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Palladium Production


The first step in the technology of precious metals dissolving in aqua regia is Gold and Platinum precipitation, then Pd(NH3)2Cl2 is extracted from the solution, refined by recrystallization, from HCl ammonia solution, dried and roasted in reducing atmosphere. Palladium powder is then remelted. Palladium microcrystal is produced by reducing Palladium salts solutions. Electrodepositing is processed from nitrite and phosphate electrolytes, using, particularly, Na2[Pd(NO2)4].

Only 10% of the world Palladium had been extracted from secondary raw materials 2/3 of which had been produced in USSR.

Preparation of Palladium

Preparation of Metallic palladium is carried by reduction of its salts. To this end ammonium chlor-palladite, (NH4)2PdCl4, is very suitable. Upon ignition in hydrogen, ammonium chloride and hydrogen chloride volatilise, leaving the metal behind in a porous, spongy condition. In order to avoid occlusion of hydrogen the metal may be cooled in a current of carbon dioxide.

If the metal is required in a compact form, it may be fused in the oxyhydrogen flame, or simply raised to bright red heat and welded under pressure.

Of the methods, an interesting one consists in precipitating the metal as iodide by addition of potassium iodide to a solution containing a soluble palladium salt. To obtain the free metal the iodide is ignited in air, the last traces of iodine being removed by heating in a current of hydrogen. By this method Bunsen isolated pure palladium from residues from the Russian Mint at Petrograd, the residues containing a mixture of all the platinum metals. A convenient method consists in precipitating palladium from solution as palladous cyanide by addition of mercuric cyanide. Palladous cyanide is insoluble in water and in dilute acids, and is the only cyanide of the platinum metals that can be thrown out in this manner. Upon ignition the salt decomposes, leaving a residue of metallic palladium.

Palladium may be obtained from porpesite or palladium-gold by fusion of the latter with silver, and digestion after granulation with dilute nitric acid. This effects the solution of the silver and palladium, metallic gold being left behind as residue. Addition of sodium chloride to the filtered solution precipitates the silver as chloride, and metallic palladium is obtained from the liquid by addition of metallic zinc.

Metallic palladium may also be prepared by precipitation from solutions of its salts with reducing agents such as formic acid or even strips of metallic zinc. It is thrown out in the finely divided condition, and may be rendered compact either by fusion or welding, as indicated above.

Pure Palladium may be prepared from the commercial foil by dissolving in aqua regia and evaporating to dryness. The dichloride thus obtained is dissolved in water acidulated with hydrochloric acid, and the filtered solution treated with ammonia on the water-bath until the precipitate first formed has redissolved. Filtration removes iron, etc., but traces of copper may still remain, colouring the solution blue. To effect its removal, pure hydrogen chloride is passed into the solution, whereby the di-ammoniate or palladosammine, PdCl2.2NH3, is precipitated. This is washed, redissolved in ammonia, and reprecipitated. By repeating this several times, all traces of iron, rhodium, copper, etc., are removed. The pure di-ammoniate is now reduced by ignition in hydrogen.

If traces of gold are suspected, the metal is now dissolved in aqua regia, repeatedly evaporated with hydrochloric acid to expel nitric acid, and the chloride dissolved in dilute hydrochloric acid. Pure sulphur dioxide is passed through the solution, and the whole allowed to stand. If there is no precipitate, gold is absent. When such is the case the chloride is converted into the di-ammoniate and reduced to the pure metal.

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