Method for determining calcium sulfate dihydrate and anhydrous calcium sulfate

Abstract

The invention provides a method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate, which comprises the following steps: step 1, carrying out TGA test on a sample to obtain the content of crystal water and the thermal weight loss ratio in the sample; 2, pretreating the sample subjected to the TGA test in the step 1 with sodium carbonate; 3, feeding the supernatant of the upper layer of the sample pretreated in the step 2 to ion chromatography to test the content of sulfate radicals; step 4, performing microwave digestion on the lower-layer precipitate of the sample pretreated in the step 2 to release calcium ions, performing digestion with nitric acid, and then performing ICP (Inductively Coupled Plasma) test to measure the content of the calcium ions; and step 5, calculating the contents of calcium sulfate dihydrate and anhydrous calcium sulfate in the sample by combining the content of sulfate radicals, the content of calcium ions and the thermal weight loss ratio. According to the detection method, after thermal weight loss and sodium carbonate pretreatment are combined, the concentration of sulfate radicals and calcium ions is measured through ion chromatography and ICP, the content of calcium sulfate dihydrate and the content of anhydrous calcium sulfate are accurately determined, and meanwhile the content of crystal water in a sample is further tested in a thermal weight loss test.

Description

Technical Field

[0001] The invention relates to the technical field of testing calcium sulfate content, and in particular to a method for determining calcium sulfate dihydrate and anhydrous calcium sulfate. Background Art

[0002] Calcium sulfate, including calcium sulfate dihydrate and anhydrous calcium sulfate, has important application value in cement production and construction. As admixtures, they can significantly affect the performance of cement and the quality of building materials. Accurate determination of calcium sulfate content is crucial to ensure product quality and engineering results. However, the insolubility of calcium sulfate in most acids poses a challenge to its content determination. Traditional determination methods such as ICP (inductively coupled plasma) and IC (ion chromatography) have certain limitations in application. Although titration and gravimetric methods have applications, they also face some problems.

[0003] ICP is a commonly used elemental analysis technique with advantages such as high sensitivity and simultaneous analysis of multiple elements. However, ICP faces difficulties in the determination of calcium sulfate. Since calcium sulfate is insoluble in most acids, it is difficult to completely decompose it and convert it into an ionic state that can be detected by ICP. Even if some strong acids or mixed acids are used for digestion, it may not be possible to ensure that calcium sulfate is completely dissolved, resulting in the inability to accurately determine the calcium sulfate in the sample. In addition, other impurities may be introduced during the digestion process, interfering with the determination results of calcium sulfate, further reducing the accuracy and reliability of the analysis. Ion chromatography is mainly used to separate and determine ionic compounds. For calcium sulfate, although it can be analyzed through principles such as ion exchange, it is also limited by its poor solubility. Under conventional ion chromatography conditions, calcium sulfate is difficult to fully dissolve and ionize in the mobile phase, and it is difficult to achieve effective separation and detection on the chromatographic column. Moreover, ion chromatography has high requirements for sample purity and pretreatment. Impurities in calcium sulfate samples may affect the performance of the chromatographic column and the analysis results, which greatly limits the application of IC in the determination of calcium sulfate content. When the disodium ethylenediaminetetraacetic acid (EDTA) titration method is used to determine the calcium sulfate content, its principle is to use the complexation reaction of EDTA and calcium ions. Under appropriate pH conditions, EDTA can form a stable complex with calcium ions in the solution. First, the calcium sulfate sample is dissolved in a certain solvent, and then the indicator is added and titrated with an EDTA standard solution. When the calcium ions in the solution are completely complexed with EDTA, the color of the indicator changes, indicating the titration endpoint. By recording the volume of the consumed EDTA standard solution, the calcium sulfate content in the sample is calculated according to the complexation ratio and concentration of EDTA and calcium ions. When the calcium sulfate content is high, the recovery efficiency of the EDTA titration method will decrease. This may be due to the enhanced interaction between calcium ions and other ions or substances at high concentrations, resulting in some calcium ions being unable to fully complex with EDTA. The principle of gravimetric determination of calcium sulfate content is based on the formation of barium sulfate precipitation. The calcium sulfate sample is reacted with the barium chloride solution, and the sulfate ions combine with the barium ions to form a barium sulfate precipitate. After filtering, washing, drying and other operations, a pure barium sulfate precipitate is obtained. Then, according to the mass of the barium sulfate, the content of calcium sulfate is calculated by the conversion coefficient. The obvious disadvantage of the gravimetric method is that the steps are cumbersome. The whole process requires multiple operations, including dissolution, precipitation, filtering, washing and drying, and each step may introduce errors.

[0004] In summary, calcium sulfate is widely used in cement production and building materials, and accurate determination of its content is crucial. However, the commonly used determination methods currently have certain problems. Therefore, it is necessary to propose a method for determining calcium sulfate dihydrate and anhydrous calcium sulfate that is more accurate than the existing methods. Summary of the invention

[0005] Purpose of the invention:

[0006] The invention aims to provide a method for determining calcium sulfate dihydrate and anhydrous calcium sulfate, so as to solve the problems that the existing measuring method is inaccurate and the steps are complicated.

[0007] The technical solution of the present invention:

[0008] A method for determining calcium sulfate dihydrate and anhydrous calcium sulfate, characterized in that it comprises the following steps:

[0009] Step 1: Perform TGA test on the sample to obtain the content of crystal water and thermal weight loss rate in the sample;

[0010] Step 2: pre-treating the sample after TGA test in step 1 with sodium carbonate;

[0011] Step 3: Testing the sulfate content of the sample pretreated in step 2 by ion chromatography;

[0012] Step 4: The sample pretreated in step 2 is subjected to microwave digestion to release calcium ions, digested with nitric acid, and then subjected to ICP testing to measure the calcium ion content;

[0013] Step 5: Calculate the content of calcium sulfate dihydrate and anhydrous calcium sulfate in the sample by combining the sulfate content, calcium ion content and thermal weight loss rate.

[0014] Furthermore, the TGA test uses a thermogravimetric analyzer model of US TATGA500; the TGA test is set to a temperature range of 40-800°C, and a heating rate of 20-50°C / min; the TGA test atmosphere is nitrogen, and the flow rate is 60mL / min.

[0015] Furthermore, the sodium carbonate pretreatment comprises the following steps: placing an appropriate amount of the sample after TGA test into a beaker, adding sodium carbonate solution and stirring evenly, and then standing to obtain an upper supernatant and a lower precipitate.

[0016] Furthermore, the concentration of the sodium carbonate solution is 1-5 mol / L.

[0017] Furthermore, the amount of sodium carbonate solution added is 0.85-10.5 times the mass of the sample.

[0018] Furthermore, the ion chromatography instrument model is Thermo Fisher AQUION; in the ion chromatography test, the concentration of the sulfate standard solution is 1-10 mg / L, the injection volume is 15-20 mL, the temperature is 20-30°C, the stationary phase of the ion chromatography is ion exchange resin, the mobile phase is aqueous solution, and the mobile phase flow rate is 0.8-1 mL / min.

[0019] Furthermore, the microwave digestion test conditions are heating to 180-200 degrees Celsius for 30-40 minutes, maintaining for 25-30 minutes, and then cooling for 2-3 hours.

[0020] Furthermore, the amount of nitric acid added during the nitric acid digestion is 7-28 times the mass of the sample.

[0021] Furthermore, the ICP test power is 1200-1500W, the plasma gas flow rate is 12-15.0L / min, the auxiliary gas flow rate is 0.15-0.25L / min, the atomization gas flow rate is 0.50-0.80L / min, the injection volume is 10-15μL, and the concentration of the calcium ion standard solution is 1000μg / mL.

[0022] Furthermore, the content of calcium sulfate dihydrate is: M=c / 36*172, and the content of anhydrous calcium sulfate is:

[0023] Z = (aM / 172*97) / 97*136 or (bM / 172*40) / 40*136, wherein a% is the sulfate content, b% is the calcium ion content, and c% is the weight loss rate of calcium sulfate dihydrate TGA test at 128°C.

[0024] Beneficial effects:

[0025] The present invention discloses a method for determining calcium sulfate dihydrate and anhydrous calcium sulfate. Compared with the existing method, the method has the feature of accurately quantifying the content of crystal water in a sample. Meanwhile, the concentrations of sulfate radical and calcium ion can be accurately determined by ion chromatography and ICP through pretreatment, thereby accurately determining the content of calcium sulfate dihydrate and anhydrous calcium sulfate. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : TGA graph of cement sample 1;

[0027] Figure 2 :Ion chromatogram data of cement sample 1;

[0028] Figure 3 : Sulfate standard curve;

[0029] Figure 4 : Calcium ion standard curve;

[0030] Figure 5 : TGA graph of cement sample 2;

[0031] Figure 6 : TGA graph of cement sample 3. DETAILED DESCRIPTION

[0032] The present invention will be described below in conjunction with specific embodiments. It should be noted that the following embodiments are examples of the present invention and are only used to illustrate the present invention, but not to limit the present invention. Other combinations and various modifications within the concept of the present invention may be performed without departing from the spirit or scope of the present invention.

[0033] Unless otherwise specified, the chemical reagents used in the present invention are all commercially available analytically pure.

[0034] Ion exchange resins were purchased from Anhui Samsung Resin Technology Co., Ltd.

[0035] The method for determining calcium sulfate dihydrate and anhydrous calcium sulfate of the present invention comprises the following steps:

[0036] Step 1: Perform TGA test on the sample to obtain the content of crystal water and thermal weight loss rate in the sample;

[0037] Step 2: pre-treating the sample after TGA test in step 1 with sodium carbonate;

[0038] Step 3: Testing the sulfate content of the sample pretreated in step 2 by ion chromatography;

[0039] Step 4: The sample pretreated in step 2 is subjected to microwave digestion to release calcium ions, digested with nitric acid, and then subjected to ICP testing to measure the calcium ion content;

[0040] Step 5: Calculate the content of calcium sulfate dihydrate and anhydrous calcium sulfate in the sample by combining the sulfate content, calcium ion content and thermal weight loss rate.

[0041] Embodiment 1:

[0042] Step 1: Weigh 0.5g of cement sample 1 and put it into a crucible. Place the crucible containing cement sample 1 on the sample holder in the heating furnace of the thermogravimetric analyzer within the measuring range of the balance. Start the thermogravimetric analyzer with a starting temperature of 40°C, an end temperature of 400°C, a heating rate of 20°C / min, and a nitrogen flow rate of 60mL / min. End the experiment and calculate the content of crystalline water in the sample. Theoretical: The proportion of crystalline water in calcium sulfate dihydrate is 20.9%, and the weight loss of calcium sulfate dihydrate at 80-160°C is 20.9%. The weight loss rate of cement sample 1 is calculated to be 0.6957%, and the content of crystalline water in cement sample 1 is 0.6957%. The content of calcium sulfate dihydrate in cement sample 1 is 0.6957 / 20.9*100%≈3.3%. Figure 1 is the TGA graph of cement sample 1;

[0043] Step 2: Put 0.4 g of cement sample 1 after TGA test into a 50 mL beaker, add 4 g of 1 mol / L sodium carbonate solution, stir evenly, and let stand to obtain the upper supernatant and lower sediment of the sample;

[0044] Step 3: Prepare the ion chromatograph (model: Thermo Fisher AQUIO), eluent: potassium hydroxide solution, ion chromatographic stationary phase: ion exchange resin, mobile phase: aqueous solution; then start the instrument, set the program: gradient elution, initial concentration of 15mmol / L, retain for 10min, increase to 40mmol / L after 1min, retain for 25min, let the workstation collect signals after the baseline is stable, draw 1.5mL of sample with a syringe, inject the upper supernatant of the sample prepared in step 2 from the injection port, the temperature is 25℃, the flow rate is 1mL / min, the dilution factor is 10000, the workstation automatically collects signals, according to the collected sample 1 ion chromatogram data diagram ( Figure 2 ) Analyze the data and draw the sulfate standard curve y = 1.7244x-0.0547 ( Figure 3 ), calculate the sulfate content in the sample, sulfate content = x * dilution factor * 10 -4 %, the sulfate content in cement sample 1 is calculated to be 5.83%. Figure 2 This is the ion chromatogram data of cement sample 1, attached Figure 3 is the sulfate standard curve;

[0045] Step 4: Put 0.1 g of the lower sediment of the sample pretreated in step 2 into the microwave, heat it to 200 degrees Celsius for 40 minutes, keep it for 25 minutes, then cool it down for 3 hours, add 0.7 g of nitric acid to digest it, and then perform ICP test. Establish ICP in the ICP tester, set the power to 1300 W, the plasma gas flow rate to 15.0 L / min, the auxiliary gas flow rate to 0.20 L / min, the nebulizer gas flow rate to 0.70 L / min, the injection volume to 15 μL, the dilution factor to 10000, and the concentration of the calcium ion standard solution to 1000 μg / mL. Draw the calcium ion standard curve based on the data, and calculate the calcium ion content = x*dilution factor*10 -4 %, the calcium ion content in the cement sample was measured to be 2.41%, Figure 4 is the calcium ion standard curve;

[0046] Step 5: Calculate the contents of calcium sulfate dihydrate and anhydrous calcium sulfate in cement sample 1 according to the following formula, calcium sulfate dihydrate content M=c / 36*172, anhydrous calcium sulfate content: Z=(aM / 172*97) / 97*136 or Z=(bM / 172*40) / 40*136, wherein sulfate content a=5.83%, calcium ion content b=2.36%, weight loss rate c=0.6957% at 128°C in TGA test of cement sample 1, calculated that the content of calcium sulfate dihydrate in cement sample 1 is 3.32%, and the content of anhydrous calcium sulfate in cement sample 1 is 5.6%.

[0047] Embodiment 2:

[0048] According to the experimental steps and calculation method of Example 1, the sulfate content, calcium ion content and thermal weight loss rate in cement sample 2 were tested, and the dihydrate calcium sulfate content in cement sample 2 was measured to be 13.50%, the crystal water content in cement sample 2 was 2.842%, and the anhydrous calcium sulfate content in cement sample 2 was 17.82%. Figure 5 This is the TGA diagram of cement sample 2.

[0049] Embodiment 3:

[0050] According to the experimental steps and calculation method of Example 1, the sulfate content, calcium ion content and thermal weight loss rate in cement sample 3 were tested, and the dihydrate calcium sulfate content in cement sample 3 was 85.55%, the crystal water content in cement sample 2 was 17.88%, and the anhydrous calcium sulfate content in cement sample 2 was 89.78%. Figure 6 This is the TGA diagram of cement sample 3.

[0051] Comparative Example 1:

[0052] The contents of calcium sulfate dihydrate and calcium sulfate anhydrous in cement sample 1 in Example 1 were analyzed according to the barium sulfate weight method in "Chemical Analysis Method for Gypsum" (GB / T 5484-2012). The result showed that the content of calcium sulfate dihydrate in cement sample 1 was 3.39%, and the content of calcium sulfate anhydrous in cement sample 1 was 5.65%.

[0053] Comparative Example 2:

[0054] The content of calcium sulfate dihydrate and anhydrous calcium sulfate in cement sample 1 was analyzed by barium sulfate weight method according to "Chemical Analysis Method for Gypsum" (GB / T 5484-2012) using cement sample 2 in Example 2. The result showed that the content of calcium sulfate dihydrate in cement sample 1 was 13.70%, and the content of calcium sulfate anhydrous in cement sample 1 was 18.12%.

[0055] Comparative Example 3:

[0056] The content of calcium sulfate dihydrate and calcium sulfate anhydrous in cement sample 3 in Example 3 was analyzed according to the barium sulfate weight method in "Chemical Analysis Method for Gypsum" (GB / T 5484-2012). The result showed that the content of calcium sulfate dihydrate in cement sample 1 was 85.75%, and the content of calcium sulfate anhydrous in cement sample 1 was 90.20%.

[0057] From the above data, it can be seen that the method for determining calcium sulfate dihydrate and anhydrous calcium sulfate of the present invention has more accurate quantification of the crystal water content in the sample than the existing method. At the same time, the sulfate and calcium ion concentrations can be accurately determined by ion chromatography or ICP through pretreatment, and the content of calcium sulfate dihydrate and anhydrous calcium sulfate can be determined by sulfate or calcium ion concentration. From the comparison between Example 1 and Comparative Example 1, it can be seen that the relative error between 3.39% and 3.32% is about 2.11%, and the relative error between 5.65% and 5.6% is about The relative error between 13.70% and 13.50% is about 1.48%, and the relative error between 17.82% and 18.12% is about 1.68%; the relative error between 85.55% and 85.75% is 0.23%, and the relative error between 89.78% and 90.20% is 0.47%; in summary, it can be seen that the relative error between the present test method and the barium sulfate weight method for chemical analysis of gypsum (GB / T 5484-2012) is 2.11% at most. The method for determining calcium sulfate dihydrate and calcium sulfate anhydrous of the present invention can be used to test the content of calcium sulfate dihydrate and calcium sulfate anhydrous in a sample.

[0058] The present invention can also have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications based on the present invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the present invention.

Claims

1. A method for determining calcium sulfate dihydrate and anhydrous calcium sulfate, characterized in that: The following steps are involved: Step 1: Perform TGA test on the sample to obtain the content of crystal water and thermal weight loss rate in the sample; Step 2: pre-treating the sample after TGA test in step 1 with sodium carbonate; Step 3: sending the upper supernatant of the sample pretreated in step 2 to an ion chromatograph to test its sulfate content; Step 4: The lower sediment of the sample pretreated in step 2 is subjected to microwave digestion to release calcium ions, digested with nitric acid, and then subjected to ICP testing to measure the calcium ion content; Step 5: Calculate the content of calcium sulfate dihydrate and anhydrous calcium sulfate in the sample by combining the sulfate content, calcium ion content and thermal weight loss rate.

2. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 1, characterized in that: The TGA test uses a thermogravimetric analyzer model of US TATGA500; the TGA test is set to a temperature range of 40-800°C, and a heating rate of 20-50°C / min; the TGA test atmosphere is nitrogen, and the flow rate is 50-60mL / min.

3. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 1, wherein The sodium carbonate pretreatment comprises the following steps: placing an appropriate amount of the sample after TGA test into a beaker, adding sodium carbonate solution, stirring evenly, and standing to obtain an upper supernatant and a lower precipitate.

4. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 3, wherein: The concentration of the sodium carbonate solution is 1-5 mol / L.

5. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 3, wherein: The amount of sodium carbonate solution added is 0.85-10.5 times the mass of the sample.

6. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 1, characterized in that, The ion chromatograph model is Thermo Fisher AQUION; in the ion chromatographic test, the injection volume is 15-20 mL, the temperature is 20-30° C., the stationary phase of the ion chromatogram is ion exchange resin, the mobile phase is aqueous solution, and the mobile phase flow rate is 0.8-1 mL / min.

7. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 1, characterized in that, The microwave digestion test conditions are heating to 180-200 degrees Celsius for 30-40 minutes, maintaining for 25-30 minutes, and then cooling for 2-3 hours.

8. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 7, characterized in that, During the nitric acid digestion, the amount of nitric acid added is 7-28 times the mass of the sample.

9. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 1, characterized in that: The ICP test power is 1200-1500W, the plasma gas flow rate is 12-15.0L / min, the auxiliary gas flow rate is 0.15-0.25L / min, the atomization gas flow rate is 0.50-0.80L / min, the injection volume is 10-15μL, and the concentration of the calcium ion standard solution is 1000μg / mL.

10. The method for measuring calcium sulfate dihydrate and anhydrous calcium sulfate according to claim 1, characterized in that: The calcium sulfate dihydrate content: M=c / 36*172, the anhydrous calcium sulfate content: Z=(aM / 172*97) / 97*136 or (bM / 172*40) / 40*136, wherein a% is the sulfate content, b% is the calcium ion content, and c% is the weight loss rate of calcium sulfate dihydrate TGA test at 128°C.

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