Gold parting
Gold parting is the separating of gold from silver (and other metallic impurities). Gold and silver are often extracted from the same ores and are chemically similar and therefore difficult to separate. The alloy of gold and silver is called electrum.[1]
Contemporary technology
Two technologies are dominant. They both start with relatively pure gold.
- The Miller process affords gold up to 99.5% purity. The process involves blowing a stream of chlorine through molten gold. Impurities in the gold form chlorides, which form a slag that floats on the molten gold.[2][3]
- Invented by Emil Wohlwill in 1874, the Wohlwill process produces the highest purity gold (99.999%). It is an electrolytic process using pure gold for the cathode (or titanium as a starter cathode) and chloroauric acid (gold chloride-hydrochloric acid) as the electrolyte; this is made by dissolving gold with chlorine gas in the presence of hydrochloric acid. Gold is dissolved at the anode, and pure gold, traveling through the acid by ion transfer, is plated onto the cathode. Silver forms an insoluble chloride slime and copper and platinum form soluble chlorides that are removed. This procedure is used on a very large industrial scale and has a large set up cost due to the amount of gold that needs to be permanently dissolved in the electrolyte.[2][3]
Other
Alternative methods exist for parting gold. Silver can be dissolved selectively by boiling the mixture with 30% nitric acid, a process sometimes called inquartation. Affination is a largely obsolete process of removing silver from gold using concentrated sulfuric acid.[4]
The Acidless Separation (ALS) has been investigated to pre-refine doré and jewelry alloys even when there is high silver content, which is normally a problem for existing chemical pre-refinement processes. The metals are separated by distillation.[5]
Cupellation removes gold and silver from mixtures containing lead and other metals, but silver cannot be separated from gold by this process alone.
History
Gold parting as a process was invented specifically to remove silver. The advent of coinage required methods to remove impurities from the gold. Over the centuries special means of separation have been invented.
The main ancient process of gold parting was by salt cementation, of which there is archaeological evidence from the 6th century BC in Sardis, Lydia. In the post-medieval period parting using antimony, sulfates and mineral acids was also used.
Early history
The very earliest attempts at refining gold can be shown by the surface enhancement of gold rings. Gold quality was increased at the surface by 80–95% gold compared to 64–75% gold at the interior found in Nahal Qanah Cave dated to the 4th millennium BC. Further evidence is from three gold chisels from the 3rd Millennium BC royal cemetery at Ur that had a surface of high gold (83%), low silver (9%) and copper (8%) compared with an interior of 45% gold, 10% silver and 45% copper. The surface was compacted and heavily burnished and indicates early use of depletion gilding.
Ancient and medieval world
Separation of gold from silver was not practised in antiquity prior to the Lydian Period (12th century BC to 546 BC).[6] Material from Sardis (in modern Turkey) is evidence of the earliest use of gold and silver parting in the 6th century.[7] Literary sources and the lack of physical evidence suggest that gold-silver parting was not practised before the mid first millennium BC. Gold parting came with the invention of coinage and there is no evidence for the use of a true refining processes before the introduction of coinage. As refining gold (as opposed to surface enhancement) results in a noticeable loss in material, there would have been little reason to do this before the advent of coinage and the need to have a standard grade of material.
The first possible literary reference to the salt cementation parting process is in the
Gold parting had been well used throughout the ancient times but only in the
break into tiny pieces a tile or piece of burnt and reddened furnace-clay and when it is powdered, divide it into two equal parts by weight and add to it a third part of salt of the same weight. It should then be lightly sprinkled with urine and mixed so that it does not stick together but is just moistened.
— Theophilus, [15]
This mixture is then added to an earthenware pot and layered with thin sheets of
Then put the fire and wood below and see that a plentiful fire is not lacking for the space of a day and a night. In the morning, however, take out the gold and melt it again, hammer it, and put it into the furnace as before. After another day and night take it out again, mix a little red copper with it, melt as before, and put it back into the furnace. And when you have taken it out a third time, wash it and carefully dry it. Weigh it, when dried, and see how much has been lost, then fold it up and keep it.
— Theophilus, [16]
It was during the medieval period that
Post medieval to modern period
Comprehensive accounts of the salt cementation processes is given by
Historic processes
Salt cementation
This process was used from Lydian to post-medieval times. It is a solid state process relying on common salt as the active ingredient but it is possible to use a mixture of saltpetre (KNO3) and green vitriol (FeSO4). The basic process involved the mixing of argentiferous gold foil (in later periods granules were used), common salt and brick dust or burnt clay in a closed and sealed container. Theophilus mentions the addition of urine to the mix. With heating, the silver reacts with the salt to form silver chloride which is removed leaving a purified gold behind. Conditions needed for this process are below 1000 °C as the gold should not melt. Silver can be recovered by smelting the debris.[20] Heating can take 24 hours. Hoover and Hoover[21] explains the process thus: under heating salt (sodium chloride, NaCl) decomposes in the presence of silica and alumina (from the brick dust or clay) to produce hydrochloric acid and also some chlorine. This reacts with the silver to produce silver chloride (AgCl). The urine is acidic and aids decomposition. Silver chloride is volatile and would be removed from the metal. And the container is sealed to stop the escape of the silver which can be recovered later. Notton in experiments found that with one heating the gold content could be taken from 37.5% to 93%[22]
Sulfur and antimony processes
This is similar to the salt cementation process but creates sulfides instead of chlorides. Finely divided impure gold and elemental sulfur are reacted together under moderate heat in a sealed crucible. The impurities form metal sulfides and the gold is left unreacted. The gaseous sulfide condenses on the crucible fabric. The antimony process is the same but uses stibnite (Sb2S3) instead of sulfur because stibnite is stable at a higher temperature than sulfur. This is much quicker than the salt process and gave a purer gold, but it could dissolve some of the gold as well. This process is first described in the Probierbuchlein.[23]
Acid parting
The distillation was used in the 12th century Europe after its introduction from the East[24] and after that period more powerful acids could be created. Nitric acid (aqua fortis, called by Agricola aqua valens) could be made by the distillation of saltpetre (KNO3) with water and alum (KAl(SO4)2) or vitriol (FeSO4).[18][19]
2KNO3 + H2O + FeSO4 → FeO + K2SO4 + 2HNO3
Nitric acid, after distillation to increase the acid strength, is capable of dissolving the silver but it will not (by itself) dissolve the gold. However, nitric acid is not able to (fully) extract silver and other impurities from an alloy with a high content of gold. Therefore, one part of scrap gold was typically alloyed with three parts of copper (quartering) before parting with nitric acid. Another method uses Sterling silver instead of copper. One part pure gold is alloyed with three parts Sterling silver (inquarting). The resulting six karat (6K) gold can then be parted with dilute nitric acid (one part 68–70% nitric acid to one part distilled water). With the karat gold this low (6K), and over medium high heat, the dilute nitric acid will dissolve the Sterling silver (and other base metals in the karat gold) starting on the outside surface of the 6K gold alloy, working its way into the gold alloy, forming a honeycomb structure as it works its way into the metals. Since nitric acid will not dissolve gold, nearly pure gold (very close to 99.5% pure) will be left behind after the reaction is complete. After removing the solid gold other elements like silver and copper may be extracted from the liquid. To get the gold to a very high level of purity (999 fine gold) it is sometimes processed further with aqua regia to effectively remove all the impurities.
Aqua regia was also used for parting. It was made by adding sal ammoniac to nitric acid which produced a mixture of hydrochloric acid and nitric acid. This acid dissolved the gold to a soluble chloride and the silver was attacked and precipitated as an insoluble chloride. Silver was removed by filtering and gold was then recovered by evaporating the liquid and heating the residue. Nitric acid was suitable for separating small quantities of gold from silver and aqua regia used to separate small quantities of silver from gold. Aqua regia acid process is used by refiners of scrap gold used in jewelry manufacturing. This process is also well suited to recycling consumers' used or broken jewelry directly back onto the global market 24kt inventory.[17]
See also
Citations
- ISBN 3527306730.
- ^ ISBN 978-0-442-31797-3.
- ^ .
- ISBN 978-0471238966.
- ^ Boscato, M. (2016). "Presentation of a new acid-free refinement process for gold and silver". Jewel Technology Forum.
- ^ Craddock, P. T. 2000a Historical Survey of Gold Refining: 1 Surface Treatments and Refining Worldwide, and in Europe Prior to AD 1500. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press pp27
- ^ Rehren, T. 2003. Crucibles as Reaction Vessels in Ancient Metallurgy. In Craddock, P. T and Lang, J. (eds.) Mining and Metal Production through the Ages. London: British Museum Press pp207
- .
- ^ Craddock, P. T. 2000a Historical Survey of Gold Refining: 1 Surface Treatments and Refining Worldwide, and in Europe Prior to AD 1500. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press. Pp38
- ^ Bayley, J. 1990. Archaeological Evidence for Parting. In E. Pernicka and G. A. Wagner (eds) Archaeometry '90. Basel ; Boston : Birkhäuser Verlag
- ^ Bayley, J. 2008. Medieval precious metal refining: archaeology and contemporary texts compared. In Martinon-Torres, M. and Rehren, T. (eds) Archaeology, history and science: integrating approaches to ancient materials. Walnut Creek: Left Coast Press. Pp142-3
- ^ Bayley, J. 1992. Non-Ferrous Metalworking at 16–22 Coppergate. The Archaeology of York 17/7. London:CBA
- ^ Bayley, J. 2008. Medieval precious metal refining: archaeology and contemporary texts compared. In Martinon-Torres, M. and Rehren, T. (eds) Archaeology, history and science: integrating approaches to ancient materials. Walnut Creek: Left Coast Press pp143
- ISBN 978-0-486-23784-8.
- .
- ^ Craddock, P. T. 2000a Historical Survey of Gold Refining: 1 Surface Treatments and Refining Worldwide, and in Europe Prior to AD 1500. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press. Pp38–39
- ^ a b Taylor, F. S. 1956. Pre-scientific Industrial Chemistry. In C. Singer, E.J. Holmyard, A.R. Hall and T. I. Williams (eds) A history of technology: Vol.2, The Mediterranean civilizations and the Middle Ages; c700 B.C. to c,A.D. 1500. Oxford : Clarendon Press pp356-7
- ^ a b Bayley, J. 2008. Medieval precious metal refining: archaeology and contemporary texts compared. In Martinon-Torres, M. and Rehren, T. (eds) Archaeology, history and science: integrating approaches to ancient materials. Walnut Creek: Left Coast Press pp145
- ^ a b Craddock, P. T. 2000b Historical Survey of Gold Refining: 2 Post-medieval Europe. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press pp69
- ^ Rehren, T. 2003. Crucibles as Reaction Vessels in Ancient Metallurgy. In Craddock, P. T and Lang, J. (eds.) Mining and Metal Production through the Ages. London: British Museum Press 207
- ^ Hoover, H.C. and Hoover, L. H. 1950. Georgius Agricola: De re metallica New York : Dover pp456
- .
- ^ Craddock, P. T. 2000b Historical Survey of Gold Refining: 2 Post-medieval Europe. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press. pp68
- ^ Taylor, F. S. 1956. Pre-scientific Industrial Chemistry. In C. Singer, E.J. Holmyard, A.R. Hall and T. I. Williams (eds) A history of technology: Vol.2, The Mediterranean civilizations and the Middle Ages; c700 B.C. to c,A.D. 1500. Oxford : Clarendon Press
References
- Faoro, G. (2015). "Acidless Separation: New Technology for Refining Gold and Silver". Alchemist. 79: pp 9–11. https://www.lbma.org.uk/alchemist/issue-79/acidless-separation
- Bayley, J. 1990. Archaeological Evidence for Parting. In E. Pernicka and G. A. Wagner (eds) Archaeometry '90. Basel; Boston : Birkhäuser Verlag 19–28.
- Bayley, J. 1992. Non-Ferrous Metalworking at 16–22 Coppergate. The Archaeology of York 17/7. London:CBA
- Bayley, J. 2008. Medieval precious metal refining: archaeology and contemporary texts compared. In Martinon-Torres, M. and Rehren, T. (eds) Archaeology, history and science: integrating approaches to ancient materials. Walnut Creek: Left Coast Press, 131–150.
- Craddock, P. T. 2000a Historical Survey of Gold Refining: 1 Surface Treatments and Refining Worldwide, and in Europe Prior to AD 1500. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press, 27–53.
- Craddock, P. T. 2000b Historical Survey of Gold Refining: 2 Post-medieval Europe. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press, 54–71.
- Dodwell, C. R. (1971). "Gold metallurgy in the twelfth century". Gold Bulletin. 4 (3): 51–55. .
- Hawthorne, J. G .; Smith, C. S. (1979). On divers arts: the foremost medieval treatise on painting, glassmaking, and metalwork. New York: Dover Publications. ISBN 978-0-486-23784-8.
- Hoover, H.C. and Hoover, L. H. 1950. Georgius Agricola: De re metallica New York : Dover.
- La Niece, S. (1995). "Depletion Gilding from Third Millennium BC Ur". Iraq. 57: 41–47. JSTOR 4200400.
- Notton, J. H. F. (1974). "Ancient Egyptian gold refining". Gold Bulletin. 7 (2): 50–56. .
- Rapson, William S. (1992). "Mining, Extraction and Refining of Gold". Interdisciplinary Science Reviews. 17 (3): 203–212. .
- Rehren, T. 2003. Crucibles as Reaction Vessels in Ancient Metallurgy. In Craddock, P. T and Lang, J. (eds.) Mining and Metal Production through the Ages. London: British Museum Press. pp 207–215. ISBN 978-0-714-12770-5
- Taylor, F. S. 1956. Pre-scientific Industrial Chemistry. In C. Singer, E.J. Holmyard, A.R. Hall and T. I. Williams (eds) A history of technology: Vol.2, The Mediterranean civilizations and the Middle Ages; c700 B.C. to A.D. 1500. Oxford : Clarendon Press. pp 347–382.
- Yannopoulos, J. C. (1991). The extractive metallurgy of gold. New York: Van Nostrand Reinhold. ISBN 978-0-442-31797-3.
- A. I. Khlebnikov (2014). "Modern industrial experience of application of vacuum silver distillation for separation of gold-silver alloy". Non-ferrous Metals. 2014 #2, 25-28 ISSN 2414-0155
- M. B. Mooiman (2016). "Gold Ore Processing". Chapter 34–7, Future developments in dorè refining. 613 Edited by Mike D. Adams. ISBN 978-0-444-63658-4.
- Boscato, M. (2016). "Presentation of a new acid-free refinement process for gold and silver". Jewel Technology Forum.