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1. The preparation of a sample for analysis from the final sample received at the laboratory is a series of operations, usually sieving, grinding and mixing, carried out in such a way that the smallest amount weighed, as prescribed by the method of analysis chosen, is representative of the final sample. The sample should be ground to the fineness required by the method of analysis. (Over grinding must be avoided in cases where this will affect the solubility in various reagents). With some materials, fine grinding may lead to loss or gain of moisture and allowance for this must be made.
2. This method is applicable to fertilisers in Groups 1(b), 1(c), 2(b), 2(c), 2(d), 3(b), 3(c), 4(a), 4(b), 4(c) and 5(b) of Section A and Group 5 and 6 of Section B of the Table in Schedule 1 to the Fertilisers Regulations (Northern Ireland) 1992(1).
This method is also applicable to fluid fertilisers.
The determination of the fineness of fertilisers is carried out on the sample as received.
3.—3.1 the whole final sample is ground to the required fineness. All the ground sample is thoroughly mixed before each test portion is taken.
3.2 the final sample is thoroughly mixed before each test portion is taken.
4.—4.1 Sample grinder capable of grinding the fertiliser to pass the specified sieve.
4.2 Mortar and pestle of suitable material and size.
4.3 Sieves having square apertures of 0.18 mm, 0.5 mm and 1.0 mm. Test sieves conforming to British Standard 410: 1986 are suitable.
4.4 Sample containers of non-corrodible materials, with air-tight closures.
ALL OPERATIONS CONNECTED WITH THIS PROCEDURE SHOULD BE CARRIED OUT AS QUICKLY AS POSSIBLE TO MINIMISE ABSORPTION OR LOSS OF WATER. CARE SHOULD BE TAKEN DURING GRINDING THAT THE TEmperature of the fertiliser does not rise above 45°C to avoid loss of volatile constituents. Grinding beyond the fineness required must in all cases be avoided.
5.1 The procedure in 5.1.1 should be followed except when a grinding machine is not available, in which case 5.1.2 is applicable.
5.1.1Grind the final sample until all the sample has passed through, or for the specified time, depending on the type of grinder (4.1). To check that the grinding has been adequate, sieve a small portion of the ground sample through a 0.5 mm sieve (4.3) and discard it. If the whole of this portion does not pass the sieve, return the remainder of the sample to the grinder and repeat the grinding until satisfactory grinding is achieved.
5.1.2Sieve the whole final sample through the 0.5 mm sieve (4.3). Grind the residue on the sieve, using the pestle and mortar (4.2), until all the material passes through the sieve. Carefully mix the sample.
5.2 Place the prepared sample in a clean container (4.4) and seal it until required for analysis.
5.3 Before taking each test portion for analysis, the whole sample must be well mixed. Form the material into a flattened cone and using a spatula take the required test portion at random in small increments.
5.4 If the sample contains foreign matter which cannot be ground this shall be removed, weighed and allowed for in the results of the analysis. This material shall be retained and if possible its nature recorded.
6.—6.1 For those samples where the fineness of grinding is to be determined it shall be carried out on an unground sample. The sample should be well mixed (soft lumps may be disintegrated by lightly crushing) and divided into two parts, which are as identical as possible. All other determinations shall be carried out on the sample prepared in accordance with the directions in paragraph 5.1.
6.2 Some products such as superphosphates may become doughy if ground mechanically. In these cases crush the sample in a mortar (4.2) so that all the material passes through a 1.0 mm sieve (4.3). Place the material so crushed in a clean container (4.4) and seal it until required for analysis.
6.3 Some organic materials may be of such a nature that the procedures given above cannot be used (for example fresh guano, leather, wool and animal residues). In these cases the analyst should use the best practicable means to obtain a representative sample.
6.4 These fertilisers include materials with marked differences in texture or mechanical properties (hardness, density, etc). They may be difficult to grind entirely (for example mixtures of organic and inorganic materials) or they may segregate during handling (for example “Kalimagnesia”). Special procedures are necessary in these cases:
6.4.1for mixtures other than those in 6.4.2, follow the procedure in 5.1.1, replacing the 0.5 mm sieve by one with apertures of 0.18 mm. A grinding machine, capable of grinding the whole of the sample to the required fineness in one pass, is strongly recommended;
6.4.2in the case of mixtures containing one or more very hard components, or mixtures containing organic materials, it may be difficult to grind and homogenise all the components. To avoid overgrinding some of the softer components proceed as follows:—
grind the sample as in 5.1.1 or 5.1.2 to pass a 0.5 mm sieve. Re-sieve the sample through a 0.18 mm sieve and reduce the residue to a convenient size by further grinding or other practical means. Thoroughly remix the sample and place in a clean container (4.4).
7. Mix thoroughly by shaking, ensuring that any insoluble matter, particularly crystalline material, is thoroughly dispersed, immediately before drawing a portion of the sample for analysis.
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