All about Iridium Production, Preparation and Purification

Anode slime of copper-nickel metallurgy is the main source of Iridium production. Initially gold, paladium and platinum are isolated from the platinum group metals concentrates. The Ruthenium, Osmium and Iridium residue is fused with KNO₃ and KOH (the mentioned potash). The melt is leached by water, after which the solution is oxidized by chlorine and the OsO₄ and RuO₄ are sublimated. Meanwhile the iridium containing residue is melted with Na₂O₂ and NaOH, the fusion treated by aqua regia and, after that, by NH₄Cl solution. (NH₄)₂[IrCl₆] settled out after this treatment is then baked with metal iridium output.

Another method of iridium isolation from solutions includes extraction of hexachloroiridates by higher aliphatic amines. Iridium may be separated from basic metal by ion exchange methods. Iridium is obtained from minerals of osmiridium group by melting with BaO₂ through hydrochloric acid and aqua regia treatment, OsO₄ sublimation and, finally, (NH₄)₂[IrCl₆].

Iridium preparation may be conveniently performed from osmiridium. This alloy is fused in turn with zinc, and then with barium dioxide and nitrate. The residue, after treatment with acid and removal of osmium by distillation, consists of iridium and a small quantity of ruthenium. The liquid is reduced to small bulk, ammonium chloride and a little nitric acid added, and the whole taken to dryness on the water-bath. A violet-black crystalline residue is obtained, consisting mainly of ammonium chloriridate. This is washed with semi-saturated ammonium chloride solution until the washings are colourless, dried, and heated to redness, a current of hydrogen being finally passed over to remove the last traces of chlorine and oxygen. On cooling, the metallic residue, consisting mainly of iridium with a little ruthenium, is treated with aqua regia, which removes a little platinum and osmium. Fusion with potassium nitrate and hydroxide and subsequent washing removes any ruthenium as soluble potassium ruthenate, the insoluble residue being finally heated to whiteness until every trace of osmium has been volatilised. The residue gradually melts to a button of fairly pure iridium.

In practice the purest iridium obtained by the Purification of Iridium by foregoing process invariably contains small quantities of platinum, rhodium, ruthenium, and iron. In order to remove these, Matthey treats the metal as follows. The iridium, in a fine state of division, is fused with ten times its weight of lead, and kept in the molten condition for some hours. On cooling, the lead is removed with nitric acid, and the residue subjected to prolonged digestion in aqua regia, whereby the platinum is removed, and a mass of iridium, with small quantities of rhodium, ruthenium, and iron, results.

This is fused with potassium hydrogen sulphate at a high temperature, which treatment removes practically all the rhodium, any remaining traces being taken out with the iron at a later stage.

The resulting iridium is fused with ten times its weight of potassium hydroxide and three times its weight of potassium nitrate in a gold crucible. This operation takes a considerable time, the iron being oxidised, the iridium converted into insoluble potassium iridate, and the ruthenium to soluble potassium ruthenate.

The whole is extracted with water, and the precipitate washed with water containing alkaline sodium hypochlorite in solution, until the washings are no longer coloured, signifying the removal of most of the ruthenium. The residue, consisting mainly of blue oxide of iridium, is now allowed to remain for a time suspended in a cold solution of sodium hypochlorite, after which it is gradually raised to the boiling-point and heated until the distillate no longer reddens a solution of hydrochloric acid in diluted alcohol. The residue is again heated with potassium nitrate and alkaline sodium hypochlorite solution charged with chlorine, until the last trace of ruthenium has disappeared.

It now remains to remove the iron and traces of rhodium. This is effected by dissolving in aqua regia, evaporating to dryness, and re- dissolving in water. After filtering, the dark-coloured solution is slowly poured into a concentrated solution of sodium hydroxide containing sodium hypochlorite, and should remain clear without any perceptible precipitate. The solution is now distilled, and chlorine gas passed through; the distillate on being collected in a solution of hydrochloric acid in dilute alcohol should yield no coloration, showing that ruthenium is entirely absent. The chlorine precipitates out the greater part of the iridium as blue oxide, which is washed and dried and reduced by ignition in a porcelain tube in a current of carbon monoxide mixed with dioxide, as obtained by the action of sulphuric acid upon oxalic.

Metallic iridium is thus obtained together with oxide of iron. The whole is heated to redness with potassium hydrogen sulphate, which removes the iron and any remaining traces of rhodium. The residue is well washed with water, then with chlorine water to remove any traces of gold, and finally with hydrochloric acid to take out any silica which may have accidentally been introduced with the alkalies or have come from the vessels employed. The resulting iridium is calcined with charcoal and melted into an ingot.

Matthey states that the metal prepared in this manner, even with the utmost care, will still contain very minute, though almost inappreciable, traces of oxygen, ruthenium, rhodium, and possibly iron.

Leidie recommends the following method of Purification of Iridium as obtained by the process first described. The metal in a fine state of division is heated to dull redness with twice its weight of fused sodium chloride in a current of dry chlorine. On cooling, the mass is extracted with water containing a little hydrochloric acid. The solution is filtered, raised to 50° C., and sodium nitrite added until nitrous fumes cease to be evolved. Sufficient sodium carbonate is now introduced to render the solution decidedly alkaline, then a slight excess of sodium nitrite, and the whole raised to boiling. The precipitate obtained contains any iron, lead, and gold originally present, whilst in solution are present iridium, together with any ruthenium, rhodium, and osmium, in the form of the following compounds:

Iridium as Ir₂(NO₂)₆.6NaNO₂
Ruthenium as Ru₂(NO₂)₆.4NaNO₂
Rhodium as Rh₂(NO₂)₆.6NaNO₂
Osmium as Na₂OsO₄

The ruthenium and osmium are best removed by conversion into their volatile oxides, RuO₄ and OsO₄. This is effected by addition of excess of sodium hydroxide to the solution, and passing in chlorine whereby hypochlorites are produced. On continuing to pass in chlorine and gradually raising the temperature, the tetroxides of ruthenium and osmium volatilise, and may be condensed in a cooled receiver. The iridium and rhodium remain behind in solution as double chlorides, namely, sodium chloriridate, Na₂IrCl₆, and sodium chlor-rhodite, Na₃RhCl₆, which are obtained, together with excess of sodium chloride, on evaporating to dryness. The crystalline mass is dissolved in chlorine water and recrystallised, whereby the excess of sodium chloride is removed, the pure double salts separating out. These are dried and heated to 440° C. in a current of dry chlorine, cooled and extracted with chlorine water. The rhodium salt has been converted by this treatment into the insoluble trichloride and remains behind, the sodium chloriridate passing into solution. Addition to the last named of excess of ammonium chloride causes the precipitation of ammonium chloriridate, (NH₄)₂IrCl₆. This is washed in a concentrated solution of ammonium chloride, in which it is insoluble, dried, and reduced at red heat in a current of pure, dry hydrogen. On cooling, the hydrogen is replaced by carbon dioxide, pure iridium remaining and Purification of Iridium process finished.

Antony prepares pure iridium by precipitation from an acidulated solution of the chloride with metallic zinc. The precipitated metal is treated with nitric acid (density 1.35) containing nitrous acid, whereby osmium and palladium are removed. Repeated extraction with dilute aqua regia (3 parts HCl, 3 parts H₂O, and 1 part HNO₃) dissolves out the bulk of the platinum and any traces of osmium and palladium still remaining.

The metal is now fused with potassium hydrogen sulphate whereby a soluble rhodium compound is produced, and the insoluble residue of iridium sesquioxide, Ir₂O₃, is reduced in a current of hydrogen, and converted into potassium chloriridate by heating with potassium chloride in a current of chlorine. Repeated crystallisation to remove any chlor-ruthenate yields the purer salt, still contaminated, however, with traces of platinum. These are removed by reducing the salt to the metallic condition and heating in a current of chlorine and carbon monoxide to 250° C., whereby the platinum volatilises, leaving a dark green mass of iridium, containing a little chloride. The pure metal is readily obtained from this by reduction.