PGM’s in Fire Assay and accurate Gold determination.

Chris WalneLaboratory Manager

The Goldsmiths’ Company Assay Office

SCOPE OF PGM CONTENT

0.01 - 0.5% Rh Ru Ir Os

0.1 - 10% Pt

0.1 - 25% Pd

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ORIGINS OF PGM’s

Refined gold can originate from several different sources:

1 Mining operations

2 Alluvial deposits

3 Scrap Jewellery

4 Electronic components

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ISO 11426

For specific Au Ag Cu Zn matrix with modifications for samples containing Ni Pd or high Ag content.

Accuracy <1ppt easily achieved on these alloy types.

( Assuming closely matrix matched proofs are used with the appropriate elements added)

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PGM Visual Indications• Pt – frosted appearance, higher the Pt content the flatter

the bead and dull grey in colour.• Pd – similar to Pt except more Pd content can be present

before indications are noticable.

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Ir – Difficult to see, slightly soluble in silver and gold at cupellation temperature. Black spots on bottom of cupel staining it black due to the higher density. Crystal boundaries are visible even though it tends to look a finer texture.

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Rh – Small amounts are easy to see as the surface of the bead assumes a bluish grey colour.Ru – Black crystalline deposit particularly on the bottom edge.

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Appearance of cornets • Pt & Pd – No discolouration but shiny surfaces as opposed to a matt

effect.

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• Ir – Distorted surfaces with raised areas. Cornet edges uneven.• Rh – Black finely divided deposits spread on surface of cornet.• Ru – Cleaner cornet but black deposits on the edge.

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Iridium presence in cornet• Dissolve the weighed cornet in dilute Aqua Regia leaving Ir as the

residue ( 1 part AR with 4 parts De-ion H2O)

• Filter and wash in dilute AR, De-ionised H2O and dilute NH3

• Dry the residue and ignite in a porcelain crucible at 600-700degrees c

• Combined Au/Ir cornet weight – Ir residue weight = true Au weight

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• Palladium – readly soluble in HNO3 forming Pd(NO3)2.• Two parting stages performed i.e. re-alloy, re-cupel to facilitate removal

of all the Pd content.• Matrix matched proofs treated in the same way as per ISO 11426.

• Platinum – Not possible to separate Pt content entirely from the gold inone HNO3 treatment.

• Addition of fine gold to the sample to decrease proportionately the Ptcontent. Two parting stages including matrix matched proofs.

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METHODOLGY• Samples: 100-120mg / >120mg if Pt content <3%• Higher the Pt content, the lower the sample weight• Fine gold addition: 115-120mg 999.9 Au• Inquartation Ag: x 2.2 total Au + Pt• Copper addition: 20mg• Pb addition: 8g• Proofs matrix matched + or – 1mg based on XRF initial analysis.• Cupellation temp: 1150 degrees c

• Parting procedure: 20 minutes in both strength acids @90 degrees cwith the lower surface of the cornet on the outside so that this area isattacked directly.

• Samples and proofs to be re-cupelled with the appropriate amount ofsilver and copper prior to a second parting stage.

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CALCULATION

Au ppt = (CM2 – Fm) + (TsT / TmP) * CM2-------------------------------------------- X1000

CM1

Au ppt = Gold content in parts per thousandCM1 = Mass of sample in mgCM2 = Mass of sample cornet in mgFm = mg fine gold added * fine gold fineness / 1000TsT = Total proof surcharge in mgTmP = Total cornet mass of proofs in mg

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EXAMPLEGold sample containing 755.03ppt Au and 5.4% Pt(Ag 9.4% Cu 7.6% Zn 2.1%)

Two proofs exhibiting losses of 0.091mg & 0.101mg after double parting.

190.676 – (115.134 * 999.9 / 1000) + (0.192 / 380.731) * 190.676------------------------------------------------------------------------------------ x 1000

100.185

= 755.09 Au ppt( Retained Pt is 0.006% or 60ppm)

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Pt and Pd content retained in the cornetSAMPLE CONTENT( 18ct Au alloys )

PPM AFTER 1 PARTING PPM AFTER 2 PARTINGS

10% Pt 8281122901955844

71110668061

12% Pd 277267241199255

N/D (<1PPM)N/DN/DN/DN/D

25% Pd 698732802783772

N/DN/DN/DN/DN/D

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