution as a reducing agent in selective catalytic reduction (SCR) technology, the freezing point of the 32.5% urea aqueous solution is -11°C, while the winter temperature in many parts of the world is below -11°C, which causes difficulties in the filling and transportation of urea aqueous solution in winter.

[0003] To address the aforementioned issues, Chinese patent CN102213126B discloses a low-temperature urea reducing agent composition. This composition comprises high-purity urea, analytical grade ammonium formate, analytical grade formamide, analytical grade ethanol, and high-purity water. The resulting low-temperature urea reducing agent composition has a freezing point as low as -28°C. Furthermore, under ESC conditions, this reducing agent, when injected at its rated volume and treated by an SCR aftertreatment system, achieves nitrogen oxide emissions that meet China IV emission standards. Chinese patent CN102213124B discloses a cryogenic urea reducing agent composition. This composition includes high-purity urea, analytical-grade ammonium formate, analytical-grade formamide, analytical-grade ethanol, and high-purity water. The resulting cryogenic urea reducing agent composition has a freezing point as low as -38°C. ESC testing was conducted on an 8.6-liter diesel engine bench. Under rated injection conditions, after treatment by the SCR aftertreatment system, the nitrogen oxide emissions of this reducing agent meet the China IV emission standard. (Effect of Ammonium Formate and Mixtures of Urea and Ammonium Formate on Low Temperature Activity of SCR Systems. Anu Solla, etc. SAE Transactions Vol.114, Section 4: JOURNAL OF FUELSAND) LUBRICANTS (2005), pp. 661-668, discloses that a mixture of ammonium formate and urea lowers the freezing point of urea solution to -30°C and improves the operability of SCR systems under winter conditions. The mixture of ammonium formate and urea (Denoxium product) is also comparable to commercial urea in terms of environmental friendliness and treatment safety. The literature tests two specifications of Denoxium product: Denoxium-20 and Denoxium-30. Denoxium-20 contains 18.9% ammonium formate and 23.6% urea, with a freezing point of -20°C. Denoxium-30 contains 26.2% ammonium formate and 20.1% urea, with a freezing point of -30°C. Therefore, ammonium formate, formamide, and small molecule alcohols can all lower the freezing point of automotive selective catalytic reduction (SCR) solutions.

[0004] Because formamide is considered a teratogenic compound, and small-molecule alcohols have limited effect on lowering the freezing point, the applicant chose to use high-purity urea, analytical-grade ammonium formate, and high-purity water as raw materials for experiments to prepare a low-temperature selective catalytic reduction agent solution for automobiles. Currently, when producing a 32.5% urea aqueous solution, the applicant uses industrial urea and tertiary water as raw materials, sequentially undergoing hydrolysis crystallization, dissolution, filtration, fine filtration, and ultrafiltration to prepare a urea aqueous solution conforming to the national standard GB 29518-2013, thereby reducing raw material costs. Therefore, in the pilot production of the low-temperature selective catalytic reduction agent solution for automobiles, the applicant tried using industrial urea, industrial ammonium formate, and tertiary water as raw materials, sequentially undergoing hydrolysis crystallization, dissolution, filtration, fine filtration, and ultrafiltration to obtain a low-impurity low-temperature selective catalytic reduction agent solution for automobiles. The specific pilot production method is as follows:

[0005] First, industrial urea is hydrolyzed using tertiary water at 70-75℃ to obtain an industrial urea solution. Then, the solution is cooled to the crystallization temperature to crystallize, resulting in high-purity crystalline urea. Through multiple hydrolysis and crystallization processes, high-purity urea is obtained. Then, high-purity urea and industrial ammonium formate are dissolved in tertiary water in a specific ratio. After fine filtration and ultrafiltration, a low-temperature automotive selective catalytic reducing agent solution is obtained.

[0006] However, during pilot production, it was found that fine filtration and ultrafiltration could only remove precipitates, with little impact on the content of other soluble impurities (see Research Summary on Impurities in Automotive Urea Solution. Wang Pan, Li Hongming, Chen Pengjiao, Ren Mengwei. Shandong Chemical Industry. September 2020). The industrial ammonium formate used contained inorganic metal ion impurities, leading to a high metal ion content in the low-temperature automotive selective catalytic reduction agent solution. To solve this problem, the applicant attempted to purify ammonium formate using multiple hydrolysis and crystallization methods. However, ammonium formate has a higher binding capacity for metal ions than urea, and it easily adsorbs metal ions during crystallization, resulting in a high metal ion content in the low-temperature automotive selective catalytic reduction agent solution. According to research, although montmorillonite-type clay minerals can adsorb metal ions (see Research Progress on Heavy Metal Ion Adsorption by Montmorillonite. Zang Yunbo. Anhui Agricultural Sciences. October 2008), if clay minerals are added during the crystallization of ammonium formate, there is an interaction between the clay minerals and ammonium formate. Ammonium formate will reduce the hydration and adsorption capacity of the clay mineral surface (see Study on the Synergistic Enhancement Mechanism of Ammonium Formate in the Leaching Process of Weathered Crust-Effected Rare Earth Minerals. Huang Suhua. Master's Thesis, Wuhan University of Technology. May 2022). Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for preparing a low-temperature selective catalytic reduction agent solution for automobiles. The prepared solution has a low freezing point and low metal ion content.

[0008] To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing a low-temperature automotive selective catalytic reducing agent solution comprises the following steps: purifying urea, purifying ammonium formate, dissolving, filtering, fine filtering, and ultrafiltration;

[0010] The process for purifying urea involves adding industrial urea and tertiary water into a reactor, controlling the reactor temperature to 70-72°C, and the stirring speed to 300-350 rpm for 30-40 minutes. The stirring speed is maintained constant, and the process is repeated with the first, second, and third stages of cooling. The mixture is then transferred to a centrifuge for centrifugation, and the precipitate is collected and added to a vacuum dryer for vacuum drying to obtain purified urea.

[0011] In the purified urea, the mass ratio of industrial urea to tertiary water is 730-750:270-280;

[0012] The first stage of cooling involves cooling at a rate of 0.05℃ / min for 100-110 minutes.

[0013] The second stage of cooling involves cooling at a rate of 0.1℃ / min for 70-80 minutes.

[0014] The third stage of cooling involves cooling at a rate of 0.2℃ / min for 45-50 minutes.

[0015] The centrifugation speed is 3000-3500 rpm, and the centrifugation time is 10-15 min;

[0016] The vacuum drying temperature is 50-55℃, and the vacuum drying time is 12-14 hours.

[0017] The purification of ammonium formate involves adding ammonium formate and tertiary water to a reactor, controlling the reactor temperature to 78-80℃, and the stirring speed to 300-350 rpm for 30-40 minutes. The stirring speed is maintained constant during the first stage of cooling. Then, treated montmorillonite is added for the second and third stages of cooling. The mixture is then transferred to a centrifuge for centrifugation. The precipitate is collected and added to a vacuum dryer for vacuum drying to obtain purified ammonium formate.

[0018] In the purified ammonium formate, the mass ratio of ammonium formate to tertiary water is 950-980:200-210;

[0019] The mass ratio of ammonium formate to treated montmorillonite is 950-980:5-5.5;

[0020] The first stage of cooling involves cooling at a rate of 0.05℃ / min for 300-310 minutes.

[0021] The second stage of cooling involves cooling at a rate of 0.1℃ / min for 100-110 minutes.

[0022] The third stage of cooling involves cooling at a rate of 0.2℃ / min for 100-110 minutes.

[0023] The centrifugation speed is 3000-3500 rpm, and the centrifugation time is 10-15 min;

[0024] The vacuum drying temperature is 50-55℃, and the vacuum drying time is 14-15 hours.

[0025] The method for preparing the treated montmorillonite is as follows: polyethylene glycol 400 and polyethylene glycol 2000 are respectively added to a vacuum dryer for vacuum drying to obtain dried polyethylene glycol 400 and dried polyethylene glycol 2000. Then, the dried polyethylene glycol 400, dried polyethylene glycol 2000 and anhydrous ethanol are added to a reactor. The temperature of the reactor is controlled at 60-65℃, the stirring speed is controlled at 200-400 rpm, and the stirring is carried out for 30-40 minutes. Sodium-based montmorillonite is added, and the mixture is stirred for 3-4 hours. The mixture is then centrifuged, the precipitate is collected, and the precipitate is added to a vacuum dryer for vacuum drying. The precipitate is then ground, sieved, and the sieved material is collected to obtain the treated montmorillonite.

[0026] In the preparation of the treated montmorillonite, the vacuum drying temperature for polyethylene glycol 400 and polyethylene glycol 2000 is 50-55℃, and the vacuum drying time is 12-14h.

[0027] The mass ratio of dried polyethylene glycol 400, dried polyethylene glycol 2000, anhydrous ethanol, and sodium montmorillonite is 6-6.5:12-13:3800-4000:50-60.

[0028] The centrifugation speed was 4000 rpm and the centrifugation time was 5 min.

[0029] The vacuum drying temperature for the precipitate is 50-55℃, and the vacuum drying time is 9-10 hours.

[0030] The sieve mesh size during sieving is 325 mesh;

[0031] The dissolution process involves adding purified urea, purified ammonium formate, and tertiary water to a reactor, controlling the reactor temperature to 40-42℃, controlling the stirring speed to 200-300 rpm, and stirring for 30-40 minutes to obtain a reducing agent solution.

[0032] In the dissolution process, the mass ratio of purified urea, purified ammonium formate, and tertiary water is 23.6-26.2:18.9-20.1:53.7-57.5.

[0033] The filtration process involves using a bag filter to filter the reducing agent solution, resulting in a filtered reducing agent solution.

[0034] The bag filter has a filtration accuracy of 1μm and uses polyethylene as the filter material.

[0035] The fine filtration involves using a fine filter to finely filter the filtered reducing agent solution to obtain a finely filtered reducing agent solution.

[0036] The fine filter has a filtration accuracy of 0.25μm and uses a filter element made of polypropylene.

[0037] The ultrafiltration process involves using an ultrafilter to ultrafilter the finely filtered reducing agent solution to obtain a low-temperature automotive selective catalytic reducing agent solution.

[0038] The ultrafiltration unit has a filtration precision of 0.01 μm, uses a polysulfone membrane, and has a stable pure water flux of 100 L / (m²) at 25℃ and 0.1 MPa. 2 •h);

[0039] The pressure during ultrafiltration is 0.25 MPa.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] The low-temperature selective catalytic reductant solution for automobiles prepared by this invention has a low freezing point, ranging from -30°C to -20°C, and low metal ion content. Specifically, the solution contains 1.5-1.8 mg / kg of aldehydes (calculated as HCHO), 0.10-0.13% biuret, 2.7-3.2 mg / kg of insoluble matter, 0.26-0.30 mg / kg of phosphates (calculated as PO4), and 0.08-0.10 mg / kg of calcium. The content of iron is 0.07-0.08 mg / kg, copper is 0.07-0.09 mg / kg, zinc is 0.09-0.11 mg / kg, chromium is 0.04-0.05 mg / kg, nickel is 0.05-0.07 mg / kg, aluminum is 0.08-0.09 mg / kg, magnesium is 0.10-0.12 mg / kg, sodium is 0.11-0.15 mg / kg, and potassium is 0.04-0.05 mg / kg. Detailed Implementation

[0042] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention are now described.

[0043] In Examples 1-2 and Comparative Examples 1-3, the industrial urea, industrial ammonium formate, and tertiary water used were all from the same production batch. The total nitrogen (N) content (on a dry basis) of the industrial urea used was 46.4%, and the purity of the industrial ammonium formate was 98%.

[0044] In Examples 1-2 and Comparative Examples 1-3, the conductivity of the grade III water used at 25°C was 0.28 mS / m, which meets the national standard GB / T 6682-2008 for grade III water.

[0045] In Examples 1-2 and Comparative Examples 1-2, the sodium-based montmorillonite used had an ion exchange capacity of 120 mmol / 100g.

[0046] In Examples 1-2 and Comparative Examples 1-3, the content of aldehydes, biuret, insoluble matter, phosphates, calcium, iron, aluminum, magnesium, potassium, sodium, potassium, copper, zinc, chromium, and nickel in the reducing agent solution was tested according to the detection methods disclosed in the national standard GB29518-2013.

[0047] Example 1

[0048] A method for preparing a low-temperature selective catalytic reducing agent solution for automotive applications, specifically comprising:

[0049] 1. Purification of urea: Add 730g of industrial urea and 270g of tertiary water to a reactor. Control the reactor temperature to 70℃ and the stirring speed to 300rpm. Stir for 30min, keeping the stirring speed constant. Cool at a rate of 0.05℃ / min for 100min, then at a rate of 0.1℃ / min for 70min, and finally at a rate of 0.2℃ / min for 45min. Transfer to a centrifuge and centrifuge at 3000rpm for 10min. Take the precipitate and add it to a vacuum dryer. Control the temperature of the vacuum dryer to 50℃ and vacuum dry for 12h to obtain purified urea.

[0050] 2. Purification of ammonium formate: Add 950g of ammonium formate and 200g of tertiary water to the reactor. Control the reactor temperature to 78℃ and the stirring speed to 300rpm. Stir for 30min, keeping the stirring speed constant. Cool at a rate of 0.05℃ / min for 300min. Add 5g of treated montmorillonite and cool at a rate of 0.1℃ / min for 100min. Then cool at a rate of 0.2℃ / min for 100min. Transfer to a centrifuge and control the centrifugation speed to 3000rpm. Centrifuge for 10min. Take the precipitate and add it to a vacuum dryer. Control the temperature of the vacuum dryer to 50℃ and vacuum dry for 14h. Separate the treated montmorillonite to obtain purified ammonium formate.

[0051] The preparation method of the treated montmorillonite is as follows: Polyethylene glycol 400 and polyethylene glycol 2000 are added to a vacuum dryer, the temperature of the vacuum dryer is controlled at 50°C, and vacuum drying is carried out for 12 hours to obtain dried polyethylene glycol 400 and dried polyethylene glycol 2000. Then, 6g of dried polyethylene glycol 400, 12g of dried polyethylene glycol 2000 and 3800g of anhydrous ethanol are added to a reactor, the temperature of the reactor is controlled at 60°C, the stirring speed is controlled at 200rpm, and stirring is carried out for 30 minutes. 50g of sodium-based montmorillonite is added, and stirring is carried out for 3 hours. The mixture is then transferred to a centrifuge, the centrifugation speed is controlled at 4000rpm, and centrifuged for 5 minutes. The precipitate is collected and added to a vacuum dryer, the temperature of the vacuum dryer is controlled at 50°C, and vacuum drying is carried out for 9 hours. After grinding, the mixture is passed through a 325-mesh sieve, and the sieve-passing material is collected to obtain the treated montmorillonite.

[0052] 3. Dissolution: Add purified urea, purified ammonium formate and tertiary water to the reactor at a mass ratio of 23.6:18.9:57.5. Control the reactor temperature to 40℃ and the stirring speed to 200 rpm. Stir for 30 minutes to obtain the reducing agent solution.

[0053] 4. Filtration: The reducing agent solution is filtered using a bag filter to obtain the filtered reducing agent solution;

[0054] The bag filter has a filtration accuracy of 1μm and uses polyethylene as the filter material.

[0055] 5. Fine filtration: The filtered reducing agent solution is further filtered using a fine filter to obtain a finely filtered reducing agent solution;

[0056] The fine filter has a filtration accuracy of 0.25μm and uses a filter element made of polypropylene.

[0057] 6. Ultrafiltration: The finely filtered reducing agent solution is ultrafiltered using an ultrafilter to obtain a low-temperature automotive selective catalytic reducing agent solution;

[0058] The ultrafiltration unit has a filtration precision of 0.01 μm, uses a polysulfone membrane, and has a stable pure water flux of 100 L / (m²) at 25℃ and 0.1 MPa. 2 •h);

[0059] The pressure during ultrafiltration is 0.25 MPa.

[0060] The low-temperature selective catalytic reducer solution for automotive applications prepared in this embodiment has a freezing point of -20°C, an aldehyde content (calculated as HCHO) of 1.8 mg / kg, a biuret mass fraction of 0.13%, an insoluble matter content of 3.2 mg / kg, a phosphate content (calculated as PO4) of 0.30 mg / kg, a calcium content of 0.08 mg / kg, an iron content of 0.07 mg / kg, a copper content of 0.07 mg / kg, a zinc content of 0.09 mg / kg, a chromium content of 0.04 mg / kg, a nickel content of 0.05 mg / kg, an aluminum content of 0.08 mg / kg, a magnesium content of 0.10 mg / kg, a sodium content of 0.14 mg / kg, and a potassium content of 0.05 mg / kg.

[0061] Example 2

[0062] A method for preparing a low-temperature selective catalytic reducing agent solution for automotive applications, specifically comprising:

[0063] 1. Purification of urea: Add 750g of industrial urea and 280g of tertiary water to a reactor. Control the reactor temperature to 72℃ and the stirring speed to 350rpm. Stir for 40min, keeping the stirring speed constant. Cool at a rate of 0.05℃ / min for 110min, then at a rate of 0.1℃ / min for 80min, and finally at a rate of 0.2℃ / min for 50min. Transfer to a centrifuge and centrifuge at 3500rpm for 15min. Take the precipitate and add it to a vacuum dryer. Control the temperature of the vacuum dryer to 55℃ and vacuum dry for 14h to obtain purified urea.

[0064] 2. Purification of ammonium formate: Add 980g of ammonium formate and 210g of tertiary water to the reactor. Control the reactor temperature to 80℃ and the stirring speed to 350rpm. Stir for 40min. Keep the stirring speed constant and cool at a rate of 0.05℃ / min for 310min. Add 5.5g of treated montmorillonite and cool at a rate of 0.1℃ / min for 110min. Then cool at a rate of 0.2℃ / min for 110min. Transfer to a centrifuge and centrifuge at 3500rpm for 15min. Take the precipitate and add it to a vacuum dryer. Control the temperature of the vacuum dryer to 55℃ and vacuum dry for 15h. Separate the treated montmorillonite to obtain purified ammonium formate.

[0065] The preparation method of the treated montmorillonite is as follows: Polyethylene glycol 400 and polyethylene glycol 2000 are added to a vacuum dryer, the temperature of the vacuum dryer is controlled at 55°C, and vacuum drying is carried out for 14 hours to obtain dried polyethylene glycol 400 and dried polyethylene glycol 2000. Then, 6.5g of dried polyethylene glycol 400, 13g of dried polyethylene glycol 2000 and 4000g of anhydrous ethanol are added to a reactor, the temperature of the reactor is controlled at 65°C, the stirring speed is controlled at 400rpm, and stirring is carried out for 40 minutes. 60g of sodium-based montmorillonite is added, and stirring is carried out for 4 hours. The mixture is then transferred to a centrifuge, the centrifugation speed is controlled at 5000rpm, and centrifuged for 10 minutes. The precipitate is collected and added to a vacuum dryer, the temperature of the vacuum dryer is controlled at 55°C, and vacuum drying is carried out for 10 hours. After grinding, the mixture is passed through a 325-mesh sieve, and the sieve-passing material is collected to obtain the treated montmorillonite.

[0066] 3. Dissolution: Add purified urea, purified ammonium formate and tertiary water to the reactor at a mass ratio of 20.1:26.2:53.7. Control the reactor temperature to 42℃ and the stirring speed to 300rpm. Stir for 40min to obtain the reducing agent solution.

[0067] 4. Filtration: The reducing agent solution is filtered using a bag filter to obtain the filtered reducing agent solution;

[0068] The bag filter has a filtration accuracy of 1μm and uses polyethylene as the filter material.

[0069] 5. Fine filtration: The filtered reducing agent solution is further filtered using a fine filter to obtain a finely filtered reducing agent solution;

[0070] The fine filter has a filtration accuracy of 0.25μm and uses a filter element made of polypropylene.

[0071] 6. Ultrafiltration: The finely filtered reducing agent solution is ultrafiltered using an ultrafilter to obtain a low-temperature automotive selective catalytic reducing agent solution;

[0072] The ultrafiltration unit has a filtration precision of 0.01 μm, uses a polysulfone membrane, and has a stable pure water flux of 100 L / (m²) at 25℃ and 0.1 MPa. 2 •h);

[0073] The pressure during ultrafiltration is 0.25 MPa.

[0074] The low-temperature selective catalytic reducer solution for automobiles prepared in this embodiment has a freezing point of -30°C, an aldehyde content (calculated as HCHO) of 1.5 mg / kg, a biuret mass fraction of 0.10%, an insoluble matter content of 2.7 mg / kg, a phosphate content (calculated as PO4) of 0.26 mg / kg, a calcium content of 0.10 mg / kg, an iron content of 0.08 mg / kg, a copper content of 0.09 mg / kg, a zinc content of 0.10 mg / kg, a chromium content of 0.05 mg / kg, a nickel content of 0.07 mg / kg, an aluminum content of 0.09 mg / kg, a magnesium content of 0.12 mg / kg, a sodium content of 0.17 mg / kg, and a potassium content of 0.04 mg / kg.

[0075] Comparative Example 1

[0076] Based on the preparation method of the low-temperature automotive selective catalytic reducing agent solution in Example 1, the preparation method of treating montmorillonite in the second step of purifying ammonium formate is modified as follows:

[0077] Polyethylene glycol 400 was added to a vacuum dryer, and the temperature of the vacuum dryer was controlled at 50℃. Vacuum drying was carried out for 12 hours to obtain dried polyethylene glycol 400. Then, 18g of dried polyethylene glycol 400 and 3800g of anhydrous ethanol were added to a reactor, and the temperature of the reactor was controlled at 60℃. The stirring speed was controlled at 200rpm, and the mixture was stirred for 30 minutes. 50g of sodium montmorillonite was added, and the mixture was stirred for 3 hours. The mixture was then transferred to a centrifuge, and the centrifugation speed was controlled at 4000rpm. The mixture was centrifuged for 5 minutes. The precipitate was collected and added to a vacuum dryer, and the temperature of the vacuum dryer was controlled at 50℃. Vacuum drying was carried out for 9 hours. After grinding, the mixture was passed through a 325-mesh sieve, and the sieve-passing material was collected to obtain treated montmorillonite.

[0078] The remaining technical solutions are consistent with those in Example 1.

[0079] The low-temperature selective catalytic reducer solution for automobiles prepared in this embodiment has a freezing point of -20°C, an aldehyde content (calculated as HCHO) of 1.9 mg / kg, a biuret mass fraction of 0.15%, an insoluble matter content of 3.5 mg / kg, a phosphate content (calculated as PO4) of 0.31 mg / kg, a calcium content of 0.38 mg / kg, an iron content of 0.21 mg / kg, a copper content of 0.20 mg / kg, a zinc content of 0.17 mg / kg, a chromium content of 0.10 mg / kg, a nickel content of 0.12 mg / kg, an aluminum content of 0.24 mg / kg, a magnesium content of 0.32 mg / kg, a sodium content of 0.21 mg / kg, and a potassium content of 0.17 mg / kg.

[0080] Comparative Example 2

[0081] Based on the preparation method of the low-temperature automotive selective catalytic reducing agent solution in Example 1, the preparation method of treating montmorillonite in the second step of purifying ammonium formate is modified as follows:

[0082] Polyethylene glycol 2000 was added to a vacuum dryer, and the temperature of the vacuum dryer was controlled at 50℃. Vacuum drying was carried out for 12 hours to obtain dried polyethylene glycol 2000. Then, 18g of dried polyethylene glycol 2000 and 3800g of anhydrous ethanol were added to a reactor, and the temperature of the reactor was controlled at 60℃. The stirring speed was controlled at 200rpm, and the mixture was stirred for 30 minutes. 50g of sodium montmorillonite was added, and the mixture was stirred for 3 hours. The mixture was then transferred to a centrifuge, and the centrifugation speed was controlled at 4000rpm. The mixture was centrifuged for 5 minutes. The precipitate was collected and added to a vacuum dryer, and the temperature of the vacuum dryer was controlled at 50℃. Vacuum drying was carried out for 9 hours. After grinding, the mixture was passed through a 325-mesh sieve, and the sieve-passing material was collected to obtain treated montmorillonite.

[0083] The remaining technical solutions are consistent with those in Example 1.

[0084] The low-temperature selective catalytic reducer solution for automobiles prepared in this embodiment has a freezing point of -20°C, an aldehyde content (calculated as HCHO) of 1.6 mg / kg, a biuret mass fraction of 0.12%, an insoluble matter content of 3.1 mg / kg, a phosphate content (calculated as PO4) of 0.27 mg / kg, a calcium content of 0.31 mg / kg, an iron content of 0.25 mg / kg, a copper content of 0.20 mg / kg, a zinc content of 0.18 mg / kg, a chromium content of 0.15 mg / kg, a nickel content of 0.18 mg / kg, an aluminum content of 0.29 mg / kg, a magnesium content of 0.27 mg / kg, a sodium content of 0.23 mg / kg, and a potassium content of 0.20 mg / kg.

[0085] Comparative Example 3

[0086] Based on the preparation method of the low-temperature automotive selective catalytic reducing agent solution in Example 1, the addition of montmorillonite treatment is omitted in the second step of purifying ammonium formate.

[0087] The remaining technical solutions are consistent with those in Example 1.

[0088] The low-temperature selective catalytic reducer solution for automobiles prepared in this embodiment has a freezing point of -20°C, an aldehyde content (calculated as HCHO) of 2.2 mg / kg, a biuret mass fraction of 0.14%, an insoluble matter content of 3.6 mg / kg, a phosphate content (calculated as PO4) of 0.36 mg / kg, a calcium content of 0.73 mg / kg, an iron content of 0.45 mg / kg, a copper content of 0.43 mg / kg, a zinc content of 0.40 mg / kg, a chromium content of 0.38 mg / kg, a nickel content of 0.41 mg / kg, an aluminum content of 0.47 mg / kg, a magnesium content of 0.58 mg / kg, a sodium content of 0.26 mg / kg, and a potassium content of 0.40 mg / kg.

[0089] Comparing Example 1 and Comparative Examples 1-3, it was found that the freezing point of the low-temperature automotive selective catalytic reductant solution prepared in Comparative Examples 1-3 did not change compared to Example 1, but the metal ion content was higher. This indicates that adding treated montmorillonite prepared from polyethylene glycol 400, polyethylene glycol 2000, and montmorillonite as raw materials during the ammonium formate purification step can reduce the metal ion content in the prepared low-temperature automotive selective catalytic reductant solution. Analysis revealed a synergistic effect between polyethylene glycol 400 and polyethylene glycol 2000 in the treated montmorillonite prepared during the ammonium formate purification step. The specific mechanism of action is as follows:

[0090] First, when treating montmorillonite containing both polyethylene glycol 400 and polyethylene glycol 2000, the molecular weights of both PEG 400 and PEG 2000 coil up after being added to the crystallization system, causing montmorillonite to aggregate (see Interactions between low molecular weight water-soluble polymers and montmorillonite and their environmental response behavior. Zhao Chunhua. Doctoral dissertation, Shandong University. May 2014). PEG 400, with its smaller molecular weight and lower coiling degree, plays a major role in enhancing the adsorption of metal ions by montmorillonite. Furthermore, because ammonium formate binds to the montmorillonite surface via electrostatic interactions, the electrostatic interaction between ammonium formate and montmorillonite is weak at high temperatures. Ammonium formate has a relatively weak inhibitory effect on the hydration of montmorillonite. Montmorillonite can rapidly adsorb metal ions, but polyethylene glycol 400 has a small molecular weight and a weak ability to bind metal ions. As the temperature decreases, the molecular weight of polyethylene glycol 2000 expands, and montmorillonite is completely dispersed. Although ammonium formate is still adsorbed on the surface of montmorillonite to inhibit its adsorption, montmorillonite has already achieved a certain degree of hydration. Furthermore, the presence of polyethylene glycol 2000 can further enhance the metal ion binding capacity of montmorillonite, thus ensuring the adsorption of metal ions. Then, by centrifugation and vacuum drying, the treated montmorillonite is separated using particle size differences to obtain purified ammonium formate.

Claims

1. A method for preparing a low-temperature selective catalytic reducing agent solution for automotive applications, characterized in that, It consists of the following steps: purifying urea, purifying ammonium formate, dissolving, filtering, fine filtering, and ultrafiltration; The purification of urea involves mixing industrial urea with tertiary water, stirring at 70-72°C, performing a first-stage cooling, a second-stage cooling, and a third-stage cooling, centrifuging, and vacuum drying to obtain purified urea. The purification of ammonium formate involves mixing ammonium formate and tertiary water, stirring at 78-80°C, performing a first-stage cooling, adding treated montmorillonite, performing a second-stage cooling and a third-stage cooling, centrifuging, and vacuum drying to obtain purified ammonium formate. The method for preparing the treated montmorillonite is as follows: dry polyethylene glycol 400, dry polyethylene glycol 2000 and anhydrous ethanol are mixed and stirred at 60-65℃ for 30-40 min. Sodium-based montmorillonite is added and stirred for 3-4 h. After centrifugation, the precipitate is collected, vacuum dried, ground, sieved, and the sieve-passing material is collected to obtain the treated montmorillonite. The dissolution process involves mixing purified urea, purified ammonium formate, and tertiary water, and then stirring at 40-42°C to obtain a reducing agent solution.

2. The method for preparing the low-temperature automotive selective catalytic reducing agent solution according to claim 1, characterized in that, In the purified urea, the mass ratio of industrial urea to tertiary water is 730-750:270-280; The first stage of cooling involves cooling at a rate of 0.05℃ / min for 100-110 minutes. The second stage of cooling involves cooling at a rate of 0.1℃ / min for 70-80 minutes. The third stage of cooling involves cooling at a rate of 0.2℃ / min for 45-50 minutes.

3. The method for preparing the low-temperature automotive selective catalytic reducing agent solution according to claim 1, characterized in that, In the purified ammonium formate, the mass ratio of ammonium formate to tertiary water is 950-980:200-210; The mass ratio of ammonium formate to treated montmorillonite is 950-980:5-5.5; The first stage of cooling involves cooling at a rate of 0.05℃ / min for 300-310 minutes. The second stage of cooling involves cooling at a rate of 0.1℃ / min for 100-110 minutes. The third stage of cooling involves cooling at a rate of 0.2℃ / min for 100-110 minutes.

4. The method for preparing the low-temperature automotive selective catalytic reducing agent solution according to claim 1, characterized in that, In the preparation of the treated montmorillonite, the mass ratio of dried polyethylene glycol 400, dried polyethylene glycol 2000, anhydrous ethanol, and sodium-based montmorillonite is 6-6.5:12-13:3800-4000:50-60. The sieve mesh size during sieving is 325 mesh.

5. The method for preparing the low-temperature automotive selective catalytic reducing agent solution according to claim 1, characterized in that, In the dissolution process, the mass ratio of purified urea, purified ammonium formate, and tertiary water is 23.6-26.2:18.9-20.1:53.7-57.

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6. The method for preparing the low-temperature automotive selective catalytic reducing agent solution according to claim 1, characterized in that, The filtration process involves using a bag filter to filter the reducing agent solution, resulting in a filtered reducing agent solution. The bag filter has a filtration accuracy of 1μm and uses polyethylene as the filter material.

7. The method for preparing the low-temperature automotive selective catalytic reducing agent solution according to claim 1, characterized in that, The fine filtration involves using a fine filter to finely filter the filtered reducing agent solution to obtain a finely filtered reducing agent solution. The fine filter has a filtration accuracy of 0.25μm and uses a filter element made of polypropylene.

8. The method for preparing the low-temperature automotive selective catalytic reducing agent solution according to claim 1, characterized in that, The ultrafiltration process involves using an ultrafilter to ultrafilter the finely filtered reducing agent solution to obtain a low-temperature automotive selective catalytic reducing agent solution. The ultrafiltration unit has a filtration precision of 0.01 μm, uses a polysulfone membrane, and has a stable pure water flux of 100 L / (m²) at 25℃ and 0.1 MPa. 2 •h); The pressure during ultrafiltration is 0.25 MPa.