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Recovery of Platinum from Laboratory Residues

G.B. Kauffman, L.A. Teter and R.N. Rhoda
Inorganic Syntheses 7, 232-236 (1931)

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Pt compounds → Pt

Pt + 4 HNO3 + 6 HCl → H2[PtCl6] + 4 NO2 + 4 H2O
H2[PtCl6] + 2 NaCl → Na2[PtCl6] + 2 HCl
2 NH4Cl + Na2[PtCl6] → (NH4)2[PtCl6] + 2 NaCl
3 (NH4)2[PtCl6] → 3 Pt + 2 N2 + 2 NH4Cl + 16 HCl
H2[PtCl6] + 3 Zn → Pt + 3 ZnCl2 + H2

Although many procedures for recovering platinum appear in the literature, they are not generally applicable to laboratory residues. Since those intended for ores assume the presence of other platinum metals, they are unnecessarily complicated. On the other hand, simpler methods involving only treatment with formaldehyde, formic acid, sodium dithionite, iron, magnesium, zinc, or other reducing agents reduce other inactive metals such as copper or silver. Furthermore, standard reduction methods are seldom directly applicable to the more stable platinum complexes.

The following procedure, modified from Gilchrist's method,1 is intended for the recovery of platinum from residues containing base metals and noble metals (other than those of the platinum group) as well as strong complexing agents. A preliminary separation of base metals as hydrated oxides considerably reduces the time required to obtain pure platinum by the precipitation of ammonium hexachloroplatinate(IV). The authors have tested the procedure both with actual laboratory residues and with synthetic platinum mixtures containing as much as 50% of the following combined impurities: aluminum, ammonium, cobalt, chromium, copper, iron, mercury, potassium, silver, and sodium ions as well as ethylenediamine, diethyl sulfide, pyridine, tri(n-butyl)phosphine, urea, thiourea.

Experimental

The dried residue is heated to redness in a large porcelain casserole or evaporating dish over a Meker burner, and ignition is continued until fuming has ceased.2 Most of the soluble salts are now removed by thoroughly washing and decanting the ignited and powdered residue several times with five times its volume of boiling water for each operation.3 Since the washings may contain some finely divided platinum, they should be filtered. After the filter paper has been charred in a covered crucible, the residue should be added to the ignited and washed mixture.

The residue is treated with five times its volume of aqua regia (1HNO3:4HCl). After the initial effervescence has subsided, the mixture is gently heated on a hot plate for about half an hour. It is then allowed to settle, and the liquid is carefully decanted into a beaker. The residue is similarly treated with 5-volume portions of aqua regia until all the platinum has dissolved, i.e., until the decanted liquid, originally dark reddish orange, is a pale straw color. A total of about 30 mL of aqua regia per gram of platinum should be sufficient. The combined portions of the aqua regia solution are cooled with running water (not an ice bath),4 and the precipitate of insoluble chlorides is removed by suction filtration through a sintered-glass funnel (medium porosity). The insoluble chlorides are added to the residue insoluble in aqua regia and boiled for a few minutes with twice the total volume of water. The mixture is filtered by suction through coarse paper, and the yellow filtrate, which contains potassium hexachloroplatinate(IV), is added to the aqua regia solution. The entire process is repeated until the filtrate is colorless, whereupon the residue is discarded.

The aqua regia filtrate is transferred to a very large beaker, sodium chloride (commercial salt) (1 g/g of platinum) is added, and the solution is evaporated to near dryness on a steam bath.5 An air stream directed across the surface of the liquid increases the rate of evaporation and reduces foaming. The residue is barely covered with hydrochloric acid and evaporated to dryness in order to destroy any nitro complexes. It is next just covered with water, again evaporated on the steam bath, and dried in an oven at 110°C for one hour.6

The residue is dissolved in sufficient water to yield a concentration of about 50 g of platinum per liter, the solution heated nearly to boiling, and sufficient sodium hydrogen carbonate (commercial baking soda) added (caution: effervescence) until the resulting solution just turns red litmus blue. After being cooled with running water for about 10 minutes, the mixture is filtered through a Büchner (not sintered-glass) funnel to remove the precipitate of hydrated base metal oxides which has settled. Any platinum-containing solution is removed by washing the precipitate twice with two times its volume of cold water for each operation. The washings are added to the filtrate.

A slight excess of solid ammonium chloride (0.6g/g of platinum) is then added to the solution to precipitate the platinum. The solubility of the ammonium hexachloroplatinate(IV) is reduced by adding an amount of 95% ethanol equal to about one-tenth the volume of the solution and by cooling in an ice bath for about 10 minutes. The yellow precipitate is collected on a Büchner funnel, and the filtrate is tested for complete precipitation by adding more ammonium chloride and setting it aside. The ammonium hexachloroplatinate(IV) is washed with several one-volume portions (relative to the filtrate) of ice-cold 20% ammonium chloride solution, 95% ethanol, and ethyl ether and then air-dried. The precipitate is removed from the filter paper, placed in a partially covered crucible, and ignited to metallic platinum over a Meker burner, gently at first, but then more strongly. The residue is washed several times with 5-volume portions of 6 N nitric acid and water to remove any coprecipitated sodium chloride. The platinum is dried in an oven at 110°C for one hour.

Some platinum is still present as hexachloroplatinate(IV) in the light yellow filtrate. It is recovered by strongly acidifying this solution with hydrochloric acid (pH about 1),7 adding excess mossy zinc, and allowing the mixture to stand until reduction appears complete (about one hour), with occasional stirring if necessary to remove precipitated platinum from the zinc and expose fresh metal surface. Additional acid is added, if necessary, to dissolve any excess zinc, and the platinum is collected on a Büchner funnel and washed and dried as was the previously recovered platinum. Since the metal produced by reduction with zinc is not as pure as that produced by ignition of ammonium hexachloroplatinate(IV), the products should be kept separate.

In general, the percentage recovery of platinum increases with increasing percentage of platinum in the residue. For example, with a mixture containing 50% platinum, 77.1% recovery (53.6% by precipitation and 23.5% by reduction) was attained, while with a mixture containing 75% platinum, the recovery rose to 95.7% (80.7% by precipitation and 15.0% by reduction). If platinum of higher purity is desired, the recovered material may be dissolved in aqua regia, reprecipitated as ammonium hexachloroplatinate(IV), and reignited.

References and Notes

  1. R. Gilchrist, Chem. Revs., 32, 306 (1943)
  2. Ignition removes ammonium salts and volatile substances and decomposes platinum complexes to metallic platinum. All ignitions and evaporations should be carried out under the hood!
  3. If potassium salts are present in considerable amounts, much potassium hexachloroplatinate(IV) will be precipitated when the mixture is boiled with aqua regia.
  4. If the solution becomes too cold, a considerable amount of sparingly soluble potassium hexachloroplatinate(IV) may precipitate.
  5. A Meker burner or hot plate, although faster, would require constant stirring and swirling to prevent spattering. Sodium hexachloroplatinate(IV) is easier to handle than hexachloroplatinic(IV) acid, because the latter may decompose to insoluble products on being heated to dryness. If a reddish orange sirup rather than a yellowish green crystalline mass is obtained, the amount of sodium chloride was insufficient.
  6. This step is intended to remove most of the hydrochloric acid in order to avoid excessive consumption of sodium hydrogen carbonate and subsequent excess foaming, as well as contamination of ammonium hexachloroplatinate(IV) with coprecipitated sodium chloride.
  7. Use of an acid solution considerably reduces the time required for reduction and avoids evolution of ammonia and precipitation of basic salts.