3 Coal sampling and sample preparation
3.4 Sample preparation
3.4.4 Sample division
Here the sample is reduced in weight without significant change in particle size. The reduction and division of the gross sample can be carried out either by an online mechanical process, or an offline mechanical or manual process, to a top size and mass suitable for subsequent testing.
Mechanical methods
Mechanical dividers are designed to extract one or more parts of the coal in a number of cuts of relatively small mass. Various types of mechanical divider are commercially available and examples of them are shown in Figure 11. The design of the mechanical dividers should meet the criteria specified by relevant standards. Accurate division can be achieved by extracting and compositing a large number of increments from the sample. ASTM D2013 specifies that at least 60 increments be taken at each stage of division.
ISO 13909:4 specifies the minimum number of cuts for dividing an increment, which should be determined as follows:
• for fixed-‐mass division, a minimum of four cuts should be made when dividing primary increments.
An equal number of cuts should be taken from each primary increment in the sub-‐lot;
• for fixed-‐ratio division, a minimum of four cuts should be made when dividing a primary increment of mean mass;
• for subsequent division of individual divided primary increments, a minimum of one cut should be taken from each cut from the preceding division.
Examples of procedures recommended by ISO for division of individual increments, and subsequent sample division are shown in Figure 12. The combined mass of all the divided increments in the sub-‐lot should, at each stage, have a minimum mass greater than the mass given in Table 5 corresponding to the type and top size of the sample. If the increment masses are too low to satisfy this requirement, the divided increment should be crushed prior to further division.
Figure 11 – Examples of mechanical dividers (ISO 13909:4)
1 – feed 2 – rotating cone 3 – adjustable slot 4 – divided sample 5 – reject
1
2
3
4 5
1 – slotted belt 2 – feed 3 – inclined shute 4 – divided sample 5 – reject
1
2
3
4 5
1 – feed
2 – diverted sample in rotating recievers
1 – feed 2 – reject
3 – diverted sample
1 1
2
1
2
3 a) rotating cone type
b) container type
c) slotted belt type
d) cutter chute type
Figure 12 – Examples of procedures for division of increments and samples (ISO 13909:4)
cuts (4 )
cuts (10 ) primary increment 1
cuts (4 min) one cut from each
previous cut divided increment 1
primary increment 2
cuts (4 min) one cut from each
previous cut divided increment 2
primary increment n
cuts (4 min) one cut from each
previous cut divided increment n
constituted sample sample division (minimum 60 cuts)
divided sample (minimum mass as
given in table 1)
a) Example of division of individual increments (minimum number of cuts)
primary increment 1
divided increment 1
divided increment 1
cuts (4 ) primary increment 2
divided increment 2
cuts (4 ) primary increment n
divided increment n particle size
reduction (if required)
cuts (10 )
divided increment 2 particle size reduction (if required)
cuts (10 )
divided increment n particle size reduction (if required)
constituted sample sample division (minimum 60 cuts)
divided sample (minimum mass as
given in table 1)
a) Example of two-stage division of individual increments
For dividing a sample constituted from all increments, or divided increments, ISO recommends that a minimum of 60 cuts be taken.
The minimum mass of divided samples is dependent on the nominal top size of the coal, the precision required for the parameter concerned and the relationship of that parameter to particle size. It should be noted that the attainment of the required minimum mass after division will not, in itself, guarantee the required precision, because division precision is also dependent on the number of cuts taken during division. For different types of test sample, the values for the minimum mass of divided samples of varying top size specified by ISO and ASTM are given in Table 5 and Table 6, respectively.
A complete sampling system generally includes a primary sampler, a secondary sampler, a crusher/crushers, and sometimes a tertiary sampler, which is known as a two-‐ or three-‐stage sampling system. The primary increment is taken from the total lot of coal. The second increment is a full cross-‐stream cut of the primary increment. A crusher is used before and/or after the secondary sampler for sample reduction. Sample division may be accomplished by a tertiary sampler.
Table 5 – Values for the minimum mass of divided samples (ISO 13909:4) Nominal top size of
coal General-‐analysis and
common samples Total-‐moisture analysis
samples Size-‐analysis samples
mm kg kg 1% 2%
300 15000 3000 54000 13500
200 5400 1100 16000 4000
150 2600 500 6750 1700
125 1700 350 4000 1000
90 750 125 1500 400
75 470 95 850 210
63 300 60 500 125
50 170 35 250 65
45 125 25 200 50
38 85 17 110 30
31.5 55 10 65 15
22.4 32 7 25 6
16 20 4 8 2
11.2 13 2.5 3 0.7
10 10 2.0 2 0.5
8 6 1.5 1 0.25
5.6 3 1.2 0.5 0.25
4 1.5 1.0 0.25 0.25
2.8 0.65 0.65 0.25 0.25
2.0 0.25
1 0.1
<0.5 0.06
Table 6 – Preparation of laboratory sample (ASTM D2013) Crush to pass at least 95% through
sieve Divide to a minimum weight of, g*
group A group B
No. 4 (4.75 mm) 2000 4000
No. 8 (2.36 mm) 500 1000
No. 20 (850 μm) 250 500
No. 60 (250 μm)
(100% through) 50 50
*if a moisture sample is required, increase the quantity of No. 4 (4.75 mm) or No.8 (2.36 mm) sieve subsample by 500 g.
Manual methods
A riffle is a sample divider that divides a sample stream into halves, one of which is discarded and the other remains as a sample for further splitting or analysis. For sample division, the coal is usually fed manually along its length, and as illustrated in Figure 13, adjacent slots feed opposite receivers. The riffle should be fed using a special pan that is exactly the same width as the top of the chute. It is important that the riffle slot width be at least 3 times the nominal top size of the coal. Each half of the riffle should have the same number of slots (at least eight and preferably more). All the surfaces on which the coal might rest shall have a slope of at least 60° to the horizontal. The coal should be fed into the riffle in a slow, uniform stream covering all of the slots of the riffle. The coal should not be allowed to build up in or above the riffle slots, and it should flow freely through the slots. When a staged sample division requires two or more steps or passes, the sample retained at each step should be taken alternately from each side of the riffle.
Figure 13 – A riffle sample divider
A