Iridium, separation from platinum metals

Iridium metal is separated from its other metal ores when the combined minerals are dissolved with a strong acid know as aqua regia, which is a mixture of 25% nitric acid and 75% hydrochloric acid. Aqua regia is the only acid that will dissolve platinum and gold. Once the platinum and other metals are dissolved, the iridium, which is insoluble in this strong acid, becomes the residue. The refined iridium ends up in the form of either powder or crystals. 

The initial steps are similar to any other mineral extraction process. This involves crushing mineral, froth flotation, gravity concentration and other steps to obtain platinum metal concentrates that may contain about 30 to 40 wt% of platinum group metals. The concentrate is treated with aqua regia to separate soluble metals, gold, platinum, and palladium from other noble metals such as ruthenium, rhodium, iridium, osmium, and silver that remain in... 

The four metals rhodium, palladium, osmium and iridium, share the same centennial and have been dealt with together.147-149 Wollaston separated palladium from platinum ore in 1803 but concealed the identity of the metal until 1804.150 Osmium was isolated from crude platinum by Smithson Tennant in 1804.151 There are accounts of the discoveries of niobium (by Hatchett)152 and ruthenium.153,154... 

At Hanau the method adopted is similar to the foregoing. The crude Russian platinum is treated with a mixture of aqua regia (one part) and water (two parts) in a glass retort under a pressure of 12 inches of water. The resulting solution is evaporated to dryness and heated to 125° C., whereby the palladium is converted to the palladous condition. The product is extracted with water, and acidified with hydrochloric acid. On addition of ammonium chloride a precipitate of ammonium chlor-platinate is obtained, contaminated with some ammonium chlor-iridate. On concentration of the mother-liquor a further small amount of ammonium chlor-iridate separates out. The liquid is now treated with scrap-iron, which precipitates all the other platinum metals, together with any platinum and iridium not hitherto precipitated. The deposit is washed free from undissolved iron by treatment with hydrochloric acid, and in turn dissolved in aqua regia. From this... 

Iridium and the other platinum metals tend to occur together. A series of chemical reactions is used to separate one metal from the other. The other metals are then removed by other techniques. Very little iridium... 

Porous anode 1, used in SPE electrolysis (fig.1) consists of mixtxjre of ruthenium dioxide (75%) and iridium oxide bound to the graphite layer. Thickness of such layer depends on the amount of aphitc, covering the anode surface unit. In particular, if this amount is 40 g/m. correspondent thickness reaches 100 0jn [12]. Current feeding to anode is held with the help of point collector 6 (the metal net can sei e as this collector). Anode space is separated from the cathode one with the help of membrane SPE (2). Platinum black, serving as an anode 1, is 100 jim v/ide. Cathode point collector 3 is connected with graphite plate 5, that maintain direct contact with cathode. 

A hydrochloric acid solution contains platinum, palladium, and iridium as chloro-complex ions. The solution is a constant 1.0 AT in chloride ion and 0.020 M in each complex ion. Is it feasible to separate the three metals from this solution by electrolysis (Assume that 99% of a metal must be plated out before another metal begins to plate out.)... 

Fire assay describes a group of separation methods in which precious metals (PMs) are separated from other species by dissolution in other molten metals, usually lead, nickel, or tin. Fire assay has been and continues to be fundamental to the determination of PMs. These are defined as gold (Au), silver (Ag), and the platinum group metals (PGMs) platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os). In the context of fire assaying other elements are referred to as base elements or metals. 

The platinum group metals occur jointly as alloys and as mineral compounds in placer deposits of varying compositions. Ru and Os are separated from the PGM mix by distillation of their volatile oxides, whereas platinum, iridium, palladium, and rhodium are separated by repeated solution and precipitation as complex PGM chlorides, or by solvent extraction and thermal decomposition to sponge or powder. PGM scrap is recycled by melting with collector metals (lead, iron, or copper) followed by element-specific extraction. 

Methods for separating the different platinum metals are comphcated and are partly kept secret. The traditional method with dissolution in aqua regia is still used, in which platinum, palladium and gold are dissolved while the other platinum metals stay undissolved. Gold is obtained from the solution by reduction, platinum is precipitated as ammonium hexachloroplatinate and palladium as a dichlorodiammine compound. The residue after the first aqua regia treatment contains iridium, rhodium, osmium and ruthenium. They are separated in several complicated steps. 

Cobalt, rhodium, and iridium are also characterised by their power of yielding complex ammino derivatives, and in this manner resemble palladium and platinum. These derivatives are a remarkable series of substances entirely distinct in most of their properties from the more usual inorganic salts of the metals in question, and are dealt with separately in Volume X of this Series. 

Rhodium is insoluble in acids, even in aqua regia, although when its alloys are attacked by this latter mixture a portion of the rhodium passes into solution. When fused -with potassium hydrogen sulphate, rhodium dissolves, yielding the sulphate. This reaction is interesting as affording a convenient method of separating the metal from iridium and platinum (see p. 34-1). 

Separation of Osmium from ruthenium, rhodium, iridium and platinum may be effected by addition of zinc or magnesium to the solution containing these metals, whereby they are all precipitated as a black deposit.. 

S. Tennant, like W. Wollaston, dissolved crude platinum in aqua regia. At the bottom of the retort he discovered a black precipitate with metallic lustre. This phenomenon had been observed previously in experiments with platinum, but the precipitate was believed to be graphite. In summer 1803 Tennant suggested that the precipitate most likely contained a new metal. In autumn of the same year the French chemist H. Collet-Descoties also concluded that the precipitate contained a metal that precipitated from ammonium platinum salts and yielded red colour. In his turn, L. Vauquelin heated the black powder with alkali and obtained a volatile oxide. Vauquelin believed that it was an oxide of the metal mentioned by H. Descoties. Tennant s experiment set off a series of investigations. Tennant himself continued his research and in spring 1804 he reported to the British Royal Society that the powder contained two new metals which could be separated fairly easily. In 1805 he published the article On Two Metals Found in the Black Powder Formed after Dissolution of Platinum . The names osmium and iridium were mentioned in the article for the first time. 

K. Klaus began his research in 1840. The then Minister of Finance of Russia E.F. Kankrin, a competent and energetic person, rendered him great assistance Klaus obtained 2 pounds of crude platinum residue and extracted a considerable amount of iridium, rhodium, osmium, and palladium from it, apart from 10% platinum. In addition, Klaus separated a mixture of metals which, in his opinion, had to contain a new substance. 

The metals osmium and ruthenium can be selectively oxidized to their tetroxides and separated by extraction with carbon tetrachloride [24]. Reference 24 also deals with enrichment procedures of palladium, rhodium, iridium, and platinum chloro complexes from hydrochloric acid solutions... 

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