Precipitation of Precious Metals Concentrates from Post-Elution Solutions from Ion-Exchange Processes

: Precious metals have long been considered as critical raw materials in many countries. There is a growing emphasis on recovering these metals from secondary sources such as automotive catalysts or WEEE (waste of electrical and electronic equipment). During the leaching process of these materials, solutions with low concentrations of precious metals are obtained, which necessitates the use of ion-exchange methods. Following sorption and elution, a post-elution solution called eluate is produced, containing precious metals and no impurities. This eluate must undergo further processing to obtain pure metals or its compounds. The objective of this study was to explore the feasibility of recovering precious metals from post-elution solutions through cementation, reduction, precipitation, or refining techniques. The analysis of the research results indicated that metallic zinc powder is the most effective cementing agent for platinum, palladium, rhodium, and gold. Metallic aluminum and copper powders can selectively cement gold and palladium, separating them from platinum and rhodium. Aqueous hydrazine hydrate solution is the best-reducing agent for precious metals, while an aqueous hydrogen peroxide solution can selectively reduce platinum and palladium, separating them from gold and rhodium.


Introduction
Precious metals, including platinum, palladium, rhodium, and gold, have been recognized by the European Union as critical materials for many years; therefore, the recovery of these metals is very important for the sustainable development of the European and global economy [1][2][3][4][5].Nowadays, technologies for obtaining precious metals increasingly use waste as base raw materials.This is caused by decreasing resources of primary raw materials and their centralized occurrence in South Africa, Russia, and China [6][7][8][9][10][11]. Automotive catalysts or WEEE (waste of electrical and electronic equipment) are the most often used secondary raw materials to recycle and recover precious metals [11][12][13].The main part of every recovery technology is leaching, which dictates the future steps of the process.In terms of precious metals, solutions of chlorides are used, especially for multi-component materials, due to the high efficiency of leaching of platinum, palladium, rhodium, and gold [14][15][16][17][18].The solutions obtained after the leaching of waste materials contain low concentrations of precious metals (<100 mg/dm 3 ) and often high concentrations of other metals (e.g., Cu, Zn, Fe, Co, Ni, Al) [19].In such cases, the use of ion-exchange methods may be the only economic and technologically viable solution for the selective recovery of precious metals, when compared to other techniques like precipitation or solvent extraction, which use solutions with high concentrations of precious metals (often >1 g/dm 3 ) [20][21][22].
Ion-exchange consists of three stages, between which, a resin is washed with water: Minerals 2024, 14, 625 2 of 18 -Conditioning-performed only at the beginning of the bed operation; this process is replaced by regeneration at a later time; -Sorption; -Elution/regeneration.
One possible method for recovering precious metals from eluates is cementation.Cementation, or contact replacement, is the process of replacing (displacing) metals from their salt solutions with other metals having a lower standard electrode potential.It is a process that is very often used in metallurgy to refine solutions.It occurs at a solid metallic interface (cementing agent), resulting in the reduction of ions to their zero-valence state, which can be described by the following equation: PM n+  (aq) + CA (s) → PM (s) + CA n+   (aq)   where PM refers to precious metal and CA refers to the cementing agent.
Therefore, the aim of this research is to determine the possibility of recovery and/or separation of platinum, palladium, rhodium, and gold, contained in the post-elution solutions by precipitating them in the form of concentrates and/or compounds, which would help assess the possibility of implementation of this process in the industry.An attempt was also made to check if a refining process applies to this type of solution.A solution containing thiourea in hydrochloric acid, obtained during the elution of precious metals from three different commercial resins, was used during the experiments.To our knowledge, such extensive experiments on processing post-elution solutions containing precious metals have not been researched and published to date.
The names "metallic powder" and "dust" were used interchangeably.

Methods
The scheme of the research process is presented in Figure 1.The publication covers the steps in red brackets.

Preparation of the Eluate
A mixture of 2 mol/dm 3 of thiourea in 1 mol/dm 3 of HCl was used for the elution process.To obtain a solution with a significant concentration of precious metals, post-sorption resins from the previous works were used [43,44].The prepared eluent was mixed with the  The publication covers the steps in red brackets.

Preparation of the Eluate
A mixture of 2 mol/dm 3 of thiourea in 1 mol/dm 3 of HCl was used for the elution process.To obtain a solution with a significant concentration of precious metals, postsorption resins from the previous works were used [43,44].The prepared eluent was mixed with the resins using a ratio of V r :V e = 1:10 (V r -resin volume, V e -eluent volume) for 1 h at room temperature.Subsequently, the solution was filtered.This process was repeated for three ion-exchange resins (Puromet MTS9200, Puromet MTS9850, and Lewatit MonoPlus MP 600) and the obtained solutions were combined.A total volume of 13 dm 3 of postelution solution consisting of 2 mol/dm 3 of thiourea in 1 mol/dm 3 of HCl was obtained, with the composition shown in Table 2.The pH of the solution was 0.16.Tests were conducted to determine the possibility of cementing precious metals from the post-elution solution using various cementing agents.Metallic zinc powder (industrial and commercial), metallic magnesium powder, metallic aluminum powder, metallic copper powder, metallic nickel powder, and metallic cobalt powder were selected for testing.For this purpose, a measured amount of eluate (25 cm 3 ) was mixed with a specific amount of cementing agent (0.10 g; 0.25 g; 0.50 g; 0.75 g; 1.00 g; 1.50 g) for 1 h, at room temperature.After the given time, the samples were filtered, the pH values of the solutions were measured, and then they were analyzed for the contents of Pt, Pd, Rh, and Au.The solid precipitates were weighed while wet and then again after drying at 100 • C.

Reduction and Precipitation Experiments
Subsequent experiments were performed to determine the possibility of reducing or precipitating precious metals from the post-elution solution.A 40% aqueous hydrazine hydrate solution, 80% aqueous formic acid solution, 25% aqueous ammonia solution, oxalic acid, and 30% aqueous hydrogen peroxide solution were used in the tests.A measured amount of eluate (25 cm 3 ) was mixed with a specific amount of reducing agent (when using solutions-2.5cm 3 ; 3.5 cm 3 ; 5.0 cm 3 ; 7.5 cm 3 ; 10.0 cm 3 ; 15.0 cm 3 ; when using solid materials-0.5 g; 1.5 g; 3.5 g; 5.0 g; 7.5 g; 10.0 g) for 1 h, at room temperature.After the given time, the samples were filtered, the pHs of the solutions were measured, and then they were analyzed for the contents of Pt, Pd, Rh, and Au.The solid precipitates were weighed while wet and then again after drying at 100 • C. While using formic acid and oxalic acid solutions, the pH was corrected during the process with solid NaOH.

Refining Experiments
Using the post-elution solution, the precious metal refining test was carried out according to the methodology developed by the Łukasiewicz Research Network-Institute of Non-Ferrous Metals, Centre of Hydroelectrometallurgy.The process consisted of the following steps:

•
Concentration of the solution-the eluate was concentrated tenfold to increase the concentration of precious metals.A total of 1000 cm 3 of the concentrated solution was obtained, from which a sample of 20 cm 3 was taken, which was then analyzed for the contents of Pt, Pd, Rh, and Au; • Au extraction-980 cm 3 of the solution was mixed with 500 cm 3 of a 50% Carbitol solution in toluene (v/v) at a temperature of about 70 • C for 1 h.After the extraction process, the phases were separated.The volumes of the organic phase (485 cm 3 ) and the aqueous phase (970 cm 3 ) were measured.A sample of 20 cm 3 was taken from the aqueous phase, which was then analyzed for the contents of Pt, Pd, Rh, and Au; • Pd extraction-the aqueous phase from the previous step (950 cm 3 ) was mixed with 500 cm 3 of 50% di-n-octyl sulfide solution in toluene (v/v) at room temperature for 1 h.
After the palladium extraction process, the phases were separated.The volumes of the organic phase (500 cm 3 ) and the aqueous phase (945 cm 3 ) were measured.A sample of 20 cm 3 was taken from the aqueous phase, which was then analyzed for the contents of Pt, Pd, Rh, and Au.The organic phase was then used in the Pd re-extraction process;

•
Pd re-extraction-the organic phase from the Pd extraction step (500 cm 3 ) was mixed with a 25% aqueous ammonia solution (500 cm 3 ) at room temperature for 1 h.After the process, the phases were separated.The volumes of the organic phase (485 cm 3 ) and aqueous phase (485 cm 3 ) were measured.A sample of 25 cm 3 was taken from the aqueous phase, which was then analyzed for the contents of Pt, Pd, Rh, and Au; • Pd precipitation-the aqueous phase from the Pd re-extraction step was used to precipitate palladium.Concentrated hydrochloric acid (150 cm 3 ) was added at room temperature to the stirred solution in portions until a slightly brown precipitate was formed.The next day, the solution was filtered, its volume was measured (495 cm 3 ), and a sample of 25 cm 3 was taken, which was then analyzed for the contents of Pt, Pd, Rh, and Au.The precipitate was weighed while wet (0.4618 g) and then again after drying at 100 • C (0.0820 g);

•
Precipitation Pt-the aqueous phase from the Pd extraction step was used for platinum precipitation.A concentrated aqueous solution of ammonium chloride (300 g/dm 3 , 200 cm 3 ) was added at room temperature to the stirred solution in portions.During the process, significant amounts of yellow precipitate were formed, which prevented filtration.We decided to perform the following two tests: Pt precipitation using an aqueous solution of NH 3 -for this purpose, 50 cm 3 of a 25% aqueous ammonia solution was added to 100 cm 3 of the heated solution.
A brown precipitate formed.The next day, the solution was filtered, its volume was measured (160 cm 3 ), from which a sample of 20 cm 3 was taken and the contents of Pt, Pd, Rh, and Au were analyzed.The precipitate was weighed while wet (0.7038 g) and then again after drying at 100 • C (0.0948 g); Pt precipitation using an NH 4 Cl solution-for this purpose, 200 cm 3 of an aqueous ammonium chloride solution (300 g/dm 3 ) was added to 100 cm 3 of the heated solution.The next day, the solution was filtered, its volume was measured (280 cm 3 ), from which a sample of 20 cm 3 was taken and the contents of Pt, Pd, Rh, and Au were analyzed.The precipitate was weighed while wet (0.5039 g) and then again after drying at 100 • C (0.1048 g).

Calculations
The efficiency of cementation, reduction, and refining was calculated according to the following formula: C i -initial concentration of the metal in the solution [mg/dm 3 ], C f -final concentration of the metal in the solution [mg/dm 3 ], V i -initial volume of the solution [dm 3 ], V f -final volume of the solution [dm 3 ].

Analytical Methods
The analyses were carried out by the Łukasiewicz Research Network-Institute of Non-Ferrous Metals, Centre of Analytical Chemistry, and Centre of Functional Materials (Gliwice, Poland).The concentrations of platinum, palladium, rhodium, and gold in the collected solution samples were determined by inductively coupled plasma mass spectrometry (ICP-Minerals 2024, 14, 625 6 of 18 MS; NexION 300D, PerkinElmer, Waltham, MA, USA).To analyze the composition of the precipitates, a MiniFlex 600 diffractometer (Rigaku, Tokyo, Japan) equipped with a CuKα copper lamp (λ = 1.5406Å) and a ZSX Primus WDXRF (Rigaku, Tokyo, Japan) were used.The average error of the method was around 5%, depending on the dilution of the samples and the concentration of precious metals.

Cementation Experiments
Metallic zinc powder is one of the most common cementing agents for precious metals [36,45,46].Most of the time, a chemically pure, commercial form of it is used; therefore, the possibility of using an industrial one was researched.Commercial zinc powder is a pure substance without any impurities or unwanted metals.On the other hand, industrial zinc powder, typically a by-product of industrial processes of variable composition, may contain impurities, especially of metals processed in that specific industrial plant.This research was conducted to determine if industrial plants could use their own metallic zinc powder to cement precious metals instead of having to use commercially manufactured zinc powder.
The results of the experiments of the cementation with zinc are shown in Figure 2. Analysis of the data in Figure 2 indicates that the metallic zinc powder can be used for the cementation of platinum, palladium, rhodium, and gold from eluates, as the cementation efficiency of these metals reached results >99% when adding 0.75 g of Zn.It is important to note that when using cementation, the metal dust first reacts with the free acid and only then with the precious metals.The process becomes even more complicated when the forms in which precious metals occur in solutions are considered.Taking into account only the dominant complexes in the chloride solution, the cementation process can be represented using the following equations: However, such a situation would only occur if the eluate contained only dominant forms; in addition, there may also be less popular complexes in chloride solutions, such as More importantly, precious metals can also form complexes with thiourea, which due to the high affinity of sulfur to platinum, palladium, rhodium, and gold, can be more dominant and stable complexes than chloride ones.Possible chloride complexes of platinum with thiourea are presented in Figure 3. Analysis of the data in Figure 2 indicates that the metallic zinc powder can be used for the cementation of platinum, palladium, rhodium, and gold from eluates, as the cementation efficiency of these metals reached results > 99% when adding 0.75 g of Zn.It is important to note that when using cementation, the metal dust first reacts with the free acid and only then with the precious metals.The process becomes even more complicated when the forms in which precious metals occur in solutions are considered.Taking into account only the dominant complexes in the chloride solution, the cementation process can be represented using the following equations: However, such a situation would only occur if the eluate contained only dominant forms; in addition, there may also be less popular complexes in chloride solutions, such as More importantly, precious metals can also form complexes with thiourea, which due to the high affinity of sulfur to platinum, palladium, rhodium, and gold, can be more dominant and stable complexes than chloride ones.Possible chloride complexes of platinum with thiourea are presented in Figure 3.
However, such a situation would only occur if the eluate contained only dominant forms; in addition, there may also be less popular complexes in chloride solutions, such as More importantly, precious metals can also form complexes with thiourea, which due to the high affinity of sulfur to platinum, palladium, rhodium, and gold, can be more dominant and stable complexes than chloride ones.Possible chloride complexes of platinum with thiourea are presented in Figure 3.This means that it is difficult to calculate how much excess metallic zinc powder was used in a given experiment.However, a good indicator of whether cementation has taken place with full efficiency is the measurement of the pH of the solution on an ongoing basis (Table 2).When using industrial Zn, at the beginning of the process, the pH is within the range of 0.16-0.53,but it increases rapidly to a pH of ~5.3 with the addition of approximately 0.75 g of Zn, which would indicate a complete conversion of free acid.This value may be the limit point for carrying out the cementation process using industrial metallic zinc powder, as an increase in the amount of added Zn is not required.An XRD pattern analysis of the selected precipitate after cementation was performed (with the addition of 0.75 g of Zn).
The data shown in the diffractogram in Figure 4 indicate that the sample contains precious metals such as Pt, Pd, and Rh in an unbounded form.In addition, peaks for Zn are also visible.When using Au, its concentration was probably too low for the signal to be visible in the diffractogram.The shape of the diffractogram and intensities imply that the sample was mostly filled with amorphous material, which makes XRD analysis difficult.This means that it is difficult to calculate how much excess metallic zinc powder was used in a given experiment.However, a good indicator of whether cementation has taken place with full efficiency is the measurement of the pH of the solution on an ongoing basis (Table 2).When using industrial Zn, at the beginning of the process, the pH is within the range of 0.16-0.53,but it increases rapidly to a pH of ~5.3 with the addition of approximately 0.75 g of Zn, which would indicate a complete conversion of free acid.This value may be the limit point for carrying out the cementation process using industrial metallic zinc powder, as an increase in the amount of added Zn is not required.An XRD pattern analysis of the selected precipitate after cementation was performed (with the addition of 0.75 g of Zn).
The data shown in the diffractogram in Figure 4 indicate that the sample contains precious metals such as Pt, Pd, and Rh in an unbounded form.In addition, peaks for Zn are also visible.When using Au, its concentration was probably too low for the signal to be visible in the diffractogram.The shape of the diffractogram and intensities imply that the sample was mostly filled with amorphous material, which makes XRD analysis difficult.Figure 5 shows the results of the influence of the amount of added metallic magnesium, aluminum, nickel, cobalt, and copper powder on the cementation process.Figure 5 shows the results of the influence of the amount of added metallic magnesium, aluminum, nickel, cobalt, and copper powder on the cementation process.Figure 5 shows the results of the influence of the amount of added metallic magnesium, aluminum, nickel, cobalt, and copper powder on the cementation process.The pH of the solutions after the cementing process and the weights of the precipitates are presented in Table 3.The pH of the solutions after the cementing process and the weights of the precipitates are presented in Table 3.  Figure 5a shows the changes in the cementation efficiencies of precious metals depending on the amount of added magnesium dust.An increasing trend is visible for each precious metal tested.However, it can be concluded that the highest cementing efficiencies were obtained for gold and the lowest for platinum.When adding zinc dust, the pH of the solution also changed during the process.The pH reached a value of approximately 9.50 after adding 0.5 g of Mg, which correlates with a visible increase in the efficiency of gold and palladium cementation.Although magnesium has a lower standard electrode potential (Mg|Mg 2+ −2.37 V) compared to zinc (Zn|Zn 2+ −0.76 V), it is less effective as a reductant for precipitating precious metals from the acidic thiourea solution.This could be attributed to the elevated final pH of the solution, causing magnesium to precipitate in various complex forms, thereby diminishing its ability to cement precious metals.This can be observed by the increased amount of non-precious metal precipitate formed at higher pH levels (>9.0) after the process, indicating the presence of additional complexes.The precipitate obtained after cementation contains a significant amount of oxygen, suggesting the formation of hydroxides or oxides.
When the metallic aluminum powder is used as the cementing agent (Figure 5b), there is a visible tendency to selectively precipitate gold and palladium in the form of a concentrate.Additionally, in this case, there was a shift in the pH solution to a level of approximately 3.31 (with the addition of 0.75 g of Al).This is the point at which the efficiency of gold and palladium cementation is practically the highest and the addition of more aluminum dust is not required.This metal can be used to selectively separate Au and Pd in the form of their combined concentrate from Pt and Rh by leaving them in the solution.Similarly, when using magnesium, at a pH > 3 additional amounts of precipitate were formed.However, this did not appear to hinder the cementation of Pd and Au.This could be attributed to the lower pH of the solution.The presence of unneutralized H + ions in the solution was likely crucial for the cementation reaction of various complexes of precious metals, as can be seen in the cementation equations with Zn.
Analyzing the data in Figure 5c, it can be concluded that nickel dust does not cement precious metals such as platinum, palladium, and rhodium.However, this metal can be used to recover gold from eluates.An additional important aspect of the process is the magnetic nature of the metallic nickel powder.The concentrate can be easily separated from the solution by using a magnet-this allows us to omit the traditional filtration processes.
The graph for cobalt (Figure 5d) looks very similar to the graph for nickel.This means that the metallic cobalt powder can also be used to selectively cement gold and, thus, separate it from the rest of the precious metals.
Both cobalt and nickel have higher standard electrode potentials compared to other metals used as cementing agents (Ni|Ni 2+ −0.257 V and Co|Co 2+ −0.28 V), resulting in the lowest efficiencies of cementing platinum, palladium, and rhodium.Despite this, the pH of the solution remained unchanged (<0.4 for both nickel and cobalt), indicating that free acid was not completely neutralized.It is conceivable that these metals could precipitate precious metals at higher pH levels.However, the quantity required to neutralize the solution economically would not be feasible for industrial-scale processes.
Copper is often used in metallurgy, although it is more often recovered from solutions through cementation than being actively used in them.However, for example, it can be used to recover silver from acidic solutions.Figure 5e also shows the possibilities of using copper dust for cementing precious metals.The data indicate that the addition of excess dust (minimum 1.5 g of Cu per sample of 25 cm 3 ) can be used to selectively separate Au and Pd from Pt and Rh by separating them in the form of a concentrate.When using copper, the formation of additional precipitate can be seen.Copper has a strong affinity to sulfur found in thiourea, causing it to precipitate from the solution as thiourea complexes.These complexes were unable to efficiently reduce precious metals to their solid state, resulting in low cementation efficiencies.At higher concentrations of copper, unbound copper likely began to precipitate precious metals, leading to a gradual change in the pH from 0.3 to 0.7.

Reduction and Precipitation Experiments
The results of testing the influence of the added amount of reducing or precipitating agents on the process are shown in Figure 6.The pH of the solutions after the reduction process and the weights of the precipitates are presented in Table 4.The pH of the solutions after the reduction process and the weights of the precipitates are presented in Table 4. Based on the data in Figure 6a, it can be concluded that an aqueous hydrazine hydrate solution can be used to reduce precious metals from eluates.Hydrazine hydrate can reduce precious metals (PM) according to Equation [47]: 4PM m+ + mN 2 H 4 → 4PM (s) + mN 2 + 4mH + Practically, the addition of 3.5 cm 3 to 25 cm 3 of eluate allowed reducing metals with a high efficiency (>90% for Pt, >99% for Pd, >90 for Au, and >80% for Rh).At this point, a significant increase in the pH was also visible (from approximately 0.99 to 7.00).This means that the pH could be an indicator to assess at what point the reduction process occurs with optimal efficiency; above which, adding excess hydrazine hydrate is not advisable.This would be in accordance with hydrazine hydrate-reducing precious metals in alkaline conditions.For example, the equation for platinum would look like [47], as follows: According to the equation, hydrazine hydrate can reduce precious metals in the presence of OH − ions with better efficiency and, therefore, at a higher pH of the solution, which happened in this case.
XRD pattern analysis of the precipitate after adding 5 cm 3 of aqueous hydrazine hydrate solution was also performed and is shown in Figure 7.
Minerals 2024, 14, x FOR PEER REVIEW 14 of 19 significant increase in the pH was also visible (from approximately 0.99 to 7.00).This means that the pH could be an indicator to assess at what point the reduction process occurs with optimal efficiency; above which, adding excess hydrazine hydrate is not advisable.This would be in accordance with hydrazine hydrate-reducing precious metals in alkaline conditions.For example, the equation for platinum would look like [47], as follows: According to the equation, hydrazine hydrate can reduce precious metals in the presence of OH − ions with better efficiency and, therefore, at a higher pH of the solution, which happened in this case.
XRD pattern analysis of the precipitate after adding 5 cm 3 of aqueous hydrazine hydrate solution was also performed and is shown in Figure 7. Analysis of the data in the diffractogram in Figure 6 indicates that Pt and Pd compounds are formed in the sample following reduction with hydrazine hydrate, mainly oxides such as Pt3O4, Pd3O4, PtO2, and PdO2.In addition, rhodium is visible in the precipitate, unbound and bound to nitrogen from hydrazine hydrate, forming RhN.When using Au, its amount was probably too low for the signal to be visible in the diffractogram.
Based on the data in Figure 6b, it can be concluded that selective precipitation of gold, palladium, and rhodium is possible while using an aqueous ammonia solution as the precipitating agent.Practically, even a small addition of the ammonia solution causes a high pH increase (>8); however, only after reaching a pH above 9 is the recovery of precious metals carried out with high efficiencies (>98% for Pd, >87 for Au and >86% for Rh).The platinum reduction efficiency remains virtually unchanged throughout the entire process (50-60%).Ammonia solution is very often added to the refining process of precious metals, especially in the presence of chloride ions, as it can precipitate precious metals in the forms of:  Analysis of the data in the diffractogram in Figure 6 indicates that Pt and Pd compounds are formed in the sample following reduction with hydrazine hydrate, mainly oxides such as Pt 3 O 4 , Pd 3 O 4 , PtO 2 , and PdO 2 .In addition, rhodium is visible in the precipitate, unbound and bound to nitrogen from hydrazine hydrate, forming RhN.When using Au, its amount was probably too low for the signal to be visible in the diffractogram.
Based on the data in Figure 6b, it can be concluded that selective precipitation of gold, palladium, and rhodium is possible while using an aqueous ammonia solution as the precipitating agent.Practically, even a small addition of the ammonia solution causes a high pH increase (>8); however, only after reaching a pH above 9 is the recovery of precious metals carried out with high efficiencies (>98% for Pd, >87 for Au and >86% for Rh).The platinum reduction efficiency remains virtually unchanged throughout the entire process (50-60%).Ammonia solution is very often added to the refining process of precious metals, especially in the presence of chloride ions, as it can precipitate precious metals in the forms of: [Pd(NH 3 ) 4 ](OH) 2 , or (NH 4 ) 3 RhCl 6 .The semi-quantitative analysis results showed that in addition to the mentioned complexes, other sulfur-containing compounds were also precipitated due to the strong affinity between nitrogen and sulfur.A combination of amino-chloro-thiourea complexes was likely formed during the precipitation.
The use of formic acid (Figure 6c) enables the recovery of all precious metals with a very high efficiency (>80%), with only a small amount of reducing agent added.However, the formation of excess precipitate is visible when the concentration of formic acid and sodium hydroxide exceeds a certain threshold.It is probably a mixture of hydroxide, formic acid, and thiourea complexes.This means that when formic acid is used, special attention must be paid to the reaction balance, which, unfortunately, is not indicated by a change in the pH of the solution throughout the process, as NaOH had to be added to keep the reaction at high efficiency.It is possible that the solid form of precious metals, once precipitated, could potentially oxidize formic acid, leading to the production of more compounds and a significant increase in the amount of precipitate.
Application of the aqueous solution hydrogen for the recovery of platinum and palladium at a high level (with the efficiencies being >90% for Pt and >89% for Pd), selectively separating them from gold and rhodium.The pH change is also not very visible in this case, as it remains <1 throughout the process.Nevertheless, the addition of approximately 3.5 cm 3 of hydrogen peroxide to 25 cm 3 of eluate enables reducing high amounts of Pt and Pd.In this case, a significant amount of sulfur was present in the precipitate, likely forming compounds with precious metals.It is conceivable that instead of pure precious metals, various sulfides were produced.
The data in Figure 6e indicate that precious metals can be recovered with high efficiencies (over 80% for each metal) using oxalic acid.It should be noted that in this case, a large amount of additional precipitate is formed, which is not a compound made entirely of precious metals, but is likely a mixture of hydroxide, oxalic acid, and thiourea complexes, although the precipitation of precious metal oxalates is also possible.Also, the pH in this case cannot be an indicator that determines the maximum reduction efficiencies of Pt, Pd, Rh, and Au as NaOH had to be added to the process to obtain high efficiencies of the precipitation of precious metals.

Refining Experiment
The results of the precious metal-refining tests are presented in Table 5.The results presented in Table 5 indicate that it is not possible to carry out this type of precious metal-refining process to obtain pure compounds.The reason for low recovery efficiencies is probably the greater affinity of precious metals for thiourea than for extractants and precipitating agents.This means that the use of solvent extraction techniques is not possible and applications of different selective precipitation techniques will be required.

Conclusions
During the research, various tests were performed to extract precious metals from the post-elution solutions.Different methods such as cementation, reduction, precipitation, and refining were used to achieve this objective.The study examined the potential of using metallic powders like zinc, copper, aluminum, magnesium, nickel, and cobalt to cement platinum, palladium, rhodium, and gold.Additionally, the effectiveness of using aqueous hydrazine hydrate solution, aqueous hydrogen peroxide solution, aqueous ammonia solution, aqueous formic acid solution, and oxalic acid in reducing and precipitating precious metals was also investigated.The test results are summarized in Table 6, which shows what agents can be used to precipitate specific precious metals.Based on the conducted research, the following additional conclusions can be drawn: • The best cementing agent for platinum, palladium, rhodium, and gold is metallic zinc powder, whereas commercial metallic zinc powder can be replaced with an industrial one; • Metallic aluminum and copper powders can be used to selectively cement gold and palladium from the post-elution solutions of thiourea and HCl, separating them from platinum and rhodium; • Gold can be cemented with metallic powders of zinc, magnesium, aluminum, copper, nickel, and cobalt; • The best-reducing agent for platinum, palladium, rhodium, and gold is an aqueous solution of hydrazine hydrate; • Aqueous hydrogen peroxide solution can be used to selectively reduce Pt and Pd from the post-elution solutions of thiourea and HCl, separating them from Au and Rh.• The research findings suggest that it is possible to selectively precipitate precious metals from thiourea post-elution solutions by using a combination of various cementing, reducing, and precipitating agents in a specific combination.

Figure 1 .
Figure 1.Scheme of the research plan.

Minerals 2024 , 19 Figure 2 .
Figure 2. Dependence of the cementation efficiency on the initial amount of added metallic zinc powder (m0).

Figure 2 .
Figure 2. Dependence of the cementation efficiency on the initial amount of added metallic zinc powder (m 0 ).

Figure 4 .
Figure 4. XRD pattern for the sample after cementation with the addition of 0.75 g of metallic Zn powder.

Figure 4 .
Figure 4. XRD pattern for the sample after cementation with the addition of 0.75 g of metallic Zn powder.

Figure 4 .
Figure 4. XRD pattern for the sample after cementation with the addition of 0.75 g of metallic Zn powder.

Figure 5 .
Figure 5. Dependence of the cementation efficiency on the initial amount of added metallic magnesium, aluminum, nickel, cobalt, and copper powder (m0).

Figure 5 .
Figure 5. Dependence of the cementation efficiency on the initial amount of added metallic magnesium, aluminum, nickel, cobalt, and copper powder (m 0 ).

Figure 6 .
Figure 6.Dependence of the reduction efficiency on the initial amount of added aqueous hydrazine hydrate solution, aqueous ammonia solution, aqueous formic acid solution, aqueous hydrogen peroxide solution (V0), and oxalic acid (m0).

Figure 6 .
Figure 6.Dependence of the reduction efficiency on the initial amount of added aqueous hydrazine hydrate solution, aqueous ammonia solution, aqueous formic acid solution, aqueous hydrogen peroxide solution (V 0 ), and oxalic acid (m 0 ).

Figure 7 .
Figure 7. XRD patterns for the sample following reduction with the addition of 5 cm 3 of aqueous hydrazine hydrate solution.

Figure 7 .
Figure 7. XRD patterns for the sample following reduction with the addition of 5 cm 3 of aqueous hydrazine hydrate solution.

Table 1 .
Characteristic of the ion-exchange resins.

Table 2 .
Composition of the eluate.

Table 3 .
Results of the research on cementing the precious metals from the post-elution solution.

metallic Zn powder Amount of the added ce- menting
agent [g] Final pH of the solution Wet mass of the precipitate [g] Dry mass of the precipitate [g] Semi-quantitative analysis of the precipitate after the addition of 0.75 g of dust [%]

Table 3 .
Results of the research on cementing the precious metals from the post-elution solution.

Table 4 .
Results of the research on reducing and precipitating the precious metals from the postelution solution.

Table 5 .
Results of the research on refining the precious metals from the post-elution solution.

Table 6 .
Summary of the test results, + means cementation/reduction/precipitation is possible, − means cementation/reduction/precipitation is not possible, +/− means cementation/reduction/precipitation is possible, but the use of other agents is recommended.