Method for measuring content of calcium fluoride in lithium iron phosphate hydrometallurgical waste residues

Abstract

The present disclosure relates to the technical field of analytical chemistry, and in particular to a method for measuring the content of calcium fluoride in lithium iron phosphate hydrometallurgical waste residues. The method comprises: using citric acid to treat lithium iron phosphate hydrometallurgical waste residues to dissolve iron and aluminum in the waste residues, using the reducibility of the citric acid to reduce Fe3+ into Fe2+ to avoid complexation with fluoride ions, and due to calcium fluoride being insoluble in the citric acid, separating iron and aluminum from the lithium iron phosphate hydrometallurgical waste residues by means of a solid-liquid separation method; and using inorganic strong acid to dissolve the solid material obtained after solid-liquid separation to obtain a solution under test, using a fluorine ion selection electrode to accurately measure the content of fluorine in said solution, and then calculating the content of calcium fluoride in the lithium iron phosphate hydrometallurgical waste residues. The measurement method provided by the present disclosure does not require the addition of iron and aluminum masking agents such as triethanolamine, has the advantages of involving simple and convenient operation and low costs, and can be used for quickly and accurately measuring the content of calcium fluoride in lithium iron phosphate hydrometallurgical waste residues on a large scale.

Description

A method for detecting calcium fluoride content in lithium iron phosphate hydrometallurgical waste slag Technical Field

[0001] The present disclosure belongs to the technical field of analytical chemistry, and in particular relates to a method for detecting the content of calcium fluoride in lithium iron phosphate wet smelting waste slag. Background Art

[0002] The wet recycling of discarded lithium iron phosphate batteries produces a large amount of high-fluorine sodium sulfate acidic wastewater containing iron and aluminum, which is not only harmful to human health but also has persistent environmental impacts. Chemical precipitation is typically used to remove elements such as iron, aluminum, and fluorine from the wastewater by adding calcium hydroxide. The reaction produces waste residue containing calcium fluoride, iron hydroxide, aluminum hydroxide, calcium sulfate, and calcium hydroxide.

[0003] Calcium fluoride, primarily derived from fluorspar, is widely used in the metallurgical, building materials, and chemical industries, serving as the primary source of fluorine for fluorine chemical companies. However, overexploitation of fluorspar ore has severely impacted its supply and sustainable development. Calcium fluoride is present in the waste residue from wet process wastewater treatment, and after purification, it can be used as a fluorspar alternative. Therefore, effective determination of the calcium fluoride content in lithium iron phosphate wet smelting waste residue is crucial for its resource utilization.

[0004] The ion selective electrode method has attracted widespread attention because it can quickly and accurately determine the fluorine content. However, when iron and aluminum are present, they will form complexes with fluorine, reducing the free fluorine in the sample and causing inaccurate test results. The dissolution of solid samples currently used for fluoride analysis includes strong acid digestion or fusion with various fluxes, such as sodium hydroxide, sodium peroxide, sodium carbonate, etc. The fusion step is usually followed by dissolution with water or dilute acid. All of the above methods will lead to the dissolution of aluminum and iron. Since the waste slag from the wet process of lithium iron phosphate smelting contains iron hydroxide and aluminum hydroxide and has a high iron and aluminum content, the direct use of strong acid or alkali melting-strong acid dissolution will result in the Fe in the slag. 3+ and Al 3+ The presence of fluorine in the solution has a great influence on the fluorine content tested by the ion selective electrode method.

[0005] Therefore, the currently commonly used testing methods cannot accurately test the calcium fluoride content in lithium iron phosphate wet smelting waste slag.

[0006] In view of this, the present disclosure is proposed.

[0007] Summary of the Invention

[0008] The purpose of the present disclosure includes providing a method for detecting the calcium fluoride content in lithium iron phosphate wet smelting waste slag, aiming to accurately detect the calcium fluoride content in lithium iron phosphate wet smelting waste slag.

[0009] In order to achieve the above-mentioned purpose of the present disclosure, the following technical solutions can be adopted:

[0010] The solution provided by the present disclosure includes a method for detecting the calcium fluoride content in lithium iron phosphate wet smelting waste slag, comprising: mixing the lithium iron phosphate wet smelting waste slag and a citric acid solution for reaction, then performing solid-liquid separation to obtain a solid material, mixing and dissolving the solid material with an inorganic strong acid solution to obtain a test liquid, and using a fluoride ion selective electrode to test the fluorine content in the test liquid to calculate the calcium fluoride content in the lithium iron phosphate wet smelting waste slag.

[0011] In some embodiments of the present disclosure, the reaction temperature of lithium iron phosphate wet smelting waste slag and citric acid solution is 50° C.-80° C., and the reaction time is more than 15 minutes.

[0012] In some embodiments of the present disclosure, the reaction temperature of lithium iron phosphate wet smelting waste slag and citric acid solution is 55° C.-65° C., and the reaction time is 25 min-40 min.

[0013] In some embodiments of the present disclosure, the concentration of the citric acid solution is 180 g / L-250 g / L, and the amount of the citric acid solution used per gram of lithium iron phosphate wet smelting waste residue is 8 mL-30 mL.

[0014] In some embodiments of the present disclosure, the concentration of the citric acid solution is 200 g / L-220 g / L, and the amount of the citric acid solution used per gram of lithium iron phosphate wet smelting waste residue is 10 mL-20 mL.

[0015] In some embodiments of the present disclosure, the lithium iron phosphate hydrometallurgical waste slag contains calcium fluoride, ferric hydroxide, aluminum hydroxide, calcium sulfate and calcium hydroxide.

[0016] In some embodiments of the present disclosure, after the lithium iron phosphate wet smelting waste residue and the citric acid solution are mixed and reacted, solid-liquid separation is performed by suction filtration, and the precipitate is collected to obtain a solid material.

[0017] In some embodiments of the present disclosure, the inorganic strong acid solution is a hydrochloric acid solution, and the concentration of the hydrochloric acid solution is 6 mol / L-12 mol / L.

[0018] In some embodiments of the present disclosure, after the solid material is mixed and dissolved with the hydrochloric acid solution, the volume is constant to obtain a test solution, and the concentration of the solid material in the test solution is 0.1 g / L-1 g / L.

[0019] In some embodiments of the present disclosure, the process of testing using a fluoride ion selective electrode includes: first using a fluoride ion selective electrode and a reference electrode to test the potential values ​​of fluoride ion solutions of different concentrations, and obtaining a standard curve using the logarithmic concentration of fluoride ions and the corresponding potential values; sampling the test solution and configuring it into a sample solution, taking a portion of the sample solution for testing using a fluoride ion selective electrode and a reference electrode, and using the obtained potential value to calculate the logarithmic concentration of fluoride ions in the sample solution according to the standard curve, obtaining the fluoride ion content c in the sample solution after calculation, and calculating the calcium fluoride concentration in the lithium iron phosphate wet smelting waste slag based on the fluoride ion content c.

[0020] In some embodiments of the present disclosure, the process of obtaining a standard curve includes: taking different volumes of fluoride ion standard solution and mixing with a buffer solution, fixing the volume to obtain fluoride ion solutions of different concentrations, using a fluoride ion selective electrode and a reference electrode to test the fluoride ion solutions of different concentrations, respectively, and measuring the potential value at equilibrium under stirring; based on the linear relationship between the potential value and the logarithmic concentration of fluoride ions, a standard curve is drawn with the potential value as the ordinate and the corresponding logarithmic concentration of fluoride ions as the abscissa.

[0021] In some embodiments of the present disclosure, the process of drawing a standard curve with the potential value as the ordinate and the corresponding logarithmic concentration of fluoride ions as the abscissa includes: according to the linear regression equation, y=kx+b, then x=(yb) / k; where y is the potential value, x is the logarithmic concentration value of fluoride ions, k is the electrode slope, and b is the intercept;

[0022] Substitute the potential values ​​corresponding to the logarithmic concentration values ​​of at least two groups of fluoride ions into the expression for x to obtain the values ​​of k and b.

[0023] In some embodiments of the present disclosure, the buffer solution is a buffer solution system with a pH value of 5-6, and the concentration range of the prepared fluoride ion solution is 0.1 mg / L-10 mg / L.

[0024] In some embodiments of the present disclosure, the buffer solution is a citric acid-sodium citrate solution, and the mass ratio of citric acid to sodium citrate used in preparing the buffer solution is 1:10-13.

[0025] In some embodiments of the present disclosure, when preparing the fluoride ion solution, the ratio of the volume of the added buffer solution to the volume after constant volume is (30-50):100.

[0026] In some embodiments of the present disclosure, the process of preparing the sample solution includes: taking 3 mL-5 mL of the test solution, adjusting the pH value to 6.8-7.2, mixing with a buffer solution, and fixing the volume; the ratio of the volume of the added buffer solution to the volume after fixing the volume is (30-50):100.

[0027] The lithium iron phosphate wet smelting waste slag is first treated with citric acid to dissolve the iron and aluminum in the waste slag, and the reducing property of citric acid is used to dissolve the Fe 3+ Reduction to Fe 2+ To avoid complexation with fluoride ions, and because calcium fluoride is insoluble in citric acid, iron and aluminum can be separated from lithium iron phosphate hydrometallurgical waste residues through solid-liquid separation. The solid material obtained after solid-liquid separation is dissolved in an inorganic strong acid solution to obtain a test solution. The fluorine content in the test solution can be accurately measured using a fluoride ion selective electrode, and the calcium fluoride content in the lithium iron phosphate hydrometallurgical waste residue can be calculated. The detection method provided by the present disclosure does not require the addition of iron and aluminum masking agents such as triethanolamine, has the advantages of simple operation and low cost, and can quickly and accurately test the calcium fluoride content in lithium iron phosphate hydrometallurgical waste residues in large quantities. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0029] FIG1 is a flow chart of detecting the calcium fluoride content in lithium iron phosphate hydrometallurgical waste slag provided by the present disclosure;

[0030] Figure 2 is an electron microscope image of waste residue from lithium iron phosphate hydrometallurgy;

[0031] Figure 3 is a standard curve diagram. DETAILED DESCRIPTION

[0032] The embodiments of the present disclosure will be described in detail below with reference to the examples. However, those skilled in the art will appreciate that the following examples are intended only to illustrate the present disclosure and should not be construed as limiting the scope of the present disclosure. Where specific conditions are not specified in the examples, the experiments were performed under conventional conditions or the conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, all are commercially available conventional products.

[0033] The endpoints of the ranges and any values ​​disclosed in this disclosure are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0034] The present disclosure provides a method for detecting the calcium fluoride content in lithium iron phosphate hydrometallurgical waste slag. By optimizing the detection process and introducing a pre-treatment step, the detection accuracy can be significantly improved. As shown in Figure 1, the method includes the following steps:

[0035] S1. Pre-treatment

[0036] Waste residue from the wet process of lithium iron phosphate smelting is mixed with a citric acid solution for reaction, followed by solid-liquid separation to obtain a solid material. Citric acid can dissolve the ferric hydroxide and aluminum hydroxide in the slag under heating conditions. Simultaneously, the reducing properties of citric acid reduce trivalent iron to divalent iron, thereby preventing complexation with fluoride ions. At this time, the calcium fluoride in the slag is insoluble in weak acids. Subsequently, solid-liquid separation is used to separate the calcium fluoride and the complex formed between the iron and aluminum in the slag and the citric acid, effectively preventing the iron and aluminum from affecting subsequent testing.

[0037] Specifically, the lithium iron phosphate wet smelting waste slag comes from the wet process of discarded lithium iron phosphate batteries. Its production process generally includes the following steps: discarded lithium iron phosphate batteries and concentrated sulfuric acid are mixed, and sodium carbonate is added to adjust the pH value to generate sodium sulfate, copper, iron, aluminum and fluoride, etc., and produce iron-aluminum-containing fluoride-containing sodium sulfate acidic wastewater; calcium hydroxide is added to the above-mentioned fluoride-containing sodium sulfate acidic wastewater, and the iron, aluminum, fluorine and other elements in the wastewater are removed by chemical precipitation. During the reaction process, waste slag containing calcium fluoride, iron hydroxide, aluminum hydroxide, calcium sulfate, calcium hydroxide, etc. will be obtained.

[0038] In some embodiments of the present disclosure, the reaction temperature of lithium iron phosphate hydrometallurgical waste slag and citric acid solution is 50°C-80°C, and the reaction time is at least 15 minutes; preferably, the reaction temperature is 55°C-65°C, and the reaction time is 25 minutes-40 minutes. By controlling the reaction temperature and reaction time, the iron and aluminum in the waste slag are fully dissolved.

[0039] Specifically, the reaction temperature of lithium iron phosphate wet smelting waste slag and citric acid solution can be 50°C, 60°C, 70°C, 80°C, etc., and the reaction time can be 15min, 20min, 25min, 30min, 35min, 40min, 50min, etc.

[0040] In some embodiments of the present disclosure, the concentration of the citric acid solution is 180g / L-250g / L, and the amount of the citric acid solution used per gram of lithium iron phosphate wet smelting waste residue is 8mL-30mL. Preferably, the concentration of the citric acid solution is 200g / L-220g / L, and the amount of the citric acid solution used per gram of lithium iron phosphate wet smelting waste residue is 10mL-20mL. The amount of citric acid solution needs to be obtained according to the content of iron and aluminum in the lithium iron phosphate wet smelting waste residue. The amount of citric acid solution can be controlled to be slightly excessive to allow the iron and aluminum to react fully.

[0041] Specifically, the concentration of the citric acid solution can be 180g / L, 190g / L, 200g / L, 210g / L, 220g / L, 230g / L, 240g / L, 250g / L, etc., and the amount of citric acid solution per gram of lithium iron phosphate wet smelting waste slag can be 8mL, 10mL, 15mL, 20mL, 25mL, 30mL, etc.

[0042] In some embodiments of the present disclosure, after the lithium iron phosphate wet smelting waste residue and the citric acid solution are mixed and reacted, solid-liquid separation can be performed by suction filtration, and the precipitate can be collected to obtain a solid material, but the solid-liquid separation method is not limited to suction filtration.

[0043] S2. Redissolution

[0044] The solid material is mixed and dissolved with an inorganic strong acid solution to obtain a test solution, and the filter residue is digested with a strong acid solution so that the fluorine content can be quickly and accurately determined using a fluoride ion selective electrode.

[0045] In some embodiments of the present disclosure, the inorganic strong acid solution may be, but is not limited to, a hydrochloric acid solution. The concentration of the hydrochloric acid solution may be 6 mol / L-12 mol / L, and the amount of the hydrochloric acid solution used is not limited, as long as it can fully dissolve the solid material.

[0046] In some embodiments of the present disclosure, after the solid material is mixed and dissolved with the hydrochloric acid solution, the volume is constant to obtain a test solution, and the concentration of the solid material in the test solution is 0.1 g / L-1 g / L, which is controlled within this range so that the detection value of the diluted test solution can be used to determine the fluoride ion concentration on the obtained standard curve.

[0047] S3. Calculate the standard curve

[0048] The process of testing using a fluoride ion selective electrode includes: first using a fluoride ion selective electrode and a reference electrode to test the potential values ​​of fluoride ion solutions of different concentrations, and using the logarithmic concentration of fluoride ions and the corresponding potential values ​​to obtain a standard curve, so that the logarithmic concentration of fluoride ions corresponding to different potential values ​​can be obtained subsequently through the standard curve.

[0049] In some embodiments of the present disclosure, the process of obtaining a standard curve includes: taking different volumes of fluoride ion standard solution and mixing with buffer solution, constant volume, to obtain fluoride ion solutions of different concentrations, using fluoride ion selective electrode and reference electrode to test the fluoride ion solutions of different concentrations, respectively, and measuring the potential value at equilibrium under stirring; according to the Nernst equation E=E θ -(2.303RT / F)lg(α(F -)) shows that the potential value and the logarithmic concentration of fluoride ions are linearly related. A standard curve is plotted with the potential value (mV) as the ordinate and the corresponding logarithmic concentration of fluoride ions as the abscissa. Specifically, according to the linear regression equation, y = kx + b, then x = (yb) / k; where y is the potential value, x is the logarithmic concentration of fluoride ions, k is the electrode slope, and b is the intercept. Substitute the potential values ​​corresponding to at least two sets of fluoride ion logarithmic concentration values ​​into the expression for x to determine the values ​​of k and b.

[0050] In some embodiments of the present disclosure, the concentration range of the configured fluoride ion solution is 0.1 mg / L-10 mg / L, and the fluoride ion solution can have multiple concentrations, such as 0.1 mg / L, 1 mg / L, 2 mg / L, 4 mg / L, 6 mg / L, 8 mg / L, and 10 mg / L.

[0051] In some embodiments of the present disclosure, the buffer solution is a buffer solution system with a pH value of 5-6. The specific type is not limited, as long as the pH value of the system can be maintained at 5-6. The buffer solution can be a citric acid-sodium citrate solution. The mass ratio of citric acid to sodium citrate used in preparing the buffer solution is 1:10-13, so that the buffer solution can maintain the pH value of the system at 5-6.

[0052] In some embodiments of the present disclosure, when preparing the fluoride ion solution, the ratio of the volume of the added buffer solution to the volume after constant volume is (30-50):100 to better maintain the pH value of the system. Specifically, the ratio of the volume of the added buffer solution to the volume after constant volume can be 30:100, 35:100, 40:100, 45:100, 50:100, etc.

[0053] S4. Calculate the calcium fluoride content

[0054] The content of calcium fluoride in the lithium iron phosphate wet smelting waste slag is calculated by testing the fluorine content in the test liquid using a fluorine ion selective electrode. The molar amount of fluoride ions, i.e., the molar amount of calcium fluoride in the lithium iron phosphate wet smelting waste slag, is determined by the fluorine content in the test liquid. The calcium fluoride content can be calculated by multiplying the molar amount of calcium fluoride in the lithium iron phosphate wet smelting waste slag by the relative molecular mass of calcium fluoride (78.08).

[0055] In actual operation, the liquid to be tested is sampled and prepared into a sample solution. A portion of the sample solution is tested using a fluoride ion selective electrode and a reference electrode. Under constant stirring with a magnetic stirrer, the equilibrium potential value is measured. The obtained potential value is used to calculate the logarithmic concentration of fluoride ions in the sample solution according to the standard curve. After calculation, the fluoride ion content c in the sample solution is obtained. The formula for calculating the percentage of calcium fluoride in lithium iron phosphate wet smelting waste slag based on the fluoride ion content c is as follows:

[0056] Where c is the fluoride ion content in the sample solution, in mg / L;

[0057] m is the mass of the weighed lithium manganese iron phosphate hydrometallurgical waste slag sample, in g;

[0058] V1 is the volume of the liquid to be tested, in mL;

[0059] V2 is the volume of the sample solution removed during potential measurement, in mL;

[0060] V3 is the volume of the prepared sample solution, in mL.

[0061] In some embodiments of the present disclosure, the process of configuring the sample solution includes: taking 3mL-5mL (such as 3mL, 4mL, 5mL, etc.) of the liquid to be tested, adjusting the pH value to 6.8-7.2 and mixing it with a buffer solution, and constant volume. In the process of configuring the sample solution, the ratio of the volume of the buffer solution added to the volume after constant volume is (30-50): 100, so as to better stabilize the pH value. The volume of the constant volume can be 100mL, and the volume of the buffer solution added can be 30mL, 40mL, 50mL, etc.

[0062] Specifically, the pH value can be adjusted to 6.8-7.2 by using hydrochloric acid or sodium hydroxide solution.

[0063] In some embodiments of the present disclosure, the reference electrode may be a saturated calomel electrode, but is not limited thereto.

[0064] The features and performance of the present disclosure are further described in detail below with reference to the embodiments.

[0065] It should be noted that the source of the lithium iron phosphate wet smelting waste residue tested in the following examples and comparative examples is as follows: 10 kg of discarded lithium iron phosphate batteries and 2 kg of 98% by mass concentrated sulfuric acid are mixed, sodium carbonate is added to adjust the pH value to 4-6, and after reacting for 5 hours, the residue is filtered out to produce iron-aluminum-containing fluorine-containing sodium sulfate acidic wastewater; calcium hydroxide is added to the above-mentioned fluorine-containing sodium sulfate acidic wastewater for chemical precipitation to remove elements such as iron, aluminum, and fluorine in the wastewater, the mass ratio of the amount of calcium hydroxide added to the fluorine-containing sodium sulfate acidic wastewater is 4:1, and after reacting for 3 hours, filtration is obtained to obtain waste residue containing calcium fluoride, iron hydroxide, aluminum hydroxide, calcium sulfate, calcium hydroxide, etc.

[0066] The electron microscope image of the waste slag from the wet process of lithium iron phosphate smelting is shown in Figure 2. It can be seen that the waste slag sample is composed of secondary agglomeration of irregular small particles.

[0067] Example 1

[0068] This embodiment provides a method for detecting the content of calcium fluoride in lithium iron phosphate hydrometallurgical waste slag, comprising the following steps:

[0069] (1) Solution preparation

[0070] Prepare citric acid solution: weigh 210.14 g of citric acid and dissolve it in water and dilute to 1000 mL.

[0071] Prepare hydrochloric acid solution: measure 50mL of hydrochloric acid solution into a 250mL beaker, add 50mL of tertiary water into the same beaker and mix well to obtain a hydrochloric acid solution with a concentration of 6mol / L.

[0072] Prepare citric acid-sodium citrate buffer solution: weigh 24.00 g citric acid and 270.00 g trisodium citrate, dissolve in water and dilute to 1000 mL, mix well.

[0073] (2) Pretreatment

[0074] Weigh 0.5g of lithium iron phosphate hydrometallurgical waste slag sample into a 100mL polytetrafluoroethylene beaker, add 20mL of the prepared citric acid solution, and stir in a 60°C magnetic stirring water bath for 30 minutes. After the sample is fully dissolved, filter it and collect the precipitate.

[0075] (3) Redissolution

[0076] Transfer the collected precipitate to a 100 mL beaker and add 20 mL of the prepared hydrochloric acid solution. Stir with a polytetrafluoroethylene rod to fully dissolve the precipitate, then add grade 3 water to make up the volume to a 500 mL volumetric flask.

[0077] (4) Obtaining the standard curve

[0078] Accurately measure 0.1mL, 1mL, 2mL, 4mL, 6mL, 8mL, and 10mL of fluoride ion standard solution (100mg / L) into a 100mL volumetric flask, add 40mL of the prepared citric acid-sodium citrate buffer solution, dilute to the scale with tertiary water and shake well to obtain fluoride ion solutions with concentrations of 0.1mg / L, 1mg / L, 2mg / L, 4mg / L, 6mg / L, 8mg / L, and 10mg / L. Take 50mL of fluoride ion solution of different concentrations and insert a fluoride ion selective electrode and a saturated calomel electrode into a polytetrafluoroethylene beaker. Under constant stirring with a magnetic stirrer, measure the equilibrium potential value. According to the Nernst equation E=E θ -(2.303RT / F)lg(α(F -)) shows that the potential value and the logarithmic concentration of fluoride ions are linearly related. A standard curve (as shown in Figure 3) was drawn with the potential value (mV) as the ordinate and the corresponding logarithmic concentration of fluoride ions as the abscissa. According to the linear regression equation, y = kx + b, so x = (yb) / k; where y is the potential value, x is the logarithmic concentration of fluoride ions, k is the electrode slope, and b is the intercept. Substituting the logarithmic concentration of fluoride ions and the corresponding potential value into the expression for x, we obtain k = -59.284 and b = 231.42.

[0079] (5) Calculation of calcium fluoride content

[0080] Take 2mL of the solution in step (3) and adjust the acidity to pH 7.0 in a 100mL volumetric flask, then add 40mL of the prepared citric acid-sodium citrate buffer solution, and dilute with water to the scale mark to shake up to obtain a sample solution. Take 50mL of the prepared sample solution and insert a fluoride ion selective electrode and a saturated calomel electrode in a polyvinylidene fluoride beaker. Under constant stirring of a magnetic stirrer, measure the potential value at equilibrium. According to the standard curve calculation of step (4), the fluoride ion logarithmic concentration 0.639 in the sample solution taken is obtained, and the inverse logarithmic coordinate is checked to find that the fluoride ion mass concentration is 4.362mg / L.

[0081] The formula for calculating the percentage of calcium fluoride in lithium iron phosphate wet smelting waste slag based on the fluoride ion content c is as follows:

[0082] Where c is the fluoride ion content in the sample solution, in mg / L;

[0083] m is the mass of the weighed lithium manganese iron phosphate hydrometallurgical waste slag sample, in g;

[0084] V1 is the volume of the liquid to be tested, in mL;

[0085] V2 is the volume of the sample solution removed during potential measurement, in mL;

[0086] V3 is the volume of the prepared sample solution, in mL.

[0087] After calculation, it can be concluded that the percentage of calcium fluoride in lithium iron phosphate wet smelting waste slag is 44.8%.

[0088] Example 2

[0089] This embodiment provides a method for detecting the calcium fluoride content in lithium iron phosphate hydrometallurgical waste slag, which differs from Example 1 mainly in the sampling amount. It includes the following steps:

[0090] (1) Solution preparation

[0091] Same as Example 1.

[0092] (2) Pretreatment

[0093] The only difference from Example 1 is that the sampling amount of the lithium iron phosphate wet smelting waste slag sample is different.

[0094] Weigh 1g of lithium iron phosphate hydrometallurgical waste slag sample into a 100mL polytetrafluoroethylene beaker, add 20mL of the prepared citric acid solution, and stir in a 60°C magnetic stirring water bath for 30 minutes. After the sample is fully dissolved, filter it and collect the precipitate.

[0095] (3) Redissolution

[0096] Same as Example 1

[0097] (4) Obtaining the standard curve

[0098] Same as Example 1.

[0099] (5) Calculation of calcium fluoride content

[0100] The only difference from Example 1 is that 1 mL of the solution obtained in step (3) was sampled, and the remaining steps were the same as in Example 1.

[0101] In this example, the logarithmic concentration of fluoride ions in the sample solution was calculated based on the standard curve in step (4) to be 0.702. The inverse logarithmic coordinates were then used to determine the fluoride ion content to be 5.042 mg / L. The calculated percentage of calcium fluoride in the lithium iron phosphate hydrometallurgical waste residue was 51.8%.

[0102] Example 3

[0103] This embodiment provides a method for detecting the calcium fluoride content in lithium iron phosphate hydrometallurgical waste slag, which differs from Example 1 mainly in the sampling amount. It includes the following steps:

[0104] 1) Solution preparation

[0105] Same as Example 1.

[0106] (2) Pretreatment

[0107] The only difference from Example 1 is that the sampling amount of the lithium iron phosphate wet smelting waste slag sample is different.

[0108] Weigh 2g of lithium iron phosphate hydrometallurgical waste slag into a 100mL polytetrafluoroethylene beaker, add 20mL of the prepared citric acid solution, and stir in a 60°C magnetic stirring water bath for 30 minutes. After the sample is fully dissolved, filter it and collect the precipitate.

[0109] (3) Redissolution

[0110] Same as Example 1

[0111] (4) Obtaining the standard curve

[0112] Same as Example 1.

[0113] (5) Calculation of calcium fluoride content

[0114] The only difference from Example 1 is that the solution obtained in step (3) is sampled to 0.5 mL, and the remaining steps are the same as in Example 1.

[0115] In this example, the logarithmic concentration of fluoride ions in the sample solution was calculated based on the standard curve in step (4) to be 0.684. The inverse logarithmic coordinates were then used to determine the fluoride ion content to be 4.841 mg / L. The calculated percentage of calcium fluoride in the lithium iron phosphate hydrometallurgical waste residue was 46.5%.

[0116] Comparative Example 1

[0117] This comparative example provides a method for detecting the calcium fluoride content in lithium iron phosphate hydrometallurgical waste slag. The only difference from Example 2 is that the lithium iron phosphate hydrometallurgical waste slag is directly dissolved with a strong acid without pretreatment. The specific steps are as follows:

[0118] (1) Solution preparation

[0119] Prepare hydrochloric acid solution: measure 50mL of hydrochloric acid (12mol / L) solution into a 250mL beaker, add 50mL of tertiary water into the same beaker and mix well to obtain a hydrochloric acid solution with a concentration of 6mol / L.

[0120] Prepare citric acid-sodium citrate buffer solution: weigh 24.00 g citric acid and 270.00 g trisodium citrate, dissolve in water and dilute to 1000 mL, mix well.

[0121] (2) Strong acid dissolution of lithium iron phosphate wet smelting waste slag

[0122] Weigh 1 g of lithium iron phosphate wet smelting waste residue sample into a 100 mL polytetrafluoroethylene beaker, add 20 mL of prepared hydrochloric acid solution, use a polytetrafluoroethylene rod to stir until the waste residue is completely dissolved, and then add water to make up to a 100 mL volumetric flask.

[0123] (3) Obtaining the standard curve

[0124] Same as Example 1.

[0125] (4) Calculation of calcium fluoride content

[0126] The only difference from Example 2 is that 2 mL of the solution obtained in step (2) was sampled, and the remaining steps were the same as in Example 1.

[0127] In this comparative example, the logarithmic concentration of fluoride ions in the sample solution was calculated based on the standard curve in step (3) to be 0.769. The inverse logarithmic coordinates were then used to determine the fluoride ion content to be 5.88 mg / L. The calculated percentage of calcium fluoride in the lithium iron phosphate hydrometallurgical waste residue was 30.2%.

[0128] Comparative Example 2

[0129] This comparative example provides a method for detecting the calcium fluoride content in lithium iron phosphate hydrometallurgical waste slag. The only difference from Example 2 is that the lithium iron phosphate hydrometallurgical waste slag is first subjected to alkali melting, and then the melt is dissolved in hydrochloric acid solution. The specific steps are as follows:

[0130] (1) Solution preparation

[0131] Prepare hydrochloric acid solution: measure 50mL of hydrochloric acid solution into a 250mL beaker, add 50mL of tertiary water into the same beaker and mix well to obtain a hydrochloric acid solution with a concentration of 6mol / L.

[0132] Prepare citric acid-sodium citrate buffer solution: weigh 24.00 g citric acid and 270.00 g trisodium citrate, dissolve in water and dilute to 1000 mL, mix well.

[0133] (2) Alkali melting of waste residue from lithium iron phosphate hydrometallurgy

[0134] 1 g of lithium iron phosphate wet smelting waste residue sample was weighed into a 30 mL nickel crucible, 10 g of high-grade pure sodium hydroxide was added and evenly covered on the surface of the waste residue sample, the sample was covered with a lid, and then melted in a muffle furnace at 600°C for 10 minutes, taken out and cooled.

[0135] (3) Dissolution of the melt

[0136] Add 20 mL of hot water to the nickel crucible to dissolve the melt, transfer it to a volumetric flask, add 20 mL of the prepared hydrochloric acid solution, and then add water to make up to 100 mL of the volumetric flask.

[0137] (4) Drawing of fluoride ion standard curve

[0138] Same as Example 1.

[0139] (5) Calculation of calcium fluoride content

[0140] The only difference from Example 2 is that 2 mL of the solution obtained in step (3) was sampled, and the remaining steps were the same as in Example 2.

[0141] In this comparative example, the logarithmic concentration of fluoride ions in the sample solution was calculated based on the standard curve in step (4) to be 0.844. The inverse logarithmic coordinates were then used to determine the fluoride ion content to be 6.98 mg / L. The calculated percentage of calcium fluoride in the lithium iron phosphate hydrometallurgical waste residue was 35.8%.

[0142] Comparative Example 3

[0143] This comparative example provides a method for detecting the calcium fluoride content in lithium iron phosphate wet smelting waste slag, which differs from Example 2 only in that the citric acid solution is replaced by glacial acetic acid during the pretreatment process.

[0144] (1) Solution preparation

[0145] Prepare glacial acetic acid solution: Take 50 mL of analytical grade glacial acetic acid (17.5 mol / L) in a 250 mL volumetric flask and add water to the mark to obtain 3.5 mol / L glacial acetic acid.

[0146] (2) Pretreatment

[0147] The only difference from Example 2 is that the weak acid type is different. In this example, glacial acetic acid is selected.

[0148] Weigh 1g of lithium iron phosphate hydrometallurgical waste slag sample into a 100mL polytetrafluoroethylene beaker, add 20mL of prepared glacial acetic acid solution, and stir in a 60°C magnetic stirring water bath for 30 minutes. After the sample is fully dissolved, filter it and collect the precipitate.

[0149] (3) Redissolution

[0150] Same as Example 2

[0151] (4) Obtaining the standard curve

[0152] Same as Example 2.

[0153] (5) Calculation of calcium fluoride content

[0154] Same as Example 2

[0155] In this example, the logarithmic concentration of fluoride ions in the sample solution was calculated based on the standard curve in step (4) to be 0.586. The inverse logarithmic coordinates were then used to determine the fluoride ion content to be 3.86 mg / L. The calculated percentage of calcium fluoride in the lithium iron phosphate hydrometallurgical waste residue was 39.7%.

[0156] The test results of the embodiments and comparative examples are shown in Table 1:

[0157] Table 1 Test results of the embodiments and comparative examples

[0158] The results show that the strong acid dissolution in Comparative Example 1 and the alkali melting-strong acid dissolution in Comparative Example 2 both have the problem of complexation between iron and aluminum ions and fluoride ions, resulting in a low test content of calcium fluoride; although citric acid was used to pre-treat the sample in Comparative Example 3, the masking effect of iron and aluminum was poor, resulting in a low test result. In the embodiment of the present disclosure, a weak acid, citric acid, was added to dissolve the iron and aluminum in the lithium iron phosphate wet smelting waste slag, and at the same time, the Fe 3+ Reduction to Fe 2+ To avoid complexation with fluoride ions, and because calcium fluoride is insoluble in weak acid, iron and aluminum can be separated from lithium iron phosphate wet smelting waste slag by filtration, ensuring the accuracy of determining the calcium fluoride content by testing the fluorine content using the ion-selective electrode method.

[0159] The effects of the present disclosure are illustrated below through precision experiments, spiked experiments, and consistency experiments.

[0160] (1) Precision experiment

[0161] Six samples of lithium iron phosphate wet smelting waste residue were weighed in parallel and the calcium fluoride content was determined according to the method of Example 2. The results are shown in Table 2.

[0162] Table 2 Precision test table

[0163] It can be seen that the calculated result of the calcium fluoride content in lithium iron phosphate wet smelting waste slag detected by the method disclosed in the present invention is 51.73%, the standard deviation of the detection result is 0.094%, and the relative standard deviation is 0.182%. The method has good precision.

[0164] (2) Spiking experiment

[0165] To demonstrate the feasibility of the disclosed determination method, 3 g of analytically pure calcium fluoride sample was ground in an agate mortar for 10 to 30 minutes. 0.3 g of lithium iron phosphate hydrometallurgical waste slag and 0.08 g of analytically pure calcium fluoride sample were placed in a 100 mL beaker and the calcium fluoride content was calculated according to the above method. The test results are shown in Table 3.

[0166] Table 3 Spike recovery test table

[0167] It can be seen that the spiked recovery rate of the detection method disclosed in the present invention is 101.7% to 102.2%, which meets the range specified by the national standard, indicating that this method has high accuracy in testing the calcium fluoride content in lithium iron phosphate wet smelting waste slag.

[0168] (3) Consistency experiment

[0169] The calcium fluoride content of three fluorite standards (YSBC28731-2014, YSBC28732-2014, and YSBC28733-2014) was determined according to the method of Experimental Example 2. The results are shown in Table 4.

[0170] Table 4 Consistency test table

[0171] The test results of this method are in good agreement with those of the standard, indicating that this method is feasible for the determination of calcium fluoride content. Industrial Applicability

[0172] The present invention uses citric acid to treat the waste residue of lithium iron phosphate wet smelting to dissolve the iron and aluminum in the waste residue, and at the same time uses the reducing property of citric acid to dissolve the Fe 3+ Reduction to Fe 2+ By avoiding complexation with fluoride ions, and because calcium fluoride is insoluble in citric acid, iron and aluminum can be separated from lithium iron phosphate hydrometallurgical waste residues through solid-liquid separation. Using a fluoride ion-selective electrode, the fluorine content in the test liquid can be accurately measured. The detection method provided by the present disclosure does not require the addition of iron and aluminum masking agents such as triethanolamine, has the advantages of simple operation and low cost. It can quickly and accurately measure the calcium fluoride content in lithium iron phosphate hydrometallurgical waste residues in large quantities, and has excellent industrial applicability.

Claims

1. A method for detecting the content of calcium fluoride in lithium iron phosphate hydrometallurgical waste slag, characterized in that: include: The lithium iron phosphate wet smelting waste slag and citric acid solution are mixed for reaction, and then solid-liquid separation is performed to obtain a solid material, the solid material is mixed and dissolved with an inorganic strong acid solution to obtain a test liquid, and the fluorine content in the test liquid is tested using a fluoride ion selective electrode to calculate the content of calcium fluoride in the lithium iron phosphate wet smelting waste slag.

2. The detection method according to claim 1, characterized in that: The reaction temperature of the lithium iron phosphate wet smelting waste slag and the citric acid solution is 50° C.-80° C., and the reaction time is more than 15 minutes.

3. The detection method according to claim 1 or 2, characterized in that: The reaction temperature of the lithium iron phosphate wet smelting waste slag and the citric acid solution is 55° C.-65° C., and the reaction time is 25 min-40 min.

4. The detection method according to any one of claims 1 to 3, characterized in that: The concentration of the citric acid solution is 180g / L-250g / L, and the amount of the citric acid solution used per gram of the lithium iron phosphate wet smelting waste slag is 8mL-30mL.

5. The detection method according to any one of claims 1 to 4, characterized in that: The concentration of the citric acid solution is 200g / L-220g / L, and the amount of the citric acid solution used per gram of the lithium iron phosphate wet smelting waste slag is 10mL-20mL.

6. The detection method according to any one of claims 1 to 5, characterized in that: The lithium iron phosphate wet smelting waste slag contains calcium fluoride, ferric hydroxide, aluminum hydroxide, calcium sulfate and calcium hydroxide.

7. The detection method according to any one of claims 1 to 6, characterized in that: After the lithium iron phosphate wet smelting waste residue and the citric acid solution are mixed for reaction, solid-liquid separation is performed by suction filtration, and the precipitate is collected to obtain the solid material.

8. The detection method according to any one of claims 1 to 7, characterized in that: The inorganic strong acid solution is a hydrochloric acid solution, and the concentration of the hydrochloric acid solution is 6 mol / L-12 mol / L.

9. The detection method according to any one of claims 1 to 8, characterized in that: After the solid material and the hydrochloric acid solution are mixed and dissolved, the volume is fixed to obtain the test solution, and the concentration of the solid material in the test solution is 0.1 g / L-1 g / L.

10. The detection method according to any one of claims 1 to 9, characterized in that: The process of testing using the fluoride ion selective electrode includes: firstly using the fluoride ion selective electrode and the reference electrode to test the potential value of fluoride ion solutions of different concentrations, and obtaining a standard curve using the logarithmic concentration of fluoride ions and the corresponding potential value; sampling the test solution and configuring it into a sample solution, taking a part of the sample solution for testing using the fluoride ion selective electrode and the reference electrode, using the obtained potential value to calculate the logarithmic concentration of fluoride ions in the sample solution according to the standard curve, obtaining the fluoride ion content c in the sample solution after calculation, and calculating the calcium fluoride concentration in the lithium iron phosphate wet smelting waste slag according to the fluoride ion content c.

11. The detection method according to claim 10, characterized in that: The process of obtaining the standard curve includes: taking different volumes of fluoride ion standard solution and mixing with buffer solution, fixing the volume to obtain fluoride ion solutions of different concentrations, using fluoride ion selective electrode and reference electrode to test the fluoride ion solutions of different concentrations respectively, and measuring the potential value at equilibrium under stirring; according to the linear relationship between the potential value and the logarithmic concentration of fluoride ions, a standard curve is drawn with the potential value as the ordinate and the corresponding logarithmic concentration of fluoride ions as the abscissa.

12. The detection method according to claim 11, characterized in that: The process of drawing a standard curve with the potential value as the ordinate and the corresponding logarithmic concentration of fluoride ions as the abscissa includes: according to the linear regression equation, y=kx+b, then x=(yb) / k; wherein y is the potential value, x is the logarithmic concentration value of fluoride ions, k is the electrode slope, and b is the intercept; Substitute the potential values ​​corresponding to the logarithmic concentration values ​​of at least two groups of fluoride ions into the expression of x to obtain the values ​​of k and b.

13. The detection method according to claim 11, characterized in that: The buffer solution is a buffer solution system with a pH value of 5-6, and the concentration range of the configured fluoride ion solution is 0.1 mg / L-10 mg / L.

14. The detection method according to any one of claims 11 to 13, characterized in that: The buffer solution is a citric acid-sodium citrate solution, and the mass ratio of citric acid to sodium citrate used in preparing the buffer solution is 1:10-13; When preparing the fluoride ion solution, the ratio of the volume of the added buffer solution to the volume after constant volume is (30-50):

100.

15. The detection method according to any one of claims 11 to 14, characterized in that: The process of preparing the sample solution includes: taking 3mL-5mL of the test solution, adjusting the pH value to 6.8-7.2, mixing with the buffer solution, and fixing the volume; the ratio of the volume of the buffer solution added to the volume after fixing the volume is (30-50):100.

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