**1. Introduction**

Platinum group elements (PGE) have found their use in numerous applications including catalysts, electronic components, jewelry, chemicals, and drugs [1]. Due to their essential role in future modern and green technology, and the fact that the natural resources of these elements exist only in a handful of locations (e.g., South Africa, Russia, and Canada [2]), these elements are categorized as critical elements [3]. These reasons further encourage the effort to explore new research (i.e., secondary resources through recycling processes). Aside from maintaining a steady supply, the processing of secondary resources would reduce the pressure to the environment.

One potential secondary resource for further development is catalytic converters. The majority of PGE (i.e., Pd, Pt, and Rh) would be used as catalysts in vehicles to convert hazardous gases, formed as a result of combustion, into less toxic gases, such as carbon monoxide into carbon dioxide. The content of PGE in catalytic converters varies [4] and could reach 0.5% [5], while the rest would be support, consisting of silicate (e.g., cordierite) and oxide (e.g., alumina, titanium dioxide, and silica).

Several processes had been proposed in order to extract PGE in catalytic converters, which could be grouped into a high temperature process (pyrometallurgy), a low temperature process by leaching (hydrometallurgy), or a combination of the two processes. In pyrometallurgical approaches, the most notable one is metal smelting collection and volatilization [6–11]. In the smelting method, the raw materials containing PGE would be mixed with fluxes, a reductor, and a collector and further heated until reaching the melting point. In this process, PGE would be reduced into metals and incorporated into the collector phase (e.g., Pb, Cu, and Fe). The PGE could be further separated from the collector using an electrochemistry method, while the flux would bind to the gangue phase, such as silicate. The advantages of the smelting method are the simple process of separating PGE from the gangue phase and the high recovery of PGE, with somewhat lower Rh recovery [5]. The other pyrometallurgical approach is volatilization, which transforms the PGE into chloride compounds at a high temperature and recovers as vapor/volatile compounds [12]. In general, the pyrometallurgy approach requires a considerably large energy input and may involve toxic and corrosive gases and compounds (especially chlorine and lead oxide).

In the hydrometallurgy approach, PGE were recovered by a dissolution process using a strong acid (e.g., hydrochloric acid, nitric acid, and sulfuric acid) or a combination of two or more of these acids (e.g., aqua regia), or by the addition of oxidants into these acids (e.g., iodine, bromine, chlorine, and hydrogen peroxide) [13–17], or using a complexant (e.g., cyanide) [18,19]. Aside from the direct leaching of PGE, another approach in hydrometallurgy includes upgrading the PGE in raw materials by leaching out the supporting materials. Mishra (1987) [20] successfully removed the alumina support by exploiting the amphoteric nature of alumina, which could be leached using sulfuric acid. Although hydrometallurgy could effectively deal with low grade raw materials, the consumption of hazardous and highly corrosive chemicals is substantial, due to the relatively stable nature of PGE.

Apart from the pyrometallurgical approach and the hydrometallurgical approach, there is another approach: the fusion method. This basic approach is to transform the insoluble phase into a soluble phase through reaction with an additive (fusing agent) during moderate heat treatment (in general, less than 800 ◦C), meaning a lower energy input in comparison to pyrometallurgy. The soluble phase produced could be leached in relatively mild conditions (less hazardous chemicals used compared to hydrometallurgy). Generally, the fusion method was applied for PGE upgrading (i.e., matrix decomposition) using alkalies (e.g., sodium hydroxide) [21] or an acidic fusing agent (e.g., potassium bisulfate). Subsequent leaching would remove the matrix and leave the PGE as insoluble residue for further processing stages.

Fusion using bisulfate has attracted attention from several research groups [22–25]. Batista and Afonso [22] intended to upgrade the PGE from a used catalyst by matrix (alumina) decomposition, believing that Pt would not react with bisulfate during fusion and would be accumulated in the residue. The other researchers mentioned, with the same confidence, that bisulfate alone would be ineffective to transform Pt into the soluble phase, so they combined the bisulfate with a strong oxidant (e.g., perchlorates) during fusion to make the transformation applicable. However, none of the studies cited above clearly defined the interaction between fusing agents and PGE or the interaction between fusing agents (i.e., bisulfate and chlorate).

This belief was probably rooted in the fact that PGE (especially Pt) are some of the most inert metals, and Pt is extensively used as a material of apparatus in highly corrosive conditions. For example, the standard procedure of titanium oxide decomposition using potassium pyrosulfate is carried out in a platinum crucible [26]. However, several researchers have documented that Pt could be attacked by a sulfoxide compound at an elevated temperature to form Pt sulfate salt. For example, Pt2(HSO4)2(SO4)2 was produced by reacting Pt metal and sulfuric acid at 350 ◦C [27], and [Pt(S2O7)3] <sup>2</sup><sup>−</sup> was produced by reacting Pt metal with oleum (65% SO3) at 160 ◦C [28]. In the case of Rh, researchers have successfully produced Rh2(SO4)3·2H2O by reacting Rh metal with sulfuric acid at 465 ◦C [29].

Our preliminary observation showed that potassium bisulfate alone would effectively alter Pt, Pd, and Rh (Figure 1). Based on observation on the color of the compound produced, the compounds formed were assumed to be PGE-sulfates, which were amenable for further leaching using dilute hydrochloric acid. The hydrochloric acid was chosen in order to stabilize the PGE in an aqueous solution as a chloro-complex. In our studies, the hypotheses we proposed were the transformation of

potassium bisulfate into potassium pyrosulfate (K2S2O7) during thermal decomposition and, further, the pyrosulfate as a strong oxidant would react to PGE to form PGE-sulfate, according to this reaction:

$$\text{2KHSO}\_4 \rightarrow \text{K}\_2\text{S}\_2\text{O}\_7 + \text{H}\_2\text{O} \tag{1}$$

$$\text{Pd} + 2\text{K}\_2\text{S}\_2\text{O}\_7 \rightarrow \text{PdSO}\_4 + \text{K}\_2\text{SO}\_4 + \text{SO}\_2 \tag{2}$$

$$\text{Pt} + 4\text{K}\_2\text{S}\_2\text{O}\_7 \rightarrow \text{Pt(SO}\_4)\_2 + 4\text{K}\_2\text{SO}\_4 + 2\text{SO}\_2 \tag{3}$$

$$2\text{Rh} + 6\text{K}\_2\text{S}\_2\text{O}\_7 \rightarrow \text{Rh}\_2(\text{SO}\_4)\_3 + 6\text{K}\_2\text{SO}\_4 + 3\text{SO}\_2 \tag{4}$$

**Figure 1.** PGE salts from left (Pd, Pt, and Rh) produced by reacting PGE metal powder with potassium bisulfate in a muffle furnace at 550 ◦C for 3 h.

Based on the above hypotheses, the feasibility of recovering PGE from catalytic converters, using potassium bisulfate as the sole fusing agent, followed by hydrochloric acid leaching, would be tested. The parameters investigated included: fusion temperature, mass ratio between raw materials and fusing agents, fusion duration, and leaching parameters (pulp density, HCl concentration, leaching time, and temperature). The characterizations would also be carried out to confirm the fusion and leaching efficacy using X-Ray Diffraction and metallographic observation.

#### **2. Method, Material, and Instrumentation**

#### *2.1. Material and Instrumentation*

A used catalytic converter sample was obtained from a local scrapyard in Sapporo, Japan. The honeycomb-structured converter was then ball milled and sized with a 270 mesh (53 μm) screen that was used in fusion and leaching studies. Potassium bisulfate, hydrochloric acid, nitric acid, sulfuric acid, and sodium hydroxide were obtained from Merck, Darmstadt, Germany all in analytical grade. Deionized water (MilliQ) was used throughout the experiment.

The leaching experiment was carried out in the Research Unit for Mineral Technology of Indonesian Institute of Sciences, Lampung Selatan, Indonesia. Converter sample total decomposition was carried out using alkali fusion and subsequent HCl-HNO3-H2SO4 digestion, followed by measurement using ICP-OES (Analytik Jena, Plasma Quant 9000 Elite, Jena, Germany) to determine Pd, Pt, and Rh content in the sample after the decomposition (Table 1) or in the liquid phase after the leaching test was completed, in order to calculate the recovery (*R*, %) according to Equation (5). Characterization on materials both before and after treatment was conducted using XRD (Panalytical, Expert3 Powder, Malvern, UK) in Research Unit for Mineral Technology, Indonesian Institute of Sciences. Metallographic microscopy (Reichert MEF 4M microscope equipped with AxioCam MRc5 camera from Carl-Zeiss, Oberkochen, Germany) was conducted in Dept. of Metallurgical and Materials Engineering, Colorado School of Mines.

$$R = \frac{C\_E \times V}{C\_o \times m} \times 100\% \tag{5}$$

### where:


**Table 1.** Chemical composition of PGE in used catalytic converters.


#### *2.2. Method*

Acidic fusion-leaching studies were carried out by a batch method. Typically, 0.5 g of catalytic converter powder was mixed with potassium bisulfate with certain mass ratio in a 30 mL porcelain crucible. The mixture was then introduced into a muffle furnace with a temperature higher than 350 ◦C. After the fusion was completed, the solid product was put into a conical flask, hydrochloric acid was added, and the mixture was homogenized using an orbital shaker (speed 200 rpm). After leaching was concluded, the supernatant solution was separated by centrifugation and filtration, and the PGE content was determined using ICP-OES. All fusion and leaching data was obtained in duplicates.

## **3. Results and Discussion**

#### *3.1. Characterization Results*

Characterizations were conducted on materials before and after treatment, and they were carried out using XRD combined with a metallographic analysis, in order to confirm the efficacy of the fusion and leaching process. The XRD results (Figure 2) on powdered raw materials before and after fusion (fusion temperature 550 ◦C, KHSO4/raw material mass ratio 2, and fusion time 3 h) show that the cordierite matrix of catalytic converters was partially affected by the fusion process to form potassium alum (KAl(SO4)2·12H2O). However, based on the metallographic observation of catalytic converters, both before and after fusion, and subsequent leaching using hydrochloric acid 5 M, the cordierite matrix was unaffected, while the PGE coating was clearly washed out (Figure 3). This confirms the efficacy of potassium bisulfate as a fusing agent and its capability as the sole oxidant in transforming PGE into soluble species.
