How to reduce the error in the use of coal quality analytical instruments

1 Coal quality analysis concept and content Coal is a kind of solid combustible material with very complex nature. Due to the difference of the original material and deposition environment, the nature and composition of coal are also different. In order to deeply study the composition of coal, in order to fully understand the nature of coal and the law of coal quality changes; to determine the composition and properties of coal processing products, and to adapt to the needs of coal production and processing and use to establish a rapid and online analysis, to achieve rapid Accurately controlling the quality of coal processing products requires the analysis of coal quality and the testing of coal products in order to use coal reasonably and efficiently.

The use of coal is extensive, and various coal utilization methods, processes, and equipment for coal use have specific technical requirements for the properties of coal. Therefore, coal analysis and test methods should have the ability to accurately reflect the nature of the test object; can clearly distinguish the samples of different properties; method accuracy and precision, repeatability, reproducibility; test methods and equipment simple and easy , conducive to standardization. In addition to meeting the above basic requirements, the coal testing methods, due to the complexity and variability of coal properties, have many specific requirements and characteristics, such as strong test methods, large sample inhomogeneity, sample composition and properties. The scope of change is large, and the test items and test methods are numerous and inconsistent.

In view of the complexity, variability and various requirements of various uses of coal, in order for people to have a common understanding of the results and data of various coal quality analyses, and without misunderstandings, all countries have made uniform regulations for coal quality analysis tests. China has also established a national standard for coal quality analysis test methods. The national standards have all been unified from the determination methods of coal samples, reagents, reagent solution preparation, calculation and expression of analysis results, precision, symbol, analysis value, and the conversion of reported values ​​and conversion of various “bases”. Provisions.

1.1 Coal-like coal sample refers to a representative sample taken to determine certain characteristics of coal according to the prescribed method, that is, taking a small representative sample from a large number of coal products for analysis and testing to obtain the batch of coal. Average quality. Sampling, sample preparation and analytical tests must be carried out in accordance with regulations in order to obtain specific indicators for various analytical tests. In the analysis of coal quality, due to the specific nature of coal and different indicators, there are different technical requirements for coal samples. To obtain representative and more accurate analysis results, strict methods of operation are required for the adoption and preparation of coal samples. .

1.2 Determination In the analysis of coal quality, except for special requirements, each analysis test shall perform two parallel determinations on the same coal sample, which is generally referred to as repeated measurement or parallel determination. If the difference between the two measurements does not exceed the specified limit, that is, the same laboratory allowable error “T”, then take the arithmetic average as the measurement result; otherwise, perform the third determination; if the three measurements are less than 1.2T , Take the arithmetic mean of three measurements as the measurement result, otherwise it is necessary to perform the fourth determination; if the extreme value of four measurements is less than 1.3T, then take the arithmetic average of four measurements as the result; If the range difference is greater than 1.3T, and the range difference among the three measured values ​​is less than 1.2T, the arithmetic average of the three measured values ​​may be taken as the result. If none of the above conditions is met, all measurement results should be discarded and the instrument and operation checked, and then remeasured.

1.3 Reagents Chemical reagents are usually divided into chemical reagents, analytical reagents, pure reagents and reference reagents. As with general chemical analysis methods, reagents used in the analysis of coal quality generally use analytical reagents unless otherwise specified; if the reagents used in analytical methods do not require high purity, chemical reagents may be used; for example, standard solutions for titration are prepared. It should be formulated using a reference reagent. The water used in the analysis of coal quality is distilled water or water of equivalent purity. In the rigorous analysis, ionized water treated with anion and cation exchange resins is sometimes used.

1.4 Solution In the analysis of coal quality, all solutions except those that have been indicated as solute are aqueous solutions. A solution containing a certain amount of solute should be a solution with a defined concentration. The concentration of a solution commonly used in the analysis of coal quality is the concentration of the substance. The amount of material contained in a unit volume of solution, in mol/m3 or mol/L, or as a percentage concentration, expressed as mass fraction (%) or volume fraction (%).

1.5 Precision of the determination method The accuracy of the determination method in the analysis of coal quality is expressed in terms of repeatability and reproducibility.

Reproducibility, that is, the allowable error of the same laboratory, refers to the difference between the measured results in the same laboratory, by the same operator, the same instrument, and the same analysis of coal samples, in the short-term results (at 95% Probability) The critical value.

Reproducibility, which is the allowable error of different laboratories, refers to the average of the results of repeated measurements made on representative samples taken from the same coal sample at the final stage of coal reduction in different laboratories. The critical value of the difference (at a certain probability).

1.6 Calculation and expression of results The determination results of coal quality analysis are generally carried out according to the rounding rules of rounding, where the first digit after the last significant digit is greater than 5 is increased by 1 in the former digit, and less than 5 is discarded; If the first digit after the last significant digit is equal to 5, and the digits following 5 are not all zero, then 1 is added to the previous 5 digit; if the digits following 5 are all zero, and 5 is the previous digit If the number is odd, then 1 is added to the first digit of 5; if the previous digit is even (including zero), 5 is discarded. If there are two or more digits in the number to be discarded, multiple rounds of repairs cannot be carried out successively. The measurement results should be revised once according to the above-mentioned provisions according to the size of the first digit on the left in the number to be discarded.

2 Causes of Error Although modern analytical instruments and techniques have been widely used in coal quality analysis, in the analysis and analysis of coal quality, the laboratory personnel use instruments and drugs, and undergo certain steps such as weighing, Melting, dissolving and separating, then the results of the determination of coal quality can be obtained. In the above process, even the most skilled laboratories use the most sophisticated instruments and the reagents with the highest purity, due to limitations in the accuracy of the measuring instruments, limitations in the sensitivity of the human sense organs, and the relative purity of the reagents, etc. The reason, but can not get absolutely accurate test results. This means that there must always be a difference between the result of the measurement and the actual value, which is the error of the measurement. The analysis error can be roughly classified into two categories: system error and accidental error.

2.1 Systematic errors Systematic errors are the differences that result from fixed causes. These errors are close to each other and are of the same sign (positive or negative) and are often repeated. There are three main causes of system error: First, the instrument. For example, due to the use of uncalibrated weights, or unequal lengths of the arms of the analytical arm of the arm; the use of uncorrected burets, for example, can lead to systematic errors. The second is reagents. For example, impurities such as impure reagents or impurities in distilled water cause repetitive errors of the same sign and the same value. The third is the measurement method. For example, in the acid-base titration, the error caused by the inconsistency of the equal point and the end point.

2.2 Occasional errors Occasional errors are not fixed, but can vary. In several measurements, they are large, small, positive, and negative. Although accidental errors do not repeat regularly as systematic errors do, but if you look closely at a large number of experiments, you can find that accidental errors are also regularly followed, such as a small number of large errors, a small number of small errors, positive errors and The chance of negative errors is almost equal. The cause of accidental errors is roughly the following two factors: First, the operator's negligence. For example, when looking at a burette reading, because the last reading (less than 0.1 mL) is estimated, the difference in multiple estimates can cause accidental errors. The second is the unexpected factor. For example, changes in ambient temperature, current, voltage instability, etc., are all conditions that cannot be controlled by the operator. They will cause accidental errors.

3 Methods to Reduce Errors To understand the causes of errors in coal quality analysis, we can take targeted measures to reduce errors.

3.1 Systematic Errors Since the systematic errors are caused by instruments, reagents, and measuring methods, we must use calibrated instruments when testing. Such as the weight of the balance, the arm length of the arm balance, and electronic balances, thermocouples, etc., are regularly identified, and calibration samples are regularly tested to determine the operation of the instrument. Reagents must use analytical reagents, the container must be clean, to ensure that distilled water does not contain impurities. In the measurement methods, such as the acid-base titration, due to the use of different indicators, different end points can be obtained. Of course, the titration end point will not coincide with the isocenter point. Therefore, the selection of the indicator becomes an important issue. This requires us to test many times in the experiment. Selecting the range of the pH value of those pH values ​​and the indicator that the pH is close to the equilibrium point can reduce the titration error.

3.2 Occasional errors Due to the specificity of coal itself and different indicators, there are different technical requirements for coal samples. To obtain representative and accurate analysis results, coal samples should be taken strictly in accordance with national regulations for the preparation and preparation of coal samples. , preparation, testing, such as in the wagon, coal flow and other sampling, must be done evenly, not leaking points, sub-sample weight is not less than the prescribed weight. Because coal is a porous structure of hygroscopic substances, the air and the water in the environment have a great influence on the actual quality of the sample. The coal sample used in the analysis of coal quality has special requirements except for the use of large-size coal. In addition, it should generally be air-dried coal samples after crushing and shrinkage treatment. In order to avoid significant differences in air humidity at different times and in different regions, the results of coal quality analysis are significantly different. When various coal quality analysis items such as ash, volatile matter, elemental analysis, and calorific value are used, it is best to simultaneously Measurement results to ensure the accuracy of the measured values ​​of the indicators. If actual measurements cannot be performed at the same time, it should be carried out within the shortest possible time, that is, the period during which the coal sample water does not change significantly (up to 7 days at most). After the coal sample is made, it should be packed into a tight container, usually a glass bottle with a fine-grinded glass stopper or a plastic stopper. Before weighing, the coal sample should be fully mixed, and then weighed and tested. At the same time, the coal sample must be operated according to the regulations when it is broken or reduced.

At present, due to the use of modern instruments, subjectively estimated data may need to be reduced. However, data may still need to be estimated. For example, when looking at a buret reading, for the final reading, multiple estimates may cause accidental errors. This requires our operators to read with a standard of inertia to minimize errors.

Due to errors caused by environmental changes, currents, voltages and other unexpected factors, we can try our best to keep the laboratory in a relatively closed environment. With the current more advanced equipment, the temperature and humidity of the laboratory are all in a relatively stable environment. The errors caused by this can be reduced.

Accidental errors and systematic errors are different. After repeated measurements, it can be found that the chances of occurrence of absolute signs are the same. Therefore, accidental errors can be reduced to near elimination through multiple parallel measurements.

Accuracy refers to the degree of coincidence between the measured value and the true value. We must obtain a high degree of accuracy. The system error must be reduced to the minimum. On the premise of the minimum system error, the accidental error is minimized, so that the decrease can be achieved. Coal quality analysis error.

Due to the special nature of coal, the coal quality analysis test method is more normative. Although errors are difficult to avoid during the measurement process, errors can be minimized as long as you master the operating techniques and work in strict compliance with national standards. Therefore, coal quality analysts should be good at judging the correctness of the analysis results, find out the cause of the error, and correct it to minimize the error in the analysis of coal quality.