Process for the extraction of platinum and palladium from silver electrolytes

Abstract

The invention relates to the field of chemistry, in particular to the method of silver electrowinning in the production of high-purity silver and platinum and palladium. The proposed method of extracting platinum and palladium from silver electrolyte involves dissolution of raw materials, adsorption of platinum metal from silver electrolyte using anion exchanger, desorption with ammonia solution. The process of adsorption of platinum and palladium from silver electrolyte solution is carried out simultaneously with the process of silver electrowinning. Silver electrolyte is circulated through a fixed adsorbent layer at a rate of 1 CV / h to 15 CV / h, while saturation of the adsorbent is carried out until a specified critical value of palladium content in silver electrolyte is fixed. The method is allowed to be carried out in an automatic mode, and the concentration of palladium is photometrically determined, the specified critical value of palladium content in silver electrolyte is fixed at a value of 0 to 200 mg / l. Technical effect includes increasing the productivity of the process of obtaining high-purity cathode silver by increasing the current density of the electrolytic refining process and expanding the range of allowed platinum and palladium content in the anode feedstock.

Description

[0001] This invention relates to chemistry, and more particularly to a method for electrolytic refining of silver using an anode containing platinum and palladium, which can be used to produce high-purity silver as well as platinum and palladium.

[0002] Modern precious metal refineries face the need to comply with restrictions on the platinum and palladium content in the anode feedstock during the electrolytic refining of silver. The crux of the problem lies in the fact that during anodizing, platinum and palladium are partially transferred to the nitric acid solution (silver electrolyte) used in the silver electrolytic refining process. The reduction potentials of platinum, palladium, and silver are similar, leading to their combined recovery and the production of finished silver products containing platinum and palladium impurities. In addition to technical restrictions on the platinum and palladium content supplied to the silver anode for electrolytic refining, there are also limitations on current density in the silver electrolytic refining process.

[0003] Known methods include separating palladium from solution by treating nitric acid solution with complexing agents and / or precipitating agents.

[0004] The method for selecting palladium [1. Ginsburg. Guide to the chemical analysis of platinum metals and gold. –M.: –Nauka, 1995, p. 112] requires an excess of dimethylglyoxime to precipitate palladium. Specifically, 25 ml of a 1% alcoholic solution is required for every 10 mg of palladium. Co-precipitation of platinum at levels of 5% to 10% relative to the amount of palladium was observed. As a drawback, the high consumption of precipitant used to separate platinum and palladium should be noted.

[0005] The method for extracting palladium [2. WU, C., LIN, Y., JIANG, LJNucl. radiochem. 8, 3 (1986), p. 147] involves introducing sodium formaldehyde sulfoxylate into a palladium-containing nitric acid solution with an acidity of 0.01 M to 1.4 M, and then heating the mixture to 70°C to 80°C, which is nearly impossible under industrial production conditions.

[0006] The palladium extraction method [3.RU 2228380, 10.05.2004] involves precipitating metallic palladium from a nitric acid solution with carbon monoxide. Due to the use of toxic carbon monoxide, this operation requires sealed process equipment and high safety measures.

[0007] The methods listed for extracting platinum and palladium from silver electrolytes appear unsuitable for large-scale silver production via electrolytic refining, as they have common and significant drawbacks related to changes in the qualitative composition of the electrolyte due to the use of reagents soluble in the electrolyte, and require repeated operations for sludge separation.

[0008] A set of methods for extracting palladium from silver electrolytes by adsorption is also known.

[0009] Method [4.JP 3199392 20.01.1989] involves passing a silver electrolyte (Ag 90 g / L, Cu 1.4 g / L, Pd 0.16 g / L) through a stationary layer of activated carbon modified with amine chelate groups. The residual concentration of palladium is less than 1 mg / L. Desorption is carried out using nitric acid and hydrochloric acid. A disadvantage of this method is the relatively low peak concentration of palladium in the eluent: approximately 6 g / L, and the fact that the eluent contains up to 13 g / L of silver, thus requiring additional steps to separate the silver from the palladium during further processing of the eluent.

[0010] A method for preparing platinum metal compounds [5.DD 299876, 05 / 14 / 1992] involves performing ion exchange between an electrolyte and an adsorbent containing iminodiacetic acid groups. The electrolyte composition contains 190 g / L Ag, 35 g / L Cu, and 1.3 g / L platinum metal, wherein the solution acidity is in the pH range of 0.5 to 1.5. Desorption is performed with a 2% to 5% ammonia solution, followed by the addition of 1M to 3M sulfate or ammonium nitrate / sodium. As a disadvantage of this method, one can note the high salt background of the desorbate, the presence of a large amount of silver in the adsorbed form and its accumulation in the circulating desorbate, and the contamination of the electrolyte by sulfate ions.

[0011] The closest technically to the claimed invention is a method for extracting platinum metal [6.RU 2111272, 05 / 20 / 1998], in which a silver-containing material is dissolved in nitric acid and adsorbed onto an anion exchanger, said anion exchanger being a copolymer of vinylpyridine and divinylbenzene, and using a concentration of 3 mol / dm³. 3 Up to 8 mol / dm 3 The process involves desorption of an ammonia solution, followed by separation of a platinum metal concentrate from the resulting eluent. This method was chosen as a prototype.

[0012] The main drawback of the prototype method is that platinum and palladium are fully involved in the adsorption process. This is because platinum and palladium are quantitatively converted into a silver-containing nitric acid solution (silver electrolyte), which in turn limits the maximum content of PGM in the original silver-containing raw material. From the perspective of technical control, this situation also leads to practical technical difficulties in implementing the silver electrolyte adsorption purification method.

[0013] The purpose of this invention is to extract platinum and palladium from silver electrolytes during the electrolytic refining of silver.

[0014] The technical advantage of this invention is that it increases the productivity of the process for obtaining high-purity cathode silver by increasing the current density of the electrolytic refining process and expanding the range of permissible platinum and palladium contents in the anode raw materials.

[0015] The technical effect is achieved through the following fact: In the described method for extracting platinum and palladium from a silver electrolyte during the electrolytic production of silver, the process includes forming a silver electrolyte by dissolving the raw material in a nitric acid solution, adsorbing and extracting platinum metal from the silver electrolyte using an anion exchanger, and desorbing with an ammonia solution. According to the invention, the process of adsorbing and extracting platinum metal from the silver electrolyte solution is carried out simultaneously with the silver electrolytic refining process. The silver electrolyte is circulated through a fixed adsorbent layer at a rate of 1 CV / h to 15 CV / h, while the adsorbent is saturated until a specified critical value of palladium content in the silver electrolyte is fixed.

[0016] The adsorbent is saturated until the specified critical value of palladium content in the silver electrolyte is fixed at a value between 0 mg / L and 200 mg / L.

[0017] The specified critical value for palladium content in the fixed silver electrolyte was determined by spectrophotometric measurement.

[0018] The adsorption purification of palladium and platinum impurities in the electrolyte, combined with the use of a selective ion exchanger, makes it possible to avoid co-precipitation of palladium and platinum impurities with silver on the cathode during electrolytic refining, as critical concentrations are not reached at selected current intensities in the electrolyte. Furthermore, compared to complete dissolution of the silver-containing material according to the prototype, up to 80% of palladium and up to 97% of platinum are removed to the sludge without entering the electrolyte, and no adsorbent separation is required. The permissible current density of the electrolytic refining process depends directly on the degree of purification of the silver electrolyte, which in turn allows for improved electrolytic refining performance and thus reduces the refining time for silver, palladium, and platinum. The possibility of using silver-containing feedstocks with high platinum and palladium contents leads to a significant reduction in reagent consumption in the process prior to anode feedstock production. Compared to electroextraction processes with similar productivity, operating the electrolytic refining process to produce cathode silver reduces the one-time requirement for adsorbents. Simultaneously, it ensures the high purity of the resulting product. Furthermore, the adsorption and extraction of platinum and palladium from silver electrolytes makes it possible to avoid the need to partially extract the electrolyte when the critical concentration of platinum or palladium is reached, to process the extracted electrolyte, and to prepare the electrolyte and add it to the electrolytic refining system to replace the extracted electrolyte.

[0019] The continuous use of the process for extracting platinum metal from silver electrolytes, combined with the required performance of the adsorption equipment, ensures the continuous maintenance of subcritical concentrations of platinum and palladium in the cathode space. This allows for a wider range of permissible platinum and palladium contents in the anode feedstock, including increases in current density values. Continuous monitoring of palladium concentration at wavelengths of 380 nm and 510 nm using a flow spectrophotometer, and setting of critical concentration parameters for switching columns and the output of the saturated column for desorption, enable the possibility of continuous automation. The required capacity of the adsorption equipment is selected based on a standard circulation rate of silver electrolyte through a fixed adsorbent layer ranging from 1 CV / h to 15 CV / h, where CV is column volume and h is hours. Column volume is understood as the volume of adsorbent involved in the technical phase / operation.

[0020] Example 1. Key parameters of an electrolytic refining process for purifying platinum and palladium without adsorption from silver electrolyte.

[0021] Under conditions of long-term electrolytic refining operation and variability in the composition of anode feedstock, the time dependence of palladium accumulation in silver electrolyte was established by repeated measurement and averaging of relevant indicators. Figure 1 These dependencies are shown under constant current density and different palladium contents in the anode feedstock. The horizontal dashed line indicates the critical value of palladium content in the silver electrolyte. In textbooks used by students in higher education institutions [7. I.N. Maslenitsky, L.V. Chugaev, V.F. Borbat, et al., Metallurgy of noble metals, M.: Metallurgy, 1987, p. 432], the critical range of palladium concentration in silver electrolyte is 100 mg / L to 200 mg / L, and excess requires removal of the electrolyte for processing.

[0022] The aforementioned dependence indicates that the higher the accumulation rate of palladium in the electrolyte, the higher the palladium content in the initial negative electrode feedstock. Exceeding the critical palladium content in the electrolyte leads to defects in the cathode silver. Such silver is then sent for re-electrolysis.

[0023] Regarding the value of current density, the critical palladium concentration in the electrolyte is related to the mode of electrolytic refining process. Current density determines the productivity of the process and is inversely proportional to the critical palladium concentration in the electrolyte.

[0024] The re-electrolysis of defective cathode silver, the frequent processing of the extracted electrolyte, and the preparation of fresh electrolyte to replace the extracted electrolyte adversely affect the economic efficiency and productivity of the electrolytic refining process.

[0025] Currently, the most effective way to organize the electrolytic refining process is to technically limit the content and density values ​​of palladium and platinum in the anode feedstock. This limitation minimizes the risk of obtaining defective cathode silver and reduces the volume and frequency of electrolyte replacement to an acceptable minimum. Specifically, at 100-300 A / m... 2 At the given current density, the permissible palladium content in the anode material shall not exceed a fraction of one tenth by mass (see Table 1).

[0026] Example 2. Selecting the optimal operating mode for the adsorbent

[0027] The situation changes entirely when the electrolytic refining process is carried out simultaneously with the adsorption purification of platinum and palladium from the silver electrolyte used in the electrolytic refining. To achieve the desired technical effect, the optimal operating mode of the adsorbent needs to be selected. The importance of choosing the mode of the electrolytic refining process in terms of selecting the current density and introducing constraints on the composition of the anode feedstock under the conditions of parallel electrolyte adsorption purification is reduced to a background issue, as adsorption units with arbitrary excess capacity can be established.

[0028] At a current density of 400 A / m 2 Furthermore, during the electrolytic refining process where the palladium content in the anode feedstock is 4.9% by mass, the supply rate of silver electrolyte used for adsorption purification varies. The residual concentration of palladium in the purified electrolyte was estimated using inductively coupled plasma atomic emission spectrometry (ICP-AES). Figure 2 The corresponding correlations are shown. Here, CV represents the column volume, and h represents hours. The column volume corresponds to the volume of adsorbent involved in the technical stage / operation.

[0029] Quantitative recovery of palladium was observed in the range of 1 CV / h to 8 CV / h. Quantitative recovery refers to the residual concentration of palladium being below the sensitivity threshold of the analytical method. In the range of 8 CV / h to 15 CV / h, the Pd content in the silver electrolyte was observed to be no more than 30 mg / L to 35 mg / L, significantly lower than the critical value described in Example 1.

[0030] The practice of combining adsorption and electrolytic refining processes simultaneously allows for a range of palladium content in the anode feedstock composition to be expanded by at least 10 times, and a current density to be increased by at least 3 times. In some cases, the total platinum and palladium content in the anode can reach 10%, and current densities can be as high as 650 A / m. 2 Up to 700A / m 2 High-purity cathode silver can be obtained at current densities of [specific values] (see Table 1).

[0031] The boundaries of the range of circulation rates of the selected silver electrolyte through the fixed adsorbent layer are explained below. At rates below 1 CV / h, the depth of palladium recovery is maintained; however, the volume of adsorbent involved in the process becomes excessive, and its operation is economically unfeasible. At circulation rates above 15 CV / h, incomplete palladium extraction is observed, which may lead to the risk of obtaining defective cathode silver under high-density conditions during electrolytic refining, and the pressure in the working volume of the adsorption unit also increases significantly. Therefore, the volume of adsorbent used and the performance of the adsorption unit are selected based on the composition of the anode feedstock and the performance of the electrolytic refining process. Table 1 thus shows the average values ​​of the allowable current density for obtaining high-purity cathode silver and the palladium content in the anode feedstock for adsorption purification processes with and without silver electrolyte.

[0032] Table 1. Current density (j) in the anolyte used to obtain cathode silver corresponding to CpA-1 grade according to GOST 28595-2015 阳极 ) and palladium content values

[0033] Adsorption <![CDATA[j 阳极 ,A / m 2 ]]> C(Pd), mass% Silver grade in geochemistry no 150 0.30 СрΑ-1 no 300 0.15 СрΑ-1 yes 400 6.0 СрΑ-1 yes 600 4.0 СрΑ-1

[0034] Example 3. Complete Adsorption Cycle

[0035] Before use, the adsorbent (a copolymer of vinylpyridine and divinylbenzene) was placed in four adsorption columns. For each column, the adsorbent was transferred to the working state by passing a 1% to 5% (by mass) nitric acid solution upwards. The solution feed rate was 10 CV / h; the operation duration was 2 hours.

[0036] Then, under dynamic conditions, the treated solution is passed through a fixed adsorbent layer at a rate of 1 CV / h to 15 CV / h to extract palladium and platinum from the silver electrolyte by flowing downward through three columns with adsorbents. A fourth column is reserved.

[0037] The electrolyte is supplied to the adsorption unit by withdrawing solution from the circulation tank of the silver electrolytic refining unit during the electrolytic refining process. The return of the electrolyte, obtained as a result of the adsorption extraction of platinum and palladium, is carried out in the circulation tank of the electrolytic refining unit. The duration of this operation depends on the currently implemented solution supply rate and the initial concentration of palladium. Adsorbent saturation is carried out until a specified critical value of palladium content in the silver electrolyte is recorded in the range of 0 to 200 mg / L.

[0038] In the next stage, the adsorption unit is switched back: the most saturated column is removed from the adsorption unit, and a spare column is connected to it. The electrolyte adsorption purification is paused for the time required for the switching step. The switching time should not exceed 3 minutes. At the end of the switching, the adsorption mode is resumed, and the column with saturated adsorbent removed from the device is desorbed and then transferred to a spare column.

[0039] Desorption involves a series of sequential technical steps: 1- Washing the saturated adsorbent with downward-flowing water (10 CV / h, 1 hour), replacing the electrolyte in the circulating tank of the silver electrolytic refining unit, and collecting the wash water in the corresponding tank; 2- Washing the saturated adsorbent with downward-flowing water at a concentration of 3 mol / dm³. 3 Up to 8 mol / dm 3 The saturated adsorbent was washed with an ammonia solution (2 CV / h, 2 h), and the resulting desorbate was collected in a suitable container; 3- The adsorbent was washed with an upward flow of a nitric acid solution with a concentration of 1% to 5% (mass) (10 CV / h, 2 h), and the used regeneration solution was collected in a suitable container.

[0040] Table 2 lists the composition of the solutions involved in the adsorption cycle.

[0041] Table 2. Solution composition of the silver electrolyte adsorption purification cycle

[0042]

[0043] Importantly, it should be noted that the examples focus primarily on the concentration of palladium in the silver electrolyte, while the claims and specification also refer to platinum. This is because the extent to which platinum is transported to the silver electrolyte is very small, and despite some differences in chemical properties, it is co-extracted with palladium very extensively.

[0044] The described operation can be performed in both manual and automatic modes of the equipment. In automatic mode, the flow photometer plays a crucial role, continuously measuring the palladium content in the electrolyte at wavelengths of 380 nm and 510 nm. Based on the photometric data, exceeding a critical value for the palladium content in the electrolyte allows for the initiation of a switchback of the adsorption unit and the desorption of the saturated adsorbent without human intervention. Integrating the photometer's analytical signal for 1 to 2 minutes stabilizes the measurement results and minimizes the risk of premature switchback and desorption of the adsorbent. Using the flow photometer is also useful when operating the adsorption equipment in manual mode, as it significantly improves the efficiency of electrolyte composition estimation without incurring operational costs.

Claims

1. A method for extracting platinum and palladium from a silver electrolyte during the electrolytic production of silver, the method comprising dissolving raw materials in a nitric acid solution to form a silver electrolyte, adsorbing and extracting platinum metal from the silver electrolyte using an anion exchanger, and desorbing the adsorbent, characterized in that, By circulating the formed silver electrolyte through the adsorbent layer and returning the electrolyte obtained as a result of platinum metal extraction from the electrolyte to the silver electrolytic refining process, the process of extracting platinum metal from the silver electrolyte solution and the electrolytic refining process are carried out in parallel. The silver electrolyte is circulated through the adsorbent layer at a rate of 1 column volume / hour to 15 column volumes / hour, where the column volume is the volume of the adsorbent involved in the technical stage / operation, and the adsorbent is saturated until a specified value of palladium content in the silver electrolyte is fixed.

2. The method according to claim 1, characterized in that, The adsorbent layer is fixed.

3. The method according to claim 1, characterized in that, The adsorbent is saturated until the specified value of palladium content in the silver electrolyte is fixed in the range of 0 mg / l to 200 mg / l.

4. The method according to claim 1, characterized in that, The specified value of palladium content in the silver electrolyte is determined by photometric measurement.

5. The method according to claim 1, characterized in that, A copolymer of vinylpyridine and divinylbenzene is used as an adsorbent.

6. The method according to claim 1, characterized in that, Desorption was performed using an ammonia solution.

7. The method according to claim 1, characterized in that, Desorption occurs after the column containing the adsorbent is removed during the process of circulating the silver electrolyte through the adsorbent layer.

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