Key points of coal sample mining technology in coal assay, the core link to avoid errors

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Release time : 2026-04-18

The accuracy of coal testing depends not only on the detection instrument and operation process, but also on the collection and preparation of coal samples - coal samples are the basis of testing. If the coal samples are not representative and the preparation is not standardized, even if the follow-up testing operation is accurate, the test results will be distorted, which will mislead the production and trade decisions of enterprises. According to industry statistics, the measurement errors of calorific value, sulfur, ash and other indicators caused by coal sample mining account for about 96% of the total error. It can be seen that coal sample mining is an indispensable core link in coal testing. This article details the core technical points, standard procedures, and common problems and avoidance methods of coal sample mining, helping enterprises improve the accuracy of testing from the source.

The core principle of coal sample collection is "representativeness", that is, the collected coal samples can truly reflect the quality characteristics of the whole batch of coal, and the prepared coal samples can accurately reflect the composition and properties of the original coal samples. Therefore, the standard requirements of GB/T 474-2008 "Preparation Methods of Coal Samples" and GB/T 475-2008 "Manual Methods for Commercial Coal Samples" must be strictly followed, and the whole process should be standardized to avoid human errors.

The first is the main points of coal sample collection technology. The core of the collection process is "uniform distribution and scientific sampling". Different transportation methods (trains, automobiles, coal piles) have different sampling methods. The specific requirements are as follows:

Train coal sampling: each car is a sampling unit, the sampling points are evenly distributed in the four corners of the car, the center of the 5 positions, the depth is not less than 0.4m, to avoid sampling only from the surface (surface coal samples are easy to oxidize, water loss, unable to reflect the internal coal quality of the car); the sampling amount of each car is not less than 1kg, and the total amount of coal sampling in the whole batch is determined according to the batch (when the batch is ≤ 1000t, the total amount of sampling is not less than 10kg; when the batch is > 1000t, the amount of sampling is increased by 5kg for each additional 1000t).

Vehicle coal sampling: each vehicle is a sampling unit, the sampling points are distributed in the front, middle, and back of the car, and the depth is not less than 0.3 meters; the sampling amount of each vehicle is not less than 0.5 kilograms, and the total amount of the whole batch is not less than 10 kilograms.

Sampling of coal piles: Avoid sampling at the top, bottom, and edge of the pile (where coal is easily contaminated or oxidized). The sampling points are evenly distributed in a grid shape, with a depth of not less than 0.5m. The number of sampling points is determined according to the volume of the coal pile (when the volume is less than 100m ³, no less than 5 sampling points; when the volume is more than 100m ³, each additional 50m ³ is increased by 1 sampling point). The amount of sampling at each sampling point is not less than 1kg, and the total sampling amount is not less than 15kg.

When sampling, it is necessary to pay attention to the use of stainless steel or copper samplers for sampling tools to avoid contamination of coal samples with iron tools (iron filings can affect sulfur and ash detection results). After sampling, the coal samples should be promptly loaded into a sealed bag and marked with information such as sampling time, location, and batch size to avoid oxidation, moisture loss, or mixing of coal samples with impurities.

The second is the main points of coal sample preparation technology. The core of coal sample preparation is "crushing, screening, mixing, and shrinking". The purpose is to process the original coal sample into a uniform and representative analytical coal sample (particle size ≤ 0.2mm). The specific process and technical requirements are as follows:

Crushing: The original coal sample collected is crushed to a particle size less than or equal to 25mm to remove gangue and impurities from it to avoid impurities affecting the test results; during the crushing process, avoid excessive crushing of the coal sample (excessive crushing will lead to oxidation of the coal sample), and avoid incomplete crushing (large particle coal samples cannot be evenly mixed).

Screening: Use a standard sieve (25mm, 13mm, 6mm, 3mm, 0.2mm) to screen the crushed coal samples, remove the coal blocks that do not meet the particle size requirements, and ensure that the particle size of the coal samples is uniform; during the screening process, if there are many coal blocks on the screen, it is necessary to re-crush and then sieve until all of them pass through the corresponding sieve.

Mixing: Put the sieved coal sample into the mixer and mix it evenly to ensure that the components in the coal sample are evenly distributed to avoid errors caused by local coal quality differences. The mixing time should not be less than 5 minutes. When mixing by hand, it needs to be repeatedly turned to ensure uniform mixing.

Shrinkage: Shrinkage is to reduce the mixed coal sample according to a certain proportion to obtain a representative analytical coal sample. The commonly used method of shrinkage is the four-point method. Shrinkage step: Lay the mixed coal sample into a round cake with uniform thickness, divide it into four equal parts with a cross sample, remove the diagonal two parts, keep the other two parts, and repeat the shrinkage until the quality of the coal sample meets the analysis requirements (generally the quality of the analytical coal sample is not less than 100g). During the shrinkage process, it is necessary to ensure that the coal sample is evenly tiled and the sample is accurately divided to avoid the underrepresentation of the coal sample caused by the deviation of the shrinkage ratio.

Drying: The coal sample after shrinking needs to be air-dried, and the coal sample is tiled in a shallow plate with a thickness of not more than 10mm. It is placed in an environment at room temperature (20 ± 5 ° C) and a relative humidity of less than 60%. It is naturally dried to a constant weight. If the moisture content of the coal sample is too high, it can be dried in a drying oven at 45-50 ° C to avoid high temperature drying, which will lead to oxidation of the coal sample and decrease in calorific value.

The common problems and avoidance methods in the process of coal sample mining are as follows:

Problem 1: The distribution of sampling points is unreasonable, and the coal samples are not representative. For example, only sampling from the surface of the coal pile and the top of the carriage ignores the internal coal quality differences, resulting in the test results not matching the actual coal quality. Avoidance method: strictly follow the requirements of the national standard, according to the mode of transportation and the volume of the coal pile, evenly arrange the sampling points to ensure that the sampling points cover different positions of the whole batch of coal, and the depth meets the requirements.

Problem 2: Impurities or oxidation are mixed in during the preparation of coal samples. For example, iron filings are generated by the wear of crushing tools, contact with impurities during the shrinking process, or coal samples are exposed to air for a long time to oxidize, resulting in deviations in the detection results of sulfur, ash, and calorific value. Avoidance methods: Use qualified sampling and crushing tools to avoid iron filings contamination; seal coal samples in time during preparation to shorten exposure time; control temperature and humidity during drying to avoid oxidation.

Problem 3: Improper proportion of shrinkage and insufficient representation of coal samples. If the proportion of shrinkage is too large or too small, the coal samples will not reflect the original coal quality. For example, if the proportion of shrinkage is too small and the amount of coal samples is insufficient, the test results will fluctuate too much. Avoidance method: Strictly follow the four-point method of shrinkage, determine the number and proportion of shrinkage according to the total amount of coal samples, and ensure that the quality of the coal samples after shrinkage meets the analysis requirements.

Problem 4: Improper storage of coal samples, resulting in changes in properties. After the coal sample is collected and prepared, if it is not sealed and stored, it will lead to water loss and oxidation, which will reduce the calorific value and increase the ash content relatively. Avoidance method: Analysis of coal samples sealed and stored in a dryer, storage time does not exceed 7 days; original coal samples sealed and stored in a cool and dry place, storage time does not exceed 30 days, avoid direct sunlight and humid environment.

Coal sampling is the "first pass" for coal testing. Only by doing a good job of coal sampling can the accuracy of subsequent testing results be ensured. Enterprises should strengthen the professional training of laboratory personnel, standardize the mining process, regularly check the mining tools and equipment, avoid errors from the source, and provide reliable testing data support for production and trade.

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Key points of coal sample mining technology in coal assay, the core link to avoid errors

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