ISO 4077:2023

INTERNATIONAL ISO
STANDARD 4077
First edition
2023-06
Coal — Guidance for sampling in coal
preparation plants
Charbon — Recommandations relatives à l'échantillonnage dans les
ateliers de préparation du charbon
Reference number
© ISO 2023
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ii
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General principles and considerations . 2
4.1 General . 2
4.2 Principles of sampling . 3
4.3 Objectives of sampling in coal preparation plants . 3
4.3.1 General . 3
4.3.2 Determination of scope of sampling using a sampling decision tree . 4
5 Design considerations . 6
5.1 Principles . 6
5.1.1 Solids sampling . 6
5.1.2 Slurry sampling . 6
5.2 Systems for new plants and retrofitting . 7
5.2.1 New plant mechanical sampling systems . 7
5.2.2 New plant manual sample points . 8
5.2.3 Existing plant with no mechanical system . 8
5.2.4 Manual sampling points in existing plants . 8
6 Planning for sampling . 9
6.1 Pre-sampling inspection . 9
6.2 Personnel . 9
6.3 Containers . 9
6.4 Method . 10
6.4.1 Overview . 10
6.4.2 Sampling Time . 10
6.4.3 Sampling for feed quality characterization . 10
6.4.4 Sampling for quality monitoring and control . 11
6.4.5 Sampling for equipment performance .12
6.4.6 Sample mass . 13
7 Sampling management .21
7.1 Consideration of process . 21
7.2 Handling of samples after collection . 21
8 Sampling from a slurry stream .21
8.1 Slurry flow regimes. 21
8.2 Sampling locations . 22
8.3 Slurry sampling methods . 23
8.3.1 Considerations for sampling of slurry streams .23
8.3.2 Manual sampling .25
8.3.3 Automatic slurry samplers .30
8.4 Secondary sampling of slurry streams .38
9 Considerations for screen discharge sampling .39
10 Sampling of magnetite received in bulk .41
11 Sampling report .41
Annex A (informative) Recommended manual sampling locations and options .42
Annex B (informative) Checklist examples .48
Annex C (informative) Recommended laboratory analysis .54
Annex D (informative) Example of sampling plan .59
iii
Bibliography .63
iv
Foreword
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This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 1,
Coal preparation: Terminology and performance.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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v
INTERNATIONAL STANDARD ISO 4077:2023(E)
Coal — Guidance for sampling in coal preparation plants
1 Scope
This document specifies recommended practices for sampling in coal preparation plants (CPPs).
The document is applicable to sampling of all coal product(s), reject material(s) and magnetite. The coal
and mineral matter size covered by this document ranges from a nominal top size of 63 mm to 0,1 mm.
This document also covers larger sizes in the case of mechanical sampling. Manual sampling is not
recommended for particle size larger than 63 mm.
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, Coal and coke — 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 7936, Coal — Determination and presentation of float and sink characteristics — General directions
for apparatus and procedures
ISO 8833, Magnetite for use in coal preparation — Test methods
ISO 13909 (all parts), Hard coal and coke — Mechanical sampling
ISO 18283, Coal and coke — Manual sampling
ISO 20904, Hard coal — Sampling of slurries
AS 1038.21.1.1, Coal and coke — Analysis and testing, Part 21.1.1: Higher rank coal and coke — Relative
density — Analysis sample/density bottle method
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1213-1, ISO 1213-2,
ISO 13909 (all parts), ISO 18283, ISO 20904 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
boil-box
box or compartment installed in a piped flow stream, designed for very short residence time and
vigorous turbulence of the flow-through stream, with a fully accessible weir overflow arrangement that
the full stream shall pass over
3.2
by-line
side-stream or branch line that only accommodates a portion of the total stream flow
3.3
diverter type sampler
device that temporarily diverts the full stream to a position accessible to full-stream sampling
3.4
full-stream sampler
sampling device that traverses the full extent of a flowing stream at constant speed
3.5
hindered bed separator
beneficiation device based on the principles of hindered bed settling
3.6
hydraulic separator
coal beneficiation device that uses water as the separation medium
EXAMPLE Spirals, hindered bed separators (3.5) and water washing cyclones.
3.7
partition curve
curve indicating each density (or size) fraction, expressed as a percentage, contained in one of the
products of the separation
3.8
point sampler
device that collects a sample from only one point within the flowing stream
3.9
pressure pipe sampler
variation of a point sampler (3.8)
3.10
RD
cut-point being the exact relative density at which a separation into two fractions is desired or achieved
3.11
supervisory control and data acquisition
SCADA
user software interface for accessing process control setpoints, current and historical on-line parameter
data
Note 1 to entry: Data come from belt scales, pressure and level transducers, on-line ash analysers, motor
amperages, etc.
3.12
sampling implement
device used to collect or extract a sample increment
3.13
two-in-one slurry sampler
device that includes both primary and secondary slurry sampling apparatus
4 General principles and considerations
4.1 General
The objective of sampling is to collect a manageable quantity of material and use it to represent the total
amount of material from which it was collected. This manageable quantity of material is called a sample.
As the sample will be used to estimate the characteristics of the whole material from which it was
collected, some important rules should be followed to ensure the sample is statistically representative
of the population. This includes consideration of the location and time of sampling; type of sampling
implements and volume of sample.
Results are required to be precise (of minimum scatter) and accurate (as close as possible to the true
value) to generate information for decision making.
Table A.1 in Annex A shows all major equipment found in coal preparation plants, the manual sampling
technique that should be used for each, and where to find details on the technique in this document.
WARNING — This document does not purport to address safety issues that can be associated with
its use. It is the responsibility of the user to establish appropriate safety and health practices in
line with site safety regulations and work health and safety legislation in the country where it
is being used. It is highly recommended that clear safety instructions be provided to all staff
involved, and a risk assessment be undertaken prior to conducting any sampling exercise.
4.2 Principles of sampling
Correct sampling in a coal preparation plant (CPP) should ensure that every particle and associated
entity (e.g. water and medium) in the stream have an equal chance of reporting to the collected sample
during the sampling process.
The full stream should be accessible to the sampling implement. It should be noted that incorrect
sampling methodology will adversely affect the accuracy of the measured result. Depending on the
stream nature, sampling methods can be categorized as follows:
a) sampling of dry or moist solids stream, e.g. screen discharge;
b) sampling of slurry stream, e.g. correct medium.
In addition, the sampling methods can be categorized depending on the purpose of sampling as:
— sampling for feed quality characterization;
— sampling for quality monitoring and control;
— sampling for equipment/process performance evaluation, i.e. “special case” sampling.
It is recommended that each CPP maintain a sample point register, listing each sample point, the
sampling implement required (photographs are helpful), the volume of sample collected per implement
cut, and the usual number of cuts (increments) per sample. If special sampling implements are required,
the fabrication drawings should be referenced in the register and filed for re-ordering purposes.
4.3 Objectives of sampling in coal preparation plants
4.3.1 General
Reasons for sampling include:
a) identification of process problems to assist formulation of solutions;
b) process auditing;
c) measuring process efficiency;
d) generating data for process modelling;
e) assessing coal quality;
f) providing reliable results for decision-making;
g) process control;
h) inventory accounting and reconciliation;
i) process evaluation.
The sampling method (location and time, sample mass, procedure etc.) will depend on the reason for
sampling. Hence, the sampling objective(s) should first be clearly established. A decision tree will assist
with choosing and implementing the best sampling method.
A sample is subject to certain preparation procedures that render it suitable for either physical
testing or laboratory analysis. The type of tests or analyses that are performed are dependent on the
characteristics required to categorize the material.
4.3.2 Determination of scope of sampling using a sampling decision tree
Before planning and carrying out sampling, it is necessary to determine the scope of the sampling
exercise. The methods used, duration of sampling and sample volume will each depend on the sampling
goal, i.e. what the user is looking to achieve. The decision tree in Figure 1 will assist with planning.
If sampling is for quality control, smaller sample masses may be used since individual samples may
be analysed separately, for example, as a shift production sample, thereby generating many individual
results over time. However, in the case of a process audit where only a single sample of each stream is
collected, and the result of its analysis considered as final, then the sample taken should be larger, and
will correspond to a composite of increments. Therefore, the sample requirement depends on whether
the results of analysis are accumulated or singular.
When sampling for process performance investigations, requiring the calculation of size and/or density
partition data, larger samples are required so that enough material is present for size analysis and/or
float-sink testing.
For partition curve determinations, density tracers offer an alternative to methods based on coal
sampling. Density tracers are synthetic particles of precise sizes, shapes, and densities. For separators
with feed top size greater than 63 mm, they usually provide the only economically and practically viable
technique. Known numbers of tracer particles of known sizes, shapes and densities are added to the
feed of a density separator. After partitioning, they are collected from, or detected in, the product and
reject streams, and the partition number for each density class is calculated for reporting in a partition
curve.
Coal sampling offers the following advantages:
a) sampling facilitates measurement of process impacts for each size class of particles;
b) sampling facilitates fractionation and analyses of the resulting samples for any relevant coal quality
parameter.
Density tracer tests typically only comprise a single size of tracer for any given test, but offer the
following advantages:
— tracers facilitate a rapid result (no analysis requirements);
— tracers facilitate a rapid assessment of validity and possible error-range of result (based on tracer
losses).
Figure 1 — Sampling decision tree
5 Design considerations
5.1 Principles
5.1.1 Solids sampling
When designing a sampling system for solids, the following aspects need to be considered:
a) in all stages of the design, consideration shall be given for the safety of operators, both in completing
their tasks and egress;
b) the mass and number of primary increments required is calculated as for bulk coal in accordance
with the ISO 13909 series;
c) sub-lot samples may be used;
d) cutter speed to be 0,6 m/s or less; some bias can be introduced if speeds exceed this value;
e) ensure the sample is not being contaminated;
f) plant should be designed to eliminate spillage or loss in any way, eliminate build ups in equipment
and ensure that cutters do not choke the feed causing a “reflux” effect in which some material can
be rejected from the cutter;
g) facilities for duplicate sampling should be incorporated into the plant to allow for checks on
sampling precision.
5.1.2 Slurry sampling
When designing a slurry sampling system, in addition to the considerations listed for solids sampling,
attention should be given to the following:
a) The mass of solids/volume of slurry contained in each increment obtained in one pass of the sample
cutter is calculated from the mass of slurry collected and mass fraction of solids, expressed as a
percentage.
b) When a reference sample is needed, divert the total stream into a container for a brief period.
c) 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 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.
d) Sampling should be undertaken at a point in the handling system where there is no apparent risk of
errors due to a periodic variation in material feed or quality, e.g. away from pulsating slurry pumps
or control valves.
e) The cutter should be of sufficient capacity to accommodate the entire increment at the maximum
flow rate of the stream without any slurry loss due to reflux from the cutter aperture. Avoid spillage
of the sample or loss of material due to dribbles or run-back on the outside of a cutter.
f) Sampling of moving slurry streams using probes, spears or by-line samplers is not recommended
because they do not intercept the full cross-section of the slurry stream.
g) Sampling part of the stream with an in-stream point sampler or probe within a pipe or channel is
always incorrect.
h) The cutter aperture should be at least three times the nominal top size of the particles in the slurry,
subject to a minimum of 20 mm.
i) Restriction of the flow of the slurry increment through any device causing reflux and overflow
should be avoided. This precaution is particularly important for reverse spoon cutters where the
falling slurry stream is forced to change flow direction as it strikes the inside surface of the spoon.
j) Ascertain the nominal top size and particle density of the solids in the slurry for determining the
minimum volume of slurry increment and the minimum mass of the solids in the sample.
k) Extract slurry increments of volume proportional to the slurry flow rate at the time of taking each
increment.
l) Consider nominal top size, the expected solids mass concentration, density of the solids in the
slurry, in the design to avoid blockages.
5.2 Systems for new plants and retrofitting
5.2.1 New plant mechanical sampling systems
It is recommended that during the design phase of coal preparation plants, mechanical sampling
systems be included in the design to cover coal preparation plant feed, product, and total reject streams.
Mechanical systems shall be in accordance with the following minimum criteria:
a) that all cutters are taking full (stream) cuts from each stream [feed, product(s) and reject(s)] and
feeding the cuts preferably to a sample conveyor belt;
b) that each sample conveyor should be capable of operating in both directions.
1) Normal direction feeding an online crusher and secondary cutter to produce quality control
samples.
2) Reverse direction to produce uncrushed (physical) samples.
Sample containment should be provided to minimize evaporation of moisture, or ingress of rainfall, or
contamination.
Other sampling plant designs are permissible if the system can produce both uncrushed samples and
crushed samples for quality control from each of the feed, product and reject streams.
It is also recommended that automatic-mechanised, or mechanically assisted sampling systems be
incorporated for unit processes within the CPP, especially for streams that are difficult to sample
manually, and critical to monitoring coal and/or magnetite losses. Table 1 lists the systems that should
be considered.
Table 1 — Streams recommended for automatic or mechanically assisted sampling
Stream Sampling device
Desliming and drain and rinse (D&R) screen Slide or swing bucket (lever operated) with
overflows means to discharge sample or mechanical lift to
raise bucket out of discharge chute
D&R screen underflows (drain media only) Full-stream sampler or boil-box full-flow weir
a
overflow
Sump inflow (e.g. hydrocyclone or flotation feed Direct all inflows via a singular full-stream
sump) sampler or boil-box full-flow weir overflow, with
a
room for safe personnel access
Tailings Full-stream sampler or boil-box full-flow weir
a
overflow
Flotation and hydraulic separator streams Full-stream sampler or boil-box full-flow weir
a
overflow
a
It is critical to use a full-stream sampler on the full primary slurry stream in order to procure a
representative primary sample. It is far less useful to employ full-stream samplers for secondary or
subsequent cuts in circumstances where the primary sample is not itself collected in a representative manner.
5.2.2 New plant manual sample points
In addition to mechanical sampling systems, it is recommended that other streams within the plant
require safe access to representative sampling points and these sampling points should be included in
the CPP design with examples as follows:
a) increase the width between the falling stream and launder at the discharge end of all screens to
allow easier manual sampling;
b) have access doors on both sides of conveyor/discharge chutes at transfer points of intermediate
products and rejects;
c) install tracks and make sampling scoops to fit, which will allow cuts to be taken manually at
transfer points without the need to manually support the sampling scoop;
d) ensure sampling platforms are built adjacent to transfer points to provide safe access for sampling
and sample handling;
e) where possible on slurry streams, install by-pass systems along pipelines to allow full-stream
sampling (see 8.3.2.2.1).
Manual sampling methods represent a compromise from the point of view of precision. Wherever
possible, mechanical sampling systems should be installed.
5.2.3 Existing plant with no mechanical system
It is recommended that mechanical systems be retrofitted as described in 5.2.1.
5.2.4 Manual sampling points in existing plants
It is recommended where possible that improvements be made to existing plants to meet the sampling
criteria outlined in 5.2.2.
6 Planning for sampling
6.1 Pre-sampling inspection
To ensure an outcome of best possible sampling practice and adhere to relevant work health and safety
laws, it is recommended to conduct a pre-sampling inspection. A pre-sampling inspection checklist list
should consider the following:
a) is it possible to collect representative samples of all the samples nominated in the sampling plan;
b) is there safe access to all the sampling points;
c) do sample points allow for full-stream sampling;
d) can the sample be safely removed from the plant;
e) determine the sequence that samples should be collected, to facilitate simultaneous sampling
where appropriate, and otherwise allow for the correct residence time (lag) between feed, product
and reject streams, so as to generate the best possible data for mass-balance purposes;
f) check that the calculated sample increment masses/volumes are correct, and that the sampling
implements for each sample point are fit for the purpose of taking the correct full-stream
increments;
g) reach an agreement between relevant parties on expectations of precision and what is required to
be achieved from the sampling and analysis program;
h) revise and finalize the sampling plan based on the pre-sampling inspection.
See example pre-inspection checklist in Annex B.
6.2 Personnel
It is recommended that the following criteria relating to numbers of personnel required, their training
and supervision need to be considered:
a) determine the number of sampling personnel required based on technical and safety requirements;
b) ensure all the samples that need to be taken simultaneously are physically able to be taken at the
same time;
c) ensure that the personnel are adequately trained for purpose, and each has clearly written
instructions for the exercise to be carried out;
d) ensure adequate supervision during the sampling process;
e) ensure adequate and timely communications are available to notify sampling personnel in case of
process upsets.
6.3 Containers
It is also recommended that the following criteria relating to the type of sample containers required,
their labelling, their lids, and their handling be considered:
a) select suitable containers with respect to capacity to ensure the sample integrity is maintained;
b) prepare labels prior to sampling. both labels and ink should be water resistant;
c) liners are required for samples placed in drums;
d) drum lifters are recommended to handle drums;
e) ensure the containers are clean with closely fitting lids. lids on buckets should be taped in position
prior to sample transport;
f) mass of full containers (buckets) should be in accordance with manual handling requirements to
prevent injuries when they are moved or carried.
6.4 Method
6.4.1 Overview
It is recommended that the variability of the coal should be considered when establishing a sampling
plan, since quality fluctuations typically exist in all streams in a CPP. Much of the variation is ultimately
due to the variability in the CPP feed itself. The variability may be both short-term and long term, for a
given CPP. The presence of periodic variations can be determined by a variogram (see ISO 13909-7). If
periodic variability is present, then stratified random sampling should be performed. An alternative is
to significantly reduce the source of the periodic variations.
The number of increments taken from a lot to achieve a specified precision is a function of the variability
of the quality of the stream irrespective of the mass of the lot. The minimum mass in a sample is
dependent on the nominal top size of the coal, the precision required for the parameter concerned and
the relationship of that parameter to the particle top size. The attainment of this mass will not, of itself,
guarantee the required precision, because precision is also dependent on the number of increments in
the sample and the variability. Hence the required sample mass can be significantly higher than the
minimum masses listed in this document. The precision achieved for a lot may be measured using the
procedures given in ISO 13909-7.
An example of a sampling plan is given in Annex D.
6.4.2 Sampling Time
The minimum sampling time depends on the purpose of sampling. It can be differentiated between:
a) sampling time for coal quality data;
b) sampling time for determination of equipment performance.
It is also recommended that, for assessing equipment performance, the sampling time should be kept
as short as possible to minimize the effect of any feed coal cycle time or accumulation/depletion within
the unit operation itself (e.g. hindered bed separator (HBS) with intermittent reject discharge). The
sampling time for coal quality analysis will be longer and largely influenced by CPP feed configuration
and stockpile reclamation system.
The sampling time will also depend on:
— how quickly the required mass can be collected (personnel and equipment resources);
— how much sample is needed.
6.4.3 Sampling for feed quality characterization
Several factors need to be considered when sampling the coal feed to a preparation plant. Figure 2
outlines that there can be various locations where the raw coal is sourced. Each location will have
its own degree of variability with regards to sampling, and the sampling method should be adjusted
accordingly.
NOTE ROM means Run of mine.
Figure 2 — CPP feed sample source considerations
6.4.4 Sampling for quality monitoring and control
In coal preparation plants, there are typically two types of sampling as outlined in the flow chart in
Figure 3. As previously given in 6.4.3, several factors need to be considered when sampling the coal
feed to a preparation plant. Figure 3 outlines that there can be various locations where the raw coal
is sourced. Again, each location will have its own degree of variability with regards to sampling and
the sampling method should be adjusted accordingly. Some factors to be considered are common to all
forms of sampling while others refer only to one form of sampling.
a
Manual sampling apart from stop-belt sampling should only be used for non-critical streams and as a back-up
for failures of a mechanical sampler.
b
See ISO 13909-2 and ISO 18283 for additional information on the factors that affect frequency and increment
mass.
c
See ISO 20904 for more information on material variability.
Figure 3 — Sampling types for quality monitoring and control
Annex A provides further information on manual sampling locations and options.
6.4.5 Sampling for equipment performance
In the case of equipment performance, the goal of sampling is to measure the separation efficiency due
to the unit operation(s) as opposed to understanding a particular coal quality. Hence, the sampling
duration needs to be as short as possible (target 30 min or less). The sampling duration is limited only
by the number of sampling locations, number of sampling personnel and sample volumes required.
Consideration should be given to the following:
a) Is a particular feed type required (e.g. low or high density cut, low or high near-gravity material
assessment)? If so, schedule accordingly.
b) How variable is the CPP feed? It is necessary to arrange the feed coal to be as constant as possible
for the sampling duration. Note that a CPP which receives feed from a coal valve/dozer push
system is likely to exhibit ongoing variation in particle size distribution which will impact on the
assessment.
Many coal preparation plants source feed coals from different areas and each can have different
qualities and washing characteristics. It is recommended that when sampling for equipment evaluation,
the plant draw raw coal from one area only.
6.4.6 Sample mass
6.4.6.1 General
The mass of sample to be collected strongly depends on the top size of coal, its heterogeneity and the
sampling purpose. In addition, the mass will be affected by the type of analysis to be conducted, namely:
a) general analysis and total moisture sample only;
b) sizing only;
c) float-sink analysis (f/s);
d) analysis combinations.
In circumstances where multiple types of samples are required, e.g. general analysis, sizing and float/
sink, then the actual gross sample mass requirement needs careful planning. Subsampling for different
types of analyses may be considered. However, typically, the best approach is to avoid any need for
subsampling of samples with a top size larger than 16 mm.
NOTE In this document, “general analyses” refers to analysis carried out on a minus 212 μm sample in the
laboratory such as proximate, calorific value and total sulfur.
6.4.6.2 General analysis and total gross sample mass
The minimum mass of gross sample for general analysis should be determined from Table 2, which
requires the nominal top size (95 % passing aperture) of the coal to be known or determined.
Alternatively, if the coal has been previously characterized by float-sink analysis on every standard
size fraction (≥ 63 mm, −63 mm + 31,5 mm, −31,5 mm + 16 mm, −16 mm + 8 mm, etc.), the technique of
[3]
Lyman described in Monograph Volume II, Australian Coal Preparation Society, may be used.
The sample masses for general analysis and total moisture listed in Table 2 are derived from Gy’s
formula [see Formula (1)] and are based on low ash (washed product of low heterogeneity), low density
coal particles and assume an acceptable relative sampling error (at one standard deviation) of 0,01 %,
which, for a coal with a mass fraction of 10 % ash, is 0,1 % ash. Given precision is normally defined as
plus and minus two standard deviations, the absolute precision of the ash mass fraction result would be
expected to be within ±0,2 % in this case.
Gy’s formula is as follows:
 
clfgd
M = (1)
 
S
 2 
σ
 
FE
where
M
is the sample mass in g;
S
c is the mineralogical composition factor, in g/cm , given by:
()1− a
L
[]()1− aaρρ+
LC Lmm
a
L
a is the mass fraction of the component of interest, e.g. ash = 0,1 (10 %);
L
ρ is the density of the critical component, e.g. ash in coal (typically 1,2) g/cm ;
C
ρ is the average density of the non-critical component, i.e. the mineral matter (typically 2,8) g/cm ;
mm
L is the liberation factor when d > d ;
d
l
= when the nominal topsize, d is less than the liberation size d in cm, liberation is complete
d
with respect to the component of interest.
if the material is not fully liberated, d being the nominal top size (cm) for complete liberation
l
of the critical component from the non-critical component (coal from mineral matter);
= 1 (dimensionless);
if the material is fully liberated or d is unknown (a conservative assumption);
l
f is the particle shape factor of particles (usually 0,5 for rounded particles) (dimensionless);
g is the size range (granulometric) factor (dimensionless);
= 0,25 for a wide distribution from fine to large particle sizes;
d is the nominal top size of the material being sampled (linear dimension factor), in cm;
σ is the fundamental error for sampling as a fraction.
FE
For sampling in coal preparation plants, streams other than low-ash products will be more
heterogeneous, which means that:
a) larger sample masses are required to achieve a similarly narrow range of precision; or
b) larger ranges of precision (wider error bars) need to be accepted.
Every coal is different, so its contribution to heterogeneity will vary between different coals. It is
therefore not always possible to provide separate mass tables to cover specific situations. Table 2 should
be used as a guide for minimum masses, and the method of Lyman or Gy should be used if relevant
minimum masses are required to be quantified for a particular coal.
As an indication, the precision range can easily increase to a magnitude of ±1,5 % ash (absolute) for
very heterogeneous samples such as a 40 % ash raw coal, which can contain a wide distribution and
quantity of particle types - from low-ash, low density “coal” particles, through to composite (middling)
particles, and to high-ash, high density mineral matter particles.
If the material to be sampled is very heterogeneous, such as feed coal to a separator or a coal preparation
plant, then the sample mass will need to be increased significantly to achieve the same level of sampling
precision. If the decision is made to reduce the sample mass significantly compared to that given in
Table 2, then the relative error of the subsequent analysis, can be significantly affected. This takes
on more importance when the analytical results are used to calculate mass balances and those mass
balances are used to estimate separator efficiencies and unit operation yields.
Conversely, if a higher level of measurement uncertainty is acceptable, then the sample mass may be
reduced.
It should also be noted that the relative error associated with the minimum masses listed in Table 2
relates to the sampling error only and does not include sample preparation error and analytical error.
22 2
σσ=+σσ+ (2)
Total sP T
where
σ is the standard deviation of total relative error (total measurement uncertainty, as a fraction
Total
of the true value);
σ is the standard deviation of sampling error;
S
σ is the standard deviation of sample preparation error;
P
σ is the standard deviation of testing and analysis error.
T
NOTE The measurement uncertainty, or the error-bar extent, is usually represented as twice the standard
deviation, which in turn encompasses a 95-percentile confidence interval for a normal distribution.
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