Method for measuring specific surface area of calcium carbonate through gas concentration change

Abstract

The invention discloses a method for determining the specific surface area of calcium carbonate through gas concentration change. The method comprises the steps of sample preparation and equipment installation, exhaust treatment, research on the relationship between gas concentration and pyrolytic reaction rate, research on the relationship between the initial pyrolytic reaction rate and the specific surface area of calcium carbonate, k value calibration, specific surface area determination and the like. Calcium carbonate thermogravimetric analysis and TCD carbon dioxide concentration measurement are combined for the first time, in-situ SSA measurement in the high-temperature pyrolysis process is achieved by innovatively designing a gas treatment system and a kinetic model, the specific surface area is directly associated through the gas concentration change rate, and the problem that the high-temperature reaction activity of a material cannot be represented through a traditional method is solved; the characteristics of no standard sample, interference resistance and high efficiency provide key technical support for performance optimization of calcium carbonate in high-temperature application scenes such as ceramics, environmental protection and medicines.

Description

Technical Field

[0001] The invention relates to the technical field of testing methods for high specific surface area solids, and in particular to a method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration. Background Art

[0002] Specific surface area (SSA) is a key parameter for evaluating the performance of materials such as calcium carbonate, directly affecting their adsorption, catalytic and reaction activities. Traditional measurement methods, such as the BET nitrogen adsorption method, have obvious limitations: 1) Limited applicability: It is only applicable to physical adsorption at low temperatures (-196°C) and cannot characterize the true specific surface area of ​​the material in high-temperature reactions (such as thermal decomposition); 2) Complex pretreatment: deep degassing is required (>150°C vacuum treatment), which can easily change the surface structure of the material; 3) Time-consuming and expensive: A single test takes several hours and relies on standard calibration.

[0003] Although the high-temperature pyrolysis method can simulate the actual reaction process, it faces the problem of gas concentration monitoring: 1) The temperature of pyrolysis exhaust gas is high (>800℃), and direct detection will damage the sensor; 2) Water vapor interference causes distortion of thermal conductivity detector (TCD) readings; 3) Pipeline transmission delay causes concentration signal lag, making it difficult to capture the initial reaction rate.

[0004] Therefore, there is an urgent need to develop an in-situ, high-precision, standard-free high-temperature specific surface area determination method. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and to propose a method for measuring the specific surface area of ​​calcium carbonate by changing the gas concentration.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: A method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration comprises the following steps: 1) Sample preparation and equipment installation ① Sample preparation Grind the calcium carbonate into 50-80 mesh, dry at 105-110°C until the moisture content is less than 0.2%, obtain a sample, and measure the initial mass m; ②Equipment installation A thermogravimetric analyzer (which can be used for weighing), a heat exchanger (the circulating water pipe is cooled through the gas channel), an electronic condenser, and a TCD detector (a thermal conductivity detector that uses the difference in thermal conductivity between He and CO2 to test CO2 concentration) are set up. The exhaust gas discharged from the thermogravimetric analyzer cavity is cooled to 200-250°C by heat conduction through the water wall of the heat exchanger, and then cooled to 60-70°C by the electronic condenser to remove moisture. The CO2 concentration is detected in the TCD detector. 2) Exhaust treatment Prior to analysis, exhaust the sample and pipelines of the thermogravimetric analyzer, heat exchanger, electronic condenser, and TCD detector using carrier gas at room temperature at an inlet flow rate of 10-15 mL / min. Record the time from the start of ventilation to the TCD detector indicating the characteristic thermal conductivity of He in the carrier gas. This time is the delay time τ (representing the time difference between the TCD detection concentration and the concentration in the thermogravimetric analyzer cavity). Continue exhausting until the concentration in the TCD detector remains constant (although He is present before this, air is still the main gas, so the CO2 concentration is low, 400-800 ppm). Exhaust is then complete. 3) Study on the relationship between gas concentration and pyrolysis reaction rate The carrier gas was continuously introduced at an inlet flow rate of 10-15 mL / min, and the temperature was raised to 1000°C at a heating rate of 10-100°C / min and kept at that temperature for 2 hours. The calcium carbonate weight loss rate Δm in the thermogravimetric analyzer and the CO2 concentration C in the TCD detector were respectively measured. The C-Δm curve was drawn and fitted. The C-Δm curve was an ascending slope, indicating that the calcium carbonate weight loss rate Δm was proportional to the CO2 concentration C. Taking the weight loss rate of 2.5g standard sample as 1% in 1min as an example, the volume of CO2 produced is 2.5×1%×44 / 100÷44×22.4=5.6ml, and the inner cavity volume is 60000cm 3 , then the CO2 concentration in the thermogravimetric analyzer cavity is 5.6÷60000×10 6 +1000=1093ppm; 4) Study on the relationship between the initial pyrolysis reaction rate and the specific surface area of ​​calcium carbonate Using different specific surface areas (m 2 / g) of calcium carbonate as the standard sample, followed by 5m 2 / g、10m 2 / g, 20m 2 / g、30m 2 / g, 40m 2 / g, 50m 2 / g, 60m 2 / g, and the temperature was raised according to the steps in 3), and the weight loss rate of calcium carbonate in the thermogravimetric analyzer of each standard sample was recorded within 1 minute at the beginning of pyrolysis (i.e., the instantaneous start of the change in the CO2 concentration C in the TCD detector). The curve of the weight loss rate of calcium carbonate Δm and the specific surface area SSA of calcium carbonate was fitted and drawn. Δm-SSA is an ascending slope, indicating that the weight loss rate of calcium carbonate Δm is proportional to the CO2 concentration C, thereby indicating that the CO2 concentration C in the TCD detector at the beginning of pyrolysis is proportional to the specific surface area SSA of calcium carbonate; This conclusion is consistent with the following mechanism: using the proportional relationship between the initial decomposition rate of calcium carbonate pyrolysis reaction (CaCO3→CaO+CO2) and the specific surface area of ​​the sample: The larger the specific surface area, the more exposed reaction sites there are, and the faster the rate of CO2 production per unit time. By monitoring the initial rate of CO2 concentration increase in a closed system in real time with high precision, combined with a thermal decomposition kinetic model, the SSA can be directly calculated without the need for standard sample calibration. According to the concentration value of the TCD detector, a Ct concentration decay curve with high time resolution is obtained, and the obtained slope is the concentration change rate: Formula (1) Where C is the CO2 concentration detected in the TCD detector; t is the reaction time (s); V is the volume of the thermogravimetric analyzer cavity (cm 3 ); is the rate of change of gas production volume (cm 3 ·s -1 ); At the initial stage of the reaction (t is close to 0), the reaction is controlled by surface reactions and has little influence from diffusion. At this time, the rate of increase of gas concentration dC / dt is the largest and mainly depends on the number of reactive sites on the total surface area of ​​the sample. Calculate the instantaneous concentration change rate in the initial stage of the reaction (e.g., the first few seconds to tens of seconds, the specific time depends on the reaction rate) , obtained by differentiating the Ct curve around t = 0 (or fitting early data points to find the slope); Formula (2) Where C is the CO2 concentration detected in the TCD detector; t is the reaction time (s); V is the volume of the thermogravimetric analyzer cavity (cm 3 ); is the instantaneous concentration change rate at the initial stage of the reaction (cm 3 ·s -1 ); k is the apparent reaction rate constant (related to temperature, gas type, and surface reactivity) (cm·s -1 ); SSA is the specific surface area (cm 2 ·g -1 ); m is the sample mass (g); 5) k value calibration Test and draw different specific surface areas (m 2 / g) of the standard sample, and calculate the Ct concentration decay curve of each standard sample according to the slope of the curve , and calculate the k value of each standard sample according to formula (2), and take the number average value, which is the standard k value; 6) Specific surface area determination The samples to be tested are tested according to steps 3) and 4). , after converting formula (2), we get the following formula (3). According to formula (3), we can get the specific surface area of ​​the sample to be tested: Formula (3).

[0007] Preferably, a circulating water pipe at 5-30°C is provided in the heat exchange tube to conduct heat through the heat transfer wall and cool down the high temperature quickly.

[0008] Preferably, the electronic condenser adopts a three-stage Peltier refrigeration plate combination structure, and the temperatures of the three-stage Peltier refrigeration plates are 10°C, -5°C and -15°C respectively, which have condensed water to avoid inaccuracy in the concentration test of the TCD detector due to moisture.

[0009] Preferably, by dragging the Ct concentration decay curve obtained by the TCD detector toward the positive direction of the time axis t through the delay time τ tested in the exhaust stage, a one-to-one correspondence is basically formed with the concentration in the inner cavity of the thermogravimetric analyzer.

[0010] Preferably, the carrier gas of the thermogravimetric analyzer is a mixture of He and CO2, wherein the CO2 accounts for 0.1% (1000 ppm) and the inner cavity volume is 60000 cm 3 The sample input is 2-3g. The carrier gas is used to discharge the gas to the TCD detector. The difference in thermal conductivity between He and CO2 can be used to measure the CO2 concentration. In addition, an anaerobic protection is provided to prevent the interference of excessive heteroatoms in the test results.

[0011] Preferably, the calcium carbonate is crushed by gravity using a 5 kg roller for 10 minutes, and the grinding is repeated, the proportion of particles meeting the particle size is greater than 97%, and the porosity retention is greater than 95%.

[0012] Preferably, in order to increase the instantaneous concentration change rate in the initial stage of the reaction To improve the accuracy, an electronic condenser is connected in parallel with multiple TCD detectors. The detection time (test time point) between adjacent TCD detectors differs by 1s. By using them in a cycle, the Ct curve can be drawn by seconds, avoiding the loss of accuracy in detecting the instantaneous concentration change rate due to long TCD test time.

[0013] Compared with the prior art, the present invention has the following beneficial effects: This invention combines thermogravimetric analysis of calcium carbonate with TCD carbon dioxide concentration determination for the first time. Experiments verify that the specific surface area of ​​calcium carbonate is directly proportional to the decomposed CO2 concentration. Through the innovative design of the gas processing system and kinetic model, high-temperature, real-time, and direct SSA determination is achieved. The specific advantages are as follows: 1) Timely detection can be performed at high temperatures, and the gas cooling process can eliminate the influence of moisture on gas concentration detection; 2) By measuring the pipeline delay τ through carrier gas exhaust, the Ct curve is shifted to eliminate the time difference, and a parallel TCD detector (1s interval) is used to draw a second-level Ct curve, which can accurately capture the initial reaction rate and thus obtain a Ct curve with high time resolution; 3) The reaction constant k is verified and calibrated using standard samples. A kinetic model can be established based on the proportional relationship between the initial decomposition rate and SSA, and SSA can be directly calculated without the need for standard samples. 4) The subsequent tests of this invention do not require comparison with standard samples (which is required by the BET method). The samples only need to be condensed and dehydrated at normal pressure. A single test takes ≤2.5 hours (including the heating process). The test temperature does not require ultra-low temperature adsorption conditions. The carrier gas is protected by oxygen-free protection, which can isolate sensitive water vapor and impurities, and has strong resistance to impurity interference. This method, for the first time, enables in-situ SSA measurement during high-temperature pyrolysis. By directly correlating the rate of change in gas concentration with specific surface area, it overcomes the inability of traditional methods to characterize a material's high-temperature reactivity. Its standard-free, interference-resistant, and highly efficient characteristics provide key technical support for optimizing the performance of calcium carbonate in high-temperature applications such as ceramics, environmental protection, and medicine. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a process flow chart of a method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration proposed by the present invention; Figure 2 This is a TGA graph tested by a thermogravimetric analyzer. DETAILED DESCRIPTION

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the existing known technologies. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0016] 1. Basic Methods Reference Figure 1 , a method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration, comprising the following steps: 1) Sample preparation and equipment installation ① Sample preparation Grind the calcium carbonate into 50-80 mesh, dry at 105-110°C until the moisture content is less than 0.2%, obtain a sample, and measure the initial mass m; ②Equipment installation A thermogravimetric analyzer (which can be used for weighing), a heat exchanger (the circulating water pipe is cooled through the gas channel), an electronic condenser, and a TCD detector (a thermal conductivity detector that uses the difference in thermal conductivity between He and CO2 to test CO2 concentration) are set up. The exhaust gas discharged from the thermogravimetric analyzer cavity is cooled to 200-250°C by heat conduction through the water wall of the heat exchanger, and then cooled to 60-70°C by the electronic condenser to remove moisture. The CO2 concentration is detected in the TCD detector. 2) Exhaust treatment Prior to analysis, exhaust the sample and pipelines of the thermogravimetric analyzer, heat exchanger, electronic condenser, and TCD detector using carrier gas at room temperature at an inlet flow rate of 10-15 mL / min. Record the time from the start of ventilation to the TCD detector indicating the characteristic thermal conductivity of He in the carrier gas. This time is the delay time τ (representing the time difference between the TCD detection concentration and the concentration in the thermogravimetric analyzer cavity). Continue exhausting until the concentration in the TCD detector remains constant (although He is present before this, air is still the main gas, so the CO2 concentration is low, 400-800 ppm). Exhaust is then complete. 3) Study on the relationship between gas concentration and pyrolysis reaction rate The carrier gas was continuously introduced at an inlet flow rate of 10-15 mL / min, and the temperature was raised to 1000°C at a heating rate of 10-100°C / min and kept at that temperature for 2 hours. The calcium carbonate weight loss rate Δm in the thermogravimetric analyzer and the CO2 concentration C in the TCD detector were respectively measured. The C-Δm curve was drawn and fitted. The C-Δm curve was an ascending slope, indicating that the calcium carbonate weight loss rate Δm was proportional to the CO2 concentration C. TGA chart Figure 2 As shown, the calcium carbonate weight loss rate Δm can be calculated based on the point value. The C output value derived by TCD is imported into the MATLAB software through the counter and fitted into the C-Δm curve. The subsequent Δm-SSA curve and Ct concentration decay curve are also fitted by the software. Taking the weight loss rate of 2.5g standard sample as 1% in 1min as an example, the volume of CO2 produced is 2.5×1%×44 / 100÷44×22.4=5.6ml, and the inner cavity volume is 60000cm 3, then the CO2 concentration in the thermogravimetric analyzer cavity is 5.6÷60000×10 6 +1000=1093ppm; 4) Study on the relationship between the initial pyrolysis reaction rate and the specific surface area of ​​calcium carbonate Using different specific surface areas (m 2 / g) of calcium carbonate as the standard sample, followed by 5m 2 / g、10m 2 / g, 20m 2 / g、30m 2 / g, 40m 2 / g, 50m 2 / g, 60m 2 / g, and the temperature was raised according to the steps in 3), and the weight loss rate of calcium carbonate in the thermogravimetric analyzer of each standard sample was recorded within 1 minute at the beginning of pyrolysis (i.e., the instantaneous start of the change in the CO2 concentration C in the TCD detector). The curve of the weight loss rate of calcium carbonate Δm and the specific surface area SSA of calcium carbonate was fitted and drawn. Δm-SSA is an ascending slope, indicating that the weight loss rate of calcium carbonate Δm is proportional to the CO2 concentration C, thereby indicating that the CO2 concentration C in the TCD detector at the beginning of pyrolysis is proportional to the specific surface area SSA of calcium carbonate; This conclusion is consistent with the following mechanism: using the proportional relationship between the initial decomposition rate of calcium carbonate pyrolysis reaction (CaCO3→CaO+CO2) and the specific surface area of ​​the sample: The larger the specific surface area, the more exposed reaction sites there are, and the faster the rate of CO2 production per unit time. By monitoring the initial rate of CO2 concentration increase in a closed system in real time with high precision, combined with a thermal decomposition kinetic model, the SSA can be directly calculated without the need for standard sample calibration. According to the concentration value of the TCD detector, a Ct concentration decay curve with high time resolution is obtained, and the obtained slope is the concentration change rate: Formula (1) Where C is the CO2 concentration detected in the TCD detector; t is the reaction time (s); V is the volume of the thermogravimetric analyzer cavity (cm 3 ); is the rate of change of gas production volume (cm 3 ·s -1 ); At the initial stage of the reaction (t is close to 0), the reaction is controlled by surface reactions and has little influence from diffusion. At this time, the rate of increase of gas concentration dC / dt is the largest and mainly depends on the number of reactive sites on the total surface area of ​​the sample. Calculate the instantaneous concentration change rate in the initial stage of the reaction (e.g., the first few seconds to tens of seconds, the specific time depends on the reaction rate) , obtained by differentiating the Ct curve around t = 0 (or fitting early data points to find the slope); Formula (2) Where C is the CO2 concentration detected in the TCD detector; t is the reaction time (s); V is the volume of the thermogravimetric analyzer cavity (cm 3 ); is the instantaneous concentration change rate at the initial stage of the reaction (cm 3 ·s -1 ); k is the apparent reaction rate constant (related to temperature, gas type, and surface reactivity, cm·s -1 ); SSA is the specific surface area (cm 2 ·g -1 ); m is the sample mass (g); 5) k value calibration Test and draw different specific surface areas (m 2 / g) of the standard sample, and calculate the Ct concentration decay curve of each standard sample according to the slope of the curve , and calculate the k value of each standard sample according to formula (2), and take the number average value, which is the standard k value; 6) Specific surface area determination The samples to be tested are tested according to steps 3) and 4). , after converting formula (2), we get the following formula (3). According to formula (3), we can get the specific surface area of ​​the sample to be tested: Formula (3).

[0017] A circulating water pipe with a temperature of 5-30℃ is installed inside the heat exchange tube to conduct heat through the heat transfer wall and cool down the temperature quickly.

[0018] The electronic condenser adopts a three-stage Peltier refrigeration plate combination structure. The temperatures of the three-stage Peltier refrigeration plates are 10℃, -5℃ and -15℃ respectively. It has condensed water to avoid inaccuracy in the concentration test of the TCD detector caused by moisture.

[0019] By dragging the Ct concentration decay curve obtained by the TCD detector toward the positive direction of the time axis t by the delay time τ tested in the exhaust stage, a one-to-one correspondence is basically formed with the concentration in the inner cavity of the thermogravimetric analyzer.

[0020] The carrier gas of the thermogravimetric analyzer is a mixture of He and CO2, of which CO2 accounts for 0.1% (1000ppm) and the inner cavity volume is 60000cm3 The sample input is 2-3g. The carrier gas is used to discharge the gas to the TCD detector. The difference in thermal conductivity between He and CO2 can be used to measure the CO2 concentration. In addition, an anaerobic protection is provided to prevent the interference of excessive heteroatoms in the test results.

[0021] The calcium carbonate was crushed by gravity using a 5kg roller for 10 minutes. The particles that met the particle size requirements accounted for >97% and the porosity retention was >95%.

[0022] To increase the instantaneous concentration change rate in the initial stage of the reaction To improve the accuracy, an electronic condenser is connected in parallel with multiple TCD detectors. The detection time (test time point) between adjacent TCD detectors differs by 1s. By using them in a cycle, the Ct curve can be drawn by seconds, avoiding the loss of accuracy in detecting the instantaneous concentration change rate due to long TCD test time.

[0023] 2. Test and Verification The following examples and comparative examples were designed according to the above basic method and tested and verified: Example 1 Standard specific surface area determination (BET method test SSA=20m 2 / g): Sample preparation: Take calcium carbonate ore, crush it by gravity in a 5kg drum for 10 minutes, sieve out 50-80 mesh particles (particle size 0.18-0.3mm), dry it at 105℃ to a moisture content of 0.15%, and weigh m=2.50g.

[0024] Equipment parameters: Carrier gas: He / CO2 mixed gas (CO2 1000ppm), flow rate 12mL / min; Heat exchanger: circulating water temperature 10℃, exhaust gas temperature reduced to 220℃; Electronic condenser: three-stage Peltier (three-stage temperatures are 10°C, -5°C, and -15°C respectively), outlet gas temperature is 65°C; TCD detection: 3 TCDs connected in parallel, outputting concentration data cyclically every second.

[0025] Testing process: Exhaust treatment: flush with carrier gas at room temperature for 15 minutes, and the measured delay time τ = 45s; Decomposition by heating: heating to 1000℃ at 50℃ / min, keeping warm for 2h; Data collection: After the TCD detection Ct curve is corrected by τ, dC / dt=12.3ppm / s in the initial 1min at t=0; Thermogravimetric analysis showed a weight loss rate of m = 0.98% (corresponding to a CO2 production of 5.488 mL); Calibration k=1.85×10 -3 cm / s (fitted by standard samples); SSA calculation:

[0026] Result: Measured value 19.8m 2 / g (nominal value 20m 2 / g), error 1%.

[0027] Example 2 High specific surface area samples (BET method test SSA = 50m 2 / g): Change the conditions based on Example 1: Sample: Nano calcium carbonate (nominal SSA = 50m 2 / g), m=2.52g; Heating rate: 100℃ / min (accelerated initial reaction monitoring) Initial dC / dt = 31.6 ppm / s (after correction for t = 0); Substituting into the formula:

[0028] Results: The error was 1.6%, verifying the applicability of high SSA samples.

[0029] Example 3 Low quality sample testing: Change the conditions based on Example 1: Sample weight m = 1.0 g (nominal SSA = 30 m 2 / g); Carrier gas flow rate: 10 mL / min (to reduce dilution effect).

[0030] Initial dC / dt = 7.4 ppm / s; calculate:

[0031] Results: The error was 0.3%, proving the feasibility of the test with low sample volume.

[0032] Comparative Example 1 Ignore the delay time τ correction: Change conditions: The sample was the same as that in Example 1, but the TCD Ct curve was not time-shifted by τ=45s. Other steps were the same as those in Example 1.

[0033] question: The initial dC / dt decreases from 12.3ppm / s to 8.1ppm / s, with hysteresis causing the slope to be low; Calculate SSA = 13.1m 2 / g, with an error of 34%.

[0034] Conclusion: τ correction is the key to high-precision monitoring of initial reactions.

[0035] Comparative Example 2 Electronic condenser failure (no water removal): Change conditions: The Peltier refrigeration was turned off, the gas at the condenser outlet contained water and the temperature was greater than 70° C., and the rest was the same as in Example 1.

[0036] question: Fluctuations in TCD values ​​> ±200 ppm indicate that moisture interferes with thermal conductivity; The dC / dt reading cannot be stable, and the SSA calculation fails.

[0037] Conclusion: Three-stage condensation and water removal ensures the stability of TCD data.

[0038] Comparative Example 3 Abnormal carrier gas flow: Change conditions: The carrier gas flow rate was 5 mL / min, which was lower than the standard of 10-15 mL / min in Example 1-3. The other steps were the same as in Example 1.

[0039] Problem: The exhaust phase delay time τ increases to 120s; At the initial stage of the reaction, CO2 was retained in the pipeline, and dC / dt = 6.8 ppm / s, which was only 55% of that in Example 1; SSA calculation result 11.2m 2 / g, with an error of 44%.

[0040] Conclusion: Too low a flow rate results in delayed gas delivery and distortion of the initial rate.

[0041] Comparative Example 4: Parallel TCD detection not used Change conditions: Only a single TCD is used, the data interval is 10s (1s in Example 1), and the rest is the same as in Example 1.

[0042] question: The Ct curve data points are sparse, and the fitting dC / dt is 10.5±2.1ppm / s. The error between the slope and dC / dt calculated at each point is >15%. The curve acquisition and test accuracy have certain instability. SSA=16.8m 2 / g, with an error of 16%.

[0043] Conclusion: This shows that high time resolution is a necessary condition for accurately capturing the initial reaction rate.

[0044] Example 4 Verification steps of the k value calibration process of Example 1: Test 5-60m 2 / g standard sample, draw the dC / dt-SSA curve (where the linear fitting R 2 =0.998); The k value of each sample was calculated according to formula (2), and the range was 1.82-1.87×10 -3 cm / s; Take the average value k=1.85×10 -3 cm / s, standard deviation ±1.5%.

[0045] Significance: Verifies the stability of the k value and supports the reliability of standard-free calculations.

[0046] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration, characterized in that: The following steps are involved: 1) Sample preparation and equipment installation ① Sample preparation Grind the calcium carbonate into 50-80 mesh, dry at 105-110°C until the moisture content is less than 0.2%, obtain a sample, and measure the initial mass m; ②Equipment installation A thermogravimetric analyzer, heat exchanger, electronic condenser, and TCD detector are set up. The tail gas discharged from the inner cavity of the thermogravimetric analyzer is cooled to 200-250°C by heat conduction through the water wall of the heat exchanger, and then cooled to 60-70°C by the electronic condenser while removing moisture. The CO2 concentration is detected in the TCD detector. 2) Exhaust treatment Prior to analysis, exhaust the sample and pipelines of the thermogravimetric analyzer, heat exchanger, electronic condenser, and TCD detector using carrier gas at room temperature at an inlet flow rate of 10-15 mL / min. Record the time from the start of ventilation to the TCD detector showing the characteristic thermal conductivity of He in the carrier gas. This time is the delay time τ. Continue exhausting until the concentration on the TCD detector becomes constant. 3) Study on the relationship between gas concentration and pyrolysis reaction rate The carrier gas was continuously introduced at an inlet flow rate of 10-15 mL / min, and the temperature was raised to 1000°C at a heating rate of 10-100°C / min and kept at that temperature for 2 hours. The calcium carbonate weight loss rate Δm in the thermogravimetric analyzer and the CO2 concentration C in the TCD detector were respectively measured. The C-Δm curve was drawn and fitted. The C-Δm curve was an ascending slope, indicating that the calcium carbonate weight loss rate Δm was proportional to the CO2 concentration C. 4) Study on the relationship between the initial pyrolysis reaction rate and the specific surface area of ​​calcium carbonate Calcium carbonate with different specific surface areas was used as standard sample, which was 5m 2 / g、10m 2 / g, 20m 2 / g、30m 2 / g, 40m 2 / g, 50m 2 / g, 60m 2 / g, and the temperature was raised according to the steps in 3), and the weight loss rate of calcium carbonate in the thermogravimetric analyzer of each standard sample within the initial 1 minute of pyrolysis was recorded. The curve of the weight loss rate of calcium carbonate Δm and the specific surface area SSA of calcium carbonate was fitted and drawn. Δm-SSA is an ascending slope, indicating that the weight loss rate of calcium carbonate Δm is proportional to the CO2 concentration C, thereby indicating that the CO2 concentration C in the TCD detector at the initial stage of pyrolysis is proportional to the specific surface area SSA of calcium carbonate; According to the concentration value of the TCD detector, the Ct concentration decay curve is obtained, and the obtained slope is the concentration change rate: Formula (1) Where C is the CO2 concentration detected in the TCD detector; t is the reaction time; V is the volume of the thermogravimetric analyzer cavity; is the rate of change of gas production volume; Calculate the instantaneous concentration change rate in the initial stage of the reaction , obtained by differentiating the Ct curve near t=0; Formula (2) Where C is the CO2 concentration detected in the TCD detector; t is the reaction time; V is the volume of the thermogravimetric analyzer cavity; is the instantaneous rate of change of concentration at the initial stage of the reaction; k is the apparent reaction rate constant; SSA is the specific surface area; m is the sample mass; 5) k value calibration Test and draw the Ct concentration decay curve of standard samples with different specific surface areas, and calculate the Ct concentration of each standard sample according to the slope of the curve. , and calculate the k value of each standard sample according to formula (2), and take the number average value, which is the standard k value; 6) Specific surface area determination The samples to be tested are tested according to steps 3) and 4). , after converting formula (2), we get the following formula (3). According to formula (3), we can get the specific surface area of ​​the sample to be tested: Formula (3).

2. A method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration according to claim 1, characterized in that: The heat exchange tube is provided with a circulating water pipe at 5-30°C to conduct heat transfer on the heat transfer wall and cool down the temperature quickly.

3. A method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration according to claim 1, characterized in that: The electronic condenser adopts a three-stage Peltier refrigeration plate combination structure, and the temperatures of the three-stage Peltier refrigeration plates are 10°C, -5°C and -15°C respectively. It has a condensation and dehydration function, avoiding inaccuracy in TCD detector concentration testing caused by moisture.

4. A method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration according to claim 1, characterized in that: By dragging the Ct concentration decay curve obtained by the TCD detector in the positive direction of the time axis t by the delay time τ tested in the exhaust stage, a one-to-one correspondence is formed with the concentration in the inner cavity of the thermogravimetric analyzer.

5. A method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration according to claim 1, characterized in that: The carrier gas of the thermogravimetric analyzer is a mixture of He and CO2, of which CO2 accounts for 0.1%, and the inner cavity volume is 60000cm 3 , the sample input amount is 1-3g.

6. A method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration according to claim 1, characterized in that: The grinding is performed by gravity crushing the calcium carbonate using a 5kg roller, and the grinding is repeated for 10 minutes. The proportion of particles meeting the particle size requirements is greater than 97%, and the porosity retention is greater than 95%.

7. A method for measuring the specific surface area of ​​calcium carbonate by changing gas concentration according to claim 1, characterized in that: By connecting an electronic condenser and multiple TCD detectors in parallel, the detection time difference between adjacent TCD detectors is 1s. By using them in a cycle, the Ct curve can be drawn by seconds, avoiding the impact of long TCD test time on the detection accuracy of the instantaneous concentration change rate.

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