Kroll reduction: ZrCl4 + 2 Mg -> Zr + 2 MgCl2, run under argon at about 800-900 C. Molar masses: Zr 91.224, Cl 35.45, Mg 24.305, so M(ZrCl4) = 233.02 and M(MgCl2) = 95.21 g/mol.

Per tonne of Zr: m(ZrCl4) = 233.02/91.224 = 2.554 t, m(Mg) = 2 x 24.305/91.224 = 0.533 t, m(MgCl2) = 2 x 95.21/91.224 = 2.087 t. Mass is conserved: 2.554 + 0.533 = 1.000 + 2.087.

The sponge keeps trapped Mg and MgCl2; they are removed by vacuum distillation at about 1000 C. Mg vapour pressure (Clausius-Clapeyron, dH = 127.4 kJ/mol, Tb = 1363 K): P = 101325 exp[-dH/R (1/T - 1/1363)]. Impurity removal is modelled as first order, c = c0 exp(-k t) with k proportional to P(T). This is a teaching model: real retorts have heat and mass-transfer limits, Fe/O/N pickup and batch-specific kinetics. Educational only, no affiliation with any producer.