Method of test for length distribution (wet classification of fibre length) of chrysotile asbestos fibre using Bauer Mcnett classifier
1980 Edition

Per the guidelines in Clause 4.1 of IS 9691, the sample weight, sieve sizes, and test times vary by fibre group as follows:

Additional notes include employing filtered water for washing fibres (Clause 4.2.5) and determining fibre retention percentages by subtracting cumulative retained masses from the total sample mass (Clause 5.1). Appendix A provides a sample computation.

To correctly set up the Bauer-McNett classifier as per IS 9691 for precise fibre length distribution analysis:

• Arrange four elliptical tanks in series, each fitted with a mesh screen of decreasing size to separate fibres by length.
• Equip each tank with vertical cylindrical agitators having vertical fins and midfeathers aligned parallel to the screens to maintain a consistent, high-velocity circulation.
• Use vacuum suction cups with filter cloths (75 µm IS sieve or 45 µm cloth for float products) for drainage.
• Essential accessories include a precision balance (0.01 g sensitivity), 1000 ml beaker, filter papers, T-shaped rubber scraper, rinsing hose with control valve, drying oven or IR unit, overflow alarm, and optionally an automatic clearing device on the fourth tank.

During operation, introduce the sample into the highest tank and allow it to flow by gravity through the successive tanks. The agitation keeps fibres aligned and avoids screen clogging. Screen cleaning should be avoided during classification unless overflow risks occur, as frequent cleaning impairs reproducibility (Clause 6.6).

IS 9691 identifies the following sieve sizes for sorting chrysotile asbestos fibres in Group a (Groups 3, 4, 5, 6 & 7 D):

• 4.75 mm
• 1.18 mm
• 425 micron (µm)
• 75 micron (µm)

The sample mass recommended is 10 g with a test duration of 20 minutes. These correspond to ASTM and Tyler sieve standards as follows:

This ensures consistent particle size separation for chrysotile asbestos fibre analysis.

To determine the fibre length distribution percentages as per IS 9691:

1. Measure the mass of fibres retained on each sieve after wet classification.

2. Deduct the mass of the filter paper to obtain the net fibre mass per fraction.

3. Calculate the percentage retained on each sieve:

   % Retained = (Mass retained on sieve / Original sample mass) × 100

4. Compute the percentage passing the last sieve by subtracting the total retained percentages from 100%:

   % Passing = 100 - Sum of % Retained on all sieves

5. If duplicate test results differ by more than 5% for any fraction, conduct a third test and report the average of the two closest results.

Refer to Appendix A for a detailed example.

To preserve reproducibility and protect the classifier from damage according to IS 9691:

• Keep the classifier tanks filled with water when not in use to prevent residue build-up on screens (Clause 6.5).
• Avoid cleaning the screens during classification tests unless overflow occurs, as cleaning during tests reduces reproducibility (Clause 6.6).
• Carefully follow initial tank filling and preparation procedures outlined in Clauses 4.2.6 and 4.2.7.
• Ensure agitators and midfeathers operate correctly to maintain high uniform velocity, keeping fibres parallel to screens and preventing clogging.

These measures help maintain equipment integrity and ensure reliable fibre length distribution results.