A method for preparing ultra-fine geological samples by variable speed ball milling

Abstract

The application belongs to the technical field of superfine powder preparation, and more particularly relates to a variable-speed ball milling method for preparing superfine geological samples. A geological sample is placed in a ball mill tank containing grinding balls, and a superfine geological sample is obtained through multiple ball milling, wherein the rotation speed of the ball mill in the multiple ball milling process is sequentially increased or sequentially decreased; for a geological sample with a dosage of less than 15 g, the ball milling method can obtain a superfine geological sample with a particle size D90 of 800-1000 mesh, thereby overcoming the technical problem that a superfine powder cannot be obtained through traditional fixed-speed dry ball milling. The method is particularly suitable for ball milling of laboratory analysis geological samples, and meets the requirements of modern geological sample analysis accuracy and accuracy level.

Description

Technical Field

[0001] This invention belongs to the field of ultrafine powder preparation technology, and more specifically, relates to a method for preparing ultrafine geological samples by variable speed ball milling. Background Technology

[0002] With the rapid development of modern analytical instruments, analytical methods such as ICP-OES and ICP-MS, based on inductively coupled plasma atomization (ICP-OES), are widely used for the simultaneous determination of multiple elements in geological samples due to their low detection limits, high analytical throughput, and wide linear range. Compared with other samples, geological samples are characterized by poor macro / microscopic elemental uniformity, high particle hardness, and poor solubility / melting properties. Sample homogeneity has become a key factor affecting analytical results. Currently, most samples have a particle size of 200 mesh and a sample volume of 100 mg. For larger sample volumes, the digestion of samples requires a large amount of chemical reagents such as acids, alkalis, and / or salts, leading to a significant increase in the blank value in the sample solution, which in turn seriously affects the detection limit of the analytical method. Therefore, there is an urgent need to research and develop sample processing and preparation methods that reduce sample particle size and increase sample homogeneity, so as to minimize the sample volume (e.g., 2 mg) and meet the needs of ultra-trace element detection.

[0003] Currently, dry ball milling is commonly used in my country to prepare geological samples. This method ensures that the sample particle size reaches 200 mesh. However, with the continuous development of modern analytical techniques, the particle size of samples prepared by dry ball milling has been challenged by the development of new technologies and environmental requirements, and can no longer meet the particle size requirements of modern analytical techniques. Studies have shown that since the 1970s, my country has used 200 mesh as the technical basis for geological samples, and this has continued to this day, resulting in the problem of "geological sample particles being too large." Wang Yimin et al. studied the current situation where the 100mg sampling amount of standard material (200 mesh) is incompatible with the mainstream analytical techniques of modern geological analysis, and called on geological analysts, standard material developers, and experimental management departments to pay attention to and solve this problem. Large sample particles present several challenges. First, to ensure sample representativeness, a larger sample volume is required. For a 200-mesh sample with a D90 of 70 μm, a minimum sample weight of 100 mg is needed to guarantee reliable test results. Second, in liquid digestion, the large sample weight due to the coarse particle size hinders mass transfer, necessitating the use of more inorganic acid and longer digestion times. This results in significant inorganic acid emissions, increasing costs and causing environmental pollution. Ultrafine samples will be crucial for improving the precision and accuracy of modern geological sample analysis. Current laboratory ball milling techniques present significant challenges in preparing ultrafine samples, necessitating the exploration of new ultrafine processing technologies.

[0004] In dry ball milling for sample preparation, extending the milling time is beneficial for sample refinement, but it can easily lead to contamination by characteristic elements of the milling jar material. Furthermore, a large amount of sample adheres to the inner wall of the milling jar, significantly reducing milling efficiency and making the milled sample less homogeneous. On the other hand, longer milling times are not always better in dry ball milling. Longer milling times result in more severe powder agglomeration, which may not yield ultrafine powder. Additionally, as the milling process progresses, the powder particles become finer and less dense, making them more prone to adhering to the milling jar wall, causing sample loss. Currently, the particle size of geological samples obtained through dry ball milling is generally around 200 mesh.

[0005] Another existing technique for large-scale preparation of ultrafine powders from geological samples is air jet milling. However, air jet milling is generally suitable for preparing large quantities of ultrafine powders, such as pulverizing samples weighing tens or even hundreds of kilograms. Furthermore, the powder obtained directly from air jet milling has an uneven particle size, usually requiring an additional mixing step. For geological sample standards, due to the need for large-scale production, existing techniques generally use air jet milling for preparation. However, for laboratory analysis of geological samples, air jet milling is not suitable for ultrafine processing.

[0006] There is an urgent need for a dry ball milling method for ultrafine processing of laboratory geological samples, which can obtain ultrafine geological samples with a particle size D90 as low as 800-1000 mesh with the minimum sample volume, so as to facilitate testing after digestion. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a variable-speed ball milling method for preparing ultrafine geological samples. By employing a gradient speed increase or decrease during the ball milling process, ultrafine geological samples with a D90 particle size as low as 800-1000 mesh are prepared. This method is particularly suitable for dry ball milling ultrafineness of laboratory geological samples, enabling the acquisition of ultrafine geological samples with a particle size as low as 800-1000 mesh with minimal sample volume, facilitating direct testing after digestion.

[0008] To achieve the above objectives, the present invention provides a method for preparing ultrafine geological samples by variable-speed ball milling. The geological sample is placed in a ball milling jar containing grinding balls, and ultrafine geological samples are obtained by continuous ball milling multiple times. In the multiple ball milling processes, the ball milling speed is increased or decreased sequentially.

[0009] Preferably, the ball milling speed during the ball milling process is less than or equal to 650 rad / min.

[0010] Preferably, in the multiple ball milling processes, the time for each ball milling is less than or equal to 10 minutes, and more preferably less than or equal to 5 minutes.

[0011] Preferably, in the multiple ball milling processes, the rotation directions of any two adjacent ball milling operations are opposite.

[0012] Preferably, the multiple ball milling is 2-5 times, more preferably 3-4 times, and the rotation speed used in the multiple ball milling is successively increasing or decreasing between 200-600 rad / min.

[0013] Preferably, the multiple ball milling is a three-stage ball milling process. The first ball milling uses a rotation speed of 550-650 rad / min and a milling time of 1-5 min. The second ball milling uses a rotation speed of 400-500 rad / min and a milling time of 1-5 min. The third ball milling uses a rotation speed of 250-350 rad / min and a milling time of 1-5 min. The first and third ball milling processes rotate in the same direction, but in the opposite direction to the second ball milling process.

[0014] Preferably, the multiple ball milling is a three-stage ball milling process. The first ball milling uses a rotation speed of 250-350 rad / min and a milling time of 1-5 min. The second ball milling uses a rotation speed of 400-500 rad / min and a milling time of 1-5 min. The third ball milling uses a rotation speed of 550-650 rad / min and a milling time of 1-5 min. The first and third ball milling processes rotate in the same direction, but in the opposite direction to the second ball milling process.

[0015] Preferably, the geological sample is carbonate rock, quartzite, limestone, soil, or aquatic sediment.

[0016] Preferably, the ball-to-material ratio in the ball milling process is (2-6):1, more preferably (3-4):1; the grinding balls comprise multiple grinding balls with different diameters, more preferably, the diameter of the largest grinding ball is 14-20mm, and the diameter of the smallest grinding ball is 2-5mm; the grinding balls are made of agate and / or zirconium oxide.

[0017] Preferably, three different diameter grinding balls are used in the ball milling process. The largest grinding ball has a diameter of 14-20 mm, the smallest grinding ball has a diameter of 2-5 mm, and the middle grinding ball has a diameter of 6-10 mm. The ratio of the three different diameter grinding balls from largest to smallest is (1-5):(5-25):(10-30).

[0018] In summary, compared with the prior art, the above-described technical solutions conceived by this invention have the following advantages:

[0019] Beneficial effects:

[0020] (1) In view of the fact that the density of geological samples gradually decreases as the sample particles become smaller during the dry ball milling process, this invention proposes to use a gradient speed to perform multiple ball millings during the dry ball milling process, or to increase the ball milling speed in sequence or decrease the ball milling speed in sequence. In the preferred embodiment, the forward and reverse rotation are alternated. Finally, for geological samples with a dosage of less than 15 grams, the dry ball milling can obtain ultrafine geological samples with a particle size D90 of 800-1000 mesh, which overcomes the technical problem that the traditional fixed speed dry ball milling cannot obtain ultrafine powder or that the powder adheres to the wall of the ball milling jar, resulting in sample loss.

[0021] (2) The rotational speed gradient used in the ball milling method of the present invention is either high speed, medium speed, and low speed; or low speed, medium speed, and high speed. For different geological samples, the particle size can be greatly reduced compared with single-speed ball milling.

[0022] (3) The variable speed ball milling method for preparing ultrafine geological samples of the present invention is particularly suitable for ball milling of geological samples for laboratory analysis. Only a small amount (about ten grams) is needed to mill to 800-1000 mesh. When digesting samples of this particle size, there is no need to use a large amount of inorganic acid. Only a small amount of acid and a very short time are needed to completely digest the samples, which meets the requirements of precision and accuracy of modern geological sample analysis.

[0023] (4) The ultrafine geological samples of this invention have a D90 of less than 19 μm and a D50 of less than 10 μm. Only a few milligrams of sample weight are needed to ensure the reliability of the test results. The sample weight is greatly reduced. Correspondingly, the amount of acid used for digestion is greatly reduced during liquid digestion sample preparation, and the digestion time is significantly shortened, making it convenient to directly digest and test on the instrument. Attached Figure Description

[0024] Figure 1 This is a graph showing the particle size distribution test results of the powder obtained by ball milling the soil sample from Example 1 under the conditions of a gradient decrease in speed of 600 r / min-450 r / min-300 r / min and a forward rotation of 3 min-reverse rotation of 3 min-forward rotation of 3 min;

[0025] Figure 2 This is a graph showing the particle size distribution of powder obtained by ball milling the aquatic sediment sample from Example 2 under the following conditions: a gradient decrease in speed from 600 r / min to 450 r / min to 300 r / min, followed by 3 min of forward rotation, 3 min of reverse rotation, and 3 min of forward rotation.

[0026] Figure 3 The graph shows the particle size distribution of the powder obtained by ball milling the carbonate sample in Example 3 under the following conditions: a gradient speed increase of 300 r / min-450 r / min-600 r / min and a forward rotation of 3 min-reverse rotation of 3 min-forward rotation of 3 min;

[0027] Figure 4 The figure shows the particle size distribution test results of the powder obtained by ball milling the quartzite sample in Example 4 under the conditions of gradient speed increase of 300r / min-450r / min-600r / min and forward rotation for 3min-reverse rotation for 3min-forward rotation for 3min. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0029] The present invention provides a method for preparing ultrafine geological samples by ball milling at variable speed. The geological sample is placed in a ball milling jar containing grinding balls and ball milling is performed repeatedly to obtain an ultrafine geological sample. In the process of ball milling, the ball milling speed is increased or decreased sequentially.

[0030] In some embodiments, the ball milling speed during the ball milling process is less than or equal to 650 rad / min. Experiments of this invention have shown that performing multiple ball millings on the same geological sample, with a gradient increase or decrease in the ball milling speed, can reduce the particle size D50 of the geological sample to below 10 μm and D90 to below 20 μm. The ball milling effect is further improved when the rotation direction of the ball mill is changed. This invention refers to a ball milling process performed at the same ball milling speed and rotation direction as a single ball milling operation. In the embodiments of this invention, multiple consecutive ball millings are required for the same geological sample to obtain ultrafine powder with a particle size D90 reduced to below 20 μm and D50 less than 10 μm.

[0031] The ball mill speed here actually refers to the motor speed of the planetary ball mill. When the ball mill is working, there is a certain conversion relationship between the motor speed and the speed of the grinding jars of different ball mills. The ball mill speed defined in this invention is the maximum speed when the ball mill motor rotates and drives the symmetrically arranged grinding jars to rotate.

[0032] In some embodiments, during the multiple ball milling processes, the time for each ball milling session is less than or equal to 10 minutes, preferably less than or equal to 5 minutes, and more preferably less than or equal to 3 minutes.

[0033] During the experiment, we tried to extend the ball milling time at the same ball milling speed to reduce the particle size of the sample. However, the experiment found that after the ball milling time was extended to a certain extent, such as after 10 minutes, the particle size of the material did not change much when the ball milling time was extended further.

[0034] In a preferred embodiment, the rotation directions of any two adjacent ball milling processes are opposite.

[0035] In a preferred embodiment, the multiple ball milling is 2-5 times, more preferably 3-4 times, and the rotation speed used in the multiple ball milling is successively increasing or decreasing between 200-600 rad / min.

[0036] In some embodiments, the multiple ball milling is three ball milling processes. The first ball milling uses a rotation speed of 550-650 rad / min and a milling time of 1-5 min, preferably 2-3 min. The second ball milling uses a rotation speed of 400-500 rad / min and a milling time of 1-5 min, preferably 2-3 min. The third ball milling uses a rotation speed of 250-350 rad / min and a milling time of 1-5 min, preferably 2-3 min. The first and third ball milling processes rotate in the same direction, but in the opposite direction to the second ball milling process.

[0037] In other embodiments, the multiple ball milling is three-stage ball milling. The first ball milling uses a rotation speed of 250-350 rad / min and a milling time of 1-5 min, preferably 2-3 min. The second ball milling uses a rotation speed of 400-500 rad / min and a milling time of 1-5 min, preferably 2-3 min. The third ball milling uses a rotation speed of 550-650 rad / min and a milling time of 1-5 min, preferably 2-3 min. The first and third ball millings rotate in the same direction, but in the opposite direction to the second ball milling.

[0038] This invention, through experiments, reveals that a dry ball milling method, employing alternating forward and reverse rotation combined with multiple milling cycles at gradient speeds, can reduce the particle size (D90) of samples obtained from single-speed ball milling in the same timeframe from 200 μm in existing techniques to below 25 μm, and even as low as 13 μm (1000 mesh) and below. This yields ultrafine geological samples with particle sizes as low as 800-1000 mesh, facilitating direct testing after digestion. The reason gradient speed ball milling can reduce sample particle size is likely because during the high-speed centrifugal motion of the abrasive and grinding balls, the primary force is centrifugal force, and the sample density gradually decreases. By successively decreasing or increasing the ball milling speed, the relative velocity between the material and the grinding balls increases, promoting collisions between the material and the grinding balls, thus reducing the ball milling particle size.

[0039] The ball milling method of the present invention can be applied to different types of geological samples, including but not limited to carbonate rocks, quartzite, limestone, soil or aquatic sediments.

[0040] For quartz with high hardness and silica content, experiments have shown that unilateral gradient ball milling (i.e., without changing the direction of ball milling during gradient ball milling) is applicable; however, for samples with certain viscosity, such as soil or carbonates, experiments have shown that it is necessary to simultaneously change the direction of ball milling to promote finer grinding of the sample.

[0041] In some embodiments, the ball-to-material ratio in the ball milling process is (2-6):1, more preferably (3-4):1; the grinding balls comprise a plurality of grinding balls with different diameters, preferably, the diameter of the largest grinding ball is 14-20 mm, and the diameter of the smallest grinding ball is 2-5 mm; the grinding balls are made of agate and / or zirconium oxide.

[0042] In some embodiments, three different diameter grinding balls are used in the ball milling process. The largest grinding ball has a diameter of 14-20 mm, the smallest grinding ball has a diameter of 2-5 mm, and the middle grinding ball has a diameter of 6-10 mm. The ratio of the three different diameter grinding balls from largest to smallest is (1-5):(5-25):(10-30).

[0043] In the ball milling process of this invention, grinding balls of different materials are used to achieve different densities; preferably, the grinding balls include agate grinding balls and also include zirconia grinding balls.

[0044] The grinding jar used in the following embodiments of the present invention is an agate jar. The volume of the agate jar is 100-500mL.

[0045] In this invention, D50 represents the particle size corresponding to a sample when the cumulative particle size distribution percentage reaches 50%. D90 represents the particle size corresponding to a sample when the cumulative particle size distribution percentage reaches 90%. D97 represents the particle size corresponding to a sample when the cumulative particle size distribution percentage reaches 97%.

[0046] Example 1

[0047] Soil samples were air-dried, pre-ground, and then passed through a 10-mesh sieve for experimental use.

[0048] 15g of soil sample was weighed into a 150mL agate jar. Gradient decreasing speeds of 600r / min-450r / min-300r / min and gradient increasing speeds of 300r / min-450r / min-600r / min were used for the experiment. The running time for each speed was 1min, 2min, or 3min for either forward or reverse rotation, alternating between forward and reverse rotation. A control group with a single speed or a single milling direction was also set up. The number of agate milling beads (size 2, 14mm in diameter), size 3, 10mm in diameter, and size 4, 5mm in diameter) were 5, 25, and 10, respectively. The experimental setup and sample particle size at each stage are shown in Table 1. The particle size analysis results for the milled sample in experiment number 5 are shown in [Table 1]. Figure 1 .

[0049] Table 1. Test results of soil samples under different conditions

[0050]

[0051]

[0052] Example 2

[0053] After being air-dried and initially ground, the sediment samples from the water system were passed through a 10-mesh sieve for experimental use.

[0054] 15g of aquatic sediment sample was weighed into a 150mL agate jar. Three speed gradients (600 / 450 / 300 r / min) were selected for the milling test, with each speed gradient lasting 3 minutes in both directions. A control group was also set up with a single speed or a single milling direction. The number of agate milling balls (size 2, 14mm in diameter), 3 (size 3, 10mm in diameter), and 4 (size 4, 5mm in diameter) were 3 / 25 / 10, and 20 zirconia balls (size 4, 5mm in diameter) were used. The experimental setup and sample particle size at each stage are shown in Table 2. The particle size analysis results for the milled sample in experiment number 5 are shown in [Table 2]. Figure 2 .

[0055] Table 2. Test results of aquatic sediment samples under different conditions.

[0056]

[0057]

[0058] Example 3

[0059] Carbonate rocks are a general term for rocks whose main component is metallic carbonate, and their mineral composition includes calcite (CaCO3) and dolomite (CaMg(CO3)2). Carbonate rock samples are initially ground and then passed through a 10-mesh sieve for experimental use.

[0060] 15g of carbonate rock was weighed into a 150mL agate jar. Three speed gradients (600 / 450 / 300 r / min) were selected for the milling test. The running time for each speed gradient was 1min / 2min / 3min in both directions. A control group with a single speed or a single milling direction was also set up. The number of agate milling balls (size 2, 14mm in diameter), 3 (size 3, 10mm in diameter), and 4 (size 4, 5mm in diameter) were 3 / 25 / 10, and 20 zirconia balls (size 4, 5mm in diameter) were used. The experimental setup and sample particle size at each stage are shown in Table 3. The particle size analysis results of the milled sample in experiment number 6 are shown in Table 3. Figure 3 .

[0061] Table 3. Test results of carbonate samples under different conditions

[0062]

[0063]

[0064] Example 4

[0065] Quartzite is a rock whose main component is quartz, with a SiO2 content greater than 85%. Because quartzite is hard and difficult to crush, the sample is first calcined at high temperature, then rapidly cooled in pure water to cause it to split before preparation.

[0066] 15g of quartzite sample was weighed into a 150mL agate jar. Three speed gradients of 600 / 450 / 300 r / min were selected for the milling test. The running time for each speed gradient was 1min / 2min / 3min in both directions. A control group with a single speed or a single milling direction was also set up. The number of agate balls (No. 2 / No. 3 / No. 4) was 3 / 25 / 10, and the number of zirconia balls (No. 4, diameter 5mm) was 20. The experimental setup and sample particle size at each stage are shown in Table 4. The particle size analysis results of the milled sample in test number 6 are shown in... Figure 4 .

[0067] Table 4. Test results of quartzite samples under different conditions.

[0068]

[0069] Tables 1 to 4 show that for soil, stream sediments, and carbonate samples, using gradient speed increase or decrease, and simultaneously switching the ball mill rotation direction during speed changes, ultrafine powder with a D90 of less than 19 μm (800-1000 mesh) can be obtained within 6 or 9 minutes of ball milling. The inherent characteristics of quartzite allow for the production of ultrafine powder with a D90 of less than 19 μm (800-1000 mesh) simply by using gradient speed increase or decrease, without changing the ball mill rotation direction. Regardless of the geological sample, using a single rotation speed for the same ball milling time results in significantly coarser particle sizes, failing to meet the requirements for ultrafine samples.

[0070] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing ultrafine geological samples by variable-speed ball milling, wherein the preparation method employs a dry ball milling method, characterized in that, Geological samples are placed in a ball mill jar containing grinding balls and milled repeatedly to obtain ultrafine geological samples. The ball milling speed is either increasing or decreasing sequentially during the multiple milling processes. Any two adjacent milling processes have opposite rotation directions. The geological samples are carbonate rocks, soils, or stream sediments; The multiple ball milling refers to three ball milling processes. The first ball milling uses a rotation speed of 550-650 r / min and a milling time of 2-3 min; the second ball milling uses a rotation speed of 400-500 r / min and a milling time of 2-3 min; the third ball milling uses a rotation speed of 250-350 r / min and a milling time of 2-3 min; the rotation directions of the first and third ball milling processes are the same, but opposite to the rotation direction of the second ball milling process; or, The multiple ball milling refers to three ball milling processes. The first ball milling uses a rotation speed of 250-350 r / min and a milling time of 2-3 min. The second ball milling uses a rotation speed of 400-500 r / min and a milling time of 2-3 min. The third ball milling uses a rotation speed of 550-650 r / min and a milling time of 2-3 min. The rotation direction of the first and third ball milling processes is the same, but opposite to the rotation direction of the second ball milling process.

2. The ball milling preparation method according to claim 1, characterized in that, The ball-to-material ratio in the ball milling process is (2-6):1; the grinding balls comprise multiple grinding balls with different diameters, and the grinding balls are made of agate and / or zirconium oxide.

3. The ball milling method as described in claim 2, characterized in that, The largest grinding ball has a diameter of 14-20 mm, and the smallest grinding ball has a diameter of 2-5 mm.

4. The ball milling method as described in claim 1, characterized in that, The grinding balls used in the ball milling process are made of different materials to give them different densities.

5. The ball milling method as described in claim 4, characterized in that, The grinding balls include agate grinding balls and also include zirconium oxide grinding balls.

6. The ball milling preparation method according to claim 1, characterized in that, The grinding jar is an agate jar with a volume of 100-500 mL.

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