ISO 7936:2022

INTERNATIONAL ISO
STANDARD 7936
Second edition
2022-08
Coal — Determination and
presentation of float and sink
characteristics — General directions
for apparatus and procedures
Charbon — Détermination et présentation des caractéristiques
de flottation et d'enfoncement — Principes directeurs relatifs à
l'appareillage et aux modes opératoires
Reference number
ISO 7936:2022(E)
© ISO 2022

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ISO 7936:2022(E)
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© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
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ISO 7936:2022(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and symbols .1
4 Sampling . 1
4.1 General . 1
4.2 Sample mass . 2
4.3 Coal preparation plant products . 3
4.4 Plant control testing . 4
4.5 Comprehensive plant efficiency test . 4
4.6 Core samples. 4
4.7 Preliminary treatment . 4
4.8 Size analysis . 5
4.9 Pilot testing . 5
5 Separation media . 6
5.1 General . 6
5.2 Organic solutions . 6
5.2.1 General . 6
5.2.2 Limitations on accuracy . 6
5.3 Inorganic solutions . 7
5.3.1 General . 7
5.3.2 Formate solutions . 7
5.4 Aqueous suspensions . . 8
5.4.1 General . 8
5.4.2 Zirconium dioxide . 8
6 Apparatus .10
6.1 General . 10
6.2 Coarse coal apparatus . 12
6.3 Fine coal apparatus . 16
7 Float and sink testing procedures.16
7.1 General . 16
7.2 Relative densities of test media . 16
7.3 Testing of coarse size fractions . 17
7.3.1 General . 17
7.3.2 Procedure . 18
7.3.3 Air drying. 19
7.4 Testing of fine size fractions . 19
7.4.1 General . 19
7.4.2 Procedure . 19
8 Test report .22
Annex A (informative) Drop shatter .24
Annex B (informative) Wet tumbling .36
Annex C (informative) Sample masses for float and sink testing .38
Annex D (normative) Validation of data .43
Annex E (informative) Interpretation of data .45
Annex F (informative) Guide to the safe use of organic solutions .49
Annex G (informative) Calibration of hydrometers .52
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ISO 7936:2022(E)
Bibliography .54
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ISO 7936:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/
iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 1,
Coal preparation: Terminology and performance.
This second edition cancels and replaces the first edition (ISO 7936:1992), which has been technically
revised.
The main changes are as follows:
— addition of new procedures for the use of inorganic solutions, such as caesium and potassium
formates, and for aqueous suspensions, such as zirconium dioxide for float and sink analysis.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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ISO 7936:2022(E)
Introduction
The results of float and sink testing, presented in tabular and graphical form, are the basis for the
provision of washability data.
The results of float and sink data from coal seam samples provide an estimation of the future quality
and yield of washed coal from the area of the coal lease where the samples were taken.
The results of float and sink data from coal seams and preparation plants are also used when designing
a new plant and /or redesigning an existing plant, and also in predicting, controlling and assessing the
performance of an existing plant in total or in part.
Where tests other than those for routine control purposes are carried out, it is essential that there
is precise instruction regarding size ranges and relative density fractions to establish the scope of
information and accuracy required.
The following annexes provide new additional information in this revision as follows:
Annex A  Drop shatter – A pre-treatment of samples for float and sink testing;
Annex B  Wet tumbling – A pre-treatment of samples for float and sink and testing;
Annex C  Sample masses for float and sink testing;
Annex D  Validation of data from float and sink analysis;
Annex E  Interpretation of data from float and sink analysis;
Annex F  Guide to the safe use of organic solutions.
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INTERNATIONAL STANDARD ISO 7936:2022(E)
Coal — Determination and presentation of float and sink
characteristics — General directions for apparatus and
procedures
1 Scope
This document specifies general directions for the apparatus and procedures, using relative density
separation methods, for determining the float and sink characteristics of samples from coal seams and
of feed, products and rejects from coal preparation plants.
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 1953, Hard coal — Size analysis by sieving
ISO 13909-1, Hard coal and coke — Mechanical sampling — Part 1: General introduction
ISO 13909-2, Hard coal and coke — Mechanical sampling — Part 2: Coal — Sampling from moving streams
ISO 13909-3, Hard coal and coke — Mechanical sampling — Part 3: Coal — Sampling from stationary lots
ISO 13909-4, Hard coal and coke — Mechanical sampling — Part 4: Coal — Preparation of test samples
ISO 18283, Coal and coke — Manual sampling
3 Terms and symbols
For the purposes of this document, the terms and definitions given in ISO 1213-1 and ISO 1213-2 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/
4 Sampling
4.1 General
Samples for float and sink testing are mainly sourced from three major areas:
a) coal seams from underground and open cut mines;
b) coal preparation plants;
c) bore core.
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ISO 7936:2022(E)
4.2 Sample mass
Sampling shall be carried out in accordance with ISO 13909-1, ISO 13909-2, ISO 13909-3 or ISO 18283.
The following standard sampling guides should also be considered:
a) ISO 14180: Typical samples are bulk raw coal samples, channel samples, rotary drilled cuttings and
core samples of various diameters;
b) ISO 4077: Typical samples are raw feed, clean coal and reject from the plant in total or from various
parts of the plant such as cyclones.
The minimum mass of sample from coal seams (raw coal) and coal samples from a coal preparation
plant required for float and sink testing are outlined in Table 1. The number of discrete particles to
be aimed for in any size fraction of the sample should not be less than 2 000. The masses given in
Table 1 generally ensures that the number of particles is adequate. However, these masses may not be
practicable in the case of bore cores or some coal preparation plant products.
The mass of the coal seam bulk sample or large plant sample should be enough to contain the minimum
quantities in each fraction as listed in Table 1. Where taking a coal seam bulk sample or a large sample
from a plant, it is better to over-sample than to have insufficient material. However, in order to carry
out testing on a coal seam raw coal bulk sample at the larger sizes in Table 1, the sample may have to
be the order of 10 tonnes, or even greater. For example, in a newly opened mine, a trial shaft or other
appropriate location, the mass of bulk sample taken should not be less than 10 tonnes.
For cores, particularly small diameter cores the masses recommended in Table 1 are not often obtained.
For this reason, core plies or sections should be selected as large as possible, and subdivision of the
crushed ply or section prior to float and sink testing should be avoided. If these requirements cannot be
met, this fact shall be noted in the test report.
In coal preparation plants, some coals may give low yields in the intermediate relative density
fractions. Consequently, there may be insufficient material for analytical requirements. In addition,
the recommended mass of the sample may have to be substantially increased to meet the following
requirements: not less than 20 g and not less than 10 particles in each relative density fraction. Refer to
ISO 4077 for further guidance in this area.
This document strongly recommends that the sampling and preparation protocols and advice given in
this clause, particularly those relating to the mass of sample for float and sink analysis, are followed
carefully as, if not, the resultant results of any float and sink analysis can be compromised.
Samples with different particle sizes contain particles with different proportions of mineral matters
and organic matters, which produces different washability (different float and sink distribution).
Consequently, it is critical that a representative sample of the relevant size distribution is provided for
float and sink testing.
It is assumed that square-mesh particle sizes are used; if wedge-wire or round-hole sizes are used, this
fact should be reported. When a bulk sample is being taken, it is better to over-sample than to have
insufficient material.
For testing on the top-sizes shown in Table 1, the bulk sample mass may be up to 20 t, and for other
sizes the mass is reduced according to the decrease in nominal top-size.
NOTE The importance of enough sample mass and a method for the determination of the required mass
of a bulk sample is given in Annex C. For further information on sample masses for float and sink testing and
examples of calculations to determine masses needed at various size distributions, see Annex C. Refer to ISO 4077
for further guidance in this area.
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ISO 7936:2022(E)
Table 1 — Minimum mass for a given size fraction
a,b
Sample mass
Size fraction
kg
mm Raw coal Clean coal Reject
−125 + 63 2 150 1 810 2 680
−63 + 31,5 300 250 370
−50 + 31,5 230 190 280
−31,5 + 16 40 34 50
−16 + 8 5,2 4,4 6,5
−8 + 4 2,0 2,0 2,0
−4 + 2 2,0 2,0 2,0
−2 + 1 2,0 2,0 2,0
−1 + 0,5 2,0 2,0 2,0
−0,5 + 0,25 1,0 1,0 1,0
NOTE  The basis for calculating the number of particles was as follows:
A Rosin and Rammler (Weibull function) size distribution was applied to the default sample, using
parameters of x (size constant) = 30 mm, and n (slope) = 0,60. The number of particles within each size
fraction was calculated by fractionating each individual size fraction by mass into 1 mm (or smaller) sub-
fractions. The volume of each particle in each sub-fraction was calculated using the particle RD stated
above, and a shape factor of 1,25. Thus, if the size sub-fraction was – 60,5 + 60 mm, the particle in the sub-
fraction was assumed to have the following dimensions: 60 mm × 60 mm × 75 mm.
a
For control samples from a preparation plant as an example, where successive test results can be
averaged, the mass shown in Table 1 may be reduced by approximately one-half.
b
The sample masses in Table 1 are calculated from the required number of particles and have been
calculated based on the following assumed particle relative densities (RD): Raw Coal 1,60, Clean coal 1,35,
Reject 2,00 (see Annex C for calculation examples to determine bulk sample masses).
Both the size distribution and the ash mass fraction of the raw coal coming from a working face or mine
will vary during a shift, as well as from day to day. It is essential that the duration of sampling be long
enough to cover such variations.
The total sample mass, m , in kg required for a float and sink test is given by Formula (1):
t
m
r
m =×   100 (1)
t
w
s
where
m is the recommended mass of coarsest size fraction (from Table 1), kg;
r
w is the mass fraction of the coarsest size fraction in the sample, %.
s
4.3 Coal preparation plant products
Since the relative densities of some components, such as reject and middlings, are greater than that
of clean coal, the minimum masses of samples containing these components should be increased
proportionately. This ensures that these samples contain approximately the same number of particles
as the corresponding clean coal sample, and consequently a similar degree of accuracy will be obtained
in the test.
Samples should be taken as soon as practicable after the material leaves the cleaning unit, in order to
minimize breakage. Testing should then commence as soon as possible.
In sampling pulp, the mass of the (dried) solids should be in accordance with the requirements of
Table 1. Increments shall be taken at regular time intervals over the total cross-section of the pulp
stream, either manually or by mechanical means, using a sampling device having a capacity equal to at
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ISO 7936:2022(E)
least twice that of the recommended minimum mass of increment. Care should be taken to ensure that
none of the sample is lost by splashing.
For 4.3, 4.4 and 4.5, see also ISO 4077 which provides further advice on masses required for plant
products, control testing and efficiency tests and various combinations of all three items.
4.4 Plant control testing
Routine samples are taken regularly for the purpose of determining the average efficiency of a cleaning
plant. They may represent daily, weekly or longer periods of running. The mass taken may be less
than that given in Table 1, depending on the reason for the test. However, if any dispute arises over the
accuracy of the results, sample masses in accordance with Table 1 should be used.
4.5 Comprehensive plant efficiency test
A comprehensive cleaning plant efficiency test involves a systematic mass balance of all materials
entering and leaving the plant. In this case, the mass and moisture fractions of the raw feed, the mass
and moisture fractions “as determined” of all cleaned products, discard, etc., and the volume and solids
mass fractions of the effluent will be required. The mass of all materials is calculated to a uniform
moisture basis, and the feed entering and products leaving the plant are balanced against each other.
The efficiency of the cleaning plant is assessed from the actual and theoretical yields and ash mass
fractions. The analysis of the raw feed by computation from the masses and analyses of all the products
is more reliable than that obtained by direct examination, and it is therefore used for the calculation of
the theoretical yields.
When a screen analysis of a plant product is made in connection with a cleaning plant efficiency test, it
will be found that there is some material below the nominal bottom size being treated in the cleaning
unit. The mass and particle size range of this undersize material should be recorded.
4.6 Core samples
For core samples, guidance is given in Annex C and ISO 14180.
4.7 Preliminary treatment
Many coal samples, such as strip and core samples require pre-treatment to better simulate the size
distribution of the raw coal feed to a coal preparation plant. This pre-treatment ensures more accurate
representation of fines mass fraction, which in turn affects washability results.
The pre-treatment process can involve any or a combination of the following.
a) Drop shatter — The picking up and dropping of a sample onto a steel plate under specific conditions.
b) Top-size reduction — This process requires oversize material to be reduced to pass a nominated
screen, with a minimal amount of fine material being produced. Top-size reduction does not
simulate the size distribution of coal preparation plant feed, because the coal particles are not
selectively broken.
c) Various methods can be utilized to perform this procedure, including the following:
1) Jaw crusher — The sample is choke-fed to the crusher with the aim of producing the nominated
size;
2) Hand knapping — The sample is broken using hand-held implements. Done carefully, this
procedure can yield the least amount of fine material.
d) Hammermill type crushers shall not be used for size reduction, because of the excess amount of
fine material produced apart for final crushing to minus 212 μm for analysis.
e) Dry tumbling — The sample is tumbled end over end in a drum under specified conditions.
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ISO 7936:2022(E)
f) Wet tumbling — The sample is tumbled end over end after the addition of water and under specified
conditions.
NOTE See Annexes A and B for more information on drop shatter and wet tumbling.
4.8 Size analysis
The sample should be spread out on an impervious base, preferably under shelter, and allowed to
dry sufficiently for screening purposes. After the sample has been dried, the sample should then be
screened using a suitable range of apertures (typical sizes are given in Table 2). Oversize material may
be broken by hand or machine-crushed according to the nominal top-size required. If applicable, the
relevant part of the crusher circuit may be simulated.
The quantity of material passing the 63 mm screen is usually more than the amount required and it
can be divided before proceeding to the next size of screen. Further division may be necessary at lower
sizes.
Wet screening should be used, to ensure that fine particles adhering to larger particles are included in
the proper size fraction.
NOTE Pulp and reject samples are screened promptly to avoid excessive shale breakdown.
Table 2 — Size analysis
Size fraction Material Material
Mass fraction
(square hole) retained passing
mm % % %
+125,0 Nil Nil 100,0
-125,0 +63,0 11,9 11,9 88,1
-63,0 +31,5 12,1 24,0 76,0
-31,5 +16,0 12,8 36,8 63,2
-10,0 +8,0 15,7 52,5 47,5
-8, 0 +4,0 12,5 65,0 35,0
-4,0 +2,0 10,2 75,2 24,8
-2,0 +1,0 7,5 82,7 17,3
-1,0 +0,5 5,6 88,3 11,7
-0,5 11,7 100,0 Nil
Total 100,0
4.9 Pilot testing
Pilot testing is frequently carried out on a representative sample, in order to determine how the bulk
material will behave. This knowledge enables the operator to plan the actual test in such a way that
unnecessary operations are avoided, so that the test is carried out more expeditiously and with less
effort. The pilot test, or previous experience, may indicate that it is advantageous to commence the
separation at either the highest or the lowest relative density.
A sample that will give a high yield at either of these densities should be separated at that density, so
that the
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