Patent GB-419819-A (GB)
Title: Improvements relating to the dissolving of metals of the platinum group
IPC: C22B11/08
Filed: 1933-05-12
Abstract: Metals of the platinum group are stripped from a metal foundation by immersing the article to be stripped in a bath comprising molten sodium cyanide and/or potassium cyanide, the platinum group metal and the foundation metal being such that the platinum group metal is preferentially attacked.  To accelerate the action of the bath, an electric current may be used, the article to be stripped constituting the anode.  In addition, one or more metals of the platinum group may be transferred from one or more articles to another article by making the article or articles containing or consisting of platinum &c. the anode, and the article on which the platinum &c. is to be deposited the cathode in an electrolytic bath comprising molten sodium and/or potassium cyanide.  In the electrolytic forms of the process, the platinum &c. goes into solution at the anode at a greater rate than it is deposited at the cathode, a complex cyanogen compound of the platinum &c. accumulating in the bath.  Inactive compounds such as sodium hydroxide or potassium hydroxide may be added to the bath to lower its melting temperature; potassium or sodium ferrocyanide may also be added to the bath, the effect of these additions being to lower the melting point and to yield a little iron in the cathode deposit.  The process is preferably conducted under non-oxidizing conditions, say by using an inert atmosphere or by covering the bath with a protective layer such as a mixture containing graphite and plumbago.  When cathodically depositing the platinum metals, a small quantity of the complex alkali metal cyanide of the platinum metal or metals under treatment is preferably added to the bath in order to accelerate the deposition.  According to the examples, rhodium is stripped from silver by immersion in a molten bath at 500 DEG  C. consisting of sodium and potassium cyanides; rhodium is stripped from copper in a bath at 440 DEG  C. consisting of sodium and potassium cyanides and sodium hydroxide, the stripping being accelerated if an anodic current is passed through the metal article; a bath consisting of sodium and potassium cyanide is used to strip palladium or platinum from a silver foundation, ruthenium from copper and iridium from gold by immersion or anodic treatment; ruthenium is deposited on a gold cathode from an electrolyte consisting of sodium cyanide, potassium cyanide and potassium ruthenocyanide in a porcelain crucible in an atmosphere of nitrogen at 520--550 DEG  C., a ruthenium anode being employed.  Processes in which an electrolytic bath for the deposition of a platinum metal is made and periodically replenished by addition of an oxide of a platinum group metal to fused potassium cyanide, a platinum group metal being employed as an anode, are disclaimed.  The Provisional Specification describes also the employment of molten baths comprising alkaline earth metal cyanides alone or in admixture with alkali metal cyanides and the use of sodium chloride as an addition to lower the melting point of the bath when articles having a silver foundation are treated.