Method for determining fluorine in cryolite

Abstract

The invention relates to the technical field of chemical component analysis, in particular to a method for measuring fluorine in cryolite, and aims to solve the problems that when the fluorine content in the cryolite is measured by adopting a traditional distillation-thorium nitrate titration method or lead fluoride chloride precipitation-mercuric nitrate volumetric method and the like at present, the operation is time-consuming and labor-consuming, the reagent is relatively high in harmfulness and the detection precision is relatively low. The method comprises the following steps: decomposing a cryolite sample by using a sodium carbonate-sodium silicate mixed solvent to enable fluorine to become water-soluble fluoride, generating a lead fluorochloride precipitate by using hydrochloric acid and lead acetate as precipitants, filtering out the precipitate, dissolving the precipitate in nitric acid, precipitating chloride ions by using a silver nitrate standard solution, and measuring the concentration of the excessive silver nitrate by using ferric sulfate as an indicator. Compared with traditional fluorine content determination, the method has the advantages that the detection difficulty is reduced, the use of highly toxic reagents is effectively avoided, the test safety risk is greatly reduced, and the method is widely applicable to determination of fluorine in cryolite for factory inspection.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical composition analysis, and in particular to a method for determining fluorine in cryolite. Background Art

[0002] Cryolite, a key flux in aluminum electrolysis, plays an irreplaceable role in the aluminum industry. Its stable properties effectively lower the melting point of alumina, significantly improving the efficiency and current efficiency of the electrolysis process, profoundly impacting aluminum production quality and cost control. Among the many properties and components of cryolite, the precise determination of its fluorine content is particularly crucial. Fluorine is not only the core element that enables cryolite to function as a flux, but its content also directly influences its performance, in turn impacting various parameters of aluminum electrolysis. For example, insufficient fluorine content in cryolite reduces its ability to dissolve alumina, resulting in poor melt fluidity in the electrolytic cell and reduced current efficiency. This can also cause cell voltage fluctuations, increasing energy consumption and production costs. Excessive fluorine content, on the other hand, can generate excessive harmful gases during the electrolysis process, polluting the environment and affecting aluminum purity and quality. Currently, the main methods for determining fluorine in cryolite include distillation-thorium nitrate titration, lead fluorochloride precipitation-mercuric nitrate volumetry, and ion-selective electrode methods. The distillation-thorium nitrate titration method has a complicated operation process, a long analysis cycle, and is easily interfered with by other impurity ions. It places extremely high demands on the operator and is difficult to test in large-scale factory production. The lead fluorochloride precipitation-mercuric nitrate volumetric method is suitable for large-scale factory production testing, but mercury nitrate is a highly toxic reagent that is extremely harmful to the human body and the environment, and poses a large safety risk. Although the ion-selective electrode method has the advantage of fast response speed, the electrode selectivity and stability are poor, and the error is large when measuring complex samples. Summary of the Invention

[0003] The present invention provides a method for determining fluorine in cryolite, so as to solve the problems existing in the above background that the conventional distillation-thorium nitrate titration method or lead fluoride chloride precipitation-mercuric nitrate volumetric method for determining the fluorine content in cryolite is time-consuming and labor-intensive, the reagents are highly hazardous, and the detection accuracy is low.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A method for determining fluorine in cryolite comprises the following steps:

[0006] Step 1. Weigh 0.20g-0.30g of dried cryolite sample and place it in a 400mL-500mL beaker. Add 90mL-110mL of boiling water and boil for 18min-22min to ensure the volume is 45mL-55mL.

[0007] Step 2: Add 7g-9g of potassium carbonate to the reagent obtained in step 1, and boil for 8min-12min to clarify the sample.

[0008] Step 3: Add 3g-5g of sodium silicate to the reagent obtained in step 2, shake well, heat to a slight boil, then add 90mL-110mL of boiling water, keep it at a slight boil for 4min-6min, and wash it into a 500mL volumetric flask, cool and dilute to the scale and mix well.

[0009] Step 4: Filter the solution prepared in step 3 into a beaker using a funnel and absorbent cotton, and take 100 mL of the filtrate into a 400 mL-500 mL beaker. Add 75 mL-85 mL of 0.10 mol / L hydrochloric acid solution and 3-4 drops of 1 g / L methyl orange indicator. Heat to 38°C-42°C, and then neutralize with 2 mol / L nitric acid solution until it just turns red, with an excess of 2 mL.

[0010] Step 5: Add 1 mL-2 mL of concentrated glacial acetic acid to the solution obtained in step 4, stir immediately, and add 28 mL-32 mL of lead acetate with a concentration of 100 g / L, stir until a precipitate appears, stir for another 2 min-3 min, cool to room temperature, and stand for 3 h-12 h.

[0011] Step 6: Dry filter the solution obtained in step 5 with a semi-medium-speed quantitative filter paper, wash the beaker 5-8 times with saturated lead chloride fluoride detergent, wash the filter paper 7-10 times, and finally rinse the beaker and filter paper once with 2 mL-3 mL of water. Place the filter paper in the original beaker, add 100 mL of nitric acid solution with a ratio of (5+95), and heat to completely dissolve the precipitate.

[0012] Step 7: Add 20 mL to 25 mL of 0.10 mol / L silver nitrate standard solution to the solution obtained in step 6 using a burette, heat to a slight boil, cool, filter with qualitative filter paper into a 400 mL to 500 mL conical flask, wash with cold water, and maintain the volume at 240 mL to 260 mL.

[0013] Step 8: Add 5 mL to 6 mL of 50 g / L ferric sulfate to the solution obtained in step 7, and then titrate with 0.10 mol / L potassium thiocyanate standard solution until the solution turns orange. Record the volume of potassium thiocyanate standard solution consumed during the titration.

[0014] Step 9: Substitute the volume of potassium thiocyanate standard solution consumed in step 8 into Formula, where is the molar concentration of the silver nitrate standard solution, is the volume of silver nitrate standard solution added, C KCNSis the molar concentration of potassium thiocyanate standard solution, V KCNS is the volume of potassium thiocyanate standard solution consumed during titration, G is the number of grams of sample taken, and 0.019 is the millimolar mass of fluorine, so as to calculate the percentage of fluorine in cryolite.

[0015] Furthermore, the water in step 1 and step 3 is deionized water.

[0016] Furthermore, the potassium carbonate or anhydrous sodium carbonate in step 2 is analytically pure.

[0017] Furthermore, the sodium silicate in step 3 is solid Na2SiO3·9H2O.

[0018] Furthermore, the saturated lead fluoride washing solution in step 6 is as follows: 0.2 g of sodium fluoride is dissolved in 500 mL of water, 100 mL of 0.1 mol / L sodium chloride solution is added, 2 mL of 2 mol / L nitric acid and 1 mL of glacial acetic acid are added, the solution is heated to 40° C., 30 mL of 100 g / L lead acetate solution is slowly added under constant stirring, and the mixture is stirred until a large amount of precipitate is precipitated, then allowed to stand for 1-2 hours, filtered with slow filter paper, washed with hot water 6-7 times, and the precipitate is transferred to a 5000 mL narrow-necked bottle, 2000 mL of water is added, and the mixture is vigorously shaken for 15 minutes, and then allowed to stand for more than 12 hours to saturate it. The mixture is vigorously shaken once before use, and after the precipitate is clarified, it is filtered with medium-speed quantitative filter paper before use.

[0019] The present invention has the following beneficial effects:

[0020] The present invention provides a method for determining fluorine in cryolite. The method comprises the following steps: decomposing a cryolite sample by using a sodium carbonate-sodium silicate mixed solvent to convert fluorine into a water-soluble fluoride; then using hydrochloric acid and lead acetate as precipitants to generate a lead fluoride chloride precipitate; filtering the precipitate and dissolving it in nitric acid; precipitating chloride ions with a silver nitrate standard solution; back-titrating the excess silver nitrate with a potassium thiocyanate standard solution using ferric sulfate as an indicator; and finally calculating the fluorine content based on the consumption of the standard solution. Compared with traditional fluorine content determination, the method reduces the difficulty of detection, effectively avoids the use of highly toxic reagents, greatly reduces the safety risk of the test, and is widely applicable to the determination of fluorine in cryolite for factory inspection. The method solves the problems of time-consuming and labor-intensive operation, greater reagent hazards, and low detection accuracy in the current determination of fluorine content in cryolite by using a traditional distillation-thorium nitrate titration method or a lead fluoride chloride precipitation-mercuric nitrate volumetric method. DETAILED DESCRIPTION

[0021] The present invention will be further described below with reference to specific embodiments.

[0022] Example 1

[0023] A method for determining fluorine in cryolite comprises the following steps:

[0024] Step 1: Weigh 0.20 g of dried cryolite sample and place it in a 400 mL beaker. Add 90 mL of boiling water and boil for 18 minutes to ensure that the volume is 45 mL.

[0025] Step 2: Add 7 g of potassium carbonate to the reagent obtained in step 1 and boil for 8 minutes to clarify the sample.

[0026] Step 3: Add 3 g of sodium silicate to the reagent obtained in step 2, shake well, heat to a slight boil, add 90 mL of boiling water, keep it at a slight boil for 4 minutes, and wash it into a 500 mL volumetric flask, cool and dilute to the scale and mix well.

[0027] Step 4: Filter the solution prepared in step 3 into a beaker using a funnel and absorbent cotton, and take 100 mL of the filtrate into a 400 mL beaker. Add 75 mL of 0.10 mol / L hydrochloric acid solution and 3 drops of 1 g / L methyl orange indicator. Heat to 38°C, and then neutralize with 2 mol / L nitric acid solution until it just turns red, with an excess of 2 mL.

[0028] Step 5: Add 1 mL of concentrated glacial acetic acid to the solution obtained in step 4, stir immediately, and add 28 mL of lead acetate with a concentration of 100 g / L, stir until a precipitate appears, stir for another 2 minutes, cool to room temperature, and let stand for 3 hours.

[0029] Step 6: Dry filter the solution obtained in step 5 with a semi-medium-speed quantitative filter paper, wash the beaker 5 times with saturated lead fluoride washing solution, wash the filter paper 7 times, and finally rinse the beaker and filter paper once with 2 mL of water. Place the filter paper in the original beaker, add 100 mL of nitric acid solution with a ratio of (5+95), and heat to completely dissolve the precipitate.

[0030] Step 7: Add 20 mL of 0.10 mol / L silver nitrate standard solution to the solution obtained in step 6 using a burette, heat to a slight boil, cool, filter with qualitative filter paper into a 400 mL conical flask, wash with cold water, and keep the volume at 240 mL.

[0031] Step 8: Add 5 mL of 50 g / L ferric sulfate to the solution obtained in step 7, and then titrate with 0.10 mol / L potassium thiocyanate standard solution until the solution turns orange. Record the volume of potassium thiocyanate standard solution consumed during the titration.

[0032] Step 9: Substitute the volume of potassium thiocyanate standard solution consumed in step 8 into The formula is used to calculate the percentage of fluorine in cryolite.

[0033] Example 2

[0034] A method for determining fluorine in cryolite comprises the following steps:

[0035] Step 1: Weigh 0.25 g of dried cryolite sample and place it in a 450 mL beaker. Add 100 mL of boiling water and boil for 20 minutes to ensure that the volume is 50 mL.

[0036] Step 2: Add 8 g of potassium carbonate to the reagent obtained in step 1 and boil for 10 minutes to clarify the sample.

[0037] Step 3: Add 4 g of sodium silicate to the reagent obtained in step 2, shake well, heat to a slight boil, add 100 mL of boiling water, keep it at a slight boil for 5 minutes, and wash it into a 500 mL volumetric flask, cool and dilute to the scale and mix well.

[0038] Step 4: Filter the solution prepared in step 3 into a beaker using a funnel and absorbent cotton, and take 100 mL of the filtrate into a 450 mL beaker. Add 80 mL of 0.10 mol / L hydrochloric acid solution and 3 drops of 1 g / L methyl orange indicator. Heat to 40°C, and then neutralize with 2 mol / L nitric acid solution until it just turns red, with an excess of 2 mL.

[0039] Step 5: Add 1.5 mL of concentrated glacial acetic acid to the solution obtained in step 4, stir immediately, and add 30 mL of lead acetate with a concentration of 100 g / L, stir until a precipitate appears, stir for another 2.5 minutes, cool to room temperature, and let stand for 7 hours.

[0040] Step 6: Dry filter the solution obtained in step 5 with a semi-medium-speed quantitative filter paper, wash the beaker 5 times with saturated lead fluoride washing solution, wash the filter paper 7 times, and finally rinse the beaker and filter paper once with 2.5 mL of water. Place the filter paper in the original beaker, add 100 mL of nitric acid solution with a ratio of (5+95), and heat to completely dissolve the precipitate.

[0041] Step 7: Add 23 mL of 0.10 mol / L silver nitrate standard solution to the solution obtained in step 6 using a burette, heat to a slight boil, cool, filter with qualitative filter paper into a 450 mL conical flask, wash with cold water, and keep the volume at 250 mL.

[0042] Step 8: Add 5.5 mL of 50 g / L ferric sulfate to the solution obtained in step 7, and then titrate with 0.10 mol / L potassium thiocyanate standard solution until the solution turns orange. Record the volume of potassium thiocyanate standard solution consumed during the titration.

[0043] Step 9: Substitute the volume of potassium thiocyanate standard solution consumed in step 8 into The formula is used to calculate the percentage of fluorine in cryolite.

[0044] Example 3

[0045] A method for determining fluorine in cryolite comprises the following steps:

[0046] Step 1: Weigh 0.30 g of dried cryolite sample and place it in a 500 mL beaker. Add 110 mL of boiling water and boil for 22 minutes to ensure that the volume is 55 mL.

[0047] Step 2: Add 9 g of potassium carbonate to the reagent obtained in step 1 and boil for 12 minutes to clarify the sample.

[0048] Step 3: Add 5 g of sodium silicate to the reagent obtained in step 2, shake well, heat to a slight boil, add 110 mL of boiling water, keep it at a slight boil for 6 minutes, and wash it into a 500 mL volumetric flask, cool and dilute to the scale and mix well.

[0049] Step 4: Filter the solution prepared in step 3 into a beaker using a funnel and absorbent cotton, and take 100 mL of the filtrate into a 500 mL beaker. Add 85 mL of 0.10 mol / L hydrochloric acid solution and 4 drops of 1 g / L methyl orange indicator. Heat to -42°C, and then neutralize with 2 mol / L nitric acid solution until it just turns red, with an excess of 2 mL.

[0050] Step 5: Add 2 mL of concentrated glacial acetic acid to the solution obtained in step 4, stir immediately, and add 32 mL of lead acetate with a concentration of 100 g / L, stir until a precipitate appears, stir for another 3 minutes, cool to room temperature, and let stand for 12 hours.

[0051] Step 6: Dry filter the solution obtained in step 5 with a semi-medium-speed quantitative filter paper, wash the beaker 8 times with saturated lead fluoride washing solution, wash the filter paper 10 times, and finally rinse the beaker and filter paper once with 3 mL of water. Place the filter paper in the original beaker, add 100 mL of nitric acid solution with a ratio of (5+95), and heat to completely dissolve the precipitate.

[0052] Step 7: Add 25 mL of 0.10 mol / L silver nitrate standard solution to the solution obtained in step 6 using a burette, heat to a slight boil, cool, filter with qualitative filter paper into a 500 mL conical flask, wash with cold water, and keep the volume at 260 mL.

[0053] Step 8: Add 6 mL of 50 g / L ferric sulfate to the solution obtained in step 7, and then titrate with 0.10 mol / L potassium thiocyanate standard solution until the solution turns orange. Record the volume of potassium thiocyanate standard solution consumed during the titration.

[0054] Step 9: Substitute the volume of potassium thiocyanate standard solution consumed in step 8 into The formula is used to calculate the percentage of fluorine in cryolite.

[0055] Then conduct the following tests respectively:

[0056] (1) Repeatability test

[0057] The same cryolite was crushed, and 11 samples were randomly selected from the processed samples and measured according to the above method. The fluorine content measurement results and relative standard deviation (RSD) are shown in Table 1:

[0058] Table 1 Repeatability test results

[0059]

[0060] As shown in Table 1, the standard deviation of the fluorine content determination results in cryolite is 0.17, and the relative standard deviation (RSD) is 0.33%, which meets the requirement of RSD ≤ 2% for parallel samples in routine laboratory tests, indicating that the determination results of this method are accurate, reliable and highly reproducible.

[0061] (2) Accuracy test

[0062] Five prepared cryolite samples were randomly selected and measured according to the above method. The measured values were compared with the measured values provided by the raw material factory. The comparison results are shown in Table 2:

[0063] Table 2 Comparison of the measured values of this method and the measured values provided by the raw material factory

[0064] Sample number This method determines the fluorine content value / % Fluorine content measured by the raw material factory / % Difference / % 12 52.14 52.35 0.21 13 52.41 52.30 0.11 14 52.28 52.46 0.18 15 52.40 52.52 0.12 16 52.54 52.27 0.27

[0065] As shown in Table 2, the results measured by this method are within 0.5% of the values provided by the raw material manufacturer, indicating that the measurement results of this method are highly accurate and meet the test requirements.

[0066] Weigh 5 portions of cryolite standard material (GFC04) and measure them according to the above method. Compare the measured values with the standard values of cryolite standard material (GFC04). The comparison results are shown in Table 3:

[0067] Table 3 Comparison results between the measured values of this method and the standard values

[0068]

[0069] As shown in Table 3, the results measured by this method are consistent with the standard value of cryolite standard material (GFC04), indicating that the measurement results of this method are highly accurate and meet the test requirements.

[0070] (3) Comparative test

[0071] Five cryolite samples were randomly selected and measured using the method of the present invention and the distillation-thorium nitrate titration method, respectively, and then compared. The comparison results are shown in Table 4:

[0072] Table 4 Comparison of the results of this method and distillation-thorium nitrate titration method

[0073]

[0074] As shown in Table 4, the results measured by this method are within 0.5% of the values measured by the distillation-thorium nitrate titration method, indicating that the measurement results of this method are highly accurate and meet the test requirements. In addition, this method is simpler to operate than the distillation-thorium nitrate titration method, requires lower operator skills, and is suitable for factory testing.

Claims

1. A method for determining fluorine in cryolite, characterized in that: The following steps are involved: Step 1: Weigh 0.20g-0.30g of dried cryolite sample and place it in a 400mL-500mL beaker. Add 90mL-110mL of boiling water and boil for 18min-22min to ensure the volume is 45mL-55mL. Step 2: Add 7g-9g of potassium carbonate to the reagent obtained in step 1 and boil for 8min-12min to clarify the sample; Step 3: Add 3g-5g of sodium silicate to the reagent obtained in step 2, shake well, heat to a slight boil, then add 90mL-110mL of boiling water, keep it at a slight boil for 4min-6min, and wash it into a 500mL volumetric flask, cool and dilute to the scale and mix well; Step 4: Filter the solution prepared in step 3 into a beaker using a funnel and absorbent cotton, and take 100 mL of the filtrate into a 400 mL-500 mL beaker. Add 75 mL-85 mL of 0.10 mol / L hydrochloric acid solution and 3-4 drops of 1 g / L methyl orange indicator. Heat to 38°C-42°C, and then neutralize with 2 mol / L nitric acid solution until it turns red, with an excess of 2 mL. Step 5: Add 1 mL to 2 mL of concentrated glacial acetic acid to the solution obtained in step 4, stir immediately, and add 28 mL to 32 mL of lead acetate with a concentration of 100 g / L, stir until a precipitate appears, stir for another 2 min to 3 min, cool to room temperature, and let stand for 3 h to 12 h; Step 6: Dry filter the solution obtained in step 5 with a semi-medium-speed quantitative filter paper, wash the beaker 5-8 times with a saturated lead chloride fluoride detergent, wash the filter paper 7-10 times, and finally rinse the beaker and filter paper once with 2 mL-3 mL of water. Place the filter paper in the original beaker, add 100 mL of nitric acid solution with a ratio of (5+95), and heat to completely dissolve the precipitate; Step 7: Add 20 mL to 25 mL of 0.10 mol / L silver nitrate standard solution to the solution obtained in step 6 using a burette, heat to a slight boil, cool, filter with qualitative filter paper into a 400 mL to 500 mL conical flask, wash with cold water, and maintain the volume at 240 mL to 260 mL. Step 8: Add 5 mL to 6 mL of 50 g / L ferric sulfate to the solution obtained in step 7, and then titrate with 0.10 mol / L potassium thiocyanate standard solution until the solution turns orange. Record the volume of potassium thiocyanate standard solution consumed during the titration. Step 9: Substitute the volume of potassium thiocyanate standard solution consumed in step 8 into Formula, where is the molar concentration of the silver nitrate standard solution, is the volume of silver nitrate standard solution added, C KCNS is the molar concentration of potassium thiocyanate standard solution, V KCNS is the volume of potassium thiocyanate standard solution consumed during titration, G is the number of grams of sample taken, and 0.019 is the millimolar mass of fluorine, so as to calculate the percentage of fluorine in cryolite.

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