Recovery of iridium from scrap and residues

Abstract

Abstract In conventional hydrochemical iridium refining the scrap is alloyed with nickel to form a 70Ni-30Ir alloy by melting and the iridium is recovered as an impure powder (70Ir-30Ni) by acid leaching of this alloy. The iridium powder is then subjected to several complex oxidation, dissolution and purification steps for further refining. The use of nickel has several disadvantages and contributes to iridium losses. To find a more suitable metal solvent for iridium, several metals and alloys were alloyed with scrap iridium by induction melting. The ingots were leached in acid and the resulting powder size and purity were studied. Both pure manganese and an Mn-Cu alloy were found to be superior solvents when compared with nickel. The Mn-20Cu-20Ir alloy was very ductile and after dissolution resulted in an iridium powder with about 95% purity after acid leaching. The Mn-(20–30)Ir alloys were somewhat harder and brittle but resulted in a very fine iridium powder with 98%–99% purity after acid leaching. Both the manganese and the Mn-Cu alloys form homogeneous solid solutions with iridium, have low melting points, dissolve readily in dilute HCl and are inexpensive compared with nickel. Leaching of these alloys in acid resulted in a relatively pure and very fine (100 mesh) iridium powder with nearly 100% recovery. The good quality of the resulting iridium powder simplifies its further processing and reduces the cost of hydrochemical refining. In comparison, the Ni-Ir alloy has a much higher melting point and a very slow leaching rate in acids and results in a coarse (40 mesh) and impure (70Ir-30Ni) powder.