This valuable metal occurs in most-platinum ores and in alloy with gold. It may be separated from other platinum metals by taking advantage of the fact that, in neutral solutions, mercuric cyanide gives an insoluble precipitate of palladium di-cyanide. In preparing pure palladium from the platinum residues of the St. Petersburg mint, Bunsen adds potassium iodide to a solution of palladium dichloride, when palladium di-iodide separates out as a black insoluble precipitate. For preparing the commercial metal on the large scale from the residues of platinum works, the following process is most convenient:-
Philipp describes the following method for recovering the palladium, rhodium, ruthenium, osmium, and iridium contained in the mother-liquors of the platinum manufacture. These liquors are boiled down, by which iridium chloride separates out with a little platinum. The concentrated liquor is stood aside for a time, poured off from the deposit of iridium, diluted, and precipitated by zinc. The precipitate is digested with hydrochloric acid, washed, 3 and ignited. Aqua-regia dissolves the palladium and a little gold, while impure rhodium remains behind. The solution is supersaturated with ammonia, and the palladium is precipitated by hydrochloric acid. The residue from the platinum solutions, which in Russian platinum averages 8 per cent., is ignited, ground, and washed, to remove the greater part of the iron, etc. The pulp, now amounting to 2 or 3 Per cent. of the original material, is smelted with an equal quantity of a mixture of borax and nitre till the mass flows smoothly. After treating with hydrochloric acid and water, the metals remain behind. They are alloyed with twice their bulk of zinc in a graphite crucible; the alloy is broken up and powdered, and freed from zinc by adding hydrochloric acid.
The residue is treated with chlorine in a Hessian tube, and iridium and osmium chlorides, with a little platinum, are thus obtained. The remaining mass is ignited in a current of hydrogen, and smelted with caustic potash and nitre to afford its ruthenium.
Commercial palladium may be purified by Bunsen's process, already described, or in the following manner:-To a solution of the impure metal, as free from acid as possible, ammonia is added till the precipitate formed re-dissolves; hydrochloric acid gas is then passed into the solution, and throws down palladio-ammonium chloride, leaving copper and iron in solution; the precipitate, washed and ignited, yields spongy palladium. The same result is attained by igniting palladium cyanide. Metallic palladium is rapidly dissolved in hot nitric acid, especially when nitrous acid, copper, or silver is present; the spongy metal dissolves in hydrochloric acid in contact with the air, and the compact when chlorine is passed in; also in boiling concentrated sulphuric acid, and in fused potash bisulphate; it has a sp. gr. of 11.4, a silvery colour, and a fusing-point about equal to that of wrought iron; its colour and appearance are unalterable by exposure to the air or to sulphuretted hydrogen. It has been chiefly used as a substitute for gold in dentistry, and fox graduated surfaces on scientific instruments.