ISO 7936:2022
(Main)Coal — Determination and presentation of float and sink characteristics — General directions for apparatus and procedures
Coal — Determination and presentation of float and sink characteristics — General directions for apparatus and procedures
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.
Charbon — Détermination et présentation des caractéristiques de flottation et d'enfoncement — Principes directeurs relatifs à l'appareillage et aux modes opératoires
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Standards Content (Sample)
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 2022
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ii
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
iii
Bibliography .54
iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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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
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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.
v
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.
vi
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.
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.
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
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.
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 bulk of the sample can be removed in one operation.
In cases where there is only a small yield at one or two consecutive relative density fractions, it is
better to combine these fractions before going through a full treatment process. Within these limits it
is possible to vary the procedure without affecting the outcome of the test; in many cases its accuracy
will be improved, and the time and labour involved will be reduced.
5 Separation media
5.1 General
The medium which is to be used for the separation is generally a mixture of organic liquids as described
in Table 3. Aqueous solutions of inorganic salts (see Table 4), or solids in aqueous suspensions (see
Table 5) are acceptable but shall be validated prior to use.
The most suitable type of medium is determined by the type of testing required, particle size of the
sample and relative densities required.
Where tests are conducted in exposed situations, samples should be protected from airborne
contaminants and wind loss.
5.2 Organic solutions
5.2.1 General
WARNING — Particular attention is drawn to the fact that organic liquids and their vapours are
toxic and present a danger to health. The user of such liquids shall conform to the relevant safety
data sheet, and be aware of any statutory regulations.
Where relative densities of 1,6 and less are required, a mixture of perchloroethylene and white spirit
shall be used.
Where relative densities in the range of 1,6 to 2,9 are needed, tetrabromoethane (TBE), acetylene
tetrabromide or bromoform may be mixed with perchloroethylene.
Organic liquids shall be used sparingly, and liquid recovery shall be undertaken, particularly by
drainage, after removal of the sample from the separation tank.
Organic liquids are hazardous but are preferred to alternative media because of their low viscosity,
ease of use and the fact that they are inert towards shales. Prolonged washing and drying times are
unnecessary for the products of the separation owing to the generally high volatility of these liquids.
To accelerate drying and to minimize contamination, float and sink fractions separated by mixtures
containing TBE, shall be rinsed with a more rapidly evaporating compatible organic liquid, such as
white spirit.
NOTE The physical properties of the organic liquids used in float and sink testing are shown in Table 3.
Table 3 — Physical properties of organic liquids
Vapour
Relative Distillation range Viscosity at
Organic liquid pressure at
density or boiling point 20 °C
Flammable
20 °C
at 20 °C °C at 100 kPa mPa.s kPa
White spirit 0,77 30,0 to 200,0 — — Yes
Bromoform (tribromomethane) 2,89 149,5 2,152 (15 °C) 0,70 No
Tetrabromoethane (TBE, 2,96 239,0 12,00 0,01 No
acetylene tetrabromide)
Perchloroethylene 1,61 121 0,89 1,83 No
NOTE Refer to the vendor's Safety Data Sheet for current information regarding physical properties.
5.2.2 Limitations on accuracy
Organic liquids may react with the coal and despite thorough drying after float and sink testing,
some component of organic liquids could remain. This retention may influence subsequent analyses
and tests, for example, chlorine, trace elements, fly-ash precipitability, moisture-holding capacity and
caking properties such as fluidity.
Absorption of organic liquid into the coal may also affect the apparent relative density of the particle,
which can lead to inaccuracies in the separation.
5.3 Inorganic solutions
5.3.1 General
A water-soluble salt (e.g. caesium or potassium formate) shall be used as a dense medium for float sink
testing because of the lower density of potassium formate, its application is restricted to low density
separation or its use in a blend with caesium formate.
5.3.2 Formate solutions
5.3.2.1 General
The float sink procedure for water soluble salt solutions shall be the same as that outlined in Clause 6.
The major differences compared to using organic liquids are the preparation of the dense media, sample
rinsing and recovery of dilute media.
NOTE The physical properties of the inorganic solutions used in float and sink testing are shown in Table 4.
Table 4 — Physical properties of inorganic solutions
Distillation range or Viscosity at Vapour pressure
Relative
Formate
boiling point 20 °C at 20 °C
density at Flammable
solution
20 °C
(°C at 100 kPa) mPa.s kPa
Caesium for- 2,20 137 2,30 0,63 No
mate
Potassium for- 1,57 142 13,2 0,62 No
mate
NOTE Refer to the vendor's Safety Data Sheet for current information regarding physical properties.
5.3.2.2 Limitations on accuracy
The use of an inorganic solution such as caesium formate shall be thoroughly validated before its use to
ensure washability results are accurate and any subsequent laboratory testing of float sink fractions is
not compromised.
Particular attention shall be given to the following concerns, which can limit the use of caesium formate.
a) Caesium formate may contaminate coal particles even after rinsing. This can affect subsequent
properties to be tested, in particular ash.
b) Some coal or mineral matter particles can disintegrate in a water-based solution.
c) Density stability — Formate solutions with densities above RD 1,70 absorb moisture from the
air resulting in a decrease in the density. Solutions with densities below RD 1,70 lose moisture
resulting in a gradual increase in density.
5.3.2.3 Preparation of solutions
Caesium formate is readily soluble in water and therefore the solution can be diluted with potable tap
water to give densities in the normal working range of float and sink testing.
5.3.2.4 Sample rinsing
Particles shall be rinsed with water while vacuum filtering to remove caesium formate. The dilute
rinsings may then be processed to recover the caesium formate.
5.3.2.5 Recovery of media
If required, the recovery of caesium formate from dilute washings can be achieved by either distillation
(e.g. using an immersion heater), vacuum evaporation or reverse osmosis.
5.4 Aqueous suspensions
5.4.1 General
An insoluble material with a high relative density and correct particle size distribution shall be used to
give a relatively stable suspension of low viscosity.
Zirconium dioxide is an example of an aqueous suspension that is suitable.
5.4.2 Zirconium dioxide
5.4.2.1 General
The float and sink procedure for aqueous suspensions shall be the same as that outlined in Clause 6.
The major differences compared to using organic liquids or inorganic solutions are the preparation of
the dense media, sample rinsing and recovery of dilute media. Zirconium dioxide is chemically inert
and has a particle density of 5,75 allowing a range of aqueous suspensions to be prepared.
NOTE The physical properties of the aqueous suspensions used in float and sink testing are shown in Table 5.
5.4.2.2 Limitations on accuracy
The use of an aqueous suspension such as zirconium dioxide shall be thoroughly validated before its
use to ensure washability results are accurate and any subsequent laboratory testing of float and sink
fractions is not compromised.
Particular attention shall be given to the following concerns, which may limit the use of zirconium
dioxide.
a) Zirconium dioxide may contaminate particles even after rinsing. This contamination could affect
subsequent properties to be tested, in particular the ash in the sample.
b) Some coal or mineral matter particles may disintegrate in a water-based solution.
c) Zirconium dioxide suspensions are opaque. Therefore, the time required for the completion
of the separation of floats from sinks cannot be judged by visual observation and has to be pre-
determined by experiments combined with theoretical calculations.
d) The stability of the aqueous suspension should be known and monitored. Note that lower density
suspensions are the least stable.
e) Zirconium dioxide is not a suitable media for separation of fine particles (< 1 mm).
Table 5 — Physical properties of aqueous suspensions
Distillation range or Viscosity at Vapour pressure
Relative
Aqueous
boiling point 20 °C at 20 °C
density at Flammable
suspension
20 °C
°C at 100 kPa mPa.s kPa
Zirconium 1,30 to 2,20 — 11,2 — No
dioxide
NOTE Refer to the vendor's Safety Data Sheet for current information regarding physical properties.
5.4.2.3 Preparation of suspension
A very fine zirconium dioxide powder shall be prepared with water and electrostatically stabilized
using a suitable additive, such as a polyelectrolyte. The preparation procedure shall be as follows.
a) Prepare a water solution containing the stabilization agent.
b) Add sufficient zirconium dioxide powder to the agitated solution prepared in step a) to produce
suspensions with a relative density of 2,00 to 2,20.
c) Pass the suspension prepared in Step (b) twice through a colloid mill.
d) Measure the density of the stock suspension and store in a sealed container to avoid the evaporation
of water.
NOTE Zirconium dioxide suspension can be diluted with potable tap water to give densities in the normal
working range of float and sink testing.
5.4.2.4 Sample rinsing
Particles shall be washed by spraying with water on a vibrating sieve or screen.
NOTE The amount of suspension retained on the surface of the particles increases as the particle size
decreases because of the higher specific surface area. Further, the amount of suspension retained on the particles
increases as the viscosity increases, as the suspension can fail to drain completely.
5.4.2.5 Recovery of media
Dilute suspension media shall be recovered by concentration using one of the following options.
a) Recovering zirconium dioxide particles from a dilute suspension by reducing its pH to 6 or less to
allow fast settling; or
b) Using a ceramic membrane to re-concentrate diluted suspensions; or
c) Leaving dilute suspensions in a container for a long period of time to allow slow settling.
a c
Table 6 — Other examples of solids for aqueous suspensions
Nominal
Relative
top-size
Material Comments
b
density
µm
Finely ground shale 2,4 to 2,6 250 Discard from a coal
preparation plant
Brickwork shales
Froth flotation tailings 2,4 to 2,6 250
Barytes 3,7 to 4,1 63 Commercial barium
sulfate
Magnetite 5,0 38 As used in coal
preparation plants
Ferrosilicon 6,0 38 Ground or atomized
An alloy containing
85 % iron and 15 %
silicon
a
All of these solids can be used separately or in mixtures. Bentonite can be used to stabilize a
suspension.
b
For separations with relative densities above 1,5, ground shale or froth flotation tailings will
require an addition of higher relative density materials to avoid viscosity problems.
c
Aqueous suspensions require continuous mixing to keep the solids from settling and to
keep the mixture homogeneous, which in turn can affect the gravimetric separation of the coal
particles.
Aqueous suspensions are non-toxic and non-volatile, and therefore fume extraction is not required.
Provided that products are washed free of medium, no adverse effects on product quality occur.
Aqueous suspensions are not recommended for testing particles below 4 mm.
6 Apparatus
6.1 General
The testing apparatus shall be unaffected by the test media, robust, suitable for the purpose, convenient
and safe to use. A variety of apparatus is used, depending primarily on the particle size distribution of
samples being processed (i.e. coarse coal or fine coal).
Figure 1 and Figure 2 show common arrangements for a coarse float and sink cabinet and a typical float
and sink tank arrangement. Figures 3, 4 and 5 show typical float and sink laboratory equipment.
Mechanical handling is recommended where large quantities are to be treated.
Key
1 interior
2 shelf
3 slats
4 floor
5 exterior
Figure 1 — Float and sink cabinet
Key
1 drying hood 5 duct
2 rope pulleys 6 fan
3 plan view 7 elevation view
4 inline fan
Figure 2 — Example of float and sink tank arrangement
6.2 Coarse coal apparatus
Equipment for coarse float and sink test work typically consists of the items listed in 6.2.1 to 6.2.12.
6.2.1 Rectangular or cylindrical tank, with an inner mesh basket is shown in Figure 3. This
arrangement provides a convenient means of recovering floats and sinks separately.
Figure 3 — Coarse coal float and sink apparatus
6.2.2 Inner mesh basket.
Sinks are recovered by raising the inner basket which is then supported above the tank to allow liquid
to drain. Baskets should be fitted with suitable lifting handles. Mesh apertures shall be suitable to
contain all particles within the basket.
6.2.3 Mesh scoop of suitable aperture.
Floats are skimmed off the surface of the liquid using a mesh scoop of suitable aperture as shown in
Figure 4.
Figure 4 — Float and sink meshed scoops
6.2.4 Stirring rods, such as those shown in Figure 5, are used to agitate the contents of the separation
tank. They shall be made from materials impervious to the dense media.
Figure 5 — Float and sink stirring apparatus
6.2.5 Hydrometers.
Maintaining the required relative densities of the media is a critical aspect of the test and requires
monitoring and control throughout the whole float and sink operation. Methods for calibration of
hydrometers are described in Annex G. An example of appropriate storage of hydrometers is shown in
Figure 6. Recommendations on the types of hydrometers suitable for this purpose and requirements
for use are as follows:
a) types M100 or M50 having scale subdivisions of less than or equal to 0,002;
b) under normal working conditions, there is no need to apply temperature, meniscus or surface
tension corrections;
c) readings on the hydrometer stem shall correspond to the bottom of the meniscus.
Figure 6 — Hydrometer storage
6.2.6 Sample containers, such as plastic buckets may be used to store samples prior to testing.
6.2.7 Drying trays, shall be made from materials impervious to the dense media used. They shall
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