RU2797800C1 - Rhodium refining method - Google Patents

Abstract

FIELD: hydrometallurgy.

SUBSTANCE: invention can be used in the refining of rhodium, in particular in the precipitation of rhodium from hydrochloric acid solutions containing impurities. Rhodium is deposited from hydrochloric acid solutions by reduction at the electrolyser cathode and the resulting rhodium powder is purified from impurities by thermochlorination. The deposition is carried out at a current density of 700-1200 A/m², and in the process of deposition, the polarity of the electrodes is periodically changed. The duration of direct polarity is 10-100 s, and the duration of reverse polarity is 5-10 s.

EFFECT: invention makes it possible to obtain metallic rhodium of increased purity.

1 cl, 1 tbl

Description

The invention relates to the field of hydrometallurgy of precious metals and can be used to obtain refined rhodium during the processing of PGM concentrates.

At the final stages of processing of said raw materials, after the extraction of platinum and palladium in the form of sparingly soluble salts, rhodium and iridium remain in the mother liquors, platinum and palladium, as well as base metals, are present in residual quantities. Obtaining pure rhodium from this intermediate product is difficult due to the similarity of the properties of platinum metals, primarily the similarity of the features of the complex formation of rhodium and iridium. Only after repeated precipitation of rhodium from intermediate technological solutions, transfer to the metallic state, repeated dissolution and precipitation in one way or another, it is possible to achieve the required purity of powdered rhodium.

The traditional technology of rhodium refining is based on the nitration of hydrochloric acid solutions. The purpose of nitration is the conversion of chloro complexes of platinum metals in solutions into nitro complexes. Slightly soluble compounds of rhodium and iridium can be precipitated from nitrite solutions with ammonium chloride, rhodium-iridium concentrate: ammonium-sodium hexanitrorhodiaate (III) and hexanitroiridate (III) (NH ₄ ) ₂ Na [Rh (NO ₂ ) ₆ ], (NH ₄ ) ₂ Na[Ir(NO ₂ ) ₆ ] (ANG), while platinum and palladium remain in solution. In addition, during nitration, the pH of the solution changes and it is possible to purify the solution from base metals: copper, iron, nickel, selenium, tellurium, antimony, lead, which are released in the form of hydrates. The resulting ANG salts are dissolved and sent for extraction purification from residual amounts of PGM, then rhodium is transferred to a powdered metal state by one method or another, and additionally subjected to high-temperature oxidative treatment.

Known methods for extracting rhodium from hydrochloric acid solutions using ion exchange (1. RJ VINITI, 15 "Metallurgy", 1995, 3, 3G176P; 2. RF 2479651 dated 2013.04.20). Such methods are highly operational and do not allow obtaining rhodium of the required purity.

There is a known method for the regeneration and purification of rhodium from solutions of rhodium in hydrochloric acid containing a large amount of base metals and a small amount of rhodium ( 3. Application of Japan 3-285029). Tin chloride is added to the rhodium solution, after which it is brought into contact with porous rubber impregnated with dialkyl sulfide. The porous rubber is washed with dilute hydrochloric acid to remove tin and other elements and calcined at 800 ^o C. The resulting rhodium metal is subjected to additional refining. Ultimately, rhodium is obtained in the form of a soluble sodium hexochlororhodiate compound. The disadvantages of the known method include laboriousness, high material and time costs, as well as environmental pollution. In addition, the known method is effective only when extracting rhodium from hydrochloric acid solutions containing rhodium in small quantities.

In another group of methods for the extraction of rhodium after the isolation of platinum with carbon monoxide, the solution is treated with alkali to a pH value of ≥8, which leads to the precipitation of rhodium in the form of a fine powder, which, after boiling, is separated from the solution ( 4. RF No. 2693285; 5. RF No. 2742994) . These methods are technologically very complex and involve the use of toxic gas.

Methods for extracting noble metals from a hydrochloric acid solution, including rhodium, are widespread, including electrochemical treatment of the solution and deposition of noble metals on a three-dimensional flow cathode made of graphite material ( 6. RF No. 2164554). The resulting rhodium powder is not a commercial product and requires additional refining.

A known method of purification and extraction of rhodium, selected by the prototype ( 7. RF No. 2199612 dated 02.07.2003) and including the deposition of rhodium from solutions of rhodium in hydrochloric acid containing impurities, on the cathode of an electrolytic cell with an insoluble anode by applying a direct electric current with a density of 25-400 A / m ² from a solution with a concentration of hydrochloric acid 1-6 mol / dm ³ . In the process of electrolysis on the cathode, iron (Fe), copper (Cu), iridium (Ir) and other impurities can be deposited together with rhodium, but the ratio of their concentrations in the cathode deposit is not proportional to the ratio of concentrations in the electrolyte. The amount of co-precipitated impurities depends on the parameters of electrolysis: the concentration of hydrochloric acid in the electrolyte and the current density. Practice shows that even with a small content of impurities in the electrolyte, it is impossible to exclude their co-deposition on the cathode. Characteristically, with an increase in the current density, the content of impurities in the cathode deposit increases. In this regard, the maximum allowable current density and the performance of the electrolysis bath are limited.

In practice, to achieve refining purity, the cathode precipitate is treated by thermochlorination, during which impurities interact with chlorine to form easily volatile chlorides and sublime into gaseous products when the temperature rises.

The technical problem to be solved by the present invention is the noticeable content of impurities in the cathode deposit during the electroextraction of rhodium from hydrochloric acid solutions, the non-compliance with GOST of the obtained refined rhodium and the low speed of the process.

The technical result is to obtain metallic rhodium of high purity by changing the conditions of cathodic deposition of rhodium.

The technical result is achieved by the deposition of rhodium from hydrochloric acid solutions by applying a direct electric current. Unlike the prototype, the deposition of rhodium is carried out at a current density of 700-1200 A/m ² , and during the deposition process, the polarity of the electrodes is periodically changed, while the duration of the direct polarity is 10–100 seconds, and the duration of the reverse polarity is 5–10 seconds.

Evidence of the determining influence of the distinctive features of the proposed method on the achievement of the technical result is a set of theoretical foundations and the results of special studies.

As noted above, the cathodic deposition of rhodium is carried out from an electrolyte in which the rhodium content is many times higher than the content of impurity metals. The results of targeted studies show that, under these conditions, the main reason for the presence of impurity metals in the powdery cathode deposit of rhodium is the accumulation of impurity ions in the near-electrode layer. In the liquid phase of the powdery precipitate of rhodium, the content of impurities becomes noticeably higher than in the electrolyte as a whole, and impurities are reduced, primarily platinoids, the satellites of rhodium. To minimize this negative process, continuous renewal of the liquid phase in the volume of the cathode deposit is required; mechanical mixing is technically not feasible. An effective technique for restructuring the cathode dispersed deposit in practice is the use of an asymmetric current. For this purpose, the polarity of the electrodes is changed for a short period of time. At the moment of polarity reversal, the content of impurity metals in the near-electrode region decreases. Characteristically, during anodic polarization, platinum and base metals freshly deposited together with rhodium are oxidized and go into solution, while rhodium remains in the cathode deposit. On the whole, the cathode deposit becomes more compact, and the capture of impurity ions is minimized.

The achievement of the above goals is influenced by the composition of the electrolyte, the cathode current density and the quantitative indicators of the polarization chronogram. For the system under study, the optimal duration of the direct pulse, during which recovery occurs on the working electrode, is 10–100 seconds, and the duration of the reverse polarity is 5–10 seconds. Experiments show that with this type of alternating current polarization, a high purity of the rhodium cathode deposit and an optimal final rate of the process are ensured. A shorter duration of reverse polarity reduces the efficiency of polarity reversal. Longer polarization with reverse polarity reduces the overall speed of the process.

The current density determines the speed of the process and the performance of the electrolysis bath. In the case of an insufficiently high current density, while maintaining the required purity of the cathode deposit, the process rate is low. At an excessively high current density, the electrolyte is intensively heated, and the current efficiency decreases.

Thus, the set of distinguishing features of the proposed method:

- periodic change of polarity of electrodes;

- the duration of the forward pulse is 10-100 seconds and the duration of the reverse pulse is 5-10 seconds;

- current density at the cathode 700-1200 A / m ² ,

provide a reduction in the content of impurities in the cathode deposit of rhodium and an increase in the performance of the cell.

An example of the implementation of the proposed method are the results of the following experiments.

The ANG salt released during the processing of PGM concentrates was dissolved in hydrochloric acid. The resulting solution was purified from impurities by extraction. Raffinate containing Ir - 40 mg/l, Pt - 5 mg/l, Pd - 7 mg/l, Ru - 3 mg/l and Rh - 55 g/l, as well as base metals in the amount of 18 mg/l were subjected to electroextraction . The electrolytic deposition of rhodium was carried out in a 1 dm ³ cell at room temperature. Polished titanium was used as the cathode, and rhodium-plated titanium was used as the anode. The duration of electrolysis is 1 hour. In the experiments, we varied the current density at the cathode, as well as the duration of the forward and reverse pulses.

Upon completion of the experiment, the cathode deposit was crushed, averaged, and analyzed for the content of impurities. The bulk of the cathode deposit was subjected to thermochlorination according to a known method. Additionally, the purity of refined rhodium was evaluated by spectral methods.

For comparison, the results of the experience according to the method of the prototype. In this case, electrolysis was carried out at direct current.

The results obtained are shown in table 1.

A comparative analysis of known technical solutions, including the method chosen as a prototype, and the proposed invention allows us to conclude that it is the totality of the claimed features that ensures the achievement of the perceived technical result. The implementation of the proposed technical solution by changing the polarity of the electrodes in the recommended modes allows you to increase the purity of the refined to 99.97%, and the speed of the process by 2-3 times in comparison with the analogue.

Table 1.

Influence of electrochemical deposition conditions

on the purity of rhodium and the speed of the process

Current density, A / m ² Duration of direct polarity, s Duration of reverse polarity, s Impurity content (total) in the cathode deposit, % Purity of rhodium after thermochlorination
% rhodium performance,
g/hour 1 500 5 3 0.32 99.92 4.75 2 700 10 5 0.27 99.95 6.83 3 1000 50 7 0.15 99.96 7.72 4 1200 100 10 0.13 99.97 8.46 5 1300 150 15 0.20 99.97 8.48 6 400 Prototype,
No polarity reversal 0.33 99.95 3.85 7 700 0.75 99.92 4.31

Claims (1)

The method of refining rhodium, including the deposition of rhodium from hydrochloric acid solutions by reduction on the cathode of the electrolyzer, the purification of the obtained rhodium powder from impurities by thermochlorination, characterized in that the deposition is carried out at a current density of 700-1200 A/m ² , and during the deposition process, the polarity of the electrodes is periodically changed, at In this case, the duration of direct polarity is 10-100 s, and the duration of reverse polarity is 5-10 s.