A Process for the Recovery of Platinum from Analytical Residues

Abstract

This paper describes a process for the recovery of platinum from the analytical residue left after the determination of oxygen and hydrogen in refractory metals and alloys by vacuum fusion method. The residue, which consists of platinum alloyed with a host of other refractory metals and alloys like Ti, Zr, zircaloy etc. embedded in a graphite matrix, is calcined in air to eliminate graphite by oxidation. This is followed by (i) alloying the calcined material with a suitable metal and its subsequent removal to yield active platinum powder, (ii) acid leaching of the powder to solubilise platinum metal value, (iii) precipitation of ammonium chloroplatinate salt, (iv) thermal decomposition of the salt to platinum metal sponge, and (v) final purification and consolidation. In the entire flowsheet, the activation step is the most important part, as without it the scrap metal refuses to dissolve in any acidic media. The paper discusses the role of zinc and aluminum as alloying agent in the activation treatment. Dissolution of a noble metal like platinum could be effected only in a strongly oxidising acid mixture like aqua regia that liberates nascent chlorine to form chloroplatinic acid. The leaching step has been optimised with respect to amount of acid, temperature and time. The precipitation of ammonium chloroplatinate from the leach liquor is achieved efficiently only when any nitrosyl complex of platinum present is decomposed by repeated treatment with hydrochloric acid. The crude platinum sponge is subjected to purification to obtain 99.9%pure platinum with an overall recovery of 97.2 percent.