Recovery of Noble Metals (Au, Pt, Ir, and Ta) from Spent Single-Use Medical–Technological Products

Abstract

Due to their unique properties, i.e., fluoroscopy response and inertness, noble metals and alloys are present in several widespread medical–technological products, such as catheters, guide-wires, and stents. Despite their value, these products serve as single-use consumables, following a fate of solid waste disposal and loss of their valuable metals. This work studies the development of a treatment methodology to recover noble metals such as Pt, Ir, Au, and Ta from certain commercial products commonly used for medical practices. In particular, a sequence of preliminary pyrolysis, aiming at polymer elimination, as well as an acid digestion step for selective metals dissolution, is suggested. Pyrolysis was capable of enriching samples with the targeted metals, though a small change in their oxidation states was observed. Still, acid digestion was fully able to successfully separate Au using a 50% v/v aqua regia solution for 30 min at room temperature and the Pt/Ir using concentrated aqua regia for 72 h under heating. Dissolution of Ta required a different leaching solution, i.e., a 50% v/v HF/H₂SO₄ mixture for 10 h under heating. According to the developed method, selective extraction of such noble metals in a concentrated slurry provides a high potential for the complete recovery and valorization of otherwise disposed medical wastes.

1. Introduction

Alongside commonly produced municipal and industrial wastes, for which numerous studies have been carried out concerning novel methods of management, the handling of specific medical and hospital waste streams also has also attracted the interest of scientific research. Nowadays, more and more people have gained access to the new technological products of advanced medicinal materials, and this implies a proportional increase in the amount of corresponding waste volumes [1]. Medical wastes are generally considered to carry a significant biological load, and for this reason, their management and disposal are governed by a large number of specific regulations, while any effort to reuse any of their constituents is currently prohibited or avoided [2]. The most-common methods of managing these wastes include sterilization/pasteurization and landfill disposal or incineration [3]. Nevertheless, some of these medical–technological products are specialized devices containing valuable metals; thus, different managing approaches to those applied for the other/general medical wastes should be considered [4].

Noble metals are an important category of valuable materials appearing in these single-use medicinal products. Their extensive use is related to advanced diagnostic techniques where their unique chemical characteristics, such as their inertness and visibility through fluoroscopy by an external observer/operator, are required [5]. Τhe most widespread medical–technological products containing noble metals are diagnostic electrophysiology catheters, diagnostic guide wires, and self-expanding stents. Such products contain Pt/Ir alloy marker bands [6], Au-coating media [7], and Ta marker dots at each edge [8], respectively.

Recovering noble metals from secondary sources (i.e., through properly recycled devices) can be an means of supplying a significant percentage of market needs since mining operations are struggling to match the corresponding high and ever-growing demand [9]. Predicting a further increasing need for noble metals to support technological advancements promotes the exploitation of alternative resources. To this end, the European Union’s directives to face the problem of critical metals scarcity suggest the consideration of secondary sources as primary ones [10,11]. The problem has become more intense since efforts to replace noble metals in some of their applications, e.g., in catalysts [12,13], had an insignificant impact on the commercial demand. Therefore, many studies focus on their recovery from industrial sludges [14], catalysts [15], and electric wastes [16]. The recovery of noble metals from medical devices has been limited to the recovery of Au from implantable devices that contain printed circuit boards (i.e., pacemakers) [17]. On the other hand, a multitude of methods have been developed to recover gold from electronic waste (e-waste) [18]. To the best of our knowledge, medical–technological products, such as catheters, guide wires, and stents, have not been yet included in this research trend.

Regarding the general medical waste streams, the replacement of incineration by pyrolysis, an alternative sterilization method, was formerly studied, proving that this treatment method is an environmentally friendly option that combines lower temperatures and heating periods, lower emission of pollutants, and a residual ash/material complying with the stringent health requirements for subsequent safe disposal [19]. On the other hand, pyrolysis has already been applied in the recovery of noble metals from electronic wastes using the remaining ash. The main advantage of this process is the absence of polymeric fractions in the residual ash, so the metals’ recovery may be achieved via the application of common hydrometallurgical or pyrometallurgical processes [20].

The most popular scenarios regarding the recovery of noble metals from the residual ash or directly from the raw waste are selective leaching and digestion, a sequence commonly included in the framework of hydrometallurgical processes. Specific leaching reagents may efficiently extract noble metals from other co-existing substances of the solid waste stream, especially when only one or two of them are targeted. Typically, the noble metals are dissolved in aqua regia, cyanide, or strong acids such as HNO₃, HCl, and H₂SO₄ (

Table 1. Dissolution of noble metals in various case studies.

Reagent	Waste Type	Metals of Interest				Other Metals	Ref.
		Au	Pt	Ir	Ta
Aqua regia	Electrical and electronic equipment	✓	-	-	-	Ni, Co, Fe	[22]
HCl/H2O2	Anode slimes	✓	✓	-	-	Pd, Cu	[14]
Aqua regia	Spent catalysts	-	✓	✓	-	Pd, Rh	[23]
Aqua regia	Spent catalysts	-	✓	✓	-	Pd, Rh	[15]
HF/H2SO4	Slags/Mining tailings	-	-	-	✓	Nb	[24]
Aqua regia and HF/H2SO4	Medical–technological products	✓	✓	✓	✓	Fe, Cr, Ni, Ti	Present study

✓: The symbol indicates the metals under investigation.

), while the addition of oxidizing agents increases leaching efficiency [15]. However, when targeted metals are more than one, the acid digestion process is initially preferred before the implementation of selective leaching [16]. This preliminary dissolution results in higher recovery rates for all metals of interest, whereas the subsequent leaching reaches a higher purity level [9]. Selective extraction and adsorption via activated carbon and membranes are the most realistic for implementation technologies that were tested at a laboratory scale [21].

This work aimed to establish a sequential process for the recovery of noble metals contained in diagnostic medical–technological products such as catheters, guide wires, and stents. For this purpose, samples were first subjected to pyrolysis to remove the polymer coating and enrich noble metals, whereas an acid digestion/selective leaching process was optimized for their efficient separation from the solid waste. Various leaching solutions and experimental conditions were tested, aiming to obtain separate liquid streams of concentrated Pt, Ir, Au, and Ta solutions.

2. Materials and Methods

2.1. Samples

New (unused) commercially available medical–technological products were selected as reference samples for this study, including a diagnostic electrophysiology catheter with Pt/Ir marker bands (Boston Scientific, model Viking), a diagnostic guide wire with a Au coating on one edge (ev3 company, model Nitrex), and a self-expanding stent with Ta dots in both edges (ev3 company, model Protégé) (Figure 1). The mentioned samples supplied all the metals of interest in the simulated waste streams examined in this work. As a pre-treatment step, the parts containing the noble metals were separated by cutting them from the rest of the product in order to increase the percentage of wastes in Pt, Ir, Au, and Ta.

2.2. Pyrolysis

A digital laboratory WITEG muffle furnace model FHX-05(Witeg Labortechnik GMBH, Wertheim, Germany), has been used for the initial pyrolysis step. During the experiments, approximately 60 mg of Au-coated guide wire, 60 mg of the diagnostic catheter (including one Pt/Ir marker band of 11 mg), and 3 mg of the stent (including 2 Ta dots) were used as the simulated mixed medical waste stream. The initial content of noble metals in this waste stream was not possible to determine before digestion. Pyrolysis was carried out by placing each of the three samples in separate porcelain capsules covered by lids and equipped with an exhaust gas outlet hole, ensuring anoxic conditions. The temperature was set at 550 °C for 60 min, following the optimal conditions of our previous work [25].

2.3. Characterization

The JEOL JSM-7610F Plus Scanning Electron Microscope (SEM-JEOL, Tokyo, Japan), with an integrated X-ray energy dispersive spectrometer (EDS-JEOL, Tokyo, Japan) and Oxford Instruments AZTEC (Oxford Instruments, High Wycombe, UK) analytical system, was used in the present study to acquire the images of the morphology and the distribution of elements regarding the structure of samples before and after pyrolysis. X-ray photoelectron spectroscopy (XPS) measurements were also performed before and after the pyrolysis experiments using the Axis Ultra DLD system (Kratos Analytical Ltd., Wharfside, UK); monochromatic Al Ka radiation was used as the X-ray source. The pass energy was kept constant at 40 eV, and it was calibrated for charge-induced shifts, considering the C1s peak (originating from the surface contamination of carbon) to be at 284.6 eV. The concentrations of noble metals, as well as of the other measured metals, have been determined via flame atomic absorption spectrophotometry by using the Perkin–Elmer AAnalyst 800 instrument (Perkin-Elmer, Waltham, MA, USA).

2.4. Acid Digestion

The pyrolyzed samples were placed in a beaker under reflux conditions (fitting to the top of the beaker of a spherical flask containing water to act as a condenser) when longer periods of treatment time were needed, and a sand bath was used as a heating source. A volume of 10 mL (i.e., 1.23% w/v) of different inorganic acids or their mixtures was tested, including HNO₃ (69% w/w by Supelco), HCl (37% w/w by Supelco), H₂O₂ (30% w/w by Supelco), HF (40% w/w by Supelco), and H₂SO₄ (95–97% w/w by Supelco), for the selective digestion of noble metals. This step aimed for the complete solubilization of the studied metals: Au, Pt, Ir, and Ta; hence, the experimental conditions, i.e., the combination and concentrations of the examined acids, time, and temperature, were adjusted, with 100% of them in the liquid phase. Complete dissolution was achieved by repeating the digestion process under extreme conditions, leading to zero concentrations of noble metals in the liquid phase. In addition, other potential metallic impurities, e.g., Fe, Cr, Ni, and Ti, were also taken into account; however, in this case, they were not considered as a criterion for the determination of optimal separation conditions. The conditions wherein the highest separation between the noble metals was achieved were considered to be optimal and selective.

3. Results and Discussion

3.1. Characterization of Samples

Although the specification data provided by the products’ manufacturers are commercially available, they was also verified at the laboratory by applying EDS analysis. According to the obtained data, the diagnostic catheter contains Pt/Ir alloy bands, with a metals concentration ratio equal to 10/1. These bands are wrapped over a supporting steel grid and coated with a plastic cover. EDS analysis (

Table 2. EDS analysis on specific areas of samples containing the noble metals.

Medical Product	Specific Area	Pt	Ir	Au	Ta	O
		wt%
Diagnostic catheter	Pt/Ir marker band	77.6 ± 4.7	8.3 ± 1.6	-	-	14.1 ± 3.2
Guide-wire	Au-coated guide wire	-	-	86.6 ± 5.4	-	11.3 ± 2.7
Self-expanding stent	Ta marker dot	-	-	-	81.9 ± 4.8	18.1 ± 3.8

) revealed that the above-mentioned ratio was equal to 9.3 along with the existence of a significant amount of oxygen (14.1 ± 3.2 wt%), indicating the presence of oxidized forms of metals. XPS analysis confirmed that catheter bands contained both elemental and oxidized forms of Pt (Figure 2a) and Ir (Figure 1c). Pt peaks at 70.0 and 73.4 eV are attributed to the elemental form (88.9 ± 3.1 wt%); whereas at 71.5 and 74.9 eV, they are attributed to the respective bivalent form (11.1 ± 2.7 wt%) [26]. Moreover, Ir peaks at 59.5 and 62.4 eV are attributed to the elemental form (73.4 ± 3.5 wt%), while those at 60.6 and 63.9 eV are attributed to the trivalent form (26.6 ± 2.8 wt%) [27].

Regarding the diagnostic guide wire, this is based on a Ni/Ti alloy (nitinol) wire which was gold-plated only on one end; the rest was enclosed in a plastic cover. As before, the presence of oxygen was confirmed via EDS analysis (11.3 ± 2.7 wt%). According to XPS diagrams (Figure 2e), peaks at 84.0 and 87.7 eV are attributed to the Au elemental form (79.9 ± 2.7 wt%), while those at 84.9 and 88.4 eV are attributed to the monovalent form (20.1 ± 3.5 wt%) [28]. The self-expanding stent is also based on a nitinol grid with four Ta dots to be placed at each end. The oxygen content was obtained via EDS analysis (18.1 ± 3.8 wt%). XPS peaks of Ta (Figure 2g) at 21.6 and 23.5 eV are attributed to the elemental form (67.4 ± 3 wt%), while those at 22.4 and 24.9 eV are attributed to the bivalent form (32.5 ± 4.7 wt%) [29].

It is worth noting that the aforementioned description of the samples’ structure and metals distribution, alongside the presence of noble metals, was also confirmed via SEM-EDS analysis. Figure 3a shows the SEM images on the surface of Pt/Ir bands. The morphology of this material appears to be quite smooth, with few surface scratches, due to the metallurgical production process. EDS analysis verified the presence of Fe, C, and Cr in the steel grid of the catheter’s cable. The guide-wire-related SEM image in Figure 3c shows the presence of a lamina with spiral geometry and relatively good uniformity in the gold-plated area. The SEM image in Figure 3e shows the Ni/Ti alloy grid (outer circle); whereas the composition was verified via EDS and the Ta dots (inner circle).

3.2. Characterization of Pyrolyzed Samples

The percentage of solid residue and the corresponding mass loss of the samples after pyrolysis were calculated via Equations (1) and (2), and the results are summarized in

Table 3. Results of pyrolysis experiments (conditions: 550 °C for 60 min).

Medical Product	Solid Residue (wt%)	Mass Loss (wt%)
Guide wire, metal of interest: Au Plastic part/Metallic part	1.06/100	98.9/0
Diagnostic catheter, metals of interest: Pt/Ir	46.36	53.6
Self-expanding stent, metal of interest: Τa	93.0	7.0

.

$$\%\mathrm{solid} \mathrm{residual}=\frac{{\mathrm{m}}_{\mathrm{p}}}{{\mathrm{m}}_{\mathrm{i}}}\times 100,$$

(1)

$$\%\mathrm{mass} \mathrm{loss}=\frac{{\mathrm{m}}_{\mathrm{i}}-{\mathrm{m}}_{\mathrm{p}}}{{\mathrm{m}}_{\mathrm{i}}}\times 100,$$

(2)

where m_p is the mass in (g) of the solid product after pyrolysis, and m_i the mass in (g) of the initial solid sample.

Considerable mass loss is noticed for the samples containing plastic parts, while the mass loss for the metallic parts of samples ranged from zero (for the case of Au) to 7.02 wt% (for Ta). The observed variation in the color of the Ni/Ti alloy grid, which supports the Ta dots, from silver to violet after the application of pyrolysis, along with the possible oxidation of Ta, may be the reason for the relatively high mass loss of this sample. According to other researchers, the surface layer of the Ni/Ti alloy mainly consists of titanium oxide. The temperature, time, and heating rate used during this moderate-heat pyrolytic treatment have a direct effect on the characteristics of the titanium oxide layer, resulting in color variations in the examined metal part and, by extension, in the weight change [30]. A solid ash residue was obtained from the plastic parts of the examined samples, while the parts of samples containing the metals of interest seemed, macroscopically, to be unaffected. The SEM images of samples after pyrolysis (Figure 3b,d,f) revealed no distinguished differences compared to the SEM images of the initial samples (Figure 3a,c,e)—apart from some plastic residue—remaining on the surface of diagnostic guide wire (Figure 3d).

EDS analysis of pyrolyzed samples is summarized in

Table 4. EDS analysis at specific areas of pyrolyzed samples containing the noble metals.

Medical Product	Specific Area	Pt	Ir	Au	Ta	O
		wt%
Diagnostic catheter	Pt/Ir marker band	74 ± 4.5	7.5 ± 0.4	-	-	15.9 ± 2.6
Guide-wire	Au-coated guide wire	-	-	89.9 ± 1.2	-	7.4 ± 1.4
Self-expanding stent	Ta marker dot	-	-	-	71.2 ± 5.1	24.7 ± 4.8

. According to the measurements, oxygen content decreased only in the case of Au, i.e., its elemental form increased. Metals distribution in the XPS diagram (Figure 2f) verified the elemental Au percentage increase of up to 94.1 ± 1.6 wt% by the corresponding peaks at 84.0 and 87.7 eV. Its monovalent form decreased to 5.9 ± 1.8 wt%, as defined by the corresponding peaks at 84.8 and 88.4 eV. On the other hand, the trivalent (oxidized) form of Ir increased to 63.2 ± 3.6 wt%, as presented by the corresponding peaks at 60.4 and 63.4 eV; while its elemental form decreased to 36.8 ± 1.6 wt%, as shown by the corresponding peaks at 59.7 and 62.7 eV (Figure 2d). Pt and Ta also presented an increase in their oxidized forms, but a new metallic form was introduced in both cases. Regarding Pt (Figure 2d), 72.1 ± 2 wt% of its distribution is attributed to the elemental form (corresponding peaks at 70.9 and 73.6 eV), 17.2 ± 1.5 wt% to the bivalent form (corresponding peaks at 71.3 and 74.6 eV), and 10.7 ± 2.4 wt% to the tetravalent form (corresponding peaks at 73.0 and 76.5 eV) [16]. Moreover, 31.2 ± 2.8 wt% of Ta distribution is attributed to the monovalent form (corresponding peaks at 21.7 and 23.8 eV) (Figure 2h), 28 ± 2.6 wt% to the bivalent form (corresponding peaks at 22.7 and 24.8 eV), and 40.8 ± 2.6 wt% to the pentavalent form (corresponding peaks at 26.3 and 28.2 eV) [29]. Τhe increase in the oxidized forms of the metals, except for gold, was attributed to the anoxic conditions applied during pyrolysis and the presence of oxygen [31,32].

3.3. Separation of Metals

Pyrolyzed samples were digested via the addition of acidic mixtures, applying different experimental conditions and aiming to obtain separate streams with respect to the metals of interest. Initially, various acidic solutions were prepared to digest Au since it presents the lowest solubility among the examined metals. According to

Table 5. Acid digestion of the samples, aiming for Au separation. Conditions: 30 min, no heating.

Acidic Solution	Au	Pt	Ir	Ta	Fe	Cr	Ni	Ti
(50% v/v)	mg/L
HCl	62	-	-	-	114	30	8	3
HNO3	42	-	-	-	98	21	5	2
HCl/H2O2 (3:1)	102	-	-	-	156	42	37	28
HCl/HNO3 (3:1)	208	-	-	-	270	81	306	150
HCl/HNO3-concentrated	210	0.8	-	-	392	115	431	182

, the optimum acidic mixture was proved to be aqua regia 50% v/v. Nitric acid oxidized the elemental Au to its trivalent form, and the presence of hydrochloric acid fed Au³⁺ with chloride anions to form stable (dissolved) chloro-complexes, following reaction 3 [33]. The 100% dissolution yield of the optimum experimental conditions was verified via a second digestion step of resulting solid residue from the first stage using this time concentrated aqua regia; in this case, Au was not determined in the liquid phase. On the other hand, when concentrated aqua regia was used as the main digestion solution, although 100% of the Au was dissolved, a low concentration of Pt was detected (i.e., the concentrations of these metals in the liquid phase found 208 mg/L Au and <1 mg/L Pt, respectively). When hydrochloric and nitric acid were applied individually, the yield was found to be much lower, i.e., 30% and 20%, respectively. Based on the abovementioned mechanism, a second oxidizing reagent was tested, namely, H₂O₂, along with HCl [34]. In this case, only half of the Au was dissolved under these experimental conditions, noting, however, that the application of higher concentrations of acidic solution, as well as the reaction time, may increase the HCl/H₂O₂ digestion yield [35].

The free-from-Au digestion residual material was then subjected to the second dissolution process. The main acidic reagent was also aqua regia (HCl/HNO₃), but the experimental conditions were different [36,37]. Since Pt and Ir solubility in acids is lower than Au, the concentrated aqua regia was used in this case, while the system was also subjected to heating at 90 °C [38]. The dissolution rate was rather low due to the small reactive surface area of examined catheter marker bands and to the side reaction of HCl with HNO₃ [39]. As a result, the complete dissolution of Pt and Ir required multiple additions of concentrated aqua regia under these intensive conditions, and it was achieved after 72 h, following reactions 4 and 5, respectively [10]. In detail, every 6 h, 5 mL of the acidic reagent was added, while the reflux conditions did not allow the solution’s level to drop below 10 mL. The metals concentration in the liquid phase was found to be 653 mg/L Pt and 61 mg/L Ir, respectively. Regarding dissolution kinetics, metals concentration was also determined in 48 h (

Table 6. Acid digestion conditions of the samples’ residue, aiming for Pt, Ir, and Ta separation.

Acidic Solution	Material	T	Time	Pt	Ir	Ta	Fe	Cr	Ni	Ti
		°C	h	mg/L
HCl/HNO3-concentrated	Au digestion residue	25	0.5	0.8	-	-	392	115	431	182
HCl/HNO3-concentrated	Au digestion residue	90	48	449	43.6	5	584	202	1587	1372
HCl/HNO3-concentrated	Au digestion residue	90	72	653	61	8	587	200	1672	1411
HF 50% v/v	Pt/Ir digestion residue	90	10	-	-	38	-	-	-	-
HF/H2SO4 50% v/v	Pt/Ir digestion residue	90	10	-	-	175	-	-	-	-
HF/H2SO4 50% v/v	Pt/Ir marker band	90	72	16	30	-	-	-	-	-

). In that case, the corresponding measurements were 449 mg/L and 43.6 mg/L, respectively. Pt and Ir are used as an alloy in marker bands and their dissolution is a slow reaction, so selective separation was not reached [23].

2Au + 11HCl + 3HNO₃ → 2HAuCl₄ + 3NOCl + 6H₂O

(3)

3Pt + 18HCl + 4HNO₃ → 3[PtCl₆]²⁻ + 6H⁺ + 4NO + 8H₂O

(4)

3Ir + 18HCl + 4HNO₃ → 3[IrCl₆]²⁻ + 6H⁺ + 4NO + 8H₂O

(5)

Ta₂O₅ + HF/H₂SO₄ → TaF₆²⁻/TaF₇²⁻

(6)

The other examined materials in Pt/Ir digestion residue—besides the marker bands (i.e., the steel grid and nitinol)—were dissolved, and the only residue that was still left insoluble was the Ta dots from the stent. Aqua regia managed to dissolve a very small amount of Ta (<5%). This can be attributed to the precipitation of Ta again in the form of hydrated oxide (Ta₂O₅.5H₂O) [40]. On the other hand, the combination of HF and H₂SO₄ (acid rate, 1:1; concentration, 50% v/v; temperature, 90 °C; and reaction time, 10 h) proved capable of its subsequent dissolution following reaction 6, as the metals concentration in the liquid phase was found to be 175 mg/L (Table 6) [24]. Tantalum forms stable complexes with fluoride, but when only HF 50% v/v was applied, the dissolution was very difficult (22%) [41]. Moreover, the use of the previous HF/H₂SO₄ mixture in a catheter’s Pt/Ir marker band (i.e., without the presence of other materials) can also dissolve some of the iridium (47.4%), but not the platinum (2.4%), as chloride ions were not present to form the respective stable chloro-complexes [42].

3.4. Economic and Environmental Aspects

The economically attractive approach of the separate management of single-use medical products containing precious metals has to overcome the lack of appropriate legislation and the lack of appropriate market-driven solutions. These constraints have encouraged the prevailing single-use and discharge culture. Therefore, such research efforts could motivate the policy changes and market-driven solutions needed to transform the way in which such single-use medical products are being managed.

With growing environmental awareness, increasing costs for raw materials, and the constant pressure on the health sector to reduce its operational costs, the existing linear management approach for single-use medical products, discharged without being further treated for precious metals recovery (Table 7), is reaching its limits [43].

Considering the weight of the precious metals-bearing parts examined in this study, their respective metal content (79–90% for Au and Ta; 90% for Pt and 10% for Ir), and the current market price for trading such metals, it is estimated that since 100% of the metal content is recovered, the money revenue in USD could be of 3.5 USD for each diagnostic electrophysiology catheter, 4 USD for each diagnostic guide wire, and 0.1 USD for each self-expanding stent. The money revenue could come to millions of US dollars each year, taking into consideration that every year, only the catheter ablation procedures that are performed worldwide are estimated to be more than 1 million [44]. The realization of the economic benefit of recovering precious metals such as Au, Pt. Ir and Ta from single-use medical products, as presented in this study, may lead to new and innovative sustainable circular management business models in the future.

Table 7. Relative dissolution percentages of noble metals from various waste streams.

Waste	Acidic Solution	Metal	% Dissolution	Ref.
Electrical and electronic equipment	Aqua regia	Au, Ta	80–100%, 0%	[22]
Anode slime	Aqua regia	Au	100%	[35]
Printed circuit boards	Aqua regia	Au	100%	[45]
Spent automotive catalysts	HCl/AlCl3/ HNO3	Pt	97–98%	[46]
Epoxy-coated solid electrolyte tantalum capacitors	HF	Ta	100%	[47]
Slags/Mining tailings	HF/H2SO4	Ta	87%	[24]
Medical–technological products	Aqua regia HF/H2SO4	Au, Pt, Ir, Ta	>95% in all metals	Present study

Table 7. Relative dissolution percentages of noble metals from various waste streams.

Waste	Acidic Solution	Metal	% Dissolution	Ref.
Electrical and electronic equipment	Aqua regia	Au, Ta	80–100%, 0%	[22]
Anode slime	Aqua regia	Au	100%	[35]
Printed circuit boards	Aqua regia	Au	100%	[45]
Spent automotive catalysts	HCl/AlCl3/ HNO3	Pt	97–98%	[46]
Epoxy-coated solid electrolyte tantalum capacitors	HF	Ta	100%	[47]
Slags/Mining tailings	HF/H2SO4	Ta	87%	[24]
Medical–technological products	Aqua regia HF/H2SO4	Au, Pt, Ir, Ta	>95% in all metals	Present study

4. Conclusions and Prospects

In this study, it was proved that medical–technological products can be a valuable secondary (alternative) source of noble metals such as Au, Pt, Ir, and Ta. The incineration-landfill processes, being the main management methods currently applied, can be replaced efficiently by the proposed pyrolysis–acid leaching recovery of metals content. Through the initial pyrolysis at 550 °C, alongside disinfection (although this is not part of the presented research), the plastic parts can be completely discarded, and the overall weight of waste can be substantially decreased. Separate streams of the metals of interest were obtained by applying different acidic mixtures and experimental conditions during the following acidic digestion process. The proposed methodology is considered to be highly selective among the metals of interest, since only in the Au dissolution step were some minor Pt concentrations detected in the liquid phase, and the Au recovery reached almost 100%. On the other hand, the resulting streams of noble metals also contain significant concentrations of other metallic impurities (e.g., Fe, Cr, Ni, Ti, etc.) existing in the initial products, so further investigation is needed, and will be aimed at their selective separation and precipitation from the liquid phase. In addition, Pt and Ir should also be further selectively separated to be potentially re-utilized in other applications.

The results obtained here, however limited, could be seen as a basis for a follow-up pilot study where the technoeconomic analysis of the process could be studied to acquire an idea of a full-scale industrial application of the process. All the products and by-products, such as metal mixtures, solid residue, and acid leachates, can be useful, which makes the reported process eco-friendly in its utilization of single-use medical products as a resource rather than discarding them as waste material.