Method for detecting chloride ions in ammonium sulfate mother liquor in process of preparing ammonium sulfate from waste acid

Abstract

The invention discloses a method for detecting chloride ions in an ammonium sulfate mother liquor in a process of preparing ammonium sulfate from waste acid, which has the advantages that the influence of pyridinium salt-based organic matters and high-concentration ammonia in the ammonium sulfate mother liquor on titration determination is eliminated by a treatment method of firstly layering to remove pyridine under a weak alkali condition and then converting and removing ammonia under a strong alkali condition. Through step-by-step alkali addition, the splashing risk possibly caused by violent reaction of organic matters or instant release of a large amount of ammonia gas in a strong alkali environment is avoided. The ammonia removal end point is judged by using wet pH test paper, and the method is simple, intuitive, low in cost and easy to popularize and implement in laboratories of production fields. The method creatively solves the long-standing problem of accurate detection of chloride ions in the ammonium sulfate mother liquor in the industry of preparing ammonium sulfate from waste acid, can help a production department to optimize a mother liquor discharge strategy by providing accurate chloride ion data, and minimizes loss of ammonium sulfate products and high-value chemicals while ensuring equipment safety.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis, specifically to a method for detecting chloride ions in ammonium sulfate mother liquor during the process of producing ammonium sulfate from waste acid. Background Technology

[0002] Ammonium sulfate (abbreviated as "ammonium sulfate") is an important by-product of the coking and fertilizer industries. In the coking process, ammonia (NH3) from coke oven gas is absorbed and neutralized by sulfuric acid in the ammonium sulfate section, subsequently crystallizing to form ammonium sulfate. The core reaction medium in this process is ammonium sulfate mother liquor, which is circulated within a spray-type saturator. Its main components are free sulfuric acid and various impurity ions introduced from raw materials and gas, among which chloride ions (Cl-) are the most abundant. - Chloride ions are among the most harmful impurity ions. The sources of chloride ions mainly include two aspects: first, when using waste sulfuric acid, a byproduct of the chlor-alkali industry, as a raw material, even after dechlorination treatment, a certain amount of chlorine will remain, resulting in the raw sulfuric acid itself containing some chlorine; second, trace amounts of chloride may be carried in the coal gas. Because the ammonium sulfate mother liquor system is a continuously circulating, evaporating, and concentrating system, chloride ions do not have volatility or reactive consumption pathways, and therefore will continuously accumulate in the system.

[0003] In the high-temperature, high-salt, and highly acidic environment of ammonium sulfate mother liquor, concentrated chloride ions are extremely corrosive to carbon steel and stainless steel equipment (such as saturators, circulating pumps, pipelines, and valves), easily inducing pitting corrosion and stress corrosion cracking. This seriously threatens the long-term safe and stable operation of production facilities and may lead to safety accidents, environmental pollution, and unplanned shutdowns due to equipment leaks. Furthermore, the chloride ion concentration in the mother liquor is a key parameter for process control, directly determining the amount of mother liquor discharged to maintain the system's chloride balance, thus affecting material consumption and production costs. Therefore, accurate and reliable monitoring of the chloride ion content in ammonium sulfate mother liquor is a crucial prerequisite for achieving early warning of equipment corrosion, optimized process control, and refined management of production costs.

[0004] However, ammonium sulfate mother liquor is a complex analytical matrix. Besides high concentrations of ammonium salts and free acid, it typically contains organic amines such as pyridine groups and high concentrations of ammonia. These coexisting components severely interfere with the classic chloride ion detection method—the silver nitrate titration method (Mohr's method)—for the following reasons: pyridine groups, as organic bases, consume the titration acid; the presence of ammonia leads to significant deviations in endpoint determination; and conventional sample pretreatment methods (such as simple alkali distillation or one-step neutralization) are insufficient to effectively and thoroughly separate these interfering substances, resulting in poor accuracy and low reproducibility of the final detection results, failing to provide reliable data guidance for production.

[0005] Therefore, there is an urgent need for a chloride ion detection method that can specifically address the interference from the complex matrix of ammonium sulfate mother liquor during the production of ammonium sulfate from waste acid. Summary of the Invention

[0006] To address the aforementioned problems, the present invention aims to provide a method for detecting chloride ions in the mother liquor of ammonium sulfate produced from waste acid. This method utilizes a systematic pretreatment process to effectively separate organic matter and ammonia, thereby achieving a specialized analytical method for accurate and stable determination of chloride ions. This method is of significant practical importance for ensuring the safe, stable, and economical operation of ammonium sulfate production facilities using chlorine-containing raw materials (such as waste sulfuric acid from chemical processes).

[0007] This invention is implemented by the following technical solution: A method for detecting chloride ions in ammonium sulfate mother liquor during the production of ammonium sulfate from waste acid includes the following steps: (1) Sampling and dilution: Take a certain mass of the initial ammonium sulfate mother liquor and add a certain mass of pure water for dilution. The mass ratio of the pure water to the ammonium sulfate mother liquor is (0.5:1) to (5:1) to obtain the diluted ammonium sulfate mother liquor. (2) Preliminary alkalization and separation: Add an alkaline regulator to the diluted ammonium sulfate mother liquor obtained by sampling and dilution in step (1) to adjust the pH value to 7.0-9.0, so that the solution is separated into layers, and the lower aqueous phase is separated and retained; Chloride ions in ammonium sulfate mother liquor often exist in the form of ammonium salts (such as NH4Cl), but also contain organic impurities such as pyridine hydrochloride (C5H5N·HCl). Upon addition of NaOH, the pH of the system increases, neutralizing the pyridine hydrochloride and releasing free pyridine. Free pyridine has limited solubility in water and a density less than water, so it precipitates as an oily substance and floats to the top. By separating and removing the free pyridine from the top layer, the organic base (pyridine) that interferes with subsequent titrations can be released from its salt form and physically separated, preventing it from consuming acid or causing interference in subsequent steps.

[0008] (3) Deep alkalization and secondary separation: Add alkaline regulator to the lower aqueous phase obtained after the initial alkalization and separation in step (2) to adjust the pH value to 12.0-14.0. If an organic phase is generated, separate it again and retain the lower clear liquid. The ammonium sulfate and residual ammonium chloride in the mother liquor are completely converted into free ammonia in a strongly alkaline environment, which prepares for subsequent heating to remove ammonia.

[0009] The corresponding chemical reaction formula is: (NH4)2SO4+2NaOH→2NH3↑+Na2SO4+2H2O NH4Cl + NaOH → NH3↑ + NaCl + H2O (4) Ammonia removal treatment: The lower clear liquid obtained from the deep alkalization and secondary separation in step (3) is heated to boiling and heated continuously until the ammonia gas is fully removed, resulting in a solution after ammonia removal; taking advantage of the volatility of ammonia, the solution is heated to allow the free ammonia generated in the above reaction to escape from the solution, thereby eliminating the largest interfering ion—ammonia ions. If ammonia is not removed, it will consume the acid standard solution during subsequent acid titration, leading to an increase in the amount of acid used for neutralization, which seriously interferes with the determination of the chloride ion titration endpoint and the calculation of the result.

[0010] (5) Volume adjustment and filtration: Add water to the ammonia-removed solution obtained after the ammonia removal treatment in step (4) to adjust the volume to the preset total mass, and filter to obtain a clear filtrate; the ammonia-removed solution may produce flocculent matter due to the generation of insoluble hydroxides (such as Mg(OH)2, Fe(OH)3). Adjusting the volume to a fixed mass can ensure that the sample concentration is traceable. Filtration can obtain a clear and uniform test solution and avoid solid particles from interfering with titration observation.

[0011] (6) Neutralization and titration: Take a portion of the clear filtrate obtained from step (5) by volume adjustment and filtration, add an acid-base indicator and titrate with a standard acid solution to 6.5-8.5 to avoid premature or no formation of silver chromate precipitate in the subsequent Mohr process due to an alkaline environment. Then add an argentometric indicator and titrate with a standard silver nitrate solution. In a neutral to weakly alkaline environment, a white precipitate of silver chloride with lower solubility is formed first. After the chloride ions are completely precipitated, the excess silver ions react with the indicator potassium chromate to form a brick-red precipitate of silver chromate, indicating that the endpoint has been reached.

[0012] (7) Content calculation: Based on the amount of silver nitrate standard solution consumed in the neutralization and titration in step (6), calculate the chloride ion content in the ammonium sulfate mother liquor.

[0013] Furthermore, the alkalinity regulator is at least one of sodium hydroxide and potassium hydroxide.

[0014] Furthermore, in step (4) of the ammonia removal process, the method for determining whether ammonia has been fully removed is as follows: place a wide-range pH test paper moistened with pure water at the steam outlet of the boiling solution. If the color of the wide-range pH test paper no longer changes, it indicates that the ammonia has been fully removed. The principle is that the moistened pH test paper turns blue (green) when it comes into contact with alkaline ammonia. When the test paper does not change color, it indicates that the ammonia has been basically removed.

[0015] Furthermore, in step (5) of volume adjustment and filtration, the ratio of the total mass after volume adjustment to the initial ammonium sulfate mother liquor sample mass is (2:1) to (5:1).

[0016] Furthermore, in step (6) during neutralization and titration, the acid-base indicator is phenolphthalein indicator, and the amount of acid-base indicator added is 0.01-0.1% of the total volume of the clarified filtrate to be titrated.

[0017] Furthermore, in step (6) during neutralization and titration, the acid standard solution is a sulfuric acid standard solution with a concentration of 0.05-0.2 mol / L.

[0018] Furthermore, in step (6) and during titration, the argentometric indicator is a potassium chromate indicator, and the amount of the argentometric indicator added is 0.01-0.1% of the total volume of the filtrate to be titrated.

[0019] Furthermore, in step (6) during neutralization and titration, the concentration range of the silver nitrate standard solution is 0.05-0.2 mol / L.

[0020] Furthermore, in the content calculation of (7), the chloride ion content in the ammonium sulfate mother liquor is calculated. The calculation formula is as follows:

[0021] In the formula: C Ag Concentration of silver nitrate standard solution, unit: mol / L; V Ag : Volume of silver nitrate standard solution consumed in the titration, in mL; M Cl The molar mass of chlorine is 35.45 g / mol. D: Total dilution factor from initial ammonium sulfate mother liquor to clarified filtrate; V s The volume of the clarified filtrate taken in step (6) and titration is in mL.

[0022] Advantages of this invention: This invention eliminates the influence of pyridine-based organic matter and high concentrations of ammonia in ammonium sulfate mother liquor on titration determination by first removing pyridine in a layered manner under weakly alkaline conditions, followed by conversion and ammonia removal under strongly alkaline conditions. Stepwise alkali addition avoids the risk of splashing caused by violent reactions of organic matter or the instantaneous release of large amounts of ammonia in a strongly alkaline environment. The endpoint of ammonia removal is determined using moistened pH test paper. The method is simple, intuitive, low-cost, and easily implemented in production site laboratories.

[0023] This method creatively solves the long-standing problem of accurate chloride ion detection in ammonium sulfate mother liquor in the waste acid-to-ammonium sulfate industry. By providing accurate chloride ion data, it can help production departments optimize their mother liquor discharge strategies, minimizing the loss of ammonium sulfate products and high-value chemicals while ensuring equipment safety. It is a specialized detection solution with excellent targeting, accuracy, and reliability, and has significant industrial application value. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 A method for detecting chloride ions in ammonium sulfate mother liquor during the production of ammonium sulfate from waste acid, the specific steps of which are as follows: (1) Sampling and dilution: Accurately weigh 100.0g of the initial ammonium sulfate mother liquor sample into a 1000mL beaker using an electronic balance, add 100.0g of pure water for dilution and stir evenly.

[0026] (2) Preliminary alkalization and separation: Slowly add about 15g of solid sodium hydroxide to the diluted mother liquor while stirring, and finally monitor the pH with a pH meter until it reaches 7.5. At this time, the solution separates into layers, and a black oily substance (pyridine and other organic matter) precipitates in the upper layer. Transfer the entire mixture to a 500mL separatory funnel, let it stand for 10 minutes, and after it has fully separated into layers, carefully separate and retain the lower aqueous phase in the original beaker.

[0027] (3) Deep alkalization and secondary separation: Add about 10g of solid sodium hydroxide to the lower aqueous phase, stir until completely dissolved, and adjust the pH to 13.0. Observe that there is still a small amount of oily substance in the upper layer of the solution. Transfer the solution to the separatory funnel again, let it stand for 10 minutes, and then separate, retaining the lower clear liquid.

[0028] (4) Ammonia removal treatment: Place the lower clear liquid on a hot plate and heat to boiling. Suspend a wide-range pH test paper moistened with pure water above the mouth of the beaker where the steam is present. The test paper will quickly turn blue in the initial stage of heating. Continue heating for about 50 minutes, during which time pure water can be added as needed to maintain the total volume of the solution. When the color of the moistened pH test paper no longer changes (remaining as pale yellow when moistened), it is determined that the ammonia has been fully removed, and heating should be stopped.

[0029] (5) Volume adjustment and filtration: After the ammonia is removed, the solution is cooled to room temperature and weighed. Add pure water to the beaker to make the total mass of the solution accurately reach 300.0g (the ratio of the total mass after volume adjustment to the initial sample mass is 3:1). Filter with medium-speed quantitative filter paper, discard the initial filtrate, and collect the clear filtrate in a dry container for later use.

[0030] (6) Neutralization and Titration: Accurately pipette 10.00 mL of the clear filtrate into a 250 mL Erlenmeyer flask and dilute with approximately 50 mL of pure water. Add 1 drop of phenolphthalein indicator (approximately 0.02% of the total volume of the solution to be titrated), and the solution turns pink. Titrate with a 0.1 mol / L sulfuric acid standard solution, shaking continuously until the pink color just fades and remains unchanged for 30 seconds (at which point the pH of the solution is 8.3). Add 3 drops of potassium chromate indicator (approximately 0.05% of the total volume of the solution to be titrated) to the neutral solution, and the solution turns pale yellow. Finally, titrate with a 0.1 mol / L silver nitrate standard solution, shaking vigorously continuously until a stable pale brick-red precipitate appears. Record the volume V of silver nitrate standard solution consumed. Ag It is 12.45 mL.

[0031] (7) Content calculation: Total dilution factor D = Total mass after volume adjustment / Initial sample mass = 300.0g / 100.0g = 3; Titration sampling volume V s =10.00mL; Concentration C of silver nitrate standard solution Ag =0.1000mol / L; Consumption volume V Ag =12.45mL; Molar mass M of chlorine Cl =35.45 g / mol; Substitute into the calculation formula:

[0032] The calculated chloride ion content in the ammonium sulfate mother liquor sample was 13248 mg / L.

[0033] Comparative Example 1 The difference between this comparative example and Example 1 is that after sampling and dilution in step (1), sufficient solid sodium hydroxide is added directly at once to adjust the pH of the solution to 13.0, and then the ammonia removal treatment in step (4) is carried out directly. That is, the preliminary alkalization and separation in step (2) and the deep alkalization and secondary separation in step (3) are cancelled, and the remaining steps are the same as in Example 1.

[0034] Observations revealed that the solution became abnormally turbid and viscous under strongly alkaline conditions, with severe emulsification of the organic phase. After standing, the layers separated very slowly and the interfaces were unclear, making it difficult to obtain a clear lower layer. Subsequent titrations resulted in a prolonged endpoint and a lack of sharp color changes.

[0035] Comparative Example 2 Except for step 4, the other steps in this comparative example are the same as in Example 1. In this comparative example, after obtaining the clear liquid through deep alkalization and secondary separation in step (3), step (5) of volume adjustment and filtration is performed directly. During neutralization and titration in step (6), it was found that due to the presence of a large number of ammonium ions in the solution, too much sulfuric acid standard solution was consumed for neutralization, and during subsequent titration with silver nitrate, the brick-red precipitate that appeared at the endpoint quickly faded or was difficult to observe, resulting in poor repeatability.

[0036] Experimental Example To verify the inventiveness, accuracy, and anti-interference ability of the method of this invention, the following comparative experiment was designed: Samples of ammonium sulfate mother liquor from the same batch, with a known chloride ion content (calibrated to 13000±200 mg / L by ion chromatography), were subjected to six parallel determinations using Example 1, Comparative Example 1, and Comparative Example 2, respectively, and spiked recovery experiments were conducted. The results are shown in Table 1: Table 1 Comparative Experimental Results

[0037] Furthermore, to simulate the extreme case of higher concentrations of pyridine groups in actual production, a certain amount of pyridine hydrochloride was added to another sample, increasing the pyridine group concentration by approximately 50%, and the test was conducted again. The results are as follows: Example 1: Due to the effective precipitation and separation of most of the newly added pyridine by stepwise alkalization (first pH 7.5), the RSD of the detection results remained within 1.5%, and the recovery rate was normal.

[0038] Comparative Example 1: Due to severe emulsification of the organic phase, effective separation could not be achieved, and the experiment failed.

[0039] Furthermore, long-term stability tests show that when a sample of ammonium sulfate mother liquor with a chloride ion content of approximately 11,000 mg / L was continuously monitored for 30 days using the method of Example 1, the RSD of the daily test results was less than 1.5%, providing stable and reliable data support for the precise control of chloride ion balance in the production process.

[0040] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for detecting chloride ions in a sulphammonium mother liquor in a process for the production of sulphammonium from spent acid, characterized in that, The method comprises the following steps: (1) Sampling and dilution: a certain mass of initial ammonium sulfate mother liquor is taken and diluted with a certain mass of pure water, the mass ratio of the pure water to the ammonium sulfate mother liquor being (0.5:1) to (5:1), to obtain diluted ammonium sulfate mother liquor; (2) Preliminary alkalization and separation: an alkaline adjusting agent is added to the diluted ammonium sulfate mother liquor obtained in the step (1) sampling and dilution, the pH value is adjusted to 7.0-9.0, the solution is separated into layers, and the lower aqueous phase is separated and retained; (3) Deep alkalization and secondary separation: an alkaline adjusting agent is continuously added to the lower aqueous phase obtained after the preliminary alkalization and separation in the step (2), the pH value is adjusted to 12.0-14.0, if an organic phase is generated, the lower clear liquid is separated again and retained; (4) Ammonia removal treatment: the lower clear liquid obtained after the deep alkalization and secondary separation in the step (3) is heated to boiling, and the heating is continued until ammonia gas is sufficiently removed, to obtain an ammonia-removed solution; (5) Volume setting and filtration: the ammonia-removed solution obtained after the ammonia removal treatment in the step (4) is set to a predetermined total mass by adding water, and a clear filtrate is obtained by filtration; (6) Neutralization and titration: part of the clear filtrate obtained in the step (5) volume setting and filtration is taken, an acid-base indicator is first added, and an acid standard solution is used for titration to 6.5-8.5, then a silver amount method indicator is added, and a silver nitrate standard solution is used for titration; (7) Content calculation: according to the consumption amount of the silver nitrate standard solution in the step (6) neutralization and titration, the content of chloride ions in the ammonium sulfate mother liquor is calculated.

2. The method according to claim 1, wherein the method is characterized by, The alkaline adjusting agent is at least one of sodium hydroxide and potassium hydroxide.

3. The method according to claim 1, wherein the method is characterized by, In the step (4) ammonia removal treatment, the judgment method that ammonia gas is sufficiently removed is that a widely used pH test paper wetted with pure water is placed at the steam outlet of the boiling solution, and the color of the widely used pH test paper no longer changes, that is, it is judged that ammonia gas is sufficiently removed.

4. The method according to claim 1, wherein the method is characterized by, In the step (5) volume setting and filtration, the ratio of the total mass after volume setting to the sampling mass of the initial ammonium sulfate mother liquor is (2:1) to (5:1).

5. The method according to claim 1, wherein the method is characterized by, In the step (6) neutralization and titration, the acid-base indicator is a phenolphthalein indicator, and the addition amount of the acid-base indicator is 0.01-0.1% of the total volume of the clear filtrate to be titrated.

6. The method according to claim 1, wherein the method is characterized by, In the step (6) neutralization and titration, the acid standard solution is a sulfuric acid standard solution, and the concentration thereof is 0.05-0.2 mol / L.

7. The method according to claim 1, wherein the method is characterized by, In the step (6) neutralization and titration, the silver amount method indicator is a potassium chromate indicator, and the addition amount of the silver amount method indicator is 0.01-0.1% of the total volume of the filtrate to be titrated.

8. The method according to claim 1, wherein the method is characterized by, In the step (6) neutralization and titration, the concentration of the silver nitrate standard solution is 0.05-0.2 mol / L.

9. The method according to claim 1, wherein the method is characterized by, In the step (7) content calculation, the calculation formula of the content of chloride ions in the ammonium sulfate mother liquor is as follows: In the formula: C Ag : Concentration of silver nitrate standard solution, unit: mol / L; V Ag : Volume of titrated silver nitrate standard solution consumed, units: mL; M Cl : molar mass of chlorine, 35.45 g / mol; D: total dilution multiple of the initial ammonium sulfate mother liquor to the clear filtrate; V s : the volume of the clear filtrate taken in the step (6) and titration, unit: mL.

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