2.1 Treatment of Lean Bulk pgm Concentrates

For this experiment, chloride and the chloride–sulfate solutions of industrial copper and nickel anode sludges were used. Their contents are listed in Tables I and II.

2.2 Treatment of Industrial pgm Rich Bulk Concentrates

Section 2.1 demonstrated the capability of extracting and concentrating pgms from the solutions of their lean bulk concentrates, for example copper and nickel anode sludges. At the same time, the capability of using hydrocarbonylation to treat the rich bulk concentrates of pgms is also of great technological interest. These concentrates are represented by, in particular, three types (pgm concentrates predominantly containing: (a) platinum and palladium, M-1; (b) rhodium, ruthenium and silver, M-2; (c) iridium, M-3; and (d) speiss alloy). At present, the refinement of such concentrates is conducted under separate processes (1).

To conduct an experimental treatment through hydrocarbonylation, a total chloride solution obtained by the hydrochloration of concentrates M-1, M-2 and M-3 at the ratio of 4:2:1 was used. During the hydrochloration process of the concentrate mixture, silver was removed as AgCl precipitate. Thereafter, from the obtained solution, gold was extracted by treating it with Fe₂(SO₄)₃ solution. Table VII describes the content of the obtained solution. Within the experiment, carbon monoxide was bubbled through 20 ml of the original solution (see Table VII) under atmospheric pressure and temperature of 70°C while stirring vigorously for 3.33 h.

The solution was observed to acquire black colour as a result of the generation of finely dispersed powders of palladium and platinum, which were easily filtered out with a vacuum filter. The filtrate was red and brown. The obtained precipitate was washed with 2 M HCl, dried and weighed. 0.5515 g of black-coloured powder was generated. The precipitate possessed large specific surface area and high absorbing capacity, thereby required thorough washing.

The analysis of the powder and filtrate for the content of palladium, platinum, gold, rhodium, ruthenium and iridium showed almost full precipitation of palladium, platinum and gold (none of the elements were found in the filtrate), no ruthenium and iridium were found in the powder, and the extraction rate of rhodium from the solution amounted to approximately 1%.

It is worth noting that platinum from the solution is not recovered as metal, but as its oligomeric dicarbonyl [Pt(CO)₂]_n, where n is divisible by three. Palladium is extracted in its native form, together with its amorphous carbon phase (4).

In another experiment, 50 ml of the original solution (see Table VII) was treated with carbon monoxide under atmospheric pressure and temperature of 95°C for 4.5 h while stirring vigorously. Finely dispersed black powder was produced. After filtering, washing with 2 M HCl and air drying, 1.3501 g of a black powder were recovered, which was analysed for the content of noble metals (Table VIII).

The filtrate, which was yellow and green, was extracted twice with 10 ml of isoamyl alcohol. The organic phase coloured in yellowish was boiled dry without calcination. The boiled organic phase was analysed for the content of noble metals (Table VIII).

The results of the laboratory treatment of the mixture of concentrates M-1, M-2 and M-3 show that the hydrocarbonylation process can be successfully used at the primary stage of refinement, and this allows to separate rhodium, ruthenium and iridium from other noble metals and makes the subsequent extraction of platinum and palladium, as well as rhodium, ruthenium and iridium easier.

To confirm the possibility of extracting rhodium, ruthenium and iridium using the traditional technology of the nitration of their carbonyl chloride solutions, which are left after extracting the concentrate of platinum, palladium, gold, tellurium and selenium by hydrocarbonylation, a process solution of speiss with the following content (g l^–1) was used: platinum, 20.02; palladium, 53.00; rhodium, 6.50; ruthenium, 0.50; iridium, 2.70; gold, 0.82; tellurium, 3.00; selenium, 2.30; copper, 16.40; lead, 3.70; bismuth, 3.85; nickel, 2.90; iron, 1.55; HCl, 100.00. Carbon monoxide was bubbled through 500 ml of the specified solution at 60°C under atmospheric pressure for 4 h while stirring vigorously. This generated the precipitate of Σ platinum, palladium, gold, selenium, tellurium, which was washed with 2 M HCl after the separation. The precipitate and filtrate were analysed for the content of original components. It was concluded that gold, palladium, selenium and tellurium precipitated fully whereas platinum precipitated by 98%. The solution after the precipitation of Σ platinum, palladium, gold, selenium, tellurium was nitrated with the precipitation of rhodium in the form of its ammonium–sodium hexanitrate: (NH₄)₂NaRh(NO₂)₆.

Thus, the compatibility of hydrocarbonylation and the subsequent processes of the traditional saline extraction technology for rhodium, ruthenium, iridium (1) has been shown.

2.3 Sorptive Extraction of Rhodium, Ruthenium and Iridium

Technologies for the extraction of pgms from their chlorocomplexes utilise sorption on ion exchange resins (1). Thus, exploring the opportunity of using sorption for the solutions of carbon chloride anionic complexes Rh(I)–Rh(CO)₂Cl₂⁻, Ru(II)–Ru(CO)₂Cl₄⁻² and Ir(I)–Ir(CO)₂Cl₂⁻ to extract rhodium, ruthenium, iridium was appropriate.

To this extent, we used a solution of chlorocomplexes RhCl₆⁻³, RuCl₆⁻² and IrCl₆⁻² containing (mg l^–1): rhodium, 260.3; ruthenium, 83.0; iridium, 63.8 and HCl, 2 mol l^–1, and a solution of carbonyl chloride anionic complexes rhodium(I), ruthenium(II) and iridium(I), obtained by treating a similar solution of chlorocomplexes RhCl₆⁻³, RuCl₆⁻² and IrCl₆⁻² with carbon monoxide at 80°C for 2 h, which led the solution to change its colour from red and brown to yellowish, which is a distinct feature of carbonyl chloride anions rhodium(I), ruthenium(II) and iridium(I).

Both solutions were engaged with gel anion exchange resin based on the copolymer of styrene and divinylbenzene with benzene–pyridinium functional groups (ammonium molybdophosphate (AMP)) while stirring for 2 h. The solutions were then analysed for the content of rhodium, ruthenium and iridium. Results for the observed sorption rate are given in Table IX. The results given in Table IX show that the processes of hydrocarbonylation and sorption are sufficiently technologically compatible.

2.4 Selective Extraction of Rhodium, Ruthenium and Iridium

Given the specific properties of carbonyl chloride complexes rhodium(I), ruthenium(II), iridium(I) (2), it was of interest to obtain the experimental results of their reaction with hydrogen under atmospheric pressure, as it is known that chlorocomplex solutions rhodium(III), ruthenium(IV), iridium(IV) only react with hydrogen at high pressure (5). For this experiment, a standardised test solution of chlorocomplexes rhodium(III), ruthenium(IV), iridium(IV) in 2 M HCl with the following metal concentration (mg l^–1): 168.0; 119.0; 70.0, respectively, was treated with carbon monoxide under atmospheric pressure and temperature of 95°C for 2 h. The solution was observed to quickly lose its original red and brown colour and turned a yellowish colour, which is a characteristic of carbonyl chloride complexes rhodium(I), ruthenium(II), iridium(I). Thereafter, at the same conditions, hydrogen was fed to the reactor instead of carbon monoxide. This provoked the solution to gain black colouration and precipitate. The hydrogen treatment lasted 6 h. The obtained precipitate was filtered out and dissolved in the mixture of HCl and H₂O₂. The atomic absorption analysis of the precipitate and filtrate has shown that the resulting black precipitate comprised rhodium without ruthenium and iridium content, and that the filtrate contained almost no rhodium. The filtrate was neutralised to pH 5 and treated with hydrogen under the same conditions. The solution gained black colour and released a black precipitate. The hydrogen treatment lasted 6 h. The black precipitate was filtered out and dissolved in the mixture of HCl and H₂O₂. The atomic absorption analysis of this solution showed that it contained ruthenium only, whereas the analysis of the solution after filtering out black ruthenium showed that there was almost no ruthenium in it. The yellowish filtrate was treated with chlorine gas, which produced intense red colouration characteristic for chlorocomplex iridium(IV). NH₄Cl in the form of saturated aqueous solution was added to this solution. This resulted in the black precipitation, characteristic for (NH₄)₂IrCl₆. The above-precipitate solution analysis showed trace amounts of iridium.

The described experiment has shaped a foundation for conducting a series of experiments aimed to optimise pgm reduction processes, which allowed to formulate the method for selective extraction of rhodium, ruthenium and iridium from rhodium(I), ruthenium(II) and iridium(I) carbonyl chloride solutions by treating them with hydrogen under atmospheric pressure (6).

2.5 Impact of Carbon Monoxide Content in the Gas Mixture on pgm Reduction Rate

Reactions (vi)–(xiii) have some induction period (T_ind.), i.e. time from the beginning of the treatment of the pgm chloride solution with carbon monoxide until black pgm precipitation or change in original colour. The induction period of the hydrocarbonylation is impacted by temperature, stirring speed, partial pressure of carbon monoxide and H⁺ and Cl⁻ ion concentration (2).

Studies were conducted to identify the impact of the carbon monoxide content in the reaction gas (P_CO) on the induction period of a hydrocarbonyl process. As an example, we provide the results of the studies on the impact of P_CO on the induction period of Reaction (vi) (Table X). The original H₂PdCl₄ solution contents were as follows: [Pd(II)], 167 mg l^–1; HCl, 2 mol l^–1; solution volume, 75 ml; CO + N₂ mixture bubbling rate, 100 ml min^–1; stirring speed, 1000 rpm; temperature, 50°C. The length of the induction period of Reaction (vi) was calculated based on the automatic logging of changes in the light transmission value of the solution.

The experimental data provided in Table X indicate that air-producer gas can be used for conducting hydrocarbonyl processes in technological operations, as the significant decrease in the hydrocarbonylation rate is observed when carbon monoxide content in the CO + N₂ mixture does not exceed 20% whereas air-producer gas contains 25% of carbon monoxide.