Dissolution of rhodium and rhodium-iridium alloy dissolution in titanium autoclave equipment

The features of rhodium and rhodium-iridium alloy dissolution in hydrochloric acid media in titanium autoclave equipment have been studied under elevated oxygen pressure. Scanning electron microscopy and low-temperature nitrogen adsorption showed that “the spongy-black Rh” sample consisted of spherical granules 1–3 μm in diameter and had a specific surface area of 2.8 m2/g. According to X-ray phase analysis (XPA), the sample is a single phase of rhodium(0). The Rh–Ir alloy material has significant morphological differences from the previous one and consists of irregularly shaped particles ranging in size from several micrometers to tenths of a millimeter with a specific surface area of about 1 m2/g. According to XPA, the alloy is comprised of metallic rhodium (57%), rhodium chloride, and a water-soluble halite phase (17%). The Rh:Ir mass ratio in the alloy is 9:1, but iridium is probably in an am orphous state. It was found that in a hydrochloric acid media (3 mol/l) at a temperature of 190 oC and oxygen pressure (1 MPa) “spongy-black Rh” sample dissolved quantitatively in 2 hours, and at 170 oC about 20% of rhodium passed into solution. It was shown that, after 8 hours of contact of “Rh–Ir alloy” with 3 mol/l HCl (t = 190 oC, p(O2) = 1 MPa), 87% of rhodium and 95% of iridium dissolved. After leaching, the presence of polymorphic poorly soluble modifications of rhodium(III) chloride was revealed in the cake. It has been established that titanium has a low corrosion rate under the same conditions, which allows us to recommend its as a material for reactors.The study was supported by the Russian Science Foundation, Project No. 25-29-00159.