Method for determining the molecular ratio of an aluminum electrolyte based on fluorinated salts

Abstract

The application discloses a method for calculating the molecular ratio of aluminum electrolyte based on fluorinated salt determination, which comprises the following steps: preparing a test sample solution; calibrating an AlCl3 solution; determining; and data processing. The prepared solution is diluted and dried and filtered, so that there is no solid impurity in the solution to interfere with the analysis, the analysis solution is clear and it is easier to accurately determine the titration end point; the end point color is obtained sensitively by using the fluorinated substitution and complexation back titration method, 1 drop of color change is obtained at the near end point, the color difference before and after is obvious, the end point is easy to determine, so that the result error obtained by the complexation back titration method is smaller and closer to the actual value of the sample.

Description

Technical Field

[0001] This invention belongs to the field of aluminum electrolysis technology, specifically relating to a method for calculating the molecular ratio of aluminum electrolytes based on fluoride salt determination. Background Technology

[0002] The molecular ratio of aluminum electrolyte is a key parameter for normal aluminum electrolysis production, significantly impacting electrolysis condition adjustment, material balance calculations, and the quality of electrolytic aluminum. The determination of free aluminum fluoride in the electrolyte is crucial for calculating the molecular ratio. Let f be the percentage of excess AlF3 in the aluminum electrolyte, then the molecular ratio R is:

[0003]

[0004] The current method for determining free aluminum fluoride follows the standard YS / T 739.2-2020, Chemical Analysis Methods for Aluminum Electrolytes, Part 2: Determination of Molecular Ratio. This method first determines the standard reaction volume N between NaF standard solution and AlCl3 solution. Then, 1.0000g of electrolyte is weighed and treated to form a solution. An excess of NaF standard solution is added to the test solution, and the excess NaF is back-titrated with AlCl3 solution. The amount of free aluminum fluoride in the test solution is calculated based on the titration volume n and the standard volume N.

[0005]

[0006] This method has the following drawbacks during use:

[0007] 1. The electrolyte contains insoluble substances such as CaF2 and C slag, which make the prepared solution extremely turbid. In particular, when the C slag content is high, the solution is grayish-black. This will seriously affect the determination of the titration endpoint color in subsequent determinations, thus producing a large error.

[0008] 2. During the determination process, ascorbic acid and o-phenanthroline are added to mask iron. Since the determination requires maintaining a certain temperature range, the addition of a large amount of o-phenanthroline has a great impact on the temperature of the measuring solution, which affects the determination of the titration endpoint.

[0009] 3. The o-phenanthroline added during the determination process will form a red complex with ferrous ions, affecting the determination of the endpoint color;

[0010] 4. The titration endpoint is determined by the complexation color reaction of chromium azurite S with aluminum ions. The complexation ratio is 1:1 or 1:2. The pH buffer is not added to adjust the pH in the method. However, the color reaction of chromium azurite S needs to be carried out within a strict pH range. Otherwise, a single aluminum complex can not be formed, which will affect the determination of the titration endpoint.

[0011] 5. Due to the complexity of the synthesis process of chromium azurite S reagent, the quality of chromium azurite S reagent produced by different manufacturers or different batches varies greatly. The purchase channels are limited, and changing reagents will have a great impact on the colorimetric reaction, and the stability of the determination cannot be guaranteed.

[0012] 6. At lower temperatures, the reaction between AlCl3 and NaF is a stepwise reaction, which is slow. This can lead to drawbacks during titration, such as easy color reversion at the endpoint, slow color transition, and long titration time. Summary of the Invention

[0013] To address the aforementioned shortcomings, this invention provides a method for calculating the molecular ratio of aluminum electrolytes based on fluoride salt determination, which makes it easier to determine the endpoint and provides more accurate results.

[0014] The technical solution adopted in this invention is a method for calculating the molecular ratio of aluminum electrolytes based on fluoride salt determination, comprising the following steps:

[0015] (1) Preparation of the test sample solution

[0016] Weigh Gg of electrolyte sample into an Erlenmeyer flask, add NaOH solution, heat to boiling for 8-10 min, and after the solution cools to room temperature, add NaF standard solution, add phenolphthalein, titrate with HCl until the solution is colorless, then titrate with NaOH until a faint red color appears, add sodium chloride solid, and rinse the flask wall with water. Boil the solution for 12-15 min, then remove and cool to room temperature; transfer the solution to a volumetric flask, filter the solution dry to obtain the test solution;

[0017] (2) AlCl3 solution standardization

[0018] Measure 10 ml of AlCl3 solution into a beaker, add 3-5 glass boiling beads, rinse the beaker walls with water until the solution volume is 80-90 ml, add 1 drop of methyl orange, adjust the solution to a pale yellow color with NaOH + triethanolamine and HCl, add 20 ml of buffer solution, add 10 ml of EDTA standard solution, boil the solution for 3-5 minutes, then remove and cool to room temperature; add 3 drops of xylenol orange, titrate with Zn(NO3)2 until a rose-red endpoint is reached, and record the titration volume V. Zn Then the molar concentration of the AlCl3 solution is C. Al for:

[0019]

[0020] Among them, C E C represents the molar concentration of the EDTA standard solution. ZN The molar concentration of the Zn(NO3)2 standard solution;

[0021] (3) Measurement

[0022] Measure V of the test liquidC Add 5 ml of AlCl3 solution to an Erlenmeyer flask, add 3-5 glass boiling beads, rinse the flask walls with water until the solution volume is 80-90 ml, and heat to boiling for 10-12 min. Add 1 drop of methyl orange, adjust the solution to a pale yellow color with NaOH + triethanolamine and HCl, add 20 ml of buffer solution, and accurately add 5 ml of EDTA standard solution. Boil the solution for 3-5 min, then remove from heat and cool to room temperature. Add 3 drops of xylenol orange, and titrate with Zn(NO3)2 until a rose-red endpoint is reached. Record the titration volume V. Zn0 ;

[0023] (4) Data processing

[0024] 1) Determining the acidity / alkalinity (K) of electrolytes:

[0025]

[0026] If K > 0, the electrolyte is acidic; if K < 0, the electrolyte is alkaline.

[0027] 2) Calculation of free aluminum fluoride and sodium fluoride:

[0028] If K > 0,

[0029] If K < 0,

[0030] Electrolyte molecular ratio R calculation:

[0031] If K > 0,

[0032] If K < 0,

[0033] If K = 0, R = 3.

[0034] Preferably, in step (1), the concentration of NaOH added for the first time is 20 g / L, the mass ratio of the NaOH solution to the electrolyte sample and the sodium chloride solid is 0.8:1:15, and the volume ratio of the NaOH solution to the NaF standard solution is 4:1; the concentration of NaOH used for titration is 10 g / L, and the concentration of phenolphthalein is 0.1%.

[0035] Preferably, the concentration of methyl orange is 0.1%, the concentration of xylenol orange is 0.5%, and the concentration of Zn(NO3)2 is 0.03 mol / L.

[0036] Preferably, in step (3), the volume ratio of the test solution to the AlCl3 solution is 2 to 3:1.

[0037] Preferably, the buffer solution is an acetate-sodium acetate solution with a pH of 5.5-5.7.

[0038] Preferably, the preparation method of NaOH + triethanolamine is as follows: weigh 50g NaOH into a 1L beaker, add 300ml of water to dissolve until clear, cool, add 5ml of triethanolamine, and dilute with water to 500ml.

[0039] Preferably, the HCl solution is obtained by mixing hydrochloric acid with a purity of 12N and water at a mass ratio of 1:1.

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

[0041] 1) After diluting the prepared solution, dry filtration was performed. There were no solid impurities in the solution, and the analytical solution was clear and easy to determine the titration endpoint.

[0042] 2) o-phenanthroline can be omitted to reduce temperature fluctuations in the measuring solution and prevent the formation of a red complex that could affect the endpoint color.

[0043] 3) The use of fluorination substitution and complexation back titration methods provides a sharp endpoint color and makes the endpoint easy to determine;

[0044] 4) Both the fluorination substitution and complexation back titration methods use commercially available conventional reagents, which are widely available and of stable quality;

[0045] 5) The color development reaction is rapid, the complexation is more complete and the color is most sensitive. Near the endpoint, the color changes with just one drop. The color difference before and after is obvious and the endpoint is easy to judge.

[0046] The method used to determine the content of aluminum fluoride and sodium fluoride in aluminum electrolytes and the calculated molecular ratios play a positive role in adjusting electrolysis conditions, calculating material balance, and improving the quality of electrolytic aluminum in actual production. Detailed Implementation

[0047] The present invention will be further explained and illustrated below with reference to embodiments, so as to enable those skilled in the art to better understand it.

[0048] To verify the accuracy of this method, the molecular ratio of electrolyte standard samples provided by Guiyang Aluminum and Magnesium Design Institute was determined using this method and existing methods. Each standard sample was measured in parallel five times using different methods.

[0049] Example 1

[0050] Reagent preparation and standards:

[0051] NaOH + Triethanolamine: Weigh 50g NaOH into a 1L beaker, add 300ml of water to dissolve until clear, cool, add 5ml of triethanolamine, dilute with water to 500ml, mix well, and transfer to a plastic bottle for later use;

[0052] HCl: obtained by mixing hydrochloric acid with a purity of 6N and water in a mass ratio of 1:1;

[0053] Sodium chloride: AR grade;

[0054] AlCl3 solution: 7.4 g / L;

[0055] NaF standard solution: C Na =20g / L;

[0056] EDTA standard solution: C E =0.098Mol / L;

[0057] Zinc nitrate standard solution: C Zn =0.03Mol / L;

[0058] Aluminum electrolyte sample: G = 1.0000g, error not exceeding ±0.0005g;

[0059] Standard sample 1: Molecular ratio R = 3.00.

[0060] A method for calculating the molecular ratio of aluminum electrolytes based on fluoride salt determination includes the following steps:

[0061] (1) Preparation of the test sample solution

[0062] Weigh 1.0000g of electrolyte sample into an Erlenmeyer flask, and record the sample mass as Gg. Add 40ml of NaOH (20g / L) solution, heat to boiling for 10min, and after the solution cools to room temperature, accurately add 10ml of NaF standard solution. Add 1 drop of 0.1% phenolphthalein, titrate with HCl (6N) until the solution is colorless, and then titrate with NaOH (10g / L) until a faint red color appears. Add 15g of sodium chloride solid, and rinse the flask wall with water. Boil the solution for 15min, and then remove it and cool to room temperature. Transfer the solution to a 250ml volumetric flask, filter the solution dry, and obtain the test solution.

[0063] (2) AlCl3 solution standardization

[0064] Measure 10 ml of AlCl3 solution into a beaker, add 3-5 glass boiling beads, rinse the beaker walls with water until the solution volume reaches 80-90 ml, add 1 drop of 1% methyl orange, adjust the solution to a pale yellow color with NaOH + triethanolamine and HCl, add 20 ml of acetate-sodium acetate buffer solution (pH 5.5-5.7), accurately add 10 ml of EDTA standard solution, boil the solution for 3 minutes, then remove and cool to room temperature; add 3 drops of 0.5% xylenol orange, titrate with 0.03 mol / L Zn(NO3)2 until a rose-red endpoint is reached, and record the titration volume V. Zn Then the molar concentration C of the AlCl3 solution is... Al (Mol / L) is:

[0065]

[0066] Among them, C E C represents the molar concentration of the EDTA standard solution. ZN The molar concentration of the Zn(NO3)2 standard solution;

[0067] (3) Measurement

[0068] Accurately measure the test liquid V C Add 10ml (5ml) of AlCl3 solution to an Erlenmeyer flask, add 3-5 glass boiling beads, rinse the flask walls with water until the solution volume reaches 80-90ml, and heat to boiling for 10min. Add 1 drop of 0.1% methyl orange, adjust the solution to a pale yellow color with NaOH + triethanolamine and HCl, add 20ml of pH 5.5-5.7 buffer solution, accurately add 5ml of EDTA standard solution, boil the solution for 3min, and then remove from heat and cool to room temperature. Add 3 drops of 0.5% xylenol orange, and titrate with Zn(NO3)2 until a rose-red endpoint is reached. Record the titration volume V. Zn0 (4) Data processing

[0069] 1) Determining the acidity / alkalinity (K) of electrolytes:

[0070]

[0071] If K > 0, the electrolyte is acidic; if K < 0, the electrolyte is alkaline.

[0072] 2) Calculation of free aluminum fluoride and sodium fluoride:

[0073] If K > 0,

[0074] If K < 0,

[0075] Electrolyte molecular ratio R calculation:

[0076] If K > 0,

[0077] If K < 0,

[0078] If K = 0, R = 3.

[0079] The standard values ​​and test results for the samples are shown in the table below:

[0080] Table 1. Measurement Results

[0081]

[0082] Example 2

[0083] The difference between this embodiment and Embodiment 1 is that:

[0084] The standard sample used had a 2-molecule ratio R = 2.65; when preparing the test solution, 1.0002 g of electrolyte sample was weighed; and 15 ml of the test solution was taken.

[0085] The standard values ​​and test results for the samples are shown in the table below:

[0086] Table 2. Measurement Results

[0087] Example 3

[0088] The difference between this embodiment and Embodiment 1 is that:

[0089] The standard sample used had a 3-molecule ratio R = 2.75; when preparing the test sample solution, 1.0005 g of electrolyte sample was weighed; and 12 ml of the test solution was taken.

[0090] The standard values ​​and test results for the samples are shown in the table below:

[0091] Table 3. Measurement Results

[0092]

[0093] Example 4

[0094] The difference between this embodiment and Embodiment 1 is that:

[0095] The standard sample used had a 4-molecule ratio R = 3.17; when preparing the test sample solution, 0.0095 g of electrolyte sample was weighed; and 10 ml of the test solution was taken.

[0096] The standard values ​​and test results for the samples are shown in the table below:

[0097] Table 4. Measurement Results

[0098]

[0099] The above experimental data shows that:

[0100] 1) Existing methods are prone to interference with color judgment during titration due to the hysteresis of the chromium-based colorimetric reaction and the lack of obvious color difference. This can cause fluctuations in titration volume during parallel determinations, easily leading to over-tipping, resulting in larger measured values ​​and greater numerical fluctuations.

[0101] 2) The standard deviation (SD) of multiple parallel measurements using the existing method is greater than 0.02, indicating that each measured value deviates significantly from the standard value. When the number of measurements is small, numerical accuracy and repeatability cannot be guaranteed. An RSD greater than 0.6 indicates large fluctuations and dispersion in the test values, resulting in poor test precision.

[0102] 3) Using this method, the complexometric back titration in solution results in a rapid color change at the reaction endpoint, with a sharp and easily identifiable color. Parallel measurements are stable, with small fluctuations in the measured values, and are closer to the actual values ​​of the samples (SD < 0.02 indicates small deviations in the measured values, and RSD% < 0.6 indicates high test precision); 4) The standard deviation of multiple parallel measurements using the existing method is SD < 0.02, indicating that each measured value deviates little from the standard value, ensuring numerical accuracy and repeatability even with a small number of measurements. RSD < 0.6 indicates small dispersion in the measured values ​​and high test precision.

[0103] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of protection of the claims.

Claims

1. A method for determining the molar ratio of a fluorinated salt based electrolyte for aluminum electrolysis, characterized by, It comprises the following steps: (1) Preparation of the solution of the sample to be tested The sample of electrolyte Gg is weighed in a conical flask, NaOH solution is added, and heated to boil for 8-10 minutes. After the solution is cooled to room temperature, 10 ml of sodium fluoride standard solution with a concentration of 20 g / L is added, phenolphthalein is added dropwise, and the solution is titrated with HCl until it is colorless, and then with NaOH until it is micro-red. Sodium chloride solid is added, and the cup wall is washed with water. The solution is boiled for 12-15 minutes and then cooled to room temperature. The solution is transferred to a 250 ml volumetric flask, and the solution is dried and filtered to obtain the solution to be tested; (2) Calibration of AlCl3 solution Take 10 ml AlCl3 solution in a beaker, add 3-5 glass boiling beads, rinse the cup wall with water to 80-90 ml of solution volume, add 1 drop of methyl orange, adjust the solution to light yellow with NaOH + triethanolamine and HCl, add 20 ml buffer, add 10 ml EDTA standard solution, boil the solution for 3-5 min, then take it out and cool to room temperature; add 3 drops of dimethyl phenol orange, titrate to rose red as the end point, record the titration volume V Zn The molar concentration C of the AlCl3 solution is: Al ​ Wherein, C E is the molar concentration of the EDTA standard solution, C ZN is the molar concentration of the Zn(NO3)2standard solution; (3) Measurement V of the sample solution C ml, 5ml AlCl3 solution in a beaker, add 3-5 glass boiling beads, rinse the wall of the cup to the solution volume of 80-90ml, heating boiling 10-12min; add 1 drop of methyl orange, NaOH+triethanolamine and HCl adjust the solution to light yellow, add 20ml buffer, accurately add 5ml EDTA standard solution, solution boiling 3-5min after taking off cooling to room temperature; add 3 drops of dimethyl phenol orange, with Zn(NO3)2 titration to rose red as the end point, record the volume V Zn0 ; (4) Data processing 1) K judgment of the acidity and alkalinity of the electrolyte: If K>0, the electrolyte is acidic, and if K<0, the electrolyte is alkaline. 2) Calculation of free aluminum fluoride and sodium fluoride: If K > 0, If K < 0, 3) Calculation of the molecular ratio R of the electrolyte: If K > 0, If K < 0, If K=0, R=3.

2. A method of determining the molar ratio of a fluorinated salt based on an aluminum electrolyte according to claim 1, characterized in that, In step (1), the concentration of the first NaOH added is 20 g / L, the mass ratio of the NaOH solution to the electrolyte sample and sodium chloride solid is 0.8:1:15, and the volume ratio of the NaOH solution to the NaF standard solution is 4:

1. The concentration of NaOH used for titration is 10 g / L, and the concentration of phenolphthalein is 0.1%.

3. A method of determining the molar ratio of fluorinated salts in an aluminum electrolyte according to claim 1, characterized in that, The concentration of methyl orange is 0.1%, the concentration of dimethyl phenol orange is 0.5%, and the concentration of Zn(NO3)2 is 0.03 Mol / L.

4. The method of claim 1, wherein the aluminum electrolyte is based on a fluorinated salt. In step (3), the volume ratio of the solution to be tested to the AlCl3 solution is 2-3:

1.

5. The method of claim 1, wherein the aluminum electrolyte is based on a fluorinated salt. The buffer solution uses acetic acid-sodium acetate with a pH of 5.5-5.

7.

6. The method of claim 1, wherein the aluminum electrolyte is based on a fluorinated salt. The preparation method of NaOH+triethanolamine is as follows: 50 g of NaOH is weighed in a 1 L beaker, dissolved in 300 ml of water to be clear, and after cooling, 5 ml of triethanolamine is added and diluted with water to 500 ml.

7. A method of determining the molecular ratio of a fluorinated salt based on the electrolyte of aluminum according to claim 1, characterized in that, The HCl solution is obtained by mixing 12N hydrochloric acid with water in a mass ratio of 1:1.

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