This invention relates to a method of dissolving values of platinum group metals from ores and concentrates, for subsequent treatment for recovery of the metal. The "platinum group" of metals is: ruthenium, rhodium, palladium, osmium, Iridium and

05 platinum. The method is especially applicable to palladium and platinum, which are normally regarded, when in ore, as difficult to raise to an oxidation number at which they will form water-soluble compounds.

Although alluvial and other mineral deposits of platinum

10 group metals are the result of natural concentration processes involving weathering and gravity-separation phenomena, considerable additional treatment 1s required to yield a product suitable for marketing. After treatments which might typically include crushing, froth flotation and gravity separation, part of

15 the resultant material might be sent directly to the refinery. The remainder 1s concentrated further by smelting, oxygen blowing, magnetic separation, and pressure leaching (removing e.g. nickel which has been solubllised) to yield a final concentrate, a 60% residue of platinum group metals.

20 Most refining procedures take advantage of the ready solubility of platinum and palladium (and gold) in aqua regla, which does not dissolve the other platinum group metals (ruthenium, rhodium, osmium and 1r1d1um) and Indeed 1t is conventional wisdom that only highly oxidising substances of this

25 type will dissolve elemental palladium and platinum. (US Patent 4337226 suggests H ₂ S0 ₄ plus either NaOCl or H ₂ 0 ₂ .) Platinum 1s Indeed known for Its property of remaining unattacked by for

* example hot sulphur dioxide, and functioning for years as a catalyst for oxidising that gas at 600°C. It is therefore most

• 30 unexpected that sulphur-oxygen compounds can be used to dissolve values of platinum group metals.

According to the present invention, a method of dissolving platinum group metals from ores or concentrates comprises lixiviating the ore or concentrate in the presence of an oxldant with an aqueous solution containing a material being a compound or polyelemental anion having an atom in a lower oxidation state, and which material 1s stable in the presence of the oxidant and can co-ordinate a platinum group cation through the said atom. A "lower oxidation state" 1s any non-zero oxidation state lower than the highest normally attained, which in the case of for example sulphur is taken as 6, and arsenic 5. The counter ion of the anion if present may be an alkaline earth or alkali metal, conveniently sodium. The solution is preferably at least 10~ ⁴ M in said atom, preferably at least 10~ ³ M, most preferably at least 10~ ² M, and usefully at least 0.04M. It is preferably under 1M, more preferably under HM.

Examples of such materials are: As 0H)3 or salts thereof; organic sulphides and thio derivatives e.g. thioketones such as thiourea H ₂ NCSNH2; thlosulphate S ₂ θ3~; dithionate S ₂ 0 ₆ ~ ^" and polythionate (0 ₃ S-S _X -S03) ^" ", and the selenium and tellurium analogues (where they exist) of these sulphur materials. As lixiviants in extractive metallurgy go, some of these are not costly and some are relatively mild environmentally.

In the case of (for example) thlosulphate, it is postulated that because the thiosulphate ion is a soft donor (via sulphur), it effectively lowers the redox potential at which the metals are oxidized. An important cooperative effect in practice is its fast rate of oxidation of the metals, especially palladium and platinum, particularly in the presence of the appropriate ligands, to form coordination complexes under ambient conditions.

This discovery represents a novel way of extracting platinum group elements from their ores and concentrates by a simple leaching procedure using cheap and readily available reagents.

It 1s also the case that since bulk leaching operations are relatively inexpensive, lower grade ores than have been hitherto thought to be economic and deposits which have hitherto not been mined for their platinum group metals content could now be exploited, such as the low-grade deposits of Pt and Pd concentrated during weathering of mafic and ultramafic rocks su_ciι as in the Fifield, NSW area and in weathered sulphide ores

(gossans and the like) overlying primary sulphides 1n certain of the deposits of the West Australian shield.

Arsenic (III) species preferably 1n acid solution (more preferably pH 3-4) may be used especially for mobilizing palladium.

Preferably, means are employed to inactivate microbes such as Thiobacini which could mediate the concentrations of the lixiviants during the dissolution.

Preferably other llgating anions are additionally present such as hallde. Chloride 1s convenient, and bromide and iodide are better although in practice disproportionately more costly.

The oxidant which must be present 1s most conveniently oxygen in air at ambient temperature and pressure.

The duration of the lixiviation is preferably at least 7 days, more preferably at least 12 days.

The resulting solution of platinum group metal(s) 1s then normally separated from the residual solIds, and then treated to recover the metal(s).

The invention will now be decribed by way of example. EXAMPLE 1

A fixed mass of comminuted ore containing platinum and palladium was leached with a fixed volume of 0.001 molar aqueous sodium thiosulphate under recycling conditions at ambient temperatures (ca 20°C) 1n the presence of air. The pH of the solution was maintained at a fixed value above 6 (otherwise the thiosulphate would have decomposed) via the use of sodium carbonate Na ₂ C0 ₃ , sodium hydrogen carbonate NaHC0 ₃ , sodium hydroxide NaOH, lime CaO or Ca(0H) ₂ and/or calcium carbonate CaC0 ₃ . The rate of dissolution of platinum and palladium was monitored from time to time by taking samples and analysing for dissolved metals with the results below. When amounts in solution were judged to be sufficient, the solution was filtered to remove solids and the dissolved values recovered by (i) oxidation of the contained thiosulphate to produce a precipitate containing the values or (11) reduction of the values with an appropriate reducing agent e.g. zinc to precipitate the platinum group metals as a concentrate ready for further refining. Palladium Concentration

Thiosulphate In Solution concentration after 14 davs after 23 davs O.OOIM 8.9 35 ppm

0.01M 8.9 32 ppm 41 ppm

0.05M 8.5 44 ppm 56 ppm

0.1M 8.5 70 ppm 97 ppm (32 days)

EXAMPLE 2

Leaching is made more effective in certain instances for certain metals by addition of auxiliary ligands. Example 1 was repeated with the additional reagent sodium chloride present in solution at a concentration of 0.01 molar. Iridium was leached from the ore.

EXAMPLE 3

Palladium was leached from a sample of ore in a manner analogous to tha't described above, but using (instead of thiosulphate) sodium arsenite or arsenic (III) oxide dissolved in water (0.05 molar in arsenic in either case) and which was made acid to a fixed pH below 5 with hydrochloric acid, in the presence of air at ambient temperatures. At higher values of pH, arsenic (III) can form As(0H)4~, a hard ligand in which moreover the arsenic atom is "buried" and no longer available to ligate palladium. The HC1 both prevented this and provided Cl~ ions as auxiliary ligands.

After 15 days in each case, the palladium concentration in the solution was measured, and compared with the result when arsenic was omitted. pH held at ppm Pd using arsenic ppm Pd omitting arsenic

2 90 20

3 175 4 4 16 not detectable

5 5 not detectable