*3.3. Acidic Leaching using Hydrochloric Acid after KHSO4 Fusion*

The effect of leaching parameters was studied at constant/optimum fusion parameters (temperature 550 ◦C, fusion agent/raw material mass ratio 2.5, and fusion time 30 min). Leaching parameters studied included pulp density, HCl concentration, temperature, and time.

#### 3.3.1. Effect of Leaching Pulp Density

The effect of pulp density in the leaching stage was studied in the range of 5 to 25 mL/g. The pulp density (volume/mass) in this case was the ratio of lixiviant added to the mass of solid phase/fusion product. The constant leaching variables were HCl concentration 5 M, temperature 30 ◦C, and duration 6 h. The results in Figure 7 show the optimum recovery was attained at pulp density 15–20 mL/g.

**Figure 7.** The effect of leaching parameter pulp density on PGE recovery.

#### 3.3.2. Effect of Hydrochloric Acid Concentration

To study the effect of hydrochloric acid as a lixiviant, the leaching temperature, leaching time, and pulp density were fixed constant at 30 ◦C, 6 h, and 20 mL/g, respectively. Hydrochloric acid concentration as an independent variable was set from 0 to 5 M. The results depicted in Figure 8 show the recovery of Pd and Rh at control conditions (HCl concentration 0/leaching with deionized water) were quite significant (Pd 56.4% and Rh 73.6%), while Pt recovery was only 12%. These results also confirmed the efficacy of potassium bisulfate as the sole fusing agent to transform PGE into a soluble PGE compound. In general, excluding Rh, the PGE recovery increased as the hydrochloric acid concentration increased; this was caused by the stabilization of PGE as chloride complexes in pregnant leach solution. The Rh recovery showed a different trend; the recovery decreased as the hydrochloric acid concentration increased. This was possibly due to the formation of a less soluble Rh chloride complex (K2RhCl5) in a higher chloride concentration compared to Rh sulfate [32], Reaction (8). In the case of Pd, the increasing recovery at the higher hydrochloric acid concentration was due to the stabilization of Pd as a chloride complex.

$$\text{Rh}\_2(\text{SO}\_4)\_3 + 2\text{K}\_2\text{SO}\_4 + 10\text{HCl} \to 2\text{K}\_2\text{RhCl}\_5 + 5\text{H}\_2\text{SO}\_4\tag{8}$$

**Figure 8.** The effect of hydrochloric acid concentration as a lixiviant on the PGE recovery.

Efficient leaching of Pt was only attained at a high concentration of hydrochloric acid. The concentration of potassium seemed to play a major role in the stabilization of the Pt complex in the liquid phase. At lower HCl concentrations, Pt would tend to exist as the potassium complex K2PtCl6 (9), which has a very low solubility. As the HCl concentration increased, the Pt complex would be transformed into the highly soluble species H2PtCl6 (10), since potassium concentration in the liquid phase would decrease, due to the formation of KHSO4 (11), which has a very low solubility.

$$\text{Pt(SO}\_4\text{)}\_2 + \text{K}\_2\text{SO}\_4 + 6\text{HCl} \rightarrow \text{K}\_2\text{PtCl}\_6 \downarrow + 3\text{H}\_2\text{SO}\_4 \tag{9}$$

$$\text{K}\_2\text{PtCl}\_6 + 2\text{HCl} \rightarrow \text{H}\_2\text{PtCl}\_6 + 2\text{KCl} \tag{10}$$

$$\rm K\_2SO\_4 + \rm H^+ \to \rm KHSO\_4 \downarrow + \rm K^+ \tag{11}$$

Figure 9 represents the recovery of PGE based on two leaching variables: HCl molarity (1, 2 and 5 M) and leaching temperature (30, 50 and 80 ◦C), with constant variable pulp density and leaching time (20 mL/g and 1 h, respectively). The temperature clearly increased the recovery of PGE, which was due to increasing solubility of PGE complex in liquid phase during acidic leaching. This was pronounced in the case of Pt.

#### 3.3.3. Effect of Leaching Time

The effect of leaching time (0–60 min) was studied at various temperatures (30, 50, and 80 ◦C) while hydrochloric acid concentrations and pulp density were set constant at 5 M and 20 mL/g, respectively (Figure 10). It is demonstrated that the leaching reaction occurred fast, and the maximum recovery was attained within 20 min.

Figure 10 also shows that the saturation value of recovery increased, as a result of increasing the leaching temperature. In the case of Pt, the leaching efficiency reached 99.9% ± 2.8% within 5 min at 80 ◦C, while, for Rh, the recovery increased from 87.2% ± 1.4% (50 ◦C) to 101.4% ± 6.8% (80 ◦C). For Pd, the effect of increasing temperature was less significant than it was for Pt and Rh.

**Figure 9.** PGE recovery as a function of different hydrochloric acid concentrations and leaching temperatures (**a**) Pd, (**b**) Pt, and (**c**) Rh.

**Figure 10.** The effect of leaching duration and temperature on PGE recovery (**a**) Pd, (**b**) Pt, and (**c**) Rh at a constant hydrochloric acid concentration.

#### **4. Conclusions**

It was demonstrated that potassium bisulfate as the sole fusing agent effectively transformed PGE in catalytic converters into species which were amenable for mild condition leaching processes (HCl < 5 M, T < 80 ◦C). The advantages of using potassium bisulfate for PGE recovery were lower energy, lower chemical consumption, and selectivity, since potassium bisulfate only reacted with PGE (it did not react with cordierite). This was confirmed by XRD and metallography characterization results.

Generally, all three PGE showed the same trend according to all fusion parameters. The optimum recovery of PGE was achieved at a fusion temperature of 550 ◦C, a fusion agent-catalytic converters mass ratio of 2.5, and a fusion time within 30 min. In the case of leaching, Pd and Pt showed the same trend, which was that the increase of leaching temperature and HCl concentration would be beneficial. In this case, the recovery of Pd and Pt were 106% ± 1.7% and 93.3% ± 0.6%, respectively. On the other hand, Rh exhibited a different trend, which was that when leaching at room temperature, the increase of HCl concentration led to a recovery decline. However, at higher leaching temperatures, the trend followed the other PGE. At the optimum conditions (temperature 80 ◦C and HCl 5 M), the Rh recovery reached 94.3% ± 3.9%. A study on the effect of leaching time generally demonstrated that maximum recovery was attained within 20 min.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2075-4701/10/4/485/s1, Figure S1: XRD profile obtained from heated KHSO4 powder at 350, 450, 550, 650 and 750 ◦C.

**Author Contributions:** E.P. contributed in conceptualization, methodology, funding acquisition, investigation, data analysis and manuscript writing and review. C.A. contributed in conceptualization, funding acquisition, resources and manuscript review. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded and supported by the Indonesian Institute of Sciences through the "Penelitian Mandiri" scheme FY 2019 and the Fulbright Program through Indonesian Visiting Scholar 2019.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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*Correction*
