ISO 12740:1998

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
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
© ISO
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.
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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.
© ISO
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.
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).
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.
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.
© ISO
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.
© ISO
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
© ISO
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 conditi
...