Method of test for length distribution (wet classification of fibre length) of chrysotile asbestos fibre using Turner and Newall classifier
1985 Edition

IS 11267 encompasses chrysotile asbestos fibre grades ranging from 4A to 7D inclusive. Specifically, it excludes ultrafine powders with negligible retention on a 75 micrometer IS sieve and covers grades 4A, 4B, 4C, 5A, 5B, 6A, 6B, 7A, 7B, 7C, and 7D. This ensures consistent testing and quality control for fibres utilized in asbestos cement products.

The Turner and Newall classifier comprises four vertically stacked rotating plastic vessels, each 127 mm in diameter, fitted concentrically with 98 mm diameter screen cloths of decreasing mesh size. IS sieve sizes used include 2.36 mm, 1.18 mm, 600 µm, 300 µm, and a base 75 µm sieve measuring 203.2 mm in diameter. The apparatus is equipped with stirrers to prevent clogging and discharge spouts directing flow between vessels. Operation involves wetting the sieve discs, assembling them in the correct order, placing a wetted 75 µm test sieve at the base, initiating water flow at 80 ml/s, positioning the water jet 3 mm from the inner edge of the lower bevelled screen part, and ensuring leak-free clamping. This setup allows precise particle size classification through controlled water flow and mechanical agitation.

Key parameters to control during wet classification include maintaining the water flow rate at 80 ml per second (equivalent to 64 graduations on the flowmeter), ensuring the water jet strikes approximately 3 mm from the inner edge of the vessel’s lower beveled screen area, thoroughly wetting all sieve discs before use, daily verifying tare masses of sieve discs, soaking the test specimen adequately to prevent clogging, and performing drainage and washing of vessel walls opposite to rotation after 300 seconds. These measures guarantee accuracy and repeatability in fibre size separation.

Fibre length fractions are separated using a stacked set of sieve discs with designated mesh sizes. Fibres retained on the 75 µm diameter sieve are washed onto a 63 µm sieve via water and vacuum filtration assisted by a venturi water-jet suction pump. Each sieve with retained fibres is weighed, subtracting its tare mass to obtain the net fibre mass. The amount passing the 75 µm sieve is calculated by subtracting the total retained mass from the original sample mass. The results are expressed as percentage retained on each sieve and percentage passing the final sieve, ensuring precise measurement of fibre length distribution.

Sampling must comply with IS 4844-1968 procedures. Two specimens, each approximately 15 grams, are drawn and then reduced by coning and quartering to a test sample size of 2 ± 0.005 grams. A single 2 g specimen is dispersed in 400 ml of water within a 500 ml beaker, soaked for four minutes, and gently stirred by hand for one minute to avoid fibre damage. Stirrer clearances on the apparatus are adjusted as per specifications to ensure proper agitation during testing.