A method for measuring specific surface area based on microwave drying

Abstract

The present invention discloses a specific surface area determination method based on microwave drying, which includes the following steps: S1: Establishing a specific surface area discrimination model: Using a known soil body as a sample for establishing a specific surface area discrimination model, and constructing a discrimination model of the specific surface area discrimination model; S2: Determining a sample to be tested: Preparing a sample to be tested, subjecting the sample to be tested to microwave drying, determining the water content of the sample to be tested, fitting and calculating to obtain a dehumidification parameter of the sample to be tested, and discriminating the specific surface area of ​​the sample to be tested according to the discrimination model. The specific surface area determination method of the present invention combines microwave drying with a soil-water characteristic curve, extracts soil-water characteristic curve parameters of soil materials with different specific surface areas, analyzes and obtains a functional relationship between the specific surface area of ​​the soil material and the soil-water characteristic curve, and calculates the specific surface area of ​​the sample using the soil-water characteristic curve parameters of the sample obtained by the experiment.

Description

Technical Field

[0001] The invention belongs to the technical field of geotechnical engineering investigation, and particularly relates to a specific surface area determination method based on microwave drying. Background Art

[0002] The properties of geotechnical materials are mostly related to the specific surface area of ​​the particles. The size of the specific surface area can not only reflect the size, shape and physical and mechanical properties of the particles, but also serve as an auxiliary means of mineral identification. Therefore, the determination of specific surface area is of great significance for evaluating the engineering properties of soil and guiding engineering practice.

[0003] At present, the commonly used methods for determining the specific surface area of ​​soil include adsorption method, visible near infrared spectroscopy, thermogravimetry, mercury intrusion method and empirical formula method. Among them, the most commonly used method for scientific research is adsorption method, the principle of which is to cover the surface of soil particles with a layer of adsorption medium molecules, measure the mass of soil particles after adsorption, and convert the surface area of ​​the medium, i.e. the surface area of ​​the particles, through the mass difference before and after adsorption and the density of the adsorption medium. Depending on the adsorption medium, the adsorption method includes nitrogen adsorption method, water vapor adsorption method, glycerol method, ethylene glycol ethyl ether method (EGME method), ethylene glycol method (EG method), methylene blue method (MB method), etc. Although the adsorption method is widely used in scientific research due to its strong applicability and reliable results, the experiment is time-consuming; the ethylene glycol ethyl ether method (EGME method) is the shortest experimental method in the adsorption method, but it still takes 15 days of experimental time and requires professional experimental equipment; although the air-dried water content does not require professional equipment to measure the specific surface area, the experimental time required for the determination of air-dried water content is also not short, which cannot meet the needs of the project.

[0004] In general, the common specific surface area determination methods currently have certain shortcomings. Either the test equipment and test methods have high requirements, such as visible near-infrared spectroscopy and thermogravimetry; or the test time is long, such as air-drying moisture content method and adsorption method; and all of the above methods cannot determine the specific surface area of ​​the sample in a short time. Summary of the invention

[0005] In view of this, the present invention provides a specific surface area measurement method based on microwave drying. The measurement method combines microwave drying with the soil-water characteristic curve, extracts the soil-water characteristic curve parameters of soil materials with different specific surface areas, analyzes the functional relationship between the specific surface area of ​​the soil material and the soil-water characteristic curve, and calculates the specific surface area of ​​the sample based on the soil-water characteristic curve parameters of the sample obtained through the experiment.

[0006] The present invention is achieved by the following technical solutions:

[0007] A method for measuring specific surface area based on microwave drying includes two experimental stages, specifically including the following steps:

[0008] S1: Establishing a specific surface area discrimination model: Using a known soil as a sample for establishing a specific surface area discrimination model, testing the specific surface area and dehumidification parameters of the known soil, and constructing a specific surface area discrimination model;

[0009] S2: Determine the specific surface area of ​​the sample to be tested: prepare the sample to be tested, and discriminate the specific surface area of ​​the sample to be tested according to the discrimination model.

[0010] Further: Step S1 specifically includes the following steps:

[0011] S11: Determine the specific surface area of ​​the sample;

[0012] S12: preparing a specific surface area discrimination model sample;

[0013] S13: drying the sample with microwaves, measuring the mass moisture content ω of the sample as the microwave time step increases, drawing a curve of the mass moisture content of the sample and the microwave time step, fitting and calculating an expression of the change curve, and calculating the dehumidification parameter K of the discriminant model sample according to the expression;

[0014] Furthermore, step S2 specifically includes the following steps:

[0015] S21: preparing the sample to be tested;

[0016] S22: performing microwave drying on the sample to be tested, determining the moisture content of the sample to be tested as the microwave time step increases, drawing a variation curve of the moisture content of the sample to be tested and the microwave time step, obtaining an expression of the variation curve by fitting calculation, and obtaining a dehumidification parameter K of the sample to be tested according to the expression;

[0017] S23: Determine the specific surface area of ​​the sample to be tested.

[0018] Further, in step S13, the mass water content ω of the sample is calculated by the following formula:

[0019]

[0020] Where M j is the combined weight of the sample and the sample box at the jth time step, m b is the mass of the sample box, m s is the initial mass of the sample in a dry state.

[0021] Further, in step S14, the expression of the dehumidification parameter K is as follows:

[0022]

[0023] Where P is the power of microwave drying, y is the specific surface area of ​​the sample, k 1 y+b 1Represents the linear function relationship between residual moisture content and specific surface area; k 2 y+b 2 Represents the linear function relationship between the intake value and the specific surface area; θ s is the saturated volume water content, ψ r is the residual water content.

[0024] Furthermore, in step S23, the dehumidification parameter K of the sample to be tested is substituted into the expression of the discrimination model to calculate the specific surface area of ​​the sample to be tested.

[0025] Furthermore, in step S11, the specific surface area of ​​the sample is measured by ethylene glycol ethyl ether method.

[0026] Further, in step S12, the mass of the sample box is measured as m b , the mass is m s Put the sample into a sample box, add water to seal it and soak it for 7 days, then weigh the initial combined weight M of the sample and the sample box.

[0027] Furthermore, in step S13, the sample is subjected to microwave drying for multiple times, and the sample is weighed after each microwave drying until the mass difference between the two microwave dryings of the sample is less than 0.5 g.

[0028] Furthermore, before the sample is weighed, the sample box is cooled in a water bath. After the sample is dried by microwave, the temperature is high, and the mass measurement under high temperature conditions will produce a large error, so before weighing, the sample and the sample box are cooled in a water bath.

[0029] The experimental principle of the present invention is: according to the relationship between the soil-water characteristic curve of unsaturated soil and its specific surface area, a known soil body is used as a sample, and the sample is microwave-dried, the soil-water characteristic curves of soil materials with different specific surface areas are extracted, and the relationship between the specific surface area of ​​the sample and the soil-water characteristic curve is analyzed, and a dehumidification parameter is introduced to establish a specific surface area discrimination model, and the test soil body is microwave-dried to obtain the dehumidification parameter of the test soil body, which is brought into the specific surface area discrimination model, so as to calculate the specific surface area of ​​the test soil body.

[0030] According to existing research, the microwave soil-water characteristic curve fitting model is derived as follows:

[0031] Based on the law of conservation of energy, in a closed microwave device with stable output, the temperature and mass of the soil-water system are as follows:

[0032] Pt=ΔmC w (T 1 -T 0 )+Δmγ+(m-Δm)C(T 2 -T 1 )+ΔWf (1)

[0033] Where: P is the effective power input to the microwave oven; t is the microwave cumulative time step; m represents the mass of the dried soil sample; Δm is the amount of water evaporated from the sample after drying time t; m-Δm is the mass of the sample after drying time t; C w is the specific heat capacity of water; C is the specific heat capacity of the sample; γ is the latent heat of vaporization of water at normal pressure; T 0 is the initial temperature of the sample; T 1 is the saturation temperature of water at normal pressure, which is 100℃ in the test; T 2 ΔW is the temperature of a certain measuring point in the microwave heating test; f It represents the change in free energy.

[0034] The free energy of water in the soil is characterized by the change in soil suction. The change in the free energy of unsaturated soil is the change in suction:

[0035] ΔW f =ψ-ψ 0 (2)

[0036] Where: ψ is the matrix suction, ψ 0 is the initial matrix suction. When the sample starts to dry from a saturated state, the initial matrix suction is 0.

[0037] According to the study of Fredlund et al. on the pore size distribution curve of soil, the soil-water characteristic curve expression of any soil type was obtained. Bao Chenggang et al. studied and divided the gas phase of unsaturated soil and compared it with the soil-water characteristic curve of Fredlund et al., and found that the soil-water characteristic curve (SWCC) is almost a straight line between the two characteristic points of air intake value and residual moisture content. Therefore, the soil-water characteristic curve (SWCC) is characterized by a logarithmic equation and simplified as follows:

[0038]

[0039] Where: ψ is the matrix suction, ψ b is the air intake value of the soil, θ is the volumetric water content, θ s is the saturated volume water content, θ r is the residual water content, ψ r is the residual water content θ r The corresponding suction value.

[0040] Formula (3) is transformed into:

[0041] ψ=ψ r e λ (4)

[0042] Where: e is the natural logarithm,

[0043] According to unsaturated soil mechanics, the relationship between volumetric water content and mass water content is:

[0044]

[0045] In the formula, G s is the specific gravity of soil, ω is the mass water content, and e is the porosity.

[0046] Ignoring the heat energy change of the sample temperature rise and the phase change of water in the soil during microwave drying, only considering the change of free energy, substituting equations (2), (4) and (5) into equation (1), the following is obtained:

[0047]

[0048] Simplifying the above equation, the expression of the soil-water characteristic curve fitting model is:

[0049] ω=Aln(x)+B (7)

[0050] Where:

[0051] In order to eliminate the influence of the change of void ratio on the parameters during the test, the dehumidification parameter K is introduced as shown in the following formula:

[0052]

[0053] In addition, the air intake value and residual moisture content of the soil-water characteristic curve are both linear functions of the specific surface area. Therefore, the above formula can be changed to:

[0054]

[0055] Where: y is the specific surface area; k 1 y+b 1 Represents the linear function relationship between residual moisture content and specific surface area; k 2 y+b 2 It represents the linear function relationship between the air intake value and the specific surface area.

[0056] The function of the dehumidification parameter K and the specific surface area is shown in formula (9). In order to simplify the fitting operation, the present invention uses a quadratic function relationship to perform fitting analysis on the specific surface area and the dehumidification parameter K.

[0057] The present invention has the following beneficial effects:

[0058] The present invention combines microwave drying with the soil-water characteristic curve to construct a specific surface area discrimination model. The dehumidification parameter K of the soil sample to be tested is directly introduced into the discrimination model to obtain the specific surface area of ​​the soil sample to be tested. This process has a fast measurement speed, can effectively reduce the test time, and does not require chemical reagents, thereby achieving the purpose of rapid measurement of the specific surface area of ​​the soil.

[0059] The present invention expands the application field of microwave technology, provides a new test method for the determination of specific surface area, opens up new horizons for the study of microscopic properties such as clay dehumidification characteristics and mineral specific surface area, and also has important implications for the evaluation of clay engineering properties and the study of soil engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 This is a schematic diagram of microwave drying curve of a sample with 10% montmorillonite content;

[0061] Figure 2 It is the specific surface area discrimination model;

[0062] Figure 3 It is a schematic diagram of microwave drying curve of granite residual soil. DETAILED DESCRIPTION

[0063] In order to make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings.

[0064] This embodiment provides a method for determining the specific surface area of ​​a soil body based on microwave drying, and microwave drying tests are carried out on a mixed soil of montmorillonite and commercial kaolinite and granite residual soil. The mixed soil obtained by mixing montmorillonite and commercial kaolinite in different blending ratios is used as a soil sample for establishing a specific surface area discrimination model, and the granite residual soil is used as a test soil body. The test results are fitted to further illustrate the technical effect of the present invention. However, the soil samples for establishing a specific surface area discrimination model in the present invention are not limited to montmorillonite and commercial kaolinite, and other clays with different components and different specific surface areas are also applicable.

[0065] This embodiment provides a method for measuring the specific surface area of ​​soil based on microwave drying, which includes two test stages:

[0066] The first experimental stage: Establishing a specific surface area discrimination model, which specifically includes the following steps:

[0067] 1) Determination of the specific surface area of ​​the discrimination model sample

[0068] After drying the mixed soil, pass it through a 2 mm sieve, take the sieved soil material, and use the ethylene glycol ether method to determine the specific surface area of ​​the mixed soil with montmorillonite content of 0% (pure kaolin), 2%, 4%, 6%, 7%, 8%, 10%, 12%, 14%, and 18%. The results are 20m 2 / g, 32.4m 2 / g, 44.8m 2 / g, 57.2m 2 / g, 63.4m 2 / g, 69.6m 2 / g, 82m 2 / g, 94.4m 2 / g, 106.8m 2 / g, 131.6m 2 In other embodiments, other conventional specific surface area testing methods may be used to measure the specific surface area of ​​a known soil body.

[0069] 2) Preparation of discriminant model samples

[0070] (1) Take 8 plastic sample boxes with a diameter of 10 cm, number them and weigh the mass m of each sample box. b , weigh 100g±0.2g of dry soil as the sample, and record the sample mass m s Then put it into the sample box, add 80g of distilled water, seal it and soak it for 7 days;

[0071] (2) Use an analytical balance to measure the initial combined weight M of the eight sample boxes and the sample. ij (i is the number of each sample box, i=1, 2, ..., 8; j is the microwave time step, j=0).

[0072] 3) Test the specific surface area discrimination model sample

[0073] (1) Microwave heating: Take 4 samples as a group, divide them into two groups and place them in two microwave ovens of the same specifications. Turn on the microwave oven switch and heat them at a microwave power of 900W for 150 seconds. When placing the samples, the relative positions of the four samples in the two microwave ovens and the microwave ovens should be kept consistent as much as possible, and ensure that the placement position of each sample remains unchanged as much as possible during the subsequent microwave heating process;

[0074] (2) Weighing: Place each plastic sample box in a 0.6m 3 And equipped with 0.5m 3 The sample is placed in a water bath with water temperature of 20℃. After the sample has been in the water bath for 180 seconds, the moisture on the outside of the sample box is wiped off, the temperature of the sample box is lowered, and then the combined weight M of the 8 sample boxes and the soil sample is tested using an analytical balance. ij(i=1, 2, ..., 8; j=1, 2, ..., N), denoted as the mass of the i-th sample box at the j-th time step;

[0075] (3) Repeat steps (1) and (2) until the mass difference between the sample box and the soil material before and after the two microwave heatings is less than 0.5 g. The test is then completed.

[0076] (4) Calculate the water content of each sample box at each time step.

[0077] (5) Calculate the average water content of the eight samples at each time step

[0078] (6) After normalizing the water content, the trend line is drawn in logarithmic form and the fitting equation is obtained according to the principle of maximizing the fitting ratio and the fitting data must not be less than 10 points. The expression of the unsaturated section of each sample and the values ​​of parameters A and B are obtained, as shown in the attached figure. Figure 1 As shown in the figure, the fitting equation y = -1.367ln(x) + 4.1816 is obtained by fitting the sample with montmorillonite content of 10%. 2 =0.9991.

[0079] 4) Establish a digital discrimination model for the dehumidification parameter K

[0080] Fit the K value to the specific surface area of ​​each sample, as shown in the attached figure. Figure 2 As shown, the expression of the specific surface area discrimination model is obtained as follows:

[0081]

[0082] Where: A S is the specific surface area.

[0083] The second test stage: Determine the specific surface area of ​​the sample to be tested, which specifically includes the following steps:

[0084] 1) Preparation of test samples

[0085] Take the granite residual soil in a certain area as the test sample, take 8 plastic sample boxes with a diameter of 10 cm, number them and weigh the mass m of each sample box b , weigh 100g±0.2g of dry soil as the test sample, and record the mass of the test sample m s Then put it in a sample box, add 80g of distilled water, seal it and soak it for 7 days.

[0086] 2) Microwave drying of the test sample

[0087] Referring to the first test stage, the steps (1) to (6) of the test on the specific surface area discrimination model sample are carried out to obtain the water content and time curve of the test sample as shown in the attached figure. Figure 3As shown, the fitting results show that the A value of the sample to be tested is 1.401 and the B value is 4.1618.

[0088] 3) Determination of specific surface area of ​​the sample to be tested

[0089] According to the fitting result of step 2), the A and B values ​​are substituted into formula (8) to calculate the K value of granite residual soil to be 2.9706. According to the discriminant model obtained in the stage of establishing the specific surface area discriminant model, the specific surface area of ​​the test soil is calculated to be 130.9m 2 / g.

[0090] Finally, the specific surface area of ​​the granite residual soil measured by the ethylene glycol ether method is 134.6 m2 / g, and the relative error is 2.7%, indicating that the specific surface area determination method based on microwave drying described in this embodiment is feasible, the test time is short, the test speed is fast, and the required index data such as soil moisture content can be easily obtained through indoor tests. The specific surface area of ​​the soil can be tested more accurately, and the error of the established discrimination model is small. After the model is established, when the other test conditions remain unchanged, the specific surface area of ​​other soils can be tested using this model, and there is no need to re-establish the discrimination model, which can greatly shorten the test time. It also has important implications for the evaluation of clay engineering properties and the research on soil engineering.

[0091] The above descriptions are only some embodiments of the present invention, and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should be aware that any solutions obtained by equivalent substitutions and obvious changes made using the contents of the specification of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for measuring specific surface area based on microwave drying, characterized in that: The following steps are involved: S1: Establish a specific surface area discrimination model: S11: Determine the specific surface area of ​​the sample; S12: Prepare a specific surface area discrimination model sample; S13: Perform microwave drying on the sample, determine the mass water content ω of the sample as the microwave time step increases, draw a curve of the mass water content of the sample and the microwave time step, obtain the expression of the variation curve by fitting calculation, and obtain the dehumidification parameter K of the discrimination model sample according to the expression; S14: Fit the dehumidification parameter K with the specific surface area of ​​the sample to obtain a specific surface area discrimination model; the expression of the dehumidification parameter K is as follows: Where P is the power of microwave drying, y is the specific surface area of ​​the sample, k1y+b1 represents the linear function relationship between residual moisture content and specific surface area; k2y+b2 represents the linear function relationship between air intake value and specific surface area; θ S is the saturated volume water content, ψ r is the residual water content; S2: Determine the specific surface area of ​​the sample to be tested: S21: Prepare the sample to be tested; S22: Perform microwave drying on the sample to be tested, determine the water content of the sample to be tested as the microwave time step increases, draw a curve of the change of the water content of the sample to be tested and the microwave time step, obtain the expression of the change curve by fitting calculation, and obtain the dehumidification parameter K of the sample to be tested according to the expression; S23: Substitute the dehumidification parameter K of the sample to be tested into the expression of the discriminant model to calculate the specific surface area of ​​the sample to be tested.

2. The method for determining specific surface area based on microwave drying according to claim 1, characterized in that: In step S13, the mass water content ω of the sample is calculated by the following formula: Where M j is the combined weight of the sample and the sample box at the jth time step, m b is the mass of the sample box, m s is the initial mass of the sample in a dry state.

3. The method for determining specific surface area based on microwave drying according to claim 1, characterized in that: In step S11, the specific surface area of ​​the sample is measured by using the ethylene glycol ethyl ether method.

4. The method for determining specific surface area based on microwave drying according to claim 1, characterized in that: In step S12, the mass of the sample box is measured as m b , the mass is m s Put the sample into a sample box, add water to seal it and soak it for 7 days, then weigh the initial combined weight M of the sample and the sample box.

5. The method for measuring specific surface area based on microwave drying according to claim 1, characterized in that: In step S13, the sample is subjected to microwave drying for multiple times, and the sample is weighed after each microwave drying until the mass difference between the two microwave dryings of the sample is less than 0.5 g.

6. The method for measuring specific surface area based on microwave drying according to claim 5, characterized in that: In step S13, before the sample is weighed, the sample box is cooled in a water bath.

Images