This method is for the determination of the fineness of grinding by the dry method.
All fertilisers in Schedule 1 of the Fertilisers Regulations 1991 for which requirements are given of fineness of grinding using 0.630 mm and 0.160 mm sieves.
By mechanical sieve shaking, the quantities of product with a granule size greater than 0.63 mm and those with a granule size between 0.16 mm and 0.63 mm are determined and the percentage of fineness of grinding is calculated.
Mechanical sieve shaker.
Sieves with apertures of 0.160 mm and 0.630 mm respectively of standard ranges (diameter 20 cm, height 5 cm).
5. Weigh to the nearest 0.05 g, 50 g of the sample. Assemble the two sieves and the collecting container on the shaker (4.1), the sieve with the larger apertures being placed on top. Place the sample for analysis on the top. Sieve for ten minutes and remove the part collected on the bottom. Sieve again for one minute and check that the amount collected on the bottom during this time is not more than 250 mg. Repeat the process (for one minute each time) until the amount collected is less than 250 mg. Weigh the residual material on both sieves separately.
6. Percentage of material passing sieve of 0.630 mm apertures = (50-M1) × 2
Percentage of material passing sieve of 0.160 mm apertures = [50-(Ml+M2)] × 2
where:
Ml = mass in g of residue on the sieve with 0.630 mm apertures
M2 = mass in g of residue on the sieve with 0.160 mm apertures
The results are to be rounded up to the nearest unit.