Determination of light transmittance

Light transmittance refers to the light transmittance (%) of the solution obtained by dissolving lignite and long-flame coal with mixed acid under specified conditions. In China coal classification, it is one of the indexes to distinguish long-flame coal from lignite and fine classification of lignite.

Low-rank coal reacts with dilute nitric acid in mixed acid under specific conditions to generate light yellow to brownish red solution. According to the color depth of the solution, the light transmittance Pm(%) of coal samples was determined by visual colorimetry with different concentrations of potassium dichromate sulfuric acid solution as the standard. Phosphoric acid in mixed acid mainly covers the interference of trivalent iron on the color of colorimetric solution.

The advantages of this method are simplicity, rapidity, good reproducibility and high identification sensitivity.

equipment

The colorimetric tube is 25mL, the inner diameter is (17 0.5) mm, and the scale is 10mL, with grounding plug.

The water bath temperature can be controlled at 0 ~ (100 0.2)℃.

For some coal samples that are easy to generate bubbles during heating treatment, special long-diameter volumetric bottles should be used, with the following dimensions: the inner diameter of the bottle mouth is (151) mm; The height from the scale line to the bottle mouth is (13.5 0.5) mm.

The inner diameter of the small glass funnel mouth is 30mm; The neck is about 40mm long and the inner diameter is 4 ~ 5 mm

reagent

Sulfuric acid (1+9).

Phosphoric acid (1+9).

Mixed acid 1 volume of nitric acid (not used when it is yellow), 1 volume of phosphoric acid and 9 volumes of water are mixed.

Weigh 2.5000g (accurate to 0.000 1g) potassium dichromate standard solution, dry it at 10 ~ 120℃ for 2 hours, and prepare 250mL solution with (1+9)H2SO4 in a volumetric flask as the light transmittance. Weigh 5.0000g (accurate to 0.000 1g) of potassium dichromate powder, dry it at10 ~120℃ for 2 hours, and prepare 250mL solution with (1+9)H2SO4 in a volumetric flask. The light transmittance of this solution is 65438.

Light transmittance (Pm) standard series solution According to Table 73.23, add the required volume of standard solution with potassium dichromate standard solution into a 50mL volumetric flask in turn, and then dilute it to scale with (1+9)H2SO4. Rinse the dry colorimetric tube 1 time with the prepared standard series solution, and then pour the solution into the colorimetric tube 10mL scale. The height of the liquid column in the colorimetric tube is consistent.

The standard series solution marked with * in the table is the standard point directly determined by repeated comparison of coal samples, and the rest points are calculated on the standard curve drawn according to the standard point or on its extension line. Standard series solutions are usually used for 2 months. If the average room temperature when preparing the standard series solution is higher than 65438 00℃, the standard series solution should be prepared again.

Table 73.23 Standard Series Solutions

Analytical method

Weigh 1g (accurate to 0.000 1g) air-dried clean coal with particle size less than 0.2mm (young lignite is easy to be slimed in water, so only raw coal samples can be used) and put it in a dry 100mL volumetric flask. When the water temperature rises to (99.5 0.5)℃ [in plateau areas, a certain amount of glycerol can be added to the water to keep the water temperature at (99.5 0.5)℃]. Pipet 25.0 ml of mixed acid and add it to the volumetric flask. While adding acid, shake the volumetric flask by hand to soak the coal sample. Immediately put the acid-added volumetric flask into the water bath pot and add the small glass funnel. It is required that the water bath temperature should be raised to (99.5 0.5)℃ within 5 minutes. After heating for 90 minutes, immediately take out the volumetric flask from the water bath, quickly cool it to room temperature, dilute it to scale with (1+9)H3PO4, plug it tightly and shake it well. After standing for 15 minutes, filter it into a dry beaker of 100mL with a dry funnel and filter paper (if a very fine coal sample is filtered, it should be filtered again), and discard a small amount of solution filtered for the first time. Discard the residue after filtration, pour the filtrate into the scale of 10mL of 25mL colorimetric tube (the height of liquid column should be adjusted to be consistent with the standard series solution), and then visually inspect the sample solution and the standard series solution.

Colorimetry should be done in a bright place, but not in direct sunlight. When comparing colors, several pieces of pure white filter paper should be lined on the lower part of the colorimetric tube, and the distance between the filter paper and the colorimetric tube should be kept at about 3cm. When comparing colors, you should look down vertically from the top of the colorimetric nozzle, and exchange the positions of the left and right sides of the standard solution and the colorimetric solution for correct judgment. If the color depth of the colorimetric solution is between two standard solutions or equivalent to a standard series of solutions, the light transmittance (Pm) of the coal sample can be obtained. For coal samples with low light transmittance, because the hue of standard series solution is inconsistent with that of coal sample solution, the light and shade of solution can be used as the standard to determine the light transmittance of coal samples. For coal samples with light transmittance less than 16%, all results are filled as less than 16%.

Matters needing attention

1) This method is suitable for lignite and low rank bituminous coal.

2) Because the open water bath temperature can't reach (99.5 0.5)℃, it can't be used.

3) Some coal samples with high pyrite content and some young lignite with high organic sulfur are oxidized into SO2 gas within ten minutes after heating, and a small amount of coal samples are carried to form foam, which bubbles at the bottle mouth; Coal samples containing more carbonate minerals will also produce CO2 gas, which will carry a small amount of coal samples to form foam and run to the bottle mouth. In this case, the volumetric flask can be shaken back and forth to reduce the generation of foam. After the coal sample is treated, use (1+9)H3PO4 to flush all the pulverized coal on the bottle mouth into the volumetric flask.

4) The light transmittance of coal samples shall be determined within 7d after Mad determination.

5) The measured light transmittance of young coal is easy to be oxidized and gradually decreased, so the light transmittance should be measured within 7 days after the analysis sample is made. Old lignite and long-flame coal are less affected by oxidation, and can generally be determined within 15d.