ISO 12740:1998
(Main)Lead sulfide concentrates — Determination of silver and gold contents — Fire assay and flame atomic absorption spectrometric method using scorification or cupellation
Lead sulfide concentrates — Determination of silver and gold contents — Fire assay and flame atomic absorption spectrometric method using scorification or cupellation
This International Standard specifies a fire assay and flame atomic absorption spectrometric procedure for the determination of silver and gold contents of lead sulfide concentrates. The method is applicable to the determination of silver and gold in lead sulfide concentrates containing 10 % (m/m) to 80 % (m/m) lead. The method is applicable to silver contents from 200 g/t to 2 000 g/t and gold contents from 0,1 g/t to 25 g/t.
Concentrés sulfurés de plomb — Dosage de l'argent et de l'or — Méthode par voie sèche et spectrométrie d'absorption atomique dans la flamme à partir d'une scorification ou d'une coupellation
Koncentrati svinčevega sulfida - Določevanje srebra in zlata - Talilna analiza in plamenska atomska absorpcijska spektrometrijska metoda s taljenjem in kupelacijo
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INTERNATIONAL ISO
STANDARD 12740
First edition
1998-12-01
Lead sulfide concentrates — Determination
of silver and gold contents — Fire assay
and flame atomic absorption spectrometric
method using scorification or cupellation
Concentrés sulfurés de plomb — Dosage de l’argent et de l’or — Méthode
par voie sèche et spectrométrie d’absorption atomique dans la flamme à
partir d’une scorification ou d’une coupellation
A
Reference number
ISO 12740:1998(E)
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ISO 12740:1998(E)
Contents Page
1 Scope .1
2 Normative references .1
3 Principle.1
4 Reagents.2
5 Apparatus .4
6 Sample .5
7 Procedure .5
8 Expression of results .10
9 Precision.10
10 Test report .13
Annex A (normative) Procedure for the preparation and determination of the mass of a predried
test portion .14
Annex B (normative) Trial fusion.16
Annex C (normative) Blank determination .17
Annex D (normative) Flowsheet of the procedure for the acceptance of analytical values for the test
samples.18
Annex E (informative) Derivation of precision equations .19
Annex F (informative) Bibliography.26
© ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
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© ISO
ISO 12740:1998(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.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 12740 was prepared by Technical Committee ISO/TC 183, Copper, lead and zinc ores.
Annexes A to D form an integral part of this International Standard, annexes E and F are for information only.
iii
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INTERNATIONAL STANDARD © ISO ISO 12740:1998(E)
Lead sulfide concentrates — Determination of silver and gold
contents — Fire assay and flame atomic absorption spectrometric
method using scorification or cupellation
1 Scope
This International Standard specifies a fire assay and flame atomic absorption spectrometric procedure for the
determination of silver and gold contents of lead sulfide concentrates.
The method is applicable to the determination of silver and gold in lead sulfide concentrates containing 10 % (m/m)
to 80 % (m/m) lead.
The method is applicable to silver contents from 200 g/t to 2 000 g/t and gold contents from 0,1 g/t to 25 g/t.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
International standard. At the time of publication, the editions indicated were valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain
registers of currently valid International Standards.
ISO 385-1:1984, Laboratory glassware — Burettes — Part 1: General requirements.
ISO 648:1977, Laboratory glassware — One-mark pipettes.
ISO 1042:1998, Laboratory glassware — One-mark volumetric flasks.
ISO 3696:1987, Water for analytical laboratory use — Specification and test methods.
ISO 4787:1984, Laboratory glassware — Volumetric glassware — Methods for use and testing of capacity.
ISO 9599:1991, Copper, lead and zinc sulfide concentrates — Determination of hygroscopic moisture in the
analysis sample — Gravimetric method.
3 Principle
3.1 Scorification
Fire assay fusion of a test portion to produce a lead button, which is scorified to reduce it to a mass of 2 g to 5 g.
Retreatment fusion of the primary fusion and scorification slags to produce a low-silver content lead button which is
scorified to approximately 2 g to 5 g.
Dissolution of both lead buttons in nitric acid and filtration of the solution. Dissolution of the filter paper plus gold and
determination of silver and gold by flame atomic absorption spectrometry.
1
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© ISO
ISO 12740:1998(E)
3.2 Cupellation
Fire assay fusion of a test portion to produce a lead button, which is cupelled to produce a bead of silver and gold.
Retreatment fusion of the primary fusion slag and spent cupel to produce a second bead of silver and gold.
Dissolution of both beads in dilute nitric acid and filtration of the solution. Dissolution of the filter paper plus gold and
determination of silver and gold by flame atomic absorption spectrometry.
4 Reagents
During the analysis, use only reagents of a recognized analytical grade and water that complies with grade 2 of
ISO 3696. Wash all glassware with aqua regia followed by water and 25 % ammonia. A chloride-free environment is
recommended.
4.1 Sodium carbonate, anhydrous
4.2 Litharge, assay reagent grade having silver content less than 0,2 g/t, and gold content less than 0,01 g/t
4.3 Silica, precipitated grade
4.4 Potassium nitrate or sodium nitrate
4.5 Flour
4.6 Nitric acid (ρ 1,42 g/ml), chloride content < 0,5 μg/ml.
20
4.7 Nitric acid wash solution, (5 ml/l)
To 995 ml of water add 5 ml of nitric acid (4.6).
4.8 Nitric acid, (250 ml/l)
To 1 500 ml of water carefully add, with stirring, 500 ml of nitric acid (4.6) and cool.
4.9 Hydrochloric acid, (ρ 1,16 g/ml to 1,19 g/ml).
20
4.10 Borax, fused anhydrous sodium tetraborate
4.11 Silver metal, minimum 99,99 % purity
4.12 Gold metal, minimum 99,99 % purity
4.13 Aqua regia
Mix 3 parts of hydrochloric acid (4.9) with 1 part of nitric acid (4.6); prepare freshly as required.
4.14 Ammonia solution
Add 500 ml of ammonia solution (ρ 0,89 g/ml) to 500 ml of water.
20
4.15 Sodium chloride
4.16 Standard solutions
NOTE Standard solutions should be prepared at the same ambient temperature as that at which the determinations will be
conducted.
2
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© ISO
ISO 12740:1998(E)
4.16.1 Silver standard solutions
4.16.1.1 Silver standard solution, (1 000 μg/ml)
Weigh 0,500 0 g of silver metal (4.11) into a 400 ml beaker, add 100 ml of nitric acid (4.8), cover and heat gently
until the metal dissolves. Continue heating to remove oxides of nitrogen. Cool, and transfer to a 500 ml volumetric
flask. Dilute to volume with water and mix thoroughly.
This solution should be stored in a brown bottle.
4.16.1.2 Silver standard solution, (100 μg/ml)
Pipette 20 ml of silver standard solution (4.16.1.1) into a 200 ml volumetric flask, add 40 ml of nitric acid (4.8), dilute
to volume and mix thoroughly.
This solution should be stored in a brown bottle.
4.16.1.3 Silver standard solution, (10 μg/ml)
Pipette 20 ml of silver standard solution (4.16.1.2) into a 200 ml volumetric flask, add 40 ml of nitric acid (4.8), dilute
to volume and mix thoroughly.
This solution shall be freshly prepared.
4.16.2 Gold standard solutions
4.16.2.1 Gold standard solution, (1 000 μg/ml)
Weigh 0,100 g of gold metal (4.12) into a 50 ml beaker, add 5 ml of aqua regia (4.13), cover and heat to dissolve the
gold. Wash and remove the cover, add 0,1 g of sodium chloride (4.15) and evaporate to near dryness (do not allow
to go to dryness, as gold may precipitate.) Cool, add 10 ml of hydrochloric acid (4.9), transfer the solution
quantitatively to a 100 ml volumetric flask, dilute to volume with water, stopper and mix thoroughly. Store the
solution in a brown bottle.
(100 μg/ml)
4.16.2.2 Gold standard solution,
Pipette 10 ml of gold standard solution (4.16.2.1) into a 100 ml volumetric flask, add 5 ml of hydrochloric acid (4.9),
dilute to volume with water, stopper and mix thoroughly.
4.16.2.3 Gold standard solution, (10 μg/ml)
Pipette 20 ml of gold standard solution (4.16.2.2) into a 200 ml volumetric flask, add 10 ml of hydrochloric acid (4.9)
dilute to volume with water, stopper and mix thoroughly.
This solution shall be freshly prepared.
4.17 Calibration solutions
NOTE Calibration solutions should be prepared at the same ambient temperature as that at which the determinations will
be conducted.
4.17.1 Silver calibration solutions
To six 100 ml volumetric flasks, add from a burette (5.6) 0 ml, 5 ml, 10 ml, 20 ml, 30 ml and 40 ml of silver standard
solution (4.16.1.3); add 20 ml, 19 ml, 18 ml, 16 ml, 14 ml and 12 ml of nitric acid (4.8), dilute to volume with water
and mix thoroughly.
These standards contain 0 μg, 0,5 μg, 1 μg, 2 μg, 3 μg and 4 μg of silver per ml and shall be freshly prepared.
3
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© ISO
ISO 12740:1998(E)
4.17.2 Gold calibration solutions
To six 100 ml volumetric flasks, add from a burette (5.6) 0 ml, 10 ml, 20 ml, 30 ml, 40 ml and 50 ml of gold standard
solution (4.16.2.3); add 20 ml of aqua regia solution (4.13), dilute to volume with water and mix thoroughly.
These standards contain 0 μg, 1 μg, 2 μg, 3 μg, 4 μg and 5 μg of gold per ml and shall be freshly prepared.
5 Apparatus
Ordinary laboratory equipment plus the following.
5.1 Volumetric glassware, of class A complying with ISO 385-1, ISO 648 and ISO 1042 and used in accordance
with ISO 4787.
5.2 Conventional fire assay equipment
5.2.1 Assay crucible furnace, having a maximum required operating temperature of 1 200°C.
5.2.2 Muffle furnace, having a maximum required operating temperature of 1 100°C; temperature indication,
automatic temperature control and controlled air flow are desirable.
5.2.3 Crucibles, made of fire clay, of nominal capacity 300 ml to 500 ml, capable of withstanding corrosion by the
sample and fluxes at 1 100°C. The crucible shall be of such a size that the charge does not fill the crucible to a
depth of greater than 3/4 the depth of the crucible.
5.2.4 Scorifiers, made of fire clay, of approximately 75 mm diameter, preferably not shallow form.
5.2.5 Cupels, made of magnesium oxide, or bone ash cupels having a nominal capacity of 50 g of molten lead.
The inside bottom of the cupel shall be concave.
5.2.6 Conical mould, made of cast iron, of sufficient capacity to contain all of the molten lead plus slag from the
crucible fusion.
5.2.7 Pulverizer
5.3 Balances
5.3.1 Top loading, capable of being read to 1 mg
5.3.2 Precision analytical, capable of being read to 0,1 mg
5.4 Atomic absorption spectrometer (AAS), equipped with background correction and a glass bead in the spray
chamber rather than a flow spoiler
5.5 Filtration equipment
5.5.1 Membrane filter - cellulose nitrate/cellulose acetate, diameter 25 mm, mesh size 0,45 μm
5.5.2 Vacuum flask, of 250 ml capacity, buchner filter flask having a wide neck
5.5.3 Membrane filter funnel, suitable for use with 25 mm membrane filters
5.6 Burette, A grade 50 ml capacity, capable of being read to 0,1 ml
4
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© ISO
ISO 12740:1998(E)
6 Sample
6.1 Test sample
Prepare an air-equilibrated test sample in accordance with ISO 9599.
NOTE A test sample is not required if predried test portions are to be used (see annex A).
6.2 Test portion
Taking multiple increments, extract a test portion from the test sample in such a manner that it is representative of
the whole contents of the dish or tray. Weigh to the nearest 0,1 mg approximately 10 g of test sample. At the same
time as the test portion is weighed, weigh test portions for the determination of hygroscopic moisture in accordance
with ISO 9599.
Alternatively, the method specified in annex A may be used to prepare predried test portions directly from the
laboratory sample.
7 Procedure
7.1 Number of determinations
Carry out the determinations at least in duplicate, as far as possible under repeatability conditions, on each test
sample.
NOTE Repeatability conditions exist where mutually independent test results are obtained with the same method on
identical test material in the same laboratory by the same operator using the same equipment within short intervals of time.
7.2 Trial fusion
Carry out a trial fusion as described in annex B to ensure that the mass of the lead button is between 28 g and 35 g.
NOTE It is essential that this trial fusion be performed.
7.3 Blank tests
Carry out a duplicate reagent blank test as described in annex C in parallel with each batch of samples fired, using
the same quantities of all reagents with the addition of sufficient flour (4.5) to the flux to give a button size of 28 g to
35 g. Omit the test portion and the potassium nitrate.
NOTE If the blank solutions (annex C) exceed 0,05 μg/ml for gold, the reagents should be checked and the problem
rectified. Otherwise the blank is subtracted from the result.
7.4 Charge preparation
Determine the mass of potassium nitrate (4.4) required in the charge as indicated by the trial fusion (see annex B)
and include this reagent in the flux mixture (see table 1).
Thoroughly mix the test portion with a flux of the composition specified in table 1.
5
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© ISO
ISO 12740:1998(E)
Table 1 — Recommended masses of flux components for preparation of charges
Flux components Mass
g
Sodium carbonate (4.1) 30
Litharge (4.2) 70
Silica (4.3) 10
Borax (4.10) 10
Potassium nitrate (4.4) –
Place the mixture in the assay crucible (5.2.3).
NOTE 1 The intimate mixing of fluxes and samples is very important. All flux components should be in a finely divided state,
with a preferred particle size of less than 0,5 mm.
NOTE 2 The quantities of oxidizing reagent in the mixture are dependent upon the reducing power of the test sample. The
trial fusion (see annex B) will determine the mass of potassium nitrate necessary to yield a button of mass 28 g to 35 g. The
oxidizing reagent should not be included in a bulk flux. It should be weighed up individually to ensure the repeatability of the
mass of the lead button, which, if fusion conditions are uniformly repeated, should agree within ± 1 g of lead, thus decreasing
the variability of finishing times of scorification or cupellation.
NOTE 3 If difficulties are experienced in producing repeatable and predictable masses of lead button, a lid for the crucible
may be of assistance.
7.5 Primary fusion
Place the crucible in the furnace (5.2.1) preheated to approximately 900 °C.
Slowly raise the furnace temperature to 1 050°C. Maintain this temperature until fusion has been calm for at least
10 min.
Pour the fusion into a dry conical mould (5.2.6), taking care to ensure that no loss of lead or slag occurs. Reserve the
crucible for retreatment fusion.
Allow the mixture to cool and carefully separate the lead button from the slag. Hammer the lead button as necessary to
remove any small particles of adhering slag. Reserve the slag for retreatment.
Weigh the lead button. If the button weighs less than 28 g or more than 35 g, discard the button and slag and repeat
the assay using less oxidizing agent or adding flour (see annex B).
NOTE 1 If oil-fired or gas-fired furnaces are used, the fuel should be turned off immediately before opening the furnace.
NOTE 2 To minimize crucible corrosion and build-up of impurities in the lead button, the overall fusion time should not
exceed 40 min.
NOTE 3 Buttons weighing less than 28 g show poor collection efficiencies, and those exceeding 35 g contain higher
amounts of impurities such as copper and may also cause problems in obtaining the required mass of lead after scorification.
7.6 Selection of recovery procedure
The recovery of the silver and gold in the lead button from the primary fusion can use either scorification (see 7.7)
or cupellation (see 7.8). Individual analysts should select the technique most familiar to their respective laboratories.
The final determination of silver and gold by AAS is the same in each case.
6
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© ISO
ISO 12740:1998(E)
7.7 Scorification
7.7.1 General
Place the lead button from the primary fusion (see 7.5) in a preheated scorifier containing approximately 0,5 g of
silica (4.3) in a muffle furnace (5.2.2) at 900 °C.
Allow the scorification to proceed at a muffle temperature of approximately 900 °C with steady air flow until the visible
lead melt is approximately 10 mm in diameter (approximately 30 min depending on the mass of the lead button).
Toward the end of the scorification, increase the muffle temperature to 950 °C to decrease the viscosity for clean
pouring. The required lead mass is 2 g to 5 g.
Pour the mixture into a dry conical mould (5.2.6), allow to cool, remove and weigh the lead button. Reserve the slag for
retreatment fusion. Discard the scorifier.
7.7.2 Retreatment fusion
Place the slags from the primary fusion (see 7.5) and the scorification (see 7.7.1) in a grinder (5.2.7) and pulverize
for a period of about 20 s. Thoroughly clean the pulverizer by grinding a silver- and gold-free medium between each
sample.
Intimately mix the pulverized slag with a flux of the following composition:
litharge (4.2) 50 g;
sodium carbonate (4.1) 50 g;
silica (4.3) 50 g;
borax (4.10) 50 g;
flour (4.5) sufficient to produce a 28 g to 35 g lead button (usually approximately 4 g).
Place the mixture into the original assay crucible from 7.5.
Carry out a fusion as detailed in 7.5.
Pour the fusion into a dry conical mould (5.2.6) taking care that no loss of lead occurs. Discard the crucible and slag.
7.7.3 Scorification
Place the lead button from the retreatment fusion, 7.7.2, in a new preheated scorifier containing approximately 0,5 g
of silica (4.3) in the muffle furnace (5.2.2) at 950 °C. The button is added to the scorifier in the muffle furnace.
Proceed as in 7.7.1. Discard the slag and scorifier.
7.8 Cupellation
7.8.1 General
Place the lead button from the primary fusion, 7.5, into a preheated cupel in a muffle furnace (5.2.2) at 900 °C. The
button is added to the cupel in the muffle furnace.
Allow the cupellation to proceed at a lower muffle temperature of approximately 860 °C with steady air flow until the
visible lead melt is approximately 10 mm in diameter (approximately 30 min depending on the mass of lead button).
Raise the muffle furnace temperature to 900 °C to finish the cupellation.
Remove the cupel and allow to cool. Take the silver and gold bead and remove any attached cupel material with a
brush. Weigh the bead. Reserve the cupel for retreatment fusion.
7
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© ISO
ISO 12740:1998(E)
7.8.2 Retreatment fusion
Place the slag from the primary fusion, 7.5, and the cupel from the recovery procedure, 7.8.1 in a grinder (5.2.7) and
pulverize for about 20 s to reduce the material to minus 150 μm. Longer grinding may cause caking of the material.
Thoroughly clean the pulverizer by grinding a silver- and gold-free medium between each sample.
Intimately mix the pulverized slag and cupel with a flux of the following composition:
Litharge (4.2) 50 g;
Sodium carbonate (4.1) 50 g;
Silica (4.3) 50 g;
Borax (4.10) 50 g;
Flour (4.5) sufficient to produce a 28 to 35 g lead button (usually approximately 4 g).
Place the mixture into the original assay crucible from 7.5.
Carry out a fusion as detailed in 7.5.
Pour the fusio
...
SLOVENSKI STANDARD
SIST ISO 12740:2000
01-junij-2000
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SODPHQVNDDWRPVNDDEVRUSFLMVNDVSHNWURPHWULMVNDPHWRGDVWDOMHQMHPLQ
NXSHODFLMR
Lead sulfide concentrates -- Determination of silver and gold contents -- Fire assay and
flame atomic absorption spectrometric method using scorification or cupellation
Concentrés sulfurés de plomb -- Dosage de l'argent et de l'or -- Méthode par voie sèche
et spectrométrie d'absorption atomique dans la flamme à partir d'une scorification ou
d'une coupellation
Ta slovenski standard je istoveten z: ISO 12740:1998
ICS:
73.060.99 Druge rude Other metalliferous minerals
SIST ISO 12740:2000 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST ISO 12740:2000
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SIST ISO 12740:2000
INTERNATIONAL ISO
STANDARD 12740
First edition
1998-12-01
Lead sulfide concentrates — Determination
of silver and gold contents — Fire assay
and flame atomic absorption spectrometric
method using scorification or cupellation
Concentrés sulfurés de plomb — Dosage de l’argent et de l’or — Méthode
par voie sèche et spectrométrie d’absorption atomique dans la flamme à
partir d’une scorification ou d’une coupellation
A
Reference number
ISO 12740:1998(E)
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SIST ISO 12740:2000
ISO 12740:1998(E)
Contents Page
1 Scope .1
2 Normative references .1
3 Principle.1
4 Reagents.2
5 Apparatus .4
6 Sample .5
7 Procedure .5
8 Expression of results .10
9 Precision.10
10 Test report .13
Annex A (normative) Procedure for the preparation and determination of the mass of a predried
test portion .14
Annex B (normative) Trial fusion.16
Annex C (normative) Blank determination .17
Annex D (normative) Flowsheet of the procedure for the acceptance of analytical values for the test
samples.18
Annex E (informative) Derivation of precision equations .19
Annex F (informative) Bibliography.26
© ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
---------------------- Page: 4 ----------------------
SIST ISO 12740:2000
© ISO
ISO 12740:1998(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.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 12740 was prepared by Technical Committee ISO/TC 183, Copper, lead and zinc ores.
Annexes A to D form an integral part of this International Standard, annexes E and F are for information only.
iii
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SIST ISO 12740:2000
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SIST ISO 12740:2000
INTERNATIONAL STANDARD © ISO ISO 12740:1998(E)
Lead sulfide concentrates — Determination of silver and gold
contents — Fire assay and flame atomic absorption spectrometric
method using scorification or cupellation
1 Scope
This International Standard specifies a fire assay and flame atomic absorption spectrometric procedure for the
determination of silver and gold contents of lead sulfide concentrates.
The method is applicable to the determination of silver and gold in lead sulfide concentrates containing 10 % (m/m)
to 80 % (m/m) lead.
The method is applicable to silver contents from 200 g/t to 2 000 g/t and gold contents from 0,1 g/t to 25 g/t.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
International standard. At the time of publication, the editions indicated were valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain
registers of currently valid International Standards.
ISO 385-1:1984, Laboratory glassware — Burettes — Part 1: General requirements.
ISO 648:1977, Laboratory glassware — One-mark pipettes.
ISO 1042:1998, Laboratory glassware — One-mark volumetric flasks.
ISO 3696:1987, Water for analytical laboratory use — Specification and test methods.
ISO 4787:1984, Laboratory glassware — Volumetric glassware — Methods for use and testing of capacity.
ISO 9599:1991, Copper, lead and zinc sulfide concentrates — Determination of hygroscopic moisture in the
analysis sample — Gravimetric method.
3 Principle
3.1 Scorification
Fire assay fusion of a test portion to produce a lead button, which is scorified to reduce it to a mass of 2 g to 5 g.
Retreatment fusion of the primary fusion and scorification slags to produce a low-silver content lead button which is
scorified to approximately 2 g to 5 g.
Dissolution of both lead buttons in nitric acid and filtration of the solution. Dissolution of the filter paper plus gold and
determination of silver and gold by flame atomic absorption spectrometry.
1
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SIST ISO 12740:2000
© ISO
ISO 12740:1998(E)
3.2 Cupellation
Fire assay fusion of a test portion to produce a lead button, which is cupelled to produce a bead of silver and gold.
Retreatment fusion of the primary fusion slag and spent cupel to produce a second bead of silver and gold.
Dissolution of both beads in dilute nitric acid and filtration of the solution. Dissolution of the filter paper plus gold and
determination of silver and gold by flame atomic absorption spectrometry.
4 Reagents
During the analysis, use only reagents of a recognized analytical grade and water that complies with grade 2 of
ISO 3696. Wash all glassware with aqua regia followed by water and 25 % ammonia. A chloride-free environment is
recommended.
4.1 Sodium carbonate, anhydrous
4.2 Litharge, assay reagent grade having silver content less than 0,2 g/t, and gold content less than 0,01 g/t
4.3 Silica, precipitated grade
4.4 Potassium nitrate or sodium nitrate
4.5 Flour
4.6 Nitric acid (ρ 1,42 g/ml), chloride content < 0,5 μg/ml.
20
4.7 Nitric acid wash solution, (5 ml/l)
To 995 ml of water add 5 ml of nitric acid (4.6).
4.8 Nitric acid, (250 ml/l)
To 1 500 ml of water carefully add, with stirring, 500 ml of nitric acid (4.6) and cool.
4.9 Hydrochloric acid, (ρ 1,16 g/ml to 1,19 g/ml).
20
4.10 Borax, fused anhydrous sodium tetraborate
4.11 Silver metal, minimum 99,99 % purity
4.12 Gold metal, minimum 99,99 % purity
4.13 Aqua regia
Mix 3 parts of hydrochloric acid (4.9) with 1 part of nitric acid (4.6); prepare freshly as required.
4.14 Ammonia solution
Add 500 ml of ammonia solution (ρ 0,89 g/ml) to 500 ml of water.
20
4.15 Sodium chloride
4.16 Standard solutions
NOTE Standard solutions should be prepared at the same ambient temperature as that at which the determinations will be
conducted.
2
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SIST ISO 12740:2000
© ISO
ISO 12740:1998(E)
4.16.1 Silver standard solutions
4.16.1.1 Silver standard solution, (1 000 μg/ml)
Weigh 0,500 0 g of silver metal (4.11) into a 400 ml beaker, add 100 ml of nitric acid (4.8), cover and heat gently
until the metal dissolves. Continue heating to remove oxides of nitrogen. Cool, and transfer to a 500 ml volumetric
flask. Dilute to volume with water and mix thoroughly.
This solution should be stored in a brown bottle.
4.16.1.2 Silver standard solution, (100 μg/ml)
Pipette 20 ml of silver standard solution (4.16.1.1) into a 200 ml volumetric flask, add 40 ml of nitric acid (4.8), dilute
to volume and mix thoroughly.
This solution should be stored in a brown bottle.
4.16.1.3 Silver standard solution, (10 μg/ml)
Pipette 20 ml of silver standard solution (4.16.1.2) into a 200 ml volumetric flask, add 40 ml of nitric acid (4.8), dilute
to volume and mix thoroughly.
This solution shall be freshly prepared.
4.16.2 Gold standard solutions
4.16.2.1 Gold standard solution, (1 000 μg/ml)
Weigh 0,100 g of gold metal (4.12) into a 50 ml beaker, add 5 ml of aqua regia (4.13), cover and heat to dissolve the
gold. Wash and remove the cover, add 0,1 g of sodium chloride (4.15) and evaporate to near dryness (do not allow
to go to dryness, as gold may precipitate.) Cool, add 10 ml of hydrochloric acid (4.9), transfer the solution
quantitatively to a 100 ml volumetric flask, dilute to volume with water, stopper and mix thoroughly. Store the
solution in a brown bottle.
(100 μg/ml)
4.16.2.2 Gold standard solution,
Pipette 10 ml of gold standard solution (4.16.2.1) into a 100 ml volumetric flask, add 5 ml of hydrochloric acid (4.9),
dilute to volume with water, stopper and mix thoroughly.
4.16.2.3 Gold standard solution, (10 μg/ml)
Pipette 20 ml of gold standard solution (4.16.2.2) into a 200 ml volumetric flask, add 10 ml of hydrochloric acid (4.9)
dilute to volume with water, stopper and mix thoroughly.
This solution shall be freshly prepared.
4.17 Calibration solutions
NOTE Calibration solutions should be prepared at the same ambient temperature as that at which the determinations will
be conducted.
4.17.1 Silver calibration solutions
To six 100 ml volumetric flasks, add from a burette (5.6) 0 ml, 5 ml, 10 ml, 20 ml, 30 ml and 40 ml of silver standard
solution (4.16.1.3); add 20 ml, 19 ml, 18 ml, 16 ml, 14 ml and 12 ml of nitric acid (4.8), dilute to volume with water
and mix thoroughly.
These standards contain 0 μg, 0,5 μg, 1 μg, 2 μg, 3 μg and 4 μg of silver per ml and shall be freshly prepared.
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4.17.2 Gold calibration solutions
To six 100 ml volumetric flasks, add from a burette (5.6) 0 ml, 10 ml, 20 ml, 30 ml, 40 ml and 50 ml of gold standard
solution (4.16.2.3); add 20 ml of aqua regia solution (4.13), dilute to volume with water and mix thoroughly.
These standards contain 0 μg, 1 μg, 2 μg, 3 μg, 4 μg and 5 μg of gold per ml and shall be freshly prepared.
5 Apparatus
Ordinary laboratory equipment plus the following.
5.1 Volumetric glassware, of class A complying with ISO 385-1, ISO 648 and ISO 1042 and used in accordance
with ISO 4787.
5.2 Conventional fire assay equipment
5.2.1 Assay crucible furnace, having a maximum required operating temperature of 1 200°C.
5.2.2 Muffle furnace, having a maximum required operating temperature of 1 100°C; temperature indication,
automatic temperature control and controlled air flow are desirable.
5.2.3 Crucibles, made of fire clay, of nominal capacity 300 ml to 500 ml, capable of withstanding corrosion by the
sample and fluxes at 1 100°C. The crucible shall be of such a size that the charge does not fill the crucible to a
depth of greater than 3/4 the depth of the crucible.
5.2.4 Scorifiers, made of fire clay, of approximately 75 mm diameter, preferably not shallow form.
5.2.5 Cupels, made of magnesium oxide, or bone ash cupels having a nominal capacity of 50 g of molten lead.
The inside bottom of the cupel shall be concave.
5.2.6 Conical mould, made of cast iron, of sufficient capacity to contain all of the molten lead plus slag from the
crucible fusion.
5.2.7 Pulverizer
5.3 Balances
5.3.1 Top loading, capable of being read to 1 mg
5.3.2 Precision analytical, capable of being read to 0,1 mg
5.4 Atomic absorption spectrometer (AAS), equipped with background correction and a glass bead in the spray
chamber rather than a flow spoiler
5.5 Filtration equipment
5.5.1 Membrane filter - cellulose nitrate/cellulose acetate, diameter 25 mm, mesh size 0,45 μm
5.5.2 Vacuum flask, of 250 ml capacity, buchner filter flask having a wide neck
5.5.3 Membrane filter funnel, suitable for use with 25 mm membrane filters
5.6 Burette, A grade 50 ml capacity, capable of being read to 0,1 ml
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6 Sample
6.1 Test sample
Prepare an air-equilibrated test sample in accordance with ISO 9599.
NOTE A test sample is not required if predried test portions are to be used (see annex A).
6.2 Test portion
Taking multiple increments, extract a test portion from the test sample in such a manner that it is representative of
the whole contents of the dish or tray. Weigh to the nearest 0,1 mg approximately 10 g of test sample. At the same
time as the test portion is weighed, weigh test portions for the determination of hygroscopic moisture in accordance
with ISO 9599.
Alternatively, the method specified in annex A may be used to prepare predried test portions directly from the
laboratory sample.
7 Procedure
7.1 Number of determinations
Carry out the determinations at least in duplicate, as far as possible under repeatability conditions, on each test
sample.
NOTE Repeatability conditions exist where mutually independent test results are obtained with the same method on
identical test material in the same laboratory by the same operator using the same equipment within short intervals of time.
7.2 Trial fusion
Carry out a trial fusion as described in annex B to ensure that the mass of the lead button is between 28 g and 35 g.
NOTE It is essential that this trial fusion be performed.
7.3 Blank tests
Carry out a duplicate reagent blank test as described in annex C in parallel with each batch of samples fired, using
the same quantities of all reagents with the addition of sufficient flour (4.5) to the flux to give a button size of 28 g to
35 g. Omit the test portion and the potassium nitrate.
NOTE If the blank solutions (annex C) exceed 0,05 μg/ml for gold, the reagents should be checked and the problem
rectified. Otherwise the blank is subtracted from the result.
7.4 Charge preparation
Determine the mass of potassium nitrate (4.4) required in the charge as indicated by the trial fusion (see annex B)
and include this reagent in the flux mixture (see table 1).
Thoroughly mix the test portion with a flux of the composition specified in table 1.
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Table 1 — Recommended masses of flux components for preparation of charges
Flux components Mass
g
Sodium carbonate (4.1) 30
Litharge (4.2) 70
Silica (4.3) 10
Borax (4.10) 10
Potassium nitrate (4.4) –
Place the mixture in the assay crucible (5.2.3).
NOTE 1 The intimate mixing of fluxes and samples is very important. All flux components should be in a finely divided state,
with a preferred particle size of less than 0,5 mm.
NOTE 2 The quantities of oxidizing reagent in the mixture are dependent upon the reducing power of the test sample. The
trial fusion (see annex B) will determine the mass of potassium nitrate necessary to yield a button of mass 28 g to 35 g. The
oxidizing reagent should not be included in a bulk flux. It should be weighed up individually to ensure the repeatability of the
mass of the lead button, which, if fusion conditions are uniformly repeated, should agree within ± 1 g of lead, thus decreasing
the variability of finishing times of scorification or cupellation.
NOTE 3 If difficulties are experienced in producing repeatable and predictable masses of lead button, a lid for the crucible
may be of assistance.
7.5 Primary fusion
Place the crucible in the furnace (5.2.1) preheated to approximately 900 °C.
Slowly raise the furnace temperature to 1 050°C. Maintain this temperature until fusion has been calm for at least
10 min.
Pour the fusion into a dry conical mould (5.2.6), taking care to ensure that no loss of lead or slag occurs. Reserve the
crucible for retreatment fusion.
Allow the mixture to cool and carefully separate the lead button from the slag. Hammer the lead button as necessary to
remove any small particles of adhering slag. Reserve the slag for retreatment.
Weigh the lead button. If the button weighs less than 28 g or more than 35 g, discard the button and slag and repeat
the assay using less oxidizing agent or adding flour (see annex B).
NOTE 1 If oil-fired or gas-fired furnaces are used, the fuel should be turned off immediately before opening the furnace.
NOTE 2 To minimize crucible corrosion and build-up of impurities in the lead button, the overall fusion time should not
exceed 40 min.
NOTE 3 Buttons weighing less than 28 g show poor collection efficiencies, and those exceeding 35 g contain higher
amounts of impurities such as copper and may also cause problems in obtaining the required mass of lead after scorification.
7.6 Selection of recovery procedure
The recovery of the silver and gold in the lead button from the primary fusion can use either scorification (see 7.7)
or cupellation (see 7.8). Individual analysts should select the technique most familiar to their respective laboratories.
The final determination of silver and gold by AAS is the same in each case.
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7.7 Scorification
7.7.1 General
Place the lead button from the primary fusion (see 7.5) in a preheated scorifier containing approximately 0,5 g of
silica (4.3) in a muffle furnace (5.2.2) at 900 °C.
Allow the scorification to proceed at a muffle temperature of approximately 900 °C with steady air flow until the visible
lead melt is approximately 10 mm in diameter (approximately 30 min depending on the mass of the lead button).
Toward the end of the scorification, increase the muffle temperature to 950 °C to decrease the viscosity for clean
pouring. The required lead mass is 2 g to 5 g.
Pour the mixture into a dry conical mould (5.2.6), allow to cool, remove and weigh the lead button. Reserve the slag for
retreatment fusion. Discard the scorifier.
7.7.2 Retreatment fusion
Place the slags from the primary fusion (see 7.5) and the scorification (see 7.7.1) in a grinder (5.2.7) and pulverize
for a period of about 20 s. Thoroughly clean the pulverizer by grinding a silver- and gold-free medium between each
sample.
Intimately mix the pulverized slag with a flux of the following composition:
litharge (4.2) 50 g;
sodium carbonate (4.1) 50 g;
silica (4.3) 50 g;
borax (4.10) 50 g;
flour (4.5) sufficient to produce a 28 g to 35 g lead button (usually approximately 4 g).
Place the mixture into the original assay crucible from 7.5.
Carry out a fusion as detailed in 7.5.
Pour the fusion into a dry conical mould (5.2.6) taking care that no loss of lead occurs. Discard the crucible and slag.
7.7.3 Scorification
Place the lead button from the retreatment fusion, 7.7.2, in a new preheated scorifier containing approximately 0,5 g
of silica (4.3) in the muffle furnace (5.2.2) at 950 °C. The button is added to the scorifier in the muffle furnace.
Proceed as in 7.7.1. Discard the slag and scorifier.
7.8 Cupellation
7.8.1 General
Place the lead button from the primary fusion, 7.5, into a preheated cupel in a muffle furnace (5.2.2) at 900 °C. The
button is added to the cupel in the muffle furnace.
Allow the cupellation to proceed at a lower muffle temperature of approximately 860 °C with steady air flow until the
visible lead melt is approximately 10 mm in diameter (approximately 30 min depending on the mass of lead button).
Raise the muffle furnace temperature to 900 °C to finish the cupellation.
Remove the cupel and allow to cool. Take the silver and gold bead and remove any attached cupel material with a
brush. Weigh the bead. Reserve the cupel for retreatment fusion.
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7.8.2 Retreatment fusion
Place the slag from the primary fusion, 7.5, and the cupel from the recovery procedure, 7.8.1 in a grinder (5.2.7) and
pulverize for about 20 s to reduce the material to minus 150 μm. Longer grinding may cause caking of the material.
Thoroughly clean the pulverizer by grinding a silver- and gold-free medium between each sample.
Intimately mix the pulverized slag and cupel with a flux of the following composition:
Litharge (4.2) 50 g;
Sodium carbonate (4.1) 50 g;
Silica (4.3) 50 g;
Borax (4.10) 50 g;
Flour (4.5) sufficient to produce a 28 to 35 g lead button (usually approximately 4 g).
Place the mixture into the original assay crucible from 7.5.
Carry out a fusion as detailed in 7.5.
Pour the fusion into a dry conical mould (5.2.6), taking care that no loss of lead occurs. Discard the crucible and the
slag.
7.8.3 Cupellation
Place the lead button from the retreatment fusion, 7.8.2, into a new preheated cupel in a muffle furnace (5.2.2) at
900 °C. The button is added to the cupel in the furnace.
Proceed as in 7.8.1. Discard the cupel.
7.9 Treatment of the lead buttons or silver/gold beads
Place the cleaned lead buttons (see 7.7.1 and 7.7.3) into one 250 ml beaker the silver and gold beads (see
either or
7.8.1 and 7.8.3) into one 250 ml beaker.
Add 100 ml of nitric acid (4.8), cover and heat to dissolve all the lead or the bead. Boil gently to remove oxides of
nitrogen.
NOTE Any black material remaining after the button or bead has been dissolved is due to precious metals other than
silver.
Remove the beaker from the hotplate, allow to cool slightly, wash and remove cover, and vacuum filter through a
25 mm, 0,45 μm filter (5.5.1) into a 250 ml buchner flask (5.5.2). Use approximately 50 ml of hot nitric acid (4.7) to
wash the beaker and filter paper. Give one 10 ml wash with warm ammonia solution (4.14) to dissolve any traces of
silver chloride and one water wash.
Remove the top section of the filter funnel and carefully wash around the base of the top section of the filter funnel
into the original 250 ml beaker to remove any adhering particles of gold. Transfer the filter paper to the original
250 ml beaker, add 2 ml of nitric acid (4.6), place on a low heat hotplate and evaporate to near dryness. Do not
bake. Reserve the beaker for the determination of gold.
7.10 Determination of silver in the test solution by flame atomic absorption spectrometry
Transfer the filtrate (see 7.9) quantitatively to a 500 ml volumetric flask using nitric acid (4.7). Dilute to volume and
mix thoroughly.
Dilute by pipetting 25 ml of the above test solution into a 250 ml volumetric flask, add 25 ml of nitric acid (4.8). Dilute to
volume with water and mix thoroughly.
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Determine the silver concentration by flame atomic absorption spectrometry using calibration solutions in
accordance with 4.17.1. As a guide, the following atomic absorption settings are recommended; however, the
instrument should be optimized to give maximum sensitivity and as near as practical to a linear relationship between
absorbance and concentration.
flame: air-acetylene (oxidizing);
wavelength: 328,1 nm;
lamp current: 5 mA;
background corrector: off;
aspiration rate: optimize for maximum signal;
integration time: 3 s;
number of integrations: 5.
Perform three measurements on each standard solution. Calculate, to three significant figures, the mean
absorbance for each standard solution provided that the range of values does not exceed 0,003 absorbance units. If
this range is exceeded, repeat the calibrations.
The test solutions should be treated in the same manner. Plot a calibration graph of absorbance versus
concentration.
NOTE During all FAAS determinations, the test solutions and calibration solutions should have the same temperature as
well as the same acid concentrations.
7.11 Determination of gold in the test solution by flame atomic absorption spectrometry
Add 5 ml of aqua regia (4.13) to the beaker containing the gold (see 7.9), cover and gently heat to dissolve the gold.
Transfer the test solution quantitatively to a 25 ml volumetric flask. Dilute to volume with water and mix thoroughly.
For samples containing 10 g/t to 25 g/t of gold, dilute to 50 ml volume after adding 10 ml of aqua regia (4.13).
Determine the gold concentration by flame atomic absorption spectrometry using calibration solutions in accordance
with 4.17.2. As a guide, the following atomic absorption settings are recommended; however, the instrument should be
optimized to give maximum sensitivity and as near as practical to a linear relationship between absorbance and
concentration.
flame: air/acetylene oxidizing;
wavelength: 242,8 nm;
lamp current: 4 mA;
background corrector: on;
aspiration rate: optimize for maximum signal;
integration time: 3 s;
number of integrations: 5.
Perform three measurements on each standard solution. Calculate to three significant figures the mean absorbance
for each standard solution, provided that the range of values does not exceed 0,003 absorbance units. If this range
is exceeded, repeat the calibrations.
The test solutions should be treated in the same manner. Prepare a calibration graph of absorbance versus
concentration.
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NOTE During all FAAS determinations, the test solutions and calibration solutions should have the same temperature as
well as the same acid concentrations.
8 Expression of results
8.1 Silver content
The silver content of the test portion w , expressed in grams per tonne, is given by the following equation:
Ag
CB−×500 250
() 100
11
w = × . . . (1)
Ag
× −
mH25 100
where
C is the silver content of the analysis solution, in micrograms per millilitre;
1
B is the silver content of the blank solution, in micrograms per millilitre;
1
m is the mass of the test portion, in grams;
H is the hygroscopic moisture content, as a percentage of the test portion (in the case of a predried test
portion being used, H = 0).
8.2 Gold content
The gold content of the test portion w , expressed in grams per tonne, is given by the following equation:
Au
CB− V
() 100
22
w = × . . . (2)
Au
−
mH100
where
C is the gold content of the analysis solution, in micrograms per millilitre;
2
B is the gold content of the blank solution, in micrograms per millilitre;
2
V is the volume of the analysis solution (see 7.11), in millilitres;
m is the mass of the test portion, in grams;
H is the hygroscopic moisture content, as a percentage of the test portion (in the case of a predried test
portion being used, H = 0).
9 Precision
9.1 Expression of precision
The precision of this analytical method is expressed by the following equations:
9.1.1 Silver
= 0,003 6 + 3,202 4 . . . (3)
s X
r
s = 0,013 4 X + 8,642 1 . . . (4)
L
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