This metal occurs almost exclusively in the native state, but is seldom pure. Analyses of samples from Australia, California, the United States of Colombia, and the Urals, show 61) to 86$ Per cent. of the metal, combined with varying proportions of copper ( 1/2 to 4), gold ( 1/2 to 1 1/4), iridium (3/4 to 4 1/4 iron (4 1/2 to 11 3/4), os-miridium (} to 26), palladium (1/2 to 1 3/4), and rhodium (1/4 to 1 3/4). The preparation of the malleable metal from the raw material is generally effected in the wet way. As first conducted, this was as follows:-The crude mineral is dissolved in 15 times its weight of aqua-regia (equal parts of nitric and hydrochloric acids), and precipitated by sal-ammoniac; this precipitate of platin-ammonium chloride is dried, pressed into a conical fire-clay mould, and ignited; thus the platinum is obtained as a coherent mass. The ultimate ductility of the product is found to depend upon gently heating the double chloride at a temperature just sufficient to expel all the sal-ammoniac, and occasion the least possible cohesion of the platinum particles.
This metallic powder is rubbed in the hands till it will pass through a fine lawn sieve, then well levigated, and the cold pulp filled into a conical brass barrel, and pressed very hard; the hardened cake is removed, heated to redness on a charcoal fire to expel moisture and increase the cohesion, then subjected to about 20 minutes' ignition in the highest attainable heat of a blast furnace, and, while still hot, placed on an anvil and hit with a heavy hammer, when the ingot of metal can be applied to the usual purposes where ductility is required. This process is still very commonly adopted, in spite of its tediousness.
Several dry methods have been introduced with more or less success. The first step in this direction was the application of the oxy-hydrogen blowpipe to the fusion of large masses of platinum. The arrangement mostly employed is shown in Fig. 157; it consists of 2 wellfitting lumps of quicklime a b, the upper one a having a hole drilled in the centre for the insertion of the blowpipe c, while a side opening d serves as an exit for the products of combustion and for the molten metal. The hydrogen or common coal-gas (the latter being more generally used) enters by the tube e, while the oxygen is conveyed by /, meeting in the blowpipe c, whose upper part is of copper and lower part of platinum. The lime crucible well withstands a high temperature, and absorbs the slags of the iron and silicon oxides and other matters formed during the fusion. Based upon this principle Deville et Debray have proposed 2 dry ways of treating platinum minerals, as follows:-(a) The ore, mixed with lime, is merely fused, when a platinum-iri-diura-rhodium alloy is produced; this is well adapted for melting down old platinum vessels, as the impurities (gold, iron, lead, phosphorus, sulphur) are either volatilized or absorbed by the lime. (b) This takes advantage of the fact that lead alloys with platinum but not with osmiridium; the platinum ore is mixed with galena, and some glass and borax as fluxes, and treated in an ordinary melting crucible; the osmiri-dium goes to the bottom, and litharge is gradually added till no more sulphur dioxide is evolved; when the mass is cool, the lead alloy at the top is removed and cupelled, and the platinum residue (containing a little iridium and rhodium) is fused in a lime crucible by the oxy-hydrogen flame.
But it is difficult to ensure by these means that the platinum obtained is either homogeneous or free from foreign matters.
Heraus, at Hanau, Germany, employs the following process:- The raw ore is dissolved in a mixture of 1 part aqua-regia and 2 of water in a glass retort under a pressure of 12 in. of water (Dullo having shown that the solution of platinum is facilitated by pressure); the solution is evaporated down, and the dried mass is heated to 257° F. (125° C), when the palladium and iridium salts are reduced to chlorides. From the clear solution, acidified by hydrochloric acid, pure platinum chloride is precipitated by adding sal-ammoniac, whilst iridium chloride is recovered by evaporating down the mother-liquor. The solution remaining after precipitation of the platinum chloride is treated with scrap-iron, and the precipitate, freed from excess of iron by adding hydrochloric acid, is re-dissolved in aqua-regia; from this solution,further quantities of platinum and iridium chlorides are thrown down by sal-ammoniac. From the mother-liquors, and from the residue left on dissolving the ore in aqua-regia, palladium, rhodium, ruthenium, osmium, and iridium are obtained.
The platinum sponge produced by igniting the chloride is compressed, broken up, and fused into a mass by heating in an excess of oxygen in a lime crucible.
Commercial platinum is rarely pure, containing generally a considerable proportion (about 2 per cent.) of iridium, and a little rhodium, with sometimes other bodies. Johnson and Matthey prepare pure platinum in the following manner : -The ordinary commercial platinum is melted with 6 times its weight of pure lead; the mass is granulated, and slowly dissolved in dilute nitric acid (1 vol. acid to 8 water), being placed for this purpose in porcelain baskets, such as are used for. holding the manganese oxide when making chlorine. When the first charge of acid is sufficiently saturated, a fresh quantity should be added till no more action is apparent; at this stage, the greater part of the lead will have been dissolved out, together with a portion of any copper, iron, palladium, or rhodium that' may have been present. These metals are subsequently extracted from the mother-liquors, the lead nitrate by crystallization, and the others by well-known means. The residue is an amorphous black powder, consisting of platinum, lead, and small portions of other metals originally present, the iridium existing as a brilliant crystalline substance insoluble in nitric acid.
After digesting this compound in weak aqua-regia, immediate solution of the platinum and lead takes place, leaving the iridium still impure, but effecting complete separation of the platinum. To the platinum and lead chlorides, after evaporation, is added sufficient sulphuric acid to effect the precipitation of the whole of the lead as sulphate; and the platinum chloride, after solution in distilled water, is treated with excess of ammonium-sodium chloride, the excess being to ensure that the precipitated yellow double salt may remain in a saturated solution of the precipitant. The whole is then heated to about 176° F, (80° C), and allowed to stand for some days; the ammonio-chloride of platinum settles down as a firm deposit at the bottom of the vessel, whilst any rhodium present will colour the surface liquor a rose tint by combination of the salts of the 2 metals. The precipitate is repeatedly washed with saturated solution of ammonium chloride, and subsequently with distilled water charged with pure hydrochloric acid. The small quantity of the double salt taken up and held in solution is recovered afterwards.
Rhodium may still exist in the washed precipitate, which must therefore not be reduced to the metallic state until its separation is completed; this is best effected by mixing with the dried compound salts of ammonia chloro-platinate and chloro-rhodiate, potash-bisulphate and a small proportion of ammonia bisulphate, and subjecting to a gradual heat, brought by degrees up to a dull-red, in a platinum capsule, over which is placed an inverted glass funnel. The platinum is thus slowly reduced to a black, spongy, porous condition, free from water, nitrogen, ammonia sulphate, and hydrochloric acid, the rhodium remaining in a soluble state as rhodium bisulphate and potash, which can be dissolved out completely by digesting in boiling distilled water; a small quantity of platinum will have been taken up in a state of sulphate, but is regained by heating the residue (obtained on evaporation) to redness, at which heat it is reduced to the metallic condition, the rhodium salt remaining un-decomposed. The resulting platinum is absolutely pure.
Deville et Debray prepare pure platinum on the large scale by the following process:-The crude metal, containing iridium and rhodium, is fused with 6 to 10 times its weight of pure lead (obtained by calcining pure lead acetate); the cooled mass is treated with nitric acid, and the residue with dilute aqua-regia, when a crystalline alloy of iridium, ruthenium, and iron forms, and remains behind, while the lead, platinum, and rhodium (partially) are dissolved. [If the platinum is rich in rhodium, this is not dissolved by the aqua-regia, but will be separated from the iridium-ruthenium-iron alloy by concentrated boiling sulphuric acid.] The lead-platinum solution is treated with sal-ammoniac, when the double chloride of ammonium and platinum is precipitated, leaving the rhodium in solution. The precipitate is washed in water soured with hydrochloric acid, and ignited; the resulting metal is heated in a lime crucible till fusion is complete, when the heating is abruptly stopped, so that solidification proceeds from without, and thus avoids the formation of bubbles.
Platinum possesses properties which render it practically indispensable in chemical and metallurgical pursuits. It is inozidizable at any temperature by oxygen, water, or sulphuric or nitric acids; but it is attacked by aqua-regia, by sulphur in presence of alkalies, by alkalies themselves, by nitre, and by alkaline cyanides; the silicon reduced from the ash of burning charcoal combines with the metal, rendering it brittle and liable to crack; the heating of platinum vessels in a flame where the combustion is incomplete causes formation of platinum carbide, which oxidizes, and produces blisters; arsenic and phosphorus alloy with platinum when heated in contact, as also do the easily-reducible metals at a high temperature. The commercial platinum, containing about 2 Per cent. of iridium as an impurity, is better than the pure metal for making chemical apparatus, as it withstands acids in a superior degree; new platinum crucibles may be tested by boiling in hydrochloric acid and then in pure nitric acid, when none of the metal should be dissolved; the cleansing of crucibles is best effected by fusing potash bisulphate in them.
Pure platinum has a sp. gr. of 21.5, a tin-like colour, the softness of copper, and malleability ranking next to gold and silver; it welds readily at a white heat, but is infusible in mass, even in the highest heat of the blast furnace, succumbing only to the oxy-hydrogen flame; this latter is now utilized for joining platinum seams by autogenous soldering, instead of using gold as was formerly the case. In the forms known as "spongy platinum" and " platinum black," the metal is much used as an oxidizing agent. The preparation of the former has already been described; the latter is best obtained by warming a solution of platinum bichloride in potash with alcohol.