Behaviour of platinum metals sorbed on iron hydroxide during dissolution of the latter by sulfur dioxide

The behavior of ruthenium, iridium, and rhodium sorbed by ferric hydroxide was examined at the first stage of the process, during dissolution of the hydroxide by the action of sulfur dioxide. It was found that significant separation of the rare platinum group metals from iron is not achieved. The initial sulfuric acid content has a strong influence on the distribution of rare platinum metals between the solid and liquid phases during the dissolution. Proportional dissolution of iron and the platinum metal is observed at an H2SO4:Fe ratio of 0.35 mole/mole in the case of ruthenium, and 0.6-0.7 in the case of rhodium and iridium. At lower values of the ratio the platinum metals dissolve more slowly than iron, and in the cases of Rh and Ir they can be concentrated in the solid residue in a four- or fivefold ratio. Addition of copper sulfide to the slurry raises the relative content of platinum metals in the insoluble residue somewhat