Open-cell porous silicon is gaining attention for use as a negative electrode to increase the theoretical capacity of lithium-ion batteries. Open-cell porous silicon is prepared by eluting Mg from Mg-Si alloy in molten Bi. To reuse the expensive Bi, separation of trace Mg from Bi is indispensable. Although metallic Mg and Bi form a uniform melting state, MgCl2 floats on the Bi melt. To separate Mg from Mg-Si alloy, effects of different types of chlorinating agents, reaction temperatures, and reaction times on chlorination behavior of Mg and Bi were investigated in this study. When the alloy was heated at 600 degrees C for 21 h, as accumulated reaction time under the decomposition of NH4Cl gas, the Mg content in the sample decreased from the initial 0.9 to 0.1% and 90% of Bi was recovered as melt.