oxygen and silver - lbma · oxygen and silver relevance to good delivery bars stewart murray, ......
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OXYGEN AND SILVER
Relevance to Good Delivery Bars
Stewart Murray, LBMA Good Delivery ConsultantQuentin Bochud, Laboratory Manager, Metalor
LBMA Assaying and Refining Conference
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PREAMBLE
What is the Oxygen problem for Silver?
• It is not (mainly) the financial one, that oxygen counts as silver(even at a high level of 100 ppm the loss would be only ~£1 per bar)
• Instead it is that Oxygen is readily soluble in molten silver…
The LBMA Assaying & Refining Conference - London 2017
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Starter QuestionGay-Lussac discovered in 1830 how much oxygen a bath of molten silver could contain in solution.
Question:
For a 30 kg bath of molten silver (enough for one GD Bar) how much oxygen at 1 atmosphere pressure could it hold?
5litres
10litres
20litres
40litres
60litres
The LBMA Assaying & Refining Conference - London 2017
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PREAMBLEWhat is the Oxygen problem for Silver?
• It is not (mainly) that oxygen counts as silver(even at a high level of 100 ppm the loss would be only ~£1 per bar)
• Oxygen is very soluble in molten silver ……
• but not in solid silver
• It can therefore spoilphysical appearanceof GD bars
The LBMA Assaying & Refining Conference - London 2017
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PREAMBLE
What is the Oxygen problem for Silver?
• It is not (mainly) that oxygen counts as silver(even at a high level of 100 ppm the loss would be only ~£1 per bar)
• Oxygen is very soluble in molten silver ……but not in solid silver
• It can therefore spoil physical appearance of GD bars
The Challenge
Preventing absorption by or removal of oxygen from molten silver
Lessons for Silver from Poling of Copper?
The LBMA Assaying & Refining Conference - London 2017
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COPPER POLINGWhat is (was) poling and can it be used for silver?
Wood + Heat = CO + H2
Oxygen (soln. or Cu2O) + CO + H2 = CO2 + H2O
The LBMA Assaying & Refining Conference - London 2017
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PREAMBLE
What is the Oxygen problem for Silver?
• It is not (mainly) that oxygen counts as silver
• Oxygen is very soluble in molten silver but not in solid silver
• It can spoil physical appearance of GD bars
Preventing absorption or removal of oxygen from silver
Lessons for Silver from Poling of Copper?
Attempts to find a speaker from a GD Copper Refiner
Similarities and differences between copper and silver etc
The LBMA Assaying & Refining Conference - London 2017
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PREAMBLE
What is the Oxygen problem for Silver?
• It is not (mainly) that oxygen counts as silver
• Oxygen is very soluble in molten silver but not in solid silver
• It can spoil physical appearance of GD bars
Preventing absorption or removal of oxygen from silver
Lessons for Silver from Poling of Copper?
Attempts to find a speaker from a GD Copper Refiner
Similarities and differences between copper and silver etc
Failed attempts! That is why I am here.
The LBMA Assaying & Refining Conference - London 2017
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SURVEY OF GD REFINERS
Questions
• Raw material for melting?
• Melting method (ie furnace type and capacity)?
• Methods of minimisation of oxygen absorption?
• Use of flame polishing during casting?
• Need for physical treatment of cast bars?
• Analysis of oxygen in molten and solid silver?
Results
• Many similarities to obviating problems
• Some unique solutions
Managing Oxygen in Silver
The LBMA Assaying & Refining Conference - London 2017
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SCOPECosmological Digression - Oxygen and MetalsPhysical Chemistry of the Silver-Oxygen System• Molten silver
• Solid silverGood Delivery Issues• Why oxygen counts as silver
• Analysis is not easy (or necessary?)• Possible impact on bar qualityGD Refiner Strategies to avoid Oxygen Problems
• Oxygen absorption – sources and prevention• Removal of oxygen from molten silver• Casting Methods
The LBMA Assaying & Refining Conference - London 2017
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Let’s talk about OXYGENFrom the stellar forge to the Earthly anvil
Oxygen Occurrence
• Milky Waythe Milky Way consists of• 75% Hydrogen• 23% Helium• 1% Oxygen• 1% Everything else
Excluding Dark Matter,
The LBMA Assaying & Refining Conference - London 2017
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Oxygen’s Nuclear and Chemical StabilityWhy Oxygen is Number 3 in Cosmic Abundance
The LBMA Assaying & Refining Conference - London 2017
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Let’s talk about OXYGENFrom the stellar forge to the Earthly anvil
Occurrence
• Milky Way
• Solar System
The Milky Way consists of• 75% Hydrogen• 23% Helium• 1% Oxygen• 1% Everything else
Due to gas giants, H and He still dominate.But note the peaks for oxygen and iron
The LBMA Assaying & Refining Conference - London 2017
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Let’s talk about OXYGENFrom the stellar forge to the Earthly anvil
Occurrence
• Milky Way
• Solar System
• Earth
EarthA distilled planet: small size, low gravity and proximity to the sun.Mainly heavy elements retained.Fe 32%O 30%Si 15%Mg 14%
The LBMA Assaying & Refining Conference - London 2017
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Let’s talk about OXYGENFrom the stellar forge to the Earthly anvil
Occurrence
• Milky Way
• Solar System
• Earth
• Earth’s Crust(the slag layer)
the Milky Way consists of• 75% Hydrogen• 23% Helium• 1% Oxygen• 1% All other elements
Excluding Dark Matter
The LBMA Assaying & Refining Conference - London 2017
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Let’s talk about OXYGENFrom the stellar forge to the Earthly anvil
Occurrence
• Milky Way
• Solar System
• Earth
• Earth’s Crust
• Fire, water, ironandcyanobacteria
• The Iron Ore Age ends
• The Air Age begins
Source Graeme Churchard
The LBMA Assaying & Refining Conference - London 2017
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OXYGEN AND METALSMessage from Metals: Who wants my excess electrons?
Answer from Oxygen: I do!
• Free Energy Diagram
• Ag2O vs Cu2O vs MgO
Source C. Bodsworth The Extraction
and Refining of Metals (1994)
The LBMA Assaying & Refining Conference - London 2017
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OXYGEN SOLUBILITY IN SILVER
Effect of pressure - Sievert’s Law
NAg=KpO21/2
Solubility in molten silver
Solubility in solid silver
Supersaturation?
Oxygen Solubility in Silver At Pressure of 1 atmosphere O2
Temperature (°C) Solubility (ppm)
300 0.3
900 61.4
973 3,050
1024 2,950
The LBMA Assaying & Refining Conference - London 2017
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SUMMARY OF SURVEY RESULTSResponses: 18 GD Silver Refiners from 12 countries – Thanks!
Raw material for furnace Crystals (17); grain (2); scrap (2); bars (1) unrefined bullion (1)
Melting furnace type Induction (13); Gas fired crucible (3); Resistance (1); Reverberatory (1)
Limiting O2 absorption during melting
Carbon* cover (11); Reducing flame (3); Carbon & Flame (1); Carbon rod “poling” (2); Nitrogen purge (1)
Analysis of oxygen in molten silver 4 answered “yes” – mostly occasionally finding ~20-40 ppm
Use of flame polishing for casting Yes (12); No (6); Graphite block covers on mould (2)
Need for physical treatment of bars Yes (13) not all related to needle removal; No (5)
Physical treatment of cast bars Hammering (4); Hand Grinding (5); Wire brushing (4) milling (2) Polishing or buffing (2) (incl some combinations
Analysis of oxygen in cast bars 5 answered “yes” – but not routinely, finding variously from 20-235 ppm
* Carbon included graphite blocks, activated charcoal, briquettes and lump coal
The LBMA Assaying & Refining Conference - London 2017
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