Hard coal — Sampling of slurries
This document sets out the basic methods for sampling fine coal, coal rejects or tailings of nominal top size The procedures described in this document primarily apply to sampling of coal that is transported in moving streams as a slurry. These streams can fall freely or be confined in pipes, launders, chutes, spirals or similar channels. Sampling of slurries in stationary situations, such as a settled or even a well-stirred slurry in a tank, holding vessel or dam, is not recommended and is not covered in this Document. This document describes procedures that are designed to provide samples representative of the slurry solids and particle size distribution of the slurry under examination. After draining the slurry sample of fluid and measuring the fluid volume, damp samples of the contained solids in the slurry are available for drying (if required) and measurement of one or more characteristics in an unbiased manner and with a known degree of precision. The characteristics are measured by chemical analysis or physical testing or both. The sampling methods described are applicable to slurries that require inspection to verify compliance with product specifications, determination of the value of a characteristic as a basis for settlement between trading partners or estimation of a set of average characteristics and variances that describes a system or procedure. Provided flow rates are not too high, the reference method against which other sampling procedures are compared is one where the entire stream is diverted into a vessel for a specified time or volume interval. This method corresponds to the stopped-belt method described in ISO 13909-2.
Houille — Échantillonnage des schlamms
Standards Content (Sample)
Hard coal — Sampling of slurries
Houille — Échantillonnage des schlamms
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1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles of sampling slurries . 2
4.1 General . 2
4.2 Sampling errors . 3
4.2.1 General. 3
4.2.2 Preparation error . 4
4.2.3 Delimitation and extraction errors . 4
4.2.4 Weighting error, E . 5
4.2.5 Periodic quality fluctuation error, E . 6
4.3 Sampling and overall variance . 6
4.3.1 Sampling variance. 6
4.3.2 Overall variance . 6
5 Sampling schemes . 7
6 Minimization of bias and unbiased increment mass .13
6.1 Minimizing bias .13
6.2 Volume of increment for falling stream samplers to avoid bias .14
6.3 Volume of increment for manual sampling to avoid bias .14
7 Precision of sampling and determination of increment variance .15
7.1 Overall precision .15
7.2 Primary increment variance .15
7.3 Preparation and testing variance .16
8 Number of sub-lots and number of increments per sub-lot .16
9 Minimum mass of solids in lot and sub-lot samples .17
9.1 General .17
9.2 Minimum mass of solids in lot samples .17
9.3 Minimum mass of solids in sub-lot samples .17
9.4 Minimum mass of solids in lot and sub-lot samples after size reduction .17
10 Time-basis sampling .18
10.1 General .18
10.2 Sampling interval .18
10.3 Cutters .18
10.4 Taking of increments .18
10.5 Constitution of lot or sub-lot samples .19
10.6 Division of increments and sub-lot samples .19
10.7 Division of lot samples .19
10.8 Number of cuts for division .19
11 Stratified random sampling within fixed time intervals.19
12 Mechanical sampling from moving streams .20
12.1 General .20
12.2 Design of the sampling system .20
12.2.1 Safety of operators .20
12.2.2 Location of sample cutters .20
12.2.3 Provision for duplicate sampling .20
12.2.4 System for checking the precision and bias.20
12.2.5 Minimizing bias .21
12.3 Slurry sample cutters .22
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12.3.2 Falling-stream cutters .22
12.3.3 Cutter velocities .22
12.4 Mass of solids in increments .22
12.5 Number of primary increments .23
12.6 Routine checking .23
13 Manual sampling from moving streams .23
13.1 General .23
13.2 Choosing the sampling location .23
13.3 Sampling implements .24
13.4 Mass of solids in increments .24
13.5 Number of primary increments .24
13.6 Sampling procedures .24
14 Sampling of stationary slurries .25
15 Sample preparation procedures .25
15.1 General .25
15.2 Reduction mills .25
15.3 Sample division.25
15.4 Chemical analysis samples . .25
15.5 Physical test samples .25
16 Packing and marking of samples .26
Annex A (informative) Examples of correct slurry devices .27
Annex B (informative) Examples of incorrect slurry sampling devices .30
Annex C (normative) Manual sampling implements .34
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This document was prepared by Technical Committee ISO/TC 27, Solid mineral fuels, Subcommittee
SC 4, Sampling.
This second edition cancels and replaces the first edition (ISO 20904:2006), of which it constitutes a
minor revision. The changes compared to the previous edition are as follows:
— an amendment to Figure 6 b) to read ― incorrect;
— correction to Figure 7 b).
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© ISO 2020 – All rights reserved v
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INTERNATIONAL STANDARD ISO 20904:2020(E)
Hard coal — Sampling of slurries
This document sets out the basic methods for sampling fine coal, coal rejects or tailings of nominal top
size <4 mm that is mixed with water to form a slurry. At very high ratios of fine solids to water when the
material assumes a soft plastic form, the mixture is correctly termed a paste. Sampling of pastes is not
covered in this document.
The procedures described in this document primarily apply to sampling of coal that is transported
in moving streams as a slurry. These streams can fall freely or be confined in pipes, launders, chutes,
spirals or similar channels. Sampling of slurries in stationary situations, such as a settled or even a well-
stirred slurry in a tank, holding vessel or dam, is not recommended and is not covered in this Document.
This document describes procedures that are designed to provide samples representative of the slurry
solids and particle size distribution of the slurry under examination. After draining the slurry sample of
fluid and measuring the fluid volume, damp samples of the contained solids in the slurry are available
for drying (if required) and measurement of one or more characteristics in an unbiased manner and
with a known degree of precision. The characteristics are measured by chemical analysis or physical
testing or both.
The sampling methods described are applicable to slurries that require inspection to verify compliance
with product specifications, determination of the value of a characteristic as a basis for settlement
between trading partners or estimation of a set of average characteristics and variances that describes
a system or procedure.
Provided flow rates are not too high, the reference method against which other sampling procedures
are compared is one where the entire stream is diverted into a vessel for a specified time or volume
interval. This method corresponds to the stopped-belt method described in ISO 13909-2.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 1213-1, Solid mineral fuels — Vocabulary — Part 1: Terms relating to coal preparation
ISO 1213-2, Solid mineral fuels — Vocabulary — Part 2: Terms relating to sampling, testing and analysis
ISO 13909-1, Hard coal and coke — Mechanical sampling — Part 1: General introduction
ISO 13909-4, Hard coal and coke — Mechanical sampling — Part 4: Coal — Preparation of test samples
ISO 13909-8, Hard coal and coke — Mechanical sampling — Part 8: Methods of testing for bias
3 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 1213-1, ISO 1213-2 and
ISO 13909-1 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
© ISO 2020 – All rights reserved 1
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4 Principles of sampling slurries
For the purposes of this document, a slurry is defined as fine coal, coal rejects or tailings of nominal top
size <4 mm that is mixed with water, which is frequently used as a convenient form to transport coal,
rejects or tailings though plant circuits by means of pumps and pipelines and under gravity in launders
or chutes or through long distances in slurry pipelines. Tailings from wet plants are also discharged as
a slurry through pipelines to the tailings dam. In many of these operations, collection of increments at
selected sample points is required for evaluation of the coal or rejects in the slurry.
A lot or sub-lot sample is constituted from a set of unbiased primary increments from a lot or sub-
lot. The sample container is weighed immediately after collection and combination of increments to
avoid water loss by evaporation or spillage. Weighing is necessary to determine the mass percentage
of solids in the lot or sub-lot sample. The lot or sub-lot sample can then be filtered, dried and weighed.
Alternatively, the lot or sub-lot sample may be sealed in plastic bags after filtering for transport and
drying at a later stage.
Except for samples for which their characteristics are determined directly on the slurry, test samples
are prepared from lot or sub-lot samples after filtering and drying. Test portions may then be taken
from the test sample and analysed using an appropriate and properly calibrated analytical method or
test procedure under specified conditions.
The objective of the measurement chain is to determine the characteristic of interest in an unbiased
manner with an acceptable and affordable degree of precision. The general sampling theory, which is
based on the additive property of variances, can be used to determine how the variances of sampling,
sample preparation and chemical analysis or physical testing propagate and hence determine the total
variance for the measurement chain. This sampling theory can also be used to optimize mechanical
sampling systems and manual sampling methods.
If a sampling scheme is to provide representative samples, it is necessary that all parts of the slurry
in the lot have an equal opportunity of being selected and appearing in the lot sample for testing. Any
deviation from this basic requirement can result in an unacceptable loss of accuracy. A sampling scheme
having incorrect selection techniques, i.e. with non-uniform selection probabilities, cannot be relied
upon to provide representative samples.
Sampling of slurries should preferably be carried out by systematic sampling on a time basis (see
Clause 10). If the slurry flow rate and the coal-solids concentration vary with time, the slurry volume
and the dry solids mass for each increment will vary accordingly. It is necessary to show that no
systematic error (bias) is introduced by periodic variation in quality or quantity where the proposed
sampling interval is approximately equal to a multiple of the period of variation in quantity or quality.
Otherwise, stratified random sampling should be used (see Clause 11).
Best practice for sampling slurries is to mechanically cut freely falling streams (see Clause 12), with a
complete cross-section of the stream being taken during the traverse of the cutter. Access to freely falling
streams can sometimes be engineered at the end of pipes or by incorporating steps or weirs in launders
and chutes. If samples are not collected in this manner, non-uniform concentration of coal solids in the
slurry due to segregation and stratification of the solids can lead to bias in the sample that is collected.
Slurry flow in pipes can be homogenous with very fine particles dispersed uniformly in turbulent
suspension along the length and across the diameter of the pipe. However, more commonly, the slurry in
a pipe has significant particle-concentration gradients across the pipe and there can be concentration
fluctuations along the length of the pipe. These common conditions are called heterogeneous flow.
Examples of such flow are full-pipe flow of a heterogeneous suspension or partial-pipe flow of a fine
suspension above a slower moving or even stationary bed of coarser particles in the slurry.
For heterogeneous flow, bias is likely to occur where a tapping is made into the slurry pipe to locate
either a flush-fitting sample take-off pipe or a sample tube projecting into the slurry stream for
extraction of samples. The bias is caused by non-uniform concentration profiles in the pipe and the
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different trajectories followed by particles of different masses due to their inertia, resulting in larger or
denser particles being preferentially rejected from or included in the sample.
In slurry channels such as launders, heterogeneous flow is almost always present, and this non-
uniformity in particle concentration is usually preserved in the discharge over a weir or step. However,
sampling at a weir or step allows complete access to the full width and breadth of the stream, thereby
enabling all parts of the slurry stream to be collected with equal probability.
Sampling of slurries in stationary situations, such as a settled or even a well-stirred slurry in a tank,
holding vessel or dam is not recommended, because it is virtually impossible to ensure that all parts
of the slurry in the lot have an equal opportunity of being selected and appearing in the lot sample for
testing. Instead, sampling should be carried out from moving streams as the tank, vessel or dam is
filled or emptied.
4.2 Sampling errors
The processes of sampling, sample preparation and measurement are experimental procedures, and
each procedure has its own uncertainty appearing as variations in the final results. When the average
of these variations is close to zero, they are called random errors. More serious variations contributing
to the uncertainty of results are systematic errors, which have averages biased away from zero. There
are also human errors that introduce variations due to departures from prescribed procedures for
which statistical analysis procedures are not applicable.
The characteristics of the solids component of a slurry can be determined by extracting samples from
the slurry stream, preparing test samples and measuring the required quality characteristics. The total
sampling error, E , can be expressed as the sum of a number of independent components . Such a
simple additive combination is not possible if the components are correlated. The total sampling error,
E , expressed as a sum of its components, is given by Formula (1):
TQ1Q2Q3W DE P
E is short-range quality fluctuation error associated with short-range variations in quality of
the solids component of the slurry;
E is long-range quality fluctuation error associated with long-range variations in quality of the
solids component of the slurry;
E is periodic quality fluctuation error associated with periodic variations in quality of the solids
component of the slurry;
E is weighting error associated with variations in slurry flow rate;
E is increment delimitation error introduced by incorrect increment delimitation;
E is increment extraction error introduced by incorrect increment extraction from the slurry;
E is the preparation error introduced by departures (usually unintentional) from correct prac-
tices, e.g. during constitution of the lot sample, draining and filtering away the water, and
transportation and drying of the sample.
The short-range quality fluctuation error consists of two components, as shown by Formula (2):
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E is the fundamental error due to variation in quality between particles;
E is the segregation and grouping error.
The fundamental error results from the composition heterogeneity of the lot, i.e. the heterogeneity that
is inherent to the composition of each particle making up the solids component of the lot. The greater
the differences in the compositions of particles, the greater the composition heterogeneity and the
higher the fundamental error variance. The fundamental error can never be completely eliminated.
It is an inherent error resulting from the variation in composition of the particles in the slurry being
The segregation and grouping error results from the distribution heterogeneity of the sampled
material . The distribution heterogeneity of a lot is the heterogeneity arising from the manner in
which particles are distributed in the slurry. It can be reduced by taking more increments, but it can
never be completely eliminated.
A number of the components of the total sampling error, namely E , E and E , can be minimized or
D E P
reduced to an acceptable level by correct design of the sampling procedure.
4.2.2 Preparation error
In this context, the preparation error, E , includes errors associated with non-selective