In the Pidgeon process involving a vertical pot, bonded slag pellets occasionally emerge at the bottom of the reduction pot, impeding smooth slag discharge. To reveal the formation mechanism of the bonded slag pellets, thermodynamic calculations, X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), electron probe microanalyzer (EPMA), X-ray photoelectron spectroscopy (XPS), and differential scanning calorimetry (DSC) were employed. The bonded slag pellets mainly comprise MgO, CaSi₂, CaO, and Ca₂SiO₄. CaSi₂ in the bonded slag pellets is attributed to the reduction reaction between Si and CaO, yielding liquid CaSi₂. Simultaneously, the reaction between CaSi₂ and MgO, which will typically produce Mg vapor, is inhibited, resulting in the accumulation of CaSi₂. Owing to the solid–liquid transition of CaSi₂, this process culminates in the bonding of slag pellets. This study can guide the Pidgeon process optimization, enabling mitigation of the “dead pot” issue, thereby enhancing efficiency and reducing costs.