*3.2. Acidic Fusion Using KHSO4*

To study the effect of fusion on PGE recovery, the leaching parameters were set constant i.e., hydrochloric concentration 5 M, pulp density 20 mL/g, leaching time 6 h, and leaching temperature 30 ◦C.

**Figure 2.** X-Ray powder diffraction of raw materials (**a**) before and (**b**) after fusion with potassium bisulfate.

**Figure 3.** Metallographic observation on catalytic converters mounted on resin (**a**) before and (**b**) after fusion using potassium bisulfate, followed by leaching using hydrochloric acid 5 M. (Cor: cordierite matrix; PGE: platinum group element coating).

### 3.2.1. Effect of Fusion Temperature

The effect of fusion temperature was studied between 350 and 750 ◦C, with a constant fusion variable: mass ratio (KHSO4/catalytic converters powder) 2 and fusion duration 3 h. The results were depicted in Figure 4, which showed that the recovery of PGE increased as temperature increased, until it reached the optimum fusion temperature at 550 ◦C (recovery Pd 92%, Pt 50%, and Rh 78%), and, then, decreased (Pd and Rh) or became relatively constant (Pt). The increasing recovery up to 550 ◦C, then decreasing recovery, indicates the advantage of thermal decomposition of KHSO4 into K2S2O7, which oxidized the PGE present in the raw material. A higher fusion temperature caused further decomposition of potassium pyrosulfate into potassium sulfate (T > 600 ◦C) [30], which has less oxidative power than pyrosulfate. The XRD data confirmed the transformation of potassium bisulfate into potassium pyrosulfate during fusion up to 650 ◦C and partial transformation into potassium sulfate at higher temperatures, (Supplementary Materials).

**Figure 4.** The recovery of PGE related to fusion temperature.

#### 3.2.2. Effect of Mass Ratio

To assess the effect of the amount of fusion agent (KHSO4) added relative to raw materials on the PGE recovery, the fusion was carried out with varied KHSO4/catalytic converters mass ratios from 0.5 to 3. The constant fusion variables were temperature (550 ◦C) and fusion duration (3 h). The results are depicted in Figure 5. The figure shows the recovery increased as mass ratio increased until an optimum value of 2–2.5 (Pd 93%, Pt 76.5%, and Rh 77.6%), then the recovery decreased as mass ratio increased further. The decreasing recovery at higher mass ratio was probably caused by precipitation of PGE in the form of potassium salt of chloro-complexes (e.g., K2PtCl6 [31]) during leaching, due to an excess of potassium ion introduced during the fusion at higher mass ratios (reactions 6–7).

$$\text{Pt(SO}\_4\text{)}\_2 + 6\text{HCl} \rightarrow \text{H}\_2\text{PtCl}\_6 + 2\text{H}\_2\text{SO}\_4 \tag{6}$$

$$\rm K\_2SO\_4 + \rm Pt(SO\_4)\_2 + 6HCl \rightarrow \rm K\_2PtCl\_6 \downarrow + 3H\_2SO\_4 \tag{7}$$

**Figure 5.** The effect of fusion agent/catalytic converters mass ratios to PGE recovery.

3.2.3. Effect of Fusion Time

To evaluate the effect of fusion time to the PGE recovery, the fusion was conducted at a constant variable KHSO4/raw material mass ratio 2.5 and a fusion temperature of 550 ◦C, while the fusion time varied between 5 and 240 min. The results shown in Figure 6 demonstrated that the saturation value of PGE recovery could be attained within 30 min of fusion (Pd 98.9%, Pt 63.2%, and Rh 71.3%). Longer fusion time did not significantly increase the recovery.

**Figure 6.** The effect of fusion time on PGE recovery.
