"Determination of Dust in the Air of Workplace Part 4: Free Silica Content"
The silicate and metal oxide in the dust can be dissolved in the pyrophosphoric acid heated to 245 ℃ ~ 250 ℃, and the free silica is almost insoluble, and separation can be realized. Then weigh the separated free silica and calculate its percentage in the dust.
3.2.8 Muffle furnace: 900 ℃～1200 ℃, temperature control accuracy ±3 ℃.
1.4 Collection of samples
3.5.1 Place the collected dust samples in an oven at 105 ℃ ± 3 ℃ for 2 hours, cool them slightly, and store them in a desiccator for later use. If the dust particles are large, use an agate mortar to grind until the hand twists and feels slippery.
3.5.3 If the dust sample contains coal, other carbon and organic matter, it should be placed in a porcelain crucible or platinum crucible and ashed at 800 ℃ ~ 900 ℃ for more than 45 minutes to completely ash the carbon and organic matter. After being taken out and cooled, the residue was washed into an Erlenmeyer flask with pyrophosphoric acid. If it contains sulfide minerals (such as pyrite, chalcopyrite, chalcocite, etc.), add several milligrams of ammonium nitrate to the conical flask. Then add pyrophosphoric acid for heat treatment according to 3.5.2.
3.5.5 Fold the filter paper with sediment several times, put it in a porcelain crucible weighed to a constant weight (m1), dry and carbonize on an electric furnace; cover and leave a small gap during carbonization. Then put it into a high-temperature electric furnace, ash at 800 ℃ ~ 900 ℃ for more than 45 minutes, cool to room temperature, put it in a desiccator to cool for 1 h, weigh it to a constant weight (m2) on an analytical balance, and record it.
3.6.1 Preparation method of pyrophosphoric acid: heat 85% phosphoric acid to boiling until it does not bubbling at 250 ℃ (not exceeding this temperature), let it cool, and store it in a reagent bottle. During the heating process, a glass rod should be used for constant stirring to prevent the acid from overheating locally.
3.6.2 The temperature of pyrophosphoric acid when dissolving silicate should not exceed 250 ℃, otherwise it is easy to form a jelly.
Heating equipment in the experiment: