Common problems encountered in coal mining samples

TOP1 I. Coal mining samples

As an inorganic and organic mixture, coal has the characteristics of uneven particle composition and chemical composition. The quality of coal will vary depending on management level, mode of transport, storage method, and mining method. Coking coal has high requirements for coal coke, cohesiveness, volatile matter, ash and other quality indicators. At the same time, the coking industry also has certain requirements for the content of harmful elements in coal, mainly to reduce environmental pollution during coking. Coal is a heterogeneous mixture that must be characterized for coal quality prior to coking.

The main links in coal quality analysis are coal quality testing, sample preparation and sampling. According to the relevant investigation, the error generated by the sampling section accounts for about 80% of the total error of coal quality analysis, and the error generated by the sample preparation section accounts for about 16% of the total error of coal quality analysis, while the error rate of coal quality analysis and testing is small. This is mainly because the quality index of the batch coal is obtained by a small amount of analytical samples, and the analysis samples are taken from the bulk commercial coal, and undergo a series of shrinkage and fracture. Therefore, to ensure the accuracy of coal quality analysis, it is necessary to ensure the representativeness of the coal sample, and minimize the error in the process of preparing and analyzing the sample.

TOP2 II, common problems in the sample preparation

1. Insufficient attention to coal mining samples

Sampling of coal shortage emphasis some enterprises, resulting in no position to collect and process samples were sufficient funds invest in their daily work, nor its effective supervision and inspection. Even some enterprises have not been classified as technical work, and the professional quality and education level of the sample workers are low. In order to reduce the cost of sample preparation, some companies even choose employees who have not been trained to carry out sample preparation work.

2, the sample preparation procedure is not standardized

The test unit will be affected by different body weights. Because there is no concept of the test unit and the volume, there is a lack of standardization in the process of sample preparation. When taking and preparing samples, the work of the sample preparation personnel is not subjective according to the corresponding standards. If the basic laboratory unit does not conform to the specified standards, it often does not reduce or increase the number of subsamples according to the regulations. This causes the number of subsamples in the actual sampling process to be inconsistent with the regulations, drying, shrinking, There is also no strict follow-up procedure for screening and crushing.

3. All localities have different specifications for equipment and tools for manual sampling.

According to the corresponding regulations, the diameter and size of the sampling tool should be determined according to the nominal maximum particle size of the sampled coal, so that the minimum diameter, width and length of the sampling tool are 3 times of the nominal maximum number of samples, and each The quality of the coal sample taken once meets the minimum sub-sample quality requirement of the corresponding particle size. However, in the actual operation process, the sampling tools often fail to reach the corresponding size, and it is impossible to take more than 150 mm of vermiculite and coal in the raw coal, which also leads to frequent leakage and serious influence on the representativeness of the sample.

4, the sampling method has limitations

In general, mechanized mining samples have better national standard execution, can eliminate the influence of human factors, and the samples are also more representative. However, due to the difficulty in cleaning the equipment, it is easy to appear mixed. Manual sampling is the main sampling method of most coke production enterprises. This is because the non-uniformity of coal composition will impose certain restrictions on mechanized mining samples.

TOP3 III. Common problems and corresponding countermeasures in the sampling process

1. Coal sampling

Coal sampling is common in practice, but it has a relatively low sampling representativeness. Therefore, it is not recommended to directly sample on a stationary large coal pile, mainly in the following aspects.

First, in the sampling, the sampling points can only be distributed on the surface of the coal pile, and the middle part of the coal pile cannot be taken. However, under the influence of particle size separation and segregation, the quality of the surface of the coal pile and the internal quality are quite different. Therefore, the total sample quality taken on the surface cannot represent the quality of the whole batch of coal.

Second, according to the “Manual Method of Commercial Coal Samples”, coal sample sampling samples should be distributed at the bottom, waist and top of the coal pile. Under normal circumstances, the sampling personnel have certain subjectivity for the position of the bottom, waist and top. For various reasons, it is often impossible to take the coal sample at the top of the coal pile, which seriously affects the sampling accuracy. It is worth noting that in the actual situation, the standard conical coal pile is relatively rare, which requires a flexible and diversified operation. Based on the particle size distribution and shape of the coal pile, the appropriate sampling sub-samples are selected. The ratio is generally 1:3:5.

Third, if it is a coal pile for arbitration, or a coal pile for commercial coal for settlement, it can be layered and sampled during loading and unloading, or the coal pile can be transferred, and sampling work can be completed during the migration process.

2, car sampling and train top sampling

In the top sampling of cars and trains, the manual sampling method can only sample the coal samples in the middle and surface of the car, and the coal samples at the bottom are difficult to be collected, which also results in a lower representativeness of the samples. Moreover, it is necessary to carry out high-altitude operations, which has a large labor intensity and a certain safety risk. Therefore, mechanized sampling should be used for sampling, which can improve the representativeness of the sample and reduce the labor intensity and safety risk.

After determining the number of sub-samples, the principle of “uniform distribution” should be strictly followed in the arrangement of sub-samples, so that the chances of each part of coal being mined are equal. The sub-samples should be evenly arranged according to the system sampling method or the random sampling method. This is to ensure the representativeness of the sample and to be able to react to the quality of the coal product.

3. Coal flow sampling

China has clearly defined the sampling time interval of sampling in the coal flow. However, there is a problem of inconsistent interval time in manual sampling. The sampling personnel often judge the interval time only by experience. Moreover, the coal flow has a relatively fast speed, and is not easy to sample in the process of manual transportation, and is also prone to safety hazards. The drawback of manual sampling is that a cross-section is taken in two or three times in the coal flow section depending on the amount of coal flow to take a subsample. This requires sampling in a certain order, do not repeat sampling, and avoid errors caused by granular segregation. Manual sampling in the coal flow on the belt is not recommended in GB 475-2008. Therefore, if conditions permit, install the belt middle or belt end sampler as much as possible to take the full section of the coal flow and ensure equal Interval time to improve the representativeness of the sample. In order to achieve the corresponding precision of coal flow sampling, the number of sub-samples is determined and the quality of each sub-sample is specified. When the number of subsamples or subsamples does not meet the requirements, it is not representative, and it is difficult to achieve a certain sampling precision.

TOP4 IV. Common problems and corresponding countermeasures in the sample preparation process

1. When performing manual sample preparation, it is easy to artificially simplify the sample preparation procedure, and it is not strictly in accordance with the corresponding standards in the reduction, mixing, screening and stepwise crushing. When preparing a sample having a particle size requirement, it should be crushed by a stepwise crushing method. The equipment and sample preparation tools should be cleaned before crushing and shrinking work.

2. When analyzing the preparation of coal samples, if the coal sample is too wet, the particle size should be adjusted first, and the initial drying can be carried out to ensure that it can pass through the shrinking machine , sieve and crusher smoothly. However, in the process of sample preparation, many sample preparation personnel did not strictly follow the relevant procedures for drying. The coal samples were directly shrunk, sieved and broken, and the equipment was not cleaned in time, which easily caused mixed samples.

3. There is no systematic method for drying coal samples. Some enterprises do not pay attention to controlling the drying time of coal samples, so that the coal samples are exposed to high temperature for too long and cause oxidation, which has an effect on the cohesiveness and coking property of the coal samples, so that the thickness and bonding index of the coal samples are both There have been different degrees of reduction. Normally, the coal sample can be placed in an oven with an air circulation device below 50 ° C for drying to an air dry state. There are also some companies that use the natural drying method to spread the coal sample into a uniform thin layer and balance it with atmospheric humidity at ambient temperature.

4. When preparing a full-moisture coal sample, it is necessary not only to consider the laboratory requirements, but also to consider the external moisture content of the coal sample. In order to reduce the loss of water, the preparation speed of the whole moisture coal sample should be increased as much as possible, and it should be extracted as early as possible, and it should not be repeatedly broken and shrunk. Try to obtain a full moisture sample from the original size coal sample in one crush.

5. After the preparation of the coal sample is completed, it is also allowed to stand in the air for a period of time to balance the coal sample with the air humidity. Then put it into the coal sample bottle. The amount of coal to be loaded should not exceed three-quarters of the total volume, leaving a certain amount of space for mixing and mixing.

TOP5 five, the conclusion

In the coking industry, there is a large demand for coal and a high demand for the quality of coal. Coking enterprises should pay more attention to the quality of coal, take coal quality indicators as an important production basis, and take effective measures to improve the accuracy of coal quality analysis. It is necessary to increase the emphasis on coal mining and sample preparation, increase management efforts, and reduce manual errors to ensure the quality of coal.

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