Sample preparation method for low rank coal fines wettability contact angle and / or capillary constant measurement

Abstract

The application provides a sample preparation method for measuring the wetting contact angle and / or capillary constant of low-rank coal powder, which is beneficial to improving the repeatability and reproducibility of the measurement of the capillary constant and wetting contact angle of the low-rank coal powder. The sample preparation method comprises the following steps: obtaining coal powder, preferably with a particle size less than 0.2 mm; preparing a measuring tube provided with a coal powder compacting mechanism for compacting the coal powder contained in the measuring tube; sealing the bottom opening of the measuring tube with filter paper, loading the coal powder into the measuring tube, and applying a preset torque to the coal powder compacting mechanism to compact the coal powder in the measuring tube.

Description

Technical Field

[0001] This invention relates to the field of measurement technology for wetting contact angle and / or capillary constant of powder materials, and particularly to sample preparation method and measurement method for measuring the wetting contact angle and / or capillary constant of low-rank coal powder. Background Technology

[0002] According to the 2021 BP Statistical Review of World Energy, coal accounts for 57% of my country's energy consumption, remaining a major component of the country's energy structure. Analysis of the quality and contact angle of low-rank coal can provide theoretical guidance for its processing, washing, and flotation applications, improving combustion efficiency and reducing energy waste and pollutant emissions. Understanding the hydrophilic / hydrophobic properties of low-rank coal surfaces can facilitate research on the slurry formation mechanism of coal-water slurry.

[0003] In contact angle testing methods, the Washburn formula is applied, and the powder capillary rise method for determining contact angle has extremely high accuracy. Traditional sample preparation methods determine the loading amount based on the powder filling height. However, for low-rank coal powder, the differences in coal rank and ash content lead to significant differences in the bulk density and compressibility of the samples, resulting in the inability to standardize the capillary constant of the coal powder filling the sample tube.

[0004] Document CN101398365 discloses a method for measuring the wetting contact angle of powder materials, including loading the powder material into a tubular container sealed at the lower end with a microporous plate and pressing the powder material tightly; placing the tubular container vertically so that its lower end contacts the liquid, allowing the liquid to wet part of the powder layer, and measuring the height and time of liquid wetting of the powder layer; according to formula h 2 / t=crγ1cosθ / 2η calculates the wetting contact angle of the powder material in the liquid.

[0005] The literature "Washburn Dynamic Method Test of Coal Surface Thermodynamic Properties" (Journal of Coal Science and Technology) introduces the wetting process of coal powder and the determination of surface free energy composition. Specifically, 2.00g of coal powder of various density grades is accurately weighed and placed into the sample cup of the KRUSS tensiometer. The lower end of the sample cup is sealed with filter paper, and the sample is compressed to a certain height in the sample tube. The sample compaction height needs to be strictly controlled during the compaction process to ensure that the volume factor of the packed bed is as consistent as possible, thus ensuring the accuracy and reproducibility of the test results.

[0006] The paper "Contact Angle Measurement and Analysis of Iron Ore Powder Based on Capillary Permeation Principle" (Shandong Chemical Industry) introduces a method for determining the contact angle of iron ore powder using the Washburn equation, which describes the permeation rate of a liquid in a capillary. Twenty types of iron ore powder were tested using cyclohexane and water, ensuring consistent column height, particle size distribution, and porosity. To ensure equal structural parameters for each column, the same raw material particle size distribution, consistent loading height, and consistent porosity were selected for each test column. The particle size of the 20 iron ore powders was controlled within the range of 120–240 mesh through sieving. The loading mass was determined based on the true density value of each iron ore powder. In the experiment, the ratio of the loading mass to the true density of the raw material for each test column was set to 1 to ensure that the columns had the same height and porosity. The true density of the iron ore powder was measured using the hydrostatic bottle method.

[0007] In summary, all methods for measuring the contact angle of powder samples using the Washburn method rely on a fixed compression height as the sample preparation condition to ensure the repeatability of the capillary constant of the packed bed. However, for low-rank coal powders with varying bulk density and compressibility, no definitive sample preparation method has been developed. Therefore, it is impossible to rely on the packing height to ensure the comparability of capillary constants for different coal powders. Thus, establishing a sample preparation method suitable for the contact angle of low-rank coal powders is extremely necessary. Summary of the Invention

[0008] In view of this, the present invention provides a sample preparation method for measuring the wetting contact angle and / or capillary constant of low-rank coal powder. Using this sample preparation method is beneficial to improving the repeatability and reproducibility of the measurement of the capillary constant and wetting contact angle of low-rank coal powder.

[0009] This invention provides a sample preparation method for measuring the wetting contact angle and / or capillary constant of low-order pulverized coal, comprising the following steps:

[0010] The coal powder is obtained, preferably with a particle size of less than 0.2 mm;

[0011] Prepare a measuring tube, which is equipped with a coal powder pressing mechanism for pressing the coal powder contained in the measuring tube;

[0012] The bottom opening of the measuring tube is sealed with filter paper, the coal powder is loaded into the measuring tube, and a preset torque is applied to the coal powder pressing mechanism to press the coal powder in the measuring tube.

[0013] In some embodiments, the measuring tube includes a top cover for covering the top opening of the measuring tube, and the pulverized coal pressing mechanism includes a connecting rod and a pressing element;

[0014] The top cover is provided with a mounting hole, the surface of the connecting rod is provided with a thread, the connecting rod passes through the mounting hole and is threadedly connected to the top cover, and the connecting rod can move up and down along the thread relative to the mounting hole;

[0015] The clamping element is located at one end of the connecting rod;

[0016] When it is necessary to compress the coal powder inside the measuring tube, the top cover is placed over the top opening of the measuring tube, and the clamping member is positioned inside the measuring tube. A preset torque is applied to the connecting rod, causing the clamping member to move with the connecting rod and compress the coal powder under the preset torque condition.

[0017] In some embodiments, the connecting rod is provided with a connection portion that can be adapted to a torque wrench, and the preset torque is applied by fitting the torque wrench onto the connection portion.

[0018] In some embodiments, the clamping element is a piston.

[0019] In some embodiments, the preset torque is 0.1 Nm-10 Nm, preferably 0.1 Nm-6 Nm.

[0020] In some embodiments, filter paper is laid on the upper surface of the pulverized coal before the pulverized coal inside the measuring tube is compressed;

[0021] And / or, before compressing the coal powder inside the measuring tube, place a support pad at the bottom of the measuring tube to support the filter paper that seals the bottom opening of the measuring tube;

[0022] And / or, the measuring tube is provided with a base support detachably connected to the bottom of the measuring tube for supporting the filter paper that seals the bottom opening of the measuring tube, so that the filter paper remains in a state of sealing the bottom opening of the measuring tube.

[0023] The present invention also provides a method for measuring the wetting contact angle and / or capillary constant of low-order coal powder, wherein the test sample is prepared using the sample preparation method described above.

[0024] The present invention also provides a method for measuring the wetting contact angle of low-rank coal powder, comprising the following steps:

[0025] 1) Prepare test samples according to the sample preparation method described above;

[0026] 2) The wetting contact angle of the low-rank coal powder was obtained by capillary rise method on a surface tension meter.

[0027] In some implementations, step 2) includes the following steps 2.1) and 2.2):

[0028] 2.1) First, use a liquid with a wetting contact angle of 0 to the test sample as the probe liquid to perform the test, obtain the liquid mass m in the measuring tube at different flow times t in the measuring tube, and plot m. 2 The curve between t and t is linearly fitted to the linear region of the curve to obtain the slope of the linear equation. This slope is used as m in the following formula (1). 2 The value of / t is determined, and the density, viscosity, and surface tension of the probe liquid are substituted into the following formula (1) to calculate the capillary constant c:

[0029]

[0030] In equation (1), t = the flow time of the liquid in the measuring tube, s; m = the mass of the liquid in the measuring tube at time t, g; σ = the surface tension of the liquid, mN / m; c = the capillary constant of the powder, g / cm³. 5 ρ = liquid density, g / cm³ 3 θ = wetting contact angle, °; η = liquid viscosity, mPa·s;

[0031] 2.2) Test with the target liquid, obtain the mass m of the liquid in the measuring tube at different flow times t, and plot m. 2 The curve between t and t is linearly fitted to the linear region of the curve to obtain the slope of the linear equation, and this slope is used as m in formula (1). 2 The value of / t is taken, and the density, viscosity, surface tension of the target liquid and the capillary constant obtained in step 2.1) are substituted into formula (1) to calculate the wetting contact angle of the target liquid on the test sample.

[0032] In steps 2.1) and 2.2), when the m 2 When the curve between t and t has multiple linear regions, the linear region with the larger slope is selected for fitting to obtain the slope of the linear equation, and this slope is used as m in formula (1). 2 The value of / t.

[0033] In some embodiments, when using a surface tension meter for measurement, the detection speed is set to 6 mm / min to 10 mm / min, the detection sensitivity is 0.005 g to 0.010 g, and the immersion depth is 1 mm to 2 mm.

[0034] The technical solution provided by this invention has the following beneficial effects:

[0035] The sample preparation method of the present invention helps to ensure the uniformity of the low-rank coal powder filling bed and improves the repeatability and reproducibility of the measurement of the capillary constant and wetting contact angle of low-rank coal powder. Attached Figure Description

[0036] Figure 1 The m obtained in step 7.1 of Example 1 2 The curve with respect to t.

[0037] Figure 2 The m obtained using bituminous coal 2 in step 7.2 of Example 1 2 The curve with respect to t.

[0038] Figure 3 This is a schematic diagram of a measurement control sample in one implementation method.

[0039] Figure 4 This is a top view of the base in one embodiment.

[0040] Figure 5 This is a simplified schematic diagram of a measurement being performed on a surface tension meter in one embodiment. Detailed Implementation

[0041] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0043] The directional terms such as up, down, left, right, front, back, front, back, top, and bottom mentioned or possibly used in this specification are defined relative to the structures shown in the accompanying drawings. They are relative concepts and may therefore vary depending on their location and usage. Therefore, these or other directional terms should not be interpreted as restrictive.

[0044] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0045] The endpoints and any values ​​of the ranges disclosed herein 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 endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0046] Where specific experimental steps or conditions are not specified in the examples, the corresponding conventional experimental steps or conditions in this technical field can be followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0047] The sample preparation method for measuring the wetting contact angle and / or capillary constant of low-order pulverized coal provided by this invention mainly includes the following steps:

[0048] Obtain coal powder; preferably, the particle size of the coal powder is less than 0.2 mm, which can be obtained by passing the coal powder through a 0.2 mm sieve;

[0049] Prepare a measuring tube 1, which is equipped with a coal powder compaction mechanism 2 for compacting the coal powder 12 contained within the measuring tube (see...). Figure 2 );

[0050] Seal the bottom opening of the measuring tube 1 with filter paper 9, load coal powder 12 into the measuring tube 1, and apply a preset torque to the coal powder pressing mechanism 2 to press the coal powder in the measuring tube.

[0051] In some embodiments, coal powder with a particle size less than 0.2 mm is obtained by sieving the coal powder, for example, by passing the coal powder through a 0.2 mm sieve to obtain an air-dried coal sample. Using coal powder with this particle size requirement helps ensure sample homogeneity and representativeness during analysis. In some embodiments, the sample preparation method can be referred to GB / T474-2008. Specifically, the coal sample to be tested is crushed, mixed, reduced in size, and air-dried to a dry basis ash content of less than 15 wt%, resulting in an air-dried analytical coal sample with a particle size less than 0.2 mm (all coal powder passed through a 0.2 mm sieve). Nitrogen gas is used for protection to prevent sample oxidation.

[0052] In some implementations, see Figure 3-4The measuring tube 1 includes a top cover 8 for covering the top opening of the measuring tube 1; the coal powder compaction mechanism 2 includes a connecting rod 4 and a clamping member 5. The top cover 8 has a mounting hole (not shown) for mounting the connecting rod 4. The surface of the connecting rod 4 has threads (not shown), and the connecting rod 4 passes through the mounting hole to be threadedly connected to the top cover 8. Specifically, the inner wall of the mounting hole has an internal thread that matches the thread on the surface of the connecting rod 4. By rotating the connecting rod 4, the connecting rod 4 can be moved up and down along the thread relative to the mounting hole (or relative to the top cover 8). The clamping member 5 is located at one end of the connecting rod 4. The connecting rod 4 has a connecting end 7 for connecting to a surface tension meter, through which the measuring tube 1 is suspended on the surface tension meter for measurement; specifically, the connecting end 7 and the end connected to the clamping member 5 are the two opposite ends of the connecting rod 4. When it is necessary to compact the coal powder 12 inside the measuring tube 1, the top cover 8 is placed over the top opening of the measuring tube 1; specifically, the top cover 8 and the measuring tube 1 can be threaded together. A portion of the connecting rod 4, mounted on the mounting hole of the top cover 8, extends into the measuring tube 1. Specifically, when the top cover 8 is placed over the top opening of the measuring tube 1, the end of the connecting rod 4 with the clamping member 5 is inserted into the inner cavity of the measuring tube 1, thus positioning the clamping member 5 within the inner cavity of the measuring tube 1. A preset torque is applied to the connecting rod 4, causing the clamping member 5 to move with the connecting rod 4 and, under the preset torque condition, to clamp the coal powder inside the measuring tube 1. Preferably, the clamping member 5 can be in the form of a piston.

[0053] In some specific implementations, see Figure 3 The connecting rod 4 is provided with a connecting part 6 that can be adapted to a torque wrench 3 (or torque wrench). A preset torque is applied by mounting the torque wrench 3 on the connecting part 6. Specifically, the connecting part 6 can be designed to fit the shape of the torque wrench 3. During use, by fitting the torque wrench 3 onto the connecting part 6 of the connecting rod 4 and rotating the torque wrench 3, the connecting rod 4 rotates accordingly, causing the clamping member 5 to press against the coal powder 12 inside the measuring tube 1, so that the clamping member 5 clamps the coal powder under the preset torque condition.

[0054] In some preferred embodiments, the preset torque is specifically 0.1 Nm-10 Nm, preferably 0.1 Nm-6 Nm. Measurements under the preferred torque conditions show better consistency of test results, and better repeatability and reproducibility. In some embodiments, the preset torque is 0.1 Nm, 0.5 Nm, 1 Nm, 3 Nm, 5 Nm, 6 Nm, 8 Nm, or 10 Nm. The torque wrench can be a digital torque wrench or other torque wrench capable of operating according to the preset torque.

[0055] Furthermore, in a preferred embodiment, see [link to preferred embodiment]. Figure 3-4The measuring tube 1 is also provided with a base 10 for supporting the filter paper 9 so that the filter paper 9 keeps the bottom opening of the measuring tube 1 sealed. Specifically, the base 10 is detachably connected to the bottom of the measuring tube 1. The bottom of the base 10 has a filter paper supporting area 14, and the bottom of the base 10 also has an opening 13 corresponding to the bottom opening of the measuring tube 1, so that most of the filter paper 9 is exposed. In use, the filter paper 9 is laid on the bottom of the base 10. The filter paper 9 is supported by the filter paper supporting area 14 at the bottom of the base 10. When the base 10 is installed at the bottom of the measuring tube 1, the filter paper 9 supported by the base 10 seals the bottom opening of the measuring tube 1. Specifically, the base 10 can be threaded to the bottom of the measuring tube 1; preferably, the filter paper supporting area 14 is an annular supporting surface provided at the bottom of the base 10.

[0056] In the specific sample preparation process, it is preferable to lay filter paper on the upper surface of the coal powder before pressing the coal powder in the measuring tube 1 to prevent the coal dust from overflowing during the sample preparation process and causing sample loss.

[0057] For the best results in the specific sample preparation process, please refer to [reference needed]. Figure 3 Before compressing the coal powder inside the measuring tube, a support pad 11 is placed at the bottom of the measuring tube, so that the filter paper 9 at the bottom of the measuring tube 1 fits against the support pad 11, preventing the filter paper 9 at the bottom from being torn and leaking powder during the sample preparation process. Specifically, the support pad 11 can be configured to fit the bottom support 10 at the bottom of the measuring tube 1.

[0058] This invention also provides a method for measuring the wetting contact angle and / or capillary constant of low-rank coal powder. This method uses the sample preparation method mentioned above to prepare the test sample. By using the sample preparation method of this invention to compress the coal powder with a preset torque, rather than determining the loading amount by the powder filling height, the differences in bulk density and compressibility between prepared samples can be significantly reduced, ensuring the consistency of the capillary constant of the coal powder in the measuring tube as much as possible. Preferably, the low-rank coal powder filled in the measuring tube is compressed with a torque of 0.1 Nm-10 Nm. Using the sample preparation method of this invention, the uniformity of the low-rank coal powder filling bed can be ensured, improving the repeatability and reproducibility of the measurements of the low-rank coal powder capillary constant and wetting contact angle.

[0059] The specific steps for measuring the wetting contact angle and / or capillary constant of low-rank coal powder can be referred to relevant methods in the art. When testing the wetting contact angle and / or capillary constant of low-rank coal powder, for parallel test samples, or for analysis and research on the wetting contact angle and / or capillary constant between different types of samples, a fixed preset torque can be used for sample preparation according to the sample preparation method of this invention. The good uniformity of the low-rank coal powder packed bed can improve the repeatability and reproducibility of the measurement of the capillary constant and wetting contact angle of low-rank coal powder, thus ensuring the reliability of the test results. Preferably, the preset torque is taken in the range of 0.1 Nm to 10 Nm. In some embodiments, the method for measuring the wetting contact angle of low-rank coal powder mainly includes the following steps:

[0060] 1) Prepare test samples according to the sample preparation method described above;

[0061] 2) The wetting contact angle of low-rank coal powder was measured using the capillary rise method on a surface tension meter. The capillary rise method is a commonly used method in this field for measuring the wetting contact angle, and will not be elaborated upon here.

[0062] In some preferred embodiments, step 2) of the method for measuring the wetting contact angle of low-rank pulverized coal of the present invention specifically includes the following steps 2.1) and 2.2):

[0063] 2.1) First, use a liquid with a wetting contact angle of 0 to the test sample as the probe liquid for testing. Obtain the mass m of the liquid in the measuring tube at different flow times t and plot m. 2 The curve between t and t is linearly fitted to the linear region of the curve to obtain the slope of the linear equation. This slope is used as m in the following formula (1). 2 The value of / t is determined, and the density, viscosity, and surface tension of the probe liquid are substituted into the following formula (1) to calculate the capillary constant c:

[0064]

[0065] In equation (1), t = the flow time of the liquid in the measuring tube, s; m = the mass of the liquid in the measuring tube at time t, g; σ = the surface tension of the liquid, mN / m; c = the capillary constant of the powder, g / cm³. 5 ρ = liquid density, g / cm³ 3 θ = wetting contact angle, °; η = liquid viscosity, mPa·s;

[0066] 2.2) Test with the target liquid, obtain the mass m of the liquid in the measuring tube at different flow times t, and plot m. 2 The curve between t and t is linearly fitted to the linear region of the curve to obtain the slope of the linear equation, and this slope is used as m in formula (1).2 The value of / t is determined, and the density, viscosity, surface tension of the target liquid and the capillary constant obtained in step 2.1) are substituted into formula (1) to calculate the wetting contact angle of the target liquid on the test sample.

[0067] In steps 2.1) and 2.2), when m 2 When the curve between t and t has multiple linear regions, the linear region with the larger slope is selected for fitting to obtain the slope of the linear equation, and this slope is used as m in formula (1). 2 The value of / t.

[0068] In some specific embodiments, when using a surface tension meter for testing, the detection speed is set to 6 mm / min to 10 mm / min, the detection sensitivity is 0.005 g to 0.010 g, and the immersion depth is 1 mm to 2 mm.

[0069] Surface tension meters, such as, but not limited to, the Krüz K100 surface tension meter, can be used.

[0070] Formula (1) above is the Washburn equation derived from Poixeuille's law. To determine the capillary constant c of the powder, in step 2.1), a highly wettable (spreading) liquid (e.g., n-hexane, n-heptane, etc.) is used for measurement. This type of liquid has a wetting contact angle θ of 0° (i.e., cosθ = 1) for the test sample. The mass m of the liquid in the measuring tube at different flow times t during the measurement process is obtained, and the mass m is then compared with the measured mass. 2 The slope of the linear equation is obtained by linear fitting of the linear region of the curve between t and t. The density, viscosity and surface tension of the probe liquid are substituted into the following formula (1) to determine the capillary constant c.

[0071] During the above measurement process, the mass m of the liquid in the measuring tube and the mass relative to m are obtained at different flow times t within the measuring tube. 2 The slope of the linear equation obtained by linear fitting of the curve with respect to t, as well as the related calculation process, can be automatically completed on a surface tension meter with corresponding information acquisition and data processing functions, such as the commercially available KRUSS K100 surface tension meter equipped with KRUSS Laboratory Deskop software or other surface tension meters with similar functions.

[0072] The invention will be further illustrated below with reference to examples:

[0073] The low-rank coal powder used in the following examples involves lignite and bituminous coal, and coal samples from different batches or sources are numbered, such as lignite 1, bituminous coal 1, bituminous coal 2, bituminous coal A, etc.

[0074] Example 1 (Fixed Torque Method)

[0075] This embodiment measures the capillary constant and wetting contact angle of three coal samples, labeled lignite 1, bituminous coal 1, and bituminous coal 2, respectively. Each coal sample was prepared and tested according to the following steps:

[0076] 1. Obtaining coal powder: In accordance with the sample preparation method of GB / T 474-2008, the coal sample to be tested is crushed, mixed, reduced and air-dried. The dry basis ash content is less than 15wt%. 50g of air-dried basis analytical coal sample with a particle size of less than 0.2mm (all coal powder passes through a 0.2mm sieve) is prepared and protected with nitrogen to prevent sample oxidation.

[0077] 2. Prepare the measuring tube, which is equipped with a pulverized coal pressing mechanism. See the structural diagram of measuring tube 1 and pulverized coal pressing mechanism 2. Figure 3 , 4 As shown, the clamping component 5 is specifically a piston, and the measuring tube 1 has a top cover 8 and a bottom support 10. For a detailed description of the structure of the measuring tube and the pulverized coal clamping mechanism, please refer to the preceding text; it will not be repeated here.

[0078] 3. Place a circular quantitative filter paper 9 of the same diameter as the measuring tube on the base 10 as a substrate, and fix the base 10 to the bottom of the measuring tube 1, so that the bottom opening of the measuring tube 1 is sealed by the filter paper 9.

[0079] 4. Before sampling, mix the coal powder thoroughly and weigh about 1.5g of coal powder into the measuring tube;

[0080] 5. Cover the top of the coal powder with filter paper, tighten the top cover 8 of the measuring tube, and place the measuring tube 1 on the support pad 11.

[0081] 6. Using a digital torque wrench and setting the torque to 1 Nm (i.e., the preset torque), engage the torque wrench 3 with the connecting part 6 on the connecting rod 4. Rotate the torque wrench vertically along the connecting rod. The piston 5 will compress the coal powder until the torque wrench emits a beeping sound. At this point, the coal powder in the measuring tube is compressed to the preset torque. Remove the torque wrench and the support pad to complete the sample preparation.

[0082] 7. Determination of capillary constant and contact angle:

[0083] Measurements were performed at room temperature (approximately 20°C) using a KRUSS K100 surface tension meter equipped with KRUSS Laboratory Deskop software. Settings: probe speed 6 mm / min, probe sensitivity 0.005 g, maximum test time 600 s; immersion depth 1 mm, wetting time 150 s, negative ions to remove static electricity from the surrounding air.

[0084] The measurement process is as follows: See Figure 5The measuring tube 1, filled with pulverized coal, is suspended on the measuring head clamp of the surface tension meter. A glass cup 15 contains the liquid used for testing. The surface tension meter's measurement program is run, and the glass cup 15 moves upward at a set detection speed. The upward movement stops when the liquid surface contacts the pulverized coal measuring tube (reaching the set immersion depth). The high-precision mechanical sensor 16 equipped with the surface tension meter detects the mass change of the powder filling the measuring tube after absorbing the liquid. The software records the corresponding measurement points, thus obtaining a curve showing the change in the square of the mass of the liquid entering the measuring tube over time (m). 2 The curve between t and t), the measurement points in the linear region are fitted as a straight line, and the capillary constant and contact angle are obtained based on the slope of the straight line; in steps 2.1) and 2.2), when m 2 When the curve between t and t contains multiple linear regions, the linear region with the larger slope is selected for fitting to obtain the slope of the linear equation. Based on this slope, the capillary constant and contact angle are then obtained. The specific steps are as follows:

[0085] 7.1 When determining the capillary constant of low-order pulverized coal, use n-hexane (analytical grade) with low viscosity and low surface tension at room temperature as the probe liquid. Place at least 35 mL of n-hexane in the glass beaker provided with the instrument. Input the density, viscosity, and surface tension parameters of n-hexane at room temperature into the instrument software, and perform the test according to the previously mentioned procedure to obtain m. 2 The curve between t and linearly fitted to the linear region yields m. 2 The slope of the linear equation with respect to t (the pattern of this curve can be found in [reference]). Figure 1 The instrument software obtains the capillary constant c of the test sample according to formula (1).

[0086] 7.2 When determining the wettability of low-rank coal to water, use ultrapure water (Grade I) as the target liquid, and place at least 35 mL of pure water in a glass beaker. Input the density, viscosity, and surface tension parameters of water at room temperature into the instrument software. Also input the capillary constant obtained in step 7.1. Perform the test according to the previously mentioned procedure to obtain m. 2 The curve between t and linearly fitted to the linear region yields m. 2 The slope of the linear equation with respect to t (the pattern of this curve can be found in [reference]). Figure 2 (The curve obtained from the test of bituminous coal 2 in Table 1). The instrument software obtains the wetting contact angle between water and the test sample according to formula (1).

[0087] Each sample was tested three times using the same steps and parameters as described above.

[0088] The test results are shown in Table 1 below.

[0089] Table 1. Measurement results of capillary constants and contact angles of different low-order pulverized coal.

[0090]

[0091] As shown in Table 1, the capillary constant range of the three coal samples tested using the method of this invention is less than 0.4*10. -7 g.cm -5 It has a contact angle range of less than 2°, high testing accuracy, and is applicable to both lignite and bituminous coal.

[0092] Example 2 (Comparison of Torque Differences)

[0093] The experimental procedure was carried out in accordance with Example 1, except that the torques set on the digital torque wrench were 0 Nm, 0.1 Nm, 1 Nm, 6 Nm, 10 Nm, and 20 Nm, and the coal sample used was bituminous coal A. Three parallel tests were conducted.

[0094] The test results are shown in Table 2 below.

[0095] Table 2. Measurement results of capillary constant and contact angle of bituminous coal A.

[0096]

[0097] Analysis of experimental results:

[0098] c = r eff ε 2 (πR 2 ) 2 (2)

[0099] Formula (2) is the capillary constant formula, where r eff ε is the effective radius of the capillary, in μm; ε is the porosity of the fine particle packed bed, in %; R is the radius of the packed bed, in mm. From the capillary constant formula, we know that c includes the number of capillaries and the average radius, and depends on both the powder properties and the properties of the test tube. Since the same test tube is used during testing, the magnitude of the capillary constant is only related to the powder properties.

[0100] Analysis was conducted on the same low-rank coal powder under different compaction conditions: loose packing (torque of 0 Nm) and compacted coal powder under different torque conditions of 0.1 Nm, 1 Nm, 6 Nm, 10 Nm, and 20 Nm. The results (Table 2) show that in loose packing (0 Nm), the capillary constant is an order of magnitude higher than in low-torque packing (1 Nm). However, due to the low interparticle stress, a stable bed cannot be formed, water cannot be adsorbed, and the hydrophilicity of different coal powders cannot be accurately distinguished. In low-torque packing (0.1-10 Nm), repeatable sample test results can be obtained, and the differences in hydrophilicity between different low-rank coal powders can be distinguished. With increasing torque, the capillary constant decreases. At a torque of 20 Nm, the capillary constant decreases to two orders of magnitude lower than in low-torque packing (1 Nm). The powder becomes dense after compression, with extremely small pores, resulting in an extremely low capillary constant of 4.3297*10. -9g.cm -5 The contact angle with water was measured using the Washburn method. The powder bed could not adsorb water, and the slope of the fitted straight line was extremely low, at 0.000641 g. 2 / s, the software calculates the inverse cosine function according to the formula, and the calculated arccosθ is 20.462. However, the range of the trigonometric function cosine value is between -1 and 1. 20.462 is not within the range of the trigonometric function, so the contact angle cannot be obtained by this method. The software displays N / a.

[0101] According to the above experiment, 0.1 Nm was the minimum setting value for the torque wrench, with 6 Nm and 10 Nm set, for a total of three torque measurement points. The repeatability of the measurements at all three torque points was good, and the range of the contact angle across the three tests was within 2°. Therefore, the range of 0.1 Nm to 10 Nm meets the repeatability requirements for contact angle testing. Comparing the average contact angle values ​​at the four effective torque measurement points, the results show that 0.1 Nm, 1 Nm, 6 Nm, and 10 Nm correspond to average contact angle values ​​of 68.13°, 69.62°, 69.46°, and 71.16°, respectively, indicating better consistency within the torque range of 0.1 Nm to 6 Nm. Therefore, the repeatability and reproducibility are better under the 0.1 Nm to 6 Nm condition.

[0102] Example 3 (Comparison of Oscillation Method)

[0103] Oscillation method test: The coal sample used was bituminous coal A. The difference from Example 1 was that steps 5 to 6 were adjusted as follows: After loosely packing the coal powder into the measuring tube, the measuring tube was held and vibrated 10 times on the experimental table to compact the powder. The sample was repeated for three parallel tests. The capillary constant test results for bituminous coal A are shown in Table 3 below.

[0104] Table 3. Measurement results of capillary constant and contact angle of bituminous coal A.

[0105]

[0106] Experimental results analysis: The shaking method has a good adsorption effect on easily spreadable liquids. However, when the sample comes into contact with water, it is loosely packed with large interparticle spacing, and the shaking itself can cause particle size segregation, failing to form a stable bed and thus unable to adsorb water. The contact angle result shows 90°, which is abnormal.

[0107] Example 4 (Comparison of Fixed Height Filling Method)

[0108] Fixed height filling method test: The coal sample used was bituminous coal 1. The difference from Example 1 is that step 6 is replaced by: instead of using a torque wrench, parallel experimental tests are not performed according to the preset torque. Instead, each time the sample is prepared, the connecting rod is rotated to make the piston press the coal powder in the measuring tube to a fixed height (sample filling height is 2cm). The test was repeated three times, and the test results are shown in Table 4 below.

[0109] Table 4. Measurement results of capillary constant of bituminous coal (sample tube filled to a fixed height)

[0110]

[0111] The results showed that the capillary constant measurements of the same sample differed significantly, making it impossible to guarantee the parallelism of sample preparation.

[0112] Experimental Results Analysis: Since low-rank coal powder is a compressible powder, loosely packed samples result in uneven loading within the sample tube. Different coal powders exhibit varying bulk densities and compression ratios under certain pressures, as shown in Table 5. Lignite has a compression ratio of 22.33% at 15 kPa, while the compression ratios of the two bituminous coals are also different, at 17.50% and 13.20%, respectively. Relying solely on filling mass or height cannot guarantee uniformity of filling, leading to differences in capillary constants and ultimately affecting the repeatability and accuracy of contact angle measurements.

[0113] Table 5. Bulk density test results of pulverized coal of different coal ranks (McFT4 powder rheometer)

[0114] Lignite 1 7.4019 22.33 Bituminous coal 1 6.4221 17.50 Bituminous coal 2 6.1721 13.20

[0115] Example 4 (Particle Size Difference)

[0116] The coal sample used was bituminous coal A. The sample was pulverized and divided into two portions. One portion was processed according to step 1 of Example 1, ensuring it passed entirely through a 0.2 mm sieve to obtain coal powder; the other portion was not sieved after pulverization. All other experimental steps were performed according to Example 1.

[0117] The experimental results are shown in Table 6. Unsieved samples contained more particles larger than 0.2 mm, resulting in larger contact angles compared to sieved samples. Passing all coal samples through a 0.2 mm sieve ensured sample homogeneity and representativeness during analysis. The results showed that sieved samples were homogeneous and had good repeatability; the range of the three tests was 1.01°, meeting the repeatability requirements for contact angle measurement.

[0118] Table 6. Measurement results of capillary constant and contact angle of bituminous coal A.

[0119]

[0120] Those skilled in the art will understand that modifications or adjustments can be made to the present invention based on the teachings of this specification. These modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A sample preparation method for measuring the wetting contact angle and / or capillary constant of low-rank coal powder, characterized in that, Includes the following steps: Obtain pulverized coal, wherein the particle size of the pulverized coal is less than 0.2 mm; A measuring tube is prepared, the measuring tube being equipped with a coal powder clamping mechanism for clamping coal powder contained within the measuring tube; the measuring tube includes a top cover for covering the top opening of the measuring tube, the coal powder clamping mechanism including a connecting rod and a clamping member; the top cover has a mounting hole, the surface of the connecting rod has threads, the connecting rod passes through the mounting hole and is threadedly connected to the top cover, and the connecting rod can move up and down along the threads relative to the mounting hole; the clamping member is located at one end of the connecting rod; the connecting rod has a connecting portion adapted to a torque wrench, by which a preset torque is applied by mounting the torque wrench on the connecting portion; The bottom opening of the measuring tube is sealed with filter paper, and the coal powder is loaded into the measuring tube. When it is necessary to compress the coal powder in the measuring tube, the top cover is placed over the top opening of the measuring tube, and the pressing member is positioned in the inner cavity of the measuring tube. A preset torque is applied to the connecting rod, so that the pressing member moves with the connecting rod and compresses the coal powder in the measuring tube under the preset torque condition. The preset torque is 0.1 Nm-10 Nm. Before compressing the coal powder in the measuring tube, a support pad is placed at the bottom of the measuring tube to support the filter paper sealing the bottom opening of the measuring tube. The measuring tube is provided with a base that is detachably connected to the bottom of the measuring tube. The bottom of the base is provided with a filter paper support area and an opening corresponding to the bottom opening of the measuring tube. The base is used to support the filter paper sealing the bottom opening of the measuring tube so that the filter paper remains in the state of sealing the bottom opening of the measuring tube.

2. The sample preparation method according to claim 1, characterized in that, The clamping element is a piston.

3. The sample preparation method according to any one of claims 1-2, characterized in that, The preset torque is 0.1 Nm to 6 Nm.

4. The sample preparation method according to any one of claims 1-2, characterized in that, Before compressing the coal powder inside the measuring tube, filter paper is laid on the upper surface of the coal powder.

5. A method for measuring the wetting contact angle and / or capillary constant of low-order pulverized coal, characterized in that, Test samples are prepared using the sample preparation method described in any one of claims 1-4.

6. A method for measuring the wetting contact angle of low-rank pulverized coal, characterized in that, Includes the following steps: 1) Prepare a test sample according to the sample preparation method according to any one of claims 1-4; 2) The wetting contact angle of the low-rank coal powder was obtained by capillary rise method on a surface tension meter.

7. The method according to claim 6, characterized in that, Step 2) includes the following steps 2.1) and 2.2): 2.1) First, use a liquid with a wetting contact angle of 0 to the test sample as the probe liquid for testing. Obtain the mass m of the liquid in the measuring tube at different flow times t and plot m. 2 The curve between t and t is linearly fitted to the linear region of the curve to obtain the slope of the linear equation. This slope is used as m in the following formula (1). 2 The value of / t is taken, and the density, viscosity and surface tension of the probe liquid are substituted into the following formula (1) to calculate the capillary constant c: （1） In equation (1), t = Time the liquid flows within the measuring tube, in seconds; m = Mass of liquid in the measuring tube at time t, in grams; σ = Liquid surface tension, mN / m; c = Capillary constant of powder, g / cm³ 5 ; ρ = Liquid density, g / cm³ 3 ; θ = Wetting contact angle, °; η = Liquid viscosity, mPa·s; 2.2) Use the target liquid to conduct the test, obtain the mass m of the liquid in the measuring tube at different flow times t, and plot m. 2 The curve between t and t is linearly fitted to the linear region of the curve to obtain the slope of the linear equation, and this slope is used as m in formula (1). 2 The value of / t is determined, and the density, viscosity, surface tension of the target liquid and the capillary constant obtained in step 2.1) are substituted into formula (1) to calculate the wetting contact angle of the target liquid on the test sample.

8. The method according to claim 7, characterized in that, In steps 2.1) and 2.2), when the m 2 When there are multiple linear regions between the curve and t, the linear region with the larger slope is selected for fitting to obtain the slope of the linear equation, and this slope is used as m in formula (1). 2 The value of / t.

9. The method according to claim 6 or 7, characterized in that, When using a surface tension meter for measurement, set the detection speed to 6 mm / min ~ 10 mm / min, the detection sensitivity to 0.005 g ~ 0.010 g, and the immersion depth to 1 mm ~ 2 mm.

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