Platinum Production

Placer mining platinum, platinum scraps or concentrates are dissolved in aqua regia. The excess HNO₃ is removed by adding ethanol and sugar syrup, which also reduces iridium and palladium until Ir³⁺ and Pd²⁺. On the next step after (NH₄)₂PtCl₆ is precipitated by NH₄Cl, the sediment is dried and baked at temperature gradually increased until the contaminated platinum sponge appears at 800-1000°C. Pure platinum is obtained dissolving the sponge in aqua regia, reprecipitation and baking. Platinum sponge is remelted, and platinum black is obtained after salts reduction. Phosphate or cis-diaminonitrite electrolytes are used in platinum coating.

The crude platinum is digested with aqua regia (one part HNO₃ at 35° to 36° Beaume, and three parts HCl at 20° B.) whereby the platinum and several allied elements are completely dissolved, leaving a residue of osmiridium and sand, together with a little graphite. The solution is diluted, filtered, and evaporated to a syrup. Hydrochloric acid is added to decompose nitroso compounds, the whole taken to dryness and heated to 150° C. to expel the nitric acid and to simultaneously convert the palladium to palladous chloride. The platinum is now precipitated as ammonium chlor-platinate by addition of ammonium chloride. This is separated, washed, and finally heated to redness in a muffle, whereby spongy platinum is obtained, the ammonium chloride and chlorine being expelled.

(NH₄)₂PtCl₆ = Pt + 2NH₄Cl + Cl₂.

The spongy metal is now made into a paste with water, compressed into a mould, and either forged into a homogeneous bar by heating and hammering, or melted in an oxy hydrogen furnace constructed of lime.

As obtained in this way the platinum usually contains some 2 per cent, of iridium, precipitated along with the ammonium chlor-platinate as the corresponding iridium salt. A second refining serves to remove this iridium, a fairly pure platinum resulting.

The solution from which the ammonium chlor-platinate has been separated still contains a little platinum with practically all the rhodium, ruthenium, and palladium. All are precipitated by metallic iron and dissolved in aqua regia. Addition of more ammonium chloride effects the precipitation of the remaining platinum, and the filtrate may be worked for the other metals.

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 solution platinum and iridium are thrown out as ammonium chlor- platinate and ehlor-iridate respectively, by addition of ammonium chloride. The mother-liquor contains ruthenium, rhodium, palladium, osmium, and some iridium. The ammonium chlor-platinate, mixed with small quantities of chlor-iridate, is ignited, and the spongy metal thus obtained treated in the manner described above.

Another method consists in fusing finely powdered crude platinum with two or three times its weight of zinc. The brittle alloy thus formed is powdered and treated with dilute sulphuric acid, whereby zinc is removed. The residue is heated to redness, and part of the base metals removed with dilute hydrochloric acid, and any gold dissolved out by further treatment with cold aqua regia.

The residue is heated in retorts with aqua regia, whereby most of the platinum passes into solution, together with some rhodium, osmium, palladium, and a little iridium, whilst the undissolved portion consists of osmiridium, ruthenium, and a little platinum. The solution is distilled, the osmium passing off as the volatile tetroxide, and the mother-liquor then neutralised with sodium carbonate. Addition of mercuric cyanide throws out the palladium as cyanide. Ammonium chloride added to the filtrate effects the precipitation of ammonium chlor-platinate and chlor-iridate. Ignition of these salts yields the spongy metal, which may be worked up as already indicated into the compact form.

If desired, the two ammonium salts may be separated prior to ignition, so that the pure metals may ultimately be isolated. This may be done conveniently by either of the following methods:

1. The salts are warmed gently with a solution of potassium cyanide. The ammonium chlor-iridate is reduced to chlor-iridite, which passes into solution, leaving the chlor-platinate as an insoluble residue.
2. The salts are suspended in water and sulphur dioxide passed through. The chlor-iridate is reduced to chlor-iridite and passes into solution, leaving the insoluble chlor-platinate as residue.

Ignition of the purified ammonium chlor-platinate yields a proportionately pure product of metallic platinum. The solution containing ammonium chlor-iridite may be evaporated to dryness and the residue ignited to yield metallic iridium.

This method consists in melting the crude platinum, from which the gold has been removed, with excess of lead. On cooling, the mass is treated first with nitric acid, and then with aqua regia, whereby lead, platinum, and some rhodium pass into solution; iron, ruthenium, and rhodium remaining behind as a crystalline alloy. From the solution ammonium chlor-platinate is precipitated, and from it metallic platinum is prepared as described above.

This method is not now used.

A second method described by these authors consists in repeated melting of the crude platinum in a lime crucible, whereby volatile and oxidisable impurities are removed, the latter being absorbed by the lime. The resulting metal still contains rhodium and iridium, however, if these metals were originally present.

Platinum occurs in traces in the nickel ores of Sudbury, and is to some extent recovered from these in commercial practice by more or less secret processes. During the fiscal year ending March 31, 1916, the Canadian Copper Company produced 56,405 tons of nickel matte, the average sample of which contains:

Platinum	0.10 oz. (troy) per ton
Palladium	0.15 oz. (troy) per ton

According to these figures, the 56,405 tons of matte contained:

Platinum	5640.5 oz. (troy)
Palladium	8461 oz. (troy)

The actual amount of these precious metals recovered during the year amounted to:

Palladium + platinum	1093 oz. (troy)
Other platinum metals, mainly rhodium and iridium	257 oz. (troy)

It will thus be seen that there is considerable opportunity of adding to the commercial supplies of the platinum metals from this source.

Matthey recommends the following treatment for the commercial metal whereby it may be obtained in an exceptionally high state of purity. The platinum is melted with six times its weight of pure lead and, after granulation, treated with a mixture of nitric acid (1 part) and water (8 parts). When no more action is apparent the greater part of the lead will have dissolved out, together with a portion of any copper, iron, palladium, or rhodium that may have been present. The insoluble residue is in the form of an amorphous, black powder, and consists essentially of platinum, with lead, iridium, and small proportions of the other metals originally present.

Digestion with dilute aqua regia effects the solution of the platinum and lead, leaving a residue of impure iridium. The solution is filtered, evaporated, and the lead converted into sulphate by addition of sulphuric acid in requisite quantity. The platinic chloride is extracted with water, and ammonium chlor-platinate precipitated in the usual way with excess of ammonium chloride containing sodium chloride. The whole is heated to 80° C. and allowed to stand for several days, most of the rhodium remaining in solution and imparting to the liquid a rose-coloured hue.

The precipitated ammonium chlor-platinate is repeatedly washed with a saturated solution of ammonium chloride, and subsequently with dilute hydrochloric acid. It still contains small quantities of rhodium, however, as ammonium chlor-rhodate. The salt is mixed with potassium hydrogen sulphate to which a small quantity of ammonium hydrogen sulphate has been added, and gradually heated to redness in a platinum dish. The ammonium chlor-platinate is thereby reduced to platinum sponge, whilst the rhodium remains in a soluble state as a double sulphate with potassium.

The product is digested with water at the boiling-point, leaving a residue of platinum sponge in a high state of purity.

In addition to the rhodium sulphate the water will have extracted a small quantity of platinum in the form of sulphate.

Upon evaporating the solution to dryness and heating to redness the platinum sulphate is reduced to the metallic condition, the rhodium salt being unaffected. Digestion with water leaves a residue of pure platinum which may be added to the previous residue.

The metal thus prepared is very pure, and may, if desired, be worked into the compact form by the methods already detailed.