Johnson Matthey Technol. Rev., 2021, 65, (4), 574
Technological Capabilities of Hydrocarbonyl Processes in the Concentration and Separation of Platinum Group Metals
The principal possibility of processing the industrial poor collective concentrates of platinum group metals (pgms) using a hydrocarbonyl technology with the selective concentration of pgms from poor multicomponent chloride and chloride-sulfate solutions with the subsequent production of pure pgms is shown.
The pgms can be produced both from natural raw materials and treated pgm materials, including scrap metal and waste in the form of used platinum alloy products, electronic waste or coating materials. They may also be extracted from production induced sources, such as the byproducts of copper–nickel sulfide ore treatment. In all cases of pgm raw material treatment, production involves extracting pure pgms from multicomponent systems in which their content can vary considerably.
Extraction of pgms from natural and synthetic materials uses various technological processes, which include precipitation, extraction, sorption and electrochemical processes in various forms (1).
This study investigates fundamental technological possibilities of concentrating and separating pgms based on hydrocarbonyl processes that occur during the treatment of solutions of pgm chlorocomplexes with carbon monoxide under atmospheric pressure (Figure 1).
Carbon monoxide molecules have a high thermodynamic potential of a reducing agent. For example, for Reaction (i):
the ΔG° value amounts to −314.7 kJ. However, Reaction (i) does not occur spontaneously, due to high-energy chemical bonds between carbon and oxygen atoms. Nevertheless, in the presence of catalysts, carbon monoxide molecules are activated due to a change in the energy of bonding electrons, and the mixture of carbon monoxide with ambient oxygen gains chemical reactivity as per Reaction (i). For instance, when treating H2PdCl4 solution with carbon monoxide, the first stage is the insertion of a carbon monoxide molecule into the inner sphere of the PdCl4−2 complex, thereby generating carbonyl chloride complex as per Reaction (ii):
This is described by Reaction (v):
Reaction (vi) reflects the processes occurring in chloride pgm solutions during their treatment with a gas mixture containing carbon monoxide and depicts the nature of the ‘hydrocarbonyl process’ term (2).
All these reactions are preceded by reactions in which chloride complexes are converted into various carbonyl chloride complexes. The latter can be further subjected to inner sphere redox processes generating either metals (Reactions (vi), (ix), (xii), (xiii)) or carbonyl chloride complexes in which metals have low oxidation states (Reactions (vii), (viii), (x), (xi)).
The capacity of noble metals to enter a free state is defined by the following series: gold > palladium > platinum > rhodium (2).
This study explores the opportunities of applying hydrocarbonyl processes in the separation and concentration of pgms from multicomponent systems represented by the industrial lean bulk concentrates of pgms (copper and nickel anode sludges) and rich bulk concentrates of pgms (concentrates predominantly containing: (a) platinum and gold; (b) rhodium, ruthenium and silver; or (c) iridium).
The principles of hydrocarbonyl technology for the treatment of various pgm industrial concentrates as stated in (3), where copper–nickel anode sludges with the high contents of copper, nickel, iron, selenium and tellurium were used as primary products, have become a foundation for further feasibility studies of reprocessing lean and rich bulk pgm concentrates using hydrocarbonylation.
2. Experimental Section
Laboratory experiments on hydrocarbonyl processes were conducted using pgm chlorocomplex solutions generated from corresponding chloride reactants (standardised test solutions), chloride and chloride–sulfate solutions generated through the hydrochlorination of industrial products, such as anode sludges generated during the extraction of cathode copper and nickel, and pgm concentrates used in refining.
Carbon monoxide was obtained by treating hot sulfuric acid with formic acid. After collecting, carbon monoxide was stored in gasometers. The reactions took place in glass reactors equipped with mechanical stirrers. Moreover, analytical support was provided through atomic absorption and chemical analyses.
2.1 Treatment of Lean Bulk pgm Concentrates
|Component||Content, g l–1||Component||Content, g l–1|
|Component||Content, g l–1||Component||Content, mg l–1|
2.1.1 Treatment of Copper Anode Sludge Solution
Carbon monoxide was bubbled through 500 ml of a solution (see Table I) while stirring vigorously at 97–98°C and atmospheric pressure for 7 h. A black-coloured precipitate was obtained. The suspension was cooled down in carbon monoxide atmosphere and vacuum filtered. Thereafter, the obtained residue was washed with 2 M HCl. The obtained black precipitate was calcined at 900°C until a metal sponge was formed (C-1). This was accompanied by the distinctive odour of SeO2 and TeO2. The obtained C-1 was dissolved in aqua regia and analysed for the content of noble and non-ferrous metals. Upon the analysis of the obtained solution, the content of C-1 was found to be as presented in Table III.
|Component||Content, wt%||Component||Content, wt%|
After the isolation of C-1, the filtrate was extracted with isoamyl alcohol at room temperature while stirring for 15 min in five stages with an aqueous-to-organic phase ratio of 5:1. Both extraction phases as well as wash waters from C-1 were analysed for the contents of noble metals. Their breakdown by conversion products is shown in Table IV. Table IV indicates that the consecutive processes of hydrocarbonylation and liquid extraction result in reasonably full extraction of noble metals from the copper anode sludge solution into two products: sufficiently selective C-1 and organic phase (isoamylic extract).
|Metal||Content, % of the initial amount in copper anode sludge solution (see Table I) |
|C-1 a||Wash waters||Organic phase||Aqueous phase||Σ|
|Rh||103.90 b||not detected||1.15||not detected||105.02|
|Ru||not detected||7.63||73.00||not detected||80.63|
|Ir||not detected||not detected||115.20||not detected||115.20|
|Au||100.40||not detected||not detected||not detected||100.40|
|Ag||98.25||not detected||not detected||1.15||99.40|
To define the parameters of the hydrocarbonylation process where the coprecipitation of rhodium with palladium and platinum in C-1 is eliminated, a series of experiments on the hydrocarbonylation of standardised test solutions containing chlorocomplexes platinum(IV), palladium(II), rhodium(III), ruthenium(IV) and iridium(IV) were conducted at various temperatures and hydrochloric acid concentrations. Obtained precipitates and filtrates were analysed for pgm content, which allowed the rate of pgm extraction to be defined for each product. Results are given in Table V. The analysis of these results shows that the coprecipitation of rhodium with platinum and palladium during the hydrocarbonylation of chlorocomplex solutions platinum(IV), palladium(II) and rhodium(III) does not occur at HCl concentrations ≥ 2 mol l–1 and temperatures ≤ 80°C. These conditions were taken into account during the study of the treatment process of the nickel electrolyte sludge solution.
|No. of standardised test solutiona||HCl, mol l–1||pgm concentration in standardised test solution, mg l–1 ||pgm precipitation rate, % |
2.1.2 Treatment of Nickel Anode Sludge Solution
In a series of experiments, 0.5 l to 0.7 l of nickel anode sludge solution (see Table II) was treated with carbon monoxide under atmospheric pressure, temperature of 20–60°C and HCl concentration of 2.0 mol l–1 while stirring vigorously.
Obtained black-coloured finely dispersed precipitates were separated by filtration under vacuum and washed with 2 M HCl. These precipitates were analysed without calcination. Table VI lists the results of the analysis, which shows that reasonably a full separation of pgms into two selective products occurred: black fine precipitate containing platinum, palladium, gold, silver, selenium and tellurium; and solution containing rhodium, ruthenium, iridium and non-ferrous metals.
|Content, wt%||Item no. |
|1||2||3||4||5||Mean||pgm precipitation rate, %|
|Σ Pt, Pd, Au, Ag, Rh||82.28||82.84||82.73||81.71||84.13||82.74||–|
|Σ Se, Te||15.11||15.19||15.77||14.89||13.98||14.93||95|
|Σ Ni, Fe, Cu||0.15||0.18||0.32||0.26||0.22||0.23||–|
|Σ b total||97.54||98.21||98.82||96.86||98.33||97.95||–|
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.
|Content, mg l–1 a |
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).
|Product||Breakdown of metals byproducts regarding their content in the original solution (see Table VII), % |
|Pt||Pd||Au||Rh a||Ru b||Ir b|
|Black powder (precipitate after hydrocarbonylation)||~100||~100||~100||2.31||not detected||not detected|
|Organic phase of extraction||–||–||–||98.15||65.60||92.00|
a Rh coprecipitates due to the high temperature of hydrocarbonylation. Its presence can also be explained by the inadequate washing of the precipitate
b Not the full transition of Ru and Ir to the organic phase of extraction is consistent with their breakdown ratio (2)
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 (Tind.), 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 (PCO) on the induction period of a hydrocarbonyl process. As an example, we provide the results of the studies on the impact of PCO 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.
|CO content in CO + N2 mixture, vol%||100||70||50||33||16||8|
|Reaction (vi) induction period, Tind., s||58||62||65||67.5||70.5||135|
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.
3. Discussion of Results
The provided results of laboratory studies for the processes of the generation and breakdown of carbonyl pgm complexes, which take place when the solutions of their chlorocomplexes are treated with carbon monoxide under atmospheric pressure and cause pgm chlorocomplexes to turn into carbonyl chloride complexes with low oxidation state metals or to the full reduction of pgms, allow use of these processes to conduct selective pgm concentration from multicomponent solutions with the subsequent individual extraction of metals. This can become innovative in pgm hydrometallurgy.
A significant technological advantage of the hydrocarbonyl technology is the ability to separate original chloride and chloride–sulfate multicomponent solutions containing pgm in a wide concentration range (from tens of grams per litre to some milligrams per litre) and heavy non-ferrous metals in large amounts into two products. Platinum and palladium, along with gold, silver, selenium and tellurium, precipitate quite effectively, when rare pgms (rhodium, ruthenium and iridium) along with non-ferrous metals remain in the solution, transiting from their chlorocomplexes to carbonyl chloride anionic complexes where metals have low oxidation states: rhodium(I), ruthenium(II), iridium(I) with their specific properties, which makes the subsequent separation process easier.
Another advantage of the hydrocarbonylation process is that the separation of original multicomponent chloride solutions containing pgm into two products can be conducted in one step at the very beginning of the manufacturing process using only one reactant, i.e. cheap and available air-producer gas containing approximately 25% of carbon monoxide and 75% nitrogen in volume.
Moreover, several factors affect the results of the hydrocarbonyl processes; therefore, control possibilities for these processes can be extended to ensure that they occur as desired. This significantly distinguishes the hydrocarbonyl technology from all other technologies used in this field. Thus, it was demonstrated (2) for the Reaction (vi) that the palladium reduction rate has the following functional dependence on the component concentration, Equation (xiv):
where n = 0.5 at Pco≥ 30 kPa; n = 1.0 at Pco≤ 30 kPa.
By changing the contents of the original solution along with the carbon monoxide content in the reaction gas and temperature, one can significantly influence the rate of hydrocarbonyl processes and their final result. Thus, when treating the palladium dichloride solution with carbon monoxide in 10 M HCl at 50°C, Reaction (vi) does not go to completion, and its colour changes from red and brown (original solution) to yellow and green. When caesium chloride crystals are introduced to the solution, small yellow and green crystals comprising Cs[PdCOCl3] are generated (2).
The results of this study demonstrate the technological potential of hydrocarbonyl processes for the selective concentration of pgms from multicomponent chloride–sulfate solutions with the subsequent production of pure metals. Furthermore, hydrocarbonyl processes can be combined with existing technologies, such as precipitation in the form of complex salts, extraction and sorption, which expands their technological application. This is supported by the results of our latest studies (6–9). Hydrocarbonyl processes are also of interest in the chemistry of pgms (4).
A. Yu. Kotlyar, M. A. Meretukov and L. S. Strizhko, “Metallurgy of Noble Metals”, in 2 vols., Ore and Metals Publishing House, Moscow, Russia, 2005 LINK http://www.rudmet.com/catalog/book/109/?language=ru
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I. V. Fedoseev, V. V. Vasekin, M. V. Maramygina and N. V. Rovinskaya, ‘Method for the Selective Extraction of Rhodium RH, Ruthenium RU and Iridium IR from Hydrochloric Acid Solutions of Chlorine Complexes of Platinum PT (IV), Palladium PD (II), Gold AU (III), Silver AG (I), Rhodium RH (III), Ruthenium RU (IV) and Iridium IR (IV)’, Russian Patent Appl. 2020/119,883
I. V. Fedoseev, M. V. Maramygina, N. V. Rovinskaya and V. V. Vasekin, ‘Method of Extracting Platinum from an Industrial Salt of Ammonium Hexachloroplatinate’, Russian Patent Appl. 2020/2,711,762
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I. V. Fedoseev, V. V. Vasekin, M. V. Maramygina and N. V. Rovinskaya, ‘A Method of Metal Separation from Pt-Pd-Rh Alloys’, Russian Patent 2,693,285; 2019
Professor Igor V. l. Fedoseev is Doctor of Technical Sciences and a graduate of the Chemical Faculty of the Lomonosov Moscow State University, Russia. He previously worked at Norilsk Nickel, Russia. He was engaged in the study of carbonyl chloride complexes of platinum metals and defended his thesis and doctoral dissertation in chemistry and technology of carbonyl chloride complexes of pgms. His area of scientific interest is the chemistry of carbonyl complexes of pgms and their technological use. He is the author of two monographs.
Yuriy A. l. Kotlyar, PhD (Technical Sciences), graduated from the Moscow Machine Tool Institute, Russia, with a degree in machines and foundry technology. He worked as General Director of RAO Norilsk Nickel, General Director of JSC Gipronickel Institute, Chairman of the Board of Directors of RAO Norilsk Nickel, Chairman of the Bureau of the Coordination Council of the Union of Scientifiñ-Technical Society of Scientists, specialists in the production and processing of non-ferrous metals, gold and diamonds. He is a member of the Supervisory Board of the Golden Club of Russia, a lecturer at the Moscow Institute of Steel and Alloys, a State Prize Laureate and General Consultant of PJSC Norilsk Nickel.
Vasily V. Vasekin, PhD (Chemistry), General Director of Supermetal JSC, is a graduate of the Chemical Faculty of Lomonosov Moscow State University, Russia. He is engaged in metallurgy and chemistry of platinum alloys and materials based on them. He conducts research and develops technologies using composite materials based on platinum alloys in the production of glass-melting devices and catalytic systems.
Natalya V. Rovinskaya, PhD (Chemistry), Senior Researcher, is a graduate of the Lomonosov Moscow Institute of Fine Chemical Technology, Russia. She was engaged in the problems of synthesis of semi-synthetic antibiotics. She defended her thesis on the methods of synthesis of doxycycline. She is currently engaged in technological work on the separation of pgms from industrial wastes and their purification by hydrolytic methods and the development of methods for the analysis of pgms and their alloys.